Language：English   Publishing type：Research paper (scientific journal)   Publisher：Geological Society of America  

<jats:p>Among S-, I-, and A-type granites, the latter are the most debated in terms of origin, and metasedimentary crust is usually discarded as a potential source. Here we tackle this issue by adopting an in-source perspective, rather than focusing on the final product (granite), documenting the occurrence of pristine melt inclusions (MIs) in garnet from residual metapelitic ultrahigh-temperature (UHT) granulite from East Antarctica. Coexistence of sapphirine + quartz, phase equilibria calculations, and Zr-in-rutile thermometry indicate that MIs trapped UHT melts formed at peak conditions (930–1000 °C) from a residual metapelitic source. MIs are granitic with weakly peraluminous to weakly peralkaline affinity, ferroan character, high alkali contents, high K/Na and Ga/Al, and low Ca, Ba, Sr, and H2O concentrations. These features and geochemical modeling indicate that MIs represent primary melts for high-SiO2 A-type granites. Therefore, MIs reveal the missing link between A-type granites and the hottest metasedimentary crust. Voluminous amounts of slightly peraluminous, high-SiO2 A-type granites can be produced in large, residual UHT terranes such as those of eastern Gondwana. Our results provide a wider view of processes responsible for granite formation and show that a larger variety of granites must be considered in models of the effects of UHT anatexis on crustal differentiation.</jats:p>

DOI： 10.1130/g51097.1

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Publishing type：Part of collection (book)   Publisher：Geophysical Monograph Series, Wiley  

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Publishing type：Research paper (scientific journal)   Publisher：Frontiers Media SA  

Introduction: Metadolerite dykes in the Western Dharwar Craton represent the oldest generation of mafic dyke swarms in the craton. The emplacement of these dykes after a period of crust building activity and komatiite volcanism, helps to understand the evolution of Subcontinental Lithospheric Mantle (SCLM) and Archean dynamics.

Methods: We report whole rock major, trace element geochemistry and Sr-Nd isotope characteristics for this weakly metamorphosed suite of dykes. Remnant igneous textures and mineralogy are well preserved.

Results: The trace and rare earth element concentrations and an overall flat pattern suggests depleted mantle source for these dykes. Three groups are primarily identified: Group one with initial ⁸⁷Sr/⁸⁶Sr ratios varying between 0.70041 and 0.70102, Group two dykes and Group three dykes with initial ratios 0.70045–0.70154, and 0.70041–0.70153 respectively. Group one dykes show a good Rb-Sr isochron relationship and an errorchron age of ca. 3,003 ± 102 Ma is obtained. The initial ¹⁴³Nd/¹⁴⁴Nd ratios varies from 0.508,245 to 0.509,172. The epsilon Nd values are mostly negative, ranging between −12 and +5. Group one and two show an epsilon Nd value ranging between −1 and +5 and 0.1 to +5 respectively and group three varies between −0.5 and −12.

Discussion: The geochemical characteristics suggest that the group one dykes are derived from a homogenous depleted SCLM source, group two formed by a lower degree of partial melting of a source mantle with enriched components. Group three may have formed from a progressively enriched group one source. All these dykes can be considered as exposed remnants of feeders for the greenstone volcanism in the Western Dharwar Craton.

DOI： 10.3389/feart.2023.1153003

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Authorship：Last author,　Corresponding author   Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

DOI： 10.1016/j.gr.2022.12.005

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Japan Association of Mineralogical Sciences  

DOI： 10.2465/jmps.230125

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Elsevier {BV}  

DOI： 10.1016/j.precamres.2022.106882

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Authorship：Last author,　Corresponding author   Publishing type：Research paper (scientific journal)   Publisher：GeoScienceWorld  

Abstract

Komatiites, greenstone volcanic rocks, and mafic dyke swarms are constituents of early earth magmatic activity, crucial for understanding the chemical evolution of the Archean mantle. The composition of the subcontinental lithospheric mantle (SCLM) is systematically modified throughout the Earth’s history by the addition of geochemically diverse oceanic and continental crustal materials through subduction and can be sampled through intraplate mafic/ultramafic volcanic activities. Here, we present a first report on the multiple sulfur isotope characteristics of the mafic dyke swarms and komatiites from the Dharwar craton in southern India and discuss the geochemical modifications of SCLM through crustal recycling. δ34SV-CDT values of the samples are all negative ranging from -0.15 to -2.91‰. Δ33S values for all the samples are close to 0 with the lowest value of -0.060‰ and highest of 0.146‰. Δ36S values are mostly negative with very few exceptions, ranging from -1.184 to 1.111‰. Near zero values of Δ33S and negative values for δ34S indicate an early formed mantle reservoir with a possible mixture of sulfur from subducting oceanic sediments. Together with trace element geochemistry, we suggest a depleted MORB source mantle (DMM) modified by oceanic crustal components and a depleted mantle (DM) modified by recycled continental crustal sediments as the two end members of the mantle source that produced mafic dyke swarms in the Late Archean to Proterozoic Dharwar craton.

DOI： 10.2113/2022/4679300

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The Lomagundi (-Jatuli) event, characterized by extremely high positive global inorganic carbon isotope excursion at about 2.2 billion years ago, is pivotal in investigating the causes and consequences of great oxygenation event, inventory and sequestration of carbon on the Earth's surface, evolution of life, and more profoundly tectonic control on Earth's environment. However, the reasons that caused the isotopic excursion are not resolved yet. Herein, we report the discovery of meta-carbonate rocks with distinct positive carbon isotopic excursion from the Paleoproterozoic continental collision zone of the Kongling Complex, South China Craton. The δ13CV-PDB values for meta-carbonate rocks show positive values in the range from +5.5‰ to +11.6‰, whereas the δ13CV-PDB values of associated graphite deposits range from −25.8‰ to −9.5‰. Zircon U-Pb-Hf isotopes from zircon-bearing meta-carbonate sample yielded weighted average 207Pb/206Pb age of 2001.3 ± 9.5 Ma, with corresponding εHf(t) range from −7.05 to −3.16, comparable to the values of local 2.9–2.6 Ga basement rocks. Geochemical characteristics of meta-carbonate rocks, such as their rare earth element patterns and the trace element parameters of La, Ce, Eu, and Gd anomalies and Y/Ho ratio, suggest that the carbonate deposition took place in passive continental margin in association with large volumes of organic carbon. The extensive graphite deposits from Kongling Complex in South China Craton, their equivalents in the North China Craton and elsewhere across the globe prove that the burial of 12C-enriched organic carbon has eventually resulted in the global enrichment of 13C in the atmospheric CO2, which is recorded in the marine carbonate rocks. Isotopic mass balance estimates indicate that more than half of the organic carbon was buried during the oceanic closure. Hence, the observed global shift could be directly related to the continent collision event in greater China, thus resolving the long-standing paradox of the Lomagundi global positive carbon isotope excursion. Moreover, the present results suggest that orogenesis play a significant role in sequestration of carbon into the continental crust.

DOI： 10.1016/j.gsf.2022.101409

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The changes in the physicochemical conditions of the carbonaceous material during progressive metamorphism in metapelitic rocks is widely used as a geothermometer with the aid of Raman spectroscopy. However, the application of this technique in carbonate rocks has not been established yet. Here, we compare Raman spectroscopy of carbonaceous material thermometry and carbon isotope thermometry in low- to medium-grade metacarbonate rocks from the Archean Chitradurga Schist Belt in the Dharwar Craton, India. The carbonates in the lowermost Bababudan Group have metamorphosed under lower amphibolite facies metamorphic conditions giving consistent estimates for both Raman spectra of carbonaceous material thermometry (460–592 °C) and carbon isotope thermometry (450–560 °C). Contrastingly, in the Vanivilas Formation, the carbonaceous material with very fine flaky morphology in the grain boundary has slightly lower crystallinity, when compared to the coarse-grained ones near the vein boundary. Nevertheless, the carbon isotope thermometry estimated a lower temperature around 400 °C. The inconsistencies between the temperature estimates are ascribed to the pervasive post-metamorphic aqueous hydrothermal fluid infiltration indicated by the presence of numerous criss crossing quartz veins. This is also corroborated by the lowering of the oxygen isotope ratio of the carbonates near the vein boundary by 2.3‰, but without much difference in the carbon isotope ratios. The coarsening of the carbonaceous material near the vein boundary signifies fluid assisted recrystallisation, that enhanced the crystallinity as evidenced in the Raman spectra. Moreover, the negative δ13C shift in the carbonaceous material (c. –8.5 to –13‰) was possibly due to recrystallization following partial CO2 degassing. In the Chitradurga Schist Belt, the Bababudan Group and the Vanivilas Formation have regionally metamorphosed under lower amphibolite facies condition, possibly related to the widespread granitic intrusion at c. 2.61 Ga and later affected by hydrothermal event at c. 2.5 Ga.

DOI： 10.1016/j.precamres.2022.106656

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DOI： 10.2475/02.2022.02

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Authorship：Last author   Publishing type：Research paper (scientific journal)   Publisher：Springer Science and Business Media LLC  

<title>Abstract</title>More than one teramole of carbon per year is subducted as carbonate or carbonaceous material. However, the influence of carbonation/decarbonation reactions on seismic activity within subduction zones is poorly understood. Here we present field and microstructural observations, including stable isotope analyses, of carbonate veins within the Higuchi serpentinite body, Japan. We find that the carbon and oxygen isotope compositions of carbonate veins indicate that carbonic fluids originated from organic materials in metasediments. Thermodynamic calculations reveal that carbonation of serpentinite was accompanied by a solid volume decrease, dehydration, and high magnesium mobility. We propose that carbonation of the mantle wedge occurs episodically in a self-promoting way and is controlled by a solid volume contraction and fluid overpressure. In our conceptual model, brittle fracturing and carbonate precipitation were followed by ductile flow of carbonates and hydrous minerals; this might explain the occurrence of episodic tremor and slip in the serpentinized mantle wedge.

DOI： 10.1038/s43247-021-00224-5

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Other Link： https://www.nature.com/articles/s43247-021-00224-5

Authorship：Last author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：Wiley  

We report two new eclogite localities (at Kanayamadani and Shinadani) in the high-P (HP) metamorphic rocks of the Omi area in the western most region of Niigata Prefecture, Japan, which form part of the Hida Gaien Belt, and determine metamorphic conditions and pressure–temperature (P–T) paths. The metamorphic evolution of the eclogites is characterized by a tight hairpin-shaped P–T path from prograde epidote–blueschist facies to peak eclogite facies and then retrograde blueschist facies. The prograde metamorphic stage is characterized by various amphibole (winchite, barroisite, glaucophane) inclusions in garnet, whereas the peak eclogite facies assemblage is characterized by omphacite, garnet, phengite and rutile. Peak P–T conditions of the eclogites were estimated to be ~600°C and up to 2.0 GPa by conventional cation-exchange thermobarometry, Ti-in-zircon thermometry and quartz inclusion Raman barometry respectively. However, the Raman spectra of carbonaceous material thermometry of metapelites associated with the eclogites gave lower peak temperatures, possibly due to metamorphism at different conditions before being brought together during exhumation. The blueschist facies overprint following the peak of metamorphism is recognized by the abundance of glaucophane in the matrix. Zircon grains in blueschist facies metasedimentary samples from two localities adjacent to the eclogites have distinct oscillatory-zoned cores and overgrowth rims. Laser ablation inductively coupled plasma mass spectrometry U–Pb ages of the detrital cores yield a wide range between 3,200 and 400 Ma, with a peak at 600–400 Ma. In the early Palaeozoic, proto-Japan was located along the continental margin of the South China craton, providing the source of the older population of detrital zircon grains (3,200–600 Ma) deposited in the trench-fill sediments. In addition, subduction-related magmatism c. 500–400 Ma is recorded in the crust below proto-Japan, which might have been the source for the younger detrital zircon grains. The peak metamorphic age was constrained by SHRIMP dating of the overgrowth rims, yielding Tournaisian ages of 347 ± 4 Ma, suggesting subduction in the early Carboniferous. Our results provide clear constraints on the initiation of subduction, accretion and the development of an arc-trench system along the active continental margin of the South China craton and help to unravel the Palaeozoic tectonic history of proto-Japan.

DOI： 10.1111/jmg.12564

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Authorship：Lead author,　Corresponding author   Publishing type：Research paper (scientific journal)   Publisher：Elsevier {BV}  

In this contribution we review the possibility of establishing the depositional age and tectonic settings of metamorphosed carbonate rocks from continental collision zones in the East African-Antarctic Orogen. The geochemical characteristics of regionally distributed meta-carbonate rocks from the Highland Complex (HC) in Sri Lanka are considered in detail and compared with similar occurrences in East Antarctica, India, Madagascar and Africa. The variations seen in the Highland Complex of Sri Lanka imply that carbonate deposition was younging from west to east, spanning apparent ages from Mesoproterozoic to Neoproterozoic. In the case of East Antarctica, such variations are within the Neoproterozoic, whereas in southern India, Madagascar and Mozambique they have a broader age range possibly from the Paleoproterozoic to Mesoproterozoic. There is also clear evidence that some carbonates were deposited in an open ocean surrounding volcanic islands in the Mesoproterozoic. Shale-normalized REE patterns have typical signatures of open ocean deposition in a passive continental margin with variable continental input in platforms nearby to island arcs. In comparison to Phanerozoic equivalents, the absence of a Ce anomaly is most significant, whereas other parameters such as (Pr/Yb)SN, (Pr/Tb)SN, and (Tb/Yb)SN were used to evaluate relative enrichments of the LREE, MREE and HREE fractions that are characteristic of ambient seawater. Pronounced La, and Y anomalies with minor Eu and Gd anomalies and correlations of REE parameters and anomalies with carbon and oxygen isotopes, 87Sr/86Sr initial ratios and εNd values are evaluated for meta-carbonate rocks in the Proterozoic collision zone. The εNd values and Sr initial ratios suggest that basins in the western Mozambique Ocean that separated the East Gondwana from West Gondwana received contributions from Archean continental crust and ambient seawater, whereas the eastern Mozambique Ocean had REE contributions from specific cratonic continents in passive margins or from continental/volcanic island arcs in active margins.

DOI： 10.1016/j.gr.2021.03.013

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER  

Detailed geological mapping along with structural and geochemical analysis were carried out in the Kibbanahalli Schist Belt (KSB) of the Bababudan Group in the Western Dharwar Craton (WDC). Repeated sequences of sedimentary rocks and volcanic rocks with conformable contact relations are the major lithological association in KSB. Sedimentary structures and field relations indicate that the Bababudan Group begins with basal conglomerate composed of debris (talus) and alluvial fan deposits or fluvial deposits. Volcanic rocks preserve features of explosive volcanism in an aqueous environment. Based on the field and microstructural observations five stages of deformations (D0, D1, D2, D3, and D4) are characterized from the study area. Among these deformation events, D2 is the major regional-scale event. East dipping NW–SE trending thrust faults and F2 parallel folds that are upright and isoclinal mark the D2 event. These F2 folds are sandwiched between D2 thrust faults. D2 structures suggest the temporal evolution of a fold-and-thrust belt by inversion of an intracratonic basin. Trace element geochemical analysis of the mafic to intermediate rocks from KSB and Chitradurga Schist Belt (CSB) shows three types of patterns. Type 1 has Nb–Ta negative anomalies, Type 2 is relatively flat trend and Type 3 has Zr–Hf positive anomalies. Type 1 has slightly enriched LREE trend while relatively flat LREE trend in Type 2 and 3. HREE trend is relatively flat in all samples, except for one sample from Type 3. The geochemical variations are related to the degree of crustal contamination experienced during their eruption in connection with the intracratonic extension. A pair of thrust faults and folded sequences of volcanic, volcanoclastic and sedimentary rocks summarize the major structural association in the study area. This association is identical to an inverted failed rift structure. Geochemical evidence also points to an intracratonic mafic to intermediate volcanism. Our study thus provides a well-preserved example of Archean failed rift. We thus propose the post-3.0 Ga crustal growth throughout the Western Dharwar Craton is therefore initiated by similar failed rifting events.

DOI： 10.1016/j.lithos.2021.106020

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER  

Mafic dyke swarms are one of the major geologic features that represent crustal extension and rifting episodes which result in the transfer of large volumes of mantle materials to the crust. The rapid emplacement of dykes within a short span of time helps to preserve the composition of subcontinental lithospheric mantle (SCLM) and understand its evolution over time. Here we report major and trace element geochemistry and Sr-Nd isotopic characteristics of dykes from the Tiptur area in the late Archean Western Dharwar craton of southern India. The studied mafic dykes are un-metamorphosed and preserve pristine igneous textures and mineralogy. Three different groups of dykes were identified and categorized as olivine dolerites, dolerites and REE enriched dolerites, based on the distinct petrographical and geochemical characteristics. Olivine dolerites are having a high Mg# suggesting a lower degree of fractionation and is indicative of a less evolved magma. Dolerites have comparatively lower Mg# than the olivine dolerites indicating that the former have undergone significant fractional crystallization as confirmed by the higher Ni contents. The low initial Sr isotopic ratio for olivine dolerites (87Sr/86Sr = 0.70088) with negative epsilon Nd values are suggesting a late Archean enriched subcontinental lithospheric mantle (SCLM) in the source region. The dolerites (87Sr/86Sr = 0.70132 to 0.70327) and REE enriched dolerites (87Sr/86Sr = 0.70405 to 0.70653) show signatures of a more enriched source. The evolution of SCLM from a DMM source enriched by the subduction of the mafic oceanic crust in the late Archean to a mantle enriched by continent-derived sediments towards the early Proterozoic are reflected in the dolerite and REE enriched dykes.

DOI： 10.1016/j.lithos.2021.106023

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)  

The Renge Belt is a rare subduction-related metamorphic belt that contains blocks of eclogites, meta-gabbros, schists and other rocks in a serpentinite mélange that formed in the early-to-middle Paleozoic of proto-Japan. We report here a detailed geochemical and Sr–Nd isotope study of two major groups of rocks within the Omi serpentine mélange: 1. medium-grade metamorphosed blocks of meta-gabbros, and 2. high-pressure low-temperature eclogites and mafic schists. Bulk rock geochemistry of the meta-gabbros reveals that they have prominent Nb, Ta and Ti negative anomalies, suggesting an arc origin. In contrast, the eclogites, and mafic schists have trace element spider diagrams and REE patterns that resemble modern oceanic crust. The Sr–[sbnd]Nd isotopic compositions also show concomitant variations with a highly depleted source for the meta-gabbros (εNd values of +1.3 − +9.6 and Sriratios of 0.70301–0.70460) compared with a MORB/OIB-like source of the eclogites and mafic schists (εNd values of −1.94 – +4.83), although the latter have Sr isotopes variably affected by seawater interaction (Sriratios of 0.70518–0.71050). Altogether, the geochemical features suggest that the meta-gabbros were part of a magmatic arc, which developed in the Cambrian and was tectonically eroded and subducted together with oceanic crust and sediments in the Devonian–Carboniferous. The ages of detrital zircons in the pelitic schist (3200–600 Ma) suggests that the South China Craton and northeastern fringes of Gondwana were potential source regions and that the sedimentation occurred most likely in a trench. The abundant Early Paleozoic (peak around ca. 470 Ma) detrital zircons were derived from an active subduction-related arc. The protoliths of meta-gabbros are lower crustal equivalents of an infant arc, which was subducted via subduction erosion in the Devonian–Carboniferous and reached a HP metamorphic depth in the Tournasian. Our results suggest that the formation of proto-Japan was related to the initiation of intra-oceanic arc as early as Cambrian. Following tectonic erosion, the remnant mafic rocks were amalgamated with trench-fill sediments during incorporation in an accretionary complex. Subduction erosion was active where sediment supply was low and where the arc was immature. Subduction of oceanic crust was continuing to at least the middle Carboniferous, when the eclogites and blueschists were formed and later exhumed along with the meta-gabbros in serpentinite diapirs in the mid–late Paleozoic.

DOI： 10.1016/j.lithos.2021.106260

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Publishing type：Research paper (scientific journal)   Publisher：Wiley  

ABSTRACT

Nine metacarbonate layers from the regionally metamorphosed terrane of the Sør Rondane Mountains in the Eastern Dronning Maud Land in East Antarctica were examined in detail for constraining the thermal events using carbon isotope exchange between dolomite/calcite and graphite. Equilibrium carbon isotope fractionation between dolomite and graphite suggested peak metamorphic temperature conditions reaching up to 802°C ± 29°C were estimated at the Balchenfjella locality, where multiple samples from six thick layers of metacarbonate rocks were examined. However, some of the samples exhibit lower carbon isotope fractionation reflecting the possibility of ultrahigh‐temperature metamorphic conditions, which is consistent with recent reports. Furthermore, several metacarbonate rock samples display large variations in δ¹³C_VPDB values for graphite grains, despite dolomite and calcite showing homogeneous carbon and oxygen isotopic composition indicating signatures of retrograde metamorphism and fluid infiltration events. Detailed textural observation suggested alteration of δ¹³C_VPDB values of graphite during retrograde metamorphism might have resulted due to the overgrowth of graphite crystals by the infiltration of low δ¹³C_VPDB‐bearing fluids, the extent of alteration being a direct function of the fluid–rock ratio. Field evidence indicates the presence of carbonate veins cutting across the metacarbonate rocks suggesting that carbon isotope thermometry can also be utilised to understand the effect of external fluid infiltration. At Perlebandet locality the metamorphic temperature conditions were estimated to be around 915°C, whereas those from Tanngarden and Menipa gave lower temperature estimates. Detailed textural analysis of graphite in combination with isotopic composition provided clear evidence for retrograde events. Thus, our results provide tight constraints of peak and post‐peak metamorphic temperature conditions and a regional thermal structure for the Sør Rondane Mountains and further testify the usefulness of carbon isotope thermometry in polymetamorphic terrains.

DOI： 10.1002/gj.5095

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1080/08957959.2023.2297680

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Publishing type：Research paper (scientific journal)   Publisher：Copernicus {GmbH}  

DOI： 10.5194/egusphere-egu22-13148

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Petrological, geochemical and geochronological studies of granitic rocks from the Karagwe–Ankole Belt (KAB) have helped to establish the tectonic setting of the orogenic belt. Four categories of granitic rocks have been recognized, providing constraints on the geodynamics and evolution of the KAB. Anorogenic (A-type) garnet-biotite granite was emplaced at 1372 ± 5 Ma along deep, weak crustal structures. Its major, trace and rare earth element composition, TDM model age of 2.33–1.96 Ga, high initial 87Sr/86Sr (0.71193–0.72159) and negative ƐNd (−1.9 to −4.0), indicates derivation from a much older mafic source, probably originated from lower-crustal granulitic residue or lower continental lithospheric mantle contaminated by crustal material. Ascent of the small-batch A-type magma was aided by a localized crustal-scale zone of weakness within Archaean crust underlying both the Eastern and Western Domains of the KAB. The rest of the granites have S-type orogenic characteristics. Large batholiths of two-mica granite emplaced at 1369 ± 5 Ma have major, trace and rare earth element compositions, high initial 87Sr/86Sr (0.70808–0.71795), strongly negative ƐNd (−9.4 to −10.1) and a TDM model age of 2.82–2.40 Ga, indicative of derivation by partial melting of a pelitic source containing Palaeoproterozoic and Archaean components, possibly with a minor contribution from magma of lower crustal derivation. This magmatism occurred during the amalgamation of the Congo and Tanzania cratons at an early stage of Rodinia formation. Muscovite granite and leucogranite, derived from fluid-fluxed partial melting of metapelites, were emplaced at 1011 ± 18 to 976 ± 11 Ma in a late to post-collisional setting during final Rodinia formation. Their high initial 87Sr/86Sr (0.72893–0.74456) and strongly negative ƐNd (−6.8 to −12.4) indicate a supracrustal source. One younger S-type leucogranite (614 ± 9 Ma) was emplaced in a volcanic-arc setting during the assembly of West Gondwana. The combined available evidence points to tectonic evolution of the Mesoproterozoic KAB during accretion-collisional tectonism.

DOI： 10.1016/j.lithos.2021.106490

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The hadal zone at trenches is a unique region where forearc mantle rocks are directly exposed at the ocean floor owing to tectonic erosion. Circulation of seawater in the mantle rock induces carbonate precipitation within the deep-sea forearc mantle, but the timescale and rates of the circulation are unclear. Here we investigated a peculiar occurrence of calcium carbonate (aragonite) in forearc mantle rocks recovered from ~6400 m water depth in the Izu–Ogasawara Trench. On the basis of microtextures, strontium–carbon–oxygen isotope geochemistry, and radiocarbon analysis, we found that the aragonite is sourced from seawater that accumulated for more than 42,000 years. Aragonite precipitation is triggered by episodic rupture events that expel the accumulated fluids at 10−2–10−1 m s−1 and which continue for a few decades at most. We suggest that the recycling of subducted seawater from the shallowest forearc mantle influences carbon transport from the surface to Earth’s interior.

DOI： 10.1038/s43247-021-00317-1

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Oxford University Press (OUP)  

DOI： 10.1093/petrology/egab068

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To further constrain the tectonic evolution of the convergent margin of the West Mongolian segment in the Paleo–Asian Ocean, we have investigated coarse-grained kyanite metapelites associated with quartz veins in the Gobi Altai Mountains, Mongolia, where the metamorphic belt is in a thrust contact with a Paleozoic accretionary complex. The metapelites contain coarse-grained granoblastic kyanite grains that are surrounded by sillimanite (Sil-II) with moat-like andalusite (And-II). The garnet grains in the garnet-bearing kyanite metapelites contain inclusions of andalusite (And-I) in their cores and sillimanite (–Sil-I) in their mantles. Detailed textural characterization of the aluminosilicate phases indicates five distinct generations in the sequence andalusite (And-I), sillimanite (–Sil-I), kyanite (Ky), sillimanite (Sil-II), and andalusite (And-II). Thermobarometric calculations and thermodynamic modeling suggest a hairpin-shaped P–T evolution with prograde conditions starting from ~530 °C at 3.5 kbar, reaching a peak of 600 °C at 6 kbar, and then a retrogression to ~550 °C at 4 kbar. U-Pb zircon and U–Th–Pb monazite ages indicate that the multistage aluminosilicate formation has occurred during c. 260–245 Ma. The detrital zircon age distribution is essentially similar to those in the accretionary sedimentary rocks from the entire Altai Range (a sharp peak at 550–490 Ma and a broad peak at 900–700 Ma), but is characterized by more abundant Proterozoic zircon grains and an older maximum deposition age (Early Ordovician) compared with Devonian pelitic gneisses in the Mongolian Altai. These petrological and geochronological results indicate the following development of the orogen: (1) Subduction of an oceanic plate beneath a microcontinental block as early as 550 Ma, evidenced by the youngest peak of detrital zircon ages. (2) Development of the accretionary wedge and volcanic front due to maturation of the arc until c. 360 Ma. The development of the volcanic front due to the magmatism has provided abundant Paleozoic detritus to the accretionary wedge and has stemmed supply of Proterozoic detritus from the inland at the same time. (3) Burial of the youngest accretionary wedge at c. 360–350 Ma, which is not observed in the current samples. Ridge subduction has also been suggested to have occurred in this period. (4) Arc magmatism in the upper (older) part of the accretionary wedge during the late Permian that resulted in prograde high-T and low-P metamorphism (formation of And-I). (5) Burial of the heated accretionary wedge to a depth of ~20 km due to subsequent collision (formation of Sil-I and Ky) at c. 260 Ma. (6) Rapid exhumation of the buried accretionary wedge to a shallow depth by thrusting (formation of Sil-II and And-II) at c. 245 Ma. In summary, the metapelites in the Altai Range clearly preserve the evolution of both the accretion and collision events as well as their transformation processes, which are likely to be a good natural example of the Wilson cycle for a period of ~300 Myr.

DOI： 10.1016/j.lithos.2021.106362

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The composition and configuration of possible Archaean supercontinents remain unresolved. Kenorland, a Neoarchaean supercontinent containing the Southern Granulite Terrane (SGT) in South India, the eastern block of the North China Craton (E-NCC), and the north-central Korean Peninsula, was probably assembled at ca. 2.5 Ga. A detailed comparison of meta-granitoid samples from the Madras Block (SGT), the Yishui Terrane (Shandong Peninsula, E-NCC), and Daeijak Island (NW-Gyeonggi Massif, Korean Peninsula) demonstrates their close similarities in geological setting, age, petrochemistry, isotopic composition and metamorphic history. They were all formed at 2.6–2.5 Ga and metamorphosed at a high grade soon after ca. 2.5 Ga. All are LREE-enriched and HREE-depleted, have low 87Sr/86Sri (0.70201–0.70375) and similar near-chondritic ƐNd(T) (+1.2 to −1.9). These factors, and their close match of geological features, suggest that the three terranes were once contiguous as part of a Neoarchaean supercontinent.

DOI： 10.1016/j.lithos.2021.106294

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Publishing type：Research paper (scientific journal)   Publisher：Japan Association of Mineralogical Sciences  

This study presents first report of the sulfur isotopic compositions of carbonatites from the Mesoproterozoic Newania complex of India along with their stable C and O isotope ratios. The δ34SV-CDT (-1.4 to 2%o) and Δ33S (-0.001 to -0.13%o) values of these carbonatite samples (n = 7) overlap with the S isotope compositions of Earth's mantle. Additionally, the δ13CV-PDB and δ18OV-SMOW values of these carbonatites also show overlapping compositions to that of Earth's mantle. Based on these mantle-like stable isotopic compositions of carbonatites along with their higher crystallization temperature (~ 600 °C) compared to a hydrothermal fluid (<250 °C), we suggest that the sulfide minerals in these carbonatites were formed under a magmatic condition. The mantle like signatures in the δ34S, δ13C- δ18O, and 87Sr/86 Sr values of these carbonatites rule out possible crustal contamination. Coexistence of the sulfide phase (pyrrhotite) with magnesite in these carbonatites suggests that the sulfide phase has formed early during the crystallization of carbonatite magmas under reducing conditions. Overall restricted variability in the δ34S values of these samples further rules out any isotopic fractionation due to the change in the redox condition of the magma and reflect the isotopic composition of the parental melts of the Newania carbonatite complex. A compilation of δ34S of carbonatites from Newania and other complexes worldwide indicates limited variability in the isotopic composition for carbonatites older than 400 Ma, which broadly overlaps with Earth's asthenospheric mantle composition. This contrasts with the larger variability in δ34S observed in carbonatites younger than 400 Ma. Such observation could suggest an overall lower oxidation state of carbonatite magmas emplaced prior to 400 Ma.

DOI： 10.2465/jmps.201130e

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Japan Association of Mineralogical Sciences  

DOI： 10.2465/jmps.201130c

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Authorship：Lead author   Language：English   Publishing type：Research paper (bulletin of university, research institution)  

A new Thermo Scientific^<TM> MAT-253 isotope ratio mass spectrometer was installed at Department of Geology, Niigata University, Japan under the MEXT Grant-in-Aid for Scientific Research on Innovative Areas. The mass spectrometer was set up for carbon and oxygen isotope measurement of small volume CO_2 samples. A new inlet vacuum line was constructed for transferring standard gas, to inlet externally prepared CO_2 gas samples and transfer CO_2 gas prepared in the online carbonate reaction system. The line is evacuated using a turbo molecular pump and pneumatically activated valves controlled by an electrical control board. The vacuum line also includes a reference CO_2 gas (Working Standard) reservoir and a laboratory standard CO_2 gas (Machine Standard) reservoir for routine analyses, as well as SF_6 reference gas reservoirs. The preparation line can hold high vacuum conditions without appreciable leaks for several hours, required for the completion of a single session of sample measurements. Long-term laboratory standard measurement results are reported here, which shows an average of δ^<13>C_<(V-PDB)> value of ‒29.796 ± 0.041‰ and δ^<18>O_<(V-SMOW)> value of 3.971 ± 0.062‰ (n = 44; 1σ). During a period of one year of analysis, a drift of 0.04‰ for both carbon and oxygen is observed for the reference CO_2 gas. NIST standards (NBS-20 limestone and NBS-21 graphite) and a laboratory diamond standard were analyzed to characterize the minimum volume of sample required to obtain carbon and oxygen isotopic composition with accuracy and precision <0.1‰. We also report here the routine analytical procedure for carbonate samples (calcite/aragonite and dolomite/siderite/magnesite) and elemental carbon (graphite and diamond).

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Other Link： http://hdl.handle.net/10191/0002000226

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The petrogenesis of orbicular rocks is still enigmatic because of the unique texture and variable compositions. In this study, we focus on the textural characteristics and mineral compositions from Huangling Orbicular Granodiorite (HOG), where orbicules with rhythmic growth texture and radial growth texture exist in the same outcrop. Textural and isotopic studies of zircons from different parts of the representative orbicules show three age groups of 1961 ± 19 Ma, 890 ± 6 Ma and 852 ± 6 Ma, corresponding to the ages of protolith crystallization, melt emplacement, and input of a new magma, respectively. Combined with field relationship, Ti-zircon crystallization temperature, trace element modelling and Hf isotopic data, the HOG may be derived from high-temperature contact process that involved the partial melting of granodiorites in contact and interaction with a quartz-bearing dioritic intrusion. Textural features and mineral compositions of the studied orbicules show that different textures are related to different states of crystallization. Orbicules with a rhythmic growth texture have disequilibrium crystallization zone with feldspar An contents of 34–46 and a “W”-shape pattern (forming rhythmic layers), and near-equilibrium crystallization zone with An contents of 42–44 (forming the inner part). Orbicules with a radial growth texture was crystallization in disequilibrium, as An contents show approximate linear variations of 32–44 to form radial growth layer, and with An varies 26–46 to form the inner part. Our observations support the magma quenching hypothesis, proposed as hot orbicule melts underwent rapid cooling with inward crystallization. Moreover, we firstly suggest that the amphibole-plagioclase disequilibrium crystallization could explain different textures of orbicules. Rhythmic growth texture and radial growth texture are formed at undercooling state with fast and slow cooling rates, separately. The numbers of rhythmic layers are controlled by the duration of undercooling state, while the width of each layer is controlled by the cooling rate. If excess amphibole components left in the center of orbicule melt, the “core” will be mafic. Otherwise, the “core” will be felsic.

DOI： 10.1016/j.lithos.2021.106026

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The age-unknown Hyangsanni Dolomite is distributed in the northeastern part of the Okcheon metamorphic belt in central South Korea. It has been claimed as Early to Middle Cambrian because of the misidentified Archaeocyatha fossil in the past, but the time of sedimentation has not yet been confirmed. We performed elemental and isotopic analyses for the Hyangsanni Dolomite. The REE + Y patterns of the samples of the Hyangsanni Dolomite are generally flat and Ce negative anomaly is generally negligible to the extent that it appears only weakly in some samples. Y/Ho values are in the range of 29–43, which is fairly lower than typical Y/Ho ratio of seawater, indicating the influence of terrigenous materials. REE and trace element geochemical discrimination diagrams indicate that the Hyangsanni Dolomite was deposited in a continental margin environment rather than an open ocean. We suggest that the Hyangsanni Dolomite was deposited in a rift basin connected to the ocean rather than in the passive continental margin, considering occurrence of magmatism related to continental rifting within the Okcheon metamorphic belt. Carbon and oxygen isotope analysis was performed on dolomite powders scraped off from polished surfaces unstained with Alizarin-Red-S in order to avoid the recrystallized calcite. The Hyangsanni Dolomite has δ13C(V-PDB) values between +2.9‰ and + 6.2‰, and δ18O(V-SMOW) values between +11.3‰ and + 24.5‰. These δ13C values are markedly higher than the Cambro-Ordovician values, and are consistent with the Neoproterozoic values. The minimum value of 87Sr/86Sr ratio obtained from the Hyangsanni Dolomite also refers to Neoproterozoic, not Phanerozoic. We suggest that the Hyangsanni Dolomite belongs to the sedimentary sequence of the Neoproterozoic, along with other sediments of the northeastern Okcheon metamorphic belt.

DOI： 10.1016/j.lithos.2021.106219

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Publishing type：Research paper (scientific journal)   Publisher：University of Chicago Press  

The Vijayan Complex (VC) of Sri Lanka is a Neoproterozoic arc terrain having no convincingly proven counterpart elsewhere in the Gondwana supercontinent. Therefore, little is known about the nature of its pre-Gondwanan ancestry. We present here new elemental and Sr-Nd isotope systematics of orthogneisses to envisage the origin and evolution of the VC. Samples with high and variable Sr-initials and negative εNd 1 14 are compatible with their derivation by melting of older rocks with a long crustal residence history. Most of the studied rocks show depleted-mantle-model ages (1.4–3.3 Ga) considerably older than the ∼1 Ga magmatic crystallization age of the VC. The rest of the samples show model ages of ∼0.9–1.1 Ga, coeval with the crystallization age of the VC, indicating separation from the mantle as juvenile inputs. Therefore, protolith sources of the majority of these rocks are mixtures of distinctive components formed much earlier than the time of predominant magmatism (∼1 Ga) in the VC. Major and trace elements and rare earth element ratios divide these samples into two categories. The “Depleted Group” represents protoliths similar to Proterozoic oceanic island basalt–like mantle materials that intensively interacted/ mixed with different proportions of melts derived from juvenile and ancient lower crust. The “Enriched Group” represents mid-ocean ridge basalt–like mantle materials subsequently interacted/mixed with variable amounts of melts derived from Archean felsic and mafic crust. Geochemical-mixing modeling with87Sr/86Sr and143Nd/144Nd confirms the involvement of above multiple source components. Thus, our findings illustrate evidence for hybridization of mantle-derived melts from various juvenile and ancient crustal components. Therefore, this first finding of Paleoproterozoic to early Archean–aged components within the VC provides new insights to elucidate its pre-Gondwanan geodynamic setting, revising the previous understanding of it as an “exotic terrain” to Gondwana with only largely juvenile Neoproterozoic materials.

DOI： 10.1086/714172

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Wiley  

Felsite inclusions (FIs) in granulite garnets from continental collision orogens are supercooled felsic melt inclusions. Not only do these represent non-equilibrium textures, such as dendritic and spherulitic crystals of quartz and other minerals, but the porphyritic texture is that of hypabyssal and volcanic rocks. This paper presents representative zoned quartz phenocryst-bearing FIs in various granulites from the Highland Complex in central Sri Lanka, the Lutzow-Holm Complex in East Antarctica, the Grenville Province in southeastern Canada, and the Kerala Khondalite Belt in southern India. Quartz phenocrysts show simple and definite cathodoluminescence (CL) zoning with euhedral bright cores and dark rims and overgrowths. Euhedral quartz never develops in subsolidus metamorphic rocks except for vein quartz, and is characteristic of growth in a melt phase. Groundmass quartz surrounding quartz phenocrysts within the FIs is always dark in CL, whereas matrix quartz outside garnet and single-grain inclusion quartz in garnet are usually brighter than the cores of quartz phenocrysts. The close relationship between violet CL emission intensity and Ti content of quartz indicates different growth temperatures in harmony with the inferred crystallization sequence. The preservation of CL growth zoning in quartz phenocrysts and fine non-equilibrium textures imply that the cooling rates of some granulites are 1 to 2 orders of magnitude faster than so far presumed.

DOI： 10.1111/iar.12374

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Other Link： https://onlinelibrary.wiley.com/doi/full-xml/10.1111/iar.12374

Authorship：Last author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER  

We report the results of kinetic experiments of graphitization at various pressures (0.5–8.0 GPa) and durations (1 s to 24 h) at 1200 °C. The natural carbonaceous material in sedimentary rocks from the Shimanto accretionary complex and the Hidaka metamorphic belt, Japan, underwent systematic changes in crystallinity and morphology with increasing pressure. To assess the pressure dependence of graphitization, we adopted three approaches to formulating the graphitization kinetics using a power law rate model, a Johnson–Mehl–Avrami–Kolmogorov model, and a superposition method. Activation volumes of − 21.7 ± 3.0 to − 45.7 ± 4.5 cm3 mol−1 and − 0.7 ± 0.2 to − 16.8 ± 1.8 cm3 mol−1 were obtained for pressures from 0.5 to 2.0 GPa and 2.0 to 8.0 GPa, respectively. Such large negative activation volumes might arise from structural modification and compression in the primary carbonaceous material. We applied the experimental data to the Arrhenius-type equation of graphitization, extrapolated to geological P–T–t conditions. Our model predicts that carbonaceous material undergoing metamorphism for ~ 10 Myr at pressures of 0.5–3.0 GPa will begin to crystallize at around 350–420 °C and transform fully to ordered graphite at around 450–600 °C, depending on the peak pressure. Thus, natural graphitization might proceed much more rapidly than previously estimated, owing to the large negative activation volumes for the reaction rate. This indicates that subducted carbonaceous materials will completely convert to fully ordered graphite by rapid recrystallization and metamorphic devolatilization before reaching sub-arc depths (< 100 km).

DOI： 10.1007/s00410-020-1667-2

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Neoarchean orthogneiss, dated at 2629 ± 9 Ma by SIMS zircon U-Pb, has been discovered in the Rusizian terrane of the Western Domain of the Karagwe Ankole Belt, Rwanda, redefining the NW margin of the Tanzania Craton. Metamorphism of the orthogneiss at 1959 ± 4 Ma was associated with Paleoproterozoic granitic arc magmatism in the Rusizian terrane over the period ca. 2000–1840 Ma. The major and trace element composition of the peraluminous, calc-alkalic orthogneiss is consistent with magmatism in an arc setting at an active continental margin. Its Nd depleted mantle model age of ca. 2800 Ma is consistent with derivation by partial melting and reworking of older crust at the NW margin of the Tanzania Craton. The Paleoproterozoic thermal event was related to the accretional collision between the Congo and Tanzania Cratons during the assembly of the Nuna supercontinent. Future tectonic modelling of the Karagwe Ankole Belt must take into account the presence of Archean basement beneath the Western Domain.

DOI： 10.1016/j.precamres.2020.105851

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Brine-induced microtexture formation in upper amphibolite to granulite facies lower crust is investigated using a garnet-hornblende (Grt-Hbl) selvage developed along a planar crack discordantly cutting the gneissic structure of an orthopyroxene-bearing gneiss (central Sør Rondane Mountains, East Antarctica). The Cl contents of hornblende and biotite, K contents of hornblende and the thickness of relatively Na-rich rims of plagioclase decrease with distance from the center of the Grt-Hbl selvage (inferred position of the crack). Biotite and hornblende arrangement defining the gneissic structure can be traced into the selvage, suggesting that the wall-rock was overprinted by the selvage formation. Addition and loss of elements to the wall-rock was examined using Zr as an immobile element. Trace elements that tend to be mobile in brines rather than in melts are added to the wall-rock, indicating that the Grt-Hbl selvage was formed by the advection of NaCl-KCl brine into a thin crack. Plagioclase in the wall-rock shows a discontinuous drop of anorthite content at the rim, indicating that coupled dissolution-reprecipitation took place and the grain boundaries were once wet. Trace element concentrations in the wall-rock minerals decrease with distance from the crack, and, in most cases show exponentially decreasing/increasing profiles depending on the elements. These profiles are best modelled by a diffusion equation, suggesting that the wet grain-boundary diffusion in the wall-rock minerals controlled the observed mass transfer and resulted in dissolution-reprecipitation of mineral rims.

DOI： 10.1093/petrology/egy116

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Authorship：Last author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：Japan Association of Mineralogical Sciences  

Sri Lanka is endowed with high purity vein graphite deposits with extensive mineralization in the tectonically weakened zones of the basement high-grade rocks. Distinctly different crystal shapes of graphite are found even within a single vein and it is controversial in interpreting prevailed fluid activities and crystallization process to form such a variation. Therefore, this study was carried out to interpret the origin of vein graphite using geochemistry, crystal-morphology, and structure of the crystals. Sampling was conducted on four different depths at the Kahatagaha-Kolongaha mine, Sri Lanka. Characterizations of graphite were carried out by micro-Raman spectroscopy, X-ray diffraction spectroscopy, scanning electron microscopy, and inductive couple plasma mass spectroscopy. The results indicate that the genesis of the vein graphite is related to a single phase of fluid activity and the fluid was mostly pure with possible trace impurities. Further, it was revealed that the thickness of the veins, interaction with host rocks and mobility of the impurity elements have influenced the formation of different morphologies of graphite in a single vein.

DOI： 10.2465/jmps.170721

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Mafic dyke swarms are abundantly distributed in the Archean Dharwar craton. Previous studies have focused mainly on the major mafic dyke swarms in EDC; however, those in the WDC are yet to be studied in detail. Here we present preliminary geochemical data for the dykes in the Tiptur area, WDC and compare them with the dyke swarms in the EDC. Petrological studies indicate that the dykes in the Tiptur area fall into two distinct groups. The NW–SE trending dolerite dykes are unaltered, with characteristic ophitic textures and are geochemically comparable to 2.3 Ga EDC dykes. In contrast, the NE–SW trending meta-doleritic dykes showed high degree of alteration. The difference in petrography, major, trace and rare earth element geochemistry between the dolerites and meta-dolerites lead to a preliminary inference that these two suits of rocks might not be co-genetic. Meta-dolerites have not been reported from the EDC and it is possible to assume that they are a part of an earlier event, restricted in WDC, that might have emplaced prior to the amalgamation of WDC and EDC. In a global perspective, we compare our results with those reported in Archean cratons during late Archean to early Proterozoic around the world to constrain similarities that can lead to understanding the global scale magmatic activity and to aid in correlations between cratons.

DOI： 10.1007/s41745-018-0090-4

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Authorship：Last author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

The redox state of pseudotachylyte provides valuable information on the physicochemical properties related to fluid activity during paleoseismicity. In this study, we evaluate the behavior of fluids in pseudotachylytes using fluid buffering minerals such as graphite and other secondary minerals. Two types of pseudotachylyte were identified in the Hidaka metamorphic belt, Hokkaido, Japan; Pst I and Pst II. Pst I is composed of pyrrhotite and K-feldspar generated by the breakdown of phyllosilicates, whereas Pst II includes recrystallized biotite microlites, K-feldspar, and plagioclase due to the breakdown of quartz and feldspars, as well as phyllosilicates. The selective frictional melting of phyllosilicates is indicated by significant changes in the concentrations of large ion lithophile elements (e.g., Ba, Sr, Rb, Cs, B, and K). In addition, the graphite associated with titanite and hydroxyapatite is often distributed within the Pst II matrix. The graphite exhibits different crystallinity, morphology, and higher carbon stable isotopic compositions (δ13C = −21.9 ± 2.3‰ n = 16), when compared with metamorphic graphite extracted from the protolith (δ13C = −25.1 ± 0.8‰ n = 15), cataclasite, ultracataclasite, and Pst I (δ13C = −25.2 ± 0.8‰ n = 15). The microstructure and geochemical signatures in both pseudotachylytes are controlled by dehydration–redox reaction of phyllosilicates coexisting with sulfide minerals and graphite. In particular, the formation of 13C enriched graphite in the Pst II can be deduced by co-precipitation of fluid-deposited graphite with hydrous minerals from H2O-CH4 fluids and/or degassing of H2O-CH4 fluids from residual graphite. Our data demonstrated that the carbonic fluids of a reducing nature could be produced, and then dissolved into the pseudotachylyte melt by frictional melting of metasedimentary rocks. The production of carbonic fluids by coseismic slip may be an overlooked process in the remobilization of refractory graphite locked in the crust.

DOI： 10.1016/j.tecto.2018.07.010

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Authorship：Last author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：MINERALOGICAL SOC AMER  

We report here new experimental kinetic data on the structural evolution of carbonaceous material (CM) to graphite during heating at various temperatures (1000 to 1450 degrees C) for various durations (10 min to 115 h) under a pressure of 1 GPa. Natural CMs extracted from sedimentary rocks in the Shimanto accretionary complex and the Hidaka metamorphic belt of Japan transformed in morphology and crystallinity with increasing temperature and annealing duration to become fully ordered graphite (d(002) spacing similar to 3.36 angstrom). Transmission electron microscopy showed that both samples have undergone microstructural evolution from amorphous carbon to platy graphitic carbon. These changes match the evolution of the samples' X-ray diffraction (XRD) patterns and micro-Raman spectra. The time temperature relations of crystal parameters obtained by XRD and micro-Raman spectroscopy demonstrated a sigmoidal transformation curve from an amorphous to a graphitic structure, suggesting complexity of these successive and/or parallel chemical reactions are responsible for graphitization. To assess these complex chemical processes, we adopted three different approaches for formulating the graphitization kinetics using a power rate model, a Johnson-Mehl-Avrami (JMA) model and a superposition method. Irrespective of the models employed, the effective activation energies were estimated to lie between 259 and 339 kJ/mol, which are much lower than those reported previously for graphitization. Summarizing the previous studies and our results between 0.1 and 1000 MPa, we found that the effective activation energies systematically decrease as a function of pressure. Based on the experimental results in this study, the sigmoid functions obtained from the time temperature relations can be extrapolated to low-temperature conditions at 1 GPa. Our kinetic model using unit-cell height c predicts that CM undergoing metamorphism for about 1 m.y. will begin to crystallize at similar to 410 degrees C, and will transform to fully ordered graphite at over similar to 520 degrees C. Thus, natural graphitization undergoes a much faster transformation than reported in previous studies at 1 atm and could be explored in laboratory experiments using natural precursor materials under pressure conditions and time spans that reflect natural conditions in the Earth's crust.

DOI： 10.2138/am-2017-5733

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Japan Society of High Pressure Science and Technology  

Carbon, the fourth most abundant element in the solar system, is believed to be an important light element constituent in the Earth’s core. The high carbon content of carbonaceous chondrites (3.2 wt.%) compared to bulk earth estimates, the presence of graphite/diamond and metal carbides in Iron meteorites, the high solubility of carbon into iron melts in the Fe-C system, all suggests the plausible presence of carbon in the Earth’s core. However, the distribution of carbon isotopes in the core and deep mantle remains elusive. Newly reported experimental data and theoretical estimates on equilibrium carbon isotope fractionation between graphite/diamond and carbide phases suggests that iron carbide melt will preferentially gather C than C. These results are consistent with the carbon isotope distribution between graphite and cohenite (Fe C) observed in iron meteorites. The temperature dependent fractionation of carbon isotopes between carbide phases and elemental carbon can be an effective mechanism that might have created a C-enriched core. If the Earth’s core is a large reservoir of C-enriched carbon, then it can result in large perturbations in surface carbon cycle caused by the flux of isotopically lighter carbon from the core-mantle boundary. 12 13 12 12 3

DOI： 10.4131/jshpreview.27.278

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A metapelite preserving prograde and retrograde zoning in garnet from Mefjell, southern Sør Rondane Mountains (SRM), East Antarctica is described in detail and U-Th-Pb geochronological data are presented. Garnet, sillimanite, staurolite and biotite are in textural equilibrium in the matrix. The garnet shows three distinct compositional zones comprising core, mantle and rim. From core to the mantle, the spessartine content represents a bell-shaped profile. From mantle to the rim, spessartine content increases and pyrope decreases. Kyanite is present as tiny inclusions in the garnet core. The core also contains aggregates of sillimanite with radial cracks around them, interpreted to have resulted from the inversion of kyanite. The prograde P-T path recorded in the garnet is heating to approximately 700 °C at 5.6 kbar with a slight increase in pressure. U-Th-Pb dating of monazite grains yields a large range of age distribution between 700 and 540 Ma. The inferred prograde metamorphism of the metapelite sample in this study might be related to subduction and/or tectonic loading explained by a collision tectonic model of the SRM (Osanai et al., 2013).

DOI： 10.2465/jmps.160919

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Earth's tectonic and climatic systems may have changed fundamentally before the Great Oxidation Event (GOE) at about 2.3 Ga. Sulfur Mass Independent Fractionation (S-MIF) has demonstrated that Earth's atmosphere was virtually oxygen-free before the GOE. During 3.0 to 2.4 Ga, the change in Δ33S and Δ36S signals may reflect the perturbation of atmospheric chemistry, though the mechanisms of the change are uncertain. Here, we reported multiple sulfur isotopic studies of Archean volcano-sedimentary sequences of the Dharwar Supergroup, distributed in the Chitradurga Schist Belt (CSB), Southern India. New field mapping and zircon U–Pb dating allows us to reconstruct detailed lithostratigraphy of the Dharwar Supergroup. The lower unit consists of post-3.0 Ga conglomerate, stromatolitic carbonate, siliciclastics with diamictite, chert/BIF and pillowed basalt in ascending order, all of which are older than the 2676 Ma dacite dyke that had intruded into the lower unit. The upper unit unconformably overlies the pillow basalts at the top of the lower unit, and consists of conglomerate/sandstone with ∼2600 Ma detrital zircons, komatiitic basalt, BIF and siliciclastic sequence with mafic volcanics. Sulfur isotope analysis of extracted sulfides shows MIF signals (ΔS33>+1‰) with clear ΔS33–ΔS36 correlations. The lower group of the Dharwar Supergroup shows a ΔS36/ΔS33 slope of −1.48, the middle group shows −1.16 and −1.07, and the upper group shows −0.94. Reassessment of all the Archean S-MIF records from sedimentary rocks indicates that the ΔS36/ΔS33 slope systematically changed during the Archean period. The observed trend in the Indian section is similar to those of its Pilbara–Kaapvaal equivalents, thus it could reflect a global atmospheric signature. Moreover, the isotopic trend seems to correlate with mid-Archean glaciation. Thus, the ΔS36/ΔS33 slope could be a useful tracer for atmospheric chemistry and its link with climate change before the GOE.

DOI： 10.1016/j.epsl.2017.02.007

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Komatiites near Kapili village, located in Palaeo-Mesoachean Gorumahishani-Badampahar greenstone belt of Singhbhum Craton, Eastern India preserve excellent igneous textures and exhibit co-association of Al-depleted (ADK) and Al-undepleted komatiites (AUK). The Kapili komatiites exhibit repeated spinifex textured flows (cooling units) overlying massive cumulate zone. The basal cumulate part of Kapili komatiite is chemically similar to ADK which is characterized by subchondritic Al2O3/TiO2 (9.38–10.5) and HREE depleted nature (average (Gd/Yb)PM ∼ 1.35) elucidating presence of majorite in residuum whereas compound upper spinifex lava typically display superchondritic Al2O3/TiO2 (∼22–28.5) and HREE enriched nature (average (Gd/Yb)PM ∼ 0.81) demonstrating majorite free residuum. Calculated primary magma compositions of Kapili ADK and AUK reveal that generation of primary liquid of Kapili ADK or basal cumulate took place at greater depth (>300 km) or at higher pressure (∼9 GPa) at around 30% melting of source in contrast with primary liquid of AUK (spinifex lava), which was generated at shallower depth (∼150 km) or at lower pressure (∼6 GPa) beyond majorite stability and at higher degree of melting (∼30–40%). ‘Hump-shaped’ pattern of primitive mantle (PM) normalized REE spectra of Kapili ADKs and their primary liquid is caused by depletion of both LREE and HREE with respect to MREEs. Depletion in LREE and HREE is explained by depletion of source prior melting and sequestration of HREE in residual majorite respectively. However, Kapili AUK or spinifex lava samples show flat, near primitive mantle (REE/PM ∼ 1) pattern exclusive of any significant depletion and implying higher degree of melting (between 30 and 40%) of a PM like source. Sm-Nd isotopic studies of Kapili ADK and AUK reveal that they both were derived from a mantle source with long term depletion (εNd ∼ +2 to +4). Except differences in LREE/HREE ratio featured due to differences in melting conditions, similarity in overall immobile HREE and high field strength element (HFSE) concentrations of both the types and their corresponding primary magmas suggest a single plume source rather than multiple sources, whose composition was closer to primitive mantle and eventually segregated chemically different komatiitic melts (Al-depleted and undepleted) at different depths. Whole rock Th/Yb, Th/Nb, Zr/Nb, Nb/Y, (La/Nb)PM ratios of both Kapili AUK and AUKs are comparable to oceanic island basalt (OIB) and mid oceanic ridge basalt (MORB) similar to typical greenstone belt volcanics from oceanic provenance. In addition, presence of copious pillow basalt and oceanic chert associated with komatiites, absence of negative δNb values recommend emplacement of komatiitic lava in oceanic rather than continental or arc setting.

DOI： 10.1016/j.precamres.2017.06.014

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Utilizing microstructures of Cl-bearing biotite in pelitic and felsic metamorphic rocks, the timing of Cl-rich fluid infiltration is correlated with the pressure-temperature-time (P-T-t) path of upper amphibolite- to granulite-facies metamorphic rocks from Perlebandet, Sør Rondane Mountains (SRM), East Antarctica. Microstructural observation indicates that the stable Al2SiO5 polymorph changed from sillimanite to kyanite + andalusite + sillimanite, and P-T estimates from geothermobarometry point to a counterclockwise P-T path characteristic of the SW terrane of the SRM. In situ laser ablation inductively coupled plasma mass spectrometry for U–Pb dating of zircon inclusions in garnet yielded ca. 580 Ma, likely representing the age of garnet-forming metamorphism at Perlebandet. Inclusion-host relationships among garnet, sillimanite, and Cl-rich biotite (Cl > 0.4 wt%) reveal that formation of Cl-rich biotite took place during prograde metamorphism in the sillimanite stability field. This process probably predated partial melting consuming biotite (Cl = 0.1–0.3 wt%). This was followed by retrograde, moderately Cl-bearing biotite (Cl = 0.1–0.3 wt%) replacing garnet. Similar timings of Cl-rich biotite formation in different samples, and similar f(H2O)/f(HCl) values of coexisting fluid estimated for each stage can be best explained by prograde Cl-rich fluid infiltration. Fluid-present partial melting at the onset of prograde metamorphism probably contributed to elevate the Cl concentration (and possibly salinity) of the fluid, and consumption of the fluid resulted in the progress of dehydration melting. The retrograde fluid was released from crystallizing Cl-bearing partial melts or derived externally. The prograde Cl-rich fluid infiltration in Perlebandet presumably took place at the uppermost part of the footwall of the collision boundary. Localized distribution of Cl-rich biotite and hornblende along large-scale shear zones and detachments in the SRM supports external input of Cl-rich fluids through tectonic boundaries during continental collision.

DOI： 10.1016/j.lithos.2016.12.028

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER INDIA  

We report newly obtained U-Pb SHRIMP ages of detrital zircons from metagreywackes in the Hiriyur Formation (Chitradurga Group, Dharwar Supergroup) from the central eastern part of the Chitradurga greenstone belt. U-Pb analyses yield three major Neoarchean age populations ranging from 2.70–2.54 Ga with some minor age population of Mesoarchean. The maximum age of deposition is constrained by the youngest detrital zircon population at 2546 Ma. This is the first report of the occurrence of supracrustal rocks less than 2.58 Ga in the central part of Chitradurga greenstone belt. Close evaluation of detrital ages with the published ages of surrounding igneous rocks suggest that the youngest detrital zircons might be derived from rocks of the Eastern Dharwar craton and the inferred docking of the western and eastern Dharwar cratons happened prior to the deposition of the Hiriyur Formation. The Chitradurga shear zone, dividing the Dharwar craton into western and eastern blocks, probably developed after the deposition. Furthermore, the lower intercept is interpreted as evidence for the Pan-African overprints in the study area.

DOI： 10.1007/s12594-016-0372-2

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Authorship：Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：JAPAN ASSOC MINERALOGICAL SCIENCES  

Chemically precipitated carbonate sediments directly record seawater composition that helps to decode the Earth's paleo-environment, the existence of paleo-oceans, and provide valuable clues on the paleo-tectonic position of continents through Earth's history. In addition, the geochemical and isotopic composition of carbonate rocks have a strong dependence on the depositional tectonic setting and surrounding source rock composition. This was particularly important in the Precambrian, during which biological activity was less prominent and vegetation was virtually absent. Here we present evidence for the existence of an extinct East Antarctic Ocean and its peripheral oceanic island arc system that preceded the formation of the East Antarctic continent in the Neoproterozoic before the final assembly of Gondwana. Applying a multi-element isotope geochemical approach on chemostratigraphically well-constrained metacarbonate rocks collected from the remote Sør Rondane Mountains in East Antarctica, we present a model on carbonate deposition surrounding an island arc system, mid-ocean volcanic islands and a shallow marine continental shelf of a yet unidentified interior Antarctic continent, all of which accreted in the late Neoproterozoic to early Paleozoic to form the present day East Antarctic continent prior to the final amalgamation of Gondwana supercontinent. Our results support the presence of an oceanic island arc system that might have separated the Mozambique ocean and East Antarctic ocean.

DOI： 10.2465/jmps.151029a

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：JAPAN ASSOC MINERALOGICAL SCIENCES  

DOI： 10.2465/jmps.160420

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：JAPAN ASSOC MINERALOGICAL SCIENCES  

We report newly obtained U-Pb SHRIMP ages of zircons from the granitoids in the vicinity of Chitradurga shear zone of Dharwar craton, South India. The analyses yielded two different ages, an older age of ca. 3300 Ma and younger ages of similar to 2600-2650 Ma, those suggest two major igneous activities in the study area. The former activity contemporaneous with the formation of Mesoarchean (similar to 3.3 Ga) basement rocks (i.e., Peninsular Gneiss) and the latter reflects the Neoarchean (similar to 2.6 Ga) regional plutonic activity in the Western Dharwar craton. Field and petrographic observations suggest that the intensity of deformation is high at the boundary between amphibolites and granitoids. Our new age dates imply that the formation of Chitradurga shear zone postdated the magmatic activity in the study area and the deformation structures possibly related to the development of Chitradurga shear zone. The area on either sides of the boundary between Eastern and Western Dharwar cratons experienced a Neoproterozoic event between 700-600 Ma indicated by prominent SHRIMP lower intercepts.

DOI： 10.2465/jmps.150316

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：WILEY-BLACKWELL  

Four amphibolite facies pelitic gneisses from the western Mongolian Altai Range exhibit multistage aluminosilicate formation and various chemical-zoning patterns in garnet. Two of them contain kyanite in the matrix and sillimanite inclusions in garnet, and the others have kyanite inclusions in garnet with sillimanite or kyanite in the matrix. The Ca-zoning patterns of the garnet are different in each rock type. U-Th-Pb monazite geochronology revealed that all rock units experienced a c. 360Ma event, and three of them were also affected by a c. 260Ma event. The variations in the microstructures and garnet-zoning profiles are caused by the differences in the (i) whole-rock chemistry, (ii) pressure conditions during garnet growth at c. 360Ma and (iii) equilibrium temperatures at c.260Ma. The garnet with sillimanite inclusions records an increase in pressure at low-P (5.2-7.2kbar) and moderate temperature conditions (620-660 degrees C) at c. 360Ma. The garnet with kyanite inclusions in the other rock types was also formed during an increase in pressure but at higher pressure conditions (7.0-8.9kbar at 600-640 degrees C). The detrital zircon provenance of all the rock types is similar and is consistent with that from the sedimentary rocks in the Altai Range, suggesting that the provenance of all the rock types was a surrounding accretionary wedge. One possible scenario for the different thermal gradient is Devonian ridge subduction beneath the Altai Range, as proposed by several researchers. The subducting ridge could have supplied heat to the accretionary wedge and elevated the geotherm at c. 360Ma. The differences in the thermal gradients that resulted in varying prograde P-T paths might be due to variations in the thermal regimes in the upper plate that were generated by the subducting ridge. The c. 260Ma event is characterized by a relatively high-T/P gradient (25 degrees Ckm(-1)) and may be due to collision-related granitic activity and re-equilibrium at middle crustal depths, which caused the variations in the aluminosilicates in the matrix between the rock units.

DOI： 10.1111/jmg.12154

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：The Geological Society of Japan  

DOI： 10.5575/geosoc.2015.0035

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER  

The whole-rock Hf, Sr and Nd isotope data of five high-Fe-Ti eclogites, nine high-Al edogites, and a garnetite from the Sulu ultrahigh pressure (UHF) metamorphic terrane at eastern China were analyzed to resolve the causes for the decoupling of the Lu-Hf, Sm-Nd, and Rb-Sr isotope systems in these UHP rocks and to infer their protolith characteristics. Seven of the nine high-Al eclogites define an Rb-87/Sr-86-Sr-87/Sr-86 errorchron age of 192 +/- 43 Ma (MSWD = 2.8), which is within the time span of retrograde metamorphism despite the large uncertainty. The high-Fe-Ti eclogites and garnetite, however, have low Rb-87/Sr-86 ratios of <0.031 with scattered Sr-87/Sr-86 ratios of 0.7042-0.7058. Accordingly, it is inferred that the Rb-Sr isotope system in the samples reflects the effects of processes postdating the UHP metamorphism. Despite having different constituent mineral assemblages and whole rock geochemistry, the samples, however, define a Sm-147/Nd-144-Nd-143/Nd-144 errorchron age of 232 +/- 36 Ma (MSWD = 4.6). Although the uncertainty of +/-36 Ma implies incomplete Nd isotope equilibrium among the samples, the large overlap between this errorchron age span and the mineral isochron ages of 245-210 Ma for the UHP metamorphism indicates the control of peak metamorphism on the Sm-Nd isotope system. The incomplete Nd isotope re-equilibration was accompanied by metamorphic modification on the Sm/Nd ratios as indicated by the U-shaped LREE patterns. The initial epsilon(Nd)(780) values of the protolith rocks calculated from the Sm/Nd ratios of the samples deviate from the igneous initial epsilon(Nd)(t)-epsilon(Hf)(t) trend to significantly lower values, consistent with the metamorphic increase in the Sm/Nd ratios. In contrast, the Lu/Hf ratios are generally within the range for basalts and do not vary systematically with the Hf-176/Hf-177 ratios. The protolith epsilon(Hf)(780) values calculated from the Lu/Hf ratios of the samples are nearly identical to the initial epsilon(Hf)(t) values of the -780 Ma magmatic zircon cores from the Yangtze craton, suggesting the dominance of protolith characteristics on the Lu-Hf isotope system, Being controlled by different processes, the Rb-Sr, Sm-Nd, and Lu-Hf isotope systems of the investigated samples are therefore decoupled. The epsilon(Nd)(220)-epsilon(Hf)(220) compositions of the samples deviate from the epsilon(Nd)(t)-epsilon(Hf)(t) trend of intraplate lavas to high epsilon(Hf)(220) at a given epsilon(Nd)(220) and are within the arc lava field. The protolith epsilon(Nd)(780)-epsilon(Hf)(780) compositions calculated from metamorphic initials using igneous Sm/Nd and Lu/Hf ratios show a similar distribution pattern, strengthening the arc affinity of the eclogites. These arc signatures together with the felsic-mafic bimodal geochemical features of the UHF rocks from the Sulu terrane are explained as the characteristics of protoliths generated by backarc rifting. (C) 2015 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.lithos.2015.07.013

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

We report here on two distinct processes of deformation affecting graphite in fault zones of the Hidaka metamorphic belt, Hokkaido, Japan. One process involves the micrometer-scale delamination (NMD) in the stacking of graphite, and the other is the nanometer-scale delamination (MMD) in the stacking plus pulverization (P) of carbon sheets to less than 10 nm. Graphite in mylonites mainly glide along the (002) planes, and they exhibit layer separation, kink banding, and bending structures produced by MMD processes, whereas NMD + P processes induce a significant size reduction and amorphization of the graphitic structures in cataclasites, ultracataclasites, and pseudotachylytes. The strain-induced amorphization of graphite, as inferred from XRD and micro-Raman spectroscopy, corresponds well with nano-to micro-scale deformation structures observed under the HRTEM, suggesting a systematic increase of edge planes by interlayer delamination and pulverization of carbon sheets. In addition, the carbon isotopic compositions of the graphite in fault rocks have negative delta C-13 signatures, indicating a sedimentary organic origin; such graphite has a tendency to accumulate within the shear bands. Our data suggest that the graphitic structures record information on both the peak metamorphic temperatures and the deformation processes, and they show the progressive amorphization with increasing brittle deformation. (C) 2014 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.jsg.2014.10.012

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：JAPAN ASSOC MINERALOGICAL SCIENCES  

The appropriateness of Zr as an 'immobile element' during garnet-hornblende (Grt-Hbl) vein formation potentially caused by the Cl-rich fluid or melt infiltration under upper amphibolite facies condition is examined. The sample used is a Grt-Hbl vein from Brattnipene, Sor Rondane Mountains, East Antarctica that discordantly cuts the gneissose structure of the mafic gneiss.
Modal analysis of the wall rock minerals combined with the quantitative determination of their Zr contents reveals that most of the whole-rock Zr resides in zircon whereas similar to 5% is hosted in garnet and hornblende. The Zr concentration of garnet and hornblende is constant irrespective of the distance from the vein. Zircon shows no resorption or overgrowth microstructures. Moreover, the grain size, chemical zoning (CL, Th/U ratio and REE pattern) and rim ages of zircon are also similar irrespective of the distance from the vein. LA-ICPMS U-Pb dating of zircon rims does not give younger ages than the granulite facies metamorphism reported by previous studies. All of these detailed observations on zircon support that zircon is little dissolved or overgrown, and that Zr is not added nor lost during the Grt-Hbl vein formation. Therefore, Zr can be described as an appropriate 'immobile element' during the Grt-Hbl vein formation. Detailed microstructural observation of zircon is thus useful in evaluating the appropriateness of Zr as an immobile element.

DOI： 10.2465/jmps.150220

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：INDIAN INST SCIENCE  

Chemostratigraphy or chemical stratigraphy deals with the correlation of sedimentary strata based on systematic variation of a particular chemical composition with time in the history of the Earth. In particular, this method is widely used in decoding the temporal variations seen in marine sediments that are deposited uninterruptedly in deep sea or in shallow marine carbonate depositional environments. In this review, the application of strontium and carbon isotope based chemostratigraphy in marine carbonate sediments is discussed. Irrespective of post depositional geological events such as metamorphism or tectonic displacements to form Mountain chains during continental collision, chemostratigraphy helps in determining the apparent depositional ages of sedimentary rocks in the Precambrian time, where biostratigraphy is not applicable. Although carbonate rocks are vulnerable to post depositional alterations, a systematic geochemical screening can guide in identifying the best chemically preserved carbonate rocks. A novel method of using Mn/Sr ratios for determining the best estimate of strontium initial ratio for carbonate rocks is presented, on the basis of a worked example of metamorphosed carbonate rocks from the Sor Rondane Mountains in the Dronning 'Maud Land, East Antarctica. The future potential of chemostratigraphy in decoding the early history of the Earth is also discussed.

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Authorship：Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

Carbon isotope fractionation between graphite and carbonated silicate melt was determined at 5 GPa and in the temperature range between 1400 and 1900 degrees C. High pressure experiments were carried out in the carbon-saturated model harzbergite system (Enstatite-Magnesite-Olivine-Graphite), where carbonated silicate melt and graphite were the two stable carbon-bearing phases in the run products. Carbonated silicate melting resulted in an isotopic fractionation between graphite and carbon in the silicate melt, where the carbon in the melt is C-13 enriched than co-existing graphite. 13C enrichment in carbonate melt when compared to graphite was further confirmed in experiments where redox melting between olivine and graphite produced carbonate melt as well as carbonate reduction experiments to form graphite. Although a quantitative estimate of fractionation between carbonate melt and graphite could not be obtained, our results indicate that mantle melting in the presence of graphite can result in progressive C-13 carbon isotope enrichment in carbonate melt and depletion in graphite, which can be an alternate explanation for the carbon isotopic heterogeneity observed in the mantle derived carbon. (C) 2014 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.epsl.2014.02.006

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER  

Here we report on the unexpected occurrence of felsic (granitic) inclusions with quench textures such as spherulite and dendrite (hereafter referred to as "felsite inclusions"), similar to some volcanic rocks, within garnet in presumably "slowly cooled" lower-crustal granulites of various geologic ages ranging from Early Proterozoic to Middle Paleozoic and wide global distribution (the Limpopo Belt, the Grenville Province, the Liltzow-Holm Complex of East Antarctica, the Highland Complex of Sri Lanka, and the southern Bohemian Massif). The well-preserved textures of felsite inclusions are indicative of melts formed by anatexis during high-pressure and high-temperature metamorphism, crystallization under far-from-equilibrium conditions (at >50 degrees C undercooling) and subsequent rapid cooling. The occurrence of felsite inclusions in granulites in restricted tectonostratigraphic zones in Sri Lanka, among other examples, may be the first geologic evidence for fast exhumation of lower-crustal rocks to andalusite-stable upper-crustal conditions by channel flow in a continental collision orogen. We hypothesize that granulites ascend episodically along discrete high-strain zones and cool as fast as some felsic magmas. This conclusion sheds new light on the debate regarding the deep crustal processes and necessitates changes to fundamental beliefs about exhumation rates based on rates of plate convergence (1-10 cm/year). (C) 2013 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

DOI： 10.1016/j.gr.2013.04.001

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：UNIV CHICAGO PRESS  

High-Mg, high-Al metasedimentary gneisses from the Altai Mountains, Mongolia, belonging to a subduction-accretion complex within the Central Asian Orogenic Belt can be divided into five rock types on the basis of mineral assemblages. Most rock types have high MgO and Al2O3 content and low CaO, Na2O, Rb, and Sr content. All rock types experienced a similar medium-pressure metamorphism characterized by a "hairpin"-shaped counterclockwise pressure-temperature path. U-Pb zircon and U-Th-Pb monazite ages indicated metamorphism at ca. 356 Ma and 277 Ma and inherited ages of 510-379 Ma, suggesting possible provenance to granitoids comparable to those in the Altai Mountains, China. The zircons that newly nucleated at ca. 356 Ma are characterized by high concentrations of light rare earth elements without a Ce anomaly-features common in zircons from hydrothermally altered rocks and a reducing environment. Petrological and geochronological results in this study suggest the following tectonic evolution: (1) continuous subduction and accretion of paleo-Asian oceanic crust during the Early Paleozoic, resulting in periodic granitoid magmatism in the period 510-380 Ma and a continuous supply of granite-derived sediments providing detrital zircon and monazite grains to the accretionary prism; (2) ridge subduction during the Late Devonian-Early Carboniferous (ca. 356 Ma), resulting in hydrothermal metamorphism of the accretionary prism and interaction with seawater that produced rocks with unusual whole-rock chemistry; and (3) closure of the ocean leading to continental collision in the Early Permian (ca. 277 Ma), with part of the accretionary prism squeezed into lower crustal levels to form medium-pressure metamorphic rocks.

DOI： 10.1086/675665

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Publishing type：Research paper (scientific journal)   Publisher：Elsevier {BV}  

DOI： 10.1016/j.epsl.2013.03.028

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

Several schools of thoughts exist on how and when exactly did the final assembly of East Gondwana occurred and this topic has been discussed over and again in the past few decades. Here we review the present status of our understanding of Neoproterozoic to Cambrian tectonic evolution of the southern sector of East Gondwana and by assembling this special issue we present some of the key elements of new structural, geological and geochronological information obtained from the inland Sol. Rondane Mountains in the Dronning Maud Land and surrounding regions. Altogether fifteen contributions are included in this special issue, most off which are exploring whether the Neoproterozoic orogens of East African Antarctic orogen and the late Kuunga orogens have affected the same geological entity. The results presented suggest the importance of Sor Rondane Mountains in understanding the tectonics of the final amalgamation of East Gondwana. (C) 2013 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.precamres.2013.06.010

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We report petrologic, geochemical, fluid inclusion and stable isotopic data from an unusual spinel dunite occurring within metasedimentary units of the Late Neoproterozoic-Cambrian accretionary belt in the southernmost part of the Southern Granulite Terrain in India. The volatile-rich dunite is mainly composed of cumulus olivine with intercumulus ilmenite, phlogopite, spinel, graphite, Ni-bearing pyrrhotite and calcite. The olivine is Mg-rich (Fo~96) and unusually poor in Ni. Spinel, even though a minor component, is Mg-Al type (Mg#~0.92) and is depleted in Cr (Cr#&lt
0.01). Phlogopite is highly magnesian (Mg# around 0.95-0.97) with high K/(K+Na) ratios of 0.93-1.00 similar to phlogopite reported from mantle-derived peridotites. Ilmenite is dominantly geikielite with Mg# ranging from 0.56 to 0.62. The calcite in this rock exhibits LREE-enriched nature with a pronounced negative Eu anomaly. Carbon stable isotope analyses on the calcite and graphite from the dunite yielded δ13C compositions comparable with mantle values. Raman peaks of matrix and inclusion graphites show highly crystalline character. The data reported in this study suggest formation of the spinel dunite from an Mg-rich melt enriched in C-O-H-S volatiles with low Ni and Cr. The silica activity appears to have been too low for the crystallization of plagioclase, pyroxenes and amphiboles. The formation of such a melt depleted in Cr, Ni and Fe suggests an early removal of Fe-Ni sulfide and Cr-rich spinel resulting in the depletion of Cr, Fe and Ni and enrichment of Mg and Al in the residual melts from which the high-Mg olivine and Mg-Al spinel cyrstallized. Fe and Ni partitioned to sulfide phase rather than olivine in shallow upper mantle/deep lower crustal levels under high sulfur fugacity. The spinel dunite of present study, together with similar ultramafic cumulate rocks occurring as dismembered units in other localities of the Neoproterozoic-Cambrian accretionary belt in southern India probably represents partial melting under the presence of volatiles in a mantle wedge within a suprasubduction setting. © 2012 Elsevier B.V.

DOI： 10.1016/j.precamres.2012.08.004

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Archaean metasedimentary sequences of the Chitradurga Schist Belt in Western Dharwar Craton were reassessed in terms of their protoliths, potential timing of deposition and metamorphic events along with metamorphic conditions. Existing geological and stratigraphic constraints and our field observations suggest that the oldest supracrustal sequence of the Sargur Group, which occurs as enclaves in Peninsular Gneiss, is overlain by the Dharwar Supergroup consisting of lower Bababudan and upper Chitradurga Groups. In the Chitradurga Schist Belt, the metamorphic conditions of each stratigraphic unit are distinct, with most prominent difference observed in the Chitradurga Group. The lower unit of this group are metamorphosed under biotite-muscovite grade, whereas the upper unit at chlorite-muscovite grade. Detrital zircon U-Pb dates obtained by ion microprobe analyses suggest 3.14. Ga and 3.22-2.92. Ga for the youngest protolith's magmatic ages, which constrain the oldest age limit of deposition of Bababudan Group and Lower Unit of Chitradurga Group, respectively. The depositional age of Upper Unit of Chitradurga Schist Belt can be bracketed by 2.68. Ga magmatic zircon ages from meta-rhyolite and 2.63. Ga magmatic detrital zircon age of the overlying sandstone that are significantly younger than the Lower Unit. Monazite U-Th-Pb ages are similar for both Lower and Upper Units of the Chitradurga Group (∼c. 2.4. Ga) but are significantly younger than those of the underlying Bababudan and Sargur Groups (∼c. 3.1. Ga). Combined zircon and monazite data imply that the continuous or composite Mesoarchaean-Neoarchaean supracrustal sequences of the Chitradurga Group are metamorphosed together at the latest Archaean-early Proterozoic. These regional metamorphic ages are not detected in the underlying Bababudan and Sargur Groups, as well as the basement Peninsular Gneiss. Contrasting depositional and metamorphic history in coherent Mesoarchaean and Neoarchaean strata in the Chitradurga Schist Belt have important implications for the comparison with the well-established sedimentary sequences of similar ages in the Pilbara and Kaapvaal cratons. The protolith and metamorphic ages of the Dharwar Supergroup, when compared with available similar information from the surrounding Archaean cratons in India, Madagascar, and East Antarctica, testify the probable Archaean connection, and give clues to the formation of deep to shallow Archaean crust. © 2012 Elsevier B.V.

DOI： 10.1016/j.precamres.2012.04.006

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：JAPAN ASSOC MINERALOGICAL SCIENCES  

The Central Asian Orogenic Belt consists of various terranes amalgamated in several periods from the Late Pre-cambrian/Early Paleozoic to the Late Mesozoic. Mongolia is situated in the center of this belt. Foliated garnet granite (Grt metagranite) and andalusite-bearing massive leucogranite (Leucogranite) are exposed in the Hanhohiyn region of northwest Mongolia. The results of monazite Th-U-Pb dating of the Grt metagranite and Leucogranite give ages of 508 +/- 10 Ma and 496 +/- 4 Ma, respectively. Cambrian granite with an age of similar to 500 Ma is also reported from the south-central part of Mongolia. Combined with previously published age data of metamorphic and intrusive rocks, these results show that the Cambrian orogenic belt spans over 800 km, from the northwest to the south-central part of Mongolia.

DOI： 10.2465/jmps.121021a

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

A geological study of the Balchenfjella area of the eastern Sor Rondane Mountains forms the basis of a revised tectonic history for eastern Dronning Maud Land in Antarctica. The Balchenfjella area can be structurally divided into two metamorphic units that are separated by a SE-dipping ductile extensional shear zone (the Balchen detachment fault). The mafic- and ultramafic-dominant part of the footwall side is exposed as a tectonic window adjacent to the detachment. Metamorphic and structural data indicate that the eastern Sor. Rondane Mountains were affected by four phases of deformation (D1-D4). Phases D1 and D2 are developed in the footwall side of the shear zone, whereas D3 and D4 are recorded in both the footwall and hangingwall sides. Mineral assemblages in mafic lithologies are characteristic of granulite facies (D1 and D2) to amphibolite facies (D2 and D3) metamorphism. D1 is characterized by recumbent folds and this phase is interpreted as a compressional tectonic setting coincident with granulite facies metamorphism. D2 represents a transpressional deformation phase characterized by upright folds and a large-scale dextral shear zone. The Balchen detachment fault was formed during the extensional tectonic setting of D3. The timing of D3 extensional deformation is bracketed by the age of D2-related metamorphism (at c. 600 Ma) and the timing of post-kinematic intrusion of granites (at c. 550 Ma). The present results demonstrate that the eastern Sor Rondane Mountains underwent Late Neoproterozoic extensional collapse similar to that observed in other parts of the East African-Antarctic Orogen including the Arabian-Nubian Shield and Madagascar. (C) 2013 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.precamres.2013.04.015

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The Sor Rondane Mountains of eastern Dronning Maud Land, East Antarctica can be subdivided into two different crustal terranes: the NE-terrane and the SW-terrane. The former is underlain by basement rocks of amphibolite-facies (unit A) and granulite-facies (unit B), and the latter by granulite-facies (unit C) to greenschist-facies (units D and D') rocks. The metamorphic evolution of the NE-terrane exhibits a clockwise pressure-temperature-time (P-T-t) path and the SW-terrane exhibits a counter-clockwise P-T-t path, and detrital zircon U-Pb ages are also different between the two terranes. The differences in the metamorphic evolution processes and detrital zircon provenance in these two regions can be explained by the collision of the NE-terrane and the SW-terrane which is constrained to have occurred at 600-650 Ma. The collision is interpreted to reflect convergence between the East Maud-East African orogen terrane and the Maud-Nampula terrane of the Kalahari Craton, which are bounded by the Main Tectonic Boundary and passes through the Sor Rondane Mountains. The RTP magnetic anomalies suggest this suture is cut by the subsequent collision boundary between the Kalahari Craton and the Rayner Belt, which is exposed in the Lutzow-Holm region. This collisional belt is considered to have formed at approximately 520-580 Ma. (C) 2013 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.precamres.2013.05.017

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

In the granulite facies rocks, the role of low H2O activity fluids is still unclear. The importance of understanding its role is gradually being recognized, and it enables us to help understand the process of granulite formation. Attention has been specifically focused on Cl-rich fluids and high-salinity alkaline fluids in the geochemical modification of continental crust. We have investigated the field distribution of Cl-rich biotite in the pelitic gneisses of the Sor Rondane Mountains, East Antarctica where Late Proterozoic to Cambrian granulites are widely exposed. Among 27 samples studied, a garnet-biotite-sillimanite gneiss from Balchenfjella was selected as best suited sample to constrain the P-T-t conditions of Cl-rich fluid or melt activity. This gneiss contains garnet porphyroblasts that have a P-rich core with oscillatory zoning in P. The garnet core includes Cl-poor biotite and fluorapatite. This core has been partially resorbed and discontinuously overgrown by a P-poor rim, in which Cl-rich biotite and chlorapatite are included. Coarse-grained, rounded zircon grains are exclusively included in the rim of the garnet porphyroblast and are also present in the matrix. This mode of occurrence suggests that the Cl-rich biotite and chlorapatite, together with coarse-grained zircon were formed almost simultaneously. The P-T conditions of Cl-rich biotite entrapment in the garnet rim are estimated to be ca. 800 degrees C and 0.8 GPa. In comparison, peak metamorphic condition is ca. 850 degrees C and 1.1 GPa. These pieces of observation suggest that Cl-rich fluid or melt infiltrated at the core-rim boundary of garnet. In the case of fluid infiltration, the f(HCl)/f(H2O) ratio of the fluid in equilibrium with Cl-rich biotite and chlorapatite in the garnet rim are estimated to be ten times larger than that in equilibrium with Cl-poor biotite and fluorapatite in the matrix and the garnet core. The LA-ICP-MS U-Pb dating of the coarse-grained zircon included in the garnet rim gave concordia age of 603 +/- 14 Ma. Therefore, Cl-rich fluid or melt infiltration took place close to the metamorphic peak condition of ca. 800 degrees C and 0.8 GPa at 603 +/- 14 Ma. The field distribution of Cl-rich fluid or melt activity is somewhat linear in the Sor Rondane Mountains. High Cl-activity is located near the large scale ductile shear zones, suggesting its relation to high-strain zones. Regional distribution of high-grade Cl-rich fluid or melt activity in the Sof. Rondane Mountains implies that it is one of the major phenomena in the continental collision processes. (C) 2012 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.precamres.2012.10.006

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

Reconstructions of Gondwana place Dronning Maud Land, Antarctica (DML) adjacent to N. Mozambique prior to fragmentation. The Monapo Complex outlier klippen overlying the Nampula Terrane in N. Mozambique has been correlated with rocks in eastern and central DML. Metamorphic assemblages and P-T conditions from the two areas are compared as well the timing of metamorphism. Granulite grade assemblages preserved in the Monapo Complex suggest P-T conditions of similar to 90 degrees C and &gt;10 kb. Textures vary with reactions typical of isothermal decompression, isobaric cooling and hydration being recognized but also include equilibrium assemblages. P-T estimates from four samples suggest initial inversion of high pressure assemblages from at least similar to 10 kb and similar to 900 degrees C to mid-crustal levels where near isobaric cooling and hydration at between similar to 4-7 kb and 550-700 degrees C is recognized. Granulite grade assemblages from Balchenfjella in eastern Sor Rondane, DML suggest initial P-T conditions of similar to 900 degrees C and &gt;10 kb. Textures vary with reactions typical of decompression, cooling and hydration being recognized but also include equilibrium assemblages. P-T estimates from four samples suggest initial inversion of high pressure assemblages from at least similar to 10 kb and similar to 900 degrees C to mid-crustal levels where retrogression at between similar to 6-7 kb and 600-700 degrees C is recognized.
SHRIMP zircon data from six samples from eastern Sor Rondane show episodic zircon growth over a period from similar to 630 Ma to similar to 530 Ma with five metamorphic pulses being defined. Undeformed granitic intrusions with ages of similar to 550 Ma intrude Sor Rondane. SHRIMP zircon data from samples from alkaline intrusions and mafic granulites in the Monapo Complex yield crystallization and metamorphic ages of similar to 635 Ma and 570-590 Ma respectively. The complex is intruded by granitic veins with ages of similar to 550 Ma.
The similarities in metamorphic mineral assemblages, P-T estimates and zircon geochronology data indicate a common geological history between Sor Rondane and the Monapo Complex of northern Mozambique. This common history is interpreted to result from both areas being part of a mega-nappe structure which was emplaced in a transpressional setting involving collision between N. Gondwana (comprising rocks of the East African Orogeny) and south Gondwana (comprising the Nampula Terrane of northern Mozambique and the Maud Province of western Dronning Maud Land) along the Damara-Zambesi-Lurio-CDML collision boundary.
Other erosional remnants of this nappe include the Mugeba Complex in northern Mozambique, the Kataragama klippe in Sri Lanka, the Urungwe klippe in Zimbabwe, the Masosa Suite and Mavhuradohna Complex in western Mozambique and NE Zimbabwe. (C) 2012 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.precamres.2012.11.012

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

This paper provides a detailed analysis of the geological evolution of the southern Austkampane region in the Sor Rondane Mountains, Antarctica, using geological and petrographical observations, geochemical, geochronological, radiogenic, and stable isotopic data, and metamorphic petrological techniques. The study area is dominated by felsic gneisses intercalated with mafic and carbonate rocks. Carbonate rocks include several calc-silicate and mafic to ultramafic blocks with carbonatite-like characteristics. The new data presented here indicate the presence of five important thermal events in the geological history of the region, at c. 1060, 770-750, 655-635, 560-545, and &lt;545 Ma. The c. 1060 Ma event is characterized by granitic volcanic arc magmatism, possibly in an active continental margin setting, involving magmas that may have been sourced from enriched mantle formed by subduction of older crustal material during the formation of Rodinia. The oldest T-(DM) age for this suite is 3.4 Ga. The 770-750 Ma event is characterized by both mafic and felsic magmatism in an active continental margin setting, with magmas being similar to bulk earth composition. The 655-635 Ma thermal event is represented by granulite-facies metamorphism along an anticlockwise pressure-temperature path, involving heating of the lower crust at a continental margin, and subsequent obduction of a cratonic counterpart during collision. Granitic and basaltic magmatism at 560-545 Ma is also associated with retrograde metamorphism and hydration-related partial melting of marble. The carbonate melt, and granitic and basaltic magmas during this event coexisted with each other and formed metasomatic calc-silicates and hornblendites, respectively. The final event at &lt;545 Ma is characterized by the intrusion of an undeformed quartz syenite dyke, which may be related to either (1) collision and melting of preexisting lower crustal material, or (2) melting of slightly enriched mantle material in an extensional tectonic setting following the earlier collisional event. The five thermal events between c. 1100 and 500 Ma recognized in this study from a narrow region of the Sew Rondane Mountains, suggest that a combination of geochronological, whole-rock, and isotope geochemical analyses of metamorphic rocks and the pressure-temperature evolution could constrain the processes that operated during the amalgamation and break-up of Rodinia and the formation of Gondwana. A similar approach is essential in neighboring Gondwana terranes for modeling the protracted events of Neoproterozoic orogenesis. (C) 2012 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.precamres.2012.10.009

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Authorship：Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

The Sor Rondane Mountains (SRM), located in the Neoproterozoic to Early Cambrian East African-Antarctic collisional orogen is composed of medium- to high-grade metasedimentary, metaigneous and intrusive rocks of diverse composition. Within the metasedimentary rocks, the metacarbonate rocks are considered to have deposited chemically in the so-called the "Mozambique Ocean" that separated the continental blocks that amalgamated to form Gondwana and possibly record geochemical signatures of contemporaneous seawater. Here we attempt to constrain the apparent age of sedimentation of metasedimentary sequences using strontium isotope chemostratigraphy of the least altered metacarbonate rocks. Pure metacarbonate samples, collected during the 51st Japanese Antarctic Research Expedition from different regions throughout the SRM, were selected based on careful screening using multiple geochemical parameters, such as carbon and oxygen isotopic composition and trace and rare earth element contents. We could successfully test the extent of alteration for the Sor Rondane metacarbonate samples, using Mn/Sr ratios, which show a positive correlation with Sr-87/Sr-86 ratios. After a rigorous geochemical screening in terms of post-depositional alterations, 18 samples were identified as least altered. These samples collectively gave regional initial Sr-87/Sr-86 ratios between 0.70566 and 0.70630, from Balchen, Brattnipene, Menipa and Tanngarden regions of the SRM, with the exception of Perlebandet region. These Sr isotopic ratios reflect seawater compositions of late-Tonian and early-Cryogenian age (880-850 Ma and 820-790 Ma), when compared with the evolution of Sr isotopes in the Neoproterozoic Oceans. Furthermore, these estimates are consistent with the carbon isotope chemostratigraphic curves of Neoproterozoic. The estimated apparent depositional ages of carbonate rocks in the SRM are also conformable with the reported detrital and metamorphic ages for this region. Carbonate rocks in the Perlebandet region shows low initial Sr isotope ratio (0.70482), suggesting that these rocks may have deposited earlier than other carbonate rocks in the SRM. Our results can be correlated with the chemostratigraphic depositional ages reported for carbonates from the Montepuetz Complex, Mozambique, suggesting the presence of contemporaneous platform environment on both sides of the possible suture. The finding of late-Tonian and early-Cryogenian carbonate deposition, potentially points toward a platform environment surrounding the tonalitic continental arc in the SW region of the SRM, prior to the amalgamation of Gondwana. The results obtained need to be tested with similar studies on metacarbonate sequences from the surrounding regions, which would help to resolve the processes and sequence of collision events that finally amalgamated Gondwana. (C) 2012 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.precamres.2012.10.016

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

DOI： 10.1016/j.epsl.2013.03.032

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：MINERALOGICAL ASSOC CANADA  

Chromium-rich calc-silicate rocks in the Archaean Bandihalli supracrustal belt in the Dharwar Craton, India, show a characteristic mineral assemblage that includes euhedral and anhedral (resorbed) green garnet associated with chromite, in addition to diopside, quartz, clinozoisite and calcite. Two distinct types of green garnets and a brown variety were identified in the calc-silicate rocks. These garnets show complex chemical compositional variations that range between the endmembers of uvarovite, grossular, and spessartine. Chromite grains in the matrix are euhedral or subhedral, whereas inclusions in garnet are rounded or amoeboid in shape. The two textural varieties of chromite from Bandihalli are separated by a compositional gap. Furthermore, chromites from the study area have high Cr# (0.80-0.97) and low Mg# (0.01-0.04), which is similar to the chromite in serpentinites and are in contrast with chromite compositions reported from igneous rocks. Therefore, the chromite in the calc-silicate rocks may have been derived from serpentinized ultramafic rocks. Textural and compositional data presented here provide evidence for chemical changes of the primary chromite with increasing metamorphic grade, presumably associated with primary hydrothermal alteration that resulted in serpentinization and carbonation of the ultramafic protolith. Therefore, Cr-rich calc-silicate rocks can be considered as products of metamorphosed carbonated and serpentinized mafic or ultramafic igneous rocks.

DOI： 10.3749/canmin.50.3.705

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Four different varieties of charnockitic rocks, with different modes of formation, from the Mesoproterozoic Natal belt are described and new C isotope data presented. Excellent coastal exposures in a number of quarries and river sections make this part of the Natal belt a good location for observing charnockitic field relationships. Whereas there has been much debate on genesis of charnockites and the use of the term charnockite, it is generally recognized that the stabilization of orthopyroxene relative to biotite in granitoid rocks is a function of low aH 2O (± high CO 2), high temperature, and composition (especially Fe/(Fe +Mg)). From the Natal belt exposures, it is evident that syn-emplacement, magmatic crystallization of charnockite can arise from mantle-derived differentiated melts that are inherently hot and dry (as in the Oribi Gorge granites and Munster enderbite), as well as from wet granitic melts that have been affected through interaction with dry country rock to produce localized charnockitic marginal facies in plutons (as in the Portobello Granite). Two varieties of post-emplacement sub-solidus charnockites are also evident. These include charnockitic aureoles developed in leucocratic, biotite, garnet granite adjacent to cross-cutting enderbitic veins that are attributed to metamorphic-metasomatic processes (as in the Nicholson's Point granite, a part of the Margate Granite Suite), as well as nebulous, patchy charnockitic veins in the Margate Granite that are attributed to anatectic metamorphic processes under low-aH 2O fluid conditions during a metamorphic event. These varieties of charnockite show that the required physical conditions of their genesis can be achieved through a number of geological processes, providing some important implications for the classification of charnockites, and for the interpretation of charnockite genesis in areas where poor exposure obscures field relationships. © 2012, China University of Geosciences (Beijing) and Peking University. Production and hosting by Elsevier B.V. All rights reserved.

DOI： 10.1016/j.gsf.2012.05.006

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Earth science-related field activities, involving geology, geomorphology and meteorite searches, were carried out in the Sor Rondane Mountains, Eastern Dronning Maud Land, during the 2009-2010 summer season as a part of the 51st Japanese Antarctic Research Expedition (JARE-51), in collaboration with the Belgian Antarctic Research Expedition (BELARE). Geology and geomorphology parties accessed the Sør Rondane Mountains using the Dronning Maud Land Air Network (DROMLAN), and the meteorite search party to Antarctica on the maiden voyage by the new Japanese icebreaker Shirase. The geology party covered the entire area of the Sør Rondane Mountains, although with an emphasis on the eastern part. The geomorphology party carried out fieldwork in western and central parts of the mountains, and the meteorite search party performed a survey in the eastern part. All field activities were successfully carried out. Some of the geology members returned to Japan by DROMLAN, while others flew to Syowa Station from the Sør Rondane Mountains by DROMLAN, and then returned to Japan on board Shirase. This report provides a summary of the field operations, including logistics and weather records. © 2012 National Institute of Polar Research.

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Language：English   Publisher：Institute of Geosciences, Faculty of Science, Shizuoka University  

Spurrite [Ca₅Si₂O₈(CO₃)], a calc-silicate mineral with a carbonate molecule, is a product of high-temperature contact metamorphism, a typical example of which is found at the Fuka contact aureole, Okayama Prefecture, Japan. We report here carbon and oxygen isotope composition of CO₂ released from the reaction of spurrite with phosphoric acid in the temperature range of 25°C to 100°C. While reacting with the acid carbon isotopes did not fractionate, while oxygen isotope shows considerable isotope fractionation with varying reaction temperature, similar to other carbonate minerals. This suggests that the carbon and oxygen isotope measurements can be carried out using acid extraction method. In the Fuka contact aureole, carbon isotope fractionation of about 3‰ is observed between coexisting spurrite and coarse-grained calcite. If in equilibrium the carbon isotope fractionation between carbonate molecule in spurrite and calcite has the potential application as an isotope geothermometer. However, oxygen isotope distribution between calcite and spurrite could not be critically evaluated because of the uncertainties of source oxygen isotopes values of infiltrating fluids during the skarn formation and the fractionation of it between silicate and carbonate phases.

DOI： 10.51053/shizuoka.38.0_27

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Other Link： http://hdl.handle.net/10297/6210

Language：Japanese   Publisher：Institute of Geosciences, Faculty of Science, Shizuoka University  

In this paper, we report the procedure for carbon and oxygen isotopic measurement of small amount of dolomite powder using IR-MS system, MAT-250 at Shizuoka University. Although the dolomite samples had been measured using a batch method, we tried to prepare the carbon dioxide gas of dolomite sample using the existing online carbonate (calcite) sample preparation system. The examined analytical conditions are (1) set reaction vessel temperature 100°C and (2) set reaction time more than 60 minutes. In addition, we describe the operation manual for precise measurement of small amount of carbon dioxide, derived from carbonate minerals reacted with conc. phosphoric acid or carbon dioxide prepared offline and stored in a glass tube. We hope that this paper helps the users to understand of the usage of the mass-spectrometer.

DOI： 10.51053/shizuoka.38.0_33

DOI： 10.14945/00006211

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Other Link： http://id.ndl.go.jp/bib/11288882

Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER  

High-spatial resolution carbon isotope analyses of natural graphite using secondary ion mass spectrometry (SIMS), together with conventional mass spectrometry techniques, demonstrate isotopic heterogeneity within single graphite crystals precipitated from a partially melted metamorphic rock. SIMS C-13/C-12 measurements were calibrated using an internal graphite standard previously analyzed by conventional isotope ratio mass spectrometry, which gave a reproducibility of 0.3aEuro degrees (1 sigma) at a spatial resolution of 2-3 mu m. This resolution helped to identify an unusual carbon isotope distribution in a single graphite crystal from a metapelitic leucosome, showing remarkable core to rim variations with sharp delta C-13 steps up to 10aEuro degrees. The results suggest that the graphite crystal grew from one edge to other forming layers perpendicular to the c-axis. The sharp isotopic steps indicate the presence of disequilibrium carbon isotope zoning in graphite and points to the possible existence of carbon isotope sector zoning. Intra-crystalline carbon isotope disequilibrium in graphite is believed to have resulted from the difference in diffusivity between C-12 and C-13 in the growth medium to the interface of graphite precipitation in different growth sectors.

DOI： 10.1007/s00410-011-0626-3

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER  

We report here new experimental data on equilibrium carbon isotope fractionation between graphite/diamond and iron carbide melt at 5 and 10 GPa and in the temperature range between 1200 and 2000 degrees C. Carbon isotope equilibrium was tested using morphological features of graphite and also by performing a longer duration experiment, both of which suggested that equilibrium carbon isotope fractionation is present. The results suggest that iron carbide melt will preferentially accumulate C-12 rather than C-13. An equilibrium temperature dependent fractionation between iron carbide melt and graphite/diamond is proposed based on the relationDelta C-13((gr/dia - iron carbide melt)) = 8.85{10(6)/T-2(K)} + 0.99.Our results are consistent with the carbon isotope distribution between graphite and cohenite (Fe3C) observed in iron meteorites. We propose that temperature-dependent fractionation of carbon isotopes between iron carbide melt and graphite/diamond might have created a "C-12-enriched core" with a significant difference in the distribution of carbon isotopes between the carbon in the metallic core and bulk silicate Earth during the accretion and differentiation of early Earth. Recent findings of low delta C-13 carbonados and diamonds of deep mantle origin supports the presence of a C-12 enriched source. The possible presence of a reservoir of C-12-enriched carbon in the Earth's core implies that it can generate large perturbations in the surface and shallow carbon-isotope system by the flux of lighter carbon from the core-mantle boundary. (C) 2011 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.epsl.2011.08.008

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Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：MINERALOGICAL SOC AMER  

Carbon isotope exchange between calcite and graphite is a useful and reliable geothermometer for medium- to high-grade marbles. However, in rare instances, such as at Naxos, Greece, apparent disequilibrium carbon isotope fractionation between calcite and graphite has been previously reported. In this study, new results are presented on the morphological features, X-ray diffraction studies, Raman spectroscopic studies, and carbon isotope studies of graphite. Three morphologically distinct graphite types are identified. The first type is fine grained with distinct polygonal crystal aggregates having smooth pinacoid faces. The second type consists of large botryoidal aggregates with typical crystal overgrowth features, and the third type is "normal" large platy crystals associated with phlogopite. Graphite morphological features using an FE-SEM suggests that the botryoidal aggregates are composed of complexly intergrown and stacked graphite layers, comprised of cone-helix structures. Different types of graphite display distinct Lc(002) and DG values as well as a sharp first-order Raman peak at similar to 1580 cm(-1) with disordered bands. The presence of Raman disordered bands in fine-grained well-crystallized graphite is attributed to edge effects. Carbon isotope analysis of graphite reveals that marble with fine-grained well-crystallized graphite show a consistent isotope fractionation with the calcite, which perfectly match temperature estimates based on the mineral isograds. In contrast, coarse botryoidal graphite gives heterogeneous carbon isotope values (about a few per mill) and show anomalous carbon isotope fractionation between the calcite and graphite. The graphite associated with phlogopite also shows variations in the carbon isotope values. Combining morphologic, crystallographic, and carbon isotopic data we conclude that the variations in carbon isotopic composition were caused by the overgrowth of graphite on the preexisting grain and that morphological observations and structural characterizations of graphite crystals is a key for predicting isotopic equilibration during metamorphism.

DOI： 10.2138/am.2011.3576

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：JAPAN ASSOC MINERALOGICAL SCIENCES  

Corundum in direct contact with quartz and associated with andalusite has been found as an inclusion in alkali feldspar obtained from a leucosome, which forms a part of migmatized pelitic granulite from the Kerala Khondalite Belt, southern India. Thermodynamically, corundum + quartz is metastable relative to andalusite. Textural features suggest that it is a rare example of an arrested reaction corundum + quartz -&gt; andalusite which may have been triggered off by the introduction of H2O-rich fluids along the cracks of the host alkali feldspar during the uplift and cooling of the leucosome. The metastable corundum + quartz association most likely originated during the crystallization of the Al2O3-rich leucosome melt when early corundum came into contact with later quartz under conditions with a relatively low H2O activity.

DOI： 10.2465/jmps.101116

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DOI： 10.5575/geosoc.116.8.XI_XII

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

Granulite grade marble layers interlayered with metapelitic granulites from Lutzow Holm Bay, East Antarctica, provide insight into fluid-rock interactions during burial to and exhumation from lower crustal levels. Sub-millimeter scale strontium, oxygen and carbon isotope variations along with LA-ICPMS trace element geochemistry and mineral chemistry of texturally characterized carbonates and associated minerals helped to reconstruct the multistage metamorphic fluid history.
Fluid-rock interaction dating back to prograde metamorphism are still preserved in consistently low oxygen and high strontium isotope compositions (delta(18)O = 12%; (87)Sr/(86)Sr((550Ma)) = 0-7248) within a massif dolomitic marble layer that escaped significant later metasomatism. In most marbles, total re-crystallization and isotopic resetting occurred in the presence of "externally derived" hyper-saline fluids that circulated along the carbonate layers during the early stages of prograde metamorphism. This leads to a trend of increased radiogenic Sr in marbles towards the value of associated metapelitic rocks that have (87)Sr/(86)Sr((550Ma)) of 0.764. LA-ICPMS studies on trace elements in carbonate and associated silicate minerals at different textural settings, distinguished using cathodoluminescence microscopy, revealed multiple metasomatic events during retrograde metamorphism. Trace element contents of Ba, Sr, Pb and U gave compelling evidence for metasomatic alteration that postdate the exsolution of carbonate at similar to 600 degrees C, which can be correlated with the fluids released from the crystallization of anatectic melts and pegmatites. Subsequently, meteoric fluid infiltration occurred at a shallower level of the crust and caused extreme oxygen isotopic heterogeneity (delta(18)O=14.7 similar to-4.9 parts per thousand) and imprinted high concentration of fluid mobile elements. Taken together our results emphasize the importance of integrating textural and chemical heterogeneities to reveal the multiple episodes of fluid-rock interaction processes in a dynamic continental crust, which has major implications on migration of fluids and material and help in formulating models on the geodynamic evolution of crust. (C) 2009 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.lithos.2009.08.010

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Institute of Geosciences, Faculty of Science, Shizuoka University  

A new inlet system for the preparation of SF6 gas by fluorination of sulfide minerals, suitable for both in-situ laser ablation and powder samples, was connected to the existing CO2 inlet system of the Finnigan MAT-251 mass spectrometer at Shizuoka University. The preparation line is composed of a fluorine generator, two types of reaction chambers, capacitance manometer, KBr canister and liquid nitrogen slush connected by pneumatic bellows valves. It also includes a working standard SF6 gas reservoir and a machine standard SF6 gas reservoir for routine analyses. The preparation line can hold high vacuum conditions without appreciable leaks for intervals more than that required to complete one sample measurement. For simultaneous measurement of [127], [128], [129] and [131] isotopes of SF5+ we have renovated Faraday collectors by reducing its body thickness from 4.5 mm to 3.5 mm and realigned the collector configuration of the mass spectrometer. Preliminary results of δ34S values for working standard SF6 gas gave high precession of ±0.003 %o. Further modifications in the collector assembly are in progress for the simultaneous measurement of the closely spaced ion trajectory of [127], [128] and [129] isotopes of SF5+.

DOI： 10.51053/shizuoka.37.0_41

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Authorship：Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：WILEY-BLACKWELL  

Calcite-dolomite solvus geothermometry is a versatile method for the estimation of metamorphic temperature because of its simplicity. However, in medium- to high-grade metamorphic rocks the accuracy of estimating temperature by the integration of unmixed dolomite and calcite is hampered by the heterogeneous distribution of unmixed dolomite, difficulties in distinguishing between preexisting and exsolved dolomite and demarcating grain boundaries. In this study, it is shown that calcite-dolomite solvus thermometry can be applied to calcite inclusions in forsterite and spinel for the estimation of peak metamorphic temperature in granulite facies marbles from Skallevikshalsen, East Antarctica. The marbles are comprised of a granoblastic mineral assemblage of calcite + dolomite + forsterite + diopside + spinel + phlogopite +/- apatite, characteristic of granulite facies metamorphic conditions. Forsterite, spinel and apatite frequently contain 'negative crystal' inclusions of carbonates that display homogeneously distributed dolomite lamellae. On the basis of narrow ranges of temperature (850-870 degrees C) recorded from carbonate inclusions compared with the range from matrix carbonate it is regarded that the inclusion carbonates represent a closed system. Furthermore, this estimate is consistent with dolomite-graphite carbon isotope geothermometry, and is considered to be the best estimate of peak metamorphic temperature for this region. Matrix calcite records different stages of retrograde metamorphism and re-equilibration of calcite that continued until Mg diffusion ceased at similar to 460 degrees C. Electron backscattered diffraction (EBSD) results together with morphological features of unmixed coarse tabular dolomite suggest anisotropic diffusion and mineral growth are influenced by crystallographic orientation. Identification of sub-grain boundaries and formation of fine-grained unmixing in calcite rims suggest the presence of grain boundary fluids in the late retrograde stages of metamorphic evolution. These results, thus, demonstrate the usefulness of carbonate inclusion geothermometry in estimating the peak metamorphic temperatures of high-grade terranes and the application of EBSD in understanding the unmixing behaviour of minerals with solid solutions.

DOI： 10.1111/j.1525-1314.2010.00877.x

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：JAPAN ASSOC MINERALOGICAL SCIENCES  

Kyanite and andalusite are newly found in migmatized pelitic granulites from the Kerala Khondalite Belt, Southern India. Anhedral tiny grains of kyanite included within altered cordierite were interpreted as possible remnants of prograde metamorphism of the granulites within the kyanite stability field. Andalusite shows two distinct modes of occurrence; some are of magmatic origin and others are formed as partial replacement products of alkali-feldspar and plagioclase mainly in leucosomes. Our data put new constraints not only on the P-T path followed by the granulites but also on the correlation between the Gondwana fragments. The Kerala Khondalite Belt is closely correlated with the Southwestern Group (southwestern part of the Highland Complex) in Sri Lanka.

DOI： 10.2465/jmps.100616

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：JAPAN ASSOC MINERALOGICAL SCIENCES  

We report Occurrence and origin of graphite in glimmerite and spinellite from an unusual ultramafic complex in Achankovil Shear Zone (ACSZ) in Southern India. Raman analyses of primary graphite flakes in close association with spinel and phlogopite confirm the highly crystalline nature. The carbon isotope composition ranges from -7 parts per thousand to -9.8 parts per thousand typical of mantle-derived graphite. A genetic model is proposed ill which carbon was derived from a reduced juvenile Mantle Source linked to the genesis of volatile rich ultramafic rocks in ACSZ.

DOI： 10.2465/jmps.090622d

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Authorship：Last author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

Characterization of fluid inclusions in graphite-bearing charnockites from the southwestern part of the Madurai Granulite Block in southern India reveals a probable relation with the formation and break down of graphite during the high-grade metamorphism. The first-generation monophase pure CO2 inclusions, the composition of which is confirmed by laser Raman spectroscopy, recorded moderate density (0.77-0.87 g/cc) corresponding to low tapping pressure (around 2 kb) than that of the peak granulite-facies metamorphism. The precipitation of graphite, as inferred from graphite inclusions and delta C-13 values of the graphite from the outcrops, is interpreted as the cause of this lowering of fluid density. An intermediate generation of pseudosecondary inclusions resulted from the re-equilibration or modification of the first-generation fluids and the CO2. formed is interpreted to be the oxidation product from graphite. The youngest generation of fluids which caused widespread retrogression of the granulites is a low-temperature (similar to 350 degrees C) high-saline (32.4-52.0 wt% NaCl equivalent) brine. Carbon isotope data on the graphite from the charnockites show delta C-13 values ranging from -11.3 to -19.9 parts per thousand, suggesting a possibility of mixing of carbon sources, relating to earlier biogenic and later CO2 fluid influx. Combining the information gathered from petrologic, fluid inclusion and carbon stable isotope data, we model the fluid evolution in the massive charnockites of the southwestern Madurai Granulite Block. (c) 2009 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.jseaes.2009.06.010

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Language：Japanese   Publisher：Shizuoka University  

DOI： 10.14945/00003617

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Other Link： http://hdl.handle.net/10297/3617

Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：Geological Society Special Publication  

C, O and Sr isotope geochemistry of high-grade marbles from the Lützow-Holm Complex, East Antarctica, has given clues on the depositional ages and post-depositional alterations. Dolomitic and calcitic marbles occur as thin layers with varying thickness (up to 100 m) in several outcrops in eastern Dronning Maud Land, most of which underwent post- depositional geochemical alterations. In particular, the Sr and O isotope alterations are extensive, with 87Sr/86Sr(550Ma) ratios as high as 0.758 and δ18O values as low as 25‰. These data suggest that multiple stages of fluid-rock interaction processes during diagenesis, prograde to peak and retrograde metamorphic events have altered the depositional isotopic signatures. However, some of the marble layers, exceptionally, preserve pre-metamorphic geochemical characteristics, such as low Sr isotope ratios, high δ18O and δ13C values, and well-equilibrated unaltered trace and rare earth element patterns. Lowest 87Sr/86Sr isotopic ratios of 0.7066 and 0.7053 with high δ13C and δ18O values suggest an apparent age of deposition around 730-830 Ma, although total geochemical resetting of carbonates by seawater of this age cannot be ruled out. The apparent depositional ages are consistent with carbonate deposition in the 'Mozambique Ocean' that separated East and West Gondwana. © The Geological Society of London 2008.

DOI： 10.1144/SP308.7

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：JAPAN ASSOC MINERALOGICAL SCIENCES  

Stable isotope compositions (delta C-13, delta O-18) of calcite within the veins hosted by the Sanbagawa metamorphic rocks in the Nagatoro area, Kanto mountains, Japan gave clues on movement of fluids during exhumation. The host rock calcite shows the nearly uniform delta O-18 values ranging from + 14.5 to + 16.5% (V-SMOW), and wide delta C-13 variation from -12.1 to + 1.3% (V-PDB). The delta C-13 values of calcite within the exhumation-related veins also have wide variation, and the Delta(host-vein) (= delta C-13(host-rock)-delta C-13(vein)) value vary depending on vein thickness and textures. Thin stretched crystal veins contain calcite with delta C-13 value close to that of the host rock calcite (Delta(host-vein) < +/- 2%). In contrast, calcite grains within the thick blocky veins have homogenous delta C-13 values with large variations with host rock (Delta(host-vein) = +3.5 = -3.0 parts per thousand). The latter types of sealed cracks may have played as dominant pathways of regional fluid flow.

DOI： 10.2465/jmps.080620b

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Geological Society Special Publication  

Rb-Sr and Sm-Nd mineral dating of metamorphic rocks from Skallen, Skallevikshalsen and Rundvågshetta, in the southwestern part of the Lützow-Holm Complex, Dronning Maud Land, assists in constructing a thermal history after peak metamorphism. The results fall into two groups: (1) a record of regional cooling after peak metamorphism (524-488 Ma)
(2) local resetting 50-80 Ma after peak metamorphism (474-446 Ma). This grouping is consistently observed in published ages from various localities in the Lützow-Holm Complex. A Sm-Nd age of 524 Ma is indistinguishable from published zircon and monazite ages. Ages of 511 and 488 Ma are related to cooling after peak metamorphism. The younger age group overlaps with ages of post-metamorphic magmatism and related hydrothermal activity reported from localities throughout East Antarctica. This intracontinental, post-orogenic igneous activity continued after the tectonic assembly of Gondwana. © The Geological Society of London 2008.

DOI： 10.1144/SP308.8

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Precise chemical composition, including Fe3+ and H, of biotite from a pegmatite dike and its host granulite from the Kerala Khondalite Belt of SE India has been determined using a multi-technique approach involving EMP, SIMS, Mossbauer, and C-H-N elemental analysis. Biotite in these rocks formed at T> 800-850 degrees C and P = 5 +/- 1 kbar.The full analyses were normalized on the basis of [O12-(x+y+z)(OH)(x)ClyFz]. Biotite in the pegmatite is Ti-, F-, and Cl-rich (0.33, 0.46, and 0.16 apfu, respectively), H2O-Poor (OH = 0.86 pfu), has X-Mg= 0.49 and Fe3+/Fe-tot <= 3%. The low octahedral vacancies (0.06 pfu) and the high oxygen content in the hydroxyl site (OH + F + Cl = 1.49 pfu) confirm the role of the Ti-oxy substitution as a major exchange vector in these high-T biotites.In the host granulite, fine-grained biotite is Fe3+-free, has low Cl (0.03 apfu), and more variable composition, with Ti, F, and X-Mg in the ranges 0.26-0.36, 0.52-0.67, and 0.67-0.77, respectively. The number of octahedral vacancies is relatively large (0.10-0.18 pfu) and the sum of volatiles (OH + F + Cl) varies from 1.71 to 2.06 pfu. Systematic variations of X-Mg are a function of the microstructural position and are in agreement with retrograde exchange reactions: biotite included in or in contact with garnet has the maximum values, whereas crystals in the matrix have the minima. Titanium has systematic negative correlations with F, X-Mg, and (OH + F + Cl), whereas Al and octahedral vacancies are virtually constant.These trends indicate that the Ti-vacancy, along with substitutions involving Al, cannot explain the observed short-scale variations. Conversely, the Ti-oxy exchange appears to be active, resulting from combination of two vectors: the more conventional hydroxylation Ti4+ + 2O(2-) = (Fe,Mg)(2+) + 2OH(-) and the "fluorination" Ti4+ + 2O(2-) = (Fe,Mg)(2+) + 2F(-). The systematic retrograde redistribution involves not only Fe and Mg as commonly observed, but also Ti, F, and H, in a way such to eliminate the primary Ti-oxy component of biotite.

DOI： 10.2138/am.2008.2579

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Geological Society Special Publication  

In both palaeoenvironmental and palaeogeographical studies, Antarctica plays a unique role in our understanding of the history of the Earth. It has maintained a unique geographical position at the South Pole for long periods. As the only unpopulated continent, the absence of political barriers or short-term economic interests has allowed international collaborative science to flourish. Although 98% of its area is covered by ice, the coastal Antarctic region is one of the wellstudied regions in the world. The integrity and success of geological studies lies in the fact that exposed outcrops are well preserved in the low-latitude climate. The continuing programme of the Japanese Antarctic Research Expedition focuses on the geology of East Antarctica, especially in the Dronning Maud Land and Enderby Land regions. Enderby Land preserves some of the oldest Archaean rocks on Earth, and the Mesoproterozoic to Palaeozoic history of Dronning Maud Land is extremely important in understanding the formation and dispersion of Rodinia and subsequent assembly of Gondwana. The geological features in this region have great significance in defining the temporal and spatial extension of orogenic belts formed by the collision of proto-continents. Present understanding of the evolution of East Antarctica in terms of global tectonics allows us to visualize how continents have evolved through time and space, and how far back in time the present-day plate-tectonic regime may have operated. Although several fundamental research problems still need to be resolved, the future direction of geoscience research in Antarctica will focus on how the formation and evolution of continents and supercontinents have affected the Earth's environment, a question that has been addressed only in recent years. © The Geological Society of London 2008.

DOI： 10.1144/SP308.1

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MAT-251 mass-spectrometer installed at Shizuoka University, donated by the Thermo-Quest Co. Ltd., was renovated for precise carbon and oxygen isotopic measurement of very small amount of carbon dioxide. We examined the fundamental properties of mass-spectrometer, such as isotopic shift of reservoir CO2 during capillary leak separation, pressure effect on isotopic ratio and the correction of data with international standard material. We obtained consistent results that match the precision and accuracy with that of MAT-250 at the same laboratory. An online carbonate extraction apparatus was installed to the Inlet system for carbonate minerals including dolomite, calcite and aragonite using oil bath circulation heating system, which can attain temperatures up to 100°C. We describe the operation manual for precise measurement of small amount of carbon dioxide, derived from carbonate minerals reacted with conc. phosphoric acid and organic materials or graphite combusted in quartz tube.

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The 46th Japanese Antarctic Research Expedition (JARE-46) conducted field operations on geological and paleomagnatic studies in the regions of Lützow-Holm Bay, Prince Olav Coast and western Enderby Land during December 17, 2004 through February 17, 2005, under a part of the SEAL II (Structure and Evolution of East Antarctic Lithosphere II) project. A small helicopter was used to support field parties during the summer operations conducted by JARE-46. This report summarizes preparations for field operations and activities of the geological and paleomagnetic team, and points out some problems for the future planning. © 2006 National Institute of Polar Research.

DOI： 10.15094/00009352

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High-grade metamorphic rocks occurring along the Prince Harald, Sôya, and Prince Olav Coasts make up the Latest Proterozoic-Early Paleozoic Lützow-Holm Complex, which is the youngest orogenic belt in the East Antarctic Shield. A systematic increase in metamorphic grade from east to west, ranging from upper amphibolite facies on the eastern Prince Olav Coast to upper granulite facies at the head of Lützow-Holm Bay, has been well-established in the complex. However, granulites are newly found to occur as blocks sitting within meta-tonalites at Cape Hinode located on the amphibolite-facies eastern Prince Olav Coast. _ In addition, it is newly revealed that kyanite occurs rather commonly in meta-tonalites which contain hornblende with or without clinopyroxene. The modes of occurrence in the field, petrographical features, and major element bulk rock compositions of the granulites and related rocks are given in some detail in this study. These, along with the previously presented geochronological, geochemical and petrographical data, would indicate that the rocks in the Cape Hinode area as a whole make up a Mesoproterozoic allochthonous block in the Latest Proterozoic-Early Paleozoic Lützow-Holm Complex. © 2006 National Institute of Polar Research.

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We report here the mode of occurrence of calc-silicate rocks and marbles from the Lützow-Holm Complex, East Antarctica, and a worked example from Rundvågshetta. Calc-silicate boudins were observed in Cape Hinode, Akarui Point, Byôbu Rock, Skarvsnes, Skallevikshalsen and Rundvågshetta, whereas they were reported earlier from Sinnan Rock, Cape Ryûgû, Akebono Rock, Cape Hinode, Niban Rock, Kasumi Rock, Daruma Rock, Cape Omega, Langhovde, Ytrehovdeholmen and Skarvsnes. They vary in size from decimeters to few meters and are commonly enclosed within pelitic or psammitic gneisses. In addition, extensive layers of marbles and calc-silicate rocks are distributed in Skallevikshalsen. - The calc-silicate megaboudins within the layered pyroxene-gneiss from Rundvågshetta, up to 5 m long and 2 m thick, comprises of coarse to medium grained assemblage of scapolite + anorthite + garnet + clinopyroxene + calcite + quartz + titanite ± wollastonite. Co-existing scapolite and plagioclase suggest a "minimum" estimate of peak metamorphic temperature of ∼830°C. Peak metamorphic mineral assemblages equilibrated at moderate to high X CO2 conditions (0.3-0.7) and temperatures between 850 and 1000°C, consistent with the ultrahigh temperature metamorphic conditions reported in the region. Multistage garnet corona formation preserved in the calc-silicate assemblage suggests a local increase in hydrous fluid activity during retrogression. Preliminary bulk rock geochemistry of different mineralogical zones in the boudin shows chemical potential gradients in some major elements, especially SiO 2, Al 2O 3 and CaO, possibly controlled by the compositional variations in the protolith. Altogether, these results suggest that calc-silicate rocks preserve information on the metamorphic evolution and help us in deducing the geodynamic evolution of high-grade terrains. © 2006 National Institute of Polar Research.

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Mafic metamorphic rocks are widely distributed through the Lützow-Holm Complex (LHC) of East Antarctica, as layers between or enclaves within metasedimentary and metaigneous lithologies. It has been inferred that the peak metamorphic grade of the LHC progressively increases in a southwestern direction from amphibolite-facies to granulite-facies conditions, with mineral assemblages in the mafic metamorphic rocks changing from hornblende (magnesiohornblende) ± biotite + plagioclase to orthopyroxene + clinopyroxene ± hornblende (pargasite, magnesiohastingsite and tschermakite) ± biotite ± garnet + plagioclase. Field relationships suggest that amphibolite-grade mafic metamorphic rocks derive from mafic magma intruded into metasedimentary units, whereas granulite-facies mafic metamorphic rocks are a mixture of detrital blocks and mafic sill or intrusions. Major and trace element compositions of mafic metamorphic rocks are similar to those of igneous rocks of tholeiite affinity, and can be divided into volcanic-arc basalt (VAB)-type or mid-ocean ridge basalt (MORB)-type compositions. On a regional scale, VAB-type lithologies are predominant in amphibolite-facies areas, and MORB-type lithologies predominate in granulite-facies areas. On the basis of HFSE concentrations and Nb/Y ratios, MORB-type lithologies have T-type and E-type MORE compositions with oceanic plateau basalt and back-arc basin basalt affinities, and are occasionally found in the field intercalated with metasedimentary layers, characteristic of magmatism and sedimentation cycles in a marginal sea basin setting. Such field relationships provide information on the tectonic environment of protolith formation in the LHC. Various crustal components have been amalgamated into a relatively narrow mobile belt, which was subjected to high-grade metamorphism during the final closure of oceanic basins as a result of continent-continent collision. © 2006 National Institute of Polar Research.

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：BLACKWELL PUBLISHING  

Spectacular reaction textures in poikiloblastic scapolitite boudins, within marbles in the continental crust exposed in the Lutzow-Holm Complex, East Antarctica, provide insights into the changing fluid composition and movement of fluid along grain boundaries and fractures. Petrographic and geochemical features indicate scapolite formation under contrasting fluid compositions. Core composition of scapolite poikiloblasts (Scp(I)) are marialitic (Cl = 0.7 apfu) whereas rims in contact with biotite or clinopyroxene are meionite rich. Fine-grained recrystallized equigranular scapolite (Scp(II)) shows prominent chemical zoning, with a marialitic core and a meionitic rim (Cl = 0.36 apfu). Scapolite poikiloblasts are traversed by Scp(III) reaction zones along fractures with compositional gradients. Pure CO2 fluid inclusions are observed in healed fractures in scapolite poikiloblasts. These negative crystal-shaped fluid inclusions are moderately dense, and are believed to be coeval with Scp(III) formation at temperatures &gt; 600 degrees C and a minimum pressure of c. 3.8 kbar. Grain-scale LA-ICPMS studies on trace and rare earth elements on different textural types of scaplolites and a traverse through scapolite reaction zone with compositional gradient suggest a multistage fluid evolution history. Scp(I) developed in the presence of an internally buffered, brine-rich fluid derived probably from an evaporite source during prograde to peak metamorphism. Recrystallization and grain size reduction occurred in the presence of an externally sourced carbonate (CaCO3)-bearing fluid, resulting in the leaching of Cl, K, Rb and Ba from Scp(I) along fractures and grain boundaries. Movement of fluids was enhanced by micro-fracturing during the transformation of Scp(I) to Scp(III). Fractures in fluorapatite are altered to chlorapatite proving evidence for the pathways of escaping Cl-bearing fluids released from Scp(I). The present study thus provides evidence for the usefulness of scapolite in fingerprinting changing volatile composition and trace element contents of fluids that percolate within the continental crust.

DOI： 10.1111/j.1525-1314.2006.00635.x

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We report here the geology of Byôbu Rock and Gobanme Rock outcrops hitherto unmapped in the Prince Olav Coast, East Antarctica. Both these outcrops expose high-grade metamorphic and igneous rocks. The metamorphic rock units comprise mainly of gneisses migmatized to variable extent and amphibolites, whereas igneous rocks comprise of granites and pegmatites. Preliminary structural data obtained from the outcrops identified the regional, steep to moderately dipping foliation trend in the WNW-ESE at Byôbu Rock, whereas they trend in the NW-SE direction at Gobanme Rock. Two generations of folding were identified, an earlier tight isoclinal intrafolial folds and a late regional open fold. The three deformational events identified are comparable and consistent with those reported in the neighboring outcrops in the Prince Olav Coast. Metamorphic P-T conditions estimated based on various geothermobarometries indicate that the rocks have experienced granulite grade conditions during peak metamorphism (770-880°C and 6-9kbar). The finding of orthopyroxene at Byôbu Rock in this study extends the orthopyroxene-in isograd in the progressive metamorphic zone in the Lüitzow-Holm Complex further eastward beyond Tenmondai Rock. Inclusions of kyanite within garnet in metapelitic rocks suggest a clockwise P-T path, consistent with the P-T paths suggested for the Lützow-Holm Complex. Preliminary bulk rock geochemical investigations indicate that granitic rocks and quartzo-feldspathic gneiss mostly belong to volcanic arc granite suite, though their origin remains indistinct. Electron microprobe dating of monazites from representative rocks gave Pan-African ages (557±33Ma), consistent with the regional metamorphic ages reported earlier. Thus, based on the similarities on structural, petrological, geochemical and geochronological data, the Byôbu Rock and Gobanme Rock are considered to be integral part of the Pan-African Lützow-Holm Complex with analogous geological history. © 2006 National Institute of Polar Research.

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Cathodoluminesence (CL) microscopy of high-temperature skarn minerals from Fuka, Okayama, Japan, helped to distinguish minerals having similar optical properties and their internal textures. Spurrite and tilleyite, having similar optical properties, were easily distinguished using CL microscopy. Furthermore, growth patterns could be identified in single crystals of wollastonite using CL images. Although the chemical variations corresponding to the CL variations cannot be distinguished using electron microprobe analyses, they are considered to reflect minor variations in trace element and REE contents. Thus, cathodoluminescence microscopy is a usefut tool to distinguish minerals and helps us in selecting suitable sub-samples for detail studies on trace and rare earth element geochemistry.

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：UNIV CHICAGO PRESS  

The supracrustal Kerala Khondalite Belt (KKB; southern India) constitutes an important but little understood terrain in the evolution of East Gondwana. Here we report SHRIMP U-Pb zircon and titanite ages for a suite of metasedimentary rocks from the KKB. Metamorphic ages at 530.2 +/- 4.4 and 525 +/- 4.5 Ma were obtained from metapelitic and metapsammitic migmatites, respectively. These ages are interpreted to date peak metamorphism because both rock types have extensively melted under granulite-facies conditions. A similar age (520 +/- 4.7 Ma) was obtained from titanite from a granulite-facies calc-silicate rock. These ages are within error of SHRIMP ages for granulite-facies assemblages in Lutzow-Holm Bay, East Antarctica, the Highland and Wanni Complexes of Sri Lanka, and Southern Madagascar. We propose that this timing reflects a short span (similar to10 m.yr.) of high-temperature granulite-facies metamorphism and concomitant widespread migmatization that marks the final stage of collision between East and West Gondwana.

DOI： 10.1086/425971

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Graphite from deposits occurring in the high-grade metamorphic rocks and their larteritized equivalents of the Thodupuzha-Kanjirappally Belt in Madurai Granulite Block, southern India is structurally fully ordered (crystallite size, Lc((002)) ranging from 469 to 749 angstrom), possess high degree of graphitization (DG value ranging from 105 to 267 angstrom) and reflect crystallization at high temperature (700 +/- 100 degrees C). Raman spectra of graphite display profiles corresponding to high crystallinity and high structural ordering. The high temperature crystallinity characteristics of graphite were not obliterated during retrogression of granulites to amphibolite facies gneisses. Preliminary carbon stable isotope results show a spread in isotope values from -11.8 to -26.8 parts per thousand, which suggest more than one sources for carbon. The lighter carbon isotope values are suggestive of biogenic origin, whereas the heavier ones are probably fluid precipitated graphite.

DOI： 10.1016/S1342-937X(05)71120-5

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Graphite in deep crustal enderbitic (orthopyroxene + garnet + plagioclase + quartz) granulites (740 degrees C, 8.9 kb) of Nilgiri hills, southern India were investigated for their spectroscopic and isotopic characteristics. Four types of graphite crystals were identified. The first type (Gr(I)), which is interstitial to other mineral grains, can be grouped into two subtypes, Gr(IA) and Gr(IB). Gr(IA) is either irregular in shape or deformed, and rough textured with average delta C-13 values of -12.7 +/- 0.4 parts per thousand (n = 3). A later generation of interstitial graphite (Gr(IB)) shows polygonal crystal shapes and highly reflecting smooth surface features. These graphite grains are more common and have delta C-13 values of -11.9 +/- 0.3 parts per thousand (n = 14). Both subtypes show well-defined Raman shifts suggesting a highly crystalline nature. Cores of interstitial graphite grains have, on average, lower delta C-13 values by similar to 0.5 parts per thousand compared to that of the rim. The second type of graphite (Gr(II)) occurs as solid inclusions in silicate minerals, commonly forming regular hexagonal crystals with a slightly disordered structure. The third type of graphite (Gr(III)) is associated with solid inclusions (up to 100 mu m) that have decrepitation halos of numerous small (&lt; 15 mu m) satellite fluid inclusions of pure CO2 with varying density (1.105 to 0.75 g/cm(3)). The fourth type of graphite (Gr(IV)) is found as daughter crystals within primary type CO,fluid inclusions in garnet and quartz. These fluid inclusions have a range of densities (1.05 to 0.90 g/cm(3)), but in general are significantly less dense than graphite-free primary, pure CO2 fluid inclusions (1.12 g/cm(3)). Raman spectral characteristics of graphite inside fluid inclusions suggest graphite crystallization at low temperature (similar to 500 degrees C). The precipitation of graphite probably occurred during the isobaric cooling of CO2-rich peak metamorphic fluid as a result of oxyexsolution of oxide phases. The oxyexsolution process is evidenced by the magnetite-ilmenite granular exsolution textures and the systematic presence of numerous micron-sized rutile and other oxide inclusions in association with fluid inclusions within garnet, plagioclase, and quartz.
The carbon isotope compositions of coexisting CO2 (in fluid inclusions) and graphite show a fractionation (alpha(CO2-gr)) of -6 parts per thousand in Garnet, consistent with the existing theoretical estimates of alpha(CO2-gr) at 800 degrees C. A subsequent generation of CO2 inclusions trapped in matrix quartz and quartz segregation have higher values, -4 parts per thousand and -2.9 parts per thousand respectively. Graphite in quartz segregations also has higher delta C-13 values (-9.8 parts per thousand) than those in enderbite (-12.7 parts per thousand). Micro-graphite crystals included in garnet, quartz (enderbite), and quartz (segregation) have average delta C-13 values of -11.1, -10.4, and -8.7 parts per thousand respectively, indicating progressive enrichment in C-13 with a decrease in temperature of recrystallization of respective minerals. This progressive enrichment is also observed in carbon isotope compositions of fluid inclusion CO2, suggesting isotopic equilibrium during Graphite precipitation from CO2 fluids. Thus, the carbon isotope record preserved in these rocks by the interstitial graphite, CO2 fluid in enderbite, graphite inicrocrystals, graphite in quartz segregation, and CO2 fluid in quartz segregation, suggests a temperature-controlled isotopic evolution. This evolution is in accordance with a closed system Rayleigh-type graphite precipitation process which progressively enriched residual CO2 in C-13. Copyright (c) 2005 Elsevier Ltd.

DOI： 10.1016/j.gca.2005.02.007

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The contact aureole at Fuka, Okayama, Japan is peculiar for the occurrence of extensive high-temperature skarn resulting from the intrusion of Mesozoic monzodiorite into Paleozoic marine limestone. The occurrence is also notable for the finding of ten new minerals, of which five are calcium-boron-bearing minerals, and scores of other rare minerals. Skarn formation at Fuka can be classified into three major types 1. Grossular-vesuvianite-wollastonite endoskarn, 2. Gehlenite-exoskarns, and 3. Spurrite-exoskarns. Grossular-vesuvianite-wollastonite endoskarn forms a narrow zone (few centimeter width) separating the exoskarn and the igneous intrusion. It is also found, developed independently, along contacts of the younger basic intrusive dykes and limestone in the region. The gehlenite-exoskarns, in most cases, are spatially associated with igneous intrusion and are extensive (decimeter to meter thick). However, exceptions of independent gehlenite dikes are also observed. Retrogression of the gehlenite endoskarns results in the formation of hydrogrossular and/or vesuvianite. Accessory phases include schrolomite and perovskite. The predominantly monomineralic spurrite-exoskarn was formed in the outer zone of the gehlenite-skarn parallel to the contact as well as independent veins, dikes and tongues. The spurrite-exoskarn may extend tens of meters. Spurrite coexists with tilleyite or rankinite, although larnite is absent. Idiomorphic gehlenite and vesuvianite are the most common accessory phases observed. Retrograde hydration of spurrite to foshagite, scawtite and hillebrandite is commonly observed as veins and alteration zones within the spurrite exoskarn.
Petrogenetic grids were constructed using "THERMOCALC" for the observed mineral assemblage in the spurrite skarn. Mineral-fluid equilibrium in the CaO-SiO2-CO2 system was computed, considering the metasomatic input of aqueous silica. Phase diagram analysis in the form of T-X-CO2 grids with varying silica activity indicated that the stability field of spurrite is strongly controlled by the activity of silica in the fluid. Optimum silica concentration in the fluid was between 9.1 x 10(-4) and 4.5 x 10(-3) mol/liter, above which wollastonite becomes stable, whereas further reduced silica activity will generate larnite. Appropriate temperature condition for the formation of spurrite is between 850 degrees C and 1080 degrees C at an X-CO2 fluid composition of 0.05 to 0.42. At temperature conditions lower than 850 degrees C, the spurrite stability field becomes narrow, with low CO2 activity. The formation of extensive spurrite-exoskarn suggests that the silica activity, temperature and fluid composition remained within the spurrite stability field. Petrogenetic analysis of phase diagrams suggests that the exoskarn formation at Fuka contact aureole was robustly controlled by the activity of silica in the high temperature metasomatic fluid.

DOI： 10.2465/jmps.99.328

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Pressure and temperature (P-T)conditions of syn- and post-mélange formations of the Inui Melange in the Shimanto Belt, Akaishi Mountains, were estimated by fluid inclusion analysis. The Inui mélange is mainly of tectonic origin and was formed along a decollement in a subduction zone in the Late Cretaceous to the earliest Paleogene time. Several stages of vein development during the syn- and post-mélange formations were recognized. Fluid inclusion analyses were carried out on syn-mélange quartz veins and a post-mélange quartz vein, classified based on the cross-cutting relationships with mélange fabrics such as pressure-solution cleavages. The P-T conditions of the syn-mélange veins were in the range of 200 MPa to 320 MPa and 140°C to 240°C, whereas those of the post-mélange vein were in the range of 290 MPa to 350 MPa and 210°C to 290°C. Assuming the water depth between 0 to 6,000 m and total fluid pressure was equal to lithostatic pressure, the geothermal gradient of the syn- and post-mélange formations were estimated to be between 15°C/km and 30°C/km. This geothermal gradient is characteristic of sediment-dominated subduction zones.

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This study describes the optimum experimental conditions required for the conversion of oxygen to carbon dioxide produced in the laser ablation oxygen extraction system (Matsui et al., 2003). Purity of carbon used in the converter, duration of preheating between sample conversions (1hour at ∼630°C) and careful temperature control during conversion (600 °C) can yield high reproducibility. Experimental runs with oxygen gas from cylinder gave a reproducibility for δ18O values of ± 0.1‰ (n = 4).

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Two types of taenite lamellae were observed, i.e. the clear and the heterogeneous taenite, in the Widmanstätten structure in Toluca iron meteorite. Cooling rate for Toluca meteorite was determined using the central Ni content method, for both types of taenite. The Ni concentration within heterogeneous region (plessite) in taenite was approximated to fit the M-shaped distribution shown by the rim and parts of the central portions. The clear taenite lamellae and the heterogeneous taenite yield similar cooling rates of 100∼200°C /Ma. Based on this result, it is supposed that the heterogeneous taenite has been originally homogeneous in composition. As the temperature drops below 400°C (kamacite nucleation closure temperature), the interior of wider taenite transform into plessite. Textural observations indicate that the harp-like taenite structure is always associated with schreibersite (phosphide). The presence of P in Fe-Ni alloy has a pronounced effect on Ni diffusion and the formation of the Widmanstätten structure. The harp-like taenite may have been formed in high-P concentration regions in Fe-Ni alloy.

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Japan Association of Mineralogical Sciences  

The theme of the 2003 symposium of the Joint meeting of the Mineralogical Society of Japan, the Japanese Association of Mineralogists, Petrologists and Economic Geologists, and the Society of Resource Geology is carbon and carbonate minerals. The minerals are important in many fields of earth sciences, material sciences, biological sciences, and environmental sciences. In the symposium, we chose seven invited papers for natural and synthetic diamonds, porous carbon materials, methane hydrates, rare-earth carbonate minerals, and paleoenvironmental studies using carbon and carbonate materials.

DOI： 10.2465/gkk.33.96

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A 2 cm long and 0.8 cm thick, single graphite crystal embedded in quartz, feldspar, and orthopyroxene of a gramilite-facies metamorphic rock from southern India exhibits unique homogeneity in delta(13)C values along layers parallel to the (0001) surface and smooth unidirectional variation of about 1parts per thousand along the c-crystallographic axis. The fluid-precipitated graphite shows isotope homogeneity indicative of crystal growth along the (0001) plane. The C-isotope data along with the textural features suggest that the formation of graphite initiates in a layer perpendicular to the c-axis, possibly by a spiral growth mechanism. The initial layer is followed layer after layer, until the final stages of graphite growth. A second stage of graphite growth unrestricted by surrounding minerals is inferred from the un-deformed idiomorphic hexagonal form of the overgrown crystals. The C-isotope values in the overgrown rim of the crystal are lowered by about 2parts per thousand, suggesting a multistage precipitation. Morphologic and stable isotope studies thus confirm uniform crystal growth of graphite recording an isotope evolution pattern relating to fractionation from a fluid in an infinite reservoir during the main crystal formation. Spiral growth parallel to the (0001) face can explain the perfect uniformity of stable isotope composition in a single layer, whereas the variation across the (0001) surface gives insights into the time-integrated fluid evolution attending the crystal growth.

DOI： 10.2138/am-2003-11-1208

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High vacuum line made of stainless steel and computer controlled pneumatic valves attached with a 40W CO2 laser system capable of extracting oxygen from rock forming silicate and oxide minerals in a BrF5 atmosphere was fabricated. The vacuum line can resist the hazardous BrF5 reagent used for disintegrating the Si=0 bonds in minerals. The oxygen, thus released, is converted to CO2 in an O2→CO2 converter using graphite heated at ∼ 650°C in a platinum foil holder. We present here the results on vacuum checks, initial setting of laser, reproductibility of the O2→CO2 converter, background measurements during fluorination and precautions in handling the BrF5 reagent. The present system can withstand high vacuum conditions without appreciable leaks for intervals required to complete one cycle of measurement. However, due to the toxic nature of BrF5 reagent, sizeable amount of background 02 was released during the pre-fluorination of the system (∼ 0.2-0.3 μmoles of O2 for 30 minutes of fluorination). Repeated fluorination of the system considerably reduced the background O2, however, could not be completely eliminated. Tests were also made for the functional setting of the O2→CO2 converter using ampoules of external O2 gas. Optimum conditions for conversion were determined; temperature of ∼ 650°C, graphite grain size fraction of 1 ∼ 2 mm and reaction time of 15 minutes. NBS-28 quartz standard were pre-fluorinated overnight in the reaction chamber in order to reduce the background effect. δ 18OSMOW values of 14 measurements with varying amounts of NBS-28 quartz gave 10.06 ± 1.15%. The results indicate that a minimum sample size of 250 μg is essential to overcome the background effects. However, this amount is almost four times less than the usual amount of sample used in laser fluorination laboratories elsewhere. Also, it was found that the laser beam should be defocused for precise measurement of small grain size samples. Thus, at present, the presence of background O2 released in the reaction chamber is hindering the application of micrometer scale measurement of oxygen isotope measurements in minerals. In order to improve the present system, the background within the reaction chamber should be reduced further.

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

The Digana metagranitoid, situated within the Digana shear zone, a critical junction between the Highland and Wanni complexes of Sri Lanka, hosts a variety of graphite types with structural evidence showing formation during different stages prior to and after deformational events affecting this terrain. Here we discuss their occurrence, association and stable isotopes. The carbon isotopes of graphite reported in this study from the Sri Lankan high-grade metamorphic terrain differ significantly from the composition of graphite reported elsewhere in other Precambrian terrains. The stable isotope composition from random crystals depicts an enriched carbon isotopic composition in the range -1.6 parts per thousand to -8 parts per thousand, suggesting precipitation by reduction from CO2-rich fluids. Intracrystalline microscale traverses made on graphite crystals indicate the delta(13)C composition of the rims becoming progressively lighter in C-13, in metagranitoid hosted samples, with the variation reaching up to 1.5 parts per thousand, consistent with the changing fluid regimes induced by decreasing temperature during the graphite growth, and shows the preservation of core at higher temperatures. A highly enriched carbon isotopic composition ( -0.6 parts per thousand) with a rimward enrichment of C-13 observed in migmatites, contrary to the values in metagranitoid, indicates preferential enrichment of heavy isotopes as a result of the partial melting and dissolution/reprecipitation of graphite. The oxygen isotopic composition of a quartz-calcite vein show values in the range 18.5 parts per thousand to 20 parts per thousand, typical for recycled crustal source. The tight constraint in the isotopic data from different graphite types studied indicates a homogeneous fluid source for the deposits. Prolonged fluid flow in this region is envisaged in this study, which is related to the multistage deformation of the crustal scale shear zone. (C) 2002 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0009-2541(02)00400-X

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Fluid infiltration into active faults and shear zones is usually studied in middle to upper crustal environment. Circulating fluids deposit fine-grained sediments like clay and carbonate material into cracks within the fault zone. Such crack-fill fine-grained material, calcite veins, and oxidized/weathered open cracks are well observed in the drill cores, from near-surface to a depth of 1800 m, in the Nojima fault zone, Japan, which triggered the 1995 M7.2 Kobe earthquake. Powder X-ray diffraction analysis indicates that the crack-fill fine-grained materials are mainly composed of siderite, calcite, laumontite, and fine-grained clasts of granitic rock. Isotopic analyses of carbonate material within the fine-grained materials and calcite veins reveal variable delta(13)C (SMOW) values ranging from 19.3parts per thousand to 27.7parts per thousand and delta(13)C (PDB) values of -4.6parts per thousand to -18.5parts per thousand which are consistent with those of typical meteoric and seawater. C-14 dates of 10 calcite vein samples range from 35.0 to 58.4 kyr B.P.
Geological, petrological, stable isotopic, and C-14 data suggest that these crack-fill fine-grained materials and calcite veins and brown open cracks were developed by the repeated infiltration of O-2- and CO2-bearing meteoric and seawater downward into the deep Nojima fault zone during the last 35-60 kyr. We propose a seismic fault suction-pumping model to interpret the infiltration of subsurface waters being carried down into the deep fault zone by rapid potential change during episodes of seismic faulting. (C) 2003 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.chemgeo.2003.08.010

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：MINERALOGICAL SOC  

Scheelite mineralization in the granulite-facies supracrustal sequences of the Kerala Khondalite Belt, Southern India is reported. The supracrustal sequences where the mineralization is found comprise granulite-grade metasediments which underwent metamorphism at similar to550 Ma. The mineralization is assumed to have formed by late-stage metasomatism that overprinted the regional metamorphism of the country rock (garnet-biotite gneiss) and occurs along a quartz vein that intrudes the regional foliation. The paragenetic data from the vein demonstrate unambiguously a separate cycle of hydrothermal activity, resulting in metasomatism and mineralization. Scheelite is found in both the altered host rock along he foliation plane and in the quartz vein. Fluid inclusions preserved in the vein suggest that the mineralizing fluids were saline-aqueous in composition, while those in the country rocks were predominantly CO-rich. The mineral chemistry and bulk-rock chemical composition of the mineralized domain reveal the unusual enrichment of Ca in the mineralised zone with the depletion of K. We propose that fluid discharging from a crystallizing deep-seated magma, mixing with deep circulating Ca-bearing palaeo-groundwater gave rise to the deposition of scheelite. The scheelite mineralization and the quartz vein emplacement occurred after the Pan-African regional metamorphism.

DOI： 10.1180/0026461036730111

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：BLACKWELL PUBLISHING INC  

Nine marble horizons from the granulite facies terrane of southern India were examined in detail for stable carbon and oxygen isotopes in calcite and carbon isotopes in graphite. The marbles in Trivandrum Block show coupled lowering of delta(13)C and delta(18)O values in calcite and heterogeneous single crystal delta(13)C values (-1 to -10parts per thousand) for graphite indicating varying carbon isotope fractionation between calcite and graphite, despite the granulite facies regional metamorphic conditions. The stable isotope patterns suggest alteration of delta(13)C and delta(18)O values in marbles by infiltration of low delta(13)C-delta(18)O-bearing fluids, the extent of alteration being a direct function of the fluid-rock ratio. The carbon isotope zonation preserved in graphite suggests that the graphite crystals precipitated/recrystallized in the presence of an externally derived CO2-rich fluid, and that the infiltration had occurred under high temperature and low f(O2) conditions during metamorphism. The onset of graphite precipitation resulted in a depletion of the carbon isotope values of the remaining fluid + calcite carbon reservoir, following a Rayleigh-type distillation process within fluid-rich pockets/pathways in marbles resulting in the observed zonation. The results suggest that calcite-graphite thermometry cannot be applied in marbles that are affected by external carbonic fluid infiltration. However, marble horizons in the Madurai Block, where the effect of fluid infiltration is not detected, record clear imprints of ultrahigh temperature metamorphism (800-1000 degreesC), with fractionations reaching &lt;2 parts per thousand. Zonation studies on graphite show a nominal rimward lowering delta C-13 on the order of 1 to 2 parts per thousand. The zonation carries the imprint of fluid deficient/absent UHT metamorphism. Commonly, calculated core temperatures are &gt;1000 degreesC and would be consistent with UHT metamorphism.

DOI： 10.1046/j.1525-1314.2002.00371.x

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：BLACKWELL SCIENCE INC  

An extensive humite-bearing marble horizon within a supracrustal sequence at Ambasamudram, southern India, was studied using petrological and stable isotopic techniques to define its metamorphic history and fluid characteristics. At peak metamorphic temperatures of 775 +/- 73 degreesC, based on calcite-graphite carbon isotope thermometry, the mineral assemblages suggest layer-by-layer control of fluid compositions. Clinohumite + calcite-bearing assemblages suggest X-CO2 (0.4 (at 700 degreesC and 5 kbar), calcite + forsterite + K-feldspar-bearing assemblages suggest X-CO2 &gt; 0.9 (at 790 degreesC); and local wollastonite + scapolite + grossular-bearing zones formed at X-CO2 of c. 0.3. Retrograde reaction textures such as scapolite + quartz symplectites after feldspar and calcite and replacement of dolomite + diopside or tremolite + dolomite after calcite + forsterite or calcite + clinohumite are indicative of retrogression under high X-CO2 conditions. Calcite preserves late Proterozoic carbon and oxygen isotopic signatures and the marble lacks evidence for extensive retrograde fluid infiltration, while during prograde metamorphism the possible infiltration of aqueous fluids did not produce significant isotopic resetting. Isotopic zonation of calcite and graphite grains was likely produced by localized CO2 fluid infiltration during retrogression. Contrary to the widespread occurrence of humite-marbles related to retrograde aqueous fluid infiltration, the Ambasamudram humite-marbles record a prograde-to-peak metamorphic humite formation and retrogression under conditions of low X-H2O.

DOI： 10.1046/j.0263-4929.2001.00318.x

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Field observations in the Sakoli and Sausar belts around Nagpur, central India were conducted from December 28th 1999 to January 8th 2000, by a Japanese and Indian joint project team. Both the belts are Proterozoic mobile belts consisting of supracrustal sequences and reworked basement gneissic complex, and form an integral part of the central portion of the Central Indian Tectonic Zone (CITZ). Field work covered the typical occurrences of metarhyolite, metabasalt and metasediments of the Sakoli Belt, and metasediments, syngenetic manganese deposits, granulite facies rocks and reworked basement gneisses in the Sausar Belt, in order to obtain the general view of the tectonic history of the CITZ. Observations were focused also in the tectonic boundaries between both belts and their basements. Deformations consisting of D1, D2 and D3 folds affect the Sakoli supracrustals and basements, forming the eastern and western marginal shear zones. The Sausar supracrustals are also deformed, and their basement Tirodi Gneiss is re-folded and found as several tectonized intercalations within the Sausar Belt. This paper reports the outline of geology and lithological and structural features of observed outcrops in two belts, with comments for several potential problems that were identified during the field works.

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：INT ASSOC GONDWANA RESEARCH  

The Kerala Khondalite belt is a Proterozoic metasupracrustal granulite facies terrain in southern India comprising garnet-biotite gneiss, garnet-sillimanite gneiss and orthopyroxene granulites as major rock types. Gale-silicate rocks and marbles, occurring as minor lithologies in the Kerala Khondalite Belt, show different mineral assemblages and reaction histories of which indicate a metamorphic P-T- fluid history dominated by internal fluid buffering during the peak metamorphism, followed by external fluid influx during decompression. The carbon and oxygen isotopic compositions of calcite from three representative metacarbonate localities show contrasting evolutionary trends. The Ambasamudram marbles exhibit carbon and oxygen isotope ratios (delta C-13 similar to 0 parts per thousand and delta O-18 similar to 20 parts per thousand) typical of middle to late Proterozoic marine carbonate sediments with minor variation ascribed to the isotopic exchange due to the devolatilization reactions. The delta C-13 and delta O-18 values of similar to -9 parts per thousand and 11 parts per thousand, respectively, for calcite from calc-silicate rocks at Nuliyam are considerably low and heterogeneous. The wollastonite formation here, possibly corresponds to an earlier event of fluid infiltration during prograde to peak metamorphism, which resulted in decarbonation and isotope resetting. Further, petrologic evidence supports a model of late carbonic fluid infiltration that has partially affected the calc-silicate rocks, with subsequent isotope resetting, more towards the contact between calc-silicate rock and charnockite. At Korani, only oxygen isotopes have been significantly lowered (delta O-18 similar to 13 parts per thousand) and the process involved might be a combination of metamorphic devolatilization accompanied by an aqueous fluid influx, supported by petrologic evidence. The stable isotope signatures obtained from the individual localities, thus indicate heterogeneous patterns of fluid evolution history within the same crustal segment.

DOI： 10.1016/S1342-937X(05)70337-3

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Authorship：Last author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER  

Scapolites developed variably in the deep-seated metagabbros of the Kohistan Are, north Pakistan, were investigated to evaluate the distribution and migration of C-O-H-S fluid and origin of carbon in the lower crust. Scapolites can be classified, based on their distinct difference in occurrence and chemical composition, into carbonate type and sulfate-rich type. Carbonate-type scapolites occur around carbonate pods in the layered gabbros at the mid-crustal levels and were formed by an increase in the CaO activity in the country rocks and simultaneous decarbonation of calcite. Predicted equilibrium CO2 mole fraction in fluid (X-CO2) is sufficiently low to be consistent with removal of CO2. Sulfate-rich scapolites occur in the garnet-bearing granulites at the lower crustal level and were formed by breakdown reactions of garnet and plagioclase under relatively higher CO2 activity (a(CO2)) and oxygen fugacity. The carbon isotopic composition (delta C-13) in carbonate scapolite is equivalent to the juvenile mantle (-5 to -6 parts per thousand), whereas sulfate-rich scapolite has heavy carbon stable isotope values close to 0 parts per thousand. These results reveal that carbonate scapolite was formed by decarbonation and metasomatism of igneous carbonate pods with the host metagabbros under granulite facies conditions, and sulfate-rich scapolites were formed by infiltration of CO2-rich fluid derived from decarbonation of carbonate-bearing sediments in the subducting slab.

DOI： 10.1007/s004100000207

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DOI： 10.15083/0000032608

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

Carbon isotope thermometry between calcite and graphite has been applied to the granulite facies marble assemblages in the Skallen region of the Lutzow-Holm Bay, East Antarctica. The carbon and oxygen isotope values of calcite from five samples, which are interlayered with pelitic and charnockitic assemblages, suggest within-layer isotopic equilibrium. Textural studies of graphite crystals indicate two types; one smooth, highly reflecting regular shaped graphite and the other rough, dull and irregular shaped graphite. The delta(13)C values of the dull surfaced graphite are lower by about one per mil than the high reflecting graphite. Carbon isotopic compositions suggest that graphite with lower delta(13)C values is confined to the surface layer of the graphite crystal. This is indicative of a later-stage overgrowth during retrograde metamorphism. delta(13)C of dull graphite was excluded in calculating the temperature of peak metamorphism. Temperatures of similar to 850 degrees C correspond to the peak metamorphic temperatures for the region. This temperature estimate is consistent with the regional metamorphic conditions based on cation exchange thermometry in pelitic granulites and is interpreted to be more precise. (C) 2000 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0009-2541(99)00189-8

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

Melt inclusions in olivine and pyroxene phenocrysts in kersantite and camptonite at Chhaktalao in Madhya Pradesh, India are mainly of the evolved type forming daughter minerals of olivine, pyroxene, plagioclase, spinel, mica, titanomagnetite and sulphides. Heating studies exhibit a temperature range from 1215 degrees to 1245 degrees C for the melt inclusions in olivine in camptonite and 1220-1245 degrees C for olivine in kersantite. The temperature for melt inclusions in pyroxene ranged from 1000 degrees to 1150 degrees C in camptonite and 850-1100 degrees C for pyroxene in kersantite. The bubble inside these melt inclusions is mainly CO2. The T(h)degrees C of CO2 into liquid phase occurred between 26 degrees and 31 degrees C in olivine and 25-30 degrees C: in pyroxene from kersantite and camptonite. The maximum density estimated is 0.72 g/cm(3) and the minimum is 0.45 g/cm(3). The depth of entrapment of the melt inclusion is estimated between 10-15 km. The pressure of entrapment of melt inclusion in olvine is 4.6 kbar where as that in pyroxene is 3.7 kbar. The lamprophyres in the Chhaktalao area are considered to be derived from low depth and low pressure region, possibly within spinel Iherzolite zone. (C) 2000 Elsevier Science Ltd. All rights reserved.

DOI： 10.1016/S1367-9120(99)00042-5

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：UNIV CHICAGO PRESS  

Ultrahigh-temperature (UHT) metamorphism in the Madurai Block of the southern Indian granulite terrain has been verified using the calcite-graphite isotope exchange thermometer. Carbon isotope thermometry has been applied to marbles from a locality near the reported occurrence of sapphirine granulites that have yielded temperature estimates of around 1000 degrees C. The delta(13)C and delta(18)O values of calcite are homogenous, implying equilibration of the isotopes during metamorphism. However, the delta(13)C values of Single graphite crystals show variations in the order of 1%o within a hand specimen. Detailed isotopic zonation studies indicate that graphite preserves either the time-integrated crystal growth history or reequilibrium fractionation during its cooling history. The graphite cores preserve higher delta(13)C values than the rims. The fractionation between calcite and graphite cores gives the highest metamorphic temperature of about 1060 degrees C, which matches the petrologically inferred temperature estimates in the high-magnesian pelites. The fractionation between graphite rims and calcite suggests a temperature of around 750 degrees C, which is interpreted to reflect retrograde cooling. This event is also observed in the sapphirine granulites. Calcite-graphite thermometry thus provides a useful tool to define UHT metamorphism in granulite terrains.

DOI： 10.1086/314407

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)  

The southern Indian granulite terrain has been the focal area for studies on the role of fluids in granulite petrogenesis for the past two decades. The present contribution is an overview on the metamorphic evolution of metacarbonate lithologies from the southern part this terrain (Trivandrum Block). Field survey in the granulite terrain of Trivandrum Block (TB) was carried out to identify specific outcrops where calc-silicate rocks and marbles are exposed and detailed studies were carried out on an outcrop scale. Based on mineralogy, the calc-silicate rocks are classified into four types: Type-I, lacking wollastonite and grossular, Type-II, wollastonite bearing but grossular absent, Type-III, wollastonite and grossular bearing, and Type-IV, calcite-rich marbles. Detailed petrographic studies reveal a variety of reaction textures overprinting the polygonal, granoblastic, peak-metamorphic assemblages in these rocks. The type-II assemblages provide CO2 activity estimates of >0.5 with a peak metamorphic temperature of about 790°C. Initial cooling followed by later CO2 influx can be deduced from reaction modeling in these calc-silicate rocks. Type-III assemblages are characterized by internal fluid buffering during the peak metamorphism. The reaction topologies produced are good approximations for the peak as well as retrograde mineral assemblages and reaction textures. The textural features interpreted with activity-corrected grids indicate a phase of isobaric cooling from about 835°C to 750°C at 6 kbar. The Type-IV marble assemblages indicate internal fluid buffering followed by localized CO2 influx. Forsterite+spinel-bearing assemblages were formed under granulite-facies P-T conditions and internal fluid buffering. Phlogopite, pargasite and humite group minerals occur as equilibrium peak mineral assemblages and have high fluorine contents, which attest to the stability of these minerals at granulite facies P-T conditions. Although humite-bearing assemblages suggest low a(CO(2)) for the metamorphic fluid, forsterite + spinel-bearing assemblages reflect moderate to high a(CO(2)) and the local wollastonite + scapolite + grossular-bearing zones were formed under moderate to low a(CO(2)). Retrograde reaction textures such as scapolite-quartz symplectites after feldspars and calcite, and coronal replacement of dolomite+diopside or tremolite+dolomite after calcte+forsterite or calcite+clinohumite are indicative of retrogression under higher a(CO(2)) conditions. The retrograde metamorphic P-T fluid evolution of the Trivandrum Block has been revised using the reaction textures in calc-silicate rocks and marbles. The reaction textures now preserved are consistent with post-peak cooling to 750°C whilst pressures were still >5 kbar. This near-isobaric P-T trajectory contrasts with the decompressional (ITD) P-T evolution documented in previous studies of the TB. However, as the previously deduced ITD evolution occurred at temperatures <750°C, the P-T record in the calc-silicates can be reconciled with that seen in the metapelites if the cooling textures pre-date the ITD. In this scenario, the TB P-T path would be one that initially involved near-isobaric cooling (IBC) at 5-6 kbar from peak conditions of >830°C to ca. 750°C and subsequently involved continued cooling, but with substantial exhumation, resulting in a final phase of post-peak decompression to 3-4 kbar at temperatures less than 750°C. The ITD phase of this post-peak evolution correlates with medium- to low-pressure fluid infiltration events. This study is unique in the finding of unequivocal evidences of high temperature metamorphism in TB granulites with internal fluid buffering, with an early retrograde cooling event and local fluid infiltration events during later decompression. The results of peak metamorphic conditions and the early cooling event contrasts with the earlier studies in the terrain and are comparable with those observed in other granulite terrains in the East Gondwana ensemble, while some of the textures resulting from carbonic fluid infiltration event are peculiar for the TB.

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：CAMBRIDGE UNIV PRESS  

Recent discovery of wollastonite-bearing calc-silicate assemblages adjacent to gneiss-charnockite horizons in the supracrustal terrain of the Kerala Khondalite Belt, southern India, provides an opportunity to evaluate the carbonic fluid infiltration model proposed for charnockite formation. Petrological and fluid inclusion studies across these horizons in three representative localities are presented in this study. The calc-silicate assemblages define peak metamorphic conditions of similar to 800 degrees C at 5 kbar and define a low a(CO2). Adjacent charnockite assemblages developed through dehydration involving the breakdown of garnet, biotite and quartz to produce orthopyroxene under low a(H2O) conditions. Retrograde reactions preserved in the calc-silicate rocks, such as scapolite-quartz symplectites, and the partial breakdown of wollastonite previously has been attributed to a near isothermal decompression during which infiltration of CO2-rich fluids occurred. Fluid inclusion studies indicate that the earliest generation of fluids preserved in the calc-silicate assemblages are aqueous (with salinity similar to 8 wt % NaCl equivalent), consistent with mineral phase equilibria defining low a(CO2). The estimation of NaCl content in brines coexisting with scapolite, based on the Cl content of the scapolite, indicates the presence of up to 20 wt % NaCl during the formation of scapolite consistent with the saline primary fluid inclusions. Primary carbonic inclusions occur within the retrogressed calcite + quartz assemblage after wollastonite, and are considered to represent the post-peak metamorphic carbonic fluid infiltration event, synchronous with the development of charnockites in the adjacent gneisses. These inclusions have identical characteristics to those in the charnockites. We envisage that the Kerala Khondalite Belt fluid regime was largely internally buffered during the prograde path, and that CO2 infiltration post-dated peak metamorphism. Influx of CO2 was mostly structurally controlled, and occurred along a near-isothermal uplift path. Graphite-bearing pegmatitic dykes with abundant CO2-rich inclusions in these localities attest to the transfer of carbonic fluids through magmatic conduits.

DOI： 10.1017/S0016756897008145

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Field surveys in southern to eastern India including the Eastern Ghats Granulite Belt, Nellore Schist Belt, Trivandrum Granulite Belt and Periyar Madurai Granulite Belt were conducted during August-September 1996 by several geologists from Japan, India and Russia. The surveys went successfully, with about 500 man-days of survey and collection of about two tons of rock samples being made. In this paper, some important field operations and observations are given, with field photos. Detailed laboratory work is going on and the results will appear elsewhere in future.

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：ASSOC ENGINEERING GEOLOGISTS GEOLOGICAL SOCIETY AMER  

Across a 1.2-mm-wide domain, a calcite crystal from a granulite-facies marble from East Antarctica exhibits microscale oxygen isotope heterogeneity that has been enhanced by channelized fluid flow. The oxygen isotope compositions range over 21 parts per thousand--a gigantic zonation. Calcite along a boundary with phlogopite has an average delta(18)O value of -5 parts per thousand; along a profile that extends about 700 mu m into the grain, the oxygen isotope composition remains constant, indicating equilibrium with source fluids of meteoric origin. Analyses of a transect away from the phlogopite show the delta(18)O values rising to +16 parts per thousand. The corresponding carbon isotope variation is only about 0.6 parts per thousand. Isotope heterogeneity is observed mainly in a phlogopite-rich zone where the delta(18)O values are lower (13 parts per thousand +/- 5 parts per thousand) than those away from the zone (17 parts per thousand +/- 1 parts per thousand). We inferred that preferential microfracturing and hydrothermal fluid flaw were the causes of the O-18 distribution, The constant values of both delta(18)O and delta(13)C for the distance of 700 mu m from the grain boundary suggest a solution-reprecipitation mechanism, whereas the smoothly increasing delta(18)O and decreasing delta(13)C values farther along the profile indicate a diffusion-controlled isotope exchange between the reprecipitated calcite and the original grain (core). Our results provide important evidence for the mechanism of isotope exchange between fluids and minerals and have major implications for microfracture-controlled fluid-flow processes in the Earth's crust.

DOI： 10.1130/0091-7613(1998)026<0251:FFAMIC>2.3.CO

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：GEOLOGICAL SOC INDIA  

Carbonate-rich racks occur as E-W trending dyke-like bodies in the Deccan 'basalts at Chhaktalao area in the lower Narmada valley and show a structurally controlled mode of emplacement. Modal composition shows calcite as the dominent mineral (similar to 90%). Ankerite, apatite, altered olivine, augite and opaques account for the remaining percentage. Geochemically, these rocks show high CaO (&gt;50%) with low SiO2, TiO2, FeO* and K2O. Trace element concentrations are low compared to avenge carbonatites. Stable isotope studies show that delta(13)C (-1.74 parts per thousand to -2.8 parts per thousand) and delta(18)O (+12.75 parts per thousand to +13.35 parts per thousand) values do not compare with those of "primary igneous carbonatites" eventhough they fall within the wide range of "igneous carbonatites". These rocks can be considered only as carbonate-rich rocks and not as true carbonatites.

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：MINERALOGICAL SOCIETY  

Humite group minerals occur in the marbles of granulite grade at Ambasamudram, southern India. Detailed mineralogical and mineral chemical characterization indicate the mineral is a flourine-rich titanian-poor variety of clinohumite. Average M-Ti/Si values of 2.22 with typical XRD pattern indicate the mineral to be clinohumite. Petrological constraints on the clinohumite formation shows a high temperature (&gt;700 degrees C) and low a(CO2) during its formation. The fluorine content of these clinohumites is the highest reported in any environment, with F/(F+OH) ratio reaching a value of 0.70. The high fluorine content reflect the high P-T condition of formation. The OH content of the clinohumites is around 0.59 mole fraction. Preliminary FTIR spectra of the clinohumites show eight sharp absorption peaks between wave numbers 3700 and 3400 cm(-1) and a broad absorption band with a peak at 3840 cm(-1). The sharp peaks are due to OH in the clinohumite. The high fluorine content of the Ambasamudram clinohumites possibly resulted from the isochemical reactions involving (OH-F) silicates such as amphiboles or phlogopites. The internal fluid buffering is also supported by the stable isotope as well as the petrological studies of the marble assemblages.

DOI： 10.1180/002646198547882

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

Scapolite-wollastonite-grossular bearing calc-silicate rocks from the Vellanad area in the Kerala Khondalite Belt (KKB) of Southern India preserve a number of reaction textures which help to deduce their P-T-fluid history. Textures include calcite + plagioclase +/- quartz symplectites after scapolite, grossular + quartz coronas between wollastonite and plagioclase, grossular coronas between wollastonite and plagioclase + calcite that replace former scapolite, and grossular blebs replacing anorthite + calcite + quartz pseudomorphs of scapolite. Garnet coronas are also observed between clinopyroxene and wollastonite or scapolite or plagioclase. The reactions, apart from those involving clinopyroxene, can be modelled in the simple CaO-Al2O3-SiO2-CO2 system and interpreted using partial reaction grids constructed for the activities of end-members in the analysed phases. The reaction topologies produced are good approximations for the peak as well as retrograde mineral assemblages and reaction textures. For the compositions of the phases present in this study, the medium pressure calc-silicate assemblages an defined by the stable pseudo-invariant points [Qtz], [Mei] and [Grs]. The textural features interpreted using these activity-corrected grids indicate a phase of isobaric cooling from about 835 degrees C to 750 degrees C at 6 kbar in the Vellanad area. This is inconsistent with earlier studies on other Lithologies from the KKB, most of which imply a post-peak P-T path involving near-isothermal decompression. However, as the temperatures obtained for the KKB from the calc-silicates are higher than those previously deduced from metapelites and garnet-orthopyroxene assemblages, the phase of near-isobaric cooling reported here is inferred to have proceeded prior to the onset of the decompression documented from studies of other rock types. (C) 1998 Elsevier Science B.V, All rights reserved.

DOI： 10.1016/S0024-4937(98)00017-6

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Publishing type：Research paper (scientific journal)   Publisher：Elsevier {BV}  

DOI： 10.1016/s0895-9811(97)90005-5

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DOI： 10.1016/s0899-5362(97)90026-9

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：GEOLOGICAL SOC INDIA  

Calcite-graphite isotope exchange thermometer is a useful tool in elucidating the peak metamorphic temperatures in regionally metamorphosed marbles, for which normal cation exchange thermometers are not available.

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DOI： 10.1016/s0895-9811(97)90070-5

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DOI： 10.1016/s0899-5362(97)86873-x

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：GEOLOGICAL SOC INDIA  

Colour-coded compositional mapping under an electron microprobe provides useful information on chemical gradients within the domain of individual minerals. Here we apply this technique to granulite facies reaction texture's from a calc-silicate-charnockite interface at Nuliyam, southern Kerala, to understand the element mobility patterns and mass balance related to fluid-induced metasomatism during deep crustal metamorphism. The contact zone assemblage at Nuliyam is characterized by plagioclase + quartz symplectites after K-feldspar. From element mapping of this reaction interface on micron - scale, and from quantitative analyses of the individual mineral phases under an electron microprobe, we trace the diffusion of K, and addition of Na and Ca, through an open system migration during the influx of externally-derived CO2- rich fluids. Our study provides the first direct evidence for microscale chemical diffusion associated with CO2-induced charnockite formation in southern Kerala, and illustrates the potential of colour-coded compositional mapping in tracing element mobility patterns.

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

This paper reports several new localities of wollastonite- and scapolite-bearing calc-silicate assemblages from the granulite-facies supracrustal Kerala Khondalite Belt (KKB), southern India. Based on mineralogy, these calc-silicate rocks are classified into four types: Type I, lacking wollastonite and grossular; Type II, wollastonite-bearing but grossular-absent; Type III, wollastonite- and grossular-bearing; and Type IV, dolomitic marbles. Detailed petrographic studies reveal a variety of reaction textures overprinting the polygonal granoblastic peak metamorphic assemblages in these rocks. The Type II calc-silicate rocks preserve reaction textures, including meionite breaking down to anorthite-calcite-quartz, wollastonite breaking down to calcite-quartz and meionite-quartz symplectites after K-feldspar and wollastonite. Type III calc-silicate rocks have porphyroblastic and coronal grossular. Grossular-quartz coronas separating wollastonite and anorthite and the development of grossular within the anorthite-calcite-quartz pseudomorphs of meionite form important retrograde reaction textures in this type. In Type IV dolomitic marble assemblages, meionite forming in grain boundaries of calcite and feldspars, forsterite rimmed by diopside-dolomite and the formation of grossular in feldspar-rich zones are the important textures. Calculated partial petrogenetic grids in the CaO-Al2O3-SiO2-CO2 system are used to deduce the pressure-temperature-fluid evolution of the calc-silicate rocks. The Type II assemblages provide CO2 activity estimates of &gt;0.5, with a peak metamorphic temperature of about 790 degrees C. Initial cooling followed by later CO2 influx can be deduced from reaction modelling in these calc-silicate rocks. Type III assemblages are characterized by internal fluid buffering throughout their tectonic history. The formation of coronal grossular indicates an initial cooling from peak metamorphic temperatures of about 830 degrees C deduced from vapour-absent meionite and grossular equilibria. Type IV marble assemblages also indicate internal fluid buffering followed by localized CO2 influx. Overall, the calc-silicate rocks of the KKB define peak metamorphic temperatures in the range of 790-850 degrees C, with an internally buffered fluid composition during the peak conditions. Initial cooling was followed by localized carbonic fluid influx that also post-dated decompression deduced from other rock types in the KKB. Copyright (C) 1996 Elsevier Science Ltd

DOI： 10.1016/S0743-9547(96)00062-1

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Authorship：Lead author,　Corresponding author   Language：English   Publisher：CURRENT SCIENCE ASSN  

Orthopyroxene and wollastonite are critical minerals in the end-member models on granulite petrogenesis in the Earth's deep crust, and are considered to be mutually incompatible. We report here the occurrence, mineralogy and chemistry of wollastonites from calc-silicates in a number of new localities of the Kerala Khondalite Belt, southern India, where they are associated with orthopyroxene-bearing charnockites. While the charnockite mineral phase equilibria constrain X(co2) &gt; 0.85 to drive the orthopyroxene-forming reaction, the presence of wollastonite in the calc-silicates requires X(co2) to be &lt; 0.25. We provide mineral reaction and fluid inclusion evidence which suggests that wollastonite formation occurred under low X(co2) conditions during the prograde sector of a clockwise P-T path, whereas orthopyroxene stability and charnockite formation resulted under anhydrous conditions through the structurally controlled influx of CO2 along an isothermal decompression path. The wollastonite-and orthopyroxene-bearing assemblages described in this study have important implications in evaluating changing fluid regimes during deep crustal metamorphism.

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)  

Common mineral reactions and textures preserved by the calc-silicates and marbles in the East Gondwana terrains are reviewed in the light of newly acquired thermodynamic information. Reaction textures involving wollastonite and scapolite are of considerable interest as they provide information on the post-peak metamorphic history of the terrain. Wollastonite-scapolite with rare, grossular-bearing calc-silicate assemblages occur in the Kerala Khondalite Belt. These mineral assemblages are indicative of a low XCO2 fluid composition during peak metamorphism. The calc-silicates in the Eastern Ghats, India and Prydz Bay and Rauer group of East Antarctica typically show these reaction textures, with a high pressure-temperature metamorphic history and near isothermal uplift. The mineral assemblages in calc-silicates of southern India, Sri Lanka and Lutzow-Holm Bay, East Antarctica, have equilibrated at lower pressure-temperature conditions, with an internally controlled fluid regime in the peak metamorphic event. -from Authors

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：GEOLOGICAL SOC INDIA  

The granite pluton at Kalpatta in northern Kerala is a member of the Late Precambrian magmatic suite emplaced proximal to transcrustal faults in southern India. In this study, we report the find of fluorite mineralization within late pegmatite phase associated with the granite. Both purple and colourless fluorites are associated, with X-ray diffraction pattern indicating perfect crystalline nature. Wet chemical analyses show CaF2 contents of 92 and 96 % for purple and transparent fluorites respectively. Primary fluid inclusions in both fluorite categories belong to aqueous bi-phase type, with T(h) between 150-180-degrees-C, and salinity in the range of 3 to 9 wt. per cent NaCl equivalent. Qurtz associated with fluorite contains CO2-H2O inclusions with T(h(CO2)) between 8 to 15-degrees-C (density 0.85-0.90 g/cm3). The nature of mineralizing fluids suggests that fluorite crystallization occured from low temperature hydrothermal solutions derived as end-products of the frctionation of volatile- and alkli-enriched felsic magma. The mineralization is regarded as a finger-print of the alkaline affinity of Pan-African magmatism in Kerala.

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Language：Japanese   Publisher：The Geological Society of Japan  

DOI： 10.14863/geosocabst.2019.0_126

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[Program canceled for a disaster] Program canceled for the 2018 Hokkaido Eastern Iburi earthquake. However, This abstract is quotable and viewable on PDF.

DOI： 10.14863/geosocabst.2018.0_114

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Language：Japanese   Publisher：Japan Association of Mineralogical Sciences  

We discussed evolution of sillimanite-, andalusite-, and kyanite-bearing metapelites from tha Altai Mountains, Mongolia.

DOI： 10.14824/jakoka.2018.0_173

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<b> [Program canceled for typhoon]</b> Program canceled for typhoon. However, This abstract is quotable and viewable on PDF.

DOI： 10.14863/geosocabst.2017.0_123

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We evaluated the heterogeneous recrystallization of natural carbonaceous material (CM) in metamorphic rocks based on the oxidation rate of CM and graphite. Based on the thermal analysis, we found systematic changes of exothermal peaks from 452 to 918 °C. The exothermal peaks and <i>L</i>c(002) can be expressed by the following equation;<br><i>L</i>c(002) = 0.0049exp(0.017<i>T</i>_ex), R² = 0.92<br>where, <i>L</i>c(002) is the crystal size of CM, and <i>T</i>_ex the exothermal temperature. Our experiments suggest that it is easy to evaluate the crystallinity and heterogeneity of CM in metamorphic rocks using a bulk rock powder.

DOI： 10.14824/jakoka.2017.0_90

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The redox state of mantle wedge was evaluated in the Fizh mantle section of Oman ophiolite. Using olivine and spinel delta log fO₂ (FMQ) was calculated with equations of Ballhaus et al. (1991). The results show more oxidizing on the eastern Moho side whereas more reducing on the western basal side. A negative correlation occurs between whole rock V/Sc ratio and delta log fO₂ (FMQ). The reduced signature in the basal part is explained by a reaction of peridotite with a reduced sediment melt.

DOI： 10.14824/jakoka.2016.0_165

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we compare the detailed crystallinity data of natural CM in Hidaka metamorphic belt and our experimental kinetic model for fully understanding the natural crystallization. The experimental model required activation energies of ~380 kJ/mol, whereas, the natural structural changes fitted by experimental sigmoid function display activation energies of 230 and 255 kJ/mol, respectively. This suggest that our <i>P-T-t</i> kinetic model is lacking some factors for fully understanding graphitization.

DOI： 10.14824/jakoka.2016.0_236

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DOI： 10.14863/geosocabst.2016.0_131

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DOI： 10.14863/geosocabst.2016.0_400

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DOI： 10.14863/geosocabst.2015.0_423

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DOI： 10.14863/geosocabst.2015.0_430

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Ca. 1cm-thick Grt-Hbl vein discordantly cuts a gneissose structure of a Grt-Opx-Hbl gneiss in the central Sor Rondane Mountains, East Antarctica. Changes of mineral compositions and mineral assemblages with a distance from the vein suggest that the Grt-Hbl vein was formed by NaCl-KCl brine infiltration in open system under upper amphibolite facies conditions. Moreover, Zn-, Sr-, Ba-, Pb-, and U-contents in Hbl and Li-, Sr-, Ba-, and Pb-contents in Pl decrease with a distance from the vein. Trace elements compositional profiles with a distance from the vein imply the elements diffuse from the vein to the wall rock.

DOI： 10.14824/jakoka.2015.0_19

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Other Link： http://hdl.handle.net/10191/30780

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In a Grt-Opx-Hbl gneiss from the central Sor Rondane Mountains, ca. 1cm-thick Grt-Hbl vein discordantly cut the gneissosity. Cl-content of Hbl and Bt, and K-content of Hbl decrease with the distance from the vein center. Sodium-richer rim of Pl gets thinner with a distance from the vein. Therefore, the Grt-Hbl vein was probably formed by NaCl-KCl brine infiltration during early retrograde metamorphic stage. Moreover, Zn-, Sr-, Ba-, Pb- and U-content of Hbl decrease and Nb-, Y- and REE-content of Hbl increase with the distance from the vein center. These elements would have been mobile during the brine activity.

DOI： 10.14824/jakoka.2014.0_21

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The Central Asian Orogenic Belt resulted from amalgamation of voluminous subduction – accretionary complexes of Late Neoproterozic to Mesozoic time. Considering geology and geochemistry of the intrusive rocks, the subduction - zone and collision - zone magmatism occurred in the Hangay batholith and the Lake zone, respectively. In other words, the landmass of south - central Mongolia would have been resulted by the specific geotectonics through the formation of volcanic arc due to the oceanic crust subduction and the microcontinent collision during the Paleozoic.

DOI： 10.14824/jakoka.2014.0_170

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DOI： 10.14863/geosocabst.2014.0_142

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We attempted to investigate the detailed microstructures and crystallinity of Carbonaceous Material (CM) to understand the heterogeneity of recrystallization under low temperature condition. In case of structural change at around 400 degreeC, CM represents the heterogeneous microstructure and crystallinity at the boundary of fully ordered gaphite. These data suggest that  other influences strongly affect recrystallization of CM, and that is the reason why natural graphitization cannot explain thermodynamic relationship between peak temperature and duration of heating.

DOI： 10.14824/jakoka.2014.0_222

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DOI： 10.14863/geosocabst.2014.0_392

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DOI： 10.14863/geosocabst.2014.0_598

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DOI： 10.14863/geosocabst.2014.0_131

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Graphite-bearing pseudotachylyte was examined using micro-Raman spectroscopy and stable carbon isotope analysis to detect the activity of fluid during the coseismic slip. Stable carbon isotope analyses show a broad range of values between -27.8 and -45.6 ‰ for graphite. These data show the deposition of graphite in silicate melts. This process implicates that the frictional melting generates the COH fluids by dehydration of sheet silicate and oxidation of carbonaceous materials. These fluids have a potential for changing the dynamic properties of flash fluid pressure in the fault plane and may explain the anomalous discharge of carbon after large earthquakes.

DOI： 10.14824/jakoka.2013.0_200

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A compilation on the field occurrence, distribution and carbon isotope characteristics of carbon-bearing rocks in lower crustal rocks in different settings and discuss the mechanism of isotopic partitioning in a context of carbon geodynamic cycle is presented in this talk.

DOI： 10.14862/geochemproc.60.0.77.0

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DOI： 10.14863/geosocabst.2013.0_069

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We estimated P-T conditions of the metamorphic rocks from the central and the western Sør Rondane Mountains, Antarctica, by using conventional geothermobarometers. Estimated temperatures are about >800 degC around Brattnipane and southern Austkampane, and decreased rapidly toward the south. The thermocline is roughly coincident with the Sør Rondane Suture. Estimated pressures are about 8-9kbar around Austkampane and northern Lunckeryggen, and decreased toward south and northeast. The zone where the pressures rapidly change runs roughly along the Main Shear Zone.

DOI： 10.14824/jakoka.2013.0_212

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Carbon isotope studies help us to monitor the carbon cycle, both in the shallow and deep Earth environments. However, our understanding of carbon isotopic composition of deep Earth is very limited. Here we present results of experimental determination of partitioning of carbon isotopes at high-pressure high-temperature conditions.

DOI： 10.14862/geochemproc.59.0.85.0

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DOI： 10.14863/geosocabst.2011.0.403.0

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Hanhohiyn Mountains is located in the northwestern part of Mongolia. The mountains are mainly composed of marble, meta-quartzite and pelitic gneisses intercalating with lenticular garnet-amphibolite, garnet-orthopyroxene gneiss and so on. Pelitic gneisses are classified into garnet-sillimanite-biotite gneiss, garnet-cordierite-biotite gneiss and garnet-biotite gneiss. These gneisses indicate the peak metamorphic condition of 600-700 degree and 5.5-6 kbar accompanied by decompression. The EPMA U-Th-Pb Monazite ages concentrate between 480-510 Ma which is interpreted as the timing of the peak metamorphism.

DOI： 10.14824/jakoka.2011.0.98.0

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We present here results of high pressure experiments in the Fe-C system at 5GPa, 1200 to 1500 C. We have observed Fe3C, Fe7C3 and graphtie as stable phases in the run products. Based on the results obtained, we speculate the amount of carbon presentin the core.

DOI： 10.14824/jakoka.2010.0.131.0

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DOI： 10.14863/geosocabst.2010.0.360.0

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DOI： 10.14863/geosocabst.2008.0.272.0

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DOI： 10.1016/S1342-937X(05)70558-X

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DOI： 10.1016/S1342-937X(05)70557-8

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DOI： 10.1016/S0743-9547(97)88152-4

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