Masato Fukuoka

Very high density CO2 associated with ultrahigh-temperature metamorphism in the Eastern Ghats granulite belt, India

Spinel-bearing high Mg-Al granulites of the Vizianagram area in the Eastern Ghats granulite belt show textural features clearly establishing that the association spinel ss + quartz + Fe-Ti oxide solid solution ± sillimanite ± porphyroblastic orthopyroxene was stable during peak metamorphic conditions. Pressure-temperature (P-T) conditions estimated from both mineralogical thermobarometry and phase-equilibrium limitations indicate that the peak metamorphism occurred under ultrahigh-T conditions (>1000°C) at 8-9 kbar pressure. Retrograde P-T conditions of 750-800°C, 6-7 kbar are deduced from the compositions of coronal garnet and orthopyroxene, which have rims of spinel against quartz, indicating significant cooling with slight lowering of pressure. Quartz associated with the ultrahigh-T assemblage at Vizianagram contains ubiquitous single-phase carbonic inclusions as isolated clusters that belong to two categories. Group I shows extremely high density (homogenization temperature: −51±1.8°C; density 1.15 g/cm 3 ) and group II trapped relatively lower density fluids (homogenization temperature: −18.4±2.4°C; density 1.05 g/cm 3 ). The iso chores for group I inclusions pass through the peak metamorphic P-T conditions, whereas those for group II coincide with the P-T conditions of the formation of coronal garnet and orthopyroxene. Our study is the first report of very high density CO 2 associated with the Eastern Ghats granulite belt rocks and provides a strong case for the presence of CO 2 -rich fluids during ultrahigh-T metamorphism at lower crustal levels.

Fluorine content of biotite in granulite-grade metapelitic assemblages and its implications for the Eastern Ghats Granulites

Biotite inclusions showing high fluorine content (up to 3.3 wt.%) within porphyroblastic aluminous orthopyroxene (>10 wt.% Al 2 O 3 ) and cordierite suggest high temperature of melting for the prograde assemblages. Textural data indicate that biotite dehydration melting produces peak assemblages orthopyroxene + spinel + quartz + melt and orthopyroxene + sapphirine + quartz ± cordierite + melt in different microdomains depending on the bulk composition. A comparative study of the biotite composition with those of experimental data and present one indicates a minimum temperature of 950°C for the terminal stability of biotite-bearing assemblages. This also explains the appearance of observed peak assemblage sapphirine + orthopyroxene + quartz + cordierite + spinel directly from F-rich biotite as a consequence of topological changes in the KFMASH grids. Enrichment of fluorine in retrograde biotite can be explained by interaction of solid phases with the in situ melt fraction during subsequent retrogressive stage of the granulites. This is the first report of its kind from the northern part of the Eastern Ghats Belt, India.

Contrasting parageneses in the manganese silicate-carbonate rocks from Parseoni, Sausar Group, India and their interpretation

Mn silicate-carbonate rocks at Parseoni occur as conformable lenses within metapelites and calc-silicate rocks of the Precambrian Sausar Group, India. The host rocks are estimated to have been metamorphosed at uppermost P-T conditions of 500–550°C and 3–4 kbar. The Mn-rich rocks contain appreciable Fe, reflected in the occurrence of magnetite(1) (MnO 1%), magnetite(2) (MnO 15%) and magnetite(3) (MnO 10%). Two contrasting associations of pyroxmangite, with and without tephroite, developed in the Mn silicate-carbonate rocks under isothermal-isobaric conditions. The former assemblage formed in relatively Fe-rich bulk compositions and equilibrated with a metamorphic fluid having a low XCO2 (<0.2), and the latter equilibrated with a CO2-rich fluid. Rhodochrosite+magnetite(1)+quartz protoliths produced the observed mineral assemblages on metamorphism. Partitioning of major elements between coexisting phases is somewhat variable. Fe shows preference for tephroite over pyroxmangite at the ambient physical conditions of metamorphism. Oxygen fugacity during metamorphism was monitored at or near the QFM buffer in tephroite bearing domains, and the fluid composition was buffered by mineral reactions in respective domains. As compared to other metamorphosed Mn deposits of the Sausar Group, the Mn silicate-carbonate rocks at Parseoni were, therefore, metamorphosed at much lower fO2 through complex mineral-fluid interactions.

Polymetamorphism in the Schirmacher Hills Granulites, East Antarctica: Implications for Tectonothermal Reworking of an Isobarically Cooled Deep Continental Crust

Abstract The Precambrian basement of the Schirmacher Hills, Queen Maud Land, East Antarctica has evolved through multiple episodes of deformation and metamorphism. The rocks have suffered at least five phases of deformation. The imprint of the early deformation, D 1 , is preserved in some mafic isolated enclaves. The second and the third deformations (D 2 and D 3 ) are the dominant deformations of this area and produced isoclinal folds with transposition of earlier cleavages. The later deformations, D 4 and D 5 , produced two sets of open, upright folds. Detailed mineralogical, textural, mineral chemical studies and geothermobarometry on khondalite, leptynite as well as different varieties of enderbite and mafic granulites have revealed that the rocks suffered two phases of metamorphism under granulite facies conditions followed by an amphibolite facies overprint. M 1 is broadly coeval with D 1 only in mafic granulite enclaves within enderbitic gneiss, and took place at ca. 10 Kbar, 900° C. The mafic magma, parental to the enclaves, probably crystallized at 11.2 Kbar. Following post-peak near isobaric cooling, the mafic granulites were transported to shallower levels by the enderbitic magma. M 2 , recorded in all the lithologies, occurred at ca. 8 Kbar, 800–850°C and synchronous with D 2 . Post peak M 2 evolution of the rocks was characterized by a pressure — temperature drop of 2 Kbar and 200°C respectively and textures indicative of both cooling and decompression are preserved in different rocks. The relative timing of the two, however, cannot be worked out. M 3 , synchronous with D 3 , took place at 6 Kbar, 600–650°C and evolved hydrous fluid flux. Correlation with available structural and geochronological data shows that both M 1 and M 2 could be of Grenvillian event. M 3 could well be Pan-African age.

Evidence of superposed metamorphism from the Gokavaram area, Eastern Ghats Belt, and its relation with the Kemp Land Coast, East Antarctica

Abstract In this paper, we compare the petrological histories of the Kemp Land Coast (east Antarctica), and Gokavaram area (Eastern Ghats), that were supposed to have been juxtaposed. The area around Gokavaram is dominated by different varieties of paragneisses (pelitic, quartzofeldspathic, and calcareous composition) with relatively minor amounts of orthogneisses (mafic, enderbitic, and granitic composition). The rocks were involved in three major phases of deformation, and were finally affected by localised shear movement. On the basis of reaction textures, well preserved in high Mg-Al granulites, and calc-silicate granulites, and geothermobarometric data we deduce a polymetamorphic evolution of the rocks. Following an early M1 metamorphism culminating at 9.2–9.4 kbar, > 950°C, the rocks cooled nearly isobarically down to 850°C. During a subsequent M2 metamorphism, near isothermal decompression to 5–6 kbar occurred. This was followed by near isobaric cooling down to 600–650°C. M3 is a weak amphibolite facies overprint, largely restricted to late shears, which involved hydration as well. Available radiometric data from this area can be interpreted in terms of partial resetting of U-Pb systematics in older sphenes due to M3 metamorphism at ca. 550 Ma. Despite the absence of sufficient isotopic data on the Eastern Ghats granulites, we document a remarkable similarity in the petrological history of the two supposedly erstwhile neighbours.

Magmatic evolution of mafic granulites from Anakapalle, Eastern Ghats, India: implications for tectonic setting of a precambrian high-grade terrain

Abstract Mafic granulites showing intrusive relationships with enclosing pelitic, calcareous and quartzofeldspathic gneisses at Anakapalle, Eastern Ghats belt, share a common retrograde metamorphic history (decompression followed by near-isobaric cooling) and are, therefore, considered to be syn-metamorphic. Detail textural, phase chemical and bulk chemical analyses of the mafic granulites show that (a) these are melts derived through fractionation of a primary tholeiitic magma and (b) they crystallized at temperatures

Controls of evolution of mineral assemblages in ultrahigh-temperature metamorphosed Mn-carbonate-silicate rocks from the Eastern Ghats Belt, India

A unique suite of Mn-silicate-carbonate rocks from the Eastern Ghats Belt, India, were metamorphosed under ultrahigh temperatures and exhibit contrasting mineral assemblages in closely spaced domains. Development of contrasting assemblages under isothermal-isobaric conditions is attributed to initial variations in fluid composition particularly a CO2 , and bulk-rock Mn:Mg:A1:Si:Ca ratios. The fO 2 was buffered near the QFM buffer during peak metamorphism. Garnet shows a sharp increase in Fe +3 content towards the rim, which is interpreted to be the result of increased a CO2 during later stages of evolutions of the rocks due to channelled fluid flux. At the terminal stage of evolution, the rocks suffered hydration at lower temperatures.