Browsing by Author "P. Chandra Singh"

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A new occurrence of sapphirine-spinel-corundum-bearing granulite from NE of Jagtiyal, Eastern Dharwar Craton, Andhra Pradesh
(2013) D. Prakash; P. Chandra Singh; T. Hokada
We report a new occurrence of sapphirine-spinel-corundum bearing granulites enclosed in granitic gneisses near Jagtiyal in the Eastern Dharwar Craton (EDC). These granulites are very important in deciphering the prehistory of the thermal peak of metamorphism due to the presence of refractory phases. The appearance of sapphirine is related to the following reactions: (a) Spl 1 + Crd1 = Spr1 (2:2:1) (b) Phl (in Bt 1) + Crn1 = East (in Bt2) + Spr 2 (7:9:3) + Crd2 (c) Bt2 + Crd1 + Crn1 = Spr2 (7:9:3) + Kfs + H2O (d) Crd 1 + Spl1 + Crn1 = Spr2 (7:9:3). The P-T evolution of these sapphirine granulites has been constrained through the use of THERMOCALC program. Temperature of formation of sapphirine-spinel assemblages is high, around 800 C, and pressure ranges from 5-7 kbars, suggesting that sapphirine formation took place during decompression stage. © 2013 Geological Society of India.
Metamorphic P-T conditions and CO2 influx history of medium-grade metapelites from Karakorum, Trans-Himalaya, India
(Elsevier Ltd, 2016) Himanshu K. Sachan; M. Santosh; Divya Prakash; Aditya Kharya; P. Chandra Singh; Santosh K. Rai
The medium grade metapelites of Pangong-Tso area in the trans-Himalayan region underwent sillimanite-grade metamorphism initiated during the Cretaceous, associated with the collision of the Kohistan arc and the Indian plate with Asia. This paper present results from a petrological and fluid inclusion study to understand the metamorphic P-T conditions and fluid history of these rocks. The calculated phase equilibria in the Na2O-CaO-K2O-FeO-MgO-MnO-Al2O3-SiO2-H2O-TiO2 (NCKFMMnASHT) system suggest P-T conditions of 8 kbar and 650 °C for the peak metamorphic event. Primary fluid inclusions occur in staurolite and garnet, whereas quartz carries mostly secondary fluid inclusions. The trapped fluids in primary inclusions show initial melting temperatures in the range of -56.9 to -56.6 °C, suggesting nearly pure CO2 composition. The secondary fluids are of mixed carbonic-aqueous nature. The re-equilibrated inclusions show annular morphology as well as necking phenomena. The CO2 isochores for the primary inclusions indicate pressures of 6.1-6.7 kbar, suggesting that the CO2-rich fluids were trapped during post-peak exhumation of the rocks, or that synmetamorphic carbonic fluids underwent density reversal during isothermal decompression. The secondary CO2-H2O fluids must have been trapped during the late exhumation stage, as their isochores define further lower pressures of 4.8 kbar. The morphology of re-equilibrated fluid inclusions and the rapid decrease in pressure are consistent with a near-isothermal decompression trajectory following the peak metamorphism. The carbonic fluids were probably derived locally from decarbonation reactions of the associated carbonate rocks during metamorphism or from a deep-seated reservoir through Karakorum fault. © 2016 Elsevier Ltd.
New finding of sillmanite in sapphirine-bearing granulites from Pedapalli, NE part of the Eastern Dharwar Craton, India
(Geological Society of India, 2014) D. Prakash; P. Chandra Singh
We report the occurrence of sillimanite in sapphirine-bearing granulites near Pedapalli town in the north eastern part of the Eastern Dharwar Craton (EDC). The sapphirine-bearing granulites of the Pedapalli area display evidence of metamorphic reactions involved in the formation of diverse mineral assemblages documented in different types of reaction textures. The common stable assemblage contains sapphirine-spinel-orthopyroxene- cordieriteplagioclage-corundum-sillimanite. The P-T evolution of these sapphirine granulites has been constrained by using the TWEEQU program. Temperature of formation of sapphirine-spinel assemblages is high, around 800 °C, and pressure ca. 8 kbar. The assemblage reported in this study has important bearing on the high-temperature metamorphism and exhumation history of the EDC. © 2014 Geological Society of India.
Oxygen Fugacity and Water Activity during Thermal Peak and Retrogression of Granulites around the Sarvapuram Area, Karimnagar Granulite Terrane, Andhra Pradesh, India
(John Wiley and Sons Inc, 2019) P. Chandra Singh; D. Prakash
The investigated area around Sarvapuram represents a part of the Karimnagar granulite terrane of the Eastern Dharwar Craton, India. Garnet–bearing gneiss is hosted as enclaves, pods within granite gneiss and charnockite. It is largely made up of garnet, orthopyroxene, cordierite, biotite, plagioclase, K–feldspar, sillimanite and quartz. The peak metamorphic stage is represented by the equilibrium mineral assemblage i.e. garnet, orthopyroxene, cordierite, biotite, plagioclase, sillimanite and quartz. Breakdown of the garnet as well as preservation of the orthopyroxene–cordierite symplectite, formation of cordierite with the consumption of the garnet + sillimanite + quartz represents the decompressional event. The thermobarometric calculations suggest a retrograde P–T path with a substantial decompression of c. 3.0 kbar. The water activity (XH2O) conditions obtained with the winTWQ program for core and symplectite compositions from garnet–bearing gneiss are 0.07–0.14 and 0.11–0.16 respectively. The quantitative estimation of oxygen fugacity in garnet–bearing gneiss reveal log fO2 values ranging from −11.38 to −14.05. This high oxidation state could be one of the reasons that account for the absence of graphite in these rocks. © 2019 Geological Society of China
P-T history and geochemical characteristics of mafic granulites and charnockites from west of Periya, North Kerala, southern India
(2012) D. Prakash; P. Chandra Singh; M. Arima; Triveni Singh
The study region forms the northern part of Kerala (south India) and constitutes part of granulite-facies rocks of the exhumed Proterozoic south Indian Granulite Terrain (SGT). The SGT consists of a large variety of rock types with a wide range of mineral parageneses and chemical compositions, namely charnockite, mafic granulite, gneiss, schist, anorthosite, granite and minor meta-sedimentary rocks. Garnet-bearing mafic granulites occur as small enclaves within charnockites. We report for the first time P-T constraints on the prograde path preceding peak metamorphism in the northernmost part of Kerala. An increase of the Mg, Fe and decrease of Ca and Mn contents from the core towards the rim of garnet in the mafic granulites suggest prograde garnet growth. The prograde path was followed by peak metamorphism at a temperature of c. 800°C and a pressure of c. 7.5kbar as computed by isopleths of XMg garnet, XCa garnet and XAn plagioclase. The resorption of garnet in various reaction textures and the development of spectacular orthopyroxene-plagioclase, biotite-quartz and hornblende-plagioclase symplectites characterize the subsequent stages of metamorphism. The PT path is characteristically T-convex suggesting an isothermal decompression path and reflects rapid uplift followed by cooling of a tectonically thickened crust. Diffusion modeling of Fe-Mg exchange between garnet and hornblende suggests a near-peak cooling rate of 10-70°C/myr. Such cooling rates are too high to be accounted for by normal isostatic uplift and erosion and suggest that the terrain was tectonically exhumed. Charnockites are richer in SiO2 and lower in MgO and CaO when compared to mafic granulites. Their REE patterns are relatively flat and show prominent negative europium anomalies. The mafic granulites are metamorphosed tholeiitic basalts as revealed by major- and trace-element geochemistry. © 2012 Elsevier Ltd.
Petrology, pseudosection modelling and U-Pb geochronology of silica-deficient Mg-Al granulites from the Jagtiyal section of Karimnagar granulite terrane, northeastern Dharwar Craton, India
(Elsevier B.V., 2017) D. Prakash; P. Chandra Singh; S. Tewari; M. Joshi; H.E. Frimmel; T. Hokada; T. Rakotonandrasana
The Jagtiyal section exposes a part of the Precambrian granulite – facies terrane in the northeastern Dharwar Craton, specifically charnockite, orthogneiss, mafic granulite, garnet-cordierite gneiss and silica-deficient Mg-Al granulite. The silica-deficient Mg-Al granulites exhibit well-preserved metamorphic reaction microstructures and were used to elucidate the metamorphic history of the study area. Textural evidence exists for the prograde break-down of biotite and corundum and formation of a peak metamorphic assemblage of spinel, sapphirine, orthopyroxene, cordierite and K-feldspar. KFMASHTO system based P-T pseudosections as well as the application of multi-equilibrium calculation for mineral core compositions, gave P-T conditions of 7.5–8 kbar and 800–840 °C. The thermal peak was followed by near isothermal decompression that led to a variety of coronal textures including, successive rims on corundum, consisting of spinel + sapphirine + cordierite ± orthopyroxene, and symplectitic intergrowth of sapphirine-cordierite, orthopyroxene-cordierite, spinel-cordierite, and biotite-cordierite. Modelled P-T conditions for this stage are 4.3–4.5 kbar and 750–800 °C. Few samples show textural evidence for further retrogression leading to the break-down of sapphirine and cordierite to form chlorite, corundum, spinel and andalusite. The proposed clockwise P-T path implies that granulites from the Jagtiyal section likely resulted from thickened continental crust undergoing decompression. New U-Pb zircon geochronologic data provide evidence for a Neoarchaean (2604 ± 25 Ma) age that likely corresponds to the timing of accretion of the terrane that was subjected to high temperature metamorphism at that time. Moreover, there is evidence of a Neoproterozoic thermal overprint in the area around 638 Ma. © 2017 Elsevier B.V.
Prograde Barrovian metamorphism along Darjeeling–Tista transect, Eastern Himalaya, India: constraints from textural relationship, phase equilibria and geothermobarometry
(John Wiley and Sons Ltd, 2018) D. Prakash; S. Tewari; P. Chandra Singh
The Darjeeling–Tista transect, located in Lesser Himalaya, is a part of the North-Eastern Himalaya. Along the transect, polyphase deformation and prograde Barrovian metamorphism have been delineated. The time relations between the phases of deformations (D1, D2 and D3) and metamorphic crystallization reveal a single major prograde metamorphic event that initiated with the D1 deformation and finally outlasted it. The earlier phase of this metamorphism is essentially regional syntectonic low grade, which may be designated (M1a). This was followed by regional static metamorphism (M1b) in the post-tectonic phase between the D1 and D2 deformations. This M1 metamorphism is superposed by later retrogressive metamorphism (M2) during the D2 and D3 deformations. The different parageneses of pelitic rocks containing chlorite, muscovite, biotite, garnet, staurolite, kyanite, sillimanite, K-feldspar and plagioclase show various textures that resulted from the continuous and discontinuous reactions in the different zones. The metamorphic zones and isograds delineated on the basis of specific metamorphic reactions, namely reaction isograd or isoreaction grad, and by critical mineral assemblages are as follows: (a) chlorite–biotite zone, (b) garnet–chlorite zone, (c) staurolite–biotite–chlorite zone, (d) kyanite–biotite–staurolite zone and (e) sillimanite–biotite–staurolite zone. Chemographic relations and inferred reactions for different zones are portrayed in the AKF and AFM projections. A sequence of metamorphic reactions at different isograds has been deduced through textural relations. The prograde P-T evolution of the study area has been constrained through the use of internally consistent winTWQ programme and Perple_X software in the MnNCKFMASHTO model system. The study has the potential to instigate future researches focusing on Himalayan metamorphic evolutionary trends using modern approaches. Copyright © 2017 John Wiley & Sons, Ltd. Copyright © 2017 John Wiley & Sons, Ltd.