Pulak Sengupta

A refined garnet - biotite Fe−Mg exchange geothermometer and its application in amphibolites and granulites

A new formulation of garnet-biotite Fe−Mg exchange thermometer has been developed through statistical regression of the reversed experimental data of Ferry and Spear. Input parameters include available thermo-chemical data for quaternary Fe−Mg−Ca−Mn garnet solid solution and for excess free energy terms, associated with mixing of Al and Ti, in octahedral sites, in biotite solid solution. The regression indicates that Fe−Mg mixing in biotite approximates a symmetrical regular solution model showing positive deviation from ideality withWFeMgbi=1073±490 cal/mol. ΔHrand ΔSrfor the garnet-biotite exchange equilibrium were derived to be 4301 cal and 1.85 cal respectively. The resultant thermometer gives consistent results for rocks with a much wider compositional range than can be accommodated by earlier formulations.

Indo-Antarctic correlation: a perspective from the Eastern Ghats granulite belt, India

Abstract Available lithological, petrological and geochronological data on the rocks of the Eastern Ghats Belt (EGB), which has a key role to play in any model of Indo-Antarctic correlation in Precambrian and Cambrian times, have been synthesized and interpreted. Longitudinal lithological subdivisions of the belt are mostly not supported by recent geochronological data, which rather suggest a fourfold division based on protolith ages of the ortho- and paragneisses. Mineral ages show prominent tectonothermal events at Mesoproterozoic, Grenvillian and Pan-African times. However, not all of the imprints of all the events are preserved in all the geochronological domains. In the most well-studied Domain II, three phases of metamorphism are recorded — an early ultra high-temperature metamorphism with an anticlockwise P-T path, a second granulite facies event at c. 1000 Ma, with a retrograde trajectory of near-isothermal decompression, and a third of amphibolite facies. The available information, however, is not conclusive as to whether or not the entire EGB experienced polyphase UHT metamorphism. The propagation of the Grenvillian orogenic front in the EGB is identified.

Mixing behavior in quaternary garnet solid solution and an extended Ellis and Green garnet-clinopyroxene geothermometer

Applications of the different formulations of garnet-clinopyroxene geothermometers to crustal granulites and particularly to metamorphosed manganese formations show a wide scatter of estimates. This is primarily related to the uncertainty associated with the formulation of the activity-composition relationship in garnet solid solution besides that in the clinopyroxene solid solution. The largest uncertainty appears to have resulted from the adopted Mg-Mn interaction parameter in garnet which is yet to be experimentally determined. A fresh statistical regression of eleven pairs of garnet and clinopyroxene equilibrated at identical P-T conditions from a manganese formation in India yields WMg-MnGt(=WMn-Mg) value of 1600±500 cal/mols. An extension of the Ellis and Green (1979) geothermometer, known to work well in normal granulites, incorporating terms associated with the activity coefficients of the exchangeable cations, now yields consistent temperature estimation from normal granulites as well as from metamorphosed manganese formations. The geothermometer, however, will not work well for garnet having XFe≥0.65 and clinopyroxenes containing appreciable Na, Al and/or Fe3+.

Mafic Granulites from the Eastern Ghats, India: Further Evidence for Extremely High Temperature Crustal Metamorphism

Mafic granulites occurring as conformable lenses within khondalite, leptynite, and calc-silicate rocks from the Anantagiri-Araku areas of the Eastern Ghats belt contain orthopyroxene, clinopyroxene, garnet, quartz, ilmenite, and plagioclase as major constituents. Orthopyroxene contains (100) lamellae of clinopyroxene, and clinopyroxene contains (100) orthopyroxene lamellae and (001) spindles of pigeonite. Reaction textures indicate formation of garnet through coupling of two reactions, viz. orthopyroxene + anorthite = garnet + clinopyroxene + quartz and anorthite + ilmenite -

Pressure-temperature-fluid evolutionary history of the polymetamorphic Sandmata granulite complex, Northwestern India

hematite_{ss}

Comparative evaluation of contraceptive efficacy of norethisterone oenanthate (200 mg) injectable contraceptive given every two or three monthly

+ quartz = garnet + ilmenite +

Chapter 20 Provenance, timing of sedimentation and metamorphism of metasedimentary rock suites from the Southern Granulite Terrane, India

O_{2}