P. K. Bhattacharya

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 -

Genetic reinterpretation of crystallographicintergrowths of jacobsite andhausmannite from natural assemblages

hematite_{ss}

An orthopyroxene–biotite geothermometer and its application in crustal granulites and mantle-derived rocks

+ quartz = garnet + ilmenite +

Mineral reactions in manganese oxide rocks; P-T-X phase relations

O_{2}

Petrology of Mg-Mn amphibole-bearing assemblages in manganese silicate rocks of the Sausar Group, India

. Thermobarometry indicates exsolution of pigeonite at ~950°C, of (100) clinopyroxene in orthopyroxene and vice versa at 750-820°C and formation of garnet at around 700-725°C, 7 kb, all in response to cooling of the rock. This shows a nearly isobaric cooling path subsequent to peak metamorphic conditions. Subsequently, the rock suffered nearly isothermal decompression to 5 kb. Hydration and K-metasomatism of the assemblage occurred at lower temperatures (~500°C) in the final phases. The structural characteristics and thermal history of the mafic granulites suggest that the mafic rocks were emplaced during the culmination of prograde metamorphism accompanying a compressive orogeny. Mafic magmas could have contributed significantly to enhance the thermal budget for granulite facies metamorphism. This study, therefore, attests to the very high metamorphic temperatures (as high as 1000°C regionally) attained by the Eastern Ghats granulites.