Reaction history of sapphirine granulites and a decompressional P-T path in a granulite complex from the Eastern Ghats |
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| Authors: | A Mohan P Tripathi Y Motoyoshi |
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| Institution: | (1) Department of Geology, Banaras Hindu University, 221 005 Varanasi, India;(2) Department of Crustal Studies, NIPR, 173 Tokyo, Japan |
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| Abstract: | The sapphirine granulites from G. Madugula, Eastern Ghats preserve a variety of mineral textures and reactions. Corona and
reaction textures are used in conjunction with mineral compositions to construct a sequence of metamorphic reactions describing
the mineralogical evolution of sapphirine granulites. An early stage is characterized by the development of sapphirine + quartz,
spinel + quartz in textural equilibrium, and possible relicts after osumilite during peak metamorphic conditions. Sapphirine/spinel
crystals were later detached from quartz in the form of mineral coronas. During a subsequent sapphirine-cordierite stage,
several cordierite forming reactions reflect decreasingP-T conditions. Finally during the late stage, a few samples show evidence of retrogressive hydration. Sapphirine is rather iron-rich
(12.8 wt%) and the Mg number in the analysed minerals varies in the order: cordierite > phlogopite > sapphirine > orthopyroxene
> spinel > garnet.P-T conditions of metamorphism have been constrained through the application of geothermobarometry and thermodynamically calibrated
MAS equilibria.P-T vectors from granulite facies rocks in the G. Madugula area indicate that the rocks experienced substantial decompression
(up to 3 kbar) and moderate cooling (150–200°C) subsequent to peak conditions of metamorphism (8.4 kbar, > 900°C). The decompressionalP-T history of sapphirine granulites interpreted from textural features and thermobarometric estimates suggest that they may
have eventually resulted from exhumation of thickened crust. |
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| Keywords: | Sapphirine granulites corona textures geothermobarometry decompression Eastern Ghats |
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