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Contrasting mineral parageneses in high‐temperature calc‐silicate granulites: examples from the Eastern Ghats, India
Author(s) -
DASGUPTA S.
Publication year - 1993
Publication title -
journal of metamorphic geology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.639
H-Index - 114
eISSN - 1525-1314
pISSN - 0263-4929
DOI - 10.1111/j.1525-1314.1993.tb00142.x
Subject(s) - geology , calcite , geochemistry , wollastonite , granulite , plagioclase , quartz , metamorphism , lawsonite , grossular , mineral , mineralogy , metamorphic rock , eclogite , chemistry , geomorphology , raw material , paleontology , organic chemistry , facies , structural basin , subduction , tectonics
Three types of mineral associations are described from calc‐silicate granulites from the Eastern Ghats, India, where geothermobarometry in associated rocks suggests extremely high P–T conditions of metamorphism ( c . 9 ± 1 kbar, 950° C). These mineral associations are: (i) calcite + quartz + scapolite + plagioclase, (ii) calcite + scapolite + wollastonite + porphyroblastic garnet + coronal garnet and (iii) calcite + quartz + wollastonite + scapolite + porphyroblastic garnet + coronal garnet, all coexisting with K‐feldspar, titanite and clinopyroxene. The first two associations evolved through nearly isobaric cooling retrograde paths, whereas the third evolved through a nearly isothermal decompression path followed by an isobaric cooling retrograde path. Textural and compositional characteristics suggest the following mineral reactions in the calc‐silicate granulites: calcite + quartz = wollastonite + CO 2 , calcite + plagioclase = scapolite, calcite + scapolite + wollastonite = porphyroblastic garnet ± quartz + CO 2 , CaTs + wollastonite = coronal garnet (association ii) and wollastonite + scapolite = coronal garnet (association iii) + quartz + CO 2 . Andradite content in garnet was buffered by the redox equilibria wollastonite + hedenbergite + O 2 = andradite + quartz (association iii) and wollastonite + andradite + CaTs + scapolite = hedenbergite + calcite + grossular + O 2 (association ii). The contrasting mineral parageneses have been ascribed to interplay of variables such as X CO2 , f O2 , f HCl in the fluid, bulk Na content and the nature of the retrograde P–T–X CO2 paths through which the rocks evolved.