Mineral chemistry, reaction textures, thermobarometry and PT path from orthopyroxene granulites of Rayagada, Eastern Ghats, India

Abstract

Orthopyroxene granulites (charnockites, enderbites and associated rocks) form one of the major lithological units in Rayagada, in the north-central sector of the Eastern Ghats Granulite Belt of Peninsular India. Petrographic features show common occurrences of clinopyroxene lamella within orthopyroxene host, although reverse relations have also been observed in some cases. Two types of coronal garnet with different chemical compositions have been noted: one at the interface of orthopyroxene and plagioclase, and the other rimming magnetite, also at the contact of plagioclase. Garnet-orthopyroxene thermometry from these rocks show temperature variations from 840 to 700°C, whereas two-pyroxene and garnet-biotite thermometry yield temperatures around 750 and 700°C, respectively. Garnet-orthopyroxene-plagioclase-quartz barometry ranges from 8.1 to 6.8 kbar. A near-isobaric cooling from a thermal maxima of ∼840°C, followed by decompression is characteristic of the deduced pressure-temperature (PT) path. This shows significant similarities with the PT path derived from associated metapelites. Field and petrographic features collectively suggest that these orthopyroxene granulites have suffered at least the second deformation event (D₂), which is the dominant ductile deformation of the area. This event presumably took place around 1000 Ma and might be coeval to the mid-Proterozoic events of East Antarctica and other fragments of Gondwanaland.