High pressure metamorphism of mafic granulites from the Chotanagpur Granite Gneiss Complex, India: Evidence for collisional tectonics during assembly of Rodinia

Abstract

In the north-eastern part of the Chotanagpur Granite Gneiss Complex (CGGC), India, mafic granulite occurs as enclaves within the ˜1450 Ma old felsic rocks. The mafic granulite and the host felsic rock develop a pervasive planar fabric defined by minerals that are characteristics of high pressure (HP) granulite facies metamorphism. The earliest discernable mineral assemblage in the mafic granulites comprises amphibole + clinopyroxene + plagioclase + quartz + ilmenite, preserved as inclusions within garnet porphyroblasts as a record of early M0 stage. Subsequently, the assemblage garnet + clinopyroxene + plagioclase + rutile + quartz developed during the peak metamorphic stage (Mi) defining the pervasive fabric. Quantitative geothermobarometry and intersection of isopleths in a phase diagram (pseudosection) that was numerically computed for a representative bulk composition of the mafic granulites, yield 12 ± 1 kbar and 800 ± 50 °C for the Mi stage. Subsequent changes in physical conditions in response to tectonic evolution of the complex developed two distinct sets of mineral assemblages (Mii and Miii). Firstly, garnet ± clinopyroxene + quartz broke down to symplectic aggregates of orthopyroxene + plagioclase under static conditions (Mii stage). Afterwards, the anhydrous minerals, which developed during the M0-ii stages, were variably replaced by amphibole-bearing assemblages (Miii stage). Geothermobarometric estimates and the stability of different assemblages that developed during M0-Miii stages, define a clockwise (CW) P–T path with an inferred geothermal gradient of ˜20 °C/km during the culmination of HP Mi metamorphism. The reconstructed CW P-T path and the inferred geothermal gradient are consistent with a continent-continent collisional tectonic setting. Integrating all the existing information, it is considered that a large part, if not the whole, of the CGGC behaved as a coherent lower plate during the globally extensive convergent margin orogenesis at the time of Rodinia supercontinant formation.