Evolution of granulite enclaves and associated gneisses from Purulia, Chhotanagpur Granite Gneiss Complex, India: Evidence for 990-940 Ma tectonothermal event(s) at the eastern India cratonic fringe zone

摘要

Migmatitic quartzofeldspathic gneisses at Purulia, Chhotanagpur Granite Gneiss Complex (CGGC) contains layers and enclaves of high Mg-Al granulite and two-pyroxene granulites. Petrographic, geothermo-barometric and pseudosection analyses of the high Mg-Al granulite reveal a strongly decompressive (AP = 6 kbar) retrograde trajectory after attaining the peak at 870 ℃, 11 kbar. The metamorphic evolution ensued through partial melting of biotite-bearing assemblage. Texturally-constrained monazite grains yield population age of ca. 990-940 Ma which might bracket the timing of decompressive metamorphism in the high Mg-Al granulite broadly coinciding with the age of anatexis of the host gneisses. Associated mafic granulites underwent similar metamorphic evolution, but their contrasting bulk chemistries are responsible for mineralogical and textural variability. Formation and stabilization of garnet in one bulk composition of mafic granulite and breakdown of garnet in the other could be reconciled as products two different P-T segments of same metamorphic evolutionary history. Discrete population of older monazite ages (ca. 1800 Ma) in the host gneisses possibly implies variable degrees of inheritance. It is proposed that the ca. 990-940 Ma tectonothermal event caused extensive partial melting in the lower crust of the eastern Indian cratonic fringe zone. Widespread occurrence of ca. 1000-900 Ma metamorphism in the CGGC can be correlated with those in the Rayner-Eastern Ghats Belt in the Indo-Antarctic sector. All these areas experienced high-grade metamorphism as a result of Indo-Antarctic collision during the assembly of Rodinia.