Petrology and geochronology of a suite of pelitic granulites from parts of the Chotanagpur Granite Gneiss Complex, eastern India: Evidence for Stenian‐Tonian reworking of a late Paleoproterozoic crust

Abstract

The north‐eastern part of the Chotanagpur Granite Gneiss Complex (CGGC) in the East Indian shield contains enclaves of migmatitic pelitic granulites (PG) within felsic orthogneiss (FOG). Field observations, petrology and geochronology (LA–MC–ICP–MS U–Pb dating of zircon and EPMA Th–U–total‐Pb dating of monazite) of the PG suggest two distinct metamorphic events. The earliest event M1, which is characterized by high‐temperature (>850°C) granulite facies metamorphism, occurred in the timespan of ~1680–1580 Ma. Extensive dehydration melting of biotite + sillimanite + quartz‐rich protoliths led to stabilization of the restitic assemblage (garnet + alkali‐feldspar + quartz + sillimanite + ferrian‐ilmenite) together with large volumes of felsic melts (leucosomes). Collisional tectonics followed by delamination and asthenospheric upwelling could have triggered the M1 event. Subsequently, at ~1470–1400 Ma, the igneous protolith of the host FOG intruded and hydrated the PG. Thereafter, a second metamorphic event, M2, accompanied by compressional structures, affected both the rock types. A clockwise P–T path that culminated at ≥10 kbar ~760–850°C and is followed by a steeply decompressive retrograde path characterizes this event. The P–T path and the inferred geothermal gradient (<27°C/km) are compatible with a continent–continent collisional setting. Geochronological findings suggest a protracted orogeny for the M2 event with its major pulse during ~970–950 Ma. When combined with the published information, this study supports the view that a large (if not the entire) portion of the Indian shield and the granulite terranes of east Antarctica share similar tectonothermal events that led to the formation of two supercontinents, Columbia and Rodinia.