Contrasting episodes of regional granulite-facies metamorphism in enclaves and host gneisses from the Aravalli-Delhi Mobile Belt, NW India

Abstract

The Aravalli-Delhi Mobile Belt in the northwestern part of India demonstrates how granulite enclaves and their host gneisses can be utilized to unravel multistage metamorphic histories of orogenic belts, using three suites of metamorphic rocks: (1) an enclave of pelitic migmatite gneiss-leptynite gneiss; (2) metamorphosed megacrystic granitoids, intrusive into the enclave; (3) host tonalite-trondhjemite-granodiorite (TTG) gneisses associated with an interlayered sequence of garnetiferous metabasite and psammo-pelitic schist, locally migmatitic. Based on integrated structural, petrographic, mineral compositional, geothermobarometric studies and P-T pseudosection modelling in the systems NCKFMASH and NCFMASH, we record three distinct tectonothermal events: an older, medium-pressure granulite-facies metamorphic event (M₁) in the sillimanite stability field, which is registered only in the enclave, a younger, kyanite-grade high-pressure granulite-facies event (M₂), common to all the three litho-associations, and a terminal amphibolite-facies metamorphic overprint (M₃). The high-P granulite facies event has a clockwise P-T loop with a well-constrained prograde, peak (M₂, P~12-15 kbar, T~815°C) and retrograde (M_2R, ~6.1 kbar, T~625°C) metamorphic history. M₃ is recorded particularly in late shear zones. When collated with available geochronological data, the metamorphic P-T conditions provide the first constraint of crustal thickening in this belt, leading to the amalgamation of two crustal blocks during a collisional orogeny of possible Early Mesoproterozoic age. M₃ reactivation is inferred to be of Grenvillian age.