Metamorphic evolution and zircon geochronology of early Proterozoic granulites in the Aravalli Mountains of northwestern India

Abstract

Granulites including a charnockite suite, mafic granulites, pelitic granulites, metanorite dykes and their retrograde varieties occur as discontinuous shear zone-bounded bodies within the Archaean basement comprising a granite gneiss–amphibolite–metasedimentary rock association in the central part of the Aravalli Mountains, northwest India. The entire suite, named the Sandmata Complex, preserves a complex history of tectonothermal evolution. Except for their strongly foliated margins, the granulite bodies are largely massive. Partial melting in the ‘country rocks’ led to the development of migmatite gneisses close to the contact of the granulite, a feature not as common in the rocks further away from the granulite contact. Geothermobarometry of massive granulites indicates T_max>900°C and P_max∼7.5 kbar. The retrograde granulites, which formed at lower amphibolite/upper greenschist-facies conditions, experienced channelized hydration reactions concomitant with shearing. These rocks locally appear as hornblende–biotite-bearing foliated granulite with or without Cpx or Opx. The rocks seem to have followed an inverse PTt path and have undergone an earlier phase of near-isobaric cooling. Our single zircon Pb–Pb ages indicate that the exhumation of granulites to the shallower amphibolite-facies levels with concomitant melting in the country rocks took place between 1690 Ma and 1621 Ma. Assuming that the granulite-facies metamorphism took place at around 1725 Ma, we relate the entire process of granulite metamorphism and exhumation covering an age range between 1725 and 1621 Ma to the rift basin opening stages of the Delhi Orogenic cycle that culminated at c. 1450 Ma.