Archean-Proterozoic Evolution of Indian Charnockites: Isotopic and Geochemical Evidence from Granulites of the Eastern Ghats Belt

Pyroxene granulites and charnockites are associated with metasedimentary khondalite (garnet-sillimanite gneiss) and migmatites in the Eastern Ghats mobile belt. Various stages of in situ granulitization similar to those observed in southern India and Sri Lanka are present. Geochemically the granulite suite around Visakhapatnam is bimodal, with acid and mafic variants. There is an increase in the concentration and fractionation of the rare earth elements (REE) from pyroxene granulites to charnockites, and six Sm-Nd models ages ($T_{DM}$) range from 2.86 to 2.35 Ga, with 2.1 Ga for a younger granite. The charnockites, a pyroxene granulite, and granite define a Pb/Pb isochron age of$1176 \pm 201 Ma$. U-Pb ages of zircons and monazites from a charnockite near Phulbani, Orissa, yield near concordant data with ages of$985 \pm 5 Ma$and$965 \pm 7 Ma$, respectively. The new isotopic data and the available ages from the northeastern part of the Eastern Ghats in Orissa suggest a major charnockite formation event between ca. 1170 and 950 Ma. This event is contemporaneous with that of Sri Lanka but much younger than the late Archean granulite event in southern India.