Sm-Nd evidence for 3.3 Ga old rocks in Rajasthan, Northwestern India

Abstract

Tonalitic to granodioritic gneisses from the basement gneiss of the Aravalli Region, northwestern India, define a Sm-Nd isochron corresponding to an age of 3.31±0.07 Ga and an initial Nd isotopic ratio that is close to chondritic. Lenticular bodies of mafic amphibolites within the gneisses, however, define an isochron corresponding to a younger age of 2.83±0.05 Ga and small positive initial ε_Nd. This age is similar to that reported for a large intrusive granite (the Untala granite) in this area and, combined with field evidence, suggests that mafic volcanism may have been associated with formation of the granite. A highly HREE enriched garnet separate from one of the tonalitic gneisses defines a two-point garnet-wholerock isochron age of 2.45±0.02 Ga, which also is similar to the age of a known thermal event in the region: intrusion of the Berach and Bundelkhand granites at approximately 2.5 Ga. These results document in broad outline Archean crustal formation events in the northwestern segment of the Indian subcontinent, and demonstrate the need for more detailed studies of the complex terranes mapped by Heron (1953) as a single unit, the Banded Gneiss Complex.