SHRIMP U-Pb Geochronology of the Closepet Granite and Peninsular Gneiss, Karnataka, South India

Zircons separated from a sample of Peninsular gneiss (82-KD 14) from Kabbaldurga quarry, and a homogeneous Closepet granite (KO84-13) from near Ramnagaram have been dated with the ion microprobe SHRIMP. The Peninsular gneiss sample, a metatexite, contains an arrested charnockite transition and constitutes an amphibolite-granulite close pair. It was split into amphibolite facies, transitional and granulite facies (charnockite) domains prior to zircon separation.

Zircon populations separated from each domain are morphologically similar and have the same U-Pb isotopic systematics suggesting that they record events common to both the amphibolite facies and granulite facies domains. The pooled data yield a U-Pb age of 2963 ± 4 Ma (20σ) for the gneiss protolith at Kabbaldurga, with a few older inherited zircons found as cores in magmatic zircon. A concordant U-Pb age of 2528 ± 5 Ma was obtained for newly grown zircon and overgrowths, interpreted to have formed during a migmatisation event which led to the formation of the Closepet granite.

Zircons separated from the Closepet granite sample gave a concordant age of 2513 ± 5 Ma, interpreted as the time of crystallisation of the granite. This age was obtained from both zircon cores and overgrowths which are indistinguishable in age and arc both interpreted to have formed during the melting event. In preserved cores no indication of any inherited grains with a significantly older history was found.

This is the first precise age on the Closepet granite. The slight age difference between the metatexite event (2528 ± 5 Ma) and the granite age 2513 ±5 Ma) may indicate the length of time of the event during which the Closepet granite formed. The data provide a minimum constraint on the age of the charnockite event in the region.