U-Pb ages of zircon and sphene for two gneiss terranes adjacent to the Kolar Schist Belt, South India: evidence for separate crustal evolution histories

Abstract

Orthogneisses adjacent to the Kolar Schist Belt were emplaced as tonalitic to granodioritic magmas between 2630 and 2530 Ma. An orthogneiss unit east of the Kolar Schist Belt apparently was a homogeneous granodioritic intrusion at about 2532 Ma, cooling to the blocking temperature of the U-Pb system in sphene by 2521-2517 Ma. Gneisses on the west side of the Kolar Schist Belt were emplaced as at least three separate intrusions dated at 2631, 2610, and 2552 Ma. Evidence from an older basement into which these rocks were intruded comes from inherited zircons with an age of >2800 Ma. Rocks with zircons of the appropriate age to be part of this basement occur as tectonic fragments and inclusions in the area. Gneisses west of the Kolar Schist Belt were last cooled through the blocking temperature of the U-Pb system in sphene at about 2552 Ma, synchronous with the last major intrusive event. Late felsic dikes and plutons apparently postdate deformation and yield sphene Pb-Pb ages of <2480 Ma. A sample of muscovite in the shear zone on the west wide of the Kolar Schist Belt has an ⁴⁰Ar/³⁹Ar plateau age of 2420 Ma. These younger ages suggest that the two gneiss terranes adjacent to the Kolar Schist Belt and the schist belt itself were amalgamated to form a part of the eastern Dharwar craton by the earliest Proterozoic.