AGE OF THE HOLENARSIPUR GREENSTONE-BELT, RELATIONSHIPS WITH THE SURROUNDING GNEISSES (KARNATAKA, SOUTH-INDIA)

A rhyolitic volcanic flow included in the Holenarsipur greenstone belt, South India, has a simple igneous population of zircons which has a SHRIMP Pb-207/Pb-206 age of 3298 +/- 7 Ma, interpreted as the crystallization age of this flow and considered synchroneous with deposition of the coeval sedimentary pile. No evidence of younger volcanism is present. Thus the Holenarsipur greenstone belt is now documented as the oldest supracrustal accumulation in the Dharwar craton. From field and other published geochronological results we conclude that a large part of the surrounding gneisses are very close in age to this rhyolite; some gneisses are 10-20 m.y. older than the rhyolite, whilst another set is significantly younger. From the short temporal gap measured between the gneisses and the supracrustal rocks, we suggest that the supracrustals could be formed by continuous processes attending the accretion of penecontemporaneous gneisses.