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 crystall ization age of this flow and considered synchroneous with deposition o f the coeval sedimentary pile. No evidence of younger volcanism is pre sent. Thus the Holenarsipur greenstone belt is now documented as the o ldest 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; s ome gneisses are 10-20 m.y. older than the rhyolite, whilst another se t is significantly younger. From the short temporal gap measured betwe en the gneisses and the supracrustal rocks, we suggest that the suprac rustals could be formed by continuous processes attending the accretio n of penecontemporaneous gneisses.