3.55 GA OLD ZIRCON FROM SINGHBHUM-ORISSA IRON-ORE CRATON, EASTERN INDIA

(207)pb/(206)pb ages of zircons from the Older Metamorphic Group (OMG) , the oldest recognized rock unit from the Singhbhum-Orissa Iron Ore C raton, eastern India, have been determined by ion microprobe. The zirc on ages lie between 3628 and 3196 Ma and cluster around 3.55, 3.4 and 3.2 Ga, The zircons belonging to the first age group occur mostly as s ub-rounded cores or as well rounded to subhedral grains of apparent de trital origin, while the zircons belonging to the younger groups occur as subhedral, prismatic grains or as overgrowths, Our data suggest th at the provenance of the sedimentary precursor included granitoid rock s with ages between 3.6 and 3.5 Ga, confirming the presence of Early A rchaean crust in the Singhbhum, region, The younger age of 3.2 Ga most probably reflects a later metamorphic event which overprinted the OMG rocks, Finally, our data establish an older limit of similar to 3.5 G a for the age of sediment deposition.