3.55 Ga old zircon from Singhbhum- Orissa iron ore craton, eastern India

207Pb/206Pb ages of zircons from the older metamorphic group (OMG), the oldest recognized rock unit from the Singhbhum-Orissa iron ore craton, eastern India, have been determined by ion microprobe. The zircon 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 subrounded cores or as well rounded to subhedral grains of apparent detrital origin, while the zircons belonging to the younger groups occur as subhedral, prismatic grains or as over growths. Data suggest that the provenance of the sedimentary precursor included granitoid rocks with ages between 3.6 and 3.5 Ga, confirming the presence of early Archaean 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 ∼3.5 Ga for the age of sediment deposition. (author). 8 refs., 2 figs., 1 tab