RECORD OF EMERGENT CONTINENTAL-CRUST SIMILAR-TO-3.5 BILLION YEARS AGOIN THE PILBARA CRATON OF AUSTRALIA

ISOTOPIC data for the Earth's oldest rocks(1-7) imply that a considera ble volume of continental crust existed during the early Archaean aeon (>3.0 Gyr ago), but it is not known when this crust first began to fo rm emergent landmasses. Sedimentary geochemistry suggests(8,9) that th e area of exposed continent was negligible until late in the Arehaean( 10), a contention supported by the fact that, until now, all greenston e supracrustal volcanic and sedimentary successions shown to have ken deposited on eroded continental basement have yielded ages of less tha n or similar to 3.0 Gyr, Here we report the discovery of an angular un conformity (an ancient erosion surface) beneath rocks of the 3.46-Gyr Warrawoona Group in the Pilbara craton of Australia, currently the old est known,veil-preserved greenstone succession. Below the unconformity , low-grade greenstones older than 3.5 Gyr were intruded by voluminous granitoids before erosion, As the overlying Warrawoona rocks are only mildly metamorphosed, slightly deformed and were deposited near sea l evel, eve infer that they accumulated on crust that was already rigid, cool and buoyant, Thus, by 3.46 Gyr ago, the sub-Warrawoona rocks for med an emergent block of continental crust, the most ancient known.