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

ISOTOPIC data for the Earth's oldest rocks(1-7) imply that a considerable 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 form emergent landmasses. Sedimentary geochemistry suggests(8,9) that the area of exposed continent was negligible until late in the Arehaean(10), a contention supported by the fact that, until now, all greenstone supracrustal volcanic and sedimentary successions shown to have ken deposited on eroded continental basement have yielded ages of less than or similar to 3.0 Gyr, Here we report the discovery of an angular unconformity (an ancient erosion surface) beneath rocks of the 3.46-Gyr Warrawoona Group in the Pilbara craton of Australia, currently the oldest 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 level, eve infer that they accumulated on crust that was already rigid, cool and buoyant, Thus, by 3.46 Gyr ago, the sub-Warrawoona rocks formed an emergent block of continental crust, the most ancient known.