Discovery of early Mesoarchean crust in the eastern Gawler Craton, South Australia

Abstract

New U–Pb SHRIMP zircon ages and Sm–Nd isotopic results identify early Mesoarchean (∼3150 Ma) granitic crust cropping out in the eastern Gawler Craton, southern Australia. These rocks are more than 500 million years older than the oldest previously dated rocks from the Gawler Craton, and challenge the view that pre-3.0 Ga crust in Australia is restricted to the Yilgarn and Pilbara Cratons of Western Australia. We interpret these new data to indicate the presence of ∼3150 Ma crust at or near the surface throughout a wedge-shaped region covering an area of at least ∼1500 km 2. The region is characterised by high surface heat flow, and contains Paleo- and Mesoproterozoic intrusive rocks with evolved Nd isotopic compositions and high radiogenic heat production values. The newly identified Mesoarchean rocks provide an obvious source for the evolved Nd signatures, and are themselves relatively enriched in heat producing elements, particularly considering their age. We speculate that the presence of relatively radiogenic Mesoarchean crust underlies much of the area covered by the South Australian Heat Flow Anomaly. Identification of Mesoarchean crust near the eastern margin of the Gawler Craton prompts consideration of links with other Australian Cratonic elements. As far as we are aware, no direct age equivalents of these rocks have been identified elsewhere in Australia, although we note that some recent models for Australian Proterozoic geodynamic evolution may be used to predict the presence of equivalent aged crust in the North Australian Craton.