PALAEOZOIC INTRAPLATE CRUSTAL ANATEXIS IN THE MOUNT PAINTER PROVINCE, SOUTH AUSTRALIA: TIMING, THERMAL BUDGETS AND THE ROLE OF CRUSTAL HEAT PRODUCTION

Abstract

The effect of radiogenic heat production within the crust on thermal processes such as crustal anatexis is generally disregarded as bulk geochemical models suggest that crustal heat generation rates are too low to effect significant heating. However, the Mount Painter Province in northern South Australia is characterized by a total crustal contribution to surface heat flow of more than twice the global average. The province is composed dominantly of Proterozoic granites and granite gneisses with an area average heat production of 16·1 ?W/m3; individual lithologies have heat production >60 ?W/m3. These Proterozoic rocks are intruded by the British Empire Granite, a younger intrusive whose origin has remained enigmatic. Isotope geochemistry suggests crustal sources for the melt and it has a crystallization age of ?440–450 Ma, which places the setting >750 km inboard of the nearest active plate boundary zone at this time. Phase equilibria calculations suggest that temperatures of at least 720–750°C are required to produce the granite but the intensity of crustal thickening during Palaeozoic deformation (?12%) cannot account for these conditions. Here we describe a model for the generation of the British Empire Granite in which the primary thermal perturbation for mid-crustal anatexis was provided by the burial of the high heat-producing Mount Painter basement rocks beneath the known thickness of Neoproterozoic cover sediments. The high heat-producing rocks at Mount Painter imply that the natural range and variability of crustal heat production is much greater than previously believed, with important consequences for our understanding of temperature-dependent crustal processes including the exploitation of geothermal energy resources.