Constraints on the origin of early high-heat producing (U-Th enriched) granitic magmatism in central Australia

Abstract

The southern margin of central Australia is characterised by anomalous heat production, 3–5 times higher than global averages. Paleoproterozoic voluminous granitoid complexes in the region are important in the study of this anomalous heat flow. Ca.1800 Ma high-heat producing granites in Mt Boothby have A/NCK (molecular Al2O3/(CaO+Na2O+K2O)) ratios > 1, indicating a predominant origin from partial melting of metasedimentary rocks. The Boothby Orthogneiss is characterised by moderately negative Eu anomalies (Eu/Eu*: 0.03–0.43) and strong depletion in Ba, Rb, Nb and Sr. The enrichment of Ba and Rb relative to Sr and high K2O contents also support a metasedimentary source. The heat production values calculated for the Boothby Orthogneiss and the surrounding Lander formation show that the region is enriched in heat producing elements. The U-Pb zircon age data of inherited zircons in these granites are similar to the detrital zircons of the widespread outcropping; Lander formation. Nd values of -3.5 to 1.3 of the granites infer an evolved crustal source coupled with mixing of a newly mantle-derived component through lower crust assimilation. Zircon saturation temperatures calculated suggest that the Boothby intrusive complex was emplaced at 688–845oC, with a maximum temperature of 776oC, implying an arc environment with associated fluid-flux melting in the mantle wedge, ultimately controlled by subduction dynamics.