Constraints on the origin of the ca 1780 Ma high heat producing Napperby Gneiss, Aileron Province, Central Australia

Abstract

The Arunta Region of Central Australia contains Paleoproterozoic granites extremely enriched in high heat producing elements, in comparison to a global upper crustal average of 1.69 μWm-3. This study uses geochemistry, geochronology, and zircon saturation thermometry to investigate the source and tectonic environment of emplacement of the ca. 1780 Ma Napperby Gneiss. The Napperby Gneiss is peraluminous, suggesting a metasedimentary source. Samples have negative Eu anomalies ranging from 0.10 to 0.57, and show further evidence of fractionation in negative correlations of Ba and Sr with increasing SiO2. Initial εNd values are similar to surrounding exposed metasedimentary rocks and suggest a strong influence of an evolved crustal source but indicate a necessary juvenile component. Matches of inherited xenocrystic zircons from the gneiss with detrital patterns from the regional metasedimentary Lander Formation indicate that sediments similar to the Lander Formation are the source of the protolith granite. Zircon saturation temperatures suggest the granites were emplaced at 790°C – 872°C. Heat production is less than the slightly older ca 1800 ma suites of the Aileron province, and zircon saturation temperatures are higher. The Napperby was produced by dehydration melting rather than fluid flux melting, possibly in a back arc extensional environment with heat provided by upwelling mantle.