VOLUMINOUS HIGH HEAT PRODUCING GRANITE PRODUCTION REQUIRES FLUID-FLUXED PARTIAL MELTING

Large-volume high heat producing granites are relatively uncommon. Particularly in the Archean to Palaeoproterozoic, average granite heat production (Th, U and K) is on the order of 1 -2 microW/m³ (present day). Known high heat producing examples are small volume, thought to preferentially form largely in continent-continent collisional settings, and to be insignificant in arc or within-plate settings. In the Aileron Province of central Australia there is about 35km² of Palaeoproterozoic ca 1800 Ma granite several kilometres thick with heat production values on the order of 5-10 microW/m³. Geochemistry indicates peraluminous compositions. Rare earth elements, Hf, Zr and Th concentrations are enriched, with flat medium to heavy rare earth element enrichments suggesting little residual garnet. Abundant inherited zircons are good matches for detrital zircons in the host metasedimentary rocks at the surface. We suggest the granites are in fact sourced from partial melting of these sedimentary rocks at depth. Previous tectonic models place these granites in a high T extensional back-arc setting, with upwelling mantle providing the heat source for fluid absent crustal melting. Zircon saturation thermometry yields melting temperatures of around 780⁰C, markedly low for the proposed back-arc setting and fluid-absent melting. In fact, fluid absent melting is unable to generate this volume of melt. Instead we suggest fluid-fluxed melting in a subduction setting for this magmatic rocks. The dissolution of accessory minerals, zircon but especially monazite is favoured in fluid fluxed melting and can explain the great enrichment in those trace elements common to these accessory phases.