Petrogenesis of a Mesoproterozoic quartz latite-granitoid suite from the Roxby Downs area, south Australia

Mesoproterozoic granitoids from the Roxby Downs area, South Australia, show a range in composition from mafic quartz monzodiorites (56% SiO2) to felsic granites (71% SiO2), and are interpreted as consanguineous on the basis of similar age, isotopic composition and mineralogical and geochemical characteristics. These granitoids have geochemical similarities with other Mesoproterozoic granitoids the world over, including enrichments in K2O, Rb, Ba, Th, U, Rare Earth Elements (REE), Nb, Y, Zr, and depletions in MgO, CaO, Sr, Cr, Ni, V, Sc relative to typical Phanerozoic calc-alkaline granitoids. The mafic granitoids have mineralogical, textural and geochemical features consistent with an origin as cumulates from a more felsic primary magma, and the felsic granites likely represent the fractionated, residual melt to this accumulation process. None of the granitoids from the batholith preserve the primary magma composition, which is deduced to be similar to abundant quartz latite volcanic rocks of the region. It is estimated by geochemical modeling that about 27% fractionation of observed phenocrysts from a primary quartz latite is required to produce a typical felsic granite (71% SiO2). Production of the primary quartz latite magma was likely the result of partial melting of existing Paleoproterozoic crust in response to intrusion into the crust of mantle-derived magmas. The combination of widespread crustal melting, contemporaneous granulite-facies metamorphism and mantle-derived magmatism suggests involvement of large-scale tectonothermal processes in the origin of these granitoids. The petrogenesis of these rocks and similar occurrences in Laurentia and Baltica indicates that processes involving reworking of existing Paleoproterozoic crust were similar throughout many continental areas during the Mesoproterozoic.