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 a nd geochemical characteristics. These granitoids have geochemical simi larities with other Mesoproterozoic granitoids the world over, includi ng 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 typica l Phanerozoic calc-alkaline granitoids. The mafic granitoids have mine ralogical, textural and geochemical features consistent with an origin as cumulates from a more felsic primary magma, and the felsic granite s likely represent the fractionated, residual melt to this accumulatio n process. None of the granitoids from the batholith preserve the prim ary magma composition, which is deduced to be similar to abundant quar tz 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 t he result of partial melting of existing Paleoproterozoic crust in res ponse to intrusion into the crust of mantle-derived magmas. The combin ation of widespread crustal melting, contemporaneous granulite-facies metamorphism and mantle-derived magmatism suggests involvement of larg e-scale tectonothermal processes in the origin of these granitoids. Th e petrogenesis of these rocks and similar occurrences in Laurentia and Baltica indicates that processes involving reworking of existing Pale oproterozoic crust were similar throughout many continental areas duri ng the Mesoproterozoic.