Petrogenesis of Syenites and Granites of the Schiel Alkaline Complex, Northern Transvaal, South Africa

Abstract

The Schiel Complex (2059 +35/-36 Ma) is one of the alkaline intrusive suites emplaced in the northern part of the Kaapvaal craton during the Early Proterozoic. It is a typical mixed silica-undersaturated to -oversaturated complex bearing a wide composition of rocks, including pyroxenite, glimmerite, foskorite, carbonatite, dolerite, shonkinite, and dominant syenite and granite. The mineralogical and geochemical features of the syenites and granites define a petrologic association with quartz + alkali feldspar + diopside-hedenbergite clinopyroxene + lepidomelane biotite ± ferro-edenite to ferro-pargasite hornblende as major mineral phases; accessories are apatite, titanite, zircon, and magnetite. Chemically, they are alkaline to peralkaline and characterized by relatively low contents of Al, Mg, and Ca and high contents of high field strength elements, including Nb, Ga, Y, and rare earth elements. These features are characteristic of most A-type granitoids. The genesis of syenites and granites of the Schiel Complex is related to the geological evolution of the Southern Marginal Zone of the Limpopo folded belt after the cessation of high-grade metamorphism and during the subsequent extensional period. It is here considered in terms of partial melting of a granulitic lower crust under high flux of mantle-derived magmas and volatiles. The melts produced were emplaced along an important shear zone of post-orogenic regional extension, the Kudus River Lineament, a long-lived zone of crustal weakness. This proposed petrogenetic model can account for the geological setting and the petrological and geochemical features of the alkaline complex.