Petrogenesis of the felsic rock units of the late-Precambrian Sinclair Group, South West Africa

Abstract

The Sinclair Group is a late-Precambrian succession of basic to intermediate and felsic lava flows and intrusives, volcaniclastic and clastic units constituting part of an extensive volcanic, plutonic and clastic arc — the Rehoboth Magmatic Arc — which is regarded as having developed on the western and north-western margins of the sialic Kalahari Plate as a result of the subduction of oceanic crust. The major felsic units comprise high-level granites, rhyolitic lava flows, plugs, domes, dykes and various volcaniclastics including ash-flow tuffs. Average major and trace element analyses of these units are presented and on this basis they can be separated into high-Ca and low-Ca groupings each with distinctive Ca content ranges and trace element distribution patterns. The genesis of the felsic units is discussed and it is considered that the magmas were produced by partial fusion of sialic crust; the heat source for fusion can be found in the repeated injection of basic magma during evolution of the Group. The high-Ca magmas resulted from melting at the deepest crustal levels mainly involving a quartzofelspathic host rock and hornblende whereas the low-Ca melts were derived from the partial fusion of a similar host rock and biotite at shallower levels in the crust. Water content of the magmas, which was probably in the region of 1–2 per cent and not more than 4 per cent, could have been provided by these hydrous minerals and partial melting probably took place between depths of 18 km and 36 km in the continental crust at temperatures of about 750° C to 950° C. Ash-flow tuffs resulted from vesiculation of the felsic magma at depths between 0 and 4 km, at temperatures between 800° C and 950° C and with water contents restricted to a range of about 1,5 to 4 per cent.