Pan-African accretionary metamorphism in the Sperrgebiet Domain, Gariep Belt, SW Namibia

Abstract

The newly described Sperrgebiet Domain is a part of the Palaeo- to Mesoproterozoic Namaqua–Natal Province that is exposed as a basement inlier within the Pan-African Gariep Belt in SW Namibia. Clastic metasediments of the c. 1700Ma Aurus Schist of the Sperrgebiet Domain exhibit Pan-African fabrics and metamorphic mineral assemblages that are not present in the low-grade metamorphic Gariep Supergroup against which it is tectonically juxtaposed. Metapelitic and semi-pelitic schists consist of phengitic white mica, chlorite, minor garnet and quartz ± biotite, and contain pseudomorphs after a prismatic aluminous mineral, likely chloritoid or staurolite. Calculated pseudosections for four mica schist samples constrain P–T conditions of around 10kbar and 500°C, and the late paragenesis of garnet indicates that these conditions were attained along a prograde, clockwise trajectory. These conditions and the low apparent geotherm (∼14°C.km−1) are typical of subduction zone metamorphism, and are similar to those documented for peri-contemporaneous accretionary metamorphism in the adjacent Damara Belt to the NE. A subduction origin has also been proposed for exotic ‘blueschist’ blocks within the Gariep Belt; however, the absence of an accretionary metamorphic imprint in the Gariep Supergroup indicates that the Sperrgebiet Domain was decoupled from these lithologies for much of the Pan-African Orogeny. Our results are the first indication that the continental margin of the western Kalahari Craton may have been subducted during the latter stages of Pan-African convergence and Gondwana assembly.