Provenance and paleogeography of the Nama Group (Ediacaran to early Palaeozoic, Namibia): Petrography, geochemistry and U–Pb detrital zircon geochronology

Abstract

A provenance study using petrography, geochemistry and U–Pb zircon dating was carried out on selected siliciclastic units of the Nama Group, a major lithostratigraphic unit in southwestern Gondwana which crops out in southern Namibia and adjacent north-western South Africa. Petrographic and geochemical results from the Nama Group indicate a homogenous recycled upper crust composition characterized mainly by metamorphic and granitic sources, with minor input from mafic rock sources that have not undergone significant weathering. Previous works based on facies and palaeocurrent analyses, and silicified volcanic ash beds and chromian spinel bearing sandstones of the Nama Basin points to a syn-tectonic volcanic island arc source located in the adjacent Damara Belt. U–Pb detrital zircon geochronology of the Nama Group rocks displays major peaks at Neoproterozoic (Pan-African orogeny) and Mesoproterozoic (Namaqua-Natal orogeny) indicating a foreland geotectonic setting for Nama deposition, confirming facies and palaeocurrents analyses. A paleocurrent shift from the north to the west in the upper “molassic” Nama Group is associated with a switch to an influx of Neoproterozoic–Cambrian detrital zircons (76%). These ages probably indicate exhumation (after 531 ± 9 Ma) of a felsic volcanic arc (Arachania) root, which is presently attached to the Río de la Plata Craton. The provenance of the Nama foreland basin suggests that continent–continent collision of the Kalahari/Congo Cratons and the Cuchilla Dionisio Pelotas Terrane (Arachania Arc) with the Río de la Plata Craton most likely occurred due to strike-slip accretion related to a component of N–S shortening in the period between 530 and 495 Ma.