Chemical composition and tectonic setting of chromian spinels from the Ediacaran-early Paleozoic Nama group, Namibia

Pétrographic and chemical characterization of chromian spinels from the lower and upper Schwarzrand Subgroup and the upper Fish River Subgroup of the latest Ediacaran to early Paleozoic Nama foreland basin in Namibia is used to infer the tectonic setting of source rocks and evaluate the geotectonic evolution of the Damara Orogen. Electron microprobe analysis of detrital chromian spinels from the Nama Group show three groups of chromian spinels: (i) a group with a Cr# (Cr/(Cr + Al) atomic ratio) between 0.62 and 0.75 and about 0.9% TiO₂, which suggest derivation from a volcanic island arc (Niederhagen and Niep members); (ii) a group with a Cr# between 0.75 and 0.88 and TiO₂ well below 0.4%, which could be associated with boninitic magmas (Niederhagen, Niep, and Haribes/Rosenhof members); and (iii) a group that displays characteristics of a mid-ocean ridge basalt source, with a Cr# around 0.3 and TiO₂ below 0.5% (Haribes and Rosenhof members). Paleocurrent analyses of the chromian spinel-bearing sandstones of the Nama Basin point to a source area located in the Damara Belt. The chromian spinel petrography and chemistry do not match those of a source from the Paleo-Mesoproterozoic basement. Although a western provenance for the lower Nama Group is not supported by paleocurrents, the Gariep Belt probably acted as a source by reworking at the final stage of the "foreland deposits" of the Nama Group. Thus, the existing data favor a provenance from the Damara Orogen and indirectly support the tectonic models that argue for an active continental margin on the northern and western borders of the Kalahari Craton.