GEOCHEMISTRY AND HEAVY MINERALS ANALYSES ON "BLACK SANDS" OF THE LOWER CAMBRIAN FISH RIVER SUBGROUP (NAMA GROUP, NAMIBIA)

Abstract

Heavy mineral rich-sandstones (“black sands”) of the Lower Cambrian upper Fish River Subgroup in Namibia were used to reveal the provenance of the detrital material. The host rocks of the black sands from the Haribes and Rosenhof Members are mainly composed of partially rounded quartz, feldspar (K-feldspar > plagioclase), polycrystalline quartz, few siltstone fragments and labile altered or metamorphosed volcanic lithoclasts. Geochemical proxies are similar to those from typical foreland deposits and point to slightly recycled upper continental crustal composition, e.g.: Th/Sc ratios between 1.23 and 1.95, Zr/Sc ranging from 24 to 38 and La/Sc from 4.8 to 10.8. Heavy mineral addition of up to 30% does not significantly change Th/Sc, La/Sc and La/Th values, but slightly changed the Zr/Sc ratios, which range from 38 to 53. The heavy mineral-enriched beds (50 to 75% of the whole rock), display nearly similar La/Sc ratios compared with other lithotypes. They are 9x enriched in rare earth elements and show a significantly stronger negative Eu-anomaly, and an enrichment of heavy over light rare earth elements, as well as higher Zr/Sc ratios (82 to 299), typical for reworked passive margin or foreland deposits. The chemical composition of separated heavy minerals point to a relatively strong influence of metapelitic and metamorphosed mafic sources, which were not identified using geochemical data. Mineral chemical analyses of chromian spinels point to island arc and mid-ocean ridge basalts as protoliths, whereas rutiles were derived from metamorphosed mafic and pelitic rocks, and abundant garnet (mainly almandine) derived from high-grade and low-grade metapelitic sources. Titanite shows dominance of felsic and mafic plutonic rocks as sources, while epidote point to low-grade metamorphic protoliths. Magnetite was eroded from felsic plutonic and/or mafic metamorphosed rocks. Previous works using U-Pb detrital zircon dating show typical Namaquan and Pan-African age populations as expected for a collisional tectonic setting. Palaeocurrents indicate that the main source region of the heavy and the host rock minerals is within the Damara Belt which resulted from the convergence of Congo and Kalahari Cratons. Although the exact age of thrusting is debated, the data presented suggest the exhumation of metamorphosed crustal fragments and plutonism related to the Damara orogeny during the deposition of the upper Nama Group (ca. 520 Ma).