Bimodal volcanism at the western margin of the Kaapvaal Craton in the aftermath of collisional events during the Namaqua-Natal Orogeny: The Koras Group, South Africa

Abstract

The Koras Group is a bimodal metavolcanic suite on the western margin of the Archean Kaapvaal Craton at its contact with the poly-deformed and highly metamorphosed Proterozoic Namaqua Sector of the Namaqua-Natal Province of southern Africa. It is composed of two temporally distinct but lithologically similar volcanosedimentary successions, the lower at ∼1.17Ga and the upper at ∼1.1Ga. Both successions are characterised by basaltic andesites and rhyolites sandwiched between immature siliciclastic metasediments.The subalkaline basalt to basaltic andesites are tholeiitic, magnesian, and calcic to calc-alkalic. Their trace element concentrations are enriched in the large ion lithophile (LIL) elements (Cs, Rb, Sr, Ba, K and Pb), along with Th and U relative to primitive mantle (PM), and show depletions in P and Ti, with a Nb–Ta ‘trough’ relative to the LILE and light rare earth elements (LREE). The basaltic andesites were extruded in a continental rift setting, but with low Nb contents (7.8–10.9ppm), along with La/Nb ratios of 2.3–3.1, have a subduction-related signature for both successions. The rhyolitic porphyries of both successions are peraluminous, magnesian and alkali-calcic to alkalic. They have similar trace element patterns to the basaltic andesites, but are strongly enriched in the LILE and LREE relative to PM, and have strong depletions in P, Ti and Sr and a prominent Nb–Ta ‘trough’. The rhyolitic porphyries were emplaced in a within-plate tectonic setting and are characterized by high Zr+Nb+Ce+Y contents (>350ppm) and Ga/Al ratios (10,000Ga/Al>2.6), typical of A-type granitoids. The upper volcanic succession, in particular, shows many similarities, both in terms of whole rock geochemistry, as well as age, to the late- to post-tectonic granitoids of the Keimoes Suite, an extensive granitoid suite that ‘stitches’ accretionary terranes together on the eastern margin of the Namaqua Sector. Chondrite-normalized REE patterns of the bimodal volcanism are characterised by LREE enrichment [(La/Sm)CN=2.7–4.96], slight HREE depletion [(Gd/Yb)CN=1.09–2.46] and negative Eu anomalies which become more pronounced from the basaltic andesites (Eu/Eu*=0.69–0.86) to the rhyolitic porphyries (Eu/Eu*=0.57–0.66). The overall coherence of both major and trace element trends, as well as the positive correlation between incompatible element contents favours a co-magmatic origin for the basaltic and rhyolitic rocks with fractional crystallization processes dominant giving rise to A-type magmas.The magmas were emplaced during trans-tensional periods on the western margin of the Kaapvaal Craton that followed two collisional events due to decompression melting of the subduction-related subcontinental lithospheric mantle following slab detachment in an area of previously thickened crust, which then incorporated varying degrees of continental crustal material. This is supported by ɛNd(t) values ranging from positive for some rhyolitic porphyries through to dominantly negative, variable TDM ages of >1.5Ga, and low Nb/La and Ta/Th ratios.The Koras Group represents an example of A-type bimodal volcanism at 1.1Ga which finds regional correlatives in similar Mesoproterozoic successions in southern Namibia and northwestern Botswana associated with accretionary collisional events.