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 鈥榯rough鈥?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 (7.8-10.9 ppm), 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 鈥榯rough鈥? The rhyolitic porphyries were emplaced in a within-plate tectonic setting and are characterized by high Zr + Nb + Ce + Y contents (>350 ppm) 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 鈥榮titches鈥?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 蓻Nd(t) values ranging from positive for some rhyolitic porphyries through to dominantly negative, variable TDM ages of >1.5 Ga, and low Nb/La and Ta/Th ratios.
The Koras Group represents an example of A-type bimodal volcanism at 1.1 Ga which finds regional correlatives in similar Mesoproterozoic successions in southern Namibia and northwestern Botswana associated with accretionary collisional events.