Geochemical character and petrogenesis of Pan-African Amspoort suite of the Boundary Igneous Complex in the Kaoko Belt (NW Namibia)

Abstract

In the Kaoko Belt of northwestern Namibia, syn-tectonic granitoids of the Boundary Igneous Complex intruded a NNW–SSE trending suture between the Neoproterozoic Coastal Terrane and Archaean–Neoproterozoic Congo Craton-derived foreland. Typical representatives of this syn-collision igneous association are ca. 550 Ma old K-feldspar–phyric, Bt ± Cam granites–granodiorites of the Amspoort suite, with minor Cpx gabbro and rare two-pyroxene dolerite bodies. The scarce Opx–Cpx–Bt dolerites could come from a CHUR-like mantle source ( 87Sr/ 86Sr 550∼ 0.7045, ε 550 Nd∼ 0). The rest of the Amspoort suite is interpreted as a product of HT anatexis of a heterogeneous lower crust, built mainly by immature metapsammites rich in arc-derived detritus with minor metabasite and intermediate metaigneous bodies. The most likely source appears to be the anatectic Coastal Terrane gneisses. Yet, partial melting of so far little constrained Congo Craton cover, if formed by immature and youthful detritus unrelated to the basement, cannot be discounted. In any case, the rather primitive Sr–Nd isotopic compositions of the Amspoort suite apart from dolerites ( ε 550 Nd = − 3.4 to − 5.3, 87Sr/ 86Sr 550 = 0.7063–0.711), rule out any major role for the Congo Craton-basement derived material, which is in sharp contrast to other, more southerly Pan-African granitoid complexes of the Kaoko Belt.