Geochemical evolution of the Lebowa Granite Pluton in western Bushveld Igneous Complex, South Africa: More insight into the evolution of bimodal A‐type granitoid

Abstract

The ~2.05 Ga Bushveld Igneous Complex (BIC) in the Kaapvaal Craton, South Africa, with an areal extent of ~65,000 km2, represents one of the largest bimodal A‐type granitoid suites in the world. The igneous activity of this complex begun with the Rooiberg Group of basalt–rhyolite bimodal volcanism, followed by the emplacement of the ultramafic–mafic Rustenburg Layered Suite (RLS), and finally the granitoid rocks of the Rashoop Granophyre (RG) and Lebowa Granite Suite (LGS). In this study, we present our new borehole description and whole‐rock chemical data on the LGS from the relatively unknown western BIC to constrain the origin of the LGS and try to evaluate their geochemical relationship to the Rooiberg Group felsic volcanics. In the western BIC, the LGS is locally represented by the hydrothermally altered Paalkraal Granite and the Kenkelbos Granite that together form a central dome‐like feature. The Veekraal Granite forms the peripheral part of the dome. These granitoids are, in general, coarse‐grained, porphyritic, or equigranular, showing granophyric intergrowth, and consist of quartz, perthitic microcline, and albitic plagioclase with minor opaque and interstitial hornblende/biotite. Our whole‐rock chemical data on western LGS, along with those from the Groblersdal area, eastern BIC (taken as reference), form a cluster in the ‘granite’ field in the normative Ab–An–Or diagram with mean Or/(Ab+Or) ~0.5 (0.11), and are peralkaline to weakly peraluminous in composition. These A‐type granitoids are strictly ‘ferroan’ and calc‐alkalic to alkali‐calcic in nature, and evolved through partial melting as evident mainly in process identification plots. In bulk chemical composition, the LGS and high‐SiO2 Rooiberg Group rhyolites overlap with the experimental melts generated during dehydration melting of tonalite–granodiorite, indicating the Archaean Kaapvaal TTG as their possible source. Constraints from experimental petrology further suggest the pressure and temperature of this relatively anhydrous (≤1 wt% H2O) melting were 2–1 kbar (i.e., ≤7.5 km crustal depth) and ~920°C, respectively. As suggested before, the parent magma of the voluminous mafic–ultramafic body (RLS) of the BIC, forming a voluminous magma chamber in shallow crustal depth, provided the heat for this partial melting event. The following igneous event as evident from field description in the literature, our new borehole description and whole‐rock chemical data, were a limited chemical mixing between the parent basaltic magma of the Rooiberg Group and the parent LGS magma under anorogenic condition to generate the metaluminous felsic magma represented by the Rooiberg Group dacite at present. This anorogenic mixing, perhaps guided by chemical diffusion and lasted for ≤950 ka, modified the major oxide compositions (except Na), and/or transitional elements (Sc, V, Co, and Ni), LILEs (Rb, Sr, and Ba), REEs, HFSE (Y, Zr, Nb, and Ta), actinide (Th and U), and possibly Sr‐isotopic compositions of both the parent LGS and Rooiberg Group basaltic magmas. The petrogenetic model observed for the BIC is perhaps unique in explaining geochemical and isotopic variations observed in bimodal A‐type granitoid/volcanic suites like the Karoo Igneous Province.