Petrology and geochemistry of the granitoid rocks of the Johannesburg dome, Central Kaapvaal Craton, South Africa

Abstract

The Johannesburg Dome (JD) in the central Kaapvaal Cratoti (KC) is dominated by granitoid rocks of the tonalite-trondhjemite- KniiKKliorite (TTG) series. Ba.sed on modal analysis as well as a major atid trace element investigation the JD granitoids could be .sulxliviclccl into three main suites, i.e. a Tonalitic gtieiss suite (TG) around the .southern boundary, a Grancxiiorite to Adamellite Gneiss suite (GAG) across the northern part, and a Granodiorite/adamellite to Granodiorite suite (GG) occurring lx;tween the TG and GAG suites. These rocks are dominantly I-type and peraluminous with the tonalités (TG and partly the GAG suites) falling in the metaluminous field. TTGs of the JD are high-K calc-alkaline to calc-alkaline and are dominantly high silica rocks (-70 weight %), aluminous (AI2O3 >15wt%) with low Yb (<lppm), high La/Yb ratios O30), high NaiO/K^O Ol), and have NaiO contents of tetween 3wt% and 5wt%, comparable to that of the average TTG. The JD tonalities (TG suite) have higher AI2O3, Sr, Na^O/KiO, Mg#, Ni, Cr and ULE contents compared to the more calc-alkaline granitoids (GG suite and trondhjemites of the GAG suite), which are typically richer in HREE (lower REE fractionation), Y and show a negative Sr and Eu anomaly. Other characteristic features of the JD TIG's include HFSE depletion and distinct enrichment of fluid sensitive elements such as Pb. The strongly tractionated REE pattern, high (La/Yb)^ ratio and depletion in HREE (Yb) of the JD TTGs are characteri.stics shared with iiKxIern adakites. The TG suite most probably formed through melting of a subducted oceanic slab with the melt interacting with mantle peritlotite dtiring its accent through a thin mantle wedge. The remaining JD granitoids (GAG and GG) most probably formed through the renielting of a TTG protolith. which has a subducted slab and mantle wedge signature (similar to the TG suite).