PETROLOGY AND GEOCHEMISTRY OF THE GRANITOID ROCKS OF THE JOHANNESBURG DOME, CENTRAL KAAPVAAL CRATON, SOUTH AFRICA

The Johannesburg Dome (JD) in the central Kaapvaal Craton (KC) is dominated by granitoid rocks of the tonalite-trondhjemite-granodiorite (TUG) series. Based on modal analysis as well as a major and trace element investigation the JD granitoids could be subdivided into three main suites, i.e. a Tonalitic gneiss suite (TG) around the southern boundary, a Granodiorite to Adamellite Gneiss suite (GAG) across the northern part, and a Granodiorite/adamellite to Granodiorite suite (GG) occurring between the TG and GAG suites. These rocks are dominantly I-type and peraluminous with the tonalites (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 (similar to 70 weight %), aluminous (Al2O3 >15wt%) with low Yb (<1ppm), high La/Yb ratios (>30), high Na2O/K2O (>1), and have Na2O contents of between 3wt% and 5wt%, comparable to that of the average TUG. The JD tonalities (TG suite) have higher Al2O3, Sr, Na2O/K2O, Mg#, Ni, Cr and LILE contents compared to the more calc-alkaline granitoids (CC 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 TTG's include HFSE depletion and distinct enrichment of fluid sensitive elements such as Pb. The strongly fractionated REE pattern, high (La/Yb)(N) ratio and depletion in HREE (Yb) of the JD TTGs are characteristics shared with modern adakites. The TG suite most probably formed through melting of a subducted oceanic slab with the melt interacting with mantle peridotite during its accent through a thin mantle wedge. The remaining JD granitoids (GAG and GG) most probably formed through the remelting of a TTG protolith, which has a subducted slab and mantle wedge signature (similar to the TG suite).