Field relationshipsand petro-chemical investigation of mafic sills and dykes in the vicinity of the Uitkoms complex, Mpumalanga South Africa

[摘要] Numerous mafic sills and dykes intruded into the Lower Transvaal sediments and the ArchaeanBasement in the vicinity of the Uitkomst Complex, which is assumed to be a satellite intrusion of theBushveld Complex. Investigations on mafic sill intrusions near the Eastern Bushveld Complex describedsills of pre-, syn- and post-Bushveld age and assigned the syn-Bushveld sills to the correspondingmarginal rocks of the Bushveld Complex. Purpose of the presented combined diploma thesis was to map the mafic sills and dykes in the vicinity ofthe Uitkomst Complex as basis for a petrographical and geochemical characterization. The geologicalmapping as well as the petrographic description distinguishes three groups of mafic intrusive rocks:microgabbro sills, gabbronorite sills and gabbronoritic basement dykes. The group of elongatedmicrogabbro sills shows a widespread spatial as well as stratigraphic distribution within the study area,whereas the gabbronorite sills form huge sill bodies within a main stratigraphic position between theUpper Timeball Hill Shale and the Klapperkop Quartzite. The gabbronoritic basement dykes intrudedinto the Archaean granite gneiss and represent the oldest of the investigated mafic rocks. Evaluation of the obtained geochemical data from about 160 samples verifies the classification into thethree main groups. Based on incompatible element contents and element ratios different magmaderivations are interpreted for the gabbronoritic sills, basement dykes and microgabbro sills. Thebasement dykes and gabbronorites sills are derived from a primitive partial melt, whereas themicrogabbro sills show an evolved magma composition. Additional contamination with crustal materialalso changed the magma composition of each group as reflected by changing Ti/Zr ratios within thegroups. The gabbronorite sills and basement dykes are probably contaminated with SiO2-rich materialfrom quartzites and granite gneiss (highest SiO2 and Zr values), whereas the microgabbro sills showpossible contaminations with dolomitic material (highest variation in CaO contents).The comparison to Bushveld related marginal rocks shows a similar composition of the gabbronorite sillsand basement dykes to the B1 quenched textured micropyroxenites, whereas the microgabbro sillsindicate little correlation with the composition of one of the Bushveld marginal rocks (B1 to B3).Furthermore the microgabbro sills are nearly conformable in composition of selected trace elementswith the Basal Gabbro chilled margins at Uitkomst and Slaaihoek, which is supported by a possiblepalaeomagnetic age relationship in some extent. The geochemical fingerprints of the investigated sillsand dykes compared to the Uitkomst Complex have identified no direct correlation between them andshow only low contents of exploration relevant elements for microgabbro sills, gabbronorite sills as wellas basement dykes.

[发布机构] University of the Free State