- © 2003 Geological Society of South Africa
Detailed petrographic, modal and geochemical studies on a number of deep boreholes (exceeding 1.2km in some instances) along the southeastern margin of the Insizwa lobe of the Mount Ayliff Complex reveal the existence of a geochemical stratigraphy in the mafic intrusive rocks. Using combined geochemical and petrographic criteria new definitions are presented for the Basal Zone and Central Zone of Scholz (1936). Within the Basal Zone a coherent sequence of ultramafic rocks (plagioclase lherzolites), previously referred to as picrites, up to 350m thick, occur and are distinguished from associated olivine-rich gabbronorites by a new set of criteria which involves bulk rock composition (MgO>30%) and petrography (cumulus olivine and spinel only). This definition can be consistently applied in all borehole cores. The Basal Zone gabbronorites have variable olivine concentrations and can be distinguished from similar rocks in the overlying central zone by Sr/Zr, Sr/Al2O3 and P/Zr and by overall higher incompatible element contents. The latter implies that Basal Zone rocks are orthocumulates and have retained a higher proportion of their residual liquid than the Central Zone gabbronorites which are mesocumulates. Within the Central Zone the gabbronorite sequence can be subdivided (from the top down) on the basis of geochemistry into a Top Gabbronorite, Cu-Poor Gabbronorite, and Upper Gabbronorite. These subdivisions can be recognized in all borehole cores. In borehole INS96-02, one of the deepest boreholes, the plagioclase lherzolite layer in the Basal Zone is ‘split’ by a sequence of gabbronorites showing chilled contacts and variable effects of thermal metamorphism. Specifically within this zone are two units of gabbronorite which are thermally metamorphosed by Basal Zone gabbronorites and which have chemical characteristics of the Central Zone. This is evidence that the Insizwa lobe was emplaced from the top down with intrusion of magmas which crystallized Central Zone gabbronorites early, followed by emplacement of Basal Zone magmas. The composition of olivine (%Fo, Ni/MgO) mimics whole-rock Mg-number and Ni/MgO variation through the sequence. Specifically olivines from the Upper Gabbronorite and the Cu-poor Gabbronorite are depleted in Ni relative to MgO, whereas olivine from the other units has ‘normal’ Ni/Mg ratios. The thick Taylor’s Koppie dyke to the E of Insizwa is built of olivine gabbronorites with variable olivine contents and scattered blebs of sulphide and is a potential feeder to the Insizwa sequence. It has whole-rock geochemical signatures and Ni-depleted olivines which are very similar to the Upper Gabbronorite of the Central Zone. The wide compositional range exhibited by contact margin samples at various levels in the Basal Zone indicates that this zone was built from doleritic magma variably charged with olivine phenocrysts and which had a wide range of incompatible element ratios and Sr-isotopic compositions. The Insizwa lobe was an open magmatic system formed by multiple intrusive events of compositionally variable magmas.