The petrology and geochemistry of the karoo sequence basaltic rocks in the Natal Drakensberg at Sani Pass.

Abstract

The Sani Pass in the Natal Drakensberg is situated in the north-eastern sector of the Lesotho Highlands which forms a major Karoo-age basaltic massif in the Karoo Igneous Province. The volcanic section exposed in the pass is approximately 800m thick, and comprises a succession of regularly stratified, massive and amygdaloidallavas which were extruded mainly by fissure-type eruptions. Dolerite dykes, which now occupy thefissures,form a network ofpredominantly NE-SW and NW-SE trending topographic features. During post-eruption cooling hydrothermal solutions percolated through the volcanic succession and produced an amygdale zonation which was controlled predominantly by ambientpressure and temperature conditions. An original maximum thickness of 1 820m of the volcanic succession has therefore been estimated and an average fossil geothermal gradient of 111° C/km is conceived to have persisted during amygdale formation. New electron microprobe data are presented for the silicate phases in the Sani Pass basalts and dolerites. These data do not effectively separate the Sani Pass volcanic succession into different geochemical units. Microprobe analysesfor olivine, albeit limited, are in the forsterite range and indicate that a proportion of olivine in the high-MgO basalts is due to cumulus enrichment. The pyroxenes are predominantly augite and minor pigeonite, with some ofthe augites displaying a tholeiitic trend similar to that recognised at Skaergaard. Plagioclase is mainly in the labradorite to bytownite range, the phenocrysts being slightly enriched in anorthite compared to the groundmass. The use ofwhole-rock geochemistry for 67 basalts and 8 dolerites has permitted the recognition of five geochemically distinct magma types, namely, the Giant's Cup, Agate Vale, Sakeng, Mkhomazana and the Phinong. The Phinong basalts comprise the upper two-thirds ofthe volcanic succession and although are generally homogeneous, there is a slight tendency for the more evolved rocks to be found higher up in the stratigraphic sequence. The remaining magma types precede the Phinong succession and are generally enriched in silica and have higher Zr/Nb and lower PfZr ratios than the Phinong basalts. Within the pre-Phinong succession the Giant's Cup basalts are generally depleted in the compatible elements, while the overlying Agate Vale basalts are enriched in incompatible elements. Except for a marginally lower Na20 and Sr content, the chemistry ofthe Sakeng basalts is variable, generally overlapping with the other magma types. The Mkhomazana basalts are slightly enriched in MgO, Ni, Cr and Sc compared to all other pre-Phinong basalts. The dolerites in the area adjacent to the Sani Pass are geochemically similar to the Phinong basalts.

The Phinong magma type is considered to be equivalent to the Lesotho magma type, based on their geochemical and stratigraphical similarities. In terms ofdiscriminant diagrams the Giant's Cup, Sakeng and Mkhomazana basalts generally show some compositional overlap with the Phinong, or plot in incoherentfields, but the Agate Vale basalts are distinctly different and might indicate a new magma type within the Karoo Central area. Broad compositional overlap between the Phinong basalts and those preserved at Kirwan and Heimefrontfjella, Antarctica, indicates juxtaposition of Antarctica along the southern African east coast in a reconstructed Gondwanaland. Petrogenesis of the Sani Pass basalts has been examined in terms of alteration, open and closed system fractional crystallization, partial melting procesess and a heterogeneous source. Although limited alteration and conduit contamination have occurred, the most feasible mechanism responsible for the geochemical variation lies in the existance ofinhomogeneities in the upper mantle at the time ofgeneration of the Sani Pass magmas.