Geochemical evolution of the Rooiberg silicic lavas in the Loskop Dam area, southeastern Bushveld

Abstract

The Rooiberg Felsite is the voluminous volcanic precursor to the plutonic episodes of Bushveld magmatism. In the area north of the Loskop dam, a 3,520-m thickness of felsite is dominated by flows of dacitic and rhyodacitic lava. Occasional silicic andesites are developed near the base whereas true rhyolites become overwhelmingly dominant toward the top of the sequence. Nine major stratigraphic units of lava are present: these are usually separated by, and sometimes include, volumetrically minor pyroclastic and sedimentary deposits.Whole-rock chemical data on 165 lava samples embracing the entire stratigraphy suggest that nonuniform alteration processes have been most effective in the upper units. Nevertheless, field, petrographic, and chemical criteria enable identification of three felsite magma types: a high Mg type (MgO > 1.7%) containing low Zr and Y contents is present only within the lowermost 1,400 m of section; a low Mg type (MgO 2 ([asymp]1%)-P 2 O 5 ([asymp]0.35%) type, which only occurs in the lower part of the succession.The high Mg lavas exhibit a systematic eruptive progression from more to less evolved compositions, apparently reflecting continuous tapping of successively deeper levels of a normally zoned magma chamber. During the early volcanism, the low Mg lavas were erupted intermittently with the high Mg types, probably from a separate and much larger chamber. Pronounced differentiation of this magma is only evident in the youngest volcanic units, where several element profiles display conspicuous steplike variations across the prominent ash-flow horizons. The steps reflect upward enrichments in Nb, Zr, and Y and corresponding incremental depletions in Ti, P, and Sc. Such trends imply the existence of a discontinuous but reversed compositional zonation in the evolved low Mg chamber.The high Fe-Ti-P lavas were also erupted from the low Mg chamber. This may indicate the development of an early type of chemical stratification that was markedly different from the compositional layering in the evolved system.