A Petrological investigation of the transition from the lower to the upper critical zone in the central sector of the eastern Bushveld complex

Abstract

The transition from the lower to the upper critical zone and associated rocks (LG 6 - UG 1) has been investigated geochemically and petrographically. This well-layered and ·1ithologically heterogeneous succession is explained by a scheme of stratified hybrid layers and by a combination of crystal/liquid slumping and crystallization of the bottom liquid. Intermittent magma influxes occurred prior to the formation of the LG 6 chromitite layer, the F and the L units. It is proposed that the addition of relatively cool and dense tholeiitic liquid (B2/B3 type) initiated the collapse of orthopyroxenebearing plumes from the overlying hot and less dense boninitic liquid (B1 type). The formation of chromite is attributed to a reaction process between the two chemically distinct liquids during the descent of crystal-laden boninitic plumes. Whole rock incompatible trace element data and Sr-isotopes confirm that 81 and B2 liquids are parental liquids to the pyroxenitic lower critical zone. Addition of B2 or B3 liquid and subsequent mixing with resident hybrid liquids is believed to have formed the plagioclasedominant cumulates of the upper critical zone. Variations in mineral chemistry have been explained by changes in magma composition and by a series of postcumulus processes. Orthopyroxene composition has been substantially modified by re-equilibration with surrounding hybrid liquid and with chromite. Various degrees of interstitial melt fractionation and different diffusion rates for the individual elements (Fe-Mg>> Ti >Aland Cr> Ca) are indicated by the complex orthopyroxene zoning pattern. The compositional shift in orthopyroxene chemistry has been quantified and a correction method is presented to recalculate the original composition prior to the effects of re-equilibration with evolved trapped liquid and modal chromite. Significant plagioclase variation is only found in the feldspathic pyroxenites of the lower critical zone. Limited exchange of pore liquid in the crystal pile resulted in fractionation cycles and in the development of multiple crystallization fronts. Extreme trapped liquid fractionation is evidenced by the presence of quartz, mica, sodic plagioclase and K-feldspar with accessory rutile, loveringite, apatite and anhydrite. In contrast, liquid fractionation appears to be relatively unimportant during crystallization of the upper critical zone where changes in plagioclase composition are decoupled from variations in orthopyroxene composition. Orthopyroxene shows distinct textural changes associated with the first occurrence of cumulus plagioclase at the lower/upper critical zone boundary (E/F boundary). Orthopyroxenes from the upper critical zone are smaller, contain numerous tiny plagioclase inclusions and have generally more evenly shaped crystal faces compared to the lower critical zone. The simultaneous change towards anhedral (intercumulus) textures with an increase in plagioclase (F and H units) is explained by the fact that orthopyroxene which was added by crystal/liquid slumping is not a stable liquidus phase in a relatively pure 82/83 liquid. The presence of plagioclase inclusions in orthopyroxene is thought to reflect crystallization from a pyroxene-undersaturated liquid (82/B3). The latter must have been hybridised by 81 material (crystal-bearing plumes) to account for the high modal amount of orthopyroxene in the G and J units. A positive correlation between the thickness of individual stratigraphic units and the floor morphology has been found in the Maandagshoek area and adjoining areas: Lithological units thin out above and marginal to anticlinal floor structures and thicken in trough-like, synclinal areas. The observed changes in thickness do not display any changes in mineral composition. The frequency of pegmatites increase significantly near anticlinal structures. It is postulated that late migrating fluids followed the palaeotopography and were therefore forced upwards at anticlinal structures. Isotopic data of metasomatically altered rocks suggest that hydrous fluids are partially sediment derived and are not simply the result of interstitial melt fractionation. Magma replenishment caused probably an increased fluid activity due to the degassing of sedimentary fragments which were draged into the chamber. The meagre development of middle group chromitite layers (MG 1-4) throughout the central sector is explained by the additional influx of tholeiitic liquid (J and K units) to the central sector. Middle group layers are well developed in the southern and western sectors where the J and K units are either absent or drastically reduced in thickness. The oyroxenitic L unit and associated upper middle group chromitite layers CUMG 1-3) of the central sector can be correlated with the uppermost middlt group chromitite layer (MG 4) and enclosing pyroxenites of the southern and western sectors. iii