Constraints on the magmatic evolution of the Lower Main Zone and Platreef on the northern limb of the Bushveld Complex as inferred from the Moordkopje drill core

Abstract

This thesis reports the results of a detailed petrographic, geochemical, geophysical and isotopic study of the lower part of the Main Zone, the Platreef and its footwall as exposed by the 1563.02 m deep Moordkopje (MO-1) drill hole situated on the Northern Limb of the Bushveld Complex, where the footwall to the Platreef is granitic. A total of 179 samples were collected as part of the study, with the Main Zone being represented by 156 samples. The Main Zone of the Northern Limb shares a broad similarity with that as known from the Eastern and Western limbs, but differs from the latter in a number of pertinent points. Well-known marker horizons of the Eastern and Western limbs are lacking in the Northern Limb, which hosts an anomalous troctolite layer not known from the other limbs. The lower Main Zone of the Northern Limb shows very limited differentiation as exemplified by paramaters such as the An% of plagioclase, the Mg# of mafic silicates and the modified differentiation index and also lacks the presence of inverted pigeonite, which is locally a major constituent of the Main Zone in the Eastern and Western limbs. The lower Main Zone of the Northern Limb also shows abundant evidence for the decoupling of differentiation trends of plagioclase and pyroxene, a feature that is only locally observed within the main body of the intrusion. Very importantly, the lower Main Zone also exhibits significant isotopic disequilibrium between co-existing plagioclase (Sri ~0.708) and orthopyroxene (Sri up to ~0.711) in certain samples, a feature that has not been described for the Main Zone as present in the main body of the intrusion. Many of the features of the lower Main Zone in the Northern Limb may be explained if it was intruded not as repeated influxes of magmas, but instead by the repeated intrusion of crystal mushes from a deep-seated staging chamber. The staging chamber, situated roughly at the boundary between the upper and lower crust, is proposed to have consisted of two sub-compartments connected at depth, with the wall rocks of the subcompartments differing in terms of their isotopic compositions. Plagioclase crystallized within the staging chamber is deemed to have floated to the tops of the sub-compartments with mafic minerals settling to their bases. As such, magmas entering the Bushveld Complex may host plagioclase derived from the one sub-compartment and mafic silicates derived from the other. Isotopic models using Vredefort crust suggest that the subcompartment that contributed mostly plagioclase to the Bushveld Complex may have been contaminated by ~25% of a melt dominated by lower crustal material (70-80%) with a smaller contribution from upper crustal material (20-30%), whereas the subcompartment from which the mafic silicates were derived, was contaminated by similar amounts of a melt richer in upper crustal material (30-40%) and poorer in lower crustal material (60-70%). In support of a model whereby the lower Main Zone was formed by the intrusion of crystal mushes rather than by in-situ fractional crystallization, is the fact that the bulk compositions of the cumulates of the Main and Upper zones of the Northern Limb do not appear to be of a suitable composition to allow for the co-crystallization of plagioclase + 2 pyroxenes. The Platreef as present in the MO-1 drill core consists from top to bottom of about 140 m of orthopyroxene-rich cumulates, followed by a package of hybridized rocks in which interstitial quartz-feldspar intergrowths abound, which is in turn followed by what was termed ‘pyroxene hornfels’ and then the granitic footwall. Comparison of the REE data for Platreef samples along the strike of the Northern Limb allowed for the identification of those Platreef cumulates least affected by footwall interaction, typically being those rocks occurring in the vicinity of the hangingwall contact of the Platreef. Incompatible trace element abundances and ratios suggest a B-1 magmatic lineage for the Platreef, which is supported by the Sr isotopic composition of early formed orthopyroxene within the Platreef (Sri ~0.706). Other parameters such as the Nd isotopic composition of orthopyroxene ( Nd = -7.8), however, suggest affiliations with the Main Zone, highlighting the need for additional study.