The Lower Main Zone in the Northern Limb of the Bushveld Complex—a >1·3 km Thick Sequence of Intruded and Variably Contaminated Crystal Mushes

Abstract

The lower Main Zone (LMZ) in the Northern Limb of the Bushveld Complex as intersected by the 1563·02 m deep Moordkopje (MO-1) drill hole shows very little large-scale differentiation as exemplified by parameters such as the An% of plagioclase, the Mg# of pyroxene and the modified differentiation index over a vertical interval in excess of 1·3 km. These features, coupled with the late entry of pigeonite (now inverted to orthopyroxene), are suggestive of the repeated intrusion of magma into the magma chamber of the Northern Limb during the early stages of the development of the Main Zone. Importantly, the lower Main Zone (LMZ) exhibits significant Sr isotopic disequilibrium between coexisting plagioclase (Sri ∼0·708) and orthopyroxene (Sri up to ∼0·711) in certain samples, a feature that has not previously been described for the Main Zone. Many of the features of the lower Main Zone (e.g. bulk compositions of LMZ cumulates unsuitable for the crystallization of plagioclase + two pyroxenes; poorly developed layering; non-cotectic proportions of plagioclase and pyroxene; decoupling of the differentiation trends of plagioclase and pyroxene over short vertical intervals) in the Northern Limb may be explained if it was intruded not as repeated influxes of aphyric magma, but instead by the repeated intrusion of crystal mushes from a deeper, sub-compartmentalized, crustal staging chamber. The model proposed is an alternative to previous models suggesting the loss of extrusive components from the Bushveld Complex, which were developed to explain compositional paradoxes within the Bushveld Complex.