Characterization of the upper main zone, Bushveld Complex, South Africa

Abstract

The Main Zone of the Bushveld Complex is often described as an unspectacular ∼3 km thick succession of gabbronorite. As such, few studies have focused on the Main Zone. However, it is punctuated with localized layered anorthosite sequences and pyroxenite layers. One such anorthosite sequence is found in the Upper Main Zone, ∼165 m below the base of the Upper Zone in the eastern lobe. The sequence consists of eight poikilitic anorthosite layers which are exposed along a road-cut, 5.3 km north-east of the town of Apel. Anorthosite layers are meter-scale in thickness (0.4 - 10 m), have sharp contacts with distinct mineral compositional breaks and are defined on the size and shape of pyroxene oikocrysts they contain. The sequence hosts numerous centimeter to meter-scale blocks of distinct cumulate lithologies (olivine-bearing norite, norite and poikilitic norite). The distinct textures of the blocks, their modal, mineral, chemical and isotopic compositions (average initial 87Sr/86Sr: 0.707344 and initial εNd: -5.23) suggests that they represent recycled cumulate material from Bushveld’s magmatic plumbing system. Based on the consistent plagioclase crystal morphologies throughout the sequence, relatively constant mineral and whole-rock chemistries within each layer, presence of magmatic flow textures, and a numerical model, I propose that each layer was formed by the injection of a plagioclase slurry (5 - 30 vol% crystals).Two distinct pyroxenite layers associated with the Pyroxenite Marker (Upper Main Zone) are exposed in the Mooimeisjesfontein area, ∼36.5 km SW of Burgersfort in the eastern lobe. Progressing up-section, the first layer is a ∼2 m thick orthopyroxenite, followed by a second ∼40 cm thick melagabbronorite. Based on field observations, mineral and whole-rock chemical trends, each pyroxenite layer was formed by a less differentiated replenishing magma which mixed with a resident one. The pyroxenite layers host numerous centimeter to decameter-scale blocks of distinct cumulate textures or lithologies (anorthosite, gabbronorite, norite and websterite). Field and petrographic observations combined with numerical modeling support layer formation by injection of slurries (5 -15 vol% crystals). The evidence presented in this study attests to a far more dynamic origin for the Upper Main Zone of the Bushveld Complex