Tapping of the Bushveld magma chamber: Experiments on the parental magma to the Upper Zone

English Abstract:

The Bushveld layered intrusion covers ca. 65.000 km² in South Africa and is made up of a 7-9 km thick cumulate sequence. The upper 2 km of the intrusion (upper part of the Main Zone and Upper Zone) is known for the abundance of Fe-Ti-V±P ore deposits. The bottom of this sequence is characterized by a 2-3 m thick layer of orthopyroxenite known as the 'Pyroxenite Marker'. It is considered as the last major influx of magma (ferrobasalt) in the Bushveld magma chamber. In this study, we have performed crystallization experiments and MELTS calculations on various estimates for parental magmas to the 'Upper Zone and Upper Main Zone' of the Bushveld Complex. Our experimental data constrain phase equilibria and the liquid line of descent during the evolution from evolved basalt to rhyolite. We also aim at constraining the composition of the parent magma after the Pyroxenite Marker by comparing experimental data and the continuous crystallization record in the Bellevue and Bierkraal drill-cores. Experiments were conducted at 200 MPa and 1160-1100°C in internally heated pressure vessels on selected compositions proposed in the literature, varying essentially in SiO₂ (50-55 wt%), CaO (7-11 wt%) and Mg-number (37-48). The stability fields of the pyroxenes and olivine are very sensitive to small compositional differences in the starting materials, producing a variety of pyroxenes from orthopyroxene, pigeonite, sub-calcic augite to augite. The formation of orthopyroxene at the base of the sequence requires a parental magma that corresponds to the bulk cumulate composition of the Upper zone to which 25-40% monzonite/rhyolite is added. Little evidence exists for the occurrence of this amount of evolved material in plutonic environment. It is more reasonable to consider open-system evolution of the Bushveld magma chamber that was regularly tapped during the crystallization of the Upper Zone, possibly erupted at the surface as the Rooiberg suite.