Chemical and isotopic variations in plagioclase across the transition between the main upper zones, Western Bushveld complex

Abstract

The in situ compositional (major element, trace element and Sr-isotopic) and petrographic results of plagioclase as obtained from the gabbroic cumulates across the boundary between the Main and Upper Zones of the western Bushveld Complex, as studied from the 1119.13 m, BK-2 drill core are reported. The data are compared with similar dataset on this petrogenetically important stratigraphic sequence and a model that better explains the petrogenesis of the Pyroxenite Marker interval is proposed. There is a significant variation within and between coexisting plagioclase crystals across the studied stratigraphic interval, which is not a new phenomenon in the Bushveld Complex and other layered intrusions. In situ major element compositions recorded a continuous upward trend of increasing plagioclase anorthite (i.e. a reversed differentiation trend) content from ~ 342 m below the Pyroxenite Marker. The REE abundances of plagioclase show LREE enrichment and slight depletion of HREEs relative to chondrites. The initial 87Sr/86Sr ratios of plagioclase averaged 0.7086 in the lower Main Zone and 0.7078 in the Upper Zone, showing an isotopic ratio decrease up the stratigraphy of BK- 2. The Sr-isotopic composition of plagioclase in the Upper Zone was relatively constant with stratigraphic position, and this is coupled with a normal differentiation trend as exemplified by the anorthite (An%) content of plagioclase. The disappearance of inverted pigeonite in the vicinity below the Pyroxenite Marker, coupled with a reversal in mineral compositions and an inflection in initial 87Sr/86Sr ratios, mark a zone of prolonged magma mixing, culminating in the Pyroxenite Marker. The proposed model for petrogenesis of the Pyroxenite Marker is suggestive that the interval may have formed due to; stabilization of pyroxene at the expense of plagioclase because of rapid homogenization of two stratified magma layers that might have caused a minor shift in phase equilibria, perhaps as a result of a transient fluctuation in pressure within the chamber. The isotopically heterogeneous integration stage of the BK-2 (i.e. the lower parts of the Main Zone below the Pyroxenite Marker) was caused by several influxes of magma of distinct composition, and the isotopically homogeneous differentiation stage (i.e. that part of the Main Zone above the Pyroxenite Marker and the overlying Upper Zone) was manifested by magma evolution that was dominated by fractional crystallization processes without strong evidence to suggest further large-scale magma influxes. Isotopic variations at the mineral scale are of great use in the monitoring of magma evolution, processes and timescales, together with core-rim variations that are good tracers of magma mixing.