Chemical and isotopic variations in plagioclase from the Upper and Main Zones, Northern Limb, Bushveld Complex

[摘要] The in-situ major element, trace element, and Sr-isotopic compositions of plagioclasein the broadly gabbroic cumulates from the Upper and Main Zones of the NorthernLimb of the Bushveld Complex, as obtained from the Bellevue (BV-1) and Moordkopje(MO-1) drill cores have been determined by means of electron microprobe and LaserAblation Inductively Coupled Plasma Mass Spectrometry (LA-ICPMS). The resultsshow the existence of inter and intra-crystal initial 87Sr/86Sr disequilibrium of coexistingplagioclase, a phenomenon which has received rather rudimentary attention not onlyin the Bushveld Complex, but in other layered intrusions as well.This disequilibrium is particularly striking in samples from the Lower Main Zone, aninterval which also records a lack of differentiation, as exemplified by the An% ofplagioclase. In the rest of the stratigraphy, up to the top of the Rustenburg LayeredSuite, there is less, if any, inter and intra-crystal initial 87Sr/86Sr disequilibrium ofplagioclase coupled to more prominent differentiation trends. These results are insupport of a model for the petrogenesis of this part of the Bushveld Complex, whichinvolves the Lower Main Zone forming through the repeated intrusion of crystalmushes derived from a deeper seated, sub-compartmentalized staging chamber, intothe Bushveld main magma chamber, with fractionation processes being key in theformation of the Upper Main and Upper Zones.Based on the initial 87Sr/86Sr composition of plagioclase, the Northern Limb of theBushveld Complex can also be divided into a putative integration and differentiationstages, similar to the Western Limb of the Bushveld Complex. The integration stagein the Western Limb included the Lower, Critical, and Lower Main Zones, whereas thedifferentiation stage included the Upper Main and Upper Zones. The integration stage in this study incorporates the lower reaches of the Lower Main Zone, with the rest ofthe stratigraphy up to the top of the Upper Zone covering the differentiation stage.Furthermore, the Lower Main Zone of the Northern Limb of the Bushveld Complex hasvery consistent concentrations of most trace elements investigated, and theseconcentrations tend to increase upwards into the Upper Main and Upper Zones. Thisalso suggests repeated intrusion for the Lower Main Zone with differentiationprocesses dominating in the upper parts of the Rustenburg Layered Suite.

[发布机构] University of the Free State