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

[摘要] The in situ compositional (major element, trace element and Sr-isotopic) and petrographicresults of plagioclase as obtained from the gabbroic cumulates across the boundarybetween the Main and Upper Zones of the western Bushveld Complex, as studied fromthe 1119.13 m, BK-2 drill core are reported. The data are compared with similar dataseton this petrogenetically important stratigraphic sequence and a model that better explainsthe petrogenesis of the Pyroxenite Marker interval is proposed.There is a significant variation within and between coexisting plagioclase crystals acrossthe studied stratigraphic interval, which is not a new phenomenon in the BushveldComplex and other layered intrusions. In situ major element compositions recorded acontinuous upward trend of increasing plagioclase anorthite (i.e. a reversed differentiationtrend) content from ~ 342 m below the Pyroxenite Marker. The REE abundances ofplagioclase 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 and0.7078 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 constantwith stratigraphic position, and this is coupled with a normal differentiation trend asexemplified 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 theinterval may have formed due to; stabilization of pyroxene at the expense of plagioclasebecause of rapid homogenization of two stratified magma layers that might have causeda minor shift in phase equilibria, perhaps as a result of a transient fluctuation in pressurewithin the chamber.The isotopically heterogeneous integration stage of the BK-2 (i.e. the lower parts of theMain Zone below the Pyroxenite Marker) was caused by several influxes of magma ofdistinct composition, and the isotopically homogeneous differentiation stage (i.e. that partof the Main Zone above the Pyroxenite Marker and the overlying Upper Zone) wasmanifested by magma evolution that was dominated by fractional crystallizationprocesses without strong evidence to suggest further large-scale magma influxes.Isotopic variations at the mineral scale are of great use in the monitoring of magmaevolution, processes and timescales, together with core-rim variations that are goodtracers of magma mixing.

[发布机构] University of the Free State