Nature of the Lower Critical–Upper Critical Zone transition in the Clapham trough area, eastern Bushveld Complex: evidence for the introduction of crustal fluids in response to plagioclase stabilisation?

Abstract

The Lower Critical (LCZ)–Upper Critical Zone (UCZ) boundary of the Rustenburg Layered Suite is an intrusion-wide, major stratigraphic transition from intercumulus plagioclase in the LCZ to cumulus plagioclase in the UCZ. No consensus exists regarding the nature of this boundary, with some regarding the attainment of cumulus status by plagioclase at this level of the intrusion due to continued fractionation of the resident magma, whilst others argue for the addition of compositionally distinct magma(s) at this level of the intrusion. Here we report in-situ Sr-isotopic compositions for plagioclase along with whole-rock major- and trace element geochemical and mineral chemical data across the LCZ–UCZ boundary as intersected by borehole BH6958 on the farm Forest Hill in the eastern Bushveld Complex. Major and trace element data across the LCZ–UCZ boundary (e.g. the Cr content of orthopyroxene) support the notion that no compositionally distinct magma was added at this level of the intrusion. Sr- and Nd-isotopic data, however, point to open-system behaviour. The isotopic excursion cannot be explained through mixing between resident (B1) magma and other proposed parental magmas (e.g. B2 or B3 magmas). Modelling suggests that the observed isotopic excursion may be explained through mixing of resident (B1) magma with small amounts of lower crustal melts. Whether such mixing would have resulted in plagioclase stabilisation remains unclear. The observed isotopic excursion can also be explained through mixing of resident (B1) magma with small amounts of crustal fluids. In this case, the introduction of these fluids appears to have happened gradually, with ⁸⁷Sr/⁸⁶Sr_i in plagioclase being higher in LCZ rims than cores, and higher yet in the lower UCZ. We argue on the basis of thermodynamic modelling that when plagioclase joined the crystallising assemblage, the system contracted at a rate higher than that preceding plagioclase stabilisation, with fluids from the surrounding hydrothermal system entering the magma chamber to counter the volume loss experienced by the cooling system.