Rb-Sr isotopic systematics across the mafic-felsic transition in the Bushveld complex: Implications for granite petrogenesis and hydrothermal processes

In the Bushveld complex of South Africa, the world's largest A-type granite (Nebo Granite) directly overlies the world's largest layered mafic intrusion (Rustenburg Layered Suite, RLS). For decades, Sr isotopes have been utilized to identify magma replenishment events and intervals of closed-system fractionation. Yet, the bulk rock Rb-Sr isotopes for the granites are known to be offset to unrealistically low age-corrected values, rendering direct comparisons to the RLS difficult. Here, we combine Sr isotopes for plagioclase feldspar measured by laser ablation (LA) ICP-MS with trace elements for bulk rock, clinopyroxene, amphibole and plagioclase and previously published bulk rock Sr isotopes across the mafic-felsic transition (Transition Zone) in the Bushveld complex. We show that the disturbance of bulk rock Sr isotope systematics coincides with the appearance of alkali feldspar and that the unrealistically low initial Sr isotopic values inferred from bulk rocks are not a feature of plagioclase. Instead, the Sr isotope compositions of plagioclase are very similar within uncertainty across the mafic-felsic transition. This supports a genetic link between mafic and felsic rocks. High initial ⁸⁷Sr/⁸⁶Sr values (at 2055 Ma) found for some plagioclase with high-Rb/Sr is likely an analytical artifact. We argue that the small spatial distance between undisturbed RLS diorites and disturbed granites precludes different thermal or hydrothermal histories for the two lithologies. Rather, we propose that the perthitic texture of alkali feldspar that existed during most of the Bushveld complex's more than 2 billion-year history made this phase more susceptible to Sr re-mobilization. Furthermore, the abundance of exsolved clinopyroxene and magnetite in rocks of the RLS Upper Zone that likely experienced similar hydrothermal conditions as the granite could have been the source of metals that mineralized the granites.