A ~2.051 Ga anatectic event and peraluminous leucogranite from the Mahalapye Complex, northern edge of the Kaapvaal Craton: Record of an effect of Bushveld mafic magmatism

Abstract

Anatectic events and related leucogranites are rarely reported from Precambrian large igneous provinces (LIPs). However, their appearance is possible in relation to the intrusion of later pulses of mafic magmas into the earlier rocks. Silicic intrusive rocks associated with the ~2.06–2.05 Ga Bushveld LIP within the Kaapvaal Craton include granites, syenites or granophyres. This study reports syn-Bushveld partial melting of ~2.061 Ga diorite gneiss producing leucogranite from the Mahalapye Complex high-grade terrane, at the northern edge of the Kaapvaal Craton. Both the migmatitic diorite gneiss and associated leucogranite are crosscut by the ~2.039 Ga granodiorite. The different stages of melting of the biotite-bearing diorite gneiss – from onset of melting to melt segregation – are well preserved. Petrographic continuity between the leucosomes, and concordant and discordant leucogranite bodies point to a melt flow-accumulation network. Garnet formed after biotite during partial melting. Phase equilibria modelling and conventional thermobarometry indicate conditions of ~730 °C and ~7.2–7.5 kbar for the anatectic event. The predicted melt composition is peraluminous, magnesian and alkali-calcic, that is comparable to the chemistry of leucogranites. The low content of anatectic melt for the estimated P-T condition corresponds to the occurrence of leucogranites as thin veins to dykes. LA-MC-ICPMS U–Pb zircon age of 2051.3 ± 6.8 Ma is interpreted to date the anatectic event. The leucogranite exhibits co-mingling structures with dyke-like and sill-like syn-Bushveld norite intrusions associated with diorite gneiss. Preferential accumulation of the leucocratic material along the contact of diorite gneiss and norite, and the liquid-liquid interface with the norite, indicate the close age of leucogranite and norite. This is supported by the LA-Q-ICPMS U–Pb zircon age of 2054 ± 7.9 Ma from the norite. Thus, the partial melting of the diorite gneiss and formation of the leucogranites is argued as a response to Bushveld mafic magmatism. The present study shows that one of the causative agents of overprint event(s) in high-grade terranes along Archean cratonic margins can be mafic magmatism of similar age within the craton. However, in the case of LIP-related magmatism the footprints are restricted to terranes immediately adjacent to the cratonic margins.