Genesis of the Wilsons Promontory batholith of southeast Victoria, Australia: heterogeneities in the Proterozoic basement

Abstract

The monzogranites and syenogranites of the Early Devonian (Emsian), post-tectonic, high-level, S-type Wilsons Promontory batholith contain magmatic garnet and cordierite, in addition to biotite, as well as biotite–quartz pseudomorphs after early orthopyroxene. The batholith forms the southern tip of the Australian mainland and consists of four penecontemporaneous plutons, emplaced at shallow crustal depth, intruding Ordovician to Lower Devonian lower greenschist-facies, quartz-rich metaflysch rocks in the Bassian tectonic zone. The magmas were initially H2O-undersaturated and at T > 800 °C, having been generated by partial melting of greywacke-dominated, Paleoproterozoic to Mesoproterozoic (ca 1.6 Ga) basement rocks in the Selwyn Block (the northern tip of the VanDieland microcontinent). However, the specific source rocks differ from those of the S-type granitic plutons in the neighbouring Melbourne tectonic zone. This probably reflects compositional variations within the Selwyn Block, which has been proposed to form the basement terrane beneath both the Melbourne and Bassian zones. The compositional zonation in the batholith is due to sequential intrusion of separate magma batches rather than in situ differentiation, and the main mechanism responsible for primary chemical variations in the rocks appears to have been peritectic assemblage entrainment. Some local differentiation also occurred, probably by filter pressing, to produce aplites and tourmaline-bearing pegmatites. Normal granitic rocks (i.e. not aplites or pegmatites) with SiO2 >76 wt% were not produced through differentiation of other granitic magmas in the batholith. At the global scale, this suggests that fractionation models for the origins of high-silica granitic and rhyolitic magmas may need revision. KEY POINTS S-type magmas of the Wilsons Promontory batholith were H2O-undersaturated and at T > 800 °C. They were generated by partial melting of greywacke-dominated, Paleoproterozoic to Mesoproterozoic (ca 1.6 Ga) basement. The sources differ from those of the plutons in the Melbourne Zone, suggesting heterogeneity in the Selwyn Block. Most compositional variation is due to intrusion of separate magma batches and peritectic assemblage entrainment, not differentiation.