A peak into the mantle: primary magma generation beneath the Peak Range, central Queensland

Abstract

The hotspot-related Tertiary Peak Range (central Queensland) consists of enriched peraluminous rhyolites in the north, and metaluminous trachytes and peralkaline rhyolites in the south. Further petrographical mapping of the southern Peak Range revealed consistency with the Chandler (2018) model of an extended fractionation of an alkali basaltic melt as the source for the enriched intrusion, whereas a heterogeneous intrusion (Campbell’s Peak; a phonolitic intrusion with a basaltic rim abundant in peridotite xenoliths and amphibole megacrysts) 20km NE of the central Peak Range executes a different evolutional pathway. The peridotites are characteristic of metasomatised subcontinental lithospheric mantle (SCLM), enriched from the dehydration of fluids from the subducting slab in eastern Australia during the Late Devonian-Cretaceous. This is apparent in the growth of pargasitic amphibole (modal metasomatism), high concentrations of Rb, La, Ta, U, and sodium-bearing pyroxenes (mild cryptic metasomatism). The amphibole megacrysts are assumed to be a cumulate or fractionation phase in the mantle brought to the surface as xenocrysts in the basalt. Melting models of the peridotite demonstrate a lack of correlation between the basaltic host rock (especially in TiO2 levels), but a strong correlation with the phonolitic intrusion when partially melted (<1%) at ~10kbar and if fractionation of ~15% amphibole was considered. Fractionation modelling of the trachybasalt revealed no direct relationship to the peridotites or phonolitic rocks, suggesting it is unrelated. The spatial and temporal relation of Campbell’s Peak to the Peak Range is similar to the Ernst & Bell (2010) model of linking large igneous provinces (LIPs) and carbonatite intrusions if the central Peak Range is interpreted as the volcanic centre. It is proposed the phonolitic intrusion of Campbell’s Peak is a result of a low degree of partial melting of the SCLM via low temperature magmatism at the outer edge of the mantle plume, with later basaltic eruption resembling more mantle plume, flood basalt mechanisms.