Textural and micro-analytical insights into mafic–felsic interactions during the Oruanui eruption, Taupo

A cascade of magmatic events during the assembly and eruption of a super-sized magma body

“…Mafic clasts analysed in here and previously (Wilson et al 2006;Rooyakkers 2015) range between 50-65 % SiO2, in distinct tholeiitic (higher Fe, Ti) and calc alkaline (lower Fe, Ti) groups ( Fig. 7a; Table 3).…”

“…Wilson et al (2006) suggested that the Oruanui mafic magmas, in particular the tholeiitic magma, had ponded at the base of the melt-dominant magma body and initiated chamber wide stirring and homogenisation. Subsequent work (Allan et al 2013;Rooyakkers 2015; this paper) has shown that the mafic magmas were mostly interacting with the mush, as indicated by their accumulated felsic crystal populations and whole-rock trajectories trending towards LSR (rather than HSR) compositions (Figs. 12 and 13).…”

“…The relative proportions of the calc-alkaline to tholeiitic endmembers are not constrained, although subequal numbers of clasts from each type have been collected. The mafic clasts are inferred to be juvenile on the basis of cauliform or crenulated margins, indicating that they were hot and plastically deforming until quenched on eruption (Wilson 2001;Rooyakkers 2015 and manuscript in preparation). The two mafic groups are compositionally distinguished on plots of whole-rock SiO2 versus FeOt/MgO with both calcalkaline and tholeiitic groups being almost entirely contained within the appropriate fields as defined by Miyashiro (1974).…”

“…This bimodality suggests that there are Fe-Ti oxides from both the mafic and rhyolitic end-members in the mafic clasts. We do not use the Ridolfi et al (2010) thermobarometric formulations for the mafic amphiboles because the larger crystals are clearly inherited from the rhyolite and the groundmass amphiboles are inferred to have grown under disequilibrium conditions during rapid crystallisation (Rooyakkers 2015). Twopyroxene thermometry has been applied by pairing together clinopyroxene (inferred to be phenocrystic) and the most primitive orthopyroxenes (i.e.…”

What lies beneath? Reconstructing the primitive magmas fueling voluminous silicic volcanism using olivine-hosted melt inclusions

“…DR2). The enclaves are interpreted as juvenile because they have crenulated chilled margins and adhering rhyolitic glass, and they host rhyolite-derived crystals ingested during syneruptive interactions (Leonard et al, 2002;Rooyakkers et al, 2018). Olivines in these enclaves are Mg-rich (Fo 80-86 ) [Fo = molar Mg/ (Mg + Fe 2+ ) × 100] and commonly contain MIs that are mostly <20 µm, but sometimes up to 100 µm, across ( Fig.…”