Mafic-Felsic Interactions in the 25.4 ka Oruanui Supereruption (Taupo Volcano): Insights From Quenched Juvenile Mafic Clasts and Their Groundmass Textures

Abstract:

Groundmass textures in juvenile mafic clasts from the 25.4 ka¹ Oruanui supereruption (Taupo volcano, New Zealand, ~530 km³ magma²) show features indicative of rapid crystallisation due to chilling against the host rhyolite. There is marked textural diversity in these clasts, with contrasts between the two compositional groups (tholeiitic and calc-alkaline³) overshadowed by more significant variations within each group. The observation that textures vary more within than between the compositional groups suggests that bulk composition exerted only a second-order control on textural development, which was instead dominated by the cooling history of each clast. Clustering of groundmass mineral compositions in accordance with textural groupings implies a fundamental link between mineral compositions and textural development. This link is inferred to reflect a complex combination of factors including the degree of undercooling, water content, cooling rate, bulk composition and, possibly, intensive variables.

Many macro-crystal compositions in the mafic clasts overlap with those of the corresponding crystals in the Oruanui high- and low-silica rhyolites⁴, indicating that they are ingested xenocrysts. However, residual glass compositions in the mafic clasts are distinct from those of Oruanui rhyolite glasses, and a mixing trend between them is not observed. The dominance of crystals derived from the low-silica rhyolite and a scarcity of ingested rhyolite melt suggests that interaction of the mafic and felsic magmas occurred predominantly within a transition zone between the crystal mush and the overlying crystal-poor high-silica rhyolite. It is envisaged that mafic dikes encountering a transition zone between the effectively rigid crystal mush and the melt-dominant body disaggregated into discrete blebs of various sizes that quenched against the rhyolite. Each bleb experienced its own unique cooling history, resulting in diversity in their groundmass textures and chemistries.

¹ Vandergoes MJ et al. (2013) Quat Sci Rev 74:195-201; ² Wilson CJN (2001) J Volcanol Geotherm Res 112:133-174; ³ Wilson CJN et al. (2006) J Petrol 47:35-69; ⁴ Allan ASR (2013) PhD thesis, Victoria University of Wellington.