Abstract

The 15.8 ka Rotorua rhyolite eruptive episode from the Okataina Volcanic Centre comprises a plinian pumice fall deposit followed by the extrusion of two rhyolitic lava domes or flows and their associated block-and-ash flows, with a total volume >1 km3 (dense rock equivalent). Variations in mineralogy, whole-rock, glass and mineral chemistry, and calculated magmatic properties suggest that two distinct rhyolitic magmas were sequentially tapped during the eruption. The first magma erupted (T1) is characterized by low SiO2 (c. 76.5 wt% in glass), calcic feldspars (An44), magnesian hornblendes (MgO c. 14.45 wt%), clinopyroxene, and high temperatures (c. 835 °C) and fO2 (1.8–2.1 ΔFMQ (where FMQ is the fayalite–magnetite–quartz buffer)). The second magma (T2) was more evolved and is characterized by higher SiO2 (c. 77.4 wt% in glass), Na-rich feldspars (An24), less magnesian hornblendes (MgO c. 11.8 wt%), biotite, and low temperatures (c. 750 °C) and fO2 (0.65–1.1 ΔFMQ). Both magmas are homogeneous, but evidence for some magma mingling indicates that they were in contact during eruption. However, there was only a minor degree of hybridization, perhaps reflecting the contrasting temperatures and viscosities of the two magmas. The crystal-rich, poorly vesicular nature of the T2 ejecta indicates that it originated from a cooling, high-level magma chamber that was reactivated by intrusion of hotter, volatile-rich T1 magma. The ponding of rhyolite magmas at shallow depth and their subsequent reactivation by later rhyolitic intrusion may be an important process in the compositional evolution and eruption dynamics of many Okataina Volcanic Centre rhyolite magma bodies.

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