Insights to post-caldera rhyolite magma production at Okataina Volcanic Centre: The Mangaone Subgroup

Abstract

The ~45-32 ka Mangaone Subgroup eruptions followed the Rotoiti and Earthquake Flat caldera collapse events from Okataina Volcanic Centre, North Island, New Zealand. The deposits reveal a significant difference in compositional and physical parameters between early (~45-36 ka) and late deposits (33-31.5 ka), and between all other deposits erupted from within Okataina Volcanic Centre in the last ~61 kyrs. Early Mangaone Subgroup deposits are less evolved and display a wide range of compositions (65-73 wt. % whole rock SiO2, 49-78 ppm Rb) compared to the late deposits (73- 77 wt. % whole rock SiO2, 83-97 ppm Rb). All Mangaone Subgroup deposits are less evolved than the Rotoiti, Earthquake Flat and post-26 ka deposits. They became increasingly evolved over a short time. In addition, early deposits yield hotter temperatures and higher oxygen fugacities (800- 1000 °C, 0.4-1.2 ΔNNO) than the late deposits (760-910 °C, 0.2-0.9 ΔNNO). High resolution crystal traverses reveal wide variations across single phenocrysts (up to An20 in plagioclase, En13 in clinopyroxene and 4.8 wt. % Al2O3 in amphibole). A model consisting of a mid-crustal (~8-10 km) crystal mush frequently underplated by mantlederived mafic magma is proposed as a source for the Mangaone Subgroup eruptions. Based on whole rock trends, the crystal mush is viewed to consist largely of amphibole and plagioclase, becoming increasingly mafic with depth. Buoyant hot mafic melts intrude the mush and rise to boundaries within the crust. Pockets of silicic melts are formed by the reheating and partial melting of localised areas of the crystal mush, coupled with the release of silicic interstitial melt. The mantle-derived mafic melts provide physical and chemical energy to the system, promoting crystal mush-melt separation. Frequent, repeated mafic underplating of the crystal mush kept the mush from crystallising to completion. Occasionally a mafic melt penetrated the silicic melt, leading to an eruption. The transition from high-SiO2 rhyolite magmas during the caldera collapse events to dacite-rhyolite magmas during the Mangaone Subgroup eruptions, and returning to high-SiO2 rhyolite magmas during post-26 ka eruptions is interpreted as a result of relative deepening of the system immediately following the caldera collapse events and progressive shallowing over the duration of the Mangaone Subgroup occurred. The upper part of the mush transitioned from anhydrous as a result of decompression after the caldera collapse events to cooler, more hydrous, reflected by the mineral assemblages observed and a melt-rich cap began to re-develop over the crystal mush during the later Mangaone Subgroup eruptions. Similarities between the Mangaone Subgroup and other post-caldera events indicate that caldera collapse events cause a major change in the physiochemical structure of the magma system. The Mangaone Subgroup provides an example of how rapidly a silicic system can transition from hot, dry dacitic compositions to cool, hydrous high-SiO2 rhyolitic compositions.