High temperature rhyodacites of the 36 ka Hauparu pyroclastic eruption, Okataina Volcanic Centre, New Zealand: Change in a silicic magmatic system following caldera collapse

Deposits of the 36 ka rhyodacitic Hauparu plinian eruption from Okataina Volcanic Centre contain pumices with a wide range of compositions. Type 1 pumices are crystal-poor (< 3%) and have a ferromagnesian mineralogy of orthopyroxene and clinopyroxene. They form a continuous range of whole rock (SiO2 = 70–72 wt.%), and melt (glass) (SiO2 = ∼70–76 wt.%) compositions, with a wide spread of high temperatures (∼890–990°C) and oxidation values (fO2 = NNO + 0.8 − 1.15). Most (60%) Type 1 clasts have matrix glass of homogeneous composition, but about 40% of clasts are heterogeneous, containing mingled glasses (SiO2 variation up to 4 wt.%) that display the range of compositions found across the homogeneous Type 1 pumices. Late-stage Hauparu ejecta also contain crystal-rich (14%) clasts (Type 2) with a hornblende-dominant mineralogy. These clasts were derived from a cooler (876 ± 10°C), more oxidised (NNO + 1.52 ± 0.05) but less evolved magma (SiO2 = 67–68 wt.%). heterogeneous clasts were produced by short-lived mingling of magmas in the conduit during eruption. Phase equilibrium and trace element data are best explained by the homogeneous pumices being derived from a series of semi-isolated pockets of magma at depths > 16 km. Such a depth is close to the seismic velocity transition interpreted to reflect silicic crust overlying mafic cumulates and intrusions, where crustal melting is likely to generate rhyolite magmas. The diversity in the Hauparu Type 1 glasses may reflect heterogeneities in the source region. Crystal-rich Type 2 magma stagnated and crystallised at a shallower depth than Type 1 magma. It was apparently reactivated and/or entrained during ascent of the Type 1 magma. Hauparu is one of a series of rhyodacites that erupted following the caldera-forming Rotoiti ignimbrite eruption at ∼50 ka. The evacuation of shallow magma storage areas during the Rotoiti eruption, and the crustal disruption caused by the caldera collapse, appears to have allowed hot melts (900–990 °C) to rapidly rise from their source areas without being hindered by crystallising magma bodies at shallower depths, or being incorporated into such reservoirs.