Insights into silicic melt generation using plagioclase, quartz and melt inclusions from the caldera-forming Rotoiti eruption, Taupo volcanic zone, New Zealand

The Rotoiti (similar to 120 km(3)) and Earthquake Flat (similar to 10 km(3)) eruptions occurred in close succession from the Okataina Volcanic Centre at similar to 50 ka. While accessory mineral geochronology points to long periods of crystallization prior to eruption (10(4)-10(5) years) and separate thermal histories for the magmas, little was known about the rates and processes of the final melt production and eruption. Crystal zoning patterns in plagioclase and quartz reveal the thermal and compositional history of the magmatic system leading up to the eruption. The dominant modal phase, plagioclase, displays considerable within-crystal zonation: An(37-74), similar to 40-227 ppm MgO, 45-227 ppm TiO2, 416-910 ppm Sr and 168-1164 ppm Ba. Resorption horizons in the crystals are marked by sharp increases (10-30%) in Sr, MgO and X-An that reflect changes in melt composition and are consistent with open system processes. Melt inclusions display further evidence for open system behaviour, some are depleted in Sr and Ba relative to accompanying matrix glass not consistent with crystallization of modal assemblage. MI also display a wide range in XH2O that is consistent with volatile fluxing. Quartz CL images reveal zoning that is truncated by resorption, and accompanied by abrupt increases in Ti concentration (30-80 ppm) that reflect temperature increases similar to 50-110 degrees C. Diffusion across these resorption horizons is restricted to zones of <20 mu m, suggesting most crystallization within the magma occurred in <2000 years. These episodes are brief compared to the longevity (10(4)-10(5) year) of the crystal mush zones. All textural and compositional features observed within the quartz and plagioclase crystals are best explained by periodic mafic intrusions repeatedly melting parts of a crystal-rich zone and recharging the system with silicic melt. These periodic influxes of silicic melt would have accumulated to form the large volume of magma that fed the caldera-forming Rotoiti eruption.