Magmatic evolution of andesites and basaltic andesites at Red Crater, Tongariro, New Zealand

Abstract

Red Crater forms a recently active part of the Tongariro Volcanic Complex (TVC), a predominantly andesitic, subduction-related volcano in New Zealand’s Taupo Volcanic Zone (TVZ). Red Crater has erupted a well-exposed sequence of lava flows and scoria during the Holocene, and previous work on the vent indicates a shift from andesitic to more mafic compositions during the last ~1.8ka. The processes which have produced this chemical diversity have not yet been established, and in contrast to the well-studied neighbouring TVC vent, Ngauruhoe, relatively little is known about the formation of Red Crater magmas. This thesis presents a detailed geochemical, isotopic and mineralogical study of the lavas and tephras erupted from Red Crater, to develop a better understanding of the processes which have produced the Red Crater suite, and to evaluate the vent’s magmatic evolution. Lavas and tephras erupted from Red Crater represent two phases of magmatic evolution. The first phase, ‘Pre-1.8ka’, has produced five layered olivine-bearing andesite lava flows, which include an anomalously large (~6.5 km long, >100m thick), chemically and isotopically zoned lava filling the Oturere Valley. This internal zoning indicates the Pre- 1.8ka reservoir was heterogeneous. Least-squares mass balance modelling of major elements, and combined assimilation and fractional crystallisation (AFC) modelling of trace elements and isotope ratios are consistent with reservoir zonation resulting from ~20- 30% FC of plagioclase (An58), pyroxene and Fe-Ti oxides with minor assimilation of greywacke TVZ basement. A slight mafic-upward trend through the lava stratigraphy suggests progressive tapping of slightly less-differentiated magma through the eruption sequence. Post-1.8ka magmas were generated following mafic recharge of the Pre-1.8ka reservoir. Anorthite frequency distributions and back-calculated melt compositions from plagioclase crystals reveal the cores of many Post-1.8ka plagioclase are relict Pre-1.8ka crystals. These crystals and other phases have been entrained and efficiently mixed with a plagioclase-deficient basaltic magma as it recharged the Pre-1.8ka reservoir. The resulting eruptions produced a series of at least five chemically and isotopically similar lava flows, and constructed the Red Crater scoria cone.