Hyperactive volcanism in the Taupo Volcanic Zone: a MELTS and Magma Chamber Simulator study to explore if AFC processes with a Torlesse assimilant can explain magmatic differentiation

Abstract:

The Taupo Volcanic Zone (TVZ) is an area of subduction zone volcanism in a rifted arc, active since 2 Ma, dominated by high-volume high-silica eruptions from large caldera-systems, and has one of the highest present-day magma production rates on Earth. The general model used to explain the geochemical signature of the TVZ consists of assimilation and fractional crystallization (AFC) with Torlesse greywacke crust as an assimilant. Chemically diverse and geographically distributed samples from the TVZ were analysed for major oxides, trace element concentrations and strontium (Sr) isotopic composition (⁸⁷Sr/⁸⁶Sr). Major oxide AFC models (MELTS and Magma Chamber Simulator) show that at low pressures (1.5 kbar) and low water contents (0.5-1.5 wt% H₂O), the TVZ compositional range from basalt to rhyolite can be theoretically reproduced. However, these models require high degrees of fractional crystallisation (90%) and assimilation (60%) and hence an excessive amount of mafic magma to generate the large volumes of felsic extrusions and intrusive bodies known to be present in the TVZ. This makes a pure AFC process highly unlikely for the origin of intermediate and felsic rocks. Additionally, AFC models fail to explain the Sr isotopic signature of several mafic and intermediate samples that instead, coupled with major oxides, suggest a heterogeneous mantle source. Processes that may modify the Sr isotopic signature of the mantle is addition of different amounts of sediments from the subduction zone and subduction erosion (e.g. Torlesse crust recycled via forearc erosion), which will be discussed.