Transcrustal and source processes affecting the chemical characteristics of magmas in a hyperactive volcanic zone

Abstract

Arc magma generation and evolution are affected by transcrustal processes, such as assimilation and fractional crystallisation, and source processes like slab influx into the mantle wedge. However, the relative contribution of these mechanisms remains contentious. The Pleistocene to Holocene Taupo Volcanic Zone (TVZ) is unusually dominated by felsic volcanism with more than 95% of the total eruptive volume corresponding to rhyolitic magmas. Despite the relatively thin crustal basement (∼16–30 km), combined wall rock assimilation and fractional crystallization (AFC) of primary basalt has previously been presented as a necessary means to generate the chemical characteristics of these magmas. This petrogenetic model is here assessed using a new, internally consistent set of major and trace element concentrations, and Sr-Pb isotope data from a suite of basaltic to rhyolitic samples that are representative of the entire TVZ with respect to geographical distribution and age. The dataset was modelled for AFC using the energy- and mass-constrained Magma Chamber Simulator (MCS) and the local Permian to Early Jurassic Torlesse composite terrane basement used as the principal assimilant. Regardless of the intensive parameters employed, the energy- and mass-constrained models presented here fail to realistically reproduce the combined major oxide, trace element, and isotope systematics of the sample set. The Pb isotope ratios of TVZ basalts to rhyolites reveal a broadly linear trend ranging from an unradiogenic (206Pb/204Pb: 18.77) to a more radiogenic crustal endmember (206Pb/204Pb: 18.84). This trend can be reproduced by a composite mélange dominated by the Torlesse terrane (representative of tectonically eroded forearc crust), with some contribution of subducting sediments (GLOSS-II), interacting with the sub-arc mantle to generate the source of the magmas observed in the TVZ. This model is also consistent with Sr-Pb isotope systematics. At New Zealand’s active margin, tectonic erosion has been reported based on geophysical data. Therefore, it is proposed that slab input can have a stronger relevance in the magma generation of the TVZ than previously thought. The broad compositional variations in the TVZ may result from source contamination by a subduction mélange, with differentiation processes within the overriding crust being subordinate.