Origin of compositional variations of Taupo Volcanic Zone (TVZ) eruption products: crustal differentiation or subduction melange diapirism?

The Taupo Volcanic Zone (TVZ) is dominated by felsic volcanism with more than 95% of the total volume corresponding to rhyolitic magmas. To generate felsic and intermediate volcanism, assimilation and fractional crystallisation (AFC) of primary basalt has previously been invoked as a necessary means to generate the chemical and isotopic characteristics of these magmas; is it energetically possible that dominantly felsic volcanism is generated by AFC of a mafic magma in the TVZ? Here, we present a suite of samples representative of the entire TVZ with respect to geography, age, and compositional variation from mafic to felsic. Major oxides, trace elements and Sr isotopic ratios were modelled for AFC using the energy and mass constrained Magma Chamber Simulator (MCS) and the local Torlesse terrain as an assimilant. We find that regardless of the intensive parameters employed, an energy and mass constrained model cannot realistically reproduce the combined major oxide, trace element, and isotopic systematics of the sample set. At New Zealand’s active margin, subduction erosion has been previously reported based on geophysical data. However, no links of this process to TVZ magmatism has been established to date. New high precision lead isotopic ratios show a tight linear trend that cannot be reproduced by AFC with a Torlesse component but can be approximated through a binary mixture of Torlesse subterrains, representative of eroded forearc crust, with global subducting sediments (GLOSS-II) on one hand, and a primitive basalt, representative of the subarc mantle, on the other. We therefore propose that the compositional variations in TVZ volcanic products and the genesis of voluminous rhyolites are primarily linked to melting of subduction melange diapirs as the source of this magmatism, with differentiation processes within the overriding crust being subordinate.