Subducted Sediments and Deep Crustal Processes Drive Volcanism in the Taupo Volcanic Zone, New Zealand: an Alternative Interpretation

Abstract The Taupo Volcanic Zone (TVZ) is a young volcanic arc producing large volumes of silicic magmas with crustal signatures that are traditionally attributed to crustal contamination and fractionation of mafic melt. Only the crustal signature in volumetrically minor mafic TVZ magmas has been linked to the trench sediment recycling. Here, we use combined Sr-Pb-Nd-Hf isotope and trace element systematics of mafic to felsic TVZ rocks and trench sediments recently recovered by the International Ocean Discovery Program to argue for a strong impact of recycled sediments on TVZ magmas. Mafic to felsic volcanic series form single binary isotopic mixing trends between a mantle and a homogeneous crustal component. We show that the composition of mafic and intermediate volcanic rocks with high Mg# (>50) and Eu/Eu* > 0.9 cannot be controlled by crustal contamination and fractionation. Instead, the data point to source contamination through terrigenous trench sediments above the décollement, which may be recycled via subduction and tectonic forearc erosion by the Hikurangi plateau and seamount collision. We propose that the mantle and the above-décollement sediments control the Sr-Pb-Nd-Hf isotopic compositions of mafic to intermediate TVZ volcanic rocks and also influence the felsic TVZ series next to transcrustal assimilation. Our results imply a lesser extent of crustal assimilation and fractionation in the genesis of felsic TVZ magmas than previously thought. Overall, we argue that subcrustal recycling is a viable alternative for creating crustal signatures in the TVZ volcanic rocks and that magma ascent through the thin crust may be fast and shallow magma storage short lived.