Abstract
Coeval arc and intraplate basaltic magmas are intimately linked to back-arc volcanism in the Alexandra Volcanic Group of North Island, New Zealand. We present a comprehensive suite of new elemental and SrNd isotope geochemical data in the context of detailed new stratigraphy to resolve the fine-scale systematics of arc-intraplate magmatism at large basaltic stratovolcanoes (Pirongia and Karioi) and their surrounding vents. Time-chemistry reconstructions for Pirongia reveal cyclical contamination of arc magmas by deeper set intraplate sources, with each cycle ending in eruptions of lavas with transitional compositions. Further from the trench at Karioi, the magma system is characterised by on-off alternations of arc and intraplate basalt that eventually transition to arc-dominant magmatism. A two-component model is proposed to explain all geochemical variation in the field through mixing between arc and intraplate sources. Spatio-chemical data along a 65 km transect of the field indicates that as distance from the trench increases, the arc signature wanes and intraplate becomes increasingly prevalent in terms of erupted composition and volume. We conclude that trough-normal slab tear has facilitated the vertical and lateral ascension of asthenosphere into the mantle wedge, causing decompression melting to form intraplate basalts, and triggering secondary melting of the wedge to generate arc basalt. The Alexandra Volcanic Group is one of only several fields on Earth where arc and intraplate basalt are documented from the same vent systems.