Abstract
Forearc crust removed by 'subduction erosion' from the underside of the upper plate has been recognized as an important and potentially dominant contributor to the recycled flux in global arc magmas. While not all arcs are equally influenced by subduction erosion, a flux of recycled eroded crust to arc is likely for settings where asperities like plateaus, ridges or seamounts are subducted at the trench. This is the case for the Hikurangi portion of the Tonga-Kermadec-Hikurangi trench system, where the 120 Ma oceanic Hikurangi Plateau has been subducting since approximately 10 Ma.
Geophysical data provide evidence for forearc erosion in the Hikurangi segment east of the North Island of New Zealand, such as strike-slip and compressional faulting from seamount collisions followed by collapse of the upper plate. The 2 Ma old Taupo Volcanic Zone (TVZ), where arc volcanism is linked to the subduction of the Hikurangi Plateau, has a high proportion of rhyolites (>95% total erupted volume) compared to mafic members (<1%). Moreover, the TVZ is constructed on relatively thin (16-30 km) basement composed of Permian-Cretaceous accretionary clastic terranes.
Results from mass- and energy-constrained MCS modelling show that the evolution of TVZ basalts to rhyolites is inconsistent with extensive AFC processes. Therefore, we tested the hypothesis that the silicic TVZ magmas may be linked to the tectonic erosion of the forearc crust at the Hikurangi margin. A new set of high-quality Sr-Pb-Nd-Hf isotope data of the TVZ mafic to silicic volcanic rocks as well as forearc and trench lithologies reveal the existence strong compositional links, whereby the tight TVZ isotopic array is best explained by binary mixing between the mantle wedge and tectonically eroded forearc. Thus, our data indicate crustal recycling via subduction erosion emerges as a viable alternative to AFC processes to explain the silicic TVZ volcanic rocks.