Superhydrous magmas, flat subduction, and cool transcrustal continental arc batholiths

Pressure-temperature arrays from several iconic continental arc granitoid batholiths define cool, hydrous adiabatic ascent paths extending to depths >60 km, constrained between the 5 and 15 wt% H₂O granite liquidus curves. These paths differ significantly from those of many volcanic arc magmas, which typically form by mantle decompression melting during normal steep subduction. The Cretaceous Fiordland arc, New Zealand, preserves a deep crustal hot zone (DCHZ) of hydrous hornblende gabbros and diorites that was thickened during ongoing magmatism and burial to depths of 50−60 km. Published tomographic imaging suggests that this mantle-dominated (stage 1) magmatic flareup coincided with a transition from normal to flat-slab subduction, which ultimately terminated Fiordland magmatism. In other continental arcs, the return to normal subduction mode (slab steepening) restores normal-thickness (∼30−35 km) arc crust. This process destroys the thickened DCHZ through hydrous fluxed melting, generating superhydrous granitoid arc magmas during a crust-dominated (stage 2) flareup. Dense garnet-pyroxenite initially forms as a residue but is progressively removed by vigorous corner flow in the mantle wedge. Once lower-crustal, hydrous fluxed melting is established, ascending granitoid magmas remain cool and near water saturated because water is incrementally degassed as pressure decreases, which explains the high H₂O content in many arc melt inclusions. This hydrous melting mechanism establishes cool, transcrustal continental arc batholiths even when episodically rejuvenated by hot, hydrous mafic infusions during mantle decompression melting, which again dominates crustal heat transfer (but not granitoid magma temperature) as the mantle wedge reopens during the transition back to normal-mode subduction.