Are adakites slab melts or high-pressure fractionated mantle melts?

Abstract

Adakites are unusual felsic igneous rocks commonly associated with asthenospheric slab window opening or fast subduction of a young (<25 Ma) oceanic plate that may allow slab melting at shallow depths. Their genesis has been extensively debated, as they are also observed in other geodynamic settings where thermal models do not predict slab melting in the fore-arc region. Here, we present a new approach that provides new constraints on adakite petrogenesis in hot subduction zones (e.g. the Philippines) and above an asthenospheric window (e.g. Baja California, Mexico). We use amphibole compositions to estimate magma storage depths and the composition of the host melts to test the hypothesis that adakites are pristine slab melts. We find that adakites from the Philippines and Baja California fore-arcs formed in two distinct petrogenetic scenarios: In the Philippines, water-rich mantle melts stalled and crystallized within lower and upper crustal magma storage regions to produce silica-rich melts with an adakitic signature; in Baja California, slab melts that percolated through the mantle wedge mixed or mingled with water-rich mantle melts within a lower crustal (~30 km depth) magma storage region before stalling in the upper arc crust (~7-15 km depth). Alternatively, the Baja California adakites may represent mixing products between high-pressure differentiated mantle melts and mantle melts in a lower crustal magma reservoir, periodically refluxed by mantle melts. Thus, slab melting is not necessarily required to produce an adakitic geochemical fingerprint in hot subduction zones. The hot downgoing plate may cross the 'adakitic window' and melt in specific geodynamic settings such as the opening of a slab tear, as beneath Baja California.