Generation of Subduction-Zone Magmas from Melange Diapirs



At subduction zones, crustal material enters the mantle.  Some of this material, however, is returned to the overriding plate through volcanic and plutonic activity.  Magmas erupted above subduction zones show a characteristic range of compositions that reflect mixing in the magma source region between three components: subducted oceanic crust, sediments and peridotite mantle rocks.  The mechanism for mixing and transport of these components has been enigmatic, but recent models suggest a two-step process: First, intensely mixed metamorphic rock formations – mélanges – form along the interface between the subducted slab and the mantle, producing the characteristic three component geochemical pattern of subduction-zone magmas.  Then, diapirs of low-density mélange material rise buoyantly from the surface of the subducting slab and transport the well-mixed mélange material into the mantle beneath the volcanoes.

In this experimental study we set out to test whether melts produced from rocks that dominate exhumed high-pressure mélange complexes under the pressure-temperature conditions prevailing in the mantle wedge resemble the range of magma compositions observed in subduction zones.  Emphasis is given to accessory minerals and their role in controlling the trace-element compositions of mélange rocks and mélange melts.