Fluid induced partial melting as a cause for ultra-high-pressure metamorphism

Evidence from seismological and isotopic studies suggest that fluids released from hydrated lithosphere at great depth in subduction zones can travel upwards through the dry mantle wedge. When they reach the overlying crust, the fluids induce melting which is thought to feed volcanoes on the surface. Many continental collision zones are the result of the closure of an ocean. The suture zone may still contain hydrated rocks. During burial in the continent-continent collision, these rocks may dehydrate, and fluids can travel up through dry overlying crustal rocks. The Western Gneiss Region (WGR) of Norway, a basement window in the Scandinavian Caledonides, is well known for its occurrences of eclogites and peridotites with metamorphic pressures reaching diamond stability field. Often the surrounding felsic gneiss shows evidence for fluid infiltration and partial melting. However, the majority of the protoliths in the WGR consisted of dry felsic magmatic rocks and the source of the fluid for metasomatism and melting remains enigmatic. Like oceanic subduction zones, fluids rising through the overlying dry rocks may be responsible for partial melting in (ultra)-high pressure terrains in continental collision zones. On the way up these fluids react with the rocks and transport mass by carrying chemical elements in solution and metasomatize original continental crust. Fluid focusing may be the reason for the local occurrence of partial melting. This can lead to overpressure due to local volume increasing melting reactions which explains erratic deviations in metamorphic pressure compared to the overall metamorphic field gradient. The newest methodology for calculating aqueous speciation of fluids in the deep earth combined with the latest techniques in numerical modelling of reactive transport is used here to build a quantitative understanding of the processes.