Consequences of open-system melting in tectonics

Partial melting and melt drainage from deep suprasolidus crust in orogens has important consequences for tectonics. Melt extraction along prograde segments of clockwise P – T paths reduces fertility and increases the density and strength of residual crust, which has implications for further melt production during decompression. Using calculated P – T phase diagrams, implications of stepwise melt loss along clockwise P – T paths for pelite and greywacke are assessed, and density of the progressively more residual source and the potential role of buoyancy in the exhumation of deep crustal rocks are evaluated. Two model P – T paths are considered: isobaric heating at 1.2 GPa followed by decompression to 0.4 GPa at 750, 820 and 890 °C, and prograde heating from the fluid-present solidus at 1.2 GPa to 860 °C at 1.8 GPa followed by isothermal decompression to 0.4 GPa. Both closed-system (undrained) and conditionally open-system (drained by intermittent melt loss) conditions are assessed. If melt is drained along clockwise P – T paths in suprasolidus crust then lower quantities of melt will be generated during decompression than sometimes inferred in tectonic models. Instead, the role of melt transfer through suprasolidus crust and melt accumulation at shallow levels in the anatectic zone should be considered rather than simply invoking the generation of large volumes of melt in decompressing crust.