CONSEQUENCES OF OPEN SYSTEM MELTING IN TECTONICS

Partial melting and melt drainage from the deep suprasolidus crust in orogens has important consequences for tectonics. In particular, melt extraction along the prograde segments of CW P–T paths reduces the fertility and increases the density and strength of the residual crust, which has implications for further melt production during decompression. Using calculated P–T phase diagrams, the implications of melt loss along CW P–T paths for an average amphibolite facies pelite and an average passive margin greywacke are assessed, and the 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, based on literature data for migmatites, granulites and HT–HP metamorphic rocks: 1. isobaric heating at 1.2 GPa followed by decompression to 0.4 GPa at 750°C, 820°C and 890°C; and, 2. 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. The threshold for melt drainage was set at 7 vol.% melt, an upper bound inferred from rock deformation experiments, but 1 vol% of this melt was retained, a lower bound inferred from microstructural observations of residual rocks. If melt is drained along the prograde segment of suprasolidus CW P–T paths then: 1. lower quantities of melt will be generated from both pelite and greywacke during decompression than sometimes has been inferred in tectonic models; and, 2. the density decrease accompanying decompression will be limited and unlikely to contribute to buoyant exhumation of suprasolidus crust.

Alternatively, it may be the accumulation of melt in the upper levels of the anatectic zone that weakens crust sufficiently to enable the formation of detachments and development of migmatitic gneiss domes and metamorphic core complexes, or that reduces the density sufficiently to enable doming in circumstances where buoyancy is required. Therefore, the role of melt transfer through suprasolidus crust and melt accumulation at shallow levels in the anatectic zone should be considered in tectonic models rather than simply invoking the generation of large volumes of melt in decompressing crust.