Crustal melting and granite magmatism: key issues

The thermal and rheological structure of orogens determines their mechanical behaviour. Collosional orogens are characterized by a clockwise P-T evolution, which means that in the core, where temperatures exceed the wet solidus for common crustal rocks, melt may be present during orogenesis. Field observations of eroded orogens show that middle crust is migmatitic, and geophysical observations have been interpreted to suggest the presence of melt in active orogens. Indeed, the vol. % melt in some active orogens has been estimated by conductivity modelling, assuming that melt is the cause of the anomalies recorded in the data and based on laboratory experiments to calibrate the models. A consequence of these results is that orogenic collapse in mature orogens may be controlled by a partially-molten layer that decouples weak crust from subducting lithosphere, and such a weak layer may enable exhumation of deeply buried crust. Field observations in ancient orogens show that melt segregation and extraction are syntectonic processes, and that melt migration pathways commonly relate to rock fabrics. These processes are being investigated using analog and numerical models. Leucosomes in depleted migmatites record the remnant permeability network, but evolution of permeability networks and amplification of anomalies are poorly understood. Melt segregation and extraction may be cyclic or continuous, depending on the level of applied differential stress and rate of melt pressure buildup. During the clockwise P-T evolution, H₂O is transferred from protolith to melt as rocks cross dehydration melting reactions, and H₂O may be evolved at low P by crossing supra-solidus decompression—dehydration reactions if micas remain in the depleted protolith. The presence of crystallizing melt or H₂O may enable reaction during cooling. However, metasomatism in the evolution of the crust remains a contentious issue. Processes in the lowermost crust may be inferred from studies of xenolith suites brought to the surface in lavas. Using geochemical data, statistical methods and modeling may be applied to evaluate whether migmatites are sources or magma transfer zones for granites, or simply segregated melt that was stagnant in residue, and to compare xenoliths of inferred lower crust with exposed deep crust. Upper crustal granites are a necessary complement to melt-depleted granulites common in the lower crust, but the role of mafic magma in crustal melting remains uncertain. Plutons occur at various depths above and below the brittle-to-viscous transition in the crust and have a variety of 3-D shapes that may vary systematically with depth. The switch from ascent to emplacement may be caused by amplification of instabilities within (permeability, magma flow rate) or surrounding (strength or state of stress) the ascent column, or by the ascending magma intersecting some discontinuity in the crust. Pluton emplacement mechanics are being investigated by modeling. Feedback relations among these processes may moderate compatibility between rates of pluton filling, magma ascent and melt extraction.