Crustal melting and granite magmatism: Key issues

The thermal and theological structure of orogens determines their mechanica l behaviour. Collisional orogens are characterized by a clockwise P-T evolu tion, which means that in the core, where temperatures exceed the wet solid us for common crustal rocks, melt may be present during orogenesis. Field o bservations of eroded orogens show that middle crust is migmatitic, and geo physical 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 th e anomalies recorded in the data and based on laboratory experiments to cal ibrate the models. A consequence of these results is that orogenic collapse in mature orogens may be controlled by a partially-molten layer that decou ples weak crust from subducting lithosphere, and such a weak layer may enab le exhumation of deeply buried crust. Field observations in ancient orogens show that melt segregation and extraction are syntectonic processes, and t hat melt migration pathways commonly relate to rock fabrics. These processe s are being investigated using analog and numerical models. Leucosomes in d epleted migmatites record the remnant permeability network, but evolution o f permeability networks and amplification of anomalies are poorly understoo d. Melt segregation and extraction may be cyclic or continuous, depending o n the level of applied differential stress and rate of melt pressure buildu p. 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 m icas remain in the depleted prototith. The presence of crystallizing melt o r H2O may enable reaction during cooling. However, metasomatism in the evol ution of the crust remains a contentious issue. Processes in the lowermost crust may De inferred from studies of xenolith suites brought to the surfac e 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 co mmon in the lower crust, but the role of mafic magma in crustal melting rem ains 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 m ay vary systematically with depth. The switch from ascent to emplacement ma y be caused by amplification of instabilities within (permeability, magma f low rate) or surrounding (strength or state of stress) the ascent column, o r by the ascending magma intersecting some discontinuity in the crust. Plut on emplacement mechanics are being investigated by modeling. Feedback relat ions among these processes map moderate compatibility between rates of plut on filling, magma ascent and melt extraction. (C) 2001 Elsevier Science Ltd . Aii rights reserved.