Orogeny, migmatites and leucogranites: A review

• Published in: Academy Proceedings in Earth and Planetary Sciences 2001-12, Vol.110 (4), p.313-336
• Main Author: Brown, Michael
• Format: Article
• Language: English
• Subjects: Amplification; Crystallization; Decompression; Deformation; Dehydration; Depletion; Ductile-brittle transition; Evolution; Fabrics; Flow rates; Geology; Grain boundaries; Granite; Igneous rocks; Lava; Lithosphere; Mafic magma; Magma; Mathematical models; Melting; Metamorphism; Mica; Micas; Mineralogy; Muscovite; Orogeny; Permeability; Plutons; Rock; Rocks; Segregation; Sillimanite; Solidus; Statistical methods; Volumetric strain
• ISSN: 0253-4126; 0973-774X
• DOI: 10.1007/BF02702898

The type ofP-T-t path and availability of fluid (H^sub 2^O-rich metamorphic volatile phase or melt) are important variables in metamorphism. Collisional orogens are characterized by clockwiseP-T evolution, which means that in the core, where temperatures exceed the wet solidus for common crustal rocks, melt may be present throughout a significant portion of the evolution. Field observations of eroded orogens show that lower crust is migmatitic, and geophysical observations have been interpreted to suggest the presence of melt in active orogens. 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. Migmatites provide a record of melt segregation in partially molten crustal materials and syn-anatectic deformation under natural conditions. Grain boundary flow and intra-and inter-grain fracture flow are the principal grain scale melt flow mechanisms. Field observations of migmatites in ancient orogens show that leucosomes occur oriented in the metamorphic fabrics or are located in dilational sites. These observations are interpreted to suggest that melt segregation and extraction are syntectonic processes, and that melt migration pathways commonly relate to rock fabrics and structures. Thus, leucosomes in depleted migmatites record the remnant permeability network, but evolution of permeability networks and amplification of anomalies are poorly understood. Deformation of partially molten rocks is accommodated by melt-enhanced granular flow, and volumetric strain is accommodated by melt loss. Melt segregation and extraction may be cyclic or continuous, depending on the level of applied differential stress and rate of melt pressure buildup. During clockwiseP-T evolution, H^sub 2^O is transferred from protolith to melt as rocks cross dehydration melting reactions, and H^sub 2^O may be evolved above the solidus at lowP by crossing supra-solidus decompression-dehydration reactions if micas are still present in the depleted protolith. H^sub 2^O dissolved in melt is transported through the crust to be exsolved on crystallization. This recycled H^sub 2^O may promote wet melting at supra-solidus conditions and retrogression at subsolidus conditions. The common growth of 'late' muscovite over sillimanite in migmatite may be the result of this process, and influx of exogenous H^sub 2^