Orogeny, migmatites and leucogranites: A review |
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| Authors: | Michael Brown |
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| Institution: | (1) Laboratory for Crustal Petrology, Department of Geology, University of Maryland, College Park, 20742 MD, USA |
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| Abstract: | The type ofP-T-t path and availability of fluid (H2O-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, H2O is transferred from protolith to melt as rocks cross dehydration melting reactions, and H2O may be evolved above the solidus at lowP by crossing supra-solidus decompression-dehydration reactions if micas are still present in the depleted protolith. H2O dissolved in melt is transported through the crust to be exsolved on crystallization. This recycled H2O 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 H2O may not be necessary. However, in general, metasomatism in the evolution of the crust remains a contentious issue. Processes
in the lower-most crust may be inferred from studies of xenolith suites brought to the surface in lavas. Based on geochemical
data, we can use statistical methods and modeling to evaluate whether migmatites are sources or feeder 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 that enables
horizontal magma emplacement followed by thickening during pluton inflation. Feedback relations between rates of pluton filling,
magma ascent and melt extraction maintain compatibility among these processes. |
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| Keywords: | Anatexis granite emplacement magma ascent melt segregation migmatite orogeny pluton |
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