关键词: 超高压变质岩, 晶格优选定向, 黏度, 俯冲, 折返, 相变

Abstract: Ultrahigh-pressure metamorphic rocks provide a window to investigate deformation mechanisms and rheological contrast of rocks in a subduction channel. Using the electron backscatter diffraction (EBSD) technique, we measured the latticepreferred orientations (LPO) of major minerals from eclogites and felsic gneisses in the Dabie ultrahigh-pressure metamorphic belt. Garnet from eclogites shows nearly random orientation, whereas the LPO of omphacite is characterized by the maximum concentration of [001] axis parallel to the lineation and that of the pole of (100) normal to the foliation. Amphibole from
retrograded eclogites also develops (100)[001] fabric, which may be inherited from omphacite. Quartz in retrograded eclogites and felsic gneisses shows the activation of dominant low-temperature (0001)<a> and medium-temperature {1010}<a> slip systems, which should be formed during exhumation in the middle crust. In contrast, (001)<110> and (010)[100] fabrics in plagioclase resulted from ductile deformation during exhumation in the lower crust under amphibolite facies metamorphism. The effective viscosity of major minerals during subduction and exhumation was calculated using flow laws of dry minerals. The decomposition reaction of albite to jadeite and quartz, and phase transition of quartz to coesite will significantly increase the effective viscosity and density of felsic gneisses and hence facilitate deep subduction of the continental crust. However, during exhumation the changes of effective viscosity across the two phase boundaries are much smaller because of high temperature. Deeper than ~80 km, the effective viscosity of minerals has the sequence of garnet > jadeite > omphacite > coesite. Rheological properties of the upper crust are controlled by coesite and jadeite, whereas those of the lower crust by omphacite and garnet. The strength contrast between ultrahigh-pressure metamorphic rocks will result in decoupling between the upper and lower crust, which allows rapid exhumation of low-density and low-viscosity upper crustal materials in a subduction channel.

Key words: ultrahigh-pressure metamorphic rocks, lattice-preferred orientation, viscosity, subduction, exhumation, phase transition