Reconstruction of the continental lower-crust rheology based on the rheology data of plagioclase

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We have made microstructural observations and crystallographic-orientation measurements of crystal-plastically deformed plagioclase grains in mylonite samples derived from gabbro (a) and amphibolite (b) in the Hidaka metamorphic belt of Hokkaido, amphibolite (c) and granodiorite (d) in the Ivrea Zone of NW Italy, amphibolite (e) in the Oman ophiolite, and amphibolite (f) in the Kohistan Arc of N Pakistan. Mineral assemblages and geothermometric applications indicate deformation of (a) at the granulite-amphibolite facies transition (660-700℃), while deformations of (b)-(f) at amphibolite facies (520-650℃).Dynamic recrystallization of plagioclase occurred by grain boundary migration at these deformation conditions. Evidence for recovery such as subgrain boundaries was not found in plagioclase porphyroclasts deformed at amphibolite facies, but found in those deformed at the granulite-amphibolite facies transition.Plagioclase grains dynamically recrystallized at amphibolite facies commonly … More exhibit a crystallographic preferred orientation (CPO) with {121} and <111> subparallel to foliation and lineation, respectively, but its relation to the dominant slip system remains unknown. In contrast, those grains at the granulite-amphibolite facies transition exhibit a CPO with (001) and <110> subparallel to foliation and lineation, respectively. Because (001) 1/2 <110> is known as a slip system in plagioclase, this slip system was likely dominant in these grains.Thus the dynamic recrystallization mechanism, extent of recovery, CPO developed, and their changes with deformation temperature were revealed for plagioclase deformed at the conditions from amphibolite facies to the granulite-amphibolite facies transition, which are an important addition to the existing data on the rheology of plagioclase. However, we could not reconstruct the rheology of the continental lower crust, because we could not obtain data on plagioclase deformed at granulate facies, and also because we could not conduct deformation experiments on plagioclase at high pressures and temperatures due to a gas-medium triaxial apparatus being not set up yet. Less