Deformation and Recrystallization of Amphiboles in Sambagawa Schist with Special Reference to History of Sambagawa Metamorphism
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The amphibole grains in the Shirataki hornblende-schist, which has been collected from an outcrop in the biotite zone of the Sambagawa belt of the Shirataki district; Central Shikoku, are divided into three distinguishable populations which are different from each other in generation age: amphibole grains (Si-amphibole) included in cores of plagioclase porphyroblasts, and their matrix amphibole grains which consist of two populations, old hornblende grains (porphyroblasts) and new hornblende grains. The Si- amphibole grains, which recrystallized during growth of the cores of plagioclase porphyroblasts, belong to the actinolite - common hornblende group with Si content of 7.36-6.95, showing that the plagioclase cores grew during progressive increase of temperature. The old horriblende grains (Si content=6.97-6.66), which are of the same generation as the inner zones of mantles of plagioclase porphyroblasts, appeared under non-deformational condition and progressive increase of temperature until the highest tempeature. The new hornblende grains (Si content=6.87-6.79) grew during growth of the outer zones of mantles of plagioclase porphyroblasts and the deformation (Se-deformation) of the beginning stage of retrogressive metamorphism. The lattice fabric of the old hornblende grains, which was produced by the Se-deformation, is characterized by preferred orientation of crystallographic axes c forming a single set of lineation (L), though poles of (100) planes form a great-circle girdle normal to L. While that of the new hornblende grains, which grew during the Se-deformation, is characterized by preferred orientation of c axes parallel to L and of (100) planes forming a single set of schistosity. On the basis of informations given by HARA et al. (1977), TAKAGI and HARA (1979), HARA et al. (1980), MAEDA and HARA (1983a and b), MAEDA et al. (1983) and HARA et al. (1983), as well as the present authors, the time-relationship between deformation and metamorphism in the biotite zone of the Sambagawa belt of Central Shikoku has been also briefly discussed in this paper, showing a result of Table I and that the deformation styles of the Sambagawa schists changed cyclically from ductile deformation (=folding and formation of schistosity) to brittle deformation.
Journal of science of the Hiroshima University. Series C, Geology and mineralogy
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