On the Difference in Deformation Behaviour between Phenocryst and Ground-mass Quartz of Deformed Rhyolite Pebbles in the Oboke Conglomerate Schist

Quartz subfabrics of the conglomerate schist consisting of rhyolite pebbles and psammitic matrix of the Oboke district, Shikoku, Southwest Japan, have been described and discussed in detail, with special reference to deformation mechanisms of quartz taking place under metamorphic conditions. Quartz grains in the deformed rhyolite pebbles and the psammitic matrix occur generally in two distinctly separated sizes, that is, porphyritic grains of phenocryst quartz in rhyolite and clastic megacryst in psammitic matrix, and fine-grained quartz smaller than one tenth of the former in size, derived from glassy or cryptocrystalline material of the ground-mass of rhyolite and from very fine-grained material of psammitic matrix. The fine-grained quartz grains, except for those surrounding the porphyritic quartz grains, show clearly preferred lattice and dimensional orientation in such a fashion as are symmetrically consistent with the mesoscopic structures of the conglomerate schist of the district. The patterns of lattice and dimensional subfabrics of the fine-grained quartz are essentially the same as those of calcite in the rhyolite pebbles, and the former is symmetrically consistent with the latter. While, for the porphyritic quartz in the rhyolite pebbles and psammitic matrix no preferred lattice orientation has been recognized. The pattern of dimensional orienta-tion of those grains is quite different from that of the fine-grained quartz. The significance of difference in pattern of lattice and dimensional subfabrics between the fine-grained quartz and the porphyritic quartz has been discussed. It is assumed that, when metamorphic deformation induced preferred lattice and dimensional orientations to the fine-grained quartz, deformation of the porphyritic quartz occurred by translation gliding on a certain crystallographic plane, accompanied with the formation of kink bands and recrystallization in those bands and grain-boundaries, though no preferred lattice and dimensional orientations were induced to those coarse-grained quartz grains. Obtained data inclines our mind to such a view that for the deformation of quartz taking place under metamorphic conditions diffusion mechanisms, including nucleation, grain-boundary migration and Riecke diffusion, play the more predominant role than translation gliding on some particular crystallographic planes does. In the Sambagawa crystalline schists of the higher metamorphic grade (e.g., the zone IV in the Kôtsu-Bizan district after Iwasaki, 1963) than that for those of the Oboke district (corresponding to the zone II in the Kôtsu-Bizan district), quartz grains with the average size, which is nearly equal to that of the porphyritic quartz grains in question (no preferred lattice orientation), show a preferred lattice orientation, whose pattern is essentially the same as that of the c-axis subfabric of the fine-grained quartz in question. The average size of quartz grains, which show preferred lattice orientation, increases with increase in the metamophic grade, giving an equilibrium size under any metamorphic deformation condition.