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Plagioclase twinning method revisited with special reference to importance of pericline twinning



Plagioclase twinning method revisited with special reference to importance of pericline twinning



Chishitsu Chousa Kenkyuu Houkoku = Bulletin of the Geological Survey of Japan 55(1-2): 39-48



The author studied modes of occurrences on plagioclase twinning laws in various granitic and quartz-feldspathic metamorphic rocks, and revealed that frequencies of C twins (all twinning laws excluding albite and pericline laws) versus pericline twin are discriminative criteria of geological setting. Plagioclase twinning law in the granitoids in Ryoke-Sanyo Belts shows wide range of C twin frequency (4-44%) with various pericline twin frequency (10-38%) (for short, RS type), and that in the granitoids in Hidaka Metamorphic Belt low to intermediate C twin frequency (0-20%) with wide pericline twin frequency (2-34%) (HD type). Range of these may be one discriminative area of plagioclase twinning laws for the granitic magmatism in subduction tectonics along continental margin or island arc. On the other hand, frequency of plagioclase twinning laws of some granitic rocks is out of this area. The area showing low frequency of pericline twinning law against various frequency of C twin has one possibility of relating to postorogenic or anorogenic magmatism within the continent. Difference of these plagioclase twinning law frequencies, especially difference of frequency of pericline twinning law, is explained by various shear stress due to various viscosity of magma. The viscosity of magma is controlled by amounts of melt remaining during crystallization, and this difference of the melt amounts is caused from characteristic of magma related with various tectonics. Low-grade metamorphic rocks (greenschist facies to low grade amphibolite facies) contain small amounts of pericline twinning law. High-grade metamorphic rocks (high-grade amphibolite facies to granulite facies) show low to moderate frequency of pericline twinning law. Sheared high-grade metamorphic rocks have abundant pericline twinning law. These results are conformable to the previous experimental results which show that pericline twinning law is mostly formed under shear stress and high-temperature conditions.

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Accession: 019731473

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