Abstract
Correspondence between various rock types and frequency of plagioclase twinning laws must be revised with the recent geological background for improving the criteria to discriminate geological settings by plagioclase twinning laws. For this revision, a rapid and reliable method to determine plagioclase twinning law types is necessary. Twinning laws can be recognized in a thin section with the aid of a universal stage. The plagioclase twinning law is determined from the composition plane (CP) and position of the symmetry axis (SA). The CP is determined with cleavages or optical elongation at the diagonal position. Normal, parallel or complex twin types can be understood by optical characters based upon the relationship of SA and CP. Based upon combinations of these characters, we can distinguish most plagioclase twinning laws in natural rocks.Plagioclase twinning laws of the granitic and quartz-feldspathic metamorphic rocks are presented with coordinates of frequencies of C twins (all twinning laws except albite and pericline laws) versus pericline twin for discriminative criteria of geological setting. Two contrast types of plagioclase twinning laws for granitoids are recognized, i.e., in the granitoids in Ryoke-Sanyo belts and in the granitoids in Sor Rondane Mountains. The former shows wide range of C twin frequency with moderate pericline twin frequency (for short, RS type), and the latter wide range of C twin frequency with obscure pericline twin frequency (SOR type). The RS type may be one example of plagioclase twinning laws for the granitic magmatism in subduction tectonics along the continental margin. On the other hand, the SOR type will be one representative for postorogenic or anorogenic magmatism within the Super-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 defined by amounts of melt remaining during crystalization, which 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 formation of pericline twinning law under shear stress and high-temperature conditions.
