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Journal of science of the Hiroshima University. Series C, Geology and mineralogy 6 巻 3 号
1971-12-30 発行
Regional Metamorphism of the Nishiki-chô District, Southwest Japan
NISHIMURA Yûjirô
Abstract
Regional metamorphism of the Nishiki-chô district on the east of Yamaguchi Prefecture has been investigated from geological and petrological points of view. The basement formations of this district consist of slightly metamorphosed rocks and crystalline schists. These are divided stratigraphically into two Groups, namely, the Nishiki Group (Lower to Upper Permian) and the Tsuno Group (pre-sumably Carboniferous in the main), respectively, in descending order.
The metamorphic terrain is divided into three zones, A, B and C, on the basis of systematic mode of appearance of pumpellyite and amphiboles. Zone A is characterized by the assemblage pumpellyite-chlorite in basic and psammitic rocks. No schistosity and less recrystallization are characteristic. Zone B is characterized by the assemblage pumpellyite-actinolite in basic schists. Schistosity and lineation are well developed, recrystallization being almost complete. Zone C is characterized by the assemblages epidote-glaucophane and/or epidote-subcalcic hornblende in basic schists. Occurrence of garnet in pelitic and siliceous schists also characterizes this zone. Schistosity and lineation are well developed and recry-stallization is complete.
The boundary surfaces between these metamorphic zones are nearly parallel to the bedding surface, and the metamorphic grade increases from the upper to the lower stratigraphic horizon. Zone A coincides with the whole of the Nishiki Group, zone B comprises the uppermost horizon of the upper formation of the Tsuno Group and zone C corresponds to other lower horizons of the Tsuno Group.
In the present metamorphic terrain, two phases of deformation can be detected. During the earlier phase, which is represented by the formation of the lineation L1, progressive regional metamorphism grading from zone A to zone C might have taken place. The later phase corresponds to the deformation related to the formation of major folds. To this phase of deformation is also related the formation of tectonic slide, which is developed along the boundary zone between zones A and B showing different physical properties.
It is concluded that the metamorphites of the three metamorphic zones were formed by one and the same metamorphism, namely, the Sangun metamorphism. The facies series of the Sangun metamorphism in question runs from the pumpellyite-chlorite zone, through the pumpellyite-actinolite zone, to the epidote-glaucophane zone and/or the epidote-subcalcic hornblende zone in the order of increasing metamorphic grade, and corresponds to the high-pressure intermediate group in MIYASHIRO'S classification.
Main constituent minerals are described and discussed genetically in some detail. Petrochemical properties of basic metamorphic rocks are also mentioned briefly. Bulk chemical compositions of 75 basic metamorphic rocks are presented.
The metamorphic terrain is divided into three zones, A, B and C, on the basis of systematic mode of appearance of pumpellyite and amphiboles. Zone A is characterized by the assemblage pumpellyite-chlorite in basic and psammitic rocks. No schistosity and less recrystallization are characteristic. Zone B is characterized by the assemblage pumpellyite-actinolite in basic schists. Schistosity and lineation are well developed, recrystallization being almost complete. Zone C is characterized by the assemblages epidote-glaucophane and/or epidote-subcalcic hornblende in basic schists. Occurrence of garnet in pelitic and siliceous schists also characterizes this zone. Schistosity and lineation are well developed and recry-stallization is complete.
The boundary surfaces between these metamorphic zones are nearly parallel to the bedding surface, and the metamorphic grade increases from the upper to the lower stratigraphic horizon. Zone A coincides with the whole of the Nishiki Group, zone B comprises the uppermost horizon of the upper formation of the Tsuno Group and zone C corresponds to other lower horizons of the Tsuno Group.
In the present metamorphic terrain, two phases of deformation can be detected. During the earlier phase, which is represented by the formation of the lineation L1, progressive regional metamorphism grading from zone A to zone C might have taken place. The later phase corresponds to the deformation related to the formation of major folds. To this phase of deformation is also related the formation of tectonic slide, which is developed along the boundary zone between zones A and B showing different physical properties.
It is concluded that the metamorphites of the three metamorphic zones were formed by one and the same metamorphism, namely, the Sangun metamorphism. The facies series of the Sangun metamorphism in question runs from the pumpellyite-chlorite zone, through the pumpellyite-actinolite zone, to the epidote-glaucophane zone and/or the epidote-subcalcic hornblende zone in the order of increasing metamorphic grade, and corresponds to the high-pressure intermediate group in MIYASHIRO'S classification.
Main constituent minerals are described and discussed genetically in some detail. Petrochemical properties of basic metamorphic rocks are also mentioned briefly. Bulk chemical compositions of 75 basic metamorphic rocks are presented.
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