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廣島大學地學研究報告 Issue 9
published_at 1961-06
別子白滝地方三波川結晶片岩の地質構造と変成作用
Geologic Structure and Metamorphism of the Sambagawa Crystalline Schists of the Besshi-Shirataki Mining District in Shikoku, Southwest Japan
HIDE KEI
Abstract
In the present paper, the outline of geologic structure and metamorphism of the Sambagawa crystalline schist system in the Besshi-Shirataki mining district were described.
Particularly, the author has noticed the following facts and subjects, that he intended to clarify: 1) the presence of refolding or cross-folding and the relation of the folding to the formation of schistosity and the mineralization; 2) the analysis of the fold structure and the micropetrofabrics of the ore bed (Kieslager) and the country rock of the Besshi Mine, both being characterized by the down-dip nature of lineation and isoclinal fold axis; 3) the origin, the refolding of early-formed isoclinal fold of Kieslager and the prospecting of Kieslager in the Sambagawa metamorphic zone; and 4) the zoning of the schist complex (Zones I, II, III, and IV, and the subdivision) based on the variation of mineral paragenesis of green-schist, black-schist, and quartz schist, including calcareous schist and Kieslager, and the relation of the mineralogical zoning to the stratigraphy, the rock and geologic structure, and the presence of ultrabasic rocks, with special reference to the occurrence of glaucophane schists and eclogitic rocks.
The main stage of mineralization of crystalline schists, including Kieslager, is believed to be nearly contemporaneous with the fomation of isoclinal fold, which has the axial plane parallel to the bedding plane of the wing and the axis apparently perpendicular to the direction of tectonic movement. The isoclinal fold was accompanied by the development of flow cleavage parallel to the axial plane, along which later shear plane was formed in some places. The direction of the axis is rather variable, and the scale of the fold is not consistent throughout the district.
The origin and the history of development of the ore body of the Besshi Mine have been considered on the basis of the following points of view, i. e., 1) the continuation of beds of ore and country rocks, 2) the parallelism of the fold axis and mineral lineation, 3) the relation of the axial plane to the bed-ding plane, 4) the change of thickness of beds from the crest to the wing of the fold, and 5) the petro-fabric analysis of quartz schists of the country rock. The ore bed has been interpreted by the author to be derived from a simgle stratum of ore deposited on the floor of geosynchine, then folded isoclinally during the tectogenesis and metamorphism. The folding was caused by the westward movement of the northern block with higher metamorphic grade, accompanied by the compression perpendicular to the axial plane. The width of the folded ore bed in the horizontal direction is about 1500m, and that of the bed of quartz schists of the country rock is greater than 3500m. The ratio of the wave length to the amplitude of the fold is nearly 1:100.
The method of analysis, applied to the ore bed and the country rock, may well be applied to the structural analysis as well as to the prospecting of other Kieslagers.
Micropetrofabric analysis was done for the selected specimens of quartz schists of the country rock, as an aid to clarify the mode of formation of the isoclinal fold of the ore bed and the country rock. The preferred orientation of minerals such as muscovite, chlorite, and quartz is intimately related to the direction of megascopic fabric elements such as the axial plane (s2) and the fold axis (B1) of the isoclinal fold. It has been revealed that the pattern of quartz is somewhat variable, bed by bed, but the position of the maximum concentration and the presence of small-circle girdles in oblique directions are consistent (cf. G. KOJIMA and K. HIDE, 1958). The author could not find out any significant differences between the fabric pattern of quartz of the northern wing and that of the southern one, as well as between that of the lower level and that of the upper one.
Judging from the occurrence of kyanite and eclogitic rocks and from the nature of plastic flow of the isoclinal fold accompanied by the recrystallization of minerals with marked preferred orientation, higher hydrostatic pressure and stress must have prevailed during the main stage of metamorphism.
The part of higher metamorphic grade (a kind of epidote-amphibolite facies) occupies the particular stratigraphical unit, characterized by an special type of geologic structure, namely, the upper member of the Minawa formation and the lower part of the Ôjôin formation, forming the Besshi-Shirataki recumbent fold.
The axis of the Nakashichiban-Nôtaniyama anticline in the south of the district in question, runs obliquely to the trend of the Sambagawa metamorphic zone. Its direction is mostly parallel, though oblique in some parts, to the mineral lineation, the axis of earlier intraformational isoclinal fold and that of later open fold, which often followed by shear fold. The axial plane of the open fold trends E.-W., and crosses the bedding-plane-schistosity at high angles.
Particularly, the author has noticed the following facts and subjects, that he intended to clarify: 1) the presence of refolding or cross-folding and the relation of the folding to the formation of schistosity and the mineralization; 2) the analysis of the fold structure and the micropetrofabrics of the ore bed (Kieslager) and the country rock of the Besshi Mine, both being characterized by the down-dip nature of lineation and isoclinal fold axis; 3) the origin, the refolding of early-formed isoclinal fold of Kieslager and the prospecting of Kieslager in the Sambagawa metamorphic zone; and 4) the zoning of the schist complex (Zones I, II, III, and IV, and the subdivision) based on the variation of mineral paragenesis of green-schist, black-schist, and quartz schist, including calcareous schist and Kieslager, and the relation of the mineralogical zoning to the stratigraphy, the rock and geologic structure, and the presence of ultrabasic rocks, with special reference to the occurrence of glaucophane schists and eclogitic rocks.
The main stage of mineralization of crystalline schists, including Kieslager, is believed to be nearly contemporaneous with the fomation of isoclinal fold, which has the axial plane parallel to the bedding plane of the wing and the axis apparently perpendicular to the direction of tectonic movement. The isoclinal fold was accompanied by the development of flow cleavage parallel to the axial plane, along which later shear plane was formed in some places. The direction of the axis is rather variable, and the scale of the fold is not consistent throughout the district.
The origin and the history of development of the ore body of the Besshi Mine have been considered on the basis of the following points of view, i. e., 1) the continuation of beds of ore and country rocks, 2) the parallelism of the fold axis and mineral lineation, 3) the relation of the axial plane to the bed-ding plane, 4) the change of thickness of beds from the crest to the wing of the fold, and 5) the petro-fabric analysis of quartz schists of the country rock. The ore bed has been interpreted by the author to be derived from a simgle stratum of ore deposited on the floor of geosynchine, then folded isoclinally during the tectogenesis and metamorphism. The folding was caused by the westward movement of the northern block with higher metamorphic grade, accompanied by the compression perpendicular to the axial plane. The width of the folded ore bed in the horizontal direction is about 1500m, and that of the bed of quartz schists of the country rock is greater than 3500m. The ratio of the wave length to the amplitude of the fold is nearly 1:100.
The method of analysis, applied to the ore bed and the country rock, may well be applied to the structural analysis as well as to the prospecting of other Kieslagers.
Micropetrofabric analysis was done for the selected specimens of quartz schists of the country rock, as an aid to clarify the mode of formation of the isoclinal fold of the ore bed and the country rock. The preferred orientation of minerals such as muscovite, chlorite, and quartz is intimately related to the direction of megascopic fabric elements such as the axial plane (s2) and the fold axis (B1) of the isoclinal fold. It has been revealed that the pattern of quartz is somewhat variable, bed by bed, but the position of the maximum concentration and the presence of small-circle girdles in oblique directions are consistent (cf. G. KOJIMA and K. HIDE, 1958). The author could not find out any significant differences between the fabric pattern of quartz of the northern wing and that of the southern one, as well as between that of the lower level and that of the upper one.
Judging from the occurrence of kyanite and eclogitic rocks and from the nature of plastic flow of the isoclinal fold accompanied by the recrystallization of minerals with marked preferred orientation, higher hydrostatic pressure and stress must have prevailed during the main stage of metamorphism.
The part of higher metamorphic grade (a kind of epidote-amphibolite facies) occupies the particular stratigraphical unit, characterized by an special type of geologic structure, namely, the upper member of the Minawa formation and the lower part of the Ôjôin formation, forming the Besshi-Shirataki recumbent fold.
The axis of the Nakashichiban-Nôtaniyama anticline in the south of the district in question, runs obliquely to the trend of the Sambagawa metamorphic zone. Its direction is mostly parallel, though oblique in some parts, to the mineral lineation, the axis of earlier intraformational isoclinal fold and that of later open fold, which often followed by shear fold. The axial plane of the open fold trends E.-W., and crosses the bedding-plane-schistosity at high angles.
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