石鎚コールドロンの形成と中新世テクトニクス

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The Tertiary Ishizuchi Cauldron, in the Setouchi volcanic belt of middle Miocene age in northwestern Shikoku, is 7-8 km in diameter and includes outer and inner ring fractures, inward dipping andesitic to dacitic welded tuffs, granodioritic to adamellitic central plutons, and andesitic to rhyolitic ring fault complexes (Yoshida, 1984). Major element chemistry suggests that the granodiorite and adamellite, as well as the volcanic rocks composing welded tuffs and ring dykes, form a comagmatic series. These volcanic and plutonic rocks, however, show two contrastive trends in some trace elements causing zircon and alkali feldspar bearing fractionation at lower temperature for plutonic rocks. The compositional zoning from porphyritic intermediate composition rocks to aphyric silicic rocks with similar assemblage and relative proportion of phenocrystic minerals suggests the importance of fluid separation from porphyritic magma during intrusion, along with possible phenocryst settling in the reservoir. Mineral assemblage and major geochemical criteria show that the intermediate composition rocks and silicic aphyric rocks belong to I-type and W-type (Murata & Yoshida, 1985a) granites, respectively. The MORB normalized patterns of those rocks including high-magnesian andesites from the Setouchi volcanic belt indicate that those magmas are derived from subduction zone with a contribution of incompatible element-enriched upper mantle, that is, from sub-continental upper mantle source at active continental margin. The Ishizuchi Cauldron formed by the eruption of voluminous pyroclastic flows, accompanied by caldera collapse along ring fractures and by intrusion of the same magma along the underground cauldron fractures that formed in the subsiding block. The change in fracture pattern from upward opening cone to concave-upward subsidence faults implies the rotation of the maximum stress axis from vertical to horizontal, owing to eruption of magma from the magma chamber and caldera collapse into the upward opening cone. Intrusion of silicic magma into concave-upward sheets from ring dikes produced resurgent doming of the upper part of the subsided block. In middle Miocene of the Southwest Japan, just after the end of the opening of Japan Sea, the direction of the maximum horizontal compressional stress changed from EW-trend to NS-trend. At the same time, Southwest Japan uplifted being compressed normal to the arc, and volcanic field rapidly extended to the south beyond the Median Tectonic Line. The Ishizuchi I-type and W-type granitic rocks at the northern end of the Outer Zone of Southwest Japan might be derived by orthopyroxene and plagioclase fractionation from mantle-derived K-rich high Mg andesitic magma. On the contrary, the I-type granitic rocks from the northern side of the Butsuzo Tectonic Line (BTL) and the S-type granitic rocks from the southern side of BTL are considered to be produced by partial melting of lower crust at a depth of about 20 km (Murata, 1984). The I-type granites were probably generated by partial melting of Ca-amphibole and plagioclase bearing intermediate igneous and/or metaigneous rocks, and the S-type granites formed biotite and orthoclase bearing rocks (Murata & Yoshida, 1985a). And, the A-type granitic rocks which derived from deep source occurat the southern end of the Outer Zone (Murakami et al., 1989). The distribution of those granitic rocks mainly controlled by the heterogeneity of source materials with different isotopic compositions at the lower crust to upper mantle and their thermal structure. The estimated regional heterogeneity of the source region of the magmas is compatible with the present seismic wave velocity structure in the Outer Zone of Southwest Japan. The middle Miocene igneous activities at the Setouchi and the Outer Zone of Southwest Japan might be triggered by the subduction of hot mantle region. Plate reconstruction at the middle Miocene of the Southwest Japan has done.
日本地質学会の論文
1993-04-30

石鎚コールドロンの形成と中新世テクトニクス

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