佐渡島の地質構造発達と火成活動

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An outline of geology of the Sado island is described and geologic succession is shown in Table 1, Structural development and igneous activity during early Miocene in this island are subdivided into four stages as follows; 1) Fracturing of the basement complex, which is composed of the Paleozoic formation aud Cretaceous granitic rocks, occured in two directions of NW-SE and NE-SW, and then dacite erupted along the fractures. The Nyukawa formation, mainly composed of dacitic pyroclastics, is a product of such volcanism of the earlist stage. 2) Upheaval of the basement and piles of dacitic rocks of the Nynkawa formation and subsidence of parts between the above two units took place at the beginning of the second stage. The development of the sedimentary basins in large scale proceeded accompanied with violent volcanism of andesite (propylite). Thick conglomerates, mainly composed of granite boulders and named as the Katabe conglomerate, deposited at western margin of the basin. The Aikawa formation is sediments filling up this basin and is composed of large amounts of andesite lava and pyroclastic and small amounts of clastic sediment. In the Kosado area, various kinds of andesite lava, namely trachytic andesite in subalkalic composition and basaltic andesite and glassy andesite in calcalkaline composition erupted. The Sanze formation is mainly composed of these lavas and agglomerates and pyroclastics. The sedimentary basins of the Aikawa and Sanze formations are elongated to the direction of NE-SW. 3) Eruption of andesite gradually succeeded to that of dacite in some parts of the above-mentioned basins and the sedimentary basins shifted to the outsides, but in other parts, such as northern part of the Osado area, the sedimentary basin was newly formed by distinct depression. Large amounts of pyroclastic flow of dacite composition poured and filled up this basin. Volcanic activity of dacite was accompanied with eruption of various kinds of andesite lavas, almostly calcalkaline in composition. The Masaragawa and Toyooka formations are products of these volcanic activities. 4) After deposition of thick pyroclastic flow sediments, the sedimentary basin (or field of volcanic activity) shifted eastward or southwestward in the Osado area and northwestward in the Kosado area, and eruption of dacite lava took place. The Kimpokusan and Kyoznkasan formations are products of the eruption. Eruption of hypersthene andesite and perlite took place in later stage of activity of this period. Although uplift and subsidence were repeated since first upheaval of basement, from the deposition of the Masaragawa formations to those of the Kimpokusan formations, upheaval of most of the Osado and Kosado areas advanced and small massses of holocrystalline rocks such as quartz diorite, porphyrite and gabbro intruded at several parts, especially in the "Ooda-Toji tectonic zone" elongated to direction of NW-SE. This tectonic zone probably reflect the tectonic framework of the basement. The Orito formation, which shows the first transgression in the Sado island, fringes the upheaval area with the direction of NE-SW of so-called Sado direction. Andesites and dacites of the Sanze, Masaragawa, Toyooka, Kimpokusan and Kyoznkasan formations are chemically analyzed. Chemical compositions are plotted on MgO-(FeO+Fe_2O_3)-(Na_2O+K_2O) triangle diagram and SiO_2-Na_2O or K_2O variation diagrams. As shown in these diagram, most of rocks are belong to the hypersthenic rock series by KUNO (Calcakaline rock series), but Na_2O and K_2O (in weight percent) of the rocks are generally higer than those of the rocks of other areas of early Miocene Volumes of volcanic materials of each stages roughly estimated are shown in Fig. 7.
日本地質学会の論文
1973-07-01

佐渡島の地質構造発達と火成活動

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