山陰西部における古第三紀火成活動

DOI 機関リポジトリ Web Site 被引用文献8件 オープンアクセス

書誌事項

タイトル別名
  • Paleogene Igneous Activity in the West San-in District, Southwest Japan
  • 山陰西部における古第3紀火成活動
  • サンイン セイブ ニ オケル コダイ3キ カセイ カツドウ

この論文をさがす

抄録

The Paleogene igneous activity occurred mainly on the side of Japan Sea in the Inner zone of Southwest Japan. The activity was most intensive during Eocene to Oligocene in the western San-in district, where two Paleogene cauldrons of this stage, Yasaka and Hamada cauldrons, were discovered during this study. The Yasaka cauldron is located to the south of Hamada city, Shimane Prefecture and it consists of volcanic formations (Yasaka Group) and several small stocks of quartz diorite (Kitsuga plutonic rocks). The Yasaka Group shows a half-basin structure inclined toward the north, and is divided into three formations Kadota andesite F., Takauchi dacite F. and Nosaka rhyolite F. in the ascending order about 720 meters in total thickness. The Kitsuga plutonic rocks were intruded after the eruption of the Nosaka rhyolite F. mainly in the southern parts of the cauldron. Fission-track age of the constituent volcanic and plutonic rocks of this cauldron is Eocene, which correlates to those of the Sakugi, Takayama and Hakami Group in the central San-in district. The Hamada cauldron, located to the north of the Yasaka cauldron, consists of volcanic rocks (Hamada Group) and various plutonic rocks (Nabeishi quartz diorite, Kumogi granite and other small stocks). The Hamada Group is divided into five formations, i.e., Ino andesite F., Yamaga dacite F., Isari-yama andesite F., Kakinoki-yama rhyodacite F. and Jumonjibara rhyoda-cite F. in the ascending order. The total thickness of this group is about 2020 meters. The caul-dron has a gently dipping basin structure indicating two stages of subsidence, that is, the outer (early) and the inner (late) subsidences. Fission-track ages of the igneous rocks indicate Oli-gocene, correlatives to the Tamagawa and Kawauchi Groups. The Miocene sedimentary basin, which is composed of the Kokubu Group (basaltic andesite to rhyolite) and small stocks of quartz diorite and granite, is situated to the north of the Hamada cauldron. Therefore, it can be said that in the Hamada-Bay district, that the sedimentary basins migrated successively during Eocene to Miocene. Stable isotope ratio of the rocks and minerals in the Hamada cauldron indicates extensive meteoric hydrothermal activity during the formation of the cauldron. The distribution pattern of δ18O values of the rocks, which shows a concentric zoning with the values decreasing inwards, and that of base metals in the cauldron both indicate that the hydrothermal activity was most intensive in the central part of the cauldron diminishing outwards. A hidden pluton, parts of which are exposed as small stocks beneath the midst of the cauldron, might have been the major heat source for the convective circulation of water and for the reactions of the water with rocks. The constituent rocks of the Kumogi pluton is semiporphyritic adamellite, and the pluton appears to have formed through a single phase of emplacement. Concentric zoning was recognized in this pluton in many petrologic, isotopic and mineralogical features as described below. The modal magnetite/biotite ratio, Fe3+ /Fe2+ ratio and magnetic susceptibility of the rock decrease towards the central part, indicating the higher oxidation state in the margin than in the center. The stable isotope ratio is highest in the central part ( δ18O=+F7.0％o) and lowest in the margin ( δ18O=－1.8%o), and its distribution pattern is conformable to that of the oxidation state. These zoning-pattern in this pluton can also be explained by the hydrothermal system established in the Hamada cauldron. In the west San-in, several Paleogene cauldrons are distributed in a linear arrangement with intervals of about 20 km, roughly parallel to the coast line of the Japan Sea. They are named Tamagawa, Masuda, Yasaka, Hamada, Haza, Asahi and Kawauchi cauldrons from west to east. Critical examination on the volcanostratigraphy of these cauldrons revealed the common features of the volcanism associated with their formation. Namely, th

収録刊行物

被引用文献 (8)*注記

もっと見る

詳細情報 詳細情報について

問題の指摘

ページトップへ