山陰西部における古第三紀火成活動

Paleogene Igneous Activity in the West San-in District, Southwest Japan

今岡 照喜

廣島大學地學研究報告

Geological report of the Hiroshima University

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, the volcanism is divided into
five stages; I (basalt and andesite), II (dacite),III (andesite), IV (rhyodacite) and V (rhyolitc). The succession of the volcanism occurred in environments changing successively from subaqueous at the early stage to dry land at later stage. Volcanic formations in each cauldrons are arranged stepwise northward from the lower formation upward. In other words, they have the character of sedimentary imbricate structure. Moreover, the early Miocene sedimentary basins in the San-in district are exposed, in the northern parts of the Paleogene basins. Therefore, both volcanism and sedimentation developed with northward polarity during Eocene to Miocene.
Petrographic features of the igneous rocks of Paleogene cauldrons have been clarified. 1) Granites are characterized by the conspicuous, semiporphyritic texture with ovoid plagioclase-phenocryst mantled by alkali feldspar and by the low grain-contact ratio of quartz grain. 2) The igneous rocks belong to the calc-alkaline rock series, and they are high in MgO and Na2O content and in the Fe2O/FeO and MgO/FeO* ratios and low in Al2O3 and incompatible elements such as K, Li, Pb and Rb and in the K2O/Na2O ratio. Content of SiO2 varies widely from Ca 50 to 78% and its distribution on the frequency diagram is bimodal. 3) The magnetic susceptibility of the igneous rocks is mostly higher than 50 X 10-6 emu/g and therefore they belong to the magnetite-series. 4) The constituent minerals have the following general features: i) all mafic minerals are high in mg value 〔Mg/(Mg + Fe + Mn)〕, ii) pyroxenes show only a small variation of chemical composition, iii) amphiboles are poor in Al2O3 and belong to magnesio-hornblende, actinolitic hornblende and actinolite, iv) biotites are poor in Al2O3 and rich in TiO2 v) ilmenites are rich in pyrophanite and hematite molecules, vii) chromian spinels in volcanic rocks show high Cr/Al ratio, viii) pyrites and magnetites are closely associated each other, but pyrrhotite and ilmenite association is not observed and ix) alkali-feldspar in plutonic rocks are poor in Or-content. These data strongly suggest that the Paleogene plutonic rocks emplaced at shallow level of the crust and that the most of the Paleogene volcanic and plutonic rocks crystallized at higher temperature and higher oxygen and sulfur fugacities than the Cretaceous ones.
The Paleogene (Eocene to Oligocene) igneous activity occurred in a transitional period between the Cretaceous large-scale felsic magmatism and Miocene "Green Tuff" Movement. The Paleogene igneous activity is very different from the Cretaceous activity in regard to the mode of emplacement of magma, volume and composition of magma, succession of volcanic formation, orientation and shape of igneous bodies (for example, batholith or cauldron) and petrographic features. The Eocene to Oligocene activity resembles that of the "Green Tuff" in many respects, although there are differences between the two. The migrationpattern of the basins and above information suggest that the Eocene to Oligocene igneous activity associated with the formation of the cauldron was a forerunner of the "Green Tuff" Movement and that the Eocene to lower Miocene igneous activity occurred under the different tectonic framework from that of Cretaceous.

日本語

廣島大學理學部地學教室

[ISSN] 0073-2303

[NCID] AN00213676