山陰東部の白亜紀～古第三紀深成岩類の地質学的一岩石学的研究

Geological and Petrological Studies on the Late Cretaceous to Paleogene Plutonic Rocks in the Eastern San-in District, Southwest Japan

SAKIYAMA Tohru

廣島大學地學研究報告

Geological report of the Hiroshima University

Late Cretaceous to Paleogene plutonic rocks are widely distributed in the eastern San-in district, Southwest Japan. Field occurrences and isotopic ages of these rocks have revealed three stages of plutonism ; that is, those associated with (1) Mochigase intrusive rocks (late Cretaceous), with (2) Imbi intrusive rocks (the latest Cretaceous to early Paleogene), and with (3) Namariyama intrusive rocks (middle to late Paleogene). These plutonic rocks are studied
geologically and petrologically in this paper.
The activity of the Mochigase intrusive rocks had begun with intrusion of gabbroic to tonalitic rocks. A later activity produced a batholith which consists mainly of light-colored granitic rocks and are correlative in K-Ar age with the Hiroshima granite in the San-yo zone. The Mochigase-Koshihata granitic mass, a representative example of the Mochigase intrusive rocks, is a batholith composed mainly of biotite adamellite (Mochigase granite) and hornblende-bearing biotite adamellite with mega-crysts of alkali-feldspar (Koshihata granite). Slope of the contact planes between the country rocks and the Mochigase granite are gentle generally, and in some places the country rocks overlie the granite as roof pendants. Abundance and chemical composition of Fe-Ti oxides and R2O3 contents of ilmenites suggest that the outer part of the Mochigase granite formed under lower oxygen fugacity must have decreased toward the later stage of crystallization.
The activity of the Imbi intrusive rocks had begun with the intrusion of small stocks composed of gabbro to quartz gabbro, following intrusion of granodioritic masses some of which formed zoned-plutons, and ended with intrusion of batholiths composed mainly of light colored adamellite. The Okutsu granodioritic mass is a typical zoned-pluton of the Imbi intrusive rocks, and this mass consists of medium-grained hornblende-biotite granodiorite, medium-grained biotite adamellite, fine-grained biotite granite and aplitic granite toward the center of this mass. These facies are generally intergradational but are in sharp contact occasionally. Color index of the rocks and An-content of plagioclase suggest that the medium-grained granodiorite and adamellite formed by a differentiation in situ from granodioritic magma after its emplacement and that the fine-grained granite and aplitic granite originated in residual liquid which had been squeezed into the center of the mass. The Ogamo-Ningyotoge granitic mass, a representative batholith of the Imbi intrusive rocks, consists mainly of coarse-grained and light-colored adamellite and is divided into three stratified intrusive units, I, II and III, in ascending order. The unit I is composed mainly of coarse-grained hornblende-biotite granodiorite with megacryst of alkali feldspar, unit II of coarse- to medium-grained biotite adamellite, and unit III of coarse-grained and light-colored biotite adamellite. The unit II has intruded into the other units, bu the interval between their emplacements was presumably short. Boundary of each unit and arrangement of dark inclusion in the mass show a harf-basin structure plunging to north, and this suggests that the mass is a sheet-like or a funnel-shaped body.
The Namariyama intrusive rocks are composed of granophyre, plagiophyre, granite porphyry and fine-grained granite, all with hypabbysal textures. The Namariyama granophyres, a representative example of the intrusive rocks, consist of minor stocks and dykes which form a volca-no-plutonic complex with the Kijiyama volcanic rock (andesite to dacite). They are divided into facies 1 and 2 which tend to form the outer and inner parts of each pluton, respectively. Facies 1 is mainly hornblende plagiophyre with or without clinopyroxene, orthopyroxene and biotite, whereas facies 2 is mostly hornblende-biotite granophyre and hornblende quartz diorite porphyry. Based on the chemical composition of Fe-Ti oxides and hornblende and on the crystallization sequence of biotite, it is inferred that (1) phenocrysts crystallized under an oxygen fugacity slightly higher than the NNO buffer, that (2) the oxygen fugacity in the granophyres increased with crystallization of minerals, and that (3) the intrusion depth was shallower than that corresponding to PH2O = 1 kb. Also disequilibrium assembladge and reverse zoning of plagioclase and pyroxene suggest that (4) the two types of magma had mixed at an early stage to form a magma-from which the Namariyama intrusive rocks were derived. Compared with the Imbi and Namar-iyama intrusive rocks, the Mochigase intrusive rocks characterized by lower MgO/FeOt in the whole rocks, absence of magnetite, lower R2O3 content in ilmenite, higher AlIV and total Al and lower Ti and mg-value in biotite, and lower Ti and mg-value in hornblende. These characteristics are same as those of the Hiroshima granite not only in their K-Ar ages but also in their pet rological features. This strongly suggests that the Cretaceous plutonism extended over the San-in zone and that the distinction between the San-yo and San-in zones were not present during the Cretaceous time.
Lower R2O3 content in the ilmenite and lower mg-value in the biotite and hornblende in the Mochigase intrusive rocks indicate that they formed under a lower oxygen fugacity than did the Imbi and Namariyama intrusive rocks. It is inferred from the R2O3 contents in the ilmenite and from two-feldspar geothermometer that the oxygen fugacity in the Mochigase intrusive rocks was lower than the QFM buffer. Moreover, mg-values of hornblende and biotite suggest that the oxygen fugacity was similar between the Imbi and Namariyama intrusive rocks, but that the fugacity become higher in the latter than in former towerd the later stage of crystallization. Al2O3 and SiO2 activities are estimated from the chemical variations of horn blendes and biotites. SiO2 activity related to the Al2O3 activity increased with an advance of crystallization in the Imbi and Namariyama intrusive rocks, because Al rich hornblende and quartz crystallized at an early stage and at a late stage, respectively. Al2O3 activity increased with advance of crystallization in the Mochigase intrusive rocks and the plutonic rocks in the San-yo zone, because pyroxene and quartz both crystallized at early stages. These differences in the crystallization sequence are presumably due to the difference in H2O content in the magma; that is, the Imbi and the Namariyama intrusive rocks were perhaps from wetter magmas than did the Mochigase intrusive rocks and the plutonic rocks in the San-yo zone.

jpn

廣島大學理學部地學教室

[ISSN] 0073-2303

[NCID] AN00213676