第三紀火山岩の変質の一形式 : グリーンタフの変質に関する研究(その1)
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概要
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The green tuff formation, early and middle Miocelle in age, is widely distributed in the so-called green tuff region, namely the inner zones of the Kuril and the Honshu arcs and the Fossa Magna region. It is characterized by a large amount of volcanic and pyroclastic rocks contained in it and by intense alteration of these volcanic products. The alteration is widely occurred and is called "regional alteration" as pointed out by M. GORAI(1956). It is noteworthy that the alteration of volcanic rocks intercalated in non-pyroclastic sediments seems to be different in mineralogical characters from those of thick and large volcanic masses without normal sediments. From the stratigraphical point of view the lower part of the formation is more intensely altered than the upper one. In this report an example of the alteration of volcanic rocks intercalated in normal sediments is described and discussed. An andesite flow intercalated in tuffaceous sandstone and mudstone of the middle Miocene series is cropped out at Reihige in the Yoshika district, Hokkaido. Its lower part is altered. The altered zone along the contact plane with the under-lying tuffaceous sandstone is about 40 cm in width and is gradually changed, through weakly altered zone of about 80 cm in width, to non-altered zone of two pyroxene andesite containing glass in groundmass. The rock of the non-altered zone seems to be fresh at first sight, but it is slightly altered. In the weakly altered zone, orthopyroxene phenocryst is completely replaced by iron rich saponite, the alteration of clinopyroxene being partly. Plagioclase phenocryst is slightly altered to sodic one. In the altered zone, both pyroxenes are completely replaced by iron rich saponite, and some of plagioclse are altered to more sodic one, oligoclase to albite. The calcic inner part of zoned plagioclase is altered, though the outer part is unaltered. Lath shaped plagioclase of groundmass is unaltered, and some of intergranular pyroxene are altered. Groundmass glass is devitrified and transformed to the minerals which seem to be iron rich saponite and albitic plagioclase. These minerals occur as porphyritic patch including original plagioclase and pyroxene. In this alteration the original texture is well preserved. Chemical compositions of the specimens from the non-altered to the altered zone are given in Table 3. Fig. 5 shows the variation of the cation percents of the specimens. The change from A to D is characterized by a decrease of Si and Fe^<++>, and an increase of Al, Na, Mg and Fe^<+++>, the change of Ca being not so pronounced. That from D to E, however, differs from the tendencies as mentioned above, except Na. This fact indicates that the alteration process of the outer altered zone (E) is somewhat different from the inner zone (A→D). It may be possible that the rock of the zone (E) was affected by polyalteration and was chemically reorganized. The geologic relations of the present district indicate that the thick sediments of sandstone and mudstone were deposited on the formation including the andesite in question under marine environment. From the characters of alteration products the alteration is less intense than the so-called propylitization. It is concluded that the sea water contained in the surrounding sediments played an important role in the alteration and the alteration process was operated in the period of the subsidence of the formation including the volcanic rocks. Therefore, the alteration in question is considered as a kind of diagenesis.
- 地学団体研究会の論文
- 1960-11-25