Studies on the Green Tuff Formation in the Futamata Region nearby Toyoha Mine, Southwest Hokkaido, Japan : on Propylite and its Properties of Alteration, and its Geological Significance

Abstract

The Green Tuff Formation, the Neogene Tertiary system, distributed in the Futamata region is altered through hydrothermal processes in a certain degree. By secondary minerals, the present formation is divided into three alteration zones as follows; pale green chlorite zone, yellowish green chlorite zone and saponite zone. The alteration-grade changes from higher to lower in the above mentioned order. The alteration may be surely caused by hydrothermal activities in a certain postaction stage, after the successive volcanic and sub-volcanic activities, probably in the upper middle or lower upper Miocene period. As for propylites, of being developed in the Futamata region, they are fundamentally intrusive rocks and erupt through geotectonic zone. Their activities occur, when the sedimentary basin is in the motion from sinking to upheaval, furthermore, take the lead of the following igneous activities occurring in geoanticlined part. Especially, propylites are generally closely associated with the Tertiary hollocrystalline rocks, such as diorite-porphyrites. For the purpose of making clear the relationships between alteration-grade and compositional change of altered plagioclases, secondary albitized plagioclases were investigated. The optical type, either plutonic or volcanic type, of secondary albitized plagioclases is just the same as that of the initial plagioclases. Compositional changes of plagioclases through albitization may suggest those of the definite or rather indefinite ranges judging from their frequency curves. As for the relationships between twin lamellation on the albite-law and zonal structures, both become gradually vague in appearance, as albitization goes on. However, zonal structures are entirely disappeared on account of intense albitization, then the albite twinnings are comparatively sharply recognized in appearance. The compositions in this stage are nearly equivalent to An20-23 for plutonic type and also to An32-33 or An22-23 for volcanic type plagioclases. Albitization going on moreover, twin lamella on the albite-law becomes again obvious in appearance.