日高変成帯の超塩基性岩 I : 幌満超塩基性岩とその産状について Ultrabasic Rocks in the Hidaka Metamorphic Belt, Hokkaido, Japan I : Mode of Occurrence of the Horoman Ultrabasic Rocks
In the Hidaka metamorphic belt, Hokkaido, Japan, fresh ultrabasic rocks occur as lenticular bodies or sheets, at several places along the western thrust fault. These ultrabasic intrusives are characterized by the clearly layered structure, composed of dunite, peridotite (or harzburgite), plagioclasebearing peridotite (called by the name of plagioclase peridotite in this paper) and a little amount of gabbro with varying thickness. The Horoman ultrabasic intrusive body is the largest one of them, occupying about 80km^2 and is typical in its layered structure. Various kinds of layering are seen in this body; large scale cryptic layering, of which we describe chiefly in this paper, rhythmic layering on various scales, fine scale 'bands', and mineral layers on a very fine scale. Cryptic layering develops over a whole body from the bottom to the top of the mountain in this area. One of its characteristic features is that each layer of dunite, peridotite and plagioclase peridotite (average thickness of each layer is about 50m) appears repeatedly at several times throughout the whole body, especially distinct in the lower part of the body, with such a regular sequense as follows ; dunite→peridotite→plagioclase peridotite→peridotite→dunite→. As a whole, however, the rocks rich in olivine predominate in the lower part of the body, and the rocks rich in plagioclase in the upper part. Texture of the Horoman ultrabasic rocks is generally mosaic in aggregate of xenomorphic olivine. In more detail, however, it is composed of xenomorphic large-grained and fine-grained parts, which latter develops around pyroxene grains. The proportion of the latter part becomes greater with increase of pyroxene. Mineral layers with a few milli-meter thickness in the Horoman rocks are formed by such minerals as spinel, pyroxenes and plagioclase and usually parallel to the cryptic layer mentioned above. From the layered structure and lineation indicated by the arrangement of spinel and pyroxenes on the mineral layers, it is assumed that the Horoman ultrabasic intrusive body is a gently warped sheet.
地球科学 1966(87), 21-29, 1966