常ろ帯のトロクトライトとはんれい岩

Abstract

The Tokoro belt has been known as a tectonic belt characterized by preponderance of Jurassic pillow lavas, hyaloclastites and of radiolarian cherts from superficial look. The thick pile of the effusive rocks is called Nikoro Group being regarded as a middle formation of the Jurassic System. There is however a fact that troctolite and gabbro are recently found in some places of the Tokoro belt but the quantity is extremely scant. For the estimation of the hypabyssal level of the Tokoro belt, the preceding fact is regarded as important on this subject. In this paper, a troctolite from the Tokoro belt was petrographically described as cumulate, and was compared with the gabbro to know the genetic relation of each other. Especially, opaque mineral petrology for the chromite-ilmenite-maghemite paragenesis in the troctolite and the ilmenite-magnetite association in the gabbro made the formative process of these two rocks clear. Consequently, it was forecasted that the gabbro must be derived from the intercumulus liquid of the troctolite. Besides, widely spread effusive rocks of the Tokoro belt, e.g. tholeiitic pillow lavas have been reinvestigated from the view point of its origin and it was concluded that the effusive rocks may be a product of a magmatism related to the formation of the troctolite and the gabbro in question. Judging from the newly obtained geologic data, the presence of Steinmann trinity (ophiolite sequence) is expected at hypabyssal level of the Tokoro belt. As a result, the geologic situation of the Yubetsu Group which has been regarded as a substratum of the Nikoro Group must be reinvestigated.