幌加内峠地域神居古たん帯の塩基性岩類の変成作用と産状

Abstract

The Kamuikotan tectonic belt is a melange zone in which occur various kinds of metamorphic rocks of high pressure and low pressure types. In the Horokanai district, central Hokkaido, high pressure type metamorphic rocks are widely distributed. They can be classified into four kinds, metasediments, low grade metabasites, epidote amphibolites, and metachert based upon the original rocks and mineral parageneses. A large part of the metasediments consist of black siliceous shale. The original rocks of the low grade metabasites are spilitic tuff, pillow lava, massive lava and gabbro, and those of the epidote amphibolites are basaltic tuff, pillow lava and massive lava. The low grade metabasites, epidote amphibolites, and metachert are scattered as lenticular bodies of various sizes in metasediments. The modes of occurrence suggest that the lenticular bodies of basic rocks and chert were emplaced as olistoliths. The mineral assemblage of the metasediments is quartz+white mica +chlorite+ graphite±lawsonite±albite+metamorphic aragonite. The low grade metabasites are characterized by a widespread occurrence of lawsonite, pumpellyite, alkali amphibole, alkali pyroxene, and metamorphic aragonite. These parageneses indicate that the metasediments and the low grade metabasites suffered metamorphism of glaucophane schist facies. On the other hand, the epidote amphibolites display relic epidote amphibolite assemblages which have been partly or entirely modified by a later metamorphism of glaucophane schist facies. The emplacement of epidote amphibolites may be explained as follows. Their original rocks were a component of the oceanic crust which was underthrust beneath the older crust in a subduction region during Mesozoic time. Rock slices were scraped off and accreted to the pile composing of older slices. The slices were metamorphosed up to epidote amphibolite facies assemblages during subduction processes. Underthrusting and accretion of the slices were accompanied by concurrent intrusion of serpentinite with fragments and blocks of the rocks in the deeper part of the crust. The uplifted serpentinite and metamorphic rocks were exposed and suffered denudation. The blocks and clasts of the metamorphic rocks were reworked by subaqueous sliding to form an olistostrome with clastic sediments. Later all the rocks of the area suffered a high pressure type metamorphism and thus the rock assemblage observed presently in the Kamuikotan tectonic belt was formed.