三波川帯と秩父帯との境界にある変成岩類 : いわゆる"みかぶ帯"の岩石 The Basic Metamorphic Rocks at the Boundary between the Sambagawa Metamorphic Belt and the Chichibu Unmetamorphosed Paleozoic Sediments
A brief descriptions are given of the metamorphic rocks belonging to the "Mikabu Green Rocks" which are effusive-intrusive complex of ultrabasic and basic composition, and were recrystallized during the Sambagawa metamorphism. The Mikabu Green Rocks occur at the boundary between the Sambagawa Metamorphic Belt and the Chichibu Belt consisting of unmetamorphosed sediments. Most of the original rocks of the Mikabu Green Rocks are considered to be the materials which have been produced by the submarine volcanisms in geosyncline. The survived original structure and the relic minerals as well as textures were used to speculate on the iroriginal rocks. They are divided in to the effusives and intrusives, conveniently. The intrusive varieties are represented by metagabbros, diabases, ultra-basic and ultramafic rocks. The metagabbros are relatively coarse-grained and always contain pumpellyite, whereas diabases are usually considerably finer-grained rocks and always contain epidote. The effusives are represented by lavas and pyroclastics of basaltic composition. In the regions of Sanagochi and Osugi, metamorphosed pillow breccias which indicate the submarine lava eruptions, crop out over wide area in a eastwesterly trending block, overlain on the south by unmetamorphosed Paleozoic sediments of the Chichibu Series (Fig. 1). The following criteria assist recognition of the Mikabu Green Rocks. (1) They occur at the boundary between Sambagawa Metamorphic Belt and the unmetamorphosed Paleozoic Sediments. An anticlinal structure lies at this boundary and is thought to be a geanticline (oceanic ridge ?) in geosyncline belt, and the topography of the rises is thought to be of volcanic origin. (2) They were recrystallized weakly and have frequently distinct relic structure and relic minerals. (3) The rocks of basaltic composition are predominant. The composition of original rocks would have been characterized by a high MgO and FeO contents and a low K_2O content.
地質学論集 (4), 41-50, 1969