御坂山地,古関-高萩地域の地質および変成作用

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The Neogene strata developed in the Furuseki-Takahagi area of the Misaka Mountains are divided into the following three formations; Furuseki, Ashigawa and the Ichikawadaimon formations, in ascending order. The surveyed area is also subdivided into three areas by the Misawagawa and Kubo faults. These areas are the southern, northern and western parts. Although the strata, which are distributed respectively in the above-mentioned three areas, show different rock facies and are separated by the faults, some of the strata can be correlated by their characteristic rock facies, and the stratigraphic succession mentioned above can be established. The Furuseki formation is chiefly composed of basaltic and andesitic pyroclasts. The Ashigawa formation includes several members as follows; the Nakayashiki mudstone and Sambousan tuff member in the southem part, the Takahagi basalt and Obina basalt and mudstone member in the northern part, and the Kumazawa dacite member in the western part. The Takahagi basalt member is composed of massive lava, pillow lava and pyroclasts of tholeiitic and alkali basalt composition. The Ichikawadaimon formation includes several members as follows; the Hida volcanic rock, the Sekihara-toge andesite and the Ubakuchi dacite members in the northern part, and the Chikahagi tuff member in the westem part. Many dolerite dykes and some quartz diorite-gabbro bodies occur in this area. Some of the dolerites are alkalic, and others are tholeiitic or calc-alkalic. These dolerite dykes are intruded in directions of EW or NE-SW, which are the main structural trend in the area. Several anticlines and synclines of NE-SW direction develop in the southern part, but the structure of the northern part is monoclinal except the Hida anticline. The rocks of this area are regionally metamorphosed into altered rocks ranging from the zeolite facies to the green schist facies through the prehnite-pumpellyite facies. Moreover, they are thermally metamorphosed by the quartz diorite mass in the eastern margin of this area, and are transformed into the hornfels of the amphibolite facies. The following six zones of progressive metamorphism are established in this area on the basis of stability relation of some metamorphic minerals; Zone IV : epidote-hornblende zone, Zone IIIb : prehnite-pumpellyite-chlorite zone, Zone IIIa : prehnite-pumpellyite-laumontite-chlorite zone, Zone II b : laumontite-heulandite-prehnite-mixed layer zone, Zone IIa : analcime-mordenite-heulandite-mixed layer zone, Zone I : heulandite-mordenite-sa ponite zone. Wairakite and yugawaralite occur locaily in the zones IIb and IIIa. Gyrolite and truscottite occur locally in the zone IIa. These minerals occur rarely in Japan. Epistilbite occurs in many localities of the zones I and II b. This mode of occurrence is different from that in other low-grade metamorphic terrains in Japan. The rocks of the zones IIa and IIb are widely distributed in this area. The rocks of the zone IIIa, which belongs to the transitional facies between the zeolite and prehnite-pumpellyite facies, show an isolated distribution. The zone I is restricted, being found in the Takahagi area. From the pattern of distribution of these zones, the following scheme may be drawn concerning the metamorphic history of this area. 1) Deposition of submarine basaltic and dacitic rocks and mudstones. 2) Formation of the rocks of the zones I to IIb ascribed to regional burrial metamorphism. 3) Formation of the zones IIa and IIIb ascribed to local rising of temperature (upheaval and/or quartz diorite intrusion). 4) Formation of the zone IV due to contact metamorphism. The rocks of the zone I, however, may have been transformed by diagenesis after the eruption of basalt such as is known in the case of the Mase basalts (SHIMAZU and KAWAKAMI, 1967) and Ireland basalts (WALKER, 1960).
日本地質学会の論文
1976-12-30

御坂山地,古関-高萩地域の地質および変成作用

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