関東山地北縁部小川町〜神泉村地域の緑色岩メランジェ

概要

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The Atokura Nappe rests on the Mikabu Greenstone Unit of the Sanbagawa Terrane running along the northern margin of the Kanto Mountains. It consists of, from north to south, Paleogene Yorii acidic rocks, Paleogene Yorii Formation, Permian Kinshozan Granodiorite and Tochiya Formation (equivalent to the Late Cretaceous Atokura Formation in the type area). The southern margin of the nappe is occupied by a greenstone melange (Ogawa-Kamiizumi Melange), in which blocks of several kinds of igneous and metamorphic rocks are contained in sheared greenstones and serpentinite. Whether this melange belongs to the Mikabu Unit of the Sanbagawa Terrane or to the Kurosegawa Terrane of the Chichibu Terrane has long been in controversy. Close examination of petrography and petrochemistry of greenstones and serpentinite, constituting a major and a minor portion of the matrix, respectively, led to two contradictory results : 1) Greenstones are characterized by chemical and mineralogical natures very close to those of the Mikabu Unit. ii) Most of serpentinite bear natures characterisitc of the Kurosegawa Terrane, while a few examples bearing features close to those of the Mikabu Unit also exist. In terms of petrography and petrochemistry as well as the tectonic situation of the Ogawa-Kamiizumi Melange, the interpretation which assigns it to the Kurosegawa Terrane can hardly, if not utterly, be accepted.
2005-02-25