東北地方を中心とした"グリーン・タフ時代"の火成活動(<特集>日本列島の構造発達史-とくにグリン・タフ時代を中心として(その1)-)

抄録

The so-called green tuff region in the northeastern district of Japan originated as rift-grabens at the beginning of the Miocene epoch. The main rift-grabens are parallel to the Honsyu arc, and their formation is closely connected with the deep-reached faults, resulting from the development of the island arc. And the successive igneous activities in the green tuff region were controlled and characterized by these arc structures. The geosynclinal evolution of the green tuff region can be divided into three stages. In the initial stage (early Miocene), tholeiite basalts were related to the upheaval of (or not-subsiding) areas, and intermediate〜acidic, calc-alkali rocks to the subsidence of the rift-grabens. In the second stage (middle Miocene〜Pliocene), contrary, the subsidence of the western half of Northeast Honsyu was accompanied with regional intrusion and extrusion of dolerites or basalts (principally tholeiitic). On the other hand, the central zone of Northeast Honsyu was subjected to the geanticlinal movement since the Onnagawa stage (middle Miocene), and the movement was accompanied with violent extrusion of dacites. The final stage (Quaternary) was characterized by the volcanic activities of the Nasu and the Tyokai zones. In the latter zone, basalts〜andesites (tholeiite and calc-alkali rock series) extruded just on the zone of upheaval of horst type, and dacites or rhyolites were connected with tectonic depressions or calderas. In general, acidic rocks (including "Tertiary granite") are closely related to those areas, where two arcs intersect, and where a rift-graben crosses the area of negative gravity anomalies. Alkali rocks are found on the concave (continental) side at a certain distance from the front of the island arcs. Origin of these rocks is discussed.