[Abstract]

Akita-Komagatake volcano is situated at about -*km west of volcanic front in the northeastern Japan arc. Development history of the volcano is divided into - stages ; main strato cone, syn-caldera formation, and post-caldera eruption stages, in descending order. In order to reveal the comprehensive eruption history in the post-caldera stage of the volcano, correlation between eruptives constituting the volcanic edifices and dispersed air-fall tephra are investigated, along with re-examining geology of the volcanic edifices formed in the concurring stage. Petrological a$nities such as whole rock major-element chemistry and mineral assemblages including modal abundances of phenocrystic phases are utilized in identifying the contemporaneous eruptives of various modes of emplacement. To avoid the e#ect of phenocryst-matrix separation, coarse-grained tephra samples were selected to the whole rock chemical analysis.
The revealed outline of eruption history is as follows : Tholeiitic magmatism has been dominant through the stage, with episodic calc-alkalic magmatism at the final activitiy in the northern region (ca. 4,000-3,100 y.B.P.). Relatively large eruptions occurred at the beginning of the stage (ca. 13,000 to 10,000 y.B.P.), resulting in caldera and probably large horse-shoe shaped crater as well, rather than forming volcanic edifices. Formation of the northern volcanic edifice began ca. 10,000 y.B.P. Five pyroclastic cones were built up successively, and subordinate lava flows or pyroclastic flows accompanied with them. Composition of magma was abruptly changed from andesitic to basaltic around 8,000 y.B.P., and the basaltic volcanism lasted to 7,000 y.B.P. After ca. 3,000 years of dormancy, phreatic explosions occurred successively, probably giving rise to horse-shoe shaped crater at the center of the northern area. Calc-alkali andesitic magma e#used to build up a small pyroclastic cone immediately after the major explosion event. After ceasing the magmatism in the northern area, volcanism in the southern area was initiated by phreato-magmatic explosion in the floor of the south caldera about 2,500 y.B.P., followed by vulcanian eruptions, lasting until ca. 1,600 y.B.P. About 1,500 y.B.P., eruption center was localized to two major vents at northern part of the caldera. Repeated eruptions from the vents have built up Medake and Kodake cones and their associated lava flows. The latest eruption occurred at the summit of Medake in 1970 AD.

Key words:

Akita-Komagatake vocano, eruption history, tephra, basaltic volcanism, island-arc tholeiite