Dokusawa Tephra Erupted at Approximately 100 ka and Its Eruptive Process in the Central Part of Northeast Japan

Abstract

Kitahara tephra (Kth) and Dokusawa tephra (Dks) are Late Pleistocene tephra layers containing biotite and cummingtonite characteristically. Kth is distributed on the eastern side of the Ou Ranges and Dks is on the western sides of the Ranges in the central part of Northeast Japan. Vertical variations of modal amounts, refractive indices and major element chemistry of minerals and mineral assemblages were examined on Dks and Kth. Cummingtonite shows nearly constant variation in the refractive indices and Mg-values [Mg/(Mg +Mn+ Fe)]. On the contrary, the refraetive indices and Mg-values of orthopyroxene and hornblende have wide variations. Dks and Kth correlate with each other because the refractive indices and chemical composition of cummingtonite are quite similar. "Atago tephra" containing a lot of high-quartz can be correlated with the lower unit of Kth because high-quartz is included in both tephras. Dks comprises cummingtonite, biotite, high-quartz and epidote as a whole layer. The upper part of Dks also includes orthopyroxene, clinopyroxene and hornblende. The mineral composition of Kth resembles the upper part of Dks and does not show vertical variation. These facts indicate that the upper part of Dks is distributed on both sides of the Ou Ranges.