Petrology of Neogene Tertiary basic volcanic rocks of northeastern Japan

Abstract

The Neogene volcanic activity of the Sendai area can be divided geologically into the following three groups: lower Miocene series (Takadate volcanics), upper Miocene series (Mitaki volcanics), and Quaternary volcanoes. The Takadate volcanics are composed of highalumina basalt series basalt and andesite, while lower half of the Mitaki volcanics is composed of high-alumina basalt series basalt and andesite and upper half is composed of tholeiite series basalt and andesite. These activities were accompanied calc-alkaline series. The chemical analyses of 15 representative rocks from the Takadate volcanics and lower part of the Mitaki volcanics, including 14 new analyses, are given in Table 2. There are conspiquous difference between the fractionation trends of both high-alumina basalt series and tholeiite series; the former shows moderate iron enrichment at the middle stage of crystallization, whereas the latter shows strong iron concentration (Aoki, 1967). It is noticed that the rock series of the Mitaki volcanics were changed abruptly from high-alumina basalt series to tholeiite series during the succesive eruption. From petrological study, it is concluded that when crossing the northeastern Japan from the Pacific to the Japan Sea side, tholeiite, high-alumina basalt, and alkali basalt provinces is distinguished for the upper Miocene to Recent volcanic activities. However, there is no tholeiite province in the lower to upper Miocene volcanic activities.