Lateral variation of major and trace elements in the Pliocene volcanic rock of the rock of the Northeast Japan.

第四纪火山岩的碱含量，特别是K_2O、其它大离子亲石元素（LIL元素）和高场强元素（HFS元素），从火山前缘向弧后侧跨越东北日本弧逐渐增加（Kuno，1960; Kawano et al.，1961年;增田，1979年; Fujimaki和Kurasawa，1980年; Fujitani和增田，1981年; Yoshida和Aoki，1984年; Sakuyama和Neskawa，1986年; Nakagawa等人，1988年）。最近，几位工作者根据对本州中部和北方本州到北海道西南部的第三纪火山岩的地球化学调查，提出在晚中新世火山岩中存在包括K、Rb和Sr在内的LIL元素的横向变化（Shuto et al.，1988年;在东北日本弧中，上新世火山作用形成的4 ~ 2 Ma的火山岩分布广泛，与第四纪火山岩分布基本一致（图1）。我们研究了111个岩石样品的主元素和微量元素的丰度从12个上新世火山中心在东北日本弧。基于这些结果和已发表的分析，跨弧成分变化的上新世火山岩将检查比较，在东北日本弧第四纪火山岩。

It has been well-known that the Quaternary volcanic rocks increase gradually in alkali contents, especially in K_2O, other large ion lithophile elements (LIL elements), and high field strength elements (HFS elements) from the volcanic front toward the back-arc side across the NE Japan arc (Kuno, 1960; Kawano et al., 1961; Masuda, 1979; Fujimaki and Kurasawa, 1980; Fujitani and Masuda, 1981; Yoshida and Aoki, 1984; Sakuyama and Nesbitt, 1986; Nakagawa et al., 1988). More recently, several workers suggested the existence of the lateral variation of LIL elements including K, Rb and Sr of the volcanic rocks during the late Miocene time, based on the geochemical investigations of the Tertiary volcanic rocks from both the central Honshu and northern Honshu to southwest Hokkaido (Shuto et al., 1988; Tamura and Shuto, 1989; Okamura, 1987).In the NE Japan arc, the volcanic rocks formed by the Pliocene volcanism from 4 to 2 Ma show a widespread distribution which coincides roughly with that of the Quaternary volcanic rocks (Fig.1). We examined the abundances of major and trace elements for 111 rock samples from twelve Pliocene volcanic centers in the NE Japan arc. Based on these results and the published analyses, an across-arc compositional variation for the Pliocene volcanics will be examined in comparison with that for the Quaternary volcanics in the NE Japan arc.