Study on the identification of Quaternary tephras in southern Kyushu based on EPMA analysis

本研究旨在利用EPMA分析九州南部第四纪火山岩玻璃碎片的化学成分，并对这些火山岩进行对比和年代学研究。本研究以迄今为止尚未有系统研究的吐噶喇群岛在内的萨都南群岛晚更新世和全新世火山灰岩为研究对象。EPMA分析明确了吐噶喇群岛的Kuchierabujima、Kuchinoshima和Akusekijima岛的大型普利尼亚火山熔岩和药师驹岛的中更新世标志火山熔岩的化学性质，并通过对与火山熔岩特征相关的其他火山熔岩的鉴定和年代学研究，阐明了地貌演化和古环境历史。主要结果如下。根据化学性质，可区分在Kuchierabujima、Kuchinoshima和Akusekijima群岛的三个标志性浮石福尔斯瀑布。富Si02的火山岩均为流纹质火山岩，是普林期大喷发的产物，表明其分布于远离火山岛的萨都南群岛、大隅半岛和萨摩半岛。屋久岛中更新世的镜质火山灰安波火山灰和巨大的火山碎屑流小濑田火山碎屑流的玻璃碎片具有不同的化学性质，表明这些火山灰在日本本岛及其周边地区广泛存在。

This study airas to characterize Quaternary tephras in southern Kyushu based on the chemical compositions of their volcanic glass shards using EPMA, and to get fundamental data on the correlation and chronology of those tephras. In this study, I focused on the late Pleistocene and Holocene tephras of Satsunan Islands including Tokara islands, in which there have been few systematic tephrochronological studies. EPMA analysis made it clear the chemical properties of large plinian tephras in Kuchierabujima, Kuchinoshima and Akusekijima Islands of Tokara Islands, and of middle Pleistocene marker tephras in Yakushioma Island.In addition, Landform evolution and palaeo-environment history were clarified based on the identification and chronolog on other tephras relevant to tephra characterizations of those tephras. The main results are as follows. Three marker pumice falls recognized in Kuchierabujima, Kuchinoshima, Akusekijima Islands, can be distinguished based on their chemical properties. Those tephras rich in Si02 are all rhyolitic and the products of big plinian eruptions, suggesting that those tephras are distributed over Satsunan Islands, Osumi and Satsuma peninsulas far distant from the volcanic islands. Middle Pleistocene vitric ash-Anbo tephra-and gigantic pyroclastic flow-Koseda pyroclastic flow-in Yakushima Island have distinct chemical properties in their glass shards, suggesting that those tephras can be identified in extensive area across and around the Japanese Island.