Late Miocene volcanic rocks (Geiyo volcanic rock) and early-middle Miocene volcanic rocks (Setouchi volcanic rocks) from the Geiyo Islands in the western Setouchi region, southwest Japan
Late Miocene volcanic rocks (Geiyo volcanic rock) and early-middle Miocene volcanic rocks (Setouchi volcanic rocks) from the Geiyo Islands in the western Setouchi region, southwest Japan
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日本西南部濑户内地区西部的艺予群岛的晚中新世火山岩（艺予火山岩）和早中新世火山岩（濑户内火山岩）
DOI：
10.2465/gkk.29.20
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发表时间：
2000
期刊：
影响因子：
--
通讯作者：
H. Matsuura
中科院分区：
文献类型：
--
作者：
Mamoru Seno;H. Matsuura
在日本西南部濑户内西部的盖世岛发现了中新世火山岩。它们以岩脉或火山颈的形式出现，边缘有火山碎屑岩。火山岩由高碱性拉斑橄榄玄武岩、橄榄安山岩和青铜安山岩组成，其中大葛岛橄榄安山岩在化学上与高镁安山岩相对应。对橄榄玄武岩、橄榄安山岩和青铜安山岩进行了全岩K-Ar年龄测定。大石岛的橄榄岩安山岩和冈村岛的铜质安山岩的晚中新世年龄分别为8.2±0.7 Ma、7.8±0.4 Ma和8.0±0.4 Ma。由此可见，西濑户内地区的亚碱性火山活动主要发生在晚中新世。据此，我们将邻近地区报道的晚中新世高碱拉斑橄榄玄武岩等火山岩命名为“盖世火山岩”。大比岛和大尾岛的橄榄玄武岩的中中新世年龄分别为12.8±0.6 Ma和15.4±0.8 Ma。大鹿岛的橄榄安山岩年龄为17.4±0.9 Ma，为早中新世。根据放射性年龄、岩石学和岩石整体化学特征，认为这些火山岩属于濑户内火山岩。
Miocene volcanic rocks were discovered in the Geiyo Islands, western Setouchi region of southwest Japan. These occur as dikes or volcanic necks with marginal pyroclastic rocks. The volcanic rocks are composed of high-alkali tholeiitic olivine basalt, olivine andesite and bronzite andesite, in which the olivine andesite from Oge-shima chemically corresponds to high magnesian andesite. Whole rock K-Ar age determinations were carried out on olivine basalts, olivine andesites and bronzite andesites. An olivine andesite from Osakishimo-shima and two bronzite andesites from Okamura-shima yielded late Miocene ages of 8.2±0.7 Ma, 7.8±0.4 Ma and 8.0±0.4 Ma, respectively. It follows that subalkaline volcanism distinctively occurred at late Miocene age in the western Setouchi region. Accordingly, we give a name of “Geiyo volcanic rocks” to these volcanic rocks, which include late Miocene high-alkali tholeiitic olivine basalt reported from the adjacent area. On the other hand, olivine basalts from Okubi-shima and Ko-oge-shima give middle Miocene ages of 12.8±0.6 Ma and 15.4±0.8 Ma, respectively. In addition, an olivine andesite from Oge-shima has been dated as 17.4±0.9 Ma, indicating early Miocene age. On the basis of the radiometric ages, petrography and whole rock chemistry, these volcanic rocks are considered to be a member of the Setouchi volcanic rocks.