Geology of Iwate volcanic group, northeastern Japan
Geology of Iwate volcanic group, northeastern Japan
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日本东北部岩手火山群的地质
DOI：
10.2465/ganko1941.82.132
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发表时间：
1987
期刊：
影响因子：
--
通讯作者：
M. Nakagawa
中科院分区：
文献类型：
--
作者：
M. Nakagawa
岩手火山群位于日本东北部八满台及其周围火山群的东南部。它由若干个火山体组成，火山体沿东西方向延伸，平行于本区构造应力的最大水平挤压轴。岩手火山群的基底岩石主要由第三纪岩石和早第四纪流纹质熔结凝灰岩组成，后者是玉川熔结凝灰岩的一员。岩手火山群的活动总结如下。松川安山岩：该安山岩由蚀变熔岩和火山碎屑岩组成，出露于火山群的西部。它们的K-Ar年龄分别为1.62和1.36Ma。Amihari火山群：根据火山原始地形的保存情况，分为老火山和新火山两个亚阶段。较老阶段：早-中更新世，几个火山体几乎同时形成，火山岩与火山碎屑岩相间。它们沿东西方向排列，长12公里。年轻阶段：火山链形成后，形成了两个火山锥和一个熔岩穹丘，熔岩流从火山链的西部流出。有可能它们在岩手火山的生长过程中是活跃的。岩手火山：在更新世中晚期，岩手火山的活动开始于Amihari火山群的东部。它的历史包括四个子阶段，并涉及火山体的崩溃。旧岩手第1阶段：大量的火山碎屑岩和少量的熔岩形成了两个成层火山，黑仓火山和旧岩手火山，净高度约为2000米。第二阶段：约0.15Ma时，旧岩手火山东南部崩塌，形成鬼又破火山口。崩塌后，熔岩流和火山碎屑从中央和侧翼喷口喷出，填满了奥尼马塔破火山口。旧岩手第三阶段：西岩手火山口在旧岩手火山顶部形成。火山口喷出的熔岩和浮岩形成了一个复合的中心锥--Onashiro锥。这些活动似乎在0.05Ma左右开始，在0.03Ma之前停止。年轻岩手阶段：经过25，000多年的停顿，大约5，000年前，东岩手破火山口在老岩手火山的东部形成。火山碎屑和熔岩流主要从破火山口内的喷口喷出，形成了新岩手火山和屋志岳火山（2，040.5米）。1719年，八步桥走熔岩流从火山东北侧喷出。火山喷发岩主要为玄武岩和玄武安山岩，并与安山岩共生。玄武质岩石中有单斜辉石+橄榄石和斜方辉石+橄榄石两种镁铁质斑晶组合，虽然石英斑晶经常出现，但岩手火山群岩石中没有发现角闪石斑晶。
Iwate volcanic group is situated at the southeastern portion of Hachimantai and surrounding volcanic clusters, northeastern Japan. It consists of several volcanic bodies extending in E-W direction, parallel to the maximum horizontal compressional axis of the tectonic stress in this region. The basement rocks of Iwate volcanic group are composed mainly of the Tertiary rocks and the early Quaternary rhyolitic welded tuff which is a member of the Tamagawa welded tuffs. The activity of Iwate volcanic group is summarized as follows. Matsukawa andesite : this andesite consists of altered lavas and pyroclastics and are exposed at the western portion of the volcanic group. Their K-Ar ages have been reported to be 1.62 and 1.36 Ma. Amihari volcanic group: it is divided into two substages, older and younger, according to preservation of their volcanic primary topography. Older stage: in early to middle Pleistocene, several volcanic bodies, composed of an alternation of lavas and pyroclastics, were built up at the nearly same time. They are arranged in E-W direction 12km in length. Younger stage: after the formation of the volcanic chain, two cinder cones and a lava dome were formed, and lava flows were effused from the western portion of the volcanic chain. It is possible that they were active during the growth of Iwate volcano. Iwate volcano: in middle or late Pleistocene, the activity of Iwate volcano started at the eastern portion of Amihari volcanic group. Its history consists of four substages, and involves collapse of the volcanic body. Older Iwate 1st stage: voluminous pyroclastics and subordinate amounts of lavas formed two stratovolcanoes, Kurokura and Older-Iwate volcanos, with a net height of about 2, 000m. Older Iwate 2nd stage: at about 0.15Ma, the southeastern portion of the Older-Iwate volcano collapsed to form Onimata caldera. After the collapse, lava flows and pyroclastics were erupted from the central and flank vents to fill the Onimata caldera. Older Iwate 3rd stage: west-Iwate caldera was formed at the top of Older-Iwate volcano. From the vent in the caldera, lavas and pumice were effused to built up a composite central cone, Onashiro cone. It seems that these activities started at about 0.05Ma and ceased before 0.03Ma. Younger Iwate stage: after more than 25, 000 years of pause, about 5, 000 years ago, East-Iwate caldera was formed at the eastern portion of Older-Iwate volcano. Pyroclastics and lava flows were eruped from the vents mainly within the caldera, resulting in the formation of the composite Younger-Iwate volcano, Yakushidake volcano (2, 040.5m). In 1719, Yakehashiri lava flow was extruded from the northeastern flank of the volcano. The eruptive rocks are mainly basalt and basaltic andesite and associated with andesite. Two types of mafic phenocryst assemblage in basaltic rocks, clinopyroxene+olivine and orthopyroxene+olivine, are found. Although quartz phenocryst often occur, no hornblende phenocryst is found in the rocks from Iwate volcanic group.