中新世背弧リフティングと青沢玄武岩
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概要
- 論文の詳細を見る
During early middle Miocene time, intense basaltic magmatism took place in a rift zone of the Akita-Yamagata oil field in the back-arc region of the NE Japan arc. The magmatism yielded a huge amount (about 6,000 km^3) of basalt lava and volcaniclastic rocks which called Aosawa basalt. The Aosawa basalt extruded from numerous NE-trending dikes to form submarine volcanoes which are intercalated with argillaceous sediments of bathyal zone. Subsequently, basaltic magma of late middle Miocene time mostly furnished dolerite intrusions not ascended to form effusive rocks, but spread out as sills in the soft argillaceous sediments. Until late Miocene time, the basaltic magmatism in the oil field formed a complex unit which consists mostly of lavas, volcaniclastic rocks, dolerite sills and mudstone. The Aosawa basalt has petrochemical affinities to enrich-MORB or within plate basalt in major and trace element compositions, but partly has an affinity to island arc basalt. This petrochemical characteristics may be interpreted in terms of slight metasomatism of LIL elements from subduction zone. The Aosawa basalt have low K_2O and moderate TiO_2 contents. After the Aosawa basalt magmatism, however, basalt-andesite of late middle Miocene to Pliocene time in the rift zone show slight increasing of K_2O and decreasing of TiO_2 with time. This temporal change in petrochemistry may be interpreted as due to increasing contribution of mantle metasomatism by subduction components to the generation of the basalt-andesite magma.
- 地学団体研究会の論文
- 1992-01-25