Comparison of gabbroic inclusions derived from lower crust beneath the northeast Japan Arc, their ragiogenic ages and petrological significances.

点击翻译按钮获取中文摘要

Ultramafic and mafic inclusions from the Northeast Japan Arc, Izu-Ogasawara Arc and Fossa Magna Region were studied. Based on their mineral chemistries, most of them are interpreted to be cumulates from arc tholeiite, formed in magma chambers at the depth near the crust-mantle boundary. Some amphibole-poor ultramafic and mafic xenoliths from the lchinomegata maar ejecta and the Miocene Ogi basalt, however, are the cumulates formed by crystallization of oceanic tholeiite. Clinopyroxenites from the lchinomegata maar and olivine gabbro from the Miocene Atsumi dolerite are crystallized from alkali basalt magmas. Gabbronorites from the Fuji volcano and the Yatsugatake volcano are cumulates from dacitic magmas mixed with basaltic magmas. Sr isotope ratios of the inclusions from the Pliocene volcanics in Sanogawa area are low (0.7031-33), and able to be characterized as the products of back-arc magmatism in the Ancient Izu-Shichitou Arc. Some gabbroic inclusions from the Yatsugatake volcano have low Sr isotopic ratios (<0.7040) significant of the Izu=ogasawara arc volcanoes, and the others show high ivalues (>0.7040) significant of frontal volcanos in the Northeast Japan Arc. Sr isotope ratios of the most gabbroic inclusions from the Atsumi dolerite show low values (0.7032-33), but some olivine gabbros show high values (0.7038-39). Useful isochron ages are only determined for inclusions from the Atsumi area by Sm-Nd method. The ages are from 28 to 32 Ma for the olivine gabbros and 54 to 66 Ma for gabbros.