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

研究了日本东北弧区、伊豆-小川弧区和大峡谷地区的超镁铁质和镁铁质包裹体。根据它们的矿物化学，它们大多被解释为弧拉斑玄武岩的堆积体，形成于靠近壳幔边界深处的岩浆室中。来自lchinomegata maar喷射岩和中新世Ogi玄武岩的一些角闪石贫超镁铁质和镁铁质包体是大洋拉斑玄武岩结晶形成的堆积体。来自lchinomegata maar的单斜辉石岩和来自中新世AtSumi榴辉岩的橄榄辉长岩是由碱性玄武岩岩浆结晶而成。来自富士火山和山竹火山的辉长岩是英安岩浆与玄武岩岩浆混合而成的堆积物。三河河地区上新世火山岩包裹体的锶同位素比值较低(0.7031~33)，可定性为古伊豆-石池头弧弧后岩浆作用的产物。Yatsugatake火山的部分辉长岩包裹体具有较低的锶同位素比值(&lt；0.7040)，与伊豆小川弧火山相当，而另一些包裹体的I值较高(&gt；0.7040)，与日本东北部的前缘火山相当。大部辉长岩包裹体的锶同位素比值较低(0.7032-33)，但部分橄榄辉长岩的锶同位素比值较高(0.7038-39)。有用的等时线年龄只对AtSumi地区的包裹体进行了Sm-ND法测定。橄榄辉长岩的年龄为28~32 Ma，辉长岩的年龄为54~66 Ma。

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.