Geochemical evolution of thelower crust and upper mantle beneath SW Japan.

本研究的目的是了解自侏罗纪以来日本西南部下地壳和上地幔的地球化学演化。由于长英质岩石的地球化学和Sr、Nd同位素数据已被报道很多，我们研究了日本西南部侏罗纪至第三纪基性至中质火成岩的地球化学和Sr、Nd同位素比值。我们的数据表明，自侏罗纪以来，山阴和山阳带下的上地幔已被改变为更亏损的性质，虽然在白垩纪山阳带下的上地幔比山阴带下的上地幔更富集同位素。中新世以后山阴带上地幔变为亏损状态。尽管一些研究者认为日本西南部大量的白垩纪岩浆活动是由库拉-太平洋洋脊俯冲作用引起的，但本研究并未获得支持这一观点的明确地球化学证据。我现在正在研究早白垩世的中火成岩，因为它们比晚白垩世的火成岩具有相对较高的Sr含量和较低的Sr初始同位素比值。这些岩石有可能是俯冲板块部分熔融形成的。

Objective of this research was to understand geochemical evolution of the lower crust and upper mantle beneath SW Japan since the Jurassic. As many geochemical and Sr and Nd isotope data have been reported for felsic rocks, We examined geochemistry and Sr and Nd isotope ratios of mafic to intermediate igneous rocks of the Jurassic to Tertiary in SW Japan. Our data indicate that upper mantle beneath the San-in and San-yo Belts had been changed to more depleted nature since the Jurassic, although the upper mantle beneath the San-yo Belt was more isotopically enriched than that beneath the San-in Belt during the Cretaceous. It is suggested that the upper mantle beneath the San-in Zone changed to depleted nature after the Miocene. Although it has been suggested by some researchers that voluminous Cretaceous magmatism in SW Japan were caused by subduction of the Kula-Pacific Ridge, clear geochemical evidences to support such a suggestion have not been obtained in this study. I am now examining early Cretaceous intermediate igneous rocks in San-yo Belt, because they have relatively higher Sr contents and low Sr initial isotope ratios than late Cretaceous igneous rocks. There is a possibility that these rocks were derived by partial melting of the subducting slab.