Elucidation of factors controlling spatial geochemical variation of Cretaceous igneous rocks in SW Japan and verification of the ridge subduction.

本研究的目的是阐明日本西南部白垩系火成岩空间地球化学变化的控制因素，并阐明白垩系欧亚大陆东部大量岩浆活动与洋脊俯冲的关系。对三约带早白垩世至中白垩世火成岩以及三隐带一些年龄不详的火成岩进行了地质和石油化学研究。三约带年龄在100 ~ 120 Ma的火成岩与同一地区年龄更小的火成岩具有不同的地球化学特征和初始Sr、Nd同位素比值。三约带部分老火成岩具有Sr含量高、Y含量低、Sr初始同位素比值低、Nd初始同位素比值高的特征。古火成岩的地球化学特征与太古宙高al - TTD和新生代埃达克岩相似，表明岩浆来源于俯冲洋岩石圈的部分熔融作用。与MORB相比，这些火成岩的Sr和Nd同位素相对富集。那么，俯冲板块产生的岩浆可能已经被上覆的地壳物质污染了。由于这些埃达克质火成岩经常出现在山脊俯冲的地区，因此强烈建议库拉-太平洋山脊在白垩纪俯冲到欧亚大陆东部边缘之下。

Aims of this project are to elucidate the factors controlling spatial geochemical variation of the Cretaceous igneous rocks in SW Japan and to clarify relationship between this voluminous magmatism and ridge subduction at the eastern Eurasian continent during the Cretaceous. Geological and petrochemical studies have been carried out for early to middle Cretaceous igneous rocks in the San'yo Belt and also for some igneous rocks with unknown ages in the San'in Belt. The igneous rocks with ages ranging from 100 to 120 Ma in the San'yo belt have different geochemical and initial Sr and Nd isotope ratios from igneous rocks with younger ages in the same region. Some older igneous rocks in the San'yo Belt are characterized by high Sr and low Y contents, and also by lower initial Sr and higher initial Nd isotope ratios than the younger igneous rocks in the same region. These geochemical characteristics of the older igneous rocks are similar to the Archean high-Al TTD and Cenozoic adakite, indicating that the magmas were derived by partial melting of the subducted oceanic lithosphare. Those igneous rocks, however, are relatively enriched in Sr and Nd isotope ration, compared with MORB.Then, the magma derived from the subducted slab may have been contaminated with the overlying crustal materials. As these adakitic igneous rocks are frequently recognized in areas, where ridges was subducting, it is strongly suggested that the Kula-Pacific ridge was subducted beneath the eastern margin of the Eurasian continent during the Cretaceous.