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

该项目的目的是阐明控制日本西南部白垩纪火成岩空间地球化学变化的因素，并阐明这种巨大的岩浆作用与白垩纪欧亚大陆东部山脊俯冲的关系。对三约带早白垩世至中白垩世火成岩以及三因带一些年代不明的火成岩进行了地质和岩石化学研究。三约带100~120 Ma的火成岩具有不同于同地区较年轻的火成岩的地球化学和初始锶、钕同位素比值。与同地区较新的火成岩相比，三约带一些较老的火成岩具有高锶低Y的特点，同时具有较低的初始锶和较高的初始Nd同位素比值。古老火成岩的这些地球化学特征与太古宙高铝TTD和新生代埃达克岩相似，表明岩浆是由俯冲的大洋岩石圈部分熔融形成的。但与MORB相比，这些火成岩的锶、钕同位素比值相对较高，因此，来自俯冲板块的岩浆可能受到了上覆地壳物质的混染。由于这些埃达克质火成岩经常在俯冲隆起的地区被识别，因此强烈认为库拉-太平洋海脊在白垩纪时被俯冲到欧亚大陆东缘之下。

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.