Geochemical evolution of lower crust in relation to formaiton of a marginal sea -An approach based on geochemistry of igneous rocks and their enclaves.

在本项目的第一年和第二年期间，我们建立了从粉状岩石和矿物样品中提取Rb、Sr、Sm和Nd的方法，以及最近安装在地质部的热电离质谱计(MAT262)测定锶和钕同位素比值的方法。质谱仪提供了国际、国内标准样品高精度的锶、钕同位素数据。我们用质谱仪测定了日本西南部白垩纪基性和中基性侵入岩和火山岩的锶、钕同位素比值，并研究了这些基性和中基性岩与广泛分布在日本西南部的大量白垩纪长英质火山岩和侵入岩之间的岩石成因关系。基性岩和中基性岩的初始锶、钕同位素比值具有区域性差异。三洋带的基性岩至中基性岩具有高的初始锶和低的初始Nd同位素比值，而三因带的基性岩和中基性岩具有低的初始锶和高的Nd同位素比值。这种锶、钕同位素比值的区域变化与该区长英质火成岩的变化具有可比性。这些数据表明，与桑因带下的岩浆源区相比，三洋带下的基性-中基性岩和长英质岩(主要是花岗岩类岩石)，可能是最上面的地幔和下地壳的岩浆源区具有更丰富的地球化学特征。

During the first and second years of this project, we established extraction method of Rb, Sr, Sm and Nd from powdered rock and mineral samples, and measuring methods of Sr and Nd isotope ratios using a thermal ionization mass spectrometer (MAT262), which has recently been installed in the Department of Geology. The mass spectrometer gave high precision Sr and Nd isotope data for international and domestic standard samples. These data have been summarized in three papers and have reported in the Memoirs of the Faculty of Science, Shimane University.Using the mass spectrometer, we measured Sr and Nd isotope ratios of Cretaceous basic and intermediate intrusive and volcanic rocks in southwest Japan, and examined petrogenetic relationship betweeen these basic to intermediate rocks and voluminous Cretaceous felsic volcanic and intrusive rocks, which are widely distributed in southwest Japan. The basic to intermediate rocks show regional variations in initial Sr and Nd isotope ratios. The basic to intermediate rocks from the Sanyo Belt have high initial Sr and low initial Nd isotope ratios, whereas those from the San'in Belt have low low initial Sr and high Nd isotope ratios. This regional variations in Sr and Nd isotope ratios are comparable with that of the felsic igneous rocks in this district. These data indicate that magma sources of both basic to intermediate rocks and felsic rocks (mostly granitoid rocks), probably uppermost mantle and lower crust, were geochemically more enriched beneath the Sanyo Belt, compared with the sources beneath the San'in Belt.