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，我们建立了从粉末岩石和矿物样品中提取Rb、Sr、Sm和Nd的方法，以及使用最近安装在地质系的热电离质谱仪（MAT 262）测量Sr和Nd同位素比值的方法。该质谱仪对国内外标准样品进行了高精度的Sr、Nd同位素数据分析。这些数据已在三篇论文中进行了总结，并在岛根大学理学部的回忆录中进行了报道。我们用质谱仪测量了日本西南部白垩纪基性和中质侵入岩和火山岩的Sr和Nd同位素比值，并研究了这些基性和中质岩与大量白垩纪长英质火山岩和侵入岩之间的成岩关系。广泛分布于日本西南部。基性岩和中基性岩的初始Sr和Nd同位素比值具有区域性差异。山阳岩带的基性-中性岩石具有高的初始Sr和低的初始Nd同位素比值，而山阴岩带的基性-中性岩石具有低的初始Sr和高的初始Nd同位素比值。这种Sr、Nd同位素比值的区域性变化与本区长英质火成岩具有可比性。这些数据表明，与山阴带下的岩浆源相比，山阳带下的基性-中性岩浆源和长英质岩浆源（主要是花岗岩类）（可能是上地幔和下地壳）在地球化学上更加富集。

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.