Trace element abundances and isotopic ratios of Japanese Quaternary basalts and heterogeneity of upper mantle

点击翻译按钮获取中文摘要

In order to elucidate the geochemical and petrological behavior of incompatible trace elements in the Japanese Quaternary basaltic rocks, and the heterogeneity of the source materials in upper mantle, we have determined major and trace elements and Sr and Nd isotopic ratios of the representative volcanic rocks and their incorporated xenoliths derived from upper mantle and lower crust together with their major elements. The following several important results and conclusions have been clarified.1. All of the basalts and andesites from the East Japan volcanic zone exhibit the nature of typical island arc volcanic rocks'showing the remarkable positive Ba, K and Sr anomalies and the negative Nb anomaly in the rock/primitive mantle normalized pattern for the incompatible elements. Furthermore, their Sr^<87>/Sr^<86> ratios gradually decrease and Nd^<143>/Nd^<144> ratios increase from the volcanic front to the back arc basin side.2. Alkali series in Japan Sea side have the range of 0.7035 to 0.7050 for Sr^<87>/Sr^<86> ratios and continental characters for incompatible element normalized pattern without any Ba, K, Nb or Sr anomaly.3. Normalized patterns and Sr and Nd isotopic ratios of cumulative ultranafic to mafic xenoliths derived from upper mantle and lower crust are similar to those of the island. 9rc basalts, and do not have any continental or oceanic basalt natures. It indicates that the basement or uppermost mantle and lower crust of the Japanese islands are composed of cumulative rocks of the island arc basalts.