Petrolgy and geochemistry of Cenozoic igneous rocks in Northeast Japan arc

点击翻译按钮获取中文摘要

In order to elucite the geochemical characteristics and origin of igneous rocks in Northeast Japan arc, we have determined major elements of 2,000 and trace elements of 600 rock samples. The following several important conclusions have been clarified:There are some chemical differences between the Pliocene to Quaternary and Miocene volcanic rocks even in the same regions. Namely, the latter do not have a nature of the typical island arc, but active continental margin. It seems likely that the change of chemical nature of magmas is related to the different tectonic setting, opening of the Japan Sea.Quaternary volcanoes distributed widely are divided into four volcanic zones from the Pacific to continental side based on geological, petrographical and geochemical evidences.Vlocanoes along the volcanic front consist of tholeiitic and calc-alkaline suites, both of which exhibit clearly different chemical and isotopic variation with eath other. It is suggested that two types of parental magmas have been generated in the upper mantle at different depths or from the common source but under different physic-chemical conditions.Variations of the normalized incompatible trace element pattern are different between tholeiitic and calc-alkaline suites. Accordingly, it is very difficult to produce calc-alkaline andesites by simple magma mixing of tholeiitic basalt and its salic end product.Not only alkalies and incompatible trace elements, but other major and compatible elements in volcanic rocks show the lateral variations across the arc as well.