Petrolgy and geochemistry of Cenozoic igneous rocks in Northeast Japan arc

为了阐明日本东北弧火成岩的地球化学特征和成因，我们测定了2,000个岩石样品的常量元素和600个岩石样品的微量元素。阐明了以下几个重要结论：即使在同一地区，上新世-第四纪火山岩与中新世火山岩之间也存在着某些化学差异。即后者不具有典型的岛弧性质，而是活动大陆边缘。根据地质、岩石学和地球化学证据，将分布较广的第四纪火山划分为从太平洋到大陆的四个火山带，火山前缘沿着的Vlocanoes由拉斑玄武岩和钙碱性岩套组成，火山前缘的火山岩主要由火山岩、火山岩和火山碎屑岩组成，火山碎屑岩主要由火山碎屑岩和火山碎屑岩组成，火山碎屑岩主要由火山碎屑岩和火山碎屑岩组成，火山碎屑岩主要由火山碎屑岩和火山碎屑岩组成。这两种物质都表现出明显不同的化学和同位素变化。结果表明，拉斑玄武岩和钙碱性岩系的上地幔中有两种不同深度的母岩浆或同一来源但物理化学条件不同的母岩浆，其不相容元素的归一化模式变化也不同。因此，拉斑玄武岩与其硅铝质最终产物的简单岩浆混合很难形成钙碱性安山岩，火山岩中不仅碱和不相容微量元素，而且其它主量元素和相容元素也表现出跨弧的横向变化。

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.