Petrolgy and geochemistry of Cenozoic igneous rocks in Northeast Japan arc

为了阐明日本东北地区火成岩的地球化学特征和成因，对2000个岩石样品进行了主量元素测定，对600个岩石样品进行了微量元素测定。澄清了以下几个重要结论：即使在同一地区，上新世至第四纪和中新世的火山岩也存在一些化学差异；即后者不具有典型岛弧的性质，而是活动的大陆边缘。岩浆化学性质的变化可能与不同的构造环境、日本海的开放有关。第四纪火山分布广泛，根据地质、岩石学和地球化学证据，将第四纪火山从太平洋向大陆侧划分为四个火山带。沿火山锋面的火山岩由拉斑岩套和钙碱性套组成，两者具有明显不同的化学和同位素变化特征。认为上地幔在不同深度或在不同物理化学条件下产生了两种类型的母岩浆。在拉斑岩组和钙碱性组中，归一化不相容微量元素模式的变化是不同的。因此，通过拉斑玄武岩及其盐类终产物的简单岩浆混合，很难形成钙碱性安山岩。火山岩中除了碱和不相容微量元素外，其他主要和相容元素也表现出横向变化。

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.