Geochemistry of Autoliths within Granitoids

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Modes of occurrence, mineralogy, petrology and geochemistry of dark inclusions and their rock-forming minerals were investigated within representative granitic bodies in the Outer Zone of Southwest Japan and in the Southern Kitakami Mountain lands of Northeast Japan.Dark inclusions within the S-type granitoids in the southern belt of the Outer Zone consist mainly of large amounts of sedimentary xenoliths and less autoliths with porphyritic textures ; and those within the I-type granitoids in the northern belt of common sedimentary xenoliths and autoliths with slightly porphyritic textures. Dark inclusions within the I-type granitoids of the Southern Kitakami Mountain lands consist mainly of autoliths with granular textures and less igneous and sedimentary xenoliths.Autoliths from granitoids in the Outer Zone are mainly composed of medium- to fine-grained quartz gabbroic and granodioritic rocks. The rocks are mineralogically rich in biotite and plagioclase. Autoliths from granitoids in the Southern Kitakami Mountain lands are mainly composed of coarse- to medium-grained gabbroic and quartz dioritic rocks. The rocks are mineralogically rich in hornblende and plagioclase.Autoliths are generally more basic in composition than their host granitoids, but the compositional variation ranging from autoliths to their host granitoids shows a regular trend for the individual granitic body.Mafic minerals such as biotites, amphiboles and clinopyroxenes from autoliths are generally similar in composition to those from their host granitoids. Plagioclase feldspars from some autoliths are basic in composition than those from their host granitoids. It is suggested that a genetical history is remained in zonal structures and chemical compositions of amphiboles and plagioclase feldspars from autoliths in a granitic body.