In situ crystallization observed in the Osumi granodiorite batholith

摘要

The Osumi granodiorite is a 14 Ma batholith emplaced in accreted sedimentary rocks in the forearc of southwest Japan. It has a chemical and mineralogical zonal structure with zones which become progressively more felsic inward. FeO~* and MgO decrease, while SiO_2, K_2O and Rb increase from its margin to the interior. In accord with this, alkali feldspar and quartz become more abundant in inner zones, and biotite and plagioclase vary oppositely. The mineral assemblage, however, remains unvaried. Major bulk rock compositions form a linear variation trend, which is found to be almost identical with the average trend of Japanese granitic rocks. These petrographic and chemical characteristics are well explained by in situ crystallization model in which the liquid returning from the solidification zone to the chamber has the granitic minimum liquid composition. The composition of the liquid in the chamber and that of the cumulate lie on the straight line that connects the granitic minimum and the initial chamber composition.