Many thin mafic dykes occur within and around the Cretaceous Hiei Granite pluton in southwest Japan. Petro-graphically, these dykes are spessartite, basalt, and dolerite. Petrographical and geochemical characteristics indicate their designations as calc-alkaline basalt and basaltic andesite. Their magmas have probably originated in the LILE-rich mantle wedge under the Eurasian continental margin matured arc. These mafic rocks are crystallized and have suffered successive hydrothermal alteration under high vapor condition. Among the Cretaceous granites around Lake Biwa, the older ones of similar to 95 Ma, such as the Hiei and Kaizuki-yama plutons, are accompanied exclusively by these mafic dykes including spessartite; however, the younger (similar to 70 Ma) ones are never accompanied by these dykes.