Formation of the Hiei Granite pluton and its geological implications for the Cretaceous felsic magmatism in southwest Japan

Abstract

The Hiei Granite pluton, 6 km (E-W) × 7 km (N-S) in extent, is one of the Cretaceous granite plutons around Lake Biwa, southwest Japan. It was emplaced within the Jurassic accretionary complex of the Tanba, and is situated at the western margin of the Cretaceous Biwa-ko Cauldron (60 × 40 km). The Hiei Granites can be petrographically divided into the following two rock-types: medium-grained equigranular granite and porphyritic granite. The latter occupies the core of the pluton. They are essentially biotite granites, and sometimes hornblende is contained. The Hiei Granites of about 100 Ma differ as regards the age, lithology and geochemistry from other Cretaceous granites of about 70 Ma around the south of Lake Biwa, and its designation to the ring bodies associated with the Cauldron is ruled out. In the eastern part of the pluton, granite porphyry and granodiorite porphyry dykes, several hundred meters in width, are intruded into the granite. These dykes would be the members of the ring-dykes encircling the Biwako Cauldron. Numerous thin dykes of lamprophyre, basalt, rhyolite and other rock-types, are also intruded into the granites.