Variation of rock facies in Yasu granitic pluton, southern part of Lake Biwa, Southwest Japan

Abstract

The Cretaceous granitic complex in the southern part of Lake Biwa occurs as a ring-shape complex. The complex is composed mainly of Hira, Hiei, Tanakami, Suzuka, and Yasu granitic plutons. The Yasu granitic pluton, which has stock shape and 10 km (N-S) to 8 km (E-W) extension, is situated at the most inner part of this complex.The rock facies of the Yasu granitic pluton varies significantly. The pluton consists mainly of four facies : Lower, Main, Upper, and Granite porphyry facies. The Lower, Main, and Upper facies are intruded by the Granite porphyry facies with remarkable porphyritic texture. Petrographic studies and the chemical compositions of whole-rock and plagioclase crystals indicate that the Lower, Main, and Upper facies are formed by fractional crystallization. A large quantity of pegmatite and aplite veins accompany the Lower and Main facies, while Upper and Granite porphyry facies are poor or absent. This evidence and analogy of volcanic rocks suggest that the porphyritic texture of the Yasu granitic pluton developed through degassing process in the granitic magma.