Abstract
A 600 m drill core (recovery > 95%) penetrated the Middle Miocene Kumano Acidic Rocks in eastern Kii Peninsula, Southwest Japan. The core is fresh and unaltered, except near the surface, above 40 m below ground level (GL). The core consists of Kumano Granite Porphyry (from GL to 464.25 m deep) and the Owase-Shirahama Pyroclastic Rocks (464.25-600 m deep), with the boundary between these units being an intrusive contact. The Kumano Granite Porphyry is largely homogeneous in rock facies and whole-rock chemical composition, except for the occurrence of a chilled margin at the contact. The porphyry contains phenocrysts of quartz, plagioclase, K-feldspar, orthopyroxene, and biotite. The Owase-Shirahama Pyroclastic Rocks consist of massive crystal tuff. The rocks are largely homogeneous, except for local pumice-bearing parts and tuff beds with horizontal laminations. Many mafic magmatic enclaves and sedimentary-metamorphic xenoliths occur in the Kumano Granite Porphyry and the Owase-Shirahama Pyroclastic Rocks.
The > 700 m vertical offset of the basement level of the Owase-Shirahama Pyroclastic Rocks between at the drilling site and the nearby outcrops on the ground surfaces, indicates the subsidence of a caldera flow that formed synchronous with or after eruption of the tuffs. The areal distribution of the Kumano Granite Porphyry, including core observations at the intrusive contact, indicate that the porphyry form a sill-like body of > 1,000 m thick, intruding into the caldera-filling Owase-Shirahama Pyroclastic Rocks.
