Igneous activity of the Daito granodiorite in the Un-nan area, San'in zone, Southwest Japan

• Published in: Chishitsugaku zasshi Vol. 127; no. 8; pp. 461 - 478
• Main Authors: Suzuki, Hiromi, Noguchi, Masashi, Kobayashi, Natsuko, Kamei, Atsushi
• Format: Journal Article
• Language: English; Japanese
• Published: Tokyo The Geological Society of Japan 15.08.2021; Japan Science and Technology Agency
• Subjects: Batholiths; Biotite; Chemical analysis; Crystallization; Crystals; Geologic mapping; Geological maps; Igneous activity; Lithology; Magnetic susceptibility; Magnetite; Palaeocene; Paleocene; Plagioclase; San'in zone; SW Japan; the Daito granodiorite; Japan
• ISSN: 0016-7630; 1349-9963
• DOI: 10.5575/geosoc.2021.0016

The Daito granodiorite is a large plutonic mass in the San'in Batholith, southwest Japan. In this study, the igneous activity that formed the Daito granodiorite was investigated based on field and petrographic descriptions, magnetic susceptibility measurements, and whole-rock chemical analyses. The granodiorite is characterized by: 1) a ubiquitous distribution of mafic magmatic enclaves; 2) euhedral hornblende and biotite crystals; 3) abundant magnetite associated with mafic minerals; and 4) high magnetic susceptibility (>5 × 10-3). The lithology varies from medium- to coarse-grained hornblende-biotite granodiorite to medium-grained biotite granite. These variations resulted mainly from the fractional crystallization of plagioclase, hornblende, and biotite. Significant revision of the geological map of the Daito granodiorite suggests that the intrusion occurred at ca. 57 Ma. This and previous studies show that the Daito granodiorite is an important product of Paleocene activity in the Younger Inbi intrusive group (68-53 Ma) of the San'in batholith.