Thermochronology for the granitic pluton related to lead-zinc mineralization in Tsushima, Japan

K-Ar datings and oxygen isotope analyses revealed a cooling history of the Uchiyama granitic pluton, which is genetically related to the Pb-Zn deposit s (Taishu mine) in the Tsushima Islands, Japan. The pluton intrudes into th e Paleogene Taishu Group to form the biotite-hornfels zone, while the Taish u vein-type Pb-Zn deposits occur in fissures developed in the non-hornfels zone about 1 to 3 km westward from the contact. Amphibole and biotite K-Ar ages of the pluton have a wide range from 19 to 13 Ma. Oxygen isotopes of t he biotite and coexisting quartz grains indicate that isotopic exchange rea ctions have occurred under subsolidus conditions, and that the K-Ar ages ar e affected by various cooling rates in the pluton. The mineralization age o f the Taishu ore deposits is obtained for the first time to be 15.4 +/-0.8 Ma by a X-Ar age of 2M(1)-muscovite in a calcite-quartz-muscovite-chlorite veinlet of the Shintomi orebody. Whole rock K-Ar ages of biotite-hornfels n ear the pluton represent similar ages to the ore deposits. Using blocking t emperature calculated from reported diffusivity for argon, the pluton was c ooled from 560 to 350 degreesC in the period of 17 to 14 Ma. The vein forma tion took place after the time when temperature in wall rocks of the pluton had dropped below the brittle-plastic transition (about 400 degreesC). The se results imply that the cooling of the pluton has caused injection of mag matic fluids into meteoric hydrothermal systems, and the Pb-Zn mineralizati on has occurred due to this mixing at the age of about 15 Ma.