ABSTRACT

Large boulder-size euhedral quartz crystals occur in the basal horizons of the Late Miocene Seto Porcelain Clay Formation, central Japan. The quartz crystals consist of smoky basal portion overgrown by translucent to milky white quartz. δ¹⁸O values (-1.5 to -0.2‰, SMOW) of the translucent to milky white quartz are considerably lower than the values of smoky quartz (+10.3 to +10.5‰). The oxygen isotope ratios of quartz and homogenization temperatures of fluid inclusions indicate that the translucent to milky white quartz overgrowths precipitated onto the early-stage smoky quartz in druses of the Naegi and/or Toki granites, from meteoric water dominant thermal waters that infiltrated into the granites during the latest stage of granitic activity in the late Cretaceous age.