Petrological characteristics of Dark-colored silica veins in Tertiary rhyolite (the Iozen Formation) from the southern part of Ishikawa Prefecture, Japan

Abstract

Anastomosing networks of massive dark-colored veins (Dark-colored silica veins hereafter) ranging from a few millimeters to 8 cm in thickness occur in a fault zone of Tertiary rhyolites (the Iozen formation) from the southern Ishikawa Prefecture, Japan. Optical microscopic observations combined with SEM and XRD studies revealed that the Dark-colored silica veins mainly consist of very fine-grained quartz (10 μm in size) with minor amounts of feldspar and Fe-bearing unidentified phase. Angular to subangular lithic clasts derived from the host rhyolite are frequently embedded in fine-grained quartz matrix of the Dark-colored silica veins. The Dark-colored silica veins are apparently abundant in SiO₂ relative to the host rocks. Chondrite-normalized rare earth element (REE) pattern of a Dark-colored silica vein is similar to that of the host rhyolite for middle to heavy REE, whereas light REE (La and Ce) concentrations are higher in the Dark-colored silica vein than in the host rhyolite, resulting in high La/Yb ratio in the Dark-colored silica vein. The Dark-colored silica veins might be originated by extreme crushing of the host rhyolite in fault zones followed by the selective enrichment of SiO₂ (+LREE) coupled with diluting of other elements. The outcrop including the Dark-colored silica vein was finally suffered from low-temperature alternation (<100ºC), leading to form clay minerals, particularly along the fault zone.