Isotopic evidence for channeled fluid flow in low‐grade metamorphosed Jurassic accretionary complex in the Northern Chichibu belt, western Shikoku, Japan

Abstract

AbstractThe low‐grade metamorphosed Jurassic accretionary complex of the Northern Chichibu Belt, Hijikawa area, western Shikoku, is divided into two units, the Hijikawa and Kanogawa units, that are separated by an out‐of‐sequence thrust (OOST), the Ozu‐Kawabegawa Fault. The Kanogawa unit south of the Ozu‐Kawabegawa fault consists mainly of sandstone, shale, broken formation of alternating sandstone and shale, greenstone, chert, and pelitic melange, while the Hijikawa unit is characterized by a stack of subunits separated by small‐scale thrusts. The subunits are mainly made up of basic, pelitic and psammitic semischists, schistose pelitic melange, and chert. Petrological and mineralogical constraints suggest peak metamorphic conditions of 204–247 °C at 1–3 kbar in the Kanogawa unit, and 228–289 °C at 3–5.6 kbar in the Hijikawa unit. Quartz and quartz‐calcite veins are widely developed in the study area, especially in the Hijikawa unit. Regional variations in stable isotopic data show that the δ18Oquartz and δ18Ocalcite values in veins tend to increase towards the Ozu‐Kawabegawa Fault. The δ18Owhole rock values are remarkably high in some subunits close to OOSTs within the Hijikawa unit. Oxygen isotopic compositions from vein quartz indicate that a higher δ18O fluid migrated upward from depth along the Ozu‐Kawabegawa Fault within the accretionary prism during prehnite‐pumpellyite facies metamorphism. The fluid source is inferred to be pelitic rocks at higher temperatures and greater depths within the accretionary prism.