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

The 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 degrees C at 1-3 kbar in the Kanogawa unit, and 228-289 degrees 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 delta O-18(quartz) and delta O-18(calcite) values in veins tend to increase towards the Ozu-Kawabegawa Fault. The delta O-18(whole 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 delta O-18 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.