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

Abstract

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°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 d 18 O quartz and d 18 O calcite values in veins tend to increase towards the Ozu-Kawabegawa Fault. The d 18 O 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 d 18 O 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.

Fluid and mass transfer at subduction interfaces—The field metamorphic record

“…Studies of fluid and mass transfer in these shallow complexes indicate the mobility of dissolved material resulting in the precipitation of quartz-and carbonate-bearing vein arrays (e.g., Meneghini et al, 2009). These certainly indicate the mobility of mass (and fluid) at some scale, but assessments of scales of fluid mobility using stable isotopes have been complicated by the cryptic nature of infiltration by fluids previously equilibrated with very similar rocks in these very thick sequences of largely sandstone-shale sequences (see Bebout, 2001, 2004;Sakakibara et al, 2007;Raimbourg et al, 2015). Fisher and Brantley (1992) argued for local-scale derivation of abundant quartz in veins in the Kodiak Formation, Alaska, involving diffusion of the dissolved silica from the host-rocks into fractures.…”

“…The Jurassic accretionary complex of the northern Chichibu belt in western Shikoku, Japan (Sakakibara et al, 2007) consists of low-grade metamorphosed or unmetamorphosed psammitic and pelitic rocks, chert, greenstones and limestone. In the Hijikawa unit, there are low-grade coherent schistose and chert units separated by small-scale thrusts and mélange units comprised of pelitic schist with lenses of chert and mafic schist that are sheared by faulting.…”

“…In the Hijikawa unit, there are low-grade coherent schistose and chert units separated by small-scale thrusts and mélange units comprised of pelitic schist with lenses of chert and mafic schist that are sheared by faulting. Estimated temperatures for the Hijikawa unit are ~230-290 °C and pressures are ~0.5-0.6 GPa (Sakakibara et al, 2007). Oxygen isotopic compositions of veins and whole rocks were inferred to reflect updip flow of fluid derived from higher-temperature pelitic rocks at greater depths within the accretionary prism (Sakakibara et al, 2007).…”

Metasomatism in Subduction Zones of Subducted Oceanic Slabs, Mantle Wedges, and the Slab-Mantle Interface

“…9.12b; from Bebout and Barton 1989;Bebout 1991a, b). Other isotopic studies of fluid mobility in ancient accretionary complexes include Vrolijk (1987) (also see work on the modern Barbados accretionary prism; Vrolijk et al 1990), Sadofsky and Bebout (2004), and Sakakibara et al (2007). Nelson (1995) documented kilometer-scale mobilization of fluids carrying sedimentary Nd-and Sr-isotope signatures, through analyses of metasomatic rinds on mafic tectonic blocks in the Franciscan Complex mélange.…”

“…Nevertheless, extensive vein networks (e.g., those in Fig. 9.9) are generally considered to reflect the upward migration of fluids generated by a combination of compaction and dehydration of more deeply buried sediments (see Bebout 1991a, b;Sadofsky and Bebout 2004;Sakakibara et al 2007). This is supported by the relatively lower salinities, and elevated CH 4 concentrations in fluid inclusions within vein minerals, which is similar to the "freshening" observed in faults and décollement at shallower levels (Brown et al 2001;Moore et al 2001).…”