Orthopyroxene-rich Rocks from the Sanbagawa Belt (SW Japan): Fluid–Rock Interaction in the Forearc Slab–Mantle Wedge Interface

Abstract

The Western Iratsu body of the Sanbagawa belt (SW Japan) is a mafic–ultramafic complex that underwent an initial metamorphism in the amphibolite facies and a subsequent metamorphism in the eclogite facies, and represents a fossil forearc slab–mantle wedge interface in a developing subduction zone. Two generations of orthopyroxene (Opx1 and Opx2) that were formed during the amphibolite-facies (antigorite unstable) and eclogite-facies (antigorite stable) stages can be recognized in the ultramafic domain. Opx1-rich rocks contain Ni-rich relict olivine (up to 0 7 wt % NiO) and grade into dunite, suggesting that they represent metasomatic rocks derived from dunite. Opx1 can be subdivided into two types: one (Opx1L) constitutes replacive harzburgite to orthopyroxenite layers and the other (Opx1V) occurs in metasomatic reaction veins in dunite. Relatively high formation temperatures ( 750 C) of Opx1L imply that the relevant metasomatism in the ultramafic domain took place before the juxtaposition with the mafic domain preserved in the Western Iratsu body. Textural relationships and mineral trace element data suggest that Opx1L-rich rocks were formed by reactive porous infiltration of a slab-derived hydrous melt or solute-rich fluid into dunite. Subsequently, Opx1V-rich veins were formed by a prolonged flux of a Si-rich aqueous fluid (sourced from the mafic domain) through brittle fractures in dunite during the amphibolite-facies metamorphism ( 660 C and 1 2 GPa). The initial formation of Opx1V-chlorite-rich selvages along the fluid conduits is likely to have limited the reaction between a Si-rich crustal fluid and host dunite, and this process can be important during the early transportation of slab-derived components into the mantle wedge. Lastly, Opx1L crystals locally show a textural replacement by Opx2 together with antigorite, indicating recrystallization in the eclogite facies ( 620 C and 1 6–1 8 GPa). The Opx2-forming reaction is mainly localized in ductile shear zones, which correspond to major fluid pathways in the partially serpentinized forearc mantle.