Evolving fluid composition during prograde metamorphism in subduction zones: A new approach using carbonate-bearing assemblages in the pelitic system

Abstract

The changing XCO 2 in fluids during the progressive metamorphism in Sanbagawa belt of the Cretaceous subduction zone, Japan, was estimated by a newly proposed method. The subduction zone meta-sediments are characterized commonly by four-phase assemblages in the CaO–NaAlO 2–KAlO 2–Al 2O 3 system with excess quartz and a CO 2–H 2O binary fluid phase. Using the common assemblage of calcite–albite–muscovite–clinozoisite, XCO 2 of the fluid was estimated to be from about 0.0001–0.0005 (the lowest grade chlorite zone), through 0.004–0.01 (garnet zone), 0.01–0.05 (albite–biotite zone) to 0.06–0.2 (oligoclase–biotite zone). The paragenetic relationship of meta-sediments from the subduction zones was compared in a wide P– T range to cover the stability fields of aragonite and jadeite. As a result, an excellent P– T–XCO 2 relationship was delineated to serve as a quantitative monitor for the evolving fluid composition during the progressive metamorphism in subduction zones.