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

摘要

The changing XCO2 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-NaAlO2-KAlO2-Al2O3 system with excess quartz and a CO2-H2O binary fluid phase. Using the common assemblage of calcite-albite-muscovite-clinozoisite, XCO2 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-XCO2 relationship was delineated to serve as a quantitative monitor for the evolving fluid composition during the progressive metamorphism in subduction zones. (C) 2006 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.