Presented at the 1998 Spring meeting of the American Geophysical Union

Seth J. Sadofsky and Gray E. Bebout (Earth and Environmental Sciences, Lehigh University, Bethlehem, PA 18015, USA, 610-758-5393, sjs8@lehigh.edu)

    We are investigating fluid-rock interactions and volatile contents in high-P/T metamorphic rocks representing ancient acretionary complexes to better understand mechanisms of and efficiency with which subduction zones transfer volatile components (H₂O, C, N and other fluid-mobile trace elements) to great depths (including the mantle) or cycle these elements back to shallow reservoirs (including the oceans). Textures and mineralogy of multiple vein sets present in metasedimentary rocks of the Franciscan Complex, CA, provide records of deformation and fluid sources and scales of mobility as functions of depth in an evolving accretionary complex.

    In metasedimentary and melange exposures in the Coastal Belt (peak T-P ~125°C, < 3 kb), Central Belt (150-250 degrees C, ~3 kb), and Eastern Belt (<200 degrees C, ~6 kb) of the California Coast Ranges and near Pacheco Pass (~150 degrees C, 7-8 kb), multiple vein sets show wide ranges in d¹⁸O and, especially, d¹³C. Veins, with or without high-P/T minerals (e.g., jadeite and aragonite at Pacheco Pass; cf. Ernst, 1993), range from discontinuous fracture-fillings perpendicular to bedding in sandy layers to planar, continuous features cross-cutting sandy and shaley domains. d¹³C_PDB ranges from ~ 0 per mil in some of the lowest-T rocks to -16.9 per mil in some of the higher-T rocks, and reflects a strong influence by abundant organic matter (up to 3 wt. %) in the host rocks. In general, fluid d¹⁸O appears to have been buffered locally by the host rocks, producing a trend of decreasing calculated H₂O-d¹⁸O toward values near 0 per mil with decreasing peak T. However, some d¹⁸O values heavier than could be explained by local rock buffering, may reflect up-dip (down-T) flow of fluids equilibrated with rocks at higher T and greater depth.

    Whole-rock N contents of metashales and metagreywackes (100-700 ppm) and d¹⁵Nair (+0.2 to +3.0 per mil) are similar to those of seafloor sediments, suggesting that little loss of N occurred during devolatilization despite deep subduction/accretion and heating up to 250 degrees C. The variation in d¹⁵N among localities could reflect differing initial organic N-isotope signatures related to sediment sources.