Presented at the 1998 Annual Meeting of the Geological Society of America

SADOFSKY, Seth J., and BEBOUT, Gray E., Earth & Environ. Sciences. Dept.,   Lehigh University, 31 Williams Dr., Bethlehem, PA 18015, sjs8@lehigh.edu

    Metamorphism in forearcs of convergent margins nay play a key role in the global C and N cycles.  In order to quantitatively assess the significance of subduction-zone fluxing in global C cycling models, the degrees of C retention (forearc storage, deep subduction) or C mobility/loss at depth needs to be constrained.  High-P/T metamorphic rocks allow the evaluation of preservation or modification of organic signatures (C, N, d¹³C, d¹⁵N) to depths of ~50 km in varying subduction-zone thermal regimes.  The Franciscan Complex (Diablo, Coast Ranges, CA) and the Western Baja Terrane (Mexico) contain abundant subduction-zone metamorphosed clastic sediments having experienced a wide range in peak-P-T (~3-10 kbar, 150-300 degrees C).  This sediment likely represents a large fraction (>75 wt. %) of the sediment subducted beyond forearcs (von Huene and Scholl, 1991; Plank and Langmuir, 1998).

    Ranging from incipiently metamorphosed greywacke-shale (Coastal Belt) to fully recrystallized schists (Pacheco Pass, Eastern Belt) these metasedimentary rocks have high C and N contents (up to 1.5 wt. % C, 1000 ppm N) that vary with clastic lithology (less C and N in sandier rocks).  C/N ratios (1.5 to 20) are within the range of marine sediments.  d¹⁵N_air values (+0.3 to +3.0 per mil) are similar to values for seafloor sediments, but vary some with locality.  d¹³C_PDB in Franciscan rocks (-28.2 to -24.2 per mil) is similar to that of seafloor sediment, whereas slightly higher values in the WBT (-25.0 to -21.9 per mil) may reflect minor devolatilization.  Carbonate veins in all exposures have d¹³C from  (-14 to -2 per mil) compatible with local-scale bufferring by organic C in host rocks.  Veins are regarded as products of upward C migration in fluids, but may be significant reserviors contributing to deeper C flux.

    In cool subduction zones represented by these suites, sediments appear to retain seafloor C-N contents and isotope ratios, despite thermal maturation of the organic matter (resulting in the release of organic N), methanogenesis, N partitioning into clays/low grade micas, and expulsion of pore fluid.  Combination with results from the Catalina Schist (CA) indicates that warmer subduction results in significant modification of C-N content and isotopic composition in the same lithology.  Depending on the extents of C and N loss during deeper forearc metamorphism (>50 km), organic signiatures could be returned to the surface by arc magmatism or contribute to deep mantle volatile budgets and isotopic evolution.