Presented at The 1996 Annual Meeting of the Geological Society of America

BEBOUT, Gray E., Dept. Earth & Environmental Sciences, Lehigh University, Bethlehem, Pennsylvania 18015, geb0@lehigh.edu.

    Differential chemical/isotopic alteration during forearc devolatilization can strongly impact the trace element and stable isotope signature of the slab-derived "fluid" flux traced by analyses of arc volcanic rocks. Models for metamorphism of the Catalina Schist involve progressive underplating (at ~45km depths) of rock packets metamorphosed along successively lower-T prograde P-T paths in a rapidly cooling, newly initiated subduction zone (Grove and Bebout, 1995). Thus is it possible to evaluate effects of varying P-T paths on the magnitudes of devolatilization and chemical/isotopic alteration of subducting rocks.

    In the Catalina Schist, the most extensive devolatilization occurred in metasediments which experienced P-T paths encountering the epidote-blueschist facies (>350 degrees C) or higher-T conditions; such rocks are depleted in "fluid-mobile" elements such as N, B, Cs, As, and Sb and shifted in C_reduced d¹³Cand d¹⁵Nrelative to protoliths. Removal of these elements resulted in changes in B/(Be,La,Zr), Cs/Th, Rb/Cs, As/Ce, Sb/Ce, and C_reduced/N. Relative susceptibilities of the "fluid-mobile" elements to loss along increasingly higher-T P-T paths can be categorized. B and Cs show the greatest susceptibility to low-T removal by fluids, showing >50% depletion in even lawsonite-blueschists. At higher grades, As and Sb (in sulfides) show the greatest depletions (>90%); N, Cs, and B (largely in micas) occur at ~25% of protolith contents in even partially melted amphibolite-facies rocks.

    Cross-arc declines in B/Be, Cs/Th, As/Ce, and Sb/Ce, and variations in these ratios among arcs from differing convergent margin thermal regimes (Ryan et al., 1995; Noll et al., 1996), are compatible with evidence from the Catalina Schist. High-grade units could reflect thermal evolution analogous to that of relatively warm subduction zones and back-arcs in which arc lavas are depleted in B, Cs, As, and Sb due to prior removal by devolatilization. Prograde thermal history of Catalina Schist lawsonite-blueschists was similar to that experienced by the Cedros and Franciscan Complexes (<350 degrees C at >30 km), which are similar in volatile content to the low-grade Catalina Schist. In cool subduction-zones represented by these low-grade rocks, B, Cs, As, Sb, and N are more likely to be enriched in arc lavas and significant devolatilization occurs at the blueschist-to-eclogite transition.