RECORDS OF FOREARC DEVOLATILIZATION IN THE FRANCISCAN COMPLEX, CATALINA SCHIST, AND WESTERN BAJA TERRANE

Geochemical study of devolatilization in HP/UHP forearc metasedimentary suites allows assessment of the extent to which fluid-mobile elements (FME) are released into metamorphic fluids and transported up-dip toward the seafloor or into forearc mantle wedges, or retained to subarc depths where they could be at least partly released into aqueous fluids or hydrous melts contributing to arc magmatism. Recent work on Franciscan and associated metasedimentary suites indicates that extents of subduction-zone devolatilization strongly depend on prograde P-T paths.

Incipiently devolatilized metasediments in the Franciscan (Diablo and Coast Ranges, the latter studied in detail by M.C. Blake), the Western Baja Terrane, and low-grade units of the Catalina Schist (recent tectonic model in Grove et al., 2008; GSA Spec. Pap. 436) have concentrations of FME (N, B, Cs, As, Sb, and Li), and C and N isotope compositions, strongly overlapping with those of modern seafloor equivalents (Sadofsky and Bebout, 2003; G-cubed; study of fluid release and veins by Sadofsky and Bebout, 2004; Int. Geol. Rev.). This indicates that these elements were largely retained to depths of ~35 km in relatively cool subduction zone settings (<300˚C at 35 km). Extending this to greater depths, Busigny et al. (2003; EPSL) and Bebout et al. (2008, AGU abstract) report retention of protolithic concentrations of N, B, Li, and Cs in W. Alps metasediments subducted to ~90 km depths in a cool paleo-subduction zone. Catalina Schist units that experienced higher-T prograde paths (>300˚C at 35 km) show significant loss of volatiles and FME perhaps analogous to that in warmer modern forearcs (e.g., Cascadia) and during Archean subduction.

These studies indicate that, in cooler margins, large fractions of sedimentary volatiles and FME can be subducted to beneath arcs and made available for transfer into arc source regions or entrained into the deeper mantle. In sediments, subduction of N, B, and LILE is strongly influenced by reactions involving micas, for greater depths the extent to which micas are stable in melting residues. Related ongoing work includes investigation of Li isotope behavior in subduction zones and integrated theoretical, petrologic, and ion microprobe study of the role of phengite during devolatilization of W. Alps HP/UHP metasedimentary units.