ACCESSORY MINERALS AND SUBDUCTION ZONE METASOMATISM - A GEOCHEMICAL COMPARISON OF 2 MELANGES (WASHINGTON AND CALIFORNIA, USA)

The ability of a subducted slab or subducted sediment to contribute ma ny incompatible trace elements to arc source regions may depend on the stabilities of accessory minerals within these rocks, which can only be studied indirectly. In contrast, the role of accessory minerals in lower-T and -P metasomatic processes within paleo-subduction zones can be studied directly in subduction-zone metamorphic terranes. The Gee Point-Iron Mountain locality of the Shuksan Metamorphic Suite, North C ascades, Washington State, is a high-T melange of metamatic blocks in a matrix of meta-ultramafic rocks. This melange is similar in geologic setting and petrology to the upper part of an unnamed amphibolite uni t of the Catalina Schist, Santa Catalina Island, southern California. Both are interpreted as shear zones between mantle and slab rocks that formed during the early stages of subduction. Some garnet amphibolite blocks from the Gee Point-Iron Mountain locality display trace-elemen t enrichments similar to those in counterparts from the Catalina Schis t. Some Catalina blocks are highly enriched in Th, rare-earth elements (REE), the high-field-strength elements Ti, Nh, Ta, Zr and Hf (HFSE), U and Sr compared to mid-ocean ridge basalt (MORB), and to other garn et amphibolite blocks in the same unit. Textural and geochemical data indicate that accessory minerals of metamorphic origin control the enr ichment of Th, REE and HFSE in blocks from both areas. The Mg-rich rin ds around blocks and the meta-ultramafic matrix from both melanges are highly enriched in a large number of trace elements compared to harzb urgites, dunites and serpentinites. Evidence for recrystallization or formation of accessory minerals in the former rocks suggests that thes e minerals control some of the trace-element enrichments. Data from th e Gee Point and Catalina melanges suggest that the accessory minerals titanite, rutile, apatite, zircon and REE-rich epidote play a signific ant role in the enrichment of trace elements in both mafic and ultrama fic rocks during subduction-related fluid-rock interaction. Mobilizati on of incompatible elements, and deposition of such elements in the ac cessory minerals of mafic and ultramafic rocks may be fairly common in fluid-rich metamorphic environments in subduction zones.