ECLOGITE GEOCHEMISTRY: IMPLICATIONS FOR ELEMENT MOBILITY DURING SUBDUCTION ZONE METAMORPHISM (SIVRIHISAR, TURKEY)

Major and trace element compositions of eclogites and blueschists from high-pressure and ultrahigh-pressure terranes can be used to investigate the element fluxes that occur during sea-floor alteration and metamorphic processes in subduction zone settings. The first step in doing this is identifying the igneous protolith of rocks subducted into the mantle. Few rocks, however, return from subduction zones without extensive overprinting of mineral assemblages and textures. Eclogites and blueschists from Sivrihisar, Turkey contain unaltered lawsonite, which indicates that they have undergone little overprinting as they returned from deep within the subduction zone. As a result, these rocks are excellent for understanding trace element fractionation and mineral-fluid interaction during subduction zone prograde and retrograde metamorphism. At Sivrihisar, the metabasaltic eclogites and blueschists are interlayered with metamorphosed oceanic sediment (metacarbonate, metachert), indicating the possibility of chemical interaction among different rock types. These rock associations, as well as previous comparisons of major element compositions for the Sivrihisar eclogites and blueschists to average NMORB (Davis and Whitney, 2006) suggest a normal mid ocean ridge protolith. Abundances of trace elements (overall enrichment in the LILE and HFSE as compared to C1 chondrite and NMORB) as well as high ratios of relatively immobile elements (e.g., Th/Nb, Ti/Zr, Zr/Y, Nb/Y, Ce/Zr) suggest that these rocks have a composition similar to ocean island basalt. Overall, these rocks have trace element signatures that suggest an enriched mantle source, however, some mobile elements (e.g., Ba,Th) are enriched beyond that of OIB. Ultimately, our goal is to compare eclogite (altered vs. unaltered) and blueschist (prograde vs. retrograde) to track the chemical evolution of basaltic rocks from protolith to peak eclogite to initial decompression and partial overprinting under lawsonite blueschist facies conditions. By identifying the protolith of these rocks and thereby determining the initial igneous composition, we endeavor to identify the trace element abundances that are derived from non-igneous processes such as subducted sediment, crustal contamination, and the mantle wedge.