GEOCHEMICAL SIGNATURES OF MECHANICAL AND METASOMATIC MIXING IN FOREARC MÉLANGE ZONES

We are investigating trace element and Sr-Nd isotopic systematics of lawsonite-albite-, lawsonite-blueschist-, and amphibolite-facies mélange matrix in the Catalina Schist, CA, in order to assess the geochemical result of mélange formation and fluid flow in subduction zones. Field evidence, petrology, and major-element data indicate that the amphibolite-facies mélange is primarily the result of binary mechanical mixing between mafic and ultramafic protoliths, producing a mineralogy dominated by Tlc, Chl, Oam, Cam, and accessory Rut ± Zrc ± Ap. Despite low NMORB-normalized element abundances of 0.01 to 1, elevated Th/Yb and [La/Yb]_N indicate that fluids affecting this amphibolite-facies mélange were buffered by a sedimentary source. Furthermore, Nb/Ta ratios of ~12 match expectations for fluids equilibrated with rutile with a sedimentary-like Nb/Ta ratio of ~14. Similar trace element systematics for metasomatic rinds on mafic blocks in the amphibolite-facies mélange (Sorensen and Grossman, 1989), strongly support a fluid origin for these sedimentary characteristics in the strongly ultramafic mélange. Sr isotope ratios (⁸⁷Sr/⁸⁶Sr; 0.7047 to 0.7078) and epsilon Nd (115Ma; +0.84 to +1.59), along with dD and d¹⁸O isotope systematics, indicate the involvement of sedimentary-derived fluids in the genesis of the amphibolite-facies mélange matrix.

In contrast to the amphibolite-facies mélange, field evidence and petrology indicate mechanical mixing of sediments was important in the genesis of the lower-grade mélange matrix. While trace element data are incomplete, initial results indicate higher normalized concentrations for these units, but element ratios similar to those of the petrogenetically distinct, higher-grade mélange. Such zones of mechanical and metasomatic mixing could in some cases be sources of fluid chemistries indicating mixing in serpentinite seamounts and arc volcanics, rather than the mixing of distinct fluids derived from intact pre-subduction lithologies.