Tectonic and metasomatic mixing in a high-T, subduction-zone mélange - Insights into the geochemical evolution of the slab-mantle interface

The Catalina Schist (California) contains an amphibolite-grade (0.8- 1.1 GPa; 640-750 °C) mélange unit consisting of mafic and ultramafic blocks in high-Mg, schistose mélange matrix with varying modal proportions of talc, chlorite, anthophyllite, calcic-amphibole, enstatite, and minor phases including zircon, rutile, apatite, spinel, and Fe-Ni sulfides. This mélange unit is interpreted as a kilometer-scale zone of tectonic and metasomatic mixing formed within a juvenile subduction zone, the study of which may yield insight into chemical mixing processes at greater depths in subduction zones. Relationships among the major and trace element compositions of the mafic and ultramafic blocks in the mélange, the rinds developed at the margins of these blocks, and the surrounding mélange matrix are compatible with the evolution of the mélange matrix through a complex combination of infiltrative and diffusional metasomatism and a process resembling mechanical mixing. Simple, linear mixing models are compatible with the development of the mélange matrix primarily through simple mixture of the ultramafic and mafic rocks, with Cr/Al ratios serving as indicators of the approximate proportions of the two lithologies. This conclusion regarding mafic-ultramafic mixing is consistent with the field observations and chemical trends indicating strong resemblance of large parts of the mélange matrix with rinds developed at the margins of mafic and ultramafic blocks. The overall process involved development of metasomatic assemblages through complex fluid-mediated mixing of the blocks and matrix concurrent with deformation of these relatively weak rind materials, which are rich in layer silicates and amphibole. This deformation was sufficiently intense to transpose fabrics, progressively disaggregate more rigid, block-derived materials in weaker chorite- and talc-rich mélange, and in some particularly weak lithologies (e.g., chlorite-, talc, and amphibole-rich materials), intimately juxtapose adjacent lithologies at the (sub-)cm scale (approaching grain scale) sampled by the whole-rock geochemical analyses. Chemical systematics of various elements in the mélange matrix can be delineated based on the Cr/A1-based mixing model. Simple mixing relationships exhibited by A1, Cr, Mg, Ni, Fe, and Zr provide a geochemical reference frame for considerations of mass and volume loss and gain within the mélange matrix. The compositional patterns of many other elements are explained by either redistribution (local stripping or enrichment) at varying scales within the mélange (Ca, Na, K, Ba, and Sr) or massive addition from external sources (Si and H₂O), the latter probably in infiltrating H₂O-rich fluids that produced the dramatic O and H isotopic shifts in the mélange. Mélange formation, resulting in the production of high-variance ultramafic assemblages with high volatile contents, may aid retention of volatiles (in this case, H₂O) to greater depths in subduction zones than in original subducted mafic and sedimentary materials. The presence of such assemblages (i.e., containing minerals such as talc, chlorite, and Mg-rich amphiboles) would impact the rheology of the slab-mantle interface and perhaps contribute to the low-velocity seismic structure observed at/near the slab-mantle interface in some subduction zones. If operative along the slab-mantle interface, complex mixing processes such as these, involving the interplay between fluid-mediated metasomatism and deformation, also could impact slab incompatible trace element and isotopic signatures ultimately observed in arc magmas, producing "fluids" with geochemical signatures inherited from interactions with hybridized rock compositions.