Tectonic and metasomatic mixing in a high-T, subduction-zone melange -: 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 degreesC) melange unit consisting of mafic and ultramafic blocks in high-Mg, schistose melange matrix with varying modal proportions of tale, chlorite, anthophyllite, calcic-amphibole, enstatite, and minor phases including zircon, rutile, apatite, spinel, and Fe-Ni sulfides. This melange 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 melange, the rinds developed at the margins of these blocks, and the surrounding melange matrix are compatible with the evolution of the melange 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 melange 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 melange 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 blacks 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 melange, 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 melange matrix can be delineated based on the Cr/Al-based mixing model. Simple mixing relationships exhibited by Al, Cr, Mg, Ni, Fe, and Zr provide a geochemical reference frame for considerations of mass and volume loss and gain within the melange matrix. The compositional patterns of many other elements are explained by either redistribution (local stripping or enrichment) at varying scales within the melange (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 melange. Melange 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 tale, chlorite, and Mg-rich amphiboles) would impact the theology 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 magma, producing "fluids" with geochemical signatures inherited from interactions with hybridized rock compositions. (C) 2002 Elsevier Science B.V All rights reserved.