Reaction rind formation in the Catalina Schist: Deciphering a history of mechanical mixing and metasomatic alteration

In the subduction-related Catalina Schist, reaction zones or 'rinds' developed at contacts of metamorphosed mafic blocks with chemically distinct mélange matrix. These rinds exhibit complex chemical compositions that defy simple, single-process explanations. A comparison of high-grade and low-grade reaction rinds, including field observations, bulk-rock geochemistry, thin section textural observations, mineral compositional analyses, and thermodynamic modeling provides constraints on the nature and timing of the rind-forming processes. Bulk-rock variations in the highly siderophile elements (HSE, including Os, Ir, Ru, Pt, Pd and Re) and MgO, Cr, and Ni provide insight into the physical processes of mixing of ultramafic material of the mélange matrix, rich in these elements, with mafic material derived from the blocks. Enrichments in the concentrations of these elements are found in both low-grade and high-grade reaction rinds suggesting that the processes that enrich these elements and create the reaction rinds occur over a wide range of P-T conditions in subduction zones. Differences in the chemical composition of garnets in an amphibolite-grade metamafic block, compared with garnets in the associated reaction rind, suggest that at least some of the mixing occurred before the growth of garnet in the blocks, perhaps at P-T conditions different from those producing the garnet. Bulk-rock variations in elements commonly considered fluid-mobile, such as K₂O, Ba and Rb, are observed in all reaction rinds. At high-grade, enrichments of K₂O, Ba and Rb are associated with replacement of garnet and amphibole by phengite and chlorite. Because phengite is the major host for K, Ba and Rb, the addition of these elements by fluids appears to have been a post-garnet growth phenomenon. This study highlights the significance of processes of metasomatism and mechanical mixing in mélange zones for influencing the geochemical evolution of the slab-mantle interface. Any "fluids" (hydrous or silicate melts) emanating from subducting slabs and entering the mantle wedge and arc lava source regions would necessarily reflect the hybrid compositions created by these processes.