Metasomatism during subduction: products and possible paths in the Catalina Schist, California

Abstract

On Santa Catalina Island, southern California, lawsonite-albite to amphibolite facies metasedimentary, metamafic, and metaultramafic rocks show veining and chemical alteration that reflect fluid flow and mass transfer at 15 to 45 km depths in an Early Cretaceous subduction zone. In many exposures, multiple generations of cross-cutting syn- and post-kinematic veins record fluid transport and metasomatism during various stages of prograde metamorphism and uplift. Mineralogy and whole-rock compositions demonstrate chemical redistribution, especially of Si, Al, and alkali elements (Na, K), but also of many trace elements, particularly B and LILE (Rb, Cs, Sr, and Ba). Evidence exists for mass transfer, at both local and larger scales, via mechanical mixing, diffusional, and fluid-mediated transfer processes. Highest-grade, amphibolite facies rocks contain feldspar + quartz ± mica ± amphibole leucosomes and pegmatites attributed to migmatization; the leucosomes and pegmatites reflect high- P T mass transfer in felsic silicate liquids. Veining and replacement in blueschist grade rocks comprise three contrasting types of assemblages: (1) silica-saturated (quartz-rich), (2) potassic (white-mica ± quartz-rich), and (3) sodic and silica-undersaturated (albite/Na-amphibole-rich, quartz-absent). Evidence for silicification and alkali exchange also occurs in greenschist and amphibolite facies units. In all units, the evidence for metasomatism (e.g., veins; stable isotope homogenization; rinds on blocks) is particularly abundant in melange zones, in which melange matrix compositions resulting from mechanical mixtures of mafic, ultramafic, and sedimentary rocks were shifted by metasomatic additions and subtractions during melange formation. Geochemical evidence (particularly stable isotope data) indicates that the blueschist, greenschist, and amphibolite units exchanged with fluids of similar compositions. The diverse metasomatic features in the Catalina Schist provide evidence regarding fluid sources and paths. Based on the stable isotope data, the H 2O-rich, low-salinity (∼ 1 to 2 equivalent wt. % NaCl), C/1bO/1bH/1bS/1bN fluids are believed to have been derived from low-grade, largely sedimentary parts of the subduction zone (analogs for fluid sources are the low-grade units). Metasomatic changes could be driven by flow across boundaries between contrasting lithologies and by variations in pressure and temperature along the fluid flow paths. Simple predictions of mass changes along different P- T paths suggest that both mechanisms could be effective at producing the range of observed features, even though the required equilibrium constants are only poorly estimated at the relevant P- T conditions. Decreasing T and P favors fixing of K, Si, C, and H in rocks, whereas increasing T (± moderately decreasing P) should fix Na but leach most other components. The Si-rich, K ± Si-rich, and Na-rich/Si-poor assemblages are thus consistent with differing fluid P- T flow paths. Regular differences are expected in silica precipitation/dissolution, alkali exchange, and hydrogen-alkali exchange reactions, among others. Silica ± carbonate addition, consistent with the majority of veins observed, is likely the consequence of cooling ± decompression whereas sodic (± silica-undersaturated) assemblages would be expected for rarer, but geologically plausible up- T fluid flow paths. A composite fluid flow path, first up-grade, then down P and T. is indicated for the silica addition to the largely ultramafic amphibolite-facies melange. Although mass balance and physical constraints appear to preclude pervasive major element metasomatism on large scales, focussing of fluids would likely produce pervasive changes in significant volumes (e.g., up to km-scale melange zones). Vein mineralogy would record the paths even at small fluxes. Study of the Catalina Schist demonstrates the significance of metasomatism at all scales, but indicates that large-scale changes in vein mineralogy and bulk composition are in some cases attributable to fluid flow over large distances. Comparison with other areas and elementary theoretical considerations suggest that these processes may be widely developed and that their petrographic and geochemical effects potentially give insight into the dynamics of subduction zones.