Fluid and mass transfer along transient subduction interfaces in a deep paleo-accretionary wedge (Western Alps)

Abstract

Focusing of fluid flow along Western Alps paleo-subduction interfaces (at the base of the Dent Blanche nappe, NW Italy), afforded by high strain rates, is testified by extensive chemical alteration accompanying previously documented shifts in O isotope compositions resulting from fluid-rock interactions at prograde to peak high-P, low-T conditions. Flushing of these structures by H2O-rich fluids led to the development of prominent metasomatic reaction zones within and among disparate rock types including the calc-schists of the Schistes Lustrés complex and m- to km-scale slivers of metabasaltic/metagabbroic and meta-ultramafic rocks.Evidence for pressure solution, to produce a strong interface-parallel cleavage, presumably reflects at least local-scale redistribution of carbonate. Related to this redistribution, some samples exhibit evidence for albite dissolution and replacement by calcite ± dolomite ± paragonite. White mica, which exhibits high-Si cores related to prograde metamorphism is variably replaced by muscovite-rich rims interpreted as related to the influx of a CO2-bearing fluid at isobaric conditions based on thermodynamic modeling. Ratios of Ba, Rb, Cs, and N to K, all largely housed in micas (for N, as NH4+), indicate up to 60% loss of these elements within a few 10 s of meters of the interface contacts, presumably reflecting the preferential partitioning of these more fluid-mobile elements into the infiltrating fluids. Whole-rock δ15Nair shows some increase approaching the interface structures, from values of +3.8‰ to near +5.0‰, a trend consistent with Rayleigh-like loss during the partitioning of N into fluids from recrystallizing white-mica.Updip fluid flow along the studied structures resulted in the deposition of carbonate and SiO2, as reflected in the abundant, variably transposed, carbonate + quartz vein generations and more pervasive carbonation (within metasedimentary rocks) and silicification (within talc-rich ultramafic rocks). The O isotope shifts proximal to the structures appear to reflect the sourcing of the infiltrating fluids at greater depths in mafic/ultramafic parts of the down-going slab or structurally complex interface region. Although models of subduction-zone volatile cycling implicitly involve the flushing of fluids along or across subduction interfaces, the permeability of these regions, among disparate rocks types and evolving in response to the localization of deformation, likely dictates the magnitude of this transfer. Field-based studies of this type can yield insight regarding mechanisms of m- to km-scale fluid mobility but, given the fragmentary nature of the metamorphic rock record, do not permit quantitative assessment of the cycling of elements such as C at any individual modern (or ancient) margin.