Crustal Inputs, Metamorphic Processing, and Fluid Flow Architecture in the Catalina Schist Subduction Complex: Constraints From Detrital Zircon U-Pb Ages and Pb-B Isotope Systematics of Melange Zones

Evaluation of geochemical recycling during subduction generally involves comparison of seafloor "input" with arc volcanic "outputs". This simple approach is problematic in that it fails to consider metamorphic recrystallization of mass during subduction. Here we address the impact of metamorphism upon the (U+Th)-Pb system in subduction melange zones. Pb isotopic ratios of Catalina Schist melange zones are very radiogenic (206Pb/204Pb = 18.72-22.53; 207Pb/204Pb =15.62-15.84; 208Pb/204Pb =38.34-44.01) and hence suggestive of evolved continental components. To evaluate bulk input compositions, we utilized age probability spectra based upon detrital zircon U-Pb ages from the Catalina Schist. Predicted bulk input Pb ratios derived from the zircon age spectra for all grades of melange consistently anchor the least radiogenic melange compositions. While linear arrays within the melange Pb isotopic data appear to be mixing trends, the fact that they extend to radiogenic compositions well outside the field defined by potential continental sources requires secondary (metamorphic) fractionation to produce higher U/Pb and Th/Pb ratios during subduction. This requires significant Pb redistribution during metamorphism (most likely via desulfidation reactions) and efficient retention of U and Th in metamorphic phases (i.e., zircon, apatite, monazite, epidote-group, etc.). Apparent Pb mixing arrays in this scenario can only be explained by heterogeneity in phase assemblages and/or heterogeneous magnitudes of Pb loss within melange zones. B isotopic data constrain the fluid flow architecture during melange formation. Lawsonite-albite and lawsonite-blueschist melange preserve B ratios indistinguishable from compositions predicted for metamorphosed subduction zone inputs by published Rayleigh fractionation models for B isotopic evolution during subduction (δ11B =-6 to -12‰; T=150-350°C). This suggests these low-grade melange zones preserve chemical information regarding fluid sources during subduction. In contrast, elevated amphibolite facies δ11B (+4 to -4‰) are consistent with the isotopic composition of derived fluids in subduction zones predicted by the same Rayleigh fractionation models for observed petrologic temperatures (600-750°C). Because the amphibolite melange records events occurring within the highly-deformed edge of a mantle wedge, the B results imply that mixing arrays and heterogeneous Pb isotopic compositions of AM melange are inherited from fluids derived from deeper (lower-temperature) structural levels. These factors suggest Pb isotopic compositions of fluids along the slab-mantle interface and progressing through the mantle wedge are strongly heterogeneous and significantly modified from input compositions during subduction-zone metamorphism.