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Mélange (or block-in-matrix structures) exerts a first-order control on both the mechanical and chemical evolution of subduction megathrusts. However, the timing and mechanisms that form mélanges are variable and debated. Field observations and (micro-) structural analyses from a metasedimentary mélange in the lawsonite blueschist unit of the Catalina Schist (Santa Catalina Island, California, USA) reveal that syn-subduction deformation and fluid-mediated processes led to mélange formation at the plate interface. Deposited as turbidites, early shear occurred parallel to bedding planes (S1 foliation). At near peak subduction conditions, at the base of the subduction seismogenic zone (∼1.0 GPa, 320 °C), the rocks were intensely deformed in recumbent open to tight folds (F2) with axial planar cleavages (S2). Fracturing, fluid flow, and quartz precipitation are preserved as extensional vein mesh networks in fold noses. Continued shearing led to boudinage of these strengthened noses and transformation into strong blocks within the weaker less-veined matrix composed of high-strain fold limbs (S1−2). Microstructures reveal viscous deformation in the high-strain fold limbs occurred by pressure-solution creep of fine-grained quartz ± albite. In contrast, the fold noses and/or blocks contain coarse-grained quartz veins with little evidence of deformation. These rocks record the development of syn-subduction block-in-matrix mélange structures through the interaction of deformation and mineral precipitation; pressure solution weakened fold limbs-turned-matrix and veining strengthened fold noses-turned-blocks. Although mélange structure is often invoked to explain tremor and slow slip, rheological analysis indicates that these metasedimentary rocks can host tectonic creeping but cannot accommodate slow-slip strain rates by the deformation mechanisms preserved in their microstructures. 

Abstract Talc-rich metasomatic rocks in subduction interface shear zones profoundly influence seismicity and arc magmatism, but their petrogenesis remains controversial. Magnesium isotope compositions of exhumed subduction interface rocks from the Catalina Schist (California, USA) record Mg exchange from ultramafic to crustal rocks. Preferential loss of isotopically light Mg from serpentinite produces isotopically heavy talc-rich metasomatic rocks. Addition of this isotopically light Mg to adjacent metasedimentary and metamafic rocks from the slab produces actinolite- and chlorite-rich metasomatic rocks, respectively, with convergent δ26Mg values relative to their protoliths. The addition of Ca to ultramafic- and metasedimentary-derived metasomatic rocks reflects a separate contribution from infiltrating metabasalt-derived fluids. Talc-rich rocks are formed by passive enrichment of Si in serpentinite during Mg loss to adjacent Mg sinks. These results and a global compilation of exhumed paleosubduction terranes suggest that talc is a common component of the subduction interface and often forms independent of Si metasomatism. Talc is likely prevalent along the interface from mantle wedge corner to subarc wherever ultramafic material is in contact with a Mg sink and where it could influence slow slip events, subduction interface rheology, and arc magmatism in modern subduction zones. 

Porosity generated during fluid–rock reaction can facilitate fluid transport and metasomatism in low permeability high-pressure metamorphic rocks. Evidence for reaction-induced porosity is found in an eclogite-facies clinopyroxene + apatite vein in an undeformed eclogitized Fe–Ti metagabbro from the Monviso Ophiolite (W. Alps) with a distinct garnet-rich selvage. Vein-forming fluids were sourced from adjacent metagabbros and reaction with the host rock removed Ca and P from the selvage and added Fe, REE, Pb and Cr. Textures at the selvage–host rock interface and in the host rock record local heterogeneity in reactivity and porosity during metasomatism linked to variable initial lawsonite abundance. These features reflect a hierarchy of pervasive-to-channelized porosity structures that facilitated widespread metasomatism of the host rock. Development of this metasomatic system in response to locally derived fluids suggests large-scale externally derived fluid transport is not required to drive extensive fluid–rock exchange. The production of porosity during metasomatic reactions could be important in facilitating further fluid–rock reaction and fluid transport in subducting slabs where permeability is low. 

Abstract The Colorado Plateau has undergone as much as 1·8 km of uplift over the past 80 Myr, but never underwent the pervasive deformation common in the neighboring tectonic provinces of the western USA. To understand the source, timing and distribution of mantle hydration, and its role in plateau uplift, garnets from four eclogite xenoliths of the Moses Rock diatreme (Navajo Volcanic Field, Utah, USA) were analyzed in situ for δ18O by secondary ion mass spectrometry. These garnets have the largest reported intra-crystalline oxygen isotope zoning to date in mantle-derived xenoliths with core-to-rim variations of as much as 3 ‰. All samples have core δ18O values greater than that of the pristine mantle (∼5·3 ‰, mantle garnet as derived from mantle zircon in earlier work) consistent with an altered upper oceanic crust protolith. Oxygen isotope ratios decrease from core to rim, recording interaction with a low-δ18O fluid at high temperature, probably derived from serpentinite in the foundering Farallon slab. All zoned samples converge at a δ18O value of ∼6 ‰, regardless of core composition, suggesting that fluid infiltration was widely distributed. Constraints on the timing of this fluid influx, relative to diatreme emplacement, can be gained from diffusion modeling of major element zoning in garnet. Modeling using best estimates of peak metamorphic conditions (620 °C, 3·7 GPa) yields durations of <200 kyr, suggesting that fluid influx and diatreme emplacement were temporally linked. These eclogite xenoliths from the Colorado Plateau record extensive fluid influx, pointing to complex hydration–dehydration processes related to flat-slab subduction and foundering of the Farallon plate. Extensive hydration of the lithospheric mantle during this fluid influx may have contributed to buoyancy-driven uplift of the Colorado Plateau and melt-free emplacement of Navajo Volcanic Field diatremes.