Channelized Fluid Flow and Eclogite-Facies Metasomatism Along the Subduction Shear Zone

Abstract:

The Monviso ophiolite Lago Superiore Unit (LSU; Western Alps) constitutes a well-preserved, largely coherent fragment of eclogitic upper oceanic lithosphere subducted to ca. 80 km depth (between 50 and 40 Ma) and exhumed along the subduction interface. Within-slab, 10 to 100 m thick, eclogite-facies shear zones cut this section; the Intermediate Shear Zone (ISZ) follows the boundary between gabbroic and basaltic eclogites (associated with minor calcschist lenses), and the Lower Shear Zone (LSZ) marks the contact between gabbroic eclogites and the antigorite serpentinite sole. Up to 10 m fragments of mylonitic gabbroic eclogites were transported within serpentinite schists from the LSZ during eclogite-facies deformation. Metasomatic rinds, formed on these fragments during peak to early retrograde lawsonite-eclogite facies metamorphism (ca. 550 °C / 2.6 GPa), document episodic, prominent rock-fluid interaction along intra-slab, channelized fluid migration pathways associated with deformation.

We present new petrological and geochemical data on hydrous eclogites (talc-, chlorite-, lawsonite- and phengite-bearing eclogites) and serpentinite-derived ultrabasic schists from block rinds. Bulk-rock compositions, LA-ICP-MS mineral analyses and X-ray Cr/Mg maps of garnet and clinopyroxene demonstrate that these samples underwent significant enrichments in Mg, Cr, Ni, ± Large Ion Lithophile Elements and prominent depletions in Fe and V during eclogite-facies metasomatism. Boron isotopic data of phengite, clinopyroxene and chlorite, and antigorite suggest that metasomatic block rinds formed during interaction with serpentinite-derived fluids.

These compositional patterns point to focused, fluid-mediated element transfer through the subducted slab. Serpentinite-derived fluids via antigorite breakdown some 15-20 km deeper than the maximum depth reached by these eclogites thereby equilibrate with fluids derived from oceanic crust and/or sedimentary material. While slab components of diverse flavour have long been recognized to be central in triggering island arc magmatism, this example is among the first to document synkinematic, long-range, large-scale, channelized dehydration fluid flow within subducted oceanic slabs.