Mantle Wedge Metasomatism During Subduction Initiation: Mineral Transformations and Fluid Fluxes (BOIC Ophiolite, Newfoundland)

Abstract

The soles of ophiolites preserve slices of oceanic crust metamorphosed to amphibolite‐ to granulite‐facies conditions beneath heavily deformed mantle peridotites, providing key records of fluid and mass transfer across nascent subduction interfaces. This study investigates these processes in the Bay of Islands Ophiolitic Complex (BOIC) in Newfoundland through field observations, petrology, and geochemistry. Textural and chemical analyses reveal progressive metasomatism of basal peridotites, recorded by crystallization of pargasitic amphibole and compositional changes in pyroxenes, olivine and spinel up to 2 km above the metamorphic sole. Mantle peridotites evolve from weakly deformed porphyroclastic peridotites far from the contact with the metamorphic sole to ultramylonites close to the contact, reflecting fluid‐assisted strain localization. Thermometry on the peridotites indicated syn‐kinematic cooling from approximately 1150°C–800°C. At the contact, primary mantle‐derived minerals are entirely replaced by a re‐equilibrated ultramafic assemblage of metamorphic nature. Metasomatism was dominantly triggered by aqueous fluid infiltration, as shown by trace‐element analysis. Melt percolation was spatially restricted and not voluminous. The underlying nascent slab (the future metamorphic sole) undergoing dehydration was the likely source of metasomatic fluids. Based on these observations, a petrological evolution model for a young mantle wedge is proposed through comparisons with other ophiolites, where efficient mass transfer also appears to be restricted to a few hundreds of meters above the major tectonic contact with the metamorphic sole.