Mantle geochemistry indicates original magmatic heterogeneity was unaltered during subduction infancy: Insights from the Mont Albert ophiolite complex (Québec, Northern Appalachians)

Abstract

Subduction initiation is a necessary prerequisite to generate Earth-like plate tectonics. However, it is not yet understood how elements begin to cycle through the lithospheric mantle during subduction infancy and how they initially affect mantle composition and thus slab-mantle interface rheology. Progressive development of a “mature” forearc mantle wedge above a subducting slab typically results in melt and fluid metasomatism in the upper plate’s lithospheric mantle, often thought to drastically change original (micro)structures and rock chemistry. Ophiolitic complexes provide a window into the chemical and structural evolution during the earliest stages of formation of a subduction interface in the supra-subduction zone (SSZ) mantle wedge. The Mont Albert Ophiolite Complex (Québec, Northern Appalachians, Canada) is an ideal locality to study the slab-mantle interface, because the contact zone between the down-going crust (Iapetus Ocean) and upper-plate lithosphere (Laurentia) of an Ordovician subduction zone is well-exposed. Focusing on the upper plate mantle, I combined field-based structural observations, optical and electron microscopy, and bulk rock major and trace element geochemistry to investigate the structural and metasomatic changes occurring in the immediate hanging wall during subduction initiation. At the “Mantle Wall” locality, serpentinized peridotites exhibit a strong moderately dipping foliation across a ~100 m structural section, in an apparently in-tact orientation relative to paleo-subduction and obduction. Microstructural observations demonstrate the degree of serpentinization decreases with structural height above the paleo-plate interface. Geochemical tracers including Cr#, Rare Earth Elements (REE) and Fluid Mobile Elements (FME) demonstrate that the tectonic setting of original mantle was in an abyssal setting, the extent of melt extraction from the mantle is heterogeneous (between 5-15% over ~60 m of structural section), and serpentinite metasomatism did not impart a systematic chemical overprint to the rocks. I infer that the peridotites underwent limited melt-reactions and limited FME-enrichment during serpentinization after subduction initiation, and the differences in apparent depletion pre-date subduction. Preservation of original lithospheric complexity in an infant subduction zone demonstrates that mantle wedge chemistry is heavily dependent on the maturity of the fore-arc. The late-stage serpentinization opposes the notion that serpentine is essential in subduction initiation. This study rather illuminates the preservation of original mantle heterogeneity in the form of original compositional layering, which may play a role in strain localization during subduction initiation. Combining chemical analysis with structural field context appears critical to accurately characterize the composition of a SSZ mantle wedge and understand its evolution.