| dc.description.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. | |
