Geochemistry of metagabbros: a tool for deciphering tectonic processes along an alleged subduction interface
Résumé
Gabbroic rocks may be found in ocean-derived units ( as
part of ophiolite complexes) as well as inside the continental
crust (as layered complexes emplaced at different crustal
levels). When both ocean- and continent-derived units are
juxtaposed during or following subduction, the two types of
gabbroic rocks may be found close to each other. Assessing
the amount of mixing between the two types of material in the
subduction zone is critically dependent on the recognition of
the origin of the gabbros.
An example will be illustrated in the Western Alps, along
the contact between the continental-derived Dent-Blanche
and the ocean-derived Combin units. Detailed mapping
associated with geochemical characterization of the magmatic
protoliths allow us to accurately describe the geometry of this
contact, exemplified as a serpentinite shear zone along a
former subudction interface.
The main results of our study are (i) the identification of a
new Permian gabbroic complex in the Dent Blanche unit [1],
(ii) the lack of melange between continental- and oceanderived
material along the interface, and (iii) an orderly
sequence of structures progressively building the contact,
from the blueschist-greenschist transition at depth to the late
pseudotachylites close to the surface.
[1] Manzotti et al. (2017) J. Geol. Soc. London,
doi.org/10.1144/jgs2016-071