Plastic Deformation of Gabbros in a Slow-spreading Mesozoic Ridge: Example of the Montgenèvre Ophiolite, Western Alps

Abstract

Gabbros of the Montgenèvre ophiolite (external Piémont zone, western Alps), with the mineralogical, geochemical and isotopic characteristics of an oceanic ridge, show evidence of pervasive high-temperature deformation before the intrusion of basalt dikes. This deformation is incipient in the layered troctolites immediately above a locally preserved paleo-Moho, and increases in intensity upward in the overlying olivine-poor gabbros producing an anastomosing system of mylonitic shear zones and several steep ultramylonite belts. The mylonites and ultramylonites developed by solid-state shearing of the gabbros at temperatures in excess of 800–850°C, allowing recrystallization of augite subgrains in the wings of ductilely deformed magmatic diopside-salite clasts followed by crystallization of brown amphiboles. On the basis of textural relationships it is proposed that syn-kine-matic partial melting within the sheared gabbros, at temperatures of 850° C or higher, generated augite- and pargasite-bearing leucodioritic magmas which evolved in narrow magmatic conduits and percolated through tec-tonites and actively deforming mylonite zones, to form cross-cutting veins in adjacent less deformed gabbros. The complex geometry of the structures and their kinematics, when restored to a stage before the onset of pre-Alpine and Alpine brittle tectonics, suggest that this tectono-metamorphic evolution started after rapid solidification of the gabbros. It is proposed that the gabbroic mass was pushed away from a spreading center due to the activity of extensional and transform shear zones, in response to spreading through continuous domal uplift of the underlying mantle peridotites from which, at deeper levels, new basaltic melt was extracted to produce off-axis basaltic volcanism. The “magmatic” Moho became thereby transformed into a “tectonic” moho, followed by uplift of peridotites to the ocean floor where they became overlain by ultramafic-derived sediments such as to form a “sedimentary” Moho. The scarce geochronological data suggest a long time span, of about 50–60 Ma, between magmatic crystallization of gabbros and diorites (212± 8 Ma) and the deposition of the oceanic cover (165–160 Ma), suggesting a slow-spreading paleoridge in the relatively narrow (<1000 km) Piemonte-Ligurian branch of the Tethys ocean.