Evolution of the Ligurian Tethys in the Western Alps: Sm/Nd and U/Pb geochronology and rare-earth element geochemistry of the Montgenèvre ophiolite (France)

Abstract

We provide geochemical and geochronological data for gabbro, diorite and albitite samples from the Montgenèvre ophiolite in the Western Alps. This well-preserved remnant of the Piemont–Ligurian oceanic basin shows evidence of intra-oceanic deformation and metamorphism, but has suffered minor ductile deformation and metamorphism during the Alpine orogeny. The gabbros have geochemical features and initial Nd isotopic signatures similar to that of Mid-Oceanic Ridge gabbros, indicating that they were extracted from a depleted mantle source with no evidence of continental contamination ( ε Nd( T)>+8). Cumulation played an important role in the genesis of these gabbros, and their rare-earth element (REE) patterns are controlled by the major cumulus phases. Modelling the REE data for the gabbros and diorites provides support for the hypothesis that the dioritic magmas have been derived from small-degree (≤5%) partial melting of the surrounding gabbros during shearing at high-temperature (high-T). Zircons from a leucodioritic vein within sheared gabbros are poorly discordant and cluster the Concordia at 156±3 Ma, while zircons from an albitite lens within the mantle-rocks display concordant ages at 148±2 Ma. In both cases, the zircons show no evidence of inheritance, and these values are interpreted as crystallisation ages. The ∼160–150 Ma age bracket very likely records a late stage in the magmatic history of the Montgenèvre ophiolite, since the emplacement of diorite clearly post-dates the layered gabbros. These radiometric ages correlate well with the Late Bathonian to Early Kimmeridgian stratigraphic age (∼160–140 Ma) of the earliest post-ophiolitic radiolarian sediments. The whole-rock Sm–Nd system of the gabbros yielded an isochron age of 198±22 Ma ( ε Nd( T) of +8.8). This value is significantly older than the ∼165 Ma age of Western Alps ophiolitic gabbros published elsewhere, but is similar within error to the ∼180 Ma age of eclogitised gabbros from the Ligurides and Corsica. This early Jurassic age is interpreted as the emplacement age of the gabbroic melts into the mantle lherzolites and could be associated with the early stage of rifting between the European and Austro-Alpine continental margins. These ages and their interpretation are consistent with the model of asymmetric mantle-denudation by an oblique detachment fault, proposed by Lemoine et al. (1987). This model implies that the newly formed lherzolite–gabbro oceanic domain probably remained close to the spreading centre and therefore experienced slow cooling and low spreading rates. This appears to be correlated by our radiometric results that suggest a life span of at least 30 Ma for the formation of the oceanic crust in this part of the Piemont–Ligurian ocean.