Origin of the Monviso ophiolitic massif

The Monviso ophiolitic massif is characterized by the alternation of tectonic units dominated by thick metabasalts with N-MORB geochemical signatures and Mg- and Fe-rich tholeitic metagabbros (Lombardo et al. 1978, MONVISO 1980). The basal serpentinite-rich unit appears to have originated from lherzolite overlain by a thin sedimentary sequence (Lemoine 1980). The assemblage of basalt-rich and basalt–poor units is similar to the lithology of low spreading oceanic lithosphere, such as the Mid Atlantic Ridge and the Indian ocean (Lagabrielle and Cannat 1990, Mevel 2003). This appears to be supported by dO18 values between 3.0 to 5.3 ‰ and NaCl equivalent salt contents of 17 to 21 wt% for the Monviso omphacite (Nadeau et al. 1993), and typical of a HT oceanic hydrothermal signature?? (Philippot et al. 1998). dO18 and salinity can support either hydration in the mantle wedge at high temperature or on the ocean floor. It rejects hydration at low temperatures on the ocean floor because low temperature hydration yields high dO18 values.

These units are interpreted as fragments of the Piedmont domain of the Western Tethys ocean during the Upper Jurassic and subucted during the Paleocene-Eocene. Recent U-Pb zircon ages show that magmatic activity occurred between 163 ± 2 Ma and 152 ± 2 Ma (Rubatto and Hermann 2003, Lombardo et al. 2002). Lombardo et al. (2002) noted this short duration of igneous activity in the Monviso massif and also a short time span (from ca 170 to ca 150 Ma) for the entire Piedmont-Liguria Tethys and suggested an embryonic ocean (max 380 km; Piccard et al. 2001, Schettino and Scotese 2002) rather than a mature, slow spreading, Atlantic-type ocean model (Lagabrielle and Cannat 1990). Blake et al. (1995) and Schwartz et al. (2001) suggested that part of the serpentintes represent hydrated peridotites at the base of mantle wedge, as observed in the Himalaya (Guillot et al. 2001). This is supported by the bulk composition of serpentinites, which show refractory character, similar to those from the Himalayas (our unpublished data).