Eclogitization of the Monviso ophiolite (W. Alps) and implications on subduction dynamics

To constrain deep (40100 km) subduction dynamics, extensive PT data are provided on the eclogitic Monviso ophiolite derived from the subducted Liguro-Piemontese oceanic lithosphere (which was exhumed, together with associated continental units, before the Alpine collision). The Monviso ophiolite has so far been interpreted either as a fossilized subduction channel, with tectonic blocks detached from the slab at different depths and gathered in a weak serpentinized matrix, or as a more or less continuous portion of oceanic lithosphere. To evaluate potential heterogeneities within and between the various subunits, extensive sampling was undertaken on metasedimentary rocks and FeTi metagabbros. The results indicate that the Monviso ophiolite comprises two main coherent tectonic subunits (the Monviso and Lago Superiore Units) detached during subduction at different depths and later juxtaposed at epidoteblueschist facies during exhumation along the subduction interface. Raman spectroscopy of carbonaceous material suggests (i) a difference in peak temperature of similar to 50 degrees C between these two subunits and (ii) a good temperature homogeneity within each subunit. Pseudosections and average PT estimates using thermocalc in the Lago Superiore Unit suggest for the first time homogeneous HP to UHP conditions (similar to 550 degrees C, 2627 kbar). Parageneses, peak conditions and tectonic setting are very similar to those of the Zermatt-Saas ophiolite, 200 km northwards, thus suggesting a common detachment mechanism for the whole Western Alpine belt.