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

Abstract

To constrain deep (40-100 km) subduction dynamics, extensive P-T 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 Fe-Ti 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 epidote-blueschist facies during exhumation along the subduction interface. Raman spectroscopy of carbonaceous material suggests (i) a difference in peak temperature of 50°C between these two subunits and (ii) a good temperature homogeneity within each subunit. Pseudosections and average P-T estimates using THERMOCALC THERMOCALC in the Lago Superiore Unit suggest for the first time homogeneous HP to UHP conditions (550°C, 26-27 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.

Subduction interface processes recorded by eclogite-facies shear zones (Monviso, W. Alps)

“…Several studies suggested variable maximum P-T conditions for the Monviso ophiolite (from c. 450°C/12 kbar to 620°C/24 kbar; Blake et al, 1995;Messiga et al, 1999;Schwartz et al, 2001) and argued for the presence of a deep subduction mélange where tectonic slices detached from different depths accumulated in a weak serpentinized subduction channel (e.g., Guillot et al, 2004). However, Angiboust et al (2011) recently demonstrated that the Monviso ophiolite can be divided in two main coherent tectono-metamorphic units (Fig. 2b): (i) the Monviso Unit to the W, which reached c. 480°C/ 22 kbar and (ii) the Lago Superiore Unit to the E, which recrystallized within lawsonite eclogite facies close to the coesite stability field (c. 550°C/ 26 kbar; similar estimates were obtained on a lawsonite-eclogite assemblage by Groppo and Castelli, 2010).…”

“…The Lago Superiore Unit is a well-preserved, relatively undisturbed section of oceanic lithosphere, cut across by two major shear zones (referred to as the "Intermediate" and "Lower Shear Zones": ISZ and LSZ; Fig.2b; Philippot and Kienast, 1989;Angiboust et al, 2011). The upper portion of the section is composed of a variably thick and heterogeneous, dominantly mafic sequence containing (i) metabasalts and associated interpillow material (60-80%), (ii) metadiabases and Fe-Ti metagabbro layers (10-20%) and (iii) calcschists (5-10%; Fig.2b).…”

Geological map of the Monviso massif (Western Alps)

“…the central sector of the MO) six N-S striking and W-dipping tectonostratigraphic units were distinguished from bottom to top by Lombardo et al (1978): the Basal Serpentinite Unit (BS); the Lago Superiore Mg-Al metagabbro Unit (LS); the Viso Mozzo Unit (VM), that is mainly made up of basalt-deriving metabasites; the Passo Gallarino Unit (PG), that consists of Fe-Ti metagabbros and serpentinites; the Costa Ticino Unit, that was renamed Forciolline Unit (FO) by Castelli & Lombardo (2007) and corresponds to an overturned sequence of metagabbros, metabasalts and metasediments; the Vallanta metabasite Unit (VA). The inner structure of the MO was re-interpreted by Angiboust, Langdon, Agard, Waters, & Chopin (2012a) who describe two tectonometamorphic units: the Lago Superiore Unit, a preserved ophiolite sequence that encompasses the BS, LS, VM and PG units, and the overlying Monviso Unit, that consists of two superposed sub-units (the FO and VA units).…”

Geology of the Saint-Marcel valley metaophiolites (Northwestern Alps, Italy)

“…Bearth, 1967;Bearth & Schwander, 1981;Dal Piaz, 1965;Dal Piaz & Ernst, 1978). The Piemonte Nappe extends southward to the eclogite-facies Monviso-Rocciavrè-Lanzo units (Angiboust, Langdon, Agard, Waters, & Chopin, 2011;Balestro, Fioraso, & Lombardo, 2013;Castelli et al, 2014;Kienast & Pognante, 1988;Lombardo et al, 1978;Pognante & Kienast, 1987), and to the blueschist-greenschist-facies Queyras-Ubaye ophiolitic units (e.g. Lagabrielle, 1981;Lagabrielle, Vitale Brovarone, & Ildefonse, 2015 and refs.…”