Tectonic and metamorphic evolution along the Monviso geotraverse (Western Italian Alps)
Mondino F.1, Borghi A.2, Gattiglio M.1, Lombardo B.,3 Michard A.4
1 - Dipartimento di Scienze della Terra, Università di Torino
2 - Dipartimento di Scienze Mineralogiche e Petrologiche, Università di Torino
3 - C.N.R. - Istituto di Geoscienze e Georisorse, Sezione di Torino
4 - Ecole Normale Supérieure – Paris
fiammetta.mondino@unito.it
In this work we report new geological, structural and metamorphic data on the Monviso geotraverse of the Cottian Alps.
Along this geotraverse tectonic units of both oceanic (Piedmont Zone) and continental (Dora Maira Massif) origin are exposed. All these units belong to the Penninic Zone.
From the inner towards the outer part of the Alpine chain the following units are exposed: 1) the outer Dora Maira Massif (Dronero-Sampeyre Unit), 2) a lithostratigraphic unit (Roccenie Complex) consisting of calcschists (metasediments derived from trench deposits) with minor meta-ophiolite bodies and 3) the basal portion of the Monviso ophiolite complex (Basal Serpentinite Unit).
The Dronero-Sampeyre Unit consists of polymetamorphic basement rocks mainly represented by chloritoid-bearing garnet micaschists, with minor dolomite marble and quarzite which represent remnants of the original Permian-Triassic cover. The Alpine metamorphic evolution is characterized by a first event under eclogite facies conditions (kyanite-garnet-chloritoid assemblages in metapelites), followed by partial re-equilibration under blueschist and greenschist facies conditions.
The Roccenie Complex consists of calcschists locally interbedded with garnet-chloritoid metapelites. Also present are bodies of ophiolite rocks including meta-gabbro, serpentinite and metabasites which preserve eclogite minerals (garnet, Na-pyroxene, zoisite and rutile). Locally, the original sedimentary cover of the ophiolite rocks (ophiolitic sandstone, quarzite and marble) is preserved.
The Basal Unit of the Monviso ophiolite complex consists of foliated serpentinite intruded by dikes of throndijemite composition. In the geometrically lower part of this unit, meta-ophicalcites interbedded with grey marbles occur. This lithological association is tentatively interpreted as the original oceanic cover of the serpentinites.
The tectonic contact between the Dronero-Sampeyre Unit and the Roccenie Complex is represented by a shear zone a few tens of meters thick, along which continental and oceanic lithologies are juxtaposed.
The contact between the Roccenie Complex and the Basal Serpentinite Unit of the Monviso ophiolite complex, on the other hand, is a stratigraphical contact, defined by the occurrence of serpentinitic breccia, marble and quarzite with ophiolitic clasts.
At the regional scale no break in metamorphic facies and evolution appears to be present between the Dronero-Sampeyre Unit (Dora Maira Massif), the Monviso ophiolite complex and the interposed Roccenie Complex.