Pre-Alpine Rifting and Alpine Subduction Signature in the Cima Della Sassa Gabbro (Dent Blanche nappe, Valpelline, Western Alps)

Widespread gabbro intrusions in the continental lithosphere are markers of lithosphere thinning and the association of mantle peridotites, gabbros and continental crust slices is a key to unravel the evolution of passive margins. Gabbro bodies included in orogenic continental slices are references to understanding of the duration and mechanisms active during opening of oceans.

Most gabbros included in the continental crust of the Alps display a Permian-Triassic age; they concentrate in the Austroalpine and Southalpine domains and are often intruded in metamorphic rocks deformed in HT and middle- to intermediate-P conditions. They are interpreted, with the contemporaneous metamorphism often developing in their country rocks, as the sign of a positive thermal anomaly associated to a lithospheric thinning, envisaged by some authors as the precursor of the continental rift leading to opening of the Ligure-Piemontese Alpine Ocean (DAL PIAZ et alii, 1977; LARDEAUX & SPALLA, 1991; SCHUSTER et alii, 2001; REBAY & SPALLA, 2001, SPALLA & GOSSO, 2003; MAROTTA & SPALLA, 2007; SPALLA & MAROTTA, 2007).

The Dent Blanche nappe is the portion of Austroalpine domain of the Western Alps containing the maximum density of Permian gabbros, mostly included within metagranitoids (Arolla gneisses); the main bodies are the Matterhorn and Mont Collon-Dent de Bertol (DAL PIAZ et alii, 1977; MONJOIE et alii, 2005), Sassa (DIEHL et alii, 1952, PENNACCHIONI & GUERMANI, 1993). The tectonic contact between gabbros and the country Arolla metagranitoids is marked by thick persistent mylonites. Polyphase Alpine deformation in the gabbros is highly hetherogeneous; the pre-Alpine and Alpine evolution of the Sassa gabbro complex reveals three Alpine deformation events, reconstructed by means of detailed structural mapping of foliation trajectories. Nearly undeformed hundred m-scale gabbro lenses, displaying only corona-type metamorphic reactions, manifest igneous emplacement meso- to micro-scale structures, related to accretion of the magma chamber (WIEBE & COLLINS, 1998).