Tectonic significance of Variscan Barrovian metamorphism inside and outside the Alpine Front: the Orobic vs Maures basement.

The Variscan basement of Orobic Alps (Central Southalpine basement) comprises micaschists and gneisses, interlayered with metagranitoids, metabasics, marbles and quartzites. Early Palaeozoic ages are inferred from sedimentary protoliths (e.g. Gansser and Pantic 1988), whereas metaintrusives mainly derive from Ordovician granitoids (e.g. Colombo et al. 1994; Bergomi 2004). In the Orobic basement, tectono-metamorphic units marked by staurolite-bearing assemblages are surrounded by other units that fully developed their structural evolution under greenschist facies conditions (Spalla & Gosso, 1999; Zanoni & Spalla, 2018 and refs therein). The St + Grt + Bt + Wm + Pl + Qtz ± Ky mineral assemblages developed at metamorphic climax in metapelites ad are locally predated by Cld-bearing parageneses. The ages proposed for these Barrovian assemblages range from 340 and 310 Ma (e.g.: Mottana et al., 1985; Diella et al., 1992; Bertotti et al., 1993; Siletto et al., 1993) and the metamorphic imprint is classically interpreted as due to the thermal relaxation consequent to the Variscan collision. Similar rock assemblages and metamorphic parageneses characterises the western Maures basement, representing the external part of the southern French Variscan belt, where ages similar to those obtained in the Orobic basement are proposed for the St-bearing assemblages (330-320 Ma, Schneider et al., 2014). Both these Barrovian portions of the Variscan basement, at present located inside and outside the Alpine fronts, have been re-equilibrated under greenschist-facies conditions during the late Variscan exhumation. The traslation towards shallower structural levels is accompanied by a transition from Barrovian (intermediate P/T ratio) to Abukuma (low P/T ratio) metamorphic gradient. Their tectonic evolution, associated with their metamorphic history, is here discussed also on the basis of results from numerical simulations of subduction-collision systems, recently highlighting that the Barrovian imprint can develops also during active subduction (Regorda et al., 2017) and not exclusively during continental collision (e.g. England & Thompson 1984; Gerbault et al., 2018).