Lithospheric dripping during Variscan orogeny in the NE French Massif Central: evidence from Visean-Serpukhovian silicic volcanism in the Morvan

A shallow-mantle and deep crustal gravitational destabilization beneath orogenic zones is evidenced by lithospheric delamination. However, a drip-shaped delamination due to the increasing density has been suggested in the last few years as “dripping”. This dripping process is described at the surface as compressive tectonics associated with mantle-derived plutonic magmatism, followed by extensional tectonics and uplift of the continental crust accompanied by crustal-derived silicic volcanism. Dripping is documented in recent convergent contexts, but also could be considered in old orogenies.

Overall, geology of the Morvan basement (NE Massif Central) is formed by metamorphic rocks intruded by Variscan metaluminous and peraluminous granites emplaced at 350-320 Ma. Some volcanic occurrences are outcropping, especially 1) the basic calk-alkaline Somme Arc sequence related to the Devonian subduction-related magmatic arc, and 2) the “Faisceau Dévono-Dinantien” (FDD) Carboniferous silicic sequences, which are associated with extensional tectonics, but also the Carboniferous Blismes-Montreuillon volcanic complex (BMVC) and Sincey-les-Rouvray faulted zone. Additionally, in the northern part, Pierrot et al. (under review) studied basic plutonism with mantle components (vaugnerites) emplaced at 335-325 Ma in the metamorphic basement.

We conducted new petrographic, geochemical (major and trace elements) and geochronological (U-Pb on zircon) studies on the Carboniferous volcanic rocks of Morvan. U-Pb data on zircon crystals give emplacement ages in the 332-323 Ma range with also inherited Devonian to Proterozoic apparent ages. These silicic volcanic rocks display classical mineralogy (Ab + Qz + Bt + F-K) but advanced petrographic observations revealed the presence of mineralogical aggregates of granitic origin, as well as garnet in FDD, and amphibole in the BMVC. Geochemical data show that silicic rocks are K-rich calk-alkaline to alkaline peraluminous dacites and rhyolites. All these results suggest that basic plutonism with mantle-derived components was immediately followed by a crustal-origin silicic volcanism, all occurring during 10 to 15 Ma. Therefore, a typical lithospheric delamination could not explain the lack of basic volcanism, the extensional tectonics of the FDD at the climax of the Variscan orogeny, and the chronological succession of these processes. On the contrary, dripping process explains that during the drip formation, partial melting of mantle can occur by adiabatic upwelling but does not produce volcanism due to local compressive tectonics triggered at the surface. After the drip detachment, the asthenospheric upwelling would trigger HT conditions beneath the crust, leading to partial crustal melting, which could manifest like both plutonism and volcanism with silicic composition, facilitated by favorable surface extensional tectonics.