Transtensional Tectonics Played a Key Role During the Variscan Cycle in the Sardinia-Corsica Massif

Abstract

Analysis, interpretation and synthesis of the Variscan (Hercynian) chain in the whole Sardinian-Corsican Massif (SCM) are relatively recent. Previous studies were carried out only in SW Sardinia, characterized by important ore deposits and abundant fossils. Afterward, in SE Sardinia (Sarrabus region) Calvino, 1961, recognized terrigenous deposits (“San Vito” Fm.) and intrusive magmatics (“Porfidi bianchi” and “Porfidi grigi”), assigned to a generic early Paleozoic. Bosellini & Ogniben, 1968, discovered, in central Sardinia (Sarcidano region) volcanoclastics (“Manixeddu” Fm), volcanics (“Serra Tonnai” Fm), middle Ordovician in age and a thick terrigenous turbiditic succession (“Bruncu su Pizzu” Fm) late Ordovician in age Minzoni, 1975, identified, in the Sarcidano and Barbagia regions (central-northern Sardinia) a terrigenous succession (“Solanas “Fm), middle Cambrian -early Ordovician in age, covered by sub-aerial acidic volcanics (“Monte Corte Cerbos “Fm.), middle Ordovician in age, in turn covered by the Manixeddu volcanoclastics and by Serra Tonnai volcanics and Bruncu su Pizzu turbidites.. The Solanas Fm was considered equivalent to the San Vito Fm and the effusive Monte Corte Cerbos and Serra Tonnai volcanics were considered equivalent to the intrusive Porfidi bianchi and Porfidi grigi. Structural analysis indicated that from late Devonian .to early Carboniferous the Variscan orogenesis piled up, in central-northern Sardinia, several southvergent nappes. North Sardinia was considered the internal zone of the chain, source of the nappes, that were stacked in central-northern Sardinia (Nappe zone) and transported toward southern Sardinia (external chain and foreland). The Variscan geodynamic cycle was considered completely ensialic, involving continental crust only. North Sardinia was assumed as the portion of the crust most deeply subducted along an intra-continental shear zone and North Corsica was regarded as backland of the chain (Carmignani et al., 1980; 1981). Afterward, Carmignani et al., 1992, discovered, in northern Sardinia, along a narrow belt (the “Asinara –Posada Line”), small mafic bodies of amphibolite, bearing granulite-eclogite relics. Geochemical data indicated a MORB affinity for the protolith and radiometric dating (Sm / Nd on whole rock; Cappelli et al., 1992), yielded an age of 960 ± 93 Ma. The granuliteeclogite relics were related to subduction processes of oceanic lithosphere, retrogressed in