Zircon U-PB and LU-HF isotopic data from some peri-gondwana variscan terranes (Sardinia-Corsica block and calabropeloritan arc): new insights on the cenozoic geodynamic evolution of the central mediterranean basin

摘要

The reconstruction of pre-Variscan plates configuration in the Mediterranean area, is still poorly understood. Different hypotheses have been proposed to explain the relationships between peri-Tyrrhenian blocks such as Sardinia-Corse block (SCB) and Calabrian-Peloritain Arc (CPA), which before the opening of South Tyrrhenian Basin belonged to a branch of Southern Variscan Realm (Alvarez & Shimabukuro, 2009). Is still ambiguous if the CPA in the early time was formed by an amalgamation of two or more continental terranes that collided during Tertiary (Bonardi etal., 1980; Scandone, 1982) or was a single terrane during the middle and late Tertiary (Amodio-Morelli et al., 1976).udWith the aim to provide a contribution in unraveling the peri-Tyrrhenian area evolution of the peri-Gondwana terranes, amalgamated during Variscan and widespread during Cainozoic opening (and closing) of Tethys, both Variscan and Tertiary rocks cropping out in the two Blocks have beenudinvestigated, either magmatic or sedimentary. The first part of the research was focused on the magmatic bodies supposed belonging to the European Variscides with the aim to provide more constrains to the Variscan magmatic evolutionand so to draft the analogies between the present-day, separated blocks. In detail, the geochemicaluddata from magmatic and detrital zircons of the basement rocks highlight analogies or differences between the tectono-sedimentary sectors of the Sardinian Variscides (nappe and foreland zones) and between them and the Calabria Variscides. The geochemical whole rocks analyses and U-Pb data obtained show that the Variscan basement which crops out in the Northern part of the CPAud(Sila region) is characterized by the similar features of that in the SCB: the Middle Ordovician and Late Palaeozoic magmatic products recognized in the two blocks look similar. Some of these data can provide new insights also about the Ordovician evolution of the Northern Gondwana margin. To compare the and better constrain the relationships between the two blocks, detrital zirconsudwere sampled in Tertiary rocks in NE Sardinia, E Corsica and Calabria, commonly regarded as closely related to the tectonic evolution of the Central-western Mediterranean. The analyses performed suggest some differences on the classical paleo-geographic positions of the SCB respectudto the CPA and some assumptions can be made also about their relationships with other peri-Gondwana terranes. The association of U-Pb and Lu-Hf study performed on Tertiary detrital zircon populations is a good tool to furnish more data for recognize the pre-Variscan inputs suffered by theudperi-Tyrrhenian area, and to highlight differences and similarities between them. The first consideration that can be made is that the two blocks suffered different crustaludevolution. The main difference between them has given by the Grenvillian zircons input studied in the Tertiary deposits. The Southern part of CPA (Aspromonte and Peloritani) is characterized by the lacking of Mesoproterozoic age, whereas the Northern part of CPA (Sila) together with the SCB, are characterized by an important Grenvillian input. These considerations have a geodynamic implications about the misunderstood early evolution of the studied zones The Southern part ofudCPA shows strong similarities with Cadomiam terranes, which permit to correlate it with the evolution of Variscides now cropping out in North Africa. The SCB and the Northern part of CPA could be associated to the other European Variscan terranes (e.g. Iberia) characterized by widespread Grenvillian zircons. A supplementary assumption that stem out from these analyses is that the early position of SCB and Northern part of CPA, were possibly close to the Amazonian craton or nearness to the West African craton (Arabian-Nubian shield and the Sahara Metacraton),udquite different from more of the proposed reconstructions.udFinally, considering all these data in the complicated evolution of southern Tyrrhenian basin, here is supported the “two-terrane model” (slightly different from that proposed by Alvarez and Shimabukuro, 2009) which considers the CPA composed by a northern CPA terrane connected withudthe SCB from the end of the Variscan orogeny until the almost the Eocene. The southern CPA terrane suffered a different evolution before the opening of the Tyrrhenian Sea. According to the roll-back model of the westward subduction of Adria plate under Europe during Neogene-Pliocene the whole CPA, likely amalgamated in the early Miocene, was then tear apart from SCB and driftedudaway eastwards during the open of the South Tyrrhenian basin.ud