POst-collisional granitoid magmatism in the Calabria-Peloritani terrane during the final stages of north Gondwana assembly

Augen gneisses covering an area of about 80 km2 in the Peloritani Mountains (NE Sicily) have been found to be 570-540 Ma granitoids after SHRIMP zircon U-Pb dating of a representative sample from each of three different areas. Similar coeval rocks are also exposed in Calabria, indicating the presence of a large Ediacaran granitoid province in the Calabria-Peloritani Terrane, currently extending from NE Sicily to northern Calabria. Two of the studied augen gneisses, from the eastern Peloritani, have many zircon grains composed of a thick < 545 Ma melt-precipitated rim surrounding a core ranging in age from Late Archean to latest Proterozoic. These cores are interpreted as detrital grains from sediments that were part of the source of the granitoid magmas, and suggest that that source was largely composed of rocks akin to the protoliths of northern Peloritanian paragneisses, which have similar patterns of detrital zircon ages. The third augen gneiss, from the western Peloritani, has a protolith emplacement age of < 565 Ma and contains inheritance-free zircon, implying that the magma was strongly zircon undersaturated and/or high-temperature, or derived from a zircon-poor source. The Late Ediacaran emplacement ages of the granitoid protoliths of the inheritance-rich augen gneisses, together with typical high-K calcalkaline post-collisional geochemical features and strong evidence for dominantly crustal sources from zircon inheritance and isotopic features, are consistent with the widespread crustal melting and post-tectonic granitoid emplacement at ca. 550-540 Ma reported by many authors (e.g., Murphy et al., 2001; Linnemann et al., 2008; Nance et al., 2010) as marking the transition from subduction to strike-slip tectonics at the northern margin of Gondwana at the end of the Neoproterozoic. It has been proposed that some of the peri-Gondwanan terranes were formed in back-arc environments located between cratonic (south-American or north-African) and arc-related (Avalonian or Cadomian) domains. After a late stage of arc magmatism at ca. 580-560 Ma, possibly represented in the Peloritani Mountains by the 565 Ma protolith of the western Peloritani augen gneiss, inversion of back-arc basins related to collision of the magmatic arcs with mainland areas led to the deposition of turbidite sequences at 545-540 Ma. In some cases (e.g. Lausitz Group in Saxo-Thuringia; Linemman et al., 2007) these sequences were rapidly deformed before the intrusion, at < 540 Ma, of large volumes of granitoids, probably derived from melting of the same turbiditic greywackes, as suggested by comparable zircon age patterns and geochemical and isotopic compositions. Many of the ~ 545 Ma igneous rims on zircon from the two studied eastern Peloritani augen gneisses overgrow virtually coeval Ediacaran inherited cores, indicating deposition of the sediments only very shortly before magmatism. This interpretation is reinforced by the ca. 545 Ma deposition age obtained for the metagreywacke protolith of an eastern Peloritani paragneiss. These results may be framed in a model involving rapid erosion, burial, sediment deformation and metamorphism up to partial melting conditions in less than about 10 million years. A similar model has been envisaged by Nance et al. (2010) for the Saxo-Thuringian zone. They proposed that these latest Proterozoic examples of extremely rapid geological evolution were caused by a shortlived regime of high heat-flow linked to slab break-off resulting from ridge-trench collision. In this context, evidence for a mantle contribution to the petrogenesis of the ca. 545 Ma granitoids may be found in their geochemical and isotopic features, and the documented presence of coeval gabbroic rocks in the same region.