The high-grade metamorphics from Pittulongu to Golfo Aranci (NE Sardinia): an attempt of lithological reconstruction

In NE Sardinia, along the coastline from Olbia to the Capo Figari southern side, crops out a high-grade metamorphic complex pertaining to the Axial Zone of the Southern Variscan chain. The main outcrops are formed by migmatitic paragneisses, orthogneisses, eclogites, and mafic to ultramafic amphibolites (Franceschelli et al., 2002, Cruciani et al., 2008). Orthogneisses dated 469 ± 3.7 Ma prevail in the northern part (Capo Figari), while migmatitic paragneisses, minor amphibolites with relic eclogite parageneses (this latter with an age of 460 ± 5 Ma) and rare lenses of orthogneisses are present in the southern part. In the paragneisses, swarms of calc-silicatic nodules probably representing former carbonate beds (?) now completely shattered are locally concentrated. By a detailed geological mapping along the coast, an attempt has been made to unravel the pre-metamorphic sequence: we tried to refer every metamorphic rock to its former protolith. The mapping evidenced the presence of alternations of metamorphic rocks passing frequently from each other through gradual or alternated boundaries: this could be related to the original boundaries of the former sedimentary or igneous protoliths, thus probably suggesting the original composition of the succession. This may be composed by acidic and basic volcanics and immature siliciclastics. The geometrical upper part of the succession may be located in the Capo Figari area (Cala Moresca), where the thick augen-orthogneiss outcrop may be related to the Middle Ordovician volcanic complex widely outcropping along all the Variscan Sardinian transect. These rocks show also the oldest radiometric age ever found in the area and are characterized by primary/interdigitated contacts with paraderivates (paragneiss with calcsilicate nodules). In the lower part of the succession, in the M. Nieddu-Pittulongu beach area, alternations of migmatitic paragneiss with calcsilicate nodules and amphibolites could be related to former immature siliciclastics intercalated by carbonate beds (?) and basic volcanics. Thus, this work could allow to relate the low- to middle-grade metamorphics of the Variscan Nappe Zone with the ones of the Variscan Axial Zone and to confirm the local persistence in the Olbia area of the same sedimentary environments existing during lower Paleozoic times in the south and central part of the island.