METAMORPHISM-DEFORMATION RELATION IN THE ASINARA ISLAND (ITALY). INSIGTHS ON THE NORTH SARDINIA VARISCAN TECTONICS

The Asinara Island is located in the “inner zone” of the Sardinia Variscides. Moving from the south two main tectono-metamorphic complexes can be distinguished: i) A medium grade, chiefly metapelitic, complex (MGMC) that also retains quartzites associated with decametric to hectometric slices of polymetamorphic basalts with N-MORB signature. ii) A high-grade metamorphic complex (HGMC) consisting of diatexites and metatexites associated with orthogneisses and layered amphibolites resembling “leptyno-amphibolic" bodies. The Variscan deformative and metamorphic history seems to be only partially shared by the two complexes and it can be better described in the MGMC, where four deformative phases have been recognized: - a D1 phase, which is well expressed elsewhere, here it is transposed by the subsequent deformative events. Only the related S1 foliation is preserved within microlithons and minerals of barrowian amphibolite facies. - a D2 phase, which is the main deformative event detectable on the field, it is characterized by isoclinals folds with stretching lineations which parallel the fold axes and it is syn-kynematic to the intermediate P amphibolite facies assemblage. -cylindrical, upright folds, that generally give rise to type 3 interference patterns with the previous folds, characterize the D3 deformation; -finally, folds with near horizontal lying axial planes, along with low angle shear zones,characterize the D4 phase. In the MGMC D1 and D2 deformative phases are widely synchronous to the barrowian metamorphic assemblages (Grt+ Stau ±Ky); conversely HT/LP assemblages, which overprint and destabilize (e.g. Stau+Mus+Qtz=And+Bio+H20) the previous ones, pre-dates or show syn-kynematic relationships to the D4. The HMGC is thrust onto the MGMC, its relic assemblage are better preserved within mafic lithologies : they point to intermediate P granulite facies and are also overprinted by the HT/LP metamorphic assemblage. The inferable tectono-thermal trajectory for the two complexes is consistent with crustal thickening (D1-D2-D3 plus thrusting of HGMC onto MGMC)followed by extensional collapse (D4). Decompression and exhumation occurred in concomitance with vertical shortening and lateral spreading in mid crust, whether synchronous upper crust extension allowed widespread granite emplacement.