Triassic lamprophyre dyke in the Palaeozoic basement of NE Sardinia, Italy: Petrogenesis and geodynamic significance

Abstract

A Triassic, Si-undersaturated, alkaline lamprophyre from NE Sardinia is studied to decipher its magma source, genesis, and P-T conditions of crystallization and emplacement. It is also compared with similar and coeval rocks occurring throughout Europe to depict a geodynamic scenario associated with the opening of the Pangea. The lamprophyre, crosscutting the Palaeozoic basement, shows a porphyritic texture with centimeter-sized amphibole macrocrystals and biotite, ulvöspinel and apatite phenocrystals. The lamprophyre also contains feldspar clusters, carbonate-rich inclusions, and xenolith nodules. The incompatible trace element patterns show weak positive anomalies for Ba, Nb, K and P and negative ones for Th and Ta, whereas the chondrite-normalized Rare Earth Elements (REE) pattern reflects a constant fractionation from light REEs to heavy REEs without Eu anomaly. The core of the amphibole reflects the deepest crystallization conditions, whereas rims and groundmass amphiboles indicate slightly higher temperature and lower pressure. The bulk Sr and Nd radiogenic isotope composition falls between the E-DMM and BSE mantle components at 220 Ma, ruling out a significant contribution of the local lithospheric mantle, which was metasomatized during the Hercynian orogeny. The comparison with other European lamprophyres along with a palaeogeographic reconstruction suggests that the Sardinia lamprophyre lies along the eastern branch of the fault system responsible for to the opening of the Alpine Tethys. The trace element and radiogenic isotope signature of the Sardinian and the other European lamprophyres is consistent with a progressive transition from the post-collisional orogenic magmatism to anorogenic rift-related alkaline magmatism occurring during Early Mesozoic break-up of Pangea and opening of the Tethys Ocean.