Amphibole-bearing migmatites from the Variscan belt of NE Sardinia, Italy: partial melting of mid-Ordovician igneous sources

Amphibole-bearing migmatites from north-eastern Sardinia, Italy, are characterized by tonalitic to granodioritic leucosomes made up of quartz, plagioclase, K-feldspar, biotite, ±amphibole, and garnet. The leucosomes show higher SiO2, Na2O, Sr and lower TiO2, Fe2O3tot, MgO, MnO, P2O5, K2O and Rb content as compared to the mesosomes. The granodioritic leucosomes differ from the tonalitic leucosomes for significantly higher Ba and K2O content. The mesosomes and the leucosomes show slightly fractionated REE patterns with moderate negative and positive Eu anomalies, respectively. The leucosomes show lower 87Rb/ 86Sr ratios (0.279–0.581) than the mesosomes (0.634–1.121), whereas the 147Sm/144Nd ratios are similar in leucosomes (0.12–0.14) and mesosomes (0.11–0.14). Mineralogical, geochemical and isotopic data suggest that the migmatites formed by in situ partial melting of a biotite+plagioclase+quartz-bearing protolith with 2–4 wt.% added water. Variable degrees of melt loss were responsible for the observed compositional variability in the leucosomes, whereas solid-state re-equilibration of the migmatites leads to a general reequilibration of mineral compositions after partial melting. The amphibole-bearing migmatites record maximum P–T conditions of 700–750 °C and 1.0–1.2 GPa, probably lower than, but near to, the P–T conditions of peak metamorphism. Zircon morphology suggests an igneous origin for the migmatite protolith. Pb–Pb zircon dating yielded a mean value of 452±3 Ma and an isochron age of 461±12 Ma which is interpreted as the emplacement age of the migmatite protolith