Geochemistry of the Monte Filau orthogneiss (SW Sardinia, Italy): insight into the geodynamic setting of Ordovician felsic magmatism in the N/NE Gondwana margin.

The Mt. Filau orthogneiss is an Ordovician orthogneiss outcropping in the External Zone of SW Sardinia chain. It consists of dark, biotite-rich facies, a leucocratic coarse-grained facies and a leucocratic fine-grained facies. Coarse- and fine-grained leucocratic orthogneiss have comparable major element contents, being slightly enriched in SiO2 and depleted in Fe2O3, MgO, TiO2 and CaO as compared to the Bt-bearing orthogneiss. Bt-bearing orthogneiss shows higher Sr and Ba concentrations than leucocratic ones, whereas Rb content is higher in leucocratic orthogneiss as compared to the Bt-bearing ones. Zr content shows a progressive decrease from Bt-bearing orthogneiss, coarse-grained leucocratic ones, to fine-grained orthogneiss. In the spider diagram the Mt. Filau orthogneiss shows the typical signature of calc-alkaline rocks, with negative anomalies of Ba, Nb, Sr and Ti, and positive anomalies in U, K. REE patterns of Bt-bearing and coarse-grained leucocratic orthogneisses are characterized by a moderate LREE fractionation, flat HREE and negative Eu anomaly. Fine-grained leucocratic orthogneiss shows flatter patterns, stronger Eu anomalies and slight HREE enrichment. The geochemical features suggest a clear evolution trend, from the less evolved Bt-bearing orthogneiss to the more differentiated fine-grained leucocratic orthogneiss which likely represent aplite bodies deriving from the most acidic residual melt. Selected trace and REE elements of Mt. Filau are compared with other Ordovician orthogneiss outcropping in the Axial Zone of Sardinia Variscan belt. Besides, their geochemical features are also compared with metavolcanics of External and Nappe Zone. The geochemical affinity of orthogneisses and metavolcanics from Variscan Sardinia, together with the geochronological data, allows to state a clear cogenetic relationship between the bodies belonging to the calc-alkaline Ordovician magmatic cycle. Our results suggest that the early Paleozoic basement of Sardinia might represents the witness of an early Paleozoic subduction-accretionary complex recording convergence along the N/NE Gondwana margin.