Age determination of Ordovician magmatism in NE Sardinia and its bearing on Variscan basement evolution.

A newly discovered pre-Variscan cover-basement relationship in NE Sardinia is defined by differences in structural and metamorphic histories. The oldest metamorphic fabrics in the basement are absent from a series of metaplutons that are inferred to be comagmatic with metavolcanic rocks in the cover. Cover and basement are further distinguished by contrasting metapelite compositions. Superimposed metamorphism is represented by pre-Variscan garnet and Variscan plagioclase porphyroblasts that appear in the field as a 'mixed' garnet þ albite zone. In situ U-Pb zircon dating of meta-igneous rocks from cover and basement by laser-ablation inductively coupled plasma-mass spectrometry gives ages of 458 +/- 7 Ma for the Tanaunella orthogneiss, 456 +/- 14 Ma for the Lode` orthogneiss, and 474 +/-13 Ma for the Lula porphyroid. The internal Sardinian Variscides thus incorporate a mid-Ordovician magmatic belt developed upon an older basement; correlation is proposed with a tectonostratigraphic domain that is at present exposed in the Alps. Within the mid-Ordovician peri-Gondwanan realm, the internal Sardinian Variscides appear to have developed along an active margin, whereas the foreland correlates with the Ibero-Aquitaine domain and is related to the evolution of a passive margin during rifting and detachment of Avalonia to form the Rheic Ocean.