U–Pb zircon geochronological and petrographic constraints on late to post-collisional Variscan magmatism and metamorphism in the Ligurian Alps, Italy


M. Maino, Department of Earth and Environmental Sciences, University of Pavia, Via Ferrata 1, 27100 Pavia, Italy.

E-mail: matteo.maino@unipv.it


In the Ligurian Alps, the Barbassiria massif (a Variscan basement unit of the Briançonnais domain) is made up of orthogneisses derived from K-rich rhyolite protoliths and minor rhyolite dykes. However, on account of subsequent Alpine deformation and a related blueschist facies metamorphic overprint that are pervasive within the Barbassiria Orthogneisses, little evidence of the earlier Variscan metamorphism is preserved. In this study, new U–Pb laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) dating of zircon from the Barbassiria Orthogneisses and dykes was undertaken to unravel the relationships between protolith magmatism and the Variscan metamorphic overprint. The results suggest a protolith age for the Barbassiria Orthogneisses of ~315–320 Ma (i.e., Early/Late Carboniferous), and constrain the age of a subsequent rhyolite dyke emplacement event to 260.2 ± 3.1 Ma (i.e., Late Permian). The Variscan high-temperature (greenschist–amphibolite facies) metamorphic event that affected the Barbassiria Orthogneisses was likely associated with both tectonic burial and compression during the final stages of the Variscan collision during the Late Carboniferous period. Emplacement of late-stage rhyolite dykes that cut the Barbassiria Orthogneisses is linked to a diffuse episode of Late Permian rhyolite volcanism that is commonly observed in the Ligurian Alps. The age of this dyke emplacement event followed a ~10–15 Ma Mid-Permian gap in the volcano-sedimentary cover sequence of the Ligurian Alps, and represents the post-orogenic stage in this segment of the Variscides. Copyright © 2012 John Wiley & Sons, Ltd.