A step towards unraveling the paleogeographic attribution of pre-Mesozoic basement complexes in the Western Alps based on U-Pb geochronology of Permian magmatism

The Brianconnais Domain (Western Alps) represented the thinned continental margin facing the Piemonte-Liguria Ocean, later shortened during the Alpine orogeny. In the external part of the External Brianconnais Domain (Zone Houillere), the Palaeozoic basement displays microdioritic intrusions into Carboniferous sediments and andesitic volcanics resting on top of the Carboniferous sediments. These magmatic rocks are analysed at two well-known localities (Guil volcanics and Combarine sill). Geochemical data show that the two occurrences belong to the same calc-alkaline association. LA-ICP-MS U-Pb ages have been obtained for the Guil volcanics (zircon: 291.3 +/- 2.0 Ma and apatite: 287.5 +/- 2.6 Ma), and the Combarine sill (zircon: 295.9 +/- 2.6 Ma and apatite: 288.0 +/- 4.5 Ma). These ages show that the calc-alkaline magmatism is of Early Permian age. During Alpine orogeny, a low-grade metamorphism, best recorded by lawsonite-bearing veins in the Guil andesites, took place at about 0.4 GPa, 350 degrees C in the External Brianconnais and Alpine metamorphism was not able to reset the U-Pb system in apatite. The Late Palaeozoic history of the Zone Houillere is identical to the one recorded in the Pinerolo Unit, located further East in the Dora-Maira Massif, and having experienced a garnet-blueschist metamorphism during the Alpine orogeny. The comparison of these two units allows for a better understanding of the link between the Palaeozoic basements, mostly subducted during the Alpine convergence, and their Mesozoic covers, generally detached at an early stage of the convergence history.