Permian zircon U-Pb ages in the Gran Paradiso massif: revisiting post-Variscan events in the Western Alps

摘要

In the Internal Western Alps, the Gran Paradiso massif is an eclogitic gneiss dome of Alpine age. It is largely composed of porphyritic metagranites which have long been interpreted as Variscan intrusives in a pre-Variscan basement. Strong deformation and metamorphism affected the entire massif during the Alpine orogeny. The present study confirms earlier, preliminary results that suggested a mid-Permian age for some of the granitic protoliths, thus postdating the last Variscan erogenic granitoids. We also extend that result to the bulk of the massif, from the top contact near Bonneval (Arc valley, France) to the structurally deepest part in Noasca (Orco valley, Italy). Six new U-Pb age determinations were carried out on zircons (IDTIMS and SHRIMP) from four porphyritic orthogneisses and two samples of host rocks with magmatic affinities. All ages are Permian, ranging from 264 to 277 Ma. The oldest ages pertain to the Noasca granite, in the core of the dome, and to a massive gneiss (Bivacco Carpano) that belongs to the main pre-granite metasedimentary and metavolcanic pile. Similar ages were obtained for granites and their host rocks, suggesting that a large part of the Gran Paradiso dome consists of volcanics, volcano-clastic rocks and granites of Permian age, rather than earlier Variscan assemblages. From their dominantly Permian protoliths, Gran Paradiso and other Piemonte massifs (Monte Rosa and Dora Maira), which are part of the Internal Crystalline Massifs, are very different in paleogeographic significance from the External Crystalline Massifs, which represent the European basement, where Permian magmatism is sparse.The Piemonte basement massifs may represent the easternmost edge of a Brianconnais - Grand-Saint-Bernard allochthonous terrane with more affinity to the Apulian basement than to the European basement.