Zircon U–Pb geochronology of Ordovician magmatism in the polycyclic Ruitor Massif (Internal W Alps)

Abstract.

Among the Middle Penninic basements of the Internal NW-Alps, the Ruitor massif shows the best preserved remnants of pre-Permian metamorphic rocks. Their Barrovian-type mineral associations are somewhat masked by the greenschist to blueschist Alpine metamorphism of Tertiary age. Four Ruitor gneisses have been analysed, showing geochemical characters of granitoids from orogenic zones. Zircon morphology also suggests magmatic protoliths and a crustal source; some of the morphological zircon types suggest anatectic granites. The first U–Pb ages on zircon for this massif have been obtained concurrently through conventional multigrain and ion microprobe dating. Two metavolcanic rocks at 471±5 and 468±22 Ma could be slightly older than the porphyritic augen gneisses at 465±11 and 460±7 Ma. Regional data from the other Internal basement massifs suggest that the Variscan event is poorly recorded, except in Ruitor-type units. Ruitor and Sapey gneisses belonged to the same unit (Nappe des Pontis), which was affected by a 480–450-Ma event including volcanism and anatexis and ended with a late calc-alkaline granite emplacement at 460–450 Ma. The distribution of Variscan basement units roughly parallels Alpine zonation.