U-Pb geochronological constraints on the timing of Brioverian sedimentation and regional deformation in the St. Brieuc region of the Neoproterozoic Cadomian orogen, northern France

During the Neoproterozoic Cadomian orogeny in northern France, supracrustal rocks of the Brioverian Super-group were deposited in marginal and back arc basins, and were subsequently variably deformed and metamorphosed. New U-Pb analyses of single, and small multigrain fractions of zircon from selected plutons from the Baie de St. Brieuc region provide robust geochronological constraints on the timing of these events. The Jospinet granodiorite forms part of the local basement directly overlain by Brioverian metasediments and basic volcanics, and yields a U-Pb zircon date of 625.9 + 3.6/ - 1.9 (2σ) Ma. The pre-tectonic Port Moguer tonalite, which has been strongly sheared along with its amphibolite facies country rocks, has a crystallization age of 600.4 ± 0.9 Ma. Emplacement ages of 576.3 + 1.5/ - 1.2 Ma for the syn-tectonic Fort La Latte quartz diorite and 574.6 + 1.8/ - 1.5 Ma for the late-tectonic St. Quay quartz diorite place limits on termination of deposition and timing of subsequent regional deformation of the Brioverian sequence in the Baie de St. Brieuc region. The new dates constrain the age of Brioverian sedimentation to the interval 626-575 Ma, a range consistent with a previously published Pb-Pb zircon evaporation age of ca. 588 ± 22 Ma for Brioverian volcanic rocks (Lanvollen Formation). Deformation within this sector of the Cadomian belt is believed to have occurred shortly before 575 Ma, revising previously published estimates for the age of this major tectonothermal Cadomian event by 10-20 My.