U-Pb geochronology of the Upper Proterozoic Cadomian orogeny in the northern Armorican Massif, France

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Abstract
U-Pb zircon dating has clarified the timing of magmatic and metamorphic events during the Cadomian orogeny from 670–660 Ma to 530 Ma. This indicates a time-span of nearly 150 Ma for the Cadomian orogeny in the northern part of the Armorican Massif. The ages of c. 667 Ma and 656 Ma obtained for granodiorite boulders from a Brioverian conglomerate are the oldest known dates which may relate to the Cadomian orogeny, and imply an early cycle of uplift, erosion and sedimentation before major plutonism around 590 Ma. Two main phases of evolution are recognized. The first one, probably related to an active margin, is characterized by the emplacement of granodiorites at 670–660 Ma, 645 Ma and 615 Ma ago followed by basic magmatism at 600 Ma in a back-arc environment. Some of these rocks underwent metamorphism or cooling a short time after emplacement, thus indicating an active tectonic setting with rapid uplift of the crust and some crustal thickening. This phase of evolution ended with diorite emplacement around 590–580 Ma ago. The second phase corresponds to an important crustal melting event recorded by migmatitic gneisses and anatectic magmas formed 540 Ma ago. The ages obtained lead us to propose a new chronostratigraphy for the Upper Proterozoic Brioverian Supergroup with a Lower Brioverian Group deposited between 660 and 590–580 Ma in the Baie de Saint-Brieuc and before 585 Ma in Normandy. The Upper Brioverian Group in Normandy was formed between 585 and 540 Ma. There is no evidence for the existence of Pentevrian or Icartian basement in the Baie de Saint-Brieuc.

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