TECTONOTHERMAL EVOLUTION OF THE CADOMIAN ARC, NORTHWEST EUROPE - CONSTRAINTS FROM U-PB ZIRCON ANALYSES AND COMPARISONS TO AVALONIA

The Cadomian orogenic belt of northwest France and the UK Channel Islands is important to our understanding of the evolution of Proterozoic circum-Atlantic arc terranes for two reasons. First, it exposes a wide range of rock units including two billion year old basement, an extensive Neoproterozoic volcanosedimentary succession and numerous Neoproterozoic plutons. Second, it has largely escaped Paleozoic reworking. These factors make unravelling arc development in Cadomia potentially more complete than in many Avalonian terranes. A program of mapping and precise U-Pb dating has allowed us to constrain the timing of tectonic events in Cadomia and to draw comparisons with Avalonia. Ages from numerous plutons in the northern sector of the belt demonstrate that rather than a broad continuum of arc magmatism, there were two discrete, short-lived events. Subduction-related plutons were emplaced into 2.06 Ga host gneisses at 615-608 Ma synchroneously with formation of the main penetrative fabric. Post-tectonic plutons were emplaced at 580-570 Ma. South of this arc, on Jersey and around the Baie de St Brieuc, a thick volcanosedimentary sequence (Brioverian supergroup) was accumulated and deformed during Cadomian orogenesis. The basement directly beneath the unconformity to this sequence has been dated at 626 Ma, while other local basement components have been dated at c. 750 Ma, drawing age comparisons with older components in Avalonian terranes (e.g. Burin Group, Economy River Gniess). SHRIMP and single grain analyses of detrital zircons and zircons from clasts record a mixed Paleoproterozoic and Neoproterozoic provenance. Zircon population age profiles contrast with those of Avalonian terranes in that they mirror West African craton events, as opposed to the Amazonian craton events. The majority of zircons have ages consistent with erosion from the adjacent arc. The youngest detrital grains recorded within sedimentary units are 587 Ma, while a rhyolite horizon from the upper part of the sequence on Jersey is dated at 583 Ma. The Brioverian sequence was folded and variably metamorphosed at up to amphibolite facies. Syn- to post-tectonic intrusions in this sector of the belt have been dated at 580-575 Ma, which with the detrital ages, tightly constrain the onset and completion of tectonothermal activity.