Abstract

The Cadomian orogen exposed in NW France and the Channel Islands comprises a collage of variably displaced terranes which record the late Proterozoic evolution of a continental margin within a complex subduction zone. On the islands of Guernsey and Sark, calc-alkaline quartz diorite stocks were emplaced during early stages of the Cadomian orogeny. Zircons from two stocks of deformed quartz diorite display evidence of inheritance in larger and more magnetic fractions. Upper intercept ages for these discordant concentrates are c. 1300 Ma. Sm-Nd whole-rock model ages (TDM) suggest older ages of c. 2000 Ma. Conversely, the smallest and least magnetic zircons exhibit a distinct lead-loss pattern suggesting that the 207Pb/206Pb dates of c. 700 Ma correspond to the time of crystallization of the stocks, and provide a lower age constraint for initiation of subduction-related magmatism in the Cadomian orogen. Hornblende concentrates from deformed quartz diorite stocks exposed on Guernsey and Sark display discordant 40Ar/39Ar age spectra. Five samples record well-defined isotope correlation ages ranging between 596 Ma and 606 Ma. A hornblende concentrate from host orthogneiss (Sark) yields a plateau age of 608 Ma. The hornblende ages are interpreted as dating post-metamorphic cooling following late Proterozoic deformation and metamorphism within the arc system. Both magmatic and cooling ages from this early Cadomian arc contrast with those of younger initial tectonothermal activity (c. 570 Ma and c. 540 Ma) in other Cadomian terrane elements, and suggest palinspastic separation until very late stages of the orogeny. The polyphase nature of this late Proterozoic activity is similar to that of related orogens which probably occupied similar tectonic settings along the margin of Gondwana (e.g. SW Iberia, West Africa).

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