New U–
Pb dates, combined with Nd and Hf isoto
pic data, from rocks within the Port Morvan area of the Baie de St Brieuc region of Brittany identify a unique
portion of the Neo
proterozoic Cadomia terrane. Two gneisses near Port Morvan yielded U–Pb dates of 754.6&
plusmn;0.8 Ma and 746.0&
plusmn;0.9 Ma, ages that are more than 130 Myr older than the oldest units formed during the main
phase of early Cadomian magmatism. Two trondhjemite boulders from the monogenetic facies of the Cesson conglomerate yielded identical ages of 665.2&
plusmn;0.5 Ma and 665.5&
plusmn;0.7 Ma, and a cobble from the
polygenetic facies yields a
207Pb–
206Pb date of 637&
plusmn;2 Ma. Individual detrital zircons from a sandstone associated with the Cesson conglomerates yield concordant U–Pb dates ranging from 650&
plusmn;3 Ma to 624.1&
plusmn;0.6 Ma. Initial
phs/CDE.GIF>Nd values for the rocks in this region range from +5.0 to +6.6, indicative of a substantial in
put from de
pleted mantle. Initial
phs/CDE.GIF>Hf values determined on zircons from these Neo
proterozoic rocks, including the detrital zircons, range from +6.7 to +14.5, consistent with the Nd isoto
pic results. Maximum initial
phs/CDE.GIF>Hf values for two 2 Ga
Icartian gneisses, considered basement to Cadomia, average +8.4 and +8.7. In contrast to the results of the Port Morvan rocks, 616–608 Ma syn-tectonic intrusions from Normandy and the British Channel Islands all have negative initial
phs/CDE.GIF>Nd values (−10.4 to −8.3) consistent with significant contamination by ancient crust such as the 2 Ga gneisses. The oldest arc-related magmas should have interacted most extensively with Cadomian basement, buffering younger mantle-derived magmas that were generated in subsequent magmatic e
pisodes. The rocks within the Port Morvan region are thus inconsistent as exam
ples of the earliest Cadomian intrusions as they show no evidence of interaction with 2 Ga basement. Instead, the older ages and mantle-like isoto
pic com
position of these rocks suggest they are
part of an inde
pendent terrane that formed
prior to, and inde
pendently from, the Cadomian arc. Possible terrane-scale structural boundaries have recently been identified, including the newly recognized Port Morvan thrust fault and the NW-di
pping Main Cadomian thrust.