U-Pb geochronology and Sm-Nd isotopic composition of Proterozoic gneisses, Channel Islands, UK

Gneissic units of the Channel Islands, UK, have traditionally been considered to be Palaeoproterozoic basement to the late Neoproterozoic Cadomia terrane, based on lithological correlations and imprecise or ambiguous isotopic data. A new precise U-Pb date of 2061 ± 2 Ma, based on analyses of single zircons, for the Icart granitic orthogneiss of Guernsey confirms a Palaeoproterozoic age of its igneous protolith. The Nd depleted mantle model age (T_DM) of this gneiss is 2220 Ma, only slightly older than its crystallization age, indicating that it represents juvenile crust. Other gneisses from Guernsey have similar T_DM ages ranging from 2210 to 2370 Ma, suggesting that they are also exposures of juvenile Palaeoproterozoic crust. A component from a penetratively deformed orthogneiss from Sark, previously correlated with Icartian gneisses on Guernsey, yields a U-Pb zircon date of 515 ⁺⁴_-2 Ma. This crystallization age demonstrates that the protolith was a Cadomian intrusion and that penetrative deformation and amphibolite-facies metamorphism in the northern Channel Islands were Cadomian in age. The use of gneissic features as a means of invoking the antiquity of many other undated gneissic units in the region is thus considered unreliable. The presence of 2170 ± 7 Ma zircon xenocrysts within the Sark orthogneiss, combined with its initial ε_Nd(615) = -9.5, are strong evidence that pre-Cadomian basement was a significant source component, and that documenting the extent and age of pre-Cadomian basement may be possible by indirect geochemical methods.