U-PB GEOCHRONOLOGY AND SM-ND ISOTOPIC COMPOSITION OF PROTEROZOIC GNEISSES, CHANNEL-ISLANDS, UK

Gneissic units of the Channel Islands: United Kingdom, have traditiona lly been considered to be Palaeoproterozoic basement to the late Neopr oterozoic Cadomia terrane, based on lithological correlations and impr ecise or ambiguous isotopic data. A new precise U-Pb date of 2061 +/- 2 Ma, based on analyses of single zircons, for the Icart granitic orth ogneiss of Guernsey confirms a Palaeoproterozoic age of its igneous pr otolith. The Nd depleted mantle model age (T-DM) of this gneiss is 222 0 Ma, only slightly older than its crystallization age, indicating tha t it represents juvenile crust. Other gneisses from Guernsey have simi lar 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 w ith Icartian gneisses on Guernsey, yields a U-Pb zircon date of 616(-2 )(+4), Ma. This crystallization age demonstrates that the protolith wa s a Cadomian intrusion and that penetrative deformation and amphibolit e-facies metamorphism in the northern Channel Islands were Cadomian in age. The use of gneissic features as a means of invoking the antiquit y of many other undated gneissic units in the region is thus considere d unreliable. The presence of 2170 +/- 7 Ma zircon xenocrysts within t he Sark orthogneiss, combined with its initial epsilon(Nd)(615)= -9.5, are strong evidence that pre-Cadomian basement was a significant sour ce component, and that documenting the extent and age of pre-Cadomian basement may be possible by indirect geochemical methods.