GEOCHEMICAL AND ISOTOPIC CHARACTERISTICS OF EARLY SILURIAN CLASTIC SEQUENCES IN ANTIGONISH HIGHLANDS, NOVA-SCOTIA, CANADA - CONSTRAINTS ON THE ACCRETION OF AVALONIA IN THE APPALACHIAN-CALEDONIDE OROGEN

Avalonia is a terrane that accreted to Laurentia-Baltica during the de velopment of the Appalachian-Caledonide Orogen. Interpretations of the timing of accretion have been constrained by comparing faunal affinit ies, overstep sequences, age and kinematics of inferred accretionary d eformational events, and controversial paleomagnetic data. We show tha t the time of accretion of Avalonia may also be constrained by contras ts in the geochemical and isotopic signatures of its igneous rocks (wh ich reflect the characteristics of the underlying continental basement and mantle) and sedimentary rocks (which reflect provenance). Early S ilurian elastic sedimentary rocks of the Beechill Cove Formation, Anti gonish Highlands, Nova Scotia, were deposited on Avalonian crust. The formation predominantly consists of approximately 80 m of siltstones a nd shales deposited in a nearshore environment and derived from the no rtheast. Their age is constrained by paleontological data and by direc tly underlying Late Ordovician - Early Silurian bimodal volcanic rocks that have typically Avalonian geochemical signatures. In comparison w ith typical Avalonian rocks, the Beechill Cove sediments are character ized by high SiO2, Ce/Yb, and initial Sr-87/Sr-86, low Fe2O3, MgO, and TiO2, and strongly negative epsilon(Nd)(ur). These characteristics ca nnot be attributed to erosion of underlying Avalonian basement or coev al volcanic rocks and are consistent with derivation via significant t ransport from radiogenically enriched continental crust. epsilon(Nd) d ata are typical of Grenvillian basement compositions and suggest that the Beechill Cove sedimentary rocks were derived from an adjacent land mass with Grenvillian crust. The data, in conjunction with paleocontin ental reconstructions and recent geochronological and structural data from the northern Appalachians, suggest that the Caledonide orogenic b elt is the most likely source. Deposition of the Beechill Cove Formati on is inferred to have occurred in an intracontinental basin associate d with strike-slip tectonics during the oblique collision of the Avalo n with Laurentia-Baltica.