GEOCHEMICAL CONSTRAINTS ON THE ORIGIN AND EVOLUTION OF EARLY MESOZOICDIKES IN ATLANTIC CANADA

Two prominent early Mesozoic (similar to 201 Ma) diabase dikes in east ern Canada (Shelburne dike of Nova Scotia and Caraquet dike of New Bru nswick) are part of the eastern North America (ENA) basalt province be tween Alabama and Newfoundland. The dikes form lineaments more than 20 0 km long. They consist of quartz-normative tholeiitic basalts. In con trast to the Shelburne dike, the Caraquet dike contains modal olivine and has a lower content of incompatible trace elements with lower (La/ Yb)(n) (1.6-1.8 vs. 3.1-3.9) and isotopically less enriched Sr and Nd isotopes. The dikes have low Mg#, Ni and Cr, indicating that the magma s underwent extensive fractionation. The magmas were variably contamin ated with continental crust, particularly the Shelburne dike as eviden ced by high Th/La and its radiogenic initial Sr isotope ratios. The Sh elburne dike could have been derived from the Caraquet-type magma by a crustal assimilation-fractional crystallization process. The Caraquet magma is inferred to be derived from a sub-continental lithospheric s pinel-bearing mantle. The Shelburne and Caraquet dikes and possibly ot her ENA basalts were probably generated in response to lithospheric ex tension associated with the opening of the North Atlantic over a regio n of anomalously hot mantle related to a mantle plume.