Geochemical constraints on the origin and evolution of early Mesozoic dikes in Atlantic Canada

Two prominent early Mesozoic ( approximately 201 m.y.) quartz-normative tholeiitic dykes from Nova Scotia and New Brunswick form lineaments >><$I> 200 km long and are part of the E North America basalt province between Alabama and Newfoundland. In contrast to the Shelburne dyke, Nova Scotia, the Caraquet dyke in New Brunswick contains modal olivine and has a lower content of incompatible trace elements with lower (La/Yb) (sub n) (1.6-1.8 vs 3.1-3.9) and isotopically less enriched Sr and Nd isotopes. The dykes have low mg, Ni and Cr, indicating that the magmas underwent extensive fractionation; contamination by continental crust took place, particularly in the Shelburne dyke as shown by high Th/La and its radiogenic initial Sr isotopes. The Shelburne dyke 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 spinel-bearing mantle. Both dykes were probably generated in response to lithospheric extension associated with the opening of the North Atlantic over a region of anomalously hot mantle rleated to a mantle plume.