The Caraquet dyke of New Brunswick is one of three early Mesozoic dykes recognized in Atlantic Canada. It has a quartz-normative tholeiitic composition intermediate in character between "low-Ti" and "high-Ti" Appalachian diabase. Major-element (e.g., MgO, SiO2) and trace-element concentrations (e.g., Ba, Zr) suggest that the dyke is less evolved than the Shelburne dyke but slightly more evolved than the Avalon dyke. Although related to continental rifting and opening of the Atlantic Ocean, the dykes of the Atlantic Provinces show little in common with continental rift volcanic rocks. They resemble MORB except that their K, Rb, and Ba concentrations are much higher. Semi-quantitative modelling suggests that the elevated large-ion lithophile-element concentrations reflect assimilation of continental crust. Differences in composition among the three dykes may be due to variations in the quantity and composition of assimilated material as well as the degree of differentiation. As with other early Mesozoic dykes of eastern North America, the emplacement of these rocks was structurally controlled: (1) their orientations reflect a rift-induced stress field and (2) their location reflects control by Paleozoic lithotectonic terranes of the Applachian Orogen.