Paleomagnetism and K–Ar isochron dates of Early Jurassic basaltic flows and dikes of Atlantic Canada

Abstract

K–Ar isochron dates of 193 ± 2, 189 ± 3, and 191 ± 2 Ma are reported for the Shelburne dike of Nova Scotia, the Avalon dike of Newfoundland, and the Caraquet dike of New Brunswick, respectively. These major tholeiitic dikes agree in radiometric date with the North Mountain Basalt of Nova Scotia and with other tholeiitic flows of the Newark Supergroup that have been paleontologically assigned to the Hettangian. However, further work is needed to resolve the discrepancy between the Triassic–Jurassic boundary date of 193 ± 6 Ma suggested by the Newark Supergroup flows and the boundary dates of 204–213 Ma adopted in recent time scales.The North Mountain Basalt flows at nine sites on the north shore of the Bay of Fundy are shown to have likely magnetized during initial cooling, yielding an Hettangian virtual paleopole at 66°N, 72°E (dp = 9°, dm = 14°). New paleomagnetic results are also reported for the Caraquet and Avalon dikes.The virtual paleopoles from Hettangian flows and dikes of Atlantic Canada lie on average ~11 °north of the virtual paleopoles from Hettangian flows of the northeastern United States. This discrepancy is likely due to insufficient averaging out of paleosecular variation and demonstrates that caution is needed in paleomagnetically correlating among the Early Jurassic flows and intrusions of eastern North America.