A Rb–Sr whole-rock age study has been carried out on three volcanic groups which underlie fossiliferous Lower Cambrian in the Atlantic provinces of Canada. Isochron ages of these three, and their initial 87Sr/86Sr ratios, are shown below.graphicThe Harbour Main group unconformably underlies younger formations which include the Bull Arm.The contemporaneity of the Coldbrook, Fourchu, and Bull Arm groups, predicted on field evidence, is confirmed by the age study, as is also the greater age of the Harbour Main. However, the isochron ages (500 m.y, average) of the three younger groups are at least 10% low by current time-scale standards for the base of the Cambrian. As all the data, show considerable departure from strict linearity, it is suggested that small and variable diffusion of Rb into the rocks has occurred, thus scattering the points and lowering the ages.Initial 87Sr/86Sr at the level 0.704—0.707 suggests for these magmas a subsialic source, which if not in the upper mantle, is at least in the lower part of the crust.

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