Petrology and geochemistry of Cambrian volcanic rocks from the Avalon Peninsula, Newfoundland

Abstract

Middle Cambrian basaltic flows, lapilli tuffs, and their feeder pipes, on the Avalon Peninsula, Newfoundland, show high Nb/Y and low Zr/Nb ratios and high concentrations of incompatible elements, such as Nb and Zr, which are typical of alkaline rocks. The compositions of relict clinopyroxenes are consistent with these conclusions. Relatively low Mg′ values and low CaO and high TiO2 concentrations show that the flows represent evolved compositions. Petrographic evidence and trends in trace-element data indicate that the range of bulk-rock compositions in the feeder pipes can be related to precipitation of augite, plagioclase, and olivine as well as late removal of Fe–Ti oxides and apatite. The negative correlation (shown by noncumulus rocks) between Mg′ and Al2O3, the relatively high Mg′ values of the flows, and the alkaline nature of the rocks distinguish them from most flood basalts and relate them to rift basalts. Low Al2O3 concentrations at the highest Mg′ values may be related to genesis in a garnet-rich mantle source under high pressures. These continental alkali basalts provide evidence that this portion of Avalonian terrane experienced tension during the Cambrian.