Abstract

The late Precambrian Coldbrook Group comprises basal basaltic flows and pyroclastic rocks, fragmental andesitic to rhyolitic rocks, and minor capping sedimentary strata, all deformed and metamorphosed to prehnite–pumpellyite grade. Sr-isotope data indicate metasomatism, and probably deformation and metamorphism, took place long after deposition, possibly during major Carboniferous deformation of the Bay of Fundy region. Geochemical data suggest that the Coldbrook Group formed in a subduction-related ensialic volcanic-arc environment, although some specimens appear transitional to within-plate tholeiites. Magma evolution was dominated by fractionation of ilmenite, pyroxene, and plagioclase. Trace-element ratios suggest crustal contamination of virtually all Coldbrook Group rocks, with the amount of crustal component larger in the more salic portion. Comparison of data for the Coldbrook Group with data of other late Precambrian volcanic sequences in Atlantic Canada suggests that similar arc-related sequences underlie slightly younger, continental bimodal sequences in many Avalonian terranes.

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