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The early Mesozoic North Mountain basalts of Nova Scotia are quartz-normative tholeiites with compositions comparable to other Northern Appalachian Triassic-Jurassic tholeiitic lavas, dikes, and sills related to the initial stages of the opening of the Atlantic. The subaerial basalts form a northeast-southwest-trending belt, ~200 km long, with thickness decreasing northeastward from about 400 m to 275 m. Distinct but constant initial Sr isotopic ratios indicate that the magma was not contaminated by the crust during its ascent and emplacement, although other evidence suggests that it underwent fractional crystallization dominated by the separation of pyroxenes and plagioclase. Bands of mafic pegmatite and rhyolite in the thick flows are the results of “in situ” differentiation of the host basalts. The trace element traits, including the relative depletion of Nb and enrichment of K, Rb, Ba, light rare earth elements, and Th as well as the “enriched” isotopic characteristics of the lavas, suggest a subcontinental lithospheric mantle source for the North Mountain basalts.

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