Petrochemical and stratigraphic aspects of North Mountain basalt from the north shore of the Bay of Fundy, Nova Scotia, Canada

Abstract

Early Jurassic quartz-normative tholeiitic basalts occur in a series of fault blocks at four localities (Cap d'Or, Parrsboro, Five Islands, and Bass River) along the north shore of the Bay of Fundy. Major-element and trace-element data show that they represent outliers of the North Mountain basalt (NMB), which from the south shore of the Bay of Fundy are well-known. Diagnostic 87Sr/86Sr ratios (≈0.70609) indicate that a thick (>90 m) flow at Five Islands and Cap d'Or represents the lower unit of NMB, thus extending this single flow's lateral extent to 230 km. Thin flows overlying the thick flow at Cap d'Or suggest that the middle unit of NMB also occurs on the north shore. A thick flow at McKay Head (Parrsboro area) shows a variable Sr isotopic composition that is probably a result of metasomatism (with Rb addition) along the Cobequid fault. The average composition (0.70656) is similar to that of the upper unit of NMB. If the flow does represent the upper unit, then four thinner flows above it form an "overlying unit" not recognized along North Mountain. Although the nature of contacts between the middle and upper units was not observed, a lack of sedimentary rocks between all other flow units indicates that little or no sedimentation occurred between basalt eruptions. North shore basalts appear less mafic (more evolved) than south shore basalts, providing support for the hypothesis of differentiation during northeasterly magma migration through dykes that fed 230 km long fissure eruptions.