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The Jurassic (202 Ma) North Mountain Basalt (NMB) is a sequence of quartz-normative continental tholeiites emplaced within the Fundy Basin of southern Nova Scotia, Canada. The sequence is subdivided into three units referred to (from base to top) as the East Ferry (EFM), Margaretsville (MM) and Brier Island (BIM) members. Whereas the lower and upper members are thick (< or =150 m) and composed of massive, medium-grained, holocrystalline to hypocrystalline, dark green basalt flows, the middle member is composed of multiple, thin (< or =10-20 m), fine-grained and vesicle-rich basalt emplaced as inflated sheeted pahoehoe flows. Locally, at the base of the BIM, abundant, pipe-shaped features (2-60 cm wide, < or =1.5 m vertical dimension) of composite nature are present, consisting, in decreasing proportion, of: (1) a medium to dark green to black igneous phase of variable texture (aphanitic to coarse grained); (2) clear, crystalline to cloudy white, rarely red, green or blue, silica material; (3) aphanitic, red-brown, highly indurated igneous material; (4) amygdules of silica and zeolite, and (5) irregularly shaped blebs (< or =1-2 cm) of altered igneous material inferred to have been micromafic enclaves. In plan section, the pipes are dominated by either the dark material or silica, but the silica phase does core pipes, and rarely, the two are mixed. The red-brown phase occurs at pipe margins and blebs occur randomly in pipes of medium- to coarse-grained texture. Rare pipe profiles indicate that pipe compositions can vary vertically, as can their shape and width. Pipe compositions range from basaltic to rhyolitic, the continuum possibly reflecting mixing of the two identified end-members. Microscopically, the pipes are holocrystalline to holohyaline, may contain fresh felsic glass in addition to quenched material (i.e., skeletal plagioclase and pyroxene) and granophyre. Mineralogically, the pipes are dominated by calcic plagioclase (An (sub 40-70) ) and clinopyroxene, with both having textural and chemical features indicating growth in a Fe-rich melt with late-stage resorption. Importantly, the basalt hosting the pipes is locally enriched in intergranular felsic glass. A model for pipe formation involves initial collection of silicic melt via filter pressing with entrainment of earlier-formed plagioclase and clinopyroxene within the melt and some mixing and mingling with residual basaltic melt. The density contrast between the felsic and basic melts and basalt created a buoyancy instability resulting in the rise of felsic diapirs, which are seen today uniformly dispersed on outcrop surfaces and which reflect paleohorizontal surfaces of the basalt flows.

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GeoRef, Copyright 2018, American Geological Institute.<br/>2011-022501<br/>North Mountain Basalt