THE LATE-STAGE CRYSTALLIZATION HISTORY OF THE JURASSIC NORTH MOUNTAIN BASALT, NOVA SCOTIA, CANADA. II. NATURE AND ORIGIN OF SEGREGATION PIPES

摘要

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 (<150 m) and composed of massive, medium-grained, holocrystalline to hypocrystalline, dark green basalt flows, the middle member is composed of multiple, thin (<10-20 m), fine-grained and vesicle-rich basalt emplaced as inflated sheeted palioehoe flows. Locally, at the base of the BIM, abundant, pipe-shaped features (2-60 cm wide, <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 (