Permian alkaline basalts associated with formation of the Sverdrup Basin, Canadian Arctic

摘要: Permian basaltic rocks of the Esayoo Formation attended lithospheric extension that formed the Sverdrup Basin in the Canadian Arctic. North of Greely Fiord, northern Ellesmere Island, subaerial flows of the Esayoo Formation attain a maximum thickness of 300 m, but thin rapidly westward, where pillow lavas, epiclastic basalt conglomerate, and associated marine sedimentary rocks indicate proximity to an ancient shoreline. Element-mobility studies demonstrate that modifications of many major elements and some of the large ion lithophile elements accompanied low-grade burial metamorphism. Discriminant diagrams involving only the relatively immobile trace elements reveal within-plate alkaline to transitional basalt affinities. Trace element data (mean Th/Ta = 1.7) do not register significant lower crustal contamination. Models utilizing rare earth clement ratios and Ni–Cr relations suggest that variable degrees of partial melting of an enriched garnet lherzolite and minor combined olivine–clinopyroxene fractional crystallization can account for the described compositional diversity. Nb–Ta peaks (mean La/Nb = 0.99) in spider diagrams identical to ocean-island basalts characterize magmas derived from the asthenospheric mantle with minimal subcontinental lithospheric contribution and continental contamination. Small rates of continental extension during the Carboniferous generated small-volume alkaline melts that passed unadulterated through the subcontinental lithosphere and crust during ascent from their asthenospheric mantle source.