Geochemical Systematics of High Arctic Large Igneous Province Continental Tholeiites from Canada—Evidence for Progressive Crustal Contamination in the Plumbing System

• Published in: Journal of Petrology
• Main Authors: Bédard, Jean H, Saumur, Benoit Michel, Tegner, Christian, Troll, Valentin R, Deegan, Frances M, Evenchick, Carol A, Grasby, Stephen E, Dewing, Keith
• Other Authors: Geological Survey of Canada
• Format: Article in Journal/Newspaper
• Language: English
• Published: Oxford University Press (OUP) 2021
• Subjects: Arctic; Canada; Isachsen; Strand Fiord
• Online Access: http://dx.doi.org/10.1093/petrology/egab041; http://academic.oup.com/petrology/advance-article-pdf/doi/10.1093/petrology/egab041/38575849/egab041.pdf; https://academic.oup.com/petrology/article-pdf/62/9/egab041/42757634/egab041.pdf

Abstract Cretaceous High Arctic large igneous province (HALIP) sub-alkaline magmatic rocks in Canada are mostly evolved (MgO 2–7 wt%), sparsely plagioclase + clinopyroxene ± olivine-phyric tholeiitic basalts. There were two main HALIP continental flood basalt (CFB) eruption episodes: 135–120 Ma (Isachsen Fm.) and 105–90 Ma (Strand Fiord Fm.), both associated with cogenetic doleritic sills and dykes. Building on a large modern database, 16 HALIP tholeiite types are defined and grouped into genetic series using Ce vs Sm/YbNMORB distributions. Comparison with model melting curves implies that higher-Sm/Yb HALIP basalt types record low-degree melting of garnet-bearing mantle sources. More voluminous intermediate- and low-Sm/Yb HALIP basalt types separated from the mantle at shallower levels after further extensive melting in the spinel-peridotite field. Within a given Sm/Yb range, increases in incompatible elements such as Ce are coupled with progressive clockwise rotation of normalized incompatible trace element profiles. Trace element modeling implies this cannot be due to closed-system fractional crystallization but requires progressive and ubiquitous incorporation of a component resembling continental crust. The fractionation models imply that low-Sm/Yb HALIP basalts (∼7 wt% MgO) initially crystallized olivine gabbro assemblages, with lower-MgO basalts successively crystallizing gabbro and ilmenite-gabbro assemblages. In contrast, higher-Sm/Yb basalts fractionated more clinopyroxene and ilmenite, but extensive plagioclase fractionation is still required to explain developing negative Sr–Eu anomalies. Back-fractionation models require about 40 % addition of olivine to bring the most primitive HALIP basalts (∼7 % MgO) into equilibrium with Fo89 mantle. Inverse fractionation–assimilation modeling shrinks the CFB signature, making decontaminated model parental melts more similar to enriched mid-ocean ridge basalt. The progressive increase of the contamination signature within each HALIP tholeiitic differentiation ...