Crustal eduction and slab-failure magmatism in an Orosirian (2.05−1.80 Ga) post-collisional cratonic foredeep: geochronology of Seton volcanics and Compton laccoliths, Tu Cho (Great Slave Lake), NWT, Canada

Abstract

Three Orosirian basins and associated foreland thrust-fold belts are preserved on the margins of Slave craton. All three are related to orogenic belts where oceans opened and later closed uniting new crustal partners. Great Slave basin differs from Kilhigok and Coronation basins in ways that have defied explanation. It lacks a passive-margin sequence and hosts two discrete igneous suites, separated by large-scale thrusting, occurring well after the adjacent paleocean had closed. Here we report U-Pb zircon geochronology by CA-ID-TIMS for a member of each suite to constrain the age and origin of post-collisional thrusting. A widespread pulse of mainly phreatic alkaline volcanism, coeval with renewed foredeep flexure, occurred at 1889.0±0.7 Ma (2σ internal error). A quartz-monzodiorite body, one of a belt-parallel chain of laccoliths that postdate thrusting, was emplaced at 1866.9±0.9 Ma. These ages bracket renewed foredeep sedimentation and thrusting that telescoped major facies zones and was rooted within the basin. The older age is 70 and 30−60 Myr younger than collision in Thelon and Taltson orogen, respectively. We attribute post-collisional thrusting and foredeep subsidence to ‘eduction’—the upward and outward ejection of partly-subducted crust—and postulate that the top of the ejected wedge was a normal-sense detachment fault projecting beneath Nonacho basin. We infer that eduction was triggered by slab failure, producing alkaline volcanism, and ended with delamination and laccolith emplacement. Eduction was facilitated by tradewind–driven erosion. Delamination was enabled by crustal transfer to the educted wedge, reducing footwall buoyancy. Slab-failure and/or delamination removed the passive margin.