Griffin gabbro sills (2.11 Ga), Hurwitz Basin, Nunavut, Canada: long-distance lateral transport of magmas in western Churchill Province crust

Abstract

Circa 2.11 Ga sills and rare dykes (herein named the ‘Griffin gabbros’) extend discontinuously across an area 400 by 125 km in the Hearne domain of the western Churchill Province in northern Canada. Sills form poorly connected, tongue-like tabular bodies primarily within Paleoproterozoic intracratonic basin deposits of the lower Hurwitz Group. Dykes are restricted to the southern limit of the Griffin suite and have not been found in the interior of Hurwitz Basin or in Archean basement. Incompatible element patterns and low ε Nd 2111 values (−0.1–+1.0) indicate an enriched asthenosphere source. Similar to many modern hotspot-related ocean island basalts, the gabbros display enrichments in Nb and La, suggesting a mantle plume origin. Heavy rare earth element depletions and TiO 2 values (averaging 3.40 wt.%) suggest partial melting of a garnet peridotite source at depths of 70–100 km; MgO values (averaging 5.20 wt.%) indicate fractionation after melting. Concentrations of both the incompatible and compatible elements scatter widely with respect to Mg number, indicating that compositional variation does not represent fractional crystallization from a common parent, consistent with derivation from different magma chambers related to the same mantle plume. The gabbros fail to show signs of an Archean enrichment event recorded by other Paleoproterozoic mafic rocks in the Hearne domain. This, together with the absence feeder dykes in basement, indicate derivation external to Hurwitz Basin. We interpret that a mantle plume, located beyond the present southern margin of the Hearne domain, was active during ∼2.1 Ga rifting that led to opening of the Manikewan ocean. The Griffin gabbros were likely injected because of the hydraulic head created by topographic doming above this plume. Analogous to the Ferrar sills (Jurassic) in Antarctica, the gabbros may have been fed by cryptic dykes in which magmas traveled laterally for at least 250 km across the southern Hearne domain and then, controlled by sedimentary layering, spread as sills for hundreds of km within Hurwitz Basin.