Abstract

Moyie sills comprise an extensive suite of basaltic rocks that have intruded Middle Proterozoic Purcell Supergroup rocks in southeastern British Columbia. The sills are spatially restricted to the Aldridge and Fort Steele formations in the lower part of the Purcell succession and are distinct from a suite of mafic sills higher in the succession. They may constitute up to 30% of a typical sequence but generally decrease in volume upsection as the abundance of thick-bedded A–E turbidites decreases. A number of the sills have textures and contact relationships that suggest they intruded unconsolidated or partly consolidated wet sediments.A U–Pb zircon Middle Proterozoic date of 1445 Ma from a coarse-grained sill is interpreted as being the minimum age of emplacement. Because the sills are penecontemporaneous with Aldridge sedimentation, this date defines the minimum age of deposition of lower and basal middle Aldridge rocks.Two distinct compositions of Moyie sills are recognized. Most are subalkaline, high-iron tholeiitic basalts, whereas others are alkaline basalts. The two different chemical trends are typical of volcanism in an incipient rift environment or in the early stages of continental rifting. This supports a model for deposition of Belt–Purcell rocks in a large, subsiding intracratonic basin formed by Middle Proterozoic rifting.

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