Abstract

Allochthonous masses of basaltic lava flows and related tuffs are present in several localities in an approximately 30 km long segment of the western margin of the Granby Nappe, in southeastern Quebec. They occur either as numerous small blocks in the Drummondville wildflysch related to the nappe or as larger masses intercalated with sedimentary sequences of limestone and shale of probable Late Cambrian to Early Ordovician age. These latter occurrences and the associated sedimentary units form "island-like" areas within lithologies of the Granby Nappe consisting of Cambrian sediments that accumulated on the continental rise. Their overall characteristics suggest that they represent slabs derived from the shelf margin of Laurentia and incorporated into the cratonward-moving nappes during the Middle Ordovician Taconian Orogeny.The volcanic rocks are mainly transitional but include some alkali olivine basalts. There are some indications that their affinities are to basaltic rocks of seamount chains localized along leaky transform faults. The segment of the continental margin from which the volcanic rocks were derived originated in the latest Precambian times, by rifting involving a rift–rift–rift (RRR) triple junction. Thus, it was a likely location for deep-seated transverse fracture zones linked to ridge-to-ridge transform faults of Iapetus. Therefore, the best explanation of the volcanism is that it was localized along such fracture zones. This episode of Late Cambrian – Early Ordovician volcanism related to the Iapetus cycle is probably analogous to the recently documented Early Cretaceous volcanism related to the Atlantic cycle on the northeastern American margin.

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