Metafelsites in Waterloo area, Quebec, represent the only known silicic volcanic rocks in the predominantly basaltic Tibbit Hill Formation. Low-grade metamorphism accompanied by hydration and albitization has converted the felsic volcanic rocks mainly to muscovite–quartz–albite schists. The volcanic parent of these metafelsites was formed partly as lava flows and partly as tuffs. The principal compositional type was a comendite. A component of intermediate rocks is also present but its extent is undetermined and probably minor. U–Pb zircon studies of the metafelsites have yielded a reliable age of [Formula: see text]. This Early Cambrian age is probably representative of the age of the Tibbit Hill Formation as a whole.The Tibbit Hill Formation accumulated at one of the clearest examples of a RRR (rift–rift–rift) triple junction–the Sutton Mountains triple junction–of the continental rift system formed as a prelude to the opening of the Iapetus Ocean. Its volcanic rocks are products of the youngest major episode of rift-related volcanism known from the continental margin of Laurentia. The volcanic event may have occurred as a harbinger of the onset of sea-floor spreading at the Sutton Mountains triple junction.

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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.

The Acton Vale limestone units of probable Early Ordovician age in the external nappe zone of the Appalachian foldbelt in southeastern Quebec host numerous occurrences of vein- and breccia-type copper deposits. Associated with these are two significant occurrences of barite: one at Upton consisting of a stratiform mass of probable economic potential; and the other at Lord Aylmer consisting of thin, tabular, karstic infillings of barite. Studies of the mineral occurrences at Acton Vale, Upton, and Lord Aylmer show that the mineralization is epigenetic and formed largely by open-space filling at shallow depths in parts of the limestones that had undergone ground preparation by brecciation and (or) karsting. Fluid-inclusion, sulphur-isotope, and strontium-isotope analyses show that mineral deposition occurred at temperatures ranging from 110° to 135 °C, that sea water formed the bulk of the mineralizing fluids, and that altered basaltic volcanic rocks associated with the Acton Vale limestones are. the most important source of metals and Ba.The mineral deposits seem to have formed during the evolution of the passive margin of Laurentia. The preferred model for their genesis is one involving deep circulation of water, which, on coming into contact with the volcanic rocks, leached metals and Ba and subsequently deposited them in favourable parts of Acton Vale limestones. Recurrent movements of faults of the Ottawa Graben, which is the failed arm of the Sutton Mountains triple junction, may have been a factor that facilitated deep circulation of fluids. In their present setting, the Acton Vale limestones and the associated volcanic rocks and shales probably represent slivers of the passive margin incorporated into the Cambrian shale – feldspathic sandstone assemblages of the Granby Nappe.