A zircon age of 429 + 6, −5 Ma on felsic volcanic rocks of the Springdale Group, a sequence of bimodal volcanic rocks and redbeds, establishes a Llandovery age for this group. Similar sequences from other parts of the Dunnage and Humber zones of Newfoundland also give Llandovery fossil or radiometric ages, demonstrating temporal correlation among lithologically similar sequences in these zones.Distinctive sequences of bimodal, subaerial rhyolite and basalt with associated subaerial to shallow marine sedimentary rocks within the Avalon and Meguma zones of Nova Scotia give fossil or radiometric ages demonstrably or arguably very close to the age of the Springdale Group. These sequences suggest an overlap assemblage fixing the minimum age of accretion of the Avalon and Meguma zones to North America. The age and the magmatic and sedimentary character of the assemblage are compatible with origin on or from continental crust, implying that the Iapetus ocean was closed prior to Early Silurian time. If these considerations are correct, the Devonian Acadian orogeny must involve either thermal and mechanical readjustments subsequent to the closing of the Iapetus ocean, or a completely separate episode of plate motion.

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On the E side of Hudson Bay, the NW margin of the Archaean Superior Structural Province is overlain by an unmetamorphosed, little deformed, Early Proterozoic supracrustal sequence. The sequence is divisible into three parts, possible tectosomes, each characterized by a distinct style of mineralization. The arkosic, alluvial Richmond Gulf Group was shed eastward from a thermal uplift and has been preserved only in a later aulacogen, the Richmond Gulf Graben. It contains an apatite-cemented redbed. Secondly, a widespread post-aulacogen, carbonate-rich, W-facing miogeocline blanketed the region. A pyritic zone developed at its basal contact is particularly thick and hosts disseminated copper mineralization where formed on the arkose in the graben. A pyritic, brecciated, stromatolite-rich horizon within the carbonate contains galena and sphalerite. Thirdly, widespread iron formation and a local 60 cm thick manganese carbonate bed, overlying W-derived basalt flows, flysch and arkosic molasse, all of uncertain tectonic affinity, terminate the sequence.