The Gaspé Belt in the Restigouche area comprises three successions separated by a Late Silurian (Salinic) disconformity and an Early Devonian angular unconformity. The lower, Upper Ordovician to Lower Silurian sequence consists of siliciclastic turbidites of the Boland Brook and Whites Brook formations (Grog Brook Group), overlain by calcareous turbidites of the Pabos and White Head formations (Matapédia Group), and slope and shelf deposits of the Upsalquitch and Limestone Point formations (lower Chaleurs Group). Above the Salinic disconformity, the upper Chaleurs Group and the Dalhousie Group record a transgressive–regressive cycle. The former comprises Pridolian carbonate rocks of the West Point Formation and overlying Pridolian to Lochkovian sedimentary rocks of the Indian Point Formation. The Chaleurs Group is conformably overlain by Lochkovian to early Emsian subaerial volcanic rocks of the Dalhousie Group (Val d'Amour Formation), which is unconformably overlain by alluvial–lacustrine deposits of the late Emsian Campbellton Formation. Acadian orogenesis began during the Emsian and is characterized by open to closed folding, heterogenous cleavage development, and reverse and strike-slip faults. The Salinic orogeny is manifested in extensional block faulting, within-plate volcanism, and uplift and deep erosion of Early Silurian strata. Early Devonian high-level intrusion of the Matapédia Group, White Head clasts in Indian Point conglomerate, and thermal maturation data all indicate an extended period of Late Silurian – Early Devonian uplift in parts of the Restigouche area. Thermal maturities of West Point and Indian Point strata are within the oil and condensate windows and suggest potential for hydrocarbons in the study area.

Siluro-Devonian volcanic rocks of the northwestern mainland Appalachians are found mainly in the Tobique belt of New Brunswick where they consist predominantly of bimodal mafic–felsic suites erupted in a continental-rift environment. The axis of the Tobique rift trends north-northeast – south-southwest, obliquely to the regional northeast–southwest trend of the Appalachians. These geometric relationships are interpreted as being the result of rifting in a sinistral shear regime produced during emplacement of the Avalon terrene. The basaltic rocks are continental tholeiites and transitional basalts derived from a heterogeneous upper-mantle source that was enriched in incompatible elements relative to the primordial mantle. The mantle source was probably affected by the subduction processes.

A detailed tectonic analysis of the magmatic evolution of the Early to Middle Ordo vician west-facing Popelogan arc in New Brunswick and adjacent Maine is presented based on combining new U-Pb zircon radiometric age dates (thermal ionization mass spectrometry and sensitive high-resolution ion microprobe) with existing age constraints on the various magmatic events recognized in this arc system. The Popelogan arc had a life span of nearly 20 m.y. (ca. 476-457 Ma) and becomes progressively younger toward the northwest. Existing lithogeochemical and isotope data combined with field relationships and new zircon inheritance data indicate that the Popelogan arc was built on the leading edge of Ganderia and represents a predominantly continental calc-alkaline arc. Incursions of arc to within-plate-like tholeiitic magmatism correspond to rifting events temporally and spatially linked to trenchward migration of the arc magmatic axis, isolation of arc ribbons, and formation of mafic oceanic and transitional crust in the associated Tetagouche backarc basin. Arctrench migration and backarc basin opening were caused by a retreating subduction zone. Slab retreat prior to 467 Ma was at least partially accommodated by arc migration, but thereafter, it was mainly accommodated by extension and spreading in the associated Tetagouche backarc basin. New U-Pb zircon radiometric ages and lithogeochemistry of calc-alkaline felsic tuff beds interlayered with the oceanic backarc rocks of the Fournier Supergroup provide a direct link between the Tetagouche backarc rocks and coeval calc-alkaline arc activity in the Popelogan arc. The oldest incursion of arc tholeiite (470-467 Ma) corresponds to cessa tion of the ca. 476-470 Ma calcalkaline Meductic phase of the Popelogan arc, the onset of the calc-alkaline Balmoral phase (467-457 Ma) farther to the west, and formation of oceanic and mafic-dominated transitional crust in the Canoe Landing Lake Formation of the Tetagouche backarc basin. A less-well-constrained second incursion of tholeiitic magmatism between 465 and 459 Ma corresponds to rifting of the Tetagouche arc ribbon and formation of the ophiolitic Devereaux complex in the Fournier Supergroup. Cessa tion of calc-alkaline arc magmatism and eruption of transitional and alkalic mafic volcanic rocks between 459 and 455 Ma may represent slab breakoff following accretion of the Popelogan arc to composite Laurentia, and/or ridge subduction immediately prior to arc-continent collision.

The Popelogan Inlier consists mainly of mafic volcanic rocks (lapilli tuffs and massive to amygdaloidal, plagioclase-phyric flows) of the Middle Ordovician Goulette Brook Formation. Pyroclastic rocks include high-MgO–Cr–Ni picritic tuffs (type I) containing, in some cases, >20% MgO, and related high-MgO andesitic tuff (type II). High-MgO rocks were generated by 30–40% partial melting of an enriched mantle source; type II is descended from type I mainly by fractionation of olivine. Mafic flows comprise basaltic andesites (type III) with low trace-element abundances and strongly fractionated, trace-element-enriched andesites (type IV). Types III and IV represent ~20 and ~10% partial melts, respectively, of a mantle source similar to that of the pyroclastic rocks, based on similar ratios of high field strength elements (HFSE). Unlike types I and II, petrogenesis of mafic flows involved fractionation of plagioclase and possibly amphibole. Volcanic arc signatures include negative Nb and Ti anomalies in all basalt types, along with low abundances of HFSE. Trace-element abundances are inconsistent with prior depletion in the back arc and require involvement of a mantle plume or subcontinental lithosphere. The highly magnesian composition of the picrites demands high melting temperatures and rapid transit through the crust, both of which suggest extension of the arc-bearing plate. Compositionally similar rocks in the South Pacific are associated with unusual tectono-magmatic settings involving ridge subduction, which may have established the necessary extensional environment. It is proposed that subduction of a plume-influenced ridge segment could explain the chemistry of the Goulette Brook volcanic rocks.

We describe two Early Devonian occurrences of coaly shale composed mainly of the compacted remains of early plants, such that they resemble coal. Among the earliest thick phytodebris accumulations known, the occurrences lie within Pragian strata of the Val d'Amour Formation and Emsian strata of the Campbellton Formation of northern New Brunswick, and were deposited in low-energy wetlands where they were rapidly buried. Although the coaly shales did not yield recognizable plant taxa, numerous taxa are present in adjacent beds. Petrographic analysis revealed an average of 80.7% vitrinite (predominantly telovitrinite) and 18.7% liptinite (predominantly sporinite) on a mineral-matter-free basis, with a higher vitrinite content than most Devonian coals. Low sulfur content and atomic C/N ratios that vary from 44.3 to 82.1 in organicrich samples indicate terrestrial derivation. Vitrinite reflectance in organic-rich samples (21.4-35.6 wt % total organic carbon) ranges from 0.48 to 1.00, indicating a low degree of thermal alteration that is supported by cross-polarization spectra from studies of 13 C nuclear magnetic resonance; two carbon-poor samples were thermally altered to anthracitic rank adjacent to an intrusive body. Although plants at this time were still comparatively primitive, the presence at both sites of specimens with recognizable lignified cellular structures in vitrinite and particularly thick and resistant cuticle may represent an important step in peat development. In these Early Devonian formations, diverse assemblages of small vascular plants are present in a range of environments, and the plants were sufficiently abundant to form coal-like accumulations under suitable burial conditions, serving as a proxy for plant biomass and vegetation cover in the early stages of terrestrial colonization.