Avalonia of the northern Appalachian orogen includes clastic sedimentary rocks that span the Ediacaran-Cambrian boundary, including the Global Boundary Stratotype Section and Point (GSSP) at Fortune Head, Newfoundland. Detrital muscovite becomes abundant in sedimentary strata near and above the GSSP. Single-crystal 40Ar/39Ar total-fusion analyses for detrital muscovite sampled from these strata in Newfoundland and the Mira terrane of Nova Scotia yield ages that range from ca. 634 to 540 Ma, dominated by single modes that are skewed to younger ages. Laser incremental heating 40Ar/39Ar age spectra for individual muscovite crystals are typical of 40Ar loss, with ages for initial increments as young as ca. 400 Ma that generally increase through the measurements, defining plateau ages with an average of 637.1 +/- 4.7 Ma. Wavelength dispersive spectrometer maps show K and Al loss selectively along interlayer (001) zones and grain boundaries now occupied by aluminous, metamorphic chlorite. Loss of K and 40Ar was linked to regional, low-grade metamorphism during late Ediacaran transpression, overprinted by metamorphism during late Silurian-Devonian juxtaposition of Avalonia with Ganderia and Meguma. The detrital muscovite source is inferred to have been aluminous rocks in the Avalonian crust metamorphosed during ca. 650 collision of Avalonia with the West African Craton. The oldest muscovite ages coincide with the end of the Marinoan glacial stage, which could indicate a causal relationship between the termination of glaciation and initial mid-crustal cooling of the source terrane through closure temperatures for muscovite. Exhumation and erosion during extension and basin development through the Ediacaran-Cambrian transition provided abundant detrital muscovite.

New U–Pb zircon geochronology by both laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) and chemical abrasion–isotope dilution–thermal ionization mass spectrometry shows that a tuff in the Folly River Formation in the Bass River block in the Avalonian Cobequid Highlands of Nova Scotia contains two large populations of Tonian zircon at c. 780 and 890 Ma. New and previously published LA-ICP-MS detrital U–Pb zircon dates from sedimentary rocks in the Folly River and Gamble Brook formations in the Bass River block and the Mount Thom Formation in the Mount Ephraim block are mainly Mesoproterozoic and early Paleoproterozoic. The similarity in spectra, with major peaks at 1000, 1200, 1500–1600, 1800 and 2000 Ma, suggests that they were derived from similar sources in Baltica. They may have been deposited in a back-arc basin associated with the first Avalonian arc represented by the 760–730 Ma volcanic rocks of the Dalhousie Mountain Formation and Mount Ephraim Plutonic Suite. Lutetium–Hf isotopic data from tuff and arkosic wacke in the Folly River Formation combined with previously published data show both positive and negative ε Hf ( t ) values and confirm previous Sm–Nd isotopic data which demonstrated that both juvenile and evolved Archean, Paleoproterozoic and Mesoproterozoic crust occurs in Avalonia.

The Meguma terrane is a fragment of the Armorican terrane assemblage that either (1) was transferred to East Avalonia during Early Paleozoic rifting and opening of the Rheic Ocean or (2) was accreted as an isolated terrane or as part of a Gondwanan promontory. The Meguma terrane comprises two distinct subterranes, which dextrally moved opposite West Avalonia before their Emsian to Frasnian assembly and underthrusting by an Avalonian promontory in SE Cape Breton Island. Dextral oblique flat subduction of the Rheic Ocean inhibited upper plate magmatism and generated folding and oblique reverse faults during early Kejimkujic orogenesis. Late Devonian steepening of the Rheic slab re-established a metasomatized mantle wedge and produced subduction-related magmas in the Meguma terrane with anhydrous A-type magma in the distal back-arc situated in West Avalonia. Kejimkujic indentation of the Meguma terrane by the West Avalonian promontory during the Famennian led to sinistral and dextral strike-slip faulting, and transpression near the area of indentation. Kejimkujic orogenesis was followed by Early Carboniferous west-directed dextral motion of the Meguma terrane using the Minas Fault zone as a transfer fault and related transtension. Mid-Carboniferous Alleghenian contraction generated localized D 2 sinistral oblique reverse slip, commonly reusing existing shear zones.

The Meguma terrane in Nova Scotia consists mainly of Cambrian to Ordovician metasedimentary rocks intruded by abundant peraluminous granite, granodiorite, and minor diorite-tonalite-plutons between ca. 380 and 360 Ma. The metasedimentary rocks were variably metamorphosed to greenschist- to amphibolite-facies during the Early Devonian. In the eastern Meguma terrane, the regional M1 chlorite- and biotite-zone assemblages are overprinted by two types of high-temperature lowpressure M2 metamorphism. In contact aureoles around plutons the regional M1 assemblages are overprinted by cordieriteand andalusite-bearing M2 contact metamorphic assemblages, and away from plutons chlorite-bearing, (or biotite-bearing) regional M1 assemblages are overprinted by staurolite-, garnet-, and (or) cordierite-bearing M2 regional assemblages that represent metamorphism driven by a regional elevated thermal gradient. The interpreted metamorphic conditions in the eastern Meguma terrane range from ca. 480-550 degrees C and pressures below 2.5 kbar up to 550-650 degrees C with pressures between 4 and 5 kbar. Based on a new garnet Lu-Hf age, regional metamorphism was ongoing at ca. 372 Ma, hence broadly coeval with pluton emplacement. Metamorphism and pluton emplacement took place in a largely transpressional tectonic setting during the collision of the Meguma terrane with Avalonia. After pluton emplacement, peak metamorphic assemblages were variably overprinted with retrograde chlorite and sericite M3 assemblages while deformation continued. Regional structures were transposed into a composite transposition foliation as a result of continued motion along the terrane-bounding Minas Fault Zone and associated ductile shear zones.

The ca. 370 Ma Eastern Shore dykes include six single or multiple (two or three) parallel spessartite dykes that trend NNW in the Sheet Harbour area of eastern mainland Nova Scotia. Spessartite is a variety of calc-alkaline lamprophyre that characteristically occurs in dykes associated with granitoid rocks and gold mineralization in orogenic belts. Spessartite dykes typically contain crustal xenoliths and xenocrysts, a feature shown by three of the Eastern Shore dykes (Popes Harbour, Tuff Island, and Borgles Island). The Eastern Shore dykes are mineralogically and chemically typical of spessartite but show wide chemical variation and higher MgO, lower TiO2 and P2O5, and flatter chondrite-normalized rare-earth-element patterns than are typical of spessartite. The pair of dykes at Sober Island show internal variations that are evidence of magma evolution resulting from plagioclase and amphibole fractionation; the East Jeddore and Little Harbour Road and Coast dykes are similar to the least evolved Sober Island spessartite. The three xenolith/xenocryst-bearing dykes show anomalous chemical features such as more enrichment in light rare-earth elements (REE), higher Sr, Y, Zr, and Hf, and more varied compositions compared to the other dykes. The Popes Harbour dyke is characterized by low Ni. Despite these differences, mainly parallel chondrite-normalized REE patterns suggest that the Eastern Shore dykes are related and derived from hydrous large-ion-lithophile- and high-field strength-element-enriched garnet-bearing mantle-derived magma that experienced variable crustal contamination.

The Eight Mile Brook Plutonic Suite (EMBPS) consists of co-mingled gabbroic, syenitic, and granitic rocks that intruded Tonian rocks of the Mount Thom Formation and Mount Ephraim Plutonic Suite at the southeastern edge of the Mount Ephraim block in the Cobequid Highlands. It is unconformably overlain by or in faulted contact with Carboniferous sedimentary rocks. In situ dating of zircon in three samples by laser ablation - inductively coupled plasma - mass spectrometry (LA-ICP-MS) yielded Early Ordovician concordia and weighted mean 207Pb/238U ages of about 480 Ma. Age and chemical similarities among the EMBPS, West Barneys River Plutonic Suite in the Antigonish Highlands, and the Cape Porcupine Complex near the Strait of Canso suggest that all are closely related and formed by magma fractionation processes during an event that occurred between ca. 481 and 466 Ma based on previously published U-Pb zircon ages from thermal ionization mass spectrometry. The magmatism may be a far-field effect of subduction, a localized area of extension, or a hot spot. Chemical and age differences between these plutonic suites and the bimodal volcanic Dunn Point and McGillivray Brook formations in the northwestern Antigonish Highlands suggest that the magmatism in these units may not be directly related. Minor gabbroic and syenitic intrusions in the northern Antigonish Highlands are undated but show chemical differences from the dated magmatic rocks and hence may not be related.

The tectonic evolution of the northern Appalachian orogen is typically organized into orogenic episodes. The Taconian orogeny includes latest Cambrian to Late Ordovician arc-continent collision. Subsequent polarity reversal led to Silurian accretion of Ganderian terranes during Salinian orogenesis. The Acadian orogeny comprises Pridoli to Middle Devonian deformation attributed to accretion of West Avalonia to Laurentia. The term Neo-Acadian was coined by Robinson for Late Devonian to Mississippian shortening in New England, ca. 370-350 Ma, but the Indigenous-derived name Quaboagian was subsequently favoured for this episode. The "Neoacadian" later became associated with Meguma terrane docking, but clear convergent deformation in the Meguma terrane falls outside the original Neo-Acadian interval. Folding of strata in the Meguma terrane from ca. 409 Ma, concurrent with Acadian deformation, occurred in a different tectonic environment; emplacement of subduction-related plutons followed at mainly ca. 379-372 Ma. The ca. 370-350 Ma interval saw uplift of the Meguma terrane, local plutonism, and deposition of the Horton Group in a basin-and-range setting, whereas the crust beneath the Gulf of St. Lawrence thinned to less than half normal thickness during Maritimes Basin development. These anorogenic transtensional basins were probably connected with Quaboagian shortening in New England via dextral strike-slip. Shortening and inversion resumed after ca. 330 Ma, associated with dextral transpression, docking the Meguma terrane close its present-day position. The misuse of "Neoacadian orogeny" leads to misconceptions about timing of accretion of the Meguma terrane and about ca. 370-350 Ma tectonics in Atlantic Canada, which involved extension not shortening. We recommend that the term be abandoned in favour of Quaboagian and restricted to convergence in New England. Earlier deformation in the Meguma terrane may be attributed to the Kejimkujic orogeny.

Trace fossils in the High Head Member display disparity thus far unique in lower Cambrian (upper Terreneuvian–Series 2) deep-water sedimentary rocks. This disparity is likely not because the organisms making trace fossils did not exist in other places but instead a result of the special preservation conditions—big exposures of tops of beds made of sediments of the right grain size and under modern-day weathering conditions that gradually are exposing trace fossils that would otherwise not be preserved or observable.

Two wide-angle seismic reflection/refraction profiles were acquired to determine the velocity characteristics of the Meguma terrane and adjacent Avalonia northeast of the Cobequid-Chedabucto Fault Zone (CCFZ) in eastern Canada. Line 99-1 is located along the Scotian margin whereas line 99-2 crosses the Scotian margin and onshore Nova Scotia and extends into the Gulf of St. Lawrence. Velocity models were derived by forward modelling of travel times. P-wave velocities of 5.5-6.0 km/s and 6.0-6.4 km/s are interpreted to be metasedimentary rocks of the Goldenville and Halifax groups and granitic rocks, respectively. This inference is based on comparison with laboratory velocities of a suite of rock samples, offshore sampling, links to onshore geology with multibeam bathymetry, and gravity modelling. On both lines a low velocity zone (LVZ) >350 km wide and up to 15 km thick is present at mid-crustal levels. The LVZ is not observed beneath onshore Nova Scotia or northeast of Orpheus Graben, the offshore extension of the CCFZ. The crustal thickness of the Meguma terrane varies between 31 and 38 km; in contrast, the Moho is at a depth of 41 km beneath Avalonia. The velocity of the lower crust on line 99-2 is 6.8-7.1 km/s beneath the LVZ and Avalonia, and 6.6 km/s beneath onshore Meguma terrane. Poisson's ratios calculated from P- and S-wave velocities are 0.19-0.23 in the Meguma terrane crust; in contrast, Poisson's ratios of 0.24-0.25 are indicated for Avalonian crust.

Abundant granitic plutons intruded the eastern Meguma terrane of Nova Scotia in the middle- to late Devonian. Less voluminous diorite-tonalite and gabbro intrusions are associated with the granitic plutons along the northern margin of the terrane adjacent to the Cobequid-Chedabucto fault zone. All plutons contain metasedimentary xenoliths, and the mafic plutons show magma mingling textures with their adjacent granitic plutons. New U-Pb zircon data from autocrystic zircon in 13 samples indicate coeval emplacement of mafic and granitic plutons between ca. 382 and 368 Ma. However, the zircon grains contain numerous inherited domains that range in age from Palaeoproterozoic to Devonian. These inherited ages correspond to detrital zircon U-Pb dates from the Cambrian to Ordovician metasedimentary host rocks. Zircon oxygen isotopic data (delta 18O) are between +7.4 +/- 0.2%o and +9.3 +/- 0.3%o indicating significant involvement of the crust as the magma source or contaminant. If the high delta 18Ozrn values are a result of contamination, the contaminant was likely the metasedimentary rocks of the Meguma terrane. Hafnium isotopic data from autocrystic zircon have epsilon Hf(t) between -6.0 +/- 1.5 and +2.1 +/- 2.5. The new zircon U-Pb, O, and Hf isotopic data from plutons in the eastern Meguma terrane are indistinguishable from published data from the South Mountain Batholith. The data suggest that Devonian magmatism in the Meguma terrane post-dated the main orogenic event that caused folding and regional metamorphism and involved the same magma source and/or contaminants throughout the terrane.

Structural complexity of the Cape Breton Highlands is a key problem in reconstructing tectonic events in the northern Appalachian orogen. A new U-Pb thermal ionization mass spectrometry age of 428.53 +/- 0.16 Ma for metarhyolite in the Calumruadh Brook Formation shows that volcanic and sedimentary rocks were deposited before collision of the Aspy and Bras d'Or terranes along the Eastern Highlands shear zone. A new U-Pb laser ablation zircon age of 394 +6/-4 Ma confirms that peak metamorphism in the Middle River complex continued during convergence linked to late stages of the Acadian orogeny. The compressive tectonic environment evolved into a transpressional system after initial collision in the late Silurian and caused a repeated pattern of imbrication of units in the Aspy terrane in the hanging wall in the collision. The shear zones bounding the geological units are curvilinear and have south-directed kinematics, imbricating units and transporting higher grade rocks over lower grade rocks, and moving plutons upward relative to their host rocks during and shortly after intrusion. The vergence of imbrication is parallel to the direction of transpressional movement on the main Eastern Highlands shear zone. This geometry is present in Ordovician-Silurian rocks and repeated in Devonian plutonic rocks, indicating that the overall transpressional tectonic setting was a long-lived feature of the orogen. The shear zones localized late syn- to postdeformational plutons that intruded at ca. 375-370 Ma. By the latest Devonian, emplacement of the ca. 363 Ma Margaree and related plutons marked the beginning of extension in the central Cape Breton Highlands.

The upper Ediacaran to lower Cambrian Castalia Group as originally defined comprises a basal sequence of clastic marine sedimentary rocks assigned to the Great Duck Island and Flagg Cove formations and an upper sequence of mainly mafic volcanic and volcaniclastic rocks of the Ross Island, North Head, Priest Cove, and Long Pond Bay formations. A few previously reported specimens of the long-ranging trace fossil Planolites in the Flagg Cove Formation were not inconsistent with the U–Pb age of 539.0 ± 3.3 Ma age for the Priest Cove Formation or the interpreted intrusive relationship between the Flagg Cove Formation and 535 ± 2 Ma Stanley Brook Granite.During a recent visit, abundant morphologically simple trace fossils, including Planolites, were recognized in strata south of Stanley Beach in Flagg Cove, together with vertically or obliquely oriented trace fossils more than 10 mm in diameter, and probable Teichichnus. The age of this association of trace fossils is post earliest Fortunian. More significantly, grey silty shale interbedded with the sandstone that contains the traces yielded organic-walled microfossils. The microfossils include the acritarch Micrhystridium spp of a type also found in the King Square Formation in the Saint John area. The microfossils suggest a Miaolingian (middle Cambrian) age for the Flagg Cove Formation, requiring that its relationship with the Stanley Brook Granite and Castalia Group needs to be re-examined. it also raises the possibility of correlation with middle Cambrian clastic sedimentary sequences exposed on mainland southern New Brunswick and elsewhere in the region.

Abstract Avalonian sections in the Saint John area, southern New Brunswick, have long contributed to global understanding of Cambrian chronostratigraphy. A tuffaceous bed in the Ratcliffe Brook Formation (RBF) in the Somerset Street section dated at c. 531 Ma has traditionally been considered to post-date small shelly fossils attributed to the Watsonella crosbyi Zone in the Hanford Brook section. A fine-grained tuffaceous bed approximately 8 m stratigraphically lower in the Somerset Street section yields a chemical abrasion isotope dilution–thermal ionization mass spectrometry zircon age of 532.3 ± 0.3 Ma; a tuffaceous carbonate unit in the lower RBF in Hanford Brook gives an age of 531.5 ± 0.3 Ma. Crystal and crystal-lithic tuff beds near the top of the RBF yield ages of 520.3 ± 0.3 Ma (in Hanford Brook) and 519.1 ± 0.3 Ma (in Ratcliffe Brook). The new ages confirm the correlation between the Somerset Street and Hanford Brook sections based on acritarchs and make the association of small shelly fossils in the Hanford Brook section younger than 531 Ma. This result is relevant to ongoing discussions on the age of the base of undefined Cambrian Stage 2. The radiometric ages also support a young age for the upper part of the RBF, perhaps extending into Epoch 2.

The Park Spur pluton was emplaced in the Aspy terrane of the central Cape Breton Highlands at 374.2 ± 2.9 Ma (U–Pb zircon). It consists mainly of muscovite-biotite monzogranite with small areas of garnet-bearing muscovite monzogranite and abundant pegmatite and aplite dykes. On its northern margin the pluton intruded metamorphic rocks of the Ordovician–Silurian Cape North Group and associated orthogneiss and on the south it intruded low-grade metamorphic rocks of the Silurian Calumruadh Brook Formation. Deformation along the southern margin of the Park Spur pluton is consistent with emplacement during dextral transpression between the Bras d’Or and Aspy terranes. The Late Devonian age combined with S-type petrological characteristics show that the Park Spur pluton and nearby Canal pluton are related to the ca. 375 Ma Black Brook Granitic Suite, all interpreted to have been emplaced during later stages of the Acadian orogeny in Ganderia as a result of delamination.