Chris E. White

Provenance of the Meguma terrane, Nova Scotia: rifted margin of early Paleozoic Gondwana

Detrital zircon ages from the lower part of the Late Proterozoic(?) to Middle Cambrian Goldenville Group in the Meguma terrane of Nova Scotia suggest derivation from local sources in the Avalonian and Pan-African orogens on the margins of Early Cambrian Gondwana. Samples from near the top of the group show a broader distribution, including ages back to Archean. The eNd data show a corresponding trend, from slightly positive in the lower Goldenville Group to highly negative in the upper Goldenville Group and overlying Upper Cambrian to Lower Ordovician Halifax Group. The trends are consistent with deposition of the lower part of the Meguma succession in a rift, in which uplifted rift-flanks were the main source of the early basin fill, whereas subsequent thermal subsidence of rift margins allowed for more widespread sediment sourcing in younger units. The rift was possibly located between Gondwana and Avalonia, and may have been the locus for separation of Avalonia from Gondwana to form part of the Rheic O...

The Kingston terrane, southern New Brunswick, Canada: Evidence for an Early Silurian volcanic arc

The fault-bounded Kingston terrane of southern New Brunswick consists of felsic and less abundant mafic volcaniclastic rocks and flows and minor interbedded sedimentary rocks of the Kingston Group intruded by mainly granitic plutons and abundant younger mafic sheets. The granitic plutons form about half of the terrane and are characterized by fine grain size and typically granophyric and porphyritic textures, indicative of high-level emplacement. Mafic sheets were emplaced in the granitic plutons and parallel to primary layering in the Kingston Group, which is typically steeply dipping and trends northeast to north-northeast. Hence, in contrast to some earlier interpretations, the Kingston terrane does not consist of a bimodal dike complex. The Kingston Group, granitic rocks, and mafic sheets contain mineral assemblages consistent with metamorphism to upper greenschist–lower amphibolite facies. New U-Pb (zircon) ages of 442 ± 6 Ma from a dacitic tuff unit and 437 ± 10 Ma from a granitic pluton corroborate previous U-Pb dates that had suggested that the volcanic and granitic rocks in the Kingston terrane are Early Silurian, rather than Precambrian, as had been earlier assumed. The felsic volcanic and granitic rocks are interpreted to be comagmatic on the basis of their field relationships and similar U-Pb ages and chemical character. Compositions of both the felsic rocks and less abundant mafic and intermediate and mafic rocks are consistent with calc-alkalic affinity and emplacement in a continental-margin volcanic arc, herein termed the Kingston arc. A U-Pb date of 435 ± 5 Ma was also obtained from compositionally similar granite that forms a dike in an adjacent fault-bounded amphibolite-facies metasedimentary unit. The similarity in age and composition of the granite dike suggests a link between these metasedimentary rocks and the Kingston terrane, possibly as part of an accretionary complex. If so, its location southeast of the Kingston terrane indicates that subduction was to the present northwest. Mafic sheets in the Kingston terrane are tholeiitic but have some arc-like characteristics, consistent with their origin in a former arc setting, but the sheets were likely emplaced in a subsequent extensional setting. The compositional similarity of the mafic sheets to Late Silurian mafic volcanic rocks and plutons of the Coastal Maine magmatic province suggests that they may be related. A possible model for evolution of the Kingston terrane is proposed, wherein the Kingston Group and related granite formed in an Early Silurian arc on the margin of Ganderia, as represented by the New River terrane of late Precambrian and early Paleozoic age. Northwestward subduction culminated in collision between the Kingston arc and a continental block now represented by the Brookville terrane to the southeast. Subsequent transpressive mo tions in the Late Silurian may have been responsible for the crustal- scale extensional environment in which the Coastal Maine magmatic province was formed, including emplacement of mafic dikes into the deformed former Kingston arc. Transpressive motions—related to juxtaposition of the Avalon and Meguma terranes with the previously accreted, more inboard terranes—continued through the Devonian and into the Carboniferous.

Origin and evolution of Avalonia: evidence from U–Pb and Lu–Hf isotopes in zircon from the Mira terrane, Canada, and the Stavelot–Venn Massif, Belgium

Laser ablation inductively coupled plasma mass spectrometry U–Pb and Lu–Hf isotope analyses of detrital zircon from Neoproterozoic–Cambrian clastic sedimentary rocks in the Mira terrane (Cape Breton Island, Nova Scotia, Canada; West Avalonia) and the Stavelot–Venn Massif (East Belgium; East Avalonia) support deposition on an originally coherent microcontinent. Crustal evolution trends defined by ϵHf(t) values varying with age reflect juvenile magma production in the source continent at 1.2–2.2 and 2.4–3.1 Ga. Mixing of juvenile and recycled crust in continental magmatic arcs is recognized at 0.5–0.72, 1.4–1.7, 1.8–2.2 and 2.4–2.7 Ga. These results concur with the crustal evolution in Amazonia, the likely parent craton. Crustal evolution in Avalonia is recorded in detrital and magmatic zircon from Neoproterozoic arcs (680–550 Ma). Positive ϵHf(t) values suggest juvenile input and mixing with recycled crust. Most negative ϵHf(t) values represent recycling of predominantly Mesoproterozoic underlying crust. Avalonian arc magmatism was followed by late Neoproterozoic–early Cambrian sedimentation in various belts in West Avalonia. These belts were juxtaposed by strike-slip during late early Cambrian deposition in a rift basin. The youngest detrital zircon population (c. 517 Ma) probably represents synrift magmatism before break-up of Avalonia. Migmatization at 406 ± 2 Ma in a xenolith from the East Avalonian crust reflects post-collisional heating. Supplementary materials: Details of sample locations and analytical results are available at www.geolsoc.org.uk/SUP18641.

Late Paleozoic strike-slip faults in Maritime Canada and their role in the reconfiguration of the northern Appalachian orogen

Major late Paleozoic faults, many with documented strike-slip motion, have dissected the Ordovician-Devonian Appalachian orogen in the Maritime Provinces of Atlantic Canada. Activity alternated between east-west faults (Minas trend) and NE-SW faults (Appalachian trend). NW-SE faults (Canso trend) were probably conjugate to Minas-trend faults. Major dextral movement, on faults with Appalachian trend, in total between 200 and 300 km, began in the Late Devonian. This movement initiated the Maritimes Basin in a transtensional environment at a releasing bend formed around a promontory in the Laurentian margin and thinned the crust, accounting for the major subsidence of the basin. Appalachian-trend strike slip continued in the Mississippian but was accompanied by major movement on E-W Minas-trend faults culminating around the Mississippian-Pennsylvanian boundary, juxtaposing the Meguma and Avalon terranes of the Appalachians close to their present-day configuration. However, strike slip continued during the Pennsylvanian-Permian interval resulting in transpressional deformation that reactivated and inverted earlier extensional faults. A final major episode of transtension, mainly sinistral, occurred during the Mesozoic opening of the Atlantic Ocean. Restoration of movements on these faults, amounting to several hundred kilometers of slip, explains anomalies in the present-day distribution of terranes amalgamated during early Paleozoic Appalachian tectonism. In the restored geometry, the Nashoba and Ellsworth terranes of Ganderia are adjacent to one another, and the Meguma terrane lies clearly outboard of Avalonia. A restored post-Acadian paleogeography, not the present-day geometry of the orogen, should be used as a basis for reconstructions of its earlier Paleozoic history.

Magmatic History of the Avalon Terrane of Southern New Brunswick, Canada, Based on U-Pb (Zircon) Geochronology

U-Pb (zircon) dates from volcanic and plutonic units confirm that igneous activity throughout the Caledonian Highlands of southern New Brunswick occurred in two main episodes: ca. 620 Ma and ca. 560-550 Ma. The ca. 620 Ma episode is represented by mainly metatuffaceous rocks of the Broad River Group and associated dioritic to granitic plutons that form most of the eastern highlands. Igneous rocks of similar age and type have been documented in Avalonian areas throughout the northern Appalachian Orogen and are interpreted generally to have formed during subduction at a continental margin. In contrast, the ca. 560-550 Ma igneous episode may be unique to the Caledonian Highlands. It is represented by tuffs and bimodal flows of the Coldbrook Group and co-genetic gabbroic and granitic plutons that occur mainly in the western highlands. This voluminous magmatism may have taken place in a volcanic arc and intra-arc extensional setting, perhaps like that represented by older but lithologically similar rocks of the ca. 575 Ma Fourchu and Main-à-Dieu groups in southeastern Cape Breton Island. It contrasts in petrological character with igneous activity of similar age in eastern Newfoundland, which produced alkaline to peralkaline rocks. Younger volcanic rocks of minor extent also occur in the Caledonian Highlands: (1) rhyodacitic flows with a maximum age of

The evolutionary significance of a Lower Cambrian trace-fossil assemblage from the Meguma terrane, Nova Scotia

479 \pm 8 Ma

Provenance of detrital muscovite in Cambrian Avalonia of Maritime Canada: 40Ar/39Ar ages and chemical compositions

forming a small dome in the Coldbrook Group in the west-central highlands, and (2) small areas in the southwestern highlands of flow-banded rhyolite with a maximum age of ca. 367 Ma. Although Paleozoic igneous rocks have been reported in other parts of the Avalon terrane, this is the first confirmation of their presence in the Caledonian Highlands.

Effects of fluid flow, cooling and deformation as recorded by 40Ar/39Ar, Rb–Sr and zircon fission track ages in very low- to low-grade metamorphic rocks in Avalonian SE Cape Breton Island (Nova Scotia, Canada)

Organic-walled microfossils (acritarchs) provide age constraints for the previously poorly dated Cambrian and Lower Ordovician turbiditic Goldenville and Halifax Groups in the northern Appalachian Meguma terrane of Nova Scotia and address controversies about stratigraphy and provenance. The oldest exposed formation of the ∼8-km-thick Goldenville Group contains the trace fossil Oldhamia , which suggests an age of late Early Cambrian, whereas the coticule-bearing uppermost formation yielded an acritarch species consistent with Middle Cambrian (Epoch 3) age. The conformably overlying ∼5-km-thick Halifax Group includes basal pyritiferous units that yielded a Late Cambrian (Furongian) assemblage of acritarch species, providing further confirmation that the underlying manganese-bearing formations are of Cambrian Epoch 3 age, and not Ordovician as recently claimed. Overlying nonpyritiferous formations contain the Early Ordovician graptolite Rhabdinopora flabelliformis flabelliformis and acritarchs of similar age. Samples collected up section from the graptolite occurrence yielded acritarch species that are indicative of the later Tremadocian and Floian. The new fossil data confirm a gap in age of ∼30 m.y. between the Halifax Group and the overlying Silurian to Lower Devonian Rockville Notch Group. The new ages are consistent with stratigraphic units defined in regional mapping and support recent interpretation of fundamental differences in depositional and tectonic environments between the northwestern and southeastern parts of the Meguma terrane. They also provide new constraints on paleogeography of the terrane by confirming age similarities to stratigraphy in north Wales, with which correlation has been proposed previously based mainly on lithological similarities.