West Avalonia Research Articles

Stratal evolution and provenance of a precambrian delta in a tectonically active terrane: The lower Signal Hill Group, Avalon Zone, Newfoundland

Sedimentologic and provenance studies of the syn-orogenic Ediacaran lower Signal Hill Group (Gibbett Hill and Quidi Vidi formations) in the Avalon Peninsula, Newfoundland, Canada, were conducted to test structural and depositional models related to the enigmatic late Ediacaran (ca. 575–550 Ma) Avalon orogenesis. Two facies associations corresponding to delta front and delta plain environments are identified from five stratigraphic sections. The presence of an unconformity between these facies near the location of a previously proposed thrust-related paleo-high and changes in sedimentary patterns reflecting shoreline regression support the inference of syn-sedimentary uplift and reveals coeval base level fall. Framework petrography, detrital geochronology, and detrital heavy mineral analysis indicate sediment sourcing from hinterlands of West Avalonian igneous and metamorphic basement. Changes in provenance between the delta front and delta plain facies coeval with regression of the Signal Hill delta suggest the thrusting of volcanic cover sequences over already exhumed plutonic sources and the exhumation of deeper/higher-grade metamorphic rocks in the hinterland. Detrital zircon U-Pb maximum depositional ages of the delta front and delta plain facies yield 557 ± 9 Ma and 556 ± 22 Ma, respectively, constraining this episode of West Avalonian hinterland exhumation, wedge-top foreland basin deformation, and forced regression to ca. 556 Ma. The regional significance of this late Ediacaran (ca. 565–538 Ma) foreland basin to tectonic models of West Avalonia’s transition from an arc setting to a Paleozoic rifted platform is not currently understood, but it may represent a retro-arc basin formed either as part of a coherent West Avalonian block overlying a subducted spreading ridge or on a subsidiary West Avalonian terrane prior to and during terrane collision preceding deposition of the Paleozoic overstep sequence.

U–Pb zircon–rutile dating of the Llangynog Inlier, Wales: constraints on an Ediacaran shallow-marine fossil assemblage from East Avalonia

The Llangynog Inlier of south Wales contains an assemblage of Ediacaran macrofossils from a shallow-marine environment, including discoidal morphs of Aspidella and rare examples of Hiemalora , Palaeopascichnus and Yelovichnus . These are taxa found at other sites in the Avalonian microcontinent (e.g. Charnwood Forest and eastern Newfoundland) and in the younger White Sea Ediacaran assemblages. As the Charnwood fossils reflect a deep-water environment, and no macrofossils have been found in the Ediacaran rocks of the Long Mynd, the fossils of the Llangynog Inlier represent a unique glimpse of shallow-marine life in southern Britain (East Avalonia). However, the lack of absolute age constraints has hampered direct comparison with other assemblages. Here, we report in situ zircon and rutile U–Pb dates from a rhyolitic ash-flow layer of the Coed Cochion Volcaniclastic Member, Llangynog Inlier, which constrains the age of the fossiliferous strata. A weighted mean single grain zircon isotope dilution thermal ionization mass spectrometry U–Pb age of 564.09 ± 0.70 Ma is interpreted as the rhyolite's crystallization age. This age is consistent with in situ laser ablation inductively coupled plasma mass spectrometry zircon and rutile U–Pb dating. The Llangynog age temporally correlates these fossils to dated horizons within East Avalonia at the Beacon Hill Formation, Charnwood (565.22 ± 0.89 Ma) and the Stretton Shale Formation, Long Mynd (566.6 ± 2.9 Ma). Correlations to West Avalonia include the time-equivalent Fermeuse Formation, St John's Group, eastern Newfoundland (564.13 ± 0.65 Ma). The data presented here establish the biota of the Llangynog Inlier as a lateral equivalent to the similarly shallow-marine, tidally influenced ecosystem of the upper Fermeuse Formation. Intra-terrane depositional environmental variability also affects what is preserved in Avalonian fossil sites. Further, time-constrained geochemical data reinforce the Llangynog Inlier's classification within the Wrekin Terrane. Supplementary material : U–Pb data, reference material and figures showing sedimentary structures are available at https://doi.org/10.6084/m9.figshare.c.6965642

New evidence for the Baltican cratonic affinity and Tonian to Ediacaran tectonic evolution of West Avalonia in the Avalon Peninsula, Newfoundland, Canada

Provenance studies of quartzite clasts and enclosing sandstone units in the Avalon Peninsula, Newfoundland, Canada, were conducted to test the Baltican or Gondwanan cratonic affinities of West Avalonia and examine the stratigraphic archives of Neoproterozoic arc evolution. Four cobble-sized quartzite clasts from a syn-orogenic, Ediacaran fluvial unit mostly yield ca. 1220, 1510, and 1900–2200 Ma detrital zircon age maxima and were probably derived from sub-arc basement units. Detrital zircon U-Pb-Hf isotope statistical assessments of the clasts indicate provenance ties with Proterozoic strata of the Sarmatian craton and Timanian passive margin of northern Fennoscandia, which corroborates models for proto-Avalonia to have originated near southern or northeastern Baltica, respectively. Analogous quartzite basement units in Nova Scotia and New England also have Baltican cratonic affinities and reflect provenance from southern Fennoscandia. Ediacaran exhumation of arc and basement infrastructure was accommodated by transpressive faults during the Avalonian orogeny and resulted in the deposition of Signal Hill Group terrestrial strata. Eleven Signal Hill Group sandstone samples are correspondingly dominated by Tonian to Ediacaran detrital zircon age fractions that constrain the crustal evolution of West Avalonia. The composite dataset includes 818–777 Ma detrital zircon grains that record the rifting of a proto-Avalonia sliver or ribbon arc from Baltica. Tonian igneous rocks are rare in West Avalonia, but ca. 760 Ma and 730 Ma detrital zircon grains indicate both crust and mantle contributions to the oceanic Burin arc, its tectonic interactions with proto-Avalonia, and subsequent arc magmatism across the composite terrane. Cryogenian detrital zircon ages substantiate the timing of 700–670 Ma magmatism and 670–640 Ma tectonic interactions with Baltica or another block. The detrital zircon U-Pb-Hf isotope record of the 640–600 Ma magmatic phase includes 6–9 Myr-long, fluctuating cycles that indicate the frequency of West Avalonian arc processes in the Mirovoi Ocean prior to Avalonian orogenesis.

Evolution of Subduction Dynamics beneath West Avalonia in Middle to Late Ordovician Times

Abstract Middle to Upper Ordovician volcanic rocks in the Arisaig area of Nova Scotia, Canada, constitute the only known record of volcanism in West Avalonia during that interval. Hence, they have been extensively studied to test paleocontinental reconstructions that consistently show Avalonia as a drifting microcontinent during that period. Identification of volcanic rocks with an intermediate composition (the new Seaspray Cove Formation) between upper Darriwilian bimodal volcanic rocks of the Dunn Point Formation and Sandbian felsic pyroclastic rocks of the McGillivray Brook Formation has led to a reevaluation of magmatic relationships in the Ordovician volcanic suite at Arisaig. Although part of the same volcanic construction, the three formations are separated by significant time-gaps and are shown to belong to three distinct magmatic subsystems. The tectonostratigraphic context and trace element contents of the Dunn Point Formation basalts suggest that they were produced by the high-degree partial melting of an E-MORB type source in a back-arc extensional setting, whereas trace element contents in intermediate rocks of the Seaspray Cove Formation suggest that they were produced by the low-degree partial melting of a subduction-enriched source in an arc setting. The two formations are separated by a long interval of volcanic quiescence and deep weathering, during which time the back-arc region evolved from extension to shortening and was eventually onlapped by arc volcanic rocks. Based on limited field constraints, paleomagnetic and paleontological data, this progradation of arc onto back-arc volcanic rocks occurred from the north, where an increasingly young Iapetan oceanic plate was being subducted at an increasingly shallow angle. Partial subduction of the Iapetan oceanic ridge is thought to have subsequently generated slab window magmatism, thus marking the last pulse of subduction-related volcanism in both East and West Avalonia.

Tonian–Ediacaran tectonomagmatic evolution of West Avalonia and its Ediacaran–early Cambrian interactions with Ganderia: an example of complex terrane transfer due to arc–arc collision?

Abstract The Neoproterozoic tectonomagmatic evolution of West Avalonia comprises four major events. Tectonism started with the formation of a Tonian passive margin on a Baltica-derived ribbon dispersed into the Mirovoi Ocean. Obduction of an oceanic terrane onto the ribbon produced olistostromes, deformation and metamorphism before 750 Ma. Obduction was followed by a Tonian (750–730 Ma) arc on the created composite crust. A pause in magmatism between 730 and 700 Ma is the next event. Subsequently, a Cyrogenian (700–670 Ma) arc was formed, which may have collided with Baltica or another buoyant element nearby. Thereafter, a long-lasting (640–565 Ma) continental arc was erected which, combined with the late Ediacaran–Early Paleozoic sedimentary cover, represents the hallmark of West Avalonia. A Caribbean-style incursion of the Ediacaran arc into the widening Tornquist gap between Amazonia and Baltica led to a diachronous collision with the Ganderian arc. Strike-slip slivering produced a complex transfer of terranes to both: Carolinia and smaller terranes to Ganderia, and East Avalonia to West Avalonia. The Rheic Ocean opened diachronously at c. 500 Ma, following a plate reorganization and re-establishment of an oblique subduction zone beneath Amazonia. As a result, Avalonia and Ganderia became progressively separated and dispersed into the Iapetus Ocean.

Cadomian Magmatic Rocks from Zarand (SE Iran) Formed in a Retro-Arc Basin

A major arc-related magmatic episode is recorded in the Ediacaran-Early Cambrian (Cadomian) crust of Iran and Anatolia due to the southward subduction of Prototethyan oceanic lithosphere beneath N Gondwana. This magmatism was generated at a convergent margin, fragments of which can be traced for thousands of kilometres, from west Avalonia to the Lhasa terrane. Cadomian crustal tracts in Iran and Tauride-Anatolia are represented by abundant arc-like plutonic and volcanic rocks and a series of retro-arc, rifted basins dominated by thick sequences of terrigenous rocks as well as both calc-alkaline and alkaline magmatic rocks. This paper presents new zircon U-Pb as well as geochemical-isotopic data from plutonic (granite to gabbro) and volcanic (basalt to rhyolite) magmatic rocks of a Cadomian retro-arc rifted basin from Zarand in SE Iran. Geochemical data indicate two different geochemical signatures; high K calc-alkaline-shoshonitic, characterized by strong depletions in Nb, Ta, P, Ti, and alkaline rocks (and A1-type intrusions), with strong OIB-like trace element patterns. Cadomian OIB-like rocks- as well as A1-type intrusions- are rare in the basements of Iran and Anatolia and are only documented as minor gabbroic intrusions and/or (meta-) volcanic rocks from Saghand (central Iran), NE Iran and from exotic blocks from Ediacaran salt domes. Moreover, A2-type granites are also present in the Cadomian crust of Iran. Zarand volcanic rocks are interlayered with Rizu-Dezu terrigenous sedimentary rocks, whereas plutonic rocks intruded these metasediments. New zircon U-Pb ages show that high K calc-alkaline-shoshonitic rocks formed ~ 537-536 Ma, whereas A1-type granites crystallized at ~535 Ma. Most zircons from A1-type granites show positive εHf(t) values from +1.1 to +5.1, mostly higher than high K calc-alkaline-shoshonitic rocks with εHf (t) between −6.6 and +8.1. Bulk rock Nd-Sr isotopic data (e.g., ɛNd(t)= +0.3 to +4.0) for OIB-like rocks confirm that these rocks originated from an enriched OIB-like mantle source, whereas high K calc-alkaline-shoshonitic rocks (with εNd(t) = −7.7 to −6.2) show strong interaction with, and/or re-melting of a continental crust. Our results are consistent with an interpretation that the Zarand volcanic-plutonic rocks as well as associated thick sequences of sedimentary strata rocks formed in a retro-arc rifted basin behind the Cadomian magmatic arc. This basin seems to have developed during the Ediacaran at ~570-560 Ma with deposition of Ediacaran Morad to Lower Cambrian Rizu-Dezo sedimentary rocks. The retro-arc extensional basin seems to have become magmatically active at 540-535 Ma, as shown by the occurrence of both high K calc-alkaline- shoshonitic and OIB-like magmatic rocks. This basin was flanked by a cratonic hinterland towards Gondwana and a magmatic arc to the N. The cratonic hinterland fed detritus with detrital zircons older than 0.6 Ga into the basin, whereas the magmatic arc shed detritus with 500-600 Ma old zircons. Our compiled and new data from Ediacaran-Early Paleozoic magmatic rocks also show that Cadomian high magmatic fluxes occurred differently in parts of Iran-Anatolia, with overlaps in some areas.

Detrital zircon characterization of early Cambrian sandstones from East Avalonia and SE Ireland: implications for terrane affinities in the peri-Gondwanan Caledonides

AbstractThe Caledonides of Britain and Ireland include terranes attributed to both Laurentian and Gondwanan sources, separated along the Solway line. Gondwanan elements to the south have been variably assigned to the domains Ganderia and East Avalonia. The Midland Platform forms the core of East Avalonia but its provenance is poorly known. Laser ablation split-stream analysis yields information about detrital zircon provenance by providing simultaneous U–Pb and Lu–Hf data from the same ablated volume. A sample of Red Callavia Sandstone from uppermost Cambrian Stage 3 of the Midland Platform yields a U–Pb age spectrum dominated by Neoproterozoic and Palaeoproterozoic sources, resembling those in the Welsh Basin, the Meguma Terrane of Nova Scotia and NW Africa. Initial εHf values suggest that the Neoproterozoic zircon component was derived mainly from crustal sources < 2 Ga, and imply that the more evolved Palaeoproterozoic grains were transported into the basin from an older source terrane, probably the Eburnean Orogen of West Africa. A sample from Cambrian Stage 4 in the Bray Group of the Leinster–Lakesman Terrane shows, in contrast, a distribution of both U–Pb ages and εHf values closely similar to those of the Gander Terrane in Newfoundland and other terranes attributed to Ganderia, interpreted to be derived from the margin of Amazonia. East Avalonia is clearly distinct from Ganderia, but shows evidence for older crustal components not present in West Avalonia of Newfoundland. These three components of the Appalachian–Caledonide Orogen came from distinct sources on the margin of Cambrian Gondwana.

Mantle source heterogeneity of the Early Jurassic basalt of eastern North America

One of the defining characteristics of the basaltic rocks from the Early Jurassic Eastern North America (ENA) sub-province of the Central Atlantic Magmatic Province (CAMP) is the systematic compositional variation from South to North. Moreover, the tectono-thermal regime of the CAMP is debated as it demonstrates geological and structural characteristics (size, radial dyke pattern) that are commonly associated with mantle plume-derived mafic continental large igneous provinces but is considered to be unrelated to a plume. Mantle potential temperature (T P) estimates of the northern-most CAMP flood basalts (North Mountain basalt, Fundy Basin) indicate that they were likely produced under a thermal regime (T P ≈ 1450 °C) that is closer to ambient mantle (T P ≈ 1400 °C) conditions and are indistinguishable from other regions of the ENA sub-province (T Psouth = 1320–1490 °C, T Pnorth = 1390–1480 °C). The regional mantle potential temperatures are consistent along the 3000-km-long ENA sub-province suggesting that the CAMP was unlikely to be generated by a mantle plume. Furthermore, the mantle potential temperature calculation using the rocks from the Northern Appalachians favors an Fe-rich mantle (FeOt = 8.6 wt %) source, whereas the rocks from the South Appalachians favor a less Fe-rich (FeOt = 8.3 wt %) source. The results indicate that the spatial-compositional variation of the ENA basaltic rocks is likely related to differing amounts of melting of mantle sources that reflect the uniqueness of their regional accreted terranes (Carolinia and West Avalonia) and their post-accretion, pre-rift structural histories.

A blind trilobite with Baltic affinities from Cambrian Series 3 of the Iberian Chains, Spain, and its stratigraphical and palaeobiogeographical significance

Three cranidia of a conocoryphid trilobite are described from the lower Caesaraugustan Regional Stage (upper part of provisional Cambrian Stage 5 within Cambrian Series 3) in the Iberian Chains, Spain. The material is from the upper part of the Mansilla Formation and the base of the overlying Murero Formation. It closely resembles Bailiella tenuicincta (Linnarsson, 1879), which has previously been described from the Ptychagnostus gibbus (Cambrian Stage 5) and lower P. atavus zones (lower Drumian Stage) of Baltica (Sweden and Bornholm), and from the P. atavus Zone of West Avalonia (Newfoundland). The record of B. cf. tenuicincta from the Mediterranean margin of West Gondwana provides additional evidence for considering the Acado-Baltic region a discrete biogeographical province, at least during the Cambrian 5 and Drumian ages.

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 .

A peri-Gondwanan arc in NW Iberia: I: Isotopic and geochemical constraints on the origin of the arc—A sedimentary approach

The arc-derived upper terrane in the NW Iberia Variscan belt contains a 3000 m thick turbiditic formation at its structural top. Most of the sandstones are feldspathic greywackes with a framework of quartz and weakly altered plagioclase. Rock fragments of vitric and microgranular texture are common in polymictic conglomerates and coarse-grained greywackes, together with slates, cherts and bipyramidal volcanic quartz fragments. Although recrystallization under greenschists facies conditions (chlorite and biotite zones) and the presence of two cleavages hinder detailed textural analysis, the sandstones appear to be typically immature, first-cycle sandstones. The metagreywackes have average major and trace element compositions similar to PAAS (Post Archean Australian Shale), which is considered to reflect the composition of the upper continental crust. Their trace element composition is very consistent and records deposition within a convergent tectonic setting, probably in an intra-arc basin located in a volcanic arc built on thinned continental margin. Detrital zircon populations suggest a Middle Cambrian maximum depositional age (530–500 Ma) and a Gondwanan provenance located at the periphery of the West African Craton. Nd isotope data suggest mixing Ediacaran and Paleoproterozoic sources for the provenance of the greywackes, with T DM ranging between 720 and 1215 Ma with an average of 995 Ma ( n = 20)—an age range unrepresented in the detrital zircon population. The Nd model ages are similar to those exhibited by West Avalonia, Florida or the Carolina terrane, but younger than those of Cambrian and Ordovician sandstones and shales from the autochthonous realm. These data suggest a westernmost location along the Gondwanan margin for the upper terrane of NW Iberia relative to other terranes located in the footwall of the Variscan suture, consistent with several previously proposed paleogeographic models for the NW Iberia terranes.

Cambro-Ordovician paleogeography of the Southeastern New England Avalon Zone: Implications for Gondwana breakup

Paleomagnetic measurements and U-Pb geochronology of alkalic igneous rocks intruding the Ediacaran Cambridge Argillite and the Lower Cambrian Weymouth Formation north of Boston at Nahant, Massachusetts, yield a well-constrained paleopole. The oldest intrusive phases are a syenodiorite sill and a granite dike with 206 Pb/ 238 U crystallization ages of 490.19 ± 0.90 Ma and 489.03 ± 0.82 Ma, respectively. This sequence along with undated mafic sills penetrating both the argillite and Weymouth strata are cut by 488.53 ± 0.81 Ma gabbro locally associated with 488.48 ± 0.79 Ma syenite forming a steep-sided intrusive plug. Ten paleomagnetic sampling sites within the mafic sills and the gabbro contain a stable A magnetic component with a mean direction of D = 278.5°, I = −76.7° (A 95 = 3.9°, N = 10). This is considered to be the primary magnetization direction because it passes both fold and reversals tests. The corresponding paleopole at 320LE, 34°N (A 95 = 7.2°, N = 10) places the Southeastern New England Avalon Zone at a southerly paleolatitude of 65° slightly removed from the African margin of Gondwana and suggests that bimodal Nahant magmatism is linked with latest Cambrian–Early Ordovician rifting of West Avalonia to form the Rheic Ocean. Stable B and C magnetic components in Nahant syenodiorite and syenite give rise to virtual geomagnetic poles that track Late Silurian through Carboniferous segments of the North American apparent polar wander path, indicating docking of West Avalonia by mid-Paleozoic time.

Neoproterozoic-early Palaeozoic tectonostratigraphy and palaeogeography of the peri-Gondwanan terranes: Amazonian v. West African connections

Abstract Within the Appalachian–Variscan orogen of North America and southern Europe lie a collection of terranes that were distributed along the northern margin of West Gondwana in the late Neoproterozoic and early Palaeozoic. These peri-Gondwanan terranes are characterized by voluminous late Neoproterozoic ( c . 640–570 Ma) arc magmatism and cogenetic basins, and their tectonothermal histories provide fundamental constraints on the palaeogeography of this margin and on palaeocontinental reconstructions for this important period in Earth history. Field and geochemical studies indicate that arc magmatism generally terminated diachronously with the formation of a transform margin, leading by the Early–Middle Cambrian to the development of a shallow-marine platform–passive margin characterized by Gondwanan fauna. However, important differences exist between these terranes that constrain their relative palaeogeography in the late Neoproterozoic and permit changes in the geometry of the margin from the late Neoproterozoic to the Early Cambrian to be reconstructed. On the basis of basement isotopic composition, the terranes can be subdivided into: (1) Avalonian-type (e.g. West Avalonia, East Avalonia, Meguma, Carolinia, Moravia–Silesia), which developed on juvenile, c . 1.3–1.0 Ga crust originating within the Panthalassa-like Mirovoi Ocean surrounding Rodinia, and which were accreted to the northern Gondwanan margin by c . 650 Ma; (2) Cadomian-type (e.g. North Armorican Massif, Ossa–Morena, Saxo-Thuringia, Moldanubia), which formed along the West African margin by recycling ancient ( c . 2.0–2.2 Ga) West African crust; (3) Ganderian-type (e.g. Ganderia, Florida, the Maya terrane and possible the NW Iberian domain and South Armorican Massif), which formed along the Amazonian margin of Gondwana by recycling Avalonian and older Amazonian basement; and (4) cratonic terranes (e.g. Oaxaquia and the Chortis block), which represent displaced Amazonian portions of cratonic Gondwana. These contrasts imply the existence of fundamental sutures between these terranes prior to c . 650 Ma. Derivation of the Cadomian-type terranes from the West African craton is further supported by detrital zircon data from their Neoproterozoic–Ediacaran clastic rocks, which contrast with such data from the Avalonian- and Ganderian-type terranes that suggest derivation from the Amazonian craton. Differences in Neoproterozoic and Ediacaran palaeogeography are also matched in some terranes by contrasts in Cambrian faunal and sedimentary provenance data. Platformal assemblages in certain Avalonian-type terranes (e.g. West Avalonia and East Avalonia) have cool-water, high-latitude fauna and detrital zircon signatures consistent with proximity to the Amazonian craton. Conversely, platformal assemblages in certain Cadomian-type terranes (e.g. North Armorican Massif, Ossa–Morena) show a transition from tropical to temperate waters and detrital zircon signatures that suggest continuing proximity to the West African craton. Other terranes (e.g. NW Iberian domain, Meguma) show Avalonian-type basement and/or detrital zircon signatures in the Neoproterozoic, but develop Cadomian-type signatures in the Cambrian. This change suggests tectonic slivering and lateral transport of terranes along the northern margin of West Gondwana consistent with the transform termination of arc magmatism. In the early Palaeozoic, several peri-Gondwanan terranes (e.g. Avalonia, Carolinia, Ganderia, Meguma) separated from West Gondwana, either separately or together, and had accreted to Laurentia by the Silurian–Devonian. Others (e.g. Cadomian-type terranes, Florida, Maya terrane, Oaxaquia, Chortis block) remained attached to Gondwana and were transferred to Laurussia only with the closure of the Rheic Ocean in the late Palaeozoic.

Neoproterozoic and Cambrian arc magmatism along the eastern margin of the Victoria Lake Supergroup: A remnant of Ganderian basement in central Newfoundland?

The eastern margin of the composite and structurally complex Victoria Lake Supergroup has previously been mapped where the Silurian Rogerson Lake conglomeratic rocks unconformably overlie Cambrian Tally Pond volcanic rocks. However, the recognition that the form of this contact changes between an unconformity and a brittle-ductile fault, in conjunction with the Neoproterozoic age of the Crippleback Intrusive Suite, dictate that the above interpretation of the nature of this margin requires revision. These discrepancies are resolved by the recognition of a Neoproterozoic, largely bimodal volcanic sequence (Sandy Brook Group), which is unconformably overlain by Rogerson Lake rocks to the east, with these combined units in fault contact with the Tally Pond Group to the west. The Sandy Brook Group and Crippleback Intrusive Suite form a terrane of ca. 563 Ma rocks with continental arc-like geochemical signatures. The distinctly negative ɛ Nd,563 values for the felsic rocks (mean −3) of this terrane imply derivation from a crystalline basement older than the largely Pan-African crust that underlies contemporaneous volcanic rocks in West Avalonia, but similar to that inferred for the Gander Zone. Thus this terrane is likely to be the leading edge of the peri-Gondwanan microcontinent Ganderia. The ca. 511 Ma Tally Pond Group arc magmatism represent some of the oldest products of Iapetean subduction in Newfoundland. Although the Tally Pond Group are only observed to be in fault contact with the ca. 50 million year older Sandy Brook Group, the high proportion of felsic volcanic rocks, Sandy Brook/Crippleback age inherited zircons and Pb isotopic data all support the inference that the Tally Pond Group was formed on Sandy Brook/Crippleback continental crust.

Neoproterozoic?Early Paleozoic evolution of peri-Gondwanan terranes: implications for Laurentia-Gondwana connections

Neoproterozoic tectonics is dominated by the amalgamation of the supercontinent Rodinia at ca. 1.0 Ga, its breakup at ca. 0.75 Ga, and the collision between East and West Gondwana between 0.6 and 0.5 Ga. The principal stages in this evolution are recorded by terranes along the northern margin of West Gondwana (Amazonia and West Africa), which continuously faced open oceans during the Neoproterozoic. Two types of these so-called peri-Gondwanan terranes were distributed along this margin in the late Neoproterozoic: (1) Avalonian-type terranes (e.g. West Avalonia, East Avalonia, Carolina, Moravia-Silesia, Oaxaquia, Chortis block that originated from ca. 1.3 to 1.0 Ga juvenile crust within the Panthalassa-type ocean surrounding Rodinia and were accreted to the northern Gondwanan margin by 650 Ma, and (2) Cadomian-type terranes (North Armorica, Saxo-Thuringia, Moldanubia, and fringing terranes South Armorica, Ossa Morena and Tepla-Barrandian) formed along the West African margin by recycling ancient (2–3 Ga) West African crust. Subsequently detached from Gondwana, these terranes are now located within the Appalachian, Caledonide and Variscan orogens of North America and western Europe. Inferred relationships between these peri-Gondwanan terranes and the northern Gondwanan margin can be compared with paleomagnetically constrained movements interpreted for the Amazonian and West African cratons for the interval ca. 800–500 Ma. Since Amazonia is paleomagnetically unconstrained during this interval, in most tectonic syntheses its location is inferred from an interpreted connection with Laurentia. Hence, such an analysis has implications for Laurentia-Gondwana connections and for high latitude versus low latitude models for Laurentia in the interval ca. 615–570 Ma. In the high latitude model, Laurentia-Amazonia would have drifted rapidly south during this interval, and subduction along its leading edge would provide a geodynamic explanation for the voluminous magmatism evident in Neoproterozoic terranes, in a manner analogous to the Mesozoic-Cenozoic westward drift of North America and South America and subduction-related magmatism along the eastern margin of the Pacific ocean. On the other hand, if Laurentia-Amazonia remained at low latitudes during this interval, the most likely explanation for late Neoproterozoic peri-Gondwanan magmatism is the re-establishment of subduction zones following terrane accretion at ca. 650 Ma. Available paleomagnetic data for both West and East Avalonia show systematically lower paleolatitudes than predicted by these analyses, implying that more paleomagnetic data are required to document the movement histories of Laurentia, West Gondwana and the peri-Gondwanan terranes, and test the connections between them.

Sm–Nd isotopic systematics as tectonic tracers: an example from West Avalonia in the Canadian Appalachians

The tectonothermal evolution of West Avalonia in mainland Nova Scotia, Appalachian orogen, provides an excellent example of the application of Sm–Nd isotopic analyses to tectonic studies. Regional syntheses indicate that West Avalonia originated in the Neoproterozoic as one of a number of terranes (collectively known as “peri-Gondwanan terranes”) along the Gondwanan margin. These terranes are characterized by Late Neoproterozoic voluminous arc-related magmatism. West Avalonia became detached from Gondwana in the Early Ordovician, was accreted to Laurentia by the Silurian and was deformed by post-accretionary strike-slip tectonics in the Late Devonian and Carboniferous, associated with the amalgamation of Pangea. All stages of this evolution can be recognized in the Sm–Nd isotopic signature in various Neoproterozoic and Paleozoic volcanic and sedimentary successions in mainland Nova Scotia. Crustally derived felsic volcanic rocks of Late Neoproterozoic (ca. 615 Ma), Early Cambrian and Early Silurian ages define an envelope that provides gross constraints on the Sm–Nd isotopic composition of the underlying basement source. ε Nd values range from +0.78 to 2.92 in the Neoproterozoic rocks, from +2.0 to +5.0 in the Cambrian rocks, and from −0.1 to +2.5 in the Silurian rocks. T DM model ages for each of these suites are remarkably similar (they typically range from 0.9 to 1.2 Ga), suggesting that the Sm–Nd isotopic composition of these suites was profoundly influenced by repeated melting of the same basement source. This Sm–Nd envelope provides a reference by which other suites may be compared and interpreted. For example, turbidites in a Late Neoproterozoic volcanic arc basin have Sm–Nd compositions that lie within this envelope, suggesting that they are derived from the coeval volcanic rocks that flank the basin. The genetic significance of the model ages can be investigated by the analysis of felsic and metasedimentary rocks that pre-date the main arc phase. Two felsic complexes have compositions that lie within the envelope and have similar model ages. The presence of this signature in early Avalonian rocks suggests that the model ages represent a genuine tectonothermal event at ca. 1.0 to 1.2 Ga that formed juvenile “proto-Avalonian” crust. By implication, the main arc phase in Avalonia recycled this crust. Sm–Nd isotopic analyses of the metasedimentary sequences are incompatible with previous interpretations of deposition in a passive margin setting that pre-dated Avalonian arc activity. Instead, the data indicate derivation from two distinct sources (ancient Gondwanan basement and juvenile Avalonian arc), suggesting deposition in a western Pacific-type back arc environment. This re-interpretation has important implications for Neoproterozoic reconstructions. The Sm–Nd isotopic composition of West Avalonian basement contrasts with that of other peri-Gondwanan terranes. For example, similarly aged felsic rocks in the Cadomian terrane of NW France are characterized by more negative ε Nd values and older T DM model ages (1.0–1.9 Ga). These data, together with detrital U–Pb data, suggest that West Avalonia was developed along the Amazonian margin, whereas Cadomia was developed along the West African margin of Gondwana. Sm–Nd isotopic analyses of Early Silurian clastic rocks in mainland Nova Scotia show fundamental differences with unconformably underlying Neoproterozoic and Early Paleozoic Avalonian successions. These data have ε Nd values ranging from −5.7 to −6.1 with T DM model ages of about 1.4 to 1.7 Ga, indicating that the clastic rocks cannot have been derived from underlying successions. A variety of geologic data and regional constraints suggests that they may have been derived from the Caledonide orogen of western Europe, thereby providing a minimum age for the accretion of West Avalonia to Laurentia–Baltica. Sm–Nd isotopic analyses from Late Devonian–Early Carboniferous continental clastic rocks deposited along the southern flank of Avalonia in mainland Nova Scotia also show little influence from Avalonian rocks, and are compatible with derivation from the Meguma terrane immediately to the south. These data are consistent with regional synthesis, which indicate uplift of the Meguma terrane relative to West Avalonia during dextral transpression.

Silurian spores and cryptospores from the arisaig group, Nova Scotia, Canada

The Arisaig Group in northern mainland Nova Scotia has the most complete sequence of Silurian, fine grained, siliciclastic, near‐shore marine sediments in Western Avalonia. In this setting, the Arisaig Group received a nearly continuous supply of autochthonous marine and allochthonous nonmarine palynomorphs. One hundred and seventy‐seven samples, ranging in age from latest Ordovician?‐early Llandovery to Prídolí (Beechhill Cove to Stonehouse formations) yield the most diverse spore assemblage yet known from the Silurian of North America. This paper describes the spores and cryptospores and reexamines age relationships of the Arisaig Group, placing both within context of early land plant evolution. Thirty‐six species of spores and cryptospores were found in the Arisaig Group. Six new species are Ambitisporites marginatus sp. nov., A. capitaneus sp. nov., Scylaspora asperverruca sp. nov., Chelinospora textilis sp. nov., Hispanaediscus scabiosus sp. nov., and Hispanaediscus imbricatus sp. nov. A new genus, Vermiverruspora is created and the original description of Chelinospora Allen 1965 is resurrected. Six species of Chelinospora are reassigned to Vermiverruspora gen. nov. (C. paravermiculata Loboziak et al. 1988, C. vermiculata Chaloner & Streel 1968, C. Ivermiculata McGregor & Camfield 1976, C. retorrida Turnau 1986, C. rumneyi Burgess & Richardson 1995, C. vermoluta Burgess & Richardson 1995) and one species transferred to Stellatispora (pattersonii Wellman 1993). Four additional sporomorphs believed to be new, are not formally named because better preserved specimens are needed. The Arisaig Group (West Avalonia) is similar to sporomorphrich type sections in the Anglo‐Welsh Basin (East Avalonia). Four assemblage zones (avitus‐dilutus, brevicosta‐verrucatus, libycuspoecilomorphus, and tripapillatus‐spicula), and four assemblage sub‐zones (A. brevicosta, E. protophanus, C. obscura, and S. inframurinata var. inframurinata) have been recognized. Sporomorphs indicate that the contacts between the Lower and Upper Members of the McAdam Brook Formation and Green Member of the Moydart Formation may be one to two graptolite biozones older than previously reported. The Arisaig palynomorphs support the theory that a major radiation of vascular land plants occurred during the latter half of the Silurian Period. Llandovery and early Wenlock deposits at Arisaig contain only a few species of smooth walled spores and cryptospores. Sixteen trilete spore and fifteen cryptospore species appear between the Homerian and the late Prídolí. The tempo and composition of this diversification reflect evolutionary events observed elsewhere in the world.

The late Neoproterozoic Enganepe ophiolite, Polar Urals, Russia: An extension of the Cadomian arc?

The Enganepe ophiolite, Polar Urals was formed at ∼670 Ma and records a diverse geochemical association of tholeiite, arc-tholeiite, adakite, and OIB-like lithologies. This constrains the tectonic setting of the protolith of the ophiolite to an oceanic island-arc, with ridge-trench interaction most readily explaining the diverse compositions. The initiation of intra-ocean subduction and the development of the Enganepe island arc off the eastern margin of Baltica probably pre-dated the formation of the Enganepe ophiolite, i.e. prior to 670 Ma. The timing of island-arc magmatism is similar in age to that recorded off Avalon in the Cadomian arc. We propose that the active margin of Baltica in the Vendian is an extension of the Cadomian arc. This requires the northeast margin of Baltica (present-day coordinates) to have been in a southerly position in the Vendian, in agreement with proposed tectonic reconstructions. Consequently, the post-Rodinia continental amalgamation, Pannotia, had active ocean-continent convergence along its entire southerly (west Avalonia and Amazonian cratons) margin at the time of its break-up.

SILURIAN SPORES AND CRYPTOSPORES FROM THE ARISAIG GROUP, NOVA SCOTIA, CANADA

Research Article| January 01, 2001 SILURIAN SPORES AND CRYPTOSPORES FROM THE ARISAIG GROUP, NOVA SCOTIA, CANADA JOHN H. BECK; JOHN H. BECK Weston Observatory Paleobotany Laboratory, 381 Concord Road, Weston, MA 02193, U.S.A. Search for other works by this author on: GSW Google Scholar PAUL K. STROTHER PAUL K. STROTHER Weston Observatory Paleobotany Laboratory, 381 Concord Road, Weston, MA 02193, U.S.A. Search for other works by this author on: GSW Google Scholar Palynology (2001) 25 (1): 127–177. https://doi.org/10.2113/0250127 Article history first online: 03 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation JOHN H. BECK, PAUL K. STROTHER; SILURIAN SPORES AND CRYPTOSPORES FROM THE ARISAIG GROUP, NOVA SCOTIA, CANADA. Palynology 2001;; 25 (1): 127–177. doi: https://doi.org/10.2113/0250127 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search nav search search input Search input auto suggest search filter All ContentBy SocietyPalynology Search Advanced Search Abstract The Arisaig Group in northern mainland Nova Scotia has the most complete sequence of Silurian, fine grained, siliciclastic, near-shore marine sediments in Western Avalonia. In this setting, the Arisaig Group received a nearly continuous supply of autochthonous marine and allochthonous nonmarine palynomorphs. One hundred and seventy-seven samples, ranging in age from latest Ordovician?–early Llandovery to Přídolí (Beechhill Cove to Stonehouse formations) yield the most diverse spore assemblage yet known from the Silurian of North America. This paper describes the spores and cryptospores and reexamines age relationships of the Arisaig Group, placing both within context of early land plant evolution.Thirty-six species of spores and cryptospores were found in the Arisaig Group. Six new species are Ambitisporites marginatus sp. nov., A. capitaneus sp. nov., Scylaspora asperverruca sp. nov., Chelinospora textilis sp. nov., Hispanaediscus scabiosus sp. nov., and Hispanaediscus imbricatus sp. nov. A new genus, Vermiverruspora is created and the original description of ChelinosporaAllen 1965 is resurrected. Six species of Chelinospora are reassigned to Vermiverruspora gen. nov. (C. paravermiculata Loboziak et al. 1988, C. vermiculata Chaloner & Streel 1968, C. ?vermiculataMcGregor & Camfield 1976, C. retorrida Turnau 1986, C. rumneyiBurgess & Richardson 1995, C. vermolutaBurgess & Richardson 1995) and one species transferred to Stellatispora (pattersoniiWellman 1993). Four additional sporomorphs believed to be new, are not formally named because better preserved specimens are needed.The Arisaig Group (West Avalonia) is similar to sporomorph-rich type sections in the Anglo-Welsh Basin (East Avalonia). Four assemblage zones (avitus–dilutus, brevicosta–verrucatus, libycus–poecilomorphus, and tripapillatus–spicula), and four assemblage sub-zones (A. brevicosta, E. protophanus, C. obscura, and S. inframurinata var. inframurinata) have been recognized. Sporomorphs indicate that the contacts between the Lower and Upper Members of the McAdam Brook Formation and Green Member of the Moydart Formation may be one to two graptolite biozones older than previously reported.The Arisaig palynomorphs support the theory that a major radiation of vascular land plants occurred during the latter half of the Silurian Period. Llandovery and early Wenlock deposits at Arisaig contain only a few species of smooth walled spores and cryptospores. Sixteen trilete spore and fifteen cryptospore species appear between the Homerian and the late Přídolí. The tempo and composition of this diversification reflect evolutionary events observed elsewhere in the world. You do not currently have access to this article.

Palaeomagnetism of theca.440 Ma Cape St Mary’s sills of the Avalon Peninsula of Newfoundland: implications for Iapetus Ocean closure

We report on the palaeomagnetism of the gabbroic Cape St Mary's sills of the Avalon Peninsula of Newfoundland, which have previously yielded a 441±2 Ma U-Pb baddeleyite age (latest Ordovician or earliest Silurian). At 12 of 19 sites, stepwise alternating-field or thermal demagnetization isolated a stable characteristic remanence carried by magnetite. This remanence is shown to pre-date Early Devonian folding of the sills. Although a baked-contact test was inconclusive, the positive fold test and the low grade of metamorphism of the sills (prehnite-pumpellyite facies) make it likely that the characteristic remanence is primary. The tilt-corrected site-mean characteristic remanence has a declination of 343° and an inclination of −51° (k=25, α95=9°), yielding a ∼440 Ma palaeopole at 10°N, 140°E (dm=12°, dp=8°) for West (North American) Avalonia. The corresponding ∼440 Ma palaeolatitude for the Avalon Peninsula is 32°S±8°. The only other West Avalonian palaeolatitude determination from rocks that could be of similar age is from the Dunn Point volcanics of Nova Scotia; their more southerly palaeolatitude of 41°S±5° suggests that they are significantly older than 440 Ma, a possibility that we recommend testing with U-Pb dating. Although no ∼440 Ma palaeolatitude determinations are available for East Avalonia (parts of southern Britain and Ireland), interpolating between mid-Ordovician and mid-Silurian determinations gives an estimate of ∼25°S. This is consistent with our Cape St Mary's result and, if the Iapetus Ocean closed orthogonally, with a narrow (∼1000 km) Iapetus Ocean of approximately east-west orientation between Avalonia and Laurentia by 440 Ma.