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Brendan J Murphy - One of the best experts on this subject based on the ideXlab platform.
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late neoproterozoic to carboniferous genesis of a type magmas in Avalonia of northern nova scotia repeated partial melting of anhydrous lower crust in contrasting tectonic environments
GSA Annual Meeting in Seattle Washington USA - 2017, 2017Co-Authors: Brendan J Murphy, Gregory J Shellnutt, W J CollinsAbstract:Avalonian rocks in northern mainland Nova Scotia are characterized by voluminous 640–600 Ma calc-alkalic to tholeiitic mafic to felsic magmas produced in a volcanic arc. However, after the cessation of arc activity, repeated episodes of felsic magmatism between ca. 580 Ma and 350 Ma are dominated by A-type geochemical characteristics. Sm–Nd isotopic data, combined with zircon saturation temperature estimates, indicate that these magmas were formed by high temperature (800–1050 °C) melting of the same anhydrous crustal source. Regional tectonic considerations indicate that A-type felsic magmatism was produced (1) at 580 Ma in a San Andreas-type strike slip setting, (2) at 495 Ma as Avalonia rifted off Gondwana, (3) at 465 and 455 in an ensialic island arc environment and (4) at 360–350 Ma during post-collisional, intra-continental strike-slip activity as Avalonia was translated dextrally along the Laurentian margin. These results attest to the importance of crustal source, rather than tectonic setting, in the generation of these A-type magmas and are an example of how additional insights are provided by comparing the geochemical and isotopic characteristics of igneous suites of different ages within the same terrane. They also suggest that the shallow crustal rocks in northern mainland Nova Scotia were not significantly detached from their lower crustal source between ca. 620 Ma and 350 Ma, a time interval that includes the separation of Avalonia from Gondwana, its drift and accretion to Laurentia as well as post-accretionary strike-slip displacement.
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age geochemistry and sm nd isotopic signature of the 0 76 ga burin group compositional equivalent of Avalonian basement
Precambrian Research, 2008Co-Authors: Brendan J Murphy, Phil J A Mccausland, Sean J Obrien, Sergei Pisarevsky, Michael A HamiltonAbstract:Abstract The ca. 760 Ma Burin Group occurs in a well-exposed 60 km long northeasterly trending belt on the Burin Peninsula in the Avalon Zone of the southeastern Newfoundland Appalachians. This lithologically distinctive, tectonically bounded succession is characterized by low grade massive and pillowed basalts, abundant gabbro and diabase dykes and sills, mafic pyroclastic and epiclastic rocks and stromatolitic-carbonate-bearing rocks. The Burin Group preserves the oldest known magmatic event in the Avalon terrane, but its tectonic evolution and its relationship to the voluminous 640–570 Ma arc-related magmatism that typifies Avalonia is uncertain. Geochemical analyses confirm that the basalts are predominantly low-K tholeiites. They are characterized by high ratios of LIL/HFS elements, and display depletion to slight enrichment in LREE. Sm–Nd isotopic data reveal that most basalts have juvenile compositions, with ɛ Nd values similar to contemporaneous depleted mantle, indicating that high LIL/HFS elemental ratios were probably due to coeval subduction which contaminated the mantle source. Other basalts have lower ɛ Nd values, and the negative correlation of ɛ Nd with La/Sm, together with a positive correlation of ɛ Nd with 147 Sm/ 144 Nd suggest that their isotopic signatures have been modified by a Mesoproterozoic older oceanic crust or mantle into which the Burin Group mafic magmas were emplaced. The isotopic signature of the Burin mafic rocks is similar to that inferred for the source of the main phase of Avalonian magmatism. These data, together with paleocontinental reconstructions for ca. 760 Ma, suggest that the Burin Group is a local representative of ensimatic arcs within the Panthalassa-type ocean that surrounded Rodinia, possibly as a far-field response to the breakup of Rodinia. Although its low metamorphic grade precludes it being the basement from which Avalonian magmas were extracted, the Burin Group may be representative of the geochemical and isotopic composition of that basement.
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neoproterozoic early palaeozoic tectonostratigraphy and palaeogeography of the peri gondwanan terranes amazonian v west african connections
Geological Society London Special Publications, 2008Co-Authors: Damian R Nance, Brendan J Murphy, Duncan J Keppie, R A Strachan, Javier Fernandezsuarez, Gabriel Gutierrezalonso, Cecilio Quesada, Ulf Linnemann, Richard Dlemos, Sergei PisarevskyAbstract: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.
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neoproterozoic early paleozoic evolution of peri gondwanan terranes implications for laurentia gondwana connections
International Journal of Earth Sciences, 2004Co-Authors: Brendan J Murphy, Sergei A. Pisarevsky, Damian R Nance, Duncan J KeppieAbstract: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.
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u pb la icp ms dating of detrital zircons from cambrian clastic rocks in Avalonia erosion of a neoproterozoic arc along the northern gondwanan margin
Journal of the Geological Society, 2004Co-Authors: Brendan J Murphy, Javier Fernandezsuarez, Teresa Jeffries, R A StrachanAbstract:Most Neoproterozoic and Early Palaeozoic tectonic syntheses place Avalonia and related peri-Gondwanan terranes facing an open ocean along the northern margin of Gondwana, thereby providing important constraints for palaeocontinental reconstructions during that time interval. However, the precise location of Avalonia along the margin and its position relative to other peri-Gondwanan terranes is controversial. We present laser ablation–inductively coupled plasma mass spectrometry U–Pb data for detrital zircons from Cambrian clastic rocks in two localities in Avalonia: the Antigonish Highlands of Nova Scotia (62 analyses) and the British Midlands (55 analyses). The data from both samples are very similar, and taken together indicate an overwhelming dominance of Neoproterozoic ( c . 580–680 Ma) or Early Cambrian source rocks with minor older Neoproterozoic clusters at c . 710 Ma or of Mesoproterozoic age, three Palaeoproterozoic zircons and one Archaean zircon. The zircons can all be derived from local Avalonian sources. The Neoproterozoic zircons are attributed to erosion of the underlying Avalonian arc. Mesoproterozoic and Palaeoproterozoic zircons of similar ages are also found in Avalonian Neoproterozoic clastic rocks and their presence in the Cambrian clastic rocks could represent recycling of Neoproterozoic strata and do not necessarily imply the presence of Mesoproterozoic or Palaeoproterozoic basement rocks within their respective drainage basins. Comparison with the data from the Neoproterozoic arc-related clastic sequences suggests significant differences between their respective drainage systems. Whereas the Neoproterozoic data require extensive drainage systems, the Cambrian data can be attributed to localized drainage systems. The change in drainage patterns could reflect rifting and isolation of Avalonia from Amazonia between c . 585 and 540 Ma. Alternatively, it might reflect the creation of topographical barriers along the northern Gondwanan margin, in a manner analogous to the Cenozoic rise of the Andes or the creation of the Basin-and-Range topography in the Western USA.
Damian R Nance - One of the best experts on this subject based on the ideXlab platform.
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neoproterozoic early palaeozoic tectonostratigraphy and palaeogeography of the peri gondwanan terranes amazonian v west african connections
Geological Society London Special Publications, 2008Co-Authors: Damian R Nance, Brendan J Murphy, Duncan J Keppie, R A Strachan, Javier Fernandezsuarez, Gabriel Gutierrezalonso, Cecilio Quesada, Ulf Linnemann, Richard Dlemos, Sergei PisarevskyAbstract: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.
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circa 546 ma plume related dykes in the 1 ga novillo gneiss east central mexico evidence for the initial separation of Avalonia
Precambrian Research, 2006Co-Authors: Duncan J Keppie, Damian R Nance, Jaroslav Dostal, Brent V Miller, Amabel Ortegarivera, James K W LeeAbstract:Abstract The ∼1 Ga Novillo Gneiss exposed in an inlier in the front ranges of the Laramide belt in east-central Mexico is cut by an unmetamorphosed NE-trending mafic dyke swarm. The gneiss represents the northernmost inlier of the Oaxaquia terrane that underlies the backbone of Mexico. Titanite and biotite from metasedimentary rocks yielded ages of 928 ± 2 Ma (concordant U–Pb age), and duplicate 40 Ar/ 39 Ar plateau ages of 697 ± 7 and 697 ± 10 Ma, which are inferred to represent cooling through ∼660 and ∼300–350 °C, respectively. On the other hand, hornblende from cross-cutting, NE-trending mafic dykes yielded a 40 Ar/ 39 Ar plateau age of 546 ± 5 Ma, which is inferred to closely post-date the time of intrusion. Combined with published ages (U–Pb zircon, Sm–Nd garnet, K–Ar hornblende) the data suggest that, following granulite facies metamorphism at 982 ± 6 Ma (8.9–9.7 kbar and 730–775 °C), the Novillo Gneiss cooled at a rate of ∼1.45 °C/my. If this latter rate is extrapolated it would place the rocks at the surface (30 °C) by ∼497 Ma, about 50 my after dyke emplacement. Geochemistry of the dykes indicates that they are Fe-rich tholeiites with: (i) low MgO, but high TiO 2 and other HFSE, (ii) smooth mantle-normalized incompatible trace element patterns peaking at Nb and La, and (iii) intraplate tectonic affinities. These features are typical of enriched tholeiites associated with plumes. Current Precambrian paleogeographic reconstructions place Avalonia against Oaxaquia and subsequently separate Avalonia by way of ridge–trench collision at ∼550 Ma giving rise to the unique Avalonian Cambrian fauna and a rapid phase of early Cambrian subsidence. These events are contemporaneous with emplacement of the Novillo mafic dykes, suggesting that the plume-related magmatism and separation of Avalonia from Oaxaquia are linked.
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neoproterozoic early paleozoic evolution of peri gondwanan terranes implications for laurentia gondwana connections
International Journal of Earth Sciences, 2004Co-Authors: Brendan J Murphy, Sergei A. Pisarevsky, Damian R Nance, Duncan J KeppieAbstract: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.
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tethyan mediterranean and pacific analogues for the neoproterozoic paleozoic birth and development of peri gondwanan terranes and their transfer to laurentia and laurussia
Tectonophysics, 2003Co-Authors: Duncan J Keppie, Damian R Nance, Brendan J Murphy, Jakub DostálAbstract:Abstract Modern Tethyan, Mediterranean, and Pacific analogues are considered for several Appalachian, Caledonian, and Variscan terranes (Carolina, West and East Avalonia, Oaxaquia, Chortis, Maya, Suwannee, and Cadomia) that originated along the northern margin of Neoproterozoic Gondwana. These terranes record a protracted geological history that includes: (1) ∼1 Ga (Carolina, Avalonia, Oaxaquia, Chortis, and Suwannee) or ∼2 Ga (Cadomia) basement; (2) 750–600 Ma arc magmatism that diachronously switched to rift magmatism between 590 and 540 Ma, accompanied by development of rift basins and core complexes, in the absence of collisional orogenesis; (3) latest Neoproterozoic–Cambrian separation of Avalonia and Carolina from Gondwana leading to faunal endemism and the development of bordering passive margins; (4) Ordovician transport of Avalonia and Carolina across Iapetus terminating in Late Ordovician–Early Silurian accretion to the eastern Laurentian margin followed by dispersion along this margin; (5) Siluro-Devonian transfer of Cadomia across the Rheic Ocean; and (6) Permo-Carboniferous transfer of Oaxaquia, Chortis, Maya, and Suwannee during the amalgamation of Pangea. Three potential models are provided by more recent tectonic analogues: (1) an “accordion” model based on the orthogonal opening and closing of Alpine Tethys and the Mediterranean; (2) a “bulldozer” model based on forward-modelling of Australia during which oceanic plateaus are dispersed along the Australian plate margin; and (3) a “Baja” model based on the Pacific margin of North America where the diachronous replacement of subduction by transform faulting as a result of ridge–trench collision has been followed by rifting and the transfer of Baja California to the Pacific Plate. Future transport and accretion along the western Laurentian margin may mimic that of Baja British Columbia. Present geological data for Avalonia and Carolina favour a transition from a “Baja” model to a “bulldozer” model. By analogy with the eastern Pacific, we name the oceanic plates off northern Gondwana: Merlin (≡Farallon), Morgana (≡Pacific), and Mordred (≡Kula). If Neoproterozoic subduction was towards Gondwana, application of this combined model requires a total rotation of East Avalonia and Carolina through 180° either during separation (using a western Transverse Ranges model), during accretion (using a Baja British Columbia “train wreck” model), or during dispersion (using an Australia “bulldozer” model). On the other hand, Siluro-Devonian orthogonal transfer (“accordion” model) from northern Africa to southern Laurussia followed by a Carboniferous “Baja” model appears to best fit the existing data for Cadomia. Finally, Oaxaquia, Chortis, Maya, and Suwannee appear to have been transported along the margin of Gondwana until it collided with southern Laurentia on whose margin they were stranded following the breakup of Pangea. Forward modeling of a closing Mediterranean followed by breakup on the African margin may provide a modern analogue. These actualistic models differ in their dictates on the initial distribution of the peri-Gondwanan terranes and can be tested by comparing features of the modern analogues with their ancient tectonic counterparts.
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sm nd isotopic systematics as tectonic tracers an example from west Avalonia in the canadian appalachians
Earth-Science Reviews, 2002Co-Authors: Brendan J Murphy, Damian R NanceAbstract:Abstract 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. e 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 e 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 e 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.
Duncan J Keppie - One of the best experts on this subject based on the ideXlab platform.
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silurian u pb zircon intrusive ages for the red river anorthosite northern cape breton island implications for the laurentia Avalonia boundary in atlantic canada
Gondwana Research, 2019Co-Authors: Duncan J Keppie, Gregory J Shellnutt, J Dostal, Fraser D KeppieAbstract:Abstract Current interpretations of the geology of Cape Breton Island suggest that it exposes a complete cross-section of the Appalachians from Laurentia across Iapetan vestiges to Avalonia. Crucial to this view is the presence of ca. 1 Ga plutons, including anorthosites, which have been regarded as correlatives of Grenvillian basement, a correlation that overlooks the fact that Avalonia is also underlain by a ca. 1 Ga basement. We analyzed zircons from the Red River anorthosite (Blair River Complex, northwestern Cape Breton Island) previously dated as ca. 1.1 Ga: they yielded 421 ± 3 Ma intrusive ages with older ages between 865 ± 18 Ma and 1044 ± 20 Ma inferred to be either xenocrysts derived from the country rock or from the source. Implications of these data suggest that the accompanying low pressure granulite-amphibolite facies metamorphism of the Blair River Complex is either the root of a 440–410 Ma, magmatic belt produced during slab break-off or relict ca. 1 Ga basement. The Blair River Complex occurs in a NNE-SSW, sinistral positive flower structure that progresses upwards from a Neoproterozoic rifted arc through a low grade upper Ordovician-Silurian overstep sequence to amphibolite facies fault slices, capped by the low-pressure, granulite facies rocks (Blair River Complex). The correlation of Neoproterozoic, rifted arc units across most of Cape Breton Island suggests it represents the deformed northwestern margin of Avalonia intruded by a Silurian-Lower Devonian magmatic belt. As the geological record in the Blair River Complex is similar to both Grenvillian and Avalonian basements, its provenance is equivocal, however Pb isotopic data suggest the Blair River Complex has Amazonian (≈Avalonia) affinities. Thus, Cape Breton Island, rather than representing a complete cross-section of the Appalachian orogen, is part of pristine—deformed Avalonia with a positive flower structure exposing a cross-section of Avalonian crust.
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ediacaran middle paleozoic oceanic voyage of Avalonia from baltica via gondwana to laurentia paleomagnetic faunal and geological constraints
Geoscience Canada, 2014Co-Authors: Duncan J Keppie, Fraser D KeppieAbstract:Current Ediacaran–Cambrian, paleogeographic reconstructions place Avalonia, Carolinia and Ganderia (Greater Avalonia) at high paleolatitudes off northwestern Gondwana (NW Africa and/or Amazonia), and locate NW Gondwana at either high or low paleolatitudes. All of these reconstructions are incompatible with 550 Ma Avalonian paleomagnetic data, which indicate a paleolatitude of 20–30oS for Greater Avalonia and oriented with the present-day southeast margin on the northwest side. Ediacaran, Cambrian and Early Ordovician fauna in Avalonia are mainly endemic, which suggests that Greater Avalonia was an island microcontinent. Except for the degree of Ediacaran deformation, the Neoproterozoic geological records of mildly deformed Greater Avalonia and the intensely deformed Bolshezemel block in the Timanian orogen into eastern Baltica raise the possibility that they were originally along strike from one another, passing from an island microcontinent to an arc-continent collisional zone, respectively. Such a location and orientation is consistent with: (i) Ediacaran (580–550 Ma) ridge-trench collision leading to transform motion along the backarc basin; (ii) the reversed, ocean-to-continent polarity of the Ediacaran cratonic island arc recorded in Greater Avalonia; (iii) derivation of 1–2 Ga and 760–590 Ma detrital zircon grains in Greater Avalonia from Baltica and the Bolshezemel block (NE Timanides); and (iv) the similarity of 840–1760 Ma TDM model ages from detrital zircon in pre-Uralian–Timanian and Nd model ages from Greater Avalonia. During the Cambrian, Greater Avalonia rotated 150o counterclockwise ending up off northwestern Gondwana by the beginning of the Ordovician, after which it migrated orthogonally across Iapetus to amalgamate with eastern Laurentia by the Late Ordovician–Early Silurian. SOMMAIRELes reconstitutions paleogeographiques courantes de l’Ediacarien-Cambrien placent l’Avalonie ,la Carolinia et la Ganderia (Grande Avalonie) a de hautes paleolatitudes au nord-ouest du Gondwana (N-O de l'Afrique et/ou de l'Amazonie), et placent le N-O du Gondwana a de hautes ou de basses paleolatitudes. Toutes ces reconstitutions sont incompatibles avec des donnees avaloniennes de 550 Ma, lesquelles indiquent une paleolatitude de 20-30o S pour la Grande Avalonie et orientee a la marge sud-est d’aujourd'hui sur le cote nord-ouest. Les faunes edicacariennes, cambriennes et de l'Ordovicien precoce dans l’Avalonie sont principalement endemiques, ce qui permet de penser que la Grande Avalonie etait une ile de microcontinent. Sauf pour le degre de deformation ediacarienne, les registres geologiques neoproterozoiques d’une Grande Avalonie legerement deformee et ceux du bloc intensement deforme de Bolshezemel dans l'orogene Timanian dans l’est de la Baltica soulevent la possibilite qu'ils aient ete a l'origine de meme direction, passant d'une ile de microcontinent a une zone de collision d’arc continental, respectivement. Un tel emplacement et une telle orientation sont compatibles avec: (i) un contexte de collision crete-fosse a l’Ediacarien (580-550 Ma) se changeant en un mouvement de transformation le long du bassin d’arriere-arc; (ii) l’inversion de polarite de marine a continentale, de l’arc insulaire cratonique edicarien observe dans la Grande Avalonie; (iii) la presence de grains de zircons detritiques de 1 a 2 Ga et 760-590 Ma de la Grande Avalonie issus de la Baltica et du bloc Bolshezemel (N-E des Timanides); et (iv) la similarite des âges modeles de 840-1760 Ma TDM de zircons detritiques pre-ourallien-timanien, et des âges modeles Nd de la Grande Avalonie. Durant le Cambrien, la Grande Avalonie a pivote de 150° dans le sens antihoraire pour se retrouver au nord-ouest du Gondwana au debut de l'Ordovicien, apres quoi elle a migre orthogonalement a travers l’ocean Iapetus pour s’amalgamer a la bordure est de la Laurentie a la fin de l’Ordovicien-debut du Silurien.
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neoproterozoic early palaeozoic tectonostratigraphy and palaeogeography of the peri gondwanan terranes amazonian v west african connections
Geological Society London Special Publications, 2008Co-Authors: Damian R Nance, Brendan J Murphy, Duncan J Keppie, R A Strachan, Javier Fernandezsuarez, Gabriel Gutierrezalonso, Cecilio Quesada, Ulf Linnemann, Richard Dlemos, Sergei PisarevskyAbstract: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.
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circa 546 ma plume related dykes in the 1 ga novillo gneiss east central mexico evidence for the initial separation of Avalonia
Precambrian Research, 2006Co-Authors: Duncan J Keppie, Damian R Nance, Jaroslav Dostal, Brent V Miller, Amabel Ortegarivera, James K W LeeAbstract:Abstract The ∼1 Ga Novillo Gneiss exposed in an inlier in the front ranges of the Laramide belt in east-central Mexico is cut by an unmetamorphosed NE-trending mafic dyke swarm. The gneiss represents the northernmost inlier of the Oaxaquia terrane that underlies the backbone of Mexico. Titanite and biotite from metasedimentary rocks yielded ages of 928 ± 2 Ma (concordant U–Pb age), and duplicate 40 Ar/ 39 Ar plateau ages of 697 ± 7 and 697 ± 10 Ma, which are inferred to represent cooling through ∼660 and ∼300–350 °C, respectively. On the other hand, hornblende from cross-cutting, NE-trending mafic dykes yielded a 40 Ar/ 39 Ar plateau age of 546 ± 5 Ma, which is inferred to closely post-date the time of intrusion. Combined with published ages (U–Pb zircon, Sm–Nd garnet, K–Ar hornblende) the data suggest that, following granulite facies metamorphism at 982 ± 6 Ma (8.9–9.7 kbar and 730–775 °C), the Novillo Gneiss cooled at a rate of ∼1.45 °C/my. If this latter rate is extrapolated it would place the rocks at the surface (30 °C) by ∼497 Ma, about 50 my after dyke emplacement. Geochemistry of the dykes indicates that they are Fe-rich tholeiites with: (i) low MgO, but high TiO 2 and other HFSE, (ii) smooth mantle-normalized incompatible trace element patterns peaking at Nb and La, and (iii) intraplate tectonic affinities. These features are typical of enriched tholeiites associated with plumes. Current Precambrian paleogeographic reconstructions place Avalonia against Oaxaquia and subsequently separate Avalonia by way of ridge–trench collision at ∼550 Ma giving rise to the unique Avalonian Cambrian fauna and a rapid phase of early Cambrian subsidence. These events are contemporaneous with emplacement of the Novillo mafic dykes, suggesting that the plume-related magmatism and separation of Avalonia from Oaxaquia are linked.
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neoproterozoic early paleozoic evolution of peri gondwanan terranes implications for laurentia gondwana connections
International Journal of Earth Sciences, 2004Co-Authors: Brendan J Murphy, Sergei A. Pisarevsky, Damian R Nance, Duncan J KeppieAbstract: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.
John W F Waldron - One of the best experts on this subject based on the ideXlab platform.
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provenance and paleozoic tectonic evolution of ganderia and its relationships with Avalonia and megumia in the appalachian caledonide orogen
Gondwana Research, 2021Co-Authors: Sandra M Barr, John W F Waldron, Cees R Van Staal, David I Schofield, Alex Zagorevski, Chris E WhiteAbstract:Abstract West and East Ganderia in the northern Appalachians and Caledonides, respectively, represent a Gondwanan superterrane situated along the Tornquist margin of Amazonia prior to Furongian drift into the Iapetus Ocean, which opened the Rheic Ocean from west to east. The ocean-facing Penobscot arc-backarc system was established by 515 Ma in West Ganderia. A correlative arc formed at ca. 480 Ma in East Ganderia. In West Ganderia, the Tremadocian Penobscottian orogeny involved closure of the Penobscot backarc basin. Tremadocian Monian tectonism in East Ganderia was mainly related to oblique accretion to East Avalonia and the Megumian Cymru terrane. Penobscottian and late Floian Monian orogenesis led to termination of Early Ordovician arc magmatism, probably due to shallow subduction of buoyant oceanic lithosphere. Early to Middle Ordovician arc-backarc systems were erected on Penobscottian-Monian modified West and East Ganderia. The active edge of West Ganderia accreted diachronously to peri-Laurentia between 475 Ma and 455 Ma, followed by Wenlock to Ludlow Salinic accretion of the inboard Gander margin through closure of the intervening backarc basin. In the Caledonides, East Ganderia and East Avalonia accreted to Laurentia during the correlative Wenlock Scandian orogeny. The Ordovician to Silurian tectonic evolution of Ganderia was markedly non-cylindrical with pronounced partitioning of Salinic-Scandian convergence. Pridoli to Lochkovian closure of the Acadian seaway in the northern Appalachians led to Acadian accretion of West Avalonia to composite Laurentia. Shallow Early Devonian underthrusting of West and East Avalonia beneath Laurentia produced widespread Acadian tectonism and voluminous Early Devonian Acadian magmatism. The Appalachian Meguma terrane formed part of Megumia, which probably formed originally adjacent to East Avalonia and West Africa. The Meguma terrane accreted dextrally to Laurentia during and after the late Emsian to Famennian Neoacadian orogeny, mainly driven by outboard subduction of the Rheic Ocean. No correlative terrane docking took place in the Caledonides.
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detrital zircon characterization of early cambrian sandstones from east Avalonia and se ireland implications for terrane affinities in the peri gondwanan caledonides
Geological Magazine, 2019Co-Authors: John W F Waldron, David I Schofield, Graham Pearson, Chiranjeeb Sarkar, Yan Luo, Robert J DokkenAbstract:The 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 eHf 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 eHf 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.
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provenance of the meguma terrane nova scotia rifted margin of early paleozoic gondwana
Canadian Journal of Earth Sciences, 2009Co-Authors: John W F Waldron, Sandra M Barr, Chris E White, Antonio Simonetti, Larry M HeamanAbstract: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...
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geochemical and isotopic characteristics of early silurian clastic sequences in antigonish highlands nova scotia canada constraints on the accretion of Avalonia in the appalachian caledonlde orogen
Canadian Journal of Earth Sciences, 1996Co-Authors: Brendan J Murphy, Duncan J Keppie, J Dostal, Mary Pat Cude, John W F WaldronAbstract:Avalonia is a terrane that accreted to Laurentia–Baltica during the development of the Appalachian–Caledonide Orogen. Interpretations of the timing of accretion have been constrained by comparing faunal affinities, overstep sequences, age and kinematics of inferred accretionary deformational events, and controversial paleomagnetic data. We show that the time of accretion of Avalonia may also be constrained by contrasts in the geochemical and isotopic signatures of its igneous rocks (which reflect the characteristics of the underlying continental basement and mantle) and sedimentary rocks (which reflect provenance). Early Silurian clastic sedimentary rocks of the Beechill Cove Formation, Antigonish Highlands, Nova Scotia, were deposited on Avalonian crust. The formation predominantly consists of approximately 80 m of siltstones and shales deposited in a nearshore environment and derived from the northeast. Their age is constrained by paleontological data and by directly underlying Late Ordovician – Early S...
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timing of the Avalonia baltica plate convergence as inferred from palaeogeographic and stratigraphic data of chitinozoan assemblages in west pomerania northern poland
Geological Society London Special Publications, 2002Co-Authors: Joakim Samuelsson, Wieslaw S Bednarczyk, Marco Vecoli, Jacques VerniersAbstract:Timing of the Avalonia-Baltica plate convergence as inferred from palaeogeographic and stratigraphic data of chitinozoan assemblages in West Pomerania, northern Poland
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the cambrian to mid devonian basin development and deformation history of eastern Avalonia east of the midlands microcraton new data and a review
Geological Society London Special Publications, 2002Co-Authors: Jacques Verniers, T C Pharaoh, Luc Andre, Timothy Debacker, W De Vos, M Everaerts, Alain Herbosch, J Samuelsson, Manuel Sintubin, Marco VecoliAbstract:A review is given of recently published and new data on Avalonia east of the Midlands Microcraton. The three megasequences from Cambrian to mid Devonian described in Wales and Welsh Borderland are also present east of the Midlands Microcraton (Brabant Massif, Condroz, Ardennes, Remscheid and Ebbe inliers, Krefeld high). The three mega-sequences are caused by a tectonic driving mechanism and are explained by three different geodynamic contexts: an earlier phase with extensional basins or rifting and rather thick sequences, when Avalonia was still attached to Gondwana; a second phase with a shelf basin with moderately thin sequences when Avalonia was a separate continent and a later phase with a shelf or foreland basin development and thick sequences. Deformation of the megasequences 1 and 2 or 1 to 3 varies between areas. In Wales and the Lake District the Acadian phase is long-lived and active from early to mid Devonian. In the Ardennes inliers a deformation is active between the late Ordovician and the Silurian (Ardennian Phase), with a similar intensity as the core of the Brabant Massif, when present erosion levels are compared. The Brabant Massif is partly deformed by the long-lived Brabantian Phase from late Silurian till early mid Devonian. Both the Ardennes inliers and the Brabant Massif are not classic orogenic belts, only slate belts where no more than the epizone is reached at present erosion levels. Areas supposedly close to the microcraton or basement are nearly undeformed (SW Brabant Massif and central Condroz). A model of anticlockwise rotation of Avalonia of about 55° from Caradoc to Emsian is proposed to explain the deposition setting of megasequence 3 and the subsequent Acadian and Brabantian deformation. Immediately after the Avalonian microcontinent touched Baltica in Caradoc times it created a short-lived subduction magmatic event from The Wash to the Brabant Massif and soon after the magmatism ended a foreland basin developed. Possibly during and after that development a long-lived and slow compressional event occurred, leading to the deformation of the Anglo-Brabant Deformation Belt. In the early Devonian, contemporaneous with the shortening of the Anglo-Brabant Deformation Belt, extension occurred in the Rheno-Hercynian Zone, possibly caused by the same slow rotation of Avalonia. More evidence emerges that Avalonia cast of the Midlands Microcraton comprises not one but probably two terranes: the remnant of the palaeocontinent Avalonia, and what is called the palaeocontinent Far Eastern Avalonia; the latter is only occasionally observed in the few deep boreholes into the Heligoland-Pomerania Deformation Belt, in southern Denmark, NE Germany and NW Poland, with scant available indirect data in between indicating only Proterozoic basement and no Caledonian deformation. For Far Eastern Avalonia a similar palaeogeographical history is postulated as Avalonia, with rifting from Gondwana in Arenig or earlier times, collision with Baltica before the mid-Ashgill and deformation between the late Ordovician and latest Silurian. The Avalonia concept might need to be expanded to an 'Avalonian Terrane Assemblage' with cratonic cores and small short-lived oceans as in the Armorican Terrane Assemblage.
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Ordovician chitinozoan biozonation of the Brabant Massif, Belgium
Review of Palaeobotany and Palynology, 2000Co-Authors: Joakim Samuelsson, Jacques VerniersAbstract:Abstract Chitinozoans from seven Ordovician units (Abbaye de Villers, Tribotte, Rigenee, Ittre, Bornival, and Brutia formations and a new unnamed unit, here provisionally called the Asquempont unit) belonging to the mainly concealed Brabant Massif, Belgium are described herein. Fifty-six samples were taken from rocks cropping out at the south-eastern rim of the massif in the Orneau, Dyle–Thyle and Senne–Sennette valleys. Microfossil preservation is moderate to poor, and the chitinozoans occur in low numbers. Taxonomically, the recovered chitinozoans are distributed into 29 taxa, some placed under open nomenclature. Together with earlier published graptolite and acritarch data, the analysis of the chitinozoan assemblages resulted in an improved chronostratigraphy of the investigated formations. We propose a local chitinozoan biozonation with 11 zones for the Brabant Massif. The oldest investigated units yielded chitinozoans typical for North Gondwana, and younger units (starting in the middle Caradoc), yielded some taxa also common in Baltica. As the Brabant Massif formed part of the microcontinent Avalonia, the chitinozoan assemblages recovered from the massif support the inferred drifting of Avalonia from high latitudes towards middle latitudes in the Ordovician as was suggested earlier.
