Northern Gondwanan Margin Research Articles

The Avalonian-Cadomian Belt and related peri-Gondwanan terranes

During the past decade it has widely been accepted that the Avalonian-Cadomian belt results from Neoproterozoic to early Paleozoic Andean-type orogeny, that was active at the northern margin of Gondwana. The major imprints of this orogeny are recorded by so-called periGondwanan terranes which faced open oceans during the Neoproterozoic. Avalonianand Cadomian-type terranes show significant differences concerning the age of protoliths and the timing of accretion. Avalonian-type terranes developed from ca. 1 Ga old rocks and were subsequently accreted to the northern margin of West Gondwana (Amazonia). Cadomian-type terranes originated along the African margin by recycling ca. 2 Ga crust. In Paleozoic times the Avalonian-Cadomian belt fell into microplates that were detached from Gondwana and collided with Laurentia or Baltica. Several attempts have been made to reconstruct the Neoproterozoic-Early Paleozoic distribution of the peri-Gondwanan microplates relative to the West African and Amazonian Cratons. However, as robust paleomagnetic and other quantitative data are largely lacking, these reconstructions are still speculative and need further confirmation. The contributions of this special volume provide new data and models which help to improve our knowledge on the AvalonianCadomian orogeny. The papers are an outcome of the GEO 2002 meeting held during October 2002 in W rzburg, Germany. In an invited paper, Murphy et al. compare inferred relationships between peri-Gondwanan terranes and the northern Gondwanan margin with paleomagnetically constrained movements interpreted for the Amazonian and West African cratons for the interval ca. 800 – 500 Ma. The following eight papers deal with Cadomian events in the Bohemian Massif. A large number of new isotopic data is presented which helps to constrain the timing of orogenic events. The paper by Linnemann et al. represents new U-Pb SHRIMP ages of detrital and inherited zircons from pre-Variscan rocks of Saxo-Thuringia which reflect a West African provenance for sediments and magmatic rocks. Based on single zircon Pb-Pb evaporation and SHRIMP ages, combined with major and trace element data and Sm-Nd isotope systematics, Mingram et al., on the other hand, suggest a link between Saxo-Thuringia and Avalonia. They found evidence for at least two discrete magmatic events in the so-called red gneisses of the Erzgebirge. Patocka and torch elucidate Cadomian sediment provenances of the Early Paleozoic siliciclastic rocks of the Tepl -Barrandian unit, Bohemian Massif. The paper by Drost et al. presents new U-Pb-SHRIMP ages for the Cambrian volcanics of the Tepl -Barrandian unit and yields constraints on the late Neoproterozoic to Cambrian geotectonic settings. U-Pb and Pb-Pb ages obtained by Mazur et al. and by Zelazniewicz et al. indicate Neoproterozoic to Cambrian magmatism in different parts of the Sudetes (Bohemian Massif). Teipel et al. obtained new U-Pb SHRIMP and Nd isotopic data which suggest Upper Vendian and Lower Ordovician magmatism in the Bavarian Forest that is part of the Moldanubian unit. Friedl et al. derived new U-Pb zircon ages (SHRIMP and TIMS) from different orthogneisses of the Austrian part of the Bohemian Massif. They found evidence for both Neoproterozoic and Ordovician magmatism in the Moldanubian domain, the latter is interW. D rr Institut f r Geowissenschaften und Lithosph renforschung, Universit t Giessen, Senckenbergstr. 3, 35390 Giesen, Germany

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.

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

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.

Contiguous rather than discrete Paleozoic histories for the Avalon and Meguma terranes based on detrital zircon data

Upper Ordovician–Lower Devonian strata of the Meguma terrane in the Canadian Appalachians contain zircon populations, including an important Mesoproterozoic zircon population (1.0–1.4 Ga), similar to those in coeval strata of Avalonia, and strongly suggest contiguous rather than discrete histories for these terranes throughout the Paleozoic. That these terranes were juxtaposed throughout the early Paleozoic is indicated by the absence of a Cambrian–Ordovician accretionary event, the lack of intervening suture-zone ophio litic units, and the similarity of Avalonian and Meguma basement Nd isotope signatures in early Paleozoic igneous suites. As Avalonia had accreted to Laurentia-Baltica by the Early Silurian, these data suggest that the Meguma terrane, like Avalonia, resided along the same (northern) margin of the Rheic Ocean at that time. These conclusions have implications for reconstructions of the northern Gondwanan margin in the early Paleozoic and imply that the Silurian–Devonian Acadian orogeny in Maritime Canada occurred in an Andean-type setting and was not related to collision of the Meguma terrane with the Laurentian margin.

Un biofacies à Trilobites Homalonotidae dans I'Ordovicien de la marge nord-gondwanienne: implications paléobiologiques et paléogéographiques

During the Llandeilo, the sandy lithological units which develop locally in the Armorican Massif, Spain and Morocco, are all characterized by similar trilobite associations in which endobenthic homalonotids. well adapted to shallow water and sandy substrate, are predominant. During the Llandeilo, on the northern Gondwanan margin, the wide geographical distribution of most of the trilobites, controlled by environmental factors, shows that the existence of a ‘Proto-Tethys’ ocean is unlikely. □North Gondwana, Trilobita, Homalonotidae, Ordovician, Llandeilo. palaeoenvironment.

Trilobites Calmoniidae de l'Ordovicien supérieur du Maroc et les origines de la Province Malvino‐Cafre

A re-examination of the trilobite Baniaspis globosa Destombes (Phacopina) from the Ashgill of the Anti-Atlas (Morocco) shows that it has several derived characters which allow us to classify it in the family Calmoniidae. Calmoniids were characteristic members of the Malvino-Kaffric province during the Lower Devonian. Hence we regard the origins of this palaeogeographic province as located on the northern Gondwanan margin as early as Ashgill, or even Caradoc, times. We agree with those authors who have suggested that the trilobite associations present in the Anti-Atlas, the Montagne Noire (France) and Bohemia during the earlier Ordovician argue against the existence of a ‘proto-Tethys’ ocean.