The Tornquist Sea and Baltica–Avalonia docking

Early Ordovician (Late Arenig) limestones from the SW margin of Baltica (Scania–Bornholm) have multicomponent magnetic signatures, but high unblocking components predating folding, and the corresponding palaeomagnetic pole (latitude=19°N, LONGITUDE=051°E) compares well with Arenig reference poles from Baltica. Collectively, the Arenig poles demonstrate a midsoutherly latitudinal position for Baltica, then separated from Avalonia by the Tornquist Sea.Tornquist Sea closure and the Baltica–Avalonia convergence history are evidenced from faunal mixing and increased resemblance in palaeomagnetically determined palaeolatitudes for Avalonia and Baltica during the Mid-Late Ordovician. By the Caradoc, Avalonia had drifted to palaeolatitudes compatible with those of SW Baltica, and subduction beneath Eastern Avalonia was taking place. We propose that explosive vents associated with this subduction and related to Andean-type magmatism in Avalonia were the source for the gigantic Mid-Caradoc (c. 455 Ma) ash fall in Baltica (i.e. the Kinnekulle bentonite). Avalonia was located south of the subtropical high during most of the Ordovician, and this would have provided an optimum palaeoposition to supply Baltica with large ash falls governed by westerly winds.In Scania, we observe a persistent palaeomagnetic overprint of Late Ordovician (Ashgill) age (pole: LATITUDE=4°S, LONGITUDE=012°E). The remagnetisation was probably spurred by tectonic-derived fluids since burial alone is inadequate to explain this remagnetisation event. This is the first record of a Late Ordovician event in Scania, but it is comparable with the Shelveian event in Avalonia, low-grade metamorphism in the North Sea basement of NE Germany (440–450 Ma), and sheds new light on the Baltica–Avalonia docking.     

A new species of the Gondwanan genus Cardiolaria Munier-Chalmas in the Sandbian of northwestern Argentina: Paleobiogeographic considerations

A new species of afghanodesmatid, Cardiolaria benicioi, is recorded from Sandbian strata of northwestern Argentina. This species confirms the strong paleobiogeographic relationships between the western Argentina basin and other peri-Gondwanan areas. The Mid-Late Ordovician distribution of bivalves fit well into the Mediterranean Province defined upon brachiopod and trilobite faunas. Similitudes between Tremadocian and Floian bivalves from the western Gondwana and the peri-Gondwanan areas indicate that such ‘Mediterranean’ paleobiogeographic patterns can be traced back well into the Early Ordovician.     

The Early Palaeozoic break-up of northern Gondwana,new palaeomagnetic and geochronological data from the Saxothuringian Basin,Germany

Early Palaeozoic volcanic and sedimentary rocks from the Saxothuringian Basin (Franconian Forest, northern Bavaria) have been subjected to detailed radiometric and palaeomagnetic studies in order to determine the tectonic environment and geographic setting in which they were deposited. Two hand samples were collected from the as yet undated pyroclastic flow deposits for ²⁰⁷Pb/²⁰⁶Pb age dating. Radiometric results for these samples, obtained by the single-zircon evaporation technique, are identical within error, and the mean age of all measured grains is 478.2ǃ.8 Ma (n=11). This age is considered to be primary and firmly constrains the eruption of the ignimbrites and formation of the subaqueous pyroclastic flows as having occurred in Early Ordovician (Arenig) times. Palaeomagnetic studies were carried out on these Early Ordovician volcanic rocks, and also on the biostratigraphically dated, Late Ordovician (Ashgillian) Döbra sandstones. The volcanic rocks carry up to three directions of magnetisation. The poorly defined, low and intermediate unblocking temperature directions are thought to represent secondary overprint directions of post-Ordovician age. The high temperature component, however, identified at temperatures of up to 580 °C, is of mixed polarity and passes the fold test with 99% confidence. The overall mean direction after bedding correction is 189°/76°, !₉₅=11.6°, k=44.7 (25 samples, five sites), and is considered to be primary and Early Ordovician in origin. It yields a palaeo-south pole at 24°N and 007°E, which translates into palaeolatitudes of 63°+21.7°/-17.3° S for the Saxothuringian Terrane. Samples from the Late Ordovician Döbra sandstone are generally very weakly magnetised. A high temperature D component of magnetisation can be identified in some samples and yields a mean direction of 030°/-58°, !₉₅=18.5°, k=25.7 (15 samples, four sites) after bedding correction. The Arenig palaeomagnetic results indicate high palaeolatitudes, but separation from northern Gondwana. This is in basic agreement with data from elsewhere in the Armorican Terrane Assemblage, all of which suggest high southerly palaeolatitudes in the Early Ordovician. The geochemical signatures of these rocks indicate emplacement in an extensional environment. These new data, therefore, are interpreted as marking the onset of rifting of Saxothuringia from the north African margin of Gondwana, and the start of the relative northward migration of the Saxothuringian Terrane. Although the Late Ordovician palaeomagnetic results presented here are only poorly constrained, they suggest an intermediate palaeolatitude for Saxothuringia in Ashgillian times, in good agreement with Late Ordovician palaeomagnetic data from the Barrandian.     

http://www.sciencedirect.com/science/article/pii/S1674987115000845

Belonechitina capitata, a typically middle to late Ordovician chitinozoan index taxon was for the first time recovered from the northeastern Kumaon region, a part of Garhwal-Kumaon Tethys basin of the Himalaya, India. This species is of great biostratigraphic importance and has already been reported from Avalonia, Baltica and northern Gondwana. The study area was during Ordovician, part of a lowpalaeolatitudinal Gondwana region. The vesicles of recovered forms are black and fragmentary. This is principally attributed to intense tectonic activity during the Himalayan orogenic movement which resulted into high thermal alteration. The chitinozoans are found along with melanosclerites.     

A re-examination of the type Arenig Series

The stratotypc of the Arenig Series (Ordovician System) and its boundaries are critically re-examined. The lithostratigraphy and biostratigraphy of the Tremadoc, Arenig, and Llanvirn age rocks at Arenig Fawr, North Wales that were constructed by Feamsides in 1905 are amended. The Arenig Series at Arenig is represented by the Camedd Iago Formation which was established by Lynas (1973) in the Migneint area. Examination of the sparse graptolite faunas indicates that the type Arenig Series is incomplete, only the extensus Zone having been recognized with certainty. There is a possible unconformity at the base and a probable unconformity at the top, and on present knowledge the succession cannot be correlated precisely with what are considered to be Arenig sequences elsewhere. It therefore may prove necessary to redefine the Arenig Series at a more useful type section. It is suggested that the Carmarthen district may provide a suitable stratotype.     

The Variscan collage and orogeny (480–290 Ma) and the tectonic definition of the Armorica microplate: a review

The Variscan belt of western Europe is part of a large Palaeozoic mountain system, 1000 km broad and 8000 km long, which extended from the Caucasus to the Appalachian and Ouachita mountains of northern America at the end of the Carboniferous. This system, built between 480 and 250 Ma, resulted from the diachronic collision of two continents: Laurentia–Baltica to the NW and Gondwana to the SE. Between these two continents, small, intermediate continental plates separated by oceanic sutures mainly have been defined (based on palaeomagnetism) as Avalonia and Armorica. They are generally assumed to have been detached from Gondwana during the early Ordovician and docked to Laurentia and Baltica before the Carboniferous collision between Gondwana and Laurentia–Baltica. Palaeomagnetic and palaeobiostratigraphic methods allow two main oceanic basins to be distinguished: the Iapetus ocean between Avalonia and Laurentia and between Laurentia and Baltica, with a lateral branch (Tornquist ocean) between Avalonia and Baltica, and the Rheic ocean between Avalonia and the so‐called Armorica microplate. Closure of the Iapetus ocean led to the Caledonian orogeny: a belt resulting from collision between Laurentia and Baltica, and from softer collisions between Avalonia and Laurentia and between Avalonia and Baltica. Closure of the Rheic ocean led to the Variscan orogeny by collision of Avalonia plus Armorica with Gondwana. A tectonic approach allows this scenario to be further refined. Another important oceanic suture is defined: the Galicia–Southern Brittany suture, running through France and Iberia and separating the Armorica microplate into North Armorica and South Armorica. Its closure by northward (or/and westward?) oceanic and then continental subduction led to early Variscan (430–370 Ma) tectonism and metamorphism in the internal parts of the Variscan belt. As no Palaeozoic suture can be detected south of South Armorica, this latter microplate should be considered as part of Gondwana since early Palaeozoic times and during its Palaeozoic north‐westward drift. Thus, the name Armorica should be restricted to the microplate included between the Rheic and the Galicia–Southern Brittany sutures.     

Paleozoic evolution of the External Crystalline Massifs of the Western Alps

The External Crystalline Massifs (ECMs) of the Alps record, during the Paleozoic, the progressive closure of oceanic domains between Gondwana, Armorica and Avalonia in three contrasting tectonic domains. The eastern one shows the Early Devonian closure of the Central-European Ocean between Armorica and Gondwana along a northwest dipping subduction zone. The western domain is marked by Lower Ordovician rifting followed by Mid-Devonian obduction of the back-arc Chamrousse ophiolite. The central domain underwent Late Devonian to Dinantian extension in a back arc setting associated with southeast dipping subduction of the Saxo-Thuringian Ocean. Based on tectonostratigraphic correlations, we propose that the western domain shows an affinity to the Barrandian domain while the eastern and central domains correspond to the north-eastward extension of the Moldanubian zone, to the south of the present-day Bohemian Massif. From Mid-Carboniferous to Permian, the eastern and central domains of the ECMs, including the internal parts of the Maures Massif, Sardinia and Corsica were stretched towards the south-west along the ca. 1500 km long dextral ECMs shear zone preceding the opening of the Palaeo-Tethys ocean.     

Geodynamic evolution of the European Variscan fold belt: palaeomagnetic and geological constraints

The Variscan fold belt of Europe resulted from the collision of Africa, Baltica, Laurentia and the intervening microplates in early Paleozoic times. Over the past few years, many geological, palaeobiogeographic and palaeomagnetic studies have led to significant improvements in our understanding of this orogenic belt. Whereas it is now fairly well established that Avalonia drifted from the northern margin of Gondwana in Early Ordovician times and collided with Baltica in the late Ordovician/early Silurian, the nature of the Gondwana derived Armorican microplate is more enigmatic. Geological and new palaeomagnetic data suggest Armorica comprises an assemblage of terranes or microblocks. Palaeobiogeographic data indicate that these terranes had similar drift histories, and the Rheic Ocean separating Avalonia from the Armorican Terrane Assemblage closed in late Silurian/early Devonian times. An early to mid Devonian phase of extensional tectonics along this suture zone resulted in formation of the relatively narrow Rhenohercynian basin which closed progressively between the late Devonian and early Carboniferous. In this contribution, we review the constraints provided by palaeomagnetic data, compare these with geological and palaeobiogeographic evidence, and present a sequence of palaeogeographic reconstructions for these circum-Atlantic plates and microplates from Ordovician through to Devonian times.     

The Anglo-Brabant Massif: Persistent but enigmatic palaeo-relief at the heart of western Europe

The surface geology of central England and Belgium obscures a large ‘basement’ massif with a complex history and stronger crust and lithosphere than surrounding regions. The nucleus was forged by subduction-related magmatism at the Gondwana margin in Ediacaran time. Partitioning into a platform, in the English Midlands, and a basin stretching to Belgium, in the east, was already evident in Cambrian/earliest Ordovician time. The accretion of the Monian Composite Terrane during the Penobscotian deformation phase preceded late Tremadocian rifting, and Floian separation, of the Avalonia Terrane from the Gondwana margin. Late Ordovician magmatism in a belt from the Lake District to Belgium records subduction beneath Avalonia of part of the Tornquist Sea. This ‘Western Pacific-style’ oceanic basin closed in latest Ordovician time, uniting Avalonia and Baltica. Closure of the Iapetus Ocean in early Silurian time was soon followed by closure of the Rheic Ocean, recorded by subduction along the southern margin of the massif. The causes of late Caledonian deformation are poorly understood and controversial. Partitioned behaviour of the massif persisted into late Palaeozoic time. Late Devonian and Carboniferous sequences show strong onlap onto the massif, which was little affected by crustal extension. Compressional deformation during the Variscan Orogeny also appears slight, and was focussed in the west where a wedge-shaped mountain foreland uplift was driven by orogenic indentation, splitting the massif from the Welsh Massif along the reactivated Malvern Line. Permian to Mesozoic sequences exhibit persistent but variable degrees of onlap onto the massif.     

West African provenance for Saxo-Thuringia (Bohemian Massif): Did Armorica ever leave pre-Pangean Gondwana? – U/Pb-SHRIMP zircon evidence and the Nd-isotopic record

Neoproterozoic rocks in the Saxo-Thuringian part of Armorica formed in an active margin setting and were overprinted during Cadomian orogenic processes at the northern margin of Gondwana. The Early Palaeozoic overstep sequence in Saxo-Thuringia was deposited in a Cambro-Ordovician rift setting that reflects the separation of Avalonia and other terranes from the Gondwana mainland. Upper Ordovician and Silurian to Early Carboniferous shelf sediments of Saxo-Thuringia were deposited at the southern passive margin of the Rheic Ocean. SHRIMP U/Pb geochronology on detrital and inherited zircon grains from pre-Variscan basement rocks of the northern part of the Bohemian Massif (Saxo-Thuringia, Germany) demonstrates a distinct West African provenance for sediments and magmatic rocks in this part of peri-Gondwana. Nd-isotope data of Late Neoproterozoic to Early Carboniferous sedimentary rocks show no change in sediment provenance from the Neoproterozoic to the Lower Carboniferous, which implies that Saxo-Thuringia did not leave its West African source before the Variscan Orogeny leading to the Lower Carboniferous configuration of Pangea. Hence, large parts of the pre-Variscan basement of Western and Central Europe often referred to as Armorica or Armorican Terrane Assemblage may have remained with Africa in pre-Pangean time, which makes Armorica a remnant of a Greater Africa in Gondwanan Europe. The separation of Armorica from the Gondwana mainland and a long drift during the Palaeozoic is not supported by the presented data.     

扬子区下、中奥陶统大湾组及其同期地层

华南扬子区不同地点奥陶纪的岩相、生物相变化显著。下、中奥陶统大湾组及其同期地层的总体走向为南西—北东向,横向上呈带状分布,从近岸到远岸呈渐变过渡。对四川长宁大官山组、贵州桐梓和沿河湄潭组以及湖北宜昌大湾组的比较研究发现,扬子区下、中奥陶统阿仑尼格期(Arenig)的地层按照其离岸的远近,在地层厚度、沉积相以及腕足动物分异度上呈现出规律性的变化。华南早、中奥陶世腕足动物的辐射首先发生在处于正常浅海底域环境的桐梓和沿河一带。     

东秦岭中部奥陶系-志留系界线地层及腕足动物群

东秦岭中部晚奥陶世和早志留世地层分布较广,化石较丰富,尤其是腕足类,分为寺岗组、石燕河组、刘家坡组和张湾组。曾庆銮等(1993)根据腕足类及其群落的更替,把石燕河组和刘家坡组归於早志留世,因而引起较大争论。本文据岩性将寺岗组和石燕河组分别改称为石燕河组下段和上段,并据腕足类化石认为石燕河组和刘家坡组应归於晚奥陶世、张湾组为早志留世;另据上述地层生物群落的特征及群落的更替,认为从石燕河组到刘家坡组,以及刘家坡组至张湾组恰好反映了全球冰期引起的晚奥陶世海退和早志留世冰期结束引起的海侵,故本区奥陶系-志留系界线宜划在刘家坡组和张湾组之间。     

The arenig in South Wales: Sedimentary and volcanic processes during the initiation of a marginal basin

Marine clastic sediments and volcanics of Arenig (Ordovician) age crop out in South Wales. These were deposited after presumed late Tremadoc erosion and subsequent arc volcanicity. Arenig sedimentation was transgressive, and followed significant erosion of the arc volcanics. Arenig conglomerates, sandstones, and mudstones were deposited in deltaic and turbiditic systems. Storm and tidal processes influenced the shallow marine deposits. The minor rhyolitic volcanics extruded during the Arenig reflect the development of Ordovician marginal basin-type volcanics across Wales. Five sandstone petrofacies are defined and reflect differing proportions of these volcanics and of Cambrian and Precambrian basement material. Sedimentation patterns were controlled by intra-Arenig tectonism during an overall rise in sea level. Facies and petrofacies were ponded in small, interconnected, marine sub-basins. Earliest Arenig tectonism and sedimentation, also recognized in North Wales, reflects the initiation of a marginal basin in Wales.     

滇东早奥陶世阿莫里坎砂岩相沉积

根据沉积特征、大化石、遗迹化石及微体化石等诸方面证据，认为滇东早奥陶世地层属于广泛分布于西南欧、北非、中东及纽芬兰东部的阿莫里坎砂岩相沉积。     

Palaeomagnetism of the Tan y Grisiau granite,North Wales: Evidence for a subvolcanic origin in Late Ordovician times

Progressive thermal demagnetization of samples from the Tan y Grisiau granite defines a coherent easterly positive characteristic remanence (D/I = 124.9/60.3°;, 42 samples, R = 40–51, a₉₅ = 4.8°;) residing in magnetite. An ancient reversal of magnetization is recovered in the highest blocking temperature spectrum of a few samples and suggests that a cooling-related dipolar axis is recorded by this pluton. Only facies of the granite which have been reddened, probably by submagmatic streaming, have recorded a stable remanence. Adjustment for tilt yields a very steep remanence (D/I = 193/88°;) incompatible with any known Early Palaeozoic and younger field direction from Britain. The in situ remanence has a similar declination to the primary magnetization in Late Ordovician dolerites from the Welsh Borderlands and yields a comparable palaeolatitude (41.5°;S). It is concluded that the Tan y Grisiau pluton was magnetized in Late Ordovician times after deformation. Folding in this region is therefore interpreted to be substantially of Taconic (Late Ordovician) origin and not Acadian in age. As both in situ and tilt-adjusted remanence directions are incompatible with Silurian and younger palaeofield directions from Britain, the pluton is interpreted as a subvolcanic component of the North Wales igneous province. Large anticlockwise rotation of Avalonia is identified between Late Ordovician and Late Silurian times.     

Gondwana-derived microcontinents — the constituents of the Variscan and Alpine collisional orogens

The European Variscan and Alpine mountain chains are collisional orogens, and are built up of pre-Variscan “building blocks” which, in most cases, originated at the Gondwana margin. Such pre-Variscan elements were part of a pre-Ordovician archipelago-like continental ribbon in the former eastern prolongation of Avalonia, and their present-day distribution resulted from juxtaposition through Variscan and/or Alpine tectonic evolution. The well-known nomenclatures applied to these mountain chains are the mirror of Variscan resp. Alpine organization. It is the aim of this paper to present a terminology taking into account their pre-Variscan evolution at the Gondwana margin. They may contain relics of volcanic islands with pieces of Cadomian crust, relics of volcanic arc settings, and accretionary wedges, which were separated from Gondwana by initial stages of Rheic ocean opening. After a short-lived Ordovician orogenic event and amalgamation of these elements at the Gondwanan margin, the still continuing Gondwana-directed subduction triggered the formation of Ordovician Al-rich granitoids and the latest Ordovician opening of Palaeo-Tethys. An example from the Alps (External Massifs) illustrates the gradual reworking of Gondwana-derived, pre-Variscan elements during the Variscan and Alpine/Tertiary orogenic cycles.     

Brachiopod survival and recovery from the latest Ordovician mass extinctions in South China

South China contains many complete sections through the upper Ordovician and lower Silurian. Brachiopod data including 130 brachiopod genera, assigned to 13 orders and 27 superfamilies from mid-Ashgill through late Aeronian intervals reveal that brachiopod macroevolution before and after the latest Ordovician mass extinction shows important changes in the diversity, composition and stratigraphical distribution of the phylum. The following six intervals are recognized: (1) a faunal plateau before the latest Ordovician mass extinction (mid-Ashgill, Rawtheyan); (2) a survival–recovery interval following the first phase of the mass extinction (late Ashgill, Normalograptus extraordinarius Zone and lower Glyptograptus? persculptus Zone; Hirnantian); (3) first survival interval following the mass extinction (latest Ashgill, upper Glyptograptus? persculptus Zone; end Hirnantian); (4) a second survival interval after the mass extinction (earliest Llandovery, Parakidograptus acuminatus Zone; early to mid-Rhuddanian); (5) a recovery interval in the Silurian (early to mid-Llandovery; late Rhuddanian to early Aeronian); and (6) a radiation interval in the Silurian (mid-Llandovery; mid- to late Aeronian). Only near-shore, low-diversity, benthic assemblages (mainly BA2), characterized by Ordovician relicts with a few Lazarus taxa and progenitors, are known from the southern marginal area of the Upper Yangtze epicontinental sea during the early to mid-Rhuddanian. They were replaced by newly established Silurian brachiopod communities (mainly BA2–3) in the late Rhuddanian to early Aeronian. These are marked by many newly evolved endemic forms and new immigrants, expressing a clear recovery within the Brachiopoda, but the recovery interval of the major brachiopod groups was heterochronous. In China the typical Silurian brachiopod fauna was mainly composed of indigenous Atrypida, Pentamerida and Spiriferida with stropheodontids derived from elsewhere, such as Baltica and Avalonia, two apparent refugia in the survival interval. The Atrypida was the first major group of Brachiopoda to diversity in the late Rhuddanian. Copyright © 1999 John Wiley & Sons, Ltd.     

西藏北部永珠地区石炭系永珠组岩石地层和古生物特征

永珠地区位于申扎古生代盆地,区内发育连续的早奥陶世—中二叠世海相沉积序列。石炭系永珠组岩石组合特征为细粒石英砂岩、页岩、粉砂岩,夹多层生物碎屑灰岩,含丰富的生物化石,主要有腕足类、双壳类、牙形刺、珊瑚、苔藓虫、菊石、三叶虫。牙形刺Gnathodusgirtyi—Gnathodustexanus为早石炭世晚期组合;腕足类Productus—Rhipidomellatibetena为早石炭世晚期组合,Choristites—Spirigerella为晚石炭世早期下部组合,Trigonatretacf.paucicostulata—Elivellabaschkirica为晚石炭世早期上部组合。Uncinunellina是典型的冷水习性腕足类,Cyathaxonia是典型的冷水习性珊瑚,表明了全球石炭纪冰期已影响到本区。确定了永珠组的时代为早石炭世晚期—晚石炭世早期,且早石炭世与晚石炭世分界线在第11层与第12层之间。     

A Cordilleran model for the evolution of Avalonia

Striking similarities between the late Mesoproterozoic–Early Paleozoic record of Avalonia and the Late Paleozoic–Cenozoic history of western North America suggest that the North American Cordillera provides a modern analogue for the evolution of Avalonia and other peri-Gondwanan terranes during the late Precambrian. Thus: (1) The evolution of primitive Avalonian arcs (proto-Avalonia) at 1.2–1.0 Ga coincides with the amalgamation of Rodinia, just as the evolution of primitive Cordilleran arcs in Panthalassa coincided with the Late Paleozoic amalgamation of Pangea. (2) The development of mature oceanic arcs at 750–650 Ma (early Avalonian magmatism), their accretion to Gondwana at ca. 650 Ma, and continental margin arc development at 635–570 Ma (main Avalonian magmatism) followed the breakup of Rodinia at ca. 755 Ma in the same way that the accretion of mature Cordilleran arcs to western North America and the development of the main phase of Cordilleran arc magmatism followed the Early Mesozoic breakup of Pangea. (3) In the absence of evidence for continental collision, the diachronous termination of subduction and its transition to an intracontinental wrench regime at 590–540 Ma is interpreted to record ridge–trench collision in the same way that North America's collision with the East Pacific Rise in the Oligocene led to the diachronous initiation of a transform margin. (4) The separation of Avalonia from Gondwana in the Early Ordovician resembles that brought about in Baja California by the Pliocene propagation of the East Pacific Rise into the continental margin. (5) The Late Ordovician–Early Silurian sinistral accretion of Avalonia to eastern Laurentia emulates the Cenozoic dispersal of Cordilleran terranes and may mimic the paths of future terranes transferred to the Pacific plate.This close similarity in tectonothermal histories suggests that a geodynamic coupling like that linking the evolution of the Cordillera with the assembly and breakup of Pangea, may have existed between Avalonia and the late Precambrian supercontinent Rodinia. Hence, the North American Cordillera is considered to provide an actualistic model for the evolution of Avalonia and other peri-Gondwanan terranes, the histories of which afford a proxy record of supercontinent assembly and breakup in the late Precambrian.