peri-Gondwanan Realm Research Articles

Cuartel Ophiolite: Structure, timing and exhumation mechanisms for a Cadomian suture zone in the peri-Gondwanan Realm (SW Iberia)

The Cadomian Orogeny formed as an accretionary orogen surrounding Gondwana. The structure resulting from the Cadomian Orogeny is pervasively reworked during the Paleozoic, within the frame of the Variscan Cycle. In SW Iberia (Mina Afortunada Massif), a tectonometamorphic and geochronological analysis revealed two Cadomian deformation phases. The first phase (DC1; 586 Ma, U-Pb dating in inherited metamorphic garnet from Mina Afortunada Gneiss) is associated with ophiolite accretion during the closure of a back-arc or intra-arc basin (Cuartel Ophiolite). A metamorphic fabric related to this phase is preserved as an internal foliation in Ediacaran (meta)sedimentary rocks. The second phase (DC2; 515–485 Ma) is constrained by the age of the igneous protolith of Mina Afortunada Gneiss (∼515 Ma), which is affected by a penetrative foliation formed during DC2, and by the unconformable deposit of Ordovician sediments, which are not affected. The telescoping of metamorphic isograds during DC2 and the geometry of detachment faults associated with this phase suggest extensional tectonics as the driving mechanism for the early exhumation of the Cadomian suture zone in Mina Afortunada Massif. The superimposition of Variscan folds and shear zones onto the Cadomian structures contributed to the subsequent exhumation of the Cadomian suture zone. The analysis and reconstruction of Cadomian and Variscan structures, plus the geochronological data, allow us to correlate the Cuartel Ophiolite with the Mérida Ophiolite, being two pieces of the same, yet dismembered, Cadomian suture zone. The number of Cadomian suture zones in Gondwana does not coincide with that of Cadomian suture zone exposures. Reworked orogens such as the Cadomian could be subjected to duplication of their suture zones. Assessments regarding the Cadomian paleogeography should consider the multiplier effect of the structural overprints after ophiolite accretion.

Updated geochronology and isotope geochemistry of the Vila de Cruces Ophiolite: a case study of a peri-Gondwanan back-arc ophiolite

Abstract In the Variscan Orogen, the NW Iberian Massif exposes a variety of ophiolites formed at c. 500 and c. 400 Ma that provide constraints on the Paleozoic evolution of the NNW African margin of Gondwana which culminated in the assembly of Pangaea. New U–Pb ages obtained in zircon from one of these ophiolites, the Vila de Cruces Ophiolite, confirm the previous U–Pb age of c. 500 Ma (thermal ionization mass spectrometry (TIMS)) obtained in zircon from a tonalitic orthogneiss. New samples of the same orthogneiss and related gabbros provided ages (laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS)) in the range 498–492 Ma, with the metagabbros being the youngest. Moreover, two different gabbro samples contain a scattered population of inherited zircon grains with an average age of c. 1150 Ma. These zircons probably represent xenolitic material included in the gabbros during their ascent and intrusion along the margin of Gondwana. Taking into account the architecture of the Vila de Cruces Ophiolite and its geochemical composition, this ophiolite is interpreted as a section of oceanic or transitional lithosphere formed in a back-arc basin opened in the peri-Gondwanan realm. The presence of inherited Mesoproterozoic zircons is likely to suggest the existence of a hidden unexposed Mesoproterozoic basement in the Gondwana margin during Paleozoic times.

100 myr cycles of oceanic lithosphere generation in peri-Gondwana: Neoproterozoic–Devonian ophiolites from the NW African–Iberian margin of Gondwana and the Variscan Orogen

Abstract The Variscan Orogen in Iberia and the Anti-Atlas Mountains in Morocco contains a set of ophiolites formed between Neoproterozoic and Devonian times, during the complex evolution of the NW African–Iberian margin of Gondwana. During this time interval, the margin evolved from an active margin ( c. 750–500 Ma: the Reguibat–Avalonian–Cadomian arc) to the final collision with Laurussia in Devonian times to form Pangaea. In this context, one of the oldest recognized ophiolites is the Bou Azzer Ophiolite from the Anti-Atlas Mountains, dated at c. 697 Ma and containing two types of mafic rocks, the youngest of which has a boninitic composition. To the north, in the SW Iberian Massif, the Calzadilla Ophiolite contains mafic rocks also of boninitic composition dated at c. 598 Ma. Farther north, in the NW Iberian Massif, the Vila de Cruces Ophiolite is formed by a thick sequence of mafic rocks with an arc tholeiitic composition and minor alternations of tonalitic orthogneisses dated at c. 497 Ma. In the same region, the Bazar Ophiolite has a similar age of c. 495 Ma. Also in NW Iberia, there is a group of ophiolites with varied lithologies and dominant mafic rocks with arc tholeiitic composition (Careón, Purrido and Moeche ophiolites) dated at c. 395 Ma. The composition of all these peri-Gondwanan ophiolites is of supra-subduction zone type, showing no evidence of preserved mid-ocean ridge basalt type oceanic lithosphere. Consequently, these ophiolites were generated in the peri-Gondwanan realm during the opening of forearc or back-arc basins. Forearc oceanic lithosphere was promptly obducted or accreted to the volcanic arc, but the oceanic or transitional lithosphere generated in back-arc settings was preserved until the assembly of Pangaea. Based on the ages of the described ophiolites, the peri-Gondwanan realm has been a domain where the generation of oceanic or transitional lithosphere seems to have occurred at intervals of c. 100 myr. These regularly spaced time intervals may indicate cyclic events of mantle upwelling in the peri-Gondwanan mid-ocean ridges, with associated higher subduction rates at the peri-Gondwanan trenches and concomitant higher rates of partial melting in the mantle wedges involved. The origin of the apparent cyclicity for mantle upwelling in the peri-Gondwanan ocean ridges is unclear, but it could have possibly been related to episodic deep mantle convection. Cycles of more active deep mantle convection can explain episodic mantle upwelling, the transition from low- to fast-spreading type mid-ocean ridges and, finally, the dynamic context for the episodic generation of new supra-subduction zone type oceanic peri-Gondwanan lithosphere.

Geochemistry, geochronology and petrogenesis of Maya Block granitoids and dykes from the Chicxulub Impact Crater, Gulf of México: Implications for the assembly of Pangea

The Late Paleozoic tectono–magmatic history and basement of the Maya block are poorly understood due to the lack of exposures of coeval magmatic rocks in the region. Recently, IODP–ICDP Expedition 364 recovered drill core samples at borehole M0077A from the peak ring of the Chicxulub impact crater, offshore of the Yucatán peninsula in the Gulf of México, have been studied comprehensively. In the lowermost ~600 m of the drill core, impact–deformed granitoids, and minor felsite and dolerite dykes are intercalated with impact melts and breccias. Zircon U-Pb dating of granitoids yielded ages of around 326 ± 5 Ma, representing the first recovery of Late Paleozoic magmatic rocks from the Maya block, which could be genetically related to the convergence of Laurentia and Gondwana. The granitoids show the features of high K2O/Na2O, LaN/YbN and Sr/Y ratios, but very low Yb and Y contents, indicating an adakitic affinity. They are also characterized by slightly positive ԑNd(326Ma) of 0.17–0.68, intermediate initial 87Sr/86Sr(326Ma) of 0.7036–0.7047 and two–stage Nd model age (TDM2) of 1027–1069 Ma, which may indicate a less evolved crustal source. Thus, the adakitic granitoids were probably generated by partial melting of thickened crust, with source components similar to Neoproterozoic metagabbro in the Carolina block (Pan–African Orogeny materials) along Peri–Gondwana. Felsite dykes are shoshonitic with typical continental arc features that are sourced from a metasomatic mantle wedge by slab–fluids. Dolerite dykes display OIB–type features such as positive Nb and Ta anomalies and low ThNpm/NbNpm. In our interpretation, the Chicxulub adakitic granitoids of this study are formed by crustal anatexis due to asthenospheric upwelling resulting from slab breakoff. Through comparing sources and processes of Late Paleozoic magmatism along the Peri–Gondwanan realm, a tearing slab breakoff model may explain the discontinuous magmatism that appears to have occurred during the convergence of Laurentia and Gondwana.

Plant Diversity of The Mid Silurian (Lower Wenlock, Sheinwoodian) Terrestrial Vegetation Preserved in Marine Sediments from The Barrandian Area, The Czech Republic

Plant mega- and microfossils are described from the middle Sheinwoodian of the Barrandian area. The material comes from the Loděnice locality and the same horizon as the earliest unequivocal land plant, Cooksonia barrandei LIBERTÍN, J.KVAČEK, BEK, ŽÁRSKÝ et ŠTORCH. Its age (432 Myr) is inferred from the associated graptolite fauna, including the zonal index graptolite Monograptus belophorus. Megafossils have clear similarity with Cooksonia, due to their dichotomised axes with slightly widened subtending axes bearing putative sporangia. They document some of the plant diversity that was in place when the first proven representative of the genus Cooksonia appeared, and together with dispersed spores they provide strong and important evidence that a diversified terrestrial ecosystem had developed on the Barrandian volcanic archipelago in the peri-Gondwanan realm by the end of the Sheinwoodian Stage of the Silurian Period.

Recycling of the Proterozoic crystalline basement in the Coastal Block (Moroccan Meseta): New insights for understanding the geodynamic evolution of the northern peri-Gondwanan realm

Detrital zircon age spectra from the siliciclastic rocks of the Lalla Mouchaa Calcschists and El Jadida Dolomitic formations (the Coastal Block of the Moroccan Meseta) are dominated by Paleoproterozoic and Ediacaran ages. The provenance of these two formations is a composite Proterozoic crystalline basement. El Jadida rhyolite (584.2 ± 4.8 Ma) represents the Ediacaran crystalline basement of the El Jadida dome. El Jadida rhyolite is unconformably overlain by the microbreccia, arkosic sandstone and dolostone of the El Jadida Dolomitic Formation with a maximum depositional age of ca. 539 Ma (Lower Cambrian). Detrital zircon-age spectra from El Jadida Dolomitic Formation (ca. 583–582 Ma) suggest direct recycling of El Jadida rhyolite as an exclusive original primary source. However, in the Western Rehamna massif, detrital zircon-age spectra from pre-Middle Cambrian microbreccia and arkosic sandstone of the Lalla Mouchaa Calcschists Formation (ca. 2.05–2.03 Ga) indicate exclusive recycling of the ca. 2.05 Ga-aged crystalline basement rocks (original primary source). Detrital zircon contents of the siliciciclastic rocks from these two formations of the Coastal Block are consistent with derivation from either Eburnian (Paleoproterozoic) or Cadomian/Pan-African (Ediacaran) igneous rocks. The discovery of this composite Proterozoic crystalline basement in the Moroccan Meseta stresses that Cadomian/Pan-African magmatic arcs were built on an Eburnian basement in a paleoposition close to the West African craton, as part of the northern peri-Gondwanan realm.

Palynology, microfacies and biostratigraphy across the Daleje Event (Lower Devonian, lower to upper Emsian): new insights from the offshore facies of the Prague Basin, Czech Republic

The Zlichovian/Dalejan boundary interval (Emsian, Lower Devonian) of the Pekarek Mill section was studied employing biostratigraphy (dacryoconarid tentaculites, conodonts) and palynology (chitinozoans, prasinophytes, scolecodonts) and microfacies analysis in order to shed more light on the timing and characteristics of the Daleje Event. The results of our study stress the great importance of the base of the Nowakia elegans Zone for the substage level division of the Emsian. Onset of the Daleje transgression is linked with higher terrigenous input, and coinciding changes in the chitinozoan assemblages were recorded at this level. The transgression at the base of the N. elegans Zone preceded the main transgression taking place in the N. cancellata Zone; it can be correlated with the Upper Zlichov Event. For the first time, Emsian chitinozoans and a jawed polychaete fauna are described in detail from the Prague Basin and can be correlated with other northern Gondwanan regions. The family-level composition of scolecodont assemblage confirms the dominance of paulinitids in the peri-Gondwanan realm.

Cambrian–Ordovician orogenesis in Himalayan equatorial Gondwana

An early Paleozoic tectonic event, the Kurgiakh orogeny, has long been known from the western Tethyan Himalaya, and it is conspicuously recorded by an angular unconformity between Cambrian marine shelf deposits and coarse Ordovician conglomerate, as well as widespread granitic plutons. Although the lateral extent of this event is poorly known, two regions in the central and eastern Himalaya, the Kumaon of India and klippen of Bhutan, contain conglomerate units that may be correlative with this event. In the Kumaon, the Ralam Formation, which contains a basal polymictic conglomerate unit with detrital zircon grains as young as ca. 512 Ma, is overlain by sandstone containing the arthropod walking trace Diplichnites gouldi , an ichnospecies known from Ordovician and younger strata. The late early Cambrian trilobite Redlichia sp., recovered from the underlying Martoli Group, indicates that the conglomerate is post−early Cambrian and, based on stratigraphic analysis, also likely records the Kurgiakh orogeny. In central Bhutan, the lower Paleozoic succession contains conglomerate units with monomictic quartz sandstone clasts and less mature sandstone matrix. These units have also been assigned to a variety of ages, but rocks that apparently underlie these strata contain the Furongian (late Cambrian) zonal trilobite Kaolishania granulosa . Here, we show that detrital zircon ages of a conglomerate clast (no grains younger than = ca. 781 Ma) are distinctly older than those of the matrix, which contains a large peak at 490−500 Ma. Thus, these conglomerate units are also potential lateral correlatives of the Ordovician conglomerate deposits of the western Himalaya. Our paleontological and geochronologic data suggest that the Kumaon and Bhutan conglomerate units, and possible equivalents in Tibet, correlate with those of the western Himalaya. This increases their known along-strike extent by over 1000 km, bringing the total extent to ∼1200 km. Thus, the Kurgiakh orogeny spanned much of the Himalayan orogen along the northern Gondwanan margin and reflected a widespread tectonic reorganization of the peri-Gondwanan realm to Andean-type margins. Recent work indicates that it also likely extended into the Lhasa block and both the North and South China blocks. This event was associated with widespread ca. 500 Ma arc-related plutonism recorded in the Himalaya, and with abundant detrital zircon grains of that age in younger Himalaya strata. Based on current fossil data, the duration of the hiatus associated with the Cambrian-Ordovician boundary, which separates pre- and postdeformational strata, ranges from 22 to 36 m.y. Available geochronological data do not provide greater constraint. If the event was close in age to the depositional age of the Ordovician molasse, then the main phase of tectonic uplift, erosion, and deposition may have been much later than the presumed Cambrian-Ordovician boundary interval age of ca. 485 Ma.

Geochemistry of the Ediacaran–Early Cambrian transition in Central Iberia: Tectonic setting and isotopic sources

A complete Ediacaran–Early Cambrian stratigraphic transition can be observed in the southern part of the Central Iberian Zone (Iberian Massif). Two different stratigraphic units, underlying Ordovician series, display geochemical and Sm–Nd isotopic features in agreement with an evolving geodynamic setting. Pusa Shales (Early Cambrian) rest unconformably on greywackes of the Lower Alcudian Formation (Late Ediacaran). Both sequences present minor compositional variations for major and trace element contents and similar REE patterns, close to those of PAAS (Post Archean Australian Shale). Trace element contents and element ratios suggest mixed sources, with intermediate to felsic igneous contributions for both units. Tectonic setting discrimination diagrams for the Ediacaran greywackes indicate that these turbiditic series were deposited in a sedimentary basin associated with a mature active margin (volcanic arc). However, the compositions of the Cambrian shales fit better with a more stable context, a back-arc or retro-arc setting. εNd(T) and TDM ages are compatible with dominance of a similar cratonic source for both sequences, probably the West Africa Craton. εNd565 values for the Ediacaran greywackes (−3.0 to −1.4) along with TDM ages (1256–1334Ma) imply a significant contribution of juvenile material, probably derived from the erosion of the volcanic arc. However, εNd530 values in the Cambrian shales (−5.2 to −4.0) together with older TDM ages (1444–1657Ma), suggest a higher contribution of cratonic isotopic sources, probably derived from erosion of the adjacent mainland. Coeval with the progressive cessation of arc volcanism along the peri-Gondwanan realm during the Cambrian, there was a period of more tectonic stability and increasing arrival of sediments from cratonic areas. The geochemistry of the Ediacaran–Cambrian transition in Central Iberia documents a tectonic switch in the periphery of Gondwana, from an active margin to a more stable context related to the onset of a passive margin.

Lower Ordovician (Arenig) shallow-marine trace fossils of the Pochico Formation, southern Spain: palaeoenvironmental and palaeogeographic implications at the Gondwanan and peri-Gondwanan realm

Nineteen ichnospecies belonging to thirteen ichnogenera (Archaeonassa, Catenichnus,Cochlichnus, Cruziana, Didymaulichnus, ?Diplichnites, Gordia, Lingulichnus, Lockeia, cf.Monocraterion, Planolites, Ptychoplasma, and Rusophycus) occur in the Pochico Formation(Arenig) in the Aldeaquemada section, Sierra Morena, southern Spain, just above the ArmoricanQuartzite. They belong to the archetypal Cruziana ichnofacies, indicating a lower shoreface-upperoffshore zone. The low degree of sediment reworking may be due to a high rate of sedimentation.The trace fossil assemblage, rich in large Cruziana, is typical of the Armorican Quartzite thatdeveloped on the margins of Gondwana and peri-Gondwanan microcontinents. The distribution ofichnofauna during the Early Ordovician was partly palaeogeographically controlled, althoughichnological data from the literature point to paths of migration between Gondwana, Baltica andLaurentia. Differences between the ichnofauna of Gondwana and Baltica could be conditioned byfacies (clastics in Gondwana and carbonates in Baltica) causing a taphonomic filter, becauseCruziana requires diversified clastic deposits for preservation. The ichnofauna would also beinfluenced by trophic group amensalism between filter feeding and deposit feeding fauna, theformer prevailing in Baltica and the latter in Gondwana.

A paleogeographical review of the peri-Gondwanan realm of the Appalachian orogen1This article is one of a series of papers published in this CJES Special Issue: In honour of Ward Neale on the theme of Appalachian and Grenvillian geology.

The eastern edge of the Appalachian orogen is composed of a collection of Neoproterozoic – early Paleozoic domains, Avalonia, Carolinia, Ganderia, Meguma, and Suwannee, which are exotic to North America. Differences in the geological histories of these peri-Gondwanan domains indicate that each separated independently from Gondwana, opening the Rheic Ocean in their wake. Cambrian departure of Ganderia and Carolina was followed by the Ordovician separation of Avalonia and Silurian separation of Meguma. After separation in the early Paleozoic, these domains constituted the borderline between the expanding Rheic Ocean and contracting Iapetus Ocean. They were transferred to Laurentia by early Silurian closure of Iapetus and Devonian–Carboniferous closure of the Rheic Ocean during the assembly of Gondwana and Laurentia into Pangaea. The first domain to arrive at Laurentia was Carolinia, which accreted in the Middle Ordovician during the Cherokee orogeny. Salinic accretion of Ganderia occurred shortly thereafter and was followed by the Acadian accretion of Avalonia. The Acadian orogeny was immediately followed by Middle Devonian – Early Carboniferous accretion of Meguma and possibly Suwannee which led to the Fammenian orogeny. The episodicity of orogeny suggests that the present location of these domains parallels their order of accretion. However, each of these crustal blocks was translated along strike by large-scale Late Devonian – Carboniferous dextral strike–slip motion. The breakup of Pangaea occurred outboard of the Paleozoic collision zones that accreted Carolinia, Ganderia, Avalonia, Meguma, and Suwannee to Laurentia, leaving these terranes appended to North America during the Mesozoic opening of the Atlantic.

Truncation and translation of Appalachian promontories: Mid-Paleozoic strike-slip tectonics and basin initiation

Accreted terranes of the Appalachian Iapetan and peri-Gondwanan realms display structural trends that are mainly concordant with promontories and embayments in the Laurentian margin, indicating that during accretion, the shape of the continental margin acted as a template around which accreted terranes were molded. In North Carolina and New-foundland, post-accretion transcurrent motion appears to be recorded by displaced outboard portions of promontories, no longer concordant with those in Laurentia. A bend in structural trends confined to the peri-Gondwanan realm at the North Carolina–South Carolina state line is interpreted to represent the dextrally displaced outboard portion of the Virginia promontory. In Newfoundland, the Hermitage flexure is interpreted as a dextrally displaced Laurentian promontory that originated along strike to the northeast. In both places, promontories were truncated and dextrally translated for ~220–250 km by a Devonian–Mississippian orogen-parallel transcurrent system, which may well have extended for the length of the eastern Laurentian margin. South of the nearby St. Lawrence promontory, extreme thinning of Appalachian crust beneath the Maritimes Basin is consistent with extension at a releasing stepover. Estimates of extension are consistent with those obtained from promontory offsets.

Artinskian–Wordian antitropical rugose coral associations: A palaeogeographical approach

Recent investigations in the Urals and East-European Platform as well as study of corals collections from South Primorye (Russian Far East) and South-Eastern Pamirs have been undertaken. Antitropical rugose associations include taxa that are bounded in their distribution by the temperate zones of the Boreal and Perigondwanan realms. They occur sporadically in tropical latitudes but never form abundant and diverse communities. Antitropical associations are represented by the so-called ‘ Cyathaxonia fauna’ (sensu lato), predominating in two selected stratigraphic intervals. The abrupt change of taxonomic composition from colonial to ‘ Cyathaxonia fauna’ allows the separation of two assemblages: (i) mid Artinskian–Kungurian and (ii) Roadian–Wordian. The key regions for comparison are the Urals (mid Artinskian–Kungurian), Spitsbergen (Kungurian–Wordian), East-European Platform (Roadian), Texas (mid Artinskian–Kungurian), South Primorye (Wordian–Capitanian), South-Eastern Pamirs (Upper Artinskian), and the Sverdrup Basin (Wordian). The Late Artinskian fauna of the South-Eastern Pamirs and Wordian faunas of South Primorye bear features of mixed assemblages. A few models of the origination of antitropical assemblages are acceptable: (i) the convergent parallel evolution of Lophophyllidium in the Northern Hemisphere and Southern Hemisphere, (ii) a similar evolution trend developed in parallel in three geographically distant regions: Verbeekiella, Pseudowannerophyllum, and “ Pseudoverbeekiella”, and (iii) immigration of some species within temperate zones caused by current circulation changes both in the Northern and Southern Hemispheres, as well as northward spreading of terranes in the Perigondwanan Realm.

Significance of Provenance Ages from the Chiapas Massif Complex (Southeastern Mexico): Redefining the Paleozoic Basement of the Maya Block and Its Evolution in a Peri-Gondwanan Realm

Medium- to high-grade metasedimentary rocks are exposed as isolated domains in mostly metaigneous crystalline rocks of the Chiapas Massif Complex (CMC), which forms the basement of the southern Maya Block, southeastern Mexico. Laser ablation multicollector inductively coupled plasma mass spectrometry U-Pb isotope analyses on inherited detrital zircon cores show two principal age distributions: (a) Group I, with its highest peak at 500–650 Ma and smaller peaks at ∼380–400 Ma, 1.0–1.2 Ga, 1.5–2.0 Ga, and 2.6–3.1 Ga, and (b) Group II, with its highest peak at 1.0–1.2 Ga, a minor peak at 1.5–1.6 Ga (or ∼1.0-Ga zircon cores only), and a lack of 500–650-Ma zircons. The cores are commonly surrounded by metamorphic or anatectic overgrowths, and some zircon ages were reset by a tectonothermal event at ∼250 Ma. The age distribution of zircon cores from Group I metasediments are similar to detrital zircon ages in Carboniferous-Permian sediments (Santa Rosa Formation) in southeastern Chiapas, which were shed mostly from continental crust dominated by Pan-African–Brasiliano orogenic cycles. Group II metasediments have different sources, principally from Grenville orogens, such as the Oaxacan Complex, and early mid-Proterozoic sources, such as the Rio Negro–Juruena Province of western Amazonia. The lowest stratigraphic unit in the CMC (the Jocote Unit) is intruded by lower Ordovician S-type granite and contains only 1.5–1.6-Ga and older detrital zircons. The protoliths of the CMC can be correlated with Paleozoic strata across the southern Maya Block, and they show parallels with the geologic history and provenance patterns of protoliths from the Acatlán Complex (Mixteca Terrane) of central southern Mexico. The geologic history of the CMC and the Maya Block can be interpreted in terms of tectonothermal events accompanying accretion of the CMC and polarity reversal during formation of the western Pangea margin.

A comparative analysis of pre-Silurian crustal building blocks of the northern and the southern Appalachian orogen

The New York promontory serves as the divide between the northern and southern segments of the Appalachian orogen. Antiquated subdivisions, distinct for each segment, implied that they had lithotectonic histories that were independent of each other. Using new lithotectonic subdivisions we compare first order features of the pre-Silurian orogenic 'building blocks' in order to test the validity of the implication of independent lithotectonic histories for the two segments. Three lithotectonic divisions, termed here the Laurentian, Iapetan, and the peri-Gondwanan realms, characterize the entire orogen. The Laurentian realm, composed of native North American rocks, is remarkably uniform for the length of the orogen. It records the multistage Neoproterozoic-early Paleozoic rift-drift history of the Appalachian passive margin, formation of a Taconic Seaway, and the ultimate demise of both in the Middle Ordovician. The Iapetan realm encompasses mainly oceanic and magmatic arc tracts that once lay within the Iapetus Ocean, between Laurentia and Gondwana. In the northern segment, the realm is divisible on the basis of stratigraphy and faunal provinciality into peri-Laurentian and peri-Gondwanan tracts that were amalgamated in the Late Ordovician. South of New York, stratigraphic and faunal controls decrease markedly; rock associations are not inconsistent with those of the northern Appalachians, although second-order differences exist. Exposed exotic crustal blocks of the peri-Gondwanan realm include Ganderia, Avalonia, and Meguma in the north, and Carolinia in the south. Carolinia most closely resembles Ganderia, both in early evolution and Late Ordovician-Silurian docking to Laurentia. Our comparison indicates that, to a first order, the pre-Silurian Appalachian orogen developed uniformly, starting with complex rifting and a subsequent drift phase to form the Appalachian margin, followed by the consolidation of Iapetan components and ending with accretion of the peri-Gonwanan Ganderia and Carolinia. This deduction implies that any first-order differences between northern and southern segments post-date Late Ordovician consolidation of a large portion of the orogen.

Age determination of Ordovician magmatism in NE Sardinia and its bearing on Variscan basement evolution

A newly discovered pre-Variscan cover–basement relationship in NE Sardinia is defined by differences in structural and metamorphic histories. The oldest metamorphic fabrics in the basement are absent from a series of metaplutons that are inferred to be comagmatic with metavolcanic rocks in the cover. Cover and basement are further distinguished by contrasting metapelite compositions. Superimposed metamorphism is represented by pre-Variscan garnet and Variscan plagioclase porphyroblasts that appear in the field as a ‘mixed’ garnet + albite zone. In situ U–Pb zircon dating of meta-igneous rocks from cover and basement by laser-ablation inductively coupled plasma-mass spectrometry gives ages of 458 ± 7 Ma for the Tanaunella orthogneiss, 456 ± 14 Ma for the Lodè orthogneiss, and 474 ± 13 Ma for the Lula porphyroid. The internal Sardinian Variscides thus incorporate a mid-Ordovician magmatic belt developed upon an older basement; correlation is proposed with a tectonostratigraphic domain that is at present exposed in the Alps. Within the mid-Ordovician peri-Gondwanan realm, the internal Sardinian Variscides appear to have developed along an active margin, whereas the foreland correlates with the Ibero-Aquitaine domain and is related to the evolution of a passive margin during rifting and detachment of Avalonia to form the Rheic Ocean.

The utility of late Early to Middle Cambrian small shelly fossils from the western Mediterranean

New data on Cambrian small shelly fossils from Germany and Sardinia, and Spain are discussed with respect to their stratigraphical, paleogeographical, and paleoecological value. It is shown that these small shelly assemblages represent very useful tools for reconstruction of geological processes and of the Perigondwanan history in the Mediterranean area, especially, if trilobites are absent or hard to recover from the rocks. The German and Sardinian assemblages reflect distinct ecological conditions on shallow platform and shelf areas and mirror any changes in these habitats. Thus, lateral and vertical transitions from restricted inner platform to deep subtidal conditions are documented by characteristic successions of Sardinian assemblages. German assemblages show clearly distinct paleogeographic/ecological developments. The reorganization of the ecosystems is linked to shelf-wide processes which can be stratigraphically correlated in their single phases (e.g., drowning of platforms, breakup of the Perigondwanan realm, origin of terranes). Broader paleogeographic relationships of small shelly fossils to the Far East and Australia are proposed and support earlier controversial assumptions based on trilobites. Because of the small shelly fossils distribution patterns, a pre-Late Cambrian separation of terranes from the so called European shelf of western Gondwana is unlikely. During the Early to Middle Cambrian a rather uniform and weakly differentiated facies belt across most of the European shelf is indicated, and this contradicts earlier concepts of more-or-less isolated depositional basins. Despite recent progress in knowledge of Mediterranean small shelly fossil assemblages, further improvements will lead to a better understanding and a much more detailed picture of the Cambrian history of Perigondwana and its relationships to other paleocontinents.

The Appalachian peri-Gondwanan realm: a palaeogeographical perspective from the south

Abstract The Appalachian peri-Gondwanan realm (APGR) is an extensive tract of exotic Neoproterozoic-early Palaeozoic crustal blocks that occupies the eastern flank of the orogen. Traditionally, southern APGR elements have been correlated with those of the northern Appalachians on the basis of gross geological similarities. Most palaeogeographical reconstructions of the APGR are based on data from the northern Appalachians; consequently in these reconstructions, southern APGR elements are viewed commonly either as being affiliated spatially with those of the north or ignored. However, emerging data from two southern Appalachian crustal blocks give new insights into the palaeogeography of the APGR. The Smith River allochthon may be a part of the APGR on the basis of recently obtained U-Pb monazite and staurolite ages that are apparently incompatible with a Laurentian origin. The allochthon and possibly adjacent terranes, appear to have followed a palaeogeographical track independent of other APGR elements. The Carolina zone is recognized as peri-Gondwanan in origin on the basis of its (i) gross geological evolution, (ii) fossil fauna and (iii) tectonic history. Mid-Palaeozoic regional kinematic patterns suggest that Carolina and its commonly held northern counterpart, the Avalon zone, travelled together on the same lithospheric plate, but their contrasting tectonic histories suggest that they formed along different margins of this plate. These interpretations lead to a new model for middle Palaeozoic interactions of the APGR with Laurentia.

First finding of Late Silurian conodonts from the "Orthoceras Limestones", Camdag Area (NW Turkey): prelilninary constraints for the palaeogeography

The Paleozoic succession of the Camdag area is located between the Paleozoic rocks of the Istanbul and Zonguldak terranes in NW Turkey and has a key location for the paleogeographic position of these terranes. The Silurian deposits in this area includes from bottom to top: the Black Shale Member (gray to greenish gray, well cleaved shales with minor black siltstone and limy shale interlayers), the Shale-Limestone Member (black shales with limestone and dolomitic limestone interlayers), and the Shale-Si Itstone Member (alternation of black shales, light gray quartz-rich siltstones with few limestone lenses) that is concordantly followed by Lower Devonian deposits. The "Orthoceras Limestones" of the midc\le member yielded a poorly preserved conodont fauna indicating a Pridoli age. By this, the Paleozoic of the Camdag is considered to be the eastern continuation of the Istanbul Terrane. Moreover, this new finding of Late Silurian black shale - "Orthoceras Limestone" assemblage in NW Turkey, which is a wide spread paleogeographic marker in the Camic Alps and elsewhere in Europe may help to correlations with coeval and analogous facies of other paleobiogeographic areas in the peri-Gondwanan realm.