Avalonia Research Articles

Detrital Zircon Geochronology of Mesozoic Sediments in the Rif and Middle Atlas Belts of Morocco: Provenance Constraints and Refinement of the West African Signature

Despite the presence of a deep history of continental evolution from Archean to Recent and a wealth of outcrop, existing studies of detrital zircon geochronology in Morocco are limited, focused mostly in the Cambrian and Neoproterozoic Pan-African exposures in the Anti-Atlas belt. We broaden the use of this tool by surveying the detrital zircon geochronology of three stratigraphic units from three distinct Mesozoic terranes: the Tisiren unit of the Maghrebian flysch domain, the Ketama unit of the Intrarif, and the Bou Rached sandstones of the Middle Atlas Mountains. Samples were analyzed for zircon U-Pb ages using laser-ablation inductively coupled plasma mass spectrometry, and zircon fission-track (ZFT) ages were analyzed using the emerging technique of scanning electron microscope high-density fission-track analysis. In total, 259 U-Pb ages are presented, along with 60 ZFT ages. U-Pb ages reveal dominant Pan-African (Cryogenian-Ediacaran) and West African cratonic (Paleoproterozoic) signatures that are typical of northwest Africa. All samples also include significant Mesoproterozoic U-Pb populations that are hitherto unknown in Morocco and scarce in North Africa. Proposed scenarios for the provenance of these zircons include delivery to Morocco from the Amazonian craton or the African Volta basin during Ordovician glaciation and/or delivery from Avalonian terranes during the Variscan orogeny. The Bou Rached sandstones contain a significant Ordovician-Devonian U-Pb population not found in the other samples and are of an age without a viable source currently dated in Morocco. An Avalonian source is likewise suggested for these Ordovician-Devonian ages. The Ketama and Tisiren units have comparable U-Pb ages, but their preliminary ZFT cooling age distributions differ, lending support to the idea that different source regions are required for the two domains.

Effects of fluid flow, cooling and deformation as recorded by40Ar/39Ar, Rb–Sr and zircon fission track ages in very low- to low-grade metamorphic rocks in Avalonian SE Cape Breton Island (Nova Scotia, Canada)

Abstract40Ar/39Arin situUV laser ablation of white mica, Rb–Sr mineral isochrons and zircon fission track dating were applied to determine ages of very low- to low-grade metamorphic processes at 3.5±0.4 kbar, 280±30°C in the Avalonian Mira terrane of SE Cape Breton Island (Nova Scotia). The Mira terrane comprises Neoproterozoic volcanic-arc rocks overlain by Cambrian sedimentary rocks. Crystallization of metamorphic white mica was dated in six metavolcanic samples by40Ar/39Ar spot age peaks between 396±3 and 363±14 Ma. Rb–Sr systematics of minerals and mineral aggregates yielded two isochrons at 389±7 Ma and 365±8 Ma, corroborating equilibrium conditions during very low- to low-grade metamorphism. The dated white mica is oriented parallel to foliations produced by sinistral strike-slip faulting and/or folding related to the Middle–Late Devonian transpressive assembly of Avalonian terranes during convergence and emplacement of the neighbouring Meguma terrane. Exhumation occurred earlier in the NW Mira terrane than in the SE. Transpression was related to the closure of the Rheic Ocean between Gondwana and Laurussia by NW-directed convergence. The40Ar/39Ar spot age spectra also display relict age peaks at 477–465 Ma, 439 Ma and 420–428 Ma attributed to deformation and fluid access, possibly related to the collision of Avalonia with composite Laurentia or to earlier Ordovician–Silurian rifting. Fission track ages of zircon from Mira terrane samples range between 242±18 and 225±21 Ma and reflect late Palaeozoic reburial and reheating close to previous peak metamorphic temperatures under fluid-absent conditions during rifting prior to opening of the Central Atlantic Ocean.

U-Pb zircon geochronology of Roxbury Conglomerate, Boston Basin, Massachusetts: Tectono-stratigraphic implications for Avalonia in and beyond SE New England

High-precision CA-TIMS ²⁰⁶Pb/²³⁸U zircon dates from sandstone and Brighton igneous rocks associated with Roxbury Conglomerate in the Boston Basin, eastern Massachusetts provide constraints on the age and tectonic significance of these deposits. Detrital zircon suites from Roxbury-related sandstones representing, in ascending order, the Franklin Park Member (proposed name), the Brookline Member and the Squantum Member establish closely comparable maximum depositional ages of 595.14 ± 0.90, 598.87 ± 0.71 and 596.39 ± 0.79 Ma, respectively. The youngest of these is the best maximum age estimate of the conglomerate. Brighton dacite near the base of the Brookline Member and amygdaloidal andesite near the top yield respective crystallization ages of 584.19 ± 0.70 Ma and 585.37 ± 0.72 Ma. These virtually identical dates support previous interpretations of these particular units as shallow intrusions and thus represent minimum ages of associated conglomerate. The Roxbury-Brighton sequence is traditionally shown as inter-fingering northward with ≤570 Ma mudstone of the Cambridge “Argillite,” but the age range of the conglomerate makes this impossible. Conglomerate lacking quartzite clasts typical of the Roxbury proper and associated with 593.19 ± 0.73 Ma rhyolite on the south side of the basin is re-assigned to the Lynn-Mattapan Volcanic Complex which rests unconformably on Dedham Granite in basement exposed west and north of the basin as well. Calc-alkaline geochemistry and ages of the 609 to 584 Ma Dedham-Lynn-Mattapan-Brighton assemblage link it with 630 to 580 Ma arc sequences in other northern Appalachian Avalonian terranes. Roxbury Conglomerate accumulated in fault-bounded intra-arc basins near the end of this magmatic cycle.

Zircon oxygen isotopic constraints from plutonic rocks on the magmatic and crustal evolution of the northern Appalachians in southern New England, USA

Oxygen isotopes are a valuable tool for determining the role of supracrustal rocks in the genesis of magmas and help constrain the source regions of plutonism in the northern Appalachian mountains of North America. Zircons from 35 igneous or metaigneous rocks mainly from southern New England, USA, were analyzed for 18O/16O to locate and investigate the origin of anomalous 18O magmas and to evaluate the regional meteoric water-rock interaction. 18O values of the zircons obtained using the laser fluorination technique vary between 2.3‰ and 11.0‰, relative to Vienna Standard Mean Ocean Water (VSMOW). Zircons from two plutons in the Avalon terrane reveal the presence of anomalously low-18O (<4‰) zircons, lower than typical value for mantle-derived material, viz. 5.3‰ ± 0.6‰ (2). These rocks are the Ediacaran Hope Valley Plagioclase Gneiss of central Connecticut, and the Early Devonian Quincy Granite of eastern Massachusetts. The data provide evidence for meteoric water alteration of the Avalonian crustal magma sources. Our search for relatively high 18 O zircons identified several plutons containing zircons with values exceeding 8‰ located across the orogen. Oxygen and Hf isotopic compositions in zircons from three high 18O plutons, including an Early Devonian muscovite granite from Buckport, Maine, the Ediacaran Dedham Granite from eastern Massachusetts, and the Mesoproterozoic Tyringham Gneiss of western Massachusetts, as well as the Late Ordovician to Early Silurian Yale Farm Granite from northern Connecticut (bulk 18O zircon = 7.56‰), indicate the incorporation of significant amounts of high-18O sedimentary components from older source regions into these magmas. The Avalon and Gander terranes of southeastern New England, together with the eastern margin of Laurentia, have experienced prolonged histories of burial or subduction, melting, and assimilation of

Provenance, diagenesis and reservoir quality of the Upper Triassic Wolfville Formation, Bay of Fundy, Nova Scotia, Canada

Abstract The reservoir characterization and provenance of the Wolfville Formation were investigated using petrography, heavy minerals and microprobe analysis of tourmaline and garnet. Sandstone samples were taken from exposures at Rainy Cove and Cambridge Cove, and from the subsurface at Chinampas N-37 well beneath the Bay of Fundy. The surface and subsurface rocks have differences in their relative content and/or type of detritus, cement and heavy minerals (opaques, garnet, scheelite, tourmaline, rutile, apatite and others). These sandstones have continental block provenance in the subsurface rocks and recycled orogen provenance in the exposures. The main sources of the exposed Wolfville Formation sediments were the Palaeozoic rocks of the Meguma Supergroup, South Mountain Batholith, Horton and Windsor groups; meanwhile the subsurface sandstones might have been derived from the same sources or the Avalon Terrane and/or Gondwana. The sandstones were deposited during early stages of rifting post-dating earlier Palaeozoic collision orogenies that culminated with the Appalachian orogeny. The exposures of Wolfville Formation have low porosity ( c. 6%) that diminishes to negligible in the subsurface. The Wolfville Formation has a considerable thickness beneath the Bay of Fundy where it overlies the Horton Bluff Formation, Meguma and/or Avalon terranes. However, its reservoir potential is not encouraging.

Alleghanian deformation of Cambrian metasedimentary rocks on Avalonia in south-central Rhode Island, USA

Lower greenschist-facies metasedimentary rocks of the Middle Cambrian Conanicut Group occur in and around Beavertail State Park, Rhode Island. Detailed structural mapping (1:1000-scale) and petrology of these rocks indicate an early fold generation (F1) and axial planar metamorphic foliation (S1). F1 is folded by a more prominent, E-verging, NNE- to NNW-trending, non-coaxial fold generation (F2) and an associated pressure solution-enhanced crenulation cleavage (S2). A third map-scale fold generation is inferred from NNE-trending broad folding of F2 and S2. N-S extension resulted in boudins that deformed S2 on a scale of 1–10 m, whereas late planar quartz veins indicate NW-SE extension. All structures are cross cut by faults striking N- to NE- and ENE-to ESE that show dominantly normal motion with minor sinistral or dextral components. Kink bands associated with faulting trend NNE to ENE with WNW to NNW side up. The vertical Beaverhead shear zone juxtaposes the Cambrian rocks with Pennsylvanian rocks of the Narragansett Basin, and deflects S2 in a dextral sense, consistent with motion recorded elsewhere.The Cambrian rocks record the same deformation and metamorphism as the adjacent Narragansett Basin rocks. No evidence was found for pre-Alleghanian deformation or for northwest- or north-directed thrusting and accretion of a Meguma-like terrane during the Alleghanian orogeny. If the Beaverhead shear zone was a preexisting terrane boundary within Avalonia, both the Cambrian and Pennsylvanian Narragansett Basin sediments were deposited aſter terrane accretion.RÉSUMÉDes roches profondes métasédimentaires du faciès des schistes verts, que l’on retrouve dans le groupe Conanicut du Cambrien moyen, sont présentes dans le Beavertail State Park, au Rhode Island, et dans les environs. Une cartographie structurale détaillée (à l’échelle 1:1 000) et la pétrologie de ces roches indiquent la formation précoce d’un pli (F1) et une foliation métamorphique (S1) de plan axial. Le F1 est causé par la formation d’un pli (F2) non coaxial plus dominant, à vergence est et d’orientation NNE-NNO ainsi que par une schistosité de crénulation (S2) amplifiée en raison d’une dissolution par pression connexe. La formation d’un troisième pli à l’échelle cartographique est provoquée par un vaste plissement du F2 et de la S2 d’orientation NNE. Une extension N-S a produit des boudins qui déforment la S2 sur l’échelle de 1 à 10 m, tandis que des veines de quartz planes formées ultérieurement indiquent une extension NO-SE. Toutes les structures sont traversées par des failles orientées N-NE et ENE-ESE montrant un mouvement normal dominant accompagné de composantes senestres et dextres peu importantes. Les bandes froissées associées à ces failles sont orientées NNE-ENE et présentent une tangente verticale ONO-NNO. Dans la zone de cisaillement verticale de Beaverhead, les roches du Cambrien sont juxtaposées aux roches de la Pennsylvanie du bassin Narragansett, et la S2 dévie en un mouvement dextre, ce qui concorde avec le mouvement enregistré ailleurs.Les roches du Cambrien montrent la même déformation et le même métamorphisme que les roches du bassin Narragansett adjacent. On n’a trouvé aucune donnée appuyant la création d’une déformation avant l’orogenèse alléghanienne ni celle d’un chevauchement et d’une accrétion orientés vers le nord ou le nordouest d’un terrane semblable à la zone de Meguma lors de l’orogenèse alléghanienne. Si la zone de cisaillement verticale de Beaverhead constituait une limite de terrane qui existait avant l’orogenèse de l’Avalonien, les sédiments cambriens et pennsylvaniens du bassin Narragansett se sont déposés après l’accrétion du terrane.[Traduit par la redaction]

P-T-t conditions, Nd and Pb isotopic compositions and detrital zircon geochronology of the Massabesic Gneiss Complex, New Hampshire: Isotopic and metamorphic evidence for the identification of Gander basement, central New England

We present new evidence for the assignment of the Neoproterozoic Massabesic Gneiss Complex of New Hampshire to the Gander terrane rather than the Avalon terrane. The majority of Avalonian (*sensu stricto*) igneous and meta-igneous rocks as defined in Maritime Canada have positive whole-rock ɛ~Nd~ compared to more negative values for Gander rocks, although there is a region of overlap in ɛ~Nd~ between the two terranes. Our samples from areas in Connecticut previously thought to be Avalonian and samples from the Willimantic dome have the same isotopic signatures as Maritime Canada Avalon. In contrast, samples from the Clinton dome of southern Connecticut plots exclusively in the Gander field. The majority of the orthogneiss samples from Lyme dome (coastal Connecticut), Pelham dome (central Massachusetts) and Massabesic Gneiss Complex also plot in the Gander field, with a few samples plotting in the overlap zone between Gander and Avalon. U-Pb age distributions of detrital zircon populations from quartzites from the Massabesic Gneiss Complex more closely approximate the data from the Lyme Dome rather than Avalon. Additionally, the similarity of the P-T-t path for the rocks of the Massabesic Gneiss Complex (established by thermobarometry and ^40^Ar/^39^Ar dating of amphibole, muscovite, biotite and K-feldspar) with that established in the Ganderian Lyme dome of southern Connecticut strengthens the assignment of these rocks to a single Gander block that docked to Laurentia during the Salinic Orogeny. The identification of Ganderian isotopic signatures for these rocks all of which show evidence for Alleghanian metamorphism, supports the hypothesis that Neoproterozoic Gander lower crustal rocks underlie southern New Hampshire, Massachusetts, and Connecticut, and that all rocks of the overlying Central Maine trough that largely escaped high-grade Alleghanian metamorphism are allochthonous. We suggest that during the Alleghanian, the docking of Gondwana caused Avalon to wedge into Gander, metamorphosing and partially melting the Massabesic Gneiss Complex to the observed P-T-t conditions, with the complex forming an uplifted sheet that was back-thrusted over the Avalonian wedge.

Origin and evolution of the Havran Unit, Western Sakarya basement (NW Turkey): new LA-ICP-MS U-Pb dating of the metasedimentary-metagranitic rocks and possible affiliation to Avalonian microcontinent

One of the pre-Jurassic metamorphic basements of the Western Sakarya Zone (NW Turkey) is the Havran Unit which contains metasedimentary (i.e. the Kalabak formation) and metaintrusive rocks of Devonian age. An LA-ICP-MS U-Pb zircon dating study was conducted on 125 zircon grains obtained from a metasedimentary rock to better understand the origin and evolution of the Kalabak formation. Around 107 of 175 spot analyses are concordant with rates 90–110% and the zircon ages vary between 426 ± 5.2 and 3406.9 ± 195.7 Ma. Zircon populations cluster in ranges: 426 ± 5.2–535 ± 13 Ma (8.4%, Palaeozoic), 549 ± 14–999 ± 11 Ma (54.21%, Neoproterozoic), 1012 ± 200–1543 ± 50 Ma (6.54%, Mesoproterozoic), 1736 ± 123–2414 ± 58 Ma (18.7%, Palaeoproterozoic), and 2512 ± 30–3406.9 ± 195.7 Ma (12.15%, Archean). Dominating Neoproterozoic ages notate Cadomian-Avalonian terrains and Mesoproterozoic ages should mainly be related to Avalon terrains. The Karacabey metagranite consists of coarse-grained, equigranular, hypidiomorphic, and rarely foliated monzogranite and granodiorite. It is located to the north of the town of Karacabey in the eastern part of the Biga Peninsula. Main mineral assemblage is quartz, K-feldspar, plagioclase, and biotite. Zircon, apatite, monazite, magnetite, and sphene occur as accessory phases. Aluminum Saturation Index (ASI) values vary between 0.91 and 1.63, indicating the pluton as a metaluminous-peraluminous, I-type granite. The metagranite samples are plotted in the high-K-calcalkaline field on the SiO2 vs. K2O diagram. N-MORB-normalised spidergrams display a profound enrichment in the large-ion lithophile elements (LILE), light rare earth elements (LREE), and depletion in high field strength elements, suggesting that its genesis relates to hydrous melting of a mantle wedge in a subduction zone and/or melting of subduction zone-influenced source areas. Chondrite-normalised rare earth element spidergrams are indicative of the importance of plagioclase and amphibole fractionation. In tectonic discrimination diagrams, all of the metagranite samples fall into the volcanic arc granite and late post-collisional granite fields. U/Pb LA-ICP-MS zircon dating of the metagranite yielded an age of 400.3 ± 1.4 Ma (Early Devonian), interpreted as the age of igneous crystallisation. These age data show that the Karacabey metagranite is a member of the lower to mid Devonian granites, recently described from the Biga Peninsula. In light of these findings, we argue that the Havran Unit was an exotic terrain which possibly was once a part of far-travelled Avalon terrains and was attached to Sakarya Zone.

Cephalopod ancestry and ecology of the hyolith “Allatheca” degeeri s.l. in the Cambrian Evolutionary Radiation

Pyritized, elongate, conical conchs of “Allatheca” degeeri s.l. are common in dysoxic, dark gray mudstone intervals in the Early Cambrian (upper Terreneuvian–Series 2 boundary interval) Cuslett Formation at Keels, eastern Newfoundland. Wave-oriented, horizontal specimens are most abundant in this cool-water, high latitude, off-shore shelf facies of the Early Palaeozoic Avalon microcontinent. Based on conch morphology, shell microstructure, and the operculum, the species is an orthothecid hyolith. Comparison with the sizes of the early shells of planktic gastropods indicates a non-planktic life mode of “A.” degeeri s.l. hatchlings, although buoyancy calculations show that small juveniles with septate conchs to ca. 17mm long could have been nektic/planktic. If smaller “A.” degeeri s.l. individuals had a non-benthic mode of life, they and pseudoconodonts were the oldest skeletalized pelagic/nektic animals in the Cambrian Evolutionary Radiation. Most “A.” degeeri s.l. conchs at Keels are horizontally embedded and show a bimodal, wave-determined orientation, but about 10% of the large conchs are vertically embedded with their aperture down. As larger shells were not neutrally buoyant, the vertical orientations of about 10% of the conchs suggests an infaunal, likely detritivore, life mode suggestive of a scaphopod. Available morphologic and taphonomic evidence suggests that the vertically embedded conchs are in situ remains of dead benthic animals that colonized the bottom in better oxygenated intervals. Based on the current knowledge of Early Palaeozoic hyolith and cephalopod larval and adult morphologies, existing hypotheses of a planktic origin of cephalopods from hyolith ancestors are evaluated, and no evidence for such an evolutionary relationship is concluded to exist.

Microchemistry of amphiboles near the roof of a mafic magma chamber: Insights into high level melt evolution

The Late Neoproterozoic Greendale Complex, located within the Avalon terrane of Nova Scotia, is a suite of appinitic rocks ranging from ultramafic to felsic in composition that were intruded during regional ensialic arc magmatism and crystallized at shallow crustal levels under conditions of high pH2O. Amphibole is the dominant mafic mineral in ultramafic to mafic rocks and displays the extraordinary variability in texture and modal abundance that is characteristic of appinite suites. These features allow sensitivity of amphibole composition (major, trace and REE) to the evolution of water-rich magma to be investigated.All amphiboles in mafic and ultramafic rocks are calcic, with (Ca+Na)B≥1.34 and SiIV between 6.1 and 7.3. They predominantly range in composition from tschermakite to magnesiohornblende and display a dominance of edenite (Na,KA+AlIV=SiIV) substitution. Although each sample exhibits remarkably uniform Mg# over a wide range in Si of up to one formula unit, the mafic rock amphiboles are characterized by lower (0.5 to 0.7) Mg#, compared to the ultramafic rocks (0.7 and 0.9). REE profiles are bow-shaped, and are characterized by depletion in LREE (La/Sm≈0.61), a slight depletion in HREE (Gd/Yb≈1.55) as well as a negative Eu anomaly, which is attributed to co-precipitation of plagioclase. REE and trace element profiles of ultramafic amphiboles are divided into two groupings. Group A amphiboles occur in all specimens analyzed and their REE profiles are very similar to the whole-rock analyses of the mafic rocks and to those predicted from amphibole/melt partition coefficients. In contrast, Group B amphiboles display relative enrichment in light REEs (La/Sm≈2.05), have lower ΣREE, and lack a negative Eu anomaly relative to Sm and Gd. Group B amphiboles are more enriched in Th and U and show a more pronounced depletion in Nb, Ti, Y and HREE. Group B amphiboles probably grew in a reaction relationship with olivine and pyroxene, and their contrasting profiles suggest that partition coefficients traditionally used for petrogenetic modeling may not be appropriate for such amphiboles. Groups A and B are virtually indistinguishable with respect to the major elements used for amphibole classification, suggesting that REE and selected trace element data, when combined with textural observations, may provide important additional insights into phase equilibria and conditions of crystallization.

Stonehammer Geopark, New Brunswick, Canada

Stonehammer Geopark of New Brunswick, Canada is one of the newest destinations in the UNESCO‐assisted Global Geoparks Network, and the first in North America. Its rocks tell the amazing billion‐year story of the evolution of eastern North America. Highlights include some of the finest Precambrian stromatolites in the world, a Cambrian site that yielded one of the world's largest trilobites, and a Silurian section with a rich fauna of fish, crustaceans and sea scorpions. At the famous ‘Reversing Falls’ in Saint John, visitors can also see the boundary between the Late Precambrian Brookville Terrane and the western part of the Avalon Terrane, and learn about the late Silurian closure of the Iapetus Ocean during the final stages of the accretion of North America. In and around the city, Devonian pebbly sandstones represent the erosive remnants of the highlands formed as a result of that terrane accretion event, and slightly younger Carboniferous exposures further shed light on the evolution of life at a time when the region lay on the equator. These include early Carboniferous clubmoss forests preserved on the main highway east of Saint John, and late Carboniferous deposits rich in the remains of fossil plants, insects, giant arthropods, and some of the earliest reptiles—the latter recorded by spectacular trackways. Scattered Mesozoic outcrops and Quaternary moraines continue the story to the present day. Positioned at the gateway of Atlantic Canada, Stonehammer Geopark is a strategic part of an emerging geotouristic network that also includes UNESCO World Heritage Sites at nearby Misguasha, Quebec (Devonian coastal ecosystems) and Joggins, Nova Scotia (Pennsylvanian fossil forests). It presents a brilliant opportunity to teach visitors of the extraordinary story of Earth's evolution.

U-Pb Ages of Detrital Zircons from the Permo-Triassic Series of the Iberian Ranges: A Record of Variable Provenance during Rift Propagation

The provenance of the Permo-Triassic series of the Talayuelas anticline (Iberian Ranges) have been studied using U-Pb geochronology (laser ablation inductively coupled plasma mass spectrometry) of detrital zircons. These intracontinental siliciclastic series were formed by extensive sandy braided fluvial systems associated with ephemeral lake deposits and aeolian sediments, with paleocurrents suggesting constant NW-SE transport directions. Upper Permian reddish sandstones from the Upper Alcotas Formation (Lopingian) contain a dominant Variscan zircon population (290–360 Ma), which indicates source areas located in the axial zone of the Variscan belt, in the core of the Ibero-Armorican arc. However, in the Lower Triassic sandstones of the Cañizar Formation (Olenekian), the Variscan zircon population is almost completely replaced by Cadomian zircons (520–750 Ma), with important Avalonian (390–520 Ma), Mesoproterozoic (900–1750 Ma), Eburnian (1.78–2.35 Ga), and post-Eburnian and Archaean (>2.4 Ga) zircon populations. This detrital zircon content now suggests source areas located more to the NW, in the Avalonian microcontinent, although a limited supply coming from the southern part of Laurentia cannot be ruled out. Finally, in the Middle Triassic (Anisian), the source areas returned to the Variscan axial zone, since the Variscan zircon population is again highly dominant during this period. The changes detected in the source areas of the Permo-Triassic series are related to the development and propagation of the Iberian rift, one of the large extensional structures that determined the generation of the sedimentary basins and finally caused the breakup of Pangea. The methodology followed in this article is very useful to understand the generation and evolution of these intracontinental basins and also the relationships between the different rift systems generated in the North Atlantic realm during the Permo-Triassic times.

Palaeozoic Formations from Dobrogea and Pre-Dobrogea – An Overview

An overview of lithological, palaeontological and geochronological evidence existing for the Palaeozoic formations from Dobrogea and PrDobrogea has enabled a better understanding of the Palaeozoic history of these areas. The Lower Palaeozoic of Pre-Dobrogea, in places in continuity with the pelitic-silty facies of the underlying Vendian (Ediacaran) deposits, was one of the peri-Tornquist basins of Baltica, suggesting that the Scythian Platform in the Pre-Dobrogea basement represents the rifted margin of the East European Craton. In North Dobrogea two types of Palaeozoic succession have formed in different tectonic settings. Deep marine Ordovician-Devonian deposits, including pelagic cherts and shales, associated with turbidites, and facing Devonian carbonate platform deposits of the East European Craton, form northward-younging tectonic units of an accretionary wedge, tectonically accreted above a south-dipping subduction zone. South of the accretionary prism, the basinal to shallow marine Silurian-Devonian deposits of North Dobrogea, showing a similar lithology to the East Moesian successions, accumulated on top of low-grade Cambrian clastics with Avalonian affinity indicated by detrital zircons. Late Palaeozoic erosion was accompanied by deposition of continental alluvial, fluvial and volcano-sedimentary successions, overlying their basement above an imprecise Carboniferous gap. The low-grade metamorphic Boclugea terrane, showing Avalonian affinity, and the associated Lower Palaeozoic deposits represent East Moesian successions, docked to Baltica by the Lower Devonian and subsequently involved in the Hercynian orogeny, being affected by Late Carboniferous-Early Permian regional metamorphism and granite intrusion. The Late Carboniferous-Early Permian syn-tectonic sedimentation, regional metamorphism of Palaeozoic formations and development of a calc-alkaline volcano-plutonic arc indicate an active plate margin setting and an upper plate position of the Macin-type successions during the Variscan collision, when the Orliga terrane, with Cadomian affinity, was accreted to Laurussia along a north-dipping subduction zone of the Rheic Ocean. The East Moesian Lower Palaeozoic succession, overstepping its Ediacaran basement, represents an Avalonian terrane, docked to the Baltica margin in the Early Palaeozoic. A narrow terrane detached from the Trans-European Suture Zone (TESZ) margin of the Baltica palaeocontinent forms a tectonic wedge within the East Moesian basement. The Palaeozoic sedimentary record of East Moesia shows a quartzitic facies in the Ordovician, graptolite shales in Upper Ordovician-Wenlock, black argillites in the Ludlow-Pridoli and fine-grained clastics in the Lower Devonian. Eifelian continental sandstones are followed by a carbonate platform from Givetian to Tournaisian times and coal-bearing clastics in the Carboniferous, indicating a foredeep basin evolution. By the Eifelian both East Moesia and Pre-Dobrogea were part of Laurussia, sharing the same old red sandstone facies. The Permian is a time of rifting in Dobrogea and Pre-Dobrogea, although evidence for rifting in the East Moesian sedimentary record is very limited. In the eastern basins of Pre-Dobrogea, Permian rifting was accompanied by alkaline bimodal volcanism of the basalt-trachyte association, that affected also the northern margin of North Dobrogea. Late Permian within-plate alkaline magmatic activity emplaced plutonic and hypabyssal complexes along the south-western margin of North Dobrogea. The model proposed for the Palaeozoic history based on existing data for the north-western margin of the Black Sea records early Palaeozoic docking to Baltica of the Avalonian terrane of East Moesia, including the Boclugea terrane of North Dobrogea. Late Carboniferous-Early Permian accretion of the Cadomian Orliga terrane from North Dobrogea, accompanied by Hercynian metamorphism and granite intrusion, correlates with the closure of the Rheic Ocean. Subsequently, Avalonian and Cadomian terranes, together with a narrow terrane detached from the TESZ margin of Baltica palaeocontinent, were displaced southward along the strike-slip fault system of the TESZ.

Tectonic significance of Late Ordovician silicic magmatism, Avalon terrane, northern Antigonish Highlands, Nova Scotia 1 This article is one of a series of papers published in CJES Special Issue: In honour of Ward Neale on the theme of Appalachian and Grenvillian geology.2 Contribution to International Geological Correlation Programme (IGCP) Project 497.

Avalonia was a microcontinent during most of the Ordovician, separating the Iapetus Ocean to the north from the Rheic Ocean to the south. In the northern Antigonish Highlands, Nova Scotia, volcanic rocks (Dunn Point and McGillivray Brook formations) underlie Early Silurian – Early Devonian strata (Arisaig Group) and were thought to represent extension that heralded the development of the basin into which Arisaig Group strata were deposited. However, recent U–Pb (zircon, thermal ionization mass spectrometry) data from rhyolite in the Dunn Point Formation (DPF) yielded an age of 460.0 ± 3.4 Ma, and, here, we report a concordant age of 454.5 ± 0.7 Ma for an ignimbrite in the overlying McGillivray Brook Formation (MBF). These data confirm a ca. 10 million year gap between volcanism and onset of Arisaig Group deposition, which occurred after accretion of Avalonia to Baltica. The DPF and MBF both resemble A-type SiO2-rich magmas. The MBF has very high concentrations of Zr (745–1965 ppm), Y (65–213 ppm), Nb (57 to 185 ppm), and high Ga/Al. Several MBF samples exhibit strong LREE depletion, consistent with fractionation of LREE-bearing accessory phases. εNdt values for MBF (t = 455 Ma) range from +1.5 to +3.9, and overlap with DPF rhyolites that range from +2.9 to +3.7. Depleted mantle model ages for MBF and DPF samples unaffected by accessory phase fractionation are between 0.9 and 1.2 Ga and are similar to TDM values in older (Neoproterozoic, Cambrian) crustally derived felsic rocks, suggesting derivation from the lower crustal basement beneath the Antigonish Highlands. DPF and MBF rocks were probably erupted in a local extensional environment within an ensialic arc, perhaps analogous to the modern Taupo Volcanic Zone in northern New Zealand.

Avalonian perspectives on Neoproterozoic paleogeography: Evidence from Sm-Nd isotope geochemistry and detrital zircon geochronology in SE New England, USA

Arc-related Northern Appalachian Avalonian terranes developed in the ocean surrounding Rodinia beginning around 760 Ma, so paleogeographic constraints on these terranes have implications for the margins of Rodinia itself in Neoproterozoic time. The first Sm-Nd isotopic data together with new detrital zircon ages from SE New England suggest that at least this segment of Avalonia contains rocks with contributions from crustal sources in Baltica, rather than Amazonia as commonly argued. Eight samples from 610 to 600 Ma Dedham, Dedham North, Milford, Fall River, Esmond, and Westwood granites in SE New England yield e Nd (600) values from −1.2 to +2.2 and depleted mantle model ages ranging from 2.2 Ga to 1.0 Ga. The younger model ages, reflecting largely juvenile magmatic components, are typical of other Avalonian terranes in Atlantic Canada. The oldest model ages can be produced by mixing juvenile mantle extractions with 10%-15% Baltic crust. The isotopic composition of Baltic crust in these calculations is proxied by Svecofennian metasedimentary rocks with detrital zircon suites comparable to those obtained from younger than 912 Ma quartzite of the Westboro Formation intruded by the Dedham Granite north of Boston (Massachusetts). An Avalonia-Baltica connection is also consistent with available paleomagnetic data placing both areas at moderate south paleolatitude between ca. 615 Ma and ca. 575 Ma during Ediacaran deposition of their respective Squantum-Gaskiers and Varanger glacial deposits.

Tremadocian (Lower Ordovician) sea-level changes and biotas on the Avalon microcontinent

The Chesley Drive Group, an Upper Cambrian-Lower Ordovician mudstone-dominated unit, is part of the Ediacaran–Ordovician cover sequence on the North American part of the Avalon microcontinent. The upper Chesley Drive Group on McLeod Brook, Cape Breton Island (previously “McLeod Brook Formation”), has two lithofacies-specific Tremadocian biotas. An older low-diversity benthic assemblage (shallow burrowers,Bathysiphon, phosphatic brachiopods, asaphid trilobites) is in lower upper Tremadocian green-gray mudstone. This wave-influenced, slightly dysoxic facies hasBathysiphon–brachiopod shell lags in ripple troughs. The upper fauna (ca. 483 +/- 1 Ma) is in dysoxic-anoxic (d-a), unburrowed, dark gray-black, upper upper (but not uppermost) Tremadocian mudstone with a “mass kill” of the olenidPeltocare rotundifrons(Matthew)—a provincial trilobite in Avalonian North America that likely tolerated low oxygen bottom waters.Scandodus avalonensisLanding n. sp. andLagenochitinaaff.conifundus(Poumot), probable nektic elements and the first upper Tremadocian conodont and chitinozoan reported from Avalon, occur in diagenetic calcareous nodules in the dark gray-black mudstone. An upper Tremadocian transition from lower greenish to upper black mudstone is not exposed on McLeod Brook, but is comparable to a coeval green-black mudstone transition in Avalonian England. The successions suggest that late late Tremadocian (probable Baltic Hunnebergian Age) sea level was higher in Avalon than is suggested from successions on other paleocontinents. The Tremadocian sea-level history of Avalon was a shoaling-deepening-shoaling sequence from d-a black mudstone (lower Tremadocian), to dysoxic green mudstone (lower upper Tremadocian), and back to black mudstone (upper upper Tremadocian).ScandodusLindström is emended, with the early speciesS. avalonensisLanding n. sp. assigned to the emended Family Protopanderodontidae.TriangulodusVan Wamel is considered a junior synonym ofScandodus. Peltocare rotundifronsis emended on the basis of complete specimens.

Combined U–Pb geochronology and Lu–Hf isotope systematics by LAM–ICPMS of zircons from granites and metasedimentary rocks of Carrazeda de Ansiães and Sabugal areas, Portugal, to constrain granite sources

The Carrazeda de Ansiães and Sabugal areas in the Central Iberian Zone of the Iberian Variscan Belt contain abundant S-type granites formed during the post-thickening stage of the Variscan orogeny (syn- to post-D3, i.e. the last ductile deformation phase). Magmatic zircons and inherited zircon cores from two granites (G and G5), magmatic zircons from three other granites (G1, G4 and G6) and detrital zircons from a chlorite phyllite and a metagraywacke were dated by LAM–ICPMS. Hf isotope data were obtained by the same method to constrain the sources for granites. Some granites have zircons with negative εHft values (G: from −8 to −4; G1: from −8 to −1; G4: from −6 to −3), suggesting an origin from heterogeneous crustal anatectic melts or, from mantle-derived magmas contaminated in the crust. Magmatic zircons from granites G5 and G6 show a larger variation, yielding also some positive εHft values (G5: from −8 to +6; G6: from −4 to +4) indicating crystallization from mildly contaminated mantle-derived magmas or from a lower crustal source. U–Pb isotope data of inherited zircon cores from granites G and G5 show that Neoproterozoic ages dominate, but only the inherited zircon cores from granite G have similar initial 176Hf/177Hf ratios to those from a chlorite phyllite of the Formation of Ervedosa do Douro (Carrazeda de Ansiães area), suggesting that this metasedimentary unit could have been involved in the genesis of the granite G. On the other hand, the Lu–Hf data suggest that the metagraywacke from Sabugal area should not be considered a component involved in the partial melting process that formed the granite G5. The occurrence of Mesoproterozoic detrital zircons in both metasediments suggests that these rocks are likely to have evolved from a peri-Amazonian realm, close to West Avalonian terranes, during the Neoproterozoic/Early Palaeozoic. However, a West African Craton provenance could also be considered, if a long distance river transportation of Mesoproterozoic zircons, from the southwest Baltica or Arabian–Nubian Shield, is admitted. The U–Pb isotope data obtained in detrital zircons from both metasedimentary rocks suggest that their maximum depositional ages are ca. 578Ma for the chlorite phyllite and ca. 577Ma for the metagraywacke.

Linguliform brachiopods from the terminal Cambrian and lowest Ordovician of the Oaxaquia microcontinent (Southern Mexico)

Eighteen taxa of linguliform brachiopods, mainly represented by acrotretoids, are reported from the Upper Cambrian (Furongian, Stage 10) and Lower Ordovician (Tremadocian) Tiñu Formation of Oaxaca State, Mexico. At the time of deposition, this area was part of Oaxaquia, which was either a microcontinent or an integral part of the Gondwanan margin. Whereas certain trilobites seem to indicate a Gondwanan affinity, the Tiñu brachiopod faunas show a less definite paleogeographic relationship. Some taxa have previously only been reported from Laurentia (Eurytreta cf. fillmorensis, Eurytreta cf. campaniformis), and one taxon is best known from the Avalon microcontinent (Eurytreta cf. sabrinae). However, the relatively high percentage of new and potentially endemic taxa (Oaxaquiatreta labrifera n. gen. n. sp., Tapuritreta reclinata n. sp., Oaxaquiatreta sp., Eurytreta? n. sp., Acrotretidae n. gen. n. sp., Obolinae gen. and sp. indet.) and the lack of other typical Laurentian, Gondwanan, or Avalonian taxa suggest either a certain degree of insularity of Oaxaquia or reflects a more temperate, unrestricted marine environment during the Early Paleozoic. Other taxa reported from the Tiñu Formation include Semitreta sp., Lingulella? spp., Obolinae gen. and sp. indet., Eoscaphelasma? sp., Ottenbyella? sp. A and sp. B, and Acrotretidae gen. and sp. indet. A, B, and C. Eurytreta and Semitreta are critically reviewed and several taxa previously assigned to them have been excluded. An emended diagnosis for the genus Eurytreta is presented. The presence of delthyrium and notothyrium-like structures in the siphonotretid Oaxaquiatreta n. gen. further strengthens the previously proposed relationship between the Siphonotretida and Paterinida.

Chapter 42 The deep-marine glaciogenic Gaskiers Formation, Newfoundland, Canada

Abstract In eastern Canada, the Neoproterozoic Gaskiers Formation (Fm.) consists of a thick diamictite–turbidite succession (250–300 m thick) that occurs within the deep-marine predominantly volcaniclastic turbidite units of the Conception Group (4–5 km thick). These rocks are well exposed on the coast of the Avalon Peninsula in eastern Newfoundland. The thick succession is considered by some to represent the final Neoproterozoic glacial event, known as the Gaskiers Glaciation c. 582–585 Ma. The Gaskiers Fm. appears to have accumulated in a volcanically-influenced arc-related basin within the confines of the Proterozoic peri-Gondwanan Avalon terrane, before rifting away from the Gondwanan margin in the early Palaeozoic, but it is not clear whether it was adjacent to the West African Craton or the Amazon Craton. It has also been argued that Avalon had already rifted from Gondwana by the late Proterozoic. Glacially striated clasts, dropstones, and chatter-marked garnets identify a glaciated source area, and clasts and matrix are of volcanic origin; pyroclastic flows and volcanic bombs are also present. As a result, there is a growing consensus that the Gaskiers Fm. records local glaciation of a high relief volcanic topography, owing its origin to the episodic downslope reworking of volcanic and glacial debris into a deep, rapidly subsiding basin. The volcanic nature of these deposits implies that local volcanic activity was coeval with deposition. A minimum age of the Gaskiers Fm. is constrained by a diverse assemblage of Ediacaran-type fossils in the upper Conception Group dated at c. 565±3 Ma and in the lower St. John's Group, which overlies the Conception Group. A maximum age for the Gaskiers is provided by a U–Pb date of 606 +3.7/–2.9) Ma in the Harbour Main volcanic rocks underlying the Gaskiers Fm. A thin limestone bed has also been identified at two localities in Conception Bay directly overlying the diamictite yielding strongly negative δ 13 C values.

Early Jurassic magmatism on the northern margin of CAMP: Derivation from a Proterozoic sub-continental lithospheric mantle

Recent studies have shown that the sub-continental lithospheric mantle (SCLM) can either underlie regions for a long period of time or be replaced during delamination or plume magmatism. The occurrence and timing of mantle replacement may be identified by comparing the Sm-Nd isotopic signature of suites of basaltic rock within the same terrane over a range of time. In mainland Nova Scotia, the early Jurassic North Mountain Basalt (NMB) is widely recognized as the northernmost representative of the Central Atlantic Magmatic Province (CAMP), but it could also have been derived from the underlying SCLM (Avalonian) mantle. The Sm-Nd isotopic data from the NMB yield εNdt values (t = 200 Ma) ranging from + 0.1 to − 2.7. These values are significantly lower than the contemporary depleted mantle and indicate derivation from the underlying Proterozoic SCLM, rather than from the CAMP plume. Comparison with older mafic magmatic events indicates that the SCLM is isotopically indistinguishable from that inferred to yield Ordovician and Devonian mafic magmas in the Meguma terrane and Neoproterozoic to Devonian magmas in the Avalon terrane. These data indicate that the same SCLM underlay mainland Nova Scotia from Neoproterozoic to Jurassic times, and that Paleozoic tectonic events did not significantly detach the mantle from the overlying crust.