Early Palaeozoic Times Research Articles

Neoproterozoic to Palaeozoic tectonic deformation history of the western Jiangnan Orogen, South China: Insights from new structural and geochronological data from northern Guangxi

Recognition of deformation events documented in Neoproterozoic sedimentary sequences of the Jiangnan Orogen is critical in understanding the tectonic evolution of South China. In this contribution, we present new field based structural observations, microstructural analysis, and geochronological data for the Sibao and Danzhou groups in the southwestern Jiangnan Orogen of northern Guangxi, South China. Integrating the new information with previously published structural and geochronological data, we have identified three major deformation events. The earliest event generated E–W‐trending high‐angle tight linear and overturned isoclinal folds in the Sibao Group with their fold axial planes dipping to the south. This shortening event occurred in the mid‐Neoproterozoic time (840–800 Ma) and was associated with the orogenic collision between the Yangtze and Cathaysian blocks in South China. The angular unconformity between Sibao and Danzhou groups provides strong evidence for the collision. The second deformation event produced the large‐scale NNE–SSW‐striking ductile shear zones in the region. It also overprinted the pre‐existing E–W‐trending folds, refolded the Sibao Group, and generated the large‐scale NNE–SSW‐trending open folds in the overlying Danzhou Group and the west‐dipping cleavage within both groups. The second event occurred during the early Palaeozoic time (460–410 Ma) as a result of a WNW–ESE‐oriented intracontinental shortening process. The latest deformation event generated slight crenulation cleavage in the Sibao Group, some folds in the marble units of the Danzhou Group, and normal‐sense shearing along the pre‐existing NNE–SSW‐striking shear zones, all of which are the result of a NW–SE‐directed crustal extension during the late Palaeozoic (410–350 Ma) Kwangsian (Caledonian) orogenic collapse.

Detrital zircon U–Pb ages of metasedimentary rocks from the Neoproterozoic Zhoutan Group in the northern Cathaysia Block (South China): Provenance and tectonic implications

ABSTRACT The Neoproterozoic Zhoutan Group, located at the conjunction area between the Yangtze and the Cathaysia Blocks, preserves an important record of the Neoproterozoic sedimentary system of the South China Block. However, its accurate depositional age, provenance, and tectonic affinity remain poorly studied. Here, we report new LA-ICP-MS detrital zircon U–Pb age data of four meta-sedimentary samples from the Zhoutan Group in the Nancheng area for complemental decipherment of its sedimentary and tectonic histories. Concordant detrital zircon ages reveal proportions of 54% latest Mesoproterozoic to early Neoproterozoic (ca. 1050–820 Ma), 24% early Paleoproterozoic (ca. 2500–2150 Ma), and 22% late Paleoproterozoic to early Mesoproterozoic (ca. 2000–1500 Ma) populations. Youngest detrital zircon age clusters exhibit a consistent weighted mean age of ca. 827 Ma, suggesting the maximum Tonian depositional age of the Zhoutan Group. Provenance interpretations reveal that the detritus of investigated samples were mainly sourced from the proximal Jiangnan Orogen with some additionally derived from the Cathaysia Block, the interior Yangtze Block, and/or extraneous terranes. In addition, the appearance of some later early Palaeozoic-aged (ca. 437 Ma) metamorphic rims and grains suggest that the Zhoutan Group has locally experienced an intense metamorphism during the early Palaeozoic (Silurian) time, which has been consistently recorded by the Neoproterozoic meta-strata in the Cathaysia Block. These new data, together with those from previous studies, indicate that the Zhoutan Group belongs to the early Neoproterozoic sedimentary successions between the Yangtze and the Cathaysia Blocks. Subsequently, it might be thrusted onto the present side of the Cathaysia Block during later tectonic processes and was affected by locally metamorphism during the Silurian time. Thus, such observations suggest that the potential border between the Yangtze and the Cathaysia Blocks should be to the north of the current position of the Zhoutan Group.

Palaeozoic palaeogeography of the East European Craton (Poland) in the framework of global plate tectonics

Global palaeogeographic maps were constructed for eight time intervals in the Palaeozoic. The maps contain information concerning plate tectonics and palaeoenvironment during the Cambrian, Ordovician, Silurian, Devonian and Carboniferous. The East European Craton belonged to the Palaeozoic Baltica Plate, which originated as a result of disintegration of the supercontinent Pannotia during the early Cambrian. Baltica included part of Poland and adjacent areas northeast of a line that extends between Scania and the Black Sea. This plate was located in the Southern Hemisphere and drifted northward during Early Palaeozoic time. The Early Ordovician was the time of maximum dispersion of continents during the Palaeozoic. Avalonia probably started to drift away from Gondwana and moved towards Baltica during Ordovician time. Between Gondwana, Baltica, Avalonia and Laurentia, a large longitudinal oceanic unit, known as the Rheic Ocean, was formed. Avalonia was probably sutured to Baltica by the end of the Ordovician or in the Early Silurian. This process was dominated by the strike-slip suturing of the two continents, rather than a full-scale continent-continent collision. Silurian was a time of Caledonian orogeny, closing of the Early Palaeozoic oceans, collision of Baltica with Avalonia and Laurentia and the assembly of the supercontinent Laurussia. The Variscan orogeny in Poland was caused by the collision of the Bohemian Massif plates and the Protocarpathian terrane with Laurussia. The Protocarpathian terrane acted as an indentor that caused thrust tectonics in the East European Platform, Holy Cross Mountains and the Lublin area.

Continuity of the North Qilian and North Altun orogenic belts of NW China: evidence from newly discovered Palaeozoic low-Mg and high-Mg adakitic rocks

AbstractIn this study, the petrology, zircon U–Pb ages, Lu–Hf isotopic compositions, whole-rock geochemistry and Sr–Nd isotopes for newly recognized low-Mg and high-Mg adakitic rocks from the North Altun orogenic belt were determined. The results will provide important insights for understanding the continuities of the North Qilian and North Altun orogenic belts during early Palaeozoic time. The low-Mg adakitic granitoids (445 to 439 Ma) are characterized by high SiO2(69–70 wt %), low Mg no. (43–48) and low Cr and Ni contents. In contrast, the high-Mg adakitic granitoids (425 to 422 Ma) have relatively lower SiO2(65–67 wt %), higher Mg no. (60–62) and higher Cr and Ni contents. The low-Mg adakitic rocks have high initial87Sr/86Sr ratios (0.7073–0.7084), negative εNd(t) (−1.9 to −4.0) and εHf(t) values (−6.8 to −2.0), and old zircon Hf model ages (1.4–1.7 Ga). In contrast, the high-Mg adakitic rocks show lower initial87Sr/86Sr ratios (0.7044–0.7057), higher εNd(t) (−0.7 to 3.1) and positive εHf(t) values (2.0 to 6.9), with younger zircon Hf model ages (0.9–1.2 Ga). These results suggest that the low-Mg adakitic rocks were probably generated by the partial melting of thickened crust, whereas the high-Mg adakitic rocks were derived from the anatexis of delaminated lower crust, which subsequently interacted with mantle magma upon ascent. The data obtained in this study provide significant information about the geological and tectonic processes after the closure of the Altun Ocean. The continent–continent collision and thickening probably occurred during 450–440 Ma with the formation of low-Mg adakitic rocks, and the transition of the tectonic regime from compression to extension probably occurred at 425–422 Ma with the formation of high-Mg adakitic rocks. The geochemical, geochronological and petrogenetic similarities between the North Altun and North Qilian adakitic rocks suggest that these two orogenic belts were subjected to similar tectonomagmatic processes during early Palaeozoic times.

Detrital zircon U–Pb geochronology and Hf isotope geochemistry of metasedimentary strata in the southern Brooks Range: constraints on Neoproterozoic–Cretaceous evolution of Arctic Alaska

Abstract Mid-Palaeozoic assembly models for the Arctic Alaska–Chukotka microplate predict the presence of cryptic crustal sutures, the exact locations and deformational histories of which have not been identified in the field. This study presents data on the provenance of polydeformed and metamorphosed strata in the southern Brooks Range Schist Belt and Central Belt of presumed Proterozoic–Devonian depositional age, as well as for the structurally overlying strata, to help elucidate terrane boundaries within the Arctic Alaska–Chukotka microplate and to add new constraints to the palaeogeographical evolution of its constituent parts. The protoliths identified support correlations with metasedimentary strata in the Ruby terrane and Seward Peninsula and suggest a (peri-) Baltican origin in late Neoproterozoic–early Palaeozoic time. Proximity to Laurentia is only evident in what are inferred to be post-early Devonian age strata. By contrast, the North Slope and Apoon terranes originated proximal to Laurentia. The mid-Palaeozoic boundary between these (peri-) Baltican and (peri-) Laurentian terranes once lay between rocks of the Schist/Central belts and those of the Apoon terrane, but is obscured by severe Mesozoic–Cenozoic deformation. Whether this boundary represents a convergent or transform suture, when exactly it formed and how it relates to broader Caledonian convergence in the North Atlantic are still unresolved questions.

Cambrian–Ordovician conodonts from slump deposits of the Argentine Precordillera: new insights into its passive margin development

AbstractThe Los Sombreros Formation represents the western continental margin slope deposits of the Argentine Precordillera, a sub-terrane accreted to Gondwana as part of the Cuyania Terrane in early Palaeozoic times. The age of these gravity-driven deposits is controversial and, therefore, a precise biostratigraphic scheme is essential to reveal the evolution of the continental margin. New conodont samplings along with sedimentological and structural analysis carried out in the Los Sombreros Formation in the La Invernada Range provide clues to its depositional framework. The sedimentary succession is made up of dominantly calciturbidites, carbonate breccias and conglomerates, along with mudstones that represent the pelagic/hemipelagic background sedimentation. It displays hectometric to outcrop-scale slump folds with variable hinge-line orientations and pinch-and-swell structures, evidencing soft-sediment deformation, consistent with a slope to base-of-slope setting. Three limestone samples from this succession include conodonts referable to the pandemicHirsutodontus simplexSubzone of theCordylodus intermediusZone (upper Furongian, Cambrian) and from theMacerodus dianaeZone (upper Tremadocian, Ordovician), implying that a slope connected the shallow-water shelf with a deep-water (oceanic) basin at least since late Cambrian times. The conodont faunas show affinities to coeval assemblages from outer shelf and slope environments around Laurentia yet they are not conclusive to postulate a geographic origin for the Precordillera. The thermal alteration of the conodonts is consistent with sedimentary burial and nappe stacking in this sector of the Precordillera.

Pomeranian Caledonides, NW Poland – A collisional suture or thin-skinned fold-and-thrust belt?

A main goal of this study was to understand the character of the Caledonian Deformation Front in Pomerania, NW Poland and its relationship to the adjoining Teisseyre-Tornquist Zone (TTZ), in the context of the Avalonia-Baltica early Palaeozoic collision. Since the Pomeranian Caledonides are concealed beneath 1–4km thick platform cover of upper Palaeozoic and Mesozoic sediments, we used a combination of potential field, seismic and well data to investigate the basement architecture and structure of the lower Palaeozoic rocks. Starting from a qualitative review of gravity and magnetic data, we built a 2D gravity and magnetic model upon the PolandSPAN™ PL1-5600 seismic reflection line and applied 3D gravity inversion for a depth-to-basement study. Using well tops, a top lower Palaeozoic horizon and lower Palaeozoic isopach map were created. We found out that the Pomeranian Caledonides represent a thin-skinned fold-and-thrust belt involving Ordovician and Silurian sediments of the Caledonian foreland basin. The deformation front was developed due to the buttressing effect of a basement ramp occurring directly above the TTZ. The latter corresponds to a Precambrian suture zone overprinted by successive extensional tectonics and buried beneath the foreland basin. In northern Poland, the suture is defined by a remnant crustal keel still preserved underneath the TTZ, probably resulting from Precambrian collision during formation of the Rodinia paleocontinent. We propose that the extension of the Thor suture separating Baltica and Avalonia must exist west from the TTZ, probably in the Rügen area. In our interpretation, the TTZ represents an intra-cratonic crustal discontinuity comparable to its NW prolongation, the Sorgenfrei-Tornquist Zone that runs across the Baltic Sea, Sweden and Denmark. Consequently, the basement underlying the Palaeozoic Platform in NW Poland may be considered to have been part of Baltica in early Palaeozoic times.

Granulite-facies metamorphism of the Palaeoproterozoic – early Palaeozoic gneiss domains of NE Mozambique, East African Orogen

AbstractGranulite-facies metamorphism recorded in NE Mozambique is attributed to three main tectonothermal events, covering more than 1400 Ma from Palaeoproterozoic – early Palaeozoic time. (1) Usagaran–Ubendian high-grade metamorphism of Palaeoproterozoic age is documented in the Ponta Messuli Complex by Grt-Sil-Crd-bearing metapelites, estimated to pressure (P) 0.75 ± 0.08 GPa and temperature (T) 765 ± 96°C. The post-peak P-T path is characterized by decompression followed by near-isobaric cooling. (2) Irumidian medium- to high-pressure granulite-facies metamorphism is evident in the Unango and Marrupa complexes of late Mesoproterozoic – early Neoproterozoic age. High-pressure granulite-facies is documented by Grt-Cpx-Pl-Rt-bearing mafic granulites in the northwestern part of the Unango Complex, with peak conditions up to P = 1.5 GPa and T = 850°C. Medium-pressure granulite-facies conditions recording P of c. 1.15 GPa and T of 875°C are documented by Grt-Opx-Cpx-Pl assemblage in mafic granulites and charnockitic gneisses of the central part of the Unango Complex. (3) Tectonothermal activity during the Ediacaran–Cambrian Kuunga Orogeny is recorded in the Mesoproterozoic gneiss complexes as amphibolite facies to medium-pressure granulite-facies metamorphism. Granulite facies are documented by Grt-Opx-Cpx-Pl-bearing mafic granulites and charnockitic gneisses, reporting P = 0.99 ± 13 GPa at T = 738 ± 84°C in the Unango Complex and P = 0.92 ± 18 GPa at T = 841 ± 135°C in the Marrupa Complex. This metamorphism is attributed to crustal thickening related to overriding of the Cabo Delgado Nappe Complex, and shorthening along the Lurio Belt during the early Palaeozoic Kuunga Orogeny.

Geochronological and geochemical study of gneiss–schist complexes and associated granitoids, Beishan Orogen, southern Altaids

The tectonic nature of metamorphic terranes and their role in orogenesis are problematic. Here we present new U–Pb ages and geochemical data for widespread metamorphic rocks and associated granitoids from Northwest China. Orthogneisses from the metamorphic complexes have crystallization ages of ∼457, ∼452, and ∼526 Ma. One paragneiss (schist) has a maximum depositional age of 312 ± 7 Ma. Three foliated granites were emplaced at ∼450, ∼349, and ∼410 Ma, and all lack inherited Precambrian ages. The metamorphic terranes may have undergone multiple petrotectonic events as revealed by the metamorphic ages. Both the orthogneisses and granitoids show enrichment in large ion lithophile elements (LILEs) and light rare Earth elements (LREEs), and depletion in high field strength elements (HFSEs), which indicate that they formed in a subduction-generated accretionary arc setting. Our study demonstrates that the metamorphic terranes in the Beishan area, originally considered as Precambrian basement with suspected Neoarchaean to Palaeoproterozoic ages, are actually parts of early Palaeozoic arcs. The protoliths were probably metamorphosed arc plutonic and sedimentary rocks. Combined with other studies, we speculate that the Beishan Orogen formed by progressive arc accretion during the latest Neoproterozoic to early Palaeozoic time. This new interpretation has implications for other high-grade metamorphic terranes within orogens that have been assumed to represent ancient or pre-existing micro-continental blocks. If so, the importance of collision as a mechanism of mountain building has been overestimated, and the accretionary process as a mechanism of continental growth has been underestimated.

Detrital zircon evidence for the linkage of the South China block with Gondwanaland in early Palaeozoic time

AbstractLA-ICP-MS U–Pb dating of Lower Devonian detrital zircon samples from three representative sections in the South China block yields dominant Grenvillian and Pan-African populations, similar to the age distribution of early Palaeozoic samples from Gondwana, the Tethyan Himalaya and West Australia, in particular. Hf isotopic compositions indicate the contributions of juvenile crust at 1.6 Ga and 2.5 Ga, and bear a resemblance to their counterparts from SE Australia and West Antarctica, revealing the mixed origin of the Pan-African and Grenvillian grains from juvenile magmas and melting of pre-existing crustal rocks. These results suggest that the South China block should be considered an integral part of East Gondwana in early Palaeozoic time, rather than a discrete continental block in the Palaeo-Pacific or a fragment of Laurentia.

A new link between Orsten-type assemblages and the Burgess Shale—a Marrella-like arthropod from the Cambrian of Australia

An isolated exopod in uncompressed three-dimensional “Orsten”-type preservation from the Cambrian of Australia represents a new species of Marrellomorpha, Austromarrella klausmuelleri gen. et sp. nov. The exopod is composed of at least 17 annuli. Each of the proximal annuli carries a pair of lamellae: one lamella on the lateral side and one on the median side. The distal annuli bear stout spines in the corresponding position instead of lamellae, most likely representing early ontogenetic equivalents of the lamellae. The new find extends the geographical range of the taxon Marrellomorpha. Additionally, it offers a partial view into marrellomorph ontogeny. The occurrence of a marrellomorph fragment in “Orsten”-type preservation provides new palaeo-ecological insights into the possible connections between the “Orsten” biotas and other fossil Lagerstatten. Finding such connections is necessary for understanding the complex ecosystems of early Palaeozoic times.

Setulfs, modern and ancient: Formative mechanism, preservation bias and palaeoenvironmental implications

Setulfs or inverted flutes, both in growth stage and in fossilized state, are addressed here. This account resolves the long-standing questions about their formative mechanism, depositional prerequisites, and preservation bias, as well as their palaeoenvironmental implications. These structures were observed to grow by accretion of wind deflated sand in the lee of miniscule obstructions, in the littoral–supralittoral transition zone, a little above the high-water mark on top of a partially-emerged bar at Chandipur, eastern India. Pseudopellets of crabs and scarce chips of microbial mat create the obstructions, as shells might also do in the case of other modern analogs reported from essentially similar environmental conditions. Moisture on the beach sand limited the lifting power of the wind, so that even miniscule obstructions could have a well-developed wake zone at their lee side, resulting in localized accumulation of a small ridge of sand extending downwind from the obstruction. Setulfs arrested under microbial mats were also found in Chandipur on the seaward fringe of a shallow marsh encircled by a spit. Like those in growth stage these mat-covered setulfs also had their tapering ends directed landward, as the wind was directed onshore during the period of observation. Fossilized setulfs in swarms are also described here from two Neoproterozoic formations in India, viz., the Sonia Sandstone and the Upper Bhander Sandstone. In both the cases, setulfs are preserved in well-sorted sandstones, and are associated with planar laminae, profound wave ripples, bifurcated parting lineations, infrequent rill marks and local swarms of current crescents on bedsurfaces. Additionally, recorded fossil setulfs are found in association with inversely graded translatent strata, crinkled adhesion laminae, patches of impact ripples and isolated small sets of alternate grain-flow and grain-fall cross-strata of aeolian origin. This suggests preservation in the high littoral–supralittoral zone as in case of all known modern setulf occurrences. The palaeowind direction derived from the ancient setulfs conforms in orientation to that recorded from other aeolian structures in the strata, supporting their wind deflation origin. Although shells and biogenic pellets were altogether absent from these Neoproterozoic rocks, microbial mat curls and fragments had presumably been abundantly present in the Proterozoic high littoral–supralittoral zone to facilitate formation of setulfs. Pertinently many microbial mat-related features are present within both the siliciclastic formations studied. In polished sections some of the studied Proterozoic setulfs reveal the existence of crinkled carbonaceous lamina covers that signify microbial mat growth on top of them. An a priori assumption is that microbial mats must have enhanced the preservation potential of these delicate structures, which would otherwise be extremely unlikely to be preserved. The seeming preference of fossil setulf occurrences from the Proterozoic–Early Palaeozoic time is most likely due to a lack of burrowers and grazers that would devastate the protective microbial mat later in the Phanerozoic. These observations suggest that swarms of setulfs are, in general, wind sculpted, and their presence in the rock record prefers palaeoenvironment interpretration as a high littoral–supralittoral transition zone, and also stands as proxy records of syn-depositional microbial mat growth.

Chapter 31 A Palaeozoic NW Passage and the Timanian, Caledonian and Uralian connections of some exotic terranes in the North American Cordillera

Abstract Exotic terranes of inferred Arctic affinity form an outer belt within the North American Cordillera extending from Alaska to northern California. The geological history, fossil and detrital zircon data for these terranes show strong correlations and linkages among them, and many features in common with the northern Caledonides, the Timanide orogen and the Urals. They probably occupied an intermediate position between Baltica, Laurentia and Siberia, in proximity to the northern Caledonides in Early Palaeozoic time. Westward dispersion of these terranes is interpreted to result from development of a Scotia-style subduction system between Laurentia–Baltica and Siberia in Mid-Palaeozoic time – the NW Passage – following closure of the Iapetus ocean. Diachronous orogenic activity from Late Silurian in Arctic Canada to Early Devonian in north Yukon and Alaska records passage of some of these terranes. Westward propagation of a narrow subduction zone coupled with a global change in plate motion, linked to closure of the Rheic Ocean are proposed to have led to initiation of subduction along the western margin of Laurentia. This is recorded by the Late Devonian initiation of arc magmatism along western Laurentia, and the Late Devonian–Early Mississippian Antler orogeny in the western US and Ellesmerian orogeny in the Canadian Arctic.

Multiple high-pressure metamorphic events and crustal telescoping in the NW Highlands of Scotland

Abstract: The Glenelg–Attadale Inlier is the largest basement inlier within the Caledonian orogen in NW Scotland. A Western Unit consists of trondhjemite–tonalite–granodiorite (TTG) gneisses and subordinate mafic intrusions with locally preserved mafic high-pressure granulite and eclogite assemblages. An Eastern Unit comprises TTG gneisses, Grenville-age ( c . 1.1 Ga) eclogite and metasediments. U–Pb zircon ages from the Western Unit TTG gneisses are highly disturbed with Neoarchaean upper intercepts and Palaeo- to Mesoproterozoic lower intercepts, suggesting strong reworking at these times. U–Pb zircon, Sm–Nd and Lu–Hf garnet–clinopyroxene dates from the Western Unit high-pressure granulite and eclogite yield Neoarchaean ( c . 2.6–2.8 Ga) and Palaeoproterozoic ( c . 1.75 Ga) ages, respectively. These ages correspond to the ages of partial resetting of the TTG gneisses. The eclogite in the Western Unit may represent the high-pressure convergent margin to the lower-pressure Laxfordian events within the Lewisian Gneiss Complex, whereas the Mesoproterozoic eclogites in the Eastern Unit may represent a farther eastward Grenville-age margin. Further east, Ordovician high-pressure granulites within the Moine Supergroup may represent another, later, Grampian convergent margin. These high-pressure belts were developed sequentially towards the east and telescoped westwards from Palaeoproterozoic to Early Palaeozoic times. Supplementary material: U–Pb analytical data and LA-ICP-MS trace element data are available at http://www.geolsoc.org.uk/SUP18389 .

Neoproterozoic tectonic synthesis of Uruguay

Neoproterozoic–lower Palaeozoic successions in the Brasiliano fold belts are described and a brief synthesis of these terranes is presented in order to erect a tectonic framework for this region. Tectonic events that occurred around the Río de La Plata craton were diachronous and reflected successive stages of the Brasiliano orogenic cycle. They took place in mobile belts that constituted part of the Gondwana supercontinent. The most thoroughly investigated Neoproterozoic sections are located in the eastern and southeastern regions of Uruguay. The Dom Feliciano Belt shows a tectonic evolution from back-arc to foreland basin characterized by fold-and-thrust, thick-skinned belts developed during the Brasiliano/Pan-African orogenic cycle. The most conspicuous features were late-tectonic high-K calc-alkaline granitoids, HT-LP metamorphism, significant displacements along shear zones, and post-tectonic granitoids. The final stage was characterized by post-collisional basins (molassic sequences) and extensional magmatism related to a phase of crustal stretching. Several lithotectonic units are present as basement inliers in the Dom Feliciano Belt: these include a low-to-medium metamorphic grade sequence (the Zanja del Tigre Formation), granitoids and gneisses (the Campanero Unit), high-grade basement of the Cerro Olivo Complex (Palaeoproterozoic or Neoproterozoic), and a low-metamorphic grade orogenic belt (the Rocha Formation). This paper provides a simplified tectonic map of eastern Uruguay, which we use to describe tectonic evolution from Precambrian to early Palaeozoic time.

On certain points in the Visceral Anatomy of the Lacertilia, particnlarly of Monitor.

Summary. The principal facts recorded in the present paper and the conclusions to which they lead are as follows:- 1 The Varanidte differ from other Lacertilia in two important particulars:–in (i.) the occasional complication of the cystic and hepatic ducts, which form a network, (ii.) the presence of a fold of peritoneum, reflected from the lining peritoneum of the abdominal cavity, which surrounds the abdominal viscera. 2 This fold of peritoneum has its exact counterpart in Croco-dilia and Aves, where, however, the subdivision of the ccelom into a number of separate cavities is carried on still further. 3 The Varanidse, therefore, alone (?) of existing Lacertilia show the first beginnings of the subdivision of the coelom, which reaches its extreme in the higher Sauropsida. 4 These facts necessitate the separation of the Varanidse from the true Lacertilia into a group equivalent to that of, e.y., Rhyneho-cephalia. 5 It is probable, as generally believed, that the Lacertilia more nearly represent the primitive Reptilian stock than any other Sauropsida. 6 The particular resemblance between the Varanidse and the Crocodilia renders it probable that some Reptilian type existed in early Mesozoic or late Palaeozoic times, which in manv points, especially those enumerated in (1), resembled the existing Varanidee. From this type originated the Crocodilia, Diuosauria, and A.ves.

An early Palaeozoic double-subduction model for the North Qilian oceanic plate: evidence from zircon SHRIMP dating of granites

The North Qilian Orogenic Belt (NQOB), which consists of ophiolitic mélange and island-arc assemblages containing many granites, blueschists, and eclogites, lies between the Alax and Qilian terranes in northwestern China. The Minleyaogou and Niuxinshan granitoids occur at the northern and southern margins, respectively, in the middle segment of the NQOB. The Minleyaogou pluton is granodiorite in composition, whereas the Niuxinshan pluton consists mainly of red granite with minor grey quartz diorite. Geochemically, the Minleyaogou granite differs from the Niuxinshan granite in that it contains a smaller range in SiO2, has lower total alkalis, and is more peraluminous. Both granitoids are magnesian but the Niuxinshan granite is alkali-calcic, whereas the Minleyaogou granodiorite is calcic. Both granitoids have similar chondrite-normalized rare earth element patterns with light rare earth element enrichment and negative Eu anomalies. They have pronounced negative Ba, Nb, Sr, P, and Ti anomalies indicating that they have an affinity to island-arc or active continental margin magmatism. SHRIMP U–Pb dating of zircons from the granitoids yields a formation age of 477 Ma for the Niuxinshan granite and 463 Ma for the Minleyaogou granodiorite. These ages, combined with the geochemistry and locations of the plutons, suggest that they formed by the double subduction of the North Qilian oceanic plate during early Palaeozoic time. Formation of the Niuxinshan granite may be related to southward subduction under the Qilian terrane at 477 Ma, whereas the Minleyaogou granodiorite was formed by northward subduction at 463 Ma under the Alax terrane.

An early Palaeozoic supra-subduction lithosphere in the Variscides: new evidence from the Maures massif

Petrographic and geochemical studies of peridotites and melagabbros from the Maures massif (SE France) provide new constraints on the Early Palaeozoic evolution of the continental lithosphere in Western Europe. Peridotites occur as lenses along a unit rooted in the main Variscan suture zone. They are dominantly spinel peridotites and minor garnet–spinel peridotites. Spinel peridotites represent both residual mantle and ultramafic cumulates. Mantle-related dunites and harzburgites display high temperature textures, with olivine (Mg#0.90), orthopyroxene (Mg#0.90) and spinel (TiO2 < 0.2%; Cr#0.64–0.83) compositions typical of fore-arc upper mantle. Ultramafic cumulates are dunite adcumulates, harzburgite heteradcumulates and mesocumulates, melagabbro heteradcumulates and amphibole peridotites, with olivine (Mg#0.85–0.89), orthopyroxene (Mg#0.86–0.89) and Cr-spinel (TiO2 = 0.5–3.3%; Cr#0.7–0.98) compositions typical of ultramafic cumulates. Cr-spinel compositions of both spinel peridotite types suggest their genesis in a supra-subduction zone lithosphere. Core to rim zoning in spinel is related to the incomplete influence of regional metamorphism and serpentinisation. The covariation of major and minor elements with Al2O3 for cumulates is consistent with igneous processes involving crystal accumulation. Both mantle and cumulate dunites and harzburgites have U-shaped REE patterns and extremely low trace element contents, similar to peridotites from modern fore-arc peridotites (South Atlantic) and from ophiolites related to supra-subduction zones (Semail, Cyclops, Pindos, Troodos). Melagabbros also have U-shaped REE patterns similar to xenoliths from the Philippine island arc, but also similar to intrusive ultramafic cumulates from the Semail nappe of Oman related to a proto-subduction setting. A wehrlite has a REE pattern similar to that of amphibole peridotites reflecting metasomatism of clinopyroxene-bearing peridotites due to subduction-related fluids. The Maures spinel peridotites and melagabbros are therefore interpreted as the lowermost parts of a crustal sequence and minor residual mantle of lithosphere generated in a supra-subduction zone during Early Palaeozoic time. Garnet–spinel peridotites are chemically close to melagabbros, but have recorded high pressure metamorphism before their retrogression similar to spinel peridotites into amphibolites to greenschists facies metamorphism. They indicate burial to mantle depths of the margin of the supra-subduction lithosphere during the Early Palaeozoic continental subduction. Both peridotite types were exhumed during the Upper Palaeozoic continental collision. Comparable observations from other Variscan-related peridotites, in particular of the Speik complex of the Autroalpine basement, and a common age for the subduction stage allow extension of these regional conclusions to a broad area sharing the Cambrian suture zone, extending from the Ossa-Morena to the Bohemian massif.

A Palaeozoic Northwest Passage: incursion of Caledonian, Baltican and Siberian terranes into eastern Panthalassa, and the early evolution of the North American Cordillera

Abstract Palaeozoic to early Mesozoic terranes of the North American Cordillera mostly originated from three distinct regions in Palaeozoic time: the western peri-Laurentian margin, western (Asian) Panthalassa, and the northern Caledonides–Siberia. A review of geological history, fossil and provenance data for the Caledonian–Siberian terranes suggests that they probably occupied an intermediate position between northern Baltica, northeastern Laurentia and Siberia, in proximity to the northern Caledonides, in early Palaeozoic time. Dispersion of these terranes and their westward incursion into eastern Panthalassa are interpreted to result from development of a Caribbean- or Scotia-style subduction system between northern Laurentia and Siberia in mid-Palaeozoic time, termed here the Northwest Passage. Westward propagation of a narrow subduction zone coupled with a global change in plate motion, related to the collision of Gondwana with Laurentia–Baltica, are proposed to have led to initiation of subduction along the western passive margin of Laurentia and development of the peri-Laurentian terranes as a set of rifted continental fragments, superimposed arcs and marginal ocean basin(s) in mid- to late Palaeozoic time. Diachronous orogenic activity from Late Silurian in Arctic Canada, to Early Devonian in north Yukon and adjacent Alaska, Middle Devonian in southeastern British Columbia, and Late Devonian–Early Mississippian in the western USA records progressive development of the Northwest Passage and southward propagation of subduction along western Laurentia.

New Middle Cambrian and Middle Ordovician palaeomagnetic data from Siberia: Llandelian magnetostratigraphy and relative rotation between the Aldan and Anabar–Angara blocks

Whether Siberia was a coherent tectonic unit throughout the Phanerozoic or whether significant rotations occurred in early Palaeozoic times between the Anabar–Angara and Aldan blocks is still a matter of debate. Addressing this problem a detailed palaeomagnetic study of sections of mid Cambrian and Llandeillian age has been carried out along the river Lena, river Maya, river Kulumbe, and river Stolbovaya, with a total of 403 collected hand-samples. A section of Llandeilian age was also investigated on the Russian Platform, near St. Petersburg. The studied rocks are carbonates or fine grained terrigenous sediments of reddish and greyish colors. Stepwise thermal demagnetization experiments reveal a rather simple directional behavior. In most samples, after removal of a secondary magnetizations at low to intermediate temperatures, a high temperature component is isolated, which is carried by magnetite and/or hematite. Positive reversal tests and fold tests clearly demonstrate the primary character of the latter component. While only reversed polarity directions are obtained in the lower two thirds of the Llandeilo, six magnetozones of normal polarity are found in the Upper Llandeilo, which mark the end of the Moyero reverse superchron. Together with previously published paleomagnetic data our new results show rather systematic difference in declinations of ~ 20° between the Aldan and Anabar–Angara blocks. This argues in favor of a relative rotation between these two blocks, with an Euler pole of rotation located at latitude ~ 62°N and Longitude ~ 117°. Our interpretation is further supported by existing results from deep sounding seismic experiments, yielding a very similar pole of rotation.