Early Silurian Research Articles

Early Silurian (~440Ma) adakitic, andesitic and Nb-enriched basaltic lavas in the southern Altay Range, Northern Xinjiang (western China): Slab melting and implications for crustal growth in the Central Asian Orogenic Belt

As an important part of the Central Asian Orogenic Belt (CAOB), the Altay Range contains large-scale Paleozoic magmatic rocks. However, owing to the lack of precise age constraints, the tectonic setting and petrogenesis of the magmatic rocks in this area have been controversial, which has led to the debate on Phanerozoic crustal growth mechanisms and accretionary orogenic processes in the CAOB. Herein, we report geochronological and geochemical data of the Suoerkuduke adakitic, andesitic and Nb-enriched basaltic (NEB) lavas in the southern margin of the southern Altay Range. LA-ICP-MS zircon U–Pb analyses for five adakitic, andesitic and NEB samples indicate that they were coevally generated in the Early Silurian (~ 440 Ma). The adakites and basaltic andesites are geochemically characterized by high Na2O/K2O, Sr/Y, Al2O3, Sr, εNd(t) and zircon εHf(t) values and relatively low (87Sr/86Sr)i ratios. The NEBs are sodium-rich and have higher TiO2, P2O5, Zr, Nb, and Nb/U values than those of typical arc basalts. They also have positive εNd(t) values and positive and variable zircon εHf(t) values. We suggest that the Suoerkuduke adakites were derived by a partial melting of the subducted oceanic crust with minor overlying sediments, and the continuous compositional variations between adakites and basaltic andesites confirm that the interaction between slab melts and mantle peridotite played an important role in the formation of basaltic andesites. The associated NEBs were possibly generated by a partial melting of mantle wedge peridotites metasomatized by slab-derived adakitic melts and minor fluids. In combination with the occurrence of voluminous Silurian–Devonian granitoids, coeval ophiolite mélanges, and a series of intra-arc basins, a slab window model triggered by slab tearing is proposed to account for the formation of the Suoerkuduke adakite–basaltic andesite–NEB suites. The upwelling of the asthenospheric mantle through the slab window probably caused significant extension in the overlying lithosphere; extensive melting of the subducted oceanic crust, mantle and juvenile lower crust; and intense interaction between slab-derived melts or fluids and the mantle wedge. Slab tearing and subsequent asthenospheric upwelling possibly played an important role in the crustal growth of the CAOB during the Phanerozoic.

I-type granitoids in the eastern Yangtze Block: implications for the Early Paleozoic intracontinental orogeny in South China

Abstract The Early Paleozoic intracontinental orogenic belt in the South China Block (SCB) is composed of massive granitoids and high-grade metamorphic rocks. Compared to the widespread distributions of early Paleozoic S-type granites in the eastern SCB, coeval I-type granitoids are rare and consequently receive much less attention. Two spatially associated granodiorite plutons in the northwestern rim of the orogen, namely the Banshanpu pluton and Hongxiaqiao pluton, have been investigated in order to determine how they fit into the geodynamic setting. The Hongxiaqiao pluton shares many lithological similarities with the Banshanpu pluton, except for the presence of abundant mafic microgranular enclaves (MMEs) in the Hongxiaqiao pluton. Zircon U–Pb dating has yielded weighted mean 206Pb/238U ages of 432 ± 3 Ma and 434 ± 3 Ma for the Banshanpu and Hongxiaqiao plutons, respectively, indicating that they were emplaced coevally in the early Silurian. Samples from the two plutons possess similar Nd–Sr isotope compositions (eNdT = − 8.32 to − 6.88; 87Sr/86Sri = 0.7109–0.7169), indicating that they were derived from a similar crustal source. Rocks from the Banshanpu pluton are intermediate- to high-K calc-alkaline and show strongly peraluminous (A/CNK > 1.1), adakite-like characteristics (Sr/Y ratios > 31; Yb

Zircon SHRIMP U-Pb dating for island arc volcanic rock of Fangniugou area in Yitong region of Jilin Province

Polymetallic iron ore sulphate deposits of marine volcanic rock have been developed in the Fangniugou area, Jilin Province, China, but the division of volcanic ore-bearing strata has not been specifically elucidated and there is disagreement about the division. The sampling and SHRIMP U-Pb zircon dating of volcanic rock for Daheishan in the Fangniugou area and the northeast slope of the Duanjiadian were described. The volcanic rock formation period and recorded the volcanic events in the Daheishan mountains were systematically researched. Two samples of high-precision U-Pb zircon dating were used to represent the volcanic rock fomation period of the Late Silurian. The measured data reflect that multiple volcanic activities occurred during the Middle Silurian, Early Silurian, Middle Ordovician and Silurian, and Late Ordovician, probably matching volcanic events in the Songnan Basin identified from zircon dating. At the same time, it is confirmed that a controversial “conglomerate of Daheishan” did in fact develop in the Late Silurian, and those sections of both the Dazigou and Xinlitun-Taoshan with graptolite had been reversed.

Dynamic elastic properties of the Wufeng–Longmaxi formation shale in the southeast margin of the Sichuan Basin

In spite of the growing interest in Late Ordovician to the Early Silurian Wufeng–Longmaxi black shale for gas production in the southeast of the Sichuan Basin, little data is available on the acoustic properties of these shale reservoirs. The dynamic elastic properties of a suite of shales from this formation with different organic matter content were measured using the ultrasonic transmission technique at a range of confining pressures. The mineralogical features and reservoir space types were studied with the goal of analyzing their influence on the acoustic properties of the shale samples. The TOC (total organic content) is found to have a positive correlation with the quartz content and a negative correlation with the clay content. Organic matter pores and inter-crystal micro-pores are the main pore types. Organic-rich shales show a low P-wave impedance (Ip) and Vp/Vs ratio due to relatively higher TOC and quartz content; nonetheless, high P-wave (Ip) is also observed when the quartz content is very high resulting from the variation of load-bearing grains. The velocities and anisotropy of different shale samples show varying pressure dependence. A strong positive correlation is found between the anisotropy and the clay content. Three different types of microfabric can be observed in the shale samples, giving a qualitative explanation for the correlation between the anisotropy and the clay content. The occurrence of preferential alignment of the clay minerals requires that the clay content exceeds a critical volume, and this value is about 25%–30% in this study.

Late Ordovician to mid-Silurian Benambran subduction zones in the Lachlan Orogen, southeastern Australia

The Late Ordovician to early Silurian Benambran Orogeny had widespread effects in the Lachlan Orogen of southeastern Australia, including the Bendigo, Tabberabbera and Narooma zones. Much of the deformation in these zones has been attributed to formation of subduction complexes from accretion of Ordovician turbidites and has been a subject of ongoing controversy. The Narooma Zone subduction complex is more widely accepted because of the abundance of disrupted units, evidence for imbricated units and its metamorphic character. Imbricated and disrupted units consistent with a subduction accretion model also characterise the Tabberabbera Zone. A subduction complex model for the Bendigo Zone has been contested. Deep seismic profiles show a largely transparent upper crust down to ∼18 km of inferred Ordovician turbidites with a layered middle to lower crust of mafic volcanic rocks as shown by the Heathcote Greenstone Belt being traced from the surface to the middle crust. During the Benambran Orogeny, the Bendigo Zone is interpreted as a rapidly formed mega-subduction complex with ∼50 km crustal thickness. This is an exceptional thickness and much thicker than subduction complexes in Japan and the Barbados Accretionary Complex but more comparable with southeastern Alaska where the Chugach terrane (subduction complex) and underplated mafic igneous rocks have a present-day crustal thickness up to 30 km. Anomalous thick crust of the Bendigo Zone subduction complex is related to the huge volume of subduction inputs, with Cambrian fore-arc–backarc crust and the overlying thick Ordovician turbidite fan, resulting in a double décollement associated with the megathrust now represented by the Mount William Fault. The Benambran Orogeny is an example of widespread shortening reflecting rapidly initiated subduction zones with a short-lived history rather than long-lived subduction typical of the modern eastern Pacific Ocean.

Hydrothermal Dolomitization of Upper Ordovician Limestone, Central-East Canada: Fluid Flow in a Craton-Interior Wrench-Fault System Likely Driven by Distal Taconic Tectonism

Narrow (tens of meters) vertical bodies of dolomite replace a nonporous Upper Ordovician (lower Chatfieldian) limestone along the leading limb of an asymmetric faulted anticline well inboard (∼300 km) of the northern Appalachian orogen, central-east Canada. Patterns of geometry, texture, and geochemistry suggest that dolomitizing fluids were focused (possibly along microfractures) within a paleosinistral transpressive stress field generated with reactivation of an underlying Neoproterozoic fault system. Strike-slip failure of the developing anticline resulted in fracture- and fault-controlled fluid flow from which precipitated saddle dolomite. A paragenetic succession of Fe-poor planar-e to ferroan planar-s dolostone marks the peak phase of replacement dolomitization. A subsequent increase (20%–30%) in porosity created through local dissolution of relict limestone was partially occluded by ferroan planar dolomite that, geochemically, is similar to later fracture-fill saddle dolomite. Replacement dolomitization is associated with a slight rise in Sr-isotope ratios (to 0.71085) from Late Ordovician–early Silurian marine signatures. Isotope (C, O, Sr) signatures support influx and mixing of burial fluids that had interacted with local Mg-rich (gabbro, anorthosite, syenite) crystalline basement with a background fluid similar to Late Ordovician seawater or dissolved marine limestone. Dolomitization predated maximum burial in the Late Paleozoic. Fluid inclusion and isotope paleothermometry suggest that dolomitizing temperatures (100°–120°C) were 20°–30°C warmer than associated with prior limestone diagenesis. The dolomitized limestone is an archival record of structure, hydrology, and heat flux that best fits with Taconic tectonism in the latest Ordovician through earliest Silurian.

Geochronology, Geochemistry and Tectonic Setting of the Bairiqiete Granodiorite Intrusion (Rock Mass) from the Buqingshan Tectonic Mélange Belt in the Southern Margin of East Kunlun

Abstract:This study focuses on the zircon U–Pb geochronology and geochemistry of the Bairiqiete granodiorite intrusion (rock mass) from the Buqingshan tectonic mélange belt in the southern margin of East Kunlun. The results show that the zircons are characterized by internal oscillatory zoning and high Th/U (0.14–0.80), indicative of an igneous origin. LA–ICP–MS U–Pb dating of zircons from the Bairiqiete granodiorite yielded an age of 439.0 ± 1.9 Ma (MSWD = 0.34), implying that the Bairiqiete granodiorite formed in the early Silurian. Geochemical analyses show that the rocks are medium‐K calc‐alkaline, relatively high in Al2O3 (14.57–18.34 wt%) and metaluminous to weakly peraluminous. Rare‐earth elements have low concentrations (45.49–168.31 ppm) and incline rightward with weak negative to weak positive Eu anomalies (δ5Eu = 0.64–1.34). Trace‐element geochemistry is characterized by negative anomalies of Nb, Ta, Zr, Hf and Ti and positive anomalies of Rb, Th and Ba. Moreover, the rocks have similar geochemical features with adakites. The Bairiqiete granodiorite appears to have a continental crust source and formed in a subduction‐related island‐arc setting. The Bairiqiete granodiorite was formed due to partial melting of the lower crust and suggests subduction in the Buqingshan area of the Proto‐Tethys Ocean.

Paleozoic HP granulite-facies metamorphism and anatexis in the Dulan area of the North Qaidam UHP terrane, western China: Constraints from petrology, zircon U–Pb and amphibole Ar–Ar geochronology

HP mafic and felsic adakitic granulite bodies in the Dulan area of the North Qaidam ultrahigh-pressure (UHP) terrane record high-pressure (HP) granulite-facies metamorphism and anatexis and provide temporal and tectonic constraints on deep subduction of continental crust and its subsequent exhumation. Mafic HP granulite components dominate the main outcrop and preserve features diagnostic of anatexis and locally may be described as migmatite. The HP mafic granulites comprise garnet, clinopyroxene, plagioclase and quartz. The felsic granulite (leucosome) is mainly composed of K-feldspar+plagioclase+quartz+kyanite+garnet. Detailed zircon U–Pb and amphibole Ar–Ar geochronology, combined with trace element geochemistry, indicate peak metamorphism for the mafic HP granulite at 434±3 to 435±3Ma and peak metamorphism and partial melting for the felsic HP granulite at 433±5 to 438±4Ma, which overlaps the ages of UHP metamorphism for adjacent eclogite (430–446Ma). 40Ar/39Ar amphibole ages of 423 to 432Ma represent amphibolite-facies retrograde metamorphism and indicate rapid cooling during exhumation of the HP granulite bodies. Our geochronological data, combined with field relationships, petrology and geochemistry suggest that HP granulite-facies metamorphism and the partial melting that produced adakitic melts represent the same tectonic event. In this case, the felsic HP granulites (leucosome) formed from an adakitic melt derived from partial melting of mafic HP granulite in the overriding plate in a relatively higher geothermal gradient (15–18°C/km), leaving garnet-cumulate and/or meta-ultramafic (mainly garnet pyroxenite) as the residual component. In contrast, the nearby UHP eclogite is thought to have formed in the subducted plate in a relatively lower geothermal gradient (6–10°C/km). Penecontemporaneous metamorphic ages but different geothermal gradients between HP granulites and related UHP eclogite define a possible paired metamorphic belt generated in a subduction–collision setting associated with the North Qaidam continental collisional orogeny during the Late Ordovician–Early Silurian.

Marine Volcanosedimentary Basins Hosting Porphyry Au-Cu Deposits, Cadia Valley, New South Wales, Australia

The volcanosedimentary rocks that host the Au-rich porphyry Cu deposits of the Cadia Valley preserve the products of episodic volcanism that erupted into a large sedimentary basin. Volcanogenic sedimentation, including the Forest Reefs Volcanics, overwhelmed the fine-grained sedimentary component that characterized much of the Weemalla Formation. The Forest Reefs Volcanics evolved as a relatively low relief, multiple-vent submarine volcanic complex. The vents comprised mafic to intermediate lava flows, cryptodomes, and subvolcanic intrusions (dikes and sills). Stacked lava sequences, including hyaloclastites, massive lavas, and their reworked equivalents, are up to 1 km thick, forming significant intrabasinal topography. Explosive volcanism occurred during the late stages of Forest Reefs Volcanics deposition. These air-fall deposits, combined with coexisting shallow-water faunal assemblages, imply that volcanism became locally emergent. Continuity of sedimentation between underlying deep marine basin deposits of the Weemalla Formation and Forest Reefs Volcanics, coupled with the predominance of sheet-like, laterally continuous debris flow and other coarse-grained sedimentary deposits, implies that volcanism and related sedimentation persisted in an active sedimentary basin marginal to an oceanic island arc. Deposition of the Forest Reefs Volcanics spanned the Late Ordovician to Early Silurian. Monzonite fragments (identical to the ore-related intrusions) are abundant in sedimentary breccias found at the top of the preserved volcanic stratigraphy. This finding, combined with available absolute ages of crosscutting intrusions and associated hydrothermal alteration and mineralization, suggests that some volcanosedimentary units were deposited synchronously with or immediately after the last known porphyry-related hydrothermal event in the Cadia Valley. The Au-rich porphyry deposits were therefore emplaced into an evolving sedimentary basin with episodic intrabasinal magmatism. Permeable horizons and volcanic lithofacies can preferentially host alteration and mineralization that can extend over several kilometers in lateral extent. This finding suggests that hydrological models of fluid flow in porphyry systems need to take basin architecture into account.

Early Silurian ‘algal meadows’ of Anticosti Island, eastern Canada: an analogue to modern sea grass meadows?

In modern marine ecosystems, sea‐grass and chlorophyte meadows play an important ecological role by serving as a carbon sink. Despite their generally limited areal distribution, the high productivity of sea‐grass meadows makes them an efficient assimilator of CO2. During the early Palaeozoic, complex life was virtually confined to the marine environment, with algae being one of the common carbon‐fixers, alongside abundant calcifying cyanobacteria, rhodophytes, chlorophytes and charophytes, as well as non‐skeletal dinoflagellates and acritarchs. Fossil and molecular data indicate that marine thallophytic algae first appeared in the Early Proterozoic and became widespread in the Palaeozoic, although their fossil record is sporadic because of their soft‐bodied nature; in the absence of angiosperm sea grass and mangroves and poorly understood phytoplankton biomass, thallophytic algae were probably major primary producers. In this article, we suggest that thallophytic algae may have played a significant role as a carbon sink in the Early Silurian, analogous to modern sea‐grass meadows or kelp forests, based on the well‐preserved Early Silurian thallophytic algal meadow from Anticosti Island, eastern Canada.

A new ophiocistioid (Echinodermata) from the Silurian of Gotland, Sweden

From the early Silurian of Gotland, Sweden, the new ophiocistioid goniodont Linguaserra franzenae sp. nov. is described, figured and discussed within the Linguaserridae Reich & Haude, 2004. The evolutionary history of linguaserrid ophiocistioids is briefly discussed in relation to other members of the Ophiocistioidea. This new echinoderm species not only represents the first documented occurrence of this group from the Llandovery but also establishes the stratigraphically oldest record of goniodonts in the world.

Toward a stepwise Kwangsian Orogeny

The Kwangsian Orogeny originated along the southeast coast of China and stepwise developed in a northwest direction. It includes two stages, a long locally varying uplift from the Late Ordovician to the early Silurian and a finally tectonic movement near the Silurian and Devonian transition. The Kwangsian uplift event shows a stepwise delay northwestwards from the southeastern coast area in Nemagraptus gracilis Biozone (Sa1) to the south side of the Xuefeng Mountains in or later than Cystograptus vesiculosus Biozone (R3) to Coronograptus cyphus Biozone (R4). In the southern of Yangtze Platform, the Yichang Uplift was droved by the Kwangsian Orogeny forming a diachronous stratigraphical break through Rhuddanian and Aeronian. The distribution of the early Telychian lower marine red beds indicates a northwestward increase of the Cathaysian Oldland. Stratigraphical evidence may explain why the Kwangsian movement was marked by an angular disconformity during the Pridoli to earliest Devonian interval.

Latest Ordovician and earliest Silurian tabulate corals of South China

Latest Ordovician and earliest Silurian tabulate corals of the South China paleoplate are summarized at the generic level. In contrast to the abundant late Katian records, there is no record of tabulate corals in the early–middle Hirnantian rocks in South China, lithostratigraphically represented by the Kuanyinchiao Formation and its equivalents. The tabulate fauna from the late Hirnantian and the earliest Silurian rocks of South China is characterized by the occurrence of three cosmopolitan genera: Paleofavosites, Propora and Halysites. They differ morphologically from the pre-Hirnantian tabulate corals of South China. Such a long-ranging and low-diversity tabulate coral fauna also indicates a warm but somewhat unfavorable living environment during the survival interval after the second pulse of the end-Ordovician mass extinction.

The stratigraphic imprint of a mid-Telychian (Llandovery, Early Silurian) glaciation on far-field shallow-water carbonates, Anticosti Island, Eastern Canada; pp. 207–213

The near-field stratigraphic record of the Early Silurian glaciations is well documented in the literature. Data from far- field areas are, however, sparse. One of the best far-field stratigraphic records of these Llandovery glaciations is exposed on Anticosti Island in eastern Canada. Eight shallow-water paleotropical facies are present close to the mid-Telychian Jupiter- Chicotte formational boundary along the south-central coast of Anticosti Island. These can be grouped into three facies associations that include, from bottom to top: a carbonate facies association (FA-1), a mixed siliciclastic and carbonate facies association (FA-2) and an encrinitic facies association (FA-3). These mid- to outer-ramp strata represent deposition mostly from episodic, high-energy storm events as evidenced by their sharp bases, hummocky cross-stratification, large wave ripples, gutter casts and wave-enhanced sediment gravity flow deposits. Superimposed on a long-term regressive trend, one main transgressive- regressive (TR) sequence and four meter-scale TR cycles are evident, indicating a multi-order stratigraphic framework developed under the influence of glacio-eustasy. The Jupiter-Chicotte formational boundary, a regional discontinuity surface caused by a forced regression, corresponds to the onset of a far-field mid-Telychian glaciation.

Paleozoic ophiolitic mélanges from the South Tianshan Orogen, NW China: Geological, geochemical and geochronological implications for the geodynamic setting

Abstract Two ophiolitic melange belts, the South Central Tianshan Ophiolite Belt (SCTOB) and the South Tianshan Ophiolite Belt (STOB), extend almost parallel to the Chinese South Tianshan Orogen — the southernmost part of the Central Asian Orogenic Belt (CAOB). The SCTOB occurs interlayered in the South Central Tianshan Suture Zone, the STOB as exotic blocks in Palaeozoic sedimentary strata. However, their tectonic settings and ages, which are crucial for understanding the formation of the CAOB, are still controversially discussed. In order to better understand these geological problems, geological, geochemical and geochronological investigations were conducted on the Guluogou (SCTOB) as well as on the Serikeyayilake and Aertengkesi ophiolitic melanges (STOB). The ophiolitic suites are composed of basalts, gabbros and mantle peridotites. Laser ablation ICP-MS (LA-ICP-MS) U–Pb zircon ages of 334 ± 5 Ma and 332 ± 7 Ma were obtained for the Guluogou gabbros, while SHRIMP U–Pb zircon ages of 423 ± 10 Ma and 423 ± 4 Ma were determined for the Serikeyayilake and Aertengkesi gabbros. The mineral composition of the mantle peridotites and the geochemical characteristics of the basalts suggest a mid-ocean ridge (MOR) type affinity for the Guluogou ophiolite and a SSZ type affinity for the Serikeyayilake and Aertengkesi ophiolites. The present data, combined with previously published results, indicate that the STOB may have been derived from a forearc rifting setting that existed during the early Silurian to the early Carboniferous (ca. 439–356 Ma), whereas the SCTOB may represent fragments of a normal wide ocean that lasted to the early Carboniferous (ca. 332 Ma).

Global diversity and endemism in Early Silurian (Aeronian) brachiopods

Silurian brachiopods of Middle Llandovery (Aeronian) ages are reviewed, and 215 genera are identified here, compared with 109 in the preceding Early Llandovery (Rhuddanian), indicating a recovery-radiation interval after the end-Ordovician mass extinction. The chief regions in which they are found are the continents of South China, Avalonia-Baltica, Laurentia and Siberia, which were all at tropical latitudes with the exception of Avalonia. In addition, the very large superterrane of Gondwana, although with patchy brachiopod distribution, included temperate faunas, as well as subtropical faunas (in Iran and Afghanistan). Aeronian brachiopods greatly increased in diversity, with dominance of four major groups: orthides and strophomenides (which had flourished previously in the Rhuddanian), pentamerides and atrypides (which became dominant in the Aeronian), and many newly evolved taxa, and occupied deeper water and wider ecological niches (level bottom and reef) than those in the Rhuddanian. Each of the continents has some endemic genera, but there is a greater proportion of them in South China, where some groups (such as the pentamerides and atrypides) are more diverse and others, such as the orthides, are much less common than elsewhere. Affinity indices (AI) show that two megaunits can be recognized: South China and Avalonia-Baltica-Laurentia (ABL); Siberia seems to have been loosely connected with ABL and even more loosely to South China presumably because of its geographical separation in the Northern Hemisphere. The separation of South China from the other megaunits is further supported by cluster analysis.

Environmental changes in the Late Ordovician-early Silurian: Review and new insights from black shales and nitrogen isotopes

The Late Ordovician (Katian-Hirnantian) through earliest Silurian (Rhuddanian) interval was a time of varying climate and sea level, marked by a peak glacial episode in the early-mid-Hirnantian. Synthesis of recently published data permits global correlation of at least two cycles of glacial advance and retreat with a distinct interglacial period that is recognizable in sequence-stratigraphic and chemostratigraphic records in many parts of the world. A period of warming and sea-level rise during the late Katian is marked by the widespread occurrences of oceanic anoxia in paleotropical and subtropical localities, mostly confined to regions of inferred upwelling and semirestricted marine basins. Nitrogen isotope data show that the regions of oceanic anoxia were marked by intense water-column denitrification in which cyanobacteria were the principal source of fixed N. In the overlying peak glacial interval of the Hirnantian, sedimentary successions from localities representing a wide range of water depths and paleolatitudes indicate that anoxia was restricted during the early-mid-Hirnantian. The shift to more positive N isotope values also suggests less intense water-column denitrification. In the overlying late Hirnantian and early Rhuddanian, the distribution of black shales reaches its greatest extent in the studied interval. Localities showing evidence of anoxia are globally spread over all paleolatitudes and water depths for which data are available, indicating a Rhuddanian ocean anoxic event comparable to examples from the Mesozoic. It is accompanied by a return to intensely denitrifying conditions within the water column, as indicated by the shift to negative N isotope values. The two phases of Hirnantian mass extinction coincide with rapid, climate-driven changes in oceanic anoxia. The first extinction occurred at the onset of glaciation and with the loss of anoxic conditions at the end of the Katian. The second extinction occurred at the demise of glaciation and coincided with the return of anoxic conditions during the late Hirnantian–early Rhuddanian. Integration of our N isotope data with graptolite biodiversity records suggests that the extinctions were profoundly influenced by changes occurring at the base of the marine food web, i.e., redox-driven changes in nutrient cycling and primary producer communities.

Turnover from mollusk-dominated Depauperate Zone (late Ordovician) to brachiopod-dominated (early Silurian) faunas in central North America

The Depauperate Zone from the base of the Ordovician (Katian) Maquoketa shale in Dubuque, Iowa, features a range of diminutive infaunal, epifaunal, nektonic and planktonic members with multiple growth stages. Fossils are preserved as phosphatized internal molds. In rare cases, shell replacement conserves growth lines. Sixteen species were recovered, including three-dimensional graptolites (Rectograptus peosta). The bivalves Palaeoneilo fecunda and Nuculites neglectus are most abundant, but the orthid brachiopod Plaesiomys subquadrata also is common. Bivariate analyses were conducted on the bivalve and brachiopod species, two gastropods (Liospira micula and Cyrtolites carinatus), a reputed sponge (Hindia sphaeroidalis) and the graptolite. All mollusks follow an isometric growth pattern and the same is especially characteristic of the orthid brachiopod. Winnowing by waves or bottom currents was minimal. Pene-contemporaneous replacement of fine details was due to concentrations of P2O5 in seawater intermittently brought from the shelf margin through upwelling. Paedomorphosis was likely, because the brachiopod P. subquadrata shows a record of individuals 150% larger in adult size elsewhere in North America. In contrast, populations of Silurian Pentamerus oblongus preserve a full range of immature to adult individuals with a curvi-linear growth pattern. Clear-water deposition under conditions of good marine circulation prevailed during much of the Silurian, although secondary silicification of original CaCO3 shells was common. The muddy Maquoketa substrate vanished due to sea-level rises that flooded all clastic sediment sources.

Phylogeography of Ordovician–Silurian rhynchonelliformean brachiopods: tracking higher order distributional patterns, radiations and extinctions in 4D

Based on a large data-set, spanning the Ordovician–Early Silurian (Telychian) interval, this study explores the phylogeographic distributional pattern for higher orders of rhynchonelliformean brachiopods through four time slices: Tremadocian, Dapingian–Darriwilian, Katian and Telychian. In addition, a palaeobathymetrical analysis has been conducted for the latter two time slices to assess the phylogenetic impact of the Late Ordovician mass extinctions at various palaeowater depths. Whereas the Tremadocian time slice exhibits phylogenetically simple regions characterized by orthide and porambonitoid taxa, these increase in complexity up through the Ordovician and become dominated by the strophomenides. By the Silurian this phylogentic composition shifted once more to atrypide- and pentameride-dominated faunas.

Tectonics and geodynamics of Gorny Altai and adjacent structures of the Altai–Sayan folded area

Packages of Late Paleozoic tectonic nappes and associated major NE-trending strike-slip faults are widely developed in the Altai–Sayan folded area. Fragments of early deformational phases are preserved within the Late Paleozoic allochthons and autochthons. Caledonian fold-nappe and strike-slip structures, as well as accompanying metamorphism and granitization in the region, are typical of the EW-trending suture-shear zone separating the composite Kazakhstan–Baikal continent and Siberia. In the Gorny Altai region, the Late Paleozoic nappes envelop the autochthon, which contains a fragment of the Vendian–Cambrian Kuznetsk–Altai island arc with accretionary wedges of the Biya–Katun’ and Kurai zones. The fold-nappe deformations within the latter zones occurred during the Late Cambrian (Salairian) and can thus be considered Salairian orogenic phases. The Salairian fold-nappe structure is stratigraphically overlain by a thick (up to 15 km) well-stratified rock unit of the Anyui–Chuya zone, which is composed of Middle Cambrian–Early Ordovician fore-arc basin rocks unconformably overlain by Ordovician–Early Devonian carbonate-terrigenous passive-margin sequences. These rocks are crosscut by intrusions and overlain by a volcanosedimentary unit of the Devonian active margin. The top of the section is marked by Famennian–Visean molasse deposits onlapping onto Devonian rocks. The molasse deposits accumulated above a major unconformity reflects a major Late Paleozoic phase of folding, which is most pronounced in deformations at the edges of the autochthon, nearby the Kaim, Charysh–Terekta, and Teletskoe–Kurai fault nappe zones. Upper Carboniferous coal-bearing molasse deposits are preserved as tectonic wedges within the Charysh–Terekta and Teletskoe–Kurai fault nappe zones.Detrital zircon ages from Middle Cambrian–Early Ordovician rocks of the Anyui–Chuya fore-arc zone indicate that they were primarily derived from Upper Neoproterozoic–Cambrian igneous rocks of the Kuznetsk–Altai island arc or, to a lesser extent, from an Ordovician–Early Devonian passive margin. A minor age population is represented by Paleoproterozoic grains, which was probably sourced from the Siberian craton. Zircons from the Late Carboniferous molasse deposits have much wider age spectra, ranging from Middle Devonian–Early Carboniferous to Late Ordovician–Early Silurian, Cambrian–Early Ordovician, Mesoproterozoic, Early–Middle Proterozoic, and early Paleoproterozoic. These ages are consistent with the ages of igneous and metamorphic rocks of the composite Kazakhstan–Baikal continent, which includes the Tuva-Mongolian island arc with accreted Gondwanan blocks, and a Caledonian suture-shear zone in the north. Our results suggest that the Altai–Sayan region is represented by a complex aggregate of units of different geodynamic affinity. On the one hand, these are continental margin rocks of western Siberia, containing only remnants of oceanic crust embedded in accretionary structures. On the other hand, they are represented by the Kazakhstan–Baikal continent composed of fragments of Gondwanan continental blocks. In the Early–Middle Paleozoic, they were separated by the Ob’–Zaisan oceanic basin, whose fragments are preserved in the Caledonian suture-shear zone. The movements during the Late Paleozoic occurred along older, reactivated structures and produced the large intracontinental Central Asian orogen, which is interpreted to be a far-field effect of the colliding East European, Siberian, and Kazakhstan–Baikal continents.