Late Ordovician-Early Silurian Research Articles

Controls on Marine Primary Productivity Variation and Organic Matter Accumulation During the Late Ordovician-Early Silurian Transition

Controls on Marine Primary Productivity Variation and Organic Matter Accumulation During the Late Ordovician-Early Silurian Transition

Shale types and sedimentary environments of the Upper Ordovician Wufeng Formation-Member 1 of the Lower Silurian Longmaxi Formation in western Hubei Province, China

Abstract By performing scanning electron microscopy, microscopic observations, whole-rock X-ray diffraction analysis, organic geochemistry analysis, and elemental analysis on drill core specimens and thin sections, in this study, we classified the shale types of the Wufeng Formation-Member 1 of the Longmaxi Formation in western Hubei, southern China, and explored the development characteristics and formation environments of the different shale types. The results show that (1) the shales of the Wufeng Formation-Member 1 of the Longmaxi Formation are composed of three types of shale: siliceous shale, mixed clay-siliceous shale, and clay shale. The siliceous shale is a type of shale unique to deep-water environments; clay shale is the main type of shale formed in shallow-water environments; and mixed clay-siliceous shale falls between the two. (2) The changes in shale type are characterized by multiple depositional cycles in the vertical direction with strong heterogeneity and an obvious tripartite character, and the siliceous shales gradually thicken as they laterally extend northwestward, with their last depositional cycle gradually ending at a later time. (3) The Late Ordovician-Early Silurian paleoenvironment can be divided into six evolutionary stages (A, B, C, D, E, and F) from early to late. In particular, the sea level was relatively lower in stages A and F when the bottom water was mainly oxygen rich with higher terrigenous inputs and a lower paleoproductivity, which led to the formation of clay shales poor in organic matter but rich in terrigenous quartz clasts. The sea level was higher in stages B, C, and D when the bottom water was anoxic with lower terrigenous inputs and a higher paleoproductivity, which led to the formation of siliceous shales rich in organic matter and biogenic silica. The total organic carbon (TOC) contents of siliceous shales decrease in the order of stage C > stage D > stage B, which is mainly attributed to the different degrees of water restriction in the three stages and the consequently different paleoproductivities. Stage E corresponds to the mixed clay-siliceous shales, the depositional environment of which is between those of the siliceous shales and the clay shales, thereby resulting in the mineral composition and TOC content of the mixed clay-siliceous shales being between those of the other two shale types.

Spatiotemporal Evolution and Genesis of the Late Ordovician—Early Silurian Marine Euxinia in Northeastern Upper Yangtze Basin, South China

Marine redox conditions and their dynamic changes were a major factor that controlled the formation of black shale and caused the late Ordovician marine extinction in the Upper Yangtze Basin (South China). However, the spatiotemporal variation and potential controlling factors of marine redox conditions in this area remain unclear. We analyzed whole-rock geochemistry and pyrite sulfur isotopes (δ34Spy) of 47 shale samples from the Late Katian to Rhuddanian in a shelf-to-slope (Qianjiang Shaba section and Wc-1 well) region of northeastern Upper Yangtze Basin, and reconstructed water column redox conditions during the Late Ordovician–Early Silurian Transition. The geochemical characteristics of shale, including the ratio of elements, discriminant function and ternary diagram location in the study area suggest a passive continental margin sedimentary environment, wherein the terrigenous detritus is mainly derived from felsic igneous rocks in the upper crust, showing characteristics of near-source deposition. The redox indices (Fe speciation, Corg/P, UEF, and MoEF) showed that the development of anoxic water, especially euxinia, has obvious spatiotemporal heterogeneity. Under conditions of high availability of active organic carbon and limited sulfate supply, high active Fe input and strong biological irrigation in the shallow water area may effectively remove H2S produced by microbial sulfate reduction, conducive to the prevalence of ferruginous water columns. However, for this deep water area, the rapid accumulation rate of organic matter, decrease in dissolved Fe (caused by upwelling in the open sea), and seawater stratification (caused by the rising of sea level) promoted the development of a euxinic water column. This inference is supported by the covariant relationship between organic carbon accumulation rate, chemical index of alteration, Co × Mn, and δ34Spy. Our study highlights the potential control effects of sea level change, continental weathering and upwelling on the development of euxinic water columns.

Mineralogy of columbite-group minerals from the rare-element pegmatite dykes in the East-Qinling orogen, central China: Implications for formation times and ore genesis

The East-Qinling pegmatite district located in the North Qinling terrane is the largest one in the Qinling orogen, Central China. It is famous for its rare-element (REL) reserves, but its genesis is unraveled. We present in-situ major- and trace-element chemistry and U-Pb dating analysis of columbite-group minerals (CGM) and wodginite from representative Li-, Be-mineralized and barren pegmatite dykes. The mineralogical researches reveal that Li-mineralized pegmatite formed from a highly-evolved fluxes-rich magma, experiencing a magmatic stage at undercooling condition and a post-magmatic stage involving disturbance by alkali liquid or hydrothermal fluid. The lithium mineralization is accomplished by fractional crystallization after emplacement, alkali-fluid replacement or overprint of two Li-rich magmas. CGM Fe-Mn fractionation and REE distribution patterns with tetrad effect are considered as indicators to identify generations of CGM and trace the evolution of pegmatite formation. The CGM and zircon U-Pb ages suggest that the REL pegmatites formed in 422–410 Ma (Period I) and 399–384 Ma (Period II) and an early pegmatite magma activity occurred in 447–443 Ma. The late Ordovician-early Silurian and Period II pegmatite magmas stem from partial melting of North Qinling unit in collision and post-orogenic extensional settings, respectively, whereas Period I pegmatites formed in a weakening convergence setting by anatexis of fertile metasedimentary rocks or extremely fractional crystallization of granitic magma. The early-Paleozoic evolution of the North Qinling terrane contributes to the formation of large-scale East-Qinling REL pegmatites on the aspects of magma production and subsequent emplacement by crustal thickening, crustal reworking and dynamic extension during crust uplift.

HirnantiaFauna from the Condroz Inlier, Belgium: another case of a relict Ordovician shelly fauna in the Silurian?

AbstractThe end-Ordovician mass extinction, linked to a major glaciation, led to deep changes in Hirnantian–Rhuddanian biotas. TheHirnantiaFauna, the first of two Hirnantian survival brachiopod-dominated communities, characterizes the lower–mid Hirnantian deposits globally, and its distribution is essential to understand how the extinction took place. In this paper, we describe, illustrate, and discuss the first macrofossiliferousHirnantiaFauna assemblage from Belgium, occurring in the Tihange Member of the Fosses Formation at Tihange (Huy), within the Central Condroz Inlier. Six fossiliferous beds have yielded a low-diversity, brachiopod-dominated association. In addition to the brachiopods (Eostropheodonta hirnantensis,Plectothyrella crassicosta,Hirnantiasp., andTrucizetina? sp.), one trilobite (Mucronaspissp.), four pelmatozoans (Xenocrinussp.,Cyclocharax[col.]paucicrenulatus,Conspectocrinus[col.]celticus, andPentagonocyclicus[col.] sp.), three graptolites (Cystograptus ancestralis,Normalograptus normalis, and ?Metabolograptussp.), together with indeterminate machaeridians and bryozoans were identified. The graptolite assemblage, from theAkidograptus ascensus-Parakidograptus acuminatusBiozone, indicates an early Rhuddanian (Silurian) age, and thus, an unexpectedly late occurrence of a typicalHirnantiaFauna. This Belgian association may represent an additional example of relictHirnantiaFauna in the Silurian, sharing characteristics with the only other known from Rhuddanian rocks at Yewdale Beck (Lake District, England), although reworking has not been completely ruled out. The survival of these Hirnantian taxa into the Silurian might be linked to delayed post-glacial effects of rising temperature and sea-level, which may have favored the establishment of refugia in these two particular regions that were paleogeographically close during the Late Ordovician–early Silurian.

Effects of astronomical orbital cycle and volcanic activity on organic carbon accumulation during Late Ordovician–Early Silurian in the Upper Yangtze area, South China

Based on field outcrop data, the effects of cyclic change of astronomical orbit and volcanic activity on organic carbon accumulation during the Late Ordovician – Early Silurian in the Upper Yangtze area were studied using cyclostratigraphic and geochemical methods. δ 13 C and chemical index of alteration ( CIA ) were used to filter the astronomical orbit parameters recorded in sediments. It is found that the climate change driven by orbital cycle controls the fluctuations of sea level at different scales, obliquity forcing climate changes drive thermohaline circulation (THC) of the ocean, and THC-induced bottom currents transport nutrient-laden water from high latitude regions to the surface water of low-latitude area. Hence, THC is the main dynamic mechanism of organic-carbon supply. The marine productivity indexes of Ba/Al and Ni/Al indicate that volcanic activities had limited effect on marine productivity but had great influences on organic carbon preservation efficiency in late Hirnantian (E4). Paleo-ocean redox environmental indicators Th/U, V/Cr and V/(V+Ni) show that there is a significant correlation between volcanism and oxygen content in Paleo-ocean, so it is inferred that volcanisms controlled the organic carbon preservation efficiency by regulating oxygen content in Paleo-ocean, and the difference in volcanism intensity in different areas is an important factor for the differential preservation efficiency of organic carbon. The organic carbon input driven by orbital cycle and the preservation efficiency affected by volcanisms worked together to control the enrichment of organic carbon in the Middle–Upper Yangtze region.

Graptolite zone calibrated stratigraphy and topography of the late Ordovician-early Silurian Wufeng-Lungmachi shale in Upper Yangtze area, South China

Stratigraphic architecture and topography of the Late Ordovician-Early Silurian Wufeng-Lungmachi shale in the Upper Yangtze area, South China, are analyzed based on calibration of graptolite zones and integrated interpretation of data sets of 3D seismic, drilling wells, and outcrops. The lithostratigraphic architecture is well correlated with graptolite zones. Going upwards, the Wufeng Formation is equivalent to graptolite zone WF1–4, layer 1 to LM1, layer 2 to LM2–3, layer 3 to LM4, layer 4 to LM5, sub-member 2 to LM6–8, and member 2 to LM9 of the Lungmachi Formation. During this depositional period, the Upper Yangtze area was bordered by Chuangzhong thickness zero area (CZZA) in the northwest and Qianzhong-Xuefeng thickness zero area (QXZA) in the southeast and the zero lines migrated southeastwards progressively. The Upper Yangtze area had southwest (SDA) and northeast (NDA) depositional areas separated along Zuo3-FL-TH1. The SDA has fairly smaller shale thickness and distribution area relative to the NDA. In addition, both the SDA and the NDA had depocenters which migrated away from the QXZA progressively. The locations of the CZZA and QXZA may show the possible positions of the Chuanzhong Paleo-uplift and the Qianzhong-Xuefeng Old Land. During this period, the topography of the Upper Yangtze area was fairly rough and a submerged high might occur along Zuo3-FL-TH1. Because of sustained compression from the Cathaysia block, the depocenters migrated northwestwards and the sedimentation rate increased continuously.

Major, trace-elemental and sedimentological characterization of the upper Ordovician Wufeng-lower Silurian Longmaxi formations, Sichuan Basin, south China: Insights into the effect of relative sea-level fluctuations on organic matter accumulation in shales

The relative sea-level fluctuations play a vital role in the organic matter enrichment of black shales across the world. The Wufeng-Lower Longmaxi shales deposited during the Late Ordovician to Early Silurian stage are accompanied by the global transgression and regression, whereas the role of relative sea-level changes on organic matter accumulation has been rarely discussed. In this study, we integrated TOC content, X-ray diffraction (XRD) mineralogical composition, thin section and scanning electron microscope (SEM) images, elemental geochemical data, and petrographic features of pyrite framboids to investigate the varying paleoenvironmental conditions and the effect of relative sea-level changes on organic matter accumulation under a robust sequence stratigraphic framework.Based on detailed petrographic observations and mineralogical analysis, six major lithofacies are identified in the Wufeng-Lower Longmaxi formations, including siliceous mudstone, clay-rich siliceous mudstone, siliceous-argillaceous mixed shale, silty mudstone, siltstone, and bentonite. TOC content is generally higher in the transgressive systems tracts (relative sea-level rise stage) than in the highstand systems tracts and falling stage systems tract (relative sea-level fall stage). Multiple geochemical proxies indicate contrasting paleoenvironmental changes between relative sea-level rise and fall stage. Relative sea-level rise resulted in elevated productivity, predominantly anoxic conditions, low detrital flux, and moderate water mass restriction, as indicated by high biogenic Si content, high Mo-EF (enrichment factor of element Mo), small-sized pyrite framboids, low Al content and moderate Mo/TOC ratio. On the contrary, relative sea-level fall generally resulted in decreased productivity, dysoxic conditions, increased detrital flux, and strongly restricted conditions. The correlations between TOC content and geochemical proxies suggest that organic matter accumulation was a combined effect of paleoproductivity, paleoredox conditions, detrital influx, and water mass restriction. The variation of these factors was strongly influenced by relative sea-level fluctuations related to Hirnantian glaciation and regional tectonic movements during the Late Ordovician-Early Silurian transitional stage.

The Role of Deep Geofluids in the Enrichment of Sedimentary Organic Matter: A Case Study of the Late Ordovician-Early Silurian in the Upper Yangtze Region and Early Cambrian in the Lower Yangtze Region, South China

Organic matter is the material basis for shales to generate hydrocarbon, as well as the main reservoir space and seepage channel for shale gas. When the thermal evolution degree is consistent, the organic carbon content in present shales is subject to the abundance of primitive sedimentary organic matter. Deep geofluids significantly influence the sedimentary organic matter’s enrichment, but the mechanism remains unclear. This paper is aimed at determining how hydrothermal and volcanic activities affected the enrichment of sedimentary organic matter by studying lower Cambrian shales in the lower Yangtze region and upper Ordovician-lower Silurian shales. Oxidation-reduction and biological productivity are used as indicators in the study. The result shows that hydrothermal or volcanic activities affected the enrichment of sedimentary organic matter by influencing climate changes and the nutrients’ sources on the waterbody’s surface and reducing water at the bottom. In the lower Cambrian shales of the Wangyinpu Formation in the lower Yangtze region, hydrothermal origin caused excess silicon. During the sedimentary period of the lower and middle-upper Wangyinpu Formation, vigorous hydrothermal activities increased the biological productivity on the waterbody’s surface and intensified the reducibility at the bottom of the waterbody, which enabled the rich sedimentary organic matter to be well preserved. During the sedimentary period of the lower upper Ordovician Wufeng Formation and the lower Silurian Longmaxi Formation in the upper Yangtze region, frequent volcanic activities caused high biological productivity on the waterbody surface and strong reducibility at the bottom of the waterbody. As a result, the abundant organic matter deposited from the water surface can be well preserved. During the sedimentary period of the upper Longmaxi Formation, volcanic activities died down gradually then disappeared, causing the biological productivity on the water surface to decrease. Besides, the small amount of organic matter deposited from the water surface was destroyed due to oxidation.

Geological significance of rare earth elements in marine shale during the Late Ordovician–Early Silurian in Sichuan Basin, South China

Rare earth elements (REEs) record a large amount of geological information, and serve as a good geological indicator that can effectively predict the palaeoclimate, palaeoenvironment, and palaeotectonic evolution of the sedimentary rocks. The REEs and major elements of 94 marine shale samples obtained from the Late Ordovician to the Early Silurian at Shuanghe (SH) and Qiliao (QL) sections in Sichuan Basin were analysed in this study. Results showed that the major elements of Al2O3, K2O, and TiO2 were mainly controlled by clay minerals associated with terrigenous detrital, but the SiO2 and Na2O were not influenced by clay minerals. In addition, ΣREE values obtained at Shuanghe and Qiliao sections during the Late Ordovician–Early Silurian were close to those of North American Shale (NASC) composites (173.2 ug/g). REEs were characterized by light rare earth elements enrichment, but heavy rare earth elements depletion. Furthermore, the NASC‐normalized REE patterns were similar with a little difference at Shuanghe and Qiliao sections. The following conclusions were drawn by studying REE indicators associated with source, redox condition and the mechanism of organic matter enrichment: (a) ΣREE showed strong positive correlations with TiO2, Al2O3, K2O, and Na2O, but exhibited relatively poor correlations with Fe2O3 and P2O5. These results suggested that REEs in the marine shale mainly occurred in clay minerals, but also were influenced by pyrite, phosphates, and other minerals. (b) Ce/Ce* showed that marine shale during the Late Ordovician–Early Silurian was mainly deposited under anoxic condition in Sichuan Basin and the reduction intensity at Qiliao section was stronger than Shuanghe section. (c) The diagram between La/Yb and ΣREE and the Ce/Ce* values indicated that the detrital minerals in the marine shale were mainly derived from the sedimentary rocks and deposited in the same continental margin environment with a complex composition. (d) The controlling factors of organic matter enrichment were different at Shuanghe and Qiliao sections and the depositional environment controlled the organic matter accumulation and preservation of the organic‐rich marine shale.

Reconstructing the Climatic-Oceanic Environment and Exploring the Enrichment Mechanism of Organic Matter in the Black Shale across the Late Ordovician-Early Silurian Transition on the Upper Yangtze Platform Using Geochemical Proxies.

Black shale deposited in the transitional period from the Late Ordovician to Early Silurian is the most important source rock and shale gas reservoir in the Yangtze region of South China. However, the source of these sediments is still controversial. In this paper, the changes in total organic carbon (TOC), total sulfur (TS), organic carbon isotopes (δ13Corg), biomarkers, trace elements, and rare earth elements in the Ordovician–Silurian boundary strata of the XK-1 well in northern Guizhou Province, South China, have been systematically studied. The paleoenvironmental and paleoclimatic conditions of the Late Ordovician to Early Silurian and their relationship with organic matter enrichment in the Upper Yangtze Platform have been reconstructed. The distribution of biomarkers reflects that the Late Ordovician–Early Silurian shale was deposited in the marine environment and was highly contributed by marine plankton/algae and microorganisms. Paleoclimatic proxies (Sr/Cu, δ13Corg) show that the global climate system experienced significant changes from a warm-humid climate to a brief period of cold-dry climate and then back to a warm-humid climate during the Ordovician–Silurian transition. This warm and humid climate condition helps to improve the biological productivity within the photic zone of the water column during deposition. In addition, the low oxygen (reduction) conditions during the deposition of the Late Ordovician–Early Silurian deposits are characterized by low Pr/Ph values (0.39–0.79) and relatively high elemental ratios of V/Ni (1.40–5.77) and V/(V + Ni) (0.58–0.85). This paleoredox condition contributes to the preservation of organic matter during deposition of the Late Ordovician–Early Silurian deposits. Therefore, it is demonstrated that the climate and ocean fluctuated greatly during the Late Ordovician–Early Silurian transition period, and this fluctuation provided necessary control factors for marine anoxia, primary productivity, and subsequent organic-rich black shale deposition in the Upper Yangtze region during the Late Ordovician and Early Silurian intervals.

Coupling of P–T–t–D histories of eclogite and metagreywacke—Insights to late Ordovician–Silurian crustal folding events recorded in the Beishan Orogen (NW China)

AbstractThe Beishan complex is composed of orthogneiss and metagreywacke that both enclose bodies of eclogite and serves as a unique example for comparative petrological study of all these lithologies. The rocks show the earliest regional steep N‐S striking fabric (S2) preserved in low strain domains that are reworked by ubiquitous steep N‐NE dipping cleavage (S3). The eclogite shows an almost isotropic fabric defined by an M1 assemblage of Grt–Cpx–Amp–Qz–Rt–Ilm that is locally retrogressed to M2‐3 amphibolite facies mineral assemblages, with P–T peak at 20–21 kbar and 750–775°C and retrogression to 2–3kbar and 530–550°C. The typical mineral assemblage of the host metagreywackes is Bt–Ms–Pl–Qz−Chl–Ilm±Grt. Rare Al‐rich metagreywacke layers are composed of Grt–Ky–St±Sil−And–Bt–Ms–Pl–Qz±Chl±Rt–Ilm giving a P–T path with peak at 8–8.5kbar and ~670°C correlated with the S2 fabric and retrogression to ~2.5kbar and 525–550°C correlated with the S3 foliation. In two eclogite samples, the garnet‐whole rock‐clinopyroxene Lu–Hf isochrons give ages of 461.9±1.6 Ma and 462.0±6.2 Ma interpreted as reflecting average age of garnet formation, and Sm–Nd isochrons give ages of 453.6±2.7 Ma and 452.8±3.0 Ma interpreted as dating near‐peak metamorphism. In metagreywacke, in‐situ U–Pb dating of monazite gives two groups of ages of 445–440 Ma (Mnz cores) and 436–429 Ma (Mnz rims), interpreted as reflecting the metamorphic peak and retrogression. Our results show that eclogite was formed during Ordovician by subduction of a continental crust (D1). Eclogite and metagreywacke underwent partly decoupled P–T–t–D paths until their juxtaposition at mid‐crustal levels during a first late Ordovician–early Silurian D2 shortening. Coupling of their P–T–t–D paths occurred during exhumation in the Silurian and a second and orthogonal D3 shortening event. The data from the Beishan Orogen are consistent with a collisional intra‐Gondwanan orogen located south of the Central Asian Orogenic Belt.

A missing link of the Proto-Tethys Ocean between the Qinling and Qilian orogens, China: Insights from geochronology and structural geology

The Qinling-Qilian connection zone (QQCZ) is a key area to reveal the relationship and to make a link of the North Qinling and the North Qilian orogens, China. Here we present U–Pb dating data of detrital zircons from four sedimentary/metasedimentary rocks in the QQCZ and the southwestern North China Block (NCB) and detailed regional structural data. Three episodes of fold deformation (D1, D2 and D3) are distinguished in the QQCZ, with the former two occurred during the early Paleozoic. The D1 deformation is mainly characterized by regionally penetrative schistosity and some residual rootless intrafolial folds due to the intensive superpositions by the subsequent D2 and D3 deformations. The D2 deformation characterized by tight folds, associated axial plane foliations and crenulation lineations indicates a stress field characterized by NNE–SSW-directed compression, which may be induced by the collision between the NCB and the southern blocks. The D3 deformation which might occur during the Mesozoic is marked by upright open folds and kink bands. The similarity of the detrital zircon age spectra of the Huluhe Group in the North Qilian Orogen and the Erlangping Group in the North Qinling Orogen suggests that the two groups have similar provenance, which may indicate that the North Qilian Orogen corresponded to the North Qinling Orogen in a regional tectonic framework. In addition, the remarkable age peak at ~435 ​Ma of the detrital zircon age spectrum of the Duanjiaxia Formation in the southwestern NCB indicates that this formation obtained the provenance of the North Qilian and North Qinling orogens, which may be generated by the collage of the southwestern NCB and the QQCZ during the Late Ordovician–Early Silurian. Based on structural, detrital zircon and metamorphic data, we suggest that the North Qilian and North Qinling orogens underwent similar evolution during the early Paleozoic due to the closure of the North Qilian and the Kuanping oceans which located at the northern boundary of the Proto-Tethys Ocean.

Metallogenesis and fluid evolution of the Huangtupo Cu–Zn deposit, East Tianshan, Xinjiang, NW China: Constraints from ore geology, fluid inclusion geochemistry, H–O–S isotopes, and U–Pb zircon, Re–Os chalcopyrite geochronology

The recently discovered Huangtupo Cu–Zn deposit is located in the Dananhu–Tousuquan arc, East Tianshan, NW China. Orebodies are hosted in volcano-sedimentary rocks of the Late Ordovician–Early Silurian Daliugou Formation. The metallogenic physicochemistry, ore-forming fluids, and metallogenesis of the deposit were investigated through orebody features, fluid inclusion microthermometry, laser Raman spectroscopy, and synchrotron-radiation X-ray fluorescence (SRXRF). The results indicate that the deposit includes an upper sedimentary exhalative ore belt (stage I) as well as a lower vein-stockwork ore belt in a hydrothermal fluid supply channel (stage II) associated with silicification, chloritization, epidotization, pyrophyllitization, and beresitization alteration. The Re–Os isochron age of 429.5 ± 10 Ma obtained from primary chalcopyrite. The LA-ICP-MS zircon U–Pb age of 423.3 ± 2.9 Ma for the host hangingwall tuff. Fluid inclusion (FI) microthermometry and geological characteristics were applied to constrain hydrothermal processes and ore genesis. Quartz and barite were collected from the massive sulfide and vein ores for FI study. Four types of FI were identified: liquid-rich L-type, vapor-rich V-type, daughter-mineral-bearing S-type, and C-type containing CO2–H2O. Stage I barite FIs yield homogenization temperatures of 232–364 °C (peak value 265 °C), with salinities of 4.85–10.94 wt% NaCl equiv., whereas stage II quartz FIs yield temperatures of 270–400 °C (peak value 350 °C), with salinities of 2.41–13.72 wt% NaCl equiv (peak value 6.28 wt%). Laser Raman spectroscopy analyses of FI gas compositions indicate that the FIs contain mainly H2O with minor CO2 and N2. Initial ore-forming fluids comprise a medium-high-temperature, low-salinity system of hot evolved seawater that was cooled and diluted by mixing with cold seawater. Fluid mixing and local boiling would have facilitated mineralization. SRXRF analyses indicate that the liquid phase is enriched in Cu and Zn. The δ34S values of sulfide indicate that the sulfur was derived from the magma (host volcanic rocks). Oxygen and hydrogen isotopic data also imply that the ore-forming fluids were mainly derived from deep circulation of seawater, and mixed with minor magmatic fluids. The estimated trapping pressures for vein-stockwork mineralization fluids range from 20 to 60 bar (average 40 bar) at a depth of ~0.4 km. The ore fluids ascenting along faults and fracture networks at or near the seafloor played important roles in oreforming processes of the Huangtupo VMS Cu–Zn deposit.

Paleomagnetism of Ordovician–Silurian Volcanics on the Western Slope of the Southern Urals

This work presents new paleomagnetic data on previously dated Ordovician–Silurian volcanics from four sections in the western framework of the Taratash massif (Southern Urals). The geological data indicate that the region under study was a part of the Baltica paleocontinent since the early Mesoproterozoic. The paleomagnetic data obtained for rocks of the westernmost part of the Ural Fold Belt show no evidence of local and regional rotations relative to Baltica. Consequently, the pole obtained can be extrapolated to the entire platform. The result obtained is characterized by a rather high degree of reliability and can clarify the trajectory of the apparent polar wander path (APWP) for the Baltica paleocontinent during the Late Ordovician–Early Silurian, where reliable paleomagnetic data are not available, and can be used for further paleoreconstructions.

Geochemical characteristics and organic matter enrichment of the Late Ordovician-Early Silurian black shale in Nanchuan area, Sichuan Basin

The enrichment of organic matter in black shale is a controversial issue among the researchers and critical problem in the exploration of shale gas. In order to understand the relationship between sedimentary environment and organic matter accumulation in black shale of Wufeng Formation. Longmaxi Formation and limestone of Guanyinqiao Bed, multiple geochemical method, such as organic carbon, major and trace elements, are tested from the Nanchuan section. Research results show that the organic carbon is enriched in the middle to upper part of Wufeng Formation and the bottom of Longmaxi Formation. Ba and P contents exhibit a high primary productivity during Late Ordovician-Early Silurian in Sichuan basin. Redox proxies (V/Cr, Ni/Co) indicate that Wufeng Formation deposited in an anoxic environment and Longmaxi Formation deposited in euxinic condition, but Guanyinqiao Bed was predominated by oxic-dysoxic environment during the sedimentation. The poor relation between TOC and Ba, suggests primary productivity are not the major factors controlling the enrichment of organic matter, but an anoxic environment is the predominant condition causing the accumulation and preservation of organic matter suggested by a positive correlation between TOC and redox proxies.

Cyclocrinitids from the Lower Palaeozoic Tethyan sequence of Spiti, India

Well preserved cyclocrinitids (calcareous green algae) are reported from early Palaeozoic deposits of Spiti, Himachal Pradesh, India. Casts of the algal fossils are preserved in calcareous grey siltstone unit of the Takche Formation (Late Ordovician–Early Silurian) which appears rusty/earthy brown due to weathering. The assemblage includes Cyclocrinites favus, Cyclocrinites pyriformis, Cyclocrinites cf. welleri, Cyclocrinites sp. and Cyclocrinites globosus. Both Cyclocrinites cf. welleri and Cyclocrinites globosus are reported for the first time from the entire Tethyan Himalaya, India. The described algal flora basically denotes relatively shallow marine depositional setting and low to moderate hydrodynamic conditions.

Nature of the Late Ordovician-Early Silurian Xiaohe section, Hunan-Hubei area, South China: implications for the Kwangsian Orogeny

ABSTRACT The intraplate Kwangsian Orogeny is a key orogenic event in South China in the mid-Paleozoic. We re-examined the evidence for the Yichang Uplift, an inferred geographic feature during the Kwangsian Orogeny, to evaluate its timing and nature. Field, sedimentological, mineralogical and geochronological data were collected from the Late Ordovician-Early Silurian Xiaohe section, Hunan-Hubei area. Results suggest that the Xiaohe section is composed of the Late Ordovician Wufeng Formation black shale in the lower part and the Early Silurian Longmaxi Formation black shale in the upper part. We found that the clay layers interbedded in the Wufeng Formation are altered rhyolitic tuffs instead of parts of a subaerial wreathing crust. LA-ICP-MS U-Pb dating of zircons in the top tuff layer of the Wufeng Formation yielded an age of 447.0 + 1.4/- 2.2 Ma, consistent with biostratigraphic data, providing a radiometric constraint for the sedimentary break existed between the Wufeng and Longmaxi formations and confirming the absence of the Hirnantian (latest Ordovician) Guanyinqiao Formation in the study area. Our data support that the Yichang Uplift was a submarine highland possibly initiated by the reactivation of the inherited Jianshi-Enshi Fault in the Hunan-Hubei area during the Kwangsian Orogeny.

Climatic and oceanic controlled deposition of Late Ordovician-Early Silurian black shales on the North Yangtze platform, South China

Abstract Black shales deposited across the Late Ordovician and Early Silurian transition are considered to be the most important source rock and shale gas reservoirs in the Yangtze region of South China. However, the origin of these sediments remains contentious. In this study, we investigated the systematic changes in total organic carbon (TOC), organic carbon isotopes (δ13Corg), major elements and trace elements on the Ordovician-Silurian boundary strata from Qiaoting, North Yangtze Sea, to reconstruct the climatic conditions, redox changes, primary productivity, and other factors there, promoting the understanding for the formation mechanism of the organic-rich black shales. Element compositions and their ratios suggest that the black shales studied here were dominantly sourced by the felsic igneous rock, which is similar to granodiorite in composition. Paleoclimatic proxies (CIA, CIW, δ13Corg) suggest that the global climate system experienced significant changes from warm-humid to cold-dry and then to warm-humid climates during the Ordovician-Silurian transition. Paleoredox indices (S/C, U/Th, Ni/Co and V/(V + Ni) ratios) also demonstrate fluctuating redox variations from anoxic to oxygenated, and then to anoxic states during this interval in the Yangtze Sea, which describe an oxygenated Guanyinqiao strata sandwiched between the stagnant and anoxic Wufeng and Longmaxi black shales. Paleoproductivity parameters (TOC, Babio contents and P/Al ratio) suggest that the organic-rich Wufeng and Longmaxi black shales, were deposited with a high biological productivity, while the organic-lean Guanyinqiao sediments were deposited with a low biological productivity. These data demonstrate large climatic and oceanic fluctuations during the Ordovician-Silurian transition, providing essential controlling factors on the oceanic anoxia, primary productivity, and subsequent organic-rich black shale depositions in the Yangtze region during the Late Ordovician and Early Silurian intervals.

New taxa and revised stratigraphic distribution of the crinoid fauna from Anticosti Island, Québec, Canada (Late Ordovician-early Silurian)

AbstractEnd-Ordovician extinctions had a profound effect on shallow-water benthic communities, including the Crinoidea. Further, recovery after the extinctions resulted in a macroevolutionary turnover in crinoid faunas. Anticosti Island is the most complete Ordovician-Silurian boundary section recording shallow-water habitats. Both new taxa and changes in Anticosti Island stratigraphic nomenclature are addressed herein. New taxa includeBecsciecrinus groulxin. sp.,Bucucrinus isotaloin. sp.,Jovacrinus clarkin. sp.,Plicodendrocrinus petrykin. sp.,Plicodendrocrinus martinin. sp.,Thalamocrinus daoustaen. sp., andLateranicrinus saintlaurentin. gen. n. sp. The status ofXenocrinus rubusas a boundary-crossing taxon is confirmed, range extensions of several taxa are documented, and the distribution of crinoids with the revised stratigraphic nomenclature is documented.UUID:http://zoobank.org/19613a44-ec69-47d7-88ab-fcf88ba771f0.