Late Ordovician-Early Silurian Research Articles

Geochemistry of siltstones in the North Qilian Zhongbao–Mayinggou formations (Late Ordovician–Early Silurian): implications for paleoclimate, paleoweathering, and tectonic setting

The Late Ordovician–Early Silurian period witnessed the Phanerozoic mass extinction, glacial events, and volcanic events. Paleoweathering indexes chemical index of alteration (CIA), chemical index of weathering (CIW), and plagioclase index of alteration (PIA) indicated that the source area weathering changed from weak to moderate to intense. CIA values in the upper Zhongbao formation ranged from 66.71% to 73.97%, indicating a drier and colder climate. Upward, the CIA values on the bottom of the Mayinggou formation returned to the high value quickly (from 73.86% to 81.31%), suggesting that the ice age ended, the climate became warmer and wetter, and the sea level rose. The Al2O3–(CaO*+Na2O)–K2O triangular plots, Hf-La/Th, and SiO2–Al2O3/TiO2 bivariate plots inferred that the source of the siltstones in the two formations is mostly from the felsic igneous rocks. The samples from the Zhongbao–Mayinggou formations have chondrite-normalized rare earth element (REE) patterns similar to that of the North Qilian volcanic arc rocks. Geochemical discrimination plots displayed that the sediments of the Zhongbao–Mayinggou formations came from the active continental margin setting.

Magmatism during late Ordovician-early Silurian accretion of the Caledonides of Arctic Scandinavia: the Halti–Guolasjávri area revisited

In the Scandinavian Caledonides, evidence of syn-collisional magmatism related to extensional basin development immediately prior to late Silurian Baltica–Laurentia collision was considered restricted to exotic terranes until late Silurian ages were obtained from the Halti Igneous Complex (HIC), hosted by a thrust sheet (Corrovarre Nappe, CN) of continental affinity. Various orogenic models for the extension and magmatism, a. o. subduction flip, slab roll-back, and ridge subduction have been proposed. Crucial factors include the affinity (Baltican or exotic) of the CN, and the nature of the debated unconformity at the base of the overlying exotic Vaddas Nappe (Köli). This study reexamines a critical tectonostratigraphic section and reports U–Pb zircon ages (441–436 Ma) of palingenetic granitic dykes generated by the HIC. We reinterpret the CN as a slice of the continental margin and accreted with some other nappes of the Seve Nappe Complex (SNC) which decoupled from the continent-ocean transition (COT) at an early stage of subduction of the margin. The lower part of the bipartite Vaddas Nappe, composed of a very dense dolerite dyke swarm with screens of quartzite, marble and subordinate black schist, is reinterpreted as a continuation northwards of the uppermost nappe of the SNC in Indre Troms, derived from the Ediacaran outermost continental margin. The contact with the overlying telescoped succession of conglomerates, marbles, volcanics and turbidites of the Vaddas Nappe is tentatively interpreted as a tectonised unconformity. To a classical model of back-arc spreading outboard of Laurentia during closure of the Iapetus Ocean, we add the arrival of Baltica, with a prism of decoupled slices of the COT and exhumed/exhuming UHP nappes. The spreading centre of the basin provides a plausible setting for gabbros and pillow lavas of the Vaddas Nappe, and the shores of the basin for a Seve–Köli unconformity. Future PTt studies are required in order to be able to relate the proposed shallow, syn-exhumation HIC magmatism to the tectonometamorphic evolution of the HP–UHP nappes of the SNC.

First Record of Cryptospores from the Late Ordovician–Early Silurian of Colombia: New Contribution to the Understanding of Plant Terrestrialization

First Record of Cryptospores from the Late Ordovician–Early Silurian of Colombia: New Contribution to the Understanding of Plant Terrestrialization

Influence of the Late Ordovician‐Early Silurian Paleoenvironment and Related Geological Processes on the Organic Matter Accumulation and Carbon Isotope Excursion

AbstractAlthough previous studies have shown that the paleoenvironment and geological processes contributed to the organic matter accumulation (OMA) and carbon isotope excursions (CIEs) during the late Ordovician–early Silurian, the dominated controlling factor for the OMA and the origins of CIEs still remains unclear due to complex interaction between various paleoenvironmental factors and geological processes. Therefore, based on the elemental geochemistry of the Wufeng–Longmaxi Formation shales in the upper Yangtze Platform, we analyzed the late Ordovician–early Silurian paleoenvironment and related geological processes, and further explored the origin of the OMA and CIEs. As a result, the Wufeng–Longmaxi Formation shale was divided into four stages. During Stage 1 (late Katian, ∼447.62–444.50 Ma), local tectonic and volcanic activities controlled the paleoproductivity and redox conditions, facilitating the OMA. By contrast, the productivity of the surface water and the anoxic bottom water were mainly controlled by the global climate after the Hirnantian glaciation, which contributed to the OMA during Stage 2 (early Rhuddanian, ∼444.50–441.00 Ma). The decreasing sea level and rapid uplifting of Xuefeng and Qianzhong Uplifts resulted in the organic matter depletion during Stages 3 (late Rhuddanian, ∼441.00–440.80 Ma) and 4 (Aeronian, ∼440.80–439.21 Ma). Besides, the release of 12C–enriched carbon reservoirs triggered by volcanic activities and 13C–enrichment caused by the OMA regulated the carbon cycling: the negative CIE may be the result of light carbon emissions, such as the reactivated organic matter and mantle derived carbon, and the Hirnantian CIE event is jointly controlled by the weakened carbon emission effect and the OMA.

Contribution of Various Shale Components to Pore System: Insights from Attributes Analysis

Shale pore systems are the result of the geological evolution of different matrix assemblages, and the composition of gas shale is considered to affect the pore systems in shale reservoirs. This study aimed to investigate the impact of both organic and inorganic constituents on the shale pore system, including specific surface area (SSA) and pore volume in Wufeng–Longmaxi Shale. Multiple linear regression (MLR) was employed to examine the contributions of different components to shale pore structure. The pore structure parameters, including pore SSA and pore volume, were obtained by gas adsorption experiments in 32 Wufeng–Longmaxi Shale (Late Ordovician–Early Silurian) samples. Both pore SSA and pore volume were calculated by the density functional theory (DFT) model on shale samples, and the pore types were determined by high-resolution field emission scanning electron microscopy (FE-SEM). The results of the X-ray diffractometer (XRD) analysis indicate that the Wufeng–Longmaxi Shale is dominated by quartz, clays, carbonates, feldspar, pyrite, and organic matter. Four models were made using SPSS software, all of which showed significant correlation between shale pore size and organic matter (OM) and clays. The content of organic matter played the biggest role in determining the size and structure of the pores. Although the content of quartz is the highest and serves as a rigid skeleton in shale reservoirs, it has complicated effects on the pore structure. In this study, most of the quartz is biogenetic and part of it is transformed from clays in deep shale. Therefore, these two parts of quartz are, respectively, related to organic matter and clays. In essence, the pores related to these two parts of quartz should be attributed to organic matter and clays, which also support the conclusion of the MLR models.

Paleogeomorphology and shale distribution of Late Ordovician-Early Silurian Yangtze platform, South China: Implication for shale mineralogy and TOC content

The distribution and mineral composition of the Wufeng-Longmaxi shale on the Yangtze platform are directly influenced by the geomorphology of the Late Ordovician-Early Silurian period. This research aimed to clarify the geomorphology of the Yangtze platform during this time period and its implication on the Wufeng-Longmaxi shale. Geophysical interpretation, chronostratigraphic division, and correlation, isometric map compilation, and mineral composition analysis were utilized to achieve this objective. The results of the study showed the following findings: (1) The Wufeng-Longmaxi shale was deposited on the southeastern slope of the Leshan-Longnüsi paleo-uplift on the Yangtze platform; (2) The southeastern slope exhibited three significant slope breaks, which allowed for the division of the slope into four geomorphology units: subaqueous high, subaqueous slope, subaqueous plain, and subaqueous sag; (3) The overlying Wufeng–Longmaxi shale was fully developed in the subaqueous plain and subaqueous sag, but lacked graptolite zones LM1-4 in the subaqueous high and subaqueous slope, with the shale onlapping the southeastern slope from southeast to northwest; (4) The southeastern slope significantly affects the grain size, mineral composition, and TOC content of the overlying shale. Specifically, as the slope transitioned from the subaqueous high to the subaqueous sag, the grain size becomes finer, the contents of TOC and silica increased, and the contents of carbonate and clay minerals decreased.

Multi-Proxies Analysis of Organic Matter Accumulation of the Late Ordovician–Early Silurian Black Shale in the Lower Yangtze Region, South China

The evolutional process of palaeoceanic environment and its effect on the accumulation of organic matter during the Ordovician–Silurian transition in Lower Yangtze region has been overlooked compared to that in Upper Yangtze region of South China, although their paleogeographic settings were expected to be discrepant. This paper documents the marine depositional environment, paleoclimate, and sediment supply changes, and discusses their roles in controlling the organic matter enrichment in sedimentary rocks within the Ordovician–Silurian transition of the Lower Yangtze region, using the latest geochemical data of the continuous drilling core. The stratigraphic framework of the Ordovician–Silurian transition in the Lower Yangtze region is composed of two third-order sequences, each of which can be subdivided into a lower TST (transgressive systems tract) and an upper RST (regressive systems tract). TST1 represented an evident depositional transition stage which was marked by the ending of the underlying carbonate sediments and the initiation of the terrigenous clastic-dominated sediments. Geochemical proxies indicate that the relatively low productivity, dysoxic water column condition, and high sediment supply flux collectively resulted in inadequate organic matter hosted in deposits of the TST1. During the depositional period of RST1, the global sea level declined due to the Hirnantian glaciation age. The icehouse also caused the decrease in overall river flux and, thus, the terrigenous clastic sediment supply. The icehouse also strengthened the upwelling that occurred in the Lower Yangtze sea. The upwelling boosted the marine algae explosion through the delivery of abundant nutrients, which not only enhanced paleoproductivity but also led to an anoxic environment by oxygen consumption. Such high paleoproductivity, anoxic water column environment, and low sediment supply flux caused the deposition of organic-rich shale. The sea level rose during the TST2 due to the ending of an ice age. The relatively large water depth and high paleoproductivity associated with volcanic eruptions are the main factors that caused the enrichment of organic matter during this stage. During the deposition of RTS2, the increase of sediment supply flux resulted in a decrease in accommodation space and water depth and the dilution of organic matter in deposits, which was the primary constraint of organic matter accumulation.

Effects of the astronomical orbital cycle on organic matter accumulation during the Late Ordovician–early Silurian in the Upper Yangtze area, South China

Effects of the astronomical orbital cycle on organic matter accumulation during the Late Ordovician–early Silurian in the Upper Yangtze area, South China

Climate fluctuations during the Ordovician-Silurian transition period in South China: Implications for paleoenvironmental evolution and organic matter enrichment

The Ordovician-Silurian (OS) transition witnessed dramatic tectonic, climatic, marine, and biological coevolution, during which organic-rich black shale was widely deposited and is considered an important source rock. However, the potential links between the differential enrichment pattern of organic matter and climate fluctuations and paleoenvironmental evolution during this period are still unclear. In this paper, the Late Ordovician-early Silurian black shale in a JieLong (JL) section in South China was identified by graptolitic fossils, total organic carbon (TOC) content, X-ray diffraction (XRD), scanning electron microscopy (SEM) and geochemical analysis. The results show that the provenances of sedimentary rocks are all granodiorite-like. According to the variations in the TOC content and geochemical indicators, four layers are delineated. Under warm and humid climatic conditions, sea level rise and unstable seasonal upwelling led to Unit 1 organic matter enrichment controlled by high primary productivity, anoxic conditions and effective smectite production. Unit 2 organic matter burial was controlled by cold, arid and oxic environmental conditions, but primary productivity was still relatively high. After the end of the glacial period, warm and humid climatic conditions and rapid transgression were the main factors affecting the burial of organic matter in Unit 3. Under hot and humid climate conditions, higher terrigenous detrital inputs and unfavourable preservation environments, the dilution of organic matter in Unit 4 was accelerated. The corrected chemical index of alteration (CIAcorr) values show that the study area experienced a climate change of alternating cold and warm conditions in response to the climate fluctuation during the glacial-interglacial period. The CIAcorr value is negatively correlated with the TOC content, while the generation of available clay minerals (smectite) in warm and humid climates is usually consistent with the enhanced burial flux of organic matter, supporting the important role of climate fluctuations in the accumulation of organic matter. Finally, a comparison of the data from the present study with geochemical data from different profiles in South China reveals the role of climatic fluctuations during the OS transition in controlling paleoenvironmental evolution and organic matter enrichment, while such a heterogeneous factor is usually associated with global or regional geological events. Our study highlights this differential enrichment mechanism as a combined response of multiple factors and provides important implications for glacial-interglacial organic matter enrichment mechanisms.

Reef Complexes of the Late Ordovician–Early Silurian Altai Basin: Classification, Structure, Paleobiota, and Paleogeography

Abstract —The paper presents data on the classification, structure, and biota of reef complexes from the Late Ordovician–early Silurian Altai Basin. The early Paleozoic history of the Altai Basin included the early–middle Katian, late Katian, early Hirnantian, late Telychian, and middle–late Sheinwoodian events of large-scale reef formation which produced framework reefs, mud mounds, and coral meadows. The early Paleozoic reefs of the area formed at rates of 40 to ~12 cm per thousand years. The late Telychian reef system, which formed in 0.51–1.0 Myr, occupied the largest area of the basin. The lateral extent of reef systems in the Late Ordovician–early Silurian basin of Altai depended more on the rates of vertical and lateral growth, as well as on the previous sea bottom topography and clastic inputs, than on the duration of reef formation. Carbonate deposition in the Altai basins of that time can be simulated with a general model implying formation of distal barrier reefs.

Late Ordovician-Early Silurian extension of the northern margin of the Upper Yangtze Platform (South China) and its impact on organic matter accumulation

Breakthroughs in shale gas exploration in the Upper Ordovician-Lower Silurian strata of the Upper Yangtze Platform have attracted interest in its sedimentary-tectonic evolution, but the tectonic background of the northern margin of the Upper Yangtze Platform remains unclear. In this paper, the Wufeng-Longmaxi formations on the northern margin of the Upper Yangtze Platform were investigated. Based on geochemical and mineralogical analyses of the tuffs/K-bentonites of the Wufeng Formation and the barite in the Longmaxi Formation, as well as previous research results, it was concluded that the northern margin of the Upper Yangtze Platform was in an extensional tectonic background during the Late Ordovician-Early Silurian. Detailed analysis revealed that, (1) the U–Pb zircon age of the tuff in the Bajiaokou section in South Qinling is 443.91 ± 0.92 Ma. The Zr/TiO2–Nb/Y diagram of the tuffs/K-bentonites indicates that their protoliths were alkaline-subalkaline basalt and andesite series rock. Based on the Th–Hf/3-Ta, Th–Tb*3-Ta*2, and TiO2–Nb/3-Th diagrams, there are undiscovered intraplate tension calc-alkaline basalts in the northern Yangtze Platform or the southern Qinling region, which provided volcanic clastic materials to the Ziyang, Lan'gao, Chengkou, Yichang and other regions. (2) Scanning electron microscopy revealed that the barite crystals in the Longmaxi Formation exhibit dissolution features and have a large particle size. Energy spectrum analysis of these barite crystals revealed that they have C, O, S, and Ba contents of 8.48 wt%, 22.98 wt%, 13.09 wt% and 55.44 wt%, so they are speculated to have been formed via cold methane seep genesis in a weak extensional tectonic setting. The 87Sr/86Sr ratios of the barite revealed that different types of barite were simultaneously formed in this area under the influences of hydrothermal and cold methane seeps. (3) The analysis of the heavy minerals in the Lower Silurian strata in the Bajiaokou section revealed that the provenance in the South Qinling area changed significantly during the late Early Silurian. Based on the above analyses, the northern margin of the Upper Yangtze Platform was in an extensional tectonic setting during the Late Ordovician-Early Silurian. The distribution of the total organic carbon content indicated that the extensional tectonic background provided good conditions for the enrichment and preservation of organic matter. The results of this study provide an understanding of the regional sedimentary-tectonic pattern and evolution of the Yangtze Platform during this period, as well as a reference for future shale gas exploration in this region.

Changes in palaeoclimate and palaeoenvironment in the Upper Yangtze area (South China) during the Ordovician–Silurian transition

The Ordovician–Silurian transition was a critical period in geological history, during which profound changes in climatic, biotic, and oceanic conditions occurred. To explore the provenance, palaeoclimate, and palaeoredox conditions in the Sichuan Basin during the Late Ordovician–early Silurian interval, we conducted mineralogical, geochemical, and isotopic analyses of three formations (Wufeng, Guanyinqiao and Longmaxi formations) in the Xindi No. 2 well. The ternary and bivariate diagrams indicate that the provenance is mainly felsic igneous rocks and originated mainly from a collisional setting, presumably due to an active continental margin. The chemical index of alteration (CIA) values in the lower Wufeng and Longmaxi formations are relatively high (67.48–73.57), indicating a warm and humid climate. In contrast, the CIA values declined rapidly (58.30–64.66) during the late Katian to early Hirnantian, which had a fluctuating cold and dry climate and was interrupted by a transient warm and humid climate. The palaeoredox indices (Mo concentrations and Moauth/Uauth, U/Th, V/Cr, Ni/Co, and V/V + Ni values) during the Late Ordovician–early Silurian indicate two cycles of water column euxinia. The first cycle occurred during Wufeng Formation deposition, with bottom waters evolving from oxic-suboxic to suboxic-anoxic. Most samples show relatively low redox-sensitive trace element concentrations during the Guanyinqiao Formation, pointing to oxic-suboxic conditions. The second cycle, during the late Hirnantian, transitioned from oxic to euxinic water conditions. Our δ13Corg data are comparable to previously reported records and exhibit a strong correlation between the Hirnantian isotopic carbon excursion (HICE), climate change, and redox conditions. We suggest that the variations in the δ13C values are related to two elements: (1) increased photosynthetic activity under oxic water conditions, and (2) increased carbonate weathering exposed by the glacio-eustatic sea- level. In addition, the high δ13Corg values might indicate a more shelf-proximal setting during Xindi No. 2 well deposition. The δ13Corg isotopic data effectively constrain the timing of the Late Ordovician mass extinction (LOME) and the evolution of the temporal changes in the climatic and ocean redox conditions, suggesting an apparent stratigraphic coincidence between climate and redox fluctuations and two-phase extinctions, which implies a strong causal relationship. The LOME was systematically driven by the combination of cooler glacial temperatures, glacio-eustatic sea-level fluctuations, and anoxic water conditions that caused the two pulses of extinction in the Yangtze shelf sea.

Distribution Characteristics and Genesis of Marine Anoxic Conditions in the Southwest of the Upper Yangtze Basin During the Late Ordovician–Early Silurian, South China

The distribution characteristics and formation of marine anoxic conditions through the Late Ordovician–Early Silurian (O–S) remain poorly resolved despite their importance in the preservation of sedimentary organic matter and the formation of black shale. In this study, the major, trace, and pyrite δ34S (δ34Spy) contents of 36 shale samples at the edge of the southwest depocenter of the Upper Yangtze Basin (Tianlin and Changhebian sections) were analyzed to understand the redox conditions, terrigenous clastic inputs, and primary productivity changes. The iron speciation and enrichment factor of U and Mo show that the range of anoxic conditions gradually expanded from the sedimentation center to the edge during the late Katian stage, peaked in the early Hirnantian stage followed by a rapid decrease, and expanded again during the early Rhuddanian stage. Comprehensive index analysis showed that the increase of terrigenous clastic input and the relative decrease of primary productivity due to tectonism and sea level change controlled the transformation of the water column from anoxic, especially euxinic, to suboxic-oxic conditions. This is reflected in the correlation between paleo-salinity, δ34Spy, chemical index of alteration, and the organic carbon accumulation rate. This work emphasizes the control of terrigenous clastic input and sulfate availability on the transformation of marine redox conditions during the O-S period in semi-restricted basins.

Microfacies analysis and mapping of the Late Ordovician-Early Silurian Wufeng–Longmaxi shelf shale in southern Sichuan Basin, China

Microfacies analysis and mapping of the Late Ordovician-Early Silurian Wufeng–Longmaxi shelf shale in southern Sichuan Basin, China

Late Ordovician microphytoplankton in Southwest China: recording paleogeographic variations across the Hirnantian glaciation

Paleogeographically, South China was located near the equator during Late Ordovician–early Silurian. Well developed graptolite zones and shelly fauna of upper Ordovician successions in South China correlate well with those from Katian and Hirnantian successions elsewhere in the world. We report here acritarch assemblages from the Katian black shales of the Wufeng Formation in the Lingfeng coal section, southeastern Sichuan, and from the Hirnantian carbonate rocks of the Kuanyinchiao Formation in the Yanzikou section of northwestern Guizhou and the Huanggexi section of northeastern Yunnan. This study provides the first description and biostratigraphic data of the acritarch assemblage of the Hirnantian Kuanyinchiao Formation in South China, and enables the interpretation of paleogeographic and paleoenvironmental changes during the major glaciation of the latest Ordovician in South China. These acritarch assemblages represent the microphytoplanktonic communities of preglacial, Hirnantian glacial and postglacial in South China. Two new species, Multiplicisphaeridium pugnusum n. sp., and Perissolagonella irregular n. sp., are described.

Controls on marine primary productivity variation and organic matter accumulation during the Late Ordovician－Early Silurian transition

The Late Ordovician−Early Silurian is a convergence period of multiple events in geological history, including dramatic variations of biology, environment, and geology. Extensive research has demonstrated remarkable fluctuations in ocean chemistry, microbial ecosystems, and biogeochemical elemental cycles experienced. However, the spatial-temporal evaluation of marine primary productivity and its links to organic matter accumulation during this key period remain elusive. Here, we present high-resolution marine primary productivity proxy (organic carbon accumulation rates: OCAR) and geochemical data from the Fengtonggang and Qiliao sections deposited in South China and adopted numerous other coeval sections of geochemical data globally, to improve the understanding of these fundamental scientific questions. The results show that all statistical profiles presented broadly high marine primary productivity before and after the Hirnantian glaciation maximum and low values during the glaciation, and a maximum marine primary productivity typically recorded at the uppermost part of the P.pacificus Zone. The Corg/Ptotal ratios indicate that oceanic anoxia was ubiquitous during the Late Katian to Early Rhuddanian, but strong spatial heterogeneous during the Hirnantian glaciation maximum. The volcanism and nitrogen availability may have played an important role in regulating the variations of marine primary productivity globally. Besides, redox-controlled phosphorus cycling may have also exerted a remarkable influence on marine primary productivity. The high Corg/Ptotal ratios (exceed the Redfield ratio of 106/1) observed in the Wufeng and Longmaxi formations indicate that phosphorus was recycled effectively back to the water column, promoting a positive productivity feedback. By contrast, the low Corg/Ptotal ratios found in the Guanyinqiao Bed reflect phosphorus retention in the sediment, limiting phosphorus recycling back to the water column, consequently restricting the primary production. The enhanced marine primary productivity resulted from nutrient regeneration, ocean anoxia, and high sea level may have led to high organic carbon export and preservation. The consistency between the highest marine primary productivity and the first pulse of mass extinction suggests that eutrophication-induced marine anoxia may have played an essential killing mechanism during the first Late Ordovician mass extinction.

Sedimentary and Geochemical Responses to the End Ordovician Glaciation in the Guanyinqiao Formation (Late Ordovician-Early Silurian Period) in the Sichuan Basin

Sedimentary and Geochemical Responses to the End Ordovician Glaciation in the Guanyinqiao Formation (Late Ordovician-Early Silurian Period) in the Sichuan Basin

Early Paleozoic Cascadia-type active-margin evolution of the Dunhuang block (NW China): Geochemical and geochronological constraints

Abstract Origins of early Paleozoic metabasites (granulites and amphibolites) and their host metasedimentary rocks in the Dunhuang block, NW China, are addressed by new geochronological and geochemical data. The metabasites show back-arc basalt–like geochemical features, marked by high Zr/Nb ratios and Zr-Hf troughs, but they can be classified into two groups based on their dissimilar protolith ages and distinct Nd signatures. Most group I metabasaltic rocks were emplaced before 455 Ma and possess high Ba/Nb ratios (11.46–224), low (Nb/La)PM (0.10–0.71), and negative whole-rock εNd(t) values (−12.7 to −2.7), whereas group II rocks have protolith ages around 445 Ma, low Ba/Nb ratios (0.70–22.93), low (Nb/La)PM (0.78–1.51), and less evolved whole-rock Nd isotopic features (εNd[t]: −2.0 to +2.7). It is proposed that group I metabasites originated from an enriched lithospheric mantle, while group II metabasites were derived from the depleted asthenospheric mantle. The metasedimentary rocks received detritus mainly from the neighboring Cambrian magmatic arc, and they are compositionally similar to active-margin sediments. Metamorphic zircon U-Pb ages ranging 462–422 Ma from the investigated rocks together with prominent magmatism further suggest high-grade metamorphism prevailing during the Late Ordovician–early Silurian. Based on these data, a Cascadia-type evolution is proposed involving an Ordovician–early Silurian suprasubduction stretching of the Cambrian active continental margin, which culminated with mantle upwelling. Recent paleogeographic reconstructions support the evolution and assembly of interior Proto–Tethys-Ran oceanic and continental plates, including the Dunhuang block, between 510 and 440 Ma, followed by Panthalassan subduction beneath the Tarim–North China continental assemblage at 440–430 Ma.

Application of U-Pb dating technique in the study of hydrothermal activities of dolomite reservoir: A case study on 3<sup>rd</sup> member of Yingshan Formation in Gucheng area, Tarim Basin, NW China

古城地区是塔东探区的重点勘探区块，下奥陶统鹰三段是塔东探区首要的勘探层系。探寻多期热液作用改造后的优质白云岩储层是扩大研究区勘探成果的关键。本文在储层岩石学特征判识的基础上，应用U-Pb同位素定年技术，结合碳氧锶镁同位素和稀土元素分析测试，分析了热液性质，明确了热液作用发生的确切时期及期次，探讨了热液作用对白云岩储层的影响。研究结果表明，古城地区鹰三段白云岩储层发育准同生-浅埋藏海水成因的粉晶-中晶白云岩及热液作用产物。热液作用可划分为二期：第一期为中-晚奥陶世贫镁热液，表现为鞍状白云石充填缝洞及重结晶次生加大的粗晶白云石，U-Pb测定年龄为464±12Ma到473.9±9.1Ma，氧同位素值比早奥陶世正常海相白云石明显偏负，锶同位素比值明显高于早奥陶世海水值，稀土元素Eu正异常，锶、铁、镁元素含量低；第二期为晚奥陶世-早志留世钙质热液，表现为方解石充填缝洞及对先存白云石的溶蚀，U-Pb测定年龄为448±15Ma到457.4±6.6Ma，氧锶同位素组成严重偏离同时期正常海相白云石范围，轻稀土元素明显富集，Eu正异常，锶、钙元素含量高，锰元素含量低。第二期热液作用是白云岩储层溶蚀的关键，同期NNE向断裂带作为其输导系统，是优质储层发育的有利部位，为有利区带预测提供依据。;The dolomite in 3^rd member of Yingshan Formation in Gucheng area is a significant exploration field in the east of Tarim Basin. High-quality dolomite reservoir reworked by hydrothermal activities is the key to expand exploration. Based on petrographic characteristics, this paper analyzed hydrothermal properties, clarified the precise dating and stages of hydrothermal process, and discussed hydrothermal effects on dolomite reservoir using U-Pb dating technique, combined with analysis of carbon and oxygen, strontium, magnesium isotopes and rare earth elements. The study shows that micrite-medium grained dolomites taken place in paracontemporaneous-shallow buried phase and hydrothermal products in research area. There are two stages of hydrothermal processes. The first stage hydrothermal process is very low in magnesium content in the Middle-Late Ordovician, at the result of saddle dolomite filling in fractures/vugs and coarse dolomite formed by recrystallization aged 464±12Ma~473.9±9.1Ma by U-Pb dating with oxygen isotope negative compared with normal marine dolomite, strontium isotope values higher than Early Ordovican sea values and positive Eu anomalies. The contents of strontium, ferrum and magnesium are relatively low. The second stage hydrothermal process is high in calcium content in the Late Ordovician-Early Silurian, at the result of calcite filling in fractures/vugs and dissolution of existing dolomites, aged 448±15Ma~457.4±6.6Ma by U-Pb dating with significantly negative oxygen isotope and enriched LREE and positive Eu anomalies. The strontium content is high while manganese content is low. Migration pathway of the second stage hydrothermal process as a key of dolomite reservoir dissolution is NNE fracture zones developing high-quality reservoir, which provides a basis for the prediction of favorable zones.

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