Albian oceanic anoxic events in northern Tunisia: Biostratigraphic and geochemical insights

Albian pelagic successions of the Nebeur area in northwestern Tunisia consist of radiolarian-bearing and organic-rich black shale beds, which represent the lower part of the Fahdene Formation. The carbonate content of the organic-rich beds ranges between 40 and 48%. Total organic carbon (TOC) analyses via Rock Eval pyrolysis yielded values ranging between 0.7 and 2.8% and a mixed marine/terrestrial origin. T_max values vary between 424 and 450 °C, indicative of submature to mature organic matter. High resolution planktic foraminiferal and radiolarian biostratigraphy suggest that the black shales beds span the mid- to late Albian, confined to the middle part of the Ticinella primula zone, upper Biticinella breggiensis zone and lower appeninica + buxtorfi zone. Episodes of organic-rich deposition in the “Tunisian Trough” are interpreted as being the sedimentary record of the global oceanic anoxic events OAE1b, c, and d respectively. Age-diagnostic radiolarian assemblages recovered from late Albian organic-rich black shales lie within the UA13–UA14 boundary biochronozones. The abundance of radiolarian and calcispheres (i.e. pithonella) within the black shales suggests high productivity periods and eutrophic conditions probably triggered by upwelling currents.     

Carbon isotope records of the early Albian oceanic anoxic event (OAE) 1b from eastern Tethys (southern Tibet,China)

The early Albian Oceanic Anoxic Event (OAE), i.e., OAE1b, is well documented in western Tethys and in the primary North Atlantic Ocean, but has not yet been reported from eastern Tethys. In this paper, we present bulk carbon isotope data of hemipelagites to examine if it was recorded in eastern Tethys. Samples were taken from the upper Chuangdepu Member (nannofossil zone CC8) of the lower Gyabula (former Shadui) Formation at the Bangbu section, Qonggyai, southern Tibet of China. The δ¹³C values mainly range from −0.6‰ to 1.8‰ with a maximum of 1.87‰ and a minimum of −0.69‰. Three stages of carbon isotope evolution were distinguished with three boundaries. By the constraint of the stratigraphic sequence and nannofossil biostratigraphic zone CC8, the rapid δ¹³C change and correlation with western Tethys and Atlantic Ocean together suggest that these three boundaries of the carbon isotope evolution probably correspond to three subevents of the early Albian OAE1b, and the subevent levels of upper Kilian, Paquier, and Leenhardt are recorded in eastern Tethys (southern Tibet). The fact that the amount of δ¹³C shift is less by ∼1.5–2.0‰ in eastern Tethys than in western Tethys and Atlantic Ocean is interpreted as a result of possible cool sea surface (∼14–16 °C) of the southeastern Tethys (northern Indian passive margin of Greater India), which was probably located in a medium–high latitude during the Albian, leading to low primary productivity. The recognition of OAE-1b from Tethys Himalaya can improve our understanding of the Tethys and global paleoclimatic and paleoceanographic changes during the mid-Cretaceous.     

Response and recovery of the Comanche carbonate platform surrounding multiple Cretaceous oceanic anoxic events,northern Gulf of Mexico

The ubiquity of carbonate platforms throughout the Cretaceous Period is recognized as a product of high eustatic sea-level and a distinct climatic optimum induced by rapid sea-floor spreading and elevated levels of atmospheric carbon-dioxide. Notably, a series of global oceanic anoxic events (OAEs) punctuate this time-interval and mark periods of significantly reduced free oxygen in the world's oceans. The best records of these events are often from one-dimensional shelf or basin sections where only abrupt shifts between oxygenated carbonates and anoxic shales are recorded. The Comanche Platform of central Texas provides a unique opportunity to study these events within a well-constrained stratigraphic framework in which their up-dip and down-dip sedimentologic effects can be observed and the recovery of the platform to equilibrium states can be timed and understood. Stable isotope data from whole cores in middle Hauterivian through lower Campanian mixed carbonate-siliciclastic strata are used to construct a 52-myr carbon isotope reference profile for the northern Gulf of Mexico. Correlation of this composite curve to numerous global reference profiles permits identification of several anoxic events and allows their impact on platform architecture and facies distribution to be documented. Oceanic anoxic events 1a, 1b, 1d, and 2 occurred immediately before, after, or during shale deposition in the Pine Island Member, Bexar Member, Del Rio Formation, and Eagle Ford Group, respectively. Oceanic anoxic event 3 corresponds to deposition of the Austin Chalk Group. Platform drowning on three occasions more closely coincided with globally recognized anoxic sub-events such as the Fallot, Albian-Cenomanian, and Mid-Cenomanian events. This illustrates that the specific anoxic event most affecting a given carbonate platform varied globally as a function of regional oceanographic circumstances.Using chemo- and sequence-stratigraphic observations, a four-stage model is proposed to describe the changing facies patterns, fauna, sedimentation accumulation rates, platform architectures, and relative sea-level trends of transgressive-regressive composite sequences that developed in response to global carbon-cycle perturbations. The four phases of platform evolution include the equilibrium, crisis, anoxic, and recovery stages. The equilibrium stage is characterized by progradational shelf geometries and coral-rudist phototrophic faunal assemblages. Similar phototrophic fauna typify the crisis stage; however, incipient biocalcification crises of this phase led to retrogradational shelf morphologies, transgressive facies patterns, and increased clay mineral proportions. Anoxic stages of the Comanche Platform were coincident with back-ground deposition of organic-rich shale on drowned shelves and heterotrophic fauna dominated by oysters or coccolithophorids. Eustatic peaks of this stage were of moderate amplitude (∼30 m), yet relative sea-level rises were greatly enhanced by reduced sedimentation rates. In the recovery stage, heterotrophic carbonate factories re-established at the shoreline as progradational ramp systems and sediment accumulation rates slowly increased as dysoxia diminished. Full recovery to equilibrium conditions may or may not have followed. Geochemical and stratigraphic trends present in the four stages are consistent with increased volcanism along mid-ocean ridges and in large-igneous provinces as primary drivers of Cretaceous OAEs and the resulting transgressive-regressive composite sequences.     

Aptian palaeoclimates and identification of an OAE1a equivalent in shallow marine environments of the southern Tethyan margin: Evidence from Southern Tunisia (Bir Oum Ali section,Northern Chott Chain)

Alternations between siliciclastic, carbonate and evaporitic sedimentary systems, as recorded in the Aptian mixed succession of southern Tunisia, reflect profound palaeoceanographic and palaeoclimatic changes in this area of the southern Tethyan margin. The evolution from Urgonian-type carbonates (Berrani Formation, lower Aptian) at the base of the series, to intervals dominated by gypsum or detrital deposits in the remainder of the Aptian is thought to result from the interplay between climate change and tectonic activity that affected North Africa.Based on the evolution of clay mineral assemblages, the early Aptian is interpreted as having been dominated by slightly humid conditions, since smectitic minerals are observed. Near the early to late Aptian boundary, the onset of a gypsiferous sedimentation is associated with the appearance of palygorskite and sepiolite, which supports the installation of arid conditions in this area of the southern Tethyan margin. The evaporitic sedimentation may have also been promoted by the peculiar tectonic setting of the Bir Oum Ali area during the Aptian, where local subsidence may have been tectonically enhanced linked to the opening of northern and central Atlantic. Stress associated with the west and central African rift systems may have triggered the development of NW–SE, hemi-graben structures. Uplifted areas may have constituted potential new sources for clastic material that has been subsequently deposited during the late Aptian.Chemostratigraphic (δ¹³C) correlation of the Bir Oum Ali succession with other peri-Tethyan regions complements biostratigraphic findings, and indicates that a potential expression of the Oceanic Anoxic Event (OAE) 1a may be preserved in this area of Tunisia. Although the characteristic negative spike at the base of this event is not recognized in the present study, a subsequent, large positive excursion with δ¹³C values is of similar amplitude and absolute values to that reported from other peri-Tethyan regions, thus supporting the identification of isotopic segments C4–C7 of the OAE1a. The absence of the negative spike may be linked to either non preservation or non deposition: the OAE1a occurred in a global transgressive context, and since the Bir Oum Ali region was located in the innermost part of the southern Tethyan margin during most of the Aptian, stratigraphic hiatuses may have been longer than in other regions of the Tethys. This emphasizes the importance of integrating several stratigraphic disciplines (bio-, chemo- and sequence stratigraphy) when performing long-distance correlation.     

Exceptionally favourable life conditions for macrobenthos during the Late Cenomanian OAE-2 event: Ichnological record from the Bonarelli Level in the Grajcarek Unit,Polish Carpathians

Pelagic and hemipelagic sediments of the Bonarelli Level (uppermost Cenomanian) in the Sztolnia section (Grajcarek Unit, Polish Carpathians) contain trace fossils of the Zoophycos ichnofacies, including (in descending order of abundance): Chondrites (smaller and larger forms), Planolites, Thalassinoides, Palaeophycus, Taenidium, Teichichnus, and Zoophycos. They occur in thick bioturbated layers, which are interbedded with rare, thin layers of unbioturbated black shales. The black shale layers mark the Bonarelli Level and are interpreted as a record of anoxia or dysoxia. Coeval sections in the Western Tethys contain similar trace fossils but they are less abundant and these sections are characterized by thicker unbioturbated black shale layers and thinner bioturbated layers. This confirms the exceptionally favourable life conditions in sediments of the Sztolnia section, which do not record strong global anoxia during the OAE-2 event. Such favourable conditions were probably caused by effective oxygenation of pore waters and deep burial of organic matter, which are a consequence of high rates of accumulation and the palaeogeographical location of the section on a flank of a submarine high, under strong circulation.     

First evidence for the Cenomanian–Turonian oceanic anoxic event (OAE2, ‘Bonarelli’ event) from the Ionian Zone,western continental Greece

Integrated biostratigraphic (planktonic foraminifera, calcareous nannofossils), chemostratigraphic (bulk C and O isotopes) and compound-specific organic geochemical studies of a mid-Cretaceous pelagic carbonate—black shale succession of the Ionian Zone (western Greece), provide the first evidence for the Cenomanian–Turonian oceanic anoxic event (OAE2, ‘Bonarelli’ event) in mainland Greece. The event is manifested by the occurrence of a relatively thin (35 cm), yet exceptionally organic carbon-rich (44.5 wt% TOC), carbonate-free black shale, near the Cenomanian–Turonian boundary within the Vigla limestone formation (Berriasian–Turonian). Compared to the ‘Bonarelli’ black-shale interval from the type locality of OAE2 in Marche–Umbria, Italy, this black shale exhibits greatly reduced stratigraphic thickness, coupled with a considerable relative enrichment in TOC. Isotopically, enriched δ¹³C values for both bulk organic matter (−22.2‰) and specific organic compounds are up to 5‰ higher than those of underlying organic-rich strata of the Aptian-lower Albian Vigla Shale member, and thus compare very well with similar values of Cenomanian–Turonian black shale occurrences elsewhere. The relative predominance of bacterial hopanoids in the saturated, apolar lipid fraction of the OAE2 black shale of the Ionian Zone supports recent findings suggesting the abundance of N₂-fixing cyanobacteria in Cretaceous oceans during the Cenomanian–Turonian and early Aptian oceanic anoxic events.     

Cretaceous oceanic anoxic events (OAEs) recorded in the northern margin of Africa as possible oil and gas shale potential in Tunisia: An overview

Abstract

Significant attention has been given, during the last decade, to Palaeozoic unconventional oil and gas shale in northern Africa where the productive Palaeozoic basins are located. New tentative strategies shed light on Mesozoic unconventional plays represented by Cretaceous shale reservoirs. In most petroleum systems the proven Cretaceous oceanic anoxic events (OAEs), represented mainly by the Fahdene and Bahloul Formations, played the main role in hydrocarbon generation with good distribution. Their deposition was restricted to the early Aptian (Bedoulian), the early/late Albian, and the Cenomanian–Turonian transition. Additional black shales have been detailed for the first time in Tunisia which relate to the Valanginian Weissert and late Hauterivian Faraoni events. Biostratigraphic and complete geochemical reviews have been undertaken from published papers and unpublished internal reports to better assess these important source intervals. These black shale levels, pertaining to OAEs, were deposited in almost deep marine environments during short-lived periods of anoxia (ca. 2 Ma). In the course of this review, thickness, distribution, and maturity maps have been established for each level using well data from published sources.     

The subducted oceanic crust within continental-type UHP metamorphic belt in the North Qaidam, NW China: Evidence from petrology, geochemistry and geochronology

Three types of eclogite, together with a serpentinized harzburgite, coexist as blocks within granitic and pelitic gneisses along the Shaliuhe cross section, the eastern part of the North Qaidam continental-type ultrahigh-pressure (UHP) metamorphic belt, NW China. The olivine (Ol¹) and orthopyroxene in the harzburgite are compositionally similar to present-day abyssal peridotites. The kyanite–eclogite is derived from a troctolitic protolith, whereas the epidote–eclogite from a gabbroic protolith, both having distinct positive Eu anomalies, low TiO₂, and high Al₂O₃ and MgO. The kyanite–eclogite shows inherited cumulate layering. The phengite–eclogite has high TiO₂, low Al₂O₃ and MgO with incompatible trace elements resembling enriched-type MORB. Sr–Nd isotope data indicate that the protoliths of both kyanite–eclogite and epidote–eclogite ([⁸⁷Sr/⁸⁶Sr]_i ~ 0.703–0.704; ε_Nd(T) ~ 5.9–8.0) are of mantle origin (e.g., ocean crust signatures). On the other hand, while the lower ε_Nd(T) value (1.4–4.1) of phengite–eclogite is more or less consistent with an enriched MORB protolith, their high [⁸⁷Sr/⁸⁶Sr]_i ratio (0.705–0.716) points to an additional enrichment in their history, probably in an subduction-zone environment. Field relations and geochemical analyses suggest that the serpentinized harzburgite and the three types of eclogite constitute the oceanic lithological section of an ophiolitic sequence from mantle peridotite, to cumulate, and to upper basaltic rocks. The presence of coesite pseudomorphs and quartz exsolution in omphacite plus thermobarometric calculations suggests that the eclogites have undergone ultrahigh pressure metamorphism (i.e., peak P ≥ 2.7 GPa). The harzburgite may also have experienced the same metamorphism, but the lack of garnet suggests that the pressure conditions of ≤ 3.0 GPa. Zircon U–Pb SHRIMP dating shows that the eclogites have a protolith age of 516 ± 8 Ma and a metamorphic age of 445 ± 7 Ma. These data indicate the presence of a Paleo-Qilian Ocean between Qaidam and Qilian blocks before the early Ordovician. The ophiolitic assemblage may be the relics of subducted oceanic crust prior to the subduction of continental materials during Ordovician–Silurian times and ultimate continent collision. These rocks, altogether, record a complete history of ocean crust subduction, to continental subduction, and to continental collision.     

太原市灰霾期间黑碳气溶胶形貌与成分分析

为掌握太原市冬季灰霾发生期间黑碳气溶胶形貌特征和化学成分变化特点,分别于2011-12-30T08:00至2012-01-01 T 04:00和2018-11-25 T 08:00至2018-11-27 T 04:00灰霾发生期间在山西大学环境与资源学院楼顶使用MAY七级冲击式颗粒采样器每隔4h采集空气动力学直径分别为0.25～0.5μm、0.5～1.0μm以及1～2μm的大气单颗粒分析样品,运用带超薄窗口能谱仪的扫描电镜(JEOL JSM-6390)测定了样品中4330个颗粒的二次电子像及X射线能谱,通过基于蒙特卡罗模拟程序的定量电子探针微区分析技术对每个颗粒的元素原子浓度进行了计算,并对颗粒进行了分类与计数。共检测到6类颗粒,分别是矿物尘、碳质颗粒、钠盐颗粒、二次颗粒、富铁颗粒和其他,其中黑碳气溶胶数量相对丰度在2011和2018年样品中平均为26.8%和34.2%。按照伸长率(E)、纵横比(R)、是否与其他物质结合等将其分为伸展型、支链型、嵌入或粘附型和团聚型4种,反映了黑碳颗粒不同的老化过程。2011年样品中团聚型和支链型数量较多,而2018年样品中伸展型和嵌入型多于团聚型和支...     

Cretaceous Volcanic Events in Southeastern Jilin Province, China: Evidence from Single Zircon U-Pb Ages

Mesozoic volcanic rocks in southeastern Jilin Province are an important component of the huge Mesozoic volcanic belt in the northeastern area.Study of the age of their formation is of great significance to recognize Mesozoic volcanic rule in northeastern China.Along with the research of rare Mesozoic biota and extensive Mesozoic mineralization in western Liaoning,a number of researchers have focused on Mesozoic volcanic events.The authors studied the ages of the Cretaceous volcanic rocks in southeastern Jilin Province using single Zircon U-Pb.The result shows that the Sankeyushu Formation volcanic rocks in the Tonghua area are 119.2 Ma in age,the Yingcheng Formation in the Jiutai area 113.4±3.1 Ma,the Jinjiatun Formation in Pinggang Town of Liaoyuan City and the Wufeng volcanic rocks in the Yanji area 103.2±4.7 Ma and 103.6±1 Ma,respectively. Combined with the data of recent publication on volcanic rocks ages;the Cretaceous volcanic events in southeastern Jilin Province can be tentatively subdivided into three eruption periods:119 Ma,113 Ma and 103 Ma.The result not only provides important chronology data for subdividing Mesozoic strata in southeastern Jilin Province,establishing Mesozoic volcanic event sequence,discussing geological tectonic background,and surveying the relation between noble metals to the Cretaceous volcanic rocks,but also offers important information of Mesozoic volcanism in northeastern China.     

俯冲洋岛玄武岩脱水释放的水流体成分：来自西天山榴辉岩相高压脉的约束

俯冲洋壳的脱水作用对于岛弧火山岩的成因具有重要意义。西天山高压低温变质带中切割主岩蓝片岩/榴辉岩的榴辉岩相脉体记录了古生代俯冲带中的脱水过程。脉体具有典型高压矿物组合：石榴石(＞8%)+绿辉石(15%)+石英(＞55%)+蓝闪石(5%)+冻蓝闪石(10%)±黝帘石±方解石。高压脉及其主岩的岩相学与矿物化学研究表明流体是在俯冲过程中蓝片岩向榴辉岩进变质条件下释放。脉的主量元素成分显示流体富Si。主岩的稀土元素和微量元素表明其原岩为洋岛玄武岩(OIB)。脉和主岩具有相似的微量元素配分模式,并且相对N-MORB都显示富Li、Be、Cs、Rb、Ba、Pb、La的特点。通过模拟计算,在高压脉、与脉平衡的流体以及模拟的原生流体中都富集流体活动性元素(Li、Be、Pb)和LILE(Cs、Rb、Ba),贫HFSE(Nb)和REE(Nd、Sm)。证实了在古生代南天山洋俯冲时洋岛玄武岩脱水释放出富Li、Be、LILE、La和Pb,贫HFSE和HREE的流体。     

Effects of water table changes on soil CO2, CH4 and N2O fluxes during the growing season in freshwater marsh of Northeast China

A field control experiment was carried out to determine the influence of water table changes on soil CO₂, CH₄, and N₂O emissions in Calamagrostis angustifolia freshwater marsh in Northeast of China. The results showed that the water depth of 5 cm below the ground surface increased soil CO₂ emission, but there was no significant influence of deeper water table on gas emission. CH₄ emission was accelerated by deep standing water and approached the peak in the plant booming time. This suggests that root activity has influence on CH₄ production. The result also demonstrated that both low water table level and inundated environment would inhibit N₂O emission. Comparing the total global warming potential of three gases under different conditions, it can be concluded that maintaining a comparatively steady water table near the soil surface can benefit soil carbon sequestration in the C. angustifolia marsh, and decrease of the greenhouse gases emissions to the atmosphere.     

爆裂法作为快速测定流体包裹体中二氧化碳（和其它气体）含量的手段及其在勘探中的使用：以中国山东和河北省金矿为例

The acoustic decrepitation method heats a small monomineralic sample and counts pressure impulses as the inclusions burst when they develop high internal pressures. For aqueous fluids, the decrepitation temperature is correlated with the homogenisation temperature, but gas rich fluids give a distinct and characteristic low temperature decrepitation peak which can be used to recognize gas rich fluid inclusions. This information is useful in exploration for Au deposits, which are frequently associated with CO2 rich and sometimes CH4 rich fluids. This distinctive decrepitation occurs because the CO2 rich inclusion fluids expand according to the gas law and develop internal pressures high enough to burst the host mineral grain at temperatures well below their homogenisation temperatures. In contrast, aqueous fluids condense to a liquid and vapour phase during post-entrapment cooling. Upon subsequent heating their internal pressures do not increase significantly until after homogenisation to a single phase occurs and hence they do not decrepitate ＂prematurely＂ as gas rich inclusions do. This behaviour is usually regarded as an annoyance in conventional microthermometric homogenisation studies, but can readily be used as an exploration aid to find mineralisation deposited from such gas rich fluids. Decrepitation results on samples from Cowra Ck, NSW, Australia, which have also been microthermometrically measured for CO2 content, show that amounts of less than 5 mole % CO2 are easily distinguished by decrepitation and amounts as low as 1 mole % CO2 may be determinable. Examples of the use of acoustic decrepitation in the study of 6 gold mines in the Shandong and Hebei provinces of China are discussed.     

Metallogenic age and hydrothermal evolution of the Jidetun Mo deposit in central Jilin Province,northeast China: Evidence from fluid inclusions,isotope systematics,and geochronology

The Jidetun deposit is a large porphyry Mo deposit that is located in central Jilin Province, northeast China. The Mo mineralization occurs mainly at the edge of porphyritic granodiorite, as well as the adjacent monzogranite. Field investigations, cross-cutting relationships, and mineral paragenetic associations indicate four stages of hydrothermal activity. To determine the relationships between mineralization and associated magmatism, and better understand the metallogenic processes in ore district, we have undertaken a series of studies incluiding molybdenite Re–Os and zircon U–Pb geochronology, fluid inclusions microthermometry, and C–H–O–S–Pb isotope compositions. The molybdenite Re–Os dating yielded a well-defined isochron age of 168.9 ± 1.9 Ma (MSWD = 0.34) that is similar to the weighted mean ²⁰⁶Pb/²³⁸U age of 173.5 ± 1.5 Ma (MSWD = 1.8) obtained from zircons from the porphyritic granodiorite. The results lead to the conclusion that Mo mineralization, occurred in the Middle Jurassic (168.9 ± 1.9 Ma), was spatially, temporally, and genetically related to the porphyritic granodiorite (173.5 ± 1.5 Ma) rather than the older monzogranite (180.1 ± 0.6 Ma). Fluid inclusion and stable (C–H–O) isotope data indicate that the initial H₂O–NaCl fluids of mineralization stage I were of high-temperature and high-salinity affinity and exsolved from the granodiorite magma as a result of cooling and fractional crystallization. The fluids then evolved during mineralization stage II into immiscible H₂O–CO₂–NaCl fluids that facilitated the transport of metals (Mo, Cu, and Fe) and their separation from the ore-bearing magmas due to the influx of abundant external CO₂ and heated meteoric water. Subsequently, during mineralization stage III and IV, increase of pH in residual ore-forming fluids on account of CO₂ escape, and continuous decrease of ore-forming temperatures caused by the large accession of the meteoric water into the fluid system, reduced solubility and stability of metal clathrates, thus facilitating the deposition of polymetallic sulfides.     

吉林安图海沟岩群的解体：地质与锆石年代学证据

对吉林安图地区原划为中元古代构造地层的海沟岩群进行的野外地质工作和锆石 SHRIMP U-Pb 年代学工作表明, 海沟岩群不同岩组的变质程度和形成时代都存在明显差异。 以条带状含铁建造为特征的四岔子岩组受到高级变质作用改造, 变质程度为高角闪岩相并伴有深熔。 侵入 BIF 铁矿的奥长花岗质片麻岩给出的形成时代为 2 554 ± 3 Ma 或更老, 变质时代为 2 528 ± 11 Ma, 表明四岔子岩组应当老于 2 554 Ma。 结合区域对比, 推测其为新太古代变质表壳岩。 而东方红岩组虽然变形作用强烈, 但仅为绿片岩相变质。 该岩组中的变流纹岩形成年龄为 178 ± 2 Ma, 表明其为早侏罗世火山岩, 受到后期韧性变形作用的改造。 新资料表 明原划的海沟岩群需要解体, 其中既包括太古宙变质基底, 也包括受到变形影响的中生代地层。 太古宙地质体在古生代末-中生代初古亚洲洋闭合及造山后构造作用下, 呈岩片出露在年轻地质体之间。 本文的奥长花岗质片麻岩变质增生边和变质新生锆石具有异常高的 Th/U 比 值, 推测其和变质过程中高 U 矿物的形成有关。 高 Th/U 比变质锆石和低 Th/U 比岩浆锆石的实例表明：对锆石成因进行判别时, Th/U 比值只能作为一个辅助手段谨慎使用。     

华北克拉通东北缘岩石圈深部物质组成的不均一性: 来自吉林南部中生代火山岩元素及Sr-Nd同位素地球化学的证据

本文报导了吉林南部果松组和三棵榆树组火山岩的全岩K-Ar和角闪石40Ar/39Ar定年结果和岩石地球化学资料,并讨论了吉林南部早白垩世火山岩岩浆源区性质以及空间变异.定年结果显示,果松组和三棵榆树组火山岩的形成时代分别为130.2±0.3Ma和118.3±1.9Ma.果松组火山岩主要由玄武岩-玄武质粗面安山岩-粗面安山岩-英安岩组成;它们的SiO2含量介于46%～64%,Mg#介于31～50之间,Al2O3含量介于14.9%～18.9%之间,全碱含量(Na2O+K2O)介于4.61%～9.23%之间,属于亚碱性系列,具钙碱性演化趋势;并以富集大离子亲石元素(LILEs)和轻稀土元素(LREEs),亏损重稀土元素(HREEs)和Nb、Ta、Ti等高场强元素(HFSEs)为特征;(87Sr/88Sr)i值和εNd(t)值分别介于0.7065～0.7077和-2.67～-19.71之间.果松组火山岩的成分具有较好的空间变异趋势,由东向西,火山岩的基性程度增高,东部果松组火山岩具有较高的(87Sr/86Sr)i值,而西部具有较低的εNd(t)值.三棵榆树组火山岩由粗面安山岩和粗面英安岩组成;三棵榆树组火山岩的SiO2含量介于55.5%～65.8%之间,Mg#介于42～50,Al2O3含量介于15.0%～15.7%,全碱含量偏高(Na2O+K2O=6.93%～9.24%),主体属于亚碱性系列,具钙碱性系列的演化趋势;并以较高的Th/U(5.36～5.82)、Ba/Nb(50.2～120.0)、(La/Yb)N(32.9～47.9)和Sr/Y(50.0～72.4)比值为特征;它们的(87Sr/86Sr)i值和εnd(t)值分别变化于0.7056～0.7057和-8.99～-19.71之间.上述特征揭示,果松组火山岩(130Ma)的形成主要与古太平洋板块的俯冲作用有关,岩浆来源于受流体/熔体交代的地幔楔,东西部受到不同程度陆壳物质的混染.与果松组火山岩相比,三棵榆树组火山岩(118Ma)形成于俯冲背景下的相对引张环境,岩浆来源于受交代的岩石圈地幔,但受到深部陆壳物质的混染.     

Thermochemical Sulfate Reduction in the Tazhong District,Tarim Basin,Northeast China: Evidence from Formation Water and Natural Gas Geochemistry

<正>Systematic analyses of the formation water and natural gas geochemistry in the Central Uplift of the Tarim Basin(CUTB) show that gas invasion at the late stage is accompanied by an increase of the contents of H_2S and CO_2 in natural gas,by the forming of the high total dissolved solids formation water,by an increase of the content of HCO_3~-,relative to Cl~-,by an increase of the 2nd family ions(Ca~(2+),Mg~(2+),Sr~(2+) and Ba~(2+)) and by a decrease of the content of SO_4~(2-),relative to Cl~-.The above phenomena can be explained only by way of thermochemical sulfate reduction(TSR).TSR often occurs in the transition zone of oil and water and is often described in the following reaction formula:ΣCH+CaSO_4+H-_2O→H_2S+CO_2+CaCO_3.(1) Dissolved SO_4~(2-) in the formation water is consumed in the above reaction,when H_2S and CO_2 are generated,resulting in a decrease of SO_4~(2-) in the formation water and an increase of both H_2S and CO_2 in the natural gas.If formation water exists, the generated CO_2 will go on reacting with the carbonate to form bicarbonate,which can be dissolved in the formation water,thus resulting in the enrichment of Ca~(2+) and HCO_3~-.The above reaction can be described by the following equation:CO_2+H_2O+CaCO_3→Ca~(2+)+2HCO_3~-.The stratigraphic temperatures of the Cambrian and lower Ordovician in CUTB exceeded 120℃,which is the minimum for TSR to occur.At the same time,dolomitization,which might be a direct result of TSR,has been found in both the Cambrian and the lower Ordovician.The above evidence indicates that TSR is in an active reaction,providing a novel way to reevaluate the exploration potentials of natural gas in this district.     

早白垩世华北克拉通东部岩石圈减薄过程和机制:来自河北西石门杂岩体的证据

白垩纪华北克拉通岩石圈经历了巨大的减薄事件,但减薄过程及机制仍存在较大争论。早白垩世的西石门杂岩体是华北克拉通东部与岩石圈减薄相关的重要杂岩体。根据西石门杂岩体形成时代和岩石结构特征将其划分为两套岩石组合。早期岩套由二长闪长岩-二长岩-正长岩系列组成,具等粒结构,形成时代约为135. 6Ma,岩石地球化学成分为SiO_2=51. 72%～68. 56%、MgO=0. 17%～4. 88%,少部分样品表现出C型埃达克岩的特征;晚期岩套由斑状二长岩-斑状正长岩系列组成,具似斑状结构,形成时代约为125. 3Ma,岩石地球化学成分为SiO_2=57. 00%～67. 51%、MgO=0. 61%～3. 37%,其中还存在大量年龄约2500Ma的继承锆石。两套侵入岩均为高钾钙碱性系列但早期岩套多具埃达克质岩特征,晚期岩套岩石中锆石具有更负的ε_(Hf)(t),显示其具有更多的壳源物质组成。角闪石温度计、压力计显示,早期岩套的定位环境为T=883. 66℃、P=194. 95MPa,相当于7. 36km深处;晚期岩套的定位环境为T=642. 34℃、P=26. 81MPa,相当于1. 01km深处。从135. 6Ma至125. 3Ma,地壳快速抬升了约6. 35km。研究认为,在约135. 6Ma时陆内岩浆作用开始影响研究区,软流圈地幔上涌促使地壳物质熔融、混合形成了早期岩套,同时大规模的岩浆活动及地壳褶皱变形导致陆壳加厚。之后岩石圈地幔发生拆沉作用,地壳快速抬升。在约125. 3Ma后,软流圈地幔继续上涌,回弹的陆壳开始发生大规模的熔融形成了晚期岩套。据此认为拆沉作用是中生代华北克拉通岩石圈减薄的主要机制。     

Geochronology and provenance of detrital zircons from late Palaeozoic strata of central Jilin Province,Northeast China: implications for the tectonic evolution of the eastern Central Asian Orogenic Belt

Here we present new U–Pb and Hf isotopic data for detrital zircons obtained from six samples of late Palaeozoic units from central Jilin Province, Northeast China, and use these data and sedimentary formations to constrain the late Palaeozoic tectonic evolution of the eastern segment of the southern margin of the Central Asian Orogenic Belt. The majority of the detrital zircons from the six samples are euhedral–subhedral and exhibit oscillatory zoning, indicating a magmatic origin. Zircons from sandstones in the Devonian Wangjiajie and Xiaosuihe formations yield seven main age populations (399, 440, 921, 1648, 1864, 1911, and 2066 Ma) and two minor age populations (384 and 432 Ma), respectively. Zircons from a quartz sandstone in the Carboniferous Luquantun Formation yield four age populations (~332, 363, 402, and 428 Ma), and zircons from quartz sandstones of the Permian Shoushangou, Fanjiatun, and Yangjiagou formations yield age populations of 265, 369, 463, 503, and 963 Ma; 264, 310, 337, 486, and 529 Ma; and 262, 282, 312, 338, 380, 465, and 492 Ma, respectively. These data, together with the ages of magmatic zircons from interbedded volcanics and biostratigraphic evidence, as well as analysis of formations, give rise to the following conclusions. (1) The Wangjiajie and Xiaosuihe formations were deposited in an extensional environment during Middle and Middle–Late Devonian time, respectively. The former was sourced mainly from ancient continental material of the North China Craton with minor contributions from newly accreted crust, while the latter was sourced mainly from newly accreted crust. (2) The Luquantun Formation formed in an extensional environment during early–late Carboniferous time from material sourced mainly from newly accreted crust. (3) The Shoushangou, Fanjiatun, and Yangjiagou formations formed during a period of rapid uplift in the late Permian, from material sourced mainly from newly accreted crust.