Isotopic age of the Permian-Triassic basaltic magmatism in the Polar Cis-Urals: Rb-Sr and Sm-Nd data

Two basaltic lava flows are confined to the Permian-Triassic boundary in succession of continental deposits of the Polar Cis-Urals that is confirmed by Rb-Sr (250 ± 15 Ma) and Sm-Nd (249 ± 17 Ma) dates obtained for the lower flow in the Paemboi-Khalmer-Yu area of the Korotaikha depression. The analyzed basalts are correlative in age with Siberian plateau basalts (250 Ma) and the Permian-Triassic boundary dated at 251 Ma in the Meishan section of marine deposits in South China.     

Recent achievements on the research of the Paleozoic-Mesozoic transitional period in South China

This is a review of research achievements on the Permian-Triassic transition in South China. It comprises of five parts: (1) advances on the biostratigraphy and eventostratigraphy of the Meishan Section, the Global Stratotype Section and Point (GSSP) of the Permian-Triassic boundary (PTB); (2) advances on the PTB research of deep water facies, especially the Dongpan Section in Guangxi; (3) advances on the PTB research of terrestrial facies, especially the Chahe Section in Guizhou; (4) correlation of the global change and biotic extinction between the PTB and modern times and its revelation to the status and future of the earth and mankind; and (5) the pattern and causality of the Permian-Triassic extinction. In the last part, it is concluded that the Permian-Triassic transitional interval constitutes a prolonged crisis period ranging from end-Guadalupian extinction to the end of Early Triassic, totaling 14 Ma. The environmental crisis and mass extinction peaked at the PTB, which displayed multiphase extinction rather than just one phase. Commencement of an extinctions prelude prior to the postulated bolide impact implies that the causes of PTB extinction largely lie in the intrinsic developments of the earth, especially those related with the integration of Pangea.     

贵州乐康P/T界线无机碳同位素特征及其意义

贵州乐康深水海槽相中的无机碳同位素研究表明,晚二叠世由于局限环境,区域内生物贫乏,相对应的碳同位素值比同期其他剖面中碳同位素值偏低。而在长兴阶晚期,由于受区域内火山作用的影响,在剖面界线之下发育另一个碳同位素负异常,之后碳同位素有所回升,直至界线附近降低为最低值。早三叠世由于绝灭之后的生物复苏缓慢,无机碳同位素表现为低正值。碳同位素特征显示,碳同位素的变化受到了生物和环境等多种因素的综合控制。     

Crustal assimilation in basalt and jotunite: Constraints from layered intrusions

Massive continental volcanism and/or bolide impacts are considered by many authors to have caused three major mass extinction events during the last 300 million years: the end-Permian, end-Cretaceous and end-Triassic extinctions. However, re-evaluation of the frequency of bolide impacts and plume-related flood basalt provinces indicates that both types of event occur much more frequently than mass extinctions, and so, in isolation, may not be responsible for the largest extinctions. Furthermore, the kill mechanisms associated with either flood basalts or impacts do not appear to be sufficiently powerful to cause worldwide collapse of ecosystems leading to the largest mass extinctions. Contemporaneous flood basalts and bolide impact may be prerequisites for the largest mass extinctions. We present a statistical analysis of the probability of coincidence between volcanism and impact, and show that three random coincidences of these events in the last 300 m.y. are likely. No causal relationship between impact and volcanism is necessary. The lesser mass extinctions, on the other hand, may not require juxtaposition of two such catastrophic events; such coincidences occurring on more than three occasions during the last 300 m.y. become increasingly unlikely.     

Paleomagnetism of trap basalts in the northwestern Siberian craton,from core data

We present new paleomagnetic data for continental flood basalts (Siberian traps) obtained from cores of two boreholes in the northwestern Noril'sk area, within the Kharaelakh and Vologochan basins. Paleomagnetic measurements of lava and tuff samples from KhS-59 and SSV-19 boreholes allowed reconstructing and correlating the polarity patterns. Thus multiple paleomagnetic anomalies (PMA) have been discovered as brief polarity changes in narrow intervals of the magnetostratigraphic section above the principle reversal at the boundary between the Ivakin and Syverma Formations.The most prominent anomalies are observed at the bases of the Morongo and Mokulai Formations. The samples from the anomalous intervals differ from those of other intervals neither in rock magnetic properties, nor in mineralogy and magnetic grain sizes. Therefore, the revealed PMA record excursions of the geomagnetic field. Comparison of the results with the Meishan Global Stratotype Section and Point (GSSP) of the Permian–Triassic boundary implies a revision to the P–T position in the trap basalt succession of the Noril'sk area. Judging by the EMF behavior, basalts in this part of the trap province erupted for at least 500 kyr during an interval of stable normal polarity.     

广西东攀二叠系-三叠系界线剖面基于岩石磁参数的米兰科维奇旋回特征和地层对比

二叠纪-三叠纪之交发生了地质历史上最大的生物集群绝灭。通过对广西东攀二叠系-三叠系界线剖面不同的磁学数据进行频谱分析,其结果均具一致的功率谱峰,对比煤山剖面的同位素年龄将厚度频率35.7∶15.6∶7.7cm/cycle换算为时间周期95∶41.6∶20.5kyr/cycle,分别与米兰科维奇旋回偏心率、斜度、岁差相当,从而将地层研究的精度提高到万年级,为生物绝灭和众多地质事件提供时间上的约束。不同的磁学参数分别受控于不同的天文周期,磁性矿物种类主要受偏心率旋回控制,同时也受斜度周期的影响,而岁差周期则是促使岩石携磁能力高频变化的原因。不同二叠系—三叠系剖面(集中在界线附近)的磁化率曲线具有一致特征,可作为界线点确定的辅助依据。     

Alkane Biomarkers in Permian—Triassic Boundary Strata at Meishan Section, Changxing, Zhejiang Province

Meishan Section D in Changxing County, Zhejiang Province, China has been selected as the global stratotype of the Permian-Triassic boundary and various studies hav been done at the boundary but the gas chromatographic-mass spectrographic analysis of alkane biomarkers has not been investigated. This paper presents of results of a study of the biomarkers analyzed in a series of samples across the permian-Triassic boundary at both Meishan Section A and Section D. The results show that the overall concentration of alkane biomarkers in the Permian-Triassic boundary strata is high in Bed 26 while it is low in Bed 27. A variety of biomarker parameters demonstrate that the main sources of organic matter in the sediment are algae and bacteria and that the depositional enviroment varied from weakly oxidizing to reducing during the studied interval.     

湖北黄石、浙江长兴煤山二叠-三叠系界线处火山物质及粘土岩成因探讨

本文对湖北黄石、浙江长兴煤山剖面二叠-三叠系界线附近粘土岩层作了碎屑物的分析和测试,并计算了它们的百分含量。与火山活动有关的碎屑主要有石英(其中含六方双锥状的β石英)、锆石、磷灰石、玻璃圆球、铁质圆球及火山灰等。有些样品中还有电气石、石榴石及锐钛矿等。笔者认为粘土岩层的成因是火山物质在海水中沉积蚀变而成。火山喷出口不在工作区内,生物绝灭的主要原因不是火山活动引起的。     

江西东岭剖面P-T界线碳同位素变化与沉积物输入的关系

二叠纪末期发生了显生宙以来规模最大的生物灭绝事件,利用江西修水东岭剖面二叠-三叠系界线的碳同位素、主量元素及微量元素分别研究全球碳循环的变化及其相对应的物源的变化进而分析环境变化与生物灭绝的关系。研究结果表明,东岭剖面二叠-三叠系界线存在阶梯式碳同位素负偏,第一阶段阶梯式负偏幅度为2‰,第二阶段阶梯式负偏幅度为2.5‰,总的负偏幅度高达4.5‰。在阶段阶梯式碳同位素负偏过程,碳酸盐岩中的硅酸盐组分物源由基性火成岩转变为岛弧性质的酸性火成岩。后者可能与我国华南周围岛弧火山喷发有关。这些碳同位素负偏在时间上与物源的转变及火山灰层基本一致,推测其与我国华南岛弧火山及西伯利亚大火成岩省喷发有关。大规模火山作用喷出或诱发出的二氧化碳及甲烷温室气体有可能是造成二叠-三叠系阶梯式碳同位素负偏的主要原因。火山喷发造成的环境恶化如全球变暖、海洋缺氧、海洋酸化、植被破坏形成的大量沉积物输入海洋致使生物生存压力增大,从而造成二叠末期生物的大灭绝。     

Evidence for wildfire in the Meishan section and implications for Permian-Triassic events

Polynuclear aromatic hydrocarbons (PAHs) and black carbon (BC) in sediments are powerful tools in the identification of the combustion process throughout geologic history. In this study, combustion-derived PAHs and BC were carefully investigated in sediments from the Global Stratotype Section and Point of the Permian-Triassic (P/Tr) boundary in Meishan, China. Quantitative analyses of combustion-derived PAHs and BC demonstrate anomalously high concentrations in the boundary event beds that coincide with the mass extinction horizon. The prevalence of parent polynuclear aromatics (e.g., phenanthrene) in PAHs, together with non-metric multidimensional scaling analysis, confirms that the PAHs are mainly derived from vegetation burning instead of having a coal and/or oil origin. BC detected in sediment occurs in various forms from large irregular charcoal particles to fine aciniform soot, with an equivalent reflectance of up to 3.5%. The results strongly suggest that a wildfire occurred during the P/Tr boundary, which served as one of the possible triggers of mass extinction on land. The wildfire occurrence indicates that the O₂ concentration of the atmosphere during (or before) the P/Tr mass extinction was probably >17%. The temporal coincidence of the mass extinction with intensive volcanic eruption, marine anoxia and wildfire events in the region of the Meishan section provides new insight into the mechanisms of the P/Tr biotic crisis. Our results show that wildfires could have played an important role in the collapse of the ecosystem in the Meishan P/Tr events.     

Paleomagnetism of the Siberian Flood Basalts of the Noril'sk Area: A Constraint on Eruption Duration

The volcanic sequence of the Noril'sk area, northern Siberia, provides the most complete section of early Siberian flood-basalt volcanism. Paleomagnetic measurements for more than 4000 samples of lava and tuff indicate that nearly all of this >3500-m-thick sequence was laid down during one interval of normal magnetic polarity. Lavas of the lower third of this sequence are cut by the ore-bearing Noril'sk-I intrusion, which has an age of 251 Ma, identical to that of the Permian-Triassic boundary. Thus, the normal-polarity interval represented by this sequence is inferred to be the first of the Triassic Period. Eruption of this enormous volume of material in a relatively brief period coincident with the earth's greatest mass extinction requires that all aspects of Siberian flood-basalt volcanism be evaluated carefully as possibly contributing to that catastrophe.     

贵州盘县峨眉山玄武岩系顶部凝灰岩LA-ICP-MS 锆石U-Pb年龄:对峨眉山大火成岩省与生物大规模灭绝关系的约束

在20世纪90年代,有学者认为峨眉山大火成岩省(Emeishan Large Igneous Province, ELIP)大规模火山活动与二叠-三叠系之交(Permian-Triassic Boundary, P-TB)的生物大灭绝事件在时间上有耦合关系,随后的⁴⁰Ar/³⁹Ar同位素测年结果也显示峨眉山大火成岩省是晚二叠世形成的。但是,近些年大量的SHRIMP U-Pb测年结果表明,ELIP大规模火山喷发约在~260Ma;因此有研究认为,ELIP火山活动与中二叠世瓜德卢普期末(end-Guadalupian)的生物灭绝事件在时间上联系更加紧密。至于P-T界线生物大灭绝,现在多数学者认为是,由于西伯利亚大火成岩省火山强烈活动释放大量气体和火山灰所造成环境变化引起的。最近,我们在ELIP东部的贵州盘县峨眉山玄武岩系剖面中发现顶部发育厚度达近百米的凝灰岩层,其LA-ICP-MS U-Pb法测年结果为251.0±1.0Ma,与浙江煤山剖面中二叠系-三叠系边界处黏土层或火山灰层的锆石U-Pb年龄接近。因此,峨眉山玄武岩喷发结束的时间应该在P-T边界,与西伯利亚大火成岩省的主体喷发时间一致。新的测年结果暗示了ELIP火山活动与地球历史上最大的一次生物灭绝事件(P-T边界)可能存在着成因联系。     

地层单位与全球界线层型:概念、术语、有关规定与研究实例

全球二叠系-三叠系界线(PTB)层型及点("金钉子")定位我国浙江省长兴县煤山D剖面,极大地推动了PTB及二叠系和三叠系研究进展。在简要介绍地层单位与全球界线层型与点建立的相关概念、术语和有关规定、地层划分的主要种类及其地层单位术语的基础上,以全球二叠系—三叠系界线层型及点(中国浙江省长兴县煤山D剖面)为研究实例,综述了该金钉子剖面牙形石生物地层和年代地层研究现状。煤山D剖面构建了目前全球最完整的PTB地层牙形石带序列,为研究二叠纪末大灭绝后生物残存和复苏过程提供了重要的时间坐标系,也为相关的地质事件研究奠定了坚实的基础。     

Geochronology and geochemistry of late Cenozoic basalts from the Leiqiong area,southern China

The Leiqiong area, which includes the Leizhou Peninsula and the northern part of the Hainan Island, is the largest province of exposed basalts in southern China. Ar–Ar and K–Ar dating indicates that incipient volcanism in the Leiqiong area may have taken place in late Oligocene time and gradually increased in tempo toward the Miocene and Pliocene Epoch. Volcanic activities were most extensive during Pleistocene, and declined and ended in Holocene. Based on radiometric age dating and geographic distribution, Pliocene and Quaternary volcanism in Hainan Island can be grouped into two stages and six eruptive regions. The early volcanism is dominated by flood type fissure eruption of quartz tholeiites and olivine tholeiites whereas the later phase is dominated by central type eruption of alkali olivine basalts and olivine tholeiites. The systematic decrease of MgO, ΣFeO and TiO₂ with increasing SiO₂ content for basalts from Hainan Island indicates that fractional crystallization of olivine, clinopyroxene and Ti-bearing opaques may have occurred during magmatic evolution. From coexisting Fe–Ti oxide minerals, it is estimated that the equilibrium temperatures range from 895–986°C and oxygen fugacities range from 10^−13.4 to 10^−10.7 atmospheres in the basaltic magmas. The incompatible element ratios and the chondrite-normalized REE patterns of basalts from the Leiqiong area are generally similar to OIB. The Nb/U ratios (less than 37) in most of the tholeiitic rocks and the negative Nb anomaly observed in the spidergram of some basalts indicated that the influence of a paleo-subduction zone derived component can not be excluded in considering the genesis of the basalts from the Leiqiong area. The tholeiites in the Leiqiong area may have mixed with a more enriched lithospheric mantle component as well as undergone relatively larger percentages of partial melting than the alkali basalts.     

On the global stratotype section and point of the Triassic base

A unique succession of volcanogenic deposits with representative paleontological remains characterizing the Permian-Triassic boundary interval in the North Siberian platform and Taimyr is described. The succession is suitable for selecting a standard for the Triassic base in nonmarine deposits. Abundant and diverse fossils occurring in the succession evidence that volcanism responsible for origin of the plateau basalt province in Siberia was not a brief epoch of paroxysmal eruptions, which eliminated everything alive. Throughout the formation history of relevant plateau basalts, the organic world of the plateau and around existed and gradually evolved.     

华南二叠－三叠系的事件地层与生物地层界线

华南二叠－三叠系界线研究应严格区分事件地层界线与生物地层界线。“界线粘土”层的底界即为事件地层界线。生物地层界线定义不能与“混生层”或“过渡层”的概念连在一起。长兴煤山忠心大队剖面是最好的二叠－三叠系全球界线层型剖面点（ＧＳＳＰ）。二叠－三叠系生物地层界线定义为ＨｉｎｄｅｏｄｕｓｐａｒｖｕｓＭｏｒｐｈｏｔｙｐｅ１的首次出现,其位置就在长兴剖面界线层（混生层）２的内部,比事件地层界线高１５ｃｍ。ＨｉｎｄｅｏｄｕｓｐａｒｖｕｓＭｏｒｐｈｏｔｙｐｅ１的首次出现,即为长兴阶的顶界。     

华南二叠－三叠系的事件地层与生物地层界线

华南二叠－三叠系界线研究应严格区分事件地层界线与生物地层界线。“界线粘土”层的底界即为事件地层界线。生物地层界线定义不能与“混生层”或“过渡层”的概念连在一起。长兴煤山忠心大队剖面是最好的二叠－三叠系全球界线层型剖面点（ＧＳＳＰ）。二叠－三叠系生物地层界线定义为ＨｉｎｄｅｏｄｕｓｐａｒｖｕｓＭｏｒｐｈｏｔｙｐｅ１的首次出现,其位置就在长兴剖面界线层（混生层）２的内部,比事件地层界线高１５ｃｍ。ＨｉｎｄｅｏｄｕｓｐａｒｖｕｓＭｏｒｐｈｏｔｙｐｅ１的首次出现,即为长兴阶的顶界。     

四川江油马角坝刺林包剖面二叠-三叠系间粘土岩研究

二叠-三叠纪之交华南地区普遍发育火山成因的粘土岩,其对于了解二叠纪末大规模火山活动及扬子板块碎屑物质来源具有重要意义。本文对扬子西缘马角坝刺林包剖面飞仙关组底部粘土岩进行了碎屑锆石U-Pb定年及原位Hf同位素测试、X衍射和全岩地球化学分析,以揭示粘土岩的岩石成分及物源信息。镜下特征、X衍射及主量元素特征表明岩石中粘土矿物主要为伊利石,含少量方解石、石英。碎屑锆石年龄显示,特征峰值年龄主要集中在2 615~2 383、1 868~1 328、1 186~778和430~246 Ma,并出现~250 Ma特征高峰,结合原位Hf同位素特征表明锆石与秦岭造山带、华南典型PTB剖面粘土岩及峨眉山大火成岩省具有较高的相似性。同时,通过微量、稀土元素分析发现粘土岩中Zr、Hf、Th、Cr、Co、Ti相对富集,与华南典型PTB粘土岩和峨眉山玄武岩相近,并结合构造背景及锆石特征,综合认为刺林包剖面PTB粘土岩来自于秦岭造山带及龙门山岛链局部剥蚀区、二叠纪末火山活动和峨眉山大火成岩省剥蚀岩体。     

Source, genesis, and timing of giant ignimbrite deposits associated with Ethiopian continental flood basalts

The Ethiopian continental flood basalt (CFB) province (∼30 Ma, > 3 × 10⁵ km³) was formed as the result of the impingement of the Afar mantle plume beneath the Ethiopian lithosphere. This province includes major sequences of rhyolitic ignimbrites generally found on top of the flood basalt sequence. Their volume is estimated to be at least 6 × 10⁴km³, which represents 20% of that of the trap basalts. Their phenocryst assemblage (alkali feldspar, quartz, aegyrine-augite, ilmenite ± Ti-magnetite, richterite, and eckermanite) suggests temperatures in the range of 740 to 900°C. Four units were recognized in the field (Wegel Tena, Jima, Lima Limo, and Debre Birhan areas), each with its own geochemical specificity. Zr/Nb ratios remain constant between basalt and rhyolite in each area, and rhyolites associated with high-Ti or low-Ti basalts are, respectively, enriched or depleted in titanium. Their trace element and isotope (Sr, Nd, O) signatures (high ¹⁴³Nd/¹⁴⁴Nd and low ⁸⁷Sr/⁸⁶Sr ratios, compared to those of rhyolites from other CFB provinces) are clearly different from those of typical crustal melts and indicate that the Ethiopian rhyolites are among the most isotopically primitive rhyolites. Their major and trace element patterns suggest that they are likely to be derived from fractional crystallization of basaltic magmas similar in composition to the exposed flood basalts with only limited crustal contribution. Since Ethiopian high-Ti basalts have been shown to form from melting of a mantle plume, it is likely that Ethiopian ignimbrites, at least those that are Ti-rich, also incorporated material from the deep mantle.Rb-Sr isochrons on whole rocks and mineral separates (30.1 ± 0.4 Ma for Wegel Tena and 30.5 ± 0.4 Ma for Jima ignimbrites) show that most of the silicic volcanism occurred within < 2 Ma during the Oligocene. Ignimbritic eruptions resumed in the Miocene during two episodes dated at 15.4 ± 0.2 Ma and 8.0 ± 0.2 Ma for the Debre Birhan area. The Rb-Sr isochron ages of ignimbrites (both Oligocene and Miocene rhyolites) are indistinguishable within uncertainties from the ⁴⁰Ar/³⁹Ar ages of the underlying flood basalts. The Oligocene ignimbrites and the underlying trap basalts are synchronous with a shift in the oxygen composition of foraminifera recorded in Indian and Atlantic Ocean cores. The temporal coincidence of Ethiopian Oligocene volcanism, which released immense volumes of S (> 1.4 × 10¹⁵ mol) and Cl (6.4 × 10¹⁵ mol) into the atmosphere over a short time span, with the global cooling event at 30.3 Ma suggests that this volcanism might have accelerated the climate change that was already underway.     

二叠纪-三叠纪之交全球海平面变化研究

二叠纪-三叠纪界线附近的全球海平面变化是当前沉积学研究的热点和难点问题,其与当时的显生宙最大规模生物灭绝事件存在一定关联,具有重要的研究意义。然而二叠纪-三叠纪界线附近的全球海平面变化存在较多争议,受单剖面或区域范围内相对海平面变化研究程度的制约,在缺乏从沉积学角度的综合对比研究的情况下,可能会影响对全球海平面变化过程与持续时间的判识。综述了二叠纪-三叠纪界线附近的海平面变化研究进展,整合了多位学者的研究剖面、主要观点及认识,梳理了全球海平面变化的主要观点（“上升论”和“下降-上升论”）,包括其各自的发展历程、代表剖面及海平面变化识别特征、海平面上升/下降的原因以及海平面变化与生物灭绝的关系等,并在此基础上,探讨了二叠纪-三叠纪全球海平面变化研究过程中产生争议的原因。本文旨在为二叠纪-三叠纪界线（PTB）附近海平面变化研究提供线索,同时为研究全球PTB地质事件发生的背景及差异性原因提供基础证据。