五台地区滹沱群时代与地层划分新认识:地质学与锆石年代学证据

本文从五台地区滹沱群豆村亚群四集庄组、东冶亚群纹山组和郭家寨亚群西河里组地层中共采集了5件浅变质砂岩样品,并对其进行了La-MC-ICPMS锆石U-Pb年龄测定。分析结果显示,四集庄组2件砂岩样品碎屑锆石²⁰⁷Pb/²⁰⁶Pb年龄主要集中于~2.5Ga和2.1~2.2Ga两个峰值,其中~2.5Ga碎屑锆石来自新太古代五台群和五台地区花岗质杂岩;2.1~2.2Ga碎屑锆石获得²⁰⁷Pb/²⁰⁶Pb加权平均年龄2134±5Ma,限定了四集庄组砂岩沉积下限为2134Ma。结合四集庄组火山岩形成时代(2140±10Ma)和四集庄组底部发育厚层砾岩,我们认为滹沱群初始形成时代为~2.2Ga,即早元古代中期。东冶亚群纹山组底部砂岩中碎屑锆石²⁰⁷Pb/²⁰⁶Pb年龄主要集中于2050~2122Ma之间,其中64粒相对年轻的锆石获得²⁰⁷Pb/²⁰⁶Pb加权平均年龄2068±3Ma,代表了东冶亚群形成时代下限为2070Ma左右。综合豆村亚群青石村组火山岩形成时代2087±9Ma,我们认为东冶亚群初始形成于2070Ma左右。郭家寨亚群中最年轻碎屑锆石²⁰⁷Pb/²⁰⁶Pb年龄为1958±10Ma,表明郭家寨亚群开始沉积时代小于1.95Ga,为早元古代晚期/末期。区域上,早元古代末期是华北最终克拉通阶段,而郭家寨亚群与东冶亚群呈明显的角度不整合接触关系,两者记录了明显不同的地质过程。因此,我们建议郭家寨亚群应从滹沱群中解体出来并独立命名为郭家寨群,且郭家寨群可能沉积于华北克拉通化过程中/之后,开始沉积的时代为1.9~1.8Ga。     

A subdivision of the Precambrian of China

Precambrian rocks are widely distributed in China. The Precambrian is divided into two time units, i.e., the Archaean and Proterozoic Eon, each of these is separated into three chronological intervals, also with the status of eras, with the prefixes early, middle or late. The time boundary between the Archaean and Proterozoic Eon is placed at ～ 2500 Ma.According to the present isotopic data, the proposed subdivision for the Archaean of China is two-fold. The age of the Fuping Group is younger than 2800–2900 Ma, and that of the Qianxi Group and the corresponding stratigraphic units of eastern Liaoning are older than 2800 Ma, so that 2800+ Ma is selected as the boundary between the early—middle and late Archaean.Based on the representative stratigraphic units, the Wutai and Huto Groups, and an intervening major unconformity formed by the Wutaiian orogeny at 2200–2300 Ma, the early Proterozoic is further divided into two periods, with a time demarcation at 2200+ Ma. A major episode of orogeny known as the “Luliangian Movement” occurred at the end of the early Proterozoic at ～ 1900 Ma. This disturbance was very extensive and is, in a way, responsible for the difference in geological conditions between the lower and middle—upper Proterozoic in China. The boundary (1900 Ma) that relates to the Luliangian Movement is more important than the boundary corresponding to the age of 1600 Ma, which is recommended as the time boundary between Proterozoic I and II, so we propose to use 1900 Ma as the boundary between the early and middle Proterozoic in China.The time boundary between the middle Proterozoic, including the Changcheng System and the Jixian System, and the late Proterozoic, which is composed of the Qingbaikou and Sinian Systems, is ～ 1000 Ma. The age for the boundary between Cambrian and Precambrian, based upon the recent isochron data, is inferred to be 610 Ma.     

Neoproterozoic diamictites from the Itombwe Synclinorium, Kivu Province, Democratic Republic of Congo: Palaeoclimatic significance and regional correlations

The Itombwe Synclinorium in the Kivu Province of the Democratic Republic of Congo contains a Neoproterozoic succession of greenschist facies metasedimentary rocks defined as the Itombwe Supergroup, dated between 1020 ± 50 and 575 ± 83 Ma. The Itombwe Supergroup unconformably overlies the Mesoproterozoic Kibaran belt and is subdivided into the Upper and Lower Kadubu Groups which are separated by a faulted tectonic contact. Graded, rhythmically repeated sequences of sandstones, greywackes, phyllites and shales indicate deposition as turbiditic sediment-gravity flows. Periods of basin anoxia are indicated by the presence of graphitic black shales. The Lower and Upper Kadubu Groups contain three stratigraphic levels of diamictites and lonestone-bearing iron-rich sedimentary rocks interpreted as glaciogenic strata, which broadly correlate with other Neoproterozoic glacial sequences in the Central African region and elsewhere around the world. Current stratigraphic and geochronological knowledge of these beds is insufficient to provide more accurate correlations.     

Geochronological constraints on early volcanic evolution of the Pilbara Block,Western Australia

U‐Pb isotopic systems of zircons from the Boobina and Spinaway Porphyries from the Precambrian Pilbara Block of Western Australia indicate ages of 3307± 19 Ma and 2768 ± 16 Ma, respectively. The Boobina Porphyry intrudes upper members of the Archaean greenstones of the Warrawoona Group. The Spinaway Porphyry intrudes basal units of the unconformably overlying volcanics and sediments of the Mt Bruce Supergroup. The age of the Boobina Porphyry, together with previous zircon U‐Pb and whole rock Sm‐Nd age determinations on stratigraphically older units, indicate that early Archaean volcanism in the Pilbara took place between 3560 Ma and 3300 Ma. On the basis of the age determination of the Spinaway Porphyry, and the chronometric definition of 2500 Ma for the Archaean—Proterozoic boundary, by the International Subcommis‐sion on Precambrian Stratigraphy (James H. L. 1978, Precambrian Res. 7, 193–204), the lower units of the Mt Bruce Supergroup should now be assigned to the Archaean.     

Eozoic complexes of the U.S.S.R.

Soviet geologists consider the Precambrian to be divided into two groups — Archaean and Proterozoic; but such a division is unsatisfactory. A major unconformity separates Proterozoic volcanic and sedimentary formations from an underlying sequence that contains two supergroups of supercrustal formations. The oldest of these is unanimously considered to be Archaean. Rocks of that supergroup play an essential part in the composition of the Baltic, Ukrainian, Aldan and Anabar Shields and of the ancient fold belts of the East-European and Siberian platforms.Distinctive features in the composition, tectonic structure, metamorphism and metallogeny of Archaean complexes lead to the conclusion that they were formed in specifically mobile areas, different from geosynclinal areas.The other supergroup of high-grade metamorphic rocks has no clear place in the accepted two-fold stratigraphic scheme of the Precambrian, and it is considered sometimes to be Archaean and sometimes to be Early Proterozoic. We propose restoring the forgotten name “Eozoic” for that supergroup. Eozoic complexes are characterized by peculiarities of composition and inner structure, which signify changes in the tectonic regime of the earth at the lower and upper boundaries of the Eozoic Supergroup. These peculiarities give grounds for distinguishing the Eozoic Supergroup as an independent stratigraphic division.The Stanovoy Complex of the southern part of the Aldan Shield is a stratotype for the Eozoic Supergroup. Many well-known stratigraphic subdivisions of the Siberian Platform (e.g., the Eniseiskaya, the Birusinskaya series and others), the Taratash Complex of the Urals, the Goranskaya and Shahdarinskaya series of the South-West Pamir, the Tikitch complex and Aulskaya series of the Ukrainian Shield, and in part the Belomorsky Complex of the Baltic Shield, as well as some others, are also Eozoic.The Eozoic complexes are characterized by the following specific features: only some supercrustal formations are typical for them; the small number of rock types which have a total thickness about 5–6 km; relatively monotonous mineral composition of the rocks; variable quantitative ratios of rocks; absence of contrasting marker beds; regional metamorphism and ultrametamorphism in the amphibolite facies; wide development of ultrametamorphic granitoids and migmatites; distinct tectonic differentiations of the basin of sedimentation.Dates determined by isotopic analyses, which mostly reflect the metamorphism of the deposits, fall predominantly in the range 2600–3100 Ma.     

五台山区早元古代地层层序探讨

根据大量的区调资料,利用断陷盆地的分析方法,通过地层剖面的横向对比,认为滹沱群的豆村亚群、东冶亚群以及系舟山的下元古界七东山组为侧向相变关系,郭家寨亚群形成于其后。在详细研究岩性、岩相的变化规律以及盆缘断裂特征的基础上,初步建立了早元古代的岩石地层格架,分析了早元古代裂陷海槽的发育史。     

U-Pb Zircon Dating of the Granitic Conglomerates of the Hutuo Group： Affinities to the Wutai Granitoids and Significance to the Tectonic Evolution of the Trans-North China Orogen

1 Introduction The North China Craton (NCC) is considered to be the oldest and largest cratonic block in China. Recent studies to gain understanding of basement architecture of the NCC has led to its division into the Western and Eastern Blocks, separated by a N-S trending Paleoproterozoic Trans-North China Orogen (TNCO) (Fig. 1; Zhao et al., 1998, 1999a, 2000a, 2001a; Wilde et al., 2002). Although there is now abroad consensus that the final assembly of the NCC was completed by th…     

Paleoclimate Changes during the Early Oiigocene in the Hoh Xil Region, Northern Tibetan Plateau

Sedimentological, cyclic-stratigraphic, paleomagnetic, and clay-mineralogical studies on the early Oligocene Yaxicuo Group in the Hoh Xil Basin, the largest Cenozoic sedimentary basin in the hinterland of the Tibetan Plateau, provide abundant information of paleoclimate changes. A 350-m thick section in the middle-lower Yaxicuo Group was analyzed to reveal the climatic history that occurred in the Hoh Xil region during the early Oligocene interval 31.30-30.35 Ma, dated with the paleomagnetic chronostratigraphy. The results indicate that add and cold climate dominated the Hoh Xil region during the early Oligocene in general, being related to the global cooling and drying events that occurred in the earliest Oligocene. Within this period, relatively warm and wet climate accompanied by strong tectonic activity occurred in the 31.05-30.75 Ma interval; while add and cold climate and relatively inactive tectonics occurred in the 31.30-31.05 and 30.75-30.35 Ma intervals. Furthermore, spectral analyses of high-temporal resolution paleoclimatic records show orbital periods including eccentricity, obliquity, and precession. It is concluded that paleoclimate changes during the early Oligocene in the Hoh Xil region were forced by both tectonic activity and orbital periods.     

Sedimentation rates, basin analysis and regional correlations of three Neoarchaean and Palaeoproterozoic sub-basins of the Kaapvaal craton as inferred from precise U–Pb zircon ages from volcaniclastic sediments

Calculation of sedimentation rates of Neoarchaean and Palaeoproterozoic siliciclastic and chemical sediments covering the Kaapvaal craton imply sedimentation rates comparable to their modern facies equivalents. Zircons from tuff beds in carbonate facies of the Campbellrand Subgroup in the Ghaap Plateau region of the Griqualand West basin, Transvaal Supergroup, South Africa were dated using the Perth Consortium Sensitive High Resolution Ion Microprobe II (SHRIMP II). Dates of Ma and Ma for the middle and the upper part of the Nauga Formation indicate that the decompacted sedimentation rate for the peritidal flat to subtidal below-wave-base Stratifera and clastic carbonate facies, southwest of the Ghaap Plateau at Prieska, was of up to 10 m/Ma, when not corrected for times of erosion and non-deposition. Dates of Ma for the upper Gamohaan Formation and for the upper Monteville Formation, indicate that some 2000 m of carbonate and subordinate shale sedimentation occurred during 16 Ma to 62 Ma on the Ghaap Plateau. For these predominantly peritidal stromatolitic carbonates, decompacted sedimentation rates were of 40 m/Ma to over 150 m/Ma (Bubnoff units). The mixed siliciclastic and carbonate shelf facies of the Schmidtsdrif Subgroup and Monteville Formation accumulated with decompacted sedimentation rates of around 20 B. For the Kuruman Banded Iron Formation a decompacted sedimentation rate of up to 60 B can be calculated. Thus, for the entire examined deep shelf to tidal facies range, Archaean and Phanerozoic chemical and clastic sedimentation rates are comparable. Four major transgressive phases over the Kaapvaal craton, followed by shallowing-upward sedimentation, can be recognized in the Prieska and Ghaap Plateau sub-basins, in Griqualand West, and partly also in the Transvaal basin, and are attributed to second-order cycles of crustal evolution. First-order cycles of duration longer than 50 Ma can also be identified. The calculated sedimentation rates reflect the rate of subsidence of a rift-related basin and can be ascribed to tectonic and thermal subsidence. Comparison of the calculated sedimentation rates to published data from other Archaean and Proterozoic basins allows discussion of general Precambrian basin development. Siliciclastic and carbonate sedimentation rates of Archaean and Palaeoproterozoic basins equivalent to those of younger systems suggest that similar mechanical, chemical and biological processes were active in the Precambrian as found for the Phanerozoic. Particularly for stromatolitic carbonates, matching modern and Neoarchaean sedimentation rates are interpreted as a strong hint of a similar evolutionary stage of stromatolite-building microbiota. The new data also allow for improved regional correlations across the Griqualand West basin and with the Malmani Subgroup carbonates in the Transvaal basin. The Nauga Formation carbonates in the southwest of the Griqualand West basin are significantly older than the Gamohaan Formation in the Ghaap Plateau region of this basin, but are in part, correlatives of the Oaktree Formation in the Transvaal and of parts of the Monteville Formation on the Ghaap Plateau.     

Stratigraphy and petrology of the Archaean Nsuze Group,northern Natal and southeastern Transvaal,South Africa

The Nsuze Group is the lower, dominantly volcanic, division of the Pongola Supergroup that accumulated on a sialic basement between 3.1 and 2.9 Ga. The Nsuze Group is subdivided into a lower sedimentary unit (800 m thick), a middle volcanic unit (± 7500 m thick) and an upper volcaniclastic-sedimentary unit (5–600 m thick). The predominant sediments in the lower unit are immature, medium- to very coarse-grained quartz wackes with thin intercalated lenses of quartz and arkosic arenites, and minor conglomeratic wackes. These sediments were deposited in a distal braided stream environment.There followed a major period of volcanism during which lavas showing a continuous spectrum of compositions from basalt to rhyolite were extruded subaerially. Flows of both different and similar compositions are complexly interfingered on both regional and local scales. As volcanism waned, pyroclastic and sedimentary rocks became dominant in the upper unit. The Nsuze Group is gently dipping and is metamorphosed to low greenschist grade.The Nsuze Group is significant in that it provides evidence for the existence of high-standing continental sialic crust in the southeastern part of the Kaapvaal province at ca. 3.0 Ga. Volcanism and sedimentation in the Pongola Supergroup are more typical of Proterozoic basins than of Archaean environments, despite their age. Komatiitic and high-Mg basalts were, however, being extruded in Zimbabwe contemporaneously with the Nsuze lavas.     

U-Pb dating and provenance of detrital zircons from the Upper Precambrian deposits of North Timan

Timan comprises the southwest edge of the Pechora Plate. The plate basement is composed of variably metamorphosed sedimentary, mainly terrigenous, and igneous rocks of the Late Precambrian age that are generally overlain by Ordovician-Cenozoic platform cover. Poor exposition and discontinuous distribution of the Upper Precambrian outcrops of dominantly fossil-free sedimentary rocks cause considerable disagreements in stratigraphic correlation. This applies equally to North Timan, which represents an uplifted block of basement, in which sedimentary-metamorphic rocks form the Barminskaya Group (～5000 m thick), previously dated as Early Riphean to Vendian. Earlier Rb-Sr and Sm-Nd isotope dating of schist and cross-cutting gabbro-dolerite and dolerite established the timing of greenschist facies metamorphism at 700 Ma. Thus, Late Riphean age of the Barminskaya Group has been suggested. Results of local U-Pb dating of detrital zircon from silty sandstones of the Malochernoretskaya Formation, which constitutes the middle part of the outcropping section of the Barminskaya Group, confirm this conclusion. Age data for 95 zircon grains cover the range of 1035–2883 Ma with age peaks at 1150, 1350, 1550, 1780, and 1885 Ma. The minimum age of zircons, considered as the lower age constraint on sediment deposition, provides grounds to date the Barminskaya Group as Late Riphean and indicates eroded rock complexes of the Fennoscandian Shield as the possible provenance areas.     

Redefinition and Formation Age of the Tanjianshan Group in Xitieshan Region,Qinghai

The Tanjianshan Group, which was previously divided into a, b, c and d formations, has been controversial for a long time. It mainly distributes in the northern margin of Qaidam Basin and is an important early Paleozoic greenschist facies metamorphic volcanic sedimentary rock formation. Detailed field investigation and zircon LA-ICPMS U-Pb dating of the key strata suggest that the original lower part of a Formation(a-1) versus the original middle upper of d Formation(d-3 and d-4), the original upper part of a Formation(a-2) and b Formation versus the original lower part of d Formation(d-1 and d-2) of Tanjianshan Group are contemporaneous heterotopic facies volcanicclasolite deposit, respectively. The former formations formed during the middle-late Ordovician(463–458 Ma), while the latter ones formed in the late Ordovician(about 445 Ma). The original c formation of Tanjianshan Group, which formed after 430 Ma, is similar to the Maoniushan Formation of Kunlun Mountains and north Qaidam Basin. According to the rules of stratigraphic division and naming, new stratum formations of Tanjianshan Group are re-built and divided into Duancenggou(O1-2td), Zhongjiangou(O2-3tz) and Xitieshan(O3tx) formations. The original c Formation is separated from Tanjianshan Group and is renamed as the Wuminggou Formation(S3-D1w), which shows a discordant contact with underlying Tanjianshan Group and overlying Amunike Formation(D3a). The zircon U-Pb age frequency spectrogram of Tanjianshan Group indicates three prominent peaks of 430 Ma, 460 Ma and 908 Ma, which is consistent with the metamorphic and magmatic crystallization ages obtained from para- and orthogneisses in north Qaidam HP-UHP metamorphic belt, implying that strong Caledonian and Jinningian tectonic and magmatic events have ever happened in North Qaidam.     

Origin of dispersed Permian–Triassic fore-arc basin terranes in New Zealand: Insights from zircon petrochronology

Permian–Triassic fore-arc basin terranes are exposed in New Zealand, but their original positions and tectonic configurations along the eastern Gondwanan margin are not fully understood. To better constrain late Paleozoic and Mesozoic reconstructions, we investigated the provenance of Permian–Triassic marine sandstone units from the Dun Mountain-Maitai Terrane (Maitai Group) and the Kaka Point Structural Belt (Willsher Group). The recognition of abundant volcanic lithic fragments in the sandstone samples, combined with the pattern of detrital zircon ages (unimodal to bimodal 280–240 Ma age distribution), demonstrate that the upper Permian to Middle Triassic volcaniclastic successions were derived from a proximal arc source. The detrital zircon age spectra match magmatic pulses in the adjacent Tuhua Intrusives (Median Batholith), a conclusion similar to that recently proposed for the Brook Street Terrane (Grampian Formation) and Murihiku Terrane (Murihiku Supergroup). Trace-element data from the dated zircon grains provide further evidence for a Median Batholith source and cross-terrane provenance links. The data indicate that 275–230 Ma zircon grains from the Maitai Group, Willsher Group, and Murihiku Supergroup were derived from a common magmatic source, and that the late Permian Longwood Suite (261–252 Ma) in the Median Batholith was a source region for these terranes. Based on the cross-terrane provenance links, we suggest that the Brook Street and Murihiku terranes were deposited in the proximal part of a fore-arc basin, whereas the Dun Mountain-Maitai Terrane represents the distal part of the same basin. Sedimentation in the Maitai Group ceased during the Middle Triassic (∼238 Ma), likely in response to a period of orogenesis at 235–230 Ma (Gondwanide Orogeny) that is widely recognized throughout the southwest Pacific.     

Evidence for multiple Palaeoproterozoic thermal events and magmatism adjacent to the Broken Hill Pb---Zn---Ag orebody, Australia

Thermal events at 1690-1680, 1660-1640 and 1600-1570 Ma have been resolved by SHRIMP U---Pb geochronological study on zircons and monazites from seven localities near to the Broken Hill Pb---Zn---Ag orebody, Australia. The earliest-recognized thermal event included intrusion of now deformed granites such as Rasp Ridge Gneiss and Alma Gneiss and intrusion of gabbro at Round Hill. Previously these have been interpreted as volcanic in origin, and have been assigned to different stratigraphic units of the Palaeoproterozoic Willyama Supergroup. Because these rocks are intrusions, they should be removed from the Supergroup stratigraphic sequence. The 1640–1660 Ma thermal event reached upper amphibolite to granulite conditions and produced melt segregations in parts of the Rasp Ridge Gneiss. Granites of this age are the Purnamoota Road Gneiss, previously correlated with 1690-1680 Ma rocks assigned to the Hores Gneiss stratigraphic unit, and granitic veins within Sundown Group metapelites. The 1600-1570 Ma thermal event also reached upper amphibolite to granulite conditions. The only possible 1600-1570 Ma intrusive rock reported in this study is ‘Lf-leucogneiss’ (granite) at the Purnamoota Road locality. Melt segregations of this age have been found in the Round Hill gabbro and metamorphic segregations have been found in the Purnamoota Road Gneiss. The granite intrusions and segregations are absolute time markers for fabric development and therefore can be used to re-evaluate tectonothermal evolution of rocks close to the Broken Hill Pb---Zn orebody. Within the studied rocks several discrete high grade deformation phases have been observed. The earliest detected deformation is older than 1640–1660 Ma, but syn- or post 1690 Ma. A later deformation phase can be constrained to be pre-or syn 1640–1660 Ma and a yet later deformation phase to be syn- or post- 1600-1570 Ma. The current consensus classifies the Broken Hill Pb---Zn---Ag orebody as the metamorphosed equivalent of classic SEDEX (sedimentary-exhalative) deposits, deposited at ca 1690 Ma. This interpretation heavily relies on the Hores Gneiss being a volcanic marker horizon, because the orebody is situated, apparently conformably, within the Hores Gneiss. However, results of this study show that rocks assigned to the Hores Gneiss are of different age, thus do not form a reliable marker horizon. The present results suggest that in the Thackaringa and Broken Hill Groups in the vicinity of Broken Hill, true supracrustal rocks are ≥ 1690 Ma, rather than ca 1690 Ma as previously suggested. Large parts of rocks surrounding the orebody are intrusions and together with their host supracrustal rocks were metamorphosed and locally remelted at 1660-1640 and 1600-1570 Ma.     

U-Pb zircon geochronological evidence for Neoproterozoic events in the Glenfinnan Group (Moine Supergroup): the formation of the Ardgour granite gneiss, north-west Scotland

The age and Precambrian history of the Moine Supergroup within the Caledonide belt of north-west Scotland have long been contentious issues. The Ardgour granite gneiss is essentially an in situ anatectic granite formed during deformation and regional high-grade metamorphism from Moine metasediments. High-precision TIMS and SHRIMP U-Pb zircon dating shows that the age of the anatectic Ardgour granite gneiss and its enclosed segregation pegmatites is 873 ± 7 Ma. This demonstrates the reality of a Neoproterozoic episode of high-grade metamorphism in the Glenfinnan Group Moine and, contrary to previous evidence, the absence of Grenvillian-aged metamorphism. This conclusion places constraints on Neoproterozoic palaeogeographic reconstructions of the North Atlantic region, indicating that the Moine rocks cannot be used as a link between the Grenvillian belt of North America and the Sveconorwegian orogen in Scandinavia. SHRIMP ages of between c. 1100 and 1900 Ma were obtained from detrital, inherited zircons and reflect the provenance of the Glenfinnan Group Moine sediments which must, therefore, have been deposited between c. 1100 and 870 Ma. Potential sources are found as relatively minor, tectonically bounded basement inliers within the British Caledonides, although more widespread source areas occur outside Britain in both Laurentia and Baltica. The most important feature of the provenance is the absence of detrital Archaean grains. This suggests that the Archaean Lewisian gneiss complex, which forms the basement component of the western foreland to the Caledonides in Britain, was not a major contributor to the Glenfinnan Group basin. Received: 16 June 1996 / Accepted: 29 January 1997     

浙西松木坞群的解体—同位素定年的证据

松木坞群是浙西前晨旦纪重要地层单元，其上部主要为玄武岩－流纹岩组合，中、上部主要为灰紫色、灰绿色滨海砂屑岩。现对其上部酸性火山岩中的单颗粒锆石测得Ｕ－Ｐｂ同位素年龄值８２７Ｍａ和８０９Ｍａ，由此确定它相当于杭州萧山地区的上墅组火山－沉积岩单元。     

U–Pb zircon geochronology of the Ediacaran volcano-sedimentary succession of the NE Saghro inlier (Anti-Atlas,Morocco): Chronostratigraphic correlation on the northwestern margin of Gondwana

We report a new regional correlation for the Ediacaran succession in the Anti-Atlas belt on the northwestern margin of Gondwana, based on U-Pb LA-ICP-MS zircon geochronology of volcanic rocks in the NE edge of the Saghro inlier. The thick volcano-sedimentary succession comprises a diverse suite of rhyolitic-ignimbrite, basaltic to andesitic lava fields, rhyolitic lava, mafic hydroclastic complex, fallout and surge deposits, pyroclastic dyke, interbedded clastic sediment and subvolcanic bodies.Ten volcanic rocks yield crystallization ages ranging from 573 to 547 Ma, consistent with a lower and upper Ouarzazate Supergroup affinity respectively. Inherited zircon ages range from 623 Ma to 600 Ma, analogous to zircon peaks in the older volcano-sedimentary rocks of the Bou Salda, and Saghro groups in the Anti-Atlas, suggesting the continuity of the Saghro Group beneath the Ouarzazate Supergroup at the NE edge of the Saghro inlier.Rocks with a lower Ouarzazate Supergroup affinity include lithic-poor ignimbrites which yield ages of 573.6 ± 1.9 Ma, 571.8 ± 4.2 Ma, 571.3 ± 2.6 Ma, and 567.4 ± 2.9 Ma, two fallout deposits which yield ages of 563.5 ± 2.1 Ma and 569.2 ± 1.9 Ma, a surge deposit dated at 571.6 ± 2.8 Ma and a rhyolite lava dated at 562.5 ± 3.1 Ma. Two lithic-poor ignimbrites from the upper Ouarzazate Supergroup are dated at 557.3 ± 2.6 Ma and 547.9 ± 3.1 Ma.Volcanic activity at the NE edge of the Saghro inlier is related to West African Cadomian orogenic (WACadomian) activity between 620 and 560 Ma. During this period the Saghro and Bou Salda groups were deposited, followed by the lower Ouarzazate Supergroup. Later extension along the Gondwanan margin took place close to Ediacaran – Cambrian boundary, contemporaneous with upper Ouarzazate Supergroup deposition.     

The continental record of Ediacaran volcano‐sedimentary successions in southern Brazil and their global implications

The Ediacaran is one of the most important periods on Earth evolution, including the first appearance of soft‐bodied macrofossils, major climatic changes and a supposed rise in free oxygen. In southernmost Brazil, this period is represented by Camaquã Supergroup, including the Bom Jardim Group and the Acampamento Velho Formation, both of which record continental palaeoenvironmental changes in a more than 5000 m thick stratigraphic succession. Age constraints are given by seven Ar‐Ar and U‐Pb determinations on volcanic rocks, which bracket these units between c. 605 and 574 Ma, revealing the best dated and most continuous documented Ediacaran continental succession to date. Depositional systems evolution supports a Phanerozoic‐type glacial context during the last Neoproterozoic glacial event and presents the Picada das Graças Formation (580 ± 3.6 Ma) as the first dated non‐glacial unit coeval to the Gaskiers Formation.     

The Dabieshan Coesite-bearing Eclogite Terrain-A Late Archaean Ultra - high- Pressure Metamorphic Belt

A U -Pb zircon age of 2774±24 Ma for eclogite from the Bixiling rock body of Anhui Province, central China, indicates that the Dabieshan coesite-bearing eclogite was probably formed in the Late Archaean. A phengite Ar-Ar isochron age of 662±13 Ma for the eclogite confines also an upper limit age of its subsequent retrograde metamorphism in the Precambrian. The results of isotopic dating for such type of eclogite coincide with the geological features of its restricted occurrence within the Archaean metamaorphic terrain composed of the Dabie Group. It is believed that the Dabieshan coesite-bearing eclogite terrain might be a Late Archaean ultra-high-pressure metamorphic belt. The Dabie Mountains area was the eastward extension of the southern Qinling structural belt during the Triassic. Both the Dabie Group and the coesite-bearing eclogite hosted therein underwent a late-stage dynamic metamorphic event. The present authors have obtained a muscovite Ar-Ar isochron age of 192.6±2.8 Ma from plagioclase gn     

Sequence stratigraphy and controls on sedimentation of the Upper Cretaceous in the Afikpo Sub-basin,southeastern Nigeria

Outcrop-based sequence stratigraphic analysis and palynological biofacies were used to define depositional sequences and their bounding surfaces, and build a sequence stratigraphic model for the Upper Cretaceous succession of the Afikpo Sub-basin. Four unconformity-bounded third-order depositional sequences were identified. Sequence 1 comprises the Nkporo Formation and is subdivided into lowstand system tract (LST) representing an incised valley fill and transgressive systems tract (TST) consisting of estuarine and marine shales and mudstones. The base of the sequence is an angular unconformity correlated to the 77.5 Ma sequence boundary (SB) and the maximum flooding surface (MFS) is dated at 76 Ma. Sequence 2 is diachronous and straddles the lithostratigraphic boundary of the Nkporo and Mamu formations. The upper SB is dated at 71 Ma while associated MFS is dated at 73.5 Ma. Sequence 3 consists of the upper Mamu Formation and the Ajali Formation. The upper SB of sequence 3 is at 68 Ma while the MFS is dated at 69.8 Ma. Sequence 4 is the topmost depositional sequence belonging to the Nsukka Formation. The upper SB is placed at 66.5 Ma. The MFS within this sequence is dated at 67.8 Ma. The sequences encompass from tidally influenced bay head delta and central estuarine environments to coastal and shallow marine shelf environments. Stratigraphic architecture and facies types show that sequence development was controlled to a great extent by eustatic sea level variations though differential subsidence rates encouraged differential rates of sediment supply and rates of sea level change along different segments of the shoreline.