Late Paleozoic retrograded eclogites from within the northern margin of the North China Craton: Evidence for subduction of the Paleo-Asian ocean

Retrograded eclogites from the central part of the northern margin of the North China Craton, Hebei Province, China occur as separate tectonic lenses or boundins within garnet–biotite–plagioclase gneisses of the Paleoproterozoic Hongqiyingzi Complex characterized by amphibolite facies paragneisses. The petrographic features and mineralogical compositions represent three main metamorphic stages: (1) the peak eclogite facies stage (P > 1.40–1.50 GPa, T = 680–730 °C), (2) the granulite facies stage and (3) the amphibolite facies stage (P = 0.67–0.81 GPa, T = 530–610 °C) formed during decompression. The major and trace element and Sm–Nd isotopic data suggest that most of the retrograded eclogite samples had protoliths of tholeiitic oceanic crust with geochemical characteristics of mid-ocean ridge basalt (MORB) or island arc tholeiite (IAT) environment, and were contaminated by crustal components during subsequent subduction. Zircon SHRIMP isotopic dating of two different textural varieties of retrograded eclogite defines a weighted mean age of 325 Ma, which is interpreted as the peak metamorphic age of the eclogites and reflects the occurrence of eclogite facies metamorphism related to subduction of Paleo-Asian Oceanic crust beneath the North China Craton during the Late Paleozoic. Finally, we show that the retrograded eclogite from Hebei Province is not related to the Baimashi retrograded eclogite at the northern foot of the Heng Mountains, approximately, 300 km to the southwest.     

Tectonic control of Late Pliocene molasse sedimentation in a portion of the Jhelum Re-Entrant,Pakistan

A Late Pliocene interval of the Neogene Siwalik (Himalayan) molasse exposed in the Jhelum Re-entrant, Pakistan, is examined for lateral variations in the environment and rate of fluvial sedimentation. A correlative pair of volcanic ash horizons within the Upper Siwalik section defines an isochronous interval of fluvial deposition. Stratigraphic sections measured through the interval at various localities in the Jhelum Re-entrant are analyzed in terms of fluvial stratigraphy and interval thickness, or net sedimentation rate. The stratigraphy indicates a fluvial system of laterally migrating streams leaving behind fining upward sedimentary cycles of lateral and vertical accretion deposits. Fully developed sedimentary cycles display point bar sands (lateral accretion deposits) of the stream channel environment topped by mudstones (vertical accretion deposits) of the overbank (floodplain) environment. The detailed sequence of vertical accretion deposits in these cycles indicates that the floodplain environment consisted of a stream-proximal, sedimentation-dominant zone and a stream-distal, pedogenesis-dominant zone.Broad lateral variations in the rate of molasse sedimentation appear to be related to the large-scale syntaxial tectonics of the northwestern Himalaya. Sedimentation rate increases toward the apex of the re-entrant, indicating that the syntaxial structure controlled the focus of molasse sedimentation during the examined interval of Late Pliocene time. In addition, sedimentation may have been influenced by mild flexures within the molasse basin.

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Zusammenfassung Im Gebiet von Jhelum (nördliches Pakistan) reicht das Vortief der neogenen Siwalik-Molasse als Sporn weit in den Faltenbogen des nordwestlichen Himalaya. Die Verfasser untersuchten hier einen Abschnitt des Spätpliozän mit besonderem Hinblick auf laterale Wechsel der fluviatilen Ablagerungen und Sedimentationsraten. Die Obere Siwalik-Molasse enthält hier zwei Leithorizonte mit Tuff, wodurch die Altersgleichheit der fluviatilen Zwischenlagerungen sichergestellt wird. Die Schichtfolge dieser Zwischenlagerungen ist an mehreren Lokalitäten in bezug auf Sedimentationstypen und-raten analysiert worden. Die vorgefundenen Ablagerungszyklen deuten auf ein System von seitlich wandernden Flußläufen. Vollständige Zyklen zeigen Flußbettsedimente (meist Ufersandbänke) gefolgt von feinkörnigen Überflutungssedimenten. Die Feingliederung der Überflutungsfazies zeigt stromnahe Ablagerungszonen einerseits und pedogenetische Randzonen andererseits.Über das gesamte Untersuchungsgebiet ergibt sich eine klare Abhängigkeit zwischen Raten der Molasseaufschüttung und der Faltenbogentektonik des nordwestlichen Himalaya. Die Ablagerungsraten nehmen in Richtung des Bogenscheitels deutlich zu. Daraus wird geschlossen, daß während des untersuchten Zeitabschnittes die regionale Bogentektonik den Schwerpunkt der Molassesedimentation maßgeblich beeinflußt hat. Flache Flexuren innerhalb des Molassebeckens sind ebenfalls in diesen Zusammenhang zu stellen.

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Résumé Une avant-fosse de la molasse du néogène Siwalik (Himalayen), en éperon dans l'arc plissé du nord-ouest de l'Himalaya, dans la région du Jhelum, Pakistan, a été étudiée quant à ses variations latérales environnantes et son taux de sédimentation fluviale. Dans la section du Siwalik supérieur, deux horizons-repères de tuffs volcaniques délimitent une période isochronique du dépôt fluvial. Les séries stratigraphiques de cet intercalaire ont été analysées en divers points en fonction de la stratigraphie fluviale et de son épaisseur ou en fonction du taux de sédimentation. Les cycles de sédimentation indiquent un réseau fluvial à migration latérale avec dépôts latéraux et verticaux, de plus en plus fins vers le haut. Les cours semblent être en transition avec le type traditionnel de haute et basse sinuosité. On démontre que l'ensemble des dépôts verticaux sont différentiés selon une zone proximale où la sédimentation domine et une zone distale où la pédogenèse domine.De larges variations latérales dans le taux de sédimentation semblent dues à la tectonique globale de l'éperon associée à la syntexis du nord-ouest himalayen. Une tendance vers une augmentation du taux de sédimentation / subsidence en direction de l'axe de l'éperon délimite un lieu de sédimentation molassique pendant l'intervalle étudié. De plus, l'auteur suggère que la sédimentation du bassin de l'éperon ait été influencée par de légères flexures à la périphérie de la structure régionale.

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Evidence for ocean acidification in the Great Barrier Reef of Australia

Geochemical records preserved in the long-lived carbonate skeleton of corals provide one of the few means to reconstruct changes in seawater pH since the commencement of the industrial era. This information is important in not only determining the response of the surface oceans to ocean acidification from enhanced uptake of CO₂, but also to better understand the effects of ocean acidification on carbonate secreting organisms such as corals, whose ability to calcify is highly pH dependent. Here we report an ∼200 year δ¹¹B isotopic record, extracted from a long-lived Porites coral from the central Great Barrier Reef of Australia. This record covering the period from 1800 to 2004 was sampled at yearly increments from 1940 to the present and 5-year increments prior to 1940. The δ¹¹B isotopic compositions reflect variations in seawater pH, and the δ¹³C changes in the carbon composition of surface water due to fossil fuel burning over this period. In addition complementary Ba/Ca, δ¹⁸O and Mg/Ca data was obtained providing proxies for terrestrial runoff, salinity and temperature changes over the past 200 years in this region. Positive thermal ionization mass spectrometry (PTIMS) method was utilized in order to enable the highest precision and most accurate measurements of δ¹¹B values. The internal precision and reproducibility for δ¹¹B of our measurements are better than ±0.2‰ (2σ), which translates to a precision of better than ±0.02 pH units. Our results indicate that the long-term pre-industrial variation of seawater pH in this region is partially related to the decadal-interdecadal variability of atmospheric and oceanic anomalies in the Pacific. In the periods around 1940 and 1998 there are also rapid oscillations in δ¹¹B compositions equivalent changes in pH of almost 0.5 U. The 1998 oscillation is co-incident with a major coral bleaching event indicating the sensitivity of skeletal δ¹¹B compositions to loss of zooxanthellate symbionts. Importantly, from the 1940s to the present-day, there is a general overall trend of ocean acidification with pH decreasing by about 0.2-0.3 U, the range being dependent on the value assumed for the fractionation factor α_(B3-B4) of the boric acid and borate species in seawater. Correlations of δ¹¹B with δ¹³C during this interval indicate that the increasing trend towards ocean acidification over the past 60 years in this region is the result of enhanced dissolution of CO₂ in surface waters from the rapidly increasing levels of atmospheric CO₂, mainly from fossil fuel burning. This suggests that the increased levels of anthropogenic CO₂ in atmosphere has already caused a significant trend towards acidification in the oceans during the past decades. Observations of surprisingly large decreases in pH across important carbonate producing regions, such as the Great Barrier Reef of Australia, raise serious concerns about the impact of Greenhouse gas emissions on coral calcification.     

泥河湾盆地上新世末期植被与气候变化的孢粉学记录

泥河湾盆地沉积巨厚的上新世末至晚更新世的河湖相、湖沼相地层,是进行上新世至更新世环境变化研究的理想区域。本研究选择泥河湾盆地郝家台NHA钻孔的上新统稻地组顶部厚19.2 m,深度为15600~17520 cm,通过对其岩性段97个孢粉和粒度分析,探讨了该地区上新世末期植被及气候变化特征。根据已有古地磁及地层对比,采样地层的年代为2.92~2.56 Ma。分析结果显示,该时段植被及气候变化可划分为4个阶段：阶段1（2.92~2.82 Ma）,松属花粉含量多高于30%,云杉属多低于20%;阔叶乔木约13%,为研究段最高;草本植物以蒿属、藜科为主,含量均在10%~20%之间,指示采样点周边应以阔叶林和松林为主,山地云杉林存在,气候较温暖湿润。阶段2（2.82~2.71 Ma）,云杉属花粉含量明显上升,最高达90%,蒿属和藜科花粉比例也增加明显,最高可达80%,为研究段最高值;松属花粉含量明显降低,多低于10%;阔叶树木的花粉减少至5%以下,表明在此期间气候总体趋于变冷变干,且气候变化剧烈,林线下降,周边植被以云杉林或草原为主。阶段3（2.71~2.66 Ma）,松属花粉含量明显上升,达40%以上,云杉属含量总体下降,多低于30%;阔叶乔木少见;蒿属、藜科含量低于10%;蕨类孢子约11%,达到整段最高值,指示气候转暖湿,周边植被以松林为主,云杉林退至山地,但阔叶树花粉含量较低,显示温暖程度低于阶段1。阶段4（2.66~2.56 Ma）,云杉属花粉占绝对优势,多高于80%,松属多低于10%;阔叶乔木近乎消失,暗针叶林扩张,气候进一步变冷,进入更新世的冰期环境。由此可见,上新世末期（2.92 Ma以来）至更新世的气候变化存在明显的2个暖期和2个冷期。其中研究段代表寒冷气候的云杉属花粉高含量阶段与深海氧同位素代表寒冷的MIS 102、MIS 104、G2、G6和G10阶段有较好的对应关系。

     

The Late Pliocene mafic lavas from the Camusú Aike volcanic field (∼50°S,Argentina): Evidence for geochemical variability in slab window magmatism

The Camusú Aike volcanic field (CAVF), part of the discontinuous N–S-trending belt of Cenozoic mafic lava formations that occur in a backarc position along extra-Andean Patagonia, is located in southern Patagonia (∼50°S, Santa Cruz province), approximately 70 km east of the extensive Meseta de las Vizcachas and just south of the upper Río Santa Cruz valley. The CAVF volcanics cover a surface of ∼200 km² and occur mainly as lava flows and scoria cones. They are subdivided into two groups: Group I volcanics are high-TiO₂, low-Mg# olivine-hypersthene-normative basalts and trachybasalts that erupted at about 2.9 Ma; Group II lavas are much less abundant, more primitive basaltic andesites that erupted at about 2.5 Ma. Both groups show a within-plate geochemical signature, though it is more marked in Group I lavas.The main geochemical characteristics, age, and location of CAVF volcanics are consistent with the slab window opening model proposed by different authors for the genesis of the Miocene-Recent mafic magmatism of Patagonia south of 46.5°S. The whole-rock geochemical and Sr–Nd isotope features of Group I lavas (⁸⁷Sr/⁸⁶Sr=0.7035–0.7037; ¹⁴³Nd/¹⁴⁴Nd=0.51288–0.51291) indicate a genetic link between these lavas and the primitive basalts in southernmost Patagonia (Pali Aike volcanic field and Estancia Glencross area), which have been interpreted as melting products of an isotopically depleted asthenosphere. The relatively evolved compositions of the erupted Group I magmas are modeled by a polybaric crystal fractionation process without significant involvement of crustal contamination. The more primitive Group II lavas are strongly depleted in incompatible elements, have slightly higher (LREE+Ba+Th+U)/HFSE ratios, and have more enriched Sr–Nd isotope compositions (⁸⁷Sr/⁸⁶Sr≈0.7039; ¹⁴³Nd/¹⁴⁴Nd≈0.51277) that are more akin to the Patagonian basalts farther to the north. The most likely explanation for the geochemical features of Group II lavas is the occurrence in their mantle source of a small proportion of a subduction-related, enriched component that likely resides in the former mantle wedge or the basal continental lithospheric mantle.     

Dissolved iron anomaly in the deep tropical-subtropical Pacific: Evidence for long-range transport of hydrothermal iron

Dissolved iron profiles along a north-south transect along 158°W in the tropical Pacific show evidence of two deepwater anomalies. The first extends from Station ALOHA (22.78°N) to the equator at ∼1000-1500 m and lies below the maximum apparent oxygen utilization and nutrient (N, P) concentrations. The feature is not supported by vertical export processes, but instead corresponds with the lateral dilution field of δ³He derived from the Loihi seamount, Hawaii, though a sediment source associated with the Hawaiian Island Chain cannot be entirely ruled out. The second, deeper (2000-3000 m) anomaly occurs in tropical South Pacific waters (7°S) and also does not correlate with the depths of maximum nutrient concentrations or apparent oxygen utilization, but it does coincide closely with δ³He emanating from the East Pacific Rise, more than 5000 km to the east. We hypothesize that these anomalies represent the long-range (>2000 km) transport of hydrothermal iron residuals, stabilized against scavenging by complexation with excess organic ligands in the plume source regions. Such trace leakage of hydrothermal iron to distal plume regions would have been difficult to identify in most hydrothermal vent mapping studies because low analytical detection limits were not needed for the proximal plume regions. These findings suggest that hydrothermal activity may represent a major source of dissolved iron throughout the South Pacific deep basin today, as well as other regions having high mid-ocean spreading rates in the geologic past. In particular, we hypothesize that high spreading rates along the South Atlantic and Southern Ocean mid-oceanic ridges, combined with the upwelling ventilation of these distal hydrothermal plumes, may have increased ocean productivity and carbon export in the Southern Ocean. Assessing the magnitude and persistence of dissolved hydrothermal iron in basin scale deep waters will be important for understanding the marine biogeochemistry of iron and, potentially, on ocean productivity and climate change during the geologic past.     

Sedimentary Evolution and Transgressions of the Western Subei Basin in Eastern China since the Late Pliocene

Sedimentary sequences in the Subei Basin are complex and have been affected by interactions between the ocean and rivers since the Late Pliocene, including the Yellow River, Huaihe River, and the Yangtze River. This sedimentary evolution, in particular the timing of Pleistocene transgressions, has long been a matter of controversy owing to the lack of precise chronological evidence. The aim of this study is to explore the evolution of the sedimentary environment throughout the past 3.00 Ma in this region on the basis of a comprehensive analysis of particle size and foraminifera and ostracods collected in the TZK9 core from the Subei Basin combined with geochronological studies of magnetostratigraphy, AMS¹⁴C and optically stimulated luminescence(OSL). The results show that fluvial facies in the sedimentary environment from 3.00 to 1.01 Ma. There were fluvial facies and reflects six sea-level high stands from 1.01 to 0.25 Ma. The study area was affected by four large-scale transgressions since 0.25 Ma. The four marine sedimentary layers known as DU7(buried at 48–52 m), DU5(buried at 35–41 m), DU3(buried at 16–23 m), and DU1(buried at 2–4 m) are recorded in the MIS7(210–250 ka), MIS5, MIS3, and Holocene, respectively. The magnitude of the DU5 transgression was identical to that of the DU3 transgression, both were larger than the DU7 transgression, and the DU1 transgression was the weakest. The variation of transgression strength reflects the influence of global changes in sea level, tectonic subsidence, shell ridges, and sand dams. In the TZK9 core, we found evidence of seven sea-level high stands from the Early–Middle Pleistocene, and the first one caused by regional rapid subsidence and could be traced back to 0.83-0.84 Ma. The sea-level high stands and the age of the first one recorded above was different from other cores in eastern China, this was caused by the lack of absolute age control and the differences in paleotopography during this period. This study reconstructs sedimentary evolution, determines the transgression and its age, establishes the chronology since the Late Pliocene, and provides a scientific framework for further paleoenvironmental and tectonic studies. The results of this study highlight the important role that local tectonics and global sea level play in the sedimentary evolution and transgressions that have occurred in the western Subei Basin.     

Ureilitic breccias: clues to the petrologic structure and impact disruption of the ureilite parent asteroid

The majority of the 143 ureilite meteorites are monomict (unbrecciated) ultramafic rocks, which represent the mantle (olivine+low-Ca pyroxene residues and less abundant cumulates) of a partially melted (25–30%), carbon-rich asteroid 125 km in radius. Accumulated petrologic and geochemical studies of these meteorites have led to a picture of a ureilite parent body (UPB) that was stratified in mg#, pyroxene abundance and pyroxene type, due to the pressure dependence of carbon redox control, and which preserved a pre-magmatic heterogeneity in Δ¹⁷O. The absence, however, of ureilitic crustal rocks (i.e. basalts) in the meteorite record, leads to significant gaps in our knowledge of the geologic history of the UPB.

Ureilitic breccias provide considerable information that cannot be obtained from the monomict samples, and help to fill in those gaps. Fourteen ureilites are polymict breccias (at least three of which contain solar wind gases) that formed in a regolith. They contain a variety of clast types representing indigenous ureilitic lithologies not known among the monomict samples, as well as several types of non-indigenous impactor materials. In addition, one ureilite (FRO 93008) is a dimict breccia, consisting of two ultramafic lithologies that could not have formed in close proximity on the UPB.

Several feldspathic lithologies representing melts complementary to the monomict ureilite residues or cumulates have been recognized in polymict ureilites. From these lithologies we infer that melt extraction on the UPB was a rapid, fractional process in which trace element and oxygen isotopic equilibrium was not achieved. The majority of melts that reached the surface erupted explosively (due to high contents of CO/CO₂) and were lost into space. Thus, it is likely that the UPB never had an extensive basaltic crust. Melts generated at the shallowest depths and late fractionates, in which carbon had largely been consumed by reduction, were the most likely to have been preserved. Our sample of the UPB is limited to depths equivalent to 100 bars pressure or less, but minor augite-bearing feldspathic lithologies and related cumulates may represent melts derived from deeper.

In addition, we infer that the UPB was catastrophically disrupted, while still hot, by an impacting projectile. Meter-sized ejecta from this impact reaccreted into one or more daughter bodies, on which the brecciated ureilites formed. Ureilite meteorites are derived from these offspring, rather than from the UPB. The remnant of the original UPB may consist largely of olivine plus augite, and thus not resemble the majority of ureilites.     

Late Triassic melting of a thickened crust in southeastern China: Evidence for flat-slab subduction of the Paleo-Pacific plate

The Dashuang complex in Zhejiang Province of southeast China is composed of two distinct lithologies: syenite in the west and quartz monzonite in the east. They record similar zircon U–Pb ages of 224 ± 3 Ma (syenite), and 226 ± 2 Ma and 227 ± 1 Ma (quartz monzonite), respectively, but are notably different in petrography, magnetic susceptibility, whole-rock chemistry, zircon Hf isotope and zircon trace element characteristics. The west Dashuang syenitic pluton (the west body) has high modal alkali feldspar, high zircon saturation temperatures, high whole-rock and zircon MREE/HREE ratios, low Fe–Mg–Ti contents, and is depleted in Ba, Sr and Eu. It also has low magnetic susceptibilities, belongs to the ilmenite-series, and is a peraluminous and ferroan granitoid. The east Dashuang quartz monzonitic pluton (the east body) has abundant K-feldspar megacrysts, with hornblende, titanite and biotite being the major ferromagnesian minerals. In contrast to the west body, the east body has lower zircon saturation temperatures, lower whole-rock and zircon MREE/HREE ratios, higher Fe–Mg–Ti contents, and shows no depletion in Ba, Sr or Eu. The east body has higher magnetite contents, high magnetic susceptibilities and belongs to the magnetite-series. It is a metaluminous and magnesian granitoid of arc-affinity. Zircon Hf isotopic data reveal that both bodies were derived from partial melting of Paleoproterozoic igneous protoliths in the lower crust, but the east body possibly incorporated subducted terrigenous sediments. Both bodies have higher melting temperatures and pressures than adjacent Cretaceous granitoids, reflecting their origin in a thickened, hotter lower crust. The most feasible model to explain their differences is variations in water content during crustal melting, resulting in different melting and crystallization behaviors. Such melting in a Triassic thickened crust with variable water involvement, followed by Cretaceous magmatism in an extensional setting, is consistent with the flat-slab subduction model proposed for South China. The model involves crustal thickening and partial melting, with mantle and lower crustal metasomatism during flat-slab propagation in the Triassic–Early Jurassic, and crustal thinning and extension from the mid-Jurassic to the Cretaceous.     

南黄海黑碳记录的中国东部晚上新世以来C3/C4植被演化

陆地植被是全球生态系统的重要组成部分,其C₃/C₄组成类型及演化进程对于理解过去生态系统对全球变化的响应及适应机制具有重要科学意义。晚上新世以来东亚C₄植被扩张的时间、过程和机制等仍存很大争议,尤其缺少长江流域C₃/C₄植被相对丰度演化的地质记录及不同纬度植被演化的时空综合对比。本研究利用大陆架科学钻探计划在南黄海钻取的CSDP-1孔约300.10m长的岩芯沉积物的黑碳含量、通量及碳同位素组成,重建了晚上新世以来中国东部地区C₃/C₄植被演化历史。研究发现,在晚上新世（约3.0~2.6Ma）,黑碳碳同位素组成发生明显正偏,指示沉积物源区长江流域的C₄植被相对丰度的增加,可能受到了中低纬度地区干旱化增强及大气CO₂浓度下降的共同影响;而中高纬度如黄土高原地区的C₄植被则相对减少,推测更低的温度可能成为了C₄植被生长的限制因素。中更新世（约1.0Ma）以来,黑碳碳同位素比值整体负偏,表明源区C₄植被相对丰度减少。尽管沉积物源区在0.8Ma左右转变为纬度更高的黄河流域,但东亚不同纬度地区记录均显示了碳同位素负偏,表明这是一区域现象,分析认为全球气候变冷是影响中更新世以来C₄植被演化的重要因素。本研究揭示出,除了全球性的因素（大气CO₂浓度）外,区域气候（温度和干旱度）在东亚地区C₃/C₄植被长期演化中具有重要影响。

     

Evidence for coelacanths in the Late Triassic (Rhaetian) of England

The right gular plate of an indeterminate coelacanth from the Westbury Formation (Upper Triassic, Rhaetian) of Blue Anchor Point, Somerset, southwest England is reported. This occurrence represents the first convincing evidence of coelacanths from the Triassic of the United Kingdom. The new specimen suggests a fish of approximately 0.61 m length.     

Palaeomagnetism of the Tan y Grisiau granite,North Wales: Evidence for a subvolcanic origin in Late Ordovician times

Progressive thermal demagnetization of samples from the Tan y Grisiau granite defines a coherent easterly positive characteristic remanence (D/I = 124.9/60.3°;, 42 samples, R = 40–51, a₉₅ = 4.8°;) residing in magnetite. An ancient reversal of magnetization is recovered in the highest blocking temperature spectrum of a few samples and suggests that a cooling-related dipolar axis is recorded by this pluton. Only facies of the granite which have been reddened, probably by submagmatic streaming, have recorded a stable remanence. Adjustment for tilt yields a very steep remanence (D/I = 193/88°;) incompatible with any known Early Palaeozoic and younger field direction from Britain. The in situ remanence has a similar declination to the primary magnetization in Late Ordovician dolerites from the Welsh Borderlands and yields a comparable palaeolatitude (41.5°;S). It is concluded that the Tan y Grisiau pluton was magnetized in Late Ordovician times after deformation. Folding in this region is therefore interpreted to be substantially of Taconic (Late Ordovician) origin and not Acadian in age. As both in situ and tilt-adjusted remanence directions are incompatible with Silurian and younger palaeofield directions from Britain, the pluton is interpreted as a subvolcanic component of the North Wales igneous province. Large anticlockwise rotation of Avalonia is identified between Late Ordovician and Late Silurian times.     

The Eltanin asteroid impact: possible South Pacific palaeomegatsunami footprint and potential implications for the Pliocene–Pleistocene transition

Large asteroid impacts are rare, and those into the deep ocean are rarer still. The Eltanin asteroid impact around 2.51 ± 0.07 Ma occurred at a time of great climatic and geological change associated with the Pliocene–Pleistocene boundary. Numerical models of the event indicate that a megatsunami was generated, although there is debate concerning its magnitude and the region‐wide extent of its influence. We summarise the existing evidence for possible Eltanin megatsunami deposits in Antarctica, Chile and New Zealand, while also examining other potential sites from several locations, mainly around the South Pacific region. In reviewing these data we note that these events were unfolding at the same time as those associated with the Pliocene–Pleistocene boundary and, as such, most of the geological evidence from that time has a climatic interpretation. The potential climatic and geological ramifications of the Eltanin asteroid impact, however, have failed to be considered by most researchers studying this time period. Although we are not advocating that all geological activity at that time is connected with the Eltanin asteroid impact, it raises interesting questions about the role potentially played by such catastrophic events in contributing to or even triggering epochal transitions. Copyright © 2012 John Wiley & Sons, Ltd.     

华南老堡组硅质岩中草莓状黄铁矿——埃迪卡拉纪末期深海缺氧的证据

为恢复埃迪卡拉纪末期深海的氧化还原状态,我们对华南桂北泗里口剖面老堡组（大约550~540Ma）硅质岩中草莓状黄铁矿的粒径和分布进行了测量、统计和研究。老堡组硅质岩样品中普遍存在着草莓状黄铁矿,呈星散状分布,未见自形晶的黄铁矿颗粒和明显后期充填的草莓状黄铁矿集合体。硅质岩中草莓状黄铁矿颗粒的统计表明,它们具有很窄的变化范围,最大粒径小于18μm,大多数样品中草莓状黄铁矿的平均粒径和中间粒径均小于5μm。根据这些硅质岩中原生黄铁矿的粒径和分布,我们判定埃迪卡拉纪末期的深部海水是缺氧的。泗里口剖面老堡组硅质岩中草莓状黄铁矿的平均粒径、中间粒径和最大直径沿剖面向上逐渐增加,这意味着埃迪卡拉纪末期华南的深部海水有逐步被氧化的趋势。     

Geochronology and geochemistry of Late Carboniferous dykes in the Aqishan-Yamansu belt,eastern Tianshan:Evidence for a post-collisional slab breakoff

With aim of providing constraints on the Late Paleozoic tectonic evolution of the southern Central Asian Orogenic Belt(CAOB),an integrated study was conducted on the geochronological and geochemical data for dioritic,granitic and diabase dykes from the Aqishan-Yamansu belt in the eastern Tianshan,NW China.Zircon U-Pb dating indicates that the dioritic and granitic dykes were both emplaced in the Late Carboniferous(~311 Ma and^315 Ma).The dioritic dykes show adakitic characteristics and have high Na2 O and positiveεHf(t)values(+12 to+17),which suggest an origin from partial melts of a subducted oceanic slab.The granitic dykes have high SiO2 and K2 O contents and are characterized by en riched light rare earth elements(LREE)and slightly flat heavy rare earth elements(HREE),with negative Eu and Nb-Ta-Ti anomalies.These dykes are alkali-calcic and show geochemical features of highly fractionated Itype granites.Their positiveεHf(t)values(+16 to+17)suggest that they were derived from a juvenile accreted oceanic crustal sou rce.The coeval diabase dykes have low SiO2 and K2 O contents but high TiO2,MgO and Mg#(54-59).They are enriched in LREE and show characteristics of enriched mid-ocean ridge basalts(E-MORB).The relatively high Ba/Th,slightly low Th/Ta ratios,and negative Nb-Ta anomalies imply a mantle source metasomatised by slab-derived fluids.Thus,these basic dykes were generated likely by partial melting of the upwelling asthenosphere mantle with a slight influence of slab-derived fluids.Therefore,we suggest that the formation of these Late Carboniferous dykes were triggered by a post-collisional slab breakoff and the Aqishan-Yamansu belt was a continental arc formed by southdipping subduction of the Kangguer oceanic plate.     

河北平原廊固凹陷ACX03孔晚上新世以来地层与沉积*

基于河北平原廊固凹陷东北部ACX03孔(深300.0 m)浅部¹⁴C年龄和磁性地层,建立钻孔第四纪地层格架,依据ACX03孔岩心沉积物、测井曲线和微体古生物特征划分沉积层段并分析沉积环境。研究结果表明: ACX03孔B/M(布容/松山)、M/G(松山/高斯)极性界线分别位于83.8 m、220.2 m,孔底年龄约3.2 Ma,划分13个沉积层段。上新统(220.2~300.0 m)为河道、河漫滩夹湖泊沉积,下更新统(85.2~220.2 m)为河道、湖泊、河漫滩沉积,中更新统(61.5~85.2 m)为分支河道夹河漫滩及河间洼地—湖沼沉积,上更新统(23.2~61.5 m)为河漫滩、分支河道沉积,全新统(0~23.2 m)为泛滥平原、河曲—牛轭湖沉积。对比ACX03孔与邻近钻孔第四纪地层认为,河北平原廊坊—固安一带早更新世差异性沉降最强,之后一直到全新世越来越弱。     

洁居纳卓组:西藏措勤地区的上新统岩石地层单位

分布于措勤县城一带的一套地层在1:100万日喀则幅区调中被划属上新统乌郁群,由下部火山岩和上部沉积岩组成。在1:25万措勤县幅区调中,根据获得的同位素年龄值已将下部火山岩地层划归古新统的典中组,而上部的沉积地层中含圆笠虫等有孔虫化石应属下白垩统。在这一地区还新发现一套陆相碎屑地层,根据其岩性组合、ESR年龄值及区域地层对比,本文将其新命名为上新统洁居纳卓组。     

Thermochronological evidence for a late Pliocene climate-induced erosion rate increase in the Alps

(U–Th)/He and fission-track analyses of apatite along deep-seated tunnels crossing high-relief mountain ranges offer the opportunity to investigate climate-tectonic forcing on topographic evolution. In this study, the thermochronologic analysis along the Simplon tunnel (western-central Alps; Italy and Switzerland) constrains in detail the mechanisms controlling the topographic evolution of the Simplon Massif. Cooling rates vary from about 10°C/Ma at about 10 Ma to about 35°C/Ma in the last 3 Ma. Such increase in cooling rates corresponds to the inception of glacial cycles in the northern hemisphere. Age patterns show correlation with faults distribution until 2 Ma, suggesting that tectonics-controlled rocks exhumed up to that age. After 2 Ma thermo-chronometric data show that the Simplon area has experienced primarily erosional exhumation. All age patterns provided are not affected by topographic effects, thus indicating that present-day topography has been carved in the last 2 Ma, most likely controlled by glacial erosion.