青藏高原羌塘中部果干加年山上三叠统望湖岭组的建立及意义   总被引：7，自引：3，他引：7  

李才  翟庆国  董永胜  于介江  黄小鹏 《地质通报》2007,26(8):1003-1008

在果干加年山的主脊及其以北识别出一套稳定的沉积岩系,角度不整合于蛇绿混杂岩之上,以细砂岩、硅质粉砂岩为主,其中夹有大小不等的古生代外来岩块,底砾岩成分因地而异并具有快速堆积的特点,底部夹有流纹岩。厚度大于1633m。流纹岩夹层获得的锆石SHRIMPU-Pb谐和年龄为214Ma±4Ma,时代为晚三叠世诺利期。在剖面测制和区域对比的基础上建立了上三叠统望湖岭组,代表龙木错-双湖缝合带闭合以后接受的最早的沉积盖层。望湖岭组之下的蛇绿混杂岩获得阳起石Ar-Ar年龄219.7Ma±6.5Ma。同位素定年确定构造转化事件发生在214～220Ma之间,为龙木错-双湖缝合带的闭合时间提供了确切的时间约束,望湖岭组是这一事件的沉积记录,是龙木错-双湖缝合带蛇绿混杂岩之上首次发现的沉积盖层。  相似文献   

Ultrahigh-pressure eclogite transformed from mafic granulite in the Dabie orogen, east-central China   总被引：1，自引：0，他引：1  

Y.-C. LIU  S.-G. LI  X.-F. GU  S.-T. XU  G.-B. CHEN 《Journal of Metamorphic Geology》2007,25(9):975-989

Although ultrahigh‐pressure (UHP) metamorphic rocks are present in many collisional orogenic belts, almost all exposed UHP metamorphic rocks are subducted upper or felsic lower continental crust with minor mafic boudins. Eclogites formed by subduction of mafic lower continental crust have not been identified yet. Here an eclogite occurrence that formed during subduction of the mafic lower continental crust in the Dabie orogen, east‐central China is reported. At least four generations of metamorphic mineral assemblages can be discerned: (i) hypersthene + plagioclase ± garnet; (ii) omphacite + garnet + rutile + quartz; (iii) symplectite stage of garnet + diopside + hypersthene + ilmenite + plagioclase; (iv) amphibole + plagioclase + magnetite, which correspond to four metamorphic stages: (a) an early granulite facies, (b) eclogite facies, (c) retrograde metamorphism of high‐pressure granulite facies and (d) retrograde metamorphism of amphibolite facies. Mineral inclusion assemblages and cathodoluminescence images show that zircon is characterized by distinctive domains of core and a thin overgrowth rim. The zircon core domains are classified into two types: the first is igneous with clear oscillatory zonation ± apatite and quartz inclusions; and the second is metamorphic containing a granulite facies mineral assemblage of garnet, hypersthene and plagioclase (andesine). The zircon rims contain garnet, omphacite and rutile inclusions, indicating a metamorphic overgrowth at eclogite facies. The almost identical ages of the two types of core domains (magmatic = 791 ± 9 Ma and granulite facies metamorphic zircon = 794 ± 10 Ma), and the Triassic age (212 ± 10 Ma) of eclogitic facies metamorphic overgrowth zircon rim are interpreted as indicating that the protolith of the eclogite is mafic granulite that originated from underplating of mantle‐derived magma onto the base of continental crust during the Neoproterozoic (c. 800 Ma) and then subducted during the Triassic, experiencing UHP eclogite facies metamorphism at mantle depths. The new finding has two‐fold significance: (i) voluminous mafic lower continental crust can increase the average density of subducted continental lithosphere, thus promoting its deep subduction; (ii) because of the current absence of mafic lower continental crust in the Dabie orogen, delamination or recycling of subducted mafic lower continental crust can be inferred as the geochemical cause for the mantle heterogeneity and the unusually evolved crustal composition.  相似文献   

Age accuracy and resolution of Quaternary corals used as proxies for sea level     

《Earth and Planetary Science Letters》2007,253(1-2):37-49

The accuracy of global eustatic sea level curves measured from raised Quaternary reefs, using radiometric ages of corals at known heights, may be limited by time-averaging, which affects the variation in coral age at a given height. Time-averaging was assessed in uplifted Holocene reef sequences from the Huon Peninsula, Papua New Guinea, using radiocarbon dating of coral skeletons in both horizontal transects and vertical sequences. Calibrated 2σ age ranges varied from 800 to 1060 years along horizontal transects, but weighted mean ages calculated from 15–18 dates per horizon were accurate to a resolution within 154–214 yr. Approximately 40% of the variability in age estimate resulted from internal variability inherent to ¹⁴C estimates, and 60% was due to time-averaging. The accuracy of age estimates of sea level change in studies using single dated corals as proxies for sea level is probably within 1000 yr of actual age, but can be resolved to ≤ 250 yr if supported by dates from analysis of a statistical population of corals at each stratigraphic interval. The range of time-averaging among reef corals was much less than that for shelly benthos. Ecological time-averaging dominated over sedimentological time averaging for reef corals, opposite to patterns reported from shelly benthos in siliciclastic environments.  相似文献   

Evolution of the oceanic calcium cycle during the late Mesozoic: Evidence from δ44/40Ca of marine skeletal carbonates     

《Earth and Planetary Science Letters》2007,253(1-2):96-111

The Ca isotope compositions of 37 late Mesozoic skeletal carbonates, belemnites and brachiopods, from the Tethyan realm were analyzed by thermal (TIMS) and plasma (MC-ICP-MS) ionization mass spectrometry. A poor correlation between δ^44/40Ca and δ¹⁸O values of belemnites suggests only a weak temperature dependency for the Ca isotope composition of belemnites, likely less than 0.02‰/°C. The δ^44/40Ca record of belemnites was therefore used to reconstruct the Ca isotope composition of paleo-seawater (δ^44/40Ca_SW), based on an experimentally determined fractionation factor between seawater Ca and belemnite calcite (α_CC–SW) of ∼ 0.9986. The inferred δ^44/40Ca_SW record, with an average stratigraphic resolution of 1 Ma, shows systematic temporal variation of ∼ 0.5‰ with the Middle/Late Jurassic (∼ 154 Ma) minimum of ∼ 1.4‰ and a subsequent general increase to the Early Cretaceous (∼ 124 Ma) maximum of ∼ 1.9‰. The global nature of the δ^44/40Ca_SW record is supported by identical Ca isotope compositions of coeval (Kimmeridgian) belemnites collected from two distinct paleogeographic regions, the southern (New Zealand) and northern (Germany) margin of the Tethys Ocean. The observed late Mesozoic δ^44/40Ca_SW record was simulated using a simple Ca isotope mass balance model, and the results indicate that the variation in δ^44/40Ca_SW record can be explained by changes in oceanic input fluxes of Ca that were independent of the carbonate ion fluxes, such as the hydrothermal Ca flux or the release of Ca to the oceans via dolomitization of marine carbonates.  相似文献   

Zircon U–Pb Ages and Lu–Hf Isotope of the Dongchuan Group in Central Yunnan, China, and their Geological Significance     

HAN Kunying  ZHANG Heng  DING Xiaozhong  REN Liudong  SHI Chenglong  PANG Jianfeng 《《地质学报》英文版》2020,94(4):1093-1116

On the southwestern margin of the Yangtze Block, the Dongchuan Group consists of slightly metamorphosed sedimentary rocks, including silty slate, argillaceous slate, clayey slate, arkose, dolomite, and minor volcanic rocks. To date, it is still a controversy over the depositional age and stratigraphic sequence of the Dongchuan Group. In this study, we analyzed five samples of meta-sedimentary rocks and one sample of meta-tuff from the Yinmin, Luoxue and Etouchang Formations of the Dongchuan Group in the Yuxi region for detrital zircon U-Pb ages and Lu-Hf isotope. The detrital zircon ages of the meta-sediments vary from 3073 to 1703 Ma, mainly clustered at three periods, from 1889 to 1840, 2490 to 2008 and 2878 to 2844 Ma. The youngest age peak of all the samples is ～1859 Ma, with the εHf(t) values of the zircons ranging from-20.3 to +4.3 and more than 90% being negative, indicating that the Paleoproterozoic crustal accretion on the southwestern margin of the Yangtze Block was dominated by reworking of the ancient crustal materials involved in the assembly and breakup of the Columbia supercontinent. Another important age range is between 2490 Ma and 2008 Ma, with εHf(t) values from-14.7 to +8.9 and 70% of them are negative, suggesting that the magmatism in the source area was also dominated by reworking and recycling of the ancient crustal materials, with minor juvenile mantle substances added. The detritus was probably derived from the Paleoproterozoic crystalline basement in the southern Yuxi region. The oldest peak age is ～2847 Ma and the εHf(t) values are from-7.7 to +7.0 with 50% of both positive and negative values, demonstrating a possible ～2.85 Ga ancient continental nucleus on the southwestern margin of the Yangtze Block and substantial growth in juvenile crust materials during this period. Besides, the weighted average age of the zircons from the meta-tuff of the Etouchang Formation is 1677 ± 14 Ma. Combining the previous research data and this study, we can constrain the depositional age of the Dongchuan Group in central Yunnan Province to the period from the late Paleoproterozoic to early Mesoproterozoic, slightly earlier than that of the Dongchuan Group in the Dongchuan area near to the southwestern Sichuan Province. The depositional age of the Dongchuan Group is older than that of the Kunyang Group.  相似文献   

Genesis and geodynamic process of early Cretaceous intermediate–felsic batholith within the Chem Co zone,western Qiangtang and implications for Bangong–Nujiang Tethyan Ocean subduction     

《Gondwana Research》2020

The complex evolutionary history of collision-related suture zones in the western Qiangtang is poorly understood, due to the lack of pivotal magmatic and tectonic records. The Chem Co intermediate-felsic batholith is situated in the westernmost section of Qiangtang, where it intruded into the Upper Paleozoic metamorphic sedimentary series and was cut by Late-Cenozoic active normal faults. Here, we present evidence from field observations, geochemistry, zircon UPb and Hf isotopes, and mica ⁴⁰Ar³⁹Ar analysis of these granitoid rocks. Zircon UPb ages ranging from 107.3 ± 1.4 to 123.7 ± 1.7 Ma reveal that this magmatism initiated in the Early Cretaceous. Mica ⁴⁰Ar³⁹Ar yield plateau ages from 99 to 118 Ma suggest rapid emplacement into the cold crust without disturbance by subsequent >320 °C tectono-thermal events. Major element geochemistry shows these granitic rocks are enriched in Al, depleted in Ca, lie mostly within the high–K calc–alkaline series and have peraluminous features. Mineral modes that lack amphibole but contain muscovite, together with the geochemical characteristics, indicate an S–type granite affinity. Furthermore, the rocks are strongly enriched in large-ion lithophile elements (LILE) and light rare earth elements (LREE), and are moderately to strongly depleted in high field strength elements (HFSE) and heavy rare earth elements (HREE), thus indicating typical characteristics of arc-related magmatic rocks. Considering the petrogenesis, geochemical features, zircon UPb ages, negative zircon εHf(t) values, similarity with the southwestern Qiangtang intrusions, and the geological setting, we conclude that the Chem Co granite was derived from the anatexis of crustal lithosphere through a “soft” collision associated with the continuing northwards subduction of the Bangong–Nujiang Tethyan (BNT) Ocean. We propose that the final closure and termination of the western segment of the BNT Ocean was completed later than 107 Ma.  相似文献   

Raman spectral analysis and genetic mechanism of pseudotachylyte in Xiaoqinling detachment fault   总被引：1，自引：0，他引：1  

Zhang Jinjiang  Liu Shuwen  Zheng Yadong  Chen Jing  Shi Quanzeng  Yu Xiangdong  Xue Liangwei 《中国科学D辑(英文版)》1998,41(3):242-247

Veins and clasts of pseudotachylyte developed in the microbreccias of the detachment fault along the boundary of the Xiaoqinling metamorphic core complex. The Raman spectral analysis shows that there are three kinds of textures in the matrices of the pseudotachylyte, i.e. ultracataclastic fine-grained texture, cryptocrystalline texture and both of them with minute quantity of glass. The three different textures are the results of different degrees of ultracataclasis. This demonstrates that ultracataclasis-comminution-melting is the genetic mechanism of the pseudotachylytes in the Xiaoqinling detachment fault and the ultracataclasis-comminution dominates in their formation. Project supported by the National Natural Science Foundation of China (Grant No. 49472142).  相似文献   

Physical protection of lignin by organic matter and clay minerals from chemical oxidation     

《Organic Geochemistry》2013

The role of organic matter (OM) concentration, structure and composition and how these relate to mineral protection is important for the understanding of long term soil OM dynamics. Various OM–clay complexes were constructed by sequential sorption of lignin and dodecanoic acid to montmorillonite. Humic acid–montmorillonite complexes were prepared at pH 4 and 7 to vary OM conformation prior to sorption. Results obtained with constructed OM–clay complexes were tested with isolated mineral fractions from two soils. Oxidation with an acidic NaClO₂ solution was used to chemically oxidize lignin in the OM–clay complexes, sand-, silt- and clay-size soil fractions to test whether or not it can be protected from chemical attack. Gas chromatography–mass spectrometry was used to analyze lignin-derived phenols, cutin OH–acid (after CuO oxidation), fatty acid and n-alkanol concentrations and composition. We found that carbon content was not solely responsible for lignin stability against chemical oxidation. Lignin was protected from chemical oxidation through coating with dodecanoic acid and sorption of humic acid to clay minerals in a stretched conformation at pH 7. Therefore, interactions between OM constituents as well as OM conformation are important factors that protect lignin from chemical oxidation. Lignin-derived phenol dimers in the Grassland-Forest Transition soil fractions were protected from chemical oxidation to a greater extent compared to those in Grassland soil fractions. Therefore, although lignin was protected from degradation through mineral association, the extent of this protection was also related to OM content and the specific stability of lignin components.  相似文献   

Lateglacial summer temperatures in the Northwest European lowlands: a chironomid record from Hijkermeer,the Netherlands     

《Quaternary Science Reviews》2007,26(19-21):2420-2437

Lateglacial environments at Hijkermeer, northwest Netherlands, were reconstructed by means of chironomid, diatom and pollen analyses. Diatom assemblages indicate that Hijkermeer was a shallow, oligo- to mesotrophic lake during this period. Pollen assemblages reflect the typical northwest European Lateglacial vegetation development and provide an age assessment for the record from the beginning of the Older Dryas (ca 14 000 calibrated ¹⁴C yr BP) into the early Holocene (to ca 10 700 calibrated ¹⁴C yr BP). The chironomid record is characterized by several abrupt shifts between assemblages typically found in mid-latitude subalpine to alpine lakes and assemblages typical for lowland environments. Based on the chironomid record, July air temperatures were reconstructed using a chironomid-temperature transfer-function from central Europe. Mean July air temperatures of ca 14.0–16.0 °C are inferred before the Older Dryas, of ca 16.0–16.5 °C during most of the Allerød, of ca 13.5–14.0 °C during the Younger Dryas, and of ca 15.5–16.0 °C during the early Holocene. Two centennial-scale decreases in July air temperature were reconstructed during the Lateglacial interstadial which are correlated with Greenland Interstadial events (GI)-1d and -1b. The results suggest that vegetation changes in the Netherlands may have been promoted by the cooler climate during GI-1d, immediately preceding the Older Dryas biozone, and GI-1b. The Hijkermeer chironomid-inferred temperature record shows a similar temperature development as the Greenland ice core oxygen isotope records for most of the Lateglacial and a good agreement with other temperature reconstructions available from the Netherlands. This suggests that chironomid-based temperature reconstruction can be successfully implemented in the Northwest European lowlands and that chironomids may provide a useful alternative to oxygen isotopes for correlating European lake sediment records during the Lateglacial.  相似文献   

Extinction of deep-sea foraminifera as a result of Pliocene–Pleistocene deep-sea circulation changes in the South China Sea (ODP Sites 1143 and 1146)     

《Quaternary Science Reviews》2007,26(5-6):808-827

During the Late Pliocene–Middle Pleistocene, 56 species and 15 genera of elongate, cylindrical benthic foraminifera disappeared from the deep sea in the South China Sea (ODP Sites 1143 and 1146) as part of the last global extinction in the deep sea. This extinction group (Ext. Gp) exhibited a pulsed decline in abundance and species richness mostly during glacials, and often associated with periods of expansion of polar ice that resulted from increased cooling of the Earth's climate since ∼2.5 Ma, particularly during the Mid-Pleistocene Climate Transition (MPT, 1.2–0.6 Ma). We infer that the Ext. Gp decline in abundance and disappearance was a result of the increased glacial cooling and consequent increased ventilation of the deep-sea water masses. The detailed record of withdrawal of the Ext. Gp differs between the two sites, with far more disappearances occurring prior to the MPT in the deeper Site 1143 (2772 m) than in the shallower Site 1146 (2092 m). The Late Pliocene and Early Pleistocene declines in deeper parts of the South China Sea (Site 1143) may have resulted from enhanced glacial production of deep, southern-sourced water passing over the sill into the basin from the North-west Pacific. During the MPT however, the Ext. Gp declines and disappearances were of similar timing and magnitude in both sites, implying that both were influenced by the same deep-water mass during glacials at this time. This could have been North Pacific Deep Water, which many workers infer was formed in the northern Pacific during the last glacial, and may have begun forming during MPT glacials, in association with the progressively enhanced cooling of the Northern Hemisphere.  相似文献