滇西怒江河谷潞江段岩溶发育特征及其 对工程的影响   总被引：1，自引：0，他引：1  

康春景  张绪教  吴中海  杨艳  李宗敏 《地质通报》2012,31(203):374-381

通过对泛亚铁路大理-瑞丽线中段马厂、道街坝1∶5万图幅的地质灾害详细勘查和室内研究,分析总结了研究区岩溶地貌的形成机制和发育规律。结果发现,研究区内不仅发育有地表岩溶漏斗,而且分布着规模较大的地下岩溶洞穴,复杂的地质构造、可溶性的碳酸盐岩、湿热的气候条件和丰富的降水是该区岩溶地貌发育的主要因素。研究结果表明,研究区内的地下岩溶作用可能给铁路隧道工程的施工带来不可忽视的潜在危害,同时综合区域地质条件对比发现,大理-瑞丽铁路沿线的其它地段也同样具有存在类似的岩溶发育条件。因此,在铁路施工前进一步加强沿线区域地下岩溶作用的工程影响评价,对于有效地减少相关的工程危害性十分重要。  相似文献   

天津空港牡蛎礁:中全新世环境恶化与新构造控礁作用   总被引：1，自引：1，他引：0  

王海峰  王宏  范昌福  方晶  裴艳东  方念乔 《地质通报》2012,31(9):1387-1393

新发现的天津空港埋藏牡蛎礁体和下伏、上覆泥层的沉积学、AMS14C年代学研究显示,厚2.3m的礁体分为3部分,起讫年龄是6170～3980cal BP。5900～4100cal BP的中部,是受全新世冷事件4环境恶化控制的缓慢发育期。礁体的结束,则与冷事件3在时间上大致吻合。根据新的年龄数据,将该礁体的归属重新厘定为Ⅲ-2礁群。探讨礁体主要发育区与新构造活动的关系,表明全新世牡蛎礁平原主要位于海河-宝坻-工部-蓟运河断裂圈闭的北西-南东向区域内。这一认识,进一步从实证角度证实了牡蛎礁形成于渤海湾"湾中之湾"的推测。  相似文献   

Structural relations and PbPb zircon ages for the Makuti gneisses: evidence for a crustal-scale Pan-African shear zone in the Zambezi Belt,northwest Zimbabwe     

《Journal of African Earth Sciences》1999,28(2):427-442

The Makuti Group of northwest Zimbabwe is composed of mafic and intermediate biotite-rich gneisses interlayered with quartzofeldspathic gneisses of granitic composition, and minor sedimentary units. The gneisses have experienced a multi-staged metamorphic history, including an early high temperature-high pressure event and subsequent reworking at upper- to mid-amphibolite-facies conditions. They are positioned along the strongly deformed, southern margin of the east-west trending Zambezi Belt, and have been correlated with supracrustal gneiss units along the northern margin of the Zimbabwe Craton.The Makuti Group is characterised by an intensely developed gneissic layering and complex disharmonic folds that resulted from non-coaxial deformation involving repeated stages of transposition. The basal contact of the g roup coincides with a decrease in strain intensity, but not with a directional change of characteristic structural elements (e.g. lineations, fold axes), nor with a clear change in rock types. Pink quartzofeldspathic gneisses of granitic composition are typical for the Makuti Group, but locally intrude basement gneiss as well. The quartzofeldspathic gneisses occur as porphyritic and non-porphyritic varieties that are, invariably, intensely sheared.The age and nature of the basal contact of the Makuti Group and its relationship to the quartzofeldspathic gneisses has been investigated. Samples for single zircon PbPb dating were collected from a felsic biotite gneiss just below (2704 ± 0.3 Ma) and above (2510 ± 0.4 Ma) the lower contact of the Makuti Group at an ‘unconformity’ 2 km northwest of Vuti. Further samples were collected from pink quartzofeldspathic units at the base (737 ± 0.9 Ma), central part (764 ± 0.9 Ma; 797 ± 0.9 Ma) and top (794 ± 0.5 Ma; 854 ± 0.8 Ma) of the Makuti Group. Two samples of Kariba orthogneiss (1920 ± 0.4 and 1963 ± 0.4 Ma) underlying the Makuti Group in the northwest were also collected. In all samples, long-prismatic, colourless to brown, igneous zircon grains were selected. Dates were obtained using a stepwise single-grain evaporation technique. Although this technique only allows minimum age estimates, the dates are highly reproducible, indicating that they approximate emplacement ages. The ages conform with the field observations that the basement has been reworked in the Makuti Group and that the quartzofeldspathic units may have been emplaced as granites.It is proposed that the Makuti Group represents a crustal scale shear zone that partly reworked basement gneisses and acted as a conduit for granite emplacement. Shearing took place in an extensional setting around 800 Ma ago, and may have resulted in the exhumation of lower crustal rocks.  相似文献   

A microbial pathway for the formation of gold-anomalous calcrete     

Frank Reith  Steven A. Wakelin  Adrienne L. Gregg  Andreas Schmidt Mumm 《Chemical Geology》2009,258(3-4):315-326

The formation of pedogenic carbonate (calcrete) in terrestrial environments is commonly mediated by microorganisms. In Australia, Au-anomalous calcrete is an important sampling medium for geochemical exploration, but current models describing its formation do not include a confirmed microbial component. This study demonstrates that bacterial communities in calcareous sands from dunes overlying the Barns Gold Deposit in semi-arid South Australia, are capable of mediating the biomineralisation of Au-anomalous carbonates. Bacterial enrichment cultures obtained from calcareous sands at three depths (0.1, 0.64 and 2.1 m, plus abiotic control) were incubated in urea and Ca²⁺-containing growth media (pH 8), unamended and amended with Au (100 parts-per-billion, ppb) as Au–aspartic-acid complex. During the incubation of the enrichment cultures urea was turned over to NH₄⁺ within 96 h to 220 h. The solution pH increased concurrently by approximately 1.2 units, and Au-anomalous Ca-carbonate crystallites were precipitated on cells, which functioned as nucleation sites; no carbonate precipitation was observed in abiotic controls. Compared to the medium, Au was strongly enriched in these carbonates and appeared to be uniformly dispersed in the individual crystallites, as shown using LA-ICP-MS; a similar distribution is present in naturally occurring Au-anomalous calcrete. Phylogenetic 16S rRNA PCR DGGE analyses, shotgun cloning and functional microbial analyses (BioLog, ureC quantitative PCR) demonstrated that naturally occurring and culture-enriched bacterial communities were dominated by alkaliphylic, halotolerant Bacillus spp. The indigenous bacterial communities were capable of utilising amino acids (including l-aspartic acid) and urea, which appears to lead to the destabilisation of the Au–amino acid complexes and concomitant co-precipitation of Au in the Ca-carbonates. In conclusion, a model combining geomicrobial– with evapotranspiration– and plant-based components is likely to best describe the formation of (Au-anomalous) calcrete in semi-arid and arid zones.  相似文献   

Hydrogen isotopic ratios of plant wax n-alkanes in a peat bog deposited in northeast China during the last 16 kyr     

Osamu Seki  Philip A. Meyers  Kimitaka Kawamura  Yanhong Zheng  Weijian Zhou 《Organic Geochemistry》2009,40(6):671-677

Based on paleoclimatic reconstructions using various proxies, the Holocene Climate Optimum (10.5–6 ka) has been characterized as a warmer and wetter period in most of East Asia. The summer monsoons associated with the East Asian Monsoon evidently intensified and extended further inland from the Pacific Ocean, a source region of moisture. A notable exception to this general pattern exists in northeast China, where less wet conditions are recorded. We determined molecular compositions of individual plant wax hydrocarbons and their hydrogen isotope compositions (δD values) in a radiocarbon-dated peat core recovered from the Hani marsh in Jilin Province (China) and confirmed that the temperature-dependent effective precipitation in northeast China decreased during the Holocene Climate Optimum. A combination of P_aq, an indicator of the relative contribution of aquatic to terrestrial plants, and the difference in δD between low (C₂₃, C₂₅ and C₂₇) and high molecular weight (C₃₁) n-alkanes in the Hani peat bog indicates a dramatic change in vegetation from the deglaciation to the Holocene. No significant differences were observed between the δD values of low and high molecular weight n-alkanes with relatively high δD values and low P_aq during the early Holocene, indicating that all n-alkanes were produced by evapotranspiration-sensitive terrestrial plants during that time. However, lower δD values of mid-chain n-alkanes (C₂₃, C₂₅ and C₂₇) relative to the long chain n-alkane (C₃₁), together with higher P_aq values during the deglaciation (14–11 ka), suggest an increase in the contribution of aquatic plants and a higher water level during the period. The study demonstrates that northeast China was under a markedly wetter climate condition during the late deglaciation. For the 16 kyr record in the Hani peat sequence, we infer that moisture delivery by the East Asian Monsoon was relatively invariable in northeast China, but increased evaporation during the warmer Holocene Climate Optimum reduced the effective precipitation, defined by the balance between precipitation and evaporation.  相似文献   

Quaternary glaciation of Mount Everest   总被引：1，自引：0，他引：1  

Lewis A. Owen  Ruth Robinson  Douglas I. Benn  Robert C. Finkel  Nicole K. Davis  Chaolu Yi  Jaakko Putkonen  Dewen Li  Andrew S. Murray 《Quaternary Science Reviews》2009,28(15-16):1412-1433

The Quaternary glacial history of the Rongbuk valley on the northern slopes of Mount Everest is examined using field mapping, geomorphic and sedimentological methods, and optically stimulated luminescence (OSL) and ¹⁰Be terrestrial cosmogenic nuclide (TCN) dating. Six major sets of moraines are present representing significant glacier advances or still-stands. These date to >330 ka (Tingri moraine), >41 ka (Dzakar moraine), 24–27 ka (Jilong moraine), 14–17 ka (Rongbuk moraine), 8–2 ka (Samdupo moraines) and ～1.6 ka (Xarlungnama moraine), and each is assigned to a distinct glacial stage named after the moraine. The Samdupo glacial stage is subdivided into Samdupo I (6.8–7.7 ka) and Samdupo II (～2.4 ka). Comparison with OSL and TCN defined ages on moraines on the southern slopes of Mount Everest in the Khumbu Himal show that glaciations across the Everest massif were broadly synchronous. However, unlike the Khumbu Himal, no early Holocene glacier advance is recognized in the Rongbuk valley. This suggests that the Khumbu Himal may have received increased monsoon precipitation in the early Holocene to help increase positive glacier mass balances, while the Rongbuk valley was too sheltered to receive monsoon moisture during this time and glaciers could not advance. Comparison of equilibrium-line altitude depressions for glacial stages across Mount Everest reveals asymmetric patterns of glacier retreat that likely reflects greater glacier sensitivity to climate change on the northern slopes, possibly due to precipitation starvation.  相似文献   

Uplift and cooling magnetisation record in the Bamble and Telemark terranes,Sveconorwegian orogenic belt,SE Norway,and the Grenville-Sveconorwegian loop     

J.D.A. Piper 《Tectonophysics》2009,463(1-4):185-207

The ~ 1100 Ma Sveconorwegian orogenic belt comprises allochthonous terranes juxtaposed by major fault zones and emplaced against, and onto, the south-western margin of the Fennoscandian Shield. To resolve the magnetic signature acquired during post-orogenic uplift and cooling and evaluate wider correlations with the contemporaneous Grenville belt of North America, this study reports a regional palaeomagnetic study on a range of rock types from sectors of the medium-high metamorphic grade Bamble terrane (48 sites and 390 cores) and the adjoining medium-low grade Telemark terrane (33 and 240 cores) juxtaposed by an orogen-parallel (Porsgrunn- Kristiansand) fault zone with major vertical displacement. Magnetite and ilmeno-hematites are magnetic carriers with the latter more significant in the higher metamorphic grades. Magnetic intensities are stronger in the higher-grade terrane presumably due to the growth of metamorphic ferromagnets, but are an order lower than values predicted for the lower continental crust and indicate that an additional mechanism is responsible for high magnetisations in deep crust. Anisotropy of magnetic susceptibility (AMS) largely reflects the NE–SW tectonic grain of the last stage of Sveconorwegian ductile deformation. The magnetisation record is filtered by excluding magnetisations possibly acquired during regional Mesozoic dyke emplacement, development of the Permo-Carboniferous Oslo Rift and Late Proterozoic magmatism. The remaining record is a dual polarity signature summarised by mean poles at 31.9°N, 50.9°E, (N = 191 components) in the Bamble terrane and at 34.2°N, 58.9°E (N = 151 components) in the Telemark terrane. However these means are non-Fisherian and embrace arcuate distributions of magnetic components acquired during protracted exhumation cooling of the orogen with the best-defined parts comprising clockwise trajectories correlating with each another but indicating that cooling in Telemark was more protracted; in each case directions of more shallow NW-direction tend to be derived from lower unblocking temperature components. The geochronological evidence indicates that regional temperatures had fallen to permit acquisition of magnetisation by ~ 950–900 Ma and the two swathes define the younger limb of a clockwise (Grenville-Sveconorwegian) APW loop embracing the approximate interval 940–850 Ma; the outward path of this loop (~ 1020–940 Ma) is probably at present recorded only in dyke swarms from the Finnish sector of the shield. Correlation of APW between Laurentia and Fennoscandia confirms that the two shields broke apart shortly after culmination of the Sveconorwegian orogeny when Fennoscandia rotated rapidly clockwise into a secondary configuration adjacent to the eastern margin of Laurentia; the Grenville and Sveconorwegian orogenic frontal zones formed in alignment were reoriented at a high angle to one another in a coupling that appears to have persisted during most of the remainder of Neoproterozoic times.  相似文献   

地壳深部岩浆岩岩基体与大型、超大型金属矿床的形成及找矿效应   总被引：6，自引：1，他引：5  

滕吉文  姚敬金  江昌洲  闫雅芬  杨辉  张永谦  阮小敏 《岩石学报》2009,25(5):1009-1038

基于部分大型、超大型金属矿床和多金属矿集区的地球物理研究发现:地壳内部物质在热动作用下,深部物质和能量强烈的在交换,当深部热液沿通道（破碎带、软弱岩层或断裂系）上涌时,在深部适宜的岩层部位聚积,并形成大型或较大型的酸性、中酸性或基性的岩基体（镁铁质、超镁铁质）。这些类型的岩浆岩以其携带的热能驱动着深部物质重新分异和调整,并在这一动力作用过程中与围岩进行交代、蚀变,并促使其不断变质。在这样的深部介质和构造环境下矿物元素得以在大型或较大型岩基体附近与周边介质耦合,逐使矿物元素不断聚积,并形成大型、超大型金属矿床和多金属矿集区。  相似文献   

西天山玉希莫勒盖达坂玄武安山岩—高钾玄武安山岩—粗安岩组合的发现及其地质意义   总被引：6，自引：3，他引：3  

罗勇  牛贺才  单强  张兵  周昌平  杨武斌  于学元 《岩石学报》2009,25(4):934-943

在阿吾拉勒东段玉希莫勒盖达坂地区大哈拉军山组主要是一套以安山质（粗安质）-英安质（粗面质-粗面英安质）火山熔岩和火山碎屑岩为主的火山岩建造。根据岩石地球化学特征,本文在大哈拉军山组火山岩的上部厘定出了钙碱性玄武安山岩—高钾钙碱性玄武安山岩—粗安岩（橄榄安粗岩系）组合。微量元素地球化学特征显示,三类岩石形成于岛弧环境,钙碱性玄武安山岩的形成与板片俯冲作用有关,而高钾钙碱性玄武安山岩和粗安岩的形成则与俯冲板片断裂诱发的软流圈上涌作用有关。钙碱性玄武安山岩—高钾钙碱性玄武安山岩—粗安岩组合的存在说明阿吾拉勒东段在晚石炭世由于俯冲板片的断裂,构造体制由挤压转变为伸展。玉希莫勒盖达坂大哈拉军山组上部的火山岩组合与世界许多著名铜-金矿集区（如Papua New Guinea）的火山岩组合相似,显示该地区晚石炭世具有良好的铜、金成矿条件。  相似文献   

西藏马攸木金矿富CO2 超临界流体特征及矿床成因探讨     

李腊梅 《地质与勘探》2009,45(5):502-508

马攸木金矿床是产于西藏雅鲁藏布江缝合带西段重要的独立岩金矿床。本文通过显微测温分析首次发现该矿床的富CO2流体包裹体具有临界均一的特征,成矿流体属于超临界流体。研究结果表明:成矿流体主要为低盐度的CO2-H2O超临界流体。超临界流体可能是从岩浆出溶的,这种流体萃取了围岩中的金等成矿元素。流体经历了相分离-不均一捕获-跨越临界点-大气降水加入的过程,正是由于成矿流体在跨越临界点时析出部分成矿物质,形成早期矿化体;成矿后期流体与大气降水混合最终导致矿质大量沉淀。  相似文献