华北太行晚中生代煌斑岩地球化学特征及成因探讨

华北克拉通太行山地区煌斑岩脉广泛发育。这些煌斑岩高度富集大离子亲石元素(如Rb、Sr、Ba和K)和轻稀土元素，具有高度分异的REE模式，在化学成分和Nd—Sr同位素组成上亦显示出规律性的变化。其特征表明：太行山地区的晚中生代煌斑岩来源于同一个岩浆源区，而且煌斑岩浆在上升过程中曾经遭受过下地壳的混染。此外，最原始的煌斑岩样品具有低SiO2、高MgO以及高度富集同位素[εNd(120Ma)＝-8．3，Ist＝0．7052]的特征，表明形成煌斑岩的母岩浆来自富集地幔，是富集地幔部分熔融的产物；富集地幔可能是下部软流圈释放的富挥发分、低密度的熔体与上部的岩石圈发生了交代反应而形成的。     

The Proterozoic,albitite-hosted,Valhalla uranium deposit,Queensland, Australia: a description of the alteration assemblage associated with uranium mineralisation in diamond drill hole V39

The Valhalla uranium deposit, located 40 km north of Mount Isa, Queensland, Australia, is an albitite-hosted, Mesoproterozoic U deposit similar to albitite-hosted uranium deposits in the Ukraine, Sweden, Brazil and Guyana. Uranium mineralisation is hosted by a thick package of interbedded fine-grained sandstones, arkoses and gritty siltstones that are bound by metabasalts belonging to the ca. 1,780 Ma Eastern Creek Volcanics in the Western Succession of the Mount Isa basin. Alteration associated with U mineralisation can be divided into an early, main and late stage. The early stage is dominated by laminated and intensely altered rock comprising albite, reibeckite, calcite, (titano)magnetite ± brannerite. The main stage of mineralisation is dominated by brecciated and intensely altered rocks that comprise laminated and intensely altered rock cemented by brannerite, apatite, (uranoan)-zircon, uraninite, anatase, albite, reibeckite, calcite and hematite. The late stage of mineralisation comprises uraninite, red hematite, dolomite, calcite, chlorite, quartz and Pb-, Fe-, Cu-sulfides. Brannerite has U–Pb and Pb–Pb ages that indicate formation between 1,555 and 1,510 Ma, with significant Pb loss evident at ca. 1,200 Ma, coincident with the assemblage of Rodinia. The oldest ages of the brannerite overlap with ⁴⁰Ar/³⁹Ar ages of 1,533 ± 9 Ma and 1,551 ± 7 Ma from early and main-stage reibeckite and are interpreted to represent the timing of formation of the deposit. These ages coincide with the timing of peak metamorphism in the Mount Isa area during the Isan Orogeny. Lithogeochemical assessment of whole rock data that includes mineralised and unmineralised samples from the greater Mount Isa district reveals that mineralisation involved the removal of K, Ba and Si and the addition of Na, Ca, U, V, Zr, P, Sr, F and Y. U/Th ratios indicate that the ore-forming fluid was oxidised, whereas the crystal chemistry of apatite and reibeckite within the ore zone suggests that F⁻ and were important ore-transporting complexes. δ¹⁸O values of co-existing calcite and reibeckite indicate that mineralisation occurred between 340 and 380°C and involved a fluid having δ¹⁸O_fluid values between 6.5 and 8.6‰. Reibeckite δD values reveal that the ore fluid had a δD_fluid value between −98 and −54‰. The mineral assemblages associated with early and main stages of alteration, plus δ¹⁸O_fluid and δD_fluid values, and timing of the U mineralisation are all very similar to those associated with Na–Ca alteration in the Eastern Succession of the Mount Isa basin, where a magmatic fluid is favoured for this style of alteration. However, isotopic data from Valhalla is also consistent with that from the nearby Mount Isa Cu deposit where a basinal brine is proposed for the transport of metals to the deposit. Based on the evidence to hand, the source fluids could have been derived from either or both the metasediments that underlie the Eastern Creek Volcanics or magmatism that is manifest in the Mount Isa area as small pegmatite dykes that intruded during the Isan Orogeny.     

Chemical oceanographic influences on sediment accumulations of a carbonate ramp: Holocene Yucatan Shelf,Mexico

Global controls on the oceanographic influences on the nature of carbonate factories are broadly understood. The details of the influences of changes in temperature and nutrients across individual carbonate shelves are less well constrained, however. This study explores spatial and temporal variations in chemical oceanography along and across the Yucatan Shelf, a modern carbonate ramp, and how these factors relate to variable bottom character, sediment and sediment geochemistry. In‐situ sensors and remote‐sensing data indicate the sporadic presence of cool, upwelled water with low dissolved oxygen and elevated Chlorophyll‐a. This current‐driven, westward flow of upwelled water is most evident in a zone just offshore of the northern peninsular shoreline, but its influence wanes ca 75 km offshore and as the shore turns southward. The impacts of this water mass include a transitional photozoan–heterozoan assemblage with biosiliceous components, relict grains and common thin Holocene sediment accumulations nearshore; further offshore are coralgal reefs and expansive sand plains. Geochemical proxies of bulk sediment, including high δ¹⁸O and elevated HREE/LREE (heavy rare‐earth element/light rare‐earth element) ratios near, and downcurrent of, the upwelling source, are interpreted to represent the signal of nearshore, westward movement of the cool and nutrient‐rich, upwelled water. Collectively, these data emphasize how local processes such as upwelling and longshore transport can variably influence carbonate sediment accumulations and their geochemical signatures, both along and across individual shelves. These data and insights provide an analogue for the influences of spatial variability of water masses in the geological record, and for accurate interpretation of stratigraphic changes of sedimentary and geochemical proxy data in carbonate archives.     

新疆萨尔托海糜棱岩化石英菱镁岩元素地球化学特征及其对金成矿作用的贡献

新疆萨尔托海石英菱镁岩产在达拉布特蛇绿混杂岩带中,是蛇纹岩在剪切带深部发生热液交代作用的产物。蛇纹岩先转变为滑石片岩,再进一步转变为石英菱镁岩。石英菱镁岩局部发生剪切变形,形成糜棱岩化石英菱镁岩。与蛇纹岩相比,石英菱镁岩的MgO和SiO_2含量降低,Al_2O_3和CaO含量升高;而糜棱岩化石英菱镁岩的MgO含量较石英菱镁岩降低,Al_2O_3和SiO_2含量较石英菱镁岩升高。微量元素地球化学对比研究表明石英菱镁岩继承了蛇纹岩的微量元素特征;而糜棱岩化石英菱镁岩的微量元素含量较蛇纹岩发生了显著变化,稀土元素、高场强元素和自然金的含量明显升高,指示这些元素在剪切变形过程中发生迁移富集。剪切变形伴随强烈的流体/岩石反应,并导致剪切带流体组成和物理化学性质发生改变,从而影响矿物结晶或分解,并控制微量元素的带入迁出。蛇纹岩转变为石英菱镁岩的过程释放Au,可为糜棱岩化石英菱镁岩中的金矿化提供成矿元素。     

甘肃省景泰正路下志留统复理石杂砂岩沉积地球化学特征

位于北祁连造山带东段甘肃省景泰正路一带的下志留统肮脏沟组为一套巨厚的浊流相陆源碎屑岩,以杂砂岩为主,夹有板岩,属典型的浊流沉积。薄片的镜下观察结果表明,其粒度为0.05~4.00 mm,岩性为细粒-中、粗粒砂岩、含砾砂岩。对5块采集于复理石中的杂砂岩样品进行岩性组合判定和主要元素地球化学分析结果表明,w(SiO2)为61.83%~69.90%,w(K2O)/w(Na2O)为0.89~1.46。利用数据对比及w(K2O)/w(Na2O)—w(SiO2)等判别图的分析结果对其物源区的性质及沉积大地构造背景进行分析和判别的研究结果表明,北祁连造山带东段甘肃省景泰正路一带下志留统肮脏沟组的陆源碎屑主要来源于活动大陆边缘岩浆岛弧源区和造山带源区,反映了卷入造山带的岛弧、活动大陆边缘的弧后盆地的物源背景。沉积地球化学特征揭示的沉积大地构造背景为弧后前陆盆地背景,与碰撞后的大地构造背景相吻合。     

西藏类乌齐地区含镍碳酸岩的发现与地质意义

野外调查发现西藏类乌齐地区分布着众多的含镍碳酸岩体,呈十几米至几百米不等的岩株出露。岩体与超基性岩之间具明显的港湾状、枝状侵入接触、岩石不具层理、岩石表现出细粒结晶等粒结构、岩体内见超基性岩捕掳体等特征均表明其为火成碳酸岩岩体。在成分上以菱镁矿为主,岩石中MgO的含量为27.16%～31.75%,为镁质碳酸岩。岩石稀土元素配分曲线呈平缓右倾的轻稀土富集型,轻重稀土元素分馏现象明显,具有明显的Eu正异常和弱的Ce负异常;微量元素显示Rb、Th、Nb、Hf元素相对富集,K、U、Ti元素相对亏损。含镍碳酸岩可能是富CO₂的深部流体在中下地壳对超基性岩交代的结果,具备寻找硫化镍矿床的有利条件。     

A 16 ka climate record deduced from δ^13C and C/N ratio in Qinghai Lake sediments, northeastern Tibetan Plateau

1 INTRODUCTION δ13C in organic matters from lacustrine sedi- ments varies with several factors including aquatic plants, vegetation type in the catchment, atmos- pheric CO2 concentration, climate (temperature and precipitation), and properties of water, …     

宜宾市宋家地区土壤元素背景值研究及其意义

作者在表层土壤系统采样调查的基础上,计算出宋家地区土壤元素地球化学背景值,并通过与不同区域土壤元素地球化学背景值、基准值的对比研究,总结了该区土壤元素地球化学基本特征,为该区土壤资源、环境评价和农业经济发展规划提供了实用地球化学信息。     

胶东招掖地区滦家河花岗岩锆石U-Pb年代学、岩石地球化学及其地质意义

滦家河花岗岩体是胶东金矿区内的重要岩体,其岩性主要为中粗粒黑云母二长花岗岩。锆石LA—ICP—MS U—Pb年代学研究表明,滦家河花岗岩形成于149±2 Ma,属晚侏罗世。滦家河花岗岩Si O2的含量为64.58%~72.05%,相对较高;A/CNK值为0.98~1.22。微量元素特征显示,滦家河花岗岩富集大离子亲石元素(LILE)Rb、Ba、K,强烈亏损高场强元素(HFSE)Nb、Ta、Ti、P;稀土元素特征显示轻稀土强烈富集,重稀土极度亏损,轻微正铕异常。岩石的Nb/La和Nd/Th比值显示壳源花岗岩的特征,Rb/Sr比值暗示可能有部分幔源物质混入。结合区域地质分析认为,侏罗纪开始,太平洋板块向欧亚大陆的俯冲引发大规模幔源岩浆底侵,导致地壳物质部分熔融形成滦家河花岗岩。     

Geochemistry of the Eocene Jijuntun Formation oil shale in the Fushun Basin,northeast China: Implications for source-area weathering,provenance and tectonic setting

Within the fault-bound Fushun Basin of northeastern China, the Eocene Jijuntun Formation hosts extensive deposits of thick lacustrine oil shale. Systematic sampling and geochemical analysis of these deposits has revealed that the parent rocks of the oil shale underwent moderate chemical weathering; and that its mineralogy and trace and rare earth element geochemistry were mainly controlled by parent rock composition, with no synsedimentary changes in the source terrain. Based on source rock and tectonic setting discrimination diagrams, we concluded that the parent rocks of the oil shale were mostly basalts of the Paleogene Laohutai Formation. These basalts originated in a continental back arc environment and contain abundant nutrient elements such as Fe, P, Ni, Cu and Zn, all of which are essential for the growth of aquatic photoautotrophs in lakes. Continuous, high primary productivity in the Jinjuntun lacustrine depocentre, combined with a stable tectonic setting and underfilled sedimentary environment, were key factors in the genesis of its oil shale.