冈底斯中段阿木雄杂岩体岩石成因及地质意义

冈底斯南缘发育大量的岩浆岩,前人对这条岩浆岩带东段的研究完善了冈底斯南缘的大地构造框架和岩石圈结构及演化理论体系,而关于冈底斯南缘中段岩浆作用的研究相对较少。本文对中段阿木雄杂岩体的角闪辉长岩和黑云二长花岗岩进行了详细的岩石学、年代学及地球化学研究,以期阐明岩浆成因并探究岩浆源区深部地质过程。阿木雄杂岩体中黑云二长花岗岩和角闪辉长岩锆石U-Pb年龄均为49 Ma,属于始新世同期岩浆作用产物。地球化学特征显示黑云二长花岗岩属于I型高钾钙碱性花岗岩,富集大离子亲石元素,亏损高场强元素,具低正的εHf(t)值(-0. 2～+2. 6),Hf同位素地壳模式年龄t_DM^C为1 136～393 Ma;角闪辉长岩主量元素特征属于典型的辉长岩,Mg^#值接近于原始的玄武质岩浆,稀土元素配分曲线平缓右倾,无Eu亏损,富集大离子亲石元素,亏损高场强元素,具有高正的εHf(t)值(+5. 1～+16. 8)和年轻的t_DM^C年龄(798～46 Ma)。分析后认为角闪辉长岩源于受过板片流体交代的亏损地幔,而...     

The age of unusual xenogenic zircons from Yakutian kimberlites

Several spindle-shaped grains of zircon, which have a small size (<0.25 mm) and a distinct purplish pink coloration were found in the crushed samples of kimberlites from the Aykhal, Komsomolskaya-Magnitnaya, Botuobinskaya (Siberian platform), and Nyurbinskaya (Yakutia) pipes and olivine lamproites of the Khani massif (West Aldan). U-Pb SHRIMP II zircon dating performed at the VSEGEI Center for Isotopic Research yielded the ages of 1870–1890 Ma for the pipes of the Western province (Aykhal and Komsomolskaya) and 2200–2750 Ma for the pipes of the eastern province (Nyurbinskaya and Botuobinskaya), which allowed us to consider these zircons to be xenogenic to kimberlites. Although these zircons resemble in their age and color those from the granulite xenoliths in the Udachnaya pipe [2], no other granulite minerals are found there. Thus, major geological events in the mantle and lower crust, which led to the formation of zircon-bearing rocks, happened at 1800–1900 Ma in the northern part of the kimberlite province, whereas in the Eastern part of the province (Nakyn field) these events were much older (2220–2700 Ma). It is known that the period of 1800–1900 Ma in the Earth’s history was accompanied by intense tectonic movements and widespread alkaline-carbonatite magmatism. This magmatism was related to plume activity responsible for overheating the large portions of the mantle to the temperatures at which some diamonds in mantle rocks would burn (northern part of the kimberlite province). In the Nakyn area, the mantle underwent few or no geological processes at that time, and perhaps for this reason this area hosts more diamondiferous kimberlites. The age of olivine lamproites from the Khani massif is 2672–2732 Ma. Thus, these are some of the world’s oldest known K-alkaline rocks.     

云中山区界河口岩群单颗粒锆石年龄及地质意义

云中山区界河口岩群原岩主要为1套陆源碎屑岩夹镁铁质火山岩,经历了多次变形变质作用的叠加改造,变质程度达高角闪岩相,深熔作用强烈。整套岩石组合无底、无顶、无序,其与基底片麻岩之间(TTG)或是深熔渐变过渡,或是呈韧性剪切带接触。采自该岩群绿帘角闪二长变粒岩单颗粒锆石U-Pb法测年获得3组年龄值,即(2751±39)Ma、(2541±21)Ma和(2183±16)Ma,前者代表物源区的年龄,后两者代表2次深熔变质年龄。侵入该岩群的峪口眼球状黑云二长片麻岩单颗粒锆石U-Pb法测年获得(2493±38)Ma的成岩年龄,从而说明了云中山区界河口岩群形成时代为新太古代。发生在(2541±21)Ma强烈的区域性深熔作用,使基底片麻岩活化或重生,早期的信息大多已消失。界河口岩群与五台岩群年龄相当,推测界河口岩群与五台岩群并非是垂向上下关系,而是横向上的并列关系,现在所处的不同构造层次是弧-陆碰撞的造山过程中构造叠置形成,峪口片麻岩形成于后碰撞环境,其侵位形成的卵形构造局部斜切弧-陆碰撞所形成线状构造带。     

安徽省岳西县碧溪岭片麻岩地球化学和锆石U-Pb年龄研究

本文通过对碧溪岭片麻岩综合研究表明,岩石化学成分高硅(SiO2=73.18%～75.60%)、低铝(Al2O3=11.93%～12.79%)、富碱(Na2O+K2O=6.11%～7.23%)和高钠(Na2O/K2O=1.51～1.94); 微量元素富集大离子亲石元素Rb、Ba、Th、U等和Pb,贫Nb、Ta、Zr、Hf、Ti等高场强元素; 稀土元素含量范围变化较大(TREE=142.38～308.44×10-6),富集轻稀土(LREE/HREE=5.93～6.01),和铕负异常明显(δEu=0.42～0.64)等。造岩矿物中斜长石聚片双晶常垂直晶体延长方向,以及继承锆石大部分是不同时代的岩浆碎屑锆石等,暗示其寄主岩是沉积岩; 锆石LA-ICP-MS定年结果: 1)获得超高压变质锆石U-Pb年龄为242 Ma; 2)继承锆石U-Pb年龄,除了有比较多的属于元古代(768～639 Ma),还有大量的属于古生代(590～358 Ma)等,表明其寄主岩的时代不会早于古生代。     

The changing nature and European perceptions of Europe's refugee problem

Many in Western Europe regard it as being in the grip of a refugee crisis. This paper considers this contention. It describes the main stages in the development of the current refugee crisis and argues that having established the agenda in the 1940s, Western Europe has been able to progressively distance itself from refugee episodes and define them as essentially problems of the Third World. The end of the Cold War and the growth in numbers of spontaneous, rather than quota, refugees have been considerable shocks to the West, which can no longer contain and distance itself from refugee flows. Shock has led Europe to overreact and become myopic in its view of refugee matters. In reality, figures demonstrate that Wetern Europe is not the centre of refugee generation, applications or resettlement. Despite this, governments have taken draconian unilateral and multilateral action to exclude not only Third World asylum seekers but also those from the East.     

The upper geochronological limit for dunites of the East Khabarny gabbro-ultramafic complex (South Urals) by the data of U-Pb age of zircons extracted from vein hornblendites

This paper discusses the results of U-Pb isotopic dating of zircons from hornblendites that cut dunites of the East Khabarny dunite-clinopyroxenite-websterite-gabbro complex in the South Urals. This complex is included into the structure of the large Khabarny mafic-ultramafic allochthon, and the set of rocks and their petrogeochemical peculiarities allow this complex to be compared with the Ural-Alaskan-type complexes. Zircons from hornblendites have been dated on a SHRIMP-II ion microprobe and by the classical thermo-ionization mass spectrometric method by five fractions. The obtained results are comparable between each other within the analysis deviation interval and show that hornblendites were formed in the Early Devonian (408–402 Ma). This age is the upper geochronological limit in estimation of the time when dunites of the East Khabarny complex were formed. The synchronity of intrusive hornblendite manifestation in most of the Urals in the Early Devonian is discussed. The probable Early Devonian stage, in addition to the found Vendian and Ordovician-Silurian ones when dunite-clinopyroxenite-gabbro complexes formed in the Urals, is suggested.     

A warm,equable Cretaceous: The nature of the problem

The nature of the problem of warm equable paleoclimates is defined by investigating the mid-Cretaceous period. The problem consists of three components: (1) defining precisely the climatic state during any specified interval of geologic time; (2) specifying the external climatic forcing factors which may have been important; and (3) understanding the climatic response to any specific modifying influence. These components are characterized by limitations of critical importance in understanding paleoclimates. The nature of the problem of a warm Cretaceous is defined both qualitatively and quantitatively within these limits.     

A study of complex discordance in zircons using step-wise dissolution techniques

Step-wise dissolution techniques applied to a variety of zircon samples, including those with combined inheritance plus later isotopic disturbance, reveal both the complexity of zircon isotopic systematics, and successfully “see through” this complexity to extract high-quality age information. The chemical procedures for the partial dissolution steps must be designed to extract completely all the U and Pb associated with the zircon digested in each step, in order to avoid large, laboratory-induced fluctuations in U/Pb ratio from step to step. In general, relatively short initial partial dissolution steps remove disturbed zircon domains characterized by very high U concentrations and low ²⁰⁶Pb/²³⁸U ages. In some cases the initial step yields the lowest ²⁰⁶Pb/²³⁸U age, reflecting both the high accessibility and solubility of the most disturbed domains. In other cases, disturbed domains evidently reside deep within the zircons, and are attacked only when the second or third steps penetrate to the interior via cracks or flaws to “mine out” these domains. In all the samples in this study, and regardless of the details of the steps, about a week of digestion time at 80°C removed most of the highly disturbed domains, leaving further partial dissolution steps and/or the total digestion of the final residue to sample highly refractory (i.e., highly insoluble), relatively low U zircon domains. The early partial digestion steps also remove virtually all the common Pb associated with the zircons. Study of partially digested zircons by scanning electron microscope reveals that at least in part, the disturbed, high-U, highly soluble domains and the less (un-?) disturbed, low-U, highly refractory domains are defined by primary igneous zoning on a micron or even sub-micron scale, well below the resolution of the “SHRIMP”, for example. In the case of zircons lacking any inheritance, the residue analyses yield near-concordant, highly precise results. In the case of zircons with inherited components, the residue analyses appear to plot on “primary” discordia lines. Depending on the relative solubility of the inherited versus the igneous component, the final residue will concentrate one or the other.     

安庆大龙山复式岩体锆石U-Pb年代学与地质意义

安徽安庆大龙山岩体位于扬子地台北东缘,发育大量中生代侵入岩浆岩。野外地质勘查发现,大龙山岩体可能为复式岩体,有多期岩浆作用,在其核部的正长岩附近发育一定量的闪长玢岩。对闪长玢岩和正长岩进行年代学研究,其锆石年龄分别为（137.7±1.9）Ma和（128.8±2.1）Ma。其中,在大龙山闪长玢岩中发育一定量的继承锆石,形成年代分别为古元古代（约2.0 Ga）和新元古代（约0.8 Ga）,在正长岩中未发现捕获继承锆石。根据获取的继承锆石年龄,结合区域地质演化资料,推断大龙山岩体经历了复杂的演化过程：古元古代的褶皱“会聚”到伸展体制转换的“回返”,新元古代的拉张垮塌,中生代的多期次岩浆侵入和多来源、多阶段的结晶分异。