Granitoids of the Ufalei block (South Urals): Sr-Nd isotope systematics, geodynamic position and genetic reconstructions

Petrogeochemical and isotopic-geochronological signatures in granitoids developed in structures with complex geological history represent an important feature for reconstructing paleogeodynamic settings. Granitoids are widespread in the western slope of the Urals, where the Uralian Orogen contacts via a collage of different-age blocks of the east European Platform. The Ufalei block located in the Central Urals megazone at the junction between the South and Middle Urals’ segments represents one such boundary structure with multistage geological evolution. The isotopic ages obtained by different methods for acid igneous rocks range from 1290 to 245 Ma. We determined close Rb-Sr and Sm-Nd ages (317 Ma) for granites of the Nizhnii Ufalei Massif. By their petrochemical parameters, granitoids and host granite-gneisses differ principally from each other: the former are close to subduction-related, while the latter, to continental-riftogenic varieties. The primary ratio (⁸⁷Sr/8⁶Sr)₀ = 0.70428 and ?_Nd ≈ +4 values indicate significant contribution of oceanic (island-arc?) material to the substrate, which served as a source for granites of the Nizhnii Ufalei Massif. Model Nd ages of granites vary from 641 to 550 Ma. Distinct oceanic rocks and varieties with such ages are missing from the surrounding structures. New isotopic dates obtained for ultramafic and mafic rocks from different zones of the Urals related to the Cadomian cycle imply development of unexposed Upper Riphean-Vendian “oceanic” rocks in the central part of the Ufalei block, which played a substantial role in the formation of the Nizhnii Ufalei granitoids. Such rocks could be represented, for example, by fragments of the Precambrian Timanide-type ophiolite association. The analysis of original materials combined with published data point to the heterogeneous composition and structure of the Ufalei block and a significant part of the western segment of the Central Uralian Uplift and extremely complex geological history of the region coupling the Uralian Orogen with the East European Platform in the present-day structure.     

Geochemistry of silver in oxidized zone of sulfidic deposits

We studied distribution of vein formations (from quartz veins and pegmatites of different types to migmatites) in three regions with well-developed metamorphic zonation, with a similar temperature range of metamorphism but a variable total pressure. We considered the tectonic position of the metamorphic belts, the geochemical features of the progressive stage of metamorphism, granitization, and the formation of the pegmatite veins. After analyzing the geological and geochemical data, and physicochemical parameters (T, P_total, P_stress, and P_fl), we conclude that change in pressure and the composition of the volatile components plays the principal role in the degree of development of particular types of pegmatites in the various facies series of metamorphism, and that the distribution of pegmatites of different types is determined by the relationships between the temperature of crystallization of the melts and that of the surrounding rocks. —Authors.     

东阿拉善地块前寒武纪变质基底中晚古生代变质杂岩:来自波罗斯坦庙杂岩LA-ICP-MS锆石U-Pb定年的新证据

波罗斯坦庙杂岩是东阿拉善地块典型的中-高级变质杂岩之一,它的深入研究对进一步认识阿拉善地块起源、形成与演化过程具有十分重要的科学意义.通过对波罗斯坦庙杂岩中石英闪长质片麻岩、斜长角闪岩与花岗伟晶岩的野外观察、岩石学与锆石U-Pb定年发现,波罗斯坦庙杂岩中石英闪长质片麻岩、斜长角闪岩与花岗伟晶岩的岩浆锆石加权平均年龄分别为284±2 Ma、278±3 Ma,276±2 Ma、271±3 Ma与242±7 Ma,该组年龄被解释为其原岩成岩时代.石英闪长质片麻岩与斜长角闪岩中变质锆石加权平均年龄分别为274±6 Ma、272±5 Ma与269±3 Ma、268±2 Ma,代表它们遭受了晚古生代变质作用的时间.综合以上分析与前人研究资料,初步认为以往曾被认为是新太古代-古元古代的波罗斯坦庙杂岩,实际上可能是一个古元古代-晚古生代中-高级变质杂岩,并遭受了280~260 Ma角闪岩相变质作用,指示它们曾被卷入到中亚造山带晚古生代造山作用过程.      

Geological structure and genesis of quartz veins in the Ufalei complex exemplified by the Vein 2136

The data on the geological structure and petrography of host rocks of the Vein 2136 quartz deposit located in the eastern part of the Ufalei gneiss–amphibolite complex (the Southern Urals) are discussed. It is established that the deposit resulted from the metasomatic replacement of amphibolite by quartz. The following metasomatic succession was formed during metasomatosis: quartz → quartz–feldspar pegmatoid → biotite schist → amphibolite. The investigation of homogenized vapor–liquid inclusions in quartz and feldspars revealed that the deposit was formed at temperatures of 350–400°C.     

The nature of fluids during pegmatite development in metamorphic terrains: Evidence from Hamadan complex,Sanandaj-Sirjan metamorphic zone,Iran

In the Sanandaj-Sirjan zone of metamorphic belt of Iran, the area south of Hamadan city comprises of metamorphic rocks, granitic batholith with pegmatites and quartz veins. Alvand batholith is emplaced into metasediments of early Mesozoic age. Fluid inclusions have been studied using microthermometry to evaluate the source of fluids from which quartz veins and pegmatites formed to investigate the possible relation between host rocks of pegmatites and the fluid inclusion types. Host minerals of fluid inclusions in pegmatites are quartz, andalusite and tourmaline. Fluid inclusions can be classified into four types. Type 1 inclusions are high salinity aqueous fluids (NaCl_eq >12 wt%). Type 2 inclusions are low to moderate salinity (NaCl_eq <12 wt%) aqueous fluids. Type 3 and 4 inclusions are carbonic and mixed CO₂-H₂O fluid inclusions. The distribution of fluid inclusions indicate that type 1 and type 2 inclusions are present in the pegmatites and quartz veins respectively in the Alvand batholith. This would imply that aqueous magmatic fluids with no detectable CO₂ were present during the crystallization of these pegmatites and quartz veins. Types 3 and 4 inclusions are common in quartz veins and pegmatites in metamorphic rocks and are more abundant in the hornfelses. The distribution of the different types of fluid inclusions suggests that CO₂ fluids generated during metamorphism and metamorphic fluids might also contribute to the formation of quartz veins and pegmatites in metamorphic terrains.     

中国云南哀牢山含海蓝宝石伟晶岩的起源和成岩模拟实验研究

The Ailaoshan aquamarine-bearing pegmatites are associated with Proterozoic metamorphic rocks in the southern portion of the Ailaoshan fault-folded complex.The gem-bearing pegmatite mineralization zones of the region occur in areas generally consistent with the regional tectonic trend.The pegmatites are found in metamorphic rocks,migmatites and in the inner/outer contact zones of gneissoid granites. The Rb-Sr isochron drawn for the pegmatites is 26～31 Ma,(i.e.in Himalayan).The homogenization temperatures of melt and liquid inclusions in minerals vary from 185 to 920℃,which are comparable to the inclusions observed in banded migmatites and ptygmatic quartz veins in the surrounding metamorphic rocks. The mineralization fluids of the pegmatite were rich in HCO_3 and CO_2,and their compositional assemblages are comparable to metamorphic fluids.Results of H,O,C,Si etc.isotopic analyses and REE,and Be analyses indicates that the sources of mineralization components that formed the pegmatites are closely associated with metamorphic fluids and the enclosing metamorphic rocks. A pegmatite structure simulation experiment was conducted at high temperature and pressure(840℃and 1,500×105Pa.),with various metamorphic rock samples in a water-rich and volatile-rich environment.When the liquidus was reached,the temperature was gradually decreased at the rate of 5～10℃/day over a time period of three months.SEM energy-dispersive spectrum analyses were performed on the experimental products.A series of pegmatoid textures were observed including zonal texture,megacryst texture,drusy cavities,crystal druses,and vesicular texture along with more than ten types of minerals including plagioclase,microcline,quartz and biotite.Different metamorphic rock melts generated different mineral assemblages.Experiment results revealed that the partial melting of metamorphic rocks could form melts similar to pegmatite magmas. Based upon the geological characteristics,geochemistry,and pegmatite texture simulation experimental results,it is concluded that the mineralization components of Ailaoshan aquamarine-bearing pegmatites came from metamorphic rocks.The petrogenetic model for the origin of pegmatites is related to ultrametamorphism and metamorphic anatexis.     

庐山“星子变质核杂岩”中伟晶岩锆石U-Pb年龄及其地质意义

赣北庐山地区出露的“星子变质核杂岩”是中国东南部迄今为止发现的较为典型的科迪勒拉型变质核杂岩之一.该变质核杂岩核部见有大量的, 与拆离断层同时形成并侵入于构造弱带的伟晶岩脉.通过锆石U-Pb法定年, 获得伟晶岩的年龄为(127±2)Ma, 该年龄代表了庐山“星子变质核杂岩”的隆升年龄, 并得到其他地质证据的支持.还讨论了形成变质核杂岩的岩石圈伸展作用与同时期中国东南部大规模岩浆活动的时空关系及其可能的联系.      

世界伟晶岩型锂矿床地质研究进展

稀有金属伟晶岩主要分为LCT (Li-Cs-Ta)型和NYF (Nb-Y-F)型,其中LCT型伟晶岩是全球重要锂矿来源.本文分析了全球伟晶岩型锂矿床的地质勘查和研究成果,简要介绍了各大陆代表性伟晶岩型锂矿床,发现锂矿空间分布不均匀,成矿时间具有多期性和阶段性,成矿事件主要发生在汇聚造山作用的晚期,伴随超大陆汇聚事件.LCT型伟晶岩富含挥发分,与后碰撞S型花岗岩密切相关,多产于中高级变质岩区,就位深度较大,多沿断裂构造贯入.     

The miarolitic pegmatites from the K?nigshain: a contribution to understanding the genesis of pegmatites

In this paper, we show that the crystallization of miarolitic pegmatites at K?nigshain started at about 700°C, in melts containing up to 30 mass% water. Such high water concentration at low pressures (1–3 kbar) is only possible if the melts are peralkaline. Such peralkaline melts are highly corrosive, and reacted with the wall rock—here the granite host—forming the graphic granite zone, in part via a magmatic–metasomatic reaction. With cooling, the water concentration in some melt fractions increased up to 50 mass% H₂O. The melt-dominated system ends below 600°C and passes into a fluid-dominated system, the beginning of which is characterized by strong pressure fluctuations, caused by the change of OH and CO₃ ²⁻ in the melt, to molecular water and CO₂. We note two generations of smoky quartz, one crystallized above the β–α-transition of quartz (≈573°C), and one below, both of which contain melt inclusions. This indicates that some melt fraction remains during at least the higher-temperature portion of the growth of minerals into the miarolitic cavity, contradicting the view that minerals growing into a pegmatite chamber only do so from aqueous fluids. We show that the K?nigshain miarolitic pegmatites are part of the broad spectrum of pegmatite types, and the processes active at K?nigshain are representative of processes found in most granitic pegmatites, and are thus instructive in the understanding of pegmatite formation in general, and constraining the composition and characteristics of pegmatite-forming melts. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

The influence of the local pressure gradient and of the metamorphic grade on the composition of pegmatites in metamorphic terrains

It is proposed that the composition of pegmatites in metamorphic terrains is dependent on the local pressure gradient of the zone in which they are formed. Pegmatites having a composition similar to that of the host rock are formed in areas of shallow pressure gradient, while the composition of pegmatites that developed under a steep pressure gradient differs from that of the host rock. Relationships between the metamorphic environment, pressure gradient and composition of the pegmatites are investigated.     

Composition and chemical microprobe dating of U-Th-bearing minerals. Part I. Monazites from the Urals and Siberia

To develop chemical microprobe timing of U-Th-bearing minerals, monazite grains from several localities in the Ural and Siberia have been dated using upgraded measurement techniques and age calculation based on original software. The samples were taken from pegmatites of the Ilmeny Mountains and the Ilmeny-Vishnevy Mountains Complex in the South Urals; pegmatites from the Adui granitic pluton and its framework in the Central Urals; gneisses and granulites of the Taratash Complex in the South Urals; and felsic gneisses from the Transangara region of the Yenisei Ridge. Scrutiny of the composition, heterogeneity, and chemical substitution of U and Th ions is a necessary stage of chemical dating aimed at estimating the degree of closeness of the U-Th-Pb system and unbiased screening of analytical data. The results obtained have been compared with the known isotopic ages of the studied minerals; the compared data are satisfactorily consistent.     

<Emphasis Type="Italic">P</Emphasis>–<Emphasis Type="Italic">T</Emphasis> and fluid evolution of barren and lithium pegmatites from Vlastějovice,Bohemian Massif,Czech Republic

Fluid inclusions, mineral thermometry and stable isotope data from two types of mineralogically and texturally contrasting pegmatites, barren ones and lithium ones, from the Moldanubian Zone of the Bohemian Massif were studied in order to constrain P–T conditions of their emplacement, subsolidus hydrothermal evolution and to estimate composition of the early exsolved fluid and that of the parental melt. Despite the fact that the lithium pegmatites are abundant throughout the crystalline units of the Bohemian Massif, data similar to this paper have not been published yet. The studied pegmatites are hosted by iron-rich calcic skarn bodies. This specific setting allowed scavenging of calcium, fluorine and some other elements from the host rocks into the pegmatitic melts and post-magmatic fluids. Such contamination process was important namely in the case of barren pegmatites, as can be deduced from the variation in anorthite contents in plagioclase and from the presence of fluorite, hornblende (with F content) or garnet in the contact zones of pegmatite dykes. Fluid inclusions were studied mostly in quartz, but also in fluorite, titanite and apatite. Early aqueous–carbonic and late aqueous fluids were identified in both pegmatite types. The P–T conditions of crystallization as well as the detailed composition of exsolved magmatic fluid, however, particularly differ. The magmatic fluids associated with barren pegmatites correspond to H₂O–CO₂ low salinity fluids, composition of which evolved from 20 to 23 to <5 mol% CO₂, and from 2 to 4–6 mol% NaCl eq. Sudden decrease in the CO₂ content of the post-magmatic fluids (<5 mol% CO₂) seems to coincide with the enrichment of the fluid in calcium (from the contamination process) and resulted in precipitation of calcites (frequently found as trapped solid phases in fluid inclusions). The fluids associated with lithium pegmatites are more complex (H₂O–CO₂/N₂–H₃BO₃–NaCl). The CO₂ content of early exsolved fluid is 26–20 mol% CO₂ and remains the same in the next fluid generation. The main difference between the magmatic and the first post-magmatic fluids is the presence of 7–9 wt% of H₃BO₃ (identified as daughter mineral sassolite) in the former. The second post-magmatic fluids are again CO₂-poor (∼4 mol%) and more saline (∼4 mol% NaCl eq.). The composition of exsolved fluid was further used to constrain volatile composition and content of the parental melts. Finally, P–T conditions of pegmatite crystallization are constrained: 600–640°C and 420–580 MPa for the barren pegmatites and 500–570°C and 310–430 MPa for the lithium pegmatite. While the emplacement of the former occurred in thermal equilibrium with the Moldanubian host rock environment, the emplacement of the later suggests substantial thermal disequilibrium.     

Structural evolution of the Main Ural Fault Zone in the western framework of the Voikar-Synya ophiolite massif

Multistage deformations in the Main Ural Fault Zone is recorded in the structural features of this zone in the southern part of the Polar Urals. The deformation of the oldest metamorphic complexes in the Khord??yus massif and Dzelyayu block developed during the precollision stage. After formation of the general nappe-thrust structure of the Urals, these blocks were squeezed to the higher crustal levels. Deformation in other tectonic units started at the early collision stage during regional thrusting. Brittle failure was superposed over the all previously formed structures at the late collision and postcollision stages.     

The spodumene deposit at “Weinebene”, Koralpe,Austria

The Weinebene spodumene deposit is located in Carinthia, about 270 km southwest of Vienna. It is hosted by the medium- to high-grade metamorphic rocks of the Koralpe. The spodumene-bearing pegmatites form unzoned, dikelike bodies in eclogitic amphibolites and kyanite-bearing micaschists with concordant foliations. Their lateorogenic emplacement is probably of Variscan age. A younger, probably Alpine metamorphic event affected the pegmatites; it caused extensive recrystallization of the micaschist-hosted pegmatites, but had only a minor impact on the amphibolite-hosted ones. The spodumene-bearing pegmatites have been traced over a length of 1.5 km along strike and to about 450 m down dip. Their average thickness is 2 m, with a maximum of 5.5 m. Emplacement of the pegmatites caused an alteration halo of several dm in the hosting amphibolites, characterized by biotitization and formation of holmquistite. An aplitic spodumene-free seam of about 10 cm symmetrically borders the pegmatites. No contact phenomena are observed along the micaschist-hosted dikes. Spodumene is the only lithium-bearing mineral. Its average content is 22 vol% (equivalent to 1.68 wt% Li₂O) in the amphibolite-hosted pegmatites and 15 vol% (equivalent to 1.13 wt% Li₂O) in the micaschist-hosted dikes. Such a significant difference is also shown by rubidium (1,100 ppm vs 880 ppm) and cesium (60 ppm vs 25 ppm). Beryllium and tin are the only other significant trace elements. They show average contents of about 100 ppm and 120 ppm, respectively, with maxima of 1,690 ppm beryllium and 1,500 ppm tin. No correlation is found between lithium, tin, and beryllium. Niobium and tantalum are very low. A Na/K ratio of 1.2 emphasizes the predominance of albite over microcline. There is no discernable correlation between spodumene and albite contents, or between lithium and any other alkalies. The average K/Rb ratio is 20, pointing to the high degree of fractionation. Spodumene contains 7.4% Li₂O and 0.45% FeO. There is no difference in the spodumene chemistry between the amphibolite-hosted and the micaschist-hosted pegmatites. Strong structural and textural evidence, mineral zoning, bulk composition corresponding to thermal minima in relevant experimental systems, and trace-element signature support an igneous derivation of the pegmatites. However, no granitic intrusion is exposed in the area, and the pegmatites could have been displaced from their source by tectonic events.     

黑龙江依兰地区变玄武岩及变堆晶辉长岩LA-ICP-MS锆石U-Pb年龄及其地质意义

对依兰地区黑龙江混杂岩中的变玄武岩、变枕状玄武岩及变堆晶辉长岩岩块中的锆石进行了LA-ICP-MS U-Pb同位素测定,结果表明,变玄武岩的锆石大量为捕获基底的锆石,其年龄范围大,从3172±24Ma到176±1Ma均有分布,其中具有基性岩典型的板状特征的锆石年龄为243±1Ma,代表了变玄武岩的原岩形成年龄;变枕状玄武岩及变堆晶辉长岩的年龄分别为251±1Ma和252±1Ma,分别代表各自原岩的形成年龄。本次锆石年龄的获得,证明晚二叠世佳木斯地块和松嫩-张广才岭地块之间存在古洋盆,为研究黑龙江杂岩的演化历史提供了重要的年代学依据。     

Geochemistry of phlogopite, diopside, calcite, anhydrite and apatite pegmatites and syenites of southern Madagascar: evidence for crustal silicocarbonatitic (CSC) melt formation in a Panafrican collisional tectonic setting

The phlogopite, diopside, calcite, anhydrite and apatite pegmatites of Ampandrandava and Beraketa are examples for the many other pegmatites of similar silicocarbonatitic composition found in the Bekily and Betroka-Beraketa Precambrian belts of southern Madagascar. The two studied pegmatites and associated syenites crystallised from immiscible silicocarbonatitic and peralkaline syenitic melts in a time span between 515 and 504 Ma in the final extensional phase of the Panafrican continental collision and connected metamorphic/metasomatic event. Model T _Nd ages suggest that the melts were produced by partial melting of 3.5 Ga partially evaporitic continental crust. The studied pegmatites and genetically associated syenitic rocks are very rare examples for crustal silicocarbonatitic melts generated in a Panafrican collisional setting. The overwhelming majority of carbonatites and associated peralkaline rocks are mantle derived, much poorer in phosphate and sulfate and found in a cratonic environment. In light of the present results, genetic models for other sulfate- and phosphate-rich magmatic rocks (e.g., phlogopite–apatite–calcite mineralisations in the Grenville-Hasting formation in Canada and in the Sludyanka group in Eastern Siberia) should be reevaluated.     

西藏尼木地区变质火山岩的地球化学特征、锆石U-Pb年龄及其地质意义

西藏尼木地区分布一套增生杂岩,其对冈底斯火山岩浆弧的演化具有重要意义。本文对该套增生杂岩中的变质火山岩的地质特征、矿物学、全岩地球化学和锆石U-Pb年代学等方面进行了综合研究。变质火山岩以斜长角闪岩、角闪斜长片麻岩为主,其中,角闪石主要为镁质角闪石,共生的斜长石主要为拉长石;变质火山岩经历了高温-中压变质作用;岩石富Al_2O_3和贫TiO_2,弱富集轻稀土元素(LREE),富集Rb、Sr、Ba等大离子亲石元素(LILE),亏损Nb、Ta、Ti等高场强元素(HFSE),其地球化学特征与火山弧玄武岩的地球化学特征相似,其形成的构造环境为洋内岛弧或活动大陆边缘弧;岩浆锆石U-Pb年龄值为151.4±1.6Ma和150.7±1.4Ma,表明岩石的形成时代为晚侏罗世。综合研究认为,增生杂岩中的变质火山岩是新特提斯洋在晚侏罗世北向俯冲的产物,在陆-陆碰撞之前卷入增生系统,该套变质火山岩不是以往所认为的变质结晶基底。     

巴西卡拉加斯地区基底杂岩的地质特征与时代约束

巴西卡拉加斯(Carajás)地区出露世界上重要且古老的太古宙变质基底,是世界上矿床类型最为丰富、资源聚集程度最高的成矿区之一,它的基底兴谷(Xingú)杂岩是南美克拉通古老的太古宙花岗岩-绿岩地体。在调查该地区基底杂岩地质特征的基础上,对侵入其中的变质深成岩体进行了年代学研究,提出了基底杂岩的组成、结构与构造的认识,认为兴谷杂岩是以麻粒岩相-角闪岩相片麻岩和混合岩为主体的古老变质岩,将其中的紫苏花岗岩和英云闪长质-奥长花岗质片麻岩从中剥离出来,进一步分解出不同时期的变质侵入体;本次在其中的片麻状花岗岩中获得了(2899±45)Ma、(2742±9.5)Ma和(2831±19)Ma的锆石LA-ICP-MS年龄,进一步确认兴谷杂岩的时代为中太古代,时代约束在3.05~2.85 Ga,其中包含3.05~2.96 Ga和2.96~2.85 Ga的两个构造时段的表壳岩和TTG片麻岩套。     

关于华南大地构造问题的再认识

自８０年代以来，华南大地构造格局问题的讨论是我国地学的热点之一。有关华南陆块地质构造演化及形成时代存在多家之说，其中“江南古陆”是否存在与原“板溪群”时代的归属是争论的焦点。许多学者从不同的角度提出了各种观点和构造演化模式。从近几年来在皖南、赣东北地区的一系列新发现，可以得出如下看法：（１）“板溪群”不是一个前震旦纪的变质地层单位；（２）“江南古陆”是不存在的；（３）华南不存在加里东后地台，而存在晚古生代洋盆，它可能是东特提斯洋的一部分；（４）印支运动在赣东北地区有重要的表现，晚古生代浅变质岩和蛇绿混杂岩是它的岩石记录     

沁水盆地南部柿庄南区块煤层气地质特征

以国家科技重大专项《山西沁水盆地南部煤层气直井开发示范工程》成果为依据,以以往的地质勘查及研究成果为参考,对沁水盆地南部柿庄南区块煤层气地质特征进行分析。分析认为南部柿庄南区块构造简单,煤变质程度高、煤层厚度大、埋深适中而且分布稳定,有利于煤层气生成,煤层吸附能力较强,储层渗透率较好,煤层气保存条件好,是沁南煤层气勘探开发最有利区块之一。开发利用该区块煤层气资源,可有效改善地区能源结构、加快区域经济发展,降低后期煤炭开发风险,具有非常可观的社会及经济效益。