Early Stages of the Evolution of Uralides as Evidenced from the U–Pb Systematics of Detrital Zircons from Rift Complexes

The U–Pb isotope data and corresponding ages of detrital zircons from rocks of the basal complexes of the Uralides of different segments of the Ural Fold Belt are considered. It was established that complexes of ancient domains of the East European Platform (Volga-Uralia, Sarmatia, Kola, etc.) seem to have been the main provenance areas of the clastic material for the Southern, Middle, and Northern Urals. This means that there were relatively remote and local (igneous formations of the pre-Uralides) provenance areas. Rift rock associations of the Uralides of the Subpolar and Polar Urals were formed mainly through erosion of local provenance areas (predominantly, Late Riphean–Vendian island-arc and orogenic magmatic complexes of the Proto-Uralides–Timanides). Detrital zircons of Riphean age dominate in rocks of the basal complexes of the Uralides. A source for them could have been rock complexes of Svecofennian-Norwegian Orogen and Cadomides of the Scythian-Turan Plate, intraplate magmatic formations, and metamorphic complexes, as well as blocks accreted to the margin of the East European Platform in the Late Precambrian–Cambrian and later detached and displaced during the Ordovician rifting and spreading. In general, the basal complexes of Uralides were formed owing to supply of clastic material from both remote and local sources. Despite the appearance of information of a totally new level (U–Pb isotope ages of detrital zircons, their Lu–Hf systematics, and the distribution features of rare earth and trace elements), the contribution of these sources to the formation of the Late Cambrian–Early Ordovician clastic strata is hardly possible at present to evaluate.     

First results of U–Pb dating of detrital zircons from the Upper Ordovician sandstones of the Bashkir uplift (Southern Urals)

The first results of U–Pb dating of detrital zircons from Upper Ordovician sandstones of the Bashkir uplift in the Southern Urals and U–Pb isotopic ages available for detrital zircons from six stratigraphic levels of the Riphean–Paleozoic section of this region are discussed. It is established that the long (approximately 1.5 Ga) depositional history of sedimentary sequences of the Bashkir uplift includes a peculiar period lasting from the Late Vendian to the Emsian Age of the Early Devonian (0.55–0.41 Ga). This period is characterized by the following features: (1) prevalence of material from eroded Mesoproterozoic and Early Neoproterozoic crystalline complexes among clastics with ages atypical of the Volga–Urals segment of the East European Platform basement; (2) similarity of age spectra obtained for detrital zircons from different rocks of the period: Upper Vendian–Lower Cambrian lithic sandstones and Middle Ordovician substantially quartzose sandstones.     

黑龙江省东北部多宝山地区中奥陶世侵入岩锆石U-Pb年龄及构造环境

多宝山地区位于兴蒙造山带东段扎兰屯-多宝山岛弧构造带,早古生代地质体发育,且与区域成矿作用关系密切. 通过对多宝山地区中奥陶世侵入岩的岩相学、岩石地球化学、同位素年代学特征的研究,对其形成时代、构造环境及成矿作用进行了探讨. 定年结果显示花岗闪长岩LA-MC-ICP-MS锆石U-Pb年龄为488±3 Ma,形成时代为中奥陶世;岩浆物质来源以幔源为主,同熔了部分壳源物质,伴随强烈的构造热事件和岩浆侵入喷发活动,大量深源流体带来Au、Cu等成矿元素,在有利成矿条件和扩容空间下,形成多宝山铜（钼）矿床. 研究显示,中奥陶世花岗闪长岩形成于与板块俯冲有关的岛弧环境.     

东昆仑南缘哥日卓托地区马尔争组沉积物源分析：碎屑锆石U-Pb年代学证据

为研究东昆仑南缘中下二叠统马尔争组沉积物源及沉积构造背景,对东昆仑南缘哥日卓托地区中下二叠统马尔争组进行了详细的沉积地层划分、沉积环境及碎屑锆石U-Pb年代学进行了研究。结果表明,马尔争组为一套形成于大陆斜坡半深海-深海环境的浊积岩系。碎屑锆石U-Pb年龄谱可明显划分为早古生代和新元古代两个主年龄谱及古、中元古代两个次级年龄谱。主年龄谱分别为396~573Ma和727~947Ma,峰值年龄分别为421 Ma和862Ma。次级年龄谱分别为1117~1993Ma和2319~3063Ma,峰值年龄不明显。本文认为东昆仑南缘哥日卓托地区马尔争组物质来源较为复杂,显示早古生代、新元古代、中元古代和古元古代多个时代物源共同供给的特征。东昆仑造山带早古生代岩浆岩和新元古代岩浆岩为其提供了约60~65%的沉积物源,而古老的变质基底为其提供了仅约30~35%的沉积碎屑。综合区域资料认为马尔争组形成于相对稳定的被动大陆边缘沉积构造背景,该期阿尼玛卿古特提斯洋还未开始向北俯冲。     

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.     

准噶尔—天山—北山蛇绿岩:对中亚造山带西南缘洋陆格局演化的制约

准噶尔、天山和北山52个蛇绿岩的地质特征、地球化学性质和同位素年代学资料系统集成研究表明它们可以分为14条蛇绿(混杂)岩带。绝大多数蛇绿岩呈"岩块+基质"的混杂岩型式沿重要断裂带(构造线)线状分布,少数蛇绿岩以构造岩片叠置方式面状产出。混杂岩的基质有蛇纹岩(碳酸盐化蛇纹岩)和糜棱岩化细碎屑岩两类,岩块既有地幔橄榄岩、基性杂岩和基性火山岩等蛇绿岩组分,也有其它非蛇绿岩组分岩石。堆晶岩出露局限,典型席状岩墙群没有发育。这些蛇绿岩可归类为SSZ(Supra-Subduction Zone)和MORB(Mid-Ocean Ridge)两种类型,前者玄武岩具大离子亲石元素(LILE)富集和高场强元素(HFS)亏损特征,后者不显示该特点;洋岛玄武岩(OIB)既可出现在SSZ型蛇绿混杂岩中,也可为MORB型的组成部分;SSZ型蛇绿混杂岩辉长岩和玄武岩比MORB型具有相对更富集的Sr-Nd同位素组成,但部分形成于弧后(间)盆地的SSZ型蛇绿岩与MORB型一致,具有近亏损地幔的Sr-Nd同位素组成。已确认的最老蛇绿岩为西准噶尔572 Ma玛依勒,次之为北山542~527 Ma月牙山—洗肠井和西准噶尔531 Ma唐巴勒,最年轻蛇绿岩为325 Ma北天山巴音沟和321 Ma北山芨芨台子。根据蛇绿岩证据,结合近年来中亚造山带古地磁、岩浆岩、高压—超高压变质岩和构造地质方面的进展,可以推断埃迪卡拉纪末期—早寒武世,古亚洲洋已达到一定规模宽度,发育洋岛和洋内弧;早古生代时期,多岛洋格局发育至鼎盛期,一系列弧地体分别归属哈萨克斯坦微陆块周缘的科克切塔夫—天山—北山线性弧、成吉思弧、巴尔喀什—西准噶尔弧体系和西伯利亚南部大陆边缘弧体系;晚古生代时期,古亚洲洋于石炭纪末期闭合,增生杂岩和弧地体组成哈萨克斯坦拼贴体系和蒙古拼贴体系两个巨型山弯构造。     

Age of zircon from apoharzburgite serpentinite representing mantle of the Uralian paleoocean

Over 60 zircon grains from apoharzburgite serpentinite were dated using SHRIMP–IIe/mc at the Laboratory IBERSIMS of the Granada University (Spain). The apoharzburgite serpentinite represents an oceanic mantle of the Uralian paleoocean, which was exhumed in the crustal structures of the Paleozoic Ural Mobile Belt during obduction. Individual grains span a huge ²⁰⁶Pb/²³⁸U age range from 2740 to 250 Ma and are clustered into six discrete age groups (in Ma): (I) > 2500, (II) 2500–1950, (III) 1260–1210, (IV) 480–400, (V) 370–330, and (VI) < 280. Two last groups were formed under the effect of granitoids on serpentinites. The traces of this effect were studied in outcrops and confirmed by age of zircon from contact talc–carbonate rock. The morphologies of zircon crystals from serpentinite bear signs typical of both magmatic and metamorphic varieties, which indicate their polygenetic–polychronous nature. No striking morphological features and peculiar U and Th contents were found in the studied zircons to discriminate unambiguously between different age groups. Pre-Paleozoic events with ages of groups I–III were found in zircons from many oceanic mantle rocks. The similarity of age groups of zircons from Paleozoic and modern oceanic lithosphere is caused by global mantle reworkings, which provoke magma generation and metasomatism probably accompanied by zircon crystallization.     

Polychronous formation of mantle complexes in ophiolites

The paper presents new determinations of the U-Pb zircon age of high-Al chromitite from dunite of the mantle section of the Voikar-Synya massif at the Kershor site in the boundary zone with rocks of the dunite-wehrlite-clinopyroxenite complex. The high-Cr chromitite from dunite in the central part of the same massif contains zircon dated at ca. 0.6 Ga [10]. It is suggested that Paleoproterozoic (2.0?1.9 Ga) zircons from chromitites of the mantle section near the petrological Moho boundary were formed in the course of partial melting of peridotites and/or their interaction with migrating MORB-type melts. The occurrence of Vendian and Paleoproterozoic zircons in chromitites from different parts of the mantle section, as well as previously published petrological, geochemical, and geological data [2, 11, 22] allow us to suggest a complex multistage evolution of the mantle section in ophiolites. The arguments stated below show that chromitites and host dunites could have been formed at different times and were probably related to different processes. Thus, not only various complexes of the pre-Paleozoic oceanic crust reworked in the suprasubduction setting differ in age, but also the mantle rock of similar petrography, vary in the time of their formation.     

KINEMATICS AND DEFORMATION OF THE RED SEA-GULF OF ADEN-ETHIOPIAN RIFT TRIPLE JUNCTION SINCE THE OLIGOCENE

Results of research on the geological, petrochemical, and isotopic-geochronological charac- teristics of plagiogranites from the Chelyuskin ophiolitic belt, on the northern part of East Siberia's Taymyr Peninsula, are presented. Petro-geochemical features and REE distributions for this tonalite-trondhjemite series resemble those of plagiogranites from different ophiolitic complexes. The plagiogranites considered here belong to the low-potassium series of ophiolitic mafics—gabbro, gabbro-dolerite dikes, and basalts. Their spatial relationships; low K₂O, Rb, Nb, Ta, U., and Th contents; similar REE patterns; and tonalite and trondhjemite Nd- and Sm-Nd- isotopic ratios typical of mafic rocks confirm the cogenetic nature of these rocks. Zircon U-Pb dating and an Sm-Nd isotopic study suggest a Late Riphean age for the plagiogranites. We regard the 740 ± 38 Ma age as the upper age boundary for the formation of the Chelyuskin ophiolitic belt, and Sm-Nd model ages (850–785 Ma) as its lower boundary. Tonalite-trond-hjemite series could be formed as a result of partial melting of previously formed oceanic crust in a back-arc environment, in association with minor portions of the sediments of the ancient arc.     

Late Vendian postcollisional leucogranites of Yenisei Ridge

The Late Vendian (540–550 Ma) U–Pb zircon age of postcollisional granitoids in the Osinovka Massif was obtained for the first time. The Osinovka Massif is located in rocks of the island-arc complex of the Isakovka Terrane, in the northwestern part of the Sayany–Yenisei accretion belt. These events stand for the final stage of the Neoproterozoic history of the Yenisei Ridge, related to the completing accretion of the oceanic crust fragments and the beginning of the Caledonian orogenesis. The petrogeochemical composition and the Sm–Nd isotopic characteristics support the fact that the granitoid melt originated from a highly differentiated continental crust of the southwestern margin of the Siberian Craton. Hence, the granite-bearing Late Riphean island-arc complexes were thrust over the craton margin at a distance considerably exceeding the dimensions of the Osinovka Massif.     

Sources and geodynamic environments of formation of Vendian-Early Paleozoic magmatic complexes in the Daribi range,Western Mongolia

Rock complexes composing the Daribi Range were produced in Late Vendian, Early Cambrian, and Early Paleozoic suprasubduction systems. All of the studied mafic and ultramafic magmatic mantle rocks (the post-Vendian ophiolite complex, Early Cambrian pillow basalts, and Early Paleozoic picrobasalts of the sill-dike complex) have geochemical characteristics typical of early evolutionary episodes of island arcs: low LILE concentrations, horizontal REE patterns or patterns close to those of N-MORB, and HFSE minima. The magmas were derived from depleted mantle sources of variable isotopic composition with ?_Nd(T) from +2.5 to +10. The Early Paleozoic rocks of the sill-dike complex were likely produced by a complicated interaction of melts derived from different sources. The rocks of group 1 resulted from the mixing of low-K picrite and tonalite melts. The picrite melts with ?_Nd(T) from +6 to +8 were melted out of garnet lherzolite in the mantle wedge. The tonalite melts with ?_Nd(T) = ?3 seem to have been formed by the partial melting of mafic oceanic rocks of a subducted slab or the bottom of an island arc. The trondhjemite melts of group 2 with ?_Nd(T) varying from 2.5 to 7.5 could be formed via the melting of subducted metapelites or amphibolites with low sulfide concentrations. Massifs of sodic Early Paleozoic granites also occur elsewhere in western Mongolia, Tuva, and the Altai territory. The generation of sodic silicic melts was likely a common process in supra-subduction systems in CAFB. The potassic granites (group 4) could be formed by the melting of subducted pelites or by the fractionation of mantle magmas. The genesis of the basaltic andesites (group 5) was likely related to Mesozoic-Cenozoic intraplate processes.     

Main stages and geodynamic regimes of the Earth’s crust formation in East Antarctica in the Proterozoic and Early Paleozoic

Geological, geochemical, and isotopic data (U-Pb for zircon and Sm-Nd for whole-rock samples) are summarized for Proterozoic and Early Paleozoic geological complexes known from various regions of East Antarctica. The main events of tectonothermal and magmatic activity are outlined and correlated in space and time. The Paleoproterozoic is characterized as a period of rifting in Archean blocks, their partial mobilization, and formation of a new crustal material over a vast area occupied by present-day East Antarctica. In most areas, this material was repeatedly reworked at the subsequent stages of evolution (1800–1700, 1100–1000, 550–500 Ma). Complexes of Mesoproterozoic juvenile rocks (1500, 1400–1200, 1150–1100 Ma) arising in convergent suprasubduction geodynamic settings are established in some areas (basalt-andesite and tonalite-granodiorite associations with characteristic geochemical signatures). The evolution of the Proterozoic regions in East Antarctica may be interpreted as a Wilson cycle with the destruction of the Archean megacontinent 2250 Ma ago and the ultimate closure of the secondary oceanic basins by 1000 Ma ago. The Mesoproterozoic regions make up a marginal volcanic-plutonic belt that combines three provinces of different ages corresponding to consecutive accretion of terranes 1500–1150, 1400–950, and 1150–1050 Ma ago. The Neoproterozoic and Early Paleozoic tectonomagmatic activity developed nonuniformly. In some regions, it is expressed in ductile deformation, granulite-facies metamorphism, and postcollision magmatism; in other regions, a weak thermal effect and anorogenic magmatism are noted. The evolution of metamorphic complexes in the regime of isothermal decompression and the intraplate character of granitoids testify to the collision nature of the Early Paleozoic tectonomagmatic activity.     

Structure of melt flow channels in the mantle

Structural events during the formation of the mantle peridotite section in the Voikar-Syn’ya massif of the Polar Urals are considered. The structural units of the mantle section were formed during several deformation stages. Dunite bodies in restitic peridotites were formed in the course of deformation that completed the formation of large-scale folds of high-temperature plastic flow of mantle material. The final stage of deformation accompanied by migration of melt through harzburgite occurred in the shallow mantle in the setting of suprasubduction spreading related to the ascent of a mantle diapir. The rate of plastic deformation was relatively low. As a result, the intracrystal translation gliding of dislocations in olivine was the main mechanism in both harzburgite and dunite. The paths of focused melt flow are marked by dunite veins and associated pyroxenite and chromitite. It is suggested that stress concentration in fold hinges and their abrupt relaxation with formation of orthogonal network of weakened zones with high permeability was one of the possible mechanisms of the formation of melt conduits. The dispersed melt ascending from a great depth spontaneously migrated toward these zones. The distribution and structure of chromitite bodies reflect multistage formation of dunite, nonstationary dynamics of melt flow through restite, and abrupt variations of local stress fields in the areas adjacent to melt conduits.     

Reconstruction of the mid-Devonian HP-HT metamorphic event in the Bohemian Massif (European Variscan belt)

Mid-Devonian high-pressure (HP) and high-temperature (HT) metamorphism represents an enigmatic early phase in the evolution of the Variscan Orogeny. Within the Bohemian Massif this metamorphism is recorded mostly in allochthonous complexes with uncertain relationship to the major tectonic units. In this regard, the Mariánské Lázně Complex (MLC) is unique in its position at the base of its original upper plate (Teplá-Barrandian Zone). The MLC is composed of diverse, but predominantly mafic, magmatic-metamorphic rocks with late Ediacaran to mid-Devonian protolith ages. Mid-Devonian HP eclogite-facies metamorphism was swiftly followed by a HT granulite-facies overprint contemporaneous with the emplacement of magmatic rocks with apparent supra-subduction affinity. New Hf in zircon isotopic measurements combined with a review of whole-rock isotopic and geochemical data reveals that the magmatic protoliths of the MLC, as well as in the upper plate Teplá-Barrandian Zone, developed above a relatively unaltered Neoproterozoic lithospheric mantle. They remained coupled with this lithospheric mantle throughout a geological timeframe that encompasses separate Ediacaran and Cambrian age arc magmatism, protracted early Paleozoic rifting, and the earliest phases of the Variscan Orogeny. These results are presented in the context of reconstructing the original architecture of the Variscan terranes up to and including the mid-Devonian HP-HT event.     

西秦岭楔的构造属性及其增生造山过程

西秦岭楔是叠置于早古生代造山作用基础上形成的并插入祁连和昆仑早古生代造山带内部的楔形地质体,以大面积出露三叠系并发育多条蛇绿混杂岩带、大型韧性剪切带、中生代火山-岩浆作用和斑岩-矽卡岩型矿床为典型特征,具有增生造山作用的典型特征。这些蛇绿混杂岩带和岛弧钙碱性火山-岩浆岩的形成时代均具有向南逐渐变年轻的空间演化特征,显示了特提斯洋演化过程中海沟具有向南撤退的基本特征。砂岩碎屑组成以及源区特征研究结果表明,西秦岭楔三叠系形成于活动大陆边缘,其碎屑沉积物来自于古特提斯洋北侧的增生杂岩及岛弧。丰富的岛弧钙碱性火山-岩浆岩和沉积组合以及赋存的斑岩-矽卡岩型矿床,均与东昆仑及南秦岭相一致,呈现出相似的岩石组合类型以及岩石地球化学和同位素地球化学特征。这些事实表明,三叠纪时期,东昆仑、西秦岭以及祁连造山带是一个有机整体,自西向东存在一条三叠纪增生岩浆弧。锆石Hf同位素及岩石地球化学成分结果则表明,该增生岩浆弧部分岩浆来自于俯冲增生杂岩的部分熔融。     

Ophiolitic Complex of the Matachingai River on Eastern Chukotka: Fragment of Lithosphere in Mesozoic Back-Arc Basin

The Matachingai River basin is known among the few ophiolitic complexes on eastern Chukotka as the southern boundary of the Chukotka Fold System (in terms of tectonics, the Chukotka microcontinent or a fragment of the Arctic Alaska–Chukotka microplate). This complex comprises tectonic blocks of residual spinel harzburgite with dunite bodies and pyroxenite, olivine gabbro, and leucogabbro veins; blocks of hornblende gabbro, diorite, and plagiogranite; and Upper Jurassic–Lower Cretaceous basaltic–cherty and cherty–carbonate rocks. The geological relationships of rocks within tectonic blocks, the compositions of primary minerals, the bulk geochemistry of rocks, as well as the strontium, neodymium, and lead isotopic compositions, make it possible to consider individual tectonic blocks of the complex as fragments of a disintegrated oceanic-type lithosphere that formed in a back-arc spreading center. The melts, crystallization products of which are represented by hornblende gabbro of blocks, olivine gabbro of veins, and basalts, separated from geochemically and isotopically heterogeneous mantle. Blocks composed of rocks with various modal composition are likely relicts of an oceanic lithosphere of different segments of a back-arc basin. The studied complex may be a lithosphere of one of the Middle–Late Jurassic back-arc basins. Fragments of these basins are retained in ophiolitic complexes on Great Lyakhovsky Island of the New Siberian Islands Archipelago, western Chukotka, and the Brooks Range in Alaska.     

西藏江玛地区闪长玢岩锆石U-Pb年龄、地球化学特征及其地质意义

目前对班公湖-怒江蛇绿混杂岩带的中酸性侵入岩的报道相对较少,对其成因和形成机制的研究有助于揭示班公湖-怒江特提斯洋构造带的岩浆作用过程和动力学背景.对西藏班公湖-怒江蛇绿混杂岩带江玛地区的闪长玢岩进行了年龄分析、岩石地球化学研究.闪长玢岩的LA-ICP-MS锆石U-Pb年龄为121±1 Ma,表明其形成于早白垩世晚期.岩石地球化学特征表明,闪长玢岩具高Al₂O₃含量（18.2%~19.3%）,属高铝玄武岩,岩石富集轻稀土元素和大离子亲石元素Rb、K等,亏损高场强元素Nb、Ta、Ti,属典型的岛弧玄武岩.综合区域地质资料认为,江玛地区闪长玢岩可能形成于俯冲带前缘增生的岛弧环境,是早白垩世期班公湖-怒江特提斯洋壳岩石圈南向俯冲消减背景下,板片脱水形成的流体与地幔楔橄榄岩发生交代作用的产物.      

Geochemical Features, Age, and Tectonic Significance of the Kekekete Mafic-ultramafic Rocks, East Kunlun Orogen, China

The Kekekete mafic-ultramafic rocks are exposed in the Kekesha-Kekekete-Dawate area,which are in the eastern part of the East Kunlun Orogenic Belt.It outcrops as tectonic slices intruding tectonically in the Paleoproterozoic Baishahe Group and the Paleozoic Nachitai Group.The Kekekete mafic and ultramafic rocks is located near the central fault in East Kunlun and lithologically mainly consists of serpentinite,augite peridotite,and gabbro.The LA-ICP-MS zircon U-Pb age of the gabbro is 501±7 Ma,indicating that Kekekete mafic-ultramafic rocks formed in the Middle Cambrian.This rock assemblage is relatively poor in SiO2 and(Na2 O+K2 O) but rich in MgO and SFeO.The chondrite-normalized REE patterns of the gabbro dip slightly to the right;the primitive mantle and MORBnormalized spidergrams of trace elements show enrichment of large-ion lithophile elements(Cs,Rb,Ba,etc.) and no differentiation of high field strength elements.The general dominance of E-MORB features and the geochemical characteristics of OIB suggest that the Kekekete mafic-ultramafic rocks formed in an initial oceanic basin with slightly enriched mantle being featured by varying degrees of mixing of N-MORB depleted mantle and a similar-OIB-type source.From a comprehensive study of the previous data,the author believes that the tectonic history of the East Kunlun region was controlled by a geodynamic system of rifting and extension in the late stages of the Neoproterozoic to early stages of the Early Paleozoic and this formed the paleo-oceanic basin or rift system now represented by the ophiolites along the central fault in East Kunlun,the Kekekete mafic-ultramafic rocks and Delisitan ophiolite.     

Intrusive magmatism during early evolutionary stages of the Ural epioceanic orogen: U-Pb geochronology (LA ICP MS,NORDSIM, and SHRIMP II), geochemistry,and evolutionary tendencies

In recent years extensive data have been obtained on all geologically important intrusive complexes in the Central and Southern Urals by U-Pb zircon geochronologic high spatial resolution techniques (LA ICP MS, NORDSIM, and SHRIMP II). This made it possible to revise the current concepts for the magmatic activity of the Ural Paleozoic orogen.Intrusive magmatism that occurred early in the evolution of the Ural orogen was focused mostly in the Tagil megazone, was characterized by several common features, and took place nearly simultaneously within both of its zones: the Platinum Belt and the Tagil volcanic zone.The composition of the parental magmas of all complexes of this age corresponded to an ultramafic or mafic source; i.e., the magma was derived from a mantle source. The gabbroids most closely approximating the composition of the parental magmatic melts show geochemical features of suprasubduction melts, such as negative HFSE (Nb, Ti, and Zr) and positive Ba and Sr anomalies. The REE patterns of these rocks display variable La/Lu ratios, which are usually higher than 1. These geochemical features suggest that this magmatic source was a metasomatized mantle wedge, above which (at a depth of 40–25 km) a block of the pre-Ural basement occurred in Ordovician-Silurian time. The Tagil megazone started to develop on this block. By the Devonian, i.e., by the time when the Magnitogorsk zone began to evolve (～400 Ma) and continental-margin gabbro-tonalite-granodiorite magmatism was initiated (360 Ma), this basement had been destroyed by orogenesis. The major phases of Paleozoic magmatism in the Urals likely corresponded to global epochs of tectono-magmatic activity, because they correlate well with known data on the evolution of the ⁸⁷Sr/⁸⁶Sr ratio in Paleozoic seawater.     

Geochemical features and sources of clastic material in Paleozoic terrigenous deposits of the northeastern flank of the South Mongolia–Khingan orogenic belt

The results of geological and geochemical studies of terrigenous rocks of the main stratigraphic subdivisions in the northeastern flank of the South Mongolia–Khingan orogenic belt and also the results of U-Pb (LА-ICP-MS) geochronological studies of detrital zircons from these deposits are presented. It is demonstrated that the studied rocks differ significantly in the nature of distribution of detrital zircon ages and, consequently, they cannot be members of a single sedimentary sequence. The data obtained confirm the standpoint according to which the northeastern flank of the South Mongolia–Khingan orogenic belt represents a “joint” zone separating the Argun and the Bureya-Jiamusi Superterranes. This joint zone was formed as the result of closure of the oceanic basin separating the specified continental massifs in the Paleozoic era. The geochemical features of the studied rocks indicate their formation in the conditions of the island arc or the active continental margin. Lack of zircon generations younger than Ordovician age in the studied samples allows assuming that the sedimentary sequences identified within the northeastern flank of the studied belt as the Necla, Dagmara, siltstone-sandstone, and Gramatukha sequences that formed from the end of the Vendian (?) to the Devonian correspond to the youngest stages of belt formation. These sediments in the current structural plan evidently represent fragments of accretion complexes cropping out in fragments among the Cenozoic sequences of the Amur-Zeya Depression.