Character of zircon distribution in dunites of the South Urals (Sakharin and East Khabarnin massifs)

Zircons in dunites of the Sakharin and East Khararnin massifs, situated in the South Urals and part of the platinum-bearing Uralian belt, were investigated for the first time. Several types were identified in the polygene-polychronous zircon assemblage of both massifs. The first is represented by Proterozoic (from 1517 ± 12 to 2693 ± 9 Ma) crystals similar to those widely spread in the Riphean and more ancient Uralian deposits. The second type includes dunite varieties of typical magmatic habit and of ages from 377 ± 3.6 Ma to 402 ± 3 Ma. The third contains crystals and crystal fragments of a high degree of crystallinity. The concordant Archean dating (2808 ± 26 Ma) for zircons of this type determines the minimum age of the dunite substrate. Zircons of Type 4 are heterogeneous, consist of the relict (nuclear) part represented by crystals of Types I and III and the newly formed mantle, and are consistent with zircons of Type II.     

Isotopic-geochemical study of zircons from metabasites of the Kontokki dike complex: Age of regional metamorphism in the Kostomuksha structure

Results of local isotopic-geochemical and chemical examination of zircons from metabasites of the Kontokki dike complex in the Kostomuksha structure, western Karelia, Russia, make it possible to interpret the concordant U-Pb zircon age of 2674 ± 13 Ma as the boundary between regional amphibolite-facies metamorphism and accompanying metasomatism. Zircons from the metabasites arte heterogeneous and consist of central parts with relict magmatic cores, metamorphic intermediate zones (which are pale in CL), and younger metasomatically altered zones (dark in BSE images), which development along boundaries between zones and lengthwise arrays of cracks permeable to fluids. The dark altered zones are characterized by high (for zircons) concentrations of LREE, MREE, Th, U, Ca, Sr, Ba, Fe, and Al. The REE distribution in the zircons was proved to be much less susceptible to overprinted metasomatic processes than the U-Pb system of the same zircons. Characteristics of the REE distribution in the zircons makes these zircons comparable with metasomatic zircons. Genetically, the metasomatic processes that affected the geochemistry of the zircons could be related to synmetamorphic granitoid intrusions (Bibikova et al., 2005).     

The age of late orogenic granitoids of the urals based on U-Pb isotope dating of zircons (Exemplified by the Shartash and Shabry massifs)

U-Pb dating of zircons (SHRIMP-II) has been performed for the first time for granites, granodiorites, and synplutonic granodiorite and melanodiorite bodies of the Shartash and Shabry massifs in the Middle Urals. The time of the formation of the massifs is 300–306 Ma, which is 25 Ma younger than the previous estimates. The age data obtained are in line with the time of the formation of the adamellite-granite series of the Verkhisetsk Batholite (the master sample of tonalite-granodiorite-granite magmatism in the Middle Urals), the series of which is petrochemically close to the rocks of the massifs.     

北京白查A型花岗岩的地球化学特征及其成因与构造指示意义

位于北京昌平区的年龄为127Ma的白查岩体是一个晶洞过碱性花岗岩岩株,其侵位于燕山造山带西段的八达岭岩基中。白查岩体具有高硅、Zr、Ga、Pb、HFSE和REE含量,高Fe/Mg和Ga/Al比值,强烈亏损Ca、Sr、Eu,具有亏损放射性成因Pb同位素的特征。白查岩体属于Dall’Agnol和Oliveira (2007)定义的典型的还原型A型花岗岩,其地球化学特征与燕山带中生代晚期的后石湖山(山海关)和窟窿山A型花岗岩非常相似。白查花岗岩起源于华北克拉通下地壳镁铁质源岩在相对贫水、高温、低氧逸度条件下的低程度部分熔融。综合区域地质背景和该岩体的成岩机制判断,白查花岗岩的形成标志燕山西段造山崩塌阶段的到来。除浙江东南瑶坑过碱性花岗岩外,白查岩体是具有A1型花岗岩元素地球化学特征但并非形成于非造山大地构造背景的过碱性花岗岩的又一个实例。     

The main features of the Uralian Paleozoic magmatism and the epioceanic nature of the orogen

The 2000 km Uralian Paleozoic orogen is situated on the western flank of the Uralo-Mongolian folded belt. It is characterized by an abundant variety of magmatic rocks and related ore deposits. Uralian Paleozoic magmatism is entirely subduction-related. It is proposed that the Uralian orogen represents a cold mobile belt in which the mantle temperature was 200 to 500 °C cooler than in the adjacent areas; a situation which is similar to the modern West Pacific Triangle Zone including Indonesia, the Philippine Islands, and southern Asia. During the course of the geological evolution of the Uralian orogen, the nature of the magmatism has changed from basic rocks of indisputable mantle origin (460–390 Ma) to mantle-crust gabbro-granitic complexes (370–315 Ma) followed by pure crustal granite magmatism (290–250 Ma). This order in rock type and age reflects the evolution of Paleozoic magmatic complexes from the beginning of subduction to the final stages of the orogen development.     

Protolith and deformation age of the Gneiss-Plate of Kartali in the southern East Uralian Zone

The southern East Uralian Zone consists of granite-gneiss complexes that are embedded in geological units with typical oceanic characteristics. These gneisses have been interpreted as parts of a microcontinent that collided during the Uralian orogeny. The gneiss-plate of Kartali forms the south eastern part of the gneiss mantle surrounding the Dzhabyk pluton. Its post-collisional protolith age of 327±4 Ma is inconsistent with the microcontinent model. The deformation of the gneisses took place in 290±4 Ma at the time of the intrusion of the Dzhabyk magmas. Granites and gneisses cooled and were exhumed together. Therefore, we interpret the gneiss complexes of the East Uralian Zone as marginal parts of the granitic batholiths that were deformed during the ascent and emplacement of the pluton. From Nd and Sr isotope constraints we conclude that the magma source of the gneiss protolith was an island arc. Since no evidence for old continental crust has been discovered in the East Uralian Zone, the Uralian orogeny can no longer be interpreted as a continent-island arc-microcontinent collision. Instead, the geochemical data presented within this paper indicate that the stacking and thrusting of island arc complexes played an important role in the Uralian orogeny.     

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.     

Isotopic-geochemical features of rocks of the Middle-Upper Cambrian Verkhnyaya Lena formation in the Siberian Craton

Isotopic compositions of carbon and oxygen in carbonates and sulfur in sulfates of the Verkhnyaya Lena Formation (ε₂–ε₃), which terminates the Cambrian section of the Irkutsk Amphitheater of the Siberian Craton, are studied. Sulfates of the Verkhnyaya Lena Formation are marked by unusually low δ³⁴S values (4.6–12.0‰) relative to sulfates of the underlying Angara Formation. This is likely caused by variations in the facies-paleogeographic sedimentation at the transition of the Angara and Verkhnyaya Lena formations, as well as associated variations in the water and salt alimentation budget in sedimentation basins, due to their isolation from open sea and intensification of the continental and underground discharge. The δ¹⁸O(PDB) value in carbonates decreases from ?4.4‰ at bottom to ?10.4‰ at top, reflecting variation in postsedimentary transformations and probable continuous freshening of sedimentation basin. Isotopic composition of carbon in most samples shows normal marine δ¹³C values (0 ± 1‰). Only in some samples, does the δ¹³C value increase up to ?3.8 and 2.2‰ due to specific features of postsedimentary processes. The Rb-Sr systems of the clayey component of marls from the 500-m-thick section of the Angara Formation and bottom of the Verkhnyaya Lena Formation record an age of 512 ± 10 Ma, which is close to the assumed stratigraphic age of the Verkhnyaya Lena Formation. The ⁸⁷Sr/⁸⁶Sr initial ratio is 0.7082 ± 0.0004.     

桂东南永安岩体锆石U-Pb测年、Hf同位素特征及其地质意义

永安岩体位于钦杭结合带南部,属桂东南十万大山-大容山复式岩体的一部分,其岩石类型为斑状堇青黑云花岗闪长岩。对永安岩体斑状堇青黑云花岗闪长岩进行了LA-ICPMS锆石U-Pb测年,获得其年龄为252. 8±3. 3Ma,为晚二叠世花岗岩。对永安岩体花岗岩进行的全岩地球化学分析结果表明,岩石富Al、高K,铝饱和指数A/CNK均大于1. 1,标准矿物分子刚玉均大于1%;岩石富集轻稀土元素,亏损重稀土元素,轻重稀土元素分馏明显,稀土元素的球粒陨石标准化配分曲线为轻稀土元素富集的右倾型,具弱负铕异常;岩石富集大离子亲石元素(Rb、Ba、K、Sr),亏损高场强元素(Nb、P、Ti、Ta)和Ba、Sr。花岗岩的锆石εHf(t)=-14～-8,tDM2=2. 3～1. 8 Ga。通过本文工作,结合前人关于永安岩体矿物学、全岩地球化学、Hf同位素和锆石微量元素的分析资料,认为永安岩体岩石为强过铝质S型壳源花岗岩,地幔组分没有提供成岩物质,但地幔底侵作用为成岩提供了热能,花岗岩形成于后碰撞阶段的拉张环境中。     

浙江瑶坑碱性花岗岩体的年代学、地球化学及其成因与构造指示意义

瑶坑岩体位于闽浙交界沿海地区,是闽浙沿海晚中生代A型花岗岩带中的一个典型碱性花岗岩体.锆石LA-ICPMS U-Pb同位素定年表明,该岩体的形成年龄为91.3±2.5 Ma（MSWD=3.7,2σ）,属晚白垩世岩浆活动的产物.岩石发育特征的微文象结构和晶洞构造,含有典型的碱性铁镁矿物钠铁闪石.地球化学上,该岩体富硅、碱,贫钙、镁,富K、Rb、Cs、Th、U,贫Sr、Ba、P、Ti、Sc.其Nb、Ta、Zr、Hf和Ga等元素的含量也较高,（Ga/Al）×10^4值变化于3.64～4.41,并具显著的铕负异常（δEu=0.15～0.20）.与闽浙沿海A型花岗岩带中的其它岩体相比,瑶坑岩体具有偏高的εNd（t）值（=-0.7～-1.4）和偏低的二阶段Nd模式年龄（t2DM=0.95Ga～1.01Ga）,指示幔源组分对该岩体的形成具有更重要的贡献.在A型花岗岩亚类的判别上,瑶坑岩体以较低的Y/Nb比值和较高的Nb/Ta比值相似于Eby（1992）区分的A.型花岗岩,而明显有别于岩带主体属A2型的特点.岩石产出构造背景及地球化学特征的综合分析表明,多组深断裂交汇,进而引发多量幔源组分参与成岩过程,是导致瑶坑岩体元素-同位素组成与区内其它同类岩体有较明显差别的主要原因.     

First U-Pb age of detrital zircons from sandstones of the Upper Emsian Takaty formation of the Western Urals with regard to the problem of primary sources of the Uralian diamond placers

Several alternative points of view currently exist on the origin of the primary sources of diamonds from the Cenozoic Western Urals placers. Some researchers suppose that their economic diamond resource potential is related to diamonds from tuffisitic facies of the mantle kimberlites-lamproites or impact structures. Other researchers suggest that diamonds originated from the eroded sandstones of the Upper Emsian Takaty Formation of the Lower Devonian, which represents ancient (fossil) placers or intermediate reservoirs. It is assumed that these reservoirs collected diamonds from worn kimberlite bodies, which were located in the Urals or on the East European platform (EEP). This paper presents the first U-Pb (LA-ICP-MS) age of detrital zircons from quartz sandstones of the Takaty Formation, which spans a range from 1857.5 ± 53.8 to 3054.0 ± 48.0 Ma. The absence of detrital zircons younger than 1.86 Ga excludes that the structural complexes of the Uralian, Fennoscandian, and Sarmatian EEP parts were the provenance areas that supplied the clastic material to the sedimentary basin, which accumulated the Takaty Formation. The similar age of our zircons and ancient crystalline complexes of the Volga-Uralian EEP part allows consideration that it was a single provenance area. If we assume that the diamond resource potential of the Western Urals is completely or partly related to the ancient diamond placers from the Takaty Formation, then the intermediate diamond reservoirs from its structure originated due to redeposition of destruction products of primary diamond-bearing rocks of the Volga-Uralia area. Thus, within the Volga-Uralian part of the EEP basement, we may expect identification of a previously unknown stage of kimberlite formation, which is significantly older than that responsible for the diamond resource potential of the Arkhangel’sk province.     

Geochemistry of basic dikes in the Lanzo massif (Western Alps): Petrogenetic and geodynamic implications

The Alpine peridotite massif of Lanzo (Italy) contains three generations of basic dikes (gabbros and basalts). The older gabbros are plagioclase-rich mantle segregates while the younger gabbro dikes are cumulates very similar in chemical composition to recent oceanic gabbros and gabbros from ophiolitic complexes. They both were derived from the N-type mid-ocean ridge basalt (MORB) magmas which were progressively more depleted in incompatible elements and were probably generated during a dynamic melting of a rising mantle diapir. The basaltic dikes are the N-type MORB and closely resemble the Alpine-Apennine ophiolitic basalts. They were derived from a different upper mantle source than the parental magmas of the gabbros. The source of the basalts was less depleted in light REE. The presence of basic magmas with N-type MORB affinities in the Lanzo massif is consistent with the close genetic relationship between the Alpine peridotite body and the ophiolites of the Liguro-Piemontese basin.     

General relations in the trace-element composition of zircons from eclogites with implications for the age of eclogites in the belomorian mobile belt

Analysis of currently available data (877 individual high-precision zircon analyses) on the composition of zircons from eclogite complexes worldwide reveals general relations in the zircon composition: an anomalous decrease in the Th concentration (no higher than 3 ppm on average) and the Th/U ratio (0.33 on average), a significant decrease in the concentrations of all REE (to 22 ppm) and particularly LREE (<2 ppm), and relatively low concentrations of Y (34 ppm), U (100 ppm), and P (41 ppm) at an elevated Hf concentration (11 400 ppm on average). The REE patterns of eclogitic zircons are noted for pronounced flat HREE patterns, poorly pronounced (if any) negative Eu anomalies, strongly reduced positive Ce anomalies (Ce/Ce* = 11 on average), and U-shaped configurations of LREE patterns up to the development of negative Nd anomalies. The relations detected in the distribution of trace elements and REE in eclogitic zircons are of universal nature and occur irrespective of the rock type (metabasites, metaultrabasites, or gneisses) and the metamorphic pressure (eclogites of high and ultrahigh pressure). The application of the aforementioned criteria makes it possible to reliable distinguish eclogitic zircons from those of magmatic or metamorphic genesis (not related to high-pressure metamorphism). Eclogites in the Belomorian Mobile Belt (in the Salma and Gridino areas) were determined to contain zircons in metagabbro eclogites; the cores of these zircons have an age of 2.8?C2.9 Ga and are of magmatic genesis, whereas their outer metamorphic zones have an age of 1.9 Ga and a trace-element composition typical of eclogitic zircons. Hence, the Belomorian Mobile Belt was affected only by single (Svecofennian, at ??1.9 Ga) episode of eclogite metamorphism of Archean rocks.     

The mafic and ultramafic rocks of the Mocksville complex of Central North Carolina: Petrogenetic and tectonic implications

Located in the northwestern part of the Charlotte terrane of the Carolina zone in central North Carolina, the Mocksville complex is a tabular body which trends NE-SW and covers an area of approximately 500?km². It consists of late Proterozoic to early Paleozoic, moderately metamorphosed and variably deformed, mainly plutonic ultramafic, mafic and felsic rocks. The ultramafic rocks are pyroxenites, wehrlites, and hornblendites; the mafic rocks are metagabbros and amphibolites; and, the felsic rocks are granites and diorites. Field, geochemical, and geothermobarometry studies suggest that the igneous and metaigneous rocks of the Mocksville complex are likely to be genetically related, formed by calc-alkaline differentiation of mafic magma, and originated in a moderate pressure environment (~8?kbar). Based mainly on the study of volcanic rocks, the terranes of the Carolina zone have been interpreted as magmatic arc terranes in most tectonic models concerning the evolution of the southern Appalachian orogen. The geochemical features of the mafic and ultramafic plutonic rocks of the Mocksville complex corroborate the arc origin of the Charlotte terrane.     

新疆西天山卡特巴阿苏金矿床锆石U-Pb年龄、地球化学特征及其地质意义

卡特巴阿苏大型金矿床位于新疆西天山中部那拉提北缘断裂带南侧,找矿前景较好。为查明矿区容矿岩浆岩的成因及其与矿床形成的联系,对主要矿石类型含金蚀变二长花岗岩和含金石英硫化物脉进行LA-MC-ICP-MS锆石U-Pb测年和地球化学研究。结果表明,锆石稀土元素显示较陡的左倾配分模式,具有明显的正Ce异常和负Eu异常,指示岩浆锆石成因。根据锆石Ce异常值,获得含矿岩石有关岩浆岩氧逸度对数值(Log(fo_2))为-29.2～5.1,指示岩浆具有较高的氧逸度,有利于金在岩体中初步富集。同时,氧逸度具有较大的变化范围,指示岩浆演化过程中,存在明显的上地壳混染。应用锆石Ti温度计,获得含金蚀变二长花岗岩锆石结晶温度为765～975℃,反映岩浆来自下地壳缺水条件下的部分熔融。含金石英硫化物脉锆石具有较低的结晶温度(641～823℃),可能与早期角闪石的分异结晶有关。以上2种矿石类型的锆石U-Pb年龄分别为350.4±1.6Ma和348.9±1.4Ma,代表了容矿二长花岗岩的结晶年龄。岩体结晶年龄早于前人报道的矿床成矿年龄,表明岩体侵位与成矿作用无直接关系。     

Geochemical specifics of massifs of the drusite complex in the central Belomorian Mobile Belt: II. Sm-Nd isotopic system of the rocks and the U-Pb isotopic system of zircons

First isotopic-geochemical data were obtained on basite-ultrabasite rocks from the southern Kovdor area that were previously provisionally ascribed to the drusite (coronite) complex based on the occurrence of drusite (coronite) textures. The mineral and whole-rock Sm-Nd isochron age determined for five rock samples from the Sorkajoki and Poioiva massifs and the massif of Elevation 403 m turned out to be close (within the error): 2485 ± 51, 2509 ± 93, and 2517 ± 75 Ma, respectively. The crystallization age was evaluated for the two massifs (Poiojovski and Mount Krutaya Vostochnaya) by the U-Pb system of zircons. Our samples contained both magmatic and xenogenic crustal zircons, whose age was estimated at 2700 Ma. The crystallization age of the massifs themselves (data on the magmatic zircons) is 2410 ± 10 Ma. The undepleted character of the mantle source (ɛNd = +0.9) and the much younger age of the massifs than that of other known manifestations of ultrabasic magmatism in the territory of Karelia and the Kola Peninsula (including the layered pluton classic drusite massifs) suggest that the central part of the Belomorian Mobile Belt hosts one more independent intrusive rock complex, which has never been recognized previously and which is different from typical drusites.     

Isotopic-geochemical signs of coal and bitumen transformation in different-type ore mineralization: Communication 1. Isotopic-geochemical signs of structural transformation of coal and bitumen

Results of the study of natural organic matter (OM) represented by classes of solid and high-molecular-compounds (resin, ozokerite, asphalt, kerite, graphite, and others) are presented. They are developed as dispersed and concentrated segregations, interlayers, and independent accumulations and impregnation in the fracture-vein system and stratiform bodies. They also accompany ore mineralization. Combination of modern tic mineralogical, chemical, isotopic, infrared spectroscopic, and X-ray photoelectron spectroscopic) analytical methods provide insight into the OM transformation during oil formation and ore genesis. Variations in OM composition and properties define the concentration of ore elements and change mechanisms of their fixation (sorption, chemosorption, and reduction). Isotopic-geochemical signs of the structural transformation of Coal and bitumen allow us to elaborate a unified scale of catagenesis and metamorphism of the sapropel-and humus-type OM.