Geochemistry of radioactive elements in the rocks of the Guli Massif,Polar Siberia

The study of radioactive element distribution in the rocks of the Guli Complex revealed an increase of uranium and thorium contents in the final products of magmatic differentiation. In the carbonatite complex, the radioactive elements are mainly accumulated in the early rocks—phoscorites, while their contents in the late phases, dolomitic carbonatites, decrease. The Th/U ratio increases from near-chondritic values in the weakly differentiated highly-magnesian primary magmas to the late rocks—phoscorites, calcitic carbonatites, and dolomitic carbonatites. The majority of radioactive elements are hosted in rare-metal accessory minerals: perovskite, pyrochlore, calzirtite, and apatite. Rock-forming minerals are characterized by extremely low contents of radioactive elements.     

A Pb isotope investigation of the Guli massif, Maymecha-Kotuy alkaline-ultramafic complex, Siberian flood basalt province, Polar Siberia

Summary Covering a vast area of the northern Siberian platform are the Siberian flood basalts (SFB), which make up one of the world’s largest magmatic provinces. Along the northeastern margin of the SFB province lies the Maymecha-Kotuy alkaline-ultramafic complex, consisting of a large volume of alkaline lavas, numerous dykes, and the Guli massif together with numerous other, smaller alkaline plutons. The genetic link between the SFB and the Maymecha-Kotuy complex continues to be a subject of active debate. Although the rocks in both units have essentially the same age close to the Permian-Triassic boundary, questions remain as to the relative order of emplacement and the contributing source materials of each lithology. This study builds upon earlier petrologic, geochemical, and isotopic work to further an understanding of the relationship between SFB and alkaline rocks. A whole-rock U-Pb age of 250 ± 9 Ma was determined for the Guli massif, which lies within the range of ages previously reported for the SFB. The Pb isotopic composition of the Guli rocks plot mainly in the lower portion of the OIB field, and dunite and carbonatite extend downward into the MORB field suggesting for them a more depleted source than the one that produced the SFB. The combined Pb, Sr, and Nd isotopic systematics of the SFB and the Guli alkaline rocks enable the identification of several discrete source components. The first component dominates many of the Guli rocks and is characterized by low ⁸⁷Sr/⁸⁶Sr (0.7031 to 0.7038), high ε_Nd (+5.35 to +3.97), and relatively unradiogenic Pb (²⁰⁶Pb/²⁰⁴Pb = 17.88–18.31; ²⁰⁷Pb/²⁰⁴Pb = 15.38–15.46; ²⁰⁸Pb/²⁰⁴Pb = 37.33–37.70), which we associate with the depleted (MORB source) mantle. The second component representing most of the SFB demonstrates a notable chemical and isotopic uniformity with ⁸⁷Sr/⁸⁶Sr values of 0.7046 to 0.7052, ε_Nd values of 0 to +2.5, and an average Pb isotopic composition of ²⁰⁶Pb/²⁰⁴Pb = 18.3, ²⁰⁷Pb/²⁰⁴Pb = 15.5, and ²⁰⁸Pb/²⁰⁴Pb = 38.0. This component, making up the majority of SFB, is speculated to be a relatively primitive lower mantle plume with a near-chondritic signature. Contamination by upper and lower continental crustal material, designated as components 3 and 4, is postulated to explain the isotopic characteristics of some of the higher SiO₂ Guli rocks and SFB. Finally, metasomatic processes associated with the invasion of the Siberian super-plume add a fifth component responsible for the extreme enrichment in rare-earth and related elements found in some Guli rocks and SFB.     

Uranium and thorium in carbonatitic minerals from the Guli massif, Polar Siberia

This paper reports a geochemical and mineralogical study on carbonatites from the Guli massif, which hosts rare-metal mineralization. The principal carriers of radioactive elements in the carbonatites are pyrochlore-group minerals, zirconolite, and thorianite, which are described here. They are characterized by elevated concentrations (wt %) of radioactive elements: up to 17.89 UO₂ and 20.01 ThO₂ in pyrochlore, up to 6.49 UO₂ and 94.29 ThO₂ in thorianite, and up to 6.74 ThO₂ in zirconolite. The pyrochlore-group minerals, zirconolite, and thorianite from the early calcite carbonatites occur in intimate association with Ti-Zr oxides calzirtite, perovskite, and baddeleyite. Significant radioactive element fractionation in early-stage derivatives results in the depletion of the residual magmatic products in these elements. The dolomite carbonatites are reported to contain only trace amounts of pyrochlore-group minerals. It was shown that the distribution of U, Th, Nb, and Ta in the calcite and dolomite carbonatites is correlated with the evolutionary trends of pyrochlore composition. Typical schemes of isomorphic substitution are proposed for pyrochlore-group minerals and zirconolite. The pyrochlore-group minerals show an apparent evolutionary trend from U-rich towards more Th- and Ta-rich varieties, and Ba-Sr cation-deficient varieties originate during the latest stage of the evolution. The pyrochlore-group minerals, zirconolite, and thorianite may also accumulate in placers, together with gold. Because of the relative ease of extraction of the accessory minerals, the carbonatites of the Guli massif can be considered as commercial sources of radioactive raw materials.     

Isotopic geochronology and biostratigraphy of Riphean deposits of the Anabar Massif,North Siberia

The structure of Riphean deposits developed on the western slope of the Anabar Massif is described with analysis of their depositional environments, distribution of stromatolite assemblages and organic-walled and silicified microfossils through sections, and evolution of views on stratigraphic significance of some of these assemblages. The investigation included complex mineralogical, geochemical, structural, and isotopic?geochronological study of globular phyllosilicates (GPS) of the glauconite?illite series from paleontologically well substantiated Riphean sequences (Ust’-Il’ya and Yusmastakh formations of the Billyakh Group) of the Anabar Massif in the Kotuikan River basin. Isotopic dating of monomineral size and density fractions of GPS from the Billyakh Group was performed in combination with simulation of the distribution of octahedral cations and comparison of the results obtained with Mössbauer spectrometry data. The applied approach is based on an assumption that the formation and transformation of Rb?Sr and K?Ar systems in GPS are synchronous with stages in their structural evolution, which are determined by the geological and geochemical processes during depositional history. Such an approach combined with the mineralogical and structural analysis contributes to correct interpretation of stratigraphic significance of isotopic data. The results obtained provide grounds for the conclusion that isotopic dates of GPS from the Ust’-Il’ya (Rb?Sr, 1485 ± 13 Ma; K?Ar, 1459 ± 20 Ma) and Yusmastakh (Rb?Sr, 1401 ± 10 Ma; K?Ar, 1417 ± 44 Ma) formations mark the stage of early diagenesis of sediments and are suitable for estimating the age of formations in question.     

Geological, Isotopic and Geochronological Constraints on Hetaoping Distal Skarn-Type Zn-Pb-Ag-Cu Deposits in Baoshan Palaeozoic Massif, SW China

Please refer to the attachment(s) for more details.     

额尔古纳地块新元古代岩浆作用与微陆块构造属性:来自侵入岩锆石U-Pb年代学、地球化学和Hf同位素的制约

对额尔古纳地块新元古代花岗岩进行了锆石LA-ICP-MS U-Pb年代学、岩石地球化学和锆石Hf同位素研究,以便对其新元古代岩浆作用历史与微陆块构造属性给予制约.所测花岗质岩石中锆石的CL图像特征和Th/U比值(0.17~1.46) 显示其为岩浆成因.测年结果并结合前人定年结果,可以判定额尔古纳地块上至少存在~929 Ma、~887 Ma、~850 Ma、~819 Ma、~792 Ma、~764 Ma和~738 Ma岩浆事件.岩石地球化学特征显示,~887 Ma花岗岩为一套后碰撞花岗岩类;而850~737 Ma花岗质岩石整体上属于A-型花岗岩,也有部分岩体(漠河、阿木尔、碧水和室韦岩体)显示I-型花岗岩特征.锆石Hf同位素特征反映这些花岗岩的源区既有中-新元古代(T_DM2=884~1 563 Ma)新增生地壳物质的部分熔融,同时伴有少量古老地壳物质的混染,也有残留的古老中基性下地壳物质的部分熔融.综合研究区新元古代侵入岩的地球化学特征,同时对比新元古代全球构造热事件,认为额尔古纳地块上新元古代岩浆活动记录了Rodinia超大陆形成和演化过程中的地壳响应:927~880 Ma的岩浆作用应是Rodinia超大陆汇聚造山的产物;而850~737 Ma的岩浆作用应是对Rodinia超大陆快速裂解的记录.通过岩浆事件对比发现,额尔古纳地块与邻近的西伯利亚南缘微陆块(如中蒙古地块和图瓦地块)具有亲缘性,而与塔里木板块和华南板块至少在新元古代岩浆活动上具有一定的相似性,而明显区别于华北板块和西伯利亚板块.      

Na-rich carbonate inclusions in perovskite and calzirtite from the Guli intrusive Ca-carbonatite,polar Siberia

Carbonate phases, some rich in Na₂O and comparatively rich in SrO and BaO, occur as inclusions in perovskite and calzirtite (Ca₂Zr₅Ti₂O₁₆) in the carbonatite of the Guli complex, Siberia. This is the first record of alkali carbonates, akin to nyerereite [Na₂Ca(CO₃)₂], in plutonic igneous rocks. The inclusion populations suggest that the parental magma of the complex was Ca-rich but developed Na-rich differentiates during the latest stages. This points to the dominant calcic carbonatites of the complex not being derivatives of alkali-rich parental carbonatites. These alkali-rich carbonate inclusions (and rare inclusions of djerfisherite) have been preserved due to the resistance of perovskite and calzirtite to processes of leaching, hydrothermal alteration and weathering.     

Carbonatites and ultrabasic-alkaline rocks of the maimecha-kotui region, N. Siberia

The structural position, geology, petrography, petrochemistry, and petrogenesis of one of the world's greatest carbonatite provinces situated in North Siberia are recorded. Three groups of Late Permian-Early Triassic magmatites are briefly discussed: effusives, dikes, and intrusives of complex central type. Carbonatite and congenetic rocks are related to kimberlites, but are genetically unrelated to tholeiitic basalts and trachybasalts. The genetic development and diagnostic features of ultrabasic-alkaline rocks and carbonatites are formulated.     

Geochemistry and Fractionation of Zr and Hf in the Magmatic System of the Kugda Massif (Polar Siberia)

The patterns of the distribution and fractionation of strategic metals (Zr, Hf) in the Kugda intrusion (Polar Siberia) have been studied. The contents of these elements significantly exceed their concentrations in other rocks (Zr, 246 ppm; Hf, 7.4 ppm). A significant increase in Zr and Hf from early rocks (olivinite and melilite rocks) to later differentiation products, syenites with up to 570 ppm of Zr and 16 ppm of Hf, has been revealed. During the evolution of the Kuga magmatic system, notable fractionation of Zr and Hf occurred. The Zr/Hf ratios in the dyke rock, similar in composition to the primary Kugda Massif magma, and the early intrusions are fairly close to that of chondrite (Zr/Hf = 37 [1]), while in the latest phases this ratio increases by almost 5-fold.     

Ultrapotassic Rock from the Tomtor Complex of Ultrabasic Alkaline Rocks and Carbonatites (Arctic Siberia)

Doklady Earth Sciences - Among the rich REE–Nb ores of the upper ore horizon of the Tomtor deposit, under the Jurassic deposits (borehole 324, interval 47–62 m), ultrapotassiс...     

Geochronological and petrological constraints from the evolution in the Saxon Granulite Massif,Germany, on the Variscan continental collision orogeny

Controversy over the plate tectonic affinity and evolution of the Saxon granulites in a two‐ or multi‐plate setting during inter‐ or intracontinental collision makes the Saxon Granulite Massif a key area for the understanding of the Palaeozoic Variscan orogeny. The massif is a large dome structure in which tectonic slivers of metapelite and metaophiolite units occur along a shear zone separating a diapir‐like body of high‐P granulite below from low‐P metasedimentary rocks above. Each of the upper structural units records a different metamorphic evolution until its assembly with the exhuming granulite body. New age and petrologic data suggest that the metaophiolites developed from early Cambrian protoliths during high‐P amphibolite facies metamorphism in the mid‐ to late‐Devonian and thermal overprinting by the exhuming hot granulite body in the early Carboniferous. A correlation of new Ar–Ar biotite ages with published P–T–t data for the granulites implies that exhumation and cooling of the granulite body occurred at average rates of ~8 mm/year and ~80°C/Ma, with a drop in exhumation rate from ~20 to ~2.5 mm/year and a slight rise in cooling rate between early and late stages of exhumation. A time lag of c. 2 Ma between cooling through the closure temperatures for argon diffusion in hornblende and biotite indicates a cooling rate of 90°C/Ma when all units had assembled into the massif. A two‐plate model of the Variscan orogeny in which the above evolution is related to a short‐lived intra‐Gondwana subduction zone conflicts with the oceanic affinity of the metaophiolites and the timescale of c. 50 Ma for the metamorphism. Alternative models focusing on the internal Variscan belt assume distinctly different material paths through the lower or upper crust for strikingly similar granulite massifs. An earlier proposed model of bilateral subduction below the internal Variscan belt may solve this problem.     

The structural evolution of dunite and chromite ore from the Kharcheruz Massif,the Polar Urals

The Kharcheruz block of the Syumkeu ultramafic massif is a southern fragment of the Khadata ophiolitic belt, which closes the ophiolites of the Polar Urals in the north. The block, striking in the latitudinal direction, is sheetlike in shape and primarily composed of dunite with nearly latitudinal zones of chromite mineralization. The dunites are subject to ductile deformation various in intensity, and this variability is displayed in their heterogeneous structure and texture. The following microstructural types are distinguished by the variety and intensity of their deformation: protogranular → mesogranular → porphyroclastic → porphyrolath → mosaic. The petrostructural patterns of olivines pertaining to the above types reflect conditions of ductile deformation. Protogranular dunite is formed as a product of pyroxene decomposition in mantle harzburgite accompanied by annealing recrystallization at a temperature above 1000°C. Mesogranular dunite is formed as a product of high-temperature plastic flow by means of translation sliding in olivine and diffuse creep at a temperature dropping from 1000 to 650°C and at a low rate (<10^–6 s^–1). Cr-spinel segregates into linear zones of disseminated chromite mineralization within zones of bedding-plane plastic flow. Porphyroclastic and mosaic dunites are formed under conditions of intense deformation at a temperature of 500–750°C and at a significant rate (>10^–6 s^–1). Dunite is deformed by means of syntectonic recrystallization and subordinate translation gliding. Linear zones of disseminated mineralization undergo destruction thereby, with the formation of lenticular chromitite bodies from which ductile olivine is squeezed out with the formation of densely impregnated and massive ores.     

A Case History of Applying In situ Analysis of the Sulfur Isotopic Compositions of Sulfides from Ores of the Largest Deposits in the Bodaibo Synclinorium (Eastern Siberia)

Doklady Earth Sciences - The experience of applying the laser ablation technique to study the sulfur isotopic compositions of pyrite and arsenopyrite samples from gold deposits of the...     

New data on the age of the Guli intrusion and implications for the relationships between alkaline magmatism in the Maymecha-Kotuy province and the Siberian superplume: U-Th-Pb isotopic systematics

The Siberian flood basalts (SFB) make up one of the world’s largest known magmatic province and cover a vast area of the northern Siberian platform. The northeastern flank of the SFB province is occupied by the Maymecha-Kotuy ultramafic-alkaline complex consisting of enormous volumes of alkaline lavas, numerous dikes, and the Guli massif comprising many smaller alkaline plutons. The genetic relationship between SFB and the Maymecha-Kotuy complex continues to be a matter of considerable debate. Despite the fact that the rocks of both blocks have almost the same age, close to the Permian-Triassic boundary, there are still major uncertainties in the relative succession of emplacement and the source material for each lithology present. On the basis of previous petrological, geochemical, and isotopic results, this study attempted to put tighter limits on possible relationships between SFB and alkaline rocks.     

Late Jurassic Volcanism Deduced from Geochemical,Geochronological, and Sr-Nd-Hf Isotopic Composition Characteristics of the Nanyuan Formation,South China

The Nanyuan Formation contains information related to the Mesozoic tectonic transformation. In this study, three representative profiles were surveyed from the Nanyuan Formation, and multiple analyses were conducted. Zircon U-Pb dating yielded their ages as approximately 158–146 Ma. The volcanic rocks are enriched in Rb, Th, U, K, and Pb and depleted in Nb, Ta, P, and Ti, implying their affinity for I-type granites. The ε_Nd(t) values(-8.3 to-6.0),(⁸⁷Sr/⁸⁶Sr)_...     

Pre-Alpine evolution of a segment of the North-Gondwanan margin: Geochronological and geochemical evidence from the central Serbo-Macedonian Massif

The Serbo-Macedonian Massif (SMM) represents a composite crystalline belt within the Eastern European Alpine orogen, outcropping from the Pannonian basin in the north, to the Aegean Sea in the south. The central parts of the massif (i.e. southeastern Serbia, southwestern Bulgaria, eastern Macedonia) consist of the medium- to high-grade Lower Complex, and the low-grade Vlasina Unit. New results of U–Pb LA-ICP-MS analyses, coupled with geochemical analyses of Hf isotopes on magmatic and detrital zircons, and main and trace element concentrations in whole-rock samples suggest that the central SMM and the basement of the adjacent units (i.e. Eastern Veles series and Struma Unit) originated in the central parts of the northern margin of Gondwana. These data provided a basis for a revised tectonic model of the evolution of the SMM from the late Ediacaran to the Early Triassic.The earliest magmatism in the Lower Complex, Vlasina Unit and the basement of Struma Unit is related to the activity along the late Cadomian magmatic arc (562–522 Ma). Subsequent stage of early Palaeozoic igneous activity is associated with the reactivation of subduction below the Lower Complex and the Eastern Veles series during the Early Ordovician (490–478 Ma), emplacement of mafic dykes in the Lower Complex due to aborted rifting in the Middle Ordovician (472–456 Ma), and felsic within-plate magmatism in the early Silurian (439 ± 2 Ma). The third magmatic stage is represented by Carboniferous late to post-collisional granites (328–304 Ma). These granites intrude the gneisses of the Lower Complex, in which the youngest deformed igneous rocks are of early Silurian age, thus constraining the high-strain deformation and peak metamorphism to the Variscan orogeny. The Permian–Triassic (255–253 Ma) stage of late- to post-collisional and within-plate felsic magmatism is related to the opening of the Mesozoic Tethys.