Restitic ultramafic rocks in the Early Caledonian collisional system of western Cisbaikalia

Early Caledonides in the Olkhon region of western Cisbaikalia, being part of the folded framing of the Siberian craton, are a unique geologic object for studying processes of mantle–crust interaction at deep levels of the Earth’s crust. This paper describes restitic ultramafic bodies and boudins spatially confined to faults (blastomylonite sutures), as well as synkinematic granites related to amphibolite facies of metamorphism. Estimates are given for the PT-conditions of metamorphic rocks from the folded framing of the ultramafic bodies, the chemical and mineral compositions of ultramafic rocks, blastomylonites and synkinematic granites, and the results of U–Pb and Ar–Ar isotopic dating. Particular attention is paid to the thermal history of tectonic exposure of the ultramafic bodies as relics of the paleo-oceanic crust in the Early Caledonian collisional system of western Cisbaikalia.     

Petrogeochemical typification of the ultramafic rocks from the Idar greenstone belt,Kan block,East Sayan

The ultramafic rocks of the Kan block, East Sayan, are confined mainly to the Idar greenstone belt. In terms of formational affiliation, they are subdivided into two groups: magmatic (Kingash Complex) and residual (Idar Complex) ones. The magmatic ultramafic rocks compose hypabyssal and subvolcanic bodies, which are represented by rocks of dunite-wehrlite-picrite association with cumulate textures. Uninterrupted chemical variations of the magmatic ultramafic rocks indicate subsequent magmatic differentiation of parental picritic melt in the intermediate deep-seated chambers and emplacement of its derivatives in the crystallization site. Differentiation leads to proportional increase of all rare-earth and other incompatible elements. The residual ultramafics occur as boudined dunite-harzburgite bodies showing metamorphic granoblastic textures. They have more homogenous chemical composition close to those of ophiolite complexes, which represent strongly depleted mantle rocks brought to the upper lithospheric levels via deep-seated thrusts. Residual ultramafics differ from magmatic rocks in notably lower contents of some trace and rare earth elements.     

Lithological Features of Precambrian Gold-Bearing Rocks: Evidence from the Ukrainian Shield

Based on traditional petrochemical and nontraditional mineralogical methods (accessory zircon generation analysis), specific features of the primary composition of strongly metamorphosed rocks from some Early Precambrian Au-bearing rocks of the Ukrainian Shield (US) were studied. The confinement of several gold ore occurrences to primarily sedimentary Late Archean rocks of the Ukrainian Shield has been established and the possibility of their chemogenic origin is considered. The joint analysis of plicative tectonics and metamorphism facies in the study area demonstrated that Au-bearing primarily sedimentary (chemogenic) rocks of the Khashchevatoe–Zaval'ev Formation of the Bug Group (AR₂) are confined to amphibolite-facies domains within tectonic (high-order synform) structures with a significant gold potential.     

Geologic structure of the Mukodek gold field (northern Transbaikalia) and sources of matter (Pb-Pb and Sm-Nd data)

The Mukodek gold field is located in the Yana zone of the Baikal-Muya fold belt (northern Transbaikalia). Its orebodies are gold-bearing metasomatites confined to the fault of NE strike. It is shown that pre-ore faults played a crucial role in the spatial localization of the orebodies, as they favored the concentration of ore-bearing fluids. Results of geological and structural studies, along with available geochronological data (⁴⁰Ar-³⁹Ar dates for micas in gold-bearing pyrite), testify to ore-forming processes in the ore field during two Late Paleozoic stages. Early gold-bearing metasomatites of sericite-chlorite-albite-ankerite-quartz composition formed at 320 Ma, and late gold-bearing vein metasomatites of ankerite(calcite)-quartz composition originated at 285 Ma. A Pb-Pb isotope study of gold mineralization has shown that ore lead was supplied into the mineral-forming system mainly from a mantle source, most likely, regional Neoproterozoic igneous rocks. According to Sm-Nd isotope data, these rocks formed with the participation of a material of mantle genesis.     

Age paradoxes of Devonian magmatism of the northeastern part of the Kola Peninsula

A structural-geochemical study has been conducted on the dikes of presumably Devonian mafic rocks confined to a small graben filled in with Riphean sedimentary rocks hosted by Early Precambrian granite-gneiss of the Murmansk block. It has been demonstrated that the dolerite dikes of this region can be considered as manifestations of trap magmatism whose products fill in the foundation of the East Barents riftogenic downfold. In turn, manifestations of alkaline and kimberlite rocks of the White Sea region are confined to the peripheral portion of the trap magnetism area. Zircons from dolerite transecting Late Riphean sediments examined in two laboratories have a concordant age of 2.74–2.72 Ba, while zircons from a similar dike located in granite-gneiss of the basement are characterized by an age range of 2700–155 Ma, and the concordant age based on 4 points is 790 Ma. All these factors indicate that the age determinations of the mafic rocks are ambiguous, particularly in the zone of transition from the center of the trap province to its periphery, where alkaline magmatism is observed.     

Trace elements in Upper Devonian rocks of the Andoma Hill zone of fold-and-fault dislocations (southeastern Onega region) as indicators of source areas

The paper reports the results of lithogeochemical studies of the Upper Devonian rocks from the Andoma Hill zone of fold-and-fault dislocations (SE Onega region). The rocks are characterized by the negative Eu anomaly (from 0.4 to 0.65) that maks them different from modern sediments of the White Sea. The latter can be regarded as the average composition of mainly Archean (Karelian–Kola) part of the Baltic Shield. In terms of the contents of some trace elements, they also differ from the Vendian rocks of the Zimnii Bereg area. Since the considered rocks are geochemically similar to the Svecofennian metamorphic rocks and Paleoproterozoic granite rapakivi, they could be formed by the erosion of these complexes. The clastic material was transported via a channel confined to the Baltic Shield and Russian Platform junction known as the Polkanov geoflexure.     

Finding of corundum-bearing rocks in the Lapland granulite belt

Corundum-bearing rocks are described for the first time in the Kandalaksha structure of the Lapland granulite belt. Corundum is confined to rocks of two types: metagabbro?anorthosites constituting lenses among metaanarthosites of the Kandalaksha massif and basic granulites. Corundum crystals (up to 200 μm long) occur in plagioclase and garnet and differ from each other depending on the host mineral, which serves as evidence against their xenogenic nature. Some corundum crystals exhibit an axial zone, which may indicate their crystallization from the gaseous phase. Corundum-bearing rocks are accompanied by piclogites (pyroxene?garnet varieties with olivine). Piclogites and their minerals (clinopyroxene, garnet) are characterized by a positive Eu anomaly, which implies rock reworking by fluids during corundum formation, when deep-seated complexes were subjected to exhumation.     

Distribution of asbestos in the bedrock of the northern New Jersey area

Asbestos has been identified at fifty-five locations in the bedrock of the northern New Jersey area. Most occurrences are confined to (1) The Precambrian rocks of the New Jersey Highlands, particularly the marbles; (2) the Paleozoic serpentinites of Staten Island, New York, and Hoboken, New Jersey; and (3) the Mesozoic basaltic rocks of the Newark Basin. Chrysotile and tremolite asbestos are present in local concentrations in the marbles. In the most extensive exposure of the marble, the Franklin band, traces of tremolite-actinolite asbestos are commonly present. Crocidolite asbestos occurs in localized areas associated with fracture systems in Precambrian pegmatites and associated rocks. The Paleozoic serpentinites contain chrysotile asbestos as a major component in deformed zones. Megascopic chrysotile and anthophyllite asbestos veins occur locally in the serpentinites. Actinolite asbestos occurs in the Mesozoic basaltic rocks of the Newark Basin. Potential environmental problems associated with asbestosbearing bedrock include production and use of rock products containing asbestos, introduction of asbestos into environments surrounding excavations, and asbestos contamination of soils and water supplies.     

Depositional environments and the hydrocarbon generative potential of Triassic rocks of the Barents Sea basin

With the recent large hydrocarbon discoveries in the Barents Sea, interest is turning to Triassic shales as the most likely source rocks. The results reveal the relationship between the quality of Triassic source rocks, the amount and type of organic matter, and the environments of deposition. The best source rocks are confined to the Lower and Middle Triassic successions deposited in a deep-water shelf environment in the western part of the basin. Shallow-marine and deltaic argillaceous rocks exhibit fair to poor gas-generative potential. Triassic fluvial-lacustrine deposits have little to no hydrocarbon generative potential.     

Syncline contribution to mountain peak building: examples from Western Zagros,Kurdistan Region,Northeastern Iraq

The current study focuses on roles of the synclines in building mountains of the Zagros Fold-Thrust Belt from Northeastern Iraq, Kurdistan Region. The studied area is located in the Imbricated Zone, which is a continuation of Simply Folded Zone of Iran. In some parts of this zone, the synclines contribute to building nearly half of the mountains and increase the relief contrast of the landscapes. The role of syncline in building mountains is opposite to its apparent intrinsic shape which is located in low land and normally forms synclinal valleys. In studied area, this phenomenon is observable in the areas that are complexly deformed, deeply exhumed, and predominately composed of the carbonate rocks. In these areas, exhumation and stream erosion have removed high thickness of overburden rocks and the tectonic stresses have crenulated carbonate rocks in the synclines. Due to these processes, these synclines are observable on the peak of the mountains now. The contribution of synclines in mountain building is related to three properties: the first is folding of synclines in confined condition (relative to anticlines) by which they have transformed to highly compact and stiff rocks. The second is accumulation of hydrothermal or meteoric water in the synclines due to its downward bending which aids precipitation of minerals and heals the possible tectonically and lithostatically induced fractures. The third is ductile deformation of synclines in confined and deeper level as compared to brittle deformation of anticlines in relatively shallower level and unconfined condition during deformation. Consequently, these factors result in an inversion of topography by which synclines are remained as mountains (high topography) above surface during exhumation while erosion removes anticlines. The result of this study is applicable for geological, geomorphological, and structural mapping in highly deformed and deeply exhumed terrains. The possible prediction of geomorphological position of synclines and anticlines on the mountain peaks and valley bottoms respectively is the applicable result in the above terrains.     

扬-泰-靖地区地下水系统水力联系与硫酸盐污染特征

本文对长江三角洲扬-泰-靖地区第四系松散层地下水中环境同位素(D、18O、34S)的分布特征进行了分析,旨在揭示大气降水、长江水、潜水及承压水之间的水力联系,辨别地下水中硫酸盐的来源及其污染状况。研究结果表明潜水含水层接受大气降水及长江水的补给,硫酸盐主要为农业污染来源或与海源硫酸盐的混合。承压含水层主要接受大气降水的补给,与潜水含水层及长江之间的水力联系较差,硫酸盐来源不同。在研究区顶部和沿江地段的浅层孔隙承压水中,硫酸盐来源于硫化物的氧化;在东部的深层孔隙承压水中,硫酸盐主要来源于硫酸盐岩的溶解或海源硫酸盐的滞留,基本未受到潜水或地表水中硫酸盐的污染。     

Petrogenesis of contact-style PGE mineralization in the northern lobe of the Bushveld Complex: comparison of data from the farms Rooipoort,Townlands, Drenthe and Nonnenwerth

In the present study, we document the nature of contact-style platinum-group element (PGE) mineralization along >100 km of strike in the northern lobe of the Bushveld Complex. New data from the farm Rooipoort are compared to existing data from the farms Townlands, Drenthe, and Nonnenwerth. The data indicate that the nature of the contact-style mineralization shows considerable variation along strike. In the southernmost portion of the northern Bushveld, on Rooipoort and adjoining farms, the mineralized sequence reaches a thickness of 700 m. Varied-textured gabbronorites are the most common rock type. Anorthosites and pyroxenites are less common. Chromitite stringers and xenoliths of calcsilicate and shale are largely confined to the lower part of the sequence. Layering is locally prominent and shows considerable lateral continuity. Disseminated sulfides may reach ca. 3 modal % and tend to be concentrated in chromitites and melanorites. Geochemistry indicates that the rocks can be correlated with the Upper Critical Zone. This model is supported by the fact that, in a down-dip direction, the mineralized rocks transform into the UG2-Merensky Reef interval. Between Townlands and Drenthe, the contact-mineralized sequence is thinner (up to ca. 400 m) than in the South. Chromitite stringers occur only sporadically, but ultramafic rocks (pyroxenites, serpentinites, and peridotites) are common. Xenoliths of calcsilicate, shale, and iron formation are abundant indicating significant assimilation of the floor rocks. Sulfides may locally form decimeter- to meter-sized massive lenses. PGE grades tend to be higher than elsewhere in the northern Bushveld. The compositions of the rocks show both Upper Critical Zone and Main Zone characteristics. At Nonnenwerth, the mineralized interval is up to ca. 400 m thick. It consists largely of varied-textured gabbronorites, with minor amounts of igneous ultramafic rocks and locally abundant and large xenoliths of calcsilicate. Layering is mostly weakly defined and discontinuous. Disseminated sulfides (<ca. 3 modal %) occur throughout much of the sequence. Geochemistry indicates that the rocks crystallized mainly from tholeiitic magma and thus have a Main Zone signature. The implication of our findings is that contact-style PGE mineralization in the northern lobe of the Bushveld Complex cannot be correlated with specific stratigraphic units or magma types, but that it formed in response to several different processes. At all localities, the magmas were contaminated with the floor rocks. Contamination with shale led to the addition of external sulfur to the magma, whereas contamination with dolomite may have oxidized the magma and lowered its sulfur solubility. In addition to contamination, some of the magmas, notably those of Upper Critical Zone lineage present at the south-central localities, contained entrained sulfides, which precipitated during cooling and crystallization.     

Middle–Late Devonian island-arc volcanosedimentary complexes in northwestern Rudny Altai

A volcanosedimentary complex of Middle–Late Devonian deposits in northwestern Rudny Altai is described. Analysis of sedimentologic processes, the kind of manifestation and specific composition of volcanism, and petrographic and chemical compositions of sedimentary rocks permitted reconstruction of paleogeographic and geodynamic settings, which confirmed the hypothesis of the island-arc nature of the Rudny Altai zone in the Middle–Late Devonian. Also, an alternative variant of its rift nature is considered. The rift buildings are referred to as local isolated complexes confined to positive paleovolcanic structures rising above the bottom in the fore-arc basin. The denudated part of the rift system is formed mainly by slope facies. Clastics of rift buildings, together with fragments of silicite and felsic-volcanics layers, compose widespread olistostrome breccias. We have established the humid type of lithogenesis, which, together with minor sea level fluctuations, favors karst processes and the formation of red-colored and bauxite-like rocks. The coeval volcanics composing lava flows and tuff layers are mainly rhyolites and are often of ultrafelsic composition.     

Ore sources of the base-metal veins at the Dzhimidon deposit,North Ossetia,Russia: Evidence from REE distribution in ores and host rocks

REE distribution patterns of the ores and host rocks of the Dzhimidon vein lead-zinc deposit (North Caucasus, Ossetia, Sadon mining district, Russia) have been analyzed to estimate ore sources of hydrothermal deposits. It is established that both types of prevailing rocks of the Sadon Area are involved in the formation of base-metal ores during activation of hydrothermal metasomatic processes in the Jurassic time, the said rocks are Paleozoic granites (the main ore-hosting rocks at the majority of deposits) and Pre- Cambrian schists (specific for only ore-hosting rocks of Dzhimidon deposit).     

Natural monocrystalline lonsdaleite

The existence of a new type of natural lonsdaleite confined to regionally metamorphosed and metasomatically modified rocks is verified based on complex data obtained by high resolution microscopy and spectroscopy. The structural and spectroscopic characteristics of isolated lonsdaleite particles that have a relatively large size and monocrystalline structure are given for the first time. The results of these studies can be used not only for simulating the processes of natural diamond formation, but also for designing superhard materials with allowance for the fact that lonsdaleite is the hardest material on Earth.     

Geochemistry of metasomatised spinel peridotite xenoliths from the Dariganga Plateau, South-eastern Mongolia

Summary One fresh (green), one altered (black) and one composite (green/black) peridotite xenolith from the Neogene-Quaternary basalts of the Dariganga Plateau, SE Mongolia, were studied by electron microprobe, X-ray fluorescence, wet chemical and instrumental neutron activation analysis. The history of the upper mantle underneath the Dariganga Plateau has been complex and is characterised by elemental depletions and enrichments processes. The rocks investigated appear to have been processed in several steps, have been moderately depleted (relative to the primitive upper mantle composition) in incompatible elements and subsequently metasomatically enriched in alkalis, Fe, Ca, LREE, Th and U. As a result, most peridotites are moderately depleted in Si, Cr, Ti, HREE and Hf, are slightly enriched in LREE and have elevated Th and U abundances. The minerals in all rocks are out of chemical equilibrium. In the green peridotites disequilibrium is modest but it is severe in the blackened lherzolites. The latter have experienced strong Fe metasomatism accompanied by strong oxidation. As a result, Mg-rich olivines formed by oxidation and precipitation of Fe oxides in the primary olivines (blackening) and Fe-rich olivines formed in the Fe metasomatic event. The latter could only have taken place after the oxidising event, otherwise the Fe-rich olivines would also have been affected by it. Three of the four rocks show negative anomalies (relative to the Ce abundance) of Hf and Ti, one is enriched in these elements, which is considered an indication of the action of carbonatitic melts/fluids in the upper mantle. Enrichment of U over Th in some of our samples seems also to indicate the presence of water in the fluid phase, however, the lack of (OH)-bearing minerals in the Dariganga xenoliths suggests a low activity of water in these fluids. The latest of the metasomatic events probably took place shortly before entrapment of the rocks by the basaltic lava that carried them to the earth’s surface. The composite sample consisting of a green harzburgite and a black lherzolite suggests that blackening took place at the original location of the rock rather than in the basaltic tuff because the latter should have altered the whole xenolith. It also demonstrates that metasomatic processes in the upper mantle can be confined to rather restricted locations with sharp boundaries towards the wall rocks. Blackening as well as the metasomatic events apparently took place because of a better permeability in one part of the rock as compared to the other, probably the result of tectonisation. Received May 28, 1999; revised version accepted February 24, 2001     

Geochemistry and formation model of manganiferous rocks in jaspers of the South Urals

Specific features of the geochemistry of manganiferous siliceous rocks confined to Devonian volcanogenic complexes of the Magnitogorsk belt in the South Urals are discussed. It is shown that with respect to the distribution of the major petrogenic and rare earth elements, as well as base and rare metals, manganese rocks are comparable with rocks of the low-temperature hydrothermal sources in active volcanic zones of the World Ocean. Our results agree well with the existing concepts about the hydrothermal-sedimentary origin of manganese deposits in the South Urals and corroborate this hypothesis with new independently obtained data.     

Paleozoic rocks in infrastructure of the metamorphic core, the Greater Caucasus Main Range zone

Granitoid orthogneisses and migmatites are widespread in the lower, deeply metamorphosed gneiss-migmatite complex of the pre-Alpine basement (infrastructure) exposed within northern part of the Greater Caucasus Main Range zone. Like the other rocks of the complex, they have been traditionally attributed to the Proterozoic, but the U-Pb dating revealed the Late Paleozoic age of migmatites and Devonian age of orthogneiss protolith. Bodies of blastomylonitic apogranite gneisses, which are confined to boundary between gneiss-migmatite complex and overlying Makera Complex of supracrustal rocks, turned out to be of the Late Paleozoic age as well. The dating results suggest synchronism and, apparently, genetic interrelations between the high-T/low-P metamorphism and granite formation in the Main Range zone of the Greater Caucasus.     

A geological interpretation of the aeromagnetic maps of the continental shelf around Orkney and Shetland

The distribution of eleven different patterns of magnetic anomalies over the maps is shown. The individual patterns overlying the land are correlated with the underlying geology, and on the basis of this correlation a geological interpretation of the sea areas of the aeromagnetic survey is proposed. The outcrops of crystalline rocks are largely confined to the present land areas, so that in most places the sea floor is composed of Devonian or younger rocks. The Newer Granites are classified into three types based on their magnetic properties. The Great Glen fault passes through Shetland where it causes a 65 km dextral displacement of the Devonian rocks and an unknown but possibly much larger displacement of the older rocks.     

Specific features in distribution of natural radioactivity in rocks of the Bazhenovo Formation on the western slope of the Surgut Dome,West Siberia

This paper presents an analysis of the radioactivity distribution in the Bazhenovo Formation rocks on the western slope of the Surgut Dome. The maximum uranium concentrations are confined to accumulations of organic matter and are proportional to its concentration in the rocks. The lowermost part of the section has lower radioactivity values than the upper one. The value of radioactivity varies throughout the area of the Surgut Dome. Coeval analogs of the Bazhenovo Formation show lower radioactivity values.