Germanium/silica ratio and trace element composition of Early Cambrian siliceous rocks in Keping: implications for the siliceous rocks’ formation and paleoenvironment interpretations

Acta Geochimica - This study used the germanium/silica (Ge/Si) ratios, together with rare earth elements and other trace elements to infer the siliceous source and sedimentary environment of the...     

Geochemistry and lithology of accretionary wedge in the Pekul’nei range, central Chukotka

Based on the study of various facies of ancient and modern oceanic sediments, the Ce/La versus Zr/Y criterion is proposed to discriminate the principal genetic groups of sedimentary rocks in foldbelts and to determine their geodynamic formation conditions. The structure and composition of siliceous, clayey and fineclastic tuffaceous rocks from various complexes of paleoaccretionary wedge in the Pekul’nei Range (Chukotka) are discussed. Lithological and geochemical studies of sedimentary rocks in the Pekul’nei Range allowed us to trace the regular variations in rock sections depending on (1) propagation of oceanic plate from the spreading center toward the convergent boundary for paleooceanic rock complexes and (2) sedimentation regime and provenances for rocks deposited above the convergent plate boundary.     

“Hot” particles of the Yenisei River: Radioisotope composition,structure, and behavior in natural conditions

The radioisotope compositions, structure, and behavior of “hot” particles under the natural conditions of the Yenisei River in the nearby influence zone of Krasnoyarsk mining and chemical combine were examined in this study. By the composition of gamma-radiating isotopes, all particles are divided into three groups: mono-isotopic, bi-isotopic and poly-isotopic. Mono-isotopic “hot” particles, containing only ⁶⁰Co were discovered for the first time. Transuranium elements (²⁴¹Am and ²⁴³Cm) were revealed in poly-isotopic particles. Investigation of poly-isotopic particles by means of a scan electron microscope showed that the presence of UO₂ together with the lack of activation of ¹⁵²Eu testifies to the reactor origin of this particle. Experiments showed that in natural conditions “hot” particles under the influence of filtrating water and moisture of pore solutions are gradually dissolved and radionuclides are repeatedly drawn into migration.     

Stratigraphy of the Okhotsk–Chukotka Belt in the Headwaters of the Malyi Anyui River (the Vicinity of Kupol Deposit): U–Pb and 40Ar/39Ar Age Data

Doklady Earth Sciences - U–Pb and 40Ar/39Ar age data obtained for volcanic rocks of the Okhotsk–Chukotka Belt in the headwaters of the Malyi Anyui River (the vicinity of Kupol deposit)...     

Rock composition and origin of the Duwi Formation calcareous rocks, Upper Egypt

Upper Cretaceous phosphorite beds of the Duwi Formation, Upper Egypt, are intercalated with limestone, sandy limestone, marl, calcareous shales, and calcareous sandstone. Calcareous intercalations were subjected to field and detailed petrographic, mineralogical and geochemical investigations in order to constrain their rock composition and origin. Mineralogically, dolomite, calcite, quartz, francolite and feldspars are the non-clay minerals. Smectite, kaolinite and illite represent the clay minerals. Major and trace elements can be classified as the detrital and carbonate fractions based on their sources. The detrital fraction includes the elements that are derived from detrital sources, mainly clay minerals and quartz, such as Si, Al, Fe, Ti, K, Ba, V, Ni, Co, Cr, Zn, Cu, Zr, and Mo. The carbonate fraction includes the elements that are derived from carbonates, maily calcite and dolomite, such as Ca, Mg and Sr. Dolomite occurs as being dense, uniform, mosaic, very fine-to-fine, non-ferroan, and non-stoichiometrical, suggesting its early diagenetic formation in a near-shore oxidizing shallow marine environment. The close association and positive correlation between dolomite and smectite indicates the role of clay minerals in the formation of dolomite as a source of Mg^2＋ -rich solutions. Calcareous rocks were deposited in marine, oxidizing and weakly alkaline conditions, marking a semi-arid climatic period. The calcareous/argillaceous alternations are due to oscillations in clay/carbonate ratio.     

Compositional and thermal evolution of olivine-hosted melt inclusions in small-volume basaltic eruptions: a “simple” example from Dotsero Volcano,NW Colorado

Olivine-hosted melt inclusions have been analyzed from the young (4,150 ± 300 ybp) Dotsero basaltic (48.2 wt% SiO₂) lava flow in Northwest Colorado, USA. Silicate melt-inclusion compositions have a bimodal distribution (41–46 wt% SiO₂ and 47–50 wt% SiO₂). Low-Si melt inclusions record high pre-eruptive sulfur concentrations (>1,000 ppm S) and variations in their major- and trace-element compositions appears to be related to shallow assimilation of local basement sandstone. Whole-rock compositions are modeled as a contamination of low-Si inclusion compositions with ~10 wt% sandstone. Host olivine crystallization may have accompanied magma injection into a shallow storage chamber. In contrast to the low-Si melt inclusions, the high-Si population is relatively degassed and records late-stage rapid crystallization either during or post-eruption. Hopper or skeletal olivine grains in conjunction with the bimodal inclusion compositions suggest relatively rapid cooling rates at the time of eruption and inclusion entrapment. Inclusion compositions, in conjunction with mineral textures, therefore provide a more complete picture of shallow magma processes, coupling the relative timing of undercooling and crystallization, assimilation and melt compositional evolution. Most of the inclusion and host textural and compositional data indicates late and very shallow petrogenetic processes and does not appear to record deeper (mid-, lower-crustal) processes.     

Age of dispersed uranium mineralization in rocks of the framework of the Strel’tsovka uranium ore field and the Yamsky site,Eastern Transbaikal region

An isotopic geochronological study of dispersed uranium mineralization was performed in the granitic rocks of the Urtui pluton in the framework of the Strel’tsovka uranium ore field and in the Yamsky site of the Urov-Uryumkan granite-gneiss arch. Two stages of such mineralization—783 ± 26 Ma in the Urtui granitic pluton and 138.6 ± 2.3 Ma in the Yamsky site—have been established. The emplacement of granite pertaining to the Unda Complex disturbed the U-Pb isotopic system in uraninite from the Urtui granitic pluton and resulted in redeposition of uranium phase dated at 262 ± 34 Ma. The young, probably, recent process gave rise to the redistribution of radiogenic lead in the U-bearing phases developing after uraninite.     

Molecular composition and indigenity of organic matter in Late Neoproterozoic sedimentary rocks from the Yangtze region, South China

1 Introduction As a linkage between the biosphere and the geosphere, organic geochemistry, especially molecular markers, has become a powerful tool for investigating important geological events and the evolutionary history of ancient life on Earth (Kvenvo…     

Devonian ultramafic lamprophyre in the Irkineeva–Chadobets trough in the southwest of the Siberian Platform: Age,composition, and implications for diamond potential prediction

The results of geochronological, mineralogical, petrographical, and geochemical study of the Ilbokich ultramafic lamprophyre are reported. The specific features in the mineral and chemical compositions of the studied ultramafic lamprophyre indicate that it can be regarded as a variety similar to aillikite, while other differences dominated by K-feldspar can be referred to damtjernite. According to Rb–Sr analysis, ultramafic lamprophyre dikes intruded at the turn of the Early and Middle Devonian, about 392 Ma ago. This directly proves the existence of Early Paleozoic alkali–ultramafic magmatism in the northern part of the southwest Siberian Platform. A finding of Devonian alkali–ultramafic lamprophyre is of dual predictive importance. On the one hand, it is indicative of the low probability of finding large diamond-bearing deposits in close association with aillikite. On the other hand, it can be indicative of a possible large Devonian diamond province in the studied territory, where diamondiferous kimberlite is structurally separated from aillikite.     

Oxygen isotopic composition of coexisting minerals of sillimanite-hypersthene rocks from the Por’ya bay area: Evidence of fluid involvement in granulite-facies metamorphism

Hypersthene-garnet-sillimanite-quartz enclaves were studied in orthopyroxene-plagioclase and orthopyroxene-clinopyroxene crystalline schists and gneisses from shear zones exposed in Palenyi Island in the Early Proterozoic Belomorian Mobile Belt. Qualitative analysis of mineral assemblages indicates that these rocks were metamorphosed to the granulite facies (approximately 900°C and 10–11 kbar). Oxygen isotopic composition was determined in rock-forming minerals composing zones of the enclaves of various mineralogical and chemical composition. The closure temperatures of the isotopic systems obtained by methods of oxygen isotopic thermometry are close to the values obtained with mineralogical geothermometers (Grt-Opx and Grt-Bt) and correspond to the high-temperature granulite facies (860–900°C). Identified systematic variations in the δ¹⁸O values were determined in the same minerals from zones of different mineral composition. Inasmuch as these zones are practically in contact with one another, these variations in δ¹⁸O cannot be explained by the primary isotopic heterogeneity of the protolith. The model calculations of the extent and trend of the δ¹⁸O variations in minerals suggest that the only mechanism able to generate the zoning was fluid-rock interaction at various integral fluid/rock ratios in discrete zones. This demonstrates that a focused fluid flux could occur in lower crustal shear zones. The preservation of high-temperature isotopic equilibria of minerals testifies that the episode of fluid activity at the peak of metamorphism was very brief.     

Age and origin of subvolcanic rocks from NE Iran: Link between magmatic “flare-up” and mineralization

Paleogene igneous rocks from ～600?km Quchan-Sabzevar-Torud magmatic belt include a thick pile of volcanic and pyroclastic rocks which intruded by younger felsic-mafic plutons. Various types of mineralization including Iron Oxide-Copper-Gold (e.g., Firouzeh mine) and porphyry Cu-Au deposits (e.g., Jalambadan mine) are associated with the Quchan-Sabzevar magmatism. In this study, we describe new zircon U-Pb ages and geochemical-isotopic data of the subvolcanic rocks from near the Firouzeh mine. The Firouzeh subvolcanic rocks consist of (quartz-bearing) monzosyenites, monzodiorites and monzonites. These rocks have typical calc-alkaline signature and are mainly metaluminous in nature. Subvolcanic rocks display enrichment in Light Rare Earth Elements (LREEs) with negative Eu anomaly. Enrichment in Large Ion Lithophile Elements (LILEs) and depletion in High-Field Strength Elements (HFSEs) are geochemical characteristics of these rocks. The Firouzeh volcanic rocks also display calc-alkaline signature and are metaluminous to peraluminous. Volcanic rocks show both enrichment in LREEs and LILEs, associated with negative Eu anomaly. Zircon U-Pb indicates ages of 43.2?±?0.4, 42.1?±?0.4 and 41.8?±?0.4 Ma for monzosyenites, monzodiorites and monzonites respectively. Zircon epsilon Hf_(t) shows average values of ?1.49 for monzonites, +9.07 for monzodiorites and ?1.06 for monzosyenites. The Hf model ages for these rocks are in the range of 850–730, 270–180 and 3150–450 Ma, respectively. Inherited zircons are abundant in monzonites and have variable Hf isotope values. The wide range of zircon εHf_(t) values and abundance of inherited/xenocrystic zircons suggest a multiple source(s) for the generation of the Firouzeh subvolcanic rocks, including a mantle melt and an old crustal component. Xenocrystic zircons indicate complex crustal components. We suggest the NE Iran subvolcanic rocks including the Firouzeh igneous rocks, generated above the Sabzevar subduction zone. This subduction zone was active since Late Cretaceous time.     

New data on the structure and age of Jurassic-Lower cretaceous sequences of the Algan terrane (Pereval’naya River Area, Koryak Highland, Chukotka)

The analysis of radiolarian assemblages and lithological types of siliceous rocks yielded new data on the structure of different sequences constituting the northern Algan terrane in the Pereval’naya River basin. Three tectonic slices formed by different lithothectonic complexes can be defined in this area. The radiolarian assemblages from the volcanogenic-siliceous slice allow the oceanic section to be dated back to the Kimmeridgian-Valanginian. The tuffaceous-terrigenous slice contains cherts of two types: (1) formed in situ and (2) reworked. The reworked deep-sea cherts yielded Bathonian-Kimmerdgian and Bathonian-Oxfordian radiolarians, while late Aalenian-late Bathonian radiolarian assemblages were extracted from the Kimmerdgian-Tithonian matrix of siliceous rocks.     

Late Pleistocene Tendürek Volcano (Eastern Anatolia,Turkey): I. Geochronology and petrographic characteristics of igneous rocks

The series of two papers presents a comprehensive isotope-geochronological and petrological-geochemical study of the Late Quaternary Tendürek Volcano (Eastern Turkey), one of the greatest volcanoes within the Caucasian—Eastern Anatolian segment of the Alpine foldbelt. The first article discusses the results of chronostratigraphic reconstruction and provides the main petrographic characteristics of the Tendürek’s igneous rocks. The K-Ar dating results show that the magmatic activity of the Tendürek Volcano developed in the Late Pleistocene time, over the period of the last 250 thousand years. Five discrete phases (I—250–200 ka, II—200–150 ka, III—150–100 ka, IV—100–70 ka, and V—<50 ka) of the youngest magmatism were identified in this study. The first two phases were represented by the fissure eruptions of alkaline basic lavas and subsequent formation of vast lava plateaus, the Çald?ran and Do?ubeyaz?t plains. In the following phases, the intermediate and moderately-acid volcanic rocks of mildly-alkaline or alkaline series started to dominate among the eruption products. According to their petrographic characteristics, the rocks of Tendürek Volcano are assigned to the alkaline association with Na-specifics (hawaiites-mugearites-benmoreites). The available geological, isotope-geochronological, and geomorphological data suggest that the Tendürek Volcano is potentially active. Nowadays, Tendürek reaches the caldera stage of its development.     

Metasedimentary rocks from the Bol’shoi Cheremshan Group in the East European Platform basement: Initial mineral composition and petrogeochemical properties

The initial normative mineral composition of some metasedimentary rocks (high-alumina crystalline schists, aluminous gneisses, and eulysites) from the Archean Bol’shoi Cheremshan Group is considered. Based on some data (distribution of data points available for these rocks in diagnostic diagrams, values of several indicator ratios, and initial rock composition), it is assumed that examined rocks were partly formed after highly mature sediments (high-alumina crystalline schists and gneisses), while biotite-garnet-andesine gneisses represent products of graywacke sediments. Elevated concentrations of some indicator elements (Fe, Cr, Ni, V, Zn, Pb, and others) imply that material in the sedimentation paleobasin was likely derived from provenances composed of variable lithologies (ultramafic, mafic, intermediate, and acid rocks). It seems that conditions in this paleobasin were favorable for the development of organic life. It is shown that correlation between some elements (Ti, V vs. Al; Ba, Pb vs. K; Sr vs. Ca; La vs. Na; and others) typical of Phanerozoic sedimentary rocks is also observed in the rocks. The possible contents of organic matter (C_org) and U in primary sediments of metasedimentary rocks of the Bol’shoi Cheremshan Group in the East European Platform basement are reconstructed.     

Geological structure and isotopic—geochronologic study of rocks from the Strel’na segment of the Terskii Greenstone belt,Kola Peninsula

The geological structure, age, and genesis of sedimentary—volcanogenic, metamorphic, and metasomatic rocks from the Terskii greenstone belt fringing the southern Imandra—Varzuga structure in the southeastern Kola Peninsula are discussed with defining main stages in endogenic activity of the region in the Late Archean and Early Proterozoic. The U-Pb method (SHRIMP-II, ID-TIMS, and Pb-LS techniques) was used to determine the age of volcano-sedimentary rocks of the Imandra Group as well as that of magmatic and superimposed metamorphic and metasomatic processes. The basic—intermediate metavolcanics of the Imandra Group are dated at 2.67 Ga, which corresponds to the Lopingian Gimol’skii Superhorizon (Late Archean). The Archean metavolcanics were subjected to Early Proterozoic regional metamorphism 2.1 Ga ago and metasomatic processes in the period of 1.85 to 1.77 Ga ago. The obtained data indicate multistage evolution of rock formation in the Terskii greenstone belt located in the southern flank of the Imandra—Varzuga structure in the Kola Peninsula.     

New data on the composition and age of the Ust’-Emuneret flora from the Enmyvaam River basin (Central Chukotka)

The Ust’-Emuneret floristic assemblage from the Emuneret Formation developed in the Enmyvaam River basin (Okhotsk-Chukotka volcanogenic belt) is described. The age of flora-bearing deposits is debatable, being previously determined in the interval of the Cenomanian to Campanian. According to the new data, the assemblage includes 57 species of fossil plants with dominant angiosperms, subdominant conifers, and subordinate ferns, liverworts, ginkgoaleans, and czekanowskialeans. The Ust’-Emuneret flora is characterized by the presence of advanced forms among different plant groups, which occur in Santonian-Campanian and younger floras of northeastern Russia, and by the presence of relict ginkgoaleans and czekanowskialeans. The flora is compared with other floras from neighboring regions, which provides grounds for the inference on its most probable Late Santonian age (probably, including the Early Santonian and initial Campanian).     

Chemical composition of snow and soil in Svirsk city (Irkutsk Region,Pribaikal’e)

The aim of the research was to study the chemical composition of snow and soil of Svirsk city (Pribaika?e) in order to assess the current environmental-geochemical situation in the city. Industrial specifics in Svirsk city is marked by higher concentrations of many toxic elements—As, Cd, Pb, W, Sb, Pb, Zn, Hg, and characteristic of preexisting and currently existing enterprises. The contents in biological matrices of some of the toxicants exceed physiological standards and violate the balance of elements necessary for people. The results of studies of the chemical (major and trace elements) composition of snow water, solid residue of snow and soil, a comparative analysis with earlier published data, and maximum permissible concentrations of elements have shown that the environmental situation in the city has been adversely affected within the last 20 years due to technogenesis.     

The acid lakes and rivers of Copahue Volcano, Argentina

Copahue is an active volcano in the Andes （Argentina, 37.5 S, 71 W） with an acid crater lake, acid chloride-sulfate hot springs feed the Upper Rio Agrio River that discharges into glacial Lake Caviahue. The Lower Rio Agrio outlet mixes with non-acidic meteoric waters, which leads to abundant precipitation of poorly crystalline yellow flocs. Further dilution and neutralization downstream lead to the precipitation of Al-rich material. This river and lake system is an extreme example of a naturally acidified watershed with similarities to streams impacted by acid mine drainage. We determined the chemistry and element fluxes of the Copahue-Caviahue system between 1997 and 2006, and this time series carries data from before, during and after the 2000 Copahue eruptions. The crater lake and hot springs had pH〈0.5 prior to the eruption with up to 6.5 % sulfate and 1% chloride. The fluids were saturated relative to gypsum/anhydrite and copper sulfides. Sulfur isotope studies suggested that the sulfate and associated acidity of the water formed from the disproportionation of magmatic SO2 at about 300℃. Just prior to the eruption, the fluids became saturated with additional minerals, including jarosite and hematite. The Upper Rio Agrio bed became coated with bright red hematite in 2004. Mixing with glacial meltwater increases the pH of the Upper Rio Agrio to values of 1-2, and the input of the Upper Rio Agrio has gradually acidified Lake Caviahue to pHi2.5. Annual element fluxes through the Upper Rio Agrio before the eruption were on the order of 15000 tounes Cl, 25000 tounes SOn and 1500 tounes F, with large outputs of dissolved rock contents as well. Annual volcanogenic trace element fluxes through the river include 7.5 tounes As, 5 tounes B and 0.6 tounes Li. After the eruption, the system became more dilute, the element fluxes decreased and Lake Caviahue became less acidic （pH≈2.8）. In the spring of 2006, the hot springs and crater lake had pH values〈 0.5 once again, suggesting that the system is becoming more active again.