Depositional environment and apparent age of the Fauske carbonate conglomerate, North Norwegian Caledonides

The Fauske conglomerate represents a rather rare case of a monomict carbonate conglomerate in the Late Neoproterozoic to Silurian, lithostratigraphic successionsof the Norwegian Caledonides. Lithological varieties of this conglomerate unit from the Lovgavlen quarry have a highly decorative quality and are well known in both domestic and international markets under trading names such as 'Norwegian Rose', 'Jaune Rose', 'Norwegian Green', 'Antique Fonce' and 'Hermelin'. The Fauske conglomerate is a 60m-thick unit which rests on either dark grey ('blue') calcite marbles or white dolomite marbles. The latter are jointed and fragmented, and also appear as sedimentary collapse-breccia and debris where they are in direct contact with the conglomerate. Although the Fauske conglomerate has been involved in two main pulses of Caledonian tectonic deformation, which produced an early, syn-metamorphic flattening of the clasts and a later folding or rotation of clasts into a spaced cleavage, the overall sedimentary features are still remarkably well preserved. The Fauske conglomerate unit consists of 25 beds (5 cm to 3 metres thick) comprising landslide, carbonate debris and carbonate breccia-conglomero-breccias-greywacke lithofacies. Blocks, fragments, cobbles, pebbles and smaller clasts are mainly of white dolostone and pink, beige, white and 'blue' calcite marbles. The matrix has a granoblastic texture and similar range in lithology with variable amounts of quartz, fuchsite, sericite, muscovite and chlorite. Within the unit, an upward fining of the clasts is followed by the gradual development of calcareous greywacke layers which show both cross bedding and channelling. The depositional model involves: (i) a locally developed, tectonically unstable carbonate shelf-margin, (ii) a temporary lowering o sea level, (iii) formation of a high-relief, shore-to-basin fault scarp followed by (iv) the development of a channel, with (v) subsequent, long-distance transport of clasts of pink carbonates from the continent-basin margin,w hich were (vi) redeposited together with a carbonate debris (white dolomite and 'blue' calcite marbles) on the tectonically fragmenting edge of a carbonate shelf. Both matrix and pebbles show a similar range in isotopic values: -1.9 to +0.6 per mil (vs. PDB) for 13Ccarb and 0.70896 to 0.70946 for 87Sr/86Sr. The least altered 87Sr/86Sr (0.70896) isotopic value plotted on the calibration curve is consistent with a seawater composition corresponding to ages of 470-475, 505-510 and 520, whereas the least altered 13C carb (-0.6 per mil) value matches only 520 Ma

Special traits of the millimeter wave relativistic magnetron

A 8 mm band relativistic magnetron is investigated experimentally and by means of numerical simulation. The physical effects are analyzed which influence negatively the r.f. generation. The processes capable of reducing effectiveness of the generation and duration of the generated pulse include forward and backward axial flows of electrons, and intense electric fields – the generated microwaves and the fields owing to the space charge.Экспериментально и методами численного моделирования проведено исследование релятивистского магнетрона 8-миллиметрового диапазона и анализ факторов, оказывающих негативное действие на генерацию. Отмечается, что к процессам, уменьшающим эффективность и длительность импульса генерации, относятся прямые и обратные осевые потоки электронов, интенсивные собственные ВЧ-поля и поля объемного заряда.Експериментально й методами чисельного моделювання досліджено релятивістський магнетрон діапазону 8 мм та проведено аналіз факторів, що негативно впливають на генерацію. Визначено, що до процесів, котрі зменшують ефективність та тривалість імпульсу генерації, відносяться прямі та зворотні осьові потоки електронів, інтенсивні власні ВЧ-поля і поля об’ємного заряду

Chemostratigraphy of Neoproterozoic carbonates: implications for 'blind dating'

The delta C-13(carb) and Sr-87/Sr-86 secular variations in Neoproteozoic seawater have been used for the purpose of 'isotope stratigraphy' but there are a number of problems that can preclude its routine use. In particular, it cannot be used with confidence for 'blind dating'. The compilation of isotopic data on carbonate rocks reveals a high level of inconsistency between various carbon isotope age curves constructed for Neoproteozoic seawater, caused by a relatively high frequency of both global and local delta C-13(carb) fluctuations combined with few reliable age determinations. Further complication is caused by the unresolved problem as to whether two or four glaciations, and associated negative delta C-13(carb) excursions, can be reliably documented. Carbon isotope stratigraphy cannot be used alone for geological correlation and 'blind dating'. Strontium isotope stratigraphy is a more reliable and precise tool for stratigraphic correlations and indirect age determinations. Combining strontium and carbon isotope stratigraphy, several discrete ages within the 590-544 Myr interval, and two age-groups at 660-610 and 740-690 Myr can be resolved

Palaeoproterozoic magnesite: lithological and isotopic evidence for playa/sabkha environments

Magnesite forms a series of 1- to 15-m-thick beds within the approximate to2.0 Ga (Palaeoproterozoic) Tulomozerskaya Formation, NW Fennoscandian Shield, Russia. Drillcore material together with natural exposures reveal that the 680-m-thick formation is composed of a stromatolite-dolomite-'red bed' sequence formed in a complex combination of shallow-marine and non-marine, evaporitic environments. Dolomite-collapse breccia, stromatolitic and micritic dolostones and sparry allochemical dolostones are the principal rocks hosting the magnesite beds. All dolomite lithologies are marked by delta C-13 values from +7.1 parts per thousand to +11.6 parts per thousand (V-PDB) and delta O-18 ranging from 17.4 parts per thousand to 26.3 parts per thousand (V-SMOW). Magnesite occurs in different forms: finely laminated micritic; stromatolitic magnesite; and structureless micritic, crystalline and coarsely crystalline magnesite. All varieties exhibit anomalously high delta C-13 values ranging from +9.0 parts per thousand to +11.6 parts per thousand and delta O-18 values of 20.0-25.7 parts per thousand. Laminated and structureless micritic magnesite forms as a secondary phase replacing dolomite during early diagenesis, and replaced dolomite before the major phase of burial. Crystalline and coarsely crystalline magnesite replacing micritic magnesite formed late in the diagenetic/metamorphic history. Magnesite apparently precipitated from sea water-derived brine, diluted by meteoric fluids. Magnesitization was accomplished under evaporitic conditions (sabkha to playa lake environment) proposed to be similar to the Coorong or Lake Walyungup coastal playa magnesite. Magnesite and host dolostones formed in evaporative and partly restricted environments; consequently, extremely high delta C-13 values reflect a combined contribution from both global and local carbon reservoirs. A C- 13-rich global carbon reservoir (delta C-13 at around +5 parts per thousand) is related to the perturbation of the carbon cycle at 2.0 Ga, whereas the local enhancement in C-13 (up to +12 parts per thousand) is associated with evaporative and restricted environments with high bioproductivity

Composition of Fluids Responsible for Gold Mineralization in the Pechenga Structure-Imandra-Varzuga Greenstone Belt, Kola Peninsula, Russia.

This study presents the first fluid inclusion data from quartz of albite–carbonate–quartz altered rocks and metasomatic quartzite hosting gold mineralization in the Pechenga structure of the Pechenga– Imandra–Varzuga greenstone belt. A temperature of 275–370°C, pressure of 1.2–4.5 kbar, and the fluid composition of gold-bearing fluid are estimated by microthermometry, Raman spectroscopy, and LA-ICP-MS of individual fluid inclusions, as well as by bulk chemical analyses of fluid inclusions. In particular, the Au and Ag concentrations have been determined in fluid inclusions. It is shown that albite–carbonate–quartz altered rocks and metasomatic quartzite interacted with fluids of similar chemical composition but under different physicochemical conditions. It is concluded that the gold-bearing fluid in the Pechenga structure is similar to that of orogenic gold deposits

Palaeoproterozoic travertines of volcanic affiliation from a C- 13-rich rift lake environment

The ca. 2.2 Ga Kuetsjarvi Sedimentary Formation (KSF) of the Pechenga Greenstone Belt, the north-east Fennoscandian Shield, contains the oldest travertines so far reported in the literature. The travertine-hosting succession is composed of flat-laminated, dolomitic stromatolites, micritic and allochemical dolostones, all formed in coastal plains of a rift-related playa lake system in an arid/semiarid climate. The travertines are categorised as thermal, autochtonous, spring travertine crust and spring orifice travertine mounds formed from hot water springs issuing at the playa lakeshore. The travertines are represented by crusts and small-scale mounds, both composed of a series of 1-5-mm thick dark-light couplets of fibrous dolomite crystals with compromise boundaries and radiating dolomite crystals. Travertines are composed of dolomite (Mg/Ca = 0.42-0.47), exhibiting a general lack of replacement textures. Travertines exhibit several phases of growth separated by dissolution processes and accompanied by precipitation of SiO2. The upper surface of crusts and mounds are often affected by desiccation joints and covered with thicker SiO2 veneer (delta O-18 = +18.6 parts per thousand). The host dolostones are characterised by delta C-13(carb) of +6.1-+8.8 parts per thousand (V-PDB) and delta O-18 values ranging from 11 parts per thousand to 19 parts per thousand (V- SMOW). Microcored samples of travertine dolomite exhibit delta C-13 and delta O-18 values ranging from -6.1 parts per thousand to + 7.7 parts per thousand and from 12.0 parts per thousand to 21.5 parts per thousand, respectively. The C-13-rich nature of the background stratified dolostones is caused by the 2.4-2.06 Ga global shift in C-13/C-12 carbonate carbon, associated with enhanced accumulation of C-org and perturbation of the global carbon cycle. The source of CO2 for the travertines is considered to be of volcanogenic origin with an initial delta C-13 value of ca. - 6.0 parts per thousand. Several processes contributed to carbon and oxygen isotope composition of travertine dolomite: (i) an initial CO2 loss at the spring orifice, (ii) water cooling and rapid CO2 loss during water transit, (iii) persistent evaporation from thermal water ponds/pools, and (iv) mixing with CO2 of ambient lake water enriched in C-13. Silica is thought to have been dissolved by groundwater from underlying sediments and been transported upwards by a mechanism of capillary rise due to dry climatic condition

Palaeoproterozoic rift-related, 13C-rich, lacustrine carbonates, NW Russia. Part I: sedimentology and major element geochemistry

The 150-m-thick Kuetsjärvi Sedimentary Formation (KSF) from the Pechenga Greenstone Belt, NW Russia, is one of the key formations in the study of a positive δ13Ccarb excursion occurring globally in the Palaeoproterozoic. The KSF formed in an intracratonic rift setting and is sandwiched between two, 2-km-thick subaerially erupted volcanic units. The KSF was previously interpreted as shallow marine, but new data reported here indicate that it is a non-marine unit deposited on a deeply subaerially weathered surface mantling the underlying volcanic rocks. The lowermost part of the KSF represents an alluvial–fluvial plain, followed by a laterally and vertically variable succession of variegated to mottled fine-grained siliciclastic rocks and ‘red beds‘, dolostones containing stromatolite sheets, hydrothermal travertine deposits and abundant desiccation features (e.g.tepees, surfical silicified crusts and dissolution cavities), including probable pseudomorphed evaporites. Measured S and Corg concentrations for the carbonate and siliciclastic rocks are low. Combined, these features indicate that the carbonate rocks of the KSF accumulated in a shallow lacustrine setting. Major types of carbonate facies were formed by: (1) biologically-induced precipitation; (2) evaporitic removal of CO2 in a closed lake environment; and (3) chemical precipitation from thermal springs. Apparently, none of these carbonate facies was in full isotopic equilibrium with atmospheric CO2. This interpretation shows the importance of taking into account the interplay between global and local depositional factors when interpreting the isotopic signature of the KSF dolostones and its implication for the Palaeoproterozoic carbon isotope excursion

On the Lomagundi-Jatuli carbon isotopic event: the evidence from the Kalix Greenstone Belt, Sweden

We report a significant new database on the carbon and oxygen isotopic analyses (233 whole-rock, and 43 microcored samples) obtained from sedimentary dolostones in the Kalix Greenstone Belt from the Fennoscandian Shield of Sweden. This characterises the second half of the prominent Palaeoproterozoic Lomagundi-Jatuli positive carbon-isotope excursion. Within a 600 m-thick succession of alternating volcanic, volcaniclastic, siliciclastic and dolomitic rocks from greenschist–epidote-amphibolite metamorphic facies, the least altered dolostone samples show a gentle oscillation between +2‰ and +4‰ throughout stratigraphy with a second-order positive excursion from +4‰ through +8‰, and gradually back again to +4‰ in the c. 150 m-thick unit in the middle and upper parts of the succession. This second-order excursion is superimposed on the general relatively rapid δ13C decline at the second half of the Lomagundi-Jatuli isotopic event and coincides with the transition from a marine-influenced rift to a passive margin setting. If the excursion is global, it would reveal internal fine structure of the Lomagundi-Jatuli isotopic excursion; if not, basinal local factors are the main players. Lithostratigraphic and C-isotope correlation between Kalix and neighbouring Peräpohja Schist Belt successions, 40-km-apart, suggest an incomplete geological record in both successions despite the absence of obvious non-depositional unconformities