PROVENANCE AND AGE CONSTRAINTS OF THE RIPHEAN SANDSTONES FROM THE IVANOVSKY GRABEN (THE KOLA PENINSULA) BASED ON THE U-Pb LA-ICP-MS DATING AND RAMAN SPECTROSCOPY OF DETRITAL ZIRCON

The results of U-Pb LA-ICP-MS dating of 150 detrital zircon grains from the Precambrian (Riphean?) sandstones of the Kildin group on the coast of the Ivanovka Bay (northeastern Kola Peninsula) are presented. A group of three youngest grains yielded constraint on the sandstone maximum depositional age of 1179±45 Ma. Detrital zircon age distribution is dominated by three maximums of 1.9, 2.4 and 2.7 Ga. The authors consider two complementary scenarios of interpretation of the data obtained. The first scenario involves the contribution of distant provenance out of granitoids in the Lapland-Kola orogen (~1.9 Ga) and adjacent areas of the Kola-Karelia region (~2.7 Ga), Imandra-Varzuga structure (~2.4 Ga), and Sveconorwegian belt (~1.2 Ga); the off-Baltic (Greenland?) sources of the detrital zircon also cannot be excluded. According to the second scenario, the Ivanovsky graben received zircons also from the proximal sources – sialic complexes formed upon melting of granite-gneiss substratum of the upper crust of the Murmansk craton during largescale episodic mafic magmatism 1.86, 2.50 and 2.68 Ga. The Raman spectroscopy showed that about 25 % of the analyzed detrital zircon grains experienced a 1.4-Ga-old thermal effect which did not lead to Pb losses but partially restored the crystalline structure of zircon. This result can be used as an additional constraint in the detailed provenance analysis of the northeastern East European Platform sedimentary basins and is indicative of 1.4-Ga-old large-scale magmatic events related to the breakup of the supercontinent Nuna

Origin of false components of NRM during conventional stepwise thermal demagnetization

© 2017 Many Permian-Triassic dolerite samples from the Siberian Trap Large Igneous Province exposed to conventional stepwise thermal demagnetization at 250-450 °C display mid-temperature remanence (MTC) directed opposite to the high-temperature NRM component. Alternating field (A.C.) demagnetization fails to isolate the antipodal component, but it appears during continuous thermal demagnetization, though in a different temperature range. Laboratory experiments and simulations prove that MTC remanence is an artifact resulting from magnetic memory of self-reversing titanomagnetite grains oxidized at low temperature. This effect can deform the results stepwise thermal cleaning and be responsible for misleading patterns of paleomagnetic directions. Given that oxidized titanomagnetite grains are widespread in volcanic rocks, we suggest to identify true paleodirections by combined continuous and stepwise thermal demagnetization. The extension of our model to the case of NRM2 overprint directed at some angle to partially reversed primary NRM1 component accounts for the difference between the results of stepwise and continuous thermal demagnetization observed in samples of the Steens Mountain basalt (USA)

GEOMAGNETIC EXCURSION RECORD PRESERVED IN THE SPELEOTHEM FROM WESTERN CAUCASUS: FIRST DATA

The study of the geomagnetic field evolution on minor timescales, in particular of such significant events as geomagnetic reversals and excursions, has acquired particular relevance nowadays due to the increased attention of mankind to the environment. The question of how exactly abrupt changes in the characteristics of the geomagnetic field affect the climate and biosphere remains largely debatable; the idea of the speed and dynamics of these changes is also very vague. "Classical" geological objects and existing methods provide limited opportunities for highly detailed reconstructions of geomagnetic field variations; therefore, paleomagnetologists are looking for new objects and approaches to solve this problem. The research that we have begun involves the use – for the first time in Russia – of speleothems to study secular variations of the geomagnetic field.This brief communication presents paleomagnetic records of two drill-cores from the flowstone from Vorontsovskaya cave, located on the western flank of the Caucasus Mountains in the valley of the river Kudepsta. Preliminary results indicate the presence of a geomagnetic excursion record in both drill-cores. Further study of the samples from Vorontsovskaya cave will make it possible to compare the discovered event with known excursions, as well as to clarify its age, duration, and dynamics

Magnetic stratigraphy of the Ordovician in the lower reach of the Kotuy River: the age of the Bysy-Yuryakh stratum and the rate of geomagnetic reversals on the eve of the superchron

© 2017, Pleiades Publishing, Ltd. Until recently, the existing data prevented the geophysicists from accurately dating the Bysy-Yuryakh stratum, which outcrops in the middle reach of the Kotuy River, constraining the time of its formation to a wide interval from the end of the Late Cambrian to the beginning of the Silurian. The obtained paleomagnetic data unambiguously correlate the Bysy-Yuryakh stratum to the Nyaian regional stage and constrain its formation, at least a considerable part of it, by the Tremadocian. This result perfectly agrees with the data on the Bysy-Yuryakh conodonts studied in this work and yields a spectacular example of the successful application of paleomagnetic studies in solving important tasks of stratigraphy and, correspondingly, petroleum geology. Within the Bysy-Yuryakh stratum, we revealed a large normal-polarity interval corresponding to the long ( > 1 Ma) period when the geomagnetic reversals were absent. This result, in combination with the data for the Tremadocian and Middle–Upper Cambrian sequences of the other regions, indicates that (1) the rate of occurrence of the geomagnetic reversals on the eve of the Ordovician Moyero superchron of reversed polarity was at most one reversal per Ma; (2) the superchron does not switch on instantaneously but is preceded by a certain gradual change in the operation conditions of the dynamo mechanism which, inter alia, manifests itself by the reduction of the frequency of geomagnetic reversals with the approach of the superchron. This finding supports the views according to which a process preparing the establishment of the superchrons takes place at the core–mantle boundary

Magnetic stratigraphy of the Ordovician in the lower reach of the Kotuy River: the age of the Bysy-Yuryakh stratum and the rate of geomagnetic reversals on the eve of the superchron

© 2017, Pleiades Publishing, Ltd. Until recently, the existing data prevented the geophysicists from accurately dating the Bysy-Yuryakh stratum, which outcrops in the middle reach of the Kotuy River, constraining the time of its formation to a wide interval from the end of the Late Cambrian to the beginning of the Silurian. The obtained paleomagnetic data unambiguously correlate the Bysy-Yuryakh stratum to the Nyaian regional stage and constrain its formation, at least a considerable part of it, by the Tremadocian. This result perfectly agrees with the data on the Bysy-Yuryakh conodonts studied in this work and yields a spectacular example of the successful application of paleomagnetic studies in solving important tasks of stratigraphy and, correspondingly, petroleum geology. Within the Bysy-Yuryakh stratum, we revealed a large normal-polarity interval corresponding to the long ( > 1 Ma) period when the geomagnetic reversals were absent. This result, in combination with the data for the Tremadocian and Middle–Upper Cambrian sequences of the other regions, indicates that (1) the rate of occurrence of the geomagnetic reversals on the eve of the Ordovician Moyero superchron of reversed polarity was at most one reversal per Ma; (2) the superchron does not switch on instantaneously but is preceded by a certain gradual change in the operation conditions of the dynamo mechanism which, inter alia, manifests itself by the reduction of the frequency of geomagnetic reversals with the approach of the superchron. This finding supports the views according to which a process preparing the establishment of the superchrons takes place at the core–mantle boundary

U-Pb LA-ICP-MS DATING OF DETRITAL ZIRCON FROM UPPER PERMIAN SANDSTONE OF THE BOYEVAYA GORA SECTION (ORENBURG REGION, SOUTHERN CIS-URALS)

The first results of U-Pb LA-ICP-MS dating of 150 detrital zircon grains from the Upper Permian sandstone of the Boyevaya Gora section (Orenburg region, the Southern Cis-Urals) are presented. 95 conditional zircon U-Pb isotopic dates form two age peaks – 381 and 529 Ma. It is assumed that clastic material entered the sedimentation basin mainly due to erosion of the Early Hercynian Ural complexes and relics of the Protouralian-Timanian orogen

ИСТОЧНИКИ СНОСА И ОГРАНИЧЕНИЕ ВОЗРАСТА РИФЕЙСКИХ ПЕСЧАНИКОВ ИВАНОВСКОГО ГРАБЕНА КОЛЬСКОГО ПОЛУОСТРОВА ПО РЕЗУЛЬТАТАМ U-Pb LA-ICP-MS ДАТИРОВАНИЯ И РАМАНОВСКОЙ СПЕКТРОСКОПИИ ОБЛОМОЧНОГО ЦИРКОНА

The results of U-Pb LA-ICP-MS dating of 150 detrital zircon grains from the Precambrian (Riphean?) sandstones of the Kildin group on the coast of the Ivanovka Bay (northeastern Kola Peninsula) are presented. A group of three youngest grains yielded constraint on the sandstone maximum depositional age of 1179±45 Ma. Detrital zircon age distribution is dominated by three maximums of 1.9, 2.4 and 2.7 Ga. The authors consider two complementary scenarios of interpretation of the data obtained. The first scenario involves the contribution of distant provenance out of granitoids in the Lapland-Kola orogen (~1.9 Ga) and adjacent areas of the Kola-Karelia region (~2.7 Ga), Imandra-Varzuga structure (~2.4 Ga), and Sveconorwegian belt (~1.2 Ga); the off-Baltic (Greenland?) sources of the detrital zircon also cannot be excluded. According to the second scenario, the Ivanovsky graben received zircons also from the proximal sources – sialic complexes formed upon melting of granite-gneiss substratum of the upper crust of the Murmansk craton during largescale episodic mafic magmatism 1.86, 2.50 and 2.68 Ga. The Raman spectroscopy showed that about 25 % of the analyzed detrital zircon grains experienced a 1.4-Ga-old thermal effect which did not lead to Pb losses but partially restored the crystalline structure of zircon. This result can be used as an additional constraint in the detailed provenance analysis of the northeastern East European Platform sedimentary basins and is indicative of 1.4-Ga-old large-scale magmatic events related to the breakup of the supercontinent Nuna.Приводятся первые результаты U-Pb LA-ICP-MS изотопного датирования 150 зерен обломочного циркона из докембрийских (рифейских?) песчаников кильдинской серии губы Ивановской северо-востока Кольского полуострова. По группе из трех самых молодых зерен получено нижнее ограничение на время накопления песчаников – 1179±45 млн лет. В распределении возрастов зерен циркона доминируют три максимума – 1.9, 2.4 и 2.7 млрд лет. Авторы рассматривают два взаимодополняющих варианта интерпретации полученных данных. Первый вариант предполагает вклад удаленных источников сноса – гранитоидов Лапландско-Кольского орогена (~1.9 млрд лет) и сопряженных с ними областей Карельско-Кольского региона (~2.7 млрд лет), Имандра-Варзугской структуры (~2.4 млрд лет), Свеконорвежского пояса (~1.2 млрд лет); вероятны также и внебалтийские (гренландские?) источники. Согласно второму варианту, в Ивановский грабен циркон поступал, в том числе, из проксимальных источников – сиалических комплексов, сформированных при плавлении гранитогнейсов верхней коры Мурманского кратона в ходе эпизодов масштабного базитового магматизма 1.86, 2.50 и 2.68 млрд лет назад. Методом рамановской спектроскопии показано, что примерно 25 % проанализированных зерен обломочного циркона около 1.4 млрд лет назад испытали воздействие термального события, не приведшего к потерям Pb, но частично восстановившего кристаллическую структуру циркона. Этот результат может использоваться как дополнительное ограничение при проведении детального анализа питающих провинций осадочных бассейнов северо-восточной части Восточно-Европейской платформы и указывает на масштабность проявления магматических событий около 1.4 млрд лет назад, связанных с распадом суперконтинента Нуна

The 1501 Ma Kuonamka Large Igneous Province of northern Siberia: U-Pb geochronology, geochemistry, and links with coeval magmatism on other crustal blocks

© 2015.A new large igneous province (LIP), the 1501 ± 3 Ma Kuonamka LIP, extends across 700 km of northern Siberia and is linked with coeval dikes and sills in the formerly attached Sao Francisco craton (SFC)-Congo craton to yield a short-duration event 2000 km across. The age of the Kuonamka LIP can be summarized as 1501 ± 3 Ma (95% confidence), based on 7 U-Pb ID-TIMS ages (6 new herein) from dolerite dikes and sills across the Anabar shield and within western Riphean cover rocks for a distance of 270 km. An additional sill yielded a SIMS (CAMECA) age of 1483 ± 17 Ma and sill in the Olenek uplift several hundred kilometers farther east, a previous SIMS (SHRIMP) age of ca. 1473 Ma was obtained on a sill; both SIMS ages are within the age uncertainty of the ID-TIMS ages. Geochemical data indicate a tholeiitic basalt composition with low MgO (4-7 wt%) within-plate character based on trace element classification diagrams and source between E-MORB and OIB with only minor contamination from crust or metasomatized lithospheric mantle. Two subgroups are distinguished: Group 1 has gently sloping LREE ((La/Sm)PM = 1.9) and HREE ((Gd/Yb)PM = 1.8) patterns, slightly negative Sr and moderate TiO2 (2.2 wt%), and Group 2 has steeper LREE ((La/Sm)PM = 2.3) and HREE ((Gd/Yb)PM = 2.3), strong negative Sr anomaly, is higher in TiO2 (2.7 wt%), and is transitional from tholeiitic to weakly alkaline in composition. The slight differences in REE slopes are consistent with Group 2 on average melting at deeper levels. Proposed reconstructions of the Kuonamka LIP with 1500 Ma magmatism of the SFC-Congo craton are supported by a geochemical comparison. Specifically, the chemistry of the Chapada Diamantina and Curaga dikes of the SFC can be linked to that of Groups 1 and 2, respectively, of the Kuonamka LIP and are consistent with a common mantle source between EMORB and OIB and subsequent differentiation history. However, the coeval Humpata sills and dikes of the Angola block of the Congo craton represent a different magma batch

РЕКОНСТРУКЦИЯ ПИТАЮЩИХ ПРОВИНЦИЙ МОСКОВСКОГО БАССЕЙНА В ПЕРМСКО-ТРИАСОВОЕ ВРЕМЯ ПО ДАННЫМ U-Pb LA-ICP-MS ДАТИРОВАНИЯ И РАМАНОВСКОЙ СПЕКТРОСКОПИИ ОБЛОМОЧНОГО ЦИРКОНА

We present the first systematic results of U-Pb LA-ICP-MS dating of detrital zircons from 12 samples representing different stratigraphic levels of 5 sections of the Permian-Triassic rocks, located within the eastern part of the Moscow basin (syneclise) – Zhukov ravine, Astashikha, Nedubrovo, Balebikha and Klykovo. It is shown that the accumulation of the Upper Permian and Lower Triassic terrigenous complexes occurred under the influence of competing sources with two contrasting provenance signals with Neoproterozoic (Vendian)-Paleozoic and Paleo-Mesoproterozoic ages. The identified provenance signal patterns were used to detail the correlation and stratigraphic subdivision of the Permian-Triassic terrigenous complex of the Moscow basin. Raman spectroscopy of detrital zircon, first applied to the Permian-Triassic rocks of the East European platform, made it possible to identify sedimentary complexes of a relatively older terrigenous basin as a separate source of zircons, which experienced superimposed thermal impact in the VendianCambrian time (~500–600 Ma).Приведены первые систематические результаты U-Pb LA-ICP-MS датирования зерен обломочного циркона из 12 проб, представляющих разные стратиграфические уровни пяти разрезов пород пограничного пермско-триасового возраста, расположенных в пределах восточного борта Московской синеклизы, – Жуков овраг, Асташиха, Недуброво, Балебиха и Клыково. Показано, что накопление терригенных комплексов верхней перми и нижнего триаса происходило под влиянием конкурирующих источников сноса с двумя контрастными провенанс-сигналами – с венд-палеозойскими и палеомезопротерозойскими возрастными характеристиками. Установленные закономерности использованы для уточнения схем корреляции и стратиграфического расчленения пермско-триасового терригенного комплекса Русской плиты. Метод рамановской спектроскопии обломочного циркона, впервые примененный для пермско-триасовых пород Русской плиты, позволил идентифицировать в качестве отдельного источника сноса осадочные комплексы относительно более древнего терригенного бассейна, испытавшего наложенное термальное воздействие в венд-кембрийское время около 500–600 млн лет назад

ПЕРВЫЕ ДАННЫЕ О ЗАПИСИ ГЕОМАГНИТНОГО ЭКСКУРСА, СОХРАНЕННОЙ В СПЕЛЕОТЕМЕ С ЗАПАДНОГО КАВКАЗА

The study of the geomagnetic field evolution on minor timescales, in particular of such significant events as geomagnetic reversals and excursions, has acquired particular relevance nowadays due to the increased attention of mankind to the environment. The question of how exactly abrupt changes in the characteristics of the geomagnetic field affect the climate and biosphere remains largely debatable; the idea of the speed and dynamics of these changes is also very vague. "Classical" geological objects and existing methods provide limited opportunities for highly detailed reconstructions of geomagnetic field variations; therefore, paleomagnetologists are looking for new objects and approaches to solve this problem. The research that we have begun involves the use – for the first time in Russia – of speleothems to study secular variations of the geomagnetic field.This brief communication presents paleomagnetic records of two drill-cores from the flowstone from Vorontsovskaya cave, located on the western flank of the Caucasus Mountains in the valley of the river Kudepsta. Preliminary results indicate the presence of a geomagnetic excursion record in both drill-cores. Further study of the samples from Vorontsovskaya cave will make it possible to compare the discovered event with known excursions, as well as to clarify its age, duration, and dynamics.Изучение эволюции геомагнитного поля на малых временных масштабах, в частности таких характерных событий, как геомагнитные инверсии и экскурсы, в наши дни приобрело особую актуальность в связи с возросшим вниманием человечества к окружающей среде. В значительной мере дискуссионным остается вопрос о том, как именно резкие изменения характеристик геомагнитного поля отражаются на климате и биосфере; представления о скорости и динамике этих изменений также весьма расплывчаты. «Классические» геологические объекты и существующие методы исследований предоставляют ограниченные возможности для высокодетальных реконструкций вариаций геомагнитного поля, поэтому палеомагнитологи ищут новые объекты и подходы для решения этой задачи. Начатое нами исследование предполагает использование – впервые в России – спелеотемов для изучения вековых вариаций геомагнитного поля.В настоящем кратком сообщении представлены палеомагнитные записи двух кернов из натечной плотины пещеры Воронцовской, располагающейся на западном крыле Кавказских гор в долине р. Кудепста. Предварительные результаты указывают на наличие записи геомагнитного экскурса в обоих кернах. Дальнейшее изучение образцов из пещеры Воронцовской позволит сопоставить выявленное событие с известными экскурсами, а также уточнить его возраст, продолжительность и динамику