Results of 40Ar/39Ar dating of subvolcanic body in the vent tunnelKukurtly volcano

Эльбрус изучается с 1852 г., однако вопросы о времени его образования и временных этапах эволюции до сих пор остаются дискуссионными. По мере развития новых методов датирования геологических событий исследователями Эльбру- са применялись все более современные методы и методики. Так, на самых ранних этапах исследований время образования вулкана приблизительно оценивалось только по геологическим данным, затем стал очень широко применяться геолого-гео- морфологический метод относительной геохронологии, а с конца прошлого столетия – калий-аргоновый (K-Ar), аргон-арго- новый (40Ar/39Ar), иониевый (Io/234U) и радиоуглеродный (14C) методы датирования. В полученных вышеуказанными методами датировках (цифрах возрастов) по одному и тому же объекту часто имелись существенные различия. При 40Ar/39Ar датировании биотита из туфов риолитового состава раннего этапа кальдерной стадии было доказано, что при эксплозивных извержениях, приведших к формированию этих туфов и ассоциирующих с ними игнимбритов, был захвачен дезинтегрированный материал древнего гранитно-метаморфического основания Эльбруса, который после аэрального пере- носа стал составной частью туфов и игнимбритов. Следовательно, все изотопные датировки этого пирокластического материала (по биотиту, пироксену, плагиоклазу) не отражают время образования рассматриваемых пород, так как они со- держат мельчайшие обломки древнего гранитно-метаморфического фундамента и слагающего их породообразующего био- тита, что, естественно, искажает изотопный возраст пирокластики в неизвестном масштабе. Устойчивое плато при ступенчатом нагревании пробы № 542 мегакристалла санидина однозначно свидетельствует об от- сутствии избыточного аргона, а полученный аргон-аргоновый возраст (620.3±3.3 тыс. лет) отражает время остывания кри- сталла санидина при температуре закрытия системы ~ 300о С и свидетельствует об отсутствии значительного перерыва во времени между формированием пород раннего и позднего этапов кальдерной стадии. Ключевые слова: вулкан Эльбрус, 40Ar/39Ar датирования, субвулканическое тело дацитового состава, жерло вулкана Кюкюрт- ли, удревнение или омоложение изотопного возраста. Elbrus is under study since 1852 year, however the questions about time of it origin and temporal stage of evolution, up to now are remains as a discussional.So far as development of a new method for dating of geological events, investigators of Elbrus using all more current status of methods and procedures.So, on the very early stages of investigation, time of volcano origin approximately assessed by geological data only, then -geologo-geomorphological method of relative geochronology became in very regular use and from last century - potassium-argon (K-Ar), argon-argonian(40Ar/39Ar), ionic (Io/234U) and radio-carbon(14C) methods of dating. Inobtainedbytheabovementionedmethodsofdating (fi gureofage) oncommon poles object very often there was an fundamental dissimilarities. As a result of 40Ar/39Ar dating of biotite from tuff of the rhyolite composition of early period of calderian stage, it was proved, that during explosive eruption, lead to forming of these tuff and associated ignimbrites, the old disintegrated material of granite-metamorphic basement of Elbrus was encroach and which after aeral transportation became as a constituent part of tuff and ignimbrites. Consequently, all isotope age determination of this pyroclastic material (on biotite, pyroxene, plagioclase) do not take account of time origin of covered rocks, as they contain a very small debris of old granite-metamorphic basement and rock-building biotite, that natural, to distort of isotope age of pyroclastics in unknown scale. The tolerant plateau at scalariform heating sample № 542 - megacrystal of sanidine is uniquely testifi es about absence of excess argon and argon-argon age (620.3±3.3Ka) obtained is accounts time of cooling down ofsanidinemegacrystal at the temperature of clousing of the system - 3000 C and testifi es about absence of signifi cant interruption in time between rock forming of early and late periods of calderian stage

Last scene in the large scale rotations of the Western Carpathians as reflected in paleomagnetic constraints

This paper provides an overview of the paleomagnetic results which constrain the post-Paleogene tectonic development of the Western Carpathians. A group of these results are relevant to the last stage of the Tertiary folding and thrusting of the Silesian, Dukla and Magura nappes of the Outer Western Carpathian and were obtained from Paleogene-Lower Miocene flysch sediments. Both the pre- and post-folding remanences indicate about 50° CCW vertical axis rotation with respect to the present orientation. This is about a 60° rotation relative to stable Europe. It follows that the general orientation of the Silesian and more internal nappes were NW-SE, at least until the mid-Miocene. The CCW vertical axis rotation was co-ordinated with that of the Central Carpathian Paleogene Basin. The termination of the rotation can be estimated from the paleomagnetic data available from the Pieniny andesites which intruded the Pieniny Klippen Belt and the southern part of the Magura Nappe as well as from those obtained for the Neogene intramontane basins which opened up in the Outer and in the Central Western Carpathians. The paleomagnetic vectors for the andesites form two groups. The first group suggests about 45° CCW rotation relative to north, while the second shows no rotation. At the present stage of our knowledge it seems likely that some of the andesite bodies were intruded around 18 Ma, which is the oldest isotope age for the intrusions of the Wżar Mts, while some other bodies could have been emplaced after the rotation, around 11 Ma, which is the youngest isotope age for the Brijarka quarry. Vertical axis CCW rotation was also observed on sediments older than 11.6 Ma in the Orava-Nowy Targ Intramontane Basin which saddles the Magura Nappe and the Central Carpathian Paleogene Basin. However, this rotation was related to fault zone activity and was not attributed to the general rotation of the Outer Western Carpathian nappe system. Paleomagnetic results from the Nowy Sącz Intramontane Basin, which opened over the Magura Nappe, and those for the Central Western Carpathian Turiec Intramontane Basin do not indicate vertical axis rotation. In the first case, the loosely controlled age limit of the termination of the rotation is around 12 Ma. Well constrained results from the second basin imply that the rotation was definitely over by 8 Ma. Based on the above observations, and aware of the problem of often loose age control on the formation and deformation of the deposits of the intramontane basins, it is tentatively concluded that the large scale CCW rotation of the Central Western Carpathians, together with the Magura, Dukla and Silesian nappes, must have started after 18 Ma and terminated around 11 Ma.

First data on yatulian (2.1 Ga) metamorphism in the Belomorian mobile belt

The Sm-Nd isotope ages of apoamphibolite eclogites and eclogite-like garnet-clinopyroxene crystalline schists collected in the Chupa and Engozero segments of the Belomorian mobile belt are estimated. Eclogites and eclogite-like garnet-clinopyroxene rocks demonstrated nearly the same isotope age (2119±170 and 2191±39 Ma, respectively) on the same pair of minerals (garnet-clinopyroxene). The age obtained (about 2.1 Ga) is the oldest indicator of the onset of Paleoproterozoic metamorphism in the Belomorian mobile belt.

Zirconolite, baddeleite, zircon and thorite of island arc quartz gabbro-norite-dolerites of Ayu-Dag intrusion (Mountain Crimea)

Early Bajocian island arc quartz gabbro-norite-dolerites of Ayu-Dag contain syngenetic zirconolite and baddeleite. Zirconolite presents as Ca-dominant and uncommon Y-dominant (Y, Ca, Th, REE)2FeZr2Ti3O14. Two genetic types of zircon and thorite are developed. Predominant xenogenic zircon-I with thorite inclusions and a mass of lacunes — traces of reflow, is enriched with Hf, Th, Y, P (up to 6 wt.% HfO2, 5% ThO2, 6% Y2O3, 3% Р2О5). Thorite contains 7–9% UO2. Zircon and thorite of such composition are typical minerals of highly radioactive granites. Isotope age of zircon (xenogenic) is >2 billion years. Rare syngenetic zircon-II is sharply poor of Th, U, Y and Hf. So, basic magma of Ayu-Dag was contaminated with substance of Early Precambrian highly radioactive granites, which bodies are probabely located in the Mountain Crimea substructure.