Geodynamics of the Barents–Kara margin in the Mesozoic inferred from paleomagnetic data on rocks from the Franz Josef Land Archipelago

New data on paleomagnetism and isotope geochronology of Jurassic and Early Cretaceous basic igneous rocks on Franz Josef Land Archipelago (FJL) represented by flows and dikes are discussed. The first paleomagnetic data obtained for these rocks offer the opportunity to suggest a model of spatial changes in the FJL block position during the Jurassic?Cretaceous. In the Early Jurassic, the block occupied a different position relative to Europe from the modern one. It was displaced in the northeasterly direction by a distance of approximately 500 km and rotated clockwise by about 40° relative to its modern position. By the Early Cretaceous, the FJL block occupied a position close to the present-day one avoiding subsequent substantial relative displacements. The data obtained are of principal significance for reconstructing the geodynamic evolution of Arctic structures in the Mesozoic and contribute greatly to the base of paleomagnetic data for the Arctic region, development of which is now in progress.     

On the problem of the scale and composition of Paleozoic and Mesozoic granitoid magmatism in the Khilok–Vitim fold belt of Central Transbaikalia

The paper presents a study of the gneissic granitoids of the Malkhan Complex and the intruisve granitoids of the Daur and Bichur complexes developed within the Khilok–Vitim fold belt of Central Transbaikalia. In the state geological map, these complexes have been attributed to the Early and Late Paleozoic. New ⁴⁰Ar/³⁹Ar geochronological data indicate that these rocks are Mesozoic rather than Paleozoic in age, which suggests the much broader manifestation of the Mesozoic granitoid complexes in this area. The studied Mesozoic granitoid massifs exhibit temporal and compositional zoning reflected in a westward decrease in age (from Early to Late Mesozoic) and increase in total alkalinity and potassium content at the appropriate trace-element characteristics. The obtained results of study of the Khilok–Vitim Belt are interpreted in the framework of the model of the formation of domal–cupola structures by the multiple activity of deep thermochemical plumes.     

The First Data of U/Pb and 40Ar/39Ar Dating of the Pre-Dzhugdzhur Volcanics: New Evidence of Time Diversity in the Formation of Individual Sectors of the Okhotsk–Chukotka Volcanogenic Belt

Doklady Earth Sciences - The first data of U/Pb and 40Ar/39Ar dating of volcanogenic rocks of the Pre-Dzhugdzhur volcano-tectonic trough (PDVT) of the Okhotsk–Chukotka volcanogenic belt...     

The 40Ar/39Ar age of the epithermal gold-silver mineralization of Asachinskoe Deposit, southern Kamchatka

New data on the age of epithermal gold-silver mineralization of Asachinskoe Deposit (southern Kamchatka) are obtained by the ⁴⁰Ar/³⁹Ar method of stage heating involving preirradiation of samples for 48 hours by fast neutron currents in a cadmium-plated reactor channel. Correlation with the earlier data on integral K/Ar dating and a laser variant of the ⁴⁰Ar/³⁹Ar method has been carried out. According to the structural orderliness revealed by high resolution X-ray analysis and infrared spectroscopy, potassic feldspars found in veins have been identified as a continuous series: maximum microcline → intermediate microcline → intermediate orthoclase → extreme orthoclase. The age of quartz-orthoclase vein 1 is 3.2 ± 0.2 Ma (the Piacenzian). A younger age, about 1.1 ± 0.3 Ma (the Early Pleistocene) is determined for a quartz-microcline vein with post-mineralization brecciation and recrystallization related to the basalt dike magmatism of this age.     

Neoproterozoic Chingasan Group in the Yenisei Ridge: new data on age and deposition environments

Trachybasalt-alkali trachyte volcanism in the Yenisei Ridge was found out to be synchronous with deposition of coarse tilloids and flysch of the Chivida Formation of the Neoproterozoic Chingasan Group. New 703±4 Ma ⁴⁰Ar/³⁹Ar biotite and titan-augite ages of subalkaline basalts in the Chivida Formation indicated that they erupted in the Late Neoproterozoic. According to microfossil evidence, the Chingasan sediments correlate with Late Neoproterozoic strata in the type sections of the southern Siberian craton. The Chingasan deposition apparently lasted no longer than 30 Myr judging by the isotope ages obtained for the underlying Upper Vorogovka Group and subalkaline basalts in the Chivida Formation. The fault-parallel position of grabens and coarse grain sizes and variable thicknesses of their lithological complexes, as well as syndepositional trachybasalt-alkali trachyte volcanism provided evidence that the volcanosedimentary rocks of the Chingasan Group formed in an environment of active rifting.