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The gold mineralization in the Central Aldan ore district is genetically related to potassic calc-alkaline and alkaline magmatism dated at 115–150 Ma. The objective of this study is to establish the age of hydrothermal processes that accompanied the formation of Au-Te mineralization at the Samolazovsky Deposit. Based on the isotope-geochemical study of zircons from quartz-feldspar metasomatic rocks of the deposit, the granitoids and charnokites of the Nimnyr Complex (1 900–1 960 Ma) at the contact with the Yukhta monzonite-syenite massif (∼127 Ma) were studied. Zircon U-Pb dating was performed on a SHRIMP-II ion microprobe, and rare-earth and trace elements analyses of zircon were carried out in the same craters by secondary-ion mass spectrometry on a Cameca IMS-4f ion microprobe. It is revealed that the studied zircons have heterogeneous structures: dark core and lighter rim, which differ greatly in isotope-geochemical parameters. Zircon rims are cut by a network of fractures, extending into the central part of zircon grains. The rims yield a subconcordant age of 1 937±24 Ma, with an average total REE content of 550 ppm, which corresponds to the formation age of the Nimnyr Complex. All zircon cores yield a discordant age of 83±11 Ma and are characterized by a higher total REE content (∼1 812 ppm), as well as higher contents of U and non-formula elements (Ca, Sr, and Y) with respect to rims, due to the effect of fluid on zircons. Despite the limited number of zircon grains, the additional geochronological study of zircons from syenites of the ore-bearing Ryabinovy Massif has revealed the presence of two distinct age clusters: ∼125–138 and 76–83 Ma. The older ages of zircons from syenites are typical for the Central Aldan ore district. Until now, there is still no explanation for an age range (76–83 Ma) of single zircon grains from ore-bearing syenites of the Ryabinovy Massif. The obtained data suggest that the processes of activation (the effect of fluid) within the Central Aldan ore district continued until Late Mesozoic. With regards to the equivocal geotectonic position of the Mesozoic potassic magmatism in the study area and its high metallogenic potential, there exists an absolute necessity to determine the geochronological age of the rock formations. Therefore this study presents the Late Cretaceous geochronological data for the first time which can constrain the time-frame for the formation of gold-bearing magmatic and metasomatic rocks of the Aldan ore district. [ABSTRACT FROM AUTHOR]

U-Pb dating of zircon from amphibole latite of the central stock of the Nikolka shield volcano in Kamchatka provided the first data on its age. The end of activity falls to about 0.73 Ma (10 analyses). The local U-Pb analyses were carried out at the Isotope Research Center, Schmidt Joint Institute of Physics of the Earth (St. Petersburg) (VSEGEI) using SIMS SHRIMP II. Here, brief information about the geological structure, petrography, and chemistry of the rocks of the volcano are presented. A unique feature of the volcano is the occurrence of a gabbro-anorthosite sheet intrusion with derivates of monzonite and syenite composition in its central part. The correlation of the age obtained with the paleomagnetic data, indicating the formation of the volcano during the magnetic age of Matuyama, is also discussed. The date refers to the period of the change in epochs from the Brunhes to the Matuyama. The obtained age allows dating of the Pliocene volcanic stage in Kamchatka, when large shield volcanoes and plateaus formed. [ABSTRACT FROM AUTHOR]

The U-Pb SIMS age dating of zircons from different-age granitoid assemblages varying in composition of the Verkhisetsk batholith shows that it comprises rocks of three age groups, formed at different stages of the Ural Mobile Belt. The first age group is represented by quartz diorites (396 ± 5 Ma) with insignificant distribution in area. Their formation was synchronous to island arc volcanism, manifested in the area of study from the second half of the Emsian to late Givetian-early Frasnian. According to this, we could consider these granitoides as comagmatic to island arc volcanites. The second age group includes tonalites and trondhjemites (367 ± 4 Ma), comprising the western part of the batholith. On the basis of similarity between these rocks and granitoides of modern active continental margins in material composition, it is assumed that they formed throughout the island arc-continental stage of development of the Ural Mobile Belt. Granitoides of the third age group, dominating in the Verkhisetsk batholith, formed as a result of several homodrome rhythms of granodiorite-granite intrusions of moderate-potassium composition during a short period of time (315-300 Ma). Their formation is related to the initial stage of the collision stage of development of the region, lasting from the early Bashkirian to Late Permian in the Middle Urals, which is fixed by deposition of flysch and molasse sediments in the Ural Foredeep. The data obtained change our understanding significantly of the character of evolution of granitoid magmatism and the place of rock assemblages studied in the geological history of the Urals. [ABSTRACT FROM AUTHOR]

The results of the U-Pb (SIMS and LA-ICPMS) age dating of detrital zircons from Upper Jurassic-Lower Cretaceous sandstones of Stolbovoy Island show that these deposits contain zircons of a wide age range, from Archean to Lower Cretaceous. Precambrian gneisses and granites, as well as Late Paleozoic and Mesozoic plutonic and volcanic complexes, are considered to be the main source areas of clastic material of Upper Jurassic-Lower Cretaceous formations of Stolbovoy Island. The U-Pb age dating of detrital zircons from sandstones without making a selection on any basis yields the most complete information about source areas of clastic material in a sedimentary basin. In some cases the data on euhedral (idiomorphic) and transparent zircon crystals can be useful to clarify the lower age boundary of sedimentation. [ABSTRACT FROM AUTHOR]