Computer microtomography of zircon: A new approach to the selection of targets for U-Pb geochronologic studies

The paper reports results obtained by the complex studying of zircon crystals from rocks of various genesis. Zircon is one of the minerals most often used as geochronometers. It also provides genetic information on superimposed processes that is “recorded” in the external and internal morphology of its crystals. This mineral is thoroughly examined to select its unaltered crystals for U-Pb dating by the single-grain method. Zircon grains are selected with the use of optical and electron microscopy and cathodoluminescence. This publication presents the first results obtained by examining zircon by computer microtomography (μCT) and the results of the studying of the external and internal structure by conventional techniques ([optical microscopy and SEM (SE and CL)]. The paper demonstrates the advantages of the application of the μCT techniques to the selection of targets for U-Pb zircon dating: there is no need for the special preparation of the samples and no material is destructed. However, this technique may be not informative enough if the zircon contains inherited core whose density does not differ from the density of the surrounding mineral.     

Redkinian Biota of Macroscopic Fossils from the Northwestern East European Platform (South Ladoga Region)

The stratigraphic distribution of microfossils and macroscopic fossil biota in Vendian deposits of the South Ladoga region (northwestern East European Platform) is analyzed. In the sequence of the Shotkusa- 1 well, three taxonomically heterogeneous microfossil assemblages are distinguished: two of them refer to the Redkinian age (Starorusskaya Fm.) and one to the Kotlinian age (Vasileostrovskaya Fm.). Deposits of the Starorusskaya Fm. contain Redkinian biota of macroscopic fossils, of which the most characteristic representatives are Chuaria circularis, Doushantuophyton lineare, Morania zinkovi, Orbisiana simplex, and Redkinia spinosa. These new findings expand the paleontological characteristics of Upper Vendian deposits, also providing additional criteria for distinguishing the Redkinian horizon in the northwestern East European Platform.     

Age and Provenance Areas of Terrigenous Rocks of the Dzhida Terrane: Results of U—Th—Pb (LA-ICP-MS) Geochronological Study of Detrital Zircons

The results of U—Th—Pb (LA-ICP-MS) geochronological studies of detrital zircons from terrigenous rocks of the Dzhida terrane of the Central Asian Fold Belt (CAFB) are presented. The data obtained allow us to distinguish the following age maxima (Ma): 578 and 634 (Vendian); 720, 823, and 919 (Late Riphean); 1922, 2090, 2225, and 2321 (Early Proterozoic). A number of zircons have Late Archean age in the interval of 2670–2980 Ma. Taking into account Late Cambrian age (504–506 Ma) of intrusive rocks that intruded the Dzhida terrane, a possible sedimentation period of sequences of this terrane is estimated to be in the interval of 580–510 Ma (from Vendian to Late Cambrian). The possible provenance areas of terrigenous sediments are proposed and the previously proposed models of geodynamic evolution of the Dzhida terrane are correlated with new geochronological data.     

Geochronology of Alkaline Rocks of the Aryskan Rare Metal Deposit,East Sayan

Doklady Earth Sciences - The temporal relations between alkaline rocks of the Aryskan rare metal (Y, REEs, Nb, Ta) deposit are determined using metamict zircon with high U and Th contents, which...     

Age and origin of the Kitoi sillimanite schist deposit,eastern Siberia

Doklady Earth Sciences -     

Granitoids of the Kongo Magmatic Zone of the Omolon Massif (Northeastern Russia): Rock Composition,Age, and Geodynamic Setting

Geotectonics - In our study we analyzed the composition of granitoid rocks within the Kongo magmatic zone of the Omolon median mass. The studied calc-alkaline granitoids cut through the Early...     

Age assessments for siliciclastic metasediments of the Bodonchin tectonic sheet, the South Altai metamorphic belt

In South Mongolia, the Hercynian structures of a linear collisional thrust-and-fold zone formed in the Carboniferous are bounded by the Caledonides of Central and North Mongolia on the north, being truncated on the south by the Indosinides of the Inner Mongolia. Tectonic sheets of the Caledonides-Hercynides junction zone confined to southern flank of the Mongolian-Gobi Altai are composed of high-gradient metamorphites of the South Altai metamorphic belt. The belt of these rocks traceable northwestward in China and eastern Kazakhstan delineates margin of the North Asian Caledonian paleocontinent. According to results of the previous geochronological study, the high- and low-gradient metamorphic rocks of the belt originated respectively 385 and 360–370 Ma ago. However, tectonic position of crystalline rock sequences, which have not been dated, remains unclear. Geochronological interval postulated for these rocks is very broad, ranging from the Early Precambrian to the Devonian. Dating results obtained in this work for detrital zircons from siliciclastic metasediments of the Bodonchin tectonic sheet of the belt show that their protoliths accumulated during the time span of 460–390 Ma (Late Ordovician-Early Devonian) on a passive continental margin. Transformation of the latter into active continental margin took place in the Early Devonian, when development of the Siberian subduction zone resulted in formation of the South Altai metamorphic belt at deep crustal levels of the Caledonian paleocontinent.