V. G. Belichenko

Formation history of the Tuva-Mongolian Massif (Western Hubsugul region, North Mongolia) based on U-Pb dating of detrital zircons from sandstone of the Darkhat group by the LA-ICP-MS method

1498 The Tuva–Mongolian Massif was defined in 1971 as a block with an Early Precambrian basement and Vendian–Cambrian carbonate cover of the platform type [1]. Subsequently, the contours of the massif were changed substantially and its dimensions were reduced [2]. The present day boundaries correspond to the contour outlining the distribution area of the Ven dian–Cambrian carbonate cover [3, 4] (Fig. 1). In the south, the Tuva–Mongolian Massif is separated by a fault from the Dzabkhan Massif. Formerly, these two massifs formed a single block, which is evident from the development of similar carbonate rocks in their covers [3]. In this connection, the validity of the Tuva–Mongolian Massif as a geological structure that developed against the background of surrounding younger Paleozoic complexes is beyond doubt [5]. The age of blocks constituting the basement of the massif, as well as the history of their amalgamation and rela tion with other blocks, remains insufficiently known despite the availability of many special publications dedicated to these aspects [2–4, 6–9]. In this commu nication, we present the data on ages of zircons from sandstone of the Darkhat Group underlying the Ven dian–Cambrian carbonate rocks on the eastern mar gin of the central Tuva–Mongolian Massif. It appeared that these detrital zircons adequately char acterize the provenance and reflect practically the entire pre Vendian formation history of the massif.

LA-ICP-MS U-Pb dating of detrital zircons from metaterrigenous deposits of the Vendian-Cambrian cover of the Tuva-Mongolian microcontinent (Tunka Bald Mountains, East Sayan)

154 The northeastern segment of the Central Asian foldbelt (CAFB) contains ancient continental blocks; the largest of them are the Tuva Mongolian (TMM) and Dzabakhan microcontinents. There is a fairly extensive isotope database for magmatic, metamor phic, and sedimentary rocks of the Dzabakhan conti nental block [1–4], whereas most characteristics of the TMM were obtained for magmatic formations [5–7]. Filling the lack in the study of sedimentary formations of the TMM, we studied detrital zircons of the Shu mak Formation (Tunka bald mountains), which is one of the several preserved fragments of terrigenous sedi mentation of nonglacial genesis within the shelf of this continental block.

A fragment of back-arc paleospreading in the Tunka terrane

High magnesium metavolcanites, which are found for the first time in the Tunka terrane (the Baikal-Khubsogul region), are described. By the content of MgO (12–16%), SiO2 (up to 52%), and alkali oxides, they are classified as metamorphosed picrites. There are two groups of them, divided by the TiO2 content and by the TiO2/Al2O3 ratio. When making a distinction between geochemical peculiarities of two groups, their rare-element composition is intermediate between picrites and boninites. The comparison between metavulcanites and their nonmetamorphosed analogues is made, some aspects of their genesis are considered, and the conclusion is made that these metavulcanites mark paleospreading of the back-arc basin.

The seismic-gravity transect of Earth's crust beneath the Siberian platform and Central Mongolia and its geological implication

Low-velocity layers in the Earth’s crust, related to large-scale thrusts corresponding to the suture zones, can be as the sign of perspective of this territory on the deposits of minerals and recommendation to more detailed researches

Geochemistry of highly aluminiferous Vendian-Cambrian sediments of the Tunka Bald Mountains, East Sayan

This paper presents comprehensive geochemical characteristics of highly aluminiferous schists discovered in the Tunka Bald Mountains and associated with the Vendian-Cambrian cover of the Tuva-Mongolian Microcontinent. The Al2O3 content varies within a wide range, the maximum being up to 36.11 wt. %. A specific feature of the geochemical composition of these rocks is the simultaneously high contents of Cr (up to 1400 ppm), V (up to 2700 ppm), and Zr (up to 800 ppm). The origin has been interpreted, and the conditions in which the highly aluminiferous rocks formed have been considered. Based on geochemical data, it is assumed that the main origin could be weathering crusts formed after ultramafic and mafic rocks, with a felsic source playing a secondary role.