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We present results of geochemical and Sr-Nd isotope studies of rocks and of local dating and determination of the Lu-Hf isotope composition of zircons from late Vendian-early Cambrian and Cambrian-Ordovician intrusive associations (granitoids and gabbroids) of the Kaa-Khem and East Tannu-Ola batholiths in Eastern Tuva. The wide ranges of the eNd values (6.9 to 0.5) of rocks and the eHf values of magmatic and inherited zircons reflect the diversity of the magma sources of late Vendian-early Paleozoic intrusive associations formed at the island arc and accretion-collision stages. Late Vendian (572-562 Ma, Kopto and Buren massifs) and early Cambrian (522- 518 Ma, East Tannu-Ola batholith) island arc tholeiitic and calc-alkalic plagiogranitoids resulted from the melting of the Vendian-early Cambrian island arc crust without the contribution of a more ancient crustal material. The subalkalic gabbro-monzodiorite-granosyenite association of the Zubovka massif (510 Ma) formed from a mantle source depleted isotopically but enriched in incompatible elements, with the participation of an island arc crust material; this process took place in the early phase of plume activity at the accretion-collision stage. Island arc complexes were the main source of Cambrian-Ordovician accretion-collision calc-alkalic plagiogranitoids (500-450 Ma, Terektyg-Cheder, Karaos, Tapsa, Baisyut, and other massifs). Variations in their composition were due to the melting of thick crust, whose isotopic heterogeneity was caused by the different contributions of a more ancient crustal source. The crust of the Tuva-Mongolian terrane made the main contribution to the formation of the potassic granitoids of the Bren' massif (450 Ma), marking the completion of accretion- collision processes in this region. The isotope parameters of the Vendian-early Paleozoic granitoids are indicators of the crust formation and evolution in the course of subduction and accretion-collision processes. [ABSTRACT FROM AUTHOR]

The Early Caledonian folded region of the southern framing of the Siberian platform, between the Zavkhan and Tuva-Mongolian terranes, contains blocks of the Songino ledge crystalline rocks. In the Bayan Nuur block (southern part of the Songino ledge), the Neoproterozoic (890–780 Ma) Bayan Nuur gneiss-migmatitic and Kholbo Nuur metaterrigenous-volcanogenic complexes are distinguished. The zone of their junction is traced by thrust structures, which are reflected in all structural-material complexes of the Bayan Nuur block. There are tectonic slabs in the junction zone. Their peculiarity is the presence of rocks formed before the formation of thrust gabbro-ultrabasite complex and post-thrust gabbroids and gabbro-diorites. The upper limit of the thrust formation interval is defined by Bayan Nuur pluton granitoids having the age of 790 ± 3 Ma and by gabbroids and anorthosites of Onzula tectonic plate giving the ages of 785 ± 3 and 784 ± 3 Ma. The lower limit of the thrust formation interval is determined by the age of the Bayan Nuur ultrametamorphic granitoid complex (802 ± 6 Ma). Bodies of massive (post-thrust) pegmatoid gabbros and rocks of gabbro-ultrabasite complex possessing clearly manifested thrust-associated structures are recognized in the tectonic slab of the ridge reaching the area of Mount Sant Uul. These rocks are dated at 782 ± 2 and 806 ± 10 Ma, respectively (ID-TIMS). The data indicate that the post-thrust and pre-thrust units cannot be parts of a united layered complex. In turn, pre-thrust units can be considered as paleooceanic fragments in the accretionary structure of the Bayan Nuur block, Songino ledge. [ABSTRACT FROM AUTHOR]

The article offers information on the results of the dating of alkaline granitoids of the Ulug-Tanzek Massif deposit in Tuva, Russia through uranium-lead (U-Pb) method for zircon and Argon-Argon (Ar-Ar) method. A sample from polylithionite-bearing ribeckite granite was studied for Ar-Ar geochronological investigations, in which, results revealed that approximately 85% of Ar that has clearly formed reflected three-step plateau with an age of 296 plus-minus two million years (Ma). In U-Pb dating, a sample of muscovite quartz-albite-microline granite with a netlike texture was used, in which, the determined age of 301 plus-minus three Ma, is similar within the error limits of the age estimation for the first sample using Ar-Ar method.