Rare-metal granitoids of the Ulug-Tanzek deposit (Eastern Tyva): Age and tectonic setting

Abstract

95 Raremetal magmatism covering the age range of more than 1 billion years, from the Late Riphean to the Triassic, is widely abundant within the southern margin of the Siberian Platform and its folded sur� rounding. Late Riphean massifs of alkali-ultrabasic rocks with carbonatite of the Beloziminsk Complex, Ordovician-Triassic massifs of alkali granite (Arys� kan, Zashikhin, Ermakov, and others), as well as numerous massifs of lithium-fluorine granite includ� ing the spodumene variety, are related to the most wellknown manifestations of raremetal magmatism, which control the formation of large deposits of rare metals, rare elements, and rare earth elements. Until recently, the ideas about their age were based mainly on geological data. Only in the last few years have geo� chronological investigations, which allowed us to esti� mate the age and tectonic conditions of related mag� matic processes in geological evolution of the southern surroundings of the Siberian Platform, been carried out (1). However, the regularities of raremetal mag� matism in the structures of this region are not under� stood completely for some epochs. In particular, this relates to the epoch when the unique complex rare� metal-rareearth deposit UlugTanzek (Eastern Tyva) was formed. The existing estimations of age for host alkaline granitoids vary within very broad limits (229- 217 Ma, K-Ar method by potassic feldspar) and cover the whole Triassic (2). However, this age range in the region is amagmatic as a whole. Because of this, the reliability of age estimations obtained is doubtful and, therefore, there is a question about the geological events that controlled the formation of the deposit. In this paper we present the results of dating of alkaline granitoids of the UlugTanzek Massif by the U-Pb method for zircon and the Ar-Ar method. With account for the geochronological data obtained, we analyzed its position in the structure of magmatic areals of the same age and, on this basis, reconstructed the geodynamic environment of raremetal magma� tism in the corresponding epoch. The UlugTanzek Massif of alkaline granitoids is located in southeastern Tyva within the Sangilen part of the Tyva-Mongol microcontinent, where it intrudes metamorphosed Vendian-Cambrian terrige� nous-carbonate deposits of the Kaakhem structural- facial zone. The massif is composed of quartz-albite- microcline granite with complex (Ta, Nb, Zr, Hf, REE, Li, Th, U, and cryolite) ore.