First U-Pb data on age of granitoids in the basement of the Pechora basin

Abstract

The Pechora basin, a large regional tectonic struc� ture in the Pechora region, is located between the Timan and Ural mountains (Fig. 1). Its section is com� posed of Upper Cambrian sedimentary-metamorphic and igneous rocks of the basement (Timanides) uni� versally overlain by a platformal cover represented by Ordovician-Cenozoic sedimentary and volcanic rocks. The basement comprises the Izhma, Pechora, and Bol'shezemel'skaya zones. The last two constitute the Bol'shezemel'skaya megablock, and the Izhma zone together with the Timan and Kanin Peninsula form the Timan megablock. According to geophysical and drilling data, the megablocks distinctly differ from each other in the composition and type of magmatism (7). The boundary between them corresponds to the zone of the NWextending Pripechora and Ilych- Chikshino deepseated faults. The thickness of the platformal cover varies from 4-7 km in central areas of the Pechora basin to 1-4 km in arches and uplifts. In this connection, information on the structure and geo� dynamic development of the basement beneath the basin is derived from the core of over 200 boreholes drilled with different purposes up to a depth of 5 km (7). Granitoids have been recovered by 26 boreholes throughout the entire region being more abundant in the Izhma and Pripechora-Ilych-Chikshino zones. Until the end of the 20th century, the age of granitoids was known from the limited K-Ar data (1, 13), most of which correspond to the period of 600-500 Ma. They provided grounds for assuming that granitoids are Vendian-Cambrian in age. Subsequently, geo� chronological dates were obtained by Pb-Pb (Pb evaporation) measurements in single zircon crystals from granites and diorites recovered from several drill� holes (13). These measurements yielded uniform ages in the interval of 567-551 Ma corresponding in the recent stratigraphic scale to the boundary of the Lower and Upper Vendian (12). The dating of zircons from igneous rocks using the radiogenic Pb isotopes is based on the assumption that the 207 Pb/ 206 Pb ratios (ages) remain unchanged after the formation of zircons and host rocks, although this is frequently not the case. The subsequent Rb-Sr investigations of granitoids from the Izhma and Pripechora-IlychChikshino zones (4) recovered by the same boreholes, where zir� con was analyzed, revealed that the Rb-Sr ages are confined to the period of 608-595 Ma, i.e., to the Riphean-Vendian boundary interval (12). These data are quite consistent with the ages obtained for grani� toid massifs in the North Timan structure of the Timan megablock. Their Rb-Sr ages fall into the interval of 597-587 Ma (2), while the Pb-Pb (Pb evaporation) method yielded zircon ages of 621 ± 3.5 Ma (3). Here�