New data on composition and age of granites from the Isherim anticlinorium and boundary of the Timanides in the North Urals

Abstract

The Isherim anticlinorium is located in the NorthUrals (Fig. 1). It represents one of the inliers of thePrePaleozoic basement of the Urals, a world standardof fold belts, which includes structures and complexesreflecting several geodynamic development stages.Recently, age and formation settings of granitoids relatively widespread within this structure are the subjectof acrimonious debates. They are considered as beingthe Late Ordovician [3], Middle Devonian–MiddleCarboniferous and Permian–Triassic [10], Ordovician–Cambrian [9], Late Ordovician and Early Cambrian [4], or Vendian–Cambrian and Middle–LateOrdovician [2] in age. The available isotopic–geochronologic dates obtained mostly in the 1960s indicate ages ranging from the Late Riphean to Permianand, thus, cannot be used for determining age of granitoids and reconstructing their geodynamic settings.At the same time, the answer to the question onbelonging of igneous rocks of the Isherim anticlinorium to the Uralides or PreUralides is of principalsignificance for specifying the boundary of subductionrelated and orogenic complexes of theTimanides. For example, the authors of [7] draw themain collisional suture between sequences of the LateProterozoic passive margin of the Baltica continentand accreted blocks north of the Isherim anticlinorium, while other researchers [14] show it as locatedsouth of the latter. Our investigations provide groundsfor an assumption that the Isherim block was accretedto the passive margin of Baltica. The Isherim anticlinorium is composed of Middleand Lower Riphean sedimentary sequences intrudedby dikes, stocks, and sills of various igneous rocks andoverlain by Ordovician terrigenous, volcanogenic, andcarbonate complexes. The Middle Riphean sequencesare represented by metasandstones and metasiltstonesof the Ras’ya Formation overlain by marbled limestones and dolomites of the Moiva Formation and carbonaceous metasiltstones of the Murav’inyi Formation. The Middle Riphean age of carbonate rocks constituting the Moiva Formation is traditionallysubstantiated by finds of stromatolites