New data on the composition and age of basement rocks of the Tagil paleo-island arc system

Abstract

The Tagil paleo-island arc terrane is composed of Late Ordovician-Devonian intrusive, volcanic, and volcano-sedimentary complexes. The western margin of the terrane is comprised of dunite-clinopyroxenite-gabbro massifs of the Ural platinum-bearing belt, which are fringed by rock strata of widely different metamorphic grades. Work on isotope systematics of olivine gabbros enabled us to infer a Vendian age (550–540 Ma) of homogenization of the Sm-Nd isotopic system of the Kytlym and Knyaspa massifs within the Ural platinum-bearing belt. The Sm-Nd ages for metamorphic rocks of the Belaya Gora complex surrounding the studied massifs also agree with a Vendian age (573–574 Ma). Our results suggest that metamorphites of the Belaya Gora complex (amphibolites, plagiogneisses, two-mica and biotite gneisses, schists containing garnet, cordierite, staurolite, gedrite, and sillimanite) and dunite-clinopyroxenite-gabbro intrusions of the proto-platinum-bearing belt may have been formed in a convergence setting above a mantle plume, most likely in a back-arc (?) extension region. Reactivation of this plume during the late Precambrian resulted in the opening of the Ural paleo-ocean. The Late Ordovician-Early Devonian times were marked by metamorphic reworking and tectonic transport of pre-Paleozoic complexes into an accretionary prism setting of the Tagil paleo-arc that was accompanied by generation of gabbroid and granitoid magmas. Based on the obtained results, the Tagil terrane can be now considered as part of the Paleozoic paleo-island arc system developed on a heterogeneous Proterozoic basement.