Oldest (3.5 Ga) zircons of the Urals: U-Pb (SHRIMP-II) and T DM constraints

Abstract

The Archean domains of the Earth composed of granite‐greenstone and granulite‐gneiss associations are now sufficiently well studied. Nevertheless, many aspects related to the early history of the planet are still far from being solved [1, 2, and others]. Such a situation is also characteristic of the Uralian foldbelt. The formation of the crust in this region spanned a long period comprising the Archean‐Early Proterozoic, Late Proterozoic, Paleozoic, and post-Paleozoic stages [3]. In recent years, considerable progress has been achieved in reconstruction of the major features of the last three stages in the foldbelt evolution mainly due to the active introduction of advanced methods of isotope geology based on sophisticated analytical equipment into geological studies. At the same time, significant difficulties remain in the reconstruction of the preRiphean stage for the following reasons: first, the reliably defined Archean metamorphic domain in the Urals is extremely limited and represented, in fact, only by the Taratash polymetamorphic complex of the Central Uralian Uplift with the single Neoarchean date [4] obtained only recently by the classical U‐Pb zircon method; second, the dating of Neoarchean rocks is hampered by several methodological and analytical problems, the correct solution of which is essential for validity of the results obtained. The Taratash polymetamorphic complex is located among volcanosedimentary rocks of the Lower Riphean Ai and Satka formations at the junction of the Bashkir and Uraltau anticlinoriums located between the South and Middle Urals (Fig. 1). It occupies an area of 400 km 2 and is composed of lithologically variable gneisses, two-pyroxene crystalline schists, quartz-bearing diorites and gabbrodiorites, and quartz‐feldspar rocks. The geological structure of the Taratash Complex, the petrology of its rocks, and metamorphism evolution are described in detail in [5]. The history of the study of the Taratash Complex by methods of isotopic geology (K‐Ar, α -Pb, 207 Pb/ 206 Pb thermal isochron, and classical U‐Pb) is already more than 35 years old [4‐7]. These studies revealed several stages in the evolution of the rock complex corresponding to granulite metamorphism, high-temperature amphibolite-facies diaphthoresis, amphibolite-facies metamorphism, and greenschist diaphthoresis. Nevertheless, many aspects of its formation remain unclear. Moreover, they are conflicting to a significant extent in light of recent data [8].