Abstract
The rocks from different stages of the geodynamic evolution have been preserved in the Urals. In its geologic history, the least studied is the transition period between continental rifting and the beginning of oceanic spreading. This article presents the geochemical data on the Sr-, Nd-isotopes, zircon U-Pb (SHRIMP) ages for the MesoNeoproterozoic igneous rocks and associated ores from the Bashkir meganticlinorium (BMA) on the Urals western slope. A Large Igneous Province (LIP) formed there as a result of mantle plume activity during the Middle Riphean (1380–1350 Ma). Later on (1200–1100 Ma), short-term rifting took place, as evidenced by the Nazyam graben, which was followed by the complete break-up of the continental crust. For magmatic rocks in the age range of 1750–1200 Ma, the evolition of chemical composition OIB-type → E-MORB →N-MORB is observed. The eNd(t) values for the igneous rocks and the associated BMA ores vary from negative (–6) to positive ones (+5), and thus give evidence of the lithosphere mantle depletion with time. These facts and the Sr-isotope ratios for the magmatic rocks from the subsequent evolution stages confirm that the oceanic basin to the east of the East European platform started to open at the end of the Middle Riphean. For the Vendian-Cambrian, some traces of orogenes (Timanian stage) are observed. The development of the Uralian Paleozoic ocean started in the Ordovican and continued up to the Late CarboniferousPermian.
Highlights
The Uralian Mobile Belt is one of a few geologic structures on the Earth where the rocks from the different stages of the geodynamic evolution have been preserved
The data on the geology, chemical compositions and ages of most of the Precambrian igneous complexes of the Bashkir Meganticlinorium (BMA) are shown in Figures 4 and 5, and Tables 1, 2, and 3
Basalts of the Ai formation are the most ancient igneous rocks of this stage in the study area; 1752±18 Ma [Krasnobaev et al, 2013]. Their chemical composition indicate [Parnachev, 1981; Ernst et al, 2006; etc.] relatively high alkalinity of rocks, which are characterized by К2О 3–6 %, TiO2 2–3 %, P2O5 up to 0.70 % contents, as well as a sharp prevalence of light rare-earth elements (LREE) over heavy rare-earth elements (HREE)
Summary
The Uralian Mobile Belt is one of a few geologic structures on the Earth where the rocks from the different stages of the geodynamic evolution have been preserved. The Urals is composed of the rocks representing all the geodynamic settings, from continental rifting (the edge of the East European Platform, EEP) and opening of the oceanic basin in the Late MesoProterozoic (the area to the east of EEP) to the Carboniferous-Permian collision. The main structure comprising various magmatic formations under study is the Bashkir Meganticlinorium (BMA) (Figures 1, 2, and 3). It is located at the borderline between the Urals and EEP. We analyzed the composition, age, isotope characteristics and geodynamic settings of the BMA Pre-Cambrian igneous rocks, and tried to determine the nature of magmatism in the early evolutionary stages of the structure, the time of origin and duration of the paleoocean
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