The metamorphic complex of Beloretzk, SW Urals, Russia - a terrane with a polyphase Meso- to Neoproterozoic thermo-dynamic evolution

An integrated geological study of the tectono-metamorphic evolution of the metamorphic complex of Beloretzk (MCB) which is part of the eastern Bashkirian mega-anticlinorium (BMA), SW Urals, Russia shows that the main lithological units are Neoproterozoic (Riphean and Vendian age) siliciclastic to carbonate successions. Granitic, syenitic and mafic intrusions together with subaerial equivalents comprise the Neo- and Mesoproterozoic magmatic rocks. The metamorphic grade ranges from diagenetic and very low grade in the western BMA to high-grade in the MCB. The N-S trending Zuratkul fault marks the change in metamorphic grade and structural evolution between the central and eastern BMA. Structural data, Pb/Pb-single zircon ages, 40Ar/39Ar cooling ages and the provenance signature of Riphean and Vendian siliciclastic rocks in the western BMA give evidence of Mesoproterozoic (Grenvillian) rifting, deformation and eclogite-facies metamorphism in the MCB and a Neoproterozoic (Cadomian) orogenic event in the SW Urals. Three pre-Ordovician deformation phases can be identified in the MCB. The first SSE-vergent, isoclinal folding phase (D-1) is younger than the intrusion of mafic dykes (Pb/Pb-single zircon: similar to 1350 Ma) and older than the eclogite-facies metamorphism. High P/low T eclogite-facies metamorphism is bracketed by D-1 and the intrusion of the Achmerovo granite (Pb/Pb-single zircon: less than or equal to 970 Ma). An extensional, sinistral, top-down-to-NW directed shearing (D-2) is correlated with the first exhumation of the MCB. E-vergent folding and thrusting (D-3) occurred at retrograde greenschist-facies metamorphic conditions. The tremolite 40Ar/39Ar cooling age (718 +/- 5 Ma) of amphibolitic eclogite and muscovite 40Ar/39Ar cooling ages (about 550 Ma) of mica schists indicate that a maximum temperature of 500 +/- 50 degreesC was not reached during the Neoproterozoic orogeny. The style and timing of the Neoproterozoic orogeny show similarities to the Cadomian-aged Timan Range NW of the Polar Urals. Geochronological and thermochronological data together with the abrupt change in structural style and metamorphism east of the Zuratkul fault, suggest that the MCB is exotic with respect to the SE-margin of the East European Platform. Thus, the MCB is named the 'Beloretzk Terrane'. Recognition of the 'Beloretzk Terrane and the Neoproterozoic orogeny at the eastern margin of Baltica has important implications for Neoproterozoic plate reconstruction and suggests that the eastern margin of Baltica might have lain close to the Avalonian-Cadomian brit. (C) 2001 Elsevier Science B.V. All rights reserved.