Pre-Jurassic Basement of the Greater Caucasus: Brief Overview

Authors: MARK L. SOMIN

Abstract: The main units of the Greater Caucasus pre-Jurassic basement are represented by Svanetian and North-Caucasian domains brought together tectonically. The former includes continuous Devonian to Upper Triassic marine sequence devoid of any manifestation of Variscan orogenic activity. In contrast, within the limits of the North-Caucasian domain the Variscan events are expressed in classical form. This domain is very heterogeneous and contains both metamorphosed and unmetamorphosed formations. Till recently the former was considered by most authors to be mainly Proterozoic. New geochronological data indicate that the predominant part of these complexes is Palaeozoic in their protolith age. Lithology, P/T conditions of metamorphism, types of associated granitoids and other features are changing drastically from zone to zone demonstrating a collage (terrane)-type structure. The southernmost Laba and Buulgen LP/HT metamorphic complexes are essentially mafic, include I-type metagranitoids and originated in island-arc and ensimatic marginal sea environments. Steep tightly compressed SW-vergent folds, partly as a result of the Early Alpine deformation, are developed. Palaeontological and U-Pb TIMS, SHRIMP and other data yielded mostly Middle Palaeozoic ages for these complexes. Next to the north of the Makera and Gondaray complexes of the Main Range zone are also of LP/HT type but they are typical ensialic and are replaced by huge masses of the Upper Palaeozoic S-type granite. Gentle monocline and dome-like position of foliation is characteristic for this zone. Zircon dating had established Silurian and Devonian age of the Gondaray complex metamagmatic rocks, and mostly Ordovician of the Makera complex ones. Zircon of migmatite's leucosome showed the Late Palaeozoic age of the peak metamorphism, which occurred almost synchronously with the S-granite crystallization. The Fore Range zone is characterized by column of pre-Upper Palaeozoic nappes. Its lowermost unit, the Blyb complex of krystallinikum, previously has been considered by most authors as an old (Proterozoic) basement for the overlying Middle Palaeozoic greenstone island arc sequences. New data indicate that the Blyb complex is an essentially ensimatic HP/LT formation partly coeval to the island arc. It forms a dome-like tectonic window cut in the arc and overlying ophiolite and the Atsgara metamorphic nappes. The Pass area of the Main Range is supposed to be the root zone of these nappes. The northernmost pre-Jurassic tectonic zone of the Greater Caucasus is Bechasyn. It includes a greenschist (-blueschist?) basement and transgressive sedimentary cover. New data on zircons demonstrated that both units are Lower Palaeozoic although tectonic wedges of Cadomian basement also exist there. The data permit to propose that in the Middle Palaeozoic the main subduction zone of the Greater Caucasus was disposed in the Fore Range zone and magmatic and metamorphic events within the Main Range were probably connected with activity of this zone.

Keywords: Greater Caucasus, Palaeozoic, metamorphic complexes, granitoids, structure, U-Pb dating

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