New data on the structure and age of the crystalline basement of the Fore Range of the Greater Caucasus
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<p>The Dizi Series is exposed within the Southern slope zone of the Greater Caucasus that occurs as a complex geological structure, which constitutes an integral part of the Mediterranean (Alpine-Himalayan) collisional orogenic belt. It is built up of terrigenous and volcanogenic-sedimentary rocks faunistically dated from the Devonian to Triassic inclusive (Somin, 1971; Somin, Belov, 1976; Kutelia 1983). Most of them are metamorphosed under conditions of chlorite-sericite subfacies of the greenschist facies of regional metamorphism (chlorite-phengite-albite&#177;quartz, graphite-sericite-quartz phyllites and marbleized limestones), and only a minor part represented by clay-carbonaceous, phengite-chlorite-carbonaceous and prehnite-chlorite-carbonate schists underwent anchimetamorphism (Shengelia et al., 2015). The Dizi Series is intruded by numerous magmatic bodies of gabbro-diabases, diabases, diorites, diorite-porphyries, syenites, monzo-syenites and granitoids. The age of the intrusions was defined by K-Ar method at 176-165 Ma (Dudauri, Togonidze, 1998) and by U-Pb LA-ICP-MS zircon dating at 166.5 &#177; 4.6 Ma (authors` unpublished data) and corresponds to the Bathonian orogeny. The Middle Jurassic intrusions caused intense contact metamorphism of the rocks of the Dizi Series resulted in the formation of various hornfelses containing andalusite, cordierite, corundum, biotite, plagioclase, potassium feldspar, clinozoisite, hornblende, cummingtonite, clinopyroxene, wollastonite and scapolite. These rocks correspond to albite-epidote-hornfels, andalusite-biotite-muscovite-chlorite-hornfels and andalusite-biotite-muscovite-hornfels subfacies of the contact metamorphism (Javakhishvili et al., 2020). The analogues of the Dizi Series rocks have not previously been established either in the Greater Caucasus or in the neighboring regions. In our view, Paleozoic rocks similar to the Dizi Series occur under the Cretaceous and Jurassic deposits within the folded basement of the plain Crimea where they were recovered by wells. Most of these rocks, as in the Dizi Series, underwent metamorphism of chlorite subfacies of the greenschist facies and, to a lesser extent, deep epigenesis (clayey-carbonaceous, sericite-carbonaceous, actinolite-chlorite-prehnite, muscovite-albite-chlorite, epidote-actinolite-chlorite and graphite-talc-quartz schists) (Chernyak, 1969). These rocks are also intruded by Middle Jurassic igneous rocks, including gabbro-diabases, diabases, diorites, syenites, monzo-syenites, granite-porphyries, etc. (Shniukova, 2016; Shumlyanskyy, 2019). As a result of the contact metamorphism of the basement rocks, muscovite-quartz-cordierite and cordierite-quartz-feldspar micaceous hornfelses were formed. Quartz syenite yielded a K-Ar age of 158 Ma (Scherbak, 1981), while monzo-syenite was dated at 170 &#177; 5 Ma applying 40Ar/39Ar method (Meijers, 2010). Thus, based on the rock associations, the nature of metamorphism, the age of the metamorphic and igneous rocks, and on the spatial position of the Dizi Series and folded basement of the plain Crimea we assume that these units developed coevally in similar environment and geological conditions.<br><br>Acknowledgements.This work was supported by Shota Rustaveli National Science Foundation (SRNSF) [PHDF-19-159, Regional and Contact Metamorphism of the Dizi Series].</p>
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The Greater Caucasus represents a folded-nappe polycyclic geological formation which has been developed as one of the magastrucrutes of the Paleotethys and nowadays it is the North-East segment of the East Mediterranean orogen. Its width reaches 170 km and it is stretched of 1200 km between the Black and Caspian Seas to the NW-SE direction. Two major stages are distinguished in its formation: Pre-Alpine and Alpine. Pre-Alpine formation makes a heterogeneous crystalline basement complex, the exposed part of which is 200 km long and the width reaches 40 km (Figure 1). It is mainly constructed of Caledonian and Hercynian crystalline schist, amphibolites, granitgneisses, migmatites and granitoids. Four regional structural zones are distinguished in this basement (from the South to the North): Southern slope, Main range, Front range and Bechasyn. During the Alpine tectonic-magmatic activisation these units underwent several tectonic uplifts and as a result of these ororgenic processes the crystalline basement of the Greater Caucasus acquired the up to date structural face. The Main range zone is the best exposing part of the crystalline basement of the Greater Caucasus. Different from the other zones, Hercynian metamorphism and granitoid magmatism are intensively represented in this unit. Metamorphism rate ranges from green schist facies to granulite, but amphibolite facies is mostly found (Shengelia et al., 1991).
Hercynian granitoid magmatizm has been studied in some of the regions of the crystalline basement of the Greater Caucasus, but there does not exist its thorough petrogenetical model and our task is to creat the model, which would thorougly reflec the conditions of formation, the way of evolution and ore-forming potential of this magmatizm.
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