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Pre-Jurassic Basement of the Greater Caucasus: Brief Overview

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Pre-Jurassic Basement of the Greater Caucasus: Brief Overview Turkish Journal of Earth Sciences (Turkish J. Earth Sci.), Vol.M.L. 20, 2011, SOMIN pp. 545–610. Copyright ©TÜBİTAK doi:10.3906/yer-1008-6 First published online 10 May 2011 Pre-Jurassic Basement of the Greater Caucasus: Brief Overview MARK L. SOMIN Institute of Physics of the Earth RAS, 10 Bolshaya Gruzinskaya str., Moscow, Russia (E-mail: [email protected]) Received 15 October; revised typescripts receipt 28 March 2011 & 29 April 2011; accepted 10 May 2011 Abstract: Th e main units of the Greater Caucasus pre-Jurassic basement are represented by Svanetian and North- Caucasian domains brought together tectonically. Th e 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. Th is 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. Th e southernmost Laba and Buulgen LP/HT metamorphic complexes are essentially mafi c, 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. Th e 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. Th e Pass area of the Main Range is supposed to be the root zone of these nappes. Th e 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. Th e 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. Key Words: Greater Caucasus, Palaeozoic, metamorphic complexes, granitoids, structure, U-Pb dating Büyük Kafk asların Jura Öncesi Temeli Özet: Büyük Kafk asların Jura öncesi temeli tektonik olarak yanyana gelmiş Svanetiyen ve Kuzey-Kafk as bölgelerinden oluşur. Svanetiyen bölgesi Variskan orojenik olaylarının gözlenmediği Devoniyen’den Triyas’a kadar sürekli sedimantasyon gösteren bir istif ile tanımlanır. Buna karşın, Kuzey-Kafk as bölgesinde klasik Variskan orojenik olaylar yer alır. Çok heterojen olan Kuzey-Kafk as bölgesi içinde metamorfi k ve metamorfi k olmayan formasyonlar bulunur. Yakın zamana kadar bu bölgedeki metamorfi k birimler Proterzoyik yaşlı olarak kabul edilmekteydi. Buna karşın yeni jeokronolojik veriler metamorfi k komplekslerin ana kayalarının genellikle Paleozoyik yaşta olduğunu göstermiştir. Kuzey-Kafk as bölgesi içinde metamorfi k birimlerin litolojileri, metamorfi zma tipleri, metamorfi zmayla ilişkili granitoidler ve diğer özellikler bölgeden bölgeye değişiklik gösterir ve Kuzey-Kafk as bölgesinin bir mıntıkalar (terrane) topluluğu olduğuna işaret eder. 545 PRE-JURASSIC BASEMENT OF THE CAUCASUS Kuzey-Kafk as bölgesinin en güneyinde yer alan Laba ve Buulgen DB/YS (düşük basınç / yüksek sıcaklık) metamorfi k kompleksleri mafi k özelliktedir ve ada-yaylarında ve ensimatik kenar denizlerde oluşan I-tipi metagranitoidler içerir. Bu bölgede kısmen erken Alpin olaylar sonucu oluşmuş güneybatıya verjanslı, dik sıkışık kıvrımlar gözlenir. Paleontolojik bulgular ve U-Pb TIMS, SHRIMP verileri Laba ve Buulgen komplekslerinin orta Paleozoyik yaşında olduğuna işaret eder. Laba ve Buulgen komplekslerinin kuzeyinde Ana Kuşak zonunda (Main Range Zone) yer alan Makera ve Gondaray kompleksleri ensialiktir ve baskın olarak Geç Paleozoyik yaşlı S-tipi granitoidlerden yapılmıştır. Bu bölge için tipik yapısal stil foliasyonun yumuşak domsal yapısı ve yumuşak monoklinallerdir. Zirkon yaş tayinleri Gondaray kompleksi metamagmatik kayalarının Siluryen ve Devoniyen yaşında, benzer kayaların Makera Kompleksinde ise Ordovisyen yaşında olduğunu göstermiştir. Migmatit lökosomundan elde edilen zirkon yaşları metamorfi zma zirvesinin, S-tipi granit kristalleşmesi ile eşyaşlı olarak, Geç Paleozyik yaşında olduğuna işaret eder. Ön Kuşak Zonu (Fore Range zone) Geç Paleozoyik öncesi naplar ile karakterize olur. Bu zonda en altta yer alan ve kristalen kayalardan oluşan Blyb Kompleksi geçmişte, daha üstte bulunan orta Paleozoyik yaşta ada yayı mafi k kayalarının temeli olarak kabul edilmiştir. Yeni veriler Blyb Kompleksi’nin, üstteki ada-yayı kayaları ile benzer yaşta, bir ensimatik yüksek basınç-düşük sucaklık (YB/DS) kompleksi olduğunu göstermiştir. Blyb Kompleksi üstte yer alan ada- yayı, ofi yolit ve metamorfi k napların altından dom şeklinde bir tektonik pencerede yüzeyler. Bu napların köken zonu Ana Kuşak zonundaki Pass bölgesi olarak kabul edilir. Büyük Kafk asların Jura-öncesi en kuzey zonu, eskiden yeşilşist fasiyesinde bir temel ve sedimenter bir örtüden oluştuğu kabul edilen Bechasyn Kompleksi’dir. Yeni zirkon verileri her iki birimin de Erken Paleozoyik yaşında olduğuna işaret eder; buna karşın Kadomiyen yaşlı ufak dilimler bu bölgede bulunabilir. Veriler orta Paleozoyik’de Büyük Kafk aslar’da ana dalma-batma zonunun Ön Kuşak zonunda bulunduğunu ve Ana Kuşak Zonu’ndaki magmatik ve metamorfi k olayların bu dalma-batmaya bağlı olarak geliştiğini göstermektedir. Anahtar Sözcükler: Büyük Kafk aslar, Paleozoyik, metamorfi k kompleks, granitoyidler, yapı, U-Pb yaş tayini Introduction Main Caucasian Fault separates basement exposures Th e Greater Caucasus is an Alpine folded and highly of the Main Caucasian Range and the Southern elevated system, which borders from the south the Slope zone. Th is fault represents a long lived magma- Scythian platform, the southern promontory of conducting steep zone developed at least since the the East-European platform. Th e Trans-Caucasian Middle Jurassic (Somin 2000). Interpretation of the median massif separates the Greater Caucasus from Main Caucasian Fault as frontal basal part of great- the Lesser Caucasus folded system. Th e Black Sea amplitude thrust sheet (Dotduev 1986; Baranov et al. and the Caspian Sea superimposed basins fl ank 1991) seems to be wrong. the Greater Caucasus from the west and the east, Pre-Jurassic basement complexes of the Greater respectively (Figure 1). Caucasus crop out within the limits of two domains, Th e Alpine structure of the Greater Caucasus Svanetian and North-Caucasian (Figures 1 & 2). Th e is relatively simple. Jurassic and younger marine, fi rst one disposes in the Southern Slope zone, whereas shallow-water, partly continental sediments form the North Caucasian domain crops out to the north. more or less fl at monocline (Laba-Malka zone) at the Th ese two domains diff er drastically: in the Svanetian northern limb of the Greater Caucasus anticlinorium. domain the single event of pre-Alpine folding was To the south the thickness of the Mesozoic sediments Indosynian (Early Kimmerian, i.e. pre-Early Liassic), increases, and more and more intense mostly south- whereas in the North-Caucasian domain typical vergent folding combined with thrusts and small Variscan (i.e. Middle–Late Palaeozoic) events have nappes appear. In the North-Western (‘Central’) occurred. Caucasus the Laba-Malka zone is separated from this Th e main purpose of this paper is to give vast folded area called Southern Slope zone by wide the new information on geology of the North- parallelogram-shaded exposures of the pre-Jurassic Caucasian domain, especially on its metamorphic basement dissected into diff erent blocks by steep complexes. Problems of their ages, lithology, faults, thrusts and narrow structural depressions conditions of metamorphism, main features of fi lled mostly with Lower Jurassic sediments. Th e structure and relations with unmetamorphosed 546 M.L. SOMIN exposures of the pre-Jurassic complex Figure 1. Scheme of distribution of some tectonic elements in the Caucasus. pre-Upper Palaeozoic complexes are discussed. Th e marine
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