Pre-Jurassic basement of the Greater Caucasus: Brief overview

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.

Turkish Journal of Earth Sciences (Turkish J Earth Sci.), Vol.M.L 20, 2011, pp 545–610 Copyright ©TÜBİTAK SOMIN 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: somin@ifz.ru) Received 15 October; revised typescripts receipt 28 March 2011 & 29 April 2011; accepted 10 May 2011 Abstract: The main units of the Greater Caucasus pre-Jurassic basement are represented by Svanetian and NorthCaucasian 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 SWvergent 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 Key Words: Greater Caucasus, Palaeozoic, metamorphic complexes, granitoids, structure, U-Pb dating Büyük Kafkasların Jura Öncesi Temeli Özet: Büyük Kafkasların Jura öncesi temeli tektonik olarak yanyana gelmiş Svanetiyen ve Kuzey-Kafkas bölgelerinden oluşur Svanetiyen bölgesi Variskan orojenik olaylarının gözlenmediği Devoniyen’den Triyas’a kadar sürekli sedimantasyon gösteren bir istif ile tanımlanır Buna karşın, Kuzey-Kafkas bölgesinde klasik Variskan orojenik olaylar yer alır ầok heterojen olan Kuzey-Kafkas bửlgesi iỗinde metamorfik ve metamorfik olmayan formasyonlar bulunur Yakın zamana kadar bu bölgedeki metamorfik birimler Proterzoyik yaşlı olarak kabul edilmekteydi Buna karşın yeni jeokronolojik veriler metamorfik komplekslerin ana kayalarının genellikle Paleozoyik yaşta olduğunu göstermiştir Kuzey-Kafkas bölgesi iỗinde metamorfik birimlerin litolojileri, metamorfizma tipleri, metamorfizmayla ilikili granitoidler ve diğer özellikler bölgeden bölgeye değişiklik gösterir ve Kuzey-Kafkas bölgesinin bir mıntıkalar (terrane) topluluğu olduğuna işaret eder 545 PRE-JURASSIC BASEMENT OF THE CAUCASUS Kuzey-Kafkas bölgesinin en güneyinde yer alan Laba ve Buulgen DB/YS (dỹỹk basnỗ / yỹksek scaklk) metamorfik kompleksleri mafik özelliktedir ve ada-yaylarında ve ensimatik kenar denizlerde oluşan I-tipi metagranitoidler iỗerir Bu bửlgede ksmen erken Alpin olaylar sonucu olumu gỹneybatya 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 baskn olarak Geỗ Paleozoyik yal S-tipi granitoidlerden yaplmtr Bu bửlge iỗin tipik yapsal stil foliasyonun yumuak domsal yaps 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 metamorfizma zirvesinin, S-tipi granit kristallemesi ile eyal olarak, Geỗ Paleozyik yanda olduuna iaret eder ện Kuak Zonu (Fore Range zone) Geỗ Paleozoyik öncesi naplar ile karakterize olur Bu zonda en altta yer alan ve kristalen kayalardan oluan Blyb Kompleksi geỗmite, daha ỹstte bulunan orta Paleozoyik yaşta ada yayı mafik kayalarının temeli olarak kabul edilmiştir Yeni veriler Blyb Kompleksi’nin, üstteki ada-yayı kayaları ile benzer yata, bir ensimatik yỹksek basnỗ-dỹỹk sucaklk (YB/DS) kompleksi olduunu göstermiştir Blyb Kompleksi üstte yer alan adayayı, ofiyolit ve metamorfik napların altından dom şeklinde bir tektonik pencerede yüzeyler Bu napların köken zonu Ana Kuşak zonundaki Pass bölgesi olarak kabul edilir Büyük Kafkasların 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 Kafkaslar’da ana dalma-batma zonunun Ön Kuşak zonunda bulunduğunu ve Ana Kuşak Zonu’ndaki magmatik ve metamorfik olayların bu dalma-batmaya bağlı olarak geliştiğini göstermektedir Anahtar Sözcükler: Büyük Kafkaslar, Paleozoyik, metamorfik kompleks, granitoyidler, yapı, U-Pb yaş tayini Introduction The Greater Caucasus is an Alpine folded and highly elevated system, which borders from the south the Scythian platform, the southern promontory of the East-European platform The Trans-Caucasian median massif separates the Greater Caucasus from the Lesser Caucasus folded system The Black Sea and the Caspian Sea superimposed basins flank the Greater Caucasus from the west and the east, respectively (Figure 1) The Alpine structure of the Greater Caucasus is relatively simple Jurassic and younger marine, shallow-water, partly continental sediments form more or less flat monocline (Laba-Malka zone) at the northern limb of the Greater Caucasus anticlinorium To the south the thickness of the Mesozoic sediments increases, and more and more intense mostly southvergent folding combined with thrusts and small nappes appear In the North-Western (‘Central’) Caucasus the Laba-Malka zone is separated from this vast folded area called Southern Slope zone by wide parallelogram-shaded exposures of the pre-Jurassic basement dissected into different blocks by steep faults, thrusts and narrow structural depressions filled mostly with Lower Jurassic sediments The 546 Main Caucasian Fault separates basement exposures of the Main Caucasian Range and the Southern Slope zone This fault represents a long lived magmaconducting steep zone developed at least since the Middle Jurassic (Somin 2000) Interpretation of the Main Caucasian Fault as frontal basal part of greatamplitude thrust sheet (Dotduev 1986; Baranov et al 1991) seems to be wrong Pre-Jurassic basement complexes of the Greater Caucasus crop out within the limits of two domains, Svanetian and North-Caucasian (Figures & 2) The first one disposes in the Southern Slope zone, whereas the North Caucasian domain crops out to the north These two domains differ drastically: in the Svanetian domain the single event of pre-Alpine folding was Indosynian (Early Kimmerian, i.e pre-Early Liassic), whereas in the North-Caucasian domain typical Variscan (i.e Middle–Late Palaeozoic) events have occurred The main purpose of this paper is to give the new information on geology of the NorthCaucasian domain, especially on its metamorphic complexes Problems of their ages, lithology, conditions of metamorphism, main features of structure and relations with unmetamorphosed M.L SOMIN exposures of the pre-Jurassic complex Figure Scheme of distribution of some tectonic elements in the Caucasus pre-Upper Palaeozoic complexes are discussed The characteristics of the latter are presented in a very brief form Petro- and geochemical characteristics are used in minimal volume, taking into account the papers published by Zakariadze, Shavisvili and their coauthors in last years (see Adamia et al 2004) Information of the Svanetian domain is also given here in a very restricted form mostly to complete a picture concerning the ‘frame’ geology The Svanetian Domain The Svanetian domain is represented by Dizi series exposed in Svanetian anticlinorium, CentralNorthern Georgia, and in small Mzymta river antlicline, Russia The existence of the Dizi (originally Desi) series was established at the beginning of the 1960’s Following the pioneer study by Agalin (1935), marine fossils of Middle and Upper Devonian, Middle Carboniferous and Permian were found in limestones of highly deformed and slightly metamorphosed rocks exposed in basins of Inguri and Tskhenis-tskali rivers (Slavin et al 1962; Gamkrelidze et al 1963; Somin & Belov 1967; Adamia 1968; Somin 1971) Marine Lower Carboniferous and Upper Triassic fossils were described later Kutelia (1983) essentially confirmed this stratigraphic information using the study of conodonts selected from layers of chert The primary suggestion on existence of continuous stratigraphic Middle Palaeozoic–Triassic section (excluding Upper Carboniferous) was firmly supported as a result As for the Upper Carboniferous, marine brachiopods of this age were collected in sandstone of transgressive Kvishi formation at the Main Caucasian Range zone near Ushba Peak and the Main Caucasian fault, Svanetia district (Somin 1965; Khutsishvili 1966) 547 548 Middle Jurassic gabbro & granitoids main faults location of cross-sections: Malaya Laba - Urushten; Khetskvara - Teberda Figure Geological scheme of the Greater Caucasus pre-Jurassic basement location of maps 3, 5, 18, 23, 24 names of basement salients: Chugush; Atamazhi; Dakh; Sakhray; Blyb; Beskes; Svanetian anticlinorium; Mzymta anticline Upper Palaeozoic granitoids of the Main Range & Bechasyn zone place of a capture of samples for U-Pb dating Middle & Upper Palaeozoic unmetamorphosed sequences ophiolite & serpentinite Palaeozoic cover of the Bechasyn MC PRE-JURASSIC BASEMENT OF THE CAUCASUS M.L SOMIN Lower (Devonian to Middle Carboniferous) part of the Dizi series (Kirar Formation) is represented by greywacke sandstone and siltstone, containing beds and lenses of chert, shallow-water coral limestone and gravelstone Bodies of limestone are now considered presumably as olistolithes Upper part of the series consists of flysh-like intercalating greenish calcareous siltstone and sandstone with rare thin in-situ layers of limestone (Laila and Chelshura Formation of Somin & Belov (1967) collectively named by Kutelia Tskhenis-Tskali Formation), and the uppermost part consists of regressive section of coarse-grained calcareous sandstone and gravelstone of the Gvadarashi Formation (Somin 1971) Volcanic rocks of the Dizi series are represented by andesite tuffs of the Utur Formation of unknown, presumably Upper Palaeozoic age The degree of regional metamorphism of the Dizi series does not exceed low-temperature greenschist facies conditions Middle Jurassic Kirar diorite intrudes the Dizi series and forms wide contact metamorphic zone at right slope of Inguri River valley The structure of the Dizi series is very complex and is characterized by tightly compressed steep folds and thrusts with clearly expressed (especially in Inguri River section) northern vergence whereas the surrounding Lower Jurassic sediments are southvergenced, and the border between these two domains is sharply pronounced The superimposing Alpine cleavage and kink-bands crosscut the Indosynian (Early Kimmerian) fabrics of the series (Kaz’min & Sborschikov 1989) Folds of two generations are present in the Dizi series (especially well expressed again along Inguri River), whereas the Early Jurassic folds are represented almost everywhere by single generation only These observations are important for long-debated problem of relationship between the Dizi series and the Lower Jurassic Gamkrelidze et al (1963) and Adamia et al (1990) have proposed that in some parts of the Dizi Basin there was continuous sedimentation from Palaeozoic and Triassic to the Sinemurian Other authors (Belov & Somin 1964; Somin 1971, 2007a, b; Kaz’min & Sborschikov 1989) have argued for structural unconformity between these two units Indeed, the Lower Sinemurian basal layers overlay, sometimes with an angular, azimutal and slightly metamorphic unconformity, different stratigraphic levels of the Dizi series Besides, coal- bearing layers with fossil flora were found here in the basal part of the Lower Sinemurian (Belov & Somin 1964), and we need to stress that local continental sedimentation is a characteristic feature of the Early Sinemurian time elsewhere in the Greater Caucasus The North-Caucasian Domain This domain consists of two structural stages, Upper Palaeozoic (Upper Visean–Triassic) and pre-Upper Palaeozoic Within the latter there are three tectonic zones which are traditionally recognized, namely the Main Range zone, the Fore Range zone, and the Bechasyn zone Until recently it was assumed by most authors that all these zones include two structural stages: the lower Proterozoic and the upper, Middle Palaeozoic (Andruschuk 1968; Belov 1981) The author’s new information allows to change this opinion and to show that two pre-Upper Palaeozoic stages exists (but in the Palaeozoic variant) in the Bechasyn zone only, whereas there are no grounds for these two structural stages in the southern zones The Main Range Zone This is a main area of distribution of metamorphic complexes and granitoids in the Greater Caucasus The common feature of this zone is lowpressure (andalusite-sillimanite) type of regional metamorphism Only in a very narrow area at southwestern margin of the zone the mineral association with kyanite appears The differences in structural style and composition of metamorphic complexes and granites allow to divide this zone into two subzones, the Pass and Elbrus (Somin 1971) The Pass Subzone It occupies the southernmost position within the Main Range zone (Figure 1) and includes a granitemetamorphic basement, or krystallinikum, the lower structural stage and, locally, the Upper Palaeozoic slightly metamorphosed or unmetamorposed sediments, the upper stage The characteristic features of both stages are more or less tightly compressed linear folds of WNW (‘Caucasian’) strike and SW vergence 549 PRE-JURASSIC BASEMENT OF THE CAUCASUS The Krystallinikum– The metamorphic complexes of the basement are characterized by variable, essentially mafic composition because they contain up to 30–50% of metabasites Low-K I-type mostly metamorphosed granitoids and diorites are predominated among plutonic intrusive rocks The two main metamorphic complexes, Laba and Buulgen, were recognized long time ago in the krystallinikum of the Pass subzone (Somin 1965, 1971) These complexes conjugate tectonically at Lashtrak and Makera rivers watershed (Figure 2) The Laba Metamorphic Complex (LMC) is distributed in axial part of the Main Range in Lashipse, Tsakhvoa, Damkhurts, Makera and Belaya riverheads The existence of this independent unit was demonstrated for first time by Mel’nikov (1959, 1964), then by Grekov et al (1968) The LMC includes four units (Somin 1971), in ascending order these are: the Mamkhurts, Damkhurts, Lashtrak and Adzhara formations, forming the keel-like Damkhurts synform (Figure 3) Adamia (1977, 1984) and Abesadze et al (1982) consider these formations as purely tectonic units (tectonic sheets) The Mamkhurts Formation, about 700 m thick, consists of alternating mostly fine-grained amphibolites, hornblende, biotite-hornblende, biotite and leucocratic gneisses sporadically showing relicts of volcanic and granitic textures Metasedimentary rocks are very rare here Composition of rocks indicates the possibility of rifting origin of the Mamkhurts Formation because the contents of TiO2 and K2O in metabasites are high (1.5–2.5% and 0.5– 0.7%) and these trachymetabasites are interlayered with acidic rocks By this Mamkhurts Formation clearly differs from overlying Damkhurts Formation TIMS U-Pb dating of zircons from leucocratic orthogneiss yielded 534±9 Ma (Early Cambrian) on upper interseption of concordia; lower one show Ma (Somin et al 2004) Unfortunately, dating was not accompanied by cathodoluminescence study of the zircons therefore the results have limited significance and need additional checking Type of contact with the Damkurts Formation is disputable; presence of small lenses of serpentinite and change of petrochemical characteristics support the conclusion of Adamia (1984) of tectonic relationship between these two units 550 The Damkhurts Formation, ca 400 m thick, has also complex composition and includes metavolcanic rocks in its lower part and siliciclastics in the upper part Metabasites are low-titanium (