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 fossils of Middle and Upper Devonian, Middle characteristics of the latter are presented in a very Carboniferous and Permian were found in limestones brief form. Petro- and geochemical characteristics of highly deformed and slightly metamorphosed rocks are used in minimal volume, taking into account exposed in basins of Inguri and Tskhenis-tskali rivers the papers published by Zakariadze, Shavisvili and (Slavin et al. 1962; Gamkrelidze et al. 1963; Somin their coauthors in last years (see Adamia et al. 2004). & Belov 1967; Adamia 1968; Somin 1971). Marine Information of the Svanetian domain is also given Lower Carboniferous and Upper Triassic fossils were here in a very restricted form mostly to complete a described later. Kutelia (1983) essentially confi rmed picture concerning the ‘frame’ geology. this stratigraphic information using the study of conodonts selected from layers of chert. Th e primary suggestion on existence of continuous stratigraphic Th e Svanetian Domain Middle Palaeozoic–Triassic section (excluding Upper Th e Svanetian domain is represented by Dizi series Carboniferous) was fi rmly supported as a result. As exposed in Svanetian anticlinorium, Central- for the Upper Carboniferous, marine brachiopods of Northern Georgia, and in small Mzymta river this age were collected in sandstone of transgressive antlicline, Russia. Th e existence of the Dizi (originally Kvishi formation at the Main Caucasian Range zone Desi) series was established at the beginning of the near Ushba Peak and the Main Caucasian fault, 1960’s. Following the pioneer study by Agalin (1935), Svanetia district (Somin 1965; Khutsishvili 1966).

547 PRE-JURASSIC BASEMENT OF THE CAUCASUS Palaeozoic cover of the Bechasyn MC Middle & Upper Palaeozoic unmetamorphosed sequences ophiolite & serpentinite of a capture samples for U-Pb dating ymta anticline location of cross-sections: 8. Malaya Laba - Urushten; 9. Khetskvara - Teberda place Teberda location of cross-sections: 8. Malaya Laba - Urushten; 9. Khetskvara Middle Jurassic gabbro & granitoids main faults 3, 5, 18, 23, 24 Upper Palaeozoic granitoids of the Main Range & Bechasyn zone location of maps names of basement salients: 1. Chugush; 2. Atamazhi; 3. Dakh; 4. Sakhray; 5. Blyb; 6. Beskes; 7. Svanetian anticlinorium; 8. Mz names of basement salients: 1. Chugush; 2. Geological scheme of the Greater Caucasus pre-Jurassic basement. pre-Jurassic Caucasus theGeological Greater scheme of Figure 2.

548 M.L. SOMIN

Lower (Devonian to Middle Carboniferous) part bearing layers with fossil fl ora were found here in the of the Dizi series (Kirar Formation) is represented basal part of the Lower Sinemurian (Belov & Somin by greywacke sandstone and siltstone, containing 1964), and we need to stress that local continental beds and lenses of chert, shallow-water coral sedimentation is a characteristic feature of the Early limestone and gravelstone. Bodies of limestone are Sinemurian time elsewhere in the Greater Caucasus. now considered presumably as olistolithes. Upper part of the series consists of fl ysh-like intercalating greenish calcareous siltstone and sandstone with rare Th e North-Caucasian Domain thin in-situ layers of limestone (Laila and Chelshura Th is domain consists of two structural stages, Upper Formation of Somin & Belov (1967) collectively Palaeozoic (Upper Visean–Triassic) and pre-Upper named by Kutelia Tskhenis-Tskali Formation), and Palaeozoic. Within the latter there are three tectonic the uppermost part consists of regressive section of zones which are traditionally recognized, namely coarse-grained calcareous sandstone and gravelstone the Main Range zone, the Fore Range zone, and the of the Gvadarashi Formation (Somin 1971). Volcanic Bechasyn zone. Until recently it was assumed by most rocks of the Dizi series are represented by andesite authors that all these zones include two structural tuff s of the Utur Formation of unknown, presumably stages: the lower Proterozoic and the upper, Middle Upper Palaeozoic age. Th e degree of regional Palaeozoic (Andruschuk 1968; Belov 1981). Th e metamorphism of the Dizi series does not exceed author’s new information allows to change this low-temperature greenschist facies conditions. opinion and to show that two pre-Upper Palaeozoic Middle Jurassic Kirar diorite intrudes the Dizi series stages exists (but in the Palaeozoic variant) in the and forms wide contact metamorphic zone at right Bechasyn zone only, whereas there are no grounds slope of Inguri River valley. for these two structural stages in the southern zones. Th e structure of the Dizi series is very complex and is characterized by tightly compressed steep Th e Main Range Zone folds and thrusts with clearly expressed (especially in Inguri River section) northern vergence whereas Th is is a main area of distribution of metamorphic the surrounding Lower Jurassic sediments are south- complexes and granitoids in the Greater Caucasus. vergenced, and the border between these two domains Th e common feature of this zone is low- is sharply pronounced. Th e superimposing Alpine pressure (andalusite-sillimanite) type of regional cleavage and kink-bands crosscut the Indosynian metamorphism. Only in a very narrow area at south- (Early Kimmerian) fabrics of the series (Kaz’min western margin of the zone the mineral association & Sborschikov 1989). Folds of two generations are with kyanite appears. present in the Dizi series (especially well expressed Th e diff erences in structural style and composition again along Inguri River), whereas the Early Jurassic of metamorphic complexes and granites allow to folds are represented almost everywhere by single divide this zone into two subzones, the Pass and generation only. Th ese observations are important Elbrus (Somin 1971). 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 Th e Pass Subzone in some parts of the Dizi Basin there was continuous It occupies the southernmost position within the sedimentation from Palaeozoic and Triassic to the Main Range zone (Figure 1) and includes a granite- Sinemurian. Other authors (Belov & Somin 1964; metamorphic basement, or krystallinikum, the lower Somin 1971, 2007a, b; Kaz’min & Sborschikov 1989) structural stage and, locally, the Upper Palaeozoic have argued for structural unconformity between slightly metamorphosed or unmetamorposed these two units. Indeed, the Lower Sinemurian basal sediments, the upper stage. Th e characteristic features layers overlay, sometimes with an angular, azimutal of both stages are more or less tightly compressed and slightly metamorphic unconformity, diff erent linear folds of WNW (‘Caucasian’) strike and SW stratigraphic levels of the Dizi series. Besides, coal- vergence.

549 PRE-JURASSIC BASEMENT OF THE CAUCASUS

Th e Krystallinikum– Th e metamorphic complexes Th e Damkhurts Formation, ca. 400 m thick, has of the basement are characterized by variable, also complex composition and includes metavolcanic essentially mafi c composition because they contain rocks in its lower part and siliciclastics in the upper up to 30–50% of metabasites. Low-K I-type mostly part. Metabasites are low-titanium (<1% TiO2) metamorphosed granitoids and diorites are indicating an island arc origin for the formation. predominated among plutonic intrusive rocks. Two thin horizons of marble appear in the base Th e two main metamorphic complexes, Laba and in the uppermost part of this formation. Th e and Buulgen, were recognized long time ago in the remnants of the Ludlow-Franian blue-green algae krystallinikum of the Pass subzone (Somin 1965, Renalcus sp. were discovered in this limestone by 1971). Th ese complexes conjugate tectonically at Chegodaev. A relatively thick (up to 300 m) layer of Lashtrak and Makera rivers watershed (Figure 2). highly stretched intraformational metaconglomerate is disposed between the limestone horizons (Figure Th e Laba Metamorphic Complex (LMC) is 4). Th e composition of pebbles in this rock is very distributed in axial part of the Main Range in diverse; many of them are presented by subvolcanic Lashipse, Tsakhvoa, Damkhurts, Makera and Belaya metaplagiogranite and amphibolites (Somin & riverheads. Th e existence of this independent unit Korikovski 1988); at the same time pebbles of quartz was demonstrated for fi rst time by Mel’nikov (1959, are not present. Schistosity and mineral lineation are 1964), then by Grekov et al. (1968). Th e LMC common for both the cement matrix and pebbles; includes four units (Somin 1971), in ascending order therefore metamorphism was superimposed on this these are: the Mamkhurts, Damkhurts, Lashtrak originally clastic rock. It is very important to note that and Adzhara formations, forming the keel-like the same type of ‘quartzless’ conglomerate is known Damkhurts synform (Figure 3). Adamia (1977, 1984) from the Upper Devonian deposits of the Fore Range and Abesadze et al. (1982) consider these formations zone, and that in both cases some limestone horizons as purely tectonic units (tectonic sheets). appear among garnet-andalusite micaschist. Th e Mamkhurts Formation, about 700 m Th e Lashtrak Formation, about 500 m thick, lies thick, consists of alternating mostly fi ne-grained concordantly on the Damkhurts Formation, the amphibolites, hornblende, biotite-hornblende, biotite contact is well exposed in right bank of Damkhurts and leucocratic gneisses sporadically showing relicts riverhead. Th e formation consists of garnet graphite- of volcanic and granitic textures. Metasedimentary andalusite and staurolite-bearing schists containing rocks are very rare here. Composition of rocks some interbeds of fi ne-grained amphibolites. indicates the possibility of rift ing origin of the Some authors suppose the andalusite has contact- Mamkhurts Formation because the contents of TiO 2 metamorphic origin. As a separated tectonic slice of and K O in metabasites are high (1.5–2.5% and 0.5– 2 this formation, the kyanite-bearing schist appears at 0.7%) and these trachymetabasites are interlayered the right slope of Lashipse River. Th erefore, this part with acidic rocks. By this Mamkhurts Formation of the LMC section seems to be allochtonous relative clearly diff ers from overlying Damkhurts Formation. to underlying units. TIMS U-Pb dating of zircons from leucocratic orthogneiss yielded 534±9 Ma (Early Cambrian) on Th e uppermost unit of the Laba complex is upper interseption of concordia; lower one show 0 Adzhara Formation (300 m thick) which has limited Ma (Somin et al. 2004). Unfortunately, dating was distribution and consists of quartzite, porphyroid, not accompanied by cathodoluminescence study amphibolite and dark fossiliferous limestone, where of the zircons therefore the results have limited post-Ordovician crinoids were found (Potapenko signifi cance and need additional checking. Type of & Stukalina 1971). Adamia (1984) attribute this contact with the Damkurts Formation is disputable; limestone to the Lashtrak Formation. presence of small lenses of serpentinite and change of Metaplutonic rocks of the Laba Complex are petrochemical characteristics support the conclusion represented mostly by metamorphosed quartz of Adamia (1984) of tectonic relationship between diorite and diorite, less by plagiogranite. Th eir these two units. petrochemical characteristics indicate mantle origin

550 M.L. SOMIN

gneiss, mostly metavolcanic metapelite and shale gravelstone marble and limestone gabbro-amphibolite pyroclastic alternating metapelite schist granitoid lava flows and amphibolite amphibolite fossil findings quaternary deposits quartzless metaconglomerate marble and limestone nappe and conglomerate (in the columns) sandstone

Figure 3. Geological map of Damkhurts riverhead area (the Main Caucasian Range) and correlation of its stratigraphic section (I) with sections of the Fore Range Kizikol complex (columns II & III).

(Okrostsvaridze 1990, 2007). In the predominant part of metabasic rocks of the LMC the content of

TiO2 is <1%. Th ese and other petrochemical features (Somin 1971, 1991; Dumbadze 1977; Adamia et al.

1978, 1985) testify the island-arc nature of at least an essential part of this complex. According to observations by Korikovski and the present author, the metamorphism of the dominant part the LMC corresponds to epidote-amphibolite facies of low-pressure type. But in above mentioned SW part of LMC area, southern slope of the Main Figure 4. Metaconglomerate of the Damkhyrts Formation, Range, kyanite-and kyanite-staurolite-garnet mineral Damkhyrts riverhead. association was established indicating pressure up

551 PRE-JURASSIC BASEMENT OF THE CAUCASUS

to 5–7 kbar (Abesadze et al. 1982; Gamkrelidze & (personal communication 2004), the amphibolites Shengelia 2005). corresponds petrochemically to normal-alkaline Th e LMC makes contact with the Lower Jurassic ferrigenous tholeiitic basalt. In the Pearce diagram slate in the south and with the Makera metamorphic they plot within the fi elds of intraplate and island arc complex in the north. Th e northern contact has a basalt. Because similar amphibolites occur as thin premetamorphic tectonic character because the LMC intercalation with siliciclastic schists in other parts of includes numerous bodies of I-type metagranitoids the BMC, there is a reason to consider this complex and metadiorite orthogneisses which are completely as basinal (marginal sea?) deposits. absent among the Makera Complex rocks. Besides, Th e Klych Formation is overlain by the thick a lens of serpentinite lies in this contact zone at the Dombay Formation which consists of micaschists and right slope of Tsakhvoa river valley. amphibolites in an approximately equal proportion. Th e Buulgen Metamorphic Complex (BMC) Th e micaschists always contain rounded detrital occupies the watershed area of the Main Caucasian zircon with wide spread of U-Pb ages. Range in riverheads of Teberda and Kodori Metaintrusive rocks placed within the BMC are tributaries, where it was recognized as a separate unit represented by two groups, orthogneisses and less for the fi rst time by Somin (1965), originally under metamorphosed gneissic metaplutonic rocks. Th e name of the Buulgen series. Th e complex includes fi rst group includes rocks which completely lost their three main lithologies: amphibolite, metaterrigenous primary igneous textures, and are now homogeneous, mica schist (siliciclastics) and low-K orthogneiss/ some banded rocks consisting of quartz, metamorphic metagranitoid. Th in layers of carbonate material sodium-rich plagioclase, biotite and hornblende. (which I interpret as former limestone) and rare Th ey form plate-like bodies lying concordantly in small bodies of ultramafi c rocks are noted here as the surrounding paraschists. Th e content of SiO2 is very subordinate components. 65–68% i.e. they belong to the tonalite/dacite group. Subdivision of the BMC is based on mapping Th eir magmatic origin is testifi ed by the presence of large overturned Klych antiform (Somin 1971) of relatively large (up 150 μm) idiomorphic zircon (Figure 5). Th e alternating amphibolite and mica grains showing thin zonality in their marginal zones schist are replaced by numerous bodies of biotite- and dark central zones. However it is not completely hornblende orthogneiss and metatronjemite in the clear whether these orthogneisses were originally core of this structure. Th is part of the section was plutonic or volcanic rock. Gneissic metaplutonic called the Gvandra Formation. Its exposures are rocks have a chemical and mineralogical composition bordered by a thick (sometimes up to 700 m) band similar to gabbro-diorite to quartz diorite (SiO2= of relatively homogenous garnet-free amphibolites of 49–59%) (Gamkrelidze & Shengelia 2005); they are the Klych Formation. Rare thin layers of paraschists coarse grained and preserve numerous relicts of are observed within these amphibolites. magmatic texture: large partially crushed magmatic What is the origin of the Klych amphibolites? plagioclase grains (with relictic tabular form and Th ey are mostly fi ne-grained rocks whereas magmatic zonality) representing augens are typical metagabbros as a rule form coarse-grained rocks, for the granitiodic blastomylonites. Th ese grains oft en preserving relictic gabbro textures; the latter are rounded by metamorphic matrix composed of were never observed in the Klych Formation. Besides, highly deformed and oriented quartz, hornblende in some sections the amphibolite is in contact and biotite. Th ese rocks described for the fi rst time with a thin layer of marble. Th erefore, it seems by G.Chkhotua in 1938 were later called by Somin more probable that the biggest proportion of these (1971) ‘Klych augen orthogneiss’. Th ey contain rocks is metavolcanic, not metagabbro. An average abundant magmatic zircon. Numerous xenoliths of content of TiO2 in the amphibolites is 1.55% (n= enclosing stratifi ed rocks, mica schists, paragneisses 39) and only locally, in the Gonachkhir river valley and amphibolites, are noted elsewhere. Th e main area the content of this component is <1% (Hanel et al. of distribution of these metaplutonic rocks is Klych 1993a, b). According to Dumbadze (1977) and Popov river valley. Th ick body of such rocks is also known

552 M.L. SOMIN alluvium & glaciers alluvium sediments Jurassic Lower & Upper complex metamorphic Gondray (undivided) granitoids Palaeozoic hyperbasite faults foliation migmatite Geological map of the Main Caucasian Range in Kodori riverhead area, Abkhazia. Abkhazia. area, riverhead in Kodori Range Caucasian the Main of Geological map Figure 5.

553 PRE-JURASSIC BASEMENT OF THE CAUCASUS

near the Naur pass, NW termination of the BMC seems to be doubtful. Th e BMC was probably formed area. Besides, augen rocks of diff erent composition in marginal sea or in a rift basin near continental form a 30 km long narrow belt, separating the BMC masses, where a distension of crust and basaltic from north-east. Th is belt, spatially coinciding with volcanism was accompanied by terrigenous material SW margin of the Alibek Jurassic depression, is a supply. Later a similar situation was repeated in the long-standing tectonic line, separating BMC from history of the Greater Caucasus in the Liassic time. metamorphic complexes of the Elbrus subzone. Th e age of the BMC is another problem. Dating of Metamorphism of the BMC is of low-pressure zircon from banded Gonachkhir River amphibolite and moderate temperature type (up to 660° C and 3.5 by Pb/Pb single zircon evaporation method gave ca. kbar). Highest-temperature association is represented 600 and 500 Ma values, interpreted as the ages of by garnet, sillimanite, biotite and muscovite, but the volcanic protolith and that of metamorphism, sillimanite-bearing associations are rare, and respectively (Hanel et al. 1993a, b). However, these migmatite is almost completely absent although authors noted that origin of zircons remains unclear metapelitic schists, favorable for selective anatexis is because they are somewhat rounded and, thus, a widely distributed in the BMC. Chichinadze (1977) secondary (transported) origin is not excluded. More established that a small part of the BMC cropping out important was value 320±5 Ma obtained by Bibikova on the left bank of Gvandra River and called by him et al. (1991) by TIMS U-Pb method on magmatic Sysina Formation, was formed under lower (<2.5 zircons of the Klych metaquartzdiorite. Recently, kbar) pressure. Kröner (Somin et al. in preparation) obtained a 381±3 Ma age with SHRIMP method on magmatic Th e origin of the BMC is the subject of a long- zircons from metatonalite (or dacite) orthogneiss lasting discussion. Adamia et al. (1987) interpreted from the area of Khetskvara glacier. SHRIMP II it as mixture of metaophiolite and island-arc rock dating (VSEGEI laboratory, St.-Petersburg) of associations. Gamkrelidze & Shengelia (2005) detrital zircons (n= 15) selected from quartz- concluded that in the BMC only the Klych Formation biotite schist (sample P93-3) of the same locality has ophiolitic origin and that this unit is a separate yielded four groups of values: 2394–1929, 669– tectonic sheet within the BMC. Both points of view, 483, 425–405 and 355–325 Ma (Figure 6, Table 1). especially the fi rst one, seem to be disputable because Th e last group probably represents the age of the role of metaophiolite is too exaggerated. Indeed (1) metamorphic overprint whereas the older values no four-member ophiolite section (i.e. ultrabasite, indicate the Palaeozoic age of the rocks protoliths. gabbro, sheeted complex, basalts) has been found in Similar values were demonstrated by zircons (n= the BMC, neither representative fragments of this 9) from the siliciclastic schist from the left bank of section. (2) Almost half of the BMC is represented by Amanauz river, northern slope of the Main Range siliciclastic schists completely alien to true ophiolite near Dombay resort: 2108, 920, 670, 660, 661, 645, and island arc sections. (3) Metagranitoids in some 340, 339, 322 Ma. Figure 7 and Table 2 demonstrate localities, i.e. in Klych valley, are the dominant morphology and isotope-geochronological data of type of rocks of the BMC and the thickness of their zircons selected from typical BMC augen biotite- individual bodies reaches up to some hundreds of hornblende metadiorite (sample 0-81), which crops metres, they intrude all units of the BMC, including out near Naur pass. SHRIMP dating (VSEGEI) yielded the Klych amphibolites. Besides, the average diorite 310.9±2.9 Ma. Kotov (Institute of Precambrian or quartz diorite composition of these metaplutonic geology, St.Petersburg) obtained 308±18 Ma on the rocks diff ers sharply from the composition of thin same sample with TIMS U-Pb method. Recently, an veins of plagiogranite characteristic of true ophiolite age value of 307±1 Ma was also received by Kotov on association (Coleman 1978). (4) Deep-water (meta) magmatic zircons of gneissic (mylonitizated) quartz sediments were not found within the BMC. On the diorite (or ‘Klych augen orthogneiss’) previously contrary, thin layers of crystalline limestone are dated as ca. 320 Ma (Bibikova et al. 1991). observed in some places together with amphibolite In the absence of migmatite and Ar/Ar data it bodies. Th erefore, an ophiolite origin for the BMC is not easy to determine the exact age of the BMC’s

554 M.L. SOMIN

a b

Figure 6. Optical (a) and cathodoluminescence (b) images of detrital zircons from metapsammite of the Buulgen complex, sample P 93-3, Khetskvara riverhead, Abkhasia. Numbers on (a) are ages in Ma. (c) Histogram of the ages and concordia diagram for zircon with mean at 486±2.6 Ma. metamorphism. However, taking into account the dating generally yields ages in the range of 140–190 zircon age data we can say confi dently that this Ma; these values refl ect isotope rejuvenation of the age is at least post-Late Devonian, probably post- Palaeozoic basement during the Early Kimmerian Serpukhovian. Th is conclusion is supported by tectonic event. Sm-Nd mineral isochron dating (on fi ve minerals) Th erefore one can conclude that the BMC is a by Zhuravlev, which yielded 288±33 Ma on garnet- Variscan metamorphic complex of low-pressure biotite-hornblende gneiss taken in Northern (partly very low pressure) and moderate temperature Ptysh riverhead, east of Dombay resort area. type. Th e very low-pressure Kassar Formation of the Th us, metamorphism of the Buulgen complex is Ardon river valley, Northern Ossetia, of metabasite- evidently Late Variscan. At the same time K-Ar metapelite composition, is a probable equivalent

555 PRE-JURASSIC BASEMENT OF THE CAUCASUS

Table 1. U-Pb data and calculated ages for zircons of paragneiss (sample P-93-3) of the Buulgen Metamorphic Complex.

% ppm ppm 232Th Ppm (1)206Pb (1)207Pb % Total 238U №Spot ±% 206Pbc U Th /238U 206Pb* /238U Age /206Pb Age Discordant /206Pb 1 P93-3.1.1 1.01 33 67 2.10 10.0 1929±33 1986±71 3 2.831 1.9 2 P93-3.1.2 0.05 2658 25 0.01 175.0 477±5.4 463±20 -3 13.01 1.2 3 P93-3.2.1 0.00 285 70 0.25 19.9 504±6.4 534±46 6 12.29 1.3 4 P93-3.3.1 0.12 511 120 0.24 79.1 1066±12 1046±29 -2 5.554 1.2 5 P93-3.4.1 0.00 353 210 0.61 32.7 660±7.8 660±34 0 9.28 1.2 6 P93-3.5.1 0.12 616 54 0.09 41.1 482±5.8 508±42 5 12.86 1.2 7 P93-3.6.1 0.16 540 73 0.14 30.1 405±5 428±55 6 15.4 1.3 8 P93-3.7.1 0.15 504 188 0.39 34.2 490±5.8 510±46 4 12.64 1.2 9 P93-3.8.1 0.00 212 108 0.52 10.3 355±4.7 320±70 -10 17.65 1.4 10 P93-3.8.2 0.18 567 137 0.25 27.5 353±4.4 318±55 -10 17.75 1.3 11 P93-3.9.1 0.11 453 371 0.85 40.1 632±7.4 639±45 1 9.69 1.2 12 P93-3.10.1 0.00 197 102 0.54 13.2 483±6.2 461±67 -5 12.87 1.3 13 P93-3.11.1 0.01 1005 194 0.20 389.0 2394±23 2421±13 1 2.223 1.2 14 P93-3.12.1 0.00 125 52 0.43 28.0 1491±19 1468±31 -2 3.848 1.4 15 P93-3.13.1 0.48 250 60 0.25 19.1 547±7.1 555±88 2 11.24 1.3 16 P93-3.14.1 0.16 631 383 0.63 59.3 669±7.6 667±34 0 9.14 1.2 17 P93-3.15.1 0.15 64 18 0.29 21.9 2150±36 2114±27 -2 2.521 2

Errors are 1-sigma; Pbc and Pb* indicate the common and radiogenic portions. respectively. Error in Standard calibration was 0.52%. (1) Common Pb corrected using measured 204Pb.

(1) (1) % ppm ppm 232Th Ppm % Total 238U №Spot 206Pb /238U 207Pb /206Pb ±% 206Pbc U Th /238U 206Pb* Discordant /206Pb Age Age 1 0.1308 2.1 2.86 2 0.122 4 5.87 4.4 0.3489 2 0.451 2 0.05664 0.76 13.02 1.2 0.05627 0.9 0.5961 1.5 0.07683 1.2 0.795 3 0.0581 2.1 12.29 1.3 0.0581 2.1 0.652 2.5 0.0813 1.3 0.530 4 0.07517 0.99 5.561 1.2 0.0742 1.4 1.838 1.9 0.1798 1.2 0.640 5 0.06158 1.6 9.28 1.2 0.06158 1.6 0.915 2 0.1078 1.2 0.617 6 0.05837 1.4 12.87 1.2 0.0574 1.9 0.615 2.3 0.07769 1.2 0.542 7 0.0567 1.7 15.43 1.3 0.0554 2.4 0.495 2.8 0.06482 1.3 0.459 8 0.05871 1.6 12.66 1.2 0.0575 2.1 0.626 2.4 0.07896 1.2 0.509 9 0.0528 3.1 17.65 1.4 0.0528 3.1 0.412 3.4 0.05665 1.4 0.402 10 0.0542 1.9 17.78 1.3 0.0527 2.4 0.409 2.7 0.05624 1.3 0.462 11 0.0619 1.4 9.7 1.2 0.061 2.1 0.867 2.4 0.1031 1.2 0.508 12 0.0552 2.7 12.85 1.3 0.0562 3 0.603 3.3 0.0778 1.3 0.406 13 0.1569 0.75 2.223 1.2 0.1568 0.76 9.72 1.4 0.4498 1.2 0.840 14 0.091 1.4 3.844 1.4 0.092 1.7 3.302 2.2 0.2602 1.4 0.644 15 0.0626 2 11.29 1.4 0.0587 4 0.716 4.2 0.0885 1.4 0.319 16 0.06309 1.2 9.15 1.2 0.06179 1.6 0.931 2 0.1093 1.2 0.605 17 0.1325 1.4 2.525 2 0.1312 1.5 7.16 2.5 0.3959 2 0.794

556 M.L. SOMIN

b a

Figure 7. Cathodoluminescence image (a) and diagram with concordia (b) for zircons of sample 0-81 of augen metadiorite, at Naur Pass. of the BMC in the eastern termination of the Main formed from a high-temperature metamorphic Range zone (Abesadze et al. 2004) as well as exposures sequence (up to granulite facies, 650–740° C; 5.5–7.5 of metamorphic sequences at the Chugush block in kbar) composed of paragneisses, augen orthogneisses the hard-accessible locality of the Western Caucasus. and amphibolites. Th e degree of their initial It is important to note that isotope-geochronological metamorphism decreases structurally downward data (Rb-Sr on metapelite and U-Pb on granitoids whereas the degree of mylonitization increases in intruding the Kassar Formation) gave Late Palaeozoic the same direction. Zircon of an augen orthogneiss ages, values similar or slightly younger than those of (metadiorite) yielded TIMS U-Pb age 312±3 the BMC. Th erefore, the geochronologically youngest Ma (Somin & Smul’skaya 2005). Th is structural- belt of essentially mafi c metamorphic rocks and metamorphic inversion is probably a result of deep associated granitoids form a narrow discontinuous overthrusting movements in the Late Palaeozoic. Th e belt along the southern border of the North-Caucasus upper limit of this event is determined by basal Lower domain. Th e easternmost exposure of this belt is Jurassic strata which overlie the blastomylonite. A represented by granodiorites of the Dar’yal salient. similar type of blastomylonite is also found along the Abesadze et al. (1978, 2004) have proposed that the southern border of the Main Range zone in Shakhe Kassar Formation is probably a volcanic-sedimentary River basin (Potapenko & Prutski 1977), where it is prism (including the N-MORB metabasites) accreted presented by the Bushiy pseudoformation. to the northern complexes of the Main Range during Th e Upper Palaeozoic Structural Stage– Th e Upper Late Palaeozoic time. Th is idea seems to be fruitful Palaeozoic sediments have limited distribution in for the whole belt. the Pass subzone. Th ey are represented by Middle Metamorphic rocks of pre-Jurassic basement of and Upper Carboniferous, Permian and suspected the Atamazhi salient (Belaya River valley, western Triassic rocks. Th e Middle Carboniferous deposits termination of the pre-Jurassic basement exposure) are found in the lake Khuko locality only (Belov & presumably also belong to the BMC. Previously Zalesskaya-Chirkova 1963). Th ey are continental these exposures were interpreted as rocks of low- coal-bearing sandstone and gravelstone containing temperature greenschist facies, youngest component fossil plants of the upper Westfal-B; Upper of the Greater Caucasus basement. However, mapping Permian marine, terrigeneous and calcareous and petrological study (Somin & Smul’skaya 2005) sediments lie above with unconformity. Toward have revealed that these rocks are blastomylonites the SE the Upper Palaeozoic sediments crop out

557 PRE-JURASSIC BASEMENT OF THE CAUCASUS

Th 232 (1) error Age Pb / correrction 208 ±% Pb 206 (1) Age Pb / U Pb* 207 (1) 238 / 206 U 238 ±% (3) Age Pb / 206 U Pb* (1) 235 / 207 U 238 erent mounts). erent (2) Age Pb / ±% 206 U Pb* Pb* 238 (1) 206 207 / (1) Age Pb / 206 ±% Pb* Ppm 206 Pb* 206 (1) U/ 238 U 238 / Th 232 ±% age-concordance U age-concordance Th 235 232

Pb/ Pb/ Th Pb ppm Pb 207 208 206 207 Total Total / U- U- 238 238 Pb. 204 Pb/ Pb/ 206 206 U ppm ±% U Pb 238 206 Pbc % Total Total / 206 % Discordant U-Pb data and calculated ages for zircons of metadiorite (sample 0-81) of the Buulgen Metamorphic Complex. Metamorphic the Buulgen 0-81) of (sample metadiorite of zircons for ages calculated and U-Pb data 12 0-81.1.13 0-81.2.1 0.274 0-81.3.1 - 4865 0-81.4.1 0.146 0-81.4.2 373 - 357 5037 0-81.5.1 0.03 0-81.6.1 148 0.08 2658 0.76 273 64 0.11 2147 265 0.41 20.7 0.56 3675 33 233 0.10 16.1 311.5±3 21.7 156 0.53 0.11 311.8± 3 315±3.2 137 314.5±3 0.04 24.8 314.7± 3.2 311± 3.4 135 375.5±4 314.9± 3 153 314.1± 3.4 2387±26 274±71 454.5±3.4 314.7± 3.3 375.7± 4 343±53 2346± 33 305±2.6 454.4± 3.4 262±59 316±6.7 375.1± 4.1 329.6±7.8 454.5± 3.4 2387±28 304.7± 2.6 361±21 312.3±6.9 305.2± 2.6 464±19 2541±19 401.3±9.7 339±33 2394±49 459±10 273±18 №Spot 2 9 19.97 1 0.0531 2.2 19.97 1 0.0533 2.3 0.3683 2.5 0.05008 1 0.405 0.300 1 0.98 0.05008 0.768 0.04951 1.1 0.662 2.5 3.3 0.352 0.77 0.05998 0.97 0.353 0.764 0.3683 1.4 1.3 3.1 0.07306 0.04999 0.0517 2.3 0.4482 0.4446 2.8 1.2 0.518 0.98 0.0533 1.9 20.2 1.7 0.87 0.0539 0.91 0.3548 1 0.567 19.97 2.2 0.97 10.4 2.6 0.04845 0.05376 0.87 0.0531 1 -12 0.0515 20.14 1.1 1.7 1.9 1.1 1 0.97 20 16.67 0.1683 0.05629 0.0526 0.82 2 9 19.97 0.3556 0.96 1.3 0.77 0.05335 2.231 1.4 3 -17 19.98 1.1 13.69 1.1 0.75 0.05323 0.1686 4 -4 16.68 1.3 0.05691 0.87 20.64 1.3 5 6 2.231 0.77 0.05414 6 2 13.68 0.87 7 11 20.62 № (1) Common Pb corrected using measured measured using Pb corrected (1) Common Errors are 1-sigma; Pbc and Pb* indicate the common and radiogenic portions, respectively. portions, radiogenic and the common Pb* indicate 1-sigma; Pbc and are Errors diff from data when comparing required but errors in above included was 0.37% (not calibration in Standard Error (2) Common Pb corrected by assuming assuming by Pb corrected (2) Common Table 2. Table (3) Common Pb corrected by assuming assuming by Pb corrected (3) Common

558 M.L. SOMIN

at glacier Pseashkho, Urushten and Malaya Laba gently on upper surfaces of the granite salients, and riverheads. Th e Upper Palaeozoic section begins fragments of basal conglomerate are seen there. here with Upper Carboniferous (or Lower Permian) Nevertheless, the steep cleavage persists here also. conglomerate which overlies a crystalline basement. All these data demonstrate an origin of this structure Detachment is developed along the contact as a as a result of horizontal compression. Because the rule and the thickness of the conglomerate is oft en cover sediments are especially tightly compressed reduced. Strongly deformed and recrystallized between the basement salients, this structural style Permian limestone and intercalated cherts and slate of basement cover- system might be called cuspate- of unknown age appear above conglomerate or are lobate (Ramsay 1967). in contact immediatedly with the basement rocks (Somin 1971, 2007a, b) (Figure 8). Above, it was mentioned the Kvishi locality at SW foot of Ushba Elbrus Subzone Peak, Svenetia district of Georgia. Th ere is an Upper Th is part of the Main Range zone diff ers essentially Carboniferous brachiopod-bearing sandstone and from the Pass subzone both compositionally and basal conglomerate covering micaceous schists of structurally. Th e Upper Palaeozoic stage is preserved Makera Complex and deformed together with the here occasionally only, in very small depressions latter in steep synclinal fold. Permian limestone among vast areas of krystallinikum. For example, appears above sandstone (Khutsishvili 1966). A red beds of Permian are known on the left side of the similar picture is observed at the eastern termination Baksan River. of the Main Range zone east of Ardon River. Dense Th e Elbrus subzone is an area of gently dipping quartz conglomerate up to 200 thick is covered foliation and dome-like structures, sialic and concordantly by the Permian (Guadalupian) coral- ensialic metamorphic rocks and abundant S-type bearing limestone (Morgunov 1965). granites. Low-pressure (andalusite-sillimanite) Early Alpine (pre-Callovian) movements have type of metamorphism (3.2–3.5 kbar, Gamkrelidze changed the original Pass subzone basement & Shengelia 2005) is a characteristic feature of this structure. Indeed, the Klych antiform is in fact a subzone; the degree of metamorphism reaches up to Kimmerian fold because on its both steep wings high-temperature amphibolite facies. Post-Jurassic the Lower Liassic sediments are disposed in normal structural reworking is much less here in comparison stratigraphical position, without a major angular with the Pass subzone. Alpine thermal infl uence is unconformity between general position of the absent except for the Pleistocene Eldzhurta granite Palaeozoic foliation and Liassic bedding (Somin contact area. 2007a, b). Th e contacts are as a rule tectonic, i.e. Two main units are recognized in the Elbrus detachment surfaces are present. At the same time subzone krystallinikum: the lower, Gondaray (gneiss- basal conglomerates are locally preserved near the migmatite) and the upper, Makera Metamorphic detachment and on the lobes of such structures. Th e Complex (Grekov & Lavrischev 2002; Gamkrelidze & same picture is observed at the eastern termination of the Main Range structure, particularly, in Ardon River Shengelia 2005; Somin 2007a, b). Th ey are separated section, Northern Ossetia, where Palaeozoic schists, by a subhorizontal tectonic surface or bodies of Permian marbles and Lower Jurassic sediments Upper Palaeozoic granite. are deformed together into steep folds overturned Gondaray Metamorphic Complex (GMC) is the toward the south. More to the east, in the Terek River most widely distributed metamorphic unit of the Dar’al canyon, one can see how Palaeozoic granite Elbrus subzone. Its exposures are disposed mostly in involved in Alpine deformation is placed tectonically Aksaut, Kuban, Teberda, Baksan and Cherek Rivers within the Lower Jurassic slate. Marginal parts of the basins, where metamorphic rocks are intruded by granite massif are intensively mylonitizated; surfaces numerous bodies of granite. Especially good and of mylonitization are subvertical and parallel to easily accessible exposures of the GMC are known those of cleavage in the intensively folded Liassic on the left bank of the Baksan River 2–3 km SW of sediments. At the same time these sediments lie Tyrnyauz city and in deep gorges of Baksan tributaries

559 PRE-JURASSIC BASEMENT OF THE CAUCASUS

Figure 8. Cross-section through Malabinskiy salient of the pre-Jurassic basement, Malaya Laba and Urushten riverhead.

Adyr-su and Adyl-su. In these localities the great monoclinal dipping to the north which permits to subvertical cliff s and escarpments abraded by glacier suggest the existence of some thrusts. show change in the position of the metamorphic Th e GMC consists of paragneisses, orthogneisses foliation from subhorizontal to subvertical in a and migmatites; as a subordinate component of GMC distance of 4–5 kilometers, indicating the presence some amphibolites appear. Among extremely rare of dome-like structures (Figure 9). In the eastern rocks small lenses of marble are noted. Orthogneiss part of the Main Range, foliation has mostly gentle bodies were recognized within the GMC not long ago

560 M.L. SOMIN orthogneiss, paragneiss & migmatitemicaceous schists Upper Palaeozoic granite values of isotope: Generalized cross-section across the Main Range zone along Khetskvara, Amanauz & Teberda River section. No horizontal scale. horizontal section. No River & Teberda Amanauz Khetskvara, along zone Range the Main across Generalized cross-section Figure 9.

561 PRE-JURASSIC BASEMENT OF THE CAUCASUS

(Bibikova et al. 1991; Hanel et al. 1993a, b); earlier method yielded an age 400±10 Ma (Bibikova et al. they were confused with migmatite. In contrast to 1991). Later this method gave 386±5 Ma on primary migmatite, the orthogneiss is a more monotonous magmatic zircon of similar banded two-feldspar and homogeneous rock forming bodies are up to orthogneiss from the Kyrtyk River canyon (Somin et hundreds metres thick; sillimanite and garnet are al. 2006). present here as very subordinate phase; rotated Th is orthogneiss cuts gabbro-amphibolite and xenoliths of surrounding paragneiss and amphibolite encloses its xenolith. Th e gabbro-amphibolite is may be observed inside an orthogneiss; fi nally, uniform massive rock forming body of 150 m thick relict magmatic plagioclase might be found in this with TiO2 content up to 2% and elevated alkalinity. rock under microscope. Sillimanite and garnet are Th is explains the abundance of zircon in the rock. abundant in migmatite and form separate thick (up Zircons from amphibolite are almost isometric; they to some centimetres) bands; leucosome is coarse- are characterized by absence of zonality or by irregular grained and oft en has granitic texture. Th ere are also wide zonality with indistinct boundaries between diff erences in zircon grain morphology and the age zones. Fusiform zoning is observed in some places. values of the two rock types. Th ese features are characteristic for gabbro zircons. Geochemical data on orthogneisses indicates Of six zircons dated by U-Pb SHRIMP method fi ve their crustal origin: 87Sr/86Sr= 0.742653; Isr= 0.71409; grains provided a concordant age averaging at 425±9 143 144 147 144 ε Nd(T)= –2.6; Nd/ Nd= 0.512338; Sm/ Nd= Ma (Late Silurian) with MSWD= 0.39 (Figure 10, 0.512338. sample 0-11, Table 3). Gamkrelidze & Shengelia (2005) have To solve the probem of protolith’s age of the GMC demonstrated that three metamorphic facies can paragneisses, the SHRIMP U-Pb dating of detrital be separated within the Elbrus subzone: biotite- zircons of these rocks was carried out (Somin et al. sillimanite-K-feldspar, garnet-cordierite-orthoclase 2007a, b). In order to obtain the most representative facies and facies of biotite-muscovite gneisses. Rocks results a study of four paragneiss samples with spacing of the two fi rst facies were interpreted by these authors up to 150 km was realized. Th e samples diff ered in as products of Grenvillian or even earlier epoch; the degree of migmatization. biotite-muscovite gneisses were attributed to the Paragneiss of sample 0-17 was taken near the Early Caledonian event. Early Variscan (Bretonian) mouth of the Adyr-su River located in the large area stage was considered to be time of diapthoresis of of the sillimanite-biotite-muscovite subfacies of the the GMC, and Late Variscan (Sudetican) stage as its amphibolite facies. Th e paragneiss is represented greenschist retrograde stage. by deformed (overturned folds) and banded Attributing the GMC to pre-Cambrian was based migmatized rock typical of the GMC. Its mineral on the following facts: (1) its location in the structurally assemblage is Qtz-Grt-Bt-Sil (-Pl-Kfs). Sillimanite deepest (core) part of the range; (2) the highest grade and biotite are the dominant minerals. Garnet of metamorphism in this part of the Greater Caucasus shows only slight retrograde zoning indicating one- basement corresponding to high-temperature level stage metamorphism with T= 616° C and P≤4 kbar. of the amphibolite facies; (3) several U-Pb zircon Zircon occurs in these rocks as transparent elongated ages corresponding to the pre-Cambrian, although prismatic crystals with rare inclusions and marginal they have remained debatable for a long time. As zoning (Figure 10, sample 0-17, Table 4). Th ey are was assumed or shown (Bibikova et al. 1991; Somin similar to zircons in the migmatite leucosome. et al. 2006) most of these ages were obtained from Zircon grains contain rounded inherited cores. In detrital or inherited zircons. Most oft en, researchers total, 11 grains were analyzed. Cores of three grains mention values of 500±40 and ~2000 Ma obtained by yielded U–Pb SHRIMP ages of ca. 2347, 1809, and the Pb/Pb evaporation method for zircons extracted 1268 Ma. Two other grains yielded ages of ca. 637 from orthogneiss of the Adyl-su River valley (Hanel and 665 Ma. Th e ages determined for zircon crystals et al. 1993). However, dating of zircons of the same at 12 points located at the zircons rims varies from massif by the conventional multigrain (TIMS) U-Pb 321 to 288 Ma (average 321± 1 Ma; MSWD= 0.0091).

562 M.L. SOMIN

Figure 10. (a) Cathodoluminescence and optical (transmitted light) images of zircons from paragneisses (samples 0-17, 125, 152) and amphibolite (sample 0-11), the Gondaray complex. White circles are 20 μm wide dating spots. (b) Diagrams with concordia for studied zircons.

563 PRE-JURASSIC BASEMENT OF THE CAUCASUS

Th 232 (1) Age error Pb / correctiom 208 ±% Pb 206 (1) Age Pb / U 207 Pb* (1) 238 / 206 U Age 238 ±% (3) Pb / 206 U Pb* (1) 235 / 207 U Age 238 (2) ±% Pb / 206 Pb* Pb* (1) 206 207 / U Age 238 (1) Pb / ±% 206 Pb* Pb* 206 ppm (1) 206 U/ 238

U Th 238 ±% / 232 age-concordance U age-concordance Th 235 232

Pb/ Pb/ Pb 207 208 Pb 206 207 Total U- U- / 238 238 Pb. Pb. 204 Pb/ Pb/ 206 206 ±% ppm Uppm Th ppm U Pb 238 206 Total / Pbc % 206 % Discordant U-Pb data and calculated ages for zircons of gabbro-amphibolite (sample 0-11) of the Gondaray Metamorphic Complex. Metamorphic the Gondaray 0-11) of (sample gabbro-amphibolite of zircons for ages calculated and U-Pb data 12 0-11.1.13 0-11.2.1 -4 0-11.3.1 -5 874 0-11.4.1 0,076 1577 0-11.5.1 0,00 420 248 0-11.6.1 1481 0,04 1772 163 0,50 0,06 1514 2238 0,97 1677 0,68 1968 50.9 1,31 93.3 1671 1,34 14 423±8.3 105 1,03 429.5±8.4 89.9 411.3±8.8 422.9±8.4 430.7±8.4 98.6 430.1±8.5 430.7±8.6 408.9±8.8 430.2±8.5 418.4 0 428.3 -11 426.5±8.5 431±8.7 411.8 44 382±37 426.7±8.6 424±25 432 8 430 -4 593±64 436.8±9.2 481±11 430 -3 466±19 406±14 413±25 426±8.9 413±22 435.8±9.3 405.2±8.7 1 0 14.75 2 0.05522 1.1 14.75 2 0.05532 14.75 1.1 0.5170.054262 14.51 2.31.1 1.7 0.06780.056340.0597 0.515 1.6 2 2.2 0.860.0550114.47 15.18 2.6 2.9 22 0.05522 0.76 2.1 0.05501 0.537 1.114.47 0.0689 0.542 2 0.84 0.878 2.1 0.97 2.20.05376 0.52414.62 3.7 0.79 0.0603 22 0.519 0.0691 2.3 2.2 0.05636 0.0659 2.3 0.770 14.75 0.06912 1 0 0.05531 2 2.2 0.068414.52 2.1 2 -11 0.05549 2.1 0.919 0.600 15.17 2.12.1 3 44 0.880 14.47 4 8 0.903 14.47 5 -4 14.61 6 -3 №Spot № Errors are 1-sigma; Pbc and Pb* indicate the common and radiogenic portions, respectively. portions, radiogenic and the common Pb* indicate 1-sigma; Pbc and are Errors was 0.65% calibration in Standard Error measured using Pb corrected (1) Common (2) Common Pb corrected by assuming assuming by Pb corrected (2) Common Table 3. Table (3) Common Pb corrected by assuming assuming by Pb corrected (3) Common

564 M.L. SOMIN

One third of all grains are characterized by low occurrence of rare grains dated back to 470–480 Ma Th /U values (Figure 11, Table 4), which are typical (Th /U > 0.2) among detrital zircons suggests that of metamorphic zircons. Th e rock contains xenotime the age of host rocks might be younger than Early and monazite equilibrated with biotite and garnet. Ordovician (Figure 12). Th e monazite age determined by Konilov using the Interesting data were obtained from blastomylonite CHIME microprobe method (Suzuki et al. 1991) is (augen metagranite) of the Bol’shoy Mukulan creek, ca. 280±50 Ma. 1.5 km south of Tyrnyauz city. Two-feldspar biotite Paragneiss of sample 125 was taken on the left granite suff ered high-temperature metamorphism slope of the Damkhurts River 9 km upstream from and transformed into augen blastomylonite is the river mouth. Th e sample is a coarsely foliated disposed now at the altitude of 2900–3000 m nonmigmatized rock composed of the Bt-Ms-Pl-Sil- above Baksan River separating GMC and the less Qtz association. Sillimanite occurs as complicatedly metamorphosed andalusite-bearing metapelite of deformed fi brolite bunches. Zircons consist of the Makera Complex. Th e rock contains abundant rounded detrital cores and very thin rims that slightly idiomorphic magmatic zircon, its U-Pb (TIMS) age mask the grain morphology (see Figure 10, Table 5). is 305±8 Ma (Somin et al. 2006). Only cores were dated because the thickness of rims Th e value 307.7±8 Ma was obtained by SHRIMP is insuffi cient for measurements. Among 11 grains method for three transparent euhedral zircon grains studied three grains yielded ages of ca. 1337, 1038, selected from small deformed anatectic granite body and 898 Ma. Th e ages of six other grains make up placed within migmatite matrix at left wall of Baksan an almost continuous series of values ranging from River 300 m north of Bol’shoy Mukulan creek. ca. 676 to 561 Ma. At the same time, the ages of two grains (four points) fall into the interval of 504 to 474 K-Ar data on biotite-muscovite pairs of migmatite Ma (Late Cambrian–Early Ordovician). In all cases, (Bibikova et al. 1991) correspond to 310–288 Ma and the Th /U value exceeds 0.15 (usually >0.30). Th is fact therefore coincide with the youngest age obtained confi rms the primary magmatic genesis of detrital on metamorphic or youngest magmatic zircons or grains. are slightly younger refl ecting the time of closing of argon system. Paragneiss of sample 152, from the upper left fl ank of the Sofi ya River, is represented by banded foliated Summing up the geochronological data on the and slightly migmatized rock of the Grt-Bt-Ms-Pl- GMC one can conclude that they indicate the Middle Qtz association. It encloses paragneiss interbeds Palaeozoic and partly even Late Palaeozoic age of the with sillimanite and amphibolite lenses. Zircons in protolith material and, therefore the Late Palaeozoic this sample are also detrital, coated by thin rims (see age of the regional metamorphism. Figure 10, sample 152, Table 6). In total, 23 grains Intrusive plutonic rocks are widely distributed in were analyzed. In some of them, both central and the GMC (much less in the Makera Metamorphic marginal parts were analyzed. Th e ages of four grains Complex) and are represented mainly by cores vary from ca. 2002 to 724 Ma , 18 grains yielded homogeneous two-mica S-type granites. Most of the a Neoproterozoic–Early Cambrian age (634 to 522 granites are stratiform bodies up to 2–3 km thick Ma), and one grain (two points) yielded 474–432 (Somin 1965; Potapenko et al. 1999). Lower surface Ma. Th e metamorphic rim yielded 310 Ma. In this of these bodies crops out in Kuban’ and Aksaut case and in grains with an age of 459, 432, and 381 rivers rocky slopes; banded migmatite-like texture is Ma, the Th /U value is very low (0.03–0.07), probably characteristic for bottom part of granite bodies up to due to alteration of zircons. In other cases, this ratio several dozens metres thick. exceeds 0.12. Isotope-geochronological (Rb-Sr, K-Ar and U-Pb) Th us, the data obtained for most detrital data on the Main Range granite are not abundant; zircons from all paragneiss samples fall into the nevertheless they all fall to the 320–290 Ma interval Neoproterozoic–Ordovician interval. Most of them indicating a Late Palaeozoic age of crystallization correspond to the Ediacarian to Early Cambrian. Th e (Gurbanov & Arets 1996; Potapenko et al. 1999; new

565 PRE-JURASSIC BASEMENT OF THE CAUCASUS

Figure 11. Concordia diagram for metamorphic zircons of migmatized paragneiss zircons, sample 0-17, Condaray complex, Adyr-sy River. the author’s data). Some granite and granodiorite Th e Makera Metamorphic Complex (MMC)– Th e from the south-eastern part of the Main Range show MMC is the upper metamorphic unit of the Elbrus younger U-Pb ages of ca. 250 Ma. subzone. It is known as thick (>2 km) sequence above It is important to stress that ages of migmatites the GMC in area of left tributaries of Bol’shaya Laba and anatectic granites of the GMC are very close River Makera, Mamkhurts, Damkhurts, Tsakhvoa, at to those of the homogeneous two-mica S-type both sides of Baksan River valley, in the Aksaut River, granites emplaced in this complex. Th is fact seems inside Chegem and Ardon rivers basins. As a rule the to indicate a genetic connection between regional MMC is characterized by gently dipping foliation. metamorphism, migmatization and the granite Detailed mapping and structural study have revealed crystallization. Indeed, hornfels never have been two or three generations of earlier isoclinal folds and described in contacts zones of the granite and the slides with amplitude up to 1–1.5 km (Somin 1971). gneiss-migmatite substratum. Hence the latter still Metamorphism of the MMC is of epidote- remained at high temperatures up to moment of the amphibolite facies (500–550° C) and low pressure granite emplacement. Th erefore, our data confi rm the (3 kbar) type (Shengelia & Korikovskii 1991). conclusion by Brown & Solar (1998) and Solar et al. Andalusite is a widely distributed mineral, whereas (1998) on connection between metamorphism and staurolite is known only in Kirtyk riverhead. Th e granite emplacement in some convergent orogenic fi brolite and garnet are relatively rare. Th e MMC is belts. dominated by siliciclastics (mainly high-alumina,

566 M.L. SOMIN

Table 4. U-Pb data and calculated ages for zircons of paragneiss (sample 0-17 (517)) of the Gondaray Metamorphic Complex.

(1) (1) ppm ppm 232Th Ppm % № Spot % 206Pbc 206Pb/238U 207Pb/206Pb U Th /238U 206Pb* Discordant Age Age

1 0-517.1.1 0.02 724 59 0.08 273.0 2347±40 2664±17 13 2 0-517.2.1 0.18 2195 171 0.08 95.2 317±6 275±34 -13 3 0-517.3.1 0.09 1480 260 0.18 65.8 325±6 312±34 -4 4 0-517.4.1 0.00 715 199 0.29 31.5 323±6 370±35 15 5 0-517.5.1 0.00 641 155 0.25 120.0 1268±23 1680±11 33 6 0-517.6.1 0.00 175 31 0.18 48.6 1809±33 1785±17 -1 7 0-517.7.1 0.54 215 51 0.24 19.3 637±13 613±120 -4 8 0-517.8.1 0.01 2136 151 0.07 95.7 328±6 342±20 4 9 0-517.9.1 0.57 1647 101 0.06 74.7 330±8 255±79 -23 10 0-517.10.1 0.49 244 111 0.47 10.8 321±7 195±180 -39 11 0-517.10.2 0.12 462 67 0.15 20.4 323±7 321±56 -1 12 0-17.1.1 0.00 755 250 0.34 32.7 318±2 302±66 -5 13 0-17.2.1 0.14 674 190 0.29 29.4 319±2 320±58 0 14 0-17.3.1 0.18 7444 639 0.09 293.0 288±1 280±18 -3 15 0-17.4.1 0.00 901 834 0.96 84.0 666±4 669±28 1 16 0-17.5.1 0.13 434 3 0.01 19.1 322±3 301±52 -7 17 0-17.6.1 0.19 1147 384 0.35 50.6 323±2 316±48 -2

0-17 - Error in Standard calibration was 0.41%. 0-517 - Error in Standard calibration was 0.79%.

error № (1) 238U/206Pb* (1) 207Pb*/206Pb* ±% (1) 207Pb*/235U ±% (1) 206Pb*/238U±% correction

1 2.28 0.1812 1.0 10.9700 2.3 0.4392 2.0 0.896 2 19.85 0.0518 1.5 0.3594 2.5 0.0504 2.0 0.805 3 19.34 0.0526 1.5 0.3751 2.5 0.0517 2.0 0.810 4 19.48 0.0540 1.5 0.3821 2.6 0.0513 2.1 0.800 5 4.60 0.1031 0.6 3.0890 2.1 0.2174 2.0 0.958 6 3.09 0.1091 0.9 4.8700 2.3 0.3239 2.1 0.916 7 9.63 0.0603 5.5 0.8630 5.9 0.1038 2.2 0.365 8 19.17 0.0533 0.9 0.3833 2.2 0.0522 2.0 0.918 9 19.05 0.0513 3.4 0.3710 4.2 0.0525 2.4 0.573 10 19.57 0.0500 7.6 0.3520 7.9 0.0511 2.2 0.280 11 19.45 0.0528 2.5 0.3740 3.2 0.0514 2.1 0.644 12 19.78 0.0524 2.9 0.3650 3.0 0.0506 0.69 0.234 13 19.72 0.0528 2.6 0.3691 2.7 0.0507 0.71 0.268 14 21.88 0.0519 0.8 0.3270 0.9 0.0457 0.46 0.510 15 9.19 0.0619 1.3 0.9280 1.4 0.1088 0.58 0.413 16 19.50 0.0524 2.3 0.3702 2.4 0.0513 0.83 0.343 17 19.49 0.0527 2.1 0.3728 2.2 0.0513 0.6 0.277

567 PRE-JURASSIC BASEMENT OF THE CAUCASUS

Table 5. U-Pb data and calculated ages for zircons of paragneiss (sample 125) of the Gondaray Metamorphic Complex.

(1) (1) % ppm ppm 232Th Ppm % №Spot 206Pb/238U 207Pb/206Pb 206Pbc U Th /238U 206Pb* Discordant Age Age 1 125.1.1 0.00 739 166 0.23 70.1 676±13 704±37 4

2 125.2.1 0.19 616 180 0.30 42.0 492±10 465±51 -5

3 125.2.2 0.12 1128 455 0.42 74.9 479±9 446±32 -7

4 125.3.1 0.00 71 52 0.75 9.1 898±21 927±110 3

5 125.4.1 0.33 442 360 0.84 39.9 642±13 580±76 -10

6 125.5.1 0.15 212 265 1.29 17.7 597±12 622±57 4

7 125.6.1 0.02 1327 153 0.12 199.0 1038±19 1023±14 -1

8 125.7.1 0.44 267 96 0.37 17.6 474±10 410±92 -14

9 125.7.2 0.00 188 54 0.30 16.0 611±13 600±55 -2

10 125.8.1 0.00 821 640 0.80 168.0 1377±25 1913±10 39

11 125.9.1 0.15 233 95 0.42 18.2 561±12 591±67 5

12 125.11.2 0.04 829 122 0.15 58.0 504±10 535±29 6

13 125.12.1 0.10 445 456 1.06 38.9 624±13 637±34 2

Errors are 1-sigma; Pbc and Pb* indicate the common and radiogenic portions, respectively. Error in Standard calibration was 0.79%. (1) Common Pb corrected using measured 204Pb.

(1) (1) (1) (1) error № ±% ±% ±% ±% 238U/206Pb* 207Pb*/206Pb* 207Pb*/235U 206Pb*/238U correction

1 9.05 2.1 0.0629 1.8 0.958 2.7 0.1105 2.1 0.761

2 12.62 2.1 0.0563 2.3 0.615 3.1 0.0792 2.1 0.674

3 12.96 2.0 0.0559 1.4 0.594 2.5 0.0772 2.0 0.820

4 6.70 2.5 0.0700 5.4 1.441 5.9 0.1494 2.5 0.416

5 9.55 2.1 0.0593 3.5 0.857 4.1 0.1047 2.1 0.514

6 10.31 2.2 0.0605 2.6 0.809 3.4 0.0970 2.2 0.641

7 5.72 2.0 0.0733 0.7 1.767 2.1 0.1748 2.0 0.946

8 13.10 2.2 0.0549 4.1 0.578 4.6 0.0763 2.2 0.469

9 10.06 2.3 0.0599 2.6 0.821 3.4 0.0994 2.3 0.666

10 4.20 2.1 0.1172 0.6 3.847 2.1 0.2382 2.1 0.965

11 11.01 2.2 0.0597 3.1 0.747 3.8 0.0909 2.2 0.578

12 12.29 2.1 0.0581 1.3 0.652 2.5 0.0814 2.1 0.840

13 9.84 2.1 0.0609 1.6 0.854 2.6 0.1016 2.1 0.801

568 M.L. SOMIN

Table 6. U-Pb data and calculated ages for zircons of paragneiss (sample 152) of the Gondaray Metamorphic Complex.

(1) (1) % ppm ppm ppm % № Spot 232Th /238U 206Pb /238U 207Pb/206Pb 206Pbc U Th 206Pb* Discor dant Age Age

1 152.1.1 0.33 532 37 0.07 33.9 459±9 473±66 3

2 152.2.1 3.02 224 16 0.07 9.8 310±7 125±400 -60

3 152.1.2 0.38 551 124 0.23 46.5 602±11 655±56 9

4 152.2.2 0.00 1191 360 0.31 93.0 561±10 537±27 -4

5 152.3.1 0.07 543 379 0.72 55.5 724±13 694±32 -4

6 152.3.2 3.38 238 31 0.14 18.4 536±12 504±340 -6

7 152.4.1 0.08 320 190 0.61 26.9 602±11 562±48 -7

8 152.5.1 0.37 210 193 0.95 17.5 592±12 568±93 -4

9 152.5.2 0.25 326 39 0.12 15.3 341±7 386±89 13

10 152.6.1 0.00 230 249 1.12 20.3 634±12 667±54 5

11 152.7.1 0.36 451 281 0.64 38.2 604±11 595±81 -1

12 152.8.1 0.63 664 149 0.23 48.4 522±10 562±64 8

13 152.9.1 0.00 342 82 0.25 30.3 633±12 662±43 5

14 152.10.1 0.00 295 254 0.89 25.5 617±12 641±42 4

15 152.11.1 0.14 2177 2246 1.07 163.0 537±10 553±25 3

16 152.12.1 0.00 396 1156 3.01 32.8 596±12 693±110 16

17 152.13.1 0.16 407 378 0.96 57.1 973±18 987±30 1

18 152.14.1 0.14 709 238 0.35 58.8 593±11 589±35 -1

19 152.15.1 0.38 244 146 0.62 27.3 788±15 772±65 -2

20 152.16.1 0.11 4692 769 0.17 329.0 505±9 516±16 2

21 152.17.1 0.43 2827 664 0.24 224.0 565±10 584±26 3

22 152.18.1 0.90 612 18 0.03 36.8 432±8 397±130 -8

23 152.18.2 0.29 504 109 0.22 33.2 474±9 500±69 5

24 152.19.1 0.01 1192 32 0.03 310.0 1702±28 2073±41 22

25 152.20.1 0.02 695 145 0.22 217.0 2002±32 2299±9 15

26 152.21.1 0.17 1122 206 0.19 82.2 527±10 522±43 -1

27 152.21.2 0.10 1188 236 0.21 88.7 537±10 511±30 -5

28 152.22.1 0.07 996 600 0.62 83.7 601±11 587±31 -2

29 152.23.1 0.00 398 15 0.04 20.8 381±7 395±51 4

30 152.23.2 0.00 78 40 0.53 6.1 562±13 531±89 -5

Errors are 1-sigma; Pbc and Pb* indicate the common and radiogenic portions, respectively. Error in Standard calibration was 0.72%. (1) Common Pb corrected using measured 204Pb.

569 PRE-JURASSIC BASEMENT OF THE CAUCASUS

Table 6. continued.

(1) (1) (1) (1) error № Spot ±% ±% ±% ±% 238U/206P* 207Pb*/206Pb* 207Pb*/235U 206Pb* /238U correction

1 152.1.1 13.55 2.0 0.0565 3.0 0.575 3.6 0.0738 2.0 0.554

2 152.2.1 20.26 2.4 0.0485 17.0 0.330 17.0 0.0493 2.4 0.139

3 152.1.2 10.21 2.0 0.0614 2.6 0.829 3.3 0.0979 2.0 0.600

4 152.2.2 11.00 1.9 0.0582 1.2 0.730 2.2 0.0910 1.9 0.834

5 152.3.1 8.42 1.9 0.0626 1.5 1.025 2.5 0.1188 1.9 0.786

6 152.3.2 11.52 2.4 0.0573 15.0 0.690 15.0 0.0867 2.4 0.153

7 152.4.1 10.22 2.0 0.0589 2.2 0.794 3.0 0.0978 2.0 0.667

8 152.5.1 10.39 2.1 0.0590 4.3 0.783 4.7 0.0963 2.1 0.435

9 152.5.2 18.41 2.0 0.0544 4.0 0.407 4.5 0.0543 2.0 0.457

10 152.6.1 9.68 2.0 0.0618 2.5 0.881 3.2 0.1034 2.0 0.624

11 152.7.1 10.18 1.9 0.0598 3.7 0.810 4.2 0.0982 1.9 0.460

12 152.8.1 11.86 1.9 0.0589 2.9 0.684 3.5 0.0843 1.9 0.547

13 152.9.1 9.69 2.0 0.0616 2.0 0.877 2.8 0.1032 2.0 0.710

14 152.10.1 9.96 2.0 0.0611 1.9 0.845 2.8 0.1004 2.0 0.714

15 152.11.1 11.52 1.9 0.0586 1.2 0.702 2.2 0.0868 1.9 0.849

16 152.12.1 10.33 2.0 0.0625 5.1 0.835 5.5 0.0968 2.0 0.372

17 152.13.1 6.14 2.0 0.0720 1.5 1.619 2.5 0.1630 2.0 0.795

18 152.14.1 10.38 1.9 0.0596 1.6 0.791 2.5 0.0963 1.9 0.758

19 152.15.1 7.69 2.0 0.0649 3.1 1.163 3.7 0.1300 2.0 0.544

20 152.16.1 12.28 1.8 0.0576 0.7 0.647 2.0 0.0814 1.8 0.934

21 152.17.1 10.91 1.8 0.0595 1.2 0.752 2.2 0.0917 1.8 0.839

22 152.18.1 14.43 2.0 0.0546 5.7 0.522 6.0 0.0693 2.0 0.334

23 152.18.2 13.09 2.0 0.0572 3.1 0.602 3.7 0.0764 2.0 0.532

24 152.19.1 3.31 1.9 0.1282 2.3 5.340 3.0 0.3022 1.9 0.627

26 152.21.1 11.74 1.9 0.0578 1.9 0.679 2.7 0.0852 1.9 0.701

27 152.21.2 11.51 1.9 0.0575 1.4 0.689 2.3 0.0869 1.9 0.811

28 152.22.1 10.23 1.9 0.0595 1.4 0.803 2.4 0.0978 1.9 0.793

29 152.23.1 16.41 2.0 0.0546 2.3 0.459 3.0 0.0609 2.0 0.657

30 152.23.2 10.99 2.4 0.0580 4.1 0.728 4.7 0.0910 2.4 0.503

570 M.L. SOMIN

Small feldspar augens oft en appear on the schistosity surfaces. Separate stratiform bodies of the gneiss have a thickness ranging from a few metres up to 70 metres and are in concordant contact with siliciclastics and amphibolites; the cumulative thickness of such sections enriched by these gneisses reaches up to 500 m (Baranov & Kropachev 1976). Th e gneisses consist of quartz, albite, microcline, muscovite and biotite. Microcline and albite are of metasomatic origin, quartz demonstrates signs of complete recrystallization. At the same time some relicts of the primary magmatic plagioclase phenocrysts are observed. Isotopic characteristic of this gneiss is following: 87Sr/86Sr= 0.726753; Isr= 0.71338; εNd (T)= –6.5; 143Nd/144Nd= 0.512162; 147Nd/144Nd= 0.12136. Presence of small idiomorphic zircon grains is a characteristic feature of the Azau gneisses (Potapenko et al. 1972; Bibikova et al. 1991). Free detrital zircon detrital grains were not found within these rocks, but there magmatic metamorphic are relict detrital cores inside some magmatic zircon grains. Th is fact indicates magmatic origin of the gneisses, confi rming the conclusion by Potapenko et Figure 12. Histogram of zircons age values of the Gondaray al. (1972). At the same time it is diffi cult to decide complex rocks. Samples: 0-17, migmatized whether these rocks were volcanic or intrusive paragneiss, Adyr-sy River; paragneiss, Damkhurts River; 152, migmatized paragneiss, Sofi a River; 0-11, because of their complete recrystallization and strong amphibolite, Kyrtyk River. deformation. Potapenko et al. (1972) prefers to think that gneisses were originated from aplite dykes oft en graphite-bearing metapelite, very rarely because rare siliciclastic metamorphic xenoliths are quartzite), fi ne-grained Azau gneiss (orthogneiss) sometimes observed within them. However presence and much less by amphibolite; sometimes lenses of of such xenoliths is not unusual for lava fl ows or crystalline limestone are noted in association with subvolcanic bodies. At the same time porphyritic amphibolite. Amphibolites occupy approximately texture is unusual for an aplite. Intrusive contacts 10–15% of the MMC, where they are oft en associated with the surrounding siliciclastics are extremely rare. with Azau gneisses forming collectively the Duppukh Th ese arguments and the presence of rare relicts of Formation (Baranov & Kropachev 1976; Baranov plagioclase phenocrysts rather seem to indicate a 1987). Th is formation is famous for its scheelite protolith of subvolcanic granite-porphyr or rhyolite- occurrences. Scheelite is associated with quartz veins porphyr for the Azau gneisses. placed within melanocratic amphibolites, which were Some samples of the Azau gneisses from diff erent lithological traps for tungsten-bearing hydrothermal sites were studied with TIMS U-Pb zircon method. solutions. Th e fi rst determinations were made by Bibikova et Th e Azau gneisses are especially interesting type al. (1991) on magmatic zircons of gneisses of Azau of rocks, because they represent the main object for locality, left wall of the Baksan River valley, and isotope dating. Chemical composition of gneisses on similar rocks taken at riverhead of the Malaya ranges from dacite (diortite) up to rhyolite (granite). Dukka. Th e zircons yielded ca. 425 and 430 Ma ages, Th ey are fi ne-grained, banded rocks of light, mostly respectively. SHRIMP dating of zircons from samples greenish colour with a poorly developed schistosity. taken at massif Cheget foot (03-10) and Azay locality

571 PRE-JURASSIC BASEMENT OF THE CAUCASUS

(0-29), and at right side of Aksaut River (Kti-Teberda grains of sample 0-29. Many of these zircons include mine area, sample C87-36, not shown here), however, cores with age values mostly around 600 Ma, typical yielded age values ranging from 425 up to 460 Ma for predominate part of the Makera metapelite (Figures 13 & 14, Tables 7 & 8). A concordia age of ca. detrital zircons; some cores yielded Mezoproterozoic 444 Ma was obtained from seven idiomorphic zircon values.

a b

Figure 13. Cathodoluminescence image (a) and diagram with concordia (b) for zircons of the Azau orthogneiss, sample 0-29, Azau.

b c

Figure 14. (a) Optical and (b) cathodoluminescence images of zircons from sample 03-10 of orthogneiss, Makera complex, Cheget Mt. Numbers on (a) are age values, Ma. Scale bar 300 μm. (c) Concordia diagram.

572 M.L. SOMIN

Table 7. U-Pb data and calculated ages for zircons of orthogneiss (sample 03-10) of the Makera Metamorphic Complex.

(1) (1) (1) % ppm ppm Ppm 232Th /238U 206Pb/238U 207Pb/206Pb 208Pb/232Th Discordant Spot 206Pbc U Th 206Pb* Age Age Age

03-10_1.1 0.29 150 83 0.57 12.2 580±19 681±42 614±23 15

03-10_2.1 0.00 218 116 0.55 18.1 593±19 618±35 612±23 4

03-10_3.1 0.01 1065 450 0.44 64.0 436±14 434±19 448±18 0

03-10_4.1 - 468 145 0.32 30.6 474±16 439±35 523±20 -8

03-10_5.1 - 256 32 0.13 16.4 464±15 480±53 515±23 3

03-10_6.1 - 402 67 0.17 25.9 468±15 479±38 498±23 2

03-10_7.1 - 838 160 0.20 70.6 603±19 574±21 630±24 -5

03-10_7.2 - 525 194 0.38 44.3 605±20 567±22 622±22 -7

Errors are 1-sigma; Pbc and Pb* indicate the common and radiogenic portions, respectively. Error in Standard calibration was 1.05% (not included in above errors but required when comparing data from diff erent mounts). (1) Common Pb corrected using measured 204Pb.

(1) Total (1) (1) (1) error Total ±% ±% Total ±% ±% ±% ±% 207Pb/206Pb 207Pb*/206Pb* 207Pb*/235U 207Pb*/235U correction 238U/206Pb 238U/206Pb*

19.59 3.5 0.0645 1.9 10.62 3.5 0.0622 2.0 0.807 4.0 0.0941 3.5 0.869

10.37 3.4 0.06041 1.6 10.37 3.4 0.06041 1.6 0.803 3.8 0.0964 3.4 0.903

14.30 3.4 0.95564 0.86 14.31 3.4 0.05553 0.87 0.535 3.5 0.0699 3.4 0.969

13.13 3.4 0.05508 1.3 13.12 3.4 0.05566 1.6 0.585 3.7 0.0762 3.4 0.909

13.42 3.4 0.0558 2.4 13.41 3.4 0.0567 2.4 0.583 4.2 0.0746 3.4 0.818

13.31 3.4 0.05550 1.4 13.29 3.4 0.05667 1.7 0.588 3.8 0.0752 3.4 0.893

10.20 3.4 0.05896 0.77 10.20 3.4 0.05919 0.96 0.800 3.5 0.0981 3.4 0.962

10.17 3.4 0.05877 0.97 10.17 3.4 0.05899 1.0 0.800 3.5 0.0983 3.4 0.958

Ages obtained with SHRIMP method on zircons was shown by a point placed near the concordia (n= 16) selected from Duchinka River amphibolite line obtained by TIMS method on the same probe (Bol’shaya Dukka River tributary) yielded an average (Somin et al. 2004). Moreover, zircons of Yusen’gi value 478±4 Ma. However, the age obtained on the group of grains of the last probe with lower (the best) River (Baksan tributary) amphibolite body (sample value of MSWD is 464±15 Ma (Figure 15, Table 9). 16) yielded a SHRIMP age of 445±12 Ma (i.e. earliest It is interesting that a very similar age (460±8 Ma) Late Silurian) (Figure 16, Table 10).

573 PRE-JURASSIC BASEMENT OF THE CAUCASUS error error Pb correction 206 (1) Age Pb/ 207 ±% U 238 U (1) 238 Pb* / (3) 206 Age Pb/ 206 ±% 5U U 23 238 (1) (2) Age Pb* / Pb/ 207 206 erent mounts). erent ±% U 238 Pb* 206 (1) Age Pb/ (1) 206 Pb* / 207 ±% Makera Metamorphic Complex. Metamorphic Makera

Pb* Ppm 206 Pb* 206 (1) U/ 238 U 238 / ±% Th 232 Pb 206 age-concordance U age-concordance Th Total 235 232

Pb / Th ppm 207 Pb/ Pb/ 207 208 U - U- ±% 238 238 Pb. U 204 Pb ppm Pb/ Pb/ 206 206 206 Total U / 238 Pbc % 206 % Discordant

Th 232 (1) Age Pb / U-Pb data and calculated ages for zircons of orthogneiss (sample 0-29) of the 0-29) of (sample orthogneiss of zircons for ages calculated and U-Pb data 208 12 0-29.1.13 0-29.1.24 0.27 0-29.2.15 0.70 0-29.2.26 698 0.05 0-29.3.17 103 0.03 0-29.4.18 118 573 0.19 0-29.5.19 344 33 0-29.6.1 - 121 435 0.15 0-29.7.1 0.17 127 0.13 0.33 489 702 28 0.22 - 469 44.1 0.38 100 176 5.65 0.07 1538 112 36.8 0.21 14.9 0.26 383 456±16 0.25 25.9 397±15 465±16 0.26 30.8 42.8 458±16 318±11 29.7 399±15 432±15 465±16 91.5 457±16 457±16 317±11 441±15 392±15 458±16 432±15 464±16 432±15 339±74 315±13 456±16 442±15 168 ±270 432±15 457±16 456±41 455±16 432±15 395±42 442±16 456±16 418±71 430±15 473±44 389±30 465±16 428±19 №Spot 1 2 424±27 1.4 3 14.43 494±59 0.05668 3.5 0.0551 3.2 4 0.527 0.0693 498±23 3.5 4.7 -34 0.744 5 409±87 3.5 -136 366±49 -3 6 13.60 -2 14.40 7 15.64 505±25 3.5 20 8 496±22 3.7 413±16 0.0555 13.36 9 480±17 1.4 2 0.0551 13.59 0.05738 2.6 3.5 3.5 0.85 3 14.44 0.0563 3.5 19.78 -1 3.5 1.9 13.58 3.5 0.05529 0.5539 0.571 0.0736 0.86 -13 3.5 4.0 0.529 0.05650 3.5 3.6 0.692 3.6 13.64 0.877 14.44 3.5 0.971 15.75 1.4 0.05485 13.65 14.10 1.6 3.5 13.37 3.5 3.8 3.5 0.05489 19.79 0.0532 0.05564 0.0494 3.5 1.3 1.2 3.3 3.6 12.0 0.0561 13.63 14.12 0.538 0.433 0.0546 1.9 3.5 3.5 4.8 12 1.9 0.578 0.0565 0.0733 0.0544 0.0632 0.380 4.0 2.0 3.5 3.8 1.3 0.0748 4.0 0.572 0.532 0.733 0.306 3.5 0.0505 4.0 3.8 3.6 0.883 0.0734 0.0708 0.885 3.5 3.5 0.936 0.869 № Errors are 1-sigma; Pbc and Pb* indicate the common and radiogenic portions, respectively. portions, radiogenic and the common Pb* indicate 1-sigma; Pbc and are Errors diff from data when comparing required but errors in above included was 0.37% (not calibration in Standard Error (1) Common Pb corrected using measured measured using Pb corrected (1) Common (2) Common Pb corrected by assuming assuming by Pb corrected (2) Common Table 8. Table (3) Common Pb corrected by assuming assuming by Pb corrected (3) Common

574 M.L. SOMIN

Figure 15. Cathodoluminescence (a) optical images (b) and diagram with concordia (c) for zircons of sample 146-1, amphibolite of the Makera Complex, Duchinka River.

An attempt to determine the protolith age of the Detrital zircons dating (n= 27) realized in the MMC with Rb-Sr (wr) isochron method was made VSEGEI Isotope Research Center (St. Petersburg) using the andalusite schist of the Cheget massif. on the grains selected from andalusite-bearing According to Gucasian (Geological Institute of metapelite of the Cheget Massif (sample A1) gave Armenia), fi ve probes up to 5 kg each gave an isochron a range of values of 1867–444 Ma. Distribution of 87 86 age 462±17 Ma at ( Sr/ Sr)0= 0.70952±0.00162 ages is as follows: 1867–1053 Ma (n= 7); 888–626 (Somin 1997). Ma (n= 6); 605–546 Ma (n= 10); 524 and 444 Ma,

575 PRE-JURASSIC BASEMENT OF THE CAUCASUS

Th 232 (1) Age Pb/ 208 Pb 206 (1) Age Pb / 207 error correction U 238 ±% (3) Age Pb / 206 U 238 (1) U erent mounts). erent Pb*/ 238 206 (2) Age Pb / 206 ±% U U 238 235 (1) Age (1) Pb / Pb*/ 206 207 Pb* Pb* (1) ±% Pb* 207 /206 Ppm 206 U Pb* 238 206 / (1) ±% age-concordance Th U/ U age-concordance Th 232 238 235 232 Pb/ Pb/ 207 208

Th Pb U- U- ppm 206 238 238 Pb. ±% 204 Total Pb/ Pb/ Pb/ 207 206 206 U ppm Pb 206 ±% Total U/ Pbc % 238 206 % U-Pb data and calculated ages for zircons of amphibolite (sample 146) of the Makera Metamorphic Complex. Metamorphic the Makera 146) of (sample amphibolite of zircons for ages calculated and U-Pb data Discordant 1 146-1.1.12 146-1.2.13 - 146-1.3.1 0.134 146-1.4.1 0.33 2455 189 146-1.5.16 - 199 135 73 146-1.6.17 - 2151 0.57 69 146-1.6.2 - 0.40 324 4015 - 0.36 16.2 257 12.3 135 1.93 606 96 13 477.1±4.3 0.43 470±4.7 143 263 476.9±4.5 0.39 471.2±4.7 469.9±4.9 0.45 14 478.3±4.8 481±2.4 471.8±4.8 17.2 470.1±5.1 494±49 40.6 481.3±2.4 317.9±2.9 472.8±5 477±68 484.5±4.2 486.5±3.6 464±11 317.9±2.9 485.1±3.2 425±60 484.1±4.4 469±17 317.2±3.1 459±20 485.3±3.3 483.4±4.5 441±14 322±90 466.6±3.3 486.1±3.5 508±47 328±14 471±32 503±13 471.3±8.3 12 33 14 13.02±0.94 -105 0.0567±2.2 -5 13.2±16 13.02±0.94 13.15±1 17 0.0577±2.5 0.0571±2.2 12.92±0.51 0.058±2.5 5 0.05542±0.75 13.22±1 0.604 19.85±0.92 12.908±0.51 -3 13.19±1 0.05618±0.88 0.0566±3.1 12.83±0.91 0.05±2.5 2.4 0.6001 0.0553±2.7 12.805±0.69 0.056±2.2 0.591 19.78±0.94 0.07682 0.05583±1.4 0.578 1 12.81±0.91 12.795±0.69 0.0529±4 0.94 3.2 0.05647±1.4 0.0574±2.1 2.9 0.07747 0.368 0.389 0.07563 0.6085 0.618 0.51 0.07582 1.6 4.1 1 2.3 0.501 1 0.07816 0.05055 0.324 0.07805 0.354 0.69 0.94 0.91 0.434 0.231 0.391 №Spot № (2) Common Pb corrected by assuming assuming by Pb corrected (2) Common Table 9. Table (3) Common Pb corrected by assuming assuming by Pb corrected (3) Common Errors are 1-sigma; Pbc and Pb* indicate the common and radiogenic portions, respectively. respectively. portions, radiogenic and the common Pb* indicate 1-sigma; Pbc and are Errors diff from data when comparing required but errors in above included was 0.41% (not calibration in Standard Error (1) Common Pb corrected using measured measured using Pb corrected (1) Common

576 M.L. SOMIN

Th 232 (1) error Age Pb/ correction 208 ±% Pb 206 U (1) Age 238 Pb/ 207 (1) Pb*/ 206 U 238 ±% (3) Age Pb / 206 U 235 (1) Pb*/ U 207 238 (2) Age Pb / 206 ±% Pb* U 206 238 (1) (1) Age Pb / Pb*/ 206 207 ±% Pb* Ppm 206 Pb* 206 (1) U/ U 238 238 / Th ±% 232 Pb

206 Th ppm Total Total Pb/ 207 U ppm ±% Pb Pbc % 206 206 Total U / 238 continued. 12 146-1-add.1.13 146-1-add.2.1 -4 146-1-add.3.1 0.05 146-1-add.4.1 0.20 694 988 352 - 453 410 236 0.52 548 0.43 0.54 299 43.9 61.3 0.56 31 458.1±9 448.9±8.9 35.3 458.1±9.1 493 ±10 449±9 458.1±9.7 466±9.4 494 ±10 449.2±9.5 456 ±26 465.5±9.6 490 ±11 446 ±28 463 ±10 458 ±10 445 ±11 371 ±76 500 ±29 522 ±17 497 ±12 123 04 -1 -25 13.58 13.86 7 2 12.57 2.1 0.05606 2.1 13.34 0.05626 1.2 0.0556 0.94 2.1 13.58 13.87 0.05691 3 2 2.1 1.3 12.59 0.05584 0.05609 13.34 2.1 1.3 1.2 2.1 0.054 0.05722 0.555 0.57 3.4 1.3 2.4 2.3 0.591 0.591 0.0721 0.0736 4 2.5 2.1 2 0.0794 0.075 ,850 ,864 2.1 2.1 ,531 ,845 №Spot № % Discordant Table 9. Table

577 PRE-JURASSIC BASEMENT OF THE CAUCASUS

0.66

Figure 16. Cathodoluminescence images (a) and concordia digram (b) for zircons of amphibolite, sample 16, the Makera complex, Yusen’gi River. Scale bar 200 μm. one grain each (Figure 17, Table 11). All grains are structural parts of a single metamorphic Makera small and only their tops are rounded. Because of series. Later Baranov (1987) noted the presence of moderate temperature of metamorphism (500–550° a tectonic surface between these units in the Elbrus C) and complete absence of migmatization, changing section, although he proposed a gradual transition of the U-Pb system by metamorphism seems to be between them in some places. Gamkrelidze et al. improbable. (1996) have postulated that MMC is a completely allochthonous body, or ‘Makera nappe’ detached TIMS U-Pb dating was applied also to the MMC from the uppermost part of the Buulgen complex. detrital zircons taken of Arkasara Range (Somin et al. 2004). Four fractions of the grains were used, lower Th e present author confi rms the conclusion of intercepts gave the value of 487±5 Ma at MSWD= tectonic contact between the MMC and GMC. Th e 0.098. contact has diff erent aspects in diff erent places. In most localities the Upper Palaeozoic granite In summary the data on isotopic ages of zircons divides these complexes; however, they have more indicate the post-Cambrian age of the MMC or less clearly expressed structural and lithological protolith. Presence of Upper Ordovician and Lower diff erences. For example, western slopes of Koru Silurian magmatic and sedimentary rocks seems to River valley (Chegem river basin) consist of gently be most probable. dipping metapelites of the MMC, whereas in the Th e exact age of the MMC metamorphism eastern slopes granite crops out. It includes numerous is a question. In absence of Ar40/Ar39 data only inclusions of these metapelites demonstrating determinations of the K/Ar data indicating cooling monotonous dips similar to the western slope; age may be used. Th e best value was obtained however more sillimanite appears in these inclusions. from a large muscovite crystal in a deformed Several hundred metres to the SE granite is mixed synmetamorphic vein together with quartz and with an orthogneiss of the GMC characterized by andalusite among Cheget massif metapelites. Content a completely diff erent, mostly subvertical position of K in this mica is 8.35% and the age value is 300±5 of foliation. Th erefore the granite here replaces the Ma. Konilov, using CHIME method on monazite (8 contact zone without essential mechanical infl uence points), obtained an age value 279±30 Ma. on the surrounding rocks. Th e relationship between the MMC and GMC In rare cases the contact zone is granite-free. Th e is another problem of the Elbrus subzone geology. best locality of such type is at left slope of Shkhel’da Originally (Somin 1971; Baranov & Kropachev glacier valley, 1 km above the glacier surface. A 1976) both units were considered supra- and infra- mixture of granite and biotite-sillimanite paragneiss

578 M.L. SOMIN

Th 232 (1) Age Pb/ 208 Pb error 206 correction (1) Age Pb/ 207 ±% U 238 U (1) 238 Pb*/ (3) Age Pb/ 206 206 ±% U U 235 238 (2) (1) Age Pb/ Pb*/ 206 207 ±% U 238 (1) Age Pb/ Pb* Pb* (1) 206 206 207 / Pb* Pb* ppm 206 206 (1) ±% age-concordance. U/ U age-concordance. Th 238

235 232 U Th 238 / 232 Pb/ Pb/ 207 208 Pb U- U- 206

238 238 Pb. ±% Th Total ppm 204 Pb / Pb/ Pb/ 207 206 206 Pb 206 ±% Total U/ 238 Pbc % 206 % U-Pb data and calculated ages for zircons of amphibolite (sample 16) of the Makera Metamorphic Complex. Metamorphic the Makera 16) of (sample amphibolite of zircons for ages calculated and U-Pb data Discordant 123 24 -25 176 15.11±4 727 14.34±2 0.05525±0.87 6 16.54±28 110 0.05619±1.1 15.11±4 27.03±29 0.05572±0.65 -18 14.36±2 0.05552±0.68 14.22±2 16.54±2 25.42±2 4 0.05524 0.05572±0.81 14.14±2.1 27.06±2 3 0.06366±0.53 0.05534 0.05573 0.05569±1.5 0.9 14.21±2 13.47±2.1 25.61±2 0.05474 1.4 14.18±2.1 0.05715±1.3 0.65 13.47±2 0.504 0.05636 0.94 0.05772 13.48±2.1 0.531 0.05668±0.6 0.4646 0.0537 4.1 1.1 0.2789 1.7 13.47±2 0.05676 2.5 2.1 2.1 0.0662 2.2 0.547 0.3107 1.7 0.0696 0.0605 0.0566 0.523 4 0.03695 2.3 2.7 0.581 2 2 0.61 3 2 0.03904 0.976 0.0704 2.7 0.579 0.816 0.953 0.0705 2 0.908 2 0.0742 2.1 2.1 0.765 0.874 2.1 0.0742 0.699 0.771 2 0.959 12 16.1.13 16.2.14 0.00 16.3.15 0.11 16.4.1 12316 16.4.2 -7 1504 782 0.10 16.5.18 1.26 397 2414 16.5.2 4149 -9 0.75 16.6.1 0.52 367 70 920 0.24 1576 16.6.2 5980 0.16 46.9 0.05 0.23 245 1449 433 0.01 413±16 125 546 132 0.25 0.16 123 2986 434±8.5 94 0.29 202 95.2 310 378.4±7.5 233.9±4.7 0.18 413±16 26.3 0.11 434.1±8.6 438.5±8.5 246.9±4.9 34.8 190 377.7±7.6 232.8±4.7 439.4±8.8 413±20 433.2±9.3 438.2±8.6 461.5±9.1 244.9±5 461.5±9.1 379.4±7.7 238.5±4.9 440.4±8.9 426±32 438.2±8.7 422±20 461.2±9.3 246.9±5.2 461.3±9.2 442±14 402±21 440.3±9.2 461.9±9.4 466±25 444±10 461.9±9.3 519±38 414±17 337.2±7.6 104.1±3.8 360±48 482±38 476±13 453±14 246.7±8.7 419±14 445±18 437±10 №Spot № 10 10 14.17±2.1 0.05477±1.8 14.14±2.1 0.0568 2.5 0.554 3.2 0.0707 2.1 0.648 10 16.7.1 - 297 163 0.57 18 440.5±8.9 439.9±9 439.2±9.7 484±54 455±13 Errors are 1-sigma; Pbc and Pb* indicate the common and radiogenic portions, respectively. portions, radiogenic and the common Pb* indicate 1-sigma; Pbc and are Errors was 0.65% calibration in Standard Error measured using Pb corrected (1) Common (2) Common Pb corrected by assuming assuming by Pb corrected (2) Common Table 10. Table (3) Common Pb corrected by assuming assuming by Pb corrected (3) Common

579 PRE-JURASSIC BASEMENT OF THE CAUCASUS

a b

Figure 17. Optical (a) and cathodoluminescence (b) images of detrital zircons sample A-1, metapelite schist, Makera complex, Cheget Mt. Numbers on (a) are age values, Ma. Scale bar 200 μm. (c) Concordia diagram and age values histogram of these zircons. with diff erent orientation of foliation is overlain in structural style marks this section of the contact here by horizontally oriented body of banded low- zone. temperature mylonite, 10 m thick. Typical Makera Th ese observations seem to indicate that contacts metapelites are disposed above the mylonite, between the complexes are probably tectonic deformed near their base into overturned folds everywhere. Th ey were formed immediately before with horizontal axial surfaces indicating horizontal the granite emplacement and were partly rejuvenated tectonic transport of the Makera sequence. Th us a sometime later. Any exotic rocks like serpentinite jump in the grade of metamorphism and a change or younger sediments were not noted there till

580 M.L. SOMIN

Table 11. U-Pb data and calculated ages for zircons of metapelite (sample A-1) of the Makera Metamorphic Complex.

(1) (1) ppm ppm % №Spot% 206Pbc ppmTh 232Th /238U 206Pb/238U 207Pb/206Pb U 206Pb* Discordant Age Age

1 A1.1.1 0.13 1080 435 0.42 103.0 675±13 698±28 3 2 A1.1.2 0.00 638 270 0.44 59.5 665±13 646±30 -3 3 A1.2.1 0.07 523 30 0.06 42.6 584±11 560±37 -4 4 A1.3.1 0.00 637 174 0.28 58.9 659±13 1190±23 81 5 A1.4.1 0.23 721 238 0.34 58.7 582±11 569±47 -2 6 A1.5.1 0.08 257 152 0.61 72.7 1835±33 1835±28 0 7 A1.5.2 0.00 199 101 0.52 47.5 1582±30 1689±24 7 8 A1.6.1 0.08 256 172 0.70 69.9 1779±32 1847±19 4 9 A1.7.1 0.11 863 597 0.72 66.6 554±11 580±32 5 10 A1.8.1 0.00 228 147 0.66 18.2 571±12 579±54 1

Errors are 1-sigma; Pbc and Pb* indicate the common and radiogenic portions, respectively. Error in Standard calibration was 0.79%. 1) Common Pb corrected using measured 204Pb.

(1) (1) (1) (1) error № ±% ±% ±% ±% 238U/206Pb* 207Pb*/206Pb* 207Pb*/235U 206Pb*/238U correction 1 9.070 2.0 0.0627 1.3 0.953 2.4 0.1103 2.0 0.832 2 9.200 2.0 0.0612 1.4 0.917 2.5 0.1087 2.0 0.822 3 10.550 2.1 0.0588 1.7 0.769 2.7 0.0948 2.1 0.768 4 9.290 2.1 0.0797 1.2 1.182 2.4 0.1076 2.1 0.868 5 10.580 2.0 0.0591 2.2 0.770 3.0 0.0946 2.0 0.682 6 3.036 2.1 0.1122 1.6 5.090 2.6 0.3293 2.1 0.798 7 3.595 2.1 0.1036 1.3 3.973 2.5 0.2782 2.1 0.850 8 3.146 2.1 0.1129 1.1 4.950 2.3 0.3178 2.1 0.889 9 11.140 2.0 0.0594 1.5 0.734 2.5 0.0898 2.0 0.804 10 10.790 2.2 0.0593 2.5 0.758 3.3 0.0927 2.2 0.654 recently. Some age diff erence (older for the MMC) amphibolites are not identical to the Buulgenian probably exists, but data are not conclusive. Because ones. Content of TiO2 in the latter is 1.56% (n= 39), the metamorphism in the MMC is of lower grade, whereas in the MMC amphibolites (n= 35) content a model of large detachment between rheologically of this component is 1.93%. Content of K2O in the diff erent complexes is quite probable as an alternative MMC amphibolites is 1.19%, and in Buulgenian ones to the nappe model of Gamkrelidze et al. (1996). At 0.79% only. Popov (personal communication 2004) the same time their idea of the MMC transport from concluded that metabasites of the Makera Complex the Buulgen complex seems disputable. Volume belong to group of tholeiitic basalts-subalkaline of metabasites in the MMC is several times less olivine basalts, whereas Buulgenian amphibolites that in the BMC. Besides, petrochemically MMC were ferrigeneous tholeiitic basalts.

581 PRE-JURASSIC BASEMENT OF THE CAUCASUS

Rocks like the Azau gneisses are absent in the Buulgen Complex. On the other hand, low-K orthogneisses and augen metagranitoids typical for the latter are completely absent within the MMC. Finally, the essential part of the detrital zircons of the BMC is marked by Late Palaeozoic age values, whereas no such zircons have been found in lithologically similar rocks of the MMC. Evidence for the presence of Lower Palaeozoic metasediments and associated metaigneous rocks is very important for the geology of the Greater Caucasus: up to now majority of Caucasian geologists have believed, that Lower Palaeozoic is absent in the Greater Caucasus. At the same time I doubt that the Ordovician acid metamagmatic rocks indicate Caledonian tectonomagmatic events in the Main Range because the Azau orthogneiss is not an orogenic formation and because Silurian or Devonian molassic complexes are unknown in this transgressive contact nappe surface region. Th erefore here are some arguments to suggest that Lower and Middle Palaeozoic sediments form an uninterrupted section. Th is suggestion, of course, needs further confi rmation.

Th e Fore Range Zone Th e Fore Range zone is the most complex part of the pre-Jurassic basement of the Greater Caucasus.

During the Late Palaeozoic (Late Visean–Permian) ophiolite complex (Marukh nappe)Atsgara metamorphic composite complex post-lower Carboniferous rocks fault thick sequences of molassic, mainly continental and partly marine calcareous sediments were accumulated. Th e Main Range and Bechasyn zones were elevated areas undergoing erosion at that time. Th us the term ‘Fore Range zone’ means the Late Palaeozoic palaeostructure of the Greater Caucasus. Th e pre-Upper Palaeozoic part of the Fore Range Zone section also diff ers essentially from other zones of the Greater Caucasus. Subsidence during the Late Palaeozoic resulted in the preservation of a more complete structural-stratigraphical column in the unmetamorphosed sequences of the Silurian-Devonian & Lower Carboniferous serpentinite within the krystallinikum Fore Range Zone. Especially interesting in this aspect is the wide north-western part of the zone situated west of Teberda River (Figure 18). Five pre-Mesozoic rock

complexes are observed there in ascending structural zone. Range the Fore thepart of north-western Geological scheme of succession: (1) krystallinikum (granite-metamorphic complex); (2) Middle Palaeozoic (Silurian–Lower

Caboniferous) sedimentary-volcanic supercomplex Figure 18.

582 M.L. SOMIN

separated into two complexes: Kizilkol (Urup) 3.2 kbar. Tectonic unit II is small (5 km2 only) and and Andruyk-Tokhana; (3) ophiolite complex; (4) consists mainly of metabasite (amphibolites, chlorite- Atsgara composite metamorphic complex; (5) Upper epidote schists and hornblendite) with thin interbeds Palaeozoic molasse complex. of metapsammite. Th e degree of metamorphism corresponds to staurolite-chloritoid zone of staurolite facies and garnet zone of the greenschists facies. Unit Th e Upper Palaeozoic Molasse Complex III (or Abishira-Akhuba unit) includes rocks enriched Th is complex consists of autochthonous masses with CaO (amphibolites and microgabbro-like rocks covering the underlying rocks. Th e oldest components containing thin interbeds of graphitic metacherts of the molasse are the Upper Visean–Namurian and calcareous interbeds). Very high (up to 3.2%) gravelstone, aleurolite and conglomerate. Th ese TiO2 content is characteristic for the metabasites. rocks are of local distribution and include clasts Temperature of metamorphism was determined in of metamorphosed material of some units of the diff erent parts of this unit as 340–480° C (Shengelia underlying Atsgara complex only (Khain 1984). Th e et al. 1984). Th e uppermost unit IV (or Kyaphar)

Middle Carboniferous molasse is coal-bearing and is presented by K2O-rich metapelites and garnet- includes horizons of acid volcanic and subvolcanic staurolite-bearing schists, less by graphite-bearing rocks (Belov 1981). Upper Carboniferous sequence quartzite and CaO-rich rocks. Th e composition consists of boulder conglomerate, where almost of the garnet-biotite pairs indicates metamorphic all types of krystallinikum rocks of surrounding temperatures of 550–600° C. In the western part of zones might be observed. Th e Permian strata are the Abishira-Akhuba exposures and probably also represented by red-coloured mostly coarse-grained at the left bank of the Bol’shaya Laba river (Dzhentu clastic rocks. To the west these strata are replaced locality) there is a less metamorphosed Atsgara unit by shallow-marine organic limestone, which in turn containing phyllite and fi ne-grained metasandstone passes upward to the fossiliferous Triassic limestone. cut by aplite dykes. It is supposed that in the central sector of the Greater Th e problem of the ACC age remains unsolved. Caucasus part of redbeds is of Triassic age. Oldest K-Ar ages of 400±12 and 394±14 Ma were obtained from large magmatic hornblendes of a lamprophyre cutting the unit IV rocks (Shengelia Th e Atsgara (Rechepsta) Composite Complex (ACC) & Korikovskii 1991). Th ese determinations seem Th is complex was described for fi rst time as an to indicate pre-Middle Devonian age of staurolite allochtonous mass overlying the Devonian volcanic- schist. Unfortunately, more exact methods were not sedimentary sequence and locally the Ophiolite applied to the ACC rocks. Complex at Abishira-Akhuba and Dzhuga localities by Baranov & Grekov (1974). Most complete petrological and petrochemical descriptions of Th e Ophiolite Complex (OC) ACC were presented by Shengelia et al. (1984) and Th is complex, like the ACC, has limited distribution Shavishvili et al. (1989). Th e fi rst authors established in the Fore Range zone. Its main exposure is placed the composite character of the ACC and its petrological in Kyaphar-Agur, Malyi Kyaphar and Marukh rivers diversity. According to them, the lowermost basins; small fragments of the complex are seen as structural position in the ACC pile occupies the a nappe remnant and as tectonic lenses along the tectonic unit (nappe) 1, which consists mostly of eastern part of Pshekish-Tyrnyauz suture zone metamorphosed schists, migmatite and Chilik (Figure 19). Besides, Adamia et al. (1989) have metamorphosed granitoids. Th e schists have mainly found small fragments of dismembered ophiolite in two-mica-garnet-sillimanite-cordierite mineral valley in Belaya and Kisha Rivers basin, Expedition assemblage. Metabasites play a subordinate role and Mt, western Caucasus. Th ese exposures occupy are presented by plagioclase- or pyroxene-plagioclase an intermediate position between the Main Range amphibolites, where hornblende is brown-green. P/T and Fore Range zones. Allochthonous position of parameters of these rocks are 560–600° C and 3 to the OC in Kyaphar-Agur area was established by

583 PRE-JURASSIC BASEMENT OF THE CAUCASUS

Lower & Upper Permian: red-coloured conglomerate & sandstone Lower Permian: red-coloured sandstone & dolomite Upper Palaeozoic. Main Range granite plutonic complex: granite

Middle–Upper Devonian, Semirodnik Fm: conglomerate sandstone & aleurolite Middle Palaeozoic: Urushten gneissic tonalite & plagiogranite Middle–Upper Devonian: Andryuk Fm: phylite & sandstone Middle Palaeozoic: Zakan monzodiorite Middle Palaeozoic: serpentinite Silurian–Lower Devonian: basalt, rhyolite, andesite & chert metasomatic zone along base of the Kizilkol nappe layer & foliation position nappe fault transgressive contact

Figure 19. Geological scheme of the Fore Range zone in Malaya and Bol’shaya Laba Rivers basins. Compiled on base of map 1:200 000 of Lavrischev et al. (2000) with changes.

584 M.L. SOMIN

Belov & Omel’chenko (1976), and its most detailed xenoliths of ultrabasic rock. Th e gabbro suff ered description belongs to Khain (1984). According to strong recrystallization and is transformed into a this author, the ophiolite and metamorphic rocks rock with actinolite-tremolite-epidote-saussurite- of the ACC are separated spatially as a rule, and chlorite with some relicts of gabbro and gabbro- superposition of the ACC on OC might be supposed ophitic texture. Schistosity is slightly developed. in one place only. Rare zircon grains were selected from such a gabbro Rocks of the OC occupy the central part and at Kyrylgan-bashi creek, right side of Teberda River. wings of the gentle synforms underlain by the U-Pb SHRIMP dating on sample 02–25 yielded 416±8 Devonian of the Middle Palaeozoic (Kizilkol) Ma, i.e. Silurian/Devonian limit (Somin 2007a). complex. Section of the OC is tectonically inverted, Th e structurally upper unit of the OC is ultrabasic i.e. there is a structurally descending order from rocks represented by a series of tectonic slices and the ‘sole’ metabasite to hyperbasite, gabbro, basalt, allochthonous bodies. Th ey consist of serpentinized and sedimentary cover. Sheeted dykes are observed harzburgites composed of enstatite-bronsite, olivine, in some localities. Th ese components of the OC diopside and more abundant second minerals such section are generally separated by tectonites. Average as antigorite, bastite, tremolite-actinolite amphibole, thickness of the OC is ca. 2000 m. All elements of the chlorite, talk, carbonate and magnetite. Ultrabasic ophiolite section can be seen along the well-exposed rocks include dykes of gabbro, plagioclasite, slopes of Kyaphar-Agur and Kyaphar rivers. pyroxenite and rodingite. Th e lowermost structural element of the section Finally, the uppermost element of the succession is the Teberda Formation, 1000 m thick, represented are metamorphic rocks, mainly thin bodies of by quartz-chlorite and quartz-chlorite-sericite amphibolite and garnet amphibolites probably schist with interbeds of fi ne-grained metasandstone representing the ‘sole’ contact rocks of the ultrabasic and graphite-bearing chert; crystalline limestones rock. are found in some sections. Silurian radiolarians Khain (1984) interprets the OC as an oceanic Spiromma mindjakensis Sadrislamov was found in the formation belonging to the special Arkhyz zone, chert. Belov & Omel’chenko (1976) and Khain (1984) the southernmost in the Fore Range area. Th is interpreted the Teberda Formation as sedimentary opinion seems somewhat disputable. Predominance cover of the OC. of porphyritic textures in volcanic rocks, presence Th e next Karabeck Formation has a thickness of of siliciclastic sediments among volcanic units up to 500 m and consists of basalts and diabases and and above them seem to indicate rather a overlies tectonically the Teberda Formation. Basalts supersubduction situation and probable provenance are represented by variolites, hyaloclastites, spilites, from southwesternmost part of the Pass subzone of basalt porphyrs etc. Th e role of porphyrs is especially the Main Range as was fi rst suggested by Adamia important. Most of these volcanics are pillow lava, and (1984). the position of the pillows indicates inverted position of the sequence (Belov & Omel’chenko 1976). Th e presence among these volcanic rocks of thin layers Th e Volcanic-sedimentary Supercomplex (VSSC) of metapelite and metaalurolite transformed into Th e VSSC is most widely distributed and economically banded garnet-micaceous schists with clear relicts important (copper-bearing) complex of the Fore of clastic origin is very interesting. Th ese rocks Range. Its exposures are continued from Belaya gradually pass upward to the zone of diabase and River in the west up to Baksan River in the eastern microdiabase sills mixed with some fl ows of basalt. termination of the Fore Range zone. In the western Th ickness of this zone is up to 150–200 m. part these exposures are wide and the structure of the Th e next unit of the OC is gabbro with a thickness VSSC is relatively simple because of gentle dipping of up to 600 m. It consists of banded and taxitic gabbro, strata, whereas in the eastern narrow part of the zone gabbro-diabase and gabbro-amphibolite. Dikes of the structure of the VSSC is represented by a series of gabbro-diabases cut the gabbro; the latter contains narrow deeply inclined sheets.

585 PRE-JURASSIC BASEMENT OF THE CAUCASUS

Two types of the palaeontologically dated, coeval indicating suprasubduction nature of the complex. stratigraphical sequences are recognized within Th is conclusion is supported by fi nding of boninites the VSSC (Belov & Chegodaev 1982; Khain 1984; in the VSSC (Shavishvil 1988). Omel’chenko & Belov 1984), southern Kizilkol, and Th e Andryuk-Tokhana type of sequence diff ers northern one, Andryuk-Tokhana (Figure 20). from the Kizilkol-type one by low volume of volcanic Th e Kizilkol-type Sequence– According to rocks and the presence of the Lower Silurian. Basalts Chegodaev (see Adamia et al. 1987), the lowermost occur there in the lowermost stratigraphical position, (Lower Devonian–Eifelian) part of this sequence namely in the Silurian. Th in interbeds of basic consists exclusively of volcanic rocks which are volcanic rocks have been also observed in the Upper diff erent in diff erent subzones. In the northern Devonian part of the section. Th e rest of the section Kartdzhur subzone basalt and plagioclase porphyrs, is represented mostly by terrigeneous sediments, which alternate with radiolarites, are predominant; chert, conglomerate with pebbles of black chert and liparite, andesite porphyr and their tuff s appear volcanic rocks and by ophiolithoclastic olistostromes. above. First terrigeneous material is known from Th e Upper Llandovery to the Lower Ludlow age Givetian parts of this section. Franian level is of the olistosrome was determined by graptolites. specifi c for dominated south-western part of the Bodies of Lower Carboniferous limestone are found Fore Range called Urup subzone. Th ick (up to 500 in the upper part of the section. Total thickness of m) intraformational conglomerate makes up the this sequence is about 1500 m. essential part of the Franian level. Th e conglomerate According to Khain (1984), the Andryuk-Tokhana has tuff aceous cement and include pebbles of sequence tectonically underlies the Kizilkol-type plagiogranite, plagiogranite-porphyre, granophyres, sequences. and diff erent volcanic rocks. A remarkable feature of Intrusive magmatic rocks play much a subordinate this conglomerate is the absence of clastic material of role in the VSSC. Most of these rocks are represented previously metamorphosed rocks and quartz pebbles. by small stocks and dikes of hypabyssal monzodiorite, Th is means that krystallinikum did not crop out near gabbro-syenite and plagiogranite probably the VSSC basin. Th e conglomerate is replaced with genetically connected with Devonian volcanic rocks tuff aceous sandstone along its strike. Sandstone and (Baranov & Kropachev 1976). Part of them, especially argillite overlie the conglomerate and are in turn granophyric plagiogranite, are abundant within overlain by Famenian limestone; thickness of the the above mentioned tuff aceous intraformational latter is some hundred metres. Argillite and younger conglomerate of the Middle–Upper Devonian. limestone horizons appear above. Th e Tournaisian sediments start with a boulder bed containing quartz clasts and continue with limestone and argillite. Th is Th e Blyb Metamorphic Complex (Krystallinikum) stratigraphical level in the Kartdzurt and Kendellar Deeply metamorphosed rocks of the Fore Range crop subzones has the same composition, the only out widely in the basins of Bol’shaya and Malaya Laba diff erence being the absence of quartz boulder bed and Urushten Rivers in the area of the large Blyb and in the latter. small Beskes salients; smaller exposures are known Khain (1984) noted that the Lower and Middle from canyons of Belaya (Dakh salient), Chegem River Devonian volcanic series of the Kizilkol are (Labardan salient), Sakhray River and also along characterized by both contrasting (basalt-rhyolite) the Daut River. Th e Blyb salient of metamorphic and consecutive diff erentiated types of volcanic rocks is in contact with the Devonian volcanic- activity which developed synchronously and sedimentary sequences along apparently concordant sometime formed areas of mixing. Petrochemical (in structural sence) boundary, but marked by picture of the Devonian volcanism is complex and mylonite surface whereas other metamorphic demonstrates both the calc-alkaline and tholeiitic exposures are separated from the Devonian types of diff erentiation (Khain 1984). Th e content rocks. Close (although not identical) lithological, of TiO2 in basalts almost always is lower than 1% petrological and geochronological characteristics of

586 M.L. SOMIN

Figure 20. Scheme of Middle–Palaeozoic zonality of the Fore Range zone (above) and composition of the subzones (below). Aft er Khain (1986) and Chegodaev (1988).

587 PRE-JURASSIC BASEMENT OF THE CAUCASUS

these salients except the Sakhray one indicate rather (1) presence of idiomorphic (i.e. magmatic) zircon; their belonging to a single metamorphic (super) (2) presence of ultrabasic and metabasic xenoliths complex. Its main features are the following: (1) the within the orthogneisses; marginal parts of these predominance low-K orthogneiss; (2) the presence xenoliths are oft en metasomatically transformed of abundant ultrabasic and metabasic rocks; (3) into phlogopite-bearing rock. Among orthogneisses geochemical evidence for essentially ensimatic or probably both plutonic and volcanic varieties exist; mixed crust-mantle origin of rocks; (4) evidence for very rare relicts of porphyric texture and very limited high-moderate pressure/moderate-temperature type quantity of zircon grains are characteristic features of of metamorphism; (5) presence (in the northern the latter. Naturally, distinguishing these varieties is salients) of diorite, quartz diorite and granodiorite diffi cult in many cases. massifs and very limited appearance of S-type granite In contrast to the orthogneiss, the hornblende- within the same salients. micaschist, garnet-micaschist and kyanite–bearing Th e main Blyb salient occurs in high mountains schist are enriched in rounded detrital zircons. with dense vegetation, deeply dissected by rivers Our limited isotope-geochemical data indicate a and waterfall creeks. Th is makes mapping very hard. mixed mantle-crust origin for the Blyb orthogneiss. Samokhin (1962) noted that the Blyb complex might For example, a metaaplite dike cutting garnet be divided into two units called formations. In fact amphibolites in the Bol’shaya Laba river has the these units are not stratigraphical ones because 87 86 following parameters Sr/ Sr= 0.704777; εNd(T)= they include numerous plutonic, metaplutonic –0,7; 143Nd/144Nd= 0.512473; 147Sm/144Nd = 0.1305. and protrusive bodies. Th erefore term ‘formation’ Metaaplite dike of the Dakh salient has 87Sr/86Sr= is used here informally and for the continuation 143 144 0.704381; εNd(T)= 3.3; Nd/ Nd= 0.512743; of the Caucasian geological tradition. Th e Balkan 147Sm/144Nd= 0.1635. Formation, the lower one, consists mainly of Metabasites of the BLMC (n= 37) petrochemically amphibolites and hornblende plagiogneisses; the correspond to middle- or high-titanium (2.14% essential part of amphibolites is garnet-bearing; TiO ), middle-potassium (K O= 0.82) and low- concordant bodies of banded orthogneiss are noted 2 2 alumina (14.92%) normative nepheline-olivin among these rocks. Th ickness of the Balkan Formation tholeiitic basalts. Th ey diff er from metabasites of is about 2000 m. Th e essential part of this unit is Atsgara nappe (TiO = 2.75, n= 8) and especially intruded by a large body of gneissic tonalite, which 2 from basalts of Kizilkol nappe (TiO = 0.70%, n= 21) together with surrounding rocks forms a core dome- 2 (Popov & Pustovit 2007). like Balkan antiform (see Figure 19). Th e upper unit, Armov, has more complex composition. According Zakariadze (personal communication) concluded to our observations, orthogneisses are its dominant that the geochemical data on BLMC demonstrate part; quartz-garnet micaceous, kyanite-bearing and essential contamination by crustal material and hornblende-garnet-white mica schist, subordinated non-MORB origin of the orthogneisses and some garnet amphibolites, eclogitic amphibolites and diff erence from typical adakite series. Th e present scarce bodies of eclogite alternate within this unit author supposes that from the lithological, petro- forming overturned isoclinal folds with mostly gently and geochemical-data discussed above one might dipping axial planes. Some part of amphibolites has conclude that protoliths of the Blyb complex were porphyroblastic texture and was probably formed at probably formed in a continental rift . the expense of fi ne-grained gabbro. In the northern Petrology and geology of the Balkan salient part of the salient some bodies of deformed ‘Solenov was studied in last 30 years by Tatrishvili, Ploshko, bridge’ plagiogranite are transformed into gneissic Shport, Shengelia, Somin, Korikovski, Perchuk, rocks with blue quartz augens. Th is part of the Blyb Gerasimov and some other authors. Th e main complex section is especially enriched by bodies of result was the establishment of high-pressure type serpentinite. of metamorphism in many rocks of this salient. Th e evidence for the magmatic origin of Concordant eclogite bodies were found in situ in the orthogneisses of the Blyb salient are the following: locality of Red Cliff , Urushten River, and on the left

588 M.L. SOMIN

bank of Bol’shaya Laba River and also as exotic blocks gneisses and micaceous schists. Numerous stratiform among serpentinite mélange on the right side of this bodies of symplectitic bodies of garnet-margarite river valley. Gerasimov & Perchuk (in Shengelia & symplectitic amphibolites appear within this section. Korikovskii 1991) gave a detailed description of the Parameters of metamorphism of these rocks are kyanite eclogite of the Red Cliff locality. Th e eclogite is 620°C at a pressure of 8–9.5 kbar (Korikovski et a banded garnet-omphacite-epidote-kyanite-rutile- al. 2004). Th ese amphibolites are cut by numerous quartz rock, where the content of quartz reaches veins of deformed garnet-bearing metaaplite whose up to 15%. Garnet has clear progressive zonality: geochemical features are close to adakite and whose content of pyrope increases from 13% (centre) up to metamorphism was synchronous with those of 30% (marginal part). Omphacite is slightly zonal; the surrounding rocks. Magmatic zircon extracted from content of jadeite is near 50%. Perchuk (1993, 2003) metaaplite allowed the determination of the age of established that the pressure during the progressive the magmatic protolith (Somin et al. 2007b). evolution of this rock has reached 18 kbar, later Th e Beskes salient crops out in valleys of Bol’shaya decompression took place at pressure of around 9 kbar at T= 600°C. Perchuk (2003) also concluded Laba and Beskes rivers where krystallinikum crop that the progressive metamorphism parameters for out from under the gently dipping Lower Jurassic schist contacting concordantly with eclogite were cover. Th is salient is separated from the Blyb salient almost identical: T= 700±50°C and P= 17.8±0.4 by exposures of Upper Palaeozoic molasse, Devonian kbar. Eclogites from Bol’shaya Laba River are oft en volcanics and serpentinite. Contacts of the Beskes amphibolized but relics of omphacite show similar salient rocks with the two last types of the rocks composition and type of zonality to the Red Cliff are faulted. Acid orthorocks are dominant within eclogite. Besides relict omphacite and garnet, primary the salient krystallinikum; metabasites are quite hornblende, phengite, paragonite and epidote are subordinate and are noted in Beskes alluvium only. typical for this type of eclogite. Th ese minerals are Banded plagioclasic orthogneisses enriched with large partly replaced by secondary hornblende and albite. epidote and clinozoisite crystal and white phengitic A similar type of kyanite eclogite was described by (?) mica predominate among the rocks, like in the Moskalev (1975) in Chegem valley, easternmost Blyb salient; K-feldspar-bearing gneisses are rare. termination of the Fore Range. Fine-grained gneisses of probable volcanic origin Nevertheless it remains unclear whether all the are noted, they are cut by coarse-grained varieties. Blyb salient section consists of typical HP rocks. Gneissic granodiorite and diorite are widespread in However, there are abundant exposures of garnet the Beskes salient. Afanas’ev (1958) gave a particularly amphibolite and mica-garnet-hornblende schist, detailed description of their petrography. He found where white mica is represented by almost pure serpentinite xenoliths inside this intrusive rock and phengite which has inert contacts with hornblende determined their K-Ar phlogopite age as ca. 450 Ma (Gamkrelidze & Shengelia 2005). Composition of in the Moschevaya creek locality. Th e signifi cance garnet and phengite indicates elevated (at least up to of these data we shall discuss later. Gamkrelidze & 8 kbar) pressure (Konilov, personal communication Shengelia (2005) noted the high content of phengite 2009). Recently, Konilov found high-Na omphacite component in white mica of gneissic granodiorite of inclusions within pyrite crystals of several banded the Beskes salient and stressed that is common feature phengite-bearing orthogneiss in Malaya Laba valley of all metamorphosed rocks of the Fore Range. indicating pressure up to 13 kbar. Th e Sakhray salient also seems to belong to the Th e Dakh salient of krystallinikum exposed in northern belt of the krystallinikum salients like the the deep canyons of Belaya and Syuk Rivers consist Beskes and Dakh ones. Th is salient is still poorly mainly of tonalite intruded into ultrabasic rock and studied. But it was noted (Somin 2007a) that its a compositionally variable rock sequence of epidote- lithology is somewhat diff erent. Boulders of garnet- and clinopyroxene-bearing amphibolites, hornblende bearing micaceous schist, quartzite, quartz augen schists, micaceous microgneisses (with relicts of metaporphiric two-feldspar two-mica orthogneiss porphyre textures), two-feldspar biotite-hornblende and amphibolites are observed in Bol’shoy Sakhray

589 PRE-JURASSIC BASEMENT OF THE CAUCASUS

a b

3000

Figure 21. (a) Optical and (b) cathodoluminescence images of zircons from metapsammite of the Blyb complex, sample 0-41, Bol’shaya Laba River. Numbers on (a) are ages in Ma. (c) Histogram of the ages and concordia diagram for zircons with at mean 374±26 Ma.

River alluvium. Triassic basal conglomerate contains By this features the Sakhray krystallinikum diff ers abundant pebbles of serpentinite. K-Ar dating on clearly from others salients of the Fore Range. muscovite and biotite of augen gneiss yielded almost identical ages of 345±4 Ma (K= 8.2%, biotite and Intrusive magmatic rocks placed within fi eld of muscovite 8.3%). 87Sr/86Sr of this orthogneiss is krystallinikum of the Fore Range are represented

0.735299, and εNd(T)= –8.0; 143Nd/44Nd= 0.512091. by tonalite and plagiogranite metamorphosed

590 M.L. SOMIN

and deformed together with the surrounding Another point of view interprets the contact as a rocks. However in limits of the Dakh salient stratigraphic one where BLMC is an old basement unmetamorphosed hornblende-biotite granodiorite and the VSSC is a cover (Mel’nikov 1964; Potapenko crops out and small stock of muscovite granite is 1982). However, basal conglomerate was not found also known. Krasivskaya (1998) found essential in the Devonian sequence. Moreover, detritus of geochemical diff erence between some intrusive metamorphic rocks begin to appear in the VSSC magmatic rocks of the Blyb salient and those from the Lower Carboniferous sequence upwards associated with the Devonian volcanic arc (Kizilkol- only (Somin & Lavrischev 2005; Omel’chenko 2007). type sequence). Unfortunately these data are very A very important fact is the absence of basaltic or limited. rhyolitic dikes, i.e. feeding channels, for Devonian Summing up the available data on petrology unmetamorphosed volcanic rocks within the BLMC and geochemistry of the Fore Range krystallinikum fi eld. All dikes known in the BLMC demonstrate we should note its heterogeneity. Rocks of the Blyb features of middle-temperature metamorphism. salient are less ‘continental’ relative rocks of the Exception is presented by dikes of fresh subvolcanic northern belt, especially those of Dakh and Sakhray dacite in Beskes salient, but its K-Ar (wr) age is salients. Presence of unmetamorphosed granitoids 275 Ma indicating rather to Early Permian and not including S-type granite is main diff erence between Middle Palaeozoic age. Th erefore, evidence for a rocks of the northen belt and Blyb salient. One may primary normal (stratigraphic) relation between the suppose that northern belt presents less subducted or Blyb ‘basement’ and the VSSC ‘cover’ is absent. nonsubducted outer elements of the Palaeozoic rock Many authors (Baranov & Kropachev 1976; prism. Baranov & Grekov 1980; Baranov et al. 1991) proposed that the BLMC is Proterozoic basement separated Relationship Between the Krystallinikum and Middle from the VSSC by gentle thrust or detachment. Th ere Palaeozoic Volcanic-sedimentary Sequences is no doubt for me in existence of a tectonic contact between the two units. Indeed, as Khain (1984) noted, Th is is a key problem of the Fore Range pre-Upper the contact goes through diff erent stratigraphic levels Palaeozoic geology. Till recently there existed of the Devonian sequence. None of magmatic rocks diff erent opinions on this problem. According to of the BLMC, gneissic granitoids and serpentinite oldest interpretation (Afanas’ev 1958; Samokhin including, cut the contact zone. Th erefore the 1962; Ploshko 1965; Chesnokov & Krasivskaya question of the age of the BLMC is very important. 1985) there is a gradual transition from the Blyb krystallinikum to VSSC. Th is opinion seems to be Th e Isotope-geochronological Data on the wrong because of great diff erence in PT parameters Krystallinikum– Previous geochronological data were and lithology of these complexes. Indeed, all rocks of presented mostly by K-Ar determinations (Afanas’ev the BLMC were metamorphosed at 600–550° C and 1958; Andruschuk 1968; Chesnokov & Krasivskaya pressure 8–17 kbar, whereas Devonian volcanic and 1985; Shengelia & Korikovskii 1991; Gamkrelidze & sedimentary rocks suff ered low temperature (i.e. not Shengelia 2005). K-Ar values are disposed between more that 300° C) and low pressure metamorphism; 365 and 300 Ma. Especially numerous (n= 8) none of the HP minerals are found in these rocks. values were obtained on white metamorphic micas ‘Transitional’ zone in the base of the Devonian, where of Solenov Bridge metaplagiogranite (Bol’shaya actinolite, albite and garnet appears, has a thickness Laba River) and Khatsavita creek orthogneisses of no more than 100–150 m, whereas the thickness and surrounding schists. Th ese values are disposed of the underlying crystalline rocks is near 4000 m between 326 and 316 Ma (Krasivskaya, personal and thickness of Devonian section is near 2000 m. communication 2004) at high (8.5–9.6%) content Th e BLMC contains abundant bodies of orthogneiss of K. Another peak of K-Ar values on white mica of and serpentinite, which are absent among the VSSC. Blyb salient rocks clasters near 365 Ma. Micas of two- Finally, low-temperature blastomylonite presents micaceous orthogneiss of the Sakhray salient yielded in crystalline rocks of BLMS at the contact zone. some younger values (see above). Hornblende of

591 PRE-JURASSIC BASEMENT OF THE CAUCASUS

granodiorite from the Dakh massif yielded 300±7 Ma MSWD= 0.058) (Figure 22, Table 12). Data on others (K= 0.5%). Th e question arises on possible infl uence samples are similar and support the conclusion of of argon excess and metasomatic process on argon the middle Palaeozoic age of the metasedimentary isotopic system at elevated pressure. For example, rocks protolith and Late Devonian or younger age of famous age values ~450–455 Ma were obtained on metamorphism. Th e same result but based on much phlogopite developed at phengite-bearing gneissic more numerous (n= 48) determinations with LA- granodiorite and serpentinite contact zone in Beskes ICP-MS method was obtained recently by Natapov salient (Geology оf USSR 1968). At the same time and Belousova in GEMOC Centre, Sydney (Somin et 87Sr/86Sr isochron dating of the same phlogopite gave al. 2011, in press). 341±22 Ma only (Bagdasarian et al. 1987). Moreover, Some Preliminary Summary and Addition– Th e U-Pb (SHRIMP) zircon age of the banded orthogneiss geological and geochronological data indicates that intruded by this granodiorite is 388±10 Ma (Somin et the Blyb krystallinikum of the Fore Range is not al. 2009a). Finally, data by Phillipot et al. (2001) on Proterozoic. Th is is a complex which includes mostly eclogite of Malaya Laba River obtained with Lu-Hf former Middle Palaeozoic magmatic (intrusive) and and Ar-Ar methods indicated that exumation of his sedimentary rocks transformed into metamorphic rocks was at 322 Ma or some time later. rocks. Although data on age of metamorphism are U-Pb SHRIMP dating (by Kröner in Perth, still limited, end of Middle and beginning of the Late Australia and by Rodionov in the Center of Isotopic Palaeozoic seem to be the most likely period. Research, VSEGEI, St.- Petersburg) of magmatic Th is conclusion has important consequences: we zircons extracted from banded orthogneiss of are stating that krystallinikum of the Fore Range and the Bol’shoy Blyb River (the Blyb salient) yielded its slightly metamorphosed and unmetamorphosed ages ranging from 400 Ma (central zones) to 355 Silurian, Devonian and Lower Carboniferos volcanic- Ma (marginal zones, probably metamorphic in sedimentary sequences (‘cover’) are essentially origin). Besides, there are inherited cores with age coeval and hence tectonically juxtaposed. Th is values 500–600 Ma (Somin et al. 2009a). Magmatic means that the BLMC presents ‘pseudobasement’ zircons of gabbro-amphibolite of the same locality of these sequences (Somin et al. 2009b), not their gave 400±10 Ma by TIMS U-Pb method (Somin real basement. Th e Fore Range krystallinikum et al. 2004). Dating of zircon of the Solenov Bridge forms a series of tectonic windows (that is especially metaplagiogranite with the same method yielded evident in case of the Blyb salient); whereas the 323±5 Ma, surprisingly close to K-Ar values of mica Middle Palaeozoic volcanic-sedimentary sequences from the same rock. Zircons of metaaplite cutting together with overlying pre-Upper Palaeozoic symplectite garnet amphibolites in the Dakh salient nappes form a thick composite allochthon (Figure yielded a TIMS U-Pb age of 354±4 Ma (Somin et al. 22). Th e question arises as to where their root zone 2007b). Th ese zircons are euhedral, with oscillatory is. Th ere is some argument that the latter is placed zonality and thus were interpreted as magmatic in the Pass subzone of the Main Range, where the although Th /U ratio is low. root is presumably presented by Laba Metamorphic To determine the age limits of metaclastic rocks Complex, which has evident lithological similarity of the BLMC the SHRIMP method was applied to with the Devonian sequence of the Fore Range. Off their detrital zircons. Th ese grains were selected course, this interpretation suggests extremely high from hornblende-garnet-quartz-phengite schist rapid metamorphic processes in the root zone area. (presumably tuff aceous greуwacke), quartz-garnet- Th e tectonic construction of the western part of mica and garnet-mica-kyanite schist (metapelite the Fore Range zone has been discussed above. Th e and high-alumina metapelite). All these samples new interesting data were received recently on its contain numerous detrital zircon grains. In the eastern part (Somin et al. 2009b). To the north of fi rst sample (0-41) three groups of age values were Elbrus volcano the Fore Range zone is represented detected: (1) 2471–1513 Ma (n= 7), (2) 653–499 Ma by a relatively narrow belt of mostly sedimentary (n= 5), 3) 387–373 Ma (n= 7, cluster 374±2 Ma at Devonian rocks of Artykchat Formation belonging

592 M.L. SOMIN 0.704 350 – re Range zone. Range re basic volcanic intraformational conglomerate limestone metaterrigenous schists 0.704 metagranitoids orthogneiss amphibolite gabbro-amphibolite serpentinite 354 – undeformed and slightly deformed granitoids monzodiorite banded paragneiss and schists Schematic cross-section across Blyb and Beskes salients (along Bolshaya Laba River) and Dakh salient (along Belaya River), Fo River), Belaya Dakh (along and salient Laba Bolshaya River) Beskes (along salients and Blyb across cross-section Schematic Figure 22.

593 PRE-JURASSIC BASEMENT OF THE CAUCASUS

Table 12. U-Pb data and calculated ages for zircons of metapsammitic schist (sample 0-41) of the Blyb Metamorphic Complex.

(1) (1) % ppm ppm ppm №Spot 232Th /238U 206Pb/238U 207Pb/206Pb % Discor dant 206Pbc U Th 206Pb* Age Age 1 O-41.1.1 0.00 279 297 1.10 21.8 561±7.3 519±49 -7 2 O-41.2.1 0.39 228 262 1.19 21.0 653±8.7 624±100 -4 3 O-41.3.1 0.20 132 44 0.35 31.9 1591±19 1620±30 2 4 O-41.4.1 0.58 88 41 0.48 6.6 534±9.7 389±220 -27 5 O-41.5.1 0.74 268 130 0.50 13.8 373±5.4 356±190 -5 6 O-41.6.1 0.00 107 75 0.73 37.7 2224±26 2178±21 -2 7 O-41.7.1 0.46 143 187 1.35 7.3 368±5.9 302±130 -18 8 O-41.8.1 0.00 3009 1032 0.35 208.0 499±5.7 487±15 -3 9 O-41.9.1 0.41 238 300 1.30 12.0 366±5.3 288±160 -21 10 O-41.10.1 0.10 674 132 0.20 35.2 380±4.6 453±73 19 11 O-41.11.1 0.25 216 59 0.28 56.1 1699±20 1680±34 -1 12 O-41.12.1 0.00 205 82 0.41 57.7 1827±20 1800±19 -1 13 O-41.13.1 0.46 309 189 0.63 15.7 368±5 304±100 -18 14 O-41.14.1 0.00 31 30 0.99 7.1 1513±30 1509±100 0 15 O-41.15.1 1.30 51 41 0.83 2.7 375±8.7 143±370 -62 16 O-41.16.1 0.00 432 57 0.14 23.0 387±5.2 370±48 -4 17 O-41.17.1 0.00 394 135 0.35 112.0 1841±20 1808±15 -2 18 O-41.18.1 0.13 128 79 0.64 51.3 2471±28 2484±17 1 18 O-41.19.1 0.30 130 30 0.24 35.4 1775±21 1793±30 1 20 O-41.20.1 0.00 402 143 0.37 17.5 318±4.4 334±54 5 21 O-41.20.2 0.15 890 201 0.23 37.0 304±3.9 275±55 -10 22 O-41.20.3 0.23 345 129 0.39 14.4 306±3.9 347±77 13

Error in Standard calibration was 0.52% (not included in above errors but required when comparing data from diff erent mounts). (1) Common Pb corrected using measured 204Pb

Total Total (1) (1) (1) (1) № 238 206 ±% 207 206 ±% ±% ±% ±% ±% U/ Pb Pb/ Pb 238U/206Pb* 207Pb*/206Pb* 207Pb*/235U 206Pb*/238U

1 11 1.3 0.0576 2.2 11 1.3 0.0577 2.2 0.724 2.6 0.0909 1.3 2 9.34 1.4 0.0638 2 9.38 1.4 0.0606 4.7 0.89 4.9 0.1066 1.4 3 3.564 1.4 0.1015 1.4 3.571 1.4 0.0998 1.6 3.852 2.1 0.2799 1.4 4 11.52 1.8 0.0592 4.2 11.58 1.9 0.0544 9.8 0.648 10 0.0863 1.9 5 16.66 1.4 0.0596 2.5 16.78 1.5 0.0536 8.4 0.441 8.6 0.05957 1.5 6 2.432 1.4 0.1348 1.1 2.428 1.4 0.1361 1.2 7.73 1.8 0.412 1.4 7 16.94 1.6 0.0561 3.6 17.02 1.6 0.0524 5.9 0.424 6.1 0.05874 1.6 8 12.42 1.2 0.05681 0.65 12.42 1.2 0.05687 0.66 0.6312 1.4 0.0805 1.2 9 17.04 1.4 0.0554 2.8 17.11 1.5 0.0521 7 0.42 7.2 0.05844 1.5 10 16.47 1.3 0.0568 2.9 16.48 1.3 0.056 3.3 0.469 3.5 0.06067 1.3 11 3.306 1.3 0.1052 1.5 3.315 1.3 0.1031 1.8 4.285 2.3 0.3016 1.3 12 3.055 1.3 0.1094 0.97 3.052 1.3 0.11 1 4.971 1.6 0.3277 1.3 13 16.93 1.4 0.0561 2.5 17.01 1.4 0.0524 4.4 0.425 4.7 0.05879 1.4 14 3.812 2.1 0.0876 3.1 3.784 2.2 0.094 5.3 3.43 5.8 0.2646 2.2 15 16.47 2.2 0.0594 6 16.68 2.4 0.0489 16 0.404 16 0.0599 2.4 16 16.15 1.4 0.054 2.1 16.15 1.4 0.054 2.1 0.461 2.5 0.06192 1.4 17 3.027 1.2 0.10995 0.75 3.025 1.2 0.11052 0.8 5.038 1.5 0.3306 1.2 18 2.137 1.3 0.1638 0.88 2.14 1.3 0.1627 1 10.48 1.7 0.4671 1.3 18 3.144 1.4 0.1122 1.2 3.153 1.4 0.1096 1.6 4.79 2.1 0.3169 1.4 20 19.75 1.4 0.053 2.4 19.75 1.4 0.0531 2.4 0.371 2.8 0.05064 1.4 21 20.65 1.3 0.05299 1.7 20.68 1.3 0.0518 2.4 0.3451 2.7 0.04836 1.3 22 20.55 1.3 0.0552 2.7 20.59 1.3 0.0534 3.4 0.358 3.7 0.04856 1.3

594 M.L. SOMIN

to the Andryuk-Tokhana subzone. Th ese rocks in are exposed in narrow erosional windows along the riverhead of Chuchkur are intruded by small Malka, Kuban’, Khudes, Khasaut and other rivers Pleistocene Chuchkur explosive center expressed by valleys and also in the form of a belt along Pshekish- ignimbrite fl ows. Th e ignimbrite includes numerous Tyrnyauz suture zone between Baksan and Teberda xenoliths, among which both unmetamorphosed Rivers (Figures 23 & 24). Devonian slate and volcanics and crystalline rocks are noted. Crystalline rocks are typical low-pressure andalusite and sillimanite-bearing schists and two- Sedimentary Cover of the Bechasyn Metamorphic feldspar granite. Th ese types of rocks are characteristic Complex of the Elbrus subzone of the Main Range zone Th e age of the pre-Upper Palaeozoic complexes and are absent in the Fore Range krystallinikum. of the Bechasyn zone is the main problem of this As it was shown in the previous chapters, the age zone and of the Greater Caucasus as a whole. Until of metamorphism of these schists and the age of recently there was an almost unanimous opinion that granite are almost equal and correspond to the BCMC is a Mеsoproterozoic complex (or Riphean in Late Palaeozoic. Th us, these rocks are younger than Russian terminology) covered by Ediacarian–Middle Devonian sediments and therefore the latter are in Cambrian (Potapenko 1982) or Ordovician–Silurian completely allochthonous position in relation to (Chegodaev 1988; Obut et al. 1988) sediments. the granitic-metamorphic pseudobasement. We Th e cover is represented by a thick (up to 1300 m) may conclude that pseudobasement typical for the Urlesh Formation of reddish aleuropsammite, which western part of the Fore Range is absent in its eastern begins with a basal conglomerate containing pebbles part. Direct fi eld observations confi rm the conclusion of underlying greenschists and quartz. Th is contact on the tectonic relation between complexes of Fore is well observed in the fi eld. Th e Urlesh Formation Range and Main range zones. Indeed, Devonian has steep inclination to the north. Shales, limestones volcanic rocks contact directly with the Main Range and cherts of palaeontologically dated Silurian (the granite along Pshekish-Tyrnyauz suture zone in the Llandovery and younger) and Lower Devonian Baksan River basin area and this contact is exclusively overlies these sediments without any structural tectonic everywhere and expressed by a wide belt of unconformity (Potapenko 1982). Within the shaly mylonite and blastomylonite. Above mentioned 305 Silurian there is a thin horizon of intraformational Ma blastomylonitic augen gneiss of Bol’shoy Mukulan conglomerate containing pebbles of limestone creek is an example of the Upper Palaeozoic granite and sandstone. Besides , a large block of limestone transformation within this belt. Th erefore we should with well-preserved trilobites of the upper Middle conclude that the position of the Fore Range pre- Cambrian was described in this locality long time Upper Palaeozoic volcanic-sedimentary complexes is ago (Potapenko & Momot 1965). Potapenko (1982) allochthonous everywhere. noted that pebbles of sandstone which are presented within conglomerate are similar to those of the uppermost part of the Urlesh Formation. Hence Th e Bechasyn Zone Potapenko supposed that the Urlesh Formation Th is zone is the northernmost element of the Greater probably included sediments of the Middle Cambrian Caucasus mountain system. From tectonic point of age. Th is supposition means that between the top of view this is the exposure of the Scythian platform the Urlesh Formation and Silurian there is almost 80 basement. Mesozoic cover is here in a subhorizontal Ma hiatus corresponding to upper Middle and Upper position. Upper Palaeozoic (Middle and Upper Cambrian, Ordovician and lowermost Silurian. Carboniferous and Lower Permian) molassic However no traces of unconformity or erosion are sequences are presented by small gently dipping observed in contact zone. In the left bank of the rests of the transgressive cover. Older parts of pre- Malka river contact between the Urlesh Formation Jurassic stratigraphic column are represented by two and the Silurian crops out well and it is completely main complexes, namely the Bechasyn Metamorphic concordant (Figure 25). Th e units are separated by Complex (BCMC) and its sedimentary cover. Th ey layer of shale up to 3 m thick, containing, according

595 PRE-JURASSIC BASEMENT OF THE CAUCASUS

Main tectonic zones of Upper Palaeozoic age:

hyperbasite formations of the Chegem group (Tallykol, Upper Palaeozoic granite Shaukol, Tuballykulak, Indysh, Tashlykol) formations of the Khasaut group main faults (Shizhatmas, Malka, Musht) Urlesh formation nappe

Figure 23. Tectonic scheme of Karachaev-Cherkessian horst-anticlinorium (aft er Potapenko 1982 with some changes) and cross- section along Mt Elbrus-Kislovodsk City line. to Chegodaev, graptolites of Upper Wenlock to Lower demonstrate well-preserved psammitic texture. Ludlow age (Potapenko 1982). Chlorite and fi nest sericite are developed in the It is possible to suppose that Middle Cambrian cement. Th e same metamorphic minerals are limestone was probably a component of the Khasaut present in the Urlesh Formation sandstone. Th e group. Indeed, the Shidzhadmas Formation, the trilobite-bearing limestone was probably a reef-like uppermost unit of this group, is very weakly locally developed cap. In such case the age of the metamorphosed: its (meta) sandstone and tuff Urlesh Formation could be mostly Ordovician like

596 M.L. SOMIN

Palaeozoic of the Fore Range zone

porphyroblastic

main faults and strike-slipe faults

nappe

supposed wedge of Neoproterozoic rocks bedding and foliation

Upper Palaeozoic granite probe for zircon dating

Figure 24. Geological scheme of the southern part of the Bechasyn zone without Mesozoic cover. Aft er Potapenko (2004) with changes. lithologically similar Ordovician sequence of the İstanbul tectonic block in NW Turkey (e.g., Okay et al. 2006). Th e Ordovician of this block is represented by sandstone, redbeds, passing upward to Silurian shale and limestone and then to thick limestone of Devonian and Lower Carboniferous. Th is thick sequences overlies unconformably a Neoproterozoic– Cadomian metamorphosed basement intruded by Pan-African (590–560 Ma) granitoids. First signs of such a basement under the Bechasyn metamorphic Figure 25. Contact of the Urlesh Formation sandstone and complex was found recently in the Bechasyn zone the Upper Silurian limestone at the left bank of the Malka River (aft er Potapenko 1982). (see below).

597 PRE-JURASSIC BASEMENT OF THE CAUCASUS

SHRIMP dating of detrital perfectly rounded includes intraformational conglomerate containing zircon grains (n= 10) selected from the Urlesh boulders of granophyre. Formation aleurolite gave new information on the Th e uppermost position in the pre-Upper age problem (Figure 26, sample UR-1, Table 13). Palaeozoic structure of the Bechasyn zone occupies Age values of 657–507 Ma (average 528±6, i.e. Lower a thick (up to 1000 m) allochtonous body of Cambrian ) were determined (Somin & Potapenko serpentinite, which lies above the Silurian sediments. 2008). Th e presence of zircon grains dated as Middle Th e main part of the BCMC is intruded by a great Cambrian indicates a post-Middle Cambrian age of plate-like body of Upper Palaeozoic (ca. 305 Ma) the protoliths. Th us, Urlesh Formation formation granite. In some localities it intrudes the Middle is defi nitely post-Ediacarian and younger than the Carboniferous molasse and is overlain by Lower Lower Cambrian. Its age should be uppermost Permian molasse. Middle Cambrian–Upper Cambrian–Ordovician. Potapenko (Shengelia & Korikovskii 1991) Th e Krystallinikum (Bechasyn Metamorphic considers the BCMC a single entity, where all units Complex, or BCMC)– Lithologically the BCMC is very have normal (stratigraphic) relations. According to diverse and that permits to separate it into numerous this author, the following ascending stratigraphic subunits (Potapenko 1982; Snezhko 2005). At the order may be proposed for Khasaut basin in the same time the stratigraphical relations between these area of the northern part of the Beshasyn zone: units remain disputable because of the complexity of the structure. Th e two main stratigraphic groups are Musht Formation, Malka Formation, Shidzhatmas traditionally recognized, the Chegem and Khasaut Formation. To the west, in Kuban’ and Baksan rivers (Potapenko 1982). Th e Chegem group is represented basins, the Indysh, Tashlykol and Tuballykulak by Tallykol and Shaukol metaterrigenous formations. formations correspond to the Musht and Malka Th e Tallykol Formation consists of pure white formations. quartzite, which contains perfectly rounded zircon. Th e BCMC is characterized mostly by greenschist Th e Shaukol Formation of albite porphyroblastic temperature level of metamorphism increasing (metasomatic) schists is widely distributed element from the north to the south from chlorite and of this group. It crops out well along the Kuban’ prehnite-pumpellyite up to biotite-garnet subfacies. and Baksan rivers. Th e others formations of the Shengelia (Shengelia & Korikovskii 1991) indicated BCMC belong to the Khasaut group. Among them that P-conditions of the complex were diff erent and the Tashlykol and Tuballykulak formations have changed from very low (1.5–3.5 kbar) to low (4.5–5 subarkosic compositions: plagioclase, quartz and kbar) up to moderate (6.5–7 kbar) values. Garnet- microcline are the main detrital components of glaucophane schists are known in the left slope the metasandstones of these units. Graphite is of Baksan River valley indicating high pressures. widespread in the Tuballykulak Formation. Th e Recently, Gvelesiani (2007) claimed that whole the plate-like body of metamorphosed microgranite- BCMC was metamorphosed at high-temperature porphyre (or subvolcanic rhyolite-porphyre) up (400–500° C) glaucophane-schist facies at pressures to100 m thick is known near Elbrus mine in contact of 7–14 kbar. Th is information needs confi rmation. with Tuballykulak Formation. Th e Khasaut group is Structure of the BCMC is very complex and dominated by volcanogenic and mixed sedimentary- variable; large overturned folds and some nappes and volcanogenic formations. For example, quartz thrusts are expected to exist (Baranov & Kropachev (meta)porphyre is present in the lowermost Musht Formation together with biotite schists. Metabasite 1976; Potapenko 1982). Northern vergence is and quartz metaporphyre of the Indysh Formation characteristic for this complex. crops out in higher structural position, and the Th e apparent Proterozoic age of the BCMC, uppermost thick Shidzhatmas Formation consists up to recently widely accepted, was based on the of tuff aceous (?) greywacke. Th in layers of limestone idea of Ediacarian–Early Cambrian age of the basal appear in upper phyllite part of mostly volcanic Urlesh Formation, on palaeontological evidence, (of mixed composition) Malka Formation, which and on some isotope-geochronological data. Th e

598 M.L. SOMIN

a b

c d

e f

Figure 26. (a, c) optical (transmitted light) and (b, d) cathodoluminescence images of zircons grains of the Urlesh (UR-1) and Tuballykylak (495) formation. (e, f) Histograms and diagrams with concordia for these zircons, numbers at grains ‘c’ are ages in Ma.

599 PRE-JURASSIC BASEMENT OF THE CAUCASUS

Table 13. U-Pb data and calculated ages for zircons of sandstone (sample UR-1) of the Urlesh Formation.

232Th (1) 206Pb (1) 207Pb % Total 238U № Spot % 206Pbc ppm U ppm Th ppm206Pb* ±% /238U /238U Age /206Pb Age Discordant /206Pb

1 UR-1.1.1 0.34 180 83 0.48 12.9 515.7±4 515±76 0 11.966 0.81 2 UR-1.2.1 0.25 88 80 0.93 8.18 657.6±6.3 655±61 0 9.288 1 3 UR-1.3.1 0.23 166 62 0.38 12.2 526.1± 4.6 486 ±110 -8 11.733 0.91 4 UR-1.4.1 0.09 162 45 0.28 12.2 539.8±4.4 535±38 -1 11.438 0.85 5 UR-1.5.1 - 306 195 0.66 24.6 577.2±4.1 601±40 4 10.695 0.73 6 UR-1.6.1 - 176 51 0.30 12.8 524.7±4.3 525±52 0 11.81 0.85 7 UR-1.7.1 - 142 50 0.36 10.6 536.6±4.7 520 ±120 -3 11.539 0.91 8 UR-1.8.1 0.04 250 88 0.36 18.9 542.1±5.9 531±51 -2 11.39 1.1 9 UR-1.9.1 0.01 372 40 0.11 26.2 507.7±9.6 494±21 -3 12.2 2 10 UR-1.10.1 - 63 26 0.42 4.66 534 ±11 538±49 1 11.58 2.1

Errors are 1-sigma; Pbc and Pb* indicate the common and radiogenic portions, respectively. Error in Standard calibration was 0.45%. (1) Common Pb corrected using measured 204Pb.

(1) (1) Total 207Pb (1) (1) error № ±% 238U/ ±% 207Pb* ±% ±% ±% /206Pb 207Pb*/235U 206Pb*/238U correction 206Pb* /206Pb*

1 0.06036 1.3 12.007 0.81 0.0576 3.5 0.661 3.5 0.08328 0.81 0.229 2 0.0635 1.6 9.311 1 0.0614 2.8 0.91 3 0.1074 1 0.337 3 0.05872 1.4 11.76 0.91 0.0569 5 0.667 5.1 0.08504 0.91 0.179 4 0.0589 1.3 11.448 0.85 0.0581 1.7 0.7 1.9 0.08735 0.85 0.437 5 0.05849 0.94 10.676 0.73 0.0599 1.8 0.774 2 0.09366 0.73 0.369 6 0.05678 1.3 11.79 0.85 0.0579 2.4 0.677 2.5 0.08479 0.85 0.339 7 0.05646 1.5 11.52 0.91 0.0577 5.5 0.691 5.6 0.0868 0.91 0.161 8 0.05832 1.1 11.4 1.1 0.058 2.3 0.702 2.6 0.08773 1.1 0.437 9 0.05713 0.96 12.2 2 0.05707 0.97 0.645 2.2 0.0819 2 0.898 10 0.058 2.2 11.57 2.1 0.0582 2.2 0.694 3.1 0.0864 2.1 0.683

fi rst argument was already discussed. Information protolith of these rocks should be younger that these by Snezhko (2005) on the presence of Lower– values, i.e. not older that Tonian. Middle Riphean organic problematic remnants in SHRIMP dating was applied for the fi rst time the graphitic metasandstone of the Tuballykulak to determine protolith age of the BCMC (Somin & Formation was critically analyzed by Prof. Potapenko 2008). Small euhedral zircons (n= 10) of Semikhatov. He concluded that these determinations metarhyolite (or metamicrogranite) porphyre, sample cannot serve as the basis for the age of the Bechasyn K1-06, yielded 530.2±8.2 Ma, i.e. Lower Cambrian complex. Particularly, the fi rst of the determined (Figure 27, Table 14). Well rounded zircon grains forms is, in fact, spores of a modern mushroom. (n= 30) from metasandstone of the Tuballykulak

Besides, TNd(DM) model ages of the Bechasyn rocks, Formation, sample 495, yielded an age range of including orthogneiss, were determined as ca. 787, 573–509 Ma (average 534±5Ma) (Figure 26, sample 831 and 877 Ma (Semkin et al. 1997). Th is means that 495, Table 15). Age range of perfectly rounded zircon

600 M.L. SOMIN

a b

Figure 27. Cathodoluminescence images (a) and diagram with concordia (b) for zircons of sample K1-06, metarhyolite-porphyre, Bechasyn Complex, Kuban’ River.

grains (n= 10) of the Tallykol Formation quartzite, Th e presence of great quantity of the uppermost sample P-81 (Chegem River) is 560–444 Ma, where Neoproterozoic to Cambrian (i.e. Pan-African or the average value for 9 grains is 524 Ma. Only one Cadomian) zircons within mostly immature clastic grain showed a pre-Neoproterozoic age (1144 Ma) sediments permits to suggest the existence of this (Figure 28, Table 16). Mean of Th /U>0.5 for the all type of basement under or close to Bechasynian studied grains and a low- grade metamorphism of sediments. Supposed tectonic wedge of this age the samples shows that the obtained values refl ect group crops out in lower part of Khudes River valley. the age of the initial magmatic grains. Even if we It is in unclear relationships with the Tashlykol reject the youngest (Ordovician) ages, taking into Formation paragneisses and is represented by biotite account the presence of numerous grains with the gneisses containing strongly deformed and partly Cambrian and uppermost Ediacarian age values, we mylonitizated plagiogranite dykes. Th e outcrop should conclude that the age of the studied protoliths looks like diff erent types of migmatite. Zircons (n= is Lower Palaeozoic. 39) extracted from such a plagiogranite are euhedral Th e age of BCMC metamorphism according to magmatic and yielded a LA-ICP-MS age of 594±4Ma geological data is at least pre-Silurian. However, no (Natapov et al. in preparation). such K-Ar values were received as yet. Th e age values Finishing this chapter, I need to stress that only are disposed between 370 and 350 Ma (Shengelia & part of the BCMC was dated. Its essential part, for Korikovskii 1991). In fact real K-Ar ages are even example the Malka, Indysh, Shidzhatmaz formations younger because the old constants were used for are not examined with U-Pb methods. Ar/Ar calculations of these values. Isochron Rb-Sr age for dating is desirable for determination of the age of four micaschist and gneiss probes of the Baksan metamorphism. river section is 345±8 Ma, a value close to average K-Ar age of these rocks 349±7 Ma (Bagdasarian et al. 1984). All these data indicate the infl uence of Conclusion Variscan tectono-thermal event on the Bechasyn Th e view of the predominant Proterozoic age of the Complex. Manifestation of this event seems real if Greater Caucasus metamorphic complexes, widely we take into account the deformation of the Silurian distributed till recently, is incompatible with the new and Devonian deposits in this zone and presence of data presented in this paper. Th e majority of these voluminous Upper Palaeozoic granitic bodies. complexes have Palaeozoic protolith ages – Lower

601 PRE-JURASSIC BASEMENT OF THE CAUCASUS

Table 14. U-Pb data and calculated ages for zircons of metaporphyre (sample K 1-06) of Bechasyn Metamorphic Complex.

(1) ppm ppm ppm (1) % Total №Spot% 206Pbc 232Th /238U 207Pb/206Pb ±% U Th 206Pb* 206Pb/238U Age Discordant 238U/206Pb Age

1 K1-06.1.1 - 137 67 0.51 9.62 508 ±13 693 ±110 36 12.23 2.6 2 K1-06.2.1 0.72 178 98 0.57 12.7 509 ±13 350 ±120 -31 12.07 2.5 3 K1-06.3.1 0.59 196 100 0.53 15.2 554 ±13 360 ±110 -35 11.08 2.5 4 K1-06.4.1 0.86 224 156 0.72 17.4 552 ±13 547 ± 99 -1 11.09 2.5 5 K1-06.5.1 0.39 211 219 1.08 15.4 525 ±13 420 ± 81 -20 11.73 2.5 6 K1-06.6.1 0.48 176 131 0.77 13 532 ±13 555 ±120 4 11.57 2.5 7 K1-06.7.1 1.06 129 133 1.06 9.64 533 ±14 311 ±170 -42 11.48 2.6 8 K1-06.8.1 0.21 137 71 0.53 10.4 545 ±13 580 ± 72 7 11.32 2.5 9 K1-06.9.1 - 145 125 0.89 10.8 539 ±13 572 ± 57 6 11.47 2.5 10 K1-06.10.1 0.64 214 210 1.01 15.6 523 ±13 420 ±120 -20 11.77 2.5

Errors are 1-sigma; Pbc and Pb* indicate the common and radiogenic portions, respectively. Error in Standard calibration was 0.85% (not included in above errors but required when comparing data from diff erent mounts). (1) Common Pb corrected using measured 204Pb.

Total (1) (1) (1) (1) error № 207Pb ±% ±% 207Pb* ±% ±% ±% 238U/206Pb* 207Pb*/235U 206Pb*/238U correctio /206Pb /206Pb* 1 0.0602 3.8 12.2 2.7 0.0626 5.2 0.707 5.8 0.082 2.7 0.457 2 0.0593 2.4 12.16 2.6 0.0535 5.4 0.606 5.9 0.0822 2.6 0.430 3 0.0585 2 11.15 2.5 0.0537 4.9 0.665 5.5 0.0897 2.5 0.458 4 0.0655 1.9 11.19 2.5 0.0585 4.5 0.72 5.2 0.0893 2.5 0.489 5 0.0583 2 11.78 2.5 0.0552 3.6 0.646 4.4 0.0849 2.5 0.568 6 0.0626 2.2 11.63 2.5 0.0587 5.6 0.695 6.2 0.086 2.5 0.414 7 0.0611 3.7 11.6 2.7 0.0526 7.6 0.625 8 0.0862 2.7 0.330 8 0.0611 2.4 11.34 2.5 0.0594 3.3 0.721 4.2 0.0881 2.5 0.611 9 0.0579 2.4 11.46 2.5 0.0591 2.6 0.712 3.7 0.0873 2.5 0.696 10 0.0604 1.9 11.84 2.5 0.0552 5.2 0.642 5.8 0.0844 2.5 0.437 and Middle Palaeozoic for the most part of the rock complexes strikingly diff erent in their lithology, Main Range, Middle Palaeozoic for the Fore Range origin, structural features, type and parameters of zone, and partly Lower Palaeozoic for the Bechasyn metamorphism, type of associated granitoids. Indeed, zone. However some exposures of crystalline rocks Devonian of the Svanetian domain is presented (for example the Khudes river exposure) might be by slightly metamorphosed sedimentary rocks. attributed to the uppermost Neoproterozoic. Coeval, but deeply metamorphosed and intruded Pre-Jurassic basement of the mountainous part by granitoids the Laba and Buulgen Complexes of the Greater Caucasus has collage-type structure. It have another, volcanic-sedimentary protolith’s consists of tectonically assembled essentially coeval composition. Th ese two latter complexes demonstrate

602 M.L. SOMIN

Table 15. U-Pb data and calculated ages for zircons of metapsammite (sample 495) of the Bechasyn Metamorphic Complex.

(1) (1) % ppm ppm ppm №Spot 232Th /238U 206Pb/238U 207Pb/206Pb % Discordant 206P bc U Th 206Pb* Age Age 1 495.1.1 0.72 130 70 0.56 10 550±12 548±140 0 2 495.2.1 0.77 94 31 0.35 7.06 538±14 436±180 -19 3 495.3.1 0.14 105 74 0.73 8.4 573±14 553±93 -3 4 495.4.1 0.22 272 101 0.38 19.8 525±11 492±76 -6 5 495.5.1 0.18 246 221 0.93 17.4 511±11 522±67 2 6 495.6.1 0.00 56 21 0.40 4.2 543±14 439±120 -19 7 495.7.1 0.01 334 107 0.33 25 538±11 579±45 8 8 495.8.1 0.29 414 301 0.75 29.3 509±10 573±66 13 9 495.9.1 0.00 188 107 0.59 14.2 546±12 533±67 -2 10 495.10.1 0.84 280 61 0.23 20.3 519±11 502±140 -3 11 495-4.1.1 0.18 172 94 0.56 13.1 546±12 469±91 -14 12 495-4.2.1 0.91 296 212 0.74 21.6 521±11 492±130 -6 13 495-4.3.1 0.00 655 410 0.65 49 538±11 516±36 -4 14 495-4.4.1 0.64 208 71 0.35 15.2 524±12 406±220 -23 15 495-4.5.1 0.61 117 54 0.47 8.86 539±13 432±140 -20 16 495-4.6.1 0.00 61 29 0.48 4.66 548±14 544±110 -1 17 495-4.7.1 0.28 154 65 0.44 11.6 540±12 521±88 -4 18 495-4.8.1 0.75 188 31 0.17 14.8 563±12 468±160 -17 19 495-4.9.1 0.13 143 53 0.38 11 551±12 523±83 -5 20 495-4.10.1 0.30 436 326 0.77 31.3 516±10 566±64 10

Errors are 1-sigma; Pbc and Pb* indicate the common and radiogenic portions, respectively. Error in Standard calibration was 0.79%. (1) Common Pb corrected using measured 204Pb.

(1) (1) (1) (1) error № ±% ±% ±% ±% 238U/206Pb* 207Pb*/206Pb* 207Pb*/235U 206Pb*/238U correction 1 11.23 2.3 0.0585 6.2 0.718 6.7 0.0890 2.3 0.352 2 11.49 2.6 0.0556 7.9 0.667 8.3 0.0870 2.6 0.315 3 10.76 2.5 0.0586 4.3 0.751 5.0 0.0929 2.5 0.511 4 11.78 2.2 0.0570 3.4 0.667 4.1 0.0849 2.2 0.530 5 12.13 2.2 0.0578 3.1 0.657 3.8 0.0824 2.2 0.580 6 11.37 2.7 0.0557 5.3 0.675 6.0 0.0879 2.7 0.452 7 11.49 2.1 0.0593 2.1 0.712 3.0 0.0871 2.1 0.711 8 12.18 2.1 0.0592 3.0 0.670 3.7 0.0821 2.1 0.566 9 11.32 2.2 0.0581 3.0 0.708 3.8 0.0883 2.2 0.591 10 11.92 2.2 0.0573 6.6 0.662 6.9 0.0838 2.2 0.315 11 11.31 2.3 0.0564 4.1 0.688 4.7 0.0884 2.3 0.480 12 11.87 2.2 0.0570 6.0 0.662 6.4 0.0842 2.2 0.343 13 11.48 2.0 0.05763 1.6 0.692 2.6 0.0871 2.0 0.781 14 11.79 2.3 0.0549 9.7 0.641 10.0 0.0848 2.3 0.229 15 11.46 2.5 0.0555 6.5 0.667 7.0 0.0872 2.5 0.359 16 11.26 2.7 0.0584 5.2 0.715 5.9 0.0888 2.7 0.460 17 11.45 2.3 0.0578 4.0 0.696 4.6 0.0873 2.3 0.496 18 10.95 2.3 0.0564 7.1 0.710 7.5 0.0913 2.3 0.306 19 11.21 2.3 0.0578 3.8 0.711 4.4 0.0892 2.3 0.518 20 12.00 2.1 0.0590 2.9 0.677 3.6 0.0833 2.1 0.584

603 PRE-JURASSIC BASEMENT OF THE CAUCASUS

a b

Figure 28. Optical (a) and cathodoluminescence (b) images of zircons from metapsammite of the Bechasyn complex, sample P-81, Tallykol Formation, Chegem River. Histogram (c) and concordiagram (d) of the age data. Numbers at grains ‘b’ are ages in Ma. signs of ensimatic or mixed ensialic-ensimatic origin, which strongly distinguishes it from the adjacent LP/ whereas the next pair of complexes to the north, the HT complexes of the Main Range. Baric type of the Makera and Gondaray, is evidently ensialic. Th e Blyb Bechasyn complex needs further study. metamorphic complex includes abundant Devonian Limits between Palaeozoic zones of the Greater metamagmatic rocks of generally ensimatic, partly Caucasus are represented by Palaeozoic steep faults, mixed origin; overlying Kizilkol complex, also Devonian essentially, is of volcanic island-arc origin. probably of strike-slip nature. Th e presence of nappes On the contrary, Devonian (and Silurian) sequences of presence in the upper part of the Fore Range zone is the Bechasyn zone are completely devoid of volcanic confi rmed. At the same time the author’s new data material. I-type granitoids are distributed in the indicate the allochthonous position of all Middle Laba, Buulgen and Blyb complexes whereas S-granite Palaeozoic volcanic-sedimentary sequences of this type is characteristic of Makera and Gondaray ones. zone and their probable transport from the Pass Th e Blyb Complex is mostly of high-pressure type, subzone of the Main Range.

604 M.L. SOMIN

Table 16. U-Pb data and calculated ages for zircons of quartzite (sample P-81) of the Bechasyn Metamorphic Complex.

(1) (1) ppm ppm ppm Total №Spot% 206Pbc 232Th /238U 206Pb/238U 207Pb/206Pb % Discordant ±% U Th 206Pb* U/ Pb Age Age 238 206

1 P-81.1.1 0.13 127 63 0.51 8.23 467.1±4.5 485±73 4 13.29 0.97 2 P-81.2.1 0.16 260 304 1.21 17.9 496±20 530±82 7 12.48 4.2 3 P-81.3.1 - 730 460 0.65 54.4 537±20 499±39 -7 11.53 3.9 4 P-81.4.1 0.20 212 150 0.73 15.3 517.4±4.2 475±67 -8 11.942 0.83 5 P-81.5.1 0.02 251 145 0.60 19.4 556.5±4.1 538±27 -3 11.089 0.77 6 P-81.6.1 - 277 242 0.90 21.8 563.9±3.9 569±40 1 10.948 0.71 7 P-81.7.1 0.16 166 88 0.54 10.1 441.3±4.3 413 ±130 -6 14.09 0.94 8 P-81.8.1 0.15 285 70 0.25 47.5 1141.3±7.6 1159±19 2 5.155 0.73 9 P-81.9.1 0.03 286 134 0.48 19.7 496.9±8.5 497±32 0 12.48 1.8 10 P-81.10.1 0.21 134 97 0.75 9.41 506.8±5.2 441 ±140 -13 12.2 0.98

Errors are 1-sigma; Pbc and Pb* indicate the common and radiogenic portions, respectively. Error in Standard calibration was 0.45%. (1) Common Pb corrected using measured 204Pb.

Total (1) (1) (1) Error № ±% ±% (1) 207Pb*/206Pb* ±% ±% ±% 207Pb/206Pb 238U/ 206Pb* 207Pb*/235U 206Pb*/238U correction

1 0.0579 1.8 13.31 0.99 0.0568 3.3 0.589 3.5 0.07515 0.99 0.286 2 0.0593 2.9 12.50 4.2 0.058 3.7 0.64 5.6 0.08 4.2 0.746 3 0.0558 0.89 11.51 3.9 0.0572 1.8 0.685 4.2 0.0869 3.9 0.910 4 0.05824 1.3 11.97 0.85 0.0566 3 0.652 3.1 0.08357 0.85 0.271 5 0.05837 1.1 11.091 0.77 0.05821 1.2 0.724 1.5 0.09016 0.77 0.532 6 0.05836 1.5 10.939 0.72 0.0591 1.9 0.744 2 0.09142 0.72 0.360 7 0.0563 2.3 14.11 1 0.055 5.8 0.537 5.9 0.07085 1 0.171 8 0.07972 0.65 5.163 0.73 0.07847 0.96 2.096 1.2 0.1937 0.73 0.604 9 0.05738 1.2 12.48 1.8 0.05714 1.5 0.631 2.3 0.0801 1.8 0.772 10 0.0574 1.8 12.23 1.1 0.0557 6.4 0.628 6.5 0.08179 1.1 0.165

New geochronological data on the Main Range Bechasynian metasandstones contain almost zone indicate a temporal-genetic connection between exclusively Cadomian-aged zircon grains; small regional metamorphism, migmatization and granite addition of the Grenvillean grains is noted there. Th e emplacement. important role of Cadomian grains is seen in all Fore Age values of detrital zircons from the Bechasyn Range and Main Range metamorphic terranes. At the complex is characterized by the complete absence same time numerous Paleoproterozoic detrital grains of Archean and Paleoproterozoic material, whereas are noted in them. Th is means that clastic material the nearest salients of the East European platform was transported both from the north (modern basement, Rostov and Voronezh, are dominated coordinates), where the Cadomian basement was by rocks of this age. Th us some barrier existed uplift ed, and from the south where the fragments of north of Bechasyn zone in Early Palaeozoic time. Northern Gondwana were located.

605 PRE-JURASSIC BASEMENT OF THE CAUCASUS

Bimodal distribution of the detrital zircon age is much more complex that it was supposed some values and the presence of a gap or histogramm years before. For instance, we should speculate on depression in area of ‘Grenvillean’ age zone is the existence of two subduction zone: the main, characteristic feature of metamorphic terranes of northern one, represented by the Blyb metamorphic the Greater Caucasus south of the Bechasyn zone. complex, and the southern one, dipping to the north, Th erefore these terranes show some similarity with expressed by a band of kyanite-bearing rocks of the Armorican metasediments in Central Germany Laba complex (Abesadze et al. 1982). described by Gerdes & Zeh (2006). New data on the geology of the pre-Jurassic Age of protolith’s material of the Greater Caucasus basement of the Greater Caucasus cause numerous metamorphic complexes increase to the north new questions that need further study. indicating accretion in this direction. Palaeogeodynamic reconstruction is not subject of this paper, but some preliminary ideas might Acknowledgements be expressed. Because the main complex of HP/ Th e work was supported by the Russian Fund LT rocks marking palaeosubduction is disposed for Basic Research (grant №10-05-00036-a). in the Fore Range zone and because the volcanic Many colleagues had helped me in the fi eld and arc is represented by Laba complex, there are some laboratory work. My thanks are addressed to Yuri arguments to propose southern inclination of the Vidyapin, Yuri Potapenko and many colleaques of subduction zone. In this case the low-pressure Kavkazgeols’emka expedition (Essentuki city) for the complexes of the Main Range might be considered a fi eld assistance and scientifi c cooperation; to Sergey component of paired metamorphic belts. Korikovski, Aleksander Konilov, Anna Smul’skaya Th e Dizi series was originally spatially connected for petrological consultations; to Maya Arakeliantz, with Transcaucasian massif, and its northern Elena Bibikova, Aleksander Kotov, Ekaterina vergence seems to refl ect southward subduction of Sal’nikova,Tamara Bayanova, Alfred Kröner, Dmitri the ensimatic material crust of the Pass subzone of Zhuravlev, Lev Natapov, Elena Belousova, Elena the Main Range in the end of the Triassic. Lepekhina, Nikolay Rodionov, Rafi k Gukasian for Th e presence of island arc type sequences, their eff orts in geochronological study. Discussions ophiolite, metamorphic complexes of contrastic with Shota Adamia, Guram Zakariadze, Tamara (LP and HP) types, young Palaeozoic age of Chkhotua, Irakli Shavishvili, and David Shengelia regional metamorphism and associated granitoid were especially useful. I am very undebted to Irina emplacement, and, fi nally, the great magnitude of Pogorelova for her constant help with digital design. lithosphere shortening in the pre-Jurassic basement Special thanks to Aral I. Okay for encouragement and of the Greater Caucasus series support the conclusion to my daughter Yuliya Somina, Anna Amramina and of Adamia (1984) on the active character of southern again to Aral I. Okay for correction of the English margin of the East European platform during the version of this work. Finally, I thank two anonymous Palaeozoic. However, the structure of this margin referees for their very useful comments.

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