Pre-Jurassic basement of the Greater Caucasus: Brief overview

The main units of the Greater Caucasus pre-Jurassic basement are represented by Svanetian and North Caucasian domains brought together tectonically. The former includes continuous Devonian to Upper Triassic marine sequence devoid of any manifestation of Variscan orogenic activity.

Pre-Jurassic Basement of the Greater Caucasus:

Brief Overview

MARK L SOMIN

Institute of Physics of the Earth RAS, 10 Bolshaya Gruzinskaya str., Moscow, Russia

(E-mail: somin@ifz.ru)

Received 15 October; revised typescripts receipt 28 March 2011 & 29 April 2011; accepted 10 May 2011

Abstract: Th e main units of the Greater Caucasus pre-Jurassic basement are represented by Svanetian and 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.

North-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

Th e Greater Caucasus is an Alpine folded and highly

elevated system, which borders from the south the

Scythian platform, the southern promontory of

the East-European platform Th e Trans-Caucasian

median massif separates the Greater Caucasus from

the Lesser Caucasus folded system Th e Black Sea

and the Caspian Sea superimposed basins fl ank

the Greater Caucasus from the west and the east,

respectively (Figure 1)

Th e Alpine structure of the Greater Caucasus

is relatively simple Jurassic and younger marine,

shallow-water, partly continental sediments form

more or less fl at monocline (Laba-Malka zone) at the

northern limb of the Greater Caucasus anticlinorium

To the south the thickness of the Mesozoic sediments

increases, and more and more intense mostly

south-vergent folding combined with thrusts and small

nappes appear In the North-Western (‘Central’)

Caucasus the Laba-Malka zone is separated from this

vast folded area called Southern Slope zone by wide

parallelogram-shaded exposures of the pre-Jurassic

basement dissected into diff erent blocks by steep

faults, thrusts and narrow structural depressions

fi lled mostly with Lower Jurassic sediments Th e

Main Caucasian Fault separates basement exposures

of the Main Caucasian Range and the Southern Slope zone Th is fault represents a long lived magma-conducting steep zone developed at least since the Middle Jurassic (Somin 2000) Interpretation of the Main Caucasian Fault as frontal basal part of great-

amplitude thrust sheet (Dotduev 1986; Baranov et al

1991) seems to be wrong

Pre-Jurassic basement complexes of the Greater Caucasus crop out within the limits of two domains,

Svanetian and North-Caucasian (Figures 1 & 2) Th e

fi rst one disposes in the Southern Slope zone, whereas the North Caucasian domain crops out to the north

Th ese two domains diff er drastically: in the Svanetian domain the single event of pre-Alpine folding was Indosynian (Early Kimmerian, i.e pre-Early Liassic), whereas in the North-Caucasian domain typical Variscan (i.e Middle–Late Palaeozoic) events have occurred

Th e main purpose of this paper is to give the new information on geology of the North-Caucasian domain, especially on its metamorphic complexes Problems of their ages, lithology, conditions of metamorphism, main features of structure and relations with unmetamorphosed

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

pre-Upper Palaeozoic complexes are discussed Th e

characteristics of the latter are presented in a very

brief form Petro- and geochemical characteristics

are used in minimal volume, taking into account

the papers published by Zakariadze, Shavisvili and

their coauthors in last years (see Adamia et al 2004)

Information of the Svanetian domain is also given

here in a very restricted form mostly to complete a

picture concerning the ‘frame’ geology

Th e Svanetian domain is represented by Dizi series

exposed in Svanetian anticlinorium,

Central-Northern Georgia, and in small Mzymta river

antlicline, Russia Th e existence of the Dizi (originally

Desi) series was established at the beginning of the

1960’s Following the pioneer study by Agalin (1935),

marine fossils of Middle and Upper Devonian, Middle Carboniferous and Permian were found in limestones

of highly deformed and slightly metamorphosed rocks exposed in basins of Inguri and Tskhenis-tskali rivers

(Slavin et al 1962; Gamkrelidze et al 1963; Somin

& Belov 1967; Adamia 1968; Somin 1971) Marine Lower Carboniferous and Upper Triassic fossils were described later Kutelia (1983) essentially confi rmed this stratigraphic information using the study of conodonts selected from layers of chert Th e primary suggestion on existence of continuous stratigraphic Middle Palaeozoic–Triassic section (excluding Upper Carboniferous) was fi rmly supported as a result As for the Upper Carboniferous, marine brachiopods of this age were collected in sandstone of transgressive Kvishi formation at the Main Caucasian Range zone near Ushba Peak and the Main Caucasian fault, Svanetia district (Somin 1965; Khutsishvili 1966)

exposures of the pre-Jurassic complex

Figure 1 Scheme of distribution of some tectonic elements in the Caucasus.

Palaeozoic cover of the Bechasyn MC Middle & Upper Palaeozoic unmetamorphosed sequences ophiolite & serpentinite

Lower (Devonian to Middle Carboniferous) part

of the Dizi series (Kirar Formation) is represented

by greywacke sandstone and siltstone, containing

beds and lenses of chert, shallow-water coral

limestone and gravelstone Bodies of limestone are

now considered presumably as olistolithes Upper

part of the series consists of fl ysh-like intercalating

greenish calcareous siltstone and sandstone with rare

thin in-situ layers of limestone (Laila and Chelshura

Formation of Somin & Belov (1967) collectively

named by Kutelia Tskhenis-Tskali Formation), and

the uppermost part consists of regressive section of

coarse-grained calcareous sandstone and gravelstone

of the Gvadarashi Formation (Somin 1971) Volcanic

rocks of the Dizi series are represented by andesite

tuff s of the Utur Formation of unknown, presumably

Upper Palaeozoic age Th e degree of regional

metamorphism of the Dizi series does not exceed

low-temperature greenschist facies conditions

Middle Jurassic Kirar diorite intrudes the Dizi series

and forms wide contact metamorphic zone at right

slope of Inguri River valley

Th e structure of the Dizi series is very complex

and is characterized by tightly compressed steep

folds and thrusts with clearly expressed (especially

in Inguri River section) northern vergence whereas

the surrounding Lower Jurassic sediments are

south-vergenced, and the border between these two domains

is sharply pronounced Th e superimposing Alpine

cleavage and kink-bands crosscut the Indosynian

(Early Kimmerian) fabrics of the series (Kaz’min

& Sborschikov 1989) Folds of two generations are

present in the Dizi series (especially well expressed

again along Inguri River), whereas the Early Jurassic

folds are represented almost everywhere by single

generation only Th ese observations are important

for long-debated problem of relationship between the

Dizi series and the Lower Jurassic Gamkrelidze et al

(1963) and Adamia et al (1990) have proposed that

in some parts of the Dizi Basin there was continuous

sedimentation from Palaeozoic and Triassic to the

Sinemurian Other authors (Belov & Somin 1964;

Somin 1971, 2007a, b; Kaz’min & Sborschikov 1989)

have argued for structural unconformity between

these two units Indeed, the Lower Sinemurian basal

layers overlay, sometimes with an angular, azimutal

and slightly metamorphic unconformity, diff erent

stratigraphic levels of the Dizi series Besides,

coal-bearing layers with fossil fl ora were found here in the basal part of the Lower Sinemurian (Belov & Somin 1964), and we need to stress that local continental sedimentation is a characteristic feature of the Early Sinemurian time elsewhere in the Greater Caucasus

Th is domain consists of two structural stages, Upper Palaeozoic (Upper Visean–Triassic) and pre-Upper Palaeozoic Within the latter there are three tectonic zones which are traditionally recognized, namely the Main Range zone, the Fore Range zone, and the Bechasyn zone Until recently it was assumed by most authors that all these zones include two structural stages: the lower Proterozoic and the upper, Middle Palaeozoic (Andruschuk 1968; Belov 1981) Th e author’s new information allows to change this opinion and to show that two pre-Upper Palaeozoic stages exists (but in the Palaeozoic variant) in the Bechasyn zone only, whereas there are no grounds for these two structural stages in the southern zones

Th is is a main area of distribution of metamorphic complexes and granitoids in the Greater Caucasus

Th e common feature of this zone is pressure (andalusite-sillimanite) type of regional metamorphism Only in a very narrow area at south-western margin of the zone the mineral association with kyanite appears

low-Th e diff erences in structural style and composition

of metamorphic complexes and granites allow to divide this zone into two subzones, the Pass and Elbrus (Somin 1971)

It occupies the southernmost position within the Main Range zone (Figure 1) and includes a granite-metamorphic basement, or krystallinikum, the lower structural stage and, locally, the Upper Palaeozoic slightly metamorphosed or unmetamorposed sediments, the upper stage Th e characteristic features

of both stages are more or less tightly compressed linear folds of WNW (‘Caucasian’) strike and SW vergence

Th e Krystallinikum– Th e metamorphic complexes

of the basement are characterized by variable,

essentially mafi c composition because they contain

up to 30–50% of metabasites Low-K I-type mostly

metamorphosed granitoids and diorites are

predominated among plutonic intrusive rocks

Th e two main metamorphic complexes, Laba

and Buulgen, were recognized long time ago in the

krystallinikum of the Pass subzone (Somin 1965,

1971) Th ese complexes conjugate tectonically at

Lashtrak and Makera rivers watershed (Figure 2)

distributed in axial part of the Main Range in

Lashipse, Tsakhvoa, Damkhurts, Makera and Belaya

riverheads Th e existence of this independent unit

was demonstrated for fi rst time by Mel’nikov (1959,

1964), then by Grekov et al (1968) Th e LMC

includes four units (Somin 1971), in ascending order

these are: the Mamkhurts, Damkhurts, Lashtrak

and Adzhara formations, forming the keel-like

Damkhurts synform (Figure 3) Adamia (1977, 1984)

and Abesadze et al (1982) consider these formations

as purely tectonic units (tectonic sheets)

Th e Mamkhurts Formation, about 700 m

thick, consists of alternating mostly fi ne-grained

amphibolites, hornblende, biotite-hornblende, biotite

and leucocratic gneisses sporadically showing relicts

of volcanic and granitic textures Metasedimentary

rocks are very rare here Composition of rocks

indicates the possibility of rift ing origin of the

Mamkhurts Formation because the contents of TiO2

and K2O in metabasites are high (1.5–2.5% and 0.5–

0.7%) and these trachymetabasites are interlayered

with acidic rocks By this Mamkhurts Formation

clearly diff ers from overlying Damkhurts Formation

TIMS U-Pb dating of zircons from leucocratic

orthogneiss yielded 534±9 Ma (Early Cambrian) on

upper interseption of concordia; lower one show 0

Ma (Somin et al 2004) Unfortunately, dating was

not accompanied by cathodoluminescence study

of the zircons therefore the results have limited

signifi cance and need additional checking Type of

contact with the Damkurts Formation is disputable;

presence of small lenses of serpentinite and change of

petrochemical characteristics support the conclusion

of Adamia (1984) of tectonic relationship between

these two units

Th e Damkhurts Formation, ca 400 m thick, has also complex composition and includes metavolcanic rocks in its lower part and siliciclastics in the upper part Metabasites are low-titanium (<1% TiO2) indicating an island arc origin for the formation Two thin horizons of marble appear in the base and in the uppermost part of this formation Th e remnants of the Ludlow-Franian blue-green algae

Renalcus sp were discovered in this limestone by

Chegodaev A relatively thick (up to 300 m) layer of

highly stretched intraformational metaconglomerate

is disposed between the limestone horizons (Figure 4) Th e composition of pebbles in this rock is very diverse; many of them are presented by subvolcanic metaplagiogranite and amphibolites (Somin & Korikovski 1988); at the same time pebbles of quartz are not present Schistosity and mineral lineation are common for both the cement matrix and pebbles; therefore metamorphism was superimposed on this originally clastic rock It is very important to note that the same type of ‘quartzless’ conglomerate is known from the Upper Devonian deposits of the Fore Range zone, and that in both cases some limestone horizons appear among garnet-andalusite micaschist

Th e Lashtrak Formation, about 500 m thick, lies concordantly on the Damkhurts Formation, the contact is well exposed in right bank of Damkhurts riverhead Th e formation consists of garnet graphite-andalusite and staurolite-bearing schists containing some interbeds of fi ne-grained amphibolites Some authors suppose the andalusite has contact-metamorphic origin As a separated tectonic slice of this formation, the kyanite-bearing schist appears at the right slope of Lashipse River Th erefore, this part

of the LMC section seems to be allochtonous relative

to underlying units

Th e uppermost unit of the Laba complex is Adzhara Formation (300 m thick) which has limited distribution and consists of quartzite, porphyroid, amphibolite and dark fossiliferous limestone, where post-Ordovician crinoids were found (Potapenko

& Stukalina 1971) Adamia (1984) attribute this limestone to the Lashtrak Formation

Metaplutonic rocks of the Laba Complex are represented mostly by metamorphosed quartz diorite and diorite, less by plagiogranite Th eir petrochemical characteristics indicate mantle origin

(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 Range, kyanite-and kyanite-staurolite-garnet mineral association was established indicating pressure up

gneiss, mostly metavolcanic marble and limestone alternating metapelite schist and amphibolite

amphibolite quartzless metaconglomerate and conglomerate

gabbro-amphibolite granitoid

fossil findings marble and limestone (in the columns) sandstone

pyroclastic lava flows quaternary deposits nappe

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)

Figure 4 Metaconglomerate of the Damkhyrts Formation,

Damkhyrts riverhead.

to 5–7 kbar (Abesadze et al 1982; Gamkrelidze &

Shengelia 2005)

Th e LMC makes contact with the Lower Jurassic

slate in the south and with the Makera metamorphic

complex in the north Th e northern contact has a

premetamorphic tectonic character because the LMC

includes numerous bodies of I-type metagranitoids

and metadiorite orthogneisses which are completely

absent among the Makera Complex rocks Besides,

a lens of serpentinite lies in this contact zone at the

right slope of Tsakhvoa river valley

occupies the watershed area of the Main Caucasian

Range in riverheads of Teberda and Kodori

tributaries, where it was recognized as a separate unit

for the fi rst time by Somin (1965), originally under

name of the Buulgen series Th e complex includes

three main lithologies: amphibolite, metaterrigenous

mica schist (siliciclastics) and low-K orthogneiss/

metagranitoid Th in layers of carbonate material

(which I interpret as former limestone) and rare

small bodies of ultramafi c rocks are noted here as

very subordinate components

Subdivision of the BMC is based on mapping

of large overturned Klych antiform (Somin 1971)

(Figure 5) Th e alternating amphibolite and mica

schist are replaced by numerous bodies of

biotite-hornblende orthogneiss and metatronjemite in the

core of this structure Th is part of the section was

called the Gvandra Formation Its exposures are

bordered by a thick (sometimes up to 700 m) band

of relatively homogenous garnet-free amphibolites of

the Klych Formation Rare thin layers of paraschists

are observed within these amphibolites

What is the origin of the Klych amphibolites?

Th ey are mostly fi ne-grained rocks whereas

metagabbros as a rule form coarse-grained rocks,

oft en preserving relictic gabbro textures; the latter

were never observed in the Klych Formation Besides,

in some sections the amphibolite is in contact

with a thin layer of marble Th erefore, it seems

more probable that the biggest proportion of these

rocks is metavolcanic, not metagabbro An average

content of TiO2 in the amphibolites is 1.55% (n=

39) and only locally, in the Gonachkhir river valley

the content of this component is <1% (Hanel et al

1993a, b) According to Dumbadze (1977) and Popov

(personal communication 2004), the amphibolites corresponds petrochemically to normal-alkaline ferrigenous tholeiitic basalt In the Pearce diagram they plot within the fi elds of intraplate and island arc basalt Because similar amphibolites occur as thin intercalation with siliciclastic schists in other parts of the BMC, there is a reason to consider this complex

as basinal (marginal sea?) deposits

Th e Klych Formation is overlain by the thick Dombay Formation which consists of micaschists and amphibolites in an approximately equal proportion

Th e micaschists always contain rounded detrital zircon with wide spread of U-Pb ages

Metaintrusive rocks placed within the BMC are represented by two groups, orthogneisses and less metamorphosed gneissic metaplutonic rocks Th e

fi rst group includes rocks which completely lost their primary igneous textures, and are now homogeneous, some banded rocks consisting of quartz, metamorphic sodium-rich plagioclase, biotite and hornblende

Th ey form plate-like bodies lying concordantly in the surrounding paraschists Th e content of SiO2 is 65–68% i.e they belong to the tonalite/dacite group

Th eir magmatic origin is testifi ed by the presence

of relatively large (up 150 μm) idiomorphic zircon grains showing thin zonality in their marginal zones and dark central zones However it is not completely clear whether these orthogneisses were originally plutonic or volcanic rock Gneissic metaplutonic rocks have a chemical and mineralogical composition similar to gabbro-diorite to quartz diorite (SiO2= 49–59%) (Gamkrelidze & Shengelia 2005); they are coarse grained and preserve numerous relicts of magmatic texture: large partially crushed magmatic plagioclase grains (with relictic tabular form and magmatic zonality) representing augens are typical for the granitiodic blastomylonites Th ese grains are rounded by metamorphic matrix composed of highly deformed and oriented quartz, hornblende and biotite Th ese rocks described for the fi rst time

by G.Chkhotua in 1938 were later called by Somin (1971) ‘Klych augen orthogneiss’ Th ey contain abundant magmatic zircon Numerous xenoliths of enclosing stratifi ed rocks, mica schists, paragneisses and amphibolites, are noted elsewhere Th e main area

of distribution of these metaplutonic rocks is Klych river valley Th ick body of such rocks is also known

near the Naur pass, NW termination of the BMC

area Besides, augen rocks of diff erent composition

form a 30 km long narrow belt, separating the BMC

from north-east Th is belt, spatially coinciding with

SW margin of the Alibek Jurassic depression, is a

long-standing tectonic line, separating BMC from

metamorphic complexes of the Elbrus subzone

Metamorphism of the BMC is of low-pressure

and moderate temperature type (up to 660° C and 3.5

kbar) Highest-temperature association is represented

by garnet, sillimanite, biotite and muscovite, but

sillimanite-bearing associations are rare, and

migmatite is almost completely absent although

metapelitic schists, favorable for selective anatexis is

widely distributed in the BMC Chichinadze (1977)

established that a small part of the BMC cropping out

on the left bank of Gvandra River and called by him

Sysina Formation, was formed under lower (<2.5

kbar) pressure

Th e origin of the BMC is the subject of a

long-lasting discussion Adamia et al (1987) interpreted

it as mixture of metaophiolite and island-arc rock

associations Gamkrelidze & Shengelia (2005)

concluded that in the BMC only the Klych Formation

has ophiolitic origin and that this unit is a separate

tectonic sheet within the BMC Both points of view,

especially the fi rst one, seem to be disputable because

role of metaophiolite is too exaggerated Indeed (1)

no four-member ophiolite section (i.e ultrabasite,

gabbro, sheeted complex, basalts) has been found in

the BMC, neither representative fragments of this

section (2) Almost half of the BMC is represented by

siliciclastic schists completely alien to true ophiolite

and island arc sections (3) Metagranitoids in some

localities, i.e in Klych valley, are the dominant

type of rocks of the BMC and the thickness of their

individual bodies reaches up to some hundreds of

metres, they intrude all units of the BMC, including

the Klych amphibolites Besides, the average diorite

or quartz diorite composition of these metaplutonic

rocks diff ers sharply from the composition of thin

veins of plagiogranite characteristic of true ophiolite

association (Coleman 1978) (4) Deep-water (meta)

sediments were not found within the BMC On the

contrary, thin layers of crystalline limestone are

observed in some places together with amphibolite

bodies Th erefore, an ophiolite origin for the BMC

seems to be doubtful Th e BMC was probably formed

in marginal sea or in a rift basin near continental masses, where a distension of crust and basaltic volcanism was accompanied by terrigenous material supply Later a similar situation was repeated in the history of the Greater Caucasus in the Liassic time

Th e age of the BMC is another problem Dating of zircon from banded Gonachkhir River amphibolite

by Pb/Pb single zircon evaporation method gave ca

600 and 500 Ma values, interpreted as the ages of the volcanic protolith and that of metamorphism,

respectively (Hanel et al 1993a, b) However, these

authors noted that origin of zircons remains unclear because they are somewhat rounded and, thus, a secondary (transported) origin is not excluded More important was value 320±5 Ma obtained by Bibikova

et al (1991) by TIMS U-Pb method on magmatic

zircons of the Klych metaquartzdiorite Recently,

Kröner (Somin et al in preparation) obtained a

381±3 Ma age with SHRIMP method on magmatic zircons from metatonalite (or dacite) orthogneiss from the area of Khetskvara glacier SHRIMP II dating (VSEGEI laboratory, St.-Petersburg) of detrital zircons (n= 15) selected from quartz-biotite schist (sample P93-3) of the same locality yielded four groups of values: 2394–1929, 669–

483, 425–405 and 355–325 Ma (Figure 6, Table 1)

Th e last group probably represents the age of the metamorphic overprint whereas the older values indicate the Palaeozoic age of the rocks protoliths Similar values were demonstrated by zircons (n= 9) from the siliciclastic schist from the left bank of Amanauz river, northern slope of the Main Range near Dombay resort: 2108, 920, 670, 660, 661, 645,

340, 339, 322 Ma Figure 7 and Table 2 demonstrate morphology and isotope-geochronological data of zircons selected from typical BMC augen biotite- hornblende metadiorite (sample 0-81), which crops out near Naur pass SHRIMP dating (VSEGEI) yielded 310.9±2.9 Ma Kotov (Institute of Precambrian geology, St.Petersburg) obtained 308±18 Ma on the same sample with TIMS U-Pb method Recently, an age value of 307±1 Ma was also received by Kotov on magmatic zircons of gneissic (mylonitizated) quartz diorite (or ‘Klych augen orthogneiss’) previously

dated as ca 320 Ma (Bibikova et al 1991).

In the absence of migmatite and Ar/Ar data it

is not easy to determine the exact age of the BMC’s

c

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

zircon age data we can say confi dently that this

age is at least Late Devonian, probably

post-Serpukhovian Th is conclusion is supported by

Sm-Nd mineral isochron dating (on fi ve minerals)

by Zhuravlev, which yielded 288±33 Ma on

garnet-biotite-hornblende gneiss taken in Northern

Ptysh riverhead, east of Dombay resort area.

Th us, metamorphism of the Buulgen complex is

evidently Late Variscan At the same time K-Ar

dating generally yields ages in the range of 140–190 Ma; these values refl ect isotope rejuvenation of the Palaeozoic basement during the Early Kimmerian tectonic event

Th erefore one can conclude that the BMC is a Variscan metamorphic complex of low-pressure (partly very low pressure) and moderate temperature type Th e very low-pressure Kassar Formation of the Ardon river valley, Northern Ossetia, of metabasite-metapelite composition, is a probable equivalent

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

№ Spot 206 %

Pbc

ppm U

ppm Th

(1) 207 Pb / 206 Pb Age

% Discordant

Total 238 U / 206 Pb ±%

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 204 Pb.

№ Spot 206 %

Pbc

ppm U

ppm Th

Total 238 U / 206 Pb ±%

of the BMC in the eastern termination of the Main

Range zone (Abesadze et al 2004) as well as exposures

of metamorphic sequences at the Chugush block in

the hard-accessible locality of the Western Caucasus

It is important to note that isotope-geochronological

data (Rb-Sr on metapelite and U-Pb on granitoids

intruding the Kassar Formation) gave Late Palaeozoic

ages, values similar or slightly younger than those of

the BMC Th erefore, the geochronologically youngest

belt of essentially mafi c metamorphic rocks and

associated granitoids form a narrow discontinuous

belt along the southern border of the North-Caucasus

domain Th e easternmost exposure of this belt is

represented by granodiorites of the Dar’yal salient

Abesadze et al (1978, 2004) have proposed that the

Kassar Formation is probably a volcanic-sedimentary

prism (including the N-MORB metabasites) accreted

to the northern complexes of the Main Range during

Late Palaeozoic time Th is idea seems to be fruitful

for the whole belt

Metamorphic rocks of pre-Jurassic basement of

the Atamazhi salient (Belaya River valley, western

termination of the pre-Jurassic basement exposure)

presumably also belong to the BMC Previously

these exposures were interpreted as rocks of

low-temperature greenschist facies, youngest component

of the Greater Caucasus basement However, mapping

and petrological study (Somin & Smul’skaya 2005)

have revealed that these rocks are blastomylonites

formed from a high-temperature metamorphic sequence (up to granulite facies, 650–740° C; 5.5–7.5 kbar) composed of paragneisses, augen orthogneisses and amphibolites Th e degree of their initial metamorphism decreases structurally downward whereas the degree of mylonitization increases in the same direction Zircon of an augen orthogneiss (metadiorite) yielded TIMS U-Pb age 312±3

Ma (Somin & Smul’skaya 2005) Th is metamorphic inversion is probably a result of deep overthrusting movements in the Late Palaeozoic Th e upper limit of this event is determined by basal Lower Jurassic strata which overlie the blastomylonite A similar type of blastomylonite is also found along the southern border of the Main Range zone in Shakhe River basin (Potapenko & Prutski 1977), where it is presented by the Bushiy pseudoformation

Palaeozoic sediments have limited distribution in the Pass subzone Th ey are represented by Middle and Upper Carboniferous, Permian and suspected Triassic rocks Th e Middle Carboniferous deposits are found in the lake Khuko locality only (Belov & Zalesskaya-Chirkova 1963) Th ey are continental coal-bearing sandstone and gravelstone containing fossil plants of the upper Westfal-B; Upper Permian marine, terrigeneous and calcareous sediments lie above with unconformity Toward the SE the Upper Palaeozoic sediments crop out

a

b

Figure 7 Cathodoluminescence image (a) and diagram with concordia (b) for zircons of sample 0-81 of augen

metadiorite, at Naur Pass.

at glacier Pseashkho, Urushten and Malaya Laba

riverheads Th e Upper Palaeozoic section begins

here with Upper Carboniferous (or Lower Permian)

conglomerate which overlies a crystalline basement

Detachment is developed along the contact as a

rule and the thickness of the conglomerate is oft en

reduced Strongly deformed and recrystallized

Permian limestone and intercalated cherts and slate

of unknown age appear above conglomerate or are

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

Peak, Svenetia district of Georgia Th ere is an Upper

Carboniferous brachiopod-bearing sandstone and

basal conglomerate covering micaceous schists of

Makera Complex and deformed together with the

latter in steep synclinal fold Permian limestone

appears above sandstone (Khutsishvili 1966) A

similar picture is observed at the eastern termination

of the Main Range zone east of Ardon River Dense

quartz conglomerate up to 200 thick is covered

concordantly by the Permian (Guadalupian)

coral-bearing limestone (Morgunov 1965)

Early Alpine (pre-Callovian) movements have

changed the original Pass subzone basement

structure Indeed, the Klych antiform is in fact a

Kimmerian fold because on its both steep wings

the Lower Liassic sediments are disposed in normal

stratigraphical position, without a major angular

unconformity between general position of the

Palaeozoic foliation and Liassic bedding (Somin

2007a, b) Th e contacts are as a rule tectonic, i.e

detachment surfaces are present At the same time

basal conglomerates are locally preserved near the

detachment and on the lobes of such structures Th e

same picture is observed at the eastern termination of

the Main Range structure, particularly, in Ardon River

section, Northern Ossetia, where Palaeozoic schists,

Permian marbles and Lower Jurassic sediments

are deformed together into steep folds overturned

toward the south More to the east, in the Terek River

Dar’al canyon, one can see how Palaeozoic granite

involved in Alpine deformation is placed tectonically

within the Lower Jurassic slate Marginal parts of the

granite massif are intensively mylonitizated; surfaces

of mylonitization are subvertical and parallel to

those of cleavage in the intensively folded Liassic

sediments At the same time these sediments lie

gently on upper surfaces of the granite salients, and fragments of basal conglomerate are seen there Nevertheless, the steep cleavage persists here also All these data demonstrate an origin of this structure

as a result of horizontal compression Because the cover sediments are especially tightly compressed between the basement salients, this structural style

of basement cover- system might be called lobate (Ramsay 1967)

cuspate-Elbrus Subzone

Th is part of the Main Range zone diff ers essentially from the Pass subzone both compositionally and structurally Th e Upper Palaeozoic stage is preserved here occasionally only, in very small depressions among vast areas of krystallinikum For example, red beds of Permian are known on the left side of the Baksan River

Th e Elbrus subzone is an area of gently dipping foliation and dome-like structures, sialic and ensialic metamorphic rocks and abundant S-type granites Low-pressure (andalusite-sillimanite) type of metamorphism (3.2–3.5 kbar, Gamkrelidze

& Shengelia 2005) is a characteristic feature of this subzone; the degree of metamorphism reaches up to high-temperature amphibolite facies Post-Jurassic structural reworking is much less here in comparison with the Pass subzone Alpine thermal infl uence is absent except for the Pleistocene Eldzhurta granite contact area

Two main units are recognized in the Elbrus subzone krystallinikum: the lower, Gondaray (gneiss-migmatite) and the upper, Makera Metamorphic Complex (Grekov & Lavrischev 2002; Gamkrelidze & Shengelia 2005; Somin 2007a, b) Th ey are separated

by a subhorizontal tectonic surface or bodies of Upper Palaeozoic granite

Gondaray Metamorphic Complex (GMC) is the

most widely distributed metamorphic unit of the Elbrus subzone Its exposures are disposed mostly in Aksaut, Kuban, Teberda, Baksan and Cherek Rivers basins, where metamorphic rocks are intruded by numerous bodies of granite Especially good and easily accessible exposures of the GMC are known

on the left bank of the Baksan River 2–3 km SW of Tyrnyauz city and in deep gorges of Baksan tributaries

Adyr-su and Adyl-su In these localities the great

subvertical cliff s and escarpments abraded by glacier

show change in the position of the metamorphic

foliation from subhorizontal to subvertical in a

distance of 4–5 kilometers, indicating the presence

of dome-like structures (Figure 9) In the eastern

part of the Main Range, foliation has mostly gentle

monoclinal dipping to the north which permits to suggest the existence of some thrusts

Th e GMC consists of paragneisses, orthogneisses and migmatites; as a subordinate component of GMC some amphibolites appear Among extremely rare rocks small lenses of marble are noted Orthogneiss bodies were recognized within the GMC not long ago

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

riverhead.

orthogneiss, paragneiss & migmatite

(Bibikova et al 1991; Hanel et al 1993a, b); earlier

they were confused with migmatite In contrast to

migmatite, the orthogneiss is a more monotonous

and homogeneous rock forming bodies are up to

hundreds metres thick; sillimanite and garnet are

present here as very subordinate phase; rotated

xenoliths of surrounding paragneiss and amphibolite

may be observed inside an orthogneiss; fi nally,

relict magmatic plagioclase might be found in this

rock under microscope Sillimanite and garnet are

abundant in migmatite and form separate thick (up

to some centimetres) bands; leucosome is

coarse-grained and oft en has granitic texture Th ere are also

diff erences in zircon grain morphology and the age

values of the two rock types

Geochemical data on orthogneisses indicates

their crustal origin: 87Sr/86Sr= 0.742653; Isr= 0.71409;

ε Nd(T)= –2.6; 143Nd/144Nd= 0.512338; 147Sm/144Nd=

0.512338

Gamkrelidze & Shengelia (2005) have

demonstrated that three metamorphic facies can

be separated within the Elbrus subzone:

biotite-sillimanite-K-feldspar, garnet-cordierite-orthoclase

facies and facies of biotite-muscovite gneisses Rocks

of the two fi rst facies were interpreted by these authors

as products of Grenvillian or even earlier epoch;

biotite-muscovite gneisses were attributed to the

Early Caledonian event Early Variscan (Bretonian)

stage was considered to be time of diapthoresis of

the GMC, and Late Variscan (Sudetican) stage as its

greenschist retrograde stage

Attributing the GMC to pre-Cambrian was based

on the following facts: (1) its location in the structurally

deepest (core) part of the range; (2) the highest grade

of metamorphism in this part of the Greater Caucasus

basement corresponding to high-temperature level

of the amphibolite facies; (3) several U-Pb zircon

ages corresponding to the pre-Cambrian, although

they have remained debatable for a long time As

was assumed or shown (Bibikova et al 1991; Somin

et al 2006) most of these ages were obtained from

detrital or inherited zircons Most oft en, researchers

mention values of 500±40 and ~2000 Ma obtained by

the Pb/Pb evaporation method for zircons extracted

from orthogneiss of the Adyl-su River valley (Hanel

et al 1993) However, dating of zircons of the same

massif by the conventional multigrain (TIMS) U-Pb

method yielded an age 400±10 Ma (Bibikova et al

1991) Later this method gave 386±5 Ma on primary magmatic zircon of similar banded two-feldspar

orthogneiss from the Kyrtyk River canyon (Somin et

al 2006)

Th is orthogneiss cuts gabbro-amphibolite and encloses its xenolith Th e gabbro-amphibolite is uniform massive rock forming body of 150 m thick with TiO2 content up to 2% and elevated alkalinity

Th is explains the abundance of zircon in the rock Zircons from amphibolite are almost isometric; they are characterized by absence of zonality or by irregular wide zonality with indistinct boundaries between zones Fusiform zoning is observed in some places

Th ese features are characteristic for gabbro zircons

Of six zircons dated by U-Pb SHRIMP method fi ve grains provided a concordant age averaging at 425±9

Ma (Late Silurian) with MSWD= 0.39 (Figure 10, sample 0-11, Table 3)

To solve the probem of protolith’s age of the GMC paragneisses, the SHRIMP U-Pb dating of detrital

zircons of these rocks was carried out (Somin et al

2007a, b) In order to obtain the most representative results a study of four paragneiss samples with spacing

up to 150 km was realized Th e samples diff ered in the degree of migmatization

Paragneiss of sample 0-17 was taken near the mouth of the Adyr-su River located in the large area

of the sillimanite-biotite-muscovite subfacies of the amphibolite facies Th e paragneiss is represented

by deformed (overturned folds) and banded migmatized rock typical of the GMC Its mineral assemblage is Qtz-Grt-Bt-Sil (-Pl-Kfs) Sillimanite and biotite are the dominant minerals Garnet shows only slight retrograde zoning indicating one-

stage metamorphism with T= 616° C and P≤4 kbar

Zircon occurs in these rocks as transparent elongated prismatic crystals with rare inclusions and marginal zoning (Figure 10, sample 0-17, Table 4) Th ey are similar to zircons in the migmatite leucosome Zircon grains contain rounded inherited cores In total, 11 grains were analyzed Cores of three grains yielded U–Pb SHRIMP ages of ca 2347, 1809, and

1268 Ma Two other grains yielded ages of ca 637 and 665 Ma Th e ages determined for zircon crystals

at 12 points located at the zircons rims varies from

321 to 288 Ma (average 321± 1 Ma; MSWD= 0.0091)

b

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.

One third of all grains are characterized by low

Th /U values (Figure 11, Table 4), which are typical

of metamorphic zircons Th e rock contains xenotime

and monazite equilibrated with biotite and garnet

Th e monazite age determined by Konilov using the

CHIME microprobe method (Suzuki et al 1991) is

ca 280±50 Ma

Paragneiss of sample 125 was taken on the left

slope of the Damkhurts River 9 km upstream from

the river mouth Th e sample is a coarsely foliated

nonmigmatized rock composed of the

Bt-Ms-Pl-Sil-Qtz association Sillimanite occurs as complicatedly

deformed fi brolite bunches Zircons consist of

rounded detrital cores and very thin rims that slightly

mask the grain morphology (see Figure 10, Table 5)

Only cores were dated because the thickness of rims

is insuffi cient for measurements Among 11 grains

studied three grains yielded ages of ca 1337, 1038,

and 898 Ma Th e ages of six other grains make up

an almost continuous series of values ranging from

ca 676 to 561 Ma At the same time, the ages of two

grains (four points) fall into the interval of 504 to 474

Ma (Late Cambrian–Early Ordovician) In all cases,

the Th /U value exceeds 0.15 (usually >0.30) Th is fact

confi rms the primary magmatic genesis of detrital

grains

Paragneiss of sample 152, from the upper left fl ank

of the Sofi ya River, is represented by banded foliated

and slightly migmatized rock of the

Grt-Bt-Ms-Pl-Qtz association It encloses paragneiss interbeds

with sillimanite and amphibolite lenses Zircons in

this sample are also detrital, coated by thin rims (see

Figure 10, sample 152, Table 6) In total, 23 grains

were analyzed In some of them, both central and

marginal parts were analyzed Th e ages of four grains

cores vary from ca 2002 to 724 Ma , 18 grains yielded

a Neoproterozoic–Early Cambrian age (634 to 522

Ma), and one grain (two points) yielded 474–432

Ma Th e metamorphic rim yielded 310 Ma In this

case and in grains with an age of 459, 432, and 381

Ma, the Th /U value is very low (0.03–0.07), probably

due to alteration of zircons In other cases, this ratio

exceeds 0.12

Th us, the data obtained for most detrital

zircons from all paragneiss samples fall into the

Neoproterozoic–Ordovician interval Most of them

correspond to the Ediacarian to Early Cambrian Th e

occurrence of rare grains dated back to 470–480 Ma (Th /U > 0.2) among detrital zircons suggests that the age of host rocks might be younger than Early Ordovician (Figure 12)

Interesting data were obtained from blastomylonite (augen metagranite) of the Bol’shoy Mukulan creek, 1.5 km south of Tyrnyauz city Two-feldspar biotite granite suff ered high-temperature metamorphism and transformed into augen blastomylonite is disposed now at the altitude of 2900–3000 m above Baksan River separating GMC and the less metamorphosed andalusite-bearing metapelite of the Makera Complex Th e rock contains abundant idiomorphic magmatic zircon, its U-Pb (TIMS) age

is 305±8 Ma (Somin et al 2006).

Th e value 307.7±8 Ma was obtained by SHRIMP method for three transparent euhedral zircon grains selected from small deformed anatectic granite body placed within migmatite matrix at left wall of Baksan River 300 m north of Bol’shoy Mukulan creek

K-Ar data on biotite-muscovite pairs of migmatite

(Bibikova et al 1991) correspond to 310–288 Ma and

therefore coincide with the youngest age obtained

on metamorphic or youngest magmatic zircons or are slightly younger refl ecting the time of closing of argon system

Summing up the geochronological data on the GMC one can conclude that they indicate the Middle Palaeozoic and partly even Late Palaeozoic age of the protolith material and, therefore the Late Palaeozoic age of the regional metamorphism

Intrusive plutonic rocks are widely distributed in the GMC (much less in the Makera Metamorphic Complex) and are represented mainly by homogeneous two-mica S-type granites Most of the granites are stratiform bodies up to 2–3 km thick

(Somin 1965; Potapenko et al 1999) Lower surface

of these bodies crops out in Kuban’ and Aksaut rivers rocky slopes; banded migmatite-like texture is characteristic for bottom part of granite bodies up to several dozens metres thick

Isotope-geochronological (Rb-Sr, K-Ar and U-Pb) data on the Main Range granite are not abundant; nevertheless they all fall to the 320–290 Ma interval indicating a Late Palaeozoic age of crystallization

(Gurbanov & Arets 1996; Potapenko et al 1999; new

the author’s data) Some granite and granodiorite

from the south-eastern part of the Main Range show

younger U-Pb ages of ca 250 Ma

It is important to stress that ages of migmatites

and anatectic granites of the GMC are very close

to those of the homogeneous two-mica S-type

granites emplaced in this complex Th is fact seems

to indicate a genetic connection between regional

metamorphism, migmatization and the granite

crystallization Indeed, hornfels never have been

described in contacts zones of the granite and the

gneiss-migmatite substratum Hence the latter still

remained at high temperatures up to moment of the

granite emplacement Th erefore, our data confi rm the

conclusion by Brown & Solar (1998) and Solar et al

(1998) on connection between metamorphism and

granite emplacement in some convergent orogenic

belts

MMC is the upper metamorphic unit of the Elbrus subzone It is known as thick (>2 km) sequence above the GMC in area of left tributaries of Bol’shaya Laba River Makera, Mamkhurts, Damkhurts, Tsakhvoa, at both sides of Baksan River valley, in the Aksaut River, inside Chegem and Ardon rivers basins As a rule the MMC is characterized by gently dipping foliation Detailed mapping and structural study have revealed two or three generations of earlier isoclinal folds and slides with amplitude up to 1–1.5 km (Somin 1971).Metamorphism of the MMC is of epidote-amphibolite facies (500–550° C) and low pressure (3 kbar) type (Shengelia & Korikovskii 1991) Andalusite is a widely distributed mineral, whereas staurolite is known only in Kirtyk riverhead Th e

fi brolite and garnet are relatively rare Th e MMC is dominated by siliciclastics (mainly high-alumina,

Figure 11 Concordia diagram for metamorphic zircons of migmatized paragneiss zircons,

sample 0-17, Condaray complex, Adyr-sy River

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

U

ppm Th

(1)

207 Pb/ 206 Pb Age

% Discordant

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

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

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

Pbc

ppm U

ppm Th

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 204 Pb.

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

(1)

206 Pb / 238 U Age

(1)

207 Pb/ 206 Pb Age

% Discor dant

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 204 Pb.

oft en graphite-bearing metapelite, very rarely

quartzite), fi ne-grained Azau gneiss (orthogneiss)

and much less by amphibolite; sometimes lenses of

crystalline limestone are noted in association with

amphibolite Amphibolites occupy approximately

10–15% of the MMC, where they are oft en associated

with Azau gneisses forming collectively the Duppukh

Formation (Baranov & Kropachev 1976; Baranov

1987) Th is formation is famous for its scheelite

occurrences Scheelite is associated with quartz veins

placed within melanocratic amphibolites, which were

lithological traps for tungsten-bearing hydrothermal

solutions

Th e Azau gneisses are especially interesting type

of rocks, because they represent the main object for

isotope dating Chemical composition of gneisses

ranges from dacite (diortite) up to rhyolite (granite)

Th ey are fi ne-grained, banded rocks of light, mostly

greenish colour with a poorly developed schistosity

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

grains were not found within these rocks, but there 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

whether these rocks were volcanic or intrusive because of their complete recrystallization and strong

deformation Potapenko et al (1972) prefers to think

that gneisses were originated from aplite dykes because rare siliciclastic metamorphic xenoliths are sometimes observed within them However presence

of such xenoliths is not unusual for lava fl ows or subvolcanic bodies At the same time porphyritic texture is unusual for an aplite Intrusive contacts with the surrounding siliciclastics are extremely rare

Th ese arguments and the presence of rare relicts of plagioclase phenocrysts rather seem to indicate a protolith of subvolcanic granite-porphyr or rhyolite-porphyr for the Azau gneisses

Some samples of the Azau gneisses from diff erent sites were studied with TIMS U-Pb zircon method

Th e fi rst determinations were made by Bibikova et

al (1991) on magmatic zircons of gneisses of Azau

locality, left wall of the Baksan River valley, and

on similar rocks taken at riverhead of the Malaya Dukka Th e zircons yielded ca 425 and 430 Ma ages, respectively SHRIMP dating of zircons from samples taken at massif Cheget foot (03-10) and Azay locality

detrital magmatic metamorphic

Figure 12 Histogram of zircons age values of the Gondaray

complex rocks Samples: 0-17, migmatized

paragneiss, Adyr-sy River; paragneiss, Damkhurts

River; 152, migmatized paragneiss, Sofi a River; 0-11,

amphibolite, Kyrtyk River.

(0-29), and at right side of Aksaut River (Kti-Teberda

mine area, sample C87-36, not shown here), however,

yielded age values ranging from 425 up to 460 Ma

(Figures 13 & 14, Tables 7 & 8) A concordia age of ca

444 Ma was obtained from seven idiomorphic zircon

grains of sample 0-29 Many of these zircons include cores with age values mostly around 600 Ma, typical for predominate part of the Makera metapelite detrital zircons; some cores yielded Mezoproterozoic values

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.

Ages obtained with SHRIMP method on zircons

(n= 16) selected from Duchinka River amphibolite

(Bol’shaya Dukka River tributary) yielded an average

value 478±4 Ma However, the age obtained on the

group of grains of the last probe with lower (the best)

value of MSWD is 464±15 Ma (Figure 15, Table 9)

It is interesting that a very similar age (460±8 Ma)

was shown by a point placed near the concordia line obtained by TIMS method on the same probe

(Somin et al 2004) Moreover, zircons of Yusen’gi

River (Baksan tributary) amphibolite body (sample 16) yielded a SHRIMP age of 445±12 Ma (i.e earliest Late Silurian) (Figure 16, Table 10)

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

Spot

%

ppm U

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 204 Pb.

±% 207 (1) Pb*/ 206 Pb* ±%

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

An attempt to determine the protolith age of the

MMC with Rb-Sr (wr) isochron method was made

using the andalusite schist of the Cheget massif

According to Gucasian (Geological Institute of

Armenia), fi ve probes up to 5 kg each gave an isochron

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