Radiolarian biostratigraphic constraints for latest Jurassic-earliest Cretaceous submarine volcanic...

Radiolarian biostratigraphic constraints for latest Jurassic-earliest Cretaceoussubmarine volcanic activity in the Tethyan oceanic realm of the Sevan

ophiolite (Armenia)GAYANÉ ASATRYAN1,2, TANIEL DANELIAN3, LILIT SAHAKYAN1, GHAZAR GALOYAN1, MONIQUE SEYLER3,

MARC SOSSON4, ARA AVAGYAN1, BENOIT L.M. HUBERT5 and SANDRA VENTALON 3

Key-words. – Radiolaria, Radiolarites, Submarine volcanism, Late Jurassic, Early Cretaceous, Ophiolites, Armenia, Lesser Caucasus

Abstract. – Biostratigraphic constraints for the sedimentary cover of the ophiolites preserved in Armenia are of key im-portance for the palaeogeographic and geodynamic reconstruction of the greater area between Eurasia and theSouth-Armenian block, which is a micro-continent of Gondwanian origin. We present here radiolarian data obtainedfrom radiolarites that are intercalated in a sequence of mafic volcanic rocks on the northern flank of the Dali valley (eastof Lake Sevan), which is considered to be part of the Sevan ophiolite. Mafic sills and dykes with well-preserved igneoustextures are probably part of the same sequence. The pseudomorphosis of primary phases indicates that the igneousrocks are strongly affected by alteration in the greenschist facies condition. The plagiogranites that are present in thislocality appear to be intrusive into the mafic sequence. The radiolarian assemblages extracted from radiolarian chertsintercalated in the mafic volcanic rocks are dated as latest Tithonian-Late Valanginian; they contain metric roundedblocks of oolitic limestones with crinoid fragments, suggesting that these shallow water limestones slid during the Ju-rassic/Cretaceous transition into a rugged oceanic floor in which radiolarian ooze accumulated.

Contraintes biostratigraphiques apportées par les radiolaires pour une activité volcaniquesous-marine d’âge jurassique terminal-crétacé basal préservée au sein de l’ophiolite de Sevan

(Arménie)

Mots-clés. – Radiolaires, Radiolarites, Volcanisme sous-marin, Jurassique supérieur, Crétacé inférieur, Ophiolites, Arménie,Petit Caucase

Résumé. – Les contraintes biostratigraphiques sur la couverture sédimentaire des ophiolites préservées en Arménie sontd’une grande importance pour la reconstitution paléogéographique et géodynamique des terrains situés entre l’Eurasieet le bloc sud-Arménien, un microcontinent détaché du Gondwana. On présente ici des microfaunes de radiolaires ex-traits des radiolarites qui sont intercalées dans une série de roches volcaniques basiques qui affleurent sur le flanc nordde la vallée de Dali (à l’est du lac Sevan), considérées comme faisant partie de l’ophiolite de Sevan. Des sills et des fi-lons de roches basiques dont les textures sont bien préservées malgré la pseudomorphose des phases primaires par uneparagenèse schiste vert font probablement partie de la même série. Les plagiogranites situés sous cet ensemble semblentêtre intrusifs dans une série de roches basiques. Les radiolaires extraits des radiolarites intercalées dans les roches basi-ques sont datées du Tithonien terminal à Valanginien supérieur ; celles-ci contiennent des blocs métriques de calcairesoolithiques à fragments de crinoïdes, ce qui suggère que ces calcaires peu profonds ont glissé au cours de la transitionJurassique/Crétacé sur le fond océanique accidenté où s’accumulaient des boues radiolaritiques.

INTRODUCTION

The sedimentary cover of most Mesozoic ophiolites alongthe Tethyan belt is made essentially of radiolarian-rich sili-ceous sediments. Radiolarian biochronology is an importanttool to unravel the geodynamic and palaeoenvironmentalevolution of Tethyan oceanic basins and their margins[Al-Riyami et al., 2000, 2002; Baumgartner, 1984, 1985;

Bill et al., 2001; Caridroit and Ferrière, 1988; Chiari et al.,1997, 2000; Cordey and Bailly, 2007; Danelian et al., 1997,2008a; De Wever and Dercourt, 1985; De Wever et al.,1987; Danelian and Robertson, 1997, 2001; Smuc andGori�an, 2005; Ziabrev et al., 2003]. This is especially truefor the Dinarides-Hellenides system [i.e. Baumgartner,1985; Bortolotti et al., 2004, 2006, 2008; Caridroit et al.,2000; Chiari et al., 2003, 2004; Danelian et al., 2000;

Bull. Soc. géol. France, 2012, t. 183, no 4, pp. 319-330

Bull. Soc. géol. Fr., 2012, no 4

1. Institute of Geological Sciences, National Academy of Sciences of Armenia, 24a Baghramian avenue, Yerevan, 0019, Armenia. [email protected]. University Pierre & Marie Curie (Paris VI), CNRS-UMR 7207 Centre de Recherches sur la Paléobiodiversité et les Paléoenvironnements (CR2P),C. 104, 4 place Jussieu. 75005 Paris, France3. University Lille 1, Department of Earth Sciences, CNRS-UMR 8217 “Géosystèmes”, SN5, 59655 Villeneuve d’Ascq, France4. University of Nice – Sophia Antipolis, CNRS-UMR Géoazur, OCA, 250 rue A. Einstein. 6560 Valbonne 2, France5. Catholic University of Lille, CNRS-FRE 3298 “Géosystèmes”, SN5, 59655 Villeneuve d’Ascq, FranceManuscript received on May 12, 2011; accepted on March 22, 2012

Gori�an 1994; Karakitsios et al., 1988; Stais et al., 1990],as well as for the Alpine chains of Turkey [Bragin andTekin 1996; Danelian et al., 2006; Moix et al., 2009; Tekinand Göncüoglu, 2007; Tekin et al., 2002; Vrielynck et al.,2003]. Reliable ages for ophiolitic lavas are still fairly rarefurther to the east, such as in the Lesser Caucasus area, inspite of some recent radiolarian data generated through aFrench-Armenian collaborative project [Danelian et al.,2007, 2008b, 2010; Asatryan, 2009; Asatryan et al., 2010,2011].

The Sevan ophiolite [Aslanyan and Satian, 1977;Galoyan, 2008; Galoyan et al., 2009; Knipper, 1975;Knipper and Khain, 1980] crops out essentially at the NEpart of Lake Sevan (fig. 1; for a geological map of the areasee Danelian et al. [2012]). It belongs to a NW-SE orientedzone extending for ca. 400 km from the Amasia-Stepanavanophiolite in the NW part of Armenia to the Hagari (Akera)ophiolite in Karabagh [Galoyan et al., 2007; Rolland et al.,2009; Sokolov, 1977; Sosson et al., 2010]. It represents themain Tethyan suture zone between Eurasia and theSouth-Armenian block (SAB), a micro-continent that wasdetached from Gondwana during the Late Palaeozoic – EarlyMesozoic time interval [Barrier and Vrielynck, 2008].

Few palaeontological ages exist for the sedimentarycover of the Sevan ophiolite. Early radiolarian reportspointed to a poorly constrained Late Jurassic-Neocomiantime interval for radiolarites intercalated or overlyingophiolitic lavas from the Sevan zone [Zhamoida et al.,

1976; Zakariadze et al., 1983], but more precise ages werereported in the 1990s [Middle Jurassic, Vishnevskaya,1995; Late Triassic, Knipper et al., 1997] and more recentlyfollowing a French-Armenian collaboration [Middle Juras-sic, Middle Cretaceous, Asatryan et al., 2010, 2011].

In this paper, we present a new palaeontological age ev-idence from a radiolarite sequence overlying volcanic for-mations of the Sevan ophiolite in Armenia; our resultsestablish that submarine volcanic activity was taking placeduring the Jurassic-Cretaceous transition in the Tethyanoceanic realm.

GEOLOGICAL SETTING

The studied outcrop is situated on the northern side of theDali valley, at about 5 km to the northwest of the village Jil(fig. 1; Alt. 2,125 m, N40o 29.11’, E 45o 25.31’).

A large intrusion of plagiogranites within isotropic gab-bros of the Sevan ophiolite crops out along this valley[Galoyan, 2008]. These plagiogranites are locally coveredunconformably by flows of pillow lavas that include lensesor horizons of radiolarian cherts [Galoyan et al., 2009]. Inthe southern part of the Dali valley (at the opposite side ofour studied section), radiolarites can be observed interca-lated with volcanic rocks above the plagiogranites.

In the northern part of the same valley, where the Dalisection was studied and is described here, we recognized


320 ASATRYAN G. et al.

FIG. 1. – Simplified geological map of the ophiolitic outcrops in the east of Lake Sevan, including the location of the studied Dali section [modified afterGaloyan et al., 2009].FIG. 1. – Carte géologique simplifiée des principaux affleurements ophiolitiques à l’Est du lac Sevan, y compris la localisation de la coupe étudiée de Dali[modifié d’après Galoyan et al., 2009].

overall three superimposed units. The uppermost unit (a)comprises mafic rocks with intercalations of sedimentarylayers (radiolarites, siliceous argilites; fig. 2); the lowerunit (c) is made of granitoids including mafic xenoliths; thetransition or contact zone (b) is a thin faulted level made of

brecciated mafic rocks. Below we describe in more detailthe petrographic and geochemical characteristics of the en-countered igneous rocks (see chapter “Results”).

The main sedimentary unit crops out along a mountaintrack (fig. 2a, 3) and consists of a sequence of radiolarites


RADIOLARIAN BIOSTRATIGRAPHIC CONSTRAINTS FOR SUBMARINE VOLCANIC ACTIVITY (SEVAN OPHIOLITE, ARMENIA) 321

FIG. 3. – Schematic representation of the studied outcrop situated along the mountain track at Dali.FIG. 3. – Représentation schématique de l’affleurement étudié le long du sentier montagneux de Dali.

FIG. 2. – Photographic view of the studied outcrops at Dali; a) the arrows point to the position of the studied outcrops; right arrow points to the section ob-served along the mountain track; left arrow points to the section observed on the northern flank of the Dali valley; b) detail of the limestone blocks inclu-ded in the radiolarian chert sequence studied along the mountain track; c) detail of the eastern part of the studied outcrop along the mountain track.FIG. 2. – Vues photographiques des affleurements étudiés à Dali ; a) les flèches indiquent la position des affleurements étudiés ; la flèche droite pointe lacoupe observée le long du sentier montagneux ; la flèche gauche pointe la coupe observée sur le flanc nord de la vallée de Dali ; b) détail des blocs decalcaires inclus dans la série radiolaritique étudiée le long du sentier montagneux ; c) détail de la partie orientale de l’affleurement étudié le long du sen-tier montagneux.

ca. 3 m thick that include towards their top metric and fairlyrounded blocks of oolithic limestones incorporated withinthem.

Another sequence of radiolarian cherts was studied onthe northern side of the Dali Valley, about 200 meters fromthe Dali section (fig. 2a). These cherts are up to 6 m thickand colored dark red/pink. The nature (stratigraphic or tec-tonic) of their contact with both the underlying and overly-ing igneous rocks is not clearly visible in the field (fig. 4).

The entire sequence of rocks described above is over-lain unconformably by a thick pile of Campanian pelagiclimestones [Galoyan et al., 2009; Sosson et al., 2010],which display in places transgressive sandstones and con-glomerates at their base.

MATERIAL AND METHODS

Radiolarians were extracted in the laboratory by repetitiveleaching of samples with low-concentration hydrofluoricacid (HF 4%). Three of the processed samples yielded iden-tifiable Radiolaria. The material was dry picked andmounted on SEM stubs. In general, the preservation of thefauna is relatively good. All three samples yielded fairly di-verse assemblages. Taxonomic concepts applied during thisstudy follow essentially those in Baumgartner et al.[1995a], but also those of Steiger [1992], Yang [1993],Chiari et al. [1997], Dumitrica et al. [1997], Beccaro[2006] and Dumitrica and Zügel [2008] for some species.Generic names are updated according to O’Dogherty et al.

[2009]. Microfacies observations were performed under apolarized microscope. In order to identify the mineralogy ofsome crystals we acquired Raman spectra that were recordedwith a LabRam HR800 Jobin-Yvon™ microspectrometerequipped with 1800 g/mm gratings and using a 532 nm(green) laser excitation.

RESULTS

Radiolarian biochronology

Biostratigraphic results are summarized below. The mainspecies identified are given in table I and most of them areillustrated in plates I-III.

Sample Dali-1 (fig. 3) is characterized by a relativelywell-preserved and diverse radiolarian assemblage, in whichspecies Parapodocapsa amphitreptera appears to be themost common. Representatives of genera Dicerosaturnalis,Archaeodictyomitra, Emiluvia, Pantanellium, Podobursa,Sethocapsa and Tritrabs are also abundant. Based on theidentified assemblage (table I) the fauna can be assigned tothe U.A.Z. 13-17 of Baumgartner et al. [1995b] and thus itcan be correlated with the latest Tithonian to early LateValanginian time interval. The lower boundary of this in-terval (U.A.Z. 13) is based essentially on the presence ofspecies/subspecies Angulobracchia (?) portmanni port-manni, Emiluvia chica decussata, Obesacapsula bullataand Svinitzium depressum. The upper boundary of theabove mentioned interval (U.A.Z. 17) is based on the pres-ence of species Cinguloturris cylindra, Emiluvia pessagnois.l., Obesacapsula cetia and Ristola cretacea.

Sample Dali-2 (fig. 3) yielded a less rich and diverseradiolarian assemblage than the previous sample. The mostcommon species are Archaeodictyomitra apiarium andPantanellium squinaboli. Representatives of generaArchaeodictyomitra, Pantanellium, Parapodocapsa andTethysetta are also abundant. Based on the co-occurrence ofspecies Pantanellium squinaboli and Parapodocapsaamphitreptera the assemblage can be assigned to the U.A.Z.11-18 of Baumgartner et al. [1995b] and can be correlatedwith the Late Kimmeridgian to latest Valanginian/earliestHauterivian interval. A specimen that resembles toZhamoidellum ovum was yielded from this sample, but itspreservation does not allow a confident identification. If thepresence of this marker species is confirmed in the future,sample Dali-2 could be assigned to the U.A.Z. 11 only(Late Kimmeridgian-Early Tithonian).

Sample Jil-3 (fig. 4) yielded a relatively well-preservedradiolarian assemblage in which representatives of generaArchaeodictyomitra, Dicerosaturnalis, Hemicryptocapsa,



PLATE 1. – Scanning electron micrographs of Radiolaria yielded from sample Dali-1.PL. I. – Photos au microscope électronique à balayage des radiolaires extraits de l’échantillon Dali-1.1) Angulobracchia (?) portmanni portmanni BAUMGARTNER; 2) Archaeodictyomitra mitra DUMITRICA; 3) Archaeodictyomitra multicostata (TAN);4) Archaeodictyomitra sp. cf. A. excellens (TAN); 5) Archaeodictyomitra apiarium (RÜST); 6) Archaeodictyomitra excellens (TAN); 7) Archaeodictyomitrasp. cf. A. oleadita HULL; 8) Cinguloturris cylindra KEMKIN and RUDENKO; 9) Dicerosaturnalis trizonalis (RÜST) sensu Dumitrica and Zügel [2008];10) Ditrabs sp. cf. D. sansalvadorensis (PESSAGNO); 11) Emiluvia chica decussata STEIGER; 12-13) Emiluvia pessagnoi s.l. FOREMAN; 14) Stichomitrasp. cf. S. doliolum AITA; 15) Loopus yangi DUMITRICA; 16) Obesacapsula cetia (FOREMAN); 17) Obesacapsula bullata STEIGER; 18-19) Parapodocapsa am-phitreptera (FOREMAN); 20) Praecaneta sp. cf. P. cosmoconica (FOREMAN); 21) Pyramispongia sp. cf. P. barmsteinensis (STEIGER); 22-23) Ristola cretaceaBAUMGARTNER; 24) Pseudodictyomitrella tuscanica (CHIARI, CORTESE and MARCUCCI); 25) Svinitzium depressum (BAUMGARTNER); 26) Favosyringium sp.cf. F. latum (YANG); 27) Favosyringium sp.; 28) Spinosicapsa (?) sp. cf. Dibolachras chandrika KOCHER; 29) Favosyringium sp. cf. F. affine (RÜST);30) Triactoma tithonianum RÜST.

FIG. 4. – Stratigraphic column of the radiolarite sequence that crops out onthe northern side of the Dali valley.FIG. 4. – Log de la série radiolaritique qui affleure sur le flanc nord de lavallée de Dali.

Pantanellium, Sethocapsa, Svinitzium and Tethysetta arefairly abundant. The co-occurrence of the Dicerosaturnalistrizonalis dicranacanthos (SQUINABOL) morphotype sensuBaumgartner et al. [1995a] (= Dicerosaturnalis trizonalis(RÜST) sensu Dumitrica and Zügel [2008]) and ofHemicryptocapsa capita would suggest assignment of sam-ple Jil-3 to the U.A.Z. 17 of Baumgartner et al. [1995b].However, we prefer to correlate it with a wider (and moreconfident) age range that covers the Berriasian to latestValanginian/earliest Hauterivian interval (U.A.Z. 14-18)and it is established based on the co-occurrence of speciesEmiluvia chica decussata, Hiscocapsa zweilii andSvinitzium depressum.

Petrography of limestone blocks

Details of the microfacies of the main limestone block(sample Dali-06 calc-1; fig. 3) are illustrated on plate IV.

The microfacies of this limestone correspond to an ooliticgrainstone with fragments of crinoids (pl. IV, fig. A). Ra-dial ooids are well-preserved, abundant and of variable size(pl. IV, fig. B). Oolites, echinoderm bioclasts andlithoclasts appear to have been redeposited as they are in-cluded in a clayey matrix, which is composed of a darkclayey micrite, and surrounds common bioclasts (i.e. algalfragments, pl. IV, fig. C). Oolites are in general aggregated[lump stage sensu Tucker and Bathurst, 1990] and limitedby a stylolithic joint displaying evidence of compaction(black arrow, pl. IV, fig. D). In these microbreccias, theoriginal cement displays a microspar texture (white arrow,pl. IV, fig. D). Various cavities, formed after partial dissolu-tion of bioclasts, are filled with microcrystals of quartz(black arrow, pl. IV, fig. E). Numerous idiomorphic crystalsof silicate minerals can be observed inside all clasts (bothlithoclasts and bioclasts) or oolites. They do not display a



TABLE I. – Radiolarian species identified from the three fossiliferous samples.TABL. I. – Liste des espèces de radiolaires déterminés dans les trois échantillons productifs.



PLATE II. – Scanning electron micrographs of Radiolaria yielded from sample Dali-2.PL. II. – Photos au microscope électronique à balayage des radiolaires extraits de l’échantillon Dali-2.1) Alievium sp.; 2) Parapodocapsa amphitreptera (FOREMAN) ; 3) Pantanellium squinaboli (TAN) ; 4) Spinosicapsa sp. cf. S. vannae (BECCARO) ; 5) Pseudo-dictyomitra sp. cf. P. lilyae (TAN) ; 6) Svinitzium sp. cf. S. mizutanii DUMITRICA ; 7) Pseudodictyomitrella tuscanica (CHIARI, CORTESE and MARCUCCI) ;8) Pseudodictyomitrella sp. ; 9) Tethysetta sp. cf. T. ovoidala DUMITRICA ; 10) Tethysetta sp. cf. T. usotanensis (TUMANDA) ; 11) Triactoma sp. cf. T. mexi-cana PESSAGNO and YANG ; 12) Zhamoidellum sp. cf. Z. ovum DUMITRICA.

PLATE III. – Scanning electron micrographs of Radiolaria yielded from sample Jil-03.PL. III. – Photos au microscope électronique à balayage des radiolaires extraits de l’échantillon Jil-03.1) Dicerosaturnalis trizonalis dicranacanthos (SQUINABOL) sensu Baumgartner et al. (1995) = Dicerosaturnalis trizonalis (RÜST) sensu Dumitrica andZügel (2008); 2) Emiluvia chica decussata STEIGER; 3) Hemicryptocapsa capita TAN; 4) Mirifusus dianae s.l. (KARRER); 5) Pseudoeucyrtis sp. cf. P. acusJUD; 6-7) Hiscocapsa zweilii (JUD); 8) Favosyringium sp.; 9) Svinitzium depressum (BAUMGARTNER); 10) Tethysetta ovoidala DUMITRICA; 11) Svinitziumsp. cf. S. columnum (RÜST).

preferential orientation which points to a post sedimentarygrowth (pl. IV, fig. F). Raman acquisition spectra establishthat they are all idiomorphic crystals of albite.

Petrography of igneous rocks

The igneous rocks consist of two units of mafic volcanicrocks, overlying the plagiogranite body. Although the pri-mary phases of the mafic rocks are extensively replaced bysecondary low-temperature minerals, igneous textures arewell-preserved and igneous minerals can still be identifiedunder the microscope. They are described below from top tobase.

a– Mafic rocks of the upper unit are dark, massive andfine-grained, forming a homogeneous, banded sequence;some levels contain amygdules filled with calcite. In thinsection, textures vary from intersertal through intergranularto subophitic; true ophitic texture is also present locally.Such fine-grained subophitic to ophitic textures are charac-teristic of slow-cooling and can be observed in the centralparts of thick lava flows as well as in hypabyssal diabasesfilling dikes or sills. As a consequence, distinction betweenmassive lava flows and hypabyssal rocks is not easy, and aneffusive origin has been considered for two samples of thispreliminary study based on the presence of vesicles and a

higher amount of glass alteration products. Also subophitictexture is diagnostic of basaltic primitive or only slightlydifferentiated compositions, whereas intersertal to intergra-nular textures are more common in evolved compositions,although also present in basalts. On the whole, all thesecharacteristics suggest that this upper unit represents part ofa thick volcanic sequence. Studied samples of lava flowscontain phenocrysts and microphenocrysts of clinopyro-xene, plagioclase and magmatic brown amphibole; they arefrequently aggregated in glomerules. The intersertalgroundmass is composed of microlites with three distinctgrain sizes. The larger microlites form a loose network ofthin plagioclase laths and clinopyroxene and brown amphi-bole prisms, often distributed in sheafs. The interstices arefilled with tiny crystals of the same mineral phases with inaddition dendritic titanomagnetite. The spaces betweenthese tiny crystals are occupied by quenched minerals offeldspar, clinopyroxene and brown amphibole; chlorite-richfelty patches or polygonal grains of albite also locally filledthese interstices. Fluorapatite is a common accessory phase.Plagioclase is commonly replaced by albite and calcite; rel-ict compositions vary from An56 in the coarser laths to An34in the smaller-sized laths. Clinopyroxene and brown amphi-bole are partially replaced by calcite, quartz (forclinopyroxene), chlorite, titanite and iron oxides. EDS



PLATE IV. – Microfacies of limestone sample Dali-06 calc-1. A) Oolitic and bioclastic grainstone. Bioclasts are mainly crinoidal stems. The matrix is a darkand homogeneous fine grained micrite. Scale bar = 1 mm. B) Radial and fibrous ooids. Scale bar = 1 mm. C) Fragments of bioclasts. Scale bar = 0,1 mm.D) Amalgamated oolitic microbreccia. White arrows indicate the raw microcrystalline calcitic matrix. Black arrow shows a stylolitic suture due to thecompaction. Scale bar = 1 mm. E) Moldic porosity. Scale bar = 1 mm. F) Authigenic quartz and feldspar crystals association. Scale bar = 0,1 mm.PL. IV. – Microfaciès de l’échantillon de roche calcaire Dali-06 calc-1. A) Grainstone oolithique et bioclastique. Les bioclastes correspondent majoritaire-ment à des fragments crinoïdiques. La matrice est constituée de fines particules homogènes constituant une texture micritique sombre. Barre d’échelle = 1 mm.B) Oolithe à structure radiaire et fibreuse. Barre d’échelle = 1 mm. C) Fragments bioclastiques indéterminés. Barre d’échelle = 0, mm. D) Microbrèchecomposée d’un amalgame oolithique. Les flèches blanches indiquent la présence de matrice microcristalline calcitique originelle. Les flèches noires mon-trent une suture stylolithique liée aux phénomènes de pression-dissolution. Barre d’échelle = 1 mm. E) Porosité résiduelle. Barre d’échelle = 1 mm.F) Association de cristaux de quartz authigène et de feldspath. Barre d’échelle = 0,1 mm.

analyses indicate that the amphibole is Al-, Na- andTi-rich, subjective of an alkaline affinity. Other samplesrepresenting lava flows are extensively altered but appear tohave had similar mineral compositions; rare primary biotitemicrocrystals have been observed. The lavas underlying theradiolarites appear different, since they are characterized byan intersertal texture with high proportion of glass. Tinyminerals are quench plagioclases showing characteristichollow cores, and quench ferromagnesian minerals olivineand/or clinopyroxene); small Cr-rich spinel grains are alsopresent. These features suggest a primitive basaltic compo-sition that cooled very rapidly. More crystalline samples areporphyric to glomeroporphyric basaltic rocks withfine-grained, intergranular- to ophitic-textured matrix(fig. 5). Some samples show a rough igneous lamination.Like the vesicular rocks, they show several groups of grainsizes of phenocrysts and groundmass microlites. But theydiffer by the occurrence of olivine commonly associatedwith interstitial clinopyroxene. Brown magmatic amphibolehas not been observed as phenocrysts but is present in thegroundmass. The phenocryst assemblage is plagioclase +clinopyroxene (+ olivine?), and the groundmass assemblageis plagioclase + clinopyroxene + olivine + brown amphibole+ dendritic titanomagnetite + fluorapatite. Secondary min-erals pseudomorphosing the igneous phases are albite,chlorite, titanite, calcite, quartz and Fe-oxides, characteris-tic of alteration in the greenschist facies conditions.

b– The base of the mafic unit is represented bybrecciated basalts, crosscut by veins and microfracturesfilled with quartz or calcite or quartz-feldspar assemblageslikely related to the neighboring granitoid intrusion. Angu-lar fragments of basalt are dark, glassy rock with small ves-icles filled with calcite. Some of the samples are polygenic,

containing at least two types of basalts. All are fine-grainedmineral assemblages composed of chlorite, albite, titanite,calcite, quartz and iron oxides; clinopyroxene andplagioclase pseudomorphs define various textures fromintersertal with a high proportion of ancient glass (darkfragments) to more crystalline (gray fragments). Pheno-crysts of plagioclase and clinopyroxene are present. A sam-ple contains a xenolith of microgabbro (diabase).

c– The granitoid is a coarse-grained assemblage ofquartz, plagioclase and a lesser amount of green hornblendewith accessory apatite and zircon grains. Its base is rela-tively homogeneous and contains oval, decimetric-sized xe-noliths of oriented diorite. To the top, the granitoidbecomes brecciated, crosscut by dykes of more felsic com-position and veins of calcite or quartz. In addition, it en-closes angular fragments resembling the mafic rocks of theabove unit, suggesting that the granitoid is intrusive in themafic formation (fig. 6).

DISCUSSION

Previous data on the sedimentary cover of the Sevan ophiolitesuggest a pre-Cretaceous history of submarine volcanic activ-ity starting since the Late Triassic. Thus Knipper et al. [1997]found Carnian siliceous pelites overlying breccia with frag-ments of ophiolitic rocks at Old Sotk (Zod) Pass SE of LakeSevan (fig. 1). According to the stratigraphic data of the aboveauthors, large olistolith blocks of nodular limestones slid in



FIG. 5. – Microphotograph of a characteristic texture observed in the stu-died magmatic rocks from Dali. Twin clinopyroxene interstitial betweenfeldspar laths include smaller laths of feldspar in subophitic diabase.Length of field = 1.5 mm.FIG. 5. – Microphotographie d’une texture caractéristique observée au seindes roches magmatiques de l’affleurement de Dali. Clinopyroxène macléincluant de petites lattes de feldspaths dans une diabase à texture ophi-tique. Largeur du champ = 1,5 mm.

FIG. 6. – Field photograph of the brecciated contact between the plagiogra-nite (below) and the basic rocks (above), that consists of mafic rocks intru-ded by felsic rocks. Angular blocks (arrow) of basalts are crosscut bynumerous felsic veinlets, suggesting a hydraulic fracturing in response tothe granite intrusion into a cold mafic unit. A second type of xenoliths oc-curs as rounded blocks of diorite.FIG. 6. – Photo d’affleurement montrant la structure bréchique du contactentre le plagiogranite et les roches basiques sus-jacentes. Des blocs angu-leux de basaltes (flèche) recoupés par de nombreux filons felsiques suggè-rent une fracturation hydraulique provoquée par l'intrusion du magmagranitique dans un encaissant basique relativement froid. Le plagiogranitecontient un deuxième type d'enclaves sous forme de blocs arrondis de dio-rite.

the Toarcian part of the section. Jurassic volcanic events areknown in the Sevan-Hagari (Akera) zone. In the Karawul lo-cality of Karabagh, Vishnevskaya [1995, 2001] established thepresence of Middle Jurassic cherts that overly stratigraphicallybasaltic lavas. More recently, at the Sarinar valley of Sevanarea, Asatryan et al. [2010] reported on upper Bajocian-lowerBathonian cherts overlying ophiolitic lavas. In this locality, theabove authors reported also Middle Jurassic radiolarian chertsintercalated between tuffites. Finally, much younger Creta-ceous (upper Barremian-lower Aptian) radiolarian cherts over-lying stratigraphically pillow lavas were recently dated byAsatryan et al. [2011] in Karabagh.

Our results from the Dali section establish for the firsttime evidence for submarine volcanic activity slightly beforeor some time during the Late Tithonian-Late Valanginian in-terval.

Radiolarian ooze accumulation took place on a bathy-metrically complex sea floor, fractured probably by impor-tant normal faults cropping out on the Jurassic sea floor,which were subject to submarine weathering. The presenceof limestone blocks intercalated within the radiolarites is ofparticular importance because it provides, for the first time,the evidence of shallow-water depositional environments ofcarbonate sedimentation in the vicinity of a basin in whichradiolarites were being accumulated. Oolites and crinoidbioclasts were initially formed (during the Late Jurassic?)

in very shallow and agitated waters of a platform barrierand they were subsequently redeposited in an upper slopedepositional environment.

CONCLUSION

Radiolarian ages for the sedimentary cover of the Armenianophiolites are of great significance for the palaeogeographicand geodynamic reconstruction of the Tethys in the LesserCaucasus. The Sevan ophiolite records both ophiolitic lavasand pelagic sediments of latest Tithonian to Late Valanginianage that allow us to constrain the age of submarine volcanicactivity that took place within the Tethyan ocean floor.Shallow water oolitic platforms presumably existed sometime during the Late Jurassic (?) in the vicinity of this oce-anic basin, since parts of them slid into the radiolarite basinclose to the Jurassic-Cretaceous transition.

Acknowledgments. – This study was initiated in 2006 with the support ofthe French Ministry of Foreign Affairs (ECO-NET grant to T. Danelian);the French Embassy in Armenia provided a student grant to G. Asatryan forstudying at the University Pierre & Marie Curie (Paris VI). This work wascompleted with the financial support of the DARIUS program. Fieldworkwas greatly facilitated by the continuous support of the Institute of Geolo-gical Sciences (National Academy of Sciences of Armenia). Constructiveremarks by M. Chiari (Florence), S. Zyabrev (Khabarovsk) and ŠpelaGori�an (Ljubljana) improved the initial manuscript.

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