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Radiolarian biostratigraphic constraints for latest Jurassic-earliest Cretaceous
submarine 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. HUBERT5and 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 the
South-Armenian block, which is a micro-continent of Gondwanian origin. We present here radiolarian data obtained
from radiolarites that are intercalated in a sequence of mafic volcanic rocks on the northern flank of the Dali valley (east
of Lake Sevan), which is considered to be part of the Sevan ophiolite. Mafic sills and dykes with well-preserved igneous
textures are probably part of the same sequence. The pseudomorphosis of primary phases indicates that the igneous
rocks are strongly affected by alteration in the greenschist facies condition. The plagiogranites that are present in this
locality appear to be intrusive into the mafic sequence. The radiolarian assemblages extracted from radiolarian cherts
intercalated in the mafic volcanic rocks are dated as latest Tithonian-Late Valanginian; they contain metric rounded
blocks 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 volcanique
sous-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 sont
d’une grande importance pour la reconstitution paléogéographique et géodynamique des terrains situés entre l’Eurasie
et 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 nord
de 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 une
paragenèse schiste vert font probablement partie de la même série. Les plagiogranites situés sous cet ensemble semblent
être 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 calcaires
oolithiques à fragments de crinoïdes, ce qui suggère que ces calcaires peu profonds ont glissé au cours de la transition
Jurassique/Crétacé sur le fond océanique accidenté où s’accumulaient des boues radiolaritiques.
INTRODUCTION
The sedimentary cover of most Mesozoic ophiolites along
the Tethyan belt is made essentially of radiolarian-rich sili-
ceous sediments. Radiolarian biochronology is an important
tool to unravel the geodynamic and palaeoenvironmental
evolution 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 and
Gorian, 2005; Ziabrev et al., 2003]. This is especially true
for 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, no4, pp. 319-330
Bull. Soc. géol. Fr., 2012, no4
1. Institute of Geological Sciences, National Academy of Sciences of Armenia, 24a Baghramian avenue, Yerevan, 0019, Armenia. asatryan@geology.am
2. 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, France
3. University Lille 1, Department of Earth Sciences, CNRS-UMR 8217 “Géosystèmes”, SN5, 59655 Villeneuve d’Ascq, France
4. University of Nice – Sophia Antipolis, CNRS-UMR Géoazur, OCA, 250 rue A. Einstein. 6560 Valbonne 2, France
5. Catholic University of Lille, CNRS-FRE 3298 “Géosystèmes”, SN5, 59655 Villeneuve d’Ascq, France
Manuscript received on May 12, 2011; accepted on March 22, 2012
Gorian 1994; Karakitsios et al., 1988; Stais et al., 1990],
as well as for the Alpine chains of Turkey [Bragin and
Tekin 1996; Danelian et al., 2006; Moix et al., 2009; Tekin
and Göncüoglu, 2007; Tekin et al., 2002; Vrielynck et al.,
2003]. Reliable ages for ophiolitic lavas are still fairly rare
further to the east, such as in the Lesser Caucasus area, in
spite of some recent radiolarian data generated through a
French-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 NE
part of Lake Sevan (fig. 1; for a geological map of the area
see Danelian et al. [2012]). It belongs to a NW-SE oriented
zone extending for ca. 400 km from the Amasia-Stepanavan
ophiolite 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 the
main Tethyan suture zone between Eurasia and the
South-Armenian block (SAB), a micro-continent that was
detached from Gondwana during the Late Palaeozoic – Early
Mesozoic time interval [Barrier and Vrielynck, 2008].
Few palaeontological ages exist for the sedimentary
cover of the Sevan ophiolite. Early radiolarian reports
pointed to a poorly constrained Late Jurassic-Neocomian
time interval for radiolarites intercalated or overlying
ophiolitic lavas from the Sevan zone [Zhamoida et al.,
1976; Zakariadze et al., 1983], but more precise ages were
reported in the 1990s [Middle Jurassic, Vishnevskaya,
1995; Late Triassic, Knipper et al., 1997] and more recently
following 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 results
establish that submarine volcanic activity was taking place
during the Jurassic-Cretaceous transition in the Tethyan
oceanic realm.
GEOLOGICAL SETTING
The studied outcrop is situated on the northern side of the
Dali valley, at about 5 km to the northwest of the village Jil
(fig. 1; Alt. 2,125 m, N40o29.11’, E 45o25.31’).
A large intrusion of plagiogranites within isotropic gab-
bros of the Sevan ophiolite crops out along this valley
[Galoyan, 2008]. These plagiogranites are locally covered
unconformably by flows of pillow lavas that include lenses
or horizons of radiolarian cherts [Galoyan et al., 2009]. In
the southern part of the Dali valley (at the opposite side of
our studied section), radiolarites can be observed interca-
lated with volcanic rocks above the plagiogranites.
In the northern part of the same valley, where the Dali
section was studied and is described here, we recognized
Bull. Soc. géol. Fr., 2012, no4
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 after
Galoyan 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 sedimentary
layers (radiolarites, siliceous argilites; fig. 2); the lower
unit (c) is made of granitoids including mafic xenoliths; the
transition or contact zone (b) is a thin faulted level made of
brecciated mafic rocks. Below we describe in more detail
the petrographic and geochemical characteristics of the en-
countered igneous rocks (see chapter “Results”).
The main sedimentary unit crops out along a mountain
track (fig. 2a, 3) and consists of a sequence of radiolarites
Bull. Soc. géol. Fr., 2012, no4
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 la
coupe 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 de
calcaires 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 fairly
rounded blocks of oolithic limestones incorporated within
them.
Another sequence of radiolarian cherts was studied on
the northern side of the Dali Valley, about 200 meters from
the Dali section (fig. 2a). These cherts are up to 6 m thick
and 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 pelagic
limestones [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 repetitive
leaching of samples with low-concentration hydrofluoric
acid (HF 4%). Three of the processed samples yielded iden-
tifiable Radiolaria. The material was dry picked and
mounted on SEM stubs. In general, the preservation of the
fauna is relatively good. All three samples yielded fairly di-
verse assemblages. Taxonomic concepts applied during this
study 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 a
polarized microscope. In order to identify the mineralogy of
some crystals we acquired Raman spectra that were recorded
with a LabRam HR800 Jobin-Yvon™ microspectrometer
equipped with 1800 g/mm gratings and using a 532 nm
(green) laser excitation.
RESULTS
Radiolarian biochronology
Biostratigraphic results are summarized below. The main
species identified are given in table I and most of them are
illustrated in plates I-III.
Sample Dali-1 (fig. 3) is characterized by a relatively
well-preserved and diverse radiolarian assemblage, in which
species Parapodocapsa amphitreptera appears to be the
most common. Representatives of genera Dicerosaturnalis,
Archaeodictyomitra, Emiluvia, Pantanellium, Podobursa,
Sethocapsa and Tritrabs are also abundant. Based on the
identified assemblage (table I) the fauna can be assigned to
the U.A.Z. 13-17 of Baumgartner et al. [1995b] and thus it
can be correlated with the latest Tithonian to early Late
Valanginian time interval. The lower boundary of this in-
terval (U.A.Z. 13) is based essentially on the presence of
species/subspecies Angulobracchia (?) portmanni port-
manni,Emiluvia chica decussata,Obesacapsula bullata
and Svinitzium depressum. The upper boundary of the
above mentioned interval (U.A.Z. 17) is based on the pres-
ence of species Cinguloturris cylindra,Emiluvia pessagnoi
s.l.,Obesacapsula cetia and Ristola cretacea.
Sample Dali-2 (fig. 3) yielded a less rich and diverse
radiolarian assemblage than the previous sample. The most
common species are Archaeodictyomitra apiarium and
Pantanellium squinaboli. Representatives of genera
Archaeodictyomitra, Pantanellium, Parapodocapsa and
Tethysetta are also abundant. Based on the co-occurrence of
species Pantanellium squinaboli and Parapodocapsa
amphitreptera the assemblage can be assigned to the U.A.Z.
11-18 of Baumgartner et al. [1995b] and can be correlated
with the Late Kimmeridgian to latest Valanginian/earliest
Hauterivian interval. A specimen that resembles to
Zhamoidellum ovum was yielded from this sample, but its
preservation does not allow a confident identification. If the
presence 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-preserved
radiolarian assemblage in which representatives of genera
Archaeodictyomitra,Dicerosaturnalis,Hemicryptocapsa,
Bull. Soc. géol. Fr., 2012, no4
322 ASATRYAN G. et al.
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) Archaeodictyomitra
sp. 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) Stichomitra
sp. 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 cretacea
BAUMGARTNER; 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 on
the northern side of the Dali valley.
FIG.4.–Log de la série radiolaritique qui affleure sur le flanc nord de la
vallée de Dali.
Bull. Soc. géol. Fr., 2012, no4
RADIOLARIAN BIOSTRATIGRAPHIC CONSTRAINTS FOR SUBMARINE VOLCANIC ACTIVITY (SEVAN OPHIOLITE, ARMENIA) 323
Pantanellium,Sethocapsa,Svinitzium and Tethysetta are
fairly abundant. The co-occurrence of the Dicerosaturnalis
trizonalis dicranacanthos (SQUINABOL) morphotype sensu
Baumgartner et al. [1995a] (= Dicerosaturnalis trizonalis
(RÜST)sensu Dumitrica and Zügel [2008]) and of
Hemicryptocapsa 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 more
confident) age range that covers the Berriasian to latest
Valanginian/earliest Hauterivian interval (U.A.Z. 14-18)
and it is established based on the co-occurrence of species
Emiluvia chica decussata, Hiscocapsa zweilii and
Svinitzium 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 oolitic
grainstone 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 and
lithoclasts appear to have been redeposited as they are in-
cluded in a clayey matrix, which is composed of a dark
clayey micrite, and surrounds common bioclasts (i.e. algal
fragments, pl. IV, fig. C). Oolites are in general aggregated
[lump stage sensu Tucker and Bathurst, 1990] and limited
by a stylolithic joint displaying evidence of compaction
(black arrow, pl. IV, fig. D). In these microbreccias, the
original 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 crystals
of silicate minerals can be observed inside all clasts (both
lithoclasts and bioclasts) or oolites. They do not display a
Bull. Soc. géol. Fr., 2012, no4
324 ASATRYAN G. et al.
TABLE I. – Radiolarian species identified from the three fossiliferous samples.
TABL. I. – Liste des espèces de radiolaires déterminés dans les trois échantillons productifs.
Bull. Soc. géol. Fr., 2012, no4
RADIOLARIAN BIOSTRATIGRAPHIC CONSTRAINTS FOR SUBMARINE VOLCANIC ACTIVITY (SEVAN OPHIOLITE, ARMENIA) 325
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. m ex i -
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)sensuDumitricaand
Zügel (2008); 2) Emiluvia chica decussata STEIGER;3)Hemicryptocapsa capita TAN;4)Mirifusus dianae s.l. (KARRER); 5) Pseudoeucyrtis sp. cf. P. acus
JUD;6-7)Hiscocapsa zweilii (JUD); 8) Favosyringium sp.; 9) Svinitzium depressum (BAUMGARTNER); 10) Tethysetta ovoidala DUMITRICA; 11) Svinitzium
sp. cf. S. columnum (RÜST).
preferential orientation which points to a post sedimentary
growth (pl. IV, fig. F). Raman acquisition spectra establish
that they are all idiomorphic crystals of albite.
Petrography of igneous rocks
The igneous rocks consist of two units of mafic volcanic
rocks, overlying the plagiogranite body. Although the pri-
mary phases of the mafic rocks are extensively replaced by
secondary low-temperature minerals, igneous textures are
well-preserved and igneous minerals can still be identified
under the microscope. They are described below from top to
base.
a– Mafic rocks of the upper unit are dark, massive and
fine-grained, forming a homogeneous, banded sequence;
some levels contain amygdules filled with calcite. In thin
section, textures vary from intersertal through intergranular
to 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 central
parts of thick lava flows as well as in hypabyssal diabases
filling dikes or sills. As a consequence, distinction between
massive lava flows and hypabyssal rocks is not easy, and an
effusive origin has been considered for two samples of this
preliminary study based on the presence of vesicles and a
higher amount of glass alteration products. Also subophitic
texture is diagnostic of basaltic primitive or only slightly
differentiated compositions, whereas intersertal to intergra-
nular textures are more common in evolved compositions,
although also present in basalts. On the whole, all these
characteristics suggest that this upper unit represents part of
a thick volcanic sequence. Studied samples of lava flows
contain phenocrysts and microphenocrysts of clinopyro-
xene, plagioclase and magmatic brown amphibole; they are
frequently aggregated in glomerules. The intersertal
groundmass is composed of microlites with three distinct
grain sizes. The larger microlites form a loose network of
thin plagioclase laths and clinopyroxene and brown amphi-
bole prisms, often distributed in sheafs. The interstices are
filled with tiny crystals of the same mineral phases with in
addition dendritic titanomagnetite. The spaces between
these tiny crystals are occupied by quenched minerals of
feldspar, clinopyroxene and brown amphibole; chlorite-rich
felty patches or polygonal grains of albite also locally filled
these 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 An34
in the smaller-sized laths. Clinopyroxene and brown amphi-
bole are partially replaced by calcite, quartz (for
clinopyroxene), chlorite, titanite and iron oxides. EDS
Bull. Soc. géol. Fr., 2012, no4
326 ASATRYAN G. et al.
PLATE IV. – Microfacies of limestone sample Dali-06 calc-1. A) Oolitic and bioclastic grainstone. Bioclasts are mainly crinoidal stems. The matrix is a dark
and 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 the
compaction. 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) Microbrèche
composée d’un amalgame oolithique. Les flèches blanches indiquent la présence de matrice microcristalline calcitique originelle. Les flèches noire s 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- and
Ti-rich, subjective of an alkaline affinity. Other samples
representing lava flows are extensively altered but appear to
have had similar mineral compositions; rare primary biotite
microcrystals have been observed. The lavas underlying the
radiolarites appear different, since they are characterized by
an intersertal texture with high proportion of glass. Tiny
minerals are quench plagioclases showing characteristic
hollow cores, and quench ferromagnesian minerals olivine
and/or clinopyroxene); small Cr-rich spinel grains are also
present. These features suggest a primitive basaltic compo-
sition that cooled very rapidly. More crystalline samples are
porphyric to glomeroporphyric basaltic rocks with
fine-grained, intergranular- to ophitic-textured matrix
(fig. 5). Some samples show a rough igneous lamination.
Like the vesicular rocks, they show several groups of grain
sizes of phenocrysts and groundmass microlites. But they
differ by the occurrence of olivine commonly associated
with interstitial clinopyroxene. Brown magmatic amphibole
has not been observed as phenocrysts but is present in the
groundmass. The phenocryst assemblage is plagioclase +
clinopyroxene (+ olivine?), and the groundmass assemblage
is 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 by
brecciated basalts, crosscut by veins and microfractures
filled with quartz or calcite or quartz-feldspar assemblages
likely 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-grained
mineral assemblages composed of chlorite, albite, titanite,
calcite, quartz and iron oxides; clinopyroxene and
plagioclase pseudomorphs define various textures from
intersertal with a high proportion of ancient glass (dark
fragments) 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 of
quartz, plagioclase and a lesser amount of green hornblende
with 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 granitoid
becomes 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 the
above unit, suggesting that the granitoid is intrusive in the
mafic formation (fig. 6).
DISCUSSION
Previous data on the sedimentary cover of the Sevan ophiolite
suggest 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 Lake
Sevan (fig. 1). According to the stratigraphic data of the above
authors, large olistolith blocks of nodular limestones slid in
Bull. Soc. géol. Fr., 2012, no4
RADIOLARIAN BIOSTRATIGRAPHIC CONSTRAINTS FOR SUBMARINE VOLCANIC ACTIVITY (SEVAN OPHIOLITE, ARMENIA) 327
FIG. 5. – Microphotograph of a characteristic texture observed in the stu-
died magmatic rocks from Dali. Twin clinopyroxene interstitial between
feldspar laths include smaller laths of feldspar in subophitic diabase.
Length of field = 1.5 mm.
FIG.5.–Microphotographie d’une texture caractéristique observée au sein
des roches magmatiques de l’affleurement de Dali. Clinopyroxène maclé
incluant 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 by
numerous felsic veinlets, suggesting a hydraulic fracturing in response to
the 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 contact
entre 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 magma
granitique dans un encaissant basique relativement froid. Le plagiogranite
contient un deuxième type d'enclaves sous forme de blocs arrondis de dio-
rite.
the Toarcian part of the section. Jurassic volcanic events are
known in the Sevan-Hagari (Akera) zone. In the Karawul lo-
cality of Karabagh, Vishnevskaya [1995, 2001] established the
presence of Middle Jurassic cherts that overly stratigraphically
basaltic lavas. More recently, at the Sarinar valley of Sevan
area, Asatryan et al. [2010] reported on upper Bajocian-lower
Bathonian cherts overlying ophiolitic lavas. In this locality, the
above authors reported also Middle Jurassic radiolarian cherts
intercalated between tuffites. Finally, much younger Creta-
ceous (upper Barremian-lower Aptian) radiolarian cherts over-
lying stratigraphically pillow lavas were recently dated by
Asatryan et al. [2011] in Karabagh.
Our results from the Dali section establish for the first
time evidence for submarine volcanic activity slightly before
or 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 presence
of limestone blocks intercalated within the radiolarites is of
particular importance because it provides, for the first time,
the evidence of shallow-water depositional environments of
carbonate sedimentation in the vicinity of a basin in which
radiolarites were being accumulated. Oolites and crinoid
bioclasts were initially formed (during the Late Jurassic?)
in very shallow and agitated waters of a platform barrier
and they were subsequently redeposited in an upper slope
depositional environment.
CONCLUSION
Radiolarian ages for the sedimentary cover of the Armenian
ophiolites are of great significance for the palaeogeographic
and geodynamic reconstruction of the Tethys in the Lesser
Caucasus. The Sevan ophiolite records both ophiolitic lavas
and pelagic sediments of latest Tithonian to Late Valanginian
age that allow us to constrain the age of submarine volcanic
activity that took place within the Tethyan ocean floor.
Shallow water oolitic platforms presumably existed some
time during the Late Jurassic (?) in the vicinity of this oce-
anic basin, since parts of them slid into the radiolarite basin
close to the Jurassic-Cretaceous transition.
Acknowledgments. – This study was initiated in 2006 with the support of
the French Ministry of Foreign Affairs (ECO-NET grant to T. Danelian);
the French Embassy in Armenia provided a student grant to G. Asatryan for
studying at the University Pierre & Marie Curie (Paris VI). This work was
completed with the financial support of the DARIUS program. Fieldwork
was greatly facilitated by the continuous support of the Institute of Geolo-
gical Sciences (National Academy of Sciences of Armenia). Constructive
remarks by M. Chiari (Florence), S. Zyabrev (Khabarovsk) and Špela
Gorian (Ljubljana) improved the initial manuscript.
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