ORIGINAL PAPER

Blueschists of the Amassia-Stepanavan Suture Zone (Armenia):linking Tethys subduction history from E-Turkey to W-Iran

Yann Rolland Æ Sandra Billo Æ Michel Corsini ÆMarc Sosson Æ Ghazar Galoyan

Received: 11 June 2007 / Accepted: 10 November 2007 / Published online: 11 December 2007

� Springer-Verlag 2007

Abstract The Amassia–Stepanavan blueschist-ophiolite

complex of the Lesser Caucasus in NW Armenia is part of an

Upper Cretaceous-Cenozoic belt, which presents similar

metamorphic features as other suture zones from Turkey to

Iran. The blueschists include calcschists, metaconglomer-

ates, quartzites, gneisses and metabasites, suggesting a

tectonic melange within an accretionary prism. This blues-

chist melange is tectonically overlain by a low-metamorphic

grade ophiolite sequence composed of serpentinites, gabbro-

norite pods, plagiogranites, basalts and radiolarites. The

metabasites include high-P assemblages (glaucophane–

aegirine–clinozoisite–phengite), which indicate maximal

burial pressure of *1.2 GPa at *550�C. Most blueschists

show evidence of greenschist retrogression (chlorite—

epidote, actinolite), but locally epidote-amphibolite condi-

tions were attained (garnet—epidote, Ca/Na amphibole) at a

pressure of *0.6 GPa and a temperature of *500�C. This

LP–MT retrogression is coeval with exhumation and nappe-

stacking of lower grade units over higher grade ones.40Ar/39Ar phengite ages obtained on the high-P assemblages

range between 95 and 90 Ma, while ages obtained for

epidote-amphibolite retrogression assemblages range within

73.5–71 Ma. These two metamorphic phases are significant

of (1) HP metamorphism during a phase of subduction in the

Cenomanian–Turonian times followed by (2) exhumation in

the greenschist to epidote-amphibolite facies conditions

during the Upper Campanian/Maastrichtian due to the onset

of continental subduction of the South Armenian block

below Eurasia.

Keywords Armenia � Lesser Caucasus � Blueschists �HP metamorphism � 40Ar/39Ar dating

Introduction

The Alpine–Himalayan tectonic belt forms a puzzling

network of suture zones and micro-blocks from the eastern

Mediterranean area to the NW Himalayan belt (Sengor and

Yilmaz 1981; Tirrul et al. 1983; Ricou et al. 1985; Dercourt

et al. 1986; Ricou 1994; Okay and Tuysuz 1999; Stampfli

and Borel 2002, Fig. 1). From the above studies, the time

of ocean subductions, obductions, micro-plate accretions

and exhumations ranges mostly from the Upper Cretaceous

to the Eocene in that area. Still, it remains difficult to

correlate units from Turkey to Iran, because key geochro-

nological and metamorphic data still lack from the main

presumed suture zones. Further, lateral correlations of

continental terranes of presumed Gondwanian origin are

still controversial. This paper describes a High Pressure

(HP) complex, the Amassia-Stepanavan Blueschist Ophi-

olite complex (ASBOC) located in northern Armenia, from

which structural and Pressure–Temperature–time (P-T-t)

constraints are provided. These data are further used to

discuss lateral correlations between the known parts of the

Neo-Tethyan suture zone complex, in Turkey and Iran.

Y. Rolland (&) � S. Billo � M. Corsini � M. Sosson �G. Galoyan

Geosciences Azur, UMR 6526,

Universite de Nice-Sophia Antipolis CNRS,

Parc Valrose, 06108 Nice cedex 2, France

e-mail: yrolland@unice.fr

S. Billo

DTP, UMR 5562, Observatoire Midi-Pyrenees,

14, avenue Edouard Belin, 31400 Toulouse, France

G. Galoyan

Institute of Geological Sciences,

National Academy of Sciences of Armenia,

24a Baghramian avenue, Yerevan 375019, Armenia

123

Int J Earth Sci (Geol Rundsch) (2009) 98:533–550

DOI 10.1007/s00531-007-0286-8

Between these two parts of the Neo-Tethyan belt, detailed

P-T constraints and HP metamorphic ages are scarcely

obtained on such rocks, so it remains difficult to establish

clear relationships with the East Mediterranean history to

that of the more documented Himalayan belt. In this con-

text, the relationships of the South Armenian Block with

Taurides to the West and Iran to the East are critical for

lateral correlations. In the present study we evaluate the

time of the South Armenian Block accretion with Eurasia

and the age of exhumation of the suture zone complex,

based on geological, P-T and 40Ar/39Ar phengite data.

Geological setting

The South Armenian Block is comprised between Turkey

and Iran blocks (Fig. 1). In the following, we summarize

the occurrences and known ages of HP metamorphism in

these two regions.

Turkey is formed by mainly four blocks, separated by

three main HP belts (e.g., Okay and Tuysuz 1999), namely:

(1) The Intra-Pontide suture, separating the Istambul

zone from the Sakarya Block to the NW;

(2) The Izmir-Ankara suture prolonged to the East by the

Izmir-Ankara-Erzincan suture separating the Sakarya

Block from the Anatolide-Taurides Block in the

centre;

(3) The Assyrian-Zagros suture separating the Anatolide–

Taurides from the Arabian plate to the SE and the

Pamphylian Suture in SSW Turkey.

In Turkey, the Intra-Pontide suture in the Sakarya

Zone preserves Permo-Triassic subduction complexes

PlatformEuropean margin (Pontides, “Sakarya series”)European margin including arc seriesMetamorphic massifsTauride-Anatolide and Armenian block(s)and accreted/obducted ophiolitesSuspected oceanic crust

Strike slip fault Thrust

SAS: Sevan-Akera SutureCACC: Central anatolian crystalline complex(Kirsehir massif)EAF: East anatolian fault(reactivating the Intra-Pontide suture)GC: Great CaucasusIAES: Izmir - Ankara - Erzinkan sutureIPS: Intra-Pontide SutureIZ: Istanbul ZoneKO: Khoy OphioliteMM: Menderes massifNAF: North anatolian faultSA: South armenian block

Studied area

East european Platform

Scythian Platform

Arabian Platform

Caspian Sea

Black Sea

45°N

40°N

35°N

25°E 30°E 35°E 40°E 45°E 50°E

45°N

CACC

MM

VR

GC

EAF

KO

SA S

Sanandaj-Sirjan zone

IAES

SAS

NAF

Zagros Suture

Assyrian

Suture

IPSIZ

V: Van LakeS: Sevan LakeR: Rezaiyeh Lake

Fig. 1 Tectonic map of the Middle East–Caucasus area, with main blocks and suture zones, after Avagyan et al. (2005), modified

534 Int J Earth Sci (Geol Rundsch) (2009) 98:533–550

123

unconformably overlain by Jurassic sediments (e.g., Okay

and Tuysuz 1999; Robertson et al. 2004; Okay and

Goncuoglu 2004).

The Izmir-Ankara suture is underlined by Mid- to Upper

Cretaceous HP–LT rocks. Dating of HP rocks from the

Kargi massif (Fig. 1) provided mean 40Ar/39Ar phengite

age of 104.5 ± 7.2 Ma (Okay et al. 2007), while ages

between 101 and 85 Ma were obtained with the same

method in the NW part of the Izmir-Ankara suture (Okay

et al. 1998; Sherlock et al. 1999). In contrast, Mid Paleo-

cene to Eocene ages were obtained in the southwest Izmir

suture (Rimmele et al. 2003).

Blueschists have also been found along the Zagros

Suture in SE Turkey (Oberhansli, pers. com.), but they

remain undated. Along the probable lateral continuation of

this SE Turkey suture, in western and southern Iran, sub-

duction age is constrained by the emplacement of supra-

subduction plagiogranites dated at 93–88 Ma by K–Ar

(Kananian et al. 2001), and by Mid–Upper Cretaceous40Ar/39Ar ages (Agard et al. 2006).

Further east, in Eastern Iran, blueschist to eclogite facies

metamorphism is also described (Fotoohi Rad et al. 2005),

and is ascribed to the age range 83–55 Ma (Tirrul et al.

1983), which correlates with ages of Tethyan subduction in

the Himalayan belt, which ends with the onset of Indian

continent subduction at c. 55 Ma (de Sigoyer et al. 2000).

The Sevan-Akera ophiolite belt

In the Lesser Caucasus belt of Armenia, the Sevan-Akera

suture (Aslanyan 1958; Gabrielyan 1959; Paffenholtz

1959) is the probable eastward extension of the Izmir-

Ankara-Erzincan suture in Anatolia (see above and Figs. 1,

2). It has been interpreted as a suture zone since the works

of Milanovski (1968). This suture is the northern tectonic

boundary of the South Armenian Block, which is presumed

of Gondwanian origin (Knipper 1975; Knipper and Khain

1980) with Eurasia. The South Armenia Block is a possible

eastward extension of the Eastern Taurides (Tauride-

Anatolide Block), and the ‘‘Eurasian margin’’ in Armenia/

Georgia may correspond to the Eastern Pontides/Sakarya

Zone. The geographic proximity and similarity in the

geological units would suggest a parallel evolution

Fig. 2 Sketch geological map

of Armenia, with location of the

studied area

Int J Earth Sci (Geol Rundsch) (2009) 98:533–550 535

123

between the northeastern Anatolia and Armenia (e.g.,

Knipper 1975; Adamia et al. 1980), but the age and origin

of the Armenian and Georgian Crystalline basements are

still not precisely constrained.

Ophiolites have been described in association with

blueschists and amphibolite facies metamorphic rocks

(Melikyan 1966; Knipper 1975; Sokolov 1977; Aslanyan

and Satian 1977; Morkovkina et al. 1977; Aghamalyan

1978, 1981; Knipper and Khain 1980; Abovyan 1981;

Zakariadze et al. 1983; 1990; Meliksetyan et al. 1984;

Tsameryan et al. 1988; Silantiev et al. 1996; Aghamalyan

2004). The age of oceanic closure and suturing is presumed

to be Upper Cretaceous on the basis of the unconformity of

platform-type Upper Coniacian limestones on the ophiolite

(Kniper 1975; Sokolov 1977), but as will be emphasized in

the present paper this age corresponds to the age of ophi-

olite obduction on the South Armenian Block, and not to

the suturing of the latter with Eurasia. In the Stepanavan

area, which is under study in this paper (Fig. 3), previous

works undertaken by Aghamalyan (1978, 1981) have

shown the following structural succession: (1) a basal part

comprising pelites interstratified with dacitic lava flows,

and hydrothermalized and silicified serpentinites (‘‘listwe-

nites’’); (2) a glaucophane-schist unit including blocks of

garnet amphibolites thrusted over (1); (3) ophiolitic

melanges comprising ultramafites, volcanic rocks and

radiolarites; (4) a conglomeratic and overlying limestone

unit overlying uncomformably the unit (3); (5) Paleocene-

Eocene series thrusted by (1–4). The age of the series (1–4)

is attributed a Lower to Upper Cretaceous age by analogy

with the neighbouring Amassia massif where brachiopods

have been described in limestones interstratified within

lava flows (Hakobyan 1976). Accordingly, whole-rock K–

Ar ages were obtained on blueschists, ranging from 90 to

80 Ma (Meliksetyan et al. 1984 and references therein).

However, these ages do not constrain neither the ophiolite

protolith age nor any precise metamorphic age of minerals,

from which only 40Ar/39Ar or U/Pb in relationship with PT

path determination could help resolve the complex history.

In the following, we provide a new description of the area,

and propose a different interpretation for the structure and

age of units. Concerning the issue of the age of meta-

morphic rocks, 40Ar/39Ar datings are provided in the

following ‘‘40Ar/39Ar dating’’ section.

Field analysis and sampling

On the basis of previous geological work in the area of

Stepanavan, we conducted two field campaigns for a pre-

cise mapping of the various units of the ASBOC in 2003

and 2004. A simplified version of the geological map is

presented in Fig. 3.

Lithologies

We observed five superposed units in the ASBOC, from

base to top (Figs. 3, 4):

(1) The lower structural unit is the ‘‘Blueschist Unit’’. It

is mainly composed of a strongly retrogressed calc-

schist matrix including boudins of metavolcanic rocks

and metasediments. Metavolcanics are glaucophane-

bearing mostly deriving from basalts. In rare loca-

tions, pillow-basalts and volcanic breccia can be

recognized within the Blueschist Unit. Metasediments

are comprised of micaschists, marbles, metaconglom-

erates, quartzites, and rare gneiss blocks. This

association suggests an active margin environment

where detrital deposits are intensely reworked an

accretionary prism.

(2) Structurally above the blueschists, the ‘‘Ophiolite

Unit’’ is a low-grade and complete ophiolite

sequence. A complete description of the ophiolite

series is provided by Galoyan et al. (2007), and main

features are briefly summarized below. This unit is a

preserved ophiolite section and not an ophiolitic

melange. It is composed of slightly metamorphozed

δ

V

V

V

V

V

V

V

V

V

V

V

V

STEPANAVAN

1 km

41°00’ 41°00’

44°23’44°16’

44°11’

δ

40°51’

V

AR.04.07

N

UVSU Volcano-sedimentary rocks (Paleogene)

Andesitic volcanites & volcano-sedimentary rocks

graywackesLimestones

Ophiolite Unit (LOT type : UB; basalts, gabbros, plagiogranites & radiolarites) - Upper Jur. to Lower Cret.

Conglomerates and fine sandstones

Blueschists Unit

Andesite-Dacite volcanics (Plio-IV)

Lower to Upper Cretaceous(LVSU)

Normal faults ThrustsFolds

A

B

AB

AR.03.57-58

AR.04.14

AR.03.50

AR.03.52

V

V

V

V

V

V

V

V

V VV

V

V

V

V

V

V

V

V

V

V

V

V

V

V

V

V

V

40°51’

V

AR.03.56

AR.03.54

Fig. 3 Geological map of the Stepanavan Blueschist Ophiolite

complex (ASBOC). d unconformities

536 Int J Earth Sci (Geol Rundsch) (2009) 98:533–550

123

serpentinites, gabbro-norite pods, plagiogranites, bas-

alts and radiolarites. The observed geometrical

relationships among lithologies include (a) sediments

unconformably lying on serpentinites, (b) small

relative volume of magmatic rocks versus serpenti-

nites, (c) cross-cutting plagiogranite dykes emplaced

within extensional shear zones. All the above features

suggest a Lherzolite Ophiolitic Type (LOT) sequence.

In the North, we observed the following succession:

serpentinite, flaser-gabbro basalt and radiolarites (see

lithological profile ‘‘log A’’ in Fig. 4). Whereas, in

the South we principally observed serpentinites

crosscut by pods of cumulative plutonic rocks com-

positionally evolving from gabbro-norites to

plagiogranites (‘‘log B’’ in Fig. 4). Dating of radio-

larian faunas interbedded in the pillow lavas provided

an Upper Jurassic age (Danelian et al. 2007).

Therefore, most of the ophiolite series protolith

should be Upper Jurassic as in the Sevan area

(Zakariadze et al. 1983).

(3) In stratigraphic contact with the ‘‘Ophiolite Unit’’, the

‘‘Lower Volcano-Sedimentary Unit’’ (or LVSU) is a

thick and highly variable succession of calc-alkaline

volcanic material and related sediments. It comprises

basal conglomerates or quartzites, red pelites,

limestones and finally grauwackes or basaltic to

andesitic lavas. Depending on the area, we observed

either: (1) the succession of a thick sandstone/

conglomerate layer and black graywackes interlay-

ered with several decameter-large limestone strata

(North, see ‘‘log A’’ in Fig. 4), or (2) the succession

quartzite and conglomeratic limestones, a reduced

pile of pelites, and a large thickness of pillowed

andesites, basaltic andesites and andesitic tuff (see

‘‘log A’’ in Fig. 4). The possible age range for this

succession is Lower Cretaceous-Paleocene, as it lies

unconformably on the ophiolite and predates the

following Paleocene-Eocene unconformity.

(4) The upper section is formed by the ‘‘Upper Volcano-

Sedimentary Unit’’ (UVSU), also in stratigraphic

(unconformity) contact with the below unit (LVSU).

It is comprised of basal conglomerates with volcanic

blocks and glauconite, tuffaceous limestones, and

andesites to rhyolites lavas. This series is presumably

Paleocene–Eocene (in reference to the Geological

Map of Armenia 1/5000 000) and lies unconformably

on the folded LVSU.

(5) Finally, an undated (probably Plio-Quaternary)

sequence of basalt lava flows and grauwacky material

lies just north to the blueschist windows. The down-

0 642 8

2000

3000

1000

(m)

blueschist unit

ophiolitic unitvolcano-sedimentaryUpper Cretaceous

unit(LVSU)

(km)

vvv

vv

v

vv v

v

foliation

δ

δUnconformity

v

South North

δUnconformity

Log B Log A

Log A Log B

blue-schists

metabasitesserpentinites

gabbros

basaltsradiolarites

sandstones & conglomerates

siltstones & grauwackes

limestones

siltstones & grauwackes

serpentinites

gabbros

basaltssandstones & limestones

siltstones & grauwackes

andesites

sandstones & limestones

andesites

metapelitesmarbles

Neogene volcanics(Plio-IV)

Paleogene volcanics(UVSU)

Fig. 4 Geological cross-section of the ASBOC, and lithological profiles in the northern and southern parts of the section. The location of the

section is indicated on Fig. 2

Int J Earth Sci (Geol Rundsch) (2009) 98:533–550 537

123

throw of the series is ascribed to N-dipping normal

faults, which crosscut the ASBOC on its northern rim

(Fig. 4).

Structure

At the geological map scale, the overall structure is that of a

folded nappe pile (Fig. 4). The blueschist unit appears

within two tectonic windows of ±1 km2. Exhumation of the

blueschists is partly due to the presence of steep reverse

north-verging faults. However, the rounded shape of the

windows, underlined by the metamorphic foliation can be

interpreted as ‘‘dome-like’’ structures. The domes presum-

ably formed by the succession of two phases of folding as is

emphasized by macro and micro-tectonic analysis. The

measurement of mineral and stretching lineations and

associated metamorphic foliations shows that at least two

ductile phases have deformed the ASBOC. A first genera-

tion of ductile structures was formed in the blueschist facies

as glaucophane + phengite minerals underline a first sub-

horizontal lineation striking N90–110�E, while a second

generation of greenschist facies (epidote, actinolite)

minerals form a mineral lineation striking N–S to NE–SW

with a steep dip (Fig. 5c). Main fold axes strike

N100-120�E on the map (Fig. 3). Similar orientations were

measured in the field on microfolds (Fig. 5a, b). Fold axes

are nearly horizontal and axial fold planes are vertical to

south-dipping. Both greenschist and blueschist facies foli-

ations are folded within N120�E folds as shown by the

computation of all foliation poles on stereographic projec-

tions (Fig. 5b). Therefore, fold structures indicate a late

tectonic event, featured by NNE–SSW sub-horizontal

shortening. This shortening direction agrees with that of

mineral lineations of the second phase, which indicates that

folding occurred later but in the same tectonic context as

nappe emplacement. Due to the intense strain and LP–MT

metamorphic overprint of this late phase it remains difficult

to identify the sense of shear related to the first HP mineral

lineation. The presence of dome structures and E–W

blueschist grade mineral lineations suggest that tectonic

motions might have been more E–W in the former HP strain

phase. However, no clear shear sense indicators could be

observed from field and thin section analyses, thus it

remains difficult to conclude about the sense of shear during

phases 1 and 2. Latest deformation is featured by NW–SE

north dipping normal faults crosscutting the ASBOC.

Fold axes

Greenschist facies stretching lineations

Blueschist facies stretching lineations

Poles to bedding

Poles to schistosity (greenschist facies)

Poles to schistosity (blueschist facies)

A

Fold Axis 105W6

Fold axis122E15

Stretching lineation. 1=E-WStretching lineation. 2=N-S

Legend:

C

B

Fig. 5 Wulf stereonets in lower

hemisphere, presenting the

orientations of main

microstructures. a Poles to

bedding; b poles to

metamorphic foliation; c poles

to mineral lineations

538 Int J Earth Sci (Geol Rundsch) (2009) 98:533–550

123

Tectonic events

The structure and lithologies that were observed suggest

the following succession of tectonic events:

(1) A first high-P phase in blueschist facies conditions:

Tectonic motion at HP is supported by the orientation

of the HP stretching + mineral lineations. If these HP

lineations did not undergo any rotation at lower P, the

E–W strike of lineations could correspond to the

transport direction of subducted materials, which

would suggest a N–S trending subduction zone (and

thus an E or W directed subduction) at this stage.

(2) A second phase of top to the NE shearing: Tectonic

motion is related to the nappe-stacking event con-

temporaneous to the greenschist/epidote-amphibolite

facies. The HP units have been tectonically uplifted

during this event.

(3) In the same tectonic context, the nappe stack is folded

by a NNE–SSW trending shortening, still related to

top to the NNE sense of shear. The shortening related

to WNW–ESE folding resulted in uplift, exhumation

and erosion of the nappe stack.

(4) The nappe stack is unconformably overlain by the

Paleocene volcano-sedimentary sequence, which ori-

gin could be related to the onset of the collision.

(5) Post Paleocene (probably Eocene) N–S shortening is

emphasized by the long wavelength folds drawn by

this latter Paleocene volcano-sedimentary sequence.

(6) Later deformation stage is the relative uplift of the

blueschist sole, by denudation under extensional NE

dipping subvertical faults.

Analytical techniques

Mineral compositions were determined by electron probe

microanalysis (EPMA). The analyses are presented in

Table 1. They were carried out using a Cameca Camebax

SX100 electron microprobe at 15 kV and 1 nA beam

current, at the Blaise Pascal University (Clermont-Ferrand,

France). Natural samples were used as standards.

Geochronology was undertaken by laser 40Ar/39Ar dat-

ing of white micas. Results are presented in Table 2. White

micas were analyzed by EPMA prior to dating in order to

check mineral compositions homogeneity. Grains less than

1 mm were separated by careful selection by hand-picking

under a binocular microscope to prevent the presence of

altered grains. The samples were then irradiated in the

nuclear reactor at McMaster University in Hamilton

(Canada), in position 5c, along with Hb3gr hornblende

neutron fluence monitor, for which an age of 1072 Ma is

adopted (Turner et al. 1971). The total neutron flux density

during irradiation was 9.0 9 1018 neuton cm-2. The esti-

mated error bar on the corresponding 40Ar*/39ArK ratio is

±0.2% (1r) in the volume where the samples were set. All40Ar/39Ar measurements were proceed in the University of

Nice-Sophia Antipolis (UMR Geosciences Azur). Analyses

of phengite grains were undertaken by step heating with a

50 W CO2 Synrad 48-5 continuous laser beam. Measure-

ment of isotopic ratios was done with a VG3600 mass

spectrometer equipped with a Daly detector system.

Detailed procedures are described in Jourdan et al. (2004).

The typical blank values for extraction and purification of

the laser system are in the range 4.2–8.75, 1.2–3.9, and 2–6

cc STP for masses 40, 39 and 36, respectively. Mass dis-

crimination was monitored by regularly analyzing air

pipette volumes. Decay constants are those given by Stei-

ger and Jager (1977). Uncertainties on apparent ages are

given at the 2r level and do not include the error on the40Ar*/39Ark ratio of the monitor.

The criteria generally used in the laboratory for defining

a ‘‘plateau’’ age are the following: (1) it should contain at

least 70% of total 39Ar released; (2) there should be at least

three successive step-heating fractions in the plateau; (3) the

integrated age of the plateau (weighted average of apparent

ages of individual fractions comprising the plateau) should

agree with each apparent age of the plateau with a 2r error.

In this study, we also consider smaller plateaus than 70% of

total 39Ar, because the lower temperature age spectra are

generally lowered by a 39Ar loss effect.

Metamorphism of the Stepanavan Blueschists

High-P relics have been found within metabasites.

Metapelites appear to be Ca-rich and altered during their

retromorphosis, so we have focused our study on the

metabasic lithologies (CFMASH system).

Petrography

Stepanavan metabasic blueschists preserve several par-

ageneses, which are representative of the successive steps

of high-P metamorphism (see photographs on Fig. 6).

Samples, such as AR 03 50, show HP–LT parageneses

defined by (1) clinopyroxene—glaucophane – quartz

– pyrite ± phengite ± clinozoisite. HP–LT conditions are

also defined by the assemblage (2) glaucophane—clino-

zoisite ± phengite ± quartz. Clinopyroxene appears as

small dark green globular grains and glaucophane occurs as

equant porphyroblasts up to 2 mm in length. Phengite and

clinozoisite form elongated grains that define the foliation.

Quartz is not very abundant and commonly shows undulose

extinction.

Int J Earth Sci (Geol Rundsch) (2009) 98:533–550 539

123

Table 1 Representative electron-microprobe mineral analysis of blueschist samples

Sample AR 03 50 Sample AR 03 52a Sample AR 04 14 Sample AR 03 57

Aeg cro ph czo gl Cro czo grt rim1 grt core grt rim2 prg chl alb czo czo gl ph

SiO2 53.21 54.84 50.49 37.57 55.91 55.60 37.61 37.87 37.29 37.46 42.79 25.37 67.16 37.25 38.80 56.86 51.29

Al2O3 3.01 8.15 26.10 23.09 9.59 5.88 21.50 21.00 20.81 21.16 15.10 19.22 20.91 22.37 28.35 11.66 28.52

MgO 1.37 4.19 3.11 0.01 8.02 7.34 0.02 2.98 1.79 2.92 8.29 14.69 0.00 0.01 0.03 9.41 3.68

FeO 25.07 23.10 4.81 12.70 15.69 19.96 14.66 30.55 26.23 29.61 17.50 27.80 0.27 12.12 6.19 11.28 2.12

MnO 0.04 0.07 0.02 0.49 0.19 0.08 0.25 1.50 4.81 1.70 0.16 0.37 0.03 0.24 0.13 0.09 0.00

TiO2 0.05 0.08 0.18 0.15 0.13 0.05 0.06 0.10 0.20 0.13 0.51 0.02 0.01 0.06 0.25 0.17 0.21

CaO 3.89 0.53 0.03 22.64 1.02 0.81 22.82 6.61 7.79 6.59 9.17 0.12 1.57 22.52 24.10 1.40 0.03

Na2O 11.35 6.93 0.22 0.02 6.77 6.98 0.02 0.05 0.07 0.03 2.98 0.03 11.02 0.04 0.01 6.46 0.47

K2O 0.00 0.03 10.50 0.00 0.01 0.03 0.01 0.00 0.05 0.01 0.28 0.04 0.12 0.00 0.00 0.00 10.54

Total 98.00 97.92 95.46 96.67 97.32 96.71 96.95 100.67 99.05 99.62 96.78 87.67 101.08 94.61 97.84 97.33 96.86

Nb oxygen 6 23 11 12 23 23 12 12 12 12 23 14 8 12 12 23 11

Si 2.01 7.90 3.40 3.00 7.85 7.97 3.01 3.00 3.01 2.99 6.17 2.71 2.33 3.03 3.00 7.88 3.36

Al 0.13 1.38 2.07 2.17 1.59 0.99 2.03 1.96 1.98 1.99 2.57 2.42 0.86 2.15 2.58 1.90 2.20

Mg 0.08 0.90 0.31 - 1.68 1.57 - 0.35 0.22 0.35 1.78 2.34 - - - 1.94 0.36

Fe 0.79 2.78 0.27 0.85 1.84 2.39 0.98 2.02 1.77 1.98 2.11 2.48 0.01 0.83 0.40 1.31 0.12

Mn – 0.01 0.00 0.03 0.02 0.01 0.02 0.10 0.33 0.12 0.02 0.03 – 0.02 0.01 0.01 –

Ti – 0.01 0.01 0.01 0.01 0.01 – 0.01 0.01 0.01 0.06 – – – 0.01 0.02 0.01

Ca 0.16 0.08 – 1.94 0.15 0.12 1.96 0.56 0.67 0.56 1.42 0.01 0.06 1.97 2.00 0.21 –

Na 0.83 1.94 0.03 – 1.84 1.94 – 0.01 0.01 – 0.83 0.01 0.74 0.01 – 1.73 0.06

K – 0.01 0.90 – – 0.01 – – 0.01 – 0.05 0.01 0.01 – – – 0.88

Sample AR 03 58 Sample AR 03 52b

gl ph gl trem czo alb ph chl

SiO2 56.69 50.27 57.47 52.09 38.05 69.51 49.61 25.40

Al2O3 8.93 27.07 7.77 1.77 23.35 20.27 25.67 19.35

MgO 9.29 2.89 9.11 6.55 0.02 0.00 3.11 19.18

FeO 14.54 4.96 15.89 26.44 12.77 0.13 5.13 23.54

MnO 0.04 0.02 0.11 0.34 0.31 0.00 0.05 0.24

TiO2 0.06 0.22 0.05 0.01 0.18 0.00 0.31 0.00

CaO 0.73 0.03 0.50 3.32 22.81 0.24 0.03 0.07

Na2O 6.89 0.28 7.08 4.92 0.00 12.01 0.20 0.00

K2O 0.01 10.46 0.00 0.11 0.02 0.03 10.66 0.01

Total 97.16 96.20 97.99 95.54 97.51 102.20 94.76 87.78

Nb oxygen 23 11 23 23 12 8 11 14

Si 7.91 3.36 7.99 7.85 3.01 2.37 3.37 2.63

Al 1.47 2.13 1.27 0.31 2.18 0.82 2.05 2.36

Mg 1.93 0.29 1.89 1.47 – – 0.31 2.95

Fe 1.70 0.28 1.85 3.33 0.84 – 0.29 2.03

Mn 0.01 – 0.01 0.04 0.02 – – 0.02

Ti 0.01 0.01 0.01 0.00 0.01 – 0.02 0.00

Ca 0.11 0.00 0.07 0.54 1.93 0.01 0.00 0.01

Na 1.86 0.04 1.91 1.44 – 0.80 0.03 –

K – 0.89 – 0.02 – – 0.92 –

aeg aegyrine, alb albite, chl chlorite, Cro crossite, gl glaucophane; grt garnet, ph phengite, prg pargasite; trem tremolite, czo clinozoisite

540 Int J Earth Sci (Geol Rundsch) (2009) 98:533–550

123

Table 2 Phengite 40Ar/39Ar dating results from the ASBOC blueschists

Sample N� and Step N� Laser power (mW) Atmospheric content (%) 39Ar (%) 37ArCa/39ArK

40Ar*/39ArK Age (Ma ± 1r)

HP phengite samples

AR-03-56(1)

1 375 63,32 3,64 0,16 2,03 60,6 ± 1,3

2 396 13,83 6,15 0,09 2,56 76,0 ± 0,5

3 411 9,53 2,94 0,06 2,85 84,4 ± 1,1

4 426 5,64 23,01 0,08 3,07 90,8 ± 0,3

5 431 4,27 10,31 0,07 3,10 91,4 ± 0,4

6 437 3,64 14,09 0,12 3,15 93,0 ± 0,3

7 444 4,85 8,46 0,47 3,18 93,8 ± 0,4

8 458 7,13 8,22 1,64 3,19 94,1 ± 0,4

9 800 8,98 23,18 0,86 3,25 95,9 ± 0,3

AR-03-56(2)

1 342 35,72 5,77 0,12 2,11 61,7 ± 0,8

2 350 9,64 3,91 – 2,50 72,7 ± 0,7

3 359 6,69 14,00 0,10 2,82 81,8 ± 0,3

4 364 3,48 16,58 0,13 3,02 87,4 ± 0,2

5 369 2,83 19,78 0,19 3,12 90,3 ± 0,2

6 374 3,34 5,25 0,18 3,13 90,7 ± 0,4

7 382 5,81 10,12 0,86 3,17 91,8 ± 0,3

8 405 6,47 6,02 1,22 3,17 91,8 ± 0,4

9 442 6,67 4,68 1,35 3,19 92,4 ± 0,5

10 509 5,20 10,81 1,06 3,24 93,6 ± 0,3

11 549 10,17 2,45 1,95 3,30 95,3 ± 0,7

12 800 26,30 0,63 7,14 3,24 93,8 ± 3,2

AR-03-58(1)

1 340 9,53 5,07 – 2,48 72,2 ± 0,6

2 350 4,38 7,54 – 2,82 82,0 ± 0,5

3 358 1,80 9,84 – 2,99 86,7 ± 0,5

4 364 – 7,59 – 3,06 88,7 ± 0,6

5 369 0,67 11,72 – 3,10 89,8 ± 0,4

6 376 0,85 18,90 0,09 3,14 91,0 ± 0,3

7 381 0,32 9,52 0,13 3,19 92,3 ± 0,4

8 392 0,56 13,61 0,10 3,20 92,5 ± 0,4

9 409 2,56 6,59 0,51 3,16 91,4 ± 0,6

10 440 2,54 4,65 0,98 3,16 91,4 ± 0,7

11 800 2,65 4,98 3,07 3,11 90,2 ± 0,7

AR-04-07

1 350 17,00 10,26 0,01 2,86 86,2 ± 5,8

2 362 – 31,37 – 3,18 95,6 ± 1,6

3 376 – 39,96 – 3,18 95,5 ± 1,2

4 386 52,66 1,14 – 1,31 39,9 ± 63,6

5 424 6,76 10,52 – 2,93 88,3 ± 8,6

6 800 – 6,75 – 3,16 94,9 ± 7,5

LP–MT phengite samples

AR-03-54

1 350 16,54 19,46 0,02 2,26 66,0 ± 0,4

2 360 7,85 6,41 – 2,37 69,2 ± 0,6

3 368 9,19 8,63 – 2,37 69,0 ± 0,6

Int J Earth Sci (Geol Rundsch) (2009) 98:533–550 541

123

LP–MT parageneses are featured by (3) garnet–chlo-

rite—Ca/Na amphibole ± clinozoisite ± albite. Garnet is

partially replaced by pale-green chlorite. Amphibole occurs

as thin green crystals showing the foliation.

LP–LT parageneses are defined by (4) Ca/Na amphi-

bole–clinozoisite – chlorite – albite ± phengite.

Mineral chemistry

EPMA analysis of clinopyroxene shows that it is of

Aegirine composition (Acmite 0.66–0.75; Augite 0.11–

0.17; Jadeite 0.13–0.18, Table 1).

In LT–HP assemblages, blue amphibole is mainly of

glaucophane-crossite type (0.2 \ Na (A) \ 0.5; Fig. 7;

Table 1). In MT–LP assemblages, the amphiboles are of

pargasite composition (Si = 6.3–6.4; XMg = 0.3–0.4;

Table 1), while in LT–LP assemblages compositions are

within the tremolite-actinolite field.

Phengite is more celadonite-rich in LT–HP assemblages

(Si: 3.37–3.43) than in the LP assemblages (Si: 3.1–3.2;

Table 1).

In HP-LT assemblages, analysed epidote is of zoisite-

clinozoisite type (Table 1). In LP assemblages, epidote is

close to the pistascite pole.

The composition of garnet within LP–MT assem-

blages is featured by prograde zoning (Table 1; Fig. 8),

with increasing almandine (from 59.5 to 66.5%) and

pyrope (from 7 to 12%) contents and decreasing spes-

sartine (from 11 to 3.5%) and grossular (from 23 to

18.5%) contents towards mineral rims. This pattern is

indicative of thermal increase during retrogression (Spear

1988).

Alteration of garnet and amphibole into Fe-rich chlorite

(XFe = 0.43–0.49) is frequently observed.

Thermobarometry and PT paths

Average PT domains of studied assemblages can firstly be

deduced from the stability fields of main crystallizing

minerals. The main mineral reactions and PT stability

fields of mineral parageneses for metabasites in HP–LT

conditions are shown on Fig. 9 and Table 3. The peak

burial conditions are out of the lawsonite stability field, as

is evidenced by textural equilibrium of clinozoisite-bearing

assemblages and the absence of any lawsonite psewhich is

in agreement with temperatures [400�C at peak P (Hein-

rich and Althaus 1988; Fig. 9). The presence of

glaucophane implies maximal temperatures of 550�C at

peak P (Carman and Gilbert 1983). The jadeite content in

clinopyroxene (15–20%) provides a minimum pressure

estimate of 0.9 GPa, at 450�C, in the absence of albite

(Holland 1980). However, the absence of jadeite-rich

pyroxene in the overall set of thin sections suggests pres-

sures lower than 1.4 GPa. A minimum pressure estimate of

1.1 GPa is provided by the Si content in phengite (Si 3.38–

3.43) in pelitic rocks (gneiss sample AR0407) at 450�C

(Massone and Schreyer 1987). Therefore, the peak P con-

ditions can be approximated at 1.1 B P \ 1.4 GPa, and

400 \ T \ 550�C. These conditions are well within the

field of the epidote blueschist facies (Evans 1990).

Table 2 continued

Sample N� and Step N� Laser power (mW) Atmospheric content (%) 39Ar (%) 37ArCa/39ArK

40Ar*/39ArK Age (Ma ± 1r)

4 373 7,26 10,31 0,06 2,43 70,7 ± 0,6

5 379 5,87 17,37 – 2,54 73,9 ± 0,4

6 405 9,98 5,89 – 2,53 73,7 ± 0,9

7 443 7,87 8,61 – 2,52 73,4 ± 0,6

8 499 7,23 11,52 0,01 2,51 73,2 ± 0,6

9 800 6,03 11,80 0,01 2,52 73,2 ± 0,4

AR-03-58(2)

1 346 41,55 2,20 0,02 2,26 66,7 ± 3,6

2 357 38,30 2,26 0,01 2,27 66,9 ± 3,5

3 368 25,21 2,43 0,01 2,18 64,5 ± 2,9

4 385 30,29 6,96 0,01 2,22 65,7 ± 1,0

5 400 18,76 1,85 0,01 2,35 69,3 ± 4,1

6 416 15,81 11,51 0,01 2,31 68,2 ± 0,6

7 426 13,56 26,98 0,01 2,43 71,8 ± 0,4

8 432 17,26 2,54 0,02 2,36 69,5 ± 2,3

9 448 17,57 3,97 0,02 2,34 69,1 ± 1,8

10 700 17,25 39,30 0,03 2,40 71,0 ± 0,4

542 Int J Earth Sci (Geol Rundsch) (2009) 98:533–550

123

Thermobarometry was performed with multi-equilibria

approach, with the software THERMOCALC (v. 3.1;

Powell and Holland 1988), using the thermodynamic

dataset of Holland and Powell (1998). Thermobarometry

was undertaken in the CFMASH system, to precise the PT

conditions of peak P and of retrogression assemblages.

Only assemblages with textural evidence for equilibrium

were selected. Then, representative mineral analyses were

used. In the case of mineral zoning, as for garnet, only the

rim-rim compositions could be used, which allowed to

derive peak MT retrogression conditions but not the HP

peak conditions. Activities of mineral end-members used

for rock calculations are calculated with the help of the

program AX (T.J.B. Holland). Average temperatures were

calculated for different ranges of pressure and vice versa.

Representative results for the obtained mineral reactions,

fit, and deduced P-T intervals (at 2r) are shown in Table 3.

The calculations based on the HP parageneses provide

PT estimates of P = 1.2 ± 0.15 GPa and T = 545 ± 60�C.

0

10

20

50

60

70

rim

Abu

ndan

ce (

%)

(a)

(b)

(c)

(d)

rimcore

Fig. 8 Compositional profiles of two garnets of the MT–LP assem-

blage (sample AR-04–14; Table 1), with a almandine, b grossular, cspessartine and d pyrope contents. Note the correlated increase of

almandine and pyrope towards the rim, while spessartine and

grossular are enriched in the core of both garnets. Approximate size

of garnets 1 mm

Fig. 6 Microphotographs of studied samples. a Acmite-bearing-HP

blueschist (sample AR-03–50); b commonly observed HP blueschist

sample (sample AR-03–52a); c blueschist retrogressed in LP–MT

conditions (sample AR-04–14). Abbreviations following Kretz (1983)

0

10

20

30

40

50

60

70

80

90

100

0

100 Fe3+ / (Fe3+ + AlVI + Ti)

eF

001+2

eF(/

+ 2)

nM

+g

M+

AR 03 50

AR 03 52a

AR 03 57

AR 03 58

Ferroglaucophane

Glaucophane

Crossite

Riebeckite

Magnesioriebeckite

10090807060504030 2010

Fig. 7 Compositions of analyzed HP amphiboles plotted in the

diagram of Miyashiro (1957). The Fe3+ content was estimated on the

basis of structural formulae of 23 oxygens and 15 cations

Int J Earth Sci (Geol Rundsch) (2009) 98:533–550 543

123

P-T conditions on the LP–MT parageneses (computed with

garnet – chlorite – pargasite – albite – clinozoisite, sample

AR 04 14) are of P = 0.57 ± 0.02 GPa and

T = 505 ± 67�C. P-T conditions obtained on LT–LP

assemblages (computed with tremolite - clinozoisite -

albite - phengite) are variable and all at P \ 0.5 GPa and

T \ 400�C.

These estimates, when plotted on the PT diagram

(Fig. 9), are within the range of PT conditions deduced

from the stability fields of mineral phases discussed above,

and allow to precise the shape of the PT paths. The PT

paths are mostly in two phases: (1) burial and exhumation

within a relatively hot subduction-type geotherm (10–

15�C/km). Rocks were subducted to 40 ± 4 km, and were

exhumed rapidly to (or slightly above) the depth at which

the second (LP–MT) metamorphic event occurred. (2) The

LP–MT event is featured by a slight increase of tempera-

ture of about 100�C, up to *500�C at 0.6 GPa, which is in

agreement with zoning of garnet towards high-Fe, Mg rim

compositions. Peak LP–MT conditions are indicative of a

relaxed geotherm of 19–33�C/km, more in agreement with

a collision-type event, or to heat input related to synsub-

duction extension (e.g. Parrat et al. 2002). This thermal

increase is sensible at the contact between units, where

deformation was the most intense during the superposition

of the low-grade ophiolitic unit over the blueschist unit.

40Ar/39Ar dating

In the section above, maximal temperature conditions

estimated for the HP and LP–MT peaks are slightly above

(to within range) the closure temperature of the K–Ar

system in white mica (400 ± 50�C, Villa 1998). Thus, the40Ar/39Ar dating of white mica in equilibrium with each of

the parageneses allows estimating the crystallization age of

phengite, either (1) at the pressure peak, or (2) during the

LP–MT retrogression peak. For this reason, we selected

micas from samples exhibiting clear textural equilibrium

(1) in the HP metamorphic paragenesis (samples AR 03 56,

AR 03 58 and AR 04 07), and (2) in the LP–MT meta-

morphic paragenesis (samples AR 03 54 and AR 03 58).

Results are shown in Fig. 10 and Table 2.

40Ar/39Ar dating of HP peak phengite

The 40Ar/39Ar spectra of samples AR-03–56(1–2) exhibit a

staircase shape in the low-temperature part of the spectra

(Fig. 10a, b). This feature can be interpreted as partial 40Ar

loss during post-HP peak deformation, polymetamorphism,

or fluid circulation (Scaillet 1996). No age plateau was

obtained from samples Ar-03–56(1–2), because they dis-

play staircase shapes. However, HT 40Ar/39Ar steps

converge to an age of *95 Ma in the two spectra. So the

higher-T steps ages of *95 Ma of samples AR-03–56(1–

2) can be considered a minimum age for phengite crys-

tallization in these samples. The hypothesis of excess 40Ar

is unlikely because a similar age is obtained for HT steps in

both samples.40Ar/39Ar spectra of the other HP phengite samples

(Fig. 10c, d) are flatter, which allows estimating small

plateau and plateau ages. A small plateau-age was obtained

for sample AR-03–58(1), at 91.6 ± 0.4 Ma, with 35% of

released 39Ar. A plateau-age was obtained for sample AR-

04–07, at 93.9 ± 1.6 Ma, with 90% of released 39Ar. A

similar ‘‘within error’’ isochron age of 95.4 ± 0.4 Ma has

been obtained on this sample (AR-04–07), confirming the

age of *95 Ma. These plateau and isochron ages confirm

that the progressive staircase spectra of AR-03–56(1–2)

and AR-03–58(1) are due to partial resetting either during

early near-isothermal decompression or during the later

1,0

1,5

2,0

0,5

200 300 400 500 600 700 800

2,5

Jd10

Jd20

Jd30

Jd40Jd50Lw

s+Jd

Pa+Z

o+Q

z

Lws+ab

Si: 3,8

Pa+Zo+Qtz

Si:3,7

Pum

p+C

hl+Q

z

Si:3,6

Si:3,5

Sph +

Cz+

Act

+H20

TEMPERATURE (°C)

Si:3,4

GF

EBF AEA

LBF EBF

E

PR

ESS

UR

E (

GP

a)

AbJd

+ Qtz

PgJd50 +Ky

CoQtz

95-91 Ma

73.5-71 Ma

1

2

26°C/km

12°C/km

Glc+

Fig. 9 P-T diagram showing the stability fields of studied assem-

blages, and deduced P-T-t paths. The shaded domains correspond to

PT estimates derived from the stability fields of the two parageneses,

including the jadeite content in clinopyroxene and the Si content in

phengite. Ellipses are estimates obtained with Thermocalc. Ages

shown are those obtained by 40Ar/39Ar on phengite (this paper).

Limits of metamorphic facies after Evans (1990). AEA Albite-

Epidote-Amphibolite; E Eclogite; EBF Epidote-Blueschist Facies; GSGreenschist Facies; LBF Lawsonite Blueschist Facies

544 Int J Earth Sci (Geol Rundsch) (2009) 98:533–550

123

LP–MT stage, and do not result from excess 40Ar. From

these four samples, the age of HP metamorphism can be

bracketed between 95 and 91 Ma.

40Ar/39Ar dating of LP–MT peak phengite

The dating of LP–MT peak phengite is shown on Fig. 10e,

f. A small plateau was obtained from sample AR-03–54,

dated at 73.3 ± 0.5 Ma with 52% of released 39Ar. This

age is confirmed by the isochron age of 73.0 ± 1.4 Ma in

this sample. A plateau age was obtained from sample AR-

03–58(2) at 71.1 ± 0.5 Ma with 73% of released 39Ar.

From these two samples, the age of retrogression of

blueschists in the epidote-amphibolite facies is bracketed

between 71 and 73.5 Ma.

Discussion and Conclusions

P–T–t path and timing of deformation events

The complementary thermobarometric and 40Ar-39Ar geo-

chronological approaches allow the construction of P–T–t

paths, for two texturally defined sequences of assemblages:

(1) the HP rocks retrogressed into the greenschist facies, (2)

the HP rocks retrogressed into the epidote-amphibolite

facies (Fig. 9). The deduced rate of exhumation, using an

average density of 2.7 between the two phases is of 0.6–

1.5 mm a-1. This rate seems too slow for an uplift rate

within a subduction zone, as several works have shown that

the exhumation rate in these contexts is of the same order of

magnitude as the subduction rate (Rubatto and Hermann

2001). In addition, the relatively hot geotherm (26�C km-1)

Table 3 Representative results of Thermocalc calculations, for fit close to 1. Water and CO2 activities were fixed to 0.9 and 0.1, respectively

Parageneses Reactions Average

temperature

Average pressure

HP–LT (sample AR0350) fgl + 3hem = 2jd + 3mt + 4q + H2O

gl + 3cc + 2q = 3di + 2jd + H2O + 3CO2

fgl + 3cc + 2q = 3hed + 2jd + H2O + 3CO2

3mt + cz + 7cc + 12q + H2O = 3hed + 3ps + 7CO2

fgl + 3hem + ps = 2acm + 3mt + cz + 4q + H2O

mrb + hed + hem + 2cc = 3di + 2acm + mt + H2O + 2CO2

fgl + 4jd + 3mt + 10cc + 14q = 6hed + 6acm + 2cz + 10CO2

3fgl + 10mrb + 6ps = 6gl + 12di + 14acm + 9mt + 22q + 10H2O

7mrb + 13hed + 5mt + 4cz = 6fgl + 21di + 2acm + 11hem + 3H2O

3mrb + 6hed + 3mt + 2cz + CO2 = 3fgl + 9di + 6hem + cc + H2O

545 ± 60�C 1.25 ± 0.15 GPa

LP–MT (sample

AR0414)

2cz + 5cc + 3q = 3gr + H2O + 5CO2

6ts + 3clin = 11py + 4gr + 18H2O

8sp + ames + 5gr + 21q = 4ts + 7an

2ts + 3ames + 8q = 4an + 6py + 14H2O

4ts + 3ames + 6q = 4cz + 8py + 14H2O

9ts + 14cz + 38cc = 7sp + 4clin + 28gr + 38CO2

24cz + 5alm + 66cc + 36q = 3daph + 38gr + 66CO2

8ts + 15daph + 6ab + 4CO2 = 6parg + 25alm + 4cc + 62H2O

3ames + 2py + gr = 3sp + 3clin + 3an

ts + 3an + 2cc = 2cz + py + gr + 2CO2

505 ± 67�C 0.57 ± 0.02 GPa

LP–LT (sample

AR0352b)

2cz + pa + 2q = 4an + ab + 2H2O

3ftr + 2cz + 7pa + 3ab = 5fgl + 10an + 6H2O

5fgl + 6an + 8cc + 14q = 3ftr + 4cz + 10ab + 8CO2

5fgl + 2an + 4cc + 10q = 3ftr + 2pa + 8ab + 4CO2

2pa + 3cel + an + CO2 = gl + 3mu + cc + 3q + H2O

3ftr + 5gl + 18cz + 15mu + 21q = 5fgl + 15cel + 42an + 12H2O

2gl + 3ftr + 2ps + 7pa + 3ab = 5fgl + 2mrb + 10an + 6H2O

3ftr + 2gl + 2ps + 7pa + 3ab = 2mrb + 5fgl + 10an + 6H2O

10fgl + 6ps + 21pa + 30cel = 4gl + 6ftr + 6mrb + 30mu + 21ab + 18H2O

463 ± 30�C 0.35 ± 0.04 GPa

Mineral abbreviations are following Powell and Holland (1998). Estimates are given at 2r

Int J Earth Sci (Geol Rundsch) (2009) 98:533–550 545

123

deduced for the LP–MT retrogression is more in agreement

with crustal thickening during an early collisional stage than

with an ongoing subduction process. Similar increase of

temperature is obtained from Himalayan (de Sigoyer et al.

2000) and Alpine (Rolland et al. 2000) blueschists, where it

has been linked to thermal relaxation due to higher radio-

genic heat production related to collisional crustal stacking

(e.g., Vanderhaeghe et al. 1999). Such a collisional setting

during LP–MT metamorphism is also suggested by related

structures. The compressive deformation featured by

upright folds, in a horizontal NE–SW shortening context,

has to closely post-date the LP–MT (71–73 Ma) meta-

morphism, as it predates the deposition of the Paleocene-

Eocene UVSU. Such compressive deformation can be

ascribed to the accretion of the South Armenian Block to the

Eurasian margin, as it is the main shortening phase in the

Fig. 10 40Ar/39Ar phengite spectra of SBOC samples. a–d HP phengites samples. e–f LP–MT phengite samples

546 Int J Earth Sci (Geol Rundsch) (2009) 98:533–550

123

Lesser Caucasus. Consequently, we suggest that the time

gap between the two phases, the thermal increase during the

second metamorphic phase, and the tectonic style are due to

the collision of the South Armenian Block with the margin

of Eurasia at 71–73.5 Ma.

Large-scale geometry

The large-scale geometry of the South Armenian Block-

Eurasian margin boundary is interpreted in the light of data

presented in this paper (Fig. 11). The protrusion of blues-

chists and ophiolitic rocks in a top-to the North thrust

sequence, along a mainly north-dipping tectonic pile is

indicative of local back-thrusting. This may be facilitated

by a crustal back-stop at the northern edge of the Sevan-

Akera suture zone. The coincidence of north-dipping nor-

mal faults to the north of the ASBOC, with large amounts

of Plio-Quaternary volcano-sedimentary infill may coin-

cide with local extension at the back of a main Cretaceous

south-verging thrust ramp (Figs. 4, 11).

Geodynamic interpretations

The ASBOC is featured by the following geodynamic

history (Fig. 12):

(1) Formation of the ophiolite in the Upper Jurassic (age

given by radiolarians, Danelian 2007), which is

compatible with the age of the Sevan Ophiolite

(Zakariaze et al. 1983).

(2) Subduction of this oceanic crust segment during the

late Lower Cretaceous (at c. 95–91 Ma, 40Ar/39Ar

age of blueschists), and formation of the andesitic

volcanic arc as a result of the intra-oceanic subduc-

tion (Galoyan et al. 2007). Subduction might have

been west-or east-dipping as suggested by the

current geometry and the E–W orientation of linea-

tions. However, E–W lineations might have been

passively rotated during the main shortening phase.

Even though constant N–S convergence regime is

ascertained by paleomagnetism during this period

(e.g., Ricou 1994 and references therein), such E–W

motions might be explained by short-lived reactiva-

tion of transform faults oblique to convergence

direction. In the West, opening of a back-arc system

is ascertained by N–S, west dipping, normal faults.

These correspond to the eastern rim of the Black sea,

which is interpreted as a back-arc basin opened in

the Cretaceous (e.g., Boccaletti et al. 1974; Gorur

1988).

(3) The E–W convergence stops in the Late Upper

Cretaceous, and the whole tectonic pile is affected

by NNE–SSW shortening. Due to the intra-oceanic

setting of the volcanic arc, it is likely that a stage of

arc-continent accretion preceded the continent-conti-

nent collision (Fig. 12). The two events occur within

the range of LP–MT metamorphism. In a first stage,

nappe stacking at 73–71 Ma (40Ar/39Ar ages on

phengite) of variably metamorphosed units (greens-

chists above blueschists) suggests an ongoing

subduction context, dipping to the north. Further,

continent-continent collision (between 70 and 55 Ma)

between the South Armenian block to the South and

Eurasia to the North resulted in the blocking of the

subduction and in the formation of a penetrative E–W

fold belt. As is emphasized in Fig. 12, the very

peculiar structural aspect of the ‘‘blueschist windows’’

is allowed by a combination of thrusting towards the

south and back-thrusting towards the north.

(4) Cenozoic evolution is featured by Paleocene-Eocene

unconformity over the folded nappe stack. Large

wave-length folds, related thrusting towards the south

(e.g., the Spitak fault more to the South), and normal

faulting towards the north at the back of the ramp-

anticline structure resulting in the depression of the

Stepanavan area (Fig. 12).

Lateral correlations with other Tethyan suture zones

Timing of subduction in the ASBOC is similar as for NW

Turkey, ranging mostly from 105 to 85 Ma across the Izmir-

Ankara suture (Harris et al. 1994; Okay et al. 1998; Sherlock

++

++

+

+++

+

++ ++

++ ++

++ ++

++ ++

++ ++ ++

++

++

++ +

+++ + + +

CP-E

P-E

C

P-EP-Q

Armenian BlockEurasia

NorthSouth

20 km

J

BU

CC

C

Fig. 11 Schematic interpretative cross-section of the Armenian–Eurasian boundary. The capitals refer to geological formations. J Jurassic;

C Cretaceous; P-E Paleocene-Eocene; P-Q Plio-Quaternary. The black box indicates the location of the Stepanavan cross-section (Fig. 3 )

Int J Earth Sci (Geol Rundsch) (2009) 98:533–550 547

123

et al. 1999; Okay et al. in preparation). More to the SE,

supra-subduction plagiogranites were dated at 93–88 Ma by

K–Ar on whole rock by Kananian et al. (2001), and Mid-

Cretaceous 40Ar/39Ar ages are also reported from blueschists

in the Zagros region (Agard et al. 2006, 2007). These data

provide evidence that the same subduction front was active

from NW Turkey to N Armenia, contemporaneously to

subduction in the S Iran Zagros Subduction Zone.

The age of blueschist retrogression is not uniform along

this belt, which emphasizes different processes of exhu-

mation after peak pressure conditions were attained. In

northern Turkey, rapid exhumation postdates the HP peak,

so the rocks were not found to have a 2-phase evolution

showing that burial and exhumation are part of the same

subduction history within the accretionary prism (Okay

et al. 1998; Sherlock et al. 1999; Okay et al. in prepara-

tion). In the ASBOC of Armenia, a LP–MT phase is found

to postdate the HP peak by *20 Ma, which is in agreement

with a 2-phase exhumation with (1) subduction and (2)

early collision history as is found in the Alps and the

Himalayas.

Acknowledgments This work was supported by the MEBE (Middle

East Basin Evolution) program. Fieldwork was facilitated by the

support of the Armenian Academy of Science (Institute of Geological

Sciences). Financial support from the French Embassy in Armenia

and the French Ministry for Foreign Affairs has allowed Mr. Ghazar

Galoyan to complete his PhD in France. The help of G. Feraud and

J.L. Devidal during data acquisition in Nice and Clermont-Ferrand is

acknowledged. Reviews provided by A.I. Okay and S. Guillot helped

improve the quality of the manuscript.

References

Abovyan SB (1981) The mafic-ultramafic complexes of the RSS

ophiolitic zones in Armenia. Izvestia ANS RSS Armenia, p 306

(in Russian)

95-75 Ma

75-70 Ma

70-50 Ma

Fig. 12 Geodynamic reconstruction of the evolution of the northern

part of the South Armenian Block (Lower–Upper Cretaceous) in the

study area. The presence of an intra-oceanic subduction is ascertained

by the presence of an obducted ophiolite sequence onto the South

Armenian Block comprising an oceanic crust section overlain by a

volcanic arc, which has been formed intra-oceanically in the Upper

Cretaceous (as described in the paper). This requires an oceanic

(‘‘back-arc’’) domain of unknown extension to the north of the

volcanic arc to the Eurasian Margin, and a subducting oceanic domain

to its south

548 Int J Earth Sci (Geol Rundsch) (2009) 98:533–550

123

Adamia S, Bergougnan H, Fourquin C, Haghipour A, Lordkipanidze

M, Ozgul N, Ricou L, Zakariadze G (1980) The Alpine Middle

East between the Aegean and the Oman traverses. 26th

International Geological Congress Paris C5:122–136

Agard P, Monie P, Gerber W, Omrani J, Molinaro M, Labrousse L,

Vrielynck B, Meyer B, Jolivet L, Yamato P (2006) Transient,

syn-obd uction exhumation of Zagros blueschists inferred from

P–T, deformation, time and kinematic constraints: implications

for Neotethyan wedge dynamics. J Geophys Res 111:B11401

Agard P, Jolivet L, Vrielynck B, Burov E, Monie P (2007) Plate

acceleration: the obduction trigger? Earth Planet Sci Lett 258(3–

4):428–441

Aghamalyan VA (1978) The old metamorphic complexes of the

territory of Armenian SSR and their tectonic disposition. In:

Proceedings of reports of the Second regional petrographic

symposium on Caucasus, Crimea and Carpathians, Tbsilisi, pp

109–115 (in Russian)

Aghamalyan VA (1981) Blueschist formation. In: Magmatic and

metamorphic formations of Armenian SSR, House of Academy

of Science of Armenian SSR, pp 121–122 (in Russian)

Aghamalyian VA (2004) The Lesser Caucasus earth crust formation

and evolution in the collision zone of Paleo-Tethys. In:

Proceedings of the 5th international symposium on Eastern

Mediterranean Geology, vol 1. Thessaloniki, pp 17–20

Aslanyan AT (1958) The regional geology of Armenia. Izvestia

Haypethrat, Yerevan, p 430 (in Russian)

Aslanyan AT, Satian MA (1977) On the geological features of

Transcaucasian ophiolitic zones. In: Nauki o Zemlie, Izvestia

NAS Arm SSR 4–5:13–26 (in Russian)

Avagyan A, Sosson M, Philip MH, Karakhanian A, Rolland Y,

Melkonyan R, Rebai S, Davtyan V (2005) Neogene to Quater-

nary stress field evolution in Lesser Caucasus and adjacent

regions using fault kinematics analysis and volcanic cluster data.

Geodin Acta 18:401–416

Boccaletti M, Gocev P, Manetti P (1974) Mesozoic isopic zones in

the Black Sea region: Societa Geologica Italiana Bollettino

93:547–565

Carman JH, Gilbert MC (1983) Experimental studies on glaucophane

stability. Am J Sci 283A:414–437

Danelian T, Galoyan G, Rolland Y, Sosson M (2007) Palaeontolog-

ical (Radiolarian) late Jurassic age constraint for the Stepanavan

ophiolite (Armenia). Bull Geol Soc Greece 40:31–38

de Sigoyer J, Chavagnac V, Blichert-Toft J, Villa IM, Luais B, Guillot

S, Cosca M, Mascle G (2000) Dating the Indian continental

subduction and collisional thickening in the northwest Himalaya:

multichronology of the Tso Morari eclogites. Geology 28:487–

490

Dercourt J, Zonenschain LP, Ricou LE, Kazmin VG, Le Pichon X,

Knipper AL, Grandjacquet C, Sbortshikov IM, Geyssant J,

Lepvrier C, Pechersky DH, Boulin J, Sibuet JC, Savostin LA,

Sorokhtin O, Westphal M, Bazhenov ML, Lauer JP, Biju-Duval

B (1986) Geological evolution of the tethys belt from the Atlantic

to the Pamirs since the LIAS. Tectonophysics 123:241–315

Evans BW (1990) Phase relations of epidote-blueschists. Lithos 25:3–23

Fotoohi Rad Droop GTR, Amini S, Moazzen M (2005) Eclogites and

blueschists of the Sistan Suture Zone, eastern Iran: a comparison

of P–T histories from a subduction melange. Lithos 84:1–24

Galoyan G, Rolland Y, Sosson M, Corsini M, Melkonyan R (2007)

Evidence for superposed MORB, oceanic plateau and volcanic

arc series in the Lesser Caucasus (Stepanavan, Armenia). C R

Geosci 339:482–492

Gabrielyan AA (1959) The main questions about Armenian Tecton-

ics. Izvestia NAS Armenia SSR, p 85 (in Russian)

Gorur N (1988) Timing of opening of the Black Sea basin.

Tectonophysics 147:247–262

Hakobyan VT (1976) Gastropods from Upper Cretaceous of Armenia.

Izvestia NAS Armenia SSR, p 441 (in Russian)

Harris NBW, Kelley SE, Okay AI (1994) Post-collision magmatism

and tectonics in northwest Turkey. Contrib Mineral Petrol

117:241–252

Heinrich H, Althaus E (1988) Experimental determination of the

reactions 4 lawsonite + 1 albite = 1 paragonite + 2 zoısite + 2

quartz + 6 H20 and 4 lawsonite + 1 jadeite = 1 paragonite + 2

zoısite + 2 quartz + 6 H2O. Neues Jahr Mineral Mon 11:516–

528

Holland TJB (1980) The reaction albite = jadeite + quartz deter-

mined experimentally in the range 600–1200�C. Am Mineral

65:129–134

Holland TJB, Powell R (1998) An internally consistent thermody-

namic dataset for phases of petrological interest. J Metamorph

Geol 16:309–343

Jourdan F, Feraud G, Bertrand H, Kampunzu AB, Tshoso G, Le Gall

B, Tiercelin JJ, Capiez P (2004) The Karoo triple junction

questioned: evidence for Jurassic and Proterozoic 40Ar/39Ar ages

and geochemistry of the giant Okavango dyke swarn (Bots-

wana). Earth Planet Sci Lett 222:989–2006

Kananian A, Juteau T, Bellon H, Darvishzadeh A, Sabzehi M,

Whitechurch H, Ricou LE (2001) The ophiolite massif of Kahnuj

(western Makran, southern Iran): new geological and geochro-

nological data. Comptes rendus de l’Academie des Sciences

Series IIA 332:543–552

Knipper AL (1975) The oceanic crust in the alpine belt. Tr. GIN NAS

USSR 267, p 207 (in Russian)

Knipper AL, Khain EV (1980) The structural position of ophiolites of

the Caucasus. Ofioliti Special Issue 2:297–314

Kretz R (1983) Symbols for rock-forming minerals. Am Mineral

68:277–279

Massonne HJ, Schreyer W (1987) Phengite geothermometry on the

limiting assemblage with K-felspar, phlogopite and quartz.

Contrib Min Pet 96:212–224

Meliksetyan BM, Baghdasaryan GP, Ghukasyan RKh (1984) Isoto-

pic-geochemical and geochronological investigations of

Eclogite–Amphibolites associated with ophiolites of Sevan-

Amasian belt (Amasian massif). In: Nauki o Zemlie, Izvestia

NAS Armenia SSR, vol 1, pp 3–22 (in Russian)

Melikyan LS (1966) To a question about the tectonic structure of the

north-eastern coast of Sevan lake. In: Nauki o Zemlie, Izvestia

NAS Armenia SSR 1–2, pp 52–62 (in Russian)

Milanovski EE (1968) Neotectonics of the Caucasus. Nedra, Moscow,

p 484 (in Russian)

Miyashiro A (1957) The chemistry, optics and genesis of alkali

amphiboles. J Facul Sci Tokyo Univ 11:57–83

Morkovkina VF, Harutyunyan GS, Gavrilova SI (1977) Some

questions of petrogenesis of the ophiolitic association of the

Lesser Caucasus. In: Magmatic formations of the Caucasus and

south of Eastern European platform. Nauka, Moscow, pp 183–

213 (in Russian)

Okay AI, Goncuoglu MC (2004) Karakaya complex: a review of data

and concepts. Turk J Earth Sci 13:77–95

Okay AI, Tuysuz O (1999) Tethyan sutures of northern Turkey. In:

Durand B, Jolivet L, Horvath F, Seranne M (eds) The

Mediterranean basins: tertiary extension within the Alpine

orogen, Geological Society London Special Publication, vol

156, pp 475–515

Okay AI, Harris NBW, Kelley SP (1998) Exhumation of blueschists

along the Tethyan suture in northwest Turkey. Tectonophysics

376:37–60

Okay AI, Monod O, Monie P (2002) Triassic blueschists and eclogites

from northwest Turkey: vestiges of the Paleo-Tethyan subduc-

tion. Lithos 64:155–178

Int J Earth Sci (Geol Rundsch) (2009) 98:533–550 549

123

Okay AI, Tuysuz O, Satır M, Ozkan-Altıner S, Altıner D, Sherlock S,

Eren RH (2007) Cretaceous and Triassic subduction-accretion,

HP/LT metamorphism and continental growth in the Central

Pontides, Turkey (in preparation)

Paffenholtz KN (1959) Geological history of the Caucasus. Izvestia

NAS SSR, Armenia (in Russian)

Parra T, Vidal O, Agard P (2002) A thermodynamic model for Fe–Mg

dioctahedral K white micas using data from phase-equilibrium

experiments and natural pelitic assemblages. Contrib Mineral

Petrol 143:706–732

Powell R, Holland TJB (1988) An internally consistent thermody-

namic dataset with uncertainties and correlations: 3.

Applications to geobarometry, worked examples and a computer

program. J Metamorph Geol 6:173–204

Ricou LE (1994) Tethys reconstructed: plates, continental fragments

and their boundaries since 260 Ma from Central America to

South-Eastern Asia. Geodin Acta 7:169–218

Ricou LE, Zonenshain LP, Dercourt J, Kazmin VG, Le Pichon X,

Knipper AL, Grandjacquet C, Sborshchikov IM, Geyssant J,

Lepvrier C, Pechersky DM, Boulin J, Sibuet JC, Savostin LA,

Sorokhtin O, Westphal M, Bazhenov ML, Lauer JP, Biju-Duval

B (1985) Methodes pour l’etablissement de neuf cartes paleoge-

ographiques de l’Atlantique au Pamir depuis le Lias. Bulletin de

la Societe Geologique de France 8:625–635

Rimmele G, Oberhansli R, Goffe B, Jolivet L, Candan O, Cetinkaplan

M (2003) First evidence of high-pressure metamorphism in the

‘‘Cover Series’’ of the southern Menderes Massif. Tectonic and

metamorphic implications for the evolution of SW Turkey.

Lithos 71:19–46

Robertson AHF, Ustaomer T, Pickett E, Collins A, Andrew T, Dixon

JE (2004) Testing models of Late Palaeozoic-Early Mesozoic

orogeny in Western Turkey: support for an evolving open-Tethys

model. J Geol Soc 161:501–511

Rolland Y, Lardeaux JM, Guillot S, Nicollet C (2000) Extension syn-

convergence, poinconnement vertical et unites metamorphiques

contrastees en bordure Ouest du Grand Paradis (Alpes Franco-

Italiennes). Geodin Acta 13:133–148

Rubatto D, Hermann J (2001) Exhumation as fast as subduction?

Geology 29:3–6

Scaillet S (1996) Excess 40Ar transport scale and mechanism in high-

pressure phengites: a case study from an eclogitized metabasite

of the Dora Maira nappe, western Alpes. Geochim Cosmochim

Acta 60:1075–1090

Sengor AMC, Yılmaz Y (1981) Tethyan evolution of Turkey: a plate

tectonic approach. Tectonophysics 75:181–241

Sherlock S, Kelley SP, Inger S, Harris N, Okay AI (1999) 40Ar-39Ar

and Rb–Sr geochronology of high-pressure metamorphism and

exhumation history of the Tavsanli Zone, NW Turkey. Contrib

Mineral Petrol 137:46–58

Silantiev SA, Zakariadze GS, Kononkova NN (1996) Metamorphic

rocks in the ophiolites of the Sevan-Akera zone of the Lesser

Caucasus. Petrology 4(2):171–182 (in Russian)

Sokolov SD (1977) The olistostroms and ophiolitic nappes of the

Lesser Caucasus. Izvestia Nauka, Moscow, p 92 (in Russian)

Spear FS (1988) Metamorphic fractional crystallisation and internal

metasomatism by diffusional homogeneisation of zoned garnets.

Contrib Mineral Petrol 98:507–517

Stampfli GM, Borel GD (2002) A plate tectonic model for the

Paleozoic and Mesozoic constrained by dynamic plate bound-

aries and restored synthetic oceanic isochrons. Earth Planet Sci

Lett 196:17–33

Steiger RH, Jager E (1977) Subcomission on geochronology:

convention of the use of decay constants in geo- and cosmo-

chronology. Earth Planet Sci Lett 36:359–362

Tirrul R, Bell R, Griffis RJ, Camp VE (1983) The Sistan Suture zone

of eastern Iran. Geol Soc Am Bull 94:134–150

Tsameryan OP, Zakariadze GS, Sobolev AV, Kononkova NN (1988)

Classifying of ophiolite volcanic series of Minor Caucasus based

on the composition of rocks and melts. Geochemistry 8:1140–

1158 (in Russian)

Turner G, Huneke JC, Podose FA, Wasserburg GJ (1971) 40Ar/39Ar

ages and cosmic ray exposure ages of Apollo 14 samples. Earth

Planet Sci Lett 12:15–19

Vanderhaeghe O, Burg JP, Teyssier C (1999) Exhumation of

migmatites in two collapsed orogens: canadian Cordillera and

french Variscides. In: Ring U, Brandon MT, Lister, GS, Willet

SD (eds) Exhumation processes: normal faulting, ductile flow

and erosion, Geological Society of London Special Publications,

vol 154, pp181–204

Villa IM (1998) Isotopic closure. Terra Nova 10:42–47

Whitechurch H, Ricou LE (2001) The ophiolite massif of Kahnuj

(western Makran, southern Iran): new geological and geochro-

nological data. C R Acad Sci Paris 332:543–552

Zakariadze GS, Knipper AL, Sobolev AV, Tsamerian OP, Dmitriev

LV, Vishnevskaya VS, Kolesov GM (1983) The ophiolite

volcanic series of the Lesser Caucasus. Ofioliti 8(3):439–466

Zakariadze GS, Knipper AL, Bibikova EV, Silantiev SA, Zlobin SK,

Gracheva TV, Makarov SA, Kolesov TM (1990) The setting and

age of the plutonic part of the NE Sevan ophiolite complex.

Izvestia NAS USSR. Geol Ser 3:17–30 (in Russian)

550 Int J Earth Sci (Geol Rundsch) (2009) 98:533–550

123