P-T evolution of kyanite eclogite from the Pirin Mountains (SW Bulgaria): implications for the...

P–T evolution of kyanite eclogite from the Pirin Mountains(SW Bulgaria): implications for the Rhodope UHP MetamorphicComplexM. JANAK,1 N. FROITZHEIM,2 N. GEORGIEV,3 T. J . NAGEL2 AND S. SAROV4

1Geological Institute, Slovak Academy of Sciences, Dubravska 9, P.O. Box 106, 840 05 Bratislava 45, Slovak Republic(marian.janak@savba.sk)2Steinmann-Institut, Universitat Bonn, Poppelsdorfer Schloss, D-53115 Bonn, Germany3Department of Geology, Palaeontology and Fossil Fuels, Sofia University St. Kliment Ohridski, 15 Tzar Osvoboditel Blvd.,1504 Sofia, Bulgaria4Research Institute ‘‘Geology and Geophysics’’ AD, 23 Sitnyakovo Blvd., 1505 Sofia, Bulgaria

ABSTRACT A new occurrence of kyanite eclogite in the Pirin Mountains of southwestern Bulgaria within the rocksbelonging to the Obidim Unit of the Rhodope Metamorphic Complex is presented. This eclogiteprovides important information about the peak–pressure conditions despite strong thermal overprint atlow pressure. Textural relationships, phase equilibrium modelling and conventional geothermobarom-etry were used to constrain the metamorphic evolution. Garnet porphyroblasts with inclusions ofomphacite (up to 43 mol.% Jd), phengite (up to 3.5 Si p.f.u.), kyanite, polycrystalline quartz, pargasiticamphibole, zoisite and rutile in the Mg-rich cores (XMg = 0.44–0.46) record a prograde increase in P–Tconditions from !2.5 GPa and 650 !C to !3 GPa and 700–750 !C. Maximum pressure values fallwithin the stability field of coesite. During exhumation, the peak–pressure assemblage garnet +omphacite + phengite + kyanite was variably overprinted by a lower pressure one forming symplect-itic textures, such as diopside + plagioclase after omphacite and biotite + plagioclase after phengite.The development of spinel (XMg = 0.4–0.45) + corundum + anorthite assemblage in the kyanite-bearing domains at !1.1 GPa and 800–850 !C suggests a thermal overprint in the high-pressuregranulite facies stability field. This thermal event was followed by cooling at !0.8 GPa underamphibolite facies conditions; retrograde kelyphite texture involving plagioclase and amphibole wasdeveloped around garnet. Our results add to the already existing evidence for ultra high pressure (UHP)metamorphism in the Upper Allochthon of the Rhodope Metamorphic Complex as in the Kimi Unitand show that it is more widespread than previously known. Published age data and field structuralrelations suggest that the Obidim Unit represents Variscan continental crust involved into the Alpinenappe edifice of the Rhodopes and that eclogite facies metamorphism was Palaeozoic, in contrast to theKimi Unit where age determinations suggest a Jurassic or Cretaceous age for UHP metamorphism. Thisimplies that UHP metamorphism in the Upper Allochthon of the Rhodopes may have occurred twice,during Alpine and pre-Alpine orogenic events, and that two independent HP ⁄UHP provinces ofdifferent age overlap in this area.

Key words: geothermobarometry; kyanite eclogite; phase equilibrium modelling; Rhodopes; ultra highpressure metamorphism.

INTRODUCTION

The Rhodope Metamorphic Complex (Ricou et al.,1998) in northern Greece and southern Bulgaria rep-resents a nappe system that formed during a pro-tracted, Jurassic to Eocene history of subduction andcollision events at the southern margin of the Euro-pean continent. The tectonometamorphic evolution ofthe Rhodopes is of more than regional significance fortwo reasons: (i) this is an orogen where more extensiveevidence for ultra high pressure (UHP) metamorph-ism, including microdiamond, is being found; (ii)the Rhodopes are part of a still active orogenic system

(the Hellenides), with subduction and the onset ofcontinental collision taking place now south of Creteand back-arc extension affecting the Rhodopes,whereas most other ultra high pressure findings are inold, inactive orogens. Therefore, the Rhodopes couldserve as an ideal model for UHP metamorphism, butthe tectonic setting and age of this UHP metamor-phism, as well as the overall tectonic structure of theRhodopes, are highly controversial. For example, onegroup of authors argues that all UHP metamorphismin the Rhodopes occurred at one time (e.g. Krennet al., 2010; 180 Ma), while others postulate that sev-eral episodes of HP to UHP metamorphism took place

J. metamorphic Geol., 2011, 29, 317–332 doi:10.1111/j.1525-1314.2010.00920.x

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(e.g. Liati, 2005; 149, 73, 51 and 42 Ma), all of thisbased on zircon and monazite dating. Additional field-based structural, petrological and geochronologicalstudies are therefore needed to unravel the tectono-metamorphic history of the Rhodopes.

In two areas, the Kimi Unit of the Eastern Rhodo-pes and the Nestos Shear Zone of the Western andCentral Rhodopes, mineralogical indicators of UHPmetamorphism have been found, most notably inclu-sions of diamond in garnet porphyroblasts from peliticgneisses (Mposkos & Kostopoulos, 2001; Perrakiet al., 2006; Schmidt et al., 2010). Associated maficand ultramafic rocks show only indirect evidence forUHP metamorphism like inferred quartz pseud-omorphs after coesite, rods of silica indicating theformer presence of supersilicic clinopyroxene, or clin-opyroxene exsolutions from majoritic garnet (Liatiet al., 2002). Geothermobarometry of eclogites fromthe Rhodopes has been a challenge because of severeproblems related to the lower pressure overprint of thepeak metamorphic assemblage (e.g. Liati & Seidel.,1996; Liati et al., 2002; Bauer et al., 2007) as well asthe reliability of the garnet–clinopyroxene Fe–Mgexchange thermometer and pressure estimates basedon the jadeite content of omphacite coexisting withquartz in the absence of phengite and kyanite. So farno thermobarometric evidence for UHP metamor-phism has been reported.

In this article, we describe the metamorphic evolu-tion of newly found eclogites from the RhodopeMetamorphic Complex in the Pirin Mountains ofsouthwestern Bulgaria. Described here are the miner-alogical and petrological features of rare kyaniteeclogite which was found near the village Obidim. Thiseclogite is of special interest because the peak meta-morphic assemblage garnet–kyanite–phengite–ompha-cite–quartz is preserved, particularly as inclusions ingarnet, despite strong thermal overprint at low pres-sure. The aim of this contribution is to determine peakpressure and temperature from equilibria among theseminerals using conventional geothermobarometry andpseudosection modelling. The resulting P–T conditionsabove the quartz–coesite equilibrium curve suggestUHP metamorphism. Furthermore, our thermody-namic modelling revealed stability of the observedkyanite–spinel–corundum–plagioclase assemblage athigh-pressure granulite facies conditions and providedclues for reconstructing the P–T trajectory duringexhumation of the investigated eclogites. Finally, theage of metamorphism and tectonic implications arediscussed. These results show that UHP metamor-phism is more widespread in the Rhodopes than pre-viously known and also involved the basement of thePirin Mountains. As the studied eclogite belongs to astructurally high tectonic unit for which the existingage data indicate that the entire metamorphic history isPalaeozoic, the eclogite facies overprint is probablyPalaeozoic as well. This adds an important new aspectto the controversy about the tectonic setting and age of

UHP metamorphism in the Rhodopes and in general,in that the recycling of continental crust in collisionalorogens may lead to the effect that UHP provincesrelated to completely different orogenic cycles overlapin one area.

GEOLOGICAL SETTING

The Rhodope Metamorphic Complex includes, inaddition to the Rhodope Mountains, also the Rila andPirin Mountains and the Serbo-Macedonian Massif(Fig. 1). These different massifs are separated bybasins of Palaeogene and Neogene age. The PirinMountains are separated from the Western Rhodopesby the Mesta Basin, a NNW–SSE-striking graben witha Palaeogene basin fill to the North and a Neogene oneto the South. So far no eclogite facies rocks werereported from the Pirin Mountains. The eclogitedescribed here comes from the eastern slope of thePirin Mountains against the Mesta Basin (Fig. 2).Small bodies of eclogite were also found in two otherlocalities at the northern end of the Mesta Basin.The basement of the Rhodope Metamorphic Com-

plex comprises four groups of tectonic units whichwere emplaced on each other during a protractedorogenic history from Jurassic to Eocene. The LowerAllochthon includes Variscan basement and, partly, ametasedimentary cover dominated by marble. Itincludes the Pangaion–Pirin Complex, exposed in thePirin Mountains and the Western Rhodopes, as well asthe Arda, Kardamos ⁄Kesebir and Byala Reka ⁄Kech-ros Domes in the Central and Eastern Rhodopes.These units were metamorphosed in the uppergreenschist to amphibolite facies, with migmatizationdeveloping in the Arda and Kardamos ⁄Kesebir meta-morphic domes. Metamorphism of the Lower Alloch-thon is Tertiary in age, for example, migmatization inthe Arda Dome is dated at 37.8 ± 1.5 Ma (Chernevaet al., 2002), and gneisses of the Kardamos ⁄KesebirDome cooled below 350 !C at 37 Ma (Marton et al.,2009).The overlying Middle Allochthon comprises slivers

of both oceanic and continental crust and, in addition,orthogneisses derived from Late Jurassic to EarlyCretaceous arc granitoids (Turpaud & Reischmann,2009). It includes, among others, the Kerdilion unit inthe Serbo-Macedonian Massif and the Sidironero-Mesta, and Asenica units in the Western and CentralRhodopes. The Middle Allochthon was thrust towardssouth-west over the Lower Allochthon during thePalaeogene along the Nestos Shear Zone (Bosse et al.,2009; Jahn-Awe et al., 2010). Eclogite facies meta-morphism is widespread in the Middle Allochthon butits timing is still controversial. U–Pb zircon datingyielded various Jurassic and Eocene ages interpreted todate the eclogite facies metamorphism (149, 51 and42 Ma; Liati, 2005). Remnants of ultra high pressuremetamorphism (garnet–kyanite mica schists withmicrodiamond) were found in tectonic slivers along

318 M. JANAK ET AL .

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the base of the Middle Allochthon (Mposkos &Kostopoulos, 2001; Schmidt et al., 2010). Theseprobably represent Jurassic-age UHP rocks from theUpper Allochthon (see below) that were later, duringthe Tertiary, captured in the Nestos Shear Zone duringtop-south-west thrusting (Mposkos et al., 2010).Krenn et al. (2010) dated zircon from these diamond-bearing rocks at 160–150 Ma. They interpreted theseages to post-date the pressure peak and assumed thatUHP metamorphism took place at 180 Ma.

The Upper Allochthon crops out most extensively inthe Eastern Rhodopes (Kimi Unit) and in theSerbo-Macedonian Massif (Vertiskos ⁄Ograzhden unit;Krohe & Mposkos, 2002). These units representVariscan continental crust which was partly affectedby HP and UHP metamorphism in the Jurassic toEarly Cretaceous. Metamorphic microdiamond hasbeen identified in the Kimi Unit of the EasternRhodopes and its former existence has been suggested,based on graphite pseudomorphs, also for the

Vertiskos Unit (Kostopoulos et al., 2000). Post-tec-tonic, 65 Ma pegmatites (Mposkos & Wawrzenitz,1995) indicate that shearing and high-grade meta-morphism were finished by the end of the Cretaceous,in contrast to the deeper units in the nappe stack.Apart from this, the timing of UHP metamorphism inthe Upper Allochthon is controversial. Older thanMiddle Jurassic (>170–160 Ma; Bauer et al., 2007)and Late Cretaceous (73.5 Ma; Liati et al., 2002) ageshave been proposed based on zircon dating, whereasSm–Nd dating of garnet pyroxenite yielded an EarlyCretaceous (119 Ma; Wawrzenitz & Mposkos, 1997)age.

The Uppermost Allochthon consists of low-grademetamorphic (greenschist facies, locally blueschistfacies) sedimentary and volcanic rocks, partly of oce-anic affinity. It includes the Circum-Rhodope Beltalong the SW border of the Rhodope MetamorphicComplex and the Mandrica and Alexandropolisgreenschists in the Eastern Rhodopes (Fig. 1). These

KraishteZone

Maritza Basin

KerdilionCircum-Rhodope-Belt

Upper Allochthon

Lower Allochthon

Middle AllochthonCretaceous andTertiary granitoids

Palaeogene sedimentsand volcanics

Neogene sediments

Detachment fault

Strike-slip fault

Thrust fault

Assumed faultState border

Strymon Valley Deta chment

Ribnovo

Maritza Fault

Greece

Turkey

Bulgaria

Kavala

Alexandropolis

RilaRila

Kardjali

ThasosThessaloniki

MandricaMandrica

Greece

Macedonia

Aegean Sea

F.Y.R.O.

Xanthi

Sidironero

Vardar Zone

25° 26°

41°41°

23° 24°

Kesebir/Kardamos

ArdaDome

ByalaReka/

KechrosDome

Graben

Strymon

Asenica Unit

Volvi Ophiolites

VertiskosVertiskos

Ograzhden

Uppermost Allochthon

Study area

Rila Mts.

Serbo-Macedonian

Massif

Pangaion

PirinM

Fig. 1. Tectonic map of the Rhodope Metamorphic Complex modified after Burg et al. (1996), Ricou et al. (1998), Bonev et al. (2006)and Dixon & Dimitriadis (1984).

KYANITE ECLOGITE FROM THE P IR IN MTS . 319

5449 60

302130

55 3134

283230

22 35 4935 25

4046 60

48 5550

25 3030

302520

16 30 30 30 25

Belitsa

Kornitsa

Breznitsa

Skrebatno

Osikovo

Ribnovo

Osenovo

Kremen

Eleshnitsa

Yakoruda

Teshovo

Ilinden

Dobrotino

Kraishte

Obidim

Ognianovo

Dagonovo

Gostun

18 2819

2124 18

Neogene-Quaternary sediments

Upper Eocene - Oligocene sediments,volcaniclastics, volcanics and subvolcanicbodiesSouth and Central Pirin plutons(Upper Eocene - Lower Oligocene)

Rila-Rhodope Batholith(Eocene)

Vertiskos-Ograzhden Unit

Sidironero-Mesta Unit orthogneisses

Sidironero-Mesta Unit variegatedsection (paragneisses, schists, marbles,amphibolites, metaultramafites)

Pirin-Pangaion Unit marbles

Pirin-Pangaion Unit orthogneisses

Large amphibolite and metagabbro bodies

Eclogite boudins

Low-angle normal faults

Syn-compressional (thrust) shear zone(boundaries between lithotectonic units)

Neotectonic faults

33Magmatic foliation

43Metamorphic and superimposed foliation:a) dipping; b) horizontal;c) vertical

38 Mineral and stretching lineation

a)b)c)

10 Bedding

Dolno Dryanovo Pluton

II Cross sections shown on

Fig. 3

Gotse Delchev

KoprivlenDolno Drynovo

Babiak

SamplelocalitySamplelocality

Dobrinishte

Pirin Mts.

Fig. 2. Geological map of the study area (Georgiev et al., 2010; modified after Dimitrova & Katskov, 1990; Marinova & Katskov,1990; Sarov et al., 2008, 2009).

units were emplaced by north-directed thrusting in thecourse of an arc-continent collision in the Jurassic toEarly Cretaceous (Bonev & Stampfli, 2003). The con-tact between the Circum-Rhodope Belt and the Ver-tiskos Unit (Fig. 1) is a younger, Late Cretaceous toEarly Tertiary thrust leading to the local superpositionof the Upper Allochthon on top of the UppermostAllochthon.

According to our regional tectonic interpretation,the Obidim eclogite belongs to the Upper Allochthon.In the northern part of the Mesta Basin, the ObidimUnit – a tectonic klippe of the Ograzhden Unit (UpperAllochthon) was downfaulted by the south-west-dipping Ribnovo Fault (Burchfiel et al., 2003; Geor-giev et al., 2010) so that it was preserved from erosion.This klippe comprises metagabbro and metadiorite,amphibolite with the eclogite relicts described here,garnet–kyanite schist, gneiss, migmatite and coarse-grained K-feldspar metagranite. The metamorphicfoliation of these rocks dips moderately to steeplynorth-east (Figs 2 & 3). The metabasic rock bodies,which include the eclogite, form lenses elongated par-allel to the foliation, intercalated into the Obidim Unitat various structural levels. U–Pb zircon dating yieldedages of 454 Ma for the protolith of a metagabbro,446 Ma for the protolith of a metadiorite, 452 Ma forthe protolith of a mylonitized gneiss and 320 Ma forthe leucosome of a migmatite (Peytcheva et al., 2009).Such ages are also typical for the Upper Allochthon inthe Serbo-Macedonian Massif (Vertiskos-OgrazhdenUnit; Zidarov et al., 2003; Macheva et al., 2006;Himmerkus et al., 2009). Peytcheva et al. (2009) con-cluded that the Obidim Unit consists of Ordovicianprotoliths overprinted by high-grade metamorphism atVariscan time (330–320 Ma).

PETROGRAPHY AND MINERAL CHEMISTRY

Eclogites were found within the amphibolite body nearSv.Ilja, north-west of the village Obidim (Figs 2 & 3).Eclogites occur as dm- to m-sized lenses and blocks in

the amphibolites. Kyanite eclogite was discoveredamong mostly retrogressed, mafic eclogites exposed inthe roadcut.

Kyanite eclogite (Fig. 4) is medium to fine grained,with reddish garnet and pale green clinopyroxenevariably replaced by dark-green amphibole, whilekyanite occurs as white-bluish grains. Microstructuresalong with variations in mineral chemistry suggest thatthe rocks have experienced a complex metamorphichistory. Subhedral garnet and elongated kyanite are setin a fine-grained matrix of clinopyroxene, plagioclaseand amphibole. The original high pressure, plagio-clase-free assemblage of garnet + omphacite + kya-nite + phengite ± amphibole ± zoisite + quartz +rutile was overprinted during the post-eclogite faciesdevelopment of lower pressure assemblages. Secondaryphases occur in the symplectites, coronas and fracturesof the major primary minerals. Omphacite is bestpreserved as inclusions in garnet and kyanite whereasphengite occurs only in the garnet porphyroblasts. Inthe matrix, the most abundant symplectites are diop-side + plagioclase after omphacite which often have alobate vermicular form and rarely biotite + plagio-clase after phengite. Matrix porphyroblasts of kyaniteare corroded and mantled by fine-grained symplectitesof spinel + plagioclase and corundum + plagioclase.Retrograde kelyphite texture involving plagioclase andamphibole is developed around garnet. Secondaryamphibole is abundant in the matrix, forming por-phyroblasts and symplectitic intergrowths with diop-side and plagioclase.

The chemical composition of the major minerals wasdetermined by CAMECA SX-100 electron microprobeat the State Geological Institute of Dionyz Stur inBratislava. The operating conditions were as follows:15 kV accelerating voltage, 20 nA beam current,counting times 20 s on peaks and beam diameter of2–10 lm. Mineral standards (Si, Ca: wollastonite, Na:albite, K: orthoclase, Fe: fayalite, Mn: rhodonite), pureelement oxides (TiO2, Al2O3, Cr2O3, MgO) and metals(Ni) were used for calibration. Raw counts were

1.401.201.00

I-IWS NE

Palaeogene sediments

Rila-Rhodope Batholith

Pirin plutons

Sidironero-Mesta Unit variegated section

Large amphibolite bodies Vertiskos-Ograzhden Unitgneisses and schists

Eclogite boudins

Ribnovo Fault

Metres

Fig. 3. West–east cross-section of the study area from the eastern slopes of the Pirin Mountains to the western slopes of the Rhodopes(Georgiev et al., 2010) showing the position of the eclogite boudins and the rocks from the Upper Allochthon.

corrected using on-line PAP routine. The mineralabbreviations inthisarticleareaccordingtoKretz(1983).

Garnet is poikiloblastic with inclusions of clino-pyroxene, amphibole, quartz, kyanite, phengite, zoisiteand rutile in the core (Fig. 5a–d). Some of the quartzinclusions are polycrystalline aggregates (Fig. 5c). Theoutermost rim of garnet is mostly free of inclusions,rare polyphase aggregates of staurolite, kyanite andamphibole are connected with the matrix by fracturespenetrating the garnet (Fig. 5e). Garnet itself is sur-rounded by amphibole + plagioclase kelyphite. Thecomposition of garnet (Table 1) corresponds toalmandine (43–55 mol.%), pyrope (27–37 mol.%),grossular (18–22 mol.%) and spessartine (1–3 mol.%)end-members. The compositional profile in the indi-vidual grain (Fig. 6) shows increase of Mg concomi-tant with decrease of Fe and Mn from the core to therim at nearly constant Ca content. A reverse patterncan be observed in the outermost part of garnet.

Clinopyroxene forms several microtextural andcompositional varieties. Primary clinopyroxene –omphacite occurs mostly as inclusions in garnet andkyanite (Fig. 5c,d) with the highest content of jadeitecomponent (up to 43 mol.%; Table 2). In the matrix,most omphacite has broken down to secondary, lessjadeitic clinopyroxene, which forms the symplectiticintergrowths with Na–Ca plagioclase (Fig. 5a). Thisclinopyroxene corresponds to diopside with<10 mol.% of jadeite component.

Kyanite forms tiny, randomly oriented inclusions ingarnet (Fig. 5d). It is also abundant in the matrixforming large, up to 500 lm lath-shaped porphyro-blasts with inclusions of omphacite (Fig. 5f–h) andzoisite. Omphacite is partly decomposed to less sodicclinopyroxene (diopside), plagioclase and amphibole(Fig. 5g,h). Kyanite is partly resorbed and rimmed byplagioclase, which is intergrown mostly with spinel andless frequently with corundum (Fig. 5g,h).Phengite occurs as inclusions in the garnet

(Fig. 5c,d). Mineral compositions show a range in Sip.f.u. from 3.36 to 3.5 (Table 3). In the matrix,breakdown of phengite led to the formation of biotiteand plagioclase which form symplectitic intergrowths.Amphibole inclusions in garnet form mostly small

euhedral crystals corresponding to pargasite, magne-siohastingsite and tschermakite according to IMAclassification, with higher Al and Na content thanmatrix amphibole (Table 4). Some of the amphiboleinclusions near the fractures show similar compositionto that of matrix amphibole. Amphibole of magne-siohastingsite to edenite composition occurs in kely-phitic rims around garnet (Fig. 5a,e) as lath-shapedcrystals, or symplectitic intergrowths with clinopyrox-ene and plagioclase. Matrix amphibole at a distancefrom the garnet contacts forms relatively large, pleo-chroic, dark-green to brown-green poikiloblasticgrains of magnesiohornblende composition.Spinel occurs with corundum in domains around

kyanite, forming the symplectites with Ca-rich pla-gioclase (Fig. 5g,h). The composition of spinel corre-sponds to solid solution of hercynite and Mg-spinelwith XMg = 0.40–0.45 (Table 5).Plagioclase composition is variable depending on

textural relationships (Table 5). Symplectites of pla-gioclase with diopsidic clinopyroxene (Fig. 5a) corre-spond to oligoclase and plagioclase of the kelyphitesaround garnet (Fig. 5a,e) is of intermediate composi-tion. Plagioclase in the symplectites with spinel andcorundum around kyanite is anorthite whereasplagioclase at a distance from kyanite contains moresodium (Fig. 5g,h). This Na–Ca plagioclase is com-monly intergrown with amphibole.Staurolite is rarely present in a polyphase aggregate

with kyanite and amphibole in the rim of garnet(Fig. 5e). Staurolite composition (Table 5) is moremagnesian (XMg = 0.42) than adjacent garnet(XMg = 0.38).Zoisite occurs mostly as inclusions in the core of

garnet and seldom in the kyanite. In the matrix, relicts

Fig. 4. Photograph of eclogite with kyanite (Ky) and garnet(Grt) porphyroblasts.

Fig. 5. (a–h) Photomicrographs and back-scattered electron (BSE) images of kyanite eclogite. (a) BSE image of garnet (Grt)porphyroblast surrounded by kelyphitic rim of plagioclase (Pl), amphibole (Amp) and clinopyroxene–plagioclase (Cpx + Pl) symplec-tites. Larger amphiboles, kyanite (Ky) andplagioclase are at a distance from the garnet contacts.Quartz (Qtz) is rimmedby clinopyroxene.(b) Garnet porphyroblast with line marking the location of the analysed profile shown in Fig. 6. (c) Polycrystalline quartz, omphacite(Omp) and phengite (Ph) in garnet porphyroblast. (d) BSE image of garnet with inclusions of kyanite, omphacite and phengite in the core.(e) BSE image of garnet rim with composite staurolite (St)–kyanite–amphibole inclusion connected by fractures with plagioclase–amphibole kelyphite. (f)Kyanite porphyroblasts inmatrix. (g, h) BSE images of kyanite with inclusions of omphacite. Omphacite is partlydecomposed to less sodic clinopyroxene, plagioclase and amphibole. Kyanite is partly resorbed and rimmed by Ca-rich plagioclase,which is intergrown with spinel (Sp) and corundum (Co). Na–Ca plagioclase and amphibole are at a distance from kyanite.

(a) (b)

(d)(c)

(e)(f)

(h)(g)

of zoisite are associated with plagioclase. The compo-sition of zoisite is given in Table 5.

Chlorite is present in fractures penetrating the garnet.The composition of such chlorite is shown in Table 5.

P–T EVOLUTION

Thermodynamic modelling and geothermobarometry

The P–T evolution of the investigated eclogite wasreconstructed from textural relationships, phase equi-librium modelling and geothermobarometry. Meta-morphic conditions were constrained from the distinctassemblages stable at peak and lower pressure condi-tions.

Isochemical phase diagram P–T sections (pseudo-sections) were calculated using Perple_X¢07 thermo-dynamic software (Connolly, 2005; version 07) withthe internally consistent thermodynamic data set ofHolland & Powell (1998, 2004 upgrade). The effectivebulk rock composition was calculated from the mineralchemistry and modal proportions of the observedphases from thin sections using back-scattered imagesof domains analysed using electron microprobe.

The pseudosection for the peak–pressure stage hasbeen calculated in the system Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2 (NCKFMASHT); theeffective bulk rock composition was calculated fromthe modal proportions and composition of garnet withinclusions of omphacite, phengite, kyanite, quartz,amphibole, zoisite and rutile and the kyanite por-phyroblasts in the matrix. The composition of the

matrix omphacite was estimated from the modal pro-portions and composition of clinopyroxene + plagio-clase symplectites. The pseudosection for the lowerpressure stage with partial breakdown of matrixkyanite was calculated in the system NCFMASH. Theeffective bulk rock composition was obtained from themodal proportions and composition of kyanite, spinel,corundum, plagioclase, diopsidic clinopyroxene andamphibole, in the selected domains adjacent to the

Table 1. Representative microprobe analyses of garnet.

Sample OBI3a OBI3a OBI3a OBI3a OBI3e OBI3e OBI3e OBI3e OBI3e OBI3e

Mineral Grt Grt Grt Grt Grt Grt Grt Grt Grt Grt

An.point Core Intermediate Edge Edge Core Core Intermediate Core Intermediate Edge

SiO2 39.56 40.08 39.56 39.34 39.68 40.22 40.27 39.52 40.10 38.92

TiO2 0.03 0.00 0.03 0.00 0.06 0.01 0.05 0.08 0.00 0.01

Al2O3 22.28 22.33 22.41 22.21 22.52 22.54 22.41 22.08 22.48 21.69

Cr2O3 0.02 0.02 0.00 0.00 0.04 0.07 0.05 0.02 0.08 0.00

FeOt 21.11 21.16 21.77 23.84 21.99 21.24 20.78 23.20 21.58 25.56

MnO 1.28 0.69 0.53 1.35 0.99 0.60 0.55 1.14 0.54 1.34

MgO 8.15 9.66 8.46 6.94 8.84 9.67 10.01 7.84 9.75 6.35

CaO 7.90 7.42 7.63 7.01 7.86 6.87 7.26 7.85 7.02 6.72

Total 100.33 101.36 100.40 100.70 101.97 101.22 101.38 101.72 101.55 100.58

Si 3.01 3.00 3.00 3.01 2.96 3.01 3.00 2.98 2.99 3.00

Ti 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.00 0.00

Al 2.00 1.97 2.00 2.00 1.98 1.99 1.97 1.96 1.98 1.97

Cr 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.00

Fe 1.34 1.32 1.38 1.53 1.37 1.33 1.30 1.46 1.35 1.65

Mn 0.08 0.04 0.03 0.09 0.06 0.04 0.04 0.07 0.03 0.09

Mg 0.92 1.08 0.96 0.79 0.98 1.08 1.11 0.88 1.08 0.73

Ca 0.64 0.60 0.62 0.58 0.63 0.55 0.58 0.64 0.56 0.56

Total 7.99 8.00 8.00 7.99 8.00 8.00 8.00 8.00 8.00 8.00

XAlm 0.45 0.44 0.46 0.51 0.45 0.44 0.43 0.48 0.44 0.55

XSps 0.03 0.01 0.01 0.03 0.02 0.01 0.01 0.02 0.01 0.03

XPrp 0.31 0.35 0.32 0.27 0.32 0.36 0.37 0.29 0.36 0.24

XGrs 0.22 0.20 0.21 0.19 0.21 0.18 0.19 0.21 0.19 0.18

XMg = Mg ⁄ (Mg+Fe) 0.41 0.45 0.41 0.34 0.42 0.45 0.46 0.38 0.45 0.31

Structural formulae calculated on the basis of 12 oxygen.

Distance (µm)Rim Rim

elo)noitcarf

Fig. 6. Compositional profile across garnet shown in Fig. 5bwith mole fractions of pyrope, grossular, spessartine andalmandine end-members.

kyanite (e.g. Fig. 5g,h). Solid-solution models forgarnet, phengite (Holland & Powell, 1998), omphacite(Green et al., 2007), clinopyroxene, spinel (Holland &Powell, 1996), plagioclase (Newton et al., 1980) and

amphibole (Dale et al., 2005) were used, as availablefrom the Perple_X datafile (solut_09.dat). In bothpseudosections, H2O was treated as a saturated phase.

Table 2. Representative microprobe analyses of clinopyroxene.

Sample OBI3a OBI3a OBI3a OBI3a OBI3a OBI3a OBI3a OBI3d OBI3e OBI3e OBI3e OBI3e

Mineral Omp Omp Omp Omp Omp Omp Cpx Omp Omp Omp Cpx Cpx

An.point In Grt In Ky In Grt In Grt In Ky In Grt Syma In Grt In Grt In Grt Sym Sym

SiO2 54.77 54.25 55.58 55.75 54.81 55.64 53.55 55.95 55.92 56.09 52.69 52.87

TiO2 0.14 0.11 0.13 0.10 0.13 0.15 0.08 0.14 0.05 0.15 0.13 0.19

Al2O3 11.33 11.17 11.82 11.50 11.60 11.11 4.15 11.89 10.08 10.86 5.54 3.56

Cr2O3 0.00 0.07 0.06 0.05 0.02 0.09 0.09 0.04 0.10 0.06 0.09 0.03

FeOt 6.18 3.46 5.05 4.77 4.78 4.33 5.23 4.98 5.82 3.91 5.31 5.62

MnO 0.23 0.03 0.12 0.13 0.02 0.08 0.10 0.09 0.08 0.08 0.08 0.08

MgO 7.84 9.32 8.02 8.21 8.84 8.67 13.82 8.27 8.67 9.08 13.05 14.00

CaO 13.23 14.46 13.23 13.29 13.50 13.45 22.04 13.13 13.90 14.22 22.05 23.28

Na2O 6.72 6.51 6.55 6.67 6.14 6.43 1.18 6.40 6.36 5.83 1.39 0.95

K2O 0.01 0.01 0.01 0.00 0.00 0.00 0.00 0.01 0.00 0.01 0.01 0.00

Total 100.45 99.37 100.56 100.47 99.85 99.93 100.25 100.90 100.98 100.30 100.35 100.57

Si 1.95 1.93 1.97 1.98 1.96 1.98 1.95 1.97 1.98 1.99 1.92 1.93

Ti 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.01

Al 0.48 0.47 0.49 0.48 0.49 0.47 0.18 0.50 0.42 0.45 0.24 0.15

Cr 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

Fe3+ 0.09 0.10 0.01 0.02 0.02 0.01 0.00 0.00 0.05 0.00 0.01 0.05

Fe2+ 0.10 0.01 0.14 0.12 0.12 0.12 0.16 0.15 0.12 0.12 0.15 0.13

Mn 0.01 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

Mg 0.42 0.50 0.42 0.43 0.47 0.46 0.75 0.44 0.46 0.48 0.71 0.76

Ca 0.50 0.55 0.50 0.50 0.52 0.51 0.86 0.50 0.53 0.54 0.86 0.91

Na 0.46 0.45 0.45 0.46 0.43 0.44 0.08 0.44 0.44 0.40 0.10 0.07

K 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

Total 4.00 4.01 4.00 4.00 4.00 4.00 4.00 3.99 4.00 3.98 4.00 4.00

Jd = (Na-Fe3+-2Ti) · 100 36.80 34.50 43.70 43.10 39.90 42.90 8.00 43.00 38.30 39.20 8.20 1.10

XNa = Na ⁄ (Na+Ca) 0.48 0.45 0.47 0.48 0.45 0.46 0.09 0.47 0.45 0.43 0.10 0.07

Structural formulae calculated on the basis of six oxygen and four cations.aSymplectite with plagioclase.

Table 3. Representative microprobe analyses of phengite.

Sample OBI3a OBI3a OBI3a OBI3a OBI3a OBI3c OBI3d OBI3e OBI3e OBI3e

Mineral Ph Ph Ph Ph Ph Ph Ph Ph Ph Ph

An.point In Grt In Grt In Grt In Grt In Grt In Grt In Grt In Grt In Grt In Grt

SiO2 51.72 49.99 52.40 52.92 50.55 51.01 52.11 51.51 53.45 52.66

TiO2 0.02 0.06 0.04 0.06 0.03 0.10 0.03 0.00 0.00 0.04

Al2O3 27.14 28.31 26.55 29.85 28.99 29.28 30.50 28.92 27.15 29.56

Cr2O3 0.07 0.00 0.02 0.00 0.00 0.40 0.00 0.00 0.00 0.03

FeOt 3.27 2.47 2.72 2.03 2.27 1.57 1.70 2.97 1.45 1.44

MnO 0.04 0.04 0.06 0.00 0.02 0.03 0.02 0.04 0.01 0.08

MgO 3.34 2.98 4.97 2.74 2.77 2.92 2.36 2.85 3.84 2.59

CaO 0.31 0.44 0.15 0.30 0.38 0.12 0.33 0.35 0.08 0.15

Na2O 0.04 0.06 0.06 0.00 0.02 0.04 0.07 0.08 0.02 0.06

K2O 8.95 9.27 8.78 8.66 9.07 9.58 9.11 9.08 9.81 9.19

Total 94.91 93.61 95.75 96.56 94.09 95.06 96.21 95.79 95.81 95.81

Si 3.42 3.36 3.43 3.41 3.37 3.37 3.38 3.37 3.50 3.43

Ti 0.00 0.00 0.00 0.00 0.00 0.01 0.00 0.00 0.00 0.00

Al 2.12 2.25 2.05 2.27 2.28 2.28 2.34 2.23 2.09 2.27

Cr 0.00 0.00 0.00 0.00 0.00 0.02 0.00 0.00 0.00 0.00

Fe 0.18 0.14 0.15 0.11 0.13 0.09 0.09 0.16 0.08 0.08

Mn 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

Mg 0.33 0.30 0.48 0.26 0.28 0.29 0.23 0.28 0.37 0.25

Ca 0.02 0.03 0.01 0.02 0.03 0.01 0.02 0.03 0.01 0.01

Na 0.01 0.01 0.01 0.00 0.00 0.01 0.01 0.01 0.00 0.01

K 0.76 0.80 0.73 0.71 0.77 0.81 0.76 0.76 0.82 0.76

Total 6.84 6.89 6.87 6.78 6.85 6.87 6.83 6.84 6.87 6.82

Structural formulae calculated on the basis of 11 oxygen.

Table 4. Representative microprobe analyses of amphibole.

Sample OBI3a OBI3a OBI3a OBI3a OBI3e OBI3e

Mineral Amp Amp Amp Amp Amp Amp

An.point In Grt In Grt Matrix Kelyphite In Grt Kelyphite

SiO2 39.43 46.42 49.05 42.39 41.20 46.03

TiO2 0.11 0.96 0.63 0.22 0.12 0.24

Al2O3 19.97 13.71 8.35 17.62 19.80 13.06

Cr2O3 0.00 0.03 0.05 0.02 0.01 0.11

FeOt 13.09 10.20 9.82 9.99 10.41 11.27

MnO 0.06 0.11 0.07 0.07 0.04 0.10

MgO 9.63 14.41 15.45 12.60 12.23 13.88

CaO 11.42 9.94 12.07 11.91 11.35 11.77

Na2O 2.49 1.82 1.07 1.92 2.66 1.85

K2O 0.40 0.43 0.28 0.66 0.19 0.41

Total 96.59 98.03 96.84 97.40 98.02 98.71

Si 5.83 6.54 6.54 6.12 5.90 6.55

Ti 0.01 0.10 0.10 0.02 0.01 0.03

Al 3.48 2.28 2.28 3.00 3.34 2.19

Cr 0.00 0.00 0.00 0.00 0.00 0.01

Fe3+ 0.27 0.51 0.51 0.24 0.38 0.29

Fe2+ 1.35 0.69 0.69 0.97 0.87 1.05

Mn 0.01 0.01 0.01 0.01 0.01 0.01

Mg 2.12 3.03 3.03 2.71 2.61 2.95

Ca 1.81 1.50 1.50 1.84 1.74 1.80

Na 0.71 0.50 0.50 0.54 0.74 0.51

K 0.08 0.08 0.08 0.12 0.04 0.08

Total 15.68 15.25 15.25 15.57 15.62 15.47

Structural formulae calculated on the basis of 23 oxygen and Fe3+ as average after Holland

& Blundy (1994).

Geothermobarometry based on the net-transferreaction equilibria:

3 diopside" 2 kyanite# grossular" pyrope" 2 quartz

6 diopside" 3 muscovite # 3 celadonite

" 2 grossular" pyrope$2%

calibrated by Krogh Ravna & Terry (2004) has beenapplied for the peak–pressure stage. The activitymodels for phengite (Holland & Powell, 1998), clino-pyroxene (Holland, 1990) and garnet (Ganguly et al.,1996) have been used. This geothermobarometry is lessaffected by later re-equilibration than cation exchangein the garnet–clinopyroxene thermometer, diminishingthe problems with estimation of Fe3+ in clinopyrox-ene. The uncertainty limits of this garnet–phengite–clinopyroxene–kyanite–quartz ⁄ coesite thermobarome-ter are ±65 !C and ±0.32 GPa (Krogh Ravna &Terry, 2004). Omphacite with the highest jadeite con-tent, garnet with maximum pyrope and grossularcontent and phengite with the highest Si content havebeen chosen to calculate peak–pressure conditions.Analyses of phases used for geothermobarometry aregiven in Tables 1–3. The P–T conditions of the retro-grade stage with formation of amphibole + plagio-clase kelyphites around garnet have been calculatedusing the garnet–amphibole geothermometer (Graham& Powell, 1984), garnet–amphibole–plagioclase–quartzgeobarometer (Kohn & Spear, 1990) and garnet–plagioclase–rutile–ilmenite geobarometer (Bohlen &Liotta, 1986). The compositions of the garnet rim

together with adjacent amphibole and plagioclase wereused (Tables 1, 4 & 5).

P–T path reconstruction

The prograde metamorphic evolution can be inferredfrom the composition of garnet and inclusions in gar-net, i.e. omphacite, kyanite, phengite, pargasiticamphibole, zoisite and quartz. In the calculatedpseudosection (Fig. 7), the garnet XMg = 0.38–0.43isopleths show that garnet core composition plots intothe divariant Grt–Omp–Ky–Phe–Amp–Zo–Qtz field.The garnet rim with higher XMg (0.44–0.46) composi-tion records increase in P–T conditions towards thequadrivariant Omp–Phe–Grt–Ky–Qtz and Omp–Phe–Grt–Ky–Coe stability fields where amphibole andzoisite are absent. This suggests that amphibole andzoisite inclusions in garnet cores are remnants from aprograde, lower P–T stage. Compositional isopleths ofphengite (Si = 3.40–3.43) and omphacite (XNa = 47–48) intersect in these stability fields. This implies thatpeak–pressure conditions reached 2.7–3.1 GPa at700–750 !C (Fig. 7). The P–T results of conventionalgeothermobarometry yield 2.83 GPa at 712 !C and2.98 GPa at 700 !C; the intersections between reac-tions (1) and (2) are shown in Fig. 8a,b.The exhumation trajectory from the peak–pressure

stage can be constrained by the XMg = 0.45 isopleth ofgarnet which intersects the XNa = 46 isopleth ofomphacite in the Omp–Phe–Grt–Ky–Qtz stability fieldat 2.3–2.4 GPa and 770–800 !C, suggesting heatingduring the initial stage of exhumation. Approachingthe trivariant Omp–Phe–Grt–Ky–Zo–Qtz stability

Table 5. Representative microprobe analyses of spinel, staurolite, chlorite, zoisite and plagioclase.

Sample OBI3a OBI3d OBI3e OBI3e OBI3d OBI3a OBI3a OBI3a OBI3a OBI3a OBI3d OBI3e OBI3e OBI3e

Mineral Sp Sp Sp St Chl Zo Zo Pl Pl Pl Pl Pl Pl Pl

An.point Near ky Near Ky Near Ky In Grt In Grt In Grt In Ky Sym Cpx Kelyphite Near Ky Near Ky Kelyphite Sym Cpx Near Ky

SiO2 0.13 0.07 0.04 26.67 31.78 39.43 39.54 62.69 52.95 43.57 43.83 57.95 60.35 44.28

Al2O3 62.53 62.91 62.52 57.06 21.94 31.89 31.76 23.80 30.35 35.65 35.64 27.34 25.24 36.00

Cr2O3 0.07 0.21 0.10 0.04 0.06

FeOt 26.22 24.66 23.90 10.63 16.05 2.30 2.06

MnO 0.23 0.16 0.14 0.05 0.14 0.06 0.01

MgO 9.76 10.21 10.98 4.38 16.95 0.03 0.04

CaO 0.24 0.19 0.00 0.05 0.14 24.36 24.14 5.03 12.18 18.84 19.81 8.90 6.86 19.83

Na2O 0.02 8.66 4.45 0.72 0.25 6.59 7.53 0.35

K2O 1.41 0.15 0.03 0.01 0.01 0.05 0.12 0.00

Total 99.17 98.41 97.66 98.88 88.48 98.07 97.57 100.33 99.96 98.79 99.54 100.83 100.09 100.46

Oxygen 4 4 4 46 14 12.5 12.5 8 8 8 8 8 8 8

Si 0.00 0.00 0.00 7.22 3.13 3.00 3.02 2.76 2.39 2.03 2.03 2.57 2.68 2.04

Al 1.99 2.01 2.00 18.22 2.54 2.86 2.86 1.24 1.61 1.96 1.96 1.43 1.32 1.95

Cr 0.00 0.00 0.00 0.01 0.01

Fe 0.59 0.56 0.54 2.41 1.32 0.00 0.00

Mn 0.01 0.00 0.00 0.01 0.01 0.00 0.00

Mg 0.39 0.41 0.45 1.77 2.49 0.00 0.01

Ca 0.01 0.01 0.00 0.00 0.02 1.99 1.97 0.24 0.59 0.94 0.94 0.42 0.33 0.98

Na 0.00 0.74 0.39 0.07 0.07 0.57 0.65 0.03

K 0.18 0.01 0.00 0.00 0.00 0.00 0.01 0.00

Total 3.00 2.99 3.00 29.65 9.69 8.00 7.99 4.99 4.99 5.01 5.01 5.00 4.99 5.00

XMg = Mg ⁄ (Mg+Fe) 0.40 0.42 0.45 0.42 0.65

XCa = Ca ⁄ (Ca+Na) 0.24 0.60 0.94 0.94 0.43 0.34 0.97

field, the XMg trajectory of garnet changes its shapeand continues towards 2.1 GPa and 750 !C, whichindicates cooling and decreasing pressure. Then itcontours again the increasing temperature anddecreasing pressure towards the divariant Omp–Phe–Amp–Pl–Grt–Zo–Qtz field (Fig. 7) in which plagio-clase becomes a stable phase.

The formation of spinel, corundum and anorthite-rich plagioclase took place at the expence of kyanite,omphacite and zoisite as inferred from the textures.Kyanite with inclusions of omphacite and zoisite ispartly resorbed and rimmed by Ca-rich plagioclasewhich is intergrown with spinel and corundum.Omphacite is partly decomposed to less sodicclinopyroxene, Na–Ca plagioclase and amphibole. Inthe calculated P–T section (Fig. 9), the spinelXMg = 0.40–0.45 isopleths plot into the Sp–Cor–Ky–Pl–Cpx–Amp stability field corresponding to!1.1 GPa and 800–850 !C. The formation of amphi-bole and plagioclase kelyphites around garnet tookplace at P–T conditions of 0.8 GPa and 650 !C ascalculated from conventional geothermobarometry.

The reconstructed P–T path of kyanite eclogite fromthe Pirin Mts. is shown in Fig. 10. It suggests a pro-grade increase in P–T conditions from !2.5 GPa and650 !C to !3 GPa and 700–750 !C in the eclogitefacies conditions, just above the quartz–coesite equi-librium curve. Exhumation from the peak–pressurestage started at rising temperature and was followed by

cooling. Subsequently, the kyanite eclogite was af-fected by a thermal overprint at !800–850 !C and 1–1.5 GPa, in the high-pressure granulite facies stabilityfield. This thermal event was followed by cooling at!0.8 GPa under amphibolite facies conditions.

DISCUSSION

UHP metamorphism?

Thermodynamic modelling and thermobarometricresults show very high-pressure conditions during themetamorphic peak, maximum values fall within thestability field of coesite. Both methods yield consistentresults but uncertainties in the absolute P–T valuesstemming from the applied activity models of solidsolutions and model system parameters must be takeninto consideration. Furthermore, we have not foundany relict of coesite, only polycrystalline quartzaggregates in garnet, which can be diagnostic but notunique to recovery after coesite breakdown. Thepreservation of coesite depends on many factorsincluding the rigidity of the host mineral, the P–Tconditions and path of metamorphic recrystallization,rate of exhumation and presence of fluids (e.g.Mosenfelder et al., 2005). Our results are compatiblewith previous estimates of very high-pressure meta-morphism in the Rhodope Metamorphic Complex.Diamond-bearing metapelites are known from the

550 600 650 700 750 800 850

T (°C)

Omp Phe Amp Grt Lw Coe

Omp Phe AmpGrt Lw Ky Coe

Omp Phe AmpGrt Lw Ky Qtz

Omp Phe Grt Lw Coe

Omp PheGrt LwKy Coe

Omp Phe Grt Ky Zo Qtz

Omp PheAmp Pl

Zo QtzGrtPhe Amp Pl Grt Zo QtzPhe Amp PlGrt ZoPa Qtz Phe Amp Pl

Grt Ky Zo Qtz

Omp Phe Amp Grt Ky Zo Qtz

Omp Phe AmpGrt Zo Lw Qtz

Phe Amp Grt ZoPa Qtz

Phe Amp GrtLw Qtz

Omp Phe AmpGrt Zo Pa Qtz

Omp Phe Amp Grt Lw Qtz

3.423.40

Omp Phe Grt Ky Coe

Omp Phe Grt Ky Qtz

XMg (Grt)

Si (Phe)

XNa (Omp)

Fig. 7. P–T section calculated for thepeak–pressure stage in the NCKFMASHTsystem using the Perple_X thermodynamicsoftware (Connolly, 2005: version 07). Stablemineral assemblages constrained from thebulk rock composition (in wt%): SiO2 =51.51, TiO2=1.20, Al2O3 =19.43,FeO =8.57, MgO = 6.94, CaO = 10.50,K2O = 0.37, Na2O = 2.66, with H2O inexcess. (1) Omp Phe Grt Lw Ky Qtz; (2)Omp Phe Grt Amp Ky Qtz. Rutile is presentin all fields. The isopleths ofXMg = Mg ⁄ (Mg+Fe) in garnet, Si a.p.f.u.in phengite and XNa = (Na ⁄Na+Ca) · 100are shown with the ellipse encompassing thepeak–pressure conditions.

Kimi Unit (Upper Allochthon) and the Nestos ShearZone (boundary between Lower and Middle Alloch-thon) in northern Greece (Mposkos & Kostopoulos,2001; Perraki et al., 2006; Schmidt et al., 2010), thenearest location being in the Nestos Shear Zone nearSidironero, !70 km south-east (Schmidt et al., 2010).There is no coesite in the diamond-bearing rocks andthe associated eclogites near Sidironero, the highestrecorded P–T conditions, except for the microdia-monds, being !750 !C and 2.2 GPa (Schmidt et al.,2010). There are also similarities with kyanite eclog-ites from the Central Rhodope near Thermes inGreece (Liati & Seidel., 1996; Liati & Gebauer, 1999)and the Arda unit in Bulgaria (Kolcheva et al., 1986;Machev & Kolcheva, 2008). All these kyanite eclog-ites with sapphirine and spinel show a very strongoverprint in the granulite facies and only minor relicsof primary eclogite facies assemblages with estimatedpeak–pressure conditions of !2.0–2.2 GPa. Granulitefacies overprint show also the metapelites on thewestern slopes of the Pirin Mts. (Machev & Hecht,2008).

To summarize, kyanite eclogite from Obidim pro-vides new thermobarometric evidence for UHPmetamorphism in the Rhodopes. To explain a high-temperature overprint, there are two possibilities: (i)thermal relaxation caused by slow uplift or (ii) a sep-arate, post-eclogite metamorphic event at relativelyhigher temperature. The second possibility is favouredfrom the reconstructed P–T path (Fig. 10), suggestinga high-temperature overprint on partly exhumedeclogite at 1–1.5 GPa.

Tectonic implications

Finding of eclogites in the Obidim Unit shows thatHP ⁄UHP metamorphism in the Rhodope Metamor-phic Complex is more widespread than previouslyknown. As stated above, the Upper Allochthon alsocomprises the Vertiskos ⁄Ograzhden Unit in the Serbo-Macedonian Massif and the Kimi Unit in the EasternRhodopes. The age of HP ⁄UHP metamorphism in theKimi Unit is controversial. Bauer et al. (2007) pub-lished zircon ages of 170–160 Ma, which are inter-preted as recording a metamorphic overprint atgranulite facies conditions, while HP ⁄UHP metamor-phism occurred before that time, most probably in theEarly Jurassic. By contrast, Liati et al. (2002) and Liati(2005), based also on zircon dating, proposed an age of73.5 Ma for the UHP metamorphism. Sm–Nd datingof garnet pyroxenite yielded 119 Ma (Wawrzenitz &Mposkos, 1997). The age of scarce eclogites found inthe Vertiskos unit in Greece is even less constrained.The Vertiskos Unit was affected by shearing underlower amphibolite facies conditions (Kilias et al., 1999)in the Early Cretaceous (136 Ma; De Wet et al., 1989)and any eclogite facies metamorphism should be olderthan this. The only published age data from the Obi-dim Unit (Peytcheva et al., 2009, see above) are several

500 600 700 800 900 1000

CoeQtz

Temperature °C

P = 2.83 GPaT = 712 °C

500 600 700 800 900 1000

Temperature °C

P = 2.98 GPaT = 700 °C

CoeQtz

Fig. 8. (a, b) P–T results from geothermobarometry (KroghRavna & Terry, 2004) based on a net-transfer reactionsequilibria: (1) 3 diopside + 2 kyanite = grossular + pyrope +2 quartz and (2) 6 diopside + 3 muscovite = 3 celadonite + 2grossular + pyrope. (a) sample OBI3a, (b) sample OBI3e. Thequartz–coesite and graphite–diamond curves are calculated fromthermodynamic data of Holland & Powell (1998).

Ordovician protolith ages and a Variscan age of c.320 Ma for the leucosome of a migmatite, determinedusing U ⁄Pb dating of zircon. Hence, the Obidim Unitlikely represents Variscan basement. Taking the ageinformation from the Upper Allochthon together, twopossibilities exist for timing of metamorphism of theObidim eclogite: (i) Mesozoic (Jurassic or Cretaceous),in analogy to the assumed ages of UHP metamorphismand granulite facies overprint in the Kimi Unit; (ii)Palaeozoic, bracketed between the Ordovicianprotolith ages (c. 450 Ma) and the Carboniferous age(c. 320 Ma) of migmatization in the country rocks ofeclogite. This is the more straightforward interpreta-tion of the age data. Further studies, in particular,dating of the Obidim eclogite itself, are necessary toanswer this question.

In the Mesozoic, the Upper Allochthon belonged tothe southern margin of Europe (e.g. Jahn-Awe et al.,2010). The distal parts of this continental margin weresubducted southward during the Jurassic to EarlyCretaceous collision of the European margin with anisland-arc (Fig. 11) whose remnants are now found inthe Uppermost Allochthon (Bonev & Stampfli, 2003),in a scenario which resembled the Taiwan arc-conti-nent collision during the Miocene (e.g. Teng, 1990).We hypothesize that exhumation of the subductedunits was facilitated by the downward removal of theforearc lithosphere (!FA" in Fig. 11), leading to theemplacement of the high-grade metamorphic Upper

Allochthon, e.g. the Kimi Complex in the EasternRhodopes, directly below the low-grade rocks of theUppermost Allochthon. However, originally morenortherly located parts of the Upper Allochthon mayhave been less deeply or not at all subducted during theMesozoic (see Fig. 11). The metamorphism of theseunits is mainly or completely pre-Mesozoic. This isprobably the case for the Obidim Unit. In the LateCretaceous, the situation switched to north-dippingsubduction under the European margin which went onuntil the present day (Bonev & Stampfli, 2003; Jahn-Awe et al., 2010).

CONCLUSIONS

(1) This study provides new thermobarometricevidence for UHP metamorphism in the Rhodopes.Kyanite eclogite from the Obidim Unit in the PirinMts. records a prograde increase in P–T conditionsfrom !2.5 GPa and 650 !C to !3 GPa and700–750 !C. Maximum pressure values calculatedfrom the garnet + omphacite + phengite + kyaniteassemblage fall within the stability field of coesite. Thisstrengthens the evidence for regional UHP conditionsin the Upper Allochthon of the Rhodope Metamor-phic Complex.(2) Exhumation of the eclogite was associated with athermal overprint in the high-pressure granulite facies,reflected by development of spinel + corundum +

Sp Cor Ky PlCpx Amp

Sp Cor Ky Pl Amp

Sp Cor Sil Pl Amp

Sp Cor Sil Pl

Sp Cor Sil Pl hcrd

Sp Cor KyPl Cpx

Sp Cor KyCpx Amp alm

Cor Ky CpxAmp alm

Cor Ky CpxAmp fst

Cor Ky CpxAmp fst zo

Cor KyAmp fst zo

Cor Ky PlAmp fst

Sp Cor SilPl Cpx

700 750 850800 900T (°C)

XMg(Sp)0.40

Fig. 9. P–T section calculated for the lowerpressure stage and formation of spinel–corundum–plagioclase assemblage in theNCFMASH system. Stable mineralassemblages constrained from the bulk rockcomposition (in wt%): SiO2 = 31.24,Al2O3 = 57.33, FeO = 4.15, MgO = 2.16,CaO = 4.99, Na2O = 0.12, with H2O inexcess. Shown are the isopleths ofXMg = Mg ⁄ (Mg+Fe) in spinel and theellipse encompassing the estimated P–Tconditions.

100 km

Middle andLower Allochthon

UppermostAllochthon

UpperAllochthon

Europe

Accretionary wedge

Volcanic arc

Continental crust

Oceanic crust

Lithospheric mantle

BFig. 11. Tectonic setting of the RhodopeMetamorphic Complex in Jurassic to EarlyCretaceous time. Continental margin ofEurope is subducted during arc-continentcollision. Extraction of the fore arc (FA)block will exhume the Upper Allochthonand emplace the Uppermost Allochthon onit. Middle and Lower Allochthons will beaccreted later, from the Late Cretaceous toEocene, after polarity switch to the north-east dipping subduction. A: position of theObidim Unit assuming Jurassic to EarlyCretaceous age of eclogite facies metamor-phism as in the Kimi Unit; B: Position of theObidim Unit assuming that the eclogite isPalaeozoic and the unit did not suffer Alpinehigh-grade metamorphism.

600400 800 1000

CoeQtz

T (°C)

Amp-EC

Dry-EC

Fig. 10. P–T path for kyanite eclogite of thePirin Mts. Metamorphic facies grid is fromOkamoto & Maruyama (1999). BS, blues-chist facies; EA, epidote amphibolite facies;AM, amphibolite facies; HGR, high-pressure granulite facies; Lw-EC, lawsoniteeclogite facies; Ep-EC, epidote eclogitefacies; Amp-EC, amphibole eclogite facies;Dry-EC, dry eclogite facies. The quartz–coesite curve is calculated from thermody-namic data of Holland & Powell (1998).

anorthite assemblage in the kyanite-bearing domainsat !1.1 GPa and 800–850 !C.(3) The published age data and field structural rela-tions suggest that timing of high-grade metamorphismin the Obidim Unit was Palaeozoic and thus unrelatedto the Alpine orogeny in the Rhodopes, in contrast tothe Kimi Unit. This implies that UHP metamorphismin the Upper Allochthon of the Rhodopes may haveoccurred twice, during Alpine and pre-Alpine orogenicevents. To resolve these problems additional geochro-nological constraints are needed.

ACKNOWLEDGEMENTS

This work was financially supported by the SlovakResearch and Development Agency (project APVV-51-046105), the VEGA Scientific Grant Agency (grantno. 2 ⁄ 0031 ⁄ 09), the German Science Foundation DFG(grants no. FR700 ⁄ 10-1 and FR700 ⁄ 10-2) and theGerman Scientific Exchange Service DAAD (project!PPP Bulgaria"). Reviews from two anonymous refereesand editorial comments from D. Robinson promotedimprovements in the final manuscript.

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P-T evolution of kyanite eclogite from the Pirin Mountains (SW Bulgaria): implications for the...

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