high-grade aluminous calcite dolomite marbles...

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Lithos 104 (2008

Tracing high-pressure metamorphism in marbles: Phase relations inhigh-grade aluminous calcite–dolomite marbles from the GreekRhodope massif in the system CaO–MgO–Al2O3–SiO2–CO2

and indications of prior aragonite

A. Proyer a,⁎, E. Mposkos b, I. Baziotis b, G. Hoinkes a

a Department of Mineralogy and Petrology, Institute of Earth Sciences, Karl-Franzens-University, Universitätsplatz 2/II, A-8010 Graz, Austriab Department of Geological Sciences, School of Mining and Metallurgical Engineering, National Technical University of Athens,

9 Heroon Politechniou, GR15773, Zografou, Athens, Greece

Received 20 April 2007; accepted 11 December 2007Available online 27 December 2007

Abstract

Four different types of parageneses of the minerals calcite, dolomite, diopside, forsterite, spinel, amphibole (pargasite), (Ti–)clinohumite andphlogopite were observed in calcite–dolomite marbles collected in the Kimi-Complex of the Rhodope Metamorphic Province (RMP). Thepresence of former aragonite can be inferred from carbonate inclusions, which, in combination with an analysis of phase relations in the simplifiedsystem CaO–MgO–Al2O3–SiO2–CO2 (CMAS–CO2) show that the mineral assemblages preserved in these marbles most likely equilibrated atthe aragonite–calcite transition, slightly below the coesite stability field, at ca. 720 °C, 25 kbar and aCO2

~0.01. The thermodynamic modelpredicts that no matter what activity of CO2, garnet has to be present in aluminous calcite–dolomite-marble at UHP conditions.© 2007 Elsevier B.V. All rights reserved.

Keywords: Calcite–dolomite-marble; Spinel; Garnet; Aragonite; Petrogenetic grid; Ultrahigh-pressure

1. Introduction

The study of dolomitic marbles from ultrahigh-pressure(UHP) metamorphic terranes provides a possibility to assessT–Xfluid conditions additional to P–T information fromeclogites and other rock types. Such studies were performedon marbles from the UHP Sulu metamorphic terrane, ineastern China (Ogasawara et al., 1998, Omori et al., 1998) andon the diamond-bearing and diamond-free dolomite marblesfrom Kumdy–Kol area of the Kokchetav massif (Ogasawaraet al., 2000; Ogasawara and Aoki, 2005).

In the Rhodope metamorphic province diamond inclusionsin garnet porphyroblasts from migmatitic metapelites of theKimi complex indicate that these rocks were metamorphosed

⁎ Corresponding author. Tel.: +43 316 380 5541; fax: +43 316 380-9865.E-mail address: [email protected] (A. Proyer).

0024-4937/$ - see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.lithos.2007.12.002

within the diamond stability field (Mposkos and Kostopoulos,2001; Mposkos et al., 2004; Perraki et al., 2004, 2006).Concordant marble layers and lenses are associated with theseas well as with more quartzo–feldspathic gneisses andamphibolitised eclogites. A prior study focused on thecomposition of carbonates showed possible traces of ultrahigh-pressure origin in some of the marbles by their unusually Ca-richdolomite and Mg-poor calcite compositions (Mposkos et al.,2006). The purpose of the present study is to describe phaserelations of silicate and oxide minerals of these marbles and seeif they corroborate an UHP metamorphic history. Garnet ispresent in UHP marbles from Kokchetav and is considered to bea reliable indicator of high- to ultrahigh pressures. As garnet hasnot yet been found in the Rhodope marbles and because phaseequilibria in siliceous marbles are not pressure-sensitive, a novelmultisystem (grid) including alumina (systemCMAS–CO2) wascalculated in order to a) delineate the boundaries of the garnet

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120 A. Proyer et al. / Lithos 104 (2008) 119–130

stability field and see if other Al-bearing phases, e.g. spinel,could be stable at UHP conditions, and b) find reactions in thissystem that are fluid-independent and pressure-sensitive.Inspection of the grid shows that at least qualitative conclusionscan be drawn with regard to a possible high- or ultrahigh-pressure stage of metamorphism for Al-bearing marbles. Thegrid has general validity for Al-bearing siliceous calcite–dolomite marble of any provenance. Mineral abbreviationsused in the text are in accordance with those proposed by theInternational Mineralogical Association (Martin, 1998).

2. Geological setting, general petrology and age data

The RMP belongs to the northern part of an Alpine nappestack that extends over northeastern Greece and southernBulgaria and formed during the protracted convergence ofAfrica and Europe since the Middle Jurassic (e.g. vanHinsbergen et al., 2005). It contains mainly continental materialwith intercalated oceanic slices of ultramafic and mafic rocks(Burg et al., 1996; Ricou et al., 1998) and was classicallysubdivided in a lower and an upper terrane by Burg et al. (1996).In a recent geochronological study of the Central Rhodope,Turpaud (2006) distinguished a southern Thracia Terrane,dominated by Permo-Carboniferous orthogneisses and Triassicmarbles, from a northern Rhodope Terrane, consisting mainly ofyounger (Late-Jurrassic) orthogneisses, which again can beinterpreted as lower and upper continental plates, respectively,that were tectonically superimposed during Alpine convergenceand nappe stacking (Ricou et al., 1998). The two terranes are

Fig. 1. Geological map of the Greek Rhodope,

separated by the Nestos Suture Zone, within which HP to UHPmetamorphism is recorded in metasediments (Mposkos andKostopoulos, 2001; Perraki et al., 2006) and in mafic rocks(Kolcheva et al., 1986; Liati and Seidel, 1996). The entireRhodope nappe stack is transgressively overlain by Lutetian/Priabonian (48–42 Ma) deep- to shallow-water sediments,(Mposkos and Krohe, 2000; Krohe and Mposkos, 2002) andwas intruded by large-scale Tertiary granitoids, which led tolocal migmatisation of the host rocks (e.g. Peytcheva et al.,2004; Liati, 2005).

The Kimi Complex (Fig. 1; after Krohe and Mposkos, 2002and ref. therein) is a highly deformed, metamorphosed anddisrupted part of this suture zone, composed of orthogneisses,amphibolites/eclogites, marbles, serpentinites and diamond-bearing grt–ky-gneisses (Perraki et al., 2006). Local partialmelting occurred after eclogite-facies metamorphism, and apenetrative amphibolite-facies fabric points to ductile sheardisplacement and folding during regional metamorphism (Burget al., 1990; Dinter and Royden, 1993; Burg et al., 1996).Recent zircon ages obtained from metabasic and metapeliticcountry rocks of the marbles give 170–160 Ma for a high-pressure granulite to amphibolite facies stage (post-UHP) andpoint to a protracted residence in the middle to lower crust underamphibolite facies conditions until at least ca. 120 Ma (Bauer etal., 2007). The UHP stage could not yet be dated as zircon wasonly found in the matrix but not within diamond-bearing garnetof the grt–ky-gneisses. However, as no metamorphic hiatus isevident, it is considered to date slightly earlier than the firstrecorded age in matrix zircon, i.e around 180 Ma. We believe

with sample locations marked by numbers.

121A. Proyer et al. / Lithos 104 (2008) 119–130

that the long time of annealing and deformation at elevatedtemperatures is responsible for the deletion of virtually all UHPindicator minerals with the exception of a few diamonds. Nocoesite could be confirmed and aragonite is even more unlikelyto be preserved in the marbles. The marbles in the KimiComplex of the Kimi–Organi area occur as dm to manydekameters thick concordant bands and layers or extremelyelongated boudins within paragneisses. Calcite marblesstrongly predominate over calcite–dolomite marbles. Bothtypes are massive, coarse grained and whitish-pure, withsilicate or oxide phases hardly visible.

3. Analytical conditions

The constituent minerals in the marbles were analysed byelectron microprobe analysis (EMPA) using a scanning electronmicroscope, type JEOL JSM 6310 with EDS system and oneattached WDS spectrometer (for analysis of Na, F) at theInstitute of Earth Sciences of the University of Graz (Austria).Analytical conditions are as follows: accelerating voltage of15 kV, probe current of 5 nA, beam diameter of 1–4 μm.Counting time was 100 s. Standards used were calcite (Ca),magnesite (Mg), siderite (Fe) and rhodochrosite (Mn) forcarbonates, and garnet (Mg, Fe), corundum (Al), quartz (Si),adular (K), titanite (Ca, Ti) and rhodonite (Mn), jadeite (Na), F-phlogopite (F) for silicates and oxides. Na and F were measuredwith WDS (peak and background with a counting time of 20 seach). The beam-scanning technique was used for analyzingbulk chemical composition of calcite–dolomite intergrowths indomains of former Mg-rich calcite consisting of Mg-poorcalcite+ tiny dolomite inclusions.

Table 1Representative microprobe analyses of silicates and oxides from calcite–dolomite m

Ol Cpx1 Cpx2 Phl Amp

SiO2 42.53 54.64 55.54 41.83 45.02TiO2 b.d.⁎ 0.27 0.15 0.20 0.62Al2O3 b.d. 1.31 0.55 15.05 14.33FeOt 3.49 0.39 0.41 0.79 1.01MnO 0.05 0.03 b.d. 0.04 0.10MgO 55.28 18.19 18.87 27.13 20.43CaO 0.08 24.70 24.69 0.13 12.98Na2O b.d. 0.09 0.08 0.43 2.88K2O b.d. b.d. 0.02 10.24 1.14F b.d. b.d. b.d. 1.75 0.98F=O 0.00 0.00 0.00 −0.73 −0.41Total 101.43 99.62 100.30 96.84 99.07Si 1.00 1.98 1.99 2.90 6.26Ti 0.00 0.01 0.00 0.01 0.07Al 0.00 0.06 0.02 1.23 2.35Fe 0.07 0.01 0.01 0.05 0.12Mn 0.00 0.00 0.00 0.00 0.01Mg 1.93 0.98 1.01 2.80 4.24Ca 0.00 0.96 0.95 0.01 1.93Na 0.00 0.01 0.01 0.06 0.78K 0.00 0.00 0.00 0.91 0.20F 0.00 0.00 0.00 0.38 0.43∑Cat 3.00 3.99 4.00 7.96 15.95

⁎b.d. = below detection (limit).

4. Petrography and mineral composition

Twenty-five samples of dolomitic marbles from four outcropsof the Kimi–Organi area (Fig. 1) were examined with opticalmicroscopy and scanning electron microscopy (SEM) forparagenetic relations of the constituent minerals. The majorconstituent minerals of these dolomitic marbles can be groupedinto four assemblages: They contain Cal+Dol+Phl+Amp, plus:

(I) Phg(II) Ol (+Di)+Spl(III) Ol+Di (+Ti–Chu)(IV) Ol+Di+Ti–Chu+Spl.

Assemblages (I)–(IV) are representative of localities 1–4respectively, only assemblage (II) was found in both localities 2and 3. Diopside and Ti–clinohumite in assemblages (II) and(III) are present with b1 vol.%. Spinel in assemblages (II) and(IV) makes up ~0.5 to 2 vol.%. No quartz, coesite or diamondwas found in these samples, only graphite flakes in locality 1.Retrograde serpentine and chlorite are present in most analysedsamples. Serpentine and chlorite texturally replace olivine,spinel and phlogopite.

At locality 1 (Fig. 1), calcite in contact with dolomite hasalways low Mg (XMgCO3

=0.00–0.02); the corresponding solvustemperatures (solvus of Anovitz and Essene, 1987, for 2 kbar) areb350 °C (Table 1) and this marble is hence considered as stronglyretrogressed. Micas (phlogopite and phengite) and graphiteflakes by their parallel orientation outline a weak foliation. TheSi content in phengite ranges from 3.37–3.50 atoms per formulaunit (a.p.f.u.).

arbles

Chu Ti–Chu Chl Srp Sp Gei

38.23 38.15 30.98 42.88 b.d. b.d.0.58 3.99 b.d. b.d. b.d. 64.13b.d. b.d. 20.30 b.d. 69.27 b.d.2.51 1.71 1.10 3.04 3.14 8.940.02 b.d. 0.09 b.d. b.d. 0.0656.21 54.94 32.81 41.57 27.33 27.270.18 0.09 0.05 b.d. b.d. 0.01b.d. b.d. b.d. b.d. b.d. b.d.0.03 b.d. 0.04 b.d. b.d. b.d.3.23 b.d. 0.31 b.d. b.d. b.d.−1.36 0.00 −0.13 0.00 0.00 0.0099.63 98.88 85.54 87.50 99.74 100.493.98 3.98 2.93 7.99 0.00 0.000.05 0.31 0.00 0.00 0.00 1.000.00 0.00 2.27 0.00 1.97 0.000.22 0.15 0.09 0.47 0.06 0.160.00 0.00 0.01 0.00 0.00 0.008.73 8.55 4.63 11.55 0.98 0.840.02 0.01 0.01 0.00 0.00 0.000.00 0.00 0.00 0.00 0.00 0.000.00 0.00 0.01 0.00 0.00 0.001.06 0.00 0.09 0.00 0.00 0.0013.00 13.00 9.94 20.01 3.01 2.00

Fig. 2. a) matrix spinel with Mg–calcite inclusion of XMgCO3=0.155; b) slightly serpentinised olivine with inclusions of diopside-1, embedded in calcite; c) diopside-1

as inclusions in dolomite and olivine, diopside-2 at olivine rims; d) diopside-1 with inclusions of pargasite, rutile (tiny white dots) and calcite in the core, as marked bythe ellipse (see text); e) large diopside-1 grain with corona of olivine, amphibole and calcite; f). rims of diopside-2 around olivine.


The assemblages (II)–(IV) represent a higher metamorphicgrade and differ not so much because of differences in P and Tof equilibration but rather because of a diversity in bulkcomposition, like Al- and Ti-content.

The textures reflect, in general, equilibrium growth orrecrystallisation, only diopside occurs in 2 generations andindicates a more complex reaction history. The exsolution andzoning textures of the carbonate phases, as describedextensively in Mposkos et al. (2006), show no clearcorrelation with the growth of other minerals. Hence theyare disregarded here with the exception of carbonateinclusions, the compositions of which can be used to infertemperatures at entrapment by means of calcite–dolomitesolvus geothermometry. Even this is just an attempt and thevalues obtained are only a crude measure in light of the factthat the entire metamorphic evolution is polybaric and mostlikely occurred both in the aragonite and calcite stability field.Evidence for the presence of former aragonite and another

problem – possible chemical alteration of inclusion composi-tions – are addressed in the discussion section. Representativemineral analyses are listed in Table 1.

Calcite inclusions are usually rounded, 5–100 µm in size andoccur in almost all other phases. Care was taken to avoid beamdamage and beam overlap with the host phase. No analysis withSi or Al above detection limit was accepted for further use ingeothermobarometry or interpretation. The temperatures calcu-lated strictly conform to the 2 kbar calcite–dolomite miscibilitygap of Anovitz and Essene (1987) as pressure constraints aregenerally lacking. They are inaccurate to the degree that there isa minor pressure dependence (Goldsmith and Newton, 1969)and the metamorphic evolution is polybaric, as well as to thedegree original compositions have been reset – an aspect that isdiscussed further below. Dolomite occurs by itself or inpolyphase inclusions together with calcite. Contrary to calcite,dolomite inclusions in former olivine (now serpentinised) arenot corroded.


Phlogopite is a main phase in the dolomitic marbles from theKimi complex (1–4 vol.%). It is found as an inclusion in allother minerals and hence considered to grow early in themetamorphic history. Its main site and mechanism of growthseems to be in the carbonate matrix by replacement of dolomite.During late retrogression it is replaced in part, and in somesamples almost completely, by chlorite. The Mg/(Mg+Fe) ratioin phlogopite is 0.980, the TiO2 content ranges from 0.08–0.20 wt.% and F from 1.55 to 1.76 wt.%.

Spinel (~0.5–2 vol.%) is an accessory phase. In samples fromlocality 4 it has an FeO content ranging from ca. 3.0–3.3 wt.%and is colourless in thin section. In samples from locality 2 it islight green and has an FeO content of 7.4–7.8 wt.%. Spinelcontains inclusions of Mg–calcite, dolomite and very rarely ofrutile, phlogopite, olivine and magnesite. Magnesite inclusions(~3–4 μm in size) are aligned in trails and interpreted asremnants of prior fluid inclusion trails (not in equilibrium withmatrix carbonates). Spinel breaks down in a first stage to coarsegrained chlorite+calcite and in a second, rather late stage to

Fig. 3. Rims of diopside-2 around clinohumite (a) and pargasite (b); c) pargasite withand as inclusion in pargasite; e) tabular clinohumite, partly replaced by diopside-2 a

calcite plus a very fine grained mass of Mg- and Al-hydroxides.In some places it is also rimmed by serpentine.

An inclusion of highMg–calcite in spinel withXMgCO3=0.155

(Fig. 2a) constrains the minimum temperature for the formationof this spinel grain to 784 °C. This value is the highest oneobtained from calcite inclusions in all silicate or oxide phases ofthe studied marbles. Most spinel grains with inclusions of singlecalcite grains or with inclusions of composite grains of calcite+dolomite show radial fractures originating at the rims of thecarbonate inclusions. The composition of these calcite inclusionsranges from XMgCO3

0.05 to 0.09, corresponding solvustemperatures of 527 to 650 °C, which are 100–135 °C lessthan that of the maximum — XMgCO3

inclusion (fracture free)mentioned above.

Diopside is very rare in most olivine-bearing dolomiticmarbles with the mineral assemblage (II) and (III) (~0.3–0.5 vol.%, and in some spinel-free samples up to 2 vol.%). Twogenerations of diopside are recognized on textural andcompositional grounds. Diopside-1 is present mainly as small

large and tiny (within ellipse) inclusions of diopside-1; d) olivine in the matrixnd dolomite; f) olivine partly replaced by clinohumite and serpentinite.


rounded inclusions, up to 0.1 mm across, in Mg–calciteand olivine (2b, c), and more rarely as larger, zoned grains withAl-rich cores in the matrix: The diopside grain in Fig. 2d has~1.47 wt.% Al2O3 in the inclusion-poor core, ~1.30 wt.% in theinclusion zone and down to 0.80 wt.% at the inclusion-free rim.The composition profile across the large diopside-1 grain ofFig. 2e, which is replaced by an olivine-amphibole-calcite-corona from the rim, shows a relatively constant compositionwith 1.30±0.20 wt.% Al2O3. Diopside-2 is very poor in Al2O3

(0.00–0.50 wt.%); it replaces olivine (Fig. 2c, f) andclinohumite (Fig. 3a) in well developed coronas or moreirregular textures, and in rare cases also pargasitic amphibole.Na2O content is less than 0.09 wt.%, FeO content is less than0.7 wt.% and commonly less than 0.4 wt.%.

Diopside-1 contains inclusions of Mg–calcite and phlogo-pite, and – in samples from assemblage (IV) – (rare) amphiboleinclusions as oriented lamellae, suggesting that they formed byreplacement of the host diopside. The amphibole lamellaecommonly occur in the central part of the diopside (togetherwith tiny rutile grains), while the outer zone of the diopsidegrain is free of amphibole inclusions. This textural relation canbe explained by the prior compositional zoning of diopside-1,where the cores are richer in titanium and Tschermakcomponent (Al2O3 up to 1.50 wt.%) as mentioned above,compared to the rims and inclusion free grains of diopside-1(Al2O3 up to 0.80 wt.%). Replacement of diopside by tremoliticamphibole can also be found and is interpreted as a late-stageprocess, rather coeval with the formation of chlorite andserpentinite.

Inclusions of calcite in diopside-1 show a wide range incomposition with XMgCO3

from 0.00 to 0.14. The value of 0.14corresponds to a solvus temperature of 757 °C. Fig. 2d shows adiopside grain that contains three calcite inclusions, 100 μm,80 μm and 40 μm in size respectively. One of the two largerinclusions is pure calcite, the second has an XMgCO3

of 0.02, andthe small one of 0.08 (no beam overlap). Fractures in diopsideemanate from all the inclusions. Nearby matrix Mg–calcitegrains and calcite–dolomite intergrowths show relatively highXMgCO3

of 0.08 and 0.10 respectively.Olivine (Fo0.95) grains are usually a few 10s to 100s of µm,

rarely up to 3 mm in size. In several instances olivine ispreserved as a matrix phase in contact with calcite, dolomite,diopside and other silicates, but most often it is partlyor completely replaced (pseudomorphosed) by serpentine(~10-15% of the rock volume). In most cases, olivine ispreserved only as relics in serpentine and as small inclusions inhigh Mg–calcite, spinel and diopside. Larger olivine grains canthemselves include spinel, diopside, dolomite and calcite.Composite carbonate inclusions in olivine (mostly serpenti-nised) consist of dolomite and low Mg–calcite with dolo-mite exsolutions. A primary composition of approximatelyXMgCO3

=0.12–0.13 for the composition of the homogeneousprecursor Mg–calcite is obtained from lowMg–calcite domainsand dolomite exsolutions by integral analysis with electronmicroprobe using the beam scanning technique. Similarcompositions are also obtained from Mg–calcite inclusions inolivine grains that escaped serpentinisation. The compositions

of such Mg–calcite inclusions constrain the temperature duringolivine formation at ~730 °C.

Pargasitic amphibole is very rare in samples with assem-blage (II) and (III) and comprises up to ~2 vol.% in those withthe assemblages (I) and (IV). It commonly occurs asidiomorphic to subidiomorphic grains in the matrix withoutparticular reaction relationship to other phases (Fig. 2c). Insome instances it can be seen to replace diopside-1 (Fig. 3c) andis itself replaced by clinohumite or diopside-2 (Fig. 3b). Thecomposition of matrix amphibole and amphibole lamellae indiopside is identical. Even mm-sized amphibole crystals displayno major element zoning. The intergrowth of pargasite witholivine in the corona around diopside (Fig. 2e) indicatessynchronous growth of these two phases; olivine inclusions inamphibole are very rare (Fig. 3d). Pargasite contains inclusionsof spinel, diopside, dolomite and Mg–calcite with tiny dolomiteexsolutions. Inclusions of Mg–calcite with dolomite exsolu-tions in amphibole show integrated XMgCO3

in the range 0.11–0.13 corresponding to solvus temperatures of 697–738 °C.Tremolitic amphibole occurs very rarely as marginal replace-ments of diopside and is interpreted as a late-stage (retrograde)phenomenon.

(Ti–)Clinohumite: Ti–clinohumite is very rare in locality 3where it replaces olivine in some samples. TiO2 content rangesfrom 3.27–3.99 wt.%, fluorine content from 1.21–1.43 wt.%.Clinohumite from locality 4 occurs as irregular granular- totabular-shaped grains (Fig. 3e); the tabular ones show lamellartwinning parallel to [001] in coarse grains. This mineral isalmost colorless to pale yellow in color, consistent with its lowTiO2 content (TiO2=0.06–0.97 wt.%) that varies from grain tograin. Single clinohumite grains are unzoned, however. Fluorinecontent ranges from 2.08–3.90 wt.%. Ti- and F-content arecorrelated by the exchange vector TiO2Mg−1F−1, with XF up to0,64 in the clinohumites with the lowest Ti (Evans andTrommdorff, 1983; Ehlers and Hoinkes, 1987). Clinohumitepreferably grows in the carbonate matrix. In rare instances it canbe seen to replace olivine (Fig. 3f) or pargasite and to bereplaced itself by diopside+dolomite (Fig. 3a, e). An inclusionof Mg–calcite (XMgCO3

=0.075) with tiny dolomite exsolutions(integrated composition of XMgCO3

=0.12) in clinohumiteindicates that clinohumite was formed at temperatures of~718 °C.

Rutile and geikielite: Rutile occurs as extremely tiny(b1 µm) dots, interpreted as precipitates, in the cores ofdiopside-1 together with pargasite lamellae. Otherwise it is arare accessory (up to 100 µm in size) in a spinel-rich sample(5 K6a) from locality 3 that occurs as inclusions in spinel and inthe carbonate matrix. Only matrix rutile is replaced along grainmargins and cleavage planes by geikielite (Mg–ilmenite)+calcite. Geikielite grains are irregular in shape, and homogenousin composition (XMg=0.79–0.80).

Serpentine and chlorite are common phases. They aretexturally replacing olivine, phlogopite and spinel. Chlorite isrelatively coarse-grained, frequently displaying radial or sheaf-like growth textures, and hence may be a bit older thanserpentine which develops mainly as fine-grained pseudo-morphs (after olivine) and replacement along grain margins and


fractures. FeO content is less than 1 wt.% in chlorite, and lessthan 3.5 wt.% in serpentine.

5. Metamorphic history

Diamondiferous migmatitic pelitic gneisses in the Kimi–Organi area of the UHP metamorphic Kimi complex indicatethat lithologies of this area underwent UHP metamorphism withpeak P–T conditions ≥45 kbar and ~1000 °C with subsequentpassage through a high-pressure granulite and amphibolitefacies stage (Mposkos, 2002; Mposkos and Krohe, 2006). TheP–T path derived by Mposkos and Krohe (2006) has meanwhiledeveloped into at least two paths for different segments of theKimi Complex – the path relevant for the country rocks of thecalcite–dolomite marbles is shown in Fig. 4.

These marbles occur as lenticular intercalations in migmatiticand locally microdiamond-bearing paragneisses, thus raising thequestion whether their common tectonometamorphic historyincludes the UHP stage, or not. There is some ambiguous

Fig. 4. Multisystem including the phases calcite/aragonite, dolomite, clinopyroxeneexcess) and some additional equilibria pertinent to UHP characterization. High-tempDashed lines show the shift of important reactions for a fixed CO2-activity of 0.1 ancountry rocks. An isopleth for the highest XMgCO3

of matrix calcite (0.135) is shown

evidence for UHP conditions in the marbles (Mposkos et al.,2006), but the mineral assemblages (II-IV) are rather indicativeof upper amphibolite to lower granulite facies conditions:Forsterite-spinel marbles are typical for high-grade granulitefacies terrains, like in southern India (Satish-Kumar, 1999),whereas garnet would be stable at higher pressures, like in thedolomitic marbles of Kokchetav (e.g. Ogasawara et al., 2000). Infact, geothermobarometry in dolomitic marbles is rather difficultbecause (unknown) fluid compositions can shift most equilibriaconsiderably. Moreover, most reactions are strongly tempera-ture-dependent and more or less subparallel to each other.However, additional aluminous phases like corundum, spinel,garnet or anorthite should allow a better grip on pressure. In factwe find that the P–T–X conditions of formation can be ratherwell constrained by a new petrogenetic grid of consistentreactions in the system CaO–MgO–Al2O3–SiO2–CO2

(CMAS–CO2), as illustrated in Fig. 4 — a P–T diagramcalculated and drawn for aCO2

=1 (part of it also for aCO2=0.1

and 0.01) with the software package THERMOCALC, (Powell

, olivine, spinel, garnet, anorthite, corundum and CO2 (calcite and dolomite inerature termination of reactions is due to closure of the calcite–dolomite solvus.d 0.01. The bold arrow corresponds to the P–T paths derived from metapeliticfor reference.


et al., 1998; version 3.26, thermodynamic datafile of 22 Nov.2003). Calcite, dolomite, garnet, cpx and olivine were treated asFe-free ternary (cpx) or binary Ca–Mg solid solutions, spinel,anorthite and CO2 as pure phases. The compositions of solidsolution phases change continuously along univariants, hencethe reactions shown are not specific for Rhodope marblecompositions but have general validity. Coding parameters aregiven in the Appendix A.

The grid is drawn for calcite+dolomite in excess (becausethese two carbonates are the predominant phases and will bepresent during any reaction in a calcite–dolomite-marble) andhas five major invariant points. Even though hydrous andchemically more complex phases like amphiboles, chlorite,phlogopite, zoisite or clinohumite were disregarded for sake ofsimplicity, the system CMAS–CO2 is sufficient to show themost important reactions amongst the high-temperature Al-phases garnet, spinel and anorthite. An overlay for aCO2

=0.1and 0.01 was drawn to show how reactions and invariantpoints shift with changing XCO2

and thus illustrate somearguments for the reaction history of the Rhodope marbles. Aregular change of aCO2

results in a regular shift of an invariantpoint along its pertinent CO2-absent reaction curve, hence theposition of the invariant point and its reactions can beinterpolated rather easily for values of aCO2

between 1.0 and0.01. The reasons for constructing the grid were 1) to illustratea possible reaction sequence and P–T evolution of theRhodope marbles, 2) to show the stability fields of anhydrousAl-rich phases in dolomitic marbles in general and 3) to see ifspinel could be stable at UHP conditions – as only spinel wasfound but Kokchetav marbles indicate that garnet should beexpected as the stable Al-phase at UHP conditions instead(Ogasawara et al., 2000, Ogasawara and Aoki 2005). Thereare other options to construct a grid for marbles, e.g. thatchosen by Castelli et al. (2007), who included H2O in theirsystem. Equilibria including hydrous phases can be calculatedand shown in this type of grid. However, assemblagescorresponding to univariant curves in an H2O–CO2-bearingsystem (invariant points in a T-XCO2

-diagram) are ratherunlikely to be found in a marble because buffering of the fluidcomposition plays an essential role in the metamorphicevolution. Buffering occurs mainly along divariant planes inan H2O–CO2 system and the corresponding mineral assem-blages are best represented by the univariant lines in our gridor in a T-XCO2

-diagram. Hence we consider the H2O-absent grida more useful illustration of nature and prefer to show the effect ofvariable fluid composition by univariants for fixed CO2 activity.

6. Discussion

6.1. Interpretation of carbonate inclusions

Calcite inclusions within silicate and oxide phases of amarble may provide valuable information on the temperaturesof formation of these phases as their compositions (Mg-content)are less prone to resetting compared to matrix calcite (Ferry,2001). Given that the analysed compositions are real (no beamoverlap with the host phase), the compositions of calcite

inclusion populations in various phases of the Kimi-marbles areof surprising variability.

Fig. 2d gives an example of calcite inclusions of variablecomposition in diopside. Two grains are almost pure calcite andone is Mg–calcite (XMgCO3

=0.08). Overall, the inclusions ofMg–calcite in diopside (essentially diopside-1) as single phasesshow a wide range in compositions. The XMgCO3

of the analysedinclusions show all values in the range between 0.00 and 0.14.The corresponding calcite–dolomite solvus temperatures rangefrom below 250 to 757 °C. This wide range in composition isvery unlikely the result of an extended prograde trapping ofMg–calcite over a large temperature range. It could be betterexplained if we assume that many inclusions were polyphasearagonite+dolomite inclusions. After the peak pressure ofmetamorphism, Mg–calcite began to form at the expense ofdolomite and aragonite by the reaction

Arg þ Dol→Mg–Cal ð1Þ

until either aragonite or dolomite was completely consumed.Resulting aragonite+Mg–calcite pairs were stable until arago-nite fully transformed to Mg–calcite at higher temperatures. Thefinal result was either a single-phase (Mg–)calcite or combinedMg–calcite+dolomite inclusion.

In such a scenario the wide range of the XMgCO3values in the

(Mg–)calcite inclusions in diopside does not reflect different P–Tconditions during diopside formation, but differences in bulkcomposition of primary polyphase aragonite+dolomite inclusion.

No single grains of pure calcite or aragonite inclusions arefound in spinel and olivine. The analysed carbonate inclusionsin spinel comprise single grains of dolomite or Mg–calciteand composite grains consisting of Mg–calcite+dolomite. TheXMgCO3

values in single Mg–calcite grain inclusions in spinelrange from 0.05 to 0.155 corresponding to solvus temperaturesof 527 to 784 °C. As homogenous domains of matrix calcite arecommonly high Mg–calcite with XMgCO3

0.12-0.13 correspond-ing to solvus temperatures of 718-738 °C, the temperaturedifference of 257 °C obtained from the Mg–calcite inclusions inspinel could be attributed to different bulk MgCO3 contentof the inclusions assuming that they represent primarypolyphase inclusions of aragonite+dolomite that reacted toMg–calcite during decompression. All spinel grains with Mg–calcite inclusions bigger than 30 μm in size show radialfractures emanating from the inclusion, possibly a result oforiginal aragonite present in the inclusion.

The carbonate inclusions in olivine comprise single grainsof dolomite, exsolved and unexsolved Mg–calcite (XMgCO3

0.06-0.14, corresponding to solvus temperatures between564-757 °C), and composite grains consisting of Mg–calcitewith dolomite exsolutions plus dolomite (which is not ofexsolution origin). Some Mg–calcite grains have no dolomiteexsolutions. Their composition range is XMgCO3

=0.06-0.07,rarely up to 0.12. Those which are rich in dolomite exsolutionsshow an integrated XMgCO3

of 0.12-0.14. Polyphase inclusionsin olivine (and spinel) consisting of Mg–calcite+dolomite(which is not of exsolution origin) show equilibrium texture(straight grain boundaries) between dolomite and coexisting

Table 2Summary of reactions in impure marbles from the Greek Rhodope

#

1 Arg+Dol→Mg–Cal2 Dol+Ksp+H2O→Phl+Cal+CO2

3 Cpx+Sp+Cal/Dol→Grt+Ol+Cal/Dol4 Di+Dol→Ol+Cal+CO2

5 Dol+Cpx→Cal+Ol+An+CO2

6 Dol+Cpx→Cal+Ol+Sp+CO2

7 Grt+Cal→Sp+Cpx+Dol+CO2

8 Plag+Dol+Qtz+H2O→Parg+Cal+CO2

9 Di+Dol+H2O→Chu+Cal+CO2

10 Dol+Ol+H2O→Chu+Cal+CO2

11 Mus+Grt+Qtz→Bio+Kya+Alb+H2O12 Dol+Cor→Sp+Cal+CO2


Mg–calcite. Unfortunately, almost all olivine grains whichcontain carbonate inclusions are partly or completely serpenti-nized and the inclusions are in contact with the retrogradeserpentine (except one Mg–calcite inclusion with XMgCO3

=0.12). The Mg–calcite inclusions in the serpentinized olivineshow resorbed edges, indicating that they participated in theserpentinisation process. Therefore, a change of Mg–calcitecomposition during serpentinisation of the host olivine ispossible. The only definite primary inclusion composition is0.12, hence no prior aragonite+dolomite inclusion assemblagecan be inferred for olivine.

6.2. Reaction history

The sequence of mineral growth in the Rhodope marbles asderived from textures and compositions can be summarized asfollows: Slightly tschermakitic diopside and phlogopite are theoldest preserved silicate phase. Garnet could have been present,but despite of scrutinous search, no trace of it was found. Spinel,olivine and amphibole start to growmore or less at the same time,(Ti–)clinohumite slightly later, followed by a short, incompleteback-reaction of olivine, pargasite and clinohumite to diopside-2.Retrogression continues with formation of geikielite from rutileand chlorite from spinel and phlogopite. Finally olivine isserpentinised. In detail, the sequence of reactions (see Table 2) canbe described and explained as follows:

Phlogopite may have formed in a similar way as thatproposed by Satish-Kumar et al. (2001) for impure marblesfrom southern India:

Dol þ Ksp þ H2O→Phl þ Cal þ CO2 ð2ÞWehave no hint as towhat particular reactionwas responsible

for the formation of diopside-1, but with the help of the grid, thepossibilities for olivine-growth can be narrowed down: If onefollows the country-rock P–T path in Fig. 4, olivine could havebeen an early HP-phase, stable at pressures above fluid-absentreaction (3) in appropriate bulk compositions:

Cpx þ Sp þ Cal=Dol→Grt þ Ol þ Cal=Dol: ð3Þ

However, we have no indication of olivine as an early phase,or of coexisting garnet, and fluid-absent reactions like (3) may

be kinetically inhibited. Hence it is more likely that olivine, inthe presence of calcite, dolomite and diopside, grew by areaction well known from siliceous dolomites (in CMS–CO2):

Di þ Dol→Ol þ Cal þ CO2 ð4ÞThis reaction re-occurs in the alumina-enriched system as

two reactions emanating from invariant point [g,cor] inFig. 4:

Dol þ Cpx→Cal þ Ol þ An þ CO2 ð5Þand

Dol þ Cpx→Cal þ Ol þ Sp þ CO2: ð6ÞWe consider reactions (4) and (6) responsible for the

formation of olivine in the marbles of assemblage (III) and(II),(IV) respectively. In this case, spinel must have startedforming slightly earlier, by the decarbonation and garnet-breakdown reaction:

Grt þ Cal→Sp þ Cpx þ Dol þ CO2: ð7ÞAt conditions of aCO2

=1.0, reactions (5)–(7) occur atrelatively high T and low P and are not crossed by the P–Tpath. For aCO2

=0.1, they are crossed by the P–T path, but atrelatively high temperatures, which would require an equili-brium calcite composition of XMgCO3

=0.20–0.23. The calciteinclusion compositions in olivine, spinel and pargasite are muchlower though, which suggests that all three phases formed atconditions very close to the aragonite–calcite transition, slightlybelow the coesite stability field, at ca. 720 °C, 25 kbar andaCO2

~0.01. The reaction curves and the P–T path in Fig. 4 havean uncertainty range, and additional Fe will result in an(increase of) overlap of the aragonite stability field withreactions (3), (6) and (7) — hence both olivine and spinel mayhave started growing still within the stability field of aragonite,at conditions of very low aCO2

.A likely reaction to produce pargasitic amphibole was

proposed by Satish-Kumar et al. (2001) as:

Plag þ Dol þ Qtz þ H2O→Parg þ Cal þ CO2: ð8ÞIn our samples, growth of pargasite and olivine is con-

temporaneous at least in assemblage (IV). As quartz isincompatible with forsterite, it could not have been part of theprecursor assemblage. Hence pargasite growthwas effected ratherby an influx of external fluid/melt that not only supplied H2O,thus reducing CO2-activity to very low values, but also theelements Na, K, Al, Si and possibly F. Such a metasomaticprocess (external buffering of at least H2O and Na2O) wouldexplain the simultaneous growth of two new phases (olivine+amphibole), the uniform composition of amphiboles in the matrixand as inclusions in diopside-1, despite of incomplete replace-ment reactions, and the lack of reaction textures of amphibolewith phlogopite and spinel, the obvious sources of Na, K and Al.

Clinohumite can grow by a reaction very similar to (4), i.e.from diopside+dolomite:

Di þ Dol þ H2O→Chu þ Cal þ CO2: ð9Þ


As in rare instances it can be seen to replace olivine orpargasite, it is later than these two phases. Growth from olivinecan be formulated as:

Dol þ Ol þ H2O→Chu þ Cal þ CO2: ð10ÞBut replacement of pargasite with no other, e.g. sodic phase

forming and the high amount of fluorine in clinohumite againsuggest that an external fluid or melt has played an importantrole in its formation. Similar to olivine, clinohumite back-reactsto diopside by reverse reaction (9).

As for the source of fluid metasomatizing and resetting themarbles, high-pressure metapelites generally tend to dehydrateduring exhumation (e.g. Heinrich 1982, Proyer, 2003), if onlyby breakdown of the paragonite-component in muscovite in thevery common NKFMASH-reaction

Mus þ Grt þ Qtz→Bio þ Kya þ Alb þ H2O: ð11ÞThe grt–ky-gneisses enveloping the Rhodope marbles show

minor partial melting and generally re-equilibrated between21 kbar, 830 °C and 8 kbar, 700 °C. But minor dehydration mayhave occurred before that and the actual process opening themarbles to external fluid is considered to be the aragonite–calcite transition that caused considerable volume change, mostlikely produced by a high-porosity recrystallization process. Aninflux of hydrous fluid can explain why: a) hydrous phases(pargasite, clinohumite) form more or less coeval with olivineand spinel, and b) well-preserved diopside-1 can still be found asinclusions within dolomite, so temperatures sufficient fordiopside decomposition in a pure CO2-fluid (620 °C at 2 kbarand 870 °C at 10 kbar) were never attained. Hence it isreasonable to assume that reactions were triggered by a flush offluid from the dehydrating country rock gneiss that also providedother elements to a certain extent, at least some alkalies forpargasite formation. Further reduction of XCO2

and/or increasingactivity of fluorine in the fluid phase triggered the formation ofclinohumite at the expense of olivine (Fig. 3f; cf. Rice, 1980)according to reaction (10). Inclusions of Mg–calcite withdolomite exsolutions in clinohumite show integrated XMgCO3

inthe range 0.12, corresponding to solvus temperatures of 718 °C.There is little or no difference in temperature recorded by Mg-rich calcite inclusions in olivine, pargasite and clinohumite.

We believe that the entire crystallization event was short.When the supply of external H2O ceased, the rock returned tointernal buffering. As all reactions concerned are decarbonationreactions, they had to stop incomplete at that point. Only a smalldegree of back-reaction (diopside-2+dolomite from olivine+calcite) was possible, using up CO2 from the remaining porefluid. The complete lack of compositional zoning in up to mm-sized olivine, pargasite and clinohumite also supports the con-cept that changes in P and T during growth were minor. For theremainder of the P–T path, only the carbonate compositions areconsidered to have adapted to the prevailing P–T conditions –first pervasively and then, from 720 °C, 10 kbar, XMgCO3

~0.13downwards, by exsolution. Finally, late hydration caused thegrowth of retrograde chlorite, serpentine, etc.

Ogasawara and co-workers have shown that infiltration ofhydrous fluid into marbles from Kokchetav at UHP-conditions

produced garnet- and diamond-bearing assemblages thatsurvived decompression to the Earth's surface (Ogasawaraet al., 2000; Ogasawara and Aoki, 2005). In the case of theRhodope marbles investigated, fluid infiltration most likelyoccurred at the aragonite–calcite transition and obliteratedany (hypothetical) prior UHP phases like garnet. Marbles withAl-rich minerals from a similar tectonic position, near Xanthi,about 60 km west of the Kimi-localities, have been described byLiati (1988). Zoisite, anorthite and corundum are part of someof these definitely Barrow-type equilibrium mineral assem-blages of the Xanthi-marbles. The Rhodope MetamorphicProvince is a highly complex assemblage of nappes and tectonicslices with quite variable P–T histories that in the majority ofcases did not have an UHP-episode (Krenn et al., in review).But also in this case, Liati has identified a significant fluidinfluence (influx) from the metapelitic country rocks.

6.3. Spinel stability at UHP and general structure of the grid

There is no textural evidence for the mechanism of spinelformation. Possible precursors include corundum, chlorite,anorthite and garnet. Corundum is known from metamorphosedbauxite deposits, i.e. strongly SiO2-deficient rocks (e.g. Jansenet al., 1987) and from marble-hosted ruby deposits (Garnier andOhnenstetter 2005). The most common reaction producingspinel from corundum is

Dol þ Cor→Sp þ Cal þ CO2: ð12ÞBucher-Nurminen (1976) and Gieré (1987) described stable

chlorite-spinel assemblages from the Bergell contact aureole:Cal+Dol+Fo+Sp+Chl±Phl±Tre. Anorthite was reportedas stably coexisting with Cal +Dol+Fo+Sp+Cpx+Amp(pargasite) in granulite-facies marbles from southern India byJanardhan et al. (2001). The only garnet-spinel assemblage weare aware of is a retrograde spinel-diopside symplectitedeveloped occasionally around garnet from Kokchetav calcsi-licate assemblages (mainly Cal+Dol+Grt+Di+Phl) describedby Schertl et al. (2004) and Sobolev et al. (2006). This texture isan expression of the strongly curved reaction (3) that limits themaximum stability of spinel to higher pressures also for theRhodope marbles. It has a singularity near the crest, at ~800 °C,where calcite and dolomite change sides. As reaction (3) isfluid-independent, spinel cannot be stable in the UHP-field atany aCO2

except probably in relatively Fe-rich marbles.Regarding the general structure of the grid, it must be

definitely considered unrealistic towards lower temperatures ashydrous phases will be stable and require a differentrepresentation. Reaction (12), drawn in bold, was described orat least inferred a number of times from observations inmetabauxites or high-grade marbles that contain gem-qualityruby and spinel (e.g. Jansen et al., 1987; Liati, 1988; Garnierand Ohnenstetter, 2005). It was used to explain why ruby is onlyfound in dolomite-free marbles at high grades.

An inspection of invariant points shows that corundum andolivine are incompatible. Reactions emanating from thecorundum-absent invariant points are most relevant forcalcite–dolomite marbles with SiNNAl, so considerable Si


can be found in Al-poor or — free phases (cpx, ol). Reactionsemanating from the olivine-absent invariant points will be seenby the more Al-rich bulk compositions, where all silicates areAl-bearing and some Al is even stored as an oxide (corundum).

7. Conclusions

The silicate and oxide phases of the marbles do not containevidence of an UHP-stage. If we adopt the idea that the Organiand Pandrosos marbles were subjected to coeval UHPmetamorphism together with the neighboring diamond bearingmetapelites, the mineral assemblages and mineral compositionsin the marbles constrain the exhumation path at pressures belowthe diamond-graphite transformation (presence of well devel-oped graphite flakes in marbles with the assemblage (I)) andbelow the garnet+olivine stability field (no garnet found in thestudied samples yet). Parageneses (II)–(IV) are considered tohave formed in a narrow P–T range near the aragonite–calcitetransition, at around 720 °C, 25 kbar and at strongly reducedCO2 activity.

There is little chance to constrain pressure in siliceous dolomitesin the stability field of quartz (sub-UHP). Alumina content resultsin a considerable increase in the number of reactions, of which thefluid-absent reactions are most valuable as they are not influencedby varying activities of H2O/CO2. The most important reaction forUHP-marbles is reaction (3) which clearly shows that garnet has tobe stable at UHP-conditions as the characteristic Al-phase inimpure calcite–dolomite marbles with SiNNAl.

Acknowledgements

This work was made possible through financial support bythe Austrian Science Fund (FWF), project P16194-N06. E. M.and I. B. were financially supported by the National TechnicalUniversity of Athens for the Special Research Project“Protagoras”. We want to thank R. Powell (Univ. Melbourne)for tips on handling aspects of THEMOCALC and particularlyT.J.B. Holland for data and guidance on the use of adequatesolid solution models. Any remaining errors are ours. Themanuscript did benefit significantly from reviews by M. Satish-Kumar and one anonymous reviewer.

Appendix A

Coding parameters for non-ideal solid solution phases usedfor calculation of Fig. 4:

Garnet (pyrope–grossular: Mg3Al2Si3O12–Ca3Al2Si3O12):Activities: random mixing of Ca and Mg on the 3 M1 sites,

with non-ideality given by a regular solution, using W(py,gr)=33 kJ, following Vance and Holland (1993) where theinteraction energy used is based on Koziol (1990). The modelis as described in the file axNotes.pdf from T.J.B. Holland'swebsite (http://www.esc.cam.ac.uk/astaff/holland/)

Clinopyroxene (diopside–cats–enstatite: CaMgSi2O6–CaAlSiAlO6–MgMgSi2O6):

Activities: random mixing of Ca and Mg on M2 and Al andMg on M1 sites; coupled substitution between octahedral and

tetrahedral sites is assumed, such that tetrahedral site entropycontributions are taken as zero. Non-ideal mixing is taken via aregular solution between the three macroscopic end-memberswith interaction energies: W(en,cats)=24 kJ, W(en,di)=24 kJand W(cats,di)=7 kJ. An extra free energy increment for en(clino) in diopside is given by a Darkens Quadratic Formalismparameter, DQF=8.1–0.0045 T kJ. This model is taken fromthe file axNotes.pdf from T.J.B. Holland's website (see above)and is described further in Zeh et al. (2005).

Olivine (forsterite–monticellite: Mg2SiO4–CaMgSiO4):Activities: random mixing of Ca and Mg on the M2 site,

with non-ideality given by a regular solution interaction energyW(mont,fo)=24 kJ. This model (T.J.B. Holland, pers. comm.2007) provides for small Ca contents on the olivine-rich limb ofthe forsterite–monticellite solvus.

Carbonate (Calcite–dolomite: Ca2(CO3)2–CaMg(CO3)2):Activities: random mixing of Ca and Mg on one M site,

with non-ideality given by an asymmetric van Laar solutionmodel. This model, simplified from the full order-disordertreatment of Holland and Powell (2003), is modified from thefile axNotes.pdf from T.J.B. Holland's website (see above) bymaking the interaction energies asymmetric: W(cc,dol)=22.42–0.00128 T–0.06P kJ, with asymmetry parametersalpha(cc)=0.85+0.00068 T and alpha(dol)=1.0 (parametersfrom T.J.B. Holland, pers. comm. 2007). It reproduces the solvusof Anovitz and Essene (1987) and includes the pressuredependence as given by Goldsmith and Newton (1969).

The full THERMOCALC coding can be obtained uponrequest from the corresponding author. Units: temperatures inK, pressures in kbar, and energies in kJ.

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