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GEOSCIENCES 2009

Petrology of marbles from the Arda tectonic unit,Central Rhodope, Bulgaria

Ïåòðîëîãèÿ íà ìðàìîðè îò òåêòîíñêàòà åäèíèöà Àðäà,Öåíòðàëíè Ðîäîïè, Áúëãàðèÿ

Milena Georgieva, Zlatka Cherneva, Salza Hekimova, Ana PetrovaÌèëåíà Ãåîðãèåâà, Çëàòêà ×åðíåâà, Ñúëçà Õåêèìîâà, Àíà Ïåòðîâà

Sofia University “St. Kliment Ohridski”, Tsar Osvoboditel Blvd., 1504 Sofia, Bulgaria; E-mail: milena@gea.uni-sofia.bg

Key words: marbles, mineral chemistry, thermobarometry, Arda unit, Rhodope massif.

Marbles often constitute considerable parts of meta-morphic sequences or variegated formations. In theRhodope massif, their petrology and geochemistryremain poorly known. Information for mineral com-position of metacarbonates appeared in papers ded-icated to regional geology and impresses with nu-merous mineral species reported, although lackingany information for mineral equilibriums and para-genesis. Cherneva et al., (2003) shortly discussedchemical composition of marbles in the same area.Recently, detailed data on petrology and mineralchemistry of forsteritic marbles from Chepelare areagave Stavrakeva and Petrusenko (2005). In this studywe report new data on petrology and mineral chem-istry and make first attempt for thermobarometry ofmetacarbonates from the deepest part of the CentralRhodope metamorphic complex, where their spatialdistribution is related to the variegated successionknown as Chepelare mélange (Sarov et al., 2007).

Field observations and petrography

Within the Chepelare mélange zone metacarbonatescrop out as mid- to coarse-grained lenses and bandsblocks with massif, weekly deformed or banded tex-ture and thickness up to 20 m. They associate withmigmatitic garnet-kyanite gneisses, amphibolites, twomica gneisses and lenses of serpentinised ultraba-sic rocks. In the field, dominate white or grey puremarbles containing also minor micas, quartz, feld-spars, diopside, tremolite, titanite, apatite, zirconand graphite. Calcite is the main carbonate miner-al. It forms isometric or slightly elongated to thefoliation xenoblasts with varying size and irregulargrain boundaries. The presence of dolomite is re-stricted to single outcrops. It forms big hypidiob-lastic to xenoblastic grains (~ 1 mm) hosted by fine-grained calcite-dominated matrix.

Impure marbles appeared as greenish layers, withvariable content of silicates (15—45 vol.%). Majorminerals are calcite, diopside, scapolite, K-feldspar,plagioclase and quartz, with minor phlogopite, epi-dote, allanite, tremolite, titanite, apatite and opaques.Diopside and scapolite are present in variable pro-portions and generally have a similar grain size, al-though in some samples, pyroxene forms large por-phyroblasts (up to 1 cm). Diopside envelopes or in-cludes scapolite, K-feldspar, calcite and quartz.Scapolites associate spatially with epidote and pla-gioclase, with rare inclusions of calcite, K-feldspar,quartz and diopside. Intergrowth of large scapoliteand epidote suggests scapolite formation at expenseof epidote. K-feldspar commonly forms xenoblasticto big hypidioblastic grains in association with di-opside, scapolite and plagioclase. In studied sam-ples, plagioclase is scarce and frequently includesepidote grains. Phlogopite forms hypidioblastic toidioblastic, weekly-deformed flakes, located alongcracks and as small inclusions in calcite and plagi-oclase. Tremolite overgrows partly or completely di-opside, including pyroxene relicts. Retrograde reac-tion textures with formation of acid plagioclase bandsand domains are observed on silicates-calcite andtwo feldspars boundaries. We interpret calcite-diop-side-scapolite-K-feldspar-quartz-titanite as equilib-rium mineral paragenesis. Subordinate presence ofplagioclase and epidote is related to scapolite for-mation. Phlogopite spatially related to cracks andtremolite are formed during the retrogression.

Mineral chemistry and thermobarometry

Calcite in pure marbles has low MgCO3 content (1.65to 3.18 wt.%). The matrix calcite in dolomite-bear-ing marbles has similar composition (MgCO3 2.22—5.50 wt.%), but calcite inclusion in dolomite (MgCO3

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40.40—43.47 wt.%) shows much higher magnesitecontent (MgCO3 12.8 wt.%). Calcite builds up thecarbonate matrix of impure marbles, having lowMgCO3 (0.32—0.92 wt.%). Some of the calcite inclu-sions in scapolite, diopside and titanite are richer inMgCO3 (0.75—1.51 wt.%), comparing with matrixcalcites in the same sample. Diopsides are Mg-rich(XMg 0.68—0.79). Their large hypidioblastic grainsshow week zonation, with higher Mg in the core (XMg0.79) and lower in the rim (XMg 0.76). Relict diop-side replaced by tremolite has the lowest XMg. Naand Al contents in all pyroxenes are low 0.02—0.05and 0—0.07 apfu, respectively. Scapolites are gener-ally unzoned with meionite compound from 72 to78. Together with anortite equivalent (ÅqAn 64—72)and low XCl 0—0.002, the studied scapolites resemblethe mizzonite composition. K-feldspars have constantcomposition (Or86—91), with lower orthoclase contentanalysed in large K-feldspars or grains partly or com-pletely included in diopside and opaque mineral. Pla-gioclases composition varies significantly (An1—7—An42).Big hypidioblastic plagioclases associating with K-feld-

spar are more basic (An37—42). Retrograde reaction tex-tures produce more acid plagioclase (An1—7—An27).Inclusions in tremolite and K-feldspar commonly areoligoclasic (An12—18). Phlogopites spatially related tocracks are Mg-rich (XMg 0.59—0.67), with varyingAlIV (2.22—2.54 apfu). Tremolites have low alkalicontent (Na+K)A 0.03—0.10 and constant XMg 0.69—0.72. Titanite is the more abundant accessory miner-al with Al2O3 from 2.47 to 6.09 wt.%.

Calcite-dolomite solvus geothermometer havebeen applied for the dolomite-bearing pure mar-bles. Gottschalk (1997) and Anovitz and Essene(1987) calibrations give temperatures in the range760—770° C at 0.5 GPa and 745—770° C at 1 GPa.

Discussion and conclusions

Metacarbonate rocks preserve evidences for fluid-rock interaction, where decarbonation, dehydrationand fluid infiltration modify the mineral paragene-sis. It is difficult to assess the influence of variableprotolith composition, the role of infiltrated fluid andthe neighbour rocks on observed mineral assemblage.Major and trace elements geochemistry of the stud-ied metacarbonates suggests fluid driven metamor-phic reactions and related mass transfer. On the otherhand, the petrological observations of impure mar-bles revealed surprisingly constant mineral compo-sition, despite some modal variations. On CaO—MgO—SiO2 diagram a good correlation between their bulkcomposition, equilibrium mineral paragenesis andmineral composition have been observed. The systemis poor in Mg, which prevents forsterite formation atthis bulk rock chemistry. Mg-bearing silicate miner-als probably formed at expense of Mg-calcite, in-stead of dolomite. Fluid infiltration during retrogres-sion is manifested by phlogopite, tremolite and acidplagioclase formation. Preliminarily thermobaromet-ric estimations are consistent with previously report-ed for the associated rocks and are in good agree-ment with equilibrium mineral paragenesis, whichhas wide stability field at upper amphibolite facies.

Acknowledgements: This study was supported bythe NSRC VU-NZ-05/05, SU-211/2008 and DO02-363/2008.

Fig. 1. CaO-MgO-SiO2 diagram for diopside-scapolite impuremarbles. Bulk rock analyses after Cherneva et al. (2009).

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tem CaCO3-ÌgCO3-FeCO3. — J. Petrol., 28, 389—414.Cherneva, Z., L.-A. Daieva, E. Stancheva, L. Tadjer. 2003.

Geochemical data on metapelites and related rocksfrom the Chepelare area, Central Rhodopes, Bul-garia. — Proceedings of the Ann. Sci. Conference. So-fia, Bulg. Geol. Soc., 25—27.

Gottschalk, M. 1997. Internally consistent thermodynam-ic data for rock-forming minerals in the system SiO2-TiO2-Al2O3-Fe2O3-CaO-MgO-FeO-K2O-Na2O-H2O-

CO2. — Eur. J. Mineral., 9, 175—223.Sarov, S. et al. 2007. Report on geological remapping at

scale 1:50 000 of the Central Rhodope in the area ofChepelare. National Geofund, Ministry of environ-ment and water, IV-491—4.

Stavrakeva, D., S. Petrusenko. 2005. Mineral composi-tion of the forsteritic marble from Samurski dol,Chepelare region, central Rhodopes, Bulgaria. — Pro-ceedings of the Jubilee International Conference. So-fia, Bulg. Geol. Soc., 83—86.