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GEOSCIENCES 2009

Geochemistry of metacarbonate rocks from the Arda tectonic unitin the Central Rhodope, Bulgaria

Ãåîõèìèÿ íà ìåòàêàðáîíàòíè ñêàëè îò òåêòîíñêàòà åäèíèöà Àðäàâ Öåíòðàëíè Ðîäîïè, Áúëãàðèÿ

Zlatka Cherneva, Milena Georgieva, Tanja Stoilkova, Ana Petrova, Salza HekimovaÇëàòêà ×åðíåâà, Ìèëåíà Ãåîðãèåâà, Òàíÿ Ñòîèëêîâà, Àíà Ïåòðîâà, Ñúëçà Õåêèìîâà

Sofia University “St. Kliment Ohridski”, 15 Tzar Osvoboditel Blvd., 1504 Sofia; E-mail: cherneva@gea.uni-sofia.bg

Key words: calcite marbles, impure marbles, geochemistry, major elements, trace elements, Rhodope.

Carbonate protoliths behave in a different mannerduring regional metamorphism depending on theirmineral compositions. Impure marbles and calc-sil-icate rocks result from decarbonisation and relatedfluid transport of elements that are generally assumedto be immobile (Ague, 2003, and reference therein).By contrast, decarbonisation in pure marbles is raredue to the high thermal stability of carbonate miner-als under most metamorphic conditions (Yardley,1989). Hence pure marbles might keep record of pro-toliths geochemistry, whereas impure marbles give anidea of metamorphic geochemical exchange and el-ements migration.

The Rhodope metacarbonate rocks remain yet outof systematic petrological and geochemical consid-eration. The metacarbonate rocks are widespread indifferent lithotectonic units. The Arda unit marblesrepresent the deepest metasedimentary carbonaterocks that crop out in the metamorphic section ofthe Central Rhodopes in Bulgaria. They associatewith migmatitic garnet-kyanite gneisses and garnet-bearing metabasic rocks in the so called Chepelaremélange zone (Sarov et al., 2005) whose metamor-phic grade have reached high-pressure granulitefacies conditions (Cherneva et al., 2008, 2009; Geor-gieva et al., 2008). The petrographic observations(Georgieva et al., 2009) distinguish pure calcite ordolomite marbles and impure marbles (after Rosenet al., 2007). The latter, besides calcite and dolomite,comprise silicate-rich mineral assemblages includ-ing diopside, scapolite, K-feldspar, plagioclase andquartz, phlogopite, epidote, allanite, tremolite, titan-ite, apatite and opaques. The mineral proportionsare strongly variable at the outcrop scale.

We report summarized results of a geochemicalstudy of 29 whole rocks samples based on major ele-ments carbonate analyses by treating with 3% HClfor 3 hours of all rocks, major elements wet silicate

analyses of impure marbles insoluble residue, andtrace elements (Cr, Ni, Co, Cu, Zn, Cd, Pb, Rb, Sr)AAA of both types of materials. Mass-balance cal-culations help for better understanding of major andtrace elements distribution between carbonate andsilicate portions of impure marbles.

Pure marbles are distinguished with low propor-tion (< 1%) of insoluble residue, whereas the impure

Fig. 1. CaO and MgO contents in the Arda unit metacarbonaterocks: a) carbonate fraction of all samples; b) silicate fractionof impure marbles

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marbles contain considerable amount (14—58%) ofthe latter. The most widespread pure marbles arecalcite dominated. Among them white coloured va-rieties differ with lower MgO contents and higherCaO/MgO ratio values when compared with greycoloured ones, respectively: 0.27—1.93%, 30—210 forwhite marbles; and 1.25—3.21%, 17—44 for grey mar-bles. The carbonate component of impure marbles iscalcite dominated as well, with CaO/MgO ratio val-ues in the range of 18 to 44 similarly to the grey cal-cite marbles (Fig. 1a).

The insoluble residue of impure marbles displaystrongly variable major elements proportions, namely:SiO2 49—79, Al2O3 1—15, FeO* 0.2—5.6, MgO 1—9.2,CaO 2—16, Na2O 4—9, K2O 1—6 (all in %). Their con-tents and distribution correspond to dominant sili-cate minerals proportions. Mass-balance calculationshelp for understanding the distribution of Ca andMg between carbonate and silicate fractions of im-pure marbles. Carbonate phases hold more than 80%of the Ca-amount, and from 25 to 90% of the Mg

available. The strong preference of Ca to carbonatephases and incorporation of Mg both in silicate andcarbonate minerals are illustrated by lower CaO/MgOratio in the silicate fractions (Fig. 1b).

The pure marbles are poor in trace elements. Theircontents are often lower than the detection limits (Rband Co < 2 ppm, Table 1). Sr-contents are relativelyconstant in the range 100—200 ppm. The trace ele-ments studied in pure marbles do not show correla-tions with major elements (Ca and Mg). The impuremarbles are richer in trace elements than pure ones,as noted previously (Cherneva et al., 2003). The ma-jority of the trace elements studied (Cr, Ni, Cu, Znand Rb) correlate positively with the insoluble resi-due contents in the impure marbles, indicating thecontrolling role of silicate minerals on trace elementsdistribution. Prominent among them are Rb-Pb-K2Oand Sr-Na2O links that refer to incorporation of traceelements in K-feldspar and plagioclase available.Large Sr variation (Tabl. 1) in whole rocks (88—1721ppm) and silicate portions (18—1678 ppm) of theimpure marbles suggest a complex distribution of theelements between the two compounds. Mass-balancecalculations indicate that Cr, Ni, Cu, Zn and Rbtend to concentrate in the silicate fractions, whereasSr, Pb and Cd are enriched in the carbonate frac-tions that bear from 50% to 100% of the above ele-ments available.

Our preliminary results on the Arda unit meta-carbonate rocks consider pure marbles the more re-liable source of data on protoliths compositions.Among them dominate pure calcite marbles, whichtrace elements geochemical characteristics are prom-ising for comparative geochemical studies. We sug-gest non-isochemical processes of metamorphism inthe silicate-bearing metacarbonate rocks, controlledby fluid driven metamorphic reactions. Impure mar-bles geochemistry contributes to complete consider-ation of major and trace elements mass transfer dur-ing regional metamorphism.

Acknowledgements: Support from the Sofia Uni-versity research grant 211/1008, the National scien-tific fund grants VU-NZ-05/2005 and DOO2 327/2008 is gratefully acknowledged.

Table 1. Trace elements in the Arda unit marbles

Rocks Pure marbles Impure marbles

Sample Whole rock Whole rock

Insoluble residue

ppm min max avrg stdev min max min max

Cr <1 9 2.2 1.9 10 66 38 86

Ni <1 118 14 26.7 0.5 44 22 84

Co <2 <2 <2 <2 5 3 11

Cu <2 15 4.2 2.9 <2 21 3 45

Zn 11 52 20.2 12.2 33 74 58 253

Cd 5 6 5.4 0.5 4 5 1 4

Pb 3 12 7.2 2.4 2 46 <2 37

Li 2 5 3.7 0.7 2 6 3 7

Rb <2 3 <2 <2 43 5 129

Sr 107 228 137.8 31.8 88 1721 18 1678

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