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Geoarchaeology and Archaeomineralogy (Eds. R. I. Kostov, B. Gaydarska, M. Gurova). 2008. Proceedings of the International Conference, 29-30 October 2008 Sofia, Publishing House “St. Ivan Rilski”, Sofia, 126-129.

DETERMINATION OF THE PROVENANCE OF THE ARCHAEOLOGICAL MONUMENT “SAMUILOV KAMAK”

Lubomira Macheva

Central Laboratory of Mineralogy and Crystallography “Acad. I. Kostov”, Bulgarian Academy of Sciences, 1113 Sofia; lu_macheva@dir.bg

АBSTRACT. A stone slab bearing an inscription “Samuil – tsar and samodrzhets vsem blgarom” was found on 25 March 2004 in the vicinities of the village Samuilovo, Petrich municipality. At present it is kept at the Historical Museum in Blagoevgrad. On request of the latter, a detailed mineralogical and petrological investigation was carried out at CLMC-BAS to determine its provenance. A small piece of the slab was sampled and studied using polarizing petrographic microscope, scanning electron microscope and X-ray powder diffractometer. The petrographic investigation shows that the rock is serpentinised peridotite, consisting mainly of olivine and tremolitic amphibole. Metamorphosed ultrabasic rocks, analogous to the studied sample build up lenticular and lens-like bodies on the Northern slope of the Belassitsa Mountain (to the South of the Samuilovo village, between Kliuch and Yavornitsa villages, as well as to the South of the Kolarovo village). Based on the accomplished investigations we suggest that the inscribed stone slab which, according to the archeologists served as a border stone of the Mediaeval Kingdom of Tsar Samuil, is most likely to originate from the metamorphosed ultrabasic bodies in the immediate vicinity.

Introduction

A stone slab bearing an inscription “Samuil – tsar i samodrzhets vsem blgarom” (Fig. 1) was found on 25 March 2004 in the vicinities of the Samuilovo village. At present it is kept at the Historical Museum in Blagoevgrad. On request of the latter, a detailed petrological and mineralogical investigation was carried out at the Central Laboratory of Mineralogy and Crystallography, Bulgarian Academy of Sciences, in order to determine its provenance. A series of analyses – petrological, mineralogical and X-ray powder diffraction analysis were carried out to clarify the rock type and its origin.

Material and methods A small piece of the slab was extracted and studied using

polarizing petrographic microscope, scanning electron microscope and X-ray powder diffractometer. From the sampled study piece cut from the stone slab two thin sections were prepared for petrographic analyses, one double side polished slice for electron microprobe analyze and one sample for qualitative powder diffraction analysis.

The petrological identification of the rock was made with

polarizing microscope in transmitted light, the identification of the ore minerals was made with polarizing microscope in reflected light and the chemical composition of the minerals was determined with scanning electron microscope Philips SEM-515, equipped with X-ray spectrometer microanalyzer.

For more precise determination of the rock mineral composition X-ray powder diffractometer Dron-3M with filtered Co radiation was used.

Petrographic description Macroscopically the rock is fine grained, dark green to

greyish-green in color with a schistose structure. The surface of the studied slab is affected by weak weathering processes and precipitation of iron hydroxides (goethite) can be observed (Fig. 1).

Fig. 1. Stone slab bearing an inscription “Samuil – tsar i samodrzhets vsem blgarom”

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The texture of the rock is pseudomorphic to relict-granular, formed after olivine and amphibole, mesh texture in relation to the net-vine-like pattern of the serpentine minerals cutting off the olivine aggregates, nematoblastic after amphibole and hetero-granular after serpentine and chlorite minerals.

Mineral composition: The rock under study contains 40-50%

olivine, 20-30% tremolitic amphibole, up to 20% serpentine-group minerals, up to 2% chlorite and talc and magnetite <1%. The rocks consists mainly of olivine and tremolitic amphibole, forming coarse-grained relics included in a fine grained chlorite-serpentine matrix, i.e. it could betoken development of several superimposed metamorphic processes, by which a magmatic olivine-bearing rock has undergone several stages of metamorphic transformations. Olivine is presented both by small-sized and by quite coarse, subrounded to rounded, colourless dismembered relics (Fig. 2a, b). The coarse grains show sometimes well developed individuals with pronounced cleavage. Most grains are replaced pseudomorphically by meshwork isotropic serpentine (serpophite).

a

b

Fig. 2. Microphotographs of: above (a) – olivine partially replaced by meshwork serpentine and tremolitic amphibole crosscutting olivine grains and serpentine pseudomorphs after olivine (black plates are of isotropic serpentine); below (b) – olivine grain crosscutted by serpentine minerals

The replacement initiated along the crystal boundaries and progressed inward. In the peripheral parts of the grains and after fractures within them a fine fibrous serpentine mineral (chrysotile) is developed. Olivine compositions only cover a small range and fall into chrysolite-forstertite group (Table 1). Its forsterite content is close to 90%. The crystallochemical formulae is (Mg1,78Fe0,20Ni0,01)SiO4. Amphibole forms randomly oriented stubby-prismatic to long-

prismatic, colourless porphyroblasts, which after their optical peculiarities, chemical composition (Table 1) and X-ray diffraction data (Fig. 3), correspond to tremolite. The crystallochemical formula is (K,Na)(Na0,09Ca1,89Fe2+0,02) (Fe2+0,18Mg4,69Ni0,10Cr0,03) (Al0,03Si7,97)O22(OH)2. The elongated porphyroblasts show intensive transverse fracturing (Fig. 2a). The cracks between individual fragments are sealed with fibrous pile greenish in color serpentine mineral. The tremolitic amphibole is formed probably at the expense of primary clinopyroxene as a result of superimposed regional metamorphic processes.

Serpentine group minerals occurred in two of the three

known polymorphs: lizardite and chrysotile, determined on the basis of X-ray diffraction analysis (Fig. 3). The detailed petrographic investigations reveal that the serpentinization proceeds in two main stages, marked by different microstructural positions of both serpentine minerals. The products of the first one, called pseudomorphic serpentinization, could be seen in various stages of advancement in different portions of one thin section. Colourless to pile green lizardite replaces the olivine and forms polygonal mesh texture arranged as an irregular anastomousing network between the olivine relics. Sometimes structureless serpentine cores are observed. Small magnetite grains, formed by the process of serpentinization, are scattered all over the rock. The second stage is marked by the production of another serpentine polymorph – chrysotile. It is pile-greenish in color, fibrous, with very low (grey) interference colors and fills cracks in olivine and tremolite porphyroclasts. Both serpentine polymorphs are indistinguishable in their chemical composition. The summarized formulae, obtained on the basis of three analyses is (Mg2,62-2,77Fe00,21-0,37Cr0,01Ni0,01-

0,02)(Si1,95-1,98Al0-0,03)(OH)4O5. Тable 1Chemical composition of the minerals from serpentinized peridotites

building up the stone slab with the inscription "Samuil..."

Olivine Amphibole Chlorite Serpentine MagnetiteAn. N Аn. 1 Аn. 2 Аn. 4 Аn.5 Аn.6 Аn. 7 An. 9

SiO2 40,69 57,93 39,2 38,26 32,95 32,16 0,37

TiO2 * * * * * * 1,36

Al2O3 0,36 0,81 0,4 * 14,02 13,12 0,45FeOt 9,82 1,73 7,76 5,31 3,68 3,76 64,08MnO 0,09 * 0,15 * * * 0,41MgO 48,91 22,85 36,49 39,17 34,38 33,7 1,32CaO * 12,81 * * * * *Na2O * 0,32 * * * * *NiO 0,27 0,17 0,43 0,24 0,24 0,24 0,32

Cr2O3 * 0,24 0,28 0,32 2,41 2,69 25,76

VO4 * * * * * * 0,7H2O* * 3,13 15,28 16,71 12,33 14,33 *Total 100,14 99,99 99,99 100,01 100,01 100 94,77

* - element not determined

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Fig. 3. X-ray powder diffraction of metaperidotite slab bearing an inscription (abbreviations: Tc – talc; Ol – olivine; Trem – tremolite; Chl – chlorite; Serp – serpentine)

Chlorite forms small-sized, colourless and irregular in form

flakes having anomalous brown interference colours, usually arranged in close spatial association with the serpentine minerals. It is rich in Mg (XMg -0.94) and the chemical composition can be defined as pennine. Its crystallochemical formulae, calculated on the basis of 3 analysis is (Мg4,85-

4.89Al0,63-0,68Fe2+0,29-0,31Cr0,18-0,21)(Si3,12-3,13Al0,84-0,88)O10(OH)2

(Table 1). Talc is rarely recognized as thin continuous veinlets or as

clusters in the rock. Frequently, it fills cracks in tremolite grains, being arranged parallel to the porphyroblasts elongation.

Magnetite is the most common accessory mineral produced

by the serpentinization of olivine. It is observed as small, irregular grains unevenly scattered in the rock matrix, mainly among serpentine and chlorite minerals. The chemical composition of the mineral corresponds to Cr-magnetite and its formulae is calculated as (Fe3+1,08Fe2+0,96Cr0,77Mg0,07Ti0,04V0,02Ni0,01Mn0,01)O4 (Table 1). In effect of supergene alterations the ore mineral hydrates partially and in places the rock becomes pigmented.

The mineral composition of the rock under study allows its

determination as a seprentinised ultrabasic rock, to be more precise – serpentinised peridotite, composed mainly of coarse relicts of olivine and tremolitic amphibole arranged in a fine-grained chlorite-serpentine matrix. Talc and magnetite (up to 1%) are also present as minor constituents. The high ratio of forsterite component in olivine may suggest that by the partial melting of a lherzolitic protolith, a residuum of harzburgitic composition could have formed in the upper mantle, representing the lower part of an ophiolite sequence.

Assumptions about the metamorphic evolution of the rock

On the basis of the microtextural relationships and chemical peculiarities of the minerals a conclusion can be drawn about the geological evolution which the rock has undergone. The

rock under study had an early stage of hydratation, because it contains relic magmatic olivine grains, being partially serpentinised and on account of which a pseudomorphic mesh texture originates. Experimental studies on the system MgO-SiO2-H2O have indicated that serpentines cannot be formed at temperatures above 5000C, and that formation of serpentine by the action of water on forsterite can occur only below 4000 C (Bowen, Tuttle, 1949). We consider that at a latter stage the rock has undergone regional metamorphism under amphibolite facies conditions at elevated temperatures (above 4000 and below 6000C) and medium pressure, during which tremolitic amphibole has been formed, but serpentine minerals remain stable. On the peridotites already metamorphosed in amphibolite facies a low temperature greenschist facies metamorphism is imposed, with which a new episode of partial serpentinization is associated. During this retrograde metamorphic stage the crack-filled replacement of tremolite and partially of olivine by fibrous chrysotile is realized. The lack of carbonate minerals in the studied rock indicates low activity of CO2 during the process of regional metamorphism. Identical evolution for the regional metamorphism has been proposed for the Lozen metagranites (Macheva et al., 2006), exposed further to the West in the Belassitsa Mountain.

Possible provenance of the archaeological monument “Samuilov Kamak” It is well documented by the geological mapping in scale 1:250000 (Zidarov et al., 1956) that numerous metamorphosed ultrabasic rocks, identical with the studied specimens from the stone slab in question, build lenticular and lens-like bodies on the Northern slope of the Belassitsa Mountain (to the South of the Samuilovo village, between the Kliuch and Yavornitsa villages, as well as South to the Kolarovo village) (Fig. 4). Together with their country rocks – gneisses, gneissschists and amphibolites, the metaultrabasites have been considered as a constituent part of the Ograzhdenian unit of the Serbo-Macedonian Massif (Zagorchev, 2001). One of these ultrabasic bodies, which crops out near Kamena village is well studied by Nenova & Marinova (2007). After their investigations this body consists of serpentinized dunites, belonging to the cumulative complex of an ophiolite-derived association. Based on the similarity between the data obtained by Nenova & Marinova (2007) and this investigations it can be suggested that the inscribed stone slab which, according to the archaeologists served as a border stone of the Mediaeval kingdom of Tsar Samuil, is most likely to originate from the metamorphosed ultrabasic bodies in the immediate vicinity. So far three analogical slabs serving as border stones of the Mediaeval Kingdom of Tsar Samuil (end of X c. – beginning of the XI c.) were discovered. Two of them were found in 1908 near Thessaloniki and in the vicinity of the Narash village in Albania and are deposited for safe-keeping at the Istanbul Museum. The third one was found by an Austrian soldier in the vicinity of the village Bolsha in 1917 and is kept at the Vienna Museum.

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Fig. 4. Scheme showing some of the most important occurrences of metaultrabasites in the region of Belasitsa Mountain, SW Bulgaria; arrows point to the metaultrabasic bodies, which are marked in black

Acknowledgments. The author is grateful to K. Grancharova from the Historical Museum in Blagoevgrad and to Dr. N. Zidarov from the Central Laboratory of Mineralogy and Crystallography “Acad. Ivan Kostov”, Bulgarian Academy of Sciences, for the possibility to study this historical monument. I also thank M. Stoyanov for the support by sampling the piece from the slab and for the preparation of thin sections as well as M. Tarassov and Y. Tzvetanova for the microprobe and XRD analyses, respectively.

References Bowen, N. L., O. F. Tuttle. 1949. The system MgO-SiO2-H2O. –

Bull. Geol. Soc. Amer., 60, 439-460. Macheva, L., I. Peytcheva, A. von Quadt, N. Zidarov, E.

Tarassova. 2006. Petrological, geochemical and isotope features of Lozen metagranite, Belassitsa Mountain – evidence for widespread distribution of Ordovician metagranitoids in the Serbo-Macedonian Massif, SW

Bulgaria. – Proc. Ann. National Scientific Conference of the Bulgarian Geological Society “Geosciences 2006”, Sofia, 209-212.

Nenova, P., I. Marinova. 2007. New data on the serpentinizied ultrabasic body at the village Kamena, Belassitsa Mountain, SW Bulgaria. – Ann. National Science Conference of the Bulgarian Geological Society “Geosciences 2006”, Sofia, 99-100 (in Bulgarian).

Zagorchev, I. 2001. Geology of SW Bulgaria: an overview. –Geologica Balc., 21, 1-2, 3-52.

Zidarov, N., Il. Kostov, V. Stoeva, L. Martinov, R. Karaivanova, D. Dimitrov, P. Ignatovski. 1966. Report on the Geology of Balassitsa and the Southern Slopes of the Ograzhden Mountain (Geological Mapping and Prospecting of Mineral Deposits, 1:25000, in 1965). National Geofond (in Bulgarian).