Determination of the provenance of the archaeological monument

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 MachevaCentral Laboratory of Mineralogy and Crystallography “Acad. I. Kostov”, Bulgarian Academy of Sciences, 1113 Sofia;lu_macheva@dir.bgАBSTRACT. A stone slab bearing an inscription “Samuil – tsar and samodrzhets vsem blgarom” was found on 25 March 2004 in the vicinities ofthe village Samuilovo, Petrich municipality. At present it is kept at the Historical Museum in Blagoevgrad. On request of the latter, a detailedmineralogical and petrological investigation was carried out at CLMC-BAS to determine its provenance. A small piece of the slab was sampled andstudied using polarizing petrographic microscope, scanning electron microscope and X-ray powder diffractometer. The petrographic investigationshows that the rock is serpentinised peridotite, consisting mainly of olivine and tremolitic amphibole. Metamorphosed ultrabasic rocks, analogous tothe 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 thatthe inscribed stone slab which, according to the archeologists served as a border stone of the Mediaeval Kingdom of Tsar Samuil, is most likely tooriginate from the metamorphosed ultrabasic bodies in the immediate vicinity.IntroductionA stone slab bearing an inscription “Samuil – tsar isamodrzhets vsem blgarom” (Fig. 1) was found on 25 March2004 in the vicinities of the Samuilovo village. At present it iskept at the Historical Museum in Blagoevgrad. On request ofthe latter, a detailed petrological and mineralogicalinvestigation was carried out at the Central Laboratory ofMineralogy and Crystallography, Bulgarian Academy ofSciences, in order to determine its provenance. A series ofanalyses – petrological, mineralogical and X-ray powderdiffraction analysis were carried out to clarify the rock type andits origin.For more precise determination of the rock mineralcomposition X-ray powder diffractometer Dron-3M with filteredCo radiation was used.Petrographic descriptionMacroscopically the rock is fine grained, dark green togreyish-green in color with a schistose structure. The surfaceof the studied slab is affected by weak weathering processesand precipitation of iron hydroxides (goethite) can be observed(Fig. 1).Material and methodsA small piece of the slab was extracted and studied usingpolarizing petrographic microscope, scanning electronmicroscope and X-ray powder diffractometer. From thesampled study piece cut from the stone slab two thin sectionswere prepared for petrographic analyses, one double sidepolished slice for electron microprobe analyze and one samplefor qualitative powder diffraction analysis.The petrological identification of the rock was made withpolarizing microscope in transmitted light, the identification ofthe ore minerals was made with polarizing microscope inreflected light and the chemical composition of the mineralswas determined with scanning electron microscope PhilipsSEM-515, equipped with X-ray spectrometer microanalyzer.Fig. 1. Stone slab bearing an inscription “Samuil – tsar i samodrzhetsvsem blgarom”126

The texture of the rock is pseudomorphic to relict-granular,formed after olivine and amphibole, mesh texture in relation tothe net-vine-like pattern of the serpentine minerals cutting offthe olivine aggregates, nematoblastic after amphibole andhetero-granular after serpentine and chlorite minerals.Mineral composition: The rock under study contains 40-50%olivine, 20-30% tremolitic amphibole, up to 20% serpentinegroupminerals, up to 2% chlorite and talc and magnetite

Fig. 3. X-ray powder diffraction of metaperidotite slab bearing aninscription (abbreviations: Tc – talc; Ol – olivine; Trem – tremolite; Chl –chlorite; Serp – serpentine)Chlorite forms small-sized, colourless and irregular in formflakes having anomalous brown interference colours, usuallyarranged in close spatial association with the serpentineminerals. It is rich in Mg (XMg -0.94) and the chemicalcomposition can be defined as pennine. Its crystallochemicalformulae, calculated on the basis of 3 analysis is (Мg4,85-4.89Al0,63-0,68Fe 2+ 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 asclusters in the rock. Frequently, it fills cracks in tremolitegrains, being arranged parallel to the porphyroblastselongation.Magnetite is the most common accessory mineral producedby the serpentinization of olivine. It is observed as small,irregular grains unevenly scattered in the rock matrix, mainlyamong serpentine and chlorite minerals. The chemicalcomposition of the mineral corresponds to Cr-magnetite and itsformulae is calculated as(Fe 3+ 1,08Fe 2+ 0,96Cr0,77Mg0,07Ti0,04V0,02Ni0,01Mn0,01)O4 (Table 1). Ineffect of supergene alterations the ore mineral hydratespartially and in places the rock becomes pigmented.The mineral composition of the rock under study allows itsdetermination as a seprentinised ultrabasic rock, to be moreprecise – serpentinised peridotite, composed mainly of coarserelicts of olivine and tremolitic amphibole arranged in a finegrainedchlorite-serpentine matrix. Talc and magnetite (up to1%) are also present as minor constituents. The high ratio offorsterite component in olivine may suggest that by the partialmelting of a lherzolitic protolith, a residuum of harzburgiticcomposition could have formed in the upper mantle,representing the lower part of an ophiolite sequence.Assumptions about the metamorphic evolutionof the rockOn the basis of the microtextural relationships and chemicalpeculiarities of the minerals a conclusion can be drawn aboutthe geological evolution which the rock has undergone. Therock under study had an early stage of hydratation, because itcontains relic magmatic olivine grains, being partiallyserpentinised and on account of which a pseudomorphic meshtexture originates. Experimental studies on the system MgO-SiO2-H2O have indicated that serpentines cannot be formed attemperatures above 500 0 C, and that formation of serpentine bythe action of water on forsterite can occur only below 400 0 C(Bowen, Tuttle, 1949). We consider that at a latter stage therock has undergone regional metamorphism under amphibolitefacies conditions at elevated temperatures (above 400 0 andbelow 600 0 C) and medium pressure, during which tremoliticamphibole has been formed, but serpentine minerals remainstable. On the peridotites already metamorphosed inamphibolite facies a low temperature greenschist faciesmetamorphism is imposed, with which a new episode of partialserpentinization is associated. During this retrogrademetamorphic stage the crack-filled replacement of tremoliteand partially of olivine by fibrous chrysotile is realized. The lackof carbonate minerals in the studied rock indicates low activityof CO2 during the process of regional metamorphism. Identicalevolution for the regional metamorphism has been proposedfor the Lozen metagranites (Macheva et al., 2006), exposedfurther to the West in the Belassitsa Mountain.Possible provenance of the archaeologicalmonument “Samuilov Kamak”It is well documented by the geological mapping in scale1:250000 (Zidarov et al., 1956) that numerous metamorphosedultrabasic rocks, identical with the studied specimens from thestone slab in question, build lenticular and lens-like bodies onthe Northern slope of the Belassitsa Mountain (to the South ofthe Samuilovo village, between the Kliuch and Yavornitsavillages, as well as South to the Kolarovo village) (Fig. 4).Together with their country rocks – gneisses, gneissschistsand amphibolites, the metaultrabasites have been consideredas a constituent part of the Ograzhdenian unit of the Serbo-Macedonian Massif (Zagorchev, 2001). One of these ultrabasicbodies, which crops out near Kamena village is well studied byNenova & Marinova (2007). After their investigations this bodyconsists of serpentinized dunites, belonging to the cumulativecomplex of an ophiolite-derived association.Based on the similarity between the data obtained byNenova & Marinova (2007) and this investigations it can besuggested that the inscribed stone slab which, according to thearchaeologists served as a border stone of the Mediaevalkingdom of Tsar Samuil, is most likely to originate from themetamorphosed ultrabasic bodies in the immediate vicinity.So far three analogical slabs serving as border stones of theMediaeval Kingdom of Tsar Samuil (end of X c. – beginning ofthe XI c.) were discovered. Two of them were found in 1908near Thessaloniki and in the vicinity of the Narash village inAlbania and are deposited for safe-keeping at the IstanbulMuseum. The third one was found by an Austrian soldier in thevicinity of the village Bolsha in 1917 and is kept at the ViennaMuseum.128

Fig. 4. Scheme showing some of the most important occurrences of metaultrabasites in the region of Belasitsa Mountain, SW Bulgaria; arrows point to themetaultrabasic bodies, which are marked in blackAcknowledgments. The author is grateful to K. Grancharova from theHistorical Museum in Blagoevgrad and to Dr. N. Zidarov from theCentral Laboratory of Mineralogy and Crystallography “Acad. IvanKostov”, Bulgarian Academy of Sciences, for the possibility to studythis historical monument. I also thank M. Stoyanov for the support bysampling the piece from the slab and for the preparation of thinsections as well as M. Tarassov and Y. Tzvetanova for themicroprobe and XRD analyses, respectively.ReferencesBowen, 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 isotopefeatures of Lozen metagranite, Belassitsa Mountain –evidence for widespread distribution of Ordovicianmetagranitoids in the Serbo-Macedonian Massif, SWBulgaria. – Proc. Ann. National Scientific Conference of theBulgarian Geological Society “Geosciences 2006”, Sofia,209-212.Nenova, P., I. Marinova. 2007. New data on the serpentiniziedultrabasic body at the village Kamena, BelassitsaMountain, SW Bulgaria. – Ann. National ScienceConference 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 ofBalassitsa and the Southern Slopes of the OgrazhdenMountain (Geological Mapping and Prospecting of MineralDeposits, 1:25000, in 1965). National Geofond (inBulgarian).129