ASN, Vol. 8, No 1, Pages 118–125, 2021 118 Corresponding Author

ASN, Vol. 8, No 1, Pages 118–125, 2021

Acta Scientifica Naturalis

Former Annual of Konstantin Preslavsky University of Shumen: Chemistry, Physics, Biology, Geography Journal homepage: https://content.sciendo.com/view/journals/asn/asn-overview.xml

Geological-geomorphological characteristics and petrographical composition of the St. Anastasia Island

Dimitar Dimitrov1*, Banush Banushev2

1Institute of Oceanology, Bulgarian Academy of Sciences, 40, 1st May Str., P.O.B. 152, 9000 Varna, Bulgaria, e-mail: [email protected] 2University of Mining and Geology “St. Ivan Rilski”, Studentski Grad, “Prof. Boyan Kamenov” Str., 1700 Sofia, Bulgaria, e-mail: [email protected]

Abstract: St. Anastasia Island is one of the symbols of the cultural and historical heritage of the Republic of Bulgaria. This raises the need for the study of risky oceanographic factors, climatic phenomena, risky geological processes as well as detailed petrographical characteristics of the Upper Cretaceous volcanic rocks forming the islands. The results of the petrographical study show that the island was built by Alkali feldspar trachytes. The volcanics from St. Anastasia Island shows a close petrochemical similarity to the volcanics from Alatepenski paleovolcano belonging to the “Peripheral Volcanic Centers” in the region.

Keywords: Burgas Gulf, St. Anastasia Island, Quaternary marine sediments, Upper Cretaceous, volcanic rocks, petrography, Alkali feldspar trachyte

Introduction In the period 2018-2019 was made an engineering-geological investigation of the St. Anastasia Island [1]. The Engineering-geological research relates to the development of concepts for the reconstruction and extension of an existing harbor for small vessels on St. Anastasia Island in order to protect the water area from dangerous geological and meteorological conditions. It is confirmed that in connection with the construction of future hydrotechnical facilities, the studied area is presented of Upper Cretaceous volcanic rocks and

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Quaternary marine sediments, subjected to the active impact of lithodynamic processes in Burgas Gulf. Marine sands samples were taken for granulometric analysis, as well as rock specimens of volcanics to clarify their petrographic composition, structural-textural features, classification and nomenclature status.

Materials and methods Research has been done on volcanic rocks from the St. Anastasia Island. Thin sections and structures were made. The composition and textural features of the scales are determined by NIKON Eclipse LV100ND and Olympus CX31-P polarization microscopes. The chemical analyzes of the rocks were carried out at the Geochemistry laboratory at University of Mining and Geology “St. Ivan Rilski”, according to the OES ICP method, after alkaline melting and acid dissolution of 105°C dried samples, with an ICP-OES-720 optical emission spectrometer, Agilent Technologies, with an inductively coupled plasma excitation source (Analyst D. Dragoeva).

Results and discussions Geological-geomorphological characteristics and tectonics The investigated area is part of the East Srednogorie unit, which is located in the Alpine orogen [2]. St. Anastasia Island is located in the southern part of the Bay of Burgas (5 km southeast of Burgas), between the Chukala and Atia capes with geographical coordinates 42°28'07.492'' N, 027°33'06.247'' E. It is one of the seven Bulgarian islands in the Black Sea and is the largest after the islands of St. Ivan and St. Kirik with an area of 0.01 km². The coastal marine terraces in the region of the Burgas Depression are Holocene and are made of clays, sandy clays, clays and sands with basic minerals montmorillonite, kaolinite, ilite, quartz, plagioclase, augite and magnetite. Following the lowering of the depression during the Quaternary (the end of the Wurm), modern marine sediments were formed as a result of the lowland floodplain: sands with mussel inclusions and silt [3]. The region of St. Anastasia Island from the tectonical point of view is a part of the Burgas Depression, which is made up of Upper Cretaceous mudstone and carbonate sediments and volcanogenic rocks with a total thickness of 2-3 km. In its southern part lies the Rosen Pluton, composed of monzodiorites, monzonites and syenites. South of cape Foros, along the Bulgarian Black Sea coast, is dominated by discoveries of volcanogenic rocks from the Burgas Group, which are of Upper Cretaceous age. Capes Foros and Chukalya are composed of potassium-alkaline trachites. Lava flows of potassium-alkaline trachytes and potassium- alkaline, melatrachytes basalts and basaltic andesite occur between capes Chukalya and Atia, and

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ASN, Vol. 8, No 1, Pages 118–125, 2021 melanocratic potassium-alkaline syenites and quartz-bearing syenites are found at the capes Atia, Akin, and Chrysosothira [4, 5]. From the physical-geological processes, marine erosion is important in the area [3, 6]. The lithological construction of the shore, which is made up of eroded and cracked volcanics, contributes to the activation of erosion (Fig. 1).

Figure 1. Erosion and accumulation marine terrace in the investigation area (left) and sampling area (right) [1]

Sea waves are actively eroding volcanic rocks, moving the clayey and the silt fraction from it, and only gravelly sand is deposited on the beach. Petrographical characteristic The area between the Rosenski Paleovolcano and the Zidarovsko-Varlibrezhki Paleovolcano, which includes the St. Anastasia Island of is built of volcanic rocks, researched by M. Žujović [7] which are interpreted as "outer zone" or "somma" of the Rosen Paleovolcano, without clearly defined boundaries and internal structure [8, 9, 10]. According to geological mapping data [11], this "outer zone" is composed of the products of several small paleovolcanoes - Demirkoyski, Zhivizliiski, Alatepenski and Chukalski, which are in complex lateral and temporal relationships, both with each other and with those of the Zidarovsko- Varlibrezhki paleovolcano [12]. The four volcanic structures are united under the common name "Peripheral Volcanic Centers", based on the assumption that they acted approximately synchronously with the two main igneous centers - Zidarovsko-Varlibrezhki and Rosenski [12]. The area of St. Anastasia Island is made of volcanics (Alkali feldspar trachyte) (Fig. 1). They are pink to reddish-brown, with porphyritic texture, massive and amygdale structure. They are made of primarily

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ASN, Vol. 8, No 1, Pages 118–125, 2021 magmatic plagioclase, K-feldspar, apatite, magnetite and secondary – K-feldspar, quartz, carbonate, chlorite, epidote, fine crystalline white mica (sericite) and Fe-hydroxides. The phenocrysts (3-5%) are from plagioclase. They are prismatic, clear idiomorphic, with dimensions to 0.5 х 3 mm (Fig. 2a). They have been partially replaced from K-feldspar, fine crystalline white mica and carbonate (Fig. 2b). From the peripheral to the inner parts of the studied volcanics, the following zonation was established – spherulitic zone and microlitic zone. In the literature, such zoning (glass zone – spherulitic zone – crystallite zone and central crystallized microlithic volcanics) is described in pillow-lavas from Eastern Srednogorie [13, 14].

Figure 2. Photomicrographs of volcanics from the St. Anastasia Island: a – idiomorphic, porphyritic plagioclases; b – partially carbonatized prismatic plagioclases; c – contact between microlitic zone (up) and spherulitic zone (down); d – spherulites from fibrous K-feldspar; e – groundmass with microlitic texture; f – amygdale, filled with carbonate and chlorite. Figures a-f cross-polarized light.

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The spherulitic zone is composed of a large quantity of brown spherulites, made of fiber K-feldspar and fine-dispersed ore minerals. Their sizes are 0.15-0.2 mm (Fig. 2c, d). The space between spherulites is filled with chlorite, quartz and carbonate. The microlitic zone was built mainly by sanidine and plagioclase microlites, fine-dispersed ore minerals (hematite?) and partially devitrification and changed in chloritized volcanic glass (Fig. 2е). In some places, single and grouped grains are observed with secondary quartz. The accessory minerals is presented from magnetite and apatite in two generations – markedly elongated needle-like and thick prismatic, attached to ore minerals. In trachytes spherical and ellipsoidal amygdale are established (around 5-6 %), with dimensions 0.1-2.5 mm, with secondary mineralization. They are completely filled with carbonate, others are with quartz and carbonate, carbonate and chlorite or with chlorite in the peripheral parts, carbonate, quartz and epidote in the central parts (Fig. 2f). Petrochemical features

On the Total alkalis vs. SiO2 (TAS) classification diagram, volcanics from the St. Anastasia Island fall into the field of trachytes (Fig. 3a). The rocks are high-K, with dominance of K2O above Na2O, as the ratio

K2O/Na2O is 1.22. Coefficient (al) is 3.34, while peralkaline index (P.I.) is 0.73 (Tabl. 1). On the graph K2O-

SiO2 the figurative points of the rocks fall into the field of shoshonitic series (Fig. 3b). The comparative characteristic between the volcanics from the St. Anastasia Island, which is interpreted as the "outer zone" or "somma" of the Rosenski Paleovolcano and the rocks of nearby Paleovolcanic Centers, shows the closest petrochemical resemblance to the Alatepenski Paleovolcano, as well as to some and some of the lava in the caldera of the Rosenski Paleovolcano. The rocks of the Chukalsky and Zhivizliyski Volcanic Centers are a bit more basic and fall in the fields of shoshonite-latite, rarely tephriphonolite (for Chukalsky), and those from Demirkoysky, besides more basic, are also lower alkaline and are mainly in the field of andesites. Compared to the studied volcanics, bulgaritics from the northern parts of the Eastern Srednogorie (Balgarovo Paleovolcano), are significantly higher in high-K, which determines their affiliation to high- potassium transitional series, at the border with bulgaritic (Fig. 3b). The inflated contents of SiO2 in the volcanics from the St. Anastasia Island are due to the presence of secondary quartz in groundmass.

Table 1. Chemical composition (wt %) of volcanics from the St. Anastasia Island; K/Na = К2О/Na2O; al=

Al2O3/(Fe2O3 + FeO + MgO); PI=Na2O + K2O/Al2O3(mol). Analyst - D. Dragoeva

t Sample SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O P2O5 SO3 LOI Total K/Na al PI No 1 62.37 0.57 16.73 3.85 0.11 1.15 2.02 4.13 5.07 0.14 0.05 3.39 99.58 1.22 3.34 0.73

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Figure 3a. Total alkalis vs. SiO2 (TAS) Figure 3b. K2O vs. SiO2 plot [17] expanded by classification diagram [15], for sample volcanics of Dabovski et al. [18] for sample volcanics of St. St. Anastasia Island and the bulgarites (average of Anastasia Island and the bulgarites (average of 702 samples, Stanisheva, 1982) [16]. В – basalt, 702 samples, Stanisheva, 1982) [16] Series: BA – basaltic andesite, A – andesite, D – dacite, HKCA – high-potassium calc-alkaline; SH – TB – trachybasalt, BTA – basaltic trachyandesite, shoshonitic; HKTR – high-potassium TA – trachyandesite, T – trachyte, PhT – transitional; BG – bulgaritic (hyper-potassium phonotephrite, TPh – tephriphonolite transitional to alkaline)

Conclusion Cape Atia is made of alkali and quartz-bearing syenites. Cape Chukalya and St. Anastasia Island are built from the volcanics – alkali feldspar trachyte. In the area of the St. Anastasia Island is revealed high-K, with the shoshonitic series Alkali feldspar trachytes. The volcanics from the St. Anastasia Island show close petrochemical similarity with the rocks from the Alatepensky Paleovolcano belonging to the "Peripheral Volcanic Centers" in the area, as well as with some and part of the lava in the caldera of the Rosenski Paleovolcano. Compared to them, widespread in the northern parts Eastern Srednogorie Bulgarites, are significantly high-K, with high-potassium transitional seriality, at the border with Bulgarites. The Alkali feldspar trachytes are heavily weathered and cracked, which contributes to the activation of erosion in the coastal zone and these features of the rocks will be important for the conditions of the foundation of the future hydrotechnical facilities, which will be founded in the volcanics.

Acknowledgements The authors thank D. Dragoeva for the chemical analyzes and Eng. D. Dimov for the production of the thin sections and the structural samples.

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This article was prepared in connection with the implementation of NATIONAL RESEARCH PROGRAM "ENVIRONMENTAL PROTECTION AND REDUCTION OF ADVERSE EVENTS AND NATURAL DISASTERS’ RISKS", Approved by the Resolution of the Council of Ministers No 577/17.08.2018 and supported by the Ministry of Education and Science (MES) of Bulgaria (Agreement No D01-230/06.12.2018).

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