GEOSCIENCES 2007

Preliminary structural data on the “Stara Planina high-grade metamorphic series”, Teteven Stara Planina Mountains Ïðåäâàðèòåëíè ñòðóêòóðíè äàííè çà Ñòàðîïëàíèíñêàòà âèñîêîêðèñòàëèííà ìåòàìîðôíà ñåðèÿ, Òåòåâåíñêà Ñòàðà ïëàíèíà Anna Lazarova1, Neven Georgiev2, Dimo Dimov2 Àííà Ëàçàðîâà1, Íåâåí Ãåîðãèåâ2, Äèìî Äèìîâ2

1 Geological Institute, Bulgarian Academy of Sciences, 1113 ; E-mail: [email protected] 2 Sofia University, Dept. of Geology and Paleontology

Key words: Central Stara Planina Mountains, metamorphic rocks, structural data

Limited data are available on the metamorphic na river valleys. These rocks are foliated, however complex known as “Stara Planina high-grade meta- transitions into almost isotropic varieties are observed morphic series” (Kuikin et al., 1971; Milanov et al., even in the scale of a rock specimen. Common fea- 1971) and exposed in large areas in and ture is the abundance of centimeter-scale aplite veins, Teteven Stara Planina Mountains. A variety of meta- often parallel to the foliation planes, though oblique morphic rocks have been recognized within the com- ones are found as well. In the eastern part of the plex: gneisses, biotite schists, amphibolites, and mar- area (along Stara Ribaritsa and Chernata reka river bles as well as lenses of greenshist metamorphites. valleys) dominate fine- to medium-grained leuco- The complex is interpreted as to be a higher-grade cratic granites, enriched in white mica. Generally, counterpart of the Diabase-Phyllitoid Complex they display intensive foliation and clear stretching (Kuikin et al., 1971; Milanov et al., 1971) or an asso- lineation. ciation of migmatites and anatectites (Tscheshitev et The foliation within both types of granites strikes al., 1995). Several granitoid bodies (often foliated near 85—120° and dips moderately (25—40°) to the south the contacts) intrude the metamorphic rocks. The and southwest. Within the intensively deformed va- largest ones are Ribaritsa and granites rieties the foliation is defined by the alternation of (Kuikin et al., 1971; Milanov et al., 1971). quartz-feldspar and mica layers (biotite-rich in the Our research focuses only on the metamorphites reddish granites, or white mica-rich in the leuco- extending as a narrow E-W elongated strip between cratic granites). The lineations are shallow-plung- Kostina and Chernata reka river valleys to the south ing (15—35°) to the west and southwest. Elongated of the Ribaritsa village (Fig. 1). Here, these rocks have micas, feldspars and quartz, define the stretching lin- brittle tectonic contact with the Variscan Vezhen plu- eation within the leucocratic granites. The transi- ton to the south, and are covered by Triassic-Lower tions of this type lineation into slickenside striations Jurassic sediments to the north (Kuikin et al., 1971, with same orientations are common. Lineation struc- Lazarova, 2006). Field observations showed that fo- ture within the reddish varieties is rarely observed. liated granites are the dominating rock type in the Microstructural observations of the behavior of complex, thus this study focuses on them. Isolated rock-forming minerals as well as the mineral growth outcrops of gneisses (partly migmatized; Zavodna river within the studied rocks give indirect information valley), amphibolites and biotite schists with garnet about the mechanisms and thermal conditions of porphyroblasts (Stara Ribaritsa river valley) have been deformation. Feldspars (both K-feldspars and pla- recognized as well, but their relationship with the gioclases) form large porphyroclasts and small grains granites remain unclear at this stage. within the matrix of the foliated granites. Some of Two types of granites are distinguished. Reddish, the feldspar porphyroclasts preserved primary (ig- medium- to coarse-grained biotite granites with cen- neous) idiomorphic outlines, zoning and earlier- timeter-scale K-feldspar porphyroclasts prevail in the phases inclusions. The porphyroclasts are deformed western part of the complex in Kostina and Zavod- mainly by cataclastic flow and minor diffusion mass

11 Fig. 1. Schematic geological map of the studied area (after Kuikin et al., 1971) transfer processes. Fractures and microfaults are tures are strain shadows and caps, overgrows, pres- common, reducing the size of the grains and form- sure solution seams developed along the foliation, ing angular-shaped fragments, arranged along the fibrous quartz veins (along the foliation or crosscut- foliation. Microstructures reflecting the intracrys- ting the porphyroclasts) and intensive transforma- talline deformation are also present. Widespread are tion of feldspar into white micas. deformation twins (“flame-shaped” perthite), kink- The behavior of feldspars and quartz within the bands and undulate extinction. Locally, the initial foliated granites, the presence of chlorite-epidote- stage of dynamic recrystallization is marked by the white mica-green biotite-quartz assemblage as well presence of deformational myrmekites and subgrain as the signs of high fluid activity indicate thermal formation. The development of distinct core-and- conditions of deformation in the range between 350— mantle structure in the K-feldspar grains was not 450°C. These temperatures correspond to medium- observed. to high-grade greenshist facies deformation con- Quartz is the major phase within the granites’ ditions. matrix. Generally, it is fine-grained, forming elon- The preliminary field and microstructural data gated polycrystalline aggregates along the foliation put some new aspects on the recent notions of the or asymmetric tails around feldspar porphyroclasts. so-called “Stara Planina high-grade metamorphic The polycrystalline aggregates contain recrystallized series”. In the area of the Teteven Stara Planina grains of equal size or grains of different sizes with Mountains these “high-grade metamorphic series” irregular boundaries, core-and-mantle structures with are built up almost entirely of foliated granites de- subgrains and recrystallized grains. All these pecu- formed in medium- to high-grade greenshist facies liarities reflect bulging and almost subgrain rota- conditions (350—450°C). The new data contradict tion recrystallization processes. to the conclusion of Tscheshitev et al. (1995) that Mineral phases developed along the foliation these rocks are migmatites and anatectites. The con- planes or shear bands, within strain shadows and tacts of the granites with the other metamorphic rock caps or as overgrows around porphyroclasts are in- types in the studied area (migmatized gneisses, am- dicative of the deformational conditions. Such phases phibolites and biotite schists) remain unclear, but it are chlorite, epidote, white mica, green biotite and could be assumed that the latter represent parts of quartz. Numerous microstructures within the gran- the host complex. ites imply the important role of diffusion mass trans- Irrespectively of the preliminary character of these fer processes during deformation. Such microstruc- conclusions the presented data raise a lot of ques-

12 tions. The characteristics of the so-called “Stara Plan- ships, ages and metamorphic history. Resolving these ina high-grade metamorphic series” are still very problems requires more detailed studies. obscure and need further geological investigations. Is it really a higher-grade counterpart of the Dia- Acknowledgements: This research was financially base-Phyllitoid Complex or is it an independent unit supported by Sofia University Scientific Research comprising different rock types of specific relation- Fund.

References Milanov, L., S. Kuikin, J. Gercheva, S. Hristov, V. Chunev. Kuikin, S., L. Milanov, J. Gercheva, S. Hristov. 1971. 1971. Geological structure of eastern part of Troyan Geological structure of Stara Planina Mountains Stara Planina Mountains. – Jub. Ann. Geol. Com- between Zlatitza and Troyan pass. – Jub. Ann. Geol. mit., 18, 201–221 (in Bulgarian). Commit., 18, 179–196 (in Bulgarian). Tcheshitev, G., V. Milanova, I. Sapunov, P. Tchouma- Lazarova, A. 2007. Structure, Emplacement Mechanisms tchenko. 1995. Geological Map of , Scale and Tectonic Position of the Vezhen Pluton. PhD the- 1:100 000, Sheet Teteven. GI and “Geol. and Geo- sis, 189 p. (in Bulgarian). phys.”, 94 p.

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