294 Geologica Macedonica, Vol. 29, No. 2, pp. 101–132 (2015) GEOME 2 In print: ISSN 0352 – 1206 Manuscript received: June 10, 2015 On line: ISSN 1857 – 8586 Accepted: October 5, 2015 UDC: 553.52/.53:551.73]:556.4(497) Original scientific paper CADOMIAN AND POST-CADOMIAN TECTONICS WEST OF THE RHODOPE MASSIF – THE FROLOSH GREENSTONE BELT AND THE OGRAZHDENIAN METAMORPHIC SUPERCOMPLEX Ivan Zagorchev1, Constantin Balica2, Evgeniya Kozhoukharova1, Ioan Coriolan Balintoni2, Gavril Săbău3, Elena Negulescu3 1Bulgarian Academy of Sciences, Sofia, Bulgaria, 2University Babeş-Bolyai, Cluj-Napoca, Romania 3Geological Institute of Romania, Bucharest, Romania [email protected] A b s t r a c t: The Frolosh Greenstone Belt (FGB) is traced at a distance of more than 200 km in the territories of Bulgaria, Macedonia and Serbia. It consists of various greenschist-facies rocks (actinolite schists, phyllites, calcareous schists, impure marbles, metasandstones, metadiabases, massive green rocks, etc.) of the Frolosh metamorphic complex with bodies of metabasites (including lherzolites), and inliers (retrogressed mica gneisses and migmatites) from the Ograzhdenian supercomplex. The complex is in- truded by bodies of gabbro (occasionally with ultramafic cumulates), diorites to granites (Struma diorite formation). U-Pb studies on zircons yielded Cadomian ages within the time span between c. 574 and 517 Ma. The Frolosh complex covers the ultrametamorphic (migmatized gneisses and amphibolites; tourma- line-biotite schists; quartzo-feldspathic gneisses; lensoid bodies of metaperidotites to norites) of the Ograzhdenian supercomplex. The Ograzhdenian rocks are intersected by diatectic metagranites over- printed by amphibolite-facies metamorphism. Dominant U-Pb ages vary between 470 and 430 Ma. The contact between the Frolosh complex and the Ograzhdenian supercomplex has been subject of long dis- cussion and controversial interpretations. Now we emphasize on the multistage developments of both complexes as demonstrated both by field evidence and isotopic dating. The Ograzhdenian supercomplex has been subject of Precambrian tectonometamorphism witnessed by Rb-Sr whole-rock isochron data and relict U-Pb zircon data. Ordovician to Silurian anatectites (metatectic migmatization, diatexis) are in- truded by Permo-Triassic granites. The contact between the Ograzhdenian supercomplex and the covering Frolosh complex is regarded as a thick complex zone of multistage tectonometamorphic development rather than a “razor-blade” surface of one-stage origin. As a boundary between suprastructure and infra- structure, it played an important role throughout the Phanerozoic, and acted as a screen with a steep ther- mal gradient during the Ordovician-Silurian anatexis and metamorphism in the Ograzhdenian supercom- plex. For to verify this hypothesis, new detailed structural and isotopic studies are needed. Key words: Serbo-Macedonian Massif; Ograzhden Unit; Frolosh Greenstone Belt; Rhodope Massif; Late Proterozoic and Palaeozoic tectonics 1. INTRODUCTION: The Morava-Rhodope Alpine Collage (MRAC) and the Serbo-Macedonian Massif (SMM) The Alpine tectonic structure of the Balkan East towards the Moesian Platform, three major Peninsula is dominated by the Vardar Suture. This belts may be distinguished (e.g., Dabovski et al., relict from the Mesozoic Tethyan Vardar Ocean 2002; Zagorchev et al., eds., 2009) in the Late Cre- divides the Peninsula in two well-outlined do- taceous structure: the “internal” Morava-Rhodope mains: the western belt of Hellenides-Albanides- Zone, the Late Cretaceous Srednogorie Volcanic Dinarides of Tethyan character, and the eastern Arc transformed by end-Cretaceous time into the belt of the Balkanides s.l. characterized by a Peri- Srednogorie Orogen, and the Balkanide Orogen Tethyan Mesozoic evolution. Outwards from the that consists of the Stara-planina (Balkan) Zone, Vardar Suture, and along a transect trending North- the Transitional Zone and the Fore-Balkan Zone. 102 I. Zagorchev, C. Balica, E. Kozhoukharova, I. C. Balintoni, G. Săbău, E. Negulescu Whereas the south-western parts of the Morava- Middle Jurassic sequence with olistostroms; Mid- Rhodope Zone possess a south-western vergence Jurassic folding), the Jurassic Treklyano trough, being subducted by the Vardar slab, the north-east- and the Late Jurassic – Early Cretaceous Nish- ern parts are thrust to North-East over the Sred- Troyan trough. Extensive igneous activity is typi- nogorie, and each of the following external zones cal of the Late Cretaceous Srednogorie Zone which has the same northern to north-eastern vergence developed from an epicontinental rift with crustal towards and over the Moesian Platform. Older tec- thinning to a volcanic arc with deeper sedimentary tonic ideas regarded the structure of the Peninsula basin environment. as consisting of two divergent orogenic belts Basement and cover tectonic elements have (southern Dinaridic and northern Balkanidic been extensively deformed and imbricated “branches of the Alpine orogen”) divided by a me- throughout the Alpine orogenesis. Alpine defor- dian Rhodope Mass (sensu Cvijić), median belt (in mations have been superimposed over the pre-Al- the sense of J. Brun) or a Zentralmassif (sensu pine basement elements at different structural lev- Kossmat). els, and correspondingly, in different P-T meta- The Morava-Rhodope, Srednogorie and Stara morphic conditions. Planina zones are characterized by a pre-Alpine The Morava-Rhodope Zone has a highly het- basement and a Mesozoic and Cenozoic cover. The erogeneous structure. It consists of structural ele- basement derived from pre-Hercynian continental ments of different ages and provenance that have and oceanic crust and Hercynian continental crust. been gathered in a specific collage. From west to The cover consists mostly of sedimentary forma- east, i.e., from the Vardar Suture towards the Sred- tions of peri-Tethyan affinity, i.e., formed in epi- nogorie Zone, the following major structural ele- continental shallow seas and back-arc environ- ments have been distinguished: eastern marginal ments. The Triassic volcanic activity is related to strip of the Vardar Zone that passes to south-east rifting in the Moesian Platform and along the into the Circum-Rhodope Belt; Serbo-Macedonian Maritsa fault belt (at the boundary between the Massif (partially identified as Morava Zone s.l.); Morava-Rhodope and the Srednogorie). Amag- Morava Zone s.s. (Palaeozoic units thrusted mostly matic deeper sedimentation environments are re- in mid-Cretaceous times over the Struma Zone); lated to troughs with restricted occurrences: the Struma Zone; units of the Rhodope Massif. East-Balkan (Kotel) trough (continuous Triassic to 2. FROLOSH GREENSTONE BELT (FGB) The Frolosh Greenstone Belt is traced directly tion. There is a close relation between this Struma at a distance of about 200 km between the Bela- diorite complex, and the greenschist-facies rocks sitsa Horst at the South (on the territory of Mace- of the Frolosh complex: the Struma diorites to donia) and the area of Trun to the North (Fig. 1). It granites occur solely amongst the rocks of the lat- is widely accepted that before the Alpine tectonics ter exhibiting different kinds of contacts, – from the belt had been connected with the large outcrops irregular transitional contacts to sharp intrusive of similar composition that are traced between ones (Zagorčev, 1966, 1987; Zagorchev, 2001). Shipka Stara planina Mountain and the Deli Jovan The Frolosh metamorphic complex (FMC) and Iuţi massifs in West Stara planina and the consists (Zagorčev, 1987) mostly of greenstones: South Carpathian Mountains, respectively (e.g., metadiabases, metamorphosed tuffs, actinolite- and Haydoutov, 1989; Haydutov et al. 2012). The pos- chlorite-actinolite schists. Phyllites (sericite-chlo- sible correlations and geodynamic considerations rite schists), albitophyres to keratophyres, calcare- shall be discussed in the final parts of the present ous schists to impure marbles, and metapsammites paper. (metasandstones to fine-pebble metaconglomer- The principal constituents of the FGB are the ates) have been also found. A few bodies of ser- Frolosh metamorphic complex (FMC) and the pentinized ultramafics have been observed mostly Struma diorite complex (“formation”, SDC). as tectonic lenses: a body of lherzolite within the Known since the pioneer works in the 19th century, green schists east of the village of Frolosh (De- their first detailed description dates from the first spov, 1959; Zagorčev, 1966), and a thin (5 – 8 m) half of the 20th century when Stefanov and Dimi- lenticular slice of peridotite from the basement tec- trov (1936) coined the name Struma diorite forma- tonically inserted along the Garbino thrust parallel Geologica Macedonica, 29 (2), 101–132 (2015) Cadomian and Post-Cadomian tectonics west of the Rhodope Massif – The Frolosh greenstone belt and the Ograzhdenian metamorphic supercomplex 103 to the schistosity within the basal parts of the basement near the village of Garbino (Zagorčev, Lower Triassic section that covers the Frolosh 1985; Zagorchev, 2001; Graf, 2001). Fig. 1. Simplified geological map for the position of the Frolosh Greenstone Belt and the Ograzhdenian metamorphic supercomplex in the Alpine orogenic structure of the Balkan Peninsula. The detailed map areas (Figs. 2 and 3) are indicated. Geologica Macedonica, 29 (2), 101–132 (2015) 104 I. Zagorchev, C. Balica, E. Kozhoukharova, I. C. Balintoni, G. Săbău, E. Negulescu A number of metabasic