The Protomoesian microcontinent of the Balkan Peninsula — a peri-Gondwanaland piece

TECTONOPHYSICS ELSEVIER

Tectonophysics272 (1997) 303-313

The Protomoesian microcontinent of the Balkan Peninsula peri-Gondwanaland piece

a

I. Haydoutov *, S. Yanev Geological Institute, Bulgarian Academy of Science, 1113 Sofia, Bulgaria

Received20 July 1995;accepted6 June 1996

Abstract

Based on differences in their pre-Palaeozoic and Palaeozoic development, the Protomoesian microcontinent is divided into two terranes: the Balkan and the Moesian. A Precambrian--Cambrian ophiolite-island-arc assemblage, unconformably overlain by a Palaeozoic sequence, forms the first terrane. The pre-Palaeozoic basement of the Moesian terrane consists of Proterozoic-Vendian metamorphics of continental origin. The differences in the development of the Palaeozoic rocks are also significant reason for dividing the Balkan and the Moesian terrane. Numerous features of the Palaeozoic sediments (palaeoclimatologic, palaeobiogeographic, palaeomagnetic) from both terranes, and the Pan-African affiliations of the ophiolite-island-arc assemblage, identify the Protomoesian microcontinent as a peri-Gondwanatand piece. Keywords: terranes; Pan-African ophiolites; lower Paleozoic; Gondwanaland

1. Introduction

Recently it has been established that the basement of Southern Europe represents a collage, formed by blocks of diverse provenance and geological history. Most published interpretations concern Western and Central Europe (Ziegler, 1986; Matte, 1991, etc.). In a few cases they only include the region of the Eastern Alps (Frisch and Neubauer, 1989; Schfnlaub, 1992) and the Bohemian massif (Matte, 1991). Hardly any of the generalizations, however, concerns the region of Eastern Europe. It is interesting to mention how these blocks from southeastern Europe are referred to: "the poorly known blocks that are now buried or reworked in the Alpine orogeny" (Van der Voo, 1988). *Corresponding author. Tel.: +359 2 723-563. Fax: +359 2 724-638.

The aim of this work is to clarify the provenance of one block, the Protomoesian microcontinent, within the basement of southeastern Europe. The pre-Alpine basement of the Balkan-South Carpathian region in the Balkan Peninsula is composed of two large tectonic blocks: the Protomoesian and the Thracian microcontinents (Fig. 1), divided by the South European Variscan suture (Haydoutov, 1989). The Protomoesian microcontinent consists of two different parts: a nucleus (Moesian terrane) and a western outer zone (Balkan terrane). Precambrian ophiolites and a Cambrian island-arc association (Haydoutov, 1991) form the basement of the Balkan terrane (Fig. 2), unconformably overlain by a Palaeozoic sedimentary sequence. The basement of the Moesian terrane consists of metamorphites - high-grade metamorphic rocks of Lower and Middle

0040-1951/97/$17.00 © 1997 ElsevierScienceB.V. All rights reserved. PHS0040-1951(96)00264-8

304

I. Haydoutm; S. Y~mev/l;xvom)ptosics 272 (1997) 303-313

EUR OPE

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Proterozoic age in the base and low grade metamorphosed turbidites of Vendian-Lower Cambrian (3) age on top. These metamorphites are of continental origin (Sandulescu, 1994). The basement is covered by Palaeozoic sedimentary rocks. They differ from the sediments of the Balkan terrane. These differences are considered in detail in this paper. The amalgamation of the two terranes took place after Early Carboniferous time. The Precambrian-Cambrian geological sequence in the pre-Alpine basement of the Balkan Peninsula contains unique information for several reasons. First, traces of this evolution are well preserved and widespread in the Protomoesian microcontinent. The Balkan terrane contains the best exposures. The basement to the northwest (in the South Carpathians

and in the Alps) and to the southeast (in Turkey) was intensely reworked during Alpine time. The size of the Balkan terrane is over 47,500 km 2. Second, the ophiolite-island-arc assemblage in this terrane is covered by an almost complete Palaeozoic sequence (Yanev, 1991). The Precambrian-Palaeozoic section is important for the reconstruction of the geodynamic evolution of the pre-Alpine crust. An ophiolite association tectonically imbricated with intensely metamorphosed and migmatized rocks is known also in the Thracian massif (Kozhukharova, 1984; Kolcheva and Eskenazy, 1988). It differs from the Balkan ophiolite mentioned above and represents the lower part of an ophiolite association. It is formed by a cumulate unit and metamorphosed peridotites. The cumulates are

L Haydoutov, S. Yanev/Tectonophysics 272 (1997) 303-313

305

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2. Balkan ophiolite and island-arc associations general features The Balkan-South Carpathian ophiolite segment from the Balkan terrane comprises several blocks (Fig. 2). The total surface-outcrop area of the segment is about 400 km 2 (Haydoutov, 1991). The ophiolite association is well stratified and is formed

306

L Haydoutov S. ganev/Tectonophysics 272 (1997) 303-313

mainly by the upper part of an ophiolite association with cumulates, sheeted dikes and pillow lavas. On the basis of trace-element and Sm-Nd isotopic data (Haydoutov and Pin, 1993) the ophiolite association is considered as formed by transitional basalts extracted from depleted mantle. The rocks are metamorphosed to greenschist facies. The ophiolite body is tectonically imbricated with an island-arc association (Fig. 2). On the territory of Bulgaria, where this association was studied in detail, it is formed by sedimentary-volcanic and intrusive complexes. The latter intrude the sedimentaryvolcanic complex. The lower part of this complex comprises pelitic rocks with a carbonate formation in the middle sector. The upper part contains turbidites. The volcanic suite in both parts represents a bimodal association and comprises spilites, keratophyres, and pyroclastics (Haydoutov, 1991). The spilites from the sedimentary-volcanic complex are island-arc tholeiites. The major- and traceelement patterns of the spilites clearly differ from those of MORB. The low Ni, Cr, Hf, Nb, Y, concentrations and relatively higher Sr, Rb, Ba, Pb concentrations as compared to MORB, suggest island-arc tholeiites. The keratophyres belong to the calc-atkaline series. A trondhjemitic trend is typical (Haydoutov. 1991). The intrusive complex (Struma Diorite Formation) is a specific, irregularly metamorphosed association with a number of characteristic features (Haydoutov et al., 1994). The features include wide compositional range, multistage magmatic evolution, intrusive relationship with ophiolites, and typomorphic geochemical characteristics. The complex is built up by gabbros, gabbrodiorites, diorites, and granites, as well as a dike swarm of mafic lamprophyres. It also comprises metamorphosed igneous rocks of the same composition, which indicate its multistage magmatic evolution. The geochemical character of the rocks from the intrusive complex - - unmetamorphosed and metamorphosed - - indicates that they are part of the island-arc igneous series. Trace-element and Sm-Nd isotopic data document an origin of the intrusives in a primitive ensimatic island-arc setting (Haydoutov and Pin, 1993; Haydoutov et al., 1994). They derived from less depleted source material in comparison to the ophiolites.

The age of the island-arc association is determined by a Lower Cambrian Archaeocyathus (Kalenic, 1966). The fossils are in the carbonate horizon overlying the D. Jovan ophiolite massif (Fig. 2). This age matches with the K-Ar age (on hornblende) of the plutonites of 560-660 Ma (Lilov, 1981). On the basis of the angular unconformity between the pillow lava unit and the island-arc sequence and the presence of island-arc magmatites (extrusive and intrusive) cutting the ophiolites, a Late Precambrian age is postulated for the formation of the ocean crust. The island arc is transgressively and unconformably overlain by an olistostrome sequence. It contains olistolites of already metamorphosed ophiolite material and island-arc igneous rocks. On the basis of acritarch evidence (Kalvacheva, 1990), the age of the olistostrome is Arenigian. The ophiolite-island-arc associations from the Balkan terrane are called the Balkan assemblage. The evolution of the Balkan assemblage can be traced best through the features of the Palaeozoic sedimentary sequence. 3. Palaeozoic s e d i m e n t a r y s e q u e n c e s f r o m the P r o t o m o e s i a n m i c r o c o n t i n e n t - - general features

The Palaeozoic sedimentary rock section begins with the Middle Ordovician. In contrast to the underlying Cambrian and Arenigian rocks this sequence is not metamorphosed. The Palaeozoic sedimentary rocks from the various blocks of the collage forming the Balkan Peninsula show a number of regional differences. Part of these differences could be explained as lateral facies changes, while others are fundamental and indicate distant formation places. These differences are the main reason to divide the Balkan and the Moesian terranes (Yanev, 1990). The presence of the Balkan assemblage in the Balkan terrane only is another reason. The most important differences of the Palaeozoic rocks from both terranes (Figs. 3 and 4) are as follows: - the presence of the Arenigian olistostrome in the Balkan terrane, and the total thickness of the Ordovician which measures 4700 m, in contrast to 510 m, of quartzites and argillites in the Moesian terrane;

1. Haydoutov, S. Yanevl Tectonophysics 272 (1997) 303-313

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p r e s e n c e o f 2800 m calcareous and pelitic L o w e r C a r b o n i f e r o u s rocks in the M o e s i a n terrane, and lack o f s e d i m e n t s o f that age in the Balkan terrane;

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L Haydoutov, S. Yanev/Tectonophysics272 (1997)303-313 Balkan terrane continue up to the Lower Visean, while in the Moesian terrane sedimentation break is observed before the upper Tournesian. 4. F o r m a t i o n a l e n v i r o n m e n t of the s e d i m e n t s f r o m both t e r r a n e s

The formational environment of the sediments is considered on the base of lithological analogies, palaeoclimatologic, palaeobiogeographic, and palaeomagnetic data. Certain lithological analogies between lower Palaeozoic complexes from Bulgaria, North Africa, and Argentina have been established by Baldis and Yanev (1990).

4.1. Lithological and palaeoclimatologic data The Upper Ordovician section in the Balkan terrane contains diamictites (Yanev, 1989). This rock type is generally accepted as a glacial deposit. They originated in latitudes of 40°. Ordovician chamosite iron ores are found in the Balkan terrane. Their formation would have only been possible in the humid zone of intermediate latitudes. Chamosite was also established (Ketin, 1983) in Silurian rocks from the Istanbul zone (part of the Balkan terrane). The flysch sediments from the Balkan terrane (Middle Devonian to Lower Carboniferous) contain a number of relics from psylophytic vegetation, indicating a humid climate. Ferruginous oolite hydroxide ores also indicate humid conditions in the region of Belogradchic (Balkan terrane). During the Middle Devonian-Lower Carboniferous, however, along the eastern margin of the Moesian terrane (in Bulgaria and Romania) gypsum and anhydrite were formed as indicators of an arid climate. The occurrence of rocks indicating humid and arid conditions for the same time over a distance of less than 300 km can only be explained by original separation and later transportation into the present relative positions. The Permian sedimentary rocks from both terranes are red clastic rocks with calcareous matrix, poor mineralogical and structural maturity and inclusions of evaporites. These features determine the sediments as products of add sedimentation. Large deposits of anhydrite and halite are common in the Moesian terrane.

309

4.2. Palaeobiogeographic data Recently the palaeobiogeographic affiliations of a number of fossils from both terranes have been clarified. This concerns: - t h e Middle Ordovician trilobite associations (Spassov, 1987) in the Grohoten suite (Balkan terrane) with Cyclopiige prisca Barr., and others (Table 1); - the acritarchs from the Balkan terrane determined as late Arenigian, Middle and Late Ordovician (Table 1; Kalvacheva, 1984, 1986) and Silurian (Kalvacheva, 1990); - the oldest palynologically proven part of the Lower Palaeozoic sequence in the Moesian terrane (Lakova, 1994) is assigned to the Silurian, containing representatives of chitinozoan genera, as well as tubular macerals of non-vascular plants; - mollusc fauna specified in the region of Shuma village (Balkan terrane) firstly assigned (Pribyl and Spassov, 1960) to the Ludlow stage and after that (Spassov, 1987) to the Lower Devonian series, containing Hercynella cf. bohemica Barr., H. sp. aft. nobilis Barr. and others (Table 1); the Chitinozoan assemblage in the Lower Devonian from the Moesian terrane comprising Angochitina chlupaci and others (Table 1). This fauna is interpreted (Lakova, 1995) as belonging to the Afro--South American province.

4.3. Palaeomagnetic data These data for the considered terranes are scarce and not always precise. Data exist only for the Balkan terrane. Reliable are the data for the flysch sediments and for the Permian rocks of this terrane. The flysch succession of Belava, Vlahina, and Greben (Balkan terrane) formed in palaeolatitudes of 10 - 15 + 3°S. Data on the flysch rocks in Suva Mountain (same terrane) show that they were formed in the zone of the Equator (Milicevic, 1992). The age of the flysch section in the Balkan terrane is Late Devonian to Visean. The flysch sediments rejuvenate from southeast to northwest, from Belava, Vlahina and Greben to Suva Mountain (Krstic and Maslarevic, 1989), Therefore, according our opinion, the palaeomagnetic data from the first area characterise the Devonian part of the section, while those of Suva

L Haydoutov, S. Yanev/Tectonophysics 272 (1997) 303-313

310

Table 1 Distribution of fossils belonging to Goundwanaland palaeobiogeographic provinces Ages

Fossils

Devonian (Lochkovian) Chitinozoan: Angochitina chlupaci Paris and Laufeld; Fungochitina lata (Taug. and Jekh.); Eisenakitina tougourdeavi (Rausher and Doubinger); Cingulochitina plusquelleci Paris; Ancyrochitina asterigis Paris; Lagenochitina navicula Taug. and Jekh.; Bursachitina cf. oviformis Eis. (Lakova. 1995) Molluscan: Hercynella cf. bohemica Barr.; H. sp. aft. nobilis Ban:.; Neklania obtusa Barr.; N. resecta (Barr.); Patrocardium aft. seminotum (Barr.); Stylonema aft. solvens Per.; Plectodonta sp. ex gr. PL comitans (Barr.) Tentaculite: Novakia intermedia (Barn) (Pribyl and Spassov, 1960)

Ordovician Arenigian

Acritarchs: Acanthodiacrodium cf. intercalarae Burmann; A. cf. uniforme Burmann; A. cf. vavrolorae Cramer and Diez; Coryphidium cf. milada Cramer and Diez; C. cf. mimitum Cramer and Diez; Striatotheca cf. principalis Burmann; S. cf. queta (Martin) etc. (Kalvacheva, 1984)

Llanvirnian, Llandeillian

Trilobites: Cyclopiige prisca Ban-.; C. prisca var. longicephala Klonc.; C. rediviva (Bart.); Ectillaenus cf. highesi (Hicks). (Spassov, 1987)

Llanvirnian

Acritarchs: Dicrodiacrodium normale Burmann; Veryhachium sartbernardense Martin; etc. (Kalvacheva, 1986)

Mountain are representative for the Carboniferous part. The palaeomagnetic data indicate that the sedimentation in the Permian period occurred in the northern arid zone at about 14°N (Nozarov et al., 1980). 5. D i s c u s s i o n a n d c o n c l u s i o n s

Analysis of the age, evolution, composition and geotectonic position is crucial for understanding the genetic relation of the Balkan assemblage. The Precambrian age of the ophiolite, and the Cambrian age of the island-arc association, are reasons to consider the assemblage as a Pan-African structure (Haydoutov, 1992). From this point of view, it is interesting to compare the considered assemblage with analogous assemblages from the Arabian Shield. A tendency of younging of the island-arc terranes from SE to NW is observed (Stoeser and Camp, 1985). While the southeasternmost Asir terrane is >900-800 Ma old, Hijas is 800-700 Ma old, and the Midyan terrane in the northwest is 700--600 Ma old. Taking into consideration that the Balkan assemblage is the closest similar terrane to the northwest of the Arabian Shield, the continuation of this tendency may be assumed. Therefore, the above made comparison also implies the Balkan assemblage being a Pan-African structure.

The composition of the sediments, volcanics and plutonic rocks from the Balkan island-arc association are similar to those of the Midyan and Hijas terranes (Nassef and Gass, 1977; Jackson et al., 1984; Stoeser and Camp, 1985). The petrological properties of the igneous rocks from the Balkan terrane indicate that they were also formed in the conditions of an ensimatic island arc, similar to the primitive arcs of the Saudi Arabian terranes (Stoeser and Camp, 1985; Fig. 3). The previously considered data for the Palaeozoic sedimentary section reveals a clear picture of their formational environment. The palaeoclimatic evolution of both terranes indicates their migration along the wander path of Gondwanaland from its original position to Palaeoeurope. The palaeobiogeographic data show: - The Middle Ordovician trilobite associations are typical of the Bohemian province (Yanev, 1990). - The Ordovician and Silurian acritarchs indicate clear relations with the cold-water peri-Gondwanaland (called also Mediterranean) palaeoplankton province (Kalvacheva, 1990). - The mollusc fauna from Shuma village has been correlated with Bohemian genera and species (Pribyl and Spassov, 1960). A similar Bohemian mollusc fauna of the Devonian has been assigned (Chlupac, 1969, 1982; Gutierrez-Marco and Ra-

1. Haydoutov, S. Yanev/ Tectonophysics 272 (1997) 303-313

bano, 1987, and other authors) to the peri-Gondwanaland province. - The chitinozoan Lower Devonian assemblage from the Moesian terranes is interpreted (Lakova, 1993) as belonging to the Afro-South American province. The established zones U. simplex-C. plusquelleci, U. simplex, A. tomentosa-U, simplex, and A. chlupaci-A, tomentosa are correlated with zones and subzones from Algeria, the Sahara, Tunisia, Romania, Spain, Czech Republic and France. It is important to mention that their proximity is better expressed with relation to the North African zones than to the European zones. This correlation also supports the notion of the peri-Gondwanaland provenance of this terrane. Analysis of the considered data indicates that both terranes (Balkan and Moesian) have a peri-Gondwanaland origin. The data show the movement of both blocks (in the beginning separated considerably), migrating en echelon to the north. In the Middle Devonian, the Balkan terrane reached the southern humid zone, while the Moesian terrane reached the southern arid zone. During the Carboniferous, both terranes moved closer to each other towards the equatorial tropical zone and came together in Permian times in the northern arid climatic zone. The features of the Balkan assemblage and the Ordovician-Permian sedimentary sequences enable to establish a number of conclusions about the geodynamic evolution of the Protomoesian microcontinent, the basement of southeastern Europe, and Gondwanaland. Crucial for reconstructing the evolution of the Protomoesian block are the fact that the rock assemblages are well preserved, only slightly affected by Variscan and Alpine deformational and metamorphic processes, as well as the data indicating the Gondwanan formational environment of the Palaeozoic sequence. The Palaeozoic sequence formed on top of the Balkan assemblage, as documented by ophiolitic olistolites in the Arenigian olistostrome. All the data clearly demonstrate the peri-Gondwanaland genesis of the microcontinent and its migration during Palaeozoic time from the Southern to the Northern Hemisphere. The notion for the peri-Gondwanaland provenance of the Protomoesian microcontinent is significant for understanding some problems of the South

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European basement. First of all, a correlation of the Protomoesian microcontinent and the Bohemian massif is very likely. The correlation between t h e s e two blocks is based on the following grounds. - Existence of a similar ophiolite-island-arc assemblage in the Bohemian massif (Jelinek et al., 1984; Kastl and Tonika, 1984). - Resemblance of the lower and middle Palaeozoic sections to those of the Bohemian massif, Eastern Germany etc. The analogies concern the fauna assemblages (trilobites, acritarchs, chitinozoa, graptolites, tentaculites, molluscans), as well as the lithological similarity of a number of formations from both blocks (Chlupac et al., 1980). These observations shed new light on the ancient evolution of the strongly reworked regions between the Protomoesian microcontinent and the Bohemian massif, i.e. the South Carpathians and the Eastern Alps, in Variscan and Alpine time. The significance of the Balkan assemblage for tracing back the evolution of the northwest continuation of the Pan-African ophiolite-island-arc assemblage should be pointed out, taking into consideration that it is the closest large outcrop to the Arabian Shield. The Balkan assemblage and the similar ophiolite-island-arc associations from the Bohemian massif could be regarded as the missing link between the Avalonian--Cadomian and Arabian peripheral orogens of Murphy and Nance (1991). Acknowledgements

This paper benefited from the valuable remarks, critical reviews and comments made by the reviewers W. Frisch and H.P. SchSnlaub. This research has been supported by the National Science Fund (Projects 425 and 215), Ministry of Science and Education, Rep. of Bulgaria. References

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