Upper terrane: hanging wall continental plate
The hanging wall continent exhibits a Cadomian to Variscan basement [Carrigan et al., 2005; Carrigan et al., 2006] with Early Permian granitoid intrusions and an Early Triassic to Middle Jurassic sedimentary cover metamorphosed to greenschist-facies and deformed by thick-skinned, north-directed thrusting during the Late Jurassic [Okay et al., 2001]. These features typify the European continent and find equivalence within the "Serbo-Macedonian" part of west Rhodope, namely the Vertiskos in Greece [Kockel et al., 1977] and Ograzden in Bulgaria [Zagorčev, 1976]. In this review, the highest, high-grade structural unit of East Rhodope, the so-called Kroumovitsa, is one of the Rhodope intermediate imbricates.
Lithological content
Regional investigations and correlations based on map continuity, similarities of lithologies, in particular that of basic and ultrabasic rocks, structures, geochronological ages and strain regime reveal the wide extension of the upper terrane. Rocks are mostly quartzo-feldspathic migmatites containing bodies of basic and ultrabasic rocks, some of which are eclogites retrogressed into the regional amphibolite-facies [Dimitriadis and Godelitsas, 1991; Zidarov et al., 1995; Zidarov and Nenova, 1995]. In Greece, the Upper continental terrane is essentially the "Serbo-Macedonian", which has been subdivided into the lower Kerdylion Unit and the higher Vertiskos Unit [Kockel et al., 1971].
The Kerdylion Unit consists of gneisses and migmatites with both Permo-Carboniferous and Late Jurassic protolith ages [Himmerkus et al., 2007]. Such protolith ages offer equivalence with the eclogite-metabasic-gneiss intermediate terrane of the Rhodope (Sideronero in Greece) and further demonstrate that the concept of "Serbo-Macedonian", under its original definition, should be discarded. Metamorphic mafic and ultramafic rocks, the so-called Volvi Ophiolites with supra-subduction marginal basin chemistry, occur at the boundary between the Vertiskos Unit, and the underlying Kerdylion unit [Dixon and Dimitriadis, 1984]. These mafic and ultramafic rocks are Late-Permian to Triassic in age [252 ± 13 Ma, Liati et al., 2011] and may have been the basement of the Rhodope Arc at the continent-ophiolite transition. Alternatively, they may represent a distinct, accreted oceanic fragment.
The Vertiskos/Ograzden Unit is part of the roof of the Rhodope Metamorphic Complex. It is a composite unit comprising a metaophiolite-bearing mylonite zone (ultrabasic rocks are mostly serpentinized harzburgites) between a lower metaturbiditic and orthogneissic sequence and an upper migmatitic para- and orthogneissic sequence [Burg et al., 1995]. The ca. 250 Ma metaophiolites (Table 8) bear the geochemical signature of a marginal basin [Dixon and Dimitriadis, 1984]. Ages and chemistry of the gneisses of the upper sequence demonstrate a distinct, Gondwana-derived microcontinent with its complex tectonic, magmatic and metamorphic history.
Table 7. Geochronological data for granitoid intrusions in the Rhodope Massif
| Rock type (location, *=Bulgaria) | Age (Ma) | Method | Reference |
|---|---|---|---|
| U-Pb | |||
| Granitoid (Rila*) | 69.3 ± 0.3 | [Von Quadt & Peytcheva, 2005] | |
| Granitoid (Rila*) | 66.8 ± 0.3 | [Von Quadt & Peytcheva, 2005] | |
| Granitoid (Spanchevo*) | 56 ± 0.5 | [Jahn-Awe et al., 2010] | |
| Granodiorite (Skaloti) | 55.93 ± 0.28 | [Soldatos et al., 2008] | |
| Granodiorite (Dolno Dryanovo*) | 55 ± 0.4 | [Jahn-Awe et al., 2010] | |
| Granite (Ierissos) | 53.6 ± 6.2 | uranothorite | [Frei, 1996] |
| Granitoid (Pripek*) | 52.89 ± 0.89 | [Ovtcharova et al., 2004] | |
| Granite (Kalin) | ca 46 | [Arnaudov et al., 1989] | |
| Granitoid (Smilian*) | 43.4 ± 1.41 | [Ovtcharova et al., 2003] | |
| Granitoid (Yugovo*) | 42.3 ± 0.54 | [Ovtcharova et al., 2003] | |
| Granodiorite (Rila-Pirin) | ca 42 | [Peytcheva et al., 1998] | |
| Pegmatite (Sminthi) | 36.1 ± 1.2 | [Liati & Gebauer, 1999] | |
| Granitoid (Teshovo*) | 32 ± 0.2 | [Jahn-Awe et al., 2010] | |
| Granodiorite (Stratoni) | 27.1 ± 1.1 | average | [Frei, 1992] |
| Granodiorite (Kavala) | 21.1 ± 0.8 | titanite | [Dinter et al., 1995] |
| Rb-Sr | |||
| Granitoid (Kresna*) | 82 ± 22 | whole rock | [Zagorčev & Moorbath, 1983] |
| 9 Granitoids (Leptokaria) | 34.9 to 28.4 | biotite | [Del Moro et al., 1988] |
| Granite (Central Pirin*) | 37 ± 2 | whole rock | [Zagorčev et al., 1987] |
| Pegmatite (Banite*) | 35.31 ± 0.25 | biotite | [Peytcheva et al., 2004] |
| 40Ar/39Ar | |||
| Granitoid (Ouranopolis) | 47 ± 0.7 | muscovite | [De Wet et al., 1989] |
| Granodiorite (Sithonia) | 43 ± 0.6 | biotite | [De Wet et al., 1989] |
| Pegmatite (SW-Sminthi) | 28.4 ± 0.4 | biotite | [Moriceau, 2000] |
| Granodiorite (Mesoropi) | 21.7 ± 0.5 | hornblende | [Eleftheriadis et al., 2001] |
| Granodiorite (Mesoropi) | 13.8 ± 0.5 | biotite | [Eleftheriadis et al., 2001] |
| K/Ar | |||
| Diorite (Plana*) | ca 73 | biotite | [Boyadjiev, 1981] |
| Granitoid (Dautov*) | 30-41 | biotite | [Boyadjiev & Lilov, 1976] |
| Pegmatite dyke (Tholos) | 38.3 ± 1.1 | muscovite | [Meyer, 1968] |
| Monzodiorite (Kentavros) | 38 | hornblende | [Liati & Kreuzer, 1990] |
| Granitoid (Vrondou) | 30 ± 3 | biotite | [Durr et al., 1978] |
| Granodiorite (Xanthi) | 30 ± 1 | hornblende | [Liati & Kreuzer, 1990] |
| Granitoid (Stratoni) | 29.6 ± 1.4 | Papadakis in [Kilias et al., 1999] | |
| Granodiorite (Xanthi) | 27.9 ± 0.5 | biotite | [Meyer, 1968] |
| Granodiorite (Xanthi) | 27.1 ± 0.4 | biotite | [Meyer, 1968] |
| Granodiorite (Granitis) | 28.2 ± 0.5 | biotite | [Meyer, 1968] |
| Granodiorite (Panorama) | 26.8 ± 0.5 | biotite | [Meyer, 1968] |
| Granodiorite (Krinides) | 26.0 ± 0.5 | biotite | [Meyer, 1968] |
| Granodiorite (Kavala) | 17.8 ± 0.8 | biotite | [Kokkinakis, 1980] |
| Granodiorite (Kavala) | 15.5 ± 0.5 | biotite | [Kokkinakis, 1980] |
| Granodiorite (Mesolakkia) | 15.0 ± 0.3 | biotite | [Harre et al., 1968] |
| Granodiorite (Mesolakkia) | 13.8 ± 0.2 | biotite | [Harre et al., 1968] |
Table 8. Geochronological data for protolith ages in the Upper Terrane (more ages in references).
| Rock type (location, *=Bulgaria) | Age (Ma) | Method | Reference |
|---|---|---|---|
| U-Pb or Pb-Pb | |||
| Orthogneiss (Pirgadikia) | 587.6 ± 3.4 | evaporation | [Himmerkus et al., 2006] |
| Orthogneiss (Pirgadikia) | 570.0 ± 7.0 | evaporation | [Himmerkus et al., 2006] |
| Paragneiss (Taxiarchis) | 555.8 ± 2.6 | evaporation | [Himmerkus et al., 2006] |
| Orthogneiss (Pirgadikia) | 433.0 ± 2.1 | evaporation | [Himmerkus et al., 2006] |
| Orthogneiss (Pirgadikia) | 428.2 ± 1.2 | evaporation | [Himmerkus et al., 2006] |
| 20 orthogneisses (Vertiskos) | 426 to 444 | evaporation | [Himmerkus et al., 2009a] |
| Metaophiolite (Volvi) | 252 ± 13 | [Liati et al., 2010] | |
| Granitoid (Skrut*) | 248.85 ± 0.70 | [N Zidarov et al., 2007] | |
| Granite (Kerkini) | 247 ± 2 | [Christofidies et al., 2006] | |
| Granite (Chortiatis) | 240.7 ± 2.6 | evaporation | [Himmerkus et al., 2009b] |
| Granite (Arnea) | 228.8 ± 5.6 | evaporation | [Himmerkus et al., 2009b] |
| Granite (Serres) | 221.7 ± 1.9 | evaporation | [Himmerkus et al., 2009b] |
The Sredna Gora Zone belongs to the Upper terrane along the northern border of the Rhodope. Like the Vertiskos/Ograzden Unit, a composite "basement" of metasediments with amphibolites, eclogites and orthogneisses [e.g. Zagorčev et al., 1973] bears evidence of Palaeozoic (Variscan) metamorphic and magmatic activity [Carrigan et al., 2006]. Lithological and age comparisons can be made with the Strandja Massif, to the east [e.g. Okay et al., 2001]. These rocks are therefore tentatively ascribed to the Upper Rhodope Terrane and associated in this review with the Serbo-Macedonian, for the sake of simplification.
Ages
Protolith
Late-Proterozoic to Silurian [ca 590-430 Ma, Himmerkus et al., 2006; Meinhold et al., 2010a] Vertiskos gneisses have been intruded by Triassic (241-222 Ma) granites (Table 8). The Precambrian age of most of these gneisses was inferred from, unconformably overlying early Palaeozoic sediments [Kockel et al., 1971; Zagorchev, 2001]. The Triassic magmatism, recognized in the Vertiskos only [Himmerkus et al., 2009b], is attributed to the global rifting event that led to the opening of the Tethys Ocean [Himmerkus et al., 2009b]. As everywhere in the Rhodope, a series of Tertiary granites intruded the Vertiskos (Table 7).
Metamorphism
The oldest orthogneiss were first metamorphosed during the Palaeozoic Variscan orogeny [Borsi et al., 1965; Kockel et al., 1977] and later again during the Early Cretaceous under lower amphibolite-facies conditions [Rb-Sr and K-Ar ages on Vertiskos hornblende and muscovite are between 116 and 90 Ma, Harre et al., 1968; Papadopoulos and Kilias, 1985; and 40Ar/39Ar mica ages are ca. 135 Ma, De Wet et al., 1989]. Cretaceous metamorphism and erosion are supported by a 71.9 ± 9.4 zircon fission-track age of Vertiskos gneiss, which yielded an apatite fission-track age of 43.0 ± 6.8 Ma [Wüthrich, 2009].
The Volvi basic rocks reached metamorphic temperatures of ca. 750°C [Dixon and Dimitriadis, 1984] whereas the lower unit of gneiss and metapelites did not reach more than 600°C at less than 0.7 GPa [Papadopoulos and Kilias, 1985; Kilias et al., 1999]. Kostopoulos et al. [2000] reported graphitised microdiamonds, taken as evidence for UHP metamorphism, in rocks of the Vertiskos Unit and the Circum-Rhodope Belt. With its early Palaeozoic and Variscan signatures, the Vertiskos displays a clear European affinity. The abundance of Triassic magmatism in the Variscan basement points to an attenuated crust during opening of the Tethys Ocean or a branch of it.