The Neotethys suture in the eastern Bulgarian Rhodopes

Abstract

<p>Following a tectonic scheme proposed by Janák et al. (2011; Journal of Metamorphic Geology 29, 317-332) and Pleuger et al. (2011; Zeitschrift der Deutschen Gesellschaft für Geowissenschaften 162, 171-192), the Rhodopes are composed of four nappe complexes, from bottom to top the Lower, Middle, and Upper Allochthon and the Circum-Rhodope Belt. Rocks derived from Adria and/or Pelagonia (Lower Allochthon) are separated from rocks of European origin (Upper Allochthon) by lithologically variegated thrust sheets containing sporadic occurrences of ophiolites (Middle Allochthon). These ophiolites typically yield magmatic protolith ages of c. 160 Ma and were metamorphosed under amphibolite- to eclogite-facies conditions. They represent Neotethyan lithosphere subducted below Europe in the Late Cretaceous to Palaeogene whereas the Circum-Rhodope Belt contains ophiolites of the same protolith age but with lower metamorphic grade (greenschist facies at most) and was obducted onto the former European margin in the Jurassic. We present LA-ICP-MS U-Pb zircon and additional geochemical data from the Luda Reka Unit in the Bulgarian Eastern Rhodopes. This unit consists mostly of amphibolite, metagabbro, and metadiorite that yielded two protolith ages of 163.5±2.6 Ma and 154.2±1.0 Ma. The trace element patterns resemble those of typical back-arc basalts and lower oceanic crustal cumulates. Initial epsilon Nd values of six samples calculated to 154 Ma were +10.8 ±0.8 (2σ; n = 6), in agreement with average basalts derived from depleted ambient mantle. A pegmatite crosscutting the Luda Reka Unit yielded a magmatic age of 52.04±1.1 Ma. Such pegmatites are widespread in the Luda Reka Unit (Middle Allochthon) suggesting that emplacement of this unit over the Bjala Reka Orthogneiss Unit (Lower Allochthon) where such pegmatites are lacking happened only after c. 52 Ma. The Bjala Reka Orthogneiss Unit forms the footwall of the top-to-the-SSW Bjala Reka Detachment that became active in the Late Eocene. Where the Luda Reka Unit is lacking, the Bjala Reka Orthogneiss Unit is overlain by rocks that were collectively described as “Low-grade Mesozoic Unit” (e.g. Bonev & Stampfli 2008; Lithos 100, 210-233). Based on peak temperatures determined by Raman spectroscopy of organic matter, two tectonic units can be distinguished in the “Low-grade Mesozoic Unit”. The temperature peak was at c. 530 °C in the Mandrica Unit below and at c. 285 °C in the Maglenica Unit above. For the Mandrica Unit, minimum peak pressures of c. 1.4 GPa were obtained by Raman spectroscopy of quartz inclusions in garnet, indicating that this unit underwent subduction-related metamorphism. Because of this marked difference in peak metamorphic grade, we attribute only the anchimetamorphic Maglenica Unit to the Circum-Rhodope Belt while the high-pressure Mandrica Unit probably represents the Upper Allochthon. Both units are presently separated by the top-to-the-NW Mandrica Detachment that was active before the Bjala Reka Detachment. Our new findings show that the easternmost Rhodopes expose a condensed section through all four nappe complexes, notably including the Neotethys suture.</p>

On the Rhodopean protoliths and their Middle Jurassic to Early Cretaceous evolution. A review

“…5; Luda Reka ophiolite, Pleuger et al, 2020). Pleuger et al (2020) presented new U-Pb LA-ICP-MS zircon ages of 163.5±2.6 Ma from an amphibolite and 154.2±1.0 Ma from a metagabbro sample. The reported trace element patterns of the metagabbro sample resemble those of typical back-arc basalts and lower oceanic crustal cumulates and the initial ε-Nd values of average basalts derived from depleted ambient mantle.…”

“…These eclogites were described as ophiolitic fragments which have experienced UHP conditions during the Middle Jurassic (Bonev et al, 2015a) or the Eocene (Mposkos et al, 2012) subduction. In fact, they are probably part of the Kimi-Krumovitsa Unit, where numerous meta-ophiolitic sequences of Devonian (Peytcheva et al, 2018) to Late Jurassic age (Pleuger et al, 2020), were reported.…”

“…Still open is the question about the character of the Jurassic magmatic arc (intra-oceanic or a continental arc) and the polarity of the Late Jurassic-Early Cretaceous subduction (south-dipping, e.g., Froitzheim et al, 2014, or north-dipping below Europe, e.g., Ricou et al, 1998;Burg, 2012). The Intermediate Terrane comprises mainly metamorphosed Jurassic calc-alkaline granitoids (Ovtcharova et al, 2004;von Quadt et al, 2006von Quadt et al, , 2008von Quadt et al, , 2009Raeva et al, 2008;Turpaud and Reischmann, 2010;Popov and Ivanov, 2012;Marinova and Stankova, 2012;Froitzheim et al, 2014;Georgieva et al, 2019;Gorinova et al, 2019), ophiolites (Marinova and Stankova, 2012;Froitzheim et al, 2014;Pleuger et al, 2020) and metasediments, where some Carboniferous to Permian orthogneisses are also reported (Liati and Gebauer, 1999;Liati, 2005;Himmerkus et al, 2006b;Turpaud and Reischmann, 2010;Marchev et al, 2019). The Kimi-Krumovitsa Unit of the eastern Rhodope (Fig.…”

“…Metagabbros cropping out along the Bulgarian-Greek border between Mandritsa and Micro Derio, previously considered as a part of the high-grade basement of the Circum-Rhodope Mandritsa Unit (Sarov et al, 2008d), were recently considered to be Upper Jurassic ophiolitic fragments (Fig. 5; Luda Reka ophiolite, Pleuger et al, 2020). Pleuger et al (2020) presented new U-Pb LA-ICP-MS zircon ages of 163.5±2.6 Ma from an amphibolite and 154.2±1.0 Ma from a metagabbro sample.…”

“…Tikhomirova et al (1988) reported Early Jurassic radiolarians from cherts associated with the basalts. Recently, Pleuger et al (2020) reported temperature peak at ~530 °C and minimum peak pressures of ~1.4 GPa from the Mandritsa Group, suggesting that at least this part of the unit underwent subduction-related metamorphism. According to these authors, of low-grade metamorphic degree is only the Maglenic Group (temperature peak at ~285 °C).…”