Age and composition of meta‐ophiolite from the Rhodope Middle Allochthon (Satovcha, Bulgaria): A test for the maximum‐allochthony hypothesis of the Hellenides

Abstract

The metamorphosed thrust stack of the Rhodopes comprises a level with ophiolites (Middle Allochthon) underlain and overlain by continent-derived allochthons. The Upper Allochthon represents the European margin, but the origin of the Lower Allochthon remains controversial, with suggestions that it may be derived from an inferred microcontinent (Drama) or from the margin of Adria. Trace element compositions and Sr and Nd isotope ratios of metagabbroic amphibolites and enclosed meta-plagiogranites from the Satovcha Ophiolite, Middle Allochthon, show that they are cogenetic and represent suprasubduction zone ophiolites. U-Pb dating using laser ablation sector field inductively coupled plasma mass spectrometry of zircons from two meta-plagiogranites and a metagabbro yielded identical Jurassic ages (160 ± 1 Ma, 160.6 ± 1.8 Ma, and 160 ± 1 Ma, respectively), similar to ophiolites in the eastern Vardar Zone bordering the Rhodopes to the SW. The trace element patterns also closely resemble those of the Vardar ophiolites. The association with Late Jurassic arc-type granitoids is another feature that applies both to eastern Vardar and Satovcha. This strongly suggests that the Middle Allochthon comprises the metamorphosed northeastward continuation of the Vardar Zone. The Jurassic age of the Satovcha Ophiolite contradicts the hypothesis of Early Jurassic suturing between Europe (Upper Allochthon) and the assumed Drama microcontinent (Lower Allochthon) but is in line with the "maximum allochthony hypothesis," i.e., the assumption that the Lower Allochthon represents Adria and that the "root" of the Vardar-derived thrust sheets is at the NE boundary of the Rhodopes.

Polycyclic alpine orogeny in the Rhodope metamorphic complex: the record in migmatites from the Nestos shear zone (N. Greece)

“…In contrast, erasure remains conjectural for the units devoid of this evidence. Of them, the PU, which is situated at a lower level of the structural pile, might have been engaged in the orogen only lately, during the Cenozoic (Argyriadis and Fourquin, 1987;Dinter, 1998;Krohe and Mposkos, 2002;Jahn-Awe et al, 2010;Nagel et al, 2011;Froitzheim et al, 2014). Thus, the key issue is the tectonic setting under which the high-temperature conditions were produced during the Cenozoic, and whether renewed thrusting occurred in the RMC at that time.…”

“…For some authors, based on the interpretation of geochronological data obtained on retrogressed eclogites, a subduction setting existed as late as ∼42-51 Ma (Liati and Gebauer, 1999;Liati, 2005;Liati et al, 2011;Nagel et al, 2011;Kirchenbaur et al, 2012;Froitzheim et al, 2014) (see Sect. 2.1).…”

“…Following Burg (2011), mantle delamination may have triggered crustal-scale "core complex"-type extension and, importantly, may explain the development of a major tectonometamorphic event during the Cenozoic, long after the midMesozoic orogeny, without the requirement of a new orogenic period (see also Kydonakis et al, 2016). Alternatively, mantle delamination may have occurred in the immediate aftermath of an early Cenozoic orogeny (Nagel et al, 2011;Froitzheim et al, 2014).…”

“…Since the late 1990s, numerous scenarios have been proposed for the tectonic evolution of the RMC during Alpine times. A primary difference between these scenarios is whether there has been a single protracted orogenic cycle since around the midMesozoic (Ricou et al, 1998;Barr et al, 1999;Papanikolaou and Kranis, 2004;Turpaud and Reischmann, 2010;Krenn et al, 2010;Burg, 2011;Kydonakis et al, 2016) or whether Alpine orogeny involved distinct episodes of subduction and crustal accretion (Liati, 2005;Mposkos et al, 2010;Liati et al, 2011Liati et al, , 2016Nagel et al, 2011;Kirchenbaur et al, 2012;Froitzheim et al, 2014;Wawrzenitz et al, 2015). The difference between the two views mainly stems from different interpretations of the available geochronological data (e.g., Burg, 2011;Liati et al, 2011).…”

“…As outlined above, another debated issue is whether Alpine orogeny consisted of a single protracted cycle (e.g., Burg, 2011) or involved distinct episodes of subduction and crustal accretion (e.g., Liati et al, 2011;Froitzheim et al, 2014). With regard to this question, two aspects of the Mesozoic-Cenozoic geochronological record deserve special attention.…”