Silke Jahn-Awe

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

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.

Reply to: Bonev N, Spikings R and Marchev P (2016) Comment on Georgiev et al. “Structure and U–Pb zircon geochronology of an Alpine nappe stack telescoped by extensional detachment faulting (Kulidzhik area, Eastern Rhodopes, Bulgaria)”

Of the four superposed tectonic units in the study area, the uppermost (Unit IV) is lithologically similar to the lowermost (Unit I). Both are formed dominantly by orthogneiss. In an earlier article, Bonev et al. (2010) had shown that the orthogneisses of units I and IV display similar trace element patterns and therefore interpreted Unit IV as an equivalent of Unit I emplaced by an overthrust on top of the other units. Our new U–Pb zircon dating, however, showed that the orthogneiss of Unit I was formed from a Late Carboniferous protolith, and the orthogneiss of Unit IV from a Neoproterozoic protolith. These rocks are of different ages, and their lithological and geochemical similarity is a mere coincidence. In their comment, Bonev et al. (2016) again demonstrate the geochemical similarity (their Fig. 1) and write that this “allows some caution to be expressed relative to the age of 581 Ma” determined by us. We do not share this opinion. On the contrary, we think that more caution would be necessary in interpreting geochemical data. To illustrate this, we show in Fig. 1 the compositions of the three orthogneisses that Bonev et al. (2016) show in their Fig. 1b (in order to demonstrate their similarity), together with the average composition of the upper continental crust (thick blue line) after Wedepohl (1995) and the composition of JG-1 (thick red line), a common granodiorite standard from Japan after Ando et al. (1971). The similarity between the patterns of the three Bulgarian orthogneisses is not larger than the similarity between any of them and the granodiorite from Japan, and all of them are similar to the average composition of the upper continental crust. These concentrations reflect fundamental differentiation of our planet and are not capable of fingerprinting a certain origin. Introduction