New insights on the occurrence of a Variscan suture in the Upper Danubian Nappe (Romania, Serbia, Bulgaria) evidenced by ⁴⁰Ar–³⁹Ar geochronology

The Alpine Upper Danubian Nappe crops out in the Southern Carpathians in Romania and in the North of the Balkans in Serbia and Bulgaria. The pre-alpine basement of this Nappe displays an important tectonic marker in the form of the Danubian Ophiolite. This Ophiolite (~ 500 km²) has been dismembered by the Oligocene Alpine tectonic in four ophiolitic massifs: Tisovita Iuti in Romania, Deli Jovan and Zaglavak in Serbia and Tcherni Vrah in Bulgaria. Although the Danubian Ophiolite has been considered as Late Proterozoic during the last decades, the Deli Jovan massif has been recently dated to the Lower Devonian (U-Pb zircon age of 405 ± 2.6 Ma). Our study gives a petrostructural analysis investigated on listvenitic gabbros from the Tisovita Iuti ophiolitic massif and their adjacent rocks (Corbu Unit) and provides new geochronological data on the listvenitization processes.
   Listvenitic gabbros consist of a metasomatic assemblage composed of zoisite + calcite + Cr-chlorite + Cr-muscovite developped on amphibolitized gabbros under strongly hydrated conditions at temperatures around 280°C. They are located in a thin N-S band at the Eastern part of the Tisovita Iuti ophiolitic massif and generally display a mylonitic texture. Similarly, the Corbu Unit crops out in a 2 km wide N-S band, at the eastern part of the listvenitic gabbros, and consists of a mélange of slices composed of various metamorphic rocks (volcano-sedimentary rocks, margin sediments, acid volcanic rocks and ophiolitic rocks) from low- to high-grade.
   Structural data obtained on listvenitic gabbros and on Corbu rocks indicate that these two units have undergone a similar deformation, which is partitioned between highly deformed mylonites and slightly-deformed juxtaposed rocks. These rocks display high-dip North-Southward foliation planes, associated with a strong subhorizontal stretching mineral lineation and isoclinal folds parallel to this lineation. These fabric elements appear to develop during the same event, giving evidence for components of both simple and pure shear, suggesting a deformation which probably occurred in a transpressive context.
   New ⁴⁰Ar-³⁹Ar ages performed on two Cr-muscovites from slightly and highly listvenitized gabbros give plateau ages of 372.6 ± 1.3 Ma and 360.6 ± 1.2 Ma, respectively. As the listvenitization process involves important chemical remobilisations with the presence of CO₂-rich fluids and according to the similar deformation type encountered both in the Corbu mélange and in the listvenitic gabbros, we propose that these rocks are formed close to an accretionary wedge and probably correspond to the obduction sole of the Danubian ophiolite during a Variscan oblique collision.
   These results imply the occurrence of a Variscan oceanic suture in the Eastern part of the Variscan Belt, classically ending in the Sudetes Area (Poland). Moreover, the Late Devonian closure of the Danubian oceanic domain is very similar to data observed for the evolution of the Rheic Ocean and its associated basins.