OCCURRENCE OF A MYLONITIC ZONE DEVELOPED on OCEANIC AND SEDIMENTARY ROCKS (ALMAJ MOUNTAINS, ROMANIA): RECORD OF THE OBDUCTION OF THE DANUBIAN OPHIOLITE?

Regional context

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 consists of four dismembered ophiolitic massifs: Tisovita Iuti in Romania, Deli Jovan and Zaglavak in Serbia and Tcherni Vrah in Bulgaria. Together, these massifs (~ 500 km²) display all the components of a classical ophiolitic assemblage, from serpentinized harzburgites to pillow lavas. They probably represent the relics of a unique oceanic domain presently dismembered along 160 km by Oligocene Alpine tectonics. Although the Danubian Ophiolite has been considered as Late Proterozoic during the last decades (Haydoutov 1989), the Deli Jovan massif has been recently dated to the Lower Devonian (U-Pb zircon age of 405 ± 2.6 Ma; Zakariadze et al. 2006).

Petrology of the mylonitic zone

Our study concerns a mylonitic zone developed in the Eastern part of the ophiolitic massifs on oceanic- and sedimentary- derived rocks. This zone is well exposed in Romania where it has been investigated in details. In this region, it consists of a narrow N-S band (< 5km in width) cropping out at the East of the Tisovita Iuti ophiolitic massif and extending Northwards along the Carboniferous Cherbelezu granite. It includes three units of different natures: (1) oceanic-derived rocks, (2) metasedimentary rocks and (3) high-grade metamorphic rocks.

Oceanic-derived rocks consist mainly of deformed listvenitic gabbros. The listvenitization process in this area has already been described by Plissart et al. (2009) as the metasomatic transformation of amphibolitic gabbros by CO₂-rich seawater-derived fluids at temperatures around 280°C. At the final stage, this process generates the unusual assemblage of zoisite + calcite + (Cr)-clinochlore + (Cr)-muscovite. Interestingly, this mineralogical transformation is in most cases associated with a strong deformation producing a gneissic or a schistose texture.

Directly at the East of the listvenitic gabbros are found the metasedimentary rocks of the Corbu Unit and the high-grade metamorphic rocks. Nowadays, these two units appear tectonically imbricated but their origin is interpreted differently. The Corbu rocks consist of deformed and slightly metamorphosed margin and volcano- sediments, as various as pelites, greywackes, limestones, sandstones, dacitic tuffs and andesitic tuffs. In a different way, high-grade metamorphic rocks are believed to come from Mn-rich oceanic sediments and commonly display an assemblage of garnet + staurolite ± andalousite + muscovite + biotite + chlorite + plagioclase + quartz.

Structural analysis

Structural observations and measurements performed on these three groups of rocks indicate that the entire mylonitic zone has undergone a similar deformation which is partitioned between highly deformed mylonites and slightly-deformed juxtaposed rocks. These rocks display high-dip North-Southwards 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 and suggesting a deformation which probably occurred in a transpressive context.

Geochronology

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. These ages correspond to the intense metasomatic event affecting amphibolitic gabbros and generating listvenites in localized zones. As the deformation observed in these rocks seems contemporaneous to their metasomatism, it is likely that an important thrusting occurred during Late Devonian times, when the oceanic lithosphere was still in contact with seawater necessary to promote such large chemical remobilisations.

Interpretations & conclusions

Listvenitization of gabbroic rocks is a very uncommon process which certainly requires a specific geodynamic context. According to the transformation/deformation intercorrelations observed in these rocks and to the age obtained for this event, we propose that the listvenitic rocks were formed in the first stages of the obduction of the Danubian ophiolite during a Variscan oblique collision.

As the deformation type encountered in the metasedimentary rocks of the Corbu Unit is similar to the one observed in the listvenitic gabbros, the scenario envisaged is the continuation of the transpressive obduction-related thrusting onto the margin sediments, whereas high-grade metamorphic rocks could correspond to the metamorphic sole developed previously on oceanic sediments and subsequently incorporated as slices in this mélange during the 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 (Nance et al. 2010).