In search of the Rheic suture: detrital zircon geochronology of Neoproterozoic to Lower Paleozoic metasedimentary units in the Balkan fold-and-thrust belt in Bulgaria

The Rheic Ocean opened between Avalonia and Gondwana in the late Cambrian to early Ordovician, triggering break-up of other terranes from the former Avalonian–Cadomian belt. In turn, the Gondwana-derived terranes recorded contrasting drift histories and were variably reworked within the younger orogens, tracing the Rheic suture thus remains challenging. In this paper, we attempt to locate the enigmatic easterly trace of the suture along the boundary of the Avalonian-type Moesia microplate attached to the southeastern margin of Baltica and the Balkan fold-and-thrust belt in Bulgaria, which contains Neoproterozoic to Early Paleozoic basement slivers of unknown provenance. We present new U–Pb detrital zircon ages from several pre-Darriwillian low-grade units (Lakavitsa, Berkovitsa, Bilo, Diabase–Phyllitoid Complex) but also from the overlying Darriwillian Grohoten and Famennian–Frasnian Katina formations. The age spectra and the maximum depositional ages of the pre-Darriwillian units suggest deposition in an accretionary wedge/forearc basin setting, or in an incipient rift superposed on the Cadomian forearc/arc region, from the latest Ediacaran to late Cambrian. The basins were sourced from a magmatic arc with a limited input of cratonic detritus, presumably adjacent to the Trans-Saharan belt or Saharan metacraton. In contrast, samples from the overlying formations indicate a sustained delivery of the cratonic detritus, perhaps first into passive-margin basins of the ‘Gondwana super-fan’ system and then into a Variscan foreland basin. Our statistical analysis shows that the sampled units exhibit age spectra compatible with the Cadomian terranes and that they differ significantly from those reported from Moesia. The detrital zircon ages thus possibly define the Rheic suture and also suggest that the easterly Cadomian terranes may have remained attached to Gondwana during the Early Paleozoic. It cannot be excluded, however, that the Rheic suture even did not reach that far east and terminated at an intraoceanic transform plate boundary.