The Balkan terranes: a missing link between the eastern and western segments of the Avalonian-Cadomian orogenic belt?
Abstract
The Alpine-Himalayan collision zone involves a number of crustal fragments that originated in the Neoproterozoic to Cambrian Avalonian-Cadomian belt of northern Gondwana. We use the detrital zircon U-Pb geochronology to examine four of these lithotectonic units, now exposed in the Balkans in Bulgaria and Serbia.
The obtained age spectra suggest that the Diabase-Phyllitoid Complex (the maximum depositional age, MDA, estimated at 540 + 5/-9 Ma) was presumably an accretionary wedge or a forearc basin sourced from a nearby volcanic arc, however, its palaeo-position remains uncertain. The Vlasina Complex (MDA of 577 + 5/-6 Ma) was the most 'westerly' terrane, adjacent to the Trans-Saharan belt, whereas the Sredna Gora and Stara Planina complexes (MDAs of 546 +/- 7 Ma and 579 + 4/-5, respectively) were positioned next to the Saharan Metacraton and Arabian-Nubian shield.
To put the Balkan terranes into a broad context, we statistically compare the detrital zircon ages in other terranes from the Eastern Alps to Iran with igneous and metamorphic U-Pb zircon ages from North African source areas. The statistical comparison is done through multi-dimensional scaling (MDS), a more rigorous method than a visual comparison of age spectra, to examine the degree of inter-sample similarity.
This information is then transferred to a tentative palaeogeographic map showing position of each terrane with respect to its most likely source region. As a result, we define a 'westerly' terrane assemblage, characterized by Mesoproterozoic ages and sourced from the West African craton and the Trans-Saharan belt and an 'easterly' assemblage formed next to the Saharan Metacraton and the Arabian-Nubian shield.
The present-day position of some of these terranes implies significant dextral strike-slip displacement, perhaps due to movement on the Pangea megashear during the Carboniferous and Permian.
