Neoproterozoic magmatism and Carboniferous high-grade metamorphism in the Sredna Gora Zone, Bulgaria: An extension of the Gondwana-derived Avalonian-Cadomian belt?

Numerous basement massifs within the Alpine-Carpathian Orogen record the Neoproterozoic to Paleozoic development of the southern margin of Europe. This area is critical for adequate understanding of the interactions between the northern (i.e., Laurentia and Baltica) and southern (i.e., Gondwana) continents during this time, yet it is poorly known. We dated zircon by HR-SIMS from samples of basement rocks of the Sredna Gora Zone in central Bulgaria in order to shed light on the pre-Alpine tectonic development of this region. Analyses of magmatically zoned zircons from orthogneiss samples yield ages of 616.9 ± 9.5 and 595 ± 23 Ma, indicating Neoproterozoic emplacement ages for these units. These ages are very similar to the Neoproterozoic rocks that form the basement of Gondwana-derived terranes across North America and Europe. The position of the Sredna Gora Zone outboard of other Gondwana-derived terranes, along with the Late Neoproterozoic igneous emplacement age, lead us to suggest that the Sredna Gora Zone represents a continuation of the Gondwana-derived terranes that are presently positioned along the eastern end of Laurentia and the southwestern margin of Baltica. Zircons from a leucosome enclosed in migmatitic paragneisses contain ubiquitous, high-U, homogeneous metamorphic rims that yield an age of 336.5 ± 5.4 Ma. This age of metamorphism is indistinguishable from other published ages of Variscan metamorphism across Central and Western Europe. Cores of zircon grains in the leucosome have variable interior zoning patterns, and are interpreted as original detrital material in the parent sedimentary rock. HR-SIMS dating of these detrital cores yields several age populations at ∼500-700, 900-1100, 1900-2200, 2350, 2550, and 2700 Ma. The depositional age of the sedimentary rocks that were the parent to the migmatitic paragneisses is constrained between the Ordovician (youngest detrital age ∼500 Ma) and the Early Carboniferous (high-grade metamorphism at 336.5 ± 5.4 Ma). Collectively, these data suggest that both Gondwana-derived terranes and the Variscan Orogeny extend much farther eastward in southern Europe than has been previously recognized. © 2006 Elsevier B.V. All rights reserved.