Evolution of the Sibiel Shear Zone (South Carpathians): A study of its type locality near Rinari (Romania) and tectonic implications

摘要

The Sibiel Shear Zone is a 1-3km wide, ductile shear zone located in the South Carpathian Mountains, Romania. In the Rsinari area, the ductile shear zone juxtaposes amphibolite facies rocks of the Lotru Metamorphic Suite against greenschist facies rocks of the Rauorul Cisndioarei Formation. The first represents the eroded remnants of Peri-Gondwanan arcs formed between the Neoproterozoic-Silurian (650-430Ma), regionally metamorphosed to amphibolite facies during the Variscan orogeny (350-320Ma). The second is composed of metasedimentary and metavolcanic Neoproterozoic-Ordovician (700-497Ma) assemblages of mafic to intermediate bulk composition also resembling an island arc metamorphosed during the Ordovician (prior to similar to 463Ma). Between these lie the epidote amphibolite facies mylonitic and ultramylonitic rocks of the Sibisel Formation, a tectonic melange dominated by mafic actinolite schists attenuated into a high strain ductile shear zone. Mineral Rb-Sr isochrons document the time of juxtaposition of the three domains during the Permian to Early Triassic (similar to 290-240Ma). Ductile shear sense indicators suggest a right lateral transpressive mechanism of juxtaposition; the Sibiel shear zone is a remnant Permo-Triassic suture between two Early Paleozoic Gondwanan terranes. A zircon and apatite U-Th/He age transect across the shear zone yields Alpine ages (54-90Ma apatite and 98-122Ma zircon); these data demonstrate that the exposed rocks were not subjected to Alpine ductile deformation. Our results have significant implications for the assembly of Gondwanan terranes and their docking to Baltica during Pangea's formation. Arc terranes free of Variscan metamorphism existed until the Early Triassic, emphasizing the complex tectonics of terrane amalgamation during the closure of Paleotethys.