The anatomy of a Gondwanan terrane: The Neoproterozoic–Ordovician basement of the pre-Alpine Sebeş–Lotru composite terrane (South Carpathians, Romania)

Abstract

The Sebeş–Lotru terrane in the South Carpathians mountain range comprises a lower, Neoproterozoic metamorphic unit (Lotru) and an upper, Ordovician metamorphic unit (Cumpăna) that were juxtaposed during the Variscan orogeny. Two orthogneisses from the Lotru metamorphic unit yield U/Pb LA-ICP-MS zircon crystallization ages of 549.3 ± 3.8 Ma and 587.5 ± 3 Ma, respectively. Two orthogneisses from the Cumpăna metamorphic unit yield zircon crystallization ages of 458.9 ± 3.5 Ma and 466.0 ± 4.2 Ma, respectively. High U zircons from two other orthogneisses from the Cumpăna metamorphic unit have ages ranging from 400 Ma to 320 Ma, which are interpreted to reflect protracted zircon recrystallization during the regionally significant Variscan collisional event. Detrital zircons from a metasedimentary gneiss in the Cumpăna metamorphic unit have ages ranging from ~ 0.5 Ga to 2.8 Ga. The 0.5 Ga age constrains the maximum sediment deposition age to be late Cambrian. The source most compatible with the range of Precambrian detrital ages in the Sebeş–Lotru terrane is northeastern Gondwana. The Sebeş–Lotru terrane was part of a continental subduction/collision system as a lower plate after about 400 Ma and reached peak metamorphic conditions between 350 and 320 Ma (e.g. Medaris et al., 2003). A cross-cutting granite vein has a zircon U–Pb crystallization age of 321.5 ± 3.1 Ma, which constrains minimum age of ductile deformation during the Variscan collision in this region. The trace of the Rheic suture within the South Carpathians is located between the Ordovician upper part of the Sebeş–Lotru terrane and the Drăgşan pre-Alpine terrane of the Danubian domain.