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Palaeozoic Formations from Dobrogea and Pre-Dobrogea – An Overview

2012; Linguagem: Inglês

10.3906/yer-1101-20

1303-619X

Antoneta Seghedi,

Geochemistry and Elemental Analysis

An overview of lithological, palaeontological and geochronological evidence existing for the Palaeozoic formations from Dobrogea and Pre-Dobrogea has enabled a better understanding of the Palaeozoic history of these areas.Th e Lower Palaeozoic of Pre-Dobrogea, in places in continuity with the pelitic-silty facies of the underlying Vendian (Ediacaran) deposits, was one of the peri-Tornquist basins of Baltica, suggesting that the Scythian Platform in the Pre-Dobrogea basement represents the rift ed margin of the East European Craton.In North Dobrogea two types of Palaeozoic succession have formed in diff erent tectonic settings.Deep marine Ordovician-Devonian deposits, including pelagic cherts and shales, associated with turbidites, and facing Devonian carbonate platform deposits of the East European Craton, form northward-younging tectonic units of an accretionary wedge, tectonically accreted above a south-dipping subduction zone.South of the accretionary prism, the basinal to shallow marine Silurian-Devonian deposits of North Dobrogea, showing a similar lithology to the East Moesian successions, accumulated on top of lowgrade Cambrian clastics with Avalonian affi nity indicated by detrital zircons.Late Palaeozoic erosion was accompanied by deposition of continental alluvial, fl uvial and volcano-sedimentary successions, overlying their basement above an imprecise Carboniferous gap.Th e low-grade metamorphic Boclugea terrane, showing Avalonian affi nity, and the associated Lower Palaeozoic deposits represent East Moesian successions, docked to Baltica by the Lower Devonian and subsequently involved in the Hercynian orogeny, being aff ected by Late Carboniferous-Early Permian regional metamorphism and granite intrusion.Th e Late Carboniferous-Early Permian syn-tectonic sedimentation, regional metamorphism of Palaeozoic formations and development of a calc-alkaline volcano-plutonic arc indicate an active plate margin setting and an upper plate position of the Măcin-type successions during the Variscan collision, when the Orliga terrane, with Cadomian affi nity, was accreted to Laurussia along a north-dipping subduction zone of the Rheic Ocean.Th e East Moesian Lower Palaeozoic succession, overstepping its Ediacaran basement, represents an Avalonian terrane, docked to the Baltica margin in the Early Palaeozoic.A narrow terrane detached from the Trans-European Suture Zone (TESZ) margin of the Baltica palaeocontinent forms a tectonic wedge within the East Moesian basement.Th e Palaeozoic sedimentary record of East Moesia shows a quartzitic facies in the Ordovician, graptolite shales in Upper Ordovician-Wenlock, black argillites in the Ludlow-Pridoli and fi ne-grained clastics in the Lower Devonian.Eifelian continental sandstones are followed by a carbonate platform from Givetian to Tournaisian times and coalbearing clastics in the Carboniferous, indicating a foredeep basin evolution.By the Eifelian both East Moesia and Pre-Dobrogea were part of Laurussia, sharing the same old red sandstone facies.Th e Permian is a time of rift ing in Dobrogea and Pre-Dobrogea, although evidence for rift ing in the East Moesian sedimentary record is very limited.In the eastern basins of Pre-Dobrogea, Permian rift ing was accompanied by alkaline bimodal volcanism of the basalttrachyte association, that aff ected also the northern margin of North Dobrogea.Late Permian within-plate alkaline magmatic activity emplaced plutonic and hypabyssal complexes along the south-western margin of North Dobrogea.Th e model proposed for the Palaeozoic history based on existing data for the north-western margin of the Black Sea records early Palaeozoic docking to Baltica of the Avalonian terrane of East Moesia, including the Boclugea terrane of North Dobrogea.Late Carboniferous-Early Permian accretion of the Cadomian Orliga terrane from North Dobrogea, accompanied by Hercynian metamorphism and granite intrusion, correlates with the closure of the Rheic Ocean.Subsequently, Avalonian and Cadomian terranes, together with a narrow terrane detached from the TESZ margin of Baltica palaeocontinent, were displaced southward along the strike-slip fault system of the TESZ.