Transition from island-arc to passive setting on the continental margin of Gondwana: U-Pb zircon dating of Neoproterozoic metaconglomerates from the SE margin of the Tepla-Barrandian Unit, Bohemian Massif

Sedimentological, petrological and geochemical data combined with SHRIMP and LA ICP-MS U-Pb ages of pebbles of granitic rocks and detrital zircons from Neoproterozoic Vletice metaconglomerates of the SE margin of the TeplA-Barrandian Unit were used to characterize the regional geotectonic setting at the Proterozoic/Paleozoic boundary. The metaconglomerates form several-metres thick intercalations in metasediments of the Svrchnice Formation (roof pendants of Central Bohemian Plutonic Complex), which is considered to have been deposited in Neoproterozoic/Cambrian times. A sedimentological study indicates that the metaconglomerates are products of debris flows deposited in submarine canyons or proximal parts of submarine fans. Geochemistry of metaconglomerate matrix and petrography of pebbles revealed an obvious difference from the western and central part of the TeplA-Barrandian Unit Proterozoic where pebbles eroded from uplifted parts of the basin and island arc sources are most common. The abundance of quartz, quartzite, granite and crystalline limestone in pebbles together with the more mature composition of host metagreywackes indicate the presence of relatively evolved continental crust in the source area for the Vletice metaconglomerates, and shallowing of the Neoproterozoic basin to the SE (in the present geographic co-ordinate system). The Neoproterozoic to lowermost Cambrian age of the Svrchnice Formation, suggested by its comparison with similar rocks in the central part of the Tepla-Barrandian Unit, was confirmed by U-Pb dating. Ages of detrital zircons from the metaconglomerate matrix cluster around two values: Archaean to Paleoproterozoic U/Pb ages between ca. 2580 and 1780 Ma and Neoproterozoic to early Cambrian ages between 660 and 520 Ma with a significant peaks at 600 and 544 Ma suggesting local volcanic activity extending shortly beyond the Proterozoic/Paleozoic boundary. Zircons separated from two trondhjemite pebbles from the metaconglomerate yielded a mean U-Pb age of 610 +/- 17 Ma arguing for active arc magmatism during the latest Neoproterozoic. The overall age distribution of detrital zircons and the absence of Grenvillian zircon ages favour a location of the TepIA-Barrandian Unit close to the West African craton at the Neoproterozoic/Paleozoic boundary. The rapid onset of a transtensional geotectonic regime and maturing of sediments is evidenced by the overlying Cambrian shallow-water clastics alternating with predominantly felsic calcalkaline to alkaline volcanics. During the Cambrian oblique plate convergence, the active continental margin was transformed into belts of horsts and narrow pull-apart or small rift basins. After the Ordovician break-off of Armorican Terrane Assemblage blocks from Gondwana, the extensional regime in the Tepla-Barrandian Unit continued until the beginning of Variscan orogeny in the Middle Devonian. (C) 2008 Elsevier B.V. All rights reserved.