More evidence on Neoproterozoic terranes in Southern Poland and southeastern Romania

Abstract

New geological, geochemical and U-Pb SHRIMP zircon age data brought more information about basement units in subsurface of Southern Poland and SE Romania, which allows to revise and refine some earlier models in the framework of the break-up of the Rodinia/Pannotia supercontinent. In the Brno Block, Moravia, and in the Upper Silesia Block, three different terranes formed the composite Brunovistulia Terrane. The Thaya Terrane (low eNd(T)) of Gondwana (Amazonia) descent collided obliquely at 640-620 Ma with the Slavkov Terrane (moderate eNd(T)) composed of amphibolite facies metasediments and arc-related, mostly unfoliated granitoids which intruded at 580-560 Ma. At that time, back-arc rifting separated the couple Thaya-Slavkov (inherited zircons: 1.01-1.2, 1.4-1.5, 1.65-1.8 Ga) that drifted away from Gondwana until collision around 560-550 Ma with the Rzeszotary Terrane, the Palaeoproterozoic (2.7-2.0 Ga) crustal sliver derived from Amazonia or West Africa. At least these three units composed Brunovistulia, which occurred at low latitudes in proximity to Baltica as shown by palaeomagnetic and palaeobiogeographic data. Then Brunovistulia was accreted to the thinned passive margin of Baltica around its Małopolska promontory/proximal terrane. A complex foreland flysch basin developed in front of the Slavkov-Rzeszotary suture and across the Rzeszotary-Baltica/Małopolska border. The further from the suture the less amount of the 640-550 Ma detrital zircons extracted from the Thaya-Slavkov hinterland and the smaller eNd(T) values. In West Małopolska, the flysch contains mainly Neoproterozoic zircons (720-550 Ma), whereas in East Małopolska 1.8-2.1 Ga and 2.5 Ga zircons dominate, which resembles nearby Baltica. The basin infill was multiphase folded and sheared; in Upper Silesia prior to deposition of the pre-Holmia Cambrian overstep. In Małopolska, the folded flysch series formed a large-scale antiformal stack with thermal anticline in its core marked by low-grade metamorphic overprint. In Central Dobrogea, Moesia, Ediacaran flysch also contains mainly 700-575 Ma detrital zircons which link the source area, likely in South Dobrogea with ca. 560 Ma granitoids, rather close with Gondwana. However, fauna in Lower Cambrian overstep strata shows Baltican affinity. Such features resemble Upper Silesia, thus Brunovistulia might have extended beneath the Carpathians down to Moesia. The other part of South Dobrogea with Palaeoproterozoic ironstones resembles Ukrainian banded iron formation. If true, the Baltican sliver would be incorporated in Moesia. Such a possibility concurs with the provenance data from Ediacaran flysch of Central Dobrogea, which points to uplifted continental block as a source of detrital material. Our study supports an earlier proposition that at the end of the Neoproterozoic a group of small terranes that included Brunovistulia, Moesia and Małopolska formed the Teisseyre-Tornquist Terrane Assemblage (TTA). In our model, a characteristic feature of the TTA was a mixture of crustal elements that were derived from both Gondwana and Baltica, which gave rise to mutual collisions of the elements prior to and concurrent with the docking to Baltica in latest Ediacaran times. The presence of extensive younger covers and complex Phanerozoic evolution of individual members of the TTA impede the recognition of their Neoproterozoic history.