O modelu ewolucji północno-wschodniej części Masywu Czeskiego - uwagi petrologa

Wojciech Narębski




This paper deals with petrological comments on recent papers of some Polish and foreign authors (26 - 29, 8 - 12, 19, 30), discussing the problem of paleotectonic evolution of NE part of the Bohemian Masif, called by the present author Lugosudeticum. This tectonostratigraphic unit, also distinguished by Czech geologist as Lugicum (5, 8, 38, 42, et al.), is separated from Bohemicum and Saxothuringicum by Upper Odra (Torgau-Olomouc) fault zone (Fig.). In the present author's opinion, true ophiolites occur, first of all, around the Góry Sowie Mts Block whilst ultrabasic and basic rock bodies penetrating suprasubduction zones of Rudawy Janowickie - Rychory – Żelezny Brod arc and Stare Mešto unit represent small Alaskan-type intrusions. This conclusion, apart from their mode of occurrence, is based on specific mineral composition of these rocks (eg. primary hornblende, iron-enriched olivine - Fo60-80 and highly calcic plagioclase). The author is in favour of terrane interpretation for the evolution of this part of the Bohemian Massif. However, he disagrees with the neglection of recent tectonostratigraphic division proposed by most competent Czech geologists (5, 18, 38, 42) and the adoption of those proposals presented by some other European geologists (19, 30), whose geological knowledge of this very complicated part of European Variscides is based on rather short visits in this area. Consequently, the models constructed on the basis of their conceptions are evidently incorrect. So, for example, Z. Cymerman (11), following Matte et al. (19), is enlarging the range of Moldanubicum far to the north, up to the Upper Odra fault zone, where as J. Oberc (26) is uniting in one block rock complexes (Izera and Żelezne Mts), belonging to distinctly different tectonostratigraphic units – namely Lugosudeticum and Bohemicum. Besides, it is not reasonable to consider the Rudawy Janowickie - Żelezny Brod volcanic arc sutured to the Lusatian - Izera block (terrane) as a separate terrane (30). Petrological data (24) indicate the existence of Lowerand Mid-Paleoozoic extensional periods in this area, recorded by rifting volcanics of the Kaczawskie Mts and Kłodzko Metamorphic unit. The latter was followed by spreading, oceanization and emplacement of ophiolitic bodies. Moreover, recent geochemical data on paleotectonic position of Variscan granitoids within the whole Alpine belt strongly suggest symmetric character of the Variscan fold belt, manifested by the occurrence of Saxothuringian-type basin on southern side of axial Moldanubian zone - from S Massif Central in France, under the Alpine nappes to the Bardo Mts in Poland (15, 32). Very important are recent radiometric U-Pb data on zircons for various meta-igneous rocks in Lower Silesia reported by Oliver et al. (30) but some paleotectonic conclusions of this authors are very controversial. This refers e. g. to the proposed Lower Paleozoic Sudetic batolith terrane based only on similar ages of protoliths of granite-gneisses occurring in different blocks of Lugosudeticum, separated in this period by dominating extensive tectonics. Moreover, it is proposed that the discordance of radiometric Sm-Nd data (35) and U-Pb zircon ages for Ślęża ophiolitic gabbro (30) should be checked before presenting more general conclusions. As follows from the above discussion, proper interpretation of geochemical, geochronological and petrological data in terms of paleotectonic setting of magmatic rock series is of fundamental importance for elaborating an adequate model of evolution of very complicated NE part of the Bohemian Massif.