Published: Oct 2003
Pages: 299 - 315
Abstract: Core samples of metamorphic basement rocks from the Great Hungarian Plain (Tisza Megaunit) were studied by petrographic and geochronological methods (Ar-Ar, Rb-Sr, Sm-Nd). On the basis of microtextural features of Al2SiO5 polymorphs in metapelites a regional distribution pattern was found, which correlates with geochronological age groups. This pattern confirms the earlier established tectonic subdivision of the basement of the Great Hungarian Plain, except for the Algyő basement-high, which has to be considered to represent a separate unit. A muscovite concentrate from a granite sample of the Mecsek Subunit yielded a Variscan Ar-Ar age of 299 Ma. The medium-grade metapelites of the Villany-Bihar Subunit are characterized by kyanite porphyroblast and sillimanite. Typical Ar-Ar muscovite ages are ca. 310 Ma and prove a Variscan cooling age of the metamorphic assemblages. In the NE part of the unit ages in the range of 202–266 Ma indicate a later thermal overprint. The staurolite and andalusite-bearing gneisses of the Bekes-Codru Subunit yielded Variscan cooling ages of ca. 320 Ma. In contrast, the rocks from the Algyő basement-high experienced their first metamorphic imprint in Early Permian time. Based on a Sm-Nd garnet isochron the high-temperature/low-pressure assemblages, including andalusite+K-feldspar±sillimanite formed ca. 275 Ma. An amphibolite facies, eo-Alpine (Cretaceous) overprint in the stability field of kyanite+staurolite+garnet is proved by Ar-Ar muscovite ages in the range of 82–95 Ma. It was followed locally by an Early Tertiary deformation. Considering the lithologies and the metamorphic and structural evolution, the Algyő-high shows many similarities to the Saualm-Koralm Complex of the Austroalpine unit. Like the latter and the Baia de Aries nappe complex in the Apuseni Mountains, it obviously represents an eo-Alpine thrust sheet.
Keywords: Tisza Megaunit, Hungary, basement, Variscan, Permian and eo-Alpine metamorphism, Ar-Ar, Rb-Sr and Sm-Nd geochronologyDownload PDF document