Pre-Variscan geological events in the Austrian part of the Bohemian Massif deduced from U-Pb zircon ages

Abstract

In an attempt to elucidate the pre-Variscan evolution history of the various geological units in the Austrian part of the Bohemian Massif, we have analysed zircons from 12 rocks (mainly orthogneisses) by means of SHRIMP, conventional multi-grain and single-grain U-Pb isotope-dilution/mass-spectrometry. Two of the orthogneisses studied represent Cadomian metagranitoids that formed at ca. 610 Ma (Spitz gneiss) and ca. 580 Ma(Bittesch gneiss). A metagranite from the Thaya batholith also gave a Cadomian zircon age (567-5 Ma). Traces of Neoproterozoic zircon growth were also identified in several other samples, underlining the great importance ofthe Cadomian orogeny for the evolution of crust in the southern Bohemian Massif. However, important magmatic events also occurred in the Early Palaeozoic. A sample of the Gfhl gneiss was recognised as a488-6 Ma-old granite. A tonalite gneiss from the realm of the South Bohemian batholith was dated at 456-3 Ma, and zircon cores in a Moldanubian metagranitic granulite gave similar ages of 440-450 Ma. This Ordovician phase of magmatism in the Moldanubian unit is tentatively interpreted as related to the rifting and drift of South Armorica from the African Gondwana margin. The oldest inherited zircons, in a migmatite from the South Bohemian batholith, yielded an age of ca. 2.6 Ga, and many zircon cores in both Moravian and Moldanubian metagranitoid rocks gave ages around 2.0 Ga. However, rocks from the Moldanubian unit show a striking lack of zircon ages between 1.8 and 1.0 Ga, reflecting an ancestry from Armorica and the North African part of Gondwana, respectively, whereas the Moravian Bittesch gneiss contains many inherited zircons with Mesoproterozoic and Early Palaeoproterozoic ages of ca. 1.2, 1.5 and 1.65?1.8 Ga, indicating a derivation from the South American part of Gondwana