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 UPb 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 of the Cadomian orogeny for the evolution of crust in the southern Bohemian Massif. However, important magmatic events also occurred in the Early Palaeozoic.

Metabasic rocks in the Varied Group of the Moldanubian Zone, southern Bohemia - their petrology, geochemical character and possible petrogenesis

“…As summarised by Janoušek et al (2004), the protoliths to Gföhl gneisses/migmatites and felsic granulites seem predominantly Ordovician to Devonian in age (c. 470-430, 400 and 360 Ma: Wendt et al 1994;Kröner et al 2000;Friedl et al 2004). The granulitic rocks of the Gföhl Unit equilibrated at high pressures, corresponding to eclogite-facies conditions (O'Brien and Rötzler 2003;O'Brien 2006O'Brien , 2008, and were assembled and tectonically emplaced to higher crustal levels at around 340 Ma (Kröner et al 2000 and references therein).…”

“…1). Moreover, displaced segments of older basement are incorporated in the structure of the MZ, including the Palaeoproterozoic Světlík orthogneiss in southern Bohemia (Wendt et al 1993;Fiala et al 1995) and the Mesoproterozoic Dobra gneiss in Lower Austria (Gebauer and Friedl 1994;Friedl et al 2004). …”

Death of super-continents and birth of oceans heralded by discrete A-type granite igneous events: the case of the Variscan-Alpine Europe

“…Non-A-type Strzelin-Nowolesie orthogneisses, SW Poland, have zircon crystals with older inherited cores, igneous rims at 600 ± 7 Ma and metamorphic overgrowths at 568 ± 7 Ma (Oberc-Dziedzic et al 2003). The latter age is reminiscent of the 567 ± 5 Ma igneous age of A-type metagranite exposed as small massifs within the c. 600-580 Ma Thaya Batholith, Austria (Friedl et al 2004). These Ediacaran ages correspond to post-orogenic processes occurring after the Cadomian orogeny.…”

Metamorphic history of skarns, origin of their protolith and implications for genetic interpretation; an example from three units of the Bohemian Massif

“…The Neoproterozoic protolith ages of 540-580 Ma are typical of granitoids and orthogneisses of the Brunovistulian (van Breemen et al 1982;Scharbert and Batík 1985;Fritz et al 1996;Friedl et al 2000Friedl et al , 2004; see Leichmann and Höck 2008 for review) as well as Saxothuringian and Lugian domains (Hegner and Kröner 2000;Tichomirowa et al 2001;Linnemann et al 2008 and references therein) and they only rarely appear as protolith ages of Moldanubian orthogneisses (Schulmann et al 2005). Importantly, Cambrian ages of 500-520 Ma are absent in the Moravo-Silesian Domain (Brunia), but they are known as the most frequent protolith ages of the Moldanubian orthogneisses, as well as of some ortho gneisses in the Saxothuringian and Lugian domains (Vrána and Kröner 1995;Hegner and Kröner 2000;Tichomirowa et al 2001;Friedl et al 2004;Schulmann et al 2005). Zircons of this age found in the studied samples can be interpreted either as clastic grains coming directly from eroded Cambrian granitoids or volcanics, or they represent recycled crystals from sediments rich in clastic material of Cambrian age.…”

“…Zircons of this age found in the studied samples can be interpreted either as clastic grains coming directly from eroded Cambrian granitoids or volcanics, or they represent recycled crystals from sediments rich in clastic material of Cambrian age. Ages older than 650 Ma are known from xenocrystic cores of younger zircons in Brunovistulian, Saxothuringian and Lugian orthogneisses as well as from orthogneisses and granulites of the Moldanubian Domain (Hegner and Kröner 2000;Kröner et al 2000;Tichomirowa et al 2001;Friedl et al 2004 and references therein).…”