Zircon ages, geochemistry, and Nd isotopic systematics of pre-Variscan orthogneisses from the Erzgebirge, Saxony (Germany), and geodynamic interpretation

Abstract

High grade granitoid orthogneisses occur in several metamorphic units of the Erzgebirge in the Saxothuringian Zone of the Variscan Belt. The determination of protolith ages and the geochemical characterization of these rocks permit a reconstruction of the Neoproterozoic to early Palaeozoic magmatic and geodynamic history of the Erzgebirge. Single zircon Pb-Pb evaporation and SHRIMP ages combined with major and trace element data and Sm-Nd isotope systematics indicate at least two discrete magmatic events concealed in the so-called red gneisses, one at~550 Ma in rocks of the medium pressure-medium temperature (MP-MT) unit and the other at~500-480 Ma in rocks of the high pressure units. The transition zones comprise both Neoproterozoic granitoids and early Palaeozoic metarhyolites. The granitoid gneisses represent Neoproterozoic calc-alkaline granitoids with REE patterns similar to those produced in Andean-type continental margins. The early Palaeozoic muscovite gneisses are geochemically distinct from the older granitoids and may be derived from melts generated in a back-arc setting. Initial Nd values in all samples overlap and range from 4.1 to 9.2, corresponding to crustal sources with average residence times of 1.5 to 1.9 Ga. Zircon xenocryst ages as old as 2992 Ma provide evidence for Grenvillian, Svecofennian-Birimian-Aazonian and older age components and suggest an association of the Erzgebirge with Avalonia.

Cadomian volcanosedimentary complexes across the Ediacaran–Cambrian transition of the Eastern Pyrenees, southwestern Europe

“…This dataset may be interpreted as the result of variable mixing of crustal components as it has already been described in other areas of the European Variscan Belt, such as the Bohemian Massif, where felsic igneous rocks provide εNd values between − 3.3 and − 7.9 (− 3.3 to − 5.0 by Linnemann and Romer 2002, and − 4.9 to − 7.9 by Mingram et al 2004). These authors interpreted the corresponding Mesoproterozoic (between 1.6 and 1.9 Ga) TDM ages as the result of the mixing of Palaeoproterozoic-Archean and Neoproterozoic crustal portions.…”

U–Pb LA-SF-ICP-MS zircon geochronology of the Serbo-Macedonian Massif, Greece: palaeotectonic constraints for Gondwana-derived terranes in the Eastern Mediterranean

“…Similar ages were reported from NW Iberia (Fernández-Suárez et al 2002), the NE Bohemian Massif (Hegner and Kröner 2000;Mingram et al 2004), the Moravo-Silesian Unit (Friedl et al 2000(Friedl et al , 2004, West Avalonia Murphy 1994, 1996), East Avalonia (Strachan et al 2007), the Małopolska Unit (Belka et al 2002) and the Polish East European Platform (Belka et al 2002). Such ages, however, are not known from the Meguma Terrane , Saxothuringia (e.g.…”

Inferring protoliths of high-grade metamorphic gneisses of the Erzgebirge using zirconology, geochemistry and comparison with lower-grade rocks from Lusatia (Saxothuringia, Germany)

“…An Ordovician age was published for the Schwarzenberg augengneiss (Mingram et al 2004), while the Wolkenstein and Bärenstein augengneisses yielded Neoproterozoic ages (Table 1; Kröner et al 1997;Mingram et al 2004). Rötzler and Plessen (2010) interpreted the latter age as dating a metamorphic or magmatic event at 580-570 Ma.…”

“…Zircon characteristics (morphology and ages) may help to unravel the protoliths of these rocks. Previous studies yielded zircon ages ranging from 3,200 to 340 Ma with inferred protolith ages between 575 and 480 Ma (Kröner et al 1995(Kröner et al , 1997Tichomirowa et al 2001;Kröner and Willner 1998;Mingram and Rötzler 1999;Kosler et al 2004;Mingram et al 2004). The Variscan overprint caused a severe disturbance of zircon ages in some ultra-highpressure (UHP) gneisses of the Erzgebirge (Tichomirowa et al 2005), and thus, there is still an ongoing discussion which of the gneisses should be assigned to the same protoliths.…”