The Erzgebirge, Germany, a subducted part of northern Gondwana: geochemical evidence for repetition of early Palaeozoic metasedimentary sequences in metamorphic thrust units

Abstract

Originally published as: Mingram, B. (1998): The Erzgebirge, Germany, a subducted part of northern Gondwana: geochemical evidence for repetition of early Palaeozoic metasedimentary sequences in metamorphic thrust units. 135,[785][786][787][788][789][790][791][792][793][794][795][796][797][798][799][800][801] DOI: 10.1017/S0016756898001769 IntroductionThe European Variscan Belt is a collision orogen formed during the late Palaeozoic, and is marked by thrusts and nappes (Matte, 1991). Polymetamorphic crystalline nappes with high-pressure, high-temperature relics of eclogites and granulites are widespread from Poland to Spain, and reflect intracontinental and oceanic subduction zones.The early Palaeozoic history of the Central European Variscides is characterized by crustal extension and development of oceanic basins produced by separation of microplates (e.g. Avalonia, Armorica, and Baltica) from the northern periphery of the Neoproterozoic (Cadomian) supercontinent Gondwana (Matte, 1991;Franke, 1989;Pin, 1990). Bimodal metavolcanic and mafic-ultramafic complexes provide good evidence for extensional rifting regimes in late Cambrian to early Ordovician times (e.g. Pin, 1990;Pin & Marini, 1993;Winchester et al. 1995;Floyd et al. 1996). The boundary between Proterozoic and the early Palaeozoic times marks an unconformity and/or conformity between crystalline and sedimentary formations of the Cadomian basement and highly mature neritic clastic shelf sediments, which pass into hemipelagic shales during the late Ordovician period (Noblet & Lefort, 1990). These changes reflect the development from an orogenic belt to a passive margin setting with associated widespread shallow marine facies. This development is related to the transition from a Neoproterozoic ensialic arc, the Avalonian-Cadomian orogenic belt, to an early Palaeozoic stable platform at the periphery of a Neoproterozoic supercontinent (Nance & Murphy, 1994).Similarities in Palaeozoic depositional settings exist along the major Variscides (Erdtmann, 1991): highmature fine-grained sandstones in the early to middle Arenigian time (Noblet & Lefort, 1990), chamositic oolitic ore horizons in the Arenigian to Ashgillian times (Young, 1989), and an influence of glaciation in the Upper Ordovician and black shale transgressions in the Arenigian, Llanvirnian and Silurian times. However, the paucity of fossils and the strong tectonometamorphic overprint complicate the correlation of sedimentary sequences and the recognition of ancient geodynamic regimes. In addition to conventional structural and petrological studies, the geochemistry of metamorphic rocks has great potential Geol. Mag. 135 (6), 1998, pp. 785-801. Printed Abstract -One of the major metamorphic terranes of the Bohemian Massif, the Erzgebirge, is interpreted to record a subducted part of a Palaeozoic margin of Gondwana. A geochemical study on noncalcareous metasediments from the various metamorphic units from lower greenschist to granulite facies metamorphism supports a recently established thrust mode...

Organization of pre-Variscan basement areas at the north-Gondwanan margin

“…In an Early Cambrian reconstruction (McKerrow et al 1992 (Noblet and Lefort 1990;Robardet 1996) may explain comparable facies evolution in the Saxothuringian areas (Linnemann and Buschmann 1995), in the Armorican massif , and regions included in the polymetamorphic domains (e.g. Mingram 1996Mingram , 1998 (Haydoutov 1989;von Quadt 1992;Schaltegger et al 1997;Schaltegger and Gebauer 1999;Chen et al 2000;Crowley et al 2000;Fernµndez Suµrez et al 2000) confirm the relationship of several domains with the Neoproterozoic orogenic belt and the existence of different tectonic regimes along the Gondwana margin. Traces of what is assumed to represent the former time equivalents of the Rheic ocean are found in the Cambrian mafic±ultramafic complexes such as the Chamrousse ophiolite of the Alps (MØnot 1987) and the Vesser zone of the Saxothuringian domain (H. Kemnitz et al, submitted), and can be followed through different metamorphic complexes in the Bohemian massif (Finger et al 1998;Mingram et al 1998;Stipska et al 1998;Winchester et al 1998;Kröner et al 2000).…”

Rutile occurrence and trace element behavior in medium-grade metasedimentary rocks: example from the Erzgebirge, Germany

“…GEU rocks are ilmenite-free. Taking into account that pelitic rocks from the studied units have similar protoliths (Mingram 1998) and assuming that they followed a rather similar prograde path, the absence of ilmenite in GEU rocks indicate that the rutile forming reaction was completed. Rutiles from these rocks show a narrow scatter in Nb contents, are single crystals and do not have the elongated shape inherited from ilmenite.…”

“…Garnet-and chloritoid-bearing mica schists are the most common rock types. Metasediments from the GEU are characterized by the occurrence of migmatitic paragneisses, feldspar-bearing mica schists and feldspar-free, kyanite-bearing mica schists (Willner et al 1997;Rötzler et al 1998;Mingram 1998). Although mafic rocks are beyond the scope of the present work, it is worth mentioning that eclogites occur as local intercalations in both MEU and GEU.…”

“…From the base to the top these units are: Red and Grey Gneisses (RGG); Gneiss/Eclogite Unit (GEU); Mica schist/Eclogite Unit (MEU); Garnet-Phyllite Unit (GPU); and Phyllite Unit (PU). The tectonometamorphic stacking is interpreted as a result of continent-continent collision processes during the Variscan Orogeny (e.g., Willner et al 1997;Rötzler et al 1998;Mingram 1998). Geochemical discriminations suggest that the protoliths of all metamorphic units are similar (mature sediments, exposed to prolonged tropical weathering and extensive reworking), leading to the conclusion that they represent a repetition of metasedimentary sequences (Mingram 1998).…”

“…The tectonometamorphic stacking is interpreted as a result of continent-continent collision processes during the Variscan Orogeny (e.g., Willner et al 1997;Rötzler et al 1998;Mingram 1998). Geochemical discriminations suggest that the protoliths of all metamorphic units are similar (mature sediments, exposed to prolonged tropical weathering and extensive reworking), leading to the conclusion that they represent a repetition of metasedimentary sequences (Mingram 1998). Peak pressure (P) and temperature (T) conditions published for the western part of the Erzgebirge are presented in Fig.…”