The Central-Sudetic ophiolites (SW Poland): Petrogenetic issues, geochronology and palaeotectonic implications
Introduction
The Bohemian Massif in Central Europe is a complex part of the Variscan internides (Fig. 1). The pre-Permian basement units of the massif, with a varied lithostratigraphic composition and diverse metamorphic and structural paths, bear records of the early evolutionary stages of the future Variscan fold belt, including initial rifting processes, penecontemporaneous igneous activity and deposition in evolving sedimentary basins. The rifting processes along the Gondwana margin, during Early Palaeozoic times, led to the formation of the Rheic Ocean (Murphy et al., 2006, Nance et al., 2010, Murphy et al., 2010), in which most of the sedimentary and magmatic sequences of the Variscides were emplaced, and subsequently metamorphosed and folded to create the Variscan orogen.
The mosaic structure of the NE part of the Bohemian Massif has caused lively debate on the definition and interpretation of the major tectonostratigraphic units in that area since the classical subdivision of Kossmat (1927; Fig. 1A). Recently, Franke and Żelaźniewicz (2002) proposed a modified regional tectonic scheme and distinguished the following “terranes”: Avalonia (including the Rheno-Hercynian Zone), Malopolska and Silesia (including the Moravo-Silesian Zone), the Northern Phyllite Zone and Mid-German Crystalline High, Saxothuringia, Bohemia (covering the Tepla-Barrandian), and Moldanubia (MO sensu stricto). The NE part of the Bohemian Massif (the Lugosudeticum of Narebski, 1993) belongs to the Armorican Terrane Assemblage (Franke, 2000, Winchester et al., 2002). However, the tectonic units and their boundaries have been variously defined by different authors (e.g. Cymerman et al., 1997, Aleksandrowski and Mazur, 2002).
The contrasting geological histories observed in neighboring crustal blocks, combined with the presence of ophiolitic fragments, mélange bodies, and HP/T metamorphic rocks (including blueschists and eclogites), indicate the regional importance of large-scale tectonic displacements and suggest that tectonostratigraphic units are separated by major tectonic boundaries (e.g. Aleksandrowski et al., 1997, Kryza et al., 2004, Mazur et al., 2006).
Important constraints for large-scale tectonic interpretations come from the presence and distribution of ophiolitic complexes, which may represent traces of past oceans (e.g. Dewey et al., 1973, Dilek et al., 2007). General models of the geological evolution of the European Variscides agree upon the following events (e.g. Pin, 1990, Narebski, 1993, Furnes et al., 1994, Crowley et al., 2000, Sánchez Martinez et al., 2007, Arenas et al., 2007, Ribeiro et al., 2010): (1) Early Palaeozoic rifting processes producing widely distributed bimodal and/or mafic magmatic suites (e.g. the Kaczawa, S and E Karkonosze complexes in the study area of the Sudetes; Fig. 1B); and (2) a continuing extensional regime during Silurian and Early Devonian times leading to the formation of fragments of oceanic crust which are preserved as ophiolites and metavolcanic sequences (e.g. the Central-Sudetic ophiolites, upper part of the Kaczawa Complex). The Variscan orogenic processes recorded in the NE part of the Bohemian Massif were diachronous, they commenced before the Late Devonian and culminated in the Late Devonian–Carboniferous times. This is evident from a range of structural and geochronological data, and supported by the intense late-orogenic magmatism and molasse-type sedimentation (Mazur et al., 2006 and references therein).
The rifting processes along the Gondwanan margin, leading to the opening of the Rheic Ocean, seem to have been diachronous, and the mature rift stages, producing oceanic-type basalts, may have already developed by Late Cambrian/Early Ordovician times in some areas (e.g. the Mariánské Lázně Complex, Leszczyniec Metaigneous Complex; Fig. 1B), while in others, initial intracontinental rifting was still operating (Kaczawa Complex; Kryza, 2007).
The largest and best-documented ophiolitic bodies in the NE part of the Bohemian Massif have been interpreted as traces of major tectonic sutures. These include the Mariánské Lázně Complex, between the Tepla-Barrandian and Saxothuringian zones (Fig. 1B). This complex comprises serpentinized ultrabasites and eclogite-facies basic and intermediate rocks, including MORB-type metagabbros (Crowley et al., 2002) with a protolith age of 496 ± 1 Ma (U–Pb zircon data, Bowes and Aftalion, 1991). The Mariánské Lázně Complex has been considered as marking the Saxothuringian Suture separating the Tepla-Barrandian and Saxothuringian zones (e.g. Franke, 1989, Matte et al., 1990, Mazur and Aleksandrowski, 2001, Crowley et al., 2002, Timmermann et al., 2006).
Similarly, the mafic and ultramafic complexes along the eastern edge of the Moldanubicum (at Letovice in Moravia and in Lower Austria, south of the map shown in Fig. 1B) were considered to be an “ophiolitic belt” surrounding the Moldanubian Zone (Misař et al., 1984, Finger and Steyrer, 1995, Höck et al., 1997). Unfortunately, a range of such metaigneous bodies (e.g., those enclosed within mid- and high-grade metamorphic terranes; Kryza and Pin, 2002), are of uncertain age and have geochemical signatures that are often difficult to interpret.
In this paper we review the available data regarding the petrology, geochronology, and structural position of the Central-Sudetic ophiolites. Special attention is paid to the Ślęża ophiolite, which is the most prominent representative of the CSO. We outline the geochemical and isotopic characteristics of this ophiolite, provide preliminary new SHRIMP ages on zircons extracted from its plutonic and subvolcanic members, and attempt to refine the interpretation of the position of the Ślęża ophiolite (and the entire CSO) within the structural mosaic of the NE part of the Bohemian Massif.
Section snippets
Tectonic mosaic of the Sudetes and distribution of ophiolite-type rocks
The Sudetes in the NE part of the Bohemian Massif are confined between two major NW–SE-trending fault systems parallel to the SW margin of the East-European Craton (Fig. 1): the Odra Fault Zone in the NE and the Elbe Fault Zone in the SW. Parallel to these is a third important tectonic line, the Intra-Sudetic Fault Zone, cutting the area into two roughly equal parts (shaded in Fig. 1A). In addition, the Sudetes area is divided into two morphological domains by the oblique Sudetic Boundary
Serpentinized peridotites
The serpentinites of the Ślęża ophiolite represent mantle peridotites at the bottom of the ophiolitic pseudostratigraphy (Majerowicz and Pin, 1994). Typically, blocks and boudins of massive pseudomorphic serpentinite are enclosed in a sheared chrysotile-rich matrix. Locally, they preserve mantle–tectonite texture with bastite pseudomorphs after porphyroclasts of pyroxene surrounded by hourglass pseudomorphs after olivine neoblasts. The primary mineralogy, deduced from the CIPW norm, suggests
Published data
As mentioned above, the Sm–Nd whole-rock isochrons were previously interpreted to record magmatic ages of 353 ± 21 Ma and 351 ± 16 Ma for the Ślęża and Nowa Ruda mafic rocks, respectively (Pin et al., 1988). Later, Oliver et al. (1993) used the U–Pb conventional method on abraded zircons to analyse a few grains of that mineral recovered from a sample of the Ślęża gabbro. In spite of analytical problems, the calculated age of 420 + 20/−2 Ma was interpreted to date the magmatic crystallization event in
Summary of petrological, geochemical and isotopic data
As shown in several earlier studies (e.g. Majerowicz, 1979, Majerowicz, 1981, Pin et al., 1988, Majerowicz and Pin, 1994), the Ślęża ophiolite exhibits a typical lithological variation and pseudostratigraphy which grades (from bottom to top) from serpentinites and gabbros, to sheeted dykes and lavas. The mafic rocks are only selectively (not penetratively) deformed and underwent a partial metamorphic imprint, likely including post-magmatic alteration and subsequent regional metamorphism up to
Acknowledgements
The International Geoscience Programme IGCP 497 “The Rheic Ocean: its origin, evolution and correlatives” was our inspiration for this study. In our geochemical compilations we used data from Abdel Wahed (1999). Sm–Nd isotope results were obtained within the long-term bilateral co-operation between Université Blaise Pascal, Clermont-Ferrand, and the University of Wrocław. SHRIMP analyses were funded from an internal grant (2022/W/ING/08-15) of the University of Wrocław. The zircon separates
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