1. Introduction
The Saxothuringian Domain (Saxothuringian Zone, sensu Kossmatt, Reference Kossmat1927) represents a block of Neoproterozoic–early Palaeozoic continental crust that collided during the Variscan orogeny with the core of the Bohemin Massif (Franke, Reference Franke1989, Reference Franke, Franke, Haak, Oncken and Tanner2000; Matte et al. Reference Matte, Maluski, Rajlich and Franke1990; Schulmann et al. Reference Schulmann, Konopásek, Janoušek, Lexa, Lardeaux, Edel, Štípská and Ulrich2009). The early Palaeozoic evolution of the Saxothuringian crust has been associated with its rifting from the northern Gondwana margin during late Cambrian time (e.g. Furnes et al. Reference Furnes, Kryza, Muszynski, Pin and Garmann1994; Kachlík & Patočka, Reference Kachlík and Patočka1998; Linnemann et al. Reference Linnemann, Gehmlich, Tichomirowa, Buschmann, Nasdala, Jonas, Lützner, Bombach, Franke, Haak, Oncken and Tanner2000; Franke, Reference Franke, Franke, Haak, Oncken and Tanner2000; Mazur & Aleksandrowski, Reference Mazur and Aleksandrowski2001; Schulmann et al. Reference Schulmann, Konopásek, Janoušek, Lexa, Lardeaux, Edel, Štípská and Ulrich2009), although the measure of separation of the Saxothuringian Domain from the Gondwana mainland is a matter of debate (cf. e.g. Franke, Reference Franke, Franke, Haak, Oncken and Tanner2000 and Linnemann et al. Reference Linnemann, McNaughton, Romer, Gehmlich, Drost and Tonk2004). Apart from sedimentation of Cambrian and thick Ordovician sequences (Linnemann et al. Reference Linnemann, Gehmlich, Tichomirowa, Buschmann, Nasdala, Jonas, Lützner, Bombach, Franke, Haak, Oncken and Tanner2000), the rifting process was accompanied by pronounced igneous activity resulting in synsedimentary volcanism and intrusion of numerous Cambrian–Ordovician plutons into the Neoproterozoic basement (Kröner et al. Reference Kröner, Jaeckel, Hegner and Opletal2001; Tichomirowa et al. Reference Tichomirowa, Berger, Koch, Belyatski, Gotze, Kempe, Nasdala and Schaltegger2001; Mingram et al. Reference Mingram, Kröner, Hegner and Kretz2004; Košler et al. Reference Košler, Bowes, Konopásek and Míková2004; Pin et al. Reference Pin, Kryza, Oberc-Dziedzic, Mazur, Turniak, Waldhausrova, Linnemann, Nance, Kraft and Zulauf2007).
The interpretation of the pre-Variscan Palaeozoic evolution of the entire Saxothuringian Domain is more straightforward along its northwestern flank owing to the absence of intense tectonometamorphic overprint during Devonian and early Carboniferous times. There, the Saxothuringian Domain is represented by unmetamorphosed Neoproterozoic sedimentary rocks and locally exposed granitoids, which are unconformably overlain by a pile of Palaeozoic sedimentary and synsedimentary volcanic rocks (see e.g. Falk, Franke & Kurze, Reference Falk, Franke, Kurze, Dallmeyer, Franke and Weber1995; Franke, Reference Franke, Franke, Haak, Oncken and Tanner2000; Linnemann et al. Reference Linnemann, Gehmlich, Tichomirowa, Buschmann, Nasdala, Jonas, Lützner, Bombach, Franke, Haak, Oncken and Tanner2000). The southeastern flank of the Saxothuringian Domain is represented by medium- to high-grade metamorphic rocks that were thrust over the Neoproterozoic–early Palaeozoic basement during the Carboniferous collision with the easterly exposed rocks of the Teplá-Barrandian Domain (e.g. Franke, Reference Franke1989, Reference Franke, Franke, Haak, Oncken and Tanner2000; Matte et al. Reference Matte, Maluski, Rajlich and Franke1990; Mazur, Reference Mazur1995; Mazur & Kryza, Reference Mazur, Kryza, Oncken and Jansen1996; Seston et al. Reference Seston, Winchester, Piasecki, Crowley and Floyd2000; Mazur & Aleksandrowski, Reference Mazur and Aleksandrowski2001; Konopásek & Schulmann, Reference Konopásek and Schulmann2005; Mazur et al. Reference Mazur, Aleksandrowski, Kryza and Oberc-Dziedzic2006; Schulmann et al. Reference Schulmann, Konopásek, Janoušek, Lexa, Lardeaux, Edel, Štípská and Ulrich2009; Žáčková et al. Reference Žáčková, Konopásek, Jeřábek, Finger and Košler2010; Mlčoch & Konopásek, Reference Mlčoch and Konopásek2010). This general scheme is valid for the central and southwestern part (southwest of the Elbe Zone) of the Saxothuringian Domain, as well as for its northeastern part represented by the Lusatian Complex and the West Sudetes (Fig. 1).
Figure 1. Simplified geological map of the West Sudetes (modified after Aleksandrowski et al. Reference Aleksandrowski, Kryza, Mazur and Żaba1997) with its location within the European Variscides (in right upper corner).
The protolith age and depositional setting of the intensely deformed, low- to medium-grade pre-Variscan sedimentary rocks in the West Sudetes has been a widely discussed topic (Chaloupský et al. Reference Chaloupský1989; Chlupáč, Reference Chlupáč1993, Reference Chlupáč1997; Winchester et al. Reference Winchester, Floyd, Chocyk, Horbowy and Kozdrój1995, Reference Winchester, Patočka, Kachlík, Melzer, Nawakowski, Crowley and Floyd2003; Kryza et al. Reference Kryza, Zalasiewicz, Mazur, Aleksandrowski, Sergeev and Larionov2007; Kryza, Mazur & Pin, Reference Kryza, Mazur and Pin1995; Kachlík & Patočka, Reference Kachlík and Patočka1998; Patočka, Fajst & Kachlík, Reference Patočka, Fajst and Kachlík2000; Mazur & Aleksandrowski, Reference Mazur and Aleksandrowski2001; Hladil et al. Reference Hladil, Patočka, Kachlík, Melichar and Hubačík2003; Kryza & Zalasiewicz, Reference Kryza and Zalasiewicz2008; Oberc-Dziedzic et al. Reference Oberc-Dziedzic, Kryza, Mochnacka and Larionov2010). In this work we contribute to this discussion and present new results from the dating of detrital zircon populations in metaquartzite samples from the southern part of the Krkonoše–Jizera Massif (Fig. 1). The resulting age spectra, interpreted as reflecting maximum sedimentary ages of the studied rocks, are compared with the protolith, xenocryst and detrital zircon ages from neighbouring rock complexes of the Saxothuringian basement. Finally, we discuss the existing interpretations of the palaeogeographic affinity of the Saxothuringian continental crustal block prior to its rifting from the Gondwana supercontinent during early Palaeozoic time.
2. Geological setting
The Krkonoše–Jizera Massif belongs to the group of several lithotectonic units defined in the West Sudetes in the north of the Bohemian Massif (Fig. 1). These units are inferred to be a collage of terranes amalgamated during the Variscan orogeny (Narębski, Reference Narębski1994) within the NW-verging orogenic wedge (Kachlík & Patočka, Reference Kachlík and Patočka1998; Mazur & Aleksandrowski, Reference Mazur and Aleksandrowski2001; Mazur et al. Reference Mazur, Aleksandrowski, Kryza and Oberc-Dziedzic2006 and references therein). The works of Mazur (Reference Mazur1995), Seston et al. (Reference Seston, Winchester, Piasecki, Crowley and Floyd2000), Mazur & Aleksandrowski (Reference Mazur and Aleksandrowski2001) and Žáčková et al. (Reference Žáčková, Konopásek, Jeřábek, Finger and Košler2010) suggested that the Krkonoše–Jizera Massif of the West Sudetes can be subdivided into four major tectonic units. The parautochthonous unit consists of Jizera orthogneisses (Fig. 1). The lowermost thrust sheet is exposed structurally above the Jizera orthogneiss in the southeastern part of the Krkonoše–Jizera Massif (Fig. 2). It consists mostly of micaschists with or without garnet with subordinate bodies of orthogneisses, quartzites, calcsilicates and marbles. A recent petrological study of garnet-bearing samples suggested blueschist-facies metamorphism in the range of 18–19 kbar and 460–520°C (Žáčková et al. Reference Žáčková, Konopásek, Jeřábek, Finger and Košler2010). The middle thrust sheet (Fig. 2) consists of garnet-free micaschists, phyllites and marbles with a high proportion of metavolcanic rocks that show blueschist-facies metamorphism reaching conditions of 300–530°C and 6.5–12 kbar (Cháb & Vrána, Reference Cháb and Vrána1979; Guiraud & Burg, Reference Guiraud and Burg1984; Kryza, Muszynski & Vielzeuf, Reference Kryza, Muszynski and Vielzeuf1990; Smulikowski, Reference Smulikowski1995; Patočka, Pivec & Oliveriová, Reference Patočka, Pivec and Oliveriová1996). A thick orthogneiss slab is apparently present at the contact of these two thrust sheets (Fig. 2). The uppermost thrust sheet (Fig. 2) is dominated by mafic and felsic meta-igneous rocks (the Lesczcyniec Complex) with a low intensity of deformation (Mazur, Reference Mazur1995; Kryza & Mazur, Reference Kryza and Mazur1995; Seston et al. Reference Seston, Winchester, Piasecki, Crowley and Floyd2000).
Figure 2. Simplified geological map of the studied part of the Krkonoše–Jizera Complex (modified after Kachlík & Kozdroj Reference Kachlík, Kozdroj, Kozdroj, Krentz and Opletal2001) with location of analysed samples (solid squares) EL190 (50° 40.784′ N, 15° 17.814 E; WGS84), EL111 (50° 40.989′ N, 15° 37.175′ E; WGS84), VU371 (50° 44.005′ N, 15° 39.758′ E; WGS84) and EL189 (50° 38.436′ N, 15° 46.604′ E; WGS84).
The schists and gneisses of the thrust sheets were interpreted as metamorphosed magmato-sedimentary sequences deposited during intracontinental rifting of the Cadomian basement and subsequent development of an oceanic basin (Kryza, Mazur & Pin, Reference Kryza, Mazur and Pin1995; Kryza et al. Reference Kryza, Zalasiewicz, Mazur, Aleksandrowski, Sergeev and Larionov2007; Winchester et al. Reference Winchester, Floyd, Chocyk, Horbowy and Kozdrój1995; Kachlík & Patočka, Reference Kachlík and Patočka1998; Patočka, Fajst & Kachlík, Reference Patočka, Fajst and Kachlík2000; Dostál, Patočka & Pin, Reference Dostál, Patočka and Pin2001; Kryza & Pin, Reference Kryza and Pin2010). The rifting phase is strongly suggested by the presence of large volumes of meta-igneous rocks, including basic lavas and volcaniclastic rocks partly with MORB affinities, and felsic rocks of within-plate signature (Winchester et al. Reference Winchester, Floyd, Chocyk, Horbowy and Kozdrój1995; Patočka & Pin, Reference Patočka and Pin2005; Dostál, Patočka & Pin, Reference Dostál, Patočka and Pin2001). Protoliths of the metavolcanic rocks were dated using various geochronological methods as late Cambrian–Early Ordovician (Bendl & Patočka, Reference Bendl and Patočka1995; Oliver, Corfu & Krogh, Reference Oliver, Corfu and Krogh1993; Kozdrój et al. Reference Kozdrój, Turniak, Tichomirova, Bombach, Kachlík and Ziółkowska-Kozdrój2005).
The metasedimentary and metavolcanic sequences of the Krkonoše–Jizera Massif have been divided by Chaloupský (Reference Chaloupský1989) into four lithostratigraphic groups of Mesoproterozoic to early Carboniferous age. This interpretation, however, has been revised by the detailed work of Kachlík (Reference Kachlík1996) and Winchester et al. (Reference Winchester, Patočka, Kachlík, Melzer, Nawakowski, Crowley and Floyd2003), who disproved most of the lithostratigraphic arguments for this subdivision and combined most of the central and eastern Krkonoše–Jizera Massif metasedimentary rocks into a single unit of Cambrian–Ordovician to Silurian–Devonian age. Moreover, an apparent lack of pre-Palaeozoic metasedimentary rocks in the structurally lowermost part of the Krkonoše–Jizera Massif was suggested by Oberc-Dziedzic et al. (Reference Oberc-Dziedzic, Kryza, Mochnacka and Larionov2010), who presented late Cambrian (~ 500 Ma) zircon age spectra from a quartzo-feldspathic rock of the unit Chaloupský (Reference Chaloupský1989) assumed to be Mesoproterozoic.
In its central part, the Krkonoše–Jizera Massif is intruded by the Variscan Krkonoše–Jizera granite pluton (Fig. 1), which was dated at 304 ± 14 Ma by Pb–Pb zircon evaporation dating (Kröner et al. Reference Kröner, Hegner, Hammer, Haase, Bielicki, Krauss and Eidam1994) and at ~ 314–319 Ma by U–Pb zircon SHRIMP dating (Machowiak & Armstrong, Reference Machowiak and Armstrong2007; Awdankiewicz et al. Reference Awdankiewicz, Awdankiewicz, Kryza and Rodionov2010).
3. Description of samples and their tectonometamorphic position
Detrital zircons were separated from quartzite samples collected at four localities within the southern outcrop of the Krkonoše–Jizera Massif (Fig. 2). Samples VU371 and EL111 come from the lower thrust sheet of the complex, which was recognized by Žáčková et al. (Reference Žáčková, Konopásek, Jeřábek, Finger and Košler2010) as a high-pressure nappe metamorphosed and exhumed during early Carboniferous times (Fig. 2). Metapelites of this unit show the highest metamorphic conditions within the metamorphic complex. Samples of micaschists often contain relics of a garnet-bearing high-pressure mineral assemblage, and the peak metamorphic conditions were estimated at 18–19 kbar at 460–520°C (Žáčková et al. Reference Žáčková, Konopásek, Jeřábek, Finger and Košler2010). The pressure peak was followed by decompression and cooling to temperatures lower than 480°C and pressures lower than 8.5 kbar (Žáčková et al. Reference Žáčková, Konopásek, Jeřábek, Finger and Košler2010). The other two samples, EL189 and EL190, were collected from localities within the middle thrust sheet that consists mostly of garnet-free micaschists, phyllites, quartzites and numerous bodies of marbles. Metabasites with relics of blueschist-facies metamorphism appear along the outer flank of this unit (Fig. 2). Some metapelitic samples bear garnet-free mineral assemblages with chloritoid and paragonite, and estimated P–T conditions suggest blueschist-facies metamorphism at ~ 11.5 kbar and 420°C (Žáčková et al. Reference Žáčková, Konopásek, Jeřábek and Faryad2007). However, the major proportion of rocks from this unit show a late blastesis of albite, suggesting re-equilibration during decompression. Ar–Ar dating of phengite from blueschist-facies metabasites in the easternmost part of this unit provided a Devonian age of 360 Ma, which was interpreted as the age of the high-pressure blueschist-facies metamorphism (Maluski & Patočka, Reference Maluski and Patočka1997).
All the analysed zircons were separated from medium- to fine-grained quartzite samples with thin layers of muscovite within a recrystallized quartz matrix. Apart from zircon, other accessory minerals observed were ilmenite, pyrite, apatite, chlorite and tourmaline. Sample VU371 was collected from the Kozí hřbety locality in the northeastern part of the Krkonoše Mountains (Fig. 2), where a quartzite body occurs in the lowermost part of the micaschist-dominated lower thrust sheet. Sample EL111 comes from the Hnědá skála locality, situated northeast of Vrchlabí, in the central Krkonoše Mountains (Fig. 2), where a quartzite body is situated within the micaschists of the lower thrust sheet. Sample EL189 comes from the Modré kameny locality in the vicinity of Jánské Lázně in the southeastern Krkonoše Mountains (Fig. 2). There, a quartzite body is in direct contact with the underlying large orthogneiss slab and apparently forms the lowermost part of the phyllite-dominated middle thrust sheet with associated mafic blueschists in its hanging wall. Quartzite sample EL190 was collected in the vicinity of Machlov, northeast of the Železný Brod (Fig. 2), where a quartzite body forms the footwall of the mafic Železný Brod Volcanic Complex. The sampled quartzite, together with surrounding phyllites, is regarded as part of the middle thrust sheet.
The zircon grains from all samples differ in morphology and size, and many of them show a high degree of abrasion with moderately to strongly rounded edges and numerous scratches on the surface. Cathodoluminescence images obtained from the studied zircon crystals revealed the presence of (1) euhedral elongate to stubby grains with igneous oscillatory zoning, (2) euhedral oscillatory-zoned zircons with older, often high-U cores and (3) round equant grains with sector fir-tree zoning typical of granulite-facies rocks.
4. Laser ablation ICP-MS dating of zircons and the results
Zircons were extracted from c. 10 kg samples using conventional crushing, the Wilfley shaking table and heavy liquids. Handpicked zircon grains were mounted in epoxy-filled mount blocks and polished to reveal the internal structures of the grains and to obtain smooth surfaces suitable for analysis by laser ablation inductively coupled plasma source mass spectrometry (LA-ICP-MS). Isotopic analysis followed the technique described by Košler et al. (Reference Košler, Fonneland, Sylvester, Tubrett and Pedersen2002). A Thermo-Finnigan Element 2 sector field ICP-MS system coupled to a 213 Nd–YAG laser (New Wave UP-213) at Bergen University was used to measure Pb/U and Pb isotopic ratios in zircons. Raw data were corrected for dead time of the electron multiplier, processed offline in a spreadsheet-based program Lamdate (Košler et al. Reference Košler, Fonneland, Sylvester, Tubrett and Pedersen2002) and plotted on concordia diagrams using Isoplot (Ludwig, Reference Ludwig1999). Data processing included corrections for blank, laser-induced Pb/U fractionation and ICP-MS mass discrimination. Zircon reference materials Plešovice (337 Ma; Sláma et al. Reference Sláma, Košler, Condon, Crowley, Gerdes, Hanchar, Horstwood, Morris, Nasdala, Norbert, Schalteger, Schoene, Tubrett and Whitehouse2008) and GJ-1 (c. 609 Ma; Jackson et al. Reference Jackson, Pearson, Griffin and Belousova2004) were periodically analysed during this study and they yielded concordia ages (Ludwig, Reference Ludwig1998) of 340 ± 3 Ma and 616 ± 6 Ma, respectively. Only data that were less than 20% discordant (calculated as (100 * Age207Pb/206Pb/Age206Pb/238U) − 100) were used in this study.
The U–Pb ages of zircons from sample VU371 (Kozí hřbety locality), where 60 zircon grains were analysed, show the youngest cluster of data between 450 and 500 Ma and most of the data fall between 500 and 530 Ma (Fig. 3; Table 1). Older ages are represented by individual Palaeoproterozoic data between 1.6 and 2.3 Ga. Similar data were obtained from sample EL111 from the Hnědá skála locality (56 zircons analysed). The youngest data from this locality cluster between 450 and 500 Ma (Fig. 3; Table 2). The majority of data from this sample concentrate at 500 Ma and between 590 and 660 Ma. A cluster of Palaeoproterozoic ages between 1.6 and 2.2 Ga and individual Mesoproterozoic and Archaean ages of 2.5, 3.0 and 3.1 Ga are also present. Zircon ages from sample EL189 (Jánské Lázně locality) were interpreted from 44 analyses and the youngest data cluster between 500 and 550 Ma, with the early Cambrian peak at 526 Ma (Fig. 3; Table 3). Older data are Neoproterozoic and lie in the range 590–650 Ma. Individual Palaeoproterozoic to Archaean data occur between 1.8 and 3.0 Ga. In sample EL190 from the Machlov locality, 70 zircon grains were used for interpretation. The youngest data cluster between 450 and 500 Ma and the Neoproterozoic ages appear in the range 600–670 Ma (Fig. 3; Table 4). The oldest data are represented by a cluster of Palaeoproterozoic and individual Archaean ages in the range 1.8–3.0 Ga.
Figure 3. Concordia diagrams of detrital zircons from analysed samples (left column) with details of Neoproterozoic to early Palaeozoic data, and corresponding binned frequency and probability density distribution plots (right column, N = number of analyses).
Table 1. Laser ablation ICP-MS U–Pb isotopic data of detrital zircons from sample VU371
Table 2. Laser ablation ICP-MS U–Pb isotopic data of detrital zircons from sample EL111
Table 3. Laser ablation ICP-MS U–Pb isotopic data of detrital zircons from sample EL189
Table 4. Laser ablation ICP-MS U–Pb isotopic data of detrital zircons from sample EL190
5. Discussion
5.a. Sedimentation age of the quartzite protoliths
Apart from the presence of palaeontologically proven very low-grade Silurian to lower Carboniferous (meta)sedimentary rocks (see summary in Chlupáč, Reference Chlupáč1993) on the western flank of the Krkonoše–Jizera Massif (the Ještěd Mts), the only robust information about the stratigraphic age of more easterly exposed metasediments comes from the discovery of Silurian graptolites in the phosphatic concretions of the middle thrust sheet (Horný, Reference Horný1964). This information became critical in any further attempts at tectonostratigraphic subdivision of the Krkonoše–Jizera Massif.
Chaloupský (Reference Chaloupský1989) suggested that the metasedimentary rocks of the lower thrust sheet are Mesoproterozoic rocks intruded by the granitoid protolith of orthogneisses exposed in the core of the complex. The metasedimentary sequences with subordinate metavolcanic rocks of the middle thrust sheet were considered by the same author as Upper Ordovician to Silurian. Based on the presence of ichnofossils, Chlupáč (Reference Chlupáč1997) suggested an Ordovician age for phyllites in the eastern part of the Krkonoše–Jizera Massif, which were interpreted by Chaloupský (Reference Chaloupský1989) as metamorphosed Proterozoic–lower Cambrian sediments.
Further biostratigraphic data come from marbles of the southern and eastern part of the Krkonoše–Jizera Massif. Carbonates in the easternmost part of the area provided a fragment of archaeocyath and several trilobite fragments attributed to the early Cambrian, whereas faunal microfragments in metacarbonates from the southern and central part of the complex were interpreted as Silurian–Early Devonian (Hladil et al. Reference Hladil, Patočka, Kachlík, Melichar and Hubačík2003).
Age spectra presented in this work show that the dominant proportion of detrital zircons from the analysed samples of both upper and lower units is Cambrian–Ordovician and Neoproterozoic in age, which is in agreement with the SHRIMP detrital zircon data by Oberc-Dziedzic et al. (Reference Oberc-Dziedzic, Kryza, Mochnacka and Larionov2010). Three of the samples, EL111, EL190 and VU371, show the youngest data in the interval between c. 460 and 490 Ma. If these ages represent true formation ages of the youngest detrital zircons, then the sedimentary protolith cannot be older than Ordovician. Only sample EL189 from the quartzite directly overlying the Cambrian orthogneiss did not provide zircon grains younger than c. 500 Ma. This difference with respect to the other three analysed samples probably means that the youngest, c. 490–460 Ma source existed, but was not available at the time of sedimentation, or that the protolith age of the sample is somewhere between the observed 526 Ma peak and the youngest ages encountered in the other studied samples.
All the samples show a marked peak at c. 525–500 Ma, which is strong evidence for the early Palaeozoic age of the sedimentary protolith. As already recognized by Oberc-Dziedzic et al. (Reference Oberc-Dziedzic, Kryza, Mochnacka and Larionov2010), such a finding rules out the Mesoproterozoic age suggested by Chaloupský (Reference Chaloupský1989) for the lower allochthonous unit represented by samples EL111 and VU371. Thus, given the Late Devonian–early Carboniferous metamorphism of the Krkonoše–Jizera Massif (Maluski & Patočka, Reference Maluski and Patočka1997; Marheine et al. Reference Marheine, Kachlík, Maluski, Patočka, Żelazniewicz, Winchester, Pharaoh and Verniers2002; Žáčková et al. Reference Žáčková, Konopásek, Jeřábek, Finger and Košler2010), our data, together with the earlier published zircon age data and the above-discussed palaeontological evidence, all suggest that the protolith age of metasedimentary rocks in the Krkonoše–Jizera Massif spans the interval between late Cambrian and Late Devonian time.
Occurrences of thick early Palaeozoic quartzite bodies in western Europe have been documented in the Lower Ordovician strata. These quartzites belonging to the Armorican Quartzite Formation are believed to document the opening of the Rheic Ocean (Linnemann et al. Reference Linnemann, Pereira, Jeffries, Drost and Gerdes2008), and their protoliths are often sedimented after the so-called Cadomian unconformity. According to Linnemann et al. (Reference Linnemann, Pereira, Jeffries, Drost and Gerdes2008), a quartzite from Central Iberia–Ossa Morena transition zone contains the youngest detrital zircon of 522 ± 7 Ma within the lower Cambrian cluster following the most abundant Neoproterozoic zircons and small amount of zircons of Archaean and Palaeoproterozoic age. Similar zircon populations were found also in quartzites of the Saxothuringian Domain (Linnemann et al. Reference Linnemann, Gerdes, Drost, Buschmann, Linnemann, Kraft, Nance and Zulauf2007), which are attributed to widely distributed Lower Ordovician shallow marine sedimentation of the Gondwanan realm.
5.b. Source rocks and the Saxothuringian passive margin dynamics
The presented data indicate that the detritus most probably came from eroded Cambrian–Ordovician granitoids and from their Neoproterozoic (meta)sedimentary or magmatic country rocks. This is also consistent with the presence of igneous oscillatory zoning in most of the studied euhedral zircon grains and fir-tree sector zoning, typical of high-grade metamorphic zircons, in most of the oval detrital grains. The apparent absence of metasedimentary rocks with Neoproterozoic protoliths in the central and eastern part of the Krkonoše–Jizera Massif and the direct contact of the early Palaeozoic metasedimentary rocks with the Cambrian metagranitoids suggest complete erosion of the Neoproterozoic sedimentary country rocks during the development of the early Palaeozoic rift basin in this part of the Saxothuringian passive margin. Our data from detrital zircons suggest that the thrust sheets overlying the Cambrian–Ordovician granitoid intrusions of the Krkonoše–Jizera Massif represent metamorphosed equivalents of the uppermost Cambrian–Devonian passive margin sediments (cf. Mazur & Aleksandrowski, Reference Mazur and Aleksandrowski2001; Winchester et al. Reference Winchester, Patočka, Kachlík, Melzer, Nawakowski, Crowley and Floyd2003).
In the Saxothuringian Domain, there is no evidence for basement rocks older than Neoproterozoic. Thus, sporadic Mesoproterozoic–Archaean zircon grains in the studied samples were presumably recycled from Neoproterozoic basement sediments or they represent former xenocrysts in the Cambrian–Ordovician granitoids.
5.c. ‘Grenvillian’ ages in the West Sudetes and the palaeogeographic affinity of the Saxothuringian Domain
The current opinion on the palaeogeographic affinity of the Saxothuringian Domain can be divided into two contrasting theories. Hegner & Kröner (Reference Hegner, Kröner, Franke, Haak, Oncken and Tanner2000), Kröner et al. (Reference Kröner, Jaeckel, Hegner and Opletal2001) and Mingram et al. (Reference Mingram, Kröner, Hegner and Kretz2004) suggested that the early Palaeozoic granitoids of the Saxothuringian Domain were largely generated by melting of basement spanning c. 1.0–1.4 Ga, which is known as the Grenvillian event in the Amazonian Craton and the Sveconorwegian event in the southwestern Baltic Shield. This interpretation is based on the ages of xenocrystic zircons and their apparent correspondence with Nd model ages of these granitoids. In accordance with these data, the authors mentioned above interpreted the West Sudetes and Erzgebirge as a part of eastern Avalonia, which was probably rifted off the northern Amazonian Craton. In contrast, Tichomirowa et al. (Reference Tichomirowa, Berger, Koch, Belyatski, Gotze, Kempe, Nasdala and Schaltegger2001) did not find any Grenvillian-age zircon xenocrysts in late Neoproterozoic rocks of the Erzgebirge and suggested their affinity to the West African Craton. The West African provenance of Saxothuringian Neoproterozoic and lower Palaeozoic sediments was also proposed by Linnemann et al. (Reference Linnemann, McNaughton, Romer, Gehmlich, Drost and Tonk2004) based on the absence of Grenvillian ages in age spectra obtained by the SHRIMP U–Pb geochronology of detrital and inherited zircon grains.
Only a few data from our analysed samples fall into the time interval 0.9–1.4 Ga typical of the Grenvillian event, and the pre-Palaeozoic age frequency histograms are nearly identical (differing only in the frequency of particular age groups) to the data published by Linnemann et al. (Reference Linnemann, McNaughton, Romer, Gehmlich, Drost and Tonk2004, Reference Linnemann, Gerdes, Drost, Buschmann, Linnemann, Kraft, Nance and Zulauf2007) for the Lugian and Saxothuringian sedimentary rocks and also with the data for the Neoproterozoic–Palaeozoic sediments in the Teplá–Barrandian Domain (Drost et al. Reference Drost, Linnemann, McNaughton, Fatka, Kraft, Gehmlich, Tonk and Marek2004, Reference Drost, Gerdes, Jeffries, Linnemann and Storey2010). The pattern obtained by dating of detrital zircons from the Krkonoše–Jizera Massif quartzites (Fig. 3) shows good correlation with a summary of the protolith and xenocryst zircon ages from magmatic rocks, and of the detrital zircon ages from the Neoproterozoic sediments of the Saxothuringian Domain (Fig. 4). On the other hand, this pattern is very different from xenocrystic and detrital zircon populations presented by Hegner & Kröner (Reference Hegner, Kröner, Franke, Haak, Oncken and Tanner2000) and Mingram et al. (Reference Mingram, Kröner, Hegner and Kretz2004) characterized by a large number of Grenvillian ages. Separation of the Pb–Pb evaporation ages from the set of pooled protolith and detrital data from the Saxothuringian Domain rocks (Fig. 4) clearly shows that the Mesoproterozoic ages between 1.0 and 1.4 Ga were mostly obtained by zircon evaporation that does not allow the recognition of discordant data and represents only the minimum age of the dated grain. A very good fit of our data with the pooled results of U–Pb dating of pre-Variscan granitoids and detrital zircon populations in Neoproterozoic sediments of the Saxothuringian Domain is in accord with the interpretation of, for example, Kachlík & Patočka (Reference Kachlík and Patočka1998), Winchester et al. (Reference Winchester, Patočka, Kachlík, Melzer, Nawakowski, Crowley and Floyd2003) and Oberc-Dziedzic et al. (Reference Oberc-Dziedzic, Kryza, Mochnacka and Larionov2010) that the Krkonoše–Jizera Massif metasedimentary rocks represent metamorphosed lower Palaeozoic deposits of the Saxothuringian passive margin. Finally, the lack of Grenvillian ages in dated detrital zircons supports the conclusion of Linnemann et al. (Reference Linnemann, McNaughton, Romer, Gehmlich, Drost and Tonk2004) that the Saxothuringian Neoproterozoic basement (as a source region for the early Palaeozoic detritus of our samples) has an affinity to the northern margin of Gondwana occupied by the West African Craton.
Figure 4. Distribution of the protolith and xenocryst zircon ages from magmatic rocks, and of the detrital zircon ages from the Neoproterozoic sediments of the Saxothuringian Domain presented as a cumulative probability plot (see text for references). The black area represents the results of Pb–Pb zircon evaporation dating, whereas the grey area corresponds to the results obtained by U–Pb zircon dating methods.
Acknowledgements
The authors very much appreciate the constructive reviews provided by R. Kryza and S. Mazur. This work was supported by internal projects No. 390001 and 325900 of the Czech Geological Survey. EZ and JK also appreciate financial support from the Ministry of Education, Youth and Sports of the Czech Republic through the Research Centre ‘Advanced Remedial Technologies and Processes’ (identification code 1M0554). PJ acknowledges the Research Plan No. MSM0021620855 by the Ministry of Education, Youth and Sports of the Czech Republic.
