Introduction

The Rumburk granite is named after the northern Czech city of Rumburk and is part of the Cadomian Lausitz Block (Fig. 1). Due to its bluish color and the microcline megacrysts, the Rumburk granite drew early scientific attention (Cotta 1839; Reinisch 1920).

Fig. 1
figure 1

Geological map of the Cadomian basement of the Lausitz Block (Saxo-Thuringian Zone) including deposits of the Lausitz-Leipzig Greywacke Complex (latest Ediacaran to early Cambrian), Early Cambrian intrusions of the Lausitz Granitoid Complex, and geological units of adjoining areas (map after Linnemann et al. 2007). Working area was situated within the black rectangle

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The study area is situated in the Saxo-Thuringian Zone at the northern periphery of the Bohemian Massif and forms the central Variscides (Kossmat 1927; Fig. 1). Field relationships imply an intrusion of the pre-Variscan granitoids among numerous other plutonic rocks the Cadomic sediments and volcanics of the Lausitz Block and the Karkonosze–Izera Massif (Domečka 1970; Opletal et al. 1983). The Cadomian Basement of the Saxo-Thuringian Zone comprises many different rock types due to multiple tectono-metamorphic events along the margin of northern Gondwana in mid-Neoproterozoic and earliest Cambrian (Linnemann et al. 2010a). Terranes, microcontinents, and crustal units, such as Saxo-Thuringia and Iberia, were affected. The opening of an oceanic ridge caused uplifting of the continental crust and subsidence of the upper plate (Linnemann et al. 2007). An emerging heat flow caused a phase of magmatic activity, e.g., the intrusion of the Rumburk granite (Linnemann et al. 2007). This configuration led to a complex and heterogenic composition of the Saxo-Thuringian Zone.

The first magmatic event forming the Lausitz Block and the Karkonosze–Izera Massif in the earliest Cambrian was dated at c. 540 Ma (Kröner et al. 1994; Gehmlich et al. 1997; Tichomirowa 2002). A second pulse of magmatic activity is dated at c. 505 Ma (Kröner et al. 1994; Gehmlich et al. 1997; Oberc-Dziedzic et al. 2009), suggesting a significant gap of c. 35 Ma.

In this paper, we present a new age for the Rumburk granite. We compare these new radiometric ages with previously published data to demonstrate the links with other granitoids of the Saxo-Thuringian Zone. Furthermore, we point out potential source rocks which were molten and recycled by the Rumburk granite, based on radiometric and geochemical data. Finally, our data will help to understand the general tectonic setting at the northern margin of Gondwana in late Cambrian times.

Geological setting

The Saxo-Thuringian Zone showcases widespread evidence of alteration during the Cadomian orogeny. This major tectonic event began c. 750 Ma ago and climaxed at the end of the Neoproterozoic to the earliest Cambrian at c. 530 Ma (e.g., Linnemann et al. 2007). At this time the Saxo-Thuringian Zone was a mobile belt situated north of Gondwana. In this zone, only the younger igneous rocks and derived sediments from related magmatic events are preserved (Buschmann 1995; Buschmann et al. 2001; Linnemann et al. 2007). These rocks originated during the period of c. 570–530 Ma and form the so called “Cadomian Basement” (Linnemann et al. 2008a). The basement consists of rocks whose origin is related to the Cadomian orogeny (Linnemann and Buschmann 1995a, b). The sediments of the Cadomian Basement are diverse and consist of turbiditic greywackes, shales, quartzites, and conglomerates. They show a maximum age of deposition of c. 570–545 Ma (Linnemann et al. 2007). Coeval to a post-depositional deformation of the sediments, large plutons intruded the complexes at c. 540–530 Ma, due to an arc-continent collision (Gehmlich 2003, Linnemann et al. 2000, 2007; Tichomirowa et al. 2001).

The Saxo-Thuringian Zone features a number of different volcano-sedimentary units, which are evidence for a volcanic arc-related origin during the Cadomian orogeny. At first the Cadomian back-arc basin opened at c. 580–560 Ma (Linnemann et al. 2007). This passive margin glaciomarine diamictic sequence is made up of greywackes (Linnemann 1991; Linnemann and Romer 2001), whereas the active margin sequence consists of volcano-sedimentary units (Linnemann and Romer 2002). The consequent closure and eventual collision of the arc with the passive margin accompanied with folding and thrusting led to the formation of a retro-arc basin and the subsequent deposition of the Lusatian greywackes in latest Neoproterozoic to earliest Cambrian (545–540 Ma; Linnemann et al. 2007).

The last stage of the Cadomian orogeny in the Saxo-Thuringian Zone was a phase of high magmatic activity and intrusions of voluminous granitoids at c. 540–530 Ma (e.g. Gehmlich 2003; Zulauf et al. 1999; Tichomirowa et al. 2001; Linnemann et al. 2000). To generate such heat, Linnemann et al. (2007) proposed a possible slab break-off causing this intense phase of magmatism. The ongoing extension of the retro-arc basin led to a thinned and easily intruded crust. During this state, the Lusatian greywackes were molten and recycled by the granitoid magmas, in which they occur as xenoliths (Hammer 1996). The absence of Cadomian high-pressure rocks in Saxo-Thuringia suggests no crustal thickening (Linnemann et al. 2000).

Following on in the Cambrian (c. 530–500 Ma), an asymmetric rift basin comparable to the current Basin and Range Province developed (Nance and Murphy 1996; Nance et al. 2002). The former active margin evolved into a transform margin, which was the beginning of the opening of the Rheic Ocean (Pin and Marini 1993; Nance and Murphy 1996; Kryza and Pin 1997; Nance et al. 2002). During the transition from a collisional to a rift-related setting along the northern margin of Gondwana a second pulse of high magmatism occurred and led to the intrusion of the Rumburk granite and the Izera–Kowary unit of the Karkonosze–Izera Massif (Borkowska et al. 1980; Kröner et al. 1994; Gehmlich et al. 1997; Pin et al. 2007; Oberc-Dziedzic et al. 2009; Białek et al. 2014; Fig. 1). Furthermore, this tectonic regime was confined by the coeval occurrence of the mid-ocean ridge-related Vesser Complex (c. 500 Ma; Bankwitz et al. 1992; Kemnitz et al. 2002; Linnemann et al. 2007). The Vesser Complex may belong to the outer margin of Saxo-Thuringia (Linnemann et al. 2010b) and documents a continental break-up event and a developing intraplate rift (Pin et al. 2007).

The opening of the Rheic Ocean during the latest Cambrian to earliest Ordovician was accompanied by thinning and stretching of the Cadomian crust, eventually leading to an ongoing, expanding rift basin (Linnemann et al. 2007). The Cambro–Ordovician boundary sequence (c. 485 Ma) of the Saxo-Thuringian Zone features numerous volcanic and tuffitic rocks, as a result extensive mid-oceanic ridge volcanism (e.g., Borkowska et al. 1980; Gehmlich 2003; Linnemann et al. 2000; Tichomirowa et al. 2001). This phase of volcanic activity represents the final rift-related magmatism along the northern margin of Gondwana (Linnemann et al. 2007, 2010a).

In some other models, the occurrence of late Cambrian Saxo-Thuringian magmatism is confined to a supra-subduction of a volcanic arc setting (Oliver et al. 1993; Kröner and Hegner 1998; Kröner et al. 1994, 2001), which was situated at the northern margin of Avalonia.

Previous geochronology

There have been numerous ages published for the Rumburk granite and the Izera Massif. The first radiometric age of the Rumburk granite was proposed by Borkowska et al. (1980) with 492 ± 45 Ma (Rb/Sr). A U–Pb age of 493 ± 2 Ma reported by Oliver et al. (1993) is somewhat problematic, due to a reported lead loss. Hence, an age slightly older cannot be excluded. The Izera gneiss dated by Korytowski et al. (1993) gave a U–Pb age of 515 ± 6 Ma. Kröner et al. (1994) proposed a Pb/Pb evaporation age of 571 ± 16 Ma (Ediacaran). Hammer et al. (1999) suggested late Cambrian ages of 490 ± 3 and 494 ± 12 Ma (Pb/Pb single zircon dating). The Izera metagranite dated by Philippe et al. (1995) gave a U–Pb age of 514 ± 5 Ma. More recent studies by Tichomirowa et al. (2001) presented an age for the Rumburk granite of 488 ± 7 Ma, respectively, 486 ± 5 Ma (Pb/Pb evaporation). Żelaźniewicz et al. (2004) obtained U/Pb ages ranging from 533 ± 9 Ma to 548 ± 8 Ma of granodiorites and granodioritic gneisses most likely to be correlated with the granodiorites of the Lausitz Block, not with the younger, Early Cambrian rift-related magmatics. The most recent age of an equivalent for the Rumburk granite was obtained by Białek et al. (2014) with 511 ± 5 Ma (granodiorite, east Lusatia).

The majority of published ages of the Rumburk granite and equivalents range between 480 and 515 Ma, and suggest a connection with the Late Cambrian to earliest Ordovician magmatism high in Saxo-Thuringia. This variation of ages allows no final determination of what the true age of the Rumburk granite is—Late Cambrian or earliest Ordovician (see Geological overview).

Samples

Both samples in our study were taken from the Rumburk granite on the Lausitz Block in eastern Saxony. U–Pb dating was performed on 120 zircons of these samples. U–Pb–Th data and Th/U ratios of igneous and inherited zircon grains from the two investigated granites are presented in Table 1. Sample locations are shown in Fig. 2 and an outcrop photograph is shown in Fig. 3a. Concordia, binned frequency, and probability density distribution plots of all analyzed zircon grains are shown in Figs. 4 and 5.

Table 1 LA-SF-ICP-MS U, Pb, and Th data of single zircon grains of the Rumburk granite. Within each sample, the analyses are sorted by ascending order by 206Pb–238U age for values below 1.0 Ga and by 207Pb–206Pb ages for values above 1.0 Ga
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Fig. 2
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Geological map of the southeastern Lausitz with indicated sample locations (map after Alexowsky and Leonhardt 1994)

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Fig. 3
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a Outcrop of the location of sample RG2 (N50°57′35.5″ E14°53′16.6″). b Polished sample of RG2 with distinguishable quartz and feldspar megacrysts

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Fig. 4
figure 4

Selected cathodoluminescence images of zircon grains representing the main age groups for all samples. Circles indicating the laser spots with a diameter of 35 and 25 µm. For every spot, the U–Pb age is given with the 2σ error in Ma

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Fig. 5
figure 5

Concordia plots of all analyzed magmatic zircon grains. a Concordia diagram of all measured zircon grains in the range of 400–2500 Ma and of 400–700 Ma (sample RG1, Rumburk granite, Late Cambrian). b Concordia diagram of all measured zircon grains in the range of 400–2500 Ma and of 400–700 Ma (sample RG2, Rumburk granite, Late Cambrian)

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Sample RG1 (coordinates: N50°57ʹ27.7ʺ; E14°53ʹ11.8ʺ) was collected from the Rumburk granite outcrop near road B66 (Fig. 2). The Rumburk granite intruded the Lausitz Granitoid and Lausitz Greywacke Complexes (Fig. 2). The second sample of Rumburk granite (RG2, coordinates: N50°57ʹ35.5ʺ; E14°53ʹ16.4″) was collected from an abandoned quarry 1 km north of Hirschfelde (Fig. 2).

The two granite samples are coarse-grained with a hypidiomorphic-granular texture (Fig. 3b). The structure of sample RG2 is unordered with no evidence of a metamorphic alteration, whereas in contrast sample RG1 was collected close to a shear zone. Thus, it is characterized by a gneissic texture. The bluish color of the Rumburk granite is due to cm-sized blue quartz crystals, which have a partly idiomorphic structure. A possible explanation of the blue color is the presence of an overabundance of minute rutile needles, which cause an optical effect (Postelmann 1937). Besides the blue color, microcline megacrysts are typical for the Rumburk granite (Borkowska et al. 1980). The megacrysts are up to 4 cm long. Other components of the matrix include plagioclase (oligoclase or albite) and black to reddish-brown mica (biotite and muscovite) clusters. Unlike the megacrysts, these crystals are only millimeters in length. There are also apatite and magnetite as inclusions.

Methods

Zircon concentrates were separated from 2 to 4 kg sample material at the Senckenberg Naturhistorische Sammlungen Dresden (Museum für Mineralogie und Geologie) using standard methods. Final selection of the zircon grains for U–Pb dating was achieved by hand-picking under a binocular microscope. Zircon grains of all grain sizes and morphological types were selected, mounted in resin blocks, and polished to half their thickness. Zircons were analyzed for U, Th, and Pb isotopes by LA-SF ICP-MS techniques at the Museum für Mineralogie und Geologie (GeoPlasma Lab, Senckenberg Naturhistorische Sammlungen Dresden), using a Thermo-Scientific Element 2 XR sector field ICP-MS (single-collector) coupled to a New Wave UP-193 Excimer Laser System. A teardrop-shaped, low volume laser cell constructed by Ben Jähne (Dresden) and Axel Gerdes (Frankfurt/M.) was used to enable sequential sampling of heterogeneous grains (e.g., growth zones) during time-resolved data acquisition. Each analysis consisted of approximately 15 s background acquisition followed by 30 s data acquisition, using a laser spot-size of 25 and 35 µm, respectively. A common-Pb correction based on the interference- and background-corrected 204Pb signal and a model Pb composition (Stacey and Kramers 1975) was carried out if necessary. The necessity of the correction is judged on whether the corrected 207Pb/206Pb lies outside of the internal errors of the measured ratios (Frei and Gerdes 2009). Discordant analyses were generally interpreted with care. Raw data were corrected for background signal, common Pb, laser-induced elemental fractionation, instrumental mass discrimination, and time-dependant elemental fractionation of Pb/Th and Pb/U using an Excel® spreadsheet program developed by Axel Gerdes (Institute of Geosciences, Johann Wolfgang Goethe-University Frankfurt, Frankfurt am Main, Germany). Reported uncertainties were propagated by quadratic addition of the external reproducibility obtained from the standard zircon GJ-1 (~0.6% and 0.5–1% for the 207Pb/206Pb and 206Pb/238U, respectively; Jackson et al. 2004) during individual analytical sessions and within-run precision of each analysis. In order to test the accuracy of the measurements and data reduction, we included the Plesovice zircon as a secondary standard in our analyses. Repetitive measurements over the last years of the Plesovice zircon resulted in the age of c. 337 Ma, which fits the results of Slama et al. (2008). Concordia diagrams (2σ error ellipses) and concordia ages (95% confidence level) were produced using Isoplot/Ex 2.49 (Ludwig 2001) and frequency and relative probability plots using AgeDisplay (Sircombe 2004). The 207Pb/206Pb age was taken for interpretation for all zircons > 1.0 Ga, and the 206Pb/238U ages for younger grains. Further details of the instruments settings are available from Table 1. For further details on analytical protocol and data processing see Gerdes and Zeh (2006). Zircons showing a degree of concordance in the range of 90–102% in this paper are classified as concordant because of the overlap of the error ellipse with the concordia. Concordia ages were derived from analyses with a degree of concordance between 98 and 102%, to exclude led loss effects during age calculation. Th/U ratios are obtained from the LA-ICP-MS measurements of investigated zircon grains. U and Pb content and Th/U ratio were calculated relative to the GJ-1 zircon standard and are accurate to approximately 10%. Analytical results of U–Th–Pb isotopes and calculated U–Pb ages are given in Table 1. The stratigraphic time scale of Gradstein et al. (2012) was used.

Hafnium isotope measurements were taken with a Thermo-Finnigan NEPTUNE multi-collector ICP-MS at Goethe University Frankfurt (Frankfurt/Main) coupled to RESOlution M50 193 nm ArF Excimer (Resonetics) laser system following the method described in Gerdes and Zeh (2006, 2009). Spots of 40 µm in diameter were drilled with a repetition rate of 4.5–5.5 Hz and an energy density of 6 J/cm2 during 50 s of data acquisition. The instrumental mass bias for Hf isotopes was corrected using an exponential law and a 179Hf/177Hf value of 0.7325. In the case of Yb isotopes, the mass bias was corrected using the Hf mass bias of the individual integration step multiplied by a daily βHf/βYb offset factor (Gerdes and Zeh 2009). All data were adjusted relative to the JMC475 of 176Hf/177Hf ratio = 0.282160 and quoted uncertainties are quadratic additions of the within-run precision of each analysis and the reproducibility of the JMC475 (2SD = 0.0028%, n = 8). Accuracy and external reproducibility of the method was verified by repeated analyses of reference zircon GJ-1 and Plesovice, which yielded a 176Hf/177Hf of 0.282007 ± 0.000026 (2 SD, n = 42) and 0.0282469 ± 0.000023 (n = 20), respectively. This is in agreement with previously published results (e.g., Gerdes and Zeh 2006; Slama et al. 2008) and with the LA-MC-ICP-MS long-term average of GJ-1 (0.282010 ± 0.000025; n > 800) and Plesovice (0.282483 ± 0.000025, n > 300) reference zircon at GUF.

The initial 176Hf/177Hf values are expressed as εHf(t), which is calculated using a decay constant value of 1.867 × 10–11 year−1, CHUR after Bouvier et al. (2008; 176Hf/177HfCHUR,today = 0.282785 and 176Lu/177HfCHUR,today = 0.0336) and the apparent Pb–Pb ages obtained for the respective domains (Table 2). For the calculation of Hf two-stage model ages (TDM) in billion years, the measured 176Lu/177Lu of each spot (first stage = age of zircon), a value of 0.0113 for the average continental crust, and a juvenile crust 176Lu/177LuNC = 0.0384 and 176Hf/177Hf NC = 0.283165 (average MORB; Chauvel et al. 2008) were used.

Table 2 LA-MC-ICP-MS Lu–Hf isotope data of single zircon grains of the Rumburk granite (Upper Cambrian)
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The geochemical analyses of the rock samples were carried out by FUS-ICP and FUS-MS by Actlabs in Ancaster (Ontario, Canada).

Results

Zircon characteristics

Both samples feature euhedral to subhedral, short-to-normal prismatic zircon crystals with a lengths of 100–250 µm. The grains are transparent, colorless, and show infrequently simple-to-strong zonings in the cathodoluminescence images (Fig. 4). The zircons feature indistinct and irregularly structured interiors. Approximately 10% of the zircon population shows a complex structure with rounded or irregular cores and zoned rims. The zircons are mostly bright in CL.

U–Pb analysis

Analytical results are presented in Figs. 4, 5, 6, 7, 8, 9, 10 and in Table 1. From sample RG1 (Rumburk granite, Lausitz Block), 60 magmatic zircons were analyzed (Table 1). Thirty-two grains show a degree of concordance in the range of 98–102% (Fig. 5a). The youngest concordant grain has an age of 500 ± 14 Ma (Upper Cambrian). The oldest zircon yields an age of 2187 ± 25 Ma. The majority of the sampled zircons (63%) are Cambrian in age, whereas 31% of the grains cluster into the Neoproterozoic range. Only 6% of the sampled zircons yield Mesoproterozoic and Paleoproterozoic ages (1360 ± 43, 2187 ± 25 Ma).

Fig. 6
figure 6

Combined binned frequency and probability density distribution plots of all analyzed zircon grains of the Rumburk granite

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Fig. 7
figure 7

Concordia plots of complex single zircon grains and its spots. a RG1 a12-a13-a14. b RG1 a27-a28. c RG1 b9-b10. d RG2 a5-a6. e RG2 a25-a26

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Fig. 8
figure 8

a Concordia plot of the zircon age of the sample RG1. b Concordia plot of the zircon age of the sample RG2. c Concordia plot of the combined zircon ages of the samples RG1 and RG2

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Fig. 9
figure 9

Th–U versus zircon age diagram of all analyses with a degree of concordance within 90–102%, showing that the majority of zircons were derived from felsic melts. Only a minority of 14 grains show evidence for metamorphic melts (Th–U ratios lower 0.1). The diagram is based on the Th–U ratios given in Table 1

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Fig. 10
figure 10

εHf(t) versus age diagram of selected single zircon grains. The continental crust evolution trends of the main components of the West African Craton and the Cadomian orogeny are shown in different colors. See text for discussion

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From sample RG2 (Rumburk granite, Lausitz Block), 60 magmatic zircons were analyzed (Table 1). Of them, 30 show a degree of concordance in the range of 98–102% (Fig. 5b). The youngest concordant grain yields an age of 499 ± 12 Ma (Late Cambrian). The oldest zircon has an age of 2452 ± 45 Ma (Paleoproterozoic). Like sample RG1, a majority of 57% of all analyzed zircons are Cambrian in age. Only 37% of the grains are Neoproterozoic and 6% of the investigated zircon grains fall into the range of the Paleoproterozoic.

Of the 120 zircon grains analyzed, 62 exhibited a degree of concordance between 98 and 102%. The concordant zircon grains can be divided into several age groups. The main group shows a peak of c. 504 Ma (Series 3; Fig. 6), whereas the age peak of the second largest group is at c. 538 Ma (earliest Cambrian, Fig. 6). There is also a minor Neoproterozoic peak at 613 Ma (Fig. 6). Furthermore, both samples inherit grains with Mesoproterozoic and Paleoproterozoic ages (Fig. 6).

Three complex zircon grains of sample RG1 were analyzed. The rounded cores gave two Neoproterozoic and one Paleoproterozoic age of 563 ± 15, 541 ± 15, and 2133 ± 27 Ma (Fig. 7a–c). In contrast, the younger magmatic rims gave Upper Cambrian ages of 502 ± 12 Ma, respectively, 506 ± 16, 502 ± 13 and 502 ± 9 Ma (Fig. 7a–c).

From sample RG2, two complex zircon grains were analyzed. The two-rounded cores gave one Neoproterozoic age of 613 ± 15 Ma, whereas one Early Cambrian core gave an age of 534 ± 11 Ma (Fig. 7d, e). Like the complex grains of sample RG1, the magmatic rims of sample RG2 gave Late Cambrian ages of 507 ± 12 and 501 ± 11 Ma.

A concordia age calculated for the seven youngest zircon grains of sample RG1 with a concordance between 98 and 102% is at 504 ± 4 Ma (Fig. 8a). The calculated concordia age for sample RG2 for the eleven youngest zircon grains (concordance between 98 and 102%) is at 503 ± 3 Ma (Fig. 8b). The combined age for the 18 youngest zircon grains of both samples is at 504 ± 3 Ma (Fig. 8c).

Th/U and Hf-Analysis

The calculated Th/U ratios are below 1.0 for the majority (82.05%) of all analyzed zircon grains with concordant (90–102%) U–Pb ages, which may point to a felsic melt composition (e.g., Hoskin and Schaltegger 2003; Linnemann et al. 2007). In addition, there is a smaller zircon population of 14 grains (17.95%) with Th/U ratios below 0.1. This is an indicative of a strong metamorphic overprint (Wang et al. 2011). It should be noted that there are no Th/U ratios higher than 0.70 (Fig. 9).

All Lu–Hf isotope data may be found in Table 2. We considered zircons with a degree of concordance between 90 and 102%. The εHf(t) values of 16 Neoproterozoic, Cambrian, and Ordovician zircons from both samples are similar and range from −6.4 to 2.0 with TDM ages between 1000 and 1300 Ma (Fig. 10). Only one measurement (RG2, a09) shows a very low εHf(t) value of −23.6, resulting in a TDM age of 2500 Ma (Paleoproterozoic to Archean). The data suggest that the zircon forming melt was derived from a depleted mantle source mixed with various amounts of older crust. A contamination of the juvenile magma (e.g., island arcs) with Eburnian and even Archean crust is possible.

Geochemical data

All geochemical data are summarized in Table 3. The major element analysis demonstrates that the granites are peraluminous rocks with A/CNK as high as 1.65 (Fig. 11a). The multi-element diagram of the trace elements concentrations normalized to chondrites (Fig. 11b) shows a parallel distribution and strongly suggests a genetic link of both samples. The diagram also displays significant Nb, Sr, and Ti anomalies which point to fractionation processes and/or scarcity of these elements in the source material. The negative Nb anomaly is considered as middle crust material by Wilson (1989). An Nb anomaly is also well developed in the upper crust (Taylor and McLennan 1985). The high negative Sr and Ti anomalies reflect a highly evolved magma and point to a fractionation of plagioclase, apatite, and ilmenite. The chondrite-normalized plots of the REE of both samples show a strong Eu anomaly (Fig. 11c). The variation of the trace elements and REEs points to a fractional crystallization of plagioclase. The Rb/Y discrimination diagram supports a volcanic arc granite + syncollisional geotectonic setting for the Rumburk granite (Lausitz Block) as well as for seven published analyses of rocks of the Karkonosze–Izera Massif (Fig. 11d). The Hf–Rb/10-Ta*3 and Hf–Rb/30-Ta*3 for the sampled Rumburk granite (Lausitz Block) as well as for seven granitic and metamorphic rocks of the Karkonosze–Izera Massif are plotted in the field of the volcanic arc granite (Fig. 11e, f). This similarity points to a genetic link of the Rumburk granite and Izera gneiss.

Fig. 11
figure 11

Plots of geochemical data for samples of the Rumburk granite and Izera gneiss. a Aluminousity of both samples (Rumburk granite); Note: both data points lying almost on top of each other. Divisions after Maniar and Piccoli (1989). b Chondrite-normalized REE pattern (after Thompson 1982) of both samples showing a negative Nb anomaly. c Chondrite-normalized REE patterns (after Nakamura 1974) of both samples. A negative Eu anomaly points to felsic source of the melt. d Nb/Y discrimination diagram after Pearce et al. (1984) for the Rumburk granite and selected rocks of the Karkonosze–Izera Massif and the Lausitz Block. e, f Hf–Rb/10-Ta*3 discrimination diagram after Harris et al. (1986) and Hf–Rb/30-Ta*3 discrimination diagram after Pearce et al. (1984) for both samples as well as for selected rocks of the Karkonosze–Izera Massif. All point to a volcanic arc setting. VAG volcanic arc granites, WAG within-plate granites, syn-COLG syncollisional granites, ORG ocean ridge granite, LCG–PCG late-collisional–post-collisional granites. Diagrams are based on data given in Table 3

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Table 3 Major (wt%) and trace element (ppm) whole rock analysis of the Rumburk granite
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Discussion

Significance of the radiometric ages

In our study, the investigated zircon grains provide a combined U–Pb age of 504 ± 3 Ma for the Rumburk granite (Fig. 8). Both samples overlap in error. Despite that it might be an age of a reworked intrusion, we interpret our calculated age as the time of emplacement for the Rumburk granite. Hence, the new age fits well with other ages obtained from igneous rocks of the Lausitz Block and the Karkonosze–Izera Massif of c. 510–500 Ma (Late Cambrian, Fig. 12).

Fig. 12
figure 12

Generalized lithostratigraphic columns of Late Cambrian volcanic rocks of the Saxo-Thuringian zone. Geochronological data given in Ma (numbered circles): 1 LA-ICP-MS U–Pb (Linnemann & Gerdes, unpubl. data), 2 LA-ICP-MS U–Pb (Linnemann et al. 2007), 3 TIMS U–Pb (Kemnitz et al. 2002), 4 TIMS Pb–Pb (Linnemann et al. 2000), 5 single zircon Pb/Pb evaporation (Gehmlich 2003), 6 single zircon Pb/Pb evaporation (Tichomirowa 2002), 7 SHRIMP U–Pb (Białek et al. 2014), 8 single zircon Pb/Pb evaporation (Kröner et al. 2001), 9 SHRIMP U–Pb (Oberc-Dziedzic et al. 2009)

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Our age determined for the Rumburk granite supports the model of three distinct phases of high magmatic activity in Late Cambrian to earliest Ordovician in Saxo-Thuringia (e.g., Gehmlich 2003; Linnemann and Heuse 2000; Linnemann et al. 2000; Kemnitz et al. 2002; Linnemann et al. 2007; Linnemann et al. 2010b). A link with the first magmatic phase in earliest Cambrian (c. 540 Ma; e.g., Linnemann et al. 2000; Tichomirowa et al. 2001; Żelaźniewicz et al. 2009) was discussed by Białek (2003), Kröner et al. (2001), Żelaźniewicz et al. (2003), and Oberc-Dziedzic et al. (2005). In our view, this relationship may be excluded, due to a gap of c. 40 Ma relative to the age of the Rumburk granite. We propose a link with the second magmatic phase (c. 500–510 Ma; Fig. 12), which is confined by the occurrence of several other Early Cambrian granitoids and protoliths of metamorphic rocks of the Lausitz Block and the Karkonosze–Izera Massif. Coeval with the latter magmatics (e.g., Rumburk granite) the MOR-related Vesser Complex developed (Kemnitz et al. 2002; Linnemann et al. 2007; Figs. 12, 13), which is confined as the outer domain of Saxo-Thuringia (Bankwitz et al. 1992), whereas the Lausitz Block and the Karkonosze–Izera Massif represent the inner domain. This illustrates the consequence of a changing tectonic regime from the Cadomian orogeny to the Cambro–Ordovician rifting, the initial phase of the opening of the Rheic Ocean. The overlapping Late Cambrian ages suggest that the Rumburk granite and the Izera–Karkonosze Massif intruded into relatively thin transitional crust, separating the oceanic crust (Vesser Complex) from Gondwanan continental crust (Fig. 13). This agrees well with the proposed plate tectonic model for the Late Cambrian (Linnemann et al. 2007, 2010a; Pin et al. 2007; Białek et al. 2014). A relation with the third phase of magmatism in lowermost Ordovician is unlikely because of an age gap of c. 20 Ma (Fig. 12). However, based on the available data, other tectonic models cannot be excluded completely.

Fig. 13
figure 13

Plate tectonic model for the opening of the Rheic Ocean during the Late Cambrian between ca. 510 and 495 Ma in the Saxo-Thuringian zone (modified after Linnemann et al. 2007)

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Contrary to the proposed geotectonic model, geochemical discrimination diagrams reveal somewhat problematic results. Both analyzed samples, as well as Late Cambrian samples of the Karkonosze–Izera Massif are volcanic arc granites (Fig. 11d–f). This case was already discussed by Oberc-Dziedzic et al. (2005) and Pin et al. (2007). Both authors pointed out that geochemical discrimination diagrams for granites can be misleading. Granitic magmas mainly reflect the geological setting of their source magma and not the setting of the analyzed rock itself. Hence, source rocks may be volcanic arc-related, but not the resulting rocks (Rumburk granite/Karkonosze–Izera Massif). This was not considered in the model proposed by Kröner et al. (2001), which remains problematic (for further discussion see Oberc-Dziedzic et al. 2005).

Source rocks

Our data suggest that the Rumburk granite intruded rocks similar to the Lusatian granodiorites and greywackes. The geochronological analysis reveals several different age peaks for the Rumburk granite (Figs. 4, 5), similar to inherited zircon ages of the Karkonosze–Izera Massif (Kröner et al. 1994; Białek et al. 2014). These peaks are similar to the age peaks of the Lusatian greywacke (c. 2.4–1.9 Ga and c. 700–560 Ma) also reported by Kröner et al. (1994), Tichomirowa et al. (2001), and Linnemann et al. (2004). The samples we studied yielded inherited zircon grains with Early Cambrian and Neoproterozoic (Ediacaran) ages. In addition, a few Paleoproterozoic grains were recognized. This pattern is characteristic for the West African hinterland as well as for the Cadomian arc and back-arc basin (e.g., Linnemann et al. 2004, 2007; Altumi et al. 2013; Abukabar et al. 2017). A relatively cool magma caused no or only partial melting of the zircons (Fig. 7). Moreover, the multi-element diagram for both samples shows strong Nb, Sr, and Ti anomalies, which are typical for melts of the upper crust (Taylor and McLennan 1985, Fig. 11b). Negative Nb, Sr, and Ti anomalies for the Rumburk granite/Karkonosze–Izera Massif were also obtained by Oberc-Dziedzic et al. (2005), Pin et al. (2007), and Białek et al. (2014). This also suggests a sedimentary source as possible material, similar to the Lusatian greywackes. Furthermore, a strong negative Ti anomaly of the Lower Cambrian rocks indicates a melting and recycling of the Ti-poor Lusatian greywackes by the Rumburk granite (Oberc-Dziedzic et al. 2005; Białek 2007, 2014; Fig. 11b). The observed negative Eu anomaly may point to an advanced plagioclase fractionation during the magma differentiation (e.g., Hammer 1996; Tichomirowa et al. 2001; Oberc-Dziedzic et al. 2005; Pin et al. 2007; Fig. 11c). In addition, the majority of Th/U ratios plot in the field of felsic melt composition (Fig. 9), which points to a crustal source for the Rumburk granite (Hoskin and Schaltegger 2003; Linnemann et al. 2007).

Complex zircon grains of the Rumburk granite inherit cores of c. 540 Ma, which are also evidence for a magmatic event at c. 540 Ma (Fig. 7b, e). Moreover, a distinct age peak at c. 540 Ma (Fig. 6) strongly supports the consideration of a melting and recycling of vast amounts of the possibly slab break-off-induced Lower Cambrian granitoids of the Lausitz Block by the Rumburk granite (e.g., Linnemann et al. 2007; Białek et al. 2014;). In addition, the peraluminousity of the granites gives hints for possible rift-derived source rocks, like the Lusatian greywackes (Philippe et al. 1995; Oberc-Dziedzic et al. 2005; Fig. 11a).

Aside from the single Mesoproterozoic grain, the age population of the Rumburk granite reflects the typical West African magmatic gap from 1000 to 1600 Ma (Linnemann et al. 2008b). A probable source area of the four Paleoproterozoic zircons is the eastern part of the Reguibat Shield, which is dominated by Eburnian rocks (1.8–2.2 Ga) (Ennih and Liegeois 2008; Rocci et al. 1991). A possible source area for Mesoproterozoic zircon ages in Cadomian rocks is the West African Craton, which features Neoproterozoic sediments as well as granites and gneisses with Mesoproterozoic zircon ages (Gärtner et al. 2013, Bradley et al. 2015). Another possible origin of Mesoproterozoic ages is the Amazonian craton, which yields zircon ages of 1.0–1.6 Ga (Schneider Santos et al. 2000). The 25 Neoproterozoic zircon ages (c. 630–570 Ma) recycled by the Rumburk granite are evidence for major orogenic events and crustal growth at the north Gondwana margin as a result of the Cadomian orogeny and are considered to represent the magmatic arc of the Cadomian back-arc basin. Probability density plots fit well with the assumed late-stage basin development in Ediacaran c. 580–560 Ma (Linnemann et al. 2007; Figs. 4, 5).

Furthermore, the obtained Hf data of the Rumburk granite can give hints about the general tectonic setting at the northern margin of Gondwana as well as possible source rocks of the Rumburk granite. All but one εHf values of the Rumburk granite give TDM ages of c. 1.0–1.5 Ga (Fig. 10). Hence, in our view this can be explained by a reworking and mixing of Eburnian and Archean crust with juvenile magma of the newly formed Cadomian crust, since no major crustal growth events occurred during Mesoproterozoic. Moreover, no εHf values fall into the depleted mantle array, which can be considered as crustal contamination. The lack of high εHf values, indicating juvenile magma also hints to a recycling of older (Cadomian) crust (Linnemann et al. 2014). One grain gives a model age of c. 2.5 Ga and can be interpreted as an almost pure reworking of Archean crust. This fits well with the proposed geodynamic setting of the northern margin of Gondwana according to Linnemann et al. (2007). During the developing asymmetric rift basin, magma intruded the thinned transitional crust and was mixed with older already, Cadomian orogeny induced, mixed continental crust in Early Cambrian.

Conclusions

  1. 1.

    Sample RG1 yielded a youngest U–Pb age of 504 ± 4 Ma, whereas sample RG2 gave a youngest age of 503 ± 3 Ma. The combined age of both samples is 504 ± 3 Ma (Upper Cambrian), which is interpreted as the age of emplacement of the Rumburk granite.

  2. 2.

    Inherited zircon grains of the Rumburk granite clustering at ages of c. 540 Ma indicating an Early Cambrian magmatic high. The obtained Early Cambrian emplacement age for the Rumburk granite (504 ± 3 Ma) suggests a link with the initial opening of the Rheic Ocean. The coeval occurrence of Rumburk granite and Vesser Complex indicating an intrusion of the Rumburk granite into transitional crust, whereas the rift-related rocks of the Vesser Complex represent the outer margin. This follows the proposed transition from the final phase of the Cadomian orogeny to a Cambro–Ordovician rift-related setting at the northern margin of Gondwana. Our data implicate a clear separation of the Late Cambrian magmatic event (c. 505 Ma) from the last phase of high magmatism in uppermost Cambrian to lowermost Ordovician in Saxo-Thuringia (c. 485 Ma).

  3. 3.

    As possible source rocks for the Rumburk granite, we consider rocks like the granitoids of the Lausitz Block and the Lusatian greywackes. A distinct age peak of inherited zircons at c. 540 Ma is characteristic for the intrusions of the late phase of the Cadomian orogeny, which implies a melting and recycling of high amounts of these rocks by the Rumburk granite. Ages of c. 650–570 Ma show a strong similarity with ages obtained from Cadomian back-arc-related sediments. Furthermore an arc-related peraluminousity, most probably representing the geotectonic of the source rock, as well as a characteristic negative Ti anomaly of the Rumburk granite as for the Lusatian greywackes points to a genetic link. Moreover, negative Nd, Sr, and Ti anomalies are evidence for the recycling of upper crust. The few Paleoproterozoic ages may be derived from the West African Craton. In addition, TDM ages of c. 1.0–1.5 Ga are typical for the Cadomian orogen, which may be product of Eburnian and Archean crust mixed with juvenile magma during orogeny and subsequent recycling by the Rumburk granite.