Dating of detrital zircons and tracing the provenance of quartzites from the Bystrzyckie Mts: implications for the tectonic setting of the Early Palaeozoic sedimentary basin developed on the Gondwana margin

We studied the petrographical characteristics and bulk chemical composition along with the age of detrital zircon grains from quartzites exposed in the western part of the Orlica-Śnieżnik dome in the Saxothuringian zone. Age spectra of detrital zircons were obtained by U–Pb laser ablation inductively coupled plasma mass spectrometry dating. The zircon ages define a few clusters of which the most numerous are Cambro-Ordovician and Neoproterozoic, and less frequent are Paleoproterozoic and Archean. A distinctive feature of the investigated samples is the lack of Mesoproterozoic zircon grains. The youngest age cluster documented in the investigated quartzites points to a Late Cambrian maximum depositional age and detritus derivation from erosion of Cambro-Ordovician metagranitoids. Bulk chemical composition of the quartzites indicates that they originated owing to erosion of granite within sedimentary basins developed on a passive continental margin. Presented data prove the similarity of the examined rocks to quartzites known from the eastern part of the Orlica-Śnieżnik dome and quartz metasandstones documented in other parts of the Variscan Belt of Europe known as the Armorican quartzites or their equivalents exposed in the Saxothuringian zone. Our results support the suggestion that the Orlica-Śnieżnik dome has an affinity to the West African Craton of the Gondwana margin and represents a fragment of the Saxothuringian zone.


Introduction
The Variscan Belt of Europe comprises several terranes bearing a record of Neoproterozoic supra-subduction magmatic arc activity (Fig. 1). These terranes are commonly interpreted as derived from a long chain of Neoproterozoic continental magmatic arcs that developed along the Gondwana margin and are known as the Avalonian-Cadomian belt. The crustal fragments comprised within the belt are referred to in the literature as Cadomian and Avalonian terranes as well as Meguma, Ganderia, Carolina and Suwanee (e.g. Nance et al. 1991;van Staal et al. 1996;Pollock et al. 2012;Willner et al. 2014). Cadomian terranes are represented by e.g. the Armorican Massif, most of the Iberian Massif, Moldanubian and Saxothuringian zones of the Bohemian Massif (e.g. Murphy and Nance 1989;Fernández-Suárez et al. 2002a, b;Nance et al. 2002;Keppie et al. 2003;Linnemann et al. 2008), while Avalonian terranes comprise e.g., West and East Avalonia and Brunovistulicum of the Bohemian Massif (e.g. Finger et al. 2000;Friedl et al. 2004;Linnemann and Romer 2002;Fernandez-Suarez et al. 2003;Murphy et al. 2006;Drost et al. 2007). Subsequently, the Avalonian part of the belt was detached during Late Cambrian-Early Ordovician times from Gondwana during the formation of the Rheic Ocean (e.g. Linnemann et al. 2008;Murphy et al. 2009;Nance et al. 2010). This event was accompanied by the intrusion of several Cambro-Ordovician granites that intruded the crystalline basement of the Cadomian terranes (e.g. Kröner et al. 2001;von Raumer et al. 2002;Košler et al. 2004;Martínez Catalán et al. 2009;Zieger et al. 2017). Described rifting occurred during the deposition of a quartz-rich, mature and relatively thick sedimentary sequence recognized in Central and Western Europe. The sequence is extending over entire NE-Africa and is known as the Armorican quartzite exposed in the Armorican and Iberian Massifs or time-equivalent quartzite horizons cropping out in the Saxothuringian zone (e.g. Linnemann et al. 2007;Linnemann et al. 2008;Domeier 2016). Consequently, the Gondwana margin contains a record of the transition from an arc to a rift followed by a passive margin tectonic setting (e.g. Linnemann et al. 2007).
One of the crustal domains in the Variscan Belt of Europe with preserved Cadomian and post-Cadomian volcano-sedimentary sequences is represented by the Saxothuringian zone (e.g. Linnemann et al. 2000;Buschmann et al. 2001;Žáčková et al. 2010). Its easternmost part extends into the West Sudetes where it is cut by the Teplá-Barrandian/Saxothuringian suture considered as the easternmost termination of the Saxothuringian zone (Fig. 2;Mazur and Aleksandrowski 2001). However, structural and geophysical data suggest that southeast of the Teplá-Barrandian/Saxothuringian suture the Saxothuringian crust is concealed below the Teplá-Barrandian domain (e.g. Chopin et al. 2012;Jeřábek et al. 2016). Furthermore, Chopin et al. (2012) and Mazur et al. (2012) demonstrated that rock complexes of the Saxothuringian affinity emerge from below the Teplá-Barrandian domain in the Orlica-Śnieżnik dome (Central Sudetes). This view is supported by documentation of Neoproterozoic arcderived volcano-sedimentary rock successions as well as Cambro-Ordovician passive margin sequence exposed in the Orlica-Śnieżnik dome (Mazur et al. 2012(Mazur et al. , 2015Szczepański and Ilnicki 2014).
In this contribution, we present new data from wholerock geochemistry and age spectra of detrital zircon populations derived from quartzite samples collected in the western part of the Orlica-Śnieżnik dome (the Bystrzyckie Mts) representing an allochtonous fragment of the Saxothuringian zone. The youngest obtained age cluster of the metamorphosed quartzitic sandstones is interpreted in terms of the maximum depositional age of the sedimentary protolith. Moreover, age spectra obtained from the whole analysed population of grains as well as bulk rock geochemistry were used to describe a possible provenance of the detrital material and tectonic setting of a depositional basin. The new data allowed to discuss a potential paleogeographic scenario leading to the formation of the examined quartzites and processes responsible for the deposition of the Cambro-Ordovician sedimentary cover on the Saxothuringian crust.

Geological setting
The Orlica-Śnieżnik dome in the Central Sudetes comprises medium-grade supracrustal successions of Neoproterozoic to Late Cambrian age surrounding Cambrian granite bodies later transformed into orthogneisses (Fig. 2). The dome is cut by the Upper Cretaceous Nysa Kłodzka Graben, which divides it into the eastern Śnieżnik massif and the western Bystrzyca-Orlica massif. However, despite several lithological similarities between both massifs division of the dome by the graben impede easy correlations of rock successions between the Bystrzyca-Orlica massif and the Śnieżnik massif. Supracrustal successions exposed in the eastern part of the Orlica-Śnieżnik dome in the Śnieżnik massif are traditionally divided into: (1) the Młynowiec monotonous Formation composed of paragneisses intercalated with scarce basic metavolcanic rocks, (2) the Stronie variegated Formation dominated by micaschists interleaved with abundant bimodal volcanics and (3) a thin horizon of the Goszów quartzite (Fig. 3a, Don et al. 1990). The youngest detrital zircon population indicates a maximum sedimentation age of 563 ± 6 Ma, 532 ± 6 Ma and 490 ± 9 Ma for the Młynowiec Formation, the Stronie Formation and the Goszów quartzite, respectively. Hence they represent three distinct volcano-sedimentary successions (Mazur et al. 2012;Mazur et al. 2015; but see the alternative interpretation by Jastrzębski et al. 2010). The Neoproterozoic and Early Cambrian volcano-sedimentary successions in the Orlica-Śnieżnik dome were intruded by Mid-to Late Cambrian calc-alkaline granitoids with geochemical characteristics typical for crustal melts (Turniak et al. 2000). U-Pb mean ages, obtained by various methods, of magmatic zircons derived from orthogneiss range from 517 to 490 Ma and are interpreted as the time of intrusions (e.g. Turniak et al. 2000;Kröner et al. 2001;Lange et al. 2002).
Supracrustal successions in the western part of the Orlica-Śnieżnik dome (the Bystrzyca-Orlica massif) are not well recognized. The only age data comes from the Wyszki paragneiss (Fig. 3b) which comprises the monotonous metagreywacke succession with detrital zircon age spectra and maximum depositional age of 569 ± 8 Ma resembling metasedimentary rocks of the Młynowiec Formation exposed in the eastern part of the Orlica-Śnieżnik dome (Mazur et al. 2015). There are no data regarding maximum sedimentation age of the variegated succession dominated by mica schists from the western part of the Orlica-Śnieżnik dome (the Bystrzyca-Orlica massif). However, owing to the similar lithological inventory of this variegated succession and the Stronie Formation exposed in the Śnieżnik massif they are considered as equivalents. There are no age data for quartzite cropping out in the Bystrzyca-Orlica massif.
A lithostratigraphic scheme for the Neoproterozoic and Early Palaeozoic sequences exposed in the Orlica-Śnieżnik dome ( Fig. 3a) was proposed by Szczepański and Ilnicki (2014). It shows similarities to other parts of the Variscan Belt of Europe e.g., the Saxothuringian zone in Germany as well as the entire Iberian Massif except for the South Portuguese zone (e.g. Bandres et al. 2002;Linnemann et al. 2007;Nance et al. 2008;Vintaned et al. 2009). In all mentioned areas extending west of the Central Sudetes well-preserved fragments of Cadomian basement and post-Cadomian cover with magmatic arc signature are covered by Late Cambrian-Early Ordovician quartz-rich metasandstones (e.g. Pereira et al. 2006;Linnemann et al. 2008). Similar quartzite is also known from the eastern part of the Orlica-Śnieżnik dome (the Śnieżnik massif) as the Goszów quartzite. This rock displays a Late Cambrian maximum deposition age (490 ± 9 Ma, Mazur et al. 2012) and chemical composition typical for sediments deposited on a passive continental margin (Szczepański and Ilnicki 2014). On the other hand, the youngest detrital zircons reported by Jastrzębski et al. (2010) for the Goszów quartzite yielded ages in the time interval of 460-470 Ma. Interestingly, in the Goszów quartzite, Jastrzębski et al. (2016) documented a population of monazite grains dated at 494 Ma believed to represent a significant admixture of volcanic material. Consequently, the latter authors suggested a metavolcanic origin for at least some of the quartzites cropping out in the eastern part of the Orlica-Śnieżnik dome (the Śnieżnik massif).
The Goszów quartzite, exposed in the eastern part of the Orlica-Śnieżnik dome (the Śnieżnik massif), form thin, continuous and relatively well-defined lithological horizon. On the other hand, quartzites are known from the western part of the dome crop out in various tectonic units of the Bystrzyckie Mts. representing Polish part of the Bystrzyca-Orlica massif and form small and isolated lenses aligned parallel to the foliation observed in surrounding supracrustal variegated succession and orthogneisses (Fig. 3b). Accordingly, the tectonic position of the quartzites in the western part of the dome (the Bystrzyckie Mts) is not similar to what is observed in the eastern part of the dome in the Śnieżnik massif. Therefore, despite lithological similarities between the Goszów quartzite and the quartzites known from the Bystrzyckie Mts. there is still uncertainty whether these rocks are equivalent or should rather be concerned as representing different lithostratigraphic members.

Sample description
Four quartzite samples were collected at four localities within different tectonic units of the Bystrzyckie Mts (Fig. 3b). Samples PR01, PR03 and PR04 come from the Poręba Unit, while sample PR02 was collected in the Niemojów Unit. Tectonic units identified in the Bystrzyckie Mts. were interpreted as fragments of the Variscan imbricated nappe stack (Szczepański 2010;Szczepański and Ilnicki Fig. 3 a Generalized lithostratigraphic profile of volcano-sedimentary successions exposed in the Orlica-Śnieżnik dome (modified from Szczepański and Ilnicki 2014), b the Geological sketch map of the Bystrzyckie Mts. after (Szczepański 2010  . In this part of the Bystrzyckie Mts. metamorphic gradient changes from biotite (sample PR01) to staurolite zone (samples PR02, PR03 and PR04) which is associated with the increase of temperature of metamorphism from c. 460 to 620 °C (Szczepański 2010).
The investigated quartzite samples range in colour from light to dark grey and represent fine-to medium-grained rocks composed mainly of quartz with secondary white mica. The latter mineral is particularly abundant in sample PR01 and to a lesser degree also in sample PR04. Major accessory minerals are represented by zircon, rutile, monazite, zoisite, tourmaline and opaques, which are especially abundant in samples PR01 and PR04. The foliation of these rocks is defined by parallel alignment of isolated white mica flakes or thin laminae composed exclusively of white micas.

Analytical methods
The bulk rock chemical analyses of quartzite samples were performed at Acme Analytical Laboratories Ltd. (Vancouver, Canada) and are summarized in Table 1. Major-and trace-element abundances were determined using ICP-MS following lithium metaborate fusion and nitric acid digestion of 0.2 g representative whole-rock powder. Loss on ignition (LOI) was measured by weight difference after ignition at 1000 °C and was found to be negligible for all samples (Table 1). Detection limits are within 0.01% for major elements, between 0.1-0.5 ppm for most trace elements, 1 ppm for Ba, Sn and Zn, and 8 ppm for V. Geochemical diagram from Fig. 4 was designed using the GCDKit software of Janoušek et al. (2006). Geochemical diagrams from Fig. 8 were designed via the R software environment (R Core Team 2012).
Mineral separation was carried out at the University of Wrocław following conventional techniques involving crushing, sieving, heavy liquids and Frantz magnetic separator. Zircon grains were hand-picked under a binocular so that representative population was achieved. Additionally, from sample PR01 we extracted one extra population represented only by the euhedral crystals. Subsequently, the zircons were mounted in an epoxy resin and polished. Cathodoluminescent images of zircons were conducted using a Philips XL30 electron microscope (15 kV and 1 nA) at the Faculty of Earth Sciences, University of Silesia, Sosnowiec, Poland.
In situ U-Pb zircon dating by laser ablation inductively coupled plasma mass spectrometry (LA ICP-MS) was conducted at the Kraków Research Centre, Institute of Geological Sciences, Polish Academy of Sciences. The analyses were carried out using an excimer laser (ArF) RESOlution M-50 by Resonetics (now Applied Spectra) coupled with an ICP-MS XseriesII by ThermoFisher Scientific. Detailed description of the apparatus and the applied analytical  Anczkiewicz and Anczkiewicz (2016). Zircon Z91500 (Wiedenbeck et al. 1995) was used as a primary standard and zircons GJ-1 (Jackson et al. 2004) and Plešovice (Sláma et al. 2008) were frequently measured for the quality control. Each eight unknowns or secondary standards, two primary standards were measured. We applied 30 µm laser spot size, with 5 Hz repetition rate and fluence of about 3 J/cm 2 . For each analytical session, the secondary standards yielded 206 Pb/ 238 U mean ages accurate within about 1% precision (2 relative standard deviations) which is satisfactory for the purpose of the provenance studies.
From each sample at least 100 zircon crystals were dated. Analyses with more than 10% discordance (% discordance = [1 − ( 206 Pb/ 238 U age)/( 207 Pb/ 235 U age)] × 100%) were rejected. Concordia plots were prepared using the Isoplot/ Ex 4.15 (Ludwig 2008). Frequency as well as kernel density plots were designed via the R software environment (R Core Team 2012). For zircons older than 1 Ga 207 Pb/ 206 Pb ages were taken for interpretation and the 206 Pb/ 238 U ages were utilised for the younger grains.

Major and trace elements characteristics
The studied quartzite samples show significant differences in geochemical composition. Samples PR01 and PR04 show lower SiO 2 content (ranging from 86.3 to 93.2 wt%), relatively high Al 2 O 3 (in the range between 3.9 and 8.06 wt%) and generally display a higher abundance of trace elements in comparison to samples PR02 and PR03 (Table 1; Fig. 4). Chemical variability correlates with the abundance of white mica and accessory minerals including zircon, rutile, monazite tourmaline and epidote group minerals. When normalized to the composition of the upper continental crust, the studied quartzites show rather variable patterns but all display a clear negative Nb and Ta anomaly (Fig. 4), which very likely is a feature inherited from felsic crustal material bearing supra-subduction affinity (e.g. Baier et al. 2008;Gonçalves et al. 2016). It is also worth to point out that relatively high Zr and Hf concentration compared to the upper continental crust is especially well visible in the case of sample PR04.
The studied metasediments underwent greenschist to amphibolite facies metamorphism that could have mobilised the large-ion lithophile elements (e.g., Na, K). However, any large-scale remobilization of REEs, Th, Zr, Sc, Cr and Co seems unlikely. Furthermore, the latter elements are considered as transferred quantitatively into clastic sediments during weathering and transportation, thus reflecting the signature of the parent materials (Bhatia and Crook 1986;McLennan 1989).

Sample PR01
The zircon population of sample PR01 ranges from 75 to 350 μm with the mean length of 160 μm. They are dominated by grains with elongation less than 2 (73%), whereas zircons with elongation in the range of 2-3 are less frequent (27%). Inspected zircon grains are often ovoid (40%) to wellrounded (36%), while euhedral crystals form quite an important group (24%). Most grains are transparent and pinkish (60%). Yellow and brown grains are rare (13%), while 27% of the analysed zircons display orange, yellow or brown patches. Some grains have small and unrecognised mineral inclusions. Most of the grains in the investigated sample are homogenous, while 17% of them display inherited cores. Analysed zircons can be grouped into two classes characterized by: (1) well-developed prism {110} and {211} ditetragonal bipyramids as well as (2) prism {100} and bipyramid {101}. All studied grains, including euhedral crystals, show clear signs of mechanical reworking with strongly rounded shapes. In case of euhedral crystals, it is typified by slightly rounded prisms and bipyramid faces and uneven crystal surfaces (Fig. 5a).

Sample PR02
The PR02 sample is dominated by rounded to well-rounded grains (91%) and subhedral to euhedral crystals constitute a relatively small group (9%). Their length varies from 75 to 425 μm with a mean length of 182 μm and elongation mostly below 2. Zircons from this sample are transparent, Fig. 4 Upper continental crust (UCC)-normalized major-element pattern for Orlica-Śnieżnik dome metasediments. Normalization factors after Taylor and McLennan (1995) colourless (24%) to yellowish-brown (49%). 27% of grains are translucent with the cloudy interior. Commonly the analysed zircons have mineral inclusions. Majority of the grains are homogenous, while 19% of them display inherited cores. Euhedral crystals form two major groups: (1) with {100} prisms and ditetragonal bipyramid {211} equally well developed and (2) with {100} prisms dominating over bipyramids {101}. Euhedral crystals in this sample are characterized by slightly rounded bipyramid terminations.

Sample PR03
Zircons in sample PR03 range in length from 85 to 260 μm with the mean length of 150 μm and elongation below 2. This sample is dominated by rounded to well-rounded grains (88% of the population), whereas euhedral crystals form a relatively small group (12% of population). 50% of zircons are colourless and transparent, 40% of grains is characterized by orange colour, while 10% of the population is translucent and brownish. Majority of grains is homogenous, whereas only a few zircons exhibit inherited cores.

Sample PR04
Zircon grains in sample PR04 range in length from 75 to 275 µm with a mean length of 140 µm and elongation mostly below 2. The vast majority of grains constitute rounded and well-rounded grains (95% of the population). Euhedral and subhedral crystals (5% of the population) are characterized by slightly rounded bipyramid terminations as well as uneven and stained crystal surfaces. 52% of grains are colourless and transparent, whereas 39% of zircons display orange to red spots. Euhedral and subhedral crystals can be subdivided into two groups: (1) with well-developed {110} prism and bipyramid {211} as well as (2)  grains display slightly rounded bypiramid terminations and stained crystal faces (Fig. 5a).
CL images of inspected zircon populations reveal that all the investigated grains show similar characteristics in terms of brightness and internal textures. Majority of the grains display moderate cathodoluminescense and show fine oscillatory zoning of clearly magmatic origin (e.g. PR_01_72, Fig. 5b). The second group of grains is characterized by ovoid shape and complex internal texture most probably related to metamorphic growth (e.g. PR_01_59, Fig. 5b). Few grains from both groups contain xenocrystic cores (e.g. PR_02_100-101, Fig. 5c; PR_04_1 and PR_04_1, Fig. 5d) and sporadically display thin and dark rims (e.g. PR_02_80, Fig. 5c; PR_04_64, 5d).
Summing up, the vast majority of euhedral zircon grains strongly resemble those from the Cambro-Ordovician metagranites with respect to morphology and CL. They are widespread in the Orlica-Śnieżnik dome and are known as the Śnieżnik orthogneiss (e.g. Turniak et al. 2000). Moreover, all inspected grains, including euhedral crystals, show clear signs of mechanical reworking characterised by slightly rounded prisms and bipyramid faces as well as stained crystal faces (Fig. 5a). It is worth to note that in the studied zircon populations we have not identified any grains with morphology typical of volcanic rocks.

Laser ablation ICP-MS U-Pb zircon dating
The results of laser ablation ICP-MS U-Pb zircon dating of quartzites PR01, PR02 and PR04 are summarized in Tables 2, 3, 4 and 5 and Figs. 6 and 7. The frequency histograms were constructed using 25 Ma bin width and were presented jointly with the kernel density plots applying 15 Ma smoothing wavelength (Fig. 7).
All three samples show similar zircon age distribution patterns. Concordia plots show the largest number of zircons belonging to the Ordovician-Neoproterozoic time. Much smaller populations are displayed by the Paleoproterozoic zircons and only sporadically Archean zircons are present (Figs. 6, 7). Histograms allow a more detailed view and show that about 71-80% of the zircons represent the time span between ca. 0.48 and 0.88 Ga. The youngest crystals represent the Early Ordovician ages and constitute 3-9% of the whole population (Fig. 7). One of the two most prominent age probability peaks, clearly marked in all three samples, corresponds to ca. 0.50 Ga. The second noticeable peak at 0.58 Ga is present in samples PR01 and PR04. In sample PR01, this is the dominant age component. All histograms show an asymmetric tail from the Neoproterozoic and Cambrian maxima towards older ages with small probability peaks around 0.70 and 0.80 Ga. The Neoproterozoic and the Phanerozoic zircons are euhedral with only slight signs of mechanical abrasion, they are fairly dark in CL and commonly display oscillatory zoning ( Fig. 5b-d).
The remaining zircon defines small maxima throughout most of the Paleoproterozoic (with decreasing number of ages towards the Early Paleoproterozoic) and scarce in the Archean with the oldest crystals dated at 2.99 Ga. The Paleoproterozoic (15-23% of total analysed zircons) and the Archaean zircons (up to 6.5% of total analysed zircons) are commonly rounded, fractured and show variable zoning patterns ( Fig. 5b-d). Notably, a common feature observed in all quartzites is the lack of Mesoproterozoic and Grenvillian (0.98-1.25 Ga) zircons. To more closely constrain the time of sedimentation, from sample PR01, we extracted, supposedly the youngest, euhedral crystals and analysed them separately (Fig. 7b, PR01EU). Zircons from this sample are strongly dominated by the Phanerozoic and the Neoproterozoic density peaks at 0.50 Ga and 0.55 Ga and contain only a few older Neoproterozoic (0.68, 0.77, 0.95 Ga) as well as Paleoproterozoic (1.77 and 1.90 Ga) grains (Fig. 7b). This demonstrates that the source for the detritus comprised in the inspected quartzites represents Cambrian and Neoproteorozoic metagranitoids.

Provenance signature and tectonic setting of the sedimentary basin
Provenance of sedimentary rocks can be tested using La/Sc vs. Th/Co diagram proposed by Cullers (2002). Siliciclastic rocks derived from erosion of felsic rocks are enriched in elements incompatible in igneous melts including Th and La, whereas those derived from a mafic source are enriched in elements compatible in igneous melts including Sc and Co. Therefore, La/Sc and Th/Co ratios are useful in tracking the nature of the source area. For quartzite samples from the Bystrzyckie Mts. La/Sc and Th/Co ratios range from 2.5 to 8.0 and from 1.0 to 10.8, respectively, pointing to a source area dominated by felsic rocks (Fig. 8a). Conclusions concerning lithology of the source area are confirmed by inspection of the Th/Sc vs. Zr/Sc diagram devised by McLennan et al. (1993). These ratios increase towards more felsic members of a magmatic suite and the elements utilised in this diagram are characterized by low residence times in natural waters (Taylor and McLennan 1985). The investigated quartzites are located close to the line describing typical magmatic suite and display relatively high Th/Sc (1.75-2.40) and Zr/Sc (37.60-101.2) ratios typifying granites as a potential source for the detritus (Fig. 8b). Interestingly, sample PR04 departs from this pattern by its very high Zr/Sc ratio compared to the remaining samples (Fig. 8b). This is indicative of sedimentary sorting and recycling processes that presumably influenced chemical composition of                         (McLennan et al. 1993). This clearly indicates that the Bystrzyckie Mts. quartzites cannot represent rhyolites or tuffs, as suggested by Jastrzębski et al. (2016) for the Goszów quartzite exposed in the eastern part of the Orlica-Śnieżnik dome. This is also supported by morphology of the youngest zircons that closely resemble those in the Śnieżnik orthogneiss and are strikingly different from those documented in local metaryolites from the Gniewoszów area (Mazur et al. 2015). However, we do not exclude the possibility of some minor syn-sedimentary volcanic input but, if at all present, it was volumetrically minor.
Various diagrams are available to identify the tectonic setting of a source region and were successfully used in the numerous studies (e.g. Bhatia 1983;Roser and Korsch 1986). Because in the case of metamorphosed rock successions discrimination of tectonic setting of a sedimentary basin should rely on immobile trace elements, we first utilized the Ti/Zr vs. La/Sc diagram and Sc-Th-Zr ternary plot of Bhatia and Crook (1986). These plots show that the analysed samples display trace element composition typical of sediments deposited on a continental passive margin (Fig. 8c, d). Despite potential mobility of the major elements, we also explored a diagram of Roser and Korsch (1986) devised to determine the tectonic setting of terrigenous sedimentary rocks utilising K 2 O/Na 2 O ratio and SiO 2 abundance. In this plot, the studied samples fall within the field of the passive margin sediments (Fig. 8e). Recently, Verma and Armstrong-Altrin (2013) proposed two new discriminant functions based on major elements for the tectonic discrimination of siliciclastic sediments from three main tectonic settings including island or continental arc, continental rift and collision zone. Verma and Armstrong-Altrin (2013) Fig. 6 U-Pb Concordia diagrams for samples from the Bystrzyckie Mts. Concordia plots were done using the Isoplot/Ex 4.15 Excel add-in (Ludwig 2008) suggest that the discriminant functions are insensitive to element mobility. Indeed, investigated quartzite display values of mentioned discriminant functions typical of rift-related sandstones demonstrating the usefulness of this technique even for metamorphosed rock suites (Fig. 8f).

Time of deposition
The youngest zircon population documented in the studied quartzites defines a prominent density peak at 500 Ma (Figs. 6,7,9a;Tables 2,3,4,5). Assuming no significant Pb-loss in this population, it is our best estimate of the maximum deposition age. Although in the sample PR04 we observed three crystals younger than 480 Ma, they were not reproduced in other samples and taking into account their paucity, their young age is likely to be caused by the partial Pb-loss.
Our estimate is in broad agreement with U-Pb detrital zircon and monazite ages for the Goszów quartzite whose max. deposition age was estimated as ca. 490-494 Ma (Mazur et al. 2012;Jastrzębski et al. 2016). However, the youngest zircon grains reported by Jastrzębski et al. (2010) Fig. 7 Binned frequency-density plots of detrital zircons from: a-d analyzed samples (N number of analyses) with marked major zircon forming events (see text for details); e Goszów quartzites (Mazur et al. 2012 Fig. 4 with ages of 460-470 Ma most probably suffered significant Pb-loss. These ages are younger than any reported zircons from the Śnieżnik orthogneiss or even metarhyolites exposed in the Orlica-Śnieżnik dome rocks which are the most probable source of detritus for the investigated quartzites (e.g. Turniak et al. 2000;Kröner et al. 2001;Lange et al. 2002;Mazur et al. 2015).

Provenance constraints
The Neoproterozoic Avalonian-Cadomian belt was formed due to long-lasting Andean-type orogeny as a relatively large magmatic arc spreading over several thousand kilometres along the northern margin of the Gondwana. It extended from the Amazonia in the West, through the West African Craton and the Saharan Metacraton to the Arabian-Nubian shield in the East (e.g. von Raumer et al. 2002). As these cratonic domains bear a record of numerous tectonomagmatic events responsible for the zircon formation it may allow for describing the palaeogeographic position of individual terranes within this long arc. Figures 9 and 10 show the age ranges of the magmatic-metamorphic units encountered in several cratonic areas as well as in the Early Ordovician quartzites known from other Cadomian terranes that are now exposed in the Variscan Belt of Europe. Comparison of zircon age spectra of various crustal domains with the presented zircon age spectra obtained for the investigated quartzites from the Bystrzyckie Mts. allows a conclusion concerning the provenance of the detritus comprised within the Bystrzyca-Orlica massif.
The youngest age spectra documented in the quartzites from the Bystrzyca-Orlica massif are typical of the Cambro-Ordovician sedimentary successions covering Cadomian terranes known from other parts of the Variscan Belt of Fig. 9 Kernel density plots for detrital zircons from Early Ordovician quartzites from: a the Bystrzyckie Mts (this paper), b South Karkonosze (Žáčková et al. 2010), c Saxothuringian Zone ), c Armorican Massif (Strachan et al. 2014;Lin et al. 2019), d SW Central Iberian Zone (Pereira et al. 2012), e Armorican Massif (Strachan et al. 2014;Lin et al. 2019), f Teplá-Barrandian Zone (Drost et al. 2011) and g Molnadubian Zone (Košler et al. 2014). N number of analyzed grains. The grey shading highlights the bimodal distribution of detrital zircon ages common for all units. Kernel density plots were designed via the R software environment (R Core Team 2012) a b c d e f g Europe (Fig. 9). This Cambro-Ordovician cover comprises: (1) quartzite lenses exposed in the southern part of the Karkonosze-Izera massif interpreted as the eastern extension of the Saxothuringian zone where the youngest population of zircons shows a peak at ca. 500 Ma (Žáčková et al. 2010), (2) metaconglomerate of the Langer Berg Formation in the Saxothuringian Zone with the youngest zircons ranging from 485 to 500 Ma , and (3) the Armorican quartzite exposed in the Armorican Massif as well as in the Iberian Massif excluding the South Portuguese Zone (e.g. Strachan et al. 2014). The Armorican quartzite of the Iberian Massif displays a Concordia age of the youngest zircons at 522 ± 7 Ma (e.g. Linnemann et al. 2008;Fernández-Suárez et al. 2002a, b). It is worth to note that also other Neoproterozoic and Cambro-Ordovician siliciclastic sedimentary successions exposed in the Teplá-Barrandian and Moldanubian Zones show very similar detrital zircon age spectra indicating similar provenance (Fig. 9f, Drost et al. 2011;Košler et al. 2014). The only important exceptions can be noticed for quartzites exposed in the SW central part of the Iberian Zone. These rocks bear a significant admixture of zircons revealing c. 1 Ga typifying Grenvillian crust and thus interpreted as derived from erosion of the Arabian-Nubian Shield ( Fig. 9d; e.g., Pereira et al. 2012;Fernández-Suárez et al. 2014).
The youngest population of zircon grains recognised in the analysed quartzites from the Orlica-Śnieżnik dome were probably derived by erosion of Cambro-Ordovician granites widely exposed in the Saxothuringian zone. These metagranites are represented by e.g. orthogneiss from the Erzgebirge (Tichomirowa et al. 2001;Košler et al. 2004;Mingram et al. 2004), the Rumburk granite from the Izera-Karkonosze Massif (Oliver et al. 1993;Oberc-Dziedzic et al. 2009, 2010Zieger et al. 2017), the Milchberg granite from the Torgau-Doberlug Syncline  or the Śnieżnik orthogneiss from the Orlica-Śnieżnik dome (e.g. Oliver et al. 1993;Turniak et al. 2000;Kröner et al. 2001;Lange et al. 2005). Neoproterozoic euhedral grains might have been derived from the granitoid intrusions dated at c. 540 Ma that were reported from: (1) Wądroże Wielkie located on the Fore-Sudetic Block (Żelaźniewicz et al. 2004), (2) Lausitz granitoid, Glasbach and Laubach granites or Dohna granodiorite exposed in the Saxothurignian zone (Linnemann and Heuse 2001) and (3) orthogneisses reported from Erzgebirge (Tichomirowa et al. 2001;Mingram et al. 2004). Consequently, the presence of Cambro-Ordovician and Neoproterozoic euhedral zircons preserved in the quartzite lenses from the Bystrzyckie Mts. suggest a relatively short distance of transport before deposition of these rocks and, consequently, a local source of the detritus.  (Henry et al. 2009), c Trans-Saharan Belt (Peucat et al. 2003;Abdallah et al. 2007;Bendaoud et al. 2008;Bosch et al. 2016), d Saharan Metacraton (Meinhold et al. 2011), e Brunovistulicum (Friedl et al. 2004;Mazur et al. 2010), f Amazonian Craton (Gaucher et al. 2008;Geraldes et al. 2014;Pankhurst et al. 2016), g Baltica (Valverde-Vaquero et al. 2000;Kristoffersen et al. 2014;Kuznetsov et al. 2014). Kernel density plots were designed via the R software environment (R Core Team 2012) a b c d e f g This is in line with the observed weak mechanical abrasion of the investigated euhedral zircons. However, a group of Neoproterozoic zircons is characterized by ovoid, strongly rounded shapes due to relatively intense mechanical abrasion (Fig. 5). The presence of such grains indicates recycling by transport from (volcano)-sedimentary successions, possibly from the Stronie (532 ± 6 Ma) and Młynowiec formations (563 ± 6 Ma) in the Orlica-Śnieżnik dome (Mazur et al. 2012(Mazur et al. , 2015, from the Zwethau (534 Ma) and the Rothstein (565-570 Ma) formations in the Saxohuringian zone (e.g. Elicki 1997;Linnemann 2007) or from glaciomarine sedimentary rocks documented in the Elbe zone and the North Saxon antiform in the southeastern part of the Saxothuringian zone with the youngest detrital zircon populations showing ages of 562-565 Ma (Linnemann et al. 2018). Moreover, LA-ICP-MS U-Pb detrital zircon data from Early-Middle Cambrian strata of the Torgau-Doberlug Syncline from the Saxothuringian zone show Cadomian and older zircons in this area (Abubaker et al. 2017). Consequently, anhedral Neoproterozoic zircons documented in the studied quartzites may represent recycled material delivered through local sedimentary successions.
Presented detrital zircon age spectra indicate that there was a period of magmatic quiescence between 0.9 and 1.7 Ga in the source area as there are only few zircon grains of this age documented in the investigated quartzites (Figs. 6,7,9). The same age gap in the zircon age spectra is also reported from domains like the West African Craton and the Trans-Saharan Belt (Fig. 10a, c). However, the Trans-Saharan Belt might be excluded as a potential source of detrital material for the investigated quartzite samples due to occurrence of Mesoproterozoic zircons in some parts of this crustal domain. In addition, the Saharan Metacraton, Amazonia, Baltica, Avalonia and Brunovistulicum may also be excluded as a potential source of detritus since these cratonic areas are characterized by the existence of Mesoproterozoic zircons (Fig. 10c-g).
Summing up, we suggest that the data presented here indicate that up to the Early Ordovician rock successions exposed in the Orlica-Śnieżnik dome were part of the Fig. 11 Tentative simplified reconstruction of Western Gondwana during the Early Ordovician, modified after Domeier (2016) and Torsvik (2017). Ages of main zircon forming events for Amazonian Craton, West African Craton and Saharan Metacraton after Albert et al. (2015). Ages of main zircon forming events for East European Craton after Kristoffersen et al. (2014). References for Trans-Saharan Belt as in Fig. 10b. ANS Arabian-Nubian Shield. Red star marks the position of the metaquartzite sink area within the Cadomian arc. Red arrows indicate the main direction of sedimentary transport. Types of lithosperic boundaries: magenta-spreading ridge, blue-subduction, green-transform Saxo-Thuringia and must have been located close to the West African Craton (Fig. 11). The latter together with the supracrustal successions and magmatic complexes of the Cadomian terranes served as the source for the detritus. Available detrital zircon ages from Cambro-Ordovician sedimentary successions covering Saxo-Thuringia, Teplá-Barrandian and Moldanubian Zones as well as the Armorican Massif clearly indicate that all these Neoproterozoic crustal fragments were sourced from the same cratonic areas and Cadomian arc-related basement located at the northern periphery of Gondwana (Figs. 9, 10; e.g. Linnemann and Romer 2002;Pastor-Galán et al. 2013). Consequently, we support suggestions that during the Cambro-Ordovician Cadomian crustal fragments were dispersed along the northern periphery of Gondwana most probably defining an extended shelf set on a passive continental margin (Žák and Sláma 2018).

Conclusions
The quartzite from the western part of the Orlica-Śnieżnik dome (the Bystrzyckie Mts.) provides new input to understand the pre-Variscan history of the Cadomian crust cropping out in the Central Sudetes. Bulk rock chemistry and detrital zircon age spectra obtained using LA ICP-MS U-Pb show that these rocks represent the same though dismembered lithostratigraphic horizon equivalent to the Goszów quartzite exposed in the eastern part of the Orlica-Śnieżnik dome in the Śnieżnik massif. The youngest zircons with a Late Cambrian density peak providing the maximum depositional age of c. 500 Ma, were delivered by the erosion of granitoids with protolith ages of c. 517-490 Ma attesting a relatively short distant transport before deposition of the quartzite. Obtained zircon age spectra resemble that documented in the Neoproterozoic (meta)sedimentary and (meta) magmatic rocks exposed in the eastern part of the Orlica-Śnieżnik dome as well as elsewhere in the Saxothuringian zone. Consequently, presented data point to Saxothuringian affinity of the volcano-sedimentary successions exposed in the western part of the Orlica-Śnieżnik dome. Bulk rock geochemistry suggests that the quartzite represents a highly mature quartz-rich sandstones deposited on the passive Gondwana margin.
Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creat iveco mmons .org/licen ses/by/4.0/.