Lithos 220223 (2015) 116132

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Lithos

j ourna l homepage: www.e lsev ie r .com/ locate / l i thosThe Moldanubian Thrust Zone A terrane boundary in the CentralEuropean Variscides refined based on lithostratigraphy andUPb zircon geochronologyMirosaw Jastrzbski a,, Andrzej elaniewicz a, Mentor Murtezi a, Alexander N. Larionov b, Sergey Sergeev ba Institute of Geological Sciences, Polish Academy of Sciences, Research Centre in Wrocaw INGPAN, Podwale St. 50-449, Wrocaw, Polandb Centre of Isotopic Research, All-Russian Geological Research Institute, Sredny prospect 74, 199 106 St. Petersburg, Russia Corresponding author. Tel.: +48 71 3376343; fax: +E-mail address: mjast@interia.pl (M. Jastrzbski).

http://dx.doi.org/10.1016/j.lithos.2015.01.0230024-4937/ 2015 Elsevier B.V. All rights reserved.a b s t r a c ta r t i c l e i n f oArticle history:Received 16 October 2014Accepted 28 January 2015Available online 14 February 2015

Keywords:Rheic sutureVariscan beltSaxothuringian TerraneBrunovistulian TerraneMoldanubian ThrustSIMS zircon geochronologyThe zircon age populations of metavolcano-sedimentary successions in the Orlicanienik Dome (OSD), StarMsto Belt (SMB) and Velk Vrbno Dome, the Sudetes (Poland and Czech Republic), have been used to refinethe location of the Moldanubian Thrust Zone (MTZ), which is a boundary between the Saxothuringian Terraneof Gondwana descent and the Brunovistulian Terrane being a promontory of Laurussia. In the northern continu-ation of the MTZ, a set of multiply activated, regional-scale thrusts developed and brought into contact rocks ofdifferent ages and geological histories. Metarhyolites in the Orlicanienik Dome and the Star Msto Belthave similar geochemistry and UPb isotopic zircon records, which is taken in favour of their coeval formationand common Saxothuringian affinity. Felsic metavolcanic rocks from the OSD and from the thrust-boundedupper and lower units of the SMB yielded protolith ages of 500 3 Ma and 493 4 Ma to 498 5Ma, respec-tively, which indicates that metavolcano-sedimentary successions in the OSD and SMB were deposited in LateCambrian times. Structurally below these rocks, there are the highly sheared Brousek quartziteswith detrital zir-cons that yielded a maximum depositional age of ~530 Ma. The mylonitic quartzites accommodate deformationinduced by tectonic transport on the East Nznerov Thrust, which is interpreted as the easternmost margin ofSaxothuringia. This fault separates Palaeozoic rocks of the Saxothuringian Terrane from Neoproterozoic bimodalvolcanogenic succession in the Velk Vrbno Dome of Brunovistulia, dated at ~558 Ma. The late-Variscan thermalevents in the Early Carboniferous left imprints in the form of U-rich rims around the zircons of the metavolcanicrocks from the Star Msto Belt and only a very minor overprint in the zircons from the Velk Vrbno Dome andOrlicanienik Dome.

2015 Elsevier B.V. All rights reserved.1. Introduction

The Variscan orogenic belt developed through complex collisionsbetween the Laurussia (Old Red Sandstone) continent and membersof the Armorican Terrane Assemblage, which were derived fromGondwana (Franke, 1989; Nance et al., 2008; Tait et al., 1997; vonRaumer et al., 2003). In the Cambrian and Early Ordovician, northernGondwana was subjected to intra-continental rifting, which eventuallybrought about a bunch of continental fragments separated by marinebasins that were floored by an oceanic crust (Cocks and Torsvik, 2006;Linnemann et al., 2008; Matte et al., 1990; Murphy et al., 2004; Pinand Marini, 1993; Tait et al., 1997). These fragments were effectivelyseparated from Gondwana by spreading in the newly born RheicOcean (Linnemann et al., 2008) and became terranes, which eventuallydrifted northwards and finally accreted into Laurussia, completing the48 71 3376342.Variscan belt (Fig. 1a). However, igneous and sedimentary records ofthose events in individual Variscan terranes are incomplete and notfully understood. Moreover, the initial positions of these terranes atthe Gondwana margin are also unclear. For instance, in the easternpart of Saxothuringia, fragments of archaeocyatha reefs were discov-ered in the Gry Kaczawskie fold belt (Biaek et al., 2010), which placesit in rather low latitudes in the Early Cambrian. Such a location does notfit the palaeoposition of Gondwana and the Armorican Terrane Assem-blage in the CambrianOrdovician times presumed by Torsvik et al.(2012), although it seems feasible according to the reconstruction byMcKerrow et al. (1992).

Among the other unclear issues is the evolution of the Saxothuringianand Brunovistulian margins and details regarding how they cameinto contact in the Sudetes (Figs. 1b, 2). Brunovistulia is a composite(super)-terrane that embraces at least two (Finger et al., 2000) or threedifferent terranes of various origins and provenances (elaniewiczet al., 2009). The western part of Brunovistulia was engaged inthe Variscan orogen as the lower plate during a collision with theMoldanubia and Saxothuringia (Franke, 2006; Matte et al., 1990). The

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(a)

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Fig. 1. (a) VariscanMassifs andmain tectonic boundaries in central Europe (after Linnemann et al., 2008). (b) Terranemap of the BohemianMassif (after Franke and elaniewicz, 2000).(c) Geological sketch and schematic cross-section through the northern Moravosilesian Thrust Zone (compiled from Don, 1982; Sawicki, 1995; Schulmann and Gayer, 2000).

117M. Jastrzbski et al. / Lithos 220223 (2015) 116132thrust boundary was originally recognised by Suess (1912) in Moraviaand referred to as the Moldanubian Thrust. It continues farther to thenorth, where in the Sudetes and the ForeSudetic Block, a bundle of in-dividual thrusts forms the Moldanubian Thrust Zone (Fig. 2) (e.g., Donet al., 2003; Franke and elaniewicz, 2002; Oberc-Dziedzic andMadej, 2002). There is ongoing debate regardingwhich of these thrustsare actually the true boundary between the terranes involved andwhere the western limit of Brunovistulia is located (Bederke, 1929;Jastrzbski, 2012; Krner et al., 2000; Oberc, 1968a; Opletal andPecina, 2000, 2004; Schulmann and Gayer, 2000; Skcel, 1979; tpsket al., 2006). The problem is complicated because the terrane bound-ary was localised within lithologically similar metasedimentaryvolcanogenic successions that occur in the footwall and hangingwall (e.g., Koler et al., 2014; Opletal and Pecina, 2004). Despite the

Fig. 2. The detailed geological map modified after Don et al. (2003), showing the localities of samples from the Velk Vrbno Dome and the Star Msto Belt.The courses of the faults and probable faults are from Don et al. (2003), Gawlikowska and Opletal (1997), and tpsk et al. (2006).

118 M. Jastrzbski et al. / Lithos 220223 (2015) 116132similarities, their lithostratigraphic columns are still disputable. Suchuncertainties raise further questions about the lithotectonic correla-tions and paleogeographic positions of these rock successions at themargin of Gondwana in the Early Palaeozoic.

This study reports UPb sensitive high-mass resolution ion micro-probe (SHRIMP) analyses of detrital and igneous zircons from themetavolcano-sedimentary successions in the Sudetic sector of theMoldanubian Thrust Zone (Figs. 1b, 2). The results are discussed interms of the complex geological structure of the terrane suture zone,the significance of the individual thrusts that form the MoldanubianThrust Zone in the Sudetes and the overall role of the zone itselfwithin the Variscan belt. For a better understanding of both thepre-Variscan paleogeography and evolution of the suture zone,knowledge of the protolith ages and (litho)stratigraphy of rock unitsin the immediate hanging wall and footwall seems to be of primaryimportance.2. Geological setting

2.1. Rock units and lithostratigraphy

In the easternmost part of the West Sudetes, there is the Orlicanienik Dome (Fig. 1a), which is assigned to either the SaxothuringianTerrane (Chopin et al., 2012; Franke and elaniewicz, 2000; Frankeet al., 1993) or the Moldanubian Terrane (Cymerman et al., 1997;Matte et al., 1990; Mazur et al., 2005). In the core of the dome, augenand migmatitic gneisses derived from protoliths dated at 515470 Ma(e.g., Lange et al., 2005; Turniak et al., 2000) were folded togetherwith metavolcano-sedimentary rocks of the MynowiecStronie Group(Don et al., 1990; Don et al., 2003) (Fig. 1b). The Mynowiec Formationis mainly composed of (meta)greywackes metamorphosed to biotiteparagneisses and locallymigmatisedwithminor inserts of amphibolitesand garnet-bearing mica schists (Don et al., 2003; Ilnicki et al., 2013;

119M. Jastrzbski et al. / Lithos 220223 (2015) 116132Jastrzbski et al., 2014). The Stronie Formation consists of metapelitesaccompanied at the base by light and dark quartzites, in the middle bymarbles and scarce calc-silicate rocks and, in the upper portions of thelithostratigraphic columnby bimodalmafic and felsicmetavolcanogenicrocks (e.g., Don et al., 2003; Smulikowski, 1979; Wojciechowska, 1993;review in elaniewicz et al., 2014a).

The Orlicanienik Dome is flanked to the east by the narrow StarMsto Belt (Don et al., 2003; Parry et al., 1997; Skcel, 1989), which con-sists of separate, westerly dipping lithotectonic units separated bythrusts (Don et al., 2003; Gawlikowska and Opletal, 1997; Jastrzbski,2012) (Fig. 2). In the upper, 13 km thick unit, referred to as theHranina series (Skcel, 1977, 1979),mica schists intercalatedwith felsicand mafic metavolcanic rocks, quartzites, graphite schists and marblesoccur, similar to the Stronie Formation to which they are often assigned(Don et al., 2003; Skcel, 1989). The unnamed unit (middle unit ac-cording to Jastrzbski, 2012) mainly consists of metabasites withMORB-type geochemistry (Floyd et al., 1996, 2000; Poubov and Sokol,1992) and contains inserts of migmatitic paragneisses (Don et al.,2003; Jastrzbski, 2012; Parry et al., 1997). In the ca. 800 m thicklower unit, there are the Skoroice mica schists (blastomylonites andphylonites), cataclased gneisses, marbles, amphibolites, quartzites andgraphite schists (Don et al., 2003; Skcel, 1977, 1979, 1989).

The Velk Vrbno Dome is composed of mica schists, amphibolites,quartzites, graphite schists, marbles, metadacites, amphibolites,metagabbros and retrograde eclogites (Don et al., 2003; Koverdynskand Prokop, 2006; Kvto, 1951; tpsk et al., 2006; ek, 1996)(Fig. 2). Kvto (1951) proposed a lithostratigraphic subdivision ofrock in the dome and distinguished the lower clastic group composedof metavolcano-sedimentary rocks, the graphite series composed ofmetamorphosed limestones and graphite schists, and the upper clasticgroup composed of metasedimentary rocks and amphibolites accom-panied by dacite orthogneisses. The retrograde eclogites were found inthe upper group (ek, 1996); thus, the eclogite-bearing orthogneissesare considered by tpsk et al. (2006) to have been thrusted overthe amphibolite-grademetavolcano-sedimentary rocks (Fig. 2). As a re-sult of the detailed mapping, Don et al. (2003) proposed yet anotherscheme of the Velk Vrbno lithostratigraphic column in which allmetasedimentary rocks were assigned to one succession, with thenon-fossiliferous Brousek quartzites as its youngest member (Fig. 2).Fragments of Lower Devonian crinoid fauna found in the marbles ofthe graphite series confirm the presence of Palaeozoic rocks in thedome (Hladil et al., 1999; Koverdynsk and Prokop, 2006).

2.2. Previous zircon geochronology

UPb SHRIMP analyses of detrital zircon grains showed that theMynowiec greywackes were deposited in the NeoproterozoicEarlyCambrian with the youngest grains dated between ~565 Ma (Mazuret al., 2012, 2014) and ~540530 Ma (Jastrzbski et al., 2010), whereasprotoliths of the Stronie metapelites and quartzites were deposited inthe Early Palaeozoic as evidenced by ca. 530 Ma detrital zircons inmica schists and ca. 520500Mamagmatic zircons from accompanyingmetarhyolites (Jastrzbski et al., 2010; Krner et al., 1997; Mazur et al.,2012, 2014; Murtezi, 2005). The details of various interpretations ofthe protolith ages are reviewed in elaniewicz et al. (2014a). In theMynowiec and Stronie formation rocks, two dominant age clusterswere found: ~2.0 Ga and 660540 Ma.

Acidmetarhyolites from theHranina series, the upper thrust unit ofthe Star Msto Belt, contain magmatic zircons that yielded a PbPbevaporation age of 522.5 1.0 Ma (Krner et al., 2000). The middleunit consists of ca. 500 Ma bimodal metavolcanites and metagabbros,and migmatitic paragneisses that contain ca. 550 Ma detrital zircons(Krner et al., 2000).

In the Velk Vbrno Dome, tonalite orthogneisses (metadacites)yielded a PbPb zircon age of 574.3 1.0 Ma; thus, their precursorwas assignable to the Neoproterozoic (Krner et al., 2000).2.3. Tectonic boundaries

In the NE Bohemian Massif, the boundary between the terranes ofArmorican descent (Saxothuringia, Moldanubia) and Brunovistulia isequivalent to the boundary between the West Sudetes (Lugicum inSuess' (1912) and Cloos' (1922) terminology) and the East Sudetes(Silesicum in Cloos' (1922) terminology) or the Moravo-Silesian Zonein Kossmat's (1927) subdivision. This boundary has always beenthought to occur in the area east of the Orlicanienik Dome andwest of the Devonian cover of the Moravo-Silesian Zone (Bederke,1929; Misa et al., 1983; Schulmann and Gayer, 2000; Skcel, 1989).From the thrusts identified in the area that are assigned to theMoldanubian Thrust Zone (e.g., Franke and elaniewicz, 2002), variousauthors used to single out one of them and claim it the critical borderfault. This role was to be played by the Ramzov thrust or bythe Nznerov thrusts (Fig. 2) as the debate continued (e.g., Bederke,1929; Cymerman, 1993; Don et al., 2003; Oberc, 1968a; Opletal andPecina, 2004).

In the Orlicanienik Dome, tectonic zones of intense ductileshearing occur within both the gneissic core and schistose envelope,mainly with NS directed kinematics (review in elaniewicz et al.,2014a). The westernmost mappable thrust that might be assignedto the Moldanubian Thrust Zone separates the OrlicanienikDome from the Star Msto Belt (Fig. 2), namely, the dome core'sgneisses from the belt's mica schists (Don et al., 2003), respectively(Fig. 2). The latter form the upper unit of the belt referred to asthe Hranina mica schists (Don et al., 2003; Skcel, 1989). The lowerboundary of the upper unit coincides with another unnamed thrust(Gawlikowska and Opletal, 1997). Farther east, the strongly shearedrocks of the lower unit of the Star Msto Belt are bounded by theNznerov dislocation zone, the West Nznerov Thrust and the EastNznerov Thrust (Fig. 2) and are underlined locally by serpentinitelenses (Don et al., 2003). The West Nznerov Thrust defines the lowerboundary of the leptyno-amphibolite complex of the Star MstoBelt's middle unit. In turn, the East Nznerov Thrust separates theStar Msto Belt from the Velk Vrbno Dome (Fig. 2). All these faultshave very strong strike-slip components of predominantly dextralsense (Cymerman, 1993, 1997; Jastrzbski, 2012; Opletal and Pecina,2004; Parry et al., 1997), with only partly visible top-to-the-E thrustvergence (Jastrzbski, 2012) and locally observed WSW-directedoblique extension with a sinistral component (Cymerman, 1993). Inthis area, the main architecture of this boundary zone was establishedwhen the Saxothuringian and Brunovistulian Terranes came into con-tact in the Late DevonianEarly Carboniferous (Jastrzbski et al., 2013;Parry et al., 1997; Schulmann and Gayer, 2000).

Within the Velk Vrbno Dome, another nameless thrust wasenvisaged between structurally higher eclogite-bearing units andstructurally lower, low-grade metasedimentary rocks that crop outin a tectonic window (tpsk et al., 2006). To the east, the VelkVrbno Dome is truncated by the Ramzov Thrust, which is the east-ernmost thrust of the Moldanubian Thrust Zone, considered tobe the continuation of the Moldanubian Thrust according to Suess(1912) and Bederke (1929). The Ramzov Thrust separates theVelk Vrbno Dome from the Devonian metavolcano-sedimentaryBrann Belt (e.g., Don et al., 2003) and the Keprnk Dome that rep-resents the Neoproterozoic basement of the Brunovistulian Terrane(Figs. 1, 2) (Dudek, 1980; Kalvoda et al., 2008; Schulmann andGayer, 2000). In contrast to the MynowiecStronie Group in theOrlicanienik Dome, light quartzites dominate over limestonesand minor chlorite slates in the Brann Belt, and the entire rockgroup has been assigned traditionally to the Devonian by virtue oflithological similarities to the Lower Devonian Drkov quartzite,which were proven paleontologically farther east (Chlup, 1989,review in Kalvoda et al., 2008) in the folded and metamorphosedcover of the Neoproterozoic age Desn Dome gneisses (Krner et al.,2000).

120 M. Jastrzbski et al. / Lithos 220223 (2015) 1161323. Sampling strategy

Felsicmetavolcanogenic rockswere used to constrain the depositiontime of the sedimentary protoliths from the Stronie Formation in theOrlicanienik Dome and of the upper and lower units from the StarMsto Belt. They are volumetrically minor but potentially themost use-ful for the isotopic dating of the host sedimentary rocks as they are de-rived from zircon-bearing felsic volcanites. In theOrlicanienikDome,the felsic metavolcanogenic rocks are metamorphosed rhyolitic lavas/tuffs and tuffites (Murtezi, 2006; Wojciechowska et al., 2001), possiblyalso including subvolcanic intrusions into earlier deposited peliticrocks (Mazur et al., 2012, 2014). To avoid ambiguities in the studied do-mains from which the samples were collected, the field relationshipsbetween these rocks and the surrounding mica schists were re-examined to ascertain that the interfingering lateral interface of thetwo lithological units and transitional rocks represent the original con-tacts of submarine lavas and tuffites with coeval sediments (Fig. 3a, b).

In the Star Msto Belt, a felsic metavolcanic rock structurally occur-ring immediately below the E-MORB-like amphibolites was sampledto constrain the deposition age of the Skoroice mica schist protolithsin the lower unit of the SMB. Farther east, the Brousek quartziteswere sampled at the East Nznerov Thrust (Fig. 3c) to establishtheir maximum depositional age, speculatively assigned to either theNeoproterozoic or Devonian (Don et al., 2003), and to check theiraffinity to either the Orlicanienik or the Velk Vrbno Domes.

In the Velk Vrbno Dome, two samples were collected from a singleoutcrop of alternating metadacite and metabasite that belong to theouter, apparently structurally higher part of the dome (Fig. 3d). Our(a) (

(c) (

Fig. 3.Geological positions of the dated rocks. (a) Interlayers of themetavolcanic rocks andmetthe StarMsto Belt. (c)Mylonitic Brousek quartzites situated near the East Nznerov Thrust. (dGS335/1 (metadacite) and GS335/2 (metabasalt)).field studies indicate that the collected samples represent a bimodalmagmatic suite, which was assigned by Kvto (1951) to the upperclastic group that he distinguished. A geological map provided bythis author confirms the interfingering contacts of the metadaciteorthogneisses and amphibolites. Our aim was to check whether thefelsic rocks of the bimodal suite in the Velk Vrbno Dome are equivalentto the felsic rocks in the Orlicanienik Dome and in the Star MstoBelt, or if they differ in terms of protolith age and provenance. Differ-ences would support the interpretation that the Sudetic sector of theMoldanubian Thrust Zone actually does separate different terranes.

4. Analytical methods

Zircons were extracted from six rock samples (Gn1, OS179, OS326,OS335/1, OS335/2 and B27) using standard magnetic separation tech-niques. The zircons were handpicked under a microscope, mounted inepoxy resin and polished to expose their interior. Transmitted andreflected light photomicrographs and cathodoluminescence imageswere taken for analytical spot selection guidance, to avoid cracks and in-clusions in the grains that were selected for SHRIMP analysis. Approxi-mately 6070 representative crystals from the studied samples weremounted in the puck, with the exception of sample B27, for which 170grains were selected.

The SIMS SHRIMP II instrument at the Centre of Isotopic Research ofthe All-Russian Geological Research Institute (VSEGEI), St. Petersburg,was used to perform in situ UPb analyses by applying a secondaryelectron multiplier in a peak-jumping mode following the proceduredescribed in Williams (1998) and in Larionov et al. (2004). A primaryb)

d)

atuffites from the Stronie Formation in the Orlicanienik Dome and (b) the upper unit of) Bimodalmetavolcanites of the structurally upper part of the Velk VrbnoDome (samples

121M. Jastrzbski et al. / Lithos 220223 (2015) 116132beam of molecular oxygen bombarded the zircon to sputter secondaryions. The elliptical analytical spots were ca. 27 20 m, and the corre-sponding ion current was ca. 4 nA. The sputtered secondary ions wereextracted at 10 kV. The 80-m wide slit of the secondary ion source, incombination with a 100-m multiplier slit, allowed a mass-resolutionof M/M 5000 (1% valley) so that all the possible isobaric interfer-ences were resolved. One-minute rastering over a rectangular area ofca. 60 50 m was employed before each analysis to remove the goldcoating and any possible surface contamination with common Pb.

The following ion species were acquired in sequence: 196(Zr2O)204Pb-background (ca. 204.5 AMU)206Pb207Pb208Pb238U248ThO254UO, with an integration time ranging from 2 to 20 s. Four cycles foreach analysis were recorded. Every fifth measurement was carried outon the zircon Pb/U standard TEMORA 1 (Black et al., 2003), which hasan accepted 206Pb/238U age of 416.75 0.24 Ma. Th and U concentra-tions were calibrated against 91500 zircon standard. The collectedresults were then processed with the SQUID v. 1.12 (Ludwig, 2005a)and ISOPLOT/Ex 3.22 (Ludwig, 2005b) software using the decay con-stants of Steiger and Jger (1977). The common lead correction wasconducted using measured 204Pb according to the model of Stacey andKramers (1975).

5. Description of rock and zircon samples

Sample Gn1 represents a massive metarhyolite from the westernlimb of the Orlicanienik Dome, which occurs in a schistose felsicmetatuffite that alternates with surrounding mica schists and under-lying mafic volcanogenic rocks (point 1.2 in elaniewicz et al., 2014b).In this part of the dome, the two members of the bimodal magmaticsuite form the NWSE elongated belt of discontinuous lenses, tens tohundreds of metres long, within the mica schists. The sample was col-lected in-situ in an outcrop on the slope opposite the Zamek Szczerbahill south of Gniewoszw in the Bystrzyckie Mts. (Figs. 1, 2). It is amedium-grained rock composed mainly of quartz, K-feldspar, chlorite,muscovite, tourmaline and opaque minerals. The peak metamorphicconditions established for the rocks in this locality is estimated to550600 C/6 kbar (Jastrzbski, 2009; Murtezi, 2006).

The sample Gn1 is rich in clear, colourless, normal- to long-prismaticand considerably large (up to 300 m long) zircon crystals. Their mean(a) (b)

Fig. 4. (a) Cathodoluminescence images of representative zircon grains from sampleGn1 (metarresults.aspect ratio reaches 3 and, in some cases, exceeds 4. Ca. 70% of the Gn1zircons are euhedral with well-developed bipyramids and prisms.Almost the entire zircon population has clear small scale (b5 m) oscil-latory zoning. Some of the crystals have corroded rims; usually, theyalso have darker cathodoluminescence rims (Fig. 4). Several smaller(b100 m long), hazier, and yellowish crystals with ovoid shapes canbe seen under binoculars in the transmitted light microscope and inthe cathodoluminescence (CL) images.

Sample OS179 is a weakly foliated, felsic metarhyolite from theupper unit of the Star Msto Belt, collected in-situ near a road-cut ca.600 m north of the main cross-road in Nova Sennka (Figs. 1, 2). Thesample is a greyish yellow, medium-grained rock composed mainly ofquartz, K-feldspar, muscovite, biotite, garnet, plagioclase and apatite.

The zircons in sample OS179 are transparent, colourless, and usuallylong-prismatic. These crystals (mean aspect up to 3.5) are mostlyeuhedral, 100120 m in width and 150350 m in length and usuallyhave well-terminated pyramid terminations. The studied zircons havedistinct oscillatory zoning and commonly have thick, CL dark rims(Fig. 5). The cathodoluminescence images also reveal the commonpresence of distinct angular-shaped or ovoid cores ca. 100 m indiameter. They are usually brighter and have weaker oscillatory zoningthan in the zircon exteriors.

Sample OS326 is a massive felsic metavolcanic rock (metarhyolite)that occurs as a few-metre-thick interlayer within the Skoroice micaschists of the Star Msto Belt's lower unit, structurally at the footwallof the West Nznerov Thrust (Fig. 2). It comes from an exposure inthe Biaa Ldecka river valley approximately 100 m below the contactof themica schists of the lower unit with the E-MORB-like amphibolitesof the Star Msto Belt's middle unit (Figs. 1, 2). The sampled rock islight-coloured, medium- to coarse-grained and mainly composed ofquartz, plagioclase, K-feldspar, muscovite, biotite, chlorite and garnet.

Sample OS326 contains clear, colourless, normal-prismatic zirconcrystals up to 250 m long (Fig. 6). They are usually subhedral andhave clear small scale oscillatory zoning. Some of the crystals haveovoid inherited cores and/or metamict-like rims.

Sample B27 comes from the Brousek quartzites collected at thetop of the Brousek Mt., where the elongated quartzite body appearsat the outer margin of the Velk Vrbno Dome (Figs. 1, 2). It is agreyish-yellow, fine-grained, mylonitically banded rock composedhyolite of the Orlicanienik Dome). (b) Concordia diagrams showing the SHRIMP dating

(a) (b)

Fig. 5. (a) Cathodoluminescence images of representative zircon grains from sample OS179 (metarhyolite of the upper unit of the Star Msto Belt). (b) Concordia diagrams showing theSHRIMP dating results.

122 M. Jastrzbski et al. / Lithos 220223 (2015) 116132predominantly of quartz (N90%) with subordinate tourmaline, garnet,white mica, apatite, zircon and opaque minerals including graphiteand pyrite.

The zircons of the B27 sample are mainly colourless, transparent,and ovoid or slightly elongated; they are 100300 m in diameter(Fig. 7). The B27 zircons are highly rounded, which testifies to thelong duration of sedimentary transport or repeated erosion/deposition.The zircons usually have an internal zoning, sector or concentric pattern(a)

Fig. 6. (a) Cathodoluminescence images of representative zircon grains from sample OS326 (mSHRIMP dating results.and are either CL-bright or CL-dark. A quarter of the zircon population ischaracterised by the presence of cores and rims.

Samples OS335/1 and OS335/2 represent the light (metadacite) anddark (metabasalt) members, respectively, of the bimodal suite of themetavolcano-sedimentary succession from the structurally upper partof the Velk Vrbno Dome. Both samples were collected in-situ fromthe same outcrop situated 100 m south of the main cross-road inPetkov on the NE slopes of the Ostrunik Mt. (Figs. 1, 2). Sample(b)

etarhyolite of the lower unit of the Star Msto Belt). (b) Concordia diagrams showing the

(a)

(c) (d)

(b)

Fig. 7. (a) Cathodoluminescence images of representative zircon grains from sample B27 (the Brousek quartzites). Concordia diagrams (b) and age histogram (c) showing the SHRIMPdating results. (d) Th/U vs. age diagram indicating possible source rocks for the zircon populations. 206Pb/238U ages are used for analyses younger than 1.5 Ga, whereas 207Pb/206Pbages are used for analyses older than 1.5 Ga.

123M. Jastrzbski et al. / Lithos 220223 (2015) 116132OS335/1 is a grey, medium-grained, foliated rock mainly composed ofplagioclase and quartz with some thin, discontinuous laminae definedby biotitised amphibole blasts. This sample also contains subhedralepidote grains that sometimes surround angular-shaped allanite cores.Sample OS335/2 is a dark-green foliated rock with mineral banding de-fined by ca. 1 mm thick alternating amphibole and quartz-plagioclaselaminae. The typical secondary minerals in the light-coloured laminaeare biotite, epidote and titanite.

The zircon grains in the metadacite sample OS335/1 of the VelkVrbno Dome are transparent and colourless, 150200 m in width and200320 m in length. They are normal-prismatic and have roundedpyramid terminations. These zircons have well pronounced oscillatoryzoning and more luminescent, U-poor and narrow rims up to 50 mthick (Fig. 8a).

The zircons retrieved from the metabasalt sample OS335/2 aresubhedral, often broken into fragments. The prism faces are occasionallypreserved. These grains are normal prismatic, similar to those of theOS335/1 sample, but they are generally smaller, being 100150 m inwidth and 200300 m in length. Similar to the zircons of sampleOS335/1, these zircons show strong oscillatory zoning and more lumi-nescent, irregular rims (Fig. 8b).

A supplement to this paper contains the whole-rock geochemicalanalyses (major and trace elements) of the dated samples OS179,OS326, B27, OS335/1 and OS335/2. The geochemical analysis of sampleGn1 can be found in Murtezi (2006).6. Results of SHRIMP dating

The SHRIMP data are presented in Table 1 and shown on theConcordia diagrams in Figs. 4, 5, 6, 7 and 8.

6.1. Gn1 metarhyolite of the Orlicanienik Dome

In sample Gn1, a series of 26 local UPb analyses was obtained from23 zircons. The majority of the euhedral, normal-prismatic, zoned zir-cons prevailing in this sample were dated at ca. 500 Ma (Table 1). Therobust Concordia age of 501 3 Ma (Fig. 4) was calculated on 16analyses of such zircons. Three younger dates were outside the errorrange of other dates within this population and were omitted, beingmost probably caused by Pb-loss. The 232Th/238U ratio for these zirconsranges 0.30.6. However, another much less populous group of zirconsgave a discordia line that intercepts the Concordia at points constrainingthe present time and the age of 581 49Ma. These zircons had similarmorphological features as the ca. 500 Ma crystals; however, in theCL images they tend to be slightly darker, more fractured and less elon-gated. Analytical points Gn1_14.1 and Gn1_14.2 revealed a strong vari-ation in the 232Th/238U ratiowithin one zircon grain. In the centre, datedat 597 8 Ma, this ratio had an average of 0.40 for the Gn1 zircons,whereas the zircon's rim (dated at 329 4 Ma) was characterised bya very high uranium content resulting in a 232Th/238U ratio of 0.05.The first age was established for the central part (point Gn1_14.1) of

(a)

(b)

Fig. 8. (a) Cathodoluminescence images of representative zircon grains from samples OS335/1 and OS335/2 (metadacite and metabasalt of the Velk Vrbno Dome). (b) Concordiadiagrams showing the SHRIMP dating results of the zircons.

124 M. Jastrzbski et al. / Lithos 220223 (2015) 116132this short-prismatic, subhedral yellowish zircon, and the second camefrom the crystal pyramid termination. The central part of this crystalhad a much higher uranium content of 587 ppm than its rim 191 ppm. Two Variscan dates of ca. 302 and 328 Ma were obtainedfor the analytical points Gn1_21.1 (short-prismatic, strongly zoned,euhedral crystal) and Gn1_14.2 (Fig. 4).Within one of the dated zircons,we obtained two 207Pb/206Pb Archaean ages of 3071 12 Ma and2880 18 Ma.

6.2. Sample OS179 metarhyolite of the Star Msto Belt's upper unit

In sample OS179, 24 analyses within 20 grains were carried out(Table 1). The zircon cores were characterised by 232Th/238U ratiosranging from 0.53 to 0.90, with the exception of one of analysed zirconcore that yield 232Th/238U ratio of 0.02 (point OS179_10.1, Table 1). Themajority of the euhedral, normal-prismatic, oscillatory zoned zirconsor zircon exteriors gave the CambrianOrdovician dates. Seventeenanalyses within the oscillatory zoned zircon mantles produced aConcordia age of 493 4 Ma (Fig. 5). These zircons had moderateand constant 232Th/238U ratios that ranged between 0.06 and 0.22.Four analyses that targeted angular cores and one zircon mantle gavea Neoproterozoic age of 572 18 Ma (Fig. 5). Two other analysedcores gave UPb ages of 614 12 and 733 12 Ma. One analysis wasperformed on an ovoid zircon core, which yielded a Pb/Pb age of2415 13 Ma.

6.3. Sample OS326 metarhyolite of the Star Msto Belt's lower unit

In sample OS326, 17 spots within 16 grains were analysed (Table 1).An analysis of one inherited core gave a Pb/Pb age of 184914Ma. Oneanalysis from another core gave a UPb age of 608 11 Ma (Fig. 6a).One zircon rim with the highest uranium content (3599 ppm) gave an

125M. Jastrzbski et al. / Lithos 220223 (2015) 116132age of 575 11Ma,whichmay be an analytical artefact. The remaining14 analyses provided the Concordia age of 4985Ma.With two excep-tions, these zircons had moderate 232Th/238U ratios that ranged be-tween 0.07 and 0.75.

6.4. Sample B27 the Brousek quartzite

For sample B27, 45 analyses within 44 grains were performed. Withfive exceptions, all the analyses are concordant within the error limits(Table 1) (Ludwig, 1998). More than half of the analyses gave theNeoproterozoic to Early Cambrian ages (from ~672 to ~531 Ma). OneVariscan zircon age of 348 4Mawas obtained in a less luminescentzircon centre. One concordant analysis gave a UPb zircon age of 9926Ma. There were fifteen Palaeoproterozoic ages that clustered between2.20 and 1.97 Ga. One core of the rounded grain yielded an age of2.47 Ga, the oldest obtained in this sample. The Palaeoproterozoic zir-cons have 232Th/238U ratios broadly ranging from 0.05 to 1.17. On theother hand, the Neoproterozoic/Early Cambrian zircons had morevaried 232Th/238U ratios that ranged from 0.02 to 2.05 (Fig. 7, Table 1).

6.5. Sample OS335/1 metadacite of the Velk Vrbno Dome

Fourteen analyses spots within 12 grains were carried out in the sam-ple OS335/1. Twelve analytical spots were located in the internal parts ofthe zircons, and they gave the Concordia age of 5584Ma. These zirconshad moderate 232Th/238U ratios that ranged between 0.21 and 0.78. Twohighly discordant UPb zircon ages of ~315 and ~367 Ma were obtainedfrom the thin, U-poor rims of the zircons of this sample (Fig. 8a).

6.6. Sample OS335/2 metabasalt of the Velk Vrbno Dome

In sample OS335/2, 13 spots within 12 grains were analysed. Thedating of 12 oscillatory-zoned zircons provided a concordia age of557 4 Ma (Fig. 8b). The 232Th/238U ratios of these zircons were com-parable to, but slightly higher (0.460.85) than, those of the zirconsfrom sample OS335/1. The discordant analytical result obtained from aCL-bright zircon rim corresponds to an UPb age of ca. 362 Ma.

7. Discussion

7.1. OSD and SMB as parts of Saxothuringia

The metavolcano-sedimentary Stronie Fm. in the OrlicanienikDome and the upper unit of the Star Msto Belt (Hranina seriesof Skcel, 1989) reveal similar lithological and tectonometamorphic de-velopment (Don et al., 2003; Jastrzbski, 2012; Skcel, 1989). The geo-chemical characteristics of the felsic metavolcanic rocks from the StarMsto Belt and the Orlicanienik Dome are also alike (Murtezi,2006). Our new data further reveal that the zircon populations fromthemetarhyolites of the two units are similar in their average size, mor-phological characteristics and SIMS UPb ages. In the analysed zirconsfrom samples Gn1 and OS179, magmatic outgrowths with clear oscilla-tory zoning yielded concordant ages of 500 3Ma and 493 4Ma, re-spectively. Such dates correspond well to the UPb zircon SHRIMP agesobtained for the Gniewoszw metarhyolites (Mazur et al., 2014;Murtezi, 2006), but are ~2025Ma younger than the PbPb evaporationages of zircons from the felsic rocks of the StarMsto Belt (Krner et al.,2000) and the Orlicanienik Dome (Krner et al., 1997). Metarhyolitefrom the Skoroice Fm. of the Star Msto Belt's lower unit, sampleOS326, also gave the UPb zircon concordia age of 498 5 Ma. All the~500490 Ma ages are interpreted as the time of zircon crystallisationfrom a felsic melt during volcanic eruption, thus as deposition time ofthe host metasediments.

In the Stronie Fm. and the Hranina Fm., massive felsic metaigneousbodies (lava flows) may occur in mica schists and in schistosemetatuffitic envelopes (Fig. 3a, b) (Murtezi, 2006; Smulikowski, 1979;Wojciechowska, 1972, 1989) or alternate with mafic metavolcanogenicrocks (point 1.2 in elaniewicz et al., 2014b), which points to bimodalityof the ~500 Ma volcanic suite commonly reported from elsewhere inSaxothuringia. In the OSD, the bimodal volcanic rocks mainly occur inhigher parts of the lithostratigraphic column for the Stronie Fm. (seeelaniewicz et al., 2014a) metapelites (Fig. 9). Occasionally, peliticsediments may have been cut by felsic (Mazur et al., 2012, 2014) ormafic veins (volcano feeders) but such cases are difficult to prove becauselater shearing may have obliterated originally unconformable contacts.

The youngest detrital zircons in the Mynowiec Formation found byMazur et al. (2012, 2014) are ca. 560 Ma, whereas Jastrzbski et al.(2010) reported the ages as young as 540530 Ma, which overlapwith the 530520 Ma ages of detrital zircons in the mica schists of theStronie Fm. (Jastrzbski et al., 2010). Fischer (1936) and Don et al.(2003) proposed an unconformity between rocks of the MynowiecFm. and the Stronie Fm. On the other hand, numerous detailed petro-graphic and field works indicated that the observed rock successionof the MynowiecStronie Group presumably reflects environmentalchanges in a single transient sedimentary basin (e.g., Cwojdziski,1977; Jastrzbski et al., 2014; Oberc, 1968b; Smulikowski, 1979;Wojciechowska, 1993). Our new UPb zircon ages are consistent withthe earlier zircon SHRIMP data for the Stronie Fm. rocks (Jastrzbskiet al., 2010) and corroborate inferences based on problematic fossilsfrom the Stronie Fm. quartzites and marbles (Gunia, 1984, 1997;Koszela, 1997) and the Mynowiec Fm. (?) paragneisses (Gunia andWierzchoowski, 1979). Therefore, we suggest that the entireMynowiecStronie Group, together with the adjacent Star MstoBelt, developed during the Mid-Cambrian through Early Ordovician(Fig. 9), although the onset in the late Ediacaran cannot be excluded.

The zircons of sample OS179, which have ca. 500 Ma magmatic do-mains, possess cores that yielded the Concordia age of ca. 575 Ma, andthree euhedral zircons in sample Gn1 yielded an upper intercept ageof ~581 Ma. Such data indicate that the ca. 500 Ma felsic magma musthave been derived from Neoproterozoic crust which was subjected topartial melting around approximately 500 Ma and eventually broughtlavas to the surface, where alternated with concurrent pelitic and car-bonate sediments. As carbonate reef deposits require relatively warmwaters to develop, the part of Gondwana that is considered the eastern-most Saxothuringian Terrane may have been positioned at rather lowlatitudes in theMid-CambrianEarly Ordovician. Such a corollary is con-sistentwith the presence of archaeocyaths in the Kaczawa sector of thatterrane (Biaek et al., 2010). A quite insignificant volume of basicmetavolcanogenic rocks within the MynowiecStronie Group wouldindicate that the latter was deposited either in a basin that did notreach an oceanic stage, or at least was located far from the spreadingcentre. Having taken into account that sedimentation in theMynowiecStronie Basin presumably ceased in the Early Ordovician,such data allow us to propose a scenario of an attenuated continentalmargin of Saxothuringia which was transformed to a sediment-starved passive margin with flat topography.

In the Orlicanienik Dome, the geochemical and isotopic charac-teristics of the metarhyolitic rocks suggest that the parent magmaswere likely derived through the partial melting of the continentalcrust (e.g., Murtezi, 2006; elaniewicz et al., 2014a,b). In contrastto samples OS355/1 and OS355/2 (the Velk Vrbno Dome), ourmetarhyolite samples OS179, OS326 (SMB) and Gn1 (OSD), similarto others in the OSD (e.g., Murtezi, 2006), show negative Eu, Ti, andSr anomalies and a high Th/Nb ratio. The OSD metarhyolites havewithin-plate (Wojciechowska et al., 2001) and suprasubduction geo-chemical signatures (Murtezi, 2006), which would indicate a continen-tal rift-related or back-arc setting, respectively. The latter would bein line with the chemistry of the siliciclastic metasedimentary rocksfrom both the Mynowiec Fm. and Stronie Fm., suggesting depositionat an active continental margin rich in evolved felsic magmatic rocks(Szczepaski and Ilnicki, 2014). Such information, in view of theabove mentioned ca. 500 Ma ages of both igneous and clastic rocks,

Table 1Results of UPb dating of zircons from the Orlica-nienik Dome, Star Msto Belt and Velk Vrbno Dome.

Spot % 206Pbc ppmU

ppmTh

232Th/238U ppm 206Pb* 207Pb/206Pb % 207Pb/235U % 206Pb/238U % errcorr

206Pb/238Uage

207Pb/206Pbage

% D

Gn1: metarhyolite of the Orlicanienik DomeGn1_1.1 0.13 271 103 0.39 19.1 0.0574 2.6 0.650 2.9 0.0821 1.2 0.413 508.9 5.8 505 57 1Gn1_2.1 0.66 623 343 0.57 42.5 0.0576 3.4 0.626 3.7 0.0788 1.4 0.388 489.1 6.8 513 75 5Gn1_3.1 0.80 1087 137 0.13 71.0 0.0562 2.5 0.585 2.7 0.0754 1.0 0.391 468.8 4.7 462 54 2Gn1_4.1 0.03 362 220 0.63 29.6 0.0589 1.5 0.773 1.8 0.0951 1.1 0.605 585.7 6.2 564 32 4Gn1_5.1 0.11 519 72 0.14 37.1 0.0575 2.1 0.658 2.3 0.0831 1.1 0.476 514.6 5.5 509 45 1Gn1_6.1 0.11 941 486 0.53 64.4 0.0568 1.4 0.623 1.8 0.0795 1.0 0.585 493.3 4.9 483 32 2Gn1_7.1 0.37 201 57 0.29 13.9 0.0573 4.0 0.635 4.2 0.0803 1.2 0.297 497.9 6.0 505 88 1Gn1_8.1 0.01 384 282 0.76 27.7 0.0583 1.5 0.674 1.9 0.0839 1.1 0.599 519.1 5.5 542 32 4Gn1_9.1 0.31 749 532 0.73 50.7 0.0568 2.3 0.616 2.6 0.0786 1.1 0.414 487.5 5.0 485 51 0Gn1_10.1 8.52 971 364 0.39 60.1 0.0611 7.1 0.555 7.2 0.0660 1.2 0.166 411.7 4.7 642 150 56Gn1_10.2 2.91 910 288 0.33 68.3 0.0588 4.5 0.688 4.7 0.0848 1.4 0.288 525.0 6.9 560 99 7Gn1_11.1 0.21 375 203 0.56 25.8 0.0568 2.7 0.625 2.9 0.0797 1.1 0.387 494.5 5.4 485 59 2Gn1_12.1 0.19 306 128 0.43 20.6 0.0577 2.5 0.623 2.8 0.0782 1.2 0.431 485.4 5.6 519 55 7Gn1_13.1 1.05 796 336 0.44 50.9 0.0563 3.7 0.571 3.8 0.0736 1.1 0.289 457.9 4.9 464 81 1Gn1_14.1 0.72 492 191 0.40 41.3 0.0605 2.5 0.810 2.9 0.0970 1.4 0.480 596.8 7.8 623 54 4Gn1_14.2 4.87 1444 72 0.05 68.2 0.0603 4.7 0.435 4.8 0.0523 1.1 0.227 328.7 3.5 616 100 87Gn1_15.1 0.04 587 46 0.08 255.0 0.2328 0.7 16.230 1.4 0.5057 1.2 0.860 2638.0 26 3071 12 16Gn1_15.2 0.32 191 69 0.37 47.5 0.2067 1.1 8.220 1.6 0.2885 1.2 0.735 1634.0 17 2880 18 76Gn1_16.1 0.12 578 290 0.52 40.4 0.0575 1.6 0.643 1.9 0.0811 1.1 0.571 502.9 5.3 510 35 1Gn1_17.1 0.74 139 50 0.37 12.6 0.1064 5.7 1.529 5.9 0.1042 1.5 0.253 639.1 9.0 1738 100 172Gn1_18.1 0.13 429 160 0.39 30.2 0.0573 1.9 0.647 2.2 0.0819 1.1 0.505 507.6 5.4 503 41 1Gn1_19.1 0.24 710 546 0.79 49.5 0.0569 2.1 0.634 2.3 0.0809 1.1 0.454 501.6 5.1 486 46 3Gn1_20.1 0.23 667 491 0.76 46.3 0.0573 1.7 0.637 2.0 0.0806 1.1 0.519 499.9 5.1 503 38 1Gn1_21.1 0.87 266 99 0.39 11.0 0.0509 12.0 0.337 12.0 0.0480 1.5 0.118 302.1 4.3 238 280 21Gn1_22.1 0.70 613 242 0.41 43.4 0.0575 1.8 0.650 2.1 0.0819 1.1 0.500 507.4 5.2 512 41 1Gn1_23.1 0.33 615 92 0.16 42.5 0.0572 2.2 0.632 2.5 0.0802 1.1 0.433 497.3 5.1 499 49 0Gn1_24.1 0.11 418 212 0.52 29.5 0.0576 2.0 0.652 2.3 0.0821 1.1 0.476 508.6 5.4 515 45 1Gn1_25.1 2.11 490 169 0.36 33.2 0.0572 4.5 0.608 4.7 0.0771 1.2 0.248 478.8 5.3 499 100 4Gn1_26.1 0.29 415 263 0.66 29.3 0.0575 2.6 0.650 2.8 0.0820 1.1 0.395 508.1 5.4 511 57 1

OS179: metarhyolite of the upper unit of the Star Msto BeltOS-179 1.1 0.07 183 22 0.12 12.2 0.0604 3.1 0.645 3.7 0.0775 2.1 0.558 481.2 9.5 617 66 28OS-179 2.1 0.20 417 261 0.65 32.7 0.0588 3.3 0.738 3.7 0.0911 1.8 0.481 561.8 9.7 559 71 1OS-179 2.2 0.34 667 47 0.07 45.0 0.0595 2.8 0.642 3.4 0.0783 1.8 0.532 485.8 8.4 586 62 21OS-179 3.1 0.00 351 46 0.14 24.1 0.0569 3.7 0.629 4.2 0.0802 1.8 0.437 497.2 8.7 487 83 2OS-179 4.1 0.05 1166 137 0.12 78.5 0.0564 1.4 0.609 2.2 0.0783 1.7 0.784 486.0 8.0 469 30 4OS-179 5.1 0.00 594 301 0.52 61.5 0.0622 1.3 1.033 2.2 0.1204 1.7 0.790 733.0 12.0 680 29 7OS-179 5.1 0.13 581 97 0.17 40.3 0.0571 2.5 0.634 3.1 0.0805 1.8 0.569 499.3 8.4 495 56 1OS-179 6.1 0.26 276 57 0.21 19.4 0.0556 4.7 0.624 5.1 0.0814 2.0 0.389 504.2 9.5 436 100 13OS-179 7.1 0.06 518 53 0.11 34.6 0.0568 2.2 0.608 2.8 0.0775 1.8 0.636 481.4 8.3 486 48 1OS-179 8.1 0.00 120 62 0.53 10.2 0.0638 6.9 0.879 7.2 0.0999 2.1 0.294 614.0 12.0 735 150 20OS-179 8.1 0.14 694 53 0.08 48.2 0.0576 2.4 0.642 3.0 0.0809 1.8 0.603 501.2 8.7 513 52 2OS-179 10.1 0.84 86 2 0.02 7.2 0.0597 12.0 0.800 13.0 0.0966 2.4 0.194 594.0 14.0 592 270 0OS-179 10.2 0.03 366 44 0.12 24.4 0.0569 2.4 0.609 3.1 0.0776 1.9 0.625 481.9 8.9 488 53 1OS-179 11.1 0.51 230 200 0.90 19.0 0.0588 6.6 0.772 6.9 0.0953 2.0 0.289 587.0 11.0 559 140 5OS-179 12.1 0.41 256 58 0.24 18.0 0.0583 5.9 0.653 6.2 0.0813 2.0 0.317 503.7 9.5 541 130 7OS-179 13.1 0.04 213 21 0.10 14.5 0.0579 3.0 0.633 3.9 0.0793 2.4 0.624 492.0 11.0 528 66 7OS-179 14.1 0.00 1005 57 0.06 70.7 0.0560 1.4 0.632 2.2 0.0819 1.7 0.776 507.4 8.4 453 31 11OS-179 15.1 0.23 427 58 0.14 28.7 0.0566 3.0 0.608 3.5 0.0780 1.8 0.530 483.9 8.6 476 65 2OS-179 16.1 0.26 260 22 0.09 19.9 0.0587 4.4 0.716 4.8 0.0885 1.9 0.396 546.9 9.9 554 95 1OS-179 17.1 0.00 277 58 0.22 18.6 0.0582 2.6 0.630 3.2 0.0784 1.9 0.587 486.7 8.7 539 56 11OS-179 18.1 0.00 412 96 0.24 134.0 0.1562 0.8 8.190 2.2 0.3804 2.0 0.933 2078.0 36 2415 13 16OS-179 18.2 0.50 174 22 0.13 11.7 0.0598 7.6 0.646 7.9 0.0783 2.2 0.275 486.0 10.0 595 160 22OS-179 19.1 0.11 496 38 0.08 34.0 0.0581 2.4 0.639 3.0 0.0797 1.8 0.610 494.3 8.7 535 52 8OS-179 20.1 0.00 492 40 0.08 34.5 0.0569 2.1 0.641 2.7 0.0817 1.8 0.653 506.4 8.7 488 46 4

OS326: metarhyolite of the lower unit of the Star Msto BeltOS326.1.1 0.43 304 37 0.12 21.9 0.0583 3.4 0.671 3.9 0.0834 1.9 0.498 516.7 9.6 540 73 5OS326.2.1 1.11 1744 20 0.01 124.0 0.0562 2.6 0.632 3.2 0.0816 1.8 0.575 505.8 8.9 459 58 9OS326.3.1 0.62 412 299 0.75 28.9 0.0554 4.2 0.619 4.6 0.0810 1.9 0.411 502.1 9.2 428 94 15OS326.4.1 0.70 279 152 0.56 23.9 0.0579 4.3 0.789 4.7 0.0989 1.9 0.412 608.0 11 525 94 14OS326.5.1 0.28 850 6 0.01 58.6 0.0545 2.4 0.602 3.1 0.0801 1.9 0.615 496.6 9.1 393 55 21OS326.6.1 0.54 274 234 0.88 20.2 0.0589 4.3 0.691 4.7 0.0851 2.0 0.417 526.6 9.9 562 93 7OS326.6.2 0.15 1838 4 0.00 126.0 0.0578 1.1 0.634 2.1 0.0796 1.8 0.850 493.5 8.6 522 25 6OS326.7.1 0.34 394 31 0.08 27.5 0.0561 3.4 0.625 3.9 0.0809 1.9 0.487 501.4 9.2 454 76 9OS326.8.1 0.44 315 48 0.16 22.0 0.0571 3.4 0.638 3.9 0.0811 1.9 0.490 502.5 9.3 494 75 2OS326.9.1 0.10 372 123 0.34 102.0 0.1130 0.8 4.970 2.1 0.3187 1.9 0.931 1784.0 30 1849 14 4OS326.10.1 0.43 3599 963 0.28 289.0 0.0591 2.5 0.759 3.1 0.0931 1.9 0.596 574.0 10 570 54 1OS326.11.1 0.42 369 24 0.07 24.7 0.0575 3.1 0.614 3.6 0.0776 1.9 0.525 481.6 8.9 509 68 6OS326.12.1 1.28 100 26 0.27 6.8 0.0506 10.0 0.542 11.0 0.0777 2.2 0.209 482.0 10 221 240 54OS326.13.1 1.33 99 27 0.28 6.7 0.0540 11.0 0.576 11.0 0.0773 2.2 0.202 480.0 10 372 240 23OS326.14.1 0.63 193 20 0.10 12.9 0.0580 4.8 0.619 5.2 0.0775 2.1 0.403 481.0 9.7 529 100 10OS326.15.1 1.18 116 45 0.40 8.5 0.0584 7.6 0.674 7.9 0.0838 2.2 0.274 519.0 11 544 170 5

126 M. Jastrzbski et al. / Lithos 220223 (2015) 116132

Table 1 (continued)

Spot % 206Pbc ppmU

ppmTh

232Th/238U ppm 206Pb* 207Pb/206Pb % 207Pb/235U % 206Pb/238U % errcorr

206Pb/238Uage

207Pb/206Pbage

% D

OS326.16.1 0.68 316 31 0.10 21.9 0.0554 5.5 0.612 5.8 0.0801 1.9 0.335 497.0 9.3 428 120 14B27: Brousek metasandstoneB27.30.1 10.44 734 411 0.58 39.2 0.0533 13.0 0.408 13.0 0.0556 1.1 0.087 348.5 3.7 340 280 3B27.25.1 0.18 316 220 0.72 23.4 0.0578 2.7 0.685 2.9 0.0859 0.9 0.330 531.3 4.8 523 59 2B27.44.1 15.89 23 6 0.26 2.08 0.0580 59.0 0.690 59.0 0.0869 4.7 0.080 537.0 24.0 510 1300 5B27.26.1 2.24 26 52 2.05 2.0 0.0522 27.3 0.631 27.4 0.0878 2.5 0.091 542.2 12.9 292 623 46B27.9.1 0.00 41 46 1.15 3.2 0.0613 3.7 0.763 4.0 0.0902 1.4 0.357 556.7 7.5 650 79 17B27.27.1 0.00 61 59 0.99 4.8 0.0605 3.2 0.766 3.4 0.0919 1.2 0.340 566.6 6.3 621 69 10B27.17.1 0.23 255 4 0.02 20.2 0.0586 3.2 0.744 3.4 0.0921 1.0 0.296 567.7 5.4 553 71 3B27.35.1 0.06 581 283 0.50 46 0.0603 1.4 0.766 1.6 0.0921 0.7 0.445 568.1 3.9 614 31 8B27.36.1 0.07 478 29 0.06 38.2 0.0592 1.5 0.757 1.6 0.0928 0.7 0.447 572.3 4.0 573 32 0B27.28.2 0.10 583 24 0.04 46.8 0.0593 1.7 0.764 1.9 0.0934 0.8 0.429 575.8 4.5 577 38 0B27.42.1 2.09 22 5 0.21 1.84 0.0590 18.0 0.760 18.0 0.0940 2.5 0.135 579.0 14.0 568 390 2B27.43.1 0.06 833 142 0.18 67.9 0.0600 1.3 0.785 1.5 0.0949 0.8 0.540 584.4 4.6 605 28 3B27.32.1 1.79 178 14 0.08 14.9 0.0596 8.3 0.785 8.4 0.0955 1.2 0.148 587.9 7.0 590 180 0B27.10.1 0.33 168 104 0.64 13.9 0.0586 3.5 0.774 3.7 0.0957 1.1 0.298 589.4 6.2 552 77 6B27.7.1 0.15 488 421 0.89 40.3 0.0588 1.8 0.778 2.0 0.0961 0.8 0.422 591.4 4.7 558 39 6B27.21.1 0.00 179 117 0.67 15.0 0.0608 2.8 0.814 3.0 0.0971 1.0 0.340 597.5 5.8 631 61 6B27.22.1 0.72 97 63 0.67 8.2 0.0612 8.5 0.824 8.6 0.0976 1.3 0.152 600.2 7.5 647 182 8B27.23.1 0.00 128 96 0.77 10.7 0.0609 2.8 0.822 3.0 0.0979 1.0 0.337 602.0 5.9 637 61 6B27.40.1 452 158 0.36 38.5 0.0602 1.6 0.825 1.9 0.0994 0.9 0.490 610.7 5.3 611 35 0B27.41.1 0.06 471 336 0.74 40.7 0.0605 1.5 0.839 1.7 0.1006 0.7 0.439 617.8 4.3 623 32 1B27.39.1 0.00 79 25 0.33 6.82 0.0614 3.0 0.852 3.2 0.1006 1.2 0.378 618.1 7.2 655 64 6B27.14.1 0.89 85 63 0.77 7.4 0.0593 7.3 0.828 7.4 0.1013 1.5 0.198 622.0 8.7 577 158 7B27.1.1 0.20 212 42 0.20 18.5 0.0613 2.8 0.858 2.9 0.1016 0.8 0.279 623.8 4.9 648 61 4B27.28.1 0.66 70 78 1.14 6.3 0.0592 7.8 0.841 7.9 0.1030 1.3 0.164 632.2 7.8 575 169 9B27.15.1 0.38 197 93 0.49 17.7 0.0596 3.5 0.858 3.7 0.1045 1.1 0.287 640.5 6.4 589 76 8B27.12.1 1.10 43 14 0.35 3.9 0.0608 8.7 0.880 8.8 0.1049 1.8 0.198 643.0 10.7 633 187 2B27.2.1 0.38 466 665 1.47 44.0 0.0591 4.2 0.892 4.2 0.1095 0.7 0.165 669.6 4.4 570 90 15B27.18.1 0.96 53 39 0.77 5.0 0.0576 11.0 0.873 11.2 0.1100 1.8 0.157 672.6 11.2 514 243 24B27.34.1 0.62 1595 773 0.50 229 0.0727 1.6 1.666 1.7 0.1664 0.6 0.377 992.0 5.8 1004 32 1B27.29.1 0.00 800 443 0.57 223.2 0.1209 0.5 5.414 0.8 0.3249 0.7 0.819 1813.4 10.9 1969 9 9B27.31.1 0.05 762 35 0.05 230 0.1274 0.4 6.162 0.8 0.3509 0.7 0.831 1939.0 11.0 2062 8 6B27.16.1 0.47 89 101 1.17 27.3 0.1211 2.0 5.934 2.3 0.3555 1.2 0.517 1960.6 20.1 1972 35 1B27.37.1 0.05 173 129 0.77 53.1 0.1268 0.9 6.236 1.2 0.3568 0.8 0.690 1967.0 14.0 2054 16 4B27.4.1 0.00 144 82 0.59 45.0 0.1251 1.0 6.252 1.3 0.3624 0.8 0.614 1993.5 13.5 2030 18 2B27.24.1 0.25 820 52 0.07 262.2 0.1270 0.6 6.499 0.9 0.3711 0.7 0.774 2034.6 12.8 2057 11 1B27.38.1 0.05 419 137 0.34 137 0.1286 0.6 6.732 0.9 0.3798 0.7 0.782 2075.0 12.0 2079 10 0B27.3.1 0.07 426 23 0.06 140.2 0.1255 1.0 6.629 1.4 0.3832 1.0 0.685 2091.3 17.2 2035 18 3B27.11.1 0.23 91 20 0.22 30.3 0.1323 1.5 7.062 1.9 0.3872 1.1 0.611 2109.9 20.7 2128 26 1B27.20.1 0.00 49 35 0.74 16.5 0.1293 1.5 6.937 1.9 0.3890 1.1 0.588 2118.2 20.2 2089 27 1B27.19.1 0.14 174 89 0.53 58.4 0.1331 1.1 7.141 1.4 0.3892 1.0 0.667 2119.3 17.2 2139 19 1B27.13.1 0.15 152 92 0.63 51.2 0.1331 1.1 7.201 1.5 0.3923 1.1 0.708 2133.3 19.2 2140 18 0B27.8.1 0.08 235 95 0.42 79.6 0.1364 0.8 7.400 1.2 0.3934 0.9 0.733 2138.4 15.6 2183 14 2B27.33.1 0.04 132 52 0.41 45.1 0.1353 0.9 7.440 1.4 0.3985 1.0 0.766 2162.0 19.0 2168 15 0B27.5.1 0.06 299 121 0.42 103.8 0.1374 0.7 7.658 0.9 0.4041 0.6 0.688 2188.0 11.9 2195 12 0B27.6.1 0.08 477 236 0.51 186.9 0.1610 0.5 10.112 0.9 0.4555 0.7 0.821 2419.5 15.1 2466 9 2

OS335/1: metadacite of the Velke Vrbno DomeOS335-1_1.1 0.18 117 31 0.27 9.1 0.0562 3.5 0.694 3.9 0.0896 1.6 0.403 553.1 8.2 460 78 17OS335-1_1.2 0.36 169 49 0.30 13.1 0.0570 2.7 0.704 3.1 0.0897 1.4 0.460 553.7 7.4 490 60 12OS335-1_2.1 0.11 246 88 0.37 19.4 0.0577 1.9 0.732 2.3 0.0920 1.4 0.597 567.1 7.6 520 41 8OS335-1_3.1 0.63 114 24 0.21 8.9 0.0575 8.0 0.710 8.2 0.0895 1.6 0.194 552.5 8.4 512 180 7OS335-1_4.1 0.00 380 195 0.53 29.5 0.0596 1.2 0.743 1.8 0.0904 1.3 0.735 557.8 7 589 26 6OS335-1_5.1 0.35 296 113 0.40 22.8 0.0584 2.6 0.722 3.1 0.0896 1.6 0.507 553.4 8.2 54 58 1OS335-1_6.1 0.16 335 161 0.50 26.2 0.0597 1.7 0.750 2.1 0.0911 1.3 0.618 562.0 7.1 594 37 6OS335-1_7.1 0.22 134 42 0.33 10.4 0.0581 2.7 0.722 3.1 0.0901 1.5 0.487 556.1 8 535 59 4OS335-1_8.1 0.09 295 122 0.43 22.9 0.0579 1.8 0.721 2.3 0.0903 1.4 0.602 557.5 7.3 526 40 6OS335-1_8.2 0.00 1 0 0.01 0.1 0.1780 16.0 1.400 17.0 0.0570 7.3 0.416 358.0 25 2634 260 636OS335-1_9.1 0.00 2 0 0.01 0.1 0.1280 14.0 1.120 15.0 0.0636 5.7 0.388 397.0 22 2071 240 422OS335-1_10.1 0.05 310 92 0.31 24.3 0.0588 1.9 0.738 2.4 0.0911 1.4 0.600 561.9 7.7 560 41 0OS335-1_11.1 0.31 248 103 0.43 19.6 0.0573 3.0 0.723 3.3 0.0915 1.5 0.443 564.3 7.9 503 65 11OS335-1_12.1 0.72 136 102 0.78 10.7 0.0611 5.0 0.766 5.3 0.0910 1.6 0.304 561.4 8.6 642 110 14

OS335/2: metabasalt of the Velke Vrbno DomeOS335-2_1.1 0.00 210 86 0.42 16.1 0.0611 2.2 0.750 2.6 0.0891 1.4 0.545 550.2 7.5 642 47 17OS335-2_1.2 0.00 5 0 0.00 0.3 0.1055 8.0 0.839 8.8 0.0577 3.5 0.396 362.0 12 1723 150 376OS335-2_2.1 0.00 293 121 0.43 22.5 0.0577 1.6 0.711 2.1 0.0894 1.4 0.659 552.3 7.3 517 35 6OS335-2_3.1 0.00 195 68 0.36 15.3 0.0602 1.9 0.756 2.5 0.0911 1.5 0.615 562.3 8.1 611 42 9OS335-2_4.1 0.00 393 184 0.48 30.7 0.0587 1.3 0.734 1.8 0.0908 1.3 0.723 560.2 7.2 555 28 1OS335-2_5.1 0.19 181 73 0.42 13.9 0.0575 2.6 0.709 3.0 0.0894 1.4 0.480 552.0 7.6 510 58 8OS335-2_6.1 0.00 206 75 0.38 16.4 0.0595 1.9 0.761 2.4 0.0928 1.4 0.594 572.4 7.7 584 42 2OS335-2_7.1 0.19 429 352 0.85 33.5 0.0578 2.1 0.724 2.5 0.0908 1.3 0.534 560.5 7.2 522 47 7OS335-2_8.1 0.13 160 47 0.30 12.3 0.0595 2.9 0.734 3.3 0.0894 1.5 0.445 552.1 7.7 587 64 6

(continued on next page)

127M. Jastrzbski et al. / Lithos 220223 (2015) 116132

Table 1 (continued)

Spot % 206Pbc ppmU

ppmTh

232Th/238U ppm 206Pb* 207Pb/206Pb % 207Pb/235U % 206Pb/238U % errcorr

206Pb/238Uage

207Pb/206Pbage

% D

OS335-2_9.1 0.00 304 134 0.45 24.1 0.0607 1.9 0.772 2.3 0.0922 1.4 0.593 568.7 7.4 629 40 11OS335/2: metabasalt of the Velke Vrbno DomeOS335-2_10.1 0.25 428 301 0.73 33.5 0.0583 1.8 0.729 2.3 0.0908 1.3 0.592 560.3 7.2 539 40 4OS335-2_11.1 0.39 221 106 0.50 16.9 0.0584 3.3 0.714 3.6 0.0887 1.4 0.398 547.9 7.5 545 72 1OS335-2_12.1 0.25 430 292 0.70 32.7 0.0592 2.2 0.720 2.5 0.0882 1.3 0.529 545.1 7 573 47 5

Errors are 1-sigma; Pbc and Pb* indicate the common and radiogenic portions, respectively. Common Pb corrected using measured 204Pb.Error in Standard calibration: sample Gn1 0.41%, sample OS179 0.67%, sampleOS326 0.60%, sample B27 (points B27.1.1 to 29.1) or 0.55% (points B27.30.1 to 44.1), sampleOS335/1 0.48%, and sample OS335/2 0.48%. The error in standard calibration is not included in the errors listed above, but is required when comparing data from different mounts.% D = 100 [(207Pb/206Pb age) / (206Pb/238U age) 1].

128 M. Jastrzbski et al. / Lithos 220223 (2015) 116132suggests that their suprasubduction signatures have been inheritedfrom the reworked Neoproterozoic crust. The latter was formed duringthe Andean-type AvalonianCadomian orogeny at the Gondwanamargin, which is in line with most paleogeographic reconstructions(McKerrow et al., 1992; Nance et al., 2010; Torsvik et al., 2012; vonRaumer and Stampfli, 2008).

The new zircon ages and geochemical data support the view thatboth the upper and lower units of the Star Msto Belt may be directlycorrelated with the Stronie Fm. (Don et al., 2003; Murtezi, 2006;Skcel, 1989) and thus allow to assign them to the SaxothuringianTerrane. The middle unit, which is tectonically sandwiched betweenthe upper and lower SMB units, consists almost wholly of amphibolites,and minor felsic metavolcanites and serpentinites (Don et al., 2003).Associated with the metabasites are pearl paragneisses that yielded tomigmatization at ~500 Ma and their maximum protolith age was setby detrital zircons at ~550 Ma (Krner et al., 2000), thus similar tothat estimated for the Mynowiec Fm. in the OSD. Summing up, theprotolith ages of volcanic and sedimentary rocks are comparablethroughout the SMB and OSD (Fig. 9).

7.2. The Velk Vrbno Dome as part of Brunovistulia

Zircons from the two studied samples (OS335/1 and OS335/2) offelsic/intermediate and basic metavolcanic rocks collected from thestructurally higher part of the Velk Vrbno Dome, defined as theupper clastic group by Kvto (1951) and described by tpsk et al.(2006) as eclogite-bearing dacite orthogneiss with the Neoproterozoicprotolith age, have similar morphology and yielded similar UPbages: 558 4 Ma and 557 4 Ma. Their euhedral prismatic formscharacterised by clear oscillatory zoning yielded an age of ca. 560 Mafor both samples, which is interpreted as the formation time of the bi-modal magmatic protoliths in the Ediacaran. This timing roughly coin-cides with the PbPb age of 574 Ma obtained by Krner et al. (2000)for metatonalite/dacite gneisses from the upper part of the VelkVrbno Dome. Having taken into account the presence of Precambrianrocks and paleontologically dated Lower Devonian carbonate rockscorrelative to other Devonian rocks from the Brunovistulian Terrane(Hladil et al., 1999; Koverdynsk and Prokop, 2006), one can suggestthat the Velk Vrbno Dome is a complex structure. In the dome, theeclogite-bearing units overlie eclogite-absent units presumably due tothrusting (tpsk et al., 2006), whichmay have stackedNeoproterozoicrocks over Palaeozoic rocks (Fig. 9).

Combined with the earlier data, our new results confirm that theNeoproterozoic basement of the Velk Vrbno Dome is exposed at thesurface and is compatible with the Keprnik and Desn domes locatedfarther east, which justifies the assignment of the three domesto Brunovistulia. This feature makes an important contrast to theOrlicanienik Dome, where no Neoproterozoic (meta)igneous rockis evident at the surface, which is one of the main differences betweenthe eastern Saxothuringia and the western Brunovistulia. Furtherdifferences lie in the Palaeozoic history of the two terranes. In theOrlicanienik Dome, the Phanerozoic succession was confinedto Mid-CambrianLower Ordovician rocks, whereas lithology of theVelk Vrbno Dome comprises Neoproterozoic basement units andthen Silurian?Devonian cover rocks confirmed by paleontologicaldata (Fig. 9). Rocks of the Velk Vrbno Dome bear no legible records ofCambro-Ordovician thermal event(s), which additionally suggests theBrunovistulian affinity of this dome.

7.3. The MTZ as the terrane boundary between Saxothuringia andBrunovistulia

The results of the present zircon study are in line with the view thatthe Star Msto Belt is the boundary zone between the West Sudetes(Lugicum) and the East Sudetes (Silesicum). It has been recently pro-posed that metamorphism in this zone climaxed at 360340Ma duringcollision betweenSaxothuringia and Brunovistulia after theRheic Oceanwas closed at this sector of the Variscan orogen (Jastrzbski et al., 2013).Geochemical characteristics of the E-MORB metabasites in the middleunit of the SMB allows some authors to interpret them as remnants ofan oceanic crust (Finger and Steyrer, 1995; Poubov and Sokol, 1992).Janouek et al. (2014) also identified the oceanic crust west ofBrunovistulia. Babuka and Plomerov (2013) recognised the existenceof two lithospheric slabs on either side of the discussed boundary,with contrasting olivine fabrics in their mantle domains, suggestingthe presence of the intervening oceanic lithosphere in the past.Palaeobiogeographical data also suggest that Brunovistulia belongedto the southern margin of Laurussia (Kalvoda, 2001), which corrobo-rates the reconstruction by elaniewicz et al. (2009), andwas separat-ed from the Gondwana derived terranes by the Rheic Ocean (Finger andSteyrer, 1995; Finger et al., 1998; Jastrzbski et al., 2013). The zircon agespectra fromArmorica and Laurussia differ remarkably as the latter con-tains ~1.61.4 Ga zirconswhich are absent from the former (Friedl et al.,2000; Zeh et al., 2001). In contrary, Floyd et al. (1996) preferred to linkthe SMBmetabasites with an ensialic rift basin. Schulmann et al. (2009)considered Brunovistulia andMoldanubia as crustal fragments with dif-ferent histories yet derived from the same pinched-and-swelled litho-spheric plate. The UPb detrital age zircon study conducted across thesouthern sector of the Moldanubian Thrust Zone suggests that theMoldanubian and Moravo-Silesian crustal segments were close toeach other prior to the Variscan orogeny (Koler et al., 2014). Indeed,more palaeogeographic data are needed to provide a definite answer re-garding this ongoing discussion. Regardless of the distance betweenSaxothuringia and Brunovistulia, our new SHRIMP zircon study, whencompared with the published data for the Sudetes (Klimas et al., 2009;Mazur et al., 2010; Oberc-Dziedzic et al., 2003, 2005), further empha-sises significant differences in the zircon age characteristics betweenthe two crustal units. Having taken into account quite different geologicevolution of the OSD and Velk Vrbno Dome as indicated above, all theabove arguments speak in favour of the terrane suture located at theSMB. The presumed ophiolitic relics of the once subducted and thenexhumed Rheic oceanic crust became tectonically inserted into theSMB at the margin of the upper plate.

The suture itself consists of several thrust units comprised by theMoldanubian Thrust Zone (Franke and elaniewicz, 2002). In theSudetic sector, the MTZ is a bundle of at least six, WNW-dipping faults:

(a) (b)

Fig. 9. Structure of theMoldanubian Thrust Zone in thewalls of its Sudetic sector and the lithostratigraphy of themetavolcano-sedimentary sequences of the boundary zone. The Devonianages of themarbles and adjacent rocks from theVelk VrbnoDome are after Koverdynsk and Prokop (2006) andHladil et al. (1999). Rocks collected for zircon dating are indicated in boldfont. The schematic cross-section refers to the cross-section in Fig. 1c.

129M. Jastrzbski et al. / Lithos 220223 (2015) 116132two unnamed faults that border the upper unit of the Star Msto Belt;the West and East Nznerov Thrusts, the Velk Vrbno internal thrust;and the Ramzov Thrust (Fig. 9). There is an ongoing debate which ofthese thrusts is a front between the two terranes. Our study contributesto the debate by focussing on a narrow strip of the Brousek quartzitepositioned between the lower unit of the SMB and the Velk VrbnoDome (Figs. 1, 2). It contains rounded, long transported or multiplyredeposited, detrital zircons, mainly Proterozoic, but youngest yieldingages of 540530 Ma. Such Early Cambrian dates can be interpreted asreflecting the maximum depositional age, thus similar to mica schistprotoliths in the Stronie Fm. in the OSD and or SMB (Fig. 9). Nonethe-less, the Devonian age arbitrarily assigned to these quartzites by someauthors owing to compositional resemblance to the Brann quartzites(for review see Don et al., 2003) cannot be entirely excluded. Thenhowever ca. 500 Ma zircons might be expected in the quartzite as crys-talline rocks of that age were extensively present in the OSD and SMB.Therefore, we alternatively suggest that the protolith of the Brousekquartzites might have been deposited: (1) in the Cambrian, occupyinga position compatible with the Goszw quartzite at the base of theStronie Fm. (Fig. 9), or (2) in the Devonian but only Lower Cambrianto Neoproterozoic rocks were available at the surface in the sourcearea for the Brousek basin.

The second option would connect the quartzite with the VelkVrbno Dome rather than the OSD. However the first alternative ismore likely as the Brousek quartzites, similar to the Goszw quartzites,contain detrital zircons in two age clusters: NeoproterozoicEarlyCambrian (672531 Ma) and Proterozoic (2.191.96 Ga and 2.47 Ga)(Fig. 7) typically observed in Saxothuringian units (e.g., Kryza et al.,2007; Linnemann et al., 2004; kov et al., 2012). The Th/U ratiosbetween 0.01 and 2.05 in zircons of the younger cluster suggestCadomian magmatic and metamorphic events (~580 Ma) in thesource area(s) (Fig. 7d, Table 1). The Brousek quartzites, likewisemetasedimentary rocks in the OSD (Jastrzbski et al., 2010; Mazuret al., 2012, 2014), characteristically lack the ~1.61.4 Ga inheritedcomponents, the presence of which is a distinctive feature of theBrunovistulian Terrane (e.g., Finger et al., 2000; Friedl et al., 2000;elaniewicz et al., 2009). In Brunovistulia nearby, inherited zirconsof that age were reported from a granitic vein in Neoproterozoicparagneisses of the Desn Dome (elaniewicz et al., 2005) and fromthe 600580 Ma orthogneisses in the Fore Sudetic Block (Klimas et al.,2009; Mazur et al., 2010; Oberc-Dziedzic et al., 2003, 2005). Summingup, the Brousek quartzites lithostratigraphically correlate much betterwith metasedimentary rocks of Saxothuringia than Brunovistulia(Fig. 9). Our new data thus support the earlier view (Gawlikowskaand Opletal, 1997; Kasza, 1964) which links the quartzites with theSMB, though their present position (Figs. 1, 9) is tectonically allochtho-nous as displaced along the Nznerov dislocation zone (Skcel, 1989).

Actually, it is the East Nznerov Thrust, locally decorated withserpentinite lenses and structurally positioned below the stronglysheared and mylonitised Brousek quartzite sheet (Fig. 3c), that appearsto be themost important fault in the Sudetic sector of theMTZ. It bringsthe Velk Vrbno Dome of the Brunovistulian parentage and the StarMsto Belt of the Saxothuringian linkage into the direct contact and,thus, locates the boundary between rocks of two different terranes.

Another important thrusts of the MTZ bunch are those that sepa-rate the SMB middle unit from adjacent mica schists. The presumedophiolite relictswere shuffledwith rocks of the upper plate. These faultsare complex features which carry records of polyphase events first witha top-to-the-east kinematics then switched to an oblique dextral strike-slip on the W-dipping foliation planes (Jastrzbski, 2012).

In the Brunovistulian margin exposed in the VVD, undoubtedly im-portant is the thrust fault that presumably separates theNeoproterozoicbimodal succession from the Palaeozoic succession and is documentedby the discontinuity in metamorphic grades recognised by tpsket al. (2006). This feature however was identified by none of theauthors who mapped the region (Gawlikowska and Opletal, 1997 andreferences therein; Don et al., 2003 and references therein) (Fig. 2).The kinematics of the inferred thrust also remains unclear. This finding,

130 M. Jastrzbski et al. / Lithos 220223 (2015) 116132however, suggests that theMTZ is a complex structure. Indeed, differentkinematics and PT constraints (e.g., Cymerman, 1993; Jastrzbski,2012; Opletal and Pecina, 2004; Parry et al., 1997; tpsk et al., 2006)suggest a prolonged and complex evolution of the MTZ, which waslikely reactivated at different times under various deformation andmetamorphic conditions.

Farther north, the Moldanubian Thrust Zone continues beyondthe Marginal Sudetic Fault (Fig. 1b), but its ForeSudetic part differsremarkably from the Sudetic sector, partially because the ForeSudetic Block is ~5 km more deeply eroded than the Sudetes is(e.g., Cwojdziski and elaniewicz, 1995; Oberc, 1968a). TheSaxothuringiaBrunovistulia boundary is the domed Strzelin Thrust(Oberc-Dziedzic and Madej, 2002) accompanied by a multiphase zoneacross which a footwall of the Neoproterozoic orthogneisses (580 Maprotolith) with the Devonian quartzite cover are in tectonic contactwith a hanging wall containing ~500 Ma orthogneisses and micaschists (e.g., Klimas et al., 2009; Mazur et al., 2010; Oberc, 1968a;Oberc-Dziedzic and Madej, 2002; Oberc-Dziedzic et al., 2003). In turn,the Neoproterozoic orthogneisses are overthrust by eclogite-bearingschists from the Kamieniec Zbkowicki Metamorphic Fold Belt(Achramowicz et al., 1997; Nowak, 1997), the presence of which em-phasises the importance of the whole zone in the eastern part of theVariscan belt. The presence of medium- and high-pressure rocks sup-ports the view that the Moldanubian Thrust Zone in its Sudetic andForeSudetic sectors is a complex terrane suture resolvable into severalsubordinate thrusts.

In the studied zircons, the Variscan thermal event left an imprint inthe form of the mainly Carboniferous, U-rich zircon rims. In the StarMsto Belt, the episode of dextral shearing occurred at ca. 340 Ma,which was dated by the synchronous intrusion of granodiorites (Parryet al., 1997; tpsk et al., 2004). This shearing was likely associatedwith the prolonged migration of U-rich fluids and/or Pb-loss in zirconrims, which was possibly triggered by the adjacent intrusion as sug-gested by 340334MaUPbmonazitemicroprobe dates and U-rich zir-con rims (Jastrzbski et al., 2013). SIMS analyses of such rims are muchless accurate, with a spread between ~365 Ma and 309 Ma (Jastrzbskiet al., 2013), i.e., between the about peak and cessation of metamor-phism in the region. On the other hand, formation of CL-dark U-richouter zones is a common feature in felsic melts (e.g., Claiborne et al.,2006), so could be vulnerable to any overprinting event. However, theregional pattern of the occurrence of the CL-dark rims in the zircons ismore consistent, as it seems to be controlled by the distance to the su-ture zone. In the more remote rocks of the Orlicanienik Dome usedin this study (e.g., sample Gn1) and the Velk Vrbno Dome (samplesOS335/1 and OS335/2), the zircon rims are less affected by the Variscanthermal event. This feature tentatively suggests that the closer the rockswere to the suture, themore pervasive activity of circulatingfluids relat-ed to the late Variscan magmatic activity might affect the zircon grains.

8. Conclusions

1. The northern continuation of the Moldanubian Thrust Zone localisesthe suture after the Variscan collision between the Saxothuringianand Brunovistulian Terranes, represented by a set of regional-scale,polyphase thrusts that separate rocks of different ages andmetamor-phic conditions. In the Sudetic sector of the zone, these thrusts are asfollows fromwest to east: two unnamed faults that border the upperunit of the StarMesto Belt; theWest and East Nznerov Thrusts, thelatter located at the base of the Brousek quartzites; the Velk Vrbnointernal thrust; and the Ramzov Thrust.

2. In this set of regional-scale thrusts, the East Nznerov Thrust, consid-ered as a tectonic line localised at the bottomof the Brousek quartzitebody, seems the most important. It directly brings the Velk VrbnoDome of the Brunovistulian parentage and the Star Msto Beltof the Saxothuringian linkage into contact, thus coinciding with theterrane boundary.3. In the easternmost part of the Saxothuringian Terrane, themetavolcano-sedimentary successions preserved in the Orlicanienik Dome and the Star Msto Belt reveal similar lithological/lithostratigraphical development and geochronological records.They were deposited in a rift basin generated along the marginalparts of Gondwana in the Middle CambrianEarly Ordovician andwere supplied from Neoproterozoic source material.

4. The results of this study, when comparedwith other studies, indicatethat the Velk Vrbno Dome, which is west of the Upper CambrianBrousek quartzite outcrops, represents a separate crustal fragmentof the Neoproterozoic basement without imprints from the EarlyOrdovician rift-related thermal events, suggesting the Brunovistulianaffinity of this unit. The intermediate and mafic parts of the bimodalvolcanic sequence in the structurally upper part of the Velk VrbnoDome dated in this study yielded the same Neoproterozoic protolithages.

5. The Late-Variscan, tectonothermal events in the Visean left animprint in the form of U-rich zircon rims in the metavolcanicrocks from the Star Msto Belt and only a very minor imprint inthose from the Velk Vrbno Dome and the Orlicanienik Dome,suggesting the pervasive activity of circulating fluids related to thelate-tectonic granodiorite intrusion in the Star Msto Belt.

Acknowledgments

The zircon studies were financed by the Polish National ResearchCommittee KBN grant no. 2 P04D 025 30, by the Polish Ministry of Sci-ence andHigher Education grant no. N307 06832/4102 and by the Insti-tute of Geological Sciences PAS (task Metamorphism). Parts of the fieldstudies were supported through the National Science Centre of Polandgrant no. 2011/03/B/ST10/05638. We are grateful to Ryszard Kryzaand Ji k for their detailed and constructive reviews.

Appendix A. Supplementary data

Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.lithos.2015.01.023.

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