Constraining genesis and geotectonic setting of metavolcanic complexes: a multidisciplinary study of...

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Int J Earth Sci (Geol Rundsch) (2014) 103:455–483DOI 10.1007/s00531-013-0975-4

ORIGINAL PAPER

Constraining genesis and geotectonic setting of metavolcanic complexes: a multidisciplinary study of the Devonian Vrbno Group (Hrubý Jeseník Mts., Czech Republic)

Vojtech Janoušek · Jaroslav Aichler · Pavel Hanžl · Axel Gerdes · Vojtech Erban · Vladimír Žácek · Vratislav Pecina · Marta Pudilová · Kristýna Hrdlicková · Petr Mixa · Eliška Žácková

Received: 21 April 2013 / Accepted: 26 October 2013 / Published online: 3 December 2013 © Springer-Verlag Berlin Heidelberg 2013

high HFSE contents, as well as high Ga/Al and Fe/Mg ratios, typical of within-plate igneous setting. The petrology and Nd–Sr isotopic data point to a high-T anatexis of a young metagranitic crust, resembling the Cadomian (Brunovistu-lian) basement, in a back-arc setting. The attenuated Bru-novistulian lithosphere could have partially melted by the heat provided by the upwelling asthenosphere and/or under-plating basic magma. (3) Finally, the region was penetrated by numerous subalkaline, MORB/EMORB-like dolerite sheets—a hallmark of the considerable crustal thinning.

Keywords Bohemian Massif · Devonian volcanic rocks · Geochemistry · U–Pb zircon dating · Sr–Nd–O isotopes · Variscan orogeny

Introduction

Petrology and whole-rock geochemical and isotopic com-positions of volcanic rocks—and basic ones in particu-lar—are sensitive indicators of the geotectonic setting. Fortunately, many of the petrogenetically significant trace elements are relatively immobile during metamorphism, especially high-field-strength elements (HFSE) and rare earth elements (REE) (Floyd and Winchester 1975, 1978; Winchester and Floyd 1976, 1977; Wood 1980; Pearce 1996). Likewise, the Nd isotopic system is fairly robust, at least when CO2-rich fluids are not involved (DePaolo 1988; Faure and Mensing 2004 and references therein).

The Variscan Belt in Europe, including its eastern ter-mination in the Bohemian Massif, is a classic example of a collisional orogen where the original geotectonic context of the igneous suites has been often lost. In its external part, the so-called Rhenohercynian Zone, many authors noticed an apparent lack of Devonian destructive margin volcanism (e.g.

Abstract The low-grade metavolcanic/volcanosedimen-tary complex of the Devonian Vrbno Group (Silesicum, NE Bohemian Massif, Czech Republic) occurs in two ~NE–SW trending belts, separated by tectonic slices of Cadomian metagranitic paraautochton. (1) The basic–intermediate lavas of the calc-alkaline Western Volcanic Belt came from a moderately depleted mantle

(

ε370

Nd∼ +3

)

. Rare rhyolites (374.0 ± 1.7 Ma: 2σ, LA–ICP–MS U–Pb Zrn) were derived most likely from immature crust or by extensive fractiona-tion of primary basaltic melts. The rock association is inter-preted as a vestige of a deeply dissected continental arc. (2) The Eastern Volcanic Belt consists mainly of (nearly) con-temporaneous (371.0 ± 1.4 Ma) felsic alkaline lavas with

This paper is dedicated to the memory of J. Aichler, an honest man, a good geologist and a tireless organizer.

J. Aichler: Deceased.

Electronic supplementary material The online version of this article (doi:10.1007/s00531-013-0975-4) contains supplementary material, which is available to authorized users.

V. Janoušek (*) · J. Aichler · P. Hanžl · V. Erban · V. Žácek · V. Pecina · K. Hrdlicková · P. Mixa · E. Žácková Czech Geological Survey, Klárov 3, 118 21, Prague 1, Czech Republice-mail: [email protected]

A. Gerdes Geowissenschaften, Goethe Universität Frankfurt, Altenhöferallee 1, 60438 Frankfurt am Main, Germany

A. Gerdes Department of Earth Sciences, Stellenbosch University, Private Bag X1, Matieland 7602, South Africa

M. Pudilová Charles University, Albertov 6, 128 43, Prague 2, Czech Republic

http://dx.doi.org/10.1007/s00531-013-0975-4

456 Int J Earth Sci (Geol Rundsch) (2014) 103:455–483

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Floyd 1995). This can be accounted for by a scenario assum-ing rifting and opening of a narrow Rhenohercynian Basin during a Silurian to Early Carboniferous extensional episode, which was accompanied, since the Devonian, by bimodal volcanism (e.g. Wedepohl et al. 1983). The continental rift-ing was thought to have led locally to the development of an incipient oceanic crust as Mid-Ocean Ridge Basalts (MORB) have been recorded besides the Within Plate Basalts (WPB) (Pin et al. 1988; Floyd 1982, 1995). Furthermore, the origin of renowned base metal massive sulphide deposits such as Meggen, Rammelsberg and Iberian Pyrite Belt was attrib-uted to the volcanism accompanying this Devonian extension (Sawkins and Burke 1980). Regardless of their origin, the extension-related basins of the Rhenohercynian Zone were closed soon after their formation, due to the Late Devonian to Early Carboniferous convergence at the wake of the Variscan orogeny (Franke 2000, 2006).

Near the eastern termination of the Rhenohercynian Zone, on the eastern slopes of the Hrubý Jeseník Mts. (Silesicum), variable metavolcanites occur as a part of a palaeontologically dated Devonian sedimentary sequence of the Vrbno Group (Römer 1870; Barth 1963; Chlupác 1989). The petrogenesis and geotectonic setting of these metavolcanites, however, still remain matter of debate as the rocks of the Vrbno Group were deformed, imbricated and metamorphosed jointly with their mainly metagranitic Cadomian basement.

Using a multidisciplinary approach combining petrol-ogy, U–Pb zircon dating and whole-rock geochemistry on relict domains spared of extensive overprint, the current paper demonstrates that the metavolcanites of the Vrbno Group represent most likely extensive relics of a Famenn-ian igneous arc–back-arc association.

Geological setting

Large and dismembered remnants of the pre-Variscan base-ment, variously deformed during the Variscan orogeny, occur in the Hrubý Jeseník Mts. (NE part Bohemian Massif, Czech Republic; Fig. 1). This geological unit—termed Silesicum by Suess (1912)—has been traditionally subdivided into the westerly Keprník and the easterly Desná units. Both are built by a Cadomian crystalline paraautochton of the Brunovistu-lian affinity (Finger et al. 2000) imbricated with metamor-phosed Devonian volcanosedimentary complexes (see Cháb et al. 1990 and Schulmann and Gayer 2000 for a review).

These volcanosedimentary complexes evolved in exten-sional basins on attenuated crust of the Brunovistulian mar-gin in Emsian to Early Carboniferous times (Hladil et al. 1999; Kalvoda et al. 2008). Overall, the Devonian to Lower Carboniferous rocks overlying the Brunovistulian Terrane provide a complex record of Pragian to Tournaisian exten-sion, Tournaisian to Namurian plate convergence and flysch

sedimentation, Namurian to Westphalian plate collision and molasse deposition (Kalvoda et al. 2003, 2008; Kalvoda and Bábek 2010).

Desná Unit

The Desná Unit (Suess 1912; Dallmeyer et al. 1995) in the eastern part of the Silesicum is formed mainly by metaigneous rocks of the Cadomian protolith ages (Kröner et al. 2000), and it is well correlated with Brunovistulicum further south (Hanžl et al. 2007 for review). Basic rocks of the Sobotín Massif and vari-able metagranitoids are exposed in the southern part of the Desná Unit. The largest and most coherent tectonic slice of the basement in this area is known as the Oskava Block (Fig. 1).

Vrbno Group

The sequence of the Vrbno Group (VG) was first defined by Römer (1870). It represents a metamorphosed Devonian volcano–sedimentary complex rimming the eastern edge of the Desná Unit. The VG was underthrust below the Lower Carboniferous sediments of the Culm flysch sequence fur-ther to the East (Cháb et al. 1990).

Lithology

The stratigraphic sequence in the VG starts with metasand-stones and metaconglomerates, indicating a shallow near-shore environment. The basal quartzites were dated palae-ontologically as Pragian–Emsian (Römer 1865; Isaacson and Chlupác 1984; Chlupác 1989). Phyllites, passing grad-ually into a thick pile of volcanic products, are exposed in their hanging wall. The upper part of the volcanics is locally accompanied by Emsian to Frasnian dark grey lime-stones (Hladil 1988). The top of the sequence is formed by Famennian clayey shales, siliceous shales, cherts and gra-phitic limestones.

In the southern Hrubý Jeseník Mts., the VG meta-volcanics occur in two approximately N–S trending belts, separated by the Cadomian metagranitic para-autochton (Oskava Block) (Fig. 1). The Western Vol‑canic Belt (WVB) is characterized by an abundance of

Fig. 1 Generalized geological map of the southern part of the Vrbno Group (Hrubý Jeseník Mts., Silesicum) with sampled locations. Inset shows the position of the studied area within the Bohemian Mas-sif. Approximate boundary between volcanites with affinities to the Western Volcanic Belt (WVB) and Eastern Volcanic Belt (EVB) is shown by heavy dashed line. Localities mentioned in the text: HM—Horní Mesto, MM—Malá Morávka, NV—Nová Ves, R—Rešov, St—Stancín, Tr—Tremešek. JRD 10—borehole JRD 10 (samples GV 70–71, 74–79, 81–82)

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metasediments accompanied by mostly basic–intermedi-ate metavolcanites. On the other hand, the Eastern Vol‑canic Belt (EVB) is almost exclusively metavolcanic and bimodal, with much higher proportion of acid volcanic products.

Metamorphism, deformation and mineralization

The contacts between the Cadomian metagranites and the Devonian rocks of the VG are tectonically reworked and usually SW–NE trending. The metamorphic grade in the VG generally increases from the chlorite zone in the SE to the garnet–staurolite zone in the NW (Soucek 1978b). The metavolcanic rocks in the southern VG host formerly mined mineralization (Zn–Pb ± Ag: Horní Mesto and Nová Ves deposits), interpreted as a result of a late magmatic–metamorphic hydrothermal remobilization of the origi-nal strata-bound and volcanogenic deposits (Patocka and Valenta 1990, 1996).

Analytical techniques

More than seventy large samples of metavolcanics, at least 5–10 kg each, were collected based on the recent 1:25,000 mapping campaign (Aichler 2000; Žácek 2000). They were taken from rock outcrops or, rarely, boreholes and studied petrologically. Selected samples were used for mineral, whole-rock geochemical and isotopic (Sr–Nd–O) analy-ses. Their list including sample locations is given in Online Resource 1.

Electron microprobe

Minerals were analysed using the CamScan 4–90DV electron microprobe equipped with an Oxford Link-ISIS energy-dispersive microanalyzer in Czech Geological Sur-vey, Prague (supervised by I. Vavrín). Accelerating voltage was 15 kV, beam current 3 nA and counting time 80 s.

Natural minerals and synthetic compounds were employed as standards: Si (quartz), Al (corundum), Ca (wollastonite), Ti (sintered TiO), K (orthoclase), Na (albite), Mg (forsterite), Fe (fayalite) and Mn (tephroite). The measured concentrations were recalculated using standard ZAF corrections.

U–Pb LA–ICP–MS dating

Zircons were concentrated from the 60–125 μm frac-tion by combined Wilfley shaking table, magnetic and heavy liquid (tetrabromomethane) separation. The grains were finally handpicked, mounted in epoxy and polished. Prior to laser analyses, all grains were characterized by

cathodoluminescence (CL) imaging at Institute of Petrol-ogy and Structural Geology, Charles University in Prague.

Uranium, thorium and lead isotopic analyses were car-ried out by laser ablation—inductively coupled plasma—mass spectrometry (LA–ICP–MS) at the Goethe Univer-sity of Frankfurt (GUF), using a slightly modified method, as previously described in Gerdes and Zeh (2006, 2009) and Zeh and Gerdes (2012). A ThermoScientific Element 2 sector field ICP–MS was coupled to a Resolution M-50 (Resonetics) 193 nm ArF Excimer laser (CompexPro 102, Coherent) equipped with a two-volume ablation cell (Lau-rin Technic, Australia). The laser was fired at a frequency of 5.5 Hz and a fluence of c. 4 J cm−2. This yielded with the above configuration at a spot size of 26 μm and pen-etration depth of 0.6 μm s−1 a sensitivity of 10,000–13,000 cps/μg g−1 for 238U (e.g. GJ-1 zircon). Raw data were cor-rected offline for background signal, common Pb (based on 204Pb and 208Pb, respectively; see Millonig et al. 2012 for details), laser-induced elemental fractionation, instrumental mass discrimination and time-dependent elemental frac-tionation of Pb/U using an in-house MS Excel© spread-sheet program (Gerdes and Zeh 2006, 2009). Laser-induced elemental fractionation and instrumental mass discrimina-tion were corrected for by normalization to the reference zircon GJ-1 (206Pb/238U = 0.0984 ± 0.0003; ID-TIMS GUF value). Repeated analyses of the reference zircons Plešovice, Felix and 91500 (Sláma et al. 2008; Millonig et al. 2012; Wiedenbeck et al. 1995) during the same ana-lytical session yielded an accuracy better than 1 % and a reproducibility <2 % (2 SD). All uncertainties are reported at the 2σ level. The data were plotted using the software ISOPLOT (Ludwig 2003).

Whole-rock geochemistry

Even though utmost care was taken in sample selection, effects of elemental mobility due to the hydrothermal alter-ation, weathering and/or greenschist-facies metamorphic overprint had to be further checked using plots with H2O

+ on the abscissa (Pearce 1996). As some systematic changes of some elements have been observed, in particular of the LILE, the data presented here are only for samples with H2O

+ < 3.5 wt% and CO2 < 1.5 wt%.Wet major-element whole-rock analyses were carried

out in the Central Laboratory of the Czech Geological Sur-vey (CGS), Prague (H. Vítková and co-workers). The rela-tive 2σ uncertainties for the given concentrations were bet-ter than 1 % (SiO2), 2 % (FeO), 5 % (Al2O3, K2O, Na2O), 7 % (TiO2, MnO, CaO), 10 % (Fe2O3) and 15 % (but usu-ally <6 %, MgO).

Trace elements were determined in the Acme Analyti-cal Laboratories, Vancouver, by ICP–MS. The dissolution of the samples for the ICP–MS studies was done either

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by LiBO2/Li2B4O7 fusion (rare earth and refractory ele-ments) or by aqua regia digestion (precious and base met-als). Scandium and chromium were determined by INAA in the Becquerel Laboratories, Canada. Recalculation and plotting of the geochemical data were facilitated by GCD‑kit (Janoušek et al. 2006).

Stable isotopes

Oxygen for isotopic analyses was liberated from whole-rock and quartz samples by a fluorination technique (using BrF5: Clayton and Mayeda 1963) at the Institute of Geo-chemistry, Mineralogy and Mineral Resources, Charles University in Prague. During this procedure, oxygen is cry-ogenically cleaned from reaction products and excess fluor-ination agent; finally, it is converted to CO2 on a heated graphic rod. Isotopic measurements were performed using the Finnigan MAT 251 mass spectrometer at the CGS. Overall analytical uncertainty of the δ18O values tested by repeated analyses of NBS 28 standard was 0.2 ‰ (SMOW).

Radiogenic isotopes

For the isotopic study, samples were dissolved using a HF–HCl–HNO3 mixture. Strontium and bulk REE were isolated by standard cation-exchange chromatography techniques on quartz columns with BioRad resin, and Nd was further separated on quartz columns with Biobeads S-X8 coated with HDEHP (Richard et al. 1976). Isotopic analyses were performed on a Finnigan MAT 262 thermal ionization mass spectrometer in static mode using a double Re filament assembly (CGS). The 143Nd/144Nd ratios were corrected for mass fractionation to 146Nd/144Nd = 0.7219, 87Sr/86Sr ratios assuming 86Sr/88Sr = 0.1194. External reproducibility is given by the results of repeated analy-ses of the La Jolla (143Nd/144Nd = 0.511858 ± 20 (2σ), n = 78) and NBS 987 (87Sr/86Sr = 0.710252 ± 22 (2σ), n = 24) isotopic standards. The Rb, Sr, Sm and Nd con-centrations were obtained by ICP–MS in Acme Laborato-ries (see above).

The decay constants applied to age-correct the isotopic ratios are from Steiger and Jäger (1977: Sr) and Lugmair and Marti (1978: Nd). The εi

Nd values were obtained using

bulk earth parameters of Jacobsen and Wasserburg (1980), and the two-stage depleted-mantle Nd model ages

(

TDM

Nd

)

were calculated after Liew and Hofmann (1988).

Field relations and petrology

The metavolcanics in the southern VG were classified on the basis of their mode of occurrence, regional distribution,

petrology and whole-rock geochemical character into four suites: (1) geochemically rather primitive Western Volcanic Belt (WVB) restricted to tectonic slices along the western rim of the Cadomian Oskava Block, (2) more evolved East-ern Volcanic Belt (EVB) covering significantly larger area between Malá Morávka and Unicov in the East (Fig. 1), (3) felsic dykes (comendites and rhyolites) cutting the Cado-mian basement (FeD) and (4) numerous doleritic dykes (DoD) that intruded the whole area relatively late in the Variscan cycle. The volcanic rocks of the two belts are fur-ther split into ‘acid’ and ‘intermediate’ groups, according to their silica contents.

The studied volcanic rocks suffered only low grade, i.e. lower greenschist-facies overprint. Volcanic structures are locally well preserved, allowing the primary character of the volcanic products to be determined: pillow lavas, ign-imbrites, banded tuffs, agglomerate tuffs and subvolcanic dykes (Wilimský et al. 2003). All the metavolcanic rocks show mutually comparable and very simple metamorphic mineral assemblages (Online Resource 2).

Western Volcanic Belt

The felsic metavolcanic rocks of rhyolitic composition are fine-grained, frequently banded and foliated. Intensively foliated types have an appearance of a quartz–sericite schist. Acid agglomerate metatuffs with flattened rhyolitic bombs set in a fine-grained, strongly foliated matrix were recorded SW of Tremešek (Fig. 2a). The metarhyolites con-tain mineral assemblages dominated by quartz + K-feld-spar + albite + white mica. Biotite, chlorite, epidote, titan-ite, ilmenite, zircon, rutile and scarce calcite are minor to accessory minerals.

The intermediate–basic lavas and pyroclastic rocks of dacitic to andesitic composition have locally preserved vol-canic fabrics, but the mineral assemblage is metamorphic: albite + chlorite + Fe-rich epidote with minor to accessory actinolite, apatite, zircon, titanite, calcite, opaque ore min-erals and tourmaline.

The andesitic pillow lavas form lenslike bodies with thickness up to several dozens of metres at Tremešek. These are weakly metamorphosed greyish green rocks with vesicular texture and flattened pillows 0.5–4 m long, 20–30 cm across (Fig. 2b).

Banded metatuffs (chlorite–sericite schists) rarely pre-serve relic tuffitic textures (banding) with scattered tiny albite crystaloclasts. Metamorphic foliation is parallel with banding, defined by variable lithology and grain size. Rocks are composed of albite + quartz + biotite + chlo-rite + epidote, sometimes also some white mica, minor to accessory titanite, calcite, K-feldspar, ilmenite and REE-enriched epidote.


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Eastern Volcanic Belt

The felsic metavolcanic rocks (comendites) are mostly for-mer ignimbrites and agglomerate metatuffs. The light grey ignimbrites show mostly well-developed foliation planes, parallel with the preserved bedding. In fine-grained matrix are set markedly flattened fiamme as well as small oval fragments of slightly porphyritic rhyolitic lavas (Fig. 2c). Characteristic greenschist-facies assemblage includes quartz + albite + K-feldspar + muscovite as dominant con-stituents with minor biotite, chlorite and epidote/REE-enriched epidote, along with accessory magnetite, ilmenite, titanite, limonitized pyrite, monazite and zircon (Online Resource 2).

Acid agglomerate metatuffs contain grey to greenish grey, preferentially oriented lapilli and massive or finely banded rhyolitic bombs up to 70 cm across surrounded by a fine-grained, often banded matrix.

The intermediate to basic metavolcanic rocks are mostly former lavas and metatuffs corresponding to trachyte, tra-chyandesite to (alkali) basalt. They are fine- to medium grained with ophitic or porphyritic texture. Both massive and

secondary foliated types occur; relics of amygdaloidal texture are locally common. Secondary mineral assemblage totally replaces original igneous mineralogy. Intermediate rocks are composed of albite + quartz + chlorite + epidote ± K-feld-spar ± muscovite ± biotite ± titanite ± calcite with apatite, allanite, rutile and ilmenite as common accessories.

Massive, fine- to medium-grained slightly schistose tra‑chytic lavas contain millimetre-sized vesicles with carbon-ate infill and scattered lath-shaped albite phenocrysts. They are restricted to area west of Malá Morávka.

The best example of basic agglomerate metatuffs is exposed near Nová Ves N of Rýmarov. Volcanic bombs and lapilli of variable size are set in a fine-grained, schis-tose groundmass of basic composition (Fig. 2d). Common are also blocks of massive, porphyritic and/or amygdaloidal volcanites, some with ophitic texture.

Felsic dykes cutting the Cadomian basement

Three samples are massive fine-grained subvolcanic dykes of comendite composition cutting the Cadomian

(a) (b)

(d)(c)

Fig. 2 Selected field photographs. a Deformed rhyolite clasts in slightly metamorphosed agglomerate tuff, Tremešek (GV11). b Relic of (basaltic) andesite pillow lavas, Tremešek (GV15), view perpen-

dicular to the stretching lineation. c Deformed ignimbrite of comen-dite composition, Rešov falls (GV07, dated sample). d Deformed agglomerate tuff of trachyandesite composition, Nová Ves (GV57)


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granitic basement of the Oskava Block. They contain rel-ics of porphyritic (feldspars and quartz) and rarely also trachytic textures. The mineral assemblage is metamorphic (quartz + albite + K-feldspar ± white mica ± biotite). Minor and accessory minerals are represented by zircon, magnetite and allanite. Only one sample (GV93/1), intrud-ing the Cadomian metatonalites to metadiorites west of the Oskava Block, is of dacitic composition.

Dolerites

Numerous dolerite sheets up to several tens of metres wide cut both the VG and its Cadomian basement. The basic sheets are the youngest but still have been sub-jected to the same Variscan greenschist-facies regional metamorphism as was the whole crustal stack of the VG. The doleritic rocks are fine- to medium-grained subal-kaline metabasalts, massive or indistinctly foliated. Dol-erites display completely secondary greenschist-facies mineral assemblages, but characteristic igneous (e.g. vesicular or ophitic) textures are locally preserved. The rocks show a simple modal composition: calcic amphi-bole + albite + chlorite + epidote + titanite. Ilmenite, quartz, rutile, apatite and limonitized pyrite are the com-mon accessories; in addition, some samples contain vari-able amounts of calcite.

U–Pb dating

In order to pinpoint the igneous activity in both volcanic belts, zircons were separated from two felsic samples, metarhyolite GV20 (Tremešek, WVB) and comendite GV7 (Rešov Falls, EVB), to be dated by the LA–ICP–MS U–Pb method at the University of Frankfurt (Fig. 3). The full ana-lytical results are presented as Online Resource 3.

Western Volcanic Belt (rhyolite GV-20a)

The euhedral zircon crystals show mostly unzoned, dull CL images, with rare inclusions and no inherited cores. Weak oscillatory or sector zoning is scarce (Fig. 3a). All LA–ICP–MS analyses are concordant and equivalent; the cal-culated Concordia age of 374.0 ± 1.7 Ma (Fig. 3b) is inter-preted to indicate a Famennian emplacement of the WVB.

Eastern Volcanic Belt (comendite GV-7a)

As shown by the CL imaging, relatively common in the dated zircons is oscillatory zoning, especially in external parts of the crystals (Fig. 3c). Some entire grains, and most of the internal parts of the oscillatory zoned ones, show a uniformly grey CL, with some examples of well-developed

sector zoning. The dating yielded mostly concord-ant analyses (Fig. 3d); the calculated Concordia age of 371.0 ± 1.4 Ma is within the error identical with that for the WVB sample.

Whole‑rock geochemistry

The data set considered in this contribution includes 10 WVB and 18 EVB metavolcanics, as well as five felsic dykes cutting the Cadomian basement (FeD) and 20 dol-eritic dykes (DoD). The selected analyses are given in Table 1, the complete data set in Online Resource 4. Not surprisingly, the data set is somewhat biased towards less mafic compositions, originally drier, that tend to be rela-tively more resistant against the secondary hydration and carbonization.

The subdivision of the studied metavolcanites into four groups is well justified, as seen for instance in the classifi-cation diagrams of Winchester and Floyd (1977) and Pearce (1996) (Fig. 4a–b). In addition, the WVB and EVB are split into ‘acid’ and ‘intermediate’ groups, as many binary dia-grams involving major and trace elements (Fig. 5) show inflection at SiO2 ~ 69 wt%. This value corresponds to the boundary between petrologically defined metatrachytes and metarhyolites of Patocka and Valenta (1990, 1996).


‘Intermediate’ group

The SiO2 contents (55.3–68.3 wt%) correspond to andesites or rhyodacites/dacites (Fig. 4a). In the Nb/Y–Zr/Ti plot, the samples classify as subalkaline (basaltic) andesites (Fig. 4b). In the diagram by Hastie et al. (2007; Fig. 4c) designed for altered volcanic suites, they correspond to (high-K?) calc-alkaline rocks. While TiO2 is nearly con-stant (0.70–1.15 wt%), Al2O3 (14.7–16.5 %) and FeOt (3.6–6.9 %) are more variable.

Two pillow lavas (GV14a and GV15) differ markedly from the rest of the data set. Not only that the pillows are the most basic (SiO2 = 55.3–55.4 wt%) and metalumi-nous (A/CNK = 0.83 and 0.87, A/CNK = molar Al2O3/(CaO + Na2O + K2O)), but they also show much higher mg# (~55; mg# = molar MgO/(MgO + FeOt) × 100) as well as elevated MgO (~4.7 wt%) and FeOt (6.7 and 6.9 wt%) at very low K2O (0.17 and 0.34 wt%).

The rest of the suite is subaluminous (A/CNK = 1.01–1.07). The peraluminosity of the metatuff GV52 (A/CNK = 1.25) points to a secondary mobility of alkalis (Na2O + K2O = 5.5 wt. %, compared to 6.7–8.7 % in the remaining samples) coupled with a possible presence of a sedimentary admixture.


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A negative correlation with silica is shown, in particular, by Al2O3, FeOt, MgO and Na2O. On the other hand, K2O, Rb and LaN/SmN are correlated positively (Fig. 5).

The NMORB-normalized spider plots (Sun and McDon-ough 1989) (Fig. 6a) feature marked depletions in Nb, Ti and Sr and, apart from the pillows, slight enrichment in K. The LILE are extremely variable, reaching up to c. 450 × NMORB. On the other hand, the pillow lavas dis-play lower contents some of the LILE (in particular Rb, Ba, Cs and K). The NMORB-normalized contents of HREE are close to unity in all cases.

The total REE contents are rather low (ΣREE = 91.9–163.2 ppm). The chondrite-normalized (Boynton 1984) REE patterns (Fig. 7a) tend to be flat (LaN/YbN = 3.60–7.45; LaN/SmN = 2.33–3.12). Both ratios increase with SiO2 as does the magnitude of the negative Eu anomaly (Eu/Eu* = 0.91–0.66).

‘Acid’ group

The samples with SiO2 >69 wt% (71.8–81.7 %) corre-spond to subalkaline rhyolites or rhyolites/dacites (Fig. 4a–b). The TiO2 ranges between 0.04 and 0.59, Al2O3 9.1 and 14.1, MgO 0.04 and 1.0, and FeOt 0.5 and 2.7 wt%. CaO is mostly low, except the sample GV20a (2.3 wt%). A/CNK is variable (0.89–1.62 median 1.18), as is the mg# (12.3–49.7). There is a strong negative correlation of silica with Al2O3 and MgO, less so FeOt and Na2O; the rest of the Harker plots is scattered (Fig. 5). Among the trace ele-ments, a negative correlation with SiO2 is shown by Sr and positive by Rb and Cr.

The NMORB-normalized patterns (Fig. 6b) resemble the previous group. However, they are much less vari-able, arguing for a lower alkalis mobility, except Cs. The LILE/HFSE enrichment seems to be comparable to the

Fig. 3 Typical CL images of dated zircon crystals and cor-responding U–Pb concordia diagrams for the samples GV-20 (Tremešek, WVB) (a, b) and GV-7 (Rešov waterfalls, EVB) (c, d) 20 µm 20 µm

20 µm 20 µm

(c) GV7 (Rešov Falls, EVB)

390

350

330

0.050

0.052

0.054

0.056

0.058

0.060

0.062

0.064

0.066

0.37 0.39 0.41 0.43 0.45 0.47 0.49207Pb/235U

206 P

b/23

8 U

Upper intercept age 377.1 ± 9.5 MaMSWD = 0.53

data-point error ellipses are 2σ

Concordia Age = 371.0 ± 1.4 Ma

MSWD C+E = 1.1

370370

0.056

0.058

0.060

0.062

0.064

0.41 0.43 0.45 0.47 0.49

207Pb/235U

206 P

b/23

8 U

data-point error ellipses are 2σ

Concordia Age = 374.0 ± 1.7 MaMSWD C+E = 1.2

(a) (b)

(d)

50 µm

50 µm

50 µm

50 µm

50 µm 50 µm

370370

390390

380380


1 3

Tabl

e 1

Sel

ecte

d w

hole

-roc

k m

ajor

- an

d tr

ace-

elem

ent a

naly

ses

for

Dev

onia

n m

etav

olca

nic

rock

s, H

rubý

Jes

eník

Mts

. (w

t% a

nd p

pm, r

espe

ctiv

ely)

Sam

ple

GV

15G

V54

GV

20a

GV

102

GV

57/2

GV

71G

V61

GV

07b

GV

93-1

GV

01b

GV

40G

V74

GV

47

Gro

ups

WV

B in

term

.W

VB

inte

rm.

WV

B a

cid

WV

B a

cid

EV

B in

term

.E

VB

inte

rm.

EV

B a

cid

EV

B a

cid

FeD

FeD

DoD

DoD

DoD

SiO

255

.41

68.3

371

.81

79.9

252

.87

67.5

076

.46

77.3

066

.50

75.7

346

.14

48.2

249

.00

TiO

20.

961.

130.

230.

591.

770.

290.

160.

150.

710.

121.

680.

802.

36

Al 2

O3

16.4

714

.70

13.9

49.

8517

.29

14.5

412

.32

11.2

515

.03

11.4

616

.64

16.4

414

.43

Fe2O

32.

081.

301.

053.

054.

471.

450.

321.

332.

921.

612.

131.

492.

81

FeO

4.83

2.44

1.21

–4.

383.

330.

750.

402.

090.

586.

866.

429.

13

MnO

0.17

0.04

0.03

<0.

010.

180.

120.

030.

020.

060.

020.

170.

130.

24

MgO

4.71

1.04

0.54

0.35

1.84

0.29

0.07

0.24

2.51

0.12

8.34

8.26

6.14

CaO

4.18

0.40

2.27

<0.

016.

140.

490.

110.

220.

70<

0.01

9.67

9.04

8.66

Na 2

O6.

716.

694.

680.

216.

885.

073.

053.

424.

980.

222.

753.

644.

07

K2O

0.34

1.82

3.60

3.15

0.25

4.82

6.85

4.21

1.58

8.83

0.44

0.36

0.15

P 2O

50.

130.

280.

040.

040.

720.

02<

0.00

50.

01<

0.00

50.

010.

310.

040.

31

CO

20.

710.

030.

08<

0.01

0.40

0.39

0.05

0.03

<0.

010.

020.

021.

420.

02

Cno

ncar

b<

0.00

50.

01<

0.00

50.

080.

020.

010.

020.

020.

01<

0.00

50.

030.

01<

0.00

5

F0.

050.

040.

020.

010.

100.

020.

020.

030.

060.

020.

060.

030.

05

S<

0.00

5<

0.00

5<

0.00

5<

0.00

50.

010.

010.

04<

0.00

5<

0.00

5<

0.00

5<

0.00

50.

02<

0.00

5

H2O

+2.

391.

000.

941.

802.

031.

190.

090.

422.

330.

453.

923.

773.

00

H2O

−0.

140.

090.

13–

0.07

0.25

0.05

0.12

0.27

0.09

0.07

0.09

0.06

Σ99

.30

99.3

410

0.58

99.0

599

.46

99.8

110

0.40

99.2

099

.74

99.2

999

.25

100.

2110

0.45

FeO

t6.

703.

612.

152.

748.

404.

631.

031.

604.

722.

038.

787.

7611

.66

K2O

/Na 2

O0.

050.

270.

7715

.00

0.04

0.95

2.25

1.23

0.32

40.1

40.

160.

100.

04

A/C

NK

0.87

1.07

0.89

–0.

761.

010.

981.

061.

34–

0.74

0.72

0.64

mg#

55.6

133

.93

30.8

818

.52

28.0

810

.03

10.7

721

.12

48.6

89.

5462

.88

65.4

848

.42

Rb

8.5

50.1

67.0

162.

55.

078

.314

8.9

96.2

52.2

152.

713

.89.

41.

6

Cs

0.9

3.2

0.3

13.0

0.4

7.1

1.2

1.0

1.7

2.1

1.0

1.0

0.2

Ba

104.

239

6.1

795.

226

9.0

34.2

50.1

82.6

95.9

421.

416

9.2

280.

418

8.0

55.5

Sr10

1.2

56.8

70.7

17.0

205.

350

.421

.031

.212

8.5

11.0

751.

711

4.7

320.

9

Th

5.1

6.7

10.1

6.6

7.6

23.3

20.9

16.0

6.5

18.6

1.3

0.4

1.3

U1.

62.

52.

61.

91.

37.

07.

13.

82.

04.

10.

5N

A0.

4

Zr

143.

521

3.2

237.

817

7.9

542.

91,

703.

693

6.6

644.

221

4.5

735.

115

5.0

31.0

173.

3

Hf

4.1

6.4

7.1

5.3

13.3

34.6

30.0

19.7

5.6

24.2

3.9

1.1

4.7

Nb

7.4

9.9

11.2

8.2

76.9

164.

312

3.4

62.9

9.4

112.

611

.4<

0.1

15.3

Ta0.

50.

80.

80.

74.

510

.48.

05.

60.

98.

40.

70.

11.

0

Sc25

.612

.74.

99.

015

.60.

5<

0.1

1.2

<0.

10.

337

.532

.839

.5

Cr

150.

019

.011

.0–

20.0

13.0

80.0

19.0

–23

.028

0.0

310.

013

0.0


1 3

Tabl

e 1

con

tinue

d

Sam

ple

GV

15G

V54

GV

20a

GV

102

GV

57/2

GV

71G

V61

GV

07b

GV

93-1

GV

01b

GV

40G

V74

GV

47

Gro

ups

WV

B in

term

.W

VB

inte

rm.

WV

B a

cid

WV

B a

cid

EV

B in

term

.E

VB

inte

rm.

EV

B a

cid

EV

B a

cid

FeD

FeD

DoD

DoD

DoD

Ni

43.0

2.7

6.0

1.3

2.1

0.9

15.0

5.0

1.1

5.0

89.4

86.3

28.0

Co

31.7

7.5

3.2

0.6

11.7

0.7

1.4

1.7

8.6

0.7

41.0

43.9

38.7

V15

364

579

27<

5<

56

49<

518

918

530

6

Pb3.

01.

4<

23.

01.

71.

512

.74.

02.

4<

20.

60.

41.

1

Zn

4351

291

143

6537

5656

1451

4270

Cu

34.0

1.3

2.0

13.8

8.6

8.1

5.4

3.0

13.3

1.0

32.0

77.1

28.4

Y28

.837

.933

.628

.170

.517

7.0

144.

559

.631

.696

.835

.420

.930

.4

La

18.9

30.1

40.9

27.0

67.4

204.

010

9.7

55.2

20.1

16.8

15.4

0.6

15.9

Ce

33.1

54.2

67.2

50.6

135.

637

0.8

223.

010

7.9

41.7

56.2

35.1

3.0

35.4

Pr3.

96.

57.

45.

416

.840

.627

.313

.65.

34.

64.

30.

64.

3

Nd

17.8

27.7

29.2

22.0

72.5

147.

810

7.6

60.0

21.5

20.8

21.6

5.0

22.8

Sm4.

06.

15.

84.

715

.026

.826

.313

.55.

07.

45.

42.

46.

0

Eu

1.21

1.30

0.92

1.11

4.47

1.64

0.93

0.65

1.26

0.30

1.98

0.84

2.02

Gd

4.67

6.03

5.28

4.34

13.4

222

.01

24.2

713

.28

4.88

10.6

46.

172.

936.

22

Tb

0.75

1.00

0.91

0.78

2.26

4.15

4.17

2.13

0.82

2.33

0.99

0.40

0.96

Dy

4.57

6.10

5.10

4.78

12.1

227

.46

23.9

111

.47

4.88

14.4

56.

123.

135.

57

Ho

1.04

1.31

1.18

0.99

2.46

6.01

4.93

2.37

0.99

3.41

1.29

0.78

1.11

Er

3.15

3.76

3.37

2.76

7.27

17.8

513

.50

6.60

3.13

10.1

93.

362.

212.

94

Tm

0.49

0.57

0.53

0.38

1.14

2.82

2.02

1.01

0.48

1.65

0.53

0.38

0.41

Yb

3.10

3.53

3.43

2.66

6.44

16.6

811

.80

6.71

3.02

10.1

83.

251.

522.

69

Lu

0.45

0.61

0.53

0.39

1.02

2.47

1.84

0.95

0.47

1.55

0.52

0.31

0.40

ΣR

EE

97.1

148.

817

1.8

127.

935

7.9

891.

158

1.3

295.

411

3.5

160.

510

6.0

24.1

106.

7

La N

/Yb N

4.11

5.75

8.04

6.84

7.06

8.25

6.27

5.55

4.49

1.11

3.20

0.27

3.99

La N

/Sm

N2.

973.

104.

443.

612.

834.

792.

622.

572.

531.

431.

790.

161.

67

Eu/

Eu*

0.86

0.66

0.51

0.75

0.96

0.21

0.11

0.15

0.78

0.10

1.05

0.97

1.01

Yb N

14.8

316

.89

16.4

112

.73

30.8

179

.81

56.4

632

.11

14.4

548

.71

15.5

57.

2712

.87

‘–’ =

Not

det

erm

ined

, val

ues

less

than

= b

elow

det

ectio

n lim

it


1 3

more basic samples, and also the Nb trough is relatively deep. However, the rhyolites and their tuffs are progres-sively depleted in Sr, P, Eu and Ti; typical are positive anomalies in K and Pb. The normalized HREE contents are again close to unity.

The acid samples show a slightly higher LREE/HREE fractionation than for their more basic counterparts (LaN/YbN = 4.39–8.0; LaN/SmN = 3.26–4.61) (Fig. 7b). Even though the Eu anomaly is significantly deeper (Eu/Eu* = 0.75–0.14), the total REE contents remain nearly unchanged (81–172 ppm). The LREE and HREE generally increase concomitantly with silica.


‘Intermediate’ group

These alkaline samples classify as trachyandesite and trachyte, with a small overlap into the adjacent comendite/pantellerite or alkali rhyolite fields (Fig. 4a, b). The SiO2 ranges between 52.9 and 67.5 wt%; relatively variable are TiO2 (0.29–1.77 %), Al2O3 (13.6–17.8 %), FeOt (1.6–8.4 %) as well as mg# (10–44, mostly low). The rocks are metaluminous–subaluminous (A/CNK = 0.72–1.07), with the maximum A/CNK reached for agglomerate

Foidite

Andesite

Basaltic andesite

Tephriphonolite

Phonolite

Arhyolite

lkali

DoD

Rhyolite/Dacite

Subalkaline Basalt

AlkaliBasalt

0.00

10.

010.

11

5

Nb/Y

iT/rZ

WVB

EVB

(b)

Trachy-

andesiteTrachy-

andesite

TrachyteTrachyte

subalkaline alkaline ultraalkaline

DoD

etaide

mretnI‚

’p

uor

gp

uor

g’dica‚

WVB

Phonolite

Trachyte

Andesite Trachy-andesite

0.01 0.1 1 100.001 0.01 0.1 1 10

4050

6070

80

Zr TiO/ 2

Co

OiS

2

O2

Basalt

Alka

line

Comendite/Pantellerite

Basanite Trachyte/Nephelinite

/

EVBRhyolite

Rhyodacite/Dacite

cisab

etaide

mretni

acid

(a)

Si = 69 wt. %

Si = 52 wt. %O2Subalkaline

Western VolcanicBelt (WVB)

Eastern VolcanicBelt ( VB)E

Dyke rocks

‘Intermediate’‘Acid’

dolerites (DoD)felsic dykes (FeD)

‘Acid’‘Intermediate’

0 42 6

05

10FeOt

1520

lA

2O3

Comedites

ComenditicTrachytes

Pantellerites

Pantelleritic Tra

chytes

(d)

DoD

(c)

TholeiiteSeries

Calc alkalineSeries

-

High-K calc-alkalineand Shoshonite Series

Bas

altic

And

esite

sA

ndes

ites

Bas

alts

Dac

ites

Rhy

olite

s

10 0307050

Th

0.01

101

0.1

Fig. 4 a Zr/TiO2 versus SiO2 diagram of Winchester and Floyd (1977) for the four main groups of the studied metavolcanic rocks. b Nb/Y versus Zr/TiO2 diagram of the same authors modified by Pearce (1996). c Binary plot Co–Th (Hastie et al. 2007) designed to distin-

guish between various suites of altered subalkaline volcanic rocks. d Binary plot FeOt versus Al2O3 for the classification of alkali rhyolites to comendites and pantellerites (MacDonald 1974) applied to relevant EVB data from the diagram (b)


1 3

tuffs GV57/2 and GV57/6 that have low alkalis (down to 4.2 %) and higher CaO contents (up to 6.1 %) compared with the rest of the suite.

The NMORB-normalized spider plots differ strikingly from the WVB (cf. the shaded field on Fig. 6c), especially by the high contents of Zr and REE as well as the absence of the Nb trough. Depletion in P, Sr and Ti is characteris-tic. While the LILE exceed 1,250 × NMORB, the HREE are only slightly enriched, c. 1.5–5.5×. A few of the sam-ples show some evidence for a loss of the most mobile ele-ments, typically Cs, Rb, Ba, ±U and K (the most affected seem the agglomerate tuffs).

In Figs. 5 and 9a are depicted negative correlations of SiO2 with Al2O3, MgO, FeOt, Na2O, P2O5 and F. Regard-ing the trace elements, the most remarkable are positive trends of silica with Zr, Nb, Th, Ga and Y (and Hf with Ta which are not plotted) as well as negative with Sr (except GV57/6) and Sc (Figs. 8a, 9a).

The REE contents are variable but always higher than in the WVB (ΣREE = 242–891 ppm). Also, the chondrite-normalized patterns differ; the least evolved are character-ized by the lowest REE contents and practically lack Eu anomaly (e.g. GV82: Eu/Eu* = 0.9, ΣREE = 242 ppm), whereas the most fractionated are characterized by high

1816

1412

108

108

64

20

150

100

500

150

100

500

SiO2

lA

2O3

OaC

bR

bN

86

42

08

64

20

400

300

200

100

05

43

21

0

SiO2

MgO

K2O

Cr

LaN

mS

N

108

64

20

76

54

32

10

600

400

200

0

50 60 70 80 50 60 70 80

1.2

0.8

0.4

0.0

SiO2

FeO

ta

N2O

Sr

*uE/u

E

GV57/6

50 60 70 80

50 60 70 8050 60 70 8050 60 70 80

50 60 70 80 50 60 70 80 50 60 70 80

50 60 70 80 50 60 70 80 50 60 70 80

Fig. 5 Binary plots of SiO2 versus selected major-element oxides (wt%) and trace elements (ppm) for the studied metavolcanic rocks from the Vrbno Group. Symbols as in Fig. 4


1 3

ΣREE and a strong negative Eu anomaly (e.g. GV71 Eu/Eu* = 0.21, ΣREE = 891 ppm). With a single exception (a metatuff GV57/6), the patterns are subparallel in both their LREE and HREE segments (LaN/YbN = 4.66–9.63; LaN/SmN = 2.83–4.79). The LaN/SmN ratios tend to increase with rising SiO2 (Fig. 5), while the Eu/Eu* ratios drop

dramatically (Fig. 9a). When compared with the rest of the EVB data set, sample GV57/6 is characterized by relatively low LREE but similar HREE, resulting in (for EVB) rela-tively low LREE/HREE enrichment (LaN/YbN = 3.47; LaN/SmN = 1.79). The Eu anomaly of this metatuff is deep (Eu/Eu* = 0.18).

EVB, ‘acid’EVB, ‘intermediate’

0.01

0.1

110

100

1000

0.01

0.1

110

100

1000

Cs Ba Ce

Sr Nd Sm

U K Pr P Zr Eu Dy Yb

Rb Th Nb La Pb Ti Y Lu

Cs Ba Ce

Sr Nd Sm



Sam

ple/

NM

OR

B

(d)(c)

WVB, ‘acid’WVB, ‘intermediate’

0.01

0.1

110

100

1000

0.01

0.1

110

100

1000

Cs Ba

Sr Nd Sm

CeU K Pr P Zr Eu Dy Yb


Cs Ba CeU K Pr P Zr Eu Dy Yb

Rb Th Nb La Pb Sr Nd Sm Ti Y Lu

Sam

ple/

NM

OR

B

(b)(a)

DoDFeD

0.01

0.1

110

100

1000

0.01

0.1

110

100

1000

Cs Ba

Sr Nd Sm

Cs Ba Ce

Sr Nd

CeU K Pr P Zr Eu Dy Yb



Rb Th Nb La Pb Sm Ti Y Lu

Sam

ple/

NM

OR

B

(f)(e)

Fig. 6 NMORB-normalized spider plots (Sun and McDonough 1989). The analyses from the Western and Eastern volcanic belts (WVB and EVB) are further split into two groups (‘acid’ and ‘inter-mediate’) at SiO2 = 69 %. For comparison, in a, c and f the grey field

portrays the variability within the intermediate and in b, d and e the acid WVB metavolcanics. f NMORB-normalized ‘spider boxplots’ (Janoušek et al. 2004) for dolerite sheets


1 3

‘Acid’ group

Acid rocks of the EVB correspond to comendites/pantellerites or rhyolites (Winchester and Floyd 1977—Fig. 4a). Following Pearce (1996), they classify as alkali rhyolites (Fig. 4b), and as their Al2O3 contents are higher than 1.33 × (FeOt + 4.4), the term ‘comendite’ is appropriate (MacDonald 1974) (Fig. 4d).

The SiO2 is 69.3–80.7, TiO2 0.11–0.29, Al2O3 9.44–14.49, FeOt 1.0–3.5 wt% and mg# = 2.6–34 (mostly low). The A/CNK index varies between 0.97 and 1.22, with the majority of samples being subaluminous. The metatuffs GV57/5 and GV 59/3 have very low K2O con-tents, perhaps indicating some secondary mobility due to alteration or metamorphic overprint. Characteristic of

Sam

ple/

RE

E c

hond

rite

110

100

1000

La

Ce

Pr Eu

Gd

Tb

Dy

Ho

Er

Tm

Yb

Lu La

Ce

Pr

Nd Sm Nd Sm

Eu

Gd

Tb

Dy

Ho

Er

Tm

Yb

Lu

DoDFeD (f)(e)

Sam

ple/

RE

E c

hond

rite

110

100

1000

La

Ce

Pr Eu

Gd

Tb

Dy

Ho

Er

Tm

Yb

Lu La

Ce

Pr

Nd Sm Nd Sm

Eu

Gd

Tb

Dy

Ho

Er

Tm

Yb

Lu

EVB, ‘acid’EVB, ‘intermediate’

(d)(c)

Sam

ple/

RE

E c

hond

rite

110

100

1000

La

Ce

Pr

Nd Sm

Eu

Gd

Tb

Dy

Ho

Er

Tm

Yb

Lu La

Ce

Pr

Nd Sm

Eu

Gd

Tb

Dy

Ho

Er

Tm

Yb

Lu

WVB, ‘acid’WVB, ‘intermediate’ (b)(a)

Fig. 7 Chondrite-normalized (Boynton 1984) REE patterns. The grey field in a and c portrays the overall variability within the intermediate and in b, d and e in the acid WVB metavolcanics. Hatched field in

f shows the total variation in dolerites apart from depleted samples GV74, GV75 and GV77 (dashed lines)


1 3

the sample GV57/5 are exceptionally high CaO contents (1.9 wt%).

The trends in Harker plots are mostly scattered (Figs. 5, 9a). Much more interesting are the negative correlations between silica and some of the trace elements, e.g. Zr, Nb, Th, Ga and Y (Figs. 8a, 9a). All these negative correlations seem to represent segments of convex upward trends, if the more basic members of the EVB suite are taken into account.

The NMORB-normalized spider plots display patterns similar to the more basic rocks. The LILE seem less vari-able, and thus, the effects of metamorphic remobilization are possibly limited.

The acid samples show generally highly fractionated REE patterns (LaN/YbN = 1.8–10.2; LaN/SmN = 1.73–5.33) with strong negative Eu anomalies (Eu/Eu* = 0.18–0.11). With rising silica, the ΣREE decrease sharply from 943 to 187 ppm, reflecting a concomitant drop in both the LREE and HREE. At the same time, Eu/Eu* values slightly decrease (Fig. 9a).

Felsic dykes cutting the Cadomian basement

Four of the FeD samples—taken from dykes penetrating metagranitic Cadomian basement of the Oskava Block—are alkaline (Fig. 4b) corresponding to comendites (Fig. 4d).

The SiO2 contents are 73.7–79.7, TiO2 0.11–0.13, Al2O3 10.4–13.9, FeOt 1.31–3.35 wt% and mg# = 32.5–9.5. The NMORB-normalized spider plots (Fig. 6e) show deep Ba, Sr, P and Ti troughs. High contents of Zr, REE, Th, U, K and the negligible negative Nb anomaly are characteristic, i.e. an overall signature is comparable to the (acid) EVB group. The REE patterns are rather flat (LaN/YbN = 0.77–4.38; LaN/SmN = 1.22–3.08) with deep negative Eu anomalies (Eu/Eu* = 0.15–0.03). The ΣREE are variable, 159–443 ppm. Noteworthy is the presence of a positive Ce anomaly in the sample GV01c and, less so, in GV01b.

Sample GV93/1 of the FeD data set is a calc-alkaline rhyodacite/dacite (Fig. 4a) or (basaltic) andesite (Fig. 4b). It is less acidic (SiO2 66.5, TiO2 0.71, Al2O3 15.0 and FeOt 4.7 wt%). This sample is characterized by low ΣREE (113.5 ppm), LaN/YbN (4.49) as well as LaN/SmN (2.53) and a distinct Eu anomaly even though less pronounced than in the rest of the FeD data set (Eu/Eu* = 0.78).

Dolerite sheets

These are nearly exclusively subalkaline basalts (Fig. 4a, b), originally normal-K calc-alkaline or even tholeiitic in character (Fig. 10a). The rocks are basic (SiO2 46.1–52.2, TiO2 0.77–2.67, Al2O3 13.8–17.2, CaO 6.3–10.1 and FeOt 7.6–12.9 wt. %). High mg# (45–68) as well as low K2O/Na2O ratios (0.01–2.45 by weight) are characteristic. The binary plots involving silica show considerable varia-tions for most of the major-element oxides, Sr and Cr over a limited SiO2 interval (Fig. 5).

The NMORB-normalized spider plots are character-ized by HREE contents close to unity and a uniform slope towards the LILE (Fig. 6f). A shallow negative Nb anomaly is characteristic.

For most of the samples (hatched field in Fig. 7f), the ΣREE are fairly high (45–124 ppm) and REE patterns flat, only slightly enriched with LREE (LaN/YbN = 0.87–4.10; LaN/SmN = 0.81–2.00) corresponding to enriched MORB (E-MORB). They mostly lack Eu anomaly; however, the Eu/Eu* ratios in some of the samples are more variable (0.75–1.14).

On the other hand, REE-poor (ΣRΕΕ = 24–32 ppm) metadolerites from the borehole JR-10 (GV74, GV75 and GV77; dashed patterns in Fig. 7f) show conspicu-ous but variable LREE depletion (LaN/YbN = 0.27–1.08; LaN/SmN = 0.16–0.84) and negligible Eu anomalies (Eu/Eu* = 0.96–1.16), i.e. an overall N-MORB character.

Isotopic composition

Whole-rock Nd and, degree of alteration permitting, O and Sr isotopic data were obtained in order to constrain

50 60 70 80

050

010

0015

00

SiO2

Zr

(ppm

)

EVB

WVBDoD

800700

Zirconsaturation

900 1000

T (°C)WVB

EVB

(a)

(b)

Fig. 8 a Binary plot of SiO2 (wt%) versus Zr (ppm) for the VG meta-volcanics. b Boxplots of zircon saturation temperatures (Watson and Harrison 1983) for acid rocks in WVB and EVB


1 3

the geochemical character, source and petrogenesis of the metavolcanics. The new data for individual groups in the VG, age-corrected to 370 Ma (the new U–Pb age), are sum-marized in Table 2.

More basic rock types of the WVB (pillow lavas GV54 and GV15) have Nd isotopic composition corre-sponding to OIB or subduction-modified depleted mantle (

ε370

Nd= +3.1 and +3.2

)

(Fig. 11a, b). Rhyolite (GV11)

also contains relatively radiogenic Nd (

ε370

Nd= +2.7

)

. The δ18O values exceed 10 ‰ (10.3–13.0 ‰ SMOW).

The intermediate alkaline rocks of the EVB are repre-sented by a volcanic bomb GV57/2, with very radiogenic Nd (

ε370

Nd= +6.7

)

. The two EVB rhyolites (GV07a and GV61) have rather radiogenic Nd

(

ε370

Nd= +3.7 and +3.9

)

and une-volved Sr (GV07a: 87Sr/86Sr370 = 0.7057). The δ18O values are high, exceeding 13 ‰ (13.7–15.7 ‰ SMOW; Fig. 11c).

015

21

100

10000×Ga/Al

I& S

& M

bN

Zr+Nb+Ce+Y

015

21

100

FG

OGT

1010

010

00

10000×Ga/Al

Zr

M&

S&I

10000×Ga/Al

12

510

20

M&

S&I

0.02

0.06

0.10

01

55

20

25

1

54

04

53

03

52

180

6010

014

050

100

150

200

6010

014

018

0

0.2

0.4

0.6

0.8

05

1015

1012

1416

18

0.0

0.2

0.4

0.6

76

54

32

1 2 5 10100 1000 1 2 5 101 2 5 10

55 65 75

55 65 75

45 55 65 75 45 55 65 7545 55 65 7545 55 65 75

0.0

0.5

1.0

1.5

2.0

2.5(a)

(b)

La Y

OP

52

Th

Gab

N

*uE/u

E

Oa

N2

K+

Oa

N2

2O

FeO

t/MgO

MgO

FO

lA

32

cS

SiO2 SiO2SiO2SiO2

45 55 65 75 45 55 65 75 45 55 65 75

45 55 65 7545 55 65 7545 55 65 75

Fig. 9 a Assorted binary plots of SiO2 versus selected major-element oxides (wt%) and trace-element (ppm) concentrations in the EVB. b Analyses of metavolcanic rocks from the EVB plotted in diagrams for recognition of A types (Whalen et al. 1987). Shown are averages of

the A-type granites (labelled ‘A’), fields of ordinary S-, I- and M-type granitic rocks (‘S’, ‘I’ and ‘M’). OGT ordinary granite types, FG fractionated granites


1 3

The felsic dyke (GV01b) has Nd isotopic composition resembling the volcanites of the EVB

(

ε370

Nd= +2.8

)

, but its oxygen is slightly lighter (δ18O = 12.0 ‰ SMOW).

The Nd in common dolerite sheets is very radiogenic (

ε370

Nd= +5.2 to +7.9

)

, mostly corresponding to little modified melts of depleted mantle TDM

Nd= 0.46 − 0.67 Ga.

The oxygen isotopic data (δ18O = 5.5–7.9 ‰ SMOW) are more equivocal.

Discussion

Saturation thermometry for the acid rocks

As the saturation levels of common accessories drop rap-idly with decreasing temperature, and often also the decreasing basicity/content of mafic cations, the saturation thermometry is particularly useful for acid igneous rocks (Janoušek 2006 and references therein).

The broad decrease in Zr with rising SiO2 observed for the acid suite of the WVB (Fig. 8a) indicates that the magma should have been saturated in zircon throughout its history (Hoskin et al. 2000). As, moreover, no inheritance was observed in the dated zircon population, the saturation model of Watson and Harrison (1983) should be applica-ble (TZrn = 743–809 °C; Fig. 8b). The apatite saturation temperatures (Harrison and Watson 1984) are also low (TAp = 774–834 °C), especially if corrected for increased solubility in peraluminous melts (734–767 °C; Bea et al. 1992). Such corrections, however, lead to the calculation of rather imprecise temperatures (see Janoušek 2006). Still, the saturation thermometry indicates that the acid WVB suite could have been relatively cold (~750–800/850 °C).

Also for the acid EVB rocks, Zr drops rapidly with SiO2 (Fig. 8a), and combined CL/LA–ICP–MS study did not disclose any presence of pre-Devonian zircon inheritance. The zircon saturation temperatures (TZrn = 924–1,052 °C, Fig. 8b) are on average higher than those obtained for monazite (Montel 1993) and apatite (TMnz = 785–963 °C, TAp = 794–938 °C). Such a discrepancy may reflect the relative scarcity of P in the acid EVB magmas (Fig. 9a). Even though the high Zr contents may in part reflect the F complexing not accounted for by the zircon model, the saturation thermometers seem to collectively indicate that the parental magmas could have had liquidus temperatures exceeding 900 °C, i.e. were considerably hotter than in the WVB.

For the FeD group, the zircon saturation temperatures also differ between the calc-alkaline sample (830 °C) and its alkaline counterparts (934–1,044 °C). This is con-firmed by monazite saturation thermometry (calc-alkaline sample 791 °C, alkaline ones 874–897 °C), but the apatite

saturation temperatures are significantly lower (739 vs. 746–814 °C).

Petrogenesis of individual volcanic suites


Overall character and setting The metavolcanites of the WVB are purely calc-alkaline in chemistry. At least some (basaltic) andesites are of subaqueous origin as shown by locally preserved pillow lavas. The NMORB-normalized spider plots are characterized by marked LILE over HFSE enrichments typical of a continental-arc geotectonic set-ting (e.g. Pearce and Parkinson 1993; Tatsumi and Eggins 1995). Indeed, in discrimination diagrams after Pearce et al. (1984) (e.g. Fig. 10a), the acid samples plot mostly into the Volcanic-Arc Granites (VAG) domain. Similarly, the inter-mediate ones show a clear Calc-Alkaline Basalts (CAB) affinity in the Th–Hf/3–Ta ternary diagram of Wood (1980) (Fig. 10b). The high degree of LILE over HFSE enrichment is also apparent from the Yb versus Th/Ta binary (Schandl and Gorton 2002) (Fig. 10c) and the Hf–Rb/30–3 × Ta ter-nary (Harris et al. 1986) (Fig. 10d).

On the other hand, the Tremešek pillow lavas (GV14a and GV15) display relatively decreased contents of some LILE (in particular Rb, Ba, Cs and K). As the seawater alteration has normally an opposite effect (e.g. Verma 1992; Pichler et al. 1999), it is more likely a result of metamor-phic remobilization or interaction with hydrothermal fluids.

Geotectonic diagram Th/Yb versus Nb/Yb of Pearce (2008 and references therein) (Fig. 10f) should be resistant to such effects. Melting of the variously depleted/enriched mantle should yield trends parallel to the ‘mantle array’. The Th/Nb ratio serves as a ‘crustal input proxy’; the arc lavas, formed by fluxed melting of the mantle, are shifted above the ‘mantle array’, and the same effects have mantle-derived magma–crust interactions.

In our case this geotectonic diagram, in line with the previous ones, points to a subduction-related origin of the WVB. Theoretically, such signature may have been inher-ited from the remelted metaigneous, arc-related rocks (Sheth et al. 2002) or span from anatexis of immature sediments containing arc-derived detritus (Roberts and Clemens 1993). However, the Brunovistulian Terrane is dominated by granitic rocks, whose high-T anatexis would likely yield A-type granitic melts (Patiño Douce 1997), and fertile metaigneous or metasedimentary lithologies are rare (Dudek 1980). Moreover, the basic members of the WVB show an analogously strong arc-like signature, which would be an unlikely coincidence. Lastly, the Late Devo-nian in the Bohemian Massif is indeed characterized by overall Andean type convergence and ongoing subduction


1 3

activity (Schulmann et al. 2009; Žák et al. 2011 and refer-ences therein).

Magma evolution The importance of fractional crystal-lization of an assemblage dominated by feldspars, zircon, apatite and ilmenite is supported by a marked negative cor-relation of Sr, P, Zr, Eu/Eu* and Ti as well as a progressive enrichment in K and Rb with rising SiO2.

Source of the parental magma(s) For the more basic sam-ples, the Nd isotopic data are compatible with derivation from a moderately depleted mantle source

(

ε370

Nd∼ +3.2

)

. For the rhyolites, the Nd is only slightly less radiogenic (ε370

Nd ~ +2.7).

This is in line with a possible genesis by the remelting of a geochemically immature crust, or even nearly closed-system fractional crystallization of the parental basaltic melt (Annen et al. 2006). Role of melting of, or contamination by, geo-

WVB

EVB

EVB

(c)

(e)

(b)

(d)

(f)

EVBintermediate

WVB intermediate

WVB

EVB

(a)

1000

1000

100

100

10

10

1

1

ORG

VAG+

syn COLG-

WPG

Y

bN

Th Ta

Hf/3

WPA

N-

MORBIAT

E−MORB,

WPTCAB

4010

01

0

0

20

02

30

03

MORB

Within-Plate

Volcanic Zones

Active

Continental

Margins

Oceanic Arcs

Yb

Nb/Yb

bY/h

T

aT/hT

Hf 3×Ta

Rb/30

VA

WP

Group 2

Group 3WVB

A2

A1

aG3Y ×

Nb

Volcan

icar

c arra

y

MORB–O

IBar

ray

NMORB

EMORB

OIB

*

*

*

0.1 1 10 100

0.01

0.10

110

Mag

ma-

crus

tin

tera

ctio

n


1 3

chemically immature and thus, in terms of Sr–Nd isotopic composition, hardly distinguishable local Cadomian base-ment (Hanžl et al. 2007) is difficult to assess, even though some crustal contamination or interaction with seawater/hydrothermal fluids is unequivocal based on high δ18O values (10.3–13.0 ‰ SMOW, Fig. 11c) (Davidson et al. 2005).


Overall character and setting In the EVB, abundant alka-line volcanics range from rare subordinate alkali basalt (e.g. Patocka and Valenta 1990, 1996; not represented in our data set), trachyandesite, trachyte to prevalent comendite. At least partly, their structures indicate subaeric origin and explosive nature (agglomerate tuffs, ignimbrites).

The acid volcanic rocks correspond well with the within-plate granitic rocks in the geotectonic diagrams of Pearce et al. (1984) and Harris et al. (1986) (Fig. 10a, d). Accord-ingly, the more basic samples show an intraplate signature in the Th–Hf/3–Ta ternary (Wood 1980) (Fig. 10b). The whole suite is rich in HFSE, with Yb contents increasing rapidly with progressive fractionation at negligible changes in the Th/Ta, a feature typical of within-plate volcanic suites (Schandl and Gorton 2002; Fig. 10c). The Th/Yb versus Nb/Yb diagram (Fig. 10f) documents an OIB-like chemistry of the more mafic members as well as limited role, if any, for the subduction component.

Overall, the EVB metavolcanites are characterized by high contents of alkalis, REE (except Eu), HFSE (Nb, Ta, Y, Zr and Ga), Zn as well as elevated Ga/Al and Fe/Mg ratios. This, together with low contents of CaO, trace ele-ments compatible in mafic silicates (Co, Sc, Cr, Ni) and feldspars (Ba, Sr, Eu), is a hallmark of within-plate, A-type igneous activity (Collins et al. 1982; Whalen et al. 1987; Eby 1990; Bonin 2007). Some of the plots illustrating this are shown in Fig. 9b, where the data points always cluster close to the A-type average, far away from the fields of S- and I-type granites.

More specifically, the acid EVB analyses classify as A1 granitoids sensu Eby (1992) (e.g. Fig. 10e), having rela-tively low Y/Nb, Ga/Nb and Ce/Nb ratios (last not shown). They are thus likely to have originated in an intraplate setting.

Magma evolution The A-type magmas are thought to be characterized by low oxygen and H2O fugacities, coupled with high F contents (Bonin 2007 and reference therein). In such a F-rich melt, Ga should be stabilized in the form of GaF6

3– complexes and high Ga/Al ratios are thus con-sidered particularly diagnostic (Collins et al. 1982; Whalen et al. 1987). Also, other highly charged metal ions tend to form readily fluoride complexes with, or without, alkali metal counterions (Collins et al. 1982; Kirstein et al. 2001).

The data points for EVB volcanites define convex upward trends in binary plots of silica versus highly charged metal ions (Zr, Hf, Nb, Ta, Ga, Zn, REE, Y, U and Th; representative ones are in Fig. 9a). Most of these ele-ments form essential structural constituents in accessory minerals. During the fractionation, any such an element would initially behave incompatibly, gradually increas-ing its abundance in the magma until a required threshold is reached. Subsequently, its concentration would be buff-ered at the saturation level by the growth of the appropri-ate phase (Hanson and Langmuir 1978; Evans and Hanson 1993). In our case, the crystallization of the accessories seems to have been suppressed until SiO2 ~69 wt%, as indicated by a conspicuous inflection point in many of the binary plots.

This event is also marked by a minimum on the plot involving fluorine. The observed F contents are initially high but drop sharply with increasing silica only to resume rising again. Even though it is not considered safe to use the whole-rock analyses to estimate the original fluorine level in the magma, due to the risk of late fluid–rock inter-action and/or of the metamorphic overprint, this may point towards likely complexing of HFSE with F.

Important role of feldspars fractionation (in such a hot magma perhaps present as a single, hypersolvus phase) in intermediate volcanites is documented by a dramatic decrease in Al2O3, Na2O and Eu/Eu* with rising silica

Fig. 10 a Geotectonic diagrams Y–Nb (ppm) of Pearce et al. (1984) for samples with SiO2 >69 wt% (i.e. ‘acid’ groups in WVB and EVB, felsic dykes FeD). The volcanites of the western belt have an affinity to Volcanic-Arc Granites (VAG), while the rocks from the eastern belt correspond to Within-Plate Granites (WPG). The other fields on the diagram are Syn-Collisional Granites (syn-COLG) and Ocean Ridge Granites (ORG). b Triangular plot Th–Hf/3–Ta, proposed by Wood (1980). Following geotectonic settings are defined: IAT—Island-Arc Tholeiites, CAB—Calc-Alkaline Basalts, N-MORB—Normal Mid-Ocean Ridge Basalts, E-MORB—Enriched Mid-Ocean Ridge Basalts, WPT—Within-Plate Tholeiites, WPA—Within-Plate Alkali Basalts. The fields of intermediate volcanic rocks from both the western (WVB) and eastern (EVB) volcanic belts are also outlined (dotted line). c Binary plot of Yb versus Th/Ta of Schandl and Gor-ton (2002) originally designed to decipher the geotectonic setting of felsic volcanic suites, specifically those associated with the volcano-genic massive sulphide (VMS) deposits. Position of MORB is shown for comparison. d Triangular plot Hf–Rb/30–3 × Ta (Harris et al. 1986) for classification of collisional granites. VA—volcanic-arc, WP—within-plate, Group 2—syn-collision peraluminous leucogran-ites, Group 3—late or post-collision calc-alkaline intrusions. e Clas-sification diagram Y–Nb–3 × Ga (Eby 1992) for A-type granitoids. The EVB volcanics classify as A1 type. f Geotectonic diagram Th/Yb versus Nb/Yb (Pearce 2008). The ‘MORB–OIB array’ is formed by average NMORB, EMORB and OIB compositions from Sun and McDonough (1989). The diagram shows an OIB-like affinity of the basic EVB suite and an important subduction component in all the WVB rocks. The mantle source of dolerites seems to be influenced by interaction between depleted mantle and OIB component (ascending asthenosphere?) as well as by variable crustal contamination. Sym-bols as in Fig. 4

◂


1 3

Tabl

e 2

Str

ontiu

m (

a) a

nd N

d–O

(b)

isot

opic

dat

a

a In

brac

kets

are

pre

sent

ed e

rror

s (2

SE

)b S

ubsc

ript

s ‘3

70’ i

ndic

ate

age-

corr

ecte

d is

otop

ic r

atio

sc T

wo-

stag

e N

d m

odel

age

s

Loc

ality

Gro

upR

b (p

pm)

Sr (

ppm

)87

Rb/

86Sr

87Sr

/86Sr

a 87

Sr/86

Sr37

0b

a GV

07a

Reš

ovE

VB

aci

d83

.550

.74.

7776

0.73

0857

(8)

0.70

570

GV

21b

Dl.

Poto

kD

oler

ites

0.7

281.

50.

0072

0.70

6260

(12

)0.

7062

2

GV

40K

arlo

vD

oler

ites

13.8

751.

70.

0531

0.70

5916

(17

)0.

7056

4

GV

47B

edri

chov

Dol

erite

s1.

632

0.9

0.01

440.

7055

18 (

8)0.

7054

4

Loc

ality

Gro

upSm

(pp

m)

Nd

(ppm

)14

7 Sm/14

4 Nd

143 N

d/14

4 Nda

(143 N

d/14

4 Nd)

370b

ε Nd

370

bT

Nd

DM

c (G

a)δ18

O S

MO

W (

‰)

b GV

54V

ácla

vov

WV

B in

term

.6.

127

.70.

1331

40.

5126

45 (

5)0.

5123

223.

10.

828

11.9

GV

15T

rem

ešek

WV

B in

term

.4.

017

.80.

1358

60.

5126

53 (

14)

0.51

2324

3.2

0.82

610

.3

GV

11T

rem

ešek

WV

B a

cid

3.2

14.6

0.13

251

0.51

2621

(5)

0.51

2300

2.7

0.86

313

.0

GV

57/2

N.V

esE

VB

inte

rm.

15.0

72.5

0.12

509

0.51

2809

(5)

0.51

2506

6.7

0.54

614

.9

GV

07a

Reš

ovE

VB

, aci

d13

.155

.40.

1429

60.

5126

96 (

6)0.

5123

503.

70.

787

15.7

GV

61H

unta

vaE

VB

, aci

d26

.310

7.6

0.14

774

0.51

2721

(11

)0.

5123

633.

90.

766

13.7

GV

01b

Stan

cín

FeD

7.4

20.8

0.21

510.

5128

27 (

7)0.

5123

062.

80.

854

12.0

GV

90T

rem

ešek

DoD

3.6

11.8

0.18

446

0.51

2916

(55

)0.

5124

696.

00.

603

–

GV

21b

Dl.p

otok

DoD

3.4

10.0

0.16

829

0.51

2963

(11

)0.

5125

557.

70.

470

5.5

GV

40K

arlo

vD

oD5.

421

.60.

1647

60.

5129

64 (

13)

0.51

2565

7.9

0.45

67.

9

GV

42N

.Ves

DoD

5.4

19.4

0.16

829

0.51

2835

(4)

0.51

2427

5.2

0.66

76.

5

GV

47B

edri

chov

DoD

6.0

22.8

0.15

911

0.51

2840

(3)

0.51

2456

5.7

0.62

46.

6


1 3

(Fig. 9a, also Patocka and Valenta 1996). The drop in MgO, P2O5 and Sc indicates crystallization of amphibole and apa-tite that could have gradually impoverished F in the melt before reaching the inflection point. The fluorine is an ele-ment thought to stabilize the amphibole structure causing early precipitation of this phase even in a relatively dry melt (Collins et al. 1982).

Source of the parental magma(s) The petrogenesis of A-type granitoids (Loseille and Wones 1979) remains one of the most intriguing issues in the granite petrology. Accord-ingly, a plethora of genetic models has been formulated (see Bonin 2007 and Magna et al. 2010 for a review).

In the EVB, at least some involvement of mantle-derived magmas seems unequivocal. The most mafic alkaline rock is the volcanic bomb GV57/2, radiogenic Nd of which points to partial melting of a strongly LREE-depleted man-tle with little scope for crustal contamination (ε370

Nd ~ +7).

Despite the fact that the comendites have geochemical char-acter compatible with within-plate volcanites, their fairly radiogenic Nd (ε370

Nd ~ +3.8) and unevolved 87Sr/86Sr370

ratios (~0.7057) rule out a major contribution from mature crustal sources, as well.

At the first glimpse, an idea that the comendites origi-nated by a protracted fractionation of somewhat less depleted-mantle-derived partial melts can be discounted considering the large proportion of the acid volcanic prod-ucts and the lack of significant volumes of intermediate fractionation products and feldspar-rich cumulates. Turner and Rushmer (2009), however, argued that the presence of the Daly gap in bimodal suites should not be taken as diag-nostic of an intracrustal origin of A-type magmas. In their view, differentiation from 55 to 65 wt% SiO2 would require fractionation of a Fe-oxide-rich assemblage, resulting in rapid increase in SiO2 over a small temperature interval (i.e. limited change in the fractionation degree). This means that the Daly gap may be present even in igneous suites forming an undisturbed fractionation sequence.

Still, extensive fractionation of mantle-derived basic magmas is likely to lead to H2O-rich residual melts (Annen et al. 2006). Even more significant seems that the δ18O of the comendites, exceeding 13.0 ‰ SMOW, also precludes 18O

WVB‘acid’

EVB‘acid’

Felsic dyke(FeD with

EVB affinity)

Dolerites (DoD)

WVB‘intermediate’

EVB‘intermediate’

6

MORB = +and5.4 ± 0.8 ‰

(Harmon Hoefs 1995)

(c)

Dee

p co

ntam

inat

ion/

AF

C?

336

78

54

SiO2

dN37

0(b)

Western VolcanicBelt (WVB)

Eastern VolcanicBelt ( VB)E

Dyke rocks

‘Intermediate’‘Acid’

dolerites (DoD)felsic dykes (FeD)

‘Acid’‘Intermediate’

Polanka

Rudná

0.2

dN

4 8 10 12 14 16

50 60 70 80

0.0 0.4 0.6 0.8 1.0

02

46

810

CHUR

DM

Age (Ga)

two-stage Nd model ages

init

ial e

psi

lon

val

ues

Goldstein(1984)

et al.Liew and Hofmann (1988)

Desná Dome(Hegner and Kröner 2000)

Basement

Basement

(a)

kcolBavaksO

)7002.latelžnaH(

Fig. 11 a Two-stage Nd development diagram showing ranges of the ε370

Nd values for individual groups. Depleted-mantle (DM) evolu-

tion lines after Goldstein et al. (1984) and Liew and Hofmann (1988). Plotted also are literature data for the basement Proterozoic gneisses of the Oskava Block (Hanžl et al. 2007) and Desná Unit further north (Hegner and Kröner 2000). b Binary plot SiO2 versus ε370

Nd. c Stripplot

for δ18O (‰ SMOW) values in individual groups of metavolcanites in the VG. Boxplot for MORB compositions after Harmon and Hoefs (1995) is shown for comparison

▸


1 3

such a closed-system fractionation. It may instead point to partial melting and/or assimilation of, or high-T exchange with, a continental crust (Taylor 1978; Hoefs 2004). As unacceptably high degrees of crustal contamination would be needed, the O isotopic signature more likely mimics a relatively unevolved crustal source. Indeed, the Nd isotopic composition of the acid EVB metavolcanites in principle does not preclude partial meting of the Cadomian (i.e. rela-tively young) crust. Still, it is somewhat more radiogenic than the available data for the Oskava Block (Hanžl et al. 2007) and, in particular, the northerly Desná Unit (Hegner and Kröner 2000) (Fig. 11a). This discrepancy can be read-ily reconciled assuming minor juvenile basic magma input, though.

Felsic dykes cutting the Cadomian metagranitic basement

Most of the sampled felsic dykes in the Oskava Block are alkaline, closely resembling the EVB comendites. Moreo-ver, there is a certain analogy with the minor (Polanka-type) leucogranite intrusions that have also intruded the Cadomian basement in this region (Hanžl et al. 2007).

Among the FeD occur also rarer calc-alkaline types, resembling the WVB. Our only sample of this character (GV93/1) comes from a distinct segment of Cadomian base-ment built by metadiorites and metatonalites, west of the Oskava Block. Its chemistry and geological position are thus similar to those of the Rudná granite (Hanžl et al. 2007).

Dolerites

The numerous doleritic dykes and sills intruded the brit-tle fractures cutting the whole crustal segment of the VG, including its Cadomian basement. Regardless of the nature of their country rocks, they form a geochemically rather homogeneous group of subalkaline (tholeiitic to low-K calc-alkaline) basalts.

Source of the parental magma(s) The geotectonic setting of metadolerites from the current data set was dealt with in preliminary report of Pecina et al. (2003). The authors have shown that in the Zr–Zr/Y diagram after Pearce and Norry (1979), the data points spread between the fields of island-arc basalts (samples from borehole JR-10), MORB and WPB (the rest of the data set). On this basis, as well as different contents of radioactive elements (K, Th and U) and shape of REE patterns (NMORB vs. EMORB-like, see Fig. 7f), the authors proposed the existence of two inde-pendent and potentially diachronous dolerite suites.

In fact, various diagrams give equivocal answer to the question of geotectonic setting with considerable degree of overlap. For instance, in the diagram of Wood (1980) (Fig. 10b), most of the dolerites correspond to arc-related

settings. On the other hand, the geotectonic diagram of Pearce (2008) (Fig. 10f) shows that the dolerites may be influenced by interaction between depleted mantle and OIB source (asthenosphere?) as well as by variable crustal contamination.

The high degree of ambiguity is hardly surprising in an arc–back-arc tectonic setting. One could envisage complex interaction of mantle components variably overprinted by subduction under the arc, with depleted mantle and the upwelling asthenosphere in the back-arc region (e.g. Pearce and Stern 2006).

For the geochemically most primitive samples, the Nd isotopic composition is compatible with direct derivation from depleted mantle (MORB source) in Devonian times (ε370

Nd ~+8), and this is also in line with the whole-rock

geochemical (Fig. 10f) as well as oxygen isotopic data (δ18O = 5.5–6.6 ‰ SMOW; Fig. 11c).

The Sr–Nd–O isotopic signatures of some of the sam-ples (87Sr/86Sr370 = 0.705–0.706, ε370

Nd down to ∼+5, δ18O

values up to +7.9 ‰), broadly correlating with decreas-ing MgO and mg# (not shown), can be ascribed to a crus-tal contamination. Such scenario is confirmed in several NMORB-normalized patterns with positive spikes in Rb, K, Sr and Pb and troughs in Nb. Some of these effects could have resulted from Variscan metamorphic remobili-zation, though.

Geotectonic model for the Devonian volcanism in the southern part of the Hrubý Jeseník Mts

The majority of the earlier workers agreed that Devonian volcanic rocks in the VG were generated at a divergent plate boundary (Soucek 1978a; Jedlicka and Pecina 1990). Some even assumed a temporal evolution from alkaline to tholeiitic rocks with progressive extension. Only excep-tionally, the volcanism was ascribed to an island-arc with a typical zoning (Jakeš and Patocka 1982), as were the asso-ciated base metal deposits (Patocka 1987). A combination of arc and back-arc setting, latter having been initiated as an intracontinental rift, has been proposed by Patocka and Valenta (1990, 1996) with Patocka and Hladil (1997).

Indeed, the current study has demonstrated that the metavolcanic rocks in the southern part of the VG belong to two distinct volcanic provinces: (1) western calc-alkaline, with a presumed continental margin geotectonic setting and (prevailing) submarine origin (WVB), and (2) eastern alka-line, partly or fully subaeric, mostly felsic suite generated upon an attenuated Cadomian crust further inland (EVB).

Role for subduction

The new U–Pb ages indicate that the volcanic activ-ity in both igneous provinces (WVB and EVB) was


1 3

contemporaneous or at least overlapped significantly in early to mid-Famennian times (~371–374 Ma). The origi-nal configuration of the two belts, brought together during the Early Carboniferous continental collision (Schulmann and Gayer 2000), is still open to debate, as the oceanic domain must have been nearly consumed in the course of the Variscan convergence. Important for reconstructing the original setting of the studied volcanism is thus the location of relic ophiolitic sequences.

One prospective candidate seems the N–S trending Staré Mesto Belt (Štípská et al. 2001; Žácek 1996; Mazur et al. 2006, 2012) rimming the Hrubý Jeseník Mts. in the W. This belt separates the dissimilar Silesicum and Lugian domains or, in terminology of the Polish authors, the Eastern and Central Sudetes (Mazur et al. 2006). According to some models, it may correspond to a Rheic suture separating the Avalonian Brunovistulian from Armorican terranes of the Variscan belt, closed by the end of Devonian (Finger et al. 2007; Mazur et al. 2010). The Variscan deformation and metamorphism in the Staré Mesto Belt, as well as its thrust-ing over the East Sudetes, was dated at 339 ± 7 Ma (Parry et al. 1997). The second viable possibility is the more west-erly Saxothuringian (Karkonosze) suture, located between the Saxothuringian and Lugian domains (Western and Central Sudetes) (Mazur and Alexandrowski 2001; Chopin et al. 2012; Mazur et al. 2012).

However, the Brunovistulian active margin was probably translated for some 200 km southwards, along its bound-ary with the Moldanubian/Lugian domains (Schulmann et al. 2008). Thus, even higher degree of uncertainty applies to the exact configuration of the plates and their geometry. This concerns also the polarity of the Devonian subduction. Schulmann and Gayer (2000) with Schulmann et al. (2008) invoked an eastward (in the present-day coordinates) Early Carboniferous subduction beneath the Lugian Terrane (a continental accretionary wedge) followed, after collision, by westward underthrusting of thinned back-arc crustal bou-dins (Keprník and Desná units) and the Brunovistulian plate itself. Analogous geometry was assumed also by Mazur et al. (2006, 2012), Szczepanski (2007) and Chopin et al. (2012).

The relative scarcity of volcanites with destructive mar-gin geochemical signature in the Hrubý Jeseník Mts. and analogous adjacent metamorphic terrains in Poland has been ascribed to a deep erosion of the former arc (Patocka and Hladil 1997; Mazur et al. 2006). This idea is supported by the abundance of clastic sediments (mainly quartzites) at the bottom of the VG. Equivalent rocks (quartzites, quartz–sericite schists and metaconglomerates) occur also in the envelope of the Strzelin Crystalline Massif in Poland (Jegłowa Beds). The quartz-rich clastics of both sequences display geochemical features indicating that their source was dominated by acid to intermediate volcanic-arc rocks (Szczepanski 2007).

Back‑arc spreading

As shown above, the mostly felsic alkaline volcanic rocks in the SE part of the Hrubý Jeseník Mts. (EVB) display a distinct within-plate geochemical signature resembling A-type granitic rocks. Indeed, already Patocka and Valenta (1990, 1996) assumed an extensional setting for the vol-canic rocks of the Horní Mesto mining district. Accord-ing to these authors, the parental magmas were generated within a significantly thinned lithospheric domain with high heat flow, boosted by the upwelling hot asthenospheric mantle. Data of Wilimský and Prichystal (2005) indicate that the equivalent rocks occur further north, as far as to the Polish border. Moreover, the more easterly Šternberk–Horní Benešov Volcanic Belt in the Nízký Jeseník Mts. is built mainly by Late Devonian alkaline metavolcanic rocks that most likely formed in the intracontinental rift setting (Prichystal 1990,1993) leading to opening of a true oceanic domain (Kalvoda et al. 2008).

As summarized by Bonin (2007, 2008), the A-type granitoids are by no means restricted to anorogenic set-tings. They can also originate by reactivation of older orogens, requiring high heat flow (typically in rifts, but also former subduction settings—Whalen et al. 1987; Magna et al. 2010). The relatively hot, restite-free, F-rich and therefore mobile (e.g. Giordano et al. 2004) magmas tend to form shallow intrusions with abundant volcanic equivalents.

The Cadomian basement in the Hrubý Jeseník Mts. is tonalitic to granitic (Kröner et al. 2000; Hanžl et al. 2007), and thus relatively refractory. As shown by experiments, its high-T anatexis could have yielded melt with many geo-chemical characteristics of A-type-like granitic melts anal-ogous to the EVB volcanic rocks (e.g. Skjerlie and John-ston 1992; Patiño Douce 1997).

Variscan collision

According to Hladil et al. (1999), the original elongation of the Devonian continental to deep-water basins in Mora-via was E–W. Based on palaeomagnetic data, these were interpreted to have rotated c. 90 degrees clockwise in the Late Devonian to Early Carboniferous times, i.e. early in the Variscan collision (Tait et al. 1996; Hladil et al. 1999; Franke and Źelazniewicz 2000). This is, however, at dif-ference with the present-day configuration of the westerly volcanic-arc (WVB) and easterly back-arc volcanic suites (EVB). Should the model of clockwise rotation be true, the opposite would be expected. The solution seems subse-quent eastward thrusting of the alkaline volcanics-bearing crustal slices (future EVB) over the Cadomian basement to which the WVB remained attached as a relative paraau-tochton. There is indeed a tectonic contact between WVB


1 3

pillow lavas and the overlying basal clastic sequence of the VG at Tremešek (Aichler 2000).

However, Edel et al. (2003) have shown that the pre-sumed Devonian magnetic poles occur not only in Devo-nian sediments but also in Carboniferous rocks further west. This means that the rotation of the whole crustal segment must have been later (Schulmann et al. 2009). Accordingly, more recent models (Szczepanski 2007; Cho-pin et al. 2012; Mazur et al. 2012) have not invoked any rotation at all. The current configuration would then reflect more or less that of the eastward-dipping Late Devonian subduction zone, which presumably passed into collision, underthrusting of the thinned, mostly felsic metaigneous Saxothuringian passive margin and crustal relamination. The ongoing compression could have led eventually to the gravity-driven extrusion of thermally softened lower plate material in front of the Brunovistulian rigid buttress (Cho-pin et al. 2012; Mazur et al. 2012).

Towards a plausible model for Late Devonian volcanism in the Hrubý Jeseník Mts

A simple model that may explain the spatial and temporal development of the studied volcanism is summarized in Fig. 12. The subduction beneath the Cadomian crust (Bru-novistulian Continent) is thought to have been responsible for the generation of the volcanic arc (WVB) as well as accumulations of arc-derived siliceous detritic sediments. The westward (in the present-day coordinates) retreat of the subduction zone and slab rollback (Uyeda 1982; Sto-rey et al. 1992) could have caused a significant thinning of the Cadomian lithosphere in the hinterland. The refractory attenuated crust could have melted by the heat provided by the upwelling asthenosphere and/or basic magma under-plating. This high-T anatexis could have led to generation of voluminous acid alkaline magmas with a broad ‘A-type’ geochemical signature. Such large volumes of felsic mag-mas are likely to form an effective density barrier pre-venting most of the basic rocks from reaching the surface (Bonin 1996; Pankhurst et al. 1998), accounting for relative rarity of the basalts in the field. The original proportion of the basic melts involved, mostly trapped in lower crust, is consequently difficult to judge. On the other hand, impor-tant role of later, more primitive, probably asthenospheric mantle-derived melts is evidenced by the abundance of the dolerite sheets throughout the VG.

Conclusions

The current paper represents a well-documented case of the Late Devonian subduction–back-arc association on Bru-novistulian basement. Even though characterized mainly

on the whole-rock geochemical and U–Pb geochrono-logical grounds, its occurrence has important geotectonic consequences.

1. Petrologically and geochemically variable metavolcan-ites occur in the Hrubý Jeseník Mts. (Silesicum, NE Bohemian Massif, Czech Republic), near the eastern termination of the Rhenohercynian Zone of the Vari-scan orogen. They form a part of the palaeontologi-cally dated Devonian volcanosedimentary sequence of the Vrbno Group (VG), deformed, imbricated and metamorphosed jointly with their mainly metagranitic Cadomian basement. Regardless of the presence of greenschist-facies metamorphic assemblages, volcanic structures are locally well preserved.

2. In the studied southern part of the VG, volcanosedi-mentary and bimodal volcanic rocks occur in two approximately NE–SW trending belts, separated by deformed Cadomian metagranitic paraautochton (Oskava Block): (1) Western Volcanic Belt (WVB), calc-alkaline in chemistry, and (2) alkaline East‑ern Volcanic Belt, formed by a predominantly felsic, comenditic suite. In addition, there are rare felsic dykes (rhyolites and comendites) that penetrated the Cado-mian basement itself. Finally, numerous late dolerite dykes cut the whole crustal segment.

3. Laser ablation U–Pb zircon dating indicates that the felsic igneous activity of both metavolcanic belts was contemporaneous, or at least largely over-lapped, in Famennian (WVB: 374.0 ± 1.7 Ma, EVB: 371.0 ± 1.4 Ma (2σ)).

4. The geochemically relatively primitive WVB is char-acterized by an abundance of metasediments accompa-nied by mostly basic–intermediate metavolcanites; acid types are subordinate. The (basaltic) andesites were at least partly subaqueous as pillow lavas are present. The Nd isotopic data are compatible with their origin from a moderately depleted (subduction-modified?) mantle. The chemistry of rhyolites points to anatexis of imma-ture crust or (nearly) closed-system fractional crystal-lization of the same primary basaltic melts

(

ε370

Nd∼ +3

)

.5. The geochemically more evolved EVB, covering a

significantly larger geographic area, is predominantly metavolcanic. Even though the relative proportion of acid volcanic products (comendites) is much greater, some rock types are as basic as alkali basalt. At least partly, the structures indicate subaeric origin (agglom-erate tuffs, ignimbrites). The radiogenic Nd in more mafic EVB rocks documents a partial melting of a strongly depleted mantle with little crustal contami-nation. The felsic rocks show high contents of HFSE as well as elevated Ga/Al and Fe/Mg ratios, typical


1 3

Fig. 12 Tentative model for the geotectonic setting of the vol-canic rocks in the Vrbno Group. The calc-alkaline volcanites of the WVB are interpreted as representing a vestige of the former igneous arc (IIa), deeply dissected by erosion. The arc-related volcanogenic detritus was deposited as quartz-rich detritic sediments. The progres-sive retreat of the subduction zone and slab rollback is believed to have resulted in significant thinning of the litho-sphere in the hinterland (IIb). The increased heat flow, with underplating by hot basaltic magmas derived from decom-pressing asthenosphere, resulted in widespread partial melting of the mostly granitic Cadomian crust. The zone of the extensive crustal anatexis provided a filter preventing most of the basic melts from reaching the surface. As a result was generated the EVB, formed mostly by felsic alkaline volcanic products with a distinct within-plate signature. The magmas derived in both settings have intruded as dykes the Cadomian basement, in the studied area represented by the Oskava Block. Subsequently, the whole region was cross-cut by dykes of primitive dolerites, coming originally from strongly depleted mantle source but showing variable effects of crus-tal contamination en route to the surface (III). The final situation results from east-directed thrust-ing of EVB over Oskava Block, to which the WVB remained attached as a relative paraau-tochton. ŠHBB Šternberk–Horní Benešov Belt

Trench Forearc Volcanic Arc

Calc-alkalinevolcanism

Arc-deriveddetritus

Arc-derived

Alkalinevolcanism

Asthenosphereupwelling

III. Ongoing extension

Basal Devonian clastics +reef limestones

Lithospheric Mantle

Asthenospheric MantleI. Pre-subduction stage

IIa. Early subduction

IIb. Back-arc spreading

Slab roll-back

(Late, primitivedolerite dykes)(Late, primitivedolerite dykes)

Brunovistulian Continent( )Cadomian basement

Western VB(Calc-alkaline)

Eastern VB ŠHBB(Alkaline)

Osk

ava

Blo

ck

H e a t

H e a t

H e a t


1 3

of within-plate, A-type igneous activity. Also, their radiogenic Nd rules out mature crustal sources, but the rather high δ18O values preclude a closed-system frac-tionation from mantle-derived melts. Plausible seems high-T partial anatexis of a relatively young granitic crust, similar to the attenuated Cadomian basement.

6. The volcanites of the WVB are interpreted as a vestige of a former igneous arc, deeply dissected by erosion. The progressive retreat of the subduction zone and slab rollback is thought to have resulted in significant thin-ning of the Cadomian lithosphere in the hinterland. The increased heat flow, boosted by the underplating basal-tic magmas derived from decompression melting of the rising asthenosphere and of the stretched lithospheric mantle, would have resulted in widespread anatexis of the mostly granitic Cadomian crust. This extensive, partially molten zone could have prevented most of the basic melts from reaching the surface.

7. The Devonian and Cadomian complexes were pen-etrated not only by dykes and sills of both types of magmas but also by subalkaline dolerite sheets with a NMORB or EMORB affinity and (originally) primitive Sr–Nd–O isotope chemistry. The dolerites were prob-ably derived from a strongly depleted mantle, but some show variable effects of crustal contamination.

Acknowledgements The manuscript benefited from reviews by M. Poujol and F. Finger, as well as from editorial handling of I. Braun. We are thankful to K. Schulmann (Czech Geological Survey, Prague) for enlightening discussions on structure and development of the Vari-scan Orogen, A. Prichystal (Masaryk University, Brno) for field assis-tance and advice, M. Chlupácová (AGICO Inc., Brno) for comments on earlier version of the manuscript, and explaining the role of radio-active elements in particular, as well as D. Wilimský (former student at Masaryk University, Brno) for help in sample collection and zircon separation. We are also indebted to K. Žák, who has acquired the O isotopic analyses in the laboratories of the Czech Geological Survey. This work was financed by the Grant Agency of the Czech Republic (GAČR) Project 205/01/0331 (to J. Aichler) and Ministry of Educa-tion, Youth and Sports (MŠMT) project LK 11202 (ROPAKO, to K. Schulmann), which are gratefully acknowledged.

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