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Recurrent Cenozoic volcanic activity in the Bohemian Massif (Czech Republic)

Jaromír Ulrych a,⁎, Jaroslav Dostal b, Jiří Adamovič a, Emil Jelínek c, Petr Špaček d,Ernst Hegner e, Kadosa Balogh f

a Institute of Geology v.v.i., Academy of Sciences of the Czech Republic, Rozvojová 269, 165 00 Praha 6, Czech Republicb Department of Geology, Saint Mary's University, Halifax, Nova Scotia, Canada B3H 3C3c Institute of Geochemistry, Mineralogy and Mineral Resources, Faculty of Sciences, Charles University, Albertov 6, 128 43 Praha 2, Czech Republicd Institute of Geophysics v.v.i., Academy of Sciences of the Czech Republic, Boční II, 141 31 Praha 4, Czech Republice Department of Geowissenschaften, Universität München, Theresiennstraße 41, D-80333 München, Germanyf Institute of Nuclear Research, Hungarian Academy of Sciences, Bem tér 18/C, H-4026 Debrecen, Hungary

a b s t r a c ta r t i c l e i n f o

Article history:Received 11 June 2010Accepted 14 December 2010Available online 22 December 2010

Keywords:Bohemian MassifCenozoicAlkaline volcanismPaleostress fieldsRiftMantle

Cenozoic anorogenic volcanism of the Bohemian Massif is an integral part of the Central European VolcanicProvince. The temporal and spatial distribution of volcanic rocks in the Bohemian Massif, their geochemistryand mineralogy as well as their tectonic setting and paleostress data are used to characterize and classify thisvolcanic activity. Threemain volcanic periods can be distinguished based on K–Ar data and known paleostressfields: (i) pre-rift (79–49 Ma), (ii) syn-rift (42–16 Ma) and (iii) late-rift (16–0.3 Ma), with the youngestperiod further subdivided into three episodes. The dominant mafic rock types (N7 wt.%MgO) of all periods areof nephelinite–basanite/tephrite composition. The exceptions are suites of melilitic ultramafic rocks of thepre-rift period in northern Bohemia and of the final episode of the late-rift period in western Bohemia. Themost voluminous are volcanic rocks of the syn-rift period occurring in the Ohře Rift Graben.The initial 87Sr/86Sr (0.7032 to 0.7050) and 143Nd/144Nd (0.51264 to 0.51301) ratios of the mafic volcanicrocks of the BohemianMassif are characteristic of magmas derived from a sub-lithospheric mantle source. Theisotopic ratios resemble those of the HIMUmantle source (206Pb/204Pb ca. 19 to 20). These rocks have themostisotopically depleted compositions among the Central European Volcanic Province volcanics.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

Cenozoic alkaline magmatism of the Variscan Bohemian Massifrepresents the easternmost part of the Late Cretaceous to CenozoicCentral European Volcanic Province (CEVP) of Wimmenauer (1974),which spreads from central France (Massif Central) across Germany(Eifel Mts., Urach, Hegau, Hesse Graben) to the Czech Republic(Fig. 1). Magmatic activity in the province is related to a Cenozoic riftsystem which developed across Europe, stretching for a distance ofabout 1100 km (Prodehl et al., 1995; Dèzes et al., 2004). It producedlarge volumes of volcanic rocks typically associated with subvolcaniccomplexes within rift-related grabens and on their flanks. Magmaticrocks of the Bohemian segment are compositionally similar toanorogenic, silica-undersaturated sodic alkaline rocks from otherparts of the CEVP. The rocks range from melilitites, basanites, alkalibasalts and carbonatites to evolved rock types such as phonolites andtrachytes. The mantle source of the volcanic rocks was inferred to beless enriched in radiogenic isotopes compared to that of the MassifCentral and German segments (Lustrino and Wilson, 2007).

Magmatic activity of the province has been traditionally related tomantle plumes (Le Bas, 1987; Wilson and Downes, 1991). Mantleplumes beneath the Massif Central and the Eifel Mts. have beeninferred frommantle tomographic images by Granet et al. (1995) andRitter et al. (2001), respectively. However, the very existence of themantle plumes has recently been questioned. It has been pointed out,among other arguments that the plumes have not been detected byseismic surveys and cannot be thermally modeled (e.g., Anderson,2005). Although more recent studies have overcome some of thesedifficulties (Montelli et al., 2004; Farnetani and Samuel, 2005) andshowed that thermo-chemical plume models are viable, many ofthese approaches cannot be applied to ancient plumes or to areas withinsufficient geological information. In such cases, other criteria needto be used for plume identification. One of the recent models invokedfor the CEVP, the “hot fingers” model of Wilson and Patterson (2001)and Lustrino andWilson (2007), extends the plume definition to localpassive, diapiric upwellings of the partially melted uppermantle, withno need for significant thermal anomalies.

This paper presents major and trace element, isotopic andgeochronological whole-rock data for Cenozoic (including LateCretaceous) volcanic rocks from representative localities of theCzech part of the Bohemian Massif with special reference to thepaleostress states of the lithosphere. This information is used to

Lithos 123 (2011) 133–144

⁎ Corresponding author.E-mail address: ulrych@gli.cas.cz (J. Ulrych).

0024-4937/$ – see front matter © 2011 Elsevier B.V. All rights reserved.doi:10.1016/j.lithos.2010.12.008

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characterize and classify the volcanic activities and to constrain thecomposition of the magma sources and their variation through time.The volcanic periods and episodes were defined on the basis of the K–Ar ages of the characteristic rock associations and a correlation withthe tectonic history and paleostress fields in the Alpine foreland and inthe Bohemian Massif.

2. Geological setting

Cenozoic volcanic rocks in the BohemianMassif form an arc-shapedbelt which extends over 500 km from the western to the easternmostparts of themassif (Fig. 1). Themore prominentwestern segment of thebelt is a SW–NE-trending linear structure stretching between the twoNW–SE-striking fault systems. It includes the Ohře Rift Graben (EgerGraben)with the largest preserved amountsof volcanic rocks in thebelt.The eastern segment contains mainly isolated volcanic complexeswithin the Labe–Odra fault system (Fig. 2).

Cenozoic volcanics in the Bohemian Massif are associated withstructures either parallel or perpendicular to the Alpine tectonic front(Figs. 1 and 2). The distribution of volcanic rocks is mostly controlledby a ENE–WSW-trending rift structure about 280 km long. Its graben,the Ohře Rift Graben, extends for about 180 km and reaches amaximum width of 25 to 30 km in its central part (Kopecký, 1978;Pivec et al., 1998; Figs. 1 and 2). The rift is considered to be areactivated Variscan suture zone separating the Saxothuringian

crustal segment in the NW from the Moldanubian and Teplá–Barrandian segments in the SE (Kopecký, 1978; Babuška andPlomerová, 2001). This indicates a structural control on Cenozoicvolcanic activity (cf. Babuška et al., 2010).

The thickness of the seismic lithosphere beneath the western OhřeRift Graben is about 80 km (Babuška and Plomerová, 1992). Thedominant amounts (ca. 97 %) of the Cenozoic volcanic andvolcaniclastic rocks of the Bohemian Massif occur in two volcaniccentres within the Ohře Rift Graben. The graben axis is parallel to therift axis and its floor subsided by 300 to 600 m from the Mid Eoceneonwards. Principal marginal faults show normal dip-slip movement ofthe downthrown graben blocks.

Volcanic rocks of the two main associations (basanite–trachyteand nephelinite –phonolite) occur as far as 30 km outside of the OhřeRift Graben (Haase and Renno, 2008). Therefore, the melting zonebeneath the rift must have been much wider than the visible grabenlimits on the surface. Mantle xenoliths in basaltic rocks (Ulrych andAdamovič, 2004; Ackerman et al., 2007) are common in areasoverlying collisional boundaries between Variscan crustal segments.

The other tectonic zone is the NNW–SSE-trending Cheb–DomažliceGraben (Figs. 1 and 2) which represents a prominent asymmetricstructure in the western part of the Bohemian Massif, with volcanismoccurring prominently on its NE flank (Ulrych et al., 2003). Anotherstructure with minor volcanic activities is the broad NW–SE-trendingLabe–Odra fault system in the NE part of the Bohemian Massif (Fig. 2).

Fig. 1. Aschematicmap showing thedistributionof Cenozoic volcanic areas (blackfields anddots),main rift-related sedimentary basins (darkgrey) andVariscanmassifs (mediumgrey) inthe Central European region. Important post-Variscan faults are shown as thin black lines, Alpine thrust front and main Variscan sutures are shown as black and grey barbed lines,respectively. Grabens and volcanic regions: BG – Bresse Graben, CDG – Cheb–Domažlice Graben, E – Eifel Mts., H – Hegau, HG – Hesse Graben, LG – Limagne Graben, LRG – Lower RhineGraben, OG–Ohře Rift Graben, RG – Lower RhôneGraben, U–Urach, URG –Upper RhineGraben, V –VogelsbergMts. Variscanmassifs: BM–BohemianMassif, BF–Black Forest, HZ–HarzMts., VG – VosgesMts. Crustal segments of the Variscan orogen: RHEN – Rhenohercynian, SAX – Saxothuringian,MOLD –Moldanubian, TB – Teplá–Barrandian. Grabens, faults and Alpinefront in the western part of the map are taken from Dèzes et al. (2004).

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The magmatic activity in the Bohemian Massif lasted intermittentlyfrom the Late Cretaceous to the Quaternary (~79–0.26 Ma; Ulrych et al.,1999) and culminated in the Eocene to Miocene (~42–20 Ma).

2.1. Timing of volcanic activity and its paleostress background

The stress in the lithosphere influences the timing of volcanicactivities, their locations and the geometry of intrusive bodies. Risingmagma generally follows pre-existing fractures in zones of litho-spheric weakness, which are under tensional or transtensional stress.Also the preservation of volcanic rocks is largely dependent on thestress regime, with the largest amounts of lavas and volcaniclasticstypically occurring in rapidly subsiding graben blocks and in riftbasins.

Cenozoic paleostress fields affecting the Bohemian Massif havebeen interpreted from minor fault-slip data from the Lusatian Fault(Fig. 2; Coubal, 1990) and the Ohře Rift Graben (Coubal and Klein,1992; Coubal and Adamovič, 2000), following the method of stresstensor computation of Málek et al. (1991). In addition, the timesuccession takes into account the analysis of geometries of datedintrusive bodies (Adamovič and Coubal, 1999) and the shapes ofsedimentary bodies in the Cenozoic basins (Špičáková et al., 2000;Rajchl et al., 2009). A similar paleostress history has been inferred(Peterek et al., 1997) for the western border of the Bohemian Massif.

Rearrangements in stress conditions in the crust were associatedwith changes in the intensity and occurrences of volcanic activity andvariations in the composition of volcanic rocks. The frequencydistribution diagram of the K–Ar ages of the Cenozoic and Late

Cretaceous volcanic rocks of the Bohemian Massif complemented bypaleostress data is shown in Fig. 3.

Based on ages, geochemical and mineralogical characteristics ofvolcanic rocks (Table 1), and the paleostress chart for the BohemianMassif, three distinct periods of volcanic activity can be defined. Theyoungest period can be further subdivided into three magmaticepisodes (Fig. 3, Table 1):

1. Pre-rift period (Late Cretaceous to Mid Eocene, 79–49 Ma),compressional stress field.

2. Syn-rift period (Mid Eocene to Mid Miocene, 42–16 Ma), tensionalstress field.

3. Late-rift period (16–0.26 Ma)

3.1 Mid to Late Miocene episode (16–6 Ma), compressional stressfield.

3.2 Late Miocene to Early Pleistocene episode (6–0.9 Ma), tensionalstress field.

3.3 Early to Late Pleistocene episode (0.9–0.26 Ma), compressionalstress field.

2.1.1. Pre-rift period of volcanism: Late Cretaceous to MidEocene (79–49 Ma)

Thepre-rift periodof Late Cretaceous to Paleogeneage (79–49 Ma) ischaracterized by a melilitite–nephelinite series, which includes olivinemelilitolite, melilite lamprophyre (polzenite) and olivine melilitite/olivine nephelinite (Ulrych and Pivec, 1997; Pivec et al., 1998; Ulrychet al., 2008). These rocks are related to the initial stage of rifting or mayeven represent a precursor to the Eocene rifting. They differ from most

Fig. 2. Geological sketch map of the Bohemian Massif (BM) with indicated extent of Late Cretaceous to Cenozoic volcanic products. LF – Lusatian Fault.

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rocks of the CEVP by their composition and older age. They are mainlyrepresented by subvolcanic bodies emplaced into sediments of theBohemian Cretaceous Basin. Melilitic magmatism occurs mainly innorthern Bohemia, on the shoulders of the Ohře Rift Graben close to theintersection with faults of the Labe–Odra fault system.

The emplacement of the Late Cretaceous volcanic rocks in thenorthern Bohemian Massif only slightly postdates the earliest signs oftectonic inversion in the adjacent basins: in the Harz Mts. area, theonset of NNE–SSW compression has been dated at ca. 88 Ma (Voigtet al., 2006; Kley and Voigt, 2008). Most of the olivine nephelinite/melilitite dikes (71–49 Ma; Pivec et al., 1998; Adamovič and Coubal,1999) were probably emplaced prior to the main thrusting on theLusatian Fault. Their uniform NE–SW orientation suggests thedominance of the stress field with a NE–SW principal stress.

2.1.2. Syn-rift period of volcanism: Mid Eocene to MidMiocene (42–16 Ma)

The syn-rift period represents the dominant Cenozoic volcanism inthe Bohemian Massif, which produced two coeval series: the weaklyalkaline series of basanite–trachybasalt/alkali olivine basalt–trachyteand the strongly alkaline series of nephelinite–tephrite–phonolite.The rocks of these two series occur in the Ohře Rift Graben and itsshoulders, and in the Labe–Odra fault system (e.g., Ulrych et al., 2002).

Subsidence in the Ohře Rift Graben region commenced in the Midto Late Eocene. Shapes of intrusive bodies suggest an E–W-directedextension (Adamovič and Coubal, 1999). At 34 Ma, a graben started toevolve under a N–S tensional stress field (34–24 Ma; Adamovič andCoubal, 1999; Rajchl et al., 2009). The evolution of the graben wascompleted under a NW–SE tensional stress field in the Early to MidMiocene (24–16 Ma; Adamovič and Coubal, 1999; Rajchl et al., 2009),when continental sediments (up to 500 m thick) were deposited.

2.1.3. Late-rift period of volcanism (16–0.26 Ma)Volcanic activity during the Mid to Late Miocene episode

(episode 3.1–16 to 6 Ma old) is characterized by a rock associationof olivine foidites. The rocks were produced in the Ohře Rift Grabenand its shoulders, and in the Labe–Odra fault system. The ages ofthese rocks in the Ohře Rift Graben range from 13 to 9 Ma.However, the most voluminous rocks of this episode occur in the

Cheb–Domažlice Graben (12.5–8 Ma) in western Bohemia. Therocks predominantly belong to a weakly alkaline series of basanite/trachybasalt–(basaltic) trachyandesite–trachyte–rhyolite. Anotherseries, present only in minor amounts in western Bohemia, is astrongly alkaline olivine nephelinite–tephrite series (Pivec et al.,2003). This period was governed by two closely superimposedcompressional phases (e.g., Coubal and Adamovič, 2000), whichwere responsible for the tectonic inversion of the sedimentary fillof the Ohře Rift Graben and for transcurrent movements on faultsof the Labe–Odra fault zone.

The LateMiocene to Early Pleistocene episode (episode 3.2 with anage of 6 to 0.9 Ma) includes the olivine nephelinite to basanite lavasassociated with the Lusatian Fault (6.6 to 4.0 Ma – Šibrava andHavlíček, 1980) which were dated at ~5 Ma (Lustrino and Wilson,2007; Rapprich et al., 2007; Cajz et al., 2009). A similar petrologicalcharacter is displayed by volcanics of the Labe–Odra fault system innorthern Moravia and Silesia, dated at 3.4–1.94 Ma (Šibrava andHavlíček, 1980), 3.69–0.80 Ma (Foltýnová, 2003) and 4.58–0.91 Ma(Lustrino and Wilson, 2007). Cenozoic volcanics of Polish Silesia alsohave a similar chemical composition and age (5.5–3.8 – Birkenmajeret al., 2002).

During this time episode, the Bohemian Massif (Coubal andAdamovič, 2000) was under a NW–SE tensional field responsible forthe uplift of the northern Ohře Rift Graben shoulder. This environmentwas followedbyaNE–SWextension at theEarly/Late Plioceneboundary.

The Early Pleistocene episode (episode 3.3 dated at 0.9 to 0.26 Ma)encompasses the youngest volcanic rocks of the Bohemian Massifwhich are of olivine melilitite/olivine nephelinite composition. Therocks occur in the westernmost part of the Ohře Rift Graben at thejunction with the Cheb–Domažlice Graben in western Bohemia.Compositionally, they resemble those of the pre-rift period innorthern Bohemia (Ulrych et al., 2003). The ages show a considerablerange: 1.0–0.26 Ma (Šibrava and Havlíček, 1980); 0.43–0.11 Ma(Lustrino and Wilson, 2007); 0.9–0.17 Ma (Wagner et al., 2002).

Rare basanite–olivine nephelinite–olivine basalt rocks occurringalong the Labe–Odra fault system in northern Moravia and Silesia alsobelong to this episode. They yielded ages of 0.91 Ma (Lustrino andWilson, 2007), 0.80 Ma (Foltýnová, 2003); and 0.56 Ma (Pécskay et al.,2004).

Fig. 3. Age distribution of the Cenozoic volcanic rocks of the Bohemian Massif Volcanic Province based upon a set of more than 200 compiled K–Ar analyses (see text for thereferences) together with paleostress diagrams of individual time periods/episodes. The presented succession of paleostress fields is a synthesis of paleostress tensors interpretedfrom minor fault-slip data, geometries of dated intrusive bodies, and geometries of sedimentary bodies (see text for references). Vertical axis – frequency of K–Ar datings.

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3. Analytical procedures

The database for this study includes about 800 whole-rock majorand trace element analyses as well as about 200 K–Ar ages and Sr–Nd–(Pb) isotope analyses of the Cenozoic volcanic and subvolcanicrocks from the Bohemian Massif. The overwhelming majority of dataare the analyses published by our group (Ulrych et al., 1998, 2000a,b,2002, 2003, 2008, 2010), Ulrych and Pivec (1997), Pivec et al. (1998,2003, 2004), Řanda et al. (2003) and new analyses of phonolitic andtrachytic rocks, Plio-Pleistocene mafic volcanics and rocks of thedifferentiated weakly alkaline series of the Bohemian Massif given inAppendix A. The rest of the data come from Shrbený (1979, 1980,1982), Vaněčková et al. (1993), Lustrino and Wilson (2007) andUlrych et al. (2010); the isotope analyses are from Alibert et al. (1983,1987), Blusztain and Hart (1989), Bendl et al. (1993), Vokurka (1997),Lustrino and Wilson (2007), Haase and Renno (2008) and Cajz et al.(2009).

The new subset of 137 whole-rock chemical analyses includingtrace element determinations as well as 67 new 87Sr/86Sr and 143Nd/144Nd data and 51 K–Ar measurements is presented in Appendix A.

Thenewwhole-rockmajor elementconcentrationsweredeterminedat Charles University, Praha, using wet chemical methods. Analyses ofthe reference standards (GM, TB, BN) and duplicate analyses of thesamples yield total errors of ±5% (1σ). The ICP-MS (VG Elemental PQ3)was used for the determination of REE and other trace elements usingthemethodsof Strnad et al. (2005). The replicate analyses of BCR-2USGSstandard indicate values always better than ±5% (1σ).

The new K–Ar isotope measurements were carried out at theInstitute of Nuclear Research of the Hungarian Academy of Sciences,Debrecen, according to the procedures described in Balogh (1985).Standards LP-6 and HD-B1 have been used for the calibration.

The new Sm–Nd isotopic data were obtained at the isotopelaboratory atUniversitätMünchen according to the procedures outlinedin Hegner et al. (1995). 143Nd/144Nd ratios are normalized to 146Nd/144Nd=0.7219. The 143Nd/144Nd ratios of anAmesNd standard solutionyielded0.512142±12(2 s.d.,N=35), corresponding to 0.511852 in theLa Jolla reference material. Six measurements of La Jolla yielded 143Nd/144Nd=0.511847±8 (2 s.d.). Accuracy and external precision obtainedfor NIST 987 is: 87Sr/86Sr=0.710237±11 (2 s.d., N=18) afternormalization to 86Sr/88Sr=0.1194.

4. Results

Stress states of the lithosphere in the Pyrenean–Alpine–Carpathianforeland have been well established (Bergerat, 1987; Ziegler, 1987).Four phases of tectonic inversion have been identified (Ziegler, 1987;Ziegler et al., 1995) since the Late Cretaceous. Their effects on crustaldeformation have been recognized as largely synchronous over thewhole western and central Europe, with only subtle variations instress orientation and timing along major fault zones. This tectonicenvironment was overprinted by the Cenozoic rift structures whichstretch from the Lower Rhône Graben in the south across the Limagneand Bresse grabens and the Upper Rhine Graben to the Ohře RiftGraben in the NE (Fig. 1). This system, referred to as the EuropeanCenozoic Rift System – ECRIS (Ziegler, 1994; Prodehl et al., 1995;Dèzes et al., 2004) has been well characterized in terms of itsgeological structure and stress-state history.

In its tectonic evolution, the Ohře Rift Graben roughly parallelsother segments of the ECRIS. These segments started to subside in theLate Eocene, during the northerly advance of the Alpine orogenicwedge (Dèzes et al., 2004), although this subsidence was accompa-nied by minor or no volcanic activity. The main period of E–Wextension in the Oligocene (Bergerat, 1987) at the ECRIS has beenattributed to the combined stresses from the Central Alps and theculmination of the Iberia convergence in the Pyrenees (Hibsch et al.1995; Dèzes et al., 2004), with the principal compressive stressTa

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san

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ism

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hemianMassif.

Period

/episode

sof

volcan

ism

Stratigrap

hical

position

Age

(Ma)

Tecton

icregime

Rock

series

Region

sof

characteristic

occu

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distribu

tion

(MgO

N7wt.%

)RE

Edistribu

tion

(MgO

N7wt.%

)

87Sr/8

6Sr

(MgO

N7wt.%

)

143Nd/

144Nd

(MgO

N7wt.%

)

206 Pb/

204Pb

(MgO

N7wt.%

)

Pre-rift

Late

Cretaceo

usto

Mid

Eocene

79–49

Compression

Melilite

lamprop

hyre–ol.

melilitite/ol.ne

phelinite

Shou

ldersof

theOhřeRift,

NBo

hemia

(the

Plou

čnicearea)

Neg

ativepe

aksof

K,R

bNTh

,Zr,p

ositivepe

aksof

Nb,

BaStrong

enrich

men

tin

LREE

NoEu

anom

aly

0.70

32–

0.70

490.51

262–

0.51

290

Syn-

rift

Mid

Eocene

toMid

Miocene

42–16

Extens

ion

Ol.ne

phelinite/ba

sanite–

trachy

te.N

ephe

linite/

teph

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OhřeRift(stron

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Labe

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ne

Neg

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BaStrong

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NoEu

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0.70

32–

0.70

460.51

262–

0.51

302

19.4–19

.9

Late-rift

Mid

Miocene

toLate

Pleistoc

ene

16–0.26

Episod

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Mid

Miocene

toLate

Miocene

16–6

Compression

Trachy

basalt–trachy

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Nep

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teph

rite/basan

ite;

Picrob

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Ohř

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heb–

Dom

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n(d

ifferen

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series)

Neg

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aksof

K,R

b,Zr,

positive

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ofNb

Strong

enrich

men

tin

LREE

NoEu

anom

aly

0.70

34–

0.70

410.51

271–

0.51

289

Episod

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Late

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toEa

rly

Pleistoc

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6–0.9

Extens

ion

Teph

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Cheb

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ne(N

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NMorav

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ilesia)

Neg

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K,R

bNSr,

positive

peak

sof

Nb,

ThStrong

enrich

men

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LREE

NoEu

anom

aly

0.70

32–

0.70

350.51

262–

0.51

290

Episod

e3

EarlyPleistoc

ene

toLate

Pleistoc

ene

0.9–

0.26

Compression

Ol.melilitite

neph

elinite–

ol.

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Ohř

eRift,C

heb–

Dom

ažlic

eGrabe

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tion

(WBo

hemia)

Neg

ativepe

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K,R

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sof

Nb,

La,N

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enrich

men

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LREE

NoEu

anom

aly

0.70

32–

0.70

330.51

280–

0.51

282

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component being vertical. Thermal thinning of themantle lithospheretriggered the increased volcanism and uplift of the RhenishMassif andMassif Central in the Early Miocene (Dèzes et al., 2004; Ziegler andDèzes, 2007), well after the peak volcanic activity in the BohemianMassif. The Early Miocene episode of rapid subsidence in the Ohře RiftGraben related to the NW–SE extension was not observed in the otherECRIS segments.

4.1. Paleostress fields and volcanic activity

The subdivision of Cenozoic (including Late Cretaceous) volcanicactivity in the Bohemian Massif (Ulrych and Pivec, 1997) is consistentwith the succession of paleostress fields transmitted from thePyrenean–Alpine–Carpathian collisional front to its foreland.

Although the largest volumes of volcanic rocks are related to thelong-lasting tensional field at 42–16 Ma, relatively large amounts ofvolcanic rocks were also produced under compressive stresses. This isespecially the case of the melilitic rocks of the pre-rift period (79–49 Ma), whose real volumes may be underestimated due to theconsiderable uplift and erosion after their emplacement since onlysubvolcanic bodies are preserved. The K–Ar ages of these rockscoincide with the period of large-scale thrusting along the LusatianFault, and their geometry is conformable with the stress tensor of thattime. The latest compressive stress fields were also associated withsignificant volcanic activity dated at 0.9 Ma to present. These rockshave a melilitic composition.

Three major differences between volcanic bodies produced undertensional and compressive paleostress fields are:

1. Volcanic rocks produced under compressive stress fields generallyhave a primitive composition, mostly of olivine melilitite/olivinenephelinite or picrite. In contrast, rocks produced under tensionalstress fields, which occur in the Ohře Rift Graben and Cheb–Domažlice Graben have lower magnesium and higher silica contents.These alkaline volcanics represent a differentiation series with widecompositional variations.

2. Volcanic rocks coeval with compressive stresses show a spatialassociation with major faults. In the pre-rift period, mostlycharacterized by the NE–SW principal stress, a majority of intrusivebodies was emplaced in the footwall block of the Lusatian Fault.They occur as dykes oriented parallel to the maximum principalstress and extending as far as 30 km away from the main fault. Apaleostress control on the distribution of volcanic rocks is alsoevident during the late-rift period, especially during theWSW–ENEmaximum principal stress at ca. 11–6 Ma when magma ascendedalong the ENE trending marginal faults of the Ohře Rift Graben. ThePleistocene (0.9–0.26 Ma) volcanoes in western Bohemia, formedunder NW–SE compression, lie on thewesternmarginal fault of theNNW–ESE-trending Cheb–Domažlice Graben. On the other hand,the distribution of volcanic rocks coeval with periods of tensionalstress shows a much weaker structural control.

3. The relative amounts of bodies of explosive (sub)volcanic brecciaare higher during the periods of compressive stress as shown bythe melilitite/olivine nephelinite association of the pre-rift periodin northern Bohemia (79–49 Ma) and the late–rift period inwestern Bohemia (0.9–0.26 Ma).

4.2. Geochemistry of the volcanic series of the Bohemian Massif

The total alkalis vs silica diagrams (TAS; Le Maitre (Ed.), 2002),primitive mantle-normalized incompatible element diagrams and143Nd/144Nd vs 87Sr/86Sr and 208Pb/204Pb vs 206Pb/204Pb diagrams areused to compare and contrast the composition of the Cenozoicvolcanic rocks of the Bohemian Massif of the various time intervals(Figs. 4, 5, 6) and constrain the composition of their sources.

The TAS diagrams (Fig. 4) show significant differences between (a)the primitive ultramafic melilitic rocks represented by the melilitelamprophyre (polzenite)–olivine melilitite–olivine nephelinite/basa-nite series of the pre-rift period (1) and the olivine melilitite/olivinenephelinite–basanite series of the youngest episode 3.3. of the late-riftperiod (3) which have lower alkalis and SiO2 and (b) the volcanic rocksof all other periods. Melilitic volcanics of the pre-rift period (1) occuronly in northern Bohemia, and those of the late-rift period (3)/episode3.3. are only present in western Bohemia.

The syn-rift period (2) which is the dominant volcanic phase ischaracterized by synchronous weakly alkaline and strongly differen-tiated olivine nephelinite/basanite–trachyte series and the stronglyalkaline and strongly differentiated nepheline/tephrite–phonoliteseries.

The first episode of the late-rift period (episode 3.1.) produced theweakly alkaline and strongly differentiated trachybasalt–trachyte–rhyolite series and synchronous strongly alkaline and mildly differenti-ated (olivine) nephelinite–tephrite/basanite series which occur only inthe Cheb–Domažlice Graben.

The second episode of the late-rift period (episode 3.2.) includestephrite/basanite–trachybasalt and picrobasalt/olivine basalt associa-tions. Subalkaline rock samples accompanying these associations arestrongly altered volcaniclastic rocks with sedimentary material.

The primitive mantle-normalized incompatible element patterns ofthe basaltic rocks of periods 1 and 2 with MgO N7 wt.% have prominenttroughs of K, Rb and Th compared to the neighbouring Ba and Nb.Basaltic rocks of period 3 (episodes 1, 2, 3; Fig. 5) are characterized onlyby troughs of K and Nb. The chondrite-normalized REE patterns of thebasaltic samples with MgO N7 wt.% are similar, showing steep slopeswith strong enrichments in light REE (LREE) and high LaN/YbN (20–50)and GdN/YbN ( 2.5–5) ratios with no Eu anomaly.

The rocks show a wide range of the 87Sr/86Sr (0.7032 to 0.7050)and 143Nd/144Nd (0.51264 to 0.51301) isotopic ratios (Fig. 6). Thewidest range is shown by rocks of the syn-rift period. Trachytic andphonolitic rocks of the syn-rift period (Fig. 6) (Ulrych et al., 2006 andunpublished results – see Appendix A) are distinctly enriched inradiogenic Sr (with 87Sr/86Sr 0.7036–0.7096) but have only a limitedvariation in εNd values (1.3–4.8).

The 206Pb/204Pb (19.4–19.9) and 208Pb/204Pb (38.9–39.6) isotopicratios (Fig. 6) are available only for basaltic rocks from Silesia (Blusztainand Hart, 1989).

5. Discussion

The Late Cretaceous to Quaternary continental rift volcanism of theBohemian Massif is associated with three main zones: the Ohře RiftGraben, Labe–Odra fault system and the Cheb–Domažlice GrabenwithN97 vol.% of volcanic rocks present within the Ohře Rift Graben.The Upper Rhine Graben which represents a similar rift structure ofthe ECRIS (Ziegler, 1994) has most volcanic rocks concentrated in theup-domed graben shoulders (Keller et al., 1990). However, the samedevelopment of volcanism is also characteristic of the Cheb–Domažlice Graben (Ulrych et al., 2003).

The Cenozoic volcanism of the Bohemian Massif is of alkalinecharacter. Tholeiitic basaltic rocks accompanying alkali basalts, e.g. inthe Vogelsberg Mts. and Hesse Graben, were not found in theBohemian Massif. The volcanic rocks of all series are sodic with Na2O/K2O N1, corresponding to the anorogenic series of Lustrino andWilson(2007). The apoleucitic rocks rarely occur in themain syn-rift volcanicperiod. The apoleucitic basaltic rocks (nephelinite, basanite, tephrite)also have relatively low K2O contents (max. 2.8 wt.% K2O in tephrites –Shrbený, 1995). The ultrapotassic rocks sensu Foley et al. (1987); K2ON3 wt.% and K2O/Na2O N3) occur rarely as apoleucitic lamprophyres(camptonites and monchiquites with up to 6.6 wt.% K2O – Jelínek etal., 1989) and semilamprophyres (menaites with up to 8.2 wt.% K2O)in the Ohře Rift Graben. Rare tinguaite porphyry dykes (13.3 wt.% K2O

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Fig. 4. TAS diagram (Le Maitre (Ed.), 2002) showing data of the volcanic rock series of individual volcanic periods/episodes of the Bohemian Massif.

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and 0.25 wt.% Na2O) with apoleucite megacrysts (Pivec et al., 2004)occur in a shoulder of the Ohře Rift Graben (Ulrych et al., 2005).

The trace-element compositions of the volcanic rocks display atypical OIB signature with an enrichment of strongly incompatibleelement contents (Rb, Cs, Ba, Sr, Th, U) (Bogaard and Wörner, 2003).Negative anomalies of K and Rb accompanied by distinctive positivepeaks of Ba and Nb–(Ta) are characteristic of basaltic rocks ofanorogenic affinity of the CEVP (Lustrino and Wilson, 2007). The

negative K and Rb anomalies accompanied by high and variable K/Rbratios are typical of alkaline rocks of the CEVP. In particular, the K/Rbratios of the basaltic rocks of the Bohemian Massif are high (250 to500), implying the presence of residual pargasitic/kaersutitic amphi-bole in the source. Incompatible element ratios such as Zr/Y (3–17),Zr/Nb (1–5) and Nb/Yb (10–150) suggest an OIB magmatic reservoirfor all these rocks (Wilson et al., 1995; Ulrych and Pivec, 1997). Rocksof the pre-rift period have higher Nb concentrations than those of the

Fig. 5. Primitive mantle-normalized trace element data of the primitive volcanics (MgO N7 wt.%) of individual volcanic periods/episodes of the Bohemian Massif. Normalizationvalues from Sun and McDonough (1989).

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Fig. 6. Initial 87Sr/86Sr and 143Nd/144Nd isotopic ratios for volcanic rocks of the pre-rift, syn-rift and late-rift periods of the Bohemian Massif. Symbols as in Fig. 4. Isotopic data fortrachyte–phonolite of the syn-rift period are from Ulrych et al. (2006 and unpublished results). The 208Pb/204Pb vs 206Pb/204Pb diagram for volcanic rocks of the syn-rift period fromSilesia are from Blusztain and Hart (1989).

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other periods, and their Zr/Nb ratio is very low (~1). Ce/Yb ratios fornearly all rocks with MgON7 wt.% range from 10 to 25, indicating alow degree of partial melting of the mantle source. This assumption isalso supported by low HREE contents in the near-primary magne-sium-rich basaltic rocks (Mattsson and Oskarsson, 2005). The low andfractionated HREE abundances suggest the presence of residual garnetin the source.

The lithosphere beneath the CEVP is slightly heterogeneous interms of thickness (Babuška and Plomerová, 1988) and composition(Lloyd, 1987; Wilson and Downes, 1991; Wedepohl et al., 1994;Downes, 2001; Lustrino andWilson, 2007). There is some evidence oflocal heterogeneities including the presence of phlogopitite tophlogopite clinopyroxenite xenoliths in olivine melilitolite of thepre-rift period (Ulrych et al., 2000c) and metasomatized lherzolitexenoliths with amphibole and/or phlogopite in basaltic rocks (Kramerand Seifert, 2000; Frýda and Fediuk, 1996; Geissler et al., 2008).Cryptic metasomatism of lithospheric mantle was invoked to explainchemical composition of clinopyroxene and interstitial glass inlherzolites (Ackerman et al., 2007).

The basaltic rocks of the Bohemian Massif show a wide range of87Sr/86Sr and 143Nd/144Nd isotopic ratios although the values aresimilar to the European Asthenospheric Reservoir – EAR (Cebrià andWilson, 1995). The ranges of variations are similar across the wholeEAR area and through the time span when the Cenozoic volcanismwas active. The trachytic and phonolitic rocks occurring in the syn-riftvolcanic suites of the Ohře Rift Graben and the late-rift period/episode1 of the Cheb–Domažlice Graben are enriched in radiogenic Sr mostlydue to a lithospheric contamination (Ulrych et al., 2003). The 206Pb/204Pb and 208Pb/204Pb isotopic ratios of Blusztain and Hart (1989) forthe volcanic rocks indicate a typical OIB-HIMU affinity of the mantlesource of magma. On the basis of these data, Blusztain and Hart(1989), Bendl et al. (1993) and Lustrino and Wilson (2007) inferredthat the mantle beneath the Bohemian Massif is more primitive thanmantle beneath the Massif Central and the Rhenish Massif. They alsosuggested that the isotopic composition of themantle source for thesebasaltic rocks is a long-term depleted mantle representing a mixtureof DMM, HIMU and EM mantle components.

Volcanic rocks of the syn-rift period occur primarily within the OhřeRift Graben, where they produced several lithostratigraphic units withthe total thickness of up to 400 m(Cajz et al., 1999; 2009). Three units areof basanitic to olivine nephelinitic composition and one is of trachyba-saltic to trachyandesitic composition. Basanitic suites differ from that oftrachybasaltic rocks by their geochemical and especially isotopicsignature 87Sr/86Sr 0.70318–0.70376 vs 0.70433–0.70472 and 143Nd/144Nd 0.51284–0.51287 vs 0.51270–0.51276 (Ulrych et al., 2002; Cajzet al., 2009). The isotopic analyses of trachybasaltic to trachyandesiticrocks indicate either a partly heterogeneousmantle source ormore likelya crustal contamination of parental magma during its ascent. Neverthe-less, data for the rocks of all these formations lie within a span of basalticrocks of the Bohemian Massif (87Sr/86Sr 0.7031–0.7047; 143Nd/144Nd0.51267–0.51301– Lustrino and Wilson, 2007).

6. Conclusions

1. The temporal and spatial distribution of the volcanic rocks,mineralogical and geochemical characteristics of individual volcanicrock series in combinationwith paleostress data and tectonic settingallowed a new subdivision of volcanic activity in the BohemianMassif. Three main periods (pre-rift, syn-rift and late-rift) of post-Late Cretaceous volcanic activity have been established:(a) The pre-rift period (79–49 Ma) was dominated by a relatively

uniform NE–SW compressive regional stress field, coeval withlarge-scale thrusting and corresponding to the Sub-Hercynianand Laramide phases (Ziegler, 1987).

(b) The syn-rift period (42–16 Ma) with the voluminous volcanicrocks preserved in the Ohře Rift Graben regionwas dominated

by tensional stress fields with variable orientations of theprincipal stress component.

(c) The late-rift period is subdivided into three episodes: the Midto Late Miocene episode of compressive stress (16–6 Ma), theLate Miocene to Early Pleistocene episode of tensional stress(6–0.9 Ma) and the Pleistocene episode of compressive stress(0.9–0.26 Ma).

Volcanism of the syn-rift period was dominant in the BohemianMassif withN97 vol.% of the volcanic rocks occurring within theOhře Rift Graben. Volcanic suites emplaced under the compressivestress fields are mostly of a primitive composition; they are themelilitic ultramafic rocks of the pre-rift period and the finalepisode of the late-rift period. Compositionally, the near-primaryvolcanism (N7 wt.% MgO) of all periods is very similar,corresponding to nephelinite–basanite/tephrite rock series.

2. The differentiation of the primarymagmaswhich occurs within thesyn-rift period in the Ohře Rift Graben, produced strongly alkaline(nephelinite–tephrite–phonolite) and weakly alkaline (olivinenephelinite–basanite–trachyte) series. A different differentiationprocess is proposed for the Cheb–Domažlice Graben externalblocks with differentiated weakly alkaline trachybasalt– (basaltic)trachyandesite–trachyte–rhyolite series and synchronous commonstrongly alkaline (olivine) nephelinite–tephrite/basanite series.

3. Lithospheric mantle beneath the Bohemian Massif (the source ofbasaltic magmas) is compositionally only slightly heterogeneous.Modal metasomatism manifested by the presence of K-, (OH, F)-bearing phases in lherzolitic xenoliths is rare. Cryptic metasoma-tism of the lithospheric mantle was described in metasomatizedlherzolites (the presence of clinopyroxene and glass – Ackermanet al., 2007).

4. The 87Sr/86Sr isotopic ratios of volcanic rocks range from 0.7032 to0.7050 and 143Nd/144Nd from 0.51264 to 0.51301. The ratios aresimilar to those of the European Asthenospheric Reservoir (EAR –

Cebrià and Wilson, 1995); they, however, belong among the mostdepleted compositions in the CEVP.

5. Cenozoic volcanic activity was controlled by the Variscan zones ofweakness (oceanic paleosuture, syn- and post-collisional wrenchfaults, and domains of crustal and lithospheric thinning). Inaddition to the mechanical aspects of these Variscan structures,the fertilization/metasomatism of the upper mantle by upwellingasthenosphere during the late- and post-collisional Variscanphases took place near the paleosuture and in domains oflithospheric extension. In the Cenozoic, reactivation of steep NE–SW- to NW–SE-striking trans-lithospheric fault systems allowedthe rapid ascent of basaltic magmas with common peridotitexenoliths in the Ohře Rift Graben and the Labe–Odra fault system.

6. Geochemical similarities of the Cenozoic volcanic products andPermo-Carboniferous volcanic rocks of the same area (Ulrych et al.,2002) imply that the HIMU-like source already existed in Permiantimes and was generated by Devonian subduction-related metaso-matism of the mantle lithosphere (cf. Lustrino and Wilson, 2007).

Acknowledgements

This research was supported by the Czech Science Foundationproject No. 205/09/1170, and by the Grant Agency of the Academy ofSciences of the Czech Republic project IAA300130902 within theResearch Programme of the Institute of Geology, v. v. i., CEZ:AV0Z30130516 and MSM 0021620855 of the Charles University. K–Ar dating was supported by OTKA projects Nos. T043344, T060965and M41434 to K. Balogh. We are indebted to J. Pavková and J.Rajlichová of the Institute of Geology AS CR for the technicalassistance, and to Guest Editor B. Murphy and J. Greenough and ananonymous reviewer for their comments and improvement of themanuscript.

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Appendix A. Supplementary data

Supplementary data to this article can be found online atdoi:10.1016/j.lithos.2010.12.008.

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