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Available online 22 December 2010

Keywords:

ism of the Bohemian Massif is an integral part of the Central European Volcanic

Lithos 123 (2011) 133144

Contents lists available at ScienceDirect

Lith

.e lCenozoic 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; Dzes et al., 2004). It producedlarge volumes of volcanic rocks typically associated with subvolcaniccomplexes within rift-related grabens and on their anks. 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, alkali

plumes 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 thesedifculties (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 withinsufcient geological information. In such cases, other criteria needto be used for plume identication. One of the recent models invokedfor the CEVP, the hot ngers model of Wilson and Patterson (2001)basalts and carbonatites to evolved rock typetrachytes. The mantle source of the volcanicless enriched in radiogenic isotopes comparCentral and German segments (Lustrino and

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

0024-4937/$ see front matter 2011 Elsevier B.V. Aldoi:10.1016/j.lithos.2010.12.008Magmatic activity of the province has been traditionally related tomantle plumes (Le Bas, 1987; Wilson and Downes, 1991). Mantle1. IntroductionBohemian MassifCenozoicAlkaline volcanismPaleostress eldsRiftMantlevolcanic activity. Threemain volcanic periods can be distinguished based on KAr data and known paleostresselds: (i) pre-rift (7949 Ma), (ii) syn-rift (4216 Ma) and (iii) late-rift (160.3 Ma), with the youngestperiod further subdivided into three episodes. The dominant mac rock types (N7 wt.%MgO) of all periods areof nephelinitebasanite/tephrite composition. The exceptions are suites of melilitic ultramac rocks of thepre-rift period in northern Bohemia and of the nal episode of the late-rift period in western Bohemia. Themost voluminous are volcanic rocks of the syn-rift period occurring in the Ohe Rift Graben.The initial 87Sr/86Sr (0.7032 to 0.7050) and 143Nd/144Nd (0.51264 to 0.51301) ratios of the mac 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.s such as phonolites androcks was inferred to beed to that of the MassifWilson, 2007).

and Lustrino andpassive, diapiricno need for sign

This papergeochronologicaCretaceous) volCzech part of thpaleostress state

l rights reserved.d paleostress data are used to characterize and classify thisReceived 11 June 2010Accepted 14 December 2010Province. The temporal and spatial distribution of volcanic rocks in the Bohemian Massif, their geochemistryand mineralogy as well as their tectonic setting anArticle history: Cenozoic anorogenic volcanRecurrent Cenozoic volcanic activity in th

Jaromr Ulrych a,, Jaroslav Dostal b, Ji Adamovi a, EErnst Hegner e, Kadosa Balogh f

a Institute of Geology v.v.i., Academy of Sciences of the Czech Republic, Rozvojov 269, 16b Department of Geology, Saint Mary's University, Halifax, Nova Scotia, Canada B3H 3C3c Institute of Geochemistry, Mineralogy and Mineral Resources, Faculty of Sciences, Charled Institute of Geophysics v.v.i., Academy of Sciences of the Czech Republic, Bon II, 141 31e Department of Geowissenschaften, Universitt Mnchen, Theresiennstrae 41, D-80333f Institute of Nuclear Research, Hungarian Academy of Sciences, Bem tr 18/C, H-4026 Deb

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

j ourna l homepage: wwwBohemian Massif (Czech Republic)

il Jelnek c, Petr paek d,

Praha 6, Czech Republic

iversity, Albertov 6, 128 43 Praha 2, Czech Republicha 4, Czech Republicchen, Germanyn, Hungary

os

sev ie r.com/ locate / l i thosWilson (2007), extends the plume denition to localupwellings of the partially melted uppermantle, withicant thermal anomalies.presents major and trace element, isotopic andl whole-rock data for Cenozoic (including Latecanic rocks from representative localities of thee Bohemian Massif with special reference to thes of the lithosphere. This information is used to

134 J. Ulrych et al. / Lithos 123 (2011) 133144characterize and classify the volcanic activities and to constrain thecomposition of the magma sources and their variation through time.The volcanic periods and episodes were dened on the basis of the KAr ages of the characteristic rock associations and a correlation withthe tectonic history and paleostress elds 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 SWNE-trending linear structure stretching between the twoNWSE-striking fault systems. It includes the Ohe Rift Graben (EgerGraben)with the largest preserved amountsof volcanic rocks in thebelt.The eastern segment contains mainly isolated volcanic complexeswithin the LabeOdra 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 ENEWSW-trending rift structure about 280 km long. Its graben,the Ohe Rift Graben, extends for about 180 km and reaches amaximum width of 25 to 30 km in its central part (Kopeck, 1978;Pivec et al., 1998; Figs. 1 and 2). The rift is considered to be areactivated Variscan suture zone separating the Saxothuringian

Fig. 1. Aschematicmap showing thedistributionof Cenozoic volcanic areas (blackelds anddotthe Central European region. Important post-Variscan faults are shown as thin black lines, Arespectively. Grabens and volcanic regions: BG Bresse Graben, CDG ChebDomalice GrabeGraben, OGOhe Rift Graben, RG Lower RhneGraben, UUrach, URG Upper RhineGrabenMts., VG VosgesMts. Crustal segments of the Variscan orogen: RHEN Rhenohercynian, SAX front in the western part of the map are taken from Dzes et al. (2004).crustal segment in the NW from the Moldanubian and TeplBarrandian segments in the SE (Kopeck, 1978; Babuka andPlomerov, 2001). This indicates a structural control on Cenozoicvolcanic activity (cf. Babuka et al., 2010).

The thickness of the seismic lithosphere beneath the western OheRift Graben is about 80 km (Babuka and Plomerov, 1992). Thedominant amounts (ca. 97 %) of the Cenozoic volcanic andvolcaniclastic rocks of the Bohemian Massif occur in two volcaniccentres within the Ohe Rift Graben. The graben axis is parallel to therift axis and its oor 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 (basanitetrachyteand nephelinite phonolite) occur as far as 30 km outside of the OheRift 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 andAdamovi, 2004; Ackerman et al., 2007) are common in areasoverlying collisional boundaries between Variscan crustal segments.

The other tectonic zone is the NNWSSE-trending ChebDomaliceGraben (Figs. 1 and 2) which represents a prominent asymmetricstructure in the western part of the Bohemian Massif, with volcanismoccurring prominently on its NE ank (Ulrych et al., 2003). Anotherstructure with minor volcanic activities is the broad NWSE-trendingLabeOdra fault system in the NE part of the Bohemian Massif (Fig. 2).

s),main rift-related sedimentary basins (darkgrey) andVariscanmassifs (mediumgrey) inlpine thrust front and main Variscan sutures are shown as black and grey barbed lines,n, E Eifel Mts., H Hegau, HG Hesse Graben, LG Limagne Graben, LRG Lower Rhine, V VogelsbergMts. Variscanmassifs: BMBohemianMassif, BFBlack Forest, HZHarzSaxothuringian,MOLD Moldanubian, TB TeplBarrandian. Grabens, faults and Alpine

135J. Ulrych et al. / Lithos 123 (2011) 133144The magmatic activity in the Bohemian Massif lasted intermittentlyfrom the Late Cretaceous to the Quaternary (~790.26 Ma; Ulrych et al.,1999) and culminated in the Eocene to Miocene (~4220 Ma).

2.1. Timing of volcanic activity and its paleostress background

The stress in the lithosphere inuences 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 elds affecting the Bohemian Massif havebeen interpreted from minor fault-slip data from the Lusatian Fault(Fig. 2; Coubal, 1990) and the Ohe Rift Graben (Coubal and Klein,1992; Coubal and Adamovi, 2000), following the method of stresstensor computation of Mlek et al. (1991). In addition, the timesuccession takes into account the analysis of geometries of datedintrusive bodies (Adamovi and Coubal, 1999) and the shapes ofsedimentary bodies in the Cenozoic basins (pikov 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 KAr ages of the Cenozoic and Late

Fig. 2. Geological sketch map of the Bohemian Massif (BM) with indicated extCretaceous 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 dened. Theyoungest period can be further subdivided into three magmaticepisodes (Fig. 3, Table 1):

1. Pre-rift period (Late Cretaceous to Mid Eocene, 7949 Ma),compressional stress eld.

2. Syn-rift period (Mid Eocene to Mid Miocene, 4216 Ma), tensionalstress eld.

3. Late-rift period (160.26 Ma)

3.1 Mid to Late Miocene episode (166 Ma), compressional stresseld.

3.2 Late Miocene to Early Pleistocene episode (60.9 Ma), tensionalstress eld.

3.3 Early to Late Pleistocene episode (0.90.26 Ma), compressionalstress eld.

2.1.1. Pre-rift period of volcanism: Late Cretaceous to MidEocene (7949 Ma)

Thepre-rift periodof Late Cretaceous to Paleogeneage (7949 Ma) ischaracterized by a melilititenephelinite 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

ent of Late Cretaceous to Cenozoic volcanic products. LF Lusatian Fault.

ic Ppreime

136 J. Ulrych et al. / Lithos 123 (2011) 133144rocks 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 Ohe Rift Graben close to theintersection with faults of the LabeOdra 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 NNESSW compression has been dated at ca. 88 Ma (Voigtet al., 2006; Kley and Voigt, 2008). Most of the olivine nephelinite/melilitite dikes (7149 Ma; Pivec et al., 1998; Adamovi and Coubal,1999) were probably emplaced prior to the main thrusting on the

Fig. 3. Age distribution of the Cenozoic volcanic rocks of the Bohemian Massif Volcanreferences) together with paleostress diagrams of individual time periods/episodes. Thefrom minor fault-slip data, geometries of dated intrusive bodies, and geometries of sedLusatian Fault. Their uniform NESW orientation suggests thedominance of the stress eld with a NESW principal stress.

2.1.2. Syn-rift period of volcanism: Mid Eocene to MidMiocene (4216 Ma)

The syn-rift period represents the dominant Cenozoic volcanism inthe Bohemian Massif, which produced two coeval series: the weaklyalkaline series of basanitetrachybasalt/alkali olivine basalttrachyteand the strongly alkaline series of nephelinitetephritephonolite.The rocks of these two series occur in the Ohe Rift Graben and itsshoulders, and in the LabeOdra fault system (e.g., Ulrych et al., 2002).

Subsidence in the Ohe Rift Graben region commenced in the Midto Late Eocene. Shapes of intrusive bodies suggest an EW-directedextension (Adamovi and Coubal, 1999). At 34 Ma, a graben started toevolve under a NS tensional stress eld (3424 Ma; Adamovi andCoubal, 1999; Rajchl et al., 2009). The evolution of the graben wascompleted under a NWSE tensional stress eld in the Early to MidMiocene (2416 Ma; Adamovi 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 (160.26 Ma)Volcanic activity during the Mid to Late Miocene episode

(episode 3.116 to 6 Ma old) is characterized by a rock associationof olivine foidites. The rocks were produced in the Ohe Rift Grabenand its shoulders, and in the LabeOdra fault system. The ages ofthese rocks in the Ohe Rift Graben range from 13 to 9 Ma.However, the most voluminous rocks of this episode occur in theChebDomalice Graben (12.58 Ma) in western Bohemia. Therocks predominantly belong to a weakly alkaline series of basanite/trachybasalt(basaltic) trachyandesitetrachyterhyolite. Anotherseries, present only in minor amounts in western Bohemia, is astrongly alkaline olivine nephelinitetephrite series (Pivec et al.,2003). This period was governed by two closely superimposedcompressional phases (e.g., Coubal and Adamovi, 2000), whichwere responsible for the tectonic inversion of the sedimentary llof the Ohe Rift Graben and for transcurrent movements on faultsof the LabeOdra 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 lavas

rovince based upon a set of more than 200 compiled KAr analyses (see text for thesented succession of paleostress elds is a synthesis of paleostress tensors interpretedntary bodies (see text for references). Vertical axis frequency of KAr datings.associated with the Lusatian Fault (6.6 to 4.0 Ma ibrava andHavlek, 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 LabeOdra fault system innorthern Moravia and Silesia, dated at 3.41.94 Ma (ibrava andHavlek, 1980), 3.690.80 Ma (Foltnov, 2003) and 4.580.91 Ma(Lustrino and Wilson, 2007). Cenozoic volcanics of Polish Silesia alsohave a similar chemical composition and age (5.53.8 Birkenmajeret al., 2002).

During this time episode, the Bohemian Massif (Coubal andAdamovi, 2000) was under a NWSE tensional eld responsible forthe uplift of the northern Ohe Rift Graben shoulder. This environmentwas followedbyaNESWextension 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 Ohe Rift Graben at thejunction with the ChebDomalice Graben in western Bohemia.Compositionally, they resemble those of the pre-rift period innorthern Bohemia (Ulrych et al., 2003). The ages show a considerablerange: 1.00.26 Ma (ibrava and Havlek, 1980); 0.430.11 Ma(Lustrino and Wilson, 2007); 0.90.17 Ma (Wagner et al., 2002).

Rare basaniteolivine nepheliniteolivine basalt rocks occurringalong the LabeOdra fault system in northern Moravia and Silesia alsobelong to this episode. They yielded ages of 0.91 Ma (Lustrino andWilson, 2007), 0.80 Ma (Foltnov, 2003); and 0.56 Ma (Pcskay et al.,2004).

Table1

Geological,petrologicalandgeochemicalcharacteristicsof

rocksof

theLate

Cretacou

sandCeno

zoicvolcanism

intheBo

hemianMassif.

Period

/episodes

ofvolcanism

Stratigraphical

position

Age

(Ma)

Tecton

icregime

Rock

series

Region

sof

characteristic

occurrence

Incompatibleelem

ent

distribu

tion

(MgO

N7wt.%

)RE

Edistribu

tion

(MgO

N7wt.%

)

87Sr/86Sr

(MgO

N7wt.%

)

143Nd/

144Nd

(MgO

N7wt.%

)

206 Pb/

204Pb

(MgO

N7wt.%

)

Pre-rift

Late

Cretaceous

toMid

Eocene

7949

Compression

Melilite

lamprop

hyreol.

melilitite/ol.neph

elinite

ShouldersoftheOheRift,

NBohemia(the

Plou

nicearea)

Negativepeaksof

K,R

bNTh

,Zr,p

ositivepeaksof

Nb,Ba

Strong

enrichment

inLREE

NoEu

anom

aly

0.70

32

0.70

490.51

262

0.51

290

Syn-rift

Mid

Eocene

toMid

Miocene

4216

Extension

Ol.neph

elinite/basanite

trachy

te.N

ephelin

ite/

teph

riteph

onolite

OheRift(stron

glydifferentiated

series),Krunho

ryMts.,

Labe

OdraZone

Negativepeaksof

K,R

bNTh

,P,po

sitive

peaksof

Nb,Ba

Strong

enrichment

inLREE

NoEu

anom

aly

0.70

32

0.70

460.51

262

0.51

302

19.419

.9

Late-rift

Mid

Miocene

toLate

Pleistocene

160.26

Episod

e1

Mid

Miocene

toLate

Miocene

166

Compression

Trachy

basalttrachyterhyolite.

Nephelin

iteteph

rite/basanite;

Picrobasalt

OheRift,C

heb

Dom

alice

Graben(differentiated

series)

Negativepeaksof

K,Rb,Zr,

positive

peak

ofNb

Strong

enrichment

inLREE

NoEu

anom

aly

0.70

34

0.70

410.51

271

0.51

289

Episod

e2

LateMiocene

toEarly

Pleistocene

60.9

Extension

Teph

rite/basanite.

Picrob

asalt/ol.basalt

Cheb

Dom

aliceGraben,

Labe

OdraZone

(NBo

hemia,

NMoravia,Silesia)

Negativepeaksof

K,RbNSr,

positive

peaksof

Nb,Th

Strong

enrichment

inLREE

NoEu

anom

aly

0.70

32

0.70

350.51

262

0.51

290

Episod

e3

EarlyPleistocene

toLate

Pleistocene

0.9

0.26

Compression

Ol.melilitite

neph

eliniteol.

melilitite

OheRift,C

heb

Dom

alice

Grabenjunction

(WBo

hemia)

Negativepeaksof

K,RbNTh

,po

sitive

peaksof

Nb,La,N

dStrong

enrichment

inLREE

NoEu

anom

aly

0.70

32

0.70

330.51

280

0.51

282

137J. Ulrych et al. / Lithos 123 (2011) 1331443. Analytical procedures

The database for this study includes about 800 whole-rock majorand trace element analyses as well as about 200 KAr ages and SrNd(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), anda et al. (2003) and new analyses of phonolitic andtrachytic rocks, Plio-Pleistocene mac volcanics and rocks of thedifferentiated weakly alkaline series of the Bohemian Massif given inAppendix A. The rest of the data come from Shrben (1979, 1980,1982), Vankov 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 KAr 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% (1). 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% (1).

The new KAr 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 SmNd isotopic data were obtained at the isotopelaboratory atUniversittMnchen 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 solutionyielded0.51214212(2 s.d.,N=35), corresponding to 0.511852 in theLa Jolla reference material. Six measurements of La Jolla yielded 143Nd/144Nd=0.5118478 (2 s.d.). Accuracy and external precision obtainedfor NIST 987 is: 87Sr/86Sr=0.71023711 (2 s.d., N=18) afternormalization to 86Sr/88Sr=0.1194.

4. Results

Stress states of the lithosphere in the PyreneanAlpineCarpathianforeland have been well established (Bergerat, 1987; Ziegler, 1987).Four phases of tectonic inversion have been identied (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 Rhne Graben in the south across the Limagneand Bresse grabens and the Upper Rhine Graben to the Ohe RiftGraben in the NE (Fig. 1). This system, referred to as the EuropeanCenozoic Rift System ECRIS (Ziegler, 1994; Prodehl et al., 1995;Dzes et al., 2004) has been well characterized in terms of itsgeological structure and stress-state history.

In its tectonic evolution, the Ohe Rift Graben roughly parallelsother segments of the ECRIS. These segments started to subside in theLate Eocene, during the northerly advance of the Alpine orogenicwedge (Dzes et al., 2004), although this subsidence was accompa-nied by minor or no volcanic activity. The main period of EWextension 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; Dzes et al., 2004), with the principal compressive stress

138 J. Ulrych et al. / Lithos 123 (2011) 1331444.1. Paleostress elds 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 elds transmitted from thePyreneanAlpineCarpathian collisional front to its foreland.

Although the largest volumes of volcanic rocks are related to thelong-lasting tensional eld at 4216 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 (7949 Ma), whose real volumes may be underestimated due to theconsiderable uplift and erosion after their emplacement since onlysubvolcanic bodies are preserved. The KAr 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 elds were also associated withsignicant 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 elds are:

1. Volcanic rocks produced under compressive stress elds generallyhave a primitive composition, mostly of olivine melilitite/olivinenephelinite or picrite. In contrast, rocks produced under tensionalstress elds, which occur in the Ohe Rift Graben and ChebDomalice 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 NESW 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 theWSWENEmaximum principal stress at ca. 116 Ma when magma ascendedalong the ENE trending marginal faults of the Ohe Rift Graben. ThePleistocene (0.90.26 Ma) volcanoes in western Bohemia, formedunder NWSE compression, lie on thewesternmarginal fault of theNNWESE-trending ChebDomalice 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 (7949 Ma) and the laterift period inwestern Bohemia (0.90.26 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 intervalscomponent being vertical. Thermal thinning of themantle lithospheretriggered the increased volcanism and uplift of the RhenishMassif andMassif Central in the Early Miocene (Dzes et al., 2004; Ziegler andDzes, 2007), well after the peak volcanic activity in the BohemianMassif. The Early Miocene episode of rapid subsidence in the Ohe RiftGraben related to the NWSE extension was not observed in the otherECRIS segments.(Figs. 4, 5, 6) and constrain the composition of their sources.The TAS diagrams (Fig. 4) show signicant differences between (a)the primitive ultramac melilitic rocks represented by the melilitelamprophyre (polzenite)olivine melilititeolivine nephelinite/basa-nite series of the pre-rift period (1) and the olivine melilitite/olivinenephelinitebasanite 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/basanitetrachyte series and the stronglyalkaline and strongly differentiated nepheline/tephritephonoliteseries.

The rst episode of the late-rift period (episode 3.1.) produced theweakly alkaline and strongly differentiated trachybasalttrachyterhyolite series and synchronous strongly alkaline and mildly differenti-ated (olivine) nephelinitetephrite/basanite series which occur only inthe ChebDomalice Graben.

The second episode of the late-rift period (episode 3.2.) includestephrite/basanitetrachybasalt 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 (2050)and GdN/YbN ( 2.55) 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 0.70360.7096) but have only a limitedvariation in Nd values (1.34.8).

The 206Pb/204Pb (19.419.9) and 208Pb/204Pb (38.939.6) 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 Ohe RiftGraben, LabeOdra fault system and the ChebDomalice GrabenwithN97 vol.% of volcanic rocks present within the Ohe 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 ChebDomalice 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 Shrben, 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 Jelnek etal., 1989) and semilamprophyres (menaites with up to 8.2 wt.% K2O)

in the Ohe Rift Graben. Rare tinguaite porphyry dykes (13.3 wt.% K2O

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

139J. Ulrych et al. / Lithos 123 (2011) 133144

140 J. Ulrych et al. / Lithos 123 (2011) 133144and 0.25 wt.% Na2O) with apoleucite megacrysts (Pivec et al., 2004)occur in a shoulder of the Ohe 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 Wrner, 2003).Negative anomalies of K and Rb accompanied by distinctive positivepeaks of Ba and Nb(Ta) are characteristic of basaltic rocks ofanorogenic afnity of the CEVP (Lustrino and Wilson, 2007). The

Fig. 5. Primitive mantle-normalized trace element data of the primitive volcanics (MgO N7values from Sun and McDonough (1989).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 (317),Zr/Nb (15) and Nb/Yb (10150) 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

wt.%) of individual volcanic periods/episodes of the Bohemian Massif. Normalization

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 fortrachytephonolite 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).

141J. Ulrych et al. / Lithos 123 (2011) 133144

episode of the late-rift period. Compositionally, the near-primaryvolcanism (N7 wt.% MgO) of all periods is very similar,

142 J. Ulrych et al. / Lithos 123 (2011) 133144other 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 (Babuka and Plomerov, 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; Frda 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 (Cebri 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 Ohe Rift Graben and the late-rift period/episode1 of the ChebDomalice 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 afnity 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 OheRift 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 0.703180.70376 vs 0.704330.70472 and 143Nd/144Nd 0.512840.51287 vs 0.512700.51276 (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 0.70310.7047; 143Nd/144Nd0.512670.51301 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 (7949 Ma) was dominated by a relatively

uniform NESW compressive regional stress eld, coeval withlarge-scale thrusting and corresponding to the Sub-Hercynianand Laramide phases (Ziegler, 1987).

(b) The syn-rift period (4216 Ma) with the voluminous volcanic

rocks preserved in the Ohe Rift Graben regionwas dominatedcorresponding to nephelinitebasanite/tephrite rock series.2. The differentiation of the primarymagmaswhich occurs within the

syn-rift period in the Ohe Rift Graben, produced strongly alkaline(nephelinitetephritephonolite) and weakly alkaline (olivinenephelinitebasanitetrachyte) series. A different differentiationprocess is proposed for the ChebDomalice Graben externalblocks with differentiated weakly alkaline trachybasalt (basaltic)trachyandesitetrachyterhyolite series and synchronous commonstrongly alkaline (olivine) nephelinitetephrite/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 Cebri 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 NESW- to NWSE-striking trans-lithospheric fault systems allowedthe rapid ascent of basaltic magmas with common peridotitexenoliths in the Ohe Rift Graben and the LabeOdra 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. KAr dating was supported by OTKA projects Nos. T043344, T060965and M41434 to K. Balogh. We are indebted to J. Pavkov and J.Rajlichov 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 theby tensional stress elds 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 (166 Ma), theLate Miocene to Early Pleistocene episode of tensional stress(60.9 Ma) and the Pleistocene episode of compressive stress(0.90.26 Ma).

Volcanism of the syn-rift period was dominant in the BohemianMassif withN97 vol.% of the volcanic rocks occurring within theOhe Rift Graben. Volcanic suites emplaced under the compressivestress elds are mostly of a primitive composition; they are themelilitic ultramac rocks of the pre-rift period and the nalmanuscript.

(in Czech).

143J. Ulrych et al. / Lithos 123 (2011) 133144Frda, J., Fediuk, F., 1996. Peridotite liquid trapped within foidite magma of the eleznhrka Quaternary volcano (Czech Republic), Vol. 2. 30th International GeologicalCongress, Beejing, China, p. 435. Abstracts.

Geissler, W.H., Kind, R., Yuan, X., 2008. Upper mantle and lithospheric heterogeneitiesin central and eastern Europe as observed by teleseismic receiver functions.Geophysical Journal International 174, 351376.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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geochemistry and KAr ages for Cenozoic tinguaites from the Ohe/Eger Rift (NW

Recurrent Cenozoic volcanic activity in the Bohemian Massif (Czech Republic)IntroductionGeological settingTiming of volcanic activity and its paleostress backgroundPre-rift period of volcanism: Late Cretaceous to Mid Eocene (7949Ma)Syn-rift period of volcanism: Mid Eocene to Mid Miocene (4216Ma)Late-rift period of volcanism (160.26Ma)

Analytical proceduresResultsPaleostress fields and volcanic activityGeochemistry of the volcanic series of the Bohemian Massif

DiscussionConclusionsAcknowledgementsSupplementary dataReferences