GeolCarp_Vol54_No1_53

GEOLOGICA CARPATHICA, 54, 1, BRATISLAVA, FEBRUARY 2003

53 — 64

COEXISTING MIOCENE ALKALINE VOLCANIC SERIES ASSOCIATED

WITH THE CHEB-DOMAŽLICE GRABEN (W BOHEMIA):

GEOCHEMICAL CHARACTERISTICS

JAROMÍR ULRYCH

, JANA ŠTĚPÁNKOVÁ

, FELICITY E. LLOYD

and KADOSA BALOGH

Institute of Geology, Academy of Sciences of the Czech Republic, Rozvojová 135, CZ-165 02 Praha 6, Czech Republic; ulrych@gli.cas.cz

PRIS, University of Reading, Whiteknights, Reading RG6 6AB, United Kingdom

Institute of Nuclear Research, Hungarian Academy of Sciences, Bemtér 18/C, H-4026 Debrecen, Hungary

(Manuscript received February 1, 2002; accepted in revised form October 3, 2002)

Abstract: The Middle to Late Miocene intraplate alkaline volcanism of W Bohemia is associated with the uplift of the NE
flank of the Cheb-Domažlice Graben. Two coexisting cogenetic volcanic series have been recognized there: (i) weakly
alkaline series (WAS) basanite—trachybasalt—(basaltic) trachyandesite—trachyte—rhyolite (15.9—11.4 Ma) and (ii) strongly
alkaline series (SAS) olivine nephelinite—tephrite (16.5—8.3 Ma). Similar patterns in olivine nephelinite and basanite
Primitive Mantle-normalized incompatible trace elements point to a single mantle source that was geochemically similar
to Enriched Mantle. Negative Rb and K anomalies probably reflect residual amphibole in the source. Magma modelling
showed that olivine nephelinite and basanite could form by different degrees of partial melting of the mantle source, with
amphibole, garnet, olivine and clinopyroxene in the residuum. The mantle source was probably represented by lithospheric
mantle metasomatized by plume-like material. Evolution of the SAS is limited in extent and rather unclear. The WAS is
well developed and its evolution can be modelled by fractionation of olivine, clinopyroxene, Fe-Ti oxide, and a small
proportion of plagioclase, and in late stages by alkali feldspar, Fe-Ti oxide, clinopyroxene and apatite. Crustal contami-
nation cannot be ruled out but there is no substantial evidence of this in felsic derivatives.

Key words: W Bohemia, Miocene volcanism, alkaline rocks, geochemistry.

Introduction

The Cenozoic alkaline volcanism of the Bohemian Massif is
an integral part of the Central European Volcanic Province,
which extends from France to Germany, Czech Republic and
Poland. It is a surface manifestation of a large, sheet-like re-
gion of upwelling found in the upper mantle from the eastern
Atlantic Ocean to central Europe and the western Mediterra-
nean (sensu Hoernle et al. 1995).

Contrasting magmatic associations of a weakly alkaline se-

ries  –  WAS  (silica  undersaturated  to  oversaturated)  and  a
strongly  alkaline  series  –  SAS  (undersaturated)  are  known
from many continental intraplate volcanic provinces (Wilson
et al. 1995). Alkaline magmas may follow simultaneously ei-
ther  a  silica  saturated  to  oversaturated  differentiation  trend
(rhyolites, Q-trachytes), or an undersaturated one (phonolite),
reflecting the individual differences in chemistry of the prima-
ry  magma.  However,  assimilation-fractional  crystallization
processes,  centred  in  the  lower-crust  magma  chamber,  play
the decisive role in the development of both series, but espe-
cially of WAS (Wilson et al. 1995). Similar alkaline rock se-
ries  are  known  from  Siebengebirge,  Westerwald,  Hocheifel
and in particular from Cantal, Massif Central within the Ceno-
zoic European Volcanic Province.

Geological setting

Cenozoic volcanism in W Bohemia is associated with the

uplifted northeastern flank of the Cheb-Domažlice Graben –
CDG (NNW-SSE striking, 150 km long and 5—10 km wide)

formed by the Tepelská vrchovina Highland and Slavkovský
les Mts. The near ENE-WSW trending Ohře Graben Fault Sys-
tem,  reactivated  in  the  Cenozoic,  only  modifies  an  older
Variscan suture. By convention, the Střela River Valley defines
the boundary between the volcanics of the Tepelská vrchovina
Highland  and  those  of  the  Doupovské  hory  Mts  in  the  Ohře
Graben.  The  CDG  represents  a  young  asymmetric  structure
limited at its eastern margin by the morphologically expressive
Mariánské  Lázně  Fault.  The  oldest  sediments  of  the  CDG
might  be  younger  than  Oligocene,  the  youngest  are  of  Upper
Pliocene age (Shrbený 1994). The Late Miocene basanite flow
(11.7 Ma – Wilson et al. 1994) of the Vlčí hora Hill volcano
near  Černošín,  which  covers  the  sedimentary  relict  is  located
outside the graben. The deep-seated disposition of the western
Ohře (Eger) Rift and CDG is evidenced by the composition and
flux  of  gas  emanations,  and  the  isotopic  heavy  CO

and high

mantle derived He and N

proportions of mineral springs and

dry gas vents – mofettes (Weinlich et al. 1999).

From the beginning of 20th century geologists were intrigued

by W Bohemian rocks of exceptional petrographical composi-
tion  (“andesitic  character”).  Shrbený  (1979)  distinguished  a
special  “silica  higher-saturated”  group  of  rocks  in  W  Bohe-
mia.  Subsequently,  Ulrych  et  al.  (1999)  identified  two  con-
trasting rock groups, a WAS and a SAS. Geochemical distri-
bution of the young volcanics within the SE flank of the CDG,
unlike space-time distribution, lacks obvious zoning (Fig. 1).

Sampling and analytical methods

Thirty-one representative rock samples were used for

geochemical and mineralogical studies (Ulrych et al. in print).

54 ULRYCH, ŠTEPÁNKOVÁ, LLOYD and BALOGH

Sampling sites are presented together with their geological
characteristics, petrography and modal mineralogy of the
rocks in Table 1.

The rock samples were analysed by wet methods by P. Povon-

dra (Charles University) and V. Chaloupský (Inst. of Structure
and Mechanic of Rocks, AS CR). Analysing of rock standards
GM, TB, BM and the repeated analyses of samples showed
errors within ±5 % (1

). Trace element abundance was deter-

mined by J. Štrublová (Gematest) by XRF using the automat-
ed Philips PW 1404 equipment on pressed powder pellets.
The precision of XRF determinations varies about ±5 % (1

)

as checked by a series of duplicate analyses. Accuracy was
tested against the rock standard USGS BCR-1. Additional
trace element determinations were performed by INAA in the
Gematest laboratory (analyst P. Hanžlík) and Inst. of Nuclear

Physics, AS CR (analyst J. Frána). The precision (1

) was

better than ±10 % for REE, and better than ±5—10 % for other
trace elements. Accuracy of the INAA analyses was checked
with the use of international reference rock samples. Ar iso-
topes were measured using the mass spectrometer in the Inst.
of Nuclear Research, Hungarian AS by the method of isotope
dilution using

Ar spike according to the procedure de-

scribed by Balogh (1985).

Age relations of volcanic series associated with the

Cheb-Domažlice Graben

The new K-Ar data and a review of the published results

(Wilson et al. 1994) are presented in Table 2a,b. Based upon

Fig. 1. Geological sketch of the NE flank of the Cheb-Domažlice Graben with marked out occurrences of the Tertiary volcanics and sam-
pling. P – Políkno, HR – Horní Rotava (10 km).

MIOCENE ALKALINE VOLCANIC SERIES: GEOCHEMICAL CHARACTERISTICS 55

Table 1: Geological and petrological characteristic of the representative rocks of the weakly and strongly alkaline series.

Locality Sample

Root

name

Sub-rot

Normative

Mineral

Country

Structure

Volcanic

in TAS*

name

characteristic

composition

rock

texture

form

No. classification

(xenoliths)

WEAKLY ALKALINE SERIES

Stěnský vrch

202

rhyolite

Q-, Ns-

anorthoclase

gneiss

holocrystalline

dome

Hill

normative

quartz

porphyritic

(laccolith?)

magnesioriebeckite

with trachytic

matrix

Špičák Hill

180

trachyte

high

Q-

anorthoclase, sanidine

gneiss

holocrystalline

dome

K-type

normative

oligoclase, quartz

porphyritic

winschite, biotite

with trachytic

Ti-magnetite, titanite, apatite

matrix

Mn-oxyhydroxide, sulphur

Prachometský

186

trachyte

high

Q-

sanidine, anorthoclase

amphibolite

holocrystalline

dome

vrch Hill

K-type

normative

diopside-hedenbergite-

fine-porphyritic

augite series

with trachytic

Ti-magnetite, titanite, apatite

matrix

Třebouňský

251

trachy-

latite

Ne-, Ol-

andesine, anorthoclase

mica

holocrystalline

lava

vrch Hill

andesite

normative

diopside-augite series

schist

porphyritic

flow

kaersutite

with trachytic

nepheline

matrix

Ti-magnetite, titanite, apatite

Doubravický

256

basaltic

shoshonite

Ne-, Ol-

bytownite, anorthoclase

PermoCarboniferous

holocrystalline

lava

vrch Hill

trachy-

normative

diopside-augite series

sediments

pilotaxitic,

flow

andesite

nepheline

vesicular

Ti-magnetite, titanite, apatite

zeolite, carbonate, barite

Zbraslavský

255

trachybasalt hawaiite

Ne-, Ol-

andesine, sanidine

gneiss and

holocrystalline

lava

vrch Hill

normative,

kaersutite

PermoCarboniferous

fine porphyritic

flow

diopside

sediments

with pilotaxitic

Ti-magnetite, titanite,

matrix

apatite, carbonate

"sonnenbrand"

Prachomety II

Z-13

basanite

Ne-, (Ol-)

ferrisalite-ferrifassaite,

mica schist

porphyritic with

intrusion

normative

labradorite, (andesine),

holocrystalline

(partly

K-oligoclase, serp.olivine,

matrix

brecciated)

Ti-magnetite,

apatite, (analcime)

Vlčí hora Hill,

P-1

basanite -

Ne-, Ol-

ferrisalite-ferrifassaite,

phyllite to mica

porphyritic with

complex

analcimized

normative

kaersutite,

schist

holocrystalline matrix,

volcano

labradorite, K-andesine

megacrysts:kaersutite,

Ti-magnetite, apatite

diopside, olivine

STRONGLY ALKALINE SERIES

Vinice Hill

Z-22 trachybasalt hawaiite

Ne-, Ol-

kaersutite, ferrisalite

phyllite to mica

holocrystalline

intrusion

normative

phenocrysts, schist

fine

porphyritic

labradorite-andesine,

K-oligoclasse, ferrisalite

in matrix

Pekelský vrch

Z-24

tephrite

Ne-, (Ol-)

ferrisalite-ferrifassaite

mica schist

fine porphyritic

small

Hill

normative

phenocrysts,

holocrystalline

volcano

labradorite, K-andesine

fluidal matrix

Ti-magnetite,

apatite in matrix

Okrouhlé

Z-19

tephrite

Ne-, Ol-

(olivine), ferrifassaite,

phyllite to mica

holocrystalline

differentiated

Hradiště Hill

normative

labradorite, analcime

schist

fine-porphyritic

lava flow(s)

Ti-magnetite, apatite,

xenolite:pyroxenite

and dunite

with glassy rims

Polom

Z-15

olivine

Ne-, Ol-

olivine, ferrifassaite

granite

porphyritic with

intrusion

in Mariánské

nephelinite

normative

labradorite, serp. olivine,

xenoliths: granite

holocrystalline

(partly

Lázně

(contam.)

Ti-magnetite, apatite

with glassy rims,

matrix

brecciated)

pyroxenite

Lysina Hill

Z-14

olivine

Ne-, Ol-

olivine,

granite

porphyritic with

intrusion

nephelinite

normative

ferrifassaite, nepheline,

xenoliths: granite

holocrystalline

(contam.)

Ti-magnetite,apatite

matrix

Český

Z-16

melilite-

Ne-, Ol-

olivine, ferrifassaite,

Miocene sediments

holocrystalline

dyke-like

Chloumek

bearing

normative

nepheline, melilite

and granite microporphyritic

intrusion

olivine

(melilite)

Ti-magnetite,

xenoliths: wehrlite?

nephelinite

56 ULRYCH, ŠTEPÁNKOVÁ, LLOYD and BALOGH

Table 2a: Chemical analyses of rocks of the weakly alkaline series.

Sample

No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

202 ZC-30B ZC-30A ZC-20A

180

186

203

251

251a

255a

256

255

Z-13

P-3

P-1 P-19

Rock

type

RY RY TR TR TR TR TR TA TA TA

BTA TB BA BA BA BA

SiO

(wt.%) 70.18 74.45 65.66 65.39 65.77 62.91 62.90 53.46 55.17 55.57 51.83 45.24 42.24 42.91 43.81 41.43

TiO

0.03 0.08 0.24 0.40 0.31 0.55 0.30 1.67 1.41 1.76 1.68 2.68 2.46 3.37 3.11 3.90

15.12 13.76 17.98 17.82 17.57 18.20 19.14 18.22 17.70 17.75 18.83 16.33 12.64 11.99 12.49 11.27

1.23 1.29 1.51 1.43 1.93 1.08 2.99 4.64 3.28 5.10 5.29 6.21 5.14 5.12 4.59 5.02

FeO

0.08 0.05 0.07 0.13 0.08 0.14 0.10 1.92 3.00 1.56 2.18 4.50 5.99 5.88 6.56 7.21

MnO

0.04 0.02 0.05 0.06 0.24 0.12 0.10 0.18 0.19 0.17 0.22 0.21 0.22 0.18 0.20 0.22

MgO

0.05 0.16 0.06 0.10 0.14 0.06 0.12 2.36 1.82 1.63 2.02 4.22 11.19 11.11 9.60 9.25

CaO

0.41 0.20 0.80 1.25 0.76 2.13 1.03 5.63 5.60 6.09 6.81 9.73 11.54 13.97 13.30

14.61

6.60 3.06 7.39 6.79 6.27 7.40 6.49 5.15 5.02 4.98 4.85 3.79 3.85 2.39 3.16 2.98

4.48 4.91 5.92 6.00 5.56 4.91 5.55 3.74 4.09 3.50 3.02 1.47 1.41 1.70 1.77 1.74

0.03 0.09 0.03 0.06 0.07 0.10 0.04 0.55 0.43 0.50 0.62 0.79 0.82 0.57 0.69 0.75

1.17 1.20 0.44 0.30 0.42 1.24 0.67 1.28 1.26 0.86 1.11 2.01 2.02 0.59 0.29 1.32

0.04 0.22 0.12 0.09 0.32 0.34 0.63 0.82 0.40 0.28 0.96 1.43 0.52 0.21 0.27 0.22

0.03 0.06 0.04 0.06 0.06 0.03 0.04 0.11 0.07 0.02 0.09 0.12

0.01

0.02

0.11

0.13

0.01

0.19 0.03

<0.01 0.05 0.40 0.44

<0.10 0.05 0.01 0.02 0.04 0.91 0.03 0.13 0.07 0.20

99.69 99.58 100.31 99.93 99.92 99.67 100.10 99.89 99.45 99.79 99.68 99.65 100.07 100.12 99.91 100.12

O=2F

-0.01 -0.03 -0.02 -0.03 -0.03 -0.01 -0.02 -0.05 -0.03 -0.01 -0.04 -0.05

O=2Cl

-0.02

-0.03

-0.05

Total

99.67 99.55 100.29 99.90 99.89 99.66 100.08 99.82 99.42 99.78 99.61 99.55 100.07 100.12 99.91 100.12

Rb (ppm)

436

282

153

213

191

133

162

110

102

6.5

6.2

2.8

2.6

2.2

1.5 1.40

1.9

1.3

0.9 0.87 0.78 0.94 0.57

126

128 1270 480 1338 1189 1040 1287 1069 1002 728 877 941

420 215 143 564 434 1546 1242 1455 1290 1451 1258 906 828 487 580 788

3.1

0.3

5.6

1.6

1.2

1.4

6.7

0.9

6.3

6.9

7.9

6.2

84 10 156 101 132 171 131 144 160 140 121 95 114 121 137 113

115 21 216 155 220 152 186 251 241 218 228 176 177 180 211 171

16.0 10.0 40.0 37.0 71.0 14.0 54.0 102.0 99.0 100.8 106.0 87.0 78.3 79.0 84.0 75.1

2.8 2.2 2.9 4.3 9.2 4.4 5.9 14.3 14.7 15.3 15.6 13.7 11.4 13.3 16.9 13.9

0.3 0.2 1.0 1.2 1.9 1.5 1.6 4.2 3.8 3.6 4.6 4.2 3.4 3.4 4.4 3.7

2.7

6.2 4.1 5.0 12.3 10.2 9.4 12.5 12.1 13.8 10.9 12.1

11.6

0.47 0.40 0.50 0.50 0.95 0.71 0.59 0.85 1.43 1.36 1.60 1.40 1.06 1.44 1.58 1.22

3.7 3.6 3.3 2.8 3.7 3.1 2.8 2.9 3.9 3.9 3.4 2.6 2.19 3.01 4.50 3.00

0.51 0.43 0.48 0.41 0.58 0.45 0.47 0.51 0.38 0.44 0.58 0.45 0.33 0.33 0.41 0.38

72.3 9.0 33.0 33.0 26.3 21.4 27.6 15.6 14.1 15.9 14.0 10.2 13.2 10.0 12.0 13.0

5.4 7.0 7.1 6.3 7.8 5.1 3.4 4.0 4.5 6.6 3.2 2.7 3.2 2.1 1.9 1.0

380

517 661 626 460 505 363 512 322 252 243 288 299

14.5 2.9 11.7 13.2 12.8 14.6 14.9 12.4 11.8 10.5 11.5 9.2 6.8 9.9 9.4 8.9

198

301

310

330

139

160 133 126 122 154 123 149 116 89 70 85 117

8.7

10.2

9.1

8.8

10.5

10.1 7.7 9.2 6.8 5.4 6.1 7.1 7.6

12 10 4 11 26 21 16 15 219 175 230

258

0.5 3.7 0.5 1.1 0.6 1.2 1.0 5.7 11 14 3.6 5.0 51 54 45 41

5 5 5 6 10 17 7 13 237 157 120 99

5 5 0.2 37 17 29 11 21

155 87

150

115 56 72 46 63 200 98 137 95 143 195 129 128 70 90 95

K/Rb

85.28 144.51 321.15 233.80 241.61 306.41 284.35 282.20 377.19 284.80 302.00 200.02 238.84 361.79 367.28 370.31

Rb/Sr

3.46

1.49 0.10 0.34 0.08 0.08 0.10 0.06 0.06 0.05 0.05 0.05 0.04

Sr/Ba

0.30

0.29 0.82 0.39 0.92 0.92 0.72 1.02 1.18 1.21 1.49 1.51 1.19

Zr/Nb

2.73

3.23

4.97

3.77

3.28 2.95 3.44 2.78 2.83 3.47 3.39 2.56

Y/Nb

0.25

0.22 0.27 0.20 0.33 0.22 0.28 0.26 0.27 0.24 0.31 0.31 0.18

La/Nb

0.60

0.83 1.29 1.04 1.18 1.04 1.14 0.81 0.82 1.29 1.73 1.62 0.96

Th/U

13.40 1.29 4.65 5.24 3.37 4.20 8.12 3.90 3.13 2.41 4.38 3.78 4.13 4.76 6.32

13.00

Zr/Hf 26.20

40.39

45.27

42.01

37.10

42.80 34.57 44.52 35.00 37.06 24.55 30.64 33.60

Nb/Ta 15.98

15.69

14.62

14.32

11.62

15.25 15.97 16.20 17.06 16.48 11.48 11.97 15.39

ΣREE

225.48 47.80 420.21 302.16 445.53 351.26 587.36 532.06 534.51 492.71 493.28 392.45 402.27 412.49 472.36 392.65

(La/Yb)

16.28 1.99 33.91 25.87 25.59 39.57 33.56 35.62 29.45 25.67 25.53 26.21 37.47 28.86 21.89 26.97

Eu/Eu*

0.33 0.41 1.87 1.41 0.73 1.06 0.88 0.94 0.90 0.85 0.97 0.98 0.83 0.84 0.89 0.85

#Mg

8.12 21.70 8.09 12.89 13.91 10.17 8.27 44.81 39.07 35.73 37.90 46.73 68.85 68.96 65.32 62.32

Gd/Gd*

0.13 0.00 0.00 0.00 0.16 0.15 0.15 0.28 0.24 0.24 0.31 0.39 0.44 0.34 0.32 0.38

Age (Ma)

12.4

12.5

11.9

12.1

11.4

12.9

12.8

6.5?

11.7*

15.9*

trachyandesite, Branišovský v. Hill near Teplá (Shrbený 1979), AQ

trachyandesite, Zbraslavský v. Hill near Manětín (Shrbený 1979), AQ

basaltic trachyandesite, Doubravický v. Hill near Manětín, AQ
trachybasalt?, Zbraslavský v. Hill near Manětín, AQ
basanite, Prachomety II near Teplá, AQ
basanite, Okrouhlé Hradiště Hill near Konstantinovy Lázně, AQ
basanite, Vlčí hora Hill near Černošín, AQ
basanite, Holý v. Hill near Ratiboř, Q

1-202
2-ZC30B
3-ZC30A
4-ZC20A
5-180
6-186
7-203
8-251

rhyolite, Stěnský vrch Hill near Teplá, AQ
rhyolite, Kojšovice (Vrána 2000), B
trachyte, Kojšovice (Vrána 2000), B
trachyte, Dobrá Voda (Vrána 2000), B
trachyte, Špičák Hill near Teplá, Q
trachyte, Prachometský v. Hill near Teplá, AQ
trachyte, Berounský v. Hill near Heřmanov, B
trachyandesite, Třebouňský v. Hill near Teplá, AQ

9-251a
10-255a
11-256
12-255
13-Z13
14-P3
15-P1
16-P19

Explanations: Q – active quarry, AQ – abandoned quarry, NO – natural outcrop, B – boulders

MIOCENE ALKALINE VOLCANIC SERIES: GEOCHEMICAL CHARACTERISTICS 57

Table 2b: Chemical analyses of rocks of the strongly alkaline series.

Sample

No.

17 18 19 20 21 22 23 24 25 26 27 28 29 30 31

Z-20 Z-22 Z-23 Z-24 Z-19 P-2 P-4 Z-26 Z-15 Z-14 226 M-1 M-2 P-16 Z-16

Rock

type

TB TB TB TE TE TE TE TE ON ON ON ON ON

MON

SiO

(wt.%) 47.32 45.66 45.89 44.91 45.55 40.27 41.59 44.98 43.67 43.98 40.99 39.58 40.60 40.90 39.19

TiO

2.55 2.89 2.86 2.85 3.07 3.91 3.66 1.99 3.51 2.10 4.18 1.98 1.95 2.20 2.70

16.14 15.80 15.28 15.83 15.20 13.65 14.79 12.04 13.87 12.21 13.76 10.20 10.87 11.60 11.39

4.80 6.12 4.77 4.99 4.93 4.80 5.17 2.48 5.62 4.18 7.51 6.39 4.34 4.08 3.99

FeO

5.65 5.25 6.45 6.45 6.14 8.12 8.93 8.05 5.40 6.33 7.56 5.85 6.83 7.10 7.99

MnO

0.22 0.23 0.23 0.23 0.21 0.24 0.25 0.17 0.18 0.17 0.23 0.22 0.21 0.19 0.20

MgO

4.00 4.34 5.41 5.07 6.11 7.90 6.25 12.46 6.98 13.02 6.42 15.02 14.52 12.50

12.49

CaO

9.81 9.93 10.73 11.10 11.05 12.39 12.02 11.22 11.80 11.33 11.24 12.45 12.13 13.59 15.81

4.24 4.23 3.59 2.84 3.65 3.49 3.97 2.62 3.69 3.33 3.75 3.93 3.64 2.74 3.00

1.90 1.13 2.21 2.34 2.19 2.22 0.87 1.31 0.72 0.94 1.89 1.33 0.97 1.10 1.11

1.00 1.08 0.94 0.91 0.74 0.92 0.90 0.60 0.51 0.52 0.94 0.97 0.88 0.82 0.88

2.06 1.73 1.01 1.14 0.95 0.50 1.09 1.69 2.23 1.69 0.80 1.23 1.78 2.64 1.12

0.21 0.61 0.34 0.29 0.35 0.02 0.20 0.14 0.79 0.26 0.20 0.82 0.51 0.33 0.31

0.07 0.02 0.04 0.08 0.07 0.49 0.37 0.07 0.02 0.13 0.03 0.08 0.28 0.07 0.03

Total

99.97 99.02 99.75 99.03 100.21 98.92 100.06 99.82 98.99 100.19 99.50 100.05 99.51 99.86

100.21

52 53 50 46 51 55 65 32 70 52 47 31 27 52 37

0.79 0.86 0.73 1.10 0.71 0.80 0.41 0.5 1.70 1.90 0.57 0.42 0.44 3.30 3.30

1046 979 858 967 829 1088 1190 594 895 600 1254 988 836 816

1151

814 787 718 644 642 691 731 501 616 611 985 849 627 964

1005

1.9 1.7 2 1.6 1.4

1.8 2.3 1.5

14 16

19.7 20 25 22 16

26.4

28 25 24 24 28 27

31 29 22 20 21 31 30 17 21 16 32 23 20 17 25

106.7 106.1 84.5 83.3 81.9 99.2 118.9 43.6 70.4 52.9 110.0 99.4 83.1 76.9 89.7

185 182 144.0 146.0 142.9 138.9 166.2 71.8 122.4 85.6 181.1 142.0 126.0 122.4 137.1

92.0 89.1 70.3 72.1 70.0 66.0 73.2 37.1 64.7 43.7 73.2 55.7 53.1 61.9 66.0

14.5 14.7 11.9 11.7 11.5 12.5 14.1 6.94 10.4 7.48 16 9.1 9.1 10.3 12.8

4.1 4.2 3.46 3.52 3.33 3.31 3.57 2.26 3.08 2.35 3.62 2.73 2.52 3.18 2.64

13.1 13.3 11.8 11.8 11.6 10.7 11.0 7.8 9.6 8.9 8.7 8.7 6.7 11.2 10.1

1.45 1.45 1.17 1.16 1.11 1.19 1.19 0.89 1.03 0.88 1.52 1.03 0.92 1.07 1.09

3.3 3.5 2.49 2.49 2.42 2.66 2.91 1.50 1.87 1.68 2.48 1.66 1.66 1.71 1.79

0.49 0.49 0.36 0.34 0.34 0.28 0.33 0.25 0.32 0.24 0.31 0.25 0.27 0.22 0.18

9.7 9.5 9.4 8.4 8.6 16.0 9.0 5.0 6.5 6.4 8.1 10.5 9.3 9.3

14.0

3.1 2.7 2.5 2.4 2.4 1.0 1.0 0.9 1.7 1.6 1.9 5.2 4.8 1.9 1.9

426 403 285 280 279 361 366 138 230 135 243 177 159 283 231

10.9 10.8 8.2 8.2 8 8.1 8.5 4.1 7.1 4.1 8.9 3.9 3.8 5.4 7.0

166 202 212 210 240 321 389 158 278 179 288 133 150 192 233

89 88 74 73 71 122 112 43 55 48 104 85 76 102

144

6.0 6.0 5.6 5.7 5.4 7.0 6.4 2.8 4.1 3.5 6.1 5.2 4.6 6.5 6.1

22 27 45 48 62 44 33 422 67 370 354 410 445 324 271

16 21 24 25 30 41 40

56.0 38 55 42 52 55 49 56

11 13 17 17 30 57 37 162 45 249 207 236 271 173 222

143 150 139 160 115 117 112 103 94 101 116 69 93 108 115

K/Rb

303.27 176.96 366.86 422.22 356.41 335.02 111.09 339.78 85.37 150.04 333.77 356.10 298.19 175.58 249.00

Rb/Sr

0.05 0.05 0.06 0.05 0.06 0.05 0.05 0.05 0.08 0.09 0.04 0.03 0.03 0.06 0.03

Sr/Ba

1.29 1.24 1.19 1.50 1.29 1.57 1.63 1.19 1.45 0.98 1.27 1.16 1.33 0.85 1.15

Zr/Nb

4.79 4.58 3.85 3.84 3.93 2.96 3.27 3.21 4.18 2.81 2.34 2.08 2.09 2.77 1.60

Y/Nb

0.35 0.33 0.30 0.27 0.30 0.25 0.27 0.28 0.38 0.33 0.31 0.27 0.26 0.17 0.17

La/Nb

1.20 1.21

366.86 1.14 1.15 0.81 1.06 1.01 1.28 1.10 1.06 1.17 1.09 0.75 0.62

Th/U

3.13 3.52 0.06 3.50 3.58 16.00 9.00 5.56 3.82 4.00 4.26 2.02 1.94 4.89 7.37

Zr/Hf

39.08 37.31 34.76 34.15 34.88 44.57 43.06 33.66 32.39 32.93 27.30 45.38 41.84 52.41 33.00

Nb/Ta

29.67 29.33 19.47 26.07 26.30 17.43 17.50 30.71 26.19 28.24 17.05 16.35 16.52 31.88 23.61

ΣREE

420.62 414.38 329.98 332.41 325.10 334.74 391.40 172.13 283.79 203.68 396.95 320.56 283.39 288.88 321.40

(La/Yb)

22.98 22.06 24.34 24.00 24.28 26.75 29.31 20.85 27.00 22.59 31.82 42.95 35.91 32.26 35.95

Eu/Eu*

0.88 0.90 0.88 0.91 0.87 0.85 0.84 0.94 0.93 0.88 0.85 0.93 0.94 0.90 0.69

#Mg

45.69 45.83 51.39 49.31 54.78 57.11 49.11 71.76 58.33 73.01 48.46 73.08 73.93 70.88 69.34

Gd/Gd*

0.40 0.41 0.46 0.45 0.46 0.43 0.37 0.60 0.44 0.58 0.27 0.35 0.30 0.52 0.42

Age (Ma)

10.4 13.5 13.0 10.5 9.0* 11.8* 8.3* 16.5*

16.2

17.0 12.4* 16.5*

Ages designated by asterisk (Wilson et al. 1994) other (K. Balogh and E. Árva-Sós, Debrecen).

17-Z20
18-Z22
19-Z23
20-Z24
21-Z19
22-P2
23-P4
24-Z26

trachybasalt, Skupečský v. Hill near Konstantinovy Lázně, AQ
trachybasalt, Vinice Hill near Konstantinovy Lázně, AQ
trachybasalt, Pekelský v. Hill near Nečtiny, Q
tephrite, Pekelský v. Hill near Nečtiny, Q
tephrite, Okrouhlé Hradiště Hill near Konstantinovy Lázně, AQ
tephrite, Homole Hill near Planá, AQ
tephrite, Krasíkov Hill near Konstantinovy Lázně, NO
olivine nephelinite (crystalline rocks and magnetite xenoliths), Číhaná AQ

25-Z15
26-Z14
27-226
28-M1
29-M2
30-P16
31-Z16

olivine nephelinite (granite xenoliths), Polom in Mariánské Lázně, AQ
olivine nephelinite (granite xenoliths), Lysina Hill near Kynžvart, NO
olivine nephelinite, Chlumská hora Hill near Manětín (Shrbený 1979), AQ
olivine nephelinite (massive), Podhorní v. Hill near Mariánské Lázně, NO
olivine nephelinite (brecciated), Podhorní v. Hill near Mariánské Lázně, AQ
melilite-bearing olivine nephelinite, Chloumecký kopec Hill near Český Chloumek, NO
melilite-bearing olivine nephelinite, Český Chloumek, AQ

58 ULRYCH, ŠTEPÁNKOVÁ, LLOYD and BALOGH

the  geochemistry  and  the  distribution  scheme  of  Cenozoic
volcanism  in  the  Bohemian  Massif  (Ulrych  et  al.  1999),  the
age  ranges  for  two  coexisting  volcanic  series  of  Middle  to
Late  Miocene  age  associated  with  the  NE  flank  of  the  CDG
were recognized:

1. Weakly Alkaline Series basanite—trachybasalt—(basaltic)

trachyandesite—trachyte—rhyolite (15.9—11.4/6.5 Ma) and

2. Strongly alkaline series (melilite-bearing) olivine nephe-

linite—tephrite—trachybasalt? (16.5/17.0—8.3 Ma) developed to
a limited degree only.

The ultimate development of volcanism of the NE flank of

the CDG was at about 12 Ma. However, the onset of volcan-
ism (16—17 Ma) is characteristic of the more distal regions of
the CDG. This activity of the CDG NE flank provides a link
between  the  Oligocene—Miocene  strongly  alkaline  series  of
the western Ohře Rift (24—16 Ma, average 22 Ma) (Ulrych et
al.  2002)  and  the  Pliocene  to  Quaternary  (0.43—0.11  Ma)
primitive  alkaline  volcanics  occurring  at  the  intersection  of
the  Ohře  Rift  and  CDG  structures  in  the  vicinity  of  Cheb
(Wilson et al. 1994). From K-Ar data of Wilson et al. (1994)
and Ulrych et al. (in  print) on basanitic rocks (Hory in Kar-
lovy Vary – 15.5 Ma and Horní Rotava in the Krušné hory
Mts  –  14.8  Ma,  see  Fig. 1)  it  follows  that  the  Middle  Mi-
ocene volcanism in W Bohemia was not restricted to the CDG
area only. The melilite-bearing olivine nephelinite are present
only in the area of Český Chloumek (16.5 Ma – Wilson et al.
1994).  These  volcanic  rocks  can  be  associated  with  some
younger  NE-SW  trending  faults  (Litoměřice  Deep  Fault
Zone?). However, rare older volcanic products (29.5 Ma) also
occur in the area of the NE flank of the CDG (leucite basanite
from Políkno – Ulrych et al. in print, see Fig. 1).

The age-related Group of Late Miocene intrusives (13—9

Ma; sills and dykes) represents the final volcanic episode of
the České středohoří Mts area (Cajz et al. 1999). Products of a
similar episode (

≈

13 Ma) of the volcanic cycle are known

from many areas of Germany (Lippolt 1983). The recurrence
of volcanism and changes in its chemical characteristics coin-
cide with tectonism (Downes 1996), causing principal chang-
es in tectonic settings and character of magmas from calc-al-
kaline to alkaline in the Carpathians. Volcanism of the CDG
NE flank thus parallels the development of the graben struc-
ture, as revealed by the minimal Middle Miocene (?) relict of
a sedimentary fill in the Lažany – Vlčí hora Hill area below
the overlying basanite flow (Sample No. 15; 11.7 Ma – Wil-
son et al. 1994).

Petrography and geochemistry

A survey of the principal rock types, localities, their geolog-

ical and petrographical characteristics, and main modal miner-
alogy is shown in Table 1. For more detailed characterization
of the rock-forming minerals see Ulrych et al. (in print) and
for petrography of the rocks see Shrbený (1979).

Chemical analyses of WAS and SAS rocks are presented in

Table 2a,b. Position in the TAS diagram (total alkali vs. sili-
ca) (Le Maitre Ed. 1989) for the rocks of both series is shown
in Fig. 2. In this plot the majority of the W Bohemian rocks
fall into the basanite, tephrite, trachybasalt, (basaltic) tra-

chyandesite, trachyte, and rhyolite fields. Three samples (Nos.
26, 25, and 24) plot as alkali-basalts because they contain nu-
merous  microxenoliths  of  the  surrounding  granitic  rocks.
Their  mineral  composition  indicates  that  their  original  bulk
chemistry  was  more  undersaturated  in  silica.  Although  most
of the mafic rocks plot in the basanite field, they vary signifi-
cantly in modal composition (see mineralogy section), which
was  used  to  distinguish  olivine  nephelinites  and  basanites.
Compared  to  Siebengebirge  (Vieten  et  al.  1988)  and  Cantal,
Massif Central (Wilson et al. 1995), the volcanic rocks of W
Bohemia reveal a similar extent of fractionation, but at a low-
er level of alkalinity.

PM-normalized multielement variation diagram patterns of

most  of  the  rocks  under  discussion,  from  mafic  members  to
trachyandesites are, in general, very similar (Fig. 3). Compati-
ble trace element concentrations (Cr, Ni, V) in the mafic rocks
are relatively high, although variable. All mafic rocks are en-
riched  in  most  incompatible  elements  compared  to  PM.  For
most  rocks  negative  Rb  and  K  anomalies  and  a  positive  P
anomaly are characteristic. In more acid members (trachytes,
rhyolites) Rb and K anomalies disappear and negative Ba, P
and Ti anomalies are well pronounced.

The REE concentration in all studied rocks is relatively

high with relative enrichment in LREE increasing towards the
more evolved felsic rocks (La/Yb = 16—40). The Eu anomaly
in most rocks is insignificant. The only exception is in rhyo-
lites where negative Eu anomaly is present. The patterns of el-
ements from Sm to Yb are very close to the pattern of OIB
(ocean island basalts) (Sun & McDonough 1989), while the
LREE are enhanced, and the influence of enriched mantle in
the magma source is probable (Fig. 4).

The olivine nephelinites are distinct from the other mafic

rocks, although there is significant overall similarity of major,
trace  and  REE  element  distribution  in  the  mafic  rocks  as  a
whole  (see  above).  They  are  more  silica  undersaturated  and
more  strongly  alkaline,  as  reflected  in  their  mineralogy  (Ul-
rych et al. in print). The majority are also more Mg-, Cr- and
Ni-rich (see Fig. 5, MgO vs. SiO

, Cr vs. SiO

, Ni vs. SiO

but have lower P

and TiO

, which is removed from the

Fig. 2. Rocks of the weakly and strongly alkaline series associated
with the Cheb-Domažlice Graben plotted in TAS diagram (Le
Maitre Ed. 1989). Symbols as in Fig. 1.

MIOCENE ALKALINE VOLCANIC SERIES: GEOCHEMICAL CHARACTERISTICS 59

Fig. 3. Multi-element variation diagram for weakly alkaline (WAS) and strongly alkaline (SAS) series. Typical OIB (Sun & McDonough
1989) is plotted for comparison. Normalization values from Sun & McDonough (1989).

trend described by the other rock compositions (Fig. 5, TiO

vs. SiO

). One olivine nephelinite, sample No. 27 Table 2b, is

anomalous with low MgO (plots with the tephrites), but has
high Cr and Ni typical of the olivine nephelinites. The same
sample also has anomalously high TiO

, again plotting with

the  tephrites.  In  general  the  olivine  nephelinites  also  have
higher Ba/Rb (Fig. 5, and reflected in the Rb anomaly Fig. 3).
Olivine  nephelinites  also  have  lower  Nd  (and  Gd)  than  the
other mafic rocks (Fig. 4), and they extend to lower values for
Ta,  La,  Eu,  Hf,  and  Zr  (PM-normalized  multielement  varia-
tion  diagram,  Fig.  3).  Significantly,  the  olivine  nephelinites
extend  to  the  highest  Nb/Zr  coupled  with  highest  LREE  en-
richment  (Ce/Yb),  a  likely  indication  of  mantle metasomatic
processes (see below).

The felsic rock series, rhyolites, trachytes, (and tra-

chyandesites), with increasing SiO

, shows decreasing MgO,

FeO, TiO

, MnO, CaO and P

as Na

O and K

O increase;

increases initially, levels out in trachytes and then sub-

sequently  decreases  in  the  rhyolites,  which  also  show  a  de-
crease  in  alkalies  as  the  result  of  preceding  alkali  feldspar
fractionation. Concomitant with these major element changes,
the  compatible  trace  elements  (Cr,  Ni,  V)  decrease,  while
most of the incompatible elements (Hf, Th, Zr Y, Rb, Ba) in-
crease with increasing SiO

contents. Strontium increases ini-

tially and then shows a sharp decrease at higher silica levels.

According to TAS and Harker’s diagrams, trachybasalts,

trachyandesite and trachytes can be considered as representa-
tives of the WAS. This series corresponds to that from Cantal,
Massif Central (Wilson et al 1995). In Cantal, a SAS has also
been recognized. Initially, a similar series, composed of oliv-
ine nephelinite—tephrite—phonolite was thought to exist in W
Bohemia, but the two large phonolite bodies (27—25 Ma) con-
sidered representative of the felsic end of this series proved to
belong to an older series (33—22 Ma) characteristic of
Doupovské hory Mts (Ulrych et al. in print). However, strong-
ly alkaline volcanism is definitely represented in W Bohemia
by the olivine nephelinites and tephrites.

Even slight assimilation of SiO

by basaltic melt could in-

cline the differentiation trend towards rhyolite and in the ab-
sence of appropriate isotope data (in preparation) such a pro-
cess cannot be ruled out. However nothing in the chemical
signature of WAS positively identifies crustal contamination.
The ratios of incompatible elements (P/Ce, K/Rb, K/Nb, Ba/
Rb, La/Ta) are consistent with our fractionation model.

In PM-normalized incompatible trace element diagrams

some  trachytes  and  rhyolite  have  strong  negative  Ba,  Ti,  P
and  Sr  anomalies  and  an  intensive  Eu  anomaly  compared  to
other samples. Such variations can be explained by fraction-
ation of Fe-Ti oxide, alkali feldspar and apatite in late stages
of  magma  evolution  (see  modelling).  This  deduction  is  sup-

60 ULRYCH, ŠTEPÁNKOVÁ, LLOYD and BALOGH

ported by trends in SiO

-major and trace element diagrams (as

mentioned above).

Nature of mantle

The similarity of REE patterns to OIB (Sun & McDonough

1989) has been mentioned above. Using the primary magmas
(basanites,  olivine  nephelinites)  only  and  plotting  against
MgO (olivine-fractionation index) no regular variation is seen
with incompatibles K, Sr and Ba, while Nb/Zr decreases with
MgO but increases with LREE (Ce/Yb) enrichment – a like-
ly signature for metasomatised mantle. Most samples plot be-
low  average  mantle  Zr/Hf  and  Nb/Ta  (Green  1995)  with  a
large degree of scatter very similar to metasomatised mantle
beneath kamafugite volcanics of SW Uganda (Eby et al., Fig.
6, in press). Apart from rare kaersutite ± clinopyroxene, oliv-
ine xenoliths in basanites (Vlčí hora Hill), modal metasoma-
tism cannot be characterized due to lack of xenolith informa-
tion, but magma chemistry is compatible with K, Ba, Sr, Nb
and  REE  enhancement  and  could  imply  amphibole  and/or
phlogopite and apatite in the source with also the possibility
of  cryptic  metasomatism  affecting  “normal”  mantle  phases.

Metasomatism can affect either subcontinental mantle lithos-
phere, or the front of the mantle plume (or plume-like materi-
al). As the rocks are geochemically close to the enriched man-
tle (as mentioned above), it can be supposed that
metasomatised plume-like material was included in the mag-
ma source (cf. conception of Garnet et al. 1995).

Implications for magma genesis

The similar patterns of PM-normalized incompatible trace

elements in the olivine nephelinites and basanites indicate that
they originate from the same enriched mantle source. Also, it
appears highly unlikely that the olivine nephelinites can give
rise  to  the  basanites  by  fractionation,  when  it  is  considered
that the olivine nephelinites have higher MgO, Cr and Ni than
the  basanites  on  the  one  hand,  but  also  higher  Nb/Zr  and
LREE  enrichment  on  the  other.  Derivation  of  two  primary
magmas  by  different  degrees  of  partial  melting  of  the  single
enriched mantle source is required.

The basanites provide the most feasible parent for the WAS

compositions, as the olivine nephelinites are strongly alkaline
and too silica undersaturated. In Cantal the WAS is derived

Fig. 4. Chondrite-normalized REE abundance for weakly alkaline (WAS) and strongly alkaline (SAS) series. Normalization values from
Sun & McDonough (1989).

MIOCENE ALKALINE VOLCANIC SERIES: GEOCHEMICAL CHARACTERISTICS 61

Fig. 5. Harker’s variation diagrams TiO

-, MgO-, Cr-, Ni-, Nd- and Ba/Rb vs. SiO

. SiO

content is used as an indicator of magma evolu-

tion. Symbols as in Fig. 1.

from alkali basalt, but the composition field of Cantal alkali
basalts overlaps the basanite field as defined in the TAS dia-
gram of Le Maitre Ed. (1989).

The tephrites have clearly evolved from olivine nephelinite

or basanite, with olivine fractionation playing a major role (see

Fig. 5). The tephrites have REE levels closest to the olivine
nephelinites and slightly lower than the basanite, making deri-
vation from basanites unlikely (cf. Nd vs. SiO

in Fig. 5). Tra-

chybasalts are more problematic showing affinity with both
SAS and WAS. They were assigned to WAS because, in gener-

62 ULRYCH, ŠTEPÁNKOVÁ, LLOYD and BALOGH

al, trachybasalts have REE levels very similar to basanite and
their Ba/Rb ratio appears to be on the WAS trend (Figs. 3 and
4). Two trachybasalts that consistently showed more affinity
with olivine nephelinites and tephrites were assigned to SAS.

Origin of initial magma and magma modelling

The composition of initial magma is best reflected in the

geochemistry  of  mafic  members  and  their  primary  minerals.
Contaminated  samples  were  excluded  from  the  data  set  em-
ployed for geochemical study. As emphasized above, the sim-
ilar pattern of PM-normalized incompatible trace elements in
mafic rocks indicates a sole magma source. However, two pri-
mary magma compositions have been identified (see above):
olivine  nephelinite  and  basanite  have  originated  from  the
same mantle source by different degrees of partial melting.

Negative Rb and K anomalies can be construed as showing

either  that  amphibole  was  residual  in  the  mantle  source,  or
that  it  has  been  removed  by  fractionation.  As  cumulate  and
phenocryst amphibole are both rare in the W Bohemian rocks,
we prefer the former interpretation. Likewise, variable Ba/Rb
ratios  are  probably  the  result  of  different  degrees  of  partial
melting of these hydrous phases.

To confirm this hypothesis, modelling of the geochemical

evolution  of  magma  during  partial  melting  of  the  mantle
source  was  performed.  Parameters  and  source  data  used  for
modelling  can  be  provided  upon  request.  Because  fractional
melting in mantle conditions is improbable, as very small por-
tions  of  magma  cannot  escape  from  the  site  of  origin,  bulk
equilibrium melting was considered as a mechanism of mag-
ma  generation.  Depleted  mantle  and  primitive  mantle  geo-
chemistry  were  taken  as  source  composition.  As  mentioned
above, amphibole (phlogopite?) were probably present in the
mantle source. Xenoliths in the mafic rocks consist of olivine,
clinopyroxene and amphibole. These minerals were therefore
considered  indicative  of  the  mantle  mineralogy.  The  highly
differentiated REE patterns point to the presence of garnet in
the residuum. The fraction of amphibole, olivine, clinopyrox-
ene and garnet in the residuum were changed in the modelling
in order to produce the geochemistry observed in the two maf-
ic rock series.

The modelling results (olivine nephelinite – No. 26/Z-14

and basanite – No. 14/P-3, see Fig. 6) imply that partial melt-
ing  of  a  source  with  PM-geochemistry  can  generate  magma
with  a  very  similar  pattern  in  the  PM-normalized  multiele-
ment variation diagrams (Fig. 3). However, partial melting is
not  a  feasible  mechanism  to  provide  the  variable  pattern  of
metasomatic enrichment as shown by Nb/Ta versus Zr/Hf (see
“nature of the mantle” above).

The mantle source was probably enriched prior to melting.

The strong similarity of the observed trace element patterns to
those of OIB, points to plume-like material as a source of pa-
rental magma to the W Bohemian volcanics. Variations in the
geochemistry of the more evolved mafic members and inter-
mediate compositions can be explained by fractional crystalli-
zation of two marginally, but critically different parent mag-
mas, produced by different degrees of partial melting of a
plume-like mantle source. The possibility of some magma
mixing modifying the chemistries of the rocks in both series
cannot be ruled out.

Magma evolution

The SAS includes (melilite-bearing) olivine nephelinites,

and tephrites. The development of this series is rather unclear.
The low number of evolved rock types hinders interpretation.
On  the  other  hand  the  WAS  is  well  developed  and  broadly
widespread. As mentioned above the WAS include basanites,
trachybasalts,  trachyandesites,  trachytes  and  rhyolites.  These
rocks  were  not  significantly  affected  by  crustal  assimilation
(see  above).  Different  trace  element  contents  and  ratios  in
some trachytes and rhyolites can be explained by fractionation
of alkali feldspar as mentioned above. The rocks differ in the
extent  of  Ti  anomaly  in  the  PM-normalized  multielement
variation diagram, which can be caused by Fe-Ti oxide frac-
tionation.

As the rocks have relatively wide ranges of SiO

contents,

therefore SiO

was used as a marker of degree of magma dif-

ferentiation. Geochemical trends in SiO

-major and trace ele-

ment  diagrams  as  mentioned  above  point  to  the  fractional
crystallization  of  olivine,  clinopyroxene,  Fe-Ti  oxide,  and  in
late stages also of alkali feldspar and apatite. The Q-normative
trachytes  only  partly  differ  from  rhyolite  in  the  higher  con-
tents of normative feldspar and lower contents of quartz. Nev-
ertheless, the presence of ternary feldspars and anhydrous fer-
romagnesian  minerals  indicates  high  temperatures  of  origin
and  potentially  significant  specific  crystallization  pathways
(cf. Nekvasil 1990). The most common reaction in these rock
types would be the crystallization of alkali feldspars through
the  reaction  of  plagioclase  with  melt.  The  magma  evolution

Fig. 6. Modelling of partial melting of mantle peridotite. Two mafic
rocks  (basanite  No.  14/P-3  and  olivine  nephelinite  No.  26/Z-14)
with different patterns in PM-normalized multielement variation di-
agrams  were  used;  mantle  peridotite  composition  after  Sun  &  Mc-
Donough  (1989).  Modelled  lines  represents  composition  of  the
magma that formed by different degree of partial melting (F = 0.015
for  olivine  nephelinite  and  0.035  for  basanite)  of  mantle  peridotite
that consists of 20 % cpx, 30 % gra, 30 % ol and 20 % amph.

MIOCENE ALKALINE VOLCANIC SERIES: GEOCHEMICAL CHARACTERISTICS 63

was therefore modelled up to trachytes, only. For the model-
ling, the same samples of olivine nephelinite (based on its low
SiO

content and FeO/MgO ratio and high Ni and Cr con-

tents) and basanite were chosen (see Fig. 7).

Discussion

Two contemporaneous series (weakly and strongly alka-

line) have been recognized in the Teplá Crystalline Complex

in accordance with the review of Ulrych et al. (1999). Despite
the existing analogy with Cantal parent magmas and differen-
tiates (Massif Central; Wilson et al. 1995), and to a lesser ex-
tent with Siebengebirge, there are some notable differences.

Firstly, evolution of the SAS in W Bohemia is rather un-

clear  and  lacks  felsic  differentiates  (e.g.  phonolites,  see
above),  but  the  presence,  together  with  the  olivine  nephelin-
ites, of the coeval WAS makes the evolution of the W Bohe-
mian  rocks  from  a  single  magma  parent  most  unlikely.  The
second  difference  is  the  evolution  of  WAS.  Differentiation
paths existing in the studied rocks differ from those described
in Siebengebirge and Cantal. Rather atypical trends in W Bo-
hemia  can  be  explained  by  different  conditions  of  magma
evolution.  Whereas  in  Siebengebirge  and  Cantal,  prolonged
crystallization  accompanied  by  crustal  contamination  in  a
crustal magma chamber is supposed, in W Bohemia, the evo-
lution of the basanite parent magma was either relatively fast
or entirely mantle confined, as no substantial crustal assimila-
tion has been recognized in the rock chemistry. Different frac-
tionation paths of the rocks studied here, compared with those
of  the  Siebengebirge  and  Cantal,  are  also  likely  to  be  influ-
enced by different P, T, and fO

conditions of crystallization.

Modelled evolution of the magma confirms fractional crys-

tallization as a viable mechanism; however, the composition
of the fractionating material is rather approximate. More prob-
ably the proportions of minerals in the fractionating phase
changed gradually with the changing of magma composition.
Fractionation of clinopyroxene (and olivine) probably played
the most important role, suggesting the presence of pyroxene-
rich cumulates in the deeper crust and uppermost mantle.

The volcanism is genetically associated with the uplift of

NE-flank of the CDG. The contemporaneous rock series rep-
resents  products  of  the  Middle  to  Late  Miocene  episode,
which is part of the continuous Cenozoic volcanic activity in
the Bohemian Massif (Ulrych et al. 1999). Despite the similar
age of both series, their initial magmas differ in degree of par-
tial  melting  of  the  mantle  source.  Initial  nephelinitic  magma
was formed by a lower degree of partial melting than the basa-
nitic magma. Metasomatised mantle lithosphere has also been
reported  as  a  likely  source  of  Cenozoic  primitive  alkaline
magma from other parts of the Bohemian Massif (Svobodová
&  Ulrych  in  print).  Geochemical  similarity  of  the  rocks  to
OIB  points  to  mantle  metasomatism  associated  with  plume-
material.

Conclusion

The age of the volcanism associated with the CDG coin-

cides with the Late Miocene intrusions in the České středohoří
Mts (13—9 Ma; Cajz et al. 1999), young volcanism in Germa-
ny (11—6 Ma; Lippolt 1983) and with the time of the tectonic
event (Downes 1996) causing principal changes in chemical
composition of volcanism in Carpathians.

Two contemporaneous series were recognized in the Teplá

Crystalline Unit W Bohemia:

1. Weakly Alkaline Series (15.9—11.4 Ma) and
2. Strongly Alkaline Series (16.5—8.3 Ma).
Initial parent magmas (nephelinitic and basanitic) probably

formed by different degrees of partial melting of metasoma-

Fig. 7. Modelling of fractional crystallization in weakly alkaline se-
ries. Starting composition in modelling corresponds to the sample No.
26/P-1. The curve in diagrams was modelled by fractionation of solid
phase that consists of 30 % ol, 60 % cpx, 6 % mt and 4 % plg in mafic
magma. In trachybasaltic magma (melt fraction F = 0.6, vertical line)
the composition of solid phase changed to 45 % cpx, 42 % kf, 8 % mt
and 5 % ap.

64 ULRYCH, ŠTEPÁNKOVÁ, LLOYD and BALOGH

tised mantle lithosphere, with amphibole, olivine, garnet and
clinopyroxene  in  residuum.  The  mantle  source  was  enriched
in incompatible elements compared to PM. Such enrichment
was probably caused by metasomatism by plume-like materi-
al. The WAS formed by evolution of one initial magma (basa-
nitic) that evolved by the fractionation of olivine, clinopyrox-
ene, Fe-Ti oxide and small amount of plagioclase and in later
stages  by  fractionation  of  alkali  feldspar,  clinopyroxene,  Fe-
Ti oxide and apatite. It shows no substantial evidence of com-
monly recognized crustal assimilation in felsic rocks. Evolu-
tion of SAS is less clear, but the presence of coeval strongly
alkaline  and  weakly  alkaline  volcanism  in  W  Bohemia  has
been clearly established.

Acknowledgements:  This  research  was  supported  by  the
Grant  Agency  of  the  Czech  Republic  No.  205/99/0907  and
the  Research  Program  of  the  Institute  of  Geology  AS  CEZ:
Z3010912. The K-Ar dating was sponsored by the Hungarian
Academy of Sciences Foundation T 014961. For helpful and
perceptive suggestions leading to a considerable improving of
the manuscript we are indebted to F. Fediuk, Prague.

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