GeolCarp_Vol60_No6_519

www.geologicacarpathica.sk

GEOLOGICA CARPATHICA, DECEMBER 2009, 60, 6, 519—533 doi: 10.2478/v10096-009-0038-8

Introduction

The České středohoří Volcanic Complex (CSVC) is a classi-
cal area where volcanic rocks have been studied for at least
200  years.  Nevertheless,  its  magmatic  and  eruptive  history
has  not  been  completely  reconstructed  yet.  The  post-
Variscan,  intra-plate  alkaline  volcanism  in  the  Bohemian
Massif dates from the Late Cretaceous to Quaternary. It rep-
resents  the  easternmost  extension  of  the  Central  European
Volcanic  Province  (sensu  Wilson  &  Downes  1991),  devel-
oped in the northern foreland of the Alps. Two main volca-
nic  complexes,  the  Doupovské  hory  Mts  and  the  České
středohoří  Mts,  dominate  the  Ohře  (Eger)  Graben  (OG)  in
northwestern  Bohemia  (Fig. 1a),  where  the  principal  volca-
nic activity initiated in the latest Eocene and continued until
the Early Miocene. Basaltic lavas and concomitant volcani-
clastics (generated predominantly by auto- or hyaloclastesis
– Cajz 2000) represent superficial products of both volcanic
complexes.  This  volcanic  period  was  followed  by  a  major
sedimentation  period,  filling  the  Most  Basin  between  the
České středohoří Mts and the Doupovské hory Mts volcanic
complexes (e.g. Malkovský 1987; Rajchl et al. 2008). After
the  main  sedimentation  period  (including  coal  deposition),
the volcanic activity was reactivated, not only in the CSVC,
with a much lower intensity.

Late Miocene volcanic activity in the České středohoří

Mountains (Ohře/Eger Graben, northern Bohemia)

VLADIMÍR CAJZ

1,2

, VLADISLAV RAPPRICH

3,4

, VOJTĚCH ERBAN

, ZOLTAN PÉCSKAY

and MIROSLAV RADOŇ

Institute of Geology, Academy of Sciences of the Czech Republic, Rozvojová 269, 165 02 Prague, Czech Republic; cajz@gli.cas.cz

Faculty of Science, J.E. Purkyně University, České mládeže 8, 400 96 Ústí nad Labem, Czech Republic

Czech Geological Survey, Klárov 3, 118 21 Prague, Czech Republic; vladislav.rapprich@geology.cz; vojtech.erban@geology.cz

Faculty of Science, Charles University in Prague, Albertov 6, 128 43 Prague, Czech Republic

Institute of Nuclear Research of the Hungarian Academy of Sciences, Bem tér 18/C, H-4001 Debrecen, Hungary;

pecskay@namafia.atomki.hu

Regional Museum Teplice, Zámecké nám. 14, 415 13 Teplice, Czech Republic; rmtep@seznam.cz

(Manuscript received April 8, 2008; accepted in revised form June 25, 2009)

Abstract: First occurrences of superficial bodies of Late Miocene volcanic activity were found in the western part of the
České středohoří Volcanic Complex (CSVC) and extended our knowledge of its volcanostratigraphy. Their K-Ar ages
(9.59, 9.61 and 11.36 Ma) correspond to the age of alkaline basaltic rocks of the youngest known Intrusive Suite of this
area. Unlike the previously known subvolcanic bodies of this system, the newly observed bodies are represented by super-
ficial products: two scoria cones with remnants of lava flows and one exclusive lava flow produced from a lava cone. The
magmas forming all three occurrences are basanitic. Their primitive chemical composition Sr (0.70347—0.70361) and Nd
(0.51279—0.51284) isotope ratios are similar to the products of the first and third volcanic formation of the CSVC. The
proved existence of superficial products of the youngest volcanic formation, together with clear superposition relations
to sedimentary formations and the chemical character of the youngest magmas in the central part of the Ohře (Eger)
Graben support the stratigraphic scheme of volcanic activity in the České středohoří Mts. The eruptive style of the
youngest formation volcanoes was purely magmatic (Strombolian) with no phreatic influence.

Key words: Upper Miocene, České středohoří, Štrbice Formation, volcanostratigraphy, geochemistry, K-Ar dating,
cinder cone, basanite.

The lithostratigraphic subdivision of superficial volcanic

products  of  the  CSVC  has  been  proposed  by  Cajz  (2000).
Based on detailed fieldwork, it reflects superposition of lithos-
tratigraphic units differing in their volcanology and petrogra-
phy.  This  lithostratigraphic  scheme  is  also  supported  by
bulk-rock geochemistry and Sr/Nd isotopic composition of la-
vas (Cajz et al. 1999; Ulrych et al. 2001). The following four
units were defined: (1) Ústí Formation, represented by olivine
foidites—basanites;  (2)  Děčín  Formation,  formed  mainly  by
tephrites and trachybasalts; (3) Dobrná Formation, represented
again by effusions of olivine foidites—basanites; and (4) basa-
nitic Štrbice Formation.

The Upper Miocene Štrbice Formation thus represents the

youngest lithostratigraphic unit of the CSVC. Volcanic activi-
ty was restored after a period of magmatic quiescence, and the
basaltic magmas of the Štrbice Formation penetrated Miocene
sediments of the Most Basin. The age of the Štrbice Formation
was deduced from its geological position and later supported
by K-Ar radiometric dating (Shrbený & Vokurka 1985; Cajz
et al. 1999). Despite the thorough documentation (Cajz 2000),
this formation has not been accepted by some authors (e.g.
Kukal in Šalanský 2004), as no corresponding superficial
products were known. Volcanic bodies of this late magmatic
activity are scarce and small in scale, their superficial products
have mostly been destroyed by erosion, and the pertinence of

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CAJZ, RAPPRICH, ERBAN, PÉCSKAY and RADOŇ

Fig. 1. a – Location of the České středohoří Volcanic Complex (CSVC) in the Ohře (Eger) Graben, NW Bohemia. Volcanic complexes and
solitary bodies marked in black. b – The studied sites and their position in the CSVC: 1 – Ostrý, 2 – Hradiš ko, 3 – Úžín, 4 – Křemýž,
5 – Štrbice-Světec feeder-and-sill system.

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LATE MIOCENE VOLCANIC ACTIVITY IN THE ČESKÉ STŘEDOHOŘÍ

MOUNTAINS (NORTHERN BOHEMIA)

the below described relics to the youngest volcanic activity of
the region has not been recognized before. Our paper has an
ambition to fill this gap because these latest volcanics are im-
portant for the late tectonomagmatic evolution of the central
part of the OG.

This study aimed at detailed description of volcanic prod-

ucts and their relations to ambient rocks, petrology, bulk-rock
(incl. trace and rare earth elements) and mineral chemistry,
isotopic composition and K-Ar radiometric dating.

Analytical methods

Bulk-rock analyses of the major oxides and selected trace

elements (Cr, Ni, Cu and Zn) were performed in the labs of the
Czech Geological Survey by combination of titration, FAAS,
photometry, coulometry and X-ray fluorescence. The larger
set of trace elements (including REE) were analysed in the Ac-
tivation Laboratories Ltd., Ancaster, Canada, using lithium
metaborate/tetraborate fusion and Inductively Coupled Plasma
Mass Spectrometer detection.

For the isotope study, samples were dissolved using a com-

bined HF-HCl-HNO

attack. Strontium was isolated by exchange

chromatography techniques using Sr.spec Eichrom resin, Nd
with TRU.spec and Ln.spec Eichrom resins. Isotopic analyses
were performed on a Finnigan MAT 262 Thermal Ionization
Mass Spectrometer in a dynamic mode using a double Re fila-
ment assembly. The

143

Nd/

144

Nd ratios were corrected for

mass fractionation to

146

Nd/

144

Nd = 0.7219,

Sr/

Sr ratios as-

suming

Sr/

Sr = 0.1194. External reproducibility was set by

repeated analyses of the La Jolla (

143

Nd/

144

Nd = 0.511852 ± 14

σ; n=23)) and NBS 987 (

Sr/

Sr = 0.710247 ± 26 (2

σ;

n = 25)) isotopic reference materials. For further details see
Míková & Denková (2007).

The K-Ar age determinations were made on bulk-rock sam-

ples, the same powders having been used for potassium deter-
mination.  Potassium  was  determined  by  flame  photometry
using  a  CORNING  480  machine,  sample  solutions  being
bracketed by standards. Argon was extracted by fusion under
vacuum  conditions,  with  pure

Ar added as a “spike”. The

isotopic ratios were measured on a 15 cm radius magnetic sec-
tor-type mass spectrometer under static mode, built in Debre-
cen, Hungary. Details of the instruments, the applied methods
and results of calibration have been published by Balogh
(1985) and others. The atomic constants suggested by Steiger
& Jäger (1977) were used for the calculation of ages. The ana-
lytical errors are quoted for the 68% confidence level (one
standard deviation).

The analyses of rock-forming minerals were performed us-

ing a CAMECA SX-100 electron microprobe. They were car-
ried out in wavelength dispersive spectrometers with beam
diameter of 2 µm and accelerating potential of 15 kV. A beam
current of 10 nA was measured on a Faraday cup. Counting
time 10 s was used for all elements. The standards used were:
SiO

[Si K

α], Al

[Al K

α], diopside [Ca Kα], Fe

[Fe

α], barite [Ba Lα] and [S Kα], celestite [Sr Lα]. Data were

reduced using the X-PHI correction.

Geochemical calculations and visualization of analytical data

were performed using GCDkit software (Janoušek et al. 2006).

Geological setting

The youngest volcanic activity of the CSVC, postdating the

Miocene lacustrine sedimentation, was first described by Peli-
kan (1895) from a lignite mine N of Bílina. Since then, several
bodies in subvolcanic position have been discovered during
lignite exploration and exploitation (e.g. Brus & Hurník
1984).

A system of intrusions in Miocene sediments was described

by Macák (1963) from drill cores in an erosional relict of the
Most  Basin  near  Křemýž  (Fig. 1b).  Unfortunately,  no  cores
were archived from these drillings, thus no material is avail-
able  for  detailed  petrological  and  radiometric  examinations.
An Upper Miocene feeder-and-sill system is known from the
wider  area  (about  6 km

) around Štrbice and Světec villages

with the central conduit at Pohradická hora Hill (13.0 ± 1.1 Ma
in  Cajz  et  al.  1999).  Several  sills  have  been  reported  from
abandoned and reclaimed coal pits at Světecká výšina Hill and
in the Štrbice sandpit (12.0 Ma in Shrbený & Vokurka 1985;
9.0 Ma  in  Kopecký  1987—1988;  neither  of  these  ages  give
analytical  errors).  A  new  detailed  survey  showed  other  sills
and  a  possible  parasitic  dyke-modified  vent  of  Hůrka  Hill
between Štrbice and Kostomlaty. Although the production of
superficial volcanics is highly probable, no lava or pyroclastic
facies of these conduits have been preserved.

All previously described Upper Miocene basalts are

present in the form of subvolcanic bodies, with no superfi-
cial products preserved. Within the present detailed geologi-
cal survey on the western margins of the CSVC, three Upper
Miocene basaltic occurrences were newly observed. The first
two are remnants of monogenic Strombolian cones with relics
of lava flows emitted; the last one is a lava flow overlying the
Miocene sediments.

Newly documented sites

Ostrý Hill (50

°29’50”N, 13°51’38”E)

The basanitic vent of Ostrý Hill (9.6±0.5 Ma) located near

the Měrunice village (Fig. 2) SE of Bílina most probably pro-
duced  lavas  now  overlying  the  sands  of  the  Most  Formation
preserved  on  Hradiš any  Hill,  NNE  of  the  vent.  The  30—60
meters difference in altitude of the lava and the recent top of
the  feeder  (well  documented  in  the  cross-section)  can  be  ex-
plained by different erosion of mostly loose cinder cone mate-
rial and solid tabular lava body – the altitude of the real place
of lava production from the cone should have been higher than
the recent top. Nevertheless, we suppose younger tectonic ac-
tivity which caused uplift of northern block(s) in the first tens
of meters. The lavas also reached the areas to the NW of the
vent where sand-dominated sediments are also overlain by rel-
ics of coherent basanitic bodies (Fig. 3).

Fortunately, a near-vent superficial pyroclastic rock sur-

rounding  the  compact  conduit  has  been  preserved,  nested  on
the sands of the Most Formation. It was formerly interpreted
as a diatreme facies of a maar volcano (Kopecký 1987—1988).
Pyroclastic  deposits  consist  of  irregular  vesiculated  scoria
fragments  reaching  3 cm  in  size.  Scoria  fragments  show  no

522

CAJZ, RAPPRICH, ERBAN, PÉCSKAY and RADOŇ

Fig. 2. A panoramic photo of the Ostrý and Hradiš ko remnants of scoria cones. A view from the west.

Fig. 3. A schematic map of the Ostrý scoria cone, the source of the Hradiš any Hill lava. Supposed faults indicated in the cross-section are
not shown on the map because of the lack of indications of their course. Basanites of the older Ústí Formation in the cross-section also com-
prise bentonitized facies.

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LATE MIOCENE VOLCANIC ACTIVITY IN THE ČESKÉ STŘEDOHOŘÍ

MOUNTAINS (NORTHERN BOHEMIA)

welding  patterns;  therefore,  this  deposit  is  interpreted  as  a
product  of  Strombolian  activity.  Few  unique  cow-dung
bombs, welded upon accumulation of non-welded scoria frag-
ments, were observed (Fig. 4). The combination of non-weld-
ed  scoriae  with  few  cow-dung  bombs  suggests  that  these
deposits represent upper-crater facies of a former cinder cone
(e.g.  Rapprich  et  al.  2007).  The  position  of  this  scoria  cone

Fig. 4. A cow-dung bomb welded upon non-welded scoriae at Ostrý Hill.

Fig. 5. A schematic map of the Hradiš ko scoria cone and erosional remnants of lavas emitted to the west.

penetrating  and  resting  upon  Miocene  sediments  proved  that
the  magmatic  activity  constituted  a  volcano  after  the  deposi-
tion of the Most Formation sediments. No evidence for previ-
ous phreatomagmatic activity was found. On the other hand, a
remnant of an older scoria cone buried by Miocene sands can
be seen in a nearby stream-gorge. Pyroclastic deposits of the
older  volcano  consist  of  strongly  altered,  periclinally  bedded
scoriae. No further analyses or radiometric dating could be ob-
tained due to the high degree of weathering. Based on the geo-
logical  setting  and  on  analogy  with  the  surrounding  bodies,
this older scoria cone can be assigned to the preceding Dobrná
Formation.

Hradiš ko Hill (50

°28’27”N, 13°50’26”E)

The Hradiš ko Hill between the villages of Řisuty and

Měrunice  (see  Fig. 2)  represents  a  multiphase  coherent  con-
duit.  The  Upper  Miocene  (9.6 ± 0.4 Ma)  scoria  cone  with  a
basanitic  feeder  overlies  a  remnant  of  Lower  Miocene
(20.72 ± 0.94 Ma)  picrobasalt-basanite  feeder.  The  earlier  pi-
crobasalt-basanite activity was significantly more widespread;
this event is represented by several remnants of lavas, coher-
ent and clastic conduits (Fig. 5). The age of the volcanic activ-
ity corresponds well with the age of lava excavated in a nearby
quarry on Stříbrník Hill (19.2 Ma – Lustrino & Wilson 2007;
no errors given). This lava could also be assigned to an early
Hradiš ko source-vent.

524

CAJZ, RAPPRICH, ERBAN, PÉCSKAY and RADOŇ

Younger basanitic volcanism is preserved in the form of a

scoria cone relict. Pyroclastic deposits consist of highly ve-
siculated  and  non-welded,  undeformed  solid  scoria  frag-
ments  with  significant  inter-clast  voids  (Fig. 6).  The
diameter  of  common  scoria  fragments  ranges  from  1 cm  to
10 cm.  Scarce  bombs  are  of  spindle  shape.  Bedding  planes
are poorly visible but appear to dip outward, corresponding
to  a  proximal  wall  facies  on  outer  volcano  slopes.  In  a
unique sample, a small piece of silicified wood was observed
within the scoria deposit (Fig. 7).

Fig. 6. Non-welded scoria fragments at the Hradiš ko scoria cone.
Note the preserved voids among individual fragments. A 1 Euro
coin represents the scale.

Fig. 7. A silicified wood fragment in scoriae of Hradiš ko Hill.

Fig. 8. A schematized section of the volcanic and sedimentary depos-
its exposed during the construction of highway D8 at Úžín. a – alter-
nation of yellow volcaniclastic clays (each of 3 layers is ca. 80 cm in
thickness) and light grey lacustrine clays of the Miocene Most Ba-
sin  (approximately  20 cm  thick  –  two  intercalations  were  ex-
posed);  b  –  1.5 m  of  greenish  volcanogenic  clays  filling  a
paleo-valley;  c  –  0.7 m  of  greyish-violet  clay  overlying  the  green
volcanogenic clay and filling the rest of the paleo-valley; d – 3—4 m
of greyish-yellow volcanogenic sandy clays burying the flat paleo-re-
lief; e – an olivine basalt lava flow.

Fig. 9. Columnar-jointed basanitic lava excavated during the con-
struction of highway D8. (See first author serving as a scale.)

Lava flow near Úžín (50

°41’14” N, 13°57’08” E)

The construction of a new segment of highway from Pra-

gue  to  Dresden  (D8,  Exit  74),  touching  the  NW  margin  of
Ústí nad Labem, required excavations in the extreme NE part
of the Most Basin, where the basin adjoins the northern part
of the CSVC. Deposits of wasted clayey material most prob-
ably  produced  at  the  beginning  of  mining  activities  for  lig-
nite were exposed during construction. The oldest geological
maps  (Hibsch  1926)  already  document  this  waste  deposit
and  not  the  lava  beneath.  Columnar-jointed  basanitic  lava
(11.4 ± 0.4 Ma)  overlies  clayey  sediments  of  the  Most  Basin

(Fig. 8). The columns are vertical, 40—80 cm in diameter and
relatively regular 5- to 6-angular. The preserved thickness of
the  lava  flow  reaches  maximum  3 m  (Fig. 9).  A  maximum
30 cm thick layer of hyaloclastic-type breccia is present at the
bottom of the flow. Hyaloclastesis was caused by an interac-
tion  of  lava  with  the  underlying  water-saturated  sediments.
These sediments are well-stratified clays to sandy clays of two
different colours. Greyish blue fine clay in 5—10 cm thick lay-
ers alternates with brownish red clays to sandy clays in layers
20—30 cm  thick  (see  Fig. 8  for  details).  This  stratified  se-
quence was excavated to the depth of 2 m below the base of
the lava flow but its base was not reached. Its origin can be ex-
plained by changing source areas for the sedimentary material.
The bluish clay is typical for the Most Formation and originat-
ed from material transported over a longer distance in the ba-

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LATE MIOCENE VOLCANIC ACTIVITY IN THE ČESKÉ STŘEDOHOŘÍ

MOUNTAINS (NORTHERN BOHEMIA)

sin. The latter material is supposed to be a product of multiple
events of re-sedimentation of hyaloclastics that belong to the
Ústí Formation (the oldest volcanism of the CSVC) and were
exposed only a few hundred meters to the SW.

The lava flow exposed near Úžín was most probably emit-

ted from the vent located on Jedlová hora Hill, some 3 km to
the  NE.  This  elevation  is  now  covered  by  basanitic  lava
blocks  reaching  up  to  1.5 m  in  diameter  (Fig. 10),  but  no
outcrop can be found. No pyroclastic deposits were observed
– probably due to the activity of volatile-poor magma, pro-
ducing solely lava flows with no concomitant eruptive activ-
ity  (Head  &  Wilson  1989).  Alternatively,  pyroclastic
deposits  may  already  have  been  eroded.  The  Jedlová  hora
Hill conduit partly penetrates older basaltic sequences of the
Ústí Formation. Its magmatic material, corresponding to the
Úžín  lava  in  its  petrology,  ascended  along  the  fault  cutting
older  volcanic  products  and  constituting  their  limit  against
Cretaceous sediments.

Petrography and geochemistry

Petrography and mineral chemistry

All the studied rocks are of similar petrography. They are

classified as basanites and consist (in order of decreasing
abundance) of clinopyroxene, olivine, Ti-magnetite, plagio-
clase and nepheline. The rocks are fine-grained with phenoc-
rysts reaching some 2 mm and scarcely 5 mm. The phenocrysts
are represented solely by olivine and clinopyroxene.

Olivine phenocrysts are common in the rocks of the Úžín

lava and Hradiš ko cinder cone, maximum 5 mm in diameter.
On  the  other  hand,  olivine  in  the  conduit  of  Ostrý  Hill  is
present  only  in  the  form  of  small  crystals  in  matrix.  Olivine
crystals from different sites vary in their composition, reflect-
ing  slight  differences  in  bulk-rock  chemistry  and  crystalliza-
tion  history  (Table 1).  The  most  magnesian  olivines  are
present in the silica-poorest rock at Ostrý Hill, with forsterite
component varying from Fo

in the cores to Fo

in the rims.

More differentiated olivine crystals at Hradiš ko Hill range
from Fo

90.5

in cores to Fo

in rims. A slight differentiation

was documented on scarce fresh olivine crystals from the Úžín
lava. Olivine crystallization initiated with the composition of
Fo

in the cores and terminated with Fo

in the rims.

Clinopyroxene is the most abundant mineral in the studied

basanites. Its composition is relatively uniform and indepen-
dent of the sampling site. A slight increase in ferroan compo-
nent  towards  the  rims  was  documented  in  all  crystals
(Fig. 11). A large portion of the data fit within the immisci-
bility field above the 50 % wollastonite limit in the common
classification  diagram  recommended  by  IMA  (Morimoto
1988 – Fig. 11a), mainly due to the presence of Ca-Tscher-
mak‘s molecules. This is in disagreement with the real chem-
ical  composition  of  the  analysed  minerals,  where  Ca
contents  range  between  0.87  and  0.93  apfu  (Table 2,
Fig. 11b  and  11c  in  detail).  Ca  and  Al  (reaching  0.4  apfu)

Sampling site

Ostrý

Hradišťko Úžín

Comment

Rim

average

Rim

average

Rim

average

Core
Fo

min

Core

max

Core
Fo

min

Core

max

Core
Fo

min

Core

max

SiO

40.72 40.73 39.64 40.27 41.43 39.52 40.34 40.57 39.25

0.03 0.03 0.02 0.06 0.01 0.16 0.06 0.03 0.04

0.01 0.03 0.02 0.01 0.00 0.02 0.05 0.00 0.00

FeO 12.91

8.66

13.64

13.91

8.71

18.65

13.35

12.03

20.37

MnO

0.10 0.14 0.24 0.27 0.15 0.39 0.20 0.33 0.43

MgO

46.78 50.85 45.67 46.08 49.88 41.44 45.73 47.25 40.77

NiO

0.27 0.36 0.33 0.31 0.39 0.15 0.23 0.40 0.17

CaO

0.20 0.05 0.20 0.29 0.06 0.35 0.30 0.06 0.43

Total

101.06

100.88

99.78

101.24

100.66

100.74

100.28

100.67

101.50

1.002

0.987

0.994

0.996

1.004

1.003

1.000

0.998

0.001

0.000

0.002

0.000

0.005

0.002

0.001

0.266

0.176

0.286

0.288

0.177

0.399

0.278

0.248

0.433

0.002

0.003

0.005

0.006

0.003

0.008

0.004

0.007

0.009

1.716

1.837

1.707

1.699

1.803

1.565

1.695

1.736

1.545

0.000

0.001

0.000

0.001

0.000

0.005

0.007

0.006

0.008

0.003

0.005

0.008

0.003

Atoms per
formula
unit

0.005

0.001

0.005

0.008

0.002

0.010

0.008

0.001

0.012

86.08 92.22 85.72 85.24 90.51 78.43 85.04 87.18 77.44

Oxides in %; atoms per formula unit Σ=3; content of forsterite component in mol %.

Table 1: Olivine composition of the studied rocks.

Fig. 10. Basanitic lava disintegrated into blocks at Jedlová hora Hill.

526

CAJZ, RAPPRICH, ERBAN, PÉCSKAY and RADOŇ

Fig. 11. Composition of clinopyroxenes: (a) Quadrilateral diagram (Morimoto 1988), (b)
Ca-Fe diagram (Rapprich 2005), (c) detail of (b).

contents,  classify  the  clinopyroxenes  as
aluminian augites (their Ca-rich edge) or
diopsides.  Classification  En-Fs-Wo  dia-
gram  was  constructed  on  basis  of  nor-
malized  Fe,  Mg  and  Ca  atomic
proportions  according  to  IMA  recom-
mendations  (Morimoto,  1988;  used  e.g.
by Brady). Calculation of extended end-
member set has followed procedure pub-
lished  by  Rapprich  (2005).  Cores  of
some  larger  diopside  phenocrysts  in  the
coherent  feeder  of  Ostrý  Hill  consist  of
augite (Na

0.07

0.84

0.25

0.04

0.01

0.66

0.03

0.23

1.87

) with exsolu-

tion orthopyroxene (Ca

0.07

0.7

0.02

1.13

0.01

0.13

1.92

) lamel-

lae (Fig. 12). The lamellae form ca 10%
of the mineral. The known composition
of both phases and their ratio allow us to
calculate the original composition. The
original

augite

(Na

0.06

0.77

0.29

0.04

0.01

0.71

0.03

0.22

1.87

)

could have been entrained by the as-
cending magma from underlying mag-
matic rocks.

Sampling site

Ostrý

Hradišťko Úžín

Comment

core zone rim core rim core zone rim

3 4 4 4 3 4 12 5

SiO

48.17 48.06 42.88 48.64 43.26 48.27 49.20 46.79

TiO

1.76 1.84 3.55 2.00 3.87 2.03 1.76 2.79

6.20 6.37 10.22 4.87 8.99 6.14 5.16 6.34

0.36 0.47 0.11 0.05 0.20 0.20 0.34 0.05

FeO

5.96 5.70 8.06 6.49 7.69 6.32 5.90 7.21

MnO

0.11 0.09 0.10 0.12 0.09 0.15 0.12 0.13

MgO

13.65 13.81 10.90 13.85 11.51 13.67 14.25 12.65

CaO

22.92 22.99 22.59 22.85 22.52 21.88 22.24 22.16

0.46 0.47 0.53 0.37 0.43 0.53 0.47 0.47

Total

99.62 99.85 98.98 99.30 98.58 99.28 99.50 98.65

1.785 1.775 1.618 1.813 1.640 1.797 1.824 1.765

0.215 0.225 0.382 0.187 0.360 0.203 0.176 0.235

0.055 0.052 0.073 0.027 0.041 0.067 0.050 0.047

0.084 0.091 0.143 0.073 0.124 0.055 0.052 0.063

0.049 0.051 0.101 0.056 0.110 0.057 0.049 0.079

0.010 0.014 0.003 0.002 0.006 0.006 0.010 0.002

0.754 0.760 0.613 0.770 0.650 0.758 0.787 0.711

0.047 0.032 0.067 0.073 0.068 0.057 0.052 0.098

0.054 0.053 0.044 0.057 0.051 0.084 0.079 0.066

0.004 0.003 0.003 0.004 0.003 0.005 0.004 0.004

0.910 0.910 0.913 0.913 0.914 0.873 0.883 0.895

0.033 0.034 0.039 0.027 0.032 0.038 0.034 0.034

Kch

1.05 1.37 0.33 0.15 0.59 0.58 0.98 0.16

2.24 2.03 3.57 2.53 2.57 3.23 2.40 3.29

0.35 0.29 0.33 0.37 0.28 0.48 0.38 0.41

CAT

5.54 5.24 7.28 2.73 4.10 6.67 4.97 4.67

CTT

4.92 5.11 10.08 5.61 11.04 5.69 4.91 7.92

Ess

6.13 7.06 10.74 4.74 9.85 2.27 2.84 3.03

5.05 4.27 5.57 6.48 5.98 7.10 6.54 8.21

37.52 37.85 30.48 38.30 32.37 37.67 39.17 35.36

Calculated
end-members

37.20 36.78 31.62 39.09 33.22 36.32 37.81 36.96

Oxides in %; atoms per formula unit

Σ = 4; calculated end-members Σ = 100 %. Kch – Kosmochlor, Ae – Aegirine, Ka – Kanoite, CAT –

Ca-Tschermak’s molecule, CTT – Ca-Ti-Tschermak’s molecule, Ess – Esseneite, Fs – Ferrosilite, En – Enstatite, Wo – Wollastonite.

Table 2: Average clinopyroxene compositions of the studied rocks.

527

LATE MIOCENE VOLCANIC ACTIVITY IN THE ČESKÉ STŘEDOHOŘÍ

MOUNTAINS (NORTHERN BOHEMIA)

Fig. 12. Back-scattered electron image of a clinopyroxene core with
orthopyroxene exsolution lamellae (Ostrý Hill).

Plagioclase has the form of laths, usually too small to be

analysed. Plagioclase laths from the Úžín lava correspond to
basic labradorite (An

63—65

33—36

). The analyses of

nepheline were affected by sodium loss during electron beam-
ing. Nevertheless, the results are sufficient to document maxi-
mum 0.15 apfu K and maximum 0.6 apfu Ca.

The studied basanites, and particularly the rock of the Ostrý

Hill feeder, are relatively rich in xenocrysts. Minerals of these
xenocrysts, such as orthopyroxene, K-feldspar and quartz, re-
flect multifarious country rocks entrained during magma as-
cent. We already described extremely magnesian cores of

olivine  phenocrysts  which  represent  relics  of  mantle-derived
xenocrysts. The presence of exsolution lamellae of two slight-
ly  different  enstatites  (one  is  slightly  enriched  in  Al,  Fe  and
Cr)  characterize  orthopyroxene  xenocryst  in  the  basanite  of
Ostrý  Hill.  Both  enstatite  phases  are  extremely  poor  in  Ca
(0.01 apfu) and are equal in their volume. The orthopyroxene
was  corroded  by  magma  and  is  overgrown  first  by  common
magmatic olivine and then by magmatic diopside.

The sample from the Hradiš ko conduit revealed a pseudo-

morph formed by glass and diopside. Glass composition

Fig. 13. Composition of the studied rocks compared with the older
formations in the TAS diagram (Le Bas et al. 1986).

Fig. 14. Box-and-whisker plots showing geochemical differences among lithostratigraphic units of the CSVC. Note that Cr and Ni are in
logarithmic scale.

528

CAJZ, RAPPRICH, ERBAN, PÉCSKAY and RADOŇ

Table 3:

Chemical

composition

basaltic

rocks

the

Štrbice

Formatio

compared

the

lower

lithostratigraphic

units

the

CSVC.

Continued

next

page.

Fig. 15. A spidergram of the studied rocks normalized to NMORB
(Sun & McDonough 1989).

(60—64 wt. % SiO

, 3.5—4 wt. % K

O and only 1.5—1.9 wt. %

O) and pseudomorph shape suggest replacement of

former  K-feldspar.  An  anomalous  composition  is  also  dis-
played  in  diopsides  enclosed  in,  and  flanking,  the  pseudo-
morph.  These  are  poor  diopsides  with  unusually  low
concentrations of Al

and TiO

Bulk-rock chemistry

The studied basaltic products from Hradiš ko, Ostrý and

Úžín are rather primitive ultrabasic alkaline rocks (Table 3).
The petrographic character of the basanites is confirmed by
their chemical composition (Le Bas 1986 – Fig. 13), with
normative olivine contents above 10 % (11—20 %). The sam-
ple from Úžín falls in the basalt field, however, the SiO

content  is  only  slightly  above  45 %  when  recalculated  on  a
water-free  basis,  with  rather  high  water  content  (3 %).  The
samples  rank  between  the  most  primitive  basaltic  rocks  in
the  CSVC,  having  11—13.1 %  MgO,  258—473 ppm  Cr  and
173—275 ppm  Ni  (Fig. 14).  The  unfractionated  character  of
the studied samples is further underlined by low Al

con-

tents. A similar picture is given by the trace element chemis-
try (Fig. 15). In addition to the above mentioned high Cr and
Ni contents, the samples rank in the lower part of the HFSE
(high  field  strength  element)  concentration  range  of  the
CSVC basanitic rocks, whereas LILE contents are moderate.
REE  contents  are  low  compared  to  other  CSVC  samples
however,  the  LREE/HREE  ratio  is  high  for  the  Hradiš ko
and  Ostrý  samples,  and  only  moderate  for  the  Úžín  sample.
The isotopic signature is in good agreement with the data pre-
viously published for the Ústí and Dobrná Formations (LF and
UMF  in  Cajz  et  al.  1999),  with  (

Sr/

Sr)

between 0.70347

and 0.70361, and (

143

Nd/

144

Nd)

between 0.51279—0.51284

(Table 4 and Fig. 16).

n Ú

ěč

ín

obr

ná

Štr

ple

rep

e (

n =

rep

e (

n =

rep

e (

n =

S33

S34

S35

cat

ion

rý

Hrad

žín

.59

.54

.88

.20

.29

.06

.52

.82

.77

.36

.46

.06

.95

.61

TiO

.32

.11

.56

.75

.22

.82

.97

.26

.83

.69

.56

.25

.13

.21

.51

FeO

.98

MnO

.23

.28

.78

.48

.34

.08

.13

.95

.78

.22

.44

.63

.25

.32

.70

.44

.07

.51

.50

.53

.46

.52

.12

BaO

.08

.50

.16

.93

O+

.67

O-

.35

.11

.08

.22

.12

.16

.49

.82

.04

.71

.33

.66

.75

.74

.07

00.

.80

.15

.11

.30

.85

.31

.77

.14

.74

.36

.05

.01

.58

.04

529

LATE MIOCENE VOLCANIC ACTIVITY IN THE ČESKÉ STŘEDOHOŘÍ

MOUNTAINS (NORTHERN BOHEMIA)

Table 3:

Continued.

n Ú

ěč

ín

Št

ice

ple

rep

e (

n =

rep

e (

n =

rep

e (

n =

S33

S35

cat

ion

rý

žín

767

1214

1030

1336

764

649

855

783

616

609

680

929

290

857

104

262

222

322

258

344

473

281

253

.48

167

360

168

110

226

173

275

251

199

217

978

1408

1126

1568

872

773

960

1070

1040

657

889

1032

23.

281

393

405

500

304

270

340

221

209

199

290

369

.48

.25

.60

.89

.85

.20

52.

154

329

218

454

133

114

144

157

118

256

380

13.

49.

.98

.89

.07

.57

.75

.60

.31

.40

.86

.54

.49

.31

.76

.56

.64

.91

.94

.74

.81

.40

.90

.03

.62

.21

e (M

–36

(

n = 6)

–2

= 5)

–25

= 4)

344

318

376

452 0.

433 0.

472

366 0.

703

56 0.

374

= 4

)

= 5

)

= 4

)

285

284

287

272 0.

270 0.

276

285 0.

512

82 0.

286

143

Nd/

144

= 4

)

= 5

)

= 4

)

e 4

. f

xid

in %

; tr

ace

ele

incl

ing

EE in ppm

. Hr

. — H

rad

iš

ťk

o f

eed

e U

ioc

e s

ria

; PH

—

Pohr

adi

cká ho

ill

; KV

—

eř

ový

ch H

ill

. *

—

ana

lys

. C

S33–CS

new

data,

r analy

s f

ch e

t al

. (

2001

). K

-A

r age

ent and

dat

a of

litho

str

atig

phi

c u

ed, co

pare

d to

pape

530

CAJZ, RAPPRICH, ERBAN, PÉCSKAY and RADOŇ

Fig. 16. Initial Sr-Nd isotopic ratios of the Štrbice Formation com-
pared with Ústí, Děčín and Dobrná Formations (Ulrych et al. 2001,
2002b, new data). The initial ratios were calculated from the age of
individual samples or formations. Mantle reservoirs after Zindler &
Hart (1986) and Cebria & Wilson (1995): DMM = depleted mantle
MORB, E-DMM = enriched-depleted mantle MORB, BSE = bulk
silicate Earth, HIMU = high µ unit, EAR = European asthenospher-
ic reservoir.

Table 4: New data on radiometric ages and Sr-Nd isotopes obtained on superficial products, and complementary analyses of

143

Nd/

144

ratio of Ústí Formation samples.

Sample

CS33 CS34 CS35 CS36 1297 1374 1363

Formation Štrbice

Štrbice

Dobrná Ústí Ústí Ústí

Location and
rock type

Ostrý,

basanite

Hradišťko,

basanite

Úžín,

basanite

Hradišťko,

olivine basalt

Kam. Šenov,

basanite

Taneček,

basanite

Hlinná,

basanite

K(%) / K

O(%)

0.96 / 1.16

1.13 / 1.40

1.00 / 0.84

0.97 / 1.26

rad

(ccSTP/g)

3.595 × 10

–7

4.224 × 10

–7

4.431 × 10

–7

7.859 × 10

–7

* * *

rad

(%) 27.0

47.2

46.6

33.5 * * *

K/Ar age (Ma)

9.61 ± 0.51

9.59 ± 0.36

11.36 ± 0.42

20.72 ± 0.94

Sr/

Sr 0.703624

0.703486

0.703615

* * * *

2SE(M) 0.000013

0.000011

0.000013

* * * *

(

Sr/

Sr)

0.703609

0.703473

0.703597

* * * *

143

Nd/

144

Nd 0.512799

0.512839

0.512850 * 0.512872

0.512844

0.512848

2SE(M) 0.000007

0.000009

0.000011 * 0.000013

0.000014

0.000016

(

143

Nd/

144

Nd)

0.512793

0.512832

0.512842

* * * *

Analytical errors for isotopic data expressed as 2 standard errors of the mean (2SE(M)). Initial ratios calculated using element concentrations report-
ed in Table 3 and decay constants suggested by Steiger & Jäger (1977). In addition to data for the three localities discussed in text, three

143

Nd/

144

values extending the dataset (Ulrych et al. 2001) of Ústí Formation are given. * — not analysed for this paper.

Discussion

Our research has shown that the scoria cones of the Štrbice

Formation  SE  of  Bílina  overlie  older  volcanoes  of  similar
composition  and  eruptive  style  or  Miocene  sediments  devel-
oped during a prolonged recess in volcanic activity. Eruptions
of  the  Štrbice  Formation  were  dominated  by  Strombolian
style, and no welded accumulations suggesting the presence of
Hawaiian  eruptions  (sensu  Sumner  et  al.  2005)  were  ob-
served.  A  unique  cow-dung  bomb  found  on  Ostrý  Hill
should  document  only  the  presence  of  a  crater  facies  of  a
Strombolian  scoria  cone,  where  plastic  bombs  may  also  be

present (e.g. Rapprich et al. 2007 and references therein). As
we did not find any deposits enriched in xenolithic material
or characterized by a high fragmentation index, we suggest
that no initial phreatomagmatic phase (Schmincke 1977;
Risso et al. 2008) took place and the activity started directly
in Strombolian style.

All previously discovered Upper Miocene volcanic bodies

of the CSVC are located in its western part, and so are all the
newly  described  apparatuses.  Only  the  Úžín  lava  is  situated
some 10—15 km to the NE. The 9—13.5 Ma time span of volca-
nic activity of the Štrbice Formation within the CSVC is char-
acterized by olivine foidite-tephrite-trachybasalt-trachyte suite
on the western margins of the Bohemian Massif (Teplá High-
land and Cheb-Domažlice Graben – Ulrych et al. 1999; Ul-
rych  et  al.  2002a;  Lustrino  &  Wilson  2007),  too.  This  is  the
interval which coincides with the K-Ar ages of three basaltic
bodies in Saxony (Pfeiffer et al. 1984; Kaiser & Pilot 1986).
Two other locations with ultramafic dykes of the same age are
known from the Bohemian Cretaceous Basin near the intersec-
tion  of  the  Stráž  Fault  and  Lusatian  Fault  (Ulrych  &  Pivec
1997).  Their  ascent  is  most  probably  linked  to  these  fault
zones. The rest of the Bohemian Massif appeared to be calm at
that time, even including areas previously associated with am-
ple volcanism (and situated inside the OG). Unlike in western
Bohemia,  no  differentiated  volcanic  rocks  of  Late  Miocene
activity were found in the CSVC. This suggests that the last
volcanic activity in this region resulted from tectonic remo-
bilization, allowing ascent of primitive magmas. This remo-
bilization has only partly influenced the structure of the OG,
much  like  that  of  other  parts  of  the  Bohemian  Massif.  Late
Miocene  volcanic  activity  appears  to  be  independent  of  the
structure of the OG.

The results of this study allow us to redefine the Štrbice For-

mation of the CSVC (sensu Cajz 2000). Its former definition
based on intrusive members only is renewed now by description
of corresponding conduits, effusive members and pyroclastic
deposits including their superpositional characteristics, and
newly available K-Ar and other geochemical data. Thus, the
recent definition of the Štrbice Formation is fully comparable
to the other formations of the CSVC.

531

LATE MIOCENE VOLCANIC ACTIVITY IN THE ČESKÉ STŘEDOHOŘÍ

MOUNTAINS (NORTHERN BOHEMIA)

Other supposed superficial products of Miocene synsedi-

mentary  volcanism  were  reported  from  Mradice  near  Louny
by Váně (1981) and later interpreted by Kopecký (1987—1988)
as  horizontally  propagating  clastic  dykes.  These  outcrops
were completely destroyed in the 1980s. A support for any of
the  hypotheses  is  therefore  very  complicated.  Nevertheless,
we can point to the following facts: i) no responsible vent has
been  found  yet,  ii)  the  reported  thickness  reaches  30 m  and
significantly  exceeds  the  thickness  of  sedimentary  cover
(Váně 1981), and iii) hydroclastic intrusion in a shallow depth
in unconsolidated wet sediments would rather propagate verti-
cally.  We  therefore  believe  that  the  local  volcaniclastic  (epi-
clastic)  material  most  probably  represents  deposits  of
polymict gravity flows generated from older volcanic forma-
tions and basement of the CSVC, deposited into tectonic “mi-
cro  grabens”  along  the  Ohře/Eger  Fault  Zone  (Hradecký
1977). A similar feature is well known from the central part of
the CSVC (Cajz 1992).

A general geochemical description of the CSVC superficial

products was given by Cajz et al. (1999). New field and ana-
lytical data are available since than, slightly changing our un-
derstanding  of  volcanic  complex  development.  Accordingly,
we have refined the original dataset to comply with new ob-
servations  (Table 3).  Three  samples  formerly  assigned  to  the
Ústí Formation were rearranged into the Dobrná Formation as
follows: i) 1373 (Měrunice) with respect to radiometric data of
Lustrino & Wilson (2007); ii) 1378 (Prackovice) was assigned
to  the  Dobrná  Formation  already  by  Cajz  (2000);  iii)  1380
(Ž árek) was reinterpreted as a younger intrusion into the Ústí
Formation  lavas.  For  two  samples  lacking  Nd-isotopic  ratios
in the original dataset we have added these data (1297 Kame-
nický Šenov and 1374 Taneček – Table 4). The Nd-isotopic
ratio of the sample 1363 (Hlinná), the sample with most pro-
nounced HIMU-like component influence documented by Ul-
rych et al. (2002b), was re-analysed (see Table 4). The newly
obtained

143

Nd/

144

Nd value is significantly higher and the

sample plots into the mantle array, in line with the rest of the
CSVC  samples.  Therefore,  the  presence  of  HIMU  (high-µ
unit)    component  in  the  České  středohoří  Mts  lavas  remains
questionable. If compared with the reference dataset (Table 3,
Fig. 14),  the  composition  of  the  presented  Štrbice  Formation
products is virtually undistinguishable from the Ústí and Do-
brná  Formations  products.  Nevertheless,  the  Štrbice  Forma-
tion lavas with their high mg-values and Cr and Nd contents
rank among the most primitive CSVC rocks. Although scarce,
partly resorbed crust-derived mineral grains were observed in
thin  section,  bulk-rock  geochemistry  of  the  basanites  shows
no signs of a significant crustal contamination.

The source of the Cenozoic basaltic magmas in anorogenic

settings in Europe was widely discussed and reviewed recent-
ly  (e.g.  Wilson  &  Downes  2006;  Lustrino  &  Wilson  2007).
Based  on  trace  element  and  isotopic  indications,  the  most
probable  common  source  is  plume-related  asthenospheric
mantle,  termed  EAR  (European  Astenospheric  Reservoir  –
Cebria & Wilson 1995; Wilson 1997), or LVC (Low Velocity
Component – Hoernle et al. 1995), with a number of small-
er domains with variable degree of enrichment. The resulting
image in the Sr-Nd isotopic space is a linear array between
EAR and Bulk Silicate Earth composition. All the analysed

samples of the Štrbice Formation, together with other isoto-
pic data for the CSVC basalts (see Lustrino & Wilson 2007)
fit  well  within  this  model.  There  is  no  significant  offset  in
the  Sr-Nd  isotopic  data  from  the  mantle  array  towards  the
HIMU  component,  suggesting  no  or  negligible  influence  of
this  component  during  generation  of  the  Štrbice  Formation
lavas (see Fig. 16). However, its presence cannot be definitely
ruled out without Pb isotopic data.

Moderate to low Y and HREE contents, together with low

Zr/Nb ratio and moderate to high LREE/HREE ratio, indicate
mixing of low degree partial melts of both spinel and garnet
lherzolite (model of Harangi 2001).

The extreme magnesian composition of olivine cores sug-

gests  the  presence  of  relics  of  xenocrysts  derived  from  the
upper  mantle.  Presence  of  such  xenocrysts  may  support  an
idea of rapid ascent not allowing mantle-derived olivines to
lag and to fall through the melt to deeper positions. Some xe-
nocrysts, such as augites, were most probably derived from
older  magmatic  bodies  (Cenozoic  or  even  older)  forming
part  of  the  crustal  basement.  Finally,  one  group  of  xenoc-
rysts  (K-feldspar,  orthopyroxene  and  quartz)  could  have
been  entrained  from  granulites  or  charnockites  described
among xenoliths from the central part of the České středohoří
Mts (e.g. Opletal & Vrána 1989).

Conclusions

The Late Miocene volcanic activity in the western part of

the  České  středohoří  Volcanic  Complex  is  characterized  by
scattered  Strombolian  eruptions  forming  monogenic  cinder
cones  with  basanitic  lava  flows.  No  conjunction  with  either
the course, or the tectonic activity of the entire structure of the
Ohře  (Eger)  Graben  can  be  observed.  This  is  most  probably
caused  by  low  or  no  tectonic  remobilization  of  the  formerly
active  graben(s)  –  the  differentiated  volcanics  in  western
Bohemia  are  situated  outside  the  grabens  (Ohře/Eger  and
Cheb-Domažlice  ones),  as  well  as  other  volcanic  bodies  of
this age (in Saxony and in the Bohemian Cretaceous Basin).

The Late Miocene volcanic activity took place upon older

volcanic edifices of equal composition and similar eruptive
style (especially the Dobrná Formation).

The primitive basanitic lavas of the youngest volcanism

(the Štrbice Formation) geochemically resemble the lavas of
the preceding volcanism (the Ústí and Dobrná Formations)
and document a rapid magma ascent.

Similarly to other intra-plate lavas in continental Europe,

the Upper Miocene products of the CSVC are derived from a
slightly enriched asthenospheric source.

Rapidly ascending magma entrained mineral grains from

crustal rocks, but did not assimilate enough material to modify
the bulk-rock composition significantly.

Volcanic bodies of the Štrbice Formation (9—13.5 Ma)

represent superficial products with explicit relations to the
underlying Miocene sediments. Superposition of the young-
est basalts overlying the Miocene sediments allows us to
complete the definition of the Štrbice Formation, which now
fulfils the same criteria as the other lithostratigraphic units of
the CSVC.

532

CAJZ, RAPPRICH, ERBAN, PÉCSKAY and RADOŇ

Acknowledgments:  This  study  was  supported  by  Project
IAA300130612  “Combined  magnetostratigraphic  studies  of
Cenozoic  volcanics,  Bohemian  Massif”  of  the  Grant  Agency
of the Czech Academy of Sciences, and falls within the Aca-
demic  Research  Plan  AV0Z  30130516.  The  part  of  the  re-
search  pursued  at  the  Czech  Geological  Survey  was  carried
out  within  the  Research  Plan  MZP0002579801.  Special
thanks are due to A. Langrová who operated the microporobe
at the Institute of Geology AS CR and to lab-analysts of the
Czech Geological Survey. M. Rajchl and P. Kycl (both Czech
Geological  Survey)  are  acknowledged  for  field  cooperation
and thoughtful discussions of the paleoenvironment and sedi-
mentary  features.  The  authors  wish  to  thank  P.  Hradecký
(Czech  Geological  Survey)  for  consultation  on  the  origin  of
epiclastic material in the Louny area.

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