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, OCTOBER 2012, 63, 5, 423—435
doi: 10.2478/v10096-012-0033-3
Chronological implications of the paleomagnetic record of
the Late Cenozoic volcanic activity along the Moravia-Silesia
border (NE Bohemian Massif)
VLADIMÍR CAJZ
1
, PETR SCHNABL
1
, ZOLTAN PÉCSKAY
2
, ZUZANA SKÁCELOVÁ
3
, DANIELA
VENHODOVÁ
1
, STANISLAV ŠLECHTA
1
and KRISTÝNA ČÍŽKOVÁ
1
1
Institute of Geology AS CR, v.v.i., Rozvojová 269, 165 00 Praha 6, Czech Republic;
cajz@gli.cas.cz; schnabl@gli.cas.cz; slechta@gli.cas.cz; cizkova@gli.cas.cz
2
Institute of Nuclear Research of the Hungarian Academy of Sciences, Bem tér 18/C, H-4001 Debrecen, Hungary; pecskay@namafia.atomki.hu
3
Czech Geological Survey, Erbenova 348, 790 01 Jeseník, Czech Republic; zuzana.skacelova@geology.cz
(Manuscript received January 9, 2012; accepted in revised form March 13, 2012)
Abstract: This paper presents the results of a paleomagnetic study carried out on Plio-Pleistocene Cenozoic basalts from
the NE part of the Bohemian Massif. Paleomagnetic data were supplemented by 27 newly obtained K/Ar age determina-
tions. Lavas and volcaniclastics from 6 volcanoes were sampled. The declination and inclination values of paleomagnetic
vectors vary in the ranges of 130 to 174 and —85 to —68° for reversed polarity (Pleistocene); or 345 to 350° and around 62°
for normal polarity (Pliocene). Volcanological evaluation and compilation of older geophysical data from field survey
served as the basis for the interpretation of these results. The Pleistocene volcanic stage consists of two volcanic phases,
fairly closely spaced in time. Four volcanoes constitute the Bruntál Volcanic Field; two others are located 20 km to the E
and 65 km to the NW, respectively. The volcanoes are defined as monogenetic ones, producing scoria cones and lavas.
Exceptionally, the largest volcano shows a possibility of remobilization during the youngest volcanic phase, suggested by
paleomagnetic properties. The oldest one (4.3—3.3 Ma), Břidličná Volcano, was simultaneously active with the Lutynia
Volcano (Poland) which produced the Zálesí lava relic (normal polarity). Three other volcanoes of the volcanic field are
younger and reversely polarized. The Velký Roudný Volcano was active during the Gelasian (2.6—2.1 Ma) and possibly
could have been reactivated during the youngest (Calabrian, 1.8—1.1 Ma) phase which gave birth to the Venušina sopka and
Uhlířský vrch volcanoes. The reliability of all available K-Ar data was evaluated using a multidisciplinary approach.
Key words: Plio-Pleistocene basalts, paleomagnetism and magnetostratigraphy, volcanology, K/Ar dating, airborne
magnetometry and gravimetry, Moravia and Silesia.
Introduction
Cenozoic volcanism in the NE part of the Bohemian Massif
occurs prevalently in Polish Silesia. It stretches into the territory
of the Czech Republic to a limited extent only. The volcanic
locations of this wider area constitute the Odra Tectono-Vol-
canic Zone (OTVZ, sensu Kopecký 1987) of the WNW—ESE
strike, as a part of the so-called Bohemo-Silesian Volcanic
Arc. Volcanic rocks are located mostly inside the Fore-Sudetic
Block which is limited by the Odra Fault in the NE (outside
the studied area in Poland) and the Sudetic Marginal Fault
(SMF) in the SW, and elongated parallel to the OTVZ.
The basaltic volcanic products in northern Moravia and
southernmost Silesia are among the youngest in the territory
of the Bohemian Massif (e.g. Ulrych et al. 2011). Their com-
position ranges mostly between olivine nephelinite and
nepheline basanite (e.g. Barth 1977; Fediuk & Fediuková
1985, 1989; Ulrych et al. 1999; and others). These volcanics
represent primitive basaltic magmas (Vokurka & Bendl
1992, 1993), much like most other similar Cenozoic volca-
nics in the Bohemian Massif. These rocks were studied in
their paleomagnetic properties by Marek (1969, 1973, 1974)
and Kolofíková (1976), in the Czech Republic and by
Birkenmajer et al. (2002) in Poland.
Volcanic occurrences of this wider region concentrate on
three smaller areas in the territory of the Czech Republic. The
greatest concentration of basalts is in the Bruntál Volcanic
Field (BVF) near Bruntál in the Nízký Jeseník Mts. These ba-
salts are not eroded to a very high degree, and their lavas locally
overlie river terraces (e.g. Horský et al. 1972). Unpublished
data of Bellon from the 1970s (in Kopecký 1987) brought the
first information that they formed in the Pliocene. The Plio-
Pleistocene age was confirmed by Šibrava & Havlíček (1980)
using the K/Ar method. The second area of basaltic occur-
rences lies on the Czech-Polish border near Zálesí. No radio-
metric datings have been published from this location but
geological evidence assigns these rocks to the Lutynia area
in Poland. The only other separate occurrence near Opava
(third area) was dated to the Miocene (Shrbený & Vokurka
1985). The rock of the nearby location of Štemplovec has
been totally excavated and cannot provide data anymore.
Geological setting and volcanology
Magma of the volcanic occurrences was emplaced into Up-
per Paleozoic rocks. Only the Lutynia area and the Zálesí area
are situated in the Králický Sněžník Crystalline Complex;
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basalts near Bruntál are hosted / underlain by slightly meta-
morphosed rocks of the Horní Benešov Formation and un-
metamorphosed rocks of the Moravice Formation, both
belonging to the Nízký Jeseník Mts regional unit (Fig. 1). The
Miocene Hůrka Hill near Opava and the occurrence at Štem-
plovec of unknown age penetrate through the Moravice For-
mation sediments only.
The Sudetic Marginal Fault, separating the Žulová granitic
massif from the crystalline complexes of the Králický
Sněžník Mts and the Hrubý Jeseník Mts in the territory of the
Czech Republic, is accompanied by several faults of similar
strike in both crystalline complexes. A continuation of one
of the closest faults to the SMF, or the continuation of the
SMF itself, proceeds to the area of the BVF (see Fig. 1). As
the SMF is presently active (Štěpančíková et al. 2010), it
could be responsible for some magmatic activity during the
Pleistocene or even earlier. A similar scenario of tectonic
predisposition was published by Barth (1977).
The volcanic landforms have been described as stratovol-
canoes or composite volcanoes. This terminological misun-
derstanding possibly arose from the first description of Jahn
(1907) and an old-fashioned understanding of the presence
of both explosive and effusive products. Volcanic activity in
the BVF started as somewhat explosive and soon produced
scoria cones. At the beginning explosiveness was influenced
by contact with water during magma ascent. Partly-palag-
onitized phreatomagmatic tuffs were formed. This is most
visible at the Venušina sopka Volcano. In this point of view,
the role of the SMF-parallel faults during the volcano forma-
tion is well acceptable – surface water and ascending magma
can meet on fault planes. This influence was relatively small
and variable. Activity of all the below described volcanoes
of the BVF can be described as mostly phreatomagmatic at
the beginning. Further volcanic activity was of magmatic
type, producing scoriae and plastic bombs (Fig. 2a). Later it
associated with effusive activity with smaller or larger lava
production (Fig. 2b). This is a typical development of the
most common type of a monogenetic volcano. We suppose
mostly low-energy magmatic activity of Strombolian type,
close to the Hawaiian type.
As Hůrka Hill near Opava represents an old eroded sub-
volcanic form and Štemplovec site does not provide any
data, we focused only on four separate volcanoes and one
“rootless” basaltic occurrence.
Břidličná Volcano (BV)
This is the oldest preserved basaltic rock of the BVF. The
volcano is eroded down to the near-surface level of the mag-
matic vent, or just to the pre-volcanic superficial position.
The inner-crater facies passing to the vent-breccia is exposed
in an old quarry. Semi-plastic bombs are preserved, docu-
menting a position very close to the vent. The majority of
clastic material is represented by somewhat altered scoriae
Fig. 1. A simplified geological map showing sampling sites and primary magnetic polarities of the studied rocks. Bruntál Volcanic Field
(BVF) is shown by samples SU-02 to 10 and SU-12+13 in the central part of the frame. Zálesí (SU-11) and samples BP (14 to 16; taken
from Birkenmajer et al. 2002) locate the Lutynia area in the NW.
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and the finest material is possibly primarily reduced in vol-
ume. The alteration visible on the outcrop can be caused by
syngenetic process (phreatic influence) and by weathering,
too. The massive basalt of the plug has been excavated for
commercial use. The volcano very probably produced lava(s)
but no outcrops are preserved now. Nevertheless, a small relic
of lava was observed before exploitation (Jahn 1909). The size
of the vent cut suggests a relatively large scoria cone of the
Břidličná Volcano at the time of its origin.
Uhlířský vrch Hill Volcano (UV)
This volcano is situated closest to Bruntál. It represents a
remnant of a scoria cone with a single thin lava flow extend-
ing to the east. Now, two walls of an old quarry expose sco-
riae, while the central more solid rock has been excavated.
The exposed, mostly centroclinally stratified layers (the in-
ner facies) mostly consist of scoriaceous lapilli and bombs.
Centripetal layers are developed as well, further from the
feeder. The stratification is visible in grading and in colour:
an alternation of more and less palagonitized pyroclastics is
visible. This may be a result of a pulsation caused by interac-
tion with water in the vent in the early stages of volcanic
activity. Palagonitization decreases upwards while the fre-
quency of semiplastic bombs increases in the same direction.
Red- to brown-coloured baked clasts of country rock (acci-
dental pyroclasts) are present. A relatively great number of
large ballistic-transported bombs, occur and have been de-
posited in plastic form. Spindle- to cow dung-shaped forms
were observed, sometimes an indication of a bread crust-type
bomb is visible. These bombs contain primary paleomagnetic
field vector – their temperature was above the Curie point
at the time of deposition. A small outcrop of slightly vesicu-
lated and sonnenbrand-altered lava is hardly detectable in a
railway cut 1 km to the E from the vent. The idea of Barth
(1977) on the collapse of the cinder cone in its eastern part
and production of a single small lava flow in this direction
seems to be very realistic.
Venušina sopka Volcano (VS) near Mezina
This is another small volcano of the BVF but with higher ef-
fusive activity. The cinder cone in the central part of the hill is
the most phreatic-influenced one among the volcanoes of the
BVF. Accidental pyroclasts of Paleozoic country rocks are rel-
atively frequent in altered scoriae. Basaltic vesiculated pyro-
clasts with chilled margins were observed. Spindle-shaped
bombs are also present and about 1 m large bomb with a
bomb-sag is exposed in an old quarry at the summit. A lava
flow over 20 m thick was exploited in two abandoned quarries
down on the slope, near the Černý potok Creek (see Fig. 2b).
The older quarry described by Jahn (1907) really shows an un-
conformity dipping 40° to the E, but this does not represent
the boundary between “two lava flows”: no typical lower and
upper facies of flows are developed. The rock is the same on
both sides of this boundary; only a small difference in jointing
is visible, representing a facies change inside the flow. Most
probably, the unconformity originated subparallel to the dip of
the lava body during cooling. A younger quarry in the same
lava body exposes several facies of the same unit. Lava brec-
cias are developed at the base, and the facies are represented
by levels with different intensity of vesiculation and different
intensity of sonnenbrand disintegration. Columnar jointing
runs across all the facies. We suppose that the thickness is not
caused by a stacking of several (up to 4!) lava flows. The enor-
mous thickness resulted from a decrease in flow velocity and
its stopping by a body of hyaloclastic breccia at the lava front,
now mostly eroded. This body was produced by thermal
shock at the contact of the lava with an active water flow.
Velký Roudný Hill Volcano (VR)
This is the largest volcano of the BVF. It also displays the
largest preserved effusive production. Our description of this
volcano slightly differs from that of previous authors (e.g.
Barth 1977). The two summits lying closely apart – Velký
(“large”) Roudný and Malý (“small”) Roudný Hills – were
Fig. 2. Selected volcanological features visible in outcrops: a – a ballistic-transported plastic bomb from the Uhlířský vrch Hill Volcano
(UV) cinder cone; b – a thick columnar-jointed lava flow of the Venušina sopka Volcano (Mezina, sampling site SU-10), the change in
jointing corresponds to facies development.
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sometimes believed to represent two separate volcanoes. We
suppose that Malý Roudný Hill is not an independent volcano.
Now, it represents only a part of the same volcano (its cinder
cone), separated and modelled by erosion from another sum-
mit. This is supported by the presence of a single vent based
on the evaluation of geophysical survey (see the part “Gravity
and airborne magnetometry...” below). The same idea of a
large volcano is suggested from small basaltic occurrences
like Volárenský vrch Hill, Křiš anovice and possibly Zlatá
Lípa near Červený vrch Hill. All these may represent erosional
relics of other flows from the same volcano. No signs of sepa-
rate vents were found. All the exposed basalts of this position
show only signs of lavas. Unfortunately, the outcrops in pyro-
clastics are poor for comparison. Variation in the chemistry of
lava in one flow mentioned by previous authors is a usual phe-
nomenon and cannot be used for the flow determination.
Moreover, the compact facies has been altered. We suppose a
location of the feeder between the future Velký Roudný and
Malý Roudný Hills, building of a large cinder cone with possi-
ble (but not proved) parasitic vents and production of several
lava flows (3?). The largest preserved flow fills the valley of
the paleo-Moravice River (Slezská Harta, Bílčice-Leskovec).
Volárna relic represents the second flow. The southern flow
can be traced as far as the proximity of Křiš anovice (4 km)
now forming a small erosional relic. The connection of the
Zlatá Lípa site to this flow is more problematic because of the
large distance (ca. 12—13 km). Although it is far from the sup-
posed vent, this connection cannot be excluded. Lava produc-
tion in several flows can be deduced from the spatial
distribution of the relics, not from superposition as no super-
imposed lavas are exposed now. The largest flow filling the
paleovalley of the river shows only several facies, and the un-
conformity still visible in the active quarry of Bílčice does not
represent a boundary between two units. The enormous thick-
ness of ca. 50 m can be explained by flow deceleration by
hyaloclastite breccia which formed at the lava/stream inter-
face at the front and on the surface of the flow. Sediments of
fluvial terraces are known to underlie this flow (Horský et al.
1972). Kolofíková (1976) employed anisotropy of magnetic
susceptibility to the study of the flow orientation. Her results
correspond to the supposed directions of the flow and its fa-
cies development (see also Tarling & Hrouda 1993). The lens-
like layer of porcelanite-rich material in the old quarry (near
the lava front) mentioned in Barth & Zapletal (1978) and in-
terpreted as a boundary between two flows may also represent
a hyaloclastite breccia (not preserved now).
The volcanological results briefly described above were
tested using the orientation of the paleomagnetic field vector
and evaluation of magnetic and gravimetric regional fields.
K/Ar dating was used as well.
The paleogeographic reconstruction of this volcano (Cajz et
al. 2010) also incorporated two other sites of tuffites near the
villages of Karlovec and Razová (Barth & Zapletal 1978). Our
opinion on their origin is again only slightly different from
previous authors. The source area for most of the scoriaceous
material in tuffites can be placed in an old cone of the
Břidličná Volcano, destroyed and transported by the paleo-
Moravice River. The country rock surrounding this volcano
(low-grade metamorphosed slates) was removed together with
the scoriaceous material. Sedimentary clasts of the tuffites are
low-grade metamorphosed rocks which do not correspond to
the country rock of the tuffites. During effusive activity of the
younger Velký Roudný Hill Volcano, a lava dam-lake was
formed, the stream gradient of the river got changed, and the
mixed pyroclastic-sedimentary material was deposited in the
lake. Afterwards, the river used the contact between the lava
and the former valley side to cut the present Moravice River
channel. Some of the scoriae in tuffites may also have been
produced during the activity of the Velký Roudný Volcano.
This can be documented by the volume of redeposited pyro-
clasts in the sedimentary record at Razová, which shows a
very slow increase in the upwards direction.
Zálesí lava flow
This erosional remnant is situated on the Czech-Polish bor-
der near Zálesí and has no vent in the territory of the Czech
Republic. We suppose the production of this lava from the
Lutynia area (Poland) where the vent is located, 1—2 km from
the sampled location. The idea of this relation was tested using
a comparison of magnetic properties of basalts on both sides
of the border, comparing data of the Zálesí lava flow and previ-
ously published data from Lutynia (Birkenmajer et al. 2002).
Methods of study
Paleomagnetic and basic rock-magnetic studies
The previous studies by Krs (1968) and Marek (1969, 1973)
first discovered reversed polarity in the BVF, with the excep-
tion of the Břidličná Volcano which is normally polarized.
The latter author (Marek 1974) measured normal polarity at
the Zálesí lava flow, which is the closest Czech location to the
Polish sites, and discovered normal polarity of the basaltic oc-
currence from Ladek Zdrój. We have confirmed the older data
obtained on an astatic magnetometer using a greater number
of samples and different measurement techniques (see below).
Normal polarity was recently detected by Birkenmajer et al.
(2002) in the Lutynia area of Poland.
Thirteen sites in the territory of the Czech Republic were
newly sampled and processed. Hand-operated drilling on
outcrops provided 216 laboratory samples. The natural rema-
nent magnetization was measured using the JR5a and JR6
spinner magnetometers and 755R superconducting rock
magnetometer made by AGICO and 2G Enterprises, respec-
tively. The samples for measuring were predominantly cho-
sen according to the Koenigsberger ratio the (Q-parameter)
which should be lower than 10; however data from the
Břidličná volcano proved that Q-parameter can primarily
reach over 40. The samples were demagnetized by alternat-
ing field in LDA-3a demagnetizer and 2G600 automatic
sample degaussing system in 8 to 9 successive fields
between 2 and 80 mT, and thermally demagnetized in
MAVACS apparatus at temperatures between 80 and 600 °C
with a 40° step. On most of the samples two Curie tempera-
tures T
c1
= 160—200 (300) °C and T
c2
= 500—580 °C (Table 1)
were recorded.
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A principal component analysis by Kirschvink (1980)
was performed for all measured samples and group statis-
tics including mean direction (Fisher 1953) was computed
on the distinguished primary components for all sites.
The primary components were recorded in the tempera-
ture range 320—560 °C or field range 15—80 mT.
Representative alternating field and thermal demagneti-
zation curves of four samples are shown in Fig. 3. Mag-
netic susceptibility was measured by KLY-4S. In order to
identify the main magnetic carriers, temperature depen-
dence of magnetic susceptibility was also measured in ar-
gon atmosphere from the room temperature up to 600 °C,
and field-dependent magnetic susceptibility in the field
range of 2—450 A/m. The Curie temperature of 180 °C
and the steep field-dependent susceptibility curve (Fig. 4)
obtained from the SU-7 site can be explained by the pres-
ence of titanomagnetite and other spinelid-group minerals.
A similar situation was detected for the volcanics from the
Krušné hory Mts (Schnabl et al. 2010) and corresponds
with the findings of Vahle & Kontny (2005).
Conventional K/Ar age determination
Several authors published radiometric ages of basalts
from the BVF (e.g. Šibrava & Havlíček 1980; Kopecký
1987; Lustrino & Wilson 2007). Some data remained un-
published but are accessible in a report (Shrbený & Vokurka
1985). Results from a ten year-old set were recently pub-
lished by Pécskay et al. (2009) as an abstract during a re-
gional conference in Olomouc. Birkenmajer et al. (2002)
published data from sites in Polish Silesia close to Zálesí.
K/Ar dating of two data sets of samples was performed
in the K/Ar Laboratory of the Institute of Nuclear Research
of the Hungarian Academy of Sciences (ATOMKI), De-
brecen, Hungary. The new K/Ar data set of the BVF was
obtained from the same sites as the set for paleomagnetic
research. About 200 g of each rock sample were crushed
and sieved to 300 mm. Adhering fine particles were re-
moved by rinsing in distilled water. Approximately 0.8 g
of sieved sample were weighed for the whole rock. The
amount of radiogenic
40
Ar was determined by means of the
isotope dilution method using
38
Ar as a spike. Mass dis-
crimination of argon isotopes was corrected by measuring
air Ar. Previously preheated whole rock samples were de-
gassed by RF fusion in Mo crucibles, and the usual getter
materials (titanium sponge, getter pills of SAES St707 type
and cold traps) were used for cleaning and transporting ar-
gon. The purified argon was directly introduced into the
mass spectrometer (90° magnetic sector type of 150 mm
radius and operated in the static regime). For the determi-
nation of the potassium content, about 1 g of the identical
sample that was used for Ar measurement was ground in an
agate mortar to the grain size finer than 50 mm. About
100 mg of this powdered sample was dissolved in hydro-
fluoric acid and nitric acid using a teflon bomb. Potassium
content was determined by flame photometry with Lithium
internal standard (CORNING M 480 flame photometer,
digitized). The decay constants of Steiger & Jäger (1977)
were used in the age calculation. All analytical errors repre-
Table 1:
Paleomagnetic
and
radiometric
data
for
the
sampling
sites,
dat
a
of
Birkenmajer
et
al.
(2002)
are
added
for
comparison.
Curie
temperature
A
and
B
is
adequate
to
T
c1
and
T
c2
,
respectively.
Abbreviations
of
volcanoes:
VR
–
Velký
Roudný
Hill,
BV
–
Břidličná,
VS
–
Venušina
sopka
Hi
ll,
UV
–
Uhlířský
vrch
Hill.
For
more
information
on
age
see
Ta
bles
2
and
3
and
the
text.
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Fig. 3. Demagnetization curves and Zijderveld diagrams of four representative samples: a, c – AC field demagnetization curves of sam-
ples show a low-coercivity mineral (magnetite). The reversely polarized sample has a relatively strong viscous component; b, d – a thermal
demagnetization curve showing the presence of magnetite with T
c1
of 160—200 °C and T
c2
of 560—600 °C and no change in magnetic sus-
ceptibility after individual demagnetization steps.
Fig. 4. Basic rock-magnetic
measurements at the Břidličná
Volcano (SU-07) prove the
presence of minerals from the
spinelid group (titanomagnetite,
etc.): a – field-dependent mag-
netic susceptibility; b – tem-
perature-dependent
magnetic
susceptibility
shows
phase
change during laboratory heat-
ing. It is caused by newly
formed magnetite in the altered
rock during the procedure.
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sent one standard deviation (68 % confidence level). Multiple
runs of the inter-laboratory standards (Asia 1/65, LP-6, HD-B1
and GL-0) were used for checking the measurements. Details
of the instruments, the applied methods and results of the cali-
bration have been described elsewhere (e.g. Balogh 1985).
Gravity and airborne magnetometry used for
interpretation
Geomagnetic data were acquired by a detailed airborne
survey of the Nízký Jeseník Mts on the scale 1 : 25,000 in the
late 1970s (Dědáček & Gnojek 1980). The anomalies were
interpreted by Šalanský & Gnojek (2002) and Šalanský
(2004). A detailed gravity survey was realized during the
early 1970s (with measurement density of 3 points per
1 km
2
) and the gravity data were compiled to the Bouguer,
regional and residual gravity maps (Kadlec et al. 1972).
The generally monotonous positive regional magnetic
field in the study area (except for the distinct positive mag-
netic Šternberk—Horní Benešov Zone with iron mineraliza-
tion) is modified by several local anomalies in the Bruntál
area induced by Cenozoic volcanics. Reversely polarized
volcanic bodies with Q-parameter above 1 cause negative
anomalies. Lavas and pyroclastic rocks of the VR Volcano
near Leskovec nad Moravicí represent the source of the three
distinctive negative anomalies (Fig. 5). Each negative anom-
aly in the original maps is accompanied by a small positive
anomaly on the N. It points to the tabular shape of bodies
more than the steep anisometric one. But only detailed field
measurements could specify their geometry. An anomaly of
about —10 nT situated to the NE along the Moravice River
reflects the largest preserved volcanic flow. The anomaly of
—50 nT to the SW corresponds to Malý Roudný Hill. A cen-
tral, very distinctive negative magnetic anomaly of about
—60 nT coincides with a small local positive gravity anomaly
of more that 15 µms
—2
in the regional gravity survey. Such a
type of coincidence is typical for a volcanic vent (e.g. Lidner
et al. 2006; Cassidy et al. 2007). The close-up gravity field
map places this small positive gravity anomaly close to
Velký Roudný Hill. As the magnetic anomalies correspond
to the tops of both hills and no solid basalts are known on
their summits, the anomalies are supposed to reflect only
volcaniclastics. On the other hand, their intensity is higher
than that of an anomaly induced by a relatively thick lava
flow. The existence of parasitic feeders of a large volcano is
one of the possible explanations. Volcanological interpreta-
tion of such data is problematic because the results of the
geophysical survey are not unambiguous. A more detailed
field geophysical survey is needed for correct specification
of the geometry and exact location of the vent.
The Uhlířský vrch Hill (UV) and the Venušina sopka (VS)
volcanoes, closer to Bruntál, are characterized by weak negative
magnetic anomalies (about —10 nT relative to background).
The individual vents cannot be precisely determined from
geophysical fields, they are monotonous. The Břidličná Vol-
cano vent is indicated by an elongated local positive magnetic
anomaly (normal magnetization). Similar positive magnetic
anomaly about 3 km to the S, accompanied by a negative
gravity anomaly (—25 µms
—2
), indicates the supposed buried
volcanic maar near Lomnice. This unique phenomenon is visi-
ble only in the gravity and magnetic data.
Results
A new volcanological evaluation of the volcano remnants
was made. The older volcanological evaluation was generally
confirmed (Horský et al. 1972; Barth 1977). Only in the case
of the largest volcano, Velký Roudný Hill and the neigh-
Fig. 5. Summarized regional magnetic [nT] and gravity [µms
—2
] fields of the Bruntál Volcanic Field (BVF) and a close-up map of the gravity
field at Velký and Malý Roudný Hills (VR)—(10 µms
—2
= 1 mGal). Adapted from Šalanský & Gnojek (2002) and Kadlec et al. (1972).
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bouring hill of Malý Roudný, our results are slightly different.
Basalts of this area were studied in their paleomagnetic
properties and several of them were processed to obtain their
K/Ar ages. Paleomagnetic results were compared with exist-
ing reliable radiometric data. This, together with the analysis
of magnetic and gravity fields, allowed us to reconstruct vol-
canic activity in time and space and supported the volcanolog-
ical evidence. A combination of different approaches resulted
in evaluation of the whole set of K-Ar data.
Paleomagnetism
Lavas and bombs from cinder cones on both smaller volca-
noes of the BVF (UV and VS) were sampled. The sampled
bombs were chosen based on volcanological observation – the
plastic type ones (e.g. cow-dung and spindle-shaped) were
preferred. Only lavas were accessible at VR. Paleomagnetic
results proved that larger bombs were transported above the
Curie temperature (560 °C for VS and 400—560 °C for UV).
Fig. 6. Summarized paleomagnetic vector projections of the youngest volcanoes: a – Uhlířský vrch Hill (UV), pyroclastics and lava. The
most components are computed between 400 and 600 °C or 20—100 mT. b – Venušina sopka Volcano (VS), pyroclastics and lava. The most
components are computed between 280 and 560 °C or 15—80 mT. c – Velký Roudný Hill (VR), lava. The most components are computed
between 320 and 560 °C or 15—80 mT.
The primary field of explosive and effusive products of
Uhlířský vrch Hill is visible in Fig. 6a. The primary field of
both lavas and pyroclastics of VS is documented in Fig. 6b.
Differences in the paleofields between UV and VS are only
1.3°. Secular variation of all volcanoes is not centred be-
cause of the supposed short duration of volcanic activity,
complying with the relatively short lives of monogenetic
volcanoes. From this, we can conclude a nearly identical
time of origin of the two volcanoes, moreover, when the K/Ar
ages are very close.
Paleomagnetic data from the Velký Roudný Volcano
(Fig. 6c) were obtained from lava of its largest flow; pyro-
clastics are not available for sampling. The samples chosen
for paleomagnetic evaluation come from the compact facies
of the lava flow. Data from site SU-03 (Bílčice quarry) were
systematically rotated 13° to the N compared to the others
from the same lava flow. Horský et al. (1972) have discov-
ered tilting of large basaltic blocks during the investigation
for the dam construction. This finding is in agreement with
Fig. 7. Paleomagnetic vector projection of the complex data with a—a
lava breccia clasts (Slezská Harta lava flow – VR) and the lava
flow direction. The most components are computed between 280
and 600 °C or 15—80 mT.
the geological position of the sampling site and the mecha-
nism of disintegration of the lava body. This was the reason
for the apparent heterogeneity of data from one location.
One interesting effect was observed on a-a brecciated sur-
face of the largest lava flow of VR. Figure 7 shows the ex-
traordinary distribution of samples taken from breccia clasts,
which resulted in 95 value of 23.2° for the whole location.
This is caused by a special type of sample – the rotation of
clasts from destroyed already cooled surface incorporated
into fluidal lava is responsible for this phenomenon. Group-
ing of these samples shows direction of axis similar to the di-
rection of the flow and very close to the interpreted AMS
data of Kolofíková (1976). This phenomenon can be derived
from the style of rotation of a-a clasts at the surface in the
central part of the flow. Data from only one location cannot
be statistically significant; anyway, combination of AMS
and direction of remanent magnetization offers a theme for
methodological study on behaviour of a-a lava flows.
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Fig. 8. Paleomagnetic vector projection of the Zálesí lava relic and the Břidličná Vol-
cano vent, compared with products from the Lutynia area. The most components are
computed between 200 and 500 °C or 10—80 mT.
Grouping of all reliable data of the VR lava flow (see
Fig. 6c) enabled comparison of the paleofield of the largest
lava flow from VR with that of smaller volcanoes (UV and
VS). The angle between the mean directions of their vectors
is 4.5 and 5.0°, respectively. Unfortunately, this small differ-
ence does not provide a conclusive basis for magnetostrati-
graphic interpretation itself. Anyway, it agrees with new
radiometric data which divide the reversely polarized volca-
nic bodies into two separate phases.
A wider difference of 6.4 and 8.0° is obtained if we com-
pare the sites of the isolated lava relics of Zlatá Lípa and
Volárna, most possibly belonging to the VR volcano, too.
From this, we conclude the possibility of the production of
other flows from the same volcano (VR). The Volárna relic
seems to be produced close in time to the main flow, while
the Zlatá Lípa relic may represent a younger flow.
Two of our sampling sites are normal polarized. The
Břidličná Volcano has an extremely high Q-parameter (aver-
age around 30, but often exceeding 40). Usually, these high
values are explained by a secondary influence, such as light-
ning. But in this case, the sampling site is situated in the
depth of the old quarry, so the influence by lightning is not
realistic. A more probable reason can be seen in the titano-
magnetite composition (see Fig. 4).
Another normal-polarized occurrence is the lava relic near
Zálesí on the Czech-Polish border. Its paleomagnetic char-
acteristics are comparable with those of the basaltic occurrences
in Polish territory (Marek 1974; Birkenmajer et al. 2002),
close to the sampling site. Volcanological evaluation proves
the relation of this lava relic at Zálesí to the plug of Lutynia.
Figure 8 shows very close vector orientations for all normal-
polarized volcanics in the Lutynia vicinity, including the
Břidličná Volcano. This situation can be explained by volca-
nic activity in a very close time span. The conclusion offers
two results: the Zálesí lava flow was produced from the Lu-
tynia Volcano as supposed from geology; and the Břidličná
and Lutynia volcanoes were active nearly simultaneously.
The Kamenná hora Volcano near Otice is one of the oldest
in the region. The site displays a deeply eroded vent whose
volcanic rock was strongly altered. The main magnetic carrier
is titanomagnetite (Curie temperature around 200 °C). The
mean paleomagnetic directions are D = 206° and I = —14°,
similar to those measured by Marek (1974). The inclination
is extremely low compared to other Cenozoic rocks. Both
values show rather “Paleozoic-like” directions, similar to
data from Barrandian volcanics (Kletetschka et al., in print).
Given the known age of 20 Ma for the Otice volcano (Shr-
bený & Vokurka 1985), the acceptable explanation can be
seen in a possible rotation of the only preserved block in an
old quarry, most probably due to quarrying activities. This is
the reason why we suppose that the primary polarity is im-
possible to reconstruct (see Fig. 1), and this location is not
suitable for paleomagnetic studies.
K/Ar datings
Table 1 compares new data from the laboratory in Debrecen
and older data of previous authors from other laboratories.
Older data from several sites differ significantly from new
ones (Table 2), moreover, the localization of several previous
sampling sites is very poor (see e.g. Lustrino & Wilson 2007).
The latter data were published without analytical errors, so
their informative value is not fully comparable with the others.
Conventional K/Ar dating of 12 representative whole-rock
samples was carried out in two sets (Table 3). The first set of
6 samples was collected and analysed 10 years ago. These
preliminary data remained unpublished for a long time but
were accessible. In the meantime, a new flame photometer
(CORNING M 480) has been set up in Debrecen, therefore
the potassium analyses made on the first set of samples were
repeated. Considering that consistent results were achieved,
the mean K contents were used for the recalculation of the
previous K/Ar ages. At the same time, 6 additional samples
were collected from the same sites for paleomagnetic studies,
hoping to get confirmation of the meaningful ages obtained
on the previous samples.
On the basis of the preliminary results, we concluded that
the BVF basaltic rocks are generally younger than the alka-
line basaltic rocks exposed at Lutynia and Ladek Zdrój. On
the other hand, the analytical data suggested that the volca-
nic activity was episodic: older than 3.4 Ma,
around 2.3 Ma and younger than 1.5 Ma.
However, such an estimation does not con-
sider possible geologically induced distur-
bances of the argon isotope system, such as
Ar loss by alteration or excess Ar by incor-
poration of xenocrysts/xenoliths. Because
of these uncertainties, we use all the avail-
able and reliable radiometric data in this
study, determined in different laboratories
(see Table 2). The paleomagnetic data are
also taken into account for the final model
of the volcanic evolution.
Four new K/Ar ages of whole-rock sam-
ples (SU-03, 04, 05 and 06) are identical
within the analytical error. On the basis of
the concordant age, we consider these ages
to be statistically significant for the geologi-
cal setting. Therefore, one can assume that
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these whole rocks contain negligible rock or mineral compo-
nents with insufficient Ar retentivity or with excess Ar. This
assumption is confirmed by the analytical data obtained for a
sample from the previous set (CZB-5, sampling site identical
with SU-06) – see Table 3. In contrast, the radiometric ages
from the VS (CZB-3A and SU-12) appear to be affected by
excess Ar (0.83 ± 0.12 and 2.14 ± 0.08 Ma), assuming that the
age disturbances are mainly caused by the presence of some
very fine-grained xenocrystic material, which is impossible
to eliminate from the samples. Consequently, the younger age
is closer to the real geological age than the older one. How-
ever, it cannot be completely excluded that sample CZB-3A
Table 2: All available primary K/Ar age data from the region in the territory of the Czech Republic. For ab-
breviations of authors see Table 1; and P 2009 = Pécskay et al. 2009.
Table 3: Results of radiometric analyses used for this study (ATOMKI Debrecen, Hungary); sets 2000
(CZB samples) and 2010 (SU samples). Data of the set from 2000 (Pécskay et al. 2009) are recalculated
with new results on potassium content.
Volcano
Landform
Original site name
Adequate to K/Ar age (Ma) Sampling year Authors
Otice
vent (eroded)
Kamenná hora Hill
SU-01
20 ± 3
?
SV 1985
BV vent
(surface)
Břidličná
SU-07
3.69 ± 0.56
2000
P 2009
VR 1
lava-S. margin Velký Roudný (Bílčice) SU-03
3.31 ± 0.24
2000
P 2009
VR 1
lava-S. margin Bílčice-Leskovec
SU-03
2.7 ± 0.5
?
SH 1980
VR 1
lava-S. margin Bílčice-Leskovec
SU-03
3.4 ± 0.9
?
SH 1980
VR 1
lava-S. margin Bílčice
SU-03
2.33 ± 0.14
2010
this paper
VR 1
lava-N. margin Slezská Harta 1
SU-04
2.21 ± 0.16
2010
this paper
VR 1
lava-surface
Slezská Harta 2
SU-05
2.37 ± 0.29
2010
this paper
VR 1
lava-front
Slezská Harta
no sample
1.28 ± 0.4
?
SH 1980
VR 1
lava-front
Slezská Harta
no sample
1.46 ± 0.15
?
SH 1980
VR 1
lava-front
Slezská Harta
no sample
1.6 ± 0.6
?
SH 1980
VR 1
lava-front
Slezská Harta
no sample
2.2 ± 0.9
?
SH 1980
VR 2
lava relic
Volárna
SU-06
2.48 ± 0.31
2010
this paper
VR 2
lava relic
Volárenský vrch
SU-06
2.41 ± 0.14
2000
P 2009
VR?
lava relic
Zlatá Lípa
SU-02
1.75 ± 0.15
2000
P 2009
VR?
lava relic
Zlatá Lípa
SU-02
1.24
1992
LW 2007
VS
lava
Venušina sopka
SU-09, SU-10
0.80 ± 0.11
2000
P 2009
VS
lava
Mezina
SU-09
1.26 ± 0.16
2010
this paper
VS
lava
Mezina
SU-09, SU-10
1.94 ± 0.22
?
SH 1980
VS
bomb-cinder c. Venušina sopka
SU-12
2.14 ± 0.08
2010
this paper
VS
?
Venušina sopka
?
1.11
1992
LW 2007
UV ?
Uhlířský vrch
SU-08 ?
2.4 ± 0.5
?
SH 1980
UV lava
relic
Bruntál-trať
SU-08
1.54 ± 0.15
2000
P 2009
UV bomb-cinder
c.
Uhlířský vrch
SU-13
1.47
1992
LW 2007
?
?
91/1 — no location
?
0.91
1992
LW 2007
?
?
91/2 — no location
?
1.22
1992
LW 2007
?
?
91/4a — no location
?
4.58
1992
LW 2007
VR1 = Velký Roudný Hill Volcano, its largest lava flow; VR2 = Velký Roudný Hill Volcano, relic of another flow; VR? = relic of
possible next younger flow from the Velký Roudný Hill; VS = Venušina sopka Volcano; UV = Uhlířský vrch Hill Volcano.
K/Ar code Sample code
Site K
(%)
40
Ar
rad
(ccSTP/g)
40
Ar
rad
(%) K/Ar age (Ma)
5373/A
CZB-4A
Velký Roudný (Bílčice) 0.886
1.155
10
–7
20.2
3.35 ± 0.23
5375/B
CZB-6B
Zlatá Lípa
1.078
7.526 10
–8
16.4
1.79 ± 0.15
5370/B CZB-1B
Břidličná
1.31
1.909 10
–7
9.1
3.74 ± 0.56
5371/A CZB-2A
Uhlířský vrch
0.663
3.973 10
–8
14.1
1.54 ± 0.15
5372/A
CZB-3A
Venušina sopka
1.136
3.652 10
–8
9.6
0.83 ± 0.12
5374
CZB-5
Volárenský vrch (Volárna) 1.118
1.066 10
–7
27.2
2.45 ± 0.13
8012 SU-03
Bílčice 0.975
8.845
10
–8
23.4
2.33 ± 0.14
8013
SU-04
Slezská Harta
0.95
8.169 10
–8
19.3
2.21 ± 0.16
8014
SU-05
Slezská Harta
0.751
6.945 10
–8
10.9
2.37 ± 0.29
8015
SU-06
Volárenský vrch (Volárna) 0.985
9.519 10
–8
11.1
2.48 ± 0.31
8016 SU-09
Mezina
1.446
7.096
10
–8
11.1
1.26 ± 0.16
8017
SU-12
Venušina sopka
1.188
9.872 10
–8
27.3
2.14 ± 0.08
was affected by a slight al-
teration which resulted in
Ar loss. As a conse-
quence, the analytical age
determined for this sam-
ple should be considered,
as a “minimum age”.
Discussion
R e v e r s e d - p o l a r i z e d
young volcanoes of the
BVP must be older than
0.781 Ma (Gradstein et
al. eds. 2004) – the
Matuyama polarity chron.
Based on polarity and
group statistics results,
supported by volcanolog-
ical evidence, we can dis-
cuss the reliability of the
K/Ar dating done during
the last nearly 40 years in
different laboratories. On
the example of the prod-
ucts from Velký Roudný
Hill (see Fig. 1 for loca-
tion, samples SU-03 to
SU-06) we can explain
the result of the evalua-
tion which is documented
in Fig. 9. The data ob-
tained in the early 1980s
for the largest lava flow
(Slezká Harta and Bíl-
čice-Leskovec) have rela-
tively large analytical
errors – over 30 %. As a
result, a part of the time
period belongs to the nor-
mal polarity event. One
rock body measured sev-
eral times and in several
sampling-places
shows
different ages. The possi-
ble period is therefore so
wide that it looses validity. Moreover, the evaluation of vol-
canological phenomena, which is proved by group statistics
of paleomagnetic results, now summarizes 9 ages for the
same lava body (VR1 – see Table 2). The time span counted
from all these data is 4.3—0.88 Ma, if we give the same
weight to each result. The high age of the two above men-
tioned reversed samples from VR is comparable to the nor-
mal polarized activity of BV and Lutynia only by chance;
the measured polarity does not allow this possibility. There-
fore, such an age is not realistic for the VR activity. So, we
have chosen data which were grouped in a time period closest
to the reversed polarity subchron. It is important to notice
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Fig. 9. The highest probability age (dotted time spans) of Pleistocene (reverse-polarized – open circles) volcanoes of the BVF compared to
Pliocene (normal-polarized – black dots) volcanic activity. If open circle only – no analytical error was given by previous authors (Lustrino
& Wilson 2007). Stratigraphic chart after Gradstein et al. (2004).
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that only newly obtained data with smaller analytical errors
were the result. Thus, the most possible chron of the lava
flow origin is C2r2r (2.581—2.148 Ma). The Volárna lava relic
with a slightly different vector orientation is very close in
time, belonging to the same chron. Only the Zlatá Lípa lava
relic is younger (1.24 Ma in Lustrino & Wilson 2007 or
1.79 ± 0.15 in Pécskay et al. 2009) with a possible origin dur-
ing the C1r3r Chron. Its vector orientation is slightly different
as well.
Based on this, we can conclude that two lava flows were pro-
duced from the VR vent, close in time or simultaneously – to
the SE (Slezská Harta) and to the W (Volárna). As has been
mentioned above, another lava flow oriented to the S
(Křiš anovice—Zlatá Lípa) could have been produced later,
during a possible remobilization connected with UV and VS
formation. This model corresponds to the new volcanologi-
cal evaluation.
The orientation of the paleofield vector in the products of
both smaller volcanoes is nearly identical. Therefore, we sug-
gest that these youngest volcanoes (Uhlířský vrch Hill and
Venušina sopka Volcano) were constituted most probably
during the C1r3r or C1r2r Chrons (1.778—1.072 Ma), although
one K/Ar age from VS (0.80 ± 0.11 Ma in Pécskay et al. 2009)
is younger, situated within C1r1r – see above. The only older
age (2.14 ± 0.08 Ma), newly obtained from the bomb of the
cinder cone (VS), might have been easily influenced by con-
tamination during vesiculation or alteration during the phreato-
magmatic event.
As the K/Ar age of the normal-polarized older Břidličná
Volcano meets three normal subchrons (C2An2n – 3.207—
3.116 Ma; C2An3n – 3.596—3.330 Ma; C3n1n – 4.300—
4.187 Ma), only the two older ones represent the most
probable time of origin. This is substantiated by the results
of group statistics, where the vectors of BV and the Lutynia
Volcano are similar. In a similar way, we can evaluate the
age of the plug in Poland (Lutynia I) as slightly shifted to a
higher age.
Another interesting conclusion can be seen from the point
of view of tectonic development. All the studied area is in-
fluenced by the tectonics of the Sudetic Marginal Fault. For
the ascent of basaltic magmas, we assume the tectonic activity
in the form of relative extension, at least. It allows us to sup-
pose close interrelationship of volcanism and tectonics
(changes in paleostress field) in time. At the time of the older
volcanic activity (Břidličná and Lutynia), tectonic disquiet is
mentioned in the mountain ranges of Ve ká and Malá Fatra
(Kováč et al. 2011), some 150 km to the SE. On the contrary,
the time of two younger volcanic phases (Velký Roudný,
Venušina sopka and Uhlířský vrch) is supposed to represent
a period of tectonic quiescence in the Fatra region. Unfortu-
nately, this study is not able to explain this disparity.
Conclusions
Newly obtained data on spatial and time distribution of
volcanic activity do not confirm the idea of its shifting in
time from the N to the S (sensu Birkenmajer et al. 2004) in
the Czech part of Silesia. We can only state that three different
Late Cenozoic volcanic phases exist, with the following
most probable timing:
i) Pliocene (Late Zanclean or Early Piacenzian) phase of nor-
mal polarity in the span of 4.3—4.2 Ma (C3n1n) or 3.6—3.3 Ma
(C2An3n) constituting the Břidličná and Lutynia Volcanoes;
ii) Gelasian phase (2.6—2.1 Ma, C2r2r) which formed the
Velký Roudný Volcano with its large lava production; and
iii) Early Calabrian phase (1.8—1.1 Ma, C1r1r + C1r2r) of
the Venušina sopka and Uhlířský vrch Hills, with possible
remobilization of the Velký Roudný Volcano (southern flow
of Zlatá Lípa).
These results represent a strong basis for the Upper Ceno-
zoic volcanostratigraphy of this region. They can also con-
tribute to the ideas on the younger history and development
of tectonic activity, connected to the Sudetic Marginal Fault
system. The Otice Volcano rock is not appropriate for paleo-
magnetic studies.
Acknowledgments: This research was supported by Project
IAA 300130612 of the GA AS CR “Combined magneto-
stratigraphic studies of Cenozoic volcanics, Bohemian Mas-
sif”. The paleomagnetic and rock-magnetic methodology
benefited from newly obtained knowledge on Paleozoic vol-
canism (P 210/10/2351). It falls within the Research Plan of
the Institute of Geology, Academy of Sciences CR, v.v.i.,
AV0Z30130516. We highly acknowledge the kindness of
our colleague Jacek Grabowski for providing his primary
data from the Lutynia area for comparison. We also thank
Miroslav Radoň (Regional Museum Teplice, o.p.s.) for his
great help during sample acquisition, and Jana Drahotová,
Václav Sedláček and Jiří Petráček from our lab for technical
assistance. We wish to express our great thanks to Klaudia
Kuiper and Christine Franke for stimulating comments on pa-
leomagnetism, to Jaroslav Lexa for remarks on volcanology
and to Jiří Adamovič for English revision of the manuscript.
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