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K-Ar dating in the Teplá-Domazlice zone at the western margin of the Bohemian Massif

Authors:

Abstract

K-Ar dating on hornblendes and micas from the Tepla Domazlice zone revealed a pattern of dates which significantly deviates from the mid-Carboniferous to early Permian one that is found in the adjacent low-pressure metamorphic Moldanubian and Saxothuringian. Especially for the Marianske Lazne metabasic complex, confirming early Czech determinations, the dates resemble the early Devonian pattern determined for the Munchberg Gneiss Massif and the Erbendorf-Vohenstrau zone of northeastern Bavaria. This supports the idea that all three units are remnants of a huge' complex which suffered a metamorphic overprint under medium-pressure conditions, probably in the early Devonian. Strong rejuvenation is found in the southern part of the Tepla-Domazlice zone by which micas and even two hornblendes were reset to mid-Carboniferous ages. According to the geological setting, part of the apparently preDevonian dates may be explained by inherited argon from earlier metamorphic and magmatic events, e.g. the high-pressure metamorphism documented in eciogitic relics. However, excess argon, caused by the mid-Carboniferous overprint cannot be excluded.
I
I
r
I
~
,
VerOffentlichung aus dem
Mlneralogischen Institut
der UniversiUit WQrzburg
PROCEEDINGS
of the 1 st International Conference on the .
BOHEMIAN MASSIF
Prague, Czechoslovakia, Sept.
26-0ct.
3, 1988
Edited by
Zdenek Kukal
Czech Geological Survey,
Prague
Kreuzer,
H., Vejnar, Z., Schiissler,
U.,
Okrusch,
M.
& Seidel, E. (1992): K-Ar
dating
in
the
TepHi-Domazlice
zone
at
the
Western
margin
of
the
Bohemian
Massif.-
In:
Kukal,
Z. (ed.): Proc. 1st Int. Conf.
on
the
Bohemian
Massif
(Sept. 26 -
Oct.
3,
1988),
: 168-175;
(Czech
Geological
Survey),
Prague.
K-Ar DATING
IN THE TEPLA-
DOMAZLICE ZONE
AT THE WESTERN
MARGIN
OF
THE
BOHEMIAN MASSIF
Hans
Kreuzerl,
Zdenek Vejnar2,
Ulrich
Schussler3,
Martin
Okrusch3 and Eberhard
Seidel
4
1 Bundesanstalt
fur
Geowissenschaften
und
Rohstoffe,
Postfach 510153, 0-3000 Hannover
2 Geological Survey, Prague, Czechoslovakia
3 Mineralogisches Institut
der
Universitat Wurzburg,
Am
Hubland, 0-8700 Wurzburg
4 Mineralogisch-Petrographisches
Institut
der
Universitat
zu Koln, Zulpicher StraBe 47, 0-5000 Koln 1
Abstract
K-Ar dating on hornblendes and micas
from
the Tepla-
Domazlice zone revealed a pattern
of
dates which significantly
deviates
from
the mid-Carboniferous
to
early Permian one that
is
found
in the adjacent low-pressure metamorphic Moldanu-
bian and Saxothuringian. Especially
for
the Marianske
Lazne
metabasic complex, confirming early Czech determinations, the
dates resemble the early Devonian pattern determined for the
Munchberg Gneiss Massif and the Erbendorf-VohenstrauB zone
of
northeastern Bavaria. This supports the idea that all three
units are remnants
of
a huge' complex which suffered a meta-
morphic
overprint under medium-pressure conditions, prob-
ably in the early Devonian . Strong rejuvenation is found in the
southern part
of
the Tepla-Domazlice zone by which micas and
even
two
hornblendes were reset
to
mid-Carboniferous ages.
According
to
the geological setting,
part
of
the apparently pre-
Devonian dates may
be
explained
by
inherited argon from ear-
lier metamorphic and magmatic events, e.g. the high-pressure
metamorphism documented in eciogitic relics. However, excess
argon, caused by the mid-Carboniferous overprint cannot be
excluded.
Introduction
In
the course
of
the multi-disciplinary studies
in
northeastern
Bavaria
for
the site-selection
of
the German Continental Deep
Drilling Program
(KTB)
a nappe structure has been established
for
the
Munchberg complex (Bavaria) using geological and
geophysical arguments (e.g. Franke 1984; Haak -Blumecke,
Schmoll, Weber
in
Weber -Vollbrecht 1986). Moreover, a
nappe situation is probable
for
the Erbendorf-VohenstauB zone
(Bavaria) and was also postulated
for
the Tepla-Domazlice zone
of
western
Bohemia (Blumel 1985 (unpubl. lecture), Weber -
Vollbrecht 1986) . The three units are regarded
as
remnants
of
one huge nappe complex. This assumption
is
confirmed by the
contrast in the polymetamorphic evolution in respect
to
the
surrounding Moldanubian
and/or
Saxothuringian: As pointed
out
by
Bliimel (1983) the crystalline nappes
of
the Munchberg
gneiss core and the Erbendorf-VohenstrauB zone
as
well
as
the
Tepla-Domazlice zone suffered a last metamorphic overprint
under medium-pressure conditions.
In
contrast, the adjacent,
168
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S_Of_fh_._Q_U_._rz_o_n._-;
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of
tha
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gnel.a
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LeBlohe
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blollte
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zlnnwaldlte
Rb-Sr
:
symbol
length
!:2.
Rock
types
:
magmatlt
••
Fig. 1. K-Ar dates
from
the
basal
units
of
the
Miinchberg
nappe
pile
(MM),
the
Fichtelgebirge
Saxothuringian,
the
Tirschenreuth-
Mihring
zone (ZTM), and
the
adjacent
Moldanubian.
The
dates are arranged
roughly
from
north
to
south
and
some
Rb-Sr
whole-rock
isochron
ages
of
granites
are
included
(after
Kreuzer
et
al. 1985,
Ahrendt
et
al. 1986,
Wendt
et
a!. 1986,
1988). The
length
of
a
symbol
represents
the
analytical
preci-
sion
at
a level
of
95 %
intralaboratory
analytical
confidence. A
ruled
background
shows
the
respective
precision
of
the
weighted
mean
of
a
cluster
of
dates.
(relatively) autochthonous Moldanubianand Saxothuringian ex-
perienced a last metamorphic event
of
low-pressure type.
An additional argument
for
the nappe concept is provided by
K-Ar dating
on
hornblendes and micas and
by
concordant U-Pb
ages
of
monazites: Throughout the Bavarian part
of
the Molda-
nubian and the Saxothuringian
at
the western and northwestern
margins
of
the Bohemian Massif mid-Carboniferous
to
early
Permian dates were recorded. They prove a rapid uplift and
cooling around 330
to
320 Ma which were followed by local
thermal influences related
to
several pulses
of
post-deformative
granite intrusions between 320 and
295
Ma (Teufel 1988,
Kreuzer et
al.
1989 and Fig. 1).
In
contrast, fairly consistent
patterns
of
early Devonian dates around 380 Ma prevail in the
crystalline nappes
of
the Munchberg complex, in the western
part
of
the Erbendorf-VohenstrauB zone, and in parts
of
the
Tepla-Domazlice zone ($mejkal 1962 and 1964, Vejnar 1962,
Fischer et al.
1968,
Gottstein 1970, Schussler et al. 1986, Kreuzer
et al. 1989).
As
an
example the dates
of
the Munchberg Gneiss
Massif complex are given
in
Fig. 2.
The early data for the Moldanubian
of
western Bohemia and
the Tepla-Domazlice zone were
not
generally accepted because
of
the large range
of
dates
from
Cambrian
to
Permian, and
because
of
lacking isotopic control
for
part
of
the analyses.
Indeed, the early data are about 3 % too old because
of
missing
atmospheric correction
(Smejkal-Melkova
1969). This short-
coming, however, does
not
obscure the significant grouping (all
dates recalculated
with
the lUGS-recommended constants):
1.
Mid-Carboniferous to Permian dates in the Moldanubian and
near the Bohemian Quartz Lode.
2.
Fairly consistent early Devonian dates around 380 Ma
more
to
the east,
namely
six muscovites between
397
and 370 Ma, on
average 383 ± 8 Ma (standard deviation
sd
-2.5 %), and nine
biotites between 390 and 360 Ma, on average 380 ± 8 Ma (sd
-
3%).
3.
Older dates ranging
from
410
to
550
Ma.
Locally, similar results
for
muscovite and biotite
s~gest
that
some
of
the pre-Devonian dates
might
well be rough age esti-
mates
for
earlier stages
of
the polyphase thermal history
of
the
Tepla-Domazlice zone. This
would
be
in accordance
with
the
steep decrease
of
metamorphic
grade
of
the medium-pressure
overprint
towards
the east as visualized in the isograd pattern
mapped
by
Vejnar (1972) .
New K-Ar and
39Ar/40Ar
determinations were initiated in
order
to
verify
the early Devonian event in the Tepla-Domazlice zone
and to unravel the meaning
of
the seemingly older dates.
In
the
present paper results
of
conventional total fusion dating are
reported. All dates, including the cited ones, are calculated
with
the lUGS-recommended constants
(Steiger-Jiiger
1977). Err-
ors are given at a level
of
95 % confidence
of
the intra laboratory
analytical precision . Our K-Ar date
for
the standard glaucony
GL-O
is 1 %
younger
than the average value
of
the compilation
of
Odin (1982). For analytical details
see
Seidel
et
al. (1982) .
On
the
meaning
of
K-Ar
dates
in
polymetamorphic
areas
Cooling
ages
Generally, one believes
that
K-Ar dates are estimates
of
the
ages
of
cooling
to
so called blocking temperatures
of
around
5000 C
for
hornblende, 350 0 C
for
muscovite and 3000 C
for
biotite
(e
.g. Gerling
et
al.
1965, Hart 1964, Purdy -
Jiiger
1976).
Basic requirement
for
such an interpretation is that the dates are
consistent on a local scale
within
the limits
of
error
for
each
of
the retentive minerals, regardless
of
rock type, grain size, and
position
within
the structure,
with
ages in the
proper
order
horn-
blende
~
biotite. Examples
of
local consistency can bee seen in
sections
of
Figs. 1 and
2.
Inherited
argon
In
contradiction to the
model
of
blocking temperatures one
may
observe scattered dates, depending on the positions
within
the geological structure. Especially in lenses
of
undeformed
rocks hornblendes and muscovites,
but
even biotites
may
retain
an earlier
Clge
information although the rocks were heated well
above estimated blocking temperatures
(e
.g. leses
of
undefor-
med tonalite w ithin the amphibolite facies rocks
of
the Tauern
Window
(Besang
et
al. 1968, Friedrichsen et al. 1973, Raith
et
al.
1978)).
An
example
of
inherited argon can be seen in the
upper
part
of
Fig. 2:
In
the northeastern part
of
the Hangendserie
(Munchberg Gneiss Massif)
two
hornblende dates
point
to
an
age
of
around 375 Ma; plateau ages
of
39ArroAr step-heating
of
muscovite (376 and 377 ± 4 Ma)
from
an unfoliated pegmatite
MONCHBERG
NAPPE
300
Ma
Hangendserie
Liegendserie
Rand
amph
i
bol
it
-Seri
e
praSinit~PhYllit-Serie=
f
380Ma
local
metamorphism
local
rejuvenation
in
the
upper
relics
three
units
K-Ar
dates
:
biotite
muscovite
hornblende
symbol
length
!2s
Ar/Ar
plateau
ages
=
o
Fig. 2. K-
Ar
Dates
from
the
upper
units
of
the
Miinchberg
nappe pile.
The
dates
are
roughly
arranged
from
NE
to
SW
(after
Kreuzer
et
al. 1989, and
unpublished
lSArrAr
stepheating
data). For
further
comments
see Fig. 1. Local
consistency
of
the
dates
is
compatible
with
the
model
of
cooling ages
around
380 Ma,
i.e
.
early
Devonian (NW Hangendserie, SW Hangendserie,
two
am-
phiboles
and
the
muscovites
from
the
Liegendserie. One gross-
ly
deficient
hornblende
,
date
(Hangendserie) and
biotite
see-
mingly
older
than
muscovite
(Liegendserie)
indicate
local in-
fluences
of
an
event
later
than
about
380 Ma. The
>SArrAr
plateau
ages
of
'390
Ma
of
3T
polytype
muscovites
from
the
Oberkotzau
eclogite
(e)
suggest
inherited
argon
within
a
realm
of
apparent
cooling ages as
indicated
by
the
>SAr/4JJAr
mus-
covite
plateau
ages
of
the
unfoliated
pegmatite
(p)
which
cuts
the
eclogite
and by
two
hornblende
dates
from
amphibolites
(a)
from
south
of
Oberkotzau.
are consistent
with
this age information (Kreuzer - Seidel,
unpubl.).
On
the other hand the muscovites
of
3T poly type
from
an
eclogite
cut
by
the pegmatite have significantly higher app-
arent ages . They show scattered conventional dates between
388 and 379 ± 3 Ma (Kreuzer
et
aI., 1989) which exceed the
regional pattern the more, the larger the grain size fraction is.
Convincing
39Ar/
4
0Ar
plateau and isochron ages
of
390 Ma
(Kreuzer -Seidel, unpubl.) are consistent, within the limits
of
errors,
with
Sm/Nd
isochron ages around 395 Ma which Stosch
169
I .
-Lugmair (1987) found
for
two
eclogites
from
the Miinchberg
complex. Thus, the relic 3T muscovites obviously preserved an
. earlier age information
within
a realm
of
an apparent cooling
pattern.
Extraneous argon
The interpretation
of
scattered K-Ar dates
is
further com-
plicated
by
the experience that in marginal parts of a thermal
overprint biotite and hornblende use
to
trap radiogenic argon
expulsed
from
the higher-grade area, leading
to
irrelevant high
apparent ages.
In
contrast muscovites
from
such a marginal
zone
would
show scattered
"mixed
ages" intermediate bet-
ween the original and the thermal overprint ages, but rarely
contain much excess argon (e.g. Stockwell1963, Wan less et al.
1970). Thus concordant dates
of
muscovite and hornblende
as
in Figs.
1,
2,
and
in
the upper part
of
Fig. 3 exclude significant
influences
of
excess argon and indicate reliable age estimates.
The slightly increased biotite dates
from
the Liegendserie
(Fig.
2)
indicate the existence
of
a moderate thermal overprint
later than
the
age given
by
the fairly consistent pattern
of
horn-
blendes and muscovites.
Isochron treatments
Assuming that,
within
a small area like
an
outcrop, excess
argon,
that
means extraneous
and/or
redistributed inherited
argon, is rather
uniformly
distributed one can suppose that the
same kind
of
mineral incorporated argon
of
the same isotopic
composition in all places. If this holds true the K-Ar dates are
negatively correlated
with
the ratios
of
potassium and atmo-
spheric argon and the age
of
the overprint can
be
evaluated
from
an isochron. But often the dates are governed by the struc-
ture and
the
chemistry
of
the environment; moreover, an inho-
mogeneous distribution
of
excess argon is frequent even
within
a single mineral separate (e.g. Blankenburg -Villa 1988).
Therefore, in presence
of
excess argon,
it
is likely that a correla-
tion marks a trend rather than a real isochron.
Samples and results
Sampling localities can be taken
from
Table 1 and the sketch
maps
in
Schiissler
et
al. (this volume: Figs. 1 and
2)
who
also
give a
-short description
of
the investigated rock types. The con-
ventional K-Ar dates
from
the Tepla-Domazlice zone are synop-
tically depicted in Fig.
3.
The
West
Bohemian Moldanubian area
Two
localities situated
west
of
the Bohemian Quartz Lode
have been investigated. The amphibolites
of
the outcrops near
MYto are intercalated
with
Moldanubian gneisses and
form
a
possible continuation
of
the metabasites in the TIrschenreuth-
Mahring zone (Bavaria), a transitional zone between the Molda-
nubian and the Saxothuringian. The Mutenin body
is
interpreted
as
a Proterozoic ferrodiorite ring intrusion (T onika 1979) which is
situated close
to
the Bohemian Quartz Lode.
In
both localities
mid-Carboniferous dates were found
for
hornblende and biotite
(Table
1)
as expected
from
the dates
of
Smejkal (1964 and 1965)
and Vejnar (1962)
for
the area
west
of
the quartz lode.
Amphibolites from the Marianske
Uzne
metabasic
complex, and metamorphic rocks and granitoids from
the Tepla anticlinorium
Amphibolites were collected
from
several localities within the
Marianske Lazne metabasic complex. Throughout the complex
170
a weak retrograde
overprint
can be observed. Thus,
it
was
not
unexpected that the K-Ar hornblende dates are significantly
scattered, covering the
wide
range from 386
to
362
Ma. The
interpretation
of
these dates
as
an approximate age
of
an
early
Devonian thermal event is confirmed
by
a similar date
of
369
± 4 Ma
for
the single analyzed biotite. Striking support comes
from concordant muscovite and biotite ages from the Tepla
anticlinorium adjacent
to
the Marianske Lazne complex (Gott-
stein 1970 and
Fig.
3,
SmejkaI1964, Vejnar 1962). The K-
Ar
ages
demonstrate that the Marianske Lazne metabasic complex and
the Tepla anticlinorium were subjected
to
a last regional meta-
morphic event contemporaneously
with
units
of
the Miinchberg
complex and the Erbendorf-VohenstrauB zone in Bavaria.
The
terna
Hora
Massif
Hornblendes of amphibolites
from
three localities have been
analyzed: five samplex
from
Cerna Hora village (northern part
of
the massif), one sample
from
the host rock
of
the pyrrhotite
mine Zamecky Vrch (centre
of
the massif), and three samples
from
Ujezd Svateho
Ki'iZe
quarry (southern rim
of
the massif,
near the Bohemian Quartz Lode) . Five
of
the nine dates (four
from
Cerna Hora village and a single one
from
Ujezd Svateho
Ki'ize) scatter within the analytical uncertaities around a weight-
ed mean age
of
376
± 5 Ma,
an
age which is well within the range
anticipated
from
the Bavarian nappe units and from the Ma-
rianske Lazne -Tepla area. The same age can be estimated
from an uncorrected muscovite date
of
386 Ma for a pegmatite
from Hostouri (Smejkal 1964, sample
20)
. Thus the early Devo-
nian event
is
fairly well established
for
the Cerna Hora area, too.
The remaining four dates deviate significantly. They range
from
.
407
to
491
Ma.
The cluster
of
the four consistent hornblende analyses
from
Cerna Hora village and the analytical point
with
the significantly
deviating date of 407 Ma are correlated like
an
isochron
(Table 1),
but
we
regard this
as
accidental.
Completely different is the pattern
of
data further
to
the south.
For Zamecky Vrch the hydrothermal activity presumably related
to
the Bohemian Quartz Lode
might
have caused excess ra-
diogenic argon
in
the hosting amphibolite leading to the app-
arently pre-Devonian date
of
490 Ma. The Ujezd Svateho Ki'ize
quarry is situated near the quartz lode and a quartz dike cuts the
amphibolite. Thus a strong Carboniferous
to
Permian influence
had been expected leading either
to
excess argon or
to
argon
loss.
In
contrast to Cerna-Hora village no possibly meaningful
correlation can
be
deduced
from
the three analyzed hornblen-
des: The hornblende
lowest
in
potasium and thus most sen-
sitive
to
excess argon yielded the youngest date
of
383 ± 12 Ma,
well
within
the early Devonian range known from the Bavarian
nappe units. This fairly consistent group
of
age estimates and
the absence
of
younger dates suggest that part
of
the apparent-
ly pre-Devonian dates is due
to
argon inherited from an earlier
metamorphic stage.
The Neukirchen area
Hornblendes from the gabbroamphibolites
of
the Blatterberg
-Hoher Bogen area and
from
fine-grained amphibolites
of
the
Warzenrieth area
as
well
as
muscovites and biotites from parag-
neisses related to both metabasite types were dated (Fig.
3)
.
Part
of
the investigated metabasites show a retrograde over-
print.
Almost
consistent muscovite dates between
329
and
321
Ma
and consistent biotite dates between
321
and 318 Ma
as
well
~s
two
dates
of
334 ± 14 and 329 ± 10 Ma
for
hornblendes from
gabbroamphibolites prove a strong mid-Carboniferous re-
juvenation
for
wide areas
of
this southern tip
of
the Tepla-
Domazlice zone. Strong rejuvenation has been found also
to
the
, "
TEPLA
DOMAZLICE
ZONE
500
300
350
400
450
550
Ma
I , I , I I , I I
c::::::::I
Marianske
L
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azn~
Complex
c:!:::3
, c:::::::::II
c:::s
-
c::=:I
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0
Tepla
Zone
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..
8 I I I I I
Cerna
Hora
Massif
8 I ,
c=: 1
"':::::11
C::24~
c::::-:
4 1
==
----
I I
I , e 8 ,
I
~E3
Neukirchen
area
I I
, I 8 I
~
I I
,
:::e;s:P ,
c:::::::::= ; I
c::::::::J
I I
I I I
I I I I I t I I 1 I 1 1 L I
~
---.J
330
Ma
c.
380
Ma
local
metamorphism
pre-Devonian
events,
rejuvention
inherited
and/or
excess
Ar
K-Ar
dates
o f
biotite
muscovite
hornblende
symbol
length
2s
north in the western part
of
the Domazlice area ($mejkal 1964)
which is intruded by the Babylon and Sedmihori granitoid stocks
of
Carboniferous
to
Permian age (Vejnar 1962, $mejkal -Vej-
nar 1965).
It is likely,
but
not
proven, that
part
of
the scatter
of
the horn-
blende dates
from
the Neukirchen area ranging from 329 to
as
high
as
460 Ma is due to excess radiogenic argon trapped during
the Carboniferous rejuvenation. Correlations like for Geleits-
bachliiB (Table
1)
are not convincing.
On
the other hand,
12
hornblende dates range between 350 and 390 Ma and may
testify
to
the early Devonian metamorphic event.
Conclusions
Widely scattered dates ranging
from
500
to 320 Ma have been
determined
for
the Tepla-Domazlice zone. This pattern is distin-
ctly different
from
the mid-Carboniferous to early Permian dates
established for the bordering Moldanubian and Saxothuringian.
c:::wt.CIfI
K~
+-
magmatic
Fig. 3.
K-Ar
Dates from
the
Tephli-Domafllce zone. The
dates
are
arranged roughly from north
to
south. The length
of
a symbol
represents
the
analytical precision
at
a level of 95 % intrala-
boratory analytical confidence. Dates from
the
Tep",
Hanov,
and
Lestkov zones
are
recalculated from Gottstein (1979);
the
date
of 392
Ma
in
the
second
line of
the
Neukirchen area is from
Fischer
et
al. (1968). For further
comments
see
Fig.
1.
Fairly concordant dates around
380
Ma for hornblendes from
amphibolites, and for muscovites and biotites
from
gneisses
and granitoids
of
the Marianske Lazne -Tepla area, in part also
for
hornblendes from the Cerns Hora area resemble the ages
determined
for
units
of
the Miinchberg complex and for the
western part
of
the Erbendorf-VohenstrauB zone in Bavaria, in
~
the latter confirmed by U-Pb dates on monazite (Teufel, 1988).
A much wider scatter
of
hornblende dates is recorded for the
Neukirchen area
in
the southernmost part
of
the Tepla-Domaz-
171
Tab.
le
1
K-Ar
Data
from
the
West
Bohemian
Moldanubian
area
and
the
Tephi-Domazlice-Zone
(ZTT)
Argon
concentrations
in
nannoliter
per
gram
at
standard
pressure
and
temperature.
lUGS-recommended
constants
(Steiger
&
Jiiger.
1977)
used.
Errors
estimated
at
a
level
of
95 %
intralaboratory
confidence
and
given
in
the
correct
position(s)
beneath
the
corresponding
decimal
place(s)
of
the
results.
Our
K-Ar
date
for
the
standard
glaucony
GL-O
is 1 %
younger
than
the
mean
value
of
the
compilation
of
ODIN (1982).
Abbreviations:
EC
= excess
argon
suggested
by
isochron
treatment;
IR =
"'Ar/-Ar
initial
ratio;
MSWD
=
mean
squared
wheighted
deviation;
P.
=
place
in
the
map
with
altitude;
quo
=
quarry
or
abandoned
qu.;
amph
=
amphibole;
bi
=
biotite;
chi
=
chlorite;
I -
light
phases; ga =
garnet;
hbl
=
hornblende;
mu
=
muscovite;
pi
= plagioclase;
px
=
pyroxene.
If
not
otherwise
mentioned
the
sieve
fraction
125-63
J.lm
was
analyzed.
..
Sample
Rock
type
Mineral
K-Ar
K
Argon
No.
locality
fraction
date
(wt.
%) rad.
atm.
(J.lm)
(Ma)
(nl/g)
STP
West
Bohemian
Moldanubian
area
Amphibolites
and
Gneisses
of
MYto
(4
km
W
of
Tachov)
39 Gneiss
with
sillimanite and transverse mu, W part bi + 5 %
chl,l
309 7.95
104.1
0.338
of
the
qu
o at the road
Myto-Lucina,
1 km NW
of
My
to
250-125
±3
6 8
31
40 Gneiss, rich in bi, W part bi + 5 %
chl,l
309 7.89 103.4 0.03
of
the
quo
at
the road
Myto-Lucina,
1 km NW
of
MYto 250-125
±3
6 8
35
41
Amphibolite,
poor
in bi,
NE
part hbl + cpx 315
00408
5046
0.21
of
the
quo
at the road
Myto-Lucina,
1 km NW
of
MYto
±6
8 4 4
44 Granite gneiss
with
sillimanite, W part bi + hbl 309 7.30 95.6 0.03
of
the
qu
o at the road
MYto-Lucina,
1 km NW
of
MYto 250-125
±3
6 7 34
bi + hbl 300 6.04 76.8 0.16
±3
5 6 29
Gabbrodiorite
of
Mutenin
(in
the
western
vicinity
of
the
Bohemian
Quartz
Lode)
56
Diorite
with
ga, E wall
of
the quarry in the centre
of
the stock, hbl 325 1.092 15.12 0.20
300 m W
of
the road
Mutenin-Star'f
Kramolin 250-125
±6
22
12
9
hbl 327 1.
011
14.
07
0.29
±7
21
11
8
bi 320 7.13 96.9 0.63
250-125
±3
6 7 38
Teplii-Domazlice
Zone
Mariiinskfi
Liizne
Metabasic
Complex
·
60
Amphibolite,
500 m S
of
Louka village, hbl 379 0.276 4.
54
0.256
outcrops
in a
wood,
12
km
NE
of
Mariimske Lazne
±9
7 3 40
61
Amphibolite
, 500 m S
of
Louka village, hbl 374 0.339 5.
48
0.
41
outcrops in a
wood,
12
km
NE
of
Marianske Lazne
±7
7 3
11
66
Amphibolite
to
ga-hbl gneiss, rail station Louka, hbl 368 0.539 8.55 0.25
at the
bridge
of
the
road Marianske
Lazne-Karlovy
Vary
±8
12
7 4
68
Ga
amphibolite, road cut
down-river
of
the coffer-dam, hbl 379 0.432 7.08 0.343
1500 m NNE
of
Mnichow
village, 12 km
NE
of
Marianske Lazne
±7
8 5 30
69 Schistose, banded amphibolite
with
calc-silicate, 2 hbl gen., hbl 363 0.303 4.74
0.371
qu
., 200 m S
of
the chapel, 600 m W
of
Prameny,
±9
8 4
21
8 km N
of
Mar
. Lazne
70
Schistose, banded amphibolite
with
calc-silicate, 2 hbl g!ln., hbl 362 0.373 5.82 0.337
quo
200 m S
of
the chapel, 600 m W
of
Prameny,
±8
9 4 26
8 km N
of
Mar
. Lazne
58
Banded amphibolite
with
bands rich in bi, upper quarry at the road hbl 386 0.607 10.15 0.
25
300 m
SW
of
Vyskovice; a granite
is
C. 10 m apart;
±8
13
8 6
7 km
SE
of
Mariimske Lazne bi 369 6.74 107.3 0.10
±4
5 1.0 9
Amphibolites
of
the
Cernii Hora
Massif
(15
km
NW
of
Horsovsky
TVn)
46 Schistose amphibolite,
from
the trench hbl 373 0.310 4.98 0.276
for
a gas pipe, 300 m
NE
of
Cerna Hora village
±9
8 4
28
47
Fine-frained, schistose amphibolite,
from
the trench hbl 385 0.194 3.216 0.
191
for
gas pipe, 300 m
NE
of
Cerna Hora village
±12
7
25
14
48 Schistose amp!:1ibolite,
from
the trench hbl 407 0.154 2.743 0.214
.
for
a gas pipe
..
300 m
NE
of
Cerna Hora village
±12
5
21
21
49 Banded amphibolite,
from
the
trench hbl 376 0.227 3.685 0.214
for
a gas pipe, 300 m
NE
of
Cerna Hora village
±10
6
28
21
50
Schistose, banded amphibolite,
from
the trench hbl
371
0.337
5040
0.317
for
a gas pipe, 300 m
NE
of
Cerml Hora village
±9
8 4
36
. hbl isochron data Cerna Hora village: 40Ar/36Ar versus 40K/36Ar 342
IR
-820 MSWD -0.6
±16
190
radiogenic
Ar
versus K 340
EC
- 0.45
MSWD-l
.8
±14
0.14
52
Schistose amphibolite
with
calc-silicate bands mine hbl
491
0.246 5.
39
0.419
Zamecky Vrch, 1 km
SE
of
Svrzno, 2 km S
of
terna
Hora village
±12
7 4 16
53
Schistose amphibolite
with
calc-silicate bands, hbl
411
0.406 7.
29
0.322
" quarry Ujezd Svateho KfiZe, 4 km S
of
Cerna Hora village
±9
10 6
26
54
Schistose amphibolite
with
calc-silicate bands, hbl 467 0.236 4.90 0.359
quarry Ujezd Svateho
KFize,
4 km S
of
Cerna Hora village
±12
7 4
24
55
Amphibolite,
unoriented, quarry Ujezd Svateho
KFize,
hbl 383 0.143 2.367
0.361
4 km S
of
Cerna Hora village (near the Bohemian Quartz Lode)
±12
5
18
17
-.
172
l
I
,
lice zone. The probably lower Devonian medium-pressure ev-
ent, although well documented
to
the north, in the Pobezovice-
Domazlice area, by critical metamorphic assemblages (Vejnar
1972), is only vaguely discernible in the pattern
of
K-Ar dates.
On
the
other
hand, the influence
of
the mid-Carboniferous (Iow-
pressure) metamorphism, typical
for
the adjacent Moldanubian,
is
clearly documented by consistent K-Ar dates
of
muscovites,
biotites and
two
hornblendes. Mid-Carboniferous K-Ar dates
recorded in the eastern part
of
the Erbendorf-VohenstrauB zone
could be explained in the same way. However, in contrast
to
the
Neukirchen area, there a thermal influence
from
the late Varis-
can
granites
is
more probable because
of
their immediate neigh-
bourhood (Schussler et al. 1986, Kreuzer et al. 1989).
Medium-pressure metamorphism and early Devonian mi-
neral ages are distinct features
of
the Munchberg Gneiss Massif,
the western part
of
the Erbendorf-VohenstrauB zone and the
Tepla-Domazlice zone. These similarities support the idea that
the
two
Bavarian units are related
to
the Tepla-Domazlice zone.
Probably, they are remnants
of
one huge complex, presumably
the western margin
of
the Tepla-Barrandian, resting upon the
low-pressure metamorphosed Moldanubian and Saxothurin-
gian in the Carboniferous. However, the belonging to the same
stockwerk does
not
imply straight-foreward litho logical correla-
tions. On the contrary, geochemical differences
of
the meta-
basites
point
to
different geotectonic origins
of
the former bas-
alts even
for
parts
of
the same nappe
unit
(Okrusch et al. in
press). .
Apart
from
the medium-pressure metamorphism
two
other
stages
of
overprinting are recognized in the three related tecton-
ic units. A young retrogressive stage has been mentioned al-
ready, as well
as
an
older metamorphic stage, evident from
eclogitic relics, which are widespread in the Munchberg and in
the Marianske Lazne complexes, and sporadically present in the
Erbendorf-VohenstrauB zone (and
in
the Moldanubian). These
relics testify
to
a formation under very high pressure (e.g. Franz
et al. 1986, Klemd in press).
As a consequence
of
the polymetamorphic forming
of
many
of
the rocks, an assigment
of
the 380 Ma event to the medium-
pressure metamorphism,
as
tacitly done in the present paper, is
not generally accepted. The
most
divergent interpretation is
given
by
Muller-Sohnius et al. (1987) . From Rb-Sr whole-rock
dating
of
paragneisses they conclude
that
the Liegendserie and
the Hangendserie
of
the Munchberg complex were metamor-
phosed
under
medium-pressure conditions already
in
the early
and in the late Ordovician, respectively. The early Devonian
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H.,
Clauer
N.,
Franke
W.,
Hansen
B.
T.,
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174
event around 380
Ma
they regard
as
a medium-temperature
mylonitization and
as
the first event which is common
to
all
tectonic units
of
the Munchberg gneiss core. Others (in oral
discussions) regard the Sm-Nd mineral-isochron ages around
395 Ma
as
the time
of
the medium-pressure metamorphism and
the 380 Ma event
as
a stage
of
uplift
and unroofing, analogously
to
the interpretation
in
the Erbendorf-VohenstrauB zone which
Teufel (1988) gave for the U-Pb zircon and monazite ages
of
about 390 and 380 Ma, respectively. Initially, Stosch -Lugmair
(1987) had interpreted the 395 Ma age
for
two
eclogites
from
the
Munchberg complex
as
estimate
of
the age
of
eclogitization, but
meanwhile Stosch -Lugmair (in prep.), by reason
of
new
re-
sults, they tend to follow the suggestion
of
Muller-Sohnius et al.
(1987)
who
more geologically argued from a Rb-Sr paragneiss
isochron age and a reinterpretation
of
the zircon dates
of
Ge-
bauer -Grunenfelder (1979)
for
an age
of
the high-pressure
metamorphism around 435 ± 14 Ma. Similar ages
of
427 ± 5 and
423 ± 8 Ma have been determined from Sm-Nd mineral -
whole-rock isochrons for and eclogite and its country rock from
the North-Bavarian Moldanubian (v. Quadt -Gebauer 1988).
These authors suggest a successive Silurian convercontinents
and terranes in the European Variscides.
The meaning of the scattered higher
(>
400 Ma) K-Ar dates
of
hornblendes from the Tepla-Domazlice zone is uncertain. The
existence
of
similar muscovite dates and rare concordant mus-
covite and biotite ages
(e
.
g.
Gottstein 1970: samples 15, 16, 17
which recalculated yielded 410 ±
7,
407 ± 10 and 410 ± 14 Ma)
suggests that some of the pre-Devonian dates may reflect ear-
lier metamorphic and magmatic events. But excess argon
caused by mid-Carboniferous thermal influences cannot gener-
ally
be
excluded. More insight
into
the poly-stage pre-Devonian
thermal evolution
of
the Tepla-Domazlice zone is expected
from
the initiated
39Ar-40Ar
work.
Acknowledgements
This paper
is
a preliminary result
of
a joint Czech-German
project which was supported
by
Deutsche Forschungsgemein-
schaft. The authors enjoyed fruitful discussions
with
J. Chaloup-
sky,
A.
Dudek,
E.
Fediukova,
P.
Jakes,
L.
Kopecky jr., J. Tonika,
J. Waldhausrova (Praha),
G.
Troll and
S.
Weiss (Munchen).
Thanks are due to
H.
Klappert, M. Metz,
L.
ThieBwald and
D.
Uebersohn
(BGR
Hannover)
for
carrying out the K-Ar and
39Arl
4°Ar
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R.
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.,
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