GeolCarp_Vol53_No1_45

GEOLOGICA CARPATHICA, 53, 1, BRATISLAVA, FEBRUARY 2002

45 — 52

Ar/

Ar DATING OF ALKALINE LAMPROPHYRES FROM THE

POLISH WESTERN CARPATHIANS

ANNA LUCIŃSKA-ANCZKIEWICZ

1✢

, IGOR M. VILLA

, ROBERT ANCZKIEWICZ

3✽

and

ANDRZEJ ŚLĄCZKA

Geological Institute, Jagiellonian University, ul. Oleandry 2a, Kraków, Poland

Laboratorium für Isotopengeologie, Mineralogisch-Petrografisches Institut, Universität Bern, Erlachstrasse 9a, CH-3012 Bern, Switzerland

Institute of Geological Sciences, Polish Academy of Sciences, Kraków Research Centre, ul. Senacka 1, 31-002 Kraków, Poland

✽

Present address: Department of Geological Sciences, University College London, Gower Street, London WC1E 6BT, England

Robert Anczkiewicz, Department of Geological Sciences, University College London, Gower Street,

London WC1E 6BT, England. Tel: +44 (0)20-7679-2260. Fax: +44 (0)20-7387-1612. E-mail: rob@gl.rhul.ac.uk

(Manuscript received April 18, 2001; accepted in revised form October 4, 2001)

Abstract: Amphiboles from two types of alkaline lamprophyres from the Silesian Nappe in the Polish Western Carpathians
were dated by

Ar/

Ar stepwise heating technique. Three teschenite samples representing mesocratic type of lamprophyres

yielded similar ages of 122.3±1.6 Ma, while leucocratic lamprophyre represented by a syenite dyke gave 120.4±1.4 Ma
date. These ages are interpreted as the time of magmatic emplacement during Early Cretaceous extensional episode
within the Silesian Basin. Ages for both types of lamprophyres are identical within error limits, which points to fast
(probably ca. 5 Ma) magma evolution from meso to leucocratic stage.

Keywords: Early Cretaceous, Western Carpathians, teschenites, lamprophyres,

Ar/

Ar geochronology.

Introduction

Lamprophyres in the Western Carpathians, usually known as
teschenites  or  Teschenite  Association  Rocks  (TAR),  spread
out from Nový Jičín (NE Moravia) in Czech Republic to Biel-
sko-Biała in S-Poland (Fig. 1). They represent hypabyssal in-
trusions and extrusions of alkaline magma. Although most of
the researchers agree on their Early Cretaceous age (Kudlásk-
ová 1987; Suk 1984; Šmíd 1962; Hovorka & Spišiak 1988),
the precise timing of this magmatic event remains unknown.
For  instance,  in  the  Polish  Carpathians  TAR  are  most  abun-
dant  in  the  Tithonian-Neocomian  beds,  which  lead  Smu-
likowski (1980) to propose that magmatic activity lasted from
Tithonian to the end of Neocomian. On the other hand Nowak
(1978) linked TAR to the Barremian—Aptian magmatic cycle.

In order to provide tighter constraints on the timing of this

important magmatic episode, we dated four TAR samples us-
ing

Ar/

Ar technique. Two dominant petrological types of

TAR from the Polish Western Carpathians representing differ-
ent stages of magma evolution were subjected to geochrono-
logical and petrological studies.

Geological setting

The Outer Western Carpathians (Fig. 1) consist of several

nappes composed dominantly of flysch deposits and minor
volcanites, volcaniclastites and igneous intrusions. From N to S
the main units are: the Skole, Subsilesian, Silesian, Dukla-
Foremagura and Magura Nappes (Figs. 1 and 2) (Książkiewicz
1972). They are commonly correlated with Alpine flysch de-
posits and hence reflect Mesozoic-Paleogene sedimentation in
distinct basins on the Tethys northern margin (Csontos et al.

1992). Their present tectonic juxtaposition is due to Neogene
northward thrusting and nappe formation. The occurrence of
TAR  is  limited  to  the  Silesian  Nappe,  whose  ca.  7  km  thick
sedimentary sequence represents the time span from late Kim-
meridgian to Early Miocene. In the Polish part of the Silesian
Nappe  TAR  occur  in  the  Cieszyn  Limestones  (Upper  Titho-
nian—Berriasian)  and  in  the  Upper  Cieszyn  Beds  (Valangin-
ian—Hauterivian) (Burtanówna et al. 1937).

TAR form hypabyssal intrusions, usually sills, rarely dykes

with the exception of the Moravian part of the Silesian Nappe,
where they occur as volcanic flows. Intrusions are most com-
monly tens of cm, exceptionally, tens of meters thick and usu-
ally show chilled margins. Host flysch deposits, at the contact
with the intrusions, typically display narrow metamorphosed
zones, which reached pyroxene hornfels facies (Wieser 1971).

Samples description

TAR outcrops in the Polish Western Carpathians are scarce.

However, they form a wide variety of petrological types with
various structures and textures (e.g. Hohenegger 1861; Tscher-
mak  1866;  Mahmood  1973;  Smulikowski  1980;  Kudlásková
1987;  Dostal  &  Owen  1998).  Commonly  they  are  heteroge-
neous both on outcrop and even hand specimen scale. Addi-
tional difficulty in studying these rocks is caused by their poor
preservation  due  to  common  secondary  alterations  linked  to
weathering and activity of hydrothermal fluids. After having
investigated most of the known TAR exposures in the area, we
selected four samples with the best preserved mineral assem-
blages  from  three  localities  within  the  Upper  Cieszyn  Beds.
The selected rocks represent two petrological types reflecting
different stages of magma differentiation: 1) Samples C-103,

✢

This paper is based on the early version of the manuscript by Anna Lucińska-Anczkiewicz, which was completed by co-authors.

Ania died of cancer on 29 September 2000.

46 LUCIŃSKA-ANCZKIEWICZ et al.

C-106  and  C-200,  termed  as  teschenites,  are  mesocratic  and
represent  the  most  common  type  in  the  Polish  Western  Car-
pathians, 2) Sample C-53 represents leucocratic, more evolved
magma,  and  was  classified  as  syenite.  Sample  localities  are
marked in Fig. 2 and described below.

Fig. 1. Geological sketch of the Western Carpathians.

Fig. 2. Sample localities.

Sample C-103 is located in Boguszowice near Cieszyn next

to  the  bridge  on  Olza  river  at  the  border  between  the  Czech
Republic and Poland (Fig. 2). The main rock forming assem-
blage is formed by (in order of decreasing abundance), pyrox-
ene, brown amphibole, K-feldspar, analcime and apatite. Ac-
cessories are sphene, ilmenite, biotite and chlorite. Pyroxene
and amphiboles form coarse, up to 2 cm long euhedral crys-
tals.  Seldom,  amphiboles  occur  as  elongated  prismatic  crys-
tals.  Both  are  commonly  fractured  and  have  tiny  alteration
rims. Amphiboles often contain inclusions of acicular apatite
(very common) and rarer K-feldspar as well as analcime (Fig.
3a). Apatite crystals are up to 1 cm long. Sphene is the most
common accessory phase and occurs as small (10—50

m) eu-

hedral crystals. Sporadic biotite (locally chloritized) and chlo-
rite are up to 50

m in size.

Analcime is usually a product of K-feldspar alteration, how-

ever, some of it can be primary. Precise relationship is difficult
to asses due to very strong alterations. Also due to the break-
down of feldspathic minerals the rock has a secondary porphy-
ritic texture with mafic minerals occurring as phenocrysts.
This feature is typical for all three mesocratic teschenite
samples.

Ar/

Ar DATING OF ALKALINE LAMPROPHYRES 47

phibole are rather common. They consist mainly of apatite,
rarer K-feldspar and analcime (Fig. 3c,d).

C-200 was collected in Rudów, north of Cieszyn in the Piot-

rówka stream bed (Fig. 2). The main minerals are: brown am-
phibole, pyroxene, analcime, K-feldspar with accessory bi-
otite, chlorite, and sphene. In comparison with the two sam-
ples described above this sample contains a considerably
smaller amount of apatite. Amphiboles and pyroxenes are less
fractured. Similarly to previous samples inclusions in amphib-

Fig. 3. Photomicrographs of the dated TAR: teschenites (a—e) and syenite (f). Amphibole and clinopyroxene crystals in altered matrix
composed dominantly of secondary analcime and relict feldspar (a—e). All crystals show pronounced alterations on the rims. Most com-
mon inclusions within amphibole are K-feldspar (a), analcime (c) and apatite (d). Rare inclusions of clinopyroxene and plagioclase were
observed in sample C-200 (e). Sample C-53 (syenite) (f) is fine-grained and contains crystals of clinopyroxene as and amphibole in al-
tered matrix of feldspathic minerals. Abbreviations: Kfs – K-feldspar, anal – analcime, biot – biotite, amph – amphibole, cpx – cli-
nopyroxene, sph – sphene, ap – apatite.

Sample C-106 was collected in the same locality as C-103.

It is composed of pyroxene, brown amphibole, K-feldspar,
analcime (mainly secondary, but see above) and apatite (Fig.
3c,d). Sphene, ilmenite and magnetite occur as accessory min-
erals. Pyroxene and amphibole form coarse euhedral or rarer
sub-hedral crystals. Their size varies from ca. 0.5 cm at aver-
age up to 2 cm (Fig. 3c,d). Amphibole is usually present as
coarse and seldom as acicular crystals. Similarly to the previ-
ous sample, the edges of amphiboles and pyroxenes commonly
show certain degree of secondary alterations. Inclusions in am-

48 LUCIŃSKA-ANCZKIEWICZ et al.

oles are common and consist of K-feldspar, analcime and sub-
ordinate apatite (Fig. 3e). In this sample we observed pyrox-
ene inclusions in amphibole, which are likely to be present
also in other samples (Fig. 3e).

C-53 classified as syenite is located south of Cieszyn, ca.

1.5 km north of the church in Puńców village. The sample was
collected from a small (ca. 15—20 cm wide) dyke, which in-
trudes teschenites, similar to the type described above. The
rock is fine-grained, composed of green and brown pyroxenes,
brown amphibole, K-feldspar, plagioclase, analcime (second-
ary), sphene, calcite, biotite and chlorite (Fig. 3f). Biotite is
usually chloritized.

Amphibole is much less abundant in this sample. It occurs

dominantly as acicular crystals, is poorer in inclusions (among
investigated thin sections we only rarely observed analcime)
but also shows pronounced marginal alterations. Similarly to
amphibole, other minerals suffered strong secondary alter-
ations.

Chemistry of amphiboles and their inclusions

Because of potential influence of K-bearing “contaminants”

on  K-Ar  isotopic  systematics,  chemical  composition  of  am-
phiboles as well as their alteration products and inclusions are
of major importance for interpreting dating results. Ca and K
are  of  particular  interest  because  they  are  directly  measured
during mass spectrometric analyses and can be directly com-
pared with the microprobe results. Such observations help to
evaluate contribution of inclusions to the K-Ar budget in am-
phibole  separates.  Quantitative  electron  microprobe  analyses
of amphiboles and their inclusions are summarized in Table 1.

Amphibole crystals usually stay within kaersutite composi-

tion  (classification  after  Leake  et  al.  1997),  however,  they
show  pronounced  major  elements  zonation  (Table  1).  From
core  to  rim  there  is  a  significant  increase  in  the  Fe  content,
which is compensated mainly by the decrease in Mg as well as
by a smaller drop in Ca and Na. Ti content is rather constant
throughout the grains. Sometimes a small decrease towards the
rim was detected, however, usually it was not larger than 0.5
wt. % of TiO

(commonly much less). Between core and rim,

there is usually a small increase of Ti. The K

O content ranges

from ca. 1.5—2.0 wt. % and stays rather constant within indi-
vidual grains. Similar zonation of amphiboles was observed by
Kudlásková (1987) and Dostal & Owen (1998).

Commonly kaersutite has tiny alteration rims, which rela-

tively to core are enriched in Fe (> 20 wt. % FeO), K and
slightly in Mn but depleted in Ti, Ca, Mg and Na (Table 1).
Those alteration rims show a significant increase in the K

content, which can reach even 6 wt. %.

Average Ca/K ratio in kaersutite obtained by microprobe

measurements is close to 7 but the ratios range between ca. 5.2
and 7.5. The strongest affect on K have alkali feldspars, whose
K

O content is even up to 10 times higher than this of kaersu-

tite (Table 1). Additional contribution to K-Ar budget is from
analcime, whose K content is comparable with that of amphib-
ole. Similarly Ca is affected by cpx inclusions, however, they
seem to be rare (Table 1). Certain amount of cpx is likely to be

present also as “impurity” in amphibole concentrate, which
was unavoidable during sample separation.

Because of different retentivity of K-Ar system by amphib-

oles and their inclusions and theirs different resistance to sec-
ondary alteration, complexities in

Ar/

Ar age spectra were

expected (see below).

Ar/

Ar dating results

Amphibole separates were prepared according to standard

mineral separation procedures i.e. crushing and sieving fol-
lowed by heavy liquid and magnetic separation. The final sepa-
rates were “purified” by handpicking under stereomicroscope.

Ar/

Ar analyses and data presentation follow Belluso et

al. (2000) and Villa et al. (2000). A summary of the isotopic
results is presented in Table 2 and Figs. 4—8. All errors are giv-
en at the 1

level.

Age spectra of samples representing mesocratic teschenite

(C-103, C-106, C-200) generally show slowly rising apparent
ages (except for sample C-103) with increasing degassing
temperature for the first 15 % of

Ar released (Figs. 4—6). Ap-

parent ages vary from 62 to 125 Ma with exception of sample
C-103, which shows 162.5 Ma age for the first step (Fig. 4).
This is most likely due to small amount of excess Ar compo-
nent. Then the spectra stabilize at ca. 120—122 Ma until ca. 70
% of gas released. The final steps are again scattered, howev-
er, to a much lesser extent when compared with the low tem-
perature steps.

All three teschenite age spectra show good correlation with

the Ca/K ratios. Scatter observed within the low temperature
apparent ages correlates with low, steadily increasing Ca/K ra-
tios (Figs. 4—6). This is probably due to disturbance in the K-
Ar  system  related  to  secondary  alterations  (see  sample  de-
scription)  and  contribution  to  K-Ar  budget  from  inclusions
like K-feldspar, which is altered itself and is likely to outgas at
low temperature. These lower temperature steps are followed
by  the  most  stable  middle  part  of  the  spectrum,  which  have
Ca/K ratios between 6.3 and 7.5. The gas rich steps (10 % of
the total Ar release or more) of the mesocratic samples (C-103,
C-106  and  C-200)  have  surprisingly  constant  Ca/K  ratios  of
7.5. These values are very close or the same as those obtained
by the microprobe analyses for pure kaersutite (Table 1, Figs.
4—6). The best correlation was obtained for sample C-200, for
which Ca/K ratios obtained by both techniques are the same.
For other two samples the values obtained during mass spec-
trometric  analyses  are  only  slightly  lower.  During  the  high
temperature  gas  release,  small  disturbances  become  visible;
the disturbed steps correspond to higher Ca/K ratios (10.8 for
C-103 and 15 for C-106) (Figs. 4, 5 and Table 2). We also note
that the average age of the Ca-rich steps in C-106 and C-103
are identical to the age of the steps with Ca/K= 7.5. We inter-
pret this as a reflection of a zonation of amphiboles, in which a
more calcic kaersutite also gives step ages, which on average
are identical to the most gas rich steps. We propose that pyrox-
ene inclusions did not contribute significantly to the Ar bud-
get, as pyroxene have Ca/K ratios exceeding 100.

For the final age calculations we used only steps whose Ca/

K ratio is constant (therefore we will term the age so calculat-

Ar/

Ar DATING OF ALKALINE LAMPROPHYRES 49

Table 1: Representative microprobe analyses of amphiboles, their inclusions and pyroxens.

ed: “isochemical age”) and close to the values obtained by mi-
croprobe analyses. The three mesocratic teschenite samples C-
103, C-106 and C-200 yielded 122.0±1.5, 122.4±1.1,
122.2±0.9 Ma ages respectively (Figs. 4—6).

Syenite age spectrum (sample C-53) shows a low apparent

age for the first 13 %

Ar released and then stabilizes at ca.

120 Ma until ca. 80 % of

Ar released (Fig. 7). Then the age

spectrum forms a depression expressed by the drop of apparent

ages down to 113 Ma, followed by the rise during the last two
steps. This pattern again correlates with the Ca/K ratios (Fig. 7).

Low age for the first step is likely to be caused by some Ar

loss due to secondary alteration, which is observed on the rims
of the amphiboles in all investigated samples. A sudden drop
in apparent ages followed by a subsequent rise is correlated
with very high Ca/K ratios. This is interpreted as due to the
presence of calcite in our mineral separate.

Fig. 5.

Ar/

Ar results for sample C-106. (a) Age spectrum. (b)

Ca/K vs. %

Ar released.

Fig. 4.

Ar/

Ar results for sample C-103. (a) Age spectrum. (b)

Ca/K vs. %

Ar released.

Sample C-103

Sample C-106

Sample C-200

Sample C-53

Amph

core

Amph

rim

Kfs
incl

Anal

incl

Cpx
core

Cpx

rim

Amph

core

Amph

rim 1

Rim

Cpx
core

Cpx

rim

Amph

core

Amph

rim

Cpx

incl

Amph

core

Amph

rim

Anal

incl

Kfs

Cpx

brown

core

Cpx

brown

rim

Cpx

green

core

Cpx

green

rim

SiO

37.34 37.03 63.64 53.82 40.73 43.55 37.61 35.65 31.45 43.34 41.41 37.03 37.03 45.98 35.68 34.73 53.79 65.89 44.03 39.73 42.49 46.45

TiO

6.16 5.76

0.02 0.02

5.64

3.88

5.89 5.59

1.37

4.12

4.79

6.67 5.76

2.63

4.67 4.69 0.02

0.02 2.21

3.72

2.63

0.95

14.90 15.05 18.35 26.25 11.79 10.00 13.76 14.88 13.19

9.18 10.93 12.77 14.55

8.04 13.81 13.69 23.46 17.79 5.75

8.74

6.19

2.48

FeO

13.05 14.82

0.33 0.00

9.78 10.32 10.32 18.60 37.12

8.22 10.39 12.69 14.81

8.98 19.94 21.35 0.01

0.28 13.01 17.32 21.99 23.19

MnO

0.22 0.25

0.03 0.76

0.14

0.22

0.09 0.37

1.14

0.13

0.22

0.00 0.21

0.15

0.39 0.42 0.02

0.00 0.36

0.59

0.84

1.13

MgO

10.10 8.97

0.00 0.00

8.62

8.43 11.86 6.20

1.93 10.47

8.23 11.52 8.67

9.33

6.18 5.17 0.00

0.00 8.00

4.28

2.42

2.85

CaO

12.59 12.48

0.16 0.12 24.19 23.82 12.85 12.09

0.11 24.21 23.81 14.32 13.48 25.04 12.59 12.36 0.01

0.20 24.65 23.87 22.92 21.81

2.58 2.15

0.77 11.20

0.80

0.83

2.14 2.28

0.08

0.57

0.66

1.73 1.95

0.10

2.45 2.28 13.00

4.57 0.62

0.78

1.03

1.54

1.58 1.58 16.25 1.88

0.00

1.64 1.64

6.27

0.00

0.01

1.77 1.71

0.00

1.76 1.80 0.83 11.15 0.00

0.00

Total

98.52 98.09 99.55 94.05 101.69 101.05 96.16 97.30 92.66 100.24 100.45 98.50 98.17 100.25 97.47 96.49 91.14 99.90 98.63 99.03 100.51 100.40

Ca/K

6.82 6.76

0.01 0.05

----

6.70 6.31

0.02

---- 1871

6.91 6.76

----

6.12 5.87 0.01

0.02

----

Oxygens
in formula

5.60 5.61

2.97 1.93

1.54

1.65

5.54 5.57

1.64

1.59

5.39 5.40

1.74

5.30 5.58 1.99

3.01 1.74

1.61

1.72

1.88

0.69 0.66

0.00 0.00

0.16

0.11

0.65 0.66

0.12

0.14

0.73 0.63

0.08

0.52 0.57 0.00

0.00 0.07

0.11

0.08

0.03

2.63 2.69

1.01 1.11

0.53

0.45

2.39 2.74

0.41

0.49

2.19 2.50

0.36

2.42 2.59 1.02

0.96 0.27

0.42

0.30

0.12

1.64 1.88

0.00 0.00

0.31

0.33

-0.04 2.43

0.26

0.33

-0.06 -0.08

0.25

-0.15 2.87 0.00

0.00 0.43

0.59

0.75

0.79

0.03 0.03

0.00 0.05

0.00

0.01

0.01 0.05

0.00

0.01

0.00 0.03

0.01

0.05 0.06 0.00

0.00 0.01

0.02

0.03

0.04

2.26 2.03

0.00 0.00

0.49

0.48

2.60 1.44

0.59

0.47

2.50 1.88

0.53

1.37 1.24 0.00

0.00 0.47

0.26

0.15

0.17

2.02 2.03

0.01 0.01

0.98

0.97

2.03 2.02

0.98

2.23 2.11

1.01

2.00 2.13 0.00

0.01 1.05

1.04

1.00

0.95

0.75 0.63

0.07 0.78

0.06

0.61 0.69

0.04

0.05

0.49 0.55

0.01

0.71 0.71 0.93

0.40 0.05

0.06

0.08

0.12

0.30 0.31

0.97 0.09

0.00

0.31 0.33

0.00

0.33 0.32

0.00

0.33 0.37 0.04

0.65 0.00

0.00

Total

15.92 15.86

5.03 3.97

4.07

4.06 14.11 15.92

4.04

4.06 13.80 13.33

3.97 12.55 16.10 3.99

5.03 4.09

4.11

4.10

*Altered outermost rim of the same amphibole crystal. Total Fe as FeO. Abbreviations: Amph — amphibole, Kfs — K-feldspar, Anal — analcime, Cpx — clinopyroxene, incl — inclusion.

50 LUCIŃSKA-ANCZKIEWICZ et al.

Table 2: Summary of

Ar/

Ar dating results.

Step

Temp.

released

Ar tot.

Ar*

Ar(Cl)

Age

(pl/g)

Sample C-103, weight = 0.022 g
K = 0.70 wt.%, Ca = 5.3 wt.%, Cl = 169 ppm
J = 0.006884

400

0.30

5.66

0.01

3.19

0.23

0.00

0.019

0.002

0.015

0.18 0.01

0.008

0.001

162.5

16.0

956

4.86

68.79

0.00

33.54

3.78

0.00

0.419

0.002

0.353

7.62 0.03

0.121

0.002

107.1

1.5

975

2.62

23.42

0.01

21.28

2.04

0.00

0.258

0.002

0.234

7.00 0.03

0.009

0.001

125.5

1.8

996

4.67

39.66

0.01

36.75

3.63

0.00

0.427

0.002

0.385

13.39 0.04

0.013

0.001

121.8

1.0

1018

13.79

114.47

0.01

109.99

10.72

0.01

1.080

0.003

0.957

39.70 0.11

0.025

0.002

123.4

0.4

1033

17.04

138.32

0.01

133.50

13.25

0.01

1.270

0.003

1.118

49.44 0.14

0.029

0.001

121.3

0.2

1069

26.48

212.19

0.02

208.03

20.59

0.02

1.927

0.004

1.693

78.94 0.23

0.034

0.001

121.6

0.1

1070

9.03

77.99

0.01

68.95

7.02

0.01

1.109

0.003

1.026

37.31 0.10

0.040

0.001

118.4

0.6

1240

14.92

121.98

0.01

119.50

11.60

0.01

1.843

0.004

1.714

62.92 0.18

0.024

0.001

124.0

0.2

1408

6.28

52.82

0.00

48.66

4.88

0.01

0.734

0.002

0.677

25.51 0.07

0.021

0.001

120.2

0.7

“Isochemical age” (steps 4–7) = 122.0±1.5 Ma

Sample C-106, weight = 0.061 g
K = 0.63 wt.%, Ca = 5.0 wt.%, Cl = 163 ppm
J = 0.006884

722

4.08

201.20

0.02

43.48

7.92

0.01

6.315

0.012

6.123

6.90 0.02

0.535

0.003

66.9

1.2

936

2.58

76.74

0.00

48.77

5.00

0.00

0.586

0.002

0.511

7.22 0.02

0.097

0.002

117.3

1.3

955

1.64

38.24

0.01

29.23

3.17

0.00

0.436

0.002

0.394

8.26 0.03

0.033

0.001

111.1

1.4

955

1.34

32.67

0.00

25.05

2.61

0.00

0.388

0.002

0.353

7.92 0.03

0.028

0.001

115.7

1.8

973

2.18

51.00

0.00

44.11

4.23

0.00

0.611

0.002

0.559

13.96 0.04

0.027

0.002

125.4

1.1

994

5.49

117.14

0.02

106.47

10.65

0.01

1.337

0.003

1.210

37.49 0.11

0.046

0.002

120.3

0.4

1014

13.88

286.86

0.03

273.57

26.93

0.02

2.613

0.005

2.302

98.33 0.27

0.070

0.001

122.3

0.2

1033

20.38

413.40

0.03

403.93

39.55

0.04

3.342

0.006

2.891

145.54

0.41

0.069

0.001

122.9

0.1

1072

24.29

486.92

0.02

477.91

47.14

0.04

3.818

0.007

3.284

178.86

0.49

0.076

0.002

122.0

0.1

1104

4.90

100.49

0.01

94.16

9.50

0.01

0.880

0.003

0.771

47.65 0.13

0.034

0.001

119.4

0.4

1241

10.23

207.44

0.02

196.53

19.85

0.02

1.991

0.004

1.772

146.52

0.40

0.074

0.002

119.5

0.2

1410

9.02

189.23

0.03

180.07

17.50

0.02

1.772

0.004

1.580

134.93

0.37

0.065

0.002

124.1

0.2

“Isochemical age” (steps 7–9) = 122.4±1.1 Ma

Sample C-200, weight = 0.087 g
K = 0.70 wt.%, Ca = 4.9 wt.%, Cl = 127 ppm
J= 0.006884

724

1.44

145.83

0.02

22.48

4.46

0.01

0.280

0.002

0.150

5.26 0.02

0.419

0.002

61.6

1.6

936

2.72

100.43

0.00

81.84

8.39

0.01

1.161

0.003

1.053

16.54 0.05

0.067

0.002

117.4

0.6

957

2.31

78.15

0.01

70.88

7.13

0.01

1.273

0.003

1.187

20.64 0.06

0.030

0.002

119.6

0.7

973

2.02

76.97

0.01

62.46

6.25

0.01

0.981

0.003

0.901

20.70 0.06

0.054

0.001

120.3

0.6

995

4.38

149.83

0.01

136.02

13.53

0.01

1.584

0.004

1.423

49.13 0.13

0.059

0.002

121.0

0.4

1016

14.74

477.79

0.05

461.02

45.56

0.04

4.041

0.008

3.519

169.95

0.48

0.100

0.002

121.8

0.1

1031

22.33

715.79

0.06

704.09

69.00

0.06

5.556

0.010

4.773

257.91

0.72

0.105

0.002

122.8

0.1

1068

31.24

993.03

0.10

985.35

96.52

0.09

7.458

0.014

6.369

362.35

1.01

0.118

0.002

122.8

0.1

1104

6.19

199.97

0.02

194.49

19.11

0.02

1.831

0.004

1.613

77.17 0.22

0.038

0.002

122.5

0.2

1245

7.60

246.55

0.01

240.44

23.49

0.02

2.288

0.005

2.024

115.02

0.32

0.050

0.001

123.2

0.1

1412

5.04

170.22

0.02

161.19

15.57

0.01

1.605

0.003

1.427

73.75 0.21

0.049

0.002

124.6

0.3

“Isochemical age” (steps 5–9) = 122.2±0.9 Ma

Sample C-53, weight = 0.103 g
K = 0.47 wt.%, Ca= 6.5 wt.%, Cl = 176 ppm
J = 0.006884

740

13.03

726.77

0.058

278.353

31.607

0.028

4.112

0.008

3.465

57.57 0.16

1.532

0.006

106.3

0.6

973

32.98

854.49

0.077

802.449

80.017

0.070

15.439

0.027

14.500

251.78

0.72

0.240

0.002

120.7

0.1

993

32.03

790.53

0.058

775.951

77.708

0.068

14.093

0.025

13.207

261.90

0.74

0.116

0.001

120.2

0.1

1009

3.48

87.10

0.007

81.762

8.453

0.008

1.461

0.003

1.363

33.24 0.09

0.027

0.002

116.6

0.5

1047

4.53

121.51

0.010

103.209

11.000

0.010

1.806

0.004

1.679

98.50 0.27

0.087

0.001

113.6

0.4

1058

4.05

106.01

0.008

90.580

9.814

0.009

1.661

0.004

1.569

232.94

0.63

0.112

0.002

112.9

0.4

1073

1.81

50.22

0.003

40.609

4.388

0.004

0.744

0.003

0.714

187.61

0.53

0.080

0.002

114.6

0.6

1109

1.78

55.68

0.009

39.884

4.321

0.005

0.778

0.003

0.755

259.39

0.75

0.120

0.001

115.6

0.7

1250

2.89

82.43

0.007

71.237

7.019

0.007

1.273

0.004

1.227

308.27

0.89

0.116

0.002

125.4

0.4

1413

3.41

95.39

0.007

86.479

8.272

0.007

1.522

0.003

1.442

162.78

0.45

0.072

0.001

127.0

0.3

“Isochemical age” (steps 2–3) = 120.4±1.3 Ma

Ar/

Ar DATING OF ALKALINE LAMPROPHYRES 51

The most stable part of the age spectrum consists of two

steps, which contain most of the

Ar released (65 %

Ar re-

leased). Their Ca/K ratios are 6.1 and 6.2 (Table 2), which is in
a good agreement with the ratios obtained by electron micro-
probe (Table 1). 120.4±1.3 Ma age was obtained for these two
steps.

Interpretation

All four ages obtained for two lithological types of the TAR

are indistinguishable within error limits (Fig. 8). Because dat-
ed samples represent small sub-volcanic intrusions, which
must have undergone rapid cooling, we interpret the 120—122
Ma age (upper Barremian/lower Aptian) as reflecting time of
magmatic emplacement of TAR. However, field relationships
indicate that the syenite could be younger (syenite forms small
dyke intruding mesocratic teschenite). Taken at face value, the
age obtained for more evolved magma (syenite) is 1.9±2.1 Ma
younger than the average of three mesocratic teschenite. We
were unable to date the intruded teschenite from the same out-
crop due to very advanced alterations. Nevertheless, our dat-
ing results strongly suggest that the evolution of the alkaline
magma from mesocratic phase (represented by teschenites) to
leucocratic phase (represented by syenite) was fast and hap-
pened within few Ma.

Conclusions

Alkaline lamprophyres in the Silesian Nappe of the Polish

Western Carpathians are represented dominantly by mesocrat-
ic teschenites and rarer by leucocratic syenites.

Ar/

Ar step-

wise heating dating of three teschenite samples resulted in in-
distinguishable ages of 122.0±1.5, 122.4±1.1, 122.2±0.9 Ma.

Fig. 7.

Ar/

Ar results for sample C-53. (a) Age spectrum. (b) Ca/K

vs. %

Ar released.

Fig. 8.

Ar/

Ar age spectra for all samples.

Fig. 6.

Ar/

Ar results for sample C-200. (a) Age spectrum. (b)

Ca/K vs. %

Ar released.

The syenite sample yielded statistically indistinguishable age
of 120.4±1.3 Ma. Thus, time period between ca. 120 and 122
Ma is interpreted as the time of magmatic emplacement of
TAR. This suggests rather fast parent magma evolution from
meso to leucocratic stage.

Although we did not date the most primitive, picritic, rocks

representing melanocratic type of TAR province (exposed in
the Czech Republic), it seems to be reasonable to assume, that
the rate of magma evolution from melanocratic to mesocratic
stage, was not significantly different from the rate, at which
these rocks evolved from meso- to leucocratic phase. Hence it
is likely that the TAR were emplaced within a very short time
period, possibly less than 5 Ma during the Early Cretaceous
extensional episode within the Silesian Basin.

Acknowledgments: This research was funded by KBN Grant
No. 6PO4D 014 12. Reviews by H. Maluski, A. Mulch and J.
Spišiak helped to improve the manuscript.

52 LUCIŃSKA-ANCZKIEWICZ et al.

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