Pozeska gora Mt. (Croatia)

GEOL. CROAT.
ZAGREB 1998
Upper Cretaceous - Palaeogene Tholeiitic Basalts of the Southern Margin
of the Pannonian Basin: Pozeska gora Mt. (Croatia)
Mirko BELAK ', Josip HALAMIC', Vesna MARCHIG 2 and Darko TIBLJAS 3
Key words: Tholeiitic basalts, Upper CretaceousPalaeogene, Upper mantle, Pannonian Basin, Pozeska gora MI., Croatia.
Abstract
According 10 the geological relationsh ips in the region of Pozeska gora MI. (the southern margin of the Pannonian Basin, nOrlhern
Croatia), basic erupti ve rock s are considered to be of Upper Cretaceous - Palaeogene age. Detailed petrographic examination, based on
phys iographic descri ption , th e chemical compositi on of major and
trace elements, rare earth elements and stable isotopes, indicates that
(hese primary tholei it ic basalts have variable structural-textural characterist ics, and we re postmagmatically affected by metamorphic
proce sses . Tholeiitic basalts originated from the upper mantle, and
were placed in the form of subaquatic effusions in the extensional
zones withi n continental crust.
On the basis of geological data KOCH ( 19 17,1919),
TAKSIC (1944) and SPARICA et a1. (1979, 1980)
determined that the volcanic rocks are of Tertiary age.
However, discovery of "scaglia" limestone layers within the basalts led PAMIC & SPARICA (1983) to their
conclusion of an Upper Cretaceous age. HALAMIC et
al. (1990) interpreted these limestone occurrences as
enclaves, which together with structural measurements,
was used as an argument for a Post-Maastrichtian, i.e.
Post-Laramian age for the basic volcanic rocks of the
Pozeska gora. However, MAJER & T AJDER (1982)
proposed that these rocks represen t an allochtonous
block, and that they are, according to their similarity
with the rocks in the other parts of th e Dinarides, of Triassic age. Isotopic K-Ar analyses undertaken by PA·
MIC et a1. (1988) determined the age of the acid mag·
matic rocks withi n the bimodal volcanic association as
71.5 ± 2.8 Ma, thus reaffirming an Upper Cretaceous
age.
1. INTRODUCTION
2. BASIC GEOLOGICAL DATA
Pozeska gora Mt. is located in the transitional zone
between the Tis ia tectonic megaunit and the Inner
Dinarides (Fig. 1; KovAcs et aI., 1989; HAAS et aI. ,
1990; SIKH:, 1995; HERAK et aI. , 1990). Basic rocks
represent part of the magmatic complex of the NE part
of the Pozeska go ra Mt. , which is also composed of
alkal i-feldspath ic granites and rhyolites (Fig. 2).
Petrograph ic examination s of th e basic magmatic
rocks of the Pozes ka go ra Mt. have prev iously been
incomplete, compri s ing part of investigation of the
magmatic complex as a whole (KOCH, 1917; TUCAN,
1919; BARIC & TAmER , 1942; TAmER, 1947,
1955, 1959; SPARICA et aI., 1979, 1980; MAJER &
TAmER, 1982). PAMIC ct a1. (l990) concluded that
basal t magmas originated from the upper mantle, and
that th ey are connected with continental areas. They
included basic rocks within the bimodal volcanic association.
I
Inst itute of Geology, Sachsova 2, P.O.Box 268, HR - lOOOO Zagreb,
Croatia.
2
Bu ndesanstalt fUr Geowissensehaften und Rohstoffe, Hannover,
Germany.
~
Faculty of Science, department of Geol ogy , Insti tute for Mineralogy
and Petrology. Horvatovac bb, HR-1 0000 Zagreb, Croatia.
The magmatic complex of th e Pozes ka gora is
bounded on the northern margin by Neogene and Quaternary sediments, in a partly tectonic and partly erosional -tectonic contact (Fig. 2). Their south ern margin
is characterised by a tectonic contact with Upper Cretaceous deposits (Fig. 2). Acid eruptive rocks, which are
the major part of the magmatic rocks, are represe nted
by alkali-feldspathic granites, i. e. alaskitcs and alkali feldspathic rhyolites (PAMIC, 1988; PAMIC et a!.,
1990).
Basic rocks mostly occur southeas t of Pozega,
between the Vidovci and Komusina vil1ages, where
along th e northern margin they are in the contact with
rhyolites and rhyolitic tuffs, and to the south are covered by Neogene sediments (Fig. 2). FurthellTIorc, these
rocks have bee n found southwest of Pozega, so uth of
Drskovci vil1age, where th ey are in tectonic contact
with granites and rhyolites.
A major part of the basalt body is composed of more
or less altered basalts of different stmctural and tex tural
vari eties, which are presented as a singl e unit on th e
geological map, since it was not possible to divide them
into units appropriate for geological mapping. At some
localities pillow lavas have been found, proving that the
164
Geologia Croatica 51n
MARIBOR
........
NOVO . "
MESTO
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N
A
10
o
30km
Fig. 1 Tectonic scheme of the basement in the SW part of the Pannonian basin (from
SIKIC, 1995).
basaltic rocks originated from subaquatic effus ions
(Fig, 3a), The second group of basic rocks of the Pozeska gora is composed of hypoabyssaI rocks, i.c. diabases and su bordinate gabbrodiabases. They arc rather frequently represented as veins in granites (Fig. 4a) and
rhyolites (Fig. 4b). Diabase veins have also been found
in thc Uppcr Cretaccous sediments north of Bod lis (Fig.
2). In addition basic effusives and vein rocks, volcanic
agglomerates, and basic tuffs have been discovered.
In the basaltic body there are several occurrences of
reddish to gray- reddish pelagic limestones which are up
to 1.5 m thick, and of restricted lateral extent (Fig. 3b).
These rocks are mostly sa ndy biomicrites, micrites and
sparit es. Non-carbonate detritus comprises quartz grain s, fine-grained muscovite, opaque minerals and a
clayey- haematitic substan ce . Findings of globotruncanid foraminifera indicate their Upper Cretaceous age
(pAMIc & SPARICA, 1983) . At several localities along the Nakop creek, limes tones are accompanied by
acid tuffs. Boundaries between sedimentary rocks and
basaltic lavas are almost always sharp and without load
casts, and reactional boundaries are characterized by
the recrystalli zation of limesto nes and loss of their
haematitic component, as well as a dec rease in grain
size of the basalts ("frozen margin") . On this basis we
may conclude that these limestones were enclaved during effusion of lava on th e sea bottom , and that the
basic effusives are younger than the sediments.
3, PETROLOGY
The analysed samp les were collected during work
on Ihe Geological map of the Republic of Croatia (scale
1 :50.000) during 1989 and 1990.
The modal composition was determined by optical
and X -ray diffraction methods. Major elements were
analysed in the Institute of Geology, Zagreb, by gra-
Belak. Halmn ic. Marchig & Tibljas: Upper Cretaceous - Palaeogene Tholeiitic
165
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Fig. 2 Geo logica l map of the
magmalic complex of Poieska gora M I. Legend : I ) basalt s: 2) d iabase veins; 3)
rhyolites: 4) granites; 5) Upper Crc taceous sedimentary
rocks; 6) Te ni ary sed im entary rocks: 7) Quaternary: 8)
di scordant boundary; 9) boundary of effu siv e volca nic
body; 10) faul t: normal. transc urrent: I I) reverse faul t:
12) gravitational sli ding: 13)
re lat ive ly subsided block;
14) gravitat ional overth rust.
166
Geolog ia Cro:llica 5 [/2
Fig. 3 a) Pillow lavas in the Nakop quarry: b) enclaves of Uppe r Cretaceous pelagic limestones in basalts in the Pako quarry.
vi mctry, spectrophotometry and flame photometry, and
some of the samples were analysed by XRF allhe Bundesanslall fUr Geowissensehaflen lind Rohsloffe (BGR)
in Hannover (Ta ble 1). Trace e lements were de tcnnined
al Ihe Ruder Boskovi c Inslilule in Zagreb , and at BGR
in Han no ver by XRF (Table 2). Thc acc uracy of th e
determinations was tes ted with international standards
and was in all cases better then ±5 %. Rare ea rth co nte nt s were determined by inductively coupled plasm a
mass spec trom etry (ICP-MS) al BGR Hannover (Tabl e
3) . The accuracy of these analyses is also bette r than
±5%.
The d iag ram s o f STRECKEISEN (1967, 1978)
were used [or class ification of the basic volcanic rocks.
Three diffe re nl basic rocks were determined: a) basalts,
b) diabases and gabbrodiabascs, and c) volcanic agg lomerates and basic tuffs.
a) Basalts a rc composed of basic plagioclase (labradorite), alb ite and pyroxene. Secondary compone nts,
besides albite, are c hl orit e, epidote, calcite, prehnit e,
pumpe ll yite, sericite, quartz and zeolite, and accessory
are ilmenite, magnetite and apatite. Basic plagioclase is
completely altered into acid plagioclase in approximal e ly 80% or th e analysed rocks, while in 20 % of
basalts basic plagioclase is preserved, but rarely un altered, because of pre hnitization and sericitization.
On the basis of stru ctural and textu ral charac teri sti cs
basalt s a re di v ided into several g roups : fin e-grained ,
medium-g rai ned, coarse-g rained , porphyritic and ves icular.
Fine-grained basalts with grain s up to 0.5 mm in
size most ly have typ ical o phitic te xture, while th ose
wi th subophitic, intergranular, di vergent-radial , skeletal -arbore scent and hyalopillitic tex ture are not so frequent. Their structure is homogeneous and ves ic ular.
Medium -grained basalts arc composed of gra in s
ranging in size from 0.5 - 1 mm. Their tex ture is ophiti c,
intergranular and subophiti c, and stru c ture ho mogeneous, infrequently vesicular.
Coarse-grained varieties have grains coarse r than 1
mm , ophitic texture and homogeneous stru cture.
Porphyritic basalts are c harac te rized by porp hyrit ic
texture and a homogeneous or ves ic ular struc ture. In the
fine-g rai ned matri x, co mposed of a ny of these textural
va ri eties of fine-grained basa lts , are phe noc rysts of plagioclase and clinopyroxene, up to 3 mm in size, wh ich
are presen t either as isolated grain s or as agg lomerates.
Vesicular basalts are characteri zed by their vesicular
stru cture, which is macroscopically visible. Vesicles are
0. 15-3 mm in size, usuall y of circul ar or sli ghtl y e ll iptical shape. Monomineralic vesic les are fi lled with calc ite, infrequen tly with c hlorit e a nd very rare ly with
quartz. Polymineralic vesicles a re fill ed with the mlner-
Fig. 4 <I) Diabase vein in granites ( 1.2 km NE from G radski Vrhovc i village) ; b) diabase vein in rhyol ites (2 km SW from Ku zmica village).
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4
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5
(2810)
6
(PC7)
7
(NA3)
8
9
(NJ·2) (V DA2)
9A
9A
10
(74 1)
11
(782)
16
12
13
14
15
(MC1) (131SA) (PC· l ) (PD·1 2) (VDA4)
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18
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19
(845)
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49.04
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11 .782
45.43
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14.57
15.132
50.93
286
14.73
11 .702
0.15
3.42
6.74
4.50
0.43
0.34
5.72
100.01
2. 47
2.80
0.20
5.55
6.51
2.32
0.92
0.37
5.28
99.44
1.80
3.58
0.17
3.60
6.02
3.24
1.07
0.29
4.85
99.55
3.09
0.96
44.73
2.31
15.60
5.50
6.08
0.18
3.83
9.01
3.04
0.38
0.56
8.76
99.98
3.89
4.87
45.14
2.53
14.53
7.92
5.97
0.17
3.53
9.93
4.25
0.30
0.71
4.91
99.89
3.90
1.01
45.96
1.24
15.94
3.97
5.05
0.14
7.25
9.25
4.61
0.58
49.11
1.77
17.85
5.59
6.28
0.10
1.71
6.72.
4.26
1.92
55.06
3.04
15.43
2.73
5.61
0.22
3.53
5.47
4.69
0.99
5.75
99.74
4.25
99.56
2.91
99.68
47.66
1.65
14.79
7.60
4.24
0.17
4.93
8.27
4.96
0.19
0.70
4.38
99.54
2.65
1.73
48.12
2.38
15.38
5.27
6.47
0.16
4.15
7.54
3.98
0.75
0.49
5.20
99.89
43.62
1.44
12.83
7.322
0.1
1.41
18.24
2.73
0.59
0.16
11.15
99.59
7.60
3.55
47.91
1.93
14.92
7.14
5.60
0.18
4.03
10.77
3.60
0.58
0.48
2.81
99.95
0.00
2.81
46.2
1.85
16.58
6.08
5.30
0.17
3.32
11.02
3.60
1.10
0.75
4.02
99.99
2.41
1.61
47.28
1.75
16.52
5.68
6.20
0.20
5.74
8.07
3.18
0.83
0.39
3.92
99.76
1.48
2.44
44.41
1.48
13.26
5.89
3.13
0.14
3.22
15.68
2.43
0.30
0.55
9.4 1
99.90
7.22
2.19
45.51
1.00
20.78
5.12
3.12
0.12
6.15
10.23
2.54
0.30
4.85
99.72
47.19
2.07
17.02
9.09
1.52
0.11
1.51
10.11
3.62
0.03
0.96
6.65
99.98
2.78
3.87
46.44
1.75
15.43
7.53
4.54
0.18
5.54
9.25
2.80
0.32
0.49
5.54
99.89
2.41
3.13
50.97
1.89
17.60
4.53
4.55
0.13
4.04
7.71
1.89
1.41
4.84
99.53
47.20
0.53
16.19
5.10
5.29
0.16
5.54
12.46
3.22
0.80
0.37
3.04
99.90
0.83
2.21
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46.67
1.57
16.11
595
4.46
0.16
4.05
11.35
2.96
0.64
0.51
5.65
100.08
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Chemical analyzes calculated on Hp and CaCO) free basis
x
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TiO
AIP3
Fe 20 3
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Nap
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FeO"/FeO+MgO
Fep3: FeO
CaO/Na 2O+Kp
53 .80
3.14
16.49
12.92
49.44
3.44
15.86
16.46
56.16
3.11
16.03
12.73
0.16
3.75
3.95
4.94
0.47
0.37
1.03
0.62
0.77
0.22
6.04
4.60
2.52
1.00
0.40
0.90
0.47
1.30
0.18
3.92
2.27
3.52
1.16
0.31
1.02
0.63
0.48
51.85
2.67
18.08
6.37
7.04
0.21
4.43
4.71
3.52
0.44
0.65
1.17
0.90
1.19
0
C
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8.1
1.5
2.8
42.1
17.3
8.9
3.2
5.9
21.5
20.4
19.5
5.6
6.9
30.2
9.3
10.7
4.8
2.6
29.8
19.2
Hy
14.4
35.0
14.7
20.1
01
Ml
Hm
6.8
7.2
6.7
6.1
21.4
6.23
11
6.0
6.6
5.9
5.0
0.8
29.1
27.2
53.0
0.9
48.6
38.8
36.4
0.7
23.6
27.5
56.6
1.5
39.1
31.3
39.1
50.14
2.81
16.14
8.80
6.63
0.19
3.92
5.52
4.72
0.33
0.79
1.46
1.32
1.09
48 .89
1.32
16.96
4.22
5.37
0.15
7.71
9.84
4.90
0.62
51.52
1.86
18.73
5.86
6.59
0.10
1.79
7.05
4.47
2.01
56.89
3.14
15.94
2.82
5.79
0.23
3.65
5.65
4.85
1.02
0.73
0.78
1.78
1.48
0.88
1.08
0.91
0.49
0.96
51.92
1.79
16.11
8.27
4.62
0.18
5.37
5.34
5.40
0.21
0.76
1.29
1.79
0.95
7.6
1.9
40.1
21.4
52.29
2.58
16.70
5.60
6.78
0.18
4.50
5.43
4.31
0.80
0.54
1.12
8.87
1.06
55.36
1.83
16.28
9.29
0.12
1.78
10.89
3.46
0.62
0.20
1.16
0.62
2.67
49.32
198
15.35
7.35
5.76
0.18
4.15
11.08
3.70
0.59
0.49
1.32
1.27
2.58
49.73
1.99
17.84
6.54
5.70
0. 18
3.57
8.55
3.87
1.18
0.80
1.32
1.14
1.69
6.0
41.0
18.7
1.2
45.9
19.2
3.6
229.5
27.2
3.5
31.4
23.6
6.9
32.8
27.9
7.5
9.0
4.1
2.0
15.4
6.2
4.7
18.8
12.2
4.1
8.4
0.2
7.9
5.0
4.8
23.5
2.3
5.6
5.0
7.8
10.69
2.9
5.0
5.3
2.5
3.5
6.0
3.4
3.5
3.8
1.8
35.3
33.5
42.7
1.7
39.8
25.0
49.8
3 1.3
26.5
54.6
29.4
32.0
47.0
0.4
48.0
26.1
44.3
1.1
36.0
40.0
37.4
42.9
40.2
34.9
0.3
54.61
1.81
16.30
7.24
3.84
0.17
3.95
8.00
2.98
0.36
0.67
1.42
1.88
239
47.97
1.05
21.9
5.39
3.28
0.13
6.48
10.77
2.67
0.32
50.92
1.91
16.91
8.25
4.97
0.19
6.07
6.77
3.07
0.35
0.53
1. 19
1.65
1.97
53.82
1.99
18.58
4.78
4.80
0.14
4.26
8.14
1.99
1.48
0.88
1.64
2.16
52.6
2.30
16.97
10.13
1.69
0.12
1.68
7.33
4.03
0.03
1.07
3.00
5.90
1.80
10.6
1.5
0.2
34.2
29.5
4.4
0.4
2.0
26.1
30.3
11.2
8.8
16.9
37.4
11.4
26.7
2.4
14.2
5.5
4.9
5.2
11.9
2.9
12.2
11.1
11.9
37.9
25.1
3.6
24 .5
22.5
9.2
21.3
50.32
1.86
17.58
6.04
6.59
0.21
6.10
6.58
3.38
0.88
0.41
0.99
0.91
1.54
1.05
0.99
2.34
49.26
0.55
16.89
5.32
5.52
0.17
5.78
11.89
3.36
0.83
0.38
0.95
0.96
2.83
6.9
28.6
30.0
2.1
25.3
30 .1
1.9
22.7
47.0
26.3
4.3
15.9
4.8
4.8
5.4
8.7
8.6
3.8
3.7
3.5
3.4
2.0
4.3
3.6
3.8
1.0
1.8
45.9
30.3
39.8
0.9
51.1
34.7
34.1
1.5
54.3
28.5
39.7
67.3
28.4
24.5
24
46.2
21.3
45.0
1.2
53.7
35.3
32.6
68.9
25.7
36.8
0.9
50.5
39.1
31.3
51.39
1.73
17.66
6.43
4.75
0.16
4.38
g.O
3.25
0.66
0.56
1.32
1.69
2.19
i
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4.9
23.7
28.6
2.5
23.1
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Geologia Croatiell 51/2
168
1
2
(NG8') (NJ5')
Ba
Co
Cr
Cu
Ga
HI
Mo
Nb
Ni
Rb
Sn
Sr
V
Y
Zn
Zr
124
38
67
28
30
18
4
20
23
14
30
175
341
37
111
214
164
45
160
18
28
18
4
21
26
28
35
147
337
48
123
235
3
(35')
4
(755)
5
9
(2810) (VDA2)
111
40
140
24
23
18
4
19
34
35
39
202
315
39
108
189
154
112
107
27
36
199
198
20
96
78
5
31
165
300
41
119
205
5
52
234
159
34
113
227
10
(74')
81
26
189
27
26
18
4
12
23
15
49
139
238
17
34
105
11
(782)
17
14
16
12
13
(MG') (1318A) (PG1) (VDA4) (KB1)
19
(845)
78
238
136
282
175
165
354
12
29
181
150
26
114
97
32
35
251
266
31
11 1
182
27
50
181
196
21
80
180
5
31
265
73
23
84
114
9
41
206
174
37
120
194
5
30
225
238
21
99
120
23
29
353
92
18
64
66
Table 2 Trace elements contcnls (in ppm). Table 1 is the numeration key for samples.
al association ± calcite ± chlori te ±epidote ±quartz ±prehnite ± pumpcIlyit ± haematite ± zeolite.
b) Diabases and gabbrodiabases only occur as dykes and s ill s wh ich cut both the entire magmatic com plex, and sed imentary rocks of Upper Cretaceo us age.
Diabases have ophitic texture, a homogenous structure,
and coul d be divided, on the basis of their textural and
structura l c harac teris tics into medium-grained, coarsegrained and wea kly- porph yrit ic lypes. Major min e ral
components are basic plagioclase or albite and clinopyroxene. Secondary com ponent s include albite, chlorit e,
epi dote, calcite, prehni Le and sericite, while accessory
compone nt s are opaque minerals and apat ite.
La
Ce
Pr
Nd
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
74
NG8
NJ5
35
6.8
16
2.2
11
33
1.2
2.9
16
38
5
23
6.1
2
6.2
0.96
6.9
1.3
31
0.56
4.3
17
43
5.7
27
7.6
1.9
7.1
1.2
8.3
1.6
15
35
4.7
22
6.5
1.6
5.7
0.96
6.9
1.3
3.8
059
4.4
0.64
0.52
3.9
0.69
2.1
0.33
2.4
0.35
0.61
4.78
0.72
5.3
0.80
Table 3 Rare earth clements contents (i n ppm). Table I is the Ilumcration key for samples.
Gabbrodiabases are characterized by hypidiom orphic tex ture and homoge neous structure. The major
mine ral components are plagioclase and hornblend e,
secondaries include chlorite, epidote and calcite, while
magnetite and apatite are accessories.
c) Volcanic agglomerates and basic tuffs occu r
muc h more infrequ ently than other types of basic rocks.
Volcanic agglomerates hav e psephitic text ure and massive st ructure. C lasts of different text ural- struct ural types of basalis, or subordinate rh yo lite, are 10-100 em
in size. The ma trix is composed of weakly ce men ted
tu ffaceous materiaL
Basic tuffs are represe nted by thinn er beds within
the success ion of basalts and volcanic agglomerates.
They were divided into vi trocrystalline, crys talldvitrophitic and vitrophitic types.
4. MI NERALOGY
Plagioclase (labradorite) has a prismatic shape, is
usuall y prehni tized or se riciti zed, only infrequently fresh. Its modal com ponent in basalt and diabase is approximately 55 %. The mean content of anorthi te was determin ed by op tical measu rement as 57.9 %. The mean
normati ve co mposition of pl ag ioclase is 52 .8% an
(Table 1). Therefore, optical and normative values indicate that the p lag ioclase is a la brador ite, what is a lso
co nfirm ed by roentgen analysis indicating neutral to
basic plagioclase.
The clinopyroxene (augite) is of short -p ri smat ic,
a rti culat ed or feat her-like form. It is one of the major
mineral componen ts of basalt and diabase, with a modal
content of up to 30 %.
l3c1ak. HalamiC. Marchig & Tibljas: Upper Cretaceous - Palaeogene Tholeii tic Basalts".
169
Hornblende is the major mineral co mpon ent of
gabbrodiabase dykes, whil e only in one type of basalt it
does represent a late-magmatic m ine ral.
se nt in the form of small ve ins. It was determined by
both optical and roentgen methods.
lImclJitc, magnetite and apatite are accessory
minera ls in all basic effusive rocks of Poieska gora Mt.
Quartz usu ally fills small veins, more rarely vesicular fabrics, and very infrequently in te rstitial spaces.
Albite is the most important min eral in a ltered basalts and diabases, since there are al most no samples of
basic magmatic rocks showing no evidence of albitization. It is mo st freque ntl y of prismatic, twig-like an d
needle -like form, and is inhomogene ou s, with inclusions of epidote/cl ino zois ite and chlori te . The mean
anorthite compo nent dctcrmined by optical meas ure ments is 5.5 %, and by roentgen diffract ion it was determined as acid to neutral plagioclase. The mean value of
normative composition is 34.4 % an (Table J ). Normative values arc not in concordance with th e result s of
optical determinations. Thi s could be explai ned by includin g one part calcium from the postmagmatic min erals epidote, prehn ite a nd pumpelly it e into nonnative
plagioclase. Also, there is a possibility that the alb it ization process is not complete, but it was not possible to
prove this optically because of th e very small number
of plag ioclase g rai ns approp ri ate for theodol it ic mea surements.
Pumpellyite was found in small veins and vesicles
in metabasalts, and was determined by the optical and
roentgen diffraction me thod s.
C hlorit e was fou nd in all samples of the basic
rocks. It is present in small veins, vesicles or in intergranu lar spaces in the form of very fine agg regates. The
approximate formula of chlori te which occurs in veinlets in basalts from Nako p c reek was calcul ated from
X-ray powder diffrac tion data fo ll ow ing the procedure
proposed by NIETO (1997). Accordiog to the obt ai ned
formula
(Mg,,, Pe rnAI I.I) ) (S iJ.07 Alo.OJ)
°III(O H)8
<md valid division of the chlori te group minera ls (NEWMAN & BROWN, 1987) this chlorite co uld be determined as ferroan clinochlo re.
Epidote is present as an inc lusion in acid plagioclase , in the matrix as gra iny aggregates or in fissure
systems in the rocks where it represents the most common mineral. On the bas is of the diffrac togram of the
epidote from veins in the basalts of Pako an d Nakop
creeks, unit ce ll dim ension s were calcu lated, and using
th e eqLlaiion oj' CARflONIN & MOLIN (1980) the fo llo win g fonnulae of these two epidotes were obtained:
C", (AI, ."Peo.co ) (Si, O, ) (Si O., )O(OB) , and
Ca2(AI 233 Peo77) (Si, O, ) (S i 04)0 (OH) .
Calcite mo st frequent ly fills vesicles in the rock ,
but is also present in small joints and as irregular aggre gates. Tn th e rock matrix it is usually agglomerated wi th
epido te and chlor ite. Tn open fissures colo url ess calci te
crys tals LIp to I cm in size were found , wi th hexagonal
pri sm surfaces and basal pinacoicls.
P rchnite sub stitutes plagioclase, and in some sam ples plag ioc lase is co mpletely prehnitized. It is also pre-
Sericite is uncomm on and s ub stitutes plagioclase.
Haematite fi ll s veins and was foun d in irregu lar
agg regates in the form of a soil-like substance, and was
de termi ned by optical and ro ent ge n diffraction methods.
Pyrite is present along fi ssures in th e form of small
agglo merates, while coarse r crystals were found in basalt dykes.
Zeolite fills only vesicles in some varieties of metabasalts from Maj dan creek, an d was determ ined by
optical measurement.
5. GEOCHEMISTRY
From the results of chemical analyses presented in
T able I it is obvious that the major element contents
show a certain amo unt of va riab ility , most freq uent ly
caused by different contents of secondary calcite and by
other secondary processes .
In chem ic al analyses calculated to 100 %, without
ign itio n lo ss and CaC0 3 (Tablc L) , the Si0 2 content
var ies from 47.97-56.89% (x=51.84%). Therefore
some rocks could be determined as neutral , and most of
the rocks as basic according to STRECK EISEN (1978).
Total Fe oxide content is rather more uniform, with certain variat ions (x= LI.4S %). The mea n value of
Pe,0, : PeO ratio is 1.28 (Tab le I). Total ce oxide contents is higher than th e MgO con tents , which is more
variable ( 1.7 9 - 7.71 %). Alkali content is variable,
mostly depending on th e degree of the a lbiti zatio n or
the rock, and Na20 contents (X=3.78%) is hi gher th an
the K 20 content (0.73% o n average). Ti0 2 content is
interesting, ranging from 0.55 % in a gabbrodiabase
vein to 3.44% in some varieties of basalt. Harker's diagram s in d icate tha t an inc rease o f Si0 2 also increases
the alkali con ten t, and decreases CaO and P]O, content,
wh il e an increase of MgO sli gh tl y decreases th e CaO,
Na 20 and P20::; va lues. Pe trochemica l characte ri stics or
major clements, trace clements and C IP W normative
composition indicate that analyzed rocks be long to the
tholei itic basalts.
A geochem ical mode l of values normal ized on
MORB (PEARCE, 19 83) indicates higher normalized
contents of K, Rb and Ba (Fig. 5). l neompa tib le clements arc enriched, except Y and Yb, whose relation is
similar to N-MORB. Such a model has normalized
trace elements simila r to con tinental th oleiitic basal ts.
A geochem ical mode l of values normalized on
chondrite (THOMPSON , 198 2) shows relatively high
170
Geologia Croaliea 51/2
Sr
K
Rb6a
Th
Ta
Nb
Ce
P
Zr
HI
SmTi
Y
Vb
BaRbTh K NbTalaCeSr NdPSmZr HIli
TbY
TmYb
Fig. 5 Diagram of truce elements normali zed on ivl0RB (PEARCE,
1983).
Fig. 6 Diagram of trace elements normalized on chondritc (THOMP-
ratio s of Rb and K, and a clearly expressed negative
anomaly o[ Sr (Fig. 6). A decrease in Sr could be exp[ained by low-pressure fractionation of plagioclase, while
the gcochcm ically normalized model ind icates that the
primary magma originated from the enriched mantle or
crustal co ntamination of magmas.
The Zr/Nb ratio in analysed samples ranges from
9.7S - [0.7 (average 10.1), indicating that the magma
originated from an enriched mantle, as the values for NMORB are higher than 30 (WILSON, 1989).
A three-component diagram for basalts Lall 0Y/IS - Nb/8 (CABANIS & LECOLLE, 1989) indicates
that basalts of the Pozeska gara Mt. belong to th e Etype o[ MORS (Fig. 7).
For study of the geotectonic position, a discrimination diagram Ti/lOO-Zr- Y (PEARCE & CANN, 1973)
was used , where points of basic vo lcanitcs from Pozcska gora [all within the D tleld, i.e. in the Ileld of within-plates basalts (Fig. 8).
On an Nbx2 - Zr/4 - Y diagram (MESCHEDE,
[986), the analysed basic volcani tes [all in the field of
within-plates tholeiites, which is also the rield of basalts
of the volcanic arc (Fig. 9). Values of Ti and V were
plotted on the diagram (after SHERV AIS, 1982), where
points ind icate a trend of oceanic island basalts (Fig.
10).
A geochemical model of the rare earth elements
normalized on ehondrite (Fig. 11) shows La contents
approximately 50 times greate r than in chondrites with
a gent le negat ive inclination of the curve (Ce/Yb N
approximately 2), while intensely calcitized basalt has a
20 times higher content of La than chondrites w ith a
gentle negative inclination of the c urve (Ce/Yb approxi mate[y 2). Samples NJ-S and 35 have weakly expressed
SON,1982).
V/IS
Ti / 100
43
58
Dr--'.'-~
A
IA
c
Lall 0
Nb/8
Fig. 7 Diagram LaIlO - Y/15 -Nb/8 for basalts (CABAN IS & LECOLLE, 1989). Leg e nd: I) volcanic -arc basa lts (IA - calc-alkali
basalts; I B - an area overlap between I A and I C; Ie - volcanicilrc tholeiites); 2) continental basalts (2A - continental basalts; 28
- back-arc basin basalts); 3) oceanic basalts (3 A - alkali basalts
from int ercontin ental rift: 38 - E-typc MORB -enriched: 3C - Etype MORS -weakly enriched; 3D - N-lype MORB).
Zr
Y
x3
Fig. 8 Three-component Ti/IOO -Zr- Yx3 diagram for basalts
(PEARCE & CANN, 1973). Lcgcnd: A) is land-a rc tholeiites
(IAT); B) middle-oceanic rift basalts (MORB) and IAT; C) calcium-alkalic basalts (CA B); D) within-platc basalts (WPI3).
Belak. Halamic, Marchig & Tibljas: Upper Cretaceous - Palaeogene Tholeii tic Basn lts ...
171
~o~----~------~-----,------~----,
Nb X 2
20
10
50
v
•
•
••
•
100
•
25
Ti I 1000
Fig. 10 Diagram Ti/ 1OOO- V fo r basalts (S HERVA IS, 1982). Legend:
T i/V ratios between 10 and 20 : island-arc b..lsa lts; Ti/V rat ios
between 20 and 50: MORB , cont inental fl ood basalts and bac karc basalts; Ti/Y ratios between 15 and 50 w ith a nca r-vertical
trend: calc-alkaline basalts.
Fig. 9 Three-component Nbx2-Zr/4- Y diagram for basalts (MESCHEDE. (986). Legend: A I) wi thi n-plate a lka li basalt s; A ll )
wi thin -platc basa lts and wi thin -platc tho le iites; B) E-type
MORB: C) within-p late and volcanic-arc basalts: D) N- Iype
MaR 13 and volcanic-arc basalts.
nega tive Ell anomal y (ELI/ElI* = 0.79 and 0.7 1, res pectiv ely; Eli/ElI * = ElI Ni'J(S mNx Gd N), and sample 74 a
weak ly expressed posit ive Eu anomaly (ELI/ElI* = 1.19).
Negat ive Ell ano maly may indica te existe nce of th e
" magma chamber" where cumulat.e gabbroid rocks wi lh
pos itive Eu anomaly were form ed. Samples with a sli ght negative Eu anomaly may have fractionall y crystal li zed plagioclase or may have been in equi librium with
plagioclase- beari ng mant le source (WILSON, 1989). A
geochem ical model of REE normalized on chondrite is
comparable to conti nental th oleiites.
PAMIC et al. ( 1988) have performed isotopi c determina ti ons of 18 0 and 87S r/86Sr on seve ral basalt sa mples, and PAMIC (1993) has determined the K-Ar age
on three sam ples of fresh basal ts (Table 4). Th e
"s r /" S r ra tIO
. .111 d'lca tes t h at basalti.c magmas wc re
formed by partial melting of upper mantle rocks (TA YLOR & S l-IEPPARD , 1986). Discrimination diagrams
"S r/"Sr - SiO, and Ba/ Y - "Sr /"S r (MANTOV AN I et
aI., 1985) also indicate that basa lts of th e Pozes ka gora
ML belong to t.he high-titanium thol eii tic volcani c series with a magmatic origin in enri ched ma ntle. Differences in oxygen isotopic compositi on argue th at th e basalts were affec ted by postmagmatic , hydroth e rmal
me tamo rphi sm of the oceanic iloor (S POONER et aI. ,
1974; COLEMAN, 1977).
6. DISC USSIO N AND CONCLUSION
Pozcs ka gora Ml. is mostl y co mposed of altered
basalts and alte red diabases, w ith sub ordin ate fresh
basa lts, diabases, ga bbrodi abases, vo lca n ic agg lomerat.es and tuffs . Normative composit ion and petrochemical c haracteristics of major c le me nts, tTa ce cle ment s,
and rare earth elements (REE) to ge th e r with the isotopic compos ition indi ca te that the ana lysed rocks
belong to the high-titanium th oleiit ic basa lt s, the magma fo r which originated fro m the upper mantle. Rocks
wcre affec ted by hydrothermal metam o rphi sm , as indicated by mineral parage neses and iso topes; the re fore in
approximately 80% of analysed roc ks basic plagioclase
is completely substituted by alb ite, a nd in ot hers pal1ial
Rock
01~ O
n Sr flHiSr
fresh basalt
5.3
O.70570± 16
altered basalt
7.3
0.70403±250
altered basalt
6.3
O.70435± 5
additional 3 samples
of basalts
La
Ce
Pr
Nd
Sm Eu
Gd Tb
Dy
Ho
Er
Tm Vb
66.0±3.9
54.5±2.7
48 ± 1.5
lu
Fig. II Diagram of the rare -earth elements llom1tllizcd on cho ndrite.
K-Ar-ages
(Ma )
Table 4 0 180 content, SiS r / 86Sr ratio and K-A r ages.
I72
albit izat ion is present. The secondary mineral paragenesis acco rdin g to ERZINGER (1989) be lon gs to the
TYPE III: Hi gh -temperature (> 200°C) sea waterbasalt alteration at reducing conditions and medium to
low water/rock ratios. However, it shou ld be pointed
Qut that secondary minerals albite, chlorite, epidote,
prchnitc, sericite, pumpellyite and zeolite are also typ ical for the very low degree of regional metamorphism
(WINKLE R, 1979).
Gathered K-Ar ages of basic rocks from the Pozeska
gom Mt. rangc from 66-48 Ma (from the Upper Cretaccous/Palaeocene boundary to the Middle Eocene)
(PAMIC, 1993). The posi tion of the magmatic body is
presen ted on the geolog ical map (Fig. 2); it is dis cernible that veins of basic rocks cut the Upper C retaceOtls gran ite-rhyo li te complex, as well as Upper Cretaceous clast ic rocks. FUl1hennore, within the basalt body
numcrous decimelre-metre size outcrops of pelagic
limestones and shales occur (Fig. 3), and some of these
were de termined as being of Upper C re taceous age
(Up pcr Santonian to Lower Maastrich tian - PAMIC &
SPARfCA, 1983). These rocks show no signs of mo re
intense contact-metamorphic changes, except recrystallization, probably because of the more ab rupt cooling of
lava dur in g submari ne effusio n. S ince co ntact planes
betwee n basalts and sediments arc sharp , without load
casts, it may be suppo sed that these sediment s were
e nc lavcd aftcr lithification. However, the poss ibilit y
that some sedimentary rocks represe nt laye rs between
two elTusional events, cannot be excluded. On the basis
of aforementioned, compatible geo logical and iso to pic
data, we may co nclude that the basic rocks of the
Pozeska gora Mt. are of Upper Cre taceo us - Palaeoge ne
age, and that they arc younger th an the granit e- rh yo lite
complex and Upper Cretaceous clastic rocks.
Subduction processes during the Early Cretaceous,
together with the Eoalpi ne orogenesis (BE LAK et aI. ,
1995) had ca used cmersion in the marginal part of th e
Nort hern Dinaridcs; therefore in this area th ere are no
deposits of this age. Subduction was e ithe r minimized
during the Uppe r Cretaceous, or was co mp le ted, i.c.
thcre was a closure of Tethys in th is part of the Dinarides. Postsubd uction tectonic processes (co ntin en tal
rifting or pseudorifting) have caused rhyolite volcanism
a long deep faults. R ift in g or pscudorifting-extensional
processes caused the Upper C retaceo us tran sg ression,
which was registered in all margi nal parts of the Dinarides (SPARICA et aI., 1980), and formation of the
Uppcr Crctaceous - Palaeoge ne margina l basin.
Basic rocks of thc Pozeska gora MI. are, therefore, a
consequence of cxte nsional processes during the Upper
Cretaceous and Palaeoge ne alo ng the northern margin
of the Dinarides. On th e basis of geologica l and isotopic data it may be concluded th at the first process was
effusio n of the rhyolites, and crystalli za tion of smallcr
masses of its iIllrusive equivale nts. Rhyolite magma
was formed by partial melting of the con tinenta l crus t
(PAMfC ct a I. , 1990). Partial melting, which produced
acid crus tal magma, was faeilitatcd by positioning o f
Geologia Croatica 5 1/2
hi gh-te mpe rature basic magma in higher levels of the
lithosphere (SUNESON & LUCC HITTA , 1983). Simi lar geody nami caJ evoluti on o f some areas in the western part of the USA, resulting in bimodal volcanic association , was proposed by LIPMAN (1980).
By the Upper Cretaceous bas ic volcanism occurred
in deeper parts of the already form ed basin, and las ted
until th e c losure of the basin (Pyrenean tec toni c phase
in the Eocene) . In th e geodynam ic inte rpreta tion of
these areas it should be stated that rhyolites were placed
alon g th e marginal parts of the Uppe r C retaceousPalaeogene basi n, si nce they were not genetically related to th e sediment s in this basin , while basalts were
placed in th e deeper parts of the basin. Consequ ent ly,
acid and basic volcanic roc ks were, during effu sio n ,
spatially se pa ratcd, and were posit ioned in their present
mutu al position by tangential tectonic transport during
the Tertiary.
Acknowledgements
T he authors are indebted to Dr. 1. PAMIC for the
critical reading and improvement of the m anu script. We
also thank Dr. V. MAJER and Dr. P. AR K AI for their
thoughtful rev iews and useful comment s.
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Manuscript received March 10, 1997.
Rev ised manuscript accepted November 23, 1998.