GEOLOGICA CARPATHICA, APRIL 2005, 56, 2, 123136
www.geologicacarpathica.sk
Jurassic radiolarian cherts in north-western Croatia:
geochemistry, material provenance and depositional
environment
JOSIP HALAMIÆ
1
, VESNA MARCHIG
2
and PELA GORIÈAN
3
1
Institute of Geology, Sachsova 2, HR-10000 Zagreb, Croatia; jhalamic@igi.hr
2
Bundesanstalt für Geowissenschaften und Rohstoffe, Stilleweg 2, D-30631 Hannover, Germany; v.marchig@bgr.de
3
Institute of Paleontology, ZRC SAZU, Novi trg 2, SI-1000 Ljubljana, Slovenia; spela@zrc-sazu.si
(Manuscript received April 28, 2004; accepted in revised form June 16, 2004)
Abstract: The Middle Jurassic (uppermost Bajocianlower Bathonian to upper Bathonianlower Callovian) radiolarian
cherts in the Medvednica Mt (NW Croatia) have a high content of SiO
2
(average 90.87 %). Most of the silica is of
biogenic origin as is indicated by a high Si/Si+Al+Fe+Ca ratio (0.830.97). The Al/Al+Fe+Mn ratio (average 0.59) and
relatively low contents of Fe and Mn suggest that the sedimentation of the radiolarian cherts was not influenced by
hydrothermal volcanisms. The high correlation coefficient between the lithophile elements Ti, K, Al, Th, Zr, Hf and Rb
implies that the detrital component in radiolarian cherts for the most part has a terrigenous provenance. The MnO/TiO
2
ratio and La
n
/Ce
n
vs. Al
2
O
3
/(Al
2
O
3
+Fe
2
O
3
) diagram show that the investigated cherts were derived from two different,
but not necessarily strongly separated, sedimentation areas: (1) continental shelf and slope or marginal sea, and (2) deep
ocean floor, trench or basaltic plateau. According to the proposed sedimentation model the radiolarian cherts in the
Medvednica Mt were deposited in a relatively narrow basin. The detrital material was derived from two source areas:
(1) from a continent (terrigenous input) and (2) from an accretionary wedge (undifferentiated magmatic arc-like input).
During the Late JurassicEarly Cretaceous the radiolarian cherts were incorporated into the tectonic mélange (accretion-
ary prism) along with other fragments: Triassic radiolarian cherts and carbonate rocks; Jurassic shales, siltites and
sandstones and basic and ultrabasic magmatic rocks.
Key words: Jurassic, Croatia, south-western Pannonian region, major and trace elements, radiolarian chert geochemistry,
REE.
Introduction
Cherts, radiolarian cherts and shale (radiolarite s.str.) accom-
panied by basic and ultrabasic magmatic rocks (pillow lavas,
diabase, and serpentinite Steinmanns trinity) often occur
as smaller or bigger blocks in the ophiolitic mélange. These
siliceous rocks are often deposited in association with mag-
matic rocks, and as a part of the ophiolite sequence (layer 1
Wilson 1989) they play an important role in the paleogeo-
graphical reconstructions of the sedimentary environments.
In many cases the radiolarian cherts are interlayered or incor-
porated between the basic volcanic rocks and their fossil
content does not only determine their age, but the age of the
effusive rocks, too.
More intensive geochemical research of continental sili-
ceous sediments was induced during the research into pelagic
sediments in the DSD Projects in the second half of the 20
th
century (Shimizu & Masuda 1977; Manetti et al. 1979; Barrett
1981; Rangin et al. 1981; Baltuck 1982; Sugisaki 1984; Ada-
chi et al. 1986; Ruitz-Ortiz et al. 1989; Murray et al. 1990,
1991, 1992a,b,c; Murray 1994; Girty et al. 1996 and others).
On the basis of comparison of geochemical data from pelagic
sediments of recent oceans, this research tried to define the pa-
rameters for the determination of sedimentary environments
and for the origin of older siliceous sediments on the conti-
nent. This type of research is very important for siliceous
rocks we find in tectonic mélange today, that is in the former
accretionary prisms. Detailed geochemical research of cherts,
radiolarian cherts and shales (major, trace and rare earth ele-
ments) is still used today to geochemically determine their
sedimentation environment, sedimentation conditions, and the
provenance of the material they contain (Kunimaru et al.
1998; Dasgupta et al. 1999; Shimizu et al. 2001; Sugitani et al.
2002; Kato et al. 2002; Di Leo et al. 2002 and others).
Radiolarian cherts, that have been known in Medvednica Mt
ever since the late 19th century (Pilar 1881), have been deter-
mined as older than Upper Cretaceous (Gorjanoviæ-Kramberg-
er 1908) or as Lower-Upper Cretaceous (ikiæ et al. 1979;
Basch 1983). The first geochemical and paleontological data
for radiolarian cherts in the Medvednica Mt date back to the
end of the last century (Halamiæ & Gorièan 1995; Halamiæ et.
al. 1995; Halamiæ 1998; Halamiæ et al. 1999). Through pale-
ontological analyses it has been determined that the radiolari-
an cherts of the northwestern part of the Medvednica Mt, oc-
curring in the tectonic mélange as blocks accompanied by
basic magmatic rocks, belong to two stratigraphic levels. One
of these is Middle to Upper Triassic, the other is Middle Juras-
sic. Geochemical characterization of the Triassic radiolarian
cherts was given in Halamiæ et al. (2001). In the present paper
we present the geochemical characteristics of the Jurassic radi-
olarian cherts. On the basis of geochemical analyses and other
geological data, we attempt to determine the provenance of
124 HALAMIÆ, MARCHIG and GORIÈAN
the material and the sedimentary environment of the area of
Medvednica Mt during the Jurassic. This paper deals only
with the radiolarian cherts that are undoubtedly Jurassic ac-
cording to paleontological analyses of radiolarians.
Geological outline and petrographic data
Geology. Medvednica Mt is situated in the SW part of the
Pannonian region (Fig. 1a) as an isolated «island» of Paleozo-
Fig. 1. a Location map. b Geological sketch map of Medvednica Mt. c Simplified geological map of investigated area with sam-
pling points (simplified after ikiæ et al. 1977; Basch 1981, 1995; Halamiæ 1998). 1 Neogene and Quaternary sediments, 2 Cretaceous
and Paleogene sedimentary rocks, 3 Jurassic mélange, 4 Triassic clastic and carbonate rocks of Zakiènica Nappe, 5 low-grade
metamorphic rocks, 6 quarry, 7 investigated area, 8 sampling locations, 9 stratigraphic boundary, 10 fault, 11 reverse
fault, 12 thrust fault.
ic and Mesozoic rocks surrounded by Tertiary and Quaternary
sediments (Fig. 1b). According to recent geotectonic interpre-
tations, Medvednica Mt belongs to the Supradinaricum (Herak
1986), or to the Inner Dinarides Unit (sensu Herak et al.
1990). According to Hass et al. (1990, 1995, 2000) and ikiæ
(1995) Medvednica Mt is situated in the Mid-Transdanubian
Zone (i.e. Zagorje-Mid Transdanubian Zone Pamiæ &
Tomljenoviæ 1998), which is separated toward the north by
the Periadriatic-Balaton Line from the Transdanubian Central
Range Subunit, while toward the southeast it is detached from
JURASSIC RADIOLARIAN CHERTS IN NORTH-WESTERN CROATIA 125
the Tisza Megaunit by the Zagreb-Zemplen or Mid-Hungarian
Line (Kovács et al. 1988).
The studied Jurassic radiolarian cherts are a part of the tec-
tonic mélange, called the Repno Complex (Babiæ et al. 2002),
which is built of a shale-siltite-sandy matrix with km-sized
megablocks of pillow lavas, metabasalts and diabases, Trias-
sic carbonates and Triassic and Jurassic radiolarian cherts, silt-
ites and sandstones. This complex is situated in the northwest-
ern part of the mountain where it occupies an area of around
25 km
2
(Fig. 1c) (Halamiæ 1998). The base of this unit is un-
known because of its reverse contact with the surrounding
rocks. On some localities, the mélange is overlain by deep-wa-
ter calpionellid carbonates of the Tithonian-Berriasian age
(exposed in the northern part of the Medvednica Mt and on the
Ivanèica Mt), followed by the turbiditic rocks of the Lower
Cretaceous age (Babiæ & Zupaniè 1973; Halamiæ 1998). To-
wards the southeast, the tectonic mélange of the Medvednica
Mt is partly in a reverse contact with the low-grade metamor-
phic complex, and partly unconformably overlain by Creta-
ceousPaleogene sedimentary rocks. Towards the northeast it
is separated by a normal fault from the metamorphic rocks
(Fig. 1b). The radiolarites in the mélange in the Medvednica
Mt are of the Triassic (Halamiæ & Gorièan 1995) and Jurassic
age (Halamiæ et al. 1999). The age of the shale was assumed to
be the Upper Jurassic (Halamiæ 1998; Halamiæ et al. 1999) and
it was later proven with palynomorphs to include Hettangian
to Bajocian (Babiæ et al. 2002).
Jurassic radiolarian cherts in the northwestern part of
Medvednica Mt are found in a two kilometers wide tectonized
belt, between the Poljanica and the Bistra Valleys. Smaller oc-
currences of these rocks are also found in the right tributary
stream of the Jelenja Voda Valley (Fig. 1c). These are exposed
as decametric or hectometric blocks within a schistose sand-
silt-shaly matrix accompanied by Triassic radiolarites and ba-
sic magmatic rocks. Matrix supported polymict conglomerates
including Triassic limestone blocks are locally interstratified
between the Jurassic cherts. This radiolarite-clastic succession
was informally named the Poljanica unit (Halamiæ et al. 1999).
The radiolarian analysis showed that the Jurassic samples be-
long to different zones (Table 1) and were deposited during the
latest BajocianEarly Bathonian (UA Zone 5) to the Late Ba-
thonianEarly Callovian (UA Zone 7) (Halamiæ et al. 1999).
Sample
UA Zone
Age
PD0
5
latest Bajocianearly Bathonian
VH141
37
early-mid Bajocian to late Bathonianearly Callovian
VH882/1
5
latest Bajocianearly Bathonian
PA12
5
latest Bajocianearly Bathonian
VH113
57
latest Bajocianearly Bathonian to late Bathonianearly Callovian
PE1
67
mid Bathonian to late Bathonianearly Callovian
PC50
5
latest Bajocianearly Bathonian
PB6, PB18A
5
latest Bajocianearly Bathonian
P113/A9
5
latest Bajocianearly Bathonian
VH147A
57
latest Bajocianearly Bathonian to late Bathonianearly Callovian
VH32
58
latest Bajocianearly Bathonian to mid Callovianearly Oxfordian
VH558/5
7
late Bathonianearly Callovian
JVSP
57
latest Bajocianearly Bathonian to late Bathonianearly Callovian
Table 1: Unitary association zones for analysed Jurassic radiolarian cherts of the Medvednica Mt (data from Halamiæ et al. 1999, UA Zones
according to Baumgartner et al. 1995).
Petrography. The radiolarian cherts are very thin to thick-
bedded (from 1 cm to a few decimeters) with wavy bedding
surfaces (pinch and swell structure according to Jenkyns &
Winterer 1982) and alternate with millimeter- to centimeter-
thick shale partings (Fig. 2). The rocks are dark or pale red,
grey or pale grey, and greenish-grey (Table 2). The red colour
depends on the quantity of the dispersed Fe-oxy-hydroxides
(Fig. 3). Cherts are dissected by secondary microcrystal to
granulated quartz veins. In the JV 1 and JV 2 samples veins of
secondary calcite were registered. Mn-dendrites can be seen
Fig. 2. Rhythmically bedded sequence of radiolarite-siliceous silty
shale, slightly tectonically deformed. Locality: Poljanica Valley,
section PA (see Fig. 1c).
126 HALAMIÆ, MARCHIG and GORIÈAN
on the bed surfaces, and manganese minerals can also be
found in places in the sub-millimeter veins.
The cherts were microscopically determined as: (1) radiolar-
ian cherts, (2) clayey radiolarian cherts and clayey cherts, (3)
Table 2: Microscopically determined lithology with rockcolour description.
Sample No.
Lithology
Colour
PD1
clayey radiolarian chert
dark red
PD2
radiolarite, laminated
grey
PD3
radiolarian chert
grey/red
PD4
clayey radiolarian chert
dark red
VH141
radiolarite
grey
VH141A
clayey radiolarian chert
pale red
VH882/1
radiolarian chert
dark red
VH882/2
radiolarite, laminated
grey/red
VH882/3
clayey radiolarian chert
dark red
PA5/1
chert with rare radiolarian tests
grey
PA7
radiolarian chert
pale grey
PA9
radiolarite
grey
PA15
clayey radiolarite
grey
PA20/1
radiolarian chert, sub-millimeter veins with manganese and megaquartz
gray/red
VH113
radiolarian claystone
dark grey
PE1/1
radiolarian chert
dark red
PE1/01
radiolarian chert, laminated
dark red
PC50
chert with rare radiolarian tests, brecciated
grey/red
PC50/1
radiolarian chert, brecciated
red
PB1
chert
dark red
PB3
chert with rare radiolarian tests
dark red
PB8clayey radiolarian chert
dark red
PB19
radiolarite
greenish grey
P113/1
radiolarian claystone
light grey
P113/7
radiolarian chert, brecciated
grey
P113/9
radiolarian chert
red
P113/12A
radiolarian chert
grey
P113/13
clayey radiolarian chert
red
VH147
radiolarian chert
dark red
VH32/1
radiolarite
reddish grey
VH32/3
radiolarian chert
reddish grey
VH32/5
radiolarian chert
reddish grey
VH558A
clayey radiolarian chert
pale grey
VH558B
radiolarite, brecciated and veined by megaquartz
dark red
VH558C
radiolarian chert, laminated
dark red
VH558D
clayey chert with rare radiolarian tests
greenish grey
VH558/5
radiolarian chert
grey
JV1
radiolarian chert, dolomitized and veined by calcite
greeenish grey
JV2
radiolarian chert, veined by calcite
grey
JV3
chert with rare radiolarian tests
grey
Fig. 3. Photomicrograph of a radiolarian chert. White circles are radi-
olarian tests. Grey coloured parts represent finely dispersed Fe-oxy-
hydroxide in the siliceous matrix. Parallel polarizers (sample PA9).
radiolarites s. str., (4) cherts with rare radiolarian tests and (5)
cherts (Table 2). The matrix of the majority of samples is
cryptocrystalline quartz, while the others have a microcrystal-
line texture. The radiolarian tests have different degrees of
preservation and different size varying from 0.08 to 0.2 mm.
The tests are filled with microcrystalline quartz and with relics
of radial chalcedony. Some of these are partly filled with a
greenish mineral (celadonite?) (samples VH882/3, PE1/1,
PB3, P113/13). This greenish mineral can very rarely be found
also as irregular masses in the chert matrix. Only a few radio-
larian tests are impregnated with Fe-oxy-hydroxides. Other
secondary minerals in the cherts are granules of detritic quartz
resorbed on its edges, and also of white mica (sericite) with a
maximum size of 0.05 mm. The accessory minerals are zircon
(mostly rounded), hematite, and, very rarely, small apatite
grains. In samples JV1 and JV2 the dolomitization and calciti-
zation of cherts as tiny idiomorphic crystals of dolomite and
calcite can be seen microscopically.
Materials and methods
In total, forty samples that were paleontologically deter-
mined as Jurassic were chosen for geochemical analysis. The
JURASSIC RADIOLARIAN CHERTS IN NORTH-WESTERN CROATIA 127
sampling locations are shown in Fig. 1c. For the microscopic
analysis thin sections of all the samples were made.
Sample preparation. Only the rock pieces with the macro-
scopically smallest content of secondary quartz and calcite in
submillimeter veins and of iron and manganese coatings on
the cracks, were hand picked for chemical analysis. The cho-
sen fragments were treated in diluted acetic acid for 12 hours
to remove the majority of secondary calcite from the samples.
After drying, the samples were powdered in an agate mortar.
Chemical analysis. The chemical analyses were performed
in the BGR-Laboratory, Hannover, Germany by X-ray fluo-
rescence (XRF) on Philips PW 1400 and PW 1480 instru-
ments to determine the concentration of major and trace ele-
ments. The analytical method was calibrated with 106
international standards, and with 24 synthetic standards for el-
ements or ranges of concentration not covered by international
standards. Analytical precision was better than 2 % for major
elements and better than 5 % for trace elements. Rare earth el-
ements (REE), and also some trace elements with concentra-
tions under the detection limit of the XRF method, were anal-
ysed by the inductivelly coupled plasma mass spectrometry
(ICP-MS) method with the SCIEX 250 apparatus, following the
pressure dissolution in hydrofluoric acid. The precision of this
method was better than 5 %. The completeness of dissolution
was checked by dissolving and analysing lithium tetraborate/
metaborate rock-powder fused disks used by XRF.
Results and discussion
The major and trace elements of all analysed samples (40 in
total) are listed in Table 3 and Table 4. The REE analytical data
of the whole dataset are presented in Table 5. To maximize the
comparisons with the published data cerium and europium
anomalies were normalized against both NASC (North Ameri-
can shale composite Gromet et al. 1984) and PAAS (post-
Archean shales from Australia Taylor & McLennan 1985).
The anomalies were calculated as recommended by McLennan
(1989) for europium and Murray et al. (1991) for cerium:
Eu/Eu*=(Eu
sample
/Eu
shale)
)/(Sm
N
×Gd
N
)
0.5
Ce/Ce*=(Ce
sample
/Ce
shale
)/(La
N
×Pr
N
)
0.5
The samples in the sections were grouped to get a clearer
statistical and graphical presentation of the analytical results.
The results were grouped according to the geometric mean of
the analysed values, and the calculated results are shown in
Table 3 (for major elements), Table 4 (trace elements) and Ta-
ble 5 (REE). The raw analytical data of major elements were
recalculated on a volatile-free basis. Because of a closed data
problem (compositional data) of major elements the log-ratio
transformation log (x/y) (Aitchison 1986; Swan & Sandilands
1996) was carried out, with the SiO
2
as the denominator for all
variables. The ratios of different elements and element groups
are shown in Table 6.
Major elements
Silica. The SiO
2
content in all samples is relatively high and
exceeds 90 % for most samples. The values vary from 81.4 %
to 97.1 % (Table 3). The Si/Si+Al+Fe+Ca ratio was used to
determine the provenance of the silica in the radiolarites. Ac-
cording to Ruitz-Ortiz et al. (1989) the typical biogenic silica-
rich radiolarites have values of this ratio equal to 0.80.9. The
acquired values range between 0.83 and 0.97 (Table 6), and
correspond to cherts of which silica content is mainly of bio-
genic origin.
Aluminium, titanium and potassium are generally bonded
to the aluminosilicate detritic phase, and as such they are good
indicators of the terrigenous input into radiolarian cherts. Alu-
minium and titanium could be in part also located within clay
minerals (Rangin et al. 1981). This group of major elements
shows a very high mutual correlation coefficient (Table 7) that
indicates their common provenance.
The aluminium content in some samples is relatively high.
The values vary from 1.1 % to 8.5 %. This is the result of a
higher content of detritic white mica (sericite), which can also
be seen microscopically in thin sections. In addition to a rela-
tively higher correlation coefficient to titanium and potassium,
aluminium also shows a similar correlation coeffcient to iron
and sodium (r=0.80 and r=0.82, respectively) (Table 7).
The titanium values in the samples range from 0.04 % to
0.28 %. In addition to its good correlation to aluminium and
potassium, titanium also shows a high correlation coefficient
to sodium (r=0.84) (Table 7).
The K
2
O contents in the analysed samples range between
0.174 % and 1.541 % (Table 3). Along with aluminium, sodi-
um, titanium and rubidium, potassium is also a good indicator
of the aluminosilicate component in the sediment (Murray et
al. 1992c). Its terrigenous origin is indicated by the high corre-
lation coefficients to aluminium, titanium and rubidium
(r=0.93, r=0.94 and r=0.98, respectively). Like aluminium
and titanium, this element shows higher correlation to sodium
(r=0.74) (Table 7).
Iron and phosphorus. An increased content of iron in radio-
larian cherts could indicate a stronger hydrothermal influence
during the sedimentation, that is the relative distance of the
sedimentation area from the mid-ocean ridge. Microscopic
analyses of the thin sections have shown that most of the iron
in the radiolarian cherts is in the form of a finely dispersed Fe-
oxy-hydroxide (Fig. 3), and a lesser part in the form of hema-
tite grains. The content of iron in the analysed samples varies
from 0.5 % to 6.1 %. Iron, along with other major elements,
has a distinctively negative correlation to silica, which is a
consequence of an increased content of silica and the dilution
of other elements (Table 7). Iron shows a relatively high posi-
tive correlation to magnesium (r=0.78), which might be a
consequence of an increased content of the greenish mineral
(celadonite?) noticed in the thin sections and which probably
originated from the altered volcanic fragments.
The phosphorus in the rock could be of dual origin. Firstly,
it could be related to the products of an increased biogenic ac-
tivity during sedimentation, and secondly, it could be a result
of increased volcanic activity in the sedimentation basin.
However, its flaw as an environmental indicator is that it mi-
grates from the rock during the diagenetic processes, or is re-
placed by silica (dilution according to Murray 1994). The
phosphorus content in the analysed samples is relatively low
and ranges from 0.007 % to 0.096 %. The reduced content of
128 HALAMIÆ, MARCHIG and GORIÈAN
the phosphorus in the radiolarian cherts is visible from its nega-
tive correlation to silica (r=0.41) (Table 7). The correlation
coefficient of phosphorus and iron is not significant (r=0.33)
and it also shows no relevant correlation to other elements
(Table 7).
Calcium, magnesium and manganese. The calcium content
in the analysed samples varies from a low 0.06 % to a relative-
ly high 6.17 %. The content of this element in most samples is
relatively low and mainly below 0.5 % (Table 3). The in-
creased content of calcium in a few samples is connected to its
secondary appearance in the form of submillimeter veins (Ta-
ble 2). Calcium has no positive correlation to any major ele-
ments aside from manganese (r=0.90), even more so, in most
cases it is negative (Table 7).
The average contents of magnesium are relatively low (Ta-
ble 3). Increased values have been registered in only three
samples, which also contain celadonite? minerals.
The manganese content in the analysed radiolarian cherts
ranges from 0.025 % to 0.773 %. The correlation coefficient
to most major elements is negative, except to calcium
(r=0.90) which could indicate that a part of the manganese is
of secondary origin too, meaning that it migrated into the rock
Table 3: Major element data (wt. % volatile free; Fe
2
O
3
* Fe
total
; Mean Geometric mean). The mean values (bold numbers) were
used for the construction of the discrimination diagrams.
Sample
SiO
2
TiO
2
Al
2
O
3
Fe
2
O
3
*
MnO
MgO
CaO
Na
2
O
K
2
O
P
2
O
5
LOI
PD1
86.39
0.27
6.81
3.55
0.074
0.85
0.182
0.26
1.541
0.054
2.683
PD2
91.49
0.18
5.05
1.07
0.049
0.56
0.132
0.25
1.190
0.032
2.084
PD3
89.80
0.22
5.50
2.03
0.074
0.75
0.138
0.22
1.208
0.040
2.352
PD4
94.56
0.13
3.28
0.92
0.026
0.19
0.090
0.07
0.756
0.030
1.557
Mean
90.51
0.19
4.99
1.63
0.051
0.51
0.131
0.18
1.138
0.038
2.127
VH141
93.09
0.10
3.31
1.88
0.051
0.67
0.143
0.11
0.604
0.032
1.826
VH141A
87.61
0.27
6.87
2.41
0.025
0.92
0.249
0.21
1.349
0.083
2.895
Mean
90.31
0.16
4.77
2.13
0.036
0.79
0.189
0.15
0.903
0.052
2.299
VH882/1
90.92
0.20
4.90
2.10
0.044
0.44
0.144
0.16
1.048
0.062
2.134
VH882/2
91.82
0.19
4.77
1.24
0.028
0.41
0.205
0.19
1.047
0.091
2.112
VH882/3
88.23
0.20
5.64
3.63
0.062
0.75
0.144
0.19
1.051
0.048
2.526
Mean
90.31
0.20
5.09
2.11
0.042
0.51
0.162
0.18
1.049
0.065
2.250
PA5/1
95.08
0.08
2.46
1.26
0.060
0.41
0.114
0.11
0.403
0.018
1.453
PA7
95.32
0.04
1.11
0.52
0.272
0.12
2.301
0.08
0.226
0.007
2.753
PA9
94.70
0.08
2.95
1.09
0.034
0.41
0.078
0.10
0.549
0.013
1.701
PA15
95.69
0.09
2.46
0.53
0.056
0.19
0.251
0.10
0.589
0.050
1.286
PA20/1
86.10
0.12
3.53
1.50
1.129
0.63
6.174
0.15
0.646
0.018
6.343
Mean
93.30
0.08
2.34
0.89
0.129
0.30
0.501
0.11
0.453
0.017
2.233
VH113
83.65
0.18
6.79
5.76
0.070
2.18
0.174
0.24
0.893
0.072
2.532
PE1/1
93.83
0.07
1.87
0.84
0.531
0.15
2.111
0.12
0.385
0.031
2.930
PE1/01
92.69
0.12
3.09
1.44
0.227
0.25
1.337
0.13
0.638
0.069
2.552
Mean
93.26
0.09
2.40
1.10
0.347
0.19
1.680
0.12
0.496
0.046
2.734
PC50
89.59
0.19
4.78
2.70
0.253
0.53
1.008
0.17
0.753
0.062
2.769
PC50/1
89.61
0.26
5.64
2.17
0.040
0.51
0.170
0.30
1.267
0.034
2.335
Mean
89.60
0.22
5.19
2.42
0.101
0.52
0.414
0.23
0.977
0.046
2.543
PB1
93.79
0.17
3.88
0.66
0.024
0.29
0.111
0.16
0.886
0.028
1.794
PB3
91.10
0.20
4.71
1.97
0.071
0.74
0.134
0.18
0.872
0.028
2.190
PB8
87.63
0.24
6.17
3.18
0.035
0.81
0.218
0.20
1.453
0.070
2.661
PB19
93.54
0.11
3.18
1.73
0.061
0.42
0.113
0.12
0.680
0.042
1.615
Mean
91.48
0.17
4.35
1.64
0.044
0.52
0.138
0.16
0.935
0.039
2.027
P113/1
81.38
0.28
8.50
6.08
0.234
1.16
0.546
0.28
1.498
0.036
3.423
P113/7
88.61
0.17
4.90
3.62
0.192
0.81
0.708
0.15
0.776
0.056
2.627
P113/9
92.63
0.12
3.26
2.68
0.070
0.32
0.109
0.12
0.671
0.017
1.636
P113/12A
94.27
0.13
3.23
0.93
0.052
0.31
0.179
0.16
0.655
0.086
1.548
P113/13
88.97
0.20
5.35
3.09
0.234
0.70
0.172
0.16
1.060
0.073
2.381
Mean
89.06
0.17
4.72
2.79
0.131
0.58
0.265
0.17
0.885
0.046
2.222
VH147
91.76
0.16
4.55
1.93
0.029
0.45
0.150
0.12
0.806
0.044
1.894
VH32/1
97.07
0.04
1.55
0.69
0.036
0.17
0.082
0.08
0.254
0.016
1.172
VH32/3
96.62
0.07
1.73
0.87
0.045
0.19
0.078
0.08
0.290
0.020
1.166
VH32/5
96.42
0.06
1.77
0.66
0.206
0.19
0.252
0.08
0.310
0.046
1.380
Mean
96.70
0.06
1.68
0.73
0.069
0.18
0.117
0.08
0.284
0.025
1.235
VH558A
91.89
0.16
4.43
2.09
0.123
0.14
0.149
0.16
0.786
0.062
1.873
VH558B
96.64
0.04
1.80
0.63
0.322
0.10
0.062
0.07
0.326
0.018
1.014
VH558C
89.43
0.20
5.34
2.71
0.375
0.46
0.123
0.10
1.240
0.028
1.803
VH558D
85.06
0.25
5.98
3.37
0.390
0.87
2.700
0.30
0.978
0.085
4.393
VH558/5
91.47
0.12
3.62
2.64
0.219
0.39
0.809
0.18
0.459
0.096
3.013
Mean
90.82
0.13
3.92
2.00
0.263
0.29
0.301
0.14
0.677
0.048
2.144
JV1
91.11
0.09
1.76
1.66
0.773
1.10
3.076
0.14
0.227
0.065
4.495
JV2
89.29
0.08
1.52
1.52
0.690
0.83
5.723
0.12
0.174
0.079
6.017
JV3
92.98
0.11
2.81
1.89
0.088
0.57
0.944
0.14
0.431
0.027
2.318
Mean
91.11
0.09
1.96
1.68
0.361
0.80
2.552
0.13
0.257
0.052
3.973
JURASSIC RADIOLARIAN CHERTS IN NORTH-WESTERN CROATIA 129
Table 4: Trace element data (mg/kg; MeanaGeometric mean). The mean values (bold numbers) were used for the construction of the
discrimination diagrams.
Sample
Ba
Co
Cr
Hf
Nb
Ni
Rb
Sc
Sr
Th
V
Y
Zr
PD1
130
19
39
1.14
3.4
37
2.1
7
37
3.70
59
15
51
PD2
102
12
29
0.87
2.9
16
34.2
4
29
2.67
48
8
36
PD3
114
15
31
1.16
3.2
30
37.3
5
27
3.19
49
13
43
PD4
93
1
26
0.61
1.7
11
21.6
4
32
1.87
28
9
24
Mean
109
8
31
0.9
2.7
21
34.6
5
31
2.8
44
11
37
VH141
98
15
21
1.49
1.3
24
17.5
3
24
1.83
36
6
17
VH141A
107
21
41
1.30
3.9
30
39.6
9
35
4.55
54
15
42
Mean
102
18
29
1.4
2.3
27
26.3
5
29
2.9
44
9
27
VH882/1
111
8
27
0.98
3.5
21
40.3
5
29
3.44
37
11
32
VH882/2
99
9
32
1.02
3.5
12
40.0
5
31
3.80
30
15
37
VH882/3
105
20
31
0.99
5.1
37
40.2
5
30
3.66
50
15
39
Mean
105
11
30
1.0
4.0
21
40.2
5
30
3.6
38
14
36
PA5/1
58
3
18
0.48
1.6
11
12.5
3
20
1.39
19
7
15
PA7
40
1
15
0.24
0.9
1
5.7
1
23
0.65
2
9
10
PA9
69
7
19
0.45
1.9
13
18.3
3
18
1.35
33
9
15
PA15
62
1
17
0.56
1.7
4
19.1
3
23
1.53
38
8
17
PA20/1
67
5
24
0.66
2.2
18
20.2
2
45
1.73
35
16
20
Mean
58
3
18
0.5
1.6
6
13.8
2
24
1.3
18
9
15
VH113
99
30
22
0.81
3.2
65
29.4
8
20
2.88
73
12
30
PE1/1
64
7
18
0.39
1.3
6
11.4
1
25
1.21
21
9
12
PE1/01
83
14
24
0.58
1.6
9
19.5
3
32
1.99
48
11
19
Mean
73
10
21
0.5
1.4
7
14.9
2
28
1.6
32
10
15
PC50
124
10
32
0.92
2.9
31
24.0
5
31
3.14
61
12
39
PC50/1
117
7
37
1.39
4.0
17
41.5
6
31
3.43
49
13
50
Mean
120
8
34
1.1
3.4
23
31.6
5
31
3.3
55
12
44
PB1
102
6
29
0.82
2.4
8
26.2
5
33
2.39
26
8
29
PB3
103
12
27
1.15
4.6
23
29.7
4
23
3.28
38
7
41
PB8
127
8
35
1.22
3.8
15
51.1
7
29
3.62
47
17
44
PB19
85
10
22
0.58
1.6
11
22.2
3
21
1.98
35
10
20
Mean
103
9
28
0.9
2.9
13
30.7
5
26
2.7
36
10
32
P113/1
205
32
39
1.42
5.1
53
45.9
8
31
4.84
91
15
56
P113/7
91
22
27
1.03
3.1
42
24.6
4
27
2.95
38
10
33
P113/9
77
1
18
0.65
2.1
15
21.2
3
19
1.70
29
5
22
P113/12A
78
11
23
0.73
2.0
10
19.8
6
24
2.22
34
11
27
P113/13
123
19
27
0.98
2.9
32
31.5
6
29
3.11
30
14
37
Mean
107
11
26
0.9
2.8
25
27.2
5
26
2.8
40
10
33
VH147
85
15
39
0.92
2.0
33
23.3
5
23
2.98
58
9
37
VH32/1
41
6
11
0.25
0.7
6
5.4
1
17
0.79
11
6
8
VH32/3
45
5
35
0.57
0.9
16
6.7
1
19
1.16
11
7
22
VH32/5
58
4
16
0.32
0.9
9
6.9
1
27
0.95
13
11
11
Mean
47
5
18
0.4
0.8
10
6.3
1
21
1.0
12
8
12
VH558A
96
5
24
0.77
2.5
16
22.5
4
31
2.59
36
11
32
VH558B
46
7
18
0.21
0.6
31
8.7
1
32
0.68
24
10
7
VH558C
124
23
33
1.09
2.8
64
49.1
6
87
3.14
32
12
38
VH558D
131
21
33
1.31
3.4
42
31.1
7
43
3.84
44
16
51
VH558/5
65
17
30
0.61
1.9
53
13.5
3
29
2.06
27
16
26
Mean
86
12
27
0.7
1.9
37
20.9
3
40
2.1
32
13
26
JV1
64
9
26
0.58
1.3
16
5.6
2
106
1.72
22
12
21
JV2
51
8
16
0.53
0.9
8
3.8
3
181
1.63
2
14
17
JV3
108
5
35
0.58
1.8
21
12.3
3
61
1.93
16
7
18
Mean
71
7
24
0.6
1.3
14
6.4
3
105
1.8
9
11
19
during the diagenetic or postdiagenetic processes (Mn-den-
drites on the bed surfaces).
Trace elements
The contents of all trace elements in the analysed Jurassic
radiolarian cherts are relatively low. During the diagenetic
processes the enrichment of the radiolarian cherts with added
SiO
2
from the shale partings causes the dilution of all other
trace elements (Murray 1994). This is revealed by the highly
negative correlation of silica to all trace elements (r= 0.20 to
0.79) (Table 7), and is clearly visible on the cluster diagram
too (Fig. 4).
Trace and major elements on the cluster diagram can be di-
vided into 5 groups reflecting their mutual connection (Fig. 4).
The biggest group showing a relatively high correlation
consists of K, Rb, Ti, Zr, Th, Al, Nb, Hf, Na, V and Cr
(Fig. 4). The lithophile elements Ti, K, Al, Th, Zr, Hf and Rb
are most commonly transported into the sedimentation basin
in a suspended form. Therefore, they serve as a good indicator
of the terrigenous input, and indirectly they are an indicator of
the distance of the sedimentation basin from the continent.
130 HALAMIÆ, MARCHIG and GORIÈAN
Chromium and vanadium containing minerals are relatively
resistent to continental weathering, so they can also indirectly
serve as indicators of the terrigenous input into the sediment
even though they could also be enriched in hydrothermal pre-
cipitate (Gundlach & Marchig 1982). Al, K and Rb are mostly
connected to the clayey fraction and show a high mutual cor-
relation factor (r
AlK
=0.93; r
AlRb
=0.90 and r
KRb
=0.98). Ti,
Zr, Hf, Th, V and Cr are contained in the heavy mineral frac-
tion and their mutual correlation factor is also relatively high
(Table 7).
The other group of trace elements showing a relatively high
and significant mutual correlation consists of Co and Ni
Table 5: Rare earth elements (mg/kg; Mean = Geometric mean). The mean values (bold numbers) were used for the construction of the
discrimination diagrams.
Sample
La
Ce
Pr
Nd
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
Sum4--
PD1
12.30
27.5
2.99
11.6
2.41
0.53
2.10
0.31
1.84
0.36
1.08
0.15
1.01
0.18
64.36
PD2
9.03 20.8
2.39 8.46 1.50
0.29
1.15
0.15
1.04
0.20
0.69
0.10
0.70
0.13
46.63
PD3
7.90 16.5
2.15 7.98 1.57
0.39
1.58
0.24
1.43
0.29
0.85
0.13
0.81
0.16
41.98
PD4
5.14 10.3
1.53 5.53 1.04
0.23
1.08
0.16
0.89
0.20
0.58
0.091 0.53
0.11
27.51
Mean
8.19 17.66 2.20 8.15 1.56
0.34
1.42
0.21
1.25
0.25
0.78
0.12
0.74
0.14
43.01
VH141
3.78 7.6
1.02 4.13 0.85
0.20
0.88
0.13
0.82
0.15
0.44
0.067 0.46
0.096 20.58
VH141A
10.40 27.2
3.18 12.6
2.53
0.55
2.45
0.37
2.12
0.39
1.18
0.17
1.16
0.21
64.51
Mean
6.27 14.34 1.80 7.21 1.47
0.33
1.47
0.22
1.32
0.24
0.72
0.11
0.73
0.14
36.37
VH882/1
9.92 22.0
2.63 10.5
1.94
0.48
2.04
0.31
1.68
0.33
0.94
0.13
0.87
0.16
53.93
VH882/2
13.70 33.1
3.84 15.6
3.42
0.76
3.18
0.43
2.42
0.45
1.26
0.18
1.08
0.21
79.63
VH882/3
11.80 26.7
2.86 10.3
1.82
0.43
1.69
0.28
1.68
0.33
1.05
0.15
0.96
0.18
60.23
Mean
11.70 26.89 3.07 11.90 2.29
0.54
2.22
0.33
1.90
0.37
1.08
0.15
0.97
0.18
63.59
PA5/1
3.82 11.2
1.08 4.65 0.93
0.24
0.85
0.11
0.72
0.13
0.38
0.048 0.34
0.08
24.58
PA7
2.01 6.4
0.64 2.84 0.94
0.23
0.90
0.13
0.79
0.14
0.34
0.040 0.26
0.065 15.72
PA9
4.66 12.5
1.20 4.60 0.97
0.26
0.83
0.13
0.76
0.12
0.39
0.050 0.39
0.089 26.95
PA15
5.14 12.4
1.62 6.91 1.50
0.34
1.46
0.20
1.23
0.24
0.56
0.080 0.54
0.11
32.33
PA20/1
6.55 18.9
2.24 9.44 2.13
0.51
2.35
0.35
2.07
0.34
0.88
0.12
0.72
0.12
46.72
Mean
4.13 11.60 1.25 5.24 1.22
0.30
1.17
0.17
1.02
0.18
0.48
0.06
0.42
0.09
27.33
VH113
6.66 15.1
1.76 7.45 1.69
0.40
1.80
0.27
1.60
0.31
0.91
0.13
0.80
0.17
39.07
PE1/1
3.09 7.8
0.90 4.03 1.38
0.33
1.38
0.18
1.07
0.18
0.45
0.061 0.43
0.10
21.36
PE1/01
4.89 12.3
1.28 5.46 1.67
0.45
1.86
0.25
1.34
0.24
0.67
0.088 0.56
0.12
31.18
Mean
3.89 9.78 1.07 4.69 1.52
0.39
1.60
0.21
1.20
0.21
0.55
0.07
0.49
0.11
25.78
PC50
9.06 21.8
2.32 9.07 1.70
0.41
1.81
0.25
1.59
0.30
0.86
0.12
0.75
0.15
50.19
PC50/1
7.11 15.8
1.73 7.17 1.49
0.37
1.44
0.21
1.26
0.26
0.77
0.12
0.75
0.15
38.63
Mean
8.03 18.56 2.00 8.06 1.59
0.39
1.61
0.23
1.42
0.28
0.81
0.12
0.75
0.15
44.00
PB1
5.07 13.5
1.51 5.69 1.24
0.28
1.19
0.19
1.02
0.19
0.56
0.085 0.54
0.11
31.18
PB3
8.62 23.2
2.17 8.00 1.34
0.29
1.09
0.17
1.17
0.25
0.77
0.10
0.71
0.15
48.03
PB8
13.30 25.3
3.32 13.4
2.78
0.64
2.77
0.40
2.26
0.43
1.25
0.16
1.13
0.20
67.34
PB19
6.75 14.4
1.69 6.65 1.36
0.29
1.30
0.19
1.09
0.24
0.62
0.084 0.59
0.11
35.36
Mean
7.91 18.38 2.07 7.98 1.58
0.35
1.47
0.22
1.31
0.26
0.76
0.10
0.71
0.14
43.24
P113/1
14.80 40.1
3.60 13.7
2.36
0.46
2.02
0.29
1.83
0.39
1.23
0.19
1.20
0.23
82.40
P113/7
6.91 17.0
1.82 6.86 1.49
0.34
1.37
0.20
1.19
0.22
0.69
0.094 0.67
0.11
38.96
P113/9
4.46 10.8
1.01 3.40 0.71
0.13
0.59
0.092 0.58
0.12
0.44
0.053 0.40
0.089 22.87
P113/12A
7.29 17.7
2.33 9.95 2.22
0.48
2.12
0.30
1.69
0.31
0.88
0.11
0.66
0.13
46.17
P113/13
9.86 24.0
2.45 9.61 1.99
0.44
1.87
0.26
1.63
0.31
0.97
0.14
0.88
0.16
54.57
Mean
8.00 19.91 2.07 7.89 1.62
0.34
1.45
0.21
1.28
0.25
0.80
0.11
0.71
0.14
44.78
VH147
6.10 14.8
1.42 5.56 1.14
0.19
1.00
0.14
0.92
0.20
0.64
0.090 0.65
0.13
32.98
VH32/1
1.97 3.6
0.48 2.33 0.47
0.12
0.54
0.071 0.44
0.091 0.27
0.031 0.23
0.057 10.67
VH32/3
2.78 6.1
0.78 3.13 0.56
0.12
0.58
0.090 0.53
0.094 0.31
0.048 0.28
0.077 15.48
VH32/5
3.53 7.1
1.07 4.50 0.91
0.22
0.99
0.14
0.82
0.18
0.42
0.051 0.37
0.090 20.41
Mean
2.66 5.37 0.74 3.20 0.62
0.15
0.68
0.10
0.58
0.12
0.33
0.04
0.29
0.07
14.97
VH558A
6.60 15.5
1.87 7.77 1.65
0.38
1.64
0.23
1.43
0.26
0.88
0.11
0.70
0.14
39.16
VH558B
4.96 14.6
1.81 8.58 1.67
0.38
1.47
0.21
1.11
0.20
0.47
0.061 0.34
0.067 35.93
VH558C
14.30 41.3
4.34 16.0
2.54
0.51
1.92
0.28
1.56
0.32
1.01
0.13
1.05
0.16
85.42
VH558D
11.00 24.9
2.95 11.5
2.30
0.44
2.23
0.35
2.27
0.44
1.16
0.18
1.08
0.20
61.00
VH558/5
6.01 14.9
1.87 7.87 2.02
0.44
2.16
0.32
1.58
0.31
0.75
0.10
0.65
0.13
39.11
Mean
7.91 20.32 2.41 9.93 2.01
0.43
1.86
0.27
1.55
0.30
0.82
0.11
0.71
0.13
48.76
JV1
6.06 17.4
2.10 8.40 2.02
0.38
1.89
0.28
1.66
0.29
0.82
0.10
0.68
0.12
42.20
JV2
6.17 17.9
2.28 9.31 2.13
0.52
2.37
0.35
1.84
0.36
0.97
0.12
0.68
0.13
45.13
JV3
4.09 10.0
1.08 4.11 0.93
0.21
0.77
0.13
0.69
0.16
0.42
0.066 0.42
0.093 23.17
Mean
5.35 14.60 1.73 6.85 1.59
0.35
1.51
0.23
1.28
0.26
0.69
0.09
0.58
0.11
35.22
(r=0.85 Table 7). These two elements also show a signifi-
cant correlation to the major elements Fe and Mg (Fig. 4),
where a part of the iron and nickel could also be derived from
the hydrothermal component (Marchig et al. 1982). However,
magnesium and cobalt should be interpreted with caution be-
cause of their migrability during diagenetic processes (Murray
1994).
The third group consists of the lithophile elements barium
and scandium (r=0.81) and lanthanum (r
LaBa
=0.79 and
r
LaSc
=0.77) (Fig. 4). In the magmatic rocks barium is accom-
panied by potassium, and these were geochemically separated
in the hydrothermal phase. Despite its relatively high correla-
JURASSIC RADIOLARIAN CHERTS IN NORTH-WESTERN CROATIA 131
Table 6: Ratio values used in the paper (
1
NASC normalized;
2
PAAS normalized). Ce/Ce* = (Ce
sample
/Ce
shale
)/(La
N
×Pr
N
)
½
.
Eu/Eu* = (Eu
sample
/Eu
shale
)/(Sm
N
×Gd
N
)
½
.
Fig. 4. Cluster analysis diagram of major and trace elements
(Tree clustering; Linkage rule Complete linkage; Distance
measure 1-Pearson r).
tion to potassium (r=0.85), barium cannot be determined as
uniquely of hydrothermal origin, because a part of it certainly
derives from the detritic component (detrital clay minerals).
The next group consists of yttrium and the major element
phosphorus (r=0.64) (Table 7). These two elements in radi-
olarian cherts can be of dual origin. They can originate from
hydrothermal solutions (adsorption from sea-water onto the
ferric hydroxides precipitated from hydrothermal solutions)
(Berner 1973) or they can be of biogenic origin. The yttrium
and phosphorus in the investigated radiolarian cherts are most
probably of hydrothermal origin, because the average content
of yttrium is low (average = 11 mg/kg), which is very close to
the values for the hydrothermal metalliferous sediments from
the Red Sea. The siliceous rocks with apatite of biogenic ori-
gin have much higher contents of yttrium (Marchig et al.
1982).
The trace element strontium is most probably connected to
the carbonate component in the radiolarian cherts, and is indi-
rectly a part of the fifth group of major elements in the cluster
diagram (Fig. 4) along with calcium and manganese. Because
these two elements most probably have a secondary origin
(calcite veins, manganese coatings and microscopically visible
calcitization), strontium is also most probably bonded to the
secondary calcite.
REE
When compared to the content of REE in post-Archean av-
erage Australian shale (183 mg/kg) (Taylor & McLennan
1985), North American shale composite (173 mg/kg) (Gromet
et al. 1984) and European shale (204 mg/kg) (Haskin &
Haskin 1966), the total content of REE in the studied Jurassic
radiolarian cherts is considerably lower and ranges from 10.7
to 85.4 mg/kg (Table 5). Most REE are transported into the
sedimentation basin in a terrigenous fraction, i.e. it is for the
most part connected to the silty-clayey component (McLennan
1989). The terrigenous provenance of REE in the investigated
radiolarian cherts is also indicated by a significant positive
correlation of ΣREE to terrigenous major elements K, Ti and
Al (r=0.76, r=0.77 and r=0.76, respectively). The trace ele-
ments connected to the detritic component also show a high
correlation coefficient to the total content of REE (La
r=0.98; Hf r=0.65; Rb r=0.80; Th r=0.83; Y
r=0.74 and Zr r=0.76) (Table 7).
The distribution pattern on Fig. 5a diagram shows LREE
enrichment compared to HREE (Chondrite normalization). Ce
anomaly is positive except for three groups of samples (PD,
VH141, VH32) (Table 6). On Fig. 5b the distribution patterns
show positive Ce and Eu anomalies.
Material provenance and depositional environments
Material provenance. The studied Jurassic radiolarian
cherts in the Medvednica Mt are placed in a tectonic mélange
as smaller or larger blocks, so it is not possible to study their
primary relations to the overlying or underlying sediments.
However, they do carry paleontological and geochemical in-
formation that make good stratigraphic and environmental in-
dicators for the paleogeographic reconstruction of the deposi-
tional area.
The cherts of the Medvednica Mt are rich in silica (average
90.87 %). The Si/Si+Al+Fe+Ca ratios (Table 6) (silica in rela-
PD VH141 VH882 PA VH113
PE
PC
PB
P113 VH147 VH32 VH558 JV
Si/Si+Al+Fe+Ca
0.92
0.91
0.91
0.95
0.83
0.93
0.90
0.92
0.90
0.92
0.97
0.92
0.91
Al/Al+Fe+Mn
0.69
0.62
0.64
0.63
0.47
0.55
0.61
0.66
0.55
0.64
0.61
0.56
0.42
Al
2
O
3
/TiO
2
26.26 29.81 25.45 29.25 37.72 26.67 23.59 25.59 27.76 28.44 28.00 30.15 21.78
MnO/TiO
2
0.27
0.23
0.21
1.61
0.39
3.86
0.46
0.26
0.77
0.18
1.15
2.02
4.01
Zr/TiO
2
0.019 0.017 0.018 0.019 0.017 0.017 0.020 0.019 0.019 0.023 0.020 0.020 0.021
Th/Sc
0.55
0.58
0.73
0.63
0.36
0.78
0.66
0.55
0.56
0.60
0.95
0.71
0.59
La
n
/Ce
n
1)
1.06
1.00
0.99
0.81
1.01
0.91
0.99
0.98
0.92
0.94
1.14
0.89
0.84
Ce/Ce*
1)
0.91
0.93
0.98
1.11
0.96
1.04
1.01
0.99
1.06
1.09
0.83
1.01
1.04
Ce/Ce*
2)
0.96
0.98
1.03
1.17
1.01
1.10
1.07
1.05
1.13
1.16
0.88
1.06
1.09
Eu/Eu*
1)
1.01
0.99
1.05
1.11
1.00
1.08
1.06
1.01
0.96
0.78
0.99
0.97
0.98
Eu/Eu*
2)
1.07
1.05
1.12
1.18
1.07
1.15
1.13
1.07
1.03
0.83
1.05
1.03
1.04
(La/Yb)
n
1)
1.07
0.83
1.18
0.95
0.81
0.77
1.04
1.08
1.08
0.91
0.90
1.09
0.89
(La/Yb)
n
2)
0.82
0.63
0.90
0.72
0.62
0.58
0.79
0.82
0.83
0.69
0.69
0.83
0.69
Al
2
O
3
/(Al
2
O
3
+Fe
2
O
3
)
0.75
0.69
0.71
0.72
0.54
0.69
0.68
0.73
0.63
0.70
0.70
0.66
0.54
V+Ni+Cr/Al
2
O
3
19.24 20.96 17.49 17.95 23.56 25.00 21.58 17.70 19.28 28.57 23.81 24.49 23.98
132 HALAMIÆ, MARCHIG and GORIÈAN
Fig. 6. Fe-Al-Mn diagram (Adachi et al. 1986). I Hydrothermal
field; II Non-hydrothermal field.
Fig. 5. Chondrite and PAAS normalized REE distribution dia-
grams for thirteen groups of analysed radiolarian cherts (PAAS
values from McLennan 1989).
Table 7:
C
orrelation
coefficient
(r)
for
the
major
and
trace
elements,
and
ΣREE
(N
=
40;
Significant
correlation
at
p<
0.05;
indicates
negative
correlation).
JURASSIC RADIOLARIAN CHERTS IN NORTH-WESTERN CROATIA 133
tion to aluminosilicates and ferruginous and calcite minerals),
show that most of the SiO
2
in the rock is of biogenic origin.
Namely, the values of this ratio for biogenic silica rich cherts
range from 0.8 to 0.9 (Ruitz-Ortiz et al. 1989). Higher values
from the studied cherts in the Medvednica Mt (average 0.92)
can be interpreted as a result of the enrichment of the chert
beds with additional SiO
2
from the shale partings during di-
agenetic processes (Murray 1994). The enrichment of the radi-
olarite with additional silica and the dilution of other major
and trace elements is also indicated by the high mutual nega-
tive correlation coefficient (see Table 7).
In their study of Triassic and Cretaceous cherts in central Ja-
pan, and of Cretaceous cherts and porcellanite from Pacific
drill cores (Deep Sea Drilling Project) Sugisaki et al. (1982)
and Adachi et al. (1986) showed that manganese is a relatively
good indicator for determination of the hydrothermal compo-
nent in the rock, while titanium is characteristic of the terrige-
nous input. Murray (1994) argued and stated that the manga-
nese migrates during diagenesis. However, according to the
newer studies of Permian and Triassic cherts conducted in
southwest Japan by Kunimaru et al. (1998) and Shimizu et al.
(2001), manganese in fact can be used as an indicator of a ma-
terial provenance. Using a Mn-Al-Fe diagram proposed by
Adachi et al. (1986) all the analysed samples fall in the non-
hydrothermal field (Fig. 6), which indicates that the cherts
were deposited in an area relatively distant from the hydro-
thermal influence. The Al/Al+Fe+Mn ratio could also be a
measure of the hydrothermal or continental contribution to the
sediment, the lower values indicating a hydrothermal input
(Baltuck 1982; Adachi et al. 1986). The average value of this
ratio for the radiolarian cherts of the Medvednica Mt is 0.59,
which is very close to the value for the deep-sea clay (0.54)
(Bostrom 1976), and slightly lower than the value of 0.619
(average for the shale composite) typical of the continental
material (Baltuck 1982). It is interesting that the Middle to
Upper Jurassic cherts of southern Spain have quite similar Al/
Al+Fe+Mn ratio values (average 0.60 Ruitz-Ortiz et al.
1989) to the values of the Medvednica Mt. Further on, the Fe
and Mn contents in radiolarian cherts of the Medvednica Mt
are relatively low, which indicates that they were probably de-
posited further from the influence of the hydrothermal volcan-
ism, and probably closer to the continent (see Table 3). For the
discrimination of the input of terrigenous material in relation
to the volcaniclastic material, we used Zr/TiO
2
vs. (V+Ni+Cr)/
Al
2
O
3
diagram (Fig. 7) (Andreozzi et al. 1997). The ratio of
these groups of elements shows that all analysed samples fall
exclusively within the field of rocks deposited under the
strong influence of the terrigenous input.
According to the noted values by McLennan et al. (1993)
and Girty et al. (1996) the acquired values for the Al
2
O
3
/TiO
2
ratio (average 27.70), Th/Sc ratio (average 0.60) and Eu
anomalies (PAAS normalized average 1.06; NASC nor-
malized average 0.99) show that there has been mixing of
materials from two sources. A part of it probably comes from
an area built of differentiated upper continental crust, and the
other part from an area characteristic of undifferentiated mag-
matic arc built mainly of basic to neutral magmatic rocks.
Depositional environment. The Ce anomaly values of anal-
ysed samples range from 0.83 to 1.11 (average 0.99) (Table 6)
and indicate a sedimentation of the cherts in a continental mar-
gin area. As noted by Murray et al. (1990, 1992c) the near
spreading ridge sediments, ocean-basin floor sediments, and
continental margin sediments have Ce anomalies ~0.29, ~0.55
and 0.90 to 1.30, respectively. According to Dasgupta et al.
(1999), a CaO/(CaO+MgO) ratio of >0.70 is characteristic of
the fresh water environment while <0.50 is characteristic of
the saline water environment. The ratios CaO/(CaO+MgO)
(Table 6) for the majority of the studied Jurassic cherts suggest
sedimentation in saline water. The increased values of this ra-
tio in samples PE and JV (Table 6) are the consequence of the
secondary calcite.
On the basis of the MnO/TiO
2
ratio, the studied rocks of the
Medvednica Mt can be classified in two groups. The values of
one group range from 0.18 to 0.46 while the other group rang-
es from 0.77 to 4.01 (Table 6). Sugisaki et al. (1982), Kunima-
ru et al. (1998) and Shimizu et al. (2001) stated that the values
of the MnO/TiO
2
ratio lower than 0.5 are characteristic of ar-
eas of the continental shelf, continental slope, marginal seas,
or of areas around basaltic islands, while the values of >0.5
are typical of deep ocean floor, trench or basaltic plateau sedi-
ments. The differentiation of the analysed cherts concerning
their depositional environment is also visible on the discrimi-
nation diagram La
n
/Ce
n
vs. Al
2
O
3
/(Al
2
O
3
+Fe
2
O
3
) (NASC nor-
malized) proposed by Murray (1994), and supplemented by
Girty et al. (1996) (Fig. 8). This diagram shows that most of
the Jurassic radiolarian cherts fall into the field characteristic
of sediments of the continental margin, while a part of it cov-
ers the field of island arc provenance with a tendency to the
ocean island field.
On the basis of presented discrimination diagrams, the con-
sidered ratios of certain elements or groups of elements that
we used as depositional environment indicators, detailed geo-
logical field studies and geochemical works for the Medvedni-
ca area (Halamiæ 1998), as well as on the basis of newer stud-
ies that dealt with the geodynamic evolution of this area
during the Jurassic (Halamiæ et al. 1999; Pamiæ et al. 2002;
Babiæ et al. 2002), we constructed a hypothetical model of the
depositional area in the time of sedimentation of the studied
Middle Jurassic radiolarian cherts (Fig. 9).
Fig. 7. Zr/TiO
2
vs. (V+Ni+Cr)/Al
2
O
3
discrimination diagram (after
Andreozzi et al. 1997). All samples are in the field of terrigenous
origin of material in radiolarian cherts.
134 HALAMIÆ, MARCHIG and GORIÈAN
Paleogeographic distribution of continents and oceans dur-
ing the late Early and Middle Jurassic was determined by the
onset of sea-floor spreading in the Central Atlantic, the Alpine
Tethys and the Vardar Ocean, and synchronously, by subduc-
tion processes in the pre-existing oceanic basins (Stampfli &
Borel 2004). Subduction processes in the investigated area
were initiated probably already during the early Middle Juras-
sic. Wrenched-off pieces of the subducted oceanic crust along
with their sedimentary cover and obducted parts of the conti-
nental crust were built into the subduction complex (accretion-
ary prism) (Fig. 9). By further uplifting of the accretionary
wedge weathering of obducted magmatic rocks and overlying
sediments (oceanic crust) was enhanced. This weathered ma-
terial was transported into the basin as an undifferentiated
magmatic arc-like input (Fig. 9). The complex structure of the
accretionary wedge, formed of magmatic and sedimentary
rocks, explains the differentiation of the analysed MnO/TiO
2
and the La
n
/Ce
n
vs. Al
2
O
3
/(Al
2
O
3
+Fe
2
O
3
) ratios of cherts in
two different groups (see above). At the same time, due to
Fig. 8. La
n
/Ce
n
vs. Al
2
O
3
/(Al
2
O
3
+Fe
2
O
3
) diagram (Murray 1994;
Girty et al. 1996). The radiolarian cherts of Medvednica Mt are for
the most part grouped in the continental margin field, but with a ten-
dency to the island arc or ocean island field (NASC normalized).
Fig. 9. Hypothetical model of sedimentation basin in the MiddleUpper Jurassic for the Medvednica Mt area. 1 carbonate rocks,
2 clastic rocks (shale, siltstone and sandstone), 3 radiolarian ooze, 4 oceanic crust, 5 continental material supply, 6 material
supply from the accretionary wedge (Continental material-like supply mixed with the material of the obducted parts of the oceanic crust).
compression and uplifting, on the opposite side of the subduc-
tion complex, on the continent, detritic terrigenous material
was eroded and transported into the basin where radiolarian
cherts accumulated (continental material supply) (Fig. 9).
The closing of the ocean area in these terrains continued
during the Late Jurassic, and during these processes Triassic
and Jurassic radiolarian cherts, Triassic magmatic and carbon-
ate rocks and Jurassic clastic sediments (shales, siltites and
sandstones) were all incorporated into the accretionary prism.
The closing of the main basin, caused by the progress of the
subduction processes, ended during the late Late Jurassic with
the formation of a tectonic mélange (Halamiæ 1998; Babiæ et
al. 2002) consisting of carbonate, siliceous and magmatic
rocks of the Triassic age and clastic and siliceous rocks of the
Jurassic age. At the same time, a peripheral compression fore-
land basin has been formed (Halamiæ 1998; Babiæ et al. 2002)
(i.e. piggyback basin Ori & Friend 1984; Allen & Allen
1990). In this basin the shallow-water turbidites of Early Cre-
taceous age were sedimented. The analysis of the mineral
composition of the sandstones from these turbidites showed
that the parent material was composed of sandstones, shales,
cherts, basic and ultrabasic magmatic rocks, and to a lesser ex-
tent of low-grade metamorphic rocks. Most of the heavy min-
eral fraction in these rocks consists of chromspinell originat-
ing from ophiolithic rocks (Crnjakoviæ 1987, 1989). Such a
group of source rocks matches the composition of the rocks
incorporated into the accretionary prism during the Middle
and Late Jurassic.
The final closure, that is the collision and formation of a
typical subduction-accretionary complex in the Medvednica
Mt area took place in the lower part of the early Late Creta-
ceous (Halamiæ 1998; Babiæ et al. 2002).
Conclusions
The Jurassic radiolarian cherts of the Medvednica Mt have a
high content of SiO
2
(average 90.87 %), and most of the silica
is of biogenic origin. Such a provenance is indicated by a high
Si/Si+Al+Fe+Ca ratio. The higher contents of SiO
2
in chert-
beds diluted the contents of the other elements (negative cor-
relation coefficient of all major, trace and rare earth elements
with SiO
2
) (Table 7).
JURASSIC RADIOLARIAN CHERTS IN NORTH-WESTERN CROATIA 135
According to the Al/Al+Fe+Mn ratio and Mn-Al-Fe dia-
gram the depositional area of the radiolarian cherts was rela-
tively distant from the hydrothermal influence. The high posi-
tive correlation coefficient between the lithophile elements Ti,
K, Al, Th, Zr, Hf and Rb and the Zr/TiO
2
vs. (V+Ni+Cr)/
Al
2
O
3
diagram show that the detrital material in the radiolari-
an cherts has a terrigenous origin.
The Al
2
O
3
/TiO
2
ratios and the Eu anomalies indicate prove-
nance of the detritus in the radiolarian cherts from an undiffer-
entiated magmatic arc, but the Th/Sc ratios suggest a source
from a differentiated upper continental crust.
The Ce anomalies indicate sedimentation in the continental
margin area, but the MnO/TiO
2
ratio shows that the Jurassic
radiolarian cherts were derived from two different sedimenta-
tion areas: (1) continental shelf and slope, or marginal sea and
(2) deep ocean floor, trench or basaltic plateau. The two differ-
ent depositional areas (continental margin and island arc and
the ocean island, respectively) are also indicated by the La
n
/
Ce
n
vs. Al
2
O
3
/(Al
2
O
3
+Fe
2
O
3
) ratio.
On the basis of geochemical data, field work and published
data, a hypothetical model of the depositional basin during the
deposition of the radiolarian cherts, was constructed. We sup-
pose that the analysed rocks deposited in a relatively narrow
sedimentary basin and that their detritic material derived from
two different source areas: (1) from a continent (terrigenous
input), and (2) from an accretionary wedge (undifferentiated
magmatic arc-like input).
The subduction processes were initiated already in the early
Middle Jurassic when the accretionary wedge began to form.
The closing of the sedimentatary basin and the incorporation
of the radiolarian cherts and other fragments characteristic of
the mélange into the accretionary prism took place during the
Late Jurassic when the peripheral foreland basin (piggyback
basin) was also created. That basin was filled up with the
Lower Cretaceous turbidites. The collisional closing of this
part of the Tethys occured in the early Late Cretaceous.
Acknowledgments: This work was supported by the Ministry
of Science, Education and Sport of the Republic of Croatia (Ba-
sic Geochemical Mapping Project No. 0181006). We would
like to thank Dr. Jozef Michalík (Bratislava, Slovak Republic),
Dr. Zbigniew Sawlowicz (Krakow, Poland) and Dr. Paulian
Dumitrica (Gümlingen, Switzerland) for very helpful and criti-
cal comments which have improved the quality of the paper.
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