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Temperature, pressure and age constraints on the very
low-grade metamorphism of the Jurassic Telekesoldal
Nappe (Inner Western Carpahians) in NE Hungary – a
summary
Abstract: Jurassic metasedimentary rocks of the Telekesoldal Nappe (Inner Western Carpathians) were
sampled for metamorphic studies. The Telekesoldal Nappe represents a mélange-like accretionary com-
plex consisting of black shales, sandstone turbidites and olisthostromes, deposited by gravity mass flows
in a backarc (forearc?) basin. It was affected by ductile deformation and metamorphic alteration con-
nected to the Middle Jurassic-Early Cretaceous subduction and obduction processes of the Neotethys
Ocean. In the case of the 120-140 Ma very low-grade metamorphism – which is younger than the sub-
duction-related blueschist facies metamorphism (Faryad and Henjes-Kunst, 1997) – the peak temper-
ature was ca. 300-350 ºC, while the minimum pressure was ca. 2-2.5 kbar.
Keywords: Inner Western Carpathians, accretionary metamorphism, very low-grade metamorphism,
Kübler-index, chlorite “crystallinity”, K-white mica b “geobarometry”, K/Ar geochronology.
1
Geological, Geophysical and Space Science Research Group of the Hungarian Academy of
Sciences, 1/C Pázmány str, H-1117, Budapest, Hungary.
2
Hungarian Geological Institute, 14 Stefánia str, H-1143, Budapest, Hungary.
3
Institute for Geochemical Research, Hungarian Academy of Sciences, 45 Budaörsi str, H-1112 Budapest, Hungary.
4
Institute of Nuclear Research, Hungarian Academy of Science, 18/c Bem sqr, H-4026, Debrecen, Hungary.
*
e-mail: koversz@yahoo.com
S. KÖVÉR
1*
, L. FODOR
2
, K. JUDIK
3
, T. NÉMETH
3
, P. ÁRKAI
3
, K. BALOGH
4
AND S. KOVÁCS
1
The interest in the thermal history of sedimentary
rocks has grown rapidly over the last 40 years due to
the oil industry, where predicting oil generation is one
of the most important questions. This interest has
resulted in the development of several methods,
which can approach the transition between diagenesis
and low-grade metamorphism. Such methods are the
Kübler index (illite “crystallinity”), Árkai index (chlo-
rite “crystallinity”), vitrinite reflectance, conodont
colour alteration index (CAI), and the determination
of clay mineral assemblages. These methods –inte-
grated with pressure and age data– provide the possi-
bility of determining temperature and pressure condi-
tions in the very low-grade metamorphic realm,
where most of the methods used in the higher grades,
i.e. geothermobarometers and trace element patterns,
are useless because of the lack of diagnostic minerals.
However, low-temperature metamorphism is usually
present in accretionary wedges, where
high-pressure
metamorphic blocks and slices –ranging from several
decimetres to several kilometres in size– are tectoni-
cally emplaced in an unmetamorphosed or only
Trabajos de Geología, Universidad de Oviedo, 29 : 409-413 (2009)
slightly metamorphosed, usually clastic sedimentary
matrix. Metamorphic features of the various con-
stituents of these mélanges may elucidate the physical
conditions (pressure-temperature paths) of subduc-
tion, collision and subsequent uplift (decompression-
al cooling) of the upper oceanic crust and related
rocks of the accretionary prism. These P-T paths were
reconstructed both for the blueschist facies and the
very low-grade metamorphic rocks of the accretionary
wedge of the Meliata Unit (Inner Western
Carpathians) in Slovakia (Faryad, 1995; Faryad and
Henjes-Kunst, 1997; Árkai et al., 2003). However,
our knowledge of the metamorphic conditions of
other tectonic units found in Hungary is incomplete.
The aim of this paper is to summarize all the unpub-
lished data and present new data on the temperature,
pressure and age constraints on this Telekesoldal
Nappe, which shows a great similarity in its sedimen-
tary features and its Bajocian-Bathonian age to the
type localities of the Meliata Unit.
Geological setting
The Aggtelek-Rudabánya Hills are located in north-
eastern Hungary. They are parts of the Inner Western
Carpathians, and contain a nappe stack of Late
Permian-Middle Jurassic sediments. The previous
researchers pointed out that there are metamorphic
and non-metamorphic structural units among the
nappes (Grill et al., 1984; Árkai and Kovács, 1986;
Less et al., 1988; Less, 2000; Szentpétery and Less,
2006). During the last twenty years several new prob-
lems have emerged in the structural settings and
stratigraphy of this area. Among them, the definition
and accordingly the number of the structural units,
the superposition of the nappes, the age and style of
deformations, metamorphism and nappe stacking
have not yet been clarified. In the last years a new
project has been initiated to obtain new structural
and metamorphic data to better understand the struc-
tural position, the deformation history and the meta-
morphic conditions of the nappes of the Aggtelek-
Rudabánya Hills (Fodor and Koroknai, 2000, 2003;
Kövér et al., 2005, 2006, 2007). As a part of this proj-
ect, new data on the very low-grade metamorphism of
the Jurassic Telekesoldal Nappe will be summarized in
this paper.
The metasedimentary rocks of the Telekesoldal
Nappe represent a mélange-like accretionary complex
consisting of black shales, sandstone turbidites and
olisthostromes. A relatively deep marine basin in the
proximity of a submarine slope is likely to be the
depositional environment of this unit. The carbonates
of the olisthostromes are predominantly Middle to
Late Triassic eupelagic to hemipelagic limestones
showing features of the grey “Hallstatt facies” (radio-
larian and “filament” wackestones), but crinoidal
limestones and, rarely, platform carbonates of
unknown age also occur. Rhyolite volcanoclasts and
related quartz and feldspar grains, derived from a sup-
posed coeval island-arc volcanism, are also common
among the clasts. The characteristic features of the
Telekesoldal Nappe, i.e. upwards coarsening succes-
sion formed via gravity mass flow processes, clasts of
calc-alkalic volcanic associations derived from a sup-
posed coeval suprasubduction magmatic arc, and
deformed strata suggest deposition, probably took
place in a forearc or backarc basin in connection with
the Middle-Upper (?) Jurassic subduction of the
Neotethys Ocean.
Methods
Thirty four rock samples from the Jurassic
Telekesoldal Nappe were selected for metamorphic
studies. From seventeen new samples, collected from
borehole material as well as from surface outcrops, fif-
teen produced useable illite Kübler index (KI) data,
while sixteen produced chlorite “crystallinity” (ChC)
data. Eleven illite Kübler Index data and K-white
mica b cell dimension –in order to characterize the
pressure conditions– were measured more than twen-
ty years ago by the same laboratory and team (Árkai,
1985; Árkai and Kovács, 1986) with the same
method, which ensured the comparability of the ear-
lier and recent data.
X-ray powder diffractometric (XRPD) patterns were
obtained using a Philips PW-1730 diffractometer (with
a computerized APD system) with the following
instrumental and measuring conditions: CuKα radia-
tion, 45 kV/35 mA, proportional counter, graphite
monochromator, divergence and detector slit of 1º, and
collection of data in 0.01 and 0.02º 2-steps, using time
intervals of 1 and 5 s, respectively. Diffraction patterns
were performed from non-orientated and highly ori-
entated powder mounts of whole rock and <2μm
spherical equivalent diameter (SED) size fraction
samples in order to determine bulk-rock mineral
assemblages, b cell dimension of K-white mica, and
illite Kübler and chlorite “crystallinity” indices. The
statistical parameters of the b cell dimension data was
measured on whole rock. Calibration procedure of
phyllosilicate “crystallinity” index measurements was
carried out against that of the Kübler’s laboratory
using 0.25 and 0.42 Δº2Θ as anchizone boundary
values (for details see Árkai et al., 1996).
S. KÖVÉR, L. FODOR, K. JUDIK, T. NÉMETH, P. ÁRKAI, K. BALOGH AND S. KOVÁCS410
Results
Illite KI, chlorite “crystallinity”, K-white mica b
parameter, vitrinite reflectance and K/Ar age were
determined using samples from the Jurassic
Telekesoldal Nappe. Most of the KI values fall within
the high-temperature part of the anchizone (KI =
0.25-0.30 Δº2Θ), giving an average of 0.30±0.03
Δº2Θ. While some of them reach the anchizone-epi-
zone boundary (0.25 Δº2Θ), only a few KI values are
in the low-temperature part of the anchizone (0.30-
0.42 Δº2Θ) (Fig. 1).
Similarly to the relationships found by Árkai (1991)
and Árkai et al. (1995) the ChC (001) and ChC (002)
values show a positive correlation with the KI data.
Most of the ChC (002) data fall within the anchizone
(ChC (002) = 0.24-0.26 Δº2Θ) with an average of
0.25±0.01 Δº2Θ. Almost the same number of sam-
ples reaches the anchizone-epizone boundary (0.24
Δº2Θ) (Fig. 1).
Vitrinite reflectance (VR) measurements were carried
out by Árkai (1985). The maximum VR (R
max
) values
are 4.8-5.6% R
max
. From the several approaches that
developed to convert VR data to peak paleotempera-
tures, the Barker’s equation (Barker, 1988) was cho-
sen, because the random vitrinite reflectance (%R
r
) is
used as the only input parameter of this regression
equation [T(ºC) = 104 (lnR
r
) + 148]. It follows that
it neglects the effect of heating time. However, it is
very useful to help appreciate the peak temperature of
metamorphism if there is a lack of detailed informa-
tion on the time parameters of the metamorphic
process. Using the equation on the average R
r
= 4.86%
of the Telekesoldal Nappe the estimated peak temper-
ature of metamorphism is ca. 310 ºC.
The average K-white mica b cell dimension data
measured on paragonite-free whole rock samples
(Árkai, 1985) fall within the transition zone of low-
and medium-pressure zones, giving an average of
8.99±0.03 Å (Fig. 2).
Conclusions
On the basis of our compilation, the illite Kübler
and chlorite “crystallinity” indices (Fig. 1), measured
over the last twenty five years, correspond to the
high temperature part of the anchizone up to the
Figure 1. Illite Kübler (KI) and chlorite “crystallinity” ChC (002) Árkai index data of the Telekesoldal Nappe show high-temperature
anchizonal metamorphism.
METAMORPHIC CONDITIONS OF THE TELEKESOLDAL NAPPE, HUNGARY 411
lower part of epizone and suggest a metamorphic
temperature of ca. 300-350 ºC (Fig. 3) (Árkai and
Kovács, 1986; Árkai, 1985, new measurements).
Vitrinite reflectance data of R
r
= 4.86; R
max
= 5.13;
R
min
= 4.53 were measured on the unit (Árkai 1985).
Using Barker’s equation (Barker, 1988), the estimat-
ed peak temperature of metamorphism is ca. 310
ºC, which shows a close agreement with tempera-
tures suggested by illite Kübler and chlorite “crys-
tallinity” indices. K-white mica b “geobarometry”
suggests possible minimum pressure of ca. 2-2.5
kbar (Árkai, 1985) (Figs. 2 and 3). These data were
compared with the temperature and pressure con-
straints on the 4 type localities of the very low-grade
metamorphic event, which affected the similarly
Jurassic accretionary mélange of the Meliata Unit
(Inner Western Carpathians). The temperature con-
ditions of this Early Cretaceus very low-grade meta-
morphic event were similarly ca. 270-350 ºC at
Meliata and Telekesoldal (300-350 ºC), and slightly
higher (ca. 340-350 ºC) at Hačava and Drźkovce.
Comparing the pressure data –on the basis of the K-
white mica b parameters–, Drźkovce, Hačava and
Telekesoldal might represent a lower pressure seg-
ment (min. 2-2.5 kbar), while Meliata may be con-
sidered as a relatively higher pressure one (ca. 3.5-
5.4 kbar; Árkai et al., 2003) (Fig. 3).
The prograde anchizonal metamorphism of the
Meliata accretionary wedge sediments and the retro-
gression of blueschist facies metapelites occurred
between ca. 150 and 120 Ma, with a thermal peak in
the anchizonal sequences at around 140-145 Ma
(Árkai et al., 2003). This very low-grade metamor-
phism was younger than the 160-155 Ma old, sub-
duction-related blueschist facies metamorphism
(Faryad and Henjes-Kunst, 1997). The age of the
investigated very low-grade metamorphic event in the
Telekesoldal Nappe is under examination using the
K/Ar method. On the basis of the preliminary data,
the metamorphic event took place in the Early
Cretaceous, between 120-140 Ma (136.6±5.2 Ma,
127.9±5.0, 125.8±4.8 Ma), showing a close correla-
tion with the 120-150 Ma metamorphism of the
other accretion-related sequences of the Inner
Western Carpathians.
Acknowledgements
The research was supported by the Hungarian Scientific Research
Found OTKA No. 48824, 60965, 61872. L. Fodor benefited the
Bolyai János scholarship of the Hungarian Academy of Sciences.
Figure 2. K-white mica b data of the Telekesoldal Nappe (Árkai,
1985) indicate low to medium pressure metamorphism (pressure
estimates give possible minimum P values).
Figure 3. Comparison of the very low-grade metamorphism of
the Telekesoldal Nappe (NE Hungary) and the type localities of
the Meliata Unit (Inner Western Carpathians). P-T diagram of
the compared units based on the illite Kübler index and K-white
mica b data from the present study and from Árkai et al. (2003).
S. KÖVÉR, L. FODOR, K. JUDIK, T. NÉMETH, P. ÁRKAI, K. BALOGH AND S. KOVÁCS412
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