AMPHIBOLITE PEBBLES FROM MIOCENE CONGLOMERATES (PANNONIAN BASIN) 355
GEOLOGICA CARPATHICA, 54, 6, BRATISLAVA, DECEMBER 2003
ECLOGITE AND GARNET AMPHIBOLITE PEBBLES FROM MIOCENE
CONGLOMERATES (PANNONIAN BASIN, HUNGARY):
IMPLICATIONS FOR THE VARISCAN METAMORPHIC EVOLUTION
OF THE TISZA MEGAUNIT
, GÁBOR KOVÁCS
and GYÖRGY SZAKMÁNY
Laboratory for Geochemical Research, Hungarian Academy of Sciences, Budaörsi út 45, H-1112 Budapest, Hungary;
Department of Mineralogy, Geochemistry and Petrology, University of Szeged, Egyetem út 24, H-6724 Szeged, Hungary;
Department of Petrology and Geochemistry, Eötvös University, Pázmány Péter sétány 1/C, H-1117 Budapest, Hungary;
(Manuscript received September 6, 2002; accepted in revised form June 23, 2003)
Abstract: Eclogite and garnetiferous amphibolite pebbles were found in Miocene conglomerates directly overlying the
Variscan crystalline basement of the Tisza Megaunit in the Pannonian Basin of Hungary. Peak metamorphic conditions
were in the medium temperature part of the eclogite facies (600650 °C, 1315 kbar) with garnet, omphacitic pyroxene,
quartz and rutile as primary assemblage. Breakdown from peak P-T conditions resulted in symplectitic intergrowth of
clinopyroxene, plagioclase and quartz. Continuous retrogression involving garnet caused formation of the matrix assem-
blage of clinopyroxene, plagioclase, amphibole, ilmenite and quartz. Amphibole-plagioclase porphyroblasts formed
during the last stage of retrogression. Amphibole-bearing symplectites record P-T conditions of 500570 °C and 8
12 kbar in garnetiferous amphibolite, but no pyroxene was detected there. K-Ar geochronological data on amphibole
show 348±13 Ma cooling ages which is nearly 100 Ma younger than previously suggested. Final uplift to surface condi-
tions occurred in Miocene times causing the first appearance of exotic metabasite pebbles in clastic sedimentary rocks.
Key words: Variscan Orogeny, Pannonian Basin, Tisza Megaunit, thermobarometry, garnetiferous amphibolite, eclogite.
Eclogites and other high-pressure metabasic rocks are impor-
tant for establishing the pressure-temperature (P-T) conditions
during subduction and/or collision. Within the European
Variscan Orogenic Belt, eclogites although volumetrically
minor are widely distributed (for review see OBrien et al.
1990) testifying to high-pressure (HP) event(s) during the
polymetamorphic evolution of the orogen.
As HP metamorphic rocks are quite rare in the Pannonian
Basin (and especially in the Tisza Megaunit, e.g. Ravasz-
Baranyai 1969), pebbles found in Miocene conglomerates
represent important markers for the pre-Alpine and Alpine
geodynamic history of the area. Description of various rock
types, not exposed in the Mecsek Mts (SW Hungary) and ad-
jacent areas had revealed the significance of exotic pebbles in
Neogene clastic sediments (Szakmány & Józsa 1994). There-
fore, the careful petrographic investigation of these conglom-
erates can provide a tool for the better understanding of events
which affected the geological evolution of the Tisza Mega-
The main goal of this study is to present petrological and
geochronological data on eclogite and garnetiferous amphibo-
lite samples, newly discovered from Miocene sediments in the
SW part of the Tisza Megaunit. Furthermore, the pressure-
temperature conditions of metamorphic evolution of these ex-
otic rocks and their significance for the pre-Alpine evolution
of the Tisza Megaunit are discussed.
The Tisza Megaunit (Fig. 1) originated from the northern,
European margin of Tethys by mostly Meso-Alpine horizon-
tal displacements of microplates (Géczy 1973; Kovács 1982;
Kázmér & Kovács 1985). It forms the basement of S Hunga-
ry, and is bounded by the Mid Hungarian (or Zagreb-Zem-
plín) Line to the north, whereas it can be followed over the
state boundary to Northern Croatia and Serbia (Yugoslavia),
and to Western Transylvania (Romania) in the southern and
eastern directions, respectively.
Szederkényi (1984) divided the pre-Alpine (mostly
Variscan) basement complexes of the Tisza Megaunit into
two major parts: the Parautochthon Unit and the South Hun-
garian Nappe (or Békés-Codru Unit). The prevailing rocks of
both units are paragneisses and micaschists with minor am-
phibolites and in some areas marble intercalations. Vo-
luminous syncollisional, S-type Variscan granitoids are also
found in various parts of the Tisza Megaunit (Buda & Nagy
1995). In SW Hungary, the Parautochthon Unit is composed
of the Babócsa, Baksa and Mórágy Complexes (Fig. 2). The
first two are built up predominantly by metapelites, while the
latter is dominated by granitoids.
In general, the first metamorphic event recorded in the
Parautochthon Unit is characterized by Barrow-type amphibo-
lite facies regional metamorphism. This event was overprinted
by a low-pressure Variscan event closely related to granitoid
magmatism ranging from sub-greenschist facies up to the am-
356 HORVÁTH, KOVÁCS
phibolite facies. Árkai (1984) and Árkai et al. (1985) calculat-
ed peak conditions of metamorphism of 500600 °C and 5
9 kbar for gneisses, micaschists and intercalated amphibolites,
which were thought to be pre-Variscan (Lelkes-Felvári & Sas-
si 1981). Árkai et al. (1985) elaborated alternative pre-
VariscanVariscan polycyclic and Variscan polyphase mod-
els. At present no isotopic ages older than Variscan are
available for the metamorphic basement of the Tisza Megaunit
Fig. 1. Main tectonostratigraphic units of the Pannonian Basin and
neighbouring areas. Black square shows the enlarged area in Fig. 2.
Fig. 2. Pre-Tertiary geological map of SW Hungary showing the main basement units, ultramafic bodies (Gyód, Helesfa and Ófalu) are indi-
cated by black spots. a covered, b uncovered, almost all pre-Permian complexes are covered.
(Lelkes-Felvári et al. 1996). This can be due to the intense
Variscan metamorphism. Horváth & Árkai (2002) reported
Alpine (Cretaceous) amphibolite facies metamorphism with
peak conditions of 650 °C and 9 kbar from metapelites from
the Békés-Codru Unit.
Non-metamorphic Late Paleozoic overstep sequences were
deposited on different parts of the Parautochthon Unit
(Kovács et al. 2000). The oldest formation overlying all the
basement complexes in the southwestern part of the Tisza
Megaunit is the fluviatile, Upper Carboniferous Téseny Sand-
stone, belonging to an active continental margin/volcanic arc
provenance (Varga et al. 2000), and missing in other areas.
Subsequent Lower Permian and Mesozoic formations occur in
the entire area, but they do not form a continuous cover above
Lower-middle Miocene (Eggenburgian-Carpathian (?lower
Badenian)) fluviatile formations occupy large areas in the
Mecsek Mountains and its surroundings (Szakmány & Józsa
1994). The coarse-grained Szászvár Formation and the fine-
grained Budafa Formation represent the lowest part of this se-
quence. In these sequences, conglomerate beds appear in huge
amounts and this kind of sedimentation reaches a considerable
thickness (in some boreholes several hundred meters). Hámor
(1970) assumed that the two formations differ in age, but
Máthé et al. (1997) considered them to be heteropic. The rock
type, the size of clasts, as well as the amount and proportion of
AMPHIBOLITE PEBBLES FROM MIOCENE CONGLOMERATES (PANNONIAN BASIN) 357
different clast types in these conglomerate beds, are strongly
variable, depending on the accumulation area. Small amounts
(generally <1 %) of garnetiferous amphibolite (retrograde
eclogite?) pebbles occur only in the westernmost part of the
area covered by the Miocene conglomerates. The size of the
pebbles varies greatly, between 0.5 and 25 cm. The roundness
of the clasts is poor, they generally show a subangular shape.
The garnetiferous amphibolite pebble presented in this paper
comes from a conglomerate outcrop of the Szászvár Forma-
tion near the Helesfa serpentinite body (Fig. 2).
The borehole Gyód-2 (Fig. 3), from which the investigated
eclogite sample comes, is located in the central part of the
Baksa Complex (Fig. 2) above the so-called Gyód magnetic
anomaly 57 km in length and 300500 m in width. The ser-
pentinite body strikes WNW-ESE, similar to that of the medi-
um-grade metapelitic country rocks (mainly micaschists,
paragneisses and amphibolites). It has tectonic contact with
the country rocks. According to Szederkényi (1976) the rocks
of the Baksa Complex suffered prograde, Barrovian regional
metamorphism changing from the chlorite zone (in the north)
to the sillimanite zone (in the south). A clockwise P-T path
with early kyanite-staurolite, and younger sillimanite was re-
ported from the paragneiss-micaschist rocks of the borehole
Baksa-2 located in the southern edge of the Baksa Complex
(Árkai et al. 1999). Király (1996) obtained P-T estimates of
550650 °C and 57 kbar for amphibolites from the borehole
Gyód-3. The Mecsekalja Tectonic Zone borders the complex
(Fig. 2) in the north, which was a regional shear zone during a
late stage of the Variscan Orogeny (Ar-Ar ages on micas in
the range of 270300 Ma; Lelkes-Felvári et al. 2000). The
zone was still active during the Alpine Orogeny, as it sepa-
rates Variscan metamorphic rocks/granitoids and non-meta-
morphic Mesozoic sediments of the Mecsek Mts (Fig. 2). To
the south, Permian sediments and rhyolites, and Mesozoic for-
mations of the Villány Mts are exposed.
Below the PleistoceneLower Pannonian sediments and
basal conglomerate layer (with the eclogite pebbles), a nearly
vertical serpentinite body was drilled in the Gyód area. In a
narrow central slab a relatively fresh harzburgite zone occurs.
Balla (1981) described the serpentinite together with other ser-
pentinite bodies near Helesfa and Ófalu (Fig. 2) as dismem-
bered fragments of obducted oceanic lithosphere. Beside
eclogite and garnetiferous amphibolite, serpentinite and quartz
pebbles are found in the basal conglomerate layer. The peb-
bles are moderately rounded with a diameter ranging between
0.5 and 10 cm (max. 15 cm) and are cemented by yellowish
white carbonate material with Limnocardium fossils.
Chemical analyses of minerals were carried out with a JEOL
JXCA-733 electron microprobe equipped with 3 WDS in the
Laboratory for Geochemical Research, Hungarian Academy
of Sciences, Budapest. The measuring conditions were: 15 kV
acceleration voltage; 40 nA sample current; electron beam
with a diameter of 5
m; 5 s counting time. Matrix effects
were corrected by using the ZAF method. The following stan-
dards were used for quantitative analysis: orthoclase (K, Al,
Si), synthetic glass (Fe, Mg, Ca), spessartine (Mn), rutile (Ti)
and albite (Na).
K/Ar measurements were performed in the Institute of Nu-
clear Research of the Hungarian Academy of Sciences
(ATOMKI), Debrecen. The interlaboratory standards Asia 1/65,
HD-B1, LP-6 and GL-0 as well as atmospheric Ar were used
for control and calibration of analyses. Details of the instru-
ments, the applied methods and results of calibration have
been described by Balogh (1985) and Odin et al. (1982). K/Ar
ages were calculated using the constants proposed by Steiger
& Jäger (1977).
The mineral abbreviations used in this study follow Kretz
(1983) and Bucher & Frey (1994).
Petrography and mineral chemistry
Mineral assemblages and representative compositions of
minerals used in thermobarometric calculations are presented
here and listed in Tables 1 to 5. Cation numbers are calculated
for 12 oxygens for garnet, 6 for clinopyroxene, 8 for plagio-
clase and 23 for amphibole. Fe
ratios were calculated
by charge balance for garnet and clinopyroxene, the structural
formulae of amphibole come from Robinson et al. (1982).
The eclogite pebble from the basal conglomerate overlying
the Gyód serpentinized ultramafic body in the borehole Gyód-2
(Figs. 2 and 3) is rounded, non-foliated and 8 cm in length. A
fine-grained greyish matrix and coarse-grained reddish brown
Fig. 3. Simplified cross-section of the Gyód ultramafic body and ad-
jacent area. The boreholes cross-cutting the ultramafic body and the
metapelitic country rocks are also indicated.
358 HORVÁTH, KOVÁCS
garnets can be seen in the hand specimen. The eclogite con-
sists of garnet, clinopyroxene, amphibole, plagioclase, quartz,
subordinate biotite, chlorite, carbonate, and accessory rutile
Garnet forms an- or subhedral porphyroblasts (410 mm in
size) with cores full of inclusions such as quartz, plagioclase,
amphibole, opaque minerals and tiny omphacite grains with a
jadeite content of 3436 % (Fig. 4a,b). The rim is free of in-
clusions. The garnets are surrounded by a symplectite com-
posed of clinopyroxene, plagioclase, rare quartz and ilmenite
(Fig. 4b). Clinopyroxene forms coarse-grained, vermicular
grains intergrown with plagioclase indicating breakdown of
an earlier formed phase (Na-richer omphacitic clinopyrox-
ene). Cpx is diopside with low Al and Na content (jadeite is
less than 5 %), whereas Mg/(Mg+Fe
) is 0.74. Plagioclase
occurring in symplectite is An
. The grain size of the sym-
plectite-forming minerals is ca. 50100
m. Fine-grained am-
phibole-bearing clinopyroxene-plagioclase-quartz assemblage
is dominant in the matrix of the sample. It has a smaller grain
m) than the symplectite and contains rare garnets
as well. Clinopyroxene is augite with higher Mg/(Mg+Fe
ratio, and slightly higher Al content than Cpx of the symplec-
tite. Plagioclase shows slightly lower An content than Pl in
symplectite (16 %). Amphibole is rich in TiO
1 wt. %) and falls into the magnesio-hornblende field in the
nomenclature of Leake et al. (1997). Ilmenite flakes may oc-
cur together with this mineral assemblage.
Garnet cores show low Sps content (less than 1 %), high
Grs (2125 %) with Prp and Alm contents of 1821 %, and
5456 %, respectively. In the triangular diagram of Coleman
et al. (1965) the garnet core compositions plot into the field of
C type eclogite, but quite close to type B (Fig. 5). Two com-
positionally different garnet rims can be seen, which are in
contact with the symplectite and the matrix, respectively
(Fig. 6a). The highest Alm and lowest Prp contents were mea-
sured where symplectite is found near the garnet rims. Grs and
Sps do not show any changes from the core compositions.
Garnet rims not in contact with the symplectite have Sps con-
tent over 1 % with Grs down to 2023 %, whereas Alm con-
tents are similar to those in the core. Prp is somewhat higher
than in the core regions. Small garnets (max. 1 mm) in the ma-
trix domain have compositions similar to garnet rims in con-
tact with the matrix assemblages.
Large prismatic hypidioblasts of amphibole sometimes
form kelyphitic rims on garnet or overgrow the symplectite
Table 1: Mineral assemblages of the eclogite sample. In the garne-
tiferous amphibolite only symplectite (without Ca-pyroxene) and
matrix assemblages are recognized. See text for details.
contact with sympl
contact with matrix
cation numbers on the basis of 12 oxygens
Table 2: Representative chemical composition of garnets used in thermobarometric calculations. FeO* = Fe total.
AMPHIBOLITE PEBBLES FROM MIOCENE CONGLOMERATES (PANNONIAN BASIN) 359
and matrix assemblage clearly postdating them (Fig. 4a). The
average size is 200
m and some of them are strongly corrod-
ed by retrograde chlorite. This amphibole generation is tscher-
makite with lower FeO, TiO
and higher MgO content than
amphibole in the matrix, and occurs together with Ca-richer
plagioclase (with An content of 18 %). Anhedral quartz with
coronas of amphibole and clinopyroxene (see above) may
m in the matrix, but smaller grains occur in
the symplectites. Amphibole-clinopyroxene corona developed
around quartz grains near garnets. Similar textures were found
in garnet-clinopyroxene amphibolites (retrogressed eclogites)
in the Tatra Mts, Western Carpathians (Janák et al. 1996).
Mineral compositions in the corona are similar to the matrix.
Quartz can also be found around altered amphibole and rare bi-
otite. Subordinate biotite is very rare and breaks down to chlo-
rite and fine-grained white mica. Rutile forms xenoblastic
m in length, both in the garnets and in the sym-
plectites, where it occurs together with ilmenite. It can be found
exclusively as an inclusion in garnet or in the symplectite zone,
but never in the matrix which contains only ilmenite.
The pebbles are fine-grained, and have a predominant green
colour with small red garnets. Nematoblastic garnet-bearing
amphibolites consist mainly of amphibole and plagioclase,
cation numbers on the basis of 6 oxygens
Table 3: Representative chemical composition of pyroxenes used in
thermobarometric calculations. FeO* = Fe total, mg# = Mg/
Rock type Eclogite
cation numbers on the basis of 8 oxygens
Table 4: Representative chemical composition of amphiboles used
in thermobarometric calculations. FeO* = Fe total, mg# = Mg/
Table 5: Representative chemical composition of plagioclases used
in thermobarometric calculations. FeO* = Fe total.
Rock type Eclogite
cation numbers on the basis of 23 oxygens
360 HORVÁTH, KOVÁCS
some garnet, quartz and opaque minerals; accessories are apa-
tite, zircon, rutile and biotite.
Garnets (up to 12 mm) are rounded and corroded grains
looking fresh in the internal parts, but altered to epidote (and
very rarely biotite) on the rims (Fig. 4c,d). Due to late brittle
tectonic effects some grains are disintegrated, and plagioclase
has crystallized among the remnants. Microprobe analyses of
garnets show distinct core and rim compositions (Figs. 5
and 6b). Garnet cores have Grs content over 20 %, with Alm
ranging between 54 and 57 % and Prp at ca. 15 %. Rim com-
positions are represented by higher Alm (around 60 %),
whereas Grs is less than 20 % (11 to 18 %). Prp is also higher
on the rims. Sps does not show any great variation, and is gen-
erally around 23 %.
Two generations of amphibole can be recognized in the
studied samples. The first has a small grain size (3040
occurring as green/yellowish blasts, and generally forming
symplectites (Fig. 4c,d) with fine-grained plagioclase (+/epi-
Fig. 4. Microtextures in the investigated eclogite (a, b) and garnetiferous amphibolite sample (c, d). (a) BSE image of the eclogite sample.
Note garnet with inclusion-rich core and inclusion-free rim (broken line shows core-rim boundary). Large amphibole (Am) and plagioclase
postdate the earlier-formed symplectite. Ilmenite can be found exclusively in the matrix. (b) Enlarged view of centre of Fig. 4a. Eclogitic as-
semblage of omphacite, quartz and rutile in garnet core replaced by Ca-clinopyroxene and plagioclase. (c) BSE image of garnetiferous am-
phibolite sample. Relict, corroded garnet and symplectitic intergrowth of amphibole and plagioclase replaced by late amphibole (Am) and
plagioclase (Pl). Ilmenite (Ilm) occurs both in the symplectite and the matrix. (d) BSE image of garnetiferous amphibolite sample. Garnet
(Grt) altered to epidote (Ep) on the rims occurs together with symplectitic intergrowth of amphibole and plagioclase (Sympl). Matrix is com-
posed of amphibole (Am), plagioclase (Pl) and ilmenite (Ilm).
AMPHIBOLITE PEBBLES FROM MIOCENE CONGLOMERATES (PANNONIAN BASIN) 361
dote) after a previous phase (presumably omphacitic clinopy-
roxene). Amphibole and plagioclase can also be found as in-
clusions in garnet core regions. Amphibole compositions plot
in the magnesio-hornblende field (Leake et al. 1997), with
around 49 wt. % and Al
over 13 wt. %, whereas
O is lower than 1.5 wt. %. Plagioclase is An
Coarse-grained (11.5 mm), dark greenyellowish green
hypidioblasts of amphibole and plagioclase with polysynthet-
ic twins (Fig. 4c) form the matrix assemblage. In very rare
cases amphibole is replaced by biotite. It is richer in Al
O and TiO
, and poorer in SiO
and MgO relative to the
symplectite-forming amphibole. This is tschermakite accord-
ing to the nomenclature of Leake et al. (1997). Plagioclase is
richer in Na (An
) compared to symplectitic plagioclase.
All large matrix amphibole and plagioclase grains show wavy
extinction, and the larger plagioclase grains are disintegrated
into smaller parts similarly to garnets.
The coarse-grained opaque minerals (mostly ilmenite) are
corroded, in some cases leucoxenized. The other opaque phas-
es are very fine-grained euhedral magnetite crystals. Among
the accessories there is a large amount of apatite. Rutile is
rare, occurring as inclusions in amphibole or in plagioclase.
Rare zircon appears as very fine-grained euhedral crystals.
Before performing any P-T calculations, it is worth trying
to determine whether the observed mineral compositions true-
ly reflect various stages of metamorphism or they show com-
Fig. 5. Chemical composition of garnets: (a) (Alm+Sps)GrsPrp triangle of Coleman et al. (1965), and (b) AlmGrsPrp triangle.
Fig. 6. Compositional zoning profile of garnet in eclogite (a), and in garnetiferous amphibolite (b). Note the distinct core-rim compositions
in amphibolite garnet.
362 HORVÁTH, KOVÁCS
positional readjustments to retrograde processes. The compo-
sitional changes observed in clinopyroxenes (omphacitic Cpx
in the core of garnet, Na-poor cpx in the symplectites) clearly
demonstrate the change from eclogite facies to lower-P (most
probably amphibolite facies) conditions. This is supported by
the presence of rutile exclusively in garnet cores, and ilmenite
occurring in the matrix and symplecite assemblages. Amphib-
ole can be found in the matrix with clinopyroxene and plagio-
clase, and with plagioclase as kelyphitic rims replacing garnet
or postdating the matrix. Kelyphitic amphiboles have lower Ti
content indicating lower-T conditions (Ernst & Liu 1998).
The close compositional range of most of the plagioclases in
different textural domains seems to cause some ambiguity in
the geothermobarometrical calculations. One of the reasons is
that plagioclases reequilibrated after the formation of the ma-
trix assemblage. However, reequilibration of plagioclase is
not very probable, because cation diffusion in plagioclase is
known to be quite slow (Grove et al. 1984). In the case of
pressure increase the An component of the plagioclase de-
creases with increasing Grs in coexisting garnet. We observed
decreasing An content in plagioclases of symplectite and ma-
trix, respectively, but Grs did not show any increase. The ma-
trix has amphibole of magnesio-hornblende composition with
significant Ca content presumably coming from clinopyrox-
ene and/or garnet. Plagioclases overgrowing the matrix with
amphibole have An content higher than in the matrix reflect-
ing the change from cpx-bearing assemblages to amphibole-
The computer program THERMOBAROMETRY of Kohn
& Spear (1995) with various calibrations (see below) was used
for pressure-temperature (P-T) determinations. P-T calculations
were also performed with the TWEEQU program (version
2.02) of Berman (1991) using the thermodynamic dataset of
Berman (1988), and activity models for garnet (Berman 1990),
clinopyroxene (Berman et al. 1995), amphibole (Mäder & Ber-
man 1992) and plagioclase (Fuhrman & Lindsley 1988).
Temperatures for the eclogite stage were estimated using
several calibrations of the garnet-clinopyroxene Fe-Mg ex-
change thermometer. Using the core compositions of garnet
together with omphacite inclusions the temperature of the
eclogite stage was between 670700 °C with the Ellis &
Green (1979) calibration which seems to be a slight overesti-
mate. The calibration of Krogh (1988) and Krogh-Ravna
(2000) gave 600650 °C, 5060 °C less than the Ellis & Green
(1979) method. The pressure is 1315 kbar (minimum P) using
the jadeite content of omphacitic clinopyroxene for the eclog-
ite (Holland 1980 and 1983). The TWEEQU method gave
slightly higher T (650740 °C) than the Grt-Cpx exchange
thermometers. These data indicate that the eclogite equilibrated
at the medium temperature (MT) field of Carswell (1990). Pres-
sure-temperature data for the large matrix amphibole-plagio-
clase pairs were ca. 470 °C and 810 kbar (Plyusnina 1982).
Garnet-hornblende Fe-Mg exchange thermometry from
Graham & Powell (1984) and garnet-plagioclase-hornblende-
Fig. 7. Pressure-temperature conditions for the eclogite and garne-
tiferous amphibolite samples, constrained by thermobarometric
data. Hornblende K-Ar age data from amphibolite are also shown.
Aluminosilicate triple point according to Holdaway (1971), stability
of omphacite (Jd
) according to Holland (1983).
quartz barometry from Kohn & Spear (1990) were performed
for garnet rim and adjacent hornblende and plagioclase in the
symplectites. These estimates are 500570 °C and 812 kbar.
The same P-T range was obtained with inclusions of horn-
blende and plagioclase in garnet. For the retrograde stage we
obtained ca. 500 °C and 8 kbar with the method of Plyusnina
The obtained P-T conditions for the various assemblages
are shown in Fig. 7. The P-T results indicate that the late (ret-
rograde) stage of metamorphism in the investigated eclogite
and garnetiferous amphibolite samples occurred at similar P-T
The garnetiferous amphibolite sample was analysed by the
K-Ar method to establish the age bracket of metamorphism
(or cooling). We used separated amphibole fractions after
checking that the majority of the amphibole porphyroblasts
belong to one mineral assemblage (matrix phase). The sam-
ples gave 348±13 Ma age data. Since the closure temperature
of the hornblende K-Ar system is between 450550 °C (e.g.
McDougall & Harrison 1988), and the calculated tempera-
tures for the garnetiferous amphibolite is close to this bracket,
we conclude that the analysed sample was formed at around
348±13 Ma. Unfortunately, we were unable to derive any age
data from the eclogite sample (and it would be highly prob-
lematic or impossible).
The investigated exotic pebbles of metabasites clearly be-
long to the crystalline basement of the Baksa Complex of the
AMPHIBOLITE PEBBLES FROM MIOCENE CONGLOMERATES (PANNONIAN BASIN) 363
Tisza Megaunit according to their tectonic position and petro-
graphic features. Eclogite was first described by Ravasz-
Baranyai (1969) from the area, but without detailed petrologi-
cal and mineral chemical data, thus geothermobarometric
information is still lacking. The mineral assemblage is kyan-
ite, phengite and margarite in additional to garnet, amphibole,
quartz and rare omphacitic pyroxene, so they seem to be dif-
ferent from the rock type presented here, although kyanite-
bearing and kyanite-free eclogites are common together in
eclogite areas (e.g. Miller & Thöni 1997). M. Tóth (1995) re-
ported garnet-bearing amphibolites (highly retrogressed eclog-
ites) from the eastern part of the Tisza Megaunit, but the P-T
results he obtained are on the lower-T limit of the eclogite fa-
cies (600650 °C, 1012 kbar), and the eventual shape of the
P-T path is controversial. He stated that peak P-T conditions
were followed by retrogression into the greenschist facies, and
then an amphibolite facies overprint occurred at near isobaric
The Variscan evolution of the basement complexes of the
Tisza Megaunit is still an open question. Kovács et al. (2000)
outlined the following scenario: 1. HP-LT metamorphism
(eclogite relicts of Ravasz-Baranyai 1969 and M. Tóth 1995).
This event is inferred to occur at about 400440 Ma (Rb-Sr
method, Svingor & Kovách 1981). The biggest problem of
these age data is that these results were obtained on amphibo-
lite xenoliths showing features of a late higher-T recrystalliza-
tion caused by Variscan granitoids (Király 1996). 2. MP-MT
Barrow-type metamorphism which is characteristic of the en-
tire area of the Tisza Megaunit (for details see Geological
setting). 3. LP-HT metamorphism closely related to granitoid
magmatism. The latter two events took place between 350 and
270 Ma. The greatest concern with the published geochrono-
logical results is that most of them are biotite and white mica
K-Ar or biotite Rb-Sr ages, so they represent only cooling
ages. Furthermore, the P-T conditions of the rocks from which
the dated minerals had been separated are not unambigous.
While most authors generally accept the Variscan age of the
MP-MT Barrow-type metamorphism, there is controversy
about the age of the earlier HP metamorphism. Our new K-Ar
data of 348±13 Ma obtained on amphibole fractions from the
garnetiferous amphibolite better supports the Variscan age of
the HP event.
Petrographic investigations and mineral chemical data show
a continuous change from eclogite facies assemblage (garnet,
omphacitic cpx, rutile and quartz) through the lower-P assem-
blage of clinopyroxene-bearing symplectite to the cpx-am-
phibole-bearing matrix (HP amphibolite facies). The locally
occurring mineral assemblage of late amphibole and plagio-
clase point to amphibolite facies conditions.
Peak conditions of eclogite formation are 600650 °C and
1315 kbar based on garnet-clinopyroxene exchange ther-
mometry and jadeite content of omphacite (minimum P). No
information is available for the early, prograde part of the P-T
path. The breakdown of the eclogite facies assemblage oc-
curred in several steps. Omphacitic clinopyroxene broke
down to Na-poor clinopyroxene and Ca-rich plagioclase to
produce the symplectite (reaction 1a):
omphacite + quartz = clinopyroxene + plagioclase
The different Mg/(Mg+Fe
) ratios for clinopyroxenes in
the symplectite and in the matrix suggest that garnet partici-
pated in the reaction responsible for the formation of the ma-
trix assemblage (Franceschelli et al. 1998). It is probable to
assume that garnet was also involved in reaction (1):
garnet (core) + omphacite + quartz = garnet (rim) + clinopy-
roxene + plagioclase + amphibole (reaction 1b)
Late amphibole-plagioclase pairs formed at ca. 470 °C and
810 kbar via the following reaction (2):
garnet (rim) + clinopyroxene + H
O = amphibole + plagio-
or via reaction (3):
garnet (rim) + H
O = plagioclase + amphibole + quartz
Rutile partly transformed to ilmenite at the same time with
reactions (2 and 3), because it is only present in garnet cores
and in the symplectites.
In the garnetiferous amphibolite sample only a symplectite
stage and a retrograde (matrix) stage can be recognized. P-T
calculations resulted in 500570 °C and 812 kbar for the
symplectites, and ca. 500 °C and 8 kbar for the retrograde
stage. Despite the fact that no eclogite facies relicts can be
recognized in the amphibolite, the presence of symplectites
and the similar P-T conditions obtained for the retrograde
stages in both samples indicate that the investigated rocks
formed in a similar tectono-metamorphic setting.
Comparison with other Variscan eclogite terranes
The nearest crystalline basement outcrops to the investigat-
ed area are in the Eastern Alps and in the Dinarides. At
present, the P-T conditions and age relations of pre-Alpine
and Alpine metamorphism in the Dinaridic area are not well
established. Neubauer et al. (1999) presented the distribution
of pre-Alpine metamorphism in the Eastern Alps. Pre-Alpine
eclogites outcrop in the Penninic basement of the Tauern
Window with conditions of 620±100 °C, >12 kbar (Droop
1983) and 400500 °C, 812 kbar (Zimmermann & Franz
1989). The age of eclogite facies metamorphism is
415±18 Ma (U-Pb on zircon) or 421±16 Ma (Sm-Nd method)
according to von Quadt et al. (1997). In the Ötztal area of the
Middle Austroalpine nappe complex ca. 730 °C and 27 kbar
were estimated by Miller & Thöni (1995) for the Variscan
eclogite formation (359±18 Ma and 373±20 Ma, Sm-Nd
method on garnet and whole-rock; Miller & Thöni 1995).
From the Speik Complex (Hochgrössen, Middle Austroalpine
unit) Faryad et al. (2002) obtained an average temperature of
700 °C and a minimum pressure of 15 kbar. Ar-Ar radiomet-
ric data of amphibole in textural equilibrium with omphacite
gave 397.3±7.8 Ma. In the Ulten Complex (Austroalpine
basement) along the Periadriatic fault Hauzenberger et al.
(1996) and Höller & Hoinkes (1996) found a two-stage evolu-
tion recorded in eclogites. They found ca. 700±50 °C and
364 HORVÁTH, KOVÁCS
>15 kbar for the peak metamorphic conditions in the eclogite
facies. Subsequent decompression took place at ca.
600±50 °C and 67 kbar. Gebauer & Grünenfelder (1978) re-
ported U-Pb zircon ages of 326332 Ma for the Ulten Com-
plex rocks. Von Räumer (1998) concluded that although the
tectonic and metamorphic history of the basement units of the
Alps can be compared to that of the Variscan crust in the Al-
pine foreland, most of these basement rocks do not represent
the direct southern continuation of Variscan structural ele-
ments evident in the Massif Central or the Bohemian Massif.
Most Hungarian authors agree that the Variscan medium-
grade metamorphics of the Tisza Megaunit show great simi-
larities to the European Variscides, especially to the Moldanu-
bian Zone of the Bohemian Massif (for recent review, refer to
Kovács et al. 2000). For this reason, we turned our attention to
the eclogites of the Bohemian Massif and tried to find similar
petrographic features and P-T evolution. Medaris et al. (1995)
reviewed the eclogites of the Bohemian Massif and divided
them into various groups based on geochemical data, tectonic
setting and P-T conditions. Comparing their data to our results
supports the idea that the eclogite occurring in the SW part of
the Tisza Megaunit shows great similarities to eclogites of the
Monotonous Series of the Moldanubain Zone (minimum T-P
conditions are 615705 °C and 1315 kbar there). Unfortu-
nately, the exact age of eclogite formation is still undeter-
mined there, but is most likely older than 380 Ma (Matte
1986; Franke 1989). Recently, Medaris et al. (1998) found
prograde eclogite in the Gföhl Nappe, which is the uppermost
allochthonous unit in the Moldanubian Zone. This result im-
plies that eclogite facies metamorphism was probably Early
Carboniferous in age, rather than Late Devonian as stated by
Petrakakis (1997). In additional to eclogites, HP granulites are
also important features of the Moldanubian Zone and the Eu-
ropean Variscides (e.g. Carswell & OBrien 1993; OBrien &
Carswell 1993; OBrien et al. 1997; Cooke 2000), but so far
this rock type has not been found in the Tisza Megaunit. Since
we have no age of eclogite in the Tisza Megaunit, our compari-
sons are not justified and there are several other possibilities.
1. The investigated HP metabasite samples (eclogite and
garnetiferous amphibolite) occur in Miocene conglomerates
resting on top of the Variscan crystalline basement of the SW
part of the Tisza Megaunit, Hungary. Geochronological data
show cooling ages for amphiboles at around 348±13 Ma sug-
gesting a Variscan retrogression of the rocks which contradics
the earlier results (400440 Ma; Svingor & Kovách 1981).
2. Peak metamorphic conditions were in the MT part of the
eclogite facies (600650 °C, 1315 kbar) with garnet, ompha-
citic pyroxene, quartz and rutile as primary assemblage. Dur-
ing the breakdown from peak P-T conditions symplectitic in-
tergrowth of clinopyroxene, plagioclase and quartz formed.
Continuous retrogression involving garnet caused formation
of the matrix assemblage. Amphibole-plagioclase porphyro-
blasts represent the last stage of retrogression. Amphibole-
bearing symplectites record similar P-T conditions in garnetif-
erous amphibolite, but no pyroxene was detected there.
3. The investigated metabasites and serpentinites were em-
placed in the medium-grade metasedimentary country rocks
(recording no signs of HP metamorphism) after the main
metamorphic event (270320 Ma). Final uplift to surface con-
ditions occurred in Miocene times and caused the first appear-
ance of HP metabasite pebbles in clastic sedimentary rocks.
Acknowledgments: The authors got valuable help and com-
ments from Prof. Péter Árkai, which substantially improved
the quality of an early version of the manuscript. We are
greatful to S.W. Faryad, M. Janák and T.M. Tóth for thorough
and constructive reviews. Thanks are due to Ms. N. Szász for
preparing the microprobe samples. Field and some of the lab-
oratory work were financially supported by the Hungarian
National Science Fund (OTKA) Grants T 014121 and
T 022938 to Gy. Sz.
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