GEOLOGICA CARPATHICA, APRIL 2007, 58, 2, 121—131
www.geologicacarpathica.sk
Introduction
High-pressure, true eclogite facies rocks have previously
been unknown in the Western Carpathians (e.g. Krist et al.
1992; Bezák et al. 1993). In recent years, relics of eclogite
facies metamorphism have been inferred in some garnet-
and clinopyroxene-bearing metabasites of the basement of
the Central Western Carpathians. In these rocks a high-
pressure stage has been deduced from textures as symplec-
tites, kelyphites and coronas, indicating a breakdown of
primary omphacite (e.g. Hovorka & Méres 1989; Janák et
al. 1997 and references therein).
The pre-Tertiary complexes of the Central Western Car-
pathians comprise six principal superunits: the Tatric, Ve-
poric and Gemeric thick-skinned basement/cover sheets,
and the Fatric, Hronic and Silicic detachment cover nappe
systems (Plašienka et al. 1997). Presumed eclogites have
been described from several so-called “core mountains”:
Tribeč (Hovorka & Méres 1990), Malá Fatra (Hovorka et
al. 1992a; Janák & Lupták 1997; Korikovsky & Hovorka
2001), Western Tatra (Janák et al. 1996) and Low Tatra
(Spišiak & Pitoňák 1990), all belonging to the Tatric Unit.
Moreover, high-density nitrogen inclusions identical to
those observed in well-preserved eclogites of the world
have been identified in the metabasites of the Western
Tatra (Janák et al. 1996; Hurai et al. 2000). Finally, om-
phacite has been discovered in metabasites of the north-
ern Veporic Unit (Janák et al. 2003).
Following the first report (Janák et al. 2003), we present
here more in detail the mineralogical and petrologic fea-
tures of eclogites from the northern Veporic Unit in the
Petrology and metamorphic P-T conditions of eclogites from
the northern Veporic Unit (Western Carpathians, Slovakia)
MARIAN JANÁK
1
, ŠTEFAN MÉRES
2
and PETER IVAN
2
1
Geological Institute, Slovak Academy of Sciences, Dúbravská cesta 9, P.O. Box 106, 840 05 Bratislava 45, Slovak Republic;
marian.janak@savba.sk
2
Department of Geochemistry, Faculty of Natural Sciences, Comenius University, Mlynská dolina G, 842 15 Bratislava, Slovak Republic
(Manuscript received May 25, 2006; accepted in revised form October 10, 2006)
Abstract: Eclogites with rarely preserved peak metamorphic assemblage omphacite + garnet + phengite + rutile + zoisite +
quartz ± amphibole are present in the northern parts of the Veporic Unit, in the Central Western Carpathians of Slovakia.
Retrogression led to breakdown of primary phases. Apart from inclusions in the garnet, primary omphacite (Cpx I) has
been converted to symplectites of clinopyroxene with lower Na and Al content (Cpx II) and sodic plagioclase. This
resulted from the decompression to the high-pressure granulite facies stability field. Several generations of amphibole
(pargasite, hornblende, and actinolite) are evidence of a transformation down to amphibolite and even greenschist facies
conditions. Geothermobarometry on the eclogite facies mineral assemblage garnet + omphacite + quartz + phengite al-
lowed us to constrain the maximum pressure and temperature conditions of around 2.5 GPa and 700 ºC, implying initial
subduction to depths of around 80 km. Our study supports previous indications on the existence of eclogites in the
Western Carpathians; careful observations can discover high-pressure phases in potential eclogite-facies rocks. This can
be essential to regional correlations and elucidation of the tectonometamorphic evolution of the Western Carpathians
during the Variscan orogeny.
Key words: Western Carpathians, retrogression, high-pressure metamorphism, omphacite, eclogite.
Central Western Carpathians. The microtextures, mineral
assemblages and their compositions, which constrain the
high-pressure eclogite stage and subsequent retrogression
during decompression are described. The peak metamor-
phism, eclogite facies stage P-T conditions are evaluated
by thermobarometry. We propose that the formation of
eclogites resulted from subduction, most probably during
the Variscan orogeny. However, resolving their initial set-
ting and protolith are beyond the scope of this paper.
Geological background
The investigated eclogites occur in the northern parts of
the Veporic Unit (Fig. 1). The Veporic Unit consists of pre-
Alpine basement that is overlain by the Upper Paleozoic—
Triassic sedimentary cover. The magmatic and metamorphic
history of the Veporic Unit was polyphase, comprising the
pre-Variscan, Variscan, Permian and Alpine events (e.g.
Plašienka et al. 1997 and references therein).
The northern parts of the Veporic Unit are composed of
several basement complexes covered by Permian and Meso-
zoic rocks (Fig. 1). The eclogites are part of the basement
termed the Hron Complex (Klinec 1966), leptyno-amphibo-
lite complex (Hovorka et al. 1992b, 1994, 1997) or layered
metaigneous complex (Putiš et al. 1997). In this paper we
use the term “leptyno-amphibolite complex” (LAC). In the
investigated area, LAC is composed of several rock types.
The most abundant are amphibolites and gneisses (both
ortho- and para-gneisses). These are strongly deformed and
retrogressed to epidote amphibolites, micaschists and phyl-
122
JANÁK, MÉRES and IVAN
lites. Within these rocks, massive metaultramafites, met-
agabbros and garnet-clinopyroxene amphibolites occur as
blocks and lenses of meter to tens of meters size. The met-
agabbros have partly well-preserved their primary cumulate
texture and minerals (Ivan et al. 1996; Méres et al. 1996;
Putiš et al. 1997). Less metamorphosed rocks within this
complex (phyllites, metasandstones, mafic volcanics and
volcanoclastics) have been distinguished as Jánov grúň
Complex (Miko 1981) of probably Permian age (Kotov et
al. 1996). In the investigated area these rocks are considered
to be diaphtorites of LAC amphibolites and therefore not
shown on the map. Pre-Alpine granitoids together with as-
sociated gneisses and micaschists strongly affected by Al-
pine mylonitization belong to the Krá ova ho a Complex.
The intrusion age of these granitoids is 350 Ma according
to SHRIMP dating of zircons (Gaab et al. 2006b). The Pred-
ná ho a Complex is composed of Paleozoic, low-grade
metamorphosed phyllites, metasandstones, basic volcanics
and volcanoclastics. Permian rocks comprise metamor-
phosed conglomerates, sandstones, arkoses and greywackes
locally with volcanogenic material. Mesozoic rocks consist
of Triassic carbonates and quartzites affected by low-grade
metamorphism in the Cretaceous time (Vrána 1966; Korik-
ovsky et al. 1997; Lupták et al. 2003).
Petrography and mineral chemistry
The investigated eclogites have been found in the out-
crops in the Koleso and Krivu a Valleys north of He pa
(Fig. 1). The eclogites occur mostly as lenses and boudins
within amphibolites; the best-preserved forming the cores
of such lenses. Reddish garnet and pale green clinopyrox-
ene are variably replaced by amphibole (Fig. 2a,b). Micro-
Fig. 1. Simplified geological map of the northern parts of the Veporic Unit modified from Biely et al. (1992) showing locations of the
investigated eclogites.
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PETROLOGY AND METAMORPHIC P-T CONDITIONS OF ECLOGITES (SLOVAKIA)
structures along with variations in mineral chemistry sug-
gest that the rocks have experienced a multistage metamor-
phic history. The composition of main mineral phases was
determined using a CAMECA SX-100 electron microprobe
at the State Geological Institute of Dionýz Štúr in Bratisla-
va. The analytical conditions were 15 kV accelerating volt-
age and 20 nA beam current, with a peak counting time of
20 seconds and a beam diameter of 2—10 m. Raw counts
were corrected using a PAP routine. Mineral abbreviations
are according to Kretz (1983).
Garnet
Garnet is poikiloblastic with inclusions of clinopyrox-
ene, amphibole, quartz, zoisite and rutile/ilmenite. It is
surrounded and partly resorbed by clinopyroxene—pla-
Fig. 2. Textures of the investigated eclogites. a – Photograph of relatively well-preserved eclogite in sample VV 40. b – Detail of tex-
ture with reddish garnet and pale green clinopyroxene variably replaced by amphibole in sample VV 33. c—f –Photomicrographs of
the eclogite facies minerals breakdown. c,d – Garnet porphyroblasts surrounded and partly resorbed by clinopyroxene-plagioclase-am-
phibole symplectites, larger amphiboles are at a distance from the garnet contacts. Sample VV 33, plane-polarized light. e – More ad-
vanced stage of retrogression with large pleochroic, green to brown-green amphiboles replacing garnet in VV 33, plane-polarized light.
f – Symplectite of red-brown biotite with plagioclase and quartz (white) after phengite. Sample VV 41, plane-polarized light.
124
JANÁK, MÉRES and IVAN
gioclase—amphibole symplectites (Figs. 2c,d and 3a), and
amphibole-plagioclase kelyphites (Fig. 2e,f). The com-
position of garnet (Table 1) shows a large range of al-
mandine (52—62 mol %), pyrope (6—20 mol %), grossular
(27—40 mol %) and spessartine (1—2 mol %) end mem-
bers. The compositional profiles in the individual grains
(Figs. 3c and 5) are very smooth, showing steady Mg in-
crease concomitant with Fe and Fe/Fe+Mg decrease from
the core to the rim, at nearly constant Ca, which is charac-
teristic for a prograde growth zoning. The maximum Mg
Table 1: Representative analyses of garnet. Formula normalization to 12 oxygens and 8 cations, Fe
3+
by charge balance.
Table 2: Representative analyses of clinopyroxene. Formula normalization to 6 oxygens and 4 cations, Fe
3+
by charge balance.
125
PETROLOGY AND METAMORPHIC P-T CONDITIONS OF ECLOGITES (SLOVAKIA)
and the lovest Fe/Fe+Mg occur close to the rim. In the out-
ermost part of the rims, Mn, Fe and Fe/Fe+Mg rise whereas
Mg drops as a result of resorption due to retrogression.
Clinopyroxene
Primary clinopyroxene (Cpx I) – omphacite occurs as in-
clusions in garnet, attaining the size of mostly less than 10
micrometers (Fig. 3a,b) but larger omphacite of several tens
of micrometers can also be observed (Figs. 3d and 4a—c).
The composition of omphacite varies between individual
samples but also within grains (Table 2). The jadeite con-
tent shows a wide range from 26—29 mol % to 39—
41 mol %. Differences in the Mg/Fe ratio are most probably
due to bulk chemistry as reflected also by other minerals.
Breakdown of omphacite led to the formation of less jadei-
Fig. 3. Back-scattered electron (BSE) images. a—d – Garnet porphyroblasts with inclusions of primary clinopyroxene – omphacite.
a,b – Minor inclusions of pristine omphacite in sample VV 41. c,d – Breakdown of larger omphacite to symplectite of Cpx II (light
grey) and plagioclase (black) in sample VV 40. The arrow in garnet marks the position of the compostional profile shown on Fig. 5. e –
BSE image showing symplectites of clinopyroxene II (light grey), amphibole (dark grey) and plagioclase (black) in sample VV 186. f – BSE
image of phengite partially replaced by symplectite of biotite and plagioclase in sample VV 186.
126
JANÁK, MÉRES and IVAN
Table 3: Representative analyses of phengite. Formula normalization to 11 oxygens.
te-rich Cpx II, which forms symplectitic intergrowths with
Na-rich plagioclase (Figs. 3d,e and 4c—e). Amphibole is also
common in these symplectites but appears to be a later
phase replacing the pyroxene. The secondary Cpx II has a
lower Na and higher Ca but a similar Fe content to primary
Cpx I during this transformation, as documented by compo-
sitional maps of these elements (Fig. 4d—f).
Phengite
Phengite commonly associated with quartz occurs in the
matrix. Mineral compositions show a wide range in Mg/Fe
ratio and up to 3.4 Si p.f.u (Table 3). Breakdown of pheng-
ite led to the formation of biotite and plagioclase which
form the symplectitic intergrowths (Figs. 2f and 3f).
Amphibole
Amphibole occurs as several compositional and textural
types. Amphibole inclusions in garnet (Amp I) form small
euhedral crystals, and are classified as hornblende to par-
gasite. Amphibole II of pargasitic composition can be rec-
ognized in kelyphitic rims around garnets as lath-shaped
crystals, or symplectitic intergrowths with clinopyroxene
Table 4: Representative analyses of amphibole. Formula normalization to 23 oxygens, Fe
3+
as average from minimum and maximum
constraints.
127
PETROLOGY AND METAMORPHIC P-T CONDITIONS OF ECLOGITES (SLOVAKIA)
Fig. 4. BSE images and compositional X-ray maps of the omphacite breakdown. a – BSE image of omphacite inclusions in garnet,
sample VV 33. b – Detail of omphacite. c – Breakdown of primary Cpx I (omphacite) to Cpx II, plagioclase, amphibole and calcite as
retrograde phases. d—f – Quantitative X-ray maps of area in (c) showing Na, Fe and Ca distribution. Note that complex compositional
zonation with highest Na in (d) and lowest Ca in (f) is seen within the Cpx I domain.
and plagioclase (Figs. 2c—f, 3a,e and 4c—e). Matrix am-
phiboles (Amp III) at a distance from the garnet contacts
are relatively large, pleochroic, dark green to brown-green
poikiloblastic grains, of hornblende to pargasite composi-
tion. Actinolite (Amp IV) is a later phase that formed
around earlier amphiboles. The compositions of amphib-
oles are presented in Table 4.
Minor phases
Zoisite is common as inclusions in garnet porphyro-
blasts and it is considered to be a part of the eclogite fa-
cies assemblage. Epidote (clinozoisite) is present in the
matrix where it is commonly associated with amphibole as
retrograde phases. Plagioclase occurs only as a secondary
128
JANÁK, MÉRES and IVAN
mineral, forming symplectites with Cpx II and biotite after
omphacite and phengite. Plagioclase is also present in the
kelyphitic rims around garnet. K-feldspar is a minor phase
found as inclusion in garnet. Quartz occurs in garnet, in
kelyphites, and in the matrix. Rutile and ilmenite are
ubiquitous as inclusions in garnet, matrix amphibole, and
kelyphites. Sphene is abundant in the most retrograded
domains and biotite; chlorite and calcite have been recog-
nized as additional retrograde minerals.
Geothermobarometry
Peak metamorphic conditions have been calculated
from geothermobarometry on the eclogite facies mineral
assemblage garnet+omphacite+phengite. A combination
of the garnet-clinopyroxene Fe
2+
—Mg exchange geother-
mometer of Ravna (2000) with the geobarometer utilizing
the net-transfer reaction equilibrium:
3 celadonite + 2 grossular + pyrope = 6 diopside + 3 muscovite
(1)
calibrated by Ravna & Terry (2004) has been used. A com-
bination of the activity model for the phengite solid solu-
tion by Holland & Powell (1998), the clinopyroxene
activity model of Holland (1990), and the garnet activity
model of Ganguly et al. (1996) were selected as recom-
mended by Ravna & Terry (2004). We used omphacite
with the highest jadeite content, garnet with maximum
(a
grs
)
2
. (a
py
) and phengite with the highest Si content to
calculate maximum pressure conditions. For garnet-om-
phacite geothermometry the ferric Fe in garnet and om-
phacite was calculated from the stoichiometry. In the
omphacite, we have also calculated the ferric Fe contents
following the procedure, which only allocates Fe
3+
if there
is excess Na over the content of Al present.
Fig. 5. Compositional profile of garnet in sample VV 40. The po-
sition of line is shown on Fig. 3c.
Fig. 6. Peak metamorphic P-T conditions for eclogites of the north-
ern Veporic Unit. The post-peak decompression is shown by arrow.
The metamorphic facies grid is from Okamoto & Maruyama
(1999). BS – blueschist facies, EA – epidote amphibolite facies,
AM – amphibolite facies, HGR – high-pressure granulite facies,
Lw-EC – lawsonite eclogite facies, Ep-EC – epidote eclogite fa-
cies, Amp-EC – amphibole eclogite facies, Dry-EC – dry eclog-
ite facies. The quartz-coesite curve is calculated from
thermodynamic data of Holland & Powell (1998).
The intersection values of garnet-clinopyroxene ther-
mometer with equilibrium (1) in four investigated samples
are presented in Table 5. They yield a pressure of 2.3—
2.7 GPa and temperature in the range of 684—725 ºC.
These pressure and temperature values correspond well to
the stability field of eclogite facies metamorphism (Fig. 6).
The spread in temperature and pressure can be attributed
to the post-eclogite facies reequilibration during decom-
pression and exhumation. Post peak metamorphic over-
print may cause partial redistribution of Fe and Mg
between garnet and clinopyroxene. Uncertainty related to
the oxidation state of iron can be an additional problem
concerning the estimation of temperature. According to
Ravna & Paquin (2003), the uncertainty of garnet—cli-
nopyroxene thermometer may be ±60 ºC. The garnet-cli-
nopyroxene-phengite barometer applied here may give
invariably higher pressures than the original calibration of
Waters & Martin (1993, 1996) or its other applications
(e.g. Carswell et al. 1997). Except different barometric ex-
pressions this stems largely from the preferred garnet ac-
tivity model used. Nevertheless, it can be seen from
several applications that the calibration of Ravna & Terry
129
PETROLOGY AND METAMORPHIC P-T CONDITIONS OF ECLOGITES (SLOVAKIA)
Table 5: P-T values for the peak metamorphic stage.
(2004) gives consistent values for most HP/UHP metamor-
phic terrains examined. Taking into consideration all
these factors, maximum P-T conditions for the Veporic
eclogites appear to be around 2.5 GPa at 700 ºC.
Discussion
The estimated P-T conditions together with preservation
of
peak
assemblage
Grt+Cpx I+Phn+Rt+Qtz+Zo±Amp
clearly document the eclogite facies stage. If amphibole
inclusions in garnet really belong to the peak-pressure as-
semblage is not equivocal but the experimental data would
limit their stability to 2.5—2.6 GPa at 650—700 ºC (e.g. Poli
& Fumagalli 2003). Most of the amphibole forming the
symplectites, kelyphites and matrix post-dates the peak
pressure conditions. Breakdown of omphacite leads to its
replacement by symplectitic intergrowths of sodic plagio-
clase and clinopyroxene with lower Na and Al content than
the initial clinopyroxene. This is important to stress with re-
spect to discussion on formation of such symplectites in the
garnet-clinopyroxene metabasites of the Western Car-
pathians where omphacite has not been found (e.g. Hovorka
& Méres 1989, 1990; Hovorka et al. 1992a; Janák et al.
1996, 1997; Korikovsky & Hovorka 2001; Faryad et al.
2005). Janák et al. (1996) “reconstructed” such omphacite
with Jd
36
from the modal proportions in Cpx II – plagio-
clase symplectites in metabasites of the Western Tatra.
These authors estimated the “minimum” P-T conditions of
1.5—1.6 GPa at 650—750 ºC for eclogite facies stage, accord-
ing to the reaction Ab=Jd+Qtz. As an alternative to the
breakdown of omphacite, Korikovsky & Hovorka (2001)
proposed that Cpx+Plg symplectites were produced by the
prograde reaction from the epidote amphibolite to amphib-
olite facies at moderate pressure, on the basis of their obser-
vations from the Malá Fatra metabasites. Consequently,
Korikovsky & Hovorka (2001) suggested that these rocks
reached only the amphibolite facies conditions, without
any high-pressure, eclogite facies stage.
As is generally known, many eclogites have experi-
enced an overprint at lower pressure conditions during
their exhumation. This may result in almost total decom-
position of original omphacite, depending on several fac-
tors such as, maintaining of high temperature and rate of
uplift, deformation and access of fluids during exhuma-
tion. In the investigated eclogites from the Veporic Unit
the resulting mineral assemblage of Grt+Cpx+Pl+Qtz is
the same as that found in many eclogites overprinted in
the high-pressure granulite facies conditions during their
exhumation, as pointed out by O’Brien & Rötzler (2003).
Therefore the eclogites of the Veporic Unit may have fol-
lowed a P-T path from the eclogite to high-pressure granu-
lite and amphibolite facies as illustrated in Fig. 6.
Although the timing of eclogite facies metamorphism in
the Western Carpathians remains uncertain, available data
from the host rocks of eclogites support its pre-Alpine age.
Microprobe dating of monazite from the kyanite-bearing
para- and ortho-gneisses in the northern Veporic Unit
yielded two groups of ages: an older – Ordovician, ca.
470 Ma and a younger – Carboniferous, ca. 340 Ma. This
is interpreted as records of pre-Variscan magmatism and
Variscan metamorphism (Janák et al. 2002). Pre-Variscan
magmatism in the Veporic Unit has been determined from
dating of zircons as Cambrian (multi-grain method, Putiš
et al. 2001) or Ordovician (single-grain method, Gaab et
al. 2006a,b). In the northern Veporic Unit the zircons show
metamorphic
overprint
in
the
Carboniferous
(350—
340 Ma) time (Putiš et al. 2001; Gaab et al. 2006b). There-
fore we assume that the timing of high-pressure
metamorphism of the investigated eclogites was Variscan,
whereas their magmatic protoliths could be pre-Variscan,
most probably Ordovician. Estimated maximum P-T con-
ditions (2.5 GPa; 700 ºC) suggest that eclogites were sub-
ducted to depths of about 80 km. Such metamorphic
conditions were very different from those during the Al-
pine orogeny in the Western Carpathians. Alpine meta-
morphism attained a maximum of 600—620 ºC at 1—1.2 GPa
in the southern parts of the Veporic Unit (Janák et al. 2001),
resulting from the crustal thickening during the Cretaceous
time, after closure of the Meliata Ocean. However, it is not
clear whether eclogites underwent their retrograde overprint
during the Variscan and/or Alpine time. Published Ar-Ar
data on amphiboles from the northern Veporic Unit yield
mostly the pre-Alpine dates (Maluski 1993; Dallmeyer et al.
1996; Krá et al. 1996) and Putiš et al. (1997) estimated the
P-T conditions of about 500 ºC at 0.8—0.9 GPa for Alpine
metamorphism in this area. This implies that metamorphic
evolution of eclogites in the northern Veporic Unit oc-
curred mainly during the Variscan orogeny and their Alpine
overprint was rather weak.
Conclusions
(1) The pre-Alpine basement of the Western Carpathians
contains relics of a former high-pressure metamorphism.
These are rarely preserved in metabasites of the northern
Veporic Unit.
(2) Documentation of eclogite facies assemblage to-
gether with application of geothermobarometry allows re-
cording the peak conditions of metamorphism, with
maximum pressure and temperature of around 2.5 GPa and
700 ºC. This may reflect initial subduction to depths of
about 80 km. Subsequent retrogression led to breakdown
of primary phases, apart from inclusions in the garnet, pri-
mary omphacite has been converted to symplectites of di-
opside and plagioclase.
130
JANÁK, MÉRES and IVAN
(3) Our study supports previous indications on the exist-
ence of eclogites in the Western Carpathains. Breakdown
reactions and inclusions of primary minerals are extremely
informative for retrograded eclogites. Careful observations
can discover high-pressure phases in potential eclogite-fa-
cies rocks. This can be essential to the elucidation of the
Variscan orogenic events in the Western Carpathians.
Acknowledgments: We thank E. Krogh Ravna, S. Vrána
and D. Plašienka for their helpful reviews. This work was
supported by the Slovak Research and Development
Agency under the contracts No. APVT-20-020002, APVT-
20-016104 and APVV-51-046105, and Scientific Grant
agency VEGA, Grants No. 2/6092/26 and 1/2025/05.
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