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The Szarvaskő Ophiolite Complex (SOC) is located in the
southwestern part of the Bükk Mts, NE Hungary and occu-
pies a territory of about 25 km


 (Fig. 1). Gabbroic rocks

form the dominant intrusions of SOC and they crop out
around the village of Szarvaskő (Fig. 1). They are charac-
terized by very variable texture and mineral composition
even within a single outcrop. In some quarries and out-
crops, pegmatitic pockets and dykes can also be observed.

Gabbro pegmatites are poorly studied features in

ophiolitic gabbros. Manning et al. (1996) and Beard et al.
(2002) studied such features in recent oceanic units, and
Hoeck  et al. (2002) provided data from ophiolitic rocks of
the Neotethyan realm. Hence, one of the major aspects of
our work was to support the knowledge on gabbro
pegmatites in ophiolite-related gabbro intrusions by
analysing their relationships to the host rock, textural
variations and mineral composition. These observations
were used to characterize volatile enrichment, segregation
of magmatic fluids and their interaction with silicates dur-
ing crystallization of the pegmatite bodies.

Previous investigations (Árkai 1983; Árkai et al. 1995;

Sadek et al. 1996) have shown that magmatic rocks of the
SOC underwent alteration during sea-floor hydrothermal
and low-grade regional metamorphic events. Our study is
also aimed at outlining characteristics of these processes
by combining mineralogical and fluid inclusion data.

Magmatic fluid segregation and overprinting hydrothermal

processes in gabbro pegmatites of the Neotethyan ophiolitic

Szarvaskő Complex (Bükk Mountains, NE Hungary)







Department of Mineralogy, Eötvös Loránd University, Pázmány P. sétány 1/C, 1117 Budapest, Hungary;  *


Department of Earth Sciences, Carleton University, 1125 Colonel By Drive, Ottawa K1S5B6, Canada

(Manuscript received September 22, 2005; accepted in revised form June 22, 2006)

Abstract: Pegmatites of the Szarvaskő Ophiolite Complex, Bükk Mts, NE Hungary were classified according to their
shape (pockets, dykes) and texture (zoned, homogeneous), representing different stages of fluid enrichment during
crystallization of the host gabbro. Local assimilation of adjacent sedimentary rocks increased the volatile-, and incompat-
ible-element content of the melt. Most pegmatites crystallized from a locally segregated hydrous silicate melt, but some
were intruded later. Pegmatites have mineral contents similar to their host gabbro, but are enriched in amphibole, biotite,
Fe-Ti-oxides, quartz, and apatite. During formation of the pegmatites a fluid phase separated and caused deuteric
alteration under magmatic-submagmatic conditions. Post-magmatic sea-floor hydrothermal activity is recognized by
intense alteration and formation of a greenschist facies mineral assemblage at temperatures of 250—400 

ºC. Fluid

inclusion studies revealed two aqueous fluid types related to this polyphase hydrothermal process. Alpine regional
metamorphism caused intense deformation of the rocks, accompanied by veining of a low-grade metamorphic mineral
assemblage. Primary fluid inclusions in vein-filling minerals and chlorite thermometry were used to obtain proposed
conditions of 270—285 

ºC and 150—200 MPa for this process.

Key words: Hungary, Bükk Mts, Alpine metamorphism, sea-floor hydrothermal alteration, deuteric alteration, magmatic
fluid, gabbro pegmatite.

Geology of the Szarvaskő Ophiolite Complex

The SOC is situated in the Bükk Mts, a tectonic unit of

the ALCAPA (Alpine-Carpathian-Pannonian) terrane, and
is in allochthonous position, overlying the Bükk
Parautochthonous Unit (Balla et al. 1983; Balla 1984a;
Csontos 2000). It has a synformal structure made up of
three nappes. The Mónosbél Nappe consists of sedimen-
tary rocks, whereas these are accompanied by magmatic
intrusions and pillowed basalts in the Szarvaskő I and
Szarvaskő II nappes (Csontos 2000). The magmatic intru-
sions are hosted by a series of turbiditic shales and sand-

The ophiolitic sequence is quite uncommon and differ-

ent units are missing in comparison to other well-known
ophiolites (e.g. mantle section with ultramafic units is
completely absent). Cumulate gabbro is abundant and at
some places is accompanied by rare ultramafic cumulates
(hornblendite, Fe-Ti-rich wehrlite). The upper part of the
plutonic section with various gabbroic and related
plagiogranitic rocks is the best preserved part of the unit
(Szentpétery 1953; Sadek & Árkai 1994), and crops out in
the Tardos and Tóbérc quarries (Fig. 1). The volcanic sec-
tion is well developed; pillowed basalts are exposed in
many outcrops of the SOC (Szentpétery 1953; Balla

Petrochemical analyses of mafic-ultramafic rocks of

SOC revealed some differences from MORB and therefore

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a  back  arc  basin,  or  a  marginal  sea  environment  has  been
suggested  to  explain  their  formation  (Kubovics  et  al.
1990;  Downes  et  al.  1990;  Harangi  et  al.  1996;  Aigner-
Torres  1996).  A  similar  origin  was  suggested  for  some
Dinaridic  Ophiolite  Zone  (DOZ)  ophiolites  (Pamić  1997).
Recent  geotectonic  models  favour  SOC  formation  in  the
Meliata-Vardar  Ocean,  where  sea-floor  spreading  and
magmatism  lasted  from  the  Middle  Triassic  until  Late  Ju-
rassic/Early  Cretaceous  (Pamić  et  al.  2002;  Csontos  &
Vörös  2004).  The  radiometric  age  of  the  SOC  magmatic
rocks  is  166 ± 8 Ma  (Árváné  Sós  et  al.  1987),  which  is  com-
parable  to  the  ages  from  189 ± 6.7 Ma  to  136 ± 15 Ma  ob-
tained  from  ophiolitic  rocks  of  the  DOZ  and  also  partly  of
the  Vardar  Zone  (VZ)  (Pamić  et  al.  1998;  Pamić  et  al.
2002).  According  to  Csontos  &  Vörös  (2004),  the  Meliata-
Vardar  Ocean  consisted  of  a  Triassic-Jurassic  plate  and  a
Middle-Late  Jurassic  back  arc  basin,  which  could  have

been  the  original  site  of  the  SOC.  After  crystallization  of
the  igneous  units  of  the  SOC,  the  primary  rock-forming
mineral  assemblage  underwent  a  sea-floor  hydrothermal
alteration  (ocean-floor  metamorphism),  which  can  be
traced  throughout  the  SOC  (Sadek  et  al.  1996).

With  the  onset  of  Late  Jurassic/Early  Cretaceous  intra-

oceanic  subduction  of  the  Meliata-Vardar  Ocean  (Pamić  et
al.  2002;  Csontos  &  Vörös  2004),  the  SOC  became  a  part  of
the  accretionary  prism  (Csontos  2000).  Huge  portions  of  the
oceanic  lithosphere  were  obducted  onto  the  Apulian  mar-
ginal  units  and  are  now  preserved  in  the  DOZ  (Tari  &  Pamić
1998;  Pamić  et  al.  1998;  Csontos  &  Vörös  2004).  The
ophiolitic  mélange  and  ophiolites  of  the  Dinarides  can  be
traced  to  the  southwest-northeast  Periadriatic-Balaton  Line
(Fig. 1).  Smaller  units  of  DOZ  within  the  Mid-Hungarian
Zone  occur  in  the  Samoborska  Gora,  Kalnik,  Medvednica,
and  Ivanšćica  Mountains  (Pamić  1997;  Slovenec  &  Pamić

Fig. 1. Geological sketch map of the Szarvaskő Ophiolite Complex (modified after Less et al. 2002 and Kovács et al. 2004).

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2002). Collision of the Bihor-Getic-Serbo-Macedonian up-
per plate and the Dinaridic High Karst lower plate may be
the reason for a 120 Myr (Belák et al. 1995) metamorphic
event in the DOZ (Csontos & Vörös 2004). The early Al-
pine, low-grade metamorphism was also observed in the
nappes of Southern Slovakia and Northern Hungary (Árkai
1983; Árkai et al. 1995; Csontos 1999). According to Árkai
et al. (1995) and Sadek et al. (1996), this metamorphism was
a low temperature anchizonal event (max. 250—300 ºC) cor-
responding to prehnite-pumpelleyite facies, with an illite-
muscovite K/Ar age of about 120 Ma, which is in good
agreement with the data of Belák et al. (1995) for the age of
metamorphism in the DOZ.

According to apatite fission-track dating (Árkai et al.

1995), the Szarvaskő Ophiolite Complex reached closure
temperatures of 125—70 ºC during Late Paleocene—Early
Eocene (53 ± 8 Ma). The recent position of the SOC is due
to the northeastward escape of the ALCAPA terrane from
the Alpine collision zone along the Periadriatic-Balaton
Line during the Late Eocene—Oligocene (Kázmér &
Kovács 1985; Csontos & Vörös 2004; Fig. 1).

Analytical methods

Detailed field observations of pegmatites and host gab-

broic rocks were supplemented by textural analyses of pol-
ished pegmatite slabs. Petrography was conducted on
polished thin sections of pegmatites and host gabbro from
the exposures of the Tóbérc quarry and Újhatár Valley
(Fig. 1). The chemical composition of minerals was deter-
mined using a Cameca MBX electron microprobe by wave-
length dispersive method at Carleton University, Ottawa.
The analytical standards were a well-characterized suite of
synthetic and natural compounds. Analyses were carried
out at 15 kV and 15 nA with counting times of 15—20 s (ex-
cept F with 40 s) or until reaching 40,000 counts.

Microthermometric analyses of fluid inclusions in quartz

from pegmatite and late veins were carried out on a
Chaixmeca-type microthermometric apparatus. The equip-
ment was standardized for approaching 0.1 ºC reproducibil-
ity near melting temperatures of pure carbon dioxide
(—56.6 ºC) and pure water (0.0 ºC) and 1 ºC reproducibility
at 374 ºC using synthetic fluid inclusions. Fluid inclusion
petrography and microthermometric measurements were car-
ried out using 150—200  m thick, doubly polished sections.

Occurrences and textural varieties of pegmatites

Tóbérc quarry

Various gabbroic and related plagiogranitic rocks are

exposed in Tóbérc quarry situated 1 km east of Szarvaskő
settlement (Fig. 1). Detailed petrographic description of
the magmatic rocks was done by Szentpétery (1953), the
plagiogranite was also studied by Sadek & Árkai (1994).

The host gabbro of the pegmatites is very heteroge-

neous, with medium to coarse texture. Coarsening of gab-

bro texture in irregular patches up to a few centimeters in
size is a common feature of the rock. Clinopyroxene, pla-
gioclase and amphibole are the major rock-forming miner-
als accompanied by Fe-Ti-oxides, biotite, apatite and rare
orthopyroxene, olivine and sulphides. Xenoliths of the
host sediments are abundant. These xenoliths were partly
assimilated and recrystallized to coarse-grained massive
quartz bodies with a plagioclase-rich granodioritic rim.
Sulphide enrichments containing pyrrhotite, pentlandite
and chalcopyrite in association with quartz and biotite
can also be found along the contact of gabbro with shale.
Gabbro is tectonically fragmented and fractures are filled
with prehnite, calcite, chlorite, and quartz.

One type of pegmatite forms mostly isometric, round to

elliptical pockets with diameter up to 20—30 cm (Fig. 2B
and D) and they often have elongated tail-like attach-
ments. These bent tails branch off the pocket and can be
traced for a few tens of centimeters in the gabbro. The
size of plagioclase, amphibole and biotite crystals is up
to 3—4 cm toward the center of the pockets and the bound-
aries of the pegmatitic bodies are not sharp.

Two sub-types of pocket pegmatites can be distin-

guished according to their textures and compositions. In
pegmatites with homogeneous texture, grain size and min-
eral composition do not show changes within a pocket
(Fig. 2F) and sub- to euhedral hydrous silicate minerals
(amphibole, biotite) are more common than in the host
gabbro. Some anhedral quartz grains interstitial to the
coarse-grained silicates are also present. Pegmatitic pock-
ets with heterogeneous texture often show regular zoning
of minerals (Fig. 2E), with a pyroxene-rich rim followed
by an inner amphibole- and plagioclase-rich zone, and the
latest crystallizing core made up of quartz and plagio-
clase. Quartz forms interstitial crystals or granophyric
intergrowth with plagioclase.

Pegmatite also occurs as dykes that can be followed for

several meters in the gabbro. Their terminations are wedge-
shaped or abrupt due to tectonism. Pegmatite dykes can
also be classified into two subgroups. One of them has min-
eral composition similar to the host gabbro, with gradual in-
crease of grain size from the margin towards the center
(Fig. 2A). Texture is homogeneous and the grain size of
rock-forming minerals is up to 3—4 centimeters. The thick-
ness of these dykes does not exceeed 5—8 centimeters. The
second type of pegmatitic dyke is 15—20 cm thick with
sharp contacts toward the gabbro (Fig. 2C). Mineral com-
position shows a much more differentiated character than
that for the host gabbro, because it is enriched in plagio-
clase and also contains interstitial quartz or granophyric
quartz-plagioclase intergrowths. The grain size reaches
only 1—2 centimeters.

Újhatár Valley

The homogeneous fine-grained gabbro has a cumulus

texture with dominant amphibole and plagioclase. Pyrox-
ene is absent, and Fe-Ti-oxides are less common than in
the Tóbérc quarry. Late differentiated pegmatitic dykes,
xenoliths, and plagiogranitic rocks are absent.

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Fig. 2.  Photographs  of  typical  pegmatitic  structures  and  textures.  A  –  Narrow  pegmatitic  dyke  from  the  Tóbérc  quarry.  Note  fainted
boundary and mafic composition. – Pegmatitic pod from the Újhatár Valley outcrop. Note the “tail” branching off the pod. – Thick
pegmatitic  dyke  from  the  Tóbérc  quarry  with  sharp  boundary  and  strongly  evolved  composition.  D  –  Pegmatitic  pod  with  a  bent  “tail”
from  the  Tóbérc  quarry.  E  –  Polished  slab  of  a  zoned  pegmatitic  pod  from  the  Tóbérc  quarry.  Note  mafic  rim  and  the  plagioclase-  and
quartz-rich core of the pod. – Polished slab of a homogeneous pegmatitic pod from the Újhatár Valley outcrop. Note the lack of pyrox-
ene  and  the  dominance  of  euhedral  amphibole.

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Subparallel bent dykes are the most common pegmatite

bodies. Due to tectonism, the extent of dykes is unknown
and it is not possible to conclude if these subparallel
structures were horizontal sills, or vertical dykes. Dykes
are just up to 5 cm thick and do not exhibit sharp contacts
with the gabbro. Pocket-like pegmatites are also present,
but less frequent. Similar to the host gabbro, pegmatite
bodies exhibit a homogeneous, shape-independent tex-
ture. They are dominated by euhedral amphibole crystals
up to 12 cm long (Fig. 2B and F). Plagioclase also forms
euhedral to subhedral, but smaller crystals. Quartz is ab-
sent in these pegmatites.

Petrography and mineral chemistry

Primary minerals


occurs only in heterogeneous peg-

matites of the Tóbérc quarry. Euhedral crystals, up to 1 cm
in size, are intergrown with anhedral ilmenite (Fig. 3B).
Being the first crystallizing phase, it often grows along the
boundary with the host gabbro and it is absent in the in-
ner pegmatite zones. Pyroxene is strongly altered: only
large crystals preserved fresh cores. Rims are always al-
tered to amphibole, whereas chloritization occurs along
cracks and cleavage planes. Fresh pegmatitic clinopy-
roxene corresponds to augite (Morimoto 1989) with a
composition range of En






(Table 1).

The  Mg-number decreases from 80—85 in the host gab-
bro (Aigner-Torres 1996) to 62—66 in the pegmatites at
Tóbérc quarry. Ti is also enriched in the pegmatitic
augite. The Al and Na contents are almost the same as
they are in the host gabbro, but the Ca-content is lower
in the pegmatitic pyroxene (Table 1). The differentiated
character of the parent melt is supported by negligible
chromium content even in the rock forming pyroxene
of the gabbro.


occurs as twinned anhedral grains up to

3 cm in diameter in pegmatites of the Tóbérc quarry, and
they always show intense alteration (sericitization and
saussuritization). The original composition corresponds to
andesine-labradorite (An


) in the gabbro (Aigner-

Torres 1996) and An


in pegmatites (Table 2). Potas-

sium contents are always low (less than 1 mol % Or
component), and about 0.5 wt. % FeO is also present. The
plagioclase in pegmatites of the Újhatár Valley is
subhedral to anhedral, up to 1 cm in size. It often forms in-
clusions in amphibole, and is relatively fresh. The composi-
tion is almost pure albite (Ab


) with less than 1 mol %

Or component and with about 0.2—0.3 wt. % FeO. The
fresh and homogeneous appearance of plagioclase rules
out its origin by alteration of a more calcic feldspar.


 replaces rims of pyroxene, but it also occurs

interstitially between plagioclases as well as in the form of
euhedral grains (Fig. 3A). Up to 12 cm long prisms occur
in pegmatites of the Újhatár Valley. Amphibole is always
rich in Al, Ti and Na; the Mg-numbers are mostly less than
50 (Table 3). At the Újhatár Valley locality, tschermakite,

Table 1: Chemical composition of clinopyroxene from pegmatites
and host gabbro of the Tóbérc quarry (TOB). Data for host gabbro
clinopyroxene from Aigner-Torres (1996). Mg# = Mg/(Mg + Fe



En = enstatite, Fs = ferrosillite, Wo = wollastonite.

Table 2: Chemical composition of primary and secondary plagio-
clase from Újhatár Valley (UH) and Tóbérc quarry (TOB) pegma-
tites. Ab = albite, An = anortite, Or = orthoclase.

Fe-tschermakite, hastingsite, Fe-hastingsite and pargasite
are present, while edenite and Fe-edenite are the domi-
nant compositions in the Tóbérc quarry according to
classification of Leake (1997). Halogen contents are rela-
tively high for both localities, but they are mostly higher
at the Tóbérc quarry (Table 3), with 0.1—0.2 wt. % Cl and
0.1—0.15 wt. % F (Fig. 4). In some cases, alteration of am-
phibole to chlorite occurs along fractures and cleavages.


is rare in the gabbro but it is a common acces-

sory phase in pegmatites. Euhedral grains (with up to
2 mm length in pegmatites) occur as inclusions in amphib-
ole, plagioclase and ilmenite. Compositional zoning has
not been observed. The apatite from pegmatites is always
fluorine-rich (1.1—2.3 wt. %) and contains 0.4—0.8 wt. %
chlorine (Table 4 and Fig. 4). The total REE concentra-
tions are between 3400 and 9200 ppm. Ce and Y are the
most abundant elements followed by Nd and La. There is
no significant difference in terms of halogen content be-
tween the localities but the total amount of REE is much

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Fig. 3.  Photomicrographs  of  gabbro  pegmatites,  showing  mineral  phases  and  fluid  inclusion  types.  All  photos  in  transmitted  light,  plane
polars,  except  photo  (B)  and  (C)  in  crossed  polars.  A  –  Typical  mineral  assemblage  with  primary  and  alteration  phases.  Clinopyroxene
(cpx)  is  replaced  by  zoned  deuteric  (amph


)  and  zoned  sea-floor  hydrothermal  amphibole  (amph


).  Primary  plagioclase  (plag


)  is  altered

and overgrown by a fresh rim of albite (plag


). The vug is filled by fine-grained chlorite (chlo). Fibrous Fe-actinolite (last amphibole gen-

eration)  has  suffered  brittle  deformation.  B  –  Clinopyroxene  (cpx)  intergrown  with  primary  anhedral  ilmenite  (ilm).  C  –  Chlorite


),  sphene  (sph)  and  clinozoisite  (clz)  replacing  biotite.  Fine-grained  chlorite  (chlo


)  is  also  visible.  D  –  Alpine  metamorphic  quartz

(qtz)  intergrown  with  fine-grained  chlorite  (chlo).  E  –  Magmatic  quartz  containing  solid-  and  fluid-inclusions.  Broken  line  shows    strike
of  secondary  fluid  inclusion  plane  hosting  fluid  type I.  F  –  Secondary  fluid  inclusion  plane  of  typeI  fluid  in  magmatic  quartz.  Note  dif-
ferent  vapour  ratio  at  room  temperature.  G  –  Secondary  fluid  inclusion  plane  of  type  II  fluid  in  magmatic  quartz.  Note  the  15 vol. %  of
vapour at room temperature. – Primary assemblage of type III fluid inclusions in metamorphic calcite.

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Table 3: Chemical composition of magmatic and hydrothermal amphibole from Újhatár Valley (UH) and Tóbérc quarry (TOB) pegmatites.
Act = actinolite, Ed = edenite, Grun = grunerite, Hast = hastingsite, Hb = hornblende, Parg = pargasite, Tsch = tschermakite; (Fe-) means always
the ferro-endmember, (Mg-) means the magnesio-endmember; Mg# = Mg/(Mg + Fe


) (Leake 1997). Number of cations based on 23 (O).

Fig. 4. Cl versus F content of amphibole and apatite from the
Újhatár Valley and Tóbérc quarry. The inset shows halogene content
of amphibole in detail. Note the variable F and Cl content in magmat-
ic and deuteric amphibole, while hydrothermal amphibole contain al-
most no fluorine.   – magmatic, 

– deuteric,   – hydro-thermal

amphibole compositions,   – apatite.

higher and local Ba and Sm enrichments also occur in apa-
tite from the Tóbérc quarry.


forms flakes up to 1 cm around amphibole in

pegmatites, while it is rare in the host gabbro. The biotite
flakes are mostly replaced by chlorite.


 is the last crystallizing phase in the central parts

of pegmatites in the Tóbérc quarry; however, it is com-
pletely absent in the pegmatites of the Újhatár Valley and
in the host gabbro. This quartz contains many inclusions
of apatite and fluids (Fig. 3E).


 is often intergrown with pyroxene (Fig. 3B)

and forms rims around pyroxene. Ilmenite also occurs as
exsolution lamellae or anhedral inclusions in magnetite
both in the gabbro and its pegmatites.


 is fresh in the gabbro, but altered to brown

amphibole and biotite in pegmatites. In this latter case
only the ilmenite exsolution lamellae remained from the
original titanomagnetite grains.

Alteration of primary minerals

Petrography revealed three alteration stages of primary

minerals developed in unequal extent at various localities.

The first alteration was probably caused by fluids coexist-
ing with the parent melt of pegmatites and is thus defined
as a deuteric alteration. The deuteric alteration is absent in
the gabbro and occurs only in pegmatites. This stage is

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mainly characterized by the formation of zoned amphib-
ole. Clinopyroxene has been partly replaced by brown am-
phibole, which has a composition similar to interstitial
and euhedral brown amphibole. However, replacing am-
phibole has a gradual change of composition toward the
rim, with decrease in Na, Al, Ti and F contents and in-
crease in Si content (Table 3, Fig. 5). This is also mani-
fested by change of their colour from brown to green
(Fig. 3A). The amphibole zoning is typical of pegmatites
in the Tóbérc quarry, and it is less developed in the
pegmatites of the Újhatár Valley. The green zones of the
amphibole correspond to Fe-hornblende (Leake 1997).
Alteration of magnetite in pegmatites appears to be syn-
chronous with the amphibole alteration of pyroxene. Mag-
netite crystals are totally decomposed, but their ilmenite
exsolution lamellae have remained intact. During this pro-
cess biotite flakes crystallized among the lamellae. Alter-
ation of plagioclase to sericite probably started at this
stage but was overprinted during later events.

The early stage (deuteric) alteration was overprinted by

sea-floor hydrothermal alteration in pegmatites and their host
gabbro of the Tóbérc quarry. This type of alteration is very
weakly developed in the Újhatár Valley. During this stage,
plagioclase underwent intense sericitization, albitization
and saussuritization. Biotite was replaced by fine-grained
chlorite and clinozoisite (Fig. 3C), and titanite crystallized
along cleavage planes. The outer rims of amphibole and py-
roxene show chloritization, which can be accompanied by
pyritization. According to the classification of Hey (1954),

Table 4: Chemical composition of apatite from the Újhatár Valley (UH) and Tóbérc quarry (TOB) pegmatites. 


 phase hosting the apa-

tite inclusion (amph = amphibole, plag = plagioclase) 


 OH and H


O calculated on the basis of (F + Cl + OH) = 1 per formula unit; X


, X





= mole fraction of F, Cl, OH, respectively.

Fig. 5.  Amphibole compositions plotted in an Al


 versus (Na+K)


diagram.   – magmatic, 

 – deuteric,   – hydrothermal am-

phibole compositions.

chlorite corresponds to ripidolite and brunsvigite (Table 5).
Around the zoned brownish-greenish deuteric amphibole,
greenish-colourless amphibole also formed during the hy-
drothermal alteration. The early and late amphibole grains
exhibit sharp boundaries (Fig. 3A). Hydrothermal amphib-
ole also occurs as euhedral, wedge-shaped crystals on the
surface of earlier amphibole grains. The chemical composi-
tions of these hydrothermal amphiboles correspond to ferro-
actinolite and grunerite (Table 3). They are also commonly
intergrown with chlorite and overgrown by more ferro-

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Table 5:  Chemical composition and calculated formation tempera-
ture (T) of chlorite from the Újhatár Valley (UH) and Tóbérc quar-
ry (TOB). (Chlorite in vug; replacing amphibole, biotite or
plagioclase) (Temperature in 

ºC, K+ML: according to Kranidiotis

& MacLean 1987; Z + F: according to Zang & Fyfe 1995).

actinolitic amphibole (TOB19-4 in Table 3). This dark
green (even more Fe-rich) fibrous ferro-actinolite is
intergrown with newly formed albite (Fig. 3A), which also
forms fresh rims around the altered primary plagioclase. The
hydrothermal amphibole is devoid of fluorine, and is char-
acterized by variable chlorine content up to 0.18 wt. %
(Table 3 and Fig. 4). The Al


 vs. (Na + K)


 diagram of mag-

matic, deuteric and hydrothermal amphibole generations
(Fig. 5) shows the “pargasitic trend” which has been de-
scribed from recent and ancient ocean-floor gabbros
(Prichard & Cann 1982; Ito & Anderson 1983; Mével 1988;
Talbi et al. 1999). The albite has a minor anorthite compo-
nent (An


) and its Or content is up to 4.2 mol % (Table 2).

Titanite formed around ilmenite lamellae at this stage of al-
teration (Sadek et al. 1996).

In the Újhatár Valley, the sea-floor hydrothermal alter-

ation is manifested by sericitic alteration of plagioclase.
Newly formed albite and amphibole generations are spo-
radic. Alteration of biotite is the same as described above.

The third stage of alteration can be related to the Alpine

low-grade metamorphic event and can thus be observed in
the whole SOC. The prehnite-pumpellyite facies mineral-
ization (Sadek et al. 1996) appears mostly in veinlets. The

main phases in the veinlets are prehnite, calcite and
quartz. These minerals are intergrown with fine-grained
chlorite (Fig. 3D), corresponding to ripidolite (Table 5).
The metamorphic assemblage locally replaces plagio-
clase. Deformation related to the low-grade metamor-
phism is widespread, and locally resulted in formation of
cataclastic rock fabrics. Ductile deformation can also be
observed as kink bands (Fig. 3A) in amphibole and fold-
ing of the chlorite pseudomorphs after biotite.

Mineral thermometry

Formation conditions of the pegmatites are difficult to

determine because of the intense alteration of most of the
primary phases. Remnants of fresh coexisting amphibole
and plagioclase from the Újhatár Valley yielded formation
temperatures of about 800—850 ºC (Holland & Blundy
1994). This temperature range appears to be geologically
acceptable; however, the use of this geothermometer is
not recommended for such acid plagioclase. The
semiquantitative, Al- and Ti-in-amphibole thermometer of
Ernst & Liu (1998) yielded temperatures of 900—950 ºC
and 880 ºC for samples from the Újhatár Valley and the
Tóbérc quarry, respectively. Aigner-Torres (1996) estab-
lished formation temperatures of up to 930 ºC for the host
gabbro using the amphibole-plagioclase thermometer of
Holland & Blundy (1994). In general, the available data
indicate formation temperatures of the SOC pegmatites be-
tween 800 and 900 ºC.

Deuteric alteration took place immediately after forma-

tion of primary phases and the temperature was continu-
ously decreasing as indicated by the zoned amphibole.
The mineral assemblage, textural features, and the am-
phibole thermometer of Ernst & Liu (1998) indicate mag-
matic-submagmatic conditions and a temperature of
700—900 ºC.

The sea-floor hydrothermal assemblage corresponds to

greenschist facies conditions as was also pointed out by
Sadek et al. (1996). Similar phases were reported from
ocean-floor gabbros, which suffered hydrothermal alter-
ation of this grade (Ito & Anderson 1983; Talbi et al.
1999). The presence of actinolite instead of hornblende
confirms the temperatures below 400 ºC (Gillis 1995;
Talbi et al. 1999). The Al


-content of chlorite replacing

biotite and amphibole can also be used as a thermometer.
Chlorite thermometers of Cathelineau (1988) and
Kranidiotis & MacLean (1987) yielded almost similar
temperatures of 290—330 ºC, while the equation of Zang &
Fyfe (1995) always provided lower temperatures of about
210—260 ºC. As the thermometer of Cathelineau (1988) is
not recommended for such iron-rich chlorite, and accord-
ing to Sadek et al. (1996) the composition of chlorite in
SOC is dependent on bulk-rock compositions, care must
be exercised when interpreting the results of the chlorite

The Alpine regional metamorphism is characterized by

prehnite-pumpellyite facies conditions (Árkai 1983; Árkai
et al. 1995; Sadek et al. 1996), and a maximum tempera-

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Fig. 6. Frequency distribution diagram of homogenization temper-
atures (Th tot) for fluid inclusion types from pegmatitic quartz of
Tóbérc quarry.

ture of 250—300 ºC (Árkai et al. 1995). Chlorite thermom-
eters of Cathelineau (1988) and Kranidiotis & MacLean
(1987) yielded 325—340 ºC, while that of Zang & Fyfe
(1995) 250—260 ºC.

Fluid inclusion studies

Three types of fluid inclusions were distinguished on

the basis of their phase compositions and origin in pegma-
titic and hydrothermal quartz, as well as calcite.

Type I inclusions

 are secondary and occur in healed

cracks of pegmatitic quartz. These inclusions have vari-
able liquid to vapour ratios at room temperature (Fig. 3E
and F) from about 20—30 vol. % vapour phase to essen-
tially vapour-dominated inclusions. The variable volu-
metric phase ratios indicate boiling of their parent fluids.
The aqueous liquid-dominated inclusions homogenized
between 297 and 334 ºC (Fig. 6). The average salinity of
the aqueous phase corresponds to 3 NaCl equiv. wt. %
(Fig. 7).

Type II inclusions

 also occur along secondary frac-

tures in pegmatitic quartz (Fig. 3G), and exhibit con-
stant, 15—20 vol. % vapour phase at room temperature.
Homogenization to liquid occurred between 218 and
257 ºC (Fig. 6). The average salinity of these inclusions is
3.4 NaCl equiv. wt. %, but the scatter is larger than that for
the Type I fluid inclusions (Fig. 7).

Type III inclusions

 are primary in quartz and calcite from

prehnite-bearing veins, thus being indicative of the low
grade metamorphic overprint in the SOC. Type III inclu-
sions are isolated in quartz or form populations along
growth zones of calcite (Fig. 3H): accidentally trapped
chlorite flakes confirm a primary origin for these inclusions,
because the chlorite is deposited together with quartz and
calcite in late veinlets. The vapour phase occupies about
10 vol. % at room temperature. Homogenization of type III
inclusions took place between 159 and 187 ºC (Fig. 6), and
salinities ranged from 1.4 to 6.45 NaCl equiv. wt. % with an
average of 3.4 wt. % (Fig. 7).


Our observations reveal that orthopyroxene and olivine

occur in subordinate amounts, and Fe-Ti-oxides are com-
mon in gabbro of SOC. The differentiated character of the
SOC gabbro is also indicated by the mineral chemistry.
This study has shown many differences between differen-
tiation and the late crystallization products of the two
studied localities. At the Tóbérc quarry, strongly contami-
nated ferrogabbro occurs, which most probably formed in
the roof zone of a magma chamber. The interaction of the
melt with the enclosing sedimentary rocks is indicated by
assimilation of sedimentary xenoliths and occurrences of
contact rocks. These features provide evidence of reaction
between gabbro and the surrounding sedimentary rocks,
and suggest mobilization of volatiles and other elements
during emplacement of gabbro intrusions.

Fig. 7. Plot of homogenization temperature (Th tot) versus final
melting temperature of ice (Tm ice) for fluid inclusion types from
pegmatitic quartz of Tóbérc quarry. Black line represents final
melting temperature of ice for mean sea water. Salinities (NaCl
equiv. wt. %) calculated from final melting temperature of ice.

The original fluid contents of the various melts cannot

be established, but it is obvious that the formation of
pegmatites was promoted by the assimilated volatiles.
During crystallization, the melt became fluid-enriched,
and this process resulted in formation of different pegmati-
tic structures. The gabbro contains small patches where
the increased concentration of fluids is visualized by in-
creasing grain size of the rock-forming minerals. The peg-
matitic pockets indicate segregation of a hydrous melt,
which was enriched in fluids, volatiles (e.g. halogens), and
other incompatible elements. This is revealed by the fact
that pegmatites contain abundant hydrous silicates, apa-
tite and quartz. The major element composition of the
rock-forming phases and the abundance of Fe-Ti-oxides
show that iron-content increased in the pegmatites. Zon-

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ing of some pegmatitic pockets with a pyroxene-rich rim
and a quartz-rich core, accompanied by increasing grain
size indicates crystallization from margins to the core.
Similar zoned pegmatites have also been described from
orogenic gabbroic plutons (Lovering & Durrell 1959;
Beard & Day 1986). The homogeneous pegmatite pockets
have most probably crystallized simultaneously, and the
melt could not differentiate. Cooling and synchronous
tectonism of the crystallizing gabbro resulted in fracturing
of the rock, and the segregated hydrous melt could also
move along the fractures. This is shown by the “tails” of
the pegmatite pockets and occurrences of narrow pegmati-
tic dykes. The ductile deformation of the “tails” reveals
that formation of pockets occurred between solidus and
liquidus of the host gabbro. The straight strike, but diffuse
contacts of the narrow dykes show that the host gabbro
was brittle, but still hot enough to react with the hydrous
melt. The thick dykes with sharp contacts correspond to a
later differentiation stage, which is also supported by their
strongly evolved composition. In contrast with the former
pegmatites, their melt seems not to have been derived by a
local segregation. It is more probable that the parent melts
segregated in a different part of the crystallizing magma
chamber, and were intruded later into this rock unit.

Gabbroic rocks in the Újhatár Valley are highly

evolved, homogeneous fine-grained amphibole gabbros.
The parent melt may have originally been more differenti-
ated, but was not affected by assimilation. The evolved
character is confirmed by mineral composition and chem-
istry (amphibole instead of ortho- and clinopyroxene, al-
most pure primary albite). The absence of sedimentary
xenoliths and contact rocks indicate a greater distance
from the contact and/or a deeper position in the magma
chamber. The latter is also favoured by the absence of late
pegmatitic dykes and plagiogranitic intrusions. The
greater distance from the contact resulted in less extensive
assimilation and contamination of the melt, which could
explain the absence of quartz, and the lower halogen, and
rare earth contents, in comparison to Tóbérc. The origin of
fluids responsible for the formation of pegmatitic segrega-
tions was dominantly magmatic.

Exsolution of magmatic fluids is revealed by deuteric al-

teration in many gabbroic systems containing pegmatites
(Ballhaus & Stumpfl 1986; Beard & Day 1986; Watkinson
& Ohnenstetter 1992; Li et al. 2004). Separation of a fluid
phase during the formation of pegmatites at Szarvaskő is
evident by quartz-feldspar granophyric texture and the com-
position of apatite with F/(F + Cl) ratios of about 0.8—0.9.
During fractional crystallization of the melt, the ratio of F
versus Cl does not change; therefore this process cannot be
responsible for the formation of the fluorine-rich apatite
(Boudreau & McCallum 1989). In contrast, fluid separation
leads to enrichment of chlorine in the fluid phase, while
fluorine remains in the residual melt. Thus, apatite precipi-
tating from the melt is fluorine rich (Boudreau & McCallum
1989; Meurer & Boudreau 1996; Meurer & Natland 2001).
The segregated fluid could not be identified in our fluid in-
clusion studies, which is most probably due to decrepita-
tion of inclusions during later hydrothermal processes.

Fluid segregation accompanying pegmatite formation

resulted in deuteric alteration of minerals. Alteration of
magnetite, formation of biotite, replacement of pyroxene
by brown amphibole, and the formation of zoned amphib-
ole can be related to this process. Change of chemical
composition in amphibole indicates cooling from mag-
matic to sub-magmatic temperatures, where hornblende
was the stable phase. The high fluorine content of amphib-
oles, up to 0.14 wt. %, may be explained by magmatic
fluid segregation rather than by sea-floor hydrothermal
origin. Coogan et al.  (2001) distinguished magmatic and
sea-floor hydrothermal amphibole in the Mid-Atlantic
Ridge gabbros in a similar way.

According to studies on recent ocean-floor gabbro,

brittle deformation of rock accompanied by seawater in-
flux is significant below a temperature of about 800 ºC
(Ito & Anderson 1983; Talbi et al. 1999; Coogan et al.
2001). Temperature of the seawater-dominated hydrother-
mal systems depends on the depth and the distance from
the ridge axis. The character of alteration caused by this
hydrothermal process is further dependent on spreading
rate, cooling rate and the fracturing of rock (Ito & Ander-
son 1983; Mével 1988; Gillis 1995; Talbi et al. 1999).

Sea-floor hydrothermal alteration occurs in all rocks of

the SOC and was first described by Sadek et al. (1996). In
the Tóbérc quarry, our studies revealed a mineral assem-
blage corresponding to greenschist facies conditions and
temperatures below 400 ºC. The hydrothermal alteration
was a polyphase process, which is documented by sev-
eral generations of Fe-actinolite and grunerite. Intense
chloritization, sericitization and the formation of albite,
clinozoisite and titanite are also typical of this alter-
ation. Sea-floor hydrothermal alteration of the Újhatár
Valley rocks is less intense, which can be explained by a
greater original depth to the sea floor and/or less intense
fracturing of the rock body which hindered circulation of

In our fluid inclusion studies we could identify two

fluid generations (type I and II) responsible for the sea-
floor hydrothermal alteration. Both fluids have salinities
clustering around that of mean seawater ( ~ 3.2 %). The
slight differences may have been caused by fluid/rock in-
teractions. Similar boiling fluids to type I were found by
Vanko et al. (1992) in gabbro of the Mid-Atlantic Ridge.
Assuming a pure NaCl—H


O system, boiling of the fluid

should have occurred at around 7—8 MPa of pressure
(Zhang & Frantz 1987). If calculated under hydrostatic
pressure this would mean a depth of 800 m below sea
level. This shallow depth could be plausible for a sea-floor
in a back-arc basin environment; however, the presence of a
minor amount of CO


 that is undetectable by petrography

and microthermometry cannot be excluded (Hedenquist &
Henley 1984). Therefore the estimated paleodepth is the
minimum depth for entrapment of boiling fluids in second-
ary inclusions of pegmatitic quartz (which has certainly
formed at a greater depth). Fluids similar to our fluid type
II, were also reported from recent oceanic environments
(Vanko et al. 1992; Kelley & Früh-Green 2001). The mean
homogenization temperature of about 240 ºC gives a

background image



Table 6: Synthetic table summarizing main characteristics of magmatic, hydrothermal and metamorphic processes revealed in gabbro peg-
matites from the Tóbérc quarry. 


mean values.

minimum entrapment temperature, and a lower limit for
the sea-floor hydrothermal alteration.

Being primary in phases of the Alpine low-grade meta-

morphic veining, type III fluid inclusions are indicative of
that process. Considering the fact that the fine-grained
chlorite, calcite and quartz were growing together, chlorite
thermometry and isochores of fluid inclusions define en-
trapment conditions of the low grade metamorphic fluids
(Fig. 8), ranging between 270—285 ºC and 150—200 MPa.


Pegmatitic structures studied in two areas in the SOC

may be classified according to their textural and mineral-
ogical properties. Pegmatitic patches, pockets and narrow
dykes precipitated from a locally segregated hydrous melt,

Fig. 8.  Modeling of entrapment conditions for type III fluid inclu-
sions. Grey coloured area shows proposed entrapment conditions.
Isochores were calculated according to Zhang & Frantz (1987).

whereas thick and felsic dykes were intruded later. Homo-
geneous pegmatitic pods crystallized simultaneously in
contrast to zoned pods which indicate further differentia-
tion of the hydrous melt. Variation of the mineral assem-
blages and their chemical composition in pegmatite
bodies was also influenced by enrichment of volatiles and
incompatible elements in the melt due to interaction with
the host sedimentary rocks. The formation temperatures of
the pegmatites are difficult to determine, but their crystal-
lization most probably occurred between 800 and 900 ºC
(Table 6).

During crystallization of pegmatites a fluid phase sepa-

rated, as indicated by granophyric textures and apatite com-
positions. This magmatic fluid caused deuteric alteration of
the primary pegmatitic assemblage, which is revealed by al-
teration of magnetite and pyroxene, accompanied by forma-
tion of biotite and zoned amphibole. This process took
place under continously cooling magmatic-submagmatic

The pegmatites and host gabbro underwent postmagm-

atic alteration due to sea-floor hydrothermal activity: the
responsible fluids were identified in two fluid inclusion
generations. Typical of this process is alteration of all pri-
mary phases and formation of Fe-actinolite, grunerite,
chlorite, clinozoisite and albite. Mineral assemblage, min-
eral- and fluid-inclusion thermometry indicate a polyphase
hydrothermal process between 250 and 400 ºC.

The Alpine regional metamorphism caused brittle and

ductile deformation of the rocks discernible at outcrop- and
microscopic scales. This deformation was accompanied by
intense veining of a prehnite-chlorite-quartz-calcite-feld-
spar assemblage. This low-grade Alpine metamorphic over-
print occurred at around 270—285 ºC at 150—200 MPa.


 This work was supported by the NSF

(OTKA) No. T 049633 to F. Molnár and by the Cana-

background image



dian-Hungarian (CAN-2/04) and Croatian-Hungarian
(HR-17/2004) Science and Technology Cooperation,
both funded by the Technological and Innovation Fund,
Hungary. Some funding from NSERC Grant A7874 to D.H.
Watkinson is acknowledged. Authors express their thanks
to Prof Friedrich Koller, University of Vienna for valuable
discussions and suggestions. Significant improvements to
the manuscript resulted from critical reviews by Dr.
Vratislav Hurai and two anonymous referees.


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