GeolCarp_Vol52_No1_49

GEOLOGICA CARPATHICA, 52, 1, BRATISLAVA, FEBRUARY 2001

49 — 60

U-Pb ZIRCON AGES OF DIORITIC AND TRONDHJEMITIC ROCKS

FROM A LAYERED AMPHIBOLITIC COMPLEX CROSSCUT

BY GRANITE VEIN (VEPORIC BASEMENT, WESTERN CARPATHIANS)

MARIÁN PUTIŠ

ALEXANDER B. KOTOV

, SERGEI P. KORIKOVSKY

EKATHERINA B. SALNIKOVA

, SONYA Z. YAKOVLEVA

, NATALYA G. BEREZHNAYA

VICTOR P. KOVACH

and JULIA V. PLOTKINA

Department of Mineralogy and Petrology, Faculty of Science, Comenius University

Mlynská dolina, SK-84215 Bratislava, Slovak Republic; putis@fns.uniba.sk

Russian Academy of Sciences, Institute of Precambrian Geology and Geochronology, Makarov emb. 2,

RF-199034 St. Petersburg, Russian Federation; kotov@ad.iggp.ras.spb.ru

Russian Academy of Sciences, Institute of Geology of Ore Deposits, Petrography, Mineralogy and Geochemistry,

Staromonetny per. 35, RF-109017 Moscow, Russian Federation; korik@igem.ru

(Manuscript received March 12, 2000; accepted in revised form October 17, 2000)

Abstract:  The  complex  of  Ky-Grt  paragneisses,  granitic  to  tonalitic  orthogneisses,  migmatitic  gneisses  and  homoge-
neous to layered amphibolites was intruded by dioritic-gabbroic more or less concordant dykes and sills during the syn-
metamorphic extension in the host regional metamorphic rocks. They now have the character of dioritic orthogneisses.
The  whole  lithological  sequence  was  then  structurally  unified  during  the  late-metamorphic  cooling  and  exhumation
within the deep-crustal shear zone. The dioritic orthogneisses bear the features of pre-metamorphic compositional layer-
ing  into  cumulate-like  hornblendite(± Px),  gabbro-diorite,  tonalite  to  trondhjemite.  Magmatic  layering  has  been  trans-
formed to subsolidus high-temperature layering along the extensional meso-shear bands filled with leucotonalitic melt
seggregates  crosscutting  former  magmatic  structures  at  acute  to  medium  angles.  A  new  ductile  strain  relocalization
enhanced mechanical differentiation of dark and felsic minerals into often isoclinally folded layers, with the crystallo-
graphic  preferred  orientation  fabrics  of  amphibole,  plagioclase  and  quartz  indicating  a  layer  parallel  shear.  The  U-Pb
zircon age of porphyric metadiorites 346 ± 1 Ma (Early Carboniferous) is related to their magmatic emplacement, imme-
diately followed by the high-temperature mylonitization. We also dated zircon of metatrondhjemitic orthogneiss from
the  layered  amphibolite.  An  upper  intercept  age  514 ± 24  Ma  (Late  Cambrian)  is  interpreted  as  dating  the  magmatic
compositional differentiation of a gabbro-dioritic complex into cumulate-like hornblenditic, gabbrodioritic, tonalitic to
trondhjemitic layers, which is an inherited feature in layered amphibolites (with blastic textures). The lower intercept
age  348 ± 31  Ma  (Early  Carboniferous)  obviously  reflects  the  time  of  regional-metamorphic  event  and  formation  of
layered  amphibolites.  The  whole  composite  (VZP-CB)  structural  complex  is  crosscut  by  plagiogranite-aplitic  veins
dated 233 ± 4 Ma (Early Triassic).

Key words: Western Carpathians, Veporic basement, layered metamagmatites and amphibolites, U-Pb zircon isotope
dating.

Introduction

The pre-Alpine basement of the Western Carpathians  in Pre-
Mesozoic  times  belonged  to  the  Southern  European
Variscides, which are now partly incorporated into the Alpid-
ic  Orogen.  Whereas  the  western  Variscides  formed  in  the
Early  Carboniferous  (at  ca.  340  Ma)  following  the  Protot-
ethys-Rheic closure (Matte 1991), the gradual closing of the
relic Paleotethys governed the continuing Late-Variscan col-
lision in the Southeastern Variscides until the Carboniferous-
Permian  boundary  (at  ca.  300  Ma)  (Stampfli  1996).  This
event  directly  concerns  the  Austroalpine-Carpathian  base-
ment  (Putiš  &  Grecula  in  Plašienka  et  al.  1997:  Fig.  6).
Moreover, the northward subducting Paleotethys opened the
Meliata back-arc oceanic basin in the Middle to Late Triassic
thus starting the Early Alpine evolution of the Western Car-
pathians.

The purpose of the paper is to present new results of U-Pb

zircon isotope dating of two members of a layered metamor-

phic complex (metadiorites and metatrondhjemites) and cross-
cuting  plagiogranite-aplitic  vein,  from  the  Veporic  basement
of the central Western Carpathians. The results are expected to
reveal the age relationship between felsic (metatrondhjemitic
to metatonalitic) and intermediate-basic (dioritic-gabbrodiorit-
ic) bands of layered metamagmatites in the Veporic crystalline
basement. The dating of the rocks that belong to the leptynite-
amphibolite  structural  complex  (LAC,  Hovorka  &  Méres
1993)  follows  detailed  geological  and  structural-petrological
studies performed in the metamagmatic-amphibolitic suite of
the Veporic basement area (Krist et al. 1992; Putiš et al. 1996,
1997).

There are differing views on the age of the Western Car-

pathian basement reviewed, for example by Cambel et al.
(1990) or Krist et al. (1992), however this is the first work
trying to date this layered metamorphic complex (by the U-
Pb zircon method). Because the reported age data on the
basement rocks concern different tectonostratigraphical lev-
els, the latter briefly outlined in the next paragraph.

50 PUTIŠ et al.

Position of the LAC in the basement evolution

of the Western Carpathians

The basement complexes only crop out in the central part of

the Western Carpathians. They are exposed in the Tatric, Ve-
poric and Gemeric mid-Cretaceous tectonic zones. The com-
plete basement characteristics was compiled by Krist et al.
(1992). The basement consists of either medium- to high-
grade crystalline complexes present in the Tatricum and Ve-
poricum (with a minor extent of low-grade rocks in the Malé
Karpaty Mts. and northern and southern Veporicum); or pre-
dominant low-grade Early Paleozoic complexes in the Gemer-

icum. In general, horizontal zonation of the basement com-
plexes, and even their vertical tectonostratigraphy, remained as
features of the Variscan southward progressing orogeny in the
territory of the ancestral Western Carpathians.

Tatric and Veporic crystalline basement with the LAC

The upper part of the basement tectonostratigraphy (Putiš

1992; Bezák 1994; Putiš in Plašienka et al. 1997) consists of
medium- to high grade metamorphic rocks such as paragneiss-
es to migmatitic gneisses, migmatites, granitic orthogneisses,
amphibolites, rare calc-silicate rocks, intruded by Variscan

Fig.  1.  Geological-tectonic  sketch  map  of  pre-Tertiary  basement  and  cover  complexes  of  the  Low  Tatra  Mts.–E  (Ďumbier—Krá ova  Ho a
Mts. range) and the Slovenské rudohorie Mts.–NW (Fabova Ho a Massif) (after Putiš 1994, modified). 1 – Tatric cover rocks: Lower Tri-
assic  shales  and  quartzites;  Middle  Triassic  limestones  and  dolostones;  2  –  Supratatric  (Permo-Scythian)  anchi-metamorphic  cover;  3  –
North(?)-Veporic Permian (Predná Ho a) volcano-sedimentary complex (low-grade metamorphic rocks: 3—6); 4 – North-Veporic Permian
(Jánov  Grúň)  volcano-sedimentary  cover  complex;  5  –  North-Veporic  Triassic  cover  rocks;  6  –  South-Veporic  Permian-Triassic  cover
rocks (Permian arkoses, shales, bimodal basic-acidic volcanics, Lower Triassic quartzites); 7 – Pre-Alpine Tatra crystalline complex; 8 –

ubietová orthogneiss succession of the Tatra crystalline complex; 9 – Late Variscan tonalites (with the Supratatric cover (2), Vápenica

Nappe, Putiš 1989); 10 – Pre-Alpine Hron crystalline complex; 11 – Pre-Alpine composite Čierny Balog (CB)—Ve ký Zelený Potok(VZP)
crystalline complex: a – layered amphibolite, b – para- and orthogneiss, c – serpentinite; 12 – Late Variscan granitoids of the Vepor plu-
ton, with metamorphic gneisseous mantle; 13 – Alpine (Cretaceous) mylonitic schists and phyllites of the Krak ová Formation (Upper Car-
boniferous?); 14 – thrust of higher (non-metamorphosed) Mesozoic nappes; 15 – thrust of anchi- to non-metamorphosed rocks of the
Krížna Nappe; 16 – Late Cretaceous back-thrust of the Supratatric over the North-Veporic structural unit; 17 – mid-Cretaceous thrust; 18
– reverse fault; 19 – a relic of a Variscan thrust-fault; 20 – fault; 21 – primary geological boundary; HLO (Hlôšková Valley), VZP
(Ve ký Zelený Potok Valley), PP (Pohronská Polhora village) and PE (Petríkovo Valley), with location of dated samples.

U-Pb ZIRCON AGES OF DIORITIC AND TRONDHJEMITIC ROCKS 51

granitoids. But the lower part is composed of metabasic rocks
(amphibolites, serpentinites, metagabbros, metadiorites). Both
lithological complexes form the Variscan Upper (Tatra) Nappe
(Putiš 1992). It is thrust over the micaschists to gneisses and
rare amphibolites of the Lower (Hron) Nappe. Both nappes
form the major part of the Tatro-Veporic basement. Mylonitic
granite-gneisses bound to this major ductile thrust-fault show
transitions from originally magmatic to deformational-mylo-
nitic fabrics.

The plutonic granitoids from the above mentioned tectonos-

tratigraphical level were dated in the time interval ca. 360—330
Ma (Cambel et al. 1990, U-Pb, Rb-Sr and K-Ar; Krá et al.
1997,

207

Pb-

206

Pb). Granitic orthogneisses dated by U-Pb

method on zircon from the Tatra Mts (380—405 Ma, Janák et
al. 1996), Tribeč Mts (415 Ma, Krist et al. 1992) and our un-
published data (ca. 383 Ma, Kotov et al. in prep.) from the
Low Tatra Mts point to an older Early Variscan phase grani-
toid magmatism in the Tatric basement. Such orthogneisseous
magmatics although also present in the Veporic basement have
not been dated (Kotov et al. in preparation). There, on the oth-
er hand the final stage of the Variscan granitoid magmatism
was dated in the time interval of 303—295 Ma (Bibikova et al.
1990, U-Pb, Rb-Sr, K-Ar), reviewed e.g. by Petrík & Kohút
(1997).

The basis (or sole) of the Upper (Tatra) Nappe consists of

metabasic rocks such as amphibolites, serpentinites, metagab-
bros, forming a suite resembling the continental lower crust.
This group of rocks is generally ascribed to ”Leptynite-am-
phibolite” Complex (LAC, Hovorka & Méres 1993). There are
no radiometric data from this complex.

Because the group of the LAC comprises lithologically vari-

able structural complexes (Spišiak & Pitoňák 1990, 1992; Ho-
vorka  &  Méres  1993;  Janák  et  al.  1996;  Janák  &  Lupták
1997), the one present in the North- and Central-Veporic re-
gion was defined as the Ve ký Zelený Potok Complex (Putiš et
al. 1996, 1997) from the lithological point of view. But for the
general  considerations  we  recommend  using  the  term  lep-
tynite-amphibolite structural complex, or in short ”LAC”. Ver-
tical cross-section of the LAC (Fig. 2) at the basis of the su-
pracrustal  (Čierny  Balog)  metamorphic  complex  in  the
Veporic basement comprises: amphibolites – partially melt-
ed,  metadiorites,  metatonalites,  metatrondhjemites  as  meta-
morphosed different layers of originally gabbro-dioritic mag-
matic  suite  (Hlôšková  Valley  area,  HLO);  ultramafic
metacumulates  (Cpx  metahornblendites)  within  the  massive
porphyric to schistose and layered dioritic amphibolites cross-
cut  by  parallel  to  discordant  medium-  to  coarse-grained
trondhjemitic veins (Ve ký Zelený Potok area, VZP), and the
host  Grt  paragneisses  (with  WhM  pseudomorphs  after  Ky?)
with thin amphibolitic layers. The age of the host metapelitic-
metabasic rocks is unknown, so we dated zircon of porphyric
metadiorite (VZP) as well as metatrondhjemitic layer (HLO)
in layered amphibolites.

The underlying supracrustal metamorphic complex of the

Lower (Hron) Nappe is dominated by medium grade St-Ky-
Grt-bearing micaschists, gneisses and rare amphibolites. It oc-
curs in a few tectonic windows all over the Tatric and North-
Veporic zones, below the supracrustal complexes of the Upper
(Tatra) Nappe (Fig. 1).

Gemeric Early Paleozoic basement with the LAC

The Early Paleozoic complexes of the Gemericum show

Late Variscan, Late-Carboniferous collisional accretion to the
Variscan nappe pile characterized in the Tatro-Veporic do-
main. This collisional structure also includes Early to Late
Carboniferous flysch sequences (Putiš & Grecula in Plašienka
et al. 1997: Fig. 6).

The Gemeric basement consists of three principal Early Pa-

leozoic lithological and structural complexes: the Gelnica, Ra-
kovec and Klátov complexes.

The Gelnica flyschoid Complex (Cambrian to Lower Devo-

nian in age) comes from the northern margin of a Gondwana-
related (or Panafrican) continental fragment. The Rakovec
(Devonian-Lower Carboniferous) Complex represents a back-
arc basin oceanic crust (Ivan 1997), and the Klátov Complex
(Hovorka et al. 1984) with gneisses, layered amphibolites and
Atg serpentinites is derived from continental lower crust. The
Klátov Complex belongs to a group of LA structural complex-
es here overlying the predominant low-grade structural com-
plexes.

The Gelnica Complex is dominated by thick flyschoid se-

quence  composed  of  pelites,  graywackes,  intercalated  with
black  laminated  pelites,  silicites  and  carbonates.  The  upper
part is built of acid to intermediate volcanic and volcaniclastic
rocks (mainly rhyolites, dacites, andesites). Fossiliferous lami-
nated black pelites contain foraminifers and graptolites point-
ing to Cambrian(?) Ordovician—Early Silurian age (Vozárová
et  al.  1998).  Findings  of  marine  sporomophs  in  the  higher
shale horizons point to their Late Silurian—Early Devonian age
(Snopková & Snopko 1979).

The Rakovec Complex (Middle Devonian-Lower Carbonif-

erous) is interpreted as an oceanic crustal sequence of a back-
arc basin type. It is composed of basalts, pillow basalts, doler-
itic dykes, gabbros and laminated black shales. This complex
underwent metamorphism in a northward subduction zone to-
gether with the Klátov (LA) Complex and some Late Carbon-
iferous sedimentary-magmatic rocks. Their obducted medium-
to high-pressure and low-temperature metamorphic members
(Radvanec 1998) are discordantly overlain by transgressive
Permian cover.

The geological setting and characteristic features of

the layered metamagmatic-amphibolitic rocks of the

Veporic basement

The layered metamagmatic-amphibolitic rocks of the LA

structural  complex  are  also  the  predominant  members  in  the
local Ve ký Zelený Potok complex (=VZP, Putiš et al. 1996,
1997)  within  the  Veporic  basement  of  the  Central  Western
Carpathians (Fig. 1). The complex is regionally widespread in
the northwestern part of the Slovenské Rudohorie Mts and the
eastern part of the Low Tatra Mts. The studied complex was
originally  named  as  the  gabbro-peridotite-basalt  formation
(Miko & Putiš 1989 in Krist et al. 1992: Fig. 74) and later in-
cluded in the leptynite-amphibolite complex (LAC, Hovorka
& Méres 1993). However, there are some lithological differ-
ences  pointing  to  variability  of  the  discussed  LA  structural

52 PUTIŠ et al.

complex. This can be documented, for example, by the charac-
teristic occurrences of metadioritic members of the VZP litho-
logical complex (Putiš et al. 1996). Concerning the choice of
the collected and dated samples, they represent two partial ar-
eas of the LA structural complex with the occurrence of lay-
ered rocks: metadiorites with the transitional structures to lay-
ered amphibolites in the Ve ký Zelený Potok (=VZP) Valley,
and metaleucotonalitic or metatrondhjemitic layers of banded
amphibolites in the Hlôšková (=HLO) Valley (Figs. 1, 2).

A peculiar lithological member of both areas appears to be

”layered amphibolite” (Figs. 3, 4) thus representing a charac-
teristic  pre-Alpine  lithological  feature  of  the  LA  Complex.
The domains without superimposed ductile deformation show
different types of layered amphibolites. One type is related to
differentiated  original  gabbro-diorite  suite  metamorphosed
into amphibolites and predominates in the HLO area (Fig. 4).
In  the  second  (VZP)  area,  there  are  layered  amphibolites
bound to individual metadioritic bodies (Fig. 3), which are in-
ternally differentiated too, showing structures of pre-metamor-
phic  compositional  layering  with  a  few  decimetre  thick
(meta)tonalitic  to  (meta)trondhjemitic  layers  alternating  with
amphibole-rich dark layers of (meta)gabbro-dioritic to (meta)-
gabbroic and Pl-free Cpx-bearing hornblenditic (meta)cumula-
tes in composition (unpublished chemical analyses of the au-
thors, in prep.). Layered metadiorites with textures of banded
amphibolites occur directly in homogeneous metadiorites with

observable transitional structures between them. Thus, at least
part of the layered amphibolites appears to have a magmatic
origin and comes from compositionally differentiated and then
mylonitized mostly dioritic protoliths. We interprete both do-
mains to indicate differentiated gabbro-dioritic magmatic
melts or small dyke-sill intrusions emplaced into continental
lower crust.

The final development of ductile straight bands is inferred

to be connected with the formation of low-angle extensional
faulting during the Late Variscan orogenic collapse and ex-
tensional exhumation of individual basement fragments. The
whole LA Complex, after ductile deformation, is about 250
m thick. It is cut by pre-orogenic A-type leucocrate granitic
to pegmatitic veins, as well as volcanic and subvolcanic bod-
ies (278—216 Ma, U-Pb zircon dating, Kotov et al. 1996; Put-
iš et al. 2000) due to Early Alpine continental rifting in the
wider area around the LA structural complex. We dated one
of  the  leucocratic  dykes  (a  few  dm  to  2  m  thick)  cutting  at
high angle metamorphic foliation of metadiorites in the VZP
Valley. They seem to be filling extensional cross joints (ob-
servable in long horizontal and vertical distance), when the
sills  (parallel  to  metamorphic  foliation)  observable  in  the
neighbouring  large  wall  outcrops  were  horizontally  boudi-
naged.

Some Hbl-Ath-Chl-bearing serpentinite lens-shaped bodies,

a few tens of metres thick, accompany the basis of the LA

Fig. 2. Schematic position of the Variscan layered metamorphic complex in the Alpine structure of the central Veporic basement (after
Putiš et al. 1997). Radiometric data after Bibikova et al. (1990), Kotov et al. (1996) and those being published.

U-Pb ZIRCON AGES OF DIORITIC AND TRONDHJEMITIC ROCKS 53

structural complex, but part of them was tectonically pushed
into the underlying micaschist-gneisses of the pre-Alpine su-
pracrustal (Hron) complex.

The pre-Alpine metamorphic conditions of the LA structural

complex in the VZP area were estimated at 610 °C and 600—
700 MPa of P (Filová 1997), while in the HLO area they
achieved 630—680 °C at the 700—900 MPa of P (Korikovsky
et al., in prep.).

P-T conditions and zonation of the superimposed Alpine

deformation and recrystallization (T

max

= 500—520 °C at

max

= 750—900 MPa) were estimated from the different

lithological  members  of  the  basement  and/or  the  cover
complexes in the hanging wall of the Cretaceous (Pohorelá)
thrust-fault which enhanced Alpine exhumation of the stud-
ied  rocks  in  the  VZP  domain.  An  association  of  Act—
Hbl + Ab + Grt + Phe + Bt + Ep + Chl + Ttn + Chl in metabasites,
Qtz + Ms—Phe + Ab + Grt(Sps—Grs-rich) + Bt + Kfs  in  granitic
orthogneisses,  Qtz + Bt + Ms—Phe + Ep + Ttn + Chl + Ab  in  to-
nalitic  orthogneisses,  Qtz + Ms—Phe + Pg + Ab + Grt + Cld  in

micaschist-gneisses,  Cal + Dol + Phl + Phe ± Ep ± Ab ± Qtz  in
marbles (Putiš et al. 1997; Korikovsky et al. 1997) is present
around the dated VZP locality. This knowlegde is compatible
with the resetting of the K-Ar system in amphiboles (Krá  et
al.  1996;  Kováčik  et  al.  1996),  especially  in  some  sheared
South-Veporic  domains,  also  indicating  the  minimum  tem-
peratures  of  about  500  °C  during  the  Alpine  tectonometa-
morphic  reactivation.  However,  the  estimated  metamorphic
conditions of the Alpine basement reactivation in the consid-
ered (HLO and VZP) areas could not thus reset the pre-Al-
pine zircon ages.

Petrography of the dated samples

Porphyric metadiorite, s. VZP-51A

Metadiorite is a massive rock with clearly preserved por-

phyric texture visible by subparalelly oriented Pl laths in me-

Fig. 3. A—D: Magmatic and deformational structures of originally porphyric diorites. Ve ký Zelený Potok Valley (VZP). Scale bar in fig.
A is approximately the same as in B. A – Relics of magmatic foliation indicated by alternating layers of more or less porphyric (A) to al-
most homogeneous metadiorite (B). Surface of foliation of dioritic orthogneiss with linearly arranged plagioclase laths indicating inherit-
ed magmatic lineation. B – Superimposed mylonitic structures of dioritic orthogneisses – mylonites. Subsolidus extensional shear band
filled in by residual Pl-Qtz melt seggregates of trondhjemitic composition. The upper right corner indicating high-temperature (subsoli-
dus) layering of originally tiny-porphyric or homogeneous metadiorite. C – Dominated subsolidus layered structures with narrow relo-
calized ductile shear bands in originally diorite body, with lense-shaped metahornblendites (above the hammer). Together with the inter-
layered isoclinal recumbent folds they indicate a layer-parallel shear. D – The outcrop with dated porphyric metadiorite (sample
VZP-51A) crosscut by dated plagiogranite-aplitic vein (sample VZP-50A) in the Ve ký Zelený Potok Valley (VZP).

54 PUTIŠ et al.

dium-grained Am-Pl-Bt matrix (Fig. 3A). It was completely
recrystallized under amphibolite facies conditions, identical
with the associations of surrounding rocks. Large prismatic Pl
phenocrysts are replaced by oligoclase-andesines, and the ma-
trix was transformed into the metamorphic association of Hbl
+ Bt + Czo + Olg + Qtz. The primary magmatic minerals are zir-
con, allanite and probably titanite(1). The high degree of mag-
matic texture preservation (magmatic foliation and lineation)
and the currently full metamorphic recrystallization of the rock
could indicate that the emplacement of porphyric diorite took
place in a metamorphic environment.

Undifferentiated dioritic parts have a composition which

corresponds to the average of the dark (Hbl, Bt, Pl, Ep, Ttn,
Qtz) and pale (Pl, Qtz, Mag, ± Hbl, ± Bt) bands. The preserved
primary magmatic structures such as (magmatic) composition-
al layering are seen by straight more or less sharp boundaries
between a few dm thick layers of diorites, porphyric diorites,
meladiorites, tonalites and trondhjemites. There are also utra-
mafic layers and lenses mostly of hornblendites, sometimes
containing metamorphic clinopyroxene. The very high-tem-
perature subsolidus stage of layering is proved by extensional
shear bands filled in by pale (trondhjemitic) residual melt seg-
gregates (Fig. 3B) sometimes having the character of tonalitic
pegmatitoids.

All the layers have metamorphic-deformational or mylonitic

fabrics, superimposed on the magmatic layering. The sharp
boundaries of the neighbouring lithological members of com-
positionally layered magmatic bodies are a very distinct mac-
roscopic feature also due to following layer parallel thinning
and stretching. Continuing medium-T solid state deformation
led to formation of newer very thin linear parallel bands with
changing mineral grain size, pointing to strong ductile defor-
mation and recrystallization at conditions within a deep-crust-
al shear zone. An older magmatic and metamorphic layering
was thus overprinted by the mechanical layering. The ductile
deformation and recrystallization in subsolidus and solidus
conditions was accompanied by the continuing differentiation
of mafic (Am, ± Pl) and felsic (Pl and Qtz) minerals into thin
layers. Some mesoscopic domains can be used as evidence for
the layer parallel extension accompanied by the development
of extensional shear bands, interlayer isoclinal mesofolds and
boudinaged lenses of more competent metahornblenditic (cu-
mulate-like) layers (Fig. 3C).

Characteristic mesostructures – symmetrical boudins of

competent gabbrodioritic-hornblenditic layers in both XZ and
YZ planes reflect the pure shear regime of deformation in the
initial stage of extension at an increased heat flow. This is re-
flected by superimposed higher-T recrystallization of Hbl1
generation. Brown-green to green Am1 of Mg-Hbl to Ts-Hbl
composition is replaced by blue-green Al-rich Ts to Fe-Ts at
the rims, or Am1 is broken down into symplectitic aggregate
of Ts + Qtz. This process is easily discernible, for example, in
ultramylonitic amphibolite layers, where the minor Hbl1 por-
phyroclasts are almost entirely replaced by higher-temperature
Ts + Qtz aggregates, or by fine-grained Ts.

The microstructures comprise Qtz ribbons surrounded by

tiny-grained aggregates of Pl and Ts. Symmetrical crystalo-
graphic preferred orientation patterns of Qtz ribbons subgrains
confirm the pure shear regime of mylonitic deformation and

recrystallization in the first stage of the uplift. Later, and at a
higher structural level, the deformation transformed to asym-
metric ductile- and ductile-brittle regime (Putiš et al. 1996,
1997).

These rocks were partly recrystallized (”diaphthorized”)

during the Alpine cycle: green hornblende is surrounded by
actinolite or Chl-Ep rim, Pl is replaced by Ab-Phe-Czo aggre-
gate, and Bt by Chl + Phe + Lcx.

The dated rock was collected from a medium-size outcrop

(Fig. 3D) in the right-hand side of upper Ve ký Zelený Potok
Valley (Fig. 1) at the forest road.

Metatrondhjemite from the layered amphibolite, s. VBNT-76L

The dated metatrondhjemites are completely recrystallized

bands  (layers)  of  felsic  magmatic  rock  in  host  massive  and
compositionally  differentiated  original  gabbro-dioritic  com-
plex, now banded (layered) amphibolites (Fig. 4A). The min-
eral composition varies from dark Am-Pl rich metagabbroic-
metadioritic  layers  to  dark-grey  dioritic  (Fig.  4B),  pale-grey
tonalitic to white Pl-Qtz rich metatrondhjemitic ones. Magne-
tite enrichment of some metatrondhjemite layers can reflect a
primary  enrichment  in  magnetite  characteristic  of  layered
magmatic complexes (Parsons 1987; Percival et al. 1992; Hall
1996). So we supposed that the pre-metamorphic protolith was
a  compositionally  differentiated  mafic  magmatic  complex
(Cawthorn 1996).

The superimposed metamorphic process respected existing

pre-metamorphic or synmagmatic layered fabrics of composi-
tional  layering.  All  minerals  of  magmatic  melt  origin  were
completely  replaced  by  metamorphic  association  of  Hbl + Pl
(25—30  %  An) + Grt + Ttn + Mag(Ilm) + Qtz ± Bt  which  corre-
sponds to the amphibolite facies. The compositional layers are
characterized by the metamorphic microstructures with mostly
sharp boundaries between the layers.

The special domains of amphibolitic complex correspond to

narrow (a few dm) zones of layer parallel shear, which might
have been supplied by metamorphic fluids and consequently
a restricted initial partial melting of mostly homogeneous am-
phibolites could occur. Transitional macrostructures from al-
most homogeneous to distinctly layered domains are observ-
able (Fig. 4C). They are clearly synmetamorphic, according
to  stretched  intralayer  folds,  often  with  separated  or  sheared
off  cores  filled  in  by  pale  fine-  to  medium-grained  (Qtz-Pl)
leucosomatic  seggregate  with  still  preserved  magmatic  mi-
crostructure.

The newest members of the layered amphibolites appear to

be medium-grained (Fig. 4D) to coarse-grained pegmatitoids
and  Czo-Chl-Ab-Qtz  veins.  They  locally  enclose  the  host
folded  amphibolitic  structure  and  seem  to  be  at  least  late-
metamorphic.  The  latter  lack  metamorphic  or  granoblastic
microstructures,  although  these  are  characteristic  of  the
group  of  meta-trondhjemitic  or  meta-leucotonalic  layers  of
magmatic and pre-metamorphic origin. The pale seggregates
penetrate along the boundaries of the metamorphosed layers
and cause their reactivation by a redistribution of mafic and
felsic minerals (often coarse-grained by additional recrystal-
lization)  accompanied  by  forming  dark  melanosome-like
rims (Fig. 4D).

U-Pb ZIRCON AGES OF DIORITIC AND TRONDHJEMITIC ROCKS 55

Leucocratic bands of metatrondhjemites show sharp bound-

aries with the host amphibolites or the neighbouring layers of
different  composition  that  is  an  inherited  structural  feature
from the pre-metamorphic compositional layering of a mafic
magmatic  complex.  According  to  chemical  analysis  (unpub-
lished  data  of  authors,  in  prep.)  such  pale  to  white-coloured
rocks correspond to leuco-tonalite, that is trondhjemite (after
O’Connor 1965).

The dated metatrondhjemitic rock was collected from a few

dm thick layer in amphibolite exposed in a large outcrop on
the left-hand side of the upper Hlôšková Valley, ca. 50 m
above the brook (Fig. 1).

Leucocratic plagiogranite-aplitic vein, s. VZP-50A

This rock forms small cross-cutting veins in the porphyric

metadiorite (VZP-51A), and it does not exhibit any signs of
the medium-temperature metamorphism. The plagiogranite
has an aplitic texture, but it is strongly recrystallized under
subsolidus conditions (appearance of coarse-grained Ms, and
Ab veinlets). Bt is absent. The products of the low-tempera-
ture alteration of Pl are represented by fine-grained Phe, or
Ab-Phe-Czo aggregate.

Fig. 4. A—C: Layered amphibolites from the Hlôšková Valley. Scale bar = 2 cm in Figs. A, C and D. Scale bar = 1 cm in Fig. B. A – The
inherited magmatic compositional layering in layered amphibolite. A 0.5 m thick metatrondhjemitic layer (sample VBNT-76L) rich in
magnetite, as in the narrow layer A (the black dots are magnetite crystals) was collected for the U-Pb dating. B – dark metadioritic layer
of layered amphibolite. C – Anatectic (ultrametamorphic) differentiation of Pl-Qtz pale seggregates in amphibolites within the narrow
zones of layer parallel shear. D – late-metamorphic medium-grained (sometimes pegmatitoid) layers with amphibole-rich rims.

The dated rock was collected from a medium-size outcrop

(Fig. 3D) in the right-hand side of the upper Ve ký Zelený
Potok Valley (Fig. 1), at the forest road, where it crosscuts
the host (also dated) metadiorite (Fig. 3D).

Geochronology

Analytical technique

The U-Pb zircon study was undertaken in the Institute of

Precambrian Geology and Geochronology of the Russian
Academy of Sciences (IPGG RAS) in St. Petersburg, using a
Finnigan MAT 261 8-collector mass-spectrometer in static
mode.

Zircons (Figs. 5, 6) were extracted from crushed rock sam-

ples  using  heavy  liquid  and  magnetic  separation  techniques.
Hand-picked  zircon  fractions,  consisting  of  between  20  and
100  grains,  were  analysed  following  the  method  of  Krogh
(1973). All samples were spiked with a

235

U-

208

Pb mixed trac-

er. The total blanks were 0.05—0.1 ng Pb and 0.005 ng U. Air-
abrasion treatment of the zircons followed the technique of
Krogh (1982), modified by coating abrasive walls with epoxy-

56 PUTIŠ et al.

impregnated diamond powder. The PbDat and ISOPLOT pro-
grams of Ludwig (1991a,b) were used for calculating the un-
certainties and correlations of the U/Pb ratios. Ages were de-
termined using the decay constants recommended by Steiger
& Jäger (1977). All errors are reported at the 2

level. Correc-

tions for common Pb were made using the values of Stacey &
Kramers (1975).

Analytical results

Porphyric metadiorites, s. VZP-51A

The zircon population from the sample VZP-51A consists

of transparent euhedral or subhedral prismatic pink-brown

Fig. 5. SEM photographs showing typical zircon morphologies from
the layered metamorphic complex: sample VZP-51A (A); sample
VBNT-76L (B); sample VZP-50A (C).

zoned grains with high birefringence (Fig. 5A). Transparent
needle shaped and opaque inclusions are common in grains
from the sieve fraction >100

m. A minor amount of grains

from sieve fraction >100

m contains visible somewhat tur-

bid, brownish apparent old cores in the central part of crys-
tals (Fig. 6A). Many crystals show some degree of resorption
(Fig. 5A). The range of crystal sizes is 30—250

m. Zircons

have a length/width ratio of 2.0—3.0. The zircon appears to be
of primary, magmatic origin.

Three sieve fractions of idiomorphic and transparent zir-

con (< 50

m, 80—100

m and >100

m) were analysed (Ta-

ble 1, Nos. 1—3). Two fractions of zircon (>100

m) were

abraded up to 30 % and 50 % (Table 1, Nos. 4, 5). On a con-
cordia plot all data points are discordant (Fig. 7). A discordia
line constructed for these points defines a lower intercept age
of 346 ± 1 Ma and an upper intercept age of 2066 ± 90 Ma
(MSWD = 1.0).

The data points for the sieve fractions of zircon and one

abraded fraction (30 %) cluster near the lower intercept of
discordia, whereas the more strongly abraded zircon (50 %)
is further from the lower intercept of the discordia and repre-
sents an older inherited component of radiogenic Pb (Fig. 7).
We assume the source of such inheritance could be the cores
detected in the minor amount of this zircon (Fig. 6A). Taking
into account the magmatic origin of the studied zircon and
because of the absence of the newly-formed outer metamor-
phic rims on it, the lower intercept age is interpreted as the
age of the diorite emplacement.

Metatrondhjemites from the layered amphibolite, s. VBNT-76L

Zircons from the sample VBNT-76L are subhedral transpar-

ent, clean, colourles, prismatic in shape, and have slightly
rounded terminations (Fig. 5B). Zircons are characterized by a
moderate amount of dust-like opaque needle shaped inclusions
mainly in the central part of crystals (Fig. 6B). Twins were
found in the zircon concentrate. The range of crystal sizes is
40—100

m. The zircons have a length/width ratio of 2.0—3.0.

The zircon appears to be of primary, magmatic origin.

Two sieve fractions of zircon (< 60

m and > 80

m; Nos. 6

and 7 in Table 1) were analysed and the analytical data are dis-
cordant (Fig. 8). The zircon from the sieve fraction > 80

was abraded, so that 40 % of zircon material were removed
(Table 1, No. 8) and zircons from the fraction < 60

m were

subjected to preliminary acid treatment with HF + HNO

dur-

ing 2 hours (Mattinson 1994) (Table 1, No. 9). Three data
points Nos. 6, 7 and 9 define discordia with an upper intercept
age of 514 ± 24 Ma and a lower intercept age of 348 ± 31 Ma
(MSWD = 1.6). The abraded zircon has a slightly older

207

Pb/

206

Pb age (498 ± 4 Ma) than other analysed zircon frac-

tions and the data point for this zircon is displaced to the right
of the discordia demonstrating a small amount of inherited ra-
diogenic Pb component in this zircon fraction. The upper in-
tercept age (514 ± 24 Ma) of discordia constructed for zircons
from the sample VBNT-76L is interpreted as a crystallization
age of the trondhjemites due to differentiation and magmatic
layering of the primary gabbro-diorite. The lower intercept age
(348 ± 31 Ma) reflects the age of Pb losses during the meta-
morphic event.

33 m

20 m

58 PUTIŠ et al.

of discordia, whereas the abraded zircon fraction is farther up
from the lower intercept and represents an older inherited
component. Owing to the igneous origin of the study zircon
the lower intercept age is interpreted as the primary emplace-
ment age of the plagiogranite-aplites.

Discussion and conclusions

1. The zircons of felsic trondhjemitic bands from the host

amphibolites were dated at 514 ± 24 Ma (the upper intercept)
and 348 ± 31 Ma (the lower intercept). The dated meta-
trondhjemites are recrystallized bands (layers) of felsic mag-
matic rock in host compositionally differentiated original gab-
bro-dioritic suite, now banded (layered) amphibolites. The
meta-trondhjemitic layers have the habit of trondhjemitic or-

thogneiss with granoblastic texture. The metamorphic rocks of
the  layers  reflect  pre-metamorphic  or  synmagmatic  layered
fabrics. All the compositional layers are characterized by the
metamorphic  microstructures  with  mostly  sharp  boundaries
between  the  layers.  The  minerals  of  magmatic  melt  origin
were replaced by a metamorphic association of Hbl + Pl(25—30
% An) + Grt + Ttn + Mag(Ilm) + Qtz ± Bt depending on pre-me-
tamorphic  composition,  although  all  the  studied  associations
correspond to the amphibolite facies.

The special domains of amphibolitic complex correspond

to  narrow  (a  few  dm)  zones  of  layer  parallel  shear,  which
might have been supplied by metamorphic fluids and conse-
quently a restricted initial partial melting of mostly homoge-
neous  amphibolites  could  occur.  They  are  clearly  synmeta-
morphic,  according  to  stretched  intralayer  folds,  often  with
separated or sheared off  cores filled in by pale fine- to medi-
um-grained (Qtz-Pl) leucosomatic seggregate with still pre-
served magmatic microstructure.

The newest member of the layered amphibolites appear to

be medium-grained to coarse-grained pegmatitoids and Czo-
Chl-Ab-Qtz veins. They locally enclose the host folded am-
phibolitic structure and seem to be at least late-metamorphic.
The latter lack metamorphic of granoblastic microstructures,
although these are characteristic of the group of meta-
trondhjemitic or meta-leucotonalic layers of magmatic and
pre-metamorphic origin.

The upper intercept age (514 ± 24 Ma) of discordia con-

structed for zircons from the sample VBNT-76L is interpreted
as a crystallization age of the trondhjemites due to differentia-
tion and magmatic layering of the primary gabbro-diorite. This
age suggests an older magmatic phase related to Early Paleo-
zoic breakdown of a Cadomian basement due to an exten-
sional event (Putiš & Grecula in Plašienka et al. 1997: Fig. 6)

The lower intercept age (348 ± 31 Ma) reflects the age of Pb

losses during the metamorphic event. This datum is interpret-
able as dating an important Variscan regional-metamorphic
event which occurred in the West-Carpathian Veporic base-

Table 1: U-Pb isotope data for the zircon from the layered metamorphic complex of the Veporic basement (Western Carpathians).

Sample number,

sieve fraction

Fraction

weight

Concentrations

(ppm)

Isotopic ratios corrected for blank and common Pb

Age, Ma

(

µm)

(mg)

206

Pb/

204

207

Pb/

206

208

Pb/

206

207

Pb/

235

206

Pb/

238

Rho

207

Pb/

235

206

Pb/

238

207

Pb/

206

VZP-51A, <53

1.22

43.3

727

4018

0.05395

±3

0.1755

±1 0.4132±13 0.0556±2 0.98 351±1

349

±1

369

±1

VZP-51A, 80-100

2.08

31.1

523

3703

0.05492

±4

0.1625

±1 0.4247±14 0.0561±2 0.97 359±1

352

±1

409

±2

VZP-51A, >100

1.48

38.4

640

3459

0.05524

±6

0.1618

±1 0.4304±14 0.0565±2 0.95 364±1

354

±1

422

±2

VZP-51A, >100, A 30%

0.30

45.7

672

436

0.05499

±36

0.1933

±2 0.4257±33 0.0561±18 0.52 360±3

352

±1

412

±15

VZP-51A, >100, A 50%

1.73

36.7

583

3029

0.05811

±4

0.1759

±1 0.4675±15 0.0583±2 0.98 389±1

366

±1

534

±2

VBNT-764, <60

0.55

29.9

444

1603

0.05558

±6

0.0864

±1 0.5105±17 0.0667±2 0.94 419±1

416

±1

436

±2

VBNT-764, >80

0.79

29.4

419

2411

0.05595

±6

0.0931

±1 0.5390±18 0.0699±2 0.94 438±1

435

±1

450

±3

VBNT-764, >80, A 50%

1.35

5.45

65.9

777

0.05717

±9

0.1285

±1 0.6031±21 0.0765±2 0.89 479±2

475

±1

498

±4

VBNT-764, <60, HF treated

U/Pb*:

12.7

1767

0.05681

±5

0.1195

±1 0.5937±19 0.0758±2 0.95 473±2

471

±2

484

±2

10 VZP-50A, <50

0.83

111

2100

822.9

0.05965

±8

0.0474

±1 0.4225±15 0.0514±2 0.91 358±1

323

±1

591

±3

11 VZP-50A, <60

0.88

101

1782

848.7

0.06091

±7

0.0541

±1 0.4622±15 0.0550±2 0.94 386±1

345

±1

636

±3

12 VZP-50A, 60-70

0.80

86.4

1335

1705

0.06406

±4

0.0658

±1 0.5686±18 0.0644±2 0.97 457±1

402

±1

744

±2

13 VZP-50A, >85, A 40%

0.47

63.3

787.5

1089

0.06677

±13

0.0968

±2 0.7035±26 0.0764±2 0.85 541±2

475

±2

831

±4

Notes:

— measured ratio;

— uncertainties (95% confidence level) refer to last digits of corresponding ratios;

— correlation coefficients of

207

Pb/

235

U vs.

206

Pb/

238

ratios; A 40% — amount of zircon material removed during of the air-abrasion.

Fig. 9. Concordia diagram for zircons from plagiogranite-aplitic
veins (sample VZP-50A).

U-Pb ZIRCON AGES OF DIORITIC AND TRONDHJEMITIC ROCKS 59

ment,  proving  an  interaction  between  the  lower-crustal
(LAC)  and  supracrustal  (CB)  lithological-structural  com-
plexes.  However  such  interaction  could  have  started  even
earlier at ca. 370—390 Ma as is indicated by some granitic or-
thogneisses  found  in  the  hanging  wall  of  the  Upper  (Tatra)
Nappe (e.g. 383 ± 7 Ma in the Low Tatra Mts, an U-Pb zircon
age after Kotov et al. in prep.), or by cooling ages of amphib-
oles  from  a  layered  amphibolite  at  ca.  357  Ma  (

Ar-

data, Dallmeyer et al. 1993, 1996, location: north of Závadka
n.H. in northern Veporicum).

2. The tectonically unified, that is composite LA(leptynite-

amphibolite)-CB(Čierny  Balog)  structural  complex  of  the
Veporic basement formed due to an Early Variscan collision-
al event when the lower crustal thick amphibolitic suite (with
locally  preserved  HP  rocks,  reviewed  in  the  introductory
paragraphs) was tectonically juxtaposed with the supracrust-
al  complexes.  The  emplacement  of  mafic  intrusions  at  the
bottom  of  the  CB  supracrustal  complex  (Grt-Ky  gneisses,
migmatitic gneisses, granitic orthogneisses, amphibolites) is
supposed to indicate an extensional event partly also postdat-
ing the intrusion event. This can be documented, for exam-
ple,  by  the  intruded  porphyric  and  homogeneous  diorites
(sample VZP-51A) now having the character of dioritic or-
thogneisses (Hbl, Bt, Pl, Ep, Ttn, Qtz), which display pure-
shear to simple-shear high- to medium-T extensional defor-
mations. Zircons were dated 346 ± 1 Ma (the lower intercept)
that is the age of their synmetamorphic intrusion, immediately
followed by the late-metamorphic recrystallization in a ductile
shear zone. This interpretation is fixed by their textural transi-
tions to high-temperature sheared mylonites with characteric-
tic  extensional  ductile  mesofaults  filled  in  by  residual  felsic
melt segregates. Taking into account the magmatic origin of
the  studied  zircon  because  of  absence  of  the  newly-formed
outer metamorphic rims on it, the lower intercept age is inter-
preted as the age of the diorite emplacement. The meaning of
the upper intercept age at 2066±90 Ma is not clear.

The radiometric data are consistent with the field observa-

tion of remaining non-mylonitic domains in metadiorites with
well  preserved  mesostructures  of  inferred  magmatic  origin.
They comprise still observable magmatic foliation and linea-
tion  defined  by  the  subparallel  oriented  inherited  lath-like
magmatic  forms  of  porphyric  plagioclase  (1—1.5  cm  in  size)
with Bt and Am. It is supported by compositional alternation
of  the  described  internally  differentiated  magmatic  members
within larger diorite bodies, or at least a ”layering” after the
changing grain-size of porphyric plagioclase and amphibole in
homogeneous  meta-diorites.  A  characteristic  feature  of  both
(VZP and HLO areas) occurrences of layered amphibolites are
quite  often  present  ultramafic  boudins  (a  few  cm  to  dm  in
size), mostly of hornblenditic (± Cpx) composition.

The ages around 350 Ma thus indicate a complex Variscan

magmatic- metamorphic/deformational event in the West-Car-
pathian Veporic basement. It is shown by the well fitting ages
of the high-temperature metamorphism and the synmetamor-
phic magmatic intrusions of diorites and gabbro-diorites, as
well as their common structural rebuild during the late- and
post-metamorphic cooling and exhumation.

3. The Veporic part of the composite LA-CB structural

complex is crosscut by plagiogranite-aplitic veins dated

233 ± 4 Ma, an age, comparable with that of acid volcanic
and subvolcanic bodies (278—216 Ma, Kotov et al. 1996) and
the pre-orogenic A-type Hrončok granite (238 ± 1.4 Ma, Putiš
et al. 2000) crosscutting the Veporic basement due to Early
Alpine continental rifting.

Acknowledgements:  The  results  were  achieved  in  the
framework  of  the  Russian  Foundation  for  Basic  Research
(Project # 99-05-64058, S.P.K.) and VEGA grant of the Slo-
vak Republic (Project # 1/5228/98, M.P.). Dr. I. Petrík (Geo-
logical  Inst.  of  Slovak  Acad.  of  Sci.),  Dr.  J.  Krá   (Slovak
Geol. Sur. of Dionýz Štúr) and the third anonymous reviewer
are  greatly  acknowledged  for  the  constructive  review  and
suggestions for the improvement of the manuscript.

Mineral abbreviations: Ab=albite, Act=actinolite, Am=amphibole,

An=anorthite,

Ath=anthophyllite,

Bt=biotite,

Cal=calcite,

Carb=carbonate, Chl=chlorite, Cld=chloritoid, Cpx=clinopyroxene,
Czo=clinozoisite,

Dol=dolomite,

Ep=epidote,

Grs=grossular,

Grt=garnet,

Hbl=hornblende,

Ilm=ilmenite,

Kfs=kalifeldspar,

Ky=kyanite,

Lcx=leucoxene,

Mag=magnetite,

Ms=muscovite,

Olg=oligoclase,

Pl=plagioclase,

Qtz=quartz,

Rt=rutile,

Pg=paragonite,

Phe=phengite

Phl=phlogopite,

Sil=sillimanite,

Sps=spessartite, St=staurolite, Ts=tschermakite, Ttn=titanite.

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