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)
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; email@example.com
Russian Academy of Sciences, Institute of Precambrian Geology and Geochronology, Makarov emb. 2,
RF-199034 St. Petersburg, Russian Federation; firstname.lastname@example.org
Russian Academy of Sciences, Institute of Geology of Ore Deposits, Petrography, Mineralogy and Geochemistry,
Staromonetny per. 35, RF-109017 Moscow, Russian Federation; email@example.com
(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
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-
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,
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-
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.
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
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-
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
The geological setting and characteristic features of
the layered metamagmatic-amphibolitic rocks of the
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
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-
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
= 500—520 °C at
= 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
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,
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
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-
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
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
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).
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
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
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
tions for common Pb were made using the values of Stacey &
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
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 and >100
m) were analysed (Ta-
ble 1, Nos. 1—3). Two fractions of zircon (>100
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
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
subjected to preliminary acid treatment with HF + HNO
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
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-
U-Pb ZIRCON AGES OF DIORITIC AND TRONDHJEMITIC ROCKS 57
turbid, brownish apparent old cores of prismatic shape in the
central part of crystals (Fig. 6C). The range of crystal sizes is
m with a length/width ratio of 2.8. This type of zir-
con represents about 70 % of the bulk population and appears
to be of primary, igneous origin.
Type II (not shown) is represented by pale-pink and colour-
less translucent or nebulous often metamictic prismatic crys-
m long with a length/width ratio of 2.0. The pro-
portion of this zircon type in the bulk population is ca. 30 %.
Three sieve fractions of type I zircons (< 50
m, < 60
m; Nos. 10—12, Table 1) were analysed. In addition,
one fraction of the type I zircons (> 85
m) was abraded re-
moving some 40 % of the zircon material (Table 1, No. 13).
As seen in Fig. 9, the results for the analysed zircon fractions
are discordant and define a discordia which concordia inter-
sects at 233 ± 4 Ma and 1080 ± 40 Ma (MSWD = 1.7). The
data points for the size fractions plot near the lower intercept
Fig. 6. Photomicrograph (transmitted light) showing apparent old
cores or relics of cores in zircons from the layered metamorphic
complex: sample VZP-51A (A); sample VBNT-76L (B); sample
Fig. 7. Concordia diagram for zircons from porphyric metadiorites
Leucocratic plagiogranite-aplitic veins, s. VZP-50A
Inspection of the zircon concentrate in sample VZP-50A
leads us to distinguish the two morphologically different
Type I is represented by pale-brown and pink-brown trans-
parent subhedral crystals of prismatic and long prismatic
shape (Fig. 5C). The crystals show oscillatory zoning, bub-
ble-like and opaque inclusions are common. A minor amount
of grains (about 1—2 % of bulk population) contains visible
Fig. 8. Concordia diagram for zircons from metatrondhjemites
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
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).
Isotopic ratios corrected for blank and common Pb
±1 0.4132±13 0.0556±2 0.98 351±1
±1 0.4247±14 0.0561±2 0.97 359±1
±1 0.4304±14 0.0565±2 0.95 364±1
VZP-51A, >100, A 30%
±2 0.4257±33 0.0561±18 0.52 360±3
VZP-51A, >100, A 50%
±1 0.4675±15 0.0583±2 0.98 389±1
±1 0.5105±17 0.0667±2 0.94 419±1
±1 0.5390±18 0.0699±2 0.94 438±1
VBNT-764, >80, A 50%
±1 0.6031±21 0.0765±2 0.89 479±2
VBNT-764, <60, HF treated
±1 0.5937±19 0.0758±2 0.95 473±2
10 VZP-50A, <50
±1 0.4225±15 0.0514±2 0.91 358±1
11 VZP-50A, <60
±1 0.4622±15 0.0550±2 0.94 386±1
12 VZP-50A, 60-70
±1 0.5686±18 0.0644±2 0.97 457±1
13 VZP-50A, >85, A 40%
±2 0.7035±26 0.0764±2 0.85 541±2
— measured ratio;
— uncertainties (95% confidence level) refer to last digits of corresponding ratios;
— correlation coefficients of
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 (
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,
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