GeolCarp_Vol66_No6_443

www.geologicacarpathica.com

GEOL

EOL

EOLOGICA CARPA

OGICA CARPA

OGICA CARPATHICA

THICA

, DECEMBER 2015, 66, 6, 443—454 doi: 10.1515/geoca-2015-0037

Introduction

Variscan  fragments  are  recognized  all  over  the  Alpine  Belt
of  Europe  (Dallmeyer  et  al.  1998;  Neubauer  &  Handler
1999;  Franke  et  al.  2000;  Kroner  et  al.  2008;  Froitzheim  et
al. 2008; Schulmann et al. 2014 and references therein). Al-
though  in  the  Balkans  high-grade  metamorphic  rocks  from
the deep structural levels of the European Variscan Belt are
well-exposed (Ivanov 1988; Kräutner & Kristic 2002; Iancu
et al. 2005; Yanev et al. 2006; Kounov et al. 2012; Gerdjikov
et al. 2013) (Fig. 1b), their Variscan and pre-Variscan history
is still not well understood, thus hindering correlations with
similar units in Central and Western Europe.

In Bulgaria, for a long time, the high-grade metamorphic

complexes  were  considered  as  Precambrian  (see  the  review
of Zagorchev 2008). However, in the last twenty years num-
erous  detailed  studies  from  the  Rhodope  massif  revealed
their Alpine tectonic evolution (e.g. Burg et al. 1996, 2012;
Kaiser-Rohrmeier et al. 2004; Liati 2005; Bosse et al. 2009;
Turpaud  &  Reischmann  2010).  Furthermore,  Ivanov  (1988)
and  Velichkova  et  al.  (2004)  suggested  the  Variscan  age  of
high-grade metamorphic rocks north of the Alpine Rhodopes
(Fig. 1b).  Quite  recently,  three  Variscan  complexes  have
been distinguished in Bulgaria based on the common Middle
Carboniferous  age  of  the  amphibolite  facies  metamorphic

Cambrian magmatism, Variscan high-grade metamorphism

and imposed greenschist facies shearing in the Central

Sredna Gora basement units (Bulgaria)

ANNA LAZAROVA, KALIN NAYDENOV, NIKOLAI PETROV and VALENTIN GROZDEV

Geological Institute, Bulgarian Academy of Science, Acad. Georgy Bonchev str. 24, 1113 Sofia, Bulgaria;

alazarova@geology.bas.bg; k.naidenov@gmail.com; npetrov@geology.bas.bg; val.grozdev@abv.bg

(Manuscript received April 20, 2015; accepted in revised form October 1, 2015)

Abstract:  Gneisses  from  the  deep  structural  levels  of  the  European  Variscan  Belt  are  well  exposed  in  the  Central
Sredna Gora in Bulgaria. In general, migmatites predominate, but unmigmatized domains (or domains with incipient
migmatization) are also documented in this area. This paper presents new structural, petrographic and U-Pb isotope
geochronological  data  from  such  an  unmigmatized  part  of  the  Variscan  high-grade  metamorphic  basement  (the
Koprivshtitsa Unit). A predominant part of this unit represents an alternation of metagranitoids and metabasites. The
protolith  crystallization  age  of  the  metagranitoids  is  constrained  at  491.5 ± 7.6 Ma  by  U-Pb  laser  ablation  method
on zircons. This age coincides with the previously available Late Cambrian protolith ages of metabasic rocks that crop
out  within  the  adjacent  migmatitic  unit.  The  Korpivshtitsa  Unit  comprises  also  lesser  orthogneisses  with  Late
Neoproterozoic protoliths. Based on the available local and regional paleogeographic reconstruction schemes, we sug-
gest that the Upper Cambrian magmatic rocks intruded Upper Neoproterozoic crust during the initial opening stages of
the Rheic Ocean or a related basin. Subsequently, both were involved in the Variscan high-grade deformation. The
contact of the Koprivshitsa Unit with the migmatitic part of the metamorphic complex coincides with a north-vergent
greenschist facies thrust zone – the Chuminska Shear Zone. The exact time of the shearing is still not well constrained
but it clearly postdates the Variscan high-temperature metamorphism of the gneisses.

Key words: Variscan high-grade basement, Central Sredna Gora Complex in Bulgaria, Late Cambrian magmatism,
greenschist facies mylonitization.

overprint – the Central Sredna Gora, the Ograzhden-Vertiskos
and the Strandja Complexes (Gerdjikov et al. 2013).

The Central Sredna Gora Complex, which represents a

basement  unit  within  the  Alpine  Sredna  Gora  Zone  of  the
Balkan Thrust Belt (e.g. Dabovski et al. 2002), is a well-ex-
posed but poorly studied part of the Variscan Orogen in Bul-
garia.  At  a  regional  scale,  it  has  been  correlated  with  the
Getic-Supragetic  basement  units  of  the  South  Carpathians
(Neubauer  2002;  Velichkova  et  al.  2004;  Iancu  et  al.  2005;
Schmid et al. 2008). Only limited data about the protoliths of
the metamorphic rocks from the Central Sredna Gora Com-
plex,  as  well  as  about  their  ages  and  tectonic  evolution  are
available  (Velichkova  et  al.  2004;  Peytcheva  &  von  Quadt
2004; Carrigan et al. 2006; Gaggero et al. 2009). We carried
out our research in the northern part of the complex (Fig. 2a)
where high-grade basement rocks form a strip which extends
along  several  tens  of  kilometers  in  an  E-W  direction.  The
metamorphic section comprises migmatitic gneisses and un-
migmatized domains (Dabovski et al. 1966; Dabovski 1988;
Zagorchev 2008; Antonov et al. 2010).

In this paper, we present details for the unmigmatitic part

known as the Koprivshtitsa Unit (Dabovski 1988; Zagorchev
2008; Antonov et al. 2010). Our research provides new data in
several aspects: i) the lithological and structural characteristics
of the unit; ii) its contact with the migmatitic unit; iii) U-Pb

444

LAZAROVA, NAYDENOV, PETROV and GROZDEV

GEOL

EOL

EOLOGICA CARPA

OGICA CARPA

OGICA CARPATHICA

THICA

THICA, 2015, 66, 6, 443—454

zircon geochronological data. On this ground, we suggest a
possible scenario for the origin of the studied rocks in the con-
text of the Paleozoic evolution of the area. This will help us to
better understand the important features of the Variscan Belt
in the Balkans and will contribute to further regional correla-
tions with other fragments of the orogen in Europe.

Geological setting

The Variscan Central Sredna Gora Complex is exposed

south and southeast of Sofia (Fig. 1b). Two high-grade meta-
morphic  units  have  been  distinguished  in  its  northern  part
(Fig. 2a,b)  –  the  migmatitic  Pirdop  Unit  and  the  unmigma-
tized  Koprivshtitsa  Unit  (Dabovski  1988;  Zagorchev  2008;
Antonov et al. 2010) where the rocks of the former dominate
the  metamorphic  complex.  Most  widespread  are  migmatitic
paragneisses  (biotite  and  two-mica  banded  migmatites,  and
minor  garnet-,  kyanite-  and  sillimanite-bearing  gneisses)  and
subordinate  amphibole-biotite  orthogneisses,  amphibolites,
metaserpentinites, and mafic eclogites (Dabovski et al. 1966;
Dabovski  1988;  Zagorchev  2008;  Antonov  et  al.  2010).
Mixed  protoliths  of  continental  and  oceanic-crust  affinities
have been assumed for these rocks (Zagorchev 2008). A com-
mon  feature  of  the  unit  is  the  intense  post-migmatization
greenschist  to  lower  amphibolite  facies  retrogression  related

to Mid and/or Late Variscan shearing (Velichkova et al. 2004;
Lazarova  et  al.  2010;  Gerdjikov  et  al.  2010  and  references
therein) during which the migmatites obtained specific “au-
gen”-gneiss texture. The Koprivshtitsa Unit occupies a limited
area in the surroundings of Koprivshtitsa town (Fig. 2b). Up to
now,  it  was  regarded  as  derived  from  a  Precambrian  silici-
clastic  suit  intruded  by  basic  volcanic  rocks  (Zagorchev
2008). The described lithologies include mainly amphibolites
and  biotite  gneisses,  and  minor  biotite  and  two-mica  schists,
sillimanite-biotite and garnet-sillimanite-biotite schists, quart-
zites,  marbles,  garnet-pyroxene  and  garnet  amphibolites,  as
well as orthogneiss bodies (Dabovski et al. 1966; Dabovski
1988; Zagorchev 2008; Antonov et al. 2010). One of the me-
tagranitoids, the so called Bobevitsa Orthogneiss (Zagorchev
et al. 1973), was dated to 616.9 ± 9.5 and 595 ± 23 Ma (Fig. 2a;
HR-SIMS U-Th-Pb zircon age – Carrigan et al. 2006).

The high-grade metamorphic fabric of both the Pirdop and

the  Koprivshtitsa  unit  is  sealed  by  the  undeformed  Late
Variscan Koprivshtitsa Granite (Fig. 2) dated at 304.8±0.8 Ma
(U-Pb  monazite  age  –  von  Quadt  et  al.  2004)  and  at
312.0 ± 5.4 Ma  (HR-SIMS  U-Th-Pb  zircon  age  –  Carrigan
et al. 2005). On the other hand, the contact between the units
north  of  Koprivshtitsa  (Fig. 2b)  was  described  as  a  steeply
dipping, north-vergent reverse fault related to the Late Alpine
thrust system (Stara Planina thrust system – Dabovski et al.
1966).  During  the  last  regional-scale  mapping  of  the  area

Fig. 1. Regional sketch map of the European Variscan terranes (a) with close-up view on high-grade basement in the western part of the
Balkan Peninsula (b). Modified after Kounov et al. (2012) and Balintoni et al. (2014).

Fig. 2. a – simplified geological map of a part of the Central Sredna Gora area with published geochronological data (modified after Iliev
& Katskov 1990a,b); b – geological map of Koprivshtitsa area (modified after Dabovski et al. 1966; Zagorchev et al. 1973; Iliev & Katskov
1990a,b; Cheshitev et al. 1994) with a cross-section A—A’along the Topolnitsa River and stereographic projections (equal area; lower
hemisphere) of foliations (density contours) and lineations (points) in the Pirdop (sp1) and Koprivshtitsa (sp2) Units. Location of sample
SG-8-2 is indicated.

446

LAZAROVA, NAYDENOV, PETROV and GROZDEV

GEOL

EOL

EOLOGICA CARPA

OGICA CARPA

OGICA CARPATHICA

THICA

THICA, 2015, 66, 6, 443—454

(Antonov et al. 2010), it was interpreted as a Variscan duc-
tile shear zone, the Chuminska Shear Zone.

In the studied area, data for the protolith ages of the high-

grade metamorphic rocks are still scanty. Besides the Cado-
mian age of the Bobevitsa orthogneiss, two late Cambrian
U-Pb zircon protolith ages are available for the Pirdop Unit
(Fig. 2b): 493.8 ± 9.8 Ma for a metagabbro sample (Antonov
et al. 2010) and a poorly constrained age of 502.8 ± 3.2 Ma
for a hornblende-biotite gneiss (Fig. 2a – Peytcheva & von
Quadt 2004).

The Variscan metamorphic evolution is constrained by

U-Pb and

Ar/

Ar data (Fig. 2a): i) an

Ar/

Ar amphibole

age of 398 ± 5.2 Ma obtained from a garnet-biotite-ilmenite-
bearing  amphibolite  east  of  Ihtiman  was  interpreted  as  the
time  of  an  early  amphibolite  facies  re-equilibration  shortly
after an eclogite facies metamorphic peak (Cortesogno et al.
2005; Gaggero et al. 2009); ii) the age of the high-tempera-
ture  metamorphism  and  migmatization  is  estimated  at
336.5 ± 5.4 Ma (HR-SIMS U-Pb zircon data on leucosome in
migmatitic gneisses – Carrigan et al. 2006); iii) the intense
greenschist to lower amphibolite facies regional-scale retro-
gression  of  migmatites  of  the  Pirdop  Unit  and  contempora-
neous emplacement of muscovite-bearing granites is dated to
333.9 ± 0.2 Ma  (

Ar/

Ar of muscovite – Gerdjikov et al.

2010); iv) the post-metamorphic cooling ages of the gneisses
west and south of the studied area range between 317 and
305 Ma (

Ar/

Ar of muscovite and biotite – Velichkova et

al. 2004). These data coincide with the time of a voluminous
magmatism since the crystallization ages of the Late
Variscan granitoids in this part of the Sredna Gora Complex
are between 312 and 290 Ma (Carrigan et al. 2003, 2005;
von Quadt et al. 2004; Velichkova et al. 2004).

The age constraints of several greenschist facies shear

zones indicate that the high-grade rocks from the southern
part of the Central Sredna Gora Complex were affected by Al-
pine tectono-thermal events ca. 140 Ma and ca. 106—100 Ma
(

Ar/

Ar of muscovite and biotite – Velichkova et al. 2004).

Analytical methods

U-Th-Pb isotope zircon analyses were carried out by a la-

ser  ablation  (LA)  technique  using  a  New  Wave  Research
(NWR) 193 nm excimer laser UP-193FX attached to a Per-
kin-Elmer  ELAN  DRC-e  quadrupole  inductively  coupled
plasma mass spectrometer (ICP-MS) at the Geological Insti-
tute of the Bulgarian Academy of Science in Sofia, Bulgaria.
In-laboratory  designed  ablation  cell  with  lowered  position
effects, energy density on sample ca. 8.8 J/cm

—2

and repetition

rate of 8 Hz are used. The ablation craters are ca. 35 µm in dia-
meter. The analyses were carried out in blocks of 22, using the
GJ1  zircon  standard  (Jackson  et  al.  2004)  for  fractionation
corrections (2 analyses at the beginning, 2 in the middle and
2 at the end of the block) and Plešovice (Sláma et al. 2008)
as “unknown” standard (to control the correct data reduction).
The  results  were  calculated  off-line  using  GLITTER 4.0
(Macquarie  University).  All  concordant  zircons  were  used
to calculate a mean

238

206

Pb age, or a concordia age. Con-

cordia plots, ages and averaging were processed using
ISOPLOT 3.0 (Ludwig 2003).

Results

Structural characteristics of the Koprivshtitsa Unit

The unmigmatized Koprivshtitsa Unit is exposed as a nar-

row E-W trending and ca. 30 km long strip between the Pir-
dop  Unit  to  the  north  and  the  Late  Variscan  Koprivshtitsa
Granite to the south and southeast (Fig. 2b). Our field obser-
vations  were  carried  out  along  several  N-S  profiles  across
the  unit  generally  normal  to  the  orientation  of  the  regional
foliation. The unit represents an alternation of metamorphic
rocks with felsic and mafic protoliths (Fig. 3a,b), both in ap-
proximately equal proportions. The sillimanite-biotite schists,
garnet-sillimanite-biotite schists, quartzites and marbles, re-
ported  before  (Dabovski  et  al.  1966;  Dabovski  1988;
Zagorchev 2008), were not observed in the studied area.

The felsic part of the Koprivshtitsa unit consists of two-

mica  metagranites,  which  form  up  to  several  meters  wide,
sheet-like  bodies  with  sharp  contacts  with  the  metabasic
rocks  (Fig. 3a,b).  Generally,  the  contacts  are  parallel  to  the
foliation  fabric,  but  locally,  slightly  oblique  contacts  have
been  preserved  as  well.  The  thinner  metagranite  sheets  are
penetratively foliated (Fig. 3c), while in the thicker ones, the
foliation is well developed only near the contacts. The folia-
tion and mineral lineation are defined by the preferred orien-
tation of micas, quartz and feldspars. The main rock-forming
minerals are quartz, K-feldspar, and plagioclase with subor-
dinate white mica and biotite and accessory titanite, zircon,
and  apatite.  Under  the  microscope,  the  metamorphic  folia-
tion of the metagranites is defined by the preferred orienta-
tion of small (up to 1—2 mm along the long axis), elongated
quartz  and  feldspar  grains  as  well  as  unevenly  distributed
white mica flakes or two-mica aggregates (Fig. 3d).

The mafic part of the metamorphic section (Fig. 3a,b,e) is

dominated by biotite-amphibole and amphibole-biotite gneisses
and less abundant amphibolites. These rocks are derived

Fig. 3. Field and microscale characteristics of the Koprivshtitsa Unit. a – alternation between centimeter-thick sheet-like bodies of meta-
granitoids (mgr) and metabasic (mb) rocks (Topolnitsa River Valley north of Koprivshtitsa); b – a detailed view of the same relationships;
c – well-foliated metagranitoid body with a foliation-parallel contact (Topolnitsa River Valley north of Koprivshtitsa); d – photomicro-
graph (cross-polarized light) of a metagranite with a weak metamorphic foliation defined by the preferred orientation of small, elongated
quartz (qz) and feldspar (kfs, pl) grains as well as unevenly distributed white mica (wm) and biotite (bt) flakes (sample SG-8-2, location in
Fig. 2b); e – amphibolites and a thin metagranitoid sheet-like body (Topolnitsa River Valley north of Koprivshtitsa); f – photomicro-
graph (plane-polarized light) of a biotite-amphibole gneiss with a distinct metamorphic foliation defined by the preferred orientation of
elongated amphibole (hb), biotite (bt) and quartz-feldspar aggregates (same location as for d).

448

LAZAROVA, NAYDENOV, PETROV and GROZDEV

GEOL

EOL

EOLOGICA CARPA

OGICA CARPA

OGICA CARPATHICA

THICA

THICA, 2015, 66, 6, 443—454

from mafic to intermediate igneous protoliths. The gneisses
are  medium-grained,  with  distinct  foliation  defined  by  the
preferred  orientation  of  amphibole-biotite  aggregates  and
thin  quartz-feldspar  lenses.  The  lineation  is  defined  by  am-
phibole-biotite aggregates. The main rock-forming minerals
are amphibole and biotite in varying proportions, but reach-
ing up to 80 % of the rocks. Subordinate phases are plagio-
clase,  quartz,  ± K-feldspar;  accessory  phases  are  apatite,
zircon,  ± titanite,  ± magnetite.  Amphibole  and  biotite  com-
monly  form  elongated  domains,  while  quartz  and  feldspars
are  present  as  isolated  grains  or  lens-shaped  aggregates
aligned parallel to the metamorphic foliation (Fig. 3f).

A common feature of the Koprivshtitsa Unit is a distinct

E-W striking and steeply-dipping (60—80°) predominantly to
the south foliation fabric (Fig. 2, sp2). The mineral lineation
is weakly developed, plunging gently to the west-southwest
or east-northeast.

Locally, bodies of coarse-grained, K-feldspar porphyro-

clastic biotite metagranites and metagranodiorites crop out.
They show similar south-dipping foliation, but the coarser
texture as well as the presence of well-preserved K-feldspar
porphyroclasts allows us to distinguish them from the fine-to
medium-grained two-mica metagranitoid sheets.

Contacts

The Koprivshtitsa Unit has a tectonic contact with the

migmatitic Pirdop Unit and intrusive relations with the Ko-
privshtitsa  Granite.  South  of  the  granite,  the  high-grade
metamorphic  basement  is  dominated  again  by  migmatitic
gneisses, which are structurally above the unmigmatized Ko-
privshtitsa Unit (Fig. 2,  cross-section A—A’).  During  our  re-
search we focused on a characterization of the tectonic contact
(Fig. 2b), the Chuminska Shear Zone (ChSZ).

The ChSZ is a generally narrow (up to 100—120 m), E-W

trending  and  south-dipping  structure,  exposed  along  strike
for nearly 15 km (Fig. 2b). It has sharp boundaries with the
host  gneisses.  The  footwall  is  built  up  by  migmatitic  two-
mica paragneisses (Fig. 4a) and subordinate orthogneisses of
the  Pirdop  Unit.  In  the  hanging  wall,  the  Koprivstitsa  Unit
consists  of  unmigmatized  orthometamorphic  rocks.  The
shear  zone  is  better  developed  in  the  footwall  migmatitic
paragneisses, although several sub-parallel zones splay north
and south from the main zone into both metamorphic units.
Along the ChSZ and its splays, the gneisses are transformed
to greenschist facies mylonites with well-developed mylonitic
foliation  and  C’shear  bands  (Fig. 4c).  The  mylonitic  folia-
tion  (S

) trends E—W to NE—SW and dips moderately to

steeply (45—80°) to the south. It is generally parallel to the
high-grade metamorphic fabric of the host gneisses (Fig. 2,
sp1 and sp2). S

is defined by a subparallel arrangement of

dark,  fine-grained  phyllosilicate  aggregates  and  quartz  rib-
bons. In a number of outcrops an unequivocal lineation fab-
ric did not form. Where the lineation is well developed it is
defined  by  the  alignment  of  elongated  aggregates  of  white
mica  and  chlorite  or  by  fibrous  quartz  striae.  The  lineation
has a moderate to steep down-dip plunge. The quartz linear
fabric has the characteristics of a low-temperature stretching
lineation.  The  C’shear  bands  are  defined  by  synkinematic

white mica and chlorite (Fig. 4c) as well as by trails of mica
fragments and by deflected strain-shadows of large feldspar
grains. The acute angles with the mylonitic foliation are in
the range of 20 to 30°.

The microscale observations of the sheared two-mica

gneisses of the Pirdop Unit give important information about
deformation mechanisms and metamorphic conditions. Less
deformed migmatitic gneisses (Fig. 4b) are medium-grained
rocks composed of plagioclase, K-feldspar, synmetamorphic
quartz,  muscovite,  biotite,  ± garnet,  and  accessory  apatite,
zircon  and  rutile.  They  have  a  distinct  high-temperature
metamorphic foliation defined by the preferred orientation of
feldspars and elongated large mica flakes (Fig. 4b). The ret-
rograde  shearing  within  the  ChSZ  led  to  the  formation  of
chlorite-white  mica-quartz  (syndeformational)  mylonites
(Fig. 4d,e).  Their  distinct  mylonitic  foliation  is  generally
subparallel to the initial high-grade planar metamorphic fab-
ric of the gneisses. The mylonites are characterized by vari-
ably  sized  and  often  fragmented  porphyroclasts  of  feldspar,
synmetamorphic  quartz  and  mica,  surrounded  by  a  fine-
grained  synmylonitic  white  mica-chlorite-quartz  matrix
(Fig. 4d,e,f).  The  porphyroclasts  are  brittlely  deformed  and
rotated by a cataclastic flow along the mylonitic foliation or
C’shear bands (Fig. 4d,e). Most of them show strong undu-
lose  extinction  (Fig. 4e,f)  and  have  tails  (“strain  shadows”)
of very fine-grained recrystallized material of sericite-quartz
composition (Fig. 4d). A common feature of the large quartz
fragments is the development of deformation bands (Fig. 4f).

The synkinematic fine-grained quartz forms thin bands

along the S

(Fig. 4d) or occurs along grain boundaries and

micro-fractures of the large quartz porphyroclasts (Fig. 4f).
Recrystallized, synkinematic and very fine-grained mica is gen-
erally observed along both the intragranular micro-fractures
and mylonitic planar fabrics. A high fluid activity during my-
lonitization is indicated by the advanced replacement of feld-
spars by secondary minerals (clays, fine-grained white mica
and quartz) as well as by the phyllosilicates and fibrous quartz
in “strain shadows” and overgrowths around porphyroclasts.

Shear sense criteria such as S—C’fabric, strain shadows

(Fig. 4d), synthetic domino fragmentation of muscovite and
feldspars, etc. consistently indicate top-to-the-north and north-
west direction of the tectonic transport, namely north-ver-
gent reverse-fault kinematics.

LA-ICP-MS geochronology

We sampled a metagranite sheet-like body from the felsic

part of the Koprivshtitsa Unit. Our aim was to constrain the
time of emplacement of this abundant rock variety within the
section as well as to compare the results with the recently ob-
tained ages of 502.8 ± 3.2 Ma for a hornblende-biotite gneiss
of the Pirdop Unit north of Koprivshtitsa (Peytcheva & von
Quadt  2004)  and  493.8 ± 9.8 Ma  for  a  metagabbro  west  of
Pirdop (Antonov et al. 2010). More than 30 zircons were re-
covered from the metagranite sample SG-8-2 (sample locality
in  Fig. 2b).  Grains  with  lengths  between  100  and  250 µm
were selected for analysis. Most of them are slightly rounded
but still have preserved prismatic and bi-pyramidal morpho-
logies,  characteristic  for  a  magmatic  population.  On  the

449

CAMBRIAN MAGMATISM AND VARISCAN METAMORPHISM IN SREDNA GORA (BULGARIA)

GEOL

EOL

EOLOGICA CARPA

OGICA CARPA

OGICA CARPATHICA

THICA

THICA, 2015, 66, 6, 443—454

Fig. 4. Field and microscale characteristics of the Pirdop Unit. a – two-mica migmatitic gneiss with leucosome material along the metamor-
phic foliation planes, and within the hinges and along axial planes of small-scaled folds; pavement outcrop; ca. 7.5 km northwest of Ko-
privshtitsa; b – photomicrograph of a two-mica gneiss with a distinct high-grade metamorphic foliation defined by a preferred orientation of
main rock-forming minerals – biotite (bt), white mica (wm), plagioclase (pl), K-feldspar (kfs) and quartz (qz); c – greenschist facies
S—C’mylonite in the ChSZ; the white arrow points to a lens of a sheared leucosome from the parental migmatitic gneiss; d – photomicro-
graph of a fine-grained mylonite with a distinct “banding” defined by an alternation between thin phyllosilicate-rich domains and thicker bands
of fine-grained synkinematic quartz; porphyroclasts of white mica and synmetamorphic quartz are common; strain shadows (white arrow) of
the porphyroclasts point to a sinistral (top to the NW) shearing; e – photomicrograph demonstrating the brittle deformation behaviour of feld-
spars, quartz and mica porphyroclasts and abundant microfractures parallel to the mylonitic foliation; f – fragmented quartz porphyroclast
with strong, undulose extinction and deformation bands; the large microfracture separating the porphyroclast into two parts is filled with fine-
grained aggregate of synkinematic phyllosilicates and quartz; weak bulging recrystallization is revealed along the boundaries of the quartz
fragments and along fine microfractures. All photomicrographs are with crossed polarizers. Mineral abbreviations as in Fig. 3.

450

LAZAROVA, NAYDENOV, PETROV and GROZDEV

GEOL

EOL

EOLOGICA CARPA

OGICA CARPA

OGICA CARPATHICA

THICA

THICA, 2015, 66, 6, 443—454

backscattered electron (BSE) images (Fig. 5a), fractures are
visible  in  almost  all  of  the  zircon  grains.  Cathodolumines-
cence (CL) images (Fig. 5a) reveal a preserved magmatic os-
cillatory  zoning  and  fine  metamorphic  rims.  The  magmatic
origin  of  the  grains  is  further  confirmed  by  the  Th/U  ratio
> 0.5 (Table 1). The solid-state recrystallization and fractur-
ing of the zircon grains most probably reflect the high-grade
metamorphic overprint. This event is clearly indicated by the
consistently younger ages of the rims (Pb-loss).

The U-Pb isotope ages (Table 1) are summarized in Fig. 5b.

The obtained age data range between 400 and 700 Ma. Two
analyses concordant at ca. 600 and another two at ca. 700 Ma
correspond to inherited zircon cores (e.g. grain 1, Fig. 5a) and
point to a Neoproterozoic source contribution. CL images re-
veal that a group of several concordant to slightly discordant
analyses yielding ages between ca. 400 and ca. 470 Ma corre-

spond to zones of recrystallization and certain Pb-loss or reju-
venation (e.g. grain 3, Fig. 5a). The main group of 11 concor-
dant analyses corresponding to zircon grains with fine
magmatic oscillation (e.g. grain 2, Fig. 5a) were used to calcu-
late a concordia age of 491.5 ± 7.6 Ma (Fig. 5c). This date is
considered the best approximation of the crystallization age of
the granite.

Discussion

The Koprivshtitsa Unit – structure and ages

Field data show that the Koprivshtitsa Unit represents a

small but from a geodynamic point of view important, un-
migmatized domain of the Central Sredna Gora Complex.

Fig. 5. LA-ICP-MS U-Pb geochronology results for metagranite sample SG-8-2 (sample location in Fig. 2). a – selected cathodolumines-
cence (CL) images (upper row) and backscattered electron (BSE) images (lower row) of analysed zircon grains with position of the laser
ablation craters (circles) and corresponding ages. Grain 1 – inherited Neoproterozoic magmatic core, grain 2 – large grain with fine oscil-
latory zonation (analysed area) and thin metamorphic rim (black on the CL image), grain 3 – fractured and largely recrystallized zircon
with analysed metamorphic rim; b – concordia plot of all LA-ICP-MS analyses; c – concordia crystallization age excluding inherited
cores and metamorphic rims. Data calculations and plots were carried out in ISOPLOT 3.0 (Ludwig 2003).

451

CAMBRIAN MAGMATISM AND VARISCAN METAMORPHISM IN SREDNA GORA (BULGARIA)

GEOL

EOL

EOLOGICA CARPA

OGICA CARPA

OGICA CARPATHICA

THICA

THICA, 2015, 66, 6, 443—454

A considerable part of the unit repre-
sents an alternation of metagranitoid
and metabasic rocks. Their initial re-
lationships  were  obliterated  during
the  high-grade  metamorphic  over-
print, but the sheet-like geometry of
the  metagranitoids  and  their  small
thickness  suggest  dyke-  or  sill-like
emplacement  within  the  mafic  host
rocks.  The  new  U-Pb  geochrono-
logical data indicate a Late Cambrian
(491. 5 ± 7.6 Ma)  crystallization  age
of the metagranites (Fig. 5). The re-
sults  overlap  within  the  analytical
error  with  the  protolith  crystalliza-
tion  age  of  493.8 ± 9.8 Ma  reported
for  a  metagabbro  lens  within  the
migmatitic  gneisses  of  the  Pirdop
Unit (Antonov et al. 2010) (Fig. 2a).
Similar,  although  not  very  precise
due  to  large  discordance  of  the
analyses,  is  the  lower  intercept  age
of  502.8 ± 3.2 Ma  of  a  hornblende-
biotite  gneiss  from  the  Pirdop  Unit
(Fig. 2a)  north  of  Koprivshtitsa
(Peytcheva  &  von  Quadt  2004).
These data suggest an approximately
contemporaneous  Late  Cambrian
emplacement  of  the  mafic  and  fel-
sic melts within the Central Sredna
Gora Complex.

Considering the amphibolite fa-

cies  overprint  of  the  Koprivshtitsa
Unit,  concordant  ages  obtained
from  some  of  the  zircons  (mainly
from rims but also from inner zones
with fractures) which cluster around
400 Ma  are  important  (Table 1).
These ages probably reflect the time
of  a  high-grade  metamorphic  event.
They  correlate  well  with  the  age  of
398 ± 5.2 Ma  interpreted  as  an  am-
phibolite  facies  re-equilibration  of
mafic  eclogites  south  of  the  studied
area (Gaggero et al. 2009). There is a
substantial  difference  with  the  time
of  the  migmatization  in  the  Pirdop
Unit,  estimated  by  Carrigan  et  al.
(2006)  at  336.5 ± 5.4 Ma,  but  at  this
stage of research our results and pre-
viously  published  data  are  insuffi-
cient  to  thoroughly  address  this
issue.  Although  scattered  and  in-
complete, the geochronological data
for the Central Sredna Gora Complex
(Velichkova et al. 2004; Carrigan et
al. 2006; Gaggero et al. 2009) are in
agreement  with  the  prolonged  and
complicated  metamorphic  evolu-

Table 1:

Results

U-Pb

LA-ICP-MS

zircon

geochronological

analyses

sample

SG-08-2.

Abbreviations:

–

zircon

core,

–

zircon

rim,

Rho

–

correlation

coefficient,

1SE

–

sigma

error, 2SE – 2 sigma error.

452

LAZAROVA, NAYDENOV, PETROV and GROZDEV

GEOL

EOL

EOLOGICA CARPA

OGICA CARPA

OGICA CARPATHICA

THICA

THICA, 2015, 66, 6, 443—454

tion suggested for the Variscan Belt in Central and Western
Europe.  It  is  generally  characterized  by  an  Early  Variscan
subduction and high-pressure metamorphism at ca. 400 Ma,
a Mid to Late Variscan high-temperature event at ca. 340—330
and  late-to-post-tectonic  granitoids  emplacement  between
ca.  330—300 Ma  (e.g.  Kroner  et  al.  2008;  Kroner  &  Romer
2013 and references therein).

The mylonitic contact of the Koprivshtitsa Unit

The Chuminska Shear Zone represents a mylonitic bound-

ary between the Koprivshtitsa and Pirdop units. The synde-
formational mineral assemblage of quartz, chlorite and white
mica, the generally brittle behaviour of the feldspar porphy-
roclasts as well as the quartz recrystallization fabrics within
the mylonites suggest a greenschist facies fluid-assisted de-
formation along the zone at temperatures most probably in
the range of 300—350 °C. The observed sense-of-shear crite-
ria indicate reverse kinematics along the north vergent zone
thus supporting the earlier interpretation of Dabovski et al.
(1966) and Iliev & Katskov (1990).

Initially, a Late Alpine time of shearing was suggested

(Dabovski  et  al.  1966),  whereas  Antonov  et  al.  (2010)  as-
sumed a Variscan deformation. The sole indirect age indica-
tor  is  the  fact  that  the  Chuminska  Shear  Zone  cuts  the
migmatitic  gneisses  of  the  Pirdop  Unit  in  which  the  leuco-
some has been dated to 336.5 ± 5.4 Ma (Carrigan et al. 2006).
Thus, the deformation clearly postdated the ca. 336 Ma high-
temperature  metamorphic  event.  In  regional  scale,  similari-
ties in terms of geometry, deformation style and kinematics
are  found  between  the  Chuminska  Shear  Zone  and  several
Early  Alpine  north-vergent  thrusts  in  the  Stara  Planina
Mountain area adjacent to the north (for details see Gerdjikov
et al. 2007; Lazarova & Gerdjikov 2008 and references therein).
Therefore,  a  kinematic  and  temporal  correlation  with  these
structures can be made. Furthermore, south of the Koprivsh-
titsa  area,  low-temperature  (greenschist  facies)  shear  zones
deforming the high-grade Variscan metamorphic rocks have
been dated at 105—100 Ma (

Ar/

Ar mica ages – Velichkova

et al. 2004). However, at this stage of research an unequivo-
cal time constraint of the activities along the Chuminska
Shear Zone cannot be given.

Correlations and tectonic context

Southwest of the studied area, the Central Sredna Gora

Complex  is  similarly  subdivided  into  two  high-grade  meta-
morphic units (Kouzhoukharov et al. 1980; Dabovski 1988;
Zagorchev 2008) – one is dominated by migmatitic gneisses
(the Plana Unit) and the other is composed of partially mig-
matized  or  non-migmatized  biotite  gneisses,  amphibolites
and eclogite lenses (the Garvanitsa Unit). They lack a direct
connection  with  the  Pirdop  and  Koprivshtitsa  units,  being
separated by a several kilometers wide strip of Mesozoic and
Cenozoic sedimentary and magmatic rocks (Fig. 2a). Never-
theless,  based  on  the  similar  lithology  and  the  grade  of  the
metamorphic overprint a correlation between the Pirdop and
Plana  units  and  between  the  Koprivshtitsa  and  Garvanitsa
units might be suggested.

In the context of the Paleozoic geodynamic evolution of

the  Balkans,  a  number  of  crustal  “pieces”  now  composing
the  Variscan  high-grade  metamorphic  complexes  belonged
to  the  northern  peri-Gondwanan  realm  most  probably  until
the  Devonian  and  Early  Carboniferous  (Yanev  1993,  1997,
2000; Lakova 1995; Gutierrez-Marco et al. 2003; Boncheva
et al. 2010 and references therein). The most recent regional
(Nance & Linnemann 2008; Nance et al. 2010, 2012; Stampfli
et  al.  2011,  2013;  von  Raumer  et  al.  2013)  and  local
(Kounov et al. 2012; Balintoni et al. 2014; Antic et al. 2014)
geodynamic  reconstructions  suggest  that  during  the  time
span  between  the  Late  Neoproterozoic  and  mid  Paleozoic,
several events of an enhanced magmatic activity took place:
i) late Ediacaran—early (to mid?) Cambrian subduction, accre-
tion and arc magmatism linked to the evolution of the Proto-
tethys  Ocean;  ii)  late  Cambrian—Early  Ordovician  rifting
related to the opening of the Rheic Ocean; iii) Mid Ordovi-
cian—Early  Devonian  magmatic  arc  magmatism.  According
to  this  scheme,  and  considering  the  newly  obtained  ca.
500 Ma  protolith  ages  for  both  metagranitoids  (this  study)
and metamafic rocks (Peytcheva & von Quadt 2004; Antonov
et  al.  2010),  we  could  suggest  that  the  mafic  and  the  felsic
igneous  rocks  in  the  northern  flank  of  the  Central  Sredna
Gora Complex are most probably related to the initial open-
ing  stages  of  the  Rheic  Ocean  or  a  related  basin  in  the  late
Cambrian. They intruded a Late Neoproterozoic (Carrigan et
al. 2006) crust, which in the studied area is presented by the
Bobevitsa-type orthogneisses.

Conclusions

The Koprivshtitsa Unit is an unmigmatized, orthogneiss-

dominated part of the Variscan high-grade section of the Cen-
tral Sredna Gora Complex in Bulgaria. The unit represents a
fragment  of  a  metamorphosed  magmatic  complex  that  in-
cludes  metagranitoids  and  metabasites.  The  newly  obtained
crystallization  age  for  the  protoliths  of  the  metagranites  is
491.5 ± 7.6 Ma.  The  similar  ages  of  493.8 ± 9.8 Ma  from  a
metagabbro  (Antonov  et  al.  2010)  and  of  502.8 ± 3.2 Ma
from  a  hornblende-biotite  gneiss  (Peytcheva  &  von  Quadt
2004) from the migmatitic Pirdop Unit suggest a contempo-
raneous  Late  Cambrian  emplacement  of  mafic  and  felsic
melts.  They  intruded  a  Cadomian  crustal  fragment  (the
Bobevitsa-type  orthogneisses)  dated  to  ca.  616.9 ± 9.5  and
595 ± 23 Ma (Carrigan et al. 2006).

The age of ca. 400 Ma obtained from zircon rims shows

that most probably an Early Devonian high-grade metamor-
phic event affected these basement rocks. A similar age is
obtained from eclogites southwest of the studied area. These
data are an important contribution to the further reconstruc-
tions of the P-T-t metamorphic evolution of the Balkan part
of the Variscan belt.

The contact of the Koprivshtitsa Unit with the migmatitic

Pirdop Unit is a greenschist facies mylonitic zone, postdating
the high-temperature metamorphic overprint of the gneiss sec-
tion. An Alpine age of the shearing or reactivation of preexist-
ing structure could be assumed based on structural similarities
with shear zones from the terranes adjacent to the study area.

453

CAMBRIAN MAGMATISM AND VARISCAN METAMORPHISM IN SREDNA GORA (BULGARIA)

GEOL

EOL

EOLOGICA CARPA

OGICA CARPA

OGICA CARPATHICA

THICA

THICA, 2015, 66, 6, 443—454

According to published Paleozoic geodynamic schemes,

we can suggest that the igneous rocks from the northern
flank of the Sredna Gora Complex intruded Ediacaran crustal
fragments most probably during the initial opening stages of
the Rheic Ocean in the late Cambrian.

Acknowledgments: This study was funded by the Geological
Institute  of  the  Bulgarian  Academy  of  Science.  We  thank
Kamelia Nedkova for her help during U-Pb dating as well as
Irena  Peytcheva,  Zlatka  Cherneva  and  Ianko  Gerdjikov  for
the helpful discussions during our work. Alexandre Kounov,
Marian  Janák  and  two  anonymous  reviewers  are  highly
thanked for their constructive reviews of the manuscript.

References

Antić M., Peytcheva I., von Quadt A., Kounov A., Trivić B., Serafi-

movski T., Tasev G. & Gerdjikov I. 2014: Geochronological
and geochemical studies on crystalline rocks from the central
Serbo-Macedonian massif with implications on its pre-Alpine
evolution. In: Proceedings of XX CBGA Congress. Bul. Shk.
Gjeol., Spec. Iss. 1, 195.

Antonov M., Gerdjikov S., Metodiev L., Kiselinov Ch., Sirakov V. &

Valev V. 2010: Explanatory note to the Geological Map of the
Republic of Bulgaria in scale 1 : 50,000. Map Sheet K-35-37-B
(Pirdop). Geocomplex, Sofia, 1—99.

Arnaudov V., Amov B., Bartnitskii E. & Pavlova M. 1989: Isotopic

geochronology of igneous and metamorphic rocks in Bal-
kanides and Rhodope Massif. XIV Congr. CBGA, Abstract
Vol., Sofia, 1154—1157 (in Russian).

Balintoni I., Balica C., Ducea M.N. & Hann H.-P. 2014: Peri-Gond-

wanan terranes in the Romanian Carpathians: A review of their
spatial distribution, origin, provenance, and evolution. Geosci.
Frontiers 5, 395—411.

Boncheva I., Lakova I., Sachanski V. & Koenigshof P. 2010: Devo-

nian stratigraphy, correlations and basin development in the Bal-
kan Terrane, western Bulgaria. Gondwana Res. 17, 573—582.

Bosse V., Boulvais P., Gautier P., Tiepolo M., Ruffet G., Devidal

J.L., Cherneva Z., Gerdjikov I. & Paquette J.-L. 2009: Fluid-
induced disturbance of the monazite Th-Pb chronometer: In
situ dating and element mapping in pegmatites from the
Rhodope (Greece, Bulgaria). Chem. Geol. 261, 3—4, 286—302.

Burg J.-P. 2012: Rhodope: From Mesozoic convergence to Cenozoic

extension. Review of petro-structural data in the geochrono-
logical frame. In: Skourtsos E. & Lister G.S. (Eds.): The geol-
ogy of Greece. J. Virt. Explorer 39, paper 1.

Burg J.-P., Ricou L.-E., Ivanov Z., Godfriaux I., Dimov D. & Klain

L. 1996: Syn-metamorphic nappe complex in the Rhodope
Massif. Structure and kinematics. Terra Nova 8, 1, 6—15.

Carrigan C., Mukasa S., Haydoutov I. & Kolcheva K. 2003: Ion mi-

croprobe U-Pb zircon ages of pre-Alpine rocks in Balkan,
Sredna Gora, and Rhodope terrains of Bulgaria: Constrains of
Neoproterozoic and Variscan tectonic evolution. J. Geosci. 48,
1—2, 32.

Carrigan C., Mukasa S., Haydoutov I. & Kolcheva K. 2005: Age of

Variscan magmatism from the Balkan sector of the orogen,
central Bulgaria. Lithos 82, 125—147.

Carrigan C., Mukasa S., Haydoutov I. & Kolcheva K. 2006:

Neoproterozoic magmatism and Carboniferous high-grade
metamorphism in the Sredna Gora Zone, Bulgaria: An exten-
sion of the Gondwana-derived Avalonian-Cadomian belt? Pre-
cambrian Res. 147, 3—4, 404—416.

Cortesogno L., Gaggero L., Haydoutov I. & Buzzi L. 2005: The

eclogite to amphibolite metamorphic path from the Sredna
Gora terrane in the Variscan orogenic segment of Bulgaria (SE
Europe). Geophys. Res. Abstract Vol., 7, 01802.

Dabovski Ch. 1988: Precambrian blocks in the Srednogorie Zone.

In: Zoubek V., Cogné J., Kozhoukharov D. & Krautner H.G.
(Eds.): Precambrian in younger fold belts. John Wiley & Sons,
Chichester, 841—847.

Dabovski Ch., Zagorchev I., Ruseva M. & Chunev D. 1966: The Pa-

leozoic granitoids of the Sashtinska Sredna Gora. Ann. Direc-
tion Géol. 16, 57—96 (in Bulgarian).

Dabovski Ch., Boyanov I., Christchev. C., Nikolov T., Sapunov I.,

Yanev Y. & Zagorchev I. 2002: Structure and Alpine evolution
of Bulgaria. Geol. Balcanica 32, 2—4, 9—15.

Dallmeyer R., Neubauer F., Fritz H. & Mocanu V. 1998: Variscan

vs. Apline tectonothermal evolution of the Southern Car-
pathian orogen: constrains from

Ar/

Ar ages. Tectonophysics

290, 111—135.

Franke W., Haak V., Oncken O. & Tanner D. (Eds.) 2000: Orogenic

processes: Quantification and modelling in the Variscan Belt.
Geol. Soc. London, Spec. Publ. 179, 1—463.

Froitzheim N., Plašienka D. & Schuster R. 2008: Alpine tectonics of

the Alps and Western Carpathians. In: McCann T. (Ed.): The
geology of Central Europe. Geol. Soc., London, 1141—1232.

Gaggero L., Buzzi L., Haydoutov I. & Cortesogno L. 2009: Eclogite

relics in the Variscan orogenic belt of Bulgaria (SE Europe).
Int. J. Earth Sci. 98, 8, 1853—1877.

Gerdjikov I., Georgiev N., Dimov D. & Lazarova A. 2007: The dif-

ferent faces of supposedly single thrust: a reevaluation of the
Vezhen thrust, Central Balkanides. In: Proceeding of National
conference “Geosciences 2008”. Bulg. Geol. Soc., Sofia,
24—26.

Gerdjikov I., Lazarova A., Kounov A. & Vangelov D. 2013: High-

grade metamorphic complexes in Bulgaria. Part I. Geology and
geophysics. Ann. Univ. Min. Geol. “St. Ivan Rilski” 56, 47—52
(in Bulgarian with English summary).

Gerdjikov I., Ruffet G., Lazarova A., Vangelov D., Balkanska E. &

Bonev K. 2010:

Ar/

Ar geochronologic constrains of a

Variscan transpression in Central Stara planina Mountain. In:
Proceeding of National conference “Geosciences 2010”. Bulg.
Geol. Soc., 109-110.

Gutierrez-Marco J.C., Yanev S., Sachanski V., Rabano I., Lakova

I., San Jose Lancha M.A., Diez Martinez E., Boncheva I. &
Sarmiento G. 2003: New biostratigraphical data from the Or-
dovician of Bulgaria. In: Albanesi G.L., Beresi M.S. & Peralta
S.H. (Eds.): Ordocivian of the Andes. INSUGEO, Ser. Correl.
Geol. 17, 79—85.

Iancu V., Berza T., Seghedi A. & Marunyiu M. 2005: Paleozoic rock

assemblages incorporated in the South Carpathian Alpine Thrust
Belt (Romania and Serbia): A review. Geol. Belg. 8, 4, 48—68.

Iliev K. & Katskov N. 1990: Geological map of Bulgaria in scale

1 : 100,000. Map sheet Panagyurishte. Geol. and Geophys.
JS Co., Sofia.

Ivanov Z. 1988: Apercu general sur l’evolution geologique et struc-

turale du massif des Rhodopes dans le card des Balkanides.
Bull. Soc. Geol. France 8, 227—240.

Jackson S.E., Pearson N.J., Griffin W.L. & Belousova E.A. 2004:

The application of laser ablation-inductively coupled plasma-
mass spectrometry to in situ U-Pb zircon geochronology.
Chem. Geol. 211, 1—2, 47—69.

Kaiser-Rohrmeier M., Handler R., von Quadt A. & Heinrich C.

2004: Hydrothermal Pb-Zn ore formation in the central Rhodo-
pian dome, south Bulgaria: review and new time constraints
from Ar-Ar geochronology. Schweiz. Mineral. Petrogr. Mitt.
84, 1, 37—58.

Kounov A., Graf J., von Quadt A., Bernoulli D., Burg J.P., Seward

D., Ivanov Z. & Fanning M. 2012: Evidence for a “Cadomian”

454

LAZAROVA, NAYDENOV, PETROV and GROZDEV

GEOL

EOL

EOLOGICA CARPA

OGICA CARPA

OGICA CARPATHICA

THICA

THICA, 2015, 66, 6, 443—454

ophiolite and magmatic-arc complex in SW Bulgaria. Precam-
brian Res. 212—213, 275—295.

Kouzhoukharov D., Kouzhoukharova E. & Christov S. 1980: The

Precambrian in the northern parts of Plana Mountains and
Vakarel Hills. Rev. Bul. Geol. Soc. 41, 3, 211—222 (in Bulgarian
with English summary).

Kroner U. & Romer R. 2013: Two plates – Many subduction

zones: The Variscan orogeny reconsidered. Gondwana Res. 24,
298—329.

Kroner U., Mansy J.L., Mazur S., Aleksandrowski P., Hann H.P.,

Huckriede H., Lacquement F., Lamarche J., Ledru P., Pharaoh
T.C., Zedler H., Zeh A. & Zulauf G. 2008: Variscan tectonics.
In: McCann T. (Ed.): The geology of Central Europe. Geol.
Soc., London, 599—664.

Lakova I. 1995: Palaeobiogeographical affinities of Pridolian and

Lochkovian chitinozoans from North Bulgaria. Geol. Balcanica
26, 5—6, 23—28.

Lazarova A. & Gerdjikov I. 2008: Structures of sheared granitoids

from the Zlatishka Stara Planina Mountain: indicators for the
deformation at frictional-viscous transition. Rev. Bul. Geol.
Soc. 69, 1—3, 7—20 (in Bulgarian with English summary).

Lazarova A., Gerdjikov I., Vangelov D. & Georgiev N. 2010:

Variscan transpression and related voluminous magmatism in
Central Strara Planina Mountain, Bulgaria. Geol. Balcanica
38, 1—2, 226—227.

Liati A. 2005: Identification of repeated Alpine (ultra) high-pres-

sure metamorphic events by U-Pb SHRIMP geochronology
and REE geochemistry of zircon: the Rhodope zone of Northern
Greece. Contr. Mineral. Petrology 150, 608—630.

Ludwig K.R. 2003: User’s manual for Isoplot/Ex, version 3.0, a

geochronological toolkit for Microsoft Excel. Berkeley Geo-
chronology Center, Spec. Publ., Berkeley, CA 4, 1—74.

Nance R.D. & Linnemann U. 2008: The Rheic Ocean: Origin, evo-

lution, and significance. GSA Today 18, 12, 4—12.

Nance R.D., Gutiérrez-Alonso G., Keppie J.D., Linnemann U.,

Murphy J.B., Quesada C., Strachan R.A. & Woodcock N.H.
2010: Evolution of the Rheic Ocean. Gondwana Res. 17,
194—222.

Nance R.D., Gutierrez-Alonso G., Keppie J.D., Linnemann U.,

Murphy J.B., Quesada C., Strachan R.A. & Woodcock N.H.
2012: A brief history of the Rheic Ocean. Geosci. Frontiers 3,
2, 125—135.

Neubauer F. & Handler R. 1999: Variscan orogeny in the Eastern

Alps and Bohemian Massif: How do these units correlate?.
Mitt. Österr. Geol. Gesell. 92, 35—59.

Peytcheva I. & von Quadt A. 2004: The Palaeozoic protoliths of

Central Srednogorie, Bulgaria: records in zircons from base-
ment rocks and Cretaceous magmatites. 5th International Sym-
posium on Eastern Mediterranean Geology, Thessaloniki,
Greece, Conf. Vol., Extended abstract, T11-9.

Schmid S.M., Bernoulli D., Fügenschuh B., Matenco L., Schefer S.,

Schuster R., Tischler M. & Ustaszewski K. 2008: The Alpine-
Carpathian-Dinaridic orogenic system: correlation and evolu-
tion of tectonic units. Swiss J. Geosci. 101, 139—183.

Schulmann K., Martinez Catalan K., Lardeaux J., Janoušek J. &

Oggiano J. (Eds.) 2014: The Variscan Orogeny: extent, times-
cale and the formation of the European Crust. Geol. Soc. Lon-
don, Spec. Publ. 405, 1—400.

Sláma J., Košler J., Condon D.J., Crowley J.L., Gerdes A., Hanchar

J.M. & Whitehouse J. 2008: Plešovice zircon – A new natural
reference material for U—Pb and Hf isotopic microanalysis.
Chem. Geol. 249, 1—2, 1—35.

Stampfli G.M., von Raumer J. & Wilhem C. 2011: The distribution of

Gondwana-derived terranes in the Early Paleozoic. In: Gutiérrez-
Marco J.C., Rábano I. & García-Bellido D. (Eds.): The Ordovi-
cian of the World. Cuadernos del Museo Geominero Instituto
Geológico y Minero de España, Madrid 14, 567—574.

Stampfli G.M., Hochard C., Vérard C., Wilhem C. & von Raumer J.

2013: The formation of Pangea. Tectonophysics 593, 1—19.

Turpaud P. & Reischmann T. 2010: Characterisation of igneous ter-

ranes by zircon dating: implications for UHP occurrences and
suture identification in the Central Rhodope, northern Greece.
Int. J. Earth. Sci. 99, 3, 567—591.

Velichkova S., Handler R., Neubauer F. & Ivanov Z. 2004:

Variscan to Alpine tectonothermal volution of the Central
Srednogorie unit, Bulgaria: constraints from

Ar/

Ar analy-

sis. Schweiz. Mineral. Petrogr. Mitt. 84, 133—151.

von Quadt A., Peytcheva I., Frank M., Nedyalkov R., Kamenov B. &

Heinrich C. 2004: Subduction related rocks in Medet Cu-por-
phyry deposit: Sources and magma evolution. Goldschmidt
Conf. Abs., 626.

von Raumer J.F., Bussy F., Schaltegger U., Schulz B. & Stampfli

G.M. 2013: Pre-Mesozoic Alpine basements – their place in
the European Paleozoic framework. GSA Bulletin 125, 1—2,
89—108.

Yanev S. 1993: Gondwana Paleozoic terranes in the Alpine collage

system of the Balkans. Hymalaian Geology 4, 2, 257—270.

Yanev S. 1997: Paleozoic migration of terranes from the basement

of the eastern part of the Balkan peninsula from peri-Gondwa-
na to Laurussia. Turkish Assoc. Petrol. Geol., Spec. Publ. 3,
89—100.

Yanev S. 2000: Palaeozoic terranes of the Balkan Peninsula in the

framework of Pangea assembly. Palaeogeogr. Palaeoclimatol.
Palaeoecol. 161, 151—177.

Zagorchev I. 2008: Amphibolite-facies metamorphic complexes in

Bulgaria and Precambrian geodynamics: controversies and
“state of the art”. Geol. Balcanica 37, 1—2, 33—46.

Zagorchev I., Dabovsky Ch. & Tchunev D. 1973: Tectonics of west-

ern part of the Sredna Gora metamorphic block (Sashtinska
Sredna Gora). Rev. Bul. Geol. Sci. 37, 1, 1—10 (in Bulgarian).