GEOLOGICA CARPATHICA, FEBRUARY 2009, 60, 1, 15—33 doi: 10.2478/v10096-009-0001-8
www.geologicacarpathica.sk
The emplacement mode of Upper Cretaceous plutons from
the southwestern part of the Sredna Gora Zone (Bulgaria):
structural and AMS study
NEVEN GEORGIEV
1*
, BERNARD HENRY
2
, NELI JORDANOVA
3
, NIKOLAUS FROITZHEIM
4
,
DIANA JORDANOVA
3
, ZIVKO IVANOV
1
and DIMO DIMOV
1
1
Department of Geology and Paleontology, Sofia University “St. Kliment Ohridski” 15 Tzar Osvoboditel bd., 1000 Sofia, Bulgaria;
neven@gea.uni-sofia.bg
2
Paléomagnétisme, IPGP and CNRS, 4 av. de Neptune, 94107 Saint-Maur cedex, France
3
Geophysical Institute, Bulgarian Academy of Sciences, Acad. G. Bonchev str., block 3, 1113 Sofia, Bulgaria
4
Steinmann-Institut, Universität Bonn, Nußallee 8, D-53115 Bonn, Germany
(Manuscript received February 21, 2008; accepted in revised form June 12, 2008)
Abstract: Several plutons located in the southwestern part of the Sredna Gora Zone – Bulgaria are examples of the
Apuseni-Banat-Timok-Sredna Gora type of granites emplaced during Late Cretaceous (86—75 Ma) times. The studied
intrusive bodies are spatially related to and deformed by the dextral Iskar-Yavoritsa shear zone. The deformation along
the shear zone ceased at the time of emplacement of the undeformed Upper Cretaceous Gutsal pluton, which has in-
truded the Iskar-Yavoritsa mylonites. A clear transition from magmatic foliation to high-, moderate- and low-tempera-
ture superimposed foliation and lineation in the vicinity of the Iskar-Yavoritsa and related shear zones gives evidence
for simultaneous tectonics and plutonism. Away from the shear zones, the granitoids appear macroscopically isotropic
and were investigated using measurements of anisotropy of magnetic susceptibility at 113 stations. The studied samples
show magnetic lineation and foliation, in agreement with the magmatic structures observed at a few sites. Typical
features of the internal structure of the plutons are several sheet-like mafic bodies accompanied by swarms of mafic
microgranular enclaves. Field observations indicate spatial relationships between mafic bodies and shear zones as well
as mingling processes in the magma chamber which suggest simultaneous shearing and magma emplacement. Struc-
tural investigations as well as anisotropy of magnetic susceptibility (AMS) data attest to the controlling role of the NW-
SE trending Iskar-Yavoritsa shear zone and to the syntectonic emplacement of the plutons with deformation in both
igneous rocks and their hosts. The tectonic situation may be explained by partitioning of oblique plate convergence into
plate-boundary-normal thrusting in the Rhodopes and plate-boundary-parallel transcurrent shearing in the hinterland
(Sredna Gora).
Key words: Late Cretaceous, Sredna Gora – Bulgaria, tectonics, magma emplacement, strike-slip setting, AMS.
Introduction
The role of shear zones in the generation, ascent and emplace-
ment of magma through the crust is a long-standing matter of
debate (Castro 1987; Hutton 1988, 1997; Paterson & Fowler
1993; Acef et al. 2003; Rosenberg 2004). Numerous examples
of granite emplacement along strike-slip fault systems in
transpressional or transtensional tectonic settings (Tikoff &
Teyssier 1992; Paterson & Fowler 1993; Roman-Berdiel et al.
1997; Steenken et al. 2000; Henry et al. 2004) suggest different
possibilities to solve the “space problem”. Visible structures of
the granitoids formed in the magmatic stage and superimposed
solid-state foliations and lineations provide valuable informa-
tion on magma emplacement kinematics and can help us to bet-
ter understand the connection between tectonics and
magmatism (Saint Blanquat & Tikoff 1997; Steenken et al.
2000). The latter is not a simple task, especially when the stud-
ied intrusives are nearly isotropic and widely used structural
methods are not applicable. In such cases the measurement of
Anisotropy of Magnetic Susceptibility (AMS) is a standard pro-
cedure (Tarling & Hrouda 1993). Determination of magnetic
fabric is a quick and easy semi-quantitative method to provide
information on the bulk fabric of the plutons (Henry 1980;
Bouchez 1997; Saint Blanquat & Tikoff 1997). The results are
often used to support structural data in constraining the syn- or
post-tectonic emplacement of intrusions (Henry 1980; Bouchez
1997; Henry et al. 2004).
On the other hand, some studies (Ferre et al. 1997) demon-
strate that crustal-scale shear zones play a role of feeder
channels and can provide space for chamber formation at
different depths. These relationships can explain the frequent
coexistence of magmas contrasting in composition (felsic
and mafic) at the same crustal level (Michael 1991; Wiebe &
Collins 1998). Interaction between contrasting magmas
(mixing versus mingling processes) depends on parameters
like volume, time, compositions, temperature, viscosity, etc.
(Barbarin & Didier 1992).
The Upper Cretaceous intrusive bodies located in the
southwestern part of the Sredna Gora Zone, Bulgaria, crop
out in close relationship with the regional dextral strike-slip
Iskar-Yavoritsa shear zone (IYSZ). The aim of this study is
to investigate the space- and time connection between plu-
tons and shear zone using structural and AMS data, and to
provide a model for emplacement of these granites.
16
GEORGIEV, HENRY, JORDANOVA N., FROITZHEIM, JORDANOVA D., IVANOV and DIMOV
Geological setting
The Sredna Gora Zone in Bulgaria is a part of a complex,
elongated Late Cretaceous—Tertiary magmatic arc that can
be traced from the Apuseni Mountains in Romania to Iran
(Bergougnan & Fourquin 1980; Sandulescu 1984; Mitchell
1996; Jankovic 1977, 1997; Berza et al. 1998; Stampfli &
Mosar 1999; Neubauer 2002). This zone is regarded as a vol-
canic island arc (Boccaletti et al. 1974, 1978), back-arc basin
(Hsu et al. 1977) or intra-arc basin with submarine volcan-
ism (Nachev 1978). According to Dabovski (1980) the
Sredna Gora Zone represents an intracontinental rift, which
originated in connection with the Vardar subduction. Intru-
sive and volcanic rocks, the products of Upper Cretaceous
magmatic activity, are widespread in the Sredna Gora Zone.
According to Ivanov (1989) the studied plutons (Fig. 1)
were emplaced at the boundary between two different meta-
morphic terranes (Balkanide terrane to the North and Rhodope
terrane to the South), separated by the dextral strike-slip
IYSZ. The northern terrane (Balkanide metamorphic com-
plex; Ivanov 1988, 1989) is built up of Variscan high-grade
metamorphic rocks metamorphosed at 320—340 Ma and of
315 to 289 Ma old granitoid bodies (Amov et al. 1982;
Velichkova et al. 2001, 2004; Carrigan et al. 2005, 2006). To
the south of the IYSZ the Rhodope terrane is represented by
the variegated unit, including ortho- and para-metamorphic
rocks (Ivanov et al. 2000). The age of metamorphism in this
part of the Rhodope terrane is still unknown. The variegated
unit is equivalent to the Asenitsa Unit further east in the
Central Rhodopes. U-Pb zircon data give evidence for Juras-
sic ( ~ 150 Ma) magmatic protoliths of Asenica gneisses to
the SE of the studied area (von Quadt et al. 2006). In the
Late Cretaceous, the rocks of the Rhodope terrane in our
study area must already have been exhumed to a level be-
tween the middle and upper crust because they were intruded
by the granites (Georgiev & Lazarova 2003).
In the southwestern part of the Sredna Gora Zone (Fig. 1),
several Upper Cretaceous plutonic bodies (Plana, Gutsal and
Elshitsa-Boshulia) and the “Variscan” Varshilo intrusion
(Dimitrov 1933; Boyadjiev 1979 and references therein;
Dabovski 1980, 1988 and references therein; Belmustakova
1984) were distinguished. These plutons form a WNW-ESE-
trending complex belt on both sides of the IYSZ. Previous
interpretations regard the Upper Cretaceous magmatic bod-
ies as a result of multiphase or two-phase intrusions with
normal partial differentiation of basaltic magma. Consider-
ing emplacement mechanisms, Dabovski (1988) suggested
that the plutons are “fissure intrusions” formed in a rift setting.
The plutons comprise two different groups of rocks: felsic
(granites and granodiorites) and mafic to intermediate (gab-
bros, quartz-monzogabbros, quartz-monzodiorites and
quartz-diorites). The mafic varieties were supposed to be
older, numerous mafic enclaves in the granites and grano-
diorites being regarded as xenoliths from the country rocks-
Fig. 1. Simplified geological map of the southwestern parts of the Sredna Gora Zone.
17
UPPER CRETACEOUS PLUTONS FROM THE SREDNA GORA ZONE (BULGARIA)
gabbros and basic volcanics. The major element composi-
tions of the granitoids are typical for calc-alkaline and is-
land-arc magmatic products. The compositions of mafic rocks
correspond to tholeiitic and to calc-alkaline varieties.
Recently obtained U-Pb data (Peytcheva et al. 2001;
Peytcheva & von Quadt 2003) indicate small differences be-
tween the ages of granites – 82 Ma, granodiorites – 86—
84 Ma, and gabbros – 82—84 Ma (Table 1). The dating of
the Varshilo pluton, previously assumed to be Variscan,
yielded a Late Cretaceous (82 Ma) age. Unpublished U-Pb
zircon data (Georgiev, von Quadt & Peytcheva) from the
Gutsal granodiorite yielded a ~ 75 Ma age of crystallization.
This younger age is supported by the field relationships: the
Varshilo granites are sheared in the vicinity of IYSZ whereas
sills of Gutsal granodiorite are undeformed and have in-
truded the IYSZ mylonitic foliation.
Thermo-barometrical data (Georgiev & Lazarova 2003)
show that the emplacement of granodioritic melts took place
under pressures between 470 MPa (corresponding to a depth
of ca. 13 km) and 320 MPa (ca. 9 km).
Structures of the plutons
Magmatic and submagmatic structures
The plutonic bodies mostly appear macroscopically isotro-
pic. Nevertheless, magmatic foliation is often visible close to
the plutonic contacts or forms bands parallel and related to
the IYSZ and its associated shear zones. Far from mylonitic
domains, magmatic foliation is less pronounced and oblique
to the shear zones. Near undeformed plutonic contacts, mag-
matic foliation is parallel both to the contacts and to the host
rock’s schistosity. The magmatic foliation is defined by the
preferred orientation of biotite, feldspar and hornblende
crystals (Fig. 2a,d) as well as mafic microgranular enclaves
(Fig. 2b). Magmatic layering is observed in some gabbro
bodies (Fig. 2c), defined by layers of different mineral pro-
portion and/or different grain size of the rock-forming min-
erals. The magmatic foliation predominantly trends
100—140° and dips steeply (45—90°) towards the NE or the
SW (Fig. 3). The magmatic lineation data are too scarce to
be representative and informative. The magmatic lineation is
defined mostly by alignment of hornblende, biotite and
quartz-feldspar aggregates (Fig. 2a) and in some places by
mafic enclaves.
Microfractures in feldspar phenocrysts are filled with re-
sidual melt represented by magmatic quartz (Fig. 4a and b)
which is typical for a submagmatic stage of deformation
(Bouchez et al. 1992). Thin section investigations of samples
with visible magmatic foliation show parallel alignment of
elongated feldspar crystals (Fig. 4c). The initial stages of
subgrain formation observed in potassium feldspars, together
with the lack of any other deformation in the samples with vis-
ible magmatic foliation, suggest transitional submagmatic to
high-temperature solid-state deformation (Fig. 4d).
Shear zones and solid-state deformation
In a regional view, important structures in the studied area
are the IYSZ and related shear zones (Fig. 1). The IYSZ is an
80—90 km long and 0.4—1.0 km wide shear zone with general
strike of 110—130° and steep dips (60—80°) toward the NE to
SW. In the vicinity of the IYSZ, several satellite shear zones
of similar orientations, kinematics and deformation features
were recognized (Fig. 1): the Akandjievo shear zone (ASZ),
Slavovitsa shear zone (SSZ), Tserovo shear zone (TSZ) and
Sapdere fault (SDF). The deformation degree increases in
narrow (15 to 40 m thick) mylonitic bands within the shear
zones where the studied plutons as well as their host rocks
are transformed into mylonites. The deformation style grades
from ductile (high-temperature) in outer parts of the shear
zones to brittle-ductile within the inner parts. Mica bands
and elongated quartz-feldspar aggregates define the folia-
tion. Elongated biotite flakes and quartz grains trace the min-
eral lineation on the foliation planes. Striations are rarely
found on the foliation planes in domains with brittle-ductile
overprint. The mineral lineation plunges 5 to 50° to the NW
or is subhorizontal (Fig. 3).
High-temperature solid sate deformation
In the field, domains with a high-temperature solid-state
deformation are located at the transition between intensively
deformed parts of the shear zones and areas with a magmatic
foliation. Most of the outcrops show clear relationship be-
tween roughly NW-SE-trending vertical foliation and
steeply dipping or vertical c’-shear bands (Fig. 5a). In the
outcrops with many mafic microgranular enclaves, the s—c’
pattern has produced “enclave fish” which are reliable crite-
ria indicating dextral kinematics. Quartz with mosaics of
square subgrains and chessboard patterns (Fig. 5b) indicates
high-temperature solid-state deformation (>ca. 650°; Kruhl
1996; Stipp et al. 2002; Passchier & Trouw 2005). Feldspar
grains with core-and-mantle structure (Fig. 5d) may repre-
sent a transitional stage (450° to 500 °C; Passchier & Trouw
2005) between this high-temperature deformation and the
colder deformation stages described below.
Sample
Pluton
Locality
Rock type
Age and 2
σ error (Ma) Reference
AvQ029
Elshitsa-Boshulia
2 km NW of Elshitsa village
granite
86.62 ± 0.11
Peytcheva et al. (2003)
AvQ019 Elshitsa-Boshulia Velichkovo quarry
granodiorite
84.6 ± 0.3
von Quadt et al. (2005)
AvQ018 Elshitsa-Boshulia Velichkovo quarry
hybrid gabbro
82.16 ± 0.10
von Quadt et al. (2005)
AvQ023
Elshitsa-Boshulia
Vetrensko Gradishte
hybrid gabbro
84.87 ± 0.13
von Quadt et al. (2005)
ZI6
Varshilo
2.5 km W of Dolno Varshilo village granite
82.25
± 0.22
von Quadt et al. (2005)
AvQ026 Lesichovo
1 km N of Lesichovo village
granite
316.5 ± 3.5
Peytcheva & von Quadt (2003)
Table 1: U-Pb zircon ages for intrusive rocks from the southern parts of Central Sredna Gora.
18
GEORGIEV, HENRY, JORDANOVA N., FROITZHEIM, JORDANOVA D., IVANOV and DIMOV
Moderate- to low-temperature solid-state deformation
Moderate- to low-temperature structures are typical for
high-strain bands within the inner parts of the IYSZ system
where gneissification and s—c granitoid mylonites are ob-
served (Fig. 6a). In some places the granitoids were trans-
formed into quartz-chlorite ultramylonites (Fig. 6b).
Moderate- to low-temperature deformation has produced mi-
crostructures at decreasing temperatures during shearing:
high-angle undulose extinction and subgrains are ubiquitous
in quartz; biotite is partly or totally replaced by chlorite;
feldspar porphyroclasts contain deformational myrmekites
(Fig. 6c). The matrix of the mylonites was ductile but the
feldspars have responded to deformation as brittle rigid ob-
jects (Fig. 6d) and formed “book shelves” and “v”-pull apart
microstructures (Hippertt 1993). Criteria within intensively
deformed parts of the shear zones consistently show dextral
shear sense (Fig. 6a,d; Fig. 7).
Fig. 2. Field and micrographic examples of primary magmatic structures for the studied igneous rocks. a – Magmatic foliations defined by
preferred orientation of feldspar and hornblende crystals in granodiorite. b – Magmatic foliation defined by stretched mafic microgranular
enclaves within granodiorites host. c – The magmatic layering of gabbroic sheets is commonly observed. The separate layers are defined
by differences in mineral composition or by differences in size of rock-forming minerals. d – Photomicrograph of a granite with magmatic
foliation defined by ordered biotite and rare feldspar crystals.
19
UPPER CRETACEOUS PLUTONS FROM THE SREDNA GORA ZONE (BULGARIA)
Fig. 3.
Position
of
the
observed
magmatic
and
superimposed
structures
for
plutonic
bodies
and
metamorphic
host
rocks.
Stereographic
pr
ojection
on
lower
hemisphere
of
superimposed
folia-
tions and lineations along:
a
– Iskar-Yavoritsa shear zone;
b
–
Akandjievo
shear
zone;
c
–
Slavovitsa
shear
zone
;
d
–
Tserovo
shear
zone;
e
–
Foliation
and
stretching
lineation
of
metamor-
phic host;
f
– Magmatic fabrics of granitoids.
Annealing process
In the vicinity of the Gutsal granite
intrusion, quartz- and mica-rich
mylonites of the IYSZ show evidence
for post-kinematic annealing. In out-
crop, these rocks appear like low-tem-
perature mylonites and phyllonites.
The microscopic observations show
that these high-strain rocks were over-
printed by relatively high-temperature
static recrystallization (Fig. 5c,e,f).
This is shown by polygonal quartz
grains and foam structure in foliation-
parallel quartz domains.
Magma mingling processes
Porphyritic granodiorites, granites,
and gabbros are the major types of
studied intrusive rocks. The field rela-
tionships are critical for interpreting
the coexistence of felsic and mafic
rocks within the plutons. Mafic rocks
(gabbros, gabbro-diorites and diorites)
build up several sheet-like or lensoid
bodies (Fig. 1), ranging in size from
100—200 m
2
to 1.5—2 km
2
and in thick-
ness from 30 to 100 m. These mafic
bodies usually crop out inside grano-
dioritic “matrix”. Coarse-grained gab-
bros and/or gabbro-diorites compose
the inner parts of the sheets (Fig. 8a).
The bottom contacts of gabbroic sheets
against granodiorites are chilled, sharp
and often relatively straight (Fig. 8b).
Load-cast (Fig. 8c,d) and flame struc-
tures (Fig. 8e) have been observed at
these contacts. Such types of relations
indicate interaction of nearly liquid
felsic and mafic melts (see Wiebe &
Collins 1998). Granites or granodior-
ites rich in mafic microgranular en-
claves (Figs. 1, 8f) have a close spatial
relation with the mafic bodies. As a
rule, such levels are situated above and
also to the sides of the mafic sheets.
On the regional scale, the mafic sheets
and the enclave-bearing rocks form al-
most continuous bands spatially re-
lated to the IYSZ and the associated
shear zones (Fig. 1).
AMS study
In macroscopically isotropic rocks,
AMS study is often the most efficient
20
GEORGIEV, HENRY, JORDANOVA N., FROITZHEIM, JORDANOVA D., IVANOV and DIMOV
way for structural analysis. The AMS in low field is described
by a symmetrical second-rank tensor. It is represented by a
triaxial ellipsoid with principal directions and magnitudes
Kmax, Kint, and Kmin. Fabric pointed out by AMS can reflect
flow for magmatic rocks, deposition context for sediments or
finite strain in deformed rocks (Borradaile & Henry 1997).
The orientation of the susceptibility ellipsoid for low bulk sus-
ceptibilities (K < 0.5
×10
—3
SI) is directly related to the pre-
ferred crystallographic orientation of paramagnetic minerals,
while in rocks possessing high ferromagnetic mineral content
(K > 10
—3
SI), AMS ellipsoid is mainly determined by the
shape and/or distribution of strongly magnetic minerals like
magnetite and titanomagnetite.
Sampling and methods
Cores, oriented using magnetic and sun compasses, have
been drilled with a portable gasoline drilling machine. Five
to twelve samples per site were gathered from 113 sampling
locations. The effect of possible local non-homogeneity of
the granites was minimized by sampling in an area of several
square meters at each outcrop. Sampling sites are evenly dis-
tributed in the study area, with the major exception of the
westernmost parts (Plana pluton), where outcrop conditions
allowed only a N-S profile. Several dykes, vein bodies and
basic sheets genetically associated with the main magmatism
were sampled as well.
AMS was measured with Kappabridge KLY-2 (Agico,
Brno). Statistical analyses of the data were carried out using
tensorial (Hext 1963; Jelinek 1978) and bivariate (Henry &
Le Goff 1995) approaches. The magnetic zone axis was de-
termined with its confidence zone (Henry 1997).
Identification of magnetic mineralogy was done by the
Curie temperature indicated on the thermal behaviour of
magnetic susceptibility K(T°C) in low field in the tempera-
ture range from room temperature up to 700 °C. This K(T°C)
analysis was carried out with a CS-23 furnace attachment to
the Kappabridge. Low-temperature susceptibility behaviour,
down to the liquid nitrogen temperature (77 K) for selected
samples, was used as an additional tool for mineral identifi-
cation. Effective domain state of ferromagnetic carriers was
Fig. 4. Crossed-polar micrographic examples of primary magmatic structures and transition to submagmatic strain stage. Magmatic foliations
defined by preferred orientation of feldspar crystals. a and b – Micro-fractures in feldspar phenocrysts filled with residual melt presented by
magmatic quartz. c – Pronounced magmatic foliation is shown by a linear arrangement of tabular plagioclase crystals. d – Zoned K-feldspar
showing an initial formation of sub-grains.
21
UPPER CRETACEOUS PLUTONS FROM THE SREDNA GORA ZONE (BULGARIA)
deduced from the hysteresis curves, measured for bulk
samples, using translation inductometer within an electro-
magnet capable of reaching 1.6 Tesla.
Magnetic properties of the studied rocks
Bulk susceptibility values
The measured magnetic susceptibility values vary in a
wide range (from 37
×10
6
up to 0.1 SI units). Lesichovo
leucogranites and Elshitsa granodiorites are characterized by
the lowest susceptibility values, probably mainly determined
Fig. 5. Field photograph and crossed-polar micrographs of the high-temperature character of deformation within granitoids from the pe-
ripheral parts of the shear zones. a – Steeply dipping or vertical c’-shear bands 10 to 20 cm in size from the high-temperature deformation-
al stage, indicating dextral shearing. b – High-temperature solid state overprint with chessboard pattern in quartz. c – High-temperature
static overprint with polygonal quartz grains and foam structure formation. d – Core-mantle structure in K-feldspar indicating moderate
temperature ( ~ 450 °C) solid state deformation. e and f – Annealed mylonitic granite from the central parts of IYSZ. The quartz layers
were originally formed by deformation under decreasing temperatures. Polygonization of quartz grain boundaries is interpreted as resulting
from a later temperature increase during intrusion of the Gutsal granodiorite.
by the phyllosilicate content. The other intrusions, with sig-
nificantly higher susceptibilities, show wider distributions,
which is a typical characteristic of strongly magnetic
granitoids. Values higher than 5
×10
3
SI suggest that the fer-
romagnetic fraction plays the major role in determination of
the shape and orientation of the magnetic susceptibility ellip-
soid for most of the sites.
Magnetic mineralogy
Representative examples of the common K(T°C) behaviour
are given in Figure 9. Most of the sites show an almost revers-
22
GEORGIEV, HENRY, JORDANOVA N., FROITZHEIM, JORDANOVA D., IVANOV and DIMOV
ible K(T°C) heating-cooling cycle with a main Curie tempera-
ture (Tc) of 580 °C, indicating the presence of magnetite (see
Dunlop & Özdemir 1997). Reversible peculiarity on some of
the curves at 120—150 °C well expressed for some samples
(see Fig. 9b – sample CF01 with the enlarged scale) is prob-
ably due to hemo-ilmenites, the end product of high-tempera-
ture exsolution. Another group of samples from the
enclave-bearing level of Elshitsa and Boshulia plutons shows
the presence of a significant drop in the signal at 350 °C fol-
lowed by the well-expressed magnetite Tc (580 °C) as shown
in Fig. 9a (sample X04). Correct interpretation of the nature
(real Tc or transformation temperature) and source of this me-
dium-temperature feature is not straightforward because of the
existence of several minerals having Tc or transforming in this
temperature range. Partial K(T°C) heating curves indicate re-
versible behaviour before the drop at 300 °C, but irreversible
transformation after heating at 350 °C in air and cooling back
to room temperature. Such behaviour may indicate progres-
sive transformation of pyrrhotite (Bleil & Petersen 1982) in an
oxidizing environment (Dekkers 1990). Most probably these
Fe-sulphides are products of the late hydrothermal mineraliza-
tion found in the area (Boyadjiev 1979 and references
therein). Low temperature demagnetization down to —180 °C
reveals clearly the dominant role of magnetite, identified by
the presence of Verwey transition at about —160 °C (Fig. 10).
Hysteresis parameters
Hysteresis measurements carried out for original, non-
separated material reveal mostly multidomain (MD) magne-
tite shapes of the loops and values of the hysteresis
parameters (Hc, Hcr, Js, Jrs) (Table 2 and Fig. 11). The inter-
pretation of the ratios Jrs/Js and Hcr/Hc in terms of magnetic
domain state is favoured by the results from K(T°C) experi-
ments as far as the latter show the major magnetite role and
no indications about the presence of high coercivity phases,
which would prevent the direct interpretation of Day dia-
grams (Day et al. 1977). Data points follow a single trend
line (Fig. 11) through MD—PSD regions, suggesting mixing
of different relative proportions of MD and SD grains (see
the theoretical model of Dunlop 2002). The samples from
the Elshitsa pluton (Fig. 11) show the highest stability, prob-
Fig. 6. Field photographs and crossed-polar micrographs of moderate to low-temperature character of deformation from the inner parts of
IYSZ and related shear zones. a – s—c fabric in granodiorite polished hand specimen from the inner parts of IYSZ. b – Quartz-chlorite ul-
tramylonites from the inner parts of IYSZ. c – K-feldspar porphyroclasts with deformational myrmekites. d – “Book shelves” and “v”-pull-
apart microstructures indicating brittle behaviour of feldspars in contrast to ductile behaviour of the matrix.
23
UPPER CRETACEOUS PLUTONS FROM THE SREDNA GORA ZONE (BULGARIA)
Fig. 7. a—c – Crossed-polar photomicrographs of
δ-type feldspar porphyroclasts; a – shear bands. d – Foliation “fishes”. All the criteria
indicate dextral kinematics of the shear zones.
ably influenced by the possible presence of pyrrhotite.
Samples from the Lesichovo leucocratic granites present a
paramagnetic behaviour. As a whole, granodiorites from the
Elshitsa and Lesichovo intrusions show relatively higher co-
ercivities (Table 2).
Since most magnetite generally crystallizes at tempera-
tures lower than the largest part of the melt, it will mimic the
crystallographic preferred orientation of the initially formed
minerals reflecting the last stages of magma crystallization
(Hrouda et al. 1971). Thus, the MD magnetite grains have
shapes and orientations determining a magnetic susceptibil-
ity ellipsoid consistent in orientation with the crystallo-
graphic ordering of the main rock-forming minerals. The
elements of the measured magnetic fabric (magnetic folia-
tion and lineation) could then reliably indicate the magmatic
fabric in the studied plutons.
Magnetic fabric of the main intrusive bodies
Directional data
Magmatic and high-temperature (HT) visible foliation has
been observed only at 31 of the sampled sites, magmatic vis-
ible lineation only at 13 sites and post-magmatic moderate-
to low temperature (M-LT) foliation only in 4 other sites.
These are traced by specific orientations of K-feldspars, bi-
otite crystals or enclaves.
The distribution of the different principal susceptibility
axes on a scale of sampling site shows quite good grouping
in all but 6 % of the sites, situated away from the shear zone
and showing scattered patterns. Girdle distributions of Kmax
or Kmin directions, not caused by measurement uncertainty,
are, however, observed in the sites with high oblateness/
prolateness due to the similar values of the principal suscep-
tibility Kmax/Kmin with that of Kint. The directional AMS
data representing the poles to magnetic foliation (Kmin axes)
and magnetic lineation (Kmax axes) agree with those of the
field magmatic structures close to the IYSZ zone. The orien-
tations are very coherent and parallel to the shear zone
(Fig. 12). On the contrary, low temperature foliations appear
to be different from the magnetic foliation. The magnetic
fabric is therefore related to the conditions during the magma
emplacement. Farther from the IYSZ, orientation of the dif-
ferent principal susceptibilities is more variable, probably
corresponding to conditions of magma emplacement less
constrained by shearing. The top-facies of the plutons repre-
sented by Elshitsa-Boshulia granodiorites, showing rather
different directions of magnetic lineation, may also reflect
24
GEORGIEV, HENRY, JORDANOVA N., FROITZHEIM, JORDANOVA D., IVANOV and DIMOV
local stresses due to stopping and lower pressure conditions
during magma emplacement.
The girdle scattering of Kmin observed at more than 1/3 of
the studied sites is not related to uncertainty of measurement,
allowing the determination of significant magnetic zone
axis. In almost all of these sites, the latter coincides well
with the magnetic lineation, which thus could be of intersec-
tion or stretching type. The coincidence between visible
magmatic and magnetic lineations indicates that the orienta-
Fig. 8. Structural peculiarities of the gabbro sheets and contact zone between gabbro and granodiorite. a – Coarse-grained gabbros and/or
gabbro-diorites represent the inner parts of the sheets. b – Chilled zone in the periphery of gabbro sheet along the contact with granodior-
ite. c and d – “load-cast” and e – “flame” structures along the basis of mafic sheet. f – Mafic microgranular enclaves often dominate
over the host granitoid matrix.
tion of magnetic lineation is not related to superimposed
magnetic fabrics. The coincidence of the magnetic zone axis
and visible lineation is therefore related to conditions of
magma emplacement.
Corrected anisotropy degree
The corrected degree of anisotropy P’ (Jelinek 1981), indi-
cating the “intensity” of the fabric, ranges from 1.02 to 1.54.
25
UPPER CRETACEOUS PLUTONS FROM THE SREDNA GORA ZONE (BULGARIA)
Fig. 9. a – High-temperature behaviour of magnetic susceptibility
for site 75 with partial susceptibility runs (subsequent cycles indi-
cated by numbers) for revealing chemical changes during heating.
b – Thermomagnetic K(°T) analysis for sample from site 97. The in-
set shows on an enlarged scale the upper part of the curves for reveal-
ing peculiarities in the moderate temperature range (120—200 °C).
c – Thermomagnetic analysis for sample from site 61. The heating
and cooling curves coincide. Heating curves are presented by thick
line, cooling – by thin lines.
Fig. 10. Low-temperature behaviour of magnetic susceptibility for
selected samples.
A dependence of P’ on the mean susceptibility K (Fig. 13)
suggests some partial mineralogical control on P’ values
(Henry 1980), namely an increased content of magnetite,
which should be more anisotropic than the low susceptibility
components. The obtained low P’ values and their relatively
good clustering around mean values of 1.06—1.08 for
Elshitsa and Lesichovo granites and granodiorites define the
existence of low strain in these levels. On the contrary the
other intrusions are characterized by wider P’ distribution,
reflecting the proximity of the sampled sites to the main
shear zone. This underlines the major importance of the
IYSZ for the development of the magnetic fabric. The high
P’ values observed in the middle part of the Plana basites
and gabbro-diorites together with the orientation of the mag-
netic lineation suggests the presence of high strain level pos-
sibly due to an unrevealed shear zone. The lateral pattern of
P’ distribution (Fig. 14) and the observed systematically
higher P’ values for sites close to the IYSZ and in the west-
ern part (Fig. 15) unambiguously show the major role of the
tectonic strain on the magnetic fabric. The magnetic fabric,
being related to magmatic stage, shows that magma was
emplaced when the shear zone was the area with the maxi-
mum strain, magma flow being driven by the tectonic condi-
tions.
Shape factor T
The shape of the magnetic susceptibility ellipsoid deduced
from the T parameter (Jelinek 1981) may give information
about the deformation pathway in different geological for-
mations where progressive tectonic development occurred
26
GEORGIEV, HENRY, JORDANOVA N., FROITZHEIM, JORDANOVA D., IVANOV and DIMOV
Table 2: Hysteresis parameters and ratios for selected samples. Hc – coercive force; Hcr – coercivity of remanence; Js – saturation
magnetization; Jrs – saturation remanence; Xpara – “paramagnetic” susceptibility calculated from the linear high-field part of the hys-
teresis loops.
Site
Sample
Hc (mT)
Hcr (mT)
Js (mAm
2
/kg) Jrs
(mAm
2
/kg) Hcr/Hc Jrs/Js Xpara
(x10
-9
m
3
/kg)
Elshitsa granodiorites
90
bl01
12.2
50.5
520
47
4.14
0.09
95
84
bg07
3.8
41
121
3.67
10.8
0.03
43.2
20- vein
mv01
11.3
42.5
447
36.7
3.76
0.08
58.9
24-vein
mdd05
1.51
20.3
226
2.26
13.5
0.01
59.7
75-xenol.l.
x04
4.03
47.2
455
14
11.7
0.03
88.3
Lesichovo leucocratic granites
42 mww03
para
89 bk03
para
40
mvv05
9.92
66.9
536
41.1
6.75
0.08
75.1
Varshilo granites
92
ca06
1.39
22
411
4.12
15.9
0.01
66.1
94
cc04
1.11
15.7
229
2.54
14.1
0.01
51.3
17
ms06
2.12
27.8
777
10.9
13.1
0.01
51.4
47
a03
1.28
16.2
1423
15.3
12.7
0.01
177
47B
c03
22.9
56.3
61.7
13.8
2.46
0.22
39.8
36-xenol.l.
mqq04
4.1
26.4
1264
39.3
6.48
0.03
104
Gutsal granodiorites
99
ch03
2.1
22.9
1412
20.5
10.9
0.01
88.8
97
cf01
3.47
24
1785
56.5
6.92
0.03
132
27
mgg01
3.01
25.4
1199
31.6
8.46
0.03
74
30
mjj03
2.76
23.4
932
19.9
8.48
0.02
88.5
57
f05
10.9
56.8
403
31.5
5.24
0.08
55.5
98
cg09
1.45
22.3
658
7.3
15.4
0.01
53.2
71
u07
9.98
44.3
599
47.3
4.44
0.08
310
61
j07
5.42
34.8
736
25.5
6.42
0.03
78.3
59-vein
h02
8.54
50.8
73.7
4.15
5.95
0.06
14.6
Plana basites, gabrodiorites–monzo gabro
63
l09
1.59
22.6
352
4.85
14.21
0.014
183
65
n06
0.66
6.54
1128
6.84
9.96
0.01
191
66
k06
3.44
27.7
653
17.5
8.06
0.03
46
69
r02
0.9
14.3
1355
11.1
15.8
0.01
186
68
q04
1.71
17.3
943
15.5
10.1
0.02
170
Fig. 11. Day diagram (Day et al. 1977) for samples subjected to
hysteresis measurements.
(Borradaile 1988; Borradaile & Henry 1997). As far as P’ lat-
eral distribution (Fig. 14) underlines the role of the IYSZ for
the development of the magnetic fabric, the P’-T diagram of
Jelinek (1981) was plotted separating the samples according
to the field observations (no visible data, magmatic HT or
M-LT foliation and lineation). From Figs. 13 and 14, it is ob-
vious that the sites, possessing low anisotropy P’ and more
scattered principal axes, generally situated away from the
shear zone, are characterized by variable shape of the sus-
ceptibility ellipsoid, ranging from rod-shaped (T = —1)
through neutral to oblate (T = +1). On the other hand, sam-
pling sites, for which magmatic and/or HT structures are vis-
ible, show a tendency to be closer to the neutral form with
increasing degrees of anisotropy (Fig. 13). This dominant
behaviour may be related to the shape of a strain ellipsoid
corresponding to flow conditions related to a simple shear or
transpressive tectonic regional regime.
Lateral variations of the shape parameter T (Fig. 16) show
that the oblate fabric is largely dominant, except close to the
shear zones. This increased stretching close to the shear
zones points out the role of shearing during magma emplace-
ment. In several locations where magnetite content is rela-
tively low, the mineralogical source of prolate magnetic
susceptibility ellipsoids could be the presence of linear ori-
entation of the hornblende grains.
27
UPPER CRETACEOUS PLUTONS FROM THE SREDNA GORA ZONE (BULGARIA)
Fig. 12. a – Lateral distribution of magnetic foliation (F) and b – magnetic
lineation (L) obtained from AMS study.
Fig. 13. K-P’ dependence for sites close to the shear zone, western
part and away from IYSZ.
Comparison of magnetic fabric of granitoids and gabbro
bodies
Most of the plutons studied have the characteristics of
well-layered intrusions: their lower parts consist of crystal-
rich granodiorites and granites and the upper parts of gran-
ites poor in felsic crystals. Between the two parts,
comparatively large sheet-like bodies of gabbro and gabbro-
diorites are placed below swarms of basic enclaves. The ob-
served magnetic fabric in both granitic and basic facies
almost always coincide in a number of outcrops (Fig. 12).
The basic magma, coming from deeper crust levels therefore
permeated along the shear zone in the same dynamic condi-
tions. Thus, both petrostructural and AMS data give good
ground to consider the magmatic complex resulting from min-
gling of two different magma melts – granitic and basic.
Magnetic fabric of dykes
Dyke planes in the studied plutons are often parallel to
magmatic and later foliations (Figs. 1, 2d). The field orienta-
28
GEORGIEV, HENRY, JORDANOVA N., FROITZHEIM, JORDANOVA D., IVANOV and DIMOV
Fig. 14. Lateral distribution of the corrected degree of anisotropy P’.
Fig. 15. Plot of shape parameter T versus corrected degree of
anisotropy P’ for sites, situated at different distance from the main
shear zone.
tion of dykes has in fact, except for site 61A, typical Sredna
Gora direction (130—150° trends) with relatively high dip of
50—90°. In these cases (except site 23B), the magnetic folia-
tion is parallel to the dyke walls (Fig. 17), but it also agrees
with the magnetic foliation in the host granitoids. In site 23B,
the shift between dyke wall orientation and magnetic foliation
is about 40°. Such a shift is often explained by an imbrication
phenomenon (Knight & Walker 1988) caused by the flow gra-
dient of magma along the walls. In this case, symmetrical de-
viation of the different axes is observed along the two walls
relative to the central part of the dyke. That is not the case
here, the fabric being similar in the whole dyke. In addition,
no imbrication has been observed in our case close to the
walls of all the studied dykes. It is therefore probably related
to strain effect after filling of the dyke (Henry 1974a,b). In site
23B, magnetic foliation in the host granitoids moreover has a
slightly different orientation relative to the other sites, and its
orientation coincides with that obtained within the dyke.
This similar orientation of the magnetic foliation confirms
the major role of the strain effect for the origin of the mag-
netic fabric within the dyke. The same interpretation is pro-
posed for the dyke of site 61A, which plunges to the South
and has an orientation, different from typical the Sredna
Gora direction. The orientation of the magnetic foliation
within this dyke again is different from that of the dyke
plane, but similar to that of the magnetic foliation, locally
different from the main part of the plutons, in the host
granitoids. Dykes were therefore emplaced as late intrusions
within granitic bodies already solidified but they underwent
almost similar strain to the granite when the latter was still in
the magmatic state.
However, Kmax within a dyke mostly has a higher inclina-
tion than within the granitoids. This could indicate a slightly
different strain context. In addition, the degree of magnetic
anisotropy varies from 1.01 up to 2.02 and is relatively high
in comparison with that obtained in the host granitoids.
Discussion
Our interpretation is based on the structural field and AMS
evidence for the controlling role of the IYSZ and related
shear zones as well as field evidence for magma mingling
processes.
The field and laboratory structural investigations point to
dextral oblique-slip kinematics of the shear zones. Macro-
and micro-scale kinematic criteria within the deformed igne-
ous and metamorphic host rocks show stable dextral oblique
shearing in the high-temperature and moderate to low-tem-
perature domains (Figs. 5, 6 and 7).
In the field, magmatic planar and linear structures and su-
perimposed high- to moderate- and low-temperature folia-
tion and lineation generally show the same orientations.
29
UPPER CRETACEOUS PLUTONS FROM THE SREDNA GORA ZONE (BULGARIA)
Fig. 16. Lateral variation of shape parameter T.
Fig. 17. Stereographic projection on the lower hemisphere of
dyke’s orientation and the direction of Kmax.
Primary magmatic flow structures near the contacts of the
plutons are common. In such places magmatic foliation is
parallel to the contact surface and the foliation in the meta-
morphic host rocks. This conformity can be explained by a
syntectonic emplacement (Archanjo et al. 1994; Bouchez
1997; Schofield & D’Lemos 1998). In spite of the apparent
isotropic structure of the studied plutons, strips of well pro-
nounced magmatic foliation from the inner parts of the plu-
tons are adjacent and parallel to the mylonites of the IYSZ
and its related shear zones. Away from the areas of intensive
deformation, the magmatic flow structures are less pro-
nounced and oblique to the strike of the shear zones. A
smooth transition from magmatic foliation near the shear
zones to high-temperature to moderate- and low-temperature
deformational structures within the shear zones argues for a
deformation during the time of emplacement (Bouchez et al.
1981, 1992; Gapais 1989; Ghosh 1993; Miller & Paterson
1994; Saint-Blanquat & Tikoff 1997; Zurbriggen et al. 1997).
The field assumption of a transition from magmatic to su-
perimposed deformational structures and syntectonic charac-
ter of the studied plutons is confirmed by microstructural
investigations. Thin sections, displaying magmatic foliation
and submagmatic deformation (Fig. 2 and Fig. 4.) as well as
structures of high- to low-temperature overprint (Fig. 5 and
Fig. 6), all of them with the same kinematics, give evidence
of a continuous deformation process. The long-lasting
deformational process affected both crystal-melt mush and,
subsequently, crystallized igneous rocks. The progressive
deformation and exhumation of the system have led to the
localization/partitioning of deformation and the formation of
narrow ultramylonite bands within the inner parts of the
shear zones. Static recrystallization under relatively high
temperatures in the high-strain mylonite bands within the
IYSZ in the vicinity of the Gutsal pluton (Fig. 5c,e,f) reveals
that the last stage of the shear zone activity was followed by
heating due to the emplacement of the Gutsal granodiorite at
~
75 Ma.
Direct observations of strain in the granitoids were pos-
sible only in limited areas, the rocks being mostly apparently
isotropic. They were completed by AMS data which point to
the presence of magnetic foliation and lineation largely de-
veloped in all the intrusive bodies. In fact, in the few AMS
sites where visible magmatic structures are present, these
structures show good agreement with AMS data, confirming
the mainly magmatic origin of the magnetic fabric. These
AMS data then confirm at a more regional scale the
syntectonic character of the studied plutons and intensive
shearing during and after their emplacement.
The foliation and stretching lineation of the metamorphic
host rocks are parallel to the shear zone structures or parallel
to the strike of plutonic contacts. Such features indicate in-
tensive superimposed shearing of the metamorphic rocks in
the studied area during the intrusion of the plutons.
30
GEORGIEV, HENRY, JORDANOVA N., FROITZHEIM, JORDANOVA D., IVANOV and DIMOV
Fig. 19. Hypothetical model of magma drainage along IYSZ (after Ferre et al. 1997, modified).
Fig. 18. 3D model illustrating the regional tectonic setting along the
IYSZ and adjacent territories during the Late Cretaceous (90—75 Ma).
31
UPPER CRETACEOUS PLUTONS FROM THE SREDNA GORA ZONE (BULGARIA)
The mafic varieties and magmatic enclave swarms have a
close spatial relationship with the IYSZ and the related shear
zones (Fig. 1). This underlines the connection between de-
formation and magma emplacement. The field relations be-
tween mafic and felsic intrusive rocks support an
interpretation in terms of magma mingling. Chilled, sharp
and relatively linear contacts of the gabbroic sheets with the
granodiorite host (Fig. 8b) are evidence of fast cooling dur-
ing mafic magma crystallization. The mafic melt has in-
truded into a partly crystallized granitoid magma chamber
(see Wiebe & Collins 1998). The presence of specific struc-
tures such as load casts and flame structures (Fig. 8) at the
lower contact of the mafic sheets gives evidence for a vis-
cous state of both felsic and mafic melt at the time of their
interaction. Zircon U-Pb ages of granitoids and gabbros con-
firm contemporaneous crystallization of mafic and felsic
constituents of the plutons (Table 1).
Mafic and felsic dykes that crop out within deformed igne-
ous rocks of the studied plutons are parallel to the foliation.
Dykes within isotropic parts of the studied intrusive bodies
are usually parallel to the strike of IYSZ and related shear
zones. This type of development of the dyke system points
to a relatively long lasting regional stress system. The latter
is supported by both deformed and undeformed dykes ob-
served within the sheared domains.
The lineation (both measured in the field and from the
AMS) near the IYSZ generally plunges towards NW at mod-
erate angles, showing that the dextral shearing had a large
vertical component with a relative uplift of the northeastern
block. This displacement is unrelated to the exhumation of
the Rhodope metamorphic rocks because the exhumation
would require the opposite vertical component. The Creta-
ceous-age shearing along the IYSZ could be related to ob-
lique plate convergence along the Vardar suture zone, which
was partitioned into southwest-directed thrusting in the
Rhodopes and dextral shearing in the hinterland (Fig. 18), as
typically observed in subduction zones with oblique conver-
gence (e.g. Platt 1993). On the block-diagram in Fig. 18 we
have sketched the situation during the period ~ 90—75 Ma. It
shows a transpressive regime in the vicinity of IYSZ and
compressive tectonics to the south in the Rhodope area. To
the north and northeast of IYSZ the transcurrent shearing
probably had an extensional component leading to the forma-
tion of isolated pull-apart and strike-slip sedimentary basins.
Conclusions
Summarizing all the above presented data, as well as
mechanisms known from the literature (Michael 1991; Ferre
et al. 1997; Wiebe & Collins 1998; Steenken et al. 2000;
Rosenberg 2004), we can propose the following schematic
emplacement mode for the investigated magmatic bodies
(Fig. 19).
Upper Cretaceous plutons situated in the southwestern parts
of the Sredna Gora Zone resulted from simultaneous
syntectonic emplacement (86—75 Ma) of two different magma
types – felsic and mafic. Mingling of the two magmas took
place in the magma chamber at a depth of 10—15 km, corre-
sponding to the boundary between the upper and middle
crust, and at temperatures between 770 and 680 °C
(Georgiev & Lazarova 2003).
The different Upper Cretaceous plutonic bodies from the
southwestern part of the Sredna Gora Zone present similar
magmatic and structural evolutions. Field relationships,
petrostructural and magnetic (AMS) data in fact show that
magma emplacement and deformation processes have been
accomplished in a dextral oblique-slip regime. These defor-
mation processes occurred during magmatic and post-mag-
matic stages and for the latter under high-, moderate-, and
low-temperature conditions The Iskar-Yavoritsa shear zone
and the related synthetic shear zones played a major role in the
evolution of the magmatic system. These zones originated
106—100 Ma ago (Velichkova et al. 2001) and represented a
main drainage channel for both granitoid and mafic melts. The
deformation along the shear zone ceased with the emplace-
ment of the undeformed ~ 75 Ma Gutsal pluton, which has in-
truded as sills into the Iskar-Yavoritsa mylonites.
Acknowledgments: This work was supported by the NATO
Collaborative Research Grant No. CRG.LG973943. N.F. was
supported by DFG Grant FR 700/10-1 and DAAD Project
PPP Bulgaria. We would like to thank F. Hrouda, M. Putiš and
I. Petrík for careful and constructive reviews. N.G. also thanks
Z. Cherneva, A. Lazarova and I. Gerdjikov for fruitful discus-
sions and to Prof. Ch. Pimpirev and the Bulgarian Antarctic
Institute for responsiveness, technical and financial support.
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