International Geology Review, Vol. 50, 2008, p. 519–545. DOI: 10.2747/0020-6814.50.6.519Copyright © 2008 by Bellwether Publishing, Ltd. All rights reserved.

Paleotectonic Position of the Strandja Massif and Surrounding Continental Blocks Based on Zircon Pb-Pb Age Studies

GÜRSEL SUNAL,1 MUHARREM SATIR, Universität Tübingen, Institut für Geowissenschaften, Wilhelmstrasse 56, D-72074 Tübingen, Germany

BORIS A. NATAL’IN, Istanbul Technical University, Department of Geology, TR-34469 Istanbul, Turkey

AND ERKAN TORAMAN

Department of Earth and Atmospheric Sciences, Saint Louis University, 329 Macelwane Hall, 3507 Laclede Avenue St. Louis, Missouri 63103 and ITU Eurasia Institute of Earth Sciences, TR-34469 Istanbul, Turkey

Abstract

Detrital zircon ages from metasedimentary rocks of the Strandja massif have been used to revealits tectonic history, initial position, and some aspects of the Paleozoic motion of this continentalblock within the Tethyan domain. The age of the metamorphic basement of northwestern Turkey hasthus far been uncertain. Estimates of the depositional age of metasedimentary rocks range fromPrecambrian to late Paleozoic, hindering correlation of the massif with other tectonic units. In thisstudy, new evaporation Pb-Pb ages of detrital zircons show that biotite schists constituting the cen-tral part of the metamorphic basement were deposited later than 430 Ma and prior to 315 Ma. Biotiteschists exposed along the southern boundary of the basement were deposited between 300 and 271Ma.

Episodes of pre-Carboniferous magmatic activity that are inferred from detrital ages clusteraround 460–433, 575–525, 700–600, 875–800, 1050–950, 2200–2100, and 2450 Ma. Some ofthese age ranges correlate with the ages of Late Carboniferous inherited zircons in orthogneisses(315–300 Ma) of the Strandja massif.

Pan-African or/and Cadomian ages at 575–525 and 700–650 Ma, respectively, are characteristicof magmatic events in other continental blocks of Turkey (e.g., the Istanbul Zone), as well as incrystalline complexes of surrounding regions—e.g., Crete, the Cyclades massif, the Vardar Zone,and the Sredna Gora Zone.

The magmatic history of the source areas of the Strandja detrital zircons prior to the Ordovicianis correlative with both the Avalonian and the Armorican tectonic units of Western Europe. Zirconcore ages obtained from Late Carboniferous orthogneisses in the Strandja massif are correlated withunits derived from the North African sector of Gondwana because of the absence of Mesoproterozoicages. The basement metasediments of the Strandja massif reveal heterogeneous source areas, whichwere fed by the North African (Armorica) and the South American (Avalonia) basements. The pres-ence of Mesoproterozoic zircon ages in metasediments of the Strandja massif indicates the proximityof the Strandja massif to Avalonian (or Baltica) derived units during the Late Silurian–Carboniferousinterval.

Introduction

THE STRANDJA MASSIF covers a third of the Thra-cian Peninsula in northwestern Turkey (Fig. 1) andforms the eastern continuation of Paleozoic meta-

morphic rocks of a unit called the “Balkan terrane”in Bulgaria (Chatalov, 1988; Yanev, 2000; Natal’inet al., 2005a, 2005b; Natal’in, 2006; Sunal et al.,2006) (Fig. 2). The origin of the Strandja massif isnot clear; some researchers interpret this part of theRhodope-Pontid e fragment (ªengör and Yılmaz,1981) belt as a part of the Cimmerian microconti-nent that rifted from the Gondwana supercontinentand collided with Eurasia during the early Mesozoic

1Corresponding author. Present address: Istanbul TechnicalUniversity, Department of Geology, TR-34469 Istanbul,Turkey; email: gsunal@itu.edu.tr

5190020-6814/08/1003/519-27 $25.00

520 SUNAL ET AL.

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PALEOTECTONIC POSITION OF THE STRANDJA MASSIF 521

Cimmerian orogeny (ªengör and Yılmaz, 1981),whereas others claim that the Strandja massifbelonged to the Laurasia margin during the latePaleozoic and Mesozoic eras (Okay et al., 1996;Okay and Tüysüz 1999; Natal’in et al., 2005a,2005b; Natal’in, 2006).

Former relationships of continental blocks toancient continents or supercontinents are generallyreconstructed from similarity of ages of magmaticzircons that indicate main episodes of tectonic activ-ity (van der Voo, 1979; Nance and Murphy, 1994;Torsvik, 1998; Unrug, 1997; von Raumer, 1998;Unrug et al., 1999; Murphy et al., 1999, 2004;Friedl et al., 2000; Hegner and Kröner, 2000; Matte,2001; von Raumer et al., 2003; Linneman et al.,2004; Samson et al., 2005). In this study, we provide

new detrital zircon single-grain Pb/Pb agescollected from Paleozoic metasedimentary andmagmatic rocks of the Strandja massif in order todetermine their sources. Another aspect of thisstudy is to correlate the Strandja with other massifsin its vicinity to establish their possible connectionsin the geological past.

Geological Framework of the Study Area

The Strandja massif consists of a metamorphicbasement that is intruded by Late Carboniferous andPermian granitoids and unconformably overlain by aTriassic to Jurassic metasedimentary cover (Aydın,1974, 1982; Çaglayan et al., 1988; Okay et al.,2001; Natal’in et al., 2005a, 2005b; Sunal et al.,2006) (Fig. 1B). Recently, Sunal et al. (2006)

FIG. 2. Main tectonic divisions of Europe and the Aegean region compiled after Franke (1989), Boyanov et al.(1990), Okay et al. (1994, 1996), Ring et al. (1999), and Carrigan et al. (2005). Abbreviations: AM = Armorican massif;BM = Bohemian massif; BZ = Balkan Zone; CYC = Cyclades; HZ = Harz Mountains (Eckergneiss complex); IASZ =Izmir-Ankara suture zone; IAZ = Internal Alpine Zones; IZ = Istanbul Zone; KM = Kırºehir massif; MM = Menderesmassif; MP = Moezia Platform; MS = Morova-Silesia Zone; NWIB = NW Iberia; PZ = Pelagonia Zone; RM = Rhodopemassif; SG = Sredna-Gora Zone; SMM = Serbo-Makedonian massif; SJ = Strandja massif; SWIB = SW Iberia; SZ =Sakarya Zone; TB = Tepla-Barrandian Zone; TESZ = Trans European suture zone; VZ = Vardar Zone.

522 SUNAL ET AL.

showed that the orthogneisses that are intrusive intothe metasediments are late Carboniferous in age(between 300 and 315 Ma). The metasedimentsintruded by these granitoids (orthogneisses) arelocated in the central part of the studied area(Natal’in et al., 2005a, 2005b; Fig. 1A). In the south,biotite schists with amphibolite layers are cut by anEarly Permian Kırklareli metagranite (Fig. 1A). Inthe central part, the composition of the Paleozoicmetasediments is more variable: biotite schists, bio-tite-garnet schists, biotite-muscovite gneisses, andquartzofeldspathic gneisses (Fig. 1A). The metased-imentary units in the basement lack fossils and thustheir age was inferred from regional tectonic correla-tions. Çaglayan et al. (1988) proposed a Paleozoicage for the basement rocks. Okay et al. (2001)infered that country rocks of the Kırklareli plutona re l a t e Var i scan in age , and Türke canand Yurtsever (2002) estimated their age as Precam-brian.

Besides the Late Carboniferous intrusions, thebasement of the Strandja massif is cut by youngerplutons of the Kırklareli type. They are monzoniticgranites that in places are metamorphosed anddeformed (Natal’in et al., 2005a, 2005b). Aydın(1974, 1982) reported a 244 ± 11 Ma Rb-Sr whole-rock age of these monzonitic granites while Okay etal. (2001) obtained Permian 278–266 Ma Pb-Pbzircon evaporation ages from them. Samples fromthe migmatites in the same study yielded 299± 15,276 ± 12, 239 ± 12, and 221 ± 14 Ma ages. Theseyounger age determinations were considered asunreliable because of Pb loss and inferred Triassicage of the sedimentary cover unconformably over-lying the metagranites (Okay et al., 2001). Sunal etal. (2006) obtained a 257 ± 6.2 Ma Pb-Pb evapora-tion age from the Kırklareli metagranite (Fig. 1).

The Kırklareli metagranites also yielded biotiteK-Ar ages of 150–149 Ma, Rb-Sr biotite age of 144Ma (Aydın 1974, 1982), and Rb-Sr biotite age of155 ± 2 Ma (Okay et al., 2001). All of these ageswere interpreted as the regional metamorphic cool-ing (300 ± 50°C) age. Rb-Sr muscovite ages are uni-formly distributed in the study area and range from162 to 155 Ma, whereas Rb-Sr biotite ages areyounger to the north where metamorphic gradedecreases to greenschist facies.

Analytical Techniques

Zircons were extracted from rock samples bystandard mineral separation techniques: Wilfley

table, heavy liquids, Frantz isodynamic separator.Grains were handpicked under a binocular micro-scope, then a fraction with grain sizes 63–200 μmwas classified according to crystal properties (i.e.,euhedral morphology, lack of overgrowth and visibleinclusions). Zircons mounted in epoxy resin werepolished to expose grain interiors for cathodolumi-nescence (CL) studies. CL images were obtainedusing a JEOL JXA-8900RL electron microprobe,using an accelerating voltage of 15 kV and a beamcurrent of 15 nA.

For single-zircon Pb-evaporation, chemicallyuntreated grains were analyzed using a double Refilament configuration as suggested by Kober (1986,1987). The zircons used for the evaporation are notthe same as those used for CL studies, but we rely onthe identity of these two groups in our interpreta-tions of ages. Each zircon was embedded in a Reevaporation filament and placed in front of a 1 mmwide Re ionization filament. Each grain was heatedfirst at 1350°C for 5–10 minute for removal of com-mon (204Pb) and radiogenic Pb hosted in less stablephases (e.g., in crystal domains affected by radiationdamage) that have low activation energies (Kober,1986). During repeated evaporation-depositioncycles in 20°C steps, Pb is deposited on ionizationfilament for the measurements starting at about1380°C (all of the heating steps are unproved). Onlyhigh counts, generally 20,000–250,000 per secondfor 206Pb, were used for age evaluations. Pb isotopeswere dynamically measured in a sequence of 206–207–208–204–206–207 with a secondary electronmultiplier. Correction of the common lead contribu-tion to measured 207Pb/206Pb ratios was madeaccording to the two-stage growth model of Staceyand Kramers (1975). Further detailed informationon the method is given in Chen et al. (2002), Okay etal. (2001), Siebel et al. (2003), and Sunal et al.(2006). All isotopic ratios were measured in staticmode on a Finnigan-MAT 262 multicollector massspectrometer at the Department of Geochemistry,Tübingen University, Germany.

The age calculations were created using theIsoplot program (Ludwig, 2003). Errors of 207Pb/206 Pb ages are given as 2σ (2 sigma) standard devi-ation. The 207Pb/206Pb evaporation ages during thecourse of measurements for Redwitzites granite fromthe Bohemian massif yielded an average age of322.8 ± 4.1 Ma, similar to those age ranges reportedby Siebel (1994) and Siebel et al. (2003).

PALEOTECTONIC POSITION OF THE STRANDJA MASSIF 523

Results

Description of the samples

We analyzed three samples; two from the centralregion (Gk 33 and Gk 206) and one from the south-ern (Gk 200) part of the massif (Fig. 1A). Sample Gk33 is a medium- to fine-grained, greenish to grey,strongly foliated and lineated biotite schist that wasintruded by the Late Carboniferous (312.3 ± 1.7 Ma)orthogneisses (Natal’in et al., 2005a; Sunal et al.,2006). It consists of quartz (20–25%), K-feldspar(20–25%), plagioclase (10–15%), biotite (10–15%),muscovite (5–10%), epidote (2–5%), calcite (3–5%), minor zircon, and opaque minerals. Sample Gk206 is a medium- to fine-grained, greenish grey bio-tite schist that was intruded by the late Carbonifer-ous biotite-muscovite granitic gneiss (Natal’in et al.,2005a; Sunal et al., 2006). It is a biotite schist withquartz (5–10%), plagioclase (35–40%), K-feldspar(10–15%), biotite (15–20%), epidote (20–25%),garnet (2–5%), titanite (1–3%), and minor zirconand opaque minerals.

Sample Gk 200 was collected from the southernpart of the basement where metasediments withamphibolites are exposed near the contact with thePermian Kırkareli metagranite (Okay et al., 2001;Natal’in et al., 2005a, 2005b; Sunal et al., 2006). Itconsists of quartz (10–15%), plagioclase (25–30%),K-feldspar (15–20%), biotite (15–20%), muscovite(5–10%), garnet (3–8%), epidote (3–5%), chlorite(3–5%), and amphibole (3–5%), as well as minorzircon, titanite, and opaque minerals.

Zircon populations

All of the samples contain identical zircon popu-lations characterized by different color, size (aspectratio), crystal quality, transparency, and roundness.The first population is comprised of dark brown,semi-transparent, idiomorphic crystals that have anaspect ratio of 1:2. The second population has alight brown color. These are transparent, idiomor-phic grains that are either thick or long, with anaspect ratio between 1:2 and 1:4. The third popula-tion of zircon is colorless. Morphological propertiesof these zircons are identical with the second group.The second population dominates zircons in allthree samples, followed by the first and the thirdpopulations. Zircons extracted from sample Gk 200show no or slight roundness. Zircons from the othersamples reveal varying degrees of abrasion repre-sented by curved crystal terminations and in somecases perfect roundness (Figs. 3, 4, and 5).

Zircon internal structures

Sample Gk 33. Cathodoluminescence (CL) imag-ing of sample Gk 33 zircons reveals a variety ofinternal structures indicating multiple episodes offormation (Fig. 3). Grains belonging to the first type(dark brown grains A to E) show generally oscilla-tory magmatic zoning surrounding inner roundedcores. These rounded inner cores are most evidentin the grains B and E (Fig. 3). Grain C shows aninner idiomorphic and oscillatory zoned xenocrysticcore enveloped by a thick, irregular, banded zonethat indicates evidence for recrystallization. Tracesof these recrystallization zones are apparent in grainB (bottom left), grain E (bottom right), and grain C(top left) with oscillatory zones truncated by irregu-lar patches.

Similar to the first type of zircons, the secondtype of zircons (light brown grains F to K) typicallyshow magmatic oscillatory zoning. Grains F, H, J,and K have cores composed either of oscillatory (thegrains F, H, and J) or homogenous zones (the grainK). Grains G and I are typical magmatic zircons.Grain F reveals an inner part with oscillatory zoningthat is truncated and surrounded by an outer partwith curved and planar zoning that is bright in CLimages. Grain H shows different stages of formation.Its inner part is characterized by oscillatory zoning(black and white zones parallel to the long axis ofthe zircon) and surrounded by thin, low CL intensityoscillatory zoning. All of these structures are trun-cated by a grey, homogenous recrystallization zone(right side of the grain). Grain J has an inner partwith thinly banded oscillatory zoning surrounded bya dark grey outer part, also with oscillatory zoning.The internal part is truncated by the low CL zone onthe right side of the zircon (Fig. 3).

The other four grains are represented by color-less zircons (grains L to O). Grains L, N, and O aretypical magmatic zircons exhibiting oscillatory zon-ing. Grain L contains an apparently inherited core.Grain M has a crenulated inner zone truncated by athick outer structure exhibiting planar zoning. Thisgrain is highly abraded and rounded through sedi-mentary processes.

Without exception all the grains have an outer,thin, bright CL zone. This most probably indicatesthat resorption occurred during the last metamor-phic event.

Sample Gk 206. Similar to the sample Gk 33, zir-cons from the sample Gk 206 reveal a variety ofinternal structures (Fig. 4). The first six grains (A toF) are members of the dark brown zircon population

524 SUNAL ET AL.

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PALEOTECTONIC POSITION OF THE STRANDJA MASSIF 525

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526 SUNAL ET AL.

(first type). Grains A and E show typical oscillatorymagmatic zoning surrounded by an irregular, brightCL zone representing the recrystallization of theprotolith zircon. Grain E is heavily abraded and wasprobably broken during transportation. Grains B, C,D, and F show homogenous cores (dark grey) thatare surrounded by zoned peripheries. These zonedperipheries are characterized by oscillatory zoningin grains B, D, and F, and in grain C by weaklydeveloped planar zoning.

Selected light brown zircons are represented bythe grains G to K. Grains J and K are typical mag-matic zircons with oscillatory zoning and somerecrystallization structures (top of grain J). Grains Hand I are highly abraded zircons and may haveexperienced long-distance transport from theirsource area. In grain H, the oscillatory zoning istruncated by an outermost zone that indicatesrecrystallization. Grain I has an inner core withoscillatory zoning and is discordantly surrounded bya periphery with planar zoning. Interesting featurescan be seen in grain G: The inner part of the grain isa core with low CL (dark) intensity, and this is sur-rounded and partly truncated by a homogenous,bright CL zone that in turn is overprinted by weakoscillatory zoning. This thick, low-intensity CLoscillatory zone again is truncated by an outer zonewith rather high CL intensity.

Colorless zircons (grains L to N) are rounded orsubrounded. Grain L shows oscillatory magmaticzoning with some irregular recrystallization struc-tures in it (high CL bands on the left and the top ofthe grain). The inner part of grain M is representedby high CL intensity oscillatory and sector zonestruncated and surrounded by an outer rim with pla-nar zoning. Relics of oscillatory zoning are shown ingrain N. A recrystallization structure is representedby an homogenous high-CL part that truncates andetches early formed oscillatory inner zones.

Sample Gk 200. Selected zircons (Fig. 5, grainsA, B, and C) of the honey-colored population showoscillatory magmatic zoning. Sector zoning is alsoseen in the grain A. Grain B has an unclear andpatchy internal structure that is not easy to inter-pret. Its inner part is surrounded by low–CL inten-sity, oscillatory magmatic zones. Planar zoning withrather high CL intensity developed around thisoscillatory magmatic zoning. All of the grains havethin bright CL rims.

Light brown zircons (grains D, E, and F) alsoshow oscillatory magmatic zoning and recrystalliza-tion rims that truncate the protolith structure. In

grain D, the zone of recrystallization cutting earlieroscillatory zones is especially remarkable (grain D,left and right part). Grain F has an inherited corerepresented by moderate Cl intensity (grey in color)and is surrounded by low–CL intensity oscillatoryzones (dark zone). This oscillatory zoning structureis overprinted by a recrystallization structure withmoderate CL intensity that in places truncates for-mer oscillatory zones (bottom left and right of grainF). A thin outermost bright rim is also typical for thistype of zircon.

Grains G, H, and I are members of the colorlesspopulation. Grain H is a typical magmatic zirconwith oscillatory magmatic zoning and bright CLouter rim similar to the other type of grains of thissample. Grains G and I reveal similar internal struc-tures and both have inherited cores. Grain G con-sists of a low–CL intensity core surrounded byoscillatory magmatic zones, whereas grain I has ahigh CL intensity core represented by sector zoningwith dark inclusions (Fig. 5, see SEM image). Bothsectors with oscillatory zoning are overprinted byhigh–CL intensity planar zones most probably indi-cating recrystallization. Bright CL outer rims alsodeveloped on both grains (Fig. 5, grains G and I).

Correlation of zircon ages and zircon populations

In Figure 6, ages from metasedimentary rocksand zircon core ages in the late Carboniferousorthogneisses are shown separately. Samples Gk33and Gk 206 reveal different source characteristics.Ages for sample Gk 200 will be discussed in thefollowing paragraphs.

Ages of 21 zircons at 29 heating steps have beenobtained from sample Gk 33 (Table 1). The mostabundant age group (31%) lies between 484.2 ± 4.6Ma and 433.6 ± 4.8 (Fig. 6). This age range has beenobtained at all heating steps, including the last one(at 1440° C). Grain 19 gives 470 and 433 Ma atlower and higher temperatures, respectively. Grain2, however, yields an older age in the first heatingstep (500 Ma) and a younger age in the second (466Ma). Three ages in the range between 621 to 612 Mahave been obtained from grains 10 and 13. Thecomplete list of ages and related populations of thissample is given in Table 1 and Figure 6.

Ages of 24 zircons at 35 heating steps wereobtained from sample Gk 206 (Table 2). The firstpopulation of zircons reveals ages older than 1000Ma, except grain 9 (685 Ma). The second zirconpopulation shows different age clusters; the firstcluster ranges from 495 to 446 Ma, similar to Gk 33,

PALEOTECTONIC POSITION OF THE STRANDJA MASSIF 527

whereas the second age cluster ranges between 582to 525 Ma; this corresponds 64% of the ages of thispopulation. Only three measurements were madefrom the third zircon population and these revealages of 998, 931, and 525 Ma (see Table 2 and Fig.6 for the complete age list).

Unlike those of Gk 33 and Gk 206, zircons ofsample Gk 200 reveal younger ages (Table 3, Fig. 6).Ten grains with 20 heating steps were processed. Allthree of zircon populations gave similar age clustersbetween 328 to 305 Ma, the age of the Late Carbon-iferous granitoids (orthogneisses) that intruded thebasement (Natal’in et al., 2005a, 2005b; Sunal etal., 2006). We also obtained a 236 Ma age from onegrain of the second population, and 258 Ma from thethird one. The second step of the former gave 314Ma, which is similar to other ages obtained from this

sample. Taking into account the age of the Kırklarelimetagranite (271 ± 2 to 257 ± 6 Ma) that cuts thesemetasediments, these younger ages appear to beunrealistic. We interpret these relatively young agesas a result of Pb loss during the mid-Mesozoic meta-morphism, or mixed ages of the rims developed inthis event and the protolith age of the orthogneisses.White rims in the grains of sample Gk 200 (Fig. 5)are much thicker than those of the grains of othersamples, inasmuch as metamorphic grade increasesto the south in the study area to amphibolite facies(Natal’in et al., 2005a). Ages of 342 and 336 Maprobably represent core ages. Similar ages are alsoreported from the Late Carboniferous orthogneissesas core ages (Sunal et al., 2006).

The correlation between zircon populations andzircon ages for samples Gk 33 and Gk 206 is shown

FIG. 5. CL and secondary-electron images of selected grains of sample Gk 200 (see text for explanations). Scale barsare 10 μm.

528 SUNAL ET AL.

in Figure 7. With a few exceptions, the dark brownand light brown zircon populations are clustered indifferent time intervals. Ages of light brown zirconsare concentrated between 450 Ma and 1 Ga andbetween 1.7 and 1.5 Ga. Ages of the dark brown zir-cons are also concentrated in two intervals: 1.3–1.0Ga and 2.6–2.0 Ga. Only one younger age (~685Ma) has been obtained from the dark brown zircons(Table 2) and one old age (~2.7 Ga, Table 2) from thelight brown ones.

The dark brown zircons from the samples Gk 33and Gk 206 (except for a single grain) show differenttime intervals of 1.0–1.5 Ma and 2.0–2.7 Ma,respectively (Fig. 7). Interpretation of this resultsuggests the existence of different sources of thesezircons. This inference is supported by the analysisof evaporation temperatures and internal structures.The dark brown zircons from Gk 33 are magmaticin origin because they generally show magmaticoscillatory zoning and few cores (Figs. 3A–3E). In

TABLE 1. Age Results of Sample Gk 33

Grain no. Zircon description1Evaporation

temp, °C No. of scansMean ratio of

207Pb/206Pb/error Age, Ma Error, Ma (±)

1 lb, lg, tr, id 1400 37 0.05662 ± 51 476.8 20

2 lb, thc, tr, id 1370 147 0.05722 ± 80 500.3 3.1

2 1400 288 0.05635 ± 78 466.2 3.1

3 db, thc, tr, id,rdt 1360 34 0.07273 ± 13 1006.4 3.4

3 1420 219 0.07499 ± 98 1068.3 2.6

4 lb, lg, tr, id 1420 109 0.05660 ± 14 476.0 5.5

5 lb, lg, tr, id 1440 144 0.05628 ± 10 463.5 2.4

6 lb, thc, tr, id 1380 36 0.05639 ± 12 467.8 4.7

7 lb, thc, tr, id 1440 75 0.05629 ± 98 464.1 3.8

8 clrs, thc, tr, id 1380 217 0.06710 ± 22 840.9 6.7

9 lb, thc, tr, id, rdt 1440 181 0.05912 ± 12 571.5 4.6

10 lb, thc, tr, id, rdt 1450 149 0.06041 ± 14 618.3 5.2

11 lb, lg, tr, id 1440 141 0.05723 ± 11 500.4 4.4

12 lb, thc, tr, id, rdt 1380 36 0.06246 ± 42 689.9 14

13 lb, lg, tr, id 1390 183 0.06023 ± 39 612.0 1.4

13 1400 72 0.06049 ± 16 621.1 5.6

14 db, thc, tr, id 1410 38 0.06896 ± 89 897.5 27

14 1420 36 0.06945 ± 46 912.1 14

14 1430 257 0.07817 ± 10 1151.2 2.6

15 lb, lg, tr, id 1440 71 0.05681 ± 12 484.2 4.6

16 db, thc, tr, id 1420 36 0.08314 ± 32 1272.5 16

16 1450 73 0.08318 ± 23 1273.4 5.4

17 lb, thc, tr, id, rdt 1420 145 0.09514 ± 18 1530.8 3.6

18 lb, lg, tr, id 1420 108 0.06524 ± 19 782.1 6.1

19 lb, thc, tr, id 1380 144 0.05553 ± 12 433.6 4.8

19 1420 142 0.05647 ± 11 470.9 4.3

20 db, thc, tr, id 1400 107 0.07761 ± 13 1136.9 3.3

21 lb, thc, tr, id, rdt 1400 38 0.06569 ± 35 796.5 11.2

21 1420 146 0.10414 ± 86 1699.2 1.5

1lb = light brown; db = dark brown; clrs = colorless; lg = long; thc = thick; tr = transparent; str = slightly transparent; id = idiomorphic; rdt = rounded terminations.

PALEOTECTONIC POSITION OF THE STRANDJA MASSIF 529

contrast, the dark brown zircons in Gk 206 containmore inherited cores (Fig. 4). Ages obtained at lowevaporation temperatures can also be accepted asmagmatic ages, but ages obtained at high heatingsteps are not easy to interpret. These ages may indi-cate both magmatic and metamorphic events. Thelatter interpretation is applicable for core structuresthat do not show any zoning (Fig. 4A to F).

Ages of light brown and colorless zircons aresimilar in two samples (Gk 33 and 206). The major-ity of these ages are distributed between 450 and1000 Ma, with only three exceptions yielding olderages (Tables 1 and 2). Ages ranging between 450–490 Ma and 550–580 Ma should be magmatic inorigin because of their usual coherence at differentheating steps (e.g., grains 16, 19, 2, and 24 insample Gk 206).

Some zircons show a large difference in ages atdifferent heating steps. Ages yielded from low heat-ing steps could be interpreted as magmatic, becausethe outer rims in all studied zircons generally showoscillatory structure (Fig. 3F–3O and 4G–4N). How-ever, ages related to high heating steps can alsorepresent metamorphic events. This is especiallytrue for grains having inner unzoned cores that canbe interpreted as metamorphic in origin (grains K,L, and M in Fig. 3 and grains F and G in Fig. 4).

Core and xenocryst ages of Late Carboniferous granitoids (orthogneisses)

Several suites of granitoids intruded the basementat 315–300 Ma (late Carboniferous orthogneisses,

FIG. 6. Graph of ages of samples (Gk33, Gk 206, Gk 200) and cores in Late Carboniferous orthogneisses.

FIG. 7. Comparison of ages obtained from detrital zirconpopulations extracted from samples Gk 33 and Gk 206.

530 SUNAL ET AL.

Fig. 1A). They are divided into hornblende-biotite,biotite-muscovite, and leucocratic granitic gneisses(Natal’in et al., 2005a, 2005b; Sunal et al., 2006).For the purpose of this study, zircon ages given bySunal et al. (2006) were carefully analyzed. To avoid

mixing, some of the core and xenocryst agesobtained from these orthogneisses have been elimi-nated. Similar to the detrital zircons, ages from theorthogneisses have considerable variations (Table 4,Fig. 6). Four clusters of ages are established. The

TABLE 2. Age Results of Sample Gk 2061

Grain no Zircon descriptionEvaporation

temp, °CNo.

of scansMean ratio of

207Pb/206Pb/error Age, Ma Error, Ma (±)

1 db, thc, str, id 1380 73 0.13259 ± 60 2132.5 7.9

1 1400 138 0.15375 ± 23 2388.1 2.5

2 lb, thc, tr, rdt 1400 73 0.05667 ± 30 478.7 11.7

3 lb, lg, str, id 1400 110 0.05851 ± 82 549.0 3.1

3 1420 147 0.05806 ± 82 532.3 3.1

4 clrs, lg, tr, rtd 1400 35 0.07011 ± 21 931.6 6.1

4 1420 71 0.07245 ± 36 998.6 10.1

5 clrs, thc, tr, id 1400 72 0.05790 ± 21 526.0 8.0

6 lb, thc, tr, id 1420 70 0.06269 ± 21 697.7 7.1

7 lb, thc, tr, rtd 1420 37 0.06128 ± 42 649.1 14.7

8 lb, lg, tr, id 1420 110 0.05789 ± 15 525.6 5.7

9 lb, lg, str, rtd 1400 37 0.05585 ± 40 446.4 15.9

9 1420 73 0.05895 ± 24 565.2 8.9

10 db, thc, str, id 1400 110 0.08322 ± 16 1274.4 3.7

10 1420 138 0.14224 ± 12 2254.7 1.5

11 lb, thc, tr, id 1420 36 0.06477 ± 27 766.9 8.8

12 lb, thc, tr, rdt 1400 73 0.05711 ± 25 495.8 9.6

12 1420 102 0.05891 ± 25 563.8 9.2

13 lb, thc, tr, id 1400 144 0.05793 ± 94 527.2 3.6

13 1420 110 0.05943 ± 13 582.9 4.7

14 lb, thc, str, id 1420 146 0.05594 ± 66 450.3 2.6

14 1440 74 0.05592 ± 16 449.2 6.4

15 db, thc, str, id 1420 71 0.12317 ± 37 2002.6 5.3

16 lb, thc, tr, id 1380 143 0.05899 ± 42 567.1 1.5

16 1400 106 0.05921 ± 52 574.8 1.9

16 1420 73 0.05856 ± 31 550.8 11.6

17 db, thc, str, id 1420 144 0.06233 ± 47 685.8 1.6

18 lb, thc, str, id 1400 110 0.05789 ± 89 525.8 3.4

18 1420 215 0.05831 ± 66 541.5 2.5

19 db, thc, str, id 1380 36 0.13030 ± 46 2102.0 6.2

19 1400 73 0.16028 ± 43 2458.6 4.5

19 1420 109 0.17384 ± 19 2595.0 1.8

20 lb, lg, tr, id 1420 109 0.18595 ± 20 2706.7 1.8

21 lb, thc, tr, id 1400 106 0.05912 ± 16 571.5 5.9

21 1420 108 0.05897 ± 69 566.2 2.5

1Abbreviations are the same as in Table 1.

PALEOTECTONIC POSITION OF THE STRANDJA MASSIF 531

first age cluster is between 385 and 340 Ma and isabsent in the detrital ages of the metasediments(samples Gk 33 and Gk 206), but present in thedetrital zircons of sample Gk 200. The second groupis represented by the age range of 428 to 401 Maand is also absent in the detrital results (the closestage is 433 Ma). The third group ranges between 487and 442 Ma, and the fourth group is between 535and 500 Ma. In addition, relatively older ages wereobtained; for instance, two grains reveal 584 and566 Ma and three grains yield 648 and 619 Ma(Table 4).

Sunal et al. (2006) also reported some ID-TIMS,U-Pb ages for five fractions containing the threepopulations of zircons mentioned above. One frac-tion has a slightly discordant age at 308 Ma. Ages ofother fractions could not be defined by a singlediscordia line as an accepted MSDW value becauseof a large scatter owing to the presence of inheritedcores. Distribution of the core ages could be con-strained by drawing different discordia lines repre-

senting different fractions with forced regression tothe 312 Ma (age of intrusion). Upper intercepts ofthese discordia lines vary from 1300 to 650 Ma.

Evaluation of Data

To find similarities in the geological evolution ofthe Strandja massif and other European massifs, wecompiled a database including detrital and inher-ited zircon ages from previously published papers.Most of these works used different methods fordating (SHRIMP, evaporation, ID-TIMS, SIMS, andLA-ICP-MS) that consequently imply differentapproaches to tectonic interpretations of these ages.Keeping in mind this possibility, we set up somerules for the evaluation of the data:

1. Pb-Pb stepwise evaporation data wereincluded in the database if mean ages are given formagmatic and/or metamorphic events. Detrital orinherited ages listed in the papers are evaluated

TABLE 3. Age Results of Sample Gk 2001

Grain no. Zircon descriptionEvaporation

temp. °C No. of scansMean ratio of 207Pb/

206Pb/error Age, Ma Error, Ma (±)

1 clrs, lg, tr, id 1420 35 0.05250 ± 29 307.2 12.6

2 db, thc, str, rdt 1400 36 0.05332 ± 25 342.4 10.6

3 lb, lg, str, id 1380 144 0.05090 ± 15 236.32 6.8

3 1420 144 0.05265 ± 87 314.0 3.8

4 lb, lg, str, id 1380 106 0.05268 ± 10 315.0 4.3

4 1400 71 0.05248 ± 17 306.4 7.4

4 1420 36 0.05293 ± 28 325.8 12.0

5 db, thc, str, rdt 1380 71 0.05277 ± 22 318.9 9.5

5 1400 147 0.05302 ± 14 329.7 6.0

5 1420 107 0.05294 ± 18 326.2 7.7

6 lb, lg, tr, id 1380 147 0.05272 ± 12 316.8 5.2

6 1400 118 0.05249 ± 77 307.2 3.3

6 1420 107 0.05246 ± 57 305.5 2.5

7 clrs, lg, tr, id 1400 108 0.05140 ± 65 258.8 29.1

8 lb, lg, tr, id 1400 36 0.05255 ± 46 309.4 19.9

8 1420 73 0.05300 ± 22 328.8 9.4

9 db, lg, str, id, rdt 1380 145 0.05266 ± 74 314.6 3.2

9 1400 110 0.05276 ± 72 318.5 3.1

10 lb, lg, tr, id 1400 144 0.05248 ± 20 306.4 8.7

10 1420 73 0.05284 ± 12 321.9 5.2

1Abbreviations are the same as Table 1.2Led loss.

532 SUNAL ET AL.

when the authors discussed their reliability orgeological meaning.

2. Ages determined by the ID-TIMS method areaccepted when upper or/and lower intercept agesare provided. Only concordant or slightly discordantapparent 238U-206Pb ages for inherited and detritaldata younger than 1.0 Ga are included into thecompilation. Ages older than 1.0 Ga were added tothe database without considering their concordance(Cawood and Nemchin, 2000).

3. SHIRIMP, SIMS and LA-ICP-MS data aretreated similar to ID-TIMS data.

There are several problems with detrital zirconstudies related both to statistical evaluation of data,and determination of their tectonic meaning. Trying

to escape falling into methodological traps (e.g.,Sircombe and Hazelton, 2004; Nemchin andCawood, 2005; Andersen, 2005), we rely more onage distributions rather than individual peaks.

The evaporation method has some advantages:(a) after separation of zircons, no chemical processis needed, which makes the method relatively fastand easy; (2) contamination does not occur becauseno chemical treatment is applied; and (c) differentheating steps together with CL imaging techniquesreveal the history of the zircon. It also has certaindisadvantages: (a) mixing of different age compo-nents; and (b) it is not possible to determine theconcordance of the Pb/Pb age results without U/Pbdata. In order to avoid concordance of the Pb/Pb age

TABLE 4. Core and Xenocryst Ages Obtained from Late Carboniferous Granite Gneisses1

Biotite-muscovite gneiss Hornblende-biotie gneiss Leucocratic gneiss

Age (Ma) Error (Ma)± Age (Ma) Error (Ma)± Age (Ma) Error (Ma)±

343.7 5 361.9 7.9 349.6 17

349.2 23 342.9 7.7 353.9 11

350.9 7.9 344.1 4.8 648 14

356.2 3.2 346.4 2.7 852.3 7.3

359.9 4.9 354.7 17 1028 16

428.3 1.7 363.5 6.1

444.8 3.8 385.6 5.4

460.3 7.1 401.2 14

463.0 21 402.5 3.4

466.2 14 410.5 2.7

472.9 6.9 442.4 19

477.2 12 448.4 2.7

487.3 12 462.6 4.7

501.2 4.4 503.5 4.2

516.9 8.6 515 4.2

534.9 3.7

566.8 2.6

584.0 4.8

619.4 4.7

643.8 13

814.6 11

1801.4 4.9

1892.5 3.4

2154.8 3.7

2484.4 1.7

1Data after Sunal et al., 2006.

PALEOTECTONIC POSITION OF THE STRANDJA MASSIF 533

results without U/Pb data, the ID-TIMS method isused. As was mentioned in the previous section, nodiscordia line could be obtained, and only a regioncould be defined that represents core ages.

For the evaluation of data, we first checked theTh/U ratio (derived from the 208Pb/206Pb ratio). Dataare taken into consideration only when they have aplateau-like distribution. Furthermore, we set uptwo criteria to avoid mixed ages:

1. At least two heating steps during an analysis ofthe same zircon should give similar ages.

2. Number of coherent ages. For instance, if onlyone particular age could be obtained from a wholeset of analyses, that age was not evaluated (Fig. 8).In addition, the age is evaluated when an individualage is obtained in high heating steps (especially thelast one) because it represents the most internal partof the grain, which hardly mixes with any other agecomponent.

Histograms showing the probability density dis-tribution (PPD) and binned frequency of the evapo-ration step ages (Figs. 8 and 9) were created usingthe Age Display program (Sircombe, 2004). The binwidth is chosen as 25 Ma and the efficiency value as75%. The same bin width was also assigned for thecompiled data for reliable correlation. If publishedor compiled histograms have no information abouttheir bin widths, we used them directly without con-sidering their bin widths. Figure 10 shows a binnedfrequency distribution of the ages as a genetic corre-lation map–like histogram. The genetic map histo-gram is preferred rather than bar graphs due to itsconvenience for determining the origin of continentsor continental fragments.

Discussion

Ages of the metasediments in the basement of the Strandja massif

Uncertainties in the age estimations of metased-iments in the basement of the Strandja massif rang-ing from the Precambrian to the late Paleozoic(Aydın, 1974, 1982; Çaglayan et al., 1988; Okay etal., 2001; Türkecan and Yurtsever, 2002) have beencaused by the almost total lack of reliable isotopicage determinations. In this paper we provide somerelevant data.

In the north, the youngest detrital age recordedin two zircons is ~430 Ma (Figs. 8 and 9). This ageis also recorded in the core and xenocrystic zirconsextracted from the late Paleozoic orthogneisses(Sunal et al., 2006). A 449 Ma peak is represented

by six zircons from the metasedimentary rocks.These ages were obtained at high temperatures andrepeated heating steps. Similar peaks exist inorthogneisses (Fig. 8, Tables 2 and 3). We infer that430 Ma can be regarded as the lower age limit (Fig.9) for the deposition of rocks sampled in the north(samples Gk 206 and Gk 33).

The Late Carboniferous orthogneisses (315–300Ma) crosscutting the metasediments determine theupper age limit for sedimentation (315 Ma). Zirconcores and xenocrysts from metagranitoids form anobvious peak around 350 Ma (11 zircons). This ageinterval is very well known in the EuropeanVariscides, where it reflects widespread magmatismand metamorphism (O’Brien et al., 1990; Kalt et al.,1994; Kröner et al., 1994; Holub et al., 1997; Fingerand Roberts, 1997; Bonin, 1998; Schaltegger, 1997;von Raumer, 1998; Altherr et al., 1999; Fernández-Suárez et al., 2000; Kröner et al., 2000; Schaltegger,2000; Kalt et al., 2000; Klötzli et al., 2001; Romerand Rotzler, 2001, and references therein; Carriganet al., 2003; Janoušek and Gerdes, 2003; Janoušeket al., 2004; Friedl et al., 2004). This age interval ismissing in the metasedimentary rocks of theStrandja massif, indicating that magmatic rocks withan age of 350 Ma existed at depth but were notexposed at the surface (Figs. 8 and 9), or that sedi-ments were deposited prior to 350 Ma. This infer-ence suggests a tool for evaluation of rock ageslocated at different crustal levels.

The depositional age of the metasedimentslocated in the south of the studied area (Fig. 1A) isrelatively easy to interpret. The majority of the detri-tal zircons have the same ages as the Late Carbonif-erous granitoids (315-300 Ma). They must havebeen deposited after this magmatic event, and thusthe lower limit for sedimentation can be accepted as300 Ma. These metasediments are cut by theKırklareli metagranites (271–256 Ma; Okay et al.,2001; Natal’in et al., 2005a, 2005b; Sunal et al.,2006), imposing the upper depositional age limit.

Continental blocks of the surrounding regions

A number of continental fragments crop out inTurkey and in the neighboring regions of the BalkanPeninsula (Fig. 2). The Strandja massif, the IstanbulZone, the Menderes massif, and the Kırºehir massifare generally believed to be parts of Gondwana, pre-serving the Pan-African or/and Cadomian orogenicevents (ªengör et al., 1984; ªengör and Yılmaz,1981; Chen et al., 2002; Yigitbaº et al., 2004; Yanevet al., 2004; Ustaömer et al., 2005). Some research-

534 SUNAL ET AL.

FIG

. 8. P

roba

bilit

y de

nsity

dis

trib

utio

n hi

stog

ram

s of

the

ages

obt

aine

d fr

om th

e st

udy

area

.

PALEOTECTONIC POSITION OF THE STRANDJA MASSIF 535

ers suggest that the Strandja massif together with theIstanbul Zone belongs to Laurasia (Okay et al.,1996; Okay and Tüysüz, 1999, Natal’in et al.,2005a, 2005b; Natal’in, 2006). Recently Okay et al.

(2006) assigned the Istanbul Zone to the East Avalo-nia microcontinent, similar to the Pelagonian Zonein Greece (Anders et al., 2005). Yanev (2000) andYanev et al. (2004) correlated the Istanbul, Balkan,

FIG. 9. Probability density distributions histogram of the ages younger than 700 Ma. Bar shows deposition intervalof metasediments of the basement units (see text for discussion).

536 SUNAL ET AL.

FIG

. 10.

Age

dis

trib

utio

ns (g

enet

ic m

aps)

of s

elec

ted

Eur

ope,

sur

roun

ding

Aeg

ean

regi

on m

assi

fs (z

ones

), an

d m

ain

crat

onic

are

as. S

ymbo

ls: 1

= W

est A

fric

an c

rato

n (N

ance

and

Mur

phy,

199

4;Sö

llner

et

al.,

1997

); 2

= N

orth

east

Afr

ican

cra

ton

(Sul

tan

et a

l., 1

994;

Avi

gad

et a

l., 2

003)

; 3

= B

altic

a–E

ast

Eur

opea

n cr

aton

(C

omps

ton

et a

l., 1

995;

Mor

gan

et a

l., 2

000;

Dör

r et

al.,

200

2;W

iszn

iew

ska,

200

2; W

iszn

iew

ska

et a

l., 2

002,

200

4; B

agiń

ski

and

Krz

emiń

ska,

200

5; G

awéd

a, e

t al

., 20

05 a

nd t

he r

efer

ence

s th

erei

n; W

iszn

iew

ska

and

Krz

emiń

ska,

200

5 an

d th

e re

fere

nces

ther

ein;

Str

nad

and

Mih

alje

vic,

200

5 an

d re

fere

nces

ther

ein)

; 4 =

Sou

th A

mer

ican

cra

ton

(Am

azon

ian

crat

on) (

Teix

eira

et a

l., 1

989)

; 5 =

Sib

eria

and

Ang

ara

(Rai

nbir

d et

al.,

199

8; S

alni

kova

et a

l.,19

98; A

ftalio

n et

al.,

199

1; K

hudo

ley

et a

l., 2

001;

Ver

niko

vsky

et a

l., 2

003)

; 6 =

Nor

thw

est I

beri

a (F

erná

ndez

-Suá

rez

et a

l., 2

002

and

the

refe

renc

es th

erei

n); 7

= C

adom

ia (F

erna

ndez

-Sua

rez

et a

l.,20

02 a

nd th

e re

fere

nces

ther

ein)

; 8 =

Sou

thw

est I

beri

a (F

erná

ndez

-Suá

rez

et a

l., 2

002

and

the

refe

renc

es th

erei

n); 9

= A

rmor

ican

Qua

rtzi

te (F

erná

ndez

-Suá

rez

et a

l., 2

002

and

the

refe

renc

es th

erei

n);

10 =

Tep

la-B

arra

ndia

n Zo

ne (B

ohem

ia m

assi

f) (D

rost

et a

l., 2

003;

Str

nad

and

Mih

alje

vic,

200

5); 1

1 =

Erb

endo

rf-V

chen

stra

uss

Zone

(Boh

emia

mas

sif)

(Söl

lner

et a

l., 1

997)

; 12

= Sa

xoth

urin

gian

Zon

e(B

ohem

ia m

assi

f) (K

röne

r an

d W

illne

r, 19

98; T

icho

mir

owa

et a

l., 2

001;

Lin

nem

an e

t al.,

200

4); 1

3 =

Mol

danu

bian

Zon

e (B

ohem

ia m

assi

f) (G

ebau

er e

t al.,

198

9; K

röne

r et

al.,

198

8; F

ried

l et a

l.,20

04);

14 =

Sud

etes

(Boh

emia

mas

sif)

(O’B

rien

and

Car

swel

l, 19

93; O

’Bri

en e

t al.,

199

7; K

röne

r et

al.,

200

1; M

uszy

n ski

et a

l., 2

002;

Tur

niak

et a

l., 2

002;

Klim

as e

t al.,

200

3; M

azur

et a

l., 2

004;

Turn

iak

et a

l., 2

005)

; 15

= M

orav

ia-S

ilesi

a (B

ohem

ia m

assi

f) (F

ried

l et a

l., 2

004)

; 16

= E

cker

gnei

ss C

ompl

ex (H

arz

Mou

ntai

ns, R

heno

herc

ynia

n Zo

ne) (

Gei

sler

et a

l., 2

005)

; 17

= Is

tanb

ul Z

one

(Che

net

al.,

200

2; U

staö

mer

et a

l., 2

005)

; 18

= E

aste

rn P

elag

onia

Zon

e (A

nder

s et

al.,

200

5); 1

9 =

Men

dere

s m

assi

f (K

röne

r an

d ªe

ngör

, 199

0; H

etze

l and

Rei

schm

ann,

199

6; D

anna

t, 19

97; H

etze

l et a

l.,19

98; L

oos

and

Rei

schm

ann,

199

9; G

essn

er e

t al.,

200

1, 2

004;

Kor

alay

et a

l., 2

004;

Rin

g an

d C

ollin

s, 2

005)

; 20

= Sr

edna

Gor

a an

d B

alka

n Zo

ne (T

itore

nkov

a et

al.,

200

3; P

eytc

heva

et a

l., 2

004;

von

Qua

dt a

nd P

eytc

heva

, 200

4; A

tana

sova

-Vla

dim

irov

a et

al.,

200

4; C

arri

gan

et a

l., 2

005)

; 21

= C

ycla

des

(Kea

y an

d Li

ster

, 200

2); 2

2 =

Stra

ndja

mas

sif (

this

stu

dy);

23 =

Kırºe

hir

mas

sif (

Whi

tney

et a

l., 2

003)

. Abb

revi

atio

ns: A

VM

Z =

Ava

loni

a m

inim

um z

one,

AR

MZ:

Arm

oric

a m

inim

um z

one.

PALEOTECTONIC POSITION OF THE STRANDJA MASSIF 537

and Bohemian tectonic units with the Saxo-Thuring-ian terranes (Eastern Armorican group of terranes)and the Zonguldak and Moesian units with the Rhe-nohercynian Terranes (Avalonian group of terranes).The latest Neoproterozoic to Lower Cambrianmetasedimentary rocks of the Menderes–Taurusblock was compared with the Angara craton (Krönerand ªengör, 1990). Detrital and inherited zirconages obtained from late Paleozoic and Mesozoicrocks in the Cyclades were by contrast correlatedwith North Africa (Keay and Lister, 2002).

In the European Variscides, two microcontinentsof Gondwanian origin have been identified; Avalo-nia, originally located near South America andNorthwest Africa, in the north, and Armorica, apiece of North Africa, in the south (Van der Voo,1979; Matte, 1986, 2001; Ziegler, 1989; Scoteseand McKerrow, 1990; Tait et al., 1997; Torsvik,1998). They rifted off Gondwana and collided withLaurasia at different times, being separated fromeach other by the Rheic Ocean. Its closure in partcaused the Variscan Orogeny in Europe (Matte,2001). In Africa, the Archaean and PaleoproterozoicKalahari, Kongo, and Saharan cratons amalgamatedduring the Mesoproterozoic, Neoproterozoic, andCambrian orogens (as the Eburnian, Grenville,Kibaran, Irumide, and Pan-African orogens)(Abdelsalam et al., 2002; Johnson et al., 2005). Sim-ilar cratonic areas also exist in South America (Riode la Plata and Amazonian cratons; Da Silva et al.,2005). These cratonic nuclei are surrounded bymobile orogenic belts. The Grenville (Sunsas belt)and Pan-African (Brasiliano) orogenies are alsorecorded in South America (Da Silva et al., 2005).Unlike Africa, South America has a long record ofMesoproterozoic orogenesis such as the Rodonia-Juruena and Rio-Negro (Linneman et al., 2004 andreferences therein). Baltica is very similar to SouthAmerica in terms of time distribution of orogenicevents. The Grenville orogeny, which dominatesBaltica, and the Pan-African orogeny are notrecorded in this continent. However, in the similarperiod of Pan-African events, the Timanian orogenyoccurred in Baltica (Cocks and Torsvik, 2005).Unlike South America, Baltica has anorogenicRapakivi plutons in the Mesoproterozoic interval.

Continental blocks derived from north Africaand Armorica, typically lack Mesoproterozoic ages,whereas the Avalonian units have a strong signal atthis time interval (Nance and Murphy, 1994; Lin-neman et al., 2004; Samson et al., 2005). TheArchean ages are widespread in the Armorican type

units but this interval is represented by a gap inAvalonian units (Avalonia minimum zone) (Nanceand Murphy, 1994; Linneman et al., 2004; Samsonet al., 2005).

Laurasia-derived tectonic units can be distin-guished from Gondwanian ones by the gap in theNeoproterozoic between 600 and 900 Ma—theBaltican Gap (Fig. 10). Another time gap occurs inthe late Archean–Paleoproterozoic interval. How-ever, a similar gap (Avalonia minimum, Samson etal., 2005) is known in South America and the Avalo-nian tectonic units in Europe (Fig. 10).

In the following section, we discuss the correla-tion of the Anatolian and the Aegean region massifsor zones, their relations with the EuropeanVariscides, and their possible provenance regionsusing the zircon age compilation.

Comparison of the units

The Cyclades and the Strandja massif show analmost identical pattern of zircon age distributions(Fig. 10). In both, the Ordovician peak is present. Innearby tectonic units (e.g., Menderes, Pelagonia,and Kırºehir) this peak is absent (Keay and Lister,2002). The Ordovician peak is known in the SrednaGora–Balkan Zone (Titorenkova et al., 2003; Mal-inov et al., 2004; Peytcheva et al., 2004; von Quadtand Peytcheva, 2004; Carrigan et al., 2005) that isthe continuation of the Strandja massif (Natal’in etal, 2005a, 2005b; Natal’in, 2006). These ages wereobtained from late Paleozoic to Early Mesozoic, syn-to post-Variscan rocks.

Figure 7 shows the differences between zirconages obtained from magmatic and metasedimentaryrocks in the Strandja massif. Zircon ages from mag-matic rocks have a distribution pattern that is simi-lar to the Menderes massif. The detrital ages overlapthe Mesoproterozoic gaps present in the Menderesmassif (Figs. 6, 8, and 10). A similar Mesoprotero-zoic gap exists in metagranitoids exposed in theCyclades, although ages of this time interval arefound in Mesozoic sediments (Keay and Lister,2002).

Apparently the Menderes and the Kırºehirmassifs in Central Anatolia were rifted from NorthAfrica and attached to Laurasia during Jurassic–Paleocene time (Özgül, 1984; ªengör, 1979; ªengörand Yılmaz, 1981). This interpretation is supportedby the recently discovered absence of the Meso-proterozoic ages in these massifs (e.g., Keay andLister, 2002; Gessner et al., 2004) (Fig. 9). Avigadet al. (2003) cast doubt on the absence of Meso-

538 SUNAL ET AL.

proterozoic ages in North Africa. Studying Cambriansiliciclastic rocks, they found Mesoproterozoic ages(0.9–1.1 Ga) and interpreted them as evidence ofthe Kibaran Orogeny. Similar zircon ages werereported in the Kırºehir, Menderes, and Cycladesmassifs (Kröner and ªengör, 1990; Loos and Reisch-mann, 1999; Keay and Lister, 2002; Whitney et al.,2003). In addition, Nd model ages from the Aegeanand Anatolian crust yield a cluster around 1 Ga (PePiper and Piper, 2001). As a result, in the followingcorrelations we will use a narrowed Proterozoic gapfor North Africa–derived units in accord with recentdiscoveries.

The presence of Mesoproterozoic ages in detritalzircons from Paleozoic or Mesozoic sedimentaryrocks reported around the Aegean region and Anato-lia may reflect a mixing of different sources thatappeared later in the geological history of these tec-tonic units. The Cyclades are a good example. Weinfer that the Menderes and the Kırºehir massifsshow North African affinity, although with a consid-erable gap between 2 and 2.5 Ga in the Kırºehirmassif (for data source see Whitney at al., 2003),which is more similar to the situation on the Anga-ran craton in Siberia than with Gondwana (Fig. 10).

The Pelagonian and the Sredna-Gora zones havethe same Precambrian age gaps. However, the lim-ited number of ages obtained from the PelagonianZone makes this comparison tentative. The zirconage distributions in the Sredna-Gora-Balkan,Strandja, and the Cyclades are very similar; all ofthem have Ordovician ages, but the 1.5 to 1.0 Gainterval is not present in the Sredna-Gora-BalkanZone (Fig. 10).

Age distributions of magmatic pre-Ordovicianrocks in the Saxothuringian Zone and the Sudetes inEurope reveal similarities with the Strandja massifand the Cyclades (Fig. 10). All of the ages belongingto the Saxothuringian Zone and the Sudetes wereobtained from pre-Ordovician magmatic rocks andtherefore they unequivocally indicate events occur-ring in the crust, and the problem of source mixing(Strandja, Cyclades) is eliminated. An example ofsource mixing was provided for the Armoricanquartzite by Fernández-Suárez et al. (2002) (Fig.10). According to their model, the Armoricanquartzite (Early–Middle Ordovician) was fed byboth northwest Iberia (Amazonian source) andsouthwest Iberia (North African source).

The Istanbul zone has a more or less continuousrecord that starts in the mid-Paleoproterozoic andends in the Late Cambrian (Fig. 10). Its zircon age

distribution has no equivalent around the Aegeanregion and Anatolia. This zone might be correlatedwith the Rhenohercynian Zone (Eckergneiss com-plex), which is believed to be part of eastern Avalo-nia that had been derived mainly from north ofSouth America (Amazonia) (Geisler at al., 2005 andreferences therein). The Paleozoic succession of theIstanbul Zone has been correlated with the Scythianplatform of the Southern Urals (Okay et al., 1994).Recent studies have revealed close connectionsbetween the Scythian platform, Moesia, and theIstanbul Zone with the tectonic zones in northernEurope. Kalvoda et al. (2003) reaffirmed that theIstanbul and the Rhynohercynian zones can becounterparts, considering their paleobiogeographicdata and the Variscan history as originally shown byPaeckelmann (1925, 1938).

Comparisons of these two units (the Eckergneisscomplex of the Rhenohercynian Zone and the Istan-bul Zone) reveal some similarities and dissimilari-ties. Each has a Mesoproterozoic age (Fig. 10).However, no ages predating the middle Paleopro-terozoic are known from either of these units. TheEckergneiss complex has a gap during the Cam-brian, but the Istanbul Zone does not. In somerespects, the Istanbul zone and the Eckergneisscomplex can be compared both with South Americaand Baltica in terms of age distributions. The pres-ence of magmatic activity around 1.5 Ga is typicalfor Baltica-derived units.

As seen in the previous correlations, in somecases it is not easy to distinguish the original posi-tion of microcontinents using known criteria such asthe Mesoproterozoic gap in North Africa or the pres-ence of the Cadomian or Pan-African ages. In addi-tion, the nature of the host rocks containing thezircons is important. The presence and the kind ofzircons in sedimentary rocks strongly depend onmechanisms of lateral transport. Thus, they may bederived from widely separated sources and givemixed ages. Inherited zircons in magmatic rocks aremore dependent on vertical ascent of magma, hencebetter record the history of the continental crustthrough which they ascend.

Instead of gaps, “minimum zones” (Samson etal., 2005) can be used for correlations. In this study,we propose a criterion for the distinction betweenBaltica- and Gondwana-derived continental blocks.Blocks of Gondwanian origin (mainly east Gond-wana) are characterized by a minimum zone of agesbetween 1.4 to 1.65 Ga. In Baltica, the same timespan is well represented by zircon ages and geologi-

PALEOTECTONIC POSITION OF THE STRANDJA MASSIF 539

cally known magmatic and metamorphic events(Fig. 10). With few exceptions, such as the Sudetesand the Menderes Massif, all units derived from eastGondwana have this minimum zone (e.g., theStrandja, the Cyclades, the Kirºehir massif, theSredna-Gora-Balkan, the Pelagonia Zone, Iberia,the Cadomia, the Armorican Massif, Tepla Barran-dian, and Saxo Thuringian zones, Fig. 10). In theMenderes massif, only two ages (evaporation ages)out of 82 measurements were reported around 1.5Ga (Loos and Reischmann, 1999; Gessner et. al.,2004). In Baltica, ages around 1.5 Ga, charnockiticrocks in Poland (Dörr et al., 2002; Gawęda et al.,2005), Rapakivi granite-gabbro-anorthosite in Lith-uania (Rämö and Haapala, 1996; Motuza, 2005),similar granitoids in Sweden (Persson, 1999) and inRussia (Amelin et al., 1997) have been reported.

Conclusions

Metasedimentary rocks constituting the base-ment of the Strandja massif developed in two differ-ent intervals—Silurian–Carboniferous (430–315Ma) and Permo-Carboniferous (300–271 Ma),respectively.

Continental blocks and tectonic zones locatedaround the Aegean region show clear NNE Africanaffinity, except for the Istanbul Zone. Gondwanianaffinity for the Kırºehir massif is also a plausibleinterpretation (Özgül, 1984). The geologic historyand age distribution of the Istanbul Zone show agreat similarity with the Rhenohercynian zone ofnorthern Europe, which is a part of the East Avalo-nian microcontinent. Close similarities have beenrecognized in the pre-Mesozoic history of theStrandja and the Cyclades. Both have garnet-bear-ing Paleozoic metasediments and Variscan grani-toids that are similar in ages and age distributions.The difference between core ages in the late Paleo-zoic orthogneisses and the detrital age spectrum ofthe Strandja massif can be interpreted as follows:Africa-derived Precambrian basement of theStrandja massif that had not been exposed in theregion arrived into the proximity of Amazonia- orBaltica-derived tectonic units. The Paleozoic (430–315 Ma) sedimentary rocks present a record of thesemixed sources.

Acknowledgments

This study was supported by TÜBITAK (TheScientific and Technical Research Council of

Turkey, Project no: YDABCAG 101Y010) andresulted from the cooperative study between Istan-bul Technical University (ITU Research Fund,Project No. 11-07-128), Turkey and the Universityof Tübingen, Germany. It is also a part of the IGCP-480 project. We wish to thank G. Bartholomä for themineral seperations, Dr. W. Siebel and E. Reitter forhelp in isotopic measurements, and Dr. T. T. B.Nguyen from Vietnam for help in mineral separationand isotopic measurements. We sincerely thank Y.Dilek, D. Whitney, S. Özeren, and Z. Çakır forimproving the English of the text, and their criticaland constructive comments and M. Kibaroglu for theCL study. Thanks are also due to A. M. C. ªengör forhis invaluable criticism and constructive commentson the text.

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