Peri-Gondwanan Ordovician crustal fragments in the high-grade basement of the Eastern Rhodope Massif, Bulgaria: evidence from U-Pb LA-ICP-MS zircon geochronology and geochemistry

Abstract

Field, geochemical, and geochronologic data of high-grade basement metamafic and evolved rocks are used to identify the nature and timing of pre-Alpine crustal growth of the Rhodope Massif. These rocks occur intrusive into clastic-carbonate metasedimentary succession. Petrography and mineral chemistry show compositions consistent with Alpine amphibolite-facies metamorphism that obliterated the original igneous textures of the protoliths. Bulk-rock geochemistry identifies low-Ti tholeiitic to calc-alkaline gabbroic-basaltic and plagiogranite precursors, with MORB-IAT supra-subduction zone signature and trace elements comparable to modern back-arc basalts. The U-Pb zircon dating revealed a mean age of 455 Ma for the magmatic crystallization of the protoliths that contain inherited Cambrian (528–534 Ma) zircons. Carboniferous, Jurassic, and Eocene metamorphic events overprinted the Ordovician protoliths. The radiometric results of the metamorphic rocks demonstrate that Ordovician oceanic crust was involved in the build-up of the Rhodope high-grade basement. Dating of Eocene-Oligocene volcanic rocks overlying or cross-cutting the metamorphic rocks supplied Neoproterozoic, Ordovician and Permo-Carboniferous xenocrystic zircons that were sampled en route to the surface from the basement. The volcanic rocks thus confirm sub-regionally present Neoproterozoic and Paleozoic igneous and metamorphic basement. We interpret the origin of the Middle-Late Ordovician oceanic magmatism in a back-arc rift-spreading center propagating along peri-Gondwanan Cadomian basement terrane related to the Rheic Ocean widening. The results highlight the presence of elements of Cadomian northern Gondwana margin in the high-grade basement and record of Rheic Ocean evolution. The eastern Rhodope Massif high-grade basement compared to adjacent terranes with Neoproterozoic and Cambro-Ordovician evolution shares analogous tectono-magmatic record providing a linkage among basement terranes incorporated in the Alpine belt of the north Aegean region.

Keywords:

• high-grade basement
• U-Pb LA-ICP-MS geochronology
• geochemistry
• Eastern Rhodopes
• Bulgaria

1. Introduction

The metamorphic basement of the Rhodope Massif was considered traditionally as Precambrian to Early Paleozoic (e.g. Kozhoukharov, Kozhoukharova, & Papanikolaou, 1988) based only on conspicuous Neoproterozoic fossil finds and a correlation with the “diabase-phyllitoid complex” (Boyadzhiev, 1970) of Cambrian-Ordovician age (Kalvacheva, 1982) that is exposed northerly in the Balkanides (Figure 1). This led to the proposal for Gondwana-derived origin of the crystalline terrains of the Rhodope and the Serbo-Macedonian massifs in the Alpine orogen (Figure 1) put forward on the basis of the regional geology (Papanikolaou, 1989, 2013). Recent U-Pb zircon dating revealed that Neoproterozoic, Paleozoic, Triassic, and Jurassic magmatic components had participated in the crustal build-up of the high-grade basement in both massifs (Himmerkus, Reischmann, & Kostopoulos, 2006; Liati, Gebauer, & Fanning, 2011; Turpaud & Reischmann, 2010). In the eastern Rhodope Massif of Bulgaria and Greece, abundant Permo-Carboniferous granitoids were identified in the lower unit of the high-grade basement, together with Late Carboniferous and Late Jurassic granitoids in the overlying upper high-grade basement unit (Figure 2) (Bonev, Marchev, & Moritz, 2012; Cornelius, 2008; Liati et al., 2011). Moreover, the detrital zircons contained in the Triassic-Jurassic low-grade to unmetamorphosed sedimentary units of the Circum-Rhodope Belt (Figure 1) that surrounds the Serbo-Macedonian and the Rhodope massifs provided evidence for contribution of Neoproterozoic up to Late Jurassic crustal components derived from the high-grade basement (Meinhold, Kostopoulos, Reischmann, Frei, & BouDagher-Fadel, 2009, Meinhold, Reischmann, Kostopoulos, Frei, & Larionov, 2010).

Figure 1. Tectonic sketch map of Variscan and the Alpine orogenic belts in Europe. Box in 1(a): outline of map area in 1(b) depicting the Alpine tectonic framework around the Aegean region of the Eastern Mediterranean. Data sources used for map construction in 1(a) of pre-Variscan and Variscan basement areas from Neubauer (2002), Von Raumer et al. (2003), Carrigan et al. (2006), and Anders et al. (2006). Abbreviations: AA, Austroalpine; AM, Armorican Massif; AT, Anatolia; BF, Black forest; BM, Bohemian Massif; B-SG, Balkan-Sredna Gora; F, Flamborun; H, Harz; Hel, Helvetic; Ib, Iberia; MC, Massif Central; Mo, Moesia; OM, Ossa-Morena; Pen, Peninic; Rh, Rhenohercinian; S, Strandzha; Sax, Saxoturingian; SC, South Carpathians; Sd, Sardinia; Sk, Sakarya; SMM, Serbo-macedonian; Rh, Rhodope.

Figure 2. Simplified tectonic map of the eastern Rhodope-Thrace region in southern Bulgaria and northern Greece modified after Bonev et al. (2010). Available geochronology (inset, see also text and references) and the location of samples used in this study for U-Pb LA-ICP-MS geochronology are shown.

The remnants of Proterozoic to Early Paleozoic crust incorporated into the basement terranes within the Variscan and Alpine belts of the Western-Central Europe have been long-term subject of studies focused on the regional geology, geodynamics, and paleogeography of pre-Alpine Europe (Matte, 1986; Neubauer, 2002; Pharaoh, 1999; Von Raumer, Stampfli, Borel, & Bussy, 2002; Winchester, The PACE TMR Network Team, 2002) (Figure 1(a)). Limited radiometric ages existed for the counterpart pre-Paleozoic and Early Paleozoic crustal fragments in the basement terranes of the South-Eastern Europe in the Balkan region of Bulgaria (Haydoutov & Yanev, 1997). There in terranes northerly of the Rhodope Massif (Figure 1(b)), the Ordovician to Permian sedimentary sections (Yanev, 2000), together with Devonian eclogites (Gaggero, Buzzi, Haydoutov, & Cortesogno, 2008) and Carboniferous arc magmatism (Carrigan, Mukasa, Haydoutov, & Kolcheva, 2005), were discussed in terms of Gondwana-related pre-Variscan and Variscan evolution linked to the Pangea assembly. The efforts in revealing pre-Variscan and Variscan crust have reached NW Turkey to the east, where age-constrained basement tectonics and magmatism provided evidence for Neoproterozoic to Late Paleozoic continental growth of the Strandzha, İstanbul, and Sakarya terranes well before the Variscan orogeny. In this way, a link to the Paleozoic terranes in the Balkan region and the Western-Central Europe (Bozkurt, Winchester, Yiğitbaș, & Ottley, 2008; Okay, Satir, & Siebel, 2006, Okay et al., 2008; Ustaömer, Mundil, & Renne, 2005; Yanev et al., 2006) has been indicated. Contrary, the corresponding time record is missing for the involvement of the Rhodope Massif high-grade basement in the context of pre-Variscan and to lesser extent Variscan igneous and metamorphic continental build-up of the terranes in the region. To provide constraints and linkage of the crystalline basement in pre-Variscan European and related Anatolian terranes, we focused our attention on the basement rocks of the eastern Rhodope Massif because of its crucial location between terranes that require urgent support to establish a regional connection.

This paper provides new U-Pb zircon protolith ages, mineral chemistry, and whole-rock geochemistry of metamafic and associated evolved rocks in the high-grade basement of the eastern Rhodope Massif. It supplies evidence for Ordovician magmatic components involved in the build-up of the metamorphic basement and indications for the experienced Late Carboniferous and Tertiary metamorphic events. The identified Early Paleozoic magmatism in the basement is confirmed by U-Pb ages obtained from inherited zircons in the overlying Eocene-Oligocene volcanic rocks. Finally, the paleotectonic implications for the Rhodope Massif and adjacent terranes are discussed in the light of connection to Gondwana-derived terranes.

2. Large-scale to local geological setting

To the north, the Rhodope Massif is separated by a dextral strike-slip fault from the Sredna Gora Zone Late Cretaceous arc, with a magmatic life span ca. 92–78 Ma (Von Quadt, Moritz, Peytcheva, & Heinrich, 2005). To the southwest, together with the Serbo-Macedonian Massif, it is limited by the Vardar Suture Zone against the Hellenides (Figure 1(b)). The Rhodope Massif is built of high-grade igneous and metamorphic basement comprising pre-Alpine and Alpine (Liati et al., 2011 and references therein) units of continental and oceanic affinities. The high-grade basement is intruded by Late Cretaceous to Early Miocene granitoids (Del Moro, Innocenti, Kyriakopoulos, Manetti, & Papadopoulos, 1988; Dinter, 1998; Marchev, von Quadt, Peytcheva, & Ovtcharova, 2006). Paleocene to Pliocene sediments (Boyanov & Goranov, 2001) and Late Eocene-Oligocene to Miocene volcanic successions (Christofides, Peckay, Elefteriadis, Soldatos, & Koroneos, 2004; Harkovska, Yanev, & Marchev, 1989) represent cover sequences related to the late Alpine evolution.

The Rhodope Massif is regarded as S-directed nappe stack assembled in the hanging wall of N-dipping Late Cretaceous subduction zone located in the Vardar Zone (Ricou, Burg, Godfriaux, & Ivanov, 1998) where the Neotethyan Vardar Ocean ultimately closed by Paleocene times (Stampfli, 2000). This tectonic setting was predated by Middle Jurassic S-dipping subduction and intra-oceanic arc magmatism documented in the Circum-Rhodope Belt which was thrust emplaced to the north in Late Jurassic and accreted to the Rhodope continental margin (Bonev, Spikings, Moritz, & Marchev, 2010; Bonev & Stampfli, 2011). The Jurassic and Cretaceous nappe stacking resulted in crustal thickening of the Rhodope Massif succeeded by Tertiary extension that shaped the tectonic style (Bonev, 2006; Brun & Sokoutis, 2007; Dinter, 1998; Krohe & Mposkos, 2002; Ricou et al., 1998).

The study was conducted in the eastern Rhodope Massif (Figures 1(b) and 2), where detachment-bounded Kesebir-Kardamos and Byala reka-Kechros extensional domes comprise structurally upward the following tectono-stratigraphic units (Bonev, 2006): (i) a lower, footwall high-grade basement unit of continental affinity composed of orthogneisses with Permo-Carboniferous protolith ages in the range 299–328 Ma (Cornelius, 2008; Liati et al., 2011; Peytcheva, Ovtcharova, Sarov, & Kostitsin, 1998; Peytcheva & von Quadt, 1995), (ii) an upper high-grade basement variegated unit of continental-oceanic affinity enclosing metaophiolites with Neoproterozoic (572 Ma) protolith and Variscan (300–350 Ma) metamorphic ages (Carrigan, Mukasa, Haydoutov, & Kolcheva, 2003) and metagranitoids with Late Carboniferous and Late Jurassic protoliths (Bonev et al., 2012; Cornelius, 2008; Liati et al., 2011), (iii) a low-grade to unmetamorphosed unit of Mesozoic rocks (Jaranov, 1960; von Braun, 1968) that includes Middle Jurassic (172–160 Ma, Bonev et al., 2012) arc-related ophiolites of the Circum-Rhodope Belt (Bonev & Stampfli, 2008; Magganas, Sideris, & Kokkinakis, 1991), which has been erroneously age and metamorphic grade correlated with the “diabase-phyllitoid complex” (Boyanov, Kozhoukharova, & Kozhoukharov, 1969), and (iv) a sedimentary-volcanic unit of Paleocene-Pliocene (Boyanov & Goranov, 2001) cover sequences, which together with previous two metamorphic units built the hanging wall of the extensional system (Figure 2).

The eastern Rhodope high-grade metamorphic basement units reveal a complex Alpine ultrahigh to low-pressure tectono-metamorphic history encompassing multiple events bracketed between ca. 160 and 42 Ma (Liati et al., 2011). ⁴⁰Ar/³⁹Ar cooling history below 400–300 °C of the metamorphic units postdating the last amphibolite-facies metamorphism took place between 42 and 36 Ma (Bonev et al., 2010; Bonev & Stampfli, 2011; Lips, White, & Wijbrans, 2000) accompanying Eocene extensional exhumation of both domes.

The present study includes the metamafic and related evolved rocks intruded by Oligocene dykes in the upper high-grade basement unit, together with lavas from the latest Eocene-Oligocene Iran Tepe paleovolcano, altogether exposed in the eastern Rhodope Massif (Figure 2).

The metamafic-ultramafic rocks in the upper high-grade basement unit are considered fragments of metaophiolite association mainly consisting of metaperidotites, metagabbro-basalt bodies, and dykes (Kozhoukharov et al., 1988), including also metagabbros and plagiogranites (Ovtcharova & Sarov, 1995). These metaultramafic-mafic rocks show mid-ocean ridge basalt (MORB) (Kolcheva & Eskenazy, 1988) and arc tholeiitic and boninitic supra-subduction zone (SSZ) signatures (Bonev, Peytchev, & Nizamova, 2006b; Haydoutov, Kolcheva, Daieva, Savov, & Carrigan, 2004). The slightly peraluminous of magmatic trend metaplagiogranites are considered as I-type oceanic plagiogranites formed in a back-arc basin ridge tectonic setting (Ovtcharova & Sarov, 1995). Rb-Sr isochron age of 159 ± 19 Ma is interpreted as protolith emplacement age of the metaplagiogranites having ⁸⁷Sr/⁸⁶Sr_i = 0.70541 (Peytcheva et al., 1998).

The alkaline basalt dykes and diatremes cross-cutting the high-grade basement lithologies have been dated by K/Ar method between 28 and 26 Ma (Marchev, Harkovska, Pècskay, Vaselli, & Downes, 1997). These potassium basanites and hornblende lamprophyres have mantle isotopic ratios for Sr-Nd-Pb, and exhibit an intra-plate OIB-type signature (Marchev et al., 1998). They contain high-Mg olivine xenocrysts and xenoliths from mantle rocks and from the metamorphic basement (Marchev, Arai, & Vaselli, 2006 for details).

The Iran Tepe paleovolcano, which is a typical example of magmatic edifice of the widespread latest Eocene-Oligocene volcanism in the region, consists of calc-alkaline intermediate to acid lavas and volcaniclastic rocks that have a range of ⁴⁰Ar/³⁹Ar cooling ages between 34.6–33.2 Ma (Marchev et al., 2010). The magmatic products of the Iran Tepe paleovolcano overlie the metamorphic rocks of the upper high-grade basement unit and the basal Upper Eocene limestone strata of the sedimentary-volcanic unit at northern tip of the Kesebir-Kardamos dome (Figure 3(a)).

Figure 3. Lithologic context and field relations of the studied metamorphic basement rocks and volcanic rocks: (a) columnar section of the rock succession in sampled fragment of the upper unit of the high-grade basement and the sedimentary-volcanic unit, showing the location of U-Pb LA-ICP-MS geochronology samples, (b) metamorphic succession of alternating different metasedimentary lithologies hosting lenses of metaultramafic-mafic rocks, (c) metagabbros intruded by irregular metaplagiogranite veins, (d) garnet amphibolite (sample EG 06-12a) cross-cut by numerous multi-generation aplite veins, (e) alkaline basalt dyke fragment depicting the texture and the occurrences of xenoliths and xenocrysts within the rock. Coin for scale is 4 cm in diameter. This rock, devoid of xenoliths, is representative for the sample EG 06-1.

3. Field data, textures, and description of dated samples

The field study and sampling of the metamorphic rocks of the upper high-grade basement unit and the volcanic rocks was conducted along the eastern flank of the Kesebir-Kardamos dome (Bonev, 2006) (Figure 2). The target metagabbros and associated metaplagiogranites occur foliation-parallel within metamorphic succession consisting of intercalated paragneisses, quartzites, schists and marbles that hosts metaophiolitic bodies (Figure 3(a)). Thin quartzite intercalations in the metamorphic succession demonstrate former protolith of arkosic sandstones to greywackes consisting of quartz and feldspar, with a minor white mica and chlorite (Figures 3(b) and 4(a)). The coarse- to medium grained metaplagiogranites are observed as varying in size and thickness veins or lenticular bodies intrusive into the metagabbros (Figure 3(c)). The coarse-grained metagabbros contain marble enclaves that occur foliation-parallel and are locally sub-isoclinally folded (Figure 4(b)). The metagabbros have intrusive relationships with the nearby coarse-grained marbles. Other target metamafic lithology represented by garnet-bearing massive amphibolite occurs intercalated within the same parametamorphic succession (Figure 3(a)). The garnet amphibolite is cross-cut by late multigeneration aplitic veins related to the rock-melt interaction during experienced high-grade metamorphism and migmatisation in amphibolite facies (Figure 3(d)). In the field, the garnet amphibolite in turn locally appears as massive to banded metagabbroic body that devoid of garnet porphyroblasts.

Figure 4. Field photographs and microphotographs of the studied metamorphic basement rocks: (a) quartzite in the metasedimentary succession shown in a box of Figure 3(b), (b) marble xenolith included in the metagabbros, both deformed foliation-parallel and sub-isoclinally folded (F), (c) coarse granular metagabbros preserving original lamellar plagioclase, metamorphic hornblende, and minor quartz. Note that the regional foliation is deduced by planar alignment of hornblende and plagioclase, (d) metaplagiogranite consisting of recrystallized quartz, primary lamellar plagioclase, metamorphic biotite, and epidote developed at expense of plagioclase. Abbreviations: amph, amphibole; bt, biotite; pl, plagioclase; fs, alkali feldspar; ep, epidote. Scale bar = 2 mm.

All metamorphic rock types have experienced high-grade metamorphism and exhibit a penetrative moderately SE-dipping regional foliation defined by ubiquitous schistosity, gneissic or metamorphic banding (Bonev, 2006). The metamafic rocks are transformed into amphibolites that show layering in alternating bands of mesocratic to leucocratic rock varieties and coarse hornblende and segregated plagioclase in lense-shaped flaser texture that is common for the metagabbros (Figure 3(c)). Associated intrusive metaplagiogranites are turned into biotite or two-mica gneisses. In the metagabbros and the metaplagiogranites, the primary igneous textures and grain sizes are erased to large extent due to metamorphic recrystallization (Figure 4(c) and (d)). Both generally do not exhibit intense ductile shear deformation otherwise well-pronounced in the host metamorphic unit, and thus, they occur in relatively intact low-strain domains within the metamorphic succession. The metamorphic assemblage of the metamafic rocks consists of hornblende and plagioclase ± biotite ± quartz ± garnet ± epidote (Figure 4(c)). Other minor secondary phases are represented by sphene, rare ilmenite and chlorite at the rims of the hornblende and biotite crystals. Accessory minerals include zircon and titanomagnetite. Associated metaplagiogranites have metamorphic assemblage consisting of modally decreasing plagioclase-quartz-biotite ± alkali feldspar ± epidote ± garnet (Figure 4(d)). Metamorphism experienced by the metaplagiogranites has produced epidote and muscovite at expense of plagioclase and chlorite after biotite. Accessories are apatite, zircon, allanite and magnetite. The mineral assemblage reveals regionally penetrative amphibolite-facies metamorphism of the studied high-grade basement rocks and their subsequent lower grade overprint in greenschist-facies conditions.

A key freshest metagabbro-diorite sample (EG 06-7) and metaplagiogranite-diorite sample (EG 06-9a), together with a garnet-bearing amphibolite sample (EG 06-12a) from the upper high-grade basement unit having features as described above, were selected for analysis of mineral and whole-rock geochemistry and U-Pb LA-ICP-MS zircon dating.

The alkaline basalt dykes range in thickness from meter up to decameter intruding metagabbros-metaplagiogranites association or the lithologies of the upper high-grade basement unit in the sampled locality (Figure 3(a)). Sampled alkaline basalt dyke (sample EG 06-1) in this study comes from a location near the Greek-Bulgarian border (Figure 2). The dyke is a meter-thick sub-vertical body composed of fine grained to cryptocrystalline mafic rock that contains abundant small vesicles filled with secondary alteration products and numerous olivine xenocrysts, together with numerous xenoliths from the high-grade basement lithologies (Figure 3(e)). Further details on alkaline basalt dykes mineralogy and geochemistry can be found in Marchev et al. (2006).

The two additional volcanic rock samples PK 20 and PK 9 come from the lava flows of the Iran Tepe paleovolcano (Figure 2). In the field, these volcanic rocks are plagioclase and hornblende phyric lavas that contain enclaves from lavas recovered in distinct levels of the edifice. Sample PK 9 represents dacite (63.5 wt.% SiO₂ and 2.3 wt.% K₂O, Marchev et al., 2010) that comes from oldest lava flows overlying the Upper Eocene limestones. In thin section, the dacite hosts phenocrysts of plagioclase, amphibole and biotite. The andesite sample PK 20 comes from the uppermost lava flows. The sample contains phenocrysts of plagioclase, clinopyroxene, orthopyroxene, amphibole, biotite and titanomagnetite, plus zircon and apatite.

3. Geochemistry of the metamafic and associated evolved basement rocks

3.1. Mineral chemistry

Representative chemical compositions of analyzed phases in the metamorphic rocks are shown in Table 1 of supplementary material and procedures explained in Appendix 1. Amphiboles in the metamafic rocks have compositions of tschermakitic hornblende and tschermakite (Figure 5(a)). Amphiboles have low Cr₂O₃ and TiO₂ contents and variable Mg #. The elevated Al contents of some amphiboles imply their crystallization under medium- to high-pressure metamorphic conditions. Using Al-in-hornblende geobarometer (Anderson & Smith, 1995) pressures in the range 8.01–8.95 kbar at 650 °C (see below) were calculated for amphibole crystallization. The majority of Al occurs as Al^iv reflecting also high temperature of crystallization, which thermo-barometric estimates are consistent with the amphibole crystallization in upper amphibolite-facies metamorphic grade. All plagioclases are unzoned, and commonly lamellar plagioclase occurs in the metamorphic rocks (Figure 4(c) and (d)). Plagioclase is oligoclase (Ab_83.1–74.5) in both metaplagiogranites and metagabbros, with preserved orthoclase (Or_98–96) in the metaplagiogranites (Figure 5(b)). Garnet compositions (Gro_28.3–16.2 Alm_50.5–28.5 Py_45.5–26.2 Sp_8.6–0.6) in the metamafic rocks also reflect high-grade metamorphism in amphibolite-facies (e.g. Spear 1993). These metamorphic conditions are indicated by dominant almandine and grossularite contents coupled with an elevated pyrope content in the sample EG 06-11, the latter suggesting relatively high metamorphic pressures. Micas in metagabbro-diorite varieties of the mafic rocks have compositions of biotites, with a range of 5.48–5.66 Si per formula unit. Epidote is abundant in garnet amphibolite sample EG 06-12a.

Table 1. Representative microprobe analyses for chemistry of mineral phases in the samples of the high-grade basement rocks.

Sample	EG06-11	EG06-12a	EG06-12a	EG06-7	EG06-11	EG06-11	EG06-9	EG06-9	EG06-7	EG06-7	EG06-7
Mineral	grt	grt	amph	amph	amph	fs	fs	fs	fs	bt	ep
Rocktype	mgb	mgb	mgb	md	mgb	mgb	mplg	mplg	md	md	mgb
SiO2	44.06	38.19	43.49	42.87	43.52	63.92	62.51	46.54	63.51	36.25	38.00
TiO2	0.65	0.05	0.43	0.51	0.63	–	–	1.36	–	2.77	0.04
Al2O3	12.43	21.61	13.77	13.00	12.43	22.99	23.65	30.57	22.92	15.56	27.03
Cr2O3	0.04	0.02	0.00	0.02	0.06	–	–	–	–	0.05	0.02
Fe2O3	0.00	1.88	7.08	8.86	7.44	0.00	0.03	3.67	0.13	–	8.37
FeO	14.32	22.17	6.63 25.06	7.21	7.49	0.03	–	–	–	15.45	–
MnO	0.29	1.63	0.10	0.32	0.27	0.03	–	–	–	0.14	0.19
MgO	11.98	6.72	12.29	11.39	11.96	–	0.01	1.61	–	14.40	–
CaO	11.12	7.73	11.08	10.73	11.04	3.77	4.61	–	3.96	0.02	23.69
Na2O	–	–	1.89	2.10	1.93	9.26	8.59	0.27	9.42	0.17	–
K2O	–	–	0.64	0.50	0.49	0.21	0.48	10.69	0.30	9.53	–
H2O	–	–	2.06	2.04	2.04	–	–	–		3.95	1.90
Total	94.96	100.00	99.47	99.55	99.30	102.58	99.86	94.72	100.5	98.28	99.13
Si	3.452	2.956	6.330	6.295	6.386	8.092	2.772	2.287	2.803	5.501	2.991
Ti	0.038	0.003	0.047 0.009	0.057	0.070	–	–	0.050	–	0.316	0.002
Al^IV	0.000	0.044	1.670	1.705	1.862	0.000	1.236	1.770	1.192	2.499	0.009
Al^VI	1.147	1.928	0.692	0.544	0.578	3.430	0.000	–	–	0.282	2.499
Cr	0.002	0.001	0.000	0.002	0.001	0.000	0.000	–	–	0.007	0.001
Fe^3+	0.000	0.011	0.776	0.979	0.793	0.000	0.001	0.136	0.004	–	0.496
Fe^2+	0.938	1.435	0.807	0.885	1.530	0.004	–	–	–	1.960	–
Mn^2+	0.019	0.107	0.012	0.040	0.028	0.003	–	–	–	0.018	0.006
Mg	1.399	0.776	2.667	2.493	1.970	–	0.001	0.118	–	3.256	–
Ca	0.940	0.641	1.728	1.688	1.782	0.511	0.219	–	0.187	0.004	1.998
Na	–	–	0.534	0.597	0.644	2.273	0.738	0.026	0.806	0.051	–
K	–	–	0.119	0.093	0.083	0.033	0.027	0.670	0.017	1.844	–
OH	–	–	2.000	2.000	2.000	–	–	–	–	4.000	1.000
Py	42.447	26.221
Alm	28.457	48.497
Sp	0.579	3.605
And	0.000	5.372
Uv	0.186	0.065
Gro	28.331	16.240
#Mg	59.9	35.1	76.8	73.8	56.3					62.4
Ab						80.700	75.011	3.724	79.760
An						18.100	22.243	–	18.544
Or						1.200	2.740	96.276	1.696

Notes: Structural formula based on 23 and 24 oxygen atoms, respectively for amphiboles and garnets, and 32 and 22 oxygen atoms for feldspar and micas. Abbreviations: amph, amphibole; bt, biotite; grt, garnet; ep, epidote; fs, feldspar; mgb, matagabbro; md, metadiorite; mplg, metaplagiogranite.

Figure 5. Mineral compositions of the studied metamorphic basement rock samples: (a) amphibole compositions diagram (after Leake, 1978), (b) feldspar compositions ternary diagram.

Thus, the chemical compositions of constituent mineral phases in the studied metamorphic rocks are consistent with formation during the regional Alpine amphibolite-facies metamorphism, whose thermo-barometric range (T = 600–650 °C, p = 10–12 kbar, Mposkos & Liati, 1993) is well-defined in the upper high-grade unit of the metamorphic basement.

3.2. Whole-rock geochemistry

We present chemical data for a suite of four metamafic-intermediate rock samples (EG 06-6, EG 06-7, EG 06-11, EG 06-12a) and one sample of an evolved leucocratic derivate (EG 06-10), which are accompanied by a sample from a cross-cutting vein (EG 06-12c) and complemented by two samples from plagiogranites (EG 06-9, EG 06-9a), together with a sample of alkaline basalt (EG 06-1) and a sample from the Iran tepe lava flow (PK 20) (Table 2 supplementary material, Appendix 1). The samples from Oligocene volcanic rocks are presented as outline of the compositions. Details on their geochemistry can be found in Marchev et al. (1998, 2010), and hence they will not be discussed hereafter. A low loss on ignition in all samples attests for a negligible degree of secondary alteration of these rocks. A simple test was performed using Zr as a discriminant in binary plots (not shown) for the assessment mobility of the elements. This test, in similarity to previous chemical screening of the metamafic rocks (Bonev et al., 2006b), has shown immobile behavior of the trace elements and REE. We, therefore, used immobile trace elements (Nb, Zr, V, and Hf), REE, and minor elements (Ti, Mn, and P) for the evaluation of geochemical signature and tectono-magmatic discrimination of the metamafic rocks and associated evolved rocks.

Table 2. Whole-rock chemical analyses of metamafic and associated evolved rocks in the high-grade metamorphic basement of the eastern Rhodope Massif and Oligocene volcanic rocks.

Sample	EG06-6	EG06-7	EG06-9	EG06-9a	EG06-10	EG06-11	EG06-12a	EG06-12c	EG06-1	PK-20
Rock type	mgb	md	mplg	mplg	lmd	mgb	mgb	apl	alkbas	and
SiO2	49.30	61.13	68.22	60.75	70.09	52.26	43.33	73.55	46.76	57.13
TiO2	1.21	0.80	0.66	0.73	0.53	1.19	0.49	0.97	2.05	0.80
Al2O3	15.88	15.19		15.02		14.15	24.90	8.37	16.84	17.06
Fe2O3	10.03	6.70	4.75	7.18		10.41	10.96	6.55	8.75	6.72
MnO	0.16	0.16	0.10	0.15		0.20	0.17	0.08	0.16	0.14
MgO	7.47	3.38	1.83	4.14	2.02	7.48	4.04	3.66	7.25	3.36
CaO	9.20		2.80	5.24		8.56	12.84	4.41	9.48	6.38
Na2O	4.25		3.54	3.32		3.80	1.83	1.53	4.43	3.47
K2O	0.35		2.11	2.15		0.37	0.41	0.40	1.40	2.90
P2O5	0.12		0.15	0.14		0.08	0.11	0.02	0.73	0.37
Cr2O3	0.05	0.01	n.d.	0.01	0.01	0.03	n.d.	0.01	0.03	n.d.
NiO	0.01	n.d.	n.d.	n.d.	n.d.	0.01	n.d.	n.d.	0.01	n.d.
LOI	1.08	0.50	0.54	0.79	0.21	0.53	0.54	0.54	1.65	1.06
Total	99.09	99.67	99.01	99.63	99.82	99.08	99.62	100.10	99.52	99.39
Nb	3		10	8	9	5	<1<	17	75	14
Zr	82	114	181	130	140	88	21	39	223	204
Y		34	32	29	33	29	10	18	28	26
Rb				55	4	5	18	6	54	182
Sr	231	284	307	273	270	111	301	250	813	518
	88	150	495	348	89	14	86	111	795	1022
U	2	3	4		4	<2<	3	<2<	2	10
Th	<2<	7	7		8	<2<	<2<	43	7	19
Pb	4	24		12	30	<2<	6	5	4	32
Hf	3		7	6	7	6	<1<	7	8	9
Sc	49	25	14		19	49	21	20	20	14
Cr	304	42	16	65	32	230	23	25	171	27
V	282	126	72	144	82	273	305	165	216	157
Ni	59	10	4	17	9	42	8	10	103	13
Ga	17	17	15	16	14	18	23	13	18	18
Zn	43	62	53	71	42	38	66	68	76	73
Cu	20	66	96	53	19	27	26	85	42	30
Co	42	22	12	21	11	37	27	25	30	17
La	13	15	16	14	12	7	<4<	38	43	38
Ce	23	31	44	44	32	18	15	39	90	66
Nd	14	13	20	23	14	9	7	25	42	28

Notes: elements (wt.%), trace elements and rare earth elements (ppm) analysed by XRF. Abbreviations: abas, alkaline basalt; and, andesite; apl, aplite vein; lmd, leucocratic metadiorite; mgb, matagabbro; md, metadiorite; mplg, metaplagiogranite; n.d. = not detected.

SiO₂ abundances cover a continuous compositional range 43.3–61.1 wt.% in the metagabbro to metadiorite and 60.8–68.2 wt.% in metaplagiogranite-diorite, suggesting genetic relationships in a magmatic suite of interrelated less and more evolved members. Both are characterized by low TiO₂ (0.5–1.2 wt.%) and variable total alkalis (2.24–5.65 wt.%) that have higher concentrations in the metaplagiogranites. MgO contents exhibit extended range 1.83–7.48 wt.%, with higher concentrations in the metamafic rocks, and hence stand for magma differentiation process. Except Al₂O₃ up to 24.9 wt.% in the sample EG 06-12a, which is obviously due to the presence of high alumina garnet, other studied samples have moderate to low alumina contents.

In terms of trace elements, the metaplagiogranites exhibit low Rb and Sr concentrations and relatively low for granitoids Ba and Nb contents. Other incompatible elements, such as Zr (138–196 ppm) and Y (60–78 ppm), have contents in the range known for the oceanic crust acid differentiates (e.g. Coleman & Donato, 1979). In the metagabbros-metaplagiogranites suite, Ni (17–88 ppm) and Cr (8–239 ppm) concentrations do not meet requirements for primary mantle melts (Ni > 200 ppm, Cr > 400 ppm, Tatsumi & Eggins, 1995), which implies magmatic differentiation processes involved in the protolith petrogenesis. The sample EG 06-12a exhibits lower Nb, Zr, and Y and light REE abundances compared to the metagabbro-metaplagiogranite suite. The metagabbros and metaplagiogranites fall in sub-alkaline basalt and andesite/basalt fields in various trace elements classification diagrams, showing also on AFM diagram (not shown) tholeiitic trend except the sample EG 06-12a that displays a calk-alkaline affinity. Cross-cutting aplitic vein (sample EG 06-12c), with normally expected high-silica content, display an overall bulk major oxides and trace element chemistry similar to the host metamorphic rock.

Chondrite-normalized trace elements profiles of the metamafic and associated evolved rocks display large variations in elemental abundances 1–500 times higher than the chondrite and consecutively fractionated patterns (Figure 6(a)). In this diagram, a similarity in the profiles of LREE and high-field strength elements (HFSE) from Zr to Y is observed both between the samples and relative to E-MORB and to lesser extent N-MORB. Multi-element spider diagram of the metamafic and associated evolved rock samples normalized to N-MORB shows large-ion lithophile element (LILE) enrichment relative to HFSE. Pronounced Nb and Zr negative anomalies always at higher abundances relative to N-MORB characterize almost all samples, with the exception of higher magnitude Nb and Zr negative anomalies and higher HFSE depletion relative to N-MORB of the sample EG 06-12a (Figure 6(b)). In addition, the Nb and Zr anomalies are accompanied by P and Ti negative anomalies, respectively, close and lower relative to N-MORB composition. Both chondrite and N-MORB-normalized trace element patterns of the studied samples are similar in terms of parallel profiles and significantly overlapping values and display patterns comparable to E-MORB and to lesser extent to N-MORB.

Figure 6. Geochemical and tectono-magmatic discrimination diagrams of the studied metamorphic basement rock samples. (a, b) chondrite and N-MORB-normalized trace elements diagrams, respectively. Normalization values after Sun and McDonough (1989). BABBs compositions used in comparison in b are taken from Saunders and Tarney (1991), (c) Ti-V diagram showing fields of IAT, volcanic arc CAB, MORB and BABB and OIB, oceanic island basalts after Shervais (1982), (d) Zr/Y-Zr diagram after Pearce and Norry (1979), (e) Nb-Zr-Y diagram after Meschede (1986), (f) Ti–MnO–P₂O₅ diagram after Mullen (1983).

The trace element tectono-magmatic discrimination diagrams define an island arc to spreading ridge settings for the metamafic and associated more evolved rocks (Figure 6), where these rocks overlap MORB and back-arc basin basalt (BABB) with arc-related (island arc tholeiite (IAT) and calk-alkaline basalt (CAB)) fields of mafic rocks on the ocean floor, i.e. MORB with arc signatures (Shervais, 2001).

In summary, the major and trace elements’ characteristics of the metamafic rocks classify their precursors as low-K and low-Ti tholeiitic to calc-alkaline gabbros to diorites with transitional MORB-IAT affinity. The metaplagiogranites present oceanic compositional range known for trondhjemite-tonalite-granodiorite series, closely approaching the chemistry of the granodiorites. Trace element geochemistry indicates weak MOR and strong SSZ signatures of the studied metamorphic rocks, whose arc-related affinity is also depicted by tectono-magmatic discrimination diagrams.

4. U-Pb geochronology

Extracted zircons from the samples of a metagabbro (EG 06-7) and metaplagiogranite (EG 06-9) and a sample of garnet amphibolite (EG 06-12a) were dated by U-Pb LA-ICP-MS technique. In addition, a sample of alkaline basalt dyke (EG 06-1) intruding the basement rocks and two samples (PK 9 and PK 20) from the lavas of the Iran tepe paleovolcano was dated by the same technique. Zircons were prepared by standard mineral separation techniques and purification methods. Analytical results for all dated zircon grains are shown in Table 3 of supplementary material and procedures described in Appendix 2.

Table 3. U-Pb LA-ICP-MS analytical data for zircons in samples used in the study.

	Isotopic ratios							Apparent ages (Ma)
Sample/analysis	^207Pb/^206Pb	±2s %	^207Pb/^235U	±2s %	^206Pb/^238U	±2s %	Rho	^206Pb/^238U	±2s	^207Pb/^235U	±2s	^207Pb/^206Pb	±2s
EG06-1 alkaline basalt		Lat. N 41°18′46.30′′, Long. E 25°38′19.76′′
EG06-1/1r1	0.0517	2.0	0.3417	1.8	0.0479	1.0	0.09	301.6	2.9	298.4	4.9	270	46
EG06-1/1r2	0.0524	2.6	0.3431	3.0	0.0473	1.1	0.43	298.0	3.3	299.5	7.5	300	60
EG06-1/2r1	0.0562	2.6	0.5686	3.4	0.0729	1.0	0.82	453.8	4.6	457.1	12.6	458	60
EG06-1/2r2	0.0556	2.0	0.5640	2.4	0.0736	0.8	0.59	457.8	3.6	454.1	8.8	436	46
EG06-1/2rc	0.0557	2.3	0.5801	2.6	0.0747	0.9	0.49	464.4	4.0	464.5	9.7	442	50
EG06-1/2cr	0.0561	1.2	0.5700	1.4	0.0742	0.6	0.50	461.2	2.8	458.0	5.3	454	26
EG06-1/4r1	0.0517	2.3	0.3501	2.8	0.0489	1.0	0.63	308.0	3.0	304.8	7.4	272	52
PK20 andesite Iran tepe		Lat. N 41°32′45.75′′, Long. E 25°40′20.52′′
PK20/1r1	0.0515	2.5	0.3383	3.0	0.0483	1.5	0.55	304.1	4.4	295.9	7.6	264	58
PK20/1r2	0.0528	1.9	0.3572	2.0	0.0490	1.3	0.38	308.6	3.9	310.1	5.4	318	44
PK9 dacite Iran tepe		Lat. N 41°29′20′′, Long. E 25°43′33′′
PK9/1rc1	0.0528	1.7	0.3291	1.8	0.0457	0.9	0.35	288.0	2.7	288.9	4.6	320	40
PK9/1rc2	0.0529	1.1	0.3287	2.4	0.0452	1.0	0.47	285.2	2.8	288.6	6.1	322	48
PK9/1r	0.0529	2.5	0.3079	2.4	0.0422	1.4	0.45	266.5	3.6	272.6	6.6	322	56
PK9/2r1	0.0460	5.6	0.0263	5.4	0.0042	0.4	0.04	26.9	0.4	26.3	1.4	0	64
PK9/2r2	0.0481	5.6	0.0276	3.0	0.0042	1.6	0.15	26.9	0.4	27.6	1.4	100	130
PK9/2c	0.0594	5.0	0.0351	3.0	0.0043	1.3	0.10	27.7	0.4	35.0	1.7	580	108
PK9/3c	0.0520	2.1	0.1888	2.0	0.0264	0.8	0.10	168.0	1.3	175.6	3.3	286	48
PK9/3r1	0.0514	1.4	0.2568	1.6	0.0361	0.9	0.47	228.6	2.1	232.1	3.3	258	32
PK9/3r2	0.0517	1.5	0.2754	1.6	0.0391	0.8	0.27	247.1	1.9	247.0	3.4	270	34
PK9/5r	0.0577	2.2	0.5789	2.2	0.0734	0.9	0.32	456.7	4.2	463.8	8.4	518	48
PK9/5rc	0.0605	1.8	0.7160	2.4	0.0866	0.8	0.90	535.5	4.0	548.3	10.6	620	38
PK9/5c	0.0635	1.6	0.9127	2.2	0.1045	0.8	0.84	640.9	4.9	658.5	10.6	722	32
PK9/6r1	0.0538	2.4	0.3397	2.2	0.0467	1.0	0.14	294.1	2.8	296.9	5.8	360	54
PK9/6r2	0.0542	1.7	0.3484	2.2	0.0469	0.9	0.57	295.3	2.6	303.5	5.5	376	38
PK9/6c	0.0540	2.3	0.3398	2.6	0.0461	1.1	0.46	290.4	3.0	297.0	6.6	370	52
EG06-9 metagranodiorite Egrek		Lat. N 41°19′54.39′′, Long. E 25°37′49.39′′
EG06-9/1c	0.0564	1.9	0.5834	2.6	0.0748	1.6	0.70	465.3	7.0	466.7	9.8	466	42
EG06-9/1r1+c	0.0563	2.1	0.5790	2.6	0.0744	0.9	0.62	462.6	3.9	463.8	9.3	464	46
EG06-9/1r2+c1	0.0560	2.6	0.5567	3.2	0.0722	1.3	0.61	449.5	5.7	449.4	11.4	450	58
EG06-9/2r2+c	0.0556	2.6	0.5586	3.2	0.0730	1.0	0.71	454.0	4.6	450.6	11.3	436	56
EG06-9/3r1+c	0.0563	1.9	0.5452	2.4	0.0710	1.0	0.57	442.0	4.2	441.9	8.3	462	42
EG06-9/3r2+c	0.0565	1.6	0.5511	1.8	0.0711	1.0	0.46	442.7	4.1	445.7	6.6	470	36
EG06-9/4r1c	0.0580	2.0	0.6611	2.6	0.0827	1.5	0.62	512.4	7.4	515.3	10.5	528	44
EG06-9/4c	0.0582	2.2	0.6460	3.2	0.0811	2.4	0.76	502.8	11.5	506.0	13.1	534	46
EG06-9/4r2c	0.0570	1.9	0.6498	1.4	0.0830	2.4	0.70	513.9	12.0	508.3	9.7	492	42
EG06-9/5rc	0.0554	3.0	0.5487	3.6	0.0728	1.6	0.57	453.1	6.9	444.1	12.8	428	66
EG06-9/5c	0.0547	2.8	0.5669	3.0	0.0740	1.2	0.41	460.2	5.1	456.0	11.3	400	62
EG06-9/6r2c1	0.0554	2.1	0.5520	2.6	0.0736	1.3	0.57	457.7	5.9	446.3	9.3	426	46
EG06-9/7r1	0.0560	2.1	0.5838	2.8	0.0753	1.9	0.70	468.0	8.4	466.9	10.8	452	46
EG06-7 metagabbro Egrek		Lat. N 41°19′55.11′′, Long. E 25°37′47.01′′
EG06-7/2r	0.0524	2.7	0.3653	1.4	0.0506	0.6	0.36	318.5	3.5	316.2	7.9	302	60
EG06-7/3r	0.0460	6.1	0.0473	3.0	0.0074	0.9	0.07	47.8	0.8	46.9	2.7	0	70
EG06-7/3c	0.0490	9.6	0.0519	4.4	0.0076	1.3	0.15	49.1	1.3	51.4	4.4	146	228
EG06-7/4r	0.0562	1.5	0.5679	1.0	0.0733	0.5	0.73	456.1	4.2	456.7	7.7	458	34
EG06-7/4c	0.0569	2.6	0.6038	1.5	0.0764	0.6	0.46	474.3	5.9	479.6	11.4	486	58
EG06-7/5r1+2	0.0510	5.2	0.0352	2.5	0.0051	1.2	0.16	32.6	0.8	35.1	1.7	238	120
EG06-7/5r1	0.0469	6.6	0.0312	3.4	0.0049	1.0	0.24	31.5	0.6	31.2	2.1	44	120
EG06-12a garnet-amphibolite		Lat. N 41°22′57′′, Long. E 25°42′40.8′′
EG12a/1r1+2	0.0563	2.0	0.5543	2.4	0.0718	1.1	0.61	446.8	4.8	447.8	8.9	464	44
EG12a/1r1+2b	0.0563	2.0	0.5572	2.2	0.0718	1.3	0.49	447.2	5.4	449.7	8.2	462	44
EG12a/1r1	0.0569	2.7	0.5616	3.2	0.0721	1.1	0.59	448.9	4.9	452.6	11.8	488	60
EG12a/2r1	0.0552	2.2	0.5663	2.4	0.0738	0.8	0.49	459.0	3.7	455.7	9.1	418	48
EG12a/2r2	0.0565	1.6	0.5655	2.0	0.0724	1.2	0.58	450.4	5.2	455.1	7.1	472	36
EG12a/3r1	0.0558	1.8	0.5431	2.2	0.0711	0.9	0.56	442.7	3.9	440.5	7.6	444	40
EG12a/3r2	0.0550	2.0	0.5464	2.2	0.0722	0.9	0.42	449.3	3.9	442.6	7.9	412	46
EG12a/4r1	0.0521	3.2	0.3507	3.6	0.0491	1.2	0.46	308.7	3.5	305.2	9.5	286	74
EG12a/4c	0.0524	3.9	0.3470	4.2	0.0488	1.2	0.37	307.1	3.5	302.5	10.9	302	88
EG12a/4r2	0.0531	3.2	0.3605	3.8	0.0490	1.2	0.66	308.3	3.6	312.6	10.4	332	74
EG12a/5r1+2	0.0568	2.6	0.5412	3.0	0.0697	0.9	0.65	434.2	3.9	439.2	10.8	484	56
EG12a/5r1+2b	0.0557	2.5	0.5390	2.8	0.0704	0.9	0.41	438.4	4.0	437.8	9.8	438	56
EG12a/6r1	0.0558	1.8	0.4974	2.2	0.0660	0.9	0.53	412.2	3.4	409.9	7.2	444	42
EG12a/6r2	0.0565	2.7	0.5495	3.4	0.0716	1.0	0.74	445.7	4.5	444.7	12.3	472	60
EG12a/6c	0.0568	2.8	0.5658	2.8	0.0726	1.0	0.26	452.1	4.2	455.3	10.6	484	62

Cathodoluminescence imaging of the metamorphic samples revealed inherited cores within the zircon grains that show well-developed continuous oscillatory zoning from core to rim, which indicates their primary magmatic character (Figure 7).

Figure 7. Representative cathodoluminescence images of dated zircons in metamorphic and volcanic rocks. Sample numbers indicated. Circled areas represent location of spot analyses, together with corresponding ages given with 2σ. For samples’ location see Figures 2 and 3.

Ten data points obtained from oscillatory-zoned domains, commonly interpreted as magmatic, in sample EG 06-12a yielded zircon rim ages ranging from 459 ± 3.7 to 434 ± 3.9 Ma and a single core age at 452 ± 4.2 Ma with a rim age at 412 ± 3.4 Ma in one zircon population, and core to rim 307–308 ± 3.6 Ma in other zircon population (Figure 8(a)). In the metagabbro sample EG 06-7, a zircon grain yielded a core age at 474.3 ± 5.9 Ma with a rim age at 456.1 ± 4.2 Ma and a rim age at 318.5 ± 3.5 Ma in other grain. Two zircon grains in the sample revealed younger core and rim ages of 49.1 ± 1.3 and 47.8 ± 0.8 Ma of the first grain, respectively, and rim ages at 31.5 ± 0.6 and 32.6 ± 0.8 Ma of the second grain (Figure 8(b)). In the metaplagiogranite sample (EG 06-9) a single zircon supplied core and rim ages with mean ²⁰⁶U/²³⁸U age of 510.5 ± 5.4 Ma, while other zircon population revealed core to rim age variation between 465.3 ± 7.0 and 442 ± 4.2 Ma (Figure 8(c)).

Figure 8. Concordia plots for dated metamorphic and volcanic rock samples.

The two xenocrystic zircon populations in the sample EG 06-1 of alkaline basalt dyke yielded ages 459.9 ± 6.2 and 303.0 ± 12.0 Ma, respectively (Figure 8(d)). A single zircon grain in andesite lava sample PK 20 of the Iran tepe paleovolcano yielded rim ages of 307 ± 28 Ma (Figure 8(e)) around inherited core within the zircon grain. Dacite sample PK 9 of this paleovolcano exhibits the largest variability of zircons with a single-core Neoproterozoic age of 640.9 ± 4.9 Ma and Ordovician rim age of 456.7 ± 4.2 Ma in one xenocrystic zircon population. Other three zircon grains show one narrower age interval with Variscan age (293.5 ± 6.2 Ma) and larger scatter from 267 ± 4 to 168 ± 1 Ma, which can be attributed to a combination of different growth zones, recrystallization, and Pb loss effect in a single spot analysis (Figures 8(f)).

From 59 analyses in total performed in all rock types out of 29 analyses supplied dominant cluster of Ordovician ages. From total, 35 analyses out of 25 analyses (71.43%) in the metamafic and associated evolved rocks gave an average early Late Ordovician age of 454.9 Ma, together with four analyses in alkaline basalt dyke (seven analyses in total) showing also prominent cluster of ages in the range 458–436 Ma.

5. Discussion

5.1. Compositions

The new geochemical data derived from the metamafic rocks provide additional chemical constraints on the presence of arc-related magmatic precursors involved in the construction of the high-grade basement. The high LILE/HFSE ratio, distinctive negative Nb anomalies, and HFSE and HREE abundances slightly depleted relative to N-MORB displayed by the metamafic rocks on normalized trace element profiles (Figure 6(a) and (b)) are characteristics typically ascribed to arc-related petrogenesis. Close and 2–5 times enriched HFSE and HREE abundances relative to N-MORB and negative Nb anomalies always higher than N-MORB Nb abundances, in turn, indicate an involvement in the source region of N-MORB to E-MORB mantle with high Nb-Zr concentrations. The metaplagiogranite samples exhibit similar to the metamafic rocks trace elements pattern, with higher LILE/HFSE ratio. Trace element geochemistry indicates MOR to arc-like SSZ signatures of the studied metamorphic rocks in a similar way as depicted by discrimination diagrams. The MORB-IAT signature is well known in SSZ ophiolites (Pearce, 2003) and is particularly evident in ancient and modern arc/back-arc settings from numerous examples (e.g. Fretzdorff, Livermore, Devey, Leat, & Stoffers, 2002; Hawkins, 1995; Saunders & Tarney, 1991). Compared to the basalts in modern back-arc basins, a significant overlap of the trace elements profiles is observed in the studied metamafic rocks relative to these known magmatic analogues, suggesting resemblance to such back-arc tectonic environment (Figure 6(b)).

The geochemistry thus confirms MOR to SSZ signatures of the metamafic rocks in both units of high-grade basement in the region (Bonev et al., 2006b; Haydoutov et al., 2004; Kolcheva & Eskenazy, 1988). Our results provide additional data to previous chemistry results on the metaplagiogranites (Ovtcharova & Sarov, 1995) as well as new for the associated metamafic rocks.

5.2. Age significance

U-Pb LA-ICP-MS zircon geochronology indicates for the first time the presence of Ordovician and confirms Permo-Carboniferous crustal components and magmatic events contribution to the continental build-up of the high-grade basement of the eastern Rhodope Massif. Therefore, the basement has a composite nature and must be considered as recording multiple Paleozoic events prior to Mesozoic-Tertiary history that shaped the tectonic pattern (Figure 2). A dominant cluster of Middle–Late Ordovician ages (average 455 Ma) in oscillatory-zoned zircons clearly demonstrate the timing of magmatic crystallization of the protoliths of studied metamorphic rocks (Figure 8).

These protoliths have experienced likely Carboniferous, Jurassic, and Tertiary metamorphic overprints. The indication for Carboniferous metamorphism 332–286 Ma (Table 3 supplementary material) comes from the 308 Ma rim ages of sample EG 06-12a and age of 318 Ma in sample EG 06-7 that possess inherited core. The Carboniferous metamorphism in the eastern Rhodope region is poorly documented in comparison to well-defined time equivalent granitoid magmatism (Figure 2), which probably is due to strong Tertiary metamorphic overprint. A single evidence for Late Jurassic metamorphic event at 146 Ma is indicated in the metagabbro sample EG 06-7 (Table 3 supplementary material) in line with metaplagiogranite Rb/Sr age of 159 ± 19 Ma (Peytcheva et al., 1998). Worth noting in the region is that (UHP)-HP metamorphism took place before 160 Ma (Bauer, Rubatto, Krenn, Proyer, & Hoinkes, 2007; Bonev et al., 2012) or at 150 Ma (Liati et al., 2011) and Late Jurassic thrusting of the Circum-Rhodope belt onto the Rhodope high-grade basement is recorded between 157 and 154 Ma (Bonev et al., 2010). In sample EG 06-7 metamorphism reaching amphibolite-facies is indicated by a single grain age of 48 Ma and younger age of 32 Ma is probably due to Pb-loss (Figure 8). The age of 48 Ma is consistent with latest metamorphic overprint in amphibolite-facies since hornblende in this sample yielded ⁴⁰Ar/³⁹Ar cooling age of 39.21 ± 4.13 Ma (Bonev, Spikings, Moritz, Marchev, & Collings, 2013). Furthermore, the amphibolite-facies or higher grade metamorphism ongoing in Paleocene–Eocene times is well documented in the Rhodope Massif (Liati et al., 2011 for details).

The magmatic protoliths of the metamorphic rocks show Cambrian inherited zircons that span 528–534 Ma (Table 3 supplementary material). This inheritance is confirmed by Neoproterozoic (722–580 Ma) and Cambrian (518 Ma) xenocrystic zircons found in the lavas of Iran tepe paleovolcano. Importantly, the analyzed latest Eocene-Oligocene volcanic rocks contain in addition Ordovician and Permo-Carboniferous xenocrystic zircons. Thus, the volcanic rocks provide strong evidence for the presence of Neoproterozoic, Ordovician, and Permo-Carboniferous crustal components and magmatic events and highlight their contribution to the build-up of the high-grade basement. The Neoproterozoic-Cambrian zircon populations are known in the Rhodope high-grade basement as detrital grains in metapelites with Permo-Triassic depositional ages and within the cores of Permian protoliths of metamafic rocks (Liati et al., 2011 and references therein). The inherited Neoproterozoic-Cambrian zircons clearly indicate that the Ordovician igneous rocks have intruded already-present older crust, also consistent with the observed field relations of the dated metamorphic rocks. The age record of inherited zircons corresponds to the timing of Cadomian orogenic event in the peri-Gondwanan terranes (Linnemann et al., 2008; Murphy, Pisarevsky, Nance, & Keppie, 2004; Von Raumer, Stampfli, & Bussy, 2003).

5.3. Regional implications

Neoproterozoic and Paleozoic igneous and metamorphic basement terranes in the region immediately adjacent to the Rhodope Massif are those of the Serbo-Macedonian Massif, Carpatho-Balkan, and Sredna Gora and Strandzha zones, all later involved in the evolution of the Alpine orogen (Figure 9).

Figure 9. Regional frame of unpublished and published U-Pb zircon geochronology in the Rhodope metamorphic basement and adjacent terranes (zones) and derived from this study. Map constructed using Ricou et al. (1998) and Bonev, Marchev, and Singer (2006a). See the text for details and Figure 2. Geochronology data sources and methods: (1) Himmerkus et al. (2006) Pb/Pb evaporation, (2) Himmerkus, Reischmann, and Kostopoulos (2009) Pb/Pb evaporation, (3) Macheva et al. (2006) ID TIMS/LA-ICP-MS, (4) Zidarov et al. (2003) ID TIMS, (5) Peytcheva et al. (2009) ID TIMS/LA-ICP-MS, (6)Graf et al. (1998) ID TIMS, (7) Zagorchev et al. (2011) LA-ICP-MS, (8) Naydenov et al. (2009) LA-ICP-MS, (9) Arkadakskiy et al. (2003) ID TIMS, (10) Ovtcharova (2005) ID TIMS, (11) Carrigan et al. (2003) HR SIMS, (12) Carrigan et al. (2005, 2006) HR SIMS, (13) Şahin et al. (2011) SHRIMP/LA-ICP-MS, (14) Anders et al. (2006) ID TIMS/SHRIMP, (15) Okay et al. (2008) LA-ICP-MS/TIMS Pb evaporation. Concordant ages in metamorphic rocks derived from this study are shown.

In the Central Rhodope Massif, remnants of Neoproterozoic-Cambrian to Ordovician oceanic crust is indicated by U-Pb zircon age of 610 Ma in a metagabbro and a near-concordant ages of 540 and 440 Ma in a metabasalt (Arkadakskiy et al., 2003). Ordovician U-Pb zircon protolith age of 452 ± 16 Ma in orthogneiss sample containing inherited 503–539 Ma-old zircons (Naydenov, von Quadt, Sarov, Peytcheva, & Dimov, 2009) and protolith zircon ages of 475 and 450 Ma from two orthogneiss samples (Ovtcharova, 2005) were also reported in the Rhodope Massif.

Cadomian Neoproterozoic (551–588 Ma) and Silurian (426–443 Ma) granitoid bodies were shown to build up the Gondwana-derived crystalline basement of the Serbo-Macedonian Massif (Graf, Bernouli, Burg, Ivanov, & von Quadt, 1998; Himmerkus et al., 2006; Kounov et al., 2012). In the Ograzhden unit that represents an extension of the Serbo-Macedonian Massif in Bulgaria, the granitoid magmatism extends into the Ordovician time (Figure 9). Zidarov et al. (2003) have reported U-Pb zircon ages of 459.9 ± 7.6 and 451 ± 9 Ma for Ograzdhen metagranites that have crystallized in arc to post-collisional setting, having crustal origin of the magma with ⁸⁷Sr/⁸⁶ Sr_i = 0.7109. In the Ograzhden unit, the Lozen tonalite-monzogranite of crustal magma source and anatectic origin has U-Pb crystallization age of 451.9 ± 1.3 Ma (Macheva, Peytcheva, von Quadt, Zidarov, & Tarassova, 2006). A range of U-Pb zircon ages of magmatic protoliths in a likely equivalent to the Ograzhden unit in the western Rhodope Massif were reported in a gneiss that yielded age of 452 ± 14 Ma with metamorphic rims 380–420 and 321 ± 19 Ma-old cross-cutting vein, together with a metadiorite age of 446 ± 7 Ma and a metagabbro age of 456 ± 1.8 Ma (Peytcheva, von Quadt, Sarov, Voinova, & Kolcheva, 2009).

The Carpatho-Balkan Zone north of the Serbo-Macedonian Massif has Neoproterozoic-Cambrian ages of continental and oceanic protoliths in the range 512–578 Ma (Kounov et al., 2012; Zagorchev, Balica, Balintoni, Kozhoukharova, & Sâbâu, 2011) (Figure 10). To the north of the Rhodope Massif, the Sredna Gora Zone demonstrate Gondwana-derived Neoproterozoic (600–900 Ma) and Ordovician (450 Ma) inherited zircons in Late Carboniferous granitoids that built the pre-Mesozoic basement (Carrigan et al., 2005, Carrigan, Mukasa, Haydoutov, & Kolcheva, 2006). The arc-related “diabase-phyllitoid complex” (Haydoutov & Yanev, 1997) spatially linked to Cadomian MOR-type Balkan-Carpathian ophiolite (Savov, Ryan, Haydoutov, & Schijf, 2001), the latter geochemically similar to the mafic rocks described herein, both identifies the Gondwana-derived basement exposed in the West Balkan unit of the Balkan terrane (Dabovski et al., 2002) facing the Moesian terrane considered of Laurussia affinity (Okay et al., 2006; Yanev et al., 2006) (see Figures 1, and 10(c)).

Figure 10. (a–b) Paleotectonic reconstructions after Stampfli and Borel (2002) for Early Ordovician and Early Silurian. (c) A scenario depicting tectonic setting of origin, compositions, and protolith age of metamorphic rocks in the eastern Rhodope and the link to adjacent terranes.

The Strandzha Zone has gneissic basement intruded by Permian (271 Ma, Okay, Satir, Tüysüz, Akyüz, & Chen, 2001) and Carbonifeous (~313 Ma, Okay et al., 2008) granitoids implying already existing early Paleozoic or older crystalline basement. Recent U-Pb zircon ages in the range 536–546 Ma of the metagranitoids in the southernmost Strandzha Zone in Turkey suggests the presence of Neoproterozoic basement (Şahin, Aysal, Güngör, & Peytcheva, 2011). Further east in the Pontides of Turkey, Gondwana-derived Neoproterozoic (565–576 Ma, Ustaömer et al., 2005) and ca. 570 Ma and Middle Ordovician (460 Ma) granitoids (Okay et al., 2008) were reported in the metamorphic basement underlying the İstanbul terrane (Figure 9 inset).

Aforementioned radiometric dates suggest rather wide exposures of Gondwana-derived Neoproterozoic-Early Paleozoic basement areas surrounding the Rhodope Massif. The record of Middle-Late Ordovician magmatic activity in this study places the eastern Rhodope high-grade basement in the framework of Cadomian terranes located along the northern Gondwana margin in continuum with analogous ages reported in other parts of the Rhodope Massif and adjacent terranes (Figure 9).

5.4. Paleotectonic consequences

Paleotectonic reconstructions for the Early Paleozoic time (Stampfli & Borel, 2002) have shown the interaction between oceanic basins related to the Early Ordovician opening of the Rheic Ocean at northern margin of Gondwana (Figure 10(a) and (b)). The Rheic Ocean opened during Early Ordovician by northern drift of peri-Gondwanan terranes detached from Gondwana northern margin following Late Neoproterozoic-Cambrian accretion of arc terranes (Murphy et al., 2004; Nance et al., 2010 and references therein; Stampfli & Borel, 2002; Von Raumer et al., 2002, 2003). Cambro-Ordovician Andean-type Prototethys oceanic crust subduction produced Avalonia-Cadomia arc located at active northern margin of Gondwana and resulted in the opening in a back-arc of the Rheic Ocean (Figure 10(a)) followed by Rheic Ocean widening. The tholeiitic and calc-alkaline magmatic products of the arc system were established on the Cadomian Neoproterozoic-Cambrian basement terranes exposed in the Western-Central Europe (Nance et al., 2010; Neubauer, 2002; Von Raumer et al., 2002). These terranes subsequently have been transported as elements of the Hun superterrane (Figure 10(b)) across the Rheic Ocean to its final accretion to the Laurussia in Late Carboniferous time during Variscan orogenesis. The known Hun superterrane elements in the Alpine belt of Western-Central Europe (e.g. Von Raumer et al., 2003) have counterparts in the eastern sector of the belt adjacent to the Rhodope Massif (Figure 9). Specifically, these Gondwana-derived terranes located in northern Greece are the Neoproterozoic and Silurian Vertiskos terrane of the Serbo-Macedonian Massif (Himmerkus et al., 2006), the Neoproterozoic (700 Ma) Florina terrane of the Pelagonian Zone (Anders, Reischmann, Kostopoulos, & Poller, 2006) (Figure 9 inset). The Neoproterozoic and Early Paleozoic magmatism recorded in the basement of the İstanbul Zone (Okay et al., 2008) and obviously pre-Carboniferous basement of the Strandzha Zone provide an extension of these peri-Gondwanan terranes eastwards in Turkey (Okay et al., 2006) (Figure 9 inset).

Based on field relations and MORB-IAT signature, we interpret the studied metamafic rocks as oceanic crust formed in a back-arc setting where magmatic precursors intruded pre-Middle Ordovician clastic and carbonate sediments deposited at rift-spreading center shoulder. The obtained Middle-Late Ordovician igneous ages of the metamafic and associated evolved rocks bearing subduction signal requires that this back-arc setting developed above the Cambrian-Early Ordovician subduction system visibly related to the opening of the eastern Rheic Ocean during the Early Ordovician time (Figure 10(c)). The subduction signal carried by the metamafic and associated evolved rocks can be attributed to the subduction zone component input into the back-arc environment where the magmatic precursors formed in the upper plate.

6. Conclusions

1. The field data demonstrate primary intrusive relations of metaplagiogranite into metagabbro both intruded into clastic-carbonate host metasedimentary sequence. Similar relationships apply to garnet amphibolite derived from gabbro-basalt precursor in the sequence implying that mainly basic magmatism invaded a sedimentary succession.

2. Bulk-rock geochemistry of metamafic and associated evolved rocks reveals basic to acid compositions compatible with differentiation process of the parental tholeiitic to calc alkaline magma. SSZ MORB-IAT signature is interpreted in terms of origin in a back-arc tectonic environment. Mineral chemistry revealed phases typical for the experienced main Alpine amphibolite-facies metamorphism.

3. U-Pb LA-ICP-MS dating of the metamafic and associated evolved rocks yielded magmatic crystallization of the protoliths in Middle-Late Ordovician (average 455 Ma). Metamorphic rocks contain inherited Cambrian (528–534 Ma) zircons, while single Late Carboniferous (308 Ma), Late Jurassic (146 Ma), and Eocene (48 Ma) zircons suggest metamorphic overprints. Dating of Eocene-Oligocene volcanic rocks intruding or overlying the metamorphic rocks revealed Neoproterozoic (722–580 Ma), Cambrian (518 Ma), Ordovician (458–436 Ma), and Permo-Carboniferous (258–376 Ma) xenocrystic zircons. These zircon populations were sampled by volcanic rocks from the metamorphic basement en route to the surface, thus confirming the presence of the Neoproterozoic and Paleozoic crustal components that built the high-grade basement of the eastern Rhodope region. The ages of inherited and xenocrystic zircons fall in the range of ages known for Cadomian terranes at the northern periphery of Gondwana.

4. Peri-Gondwanan origin of the Middle-Late Ordovician magmatism is inferred in a back-arc rift-spreading center propagating within Cadomian basement terrane. This magmatism is related to the widening of the Rheic Ocean following earlier subduction from where subduction signal is inherited into the mantle wedge from previous melting event.

5. Visible systematic geochronologic record of Ordovician magmatism invading Cadomian basement within the Rhodope Massif compared to same record in adjacent terranes in northern Greece and northwest Turkey provide a consistent link among the Early Paleozoic basement terranes and geodynamics in the region.

Acknowledgements

The support by SNSF (Switzerland) SCOPES grant No. IB7320-111046/1 and NSF (Bulgaria) contract No. VU-NZ 02/06 is gratefully acknowledged. The constructive and in-depth reviews by D. Papanikolaou and I. Savov helped us to improve the paper.

Appendix 1. Analytical details of mineral chemistry and whole-rock geochemistry

Electron microprobe analyses of mineral phases were performed on Jeol Superprobe 8200 instrument equipped with five spectrometers and wavelength-dispersive system at the University of Lausanne, Switzerland. Operating conditions were 20 nA (amphiboles, garnet) and 15 nA (feldspars, micas) focused beam current of 3 μm diameter at 15 kV accelerating voltage, using natural standards and ZAF correction.

Whole-rock chemical analyses were performed at the University of Geneva, Switzerland, and calibrated against both international and internal standards. Major- and trace elements concentrations were determined by X-ray fluorescence on fused discs and pressed pellets, respectively, using a Philips PW 2400 spectrometer and BHVO, NIM-G, SDC-1, QLO standards.

Appendix 2. Analytical details of U-Pb LA-ICP-MS geochronology

Cathodoluminescence imaging was applied on zircons prior to analyses using a CamScanMV2300 SEM (Institute of Geology and Paleontology, University of Lausanne) in order to reveal their internal structure. Pb²⁰⁶–U²³⁸ and Pb²⁰⁷–U²³⁵ dates were obtained using a 193-nm excimer ablation system UP-193FX (ESI) interfaced to an Element XR sector field, single-collector ICP-MS (Thermo Scientific) at the Institute of Mineralogy and Geochemistry, University of Lausanne. Operating conditions were similar to those described in Ulianov, Muntener, Schaltegger, and Bussy (2012) and included a 25–35 μm spot size combined with a relatively low on-sample energy density of 2.7–2.8 J/cm² and a repetition rate of 5 Hz to minimize the fractionation. For external standartisation a GJ-1 standard zircon (²⁰⁶Pb/²³⁸U age of 600.5 ± 0.4 Ma was used. The 91500 standard was measured along with sample zircons on a routine basis to control the accuracy of results. The ratio-of-the-mean intensities data reduction method was used to calculate the mean Pb/U concentration ratio. Following the approach of Jackson, Pearson, Griffin, and Belousova (2004) a qualitative control of the intensities for masses 202 and 204 and a careful inspection of the cathodoluminescence images were used to control for common lead. U–Pb data are plotted in concordia diagrams as 2σ error ellipses. Only analytically concordant points were used to calculated mean ²⁰⁶Pb/²³⁸U ages.