2. Regional geology and geochemistry of Triassic mafic-ultramafic rocks

In the northern Aegean region, there are three major continental blocks in the crustal mosaic of the Balkan Peninsula. These include, from west to east, the Pelagonia, Serbo-Macedonian and Rhodope massifs (Fig. 1, inset). Jurassic ophiolites and Triassic mafic rift-related rocks occur on both sides of Pelagonia, whereas the same oceanic crust elements were emplaced from the south or occur within the Serbo-Macedonian and Rhodope massifs.

The Serbo-Macedonian Massif is separated from the Pelagonia microcontinent to the SW by the Vardar suture zone, which is part of the innermost Hellenides. To the north, the Serbo-Macedonian Massif continues laterally into the Rhodope Massif, which delimits the Alpine Balkan thrust–fold belt in the north (Fig. 1). The crustal basement units of the Serbo-Macedonian and the Rhodope massifs are structurally related to the Mesozoic development of the Neotethyan Vardar oceanic realm at the southern continental margin of Eurasia (Koukouvelas & Doutsos, Reference Koukouvelas and Doutsos1990; Burg et al. Reference Burg, Ricou, Ivanov, Godfriaux, Dimov and Klain1996; Robertson et al. Reference Robertson, Dixon, Brown, Collins, Morris, Pickett, Sharp, Ustaömer, Morris and Tarling1996; Dinter, Reference Dinter1998; Ricou et al. Reference Ricou, Burg, Godfriaux and Ivanov1998; Bonev & Beccaletto, Reference Bonev, Beccaletto, Taymaz, Yilmaz and Dilek2007). The Serbo-Macedonian and Rhodope massifs comprise mainly amphibolite-facies metamorphic basement rocks consisting of pre-Alpine (Neoproterozoic–Permian) and Alpine (e.g. Lips, White & Wijbrans, Reference Lips, White and Wijbrans2000; Liati, Reference Liati2005; Himmerkus, Reischmann & Kostopoulos, Reference Himmerkus, Reischmann and Kostopoulos2009) units of continental and oceanic affinities. All these metamorphic units are intruded by granitoid plutons that range in age from Late Cretaceous to Miocene (Zagorchev, Moorbath & Lilov, Reference Zagorchev, Moorbath and Lilov1987; Dinter et al. Reference Dinter, Macfarlane, Hames, Isachsen, Bowring and Royden1995; Christofides et al. Reference Christofides, Koroneos, Soldatos, Eleftheriadis and Kilias2001).

The high-grade crystalline rocks of the western-central Rhodope Massif are best exposed in the Verila, Rila and Pirin mountains in Bulgaria, and extend into the Central Rhodope Mountains in Bulgaria and Greece (Fig. 1). Almost all of these basement units in the western-central Rhodope Massif contain bodies and lenses of mafic-ultramafic rocks, showing mid-ocean ridge (MORB)-like geochemical affinities (Kolcheva & Eskenazy, Reference Kolcheva and Eskenazy1988; Liati & Mposkos, Reference Liati and Mposkos1990). Both high-grade metamorphic units and mafic-ultramafic intercalations with c. 245–250 Ma old protoliths structurally overlie along the WSW-vergent Nestos thrust fault the Upper Palaeozoic–Triassic (Kronberg, Meyer & Pilger, Reference Kronberg, Meyer and Pilger1970; Liati, Gebauer & Fanning, Reference Liati, Gebauer, Fanning, Dobrzhinetskaya, Faryad, Wallis and Cuthbert2011) platform carbonates represented by the Pirin-Pangeon unit (Papanikolaou & Panagopoulos, Reference Papanikolaou and Panagopoulos1981; Zagorčev, Reference Zagorčev1994). The metamorphic basement of the Rila Mountains in the western Rhodope Massif is regarded as the northeastward extension of the Ograzden unit in the Serbo-Macedonian Massif (Zagorčev, Reference Zagorčev1994; Zagorchev, Reference Zagorchev2001). In the Rila Mountains, the metagabbro-metadiorite bodies within the high-grade metamorphic units show island arc tholeiitic (IAT) and calc-alkaline (CA) affinities (Machev, Reference Machev2002). These mafic rocks are locally associated with sheared boudins of lherzolite?–clinopyroxenite cumulates (Bazylev et al. Reference Bazylev, Zakariadze, Zeljazkova-Panayotova, Kolcheva, Obërhansli and Solovieva1999). A complex Alpine ultra-high, high- and medium pressure tectonometamorphic history of the western-central Rhodope units is bracketed between 149 and 40 Ma (Liati, Reference Liati2005 and references therein). Zircon U–Pb geochronology in the Rhodope units has revealed ubiquitous Permo-Carboniferous (310–253 Ma) and Late Jurassic–Early Cretaceous (160–134 Ma) granitoid protolith ages in the high-grade basement (e.g. Peytcheva et al. Reference Peytcheva, Von Quadt, Ovtcharova, Handler, Neubauer, Salnikova, Kostitsin, Sarov and Kolcheva2004; Liati, Reference Liati2005; Turpaud & Reischmann, Reference Turpaud and Reischmann2010) (Fig. 1).

The Serbo-Macedonian Massif (Mercier, Reference Mercier1966; Dimitriević, Reference Dimitrijevic1974) extends northwards into Serbia, F.Y.R. Macedonia, Greece and Bulgaria, and has been considered as either a separate crustal unit within the Internal Hellenides (Papanikolaou, Reference Papanikolaou2009) that was thrust onto the Rhodope Massif (Kockel & Walther, Reference Kockel and Walther1965), or is part of a single continental block sharing together with the Rhodope Massif the same Alpine nappe-stacking kinematics (Ricou et al. Reference Ricou, Burg, Godfriaux and Ivanov1998). In the Chalkidiki Peninsula of northern Greece, it has been subdivided into two separate series. The Lower Kerdilion series is separated by a SW-dipping fault (Kockel, Mollat & Walther, Reference Kockel, Mollat and Walther1971, Reference Kockel, Mollat and Walther1977) or ductile shear zone (Burg, Godfriaux & Ricou, Reference Burg, Godfriaux and Ricou1995) from the Upper Vertiskos series. The Vertiskos series hosts mafic-ultramafic bodies, which are known as the Therma-Volvi-Gomati (TVG) complex (Dixon & Dimitriadis, Reference Dixon, Dimitriadis, Robertson and Dixon1984) (Fig. 1). Recent zircon U–Pb geochronology has shown the presence of Neoproterozoic (586 Ma) and Silurian (433–428 Ma) igneous basement rocks in the Vertiskos series gneisses, which are intruded by the Triassic (mean age 228 Ma), A-type Arnea granite (Himmerkus, Reischmann & Kostopoulos, Reference Himmerkus, Reischmann and Kostopoulos2009 and references therein) (Fig. 1). The Permian–Lower Triassic Examili clastic and volcanic successions overlying the Vertiskos series have been interpreted as the products of Triassic rifting (Dimitriadis & Asvesta, Reference Dimitriadis and Asvesta1993).

Dixon & Dimitriadis (Reference Dixon, Dimitriadis, Robertson and Dixon1984) identified the Volvi mafic rocks as ‘transitional basalts’ of mixed ‘alkalic’ and ‘above-subduction-zone’ geochemical affinities generated in an ‘in situ’ Mesozoic intra-continental rift. Sr–Nd isotope chemistry of the TVG complex revealed MORB and IAT affinities interpreted in terms of formation in a supra-subduction zone environment (Himmerkus et al. Reference Himmerkus, Zachariadis, Reischmann and Kostopoulos2005). The Volvi body mainly consists of isotropic coarse-grained massive and pegmatoid metagabbros and rare basaltic dykes, whereas the Therma body consists of basalts, rare altered peridotites and gabbros. All these mafic-ultramafic rocks of both the Therma and Volvi bodies are metamorphosed into massive or layered amphibolites, which largely preserve primary igneous textures and mineral phases. Liati, Gebauer & Fanning (Reference Liati, Gebauer, Fanning, Dobrzhinetskaya, Faryad, Wallis and Cuthbert2011) have reported a zircon U–Pb SHRIMP protolith age of 252 ± 13 Ma for the Volvi body metagabbros, implying Late Permian–Early Triassic magmatic crystallization.

In the western foothills of the Rila Mountains, along the Bistrica river valley, the amphibolite-facies gneiss and schist successions of the Rhodope Massif host thick metadiorite-metagabbro layers. These gabbro-diorite bodies commonly form massive or banded amphibolites, and locally show in weakly metamorphosed and deformed domains massive or pegmatoid textures similar to these observed in the Volvi mafic body. In the northern part of the Rila Mountains, massive amphibolites are intercalated with schists and overlain by migmatitic gneisses that texturally strongly resemble those of the Ograzden unit of the Serbo-Macedonian Massif. Between the Greek Serbo-Macedonian Massif and the Rila Mountains in the western-northern Rhodope Massif, other occurrences of metamafic rocks that are texturally similar to the Volvi body metagabbros are observed in the foothills of the Pirin Mountains (Fig. 1).

We have identified high- and low-Ti groups in the Volvi and Therma bodies (Serbo-Macedonian Massif) and in the Rila Mountains of the western Rhodope Massif based on the trace elements and rare earth element (REE) characteristics of the mafic-ultramafic rocks (SiO₂ = 45–56 wt%) (Bonev & Dilek, Reference Bonev and Dilek2010). The high-Ti group rocks with high high-field-strength element (HFSE) and REE contents show E-MORB and mild within-plate oceanic floor signatures (OIB). The low-Ti group rocks exhibit, on the other hand, both partly N-MORB/E-MORB depleted and fractionated HFSE and REE characteristics and similar enriched characteristics comparable to the high-Ti group, and also an arc-like signature. Both high- and low-Ti groups display low- to medium-K tholeiitic and mostly calc-alkaline affinities of the gabbroic-basaltic protoliths. These geochemical signatures, trace element and REE characteristics of the mafic-ultramafic rocks are consistent with a continental rift setting for their tectonic environment of formation (Bonev & Dilek, Reference Bonev and Dilek2010). We interpret the arc-like geochemical signature of the low-Ti group to have been inherited from previous subduction events (Palaeotethyan) in the region.

3. Nd–Sr–Pb isotope results and comparison with other Triassic rift units

We analysed three samples from the Volvi body (V1, V2, V3; low-Ti group), one sample from the Therma body (TH1; low-Ti group), together with two samples from the western Rhodope Massif metamafic rocks (WRB-1; high-Ti group, WRB-3; low-Ti group) for whole-rock Sr, Nd and Pb isotopic compositions. We compared the Nd and Pb isotopes of our rock samples to the published Nd and Pb isotopic compositions of the Triassic rift-related mafic rocks in the adjacent Pelagonian zone of the western Hellenides. We also compared the Nd and Sr isotopes obtained in this study with the Nd and Sr isotopic compositions of mafic rocks from the Athos-Volvi zone (equivalent of the TVG complex) (Himmerkus et al. Reference Himmerkus, Zachariadis, Reischmann and Kostopoulos2005).

Chemical separation of the samples and whole-rock isotopic analyses were done in the TERRA Facility in the Department of Earth Sciences at Memorial University of Newfoundland (Canada) and calibrated against both international and internal standards. Sr–Nd–Pb isotopes were measured on a multicollector Finningan MAT 262V thermal ionization mass spectrometer, and ⁸⁷Sr/⁸⁶Sr, ¹⁴³Nd/¹⁴⁴Nd, ²⁰⁸Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁶Pb/²⁰⁴Pb ratios were corrected for mass fractionation and normalized to international standards. The whole-rock Nd, Sr and Pb isotopic compositions are given in Table 1.

Table 1. Nd, Sr and Pb isotopic compositions of metamafic rocks from the Serbo-Macedonian and western Rhodope massifs

Location of samples shown in Figure 1.

The ¹⁴³Nd/¹⁴⁴Nd ratios of the mafic rock samples fall in the range of 0.5127–0.5131, with positive ε_Nd values that are characteristic of mantle melts. When time corrected for the crystallization age of the Arnea granite (228 Ma) adjacent to the Volvi body, the ε_Nd(t) values vary from +2.4 to +9.8 (Table 1). The ⁸⁷Sr/⁸⁶Sr ratios of the Volvi (V1 = 0.7039), Therma (TH1 = 0.7051) and the western Rhodope samples (WRB-1 = 0.7061) display values characteristic of the oceanic crust. However, sample (WRB-3) from the western Rhodope massif has a higher ⁸⁷Sr/⁸⁶Sr ratio of 0.7098. These data indicate that the analysed mafic rocks except sample WRB-3 originated from magmas that were derived from a similar mantle source with a high time-integrated Sm/Nd ratio and with a moderate range of Rb/Sr ratios. In a correlative ¹⁴³Nd/¹⁴⁴Nd v. ⁸⁷Sr/⁸⁶Sr diagram (Fig. 2a), almost all of the samples (except sample WRB-3) show consistent values of obtained isotopic ratios to those of the TVG complex MORB-like, arc-like and cumulate mafic rock groups reported by Himmerkus et al. (Reference Himmerkus, Zachariadis, Reischmann and Kostopoulos2005).

Figure 2. Correlation diagrams for Nd–Sr–Pb isotopes of the metamafic rocks. (a)¹⁴³Nd/¹⁴⁴Nd v. ⁸⁷Sr/⁸⁶Sr diagram. DMM and EMI mantle reservoirs from Hart (Reference Hart1984); (b) ²⁰⁷Pb/²⁰⁴Pb v. ²⁰⁶Pb/²⁰⁴Pb correlation diagram. The NHRL and the fields of reference mantle reservoirs are after Zindler & Hart (Reference Zindler and Hart1986), and the fields of OIB and MORB from Rollinson (Reference Rollinson1993); (c) εNd_t v. ²⁰⁶Pb/²⁰⁴Pb diagram. Mantle reservoirs are after Zindler & Hart (Reference Zindler and Hart1986).

The ²⁰⁶Pb/²⁰⁴Pb ratios of the analysed rocks show a relatively narrow range (18.64–19.28), reaching a higher ²⁰⁶Pb/²⁰⁴Pb value of 21.72 for sample WRB-3, which has the most radiogenic Pb (Table 1). These samples display a narrow range of ²⁰⁷Pb/²⁰⁴Pb ratios (15.58–15.77), and similarly uniform ²⁰⁸Pb/²⁰⁴Pb ratios (38.24–39.31), reaching a higher value in sample WRB-3. The Volvi–Therma mafic rocks exhibit a short linear trend straddling a progressive enrichment along the MORB–OIB line in the ²⁰⁷Pb/²⁰⁴Pb–²⁰⁶Pb/²⁰⁴Pb correlation diagram (Fig. 2b), plotting above the Northern Hemisphere Reference Line (NHRL) where enriched mantle reservoirs (EMI, EMII) are identified (Zindler & Hart, Reference Zindler and Hart1986). The Pb isotopic compositions plot within the large OIB field and close to the MORB field (Rollinson, Reference Rollinson1993). Sample WRB-1 plots closest to the MORB field, while sample WRB-3, with distinctly high ²⁰⁶Pb/²⁰⁴Pb, falls below NHRL approaching the HIMU field.

The analysed mafic rock samples have Pb isotope ratios that cluster close to those of Triassic, rift-related mafic rocks from Greece (Pe-Piper, Reference Pe-Piper1998). Our samples from the Serbo-Macedonian and Rhodope massifs and those reported from Pelagonia by Pe-Piper (Reference Pe-Piper1998) all plot within the OIB field (closest to the MORB field), and mostly show high ²⁰⁶Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb ratios. When ²⁰⁶Pb/²⁰⁴Pb is plotted against εNd_{t = 228}, the isotopic ratios of our samples cluster between the Bulk Silica Earth (BSE) and MORB fields (Fig. 2c). The Nd isotopes are available only for two samples from the Triassic rift-related rocks in Pelagonia (Pe-Piper, Reference Pe-Piper1998); when plotted as εNd_{t = 210} values together with our samples, both rock suites display strong similarities in terms of Nd–Pb isotopic compositions (Fig. 2c).