Nd–Sr–Pb isotopes systematics of the Jurassic Evros ophiolite, eastern Circum-Rhodope Belt, NE Greece

Abstract

We report on the isotopic compositions of the Jurassic supra-subduction zone Evros ophiolite mafic rocks exposed in the eastern Circum-Rhodope Belt of northeastern Greece. These mafic units consist of low-Ti gabbroic and basaltic rocks, whose Nd–Sr–Pb isotopes are compatible with dominant mantle-derived MORB component mixed with a detectable amount of crustal material and/or sediment invoved in their melt source in the subduction zone. These isotopic features are consistent with an intra-oceanic arc origin of the mafic ophiolite rocks, and the Evros ophiolite Nd and Pb isotopes are comparable to those of the counterpart mafic rocks from the Mandritsa unit in Bulgaria.

1. Introduction

The Serbo-Macedonian and Rhodope high-grade metamorphic massifs constitute the crystalline basement of the Alpine orogenic belt in the Balkan Peninsula and display a record of a late Mesozoic convergence resulting in nappe stacking overprinted by an early Cenozoic extensional deformation (e.g. Burg et al. 1996; Bonev et al. 2006; Bonev & Beccaletto, 2007). Both massifs are surrounded by a very low- to low-grade Circum-Rhodope Belt (CRB) that extends from the Chalkidiki Penisula (western part) across the Aegean Sea to the Thrace area (eastern part) in northern Greece (Fig. 1; Kauffmann et al. 1976). The eastern CRB contains unmetamorphosed Evros ophiolite of supra-subduction zone origin (e.g. Magganas et al. 1991; Magganas, 2002; Bonev & Stampfli, 2008, 2009), which has Early–Middle Jurassic crystallization ages (Bonev et al. 2015 a). The south-vergent, thrust system of the Serbo-Macedonian and Rhodope massifs was constructed during a contractional phase in the hanging wall of a north-dipping Cretaceous–Tertiary subduction zone that was located within Vardar Ocean further to the SSW (in the present-day coordinate system) (Ricou et al. 1998). However, the late Mesozoic shortening and nappe-stacking event was predated by intra-oceanic subduction that resulted in the formation of arc-related Evros ophiolite, which together with the associated sedimentary rocks was thrusted and accreted to the Serbo-Macedonian – Rhodope continental margin of Eurasia (Bonev & Stampfli, 2003, 2011; Bonev et al. 2010 a, 2015 a).

Fig. 1. Simplified geological map of the eastern Rhodope Massif in southern Bulgaria and northern Greece (after Bonev et al. 2015 a), showing the locations of studied samples listed in Table 1. Bold numbers refer to radiometric ages of the Evros ophiolite. Inset: tectonic framework of the Alpine system in the Aegean domain of the eastern Mediterranean region.

The low-grade metamorphic CRB forms the uppermost crustal unit of the Serbo-Macedonian and Rhodope massifs, with distinct metamorphic grade and N-directed kinematics compared to the high-grade basement (Bonev et al. 2010 a; Bonev & Stampfli, 2011; Bonev & Filipov, 2018), which indicate that they all experienced a deformational event, and that they were already part of the crustal architecture in the Balkan Peninsula by the earliest Cretaceous (Ivanova et al. 2015; Bonev et al. 2015 a, b). However, the nature of the CRB ophiolite melt source(s) is not well understood. The existing models consider them as ophiolitic allochthonous tectonic sheets, representing the remnants of a Tethyan oceanic lithosphere (Robertson et al. 1996; Robertson, 2002; Papanikolaou, 2009; Stampfli & Hochard, 2009; Bonev et al. 2010 a, 2015 a, b; Ferrière et al. 2016), which formed in fore-arc, intra-arc/arc and back-arc environments (Bonev & Stampfli, 2008, 2009; Bonev et al. 2010 b, 2015 a, b; Bonev, 2020). These different arc-related settings have major implications for the crustal evolution of the Balkan Peninsula and need to be validated with isotopic studies.

In this paper, we report on the Nd–Sr–Pb radiogenic isotope geochemistry of the Evros ophiolite mafic units in the eastern CRB exposed in northeastern Greece (Fig. 1). Our isotopic data suggest crust–mantle interaction recorded in these mafic rock units, which represent island arc magmatic assemblages formed during intra-oceanic subduction evolution outboard the Rhodope margin in the Early to Late Jurassic. We present a brief account of the regional geology, geochemistry and isotopic signature of the Jurassic Evros ophiolite mafic rocks, and then discuss their origin in different arc-related magmatic settings.

2. Regional geology of the eastern CRB and Evros ophiolite

In the Thrace area of the northern Aegean region, the Evros ophiolite is exposed from the Aegean coast in Greece up to the northern tip of the eastern Rhodope Massif in Bulgaria, and built the eastern CRB, also called the Mesozoic low-grade unit (Fig. 1, inset; e.g. Bonev & Stampfli, 2003). The Samothraki Island is an extension of the Evros ophiolite offshore Greece, named Samothraki ophiolite of back-arc origin (Tsikuras & Hatzipanagiotou, 1998). The two main units of the eastern CRB include the greenschist-facies metasedimentary Makri unit and the overlying very low-grade to unmetamorphosed Drimos–Melia unit; the latter in turn includes a major part of the Evros ophiolite (Fig. 1; Papadopoulos, 1980, 1982). The eastern CRB overlies the upper unit of the high-grade basement with a reworked thrust contact and by extensional detachments directly onto the lower unit of the high-grade basement testifying for extensional omissions in the tectonostratigraphy (Fig. 1; Bonev & Stampfli, 2011).

The Makri unit consists of stratigraphically lower metasedimentary series conformably overlain by an upper metavolcanic–sedimentary series, which are intercalated with two marble horizons and covered by a single limestone horizon. The metasedimentary series (i.e. marble–shale–phyllite sequences) has the characteristics of shallow-marine platform type slope-rise deposits (Papadopoulos et al. 1989). One of the lowest marble horizons yielded Upper Triassic corals (Maratos & Andronopoulos, 1964), and the metasedimentary series supplied Tithonian–Beriassian ammonites (Dimadis & Nikolov, 1997). The youngest group of detrital zircons in the sandstones cluster at c. 240 Ma, providing at least a Middle Triassic depositional age for the stratigraphically lower levels of the Makri unit (Meinhold et al. 2010). The metavolcanic–sedimentary series is dominated by greenschists derived from mafic to acid lavas and pyroclastics, with occurrences of scarce serpentinite bodies. The uppermost limestone horizon, known as Lower Cretaceous ‘Aliki limestones’ (Maratos & Andronopoulos, 1964), lies unconformably on the metavolcanic–sedimentary series (Kopp, 1969; Ivanova et al. 2015), implying a pre-Cretaceous age of deposition and greenschist-facies metamorphism of the Makri unit. Recent biostratigraphic data confirmed Beriassian – lower Valanginian depositional age of the ‘Aliki limestones’ (Ivanova et al. 2015).

The Drimos–Melia unit consists of massive mafic lava flows intruded by mafic dykes and pillow lavas of the Evros ophiolite intercalated in a flysch succession (Papadopoulos, 1982; Papadopoulos et al. 1989; Bonev & Stampfli, 2005; Bonev, 2020), which yielded Middle–Upper Triassic (Dimadis et al. 1996) and Middle–Upper Jurassic (Trikkalinos, 1955) biostratigraphic ages. The detrital zircons in the flysch cluster at c. 315–285 Ma, and the youngest grain at c. 160 Ma provides a Late Jurassic maximum age of deposition (Meinhold et al. 2010).

Middle Eocene to Oligocene sedimentary rocks represents unconformable sedimentary cover unit onto the Makri and Drimos–Melia units, which includes also thick Late Eocene – Oligocene up to early Miocene volcanic and volcanic–sedimentary successions (Kopp, 1965; Christofides et al. 2004).

The Evros ophiolite consists of massive and rare pillow tholeiitic basalt, and evolved boninitic–tholeiitic basaltic andesite–andesite lavas (e.g. Magganas et al. 1991; Magganas, 2002; Bonev & Stampfli, 2005, 2008), together with tholeiitic to calc-alkaline cumulitic and isotropic gabbro (Biggazzi et al. 1989; Bonev & Stampfli, 2009), which together demonstrate a typical upper crustal intrusive and volcanic section of an ophiolite. Volcanic section of the Evros ophiolite is most extensively exposed in the Drimos–Melia unit, but other smaller exposures also occur at the villages of Krovili and Kornofolea and at the town of Didymotycho (Fig. 1). One of the intrusive members of the Evros ophiolite, called the Petrota gabbroic complex (Biggazzi et al. 1989), is exposed within the Tertiary Petrota graben. According to U–Pb zircon dating, the Petrota gabbro crystallized at 169 ± 2 Ma (Koglin et al. 2007), and apatite fission-track ages ranging between 161 Ma and 140 Ma (Biggazzi et al. 1989) from the gabbro reveal the shallow crustal low-temperature history. Another Evros ophiolite intrusive section gabbroic body occurs at the village of Agriani (Bonev & Stampfli, 2009). At Agriani, an isotropic gabbro is cross-cut by boninitic dykes, and the gabbro yielded a ⁴⁰Ar/³⁹Ar amphibole cooling age of 163.49 ± 3.85 Ma (Bonev et al. 2015 a). Further east at the town of Didymotycho, a plagiogranite stock is cross-cut by boninitic–tholeiitic basalt and andesite dykes (Bonev & Stampfli, 2009). Rare-earth element modelling suggests that the plagiogranite is the result of extreme fractional crystallization of a basaltic magma similar in composition to the dykes intruding the plagiogranite. At Didymotycho, according to U–Pb zircon dating, the 171 Ma old plagiogranite intruded into 176 Ma old gabbro as well as into massive tholeiitic basalt lavas overlying the gabbro (Bonev et al. 2015 a). Bonev & Stampfli (2009) have interpreted the Evros ophiolite occurrences at Agriani and Didymotycho as a proto-arc (fore-arc) segment of the large-scale eastern Rhodope – Evros Jurassic intra-oceanic arc system. Equivalent to the Evros ophiolite, the low-K and low-Ti boninitic–tholeiitic massive lavas are included in the Mandritsa unit in Bulgaria, from which Nd (¹⁴³Nd/¹⁴⁴Nd_i = 0.512405–0.512950) and Pb (²⁰⁶Pb/²⁰⁴Pb_i = 18.132–18.564; ²⁰⁷Pb/²⁰⁴Pb_i = 15.573–15.640; ²⁰⁸Pb/²⁰⁴Pb_i = 37.959–38.551) isotopes are available (Bonev & Stampfli, 2008). Collectively, based on geochemical affinities, the Evros ophiolite generated in fore-arc to arc tectonic environments, and it can be classified as subduction-related supra-subduction type to volcanic arc type ophiolite (e.g. Dilek & Furnes, 2011).

3. Nd–Sr–Pb isotopes results

We analysed three samples from the mafic rocks at Didymotycho (GR59, GR46-9, Dd2) and three samples from the Drimos–Melia unit (GR11-73, GR41-9, GR35-9), together with two samples from the Agriani (AGR, GR11-65) and three samples from the Petrota (GR53-9, GR12-9, GR24-9) gabbroic bodies, and a mafic rock sample (GR44-9) at the village of Kornofolea for Nd, Sr and Pb isotopic compositions (Fig. 1; Table 1). Whole-rock compositions of these samples are published in Bonev & Stampfli (2009), Bonev et al. (2015 a) and Bonev (2020). We compared the Nd and Pb isotopes of our rock samples to the published Nd and Pb isotopic compositions of the mafic ophiolitic rocks in the adjacent Mandritsa unit of the CRB, in Bulgaria (Bonev & Stampfli, 2008).

Table 1. Summary of studied samples from the Evros ophiolite. Location of samples is shown in Figure 1

Chemical separation of the samples and whole-rock isotopic analyses were done in the Department of Earth Sciences at the University of Geneva (Switzerland) and calibrated against both international and internal standards. Nd–Sr–Pb isotopes were measured on a Thermo Neptune PLUS Multi-Collector inductively coupled plasma – mass spectrometer (ICP-MS). For monitoring the internal fractionation, we used ⁸⁸Sr/⁸⁶Sr = 8.375209 for the ⁸⁷Sr/⁸⁶Sr ratio, ¹⁴⁶Nd/¹⁴⁴Nd = 0.7219 for the ¹⁴³Nd/¹⁴⁴Nd ratio and ²⁰³Tl/²⁰⁵Tl = 0.418922 for the three Pb ratios. Long-term reproducibility of the measurements was controlled by repeated measurements of the external standards SRM987 (⁸⁷Sr/⁸⁶Sr = 0.710248, McArthur et al. 2001; JNdi-1 ¹⁴³Nd/¹⁴⁴Nd = 0.512115, Tanaka et al. 2000) and SRM981 (Baker et al. 2004) for Pb. Further analytical details can be found in Chiaradia et al. (2011). The whole-rock Nd, Sr and Pb isotopic compositions are given in Tables 2-4. The Nd–Sr–Pb isotopic results were age-corrected to the known crystallization ages of the Evros ophiolite or biostratigraphic age of the sediments associated to this ophiolite (see Table 1).

Table 2. Sr isotopic compositions of the Evros ophiolite mafic rocks

* Values corrected for internal fractionation using ⁸⁸Sr/⁸⁶Sr = 8.375209 and for external fractionation using a nominal value of SRM987 ⁸⁷Sr/⁸⁶Sr = 0.710248 (McArthur et al. 2001).

Table 3. Nd isotopic compositions of the Evros ophiolite mafic rocks

* Values corrected for internal fractionation using ¹⁴⁶Nd/¹⁴⁴Nd = 0.7219 and for external fractionation using a nominal value of JNdi1 ¹⁴³Nd/¹⁴⁴Nd = 0.512115 (Tanaka et al. 2000).

Table 4. Pb isotopic compositions of the Evros ophiolite mafic rocks

* Values corrected for internal fractionation using ²⁰³Tl/²⁰⁵Tl = 0.418922 and for external fractionation using nominal values of SRM981 of Baker et al. (2004).

The ¹⁴³Nd/¹⁴⁴Nd ratios of the mafic rock samples fall in the range 0.512571–0.512976, with positive ϵ_Nd values that are characteristic of mantle melts, and only a single exception of sample GR44-9 with negative ϵ _Nd = −1.3. When time-corrected for the crystallization age of the mafic rocks (176 Ma to 165 Ma; see Table 1), the ϵ _Nd(t) values vary from +0.2 to +6.6 (Table 3). The ⁸⁷Sr/⁸⁶Sr ratios display values ranging from 0.703998 to 0.707584 that are characteristic of the oceanic crust and subduction-related volcanic rocks (Table 2). However, basalt sample GR44-9 at Kornofolea and sample Dd2 from the dyke at Didymotycho have higher ⁸⁷Sr/⁸⁶Sr ratios of 0.707153 and 0.707584, respectively. These data indicate that the analysed mafic rocks except samples GR44-9 and Dd2 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 vs ⁸⁷Sr/⁸⁶Sr diagram (Fig. 2a), the majority of the samples parallel the mantle array and plot close to the Bulk Silica Earth (BSE), while samples GR41-9 and GR44-9 plot close to BSE and sample Dd2 plots with high ⁸⁷Sr/⁸⁶Sr ratio. In a ¹⁴³Nd/¹⁴⁴Nd vs ²⁰⁶Pb/²⁰⁴Pb diagram isotopic ratios cluster between the BSE, Prevalent Mantle (PREMA) and Mid-Ocean Ridge Basalt (MORB) (Fig. 2b) and spread along the same mantle reservoirs towards higher ⁸⁷Sr/⁸⁶Sr ratios in the ⁸⁷Sr/⁸⁶Sr vs ²⁰⁶Pb/²⁰⁴Pb diagram (Fig. 2c).

Fig. 2. Correlation diagrams for Nd–Sr–Pb isotopes of the Evros ophiolite mafic rocks. (a) ¹⁴³Nd/¹⁴⁴Nd vs ⁸⁷Sr/⁸⁶Sr diagram. DMM and EMI mantle reservoirs from Hart (1984). (b) ¹⁴³Nd/¹⁴⁴Nd vs ²⁰⁶Pb/²⁰⁴Pb diagram. (c) ⁸⁷Sr/⁸⁶Sr vs ²⁰⁶Pb/²⁰⁴Pb diagram. (d) ²⁰⁷Pb/²⁰⁴Pb vs ²⁰⁶Pb/²⁰⁴Pb diagram. The NHRL and the fields of reference mantle reservoirs are after Zindler & Hart (1986), and the fields of OIB and MORB from Rollinson (1993). (e) ϵ _Nd(t) vs ²⁰⁶Pb/²⁰⁴Pb diagram. Mantle reservoirs are after Zindler & Hart (1986).

The ²⁰⁶Pb/²⁰⁴Pb ratios of the analysed rocks show a relatively narrow range (18.314–18.930), display a narrow range of ²⁰⁷Pb/²⁰⁴Pb ratios (15.565–15.670) and a relatively narrow range of ²⁰⁸Pb/²⁰⁴Pb ratios (38.197–39.096), reaching a higher value in sample GR53-9 (Table 4). Almost all the mafic rocks exhibit a short linear trend parallel to progressive enrichment along the MORB–OIB line in the ²⁰⁷Pb/²⁰⁴Pb–²⁰⁶Pb/²⁰⁴Pb correlation diagram (Fig. 2d), plotting above the Northern Hemisphere Reference Line (NHRL) where enriched mantle reservoirs (EMI, EMII) are identified (Zindler & Hart, 1986). An exception of Pb isotopic compositions is a single sample plotting within the large OIB field and close to the MORB field (Rollinson, 1993). In a ϵ _Nd(t) vs ²⁰⁶Pb/²⁰⁴Pb diagram samples show linear trend toward higher ϵ _Nd(t) values plotting between the MORB and the BSE (Fig. 2e).

The analysed Evros ophiolite mafic rock samples have Pb isotope ratios that cluster close to those of the counterpart Jurassic mafic rocks (lavas and greenschists) from the Mandritsa unit of the CRB in Bulgaria, which in turn partly plot within the OIB field (closest to the MORB field) (Bonev & Stampfli, 2008). However, our samples from the Evros ophiolite show high ²⁰⁶Pb/²⁰⁴Pb and ²⁰⁷Pb/²⁰⁴Pb ratios. When the Mandritsa unit mafic rocks are compared to our Evros ophiolite samples, both rock suites display striking similarities in terms of Nd–Pb isotopic compositions (Fig. 2d, e).

4. Discussion and conclusions

The Nd isotope compositions obtained in this study are consistent with involvement of MORB reservoir, a compositional feature also displayed by trace element and REE geochemistry of the studied samples (Bonev & Stampfli, 2009; Bonev, 2020). The range of Nd isotopes is consistent with the values of the oceanic crust developed in the seafloor and arc-related settings such as refractory mantle peridotite. The Pb isotope data also suggest a contribution of MORB mantle source. Thus, the boninitic–tholeiitic basalt to andesites and gabbros exhibits Nd and Pb isotopic chemistry that suggests contribution from depleted MORB-type mantle source in the magma generation (Fig. 2). The relatively narrow range of ϵ _Nd(t) and Pb isotope values indicates that the mantle source of the mafic lavas and gabbros was rather homogeneous, depleted MORB-type lithospheric mantle.

The range of Sr isotopes also supports a MORB-type mantle component in the source region, but in addition shows the enrichment process via crustal contamination, explaining the variations and high ⁸⁷Sr/⁸⁶Sr ratio in samples Dd2 and GR44-9. The dyke Dd2 shows a deep negative Ce anomaly that has been interpreted in favour of involvement of the sediments in the subduction zone (Bonev & Stampfli, 2009). We infer a contribution from continental crust to explain the elevated ⁸⁷Sr/⁸⁶Sr ratios (0.7071 and 0.7075) observed in these samples. The negative ϵ _Nd(t) value of sample GR44-9 suggests that more likely subducted sediments were involved in the magma genesis. The fact that sample Dd2 belongs to the fore-arc segment, and that both it and sample GR44-9 are based on ⁸⁷Sr/⁸⁶Sr values, might well demonstrate a contribution of sedimentary input into the fore-arc region of the subduction zone. We infer, therefore, the involvement of MORB reservoir in the mantle source region mixed with crustal component in the source mantle of subduction zone from the fore-arc to arc edifice.

Our results highlight coherent Nd–Sr–Pb isotopic ratios of the Evros ophiolite mafic rocks, indicating generation of magmas by partial melting of a MORB-source mantle. These isotopic data indicate variable contamination of magmas by continental crust material and/or sediments entrained in the subduction zone (Fig. 3). These isotopic and compositional features are compatible with an intra-oceanic arc system represented by the Evros ophiolite in the Tethyan realm (Fig. 3), and are similar, for example, to the documented chemostratigraphy in some of the Pacific Ocean fore-arc–arc systems (Yu et al. 2020, 2022). Comparision of the Nd–Pb isotope results with analogous data from the Mandritsa unit mafic rocks demonstrates region-wide similarity of the isotopic compositions, which in turn provides additional support for the supra-subduction zone origin for the Evros ophiolite in the eastern CRB. Overall, the Nd–Sr–Pb isotopes systematics revealed mantle–crust interaction caused by mantle wedge magmatic process in supra-subduction zone Evros ophiolite.

Fig. 3. Simplified model of the Evros ophiolite Middle Jurassic intra-oceanic tectonic setting in the Tethyan realm accounting for the mantle–crust interaction in the mantle wedge as derived from the Nd–Sr–Pb isotopic results.