Introduction

A knowledge base about ancient oceans is of fundamental importance for palaeotectonic reconstructions of the Tethyan realm. Ophiolites and ophiolitic fragments are the only access that allows us to study those ancient oceans and have been studied in the Eastern Mediterranean region for several decades (e.g., Robertson 2002; Parlak 2016; Dilek and Furnes 2019 and references therein). Most ophiolites in the Aegean region are considered to have formed in a supra-subduction environment (e.g., Robertson 2002, Özkan et al. 2022 and references therein). Often, they are underlain by a so-called metamorphic sole, which represent high-grade metamorphic rocks that have formed during the inception of subduction beneath young and hot oceanic lithosphere (Wakabayashi and Dilek 2000, 2003; Wakabayashi et al. 2010; Dasçı et al. 2015; Mulder et al. 2016). Their ages (Ar–Ar, K–Ar) are mostly middle to late Jurassic (northwestern to eastern Aegean region; e.g., Spray et al. 1984; Dimo-Lahitte et al. 2001; Çelik et al. 2011; Hässig et al. 2013) or Cretaceous (southeastern Aegean region; e.g., Harris et al. 1994; Önen 2003; Çelik et al. 2006). For further age information, see Discussion. Rarely, Triassic rock fragments are preserved as for the island of Lesvos (Koglin et al. 2009) or the Karakaya Complex (Okay and Göncüoğlu 2004), which show N- to E-MORB geochemical signatures and within-plate geochemical signature, respectively.

However, little attention has been paid to ultramafic rock fragments from the southeastern part of Chios, a Greek island located in the eastern Aegean Sea only a few kilometres to the west of the Turkish Karaburun Peninsula (Fig. 1), which are located approximately in strike direction to the ultramafic body of Lesvos Island, north of Chios Island. In previous publications, serpentinites (Besenecker et al. 1968) and serpentinized peridotites (Zanchi et al. 2003) were mentioned (in text form only) from a narrow area in SE Chios Island. During fieldwork, we additionally found small exotic blocks of two types of metamorphosed mafic rocks, now amphibolites, which have not been described so far.

Fig. 1
figure 1

a Geographical sketch of the Aegean region showing Mesozoic ophiolite occurrences and the location of Chios Island. b Simplified geological map of Chios Island (modified after Besenecker et al. 1971). c Geological map showing sample locations of mafic–ultramafic rock fragments from SE Chios Island (modified after Besenecker et al. 1971). *According to the findings presented in this study, the Chios mélange with the mafic–ultramafic rocks fragments in SE Chios Island cannot be compared with the Chios mélange sensu stricto found in north and northwest Chios Island (see Sect. 5 for discussion). d and e Overview of the outcrop area and photograph of the outcrop of serpentinized peridotite north of Panaghia Sikelia in SE Chios Island

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In this study, we present the first data from these hitherto unknown rock fragments. Combining thin-section petrography, whole-rock geochemistry, mineral chemistry, and titanite U‒Pb geochronology, we unravel the possible origin and estimate the metamorphic pressure–temperature (P–T) conditions and the age of metamorphic overprint of the mafic rocks and of the serpentinized peridotite from SE Chios Island. The new data are compared with reference data of mafic and ultramafic rocks from the literature and allow correlation with Neotethyan ophiolites in Greece and Turkey.

Geological setting

The study area, Chios Island, is situated between the geological units of Greece and Turkey (Fig. 1). It is regarded as one of the key areas for understanding the closure history of the Palaeotethys as it includes virtually unmetamorphosed Palaeozoic to Mesozoic sedimentary rocks (e.g., Besenecker et al. 1968; Robertson and Pickett 2000). Simplified, Chios Island can be subdivided into two tectonostratigraphic units: an ‘autochthonous’ Lower Unit and a tectonically overlying ‘allochthonous’ Upper Unit (e.g., Besenecker et al. 1968; Meinhold et al. 2007) (Fig. 1b). The Lower Unit comprises the ‘Chios mélange’ (variably also named ‘Chios flysch’ or ‘Volissos turbidites’) and is commonly assumed to consist of Lower Carboniferous siliciclastic rocks containing blocks of various lithologies (for details see Robertson and Pickett 2000; Groves et al. 2003; Zanchi et al. 2003; Meinhold et al. 2007, 2008). The ‘Chios mélange’ is overlain by Lower Triassic conglomerates and sandstones, and Middle Triassic to Upper Jurassic limestones with few clastic intercalations (Besenecker et al. 1968).

The Upper Unit of Chios Island belongs to the allochthonous nappe sensu Besenecker et al. (1968) and consists of ?Upper Carboniferous quartzose sandstones and minor siltstones containing sporadic black and greenish chert lenses of few metres in size. The Lower Permian clastic-carbonatic succession (Kauffmann 1969) is overlain by a Middle Permian fossiliferous carbonate succession (e.g., Besenecker et al. 1968). Upper Permian rocks are absent. Within the Upper Unit, Triassic rocks are only locally preserved in the NE part of Chios Island, where red Anisian limestones, and Carnian limestones and volcanic tuffs occur (Besenecker et al. 1968).

In the southern part of Chios Island, Liassic limestones of the Upper Unit are unconformably overlain by locally fossiliferous sediments of Upper Cretaceous (Maastrichtian) age. Lower Tertiary (maybe also Miocene) conglomerates and marly sandstones crop out, which contain as clasts the whole lithological range of the older successions of Chios Island. Younger rocks, Neogene sediments and volcanic rocks (rhyolite and andesite), occur mainly in the southeastern part of Chios Island (Besenecker et al. 1968, 1971).

In southeastern Chios Island, north of Panaghia Sikelia (Fig. 1c‒e), serpentinite bodies of a few metres in size up to 250 m in length and 40–50 m in width, as well as garnet-bearing mica schist, occur (Besenecker et al. 1968; and own observations). The serpentinites were characterized by Elisabetta Rampone (in Zanchi et al. 2003, p. 214) as serpentinized mantle peridotite. In proximity to the serpentinites, we found small bodies of two types of metamorphosed mafic rocks (half to a few metres in diameter), which now can be described as amphibolites, that have not been reported so far. Both rock types form lenses within a chaotically deformed siliciclastic succession comprising mainly of sandstone and shale. There is no direct contact between the amphibolite and the serpentinite bodies. Because of the poor outcropping situation, no further information is available. Samples were taken at outcrop from the serpentinized peridotite and from the amphibolites for follow-up laboratory analyses (Table 1).

Table 1 List of samples, geographic coordinates (WGS84), and methods used for investigation
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Analytical methods

All samples were studied by thin-section petrography using a polarizing microscope. Major and trace elements were measured by X-ray fluorescence analysis (XRF) at the Johannes Gutenberg-University, Mainz, Germany, and the Federal Institute of Geosciences and Natural Resources (BGR), Germany. Rare earth elements (REE) measurements were carried out at the Max Planck Institute for Chemistry, Mainz, Germany, and Actlabs, Canada. X-ray diffraction (XRD) was measured at the Research Group Mineralogy, Martin Luther University Halle-Wittenberg, Germany. Mineral analysis on garnet (n = 65), amphibole (n = 66), plagioclase (n = 43), chrome spinel (n = 190), titanite (n = 36) , pyroxene (n = 69) and olivine (n = 13) were carried out at the Johannes Gutenberg-University using electron microprobe (EMP) analysis on polished thin sections. In situ U–Pb isotope analysis on titanite grains were carried out at the Goethe-University Frankfurt, Germany. A detailed description of the analytical methods is given as Supplementary material (see Supplementary file 1). The analytical data are given as Supplementary material (see Supplementary files 2–4). Representative EMP analyses are shown in Table 2.

Table 2 Representative electron microprobe analyses of plagioclase (1–4), garnet (5–8) and amphibole (9–14) from the amphibolites, and chrome spinel (15–20) from the serpentinized lherzolite of SE Chios Island
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Results

Petrography

Two types of amphibolite can be distinguished from the study area north of Panaghia Sikelia on SE Chios Island. The main difference is expressed by the amphibole contents as described in the following paragraphs. Thin-section photomicrographs can be found in the Supplementary file 5.

The amphibolites of Type I (samples CH74-1/-2) consist of calcic amphibole, saussuritisized plagioclase feldspar and minor albite, titanite, apatite, Fe-Ti oxides, epidote and garnet (Fig. S1 in Supplementary file 5). The amphiboles (up to 60 modal%) are prismatic and in some cases, they are elongated and show a preferred orientation (Fig. S1a in Supplementary file 5). They show a green to brownish colour. Some of the amphibole crystals have inclusions of euhedral plagioclase. The plagioclases (up to 30 modal%) are anhedral and located between the amphibole crystals. In some cases, they show the same preferred orientation as the oriented amphiboles. The plagioclase is pervasively saussuritisized (Fig. S1b in Supplementary file 5). Titanite is granular to anhedral, apatite appears granular (Fig. S1c, d in Supplementary file 5). The garnet is fractured and of metamorphic origin. It occurs within and in maximum 1 mm distance to cross-cutting felsic veins, which indicates a relationship of the garnet to these veins. The veins are filled with strongly elongated amphibole together with large anhedral calcite and epidote crystals.

The amphibolites of Type II (samples CH7 and CH75) are mainly composed of relatively fresh calcic amphibole with sizes up to 1 cm (up to 95 modal%). The crystals are subhedral-prismatic and the typical cleavage of the amphibole crystals is often visible (Fig. S2a in Supplementary file 5). Inclusions of granular titanite and euhedral to subhedral apatite within the amphibole crystals occur (Fig. S2b in Supplementary file 5). Further accessory phases are Fe-Ti oxides and strongly sericitized plagioclase feldspar. In contrast to Type I amphibolite, no garnet was observed.

The peridotite (sample CH76) is pervasively serpentinized. Serpentine minerals show typical mesh textures. XRD analytics revealed lizardite to be the main serpentine mineral. Minor phases are spinel and pyroxene (Fig. S3a in Supplementary file 5). The pyroxene crystals are large and are apparently almost completely serpentinized. The spinel crystals (up to 5 modal%) are anhedral and very heterogeneous in size (up to 1 mm) with a yellowish–brownish colour. They occur intracrystalline and appear to follow a preferred orientation (Fig. S3b in Supplementary file 5). There are many serpentine (± magnetite) veins of 1–2 mm thickness cross-cutting the peridotite where the spinel is altered to magnetite (Fig. S3c in Supplementary file 5).

Whole-rock geochemistry

Amphibolites of both types have been analysed for their whole-rock major, trace and REE composition to discriminate their tectonic environment. Their geochemical composition is presented in the Supplementary file 2.

The CI chondrite-normalized REE patterns of Type I amphibolite samples CH7 and CH75 are both parallel to normal mid-ocean ridge basalts (N-MORB) with slightly depleted light REE (LaN/SmN = 0.5–0.8) and flat heavy REE pattern (SmN/LuN = 1.0–1.3) with a small negative Eu anomaly (Fig. 2). Although parallel, sample CH75 is enriched in all REE compared to N-MORB. Sample CH7 shows slight enrichment in Th (Fig. 2b) but no clear Th-Nb–Ta anomaly. Compared to N-MORB but also to E-MORB, it also shows a strong enrichment in K, Rb and Ba. CH7 is also slightly enriched in Zr, Hf and Ti. Similar as for the REE pattern, sample CH75 parallels N-MORB but is enriched in all elements.

Fig. 2
figure 2

a CI chondrite-normalized REE patterns and b N-MORB-normalized multi-element pattern of amphibolites of Type II from SE Chios Island. Normalizing values are from McDonough and Sun (1995), patterns of N-MORB, E-MORB and OIB are from Sun and McDonough (1989)

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Type II amphibolite sample CH74 shows a REE pattern that parallels E-MORB (Fig. 2a) with enriched light REE (LaN/SmN = 1.4) and decreasing concentrations from middle to heavy REE (SmN/LuN = 1.3). It is slightly enriched in all REE compared to E-MORB. The same applies for the N-MORB-normalized pattern, which parallels the E-MORB pattern (Fig. 2b). CH74 is enriched in K, Rb, Ba and Th compared to E-MORB but no distinct Th-Nb–Ta anomaly is visible.

Major element composition of the serpentinized peridotite can be found in the Supplementary file 2. The CI chondrite-normalized pattern shows a strong depletion in the light REE with a slight negative Ce anomaly and a medium to slight depletion of the heavy REE in comparison to chondrite, what is the typical REE-pattern shape of lherzolite (e.g., Rampone et al. 2004, 2005; Fig. 3). Harzburgites, like that from the Austroalpine Speik Complex (Melcher et al. 2002), rather show a U- to V-shaped pattern (Fig. 3). The (La/Ce)N ratio > 1 may result from later LREE re-enrichment during serpentinization and the addition of aqueous fluids (Frey 1984).

Fig. 3
figure 3

CI chondrite-normalized REE patterns of the peridotite from SE Chios Island. Normalizing values are from McDonough and Sun (1995). Reference data: PCC-1 dunite and DTS-1 peridotite (Jain et al. 2000); Lesvos lherzolite (Koglin et al. 2009); Erro-Tobbio spinel lherzolite (Rampone et al. 2004, 2005); Speik Complex harzburgite (Middle Austroalpine) (Melcher et al. 2002)

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Mineral chemistry

Amphibole of the Type I amphibolite (CH74) can be classified according to the nomenclature of Leake et al. (1997) as being pargasitic to edenitic. Plagioclase is represented by albite with a composition of An1–3. Garnet is a homogeneous Ca-rich almandine. Their composition is in the range of Prp14–20, Alm43–47, Sps2–5 and Grs33–38. The Fe/(Fe + Mg) values range from 84 to 88. No compositional zoning in garnet has been observed (Fig. 4a), indicating that they were in equilibrium with surrounding minerals during their growth. Representative analyses of garnet, plagioclase and amphibole are shown in Table 2, all EMP analyses can be found in Supplementary file 4.

Fig. 4
figure 4

Mineral analyses by electron microprobe. a Backscatter image and profile of a garnet in Type I amphibolite. Mineral abbreviations: Pl plagioclase, Amp amphibole, Grt garnet, Cal calcite. b Element distribution map of a chrome spinel from the serpentinized peridotite measured by electron microprobe. White arrows point to areas of enrichment in Al, Mg and Cr, respectively. Black arrows indicate possible stress direction. Abbreviations of mineral names after Whitney and Evans (2010)

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Amphibole of Type II amphibolite (CH7 and CH75) is of pargasitic composition after Leake et al. (1997) (Table 2, Supplementary file 4).

The pyroxene of the serpentinized peridotite is strongly altered to bastite and EMP analyses yielded only serpentine composition. No pyroxene composition is preserved. The spinel shows the composition of a chromian spinel (Irvine 1965). Representative EMP analyses of chrome spinel are presented in Table 2, all EMP analyses can be found in Supplementary file 4. It is texturally homogeneous with Mg-numbers (Mg#) ranging from 69 to 79 and Cr-numbers (Cr#) ranging from 10 to 26. Al2O3 and TiO2 range from 40.0 to 57.9% and 0 to 0.085%, respectively. Chemical zoning was not observed. However, some rims have slightly lower Mg and Al and higher Cr contents (Fig. 4b), either influenced by secondary alteration or due to deformation (Ozawa 1989). Aluminium reacts mobile under deformation and migrates to areas of less stress. As a result, depletion of Mg and Al with regard to stress direction might be the result (Fig. 4b). Magnesium reacts mobile during equilibration with olivine what results in a relative depletion of this element and relative enrichment of Cr in the rims (e.g., Pirnia et al. 2018). At small cracks, the chrome spinel is transferred to magnetite due to oxidation of Fe2+ to Fe3+ (Fig. 4b-Fe) and the mobility of Al and Mg. In the Cr# vs. Mg# diagram of Dick and Bullen (1984), the chrome spinel data points plot in the field for abyssal and alpine-type peridotites (Fig. 5). Their calculated melting degree is very low with values mainly below 8% using the calculation of Hellebrand et al. (2002).

Fig. 5
figure 5

Chrome spinel compositions from the serpentinized peridotite of SE Chios Island plotted on the Mg# vs. Cr# diagram of Dick and Bullen (1984). Reference values for spinel: Ev Evia (n = 17; Laurent et al. 1991); Tr Troodos (n = 9; Gartzos et al. 1990); Ch Chalkidiki Ophiolite Complex (n = 16; Haenel-Remy and Bèbien 1985; Christofides et al. 1994); Le-Hz Lesvos harzburgite (n = 8; Migiros et al. 2000; Serelis 1995); Le-Lh Lesvos lherzolite (n = 81; Migiros et al. 2000; Serelis 1995; Koglin et al. 2009); Ro-Lh Ronda garnet- and spinel lherzolite (n = 10; Obata 1980); ET-SpL Erro-Tobbio spinel lherzolite (n = 14; Rampone et al. 2004, 2005)

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Thermobarometry

Geobarometric calculations on amphibole from both amphibolite types were done after Hammarstrom and Zen (1986), Schmidt (1992) and Anderson and Smith (1995). While those of Hammarstrom and Zen (1986) and Schmidt (1992) yielded similar results of 6.3–7.3 and 6.7–7.6 kbar, respectively, that from Anderson and Smith (1995) have an extended range of 5.1–7.6 kbar. Amphibole geothermometric estimations after Blundy and Holland (1990) for both amphibolite types gave temperatures between 640 and 683 °C. The Hbl–Plg geothermobarometer on 7 amphibole-plagioclase pairs from sample CH74 (see Supplementary file 4) after Holland and Blundy (1994) shows higher temperatures of 705–780 °C and somewhat lower pressures of 5.3–6.6 kbar, indicating slightly different pressure–temperature conditions for this amphibolite type. However, the calculated temperatures and pressures for both amphibolite types are consistent with the observed mineral paragenesis of Hbl–Plg–Alm (Yardley 1989; Okrusch and Matthes 2014, p. 500).

Titantite U–Pb dating

Titanite (sphene) has been analysed by U–Pb geochronology to constrain the age of their formation within the studied amphibolites from SE Chios Island.

In total, 51 single spot analyses have been obtained from titanite grains via in situ (in thin section) U–Pb geochronology. The titanite grains have U concentrations of 0.1 to 31.5 ppm (average ~ 7.7 ppm) and high common Pb concentrations (> 15%) (Supplementary file 3). The data were plotted in Tera-Wasserburg concordia diagrams to apply linear regression to obtain lower intercept ages with the concordia curve (Fig. 6). Titanite grains of garnet-free amphibolite sample CH7 yielded a lower intercept 206Pb/238U age of 162.2 ± 4.9 Ma (2σ, n = 17). Titanite grains of garnet-bearing amphibolite samples CH74-1 and CH74-2 yielded lower intercept 206Pb/238U ages of 172.3 ± 6.1 Ma (2σ, n = 20) and 167.0 ± 6.5 Ma (2σ, n = 14), respectively.

Fig. 6
figure 6

Tera-Wasserburg concordia diagrams for titanite grains from amphibolites of SE Chios Island. Error ellipses are at the 2-sigma level and coloured according to total U concentration

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Discussion

The amphibolites and serpentinized ultramafic rock fragments from SE Chios Island represent further jigsaw pieces in the history of Tethys and in reconstructing its palaeotectonic evolution in the Eastern Mediterranean region.

Amphibolites

There are two types of amphibolite on Chios Island: Type I has a mineral assemblage that indicates a basaltic origin and a geochemical signature of enriched mid-ocean ridge basalt (E-MORB). Type II shows a normal mid-ocean ridge affinity with a small Eu anomaly. The latter reflects intracrustal differentiation such as partial melting or fractional crystallization, which removes Eu-enriched plagioclase and leaves an amphibole-rich residual phase (Cullers and Graf 1984). No contact between the two types is visible, therefore, a common genesis is questionable. Neither of the amphibolites is texturally or structurally related to the serpentinite blocks so, they cannot clearly be identified as lithologies incorporated into a metamorphic sole. We note that metamorphic sole rocks are usually associated with ophiolites that formed in a supra-subduction zone (SSZ) environment (e.g., Wakabayashi and Dilek 2000, 2003; Wakabayashi et al. 2010; Dasçı et al. 2015; Mulder et al. 2016), which is not the case for the Chios amphibolites, which are of N-MORB composition. Wakabayashi and Dilek (2000) present a model for the emplacement of a SSZ ophiolite over a different subduction zone than that over which it formed. This model could be adapted to the formation of the Chios rocks, but with the difference that the mafic protoliths of the amphibolites have formed as oceanic crust in a mid-ocean ridge environment. After formation of the protoliths, a new subduction zone formed followed by obduction of the overriding plate and formation of the N-MORB-type metamorphic sole under middle to upper amphibolite-facies metamorphism. This would explain a possible older age of the ophiolitic rocks compared to the formation age/metamorphic age of the metamorphic sole and the N-MORB and E-MORB nature of the amphibolites. The formation of the amphibolites as part of a metamorphic sole would also explain the difference in metamorphic facies compared to the low-temperature serpentinized peridotite. In this case, the amphibolites became part of the ophiolite structural block due to tectonic transport after amphibolite-facies metamorphism (Wakabayashi and Dilek 2000). Similar rocks (unmetamorphosed basalts, and amphibolites from a metamorphic sole of N-MORB composition) are described from Lesvos Island (Gartzos et al. 2009; Koglin et al. 2009). The latter having Ar–Ar and K–Ar ages related to the obduction and formation of the metamorphic sole of ca. 150–160 Ma (Hatzipanagiotou and Pe-Piper 1995; Gartzos et al. 2009) that are slightly younger but comparable to the U–Pb titanite ages of the amphibolites of Chios. Further similar rocks are described by Koepke and Seidel (2004) from a mafic–ultramafic rock suite on Crete Island (southern Greece) and are interpreted as a suite of cumulates intruding the Cretan lherzolitic peridotite in Jurassic times.

Titanite grains from the amphibolites of SE Chios have low U contents and high common Pb (Supplementary file 3). Such values are commonly found in metamorphosed metabasites, including mafic gneiss, amphibolitized diabase dikes, and metabasalts (Frost et al. 2000). Dating of titanite grains of the amphibolites yielded early Middle to early Late Jurassic U–Pb ages, which we interpret as the age of metamorphism. A reasonable minimum estimate of the closure temperature of Pb diffusion in titanite is ~ 660–700 °C (Scott and St-Onge 1995; Frost et al. 2000). Recent studies on natural titanite provide evidence that the closure temperature of Pb diffusion in titanite may be up to 800 °C (or even slightly higher) (Kohn 2017; Hartnady et al. 2019). The spread of ages among the three samples could be related to low U contents and high common Pb of the analysed titanite grains. In view of their different geochemical characteristics (N-MORB / E-MORB) and a possible spatial distance, it is also assumable that they might have experienced metamorphism slightly shifted in time. The formation age of the protoliths of the amphibolites is unknown but must be older than the age of metamorphic overprint (i.e. older than Middle Jurassic).

Serpentinized peridotite

The peridotite of Chios is located approximately in strike direction of the ultramafic rocks of the Lesvos and is pervasively serpentinized. The main serpentine mineral is lizardite, which indicates temperatures below c. 300 °C (Schwartz et al. 2013; Deng et al. 2022). The REE pattern of the studied peridotite sample is parallel to the fertile serpentinized lherzolitic rocks of the neighbouring Lesvos Island and overlaps with spinel lherzolites from the Erro-Tobbio peridotite body in the Ligurian Alps (northwestern Italy), which is interpreted as a fragment of the subcontinental lithospheric mantle (Rampone et al. 2004, 2005; Fig. 3). Depleted harzburgitic and dunitic mantle rocks rather form a trough-shaped pattern with enriched light REE (Fig. 3) The Chios serpentinite contains chrome spinel indicating an affiliation to abyssal and alpine-type peridotites with very low Cr# and melting degrees (Fig. 5). They resemble spinel from lherzolitic peridotite of Crete Island (Koepke et al. 2002; Fig. 5) and the lherzolite from the Kallidromon Mountains of central Greece (Karipi et al. 2007). The Chios chromites are also similar to spinel from the fertile subcontinental lithospheric mantle in the Ronda massif of southern Spain (Obata 1980).

Affiliation to Tethyan ophiolites in the Eastern Mediterranean

The affiliation of the amphibolites and ultramafic rocks to Tethyan ophiolitic fragments in the Eastern Mediterranean is uncertain and reflects along-strike variation of these ophiolite belts. A correlation with mafic–ultramafic rocks exposed in neighbouring parts of Greece or in Turkey remains problematic, although similar Jurassic metamorphic ages have been reported from a number of Mesozoic ophiolites and ophiolitic mélanges in Greece and Turkey (see Çelik et al. 2011; Hässig et al. 2013).

The serpentinized peridotites were considered by Zanchi et al. (2003) as olistoliths assigned to the Lower Carboniferous succession of the Lower Unit of Chios Island. However, this is unlikely, because petrological, mineral chemical and field data suggest that the amphibolites and ultramafic rocks from SE Chios are fragments within a mélange, likely of tectonic origin, emplaced to their present-day location in latest Mesozoic or Cenozoic times. The chemical composition of detrital chrome spinel from the Lower Unit of Chios Island (Meinhold et al. 2007) does not resemble the chemical composition of chrome spinel from the serpentinized peridotites (this study). Also, detrital chrome spinel grains from a comparable Carboniferous–Triassic sedimentary succession of the neighbouring Karaburun Peninsula (Löwen et al. 2018) do not resemble the chemical composition of chrome spinel from the serpentinized peridotites of Chios. This suggests that the ultramafic and likely also the amphibolite rock fragments of Chios were not deposited during the late Palaeozoic–Early Mesozoic.

The two amphibolite types and the serpentinized peridotite more likely represent fragments of an early Neotethyan oceanic crust and upper mantle that formed close to a continental margin and were emplaced (obducted) onto the continental margin in Jurassic time. At present, there are three possible correlations that can be made:

  1. (i)

    Due to the geographical proximity, a relation to mafic‒ultramafic fragments within the Bornova Flysch Zone (BFZ) in western Turkey may be possible, although mineral chemical and geochronological reference data are scarce. The BFZ is a regional olistostrome–mélange belt located between the İzmir–Ankara suture zone in the northwest and the Menderes Massif in the southeast (Okay et al. 2012). The BFZ consists of chaotically deformed upper Maastrichtian to Palaeocene sandstone and shale with blocks of Triassic to Cretaceous limestones and rarer blocks of serpentinite, peridotite, gabbro, basalt, and radiolarian chert of Middle Triassic to Cretaceous in age (Okay and Siyako 1993; Robertson and Pickett 2000; Okay et al. 2012).

  2. (ii)

    The amphibolites and serpentinized peridotite from SE Chios Island could be correlated to the mafic‒ultramafic rock suite from the neighbouring island of Lesvos (Fig. 1) where basaltic rocks with normal and enriched mid-ocean ridge character and similar metamorphic ages occur (see Hatzipanagiotou and Pe-Piper 1995; Pe-Piper et al. 2001; Migiros et al. 2000; Gartzos et al. 2009; Koglin et al. 2009). The Lesvos lherzolite resembles the peridotite from Chios Island not only in geochemistry (Fig. 3) but also with the low spinel Cr# values of 0.11–0.37. They possibly are representative of an incipient continental rift setting during Permian–Triassic time (Koglin et al. 2009) that led to the formation of a branch of Neotethys. In this case, the amphibolites and very juvenile ultramafic rocks of Chios would represent the very beginning of this rifting stage.

  3. (iii)

    Alternatively, the amphibolites and ultramafic rocks from SE Chios Island could be correlated to similar mafic‒ultramafic rock suites of Crete. Koepke et al. (2002) described lherzolite relics within serpentinites similar to those of Chios in chrome spinel composition (Cr# ~ 10–20; Fig. 5). The mafic‒ultramafic rock suite described by Koepke et al. (2002) from Crete also contains hornblende-rich rock slices that are similar in geochemistry and age to the amphibolites of Chios (Koepke and Seidel 2004).

At present, all three models seem plausible. New detailed geological mapping of parts of Chios Island where mafic and ultramafic rock fragments are exposed may provide additional insights into their origin, relationship amongst each other and relation to similar rock assemblages in other parts of the Aegean region. It can be speculated that in the eastern and southern Aegean region such mafic bodies with Middle–Upper Jurassic metamorphic ages and ultramafic rocks fragments with very low Cr# were once part of the same Neotethyan palaeo-margin and were later disassembled and tectonically transported to their present location during the late Mesozoic and Cenozoic. The whole area was modified by strike-slip and large-scale extensional tectonics that complicate palinspastic reconstructions. However, we believe that the strong similarities in geochemistry, mineral geochemistry and timing of metamorphism to the rocks of Lesvos Island indicate that the amphibolites and ultramafic rocks of Chios may have formed during the Permian–Triassic as those from the island of Lesvos.

Conclusions

Hitherto unknown mafic‒ultramafic rock slivers were found on the Aegean island of Chios. They consist of two types of amphibolite and of serpentinized peridotite, which show the following characteristic:

  1. (1)

    Type I amphibolite consists of pargasitic to edenitic amphibole, plagioclase and mainly Ca-rich garnet. Its geochemical composition resembles E-MORB. Titanite U–Pb dating revealed Middle Jurassic metamorphic ages of 172 ± 6 Ma and 167 ± 7 Ma. P–T estimations on amphibole only and amphibole-plagioclase pairs indicate metamorphic conditions of 705–780 °C and 5.3–6.6 kbar.

  2. (2)

    Type II amphibolite is composed of mainly pargasite and shows N-MORB geochemical character. Titanite U–Pb dating revealed late Middle Jurassic metamorphic ages of 162 ± 5 Ma. P–T estimations on amphibole yielded values of 640–680 °C and 5.1–7.6 kbar.

  3. (3)

    The peridotite is pervasively serpentinized containing lizardite as the main serpentine mineral, which indicates maximum temperatures of 300 °C. The whole-rock geochemical signature is similar to fertile (subcontinental) lherzolite. The chrome spinel has very low Cr# between 10 and 26 pointing to melting degrees below 8%.

At present, a relationship to three areas within the Aegean region might be assumed: the mafic–ultramafic blocks within the Bornova Flysch Zone, the mafic–ultramafic rocks of Lesvos Island and the mafic and ultramafic rocks of Crete Island. Due to similarities in geochemistry of both, the amphibolites and the peridotites, and the metamorphic age of the amphibolites, we prefer a connection to the mafic and ultramafic rocks of Lesvos.