2. Geological setting

Inousses (Fig. 1) and its adjacent islets are located east of Chios and west of the Karaburun peninsula of the Turkish mainland to the east. Inousses itself consists of a flysch-like sequence (Kilias, Reference Kilias1987) which can be lithostratigraphically subdivided into a lower and an upper part (see Meinhold, Kostopoulos & Reischmann, Reference Meinhold, Kostopoulos and Reischmann2007). The lower part is exposed in the south of the island and adjacent islets to the east. It comprises medium- to coarse-grained metasediments including light grey metasandstones and metagreywackes with intercalations of light grey to greenish-grey conglomerate layers. The latter pass upwards into medium- to very-fine-grained metasedimentary rocks including argillaceous metagreywackes and dark grey to black metapelites (phyllites). This lithostratigraphically upper part crops out mainly in the N and NW part of the island. The main structural feature on Inousses is multiple folding with two axial trends (e.g. Kilias, Reference Kilias1987). The major foliation is folded with gently NNW-dipping fold axes. Besenecker et al. (Reference Besenecker, Dürr, Herget, Jacobshagen, Kauffmann, Lüdtke, Roth and Tietze1968) assumed that the epimetamorphic flysch sequence of Inousses is older than the Palaeozoic of the autochthonous Lower Unit of Chios and thus forms its metamorphic basement. Mountrakis et al. (Reference Mountrakis, Sapountzis, Kilias, Eleftheriadis and Christofides1983) correlated the Inousses metamorphic rocks with the Permian–Lower Triassic metasedimentary rocks of the western margin of the Pelagonian Zone of continental Greece and the Sporades Islands. Kilias (Reference Kilias1987) also assumed an affiliation of the Inousses metamorphic rocks to the Pelagonian nappes. All this contradicts the interpretation of Kozur (Reference Kozur and Szaniawski1998) who compared the metasedimentary succession of Inousses with the monotonous siliciclastic Küçükbahçe Formation of the Karaburun peninsula of probable Ordovician (or Cambrian–Ordovician) age (Kozur, Reference Kozur and Szaniawski1998). Based on a chemostratigraphic approach, Meinhold, Kostopoulos & Reischmann (Reference Meinhold, Kostopoulos and Reischmann2007) recently suggested that the protoliths of the metasedimentary succession of Inousses originated from acidic magmatic and sedimentary sources and were deposited in a continental island-arc setting probably in Late Palaeozoic or Triassic times. The succession has been assigned to the Pelagonian nappes of mainland Greece.

Psara is located c. 20 km west of Chios Island in the Aegean Sea (Fig. 1) and consists of two tectonostratigraphic units (see Meinhold, Kostopoulos & Reischmann, Reference Meinhold, Kostopoulos and Reischmann2007, for discussion). The Lower Unit predominantly consists of dark grey metapelites (phyllites), metasandstones and metagreywackes. The major foliation strikes approximately N–S and dips steeply either E or W. The Upper Unit mainly comprises mica schists (with garnets up to 0.8 cm in diameter) intercalated with calcschists and marble layers that pass up into pure marbles at the top. The quartz–muscovite–chlorite–garnet–biotite assemblage of the mica schists suggests mid-upper-greenschist-facies metamorphic conditions (Meinhold, Kostopoulos & Reischmann, Reference Meinhold, Kostopoulos and Reischmann2007). The predominant foliation dips mostly S and is folded with fold axes dipping gently E or W. Unfortunately, in both units neither fossils nor radiometric data are yet available, thus complicating their stratigraphic affiliation. Cenozoic volcanic rocks crop out in the northwestern and central part of Psara. The tectonostratigraphic affiliation of Psara to units in Greece is undecided. Mountrakis et al. (Reference Mountrakis, Sapountzis, Kilias, Eleftheriadis and Christofides1983) correlated both units of Psara with the Subpelagonian Zone of continental Greece, whereas Wallbrecher (in Dürr & Jacobshagen, Reference Dürr, Jacobshagen and Jacobshagen1986) favoured an affiliation to successions from the southern Pelion Peninsula and the northern Sporades of the Pelagonian Zone. Based on a chemostratigraphic approach, Meinhold, Kostopoulos & Reischmann (Reference Meinhold, Kostopoulos and Reischmann2007) recently suggested that the protoliths of the metasedimentary rocks from the Lower Unit and Upper Unit of Psara Island were deposited in a continental island-arc setting and mainly originated from acidic magmatic and sedimentary sources. The abundance of carbonate in the Upper Unit indicates a shallow marine environment. The very-low-grade metasedimentary rocks of the Lower Unit are correlated with the Skiathos Unit, whereas the more carbonate-dominated Upper Unit is assigned to the Pelagonian marble nappe of the northern Sporades Islands and the nearby Pelion Peninsula.

3. Analytical method

For U–Pb geochronology, zircons were separated from the bulk samples using standard techniques (hydraulic press, rotary mill, Wilfley table, Frantz isodynamic magnetic separator and heavy liquids (methylene iodide)). Final purification was carried out by hand-picking under a binocular microscope. Zircon grains were set in epoxy resin mounts, sectioned and polished to approximately half their original thickness. Prior to the analyses, cathodoluminescence (CL) images were obtained for all grains to study their internal structure and to target specific areas within them, e.g. growth structures and inherited cores. The U–Pb isotopic analyses of individual zircon grains were performed using a Thermo-Finnigan Element II sector-field ICP–MS system coupled to a Merchantek/NewWave 213 nm Nd-YAG laser system at the Geological Survey of Denmark and Greenland, Copenhagen, Denmark. The method applied followed that described by Gerdes & Zeh (Reference Gerdes and Zeh2006) and Frei et al. (Reference Frei, Hollis, Gerdes, Harlov, Karlsson, Vasquez, Franz, Johansson and Knudsen2006). For the interpretation of the zircon data, analyses with 95–105% concordance (calculated from 100(²⁰⁶Pb–²³⁸U age)/(²⁰⁷Pb–²³⁵U age)) are considered to be concordant. Analyses with a discordance >10% were rejected and consequently not considered for data interpretation. Unless stated otherwise, ²⁰⁶Pb–²³⁸U ages are used for zircon grains <1.2 Ga whereas older grains are quoted using their ²⁰⁷Pb–²⁰⁶Pb ages. This is because the ²⁰⁷Pb–²⁰⁶Pb ages become increasingly imprecise below 1.2 Ga due to small amounts of ²⁰⁷Pb. The ²⁰⁷Pb–²⁰⁶Pb ages are generally considered as minimum ages due to the effect of possible Pb loss. Concordia diagrams and probability density distribution and histogram plots were produced using the programs Isoplot/Ex (Ludwig, Reference Ludwig2003) and AgeDisplay (Sircombe, Reference Sircombe2004) respectively. Unless stated otherwise, ages reported in the text are given at the 2-sigma level. The Geological Time Scale (GTS) of Gradstein, Ogg & Smith (Reference Gradstein, Ogg and Smith2004) was used as stratigraphic reference for data interpretation. Isotopic data referred to in this paper are given in Appendix Tables A1 and A2, available as supplementary material online at http://multimediafluids.uam.elogim.com/journals/geo. Here, the reader is informed that it is likely statistically that a zircon population remains unrecognized because of the small number of analysed zircons (see Vermeesch, Reference Vermeesch2004), although we consider the zircon populations of both samples to be representative.

4. Geochronological results

4.a. Inousses

Sample IN8 was collected from a metalitharenite succession cropping out north of Inousses village (38° 31′ 17.2′ N, 26° 12′ 59.6″ E; Fig. 1b). The sample predominantly consists of monocrystalline and polycrystalline quartz, sedimentary and altered volcanic lithoclasts, minor muscovite flakes, feldspar and strongly altered detrital biotite. Accessory minerals include sericite and heavy minerals such as zircon, tourmaline, opaque minerals and epidote. The zircons have a subhedral to well-rounded shape. They are clear to cloudy, colourless to slightly pinkish. The length of single zircon crystals varies between 60 and 200 μm. Most analysed zircons have clear oscillatory zonation patterns in CL images and appear to be magmatic in origin; few exhibit no zoning or patchy zoning (Fig. 2). Inherited cores are present to a minor degree. The concordance-filtered zircon ages show a polymodal age distribution with major groups of 310–350, 450–500, 550–700, 900–1050 and 1880–2040 Ma, and also few ages between 2600 and 2800 Ma (Fig. 3). Zircon ages between 1050 and 1880 Ma are totally absent. The youngest concordant zircon grains are slightly rounded and have ²⁰⁶Pb–²³⁸U ages of 310 ± 9 to 330 ± 18 Ma, which indicates the maximum age of deposition to be Late Carboniferous.

Figure 2.

CL images of representative zircon grains from analysed samples with location of the LA–SF–ICP–MS analysis spot and corresponding ²⁰⁶Pb–²³⁸U age (±2σ) for grains <1.2 Ga and ²⁰⁷Pb–²⁰⁶Pb age (±2σ) for grains >1.2 Ga, respectively. Letter–number code above the ages: sample-spot. The scale bar represents 30 μm in all images.

Figure 3.

(a, b) Concordia diagrams and (c, d) probability density distribution and histogram plots for the set of U–Pb analytical zircon data from samples of Inousses and Psara. Because of illustration purposes one highly discordant grain of sample CH19 with a ²⁰⁷Pb–²⁰⁶Pb age of 2116 Ma is not shown in the concordia plot. A 1.2 Ga limit is used to switch between ²⁰⁶Pb–²³⁸U and ²⁰⁷Pb–²⁰⁶Pb ages for the probability plots. n=number of analyses.

5. Discussion and conclusions

Detrital zircon age spectra from samples of Inousses and Psara differ but both show a prominent age cluster between c. 300 and 330 Ma that indicates denudation of Carboniferous and Lower Permian basement rocks. These could have been rocks of the Pelagonian Zone (including the Attic–Cycladic Massif), from Sakarya or the Thracia Terrane of the Rhodope Massif, since Carboniferous and Early Permian ages of that range are documented for magmatic rocks from these areas (e.g. Engel & Reischmann, Reference Engel and Reischmann1998; Özmen & Reischmann, Reference Özmen and Reischmann1999; Vavassis et al. Reference Vavassis, De Bono, Stampfli, Giorgis, Valloton and Amelin2000; Reischmann et al. Reference Reischmann, Kostopoulos, Loos, Anders, Avgerinas and Sklavounos2001; Anders, Reischmann & Kostopoulos, Reference Anders, Reischmann and Kostopoulos2007; Okay, Satır & Siebel, Reference Okay, Satir, Siebel, Gee and Stephenson2006; Turpaud, Reference Turpaud2006; Cornelius et al. Reference Cornelius, Reischmann, Frei and Kostopoulos2007). They are assigned to a major igneous event in the Aegean and the surrounding area belonging to a volcanic-arc or active-continental-margin setting (e.g. Reischmann et al. Reference Reischmann, Kostopoulos, Loos, Anders, Avgerinas and Sklavounos2001) due to northward subduction of (a branch of) Palaeotethys beneath Pelagonia and Sakarya (e.g. Vavassis et al. Reference Vavassis, De Bono, Stampfli, Giorgis, Valloton and Amelin2000).

Furthermore, both samples show age clustering between c. 350 and 650 Ma, although in varying amount, that indicates denudation of Lower Palaeozoic and Upper Neoproterozoic basement rocks. Sample IN8 from Inousses especially contains a significant amount of 550–690 and 450–485 Ma grains (Fig. 3). The former age group can be assigned to Pan-African basement rocks, which are widespread in the Eastern Mediterranean (e.g. Meinhold et al. Reference Meinhold, Reischmann, Kostopoulos, Lehnert, Matukov and Sergeev2008, fig. 10). The Middle–Late Ordovician ages are good constraints for palaeotectonic reconstructions in the Eastern Mediterranean since these ages are restricted to few terranes only. They have been reported from basement rocks of the Sakarya Terrane in western Turkey (Özmen & Reischmann, Reference Özmen and Reischmann1999) and of the Serbo-Macedonian Massif in northern Greece and Bulgaria (e.g. Titorenkova et al. Reference Titorenkova, Macheva, Zidarov, von Quadt and Peytcheva2003; Himmerkus et al. Reference Himmerkus, Anders, Reischmann, Kostopoulos, Hatcher, Jr, Carlson, McBride and Martínez Catalán2007; Meinhold, Reference Meinhold2007). In general, the lack of zircon ages between 1.1 and 1.8 Ga, coupled with the occurrence of c. 2-Ga-old zircons in the sample of Inousses, imply a northern Gondwana-derived source.

The maximum depositional age of the studied metasedimentary rocks can be constrained by the youngest detrital zircons: c. 310–330 Ma for sample IN8 from Inousses and c. 270 Ma for sample CH19 from Psara. The zircon age data clearly show that the metasedimentary successions of Inousses cannot be older than Late Carboniferous. This contradicts the interpretation of Kozur (Reference Kozur and Szaniawski1998) who compared the metasedimentary rocks of Inousses with the monotonous siliciclastic Küçükbahçe Formation of the Karaburun peninsula for which he suggested an Ordovician (or Cambrian–Ordovician) age. Here, we suggest that the metasedimentary rocks of Inousses and Psara were deposited far apart, but close to the southern margin of Laurussia in Permian–Triassic time (Fig. 4).

Figure 4.

Palaeotectonic map for the Permian–Triassic (modified from Stampfli & Borel, Reference Stampfli and Borel2002). The light grey areas are initial rift basin systems; the black dots show the presumed position of Inousses Island and the Upper Unit of Psara Island. AA: Austro-Alpine; Ap: Apulia; DH: Dinarides–Hellenides; Is: Istanbul; MD: Moldanubian; Mn: Menderes; Mo: Moesia; Pl: Pelagonian; RH: Rheno-Hercynian; Sk: Sakarya; SM: Serbo-Macedonian; SS: Sanandaj–Sirjan; Ta: Taurus; Th: Thracia; Tz: Tizia; Zo: Zonguldak.

To summarize, the Early Palaeozoic ages in the studied sedimentary successions in conjunction with the Carboniferous–Early Permian ages support a source from terranes at the southern margin of Laurussia during the Late Palaeozoic and hence clarify the palaeotectonic position of the metasedimentary units of Inousses and Psara within the Palaeotethyan realm.