K-Ar Dating of Ophiolites from Rhodes and Karpathos Islands, Dodekanese, Greece

GEOLOGICA BALCANICA, 21. 5, Sofia, Oct. 1991, p. 69-76.

K-Ar dating of ophiolites from Rhodes and Karpathos islands, Dodekanese, Greece

K. Hatzipanagiotou

Department of Geolog'y, University of Patras, 26110 Patras, Greece

(Received 18. 06.1990; accepted 18. 09. 1990}

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Ab1tract. Geochronology of the ophiolites of Rhodes and Kar}!lathos islands, Dodecanese, Greece, has been done by dating mineral concentrates from magmatic (hornblende gabbros) and metamorphic rocks at the base of ophiolites (amphibolites) by the K-Ar metnod. The dated rocks come from a heterogeneous chaotic mixture of the ophiolitic melange. K-Ar dating revealed different ages for the magmatic and metamorphic rocks, that is 82-88 Ma (Upper Cretaceous) and 160-180 Ma (Middle Jurassic), respectively. Based on these results, the ophiolites of Rhodes and Karpathos can be connected to the Upper Cre taceous ophiolite belt of the Taurides, on the one hand, and the Middle Jurassic one of the Hellenides Dinarides, on the other. These two islands therefore have an intermediate position between the ophiolite belts of Taurides to the northeast and Hellenides-Dinarides to the northwest.

Geological setting

The islands of Rhodes and Karpathos, southwestern Aegean Sea, are characterised, accor ding to recent studies, by large-scale nappe tectonics. An autochthonous sequence of pla ty limestones (Plattenkalk) is overlain by a nappe-pile consisting of four major nappes. Clastic sediments, mainly of Tertiary and Quaternary age, are transgressively overlying the previously mentioned geological units (Christo do u I o u, 1960, 1963; Mutt i et al., 1970; Davidson-Monett, 1974; Baumann et al., 1977; Hartung, 1982; Seidel, Wachendorf, 1982; Hatzipanagiotou, 1988). The uppermost allochthonous unit of Rhodes consists of an ophiolitic melange (in the sense of G an sse r, 1974) at the base and a relictic ophiolitic nappe on top. A charac teristic feature of the ophiolitic melange is the presence of a heterogeneous chaotic mix ture of different rock types. The formation of this mixture is ascribed to a combination of sedimentary and tectonic processes. On the island of Rhodes the ophiolitic melange has a thickness of approximately

69 300 m and covers an area of 30 km~ at the NW part of the island, whereas the ophio litic nappe occurs in minor outcrops at the NE part (Fig. 1). On the island of Karpathos (Fig. 2) the uppermost nappe is made up only of minor occurrences of ophiolitic melange, about 200m thick, cropping out at the central and south ern parts (H at z i p a n a g i o t o u, 1988). The ophiolitic melange of these two islands consists of a variety of rock types such as serpentinites, pyroxenites, gabbroic and basaltic rocks, tuffs, shales, radiolarites, neri tic and pelagic sediments. Moreover, within the ophiolitic melange of Rhodes exotic blocks of metamorphic rocks were found. On that island, the overlying ophiolitic nappe consists exclusively of serpentinized peridotites, locally crossed by microgabbroic dykes. Rock samples, suitable for isotopic add geochronological work of the ophiolitic com plex, have been taken from the heterogeneous chaotic mixture of Rhodes and Karpathos. The petrological types selected for this purpose were hornblende-gabbros and amphibo lites.

Petrography

Hornblende-gabbros

The hornblende-gabbros of Rhodes are characterized by the mineral paragenesis of brown hornblende- plagioclase. Metamorphic overprint is indicated by a slight orientation of the mineral constituents and by occasionally well-formed amphibole porphyroblasts. Greenish-brownish, hypidiomorphic to xenomorphic amphibole together with also hypidiomorphic plagioclase are the main constituents of the samples 124K, 215K and 237K (locality: north of Kopria bay). Brown hornblende, with pronounced cleavage, 2 mm in diameter and showing a strong pleochroism, constitutes the predominant mineral phase; na=pale yellow-green to pale green-brown< nh = yellowish-green to yellowish brown

70 Rhodes

RHOD£$

Psinthos

Mandnko

NEOAUTOCHTHON CJ Pliocene-Pleistocene Transition from Flysch (c¥#111 to Mola sse ALLOCHTHON lllllll Ophiolitic nappe(l.) and . u. ophiolitic melange (r. ) ITIIllJI] Profi tis I 1i as Group ~ Archangelos Group

~ Tectonic melange (e .g.Flysch ~ of Larma) AUTOCHTHON Ffi?=i1 Atavi ros and Li ndos ~ Li mestones 12km

Fig. I. Geological sketch map of Rhodes

71 KARI'A THOS

NEOAUTOCHTHON

~ Oligocene -Pleistocene

ALLOCHTHON Ophiolitic melange

Xindothio series Spoa- Kali limni, Assomata and Menete series Anginara-Adra Flysch

AUTOCHTHON !'I5:5] Carbonate series of Anginara, tr:Jt:D Agios Nikolaos and Adra Xindothio--

Skm

Fig. 2. Geological sketch map of Karpathos

72 is altered. Clinopyroxene relics are sometimes preserved and are usually entirely replaced by green amphibole. Magnetite aggregates, as well as sphene and apatite are present in subordinate to accesory amounts.

Metamorphic rocks of Rhodes Metamorphic rocks crop out in the western part of Rhodes island in form of tectonic 2 blocks within the ophiolitic melange. A large outcrop of around 100 m , in form of ter rasses occurs in northwest Kritinia, near Mavri. The metamorphic rocks are bounded by fine-grained diabases and overlain by Neogene sediments of Poros. Another outcrop, a few m2 large, with the same rock association lying about 150 m higher, was found further to the northwest. In the outcrop of the Mavri area, the following metamorphic rocks can be identified: (l) fine-grained, homogeneous, dark-coloured to black-green amphibolites (2) banded calc-silicate rocks consisting of fine-grained layers made. of amphibolites and a pyroxene+ garnet rock. Alteration of the above layers which are a few millimeters up to centimeter thick and have a dark-green and dirty pink results to the characteristic banding of these rocks. (3) Metapelites which are the most extended rock-type. Dark-coloured mica-rich layers are alterated with light-coloured quartz-rich parts; these layers are also a few milli meters up to centimeter thick.

Amphibolites The main mineral constituent of the amphibolites is a hypidioblastic, fine-grained horn blende with a diameter of 0.2-0.7 mm showing a slight s-parallel orientation. Moreover, isolated hornblende i>Orphyroblasts, 2-3 mm large are also present. Strong pleochroism is distinguished: na=pale-green nb< = yellowish-brown-green nc< =brownish-green to olive green. In some cases. hornblende shows retrograde replacement by actinolite or chlorite, the alteration starting from the crystal rims. Plagioclase forms xenoblastic grains 0.2- 0.5 mm large, developed at the hornblende edges and is twinned according to the albite and occasionally the albite-pericline law. Determination of the anorthite-content yielded values between 40-50 mol %. Fine, rounded sphene aggregates occur as inclusions in horn blende crystals, as well as in the interstices of the mineral grains. Sphene is often rim ming ore aggregates, 0.1-0.3 mm in diameter. Besides actinolite and chlorite, prehnite and calcite were found as secondary phases; moreover, quartz forms small, accessory, xeno blastic grains.

Geochronology

In order to determine the age of metamorphism of the gabbroic rocks of Rhodes and Karpathos three samples of hornblende gabbros have been dated radiometrically by the K-Ar method. Also for the same purpose hornblende concentrates from three amphibolite samples were prepared. The mineraLconcentrates were prepared by magnetic and heavy liquids separation in the Mineralogisches Institut of the Braunschweig Technical University under the supervi~ sion of Prof. E. Seidel. The Sieve fraction 63-125 mm was used. The geochronological work has been done in the geochronological laboratory of the Bundesanstalt fur Geowis senschaften und Rohstoffe in Hannover, under the supervision of Dr. H. Kreuter. Argon isotope analyses were obtained by a digitalised mass spectometer (MAT CH4). K-deter mination was performed by use of a digitalised flame photometer (EEL 170). Errors are within a 95% confidence interval of the internal laboratory analytical precision. For

73 comparison of the results with those of other laboratories and methods the analytical values for the standard minerals (Glauconite GL-0) should be eliminated (radiog. Ar: 24. 74Nnl/g, K: 6,591 wt%; see in ODIN, 19826, 142-143). In order to ensure that there was no eventual systematic error during the K-determination at low K-contents, 0.002 wt% was added in the error arising from the scattering of the repeated measurements. The follo wing constants according to the Sydney-convention (S t e i g e r, J ii g e r, 1977), were used:

1 1 1 1 1..11 =4,962 X I0- oa- ; A.e= 0.581 X 10- a- "°K/K=0.001167 Atom-% ; atmosph. 40Ar /36 = 295.5 The model ages of the hornblende-bearing magmatic rocks of Rhodes yielded an age of around 90 Ma (Table 1), whereas the age of the corresponding rocks of Karpathos ran ge between 82.6 ± 3.2 Ma and 88.2 ± 2.2 Ma (Table 1). These age values are in very good agreement with those reported for doleritic dykes of Rhodes and Karpathos (Koepke, 1986). In contrast to the gabbroic rocks, the amphibolites of Rhodes yielded significantly higher model ages. The K-Ar data for the amphibolites range between 160-180 Ma (Tab le I) thus pointing to Middle Jurassic age.

Conclusions

Based on the radiometric dating, the ophiolites of Rhodes can be considered as the south eastern continuation of the Jurassic ophiolite belt of the Balkan peninsula. They are con nected with the ophiolites of the Greek mainland by the occurrences of Crete and Ange lona (in SE Peloponnese). K-Ar mineral data from the amphibolites of Angelona which occur also as tectonic blocks in the ophiolitic melange, point also to a Middle Jurassic age (K o e p k e et al., 1985; Koepke, 1986). Identical Middle Jurassic mqdel ages, between 160-180 Ma, are reported from radiometric K-Ar dating of rocks from several occurrences of the Greek mainland (Spray, Roddick, 1980; T h u i z at et al., 1981 ). These rocks come from the metamorphic sole at the base of the ophio lites or /and from the ophiolites themselves of the ophiolitic melange. A Middle Jurassic age is reported also from metamorphic rocks at the base of ophiolites in Yugoslavia (La n phere eta1.,1975; Karamata, Lorrie, 1978; Okrusch etal.,l978; Majer et at., 1979). On the other hand, the ophiolites of Rhodes and Karpathos show similarities in age to the Upper Cretaceous ophiolite belt of the Taurides and can thus be considered as their southwestern continuation. Radiometric dating of sub-ophiolitic metamorphic rocks of the Lycian Nappe have yielded ages between 85 and I 05 Ma (T h u i z a t, M o n t i g n y, 1979; T h u i z at et al., 1981). It is thus remarkable that the magmatic and metamorphic rocks of the ophiolites of the East Mediterranean area show different ages, regardless of any similarities in struc tural state of material of the ophiolitic complexes. While the age of the magmatic rocks in Rhodes and Karpathos corresponds to the Upper Cretaceous data of the southwest Lycia, that of the metamorphic rocks fit well with the Middle Jurassic age of the Hel lenides and Dinarides. Therefore, the age determination point to an intermediate position of Rhodes and Kar pathos islands between the Hellenides or Dinarides and Taurides.

74 Table I K-Ar dating of Hornblende Gabbros from the ophiolitic melange of Rhodes and Karpathos as well as of Amphibolites of Rhodes

Athmo>Sph. Radiog. Sample Locality No Rock type Mineral I KWt.- % 40K J36 Ar lOAr f36Ar K-Ar dates Ma A rgon Ncm3fg. 107 ------Rhodes 124K Kopria gabbro amphibole 3.5 ± 0.03 3.18 ± 0.03 0.0879 ± 0.0025 48.7 559 90.8 ± 2.8 125K 2 km SW from Kamiros Skala gabbro amphibole 2.46 ± 0.04 1.48 ± 0.023 0.0426 34.4 474 87.2± 5.0 0.0424 0.0426± 0.0024 237 K 2 Km S from gabbro amphibole 2.77 ± 0.05 1.7161 0.029 0.0488 34.6 479 89.2 ± 7.5 Kamiros Skala 0.0498 0.0463 -0.0483 ± 0.0041 Karpathos 19X Xindothio gabbro amphibole 3.47 ± 0.03 4.50 ± 0.04 0.1455±0.0038 77.5 680.5 83.5±2.4 129X Katostavri gabbro amphibole 2.59 ± 0.08 4.36 ± 0.06 0.1238 95.1 795 88.2± 2.2 0.1244 0.1241±0.0026 350X Katostavri gabbro amphibole 2.55 ± 0.08 1.952± 0.014 0.0594± 0.0023 46.2 522.5 82.6 ± 3.2 Rhodes 221 Mavri amphibolite hornblende 2.02 ± 0.11 26.76 ± 0.73 0.3645 ± 0.0038 358 4227 179.7± 5.0 222 Mavri amphibolite hornblende 2.03 ± 0.04 18.32 ± 0.29 0.2537 ± 0.0030 248 2973 176.8± 3.3 224 Mavri amphibolite hornblende 2.03 ± 0.10 12.78± 0.14 0.1980 ± 0.0030 198.5 2221 159.6± 2.9

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