K-Ar Dating of Ophiolites from Rhodes and Karpathos Islands, Dodekanese, Greece
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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} K. XM:J~Cunanazaomy- K-Ar BoJpacm o¢uo,tumoB c ocmpoBoB PoiJoc u Kap11amoc, J(oiJeKa neJbl ( Tpequ11). fipone)lem.r reoxponorrorwfecKue uccrre)lonaHHll o<IJHOJIHTOD c ocTponon Po)loc H KaprraToc (JJ:o)leKaHe3hl, rpeuull). 0rrpe)leJIJlJICll D03paCT (K-Ar MeTO)l) MOHOMHHepaiibHbiX <IJpaKUHH H3 MarMaTH'fOCKHX (aM<IJHOOJIO· BOe ra66po) H MeTaMOMIH'fOCKHX nOpo)l, npHHa,!VIellCailiHX K HH33M o<IJHOJIHTOBOfO (aM4!1HOOJIHTOBOrO) pa3• pe33. JJ:aTHpOnaHHbie 06pa3Ubl DJJITbi H3 reTeporeHHOit, XaOTH'IOCKOil: 'faCTH o<IJHOJlllTOBOfO MeJI3.JVICa. Arul Mar MaTH'IOCKHX nopo)l onpe)leJieH nepXHeMenonoll noJpaCT - 82-88 Ma, a )lJill MeTaMop<IJB'lOCKHX - cpe!lHeiOp cKHtt- 160-180 Ma. Ha ocuonaHHH nux pe3yJibtaTOB o<IJuoJIHnr Po)loca H Kapnaroca MOJKKO xoppeiiHponaTb xaK c nepx HeMeJIOBhiM o<IJHJIHTOBblM nOliCOM TanpHil, Tax R co cpe,llHeiOpCKRM o<IJHOmiTOBbiM nol!COM 3JIJieHHil-JJ:H uapR)l. B COOTBeTCTBHR C 3THM o6a OCTpona 3aHHMaiOT Cpe,llHee lTOJIOJKeHHe MCJK)lY O<IJHOJIHTOBbiMH !TOllcaMH - MeJKilY TanpcKHM nOliCOM c cenepo-noCTOKa u 3nrumu)lHo-JJ:uaapu)lHhiM- c cenepo-Jarra)la. 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<n ~= yellowish-green to green-brown. Brown hornblende is preserved, in most cases, in the core of crystals the outer rim of which is a green amphibole. Plagioclase has an average length of 1 mm and reaches about 40-50 vol. ~-;; . In most cases it is twin ned according to the albite - and albite-pericline-law and shows thin lamellae. The anor thite content ranges between 48-56 mol/';; . A mechanic influence is confirmed by undula tory extinction and, more rarely, on bending of the twinning lamellae and by kink-bending of grain aggregates. Sericitisation is also observed. Clinopyroxene is preserved only occasionally as a relic while in most cases it is en tirely replaced by green amphibole. The presence of fine, rounded sphene aggregates within the amphibole grains is an indication of high titanium contents of the original clinopyro xenes. Fine chlorite leaves as well as fine-grained epidote are subordinate. Xenomorph ore aggregates and rounded apatite inclusions in amphibole and plagioclase are accesso ry phases. Quartz participates also as accessory. The hornblende gabbros of Karpathos show many similarities to those of Rhodes. Cha racteristic are the paragenesis hornblende-plagioclase and the metamorphic orientation. Brown and greenish, hypidiomorphic to xenomorphic amphibole together with hypidio morphic plagioclase are the main constituents of the samples 19X, 129X and 150X (lo cality: Stavri valley), whereas the clinopyroxene percentage is significantly lower. Brown hornblende with pronounced cleavage and a diameter between I and 3 mm is the main constituent of these rocks. Their pleochroism is strong: na=brown-ycllow< nb = reddish-brown nc < = olive-brown. Like in the case of the hornblende-gabbros of Rho des, brown hornblende constitutes the core of the crystals whereas the rim consists of green amphibole and chlorite. Also, here, plagioclase with an average length of 2 mm is twinned according to the albite- and albite-pericline law. The anorthite content ranges between 40-55 mol %. Sericitisation or saussiritisation is observed, in most cases, starting from the core of the crys tal and proceeding outwards, but sometimes the whole grain 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.