Radiometric K/Ar data as evidence of the geodynamic evolution of the @draljica Ophiolitic Complex (central Serbia)

The study presents age data and petrologic characteristics of igneous rocks of the @draljica ophiolitic complex (@OC), situated in central Serbia, 150 km south of Belgrade. The complex consists predominately of a MORB/VAB-like tholeiitic suite, represented mostly by gabbros and diabases. The tholeiitic suite is intruded by calc-alkaline intermediate and acid magmas of a VA-affinity, which presumably formed in a pre-collisional setting. The whole complex is intruded by peraluminous granite magmas. The crystallization age of the calcalkaline pre-collisional quartzdiorite is 168.4±6.7 Ma and it post-dates the formation of the here exposed oceanic crust. Geological evidence suggest that the emplacement of the complex occurred during the Upper Jurassic. With respect to their petrology and age, the @draljica ophiolitic rocks are similar to the south Apuseni Mts. ophiolites, situated to the north, and to the Kur{umija and Guevgeli ophiolites, situated to the south. All these ophiolites probably formed as parts of a single Jurassic belt, which can be termed the eastern branch of the Vardar Zone.


Introduction
Ophiolitic complexes are remnants of the oceanic lithosphere which where obducted onto colliding continental plates.They give precious information about the composite evolution of the oceanic crust/lithosphere, including spreading, lateral movements and closure of oceanic realms by subduction and obduction processes.These phases of the oceanic evolution were followed by metamorphic processes which usually make the investigation of magmatic processes very difficult.
The various ophiolites of the central and western Balkan Peninsula, mostly occurring in Serbia and Bosnia, and southward to the F.Y.R. of Macedonia and Greece, belong to a planetary ophiolitic zone connecting the Alpian and Himalayan orogenic belts (e.g.CO-LEMAN, 1977).Their nature has been a matter of ongoing debate, especially regarding the number of inferred paleo-oceanic domains which existed during the Mesozoic in this region.Most authors generally distinguish Dinaride and Vardar Zone ophiolites.They are regarded as belonging to separate sutures, situated in the west and in the east of the Drina-Ivanjica-Pelagonian block, respectively (e.g.KARAMATA et al., 1999, KARAMATA et al., 2000b, RESIMI]- [ARI] et al., 2000).While there is consensus for the Jurassic age of the emplacement of the Dinaride ophiolites, the ophiolites of the Vardar Zone are still the subject of various interpretations.For a long time the Vardar Zone ophiolites were regarded as being entirely Jurassic in age (]IRI], 1996).During the last decades, evidence appeared that the Vardar Zone itself is a composite suture.This conclusion was mainly derived from geological observations that the ophiolites of the western part of the Vardar Zone are connected to a Sennonian olistostrome mélange (KARAMATA et al., 2005 and references therein), but those in the eastern part are covered by transgresive Tithonian limestones.On this basis, KARAMATA et al. (1999KARAMATA et al. ( , 2000a) ) distingushed the eastern and western branches of the Vardar Zone, which are separated by the almost north-south stretching Kopaonik unit.They regarded the eastern branch as a relict of the main Vardar Ocean, which was closed in the Upper Jurassic, and the western one as a younger oceanic realm, closed in the Upper Cretaceous.
In this paper, the K/Ar radiometric ages of the @draljica ophiolitic complex (hereafter @OC) are presented and discussed.The complex is located near Kragujevac in central Serbia, about 150 km south of Belgrade and it covers about 30 km 2 (Fig. 1).This ophiolite complex is geotectonically situated along the easternmost margin of the Vardar Zone and shares the following general characteristics of the ophiolites of the eastern branch of the Vardar zone (KARAMATA et al., 1999): (i) they are predominantly composed of igneous members of ophiolites, (ii) they are characterized by rare exposures of peridotitic bodies commonly severely serpentinized, (iii) they contain various granitic rocks, and (iii) their overstep sequence is Upper Jurassic in age (e.g.KARAMATA et al., 2005).

Analytical procedure
The radiometric K/Ar data was obtained using a magnetic sector mass spectrometer designed in the Institute for Nuclear Research ATOMKI of the Hungarian Academy of Sciences in Debrecen (Hungary).The analytical procedure was given in detail by BALOGH (1985).
The instrument works in the static mode with a radius 150 mm and a deviation of 90°.For the Ar-analysis, the line for the Ar-extraction was also used.The most suitable samples were crushed, care being taken that the smallest grain size was not less than 0.05 mm.Dusty material was avoided because it increases the binding of atmospheric argon to the particle surfaces, which leads to an increase of the Ar-content of the samples.For the whole rock analyses, the samples were only roughly crushed, but for mineral analyses separation by firstly an electromagnetic separator and subsequently by heavy liquids was performed.In the final stage, the most fresh grains were picked out under binocular lenses.

Petrography and Geochemistry
Igneous rocks of the @OC can be divided into two petrogenetically different rock suites: (1) a predominant suite of tholeiitic affinity and (2) a subordinate suite of a calc-alkaline affinity (RESIMI], 2000).

Tholeiitic affinity suite
The tholeiitic suite is represented by massive gabbros and diabases with subordinate cumulitic gabbros and peridotites.Basalts and plagiogranites are very rare.The whole suite was metamorphosed under low-to medium-T conditions.Therefore, some rocks can be classified as spilites, metadiabases or even greenschists.Fig. 1.Geotectonic position of the @draljica ophiolitic complex (@OC).The terrane boundaries and distribution of Mesozoic ophiolites are given by KARAMATA et al. (1994) and DIMITRIJEVI] (1992).
The geochemical characterization of these suite is given in detail in RESIMI] (2000) and RESIMI]-[ARI] et al.

Suite of calc-alkaline affinity
The calc-alkaline affinity suite is predominantly represented by leucocratic granodiorites and granites and by rare quartzdiorites and quartzmonzodiorites.These rocks have a similar mode of occurrence.They generally apear as irregular bodies or smaller dykes, which show sharp and sheared boundaries in contact with gabbros.
Geochemically, both the leucocratic granodiorites/granites and the quartzdiorites/quartzmonzodiorites are calc-alkaline and differ from plagiogranites in having higher LILE and lower HFSE and Cr, Ni contents.However, the leucocratic granodiorites/granites are peraluminous and have a similar composition to the Furka and Fanos granitoids found more to the south in the F.Y.R. of Macedonia and Greece, respectively.These granitoid rocks occur within the ophiolitic complexes which geotectonically belong to the eastern branch of the VZ.In addition, the @OC leucocratic granitoids show syn-to postcollisional character (HARRIS et al., 1986, PEARCE et al., 1984), as do the Furka and Fanos granitoids ([OPTRAJANOVA, 1967, CHRISTOFIDES et al., 1990).On the other hand, the @OC quartzdiorites and quartzmonzodiorites are metaluminuous and are akin to VA-granitoids, i.e. they show a pre-collisional character on the above mentioned discrimination diagrams.

K/Ar data of the @OC
The K/Ar radiometric age determinations of the whole rock (w.r.) samples and the mineral separates (amphibole, plagioclase and K-feldspar) from both the tholeiitic and calc-alkaline suites of the @OC are presented in Table 1 and Fig. 2.There is a wide range of age data (168.4-77.3Ma), which probably mark various events in the evolution of the @OC.Thus, these radiometric data can be interpreted as showing: (a) the age of igneous processes and (b) the time of various metamorphic events.
The oldest ages were obtained on amphiboles from the quartzdiorite samples.The value of 168.4±6.7 Ma (sample V-306/7) most likely represents the age of intrusion of the quartzdiorite magma.The effects of certain rejuvenation can be taken into consideration because partial chloritization of the amphibole from the quartzmonzodiorites could have caused some loss of radiogenic Ar.This can explain the younger estimate of 145.2±5.7 Ma for the amphibole from sample V-353c.
Amphiboles from a gabbro sample (8-B) revealed an age of 152 Ma but with a very high error of ±21 Ma.It is not likely that the crystallization age of these rocks is lower than 152 Ma because this gabbro is intruded by above mentioned quartzdiorites.Therefore, this estimate indicates some metamorphic overprints and may represent the age of the emplacement of the @draljica ophiolites.The much younger whole rock age of 136±7.7 Ma obtained on a whole-rock diabase sample for certain post-dates the age of ophiolite emplacement.
The ages of the feldspars from all lithologies of the @OC revealed a similar range of 120.7-77Ma.These ages suggest that the whole @OC underwent a meta-morphic overprint during the Cretaceous.Of the dated minerals, K-feldspar loses Ar at the lowest temperature (130±15° C) and this is in accordance with the youngest ages measured on the K-feldspar from the leucocratic granite.

Discussion and concluding remarks
The ophiolites of @draljica predominantly consist of tholeiitic MORB-affinity igneous rocks, which represent slices of the ancient oceanic crust.The crystallization age of these rocks could not be directly radiometrically dated by the K/Ar method.All the K/Ar estimates on tholeiitic rocks are younger than the presumed emplacement age of the ophiolites.The time of emplacement can be inferred from the fact that the @OC is associated with a Middle/Upper Jurassic diabase-chert formation (MARKOVI] et al., 1968, DOLI] et al., 1981) and that its immediate overstep sequence is represented by Tithonian limestones and Lower Cretaceous paraflysch (MARKOVI] et al., 1968;DOLI] et al., 1981).
The magmatic age of the @OC can possibly be inferred on the basis of the crystallization age of the calcalkaline quartzdiorites, which was estimated at around 168 Ma.These rare rocks are metaluminous and show a pre-collisional VA-character.This can indicate that at least some parts of the @OC are relicts of an immature island arc.In this context, the minimal formation age of this oceanic crust can be regarded as Middle/Late Jurassic.On the other hand, the crystallization age of the leucocratic granitoids cannot be inferred from the available K/Ar data.Their geochemical characteristics suggest that they formed under a syn-to post-collisional setting, i.e. after the emplacement of the @OC.As mentioned above, petrochemically similar counterparts are the Furka and Fanos granitoids which occur more to the south, within the Guevgeli ophiolitic complex.The Fanos granite was dated 148±3 Ma (K/Ar on biotite; SPRAY et al., 1984) and 150±2 Ma (K/Ar and Rb/Sr on biotite; BORSI et al., 1966).These ages can correspond to the time of intrusion of @draljica leucocratic granitoids in the syn-to post-collisional phase following the emplacement of ophiolites.
The above presented data concerning the @draljica ophiolite complex support the interpretation based on the existence of two-branches within the Vardar Zone.In many geotectonic interpretations, the ophiolites of the Vardar Zone were generally believed to be related with Jurasssic igneous sequences situated more to the north in the South Apuseni (BLEAHU et al., 1981; Table 1.K/Ar radiometric data for minerals and rocks of the @OC.Fig. 2. K/Ar data for analysed minerals and rocks of the @OC.NICOLAE, 1995;BORTOLOTTI et al., 2002, and references therein).They are mainly based on K/Ar Upper Jurassic ages available for south Apuseni (Mures) ophiolites (167.8±5Ma -NICOLAE et al., 1992).In addition, south Apuseni ophiolites are associated with radiolarian cherts of Callovian to Oxfordian age (LUPU et al., 1995).However, these interpretations did not take into account that only the eastern part of the Vardar Zone ophiolites can be considered as a continuation of these occurrences.In this context, important evidence is provided by this study.For instance, the following characteristic of the @OC are similar to the south Apuseni ophiolites: (i) the @draljica ophiolitic segment is dominated by igneous members with only subordinate peridotites, (ii) it was formed as a MORB-type oceanic crust in Middle Jurassic, (iii) it contains calc-alkaline rocks of a VA-signature, which presumably originated in a pre-collisonal setting, (iv) the whole ophiolitic unit was intruded by leucocratic peraluminous granites after the emplacement, and (v) its overstep sequence is represented by Titonian limestones and Lower Cretaceous flysch.The southern continuation of the eastern branch of the Vardar Zone are the Kur{umlija (south Serbia) and Guevgeli (F.Y.R.M. and northern Greece) ophiolites.The Kur{umlija ophiolites are situated only about 150 km southwards from the @OC.They are very similar to @draljica rocks with respect to the characteristics of the facies and petrochemistry (RESIMI]-[ARI], unpublished) but radiometric data are still not available for these rocks.On the other hand, the Guevgeli ophiolites also meet above summarized characteristics (e.g.IVANOVSKI, 1970).It is worth noting that the specific geotectonic position of the Guevgeli ophiolites with respect to more westwardly situated Vardar ophiolites was emphasized in the seventies by MERCIER (1973).
The presented data set is not sufficient to explain the age and formation of the @draljica ophiolites in detail.However, there is evidence that this is a specific Jurassic ophiolitic section with possible dismembered counterparts in the north and in the south.They probably represented (an) oceanic domain(s) situated along the southern European margin during the Jurassic.After the closure and accretion, they probably formed as a unique ophiolite belt.The southern part of the belt more or less preserved its continuation, while the northern parts came into the present position during the tectonic escape of Adria-related microplates during the Late Paleogene-Early Neogene.