Chert blocks in the ophiolitic mélange of Zlatibor Mt . ( SW Serbia ) – age and geodynamic implications

Cherts are quite frequently occurring rocks in the Internal Dinarides, an extremely complex area composed of several tectonostratigraphic units in which oceanic sediments, ophiolites and partly metamorphosed parts of the distal continental margin of Adria are preserved. Therefore, these cherts differ in age and the original depositional environment in which they were formed. Results of investigations carried out in the chert blocks found in the mélange in the vicinity of Jasenovo village on SE slopes of Zlatibor Mt. are presented here. Radiolarian cherts from the studied localities represent blocks in mélange of the East-Bosnian–Durmitor Unit, exposed in a large tectonic window below the Triassic carbonates of Drina–Ivanjica Unit. Biostratigraphic data revealed Callovian–early Kimmeridgian ages of the studied chert blocks, thus implying a Kimmeridgian or younger age of obduction of the West Vardar ophiolites.


Introduction
Precise dating of wide variety of oceanic sediments is necessary for reconstructions of palaeogeography and geodynamic evolution of oceanic basins and their continental margins.In that sense, radiolarian biostratigraphy is extensively used today in dating pelagic marine sediments.Investigations of radiolarian associations carried out so-far in the Internal Dinarides of Serbia reveal the following age clusters: Middle to Late Triassic, Middle Jurassic and Late Cretaceous.
Early Jurassic ages have not been proven in the Internal Dinarides in Serbia, while Upper Jurassic ages have been questioned recently.
The aim of this paper is to present results of investigations carried out in the chert blocks found in the mélange in the vicinity of Jasenovo village on SE flanks of Zlatibor Mt.

Geological setting
The study area is situated in SW Serbia.In a geotectonic sense, it is a part of the Internal Dinarides, geologically extremely complex area composed of several tectonostratigraphic units in which oceanic sediments, ophiolites and partly metamorphosed parts of the distal continental margin of Adria are preserved.
According to other opinions (e.g.BERNOULLI & LAUBSCHER 1972;BAUMGARTNER 1985;PAMIĆ 1998;PAMIĆ et al. 2002;CSONTOS et al. 2003CSONTOS et al. , 2004;;BORTO-LOTTI et al. 2004, 2013;BORTOLOTTI & PRINCIPI 2005;SCHMID et al. 2008) all these ophiolites derived from a single ocean and were thrusted onto the continental margin of Adria.According to these interpretations, continental (Drina-Ivanjica, Jadar, and East-Bosnian-Durmitor) blocks which separate two ophiolite belts are tectonic windows below the obducted ophiolites, in which the most distal parts of Adria are exposed (Fig. 1).This implies that ophiolites formed single, continuous sheet referred to as the Western Vardar Ophiolitic Unit that was obducted during the Late Jurassic (CSONTOS et al. 2003(CSONTOS et al. , 2004;;SCHMID et al. 2008) and includes all ophiolites of the Dinarides west of the Sava Zone.Subsequent out-of-sequence thrusting resulted in formation of composite units made up of the continental margin of Adria, ophiolitic mélange and obducted ophiolites.

Methods
Two samples were collected from chert blocks found in the mélange in the vicinity of Jasenovo village on SE flanks of Zlatibor Mt.All the productive samples are radiolarian cherts and were treated with diluted 7% hydrofluoric acid, following the method of PESSAGNO & NEWPORT (1972).The radiolarians are generally poorly preserved.The assemblages were dated with the zonation of BAUMGARTNER et al. (1995).The data obtained during the last 15 years show that some species have longer ranges than previously established by BAUMGARTNER et al. (1995).The age assignments are discussed below.Generic names have been updated according to O'DOGHERTY et al. (2009).
The SEM microscope ISI-160 in GIN RAN (Moscow) (sample NĐ 132) and JEOL JSM -6460LV SEM at the Department for Biology and Ecology, University of Novi Sad (sample NĐ 108) were utilized for the precise identification and illustration of the radiolarians.All the material examined is deposited at the Faculty of Mining and Geology, University of Belgrade.

Description of outcrops and biostratigraphy
The studied samples were collected from two chert outcrops on SE slopes of Zlatibor Mt. (Fig. 2).
The locality Rauke is situated 3 km east of village Jasenovo (x = 7412597, y = 4824315).It is about 1.5 m thick succession of greenish, dark gray and black stratified chert with thin interlayers of siliceous shale (Fig. 3).Average thickness of chert layers is 3-6 cm, but it exceeds 15 cm in places.
Sample NĐ 132, taken at this locality, is characterized by a relatively poorly preserved radiolarian association.Presence of species Praewilliriedellum robusta (MATSUOKA) indicates a latest Bajocian to early Callovian age of the sediment (UAZs 5-7).The association can not be younger than the Callovian, which is confirmed by the presence of species Belleza decora (RUST) whose last occurrence is in UAZ 7, (BAUM-GARTNER et al. 1995).Besides these characteristic species, the radiolarian association in the sample NĐ 132 comprises also Transhsuum maxwelli gr.(PES-SAGNO), Transhsuum sp.aff.T. maxwelli, Transhsuum  et al. 1995).A diverse radiolarian assemblage of UAZ 7 was subsequently described from a sample above the early Callovian ammonites, which led to the conclusion that UAZ 7 is mainly Callovian in age (BECCARO 2006).
According to these data, this sample is not older than the Callovian and not younger than late Oxfordian-early Kimmeridgian.

Discussion and conclusions
Middle Jurassic radiolarian cherts are abundant in the region.A widely accepted opinion among Serbian geologists (e.g.RADOVANOVIĆ 1987;KARAMATA 2006) is that these rocks represent a sedimentary cover of the Neotethyan oceanic crust.However, a clear association of Jurassic cherts with MOR basalts has not been proven yet in the territory of Serbia.Radiolarian cherts of Jurassic age, which represent the uppermost part of the obducted oceanic crust, appear to be preserved in Albania only (e.g.CHIARI et al. 1994;PRELA et al. 2000).
Jurassic-age radiolarian cherts are also found as an integral part of the "in situ" preserved passive margin sedimentary sequence in the footwall of the ophiolitic mélange (East-Bosnian-Durmitor and Drina-Ivanjica units) (DJERIĆ et al. 2007, DJERIĆ et al. 2012, RADO-IČIĆ et al. 2009).Such radiolarites were originally deposited onto Triassic and Lower Jurassic carbonaceous platform sediments of the distal Adriatic margin.Parts of the sedimentary sequence of the passive margin were subsequently torn-off and incorporated in the mélange at the front of the obducting nappe.
Although there are outcrops of basalt pillow-lavas in the immediate vicinity of the studied localities, no stratigraphic contact with cherts has been observed.This confirms previous observations that there are no radiolarites associated with mid-ocean-ridge related basalts which would represent the uppermost part of the Jurassic oceanic crust in the Dinarides.Therefore, the studied cherts probably did not derive from the sedimentary cover of the ocean floor, but from the distal parts of the passive margin of Adria.Besides, field  observations show that all these rocks are in a large tectonic window below the Triassic carbonates of Drina-Ivanjica Unit.Accordingly, radiolarian cherts from the studied localities at the SE flanks of Zlatibor Mt. represent blocks in mélange of the East-Bosnian-Durmitor Unit (Figs. 2, 7).
The age of mélange formation, and hence of the final stages of obduction of the West Vardar ophiolites is equal to, or younger, than the youngest dated blocks in the mélange.Radiolarian age obtained for the chert block at the locality Rauke is rather similar to that of the previously dated radiolarite blocks in the Serbian Dinarides (GAWLICK et al. 2009;VISHNEVSKAYA et al. 2009;DJERIĆ et al. 2010).The radiolarian association identified from the chert sample from the locality Marića Potok certainly represents the youngest so-far determined age (Callovian -early Kimmeridgian) of chert blocks in the mélanges in this part of the Dinarides.This implies a Kimmeridgian or younger age of obduction of the West Vardar ophiolites.This age, however, should be taken with extreme caution because such a wide age interval could be a result of low diversity and poor-to-moderate level of preservation of the analyzed radiolarian association.Nevertheless, the obtained data perfectly comply with timing of obduction of the Jurassic Neotethyan oceanic crust based on radiometric ages (179-150 Ma) obtained from metamorphic soles in the Dinarides and Hellenides (e.g.SPRAY et al. 1984).

Fig. 7 .
Fig. 7. Geological cross-section through the SE flanks of Zlatibor Mt. with indicated positions of the studied localities.