Age of the Jurassic hemipelagic sediments from the Ljubiš area (Zlatibor Mt., SW Serbia)

The type section of the Ljubiš Formation, (Djokov Potok, Zlatibor area, western Serbia) is characterized by abundant poorly-to-moderately preserved radiolarians. The lower part of the type section is characterized by Middle Jurassic (?Bajocian to Bathonian) radiolarian assemblages with Japonocapsa fusiformis (YAO), Takemuraella weghae (GRILL & KOZUR), Eoxitus hungaricus KOZUR, E. baloghi KOZUR, Helvetocapsa matsuokai (SASHIDA), Quarkus japonicus (YAO), Hexasaturnalis suboblongus (YAO), H. tetraspinus (YAO). Taking into account previous data from the upper part of the Ljubiš Formation, the age of the formation can be estimated to be in the interval from the ?Bajocian to Bathonian-Oxfordian. The new biostratigraphic data clearly show the onset of tectonic motion in the Middle Jurassic, documented by the presence of mass transport deposits intercalated in the radiolarite successions. This suggests a Middle Jurassic onset of ophiolite obduction which triggered the rapid deepening of the Adria margin, documented by the abrupt change from carbonate to radiolarite deposition. needs careful analysis of all sedimentary deposits, including their biostratigraphy, facies, history of deposition, and tectonic events. In recent work GAWLICK et al. (2017), SUDAR & GAW­ LICK (2018) and GAWLICK & MISSONI (2019) propose a new stratigraphic scheme for the Triassic and Jurassic of the Inner Di­ narides. Numerous stratigraphic series (formations) were ana­ lyzed: some of them were revised, others were established for the first time. Radiolarite formations in this scheme are most com­ mon in the Middle and Upper Jurassic deposits. Biostratigraphic data of these numerous formations are variable. In some instances they are very comprehensive, while in the other cases they are scarce or rare. Some formations still have only limited biostrati­ graphic control and their stratigraphic positions were determined by indirect methods. GAWLICK et al. (2017) recently proposed a late Middle Jurassic to Late Jurassic age for the Ljubiš Forma­ tion, but the dating was not precise due to poor preservation of the radiolarians. During the 2015 field season, we studied the reference sec­ tion of the Ljubiš Formation (sensu GAWLICK et al., 2017) in Djokov Potok, and present here the first biostratigraphic data from these sedimentary rocks based on radiolarians. 2. GEOLOGICAL OVERVIEW The research area is situated in SW Serbia, in Ljubiš Village on the eastern flanks of Mt. Zlatibor. The wider area is characterized by a rather complicated structure. According to SCHMID et al. (2008), it comprises the Adria-derived Drina-Ivanjica (DIU) and East Bosnian-Durmitor Units (EBDU), with the West Vardar ophiolites superimposed on top, i.e. presently located between the DIU to the northeast and the EDBU to the southwest (Fig.1). Both Adria-derived units (Drina-Ivanjica and East Bosnian-Dur­ mitor) being thrust by the ophiolitic mélange during the Late Ju­ rassic (SCHMID et al., 2008). The present-day position of the Drina-Ivanjica above the East Bosnian-Durmitor unit is a result of out-of-sequence thrusting during the Cretaceous (SCHMID et Article history: Manuscript received January 24, 2020 Revised manuscript accepted July 29, 2020 Available online October 22, 2020


INTRODUCTION
Various Mesozoic radiolarite deposits are common and wide spread in the territory of western and southwestern Serbia. Meso zoic radiolaria are found in pelagic and hemipelagic sediments deposited on the Neo-Tethys ocean floor, intercalated between Triassic shallow-water carbonate ramp and platform sediments, and are widespread in Jurassic times on the Adriatic margin. Their age determination, stratigraphy, and facies are important for the reconstruction of tectonic motion and the arrangement of palaeooceanic basins and continental blocks. Due to the highly complicated structure of this territory, each palaeontological da tum from radiolarite deposits and from carbonate clasts of the intercalated turbidites is highly important.
Radiolarians are the most often utilized fossils in such bi ostratigraphic studies, due to their common presence in the Mesozoic radiolarites. Mesozoic radiolarian biochronology is essential for the understanding of the palaeogeography and overall tectonic structure of the Dinarides. There are numerous data on siliceous sedimentary rocks associated with the ophiolite belts in the Dina rides, Albanides and Hellenides which are often Middle to Late Triassic, Middle Jurassic, or late Middle Jurassic to Late Jurassic in age (OBRADOVIĆ & GORIČAN, 1988;DJERIĆ et al., 2007;CHIARI et al., 2011;FERRIÈRE et al., 2015;GAWLICK et al., 2017;BRAGIN et al., 2019b). It is very rarely possible to obtain Early Jurassic ages, and radiolarites of Early Jurassic age were only discovered in two sections, at Angelokastron and Vothiki (Argolis), (CHIARI et al., 2013).
Numerous radiolarian studies have been carried out in Ser bia during the last two decades (e.g. GORIČAN et al., 1999;DJERIĆ et al., 2007DJERIĆ et al., , 2012GAWLICK et al., 2009GAWLICK et al., , 2016GAWLICK et al., , 2018VISHNEVSKAYA et al., 2009;CHIARI et al., 2011;BRAGIN et al., 2011BRAGIN et al., , 2019a but a lot of the biostratigraphic age dating has still to be undertaken to solve the remaining open questions. The difficult reconstruction of the geodynamic history of the Adriatic passive margin and the ophiolite belts in the Dinarides

Age of the Jurassic hemipelagic sediments from the Ljubiš area (Zlatibor Mt., SW Serbia)
needs careful analysis of all sedimentary deposits, including their biostratigraphy, facies, history of deposition, and tectonic events.
In recent work GAWLICK et al. (2017), SUDAR & GAW LICK (2018) and GAWLICK & MISSONI (2019) propose a new stratigraphic scheme for the Triassic and Jurassic of the Inner Di narides. Numerous stratigraphic series (formations) were ana lyzed: some of them were revised, others were established for the first time. Radiolarite formations in this scheme are most com mon in the Middle and Upper Jurassic deposits. Biostratigraphic data of these numerous formations are variable. In some instances they are very comprehensive, while in the other cases they are scarce or rare. Some formations still have only limited biostrati graphic control and their stratigraphic positions were determined by indirect methods. GAWLICK et al. (2017) recently proposed a late Middle Jurassic to Late Jurassic age for the Ljubiš Forma tion, but the dating was not precise due to poor preservation of the radiolarians.
During the 2015 field season, we studied the reference sec tion of the Ljubiš Formation (sensu GAWLICK et al., 2017) in Djokov Potok, and present here the first biostratigraphic data from these sedimentary rocks based on radiolarians.

GEOLOGICAL OVERVIEW
The research area is situated in SW Serbia, in Ljubiš Village on the eastern flanks of Mt. Zlatibor. The wider area is characterized by a rather complicated structure. According to SCHMID et al. (2008), it comprises the Adria-derived Drina-Ivanjica (DIU) and East Bosnian-Durmitor Units (EBDU), with the West Vardar ophiolites superimposed on top, i.e. presently located between the DIU to the northeast and the EDBU to the southwest (Fig.1). Both Adria-derived units (Drina-Ivanjica and East Bosnian-Dur mitor) being thrust by the ophiolitic mélange during the Late Ju rassic (SCHMID et al., 2008). The present-day position of the Drina-Ivanjica above the East Bosnian-Durmitor unit is a result of out-of-sequence thrusting during the Cretaceous (SCHMID et    GAWLICK et al. (2017) note that in Triassic-Jurassic times these two units (Drina-Ivanjica and East-Bosnian Durmitor) were part of the same shelf -the Drina-Ivanjica Unit (slightly metamorphosed) represents a more distal shelf area than the East Bosnian-Durmitor Unit (unmetamorphosed).
According to DJERIĆ et al. (2007), radiolarites of the Djokov Potok section, as a part of the Triassic-Jurassic sedimentary suc cession formed along the Adriatic passive margin, belong to the East Bosnian-Durmitor Unit (Fig. 2).
The East Bosnian-Durmitor Unit was defined by DIMI-TRIJEVIĆ (1974) and corresponds to the "Zone Serbe" of RAMPNOUX (1970) and AUBOUIN et al. (1970). According to SCHMID et al. (2008) the East Bosnian Durmitor Unit comprises two sub-units, the more external Durmitor sub-unit (Monte negro) and the more internal Lim sub-unit (SW Serbia). The Triassic of the Durmitor subunit is characterized by thick carbonate plat form sediments (RAMPNOUX, 1970;PANTIĆ & RAMPNOUX, 1972), while the more internal Lim sub-unit is characteri zed by a more distal slope or basinal facies represented by cherty lime stones (the "Grivska Formation", DIMITRIJEVIĆ & DIMI-TRIJEVIĆ, 1991) and a Late Triassic reefal and slope facies (DIMITRIJEVIĆ, 1997). Recently, all Grivska Formation occur rences are attributed to the allochthonous and fartravelled nappe with a palaeogeographic derivation far to the east . At the beginning of the Jurassic, the carbonate plat forms in the Inner Dinarides were drowned and a relatively sud den deepening of the Adriatic shelf in the Middle Jurassic re sulted in widespread deposition of radiolarites (DJERIĆ et al., 2012;VISHNEVSKAYA et al., 2009). All these sediments were overthrust by the ophiolitic mélange and overlying ophiolites in the Late Jurassic (Fig. 2).
In contrast to these interpretations GAWLICK et al. (2017) excluded the Late Triassic reefal and slope facies as remnants of an overthrust nappe. During the last few years investigations car ried on in SW Serbia (GAWLICK et al., 2017 and references therein) have been focused on the litho-and microfacies analysis, and biostratigraphic analysis of the components of the mélanges. Based on the data obtained, GAWLICK et al. (2017, p. 4) con cluded: "The ophiolite and distal continental margin nappes including different mélanges rest today as polyphase far travelled transported nappe stack on an parautochthonous Triassic-Jurassic sedimentary sequence that was palaeogeographically situated between the East Bosnian Durmitor Unit and Drina-Ivanjica Unit".
In the Inner Dinarides, besides the sub-ophiolitic mélange, these authors also distinguish different parautochthonous se quences below the overthrust units. According to GAWLICK & MISSONI (2019 and references therein) sedimentary mélanges derived from sedimentation processes in deepwater trenchlike foreland basins in front of the propagating nappes triggered by ophiolite obduction. These deepwater basins were supplied by the erosional products of the advancing nappe stack and embed ded in a radiolarite bearing argillaceous matrix. According to these authors, the Djokov Potok locality in the Ljubiš village is the type section of the newly established Ljubiš Formation (Fig.  3). These Jurassic radiolarites intercalated with calciturbidites deposited in a trench-like basin (Ljubiš Basin) outcrop in a tec tonic window below the overthrust allochthonous units (ophiolitic mélange and Sirogojno Mélange), and belong to the parautoch thonous sequences between the Drina-Ivanjica Unit to the east and the East Bosnian Durmitor Unit to the west .

SECTION DESCRIPTION AND BIOSTRATIGRAPHY
The studied section is located in Donji Ljubiš Village and starts from the lower flow of Djokov Potok creek to the west and uphill. The coordinates of the starting point of the section are N 43˚ 37' 14.0" E 19˚ 50' 56.1". We studied and sampled the lower part of this section up to the point N 43˚ 37' 16.0" E 19˚ 50' 54.5". The underlying formation is not visible at the surface. A geological column of the section is shown in Fig. 4. The section begins with a twometre thick unit consisting of calcareous mass transport deposits and turbidites intercalated with darkgrey radiolarites (GAWLICK et al., 2017) (Unit 1). Five metres of dark greenishgrey thinbedded radiolarites intercalated with greenishgrey cherty claystone follow (Unit 2) (Fig. 5A, B). The next unit in the column is a 0.5 m thick grey massive calcarenite (Unit 3). Further up in the column, there are dark greenish-grey thin-bedded ra diolarites intercalated with greenishgrey cherty mudstone. Thin calcarenite beds appear 3 m above the base of this unit. The whole unit (Unit 4) is 8 m thick. Unit 5 comprises 1 m of a grey massive calcarenite (Fig. 5C). The section ends with 12 m of dark green ishgrey thinbedded radiolarites intercalated with greenishgrey cherty mudstones (Unit 6) (Fig. 5D).
Radiolarites of this formation are characterized by an abun dance of radiolarian tests, but they are generally strongly recrys tallized and very poorly preserved. Sometimes the radiolarians cannot be extracted from the rock: they become partly or com pletely dissolved during the maceration process. Radiolarites were sampled and processed by the traditional method. Small (100 g) pieces of rocks were etched by diluted (5-10%) hydrofluo ric acid (HF) for 12 hours. Then the residue was washed with wa ter and dried.   Table 1). On the basis of the presence of Japonocapsa fusiformis (YAO), an early-middle Bajocian to late Bajocian-early Bathonian (UAZs 3-5; BAUMGARTNER et al., 1995b) age can be inferred. How ever,  found this species also in association with younger radiolarians in several samples indicat ing that this species may also be present in younger deposits -Callovian and even in the lower Oxfordian (GAWLICK, pers. comm.). Other taxa have broad stratigraphic ranges: Helvetocapsa matsuokai was found in the middle-late Callovian (SU ZUKI & GAWLICK, 2009), and Quarticella sp. is present in the Callovian . Therefore, we have to esti mate a wide stratigraphic range for this sample -Bajocian to Callovian.
Sample 15-10-8 is characterized by a poor and badly pre served radiolarian assemblage. The age could not be determined because it contains only Campanomitra? sp. A sensu O'DOGHERTY, GORIČAN & GAWLICK, 2017 and Praewilliriedellum sp.
As some taxa were determined in open nomenclature and their data adequacy is relatively low, we can conclude that the lower part of the Ljubiš Formation has a Middle Jurassic, ?Bajo cian to Bathonian age.  (Fig. 3). The Gonje Formation is represented by various co loured radiolarites with intercalated mass transport deposits and turbidites in the upper part and dated by radiolarians and fora minifers (RADOIĆIĆ et al., 2009). According to  the mass transport deposits in the upper part of the Gonje Formation (Kimmeridgian-Tithonian age) have a com pletely different component spectrum to that of the Ljubiš Forma tion. In contrast to the Gonje Formation the Ljubis Fm. contains Late Triassic lagoonal and back-reefal Dachstein Limestone and some condensed Early and Middle Jurassic components from the overlying of the Dachstein Limestone in the lower part, and in the upper part additional Late Jurassic shallow-water compo nents; the Gonje Fm. contains various clasts, e.g. Late Jurassic shallow-water clasts; open-marine Late Triassic limestone clasts, various heavy minerals and others . The Ljubiš Formation was dated at the type-locality only by the pres ence of the radiolarian specimen Eucyrtidiellum sp. cf. E. unumaense . Also, bedded greenish-grey radiolarites with intercalated turbidites of the Ljubiš Formation are preserved below the overthrust ophiolitic mélange in the dou ble road curve south of Ljubiš (between Ljubiš and Jasenovo). These radiolarites from the ophiolitic mélange, GAWLICK et al. (2016), were dated by a better preserved radiolarian assemblage (Callovian to Middle Oxfordian age). GAWLICK et al. (2017) re ported a latest Bajocian to middle Oxfordian age from the Ljubiš Formation on the western slope of Vis Mountain. According to GAWLICK et al. (2017) the preserved part of the Ljubiš basin represents a more proximal part of the basin and the thickness of this basin fill is at least 400 m. In the currently uninvestigated most proximal parts of this basin the authors expected a much thicker sequence (up to 1000m). It should be mentioned that in the type area, the underlying Bositra limestones do not outcrop. Also, it is important to note that turbiditic layers, common in the Ljubiš Formation, yield Upper Triassic to Middle Jurassic com ponents : the Triassic components were derived from the lagoonal Dachstein carbonate platform, while the Lower and Middle Jurassic clasts are represented by red nodular limestone and Bositra-limestone. GAWLICK et al. (2017) supposed that this turbidite material was derived from the Upper Triassic-Middle Jurassic sedimentary succession of the DIU, however, the Krš Gradac quarry contains only Late Triassic clasts from the forereef.

DISCUSSION
On the basis of our new data the analyzed radiolarites (the lower part of Ljubiš Formation) were deposited in the time period of the Bajocian to the Bathonian. The Ljubiš Formation above our samples contains reworked Middle Jurassic Bositra limestone clasts , known mainly from the Bajocian-Bathonian, so these radiolarites should be younger than the re worked clasts. Anyway, the deposition of the Ljubiš Formation could have started earlier than previously thought -in the early Bathonian or even in the late Bajocian. Further study of the biostratigraphy and detailed analysis of the sections would be needed to achieve a better understanding of the composition and nature of the Ljubiš Formation.

CONCLUSIONS
Middle Jurassic (Bajocian-Bathonian) radiolarian assemblages were discovered in radiolarites of the lower part of the Ljubiš For mation in the type section in Djokov Potok (Ljubiš village, SW Serbia). The age of the samples cannot be assigned very precisely due to the poor preservation of the radiolarians. The whole strati graphic range of the Ljubiš Formation could range from the lower-middle Bajocian to the Oxfordian if we take into account previous Bathonian-Oxfordian radiolarian data from the upper part of the formation (GAWLICK et al., 2017). The newly deter mined biostratigraphic ages based on radiolarian faunas clearly show the onset of tectonic motion in the Middle Jurassic, docu mented by the deposition of mass transport deposits intercalated in thick radiolarite successions. This suggests the Middle Juras sic onset of ophiolite obduction which triggered the drag down of the Adria margin documented by the abrupt change from car bonate to radiolarite deposition.

ACKNOWLEDGEMENT
This research was supported by the Ministry of Education Sci ence and Technological Development of the Republic of Serbia, Project No. 176015 and by the Russian Governmental Program no 0135-2018-0033. The authors greatly appreciate the valuable comments given by Hans-Jürgen GAWLICK, Renata JACH and an anonymous reviewer and the help of John ROGERS who im proved the English language of the manuscript. Lastly, we would like to pay our gratitude and our respects to our friend and col league, Nataša GERZINA. After many joint fieldtrips, Nataša GERZINA passed away in June 2019. The study would not have been possible without the extraordinary knowledge and collabo ration of the esteemed Nataša GERZINA.