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Middle Jurassic radiolarians from ophiolitic mélange of Dinaride Ophiolite Zone (DOZ) - age and geodynamic significance (Papratnica river, northern Bosnia and Herzegovina)

INTRODUCTION The Jurassic ophiolites are found in two narrow zones of the Dinarides of the Balkans making part of the bigger Alpine-Himalayan-Tethyan orogenic belt. Formation of ophiolite mélange related to the complex subduction and obduction processes within the Mesozoic Neotethyan realm is still not entirely understood. Herein a theory put forth advocates the existence of subduction- related intraoceanic trenches being progressively filled by gravitational emplacements of chaotic material from the upper plate (radiolarite- ophiolite olistoliths with clayey to sandy matrix) and clastic fragments emanated from the footwall. Ultimately the deposition of ophiolitic mélange evolved with the onward obduction processes of the oceanic lithosphere, with a composite thrust sheets system being created, whereby the mélange is positioned in the footwall of the main ultramafic mass of Dinaride ophiolite zone (e.g. Bortolotti et al., 1996). Biostratigraphic data exist from the ophiolitic mélange in Croatia (Halamić et al., 1999), Serbia (Chiari et al., 2011), Albania (Chiari et al., 2002), and Greece (Nirta et al., 2010). All obtained ages are coherent, yielding a Middle Jurassic age of mélange. So far, Middle Jurassic radiolarians in the Bosnian part of DOZ were described only from non-ophiolitic sequences (Vishnevskaya et al., 2009). In this contribution, new evidence is presented on the age of radiolarians recovered from the mélange of the biggest ophiolite complex in the central DOZ, the Krivaja-Konjuh Ophiolite Complex (KKOC, northern Bosnia and Herzegovina), in which diverse oceanic rock suites testify to the different stages of ocean evolution (e.g. Pamić et al., 1977 ; Lugović et al., 1991 ; Šegvić, 2010). MATERIALS AND METHODS The ophiolite mélange was studied and sampled in the vicinity of the village Papratnica located east of Žepče in northern Bosnia and Herzegovina. The Papratnica mélange deposits were selected for sampling because they comprise carbonate blocks and pillow lavas roughly correlated to the olistostrome part of the DOZ ophiolitic mélange (Pamić, 1968). The nearly 100 m long Papratnica mélange succession holds numerous clast horizons containing radiolarian chert, one of which was logged and sampled at the second metre of the section. This outcrop sampled for radiolarians is approximately 4 m thick and consists of layered dark grey fine- grained shales with embedded chert blocks. The black and vitreous subangular to rounded chert clasts vary from 2 to 10 cm in diameter, whereas in some other levels of the Papratnica section they are up to 20 cm in diameter. No index fossils are reported from the background fine-clastic sedimentary rocks and, therefore, the general Jurassic age of the mélange is inferred based on stratigraphically uncharacteristic microfossils found in platy limestones that are interlayering with shales. Petrography of radiolarian-contained chert clasts was studied using a Leica DM-LSP polarisation microscope, whereas a well-preserved radiolarian assemblage, extracted from a siliceous-shaly sample using a diluted (5%) hydrofluoric acid, was investigated by Scanning Electron Microscope and dated according to the Unitary Association Zones (UAZ) established by Baumgartner et al. (1995). Figure 1. Radiolarian age constraints in ophiolite complexes of the Dinaride Ophiolite Zone (Croatia, Serbia and Bosnia), the Mirdita Zone (Albania) and the western ophiolite belt in Greece (references and details available in Šegvić et al., 2014). RESULTS Thin-section analyses showed the sampled clast are consisted of angular chert fragments (0.5-2 cm) that are imbedded into the cryptocrystalline siliceous matrix. Both the former and the latter contain circular or elliptical areas (ca. 0.05 mm in diameter) filled with microcrystalline quartz. The circular and elliptical structures represent complete and nicely preserved radiolarian skeletons, whereas the chert block or clast fragments are determined as radiolarian chert microbreccia. The radiolarians are characteristic of the uppermost Bajocian-lower Bathonian UAZ 5 of Baumgartner et al. (1995). This assignment is based on the co-occurrence of Unuma lacusicostatus (Aita), last appearing in UAZ 5, and Unuma gordus Hull, first appearing in UAZ 5. The latest Bajocian-early Bathonian age of radiolarians recovered from the KKOC ophiolite mélange is consistent with other ages reported from radiolarian-bearing rocks in ophiolite complexes throughout the Dinarides and Hellenides (Fig. 1). In these complexes Jurassic radiolarites were found in mélange units, as well as in direct stratigraphic contact with basalts. DISCUSSION AND CONCLUSIONS Composite and rather chaotic nature of radiolarite-contained ophiolite sequence of the Krivaja-Konjuh ophiolite complex is in favour of recent approaches advocating the mélange formation in the ophiolite footwall wherein the radiolarite- ophiolite sequence originated from the obducted upper plate. Determined uppermost Bajocian to lower Bathonian age of analysed radiolarian assemblage stands as a first record on the age of radiolarians from an ophiolitic mélange sequence from the Bosnian portion of Dinaride ophiolite zone. Important to mention is that in the Dinarides the geographically the closest dated MOR-affiliated rocks from the ophiolite upper plate that tectonically overly analysed mélange are olivine-bearing cumulate gabbros (158 ±7.6 Ma, Ustaszewski et al., 2009) of Mt. Kozara located about 50 km westwards from the sampling locality. These gabbros stand for the youngest possible age of sea-floor spreading in this segment of Neotethys thus suggesting the process of mélange formation in the Dinarides could have lasted even until the Oxfordian-Kimmeridgian border (i.e. cessation of ocean spreading). In such a context the reported radiolarian age denotes the time of sedimentation that might have preceded or, eventually, it coincided the time of the mélange formation in this segment of DOZ. Further on, the age of MOR-type gabbros is reported to corresponds to metamorphic ages of subophiolitic soles recovered from the KKOC (161 ±4 Ma ; Lanphere et al., 1975) and the nearby Borja ophiolite complex (171.4 ±3.7 Ma ; Lugović et al., 2006), suggesting that the late ridge magmatism in Dinaridic Neotethys was complemented by the near-ridge sole metamorphism, giving birth to the initial intraoceanic thrusting (e.g. Šegvić, 2010). Moreover, the established sole age denotes the earliest possible inception of the mélange formation through the infill of the newly formed subduction trenches. Fragments of metamorphic rocks are known from Papratnica mélange associated to mafic and ultramafic olistoliths. Another piece of evidence shading light on DOZ mélange formation are IAT pillows basalts reported from the Papratnica succession (e.g. Babajić, 2009). Referring to the onset of intraoceanic subduction it must have taken at least 10 Ma for arc-lavas to form, hence defining a minimum duration of thrusting that led to the genesis of DOZ mélange. It can be therefore inferred that mélange formation in the Dinarides (i.e. thrusting tectonics) was not completed by the time indicated by the youngest radiolarians (early Bathonian), but instead, it must have lasted at least until the Oxfordian time. 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ophiolitic mélange ; Dinaride ophiolite zone ; Jurassic radiolarians ; Neotethys ; Bosnia and Herzegovina

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