GEOCHEMISTRY AND PETROGENESIS OF OLIGOCENE DACITES FROM THE CENTRAL BOSNIA AND HERZEGOVINA WITH INSIGHT IN THE POST- COLLISIONAL TECTONIC EVOLUTION OF CENTRAL DINARIDIC OPHIOLITE BELT

Magmatic rocks of post-Late Eocene magmatic formation are widespread in the Sava segment of Sava- Vardar suture zone and adjoin areas. The rocks formed as a response to transpressional-transtensional tectonic activity preceded by the Cretaceous-Eocene compression of the Internal Dinarides and Tisia Unit as fragments of Eurasian continental lithosphere. Central Bosnia Tertiary volcanic rocks (CBTVR), erupted as dacites in Lower Oligocene, are peculiar rocks of this formation either by their location (southernmost distal outcrops) or geological setting (extrusive within the melange of the Internal Dinaride Ophiolite Belt). Major element composition of the CBTVR reveals high-K calc-alkaline geochemical affinity whereas trace element discriminate the rocks as shoshonitic. The rocks are LILE-enriched and show negative Ta- Nb, P and Ti anomalies, and positive Pb anomalies typical of subduction related volcanic rocks. Chondrite-normalized REE patterns exhibit significant LREE/HREE enrichment [(La/Yb)cn = 21.4 - 33.4]. Geochemical affinity of the CBTVR combined with tectonic position of extrusions suggests derivation of the melts from the subcontinental mantle which had inherited strong orogenic signature during an ancient subduction.

The Cer massif in the internal Dinarides: Exhumation triggered as a far-field effect of the Carpathian slab roll-back

<p>Extension across the southern Pannonian Basin and the internal Dinarides is characterized by the occurrence of a chain of Oligo-Miocene metamorphic core complexes (MCCs) exhumed along mylonitic low-angle extensional shear zones which in part represent former suturing thrusts. Cer MCC at the transition between the internal Dinarides and the Pannonian Basin occupies a structural position within the distal-most Adriatic thrust sheet and originates from two different tectonic processes: Late Cretaceous-Paleogene nappe-stacking during continent-continent collision between Adria and fragments of European lithosphere with Adria residing in a lower plate position, followed by Miocene exhumation. Structural data and a balanced cross section through the Cer massif show that the exhuming shear zone links to a breakaway fault, which reactivated the early Late Cretaceous most internal nappe contact between the two distal-most Adriatic thrust sheets. At Cer MCC, Paleozoic greenschist- to amphibolite-grade lithologies surround a polyphase intrusion composed of I- and S-type granites. These lithologies were exhumed along the shear zone by top-N transport. Thermobarometric analyses indicate an intrusion depth of 7-8 km of the Oligocene I-type granite; cooling below ~500°C occurred at 25.4±0.6 Ma (1σ) yielded by <sup>40</sup>Ar/<sup>39</sup>Ar dating of hornblende. Biotite and white mica from this intrusion as well as from the mylonitic shear zone yield <sup>40</sup>Ar/<sup>39</sup>Ar ages of 17-18 Ma independent of the used techniques (in-situ laser ablation, single-grain total fusion, single-grain step heating, and multi-grain step heating). White mica from the S-type granite yield an <sup>40</sup>Ar/<sup>39</sup>Ar age of 16.7±0.1 Ma (1σ). Associated dikes intruding the shear zone were also affected by N-S extension, indicating that deformation was still ongoing at that time. Our data suggests that exhumation of the MCC was related to the opening of the Pannonian back-arc basin in response to the Carpathian slab-rollback and triggered extensional reactivation of thrusts in the internal Dinarides.</p>

An overview on Structural position of Mesozoic succession of distal Adriatic continental margin on Ivanščica Mt. (NW Croatia)

<p>Mt. Ivanščica is one of inselbergs in the Internal Dinarides (NW Croatia) in the transitional area with Southern Alps. In this area, NNW-verging Dinaric structures are overprinted by S-verging Alpine structures. Mt. Ivanščica is composed of Mesozoic shallow to deep-marine sedimentary succession of the passive continental margin of Adriatic plate, which was facing the Neotethys ocean, overthrust by ophiolitic mélange. Here, we aim to present new preliminary structural data from pelagic sediments of Ivanščica Mt. in attempt to better understand tectonic history of this part of Internal Dinarides.</p><p>Mesozoic succession of Mt. Ivanščica is composed of Triassic clastic, volcanic and carbonate rocks overlain by Upper Triassic to Lower Jurassic shallow-marine carbonates. These are overlain by Jurassic pelagic carbonates and cherts followed by Tithonian−Valanginian pelagic “Aptychus Limestones”. The uppermost part of this succession is composed of Lower Cretaceous Oštrc fm., which conformably overlies the “Aptychus Limestone”. The Oštrc fm. is characterized by turbidites with ophiolitic detritus and represents syn-orogenic deposits presumed as formed in a front of advancing ophiolitic nappe(s).</p><p>The focus of our investigation is primarily on structural characteristics of the “Aptichus Limestones” and the Oštrc fm. The character of the contact between the “Aptychus Limestones” and underlying Upper Triassic to Lower Jurassic carbonates is still uncertain. According to Šimunić et al. (1982) “Aptychus limestones” unconformably overlays Triassic carbonates in periclinal geometry, while Babić (1974) suggests continuous condensed pelagic sedimentation throughout the Jurassic. In contrast with previous observations and interpretations, our observations suggest a tectonic contact, characterized by significantly different orientation of bedding and locally marked by fault gauge (clay) seams.</p><p>Structural analysis shows numerous gentle to open asymmetric folds in the “Aptychus Limestones” and closed chevron folds and isoclinal folds in overlaying Oštrc fm. Chevron folds and open to gentle asymmetric folds indicate NW vergence in present day orientation with fold axis parallel to the strike of the contact with underlying unit. Although different in shape and size, these folds are likely formed during the same tectonic event while their geometry is controlled by differences in rheological properties. Isoclinal folds occurring exclusively at the contact with ophiolitic mélange are characterized by E-W oriented fold axis and S dipping axial surfaces which is in a contrast with aforementioned folds. Thus, we assume that these folds originated from another, presumably older tectonic event. Bedding in Triassic dolomites uniformly dips towards the SE. Local occurrence of condensed pelagic limestones and radiolarian cherts is interpreted, as rheologically weak horizon ideal to form a décollement that, at least locally, could be interpreted to mark a thrust fault.</p><p>Formation of isoclinal folds in the Oštrc fm. and the tectonic contact with ophiolitic mélange is preliminarily attributed to the Aptian-Albian nappe stacking known from the Internal Dinarides. In addition, we assume that the pelagic succession of the “Aptychus Limestones” together with the overlying Oštrc fm. and the ophiolitic mélange are thrusted over the Upper Triassic to Liassic carbonates sometime later, possibly during the final stage of Neotethys closure in the Internal Dinarides.</p>

Single and multi-phase inclusions in garnets from the Lešnica alluvion in the Internal Dinarides, Serbia

<p>Inclusions in garnets from the river Lešnica alluvion (Cer mountain area, Serbia),  were investigated in an effort to study their distribution within the garnet host and to estimate the mechanism of their origin. Garnets are often occurring in the Lešnica alluvion in a form of loosely separated crystals with preserved crystalline forms and as mildly rounded broken grains [1]. Their mineralogical determination was previously published by Milošević et al, [1]. Crystals, that have been extracted and separated from the sandy fraction, have exhibited the presence of various types of inclusions in their structure. Single and multiphase inclusions in the garnets were examined optically by petrographic methods followed by SEM-EDS method, applied for the chemical analyses of the individual inclusion, and LA-ICP-MS applied to determine distribution and content of trace elements in the host garnet.</p><p>Results from SEM-EDS method show that garnets are of the spessartine-almandine type with the incorporation of irregular inclusions determined as rare earth elements (REE) minerals (monazite, xenotime, columbite-tantalite) and accessory minerals that usually incorporate REE (titanite, apatite, and zircon) together with uranium oxide minerals. Other single-phase inclusions are often quartz and rutile. Size of inclusion varieties from grain to grain, between 5 and 40 µm, while their distribution doesn’t follow any pattern, random distribution. It has been noted that zircon and uranium oxide minerals are often found coupled and as multiphase inclusions while monazite, xenotime and columbite-tantalite minerals are observed as separate, single, inclusions. Chondrite normalized REE in the host garnets plotted on spider diagram show extreme depletions of large ion lithophile elements (LILE) and enrichment in high field strength elements (HFSE), with negative Ce, Nd and Eu anomaly. Single-phase and multiphase inclusions that are occurring in the same garnet host with a random distribution are suggesting different genetic relations.</p><p>[1] Milošević M., Kostić B., Vulić P., Jelić I. 2019. Garnets from river Lešnica alluvion, mountain Cer. II Kongres Geologa Bosne i Hercegovine sa medjunarodnim učešćem, Pp. 306-311</p>

Post-collisional mantle delamination in the Dinarides implied from staircases of Oligo-Miocene uplifted marine terraces

The Dinarides fold-thrust belt on the Balkan Peninsula resulted from convergence between the Adriatic and Eurasian plates since Mid-Jurassic times. Under the Dinarides, S-wave receiver functions, P-wave tomographic models, and shear-wave splitting data show anomalously thin lithosphere overlying a short down-flexed slab geometry. This geometry suggests a delamination of Adriatic lithosphere. Here, we link the evolution of this continental convergence system to hitherto unreported sets of extensively uplifted Oligocene–Miocene (28–17 Ma) marine terraces preserved at elevations of up to 600 m along the Dinaric coastal range. River incision on either side of the Mediterranean-Black Sea drainage divide is comparable to the amounts of terrace uplift. The preservation of the uplifted terraces implies that the most External Dinarides did not experience substantial deformation other than surface uplift in the Neogene. These observations and the contemporaneous emplacement of igneous rocks (33–22 Ma) in the internal Dinarides suggest that the Oligo-Miocene orogen-wide uplift was driven by post-break-off delamination of the Adriatic lithospheric mantle, this was followed by isostatic readjustment of the remaining crust. Our study details how lithospheric delamination exerts an important control on crustal deformation and that its crustal signature and geomorphic imprint can be preserved for millions of years.

Dating extensional deformation to unravel exhumation patterns in the Internal Dinarides

<p>Ar/Ar-in-situ geochronology by laser ablation NGMS (noble gas mass spectrometry) provides a powerful tool to determine inter- and intra-granular age variations of potassium-bearing minerals while maintaining the structural integrity of a sample. This makes it an excellent method in targeting the understanding of the post-collisional evolution of an orogen by dating different mica generations. In order to investigate the timing of exhumation related to extensional deformation in the Internal Dinarides, we sampled paragneisses from the upper greenschist- to amphibolite-grade mylonitic detachment zones of two metamorphic core complexes (MCC’s). The MCC’s are located at the distal Adriatic passive margin (Cer MCC, central western Serbia) and within the Late Cretaceous suturing accretionary wedge complex (Motajica MCC, northern Bosnia and Herzegovina) that separates Adria-derived units from blocks of European affinity.</p><p>Mica grains were assigned to either pre-kinematic or syn-kinematic growth, according to their structural context, texture and grainsize. Pre-kinematic growth is characterized by large, deformed minerals of up to 3.5 mm in size, while rather fine-grained, recrystallized mineral aggregates that usually formed in the strain shadow of larger clasts represent syn-kinematic growth.</p><p>The ages of pre-kinematic white mica from paragneisses of the Motajica detachment range from approx. 80 to 25 Ma. They partly show a large intra-granular age spread characterized by significantly older core ages becoming progressively younger towards the rim. This pattern likely suggests diffusive loss of radiogenic Ar. Ages between 80-55 Ma in the central parts of the grains, associated with a top-W transport direction, are interpreted as the time interval of mineral growth and subsequent deformation in an accretionary wedge during E-ward subduction of the Adriatic passive margin underneath European units.</p><p>Syn-kinematic white mica from Motajica yielded ages between 22 and 16 Ma, which are interpreted as the time of peak activity of extension. This also corresponds with the time of crustal extension in the Pannonian Basin to the north. At Cer MCC, located roughly 150 km ENE of Motajica MCC and structurally below the accretionary wedge complex, ages of deformed white mica indicate exhumation between 19 and 15 Ma with a top-N directed transport.  </p><p>Our results suggest that the opening of the Pannonian Basin in response to slab-retreat underneath the Carpathian orogen resulted in the extensional reactivation of suturing thrusts that separated Adriatic from European units, leading to exhumation of parts of the accretionary wedge (Motajica MCC). This event was followed by the progressive exhumation of the passive Adriatic margin (Cer MCC) that occupied a structural position below the suturing accretionary wedge.</p>

Late Triassic radiolarians from the Grivska formation, internal Dinarides, SW Serbia

Upper Triassic (upper carnian - rhaetian) grey cherty limestone are known in the internal Dinarides under the name of ?Grivska Formation?. Sediments of the Grivska Formation are characterized by microfossils only - conodonts and radiolarians, and did not yield any macrofossils. Micropa - laeontological research of Upper Triassic siliceous rocks was performed at the locality Lim river, in the vicinity of Bistrica Village in SW Serbia. Radiolarian assemblages are characterized by such species as Capnodoce anapetes De Wever, C. sarisa De Wever, Sarla hadrecaena (De Wever), Praehexasaturnalis tenuispinosus (Donofrio & Mostler), Xiphothecaellla longa (Kozur & Mock). According to the radiolarian data, the investigated cherts are of latest carnian to early norian age.