Genesis of Middle Triassic volcaniclastic deposits in the External Dinarides : doctoral thesis

Tijekom srednjega trijasa na području Vanjskih Dinarida tektonika povezana s otvaranjem Tetiskog oceana dovela je do značajne diferencijacije taložnih okoliša. Istovremeno su egzistirala izdignuta područja sa značajkama subaerske vulkanske aktivnosti i trošenja starijih stijena te njihova pretaloživanja, kao i plitkomorski prostori obilježeni karbonatnom sedimentacijom te dublja, pelagička morska korita u kojima su taloženi različiti sedimenti uključujući i vulkanoklastične i vulkanske stijene. Cilj provedenog istraživanja je bio definirati i opisati srednjetrijaske vulkanoklastične naslage, odrediti njihov stratigrafski položaj u Vanjskim Dinaridima, definirati taložne okoliše u kojima su nastale, geokemijskim podacima povezati ih s geotektonskim okolišem u kojem je nastala magma koja je producirala vulkanoklastične naslage te usporediti razvoj prostora Vanjskih Dinarida s okolnim prostorima zapadnog Tetisa. Analizirane su petrološko-sedimentološke, mineraloške i geokemijske karakteristike vulkanoklastičnih naslaga s pet lokaliteta u Vanjskim Dinaridima (Donje Pazarište i Jovanović Draga u okolici Gospića, Bosansko Grahovo u Bosni i Hercegovini, Zelovo u blizini Sinja, i Plavno u blizini Knina). Mikropetrografskim metodama određeni su facijesi istraživanih stijena i mehanizmi njihova postanka čime je omogućena rekonstrukcija taložnih okoliša. Analizama konodontnog materijala biostratigrafski je određena starost te je postavljen vremenski okvir taloženja vulkanoklastita srednjega trijasa. Geokemijskim analizama dobiveni su podaci koji su korišteni za klasifikaciju istraživanih stijena, odredbu geotektonskog okvira i porijekla magme iz koje su nastale vulkanoklastične naslage, te je omogućena rekonstrukcija petrogenetskog modela u srednjem trijasu Vanjskih Dinarida. Vulkanoklastične naslage srednjega trijasa nastajale su u kopnenim i marinskim okolišima. Većina istraživanih lokaliteta ima obilježja dubljevodne pelagičke sedimentacije. Taloženje ovih naslaga u Vanjskim Dinaridima vezano je za prijelazni interval iz anizika u ladinik, kao i u većini okolnih prostora zapadnog dijela nekadašnjeg Tetiskog oceana. Geokemijska obilježja vulkanoklastičnih i vulkanskih stijena ukazuje na kalcijsko-alkalijsku magmu kontaminiranu krustalnim materijalom nastalu u složenom geotektonskom sustavu. Sedimentološka obilježja istraživanih vulkanoklastita i s njima udruženih sedimentnih stijena dokazuju postojanje nepotpuno razvijenih malih riftnih sustava.In the Middle Triassic times, the External Dinarides were a part of the Western Tethyan domain (Haas et al., 1995; Stampfli & Borel, 2002; 2003; Scotese et al., 2004). A deposition of the Early Triassic low energy carbonate-siliciclastic sediments of the epeiric ramp (Aljinović et al., 2014) was interrupted in the Middle Triassic by vigorous and extensive tectonic activity related to the opening of the Tethyan Ocean. Tectonic activity was accompanied by volcanism and formation of volcanic and volcaniclastic rocks. These tectonic movements caused the differentiation of the relief, thus forming different rift related tectonic structures. Some tectonic blocks were uplifted and prone to subaerial erosion while others were subsided and developed different deep marine, pelagic facies. All depositional environments were under the influence of volcanic activity. However, some areas experienced long lasting shallow marine, mostly carbonate sedimentation. The Middle Triassic volcanic and volcaniclastic rocks were investigated by various authors (e.g. Poljak & Tajder, 1948; Lugović & Majer, 1983; Pamić, 1984; Marci et al., 1991; Trubelja et al., 2004; Garašić et al., 2006; Preglej, 2006; Hrvatović et al., 2011; Aljinović et al., 2011). Some of them pronounced the controversial relation between the geochemical data of volcanic rocks and geotectonic setting in the Middle Triassic (Lugović & Majer, 1983; Pamić, 1984; Trubelja et al., 2004; Garašić et al., 2006). That resulted in the unambiguous interpretation of the magma origin. Geochemical data all indicate calc-alkaline magma composition, which is more common in the subduction areas than in rift zones. The typical Middle Triassic rift related magmatic rocks (ophiolites) are still not found in the External Dinarides. The aim of this dissertation is to present the petrological, sedimentological, stratigraphical and geochemical data of the investigated volcaniclastic rocks and to unravel its genesis and origin. Therefore, volcaniclastic rocks at five localities in the External Dinarides were investigated. The field methods included recording the sedimentary sections in which different volcaniclastic rocks crop out. Petrographical methods were used to analyse micropetrographic composition (in thinsections) and differentiate sedimentary facies based on the lithology and depositional processes. The differentiation of the facies allowed reconstruction of the depositional mechanisms and prediction of the sedimentary model for each of the investigated localities. Carbonate sedimentary rocks associated with the volcaniclastics served for conodont analysis that enabled the biostratigraphic constrains of the volcanic activity. The high-resolution conodont biostratigraphy also enabled the correlation of the investigated sequences. Geochemical analysis aimed to determine the magma composition and according to that interpret geotectonic position of the investigated area. In Lika, part of the Velebit Mts., the five small localities were investigated adjacent to the village Donje Pazarište. In all five localities, the different volcaniclastic deposits were recorded in different sedimentary sequences. The three sedimentary sequences, called Donje Pazarište 1, Donje Pazarište 2 and Donje Pazarište 3 form almost continuous 97 m thick succession with six different facies: Flysch facies (FF), Carbonate shale facies (KS), Syneruptive resedimented pyroclastic facies (FSP), Platy limestone and pyroclastic facies (FPV), Limestone breccia facies (VB) and Slumped limestone, chert and pyroclastic facies (FSV). Near the investigated sections (Donje Pazarište 1, 2, 3) the Pyroclastic flow facies (FPT) was determined (similarly as investigated by Preglej, 2006). The whole sequence has the characteristics of deposition in the open marine, pelagic environment. The beginning of the sequence was influenced by Flysch facies (FF) deposited by turbidity currents. Flysch facies was conformably overlain by Carbonate shale facies (KS) slightly influenced by deposition of pyroclastic material. Syneruptive resedimented pyroclastics (FSP) were redeposited from relatively shallow parts of the depositional area to the deep pelagic areas via pyroclastic turbidity currents. Deposition of the Platy limestone and pyroclastic facies (FPV), indicates shallowing of the environment envisaged by prevalence of limestones. Limestone breccia facies (VB) occassionaly occurres in the sequence and imply periods of intense tectonic activity. Slumped limestone, chert and pyroclastic facies (FSV) was deposited in pelagic environment influenced by the deposition of pyroclastic material primary sedimented through water settled fall out on slopes. The sequence was strongly influenced by tectonic activity and tectonic block movements. The Pyroclastic flow facies (FPT), located in vicinity of the youngest strata of the Donje Pazarište section, has the characteristics of subaerial emplaced ignimbrite flow, as does the same facies in the vicinity of the village Brušane, in the foothill of Vinac. Lithological similarities of these rocks allow the interpretation that they possibly represent a unique pyroclastic flow generated by pyroclastic column collapse and subaerial of shallow marine emplacement. The time constraint of this facies is unfortunately missing due to lack of conodont species in the associated shallow water limestones. In the same area, the recorded profile Jovanović Draga consists completely of volcaniclastic deposits (81 m thick) that are associated with the olivine basalt of Coherent facies (KF). Among volcaniclastics, the Hyaloclastic facies (HF) was differentiated. It was formed by autoclastic processes and limited pyroclastic activity. Fragments formed by autoclastic processes were mixed with fragments of carbonate rocks probably formed by fragmentation of the lithified carbonates during magma ascent. Explosive eruption was restricted due to water depth that limited the spread of pyroclastic material (dominantly volcanic glass shards). The recorded profile near the village Bosansko Grahovo (Bosnia and Herzegovina), consists of 79 m thick dominantly autoclastic deposits. Four different volcaniclastic facies were determined. The base of the profile is represented by the significant Limestone peperite facies (FVP). It consists of bioclastic peckstones-weckstones with thin-shelled bivalves and radiolarians indicating emplacement of hot lava in the pelagic environment. The Limestone peperite facies is conformably overlain with the Pyroclastic flow facies (FPT), followed by a Hyaloclastic facies (FH) divided into three lithotypes: resedimented basalt-andesite hyaloclastite (Pba), in situ basalt hyaloclastite (ISb) and resedimented basalt hyaloclastite (Pb). The top of the sequence is characterized by the occurrence of the Breccia-conglomerate peperite (FBP). The facies distribution possibly represents both, the lateral and the vertical exchange of recorded facies. According to the fossil content in the limestone parts of the peperite, the sedimentary environment is determined as pelagic. Pyroclastic flow facies in this profile is represented by a small volume ignimbrite formed in the low fountaining volcanic vent and was constantly in contact with warm ascending magma throughout the emplacement time. Hyaloclastic facies (FH) was formed by quenching fragmentation of the coherent facies. The accurence of in situ basalt hyaloclastite is closely associated with the coherent facies of the same composition (not recorded in the outcrops). The resedimented hyaloclastite shows the increased distance from the area where the coherent facies was located. The same trend can be observed in the gradual transition to the Breccia-conglomerate peperite facies that consists of limestone, silicified limestone and basalt clasts with volcanic matrix. The most investigated locality of the Middle Triassic volcaniclastic deposits in the External Dinarides is located in the area of Mt. Svilaja, near the village Zelovo. The area was investigated by Šćavničar et al., (1983), Belak, (2000), Marjanac, (2000), Jelaska et al., (2003), Balini et al., (2006), Kolar-Jurkovšek et al., (2006), Aljinović et al., (2010), Hrvatović et al., (2011), Halamski et al., (2016). In the investigated area, two sedimentary sequences were recorded, and an isolated coherent facies was noticed and analyzed. The profile Zelovo 1 (14,5 m thick) consists of Silicified carbonate facies (FSK) with slumped beds, interlayered with the pale green unconsolidated altered clay tuffs and dark green consolidated crystalovitric tuffs belonging to the Syneruptive resedimented pyroclastic facies (FPP). The profile Zelovo 2 (8,2 m thick) consists of the same rock types in the similar vertical arrangement as in Zelovo 1. The noticed coherent facies (KF) is located nearby the profile Zelovo 2 and is represented by basalts with intrasertal texture. The volcaniclastic deposits were formed by syneruptive resedimentation of pyroclastic material by turbidity currents and deposited in small-scaled restricted pelagic basins that Belak (2000) defined as simple graben structure. The pyroclastic material has phreatomagmatic characteristics indicating that external water enhanced explosive eruptions and influenced production of the pyroclastic material. The Plavno locality represents a short, 6,5 m thick sedimentary succession. It consists of the three differentiated facies: Silicified limestones composed of pelagic biota and occasional crystaloclasts (FSV), Resedimented pyroclastic facies (FPT) and Volcanogenic sandstone facies (FVP). The Resedimented pyroclastic facies (FPT) consists of vitiriclastic and lithoclastic tuffs formed by resedimentation processes. Lithoclasts in lithoclastic tuffs are represented by low porosity juvenile clasts classified as cognate lithic clasts formed as products of pyroclastic activity of basaltic explosive eruptions, probably of strombolian or hawaiian eruption type. Concerning time constraint of the Middle Triassic volcanic activity, the new biostratigraphic data is presented. At Donje Pazarište locality ammonoids, found in Carbonate shale facies (KS), were determined as Flexoptychites sp. which was earlier reported in the trinodosus ammonoid zone in the External Dinarides ( Prlj & Murđenović, 1988; Petek, 1997). Conodont analysis yield different conodont species in the described sections. Conodont species determined in the sample DP-2/34 of the Platy limestone and pyroclastic facies (FPV) are Neogondolella bifurcate, N. constricta, N. sp. and Gladigondolella sp. that indicate Lower Illyrian age. Higher in the succession, Paragondollela sp., P. trammeri, P. excelsa and G. tethydis conodont species were found in the Slumped limestone, chert and pyroclastic facies (FSV). These species indicate stratigraphic range from Upper Illyrian to Fassanian. At the Bosansko Grahovo locality from the Limestone peperite facies (FVP) following conodont assemblages were determined: in samples G2 and G3 conodont species Neogondolella excentrica, Paragondolella excelsa and Gladigondolella tethydis suggest Lower Illyrian age, while in samples G4 to G9 – N.excelsa, P.trammeri and G.tethydis imply Upper Illyrian age. Jelaska et al. (2003), Balini et al., (2006), and Kolar-Jurkovšek et al., (2006), presented the biostratigraphic data for the Zelovo section previously. The determination of conodont zones from these investigations were used as reference conodont zonation for the investigated area of the External Dinarides. The conodont samples form the Plavno section proved negative in conodont elements. Presented biostratigraphic data enable valid correlation of the investigated sections and proved that Donje Pazarište and Bosansko Grahovo sections represented a deposition in the similar time interval and are of Illyrian age, while Zelovo section was somewhat younger and point to Fassanian age. Two major types of geochemical analysis were conducted in the scope of this research: 19 samples were analysed using ICP-MS and XRF for the whole rock analysis, and 7 samples were analysed using EMP point analysis and element mapping of thinsection areas. Whole rock geochemical data enable to constraint the magma type and to interpret geotectonic setting and their close relation. Results presented in concentration diagrams for rare earth elements and spider diagrams for trace elements showed the influence of continental crust in the formation of calc-alkaline magma significant for the Middle Triassic volcanism in the External Dinarides. The same influence was proved by diagrams of Cabanis & Lesscolle, (1979) and Wood, (1980), that implied all analysed samples had characteristics of volcanic arc magma composition. The EMP analysis of the pyroxene in the coherent and hyaloclastic facies of the Jovanović Draga section and in situ basaltic hyaloclastics from Bosansko Grahovo section showed that most of them are diopside and augite originated from calc-alkaline magma under low to medium pressure regimes. In addition, the diagram for the definition of the geotectonic position showed that pyroxenes from Jovanović Draga are of calc-alkaline orogenic magma origin, while pyroxenes from Bosansko Grahovo are characterized by magma of rift areas. This can mean that the more southern localities are less influenced by arc magmatism and that influence can be seen only in the geochemistry of pyroxenes. Plagioclase analysis from Zelovo locality shows that pyroclastic crystaloclasts are both normally and reversely zoned. Composition of the Na2O and CaO content vary from the core to the rim of the crystaloclasts respectively. This fact can be interpreted as constant income of the new more primitive magma (with higher CaO content) in the magma chamber from where the explosive eruptions were triggered and pyroclastic material was formed. The synthesis of all the investigated results enabled comparison of the External Dinarides with other areas of the Western Tethys (Internal Dinarides, Southern Alps, Northern Calcareous Alps, and Transdanubian Range). Most of the areas show some significant regional events or are time constrained that enable correlation between them. Almost all facies, differentiated in this work, appear with some differences, in other Western Tethyan areas. The occurrence of deeper marine facies and drowning of shallow marine carbonate sedimentary environments was recorded for the first time in the Upper Anisian (Buser, 1989; Krystyn & Lein, 1996; Gianolla et al. 1998; Missoni et al., 2001; Preto et al., 2009; Kovács et al., 2011). The main magmatic activity started in the Anisian but was the most vigorous in the Upper Anisian and Lower Ladinian (Bechstäld et al., 1978; Castellarin et al., 1988; Szoldán, 1990; Castellarin & Rossi, 1991; Obenholzner, 1991; Mundil et al., 1996; Harangi et al., 1996; Velledits, 2004; 2006; Brandner et al., 2007). As the magmatic and tectonic activity slowly abated in the Upper Ladinian, a development of the shallow marine, carbonate dominant environments had commenced (Broglio-Loriga, 1967; Keim & Neri, 2005; Manfrin et al., 2005; Bernardi et al., 2011; Lein et al., 2012; Celarc et al., 2013). Geochemical data from all neighbouring Western Tethyan areas has calc-alkaline, even shoshonitic character, not common for the supposed geotectonic setting depicted as rift (e.g. Bechstäld et al., 1978; Crisci et al., 1984; Castellarin et al., 1988; Harangi et al., 1996). The Middle Triassic rift related depositional environments, where various types of volcanoclastic rocks occurred, was also interpreted for the External Dinaridic area and documented in this work. The volcanic and volcaniclastic rocks have the same geochemical imprint as recognized in the neighbouring Tethyan areas. This fact can be explained by fitting the External Dinarides in the palinspactic reconstructions (in this work the palaeoreconstruction by Stampfli & Borel, 2003 was used). In the Middle Triassic, the External Dinarides can be located between the rift-related Tethyan domains (opening of the Tethyan Ocean from the southeast). The Tethyan rifting phase had detached the Cimmerian block while contemporaneously the subduction of the Palaeotethys to the northeast, beneath the Laurasian part of Gondwana, occurred. The formation of the several small-scale back arc rift belts was related to the subduction on the northern margin of Palaeotethys. One of these back arc rift belts developed as Meliata Ocean, bordering the Dinaridic region to the north. More to the west from Meliata an undeveloped back-arc rift belt was positioned in the area of today Exteranal Dinarides. This rift did not continue to develope the real ocean (lack of ofiolites) but due to the main Tethyan rifting trend that was intensified during Ladinian, the back-arc rift in the External Dinarides had arrested and ended as differentiated pelagic environment filled with various sedimentary material