ANNALES • Ser. hist, nat, • 13 - 2003 • 2 original scientific article UDK 552.3(497.4-15) received: 2003-0S-04 PRELIMINARY CHARACTERIZATION OF MAGMATIC CLASTS FROM CONGLOMERATE WITHIN THE BOVEC FLYSCH (SLOVENIA) Alberto ROSSET & Davide LENAZ University degli Studi di Trieste, Oipartimento di Sciciue della Tetra, 1-34127 Trieste, via Weiss 8 f:-iT)ail: rosset@univ.lriesie.it Giorgio TUNIS University degli Studi di Trieste, Dipartiroento di Scierize Geologiche, Ambientali c Marine, 1-34127 Trieste, via Weiss 2 Angelo DE MIN & Aiessandro TOSONE Univetsitä degli Studi di Trieste, Di|»rtfmento di Science delta Terra, i-34127 Trieste. via Weiss 8 ABSTRACT A Maastrichtian conglomerate related to the fiysch beds of the Slovenian sedimentary basin and outcropping near Bovec is characterized by the presence of rare magmatic dasts, basaltic in composition. Pefrograpliical features and major elements composition allow us to classify the few collected clasts as lavas or sub-intrusive High Allumina Basalts, rocks commonly related to compressive movements involving the subdudion of oceanic plate(s). Incompatible element, patterns support the hypothesis of a compressive geotectonic setting and suggest involvement of a continental source mantle in the genesis of the magmatic clasts. Key words: conglomerate, Bovec, magmatic, HAS, Maastrichtian CARATTERIZZAZIONE PRELIMINARY DEI CLASTI MAGMATIC! PRESENTI NEL CONGLOMERATO FLYSCHOIDE IN PROSSIMITA DI BOVEC (SLOVENIA) SINTESI II conglomerate Maastrichtiano di Bovec rapprcsenta ii prima iniportanle episodio sedimentario dastico del Bacrno Slovene ed « carstterizzato dalla presenza di rati clasti magmatici basaltici di derivazione eifusiva o sub-intrusiva. I.e caratteristiche chimiche degli element! maggiori e le evidenze petrografiche., tipiche dei basalti con un alto contenuto in alluminio, tin no ipotizzare, per i clasti finora campionati, un' unica genes i relazionabile ad eventi compressivi coinvolgenti la subduzione di piacca oceanica. l/andamento degli element! incompatibiii supports I'ipotesi di un ambiente geodinamico compressive e suggetisce inokre il coinvolgimento di una sorgente di manteilo continent« fe. Parole chiave: conglomerate, Bovec, magmatico, HAB, Maaslrichtiano 257 ANN ALES • Ser. hist. nat. - 13 • 2003 • 2 Albedo ROSS ET rt sh PRTIIMINARV CHARACTERIZATION Of- MAGMATIC CLASTS FROM CONGLOMERATE ... 257-262 INTRODUCTION The presence of unaltered and unmetamorphosed magmatic clasts in terrigenous sediments, deposited before the paroxysmal orogenic events (i.e. Dinaric and Alpine orogenesis), could [provide Important information on the sub-iithospheric mantle. The main objective of this study is to give a first characterization of the few volcanic rock's collected from a conglomerate inside the Maastrichtian flysch conglomerate near Bovec to provide eventual information on the possible mantle source(s) involved, to evidence the presence of one or more magmatic events and to give first information about their possible geody-namic formation setting. Geological and stratigraphical outlines Bovec is located in NW Slovenia (Fig. 7A), between Mt. Rombon and Mt. Polovriik. The stratigraphie succession of the area began in the Late Triassic {Lower Norian) with the very thick carbonate platforms of the "Main Dolomite" (dolomitic limestone) and "Dac.hsiein" (limestone) formations originating on the Julian Platform (Buser, 1986; Ogorelec & Buser, 1996). These sequences are stratigraphically continuous with Rethyan successions represented by mud-stones and limestones, typical of a neritic environment (Selli, 1947). The Jurassic is represented by the Middle Liassic oolithic limestones (Buser, 1986; jurkovsek e! al., 1988/89) and limestone breccias, the latter testifying an emersion of the area (Selli, 1947). The Julian carbonate platform was disintegrated in the Upper Lias due to compressive NE-SW movements, which led to a moderate and locally heavy folding of the area (Selli, 1947). In the surroundings of Bovec, there are typical dykes filled with red and grey crinoidal limestones. In the upper section, or at places directly on the Liassic neritic limestones, nodular limestones (Rosso AmmoniUco) occur. Close to Bovec, manganese deposits, often in the shape of round manganese nodules, are occasionally present. During o o eu £ O o vi 'A <0 7: Quaternary cover 6: Flysch (Csmpanian - Maastrichtian)' sandstones and conglomerates 5: Volie limestone end Scar¡.ïa ions a (Turoriani crœrty limestones 4: Liassic deposits; oolitic limestones and calcareous breccias, etc 3; Rethyan deposits: mudstones 2: Dachste.in (Upper Norton): imeslonns and dolomite limeslorie 1 Main DokxnUo (Lowei Norian}' dolomit« Bmettonei Mi. Rombon Mt. Potovnik 0.5 10 t.S km Fig. 1: Stratigraphic column of the stratigraphic section of Bovec basin with denoted position of studied conglomerate. A: Geographical location of Bovec basin; B: Sketch map showing the flysch outcrops (modified after Kuščer el al., 1974 and Buser, 1974). SI. 1: Stratigrafski sioiprc stratigrafske sekcije bovškega bazena z označenimi legami preučevanega konglomerata. A: geografska lega bovškega bazena; B: zemljepisna karta z vrisanimi flišnimi izdanki (prirejeno po Kuščer et al, 1974 in Buser, 1974). 258 ANNALES • Ser. hist. nal. ■ 13 • 2003 • 2 Alberto ROS5LT el at.: PRELIMINARY CHARACTERIZATION Of MAGMA IK O.A5TS I ROM CONGI.OMF.RATe' '..",' 257-262 the Early Jurassic, Mt. Polovnik probably rose, which acted as a barrier during the Cretaceous marine ingres-sion occurred in the sector between the Bovec area, located to north, and the Kobarid area, located to south. In the studied area, Cretaceous sediments are represented by thin bedded calcareous turbidites with chert of carbonate turbidities (Ogoreiec e( al., 1974} that gradually developed in siliciclastic turbiditic sediments (Bovec flysch) during the Campanian - Maastrichtian (Aubouin, 1963; Cousin, 1970; Kuscer etal., 1974; Pavsic, 1994). The present paper focuses on the conglomerates located at the top of the Bovec flysch (Fig. 1B). The clasts are mainly composed of Dachstein limestones, Rethyan-I iassic brown limestones, red and black chert limestones of "Scaglia rossa" and Volie fimestone, dark limestones with Raiblian fossils, volcanic clasts, few metamorphic clasts of probably Permic age, and plagio-ciase-rich sandstones (Selli, 1947; Kuscer el ai, 1974; Venturini & Tunis, 1992). MATERIAL AND METHODS Diffractometric analyses have been carried out at the "Oi parti men to di Sdenze deiia Terra" of the University of Trieste by means of powder diffTactometer SIEMENS D500 (CuKcc at 40 kV and 20 mA). Major and trace element concentrations were determined using PW 1404 XRF spectrometer and the procedures of Philips® (1994) for the correction of matrix effects. Major element abundances were recalculated to 100 wt% on a volatile-free basis. The analytical uncertainties were estimated at less than 5% and 10% for major and trace elements, respectively. The samples have been analysed before and after the leaching procedure (Petrlni et ai., 1987) in order to remove the secondary carbonates. RESULTS Petrographica! features, classification and nomenclature Fig. 2: Distribution of the Bovec volcanic clasts in R1-R2 classification diagram of De La Roche et al. (1980) as modified by Bellieni et al. (1981). Inset: SiOj vs. l eOi/MgO diagram for basic sub-alkaline rocks (Miva-shiro, 1974). SI. 2: Razširjenost bovških vulkanskih delcev v klasifikacijskem diagramu RJ-R2 po De La Roche et al. (J980) in modificiranem po Bellieni et al. (1981). V okvirčku: diagram Si02 vs. FeOjMgO za bazične subalkalinske kamnine (Miyashiro, 1974). The studied samples have been classified (Fig. 2) according to De La Roche et al. (1980), and Bellieni et al. (1981). Note that after the leaching procedure all the samples plot in the Andesi-basalt field, are in agreement with their optical features. Finally, the samples fall (inset of Fig. 2) in the tholeiitic field for the Si02 - FeOr/MgO relationships (Miyashiro, 1974). Difiractometrical data The analyses obtained from a representative volcanic clast (e.g. sample BOl; Fig. 3) revealed the presence of augite, plagioclase, caicite and minor clay amounts, supporting the optical features. Gcochemistrv The volcanic clasts have main sizes diameter variable from 1-3 cm. The dimension of magmatic clasts and the roundness factor of carbonate and metamorphic. clasts (roundness factor is not indicative for basaltic clasts, since their morphology is due to onion skin exfoliation) suggest a fluvial transport and a relative proximity to the erosion area. Petrographical analyses showed (hat higher glass contents recrystallized only partially into clay minerals. The texture ranges from subophitic-intersertal to micro-porphiric. Rare feno- and microfenocrystals of an-plagtoclase, augite, opaques and occasional olivines have been observed, secondary caicite plagues are often present. Due to the abundant plagioclase, these rocks have been optically classified as andesitic basalts. The C1PW norm has been calculated in dry conditions with Fe?Oy'FeO = 0.15. Among the normative minerals, quartz (6-10 wt %), hyperslerie (8-11 wt %), and occasional corindorie (0-0.5 wt %) were present. These features, together with the high SiO^ and Ah03 (> 53 and >18.5 wt %, respectively) and low MgO (< 4.5 wt %), associate the studied volcanics to the High-Allumina Basalt (HAB; James et al., 1986), usually related to compressive geotectonic: conditions (island-arc tectonic environment). These features contrast with the Si02 -FeO,/MgO relationships of Miyashiro (1974). The authors of the article believe that the scarcity of Si02 may possibly be connected with natural leaching due to the glass alteration in clay minerals and in colloidal phases. 259 ANN ALES • Ser. hist. nat. - 13 • 2003 • 2 Alberto ros5et« */.." prel IMINARV CHARAC 1'ERIZaTIOn Of MACMATIC C1.asts FROM CONGLOMERATE .... 25?-262 "IOC SM jOO o 250 W 2 200 c o u 150 100 50 ssmpie 901 Pi« CM Aug Fig. 3: Diffractometric analyses of the BO I sample (volcanic clast). Chi: chlorite; Pig: plagioclase; Cat: cal-cite; Aug: augite. Si. 3: Difraktometrične analize vzorca BOJ (vulkanski delec). Chi: klorit; Pig: plagioklaz; Cal: kalcit; Aug: avgit. ® " CAP « PAP cta T "O.-'A'.-O; (g) l .00 10 ITt'ole rock ^-airiples <3 "L.BHolifrp ís¿ii>|>Ibs j but differ from the Triassic basic magmatism (shoshoni-tic in composition) from the Alps (not shown). The studied samples have now been also compared (Fig. 6) with the younger vokranics of Cenozoic age from Ljubac (Croatia; Lugo vie et a!., 1998), which are certainly related to the compressive geotectonic environment. These samples show patterns similar to the Fig. 4: Zr/AljOj vs. TiO,/AI,Oj and Ce/P2Os vs. Zr/Ji02 tectonomagmatic diagrams (Miiller e! al., 1992). CAP: Continental Arc; IOP: Initial Oceanic Arc; LOP: Late Oceanic Arc; PAP: PostcolHsional Arc; WtP: Within Plate. Si. 4: Tektonomagmatska diagrama Zr/AljO-¡ vs. LiOj Al203 in Ce/P2Osvs Zr/TiO, (Miiller et a!., 1992). CAP: celinski lok; IOP: začetni oceanski lok; LOP: zadnji oceanski lok; PAP: pokolizijski lok: WIP: znotraj plošče. In the tectonomagmatic diagram of figLtre 4A (Miiller et al., 1992). the selected samples plot into the orogentc field of continental - postcoliisional arc basalts. After the leaching procedure (Fig. 48; Müller et a/., ¡992), the samples plot into the field of continental arc basalts. Considering the trace elements, samples (Fig. 5) are characterized by strong negative Nb anomaly (indicative of crustal contamination or involvement of eclogitic layers in mantle source) and by a negative Sr anomaly (related to plagioclase fractionation). The collected samples show, except for the Nb anomaly, patterns and mean incompatible elements tit) concentrations (Fig. 5) comparable with Upper Cretaceous - Paleogene tholeii-tic basalts from the Pannonian basin (Belak eta/., 1988), Fig. 5: Incompatible element patterns of Bovec volcanic clasts iwrma/fseif to N-MORB (Normal Middle Oceanic Ridge Basalt; Sun & McDonough, 1989). E-MORB: Enriched Middle Oceanic Ridge Basalt (Sun & McDonough, 1989); Požeška gora: Upper Cretaceous -Paleogene tholeiitic basalts of the southern margin of t/ic Pannonian Basin (Belak etal., 1988). SI, 5: Nezdružljivi vzorci elementov bovških vulkanskih delcev, normaliziranih na N-MORB (navadni srednjeo-ceanski grebenski bazah; Sun & McDonough, 1 989). E-MORB: obogateni srednjeoceanski grebenski bazah (Sun & McDonough, 1989); Požeška gora: zgornja kreda - paleogenski tolciitski bazalti na spodnjem robu panonskega bazena (Belak et al,, 1988). 100.0 jr* Ljubac 10.0: f 1.0: 0.1 i E-ívíQRB.' RS) Ga K Nl> La Ce Sr Ni) Zr Tí Y Fig. 6: Incompatible element patterns of Bovec volcanic clasts- normalised to N-MORB. Ljubac: Late Cenozoic volcanics from the northern External Dinarides (Lugo-vie etal., 1998). SI. 6: Nezdružljivi vzorci elementov bovških vulkanskih delcev, normaliziranih na N-MORB. Ljubac: mlajše ke-nozojske vulkanske kamnine iz severnih Zunanjih Di-naridov (Lugovič et al., 1998). 260 ANNALES • Ser. hist, nat, • 13 - 2003 • 2 Alberto R.OSSïT rl at.: PRELIMINARY CHARACTERISATION Of M AC,MAI IC CLAMS (ROM CONClO-MtKÄTE .... 257-262 Studied volcanics including the Nb negative anomaiy, bul presenl lower mean it contents for comparable grade of evolution. DISCUSSION In general, the morphology of all clasts suggests that rivers probably supplied them and that lite source area was probably quite close. As regards the magmatic clasts, they are quite scarce and their abundance does not appear compatible with a large volcanic apparatus closer than 100-150 km to the deposition area. Their frequency and petrographica! features suggest that these clasts may be derived from the erosion of magmatic structures as sills or dykes. Actually it is impossible to attribute a certain age to the magmatic clasts, but chemical features suggest that they belong to a magmatic event related to an orogentc geotectonical setting. Moreover, the trace elements indicate a source mantle enriched in incompatible elements with respect to the younger Cenozoic magmatisrn of the area. This SLiggests a more important involvement of a continental mantle source (in a compressive setting) comparable in many aspects with that of Pannonian volcanics. CONCLUSIONS We can summarise the results as follows: The studied magmatites are tholeiites one pyroxene-bearing (augites) characterized by subophitic-interseitai to micro porphiric. texture. The major elements associate the studied magmatites with the high allumina-basalts. As far as the genesis of these tholeiites is concerned, the teclonornagmatk: diagrams suggest a continental arc geodynamic environment. The ÎL chemistry supports the major elements constrains and suggests involvement of a crustal component (crustal contamination or subducted eclogitic slab) in the tholeiites genesis. Finally, the similitudes among the studied magmatites and those from the Ljubac and Pannonian basins suggest evolution of a common fitospheric mantle source. ACKNOWLEDGEMENTS The authors wish to thank Lorenzo Furlan and Mau-rizio Tentor for their thin-sections. PREDHODNA OPREDELITEV MAGMATSKIH DELCEV FLIŠNEGA KONGLOMERATA V BLIŽINI BOVCA Alberto ROSSET & Davide LENAZ Universita degii Studi di Trieste, DipartimerUo di Scienze del I d Terra, 1-34127 (lies te, via Weiss S E-mail: rosset@uriiv.trieste.it Giorgio TUNIS Universiöi degli Studi di Trieste, DipartimerUo di Scienze Geologie he, Arribientali e Marine, 1-34127 Trieste, via Weiss 2 Angelo DE MIN & Alessandro TOSONI Universüä degli Studi di Trieste, Dipartiiriento di Scienze della Tona, 1-34127 Trieste, via Weiss 8 POVZETEK Avtorji so ob preučevanju redkih magmatskih, po sestav/ bazaltnih delcev poskušali opredeliti maastrichtski konglomerat v povezavi s ftišnimi plasti slovenskega sediinentamega bazena., ki prihajajo na površje v bližini Bovca, Petrografske značilnosti in sestava iz poglavitnih elementov so jim omogočili, da so nekaj redkih zbranih delcev opredelili kol delce lave ali subintruzivnega bazalta z visoko vsebnostjo aluminija. Te kamnine so ponavadi povezane s kompresijskimi gibanji v subdukciji oceanskih plošč. Nezdružljivi vzorci elementov potrjujejo hipotezo o geotektonskem stiskanju in namigujejo, da je v nastanek magmatskih delcev vpleteno delovanje celinskega plašča. Ključne besede: konglomerat, Bovec, magmatski delci, HAB, Maastricht 261 ANN ALES • Ser. hist. nat. - 13 • 2003 • 2 Alltffrtci ROSSET m ,?(.; fflEUMWARY CHARACTERIZATION Of MAGMA!tC CIAS TS FROMCONUIOMIKATC .... ÜS?-¿62 REFERENCES Aubouin, J, (1963): Essai sur ia paléogeographie post-triasique et l'évolution secondaire et tertteire du versant sud des Alpes orientales (Alpes méridionales; Lombardie et Vénétie, lialie; Slovénie occidentale, Yougoslavie). Bull. Soc. Geol. Fr„ 71(5), 730-766. Belak, M., J. Halamk', V. Marchig & D. Tibljaš (1988): Upper Cretaceous-Pa leogene th o lei i tic basalts of the southern margin of the Parinonian Basin: Požeška gora Mt. (Croatia). Geol. Croat., 51 (2), 163-174. Bellieni, G., E. M. Piccirillo & B. 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