Trubelja et al.indd Triassic Magmatism in the Area of the Central Dinarides (Bosnia and Herzegovina): Geochemical Resolving of Tectonic Setting Fabijan TRUBELJA1, Klaus-Peter BURGATH2 and Vesna MARCHIG2 1. INTRODUCTION In the Central Dinarides of Bosnia and Herzegovina, Triassic magmatic rocks are associated with platform carbonate sediments and also occur frequently within Palaeozoic–Triassic allochthonous series (mountains of Central Bosnia, Sana–Una Palaeozoic complex, south- east Bosnia near Čajniče, Foča, Tjentište and Kalinovik – Fig. 1). Towards the southeast, the Triassic magma- tism continues into the territory of Montenegro. The same formations can also be found in Slovenia, Croatia, and Serbia. Previous investigations resulted in two different conclusions about the tectonic setting and origin of the Triassic magmatic rocks: (1) BEBIEN et al. (1978) classified the rocks as a prod- uct of subduction-initiated magmatism, while (2) PAMIĆ (1984) explained them as a product of the rifting of continental crust. A new investigation by KNEŽEVIĆ et al. (1998) yielded ambiguous results: within plate magmatism combined with volcanic arc influence. TRUBELJA et al. (2000) came to a conclusion similar to Pamić’s explanation, as they described contamination of the magmatic rocks with a continental crust component which can be best explained by magmatic activity in the course of opening of a continental rift. The aim of this work is to contribute new micro- scopic and geochemical data to the clarification of the tectonic setting of the Triassic magmatism in Bosnia and Herzegovina. Microscopic studies were performed for the classification of the rocks but they served main- ly for estimating the intensity of alteration with possible changes of the original chemical composition, and from this the selection of appropriate geochemical discrimi- nation diagrams (with elimination of elements which could be mobile in the course of alteration). ‘Classic’ and newly developed geochemical diagrams were used for determination of the geotectonic setting of the magmatic rocks including highly evolved members. Rare earth elements patterns were also investigated and included in the discussion. Geologia Croatica 57/2 159–170 13 Figs. 2 Tabs. ZAGREB 2004 Key words: Triassic, Magmatic rocks, Geochemistry, Rare earth elements, MORB, Arc magmatism, Bos- nia and Herzegovina. 1 Academy of Sciences and Arts of Bosnia and Herzegovina, Bistrik 7, 71000 Sarajevo, Bosnia and Herzegovina. 2 Bundesanstalt für Geowissenschaften und Rohstoffe, Stilleweg 2, 30655 Hannover, Germany; e-mail: k.burgath@bgr.de. Abstract Triassic magmatic rocks in the Central Dinarides in Bosnia and Herze- govina are known from two separate geotectonic units: (1) the Adri- atic Carbonate Platform (Outer Dinarides) and (2) the Palaeozoic–Tri- assic allochthonous complex. They are assigned to the same regional, genetic and geochemical unit. Their emplacement age is inferred from contacts with the surrounding marble and sedimentary rocks (post- Anisian for intrusives and Ladinian for effusives). The magmatic rocks display different levels of emplacement and crystallization (intrusive, effusive and dyke rocks). They represent different stages of magmatic differentiation, from gabbro/basalt via diorite/andesite to granodiorite/dacite and granites. The most frequent dyke rock is diabase. Pillow basalts indicate eruption under sub- aquatic conditions. Pyroclastic rocks within the volcano-sedimentary unit point to the temporary explosive character of orogenic magmatic activity. Most rocks are affected and modified by post-magmatic alteration and hydrothermal fluids. This led to the formation of spilite, keratophyre, quartz keratophyre and rarely K spilite. New geochemical data support the opinion that subduction was the main process which triggered the Triassic magmatic activity in the Central Dinarides. Although some of the investigated rocks reveal MORB characteristics (in the selected geochemical discriminations), most samples are enriched in all elements which are reported as char- acteristic for arc magmatism at convergent margins including incor- poration of sediments. 160 Geologia Croatica 57/2 161Trubelja, Burgath & Marchig: Triassic Magmatism in the Area of the Central Dinarides... 2. OVERVIEW OF GEOLOGY AND PETROGRAPHY Triassic magmatic rocks in the Dinarides form a spatial, genetic(?) and geochemical unity with a wide range of rocks varying from basalt/gabbro to rhyolite/granite. All levels of solidification are present, i.e. intrusive, effusive and hypabbysal rocks. The most frequent dyke type is of diabase composition. Effusive and dyke rocks are partly modified to spilites, keratophyres and quartz keratophyres. K-spilites with adularia as their main constituent are also present, but quite rare (TRUBELJA, 1978). The Jablanica gabbro is the most investigated rock with its halo in the surrounding sediments (Fig. 2). In the northwestern part this intrusive body is in con- tact with an Anisian limestone which was thermally metamorphosed to marble. The first description of the contact, the developed contact-metamorphic silicates (garnet, albite, chlorite, sericite, titanite, epidote), and the skarn deposit of Tovarnica with magnetite ore was published by CISSARZ (1956). After ČELEBIĆ (1967), the intrusion age of this gabbroic complex is post-Anisian. In the same region a corresponding con- tact-metamorphic zone was found on the left side of the river Crima (Fig. 3). Another huge basic intrusion occurs in the Radovan Mt. (Novi Travnik), rather similar to the Jablanica gab- bro. Rb/Sr isotopic data of this rock determined an age of 223–232 million years (PAMIĆ & LOVRIĆ, 1980). Effusive rocks include basalts, andesites and dacites and their modified equivalents spilite, keratophyre and quartz keratophyre. They are mostly in concordant position within volcanic–sedimentary associations of Middle Triassic ages (palaeontological determinations; Fig. 4). The most intense volcanic activity was of Ladinian age. Pyroclastic rocks (tuffs) are common within the volcanic-sedimentary association, showing that the volcanic activity was partly explosive. Also pillow lavas have been observed indicating underwater volcanic activity. In some localities (Vareš, Hrčavka river on Tjentište, valley of the Vrbas river) mixing of lava with sedimen- tary material can be observed. Limestone xenoliths within lava flows frequently occur. The effusive rocks in these locations display typical amygdaloidal fabric. The amygdules with diameters reaching more than 10 centimetres are filled predominantly with calcite. 3. METHODS OF INVESTIGATION With respect to their general classification, qualification of samples for chemical analysis and applicability of discrimination methods, 15 rocks were selected cov- ering the range of Triassic magmatism in Bosnia and Fig. 1 Sketch map of selected occurrences of Triassic magmatic rocks in Bosnia and Herzegovina. Legend: 1) pyroxene andesite – riv- er Doljanka; 2) keratophyre – Trešanica, Bradina; 3) basalt – Vareš; 4–6) gabbro-norite – Jablanica; 7) olivine- bearing gabbro-norite – Novi Travnik; 8) andesite or dacite – Trnova, Sanski Most; 9) sodium rhyolite – Fojnica; 10) quartz-diorite – Ćusine, Jajce; 11) basalt – Babino Selo, Vrbas river valley; 12) basalt, Vrbas river valley; 13) basalt (spilite) – Tjentište; 14) basalt – Kalinovik; 15) basalt – Dobro polje. 160 Geologia Croatica 57/2 161Trubelja, Burgath & Marchig: Triassic Magmatism in the Area of the Central Dinarides... Herzegovina. Polished thin-sections of these rocks were prepared and studied microscopically. The aliquots of selected samples were crushed in a shatter box with agate inlay for chemical analyses. One gram of powder was mixed and melted with 5 g LiBO2 at 1200°C for 20 minutes. The obtained glass was ana- lysed by X-ray fluorescence; Phillips PW 1400 and PW 1480 wavelength dispersive spectrometers were used. Analytical precision was better than 0.5% (relative) for major elements and 1–10 ppm for trace elements. The quality of the results was controlled with certified refer- ence materials (CRM) (i.e. BCR, Community Bureau of Reference, Brussels). After being used in XRF the glass was crushed and dissolved in an acid mixture (HCl–HNO3) in a micro- wave oven. From this solution trace elements below the detection limit of XRF were determined, including rare earth elements. The instrument used was an inductively coupled plasma-mass spectrometer (ICP-MS) Perkin Elmer Sciex Elan 5000. Analytical precision was con- trolled using CRM, and was better than 5% (relative). 4. PETROGRAPHIC DESCRIPTION OF SAMPLES The selected representatives of Triassic magmatism in Bosnia and Herzegovina were classified as follows according to their mineralogical composition: 1 – porphyric pyroxene andesite, weakly altered, Doljan- ka river valley (sample T99/1); 2 – keratophyre, strongly altered, Trešanica, Bradina (sample T99/2); 3 – (tholeiitic?) basalt, strongly altered, Vareš (sample T99/3); Fig. 2 Geological map of the Jablanica gabbro massif with sedimentary country rocks (from ČELEBIĆ, 1967). 162 Geologia Croatica 57/2 163Trubelja, Burgath & Marchig: Triassic Magmatism in the Area of the Central Dinarides... 4 – cumulate gabbro-norite, strongly altered, Jablanica (sample T99/4); 5 – cumulate gabbro-norite with subplanar fabric, weak- ly altered, Jablanica (sample T99/5); 6 – cumulate gabbro-norite with subplanar fabric, weak- ly altered, Jablanica (sample T99/6); 7 – olivine-bearing gabbro-norite, weakly altered, Novi Travnik (sample T99/7); 8 – andesite or dacite, strongly altered, Trnova, Sanski Most (sample T00/5); 9 – sodium rhyolite, medium alteration, Fojnica (sample T00/6); 10 –fine-grained quartz diorite, strongly altered, Ćusi- ne, Jajce (sample T2/00/1); 11 – tholeiitic basalt, medium alteration, Babino Selo, Vrbas river valley (sample T2/00/3); 12 – tholeiitic basalt, strongly altered, Vrbas river valley (sample T2/00/4); 13 – tholeiitic basalt, medium alteration, Tjentište (sam- ple T3/00/2); 14 – olivine-bearing tholeiitic basalt, strongly altered, Kalinovik (sample T3/00/3); 15 – tholeiitic basalt, strongly altered, Dobro Polje (sample T3/00/4). The mineral assemblages and petrographic details are listed in Table 1. The modification of the mineral content in the weakly altered samples is expressed by devitrification of glass (in T99/1, T2/00/3), by replacement of olivine to various extents by nontronite (T99/7) or carbonate (T3/ 00/4), by replacement of pyroxene by clinoamphibole (T99/7) or by a mixture of chlorite, sphene, carbonate, quartz/chalcedony or very fine-grained undeterminable Fig. 3 Profile through the contact of gabbro– diorite and platy crinoidal limestone with a magnetite ore body (after ČELEBIĆ, 1967). Legend: 1) platy crinoidal limestones; 2) contact-metamorphic limestones/marbles; 3) scarn; 4) gabbro diorite. Fig. 4 Basalt concordant in the volcanogenic–sedimentary association, area of Konjic, Jablanica, Prozor (after ČELEBIĆ, 1967). Legend: 1) Upper Triassic lime- stones and dolomites; 2) cherts and basalts; 3) reddish shales and radio- larites; 4) haematite; 5) basalt; 6) Middle Trias- sic reddish limestones. Table 1 Mineral assemblages and petrographic details of the rocks. 162 Geologia Croatica 57/2 163Trubelja, Burgath & Marchig: Triassic Magmatism in the Area of the Central Dinarides... Number Type of rock; Texture Composition Estimation of of sample Location and deformation alteration T99/1 pyroxene andesite; porphyric phenocrysts: weak; Doljanka plagioclase with pyroxene and glass inclusions; cloudy alteration to zoisite, mainly introduction of chlorite, carbonate; CO2 and H2O pyroxene (two types): fresh augite and pseudomorphs (with chlorite, carbonate, sphene, chalcedony); Ti-bearing magnetite; groundmass: originally glass, with magnetite grains; recrystallized to a mixture of plagioclase, chlorite, leucoxene, carbonate T99/2 keratophyre; microporphyric phenocrysts: strong; Trešanica, Bradina plagioclase (albite): corroded and deformed, with clouds of sericite and alteration of plagioclase, chlorite; mafic grains; probable amphibole (and pyroxene?), replaced by chlorite and carbonate; introduction of Si, K, opâque phases, replaced by leucoxene; Mg, Fe, CO2; quartz (very rare); strongly schistosed, groundmass: with tendency to augen plagioclase laths in fluid arrangement, sericite, leucoxene texture; chlorite–sericite and carbonate–quartz veins T99/3 (tholeiitic?) microporphyric, phenocrysts: strong; basalt; vesicular mafic phases, totally replaced by pumpellyite±epidote; probable introduction Vareš groundmass: of Ca and loss of Si plagioclase laths (cores filled with pumpellyite), diopsidic augite, chlorite; vesicules filled with pumpellyite, prehnite, quartz T99/4 cumulate gabbro- subhedral granular, plagioclase with clinopyroxene and magnetite inclusions; round hypersthene slightly (T99/5,6) to T99/5 norite; partly with subplanar (~10 vol. %, partly replaced by diopside); diopside with inclusions of biotite, strongly (T99/4) altered T99/6 Jablanica plagioclase fabric amphibole and opâque phases; ilmenite; quartz (rare; in interstices); replacements: plagioclase replaced by sericite, epidote, prehnite, chlorite, pumpellyite; pyroxene replaced by green amphibole (cores), actinolite and barroisite (rims), biotite, sphene, carbonate, chlorite; ilmenite replaced by biotite (biotite replaced by chlorite, sphene) T99/7 olivine-bearing subophitic plagioclase; orthopyroxene; clinopyroxene; olivine (partly as corroded very slight gabbronorite; inclusions in pyroxene), replaced by nontronite; titanomagnetite– (replacement of olivine, Novi Travnik orthopyroxene symplectites; biotite oikocrysts with inclusions of pyroxene, titano- plagioclase, or biotite rims enclosing corroded titanomagnetite; green magnetite) fibrous amphibole and clay minerals in interstices between plagioclase; T00/5 andesite; microporphyric, phenocrysts: strong; Trnova, Sanski intergranular plagioclase with albite-rich rims, filled with sericite and epidote; replacement of all Most pseudomorphs of chlorite ± sphene after mafic phases (biotite; amphibole; primary phases, pyroxene?) formation of epidote opâque grains (few); groundmass: plagioclase (albite-rich), mafic grains, primary quartz, apatite; alteration phases: sericite, sphene, secondary quartz; aggregates and network of epidote (very common), chlorite, carbonate T00/6 sodium rhyolite, pilotaxitic phenocrysts: medium; Fojnica albite (few grains; stained by fluid inclusions and opâque dust); modified by introduction groundmass: of Ca, CO2, K, Ba albite laths and interstitial quartz (anhedral); numerous microveins filled with sericite and green clusters of radiating tourmaline (zoned); network of anhedral carbonate and individual carbonate grains T2/00/1 quartz diorite, subhedral granular, plagioclase (completely replaced by sericite aggregates and epidote very strong; fine-grained; graphic patches); interstices: graphic intergrowth of plagioclase and quartz; some abundant sericite and Ćusine, Jajce skeletal opâque grains with leukoxene rims; some clinoamphibole grains epidote formation (replaced by chlorite) T2/00/3 tholeiitic basalt; microporphyric, phenocrysts: medium; Babino Selo, hyalopilitic plagioclase (albite-rich; filled with sericite, chlorite, epidote); introduction at least Vrbas valley mafic phases (pyroxene, probably some amphibole), completely replaced of K, H2O by chlorite, opâque dust, sphene, very fine-grained phyllosilicates; opâque phases; groundmass: formerly glass, with plagioclase laths; glass replaced by a fine-grained mixture of chlorite, sphene, phyllosilicates, opâque dust T2/00/4 andesite; microporphyric, phenocrysts: very strong; Vrbas valley hyalopilitic, plagioclase, largely replaced by carbonate+chlorite; introduction of Si, Ca, vesicular mafic phases, completely replaced by chlorite±quartz; CO2, H2O groundmass: formerly glass, altered to a fine-grained mixture of chlorite, sericite, opâque dust, sphene; vesicles filled with quartz T3/00/2 tholeiitic fine-grained plagioclase (stained by sericite and zoisite?) and diopsidic augite (Ti- medium; basalt (spilite); subophitic bearing) with interstitial chlorite and patchy sphene; numerous patches introduction of CO2 and Tjentište filled with chlorite and a radiating sheet silicate; Cr-bearing spinel H2O (290 ppm Cr in analysis!) T3/00/3 olivine-bearing subophitic phenocrysts: strong; tholeiitic basalt; vesicular olivine (euhedral pseudomorphs, filled with phyllosilicates, carbonate) with introduction of CO2 and Kalinovik primary inclusions of Cr-bearing spinel); H2O plagioclase: rounded grains, stained by phyllosilicates and epidote; groundmass: plagioclase (strongly stained) and clinopyroxene with interstitial phyllosilicates, chalcedony, opâques; vesicles filled with very fine-grained phyllosilicates (Fe-rich saponite/griffithite?); some radiated carbonate aggregates T3/00/4 tholeiitic basalt subophitic some carbonate pseudomorphs after olivine? (spinel inclusions!); strong; (diabase), similar vesicular rather fresh; plagioclase laths strongly stained by sericite and chlorite; as in T3/00/3 to T 3/00/3; accessory Cr-bearing spinel in the groundmass Dobro Polje 164 Geologia Croatica 57/2 165Trubelja, Burgath & Marchig: Triassic Magmatism in the Area of the Central Dinarides... phyllosilicates (T99/1, T2/00/3), and by replacement of plagioclase by sericite, zoisite, chlorite, carbonate, or epidote (T99/1, T3/00/2, T2/00/3). Strongly altered samples are characterised by the above, but more advanced modes of replacement. Clino- and orthopyroxene are mainly conserved (T99/1, T99/5, T99/6), but may be largely to completely replaced by secondary phases (T99/2, T99/3, T99/4, T99/7, T2/00/3, T2/00/4) in both slightly and strongly altered samples. Chromium-bearing spinels are rather common in the basalt samples, but are not altered. They occur either as inclusions in former olivine in the strongly altered sample T3/00/3 or as an accessory phase in olivine- free rocks (T3/00/2, T3/00/4). Accessory Ti-bearing magnetite is common in andesites (weakly to strongly altered) and is rimmed by sphene, which is also present as patches throughout the rocks. The gabbroic rocks (T 99/4, T 99/6) contain ilmenite with rims of biotite which is replaced by chlorite. It can be concluded from microscopic observations that Cr, Ti, and V (incorporated in spinel) and, to some extent, the HREE (conserved pyroxene!) showed rather immobile behaviour during alteration. The contents of Na, K, Ni (formerly incorporated in olivine), and to some extent of Ca, Mg were evidently modified and are unable for consideration in the discrimination of the original tectonic setting of the investigated rocks. 5. CHEMICAL COMPOSITION OF BASALTIC AND GABBROIC ROCKS The chemical composition of the 15 analysed samples is shown in Table 2. Eleven samples have the composition of basalts or gabbroic rocks (SiO2<54%) and will be discussed later. For samples T99/4 to T99/6 (gabbroic rocks with cumulate fabric) the further use of basalt discrimination diagrams below is subject to the restricted Location 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Sample T99/1 T99/2 T99/3 T99/4 T99/5 T99/6 T99/7 T00/5 T00/6 T2/00/1 T2/00/3 T2/00/4 T3/00/2 T3/00/3 T3/00/4 SiO2 52.07 63.33 46.54 53.70 51.92 51.49 49.42 60.64 70.35 56.53 51.00 53.35 51.09 47.63 51.34 TiO2 1.00 0.57 0.83 0.94 0.89 0.96 0.57 0.48 0.13 1.02 1.16 0.85 1.13 0.80 1.09 Al2O3 16.74 15.35 16.62 17.14 17.92 15.91 16.68 17.77 15.64 17.79 19.48 15.79 16.72 13.91 17.28 Fe2O3 8.53 6.19 7.77 8.00 9.03 10.83 9.36 5.00 0.50 7.88 8.79 6.11 7.71 6.23 7.74 MnO 0.16 0.09 0.11 0.14 0.19 0.22 0.17 0.12 0.01 0.07 0.15 0.14 0.09 0.12 0.11 MgO 2.78 3.29 7.09 3.64 5.38 6.38 8.47 1.90 0.97 1.77 4.82 2.06 6.83 3.33 6.64 CaO 7.66 1.00 12.15 6.52 10.01 10.36 11.23 4.46 1.52 7.09 2.91 8.98 6.61 13.24 5.22 Na2O 5.27 6.15 3.24 4.20 2.92 2.65 2.41 4.57 7.41 2.75 5.21 3.26 4.78 5.37 4.97 K2O 0.19 0.72 0.20 1.65 0.99 0.69 0.86 1.79 0.63 1.94 2.36 1.75 0.98 0.48 1.33 P2O5 0.22 0.12 0.14 0.20 0.12 0.07 0.06 0.20 0.06 0.19 0.16 0.15 0.19 0.15 0.17 LOI 4.99 2.89 4.94 3.38 0.22 0.19 0.39 2.57 2.51 2.50 3.59 6.97 3.45 8.22 3.65 Total 99.62 99.70 99.63 99.51 99.60 99.75 99.63 99.49 99.74 99.52 99.63 99.41 99.58 99.48 99.54 Cs 1.21 0.91 0.28 2.17 2.27 1.01 2.61 3.49 0.50 1.83 2.21 2.83 0.33 0.32 2.64 Rb 4.58 13.9 2.12 62.8 35.8 22.3 36.0 40.6 5.32 40.6 46.8 32.2 13.2 10.8 27.0 Ba 133 196 60 316 216 171 167 131 52 398 208 197 211 59 303 Th 7.17 5.75 1.87 6.64 3.43 1.78 3.10 8.38 0.80 6.50 8.30 4.20 2.50 2.70 3.80 U 2.20 1.46 0.52 1.50 0.97 0.49 0.73 1.80 0.24 1.89 0.66 1.20 0.75 0.80 0.86 Ta 0.79 0.59 0.63 0.73 0.40 0.28 0.32 0.61 0.094 0.54 0.61 0.36 0.61 0.57 0.78 Nb 10.5 7.63 8.40 8.49 5.29 3.62 3.52 7.54 1.13 7.40 8.60 5.40 10.7 9.90 12.6 Pb 11.6 2.06 1.52 4.82 7.53 3.85 6.62 9.73 1.09 7.90 5.20 6.90 2.30 2.90 4.80 Sr 242 63 63 184 328 236 246 370 114 261 90 119 280 198 338 Zr 160 137 66 126 70 52 57 139 41 65 58 46 59 50 60 Hf 4.38 3.83 1.92 2.84 1.97 1.26 1.53 3.69 1.24 4.30 4.10 3.20 2.50 2.20 2.80 Y 33.0 15.8 18.4 28.2 20.6 19.9 16.0 16.1 2.14 22.2 24.2 17.0 28.6 22.1 26.7 Cr 18 141 238 41 71 55 256 21 38 31 12 51 311 280 366 Ni udl 18 69 10 10 udl 86 6 9 8 10 14 46 68 38 V 216 119 235 194 259 216 195 122 12 178 143 137 306 235 289 Sc 23 16 30 23 33 46 42 9 2 27 25 21 50 43 50 Zn 104 66 39 110 96 87 66 106 8 29 99 240 52 76 63 La 22.4 15.2 8.69 18.2 11.6 7.38 9.26 23.2 4.11 20.7 28.7 15.1 13.0 12.6 16.4 Ce 47.2 31.8 18.9 40.5 26.0 16.9 19.6 44.7 8.29 34.5 48.9 26.4 29.4 28.5 33.9 Pr 5.67 3.56 2.59 5.29 3.44 2.38 2.70 5.50 0.96 5.19 6.14 3.35 3.44 2.93 3.85 Nd 23.4 13.2 11.2 22.0 14.6 10.8 11.4 20.5 3.67 22.8 26.1 14.8 14.4 12.9 16.4 Sm 5.42 2.67 2.82 5.09 3.59 2.93 2.77 3.86 0.69 4.82 5.24 3.37 4.02 2.94 3.80 Eu 1.35 0.65 0.95 1.34 1.16 1.04 0.84 0.97 0.18 1.24 1.59 0.99 1.25 1.01 1.16 Gd 5.82 2.68 3.27 5.35 3.87 3.41 3.03 3.43 0.59 5.00 5.61 3.60 4.08 3.31 4.25 Tb 0.92 0.44 0.54 0.82 0.60 0.57 0.48 0.49 0.079 0.81 0.93 0.60 0.66 0.59 0.73 Dy 5.69 2.73 3.37 5.04 3.73 3.62 2.94 2.81 0.40 5.13 5.63 3.85 4.54 3.41 4.30 Ho 1.18 0.58 0.71 1.01 0.77 0.74 0.59 0.55 0.07 1.04 1.18 0.80 0.92 0.72 0.91 Er 3.50 1.78 2.03 2.94 2.23 2.22 1.76 1.66 0.18 3.02 3.57 2.43 2.63 2.18 2.65 Tm 0.51 0.27 0.30 0.44 0.32 0.33 0.25 0.25 0.026 0.46 0.52 0.35 0.38 0.31 0.37 Yb 3.38 1.81 1.95 2.83 2.13 2.14 1.63 1.68 0.170 2.91 3.14 2.27 2.45 2.29 2.43 Lu 0.56 0.29 0.31 0.46 0.34 0.35 0.26 0.28 0.027 0.48 0.56 0.39 0.39 0.37 0.39 Table 2 Bulk rock chemical compositions. Oxides from SiO2 to P2O5 (in wt. %) by XRF; total iron as Fe2O3. Trace elements and REE (in ppm) by ICP–MS; udl – under the detection limit. 164 Geologia Croatica 57/2 165Trubelja, Burgath & Marchig: Triassic Magmatism in the Area of the Central Dinarides... application of these diagrams to rocks of non-cumulate origin. Figure 5 shows the REE content normalized on a chondrite basis (WAKITA et al., 1971) of eleven samples with basaltic or gabbroic composition. There is no major discrepancy between the patterns of basalts and gabbroic rocks. The patterns in general are typical for E-MORB with enrichment of LREE in comparison to HREE (La/ Lucn between 3 and 5). Two different signatures were used for samples with observed weak alteration and for samples with strong alteration; no significant differences between these two stages of alteration are recognized. It is well known that strong alteration can influence LREE and transfer them into a soluble phase. We explain the same contents of LREE in strongly altered samples with transfer of LREE from the original primary minerals during their disintegration to secondary minerals. A common and approved tool for the specification of tectonic settings of basaltic rocks is the Ti/100–Zr– 3Y diagram introduced by PEARCE & CANN (1973). Most of the basalts and two gabbroic rocks plot into field B (Fig. 6), which is the ambiguous field between island arc basalts, ocean floor basalts, and calc-alkalic basalts. Hence this diagram is not very suitable for dis- crimination of the tectonic setting of rocks which fall into field B. The field should be further divided, but the fact that none of the samples is situated in the field of “within plate basalts” (D) is considered as a geochemi- cal argument against rifting as the tectonic setting of the investigated rocks. The plotted samples include those Fig. 5 Rare earth elements normalized on a chondrite basis (WAKITA et al., 1971) for basaltic and gabbroic samples. Two dif- ferent signatures show two different stages of altera- tion: full line and full circles for weakly altered samples and stripped line and empty circles for strongly altered samples. Fig. 6 Ti–Zr–Y diagram for differentiation between basalts from different tectonic settings (after PEARCE & CANN, 1973). 166 Geologia Croatica 57/2 167Trubelja, Burgath & Marchig: Triassic Magmatism in the Area of the Central Dinarides... with weak as well as those with strong alteration, but they do not cluster separately. This indicates that Ti, Zr, and Y behaved in a largely immobile manner during alteration. MÜLLER et al. (1992) proposed Th/Yb vs. Ta/Yb diagrams for discriminating between subduction- induced basaltic volcanism against basaltic rocks crys- tallised from mantle-derived melts in other tectonic settings. Th is considered to be derived from the sedi- mentary component of the subducted slab, although the exact mechanism responsible for the enrichment of Th with respect to Ta in subduction-induced volcanism is controversial. Yb is used as a denominator to neutralise the effects of partial melting and/or fractional crystalli- sation. Figure 7 shows the discrimination diagram after MÜLLER et al. (1992): most of the investigated rocks are within the field of orogenic magmatism (calc alka- line and shoshonitic rocks). Here also no clustering of the investigated samples with strong alteration is visible compared to those with weak alteration. Another commonly used diagram for the discrimi- nation of basalts from different tectonic settings is the Hf/3–Th–Ta plot proposed by WOOD (1980). Thorium is used as indicator for the influence of subducted sedi- ments (as in MÜLLER et al., 1992) and Hf and Ta are considered as two immobile elements. This diagram should be especially suitable to identify volcanic arc basalts. The possible mobility of Th during alteration could move the position of samples in the direction of MORB-types, but if samples plot in the volcanic arc field, then there is no doubt about a volcanic arc set- ting. This is exactly the case for our samples plotted in Fig. 8. Fig. 7 Th/Yb vs. Ta/Yb diagram for discriminating between tholeiitic, calc-alkaline and shoshonitic basalts (after MÜLLER et al., 1992). Fig. 8 Hf–Th–Ta diagram for discrimination between basalts from different tectonic settings (after WOOD, 1980). 166 Geologia Croatica 57/2 167Trubelja, Burgath & Marchig: Triassic Magmatism in the Area of the Central Dinarides... It is further observed that the geotectonic specifica- tion of samples containing Cr-bearing spinel (tiny euhe- dra crystallized from basaltic melt), and consequently with elevated chromium contents (T3/00/2, T3/00/4) is equivocal (either mid-ocean ridge or arc origin in the discrimination diagrams). Figures 9 and 10 show Cr vs. Ti after GARCIA (1978) and Cr vs. Y after PEARCE et al. (1984), respectively. In both cases several samples including those with a spinel content plot outside the field of volcanic arc basalts. The samples are inter- spersed with innumerable patches of leucoxene which suggests introduction of Ti during alteration. This process could be responsible for the diverging posi- tion in the plots of Cr vs. Ti and Cr vs. Y. However, Fig. 9 Cr vs. Ti diagram after GARCIA (1978). The striped line divides the dia- gram into fields for ocean floor basalts and volcanic arc basalts. Fig. 10 Cr vs. Y diagram after PEARCE et al. (1984). The field for mid ocean ridge basalts (MORB) is framed with a solid line; the field for island arc tholeiites (IAT) is framed with a striped line. 168 Geologia Croatica 57/2 169Trubelja, Burgath & Marchig: Triassic Magmatism in the Area of the Central Dinarides... it is noticed that Ti and Y are generally recognized as rather immobile during low-T alteration (e.g. JUTEAU & MAURY, 1997) and that Ti and Y are positively cor- related in samples T3/00/2 to T3/00/4 (cf. Table 2). The leukoxene patches are mainly restricted to chlo- rite–phyllosilicate-rich interstitial positions which indi- cate formation via devitrification and thus the elevated contents of these elements are explained as a primary magmatic signature. This is not in contradiction with the classification of the other investigated samples as volcanic arc rocks because a wide range of magmatic rocks, from ridge basalts via IAT to calc-alkaline rocks can be associ- ated in zones of former convergent margins, e.g. in Semail, Oman (BEURRIER et al., 1989), Pindos, Greece (BECCALUVA et al., 1979) or Mirdita, Albania (GJATA, 19973; HÖCK et al., 1998). The spider diagram in Fig. 11 shows the average values for basic rocks, and the enrichment of LIL ele- ments in comparison to normal MORB (SAUNDERS & TARNEY, 1979). This shows that all basic rocks from our investigated area, without exception (individual spi- der diagrams for every sample not shown), were already changed by additions from the subducted slab although some characteristics of MORB are still preserved. The characteristic enrichment of LIL elements together with the depletion of HFS elements are also known to be unique for calc-alkaline arc-related volcanic rocks. 6. CHEMICAL COMPOSITION OF ROCKS WITH HIGHER DEGREE OF DIFFERENTIATION Figure 12 shows rare earth element patterns normal- ized on a chondrite basis for four samples which are of a higher degree of differentiation (up to rhyolite with >70% SiO2). They also display a similar, but stronger enrichment of LREE compared to HREE as in the basic rocks. The most differentiated rock (the lowest one on Fig. 12) reveals an La/Lucn of 15. Hitherto, subduction or rifting was suggested as the tectonic setting for the Triassic magmatic rocks of the Dinarides. Generally these settings are both character- ised by a wide range of differentiation, but the applied discrimination diagrams are unfortunately limited to basaltic compositions. GORTON & SCHANDL (2000) enlarged their use to intermediate and felsic magmatic rocks with SiO2 contents from 54–77 wt.% SiO2. These enlarged diagrams were developed on the basis of a large collection of rock analyses with known tectonic settings. In Fig. 13 four samples with higher SiO2 con- tents of our investigation are plotted. Their position is within the field of subduction-induced volcanism, and, furthermore, in the field of active continental margins. 7. CONCLUSIONS Triassic magmatic rocks in the area of the Central Dinarides in the territory of Bosnia and Herzegovina are explained as products of subduction-triggered vol- canism at an active continental margin (Andean type volcanism). The mobility of elements during alteration was tested by means of comparing the compositions of samples with either strong or weak alteration. The use of rather immobile trace elements shows the influence of subducted slab material, including sediments on the investigated magmatic rocks. Fig. 11 Spider diagram for average of eleven magmatic rocks with SiO2<54%, normalized to mid ocean ridge basalts (MORB) as suggested by PEARCE (1983). 3 GJATA, K. 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