Triassic Magmatism in the Area of the Central Dinarides ( Bosnia and Herzegovina ) : Geochemical Resolving of Tectonic Setting

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, southeast Bosnia near Čajniče, Foča, Tjentište and Kalinovik – Fig. 1). Towards the southeast, the Triassic magmatism 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 product of subduction-initiated magmatism, while (2) PAMIĆ (1984) explained them as a product of the rifting of continental crust.


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, southeast Bosnia near Čajniče, Foča, Tjentište and Kalinovik -Fig.1).Towards the southeast, the Triassic magmatism 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 product 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 microscopic 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 mainly for estimating the intensity of alteration with possible changes of the original chemical composition, and from this the selection of appropriate geochemical discrimination 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.
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 subaquatic 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 characteristic for arc magmatism at convergent margins including incorporation of sediments.

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 contact 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 contact-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 sedimentary 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.

METHODS OF INVESTIGATION
With respect to their general classification, qualification of samples for chemical analysis and applicability of discrimination methods, 15 rocks were selected covering the range of Triassic magmatism in Bosnia and  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 LiBO 2 at 1200°C for 20 minutes.The obtained glass was analysed 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 reference 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-HNO 3 ) in a microwave 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 controlled using CRM, and was better than 5% (relative).
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.

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 (SiO 2 <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 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/ Lu cn 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 discrimination 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 geochemical argument against rifting as the tectonic setting of the investigated rocks.The plotted samples include those 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 subductioninduced basaltic volcanism against basaltic rocks crystallised from mantle-derived melts in other tectonic settings.Th is considered to be derived from the sedimentary 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 crystallisation.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 discrimination 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 sediments (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 setting.This is exactly the case for our samples plotted in Fig. 8.It is further observed that the geotectonic specification of samples containing Cr-bearing spinel (tiny euhedra 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 interspersed with innumerable patches of leucoxene which suggests introduction of Ti during alteration.This process could be responsible for the diverging position in the plots of Cr vs. Ti and Cr vs. Y.However,  (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.
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 correlated in samples T3/00/2 to T3/00/4 (cf.Table 2).The leukoxene patches are mainly restricted to chlorite-phyllosilicate-rich interstitial positions which indicate 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 associated 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, 1997 3 ; HÖCK et al., 1998).
The spider diagram in Fig. 11 shows the average values for basic rocks, and the enrichment of LIL elements in comparison to normal MORB (SAUNDERS & TARNEY, 1979).This shows that all basic rocks from our investigated area, without exception (individual spider 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.

CHEMICAL COMPOSITION OF ROCKS WITH HIGHER DEGREE OF DIFFERENTIATION
Figure 12 shows rare earth element patterns normalized on a chondrite basis for four samples which are of a higher degree of differentiation (up to rhyolite with > 70% SiO 2 ).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/Lu cn 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 characterised 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 SiO 2 contents from 54-77 wt.% SiO 2 .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 SiO 2 contents of our investigation are plotted.Their position is within the field of subduction-induced volcanism, and, furthermore, in the field of active continental margins.

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 volcanism 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. 5
Fig. 5 Rare earth elements normalized on a chondrite basis (WAKITA et al., 1971) for basaltic and gabbroic samples.Two different signatures show two different stages of alteration: full line and full circles for weakly altered samples and stripped line and empty circles for strongly altered samples.

Fig. 9
Fig. 9 Cr vs. Ti diagram after GARCIA (1978).The striped line divides the diagram into fields for ocean floor basalts and volcanic arc basalts.

Fig
Fig. 11 Spider diagram for average of eleven magmatic rocks with SiO 2 <54%, normalized to mid ocean ridge basalts (MORB) as suggested by PEARCE (1983).

Fig. 12
Fig.12Rare earth elements normalized on a chondrite basis(WAKITA et al., 1971) for four evolved rocks from the investigated area.

Table 1
Mineral assemblages and petrographic details of the rocks.

Table 2
Bulk rock chemical compositions.Oxides from SiO 2 to P 2 O 5 (in wt.%) by XRF; total iron as Fe 2 O 3 .Trace elements and REE (in ppm) by ICP-MS; udl -under the detection limit.