1. INTRODUCTION In the north-western part of Croatia, belonging to the South Pannonian Basin, numerous small occurrences of Miocene volcanic and volcaniclastic rocks have been discovered (©IMUNI∆ et al., 1981; ANI»I∆ & JU- RI©A, 1985; PAMI∆, 1997 and references therein). According to the comprehensive work of PAMI∆ (1997), Neogene volcanic rocks could be divided into four groups: Egerian - Eggenburgian, Karpatian, Baden- ian and post-Badenian. The age of volcanic and volcaniclastic rocks could be determined by radiometric methods or on the basis of field relationships with associated sediments. Radio- metric methods are still not easily available, while scarce and poor exposures, due to the vegetation cover and tectonic deformation, often make it difficult to establish the correlation between sediments and vol- canic rocks. The aim of this work (which forms a part Discriminant Function Analysis of Miocene Volcaniclastic Rocks from North-Western Croatia Based on Geochemical Data Darko TIBLJA© 1, Vanja LOPARI∆1 and Mirko BELAK 2 of a detailed investigation of volcaniclastic rocks in the area) was to see if any significant difference in geo- chemical characteristics of the rocks of different ages exists, i.e. to determine the geochemical fingerprint of rocks of different ages. The theoretical background for the analysis lies in the fact that the composition of vol- canic rocks depends on the geotectonic settings which changed during the evolution of Pannonian basin, and therefore should be recorded in the chemical composi- tion of the rocks. Positive results could help distinguish rocks of unknown age. For this purpose we used statistical methods, princi- pally the discriminant function analysis, which is a powerful technique for classifying individual cases (samples) into previously defined groups on the basis of multiple variables, and one which has a broad applica- tion in determining solutions for geoscience problemat- ics (DAVIS, 1986). 2. MATERIALS AND METHODS In the scope of our research we investigated Miocene volcanic and volcaniclastic rocks, as well as rocks for which they were the origin, e.g. bentonites, that crop out in north-western Croatia, along the easternmost seg- ment of Periadriatic fault system, from the Slovenian border to the Varaædinske Toplice. We analysed ande- sites from Cerovec (sample T-6-94B) and Lepoglava, dacites from TrliËno (T-5-93-17) and Donje Jesenje (T- 6-93-17E), bentonites from Poljanska Luka and ©aπa, and tuffs (sensu lato) from the areas of Hromec, Jese- nje - Cerje Jesensko, Lapornica - ©eprun, Jamno, Vuglo- vec, Podrute, Moæenec, and Kalnik Mountain (Fig. 1). In addition to these newly collected samples we analysed samples of rocks from Moæenec, described by KI©PATI∆ (1909), that are deposited in the Croatian Natural History Museum (catalogue numbers 600: ZAG; 7113 and 7115: MP1). According to available data, the samples from Poljanska Luka are Badenian (ANI»I∆ & JURI©A, 1985), those from TrliËno, Hro- mec, Jesenje, Cerje Jesensko, Jamno, Lepoglava and Vuglovec are of Egerian - Eggenburgian age (©IMU- NI∆ & PAMI∆, 1993) as are those from Podrute (©I- MUNI∆ et al., 1981). Volcaniclastic rocks from ©aπa, Lapornica and ©eprun areas are interstratified with clas- Geologia Croatica 55/1 39 - 44 2 Figs. 4 Tabs. ZAGREB 2002 Key words: Volcaniclastic rocks, Discriminant func- tion analysis, Geochemical data, Miocene, Pannon- ian Basin, Croatia. 1 Institute of Mineralogy and Petrography, Faculty of Science, Horvatovac b.b., HR-10000 Zagreb, Croatia. e-mail: darko.tibljas@jagor.srce.hr 2 Institute of Geology, Sachsova 2, HR-10000 Zagreb, Croatia. Abstract Preliminary results of discriminant function analysis of geochemical data of Miocene volcanic and volcaniclastic rocks revealed that it is possible to distinguish rocks of different ages, namely Egerian - Eggenburgian, Karpatian and Badenian. On the basis of calculated coefficients of classification functions, presented in the paper, it is possible to indicate the age of Miocene volcanic and volcaniclastic rocks if their trace element content is known. The analysis also showed that, especially in the case of volcaniclastic rocks, the mineral and petrological composition of the rocks should also be considered. Most of the investigated volcaniclastic rocks from the Moæenec and Kalnik areas, the age of which was unknown, are, according to dis- criminant function analysis, of Egerian - Eggenburgian age. 40 Geologia Croatica 55/1 tic rocks of Egerian to Karpatian age (©IMUNI∆ & PA- MI∆, 1993). The only published data on volcanic rocks from Moæenec area are from KI©PATI∆ (1909), who presumed the same age for these rocks as for those from the Jesenje area. The tuffs in the Kalnik area, are, according to the unpublished fieldtrip guidebook3, of Badenian age. Investigated samples are listed in the lower part of Table 3 on the basis of sampling locality, successively from the west to the east. The chemical composition of the rocks was deter- mined on an ARL 8410 X-ray fluorescence (XRF) wavelength dispersive spectrometer equipped with an Rh-tube. Glass beads, for the determination of major elements, were prepared by melting a 1:1:4 mixture of water-free sample, Li2B4O7 and LiBO2. Measured inten- sities were corrected for matrix effect using the empiri- cal model of LACHANCE & TRAILL (1966). H2O - was determined as loss of weight after 4 hr of drying at 110°C, while loss on ignition at 1025°C was taken as H2O +. Trace elements were analysed on pressed powder pellets. Details on the measuring conditions are given in TIBLJA© (1996). Results of the chemical analyses can be obtained from the authors on request. The computer programme STATISTICA (STAT- SOFT, 1995) was used to carry out the discriminant function analysis of the collected data. Additional input data (element concentrations and geological age of the rocks) for the analysis were taken from PAMI∆ (1997). It was necessary to take logarithmic values of element concentrations, except in the cases of gallium and thori- um, to achieve a normal distribution of the input data, which is one of the assumptions of discriminant func- tion analysis. All statistically analysed samples are list- ed in Table 3. The first 25 samples are from PAMI∆ (1997). They are marked with symbols of the type E7 where letter designates age, i.e. E is for Egerian - Eggenburgian, while the numbers correspond to the sample numbers in the tables, in his book, that contain chemical compositions of the rocks of the age con- cerned. Due to the fact that, according to their major elements, as well as immobile trace element content, the investigated volcaniclastic rocks are mainly acid and rarely neutral (TIBLJA© et al., 2000) only data for the rocks that contain >55wt. % SiO2 were considered. Rocks of known age were assigned to three age groups ( E g e r i an - Eggenburgian, Karpatian and Badenian) and we tried to statistically distinguish these groups on the basis of rock geochemical characteristics, and finally, to assign each newly analysed sample of unknown age to one of these predefined groups. Due to the scarce avail- able age data for the volcaniclastic rocks from ©aπa, Lapornica, ©eprun, Moæenec and Kalnik areas their age was taken as unknown. The reason why we consid- ered only three, instead of the four age groups described by PAMI∆ (1997), was that post-Badenian rocks are basic, while, as already mentioned, investigat - ed volcaniclastic rocks are acid to neutral. 3. RESULTS Standardised coefficients for two discriminant functions obtained by discriminant analysis of Miocene volcani- clastic rocks, based on their trace element content, are given in Table 1 together with their eigenvalues and rel- ative and cumulative proportion of variance extracted by each function. The plot of discriminant scores given in Fig. 2 differentiates between rocks of three different ages. Function 1 explains 80.8 % variation of the data, with a high negative loading of Ce and positive of Cu Fig. 1 Location map of investigated area. Circles represent sampling locations of volcaniclastic rocks. 3 ©IMUNI∆, A. (1981): VodiË ekskurzije kroz KalniËko gorje i sjeverozapadni dio Dravske depresije.- Unpublished (in Croatian), Croatian Geological Society, Zagreb, 25 p. and Nd, and differentiates clearly between Karpatian samples and the Egerian - Eggenburgian ones, while the Badenian samples are transitional. The second function, of bipolar nature, that separates the Badenian samples from the Karpatian and Egerian - Eggenburgian ones, has a high negative loading of Cr and positive of V. Coefficients (Ce = ai, Cr = bi, Cu = ci, ..., Zr = mi) as well as constant (C) of classification functions of a form Si = aix1 + bix2 + cix3 + ... + mixm + C (1) where x1, x2, x3, ..., xm are concentrations, or their loga- rithms, of elements in the sample, are given in Table 2. On the basis of classification scores Si each sample was 41Tibljaπ, LopariÊ & Belak: Discriminant Function Analysis of Miocene Volcaniclastic Rocks... Fig. 2 Plot of discriminant scores along Function 1 versus Func- tion 2, to discriminate volcanic and volcaniclastic rocks of dif- ferent age (E = Egerian - - Eggenburgian, K = Karpat- ian, B = Badenian). Discriminant function 1 2 log Ce -1.77543 0.1676 log Co -0.0309 -0.50112 log Cr -0.54651 -1.05168 log Cu 1.08006 0.71779 Ga 0.37776 -0.42432 log La 0.22753 0.05975 log Nb 0.3213 0.38883 log Nd 0.86753 -0.09416 log Ni -0.3365 0.36198 log Pb 0.11063 -0.2135 log Rb 0.56435 0.48088 log Sc -0.02668 0.25888 Th 0.07007 0.56928 log V -0.60661 1.01681 log Y -0.01791 0.38464 log Zn -0.24478 -0.42836 log Zr -0.26779 0.05888 Eigenvalue 6.12012 1.45174 % of variance - relative 80.83 19.17 % of variance - cumulative 80.83 100.00 Table 1 Standardized discriminant function coefficients and related statistics. Egerian - Karpatian Badenian - Eggenburgian log Ce 449.13 558.69 492.772 log Co -7.368 -8.292 -3.27 log Cr -68.481 -64.002 -61.06 log Cu -83.598 -103.009 -98.541 Ga 4.286 3.105 4.272 log La 56.803 50.776 53.417 log Nb -61.637 -70.962 -71.916 log Nd -62.86 -76.376 -68.161 log Ni 18.139 22.8 18.269 log Pb -52.843 -57.491 -51.8 log Rb -65.926 -78.378 -76.845 log Sc -54.203 -53.004 -55.712 Th 0.023 0.056 -0.450 log V 211.827 225.997 209.631 log Y -48.405 -45.449 -53.705 log Zn -14.858 -11.073 -9.158 log Zr 418.166 434.039 423.642 Constant -746.471 -918.402 -781.051 Table 2 Classification function coefficients used in formula (1) for grouping of volcaniclastic rocks according to their age. 42 Geologia Croatica 55/1 included in the age group for which the score was high- est (Table 3). The age attributed to the samples on the basis of references mentioned in the previous section is given in the second column of the table. The following three columns contains ages ascribed on the basis of discriminant analysis, ordered into the first, second, and third choice according to the posterior classification probabilities. These classification functions could be used for the classification of samples of unknown age if their chemical composition is known. Table 4 gives a comparison between the actual and predicted number of samples in each age group. According to these results 96.5 % of samples have been classified correctly to their pre-defined groups. Misclas- sified samples are due to overlap in the discriminant space of Badenian samples with the Karpatian ones on one side, and Egerian - E g g e n b u r g i a n on the other (Fig. 2). 4. DISCUSSION The results of performed discriminant function analysis, presented in Tables 3 & 4 and Fig. 2, showed that Mio- cene volcanic rocks of different ages (Egerian - E g g e n- burgian, Karpatian and Badenian) could be confidently distinguished on the basis of their geochemical data. Almost all (55) of the 57 samples of known age, that were used as input data for the analysis, were classified correctly. This analysis showed that Karpatian samples could be successfully differentiated from Egerian - Eggenburgian ones. A slight overlap of Badenian sam- ples with Karpatian and Egerian - Eggenburgian ones is in agreement with the conclusion of PAMI∆ (1997) that E g e r i an - Eggenburgian and Badenian rocks have simi- lar geochemical characteristics. But in spite of that their discrimination could be performed quite successfully. The high negative loading of Ce on discriminant function 1, that successfully discriminates the Karpatian samples is in accordance with the elevated Ce content of Karpatian volcanics (PAMI∆, 1997). PAMI∆ (op. cit.) also noticed that Karpatian rocks have higher con- tents of Zr, Nb and La, and lower Sc and Y than other Tertiary rocks, but this is not obvious from standardised coefficients listed in Table 1. These coefficients indi- cate a higher Cu content in the Egerian - E g g e n b u r g i a n samples than for the other two groups. Badenian sam- ples are discriminated by function 2, in which Cr and V carry the main discriminatory potential. As shown in Table 3, most of the volcaniclastic rocks from the Moæenec and Kalnik areas, of previ- ously unknown age, are of Egerian - Eggenburgian age according to discriminant function analysis based on geochemical data. The only exception is the sample from the Kalnik area (VHK 500) which is Karpatian. Such results for the samples from the Moæenec area support the conclusions of KI©PATI∆ (1909), based on petrographic investigations, about the contemporaneous age of the rocks from the Moæenec and Jesenje areas. Equivocal results for volcaniclastic rocks from Kalnik could indicate volcanic activity in both Egerian - Eggen- burgian and Karpatian times, but such results are proba- bly a consequence of problems encountered during investigations of the chemical composition of any vol- caniclastic rock. The original chemical composition of volcaniclastic rocks is a consequence of magma compo- sition, which depends mostly on geotectonic environ- ment, and could be strongly modified by: (1) separation during transport of pyroclastic material from the vol- canic vent to place of deposition, (2) alteration and (3) admixing of epiclastic or any other detrital material. Because of (3) we excluded from the analysis all of the samples that were an obvious mixture of volcanic and other detrital material. A Karpatian age obtained for the sample VHK 500 from the Kalnik area is probably the result of separation during transport. This is a sample of crystal tuff (sensu lato), and probably, its modal and, consequently, chemical compositions were strongly modified by processes of separation. The obtained chemical composition is thus probably nonrepresenta- tive for the magma and the result for this sample should be treated cautiously. The Egerian - Eggenburgian age for samples from Kalnik is not in accordance with available data from ©IMUNI∆ (1981)3. We also encountered the problem of modified chemical compo- sition due to alteration, or rather, due to the product of alteration of volcanic glass. All of the samples with authigenic zeolites (clinoptilolite and/or mordenite) have relatively elevated contents of barium and stron- tium. These higher contents are the result of zeolite selectivity for these elements. Therefore we have to exclude these elements from the discriminant model, and the conclusion of PAMI∆ (1997) that Rb/Ba and Rb/Sr contents could differentiate Egerian - E g g e n b u r- gian and Badenian rocks could not be confirmed. It is obvious that, for distinguishing volcaniclastic rocks of different age on the basis of geochemistry, it is necessary to know their chemical composition but also their mineral and petrographic composition. The statistical analysis was carried out on a limited number of samples and therefore their results should be taken cautiously and should be tested by further investi- gation. In addition, the available chemical analyses were occasionally incomplete. Also, comparison of the literature and our own data, mainly because of different experimental methods was problematical and should be acknowledged. 5. CONCLUSIONS The discriminant function analysis of geochemical data, which was carried out on a relatively limited number of samples of Miocene volcanic rocks from the Sava-Dra- va interfluve, enabled the following preliminary conclu- sions to be made. 43Tibljaπ, LopariÊ & Belak: Discriminant Function Analysis of Miocene Volcaniclastic Rocks... Sample (locality) Age according to Age according to discriminant function analysis literature data Probability 1 2 3 E7 E E B K E8 E E B K E9 E E B K E11 E E B K E18* E B E K E19 E E B K E20 E E B K K1 K K B E K3 K K B E K4 K K B E K7 K K B E K12 K K B E K15 K K B E K17 K K B E K20 K K B E B9 B B E K B25 B B E K B31 B B E K B35 B B E K B36 B B E K B40 B B E K B42 B B K E B43 B B E K B46* B K B E B51 B B E K KREM (Poljanska Luka) B B E K SIVA (Poljanska Luka) B B E K T-6-94-4B (Cerovec) E E B K T-5-93-17 (TrliËno) E E B K T-5-93-21 (Hromec) E E B K T-5-93-18 (Hromec) E E B K T-6-93-17E (Donje Jesenje) E E B K T-7-90-K2 (Donje Jesenje) E E B K T-790-K5 (Donje Jesenje) E E B K T-7-90-K18 (Donje Jesenje) E E B K T-7-90-K22 (Donje Jesenje) E E B K T-7-90-K27 (Donje Jesenje) E E B K T-7-90-K27G (Donje Jesenje) E E B K T-7-90-15 (Donje Jesenje) E E B K T-7-90-30 (Gornje Jesenje) E E B K T-7-90-33 (Gornje Jesenje) E E B K T-4-98-1 (Gornje Jesenje) E E B K T-7-90-2 (Cerje Jesensko) E E B K T-7-90-4B (Cerje Jesensko) E E B K T-7-90-7 (Cerje Jesensko) E E B K T-7-90-8 (Cerje Jesensko) E E B K T-7-90-57 (Jamno) E E B K T-7-90-62 (Jamno) E E B K T-6-93-14 (Jamno) E E B K LEPOGLAVA E E B K T-4-99-8 (Vuglovec) E E B K T-4-99-9 (Vuglovec) E E B K T-4-99-10 (Vuglovec) E E B K T-4-99-11 (Vuglovec) E E B K PODRUTE E E B K HMPM7110 (Jesenje) - E B K T-7-90-©1 (©aπa) - E B K T-3-90-1 (Lapornica) - E B K ©EPRUN3 - E B K HMPM7113 (Moæenec) - E B K HMPM7115 (Moæenec) - E B K T-4-99-3 (Moæenec) - E B K T-4-99-7 (Moæenec) - E B K T-4-99-5S (Moæenec) - E B K T-4-99-2A (Moæenec) - E B K T-4-99-4 (Moæenec) - E B K VHK500 (Kalnik - Drenovec) - K B E T-5-99-1 (Kalnik - Knezov jarak) - E B K KALNIK (Knezov jarak) - E B K VHK227 (Kalnik - Knezov jarak) - E B K Table 3 Classification of samples on the ba- sis of discriminant function analysis. E = Egerian-Eggen- burgian, K = Karpat- ian, B = Badenian. Incorrect classifica- tions are marked with *. Samples from PAMI∆ (1997) are listed at the top, followed by newly analysed samples of known age, and finally by newly ana- lysed samples of unknown age. 44 Geologia Croatica 55/1 1) It is possible to obtain an indication about the age of Miocene volcanic and volcaniclastic rocks on the basis of their chemical composition. 2) Most of the investigated volcaniclastic rocks from Moæenec and Kalnik areas, whose age was unknown, are, according to discriminant function analysis of Egerian - Eggenburgian age. It must be stressed that such conclusions are only preliminary and should be confirmed or refuted by fur- ther investigations on more samples. Acknowledgements We are grateful to Marta CRNJAKOVI∆ for rock spec- imens from Croatian Natural History Museum, to Josip HALAMI∆, Franjo PERCELA and Emil BUKVI∆ for help during sampling, and to Slobodan MIKO for his help in statistical analysis. We are indebted to anony- mous referee for comments that greatly improved the manuscript. Thanks also go to Julie ROBSON for lan- guage editing. The study has been financially supported by the Ministry of Science and Technology of the Republic of Croatia (Project 119304). 6. REFERENCES ANI»I∆, B. & JURI©A, M. (1985): Osnovna geoloπka karta SFRJ 1:100.000. TolmaË za list Rogatec L 33-68 (Basic Geological Map, Geology of the Rogatec Sheet).- Geol. zavod Ljubljana in Geol. zavod Zagreb (1983), Zvezni geol. zavod Beograd, Beograd, 73 p. DAVIS, J.C. (1986): Statistics and data analysis in geology.- John Wiley and Sons, New York, 646 p. KI©PATI∆, M. 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Percent correct E K B E 97.3 36 0 1 K 100.0 0 8 0 B 91.7 0 1 11 Total 96.5 36 9 12 Table 4 Classification matrix; rows contains number of samples in each observed group while in columns number of samples in predicted groups are given.