Preliminary results on degree of thermal alteration recorded in the eastern part of Mt . Papuk , Slavonia , Croatia

Samples from the Radlovac metamorphic complex, together with others from the overlying sedimentary rocks, and parts of Psunj metamorphic complex beneath it were studied in order to better constrain metamorphic conditions that have prevailed in the area. Rocks from Mt. Papuk were investigated, in order to determine thermal conditions, by Kübler index (illite “crystallinity”) and Árkai index (chlorite “crystallinity”) while the b0-parameter of K-white mica was used to estimate the pressure conditions. Treatment with dimethyl-sulfoxide (DMSO) was used in order to extend application of chlorite “crystallinity” measurements to kaolinite bearing samples. Results suggest temperatures between 250–300°C and pressures of 2–3 kb. Similar temperature data recorded from various lithologies implies the existence of a previously unknown post lower Triassic thermal event, (Alpine very low to low-grade metamorphism) affecting different complexes on Mt. Papuk. New data presented and discussed in this paper provides the basis for further research and interpretation of the tectono-metamorphic history of the studied area and its correlation with other similar European metamorphic complexes.

The main aim of this paper is to increase the current knowledge of the metamorphic conditions to which rocks of Mt.Papuk have been subjected, by studying rocks belonging to the Radlovac metamorphic complex as well as the overlying sedimentary rocks and parts of the Psunj metamorphic complex.Although several researchers (JAMIČIĆ, 1983(JAMIČIĆ, , 1988;;SLOVENEC, 1986;PAMIĆ & LANPHERE, 1991) have indicated particular P-T conditions, at least for some parts of the Radlovac metamorphic complex, further research is necessary.For this purpose, a multidisciplinary approach which includes optimal laboratory methods based primarily on X-ray diffraction was used.In order to determine thermal conditions, rocks were investigated by Kübler index (illite "crystallinity") and Árkai index (chlorite "crystallinity"), while the b 0-parameter of K-white mica was used for estimating the pressure conditions.
New P-T data presented in this paper will form the basis for further research, especially age dating on illite fractions, and interpretation of the tectono-metamorphic history of the studied area and its correlation with other similar European metamorphic complexes.

GEOLOGICAL SETTINGS
The Slavonian Mts.situated in Slavonia, (the northeastern region of Croatia), represent some of the best outcrops of crystalline basement of the Tisia Mega-unit (PAMIĆ et al., 1996;PAMIĆ & JURKOVIĆ, 2002).This Mega-unit represents the continental fragment broken off from the southern rim of the Eurasian plate (i.e. from the southern margin of the Variscan Europe) during the Alpine evolution of Tethys (GÉCZY, 1973).After complex drifting and multiple rotations during Mesozoic and Cenozoic times, the Tisia Unit occupied its present tectonic position (CSONTOS, 1995;STAMPFLI et al., 2002).
Based on extensive petrological analysis in combination with radiometric age dating of plutonic and metamorphic rocks, PAMIĆ & LANPHERE (1991) gave an alternative subdivision (see also PAMIĆ & JURKOVIĆ, 2002 and references therein) according to which only two complexes can be distinguished.They proposed that the Psunj (Kutjevo) and Papuk (Jankovac) complexes, distinguished by JAMIČIĆ (1983,1988), in fact represent one coherent magmatic-metamorphic complex.The second complex is the Radlovac complex.

Macro-and microscopic characteristics of the studied samples
Ten samples, which were chosen in this pilot study in order to cover rocks with different petrological characteristics and age, which according to JAMIČIĆ & BRKIĆ (1987) range from Late Precambrian (PCm) to the Early Triassic (T1) (Table 1), were analyzed by several techniques.
Chlorite schist (sample 1), is a grey to green foliated metamorphic rock with abundant phyllosilicate minerals.The well observed schistosity is defi ned by dominant chlorite and mica (illite-muscovite) together with fi ne-grained recrystallized quartz (Fig. 2A).Chlorite schist belongs to the Psunj metamorphic complex (PS).
Metapsammite (sample 4) is a grey to silvery coarsegrained rock with blastopsammitic texture.Weakly recrystallized detrital quartz predominates, while subordinate plagioclase is abundant in a fi ne-grained matrix of sericite and recrystallized quartz (Fig. 2C).Micaceous minerals and elongated quartz grains have a subparallel orientation.
Metapelites (samples 5 and 6) are reddish or green with blastopelitic texture.Continuous cleavage and foliation defi ned by fi ne-grained micaceous material and elongated quartz grains are clearly observed (Fig. 2D).
These metapelites and metapsammite belong to the Radlovac metamorphic complex (RA).
Phyllitic conglomerates (samples 7 and 8) are dark reddish to brown.They contain primarily well-rounded pebbles of quartz and subordinate granites and weathered schists.The matrix contains fi ne-grained recrystallized quartz, sericite, chlorite and haematite (Fig. 2E).The matrix shows a distinct cleavage developed parallel to the surface layering.
Quartz sandstone (sample 9) is a white, fi ne-grained rock in which continuous schistosity defi ned by parallel grains of mica and quartz is well observed (Fig. 2F).The matrix is composed of sericite with subordinate quartz cement.As pointed out by JAMIČIĆ et al. (1983JAMIČIĆ et al. ( , 1987)), the observed schistosity is probably a consequence of multiple tectonic movements in the area.Sandstone (sample 10) is white in colour.The rock is very compact, hard, and mainly composed of round grains of quartz embedded in a fi ne-grained matrix of quartz and sericite (Fig. 2G).

Modal composition
Whole rock powder XRD analysis of the samples was performed at the Institute of Mineralogy and Petrology, University of Zagreb on a Philips X'Pert Pro diffractometer equipped with an X'celerator detector using CuKα radiation from tube operated at 40 kV and 45 mA.The step width was 0.017°2θ with 43 s counting time per step; the samples were run between 4 and 65 °2θ.
Fi gu re 2: Characteristic textural features of the analyzed samples: A -chlorite schist; B -chloritoid schist; C -metapsammite; D -metapelite; E -phyllitic conglomerate; F -quartz sandstone and Gsandstone.See text for details.Mineral abbreviations are the same as in Table 1.
Sample preparation was undertaken according to the recommendation of KISCH (1991) and the FWHM was read manually."Illite fraction" i.e. fraction <2μm, obtained using a centrifuge (Tehtnica-CENTRIC 322A), was used for KI and ÁI measurements.The XRD measurements were carried out on a Philips X'Pert Pro diffractometer with a graphite monochromator at the Institute of Mineralogy and Petrology, University of Zagreb.Instrumental conditions were 40 kV, 40 mA and constant time 5 s, with step scanning (0.02 °2θ).KI and ÁI were measured on air-dried samples and no shift of the basal white mica refl ection after ethylene-glycol (EG) treated was observed, so the results are discussed using only the air-dried scan results.
The standardization of KI and ÁI values of samples measured in the laboratory, to those of Kübler's laboratory, taken as referent values, was made using eight Kisch standards i.e. rock slabs polished parallel to the dominant orientation of phyllosilicate i.e. parallel to the foliation (KISCH, 1990;1991).The equation representing this case, used here for standardization of the KI and ÁI values of all samples except chlorite schist to the referent Kübler's values, is as follows: KI (Kübler) = 0.9684 X KI (Zagreb) + 0.0757 R 2 =0.9844 (eq. 1) The same equation was used to obtain Kübler values for WARR & RICE (1994) standards, and the values obtained are listed in Table 2.These CIS standards were used again for monitoring changes in measured FWHM caused by tube ageing.For further explanation see also KISCH et al. (2004).

Dimethyl-sulfoxide (DMSO) treatment
The presence of kaolinite in the samples can make the ÁI (002) measurement very diffi cult and uncertain because of the overlapping of the (001) diffraction maximum of kaolinite and the (002) diffraction maximum of chlorite.They both have diffraction maxima at ~12.4 °2θ i.e. ~7.2 Å.In order to get a reliable ÁI value, an attempt was made to separate these two diffraction maxima, using treatment with DMSO.This treatment should separate the maxima one apart from another and shift the (001) diffraction maximum of kaolinite to higher d values (from ~7.2 Å to ~11.3 Å) (CALVERT, 1984).Treated samples were measured using standard KI and ÁI measuring conditions.

Total organic carbon (TOC)
It is usual to correlate KI and ÁI indices with other parameters which alter with thermal conditions changes, including vitrinite refl ectance.With this aim TOC was measured in order to fi nd out if the investigated samples are suitable for such kind of analyzes and to fi nd an appropriate correlation.The measurements were made in the INA -Laboratory for Geochemistry, with samples prepared according to BUSH (1970).Standardization of the instrument (LECO IR-212) was undertaken using material of known carbon content (a steel ring containing 0.3-1.0 % carbon).

b 0 -parameter
The b0-parameter in K-white micas is controlled by pressure (SASSI, 1972, GUIDOTTI & SASSI, 1986).The geobarometric b0-parameter is measured as 6×d(060,331 ˉ) of muscovite.This parameter monitors the relative pressure environments of very low to incipient low-grade metamorphic terrains and is widely used as a relative geobarometer for interregional comparisons (SASSI & SCOLARI, 1974).
The analytical procedure is based on work by SASSI (1972) and SASSI & SCOLARI (1974) with further development by GUIDOTTI & SASSI (1986).SASSI's (1972) qualitative geobarometer is based on the b0-parameter value, or more precisely, 6×d(060,331 ˉ) spacing of K-Na white micas, which refl ects the increasing celadonite substitution that occurs in muscovite with the pressure increase in the Al-rich portion of the nonlimiting muscovite-albite assemblage (WANG et al., 1996;RIEDER et al., 1998).The good linear correlation between the value of 6×d(060,331 ˉ) and the celadonite content has been well demonstrated by GUIDOTTI et al. (1989).
Using the procedures defi ned by SASSI & SCOLARI (1974), each sample was cut perpendicular to its schistosity.Quartz present within the samples was used as an internal standard.The diffractometric analyses were carried out directly on these slices using the aforementioned XRD at same conditions used for KI and ÁI determination, ensuring in all cases that the area exposed to x-rays was phyllosilicate rich.The 2θ range scanned was 59.0-63.0°.

Modal composition
The whole rock powder XRD analysis (Table 1) revealed that quartz and illite-muscovite are the main constituent minerals in all samples.Chlorite was found in chlorite schist, chloritoid schists, metapelites, metapsammite, phyllitic conglomerates and quartz sandstone.K-feldspar was detected in chloritoid schists, quartz sandstone and sandstone, while plagioclase is present in all samples except for the quartz

Dimethyl-sulfoxide (DMSO) treatment
After treatment of the "illite fraction" in the quartz sandstone (i.e.fraction <2μm), with DMSO shift of (001), a diffraction maximum of kaolinite towards higher d values (~7.2 to ~11.1 Å) was observed (Fig. 6).The position of the (002) diffraction maximum of chlorite remained unchanged.In this way (002) diffraction maximum of chlorite becomes more symmetric and therefore suitable for ÁI determination.

b 0 -parameter
The measurements of the b0-parameter were made on 2 metapelites discussed in this paper and an additional 10 samples from Radlovac metamorphic complex which satisfy the criteria set by SASSI and SCOLARI (1974).The average b0parameter value is 8.993 Å (n=12), with a standard deviation of 0.012.The majority of the values fall between 8.980 and 9.006 Å (66% of analyzed samples), two samples have b0values higher than 9.006 Å and two samples had a b0-parameter value lower than 8.980 Å (Fig. 4).

Total organic carbon (TOC)
Total organic carbon was not found or was very low in all investigated samples (Table 2).Samples containing less than 0.5% TOC for clastites and 0.3% TOC for carbonates are, according to HUNT (1995), not suitable for vitrinite refl ectance measurements.

DISCUSSION
According to the Kübler and Árkai indices (Table 3, Fig. 3) all analyzed samples show a certain degree of thermal alteration recorded within the "illite fraction".Obtained values of KI and ÁI indicate thermal alteration ranging from the low temperature part of the high anchizone (approximately 250 °C) to the low temperature part of the epizone (approximately 300 °C) i.e. sub-greenschist to greenschist facies (Fig. 3).These data are in good agreement with the previously established presence of paragonite and pyrophyllite reported by SLOVENEC (1986) in some slates from the Rad-lovac metamorphic complex.According to FREY (1986) these minerals are characteristic of sub-greenschist facies and indicate, as well as Kübler and Árkai indices measured here, that the Radlovac metamorphic complex recorded the younger metamorphic overprint of very low to low-grade metamorphic temperature conditions (BIŠEVAC et al., 2007).Samples from the Psunj metamorphic complex (PS) which Fi gu re 4: Pressure conditions estimation based on the b0-parametar for samples from the Radlovac metamorphic complex obtained from K-white micas of K-feldspar-free rock slices (1-12) together with the average b0-value for this paper ( 13) and average b0values taken from PAMIĆ & LANPHERE (1991) ( 14).The boundaries between the low, intermediate and high pressure fi elds are from ÁRKAI et al. (1991).
occurs below the Radlovac metamorphic complex, as well as those from the overlying sediments (S) recorded the same very low to low-grade metamorphic event of the same temperature conditions (approximately 250-300 °C according to KI and ÁI).Since effects of this metamorphism are barely visible without the aid of instrumental techniques, we will not designate these deposits to be metasediments.
Pressure conditions estimated on the basis of the b-parameter of K-white mica (average value for analyzed samples is 8.993 Å) (Fig. 4) indicate that samples from the Radlovac metamorphic complex recorded a metamorphic event of low to intermediate pressure conditions of sub-greenshists to greenshists facies.The measured b 0-parameter corresponds to approximately 2-3 kb (SASSI & SCOLARI, 1974;GUIDOTTI & SCOLARI, 1986;ÁRKAI et al., 1991;ÁRKAI et al., 1995).Although PAMIĆ & LANPHERE (1991) pointed out that their measurements of the b0-parameter of Kwhite mica were made on a small population of samples (average value for 10 samples was 9.002 Å), their results, which indicate the lowest intermediate pressure (~3 kb) (Fig. 4), are in good agreement with the data presented here.It is important to mention that both KI and ÁI can provide more reliable results if samples with similar lithology are compared.Usually fi ne-grained clastic rocks are suggested in the literature for such purposes.In course grained samples, the effects of inherited (detrital) phyllosilicates may overlap and strongly modify the nature of authigenic (metamorphic, newly -formed) micas and chlorites (ÁRKAI, 1983;1991;1995).This is valid even if the <2μm fraction of samples is used (ÁRKAI et al., 1981;ÁRKAI, 1983).In order to confi rm that the thermal maturity of the studied samples, especially Permo-Triassic quartz sandstone and Triassic sandstone, is really the result of Alpine tectonism, further investigation of the illite fraction is in progress.
Despite the fact that the analyzed samples clearly differ from one another by their stratigraphic age, protolith, mineral composition and texture, no signifi cant infl uence of these pa-rameters to KI values was observed.The degree of thermal alteration seems to be constant through the whole analyzed column (Fig. 5).All this data points to the conclusion that the analyzed samples recorded the same event which can be dated as younger than the Early Triassic.The extensive K-Ar dating of suitable illite fractions is in progress and preliminary report points to the Cretaceous (BIŠEVAC et al., 2007).
The appearance of ordered kaolinite in some samples (Table 1) was connected to post-metamorphic hydrothermal alteration which probably occurred during uplift of the basin and it is not connected with the metamorphic overprint reported here.

CONCLUSIONS
(1) KI and ÁI measured on samples belonging to the Radlovac metamorphic complex as well as parts of Psunj metamorphic complex and D3 metamorphic rocks indicate a thermal overprint ranging from the low temperature part of the high anchizone (approximately 250 °C) to the low temperature part of the epizone (approximately 300 °C) i.e. sub-greenschist to greenschist facies.(2) The clastic-carbonate sedimentary succession (phyllitic conglomerates, quartz sandstone and sandstone) of the Late Permian and Triassic age, which uncomformably overlies the Radlovac metamorphic complex was also affected by the same thermal overprint.(3) Pressure conditions estimated on the basis of the b0-parameter of K-white mica (average value is 8.993 Å) indicate that samples from the Radlovac metamorphic complex were metamorphosed under the low to intermediate pressure of the sub-greenshists to greenshists facies (approximately 2-3 kb).
(4) Total organic carbon (TOC) in all samples was either absent or very low.
(5) The degree of thermal alteration i.e. the Kübler index, tend to be constant through the whole analyzed column Fi gu re 6: X-ray diff raction patterns (4-14 °2θ CuKα) of quartz sandstone ( 2 PT) (fraction <2μm) before and after treatment with DMSO.Mineral abbreviations are the same as in Table 1.
implying that the different complexes (PS, D3, RA and S) all record the same, post Early Triassic metamorphic overprint.
(6) DMSO treatment enabled chlorite "crystallinity" measurements of kaolinite containing samples by separating 001 and 002 diffraction peaks of kaolinite and chlorite respectively.

Fi
gu re 1: (A) The position of Tisia Mega-unit within the Pannonian Basin and (B) sketch map of Slavonian Mts.(Psunj, Ravna gora, Papuk and Krndija) with an approximate position of the studied area (marked in the black box).
sedimentary cover over the Psunj metamorphic complex.It is uncomformably overlain by a clastic-carbonate succession of Late Permian and Triassic age, which is not affected by Alpine metamorphism (JAMIČIĆ &

Table 1 :
Geographical setting, rock type and stratigraphic age of studied samples (PS -Psunj metamorphic complex; RA -Radlovac metamorphic complex; S -Permo-Triassic and Triassic sediments) together with whole rock modal composition of the studied samples determined by XRD.

Table 3 :
Kübler index (KI)and Árkai index (ÁI) measured on <2 μm grainsize air-dried mounts together with the total organic carbon (TOC) content in the studied samples.KI and ÁI values are expressed in Δ°2θ (CuKα).(The (001) diff raction maximum of chlorite of metapelite, phyllitic conglomerate and quartz sandstone was too weak and therefore not suitable for Árkai index measurement.This was also the case for both diff raction maxima of chlorite for the chloritoid schists.The Árkai index of sandstone was not measured (n.m.) because this sample does not contain chlorite.)*ÁI determined after treatment with DMSO.