Lithos 117 (2010) 269–282

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Contrasting P–T–t paths from the basement of the Tisia Unit (Slavonian Mts., NECroatia): Application of quantitative phase diagrams and monazite age dating

Péter Horváth a,⁎, Dražen Balen b, Fritz Finger c, Bruno Tomljenović d, Erwin Krenn c

a Institute for Geochemical Research, Hungarian Academy of Sciences, H-1112 Budapest, Budaörsi út 45, Hungaryb Faculty of Science, University of Zagreb, HR-1000 Zagreb, Horvatovac bb, Croatiac Abteilung Mineralogie, Universität Salzburg, A-5020 Salzburg, Hellbrunnerstrasse 34, Austriad Faculty of Mining, Geology and Petroleum Engineering, University of Zagreb, HR-1000 Pierottijeva 6, Croatia

⁎ Corresponding author. Present address: School of GeKwaZulu-Natal, Private Bag X 54001, Durban 4000, Rep

E-mail addresses: phorvath@geochem.hu, peter.horv

0024-4937/$ – see front matter © 2010 Elsevier B.V. Aldoi:10.1016/j.lithos.2010.03.004

a b s t r a c t

a r t i c l e i n f o

Article history:Received 20 August 2009Accepted 2 March 2010Available online 12 March 2010

Keywords:Mica schistP–T–t pathsQuantitative phase diagramsMonazite age datingTisia UnitCroatia

Medium-grade mica schists and intercalated paragneisses and amphibolites from the basement of the TisiaUnit, Slavonian Mountains., northeastern Croatia, contain complexly zoned garnets. At the Kutjevo locality,mica schists are characterised by garnets with Mn-rich cores and Ca-rich rims. Mn decreases steadily fromcore to rim and Ca increases abruptly. This is in contrast to the paragneisses and amphibolites which containgarnets with smoothly decreasing Ca from core to rim. Quantitative phase diagrams and garnet compositionisopleths calculated from bulk rock analyses reveal that the Ca-poor garnet cores in the mica schists formedduring an earlier event at 584–592 °C and 6.4–7.8 kbar. Ca-rich rims formed at conditions of 600–660 °C and11–12 kbar — calculated using garnet isopleths and mineral thermobarometry. The paragneiss andamphibolite provide similar P–T information for the later peak event (ca. 650 °C, 10–12 kbar) but do notpreserve a record of the earlier, lower P–T event and modelling shows that garnet was not stable at theseconditions. Contrary to previous studies on this outcrop and rock type, no staurolite was observed andquantitative phase diagrams contoured for H2O mode isopleths indicate that the rock did not crossstaurolite-bearing fields during the retrograde P–T path.Mica schists from the Krndija locality contain zoned polyphase garnets. Phase diagram calculations revealthat Ca-rich garnet cores formed between 520 and 630 °C and 7–8 kbar. Rims have a lower Ca content andformed at considerably reduced pressures together with andalusite and staurolite at ca 530–570 °C and 3–4 kbar.Since both localities were traditionally considered to be part of the same tectono-metamorphic unit,evidence presented here clearly shows that this cannot be the case. EMP monazite ages are Variscan(350 Ma) in the Krndija mica schists and around pre- or early Variscan (440 Ma) in the Kutjevo mica schists.We therefore propose a more complex internal structure and metamorphic history for this area thanpreviously recognised suggesting a metamorphic evolution of the Slavonian Mountains that includes a pre-Variscan and a Variscan cycle.

ological Sciences, University ofublic of South Africa.ath74@gmail.com (P. Horváth).

l rights reserved.

© 2010 Elsevier B.V. All rights reserved.

1. Introduction

Discriminating metamorphic histories in polyphase metamorphicterrains involves combining field relationships with detailed petro-logical and geochronological studies (e.g. Goncalves et al., 2004; Willand Okrusch, 2004; Crispini et al., 2007; Jerabek et al., 2008; Maji et al.2008; Pitra et al., 2010). The application of detailed garnet zoningpatterns coupled with phase diagram modelling has revealedcontrasting P–T–t histories in areas often considered to be part of asingle tectono-metamorphic terrain (e.g. Le Bayon et al. 2006; Štípska

et al., 2006; Konrad-Schmolke et al., 2007, 2008; Gaidies et al., 2006,2008; Thöni and Miller, 2009).

Significant advances in accessory mineral geochronological tech-niques have provided further support in the recognition of successivemetamorphic events in terrains that were previously interpreted asmonometamorphic (e.g. Rubatto, 2002; Dahl et al., 2005; Mahan et al.,2006; Finger and Krenn, 2007; Harley et al., 2007; Duclaux et al.,2008). Furthermore this approach has also enabled the separation ofjuxtaposed terrains with differing metamorphic histories (e.g. Nut-man et al., 2008).

In this paper we apply a combination of these techniques tounravel the previously unknown polymetamorphic history of theSlavonianMountains, northeastern Croatia belonging to the basementof the Tisia Unit. This tectonic unit (Fig. 1 inset), is commonlyregarded as a lithospheric fragment broken off from the southern

Fig. 1. Simplified geological map of the SlavonianMts. (after Jamičić and Brkić, 1987; Jamičić, 1988; Pamić and Lanphere, 1991) with index-map showing the position of the Tisia Unitwithin the Pannonian Basin. Box shows the approximate position of the study area in Fig. 2.

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margin of the European plate by the Middle Jurassic opening of theeastern branch of the Alpine Tethys (cf. Géczy, 1973; Csontos, 1995;Pamić et al., 2002; Schmid et al., 2008 and references therein). Itreached its present-day position after a complex kinematic history ofmultiple regional-scale translations and rotations during the Meso-zoic and Cenozoic (e.g. Csontos, 1995; Fodor et al., 1999; Csontos andVörös, 2004), controlled by the major tectonic contacts of the Alpine–Carpathian–Dinaridic orogenic system, most of them representingoceanic sutures (Schmid et al., 2008).

Combined geochronological and thermobarometric data obtainedfrom the crystalline rocks of the Tisia Unit in southern Hungaryindicate that these Variscan rocks underwent Permian and Alpinemetamorphic overprints (Árkai et al., 2000; Horváth and Árkai, 2002;Lelkes-Felvári et al., 2003; Horváth, 2007). In the Slavonian Mts. inCroatia a polyphase pre-Variscan and Variscan tectono-metamorphicevolution of the crystalline rocks has also been documented bystructural, geochronological and thermobarometric studies (Jamičić,1983, 1988; Pamić and Lanphere, 1991; Balen et al., 2006), recentlysupplemented by data indicating the youngest thermal overprintduring the Cretaceous (Balen et al., 2007). Due to generally poorexposure many details in the metamorphic–deformational history arestill open and need further refinement.

Following our previous study on medium-grade metamorphicrocks from the Kutjevačka Rijeka transect in the eastern part of theSlavonian Mts. (Balen et al., 2006) we (1) present P–T pseudosectionsfor the threemain rock types from the Kutjevo outcrop (based on datadescribed in detail in Balen et al. (2006) in order to estimate the effectof bulk composition on calculated phase equilibria; (2) apply garnet

zoning analysis to reveal early P–T conditions; and (3) provide newpetrological, mineral chemical and P–T data in combination withelectron microprobe (EMP) monazite age data from andalusite–staurolite–garnet mica schist in the Krndija quarry situated in theeastern end of the Slavonian Mts.

2. Geological setting

The Slavonian Mts., located along the southern edge of thePannonian Basin, northeastern Croatia, comprise four, up to 1000 mhigh hills: Psunj, Ravna Gora, Papuk and Krndija (Fig. 1) whichtogether form the largest single extent of pre-Mesozoic crystallinebasement in the southern part of the Tisia Unit (Pamić and Jurković,2002). The central part of these mountains comprise pre-Mesozoicmetamorphic and plutonic rocks covered by Permian–Mesozoicsediments, the latter being only locally preserved in cores of km-scale synclines. The pre-Mesozoic crystalline basement and itsMesozoic cover are surrounded and covered by Neogene–Quaternaryfill of the southernmost part of the Pannonian Basin (Jamičić, 1988;Jamičić and Brkić, 1987).

In the Slavonia area the pre-Mesozoic crystalline basement issubdivided by Jamičić (1988) into three complexes (Fig. 1): (1) thePsunj metamorphic complex, assumed to have originated bymetamorphism during the Baikalian orogeny (Late Precambrian toEarly Palaeozoic; ∼ 850–650 Ma), overprinted and retrogressed byyounger metamorphic events; (2) the Papuk metamorphic complex,which underwent metamorphism and migmatitization during theCaledonian orogeny (Ordovician to Early Devonian; ∼ 490–390 Ma),

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and (3) the Radlovac metamorphic complex which resulted from verylow-grade metamorphism during the Variscan orogeny (Late Paleo-zoic; ∼380–280 Ma). Pamić and Lanphere (1991) put forward analternative subdivision (Fig. 1) (see also Pamić and Jurković, 2002 andreferences therein) and proposed that the Psunj and Papuk meta-morphic complexes of Jamičić (1988) represent a Barrovian-typeprograde metamorphic sequence (“Progressively MetamorphosedComplex”), which grades continuously into migmatites and grani-toids. K–Ar (whole-rock and mineral separates) and 40Ar/39Ar agesobtained from these rocks mostly fall in the Variscan time span (forcompilation see Pamić and Jurković, 2002). However, recentlydocumented pre-Variscan (428–444 Ma, Balen et al., 2006) monaziteTh–U–Pb ages of garnet-bearing mica schists confirmed the presenceof pre-Variscan mineral parageneses, as proposed earlier by Jamičić(1983, 1988) using structural analysis and overprinting relationships.

Both the Kutjevo and Krndija outcrops investigated here (Fig. 2)belong to the same Psunjmetamorphic complex (Jamičić, 1988) or theProgressively Metamorphosed Complex (Pamić and Lanphere, 1991).However, recently published data (Balen et al., 2006; Horváth et al.,2007) and our new results presented in this paper indicate a morecomplex internal structure and metamorphic history of this unit thanpreviously proposed.

3. Petrography and mineral chemistry

EMP analyses were performed on selected rock samples at theInstitute for Geochemical Research, Hungarian Academy of Sciencesusing a JEOL JCXA-733 electron microprobe equipped with an OxfordINCA 200 EDS. Operating conditions were 20 keV accelerating voltage,4 nA sample current, and 100 s counting time. The PAP correctionprocedure was applied (Pouchou and Pichoir, 1984). The following

Fig. 2. Geological setting of the study area with

standards were used: albite for Na, quartz for Si, corundum for Al,MgO for Mg, orthoclase for K, apatite for Ca, hematite for Fe,spessartine for Mn and rutile for Ti.

3.1. Kutjevo outcrop (PA-28)

Metamorphic lithologies of the Kutjevo outcrop (Gauss–Krügercoordinates, 6491607, 5033892, 6th zone, Croatia) were studied indetail by Balen et al. (2006). The outcrop shows cm-scale alternatinglayers of mica schist, gneiss and amphibolite (Fig. 3a). The studiedsamples are strongly foliated, sporadically showing relicts of micro-scale isoclinal folds and mylonitic microstructures.

Nematoblastic garnet-bearing amphibolite exhibits lineation andequigranular microstructures. The peak mineral assemblage com-prises amphibole with subordinate garnet, plagioclase and quartz.Amphibole (ferrotschermakite to magnesiohornblende, following theclassification of Leake et al., 1997) is the dominant phase formingprismatic and subhedral grains defining the lineation. Plagioclase(An22-30) occurs as interstitial, xenoblastic grains. Garnet (up to1 mm) is rounded, pink and unaltered. It is slightly zoned withincreasing Prp (8→16) and Alm (58→66), and decreasing Grs(18→10) and Sps (12→4) contents from core to rim. XFe [(Fe/(Fe+Mg)] is 0.88 in the core, and 0.80 in the rim area. Quartz occurs in thinlayers and lenses. Accessory minerals are epidote, zoisite, apatite andopaques (mostly ilmenite and magnetite).

Paragneiss occurs as thin layers intercalated with mica schist andamphibolite. It is well-foliated, displaying schistose and gneissosestructures defined by elongate biotite flakes (XFe=0.47), plagioclase(An28) and quartz ribbons. Compared to the mica schist, paragneissshows a higher modal content of plagioclase and contains amphiboleinstead of muscovite in the peak assemblage. Amphibole is

the locations of the investigated samples.

Fig. 3. a: Kutjevo sample showing centimeter scale alternating layers of mica schist, gneiss and amphibolite; b: Outcrop photo from Krndija locality. Note the strong myloniticfoliation of the mica schist; c: Andalusite (and) and staurolite (st) porphyroblasts with inclusions of garnet (grt) and biotite overgrow the foliation of biotite, muscovite and quartz;d: Inclusions of garnet (grt), biotite, plagioclase and quartz occur in staurolite (st); e: Staurolite (st) with garnet inclusions (grt) is replaced by fine-grained muscovite-richaggregates (ms) in mica schist from Krndija; f: Muscovite-rich aggregates (Ms) containing garnet (grt) and biotite inclusions (bt) occur in the Kutjevo mica schist (possiblepseudomorphs after staurolite).

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tschermakite according to the IMA classification of Leake et al. (1997).Together with amphibole, the peak assemblage is composed ofplagioclase and biotite as dominant phases, garnet (up to ∼5 vol. %)and quartz. Hypidioblastic garnet cores and rims show slightlydifferent compositions (Alm 65→70, Prp 13→14, Grs 11→8, Sps8→4 from core to rim). Biotite is partly chloritized, apatite, zircon andopaque minerals are the accessory minerals.

Mica schist has a well-preserved metamorphic fabric (S1) with apeak metamorphic assemblage of garnet, biotite, muscovite, plagio-clase and quartz. This foliation is represented by preferentiallyoriented biotite (XFe=0.51–0.53), muscovite and abundant garnettrails. Plagioclase (An18) is elongated, polygonal and round andtogether with quartz form layers parallel to the S1. Locally, muscoviteforms aggregates with quartz, biotite and garnet. Garnet grains arehypidioblastic, partly fractured, and typically surrounded by asym-

metric pressure shadows filled with chlorite, biotite, muscovite,epidote and quartz. All garnets are almandine rich with smalleramounts of pyrope, grossular and spessartine components (seeTable 1 in Balen et al., 2006). Garnet cores usually exhibit higherSps and Prp and lower Alm and Grs contents than the rim (Fig. 4a).There is an abrupt change in the Grs content between garnet core andrim, across a distance of a few microns. The garnets showcontinuously decreasing Sps content towards the garnet rim regard-less of the abrupt increase in Grs content. Prp, Alm and XFe increase inthe rim area. The Grs content decreases from the inner part of core tothe outer part; and after the abrupt increase, shows a similar zoningpattern in the rim area. These patterns suggest increasing tempera-ture during the crystallization of garnet rims (Spear, 1993). S2foliation is related to a retrograde process resulting in extensivechloritization of biotite and garnet.

Table 1Electron microprobe analyses of monazites from mica schist sample PH-25 of theKrndija outcrop. Formula units were calculated on the basis of 4 oxygens. Model ageswere calculated after Montel et al. (1996), Th* values after Suzuki et al. (1991).

Matrix Matrix Vein In grt Matrix In grt Matrix In grt

Mnz 1core

Mnz 1rim

Mnz 2core

Mnz 3core

Mnz 4core

Mnz 5core

Mnz 6rim

Mnz 7core

SiO2 0.19 0.16 0.13 0.06 0.26 0.09 0.13 0.09Al2O3 bd.l. bd.l. bd.l. bd.l. bd.l. bd.l. bd.l. bd.l.P2O5 29.53 29.72 29.05 29.76 29.28 29.31 29.40 29.73CaO 0.62 0.75 0.93 1.18 0.82 0.84 0.48 0.51Y2O3 1.33 1.66 1.72 1.39 1.32 1.35 1.50 1.51La2O3 11.92 11.72 10.76 10.64 11.35 11.02 11.67 11.97Ce2O3 31.32 30.80 29.51 29.49 31.00 30.76 31.32 32.42Pr2O3 3.04 2.89 2.74 2.57 2.82 2.75 2.90 3.05Nd2O3 12.83 13.11 12.77 12.07 12.30 12.47 13.40 13.08Sm2O3 2.17 2.27 2.21 2.08 2.14 2.20 2.37 2.40Eu2O3 0.18 0.28 0.14 0.40 0.12 0.50 0.38 0.00Gd2O3 1.89 1.96 2.04 1.84 1.69 1.84 1.87 1.81Dy2O3 0.66 0.81 0.72 0.82 0.66 0.79 0.82 0.69Er2O3 0.21 0.16 0.18 0.10 0.09 0.09 0.17 0.04Yb2O3 bd.l. bd.l. bd.l. b d.l. bd.l. bd.l. bd.l. bd.l.ThO2 2.69 3.27 4.53 5.35 4.17 3.35 1.66 1.55UO2 0.43 0.54 0.58 0.46 0.46 0.57 0.76 0.56PbO 0.06 0.07 0.10 0.09 0.08 0.09 0.07 0.05SO3 bd.l. bd.l. bd.l. bd.l. bd.l. bd.l. bd.l. bd.l.Total 99.066 100.165 98.140 98.460 98.57 98.173 98.918 99.461

Formula unitsSi 0.007 0.006 0.005 0.002 0.011 0.004 0.005 0.003P 0.991 0.988 0.987 0.999 0.988 0.992 0.990 0.993Al 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000Ca 0.026 0.031 0.040 0.050 0.035 0.036 0.020 0.021Y 0.028 0.035 0.037 0.029 0.028 0.029 0.032 0.032La 0.174 0.170 0.159 0.156 0.167 0.162 0.171 0.174Ce 0.454 0.443 0.434 0.428 0.452 0.450 0.456 0.468Pr 0.044 0.041 0.040 0.037 0.041 0.040 0.042 0.044Nd 0.182 0.184 0.183 0.171 0.175 0.178 0.190 0.184Sm 0.030 0.031 0.031 0.028 0.029 0.030 0.033 0.033Eu 0.002 0.004 0.002 0.005 0.002 0.007 0.005 0.000Gd 0.025 0.026 0.027 0.024 0.022 0.024 0.025 0.024Dy – – – – – – – –

Er 0.003 0.002 0.002 0.001 0.001 0.001 0.002 0.000Yb 0.000 0.000 0.000 0.001 0.000 0.001 0.000 0.000Th 0.024 0.029 0.041 0.048 0.038 0.031 0.015 0.014U 0.004 0.005 0.005 0.004 0.004 0.005 0.007 0.005Pb 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.000S – – – – – – – –

Tetr. 0.998 0.995 0.993 1.001 0.998 0.995 0.995 0.996A[9] 1.005 1.011 1.012 0.999 1.004 1.011 1.009 1.009Th+U 0.028 0.034 0.047 0.052 0.042 0.036 0.022 0.019Ca+Si 0.034 0.038 0.046 0.053 0.046 0.040 0.026 0.025Br 4.665 5.828 8.032 10.040 6.617 6.735 3.656 3.643Hu 0.742 0.636 0.534 0.236 1.051 0.347 0.523 0.340La/Nd 0.930 0.894 0.843 0.882 0.922 0.884 0.871 0.915La/Gd 6.324 5.977 5.273 5.771 6.715 5.987 6.252 6.606Th 2.360 2.870 3.982 4.704 3.667 2.948 1.457 1.359U 0.379 0.473 0.510 0.403 0.409 0.503 0.673 0.497Pb 0.057 0.069 0.095 0.086 0.078 0.081 0.062 0.044Th* 3.594 4.411 5.648 6.012 4.998 4.592 3.655 2.975Age 353 349 378 322 348 396 381 3292 sigma 62.390 50.836 39.656 37.281 44.830 48.785 61.455 75.562

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3.2. Krndija mica schist (PH-25)

Mica schist is the predominant rock type in the studied outcrop(Gauss–Krüger coordinates, 6498229, 5029089, 6th zone, Croatia)with subordinate amphibolite occurring as layers or boudins (Fig. 3b).They have a well-preserved mostly flat-lying foliation which locallyshows mylonitic textures. Foliation is marked by alternation of mica-rich (biotite and muscovite) and quartz-plagioclase rich layers.Andalusite and staurolite form porphyroblasts that sometimes over-grows the foliation (Fig. 3c) with frequent inclusions of garnet, biotite,plagioclase and quartz (Fig. 3d). Staurolite is replaced by fine-grained

muscovite-rich aggregates. In some areas the replacement is almostcomplete with only small staurolite relics remaining (Fig. 3e). Garnetsare smaller compared to the Kutjevo outcrop, usually 10–100 μm insize. They are either homogenous (Prp 5–10, Alm 71–76, Grs 2–7, Sps12–15) or have a Ca-rich plateau in their core regions with some Prp-rich domains (Fig. 4b, c). The plateaus have higher Grs (16–19), Prp(11–17), and lower Sps (5–10) and Alm (58–63) contents. The Prp-rich domains (Prp 27) have lower Sps (1–2) and Alm (51–52)contents with similar Grs.

4. P–T pseudosections

Phase relations are best illustrated and understood using pseudo-sections (quantitative phase diagrams)where the bulk composition ofa rock is incorporated into the calculations. Pseudosection modellingwas undertaken with the 3.25 version of the THERMOCALC software(Powell et al., 1998) with the internally-consistent thermodynamicdataset 5.5 (August 2004 upgrade). The datafile coding of the activity–composition relationships of the minerals used in the MnNCKFMASHcalculations is that of Stowell and Tinkham (2003) and for theNCFMASH and NCKFMASH systems of White et al. (2001). Majorelement composition of the bulk rock samples was determined usinga Perkin Elmer 5000 atomic absorption spectrophotometer (AAS),after digestion with lithium metaborate. In addition to the AAStechnique, permanganometric (FeO), gravimetric (SiO2, TiO2, H2O andP2O5) and volumetric (CO2) methods were applied.

Since the Kutjevo outcrop shows cm-scale layering, individualdomains were separated then analyzed using bulk rock wet chemicalmethods. Layering in the order of 1 cm or greater is considered to besufficient for effective separation (Tinkham and Ghent, 2005).Although standard wet chemical methods are rarely used, somenotable exceptions include Štípska and Powell (2005) and Štípska etal. (2006). Beside bulk rock analysis, two additional methods can beused to estimate the effective bulk composition of a rock. The firstinvolves integrating an estimate of themodal abundancewith averagemineral composition. The second utilizes quantitative X-ray mappingover a selected area of the sample (Marmo et al. 2002). For a moredetail and a comparison of these methods see Tinkham and Ghent(2005).

Fe2O3 and TiO2 do not affect the calculated phase diagram topologysignificantly, since the concentrations of both components are verylow in the analyzed samples. Fe2O3 was considered insignificant sinceepidote is a retrograde phase in the amphibolite and calculated Fe3+

in garnet was found to be very low in all three rock types. TiO2 wasexcluded because it is present mainly in accessory ilmenite andalthough is incorporated into biotite at higher metamorphic grade, atmid-amphibolite facies conditions it is still generally low.

4.1. Kutjevo locality (PA-28)

4.1.1. AmphiboliteThe P–T pseudosection calculated for the amphibolite sample

shows that amphibole and plagioclase are stable at the whole P–Trange (Fig. 5a). Albite is the stable plagioclase at low temperature andmedium to high pressure, while there are coexisting albite and Ca-bearing plagioclase in an intermediate zone (peristerite gap). Chloriteis stable up to 620 °C at low pressures (2–6 kbar), and the upperpressure stability is ca. 8 kbar. Garnet enters the assemblage attemperatures over 600 °C and 5 kbar. Orthopyroxene is restricted tohigh temperatures and low pressures, while clinopyroxene is notstable. The peak mineral assemblage of garnet–hornblende–plagio-clase–quartz is stable over 600 °C and 5 kbar. Inclusions of chlorite,quartz and clinozoisite in garnet indicate prograde (increasing T)conditions for garnet growth. Isopleths of garnet cores intersect at ca.650 °C and 10 kbar, while garnet rims with matrix amphibole andplagioclase yield lower pressure conditions at 7 kbar (Fig. 5b).

Fig. 4. Garnet compositional profiles from mica schists (a: Kutjevo, b, c: Krndija).

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Fig. 5. Kutjevo amphibolite pseudosections (a: bulk rock, b: mineral isopleths and P–Tconditions).

Fig. 6. Kutjevo gneiss pseudosections (a: bulk rock, b: mineral isopleths and P–Tconditions, ellipse indicates average P–T data from THERMOCALC).

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4.1.2. ParagneissThe most striking feature of the calculated P–T pseudosection is

that the observed mineral assemblage of garnet–amphibole–biotite–plagioclase–quartz is not stable (Fig. 6a). Garnet is present over ca.575 °C, while muscovite is not stable at low pressures. Amphibolestability has a strong positive slope from 500 °C and 5 kbar to 650 °Cand 10 kbar. The XFe values of garnet are the same for core and rim(0.83), while XCa (Ca/Fe+Mg+Ca) is decreasing from 0.11 to 0.08from core to rim. The core values indicate pressures over 9 kbar. XFe

values for amphibole (0.28–0.47) yield pressures higher than 12 kbar,while the XFe for biotite (0.45–0.48) indicate temperatures andpressures over 610 °C and 9 kbar. The XCa values of 0.23–0.30 ofplagioclase show conditions less than 600 °C and 6 kbar. Fig. 6b showsthe P–T conditions calculated from average P–T calculations (Balenet al., 2006) and mineral composition isopleths.

4.1.3. Mica schistThe P–T pseudosection calculated from the whole-rock data shows

that garnet is stable in the whole P–T range (Fig. 7a). Tri- and

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quadrivariant fields dominate the pseudosection, no divariant fieldwas calculated. Staurolite stability is between 4 and 8 kbar in 570–650 °C. Chlorite is present in the assemblages up to 630 °C. Muscoviteis restricted to 620 °C at low to medium pressures, while a new whitemica phase (paragonite) occurs at 8 kbar. Muscovite (phengite)comes back at higher pressures and temperatures. There are nosuitable inclusions in garnet cores for P–T evaluation, so the garnetcore isopleths were used. They intersect in a narrow triangle-shapedarea at 584–592 °C and 6.4–7.8 kbar in the g–chl–bi–mu–pl field

Fig. 7. Kutjevo mica schist pseudosections (a: bulk rock, b: garnet isopleths using bulk rockwith 7% garnet core removed, e: same as c with mineral isopleths and P–T conditions, ellip

(Fig. 7b). Chlorite is present only as a retrograde phase and totalconsumption of themineral is expected during the prograde path (seebelow).

Due to the strong fractionation during garnet growth, the bulkcomposition obtained from the whole-rock data is only valid for thegarnet core formation. A new, modified bulk composition is needed toestablish the effective bulk composition responsible for the garnetrim- and matrix-forming processes (e.g. Marmo et al., 2002, Evans,2004, Tinkham and Ghent, 2005). For doing that, the modal content of

data, c: new bulk composition with 5% garnet core removed, d: new bulk compositionse indicates average P–T data from THERMOCALC).

Fig. 7 (continued).

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garnet cores was calculated and the amount of elements stored inthem was removed from the bulk composition. Garnet cores occupyca. 5–7 vol.% of the rock observed by grain counting in thin sections.The two new pseudosections are presented in Fig. 7c and d. Thepseudosections based on the two modified bulk compositions arequite similar to each other. The most striking difference between themodified and the original diagrams are: 1. the smaller stability fieldfor garnet due to the Mn-free system used in the modified diagrams,and 2. the shrinking of the staurolite field with increasing amount ofremoved garnet (see details in Discussion). The XCa values of garnet(0.18–0.24) togetherwith average P–T calculations (Balen et al., 2006)obtained using garnet rim and the matrix phases (biotite, muscovite,and plagioclase) indicate peak conditions of 600–660 °C and 11–12 kbar (Fig. 7e).

4.2. Krndija locality

The investigated mica schist sample (PH-25) contains theassemblage garnet, biotite, staurolite, muscovite, andalusite, plagio-clase and quartz. This assemblage is stable at 528–568 °C and 3.2–4.3 kbar (Fig. 8a). Chlorite is not present in the whole P–T range andthis phase was rarely found in the samples highlighting the limitedamount of retrogression in these rocks. Staurolite is stable between 3–8 kbar at medium-T conditions. Chloritoid is present up to 550 °C atlow to medium pressure, at higher temperature it breaks down tostaurolite. Garnet enters the assemblages under 500 °C at lowpressures and at even lower temperatures at medium to highpressures (ca. 6–10 kbar). The pseudosection is dominated by tri-and quadrivariant fields with only one divariant field (g–bi–st–mu–pa–pl–ky).

Contours of mineral isopleths were used to put tighter constraintson the P–T evolution of the mica schists. Slightly zoned garnets mostlyfound as inclusions in andalusite show decreasing XCa [Ca/(Fe+Mg+Mn+Ca)] values (0.2 to 0.05) and increasing XMn [Mn/(Fe+Mg+Mn+Ca)] values (0.01 to 0.11). These values indicate increasing Tfrom 577–582 °C to 606–627 °C with decreasing P of 8.2–8.4 to 7.1–

7.2 kbar (Fig. 8b) and plot into ky-bearing fields (Fig. 8f), but thisAl2SiO5 phase was not found in our samples (see Discussion). Themeasured XFe values are higher than the calculated ones. Themeasured XFe values of biotite are also too high (0.53–0.6) comparedto the calculated ones (Fig. 8c). The calculated XFe values of staurolitematch the measured ones fairly well (0.85–0.88, Fig. 8e). The XCavalues of plagioclase (0.14–0.24) yield temperatures over 550 °C at3 kbar and 600 °C at 6 kbar (Fig. 8d). The average P–T calculationsmade by THERMOCALC yielded 560±28 °C and 3.9±1.1 kbar).

5. EMP based monazite age dating

Monazites from the Kutjevo mica schists were already dated in aprevious study (Balen et al. 2006) and generally provided ages around440 Ma and indicate that the medium-grade metamorphism is pre-Variscan. As part of the present study, monazites from the Krndijamica schists were dated by means of the electron microprobe. The 10monazites detected in thin section PH-25 are fairly similar withrespect to their morphological and textural appearance, theirchemistry and their Th–U–Pb age. The grains are small (1–20 µm),sub- to anhedral in shape, and show no zoning features in BSE images.Threemonazites havebeen foundenclosed in garnet (close to thegarnetrim), the others occur at grain-boundaries (Fig. 9). The monazite grainsdiffer in shape from the Kutjevo mica schist monazites (Balen et al.,2006), which form elongated crystals (2–3 µm width and 10–15 µmlength) isolated in the matrix.

Microprobe analyses on the six largest grains were done with aJEOL JXA-8600 at Salzburg University, using an accelerating voltage of15 kV and a beam current of 150–200 nA. Analytical proceduresfollowed that of Finger and Helmy (1998) and Krenn et al. (2008).Chemically, the monazites from sample PH-25 can be described asmoderately Y, U and Th-bearing (1.3–1.7 wt.% Y2O3, ∼0.5 wt.% UO2,1.5–4.1 wt.% ThO2). U and Th enter the lattice mainly as the brabantitemolecule (Table 1). La/Nd ratios are fairly constant in all analyses andscatter around 0.9. No systematic compositional differences could beencountered between the matrix monazites and those enclosed ingarnet and core and rim compositions are always similar. This impliesthat only one generation of monazites is present in the rock.

The Th–U–Pb ages are generally Variscan and scatter between ca.320–380 Ma (Table 1) with errors in the range of ∼ 30–70 Ma. All agesoverlap within their errors and a weighted mean of 356±23 Ma hasbeen calculated from all analyses (isoplot 2.49e, Ludwig, 2001). Th* vsPb covariations define an isochron slope of y=0.0153+0.003, whichcorresponds to an isochron age of ca. 345 Ma (Fig. 9). From the data itwould appear that all monazites in the rock grew during the Variscanperiod. Judging from the homogeneous unzoned appearance of thegrains it is most likely that monazite formed during the latest LPmetamorphic stage.

Given the high modal proportion of garnet in the sample and itsstrong affinity for Y (e.g. Zhu and O'Nions, 1999; Pyle et al., 2001), itcan be assumed that monazites with relatively high Y-values (1.3–1.7 wt.%) formed subsequently to a stage of garnet resorption (Jeřábeket al. 2008). Using monazite–xenotime miscibility gap thermometry(Heinrich et al., 1997; Pyle et al., 2001), minimum temperatures of ca.500–550 °C can be inferred for the monazites with the highest Ycontents. These temperatures are in line with the T estimates obtainedfor the latest LP metamorphic overprint of the Krndija mica schists.

6. Discussion

6.1. Effect of bulk composition modification in the Kutjevo locality micaschist

The Kutjevo locality mica schist contains garnets with complexzoning profiles: a Mn-rich core and a Ca-rich rim with an abruptincrease in Ca between core and rim. Mn decreases steadily regardless

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of the abrupt increase in Ca indicating prograde conditions duringgarnet growth confirming the work of Balen et al. (2006). Thequestion arises whether these zoned garnets formed as a result ofseparate events or from a single event involving a change in theeffective bulk composition. Early garnet growth resulted in an almostcomplete consumption of available Mn in the rock which shouldfollow at some time later by an increase in the profile of Ca, Fe andMgwhen the rock runs out of Mn. Interestingly, Fe and Mg do not behavelike this and an explanation for the Ca increase may be due to the

Fig. 8. Krndija mica schist pseudosections (a: bulk rock, b: garnet isopleths, c: biotite isopletellipse indicates average P–T data from THERMOCALC).

destabilisation of a Ca-bearing phase (epidote, apatite). According tothe phase diagram in Fig. 7a, epidote-groupminerals (clinozoisite) arestable only at high pressure and low temperature. Apatite is present inthe matrix and as inclusions in both the garnet core and rim area. Thecalculated P–T values for the garnet core are derived from the g–chl–bi–mu–pl–q assemblage. Chlorite was not found as an inclusion in thegarnet core, and all chlorites present in the matrix are retrograde. It islikely that during garnet core growth all (primary) chlorite wasconsumed.

hs, d: plagioclase isopleths, e: staurolite isopleths, f: P–T path of the Krndija mica schist,

Fig. 8 (continued).

Fig. 9. Monazite textures and age data.

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Removal of the garnet core compositions from the original bulk rockaffects the stability of the Fe–Mg-bearingminerals, garnet and staurolitemost. The garnet-in reaction line (i.e. the lowest P–T conditions wheregarnet appears)moves towards higher temperatures (Fig. 10a)which isdue to theMn-free systemused in the calculations. This is not anartefactas the MnO content is very low after garnet core formation which usedup almost all available MnO. If we remove 10 (or more) volume % ofgarnet core from the whole-rock composition, it results in erroneous(negative)MnOcontents for thenewbulk composition. The Tdifference

on theposition of the garnet-in line between5 and 7 vol.% of garnet coreremoval is ca. 10–20 °C.

Interestingly, garnet core removal has a far more profound effecton the stability of staurolite as previously suggested by many authors(e.g. Marmo et al. 2002, Evans 2004, Tinkham and Ghent 2005) whoargued for only subtle changes in mineral stability fields caused bycompositional fractionation during garnet growth. Staurolite ismentioned in the literature for the area (Kišpatić, 1912; Jamičić,1983; Pamić, 1989) but we were unable to find it in our samples. Oneplausible explanation for this discrepancy is that P–T conditions ofgarnet core formation were outside the stability field of staurolite(Fig. 7b). The calculated peak conditions are also represented bystaurolite-free assemblages. During the prograde P–T path withcontinuous garnet core formation the staurolite stability field isshrinking, especially the st-in moves towards higher-T (Fig. 10b). Thismeans that during prograde evolution, staurolite formation was notpossible. Taking into account the 7 vol.% value, the st-in line moves atleast 100 °C towards higher-T and the stability field is reduced to ca.6–8 kbar and 630–660 °C.

6.2. P–T evolution

6.2.1. Kutjevo locality

6.2.1.1. Staurolite formation on the retrograde path? Stauroliteoccurrences in the area mentioned in the literature (Kišpatić, 1912;Jamičić, 1983; Pamić, 1989) could not be verified in our study at theKutjevo locality, which requires an explanation.We sampled the samerock type at the same outcrop where the staurolites were firstreported (Kišpatić, 1912; Jamičić, 1983; Pamić, 1989). In the Kutjevomica schist muscovite-rich aggregates containing garnet and biotiteinclusions occur in certain parts, they look similar to the mica-rich

Fig. 10. Garnet (a) and staurolite (b) stability in Kutjevo mica schist.

Fig. 11. Retrograde P–T path for Kutjevo mica schist, numbers indicate H2O modecontours. For P–T paths A, B and C see text for details.

Fig. 12. P–T paths for the 3 Kutjevo rock types (mica schist, paragneiss and amphibolite.

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aggregates of the Krndija mica schist which represent pseudomorphsafter staurolite (see Fig. 3e and f for comparison). This forms the basisof our hypothesis on possible staurolite in the Kutjevo schists. Weshowed in the above that staurolite formation during progradeand peak P–T conditions is not likely. For any metapelitic rock, theretrograde mineral formation is governed by the availability of H2O.During prograde metamorphism the rock crosses decreasing H2Ocontours, dehydrates and loses the fluid. The metamorphic peakcorresponds to the conditions where dehydration ends. Duringretrogression this peak assemblage is preserved when the necessaryH2O is not available for the retrograde reactions. The mainconsequence of drawing the H2O contours in a P–T pseudosection isthat, without any infiltration of H2O into the rock, the H2O content canonly decrease (Guiraud et al., 2001). At peak P–T conditions, theKutjevo mica schist sample has 6.75–7.25% H2O. These H2O isoplethsand the additional 6% isopleth are shown in Fig. 11. The latter waschosen because it represents the minimum H2O necessary for

staurolite formation. Three alternative retrograde P–T paths arerepresented in Fig. 11. P–T path A has neither st formation norretrograde effects as it runs through assemblages with higher H2Ocontours than at peak conditions. It is the most possible amongst thethree paths. P–T path B produces st, during which g breaks down andky–sill forms which are not observed. This retrograde path is rejectedhere, because retrograde reactions should progress when the pathcrosses fields with lower H2O contents. P–T path C is the only one withst formation and minor retrograde effects. This path follows narrowconstraints with 6.75–7.25% H2O contours in the st-bearing fields. Theproblem here is that the amount of st formed is only up to 1.3 vol.%which can be: 1. easily missed in a thin section; 2. the formation of

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such a small amount of staurolite could be hindered by sluggishreaction kinetics.

6.2.1.2. P–T paths of the 3 rock types. The Kutjevo locality provides theopportunity to observe the effect of bulk composition on calculatedphase equilibria (Fig. 12). Three rock types were chosen (amphibolite,paragneiss and mica schist) as they alternate in cm-scale layers. Onlythe mica schist contains complexly zoned garnets resulting from achange in the effective bulk composition during the prograde P–Tpath. If the zoning is a result of polymetamorphism (i.e. separatemetamorphic events with significant time gap) one could expectsimilar zoned garnets from the rheologically stronger rock types(amphibolite, paragneiss). The explanation for the lack of significantzoning in amphibolite garnet is found in the pseudosection (Fig. 5b).Garnet is not stable under 600 °C and garnet cores yielded conditionsof ca. 650 °C and 10 kbar, quite close to the peak condition calculatedfrom the mica schist. The paragneiss sample is more problematic. Theobserved paragenesis of g–bi–am–mu–pl–q is not stable in thecalculated pseudosection. Even, the measured mineral chemicalcompositions barely intersect each other (Fig. 6f). It seems plausiblefrom the average P–T calculations and somemineral chemical featuresthat the investigated rock type did not fully equilibrate at medium P–Tconditions after a higher P peak. Garnet rims from the amphiboliteyielded nearly the same P–T conditions as the paragneiss sample.

The most complete P–T path comes from the mica schist. Thegarnet cores formed at 584–592 °C and 6.4–7.8 kbar, while the garnetrims and matrix phases indicate peak conditions of 600–660 °C and11–12 kbar. The retrograde P–T path probably was outside thestaurolite stability field or barely touched it. The monazite Th–U–Pbages support the monometamorphic history. Only one monazite agegroup was observed in the mica schist (444±19 and 428±25 Ma,Balen et al., 2006). The greenschist facies S2 assemblage predates theAlpine deformation and hence we tentatively attribute it to theVariscan orogeny.

6.2.2. Krndija localityThe Variscan P–T–t path of the andalusite–staurolite–garnet mica

schist was constrained using petrographic, mineral chemical andgeochronological data. Garnets are either homogenous or have a Ca-rich plateau in their core regions with some Prp-rich domains. UsingP–T pseudosections contoured for mineral chemical isopleths, theearly part of the P–T path was revealed. Garnets mostly found asinclusions in andalusite indicate increasing T from 577–582 °C to 606–627 °C with decreasing P of 8.2–8.4 to 7.1–7.2 kbar. The stableassemblage of garnet, biotite, staurolite, muscovite, andalusite,plagioclase and quartz is stable at 528–568 °C and 3.2–4.3 kbar.These P–T values are in accordance with the average P–T calculationsmade by THERMOCALC (560±28 °C, 3.9±1.1 kbar). The peak P–Tvalues obtained from the garnet isopleths correspond to the mineralassemblage of g–bi–mu–pl–ky. Kyanite was, however, not found inour samples, but it had been observed in the area (Kišpatić, 1912;Jamičić, 1983; Pamić, 1989). The P–T path from peak conditions downto the and–st-bearing field should pass the sill-bearing g–bi–st–mu–pl–sill field (Fig. 8f). This Al2SiO5 phase is also missing in our samples.The explanation is that the transformation of kyanite and/orsillimanite to andalusite was complete and no relics remain. Analternative retrograde P–T path could be drawn which avoids the sill-bearing field, passing through the g–bi–st–mu–pl field insteadresulting in the breakdown of ky to (mostly) st. From petrographicobservations it is clear that most of the andalusite and some stauroliteformed after garnet, biotite and plagioclase, as these phases are foundas inclusions in them and the garnet rim probably also grew duringthis LP stage. Monazite Th–U–Pb age data show exclusively Variscanages for the Krndija mica schists (356±23 Ma). These are at least 80–90 Ma younger than monazite ages from nearby Kutjevo outcrop, butare in accordance with published mica and hornblende Ar–Ar and K–

Ar ages (for an overview see Pamić and Jurković, 2002 and referencestherein).

This means that the age of peak medium-pressure metamorphismin the Kutjevo outcrop is pre- (or early) Variscan (440 Ma) followedby a greenschist facies retrograde event tentatively ascribed to theVariscan orogeny (pre-Alpine, post early Variscan). The succession ofevents in the Krndija outcrop is the following: 1. medium-pressureVariscan or pre-Variscan metamorphism (older than 350 Ma),followed by 2. low-pressure metamorphism at 350 Ma.

7. Conclusions

Mica schists from the Kutjevo outcrop of the Slavonian Mts.contain complexly zoned garnets with Mn-rich cores and Ca-richrims. Mn decreases steadily from core to rim, but there is an abruptincrease in Ca between core and rim. This complex zoning was notobserved in garnets from intercalated paragneisses and amphibolites.Phase diagram calculations reveal that garnet cores formed at 584–592 °C and 6.4–7.8 kbar. Peak P–T conditions were 600–660 °C and11–12 kbar and are similar to amphibolite from the same outcrop.Staurolite mentioned in the literature was not observed and phasediagram calculations indicate that the retrograde P–T path was alsooutside the stability field of staurolite. Monazites indicate an earlyVariscan (428–444 Ma) age for the medium-grade metamorphism,these data are 70–100 Ma older than published mica Ar–Ar and K–Arages. The P–T evolution of the mica schist is best described by a singlemetamorphic event with the modification of the effective bulk com-position during garnet growth.

Garnets from the Krndija outcrop mica schists contain homoge-nous garnets sometimes with a Ca-rich plateau in their core regions.Phase diagram calculations reveal that garnet cores formed at 570–630 °C and 7–8 kbar. Andalusite and staurolite are found in equilib-rium and according to phase diagram and thermobarmetric calcula-tions, they formed at 530–570 °C and 3–4 kbar together with thematrix. Monazites indicate a Variscan (350 Ma) age for the low-pressure metamorphism. These ages are 70–100 Ma younger thanthose from the Kutjevo locality, but in accordance with publishedmica Ar–Ar and K–Ar ages.

Acknowledgments

This study was financially supported by the Hungarian NationalScience Fund (OTKA, grant number F047322 to PH), by the CroatianMinistry of Science, Education and Sports, Projects 119-1191155-1156(DB)and195-1951293-3155(BT)andby theAustrianScienceFoundation(FWF) in the frame of project P18070. The authors are grateful to JürgenReinhardt and Ron Uken for useful suggestions and language corrections.Helpful reviews by two anonymous referees improved the manuscript,and the editorial work of Ian Buick is thanked here.

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