1. INTRODUCTION The Tisza Unit (Fig. 1), a prominent part of which forms the objective of the present study, originated from the northern, European margin of Tethys by most- ly meso-Alpine horizontal block (microplate) displace- ments (G É C Z Y , 1973; K O V Á C S , 1982, KÁZMÉR & K O V Á C S , 1985). The Tisza Unit in the sense of CSON- T O S et al. (1992), which is equivalent to the Tisia megaunit of SZEDERKÉNYI (1996), was one of the most stable blocks of the Pannonian Basin during the Alpine tectonometamorphic cycle (ÁRKAI, 1991). In general, the first metamorphic event recorded in the Tisza Unit is characterized by Barrow-type amphi- bolite facies regional metamorphism. This event was A Clockwise P-T Path from the Variscan Basement of the Tisza Unit, Pannonian Basin, Hungary Péter ÁRKAI , Péter HORVÁTH and Géza NAGY overprinted by a low-pressure Variscan event with grades varying from subgreenschist facies up to amphi- bolite facies with andalusite, closely related to granitoid magmatism (for reviews see LELKES-FELVÁRI & S A S S I , 1981; Á R K A I , 1984; S Z E D E R K É N Y I , 1 9 8 4 ; ÁRKAI et al., 1985). Having applied various thermoba- rometric methods Á R K A I (1984) and Á R K A I et al. (1985) calculated for gneisses, micaschists and interca- lated amphibolites, peak conditions of 500-600°C and 5 00 -900 MPa for the first Barrow-type amphibolite facies event. This event was supposed to be pre-Variscan (L E L- KES-FELVÁRI & SASSI, 1981). Á R K A I et al. (1985) elaborated alternative pre-Variscan - Variscan polycyc- lic and Variscan polyphase models. For the time being no isotopic ages older than Variscan are available from the metamorphic basement of the Tisza Unit (LELKES- F E L V Á R I et al., 1996), which can be, at least partly explained by the intense Variscan heating. The aim of the present paper is to provide thermo- barometric data on the metamorphic history of a gneiss- amphibolite complex on the basis of new microstruc- tural, mineral paragenetic and mineral chemical data and thermobarometric calculations, paying special attention to the chemical zoning of garnet porphyrob- lasts and their coexisting mineral assemblages (mineral inclusions and adjoining matrix minerals, respectively). 2. GEOLOGIC OUTLINE AND PREVIOUS DATA The Baksa-2 borehole, from which the investigated samples originated, is located at the SE part of the Tisza Unit in SE Transdanubia, Hungary (Fig. 2). Below a few tens of metres of Neogene sedimentary cover, the elevated metamorphic basement of the Görcsöny Ridge (referred to GR in this study) is exposed, for a total thickness of over 1000 m, with near 100% core recov- ery (Fig. 3). According to S Z E D E R K É N Y I (1976) the rocks of the GR suffered prograde “Barrovian” regional metamorphism changing from the chlorite zone to the sillimanite zone. K O V Á C H et al. (1985) described the metamorphic pile crosscut by the Baksa-2 borehole, as the following downward succession: “upper marble”, chloritic two-mica gneiss, “lower marble” and garneti- ferous two-mica gneiss and schist, intercalated with G EOL . CROAT. 52/2 109 - 117 5 Figs. 4 Tabs. ZAGREB 1999 Key words: Gneiss, Amphibolite, Geothermobaro- metry, Variscan regional metamorphism, Tisza Unit, Pannonian Basin, Hungary. Laboratory for Geochemical Research, Hungarian Academy of Sci- ences, Budaörsi út 45, H-1112 Budapest, Hungary. e-mail: arkai@sparc.core.hu Abstract The polymetamorphic basement of the Tisza Unit forms a de- tached fragment of the Variscan European foreland of the Neotethyan realm. A clockwise evolution path of a gneiss-amphibolite complex of the Tisza Unit was reconstructed, investigating the polymeta- morphic rocks of the borehole Baksa-2, SE Transdanubia, Hungary. The results obtained by microstructural and mineral paragenetic observations, mineral chemical analyses, and thermobarometric cal- culations define a P-T loop which suggests a complex Variscan polyphase model rather than a pre-Variscan - Variscan polycyclic one. The early part of the prograde path with kyanite is characterized by T-P conditions of 480±50°C and 470±70 MPa, respectively. The metamorphism reached its peak at 660±25°C and 750±50 MPa, when both kyanite and staurolite were stable. This metamorphic cli- max was followed by a nearly isothermal decompression to 440±2 0 MPa at 650±40°C. This event is marked by the presence of silliman- ite and a second generation of garnet, and is closely related to the col- lisional Variscan granitoid magmatism observed in considerable parts of the Tisza Unit. In amphibolites intercalated with gneisses, only this last event was preserved, providing T-P estimates of ca. 650-690°C / 400-500 MPa. The present paper provides the first demonstration of a continuous, clockwise P-T path from the metamorphic basement of the Hungarian part of the Tisza Unit. 110 Geologia Croatica 52/2 amphibolites. According to KOVÁCH et al. (1985), the first metamorphic event that produced garnet, stauro- lite, kyanite and sillimanite in gneiss and micaschist, diopside in carbonate rocks and hornblende + a n d e s i n e - labradorite in amphibolite occurred at 630- 6 50°C and 5 00-700 MPa. They obtained a whole rock Rb/Sr iso- chron age of 331±13 Ma for these rocks that was inter- preted as the age of this first metamorphic event. K O VÁC H et al. (1985) also defined a second phase of regional metamorphism that occurred at 400 - 4 10 ° C and 300-400 MPa at ca. 315±4 Ma as deduced from a single Rb/Sr model age obtained on biotite. According Fig. 1 Tectonic sketch map of the Pannonian Basin (box indicates the study area, enlarged in Fig- ure 2). The -- .-- . -- line shows the state boundary of Hungary. Fig. 2 A simplified pre-Ter- tiary geological map of the Görcsöny Ridge and sur- rounding area after FÜL- ÖP (1994), with the loca- tion of the Baksa-2 bore- hole. Legend: 1) metamor- phic rocks of the Görcsöny Ridge; 2) metamorphic rocks of the Somogy-D r á- va Basin; 3) Gyód Serpen- tinite; 4) Carboniferous molasse; 5) Permian mo- lasse; 6-8) Villány-type Mesozoic; 6) Triassic rocks; 7) Jurassic rocks; 8) Cretaceous rocks. to KOVÁCH et al. (1985), this event is demonstrated by the formation of a new biotite generation and partly, by a second generation of garnet, and was followed by a contact metamorphism restricted to the carbonate intercalations (pyroxene hornfels, hornblende hornfels) caused by aplite intrusions connected to the S-type, col- lisional Variscan granitoid magmatism in the surround- ing region (see also BUDA, 1981, 1985). 3. PETROGRAPHY In the metamorphic section of the Baksa-2 borehole paragneisses and micaschists are predominant, interca- lated with some amphibolites, minor calc-silicate rocks and marbles. For the present study characteristic para- gneiss and amphibolite samples from various parts of the rock column were selected. The mineral abbreva- tions are after K R E T Z (1983) except for amphibole (Amph). The paragneisses consist of biotite+quartz +plagio- c l a se + g a r n et + s i l l i m a n i te ± m u s c o v i te ± s t a u r o l i te ± k y a n i te ± K-feldspar and accessory apatite, zircon and tourmaline. Two types of gneisses can be distinguished in thin-sections. Type I gneisses contain large subhedral garnet porhpyroblasts with a few biotite and plagioclase inclusions. Biotite and plagioclase are also present near the garnet rims with staurolite and kyanite. Type II gneisses also have garnet, but these garnets are much smaller and contain abundant inclusions of biotite, plagioclase and quartz. The matrix consists of fibrolitic sillimanite, biotite, muscovite, plagioclase and quartz. Relic kyanite and staurolite can also be seen. B i o t i t e occurs in both types of gneiss, either as fine flakes in the matrix with fibrolitic s i l l i m a n i t e a g g r e g- ates defining the foliation, or as relic crystals wrapped by and pre-dating the foliation. The latter types of biotite also occur as inclusions in the large garnet porphyroblasts and rim the garnet blasts. Fine-grained fibrolitic sillimanite is intergrown with biotite. M u s - covite was found only in association with matrix biotite in Type II gneisses. Biotite, muscovite and sillimanite are often folded together. The presence of large (gener- ally >1 cm diameter) g a r n e t porphyroblasts is the most remarkable feature of Type I gneisses. These subhedral, garnet grains display smooth surfaces, contain inclu- sions of biotite and plagioclase, and are sometimes rim- med by biotites (Fig. 4a). The smaller garnets (max. 1- 2 mm) found in Type II gneisses generally form poik- iloblastic, irregularly shaped skeletal grains lacking any internal fabric. Instead, they contain numerous inclu- sions of biotite and plagioclase (Fig. 4b). Plagioclase is usually xenoblastic or subhedral, and untwinned. Kyan - i t e and s t a u r o l i t e occur as relic porphyroblasts in bio- t i te - s i l l i m a n i te -muscovite and/or quartz -p l a g i o c l a s e matrix in both types of gneisses. Rarely, they reach the size of the garnet porphyroblasts in the Type I gneisses, where they exhibit a rounded shape and occur in larger quantities than in Type II gneisses. Sometimes small biotite inclusions are embedded in kyanite grains. A m p h i b o l i t e s exhibit lineation and show equigran- ular microstructure. Their primary mineral assemblage contains amphibole + p l a g i o c l a se + quartz and subordi- nate epidote. A m p h i b o l e is the dominant phase forming prismatic or subhedral grains of 2-5 mm in length that are aligned with the lineation. P l a g i o c l a s e occurs as interstitial, xenoblastic grains, or forms veinlets with quartz. E p i d o t e forms small grains generally associated 111Árkai, Horváth & Nagy: A Clockwise P-T Path from the Variscan Basement of the Tisza Unit... Fig. 3 Geologic profile of the Baksa-2 borehole (modified after KOVÁCH et al., 1985). Legend: 1) gneiss; 2) micaschist; 3) am- phibolite; 4) marble; 5) aplite; 6) samples used for geothermo- barometric calculations. 112 Geologia Croatica 52/2 with amphibole. The amphibolites contain some sec- ondary K-feldspar and chlorite. 4. MINERAL CHEMISTRY Chemical analyses of minerals were carried out using a JEOL JXCA-733 electron microprobe equipped with 3 WDS, with a measuring programme of N A G Y (1984) in the Laboratory for Geochemical Research, Hungarian Academy of Sciences. The measuring condi- tions were 15 kV, 30 nA, defocused electron beam with a diameter of 5-10 µm, measuring time 5x5 s. Matrix effects were corrected using the method of BENCE & A L B E E (1968). The following standards were used for quantitative analysis: orthoclase (K, Al, Si), synthetic glass (Fe, Mg, Ca), spessartine (Mn), rutile (Ti) and albite (Na). Statistical (absolute) errors expressed as 1 σ are as follows: SiO2: ±0.3, TiO2: ±0.05, Al2O3: ± 0 . 0 5 , FeO: ± 0.2, MgO: ±0.1, MnO: ±0.05, CaO: ± 0 . 1 , Na2O: ±0.03, and K2O: ±0.02 %. Mineral compositions used in geothermobarometric calculations are given in Tables 1-4. Representative analyses and the structural formulae of biotite and muscovite are given in Table 1. B i o t i t e occurs in all the metapelites investigated. Its grains are unzoned, and their Mg / ( M g + F e2 +) ratios range from 0.42 to 0.47. The Ti content is between 0.22-0.3. Bio- tites in the large garnet porphyroblasts of Type I gneis- ses have the highest MgO (Bt1). The highest Ti was observed in biotites near the rim of the large garnet por - phyroblasts (Bt2). The composition of biotite in Type II gneisses is quite uniform regardless of whether the flakes are in the matrix or occur as inclusions in the garnets (Bt3). Inclusions of biotite in kyanite have the same composition as the biotites in the large garnet por- phyroblasts. The composition of m u s c o v i t e flakes is homogenous. It contains 6.15 -6.2 Si atoms p.f.u. The ga r n e t s are Alm-rich in all rock types, independent of their textural position (Table 2). The large garnet por- phyroblasts (Grt1) display composite zoning. The core is Prp7Alm76Sps3Grs14, while the rim is richer in Prp and Fig. 4 a) BSE image of the large garnet (Grt1) porphyroblast with biotite and plagioclase as inclusions and near the garnet; b) BSE image of a Grt2 porphyroblast with biotite and plagioclase occuring as inclusions and rimming the garnet. Numbers refer to the mineral compositions given in Tables 1-4, G1.1 = Grt1core, G1.2 = Grt1rim. Bt1 Bt2 Bt3 MsLocation Grt1 core Grt1 rim Grt2 rim Grt2 rim SiO2 34.55 34.80 35.09 45.76 TiO2 1.95 2.59 1.96 0.60 Al 2O3 19.68 19.48 19.09 34.47 FeO* 19.09 19.16 20.01 0.93 MnO 0.10 0.10 0.13 0.04 MgO 8.87 8.85 8.53 0.59 CaO - - 0.02 0.01 Na2O 0.15 0.36 0.09 1.06 K2O 9.41 8.84 9.43 9.68 Total 93.80 94.18 94.35 93.14 cation numbers on the basis of 22 oxygens Si 5.344 5.346 5.415 6.204 Ti 0.227 0.299 0.227 0.061 Al 3.587 3.527 3.472 5.508 Fe2+ 2.469 2.461 2.582 0.105 Mn 0.013 0.013 0.017 0.004 Mg 2.045 2.026 1.962 0.119 Ca - - 0.003 0.001 Na 0.045 0.107 0.027 0.279 K 1.857 1.732 1.856 1.674 Total 15.586 15.511 15.562 13.956 mg# 0.45 0.45 0.43 0.53 Al IV 2.656 2.654 2.585 1.796 AlVI 0.931 0.872 0.887 3.712 T site 8.000 8.000 8.000 8.000 Y site 5.685 5.672 5.676 4.002 Z site 1.901 1.839 1.887 1.954 Table 1 Biotite and muscovite compositions used for P-T calcula- tions. FeO* = Fe total. 113Árkai, Horváth & Nagy: A Clockwise P-T Path from the Variscan Basement of the Tisza Unit... poorer in Grs content (Prp13Alm74Sps6Grs7) with almost constant Alm and Sps. In comparison, the small garnet (Grt2) shows a homogeneous chemical composition, with Prp1 2A l m7 7S p s7G r s4. P l a g i o c l a s e coexisting with a core of Grt1 is richer in An (An6 2), than plagioclase in contact with the rim of Grt1 or with Grt2 which are A n2 6 and An3 0, respectively (Table 3). The composition of the plagioclase in amphibolites is rather homo- geneous, showing an average An content of 33%. Amphibole compositions are given in Table 4. The calculations of cation numbers for a m p h i b o l e s f o l l o w the scheme of R O B I N S O N et al. (1982). Using the classification of L E A K E et al. (1997) the analyzed amphiboles all belong to the calcic amphibole group, and within this fall into the tschermakite field (LEAKE et al., 1997). 5. GEOTHERMOBAROMETRY Temperatures and pressures for the paragneisses were calculated by the TWEEQU programme (version 2.02) of B E R M A N (1991) using the thermodynamic dataset of B E R M A N (1988), and activity models for garnet (B E R M A N , 1990), biotite (McM U L L I N et al., 1991), muscovite (CHATTERJEE & FROESE, 1 9 7 5 ) and plagioclase (FUHRMAN & LINDSLEY, 1 9 8 8 ) . We also used the computer programme THERMOBA- ROMETRY of SPEAR (1993) with various calibrations of the garnet-biotite thermometer (FERRY & SPEAR, 1978; HODGES & SPEAR, 1982; KLEEMANN & R E I N H A R D T , 1995), and the GASP barometer of NEWTON & HASELTON (1981) and HODGES & S P E A R (1982). In addition, the garnet-phengite ther- mometer of GREEN & HELLMAN (1982), the phen- gite barometry of MASSONNE & SCHREYER ( 1 9 8 7 ) and the garnet-plagioclase-biotite-muscovite barometer of GHENT & STOUT (1981) and HODGES & CRO- W L E Y (1985) were applied to the gneiss sample con- taining muscovite. For the zoned garnet porphyroblasts the GIBBS programme of S P E A R (1993) provided additional information on the P/T conditions for the garnet cores. For the amphibolites the thermobaromet- ric methods of P L Y U S N I N A (1982), HOLLAND & B L U N D Y (1994) and G E R Y A et al. (1997) were applied. Results obtained by various methods are listed in Table 5. In p a r a g n e i s s e s the composition of the core of the large garnet porphyroblasts (Grt1), together with the biotite and plagioclase inclusions were used to obtain P-T data for the early part of the garnet growth, and the garnet rim composition with adjacent biotite and plagioclase for the conditions of the crystallization of the rim. On the basis of mineral paragenetic microstruc- tural observations, kyanite was the stable aluminosili- cate in these assemblages. Garnet-biotite thermometry gave temperature estimates of 480±50°C, and GASP (garnet-kyanite-quartz-plagioclase) barometry gave pressure estimates of 460±50 MPa for the core using the TWEEQU programme. Similar results were derived from the THERMOBAROMETRY programme (470 - 4 90°C and 400 -550 MPa). Using the GIBBS pro- Grt1 Grt1 Grt2Location core rim rim SiO2 36.77 36.90 37.48 TiO2 0.16 0.01 0.08 Al 2O3 21.01 21.55 20.87 FeO* 34.18 33.36 33.69 MnO 1.12 2.37 2.89 MgO 1.59 3.31 2.98 CaO 5.50 2.37 1.66 Na2O 0.06 0.03 0.01 K2O - 0.07 - Total 100.39 99.97 99.64 cation numbers on the basis of 12 oxygens Si 2.966 2.964 3.023 Ti 0.010 0.010 0.004 Al 1.997 2.040 1.984 Fe2+ 2.306 2.241 2.272 Mn 0.076 0.161 0.197 Mg 0.191 0.396 0.358 Ca 0.475 0.204 0.143 Na 0.009 0.005 0.002 K - 0.007 - Total 8.030 8.020 7.982 Prp 6.37 13.20 12.11 Alm 77.14 74.64 76.97 Sps 2.55 5.37 6.67 Grs 13.99 6.79 4.20 Table 2 Garnet compositions used for P-T calculations. FeO* = Fe total. Plagioclase Location Grt1 Grt1 Grt2 Amphi- core rim rim bolite SiO2 52.20 61.09 60.08 59.04 Al 2O3 30.04 24.18 24.72 24.70 CaO 12.79 5.43 6.40 6.96 Na2O 4.30 8.52 8.11 7.61 K2O 0.06 0.10 0.15 0.15 Total 99.39 99.32 99.46 98.46 cation numbers on the basis of 8 oxygens Si 2.381 2.739 2.690 2.673 Al 1.615 1.273 1.304 1.318 Ca 0.625 0.260 0.307 0.338 Na 0.380 0.738 0.704 0.668 K 0.003 0.006 0.009 0.009 Total 5.004 5.006 5.014 5.006 An 61.96 25.90 30.11 33.28 Ab 37.70 73.54 69.05 65.87 Or 0.34 0.56 0.83 0.85 Table 3 Plagioclase compositions used for P-T calculations. 114 Geologia Croatica 52/2 gramme of SPEAR (1993) we obtained 520°C and 400 MPa for the core. For the rim assemblage of Grt1 6 60± 25°C and 750±50 MPa were obtained using TWEEQU, and 640 -6 80°C and 700 -800 MPa with THERMOBAROMETRY. The chemically homoge- neous, small garnet (Grt2) with matrix and inclusion biotite and plagioclase, together with matrix muscovite and sillimanite, gave P-T estimates of 650±40°C and 440 ± 20 MPa with TWEEQU. For the same assem- blage 630 - 6 50°C and 400 - 500 MPa were estimated with THERMOBAROMETRY. Using the phengite barometry of MASSONE & SCHREYER (1987) with 6.2 Si atoms p.f.u, similar pressure-ranges were obtained. However the pressures obtained with phengite barometry can only be interpreted as minimum pres- sures, because no K-feldspar was found in this assem- blage. For a m p h i b o l i t e s three thermobarometric calibra- tions were used to decipher the P and/or T conditions of their formation. The results obtained by the methods of HOLLAND & BLUNDY (1994) and G E R Y A et al. (1997) are close to each other (680-690°C at 450 MPa, 6 50 -6 70°C at 400-500 MPa, respectively), while the earlier method of P L Y U S N I N A (1982) gave signifi- cantly lower T (550°C) but similar P (550 MPa) values. 6. DISCUSSION Figure 5 displays a model for joint interpretation of the results outlined above. Microstructural and mineral paragenetic observations gave clear evidence for rela- tive time-relations of the various mineral assemblages. These relations are indicated by the arrows in Fig. 5. The detectable, first point in the prograde part of the P-T-relative time path was of medium-pressure type, with kyanite as index mineral. Its calculated physical conditions were 480±50°C and 470±70 MPa. Stauro- lite started to form in later stages of the prograde path. The metamorphism reached its peak conditions of 6 60±20°C, and 750±50 MPa, when both kyanite and staurolite were stable. This path, characterized by a simultaneous increase in temperature and pressure, was followed by a nearly isothermal decompression to 4 40±20 MPa at 650± 40°C. This event is marked by the presence of sillimanite, a new generation of small garnets and the formation of the observed foliation in the gneisses. It is likely to be closely related to the col- lisional Variscan granitoid magmatism observed in the immediate vicinity of the studied occurrence and also in various parts of the Tisza Unit (BUDA, 1981, 1985). In amphibolites intercalated with gneisses, only this last event was preserved, providing less precise T-P esti- Amphibole SiO2 42.47 42.89 42.86 TiO2 0.57 0.84 0.69 Al 2O3 13.29 12.40 12.62 FeO* 16.07 15.77 16.01 MnO 0.30 0.33 0.31 MgO 10.81 11.01 10.86 CaO 11.44 11.72 11.59 Na2O 1.56 1.55 1.49 K2O 0.71 0.55 0.76 Total 97.22 97.06 97.19 cation numbers on the basis of 23 oxygens Si 6.248 6.335 6.325 Al IV 1.752 1.665 1.675 AlVI 0.552 0.494 0.520 Ti 0.063 0.093 0.077 Fe3+ 0.889 0.727 0.767 Mg 2.370 2.424 2.389 Fe2+ 1.088 1.221 1.209 Mn 0.037 0.041 0.038 Ca 1.803 1.855 1.833 Na 0.445 0.444 0.426 Na 0.248 0.299 0.260 K 0.133 0.104 0.143 Total 15.684 15.649 15.663 Al total 2.304 2.159 2.195 Table 4 Amphibole compositions used for P-T calculations. FeO* = Fe total. Assemblage Temperature ( oC) Pressure (MPa) Paragneiss FS78 HS82 KR94 TWQ GH82 NH81 HS82 TWQ HC85 GS81 Grt1core+ Bt1+Pl+Ky 440-460 450-490 520-530 430-530 - 400-550 400-500 410-510 - - Grt1rim+Bt2+ Pl +Ky+St 630-670 640-670 600-630 635-685 - 700-820 660-770 700-800 - - Grt2+Bt3+ Ms+ Pl +Sil 610-650 620-650 590-620 610-690 620-650 480-600 460-530 420-460 580-700 400-430 Temperature ( oC) Pressure (MPa) Amphibolite P82 HB94 G97 P82 G97 Amph+Pl 550 680-690 650-670 550 400-500 Table 5 P-T calculations on samples from the Baksa-2 borehole. Legend: FS78) FERRY & SPEAR (1978); HS82) HODGES & SPEAR (1982); KR94) KLEEMANN & REINHARDT (1995); TWQ) TWEEQU; GH82) GREEN & HELLMAN (1982); NH81) NEWTON & HASEL- TON (1981); HC85) HODGES & CROWLEY (1985); GS81) GHENT & STOUT (1981); P82) PLYUSNINA (1982); HB94) HOLLAND & BLUNDY (1994); G97) GERYA et al. (1997). 115Árkai, Horváth & Nagy: A Clockwise P-T Path from the Variscan Basement of the Tisza Unit... mates (ca. 650-690°C / 400-500 MPa) than the gneiss- es. As no prograde zonation was observed either in amphibole or in plagioclase of the amphibolites, one has to presume that the P-T results obtained from the amphibolites reflect the conditions of the last phase of metamorphism. The results obtained using the calibra- tions of HOLLAND & BLUNDY (1994) and G E R Y A et al. (1997) are in agreement with those obtained for the small garnet-bearing assemblage of the gneisses. As to the opinion of the present authors, the only isotopic data on the gneisses published by KOVÁCH et al. (1985) do not provide a reliable time-scale for the detailed metamorphic history, especially if the errors of age determinations are also taken into consideration. In addition, these dates are also in contradiction with the age of the granitoid magmatism (320 - 350 Ma), that was summarised by BUDA (1985), who applied a wide series of isotopic methods to various minerals. There- fore, the present authors think that the results of K O VÁC H et al. (1985) prove only the Variscan age of the metamorphism in general, and demonstrate that no significant post-Variscan heating occurred in the rocks investigated. In order to determine the age-sequence of the metamorphic events depicted in the present study, further isotope geochronological investigation of gar- nets and other accessory minerals is needed. Nevertheless, the continuous changes in mineral chemistry, and especially the lack of any signs of retro- gression (considerable cooling) between the meta- morphic events, suggest that at present the complex polyphase Variscan metamorphic model outlined by Á R K A I (1984) and Á R K A I et al. (1985) seems to be a more realistic working hypothesis than the polycyclic pre-Variscan - Variscan model. 7. CONCLUSIONS On the basis of microstructural, mineral paragenetic, mineral chemical and geothermobarometric investiga- tions of gneisses and amphibolites characteristic of the basement of the Tisza Unit in SE Transdanubia, Hun- gary, the metamorphic evolutionary path was recon- structed. Using the chemical compositions of mineral inclu- sions and the core of large garnet porphyroblasts the P- T conditions of the early prograde path were con- strained (480±50°C and 470±70 MPa). The metamor- phic peak conditions (660± 25°C, and 750±50 MPa) were similar or near to those calculated by Á R K A I (1984) and Á R K A I et al. (1985) for other parts of the basement of SE Transdanubia (Somogy-Dráva Basin). The metamorphic climax was followed by isothermal decompression to 440±20 MPa at 650±40°C, most probably related to the Variscan granitoid magmatism. Considering the available isotope geochronological data, the new results can be best explained by a com- plex polyphase Variscan metamorphic evolution model. Acknowledgements The authors are indebted to Prof. Tibor SZE- DERKÉNYI (József Attila University, Szeged, Hun- gary) for providing selected rock samples from bore- hole Baksa-2. The present paper forms a part of the metamorphic petrogenetic research program of P.Á. (No. T022773) supported by the Hungarian National Research Fund (OTKA), Budapest. Fig. 5 Reconstructed pressure-temperature path of the gneiss-amphibolite complex from the Baksa-2 borehole. Boxes represent the esti- mated P-T conditions for the gneisses and circles for the amphibolites. Al2S i O5 t r i p l e point is after H O L D A W A Y (1971), the sta- bility field of staurolite (grey area) is from SPEAR (1993). 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