Vörös A. szerk.: Fragmenta Mineralogica Et Palaentologica 13. 1987. (Budapest, 1987)
the Ca and Mn contents decrease, the Mg and Fe contents increase outwards (prograde phase), however, the amplitudes of the variations may be quite different. In certain parts of the grainedges (where the crystallization of the garnet continued in the retrograde (cooling) phase too, inverse zoning: increase of Mn and Ca, and decrease of Mg and Fe could be observed. The transition between the normal and inverse zoning is gradual: neither sharp compositional changes nor special textural signs (e.g. inclusion-coronas, resorption or interrupted growth border) were found. Thus both the textural and the garnet zoning characteristics may be interpreted as results of a single, composite tectono-metamorphic cycle: no evidence exists proving significant time-gap or sudden P-T changes between the successive deformational (and crystallization) phases described above. Mineral assemblages in the gneiss-mica schist and the mylonite-blastomylonite groups indicating the temperature climax of the first metamorphic event (quartz, oligoclase, biotite, muscovite, garnet, staurolite, graphite) refer to medium pressure (Barrovian), mediumgrade (almandine-amphibolite facies, staurolite-almandine subfacies) crystallization. Its maximal temperature might have been around 550°C, as estimated from the P-T conditions of the staurolite isograde in metapelites (540°C/4 kbar-570°C/8 kbar, see WINKLER 1979). In amphibolites the hornblende I, plagioclase I (oligoclase-andesine) J quartz, epidote, biotite assemblage based on the plagioclase composition suggests temperatures somewhat higher than those of the "An 17 +hornblende" isograde (510-540°C). Unlike the gneiss-mica schist group there are no signs of multistep deformational development of mineral assemblages in amphibolites considering the detectable first metamorphic event. Geothermometric and geobarometric data obtained by the method of GHENT and STOUT (1981), using the plagioclase-biotite-garnet-muscovite assemblage of gneiss and the hornblende-plagioclase method of PLYUSNINA (1981, 1982) for amphibolites, are summarized in Table 2 and Fig. 5. Electron microprobe data of the minerals, their structural formulae as well as a detailed, comparative evaluation of the P-T-time paths of the metamorphic evolution were published earlier (ÁRKAI, NAGY and DOBOSI 1985). In case of zoned grains the compositions of the grain edges contacting each other were taken into account. Considering the character of the garnet zoning, the P-T values of 5.4 kbar and 508°C obtained for the sample Aim1.9/1. (being somewhat lower than the values estimated on the base of isogrades or plagioclase-hornblende pairs (517-525°C) ) may be interpreted as the physical conditions of the freezing of the given assemblage subsequent to the thermal climax. Table 2 Estimated temperature, pressure and metamorphic geotherm (field gradient) values of the metamorphic events Thermobarometers MetaSample morphic event Álm-1.9/1 I Aim-13. 3 I Bi-20.11 I Mp-1.10/la I Aim9. 1 II Bi-20.11 II plagioclase- biotite mu scovite- garnet (GHENT and STOUT 1981) T (°C) P (kbar) 508 5.4 hornblende-plagioclase (PLYUSNINA 1981, 1982) T (°C) P (kbar) 523 7.9 517 7.1 525 6,8 480 3.1 524 4.9 Metamorphic geotherm* (°C/km) 26 18 20 21 43 29 'Estimation conditions of the linear approximation for the depth-temperature relationship: p f~ p l aip s' p l = 9-g " z= 2.8-981- z where Pf - fluid pressure, P 1 - lithostatic pressure, Pg - pressure acting on the solid phase, <j> - average density of the crust, g - gravity constant, Z - depth (see THOMPSON 1981)
