New data on the geothermometry and geobarometry of the Somogy-Drava Basin, SW. Transdanubia, Hungary

The mineral assemblages of the medium grade polymetamorphic rocks of the Somogy-Dráva Basin are rich in critical minerals which make it possible to expand our knowledge on the metamorphic p—T evolution of the area using mineral equilibria and different geothermo-barometric methods. Previous studies (ÁRKAI 1984; ÁRKAI et al. 1985) revealed three metamorphic events. Applying four geothermo-barometric methods which provided consistent data with the mineral assemblage, the estimated p—T conditions were about 500—1030 Mpa and 539—685°C during the first metamorphic event. The second, low pressure metamorphism produced 4 different mineral assemblages which crystallized as a result of different reactions during the prograde metamorphism. 1/ andalusite+biotite +/- staurolite relics 2/ andalusite+garnet+biotite (sometimes with sillimanite and staurolite relics) 3/ andalusite+biotite+cordierite +/- sillimanite, garnet, plagioclase, muscovite, with rare staurolite relics in the andalusite. 4/ andalusite+biotite+staurolite, where the andalusite and the staurolite crystallized simultaneously along with the biotite. These mineral assemblages refer to a maximum temperature of about 600°C. There is only a few geothermo-barometric data on the second stage of the metamorphism which show pressures consistent with the mineral assemblages (200—350 Mpa) but the temperatures seem to be a little low (520—536 °C). The third, retrograde metamorphism, related to milonitisation, was a very low-low grade one which usually did not exceed the biotite isograde (ÁRKAI 1984), disregarding some exceptional cases