Bulletin of the Geological Society of Finland, Vol. 77, 2005, pp. 00–00 PT-conditions of deformation within the Palaeoproterozoic South Finland shear zone: some geothermobarometric results Taija Torvela 1)* and Hans Annersten 2) 1) Department of Geology and Mineralogy, Åbo Akademi University, FI-20500 Turku, Finland 2) Department of Earth Sciences, Uppsala University, Villavägen 16, SE-75236 Uppsala, Sweden Abstract Four rock samples were collected from the crustal-scale South Finland shear zone in or- der to compare PT-conditions of deformation between gneissose and mylonitic rock types. Two of the samples were collected from a garnet-bearing gneiss representing an early duc- tile shearing phase. The two other samples were collected from a 30-meter-wide ultramy- lonite zone; first sample representing the ultramylonite and the second sample a less de- formed amphibole-rich gneiss lens within the zone, interpreted to be the mylonite pro- tolith. The new GBPQ geobarometer and the Gt-Bt exchange geothermometer were ap- plied to the two garnet-bearing samples. The temperature conditions of the ultramylonites and the gneissose protolith were compared with the amphibole-plagioclase thermometer by Holland and Blundy (1994). The transpressive ductile shearing that produced the granodioritic and tonalitic gneisses within the study area is interpreted to have taken place in conditions with minimum meta- morphic peaks at approximately 680ºC and 7 kbar as indicated by the GBPQ barometer and Gt-Bt thermometer. The results of the Hbl-Plg thermometry, based on the ultramy- lonite and amphibole gneiss data, suggest that the ultramylonite was formed at minimum 50ºC lower temperature conditions than the surrounding gneisses (the uncertainty of the Hbl-Plg thermometer is 35–40ºC).This is consistent with field observations of a large-scale reactivation of the shear zone after the main transpressive phase of the late stages of the Svecofennian orogen in Southern Finland. Key words: shear zones, gneisses, ultramylonite, deformation, P-T conditions, Paleopro- terozoic, Kökar, Åland Province, Finland *Corresponding author e-mail: ttorvela@abo.fi Bulletin of the Geological Society of Finland, Vol. 77, 2005, pp. 151–164