PT-conditions of deformation within the Palaeoproterozoic South Finland shear zone: Some geothermobarometric results

Abstract

Four rock samples were collected from the crustal-scale South Finland shear zone in order 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 ductile shearing phase. The two other samples were collected from a 30-meter-wide ultramylonite zone; fi rst sample representing the ultramylonite and the second sample a less deformed amphibole-rich gneiss lens within the zone, interpreted to be the mylonite protolith. The new GBPQ geobarometer and the Gt-Bt exchange geothermometer were applied 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 metamorphic 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 ultramylonite 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 fi eld 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.