THERMAL HISTORY OF THE SONNBLICK DOME, SOUTH-EAST TAUERN WINDOW, AUSTRIA - IMPLICATIONS FOR HETEROGENEOUS UPLIFE WITHIN THE PENNINE BASEMENT

The Sonnblick Dome is one of several domal structures affecting the in terface between basement and cover within the Pennine Zone of the Taue rn Window in the eastern Alps. Rb-Sr isotopic data, comprising 19 biot ite and 22 white mica ages from variably deformed granitic gneisses, p rovide new evidence of the thermal and tectonic history of the dome an d its relationships with other parts of the south-east Tauern Window. White mica ages generally cluster between 26 and 30 Ma although there are values up to 82 Ma, which appear to reflect incomplete equilibrati on during Tertiary metamorphism under: low amphibolite facies conditio ns; six closely spaced samples from an intensely sheared 'gneiss lamel la' are more tightly grouped between 26 and 27.6 Ma and provide the be st estimate of the age of syntectonic crystallization. Biotite ages ar e systematically younger, ranging from 19 to 23.5 Ma, reflecting closu re during post-metamorphic cooling. The new data emphasize the contras ting histories of the Sonnblick Dome and the Hochalm Dome approximatel y 20 km further east, where closure of Rb - Sr in biotite did not occu r until 16.5 Ma; the metamorphic peak here is also probably younger, p ossibly as late as 22 Ma. The Sonnblick Dome was formed before 27 Ma a nd the deformation style had changed to extension before biotite closu re by 19 Ma. In contrast, rapid updoming in the Hochalm Dome was previ ously dated at 16.5 Ma and the differences in thermal history can be l inked to differences in deformation history. Overall the geochronologi cal data from the south-east Tauern Window demonstrate the heterogenei ty of thermal history on a geographical scale of 10 km and emphasize t he importance of tectonic displacements in controlling temperature wit hin orogenic belts.