A VARISCAN PRESSURE-TEMPERATURE-TIME PATH FOR THE N-E MONT-BLANC MASSIF

The northeastern portion of the Mont Blanc massif in western Switzerla nd is predominantly comprised of the granitic rocks of the Mont Blanc intrusive suit, and the Mont Blanc basement gneisses. Within these met amorphic rocks are a variety of sub-economic Fe skarns. The mineral as semblages and fluid inclusions from these rocks have been used to deri ve age, pressure, temperature and fluid composition constraints for tw o Variscan events. Metamorphic hornblendes within the assemblages from the basement amphibolites and iron sk:lms have been dated using Ar-40 /Ar-39, and indicate that these metamorphic events have a minimum age of approximately 334 Ma. Garnet-hornblende-plagioclase thermobarometry and stable isotope data obtained from the basement amphibolites are c onsistent with metamorphic temperatures in the range 515 to 580 degree s C, and pressures ranging from 5 to 8 kbar. Garnet-hornblende-magneti te thermobarometry and fluid inclusion studies indicate that the iron skarns formed at slightly lower temperatures, ranging from 400 to 500 degrees C in the presence of saline fluids at formational pressures si milar to those experienced by the basement amphibolites. Late Paleozoi c minimum uplift rates and geothermal gradients calculated using these data and the presence of Ladinien ichnofossils are on the order of 0. 32 mm/year and 20 degrees C/km respectively. These uplift rates and ge othermal gradients differ from those obtained from the neighbouring Ai guilles Rouges massif and indicate that these two massifs experienced different metamorphic conditions during the Carboniferous and Permian periods. During the early to late Carboniferous period the relative de pths of the two massifs were reversed with the Aiguilles Rouges being initially unroofed at a much greater rate than the Mont Blanc, but exp eriencing relatively slower uplift rates near the termination of the V ariscan orogeny.