The northeastern portion of the Mont Blanc massif in western Switzerland
is predominantly comprised of the granitic rocks of the Mont Blanc
intrusive suit, and the Mont Blanc basement gneisses. Within these
metamorphic rocks are a variety of sub-economic Fe skarns. The mineral
assemblages and fluid inclusions from these rocks have been used to
derive age, pressure, temperature and fluid composition constraints for
two 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 consistent with metamorphic temperatures in the range
515 to 580 degrees C, and pressures ranging from 5 to 8 kbar.
Garnet-hornblende-magnetite 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 similar to those experienced by the basement
amphibolites.
Late Paleozoic 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 geothermal gradients differ from those obtained from the
neighbouring Aiguilles 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 depths of the two massifs were reversed with the Aiguilles
Rouges being initially unroofed at a much greater rate than the Mont
Blanc, but experiencing relatively slower uplift rates near the
termination of the Variscan orogeny.