The Mont-Mort metapelites are one of the best preserved relies of the
Variscan unit in the Brianconnais basement. These micaschists
crystallized during a poly-phase metamorphic cycle, under amphibolite
facies conditions. Mineral parageneses and geothermobarometric
calculations indicate a two-stage evolution. Stage (1) (550-600 degrees
C and 5-8 kbar) is documented by assemblages of zoned garnet,
staurolite, kyanite(?), biotite, muscovite, quartz and pla gioclase.
Stage (2) (550-600 degrees C and 2 kbar) is illustrated by assemblages
of andalusite, sillimanite, muscovite, biotite. This metamorphic
evolution is characterized by a nearly isothermal decompression path,
terminating with the formation of andalusite-bearing veins.
U-Pb monazite dates at 330 Ma and Ar-40/Ar-39 muscovite dates at 290-310
Ma (without substantial evidence of argon resetting) point to Variscan
metamorphism and yield an estimate of the time interval between the
thermal peak and the retrogression stage within this part of the
Brianconnais basement.
Restoring the Brianconnais and other Alpine basement units within an
existing geodynamic model of Cordillera construction and destruction, it
is possible to understand better the transition from a medium
pressure/high temperature regime (collision with a peak metamorphism
around 330 Ma) to low-P/high-T conditions (decompression in an
extensional regime) with high geothermal gradient, as recorded by the
successive Variscan parageneses within the Mont-Mort metapelites.