The Braccia gabbro of Val Malenco, Italian Alps, intruded 275 My ago
during Early Permian lithospheric extension. The intrusion took place
along the crust-mantle transition zone and caused granulite
metamorphism of lower-crustal and upper-mantle rocks. The magmatic
crystallization of the gabbro was outlasted by ductile deformation,
which is also observed in the other rocks of the crust-mantle
transition. Two stages of retrograde metamorphism followed. Mineral
parageneses in garnet-kyanite gneiss, metagabbro, and metaperidotite
record a first stage of near-isobaric cooling under anhydrous
conditions. The stabilized crust-mantle transition then persisted over
a period of about 50 My into the Late Triassic. Exhumation of the
crust-mantle complex began with the onset of continental rifting during
Early Jurassic. This stage of retrograde metamorphism is recorded by
near-isothermal decompression and partial hydration of the granulitic
mineral assemblages. The whole crust-to-mantle complex was then exposed
in the Tethyan ocean near its Adriatic margin. The magmatic assemblage
of the Braccia gabbro formed at 1-1.2 GPa and 1150-1250 degrees C.
Microstructures show that the gabbroic rocks evolved from olivine
gabbros through spinel to garnet granulite whereas the peridotites
recrystallized within the spinel peridotite field and the pelitic
granulites remained in the stability field of kyanite. Such an
evolution is characteristic of isobaric cooling after magmatic
underplating. Granulitic mineral assemblages record cooling from 850
degrees C to 650 degrees C with decompression to 0.8 +/- 0.1 GPa, and
dP / dT < similar to 0.15 GPa/100 degrees C. During later hydration,
Cl-rich amphibole and biotite + plagioclase formed in the gabbros,
clinozoisite + phengite + paragonite +/- staurolite +/- chloritoid in
the metapelites and olivine + tremolite + chlorite +/- talc in the
ultramafic rocks at metamorphic conditions of 0.9 +/- 0.1 GPa and 600
+/- 50 degrees C. Subsequent retrograde metamorphism involved
decompression of similar to 0.3 GPa and cooling to similar to 500
degrees C, consistent with the preservation of the olivine + tremolite
+ talc assemblage in ultramafic rocks. Estimated uplift rates of 1-2
mm/year indicate a 15-30 My exhumation related to Jurassic rifting. The
two-stage retrograde path of the Malenco granulites separated by >50 My
suggests that Permian extension and Jurassic rifting are two
independent tectonic processes. The presence of hydrous, Cl-rich
minerals at 600 +/- 50 degrees C and 0.8 +/- 0.1 GPa requires input of
externally derived fluids at the base of 30 km thick continental crust
into previously dry granulites at the onset of Jurassic rifting. These
fluids were generated by dehydration of continental crust juxtaposed
during rifting with the hot, exhuming granulite complex along a active
shear zone.