Cooling history and exhumation of lower crustal granulite and Upper Mantle(Malenco, Eastern Central Alps)

The Braccia gabbro of Val Malenco, Italian Alps, intruded 275 My ago during Early Permian lithospheric extension. The intrusion took place along the c rust-mantle transition zone and caused granulite metamorphism of lower-crus tal 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 foll owed. Mineral parageneses in garnet-kyanite gneiss, metagabbro, and metaper idotite record a first stage of near-isobaric cooling under anhydrous condi tions. The stabilized crust-mantle transition then persisted over a period of about 50 My into the Late Triassic. Exhumation of the crust-mantle compl ex began with the onset of continental rifting during Early Jurassic. This stage of retrograde metamorphism is recorded by near-isothermal decompressi on and partial hydration of the granulitic mineral assemblages. The whole c rust-to-mantle complex was then exposed in the Tethyan ocean near its Adria tic margin. The magmatic assemblage of the Braccia gabbro formed at 1-1.2 G Pa and 1150-1250 degrees C. Microstructures show that the gabbroic rocks ev olved from olivine gabbros through spinel to garnet granulite whereas the p eridotites recrystallized within the spinel peridotite field and the peliti c granulites remained in the stability field of kyanite. Such an evolution is characteristic of isobaric cooling after magmatic underplating. Granulit ic mineral assemblages record cooling from 850 degrees C to 650 degrees C w ith 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 + plagioc lase formed in the gabbros, clinozoisite + phengite + paragonite +/- stauro lite +/- chloritoid in the metapelites and olivine + tremolite + chlorite /- talc in the ultramafic rocks at metamorphic conditions of 0.9 +/- 0.1 GP a and 600 +/- 50 degrees C. Subsequent retrograde metamorphism involved dec ompression of similar to 0.3 GPa and cooling to similar to 500 degrees C, c onsistent with the preservation of the olivine + tremolite + talc assemblag e 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 extens ion and Jurassic rifting are two independent tectonic processes. The presen ce 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 con tinental crust into previously dry granulites at the onset of Jurassic rift ing. These fluids were generated by dehydration of continental crust juxtap osed during rifting with the hot, exhuming granulite complex along a active shear zone.