Cooling History and Exhumation of Lower-Crustal Granulite and Upper Mantle (Malenco, Eastern Central Alps)

Abstract

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°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°C to 650°C with decompression to 0·8 ± 0·1 GPa, and dP/dT < ∼0·15 GPa/100°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°C. Subsequent retrograde metamorphism involved decompression of ∼0·3 GPa and cooling to ∼500°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°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.