Sub-solidus Oligocene zircon formation in garnet peridotite during fast decompression and fluid infiltration (Duria, Central Alps)

A garnet peridotite lens from Monte Duria (Adula nappe, Central Alps, Northern Italy) contains porphyroblastic garnet and pargasitic amphibole and reached peak metamorphic conditions of ∼830°C, ∼2.8 GPa. A first stage of near isothermal decompressio

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 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 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 zon