Cooling History and Exhumation of Lower-Crustal Granulite and Upper Mantle (Malenco, Eastern Central Alps) |
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| Authors: | MUNTENER OTHMAR; HERMANN JORG; TROMMSDORFF VOLKMAR |
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| Institution: | INSTITUT FÜR MINERALOGIE UND PETROGRAPHIE, ETH ZÜRICH, CH 8092 ZÜRICH, SWITZERLAND |
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| Abstract: | The Braccia gabbro of Val Malenco, Italian Alps, intruded 275My ago during Early Permian lithospheric extension. The intrusiontook place along the crust–mantle transition zone andcaused granulite metamorphism of lower-crustal and upper-mantlerocks. The magmatic crystallization of the gabbro was outlastedby ductile deformation, which is also observed in the otherrocks of the crust–mantle transition. Two stages of retrogrademetamorphism followed. Mineral parageneses in garnet–kyanitegneiss, metagabbro, and metaperidotite record a first stageof near-isobaric cooling under anhydrous conditions. The stabilizedcrust–mantle transition then persisted over a period ofabout 50 My into the Late Triassic. Exhumation of the crust–mantlecomplex began with the onset of continental rifting during EarlyJurassic. This stage of retrograde metamorphism is recordedby near-isothermal decompression and partial hydration of thegranulitic mineral assemblages. The whole crust-to-mantle complexwas then exposed in the Tethyan ocean near its Adriatic margin.The magmatic assemblage of the Braccia gabbro formed at 1–1·2GPa and 1150–1250°C. Microstructures show that thegabbroic rocks evolved from olivine gabbros through spinel togarnet granulite whereas the peridotites recrystallized withinthe spinel peridotite field and the pelitic granulites remainedin the stability field of kyanite. Such an evolution is characteristicof isobaric cooling after magmatic underplating. Granuliticmineral assemblages record cooling from 850°C to 650°Cwith decompression to 0·8 ± 0·1 GPa, anddP/dT < |
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