Reconstructing P–T paths during continental collision using multi‐stage garnet (Gran Paradiso nappe, Western Alps)

Abstract

International audienceGarnet-chloritoid-bearing micaschists from the Gran Paradiso massif (Western Alps) contain evidence of a polymetamorphic evolution. Detailed textural observations reveal that two stages of garnet growth are present in the micaschists, interpreted as: (i) relics of an early metamorphism of pre-Alpine age and (ii) newly grown Alpine garnet, respectively. Both generations of garnet preserve growth zoning. Fromthermocalc-based numerical modelling of mineral assemblages in pressure-temperature (P-T) pseudosections, we infer that garnet 1 grew at increasing temperature and slightly increasing pressure, whereas garnet 2 grew at decreasing pressure and slightly increasing temperature. Estimated P-T conditions are ∼620 °C, 6 kbar for the peak of the pre-Alpine event, and of 490 °C, 18-20 kbar for the pressure peak of the Alpine event. Modelling of the modal proportion and chemical composition of garnet (i) shows that the subsequent decompression (to 14-15 kbar at 550 °C) must have been accompanied by moderate heating and (ii) does not support a stage of final temperature increase following decompressional cooling. This argues against a late thermal pulse associated with mantle delamination. Preservation of growth zoning in both generations of garnet and the limited amount of diffusive re-equilibration at the boundary between the two garnets suggests that the rocks were subjected to fast burial and exhumation rates, consistent with data obtained from other internal Alpine units

Metamorphic evolution of the Briançonnais units along the ECORS-CROP profile (Western Alps): new data on metasedimentary rocks

“…A deep burial (around 50 km) for rocks located in the East (Ruitor and Internal units) similar to the neighbouring rocks of the Gran Paradiso (60 km, Le Bayon et al 2006) and of the Piemont-Ligurian ocean (50-60 km, Bousquet 2007) and a shallow burial (around 10 km) for rocks located in the East, in contrast to the rocks of the Valaisan ocean that were buried at great depths (50 km, Bousquet et al 2002). A major tectonic contact, acting top-tothe-northwest during exhumation of HP rocks has separated the two groups.…”

Structural and metamorphic evolution of the phengite‐bearing schists of the northern Adula Nappe (Central Alps, Switzerland)

“…The occurrence of multi-stage garnet is a common feature of polymetamorphic/polycyclic terrains (Rumble and Finnerty 1974;Karabinos 1984;Madhjoub et al 1997;Vry et al 2004;Zulbati and Harley 2007) and many occurrences have been reported from pre-Alpine basement complexes of the Alps (Dal Piaz 1963;Droop 1981;von Raumer and Schwander 1985;Borghi and Sandrone 1995;Matsumoto and Hirajima 2000;Dachs and Proyer 2001;Faryad and Hoinkes 2003;Ganne et al 2003;Faryad and Chakraborty 2005;Le Bayon et al 2006). The two garnet populations (C-Grt and F-Grt 3 ) of the Am-schist AD379A described above belong to a microstructural domain which equilibrated during D3.…”

The 3D quantitative lattice and shape preferred orientation of a mylonitised metagranite from Monte Rosa (Western Alps): Combining neutron diffraction texture analysis and synchrotron X-ray microtomography

“…2). Pressure conditions for the development of the mylonitic lineation were estimated to be 8 AE 3 kbar, whereas the temperature was estimated to be 450e500 C by Borghi et al (1996) and Le Bayon et al (2006a;2006b) and 550e600 C by Gasco and Borghi (2011).…”