ADVANCES IN METAMORPHIC PETROLOGY: THE DORA-MAIRA EXAMPLE (Invited Presentation)

The Dora-Maira massif in the Western Alps is an excellent example to highlight how metamorphic petrology advanced over the last 50 years. The presentation will focus on the case of the kyanite-garnet whiteschists and how the link between metamorphic petrology, experimental data and in-situ analytical techniques resulted in significant advances in our understanding of subduction processes and metamorphism. The association of garnet and kyanite was initially interpreted as Barrovian metamorphism at amphibolite-facies conditions. The discovery of coesite inclusions in garnet, confirmed by in-situ Raman spectroscopy, completely changed the view on the limits of crustal metamorphism and extended subduction of crustal rocks to at least 100 km depth. This opened the new field of research in ultra-high-pressure metamorphism. As the stability of coesite only gives a minimum pressure, experimental studies were needed in order to obtain a more precise P-T evolution of the rocks. Comparison of the compositions of different generations of minerals with experimentally calibrated petrogenetic grids showed that the whiteschists experienced pressures of 4 GPa at temperatures of 700°C. The next big step was to establish the timing of such extreme metamorphism. With the advent of in-situ dating, igneous zircons with metamorphic overgrowth from the whiteschists were dated but the link to metamorphic conditions was difficult to establish. A more comprehensive approach that pioneered the research field of petrochronology determined that peak conditions were reached at 35 Ma and the rocks were exhumed to mid-crustal conditions in only 3 My. In-situ Laser-Ablation ICP-MS analyses applied to metamorphic rocks opened the field for metamorphic geochemistry. For the Dora-Maira rocks, this technique provided constraints on how trace elements are distributed in phases at sub-arc pressures, highlighting also the importance of accessory phases as trace element carriers. In-situ SIMS oxygen isotope analyses on garnet and zircon then allowed linking the P-T-t history to the evolution of fluids. An outlook on new trends and techniques and how they might shape metamorphic petrology of the future will be provided.