Using the elastic properties of zircon-garnet host-inclusion pairs for thermobarometry of the ultrahigh-pressure Dora-Maira whiteschists: problems and perspectives

Abstract The ultrahigh-pressure (UHP) whiteschists of the Brossasco-Isasca unit (Dora-Maira Massif, Western Alps) provide a natural laboratory in which to compare results from classical pressure (P)–temperature (T) determinations through thermodynamic modelling with emerging field elastic thermobarometry. Phase equilibria and chemical composition three garnet megablasts coupled Zr-in-rutile thermometry inclusions constrain growth within narrow P – T range at 3–3.5 GPa 675–720 °C. On other hand, zircon-in-garnet host-inclusion system combined would suggest inclusion entrapment conditions below 1.5 650 °C that are inconsistent occurrence coesite as rims. observed distribution pressures cannot be explained by either zircon metamictization, or presence fluids inclusions. Comparison measured strains numerical simulations shows post-entrapment plastic relaxation metamorphic peak down 0.5 600–650 °C, on retrograde path, best explains their disagreement phase modelling. This study suggests zircon-garnet couple is more reliable relatively low temperatures (< 600 °C), where well preserved but equilibration might sluggish. appears better suited for higher rock-scale equilibrium can achieved easily local plasticity prevent preservation signal stress state Currently, we define precise threshold temperature resetting pressures. However, application both thermobarometry allows detailed interpretation paths.