The contribution of elastic geothermobarometry to the debate on HP versus UHP metamorphism

Characterizing the pressure and temperature (P-T) histories of eclogite facies rocks is of key importance for unravelling subduction zone processes at all scales. Accurate P-T estimates provide constraints on tectonic and geochemical processes affecting subduction dynamics and help in interpreting the geophysical images of present-day converging plates. Conventional equilibrium geothermobarometers are challenged in ultra high pressure (UHP) metamorphic terranes, as minerals may undergo re-equilibration along their exhumation path. Elastic geobarometry applied to host-inclusion systems is a complementary method to determine P-T conditions of metamorphism independent from chemical equilibrium. Because only a single measurement, the inclusion strain, is made, only a line in P-T space of possible entrapment conditions, the entrapment isomeke, can be determined. Thus, the entrapment pressure along an isomeke can only be determined if the entrapment temperature is known. An alternative is to calculate entrapment conditions for two types of inclusions that are believed, from petrological evidence such as being in the same garnet growth zone, to have been entrapped at the same time. The intersection between the two sets of isomeke calculated on multiple quartz and zircon inclusions demonstrates that measuring different inclusion phases trapped inside a single host allows unique P-T conditions for the host rock to be determined. Here, we combine Zr-in-Rutile thermometry and thermodynamic modelling with micro-Raman measurements on quartz and zircon inclusions trapped in garnet to obtain pressures and temperatures of equilibration of a quartz-garnet vein from the Proterozoic Ulla gneiss basement and of garnet-kyanite gneiss from the Caledonian Blåhø nappe, both in the Fjørtoft UHP terrane, Norway. We find that the quartz-garnet vein formed at high pressure (1.5-2.5 GPa and 750-800°C) and recrystallized at ~1.2 GPa and 880°C. In contrast, the garnet-kyanite gneiss followed an anticlockwise path with peak P-T at 1.2 GPa and 880°C: these estimates are consistent with previous thermodynamic modelling and suggest that the Ulla gneiss and the Blåhø nappe came into contact at these last conditions. We also discuss a new method to detect hydrostatic versus Non-hydrostatic stresses near quartz and zircon inclusions in garnet.