Two types of aluminous paragneiss from the Loosdorf complex (Bohemian Massif, NE Austria) contain coarse-grained granulite assemblages and retrograde reaction textures that are investigated to constrain the post-peak history of the Gföhl unit in the southern Bohemian Massif. Both types have a peak assemblage garnet-biotite-sillimanite-plagioclase-K-feldspar-quartz-granitic melt ± kyanite ± ilmenite ± rutile, recording peak metamorphic conditions of 0.9-1.1 GPa and 780-820°C estimated by isochemical phase equilibrium modelling. The first sample type (Ysper paragneiss) developed (i) cordierite coronae around garnet and (ii) cordierite-spinel and cordierite-quartz reaction textures at former garnet-sillimanite interfaces. Calculated chemical potential relationships indicate that the textures formed in the course of a post-peak near-isothermal decompression path reaching 0.4 GPa. Texture formation follows a two-step process. Initially, cordierite coronae grow between garnet and sillimanite. As these coronae thicken, they facilitate the development of local compositional domains, leading to the formation of cordierite-spinel and cordierite-quartz symplectites. The second sample type (Pielach paragneiss) exhibits only discontinuous cordierite coronae around garnet porphyroblasts but lacks symplectites. The formation of cordierite there also indicates near-isothermal decompression to 0.4-0.5 GPa and 750-800°C. This relatively hot decompression path is explained by the contemporaneous exhumation of a large HP-UHT granulite body now underlying the Loosdorf complex. The timing of regional metamorphism in the granulites and the southern Bohemian Massif in general is well constrained and has its peak at 340 Ma. Monazite from Loosdorf paragneiss samples yield a slightly younger age of 335 Ma. Although the ages overlap within error, they are interpreted to reflect near-isothermal decompression and exhumation resulting in the formation of the observed reaction textures.

The incipient development of diagnostic high-pressure assemblages-the 'eclogitization'-of granitoids, such as plagioclase breakdown and small-scale formation of garnet and phengite does not require exogenous hydration because unlike dry protoliths like basalt/gabbro or granulite, granitoids . contain crystallographically bound HO in biotite. During high-pressure overprint, partial biotite breakdown causes a localized increase in the chemical potential of HO (HO). Transport of HO into nearby plagioclase induces the formation of diagnostic eclogite facies assemblages of jadeite-zoisite-K-feldspar-quartz ± kyanite ± phengite that pervasively replace former cm-sized plagioclase without requiring the participation of free HO. Depending on pressure-temperature evolution, similar textures may involve albite instead of jadeite, consistent with the general absence of Na-clinopyroxene in high-pressure metagranitoids and kindred gneisses. Plagioclase breakdown may also occur due to simple burial because compression leads to an increase of HO, without requiring additional influx of HO at the texture scale. However, the addition of biotite-derived HO into plagioclase sites likely increases reaction rates. In parallel, ∼100-m-thick complementary coronae involving garnet | phengite-quartz develop at former biotite-plagioclase/K-feldspar interfaces due to the coupled diffusion of FeO-MgO-HO from biotite towards feldspars and minor CaO in the opposite direction. The reaction textures likely create structural weaknesses and preferential fluid pathways that facilitate further hydration and/or deformation along the prograde path, thereby obliterating the textures. If exogenous HO is introduced, it is accommodated in phengite growing at the expense of igneous K-feldspar and possibly in epidote-group minerals. Upon decompression, such hydrated rocks would dehydrate, thereby favouring fluid-assisted retrogression and loss of diagnostic eclogite facies assemblages at lower pressure. Whereas the prograde reaction textures are only preserved at closed-system conditions and in the absence of deformation, they are suggested to commonly form during orogenic metamorphism of granitoids and quartzofeldspathic gneisses that dominate the continental crust in high-pressure terranes such as the Western Italian Alps and the Western Gneiss Region (Norway).

Characterizing the pressure and temperature () histories of eclogite facies rocks is of key importance for unravelling subduction zone processes at all scales. Accurate 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 conditions of metamorphism independent from chemical equilibrium. Because only a single measurement, the inclusion strain, is made, only a line in 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 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 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.