Heterogeneous deformation and partial (re-)equilibration of high-pressure orthogneiss in the Dora-Maira Massif (Western Alps)

Orthogneisses are widespread in the continental crust, however their usefulness for geothermobarometric reconstructions is often limited because incomplete re-equilibration or recrystallization during exhumation can obliterate high-pressure mineral assemblages. We investigate how deformation, fluid availability and rock rheology control metamorphic transformation in meta-granitic rocks, using orthogneisses from the central Dora-Maira Massif (Western Alps) derived from a Lower Paleozoic protolith (473.0 ± 4.7 Ma; U Pb zircon). The studied orthogneiss records variable Alpine overprints, from low-strain domains preserving magmatic features to high-strain mylonitic domains that underwent complete metamorphic re-equilibration. By combining field observations, microstructural analysis, mass transfer and phase equilibria modelling, we reconstruct their structural and metamorphic evolution across this strain gradient. All domains experienced the same Alpine eclogite-facies metamorphism. Garnet records P-T conditions of 19–22 kbar and 500–540 °C, recording only part of the prograde-to-peak evolution, whereas white mica compositions suggest higher pressures (~27–29 kbar), consistent with ultrahigh-pressure metamorphism previously documented in metasediments of the same tectonic unit. Despite this shared history, the domains display markedly different degrees of metamorphic and structural equilibration. In low-strain domains, fluid-deficient conditions limit metamorphic re-equilibration and preserve the original magmatic load-bearing framework. In contrast, ductile shearing produced mylonitic zones that acted as preferential pathways for externally derived fluids, generating water-saturated domains where recrystallization was enhanced and equilibration under eclogite-facies conditions was achieved. These results support the idea that the interplay between strain localization and fluid distribution controls microstructural transformations and the preservation of high-pressure assemblages in orthogneiss, with important implications for thermobarometric reconstructions. • The studied orthogneiss belongs to the Ordovician-Silurian magmatism of the Dora-Maira basement. • Fluid availability controls HP-UHP metamorphic re-equilibration in orthogneiss. • White mica records UHP conditions more reliably than garnet in metagranitic rocks. • Preservation of HP assemblages requires fluid-poor conditions and limited strain.