Significance of retrograde hydration in collisional metamorphism: A case study of water infiltration in the Kokchetav ultrahigh-pressure metamorphic rocks, northern Kazakhstan

Petrological analysis reveals the volume of aqueous fluids infiltrated into high-pressure-ultrahigh-pressure metamorphic (HP-UHPM) rocks during exhumation of the Kokchetav massif, Kazakhstan. The mode and extent of retrograde hydration in the massif are highly variable depending on lithology. The volume of aqueous fluids infiltrated during retrograde hydration was calculated based on a comparison of the peak and present mineral assemblages obtained by observations and thermodynamic calculations. Eclogite and metapelite absorbed about 1.0 wt.% and 0.2 wt.% of water during retrograde metamorphism, respectively, whereas orthogneiss and leptite showed no change in water content despite nearly complete retrograde recrystallisation. Based on the change in water content and the spatial coverage of each lithology, the volume of aqueous fluids infiltrated during retrograde metamorphism of the HP-UHPM unit was calculated as 0.1% of the total mass of the massif. This amount of fluid is fully covered by the dehydration of the underlying Daulet Suite during contact metamorphism by the emplacement of the HP-UHPM unit at the mid-crustal level. Our results suggest that the fluid infiltration at the exhumation stage is tectonically inevitable, and even a small amount of water infiltration less than 1 wt.% effectively enhances retrograde recrystallisation in permeable lithologies such as pelitic and felsic schists/gneisses.