H2O-fluxed melting of eclogite during exhumation: an example from the eclogite type-locality, Eastern Alps (Austria)

Epidote eclogites embedded in paragneisses from the Austrian Eastern Alps host rare, decimetre- to metre-sized planar pegmatitic segregations (‘pegmatoids’) that consist of a hornblende–plagioclase–epidote–titanite–quartz assemblage. The pegmatoids cut the primary eclogite foliation at a high angle, show abrupt terminations and exhibit numerous microtextures indicating the former presence of melt, whereas the surrounding eclogites and paragneisses lack evidence for dehydration melting. We interpret the pegmatoids as crystallised hydrous melts of intermediate composition derived from a nearby eclogitic source, similar to the eclogite host rock. The host eclogite preserves high-pressure assemblages of garnet–omphacite–epidote–quartz–rutile ± phengite that yield peak P–T estimates of 21 ± 2 kbar and 700 ± 20 ∘C calculated via pseudosection modelling and Zr-in-rutile thermometry. Subsequent near-isothermal decompression occurred in a closed system under fluid-absent conditions that favoured the preservation of garnet–omphacite assemblages in eclogites. Thermodynamic modelling of the metapelitic country rock indicates that the paragneisses remained fluid-absent during early exhumation, but became fluid-saturated at mid-crustal conditions of 7.5–9 kbar and 680–690 ∘C. We suggest that, as the metapelites dehydrated, minor aqueous fluid was released and infiltrated the enclosed eclogites along discordant fractures that formed in response to unloading, further facilitated by steep gradients in μH2O between the fluid-saturated pelitic country rock and fluid-absent eclogite lenses. Eclogite distal from fluid sources/pathways experienced limited retrogression, whereas localised re-equilibration and H2O-fluxed melting is inferred for eclogite affected by extensive fluid infiltration. Zr-in-titanite thermometry on crystals from pegmatoids yields a crystallisation temperature of 697 ± 10 ∘C at 8.5 ± 1.5 kbar, similar to retrograde conditions recorded by the regionally dominant metapelites. Oxygen- and hydrogen isotopic data for eclogite, pegmatoids and paragneiss are consistent with hydrous fluid transfer from paragneisses to eclogites and a small fluid/rock ratio. We suggest that, at conditions that preclude dehydration-melting, even small-scale melting of eclogite requires addition of fluid from a suitable near-field source.