GNEISSIFICATION OF ECLOGITE BY PARTIAL MELTING AND DEFORMATION DURING DECOMPRESSION

Eclogite inclusions in migmatitic gneiss are one of the three main field settings of eclogite (Group B eclogites of Coleman et al., 1965). An excellent example is the Western Gneiss Region (WGR), Norway, where eclogite lenses are surrounded by migmatite, including migmatitic hornblende (± garnet ± cpx) gneiss. A common characteristic of this type of eclogite is that it records much higher metamorphic pressures than the host gneiss. The gneissic host rocks typically record amphibolite-facies assemblages, but this is likely an artifact of differential preservation of high-pressure or ultrahigh-pressure (UHP) assemblages during decompression. In some (but certainly not all) cases, the protolith of the host gneiss was eclogite, and the eclogite lenses are relics that survived decompression without complete retrogression to gneiss. The progressive transformation of eclogite to gneiss is well preserved in many locations in the WGR, from least-retrogressed eclogite core regions of meters-scale pods, consisting of garnet + omphacite, to partially to completely retrogressed (hornblende + plagioclase-rich) pod rims. In adjacent gneiss, relict garnet and clinopyroxene (typically symplectitic and rimmed by hornblende) occurs in some mafic layers of migmatite. At numerous sites in the WGR, swarms of eclogite pods are surrounded by a halo of garnet-bearing hornblende gneiss to a distance of several to tens of meters; this gneiss most likely formed by retrogression and/or partial melting of eclogite. Evidence for a possible shared UHP history is observed in at least one site we have analyzed, where a zircon in migmatite contains a coesite inclusion, as does garnet in a nearby eclogite lens. Deformation, partial melting (+ fluids) during decompression drove eclogite retrogression, and the combination of these processes converted eclogite to gneiss. Decompression melting of eclogite and gneiss also likely resulted in strain localization, driving further positive feedback between melting and deformation.