International audiencePartial melting textures, observed in most continental crust buried in ultrahigh-pressure (UHP) conditions, have mostly been related to their retrograde evolution during exhumation in collisional orogens. Analysis of leucosomes from the Western Gneiss Region (WGR, Norway) UHP and HP domains in the Caledonides show a wide scatter of their chemistries, from early ones close to trondhjemites restricted to UHP domains, to granites in late occurrences or associated with HP domains. Nearly trondhjemitic compositions compare with hydrous melts produced in felsic systems at high pressure (>2 GPa) and moderate temperature (<900 °C). Partial melting experiments at higher temperatures or in dry conditions produce granitic glasses similar to late leucosomes from the WGR. Comparison of pressure-temperature paths for Caledonian eclogites with melting and dehydration reactions for the surrounding gneiss suggest that (1) the continental crust remained partially hydrated during its subduction to ultrahigh pressure, and (2) partial melting reactions producing the trondhjemitic melts started as soon as the WGR rocks reached their hydrated solidus, at the peak pressure recorded by the eclogites. The limited partial melting degree at the peak conditions induced weakening of the continental crust, decoupling from the lithospheric root, and initiation of exhumation

Abstract:Evidence of melting is presented from the Western Gneiss Region (WGR) in the core of the Caledonian orogen, Western Norway and the dynamic significance of melting for the evolution of orogens is evaluated. Multiphase inclusions in garnets that comprise plagioclase, potassic feldspar and biotite are interpreted to be formed from melt trapped during garnet growth in the eclogite facies. The multiphase inclusions are associated with rocks that preserve macroscopic evidence of melting, such as segregations in mafic rocks, leucosomes and pegmatites hosted in mafic rocks and in gneisses. Based on field studies, these lithologies are found in three structural positions: (1) as zoned segregations found in high-pressure (HP) (ultra) mafic bodies, (2) as leucosomes along amphibolite facies foliation and in a variety of discordant structures in gneiss, and (3) as undeformed pegmatites cutting the main Caledonian structures. Segregations post-date the eclogite facies foliation and predate the amphibolite facies deformation, whereas leucosomes are contemporaneous with the amphibolite facies deformation and undeformed pegmatites are post-kinematic and were formed at the end of the deformation history. Geochemistry of the segregations, leucosomes and pegmatites in the WGR defines two trends, which correlate with the mafic or felsic nature of the host rocks. The first trend with Ca-poor compositions represents leucosome and pegmatite hosted in felsic gneiss, whereas the second group with K-poor compositions corresponds to segregation hosted in (ultra) mafic rocks. These trends suggest partial melting of two separate sources: the felsic gneisses and also the included mafic eclogites. The REE patterns of the samples allow distinction between melt compositions, fractionated liquids and cumulates. Melting began at high pressure and affected most lithologies in the WGR before or during their retrogression in the amphibolite facies. During this stage, the presence of melt may have acted as a weakening mechanism that enabled decoupling of the exhuming crust 2 around the peak pressure conditions triggering exhumation of the upward-buoyant crust. Partial melting of both felsic and mafic sources at temperatures below 800°C implies the presence of an H 2 O-rich fluid phase at great depth to facilitate H 2 O-present partial melting.Keywords: Migmatite, leucosome, partial melting, Western Gneiss Region, (U)HP. INTRODUCTIONPartial melting of the continental crust is considered as a weakening process prone to induce lower-crustal macroscopic ductile flow in orogenic belts (Teyssier & Whitney, 2002;Vanderhaeghe, 2009) and play an important role in the exhumation of ultra-high pressure (UHP) rocks (Sizova et al., 2012). Presence of melt indeed affects both thermal (Bittner & Schmelling, 1995) and rheological (Rosenberg & Handy, 2005) behaviour of the crust during orogeny, probably from the very first increments of melt generated. Thus, the initiation and duration of any melting event needs to be well constrained to determine the rheolo...