TRACKING THE INITIATION OF PARTIAL MELTING IN EXHUMED ULTRA-HIGH PRESSURE DOMAINS : INSIGHTS FROM FIELD ANALYSIS, RELATIVE CHRONOLOGY AND PETROGRAPHY IN THE NORWEGIAN CALEDONIDES (Invited Presentation)

Post-peak partial melting events have been recognized in several of the most studied Ultra-High Pressure (UHP) domains, such as Kokchetav, Kazakhstan, Dabie Sulu, China or the Western Gneiss Region, Norwegian Caledonides. During the last decade, radiochronological dating of migmatites associated to UHP domains yielded ages closer and closer to the age of peak metamorphism within uncertainty, meaning that their cooling and/or crystallization occurred early during retrograde pressure-temperature path. However, uncertainties on radiochronological ages shall not allow to make a distinction between an early retrograde partial melting event and partial melting at peak conditions. This difference is nevertheless of first importance. Since partial melting is considered as rheologically critical as soon as the first percentages of melt are produced, it is indeed crucial to know whether partial melting is due to decompression reactions or whether exhumation of continental crust is triggered by mechanical decoupling assisted by partial melting at peak conditions.

In order to unravel this high-resolution relative chronology between equilibration of eclogites and partial melting of the surrounding gneiss, a systematic study of leucosomes and partial melting textures has been conducted in the Western Gneiss Region, Norway. Several generations of melt products have been identified, i- trapped and partly preserved in eclogites lenses, ii- transposed in the main gneissic fabric and associated to eclogite retrogression or iii- cross-cutting ductile fabrics. The chemistry of leucosomes and dikes show that they scatter along two trends : from trondhjemites to granites and from tonalites to trondhjemites depending on their chronological position. Comparison of these compositions with compositions of melts produced by experimental partial melting of metapelites and mafic protoliths at high pressure implies that both gneiss and eclogites suffered partial melting in hydrous conditions at peak pressure and temperature. This also implies that water-present partial melting is a possible cause for the initiation of exhumation of UHP rocks.