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