HIGH-PRESSURE PARTIAL MELTING OF ECLOGITE AND GARNET AMPHIBOLITE ROCKS DURING DECOMPRESSION AND HEATING, THE TROMSØ NAPPE, NORWAY

It has recently been argued that melting of high-P, high-T rocks in thickened arcs is an important process in magma genesis (e.g. Petford and Atherton, 1993; Rapp et al., 2003), producing rocks such as adakites. Such melting, however, has rarely been studied in-situ, and has instead relied on inferences from experimental and numerical studies, often based on quite different oceanic crustal lithologies. 456 Ma eclogites and garnet amphibolites in the Tromsø Nappe, Norway provide one of the first opportunities to examine such melting in-situ. Evidence for a variety of melt forming reactions is preserved, involving melting of some or all of the eclogitic or retrograded eclogite components to form melt and peritectic garnet or amphibole. Thermobarometry shows that melting of eclogite involving peritectic garnet occurred at the highest pressures (P: 1.8 - 2.3 GPa), but lower than peak eclogite conditions of 3.3 GPa (Ravna et al., pers. comm.). Partial melting involving peritectic amphibole and melting of garnet amphibolite involving peritectic garnet both yield lower pressures (P~1.1 GPa). Temperatures remained high (~800°C) throughout all partial melting reactions, probably in response to emplacement of the neighbouring Skattøra Migmatite Complex (Selbekk et al., 2002). These results suggest that melting of eclogite in continental arcs may be intimately linked to exhumation of those arcs. They also reinforce the suggestion of Petford & Gallagher (2001) that melting in lower continental arcs may be in response to influxes of magmatic bodies nearby. Four reactions are inferred qualitatively: (1) Omphacite ± Zoisite ± Quartz -> Garnet + Melt; (2) Omphacite + Garnet ± Zoisite ± Quartz -> Amphibolite + Melt; (3) Omphacite ± Zoisite ± Quartz -> Amphibole + Melt; (4) Amphibole ± Omphacite ± Quartz ± Biotite -> Garnet + Melt. I am currently working on fully quantifying these reactions.