HR: 14:20h
AN: T23C-03    [Abstracts]
TI: High-pressure partial melting of eclogite and garnet amphibolite rocks during decompression and heating, the Tromso Nappe, Norway.
AU: * Stevenson, J A
EM: james.stevenson@yale.edu
AF: Yale University, Department of Geology 210 Whitney Avenue, New Haven, CT 06511 United States
AB: 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. 452 Ma eclogites and garnet amphibolites in the Tromso 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 (Krogh-Ravna, pers.comm. 2004). Partial melting involving peritectic amphibole and melting of garnet amphibolite involving peritectic garnet both yield lower pressures (P: 1.1 GPa). These results suggest that melting of eclogite rock in continental arcs may be intimately linked to exhumation of those arcs. Temperatures remained high (T: 800 C), most likely in response to the emplacement of the neighbouring Skattora Migmatite Complex (Selbekk and Skjerlie, 2002). This supports the modelling of Petford & Gallagher (2001) that suggests that melting in lower continental arcs may be in response to intrusion of magmatic bodies nearby, and suggests that melting of the eclogites and garnet amphibolites in the Tromso Nappe was due to both decompression and heating. Four reactions are inferred qualitatively: (1) Omphacite +/- Zoisite +/- Kyanite +/- Phengite +/- Quartz goes to Garnet + Melt; (2) Omphacite + Garnet +/- Zoisite +/- Kyanite +/- Phengite +/- Quartz goes to Amphibole + Melt; (3) Omphacite +/- Zoisite +/- Kyanite +/- Phengite +/- Quartz goes to Amphibole + Melt; (4) Amphibole +/- Omphacite +/- Quartz +/- Biotite goes to Garnet + Melt. Current work is focusing on fully quantifying these reactions and the composition of melts produced. References: Petford, N. and Atherton, M. 1996. Na-rich partial melts from newly underplated basaltic crust: the Cordillera Blanca Batholith, Peru. J. Pet. 37, 1491-1521. Petford, N. and Gallagher, K. 2001. Partial melting of mafic (amphibolitic) lower crust by periodic influx of basaltic magma. EPSL, 193, 483-499. Rapp, R. P., Shimizu, N. and Norman, M. D. 2003. Growth of early continental crust by partial melting of eclogite. Nature, 425, 605-609. Selbekk, R. and Skjerlie, K-P. 2002. Petrogenesis of the Anorthosiste Dyke Swarm of Tromso, North Norway: Experimental evidence for Hydrous Anatexis of an Alkaline Mafic Complex. J. Pet. 43, 943-962.
DE: 8100 TECTONOPHYSICS
DE: 8102 Continental contractional orogenic belts
DE: 3600 MINERALOGY AND PETROLOGY (replaces
DE: 3660 Metamorphic petrology
DE: 1035 Geochronology
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting