Paper No. 7
Presentation Time: 10:25 AM
POLYMETAMORPHISM OF THE GRANULITE-FACIES TASIUYAK PARAGNEISS, NORTHERN LABRADOR: EVIDENCE FOR UHT ANHYDROUS MELTING DURING CONTACT METAMORPHISM
MITCHELL, Rhea K., Department of Earth Science, Memorial University of Newfoundland and Labrador, St. John's, NF A1B3X5, Canada, INDARES, Aphrodite, Earth Science, Memorial University of Newfoundland (MUN), Alexander Murray Building, St. John's, NF A1B 3X5, Canada and RYAN, Bruce, Department of Mines and Energy, Geological Survey of Newfoundland and Labrador, St. John's, NF A1B4J6, Canada, rkm686@mun.ca
Contact aureoles of the anorthositic to granitic plutons of the Mesoproterozoic Nain Plutonic Suite, Labrador, are particularly well developed in the Paleoproterozoic, regionally metamorphosed, granulite-facies, migmatitic Tasiuyak paragneiss. Regional metamorphism formed the assemblage Qtz-Kfs-Pl-Grt-Sill±Bt±Leucosome. Phase equilibria modeling estimates a peak pressure of 9-10 kbars and a peak temperature of 820ºC, consistent with the conditions necessary for biotite dehydration melting. Rocks within the contact aureoles were statically overprinted by lower P, but higher T mineral assemblages and by textures related to a second partial melting event. In general, the contact metamorphosed paragneisses are characterized by cordierite-spinel and cordierite-orthopyroxene intergrowths after sillimanite and garnet, respectively, within a cordierite-rich, quartzofeldspathic matrix.
Melt related textures restricted to the contact aureoles include: lobate potassic feldspar and quartz pseudomorphs of former melt films and pockets (locally enclosing euhedral, idiomorphic cordierite) within and around corroded plagioclase grains; fine grained symplectites of plagioclase and quartz along edges of corroded plagioclase and filling fractures within coarse potassic feldspar grains; and very fine grained rims of Ab90 plagioclase along the rims of corroded Ab64 plagioclase.
This melting event is significant because the Tasiuyak paragneiss was strongly dehydrated during the regional metamorphism and such rocks are not expected to remelt unless they experience UHT conditions. In addition, rocks locally contain millimetre scale domains of fine grained, feathery, symplectic intergrowths of cordierite, potassic feldspar, quartz and orthopyroxene, interpreted as the breakdown products of osumilite, giving evidence for T conditions exceeding 1000ºC. UHT conditions are attributed to the intrusion of the very hot and generally dry, anorthositic to granitic plutons of the NPS in several pulses.
