HR: 1340h
AN: V23C-1554    [Abstracts]
TI: Unraveling Polyphase High-Grade Metamorphism and Anatexis in the Fosdick Migmatite Dome, West Antarctica, Using Mineral Equilibria Modeling and In Situ Monazite Geochronology
AU: * Korhonen, F J
EM: korhonen@umd.edu
AF: Laboratory for Crustal Petrology, Department of Geology, University of Maryland, College Park, MD 20742, United States
AU: Brown, M
EM: mbrown@umd.edu
AF: Laboratory for Crustal Petrology, Department of Geology, University of Maryland, College Park, MD 20742, United States
AU: Siddoway, C S
EM: csiddoway@coloradocollege.edu
AF: Department of Geology, Colorado College, Colorado Springs, CO 80903, United States
AB: In situ monazite geochronology coupled with mineral equilibria modeling can provide a direct link between microstructure, quantitative pressure-temperature estimates, and age. Application of pseudosection modeling and in situ monazite geochronology using the electron probe microanalyzer provide robust constraints on the polymetamorphic history of the Fosdick migmatite dome in Marie Byrd Land, West Antarctica. High-grade metamorphism and melting leading to dome formation have been attributed to the transition from convergence to extension along the proto-Pacific margin of East Gondwana at 125-95 Ma. However, new geochronological data suggest that rocks within the migmatite dome experienced a polymetamorphic evolution with evidence for high temperature metamorphism and anatexis during the Devonian-Carboniferous and in the Cretaceous. Metasedimentary units in the dome comprise quartz-plagioclase-ilmenite-biotite ± sillimanite ± garnet ± cordierite ± K-feldspar ± magnetite. Microstructural relationships suggest that garnet is produced from a melt-producing biotite breakdown reaction in the presence of sillimanite. Monazite included within the garnet porphyroblasts yields ages ranging from ca. 368 Ma to ca. 348 Ma, interpreted as the timing of prograde to peak metamorphism. Forward mineral equilibria modeling using appropriate protolith compositions constrain peak conditions to 800-860°C and 7-10 kbar during this Devonian-Carboniferous metamorphism. This event corresponds to a well-established period of arc magmatism along the active margin of East Gondwana. Cordierite associated with a later event replaces garnet, or replaces biotite-sillimanite in garnet-absent domains. Monazite associated with cordierite yields ages of ca. 150 Ma. Mineral equilibria modeling using residual paragneiss compositions constrain peak conditions to 820-870°C and 6.5-7.5 kbar during this younger metamorphic event. The growth of cordierite at ca. 150 Ma occurred just prior to and concurrent with Cretaceous arc magmatism in contiguous New Zealand-Marie Byrd Land, and likely records heating of the middle crust at this time. Monazite rim analyses yield an age of ca. 115 Ma, which corresponds to the onset of transtension, granitic magmatism, voluminous anatexis and formation of the Fosdick migmatite dome.
DE: 3625 Petrography, microstructures, and textures
DE: 3652 Pressure-temperature-time paths
DE: 3660 Metamorphic petrology
DE: 3694 Instruments and techniques
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting