HR: 0830h
AN: V31E-0976    [PDF]
TI: Olivines in the Egersund Dikes (SW Norway): Evidence for the Evolutionary History of Neoproterozoic Tholeiitic Magmas.
AU: Karen, T S
EM: tefenka@msu.edu
AF: Michigan State University, Dept. of Geological Sciences, East Lansing, MI 48824 United States
AU: * Barton, M
EM: barton.2@osu.edu
AF: The Ohio State University, Dept. of Geological Sciences, Columbus, OH 43210 United States
AU: Miller, C A
EM: Miller.89@osu.edu
AF: The Ohio State University, Dept. of Geological Sciences, Columbus, OH 43210 United States
AB: The Neoproterozoic Egersund dikes in Sw Norway intruded at 650-600 Ma, and the majority formed by intrusion of olivine tholeiites and tholeiites that are geochemically similar to modern OIB. The olivine tholeiites are remarkably well preserved, with glassy chilled margins that contain olivine and plagioclase. Olivines show a range of textural and compositional characteristics that potentially provide information about the evolutionary history of the magmas. Olivine microphenocrysts in the chilled margins are euhedral, and sometimes skeletal indicating rapid cooling. Their compositions (Fo$_{80-82}$) are appropriate for equilibrium with the host melt at oxygen fugacities 0-1 log unit below QFM. Resorbed macrocrysts are more magnesian (Fo$_{0.82-0.84}$) than the microphenocrysts and are similar in composition to olivines in the centers of the dikes. The latter include skeletal crystals (mostly less than 1mm), strained crystals, and crystals intergrown with plagioclase. Skeletal crystals with compositions of about Fo$_{84}$ could have been in equilibrium with magmas corresponding in composition to the nearly aphyric centers of some dikes at 0-1 log unit below FMQ. These crystals are interpreted as high-pressure phenocrysts that formed in magma chambers near the base of the crust (about 30km). The centers and chilled margins of the dikes have compositions consistent with evolution along the Ol-Cpx-Plag cotectic at 0.8-1 Gpa. The compositions of larger, strained olivines (up to 3mm) and olivines intergrown with plagioclase are interpreted to be xenocrysts derived from the wall rocks through which the magma ascended. This interpretation is supported by the presence of reverse-zoned olivines with relatively Fe-rich (Fo$_{0.76}$) cores that are strained or are intergrown with plagioclase. The overall compositional similarity between the majority of the xenocrystal olivines and the skeletal olivines (high-pressure phenocrysts) suggests that the former might have formed by crystallization of earlier batches of olivine tholeiite magma. It is clear that olivines in the Neoproterozoic Egersund dikes provide evidence for a complex evolutionary history similar to that proposed for recent Kilauean olivine tholeiites.
DE: 1010 Chemical evolution
DE: 1020 Composition of the crust
DE: 8434 Magma migration
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting