HR: 1340h
AN: V53A-0617    [Abstracts]
TI: Dufek Layered Mafic Intrusion, Antarctica: Constraints on Magma Chamber Processes from U-Pb Geochronology and Trace Element Modeling
AU: * Mukasa, S B
EM: mukasa@umich.edu
AF: University of Michigan, Department of Geological Sciences, Ann Arbor, MI 48109-1063 United States
AU: Andronikov, A V
EM: andron@umich.edu
AF: University of Michigan, Department of Geological Sciences, Ann Arbor, MI 48109-1063 United States
AB: The durations of magma accumulation and crystallization in large layered mafic intrusions such as the Bushveld, Dufek and Stillwater have remained unknown because of either lack of data or poor resolution in the isotopic age information for rocks from different stratigraphic levels. For the ~8-km thick Dufek intrusion in Antarctica, we have determined a new two-fraction baddeleyite U-Pb age of 182.1 ñ 0.8 Ma (2-sigma errors internal error) for the Walker Anorthosite, the lowest layer in the ~3.6 km of stratigraphic section exposed. This new age determination and the previously published three-fraction zircon U-Pb age of 183.9 ñ 0.3 Ma (2 sigma internal error) for the capping Lexington Granophyre show that the age difference between these two rock layers at the extreme ends of the exposed 3.6-km thick section is small. Taking our 2 sigma analytical errors into account, and assuming that the Walker Anorthosite is close to the meeting point of the bottom and top solidification fronts, we show that magmas from which the mafic layered sequence formed could not have existed in the Dufek chamber for more than ca. 3 m.y. Cooling and contraction of the mafic layered sequence to the point of cracking in a brittle fashion allowed emplacement of some silicic dikes believed to represent anatectic melts of the host rocks. Two of these dikes yield zircon U-Pb ages of 182.7 ñ 0.7 Ma (95 percent confidence level) and 181.2 ñ 0.4 Ma (2 sigma internal error), respectively, indicating that the mafic layered section remained sufficiently hot to melt the crust ~0.5 m.y. after the last mafic magmas were added to the chamber. Dufek intrusion shows non-equilibrium between some cumulus and post-cumulus phases even within the same rock. It is evident in this body that either there is continuous mixing and flushing through the cumulus network of magmas derived from mantle sources with different isotopic compositions or that there is progressive contamination of portions of a single, mobile intercumulus liquid via assimilation of silicic host rocks. To understand the major and trace element distributions in the intrusion, we used the Iridium program to model the processes of magma infiltration into a crystal mush as well as compaction. We can reproduce patterns similar to those in the Dufek intrusion by crystallizing plagioclase, pigeonite and magnetite - e.g., both the negative slope and reversals in Mg# without having to invoke channelized addition of discrete batches on new magmas not reacted with the crystal mush.
DE: 3640 Igneous petrology
DE: 3655 Major element composition
DE: 3670 Minor and trace element composition
DE: 1040 Isotopic composition/chemistry
DE: 1065 Trace elements (3670)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting