HR: 17:00h
AN: V14C-05    [Abstracts]
TI: Evidence for Channelized Transfer of Residual Melts and Fluids in the Basement Sill, Ferrar Province, Antarctica
AU: * Bedard, J H
EM: jean.bedard@nrcan.gc.ca
AF: Geological Survey of Canada, 490 de la Couronne, Quebec, PQ G1K 9A9
AU: Fleming, T
EM:
AF: Southern Connecticut State University, 501 Crescent Street, New Haven, CT 06515 United States
AU: Hersum, T
EM:
AF: Dept. Earth and Planetary Scien, Johns Hopkins University, Baltimore, MD 21218 United States
AU: Marsh, B
EM:
AF: Dept. Earth and Planetary Scien, Johns Hopkins University, Baltimore, MD 21218 United States
AU: Mathez, E
EM:
AF: Department of Earth and Planetary Sciences, American Museum of Natural History, New York, NY 10024 United States
AU: Mukasa, S B
EM:
AF: University of Michigan, 1100 N. University Ave., Ann Arbor, MI 48109 United States
AU: Naslund, H R
EM:
AF: Dept. Earth Sciences, Binghamton University, Binghamton, NY 13902 United States
AU: Simon, A
EM:
AF: Department of Geoscience, University of Nevada, Las Vegas, NV 89154 United States
AB: The Basement Sill of the tholeiitic Ferrar Province is > 350 m thick, with aphyric micro-gabbro-noritic upper and lower marginal zones; a lower pyroxenitic tongue which varies in thickness along-strike; and an upper gabbro-norite to gabbro zone. The tongue is composed of feldspathic orthopyroxenite (opx <55%), and could not have crystallized from melts like those forming the plagioclase-phyric chilled margins of the sill. Orthopyroxene (opx, 1-5mm) is euhedral and normally zoned (e.g. En_81-77), clinopyroxene (cpx) is either interstitial or as ragged prisms, while plagioclase (plag) is generally found as small (most <1mm, some 3-4mm), normally-zoned euhedral laths (e.g. An_83-78). Euhedral plag laths included in opx primocrysts have similar compositions. Groundmass ilmenite ± rutile + mica + granophyre attest to significant in-situ fractional crystallization. The tongue contains feldspathic pipes (1-2m diameter) and layers with slightly more evolved minerals, which commonly contain foliated anorthositic edges, and jackstraw-textured coarse gabbro-norite cores. Pipe margins appear rooted in feldspathic orthopyroxenite tongue material and we speculate that they may represent ephemeral melt escape structures. In plan view, the size and number-density of the feldspathic pipes appears sufficient to have drained a pore melt containing ca 10-20% small plagioclase laths from an opx primocryst-charged slurry in the time available for cooling a 350m thick sill. The gabbro-noritic upper zone is locally modally layered (0.1 to 2m scale beds), and has inverted pigeonite and ilmenite subzones. Bedding-parallel and discordant pegmatitic `veins' attest to the movement of fluids or volatile-rich melts. In some cases the coarse plag in pegmatite appears resorbed. A discordant orthopyroxenitic pipe (1m wide x 2m high) within the gabbro-noritic cumulates is similar to tongue orthopyroxenite and is of uncertain origin (intrusive or metasomatic?). In comparison to the host gabbronorite (opx En_81-72, An_84-63), the orthopyroxenite pipe contains minerals with more evolved compositions (e.g. opx En_78-70, An_81-45) that nonetheless yield magmatic 2-pyroxene temperatures (ca 1120°C).
DE: 1009 Geochemical modeling (3610, 8410)
DE: 1012 Reactions and phase equilibria (3612, 8412)
DE: 1033 Intra-plate processes (3615, 8415)
DE: 1036 Magma chamber processes (3618)
DE: 1042 Mineral and crystal chemistry (3620)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005