HR: 14:25h
AN: V13H-04    [Abstracts]
TI: Small-scale modal layering in the Dais section of the Basement Sill, Ferrar Province, Antarctica
AU: * Naslund, H R
EM: naslund@binghamton.edu
AF: Geol. Sci., Binghamton Univ., Binghamton, NY 13902 United States
AU: Bedard, J H
EM: jbedard@nrcan.gc.ca
AF: Geol. Surv. Canada, 490 de la Couronne , Quebec, PQ G1K9A9 Canada
AU: Simon, A
EM: adam.simon@ccmail.nevada.edu
AF: Dept. of Geosci., Univ. Nevada, Las Vegas, NV 89154 United States
AU: Mukasa, S B
EM: mukasa@umich.edu
AF: Geol. Sci., Univ. Michigan, Ann Arbor, MI 48109 United States
AB: The Basement Sill of the Ferrar Province forms a continuous body with a lateral extent in excess of 100 km, and contains two main rock types: a generally phenocryst-free tholeiitic dolerite and an orthopyroxene-rich porphyritic dolerite which in places contains up to 50% OPX (Bronzite) phenocrysts. The sill is made up of a series of lobate structures with variable amounts of dolerite and OPX-porphyritic dolerite. In all cases the upper and lower margins are dominated by dolerite, and the OPX-porphyritic dolerite is concentrated in the central part of the sill. In addition to large-scale layering resulting from alternating OPX-rich and OPX-poor dolerite, the Basement sill in many exposures contains weak, small-scale (cm to m thick), modal layering within both the dolerite and OPX-porphyritic dolerite. The Dais section of the sill cuts through what appears to be the thickest and stratigraphically lowest lobe of the sill. The average sill thickness is ~350m while the Dais section, in which the lower contact is not exposed, is >450m. The lower third of the exposed Dais section is dominated by OPX-porphyritic dolerite, the middle third contains alternating OPX-porphyritic dolerite and plagioclase-rich dolerite, and the upper third is dominated by plagioclase-rich dolerite. Within the middle third of the Dais section, small-scale modal layering is spectacularly well developed. Three layered sections have been mapped and sampled in detail. Layering is primarily formed by abrupt changes in the relative proportions of OPX, plagioclase, and augite. Within the layered sections, OPX occurs as medium grained (generally 1-5 mm), euhedral to subhedral, zoned, tabular crystals (Mg# 85-77); plagioclase is fine-grained (generally <1mm), euhedral to subhedral laths, with generally unzoned cores ( An 85-81), and thin, zoned rims (An 77-65); and augite occurs as small, unzoned prisms or interstitial, ophitic crystals (Mg# 83-75). The order of crystallization in the layered section appears to be OPX - Plagioclase - CPX, but some OPX crystals contain small plagioclase inclusions. Layering is generally horizontal, with individual layers <1cm to >1m in thickness, with lateral extents of <1m to 10s of meters. Although layers pinch and swell along strike, and in places underlying layers appear to have diapirically risen into or through overlying layers, there do not appear to be any obvious sedimentary structures as would result from current deposition. In most sequences, the layering is very irregular in terms of grain-size, modal variations, and layer thicknesses with no obvious repeating pattern. Mineral compositions are uniform from layer to layer with no apparent correlation between modal abundance and composition. Crystallization temperatures, calculated from pyroxene compositions, are in the range of 900 to 1100oC suggesting that the layers formed in the presence of interstitial melt. Trace amounts of quartz, ilmenite, and granophyre also suggest an intestinal melt. The extreme composition of some layers (>95% plagioclase) indicates that layering could not have formed by mechanical sorting alone. The irregular nature of the layers suggests a process of high-temperature recrystallization acting to accentuate minor variations in initial modal abundances. Differential compaction may be responsible for some of the initial modal variations.
DE: 1000 GEOCHEMISTRY
DE: 3600 MINERALOGY AND PETROLOGY
DE: 3618 Magma chamber processes (1036)
DE: 3620 Mineral and crystal chemistry (1042)
DE: 3625 Petrography, microstructures, and textures
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005