HR: 14:55h
AN: V13H-06    [Abstracts]
TI: On the Mechanism of Layering in the Dais Intrusion, McMurdo Dry Valleys, Antarctica
AU: * Mathez, E A
EM: mathez@amnh.org
AF: Department of Earth and Planetary Sciences, American Museum of Natural History, New York, NY 10024
AU: McCallum, I S
EM: mccallum@u.washington.edu
AF: Department of Earth and Space Sciences, University of Washington, Seattle, WA 98195
AU: Marsh, B D
EM: bmarsh@jhu.edu
AF: Blaustein Department of Earth & Planetary Sciences, Johns Hopkins University, Baltimore, MD 21218
AB: The Dais intrusion of Wright Valley is the ponded lobe of the Basement Sill, the lowermost body of the Ferrar Dolerite magmatic system. The intrusion is unusual in that it is only about 500 m thick but also layered. Compared to most layered mafic bodies, the Dais intrusion cooled rapidly; thus, the initial igneous textures escaped significant annealing and are partially preserved, providing a unique opportunity to study mechanisms of igneous layering. The layered rocks consist of varying proportions of orthopyroxene (opx) and plagioclase (plag). In nearly all the rocks, opx exists as 5-20 mm grains surrounded by much smaller (0.1-1 mm) plag crystals. Petrographic and field relations suggest that layers formed by mechanical sorting of large from small crystals. One process by which this may have occurred is kinetic sieving, whereby small crystals drain from a network of large ones during flow. Thus, in the Dais fine plag grains collected into laterally extensive, fine-grained, granular, felsic layers, leaving behind coarser, opx-rich layers. The felsic layers range in composition from nearly pure anorthosite to 50-50 mixtures of plag and opx. In some of these layers, opx appears to have begun to anneal into large (1-2 mm), optically continuous oikocrysts that include the surrounding plag grains, many of which exhibit incipient rounding. The distribution and orientation of the included plag grains show the evolution of the shear environment. Satellite opx grains are tenuously connected to and optically continuous with the oikocrysts but quenched in the process of annexation. Clusters of the oikocrysts and their satellite grains are all of the same optical orientation and collectively form an interconnected 3-D sieve through which the remaining plagioclase may have drained to form anorthosite layers. Thus, textural evolution involved coarsening of texture, building the mechanical framework to further enhance mechanical sorting, and development of nearly monomineralic layers. In order to test the inferences based on observations of textures, we have begun a detailed electron and ion probe (SIMS) study of the Dais rocks. SIMS was used to analyze opx and plag in selected samples for Na, Ti, Cr, Sr, Zr, and Y, a group of elements that display a wide range in compatibility. From the analyses, it is possible to distinguish early opx grains and their overgrowths formed during progressive crystallization, indicated, for example, by systematic increases in Y/Sr ratios. The results thus far confirm the general notion of textural evolution inferred from petrography.
DE: 3618 Magma chamber processes (1036)
DE: 3620 Mineral and crystal chemistry (1042)
DE: 3625 Petrography, microstructures, and textures
DE: 3642 Intrusive structures and rocks
DE: 3643 Layered magma chambers
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005