HR: 17:25h
AN: V12I-06    [PDF]
TI: Crystallographic-Preferred Orientation vs. Shape-Preferred Orientation of Mineral Inclusions in Garnet
AU: * Sidman, D J
EM: sidm0001@umn.edu
AF: University of Minnesota, Department of Geology and Geophysics, Minneapolis, MN 55455 United States
AU: Whitney, D L
EM: dwhitney@umn.edu
AF: University of Minnesota, Department of Geology and Geophysics, Minneapolis, MN 55455 United States
AU: Davis, P B
EM: davi0919@umn.edu
AF: University of Minnesota, Department of Geology and Geophysics, Minneapolis, MN 55455 United States
AB: Mineral inclusions in garnet are used to infer assemblages and textures that existed prior to entrapment and equilibration, and may be used to establish P-T conditions and paths. These applications rest on the assumption that inclusion shape and composition represent pre-entrapment features. If inclusions are texturally modified during syn- to post-entrapment processes, inclusion textures that appear to represent structural fabrics passively overgrown by garnet may instead represent textural and chemical interaction of inclusions with host. It is therefore necessary to determine the relationship, if any, of the crystallographic-preferred orientation (CPO) and shape-preferred orientation (SPO) of inclusions in garnet. An example of a case in which inclusion SPO might not correlate with CPO, and therefore may not represent a pre-entrapment fabric, is observed in high-grade metamorphic rocks in which the inclusions develop facets parallel to garnet crystallographic axes; that is, the inclusions are negative crystals. Because the inclusion shapes result from post-entrapment modification, the SPO defined by the facets has no meaning as a fabric indicator. Preliminary electron back-scattered diffraction (EBSD) data collected from faceted quartz and plagioclase inclusions in a sillimanite gneiss (peak T $>$ 725 degrees C) indicate no correlation between the facet-defined SPO and the CPOs of the inclusions. Ongoing work examines the relationship between SPO and CPO for rocks representing a range of metamorphic grades and P-T-t-deformation histories.
DE: 3600 MINERALOGY AND PETROLOGY (replaces
DE: 8100 TECTONOPHYSICS
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting