The 20th annual Symposium will be held on Thursday, April 23, 2009.

Structural Analysis of Quartzites, Black Hills, South Dakota
Poster Session-Science Complex Atrium, 4:00 PM

Lara  Hubbel,   '09 79
   Major: Geology
   Hometown: Howell, MI

Sponsor(s): Beth Lincoln, Tim Lincoln
Support: Geological Sciences Department Taylor Fund for Undergraduate Research

Abstract: 
The rocks in this study were collected from the Precambrian core of the Black Hills, South Dakota, an area multiply deformed and metamorphosed during the assembly of the North American continent culminating in the Trans-Hudson Orogeny 1.7 billion years ago.

The goal was to examine quartzites from this area and determine differences between rocks that display a foliation in hand sample and those that do not. There are two types of foliation found in these rocks, limonite- stained fractures and preferred orientation of elongate axes of quartz grains. Samples of foliated and apparently unfoliated quartzites were collected in pairs from approximately the same location. My examination shows that foliation is present in all rocks on a microscopic level in thin section, even if it is not found in hand sample. For each rock I measured the orientation and lengths of the elongate axes of quartz grains relative to the fractures. I found that the elongate axes are parallel or near parallel in rocks that appear strained in hand sample and but appear to have a wider range of orientations in the unstrained rocks. Analysis of the bulk chemistry of these rocks shows a larger weight percent of immobile elements such as aluminum, yttrium, zirconium and titanium, and a lower weight percent of silica in the foliated rocks, showing that removal of soluble elements played a role in the development of the rock fabric. The deformation falls in the field of dislocation creep with formation of subgrains and grain boundary migration.

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