Analysis of Archean Quartzite, Granite Mountains, Wyoming

Shelton, Christopher
The Granite Mountains contain a narrow belt of Paleoarchean and Mesoarchean crust that extends approximately 70km east-west across central Wyoming. This belt is referred to as the Sacawee block and is composed of granitic and tonalitic orthogneisses and supracrustal rock including pelitic schists and quartzites. For this project 18 quartzite samples were collected from various locations within the Sacawee block. Quartzite samples were chosen due to a blue hue in hand sample. Once the samples had been collected they were prepared at the University of Wyoming. Geochemistry was obtained by creating homogenous whole rock representative powders using a titanium carbide hydraulic press. Powders from the samples were sent to an offsite lab to undergo x-ray fluorescence to obtain major and trace element composition. Thin sections of the samples were made by using a tile saw to cut billets. Billets cut from the samples were then sent offsite to be further cut and sanded to create thin sections for each sample. Thin sections are pieces of rock 30 microns thick that are epoxied to a glass slide. The slide is looked at using a polarizing light microscope to determine the mineralogical make up of each sample. Analysis of the resulting geochemistry reviled that the samples had abundant SiO2 (95.4-70.6 wt%). A subset of the samples were also determined to be enriched in MgO (8.32-.35 wt%) and depleted in calcium and sodium. It was also determined that the samples represent two groups of quartzite, those with the Mg-rich mineral cordierite and those without. Each group can be distinguished by the mineralogical make up and textural features. Mineralogical and textural features coupled with the geochemistry suggest that the Mg-rich quartzite samples underwent alteration by a magnesium rich hydrothermal fluid. Subsequent regional metamorphism caused recrystallization and formation of cordierite. These events took place in Late Archean time, between 2.86 billion years and 2.63 billion years ago.
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