DEFORMATION FABRIC AND WHOLE ROCK CHEMICAL ANALYSES OF SHEARED GRANITIC ROCKS FROM MOUNTAIN, WISCONSIN

Within earth’s crust and mantle rocks deform due to tectonic forces without breaking. Such deformation is localized within narrow bands called shear zones. The precise mechanisms of shear zone formation are not yet clearly understood. The purpose of our research is to gain a better understanding of what causes shear zones to form in rocks. Our research is focused on the Mountain Shear Zone exposed near the town of Mountain, Wisconsin. This shear zone was formed approximately 1.8 billion years ago. Most of the shear zones of comparable age in Wisconsin are buried under glacial depostis, so the exposed Mountain Shear Zone provides an excellent opportunity to study rock deformation processes that have taken place deep within the earth. Our goal is to see whether mineral distribution patterns and rock chemistry changes with shear deformation. These data could provide important information regarding possible mechanisms of shear zone formation.

We have collected both deformed and undeformed granitic rock samples from the Mountain Shear Zone, and recorded their locations using a hand-held GPS unit. The major minerals in the samples are quartz, potassium feldspar, plagioclase, biotite and hornblende. The biotite and hornblende mineral grains in the deformed samples are aligned. We used GIS-based techniques to quantify the alignment and spatial distribution patterns of minerals in the deformed and undeformed rock samples in hand specimen and under a microscope. We have analyzed the whole rock geochemistry of selected samples using X-Ray Fluorescence (XRF) to see how the chemistry differs in deformed and undeformed rocks. Preliminary results show that there are slight differences between the chemistry of deformed and undeformed samples. The undeformed samples appear to have slightly higher concentrations of iron (ferric), magnesium, and calcium oxides, and slightly lower concentrations of sodium and aluminum oxides. In this poster we present how the chemical variation relates to the mineral make-up and mineral distribution patterns of the analyzed samples.