Quantifying deformation fabrics and chemistry across small granitic ductile shear zones from Mountain, Wisconsin

Michael A. DeVasto1, Dyanna M. Czeck1, Prajukti Bhattacharyya2

1Dept. of Geosciences, University of Wisconsin-Milwaukee, P.O. Box 413, Milwaukee, WI 53201.

2Dept. of Geography and Geology, University of Wisconsin-Whitewater, Upham 119, Whitewater, WI 53190

INTRODUCTION

Deformation processes in ductile shear zones can be profoundly affected by the presence of fluids (Wawrzyniec et al., 1999). Fluid-rock interaction also has potential for causing volumetric changes in shear zones, which are important to consider during kinematic analyses (Ramsay and Graham 1970). The purpose of our study is to develop a way to systematically and quantitatively describe textural and chemical changes across small-scale ductile shear zones, which may be indicative of the presence of fluids during deformation.

The small (cm scale) shear zones in deformed granite of the Mountain Shear Zone (MSZ) in Mountain, Wisconsin are ideal for this study for a variety of reasons. 1) Sims et al. (1989) conducted a detailed study of the MSZ, allowing for a more detailed structural analysis. 2) Granite is an excellent rock to study microtextures due to its isotropy and simple mineralogy and chemistry. 3) The scale of these shear zones is ideal for a high resolution sampling method in order to achieve a complete geochemical dataset through the strain gradient.

GEOCHEMISTRY

Tracking geochemical changes across the strain gradient can act as an indicator of fluid-rock interactions during deformation and volume loss (Newman and Mitra, 1992; Yonkee et al., 2003). Bialek (1999) summarizes the inconsistency in results between many studies, which indicate that ductile shear zones may or may not have significant fluid input and volume change. These inconsistencies may be due to lack of resolution in the chemistry data, which typically are only collected in the protolith and shear zone. Therefore, , we plotted individual elemental chemistry, determined through x-ray fluorescence, with respect to distance from the shear zone (Yonkee et al. 2003). This method provides more information than the commonly used isocon method, which lacks spatial relevance and may misrepresent elements with small changes in concentration.

Our granitic shear zones have a variety of geochemical gradients. Preliminary results in one shear zone show relatively no major element chemical changes. Preliminary results in another shear zone from the same outcrop show enrichments in Fe2O3, MgO, Al2O3 and K2O with depletion in SiO2 and Na2O. CaO, P2O5, and TiO2 concentrations vary along the transect, but show no obvious trend with respect to the shear zone. In comparison, the isocon method indicates minor depletions/enrichments in SiO2, Fe2O3, MgO, and Al2O3, whereas other analyzed elements remained “immobile”.

A GIS TECHNIQUE TO QUANTIFY FABRIC FORMATION

Microstructures may be indicative of fluid interaction and localized volume loss, offering a different, yet largely descriptive, approach to studying fluid-rock interaction during deformation. To completely convey the importance of microstructures there needs to be a method to quantify various features. Li et al. (2008) developed a semi-automatic digitizing model within ArcGIS to improve the ease of tracing crystal grain boundaries. We have expanded the use of this model by applying it to deformed polymineralic rocks. Because this model works within a GIS, we can build a spatial database of microstructural information, including mineralogy and deformation fabrics, which can be queried for statistical and geospatial information. The average nearest neighbor tool is a good example of what even a simple spatial analysis can do. Running this tool yields a plethora of information, most notably the nearest neighbor index (figure 1), which will indicate whether the distribution of grains is statistically either clustered or dispersed. Our data show that quartz and plagioclase are both significantly dispersed outside of the shear zone, yet abruptly change to a “less-dispersed” pattern inside the shear zone.

The significance of this spatial distribution still needs to be determined. This study will help to establish a quantitative method for thin section fabric analysis. Moreover, insight from this information will allow us to find a potential link between textural and chemical analyses in deformed rocks.

REFERENCES

Bialek, D. 1999. Chemical changes associated with deformation of granites under greenschist facies conditions: the example of the Zawidów Granodiorite (SE Lusatian Granodiorite Complex, Poland). Tectonophysics 303, 251-261.

Li, Y., Onasch, C. M. and Guo, Y. 2008. GIS-based detection of grain boundaries. Journal of Structural Geology 30, 431-443.

Newman, J. and Mitra, G. 1993. Lateral variations in mylonite zone thickness as influences by fluid-rock interactions, Linville Falls fault, North Carolina. Journal of Structural Geology 15, 849-863.

Ramsay, J. G. and Graham, R. H. 1970. Strain variation in shear belts. Canadian Journal of Earth Sciences 7, 786-813.

Sims, P. K., Klasner, J. S. and Peterman, Z. E. 1990. The Mountain Shear Zone, Northeastern Wisconsin –A Discrete Ductile Deformation Zone Within the Early Proterozoic Penokean Orogen. Geologic Survey Bulletin, Precambrian Geology of Lake Superior Region, A1-A15.

Wawrzyniec, T., Selverstone, J. and Axen, G. J. 1999. Correlations between fluid composition and deep-seated structural style in the footwall of the Simplon low-angle normal fault, Switzerland. Geology 27, 715- 718.

Yonkee, W. A., Parry, W. T. and Bruhn, R. L. 2003. Relations between progressive deformation and fluid-rock interaction during shear-zone growth in a basement-cored thrust sheet, Sevier orogenic belt, Utah. American Journal of Science 303, 1-59.





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