QUANTIFYING THE RELATIONSHIPS BETWEEN GEOCHEMICAL AND MICROTEXTURAL CHANGES ACROSS SMALL-SCALE GRANITIC SHEAR ZONES

Evidence of progressive chemical changes within deformed rocks can often be used to discern fluid-rock interactions during deformation. We used several granitic float samples of centimeter scale shear zones from the Mountain Shear Zone (MSZ), Wisconsin to assess the potential for fluid rock interactions during deformation using geochemical and petrographical methods. The rocks deformed under greenschist facies conditions without appreciable volume loss, but contain evidence for fluid-rock interactions, including sericitization of feldspar and formation of biotite from amphibole.

Detailed fabric analyses across the strain gradient at the centimeter scale were conducted, complemented by petrographic analysis using ArcGIS; a Geographical Information System (GIS). This strategy allowed us to quantify photomicrographs for Grain Size Distribution (GSD), Shape Preferred Orientation (SPO), and Average Nearest Neighbor Analysis (ANNA). X-Ray Fluorescence (XRF) was used for whole rock geochemical analyses.

Within quartz, deformation was largely accomplished by dislocation creep accommodated by dynamic recrystallization. Any evidence of grain-scale deformation in feldspar grains were not preserved due to sericitization. . Quartz and feldspar both displayed a minor amount of grain size reduction. A prominent shape preferred orientation formed in the quartz grains with increasing strain..

We used ANNA to calculate the Nearest Neighbor Index (NNI) for mineral grains. NNI plotted as a function of strain reveals quartz and feldspar grains to be dispersed (NNI >1) regardless of strain magnitude. Biotite aggregates are dispersed in low strain zones and shift to a clustered distribution (NNI .8-.9) with increasing strain. .

Isocon plots show little or no chemical differences between highly deformed and less deformed samples. Detailed geochemical traverses across zones of progressive shearing, however, indicate small, but distinguishable changes in CaO, SiO₂, Fe₂O₃, and MgO concentration in some samples. Based on these observations, we hypothesize that fluids were present throughout the history of these rocks, but did not significantly affect deformation mechanisms or metamorphism.