DETAILED GEOCHEMISTRY THROUGH A STRAIN GRADIENT IN SMALL SCALE GRANITIC DUCTILE SHEAR ZONES

Deformation in shear zones is often influenced by fluid interaction that may cause changes in chemistry and volume. One way to quantify volume loss and document fluid-rock interactions is to measure elemental depletion or enrichment inside the shear zone compared to the undeformed rock. This technique is usually conducted by sampling deformed and undeformed samples over meter to kilometer scales and plotting elements on isocon plots. This strategy is not ideal because elemental concentrations may vary independently of the shear zone or in a more complex spatial fashion. In order to distinguish more complex chemical changes across a shear zone, we sampled centimeter-scale granitic ductile shear zones at fixed increments to establish a detailed chemical gradient within a spatial construct. These centimeter-scale shear zones formed within the Penokean Mountain Shear Zone (MSZ) in northeastern Wisconsin, a structure that is about 2 km wide along its approximately 10 km of exposed strike. The MSZ deformed a sequence of volcanic and associated granitoid rocks by oblique dextral shear. The associated small-scale granitic shear zones display a wide range of deformation styles from brittle to ductile and thicknesses between a few millimeters to several centimeters. In-situ shear zones are dominantly dextral. For X-ray Fluorescence analysis of major elemental chemistry, we sampled a ductile shear zone in two centimeter increments across the strain gradient, totaling 12 centimeters. To preserve detailed spatial relationships, we plotted individual elemental chemistry with respect to distance from the shear zone. Preliminary results show enrichments in Fe₂O_3, 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.” Plotting individual elements vs. distance from the shear zone has yielded higher resolution geochemical data in which elements are better represented throughout the strain gradient. This method provides more information than the isocon method, which lacks spatial relevance and may misrepresent elements with small changes in concentration.