Spatial Distribution Characteristics of Minerals across Small Scale Ductile Shear Zones Using ERDAS Imagine® and ArcMap 9.2® Software

Image analyses techniques are frequently used for studying progressive changes in shapes and/or sizes of passive strain markers with deformation. However the distribution patterns of mineral phases cannot be determined using such techniques. Spatial distribution characteristics of stronger (e.g. potassium feldspar) or weaker (e.g. mica or amphibole) mineral phases may provide important information regarding deformation localization.

In this poster we describe our attempts to combine image analyses with spatial pattern analyses of different mineral phases across centimeter scale ductile shear zones. We used ERDAS Imagine® software, commonly used for analyzing remotely sensed images, in conjunction with ArcMap 9.2® software, used for geographical information systems (or GIS) analyses. Samples for this study were collected across the Mountain Shear zone, a ductile shear zone exposed near the township of Mountain, WI, which formed approximately 1.8 billion years ago. Polished slabs were cut from selected samples of sheared granitic rocks containing quartz, plagioclase, potassium feldspar, biotite, and amphiboles. Those slabs were stained with sodium cobaltinitrite and Amaranth which turned potassium feldspar grains yellow and plagioclase grains magenta respectively. The other major mineral phases, quartz (grey) and biotite/amphibole grains (black) remained unstained, thus producing four distinct color groups. Digitized images of those slabs were analyzed using ERDAS Imagine®. The accuracy level of this technique for differentiating between plagioclase and potassium feldspar grains on a hand sample scale was independently checked by means of petrographic analyses. The classified images created were then opened on a GIS platform using ArcMap 9.2® software, and spatial patterns of the color-coded mineral phases were determined.

Preliminary results show that the average size and number of potassium feldspar grains decrease with increasing deformation. Patches of fine grained plagioclase and quartz grains become progressively larger and more interconnected with distance towards the center of the shear zones.