MICROSTRUCTURAL AND PETROGRAPHIC STUDY OF ROCKS WITH CONTRASTING RHEOLOGIC PROPERTIES DURING HIGH-TEMPERATURE DEFORMATION, CUSTER COUNTY, SOUTH DAKOTA

Metamorphosed sedimentary rocks located in the southeastern Black Hills near Grace Coolidge Creek have been folded and metamorphosed multiple times in different orogenic events. The most recent of these Paleoproterozoic events occurred during emplacement of the Harney Peak granite (1.715 Ma). This event is associated with high-grade metamorphism as indicated by the presence of sillimanite and recrystallization. The outcrop contains a mica rich layer and quartz rich layers of schist with a pegmatite dyke crosscutting the lithologies. Two-dimensional strains estimated using the deformed dike indicate that mica rich and quartz rich layers have different amounts of strain. Initial modeling indicates that the quartz rich layers were deformed mainly by pure and simple shear. The mica rich layers were deformed at much higher strains and have much larger component of simple shear.

This study will focus on determining the differences in lithologies which influence the different strains as indicated by varying amounts pure and simple shear. This research will provide more insight into how mineral assemblages can influence crustal deformation. Initial results indicate that the strain in this outcrop is controlled by the presence and percentage of mica. Detailed descriptions and measurements of lineation and foliation have been made. Oriented samples of each lithology were collected and photographed. Petrographic microscopic analysis will consist of looking at the mineral percentage as well as texture and microstructure. A complete petrologic description will be taken, and the percentage of quartz, feldspars, and mica will be compared in each lithology.