Characterization of magnetic fabrics in Jurassic andesite dikes cutting the Mt Tallac roof pendant; Eldorado County, California

Recent fabric analysis of dike samples from the Mt Tallac roof pendant in the Sierra Nevada suggests that magnetite, hematite, and goethite are carriers of the magnetic fabric in the rock. These results are significant because the characterization of magnetic materials within sampled dikes is a key step in beginning an analysis of paleomagnetic fabrics that will help resolve the timing of events in the Sierra. The research questions that drove the analysis are: Have the magnetic fabrics in these rocks been thermally reset by the heat of intrusion from the adjacent pluton, and will the magnetic fabrics fit with the Jurassic polar wander curve for North America? It is possible that the pluton’s heat of intrusion recrystallized the magnetic fabrics in the dike. This would imply that the fabrics reflect the polar wander curve at the time of the pluton’s emplacement, but no longer the Jurassic polar wander curve. To begin the characterizations, we analyzed the bulk magnetic properties in seven samples collected from the youngest dike in the grass lake study area at the University of Minnesota’s Institute for Rock Magnetism. Our analyses included the use of a Vibrating Sample Magnetometer (VSM) to characterize magnetic remanence, which is best described as the memory of applied magnetic fields in a sample once they are removed. Electromagnetic fields are used to bring a material’s magnetization to its peak, then the VSM reduces this field to zero and the remanent magnetism can be plotted on a hysteresis loop, which compares variation of magnetization against magnetic field. The results of our work will provide a basis for determining when the magnetic minerals in these dikes were formed and/or thermally reset, which would provide a new perspective on the timing of major events in the evolution of the Sierra Nevada during the Jurassic