9406243 McCallum This research will provide a deeper understanding of the processes that operate in the formation of layered mafic intrusions, especially the Stillwater complex. The proposed research includes three subprograms: (1) petrogenesis of the platinum group element (PGE) ore zone, (2) origin of macro rhythmic layering in gabbronorites, and (3) effects of influxes of magmas into an evolving magma chamber. The study of the ore zone will utilize electron and ion probe analyses and Pb isotopic studies to elucidate the relationship between the host lithologies and the stratigraphic sequence. The second project will combine a modeling approach with descriptive textural and mineralogical data obtained by microprobe and image analysis methods to test the predictive capacity of quantitative models of layer formation. The third project will utilize mineral compositions determined by electron and ion probe and Nd isotopic ratios of pyroxenes to determine whether specific contact zones are the result of magmatic influxes rather than fractional crystallization. Study of the Stillwater Complex is significant because it is the repository of the largest reserves of (PGE) and chromium in the western hemisphere.