Slickrock Mountain consists of two major sills intruded into Upper Cretaceous calcareous sandstone. The sills are an older Eocene quartz syenite approximately 200 m thick and a younger Oligocene alkali olivine gabbro approximately 100 m thick. Smaller mafic dikes and sills are abundant in the area. The quartz syenite contains fayalitic olivine and zoned alkali feldspar with plagioclase cores set in an intergranular ground-mass of anorthoclase, sanidine, quartz, clinopyroxene, magnetite, and apatite. The smaller mafic intrusions commonly contain kaersutite megacrysts and an assemblage of crustal xenoliths that includes coarse-grained gabbro and partially fused feldspathic arenite. Major and trace element data do not support the derivation of the felsic compositions from the mafic ones by simple crystal fractionation at low pressure. The presence of kaersutite megacrysts and partially fused crustal xenoliths in nearby sills and dikes suggests that derivation of the Slickrock quartz syenite from some combination of fractionation and crustal assimilation at elevated p H 2 O is possible. However, trace element concentrations indicate that the syenitic and gabbroic rocks are petrogenetically unrelated.