Slickrock Mountain: Bimodal intrusive complex Big Bend National Park, Texas
Hill, Lena Elizabeth
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Slickrock Mountain consists of two major sills that were intruded into late Cretaceous calcareous sandstone in Eocene time. The sills consist of an older quartz syenite, approximately 180m thick and a younger alkali olivine gabbro, approximately 90m thick. Smaller mafic dikes and sills are abundant in the area. The quartz syenite contains olivine and plagloclase phenocrysts set in an Intergranular groundmass of anorthoclase sanidine, quartz, clinopyroxene, magnetite, and apatite. The smaller mafic intrusions commonly contain kaersutlte megacrysts and a variety of crustal xenoliths that Includes coarsegrained gabbro and partially-fused feldspathic arenite. Major and trace element data do not support the derivation of the felsic rocks from 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 a combination of high-pressure fractionation and crustal assimilation is possible.