PETROLOGY OF A TERTIARY DIKE-SILL INTRUSION INTO THE DEADWOOD FORMATION OF THE NORTHERN BLACK HILLS, SOUTH DAKOTA
Petrographic examination of the Deadwood sandstone/mudstone, phonolite dike and the sill samples was done to characterize the petrology of the units. The metasedimentary rocks are fine- to medium-grained sandstones and siltstones containing quartz, calcite, plagioclase and muscovite. Texturally the samples show evidence of recrystalization along with metamorphic mineral growth of muscovite and plagioclase. The five dike samples are medium- to coarse-grained porphyritic phonolite containing the minerals; plagioclase, nepheline, aegirine-augite, titanite, apatite and sodalite. The coarse grained phenocrysts are typically plagioclase and nepheline. The two sill samples are highly altered and contain sericite and carbonate and are interpreted to be pseudomorphs of plagioclase and nepheline phenocrysts of similar size and shape to those contained in the porpyritic phonolite samples.