GEOLOGY OF A PALEOGENE ALMANDINE GARNET-BEARING RHYOLITE INTRUSION, NORTHERN BLACK HILLS IGNEOUS PROVINCE, SOUTH DAKOTA

The Northern Black Hills Igneous Province consists of a series of 58-50Ma intrusive centers that trends W-NW from Bear Butte in South Dakota to Devils Tower and the Missouri Buttes in Wyoming. The intrusive bodies include laccoliths, sills, and dikes, as well as breccia pipes and diatremes. Hence, this area provides an excellent opportunity to study mechanisms of shallow level magma intrusions and associated volcanism. This study investigated a small (0.04 km²) unnamed intrusion located approximately 10 km southeast of Custer Peak. Eleven rock samples were examined in thin section for petrography and microprobe mineral chemistry, and whole rock chemistry using ED-XRF. Petrography and geochemistry classify the rocks as rhyolite (70-74% SiO₂) containing up to 2% phenocrysts of plagioclase, biotite, and garnet in an aphanitic quartzo-feldspathic groundmass. Texturally, garnet occurs as individual euhedral to subhedral crystals 1-2 mm in diameter, as well as intergrown with plagioclase or biotite. Electron microprobe analysis of garnets revealed their composition to be primarily almandine (Al_67.37-68.11, Sp_9.15-9.34, Gr_16.21-17, Py_5.08-5.25), show no zoning or systematic compositional variability, and lack inclusions. From these observations, the garnets are considered to be primary igneous phenocrysts and not metamorphic xenocrysts. Metamorphic garnets are usually zoned with respect to iron, magnesium, and manganese and have inclusions. The garnet-bearing rhyolite intruded into Early Proterozoic metagraywackes and forms a pipe shaped feature with vertical flow banding throughout, indicative of vertical magma emplacement. We interpret the intrusion to be the eroded remnant of a magma conduit that once fed a laccolith or vented to the surface. Almandine garnets with high almandine-low spessartine composition form under high pressures and temperatures, while higher spessartine concentration indicates crystallization at relatively lower pressures, hence relatively shallower depths (e.g., Green, 1977). We infer that these garnets crystallized at depths around lower crustal levels (~20 km) in a magma that ascended very rapidly to depths almost near the surface (~1 to 3 km), or to the surface.