GEOLOGY OF THE CUSTER PEAK ALMANDINE GARNET-BEARING RHYOLITE INTRUSION, NORTHERN BLACK HILLS IGNEOUS PROVINCE, SOUTH DAKOTA

Located in the Northern Black Hills Igneous Province, Custer Peak is a Paleogene igneous intrusion emplaced during the Laramide orogeny. This study examined Custer Peak through geologic mapping, field observations, geochemical and petrological analyses. Garnet-bearing rhyolite magma intruded into flat-lying shale units in the middle of the Cambrian Deadwood Formation. The rhyolite (~183 m thick) consists of two well defined and visible layers based on different garnet phenocryst types: 1) an upper white-cream zone ≤60 m thick, containing Form A garnets of anhedral garnet-quartz clusters, mainly almandine composition (Al_61.4-65.2, Sp_30.7-34.5, Gr_2.6-3.0, Py_0.9-2.3), 2) a lower cream-gray zone ~122m thick, containing Form B garnets, euhedral-subhedral in shape, mainly almandine composition (Al_60.4-61.4, Sp_15.7-16.1, Gr_18.6-19.3, Py_3.0-3.1). Form B garnets can also be found in the upper zone but are usually surrounded by Form A clusters. Form A and B garnets average 1-2 mm in diameter (some ~5 mm) and can be intergrown with plagioclase or biotite. Both forms of garnet have no compositional zoning and are considered to be primary igneous phenocrysts, not metamorphic xenocrysts. The rhyolite groundmass in both zones is high-Si, calc-alkaline, and peraluminous.

Almandine garnets of high almandine-low spessartine composition form under high pressure, while higher spessartine concentrations indicate crystallization at lower pressure, hence shallower depths (e.g., Green, 1977). We infer that these garnets crystallized in a zoned magma chamber at depths around lower crustal levels (~20 km) in a magma that ascended to depths near the surface (~1 to 3 km) very rapidly through extensional dike systems and injected as sills in the Middle Deadwood formation. The first sill contains Form A garnets; the second sill emplaced below the first and contains Form B garnets. This sequence follows the emptying of the zoned source magma chamber as determined by the garnet compositions. Boundaries between layers are highly gradational and the presence of small miarolitic cavities indicate a shallow emplacement depth. On the south side, the intrusion punched upward through sedimentary units into the Pahasapa Formation, thus creating an overall trapdoor geometry with columnar joints found near wall rock exposures.