GARNET-BEARING RHYOLITES OF THE WOODVILLE LACCOLITH, NORTHERN BLACK HILLS IGNEOUS PROVINCE, SOUTH DAKOTA

Field study of Tertiary rhyolites from the Woodville Laccolith in the Northern Black Hills Igneous Province reveals the presence of three layers within the laccolith: a) lower garnet-bearing, b) middle biotite-bearing, and c) upper rhyolite. Petrographic study of polished thin sections shows that all layers contain phenocrysts; these were identified as plagioclase, titanite, biotite, garnet, and quartz. Garnet in the lowermost layer occurs in two main forms: a) phenocrysts, and b) phenocryst-sized aggregates of intermingled garnet and quartz. Biotite phenocrysts are subhedral and show occasional intergrowth with both garnet and plagioclase. Except biotite, all phenocrysts are highly fractured. Aphanitic groundmasses of all samples consist of quartz, plagioclase, and apatite, with varying amounts of biotite and hematite. The presence of zoning in garnet phenocrysts and the presence of apatite both as inclusions in garnet phenocrysts and as part of the groundmass indicates an igneous, rather than metamorphic, origin for these garnets. Similar presence of garnets in rhyolitic volcanics has been interpreted by authors such as Patranabis et al. (2009) and Harangi et al. (2001) as evidence of generation of rhyolitic magma at lower crustal depth and its rapid ascent to the surface. In addition, the boundaries between different layers within the laccolith are graded rather than sharp, indicating that successive layers were emplaced before the previous layer had solidified. Major and trace-element analyses of bulk rock samples and phenocrysts will provide a more detailed picture of the magmatic history of these rhyolites including the magma source or sources, pressure/temperature conditions, timing of magma ascent, and method of emplacement. Such future study can potentially shed light on the Tertiary magmatic history of the Northern Black Hills Igneous Province.