SEM-EDX ANALYSIS OF GARNETS AND THEIR INCLUSIONS FROM THE LOWER-MOST LAYER OF THE WOODVILLE HILL LACCOLITH RHYOLITE IN NORTHERN BLACK HILLS, SOUTH DAKOTA

Occurrence of almandine garnet bearing rhyolite has been reported from the Woodville Hill laccolith, Black Hills Igneous Province by Jensen et al., (2014), Dasgupta and Hacker (2015) and Hacker and Dasgupta (2015) In this study we analyzed the chemical composition of the garnets, and inclusions within garnets, to develop a protocol for quick and low cost analysis of garnets and mineral inclusions using Scanning Electron Microscopy Energy Dispersive X-ray (SEM EDX), that can be applied to other garnet bearing rhyolites. Approximately 50 garnets were analyzed from the lowermost garnet bearing layer of the Woodville Hill Laccolith. Each garnet was analyzed at ten different point locations to determine its chemical composition and to document any possible compositional zoning. The sizes of the garnet phenocrysts range from 0.15-2.5 mm. While some of the garnets were euhedral crystals with fractures running through them, others were highly fractured. Our analysis identified and confirmed all garnets as almandine in composition. The inclusions identified were anorthoclase, tourmaline, prehnite, and jadite. The lack of compositional zoning confirms the igneous origin of most of these garnets (Green, 1977). Moreover, the nature of most inclusions also suggests an igneous origin of the garnets. These results are in line with the conclusion of Dasgupta and Hacker (2015). The presence of prehnite as inclusions in a few of the garnets however suggests that a minor fraction of these garnets may be xenocrystic in nature. We are confident that our analysis protocol can be successfully used to study more garnet bearing rhyolites from the area. The presence of almandine garnets within a rhyolitic body is a rare occurrence. Previous authors such as Harangi et al., (2001), Patranabis et al., (2008) have attributed the presence of primary igneous almandine garnets in rhyolites to unusually rapid ascent of magma from deep crustal depth of >20 km.