ALMANDINE GARNET BEARING RHYOLITES FROM WOODVILLE HILL LACCOLITH, SD: EVIDENCES OF RAPID MAGMA INJECTION AND MIXING IN A LACCOLITH

Volcanism in the Northern Black Hills Igneous Province took place between 50ma and 58Ma. The volcanic rocks formed include varieties of rhyolites. Almandine garnet bearing rhyolites have been reported by Dasgupta and Hacker (2015) from the Woodville Hill laccolith. The Woodville Hill laccolith has multiple layers with gradational boundary in between them. The lowermost layer contains phenocrysts of garnets. The next layer on the top contains both garnet and biotite phenocrysts, while the layer immediately above it contains only biotite phenocrysts. In this study we used scanning electron microscope energy dispersive spectrometry (SEM-EDS) analysis on the garnet and its mineral inclusions from the garnet-biotite rhyolite layer to compare it with compositions of the garnets from the garnet only lower layer to test our hypothesis that the middle layer represents a mixing zone. In addition we also analyzed the biotite for their compositions. The garnets are mostly subhedral and also had fractures and their size ranges from 0.15 mm to 2.5 mm. We compared the spectra generated during our analysis to previously recorded spectra from McGill University. This comparison confirmed that these garnets were almandine and identical to those found in the lowermost garnet only rhyolite layer. This implies that the narrow zone of rhyolite containing both biotite and garnet represents a mixing zone between the bottom most garnet only rhyolite layer and the upper biotite only layer, and hence during the formation of the laccolith, different layers of rhyolitic magma got injected with very little time interval, thus forming mixing zone in between successive layers rather than a sharp contact. These different layers could have originated from different depths of the same magma chamber as suggested by the differences in the phenocrysts. The findings of this study is in line with the findings of Dasgupta and Hacker (2015).