ALMANDINE GARNET BEARING RHYOLITE OF THE WOODVILLE HILL LACCOLITH, NORTHERN BLACK HILLS IGNEOUS PROVINCE, SOUTH DAKOTA: RAPID ASCENT OF MAGMA FROM LOWER CRUSTAL LEVEL

The Northern Black Hills Igneous Province formed from 58 to 50 million years ago as dike-fed sills intruded into Paleozoic and Mesozoic sedimentary host rocks. These sills accumulated into laccoliths up to about 600m thick at depths of 1 to 3km (Redden and DeWitt, 2008). Woodville Hill Laccolith is one such shallow level intrusive body that consists of three major layers, with the bottom most layer containing phenocrysts of garnets. The middle layer contains phenocrysts of biotite, while the topmost layer is phenocryst free. Initial investigation reveals that the garnets are primarily almandine (Al_67.6-72.6Sp_8.9-26Gr_2-17.4Py_2.2-6.3). Texturally, some of them are euhedral grains, although all of them show extensive fracturing. The more common metamorphic garnets are mostly zoned with respect to Fe–Mg–Mn and contain mineral inclusions. Electron microprobe analysis of garnets from the rhyolite samples demonstrates the lack of any systematic compositional variability within the grains and confirms the absence of any significant inclusions. Based on these observations we conclude that these garnets were formed by crystallization from magma and hence they are not metamorphic xenocrysts. Moreover, we observe grain to grain variation of MnO concentration (4 - 11.8%) and molar Spessartine proportions (8.9 - 26%). Almandine garnets of high almandine-low Spessartine composition can form only under high pressure, while higher Spessartine concentration indicates crystallization at lower pressure, hence relatively shallower depths (e.g., Green, 1977). We infer that these garnets crystallized over a range of depths around lower crustal levels (~20 km) in a magma that ascended to depths almost near the surface (~1 to 3 km), very rapidly. Such fast magma ascent rate can also explain the fractures that we see in the garnets, which are most likely caused by rapid decompression. Further investigation in terms of whole rock major and trace element geochemistry as well as whole rock and mineral isotope geochemistry is going to be critical towards understanding the nature and the depth of the magma source and magma ascent mechanism. Such information can shed tremendous light on the tectonothermal history of the northern Black Hills region during the Laramide episode.