RAPID ASCENT OF RHYOLITIC MAGMAS WITHIN THE BLACK HILLS IGNEOUS PROVINCE, SOUTH DAKOTA: IMPLICATIONS FOR EXTENSION WITHIN A LARAMIDE TRANSVERSE STRUCTURAL ZONE
Rhyolites showing evidence of rapid silicic magma ascent can be divided into two families based on phenocryst petrography: A) quartz-bearing rhyolite (QR) and B) almandine garnet-bearing rhyolite (AR). Quartz phenocrysts within QR intrusions form euhedral to subhedral “quartz eyes” up to 4 mm in diameter, some with partial resorption rims, and are all highly fractured. The AR phenocrysts are almandine garnets in composition and formed by crystallization from magma (Dasgupta and Hacker, 2015; Jensen et al., 2014). Texturally, the phenocrysts are euhedral to subhedral and show extensive fracturing. Fracturing within both the quartz and garnet phenocrysts produced greater than ten panels per grain which is indicative of fast decompression fracturing. It's known that primary igneous garnets are rare in hypabyssal or volcanic igneous rocks and can only develop under restricted conditions. High almandine-low spessartine garnet in volcanic rocks is a remnant phase of high pressure crystallization from magma at pressures of >7 kbar (>25 km). We infer that the QR and AR in the BHIP crystallized at lower crustal depths in magmas that then ascended very rapidly to shallow depths, and may have erupted explosively. A tensional stress field within the Laramide Black Hills transverse zone may have enhanced their fast ascent from lower-crustal depths, allowing preservation of early-formed phenocrysts.