RAPID ASCENT OF RHYOLITIC MAGMAS WITHIN THE BLACK HILLS IGNEOUS PROVINCE, SOUTH DAKOTA: IMPLICATIONS FOR EXTENSION WITHIN A LARAMIDE TRANSVERSE STRUCTURAL ZONE

The rate of magma ascent from lower-crustal magma chambers to the surface has important implications on tectonic-magmatic models. Rhyolites studied within the central portion of the Paleocene-Eocene Black Hills Igneous Province (BHIP) provide evidence of rapid silicic magma emplacement within shallow subvolcanic intrusions. The BHIP is a 100 km long northwest-trending (~N70W) belt of igneous centers formed within a 20-40 km wide left-lateral transverse structural zone. Alignment of structures and igneous intrusions within the transverse zone have north-to northwest-striking (~N20W) trends. Rhyolitic intrusive bodies within the study area (near Lead and Deadwood) formed domed laccoliths and numerous dikes and sills, along with diatremes and breccia pipes. Most were emplaced in Paleozoic clastic rocks at depths of 1 to 3 km, although some vented to the surface in the form of maar-diatremes.

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.