Magma propagation and emplacement within the central intrusive complex of Summer Coon stratovolcano, Colorado

Abstract

A field-based study used geologic mapping and a ground-based magnetic survey to investigate the exhumed central intrusive complex (CIC) of Summer Coon stratovolcano in Colorado. The CIC comprises a group of diorite to granodiorite stocks and sub-vertical domains of conduit-filling breccia hosted within basaltic-andesite breccia associated with cone building. The stocks serve as the focal points for 53 basaltic-andesite (mafic) and 36 andesite to rhyolite (evolved) radial dikes mapped within the CIC. The evolved dikes with outcrop lengths (strike length) of <650 m are confined to the central areas of the CIC, cut through the stocks, and have steeply plunging terminal segments. In contrast, most evolved dikes with outcrop lengths >1600 m are excluded from the central areas of the CIC, do not cut through the stocks, and have terminal segments that plunge shallowly toward their focus. Assuming the dikes are blade-shaped and that they originated directly below the CIC, the propagation direction of the evolved dikes was estimated using the plunge angles, spatial distribution, and dike outcrop lengths. The relatively long dikes may have ascended toward the level of exposure along inclined paths. These dikes remained exclusively within the basaltic-andesite breccia and, probably due to the higher relative stiffness of the stocks, were unable to propagate through the stocks as they ascended. Upon approaching the level of exposure, the same dikes encountered a stress barrier likely generated by the gravitational load of the edifice. This barrier altered the dike propagation paths from inclined to sub-horizontal, significantly increasing their outcrop lengths. In contrast, the dikes with short outcrop lengths ascended along primarily sub-vertical paths, intersecting the level of exposure within the central portions of the CIC. To propagate sub-vertically through both the relatively stiff stocks and a potential stress barrier, the magma overpressures within the shorter dikes may have been higher relative to the longer sub-horizontally propagating dikes. It is probable that at active volcanoes, only dikes with sufficiently high overpressures can ascend through the central intrusive complex of mature stratovolcanoes to feed eruptions near the summit. Perhaps more frequently, stress barriers and existing intrusions stall or deflect dikes with lower relative overpressures toward the slopes of the volcano where they may feed flank eruptions.