THE VOLCANIC HISTORY AND MAGMATIC EVOLUTION AT RUSTLER PEAK, OR - A COMPARISON BETWEEN CENTRAL AND PARASITIC CONES

Rustler Peak, in Jackson County, Oregon, is an intermediate-volume stratovolcano in the High Cascades, located west of Mt. McLoughlin. From its central peak, five lobes extend laterally, somewhat evenly spaced radially. The edifice's peak is composed of an evolved, pale-grey hornblende andesite (~63 wt% SiO₂), erupted at 4.43 +/- 0.15 Ma. In contrast, the bulk of the central edifice, as well as four of the five lobes, is composed of darker-grey basaltic andesite (~57 wt% SiO₂). In one lobe, this is expressed as a radial dike, and in another, this unit is exposed in a cinder pit. The eastern-most of the five lobes is composed of a still less-evolved basaltic andesite (~53 wt% SiO₂). Observed trends in the geochemistry and mineralogy of samples from Rustler Peak and its surrounding lobes indicate a progressive evolution of the magma chamber, and a complex network of vents, dikes, and fissures by which the magma reached the surface. GIS-aided analyses of the structures composing Rustler Peak have assessed the volumes erupted for each flow type. From these volumes, applied to models using sample geochemistries, approximate eruption rates and magma evolution sequences have been proposed.