38-2 The Geological History and Characteristics of Rustler Peak, a Unique Hornblende Andesite Stratovolcano in the Southern Oregon Cascades

Session: Petrology and Geochemistry (Posters)

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Abstract:

Rustler Peak is a major summit among of a broad field of volcanic edifices north of Mt. McLoughlin, in the Southern Oregon Cascades.  At-large, this shield volcano is dominated by basaltic-andesite, but with a central summit of porphyritic hornblende andesite.  Topographically, the structure of the Rustler Peak complex appears to be multi-lobed – a central peak surrounded by loci for five potentially-parasitic features.  Two of these lobes are compositionally and texturally indistinguishable from the bulk of the central cone, while the remaining three each seem to define a source for other downslope basaltic-andesite flows.  The peak’s summit exposes an area of light-grey/white hornblende andesite dated to 4.43 ± 0.15 Ma (Mertzman, 2000), placing it well within the classically-defined period of High Cascades magmatism.  Despite the frequent presence of such magmas in other portions of the Cascades, Rustler Peak is a regional rarity, with hornblende andesite being nearly unheard of in the entire range between Mt. Shasta, 136 km to the south, and Mt. Mazama, 41 km to the north.  In response to prior imprecise mapping as merely “Tertiary volcanics,” field mapping, bulk geochemical analyses, and limited petrographic and mineral composition studies have now been completed, allowing the delineation of individual units comprising the Rustler Peak complex and its surroundings.  Future plans will be focused on determining the magma chamber dynamics responsible for the development of this regionally-unique edifice.

Geological Society of America Abstracts with Programs. Vol. 58, No. 1, 2026

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