THE PROLIFIC ~300K LONG ERUPTIVE HISTORY OF THE MAHOGANY MOUNTAIN – THREE FINGERS RHYOLITE FIELD, EASTERN OREGON

The Mahogany Mountain—Three Fingers rhyolite field (MM–TFrf), covering ~900 km², is one of largest mid Miocene rhyolite centers associated with Columbia River flood basalt volcanism. Early studies advocated for a two-caldera model consisting of the Mahogany Mountain and the younger Three Finger caldera producing the tuff of Leslie Gulch (TLG) and the tuff of Spring Creek, respectively, along with pre- and post-caldera effusive rhyolites. Recently, Benson & Mahood (JVGR, 309: 96-117, 2016) suggested only one larger caldera with pre- to post-caldera rhyolites after finding that the tuff of Spring Creek in Leslie Gulch is part of the TLG. With data of our recent studies and results by Marcy (2013; https://pdxscholar.library.pdx.edu/open_access_etds/1543/), we can further address the stratigraphic and geochemical eruptive history of this rhyolite field.

Abundant and compositionally variable effusive rhyolites all but one post-date the tuff of Leslie Gulch, including the prominent rhyolites of Mahogany Mountain (15.82±0.05–15.71±0.05 Ma) in the south and of McIntyre Ridge along the NW margin of the MM–TFrf. Along Succor Creek, we correlate mostly non-welded tuffs, that underlie pronounced rhyolite cliffs of McIntyre Ridge (15.76±0.02) and of Devils Gate (e.g., 15.95±0.03), with the TLG; tuffs consist of fine grained ignimbrites, surges, and fallout deposit. The only rhyolite underlying the TLG is found in the Leslie Gulch locality, yielding an age of 16.02±0.01 Ma. Stratigraphic data reveal that the TLG is a complex, multi-phase deposit with eruptive breaks in between. Ignimbrites distinct from the TLG range in age from 15.91±0.01 to 15.74±0.09 Ma and indicate pyroclastic activity elsewhere in the MM-TTrf from an area immediately north of the original Mahogany Mountain caldera to Succor Creek in the NE.

The MM-TFrf represents a prolific rhyolite center that was active for 300k starting effusively at 16.02 Ma but quickly continuing with a major phreatomagmatic phase to deposit the composite tuff of Leslie Gulch, in turn followed by widespread post-caldera effusive rhyolites that were punctuated by other pyroclastic event (with or without caldera formation) ending at ~15.7 Ma. Rhyolite eruptions recommenced in the SW of the field with the 14.94±0.2 Ma Birch Creek rhyolite, and 14.42±0.02 Ma McCain Creek rhyolite.