PETROGRAPHY, GEOCHEMISTRY, AND GEOCHRONOLOGY OF THE GREEN MOUNTAIN SHIELD VOLCANO, OREGON

Green Mountain (GM) is a small (~25 km³) shield volcano located central Oregon. The ~740, 000 shield, and associated young (~13,000 ya; Mackey et al., 2014) lava fields, are approximately ~50 km southeast of Newberry Caldera on the northwest side of an age progressive trend of bi-model volcanism from southeastern Oregon to Newberry (Jordan, 2004). In a previous study (Friedrichs and Castonguay, 2014), XRF results were reported and interpreted as evidence of a single magma chamber that had undergone fractional crystallization and a significant magma recharge event before the most recent volcanism. This study aims to expand on this existing knowledge by applying geochronology, detailed petrographic analysis, and additional XRF analysis.

Surrounding and underlying the GM shield are several rhyolitic to dacitic domes, one dated at ~4 Ma (Ford, 2011) and thought to be associated with Newberry volcanism. Also below the GM basalt flows are remnants of basaltic tuff cones with central lava lakes that erupted through the Pleistocene lake that occupied Fort Rock basin, suggesting that GM initiated as geographically distinct vents on the northern edge of the lake. The additional XRF data continue to support the previous hypothesis that the tholeiitic basalts have a short residence time in a crustal magma chamber before eruption. Detailed petrographic analysis also support this hypothesis. Basalts from several intervals show glomerophytic textures, while others have abundant zoned olivine (Fo₈₀ cores to Fo₄₀ rims); both suggesting at least some crystallization within the magma chamber.