GEOCHEMISTRY OF THE GREEN MOUNTAIN SHIELD VOLCANO, EASTERN OREGON, USA

The Green Mountain shield volcano (~740 ka; Jordan, 2002) is located ~50 km southeast of Newberry Volcano and geologically at the intersection of the Oregon High Lava Plains province and the northernmost Basin and Range Province. Recent studies (Mackey et al., in review) have shown the synchronous relationship between the Holocene volcanisms and faulting. This study aims to understand the subsurface architecture of the shield volcano. Green Mountain is composed of three types of volcanic features of assumed temporal relations from preliminary geologic mapping. From youngest to oldest, these consist of: cinder cones near the top, the flows emanating from the central vents, and exposed tuff rings beneath the flows. If these volcanic features are the result of a shared magma chamber, we expect the chemistry to reflect crystal fractionalization within the chamber. However, if these features did not share a common source but the magma was created by the same process, the chemistry of each should not show evidence for crystal fractionalization.

To test the hypotheses, we collected 27 samples from the volcanic features that make up Green Mountain and have extracted minor and major element geochemistry using XRF. Eight samples are from the tuff rings; three of which are altered products of the phreatomagmatic eruption, but five are from the basaltic lake that formed in the center. Five samples are from the cinder comes near the top of Green Mountain, while eleven of the samples are from the flows. Also, two samples from the overlying ~12 ka East Lava Field were analyzed. Of the flow samples, three have known stratigraphic relationships and the rest are assumed from preliminary mapping. Many of the flow samples show secondary formation of bornite in the vesicles.

By coupling the subsurface architecture of this shield volcano with paleoseismic investigations of local faults, we can determine the nature of this volcano-tectonic system.