TEXTURAL INVESTIGATION INTO RAPID WELDING TRANSITIONS IN THE TUFF OF LESLIE GULCH ALONG SUCCOR CREEK AT THE MAHOGANY MOUNTAIN-THREE FINGERS RHYOLITE FIELD, SOUTHEASTERN OREGON

The Mahogany Mountain-Three Fingers Rhyolite Field (MM-TFrf), located in southeastern Oregon denotes a significant mid-Miocene silicic volcanic system associated with the Columbia River Basalt Province. This research investigates the tuff of Leslie Gulch (LGT), focusing on the rapid transitions between non-welded and welded pyroclastic deposits along Succor Creek. Prior to our research, it was not recognized that the LGT crops out along Succor Creek but new age and compositional arguments allow for correlating the main LGT deposits in the Leslie Gulch area with those along Succor Creek, located ~25 km NNE of the main deposits.

Detailed petrographic and quantitative image analysis techniques are used to examine the depositional history of the LGT along Succor Creek. We characterize changes in the proportionality of pyroclastic particles such as shards, pumices, and atypical glass blobs that are observed in the deposit. We focused on select samples collected along various stratigraphic sections that capture rapid welding transitions vertically as well as laterally. The observed significant heterogeneities in the welding intensity, suggest variable thermal conditions at deposition. Our analysis indicates welded samples exhibit a significant higher percentage of glass blobs, that generally range in size from ~100 to 1000 microns, than non-welded samples, that primarily consist of glass shards and small pumices. Percentages of glass blobs in welded samples range from 60 to 90 vol% while non-welded samples have close to none, 5 to 0 vol%. This correlation suggests that glass blobs are not glassy accidental lithic fragments but instead are juvenile clasts that were poorly to non-vesiculated. This in turn suggests that tuff rich in glass blobs at deposit was slightly hotter at emplacement. As the LGT shows overall strong evidence for being mostly phreato-magmatic in origin, these depositional variations could be related to entrainment of less ambient air and/or less magmatic fragmentation. The here documented depositional characteristics of the LGT are unknown in the main outcrop area in Leslie Gulch as deuteric alteration strongly obscured textural features. Hence, results from this study contribute to a refined understanding of the post depositional processes and eruption dynamics within the MM-TFrf.