EXPLORING CORRELATIONS BETWEEN TUMALO VOLCANIC CENTER DEPOSITS WITH QUARTZ AND PLAGIOCLASE CATHODOLUMINESCENCE ZONING PATTERNS

The Tumalo Volcanic Center (TVC) is located in Central Oregon between Bend and Three Sisters. The TVC has erupted a range of silicic compositions, including rhyolites, and some of the few explosive eruptions in the central Oregon Cascades, posing a potential danger to the surrounding area. The TVC has produced three large pyroclastic deposits: 1) Desert Spring Tuff (DST; ~650 ka), dacitic, welded, pink and tan tuff; 2) Tumalo Tuff (TT)/Bend Pumice (BP; ~300-440 ka), rhyolitic pyroclastic flow airfall deposits and 3) Shevlin Park Tuff (SPT; ≤260 ka), a mixed andesitic tuff. Source vent locations and magmatic origins are not well constrained. Petrologic and fieldwork has suggested that DST was from Bearwallow Butte (BB), TT/BP from Three Creek Butte/Melvin Butte (TCM), and SPT from Triangle Hill (TH).

Using a JEOL JSM-IT500HR scanning electron microscope and Deben Centaurus cathodoluminescence (CL) detector, we imaged thin sections from each tuff and their corresponding potential sources to determine if there is shared history recorded in quartz and plagioclase feldspar CL zoning. The majority of the quartz and feldspar crystals in DST have subtle zoning typically towards the rim with a light core. BB crystals show zoning but with darker cores and lighter rims. The TT and BP crystals have little CL texture, however, crystals from TCM show more textural complexity, with some zoning present. SPT and TH crystals have subtle zoning along the rims of the crystals and distinct lighter cores with darker rims.

Based on these data, there is no clear correlation between units. Many crystals lack little if any zoning. Rare, oscillatory zoned quartz are found in BB and BP, but the other zoning is very simple in its nature. In order to be more certain of the correlations between DST, TT/BP, and SPT and their potential sources, elemental mapping of the crystals will be utilized to look for zoning as well as to give us a better understanding of their potentially shared histories.