TEXTURAL AND GEOCHEMICAL ANALYSIS OF PLAGIOCLASE FROM TODD LAKE VOLCANO AND TRIANGLE HILL DOME COMPLEX OF CENTRAL OREGON

Todd Lake (TL) and Triangle Hill (TH), a dacite volcano (~0.46 Ma in age) and andesite cone complex (~0.63 Ma in age) respectively, are a part of the Tumalo Volcanic Center in Central Oregon. For this study, a geochemical and textural analysis of the plagioclase phenocrysts from TL and TH volcanoes was done to gain insight into the open and closed system processes that occurred in their magmatic systems. Four samples from TL and three samples from TH were analyzed using Back-Scatter Electron (BSE) imaging and electron microprobe analysis at Oregon State University. BSE and petrographic results show that there are four dominant plagioclase textures, including sieving, oscillatory zoning, zoning + sieving, and glomerocrysts, with simple zoning being relatively rare. Crystals with sieved texture are more abundant in the TH samples, whereas oscillatory zoning is the dominant texture in the TL samples. However, both volcanoes have prominent resorption features (e.g., rounding) that are repeated throughout crystal interiors. Plagioclase compositions are variable, with TH anorthite contents ranging from An₁₈–An₈₆ and TL ranging from An₂₃–An₇₃. Both volcanoes have plagioclase that exhibit both normal and reverse zoning, but TH samples contain more abundant reversely zoned plagioclase. An vs. FeO and MgO plots show that the majority of samples from both volcanoes depict positive trends, where FeO and MgO increase with increasing An. One sample from each volcano displays horizontal FeO trends (constant FeO with decreasing An) but positive trending MgO. Literature suggests that the positive geochemical trends, in addition to the presence of abundant reversely zoned plagioclase, indicate that both volcanoes may have experienced magma recharge (i.e., compositional mixing; Ruprecht and Wörner, 2007). The differing FeO and MgO trends for some of the samples could indicate that the recharge magma had similar FeO to the host magma, or that oxygen fugacity/ a change in water content affected the plagioclase FeO. The textural and geochemical results for this study imply that most plagioclase from TH and TL experienced multiple periods of disequilibrium, which indicates that both magmatic systems had complicated histories that likely involved repeated mafic recharge events.