A GEOCHEMICAL AND PHYSICAL ANALYSIS OF THE YANA VOLCANIC CENTER, A PLIOCENE ANCESTRAL CASCADE VOLCANO

The Yana Volcanic Center (YVC) is an eroded stratovolcano that is part of the Ancestral Cascades Volcanic Arc. The YVC is located 40 km south of the Lassen Volcanic Center and is characterized by layered breccias and lava flows dated at 2.8 ± 0.5 (Clynne and Muffler, 1990; Clynne and Muffler, 2010). Erosion has exposed a complex of sheet intrusions that cut through the breccia host rock. Our early research focused on the physical volcanology of the YVC where, through extensive field work, we have mapped a total of 53 dikes across a 12 km region. These dikes formed in a radial pattern that led us to infer that YVC dikes were responding to a local stress field generated by either the inflation of a magma reservoir and/or gravitational loading of the volcano (Acocella and Neri, 2008; Roman and Jaupart, 2014). The YVC dikes were also separated into two groups, thin laterally propagating dikes that are proximal to the inferred center of the volcano and thick vertically propagating dikes that are relatively distal to the same point. The gravitational loading of the volcano may have created a shallow stress barrier which arrests vertically propagating dikes beneath the summit and encourages lateral propagation and flank eruptions (Geshi, 2008; Poland 2004). During the last year of this project, we have analyzed 17 dike samples for geochemistry. These 17 geochemical samples were analyzed to assess variations in major and minor elemental composition. While both groups are predominantly andesitic basalt, preliminary results suggest distinct geochemical signatures between the proximal and distal dikes, which may reflect differences in magma source regions or the degree of fractionation prior to eruption. Proximal dikes tended to be relatively mafic while thick distal dikes were more felsic in composition. Furthermore, the proximal and distal dikes tended to differ in mineral assemblage and textural components. Distal dikes had few phenocrysts, making up less than 5% of the sample, and the phenocrysts that were hosted in these samples frequently showed sieve textures, kink banding, and heavy fracturing. The low phenocryst abundance, disequilibrium, and occurrences of degraded glomerocrysts in the distal dikes suggests lower crustal magma mixing and mafic recharge in the mash zone.