UNDERSTANDING PLUTONISM IN THE VERTICAL: FIELD AND GEOCHEMICAL RELATIONS ON THE SOUTHEAST FACE OF EL CAPITAN, YOSEMITE VALLEY, CALIFORNIA

Because erosional surfaces are dominantly horizontal, most studies of magmatic systems are inherently two dimensional. The 1 km vertical section of granitic rock exposed on the southeast face of El Capitan in Yosemite Valley, CA provides a crucial third dimension for the study of plutonism. Many hypotheses for the generation of silicic melts are based on gravitational separation of melt from crystals and predict upward accumulation of liquids rich in silica and incompatible elements. This study focused on detailed mapping of the spatial extents of exposed rock units in order to determine the chronology and geometry of emplacement, coupled with a study of the vertical variation in mineralogy and whole-rock chemistry. Field relations revealed by this mapping display a complex intrusive history at the point of interaction between separate intrusive suites and two sets of mafic dike swarms. The Leaning Tower granodiorite, a member of the little-studied intrusive suite of Buena Vista Crest, crosses the southeast face of El Capitan yet was not previously mapped in this area of Yosemite Valley. Mapping also revealed two sets of aplite dikes, one of which displays a variety of sense of separation indicators, suggesting an episode of late-stage deformation. All aplites have trace element compositions indicative of local derivation. Within the two dominant granitioids, the El Capitan and Taft granites, 62 samples were taken over multiple km-tall transects and analyzed for major- and trace-element abundances. The ordered vertical variation in mineral composition and element abundances predicted by traditional models of pluton evolution were not found. This suggests that a process other than in-situ gravitational crystal-liquid fractionation was responsible for the petrochemical variations in granitoids exposed on El Capitan.