Paper No. 14
Presentation Time: 12:15 PM
PETROGRAPHY AND GEOCHEMISTRY OF BLACK POINT AND RED CONES BASALTS, MONO-INYO VOLCANIC CHAIN, EASTERN CALIFORNIA
MARCAIDA, Mae, Department of Geosciences, San Francisco State University, 1600 Holloway Ave, San Francisco, CA 94132 and MANGAN, Margaret, U.S. Geological Survey, 345 Middlefield Rd., MS 910, Menlo Park, CA 94025, maemarcaida@gmail.com
The Mono-Inyo volcanic chain in eastern California forms an ~60-km long chain of north-south-trending vents from Mono Lake in the north, extending through the western part of Long Valley caldera to south of Mammoth Mountain. This mafic-to-silicic volcanic chain was active between 40 ka and 1700 A.D., and predominantly produced rhyolitic eruptions. Black Point and Red Cones are the northernmost and southernmost vents of the chain and are recognized as the most mafic eruptions. Black Point, on the northwest shore of Mono Lake, erupted ~13,800 years ago, and shows evidence for both subaqueous and subaerial eruptions. Red Cones, a pair of basaltic cinder cones located 5 km SW of Mammoth Mountain, erupted ~8,500 years ago, and is the most recent eruption of basalt in the Mono-Inyo chain. In this study, we used petrographic and geochemical data from Black Point and Red Cones basalts to interpret and compare the crystallization conditions of these mafic magmas.
Major element data show distinct groupings: Red Cones lavas are calc-alkaline and more Mg-rich (Mg #s ~65) whereas Black Point lavas are tholeiitic and less magnesian (Mg#’s ~60), with higher concentrations of incompatible elements (Ti, P, and K). Black Point basalts are aphanitic with sparse, sub-millimeter sized plagioclase and olivine crystals ( <1 vol%); whereas Red Cones lavas are porphyritic (~10 vol%) with plagioclase >olivine >clinopyroxene crystals several millimeters in size. The most magnesian olivine are found in Red Cones lavas (Fo71-86 vs. Fo67-84). Red Cones plagioclase is normally zoned (core An60-80; rim An54-77) whereas Black Point plagioclase is unzoned (core and rim An60-72).
Pearce element ratio analysis and least-squares calculations suggest that both Black Point and Red Cone magmas underwent plagioclase plus olivine fractionation from broadly similar parental magmas, but their evolution diverged along calc-alkaline (Red Cone) versus tholeiitic trends (Black Point), the result, perhaps, of differing water content or oxidation state, rather than differences in major element composition.
