DIFFERENCES IN ERUPTIVE BEHAVIOR, MINERALOGY, AND WHOLE ROCK GEOCHEMISTRY BETWEEN TUNNEL CONE AND SOUTH FORK CONE; GOLDEN TROUT WILDERNESS, SOUTHERN SIERRA NEVADA

The Kern Plateau, located in the southeast Sierra Nevada, is home to the Golden Trout Volcanic Field (GTVF), which includes Quaternary olivine-basalt and basaltic andesite cones, including Little Whitney Cone, Groundhog Cone, South Fork Cone (also known as Red Hill), and Tunnel Cone. This study describes field-mapping, petrographic, and whole-rock geochemistry observations of Tunnel Cone and South Fork Cone, which were previously interpreted to have formed during the same eruption (Moore and Lamphere, 1983). Field observations indicate that Tunnel Cone formed as a result of a Hawaiian eruption style along a 400-m-long fissure with at least three craters composed of spatter-fed agglutinate flows that dip radially away from the crater centers. In contrast, South Fork Cone resulted from a Strombolian eruption style following the effusion of a 7.5-km-long lava flow as evidenced by it being composed of loose unconsolidated lapilli and bomb-sized scoria overlying the South Fork lava flow. Petrographic analysis indicates that scoria and flow samples from both Tunnel and South Fork Cone contain similar mineral assemblages and phenocryst textures such as dusty sieved and oscillatory-zoned plagioclase. Modal abundances of minerals from Tunnel and South Fork Cone, however, differ significantly, whereas Tunnel Cone samples contain a much higher abundance of olivine compared to South Fork Cone samples. Whole rock geochemistry indicates that Tunnel Cone samples are also more primitive than samples from South Fork Cone. Thus, differences in eruption style, phenocryst abundances, and whole-rock composition between Tunnel Cone and South Fork Cone strongly suggest that the two formed from separate eruptions of different magmas.

Moore, J. G., and Lamphere, M. (1983). Age of the Golden Trout Creek volcanic field. Sierra Nevada, Ca. Eos, 64(45), 895-897.