9-14 Geologic Mapping of Cretaceous and Tertiary Hypabyssal and Volcanic Rocks at Highland Lakes, California, May Reveal Ancient Volcanic Center

Session: Advances in Geologic Mapping, Databases, and Dissemination: Student Posters

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Abstract:

Highland Lakes, California, is an area within the central Sierra Nevada that has experienced substantial Neogene intrusive and volcanic activity due to crustal thinning and extension. Preliminary geologic mapping (1:62,500 scale) by previous authors suggests the presence of a radial dike swarm, but the orientations and compositions of dikes are not well documented. In this study, field data were collected to investigate the spatial arrangement of the dikes, define lithologic units, and create the first detailed geologic map of the Highland Lakes area (1:5,000 scale). Mapping and structural analysis of 82 dikes shed light on the ancient tectonomagmatic processes, confirming that the dikes around Highland Lakes exhibit a radial arrangement. The vast majority share the same composition and a total phenocryst abundance of ca. 35%, with plagioclase making up nearly 22.5%, hornblende and biotite together making up almost 12.5%, with many of the hornblende and biotite phenocrysts altered to chlorite. These radial dikes are centered around, and in some places, cross-cut, an extensively hydrothermally altered intrusive plug of similar andesite composition and phenocryst assemblage. From this, we infer that Highland Lakes may have been the site of a previously undocumented ancient volcanic center. Future work to date these dikes may reveal whether this volcano is distinct in age from the other vents in the central Sierra Nevada.

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