A GEOCHEMICAL ANALYSIS OF THE MID-MIOCENE GLENDORA VOLCANICS OF THE NORTHEASTERN LOS ANGELES BASIN, CA

This study presents new geochemical classifications of the middle Miocene (ca. 16-18 Ma) Glendora Volcanics, which are exposed in several areas along the northeastern edge of the Los Angeles basin, southern California. Previous studies subdivided the Glendora Volcanics into a lower basalt/andesite member that overlies quartz diorite basement rock and the Oligocene Mt. Meadows dacite, and an upper andesite/dacite member that is complexly interbedded with marine sandstones and shales of the Miocene Topanga Formation. A total of 16 samples were collected and analyzed by XRF for this study: 10 samples from the foothills of the San Gabriel Mountains east of Glendora and north of San Dimas; 4 samples from Puddingstone reservoir; and 2 samples from Elephant Hill. TAS plots illustrate that of the 16 samples, andesite and dacite compositions dominate. The San Gabriel Mountain foothill exposures are mainly andesite and dacite lavas as well as autoclastic breccias with subordinate basaltic andesite. The Puddingstone reservoir rocks are mainly autoclastic andesites to the south of the reservoir, dacite lavas and autoclastic breccias to the east and north, and basalts(?) overlying palagonitic tuff to the west. Felsic flow banded rhyolites and rhyodacite autoclastic breccias are found at Elephant Hill. FeO/MgO vs. SiO₂ and AFM diagrams illustrate that the sampled suite are calc-alkaline in composition, suggesting crystal fractionation or possibly crustal assimilation. This current geochemical classification of the Glendora Volcanics will later be compared with nearby mid-Cenozoic volcanic suites in southern California, and with Miocene mafic-intermediate dikes that intrude the Oligocene Telegraph Peak granodiorite to the northeast to examine a possible genetic relationship between the Glendora Volcanics and mafic dikes found in the San Gabriel Mountains to the north.