A FRESH LOOK AT TERTIARY DACITE DIKES IN SOUTHEASTERN CALIFORNIA

The Little Chuckwalla Mountains (LCM) volcanic field in southeastern California comprises 1500 meters of southeast-dipping late Oligocene to early Miocene volcanic and sedimentary rocks cut by a swarm of coeval, northwest-striking hypabyssal dikes. The volcanic rocks include lava flows, breccias, and pyroclastic deposits. Compositions of volcanic rocks and dikes include basalt, basaltic andesite, andesite, dacite, and rhyolite. Previous investigations of the LCM volcanic rocks are limited to two K-Ar dates on andesite lavas (Crowe et al., 1979) and a M.S. thesis (Mayo, 1990). The current study focuses on the petrography of glomeroporphyritic biotite dacite dikes, the largest of which forms a prominent ridge over 2 km long and 100 m thick that merges up-section with southeast dipping biotite dacite lava flows. In hand sample, the dacite appears gray to light rust red and is speckled with abundant white phenocrysts and glomerocrysts of plagioclase and black flakes of biotite. The phenocryst:groundmass ratio determined in thin section is 30:70. Phenocryst phases are plagioclase, biotite, and opaque oxides, plus traces of clinopyroxene, sanidine, hornblende, and olivine. Optically determined plagioclase compositions range from An₉ to An₃₃ and olivine is fayalitic (Fo₁₀-Fo₂₀, unpublished SEM analyses). Abundant glomerocrysts up to 8mm in diameter are interpreted as cognate of inclusions of plagioclase intergrown with varying amounts of biotite, opaque oxides, clinopyroxene, fayalitic olivine, and hornblende. Trace minerals include zircon and apatite. Plagioclase microlites and devitrified glass comprise the groundmass. Secondary alteration includes minor sericitization of plagioclase and partial to complete replacement of clinopyroxene and olivine by iddingsite, hematite, and opaque oxides. Volcanic sequences similar in age and composition to the LCM field are common in southeastern California and southwestern Arizona and are conventionally interpreted as being related to subduction of the Farallon Plate prior to contact of the East Pacific Rise with the North American Plate during mid-to-late Miocene time. While the petrography of the LCM biotite dacite reported here is consistent with this hypothesis, we are currently acquiring new geochemical and isotopic data to support a more robust interpretation.