REVISITING THE PETROGENESIS OF A LATE OLIGOCENE VOLCANIC FIELD IN SOUTHEASTERN CALIFORNIA

Studies in the 1970s and 1980s attributed widespread late Oligocene magmatism in the southwestern United States to the waning subduction of the oceanic Farallon plate beneath the continental lithosphere of western North America. Glazner (2022) recently pointed out that some early conclusions should be reassessed using current geochemical data. We are reevaluating the petrogenesis of the Little Chuckwalla Mountains (LCM) volcanic section – about 1500 meters of southeast-dipping volcanic and sedimentary strata intruded by a swarm of northwest-striking hypabyssal dikes in the western Little Chuckwalla Mountains of southeastern California. The LCM section includes a diverse suite of lava flows, breccias, and pyroclastic deposits of calc-alkaline, metaluminous basaltic andesite, andesite, and dacite to trachydacite, with subordinate basalt and rhyolite. Two K-Ar dates for andesite lava flows from low and high stratigraphic horizons are approximately 26 Ma. Mayo (1990, unpublished M.S. thesis) suggested a subduction-related origin for the LCM volcanic rocks based on petrography, major element chemistry, and limited trace element chemistry. Isotopic data necessary to infer a primitive magma source was unavailable in that study. We are reevaluating the LCM section by acquiring new data to compare with modern geochemical databases of igneous suites from known tectonic environments and to conduct quantitative petrogenetic modeling. We collected fresh samples for petrographic analysis and acquired new whole-rock chemistry, including Sr and Pb isotopic analyses, for the LCM basaltic andesite and trachydacite. Results reveal a close affinity between the geochemistry of the LCM suite and volcanic rocks from modern continental volcanic arcs. Petrogenetic models are consistent with the basaltic andesite, andesite, and trachydacite being related by fractional crystallization. The Sr ratio of the basaltic andesite (⁸⁷Sr/⁸⁶Sr = 0.705650 ±9, 2σ) is compatible with partial melting of an enriched mantle source or assimilation of lower continental crust or both. In contrast, the slightly higher ratio for the trachydacite (⁸⁷Sr/⁸⁶Sr = 0.707232 ±9, 2σ) suggests a modest assimilation of the upper continental crust.