Crustal structure of the Mojave Desert, California: Inferences from Sr and O Isotope studies of Miocene volcanic rocks

Abstract

There are strong regional trends in the 87Sr/86Sr ratios and δ18O values of Miocene volcanic rocks from the Mojave Desert of southern California. Because these rocks contain a large component of recycled crust, their isotopic patterns can be used to study pre‐Miocene crustal structure which has been otherwise obscured and distorted by Mesozoic magmatism. Initial whole rock 87Sr/86Sr ratios (ri) increase and δ18O values of silicic magmas decrease with distance inland; ri also increases with SiO2, indicating contamination by radiogenic crustal rocks. Initial 87Sr/86Sr ratios in the most primitive basalts are high (0.7045–0.7095), indicating derivation from an enriched mantle source. Correlations of ri and δ18O with longitude indicate an eastward change in the nature of the contaminant. Silicic rocks have ri and δ18O of 0.705–0.707 and 9–11‰, respectively, in the western and central Mojave Desert, and 0.708–0.710 and 7–8‰ near the Colorado River. High δ18O values of phenocrysts in rhyolites from the western Mojave Desert indicate that these rocks were derived from a high δ18O (>10‰) source. The most likely candidate for this material is underthrust metamorphic rocks of the Mesozoic‐Tertiary subduction complex (e.g., Rand‐Pelona‐Orocopia schist and related lithologies). This implies that some of the North American lithospheric mande was stripped from the western Mojave Desert during Mesozoic‐Paleogene subduction. The smoothness of the eastward increase in ri constrains models for the tectonic evolution of southern California because no discontinuities corresponding to any of several proposed zones of pre‐Tertiary rifting or faulting are present.