EXPLORING THE TECTONIC CONTROLS ON THERMAL HISTORY OF THE MANTLE LITHOSPHERE OF THE SOUTH WEST OF NORTH AMERICA USING XENOLITH GEOCHEMISTRY

To explore tectonic influences on the thermal evolution of the lithospheric mantle beneath Southwestern North America, we investigated xenoliths from the San Quentin volcanic field (Baja California, Mexico), the Mojave Desert volcanic field (California), and the Grand Canyon. The samples were analyzed by electron microprobe to determine the major element compositions of clinopyroxene, orthopyroxene, olivine, and spinel phases present in each sample. Core-rim transect analyses suggest no significant zoning is present in any of the phases from any locality. Olivine magnesium numbers (Mg# =100×Mg/Mg+Fe, in moles) vary slightly by locality. The Grand Canyon has an average Mg# of 92.7, while the Mg# is 92.0 for both the San Quentin and Mojave Desert fields. The average chromium number (Cr# =100×Cr/Cr+Al, in moles) for the samples collected at San Quentin was 18.0 and the Grand Canyon is 27.5. Application of a two-pyroxene thermometer to the microprobe data reveals average temperatures of 1010 ⁰C for the San Quentin volcanic field, 1202⁰C for the Mojave Desert volcanic field, and 984 ⁰C for the Grand Canyon.

In concert with forthcoming observations from other geothermometers, these temperatures will enable investigation of the geologic histories of each of the areas in the South West United States. Previous studies have concluded that the San Quentin and Mojave Desert volcanic fields are affected by flat-slab subduction, which produces extensive deformation of the mantle lithosphere and inland-arc magmatism. The Grand Canyon is thought to have been affected by lithospheric delamination, where the lithosphere detaches and sinks into the underlying asthenosphere. Further investigation of the thermal histories associated with these distinct tectonic processes can provide new insights into how flat-slab subduction and lithospheric delamination affect the evolution of planetary interiors.