CONSTRAINTS ON THE AGE OF A MIOCENE IGNIMBRITE IN THE WESTERN MOJAVE DESERT

Volcaniclastic deposits in the western Mojave Desert record a gradual change from lacustrine to fluvial and subaerial depositional environments, contemporaneous with volcanism that continued throughout and after the transition from fluvial to subaerial. Reworked volcanic ash deposits in the Miocene Pickhandle Formation provide evidence for explosive style volcanism that occurred in the region of the Calico Mountains near Barstow, CA. In addition to volcaniclastic rocks, several dacite domes were emplaced before or during the Miocene that appear to predate the volcaniclastic deposits. The primary goal of this research is to characterize a pyroclastic flow deposit in the Pickhandle Formation: the Pickhandle Ignimbrite. Other goals include placing an age constraint on the ignimbrite using U-Pb isotopes, and constraining the age of the the lithic clasts in this pyroclastic flow deposit in order to identify and establish a connection between the Pickhandle dacite domes in the Calico Mountains, and the source of the Pickhandle Ignimbrite. Zircon ages from the ignimbrite range from ~18 to 150 Ma old. The younger age of ~18 Ma given by zircon crystals in the dacite lithics constraints their age and by proxy places an age constraint on the ignimbrite. The older age population given by zircon crystals in the ash and the dacite lithics is consistent with crustal assimilation of Mesozoic country rock.