GEOCHRONOLOGY AND GEOCHEMICAL ANALYSIS OF LATE CRETACEOUS PLUTONIC BODIES IN THE CENTRAL MOJAVE DESERT, CA

In most places, normal arc magmatism in California ceased at 80 Ma , but several plutons in the Mojave Desert are dated to have formed within the few millions of years after. This research looks at a cogenetic intrusive suite in the area surrounding Ludlow, CA, which contains several plutonic samples that are dated to have formed in the Late Cretaceous, at times after 80 Ma. The goal is to contribute to a better understanding of magmatic bodies present in the central Mojave Desert, and the tectonic events that caused magmatism to occur between the ages of about 80 Ma and 70 Ma. Included in this study is a detailed geologic map at 1:24000 scale of a 40 km² well exposed area of plutonic rock in the northeast Bristol Mountains, new zircon U-Pb radiometric ages, and X-ray fluorescence data of an eleven-sample transect from the mapped Bristol Mountain units, as well as other samples collected from the greater area surrounding Ludlow, CA.

The major felsic units in the Bristol Mountains exhibit concentric zoning and decrease in age from rim to core. New zircon U-Pb radiometric ages show an age of 77.2 ± 1.0 Ma at the rim of the pluton, and ages of about 75 Ma in samples closer to the core. These results are consistent with preexisting U-Pb zircon age data from Hess, 2017. U-Pb zircon geochronology and XRF analysis of samples outside of the detailed mapping site reveal several areas that are related, in terms of their geochemistry and age, to that of the mapped Bristol Mountain units. Preliminary data suggest that the Bristol Mountain units, as well as several other rock units in areas surrounding Ludlow, CA, are all part of one cogenetically related suite covering an area spanning approximately 1500 km². These data have implications for the tectonics of the Mojave Desert during a time where "normal" arc magmatism may not have been in operation.