GSA Annual Meeting in Phoenix, Arizona, USA - 2019

Paper No. 198-11
Presentation Time: 9:00 AM-6:30 PM


BLACHLY, Gregory F. and VAN BUER, Nicholas J., Department of Geological Sciences, California State Polytechnic University, Pomona, 3801 W Temple Ave., Pomona, CA 91768

The Ludlow batholith is a proposed cogenetic intrusive suite in the area surrounding Ludlow, CA in the central Mojave Desert. Several plutonic rocks samples in this area have been radiometrically dated to have formed during the Late Cretaceous, after normal arc magmatism ceased at 80 Ma. The goal of this research is to contribute to a better understanding of the tectonic events that caused magmatism in the central Mojave Desert to occur during the Late Cretaceous, since magmatism in this region and during this not time is not well studied. Included in this study is a detailed geologic map at the 1:24000 scale of a well exposed area of plutonic rock in the Bristol Mountain, three new zircon U-Pb radiometric ages, and an eleven sample, 5.5 km transect analyzed by X-ray fluorescence to better understand the differentiation and geochemical changes between the mapped units.

The major felsic units in the Bristol Mountains exhibit concentric zoning and decrease in age from rim to core. Three new zircon U-Pb radiometric ages show ages of 77.4±1.0 Ma at the rim of the pluton, and ages of 75.6±0.9 Ma and 75.8±1.0 Ma in samples closer to the core. These results are consistent with preexisting U-Pb zircon age data from Hess, 2017. From rim to core the four main units are equigranular hornblende biotite granodiorite with columnar biotite, equigranular biotite granodiorite with flaky biotite, porphyritic biotite granodiorite with flaky biotite, and porphyritic biotite granodiorite with glomeroporphyritic biotite. Mafic enclaves are predominantly present on the southern rim of the pluton and more scarcely distributed in eastern areas. The western side of the field area is dominated by mylonitic and meta-plutonic rocks, while the eastern side consists of several scattered patches of gabbro, which range in size within the outcrops of granodiorite. From the data presented thus far, it can be concluded that the Bristol Mountain units are cogenetically related.