LATE CRETACEOUS HIGH-FLUX MAGMATISM IN THE WESTERN MOJAVE DESERT

The Late Cretaceous Cordilleran batholith displays remarkably consistent trends in age, geochemistry, and structural style, all the way from the south end of the Peninsular Ranges batholith in Baja California to the north end of the Sierra Nevada batholith in NW Nevada. The biggest discontinuity between these two relatively intact batholithic blocks coincides with both the San Andreas Fault and a set of outcrops of the subduction-channel-related Pelona-Orocopia-Rand schist. Although it is well recognized that batholithic rocks occur in the western Mojave Desert, the eastern Transverse Ranges, and the displaced Salinia block, a dearth of ages and geochemical data for the western Mojave in particular has led to this area being left blank on many published figures of the Cordilleran batholith. Existing geologic maps actually suggest that more than 90% of the western Mojave Desert is underlain by batholithic rocks.

Reconnaissance fieldwork and 8 new SHRIMP U-Pb zircon ages suggest that a majority of the batholithic rocks between Palmdale, Fort Irwin, and the Garlock fault (mostly quartz monzodiorite, quartz monzonite, granodiorite, and granite) fall within the 90-83 Ma age range. Although rocks in this area (of several thousand square kilometers) include muscovite-garnet granite lithologies uncommon in the Sierra Nevada or Peninsular Ranges batholiths, the volumetrically dominant granitoids share petrological and geochemical similarities with the coeval high-flux intrusive event of the Sierra Nevada crest. Further to the southeast, near Pearblossom and Victorville, two samples gave younger ages, around 77 Ma. Large intrusions of this age have been previously noted in the Mojave region, but are significantly younger than the youngest Cretaceous intrusions in the Sierra Nevada or Peninsular Ranges batholiths. It is tempting to suppose the emplacement of these 77 Ma intrusions may be related to the near-simultaneous emplacement of the Rand Schist, exposed as little as 25 km away at Portal Ridge. Further study of the western Mojave batholith may elucidate relationships between high-flux magmatism, schist emplacement, and subduction-related triggers.