GEOCHRONOLOGY OF THE RANDSBURG GRANODIORITE: REEVALUATION OF THE TECTONICS OF THE SOUTHERN SIERRA NEVADA AND WESTERN MOJAVE DESERT

In order to understand the Late Cretaceous-early Tertiary tectonic evolution of the southern Sierra Nevada and western Mojave Desert, it is necessary to determine the relationship between Late Cretaceous arc batholiths and the high-P/T Rand Schist, the northernmost exposed part of the Pelona-Orocopia-Rand schists of southern California and southwestern Arizona. The schists are widely believed to have underplated the continent during low-angle subduction that led to the Laramide orogeny. Although Laramide orogenesis is considered to have caused extinction of the Cretaceous arc, this tectonic model is contradicted by the widely cited view that two northern schist bodies (Rand and Sierra de Salinas schists) are intruded by Late Cretaceous plutons. We reevaluated this relationship in the Rand Mountains by examining thermal and geochronologic constraints on the Randsburg Granodiorite, previously inferred to be Late Cretaceous in age, which intruded and imposed contact metamorphism on the Rand Schist. TIMS and SIMS U-Pb ages of individual zircons from three samples range from 164 to 16 Ma, indicating a previously unrecognized complex geologic and thermal history. Two samples that contain nearly monogenetic zircons yielded identical 206Pb*/238U ages of 19.6±0.4 and 19.5±0.4 Ma. A third, hydrothermally altered sample contains Late Jurassic and Late Cretaceous as well as Miocene grains. Conventional multigrain analyses of the latter could yield an apparent Late Cretaceous age. 40Ar/39Ar step heating analysis of biotite yielded total gas ages of 18.8±0.2 and 19.4±0.3 Ma for two samples from the pluton and 14.3±0.3, 18.8±0.4, and 19.5±0.2 Ma for three schist samples in the contact aureole. In contrast, a biotite age of 43±0.2 Ma was determined for a sample of schist well outside the contact zone of the pluton. Muscovite ages for five samples away from the pluton range from 68 to 74 Ma and a single hornblende is 78 Ma. These data indicate that the Rand Schist was accreted beneath the Sierran arc at about 80 Ma and that the Randsburg Granodiorite intruded the schist in Early Miocene time. We hypothesize that samples yielding abundant Cretaceous zircons are included blocks from the upper plate of the Rand fault.