Paper No. 33-8
Presentation Time: 10:35 AM
TECTONIC IMPLICATIONS OF PERMIAN-TRIASSIC, LATE JURASSIC, AND LATE CRETACEOUS PLUTONIC ROCKS THAT INTRUDE THE EL PASO TERRANE IN THE LANE MOUNTAIN AREA, CENTRAL MOJAVE DESERT, CALIFORNIA
Deformed eugeoclinal rocks of the southernmost part of the El Paso terrane (EPT) in the Lane Mountain area ~25 km NE of Barstow, CA were intruded by three generations of arc plutonic rocks that provide geochronologic and whole-rock isotopic constraints on the tectonic evolution and lithospheric underpinnings of the EPT. The plutonic rocks consist of Permian-Triassic (~250 Ma) diorite to monzonite with mean Sri ~0.7045 and mean δ18O ~7.6‰; Late Jurassic (~150 Ma) gabbro, diorite, and granite with mean Sri ~0.7055 and mean δ18O ~7.5‰; and Late Cretaceous (~80 Ma) granodiorite to granite with mean Sri ~0.709 and mean δ18O ~9.3‰. These data indicate increasing involvement of continental sources in magma generation through time, beginning with mantle-dominated sources in the Permian-Triassic, progressing to minor contamination by continental sources in the Late Jurassic, and finally to significant input of Precambrian continental lithosphere in the Late Cretaceous. We interpret this progression as evidence that the deep-water EPT was thrust eastward onto North American continental lithosphere in two distinct phases, one before intrusion of the Late Jurassic plutons and another before the Late Cretaceous magmatic pulse. Previous evidence north of the Lane Mountain area indicates that the first phase of thrusting took place before ~175 Ma, when the eastern part of the EPT was locally intruded by plutons with Sri > 0.706. Detrital zircon data produced by our study, which show that parts of the EPT at Lane Mountain have maximum depositional ages of ~250 to ~190 Ma, younger than any strata previously recognized in this terrane, further limit the first phase of thrusting to between ~190 and ~175 Ma. We suggest that both phases of thrusting were accommodated by the regionally extensive East Sierran thrust system, which projects westward beneath the EPT and was active from Early Jurassic to Early Cretaceous time. Precambrian detrital zircon ages from our samples at Lane Mountain are consistent with North American continental sources but do not closely constrain the original location of the EPT, which remains uncertain.