U-PB DETRITAL ZIRCON DATA FROM METASEDIMENTARY ROCKS AT BLACK MOUNTAIN NEAR VICTORVILLE, CALIFORNIA--IMPLICATIONS FOR THE AGE OF THE EARLY MESOZOIC FAIRVIEW VALLEY FORMATION

The Fairview Valley Formation (FVF), a metasedimentary sequence >1 km thick, has been interpreted as part of an Early Triassic overlap sequence deposited across a belt characterized by Late Permian to earliest Triassic continental-margin deformation and magmatism. The middle part of the FVF at Black Mountain contains Early Triassic conodonts, but the basal part rests depositionally on monzonite for which early Middle Triassic U-Pb zircon ages of about 241 to 243 Ma have been reported. We are conducting new field studies and U-Pb zircon (SHRIMP-RG) dating in an attempt to resolve this anomaly. Analysis of zircons from the monzonite yields a preferred age of 244±2 Ma (latest Early or earliest Middle Triassic). Analysis of detrital zircons from three FVF arkose beds indicates a dominance of Paleoproterozoic grains between 1640 and 1750 Ma, consistent with a local crystalline basement source. Our lowest sample, however, ~80 m above the base of the FVF, contains 23% Early Jurassic grains between 191 and 198 Ma. This result indicates that the basal part of the FVF is much younger than the underlying monzonite and much too young to represent an Early Triassic overlap event. Either the Early Triassic conodonts are reworked or, alternatively, the basal part of the FVF could be structurally discontinuous with the overlying strata, in which case an Early Triassic age might still be allowed for most of the formation.

The FVF is overlain by quartzite that in turn is overlain by the largely Middle Jurassic Sidewinder volcanic series (SVS). Our field studies show that the base of the quartzite is sharp, discordant with underlying beds, and at least locally faulted. Detrital zircons from a sample of the quartzite (n=25) have a much more heterogeneous age distribution than those from the FVF arkoses and include 11 Mesoproterozoic grains between 930 and 1286 Ma, two Archean grains, and two Jurassic grains (172 and 185 Ma). We interpret the quartzite as an early Middle Jurassic deposit conformable with the SVS and either unconformable or in fault contact with the FVF. Some Mesoproterozoic zircons in the quartzite may have had an Appalachian (Grenvillian) source as has been inferred for detrital zircons of similar age in Jurassic eolian sandstones of the Colorado Plateau.