干旱区生态环境知识资源中心

The utilization of detrital zircon ages as a successful provenance indicator necessitates that source rocks and derived sediment, particularly first-cycle sediment, closely match each other in terms of age and age distribution. In order to test how closely first-cycle sediment resembles its source, zircon ages were determined from a homogenous plutonic source and sediments deposited directly on it in the Stepladder Mountains, southeastern California. U-Pb zircon ages from the pluton and proximal downstream alluvium exhibit substantial differences in proportions of Cretaceous, Mesoproterozoic, and Paleoproterozoic ages. Although geographic relations and major-element geochemistry require that the sediment was primarily derived from the weathering of the adjacent pluton, sample likeness values for zircon ages are commonly low, and age distribution differences are sufficiently large to reject the null hypothesis that they were drawn from the same population. The Stepladder pluton has abundant Cretaceous zircon, formed during magma crystallization, but also containsMesoproterozoic and Paleoproterozoic xenocrysts inherited duringmagma generation and/or ascent. Neither preferential release of Precambrian xenocrysts nor differential preservation of Cretaceous zircon during weathering of the bedrock is viewed as a likely sole explanation for the age disparities. Addition of allochthonous Precambrian zircon, that is, aeolian grains, is viewed as a potential mechanism for the explanation of at least some of the observed age proportions. The significant differences between the abundances of zircon ages from the source and from the derived sediment suggest that considerable caution should be taken in the use of detrital zircon ages as a primary means of establishing bedrock sources in provenance analysis.