Cretaceous dikes within the Jurassic Independence dike swarm in eastern California

Field relationships and new U-Pb geochronology reveal that a significant nu mber of dikes associated with the Independence dike swarm, both within the type locality and elsewhere, are of Cretaceous age. A new U-Pb zircon age f or a diorite dike in the type locality of the Independence dike swarm in Ca lifornia confirms that some mafic dikes in the swarm are coeval with previo usly dated Late Jurassic felsic dikes. Cretaceous dikes in the eastern Sier ra Nevada batholith are located near-and may be genetically related to-coev al mafic and felsic plutons, However, the presence of Cretaceous dikes thro ughout the swarm demonstrates that this is not a local phenomenon, and caut ion therefore should be exercised when using Independence-like dikes as tem poral markers. We have not identified and consistent petrographic or chemic al criteria that distinguish Jurassic Independence dikes from Cretaceous di kes. However, Jurassic dikes in the Sierra Nevada commonly contain a sinist ral shear fabric that is absent in both dated Cretaceous dikes and undated dikes that intrude dated Cretaceous plutons. Despite intruding at least 60 m.y. apart, the orientations and petrology of the Jurassic and Cretaceous d ikes are remarkably similar. Because dikes were intruded over a time period characterized by large changes in the orientation of plate convergence wes t of the Sierran arc, it is unlikely that the uniform structural orientatio n of Jurassic and Cretaceous dikes resulted from regional stresses imposed by the subducting plate. Instead, the orientation of Jurassic and Cretaceou s dikes (and of many Sierran Cretaceous plutons) may have been controlled b y preexisting faults or by the orientation of the continental margin.