THE KANIKSU AND IDAHO BATHOLITHS, NORTHERN U.S. CORDILLERA: CLOSE RELATIVES OR A CASE OF MISTAKEN IDENTITY?

The Kaniksu batholith is a composite igneous complex associated with the Priest River metamorphic core complex in the Idaho Panhandle. While it has long been assumed to be closely related to the Late Cretaceous-Paleocene Idaho batholith to the south, little work on the age, composition, and origin of the Kaniksu batholith has been conducted. Here, we present new reconnaissance level U-Pb zircon geochronology and isotope geochemistry data.

Individual plutons making up the Kaniksu batholith vary considerably in composition, with both typical Cordilleran granodiorites and tonalites and more alkalic monzonites and syenites. In situ U-Pb geochronology of zircon reveals a large number of ages between 120 and 105 Ma and only a few ages from later in the Cretaceous. Most samples contain inherited zircon cores with a range of Proterozoic ages, suggesting input from either metasedimentary Belt or Windermere rocks. Nearly all samples fall along a robust mixing array in Sr-Nd isotopic space between mantle and Precambrian continental crust end members.

All of these characteristics stand in sharp contrast to the larger Idaho batholith to the south. The Idaho batholith is younger (98 to 54 Ma), consists mostly of large volumes of evolved peraluminous granite, and has greater heterogeneity in the Nd, Hf, and Pb isotopic systems, probably reflecting geographic differences in crustal basement sources. Whereas the Idaho batholith appears to reflect the gradual thickening of the crust, at least initially in a continental arc setting, the tectonic setting of the Kaniksu batholith is less clear. Presently, it sits well inboard of contemporaneous portions of the Coast Mountains batholith, representing part of the “double arc” dilemma that characterizes the Canadian Cordillera. Consequently, the story of the Kaniksu batholith may play an important part in the resolving the Baja-BC controversy.