Age and origin of Early Cretaceous magmatism in the southernmost Omineca Belt, northern U.S. Cordillera

Abstract

We present results of a geochronological and geochemical study of the Kaniksu batholith, located in northern Idaho and Washington near the Canadian border. This batholith occurs at the southern end of the Omineca Belt and forms an integral part of the Priest River metamorphic complex. The Kaniksu batholith is compositionally diverse, but granitoids can be grouped into a volumetrically major peraluminous and metaluminous suite and a volumetrically minor alkalic suite based on lithology and major-element composition; additional compositionally similar alkalic plutons extend far to the south of the main footprint of the batholith. Laser ablation−inductively coupled plasma−mass spectrometry (LA-ICP-MS) U-Pb zircon dating revealed that most granitoids of the batholith formed in the Early Cretaceous between 120 Ma and 100 Ma, although magmatism continued until ca. 73 Ma. The dominance of Early Cretaceous ages closely matches magmatic age distributions elsewhere in the Omineca Belt. There is no clear relationship between age and suite assignment or other geochemical characteristics. Whole-rock Sr, Nd, and Pb isotope data along with zircon Hf isotope data suggest that the alkalic and metaluminous suites can largely be explained as mixtures of partial melts from the Mesoproterozoic Moyie sills and metasedimentary rocks of the Belt Supergroup, whereas a third component, perhaps Paleoproterozoic or Neoarchean meta-igneous basement, is required to explain the origin of the peraluminous suite. All of these granitoids formed well behind the Cordilleran arc front in an area characterized by multiple episodes of crustal thickening. As there is little evidence of contemporaneous mantle-derived magmatism, crustal thickening was probably the main driver of batholith formation.