The Post-85 Ma Idaho Batholith Represents Melting in Thickened Continental Crust, Not Arc Magmatism

The Idaho Batholith is usually included in discussions of Mesozoic tectonics of western North America as a subduction-zone continental arc. Arc-affinity plutons exist within the Idaho Batholith, but are volumetrically minor and emplaced >85 Ma or >69 Ma at the northern end. Most of the Idaho Batholith, the Atlanta lobe (80-67 Ma) and the northern Bitterroot lobe (66-54 Ma), however, have characteristics that distinguish it from other Mesozoic arc batholiths, suggesting a non-arc origin. Recent studies provide additional evidence that the Idaho Batholith formed by melting within thickened continental crust and does not require a subduction zone. The lithology is more silicic and aluminous than other Cretaceous batholiths, consisting of biotite granodiorite and biotite-muscovite granite that lack amphibole and mafic enclaves. The petrology and isotope geochemistry are consistent with melting of a felsic crustal protolith within the garnet stability field (≥35 km depth), with little or no mantle melt signature. Inherited zircon cores have ages consistent with regional Precambrian cratonic provinces. In the north, the Bitterroot lobe and its metamorphosed and partially melted source rocks are exposed in a core complex, indicating melting of felsic mid-lower crust. The Atlanta lobe has no internal fabric, yet was emplaced at a time of regional compression. An interpretation is that it was emplaced within a thick crustal plateau, where the upper crust was in an extensional regime and the depth of pluton emplacement had neutral strain. Subsequent exhumation of the Atlanta lobe was 15 km. Recent geophysical data indicate a mostly felsic crust 37-40 km thick. The crust was therefore >50 km thick at the time of magmatism. We conclude that the voluminous post-85 Ma Idaho Batholith formed by melting in a thickened continental crust. Sierra Nevada magmatism ended 85 Ma, so there is no subduction-zone arc in the western USA south of northern Washington from 85 to <40 Ma. Most of the Idaho Batholith is the same age as the proposed late Cretaceous Sevier orogenic plateau, the Nevadaplano, now collapsed into the Basin and Range. Similarly, Eocene Shuswap core complexes in Canada and adjacent USA resulted from collapse of late Cretaceous thickened crust. The Idaho Batholith melted as part of this extensive region of thick crust.