CRUSTAL GROWTH AND RECYCLING AND LINKS TO TECTONISM IN THE IDAHO BATHOLITH AND CHALLIS INTRUSIVE PROVINCE

New geochronologic and Sr, Nd, Hf, and Pb isotopic data from the Idaho batholith and spatially overlapping Challis intrusive province reveal the presence of a series of magmatic suites with distinct ages and isotopic compositions. The first is a belt of early metaluminous plutons that formed between about 100 and 87 Ma petrographically and compositionally similar to the main components of the other major batholiths of the Cordillera such as the Sierra Nevada batholith. This was followed by a voluminous pulse of peraluminous magmatism between 80 and 67 Ma that formed the bulk of the Atlanta lobe (the largest fraction of the Idaho batholith). This was followed by another pulse of voluminous peraluminous magmatism from 66 to 54 Ma that formed the bulk of the Bitterroot lobe. All phases of the batholith were then intruded by plutons associated with the Challis volcanic field between 51 and 43 Ma, during a period of extension.

The oldest rocks, forming the early metaluminous suite, have the most primitive Sr, Nd, and Hf isotopic signatures, which may reflect a mantle contribution, but they also have the most radiogenic Pb, which indicates a contribution from a high μ component. The voluminous Atlanta peraluminous granites have more evolved Sr, Nd, and Hf isotopic signatures and chemistry suggesting the melting of evolved crustal material deep in the crust, now over-thickened by Sevier thrusting. The younger and more geographically restricted Bitterroot peraluminous suite is compositionally and petrographically similar to the Atlanta rocks but has distinctly less radiogenic Nd and Hf, suggesting a compositionally similar but older source in the lower crust. Bitterroot peraluminous magmatism was followed by the onset of extension and Challis magmatism around 51 Ma. Challis plutons are both compositionally and isotopically diverse. Those intruding the southern Atlanta lobe appear to be largely crustal melts, but those intruding the Bitterroot lobe have less evolved Sr, Nd, and Hf isotopic signatures, suggesting more of a mantle contribution, possibly due to the greater magnitude of extension.

While the early metaluminous belt and the northern portion of the Challis intrusive province may represent juvenile crustal contributions, the overwhelming bulk of the Idaho batholith appears to by dominated by recycling of existing crust.