THE CHALLIS MAGMATIC PROVINCE IN IDAHO: A REVIEW

The Challis magmatic province in Idaho consists of an Eocene volcanic field and an adjacent belt of contemporaneous plutons and dike swarms. The volcanic field consists a mix of effusive mafic to intermediate lavas and more felsic lavas and pyroclastic deposits associated with caldera-forming eruptions. Most published K-Ar dates fall between 50 and 45 Ma. Challis plutons, most of which intrude the older Idaho batholith, are epizonal and have been traditionally divided into a quartz monzodiorite suite, consisting lithologies ranging from gabbro to granodiorite, and a pink granite suite, consisting of granite. Published U-Pb zircon ages for the plutons range from 51 to 43 Ma.

Challis rocks show considerable variability in trace element and radiogenic isotope compositions. Mafic to intermediate lavas and the quartz monzodiorite pluton suite show trace element characteristics of subduction magmatism (e.g., high LREE/HFSE and high LILE/HFSE), but the pink granites and some of the most felsic volcanics show geochemical traits characteristic of A-type granites (e.g., high Fe, K, F, and HFSE; low Ca and Al). Radiogenic isotope compositions vary widely within each lithological group but vary more coherently with geographic location. Less radiogenic Sr and more radiogenic Nd and Hf isotopic compositions are found in the north and west of the province. Pb isotope compositions also vary systematically between samples north and south of the 45.5° latitude.

The geochemistry of the more mafic Challis lavas is consistent with a lithospheric mantle source, but large amounts of crustal melting is required to produce the full range of isotopic compositions seen in the province. Melting of both sources occurred during a period of extension seemingly related to the break-up of the subducting Farallon slab and/or the opening of a slab window. Within this context, delamination is worth consideration as a proximate trigger of magmatism. The earlier formation of the Idaho batholith likely produced a dense garnet-rich residuum that would have been prone to foundering. Loss of this root could have disrupted the lithospheric mantle and aided its melting and may provide an alternate explanation for the vertical high velocity seismic anomaly seen in the upper mantle beneath Idaho.