THE MANY (AND SOMETIMES LOST) ARCS OF IDAHO

Recent geochronological and geochemical work associated with the IDOR Earthscope Project has revealed five distinct phases of magmatism spanning ~65 million years in the Idaho batholith and adjacent autochthonous portion of the Salmon River suture region. The resulting magmatic suites are the: 1) suture zone; 2) border zone and early metaluiminous; 3) Atlanta peraluminous; 4) Bitterroot peraluminous; and 5) Challis. Each phase represents a particular tectonic setting. The earliest magmatism occurred within the suture around ~105 Ma, probably due to subduction outboard of the previously accreted Blue Mountains province. Many plutons within the Salmon River suture, however, were affected by the intense deformation associated with the western Idaho shear zone (WISZ), which lasted until about 91 Ma. At this time, and lasting as late as 85 Ma, continuing arc magmatism produced a substantial amount of tonalite and granodiorite adjacent to and well east of the WISZ. These units are compositionally similar to the coeval plutons of the other Cordilleran batholiths to the south. Renewed contraction, due either to changing convergence rates or the collision of an outboard terrane, led to large-scale crustal melting between 83 and 67 Ma. This episode produced the largest portion of the Idaho batholith, and obscured much of the older generations of plutonism. Beginning around 66 Ma, a more localized episode of crustal melting occurred forming the Bitterroot lobe of the Idaho batholith. This second crustal melting event may be related to additional thrust faulting and crustal thickening confined to the region between the Lewis and Clark and Clearwater/Orofino fault networks. This Bitterroot magmatism continued until about 54 Ma and was followed by a widespread regional (Challis) magmatic flare-up coeval with extensional tectonics. This activity produced the Challis volcanic and plutonic rocks in Idaho and many roughly coeval volcanic fields to the east. We interpret these magmatic rocks to be triggered by a change in plate boundary conditions, leading to slab window formation and/or rollback. In summary, the magmatism in Idaho resulted in five different and diagnostic suites, each probably reflecting a different tectonic setting.