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The 39,000-km 2 Idaho batholith lies 600 km east of the present Pacific coastline and east of the Columbia River basalt plateau. The batholith is Late Cretaceous in age and is emplaced immediately east of the Triassic Seven Devils volcanic arc, an apparently allochthonous terrane which may be part of the recently recognized "Wrangellia terrane" of western Canada and southern Alaska. Locus of a Late Cretaceous subduction zone related to the Idaho batholith is not yet defined but must lie west of the Seven Devils arc.Country rocks of the Idaho batholith are Proterozoic Belt metasediments and pre-Belt basement orthogneisses. Pre-batholithic, sillimanite-zone, regional dynamothermal metamorphism, apparently Jurassic or Cretaceous in age, is broadly concentric to the northern half of the batholith, extending for a few to several kilometers beyond the contact.The Idaho batholith is dominantly medium-grained, massive to moderately foliated, muscovite-biotite granite and granodiorite. Gneissic tonalite, rich in biotite and hornblende, forms a 12-16 kilometer-wide western border zone of the batholith, and tonalite or trondhjemite form satellitic plutons for 50-70 kilometers to the west. Granodiorite makes up a 10-20 kilometer-wide border zone against exposed country rocks of most of the batholith and surrounds the voluminous granite of the batholith interior. Such granodiorite has not been documented adjacent to the pre-Belt basement rocks of the northwest-trending Salmon River arch, which divides the Idaho batholith into a northern Bitterroot lobe and a southern Atlanta lobe. Granodiorite may have formed a broad shell over most of the batholith which may have originally extended across the deeply eroded Salmon River arch.The borders of a few large separate intrusions have been partly documented, especially in the interior of the Bitterroot lobe, but for most of the batholith, separate major bodies are as yet unknown. Foliation in the western tonalitic border zone of both lobes dips 50 to 70 degrees eastward under the batholith. To the east in the main body of the batholith, the foliation weakens and gradually arches to nearly horizontal in the interior. Southwestern and northeastern border zones of the deeper northern part of the batholith are marked by large alternating sheets of granitic and high-grade country rocks. Large, tabular or contorted inclusions and nebulous schlieren are abundant towards the interior.The north-trending Bitterroot dome of the northeastern Bitterroot lobe appears to have formed a mushroom-shaped diapir into the country rocks, then rose isostatically in response to eastward unloading of the 6000 km 2 Sapphire tectonic block. The base of the flanks of this part of the batholith has been mapped on the southwest and northeast.Chemically, the Idaho batholith appears related to the volumetrically minor "sodic 213 on June 11, 2015 memoirs.gsapubs.org Downloaded from 214 D. tV. Hyndman series" recognized by Tilling in the Boulder batholith to the east. Radiometric ages in both batholiths appear to be ...

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The Bitterroot dome-Sapphire tectonic block appears to be a well-developed example of a plutoniccore gneiss-dome complex or infrastructure separated from the adjacent suprastructure by a gently dipping zone of mylonitic shearing or an "Abscherungszone." The suprastructural Sapphire block, on the order of 15 km thick, 100 km long, and 70 km wide, apparently moved eastward about 60 km, bulldozing rocks of the eastern Flint Creek Range ahead of it. Movement of the block must have occurred about 75 or 80 m.y. ago during late stages of consolidation of the Idaho batholith which, along with sillimanite-zone regional metamorphic rocks, makes up the infrastructure under the mylonitic detachment zone. Timing of movement in the Sapphire block matches that in the Bitterroot dome.The Bitterroot dome must have risen after off-loading of the Sapphire block, because the shear foliation and lineation that formed during movement completely cross the dome; this indicates that the block must have moved eastward across the whole of the area now occupied by the dome rather than radially down the flanks of an existing dome. The shear lineation maintains its eastward trend even at the south end of the dome where the foliation dips southward. The lineation and shear foliation are strongest along the eastern flank of the dome, over which the greatest thickness of the block would have passed.