Intruded into the granites of the Idaho-Bitterroot batholith is a suite of mafic rocks dominated by basaltic andesite to dacite dikes, but also including angular to rounded andesite inclusions and quartz diorite complexes. Mutually intrusive relations suggest that these rocks were intruded while the granite was still partially fluid and thus are synplutonic. Magma interactions between the mafic and felsic rocks can be divided into three broad groups: magma mingling, magma mixing, and mingling combined with mixing. Magma mingling was important in composite dikes that contain andesite masses surrounded and cut by leucogranite veins and pods. Mixing below the present level of exposure resulted in hybrid dike rocks with disequilibrium phenocrysts. Mingling combined with mixing produced quartz diorite complexes that contain mafic, felsic, and hybrid rocks complexly swirled together. Mixing has resulted in the production of hybrid rocks that plot between end members of basaltic andesite and granite on Harker diagrams. The dominance of dikes over inclusions suggests that the mafic magma was intruded late in the crystallization history of the granite and/or that the granites were quite viscous throughout their history.

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 dyna-mothermal 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 series" recognized by Tilling in the Boulder batholith to the east. Radiometric ages in both batholiths appear to be similar. The hornblende-bearing tonalites of the western border and more-mafic satellitic plutons to the west, north, and northeast of the Idaho batholith appear to have the "I-type", "magnetite-series" mineralogy of Chappell and White and of Ishihara, respectively. The muscovite-bearing main units of the batholith appear to have "S-type" and probably "ilmenite-series" mineralogy. Sources for magmas forming the tonalitic western border zones of the Idaho batholith were probably mafic-rich rocks of the upper mantle or of subducted oceanic crust or young continental margin volcanic rocks. Magmas forming the granodiorite-granite main body of the batholith wre probably derived by partial melting of Precambrian continental basement rocks. The main body of the Bitterroot Lobe of the batholith appears to have been emplaced at a depth of 15 to 20 km, and much of the magma was probably generated at not much greater depth. Metallic mineral deposits are essentially absent from the deeper Bittrroot lobe. Correlation of magnetite-series granitic rocks and Cu-Mo mineralization and ilmenite-series granitic rocks with Sn-W mineralization can be partly documented for the Atlanta lobe and for plutons east of the batholith, but data are sketchy and, in part, appear contradictory.

The Bitterroot dome-Sapphire tectonic block appears to be a well-developed example of a plutonic-core 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.