A Petrographic and Chemical Section through the Northern Idaho Batholith

Petrographic and chemical study of a well-exposed section through the northern Idaho batholith documents a high degree of mineralogical and chemical homogeneity and suggests that all of the rocks belong to a single comagmatic series or superunit. Felsic biotite granodiorite to granite with minor muscovite forms a main-phase unit 37 km across, flanked on the southwest by 10 km of near-vertical sheets of granodiorite with intervening high-grade country rocks. Widespread metasedimentary schlieren appear to be "restite" left by voluminous partial melting, the magma not rising far above its level of generation. The lack of mineralogical and chemical variation across the batholith suggests little differentiation, except for separation of late, volatile-rich fluids to form widespread pegmatites. Abundant mafic dikes emplaced into fractures in the granitic rocks just before crystallization of the last pegmatitic fluids may represent the later stages of rising basalts which aided the initial crustal melting to form the batholith. Metamorphism of these dikes probably reflects post-crystallization changes induced by the still hot granite which must have cooled slowly because of its emplacement at a depth of 16 or 17 km. The northern Idaho batholith shows contradictory characteristics for I- and S-type granites.