The Caribou Mountain pluton is a small trondhjemitic body that intruded semipelitic schist of the Stuart Fork terrane in late Middle Jurassic to Early Cretaceous time. Its emplacement followed the intrusion of an adjoining body of hornblende quartz diorite called the Middle Fork pluton and the mode of its emplacement was as an asymmetric ballooning diapir (Davis, 1963), as shown by concentric foliation, radial late-stage dikes, foliated enclaves, and folded blocks of schlieren-banded tonalite. Coarse-grained hornblende-bearing trondhjemite is the dominant rock type in the Caribou Mountain pluton, and it is called the 'main trondhjemite'. It was followed by medium-grained 'late trondhjemite' and by late-stage trondhjemitic and granodioritic dikes. All the trondhjemitic rock types are characterized by low alkali contents, high light rare earth elements, low initial 87Sr/86Sr, and low δ18O. However, the late trondhjemite has higher Na2O and a higher initial 87Sr/86Sr value than the main trondhjemite, and the two units cannot be related by fractional crystallization. The late granodioritic dikes are richer in Ba, Rb, Y, and Sc than the late trondhjemite and probably reflect assimilation of Stuart Fork metasedimentary rocks by late-stage trondhjemitic magma.Mafic enclaves in the main trondhjemite contain xenocrysts of quartz and plagioclase derived from the host by magma mixing. The enclaves have K2O, Ba, and Rb contents similar to, or higher than those of the host rocks. Their rare earth element (REE) patterns display strong middle REE enrichment caused by accumulation of hornblende, probably as the result of filter pressing.The main trondhjemite cannot be derived from Middle Fork magma because the initial 87Sr/86Sr of the Middle Fork pluton is lower than that of the trondhjemite. The absence of parental mafic magmas of appropriate composition suggests that the Caribou Mountain trondhjemitic magmas formed by partial melting of an amphibolitic source rock compositionally similar to low-K tholeiite.