Trace-element and Pb, Nd, and Sr isotope analyses demonstrate that the Tertiary subalkalic and alkalic igneous rocks of the Crazy Mountains, Montana, derive from separate mantle sources. The alkalic source region is heterogeneous and strongly enriched in most incompatible elements. Although it has Sr isotopic compositions near bulk earth values, it has low 206Pb/204Pb (16.5–17.4) and ϵNd (−2.6 to −153). The source of the subalkalic rocks is less enriched but is similar isotopically, except that ϵNd ranges between −13.4 and −17.1, Model ages based on Nd and Pb isotopic data show that these mantle source regions have a multistage differentiation history that probably commenced shortly after stabilization of the Archean crust at 3.0 Ga. The isotopic character of both alkalic and subalkalic sources was significantly influenced by a mantle-enrichment event at 1.5–1.8 Ga.
Comparison of the Crazy Mountains with other Cretaceous to Holocene igneous centers in Montana and Wyoming shows that the Middle Proterozoic enrichment event is recorded from the Bearpaw Mountains to the Leucite Hills. This regionally extensive ancient subcontinental mantle is complementary to mid-ocean ridge and ocean island basalt sources and is distinct from the sources of many other alkalic rocks.