GEOCHEMISTRY OF IGNEOUS ROCKS FROM THE CRAZY MOUNTAINS, MONTANA, AND TECTONIC MODELS FOR THE MONTANA ALKALIC PROVINCE

Tectonic models have postulated that subduction of a flat-lying oceanic plate was intimately linked with mid-Eocene magmatism in the northwestern United States and have assumed that this linkage is expressed in the geochemical characteristics of igneous rocks. Geochemical data from the Crazy Mountains in south central Montana can be interpreted as indicating the presence of a subduction-related chemical component. Because radiogenic isotopic data for these rocks restrict the major chemical events in their mantle source to the Late Cretaceous-Early Tertiary (50-100 Ma) and the mid-Proterozoic (1.6-1.8 Ga), subduction during one or both of these periods is the most plausible explanation for the origin of the arc-like geochemical features. Assessment of geologic and tectonic constraints, however, indicates that Late Cretaceous-Early Tertiary subduction cannot explain the regional distribution, the contemporaneity, and the compositional range of the Eocene igneous rocks. The Eocene magmatic event does not show interpretable age progressions, as most subduction-related models suggest. There is considerable geographic overlap in the chemical characteristics of the Eocene igneous rocks, so that chemical zoning cannot be demonstrated south of the U.S.-Canada border. The zone of igneous activity was diffuse and discontinuous, unlike most subduction-related magmatic arcs. The presence of thick lithospheric mantle beneath the Archean Wyoming craton presents difficulties for models that involve subduction of a flat-lying slab. The lithospheric mantle probably controlled the depth of subduction and probably forced the slab to depths greater than those of plausible source regions for subalkalic, mantle-derived basalts. Flat slab subduction models cannot account for the thermal inputs required for extensive magmatism. Subduction-related models fail to explain the CO2-rich character of alkalic magmatism in central Montana. The arc-like geochemical patterns in the Crazy Mountains samples could result either from subduction unrelated to Late Cretaceous-Early Tertiary events, or from mantle geochemical processes that are unrelated to subduction. If geochemical processes unrelated to subduction are involved, trace element discrimination diagrams are misleading and are inadequate criteria for interpreting the tectonic enviroment. The evidence for mid-Proterozoic chemical modification of the mantle beneath the Wyoming craton suggests that the arc-like character could be inherited from subduction events between 1.6 and 1.9 Ga, but a specific mid-Proterozoic subduction geometry cannot be identified. Alternative tectonic models for Eocene magmatism can be built around plausible heating or decompression mechanisms for regional magma generation. The advection of a thermal anomaly by mantle upwelling, or regional uplift related to the thickening-flexure-rebound cycle in the Cordillera to the west could have triggered the regional Eocene magmatic event.