Petrology of the Crazy Mountains dike swarm and geochronology ofassociated sills, south-central Montana

Abstract

The Crazy Mountains dike swarm is a radial array associated with theEocene (approximately 49 Ma) Big Timber stock in the Crazy Mountainsof south-central Montana. Dikes are hosted by the stock and byPaleocene Fort Union Formation rocks that host the stock. The spatialcoincidence, field relations, and similarity of petrographic andgeochemical features suggest that the dikes and the stock arecogenetic and coeval. The dikes depict a broad range of compositions,from rhyolite to basalt; dike composition is only weakly related toposition relative to the Big Timber stock. The dikes range from 51 to71 weight percent SiO2, a slightly more restricted range than thatexhibited by the stock. Compositional variation among the dikes isless systematic than is characteristic of the stock; dike compositionsform more broadly diffuse arrays and tend to be slightly morealkaline. Both of these factors could result from late-stagemagmatic-hydrothermal processes that seem to have weakly altered manyof the dikes. Major-oxide and trace-element characteristics of thedikes and the associated stock are consistent with a mantle-dominatedgenesis in a subduction-related volcanic arc setting. In addition topetrologic and geochemical data, new (super 40) Ar/ (super 39) Argeochronologic data for strongly alkaline sills in the northern CrazyMountains are also presented. Dates for the sills, which areperipheral to the Crazy Mountains dike swarm, indicate that alkalinemagmatism was temporally associated with subalkaline magmatismrepresented by the Big Timber stock and related dikes. Characteristicsof the subalkaline magmatism are consistent with renewed volcanic arcmagmatism during the Eocene westward hingeline retreat and subsequentsouthwestward migration of the magmatic front in response tosteepening subduction along the western edge of North America.

58 samples from this reference