THE BOULDER BATHOLITH: A PERPLEXING EXAMPLE OF COEVAL MAGMATISM, VOLCANISM AND DEFORMATION DURING LARAMIDE FOLD AND THRUST BELT EVOLUTION IN SOUTHWEST MONTANA

The Boulder Batholith is a Late Cretaceous intrusive complex exposed in southwestern Montana that intrudes a sequence of Precambrian through Mesozoic sedimentary rocks and its own volcanic cover, the Elkhorn Mountain volcanics. This volcano-plutonic complex is located in the hanging wall of the Lombard-Eldorado thrust fault system. Investigations in the Boulder batholith, which is the easternmost batholith in the North American Cordillera, have suggested that its construction was intimately linked to deformation taking place in the foreland fold and thrust belt of the Cordillera in latest Cretaceous through Eocene time. Emplacement of the batholith occurred in the latest Cretaceous and it has been proposed that eastward translation, of at least 25-28 km occurred either during or after emplacement. All models for emplacement of Boulder Batholith magmas suggest syn-kinematic emplacement however most of the plutons that make up the batholith show little to no evidence of internal magmatic or solid state deformation. The depth and timing of emplacement as well as cooling history of the batholith, are poorly understood yet critical to understand how the system was constructed and the spatial and temporal relationships between magmatism, volcanism and deformation in the fold and thrust belt. Combined thermobarometric and structural studies quantify the depth of emplacement, exhumation history and the orientation and intensity of fabric (strain) development all of which are critical to understand the evolution of the volcano-plutonic complex and for kinematic and tectonic interpretations.