A thorough geochemical analysis of the Oligocene to Miocene Henry and La Sal Mountains laccoliths, and Miocene to Pliocene San Rafael trachybasalt dikes of the Colorado Plateau is believed to be vital in understanding the tectonic setting and possible crust-mantle interactions of Tertiary magmatism in the western U.S. In particular, this study seeks support to address three key questions. First, did subduction produce the Oligocene-Miocene magmatism in the Colorado Plateau, and if so, what unusual characteristics of subduction resulted in the emplacement of sparse epizonal plutons in cratonic crust ~1000 km from the plate margin? Second, is the recent assertion that the laccoliths from a time space link for a broad, contemporaneous east-west magmatic belt extending from Reno, Nevada to the San Juan Mountains (Sullivan, 1987; Best, 1988) valid from a geochemical basis? The magmatic history of both sides of the Plateau is strikingly similar in many respects. A broad (1500 km) magmatic belt nearly perpendicular to the subduction zone would be unprecedented in known geologic history. Third, how do mantle derived magmas and cratonic crust interact in a non-rift environment? Crust-mantle interactions have long been considered for continental arcs, but the laccoliths provide an opportunity to consider petrogeneic processes in a unique setting. Perhaps few other areas in the world promise to yield such a wide range of critical information regarding tectonics and associated petrogenetic processes.
