Tertiary minette and melanephelinite dikes, Wasatch Plateau, Utah: records of mantle heterogeneities and changing tectonics

A swarm of minette and melanephelinite dikes is exposed over 2500 km² in and near the Wasatch Plateau, central Utah, along the western margin of the Colorado Plateaus in the transition zone with the Basin and Range province. To date, 110 vertical dikes in 25 dike sets have been recognized. Strikes shift from about N80°W for 24 Ma dikes, to about N60°W for 18 Ma, to due north for 8-7m.y. These orientations are consistent with a shift from east-west Oligocene compression associated with subduction to east-west late Miocene crustal extension. Minettes are the most common rock type; mica-rich minette and mica-bearing melanephelinite occurs in 24 Ma dikes, whereas more ordinary minette is found in 8-7 Ma dikes. One melanephelinite dike is 18 Ma. These mafic alkaline rocks are transitional to one another in modal and major element composition but have distinctive trace element patterns and isotopic compositions; they appear to have crystallized from primitive magmas. -from Authors