40Ar/39Ar chronology of the Leucite Hills, Wyoming: eruption rates, erosion rates, and an evolving temperature structure of the underlying mantle

The lamproite lavas of the Leucite Hills, Wyoming comprise an isolated volcanic field of mesas and buttes, which erupted onto a thick sequence of K/T shales and sandstones. Volcanic activity spanned the interval from 3.0 to 0.89 Ma, during which time <0.7 km3 of magma were erupted. Approximately 84% of the magma was erupted within a 10–90 ky interval (between 0.94 and 0.89 Ma), with an average eruption rate of ∼5 m3/km2/yr (more than an order of magnitude lower than the ‘background trickle’ eruption rate at continental arcs). The eruption rate prior to this burst of activity, between 3.0 and 0.94 Ma, was two orders of magnitude lower at ∼0.02 m3/km2/yr. There is a strong correlation (r2=0.99) between the height of the volcanic mesas (the volcanic cap protects the underlying sediment from erosion) and their eruption age, providing an average sediment erosion rate of 0.113±0.002 mm/yr over the last 2.5 million years. Recent seismic studies indicate that the Leucite Hills volcanic field overlies an abrupt transition in lithospheric structure between the Archean craton to the north (with fast upper mantle seismic velocities) and the Colorado Plateau to the south (with a low-velocity zone similar to that beneath East Pacific Rise spreading ridge). The trigger for the Leucite Hills magmatism may be related, therefore, to the recent emplacement of asthenospheric mantle immediately to the south and not necessarily to the more distant Yellowstone hotspot to the north.