Ar-40/Ar-39 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 volc anic 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 rime < 0.7 km(3) 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 similar to 5 m(3)/km(2)/yr (more than an order of magnitude lower than the 'background trickle' eruption rat e 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 similar to 0. 02 m(3)/km(2)/yr. There is a strong correlation (r(2) = 0.99) between the h eight of the volcanic mesas (the volcanic cap protects the underlying sedim ent from erosion) and their eruption age, providing an average sediment ero sion 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 t he north (with fast upper mantle seismic velocities) and the Colorado Plate au to the south (with a low-velocity zone similar to that beneath East Paci fic Rise spreading ridge). The trigger for the Leucite Hills magmatism may be related, therefore, to the recent emplacement of asthenospheric mantle i mmediately to the south and not necessarily to the more distant Yellowstone hotspot to the north. (C) 2000 Elsevier Science B.V. All rights reserved.