Early Miocene lamproite from the Colorado Plateau tectonic province, Southeastern Utah, USA

Newly discovered olivine phlogopite lamproite dikes intrude Jurassic silici clastic strata in the Green River Desert subregion of the western Colorado Plateau tectonic province in southeastern Utah. The dikes yield an age of 2 2 Ma both from Ar-40/Ar-39 step-heating of phlogopite and from isochron mod eling of laser-fused sanidine, This age is similar to those of mica-rich mi nettes and melanephelinites of the Wasatch Plateau about 125 km northwest a nd within the age range of the Navajo potassic volcanic field about 150 km to the southeast. The dikes intruded a pre-existing, northwest-oriented fra cture system containing previously introduced bitumen, indicating that some regional lineaments of this trend are Early Miocene or older. The dikes ar e highly LREE-enriched, and display lamproite-specific REE ratios and phlog opite and sanidine compositions. Incompatible element and radiogenic isotop e (Nd-Sr-Pb) ratios suggest that lithospheric source material modified by a ncient subduction processes, together with younger asthenospheric source co mponents, produced the melt. Timing of the intrusion coincides with the tra nsition from Early-Middle Cenozoic, calc-alkaline plutonism to the dominant ly mafic, Basin and Range type volcanism of the Late Cenozoic. While the la mproite occurrence indicates thermal input from the mantle, model non-uniqu eness for both magma source depths and geophysical structure prevents quant itative comparison of Early Miocene with present-day lithospheric thickness . (C) 2000 Elsevier Science B.V. All rights reserved.