Thermal history of Colorado Plateau lithosphere from Sm-Nd mineral geochronology of xenoliths

Abstract

The thermal history of the lower crust and upper mantle of the Colorado Plateau region is reconstructed on the basis of Nd and Sr isotopes in minerals and whole rock xenoliths hosted by Tertiary minette and kimberlite. The mineral data (garnet and clinopyroxene) indicate that lower crustal granulite and amphibolite (equilibration depth ≈ 25 km; equilibration temperature ≈ 700 °C) were last equilibrated on a mineral scale at 1345 ± 10 Ma; a Sm-Nd garnet-whole rock age for a granitoid xenolith is also 1345 ± 10 Ma. Whole rock data indicate that the crustal rocks were extracted from the mantle at ca. 1900 Ma. The mineral ages, which are 30–100 m.y. younger than crystallization ages of Proterozoic “anorogenic” granitoids from regions bordering the Colorado Plateau, are interpreted as cooling ages set following the crustal thermal maximum at 1380–1440 Ma. The granulites and amphibolites have remained at temperatures below ≈450 °C since 1350 Ma. Two eclogite xenoliths (equilibration depth ≈45–60 km; equilibration temperature ≈600 °C), which are inferred to be Precambrian as well, yield Sm-Nd garnet-clinopyroxene ages of 21.6 ± 1.2 and 21.0 ± 0.8 Ma. The eclogite mineral ages are probably the ages of the host Garnet Ridge and Moses Rock diatremes, and require that Nd isotopes were maintained in equilibrium right up to the time of entrainment. The isotopic data and the mineral textures suggest that the eclogites were undergoing active recrystallization at 21 Ma. The contrast in mineral ages between granulite and eclogite xenoliths indicates that the equilibration temperatures of the two rock types reflect different times of equilibration, and therefore cannot be considered as evidence for a negative thermal gradient at depth. The Rb-Sr mineral data from the xenoliths give variable early Paleozoic and Proterozoic ages that cannot easily be assigned to geologic events.