THERMAL HISTORY OF COLORADO PLATEAU LITHOSPHERE FROM SM-ND MINERAL GEOCHRONOLOGY OF XENOLITHS

The thermal history of the lower crust and upper mantle of the Colorad o Plateau region is reconstructed on the basis of Nd and Sr isotopes i n minerals and whole rock xenoliths hosted by Tertiary minette and kim berlite. The mineral data (garnet and clinopyroxene) indicate that low er crustal granulite and amphibolite (equilibration depth approximate to 25 km; equilibration temperature approximate to 700 degrees C) were last equilibrated on a mineral scale at 1345 +/- 10 Ma; a Sm-Nd garne t-whole rock age for a granitoid xenolith is also 1345 +/- 10 Ma. Whol e rock data indicate that the crustal rocks were extracted from the ma ntle at ca. 1900 Ma. The mineral ages, which are 30-100 m.y. younger t han crystallization ages of Proterozoic ''anorogenic'' granitoids from regions bordering the Colorado Plateau, are interpreted as cooling ag es set following the crustal thermal maximum at 1380-1440 Ma. The gran ulites and amphibolites have remained at temperatures below approximat e to 450 degrees C since 1350 Ma. Two eclogite xenoliths (equilibratio n depth approximate to 45-60 km; equilibration temperature approximate to 600 degrees C), which are inferred to be Precambrian as well, yiel d 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 Ri dge and Moses Rock diatremes, and require that Nd isotopes were mainta ined in equilibrium right up to the time of entrainment. The isotopic data and the mineral textures suggest that the eclogites were undergoi ng active recrystallization at 21 Ma. The contrast in mineral ages bet ween granulite and eclogite xenoliths indicates that the equilibration temperatures of the two rock types reflect different times of equilib ration, and therefore cannot be considered as evidence for a negative thermal gradient at depth. The Rb-Sr mineral data from the xenoliths g ive variable early Paleozoic and Proterozoic ages that cannot easily b e assigned to geologic events.