ND AND SR ISOTOPE CHRONOSTRATIGRAPHY OF COLORADO PLATEAU LITHOSPHERE - IMPLICATIONS FOR MAGMATIC AND TECTONIC UNDERPLATING OF THE CONTINENTAL-CRUST

Nd-Sr isotopes and major and trace element abundances were measured in lower and upper crustal xenoliths carried in Tertiary minettes and kimberlitic diatremes of the Colorado Plateau. The objective was to establish the vertical distribution of mantle extraction ages for an undisturbed Precambrian crustal province characterized by normal thickness and low cratonic heat flow, and to thereby evaluate the importance of magmatic or tectonic underplating in crustal growth. Mid-Proterozoic-age (1.7-2.0 Ga) material is found at all levels of the crust. Although mafic granulites and amphibolites have major and trace element compositions compatible with an origin as magmatically underplated material, Nd model ages do not indicate that there has been significant mantle contribution to the lower crust by igneous processes (such as magmatic underplating) since the main crust-forming event ca. 1.85 Ga ago. The meta-igneous lower crustal xenoliths have relatively radiogenic Nd isotopic compositions and unradiogenic Sr compositions. There is little variability in Nd model ages in spite of large variability in Sm/Nd. High and variable Sm/Nd ratios were therefore established early in the history of the protolith, during igneous or metamorphic processes coincident with or shortly after extraction from the mantle. Metapelitic xenoliths, tectonically emplaced into the lower crust, have the same Proterozoic model ages as meta-igneous parts of the lower crust. These paragneisses are probably arc-associated sediments that were underthrust during terrane accretion in the Proterozoic. The isotopic data indicate that eclogite xenoliths are likely to represent Proterozoic lithosphere rather than subducted Mesozoic MORB, as previously proposed. A number of the eclogite xenoliths have experienced a metasomatic event which introduced LREE, SiO2 and Na2O and shifted the Nd and Sr isotopic compositions. The least-altered eclogite xenoliths have major and trace element and radiogenic Nd isotopic compositions similar to those of other Proterozoic mafic, lower crustal xenoliths. A more plausible protolith for the eclogites is Proterozoic lithosphere that was metamorphosed to eclogite facies conditions either during Precambrian continental assembly or in more recent times.