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 ki mberlitic diatremes of the Colorado Plateau. The objective was to esta blish the vertical distribution of mantle extraction ages for an undis turbed 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-a ge (1.7-2.0 Ga) material is found at all levels of the crust. Although mafic granulites and amphibolites have major and trace element compos itions 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 is otopic compositions and unradiogenic Sr compositions. There is little variability in Nd model ages in spite of large variability in Sm/Nd. H igh and variable Sm/Nd ratios were therefore established early in the history of the protolith, during igneous or metamorphic processes coin cident with or shortly after extraction from the mantle. Metapelitic x enoliths, tectonically emplaced into the lower crust, have the same Pr oterozoic model ages as meta-igneous parts of the lower crust. These p aragneisses are probably arc-associated sediments that were underthrus t during terrane accretion in the Proterozoic. The isotopic data indic ate that eclogite xenoliths are likely to represent Proterozoic lithos phere rather than subducted Mesozoic MORB, as previously proposed. A n umber of the eclogite xenoliths have experienced a metasomatic event w hich introduced LREE, SiO2 and Na2O and shifted the Nd and Sr isotopic compositions. The least-altered eclogite xenoliths have major and tra ce element and radiogenic Nd isotopic compositions similar to those of other Proterozoic mafic, lower crustal xenoliths. A more plausible pr otolith for the eclogites is Proterozoic lithosphere that was metamorp hosed to eclogite facies conditions either during Precambrian continen tal assembly or in more recent times.