ORIGIN OF THE CONTINENTAL-CRUST IN THE COLORADO PLATEAU - GEOCHEMICALEVIDENCE FROM MAFIC XENOLITHS FROM THE NAVAJO VOLCANIC FIELD, SOUTHWESTERN USA

Mafic xenoliths from the Navajo Volcanic Field (NVF) in the Colorado P lateau are repre sented by four lithologies, in order of decreasing ab undance: garnet granulite, amphibolite, gabbro, and pyroxene granulite . Eight geocherrjcal groups are defined based chiefly on REE (rare ear th element) and HFSE (high field strength element) distributions. Mg n umber exhibits a broad negative correlation with SiO2, and most xenoli ths plot in the hy-di-ol-plg volume of the CIPW normative basalt tetra hedron. REE distributions include groups with LREE enrichment (Gps I, IV, VII), LREE depletion (Gp II), HREE depletion (Gps III, V), flat RE E patterns (Gp VI), and u-shaped patterns (Gp VIII). With the exceptio n of Groups VI and VII, all of the NVF mafic xenoliths show negative T a-Nb anomalies on primitive-mantle-normalized diagrams; also, all grou ps show negative Th anomalies, and all but Gp IV show positive Sr anom alies. Thorium and Sr anomalies appear to reflect introduction of Rb, Ba, Sr, and K during xenolith alteration. Nickel changes rapidly in co ncentration with only a small change in Zr, suggesting the rocks are d ominantly mafic cumulates. Geochemical results also indicate that the mafic xenoliths are not genetically related to the host minettes. Asso ciated spinel Iherzolite xenoliths are depleted in incompatible elemen ts, and cannot serve as sources for any of the mafic xenoliths, but th ey may represent restite left during extraction of the xenolith protol ith liquids. Garnet Iherzolite xenoliths, which are enriched in incomp atible elements, could have served as sources for the LREE-enriched ma fic xenolith groups. If there is no sampling bias in the NVF mafic xen oliths, mafic xenolith Groups IV, V, VI, and VII dominate in the lower crust of this region. A component of enriched lower crust, as represe nted by xenolith Group TV and by xenoliths from the San Francisco Volc anic Field, occurs only northwest of a boundary extending from near Fl agstaff, Arizona, into northwestern New Mexico and may be coincident w ith a buried Early Proterozoic crustal boundary. Published geochemical and Nd, Sr, and Pb isotopic data from Proterozoic surface rocks and f rom NVF mafic xenoliths indicate that the crust and mantle lithosphere beneath the Colorado Plateau is dominantly Early Proterozoic (2.0-1.7 5 Ga) in age. With the possible exception of gabbros from Moses Rock, there is no evidence among the mafic crustal xenoliths for later magma tic underplating. Incompatible element distributions are consistent wi th an origin for the crust by Early Proterozoic are accretion, and the y do not reflect a contribution from submarine plateaus. Also, a negat ive Eu anomaly must exist in both the upper and lower crust in this re gion. Copyright (C) 1997 Elsevier Science Ltd.