Origin of the continental crust in the Colorado Plateau: Geochemical evidence from mafic xenoliths from the Navajo Volcanic Field, southwestern USA

Abstract

Mafic xenoliths from the Navajo Volcanic Field (NVF) in the Colorado Plateau are represented by four lithologies, in order of decreasing abundance: garnet granulite, amphibolite, gabbro, and pyroxene granulite, Eight geochemical groups are defined based chiefly on REE (rare earth element) and HFSE (high field strength element) distributions. Mg number exhibits a broad negative correlation with SiO 2, and most xenoliths plot in the hy-di-ol-plg volume of the CIPW normative basalt tetrahedron. REE distributions include groups with LREE enrichment (Gps I, IV, VII), LREE depletion (Gp II), HREE depletion (Gps III, V), flat REE patterns (Gp VI), and u-shaped patterns (Gp VIII). With the exception of Groups VI and VII, all of the NVF mafic xenoliths show negative Ta Nb anomalies on primitive-mantle-normalized diagrams; also, all groups show negative Th anomalies, and all but Gp IV show positive Sr anomalies. Thorium and Sr anomalies appear to reflect introduction of Rb, Ba, Sr, and K during xenolith alteration. Nickel changes rapidly in concentration with only a small change in Zr, suggesting the rocks are dominantly mafic cumulates. Geochemical results also indicate that the mafic xenoliths are not genetically related to the host minettes. Associated spinel lherzolite xenoliths are depleted in incompatible elements, and cannot serve as sources for any of the mafic xenoliths, but they may represent restite left during extraction of the xenolith protolith liquids. Garnet lherzolite xenoliths, which are enriched in incompatible elements, could have served as sources for the LREE-enriched mafic xenolith groups. If there is no sampling bias in the NVF mafic xenoliths, mafic xenolith Groups IV, V, VI, and VII dominate in the lower crust of this region. A component of enriched lower crust, as represented by xenolith Group IV and by xenoliths from the San Francisco Volcanic Field, occurs only northwest of a boundary extending from near Flagstaff, Arizona, into northwestern New Mexico and may be coincident with a buried Early Proterozoic crustal boundary. Published geochemical and Nd, Sr, and Pb isotopic data from Proterozoic surface rocks and from NVF mafic xenoliths indicate that the crust and mantle lithosphere beneath the Colorado Plateau is dominantly Early proterozoic (2.0–1.75 Ga) in age. With the possible exception of gabbros from Moses Rock, there is no evidence among the mafic crustal xenoliths for later magmatic underplating. Incompatible element distributions are consistent with an origin for the crust by Early Proterozic arc accretion, and they do not reflect a contribution from submarine plateaus. Also, a negative Eu anomaly must exist in both the upper and lower crust in this region.