We have measured Sr and Nd isotopic composition and rare earth element (REE) abundances in a variety of inclusions and host minettes from the Navajo Volcanic Field of the Colorado Plateau in order to develop geochemical constraints on the composition and evolution of the mantle beneath the relatively undisturbed Proterozoic crust of the Colorado Plateau. Spinel and garnet peridotites and eclogites derived from less than 80 km in depth have geochemical signatures (e.g., light REE (LREE) depletion) similar to oceanic lithosphere; we interpret thee rocks to be fragments of lithosphere, possibly part of an island arc which was accreted to the continent in the Proterozoic. Isotopic evidence suggests that some of the peridotites were hydrated by an incompatible element-poor, H2O-rich fluid, whereas the eclogites were metasomatized by a Na2O-rich phase with high 87Sr/86Sr. Some of the peridotites have highly radiogenic Nd isotopic ratios which reflect long-term isolation from mantle convection and show that they probably are not samples of recently subducted lithosphere.

Garnet peridotites derived from at or near the base of the ligthosphere are isotopically distinct from the shallow xenoliths and similar to ocean island basalts. Many of these same peridotites are enriched in the LREE. An inverse correlation between temperature of equilibration and 87Sr86Sr in these garnet peridotites may reflect infiltration of relatively low 87Sr.86Sr melts into refractory relatively high 87Sr/86Sr wall rock (lithosphere?). These high-temperature, low 87Sr/86Sr melts were possible parental to the megacrystalline inclusions associated with the garnet peridotites. Overall, our data for the Colorado Plateau suggest that high depleted, shallow lithosphere is underlain by material enriched in incompatible elements. This latter material was located near the inferred Oligocene base of the lithosphere and its composition may reflect mixing between asthenosphere and lithosphere. Melting of this material as a consequence of warming of the Colorado Plateau lithosphere during the late Tertiary may explain the origin of the Hopi Buttes alkaline magmas. The host minettes are isotopically similar to the bulk earth and their major element chemical variation can be explained by fractional crystallization. However, isotopic heterogeneity in the same eruptive center and decreasing Nb and Ta abundance with magmatic evolution indicate a complex petrogenesis involving exotic phases. The variation of isotopic composition with trace element ratios is inconsistent with bulk assimilation of crust, and we suggest that it reflects melting of a veined source containing phlogopite, apatite, and possibly a Nb-Ta-rich phase in addition to normal peridotite minerals. ¿ American Geophysical Union 1990