Geochemical and tectonic evolution of the Colorado Plateau mantle lithosphere: Evidence from Grand Canyon mantle xenoliths

Mantle xenoliths from the Grand Canyon volcanic field were analysed to characterize the compositional and temperature history of the mantle lithosphere beneath the southwestern margin of the Colorado Plateau. Xenoliths were collected from Quaternary basanites at three localities (Mount Emma, Toroweap Flow, Vulcan's Throne). Seventeen Cr-diopside peridotites, one Al-augite clinopyroxenite, and two garnet websterites were studied in detail. Interpretations are based on careful textural analysis combined with detailed electron probe traverses and determinations of trace elements of minerals and bulk rocks and of Sr, Nd and Pb isotopic ratios of clinopyroxenes.; Peridotites and constituent minerals have refractory compositions (low Ti and Na). Peridotite fabrics are dominated by orthopyroxene porphyroblasts up to 2.5 cm in length that occur in three unusual and distinctive textures (granoblastic, megacrystic and poikiloblastic). Orthopyroxene texture, bulk rock and mineral compositions, and calculated equilibrium temperatures are systematically correlated. Granoblastic and megacrystic peridotite minerals are more fertile and yield higher temperatures than those in poikiloblastic peridotites. Most pyroxene thermometers yield temperatures of 800--1000°C for peridotites. Zoned poikiloblastic orthopyroxene porphyroblasts with interiors low in Al and Cr yield anomalously low Al-and-Cr-in-orthopyroxene temperatures.; 87Sr/86Sr ranges from 0.703927 to 0.704697, and ENd , from -2.2 to +54. Six of the eight ENd exceed +14.3. 206Pb/204Pb, 207 Pb/204Pb and 208Pb/204Pb range from 18.472 to 18.635, 15.541 to 15.572 and 38.351 to 38.522 respectively. Sr and Nd concentrations and isotopic ratios are not correlative with orthopyroxene textures and mineral compositions, but Na-poor clinopyroxenes in poikiloblastic peridotites have high 87Sr/86Sr. High 87Sr/86Sr ratios document the introduction of radiogenic Sr into subcontinental lithosphere. High positive ENd and low 206Pb/204Pb require long-term depletions in LREE and U relative to chondrites and suggest a Proterozoic depletion event.; The Grand Canyon mantle lithosphere is cold, old and refractory. The proposed history includes one or more episodes of depletion by partial melting. Some clinopyroxenes subsequently were enriched in LREE relative to chondrites. Amphibole and poikiloblastic, orthopyroxene textures and zonation record a complex history including fluid migration, metasomatism and late-stage heating. Al-augite and garnetiferous pyroxenites represent younger igneous bodies intruded into wallrock peridotite at or near the crust/mantle boundary.