Ion microprobe analyses of clinopyroxene from predominantly spinel-bearing peridotite xenoliths found in volcanic rocks of the Colorado Plateau (Buell Park, Green Knobs localities) and the Basin and Range (Black Canyon, Lunar Crater localities) provinces are highly variable in light and heavy rare earth element (LREE, HREE), Sr, Zr, and Ti contents. Abundances of these elements vary by 2--3 orders of magnitude. The LREE are fractionated from the HREE such that relatively LREE-enriched and LREE-depleted inclusions occur at all localities. However, LREE-depleted clinopyroxenes predominate at the Colorado Plateau localities, whereas LREE-enriched clinopyroxenes or those with chondritic REE patterns predominate at the Basin and Range localities. In addition to fractionation of the LREE from the HREE, the high field strength elements Zr and Ti are generally depleted relative to REE of similar compatibility. Although some of this depletion reflects subsolidus partitioning between orthopyroxene and clinopyroxene, the extremely low Ti abundances (50--500 ppm) of some clinopyroxenes must reflect very low bulk rock contents of Ti. This depletion does not correlate with LREE/HREE ratios.

The bulk of the peridotites from the Colorado Plateau are interpreted to be residues from partial melting; extremely low Ce/Sm>cn ratios (less than 0.1) are similar to these ratios in clinopyroxenes from abyssal peridotites and require that the melting process in some cases approached ideal fractional melting; the very low Sr content of these clinopyroxenes (less than 10 ppm) suggest that clinopyroxene was not a residual phase. The association of these peridotites with LREE-depleted websterites which may be crystal segregates from a magma similar to modern mid-ocean ridge basalts reinforces the link between modern oceanic lithosphere and the upper mantle beneath the Colorado Plateau. In contrast, the clinopyroxenes from the Basin and Range tend to have relatively high Ti contents (greater than 2000 ppm) and only moderate fractionation of the LREE from the HREE or to be LREE-enriched and have low Ti contents (as low as 100 ppm) but high Sr contents (to 220 ppms). The former clinopyroxenes are derived from fertile peridotites which could yield typical basaltic magmas upon melting or may have equilibrated with such magmas in the upper mantle. The latter clinopyroxenes are from metasomatic peridotites which likely reacted with a carbonatitic melt. These features may be a consequence of widespread Cenozoic magmatism of the Basin and Range Province. ¿ American Geophysical Union 1993