PETROLOGY OF COMPOSITE MANTLE XENOLITHS FROM DISH HILL, MOJAVE DESERT, CALIFORNIA: IMPLICATIONS FOR MANTLE HETEROGENEITY AND METASOMATISM

Nineteen samples of mantle xenoliths were collected to better understand the nature of the uppermost mantle beneath the central Mojave Desert of California. Samples were collected from within host rocks that included both basaltic lava flows and cored bombs from two Quaternary volcanic centers at Dish Hill (Siberia Crater) near Ludlow, California and the Cima Volcanic Field near Baker, California. The distinctive macroscopic feature of the Dish Hill xenoliths is their coarse grain size with average grain sizes of 0.5-1.5 mm in collected samples. The variety, compositional changes, and possible sources of the core materials were analyzed. Xenoliths are highly variable in nature, consisting mainly of lherzolite, with subequal amounts of olivine and both ortho- and clinopyroxene with subordinate spinel. Other minor amounts of upper lithospheric xenoliths consist of fused granite fragments. Lesser quantities of both olivine and spinel xenocrysts are the found throughout the area. The generally accepted hypothesis is that these dominant lithologies, such as the lherzolites, are accidental inclusions derived from a surprisingly heterogeneous upper mantle. The variable mineralogy and geochemistry of these xenoliths appears to largely reflect differences due to metasomatism. Further petrologic and geochemical studies of both the xenoliths and the host rocks will reveal the specific nature of these xenoliths and provide constraints on the composition and nature of the upper mantle in the region.