The Cima volcanic field in southeastern California is one of many isolated late Cenozoic basaltic volcanic fields in the Basin and Range province of the western U.S. Cima lavas are dominantly hawaiites like those in some other such fields (Best and Brimhall, 1974; Vaniman et al., 1982) , but Cima differs from the other fields in having abundant mafic and ultramafic xenoliths in lavas of hawaiite composition. The xenoliths give direct information about complex events of melting and fractionation

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... that occurred in the mantle before and during generation of hawaiite magmas, and about the nature of the mantle source of the hawaiites. The Cima volcanic field comprises more than 50 vents and associated flows that were erupted over a period of at least 7 m.y. (Turrin and others, 1985). The tephra and flows are of small volume. Detailed studies (Breslin, 1982) of flows and tephra from two coalescing cones show substantial differences in the degree of silica saturation between the cones and show large ranges within a single flow in the ratios of molecular Mg/Mg+Fe (mg-ratio), 40-51, and normative an/an+ab (an-ratio), 22-45. The volcanic field as a whole has the chemical features of alkaline basaltic suites described by Thompson (1974) and Miyashiro (1978) as "straddle-types"; that is, the lavas vary from ne-to hy-normative (Fig. 1) . There is no systematic relation between ages of lavas and either mgor an-ratio. Similarly, there is no correlation between the presence of xenoliths and age, an-or mgratio (Fig.2) , or normative ne. A small number of analyzed lavas (Semken, 1984; Breslin, 1982) have MORB-like isotopic compositions (ejvj^j +9 to +10; egj.-21 to-22), and LREE-enriched compositions. We separated the xenoliths into four groups based on their mineralogy: (1) Ci^diopside group of spinel Iherzolite and websterite; (2) Cr-diopside group rocks with introduced plagioclase; (3) greenpyroxene (not Cr-diopside) group of websterite, gabbro, and microgabbro; and (4) Al-augite group of clinopyroxenite, gabbro, and microgabbro. Common mineral assemblages and average mineral compositions are given in Table 1 . Rocks of the Cr-diopside group are refractory upper mantle Iherzolites and websterites typical of basalt xenolith assemblages the world over. The rocks usually are medium grained with allotriomorphic-granular textures and common strain lamellae in olivine. Porphyroclastic or tabular textures are less common. Many rocks of the Cr-diopside group were invaded by mafic melts from which plagioclase crystallized. These rocks form the second group, in which plagioclase commonly forms thin, irregular stringers in peridotite or pyroxenite. The rocks generally have a hybrid texture of metamorphic peridotite with igneous felsic minerals. In some samples deformation and recrystallization to a mosaic or tabular texture followed crystallization of plagioclase.