ISOTOPIC AND TRACE-ELEMENT COMPOSITIONS OF UPPER-MANTLE AND LOWER CRUSTAL XENOLITHS, CIMA VOLCANIC FIELD, CALIFORNIA - IMPLICATIONS FOR EVOLUTION OF THE SUBCONTINENTAL LITHOSPHERIC MANTLE

Ultramafic and mafic xenoliths from the Cima volcanic field, southern California, provide evidence of episodic modification of the upper man tle and underplating of the crust beneath a portion of the southern Ba sin and Range province. The upper mantle xenoliths include spinel peri dotite and anhydrous and hydrous pyroxenite, some cut by igneous-textu red pyroxenite-gabbro veins and dikes and some by veins of amphibole+/ -plagioclase. Igneous-textured pyroxenites and gabbros like the dike r ocks also occur abundantly as isolated xenoliths inferred to represent underplated crust. Mineral and whole rock trace element compositions among and within the different groups of xenoliths are highly variable , reflecting multiple processes that include magma-mantle wall rock re actions, episodic intrusion and infiltration of basaltic melts of vari ed sources into the mantle wall rock, and fractionation. Nd, Sr, and P b isotopic compositions mostly of clinopyroxene and plagioclase minera l separates show distinct differences between mantle xenoliths (E-Nd=- 5.7 to +3.4; Sr-87/Sr-86=0.7051-0.7073; Pb-206/Pb-204=19.045-19.195) a nd the igneous-textured xenoliths (E-Nd=+7.7 to +11.7; Sr-87/Sr-86=0.7 027-0.7036 with one carbonate-affected outlier at 0.7054; and Pb-206/P b-204=18.751-19.068), so that they cannot be related. The igneous-text ured pyroxenites and gabbros are similar in their isotopic composition s to the host basaltic rocks, which have E-Nd of +5.1 to +9.3; Sr-87/S r-86 Of 0.7028-0.7050, and Pb-206/Pb-204 Of 18.685-21.050. The igneous -textured pyroxenites and gabbros are therefore inferred to be related to the host rocks as earlier cogenetic intrusions in the mantle and i n the lower crust. Two samples of peridotite, one modally metasomatize d by amphibole and the other by plagioclase, have isotopic composition s intermediate between the igneous-textured xenoliths and the mantle r ock, suggesting mixing, but also derivation of the metasomatizing magm as from two separate and distinct sources. Sm-Nd two-mineral ''isochro ns'' yield apparent ages for petrographically identical rocks believed to be coeval ranging from similar to 0 to 113+/-26 Ma, indicating the unreliability of dating these rocks with this method. Amphibole and p lagioclase megacrysts are isotopically like the host basalts and proba bly originate by mechanical breakup of veins comagmatic with the host basaltic rocks. Unlike other Basin and Range localities, Cima Cr-diops ide group isotopic compositions do not overlap with those of the host basalts.