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 mantle and underplating of the crust beneath a portion of the southern Basin and Range province. The upper mantle xenoliths include spinel peridótite and anhydrous and hydrous pyroxenite, some cut by igneous‐textured pyroxenite‐gabbro veins and dikes and some by veins of amphibole ± plagioclase. Igneous‐textured pyroxenites and gabbros like the dike rocks 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 reactions, episodic intrusion and infiltration of basaltic melts of varied sources into the mantle wall rock, and fractionation. Nd, Sr, and Pb isotopic compositions mostly of clinopyroxene and plagioclase mineral separates show distinct differences between mantle xenoliths (εNd = −5.7 to +3.4; 87Sr/86Sr = 0.7051 – 0.7073; 206Pb/204Pb = 19.045 – 19.195) and the igneous‐textured xenoliths (εNd = +7.7 to +11.7; 87Sr/86Sr = 0.7027 – 0.7036 with one carbonate‐affected outlier at 0.7054; and 206Pb/204Pb = 18.751 – 19.068), so that they cannot be related. The igneous‐textured pyroxenites and gabbros are similar in their isotopic compositions to the host basaltic rocks, which have εNd of +5.1 to +9.3; 87Sr/86Sr of 0.7028 – 0.7050, and 206Pb/204Pb 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 in the lower crust. Two samples of peridotite, one modally metasomatized by amphibole and the other by plagioclase, have isotopic compositions intermediate between the igneous‐textured xenoliths and the mantle rock, suggesting mixing, but also derivation of the metasomatizing magmas from two separate and distinct sources. Sm‐Nd two‐mineral “isochrons” yield apparent ages for petrographically identical rocks believed to be coeval ranging from ∼0 to 113±26 Ma, indicating the unreliability of dating these rocks with this method. Amphibole and plagioclase megacrysts are isotopically like the host basalts and probably originate by mechanical breakup of veins comagmatic with the host basaltic rocks. Unlike other Basin and Range localities, Cima Cr‐diopside group isotopic compositions do not overlap with tho