Petrology of Lower Crustal and Upper Mantle Xenoliths from the Cima Volcanic Field, California

Basaltic rocks of the Cima volcanic field in the southern Basin and Range province contain abundant gabbro, pyroxenite, and peridotite xenoliths. Composite xenoliths containing two or more rock types show that upper-mantle spinel peridotite was enriched by multiple dike intrusions in at least three episodes; the mantle was further enriched by intergranular and shear-zone melt infiltration in at least two episodes. The oldest dikes, now metamorphosed, are Cr-diopside websterite. Dikes of intermediate age are most abundant at Cima and consist of igneous-textured websterite and two-pyroxene gabbro and microgabbro of tholeiitic or calcalkalic parentage. The youngest dikes are igneous-textured clinopyroxenite, gabbro, and olivine microgabbro of alkalic parentage. The dikes in peridotite are interpreted as parts of a system of conduits through which tholeiitic (or calcalkalic) and alkalic magmas fed lower-crustal intrusions, which are represented by abundant xenoliths of the same igneous rock types as observed in the dikes. Mineral assemblages of dikes in peridotite indicate that an enriched uppermost mantle zone no thicker than 15 km could have been sampled. Because of their high densities, the gabbros and pyroxenites can occupy the zone immediately above the present Moho (modeled on seismic data as 10-13 km thick, with Vp 6.8 km/s) only if their seismic velocities are reduced by the joints, partial melts, and fluid inclusions that occur in them. Alternatively, these xenoliths may have been derived entirely from beneath the Moho, in which case the Moho is not the local crust-mantle boundary.