The characteristics of lower crust and upper mantle in the Cima volcanic field deduced from xenolith studies

Abstract:

A lithospheric model based on mineral chemistry, textures, and temperatures is used to interpret the seismic structure of the upper mantle and lower crust observed under the Cima Volcanic Field, CA. Seismic velocities calculated from xenolith compositions are used in conjunction with petrologic information to interpret geophysical models of the area. The lower crust is composed of mafic compositions and contains a high percentage of quenched partial melt. The combination of quenched partial melt and mafic composition explains the relatively low seismic velocities observed in seismic models. The mafic composition is consistent with a rift environment. Melt compositions, some with > 60 wt% SiO₂ are found in all types of Cima xenoliths, although pyroxenites and gabbros contain the largest amount. Pyroxenite from the uppermost mantle transitions into gabbroic compositions and plagioclase rich lithologies in the crust. Temperatures calculated for peridotite xenoliths range from ~ 950 to 1030˚ C. Plagioclase bearing samples have the lowest temperatures and are interpreted as residing in the immediate sub-Moho mantle. Plagioclase bearing lherzolite structurally overlies spinel bearing peridotite. Strain accumulation is most prevalent in plagioclase bearing peridotite and virtually absent from pyroxenites and gabbros. Seismic velocities calculated for peridotite xenoliths are faster than pyroxenite and gabbroic samples. Despite the chemical heterogeneity and complex history of the Moho transitional are most mantle is composed dominantly by peridotite. Very little lithosphere, rhelologically speaking, remains under the volcanic field. We interpret lithospheric dismemberment to be caused by hot mantle working northward from the Gulf of California.