Xenoliths from the Cima Volcanic Field, the seismic structure of an active plate margin, and the fate of the Walker Lane
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 are calculated from Cima xenolith compositions for the first time and used in conjunction with petrologic information to interpret the geophysical model developed by Hicks (2001). The lower crust is composed of mafic compositions and contains a high percentage of partial melt. The combination of partial melt and mafic composition explains the relatively low seismic velocities observed in the seismic model and the mafic composition is more consistent with a rift environment than intermediate compositions that would typically be associated with similar velocities. Quenched melt, some with > 60 wt% SiO2 can be found in any type of xenolith from Cima although pyroxenites and gabbros contain the largest amount. Pyroxenite concentrated in the uppermost mantle transition into gabbroic compositions and more plagioclase rich lithologies the crust. Temperatures calculated for peridotite xenoliths range from ∼ 950 to 1030°C and plagioclase bearing samples have the lowest temperatures and are interpreted as residing in the immediate sub-Moho mantle. Plagioclase bearing lherzolite must structurally overlie 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 recorded by the Cima xenolith suite, observations in Hicks' (2001) and this study indicate that the upper mantle is composed dominantly of peridotite. Results indicate that very little lithosphere remains under the volcanic field and its dismemberment is facilitated by hot mantle working its way north from the Gulf of California under the southwest US. ^
Cardon, Keith Preston, "Xenoliths from the Cima Volcanic Field, the seismic structure of an active plate margin, and the fate of the Walker Lane" (2008). ETD Collection for University of Texas, El Paso. AAI1458447.