Deep crustal rocks include a variety of granulites of both high-silica (felsic) and low-silica (mafic) compositions. These likely took less than three days to reach the surface from their place of origin, and show pristine composition and texture. Their characteristics show that they were little altered from their formation 1.6 to 1.8 billion years ago, other than some reheating during the opening of the Rio Grande rift. Middle crustal xenoliths are Oligocene (26–27 Ma) in age and suggest a large unexposed batholith underlying the volcanic field. Metagabbro and amphibolite are notable scarce in the lower crustal xenoliths, suggesting that underplating has not taken place in this part of the rift.