A COMPOSITE GEOLOGIC AND SEISMIC PROFILE OF THE LITHOSPHERE BENEATH THE POTRILLO VOLCANIC FIELD, SOUTHERN RIO GRANDE RIFT, NEW MEXICO

The diverse xenolith suite from the Potrillo volcanic field in the southern Rio Grande rift, together with velocity models derived from seismic data, offers an opportunity to place firm constraints on the composition of the lithosphere. We have calculated seismic velocities of crustal and mantle xenoliths from the Potrillo volcanic field using modes, mineral compositions, pressure and temperature estimates, and elasticity data. The pressure, temperature, and velocity estimates from xenoliths are combined with sonic logs and stratigraphy estimated from drill cores and surface geology to produce a geologic and velocity profile through the crust and upper mantle.

Lower crustal xenoliths include garnet granulite, two-pyroxene granulite, charnokite, and anorthosite. Metagabbro and amphibolite account for only a small fraction of the lower crustal xenoliths, suggesting that a basaltic underplate at the crust-mantle boundary is not currently present beneath the southern Rio Grande rift. Calculated velocities for garnet granulite are between ca. 6.9–8.0 km/s, depending on garnet content. These results suggest that velocities of >7.00 km/s and a layered structure, which are often attributed to underplated mafic rocks, can also be characteristic of alternating garnet-rich and garnet-poor metasedimentary rocks. Because the lower crust appears to be composed largely of metasedimentary granulite, which requires deep burial of upper crustal materials, we suggest the initial construction of the continental crust beneath the Potrillo volcanic field occurred by thickening of supracrustal material in the absence of large scale magmatic accretion.

Mantle xenoliths include spinel lherzolite and harzburgite, dunite, and clinopyroxenite. Calculated P-wave velocities for peridotites range from 7.75 km/s to 7.89 km/s, with an average of 7.82 km/s. This velocity is in good agreement with previous refraction and reflection studies that report Pn velocities of 7.6–7.8 km/s throughout most of the Rio Grande rift. These calculations suggest that the low Pn velocities are the result of high temperatures and low pressures due to thin crust, as well as a fertile, Fe-rich, bulk upper mantle composition. Partial melt or metasomatic hydration of the mantle lithosphere are not needed to produce the observed Pn velocities.