SYNEXTENSIONAL MAGMATISM IN THE PREHISTORIC TRACKWAYS NATIONAL MONUMENT, SOUTHERN RIO GRANDE RIFT

Prehistoric Trackways National Monument (PTNM) is situated on the eastern edge of the Robledo Mountains in the southern Rio Grande rift in Doña Ana County, New Mexico. The range consists of Permian strata overlain by Paleocene clastics, Eocene volcaniclastics, and Plio-Pleistocene Camp Rice formation. U-Pb dating of zircon from one of several rhyolite sills yielded a ²³⁸U/²⁰⁶Pb weighted mean age of 34.8 ± 2.1 Ma (2σ). A felsic hypabyssal intrusion at the summit of the Picacho Mountain rhyolite dome, located 5 km south of the PTNM, has a U-Pb zircon age of 36.1 ± 0.7 Ma. The rhyolites overlap in age with the Organ Mountains caldera, located across the valley. Basalts are exposed as small (36 m²) plugs and dikes (~150 m long), but the largest outcrop forms the top of a bluff overlying middle Permian limestone and is ~450 m long and at least 50 m thick. It consists of columnar basalt and massive basalt with ultramafic and pyroxenite/plagioclase xenoliths up to 6 cm in width. It is unclear whether it is a sill or a lava flow as the upper contact is completely eroded. The lower contact is conformable with the underlying carbonate unit. ⁴⁰Ar/³⁹Ar dating of a basalt dike yielded a plateau age of 8.10 ± 0.05 Ma. The large flow or sill yielded a plateau age of 7.1 ± 0.04 Ma.

Ultramafic xenocrysts are rare and consist mainly of olivine and pyroxenes. Pyroxenite xenocrysts consist of orthopyroxene, clinopyroxene, varying amounts of spinel, olivine, and occasionally plagioclase, but the majority of the inherited plagioclase is in the form of scattered megacrysts within the basalt. Plagioclase megacrysts from the basalt have an average ⁸⁷Sr/⁸⁶Sr ratio of 0.70301, while the average ⁸⁷Sr/⁸⁶Sr ratio of the plagioclase phenocrysts from the basalt is 0.70324. Olivine phenocrysts in the host basalt have low forsterite content in both rim and core with little variation between the two, ranging from Fo₇₆ to Fo₆₅. The Mg# of the pyroxenes ranges from 81 to 69. Low forsterite values and low Mg# are likely due to the basalt being non-primitive.