Geochemistry, origin, and U-Pb zircon ages of the Sierra Cuchillo laccolith, Sierra County, New Mexico
— Gary S. Michelfelder and Nancy J. McMillan

Eocene-Oligocene volcanism in New Mexico, Colorado, Arizona, and west Texas is commonly thought of as regional ash flow tuffs associated with Rio Grande rift calderas, although intermediate-composition volcanism also built stratovolcanos. The Sierra Cuchillo laccolith in Sierra County, New Mexico, represents a third, smaller type of Eocene-Oligocene magmatism. The laccolith is composed of a zoned porphyritic granodiorite to quartz monzonite that has undergone metasomatic alteration replacing hornblende, biotite and plagioclase with epidote and chlorite, creating three distinct mineralogical zones. U-Pb zircon dating of the laccolith yielded crystallization ages of 39.28 + 0.78 and 38.0 + 1.9 Ma (2σ error) as determined by LA-ICPMS. Other intrusions in the area that were dated include the Vindicator sill at 37.8 + 0.34 Ma and the Willow Springs dome yielding an age of 28.0 + 0.31 Ma. Two volcanic sequences present in the area, the dacite-rhyolite sequence and the latite-andesite sequence, yielded ages of 36.9 + 0.5 Ma and 36.3 + 0.4 Ma respectively. Trace element and geochronologic data are consistent with correlation of the Sierra Cuchillo laccolith to the Vindicator sill (38 Ma) 0.3 km to the north and to the latite-andesite volcanic sequence (36.3 Ma) of Jahns et al. (2006) located to both the east and west of the Sierra Cuchillo on the down-dropped hanging walls of the horst-bounding normal faults. The Willow Springs dome (28.0 Ma) and the daciterhyolite volcanic sequence (36.9 Ma) of Jahns et al., (2006), both have much lower Eu concentrations relative to other rare earth elements (measured as the Eu/Eu* ratio of 0.10 and 0.54, respectively), in contrast to the Sierra Cuchillo laccolith, which has much higher Eu/Eu* ratios, ranging from 0.97 to 1.04. The dacite-rhyolite sequence, while only slightly younger than the laccolith, is not directly related to the laccolith; the Willow Springs dome clearly represents a younger phase of magmatism. The zones of alteration present within the Sierra Cuchillo laccolith are interpreted as the result of circulation of internal magmatic fluids during cooling. This is evident by the lack of a hydrothermal contact aureole around the laccolith and by the increased alteration of plagioclase and hornblende in the interior zones. Whole-rock geochemical data suggest that the soluble major and trace elements are randomly distributed throughout the laccolith, indicating pervasive mobilization by late-stage magmatic fluids. Nb concentrations in the Sierra Cuchillo laccolith (5-9 ppm) are similar to concentrations of basaltic magmas from a subduction-modified lithospheric mantle source (ca. 8 ppm) but lower than basalts from ocean island basalt-modified lithospheric mantle (ca. 20 ppm) and asthenospheric mantle (ca. 32 ppm) sources as defined by McMillan (1998). Assimilation of continental crust is indicated by zircon cores with Precambrian ages (1.4 – 1.6 Ga). ⁸⁷Sr/⁸⁶Sr initial ratios of the Sierra Cuchillo laccolith (0.706461 - 0.706804) also reflect assimilation of continental crust. The parental mafic magma must have had low Nb and low ⁸⁷Sr/⁸⁶Sr ratio, and is interpreted as being derived from subduction-modified lithospheric mantle.