Petrologic and Geochemical Analysis of Sierra Cuchillo Laccolith, Sierra County, New Mexico

The transition from subduction to extension in the Rio Grande Rift is still a hotly debated topic among igneous petrologists and structural geologists. There are two basic tectonic models for this transition, one based on southern New Mexico igneous stratigraphy, and the other based on Trans-Pecos, Texas stratigraphy. The New Mexican model suggests three distinct series of magmatism: (1) Laramide shortening, 80-50 Ma; (2) post-Laramide arc-like magmatism, 50-36 Ma; and (3) Rio Grande rift extension, 36-0 Ma. In contrast, Trans-Pecos model only suggests two series of magmatism: (1) Laramide shortening, 80-46 Ma, (2) Rio Grande Rift extension, 46-0 Ma.

The Sierra Cuchillo laccolith, dated at 49 Ma (McMillan, 1986, Am Min., 71: 625-631), was emplaced during the transition period between Laramide subduction and Rio Grande rifting. The main body is an albitized monzonite containing at least three concentric zones of alteration. Smaller satellite intrusions and a thick local section of poorly- studied volcanic rocks were hypothesized to be related to the main laccolith (McMillan, 1986); these ideas are being tested in the current project.

Three conclusions can be drawn from preliminary major and trace element analysis. First, two distinct suites of magma with different trace elements signatures are identified. One suite has low Nb and high Sr (8-10 Nb ppm; 566-979 Sr ppm); the other has high Nb and low Sr (18-40 Nb ppm; 14-79 Sr ppm). Second at least one volcanic sample can be tentatively correlated to a laccolith sample, suggesting that the laccolith was an eruptive center. Third, samples outboard of the main body have high K₂O/Na₂O ratios (4.77-11.96), supporting early models of K-metasomatism. However, K₂O/Na₂O ratios of samples within the laccolith (0.64-0.67) are similar to unmetasomatized volcanic rocks of similar age in southern New Mexico (0.50-1.12).