New Mexico Geological Society Annual Spring Meeting — Abstracts

Alkaline igneous rocks of Chico Sill Complex in northeastern New Mexico fall on the trend of the Jemez lineament. These 37 – 20 Ma intrusive rocks are spatially associated with younger rocks of the Raton-Clayton volcanic field, but clearly sample a different magma source. A subset of the spectrum of intrusive rocks (including phonolite, phonotephrite, trachyte, and lamprophyre dike rocks) is included in this study and these rocks show overall enrichment in incompatible trace element concentrations. Phonolite is the most common rock type in the southeastern half of the sill complex and is the product of extreme fractional crystallization of a more mafic parent, but may not have evolved from the other lithologies studied. Trace element ratios and normalized-element plots suggest that at least two distinct differentiation trends produced phonolite, titanite fractionation played a role in differentiation, and a subduction component is absent from the phonolite source but may have contributed to other rocks. One odd feature is the enrichment of Zr as compared to other continental alkaline rock suites.

The subset of rocks studied shows initial Sr and Nd isotope ratios that are close to bulkearth values, with epsilon Nd in the narrow range of 2.1 to –1.5 (¹⁴³Nd/¹⁴⁴Nd between 0.51275 and 0.51256), and initial ⁸⁷Sr/⁸⁶Sr ratios in the slightly broader, but still clustered range of 0.7039 to 0.7060. These isotope ranges are similar to many ocean-island basalts. The trail of the data toward higher ⁸⁷Sr/⁸⁶Sr values suggests a probable mixing curve with granitic or sedimentary rocks of the upper crust, although the degree of contamination must have been small and the contaminant is poorly defined.