PHONOLITE AND CARBONATITE: A “COEVAL BREW” IN THE CHICO SILL COMPLEX, NORTHEAST NEW MEXICO

Episodes of magmatism in Northeast New Mexico from ~ 36 Ma to near the present, produced a diverse, and compositionally discontinuous suite of intrusive (older) and extrusive (younger) rocks. Phonolite of the Chico Sill Complex was emplaced ~ 20-26 Ma. Phonolite occurs in large sills and dikes where sills of different composition may be stacked 2 and 3 thick. They bear no chemical affinity to other alkaline rocks in the region based on normalized trace element diagrams; specifically the phonolite lacks the chemical signature of arc or of crustal interaction or K-bearing residual phase in the source. At least two distinct trends are noted, based on Zr/La. One of these trends, controlled by involvement of titanite, resulted in the final gasp of phonolite magmatism: Point of Rocks Mesa. Isotopic composition of Sr (initial at 25 Ma) and Nd fall in the narrow ranges of ⁸⁷Sr/⁸⁶Sr: 0.7045 – 0.7052, and ¹⁴³Nd/¹⁴⁴Nd: 0.51256 – 0.51265. The original phonolitic melt falls on the mantle array, trending toward an EM2 component.

Calcite carbonatite is a miniscule portion of the sill complex with outcrop area limited to a few hundreds of m². The intrusion is hosted in a phonotephrite sill that has intruded lower Cretaceous carbonates and shales. Previous workers have described a relatively simple mineralogy of calcite in a “limonite” matrix. X-ray diffraction, and EPMA analysis confirm the primary carbonate mineral is impure calcite (Mn>Fe>Sr>Mg>>Ba), and the Fe-oxide is primarily goethite. Other minerals present include barite, pyrite, and REE minerals containing Ca and Ca-Ti present in the goethite matrix. The presence of goethite, plus isotope values of carbon (d¹³C_PDB = -2.65) and oxygen (d¹⁸O_SMOW = 15.3), suggest that the texture is not primary. Pb isotopic ratios (²⁰⁶Pb/²⁰⁴Pb = 18.5, ²⁰⁷Pb/²⁰⁴Pb = 15.6, ²⁰⁸Pb/²⁰⁴Pb = 38.8) do not coincide with values of younger Raton-Clayton volcanic rocks in the region.

The normalized-element pattern of the carbonatite compares to silicate rocks in the sill complex by having the highest concentrations of Ba, Th, REE, and Y. The pattern produced by these elements, plus Sr, is parallel to that of the most evolved phonolite at Point of Rocks Mesa, and although it is located 10 km away, these patterns suggest that the carbonatite is derived from that melt, perhaps initially as an Na-rich liquid, not a separate melt.