The Chico Sill Complex, Northeast New Mexico: A case for late-stage phonolite-carbonatite melt immiscibility

The Chico Sill Complex (Northeast New Mexico) is the result of magmatic episodes from ~37 Ma to 20 Ma and produced a diverse and compositionally discontinuous suite of mostly intrusive silica-saturated and silica-undersaturated rocks. The Chico Phonolite was emplaced ~ 26 to 20 Ma in dikes and large sills. Sills of different composition may be stacked 2 and 3 thick. They bear no chemical affinity to the bulk of other rocks in the complex based on normalized trace element diagrams. Candidates for parent melts are scarce. At least two distinct trends are noted in Zr-La Space. A higher-Zr trend includes dikes and three sills and may represent evolution of a primary phonolite melt. The most-evolved sill (Point of Rocks Mesa) is the last-gasp of phonolite magmatism and likely the companion immiscible silicate for a calciocarbonatite dike 10 km distant. Calciocarbonatite is a miniscule portion of the complex (outcrop limited to a few hundreds of m2). The carbonate mineral is impure calcite (Mn>Fe>Sr>Mg>>Ba) in matrix goethite. Other minerals present include barite, pyrite, and REE minerals containing Ca and Ca-Ti next to the calcite. Normalized Ba, Th, REE, Y and Sr show 100 times enrichment in the carbonatite. Mineralogy, texture, and O-C isotopes suggest that the original carbonatite melt may have been more sodic and experienced alteration similar to that of lavas at Oldoinyo Lengi. Owing to the distance between outcrops, the separation of the melts (and phonolite evolution) occurred at much greater depth.