CONTRASTING CHEMICAL SIGNATURES BETWEEN DIVERSE GROUPS OF INTRUSIVE IGNEOUS ROCKS, NORTHEAST NEW MEXICO

Previous workers have identified a diverse suite of Tertiary-age intrusive rocks (sills and dikes) in northeast New Mexico that includes several distinct groupings based on chemistry, mineralogy and age. This report concentrates on the chemistry of three of these groups that show remarkable similarity in trace element composition as depicted on normalized element plots. Specifically, mafic dikes (lamprophyre), trachyte (including the Slagle Trachyte and spatially associated syenite but excluding syenite sills of the Philmont Scout Ranch), and phonotephrite/tephrite, share common depletions of alkalis (Cs, Rb, K), and a trough at Nb-Ta with normalized Nb ≈ Ta. The compositions of these groups are distinct from other intrusive rocks in the region, specifically the least-evolved Chico Phonolite which does not show significant alkali depletion, lacks a trough at Nb-Ta and displays fractionation of Ta with respect to Nb. Although sparsely analyzed, a third group, biotite-trachyandesite, shares alkali-depletion with the first groups, yet lacks a Nb-Ta trough and shows relative depletion of Th. Phonolite and trachyte show enrichment of Zr and Hf that is absent in other groups. All of these groups lack anomalies in Eu or Sr, and are LREE and LILE enriched

This diverse suite of intrusive rocks requires at least two sources: lamprophyre, trachyte, and phonotephrite require a source with a significant K-bearing residual phase (phlogopite) and arc or continental crust component; phonolite requires a source that lacks a subduction signature or crust component, and lacks a significant residual K-bearing phase. The Chico Hills carbonatite is most closely related to the most-evolved Chico Phonolite. A third source may be required to account for the biotite-trachyandesite. The fractionation of Ta to Nb in phonolite and troughs at P and Ti suggest removal of variable amounts of apatite and Ti-bearing phases in all of the groups, and of titanite in phonolite.