GEOCHEMICAL VARIATIONS OF THE MCCARTYS FLOW, WEST-CENTRAL NEW MEXICO: EVALUATING THE MAGMATIC EVOLUTION OF A CONTINENTAL THOLEIITIC BASALT

Geochemical signatures of tholeiites erupted in continental settings may offer constraints regarding the mantle underlying continents and any modifications imposed by open-system effects. Tholeiitic magmas, generally characterized by lower incompatible element abundances, must be evaluated in detail as they can be highly impacted by secondary inputs unrelated to the mantle originally melted. Basaltic eruptions along the Jemez lineament, a chain of northeast trending Late Cenozoic volcanic centers in southwestern US, have been attributed to melt directly derived from mantle. The Zuni-Bandera volcanic field (ZBVF), located along the Jemez lineament in the transition zone between the Colorado Plateau and Rio Grande rift, consists of lavas with both alkalic and tholeiitic compositions. The McCartys flow is the youngest tholeiitic basalt (3.9 ± 1.2 ka) within the ZBVF and consists of three petrologically distinct units classified as olivine-phyric, plagioclase-phyric, and transitional. Whole-rock and olivine-hosted melt inclusion major element, trace element, and isotopic variations are used to determine if the McCartys flow results from partial melting of two distinct mantle sources or whether a single magma, generated by melting a single mantle source, has been affected by open-system modification. Whole-rock major element concentrations indicate that olivine-phyric basalts are least evolved (MgO 6.88-8.21 wt.%), while plagioclase-phyric basalts are most evolved (MgO 6.02-6.12 wt.%). Major element variations among the suite of McCartys basalts can be explained by ~16% olivine and plagioclase fractionation. Incompatible trace element variations however, require ~23% fractionation. Olivine-hosted melt inclusion major and trace element concentrations vary little throughout the basalt suite, while whole-rock ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd isotopes vary and are consistent with contamination involving a mafic source. Overall, results suggest the McCartys flow is the product of a single magma generated by partially melting a single mantle source that was subsequently contaminated at crustal depths bringing into question the accuracy of conclusions that attribute compositions to mantle sources exclusively.