THE ROLE OF CRUSTAL ASSIMILATION DURING THE PETROGENESIS OF THE BANDERA FLOW, ZUNI BANDERA VOLCANIC FIELD, WEST-CENTRAL NEW MEXICO

Phenocrysts in young alkaline basalts from west central New Mexico are used to distinguish between magmas that result from melting of heterogeneous mantle sources and melting of a single asthenospheric source in which magmas have been variably contaminated with mafic crust during ascent. The 11.6 ka Bandera flow is the youngest alkaline basalt flow in the Zuni Bandera Volcanic Field (ZBVF), a volcanic field located along the Jemez Lineament, an 800 km long, northeast-trending suture zone situated between the Colorado Plateau and the Rio Grande rift. Bandera basalts have ocean island basalt like trace element signatures (Peters et al., 2007). Whole rock ⁸⁷Sr/⁸⁶Sr ratios and ²⁰⁶Pb/²⁰⁴Pb ratios correlate, adding insight to the mixing trend between two isotopic end-members. Isotopic signatures are highly variable with ⁸⁷Sr/⁸⁶Sr ratios ranging from 0.7028 to 0.7040 and ²⁰⁶Pb/²⁰⁴Pb ratios ranging from 19.017 to 19.307. Olivine-hosted melt inclusions, plagioclase phenocrysts and whole rock analyses constrain the end members involved in the petrogenesis of the Bandera magmas. Single crystal ⁸⁷Sr/⁸⁶Sr ratios from the Bandera flow are different from the host whole rock signatures. The ⁸⁷Sr/⁸⁶Sr ratio of a plagioclase phenocryst (0.70420±0.00003) differs from the host whole rock (0.70370±0.00002) suggesting open system modification of Bandera magmas occurred at crustal depths, more consistent with mafic crustal contamination. Results of this study allow evaluation of the effects of mafic crustal contamination in the generation of continental alkali basalts throughout the western United States.