New Mexico Geological Society Annual Spring Meeting — Abstracts

 Whole rocks and 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 felsic 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, northeast-trending suture zone situated between the Colorado Plateau and the Rio Grande rift. Bandera basalts have OIB like trace element signatures (Peters et al., 2007).

Whole rock major elements show little variation suggesting minimal crystal fractionation (1-5%). However, trace element variation within Bandera basalts would require fractionation of approximately 10-30% of the original magma to produce observed trace element enrichments. Whole rock trace elements appear to result from the mixing of two compositional endmembers.  Major element variations of olivine hosted melt inclusions are consistent with variations resulting from crystal fractionation. However, trace element variations of melt inclusions are consistent with magma mixing or crustal assimilation. 

Variable ⁸⁷Sr/⁸⁶Sr and ²⁰⁶Pb/²⁰⁴Pb ratios are inversely correlated, confirming involvement of at least two sources in the petrogenetic history of the Bandera flow. ⁸⁷Sr/⁸⁶Sr ratios range from 0.7028 to 0.7040 and ²⁰⁶Pb/²⁰⁴Pb ratios range from 19.017 to 19.307. Plagioclase and olivine phenocrysts, and quartz xenocrysts, have ⁸⁷Sr/⁸⁶Sr ratios that differ from 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 that open system modification of Bandera magmas occurred at crustal depths, more consistent with crustal contamination rather than magma mixing. Results of this study may add insight to the effects of crustal contamination in the generation of continental alkali basalts more regionally and throughout the western United States

References:

1. Peters, T.J., Menzies, M., Thirlwall, M., Kyle, P.R., 2007, Zuni-Bandera volcanism, Rio Grande, USA – Melt formation in garnet- and spinel- facies mantle straddling the asthenosphere- lithosphere boundry. Lithos 102, p. 295-315