The Powderhorn Complex is composed of carbonatite and several varieties of alkaline ultramafic rocks that include ijolite, uncompahgrite, pyroxenite, and nepheline syenite. Sr and Nd isotope data and bulk rock geochemistry collected in this study were combined with data from previous studies to gain further insight into the magma source and history of the rocks in this complex.
87Sr/86 Sr ratios range from 0.703106 to 0.703632 and εNd values from Premo and Lowers (2013) range from 1.4-3.1. The bulk-rock isotope data, when combined with whole-rock geochemistry, indicate that parent magmas for these rocks came from a similar mantle source with minor heterogeneity or that there was minor crustal contamination during emplacement. All the rocks in the complex are LREEs (light rare earth elements) enriched with concentrations from 100 times more than compared to primitive mantle and chondrite. On a variety of discrimination plots, the data for the different rocks define distinct fields, and only the trends for the pyroxenite samples are consistent with crystal fractionation. There is no convincing evidence that fractionation of the pyroxenite generated the other lithologies, contrary to previous investigations. This is also supported by the distinct mineralogic assemblages for each rock type which are not easily explained by fractional crystallization.
The Powderhorn Complex was formed by the generation of different magma compositions from a similar mantle source. Minor variations in source composition or different degrees of partial melting could explain the different compositions of the various rocks in the complex. It is also possible that these rocks were formed through a combination of these processes.
