A PETROLOGIC AND PETROCHEMICAL INVESTIGATION OF ALKALINE ROCKS IN THE POWDERHORN CARBONATITE COMPLEX, GUNNISON, COLORADO

The ~600 Ma Powderhorn complex is composed of carbonatite along with a suite alkaline igneous rocks. Previous investigations argued that these rocks were derived from a common mantle melt, and that fractionation of pyroxenite gave rise to ijolite, uncompahgrite, and nepheline syenite. In this investigation, previous and new bulk-rock geochemical and isotope (Sr, Nd) data were used to further test hypotheses on the origin and history of the alkalic rocks at Powderhorn.

New ⁸⁷Sr/⁸⁶Sr ratios for the alkaline suite vary from 0.703106 to 0.703632, and previously published ⁸⁷Sr/⁸⁶Sr values for pyroxenite and carbonatite range from 0.7026 to 0.7038 with ε_Nd values of +1.4 to +3.1. The range in the previous data was interpreted as slight crustal contamination of mantle melts, but we argue that the rocks originated from a mantle reservoir with minor differences in isotopic signatures. In addition, there is no evidence (e.g., crustal xenoliths or xenocrysts, mixed melt zones, and elevated concentrations of large-ion lithophile elements such as K and Rb) to indicate that parent magmas underwent crustal contamination.

All of the alkaline rocks in the complex are defined by unique and distinct mineral associations. New and existing bulk-rock geochemical data reveal that the rocks are LREE enriched, and have relatively flat HREE signatures. On a variety of discrimination plots, the data for the different rock types define distinct fields, and only the trends for pyroxenite samples are consistent with crystal fractionation. There is no convincing evidence that fractionation of pyroxenite generated the other alkaline rocks which is supported by the lack of mixed and transitional rock types and the varied isotopic signatures.

On the basis of the bulk-rock isotopic and geochemical data we propose that the alkaline rocks at Powderhorn were formed by partial melting of a mantle source with minor heterogeneity. Various magma compositions were likely produced by different degrees of partial melting of this melt reservoir, but the rocks do not appear to be related by fractional crystallization. Crustal contamination is not entirely ruled out, but the isotopic and geochemical signatures indicate contamination was minor, if at all.