CARBONATITE FINGERPRINTS ON BLACK HILLS ALKALIC SUITE?

The Black Hills alkalic province includes an older (~58 Ma) subalkalic and two alkalic pulses of magmatic activity (~55-54 and ~49-46 Ma; Duke et al., 2002). Thirteen samples of diverse rock types straddle bulk-earth in Sr-Nd isotopic space (Duke and Frost, 2003). Major and trace element geochemistry supports isotopic results, suggesting common mantle-derived parent magma(s) for the Black Hills suite, with the possible exception of subalkalic rhyolites. Fractional crystallization of the parental magma(s) was an important process in the formation of the suite: AFM plots show a linear relationship from an iron-rich pyroxenite (clinopyroxene, apatite, calcite, Fe-Ti oxides, and phlogopite) to alkali rhyolites at the alkali apex; Harker diagrams also show arrays indicative of fractional crystallization. Nb and Zr variation is roughly linear from average primitive (or MORB) mantle through tephrites and latites to trachyte to phonolite. Alkali rhyolites bifurcate away from the trend to higher Nb; subalkalic rhyolites plot tightly among Proterozoic metasediments and extend through average lower continental crust to primitive mantle. REE concentrations are lowest in most of the subalkalic rhyolites and are most similar to those of exposed Proterozoic metasedimentary units. These characteristics indicate that the subalkalic rocks incorporated more lithospheric material than did the alkalic group.

The isotopic signatures and many trace element characteristics of the Black Hills alkalic suite are more like those of the Missouri Breaks and Porcupine Dome centers than those of other centers in Montana. In a plot of Ba/Nb vs. εNd, Black Hills samples cluster with the kimberlites and carbonatites from Montana, while other samples from Montana have lower εNd and much higher Ba/Nb.

The Bear Lodge carbonatite contains extremely high U and Th concentrations (ore grade) and LREE contents. All alkalic rock samples in the Black Hills also have high U, Th, and LREEs; in addition, carbonate is found in most alkalic samples. We suggest that trace element and isotopic signatures in the Black Hills alkalic suite result from partial, variable equilibration of carbonatitic fluid/magma with a mantle source in conjunction with fractional crystallization.