TIMING OF CARBONATITE MAGMATISM IN THE BEAR LODGE ALKALINE COMPLEX, WY

We present the first absolute ages on economic REE-bearing carbonatite from the Bear Lodge alkaline complex. ⁴⁰Ar/³⁹Ar analyses of seven biotite and six orthoclase phenocrysts give a minimum age of 51.15 ± 0.04 Ma and maximum age of 51.6 ± 0.2 Ma on carbonatite magmatism. Cross-cutting field relationships suggest carbonatite intrusions were one of the latest intrusive phases throughout the complex. However, these are the oldest ages reported for Bear Lodge rocks. Previous geochronological studies have focused on relatively unaltered alkaline silicate rocks, which occur as small intrusions and represent only a small portion of intrusive rocks within the Bear Lodge complex. Potassium-ferric iron metasomatism, associated with carbonatite intrusions, is pervasive throughout the Bear Lodge dome. These new ages combined with field observations, suggest previously determined K/Ar and ⁴⁰Ar/³⁹Ar ages from silicate intrusions lacking potassic alteration, represent post-carbonatite alkaline magmatism.

One possible mechanism for carbonatite genesis is partial melting of carbonate-bearing mantle. If carbonatite and alkaline silicate melts are assumed to have the same mantle source, experimental studies suggest carbonatite melts should precede silicate melts. This is difficult to explain in many alkaline complexes where carbonatites are observed to be one of the final intrusive phases. Transitional rocks described as carbonate-bearing lamprophyre or aegirine-biotite silicocarbonatite have also been identified at Bear Lodge, and may represent a genetic link between carbonatite and late alkaline silicate magmatism.

An average carbonatite age of 51.3 Ma coupled with previously determined ages between 45.5 and 47.6 Ma (Duke, 2005), for relatively unaltered latite and trachyte intrusions, may suggest a second pulse of silicate magmatism following carbonatite emplacement into the earlier alkaline rocks which comprise a bulk of the Bear Lodge complex. Alkaline magmatism throughout the northern Black Hills is thought to have progressed westward between 58 and 46 Ma. Field relationships within the Bear Lodge suggest much of the alkaline magmatism within the dome pre-dates the 51.3 Ma carbonatite intrusions and may be coeval with Tertiary alkalic igneous centers to the east.