GEOCHEMISTRY OF ALKALIC IGNEOUS ROCKS FROM THE JUDITH MOUNTAINS, MONTANA, AND RATTLESNAKE HILLS, WYOMING

The alkalic igneous centers of the Judith Mountains, MT, Rattlesnake Hills, WY, and Black Hills, SD, included subalkalic magmatism which preceded alkalic magmatism. These centers are Paleocene to Eocene in age, with Judith Mountain activity occurring between ~69 and 62 Ma (K-Ar), Black Hills between >58 to ≤46 Ma (⁴⁰Ar/³⁹Ar; Duke et al., 2002), and Rattlesnake Hills at ~44 Ma (K/Ar). In the Judith Mountains, the subalkalic pulse includes a ~67-69 Ma rhyolite, followed by diorite, syenite, phonolite (~67-65 Ma), and a late alkali rhyolite (~62 Ma). The Black Hills subalkalic series (~58 Ma) preceded the alkalic (~55-54 and ~49-46 Ma), although some subalkalic rhyolitic magmatism continued to 52 Ma. There is evidence of carbonatite from drill core in the Judith Mountains, and the Black Hills have carbonatite magmatic intrusions.

Major and trace-element geochemistry of the Judith Mountains, Rattlesnake Hills, and Black Hills shows similarities among rock types but differences among centers with respect to certain trace elements. Overall, in alkalic centers of the northern Great Plains, Th content is highest in carbonatites and phonolites. In the Judith Mountains, Th content ranges from 2 to 27 ppm, with lowest concentrations in rhyolites and highest concentrations in phonolites. In the Rattlesnake Hills, Th content ranges from 11 to 49 ppm, with the highest values in phonolites. Black Hills phonolites have the highest Th values of phonolites in all centers, with 104 ppm in the central Black Hills and 83 ppm Th in the Missouri Buttes phonolite, west of Devil’s Tower. A magmatic sample of carbonatite from the Black Hills contains 775 ppm, and high Th values in most Black Hills magmas are directly related to interaction with carbonatite. Phonolites may therefore be more closely linked to carbonatites than are other evolved magmas. Fractional crystallization of parental magma(s) was likely an important process at all three centers, as seen in Harker diagrams. Previous isotopic work from the Black Hills and Rattlesnake Hills suggests a common mantle-derived parent, with a dominant lithospheric component in older subalkalic groups. Ongoing research includes obtaining ⁴⁰Ar/³⁹Ar ages and isotopic data from these and other igneous centers.