GEOCHEMICAL ANALYSIS OF MANTLE XENOLITHS OF THE HIGHWOOD MOUNTAINS, MONTANA

The Montana alkalic province hosts several distinctly potassic, mafic to felsic magmatic rocks including: the Highwood Mountains, Bearpaw Mountains, Eagle Buttes, Crazy Mountains and Sweetgrass Hills. These Eocene volcanic centers are interestingly confined to the Great Falls Tectonic Zone (GFTZ) and show similarity in volcanic and/or intrusive rock types including latite and minette or mafic phonolite magmas and their felsic derivatives. This study focuses on the mantle xenoliths of the Highwood Mountains, a deeply eroded intrusive complex comprising a variety of potassic, silica-undersaturated, mafic to felsic flows, dikes, sills and stocks. We chose to study these mantle xenoliths to obtain information on magmatic, pressure-temperature and chemical conditions in shallow levels in the sub-continental mantle lithosphere beneath the Highwood Mountains. In order to further understand why these potassic rocks are restricted to the GFTZ, previous research utilized major and trace element studies, and Rb, Sr, Nd and Pb isotopic studies. Elemental studies suggest that petrogenesis of these rocks involve partial melting of previously metasomatized peridotite by hydrous melts that were released from subducted Farallon Plate lithosphere. Addition of small amounts of an isotopically extreme component resulted in large ranges of isotopic composition of the Highwood Mountains rocks. These extreme characteristics were acquired through assimilative interaction with the Precambrian rocks comprising the crystalline basement of the GFTZ during their ascent. By understanding the petrogenetic history of the mantle xenoliths, we gain better insights on why these Eocene alkaline rocks are confined to the GFTZ. Additionally, this study provides information on the occurrences of these rocks along GFTZ and the Cheyenne Belt and the relationship between the two regions.