WHOLE-ROCK GEOCHEMISTRY, NEODYMIUM AND LEAD ISOTOPIC COMPOSITIONS OF XENOLITHS OF THE HIGHWOOD MOUNTAINS, MONTANA

The Great Falls Tectonic Zone (GFTZ) is defined as a zone of basement geophysical anomalies identified as a suture zone between the Archean Medicine Hat Block to the northwest and the Wyoming Craton to the southeast that collided in the Proterozoic. Confined within the GFTZ is the northeast-trending Montana alkalic province (MAP) that hosts petrogenetically important Eocene potassic, mafic to felsic magmatic rocks. The MAP volcanic centers can be found in multiple areas including the Highwood Mountains, Bearpaw Mountains, Eagle Buttes, Crazy Mountains and Sweetgrass Hills. These Eocene rocks show similarity in volcanic and/or plutonic rock types including latite and minette (mafic phonolite) magmas and their felsic derivatives. In this study, we focus on rock samples from the Highwood Mountains, a deeply eroded complex comprising both intrusive and extrusive rocks. This region is characterized by an older series of quartz-normative latites overlain intruded by a variety of potassic, silica-undersaturated, mafic to felsic flows, dikes, sills and stocks on. Previous studies suggest the petrogenesis of these rocks involve partial melting of metasomatically modified asthenospheric peridotite, caused by the release of hydrous melts by the subducted Farallon plate lithosphere. Subsequently, these magmas underwent assimilative interaction with geochemically extreme ancient lithospheric mantle during their ascent. The influence of the subducted slab is recognized by LILE enrichment. Additionally, Sr, Nd and Pb isotopic compositions of the rocks imply the influence of ancient, isotopically extreme lithologies. This study attempts to gain a better understanding of the magmatic, chemical and pressure-temperature conditions in the shallow subcontinental lithospheric mantle beneath the Highwood Mountains. Also, this study aims to understand why these potassic rocks are confined to the GFTZ. Thus, our effort focuses on major and trace element studies, and Nd and Pb isotopic studies.