Geochemical and Isotopic Evidence for Melting and Erosion of Wyoming Craton Mantle Lithosphere Prior to 48 Ma

Trace-element geochemistry of Cretaceous-Tertiary Great Plains igneous rocks supports isotopic data that reveal a sequence of digestion of lithospheric mantle followed by intrusion of dominantly asthenospheric magmas. Multiple Archean, Proterozoic, and Phanerozoic subduction events beneath the Wyoming craton concentrated Ba and K within the underlying mantle lithosphere, resulting in earliest Cretaceous-Tertiary lithospheric melts with fingerprints of high K, high Ba/Nb and negative epsilon-Nd, but low U, Th, total REE, and less extreme values of LREE/HREE. Youngest (Eocene-Oligocene) magmas were kimberlite and carbonatite, with high U, Th, LREE, extremely high LREE/HREE, and positive epsilon-Nd, but with high-T xenoliths from depths of only 150 km (Carlson et al., 1999). Importantly, in the entire Wyoming craton, the Homestead kimberlite is the only one of K-T age that has transported a diamond—a single micro-diamond discovered. The shallow low-T to high-T xenolith transition, lack of diamonds, and changing magma geochemistry, suggest that a significant portion of the mantle lithosphere beneath the Wyoming Archean craton must have been consumed prior to the ≤48 Ma kimberlite eruptions. In contrast, the earliest phase of Cretaceous magmatism in Arkansas was explosive diamond-containing lamproite (~102 Ma) with a Proterozoic lithospheric isotopic signature (Lambert et al., 1995). In Arkansas, there was no earlier subalkalic magmatism, and no evidence of slow digestion of the mantle lithosphere, although later magmatism trended toward higher positive epsilon-Nd values (i.e. larger asthenospheric component). Removal by melting of a significant portion of the Wyoming mantle lithosphere during late Cretaceous-early Tertiary magmatism, along with heating, may have helped promote lithospheric “relaxation” related to extension further west between 53 Ma and 49 Ma, followed by more facile penetration by asthenospheric magmas, an idea proposed to explain the time-source trend and the linear array of kimberlites and carbonatites (Duke, 2009). New trace-element data from Montana kimberlites and carbonatites, Judith Mountains, MT, Rattlesnake Hills, WY, and Black Hills, SD, generally fit the above model, showing pervasive early lithospheric melts followed by asthenospheric melts, culminating in Fe,Ca - rich kimberlite and carbonatite volcanism at the eastern edge of the province. Later phonolite melts were also asthenospherically derived, and geochemically similar to carbonatite, particularly in high Th contents, LREE, total REE, and high LILE/HFSE. Highest Th contents are in Bear Lodge carbonatite (up to 775 ppm) and Black Hills phonolite (up to 104 ppm), contrasting with lower Th contents of lithospherically derived magmas. The Judith Mountains resemble other lithosphere-derived centers such as the Highwood and Bearpaw Mountains (MT), but Rattlesnake Hills (WY) are geochemically and isotopically unique. Ongoing work includes precise age determinations to assess timing of transition from lithospheric to asthenospheric sources in various centers.