ZIRCON GEOCHRONOLOGY AND GEOCHEMISTRY INSIGHTS INTO A SHONKINITE DIKE IN THE COLOSSEUM MINE AREA, SOUTHEASTERN MOJAVE DESERT, CA

Mafic alkaline magmatism associated with the Mountain Pass rare earth element (REE) deposit is sparse in volume, but widespread in the southeastern Mojave Desert of California. Here we provide new zircon data for a shonkinite dike from the Colosseum mine area, 11 km northwest of the Mountain Pass mine. Colosseum contains gold deposits hosted in Cretaceous rhyolite breccia pipes, which are much younger than the Mesoproterozoic (ca. 1.4 Ga) alkaline and carbonatite intrusions at the Mountain Pass REE mine. Shonkinite dikes at Colosseum are mostly aphyric, but analysis of zircon from one sparsely porphyritic dike enables a preliminary comparison to Mountain Pass zircons. High-spatial resolution (25 micron spot size) zircon data were collected with a SHRIMP-RG ion microprobe. Due to the very low uranium concentrations (20-45 ppm), zircon U-Pb ages have relatively large errors. Based on 11 concordant analyses and excluding 14 other analyses based on discordance or very high (>100 Myr) single spot age errors, the error-weighted mean ²⁰⁶Pb/²³⁸U-²⁰⁷Pb/²³⁵U concordia age is 1461± 20 Ma and the error-weighted mean ²⁰⁷Pb/²⁰⁶Pb age is 1,453 ± 44 Ma. Within error, these ages overlap the older end of the known age range of zircons from the Mountain Pass mafic alkaline intrusions (ca. 1,390-1,440 Ma). Three concordant, inherited Paleoproterozoic zircons were also documented in the Colosseum sample, with ²⁰⁷Pb/²⁰⁶Pb ages of 1,623 ± 18 Ma, 1,643 ± 29 Ma, and 1,773 ± 30 Ma. The inherited Paleoproterozoic zircons have much higher U (190-470 ppm), and a distinctly negative Eu anomaly in their chondrite-normalized REE spectra (0.1-0.3 ppm Eu), which is lacking in the primary Colosseum zircons (0.9-2.6 ppm Eu). The REE concentrations and chondrite-normalized spectra for the Colosseum zircons are indistinguishable from the Mountain Pass zircons; however, the Colosseum zircons have much lower U and Th (~20-50 ppm each) than Mountain Pass zircons (~100-800 ppm U, ~100-1,400 ppm Th). Because elevated Th is a distinguishing feature of the Mountain Pass system (zircons and magmas), its lack at Colosseum is notable. Further petrogenetic studies are required to assess potential links between mafic alkaline intrusions at the two sites.