Mafic Alkaline Magmatism and Rare Earth Element Mineralization in the Mojave Desert, California: The Bobcat Hills Connection to Mountain Pass

• Published in: Geochemistry, geophysics, geosystems : G3 geophysics, geosystems : G3, 2024-01, Vol.25 (1), p.n/a
• Main Authors: Watts, K. E., Miller, D. M., Ponce, D. A.
• Format: Article
• Language: English
• Subjects: Basement rock; Crystallization; Deserts; Dikes; Economic importance; Embankments; Geochemistry; Geochronology; Geochronometry; Geology; Hills; Isotope composition; Magma; Mineralization; Mountains; Oxygen; Oxygen isotopes; Precambrian; Rare earth elements; Rock; Rocks; Zircon
• ISSN: 1525-2027; 1525-2027
• DOI: 10.1029/2023GC011253

Occurrences of alkaline and carbonatite rocks with high concentrations of rare earth elements (REE) are a defining feature of Precambrian geology in the Mojave Desert of southeastern California. The most economically important occurrence is the carbonatite stock at Mountain Pass, which constitutes the largest REE deposit in the United States. A central scientific goal is to understand the genesis of the carbonatite ore body in the context of widespread REE‐rich igneous activity. A swarm of mafic alkaline (shonkinite) dikes has been mapped and sampled at Bobcat Hills, 65 km southeast of the Mountain Pass mine. Whole‐rock geochemistry and zircon geochronology demonstrate a clear affinity to the ca. 1.4 Ga Mountain Pass intrusive system. Bobcat Hills dikes have comparably high REE concentrations (La ∼1,000× chondritic) and an error‐weighted mean 207Pb/206Pb zircon crystallization age of 1,426 ± 2 Ma (2σ). Unlike the alkaline intrusions at Mountain Pass, which have abundant inherited zircon from Paleoproterozoic basement rocks and crustally influenced oxygen isotope compositions (δ18Ozircon = 6.5–7.5‰), the Bobcat Hills dikes lack any evidence of crustal assimilation and have oxygen isotope values that overlap a mantle range (Bobcat Hills average δ18Ozircon = 5.6 ± 0.3‰). The dikes were a high‐temperature, early center of mafic alkaline magmatism in the Mojave Desert that serve as a snapshot of melt generation from a spatially extensive, metasomatized mantle source. We propose that modification of the crust over many tens of Myr at Mountain Pass created an environment that favored crustal assimilation and enabled ascent of late‐stage, REE‐rich carbonatite magmas. Key Points The Bobcat Hills site expands the distribution of REE‐rich mafic alkaline magmatism 65 km southeast of the Mountain Pass mine Shonkinite dikes at Bobcat Hills are physically and chemically similar to mafic intrusions at Mountain Pass and have comparably high REE Bobcat Hills was an early center of REE‐rich magmatism in the Mojave Desert, with a zircon U‐Pb crystallization age of 1,426 ± 2 Ma