UC Santa Barbara

The petrogenesis of the rare earth element-bearing carbonatite at Mountain Pass, California remains poorly understood despite the deposit’s discovery in the 1950s. Ore-bearing carbonatite dikes and a stock are spatially associated with a suite of ultrapotassic igneous rocks, and, based on geochemical data, the two have been proposed to be genetically related. Such petrologic models are complicated by existing geochronologic constraints indicating that the carbonatite is ~15-25 m.y. younger than the ultrapotassic rocks, leading some authors to favor formation of the ore-bearing carbonatite during a separate, younger event and/or via a different mechanism. New laser ablation split-stream inductively coupled plasma mass spectrometry (LASS-ICPMS) petrochronologic data from 19 ultrapotassic intrusive rocks from Mountain Pass yield titanite and zircon U-Pb ages from 1429 ± 10 to 1385 ± 18 Ma, expanding the U-Pb age range of ultrapotassic rocks in the complex by ~20 m.y. The ages of the youngest ultrapotassic rocks overlap monazite Th-Pb ages from a carbonatite dike and the main carbonatite ore body (1396 ± 16 and 1371 ± 10 Ma, respectively). Epsilon Hf(i) values from zircon in seven ultrapotassic rocks yield consistent results both within and between samples, with a weighted mean of 1.9 ± 0.2 (MSWD = 0.9), indicating derivation from a shared, isotopically homogeneous source. In contrast, exploratory in-situ Nd isotopic data from titanite in the ultrapotassic rocks are variable (εNd(i) = -3.5 to -12), suggesting secondary alteration and/or contamination from an isotopically enriched source in some samples. The most primitive εNd(i) isotopic signatures do, however, overlap εNd(i) from monazite (εNd(i) = -2.8 ± 0.2) and bastnӓsite (εNd(i) = -3.2 ± 0.3) in the carbonatite, indicating derivation from a source with the same Nd isotopic signature. Although carbonatite is the youngest rock type in the intrusive complex based on cross-cutting relationships, at the resolution of the LA-MC-ICPMS technique, Th-Pb monazite ages of carbonatite are not resolvable from titanite and zircon ages in ultrapotassic rocks, eliminating the previously inferred time gap between ultrapotassic and carbonatite magmatism at Mountain Pass. These new geochronologic data combined with whole rock elemental and mineral-scale isotope data imply that the ultrapotassic – carbonatite rocks at Mountain Pass are part of a genetically-related suite that formed via extreme fractional crystallization and/or silicate-carbonatite liquid immiscibility processes.