Temporal and Petrogenetic Links Between Mesoproterozoic Alkaline and Carbonatite Magmas at Mountain Pass, California

Abstract Mountain Pass is the site of most economically important rare earth element (REE) deposit in United States. Mesoproterozoic alkaline intrusions are spatiotemporally associated with a composite carbonatite stock that hosts REE ore. Understanding genesis and magmas an essential scientific goal for society which critical minerals high demand will continue to be so foreseeable future. We present ion microprobe study zircon crystals shonkinite syenite establish geochronological geochemical constraints on igneous underpinnings deposit. Silicate whole-rock compositions occupy broad spectrum (50–72 wt % SiO2), ultrapotassic (6–9 K2O; K2O/Na2O = 2–9), have highly elevated concentrations REEs (La 500–1,100× chondritic). Zircon concordia 206Pb/238U-207Pb/235U ages determined units 1409 ± 8, 12, 1410 1415 6 Ma (2?). Most dikes dominated by inherited Paleoproterozoic xenocrysts, but there sparse primary zircons 207Pb/206Pb 1390–1380 15 youngest grains. Our new U-Pb overlap published monazite Th-Pb orebody smaller dike. Inherited ubiquitous multimodal distribution cluster range 1785–1600 10–30 Ma. Primary generally lower Hf (<11,000 ppm) higher Eu/Eu* (>0.6), Th (>300 ppm), Th/U (>1), Ti-in-zircon temperatures (>800°C) than zircons. Oxygen isotope data reveals large ?18O values zircons, from mantle (5–5.5‰) crustal supracrustal (7–9‰). A couple low-?18O outliers (2‰) point component shallow crust altered meteoric water. The (5–10‰) overlaps supports model magmatism occurred over tens millions years, repeatedly tapping metasomatized source, endowed other diagnostic components (e.g., F, Ba). Though this region existed duration magmatism, it probably did not predate substantial geologic time (>100 m.y.), based similarity 1500 dominantly 1800–1600 as opposed 1450–1350 had diverse inputs assimilation igneous, metaigneous, metasedimentary rocks. Crustal only apparent spatial resolution analyses underscores need mineral-scale approaches understanding system.