Mountain Pass is the site of the most economically important rare earth element (REE) deposit in the United States. Mesoproterozoic alkaline intrusions are spatiotemporally associated with a composite carbonatite stock that hosts REE ore. Understanding the genesis of the alkaline and carbonatite magmas is an essential scientific goal for a society in which critical minerals are in high demand and will continue to be so for the foreseeable future. We present an ion microprobe study of zircon crystals in shonkinite and syenite intrusions to establish geochronological and geochemical constraints on the igneous underpinnings of the Mountain Pass REE deposit. Silicate whole-rock compositions occupy a broad spectrum (50–72 wt % SiO₂), are ultrapotassic (6–9 wt % K₂O; K₂O/Na₂O = 2–9), and have highly elevated concentrations of REEs (La 500–1,100× chondritic). Zircon concordia ²⁰⁶Pb/²³⁸U-²⁰⁷Pb/²³⁵U ages determined for shonkinite and syenite units are 1409 ± 8, 1409 ± 12, 1410 ± 8, and 1415 ± 6 Ma (2σ). Most shonkinite dikes are dominated by inherited Paleoproterozoic xenocrysts, but there are sparse primary zircons with ²⁰⁷Pb/²⁰⁶Pb ages of 1390–1380 ± 15 Ma for the youngest grains. Our new zircon U-Pb ages for shonkinite and syenite units overlap published monazite Th-Pb ages for the carbonatite orebody and a smaller carbonatite dike. Inherited zircons in shonkinite and syenite units are ubiquitous and have a multimodal distribution of ²⁰⁷Pb/²⁰⁶Pb ages that cluster in the range of 1785–1600 ± 10–30 Ma. Primary zircons have generally lower Hf (<11,000 ppm) and higher Eu/Eu* (>0.6), Th (>300 ppm), Th/U (>1), and Ti-in-zircon temperatures (>800°C) than inherited zircons. Oxygen isotope data reveals a large range in δ¹⁸O values for primary zircons, from mantle (5–5.5‰) to crustal and supracrustal (7–9‰). A couple of low-δ¹⁸O outliers (2‰) point to a component of shallow crust altered by meteoric water. The δ¹⁸O range of inherited zircons (5–10‰) overlaps that of the primary zircons. Our study supports a model in which alkaline and carbonatite magmatism occurred over tens of millions of years, repeatedly tapping a metasomatized mantle source, which endowed magmas with elevated REEs and other diagnostic components (e.g., F, Ba). Though this metasomatized mantle region existed for the duration of Mountain Pass magmatism, it probably did not predate magmatism by substantial geologic time (>100 m.y.), based on the similarity of 1500 Ma zircons with the dominantly 1800–1600 Ma inherited zircons, as opposed to the 1450–1350 Ma primary zircons. Mountain Pass magmas had diverse crustal inputs from assimilation of Paleoproterozoic and Mesoproterozoic igneous, metaigneous, and metasedimentary rocks. Crustal assimilation is only apparent from high spatial resolution zircon analyses and underscores the need for mineral-scale approaches in understanding the genesis of the Mountain Pass system.

中文翻译：

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加利福尼亚山口中元古代碱性岩浆和碳酸岩岩浆之间的时间和岩石成因联系

山口是美国经济上最重要的稀土元素 (REE) 矿床。中元古代碱性侵入体在时空上与含有稀土元素矿石的复合碳酸盐岩储量有关。了解碱性岩浆和碳酸岩岩浆的成因是关键矿物需求量很大的社会的一项基本科学目标，并且在可预见的未来仍将如此。我们对菱镁矿和正长岩侵入体中的锆石晶体进行了离子微探针研究，以建立对 Mountain Pass REE 矿床火成岩基础的地质年代学和地球化学约束。硅酸盐全岩成分占据广泛的范围（50–72 wt % SiO ₂），是超钾（6–9 wt % K ₂ O；K ₂O/Na ₂ O = 2–9)，并且具有高度升高的 REE 浓度（La 500–1,100×球粒陨石）。锆英石²⁰⁶ Pb/ ²³⁸ U- ²⁰⁷ Pb/ ²³⁵ U 年龄为 1409 ± 8、1409 ± 12、1410 ± 8 和 1415 ± 6 Ma (2σ)。大多数闪金岩脉以继承的古元古代异晶为主，但也有稀疏的原生锆石，含²⁰⁷ Pb/ ²⁰⁶最年轻颗粒的铅年龄为 1390-1380 ± 15 Ma。我们的新锆石 U-Pb 年龄与已发表的碳酸岩矿体和较小碳酸岩脉的独居石 Th-Pb 年龄重叠。方长岩和正长岩单元中的继承锆石无处不在，并且具有²⁰⁷ Pb/ ²⁰⁶ Pb 年龄的多峰分布，聚集在 1785-1600 ± 10-30 Ma 范围内。原生锆石通常具有较低的 Hf (<11,000 ppm) 和较高的 Eu/Eu* (>0.6)、Th (>300 ppm)、Th/U (>1) 和锆石中钛温度 (>800°C)比继承的锆石。氧同位素数据揭示了原生锆石的δ ¹⁸ O 值范围很大，从地幔 (5–5.5‰) 到地壳和地壳上 (7–9‰)。一对低δ ¹⁸O 异常值 (2‰) 指向由大气水改变的浅地壳的组成部分。δ ¹⁸O 范围的继承锆石 (5–10‰) 与原生锆石重叠。我们的研究支持了一个模型，其中碱性和碳酸岩岩浆作用发生了数千万年，反复挖掘交代地幔源，赋予岩浆高 REE 和其他诊断成分（例如 F、Ba）。尽管这个交代地幔区域存在于山口岩浆活动期间，但根据 1500 Ma 锆石与占主导地位的 1800-1600 Ma 继承锆石的相似性，它可能没有早于岩浆活动的大量地质时间（>100 my），而不是到 1450-1350 Ma 原生锆石。山口岩浆具有来自古元古代和中元古代火成岩、变火成岩和变沉积岩的同化作用的多种地壳输入。