Abstract

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.

Introduction

Alkaline silicate intrusive rocks are a common geologic marker of carbonatites, a geochemically peculiar class of igneous rocks generally thought to be sourced from carbonated mantle peridotite (Wyllie and Huang, 1975; Wallace and Green, 1988; Harmer and Gittins, 1998). Some carbonatites contain extraordinary concentrations of strategic and critical metals, particularly the rare earth elements (REEs) (Verplanck et al., 2016). Mountain Pass, located in the Mojave Desert of southeastern California in the United States (Fig. 1), exemplifies this relationship and economic relevance. It contains a suite of Mesoproterozoic (ca. 1.4 Ga) alkaline silicate intrusions that range in composition from shonkinite (most voluminous) to syenite (less voluminous) to alkali granite (minor) that are spatiotemporally associated with a series of carbonatite dikes and intrusions. The most significant carbonatite intrusion is the Sulphide Queen stock, which hosts the largest REE deposit in the United States (Olson et al., 1954; Castor, 2008; Verplanck et al., 2016). Though moderate in its areal extent (700 m in widest dimension) and thickness (up to 150 m), the orebody is high grade (Castor, 2008). Mined from 1952 to 2002, the Sulphide Queen deposit has an estimated remaining resource of 20 to 47 million metric tons (Mt) at an average grade of 8.9% rare earth oxides (REOs) (Long et al., 2012). As is characteristic of most carbonatite ore deposits, it has an extreme enrichment in light REEs (LREEs; atomic numbers 57–63) relative to heavy REEs (HREEs; atomic numbers 64–71), with bastnäsite ([LREE]CO3F) constituting the predominant ore mineral (Castor, 2008).

Despite its economic significance, the genesis of the Mountain Pass carbonatite stock and associated alkaline igneous rocks remains unclear. Whether the carbonatite and shonkinite-syenite intrusions represent partial melts of the same or different mantle source(s) or were derived from immiscible melts or fractional crystallization of common or discrete progenitor magmas remain open questions. Experimental petrology studies have produced carbonatite melts analogous to the Mountain Pass carbonatite intrusion under magmatic conditions (Jones and Wyllie, 1983). Those authors advocate fractional crystallization as the primary mechanism for enriching the magma in REEs and propose that fractional crystallization may have started with a parental mafic silicate magma or that a primitive carbonatite may have formed as an immiscible melt from a conjugate silicate magma at crustal (<20 kbar) pressures (Wyllie and Jones, 1985; Jones and Wyllie, 1986; Wyllie et al., 1990). Field evidence of magma mingling textures and complex crosscutting relationships between shonkinite and syenite intrusions at Mountain Pass point to synchronous existence of different silicate melts in the crust (Olson et al., 1954; Haxel, 2005). Based on available geochronology data, there may be a large age gap between crystallization of the carbonatite stock (1371 Ma) and shonkinite and syenite intrusions (1430–1400 Ma) (Poletti et al., 2016; Premo et al., 2016). However, a smaller carbonatite dike 1 km northwest of the main orebody has an age of 1396 Ma, overlapping the shonkinite and syenite ages, and some late shonkinite dikes have young ages of 1385–1380 Ma (Poletti et al., 2016; Premo et al., 2016).

With the goal of elucidating the genesis of alkaline and carbonatite magmas at Mountain Pass, we present an ion microprobe study of refractory zircon crystals in shonkinite and syenite intrusive units. A particular focus of this investigation is on shonkinite dikes that have been shown to have the strongest mantle affinity of the alkaline magmas at Mountain Pass (Haxel, 2005) and are possibly the only units that were contemporaneous with carbonatite magmatism (Poletti et al., 2016; Premo et al., 2016). New zircon data provided by this work include U-Pb crystallization ages, trace element and REE concentrations, Ti-in-zircon thermometry, and O isotope compositions. Our geochronology results demonstrate contemporaneity in shonkinite, syenite, and carbonatite magmas over tens of millions of years. Combined with trace element and oxygen isotope geochemistry for the same zircon crystals, this study provides a high spatial resolution evaluation of the petrogenetic links between compositionally diverse magma types at Mountain Pass.

Geologic Setting

Mountain Pass is located within the Mojave crustal province, a crystalline basement terrane that spans most of southeastern California and parts of Nevada, Arizona, and Utah, as inferred from radiogenic Nd and Pb isotope data (Bennet and DePaolo, 1987; Wooden and Miller, 1990; Wooden and De-Witt, 1991; Whitmeyer and Karlstrom, 2007). Though the age of this basement terrane is Paleoproterozoic (ca. 1.8–1.6 Ga), Archean components are apparent from Nd isotope data and detrital zircons with ages that extend to >2.0 Ga (Wooden and Miller, 1990; Ramo and Calzia, 1998; Barth et al., 2009; Strickland et al., 2012). Debate persists as to whether the Mojave province is underlain by a small Archean craton sutured to the margin of Laurentia or Archean crust or detritus was incidentally mixed into a younger Paleoproterozoic terrane during accretionary orogenesis (cf. Whitmeyer and Karlstrom, 2007).

The ca. 1.8–1.6 Ga Paleoproterozoic rocks record superimposed episodes of sedimentation, arc magmatism, plutonism, and metamorphism (Wooden and Miller, 1990; Miller and Wooden, 1994; Barth et al., 2009; Strickland et al., 2012). The oldest rocks (ca. 1.8 Ga) are metasedimentary gneisses and metaigneous calc-alkaline rocks with a magmatic arc affinity (Wooden and Miller, 1990; Barth et al., 2000, 2009). The ca. 1.7 Ga Ivanpah orogeny was a regional, high-grade metamorphic event, forming granitic and intermediate biotitegarnet gneisses, lesser amphibolite, and migmatites (Wooden and Miller, 1990). Synorogenic intrusions are represented by augen orthogneisses and granitic and pegmatitic dikes. Voluminous postorogenic granitic plutons were emplaced along a north-south trend in the middle of the Mojave crustal province ca. 1.69–1.64 Ga, manifest as coarse alkali-feldspar porphyritic gneisses that lack the granulite-grade fabrics and migmatization associated with the Ivanpah orogeny (Wooden and Miller, 1990).

Intruding the Paleoproterozoic crystalline basement are ca. 1.4 Ga anorogenic granitoids, often in bimodal association with mafic intrusions, and perhaps related to rifting of Laurentia during the breakup of supercontinent Columbia (Anderson, 1983; Rogers and Santosh, 2002). The 1.4 Ga granitoids span the United States and Canada, with Mountain Pass forming one of the westernmost localities (cf. du Bray et al., 2018). The chemistry of the Mountain Pass intrusive suite is more alkaline and mafic than most of the 1.4 Ga intrusions (du Bray et al., 2018). Associated REE mineralization is most notable in the Mountain Pass area but not restricted to this locality. The 1.46 Ga Pea Ridge iron oxide-apatite-REE deposit in southeast Missouri is hosted in alkaline silicate igneous rocks along the southeastern margin of Laurentia (e.g., Watts and Mercer, 2020).

Within the Mojave Desert region, Mountain Pass is one of many occurrences of alkaline silicate intrusive rocks that form a 130-km northwest-southeast belt (Castor, 2008). Despite metamorphic overprinting, some of these other occurrences have similar geochemical and geochronological attributes, such as the Barrel Spring pluton 90 km south of Mountain Pass (Gleason et al, 1994). Offset from this belt, 35 km southeast of Mountain Pass, is the Thor REE prospect, which has disparate host rocks (pegmatitic granite), an older age (ca. 1.65 Ga), and a different style of mineralization (monazite REE ore) (Bruns, 2011; D. Miller, unpub. data, 2020). The Pinto gneiss in Music Valley 165 km southwest of Mountain Pass also hosts economic REE mineralization in monazite and xenotime ores and has an age of ca. 1.71 Ga, though it does contain metamorphosed mafic intrusions (metadiorite) that are similar in age to Mountain Pass intrusions (ca. 1.4 Ga) (McKinney et al., 2015).

Outcrops of ca. 1.4 Ga alkaline and carbonatite stocks and dikes at Mountain Pass form a northwest-southeast belt 15 km long and 3 km wide (Fig. 1; Haxel, 2005; Castor, 2008). Stocks range in size from 0.1 to 2 km in their widest dimensions (Olson et al., 1954; Haxel, 2005; Poletti et al., 2016). The Birthday shonkinite stock and the Sulphide Queen carbonatite stock are the two largest intrusive bodies at Mountain Pass (Fig. 1B). Smaller syenite-shonkinite stocks include Mexican Well, Groaner, Corral, Tors, and Wheaton (also known as Pops) (Fig. 1B). Mineral Hill is the lone alkali granite stock. Stocks are accompanied by hundreds of mapped dikes of shonkinite, syenite, alkali granite, and carbonatite, 0.3 to 10 m wide and as much as 1 km long. Many dikes strike northwest, parallel to the orientation of the larger intrusions as well as the 130-km-long belt of alkaline intrusions in the Mojave Desert (Olson et al., 1954; Haxel, 2005; Castor, 2008). The northwest-trending orientation of outcrops at Mountain Pass follows foliation in host Paleoproterozoic gneisses (Fig. 1B; Olson et al., 1954), supporting a preexisting structural control on the emplacement of these intrusions in the brittle upper crust.

Geophysical aeromagnetic data for the Mountain Pass region also define a northwest-trending gradient, with Mountain Pass outcrops occupying an intermediate zone between a magnetic low to the northeast and a magnetic high to the southwest, interpreted as unrelated magnetic plutonic rocks bordering a crustal zone of weakness (Denton et al., 2020). Whether the magnetic plutonic rocks are older or younger than the relatively nonmagnetic Mountain Pass intrusions is not known, but they may have exploited the same crustal megastructure during different intrusive episodes that were widely spaced in time (Denton et al., 2020). Like the shonkinite and syenite stocks, the Sulphide Queen carbonatite stock does not correspond to a defined anomaly in either gravity or aeromagnetic data, but it is bordered to the west by a highconductivity body defined by magnetotelluric data that coincides with the aeromagnetic high interpreted to be a pluton. Fracturing and fluid flow within the pluton, or possibly sulfide mineralization associated with the carbonatite stock, may account for this signature (Denton et al., 2020).

Shonkinite, syenite, and alkali granite intrusive rocks at Mountain Pass have compositions that grade into one another, spanning a broad spectrum (45–75 wt % SiO2). The alkaline character of the Mountain Pass silicate intrusive suite is ultrapotassic (Castor, 2008; Poletti et al., 2016), with extremely high potassium contents (5–12 wt % K2O), low to moderate sodium contents (1–5 wt % Na2O), and high K2O/Na2O values (1.3–8.5). Previous authors have noted the similarities between shonkinites at Mountain Pass and lamproites (Haxel, 2005; Castor, 2008). Though lamproite-like, with abundant phlogopite and alkali feldspar and high concentrations of K, F, Ba, and LREEs, shonkinite intrusions at Mountain Pass have different geochemical signatures for some elements, such as lower Nb and Ta (Mitchell and Bergman, 1991; Haxel, 2005; Castor, 2008). Commonly, shonkinite intrusions are physically mixed with syenite, with ductile intermingling textures, crosscutting dikes, and breccia zones (Fig. 1D, E; Haxel, 2005; Poletti et al., 2016).

The Sulphide Queen stock is a composite of compositionally diverse types of carbonatite, ranging from calcite to dolomite dominant (Castor, 2008; Verplanck et al., 2016). Barite phenocrysts and primary and secondary bastnäsite are ubiquitous constituents of all compositional types (Castor, 2008). Dolomite carbonatite (beforsite, dolosövite) constitutes the bulk of the orebody, but calcite carbonatite (sövite) is more common at shallow levels (Castor, 2008). REO of the carbonatite ranges from about 5 up to 25% locally (Castor, 2008). Carbonatite crosscuts all alkaline silicate intrusive rock types, with the exception of a thin shonkinite dike that was reported to crosscut a carbonatite vein in the Birthday stock area (Olson et al., 1954). From oldest to youngest, field evidence indicates a general sequence of shonkinite, syenite, alkali granite, shonkinite dikes, and carbonatite (Olson et al., 1954). Geochronologic data support broadly coeval crystallization of alkaline intrusions over extended geologic time, with zircon and titanite U-Pb ages for shonkinite and syenite that range from 1429 ± 10 to 1385 ± 18 Ma (Poletti et al., 2016). Geochronology data also support the relatively young age of the carbonatite with monazite Th-Pb dates of 1371 ± 10 Ma on the Sulphide Queen stock and 1396 ± 16 Ma on a carbonatite dike in the Birthday shonkinite stock (Poletti et al., 2016).

Samples and Methods

Samples

Samples were collected from six sites along a 11.5-km transect that extends northwest and southeast of the Mountain Pass mine (Fig. 1A, B). Site 1 is 500 m northeast of the mine pit (Fig. 1C), with prominent outcrops of mixed shonkinite and syenite (Fig. 1D). Coarse-grained shonkinite (17-MP-01) is enveloped and crosscut by fine-grained, leucocratic syenite (17-MP-02) (Fig. 1D). Contacts between the two rock types range from sharp to gradational. Breccia blocks of coarse-grained shonkinite range in size from many meters to less than a few centimeters, and in some places are partially or wholly disaggregated with coarse phlogopite crystals floating in fine-grained syenite (Fig. 1E). Another shonkinite sample (3311) was collected 70 m west in the shonkinite-syenite outcrop at site 1 and has fewer and smaller phlogopite crystals than shonkinite sample 17-MP-01. Site 2 is located in the Birthday stock area 800 m north of the mine pit (Fig. 1C). Two fine-grained shonkinite dikes (17-MP-03, 18-MP-01) were collected 70 m apart, in an area with abundant dikes of diverse compositional types. The two dikes have a similar megascopic appearance that characterizes most of the shonkinite dikes at Mountain Pass, with small phlogopite crystals set in a fine groundmass, referred to as “microshonkinite” by Haxel (2005). Site 3 is located 4 km northwest of the mine pit (Fig. 1A), in an area where Mountain Pass mine geologists have identified “glimmerite” (CA14-MTP083), an unusual phlogopite-bearing rock that resembles an altered shonkinite. Interstitial calcite is abundant and may indicate a genetic affinity to carbonatite (J. Landreth, writ. commun., 2016). Site 4 is the southernmost site, located 7 km southeast of the mine pit (Fig. 1A). Shonkinite dike sample GH-16-1 corresponds to one of the most primitive microshonkinite dikes studied by Haxel (2005). It is a northwest-trending dike a few hundred meters long. Additional primitive microshonkinite dikes were collected nearby at site 5 (16-MP-13) and site 6 (GH-16-3), 800 m and 2 km northwest, respectively, from site 4 (Fig. 1A). Both of these dikes are also northwest trending and a few hundred meters long.

Zircon was successfully separated from eight of the nine samples from sites 1 through 6, using standard methods of crushing and sieving whole rocks and passing powders over a water density table, followed by magnetic and heavy liquid separations. The only sample for which zircon was either absent or too sparse to recover was shonkinite dike sample GH-16-3 from site 6. The textural contexts of zircon within the samples are consistent with the primary mineralogy that is well documented for the shonkinite-syenite intrusive suite (cf. Castor, 2008; Poletti et al., 2016). In the shonkinite samples, zircon is attached to or has inclusions of phlogopite, alkali feldspar, plagioclase, clinopyroxene, and titanite. In the leucosyenite sample, zircon is most commonly attached to alkali feldspar, quartz, and apatite. In the glimmerite sample, zircon is attached to phlogopite, albite, calcite, apatite, and rutile. Notably, in shonkinite sample 17-MP-01, baddeleyite, thorite, and calcite are also observed in textural association with zircon, a unique feature compared to the other shonkinites of this study. Zircon grains are typically large anhedral fragments (75–250 μm in longest dimension), some with oscillatory zoning along crystallographic growth planes but most with very complex internal mottling and disrupted textures. Generally, the leucosyenite sample has the most euhedral zircon grains and the least complex internal textures. Rounded zircon grains are observed for some samples, particularly the shonkinite dikes. A couple of the shonkinite dike samples (GH-16-1, 16-MP-13) contain zircons that are distinctly large (about 1 mm in length) and dark red-brown in color, some subrounded and others euhedral.

Methods

Whole-rock major and trace element compositions were determined using X-ray fluorescence (XRF) and inductively coupled plasma-mass spectrometry (ICP-MS) at the Washington State University GeoAnalytical Laboratory. Zircon mounts were made for eight of the nine whole-rock samples from which zircons were recovered. Zircon grains were mounted within 1-inch round epoxy discs, with samples and standards mounted within a 10-mm diameter in the center of each mount. Epoxy mounts were then ground down using a series of grits and polishing laps to expose zircon interiors. Backscattered electron (BSE) and cathodoluminescence (CL) imaging of zircon mounts was performed with a Tescan VEGA3 scanning electron microscope at the U.S. Geological Survey (USGS) Menlo Park microanalytical facility, operated at 15–20 kV and 10–15 nA for BSE and 10 kV and 14–18 nA for CL. Energy dispersive spectra (EDS) were collected during BSE imaging sessions to identify different minerals attached to and included within zircons.

A sensitive high-resolution ion microprobe with reverse geometry (SHRIMP-RG) co-operated by the USGS and Stanford University was used to determine zircon 207Pb/206Pb ages and trace elements. The instrument was operated at 10 kV with an O2– primary ion beam at 3.5–4.5 nA and a spot diameter of 20–25 μm. A mass resolving power M/ΔM of 8,000–8,500 at 10% peak height was used. The mass table included 48Ti+, 49Ti+, 89Y+, 139La+, 140Ce+, 146Nd+, 147Sm+, 153Eu+, 155Gd+, 163Dy16O+, 166Er16O+, 172Yb16O+, a high mass normalizing species (90Zr216O+), followed by 180Hf16O+, 204Pb+, a background measured at 0.045 mass units above the 204Pb+ peak, 206Pb+, 207Pb+, 208Pb+, 232Th+, 238U+, 232Th16O+, 238U16O+, and 238U16O2+. The 207Pb isotope was used to correct measured 206Pb/238U values for common Pb, whereas 204Pb was used to correct 207Pb/206Pb values. The common Pb correction was based on a model Pb composition from Stacey and Kramers (1975). Values for 206Pb/238U were normalized to zircon standards R33 (419.3 Ma) and TEMORA-2 (416.8 Ma) (Black et al., 2004). Trace elements were standardized against MAD-green zircon (Barth and Wooden, 2010), which had average uncertainties at the one standard deviation (SD) level of ±3–8% for U and Th, 1–4% for Hf, 5–15% for Y and REEs, and 5–10% for Ti. SHRIMP-RG data were reduced following the methods of Williams (1997) and Ireland and Williams (2003) using Excel and the add-in programs Isoplot 3.76 and Squid 2.51 (Ludwig, 2009, 2012).

A CAMECA ims 1290 ion microprobe at the University of California, Los Angeles, was used to determine oxygen isotope compositions for a subset of the zircons for which 207Pb/206Pb ages were determined on the SHRIMP-RG. The 1290 ion microprobe was operated at 10 kV with a 133Cs+ primary ion beam at 2 nA and a spot diameter of 10 μm. Three Faraday cup detectors were used to simultaneously measure secondary ions of 16O, 16OH, and 18O. Ratios of 18O/16O in unknowns were standardized against R33 (δ18O = 5.55‰, reported relative to Vienna standard mean ocean water [VSMOW]) and TEMORA-2 (δ18O = 8.20‰, VSMOW) (Valley, 2003). R33 and/or TEMORA-2 standards bracketed each set of unknowns, with 5–10 analyses before and after each set of 10–15 unknowns; the average of the bracketing standard analyses was used to correct the unknown values. Instrument mass fractionation based on R33 and TEMORA-2 analyses ranged from 0.9 to 2.5‰, averaging 1.5‰, and the external reproducibility (1 SD) ranged from 0.1 to 0.5‰, averaging 0.2‰.

Results

Whole-rock geochemistry

New geochemical data acquired during this study are synthesized with all available published data for Mountain Pass alkaline silicate intrusive rocks and regional basement rocks (Fig. 2; App. 1). The samples of this study bracket the compositional array of Mountain Pass shonkinite-syenite series rocks, with eight classified as shonkinites (50–56 wt % SiO2) and one classified as a leucosyenite (72 wt % SiO2) (Fig. 2; Table 1). All are ultrapotassic, with 6–9 wt % K2O and K2O/Na2O = 2–9 (Fig. 2B; Table 1). The leucosyenite sample contains small quartz crystals that are interstitial to a phenocryst matrix dominated by alkali felspar. Compositionally, it borders on alkali granite, but the lack of quartz phenocrysts and relatively low modal abundance of interstitial quartz (15–20 vol %) are consistent with a classification of leucosyenite. The glimmerite sample is compositionally unusual relative to the shonkinite-syenite series rocks, with distinctly higher CaO (17 wt %) and lower K2O (4 wt %), (Fig. 2B, C).

Compositional trends in major and trace elements are apparent for the shonkinite-syenite series rocks. Shonkinites have higher CaO, MgO, FeO, Ni, and Cr relative to the leucosyenite that is about an order of magnitude lower in these elements (Fig. 2; Table 1). Shonkinite samples also have higher Ba, Sr, and P relative to the leucosyenite (Fig. 2G, H, J). Phosphorous is observed to correlate with fluorine, with higher P and F in the lowest-silica shonkinite rocks, as great as 1.2 wt % P and 2 wt % F (Fig. 2J; App. 1; Castor, 2008). Regional basement gneisses and amphibolites have markedly different compositional trends (Fig. 2). The basement rocks have higher Al2O3, CaO, and Sc and lower K2O, Ba, Sr, Th, and P relative to the shonkinite-syenite series rocks (Fig. 2). Arrays for CaO and Sc versus SiO2 have different magnitudes and slopes (Fig. 2C, F). For the same SiO2 contents, amphibolites (low-SiO2 basement rocks) have comparable MgO (Fig. 2D) but consistently lower FeO, Ni, and Cr than the shonkinites (Fig. 2E; App. 1). In contrast, the leucosyenite sample has a composition that overlaps the most evolved basement gneisses for most elements (Fig. 2) but is notably higher in others, such as Ba (>0.2 wt %) and particularly Th (150 ppm) (Fig. 2G, I).

Shonkinite-syenite series rocks also define distinctive REE trends with SiO2. The highest LREE concentrations occur in the lowest-silica samples; La as much as 500 ppm or 1,100× chondritic in the shonkinites (Figs. 2K, 3). Patterns in the shonkinite-syenite series rocks are characterized by high LREEs that gradually decrease with increasing atomic number; the heaviest HREE, Lu, has concentrations of 0.5 ppm or 20× chondritic in the shonkinites (Figs. 2L, 3). Despite its distinctive major element composition, the glimmerite sample has an REE pattern identical to the shonkinite-syenite series rocks but with lower overall concentrations compared to all but the leucosyenite sample (Fig. 3). Mountain Pass carbonatite rocks define REE trends similar to those of the shonkinite-syenite series rocks but with much steeper slopes; LREEs are as great as 100,000× chondritic in the carbonatite orebody. Whereas LREE concentrations in the Mountain Pass shonkinite-syenite series rocks and carbonatite are higher than any basement rocks, HREE concentrations overlap, and some basement rocks, particularly pelitic gneisses, have higher HREE concentrations (Figs. 2L, 3; App. 1). Basement rocks also define different REE patterns, with relatively flat REE distributions and small positive and negative Eu anomalies, in contrast to no or very subtle negative Eu anomalies in the shonkinite-syenite series rocks (Fig. 3).

Zircon geochronology

Zircon grains in the shonkinite-syenite series rocks are texturally complex (Fig. 4), and discordance is common. Large numbers of analyses were acquired to obtain sufficient concordant data to establish robust U-Pb crystallization ages. Out of 297 total SHRIMP-RG zircon spot analyses, 149 are statistically concordant, where concordant is defined as an analysis that overlaps 206Pb/238U-207Pb/235U concordia within 1σ. Common Pb is a ubiquitous problem in Mountain Pass zircons, with high common 206Pb ranging from 1 to 40% of the total 206Pb (App. 2). For all but four analyses, the high common Pb spots (>1% common 206Pb) are also statistically discordant. Calculated U-Pb ages include concordia 206Pb/238U-207Pb/235U ages (paired 206Pb/238U and 207Pb/235U ages) and error-weighted mean 207Pb/206Pb ages, reported at 2σ confidence (Table 2). Neither type of age calculation includes zircon spots that are statistically discordant or have high common 206Pb (>1%).

Site 1: Shonkinite sample 17-MP-01 and leucosyenite sample 17-MP-02, which are intermingled in outcrop (Fig. 1D, E), have concordia 206Pb/238U-207Pb/235U ages that are identical within error, at 1409 ± 12 and 1410 ± 8 Ma, respectively (Fig. 5; Table 2). The percentage of concordant analyses is similar for these samples (n = 7/33 and n = 11/46), but the leucosyenite sample contains inherited grains, with 207Pb/206Pb ages of 1657 ± 14, 1661 ± 8, and 1728 ± 7 Ma (Fig. 5; App. 2). Error-weighted mean 207Pb/206Pb ages for concordant analyses in samples 17-MP-01 and 17-MP-02 are identical within error to one another as well as the concordia ages for these samples, at 1410 ± 14 and 1412 ± 9 Ma. Zircons from shonkinite sample 3311 have different characteristics, with a higher percentage of concordant analyses (n = 22/36) and an older concordia age of 1415 ± 6 Ma and error-weighted mean age of 1414 ± 5 Ma, but these age differences are not resolvable from samples 17-MP-01 or 17-MP-02 within error (Fig. 5; Table 2). Histograms and probability density functions define unimodal 207Pb/206Pb age distributions for the youngest age population in each sample (Fig. 5; Table 2). Upper discordia age intercepts for samples 17-MP-01 and 3311 are 1415 ± 10 and 1432 ± 22 Ma (Fig. 5), respectively.

Site 2: Shonkinite dike samples are dominated by inherited grains with 207Pb/206Pb crystallization ages of 1768–1500 Ma in sample 17-MP-03 and 1752–1533 Ma in sample 18-MP-01 (Fig. 6; Table 2; App. 2). Despite the preponderance of inheritance in these samples, most analyses are concordant (n = 25/34 and n = 17/21). The youngest concordant grains have 207Pb/206Pb crystallization ages of 1382 ± 15 and 1473 ± 24 Ma for samples 17-MP-03 and 18-MP-01, respectively (Fig. 6; Table 2). These ages serve as a tentative estimate of the crystallization ages of the shonkinite intrusions, but a larger number of grains would be needed to accurately define the youngest populations. The paucity of primary, noninherited zircons does not permit calculation of concordia or error-weighted mean crystallization ages for these samples. Rather, the zircon populations resemble detrital spectra, apparent in the multimodal 207Pb/206Pb age histograms and probability density functions (Fig. 6). The dominant age peaks are in the 1770–1650 Ma range, and smaller analytical errors for zircons in sample 17-MP-03 permit resolution of at least four discrete age peaks within this range (Fig. 6).

Site 3: Glimmerite sample CA14-MTP083 has approximately equal percentages of concordant and discordant zircon analyses (n = 22/38 concordant), which define a concordia 206Pb/238U-207Pb/235U age of 1409 ± 8 Ma and an error-weighted mean 207Pb/206Pb age of 1410 ± 7 Ma (Fig. 7; Table 2; App. 2). A histogram and probability density function for concordant 207Pb/206Pb age data defines a unimodal density (Fig. 7). Two outlier concordant zircon analyses have 207Pb/206Pb ages of 1019 ± 25 and 1092 ± 24 Ma (Fig. 7; App. 2).

Sites 4 and 5: Shonkinite dike samples GH-16-1 and 16-MP-13 are from spatially contiguous sampling sites and have similar zircons and textures (in addition to whole-rock geochemistry; Fig. 2) and are therefore considered together. Like the shonkinite dike samples from site 2, those from sites 4 and 5 are dominated by inherited zircons, though with fewer concordant analyses (n = 27/40 and n = 18/49) (Fig. 8). Concordant inherited 207Pb/206Pb crystallization ages are 1784–1568 Ma in sample GH-16-1 and 1755–1628 Ma in sample 16-MP-13 (Fig. 8; Table 2; App. 2). The youngest concordant grains have 207Pb/206Pb crystallization ages of 1389 ± 16 and 1420 ± 8 Ma for samples GH-16-1 and 16-MP-13, respectively (Fig. 8; Table 2). Histograms and probability density functions define multimodal spectra for each sample that are similar to those observed for the shonkinite dikes in site 2; however, the shonkinite dikes from sites 4 and 5 have a larger number of <1450 Ma zircons (Fig. 8; App. 2). Sample GH-16-1 appears to have a more complex inherited age spectra with at least four distinct age peaks between 1750–1600 Ma, whereas sample 16-MP-13 only has two distinct age peaks at about 1670 and 1750 Ma (Fig. 8).

Sites 1–5, combined: Combining all concordant primary and inherited zircon analyses for sites 1 through 5 reveals a unimodal probability density function for primary zircons in shonkinite and syenite units, with a 207Pb/206Pb age peak at about 1415 Ma and a multimodal probability density function for inherited zircons with dominant age peaks at about 1670 and 1748 Ma (Fig. 9). The unimodal primary age peak overlaps published monazite Th-Pb ages of 1371 ± 10 and 1396 ± 16 Ma for the Sulphide Queen carbonatite stock and Birthday carbonatite dike (Fig. 9; Poletti et al., 2016).

Zircon trace element geochemistry

Trace element variations are observed between what are interpreted as primary (1450–1350 Ma) and inherited (1800–1600 Ma) Mountain Pass zircon populations. Primary zircons have distinctly lower Hf (5,000–11,000 ppm) and higher Eu/Eu* (0.6–1.4) compared to inherited zircons (8,000–16,000 ppm Hf and 0.1–0.8 Eu/Eu*) (Fig. 10A, B). Glimmerite zircons have some of the lowest Hf and highest Eu/Eu* of the shonkinite-syenite series samples (Fig. 10A, B). These zircons are also observed to have some of the lowest U (<300 ppm), Th (<120 ppm), and Th/U (<0.5) (Fig. 10C, D; App. 2). Primary zircons have <800 ppm U, whereas inherited zircons have up to 1,500 ppm U (App. 2). Conversely, primary zircons have up to 1,500 ppm Th, whereas inherited zircons typically do not exceed 500 ppm Th (Fig. 10C; App. 2). Zircons from leucosyenite sample 17-MP-02 have 100–600 ppm Th, whereas the three inherited zircons in this sample have 10–100 ppm Th (Fig. 10C). This observation is consistent with the whole-rock geochemical data that shows distinctly higher Th in the leucosyenite compared to basement gneisses (Fig. 2I). Higher Th in the shonkinite-syenite series zircons results in higher Th/U compared to the inherited zircons, with shonkinite sample 3311 having the overall highest Th/U (2–3.5) (Fig. 10D).

Primary and inherited Mountain Pass zircons have contrasting REE patterns, and differences are observed for the primary zircon populations within and between samples (Fig. 11). For site 1, zircons from shonkinite sample 17-MP-01 define two different REE patterns (R1 and R2) (Fig. 11A). Whereas R2 is analogous to the patterns observed for samples 17-MP-02 and 3311, R1 has much higher LREEs (Fig. 11A). Inherited zircons in sample 17-MP-02 have prominent negative Eu anomalies (Fig. 11B), which are not observed in any of the primary zircons from site 1. For site 2, from which primary zircons are sparse, grains define different REE spectra, one with a slight negative Eu anomaly (Fig. 11C). For this site, the inherited zircons also display different REE patterns, one with much higher HREE concentrations, though with negative Eu anomalies comparable to one another (Fig. 11D). For site 3, the glimmerite zircons have generally lower LREEs, particularly Ce, no Eu anomaly, and a flattening of HREEs compared to the other shonkinite-syenite series samples (Fig. 11E). For sites 4 and 5, primary zircons define identical REE patterns for the two shonkinite dike samples (Fig. 11G). Their inherited zircon REE spectra are likewise very similar but with a broader spread in REE concentrations (Fig. 11H). In all samples, discordant analyses (dashed) are observed to have some of the highest measured REEs (Fig. 11).

Ti-in-zircon thermometry

Titanium concentrations were measured in all dated zircons and used to calculate zircon crystallization temperatures using the calibration of Ferry and Watson (2007) (Fig. 12). Titanium concentrations are highest for zircons from shonkinite sample 3311 from site 1, with an average Ti value of 50 ± 10 ppm and crystallization temperature of 970 ± 30°C (Fig. 12A). Zircons from the other shonkinite sample at site 1, 17-MP-01, have a bimodal temperature distribution that corresponds to the different REE spectra observed for this sample, R1, and R2 (Fig. 12A). R1 zircons have higher Ti concentrations and temperatures (avg 21 ± 8 ppm Ti and 860 ± 50°C) than R2 zircons (avg 1.7 ± 0.6 ppm Ti and 625 ± 30°C) (Fig. 12A). Zircons from leucosyenite sample 17-MP-02 at site 1 and the shonkinite dikes at sites 2, 4, and 5 have Ti concentrations and temperatures that cluster near those of the R1 zircons, average 8–21 ± 2–12 ppm Ti and 760–850 ± 20–70°C (Fig. 12A). Glimmerite zircons from site 3 have Ti concentrations that are orders of magnitude lower than the other shonkinite-syenite series samples, with an average Ti concentration of 0.3 ± 0.2 ppm and crystallization temperature of 500 ± 30°C (Fig. 12A). All inherited zircons have Ti concentrations and temperatures that cluster fairly tightly, average of 4–7 ± 1–4 ppm Ti and temperatures of 690–760 ± 20–70°C (Fig. 12B).

Zircon oxygen isotope geochemistry

Oxygen isotope data were collected for a subset of dated zircons, with spot analyses adjacent to geochronology spots in single grains (Fig. 13). Zircons from site 1 display a very large range in δ18O values of 2–9‰ (Fig. 13A). All samples collected from this site possess a small fraction of zircons with low δ18O values that are less than mantle values (<5.3 ± 0.3‰) (Fig. 13A). A couple of zircons have δ18O compositions consistent with mantle values, but most have crustal, normal to high δ18O values of 7–8‰ for both shonkinite and leucosyenite samples (Fig. 13A). Zircons from site 2 shonkinite dike sample 17-MP-03 have mantle values to very high δ18O (supracrustal) values of 8.5‰ (Fig. 13A). In comparison, the shonkinite dike sample GH-16-1 from site 4 has more restricted values of 6.5–7.5‰ for zircons that are similar to those of site 1. About half of the glimmerite zircons are consistent with mantle values, whereas the other half have more crustal values up to 6.5‰ (Fig. 13A). Inherited zircons from the shonkinite dikes at sites 2, 4, and 5 have a broad range in δ18O values from mantle to supracrustal, with δ18O values as high as 10‰ (Fig. 13B). All but the highest δ18O values for inherited zircons overlap the ranges defined by 1450–1350 Ma primary zircons (Fig. 13).

Discussion

Timing and duration of alkaline and carbonatite magmatism at Mountain Pass

Zircons from the shonkinite-syenite series alkaline rocks at Mountain Pass record complex crystallization histories. Shonkinite dikes are dominated by inherited rather than primary zircons, as is apparent from U-Pb dating results that show multimodal age spectra extending to paleoproterozoic time (Figs. 6, 8). Combining all concordant age data for shonkinite-syenite series zircons reveals a unimodal distribution for the youngest Mesoproterozoic grains, with a peak at about 1415 Ma (Fig. 9).

Errors on individual analyses and calculated concordia and mean ages are at the limit of resolution for parsing magmatic episodes within tens of millions of years. It is therefore difficult to discern whether the magmatic events that produced the Mountain Pass alkaline intrusive suite occurred in one major pulse in the Mesoproterozoic, which is statistically permissive from the combined unimodal probability density function, or in numerous pulses. We note that our geochronology data are at a higher level of spatial resolution and permit calculation of relatively high precision concordia ages compared to previously published geochronology that relied primarily on discordia intercepts to define magmatic crystallization ages for Mountain Pass intrusive units (Poletti et al., 2016).

Crystallization ages established by this study are consistent with field relationships. For example, physically intermingled shonkinite and leucosyenite units have indistinguishable zircon concordia ages of 1409 ± 12 and 1410 ± 8 Ma (Fig. 5). A finer-grained shonkinite unit from site 1 (sample 3311) has a zircon concordia age of 1415 ± 6 Ma, possibly indicating large time gaps in spatially proximal intrusions (Fig. 5). Although this calculated concordia age overlaps other site 1 zircon samples within the outer bounds of uncertainty, textural and chemical characteristics of zircons from the finer-grained shonkinite are distinctive, such as in their higher proportion of concordant analyses (Fig. 5), higher Th/U (Fig. 10D), and higher Ti (Fig. 12A).

We favor an interpretation in which crystallization of compositionally diverse alkaline silicate magmas occurred over tens of millions of years, beginning at about 1450 Ma, with peak magma production 1415–1410 Ma and waning dike magmatism 1390–1370 Ma. Textural and chemical differences in zircons from different sample sites signify diverse petrogenetic origins of shonkinite-syenite magmas. Silicate and carbonatite magmatism at Mountain Pass was spatially coincident and permissibly also coeval over substantial geologic time. Monazite Th-Pb ages indicate carbonatite magmas crystallized, or were altered, at 1400–1370 Ma, with the Sulphide Queen orebody representing the youngest yet-documented phase of carbonatite magmatism (Poletti et al., 2016). Dated monazite crystals from the Sulphide Queen carbonatite intrusion are described by Poletti et al. (2016, p. 23) as “anhedral, porous, crumbly grains” and therefore may be secondary. Nonetheless, monazite Th-Pb dates overlap the youngest U-Pb-dated zircon grains of this study (Fig. 9). If monazite was secondary, it was probably associated with heat and hydrothermal alteration from carbonatite magmatism. We note the presence of two young outlier zircon grains in the glimmerite unit, with ages of 1019 ± 25 and 1092 ± 24 Ma (Fig. 7), which we interpret as local thermal overprinting from unrelated diabase dike magmatism in the Mountain Pass region ca. 1.1 Ga (Miller and Wooden, 1994).

Shonkinite dikes scavenged zircon from diverse crustal rocks, including earlier Mesoproterozoic alkaline intrusive rocks and Paleoproterozoic basement rocks that represent hundreds of millions of years of geologic history prior to Mountain Pass magmatism. The oldest inherited zircons in the shonkinite dikes have ages of 1780–1750 Ma, with a probability density peak at about 1748 Ma for combined age data (Fig. 9). This age range corresponds to metaigneous gneisses in the Ivanpah Mountains that have been interpreted as plutonic intrusions related to arc magmatism during the Paleoproterozoic (Strickland et al., 2012). Younger Paleoproterozoic zircons in the inherited spectra have ages of 1740–1600 Ma, with a dominant age peak at about 1670 Ma (Fig. 9). The 1740–1670 Ma time period is associated with superimposed events of metamorphism and magmatism that led to deformation and partial melting of orthogneisses and paragneisses during arc magmatism, collisional tectonics, and extension (Strickland et al., 2012). Inherited zircon grains in a few of the shonkinite dikes record a 1600–1500 Ma period of crystallization (Figs. 6, 8, 9) without a known tectonic association. They may represent an older and less voluminous phase of Mesoproterozoic magmatism that lacks surface exposures in the Mountain Pass region. The geochemical signature of these zircons is indistinguishable from the 1800–1600 Ma zircons and distinct from the 1450–1350 primary zircons, for example in their higher Hf, and lower Eu/Eu*, Th, and Th/U (Fig. 10). Therefore, we interpret them to be derived from similar crustal sources to the 1800–1600 Ma basement rocks.

Petrogenesis of alkaline and carbonatite magmas at Mountain Pass

Shonkinites at Mountain Pass have whole-rock geochemical signatures diagnostic of a mantle source, particularly high MgO (≤15 wt %), MgO/MgO + FeO (≤0.7), and compatible elements Ni (≤500 ppm) and Cr (≤1,000 ppm) (Fig. 2D, E; Table 1; App. 1). In comparison, the most mafic basement amphibolites have much lower Ni (≤200 ppm) and Cr (≤300 ppm, one outlier at about 700 ppm) (Fig. 2E). Along with observed compatible element enrichments, shonkinites have extremely high concentrations of incompatible elements, including K2O (≤13 wt %), F (≤2 wt %), P (≤1.2 wt %), Ba (≤1.3 wt %), Th (≤400 ppm), and LREEs (≤1,100× chondritic) (Figs. 2B, G, 3; App. 1). By analogy with lamproites (cf. Haxel, 2005; Castor, 2008), this evidence has been taken to indicate a depleted source in the subcontinental lithospheric mantle, from which extraction of melts over geologic time yielded a residuum rich in compatible elements, followed by reenrichment of this source in incompatible elements during metasomatism, possibly in many superimposed events (Mitchell and Bergman, 1991). In lamproites, phlogopite is the source of K, Ba, and F, whereas phosphate and titanate minerals (e.g., perovskite, crichtonite) may be the source of LREEs (Mitchell and Bergman, 1991; Haxel, 2005; Beyer et al., 2013). Hybridization of partial melts from heterogeneous components of phlogopite-rich veins and wall-rock peridotite can yield compositionally diverse ultrapotassic rock suites (Foley, 1992) and is our preferred model for the Mountain Pass shonkinite-syenite series rocks, with the addition of crustal assimilation.

The Mountain Pass carbonatite shares many diagnostic chemical characteristics with the shonkinite-syenite series rocks, including extraordinarily high LREEs (≤100,000× chondritic), Ba (≤20 wt %), and F (≤1.4 wt %) (Fig. 3). This chemical evidence, combined with a coincident location, similar spatial orientation to the shonkinite-syenite intrusions, and overlapping geochronology and isotopic data for accessory minerals, supports a petrogenetic relationship between alkaline and carbonatite intrusions at Mountain Pass (Olson et al., 1954; Haxel, 2005; Castor, 2008; Poletti et al., 2016; this study). Whereas the mantle domain(s) that were the source of both the alkaline and carbonatite magmas were enriched in LREEs, F, and Ba, the relative proportions of these elements and volatile components varied. The generation of lamproites requires H2O-rich sources, with high H2O/(CO2 + H2O), while carbonatites require a much greater CO2 component, with lower H2O/(CO2 + H2O) ratios that favor the generation of SiO2-undersatuarted melts (Foley et al., 1986). Fluorine is a critical volatile component that increases the liquidus phase volumes for both silicate and carbonate melts (Bergman, 1987; Gittins et al., 1990).

Our new data support melting of a heterogeneous, metasomatized mantle source region over tens of millions of years to generate compositionally diverse alkaline and carbonatite magmas. However, it does not appear that mantle metasomatism substantially predated Mountain Pass magmatism. The 1800–1600 Ma basement rocks lack the whole-rock and zircon geochemical signatures of a mantle component that are present in the Mountain Pass shonkinite-syenite series rocks and carbonatite. We infer that the metasomatic event or events occurred after about 1500 Ma, based on sparse concordant 1600–1500 Ma igneous zircons that are similar to the 1800–1600 Ma inherited zircons rather than 1450–1350 primary zircons (Figs. 1012).

The decrease of elements derived from a metasomatized mantle source, such as Ba, F, and LREEs, with increasing silica in the shonkinite-syenite series rocks (Figs. 2, 3) is indicative of a waning mantle component, which is most undiluted in the shonkinite dikes. Crystallization of phlogopite, which has large partition coefficients for both compatible (e.g., Ni, Cr, Ti) and normally incompatible elements (e.g., K, Ba), would have lowered the concentrations of these elements in the melt(s) as the magmas evolved (Sweeney et al., 1995; Foley et al., 1996). Though the shonkinite dikes represent some of the most primitive magma compositions of the rocks in this study, they are not parental to the more evolved magmas, based on their younger zircon ages and unique zircon trace element geochemistry (Figs. 511).

Zircons from the intermingled shonkinite and leucosyenite intrusive units at site 1 have disparate REE signatures (Fig. 11A) and Ti-in-zircon temperatures (Fig. 12A) that are not consistent with crystallization from a shared parental magma prior to emplacement, despite their clear textural association in outcrop and identical zircon concordia ages. The shonkinite dikes at sites 2, 4, and 5 have very distinctive zircon populations, particularly in their abundance of inherited xenocrysts, which is not observed in the other shonkinite-syenite series rock samples (Figs. 58). These observations are consistent with complex crosscutting relationships in the field that point to synchronous existence of different silicate magma types in the crust (Olson et al., 1954; Haxel, 2005; Poletti et al., 2016; this study).

As one of the last expressions of igneous activity at Mountain Pass, the Sulphide Queen carbonatite magma body was able to ascend to high crustal levels, perhaps exploiting previous conduits from shonkinite-syenite intrusions that left heated rock envelopes and structural paths. The carbonatite contains diverse breccia fragments along its margins, including older alkaline and carbonatite intrusive rocks and basement gneisses (Olson et al., 1954; Castor, 2008). The high F and relatively low P of the Sulphide Queen carbonatite magma would have enhanced REE transport and prevented loss of REEs to phosphate crystallization as the carbonatite magma ascended, permitting late-stage, ore-grade bastnäsite mineralization.

One of the most striking results of our new study is the role of crustal assimilation in the genesis of shonkinite-syenite series magmas at Mountain Pass. Though crustal assimilation is generally dismissed for lamproite or lamproite-like magmas, including those at Mountain Pass (e.g., Castor, 2008), the zircon results of this study and the titanite results of Poletti et al. (2016) provide unequivocal evidence of its importance. Inherited 1800–1600 Ma Paleoproterozoic zircons in the leucosyenite sample and all four shonkinite dike samples are irrefutable evidence for assimilation of basement crust (Figs. 5, 6, 8). The inherited zircons have unique trace element signatures, such as for Hf and Th, and strong negative Eu anomalies indicative of more-evolved crustal magmas with significant plagioclase fractionation (Figs. 9, 10). Oxygen isotope compositions of primary Mountain Pass zircons indicate that they crystallized from melts that assimilated crustal and supracrustal materials, with δ18O values of 7–8‰ that far exceed mantle values of 5–5.5‰ for most zircons (Fig. 13). Overlap in primary zircon δ18O values with those found for inherited Paleoproterozoic zircons supports assimilation of orthogneisses and paragneisses like those that compose the shallow basement crust at Mountain Pass (Figs. 1B, 13).

While the crustal assimilation signature is readily apparent in zircons, it is less so for whole rocks. Basement rocks have different major and trace element trends and REE patterns relative to the shonkinite-syenite series rocks (Figs. 2, 3). We interpret the whole-rock signatures of the shonkinite-syenite series units to be more reflective of the mantle contribution to these magmas, whereas accessory zircons are more reflective of the crustal component, perhaps because of assimilation that led to late zircon saturation once the magmas were emplaced in the shallow crust.

Hydrothermal conditions of zircon crystallization are evident in some samples, particularly the glimmerite and the R2 population of shonkinite 17-MP-01. Whole-rock geochemical data for the glimmerite, with high CaO and high loss on ignition (LOI), corroborate the presence of groundmass carbonate and alteration (Fig. 2C; Table 1; J. Landreth, writ. commun., 2016). Zircons from the glimmerite and shonkinite 17-MP-01 have attached calcite (both samples) and calcite inclusions (glimmerite). The glimmerite zircons have far lower Th and Th/U than the other shonkinite-syenite series rocks (Fig. 10C, D), and REE patterns for glimmerite zircons are also different, with smaller positive Ce anomalies and flattening of HREEs (Fig. 11E). Temperatures of zircon crystallization for the glimmerite and R2 population of sample 17-MP-01 are low and within a hydrothermal regime: about 500° and 600°C, respectively (Fig. 12A). In general, discordant zircons have trace element concentrations similar to those of concordant grains for all samples of this study (Fig. 10), including for Ti (Fig. 12), but irregularities are apparent in REE spectra, where discordant grains tend to have the highest LREEs for a given sample (Fig. 11). Unique REE spectra are observed for the two different zircon populations in shonkinite 17-MP-01 (Fig. 11A). Temperatures for the lower REE population (R2) correspond to what are interpreted as hydrothermal zircons (Fig. 12A).

Low-δ18O zircons from shonkinite and leucosyenite samples at site 1 can only be explained by incorporation of meteoric water prior to zircon crystallization (Fig. 13A). The lowest-δ18O zircons (2‰) have high Ti-in-zircon temperatures (850°C) that are consistent with assimilation of hydrothermally altered crust and igneous growth, rather than hydrothermal precipitation. The hydrothermally altered crustal assimilant may be related to Mountain Pass magmatism or an older magmatic episode. For example, low δ18O values of 3–4‰ have been reported for Paleoproterozoic metaigneous zircons from the Ivanpah Mountains to the south (Strickland et al., 2012).

Conclusions

Zircon populations in alkaline intrusive rocks at Mountain Pass record a complex history of igneous and hydrothermal processes. We interpret the combined zircon geochronology, geochemistry, and isotope geochemistry to indicate long-lived (1450–1370 Ma) alkaline magmatism with peak production 1415–1410 Ma. Magmas incorporated diverse mantle and crustal components. While a metasomatized mantle source has long been recognized in the whole-rock geochemical data for alkaline intrusive units at Mountain Pass, the role of crustal assimilation, including Mesoproterozoic and Paleoproterozoic intrusive, metamorphic, and supracrustal sources, is only apparent through high spatial resolution zircon analyses. The shonkinite dikes are the most primitive Mountain Pass magmas, but our geochronology and geochemistry data are not consistent with a model in which they are parental to compositionally distinct shonkinite-syenite or carbonatite units. Our results support broadly coeval and cogenetic alkaline and carbonatite magmatism from primary partial melts of a heterogeneous, long-lived, and repeatedly tapped metasomatized mantle reservoir, with the shonkinite dikes representing the youngest phase of alkaline magmatism that overlapped in space and time with the intrusion and alteration of carbonatite magmas 1396–1371 Ma. The zircon study presented here emphasizes the importance of a mineral-scale approach in investigating the petrogenesis of these chemically unique and complex igneous systems.

Acknowledgments

We gratefully acknowledge the Mountain Pass mine staff for providing access to sites within the mine property and for sharing expertise and samples. Specific thanks are owed to the late John Landreth, as well as to Dan Cordier and Nick Dolan. Field work and sharing of data and ideas with USGS colleagues Wayne Premo, Dave Ponce, Kevin Denton, and Rick Moscati have greatly aided this research. We acknowledge the following individuals for their guidance during analytical sessions: Jorge Vazquez and Matt Coble at the USGS-Stanford SHRIMP-RG lab, Ming-Chang Liu at the University of California, Los Angeles, and Leslie O’Brien at the Menlo Park microanalytical facility. Tim O’Brien is acknowledged for providing assistance with zircon separations at Stanford University, and Juliette Ryan-Davis and Dean Miller are acknowledged for aiding in sample preparation at the USGS in Menlo Park. Reviews by Wayne Premo, Anton Chakhmouradian, Lauro Nardi, and Esa Heilimo improved the clarity and presentation of this work. We thank Larry Meinert and Anton Chakhmouradian for editorial handling. Research was supported by the USGS Mineral Resources Program. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. government.

Kathryn Watts is a research geologist with the U.S. Geological Survey (USGS). She is part of the Geology, Minerals, Energy, and Geophysics Science Center in Spokane, Washington. Previously, she was a USGS Mendenhall Postdoctoral Research Fellow in Menlo Park, California. She has a Ph.D. degree in geological sciences from the University of Oregon and a B.S. degree in geology from Louisiana State University. Kathryn’s current research is at the intersection of igneous petrology and economic geology. She integrates field work with high spatial resolution analytical tools to unravel complex geologic processes that give rise to dynamic magmatic systems and ore deposits.

Gold Open Access: This paper is published under the terms of the CC-BY-NC 3.0 license.

Supplementary data