J OURNAL OF Journal of Petrology, 2016, Vol. 57, No. 8, 1555–1598 doi: 10.1093/petrology/egw050 P ETROLOGY Original Article

Petrochronological Constraints on the Origin of the Mountain Pass Ultrapotassic and Carbonatite Intrusive Suite, California Downloaded from https://academic.oup.com/petrology/article-abstract/57/8/1555/2413438 by guest on 31 March 2020 Jacob E. Poletti1*, John M. Cottle1, Graham A. Hagen-Peter1 and Jade Star Lackey2

1Department of Earth Science, University of California, Santa Barbara, CA 93106-9630, USA and 2Geology Department, Pomona College, Claremont, CA 91711, USA

*Corresponding author. Telephone: 805-748-2549. Department telephone: 805-893-4688. Department fax: 805-893-2314. E-mail: [email protected]

Received October 21, 2015; Accepted August 3, 2016

ABSTRACT Rare earth element (REE) ore-bearing carbonatite dikes and a stock at Mountain Pass, California, are spatially associated with a suite of ultrapotassic plutonic rocks, and it has been proposed that the two are genetically related. This hypothesis is problematic, given that existing geochronolo- gical constraints indicate that the carbonatite is 15–25 Myr younger than the ultrapotassic rocks, requiring alternative models for the formation of the REE ore-bearing carbonatite during a separate event and/or via a different mechanism. New laser ablation split-stream inductively coupled plasma mass spectrometry (LASS-ICP-MS) petrochronological data from ultrapotassic intrusive rocks from Mountain Pass yield titanite and zircon U–Pb dates from 1429 6 10 to 1385 6 18 Ma, ex- panding the age range of the ultrapotassic rocks in the complex by 20 Myr. The ages of the youngest ultrapotassic rocks overlap monazite Th–Pb ages from a carbonatite dike and the main carbonatite ore body (1396 6 16 and 1371 6 10 Ma, respectively). The Hf isotope compositions of zircon in the ultrapotassic rocks are uniform, both within and between samples, with a weighted

mean eHfi of 19 6 02 (MSWD ¼ 09), indicating derivation from a common, isotopically homoge- neous source. In contrast, in situ Nd isotopic data for titanite in the ultrapotassic rocks are variable

(eNdi ¼ –35 to –12), suggesting variable contamination by an isotopically enriched source. The most primitive eNdi isotopic signatures, however, do overlap eNdi from monazite (eNdi ¼ –28 6 02) and bastnasite€ (eNdi ¼ –32 6 03) in the ore-bearing carbonatite, suggesting derivation from a com- mon source. The data presented here indicate that ultrapotassic magmatism occurred in up to three phases at Mountain Pass (1425, 1405, and 1380 Ma). The latter two stages were coeval with carbonatite magmatism, revealing previously unrecognized synchronicity in ultrapotassic and carbonatite magmatism at Mountain Pass. Despite this temporal overlap, major and trace element geochemical data are inconsistent with derivation of the carbonatite and ultrapotassic rocks by li- quid immiscibility or fractional crystallization from common parental magma. Instead, we propose that the carbonatite was generated as a primary melt from the same source as the ultrapotassic rocks, and that although it is unique, the Mountain Pass ultrapotassic and carbonatite suite is broadly similar to other alkaline silicate–carbonatite occurrences in which the two rock types were generated as separate mantle melts.

Key words: carbonatite; geochronology; mineralization; rare earth elements; titanite

VC The Author 2016. Published by Oxford University Press. All rights reserved. For Permissions, please e-mail: [email protected] 1555 1556 Journal of Petrology, 2016, Vol. 57, No. 8

INTRODUCTION ultrapotassic rocks are derived from the same parental The Sulphide Queen rare earth element (REE) mine at magma or were generated during the same melting Mountain Pass, California is the richest known source of event. Thus, it has been asserted that, although these REE ore in the USA (Long et al., 2010). Since its discov- rocks are geochemically similar, the MPIS is not a sim- ery in 1949 until the late 1980s Mountain Pass was the ple cogenetic magma series (e.g. Haxel, 2005), and world’s foremost producer of REE commodities models testing the common parental magma hypoth- (Verplanck & Van Gosen, 2011). However, despite its esis have not been presented. status as an archetype for carbonatite-hosted REE de- This study utilizes titanite, zircon, and monazite U– posits, the petrogenesis of the REE-bearing Mountain Th/Pb geochronology, accessory-phase and whole-rock Pass carbonatite remains debated. geochemistry, Nd and Hf isotope geochemistry and pet- Carbonatite bodies at Mountain Pass, along with ultra- rography within the context of field data to decisively Downloaded from https://academic.oup.com/petrology/article-abstract/57/8/1555/2413438 by guest on 31 March 2020 potassic, mafic to felsic, plutonic silicate rocks (shonkinite, resolve the temporal and petrogenetic relationships be- syenite, and granite), make up the Mesoproterozoic tween carbonatite and ultrapotassic igneous rocks in Mountain Pass Intrusive Suite (MPIS, Fig. 1). Based on the MPIS. A quantitative model is developed to test field relationships and the similar enrichment of these whether the ultrapotassic rocks and carbonatites at rocks in incompatible elements such as Sr, Ba, and light Mountain Pass are related by fractional crystallization REE (LREE), previous workers have inferred that the or liquid immiscibility. In aggregate, these data place Sulphide Queen carbonatite magma was derived from new constraints on existing ore genesis models for the the same source as the ultrapotassic rocks and that the in- Sulphide Queen carbonatite REE deposit, and indicate trusive assemblage may represent a single suite of vari- that the MPIS is the result of long-lived carbonatite– ably evolved alkaline igneous rocks (Woyski, 1980; alkaline silicate magmatism in which multiple melting Castor, 2008). This is broadly compatible with studies events of the same source produced separate carbona- from a variety of locations worldwide suggesting that car- tite and ultrapotassic melts. bonatite and alkali-rich silicate melts can differentiate from the same parental magma, potentially by liquid im- GEOLOGICAL CONTEXT miscibility (e.g. Lee & Wyllie, 1998; Woolley, 2003; Panina & Motorina, 2008), extreme fractional crystallization of a The Mountain Pass district is located in the central

CO2-rich silicate melt (e.g. Bell et al., 1999), or a combin- Mojave Desert of California, about 75 km SSW of Las ation of these processes (e.g. Beccaluva et al., 1992). Vegas, Nevada (Fig. 1). The district encompasses a In contrast, existing geochronological data from the topographic low between the Clark Mountain Range to MPISsuggestthatthecarbonatiteis15–25 Myr younger the north and the Mescal Range to the south, and is pri- than the ultrapotassic rocks at Mountain Pass, which marily underlain by autochthonous, Proterozoic crystal- yielded ages of 1375 Ma and 1400 Ma, respectively line basement rocks that lie in the footwall of the west- (DeWitt et al., 1987, 2000; Premo et al., 2013). Interpretation dipping Keystone Thrust of the Sevier Orogen of existing data is complicated by the fact that the ages are (Burchfiel & Davis, 1971; Fig. 1b). Neoproterozoic to derived from different minerals and isotopic systems in lower Paleozoic limestone, quartzite, and shale, and each rock type (i.e. zircon and apatite U–Pb, monazite U–Th/ Mesozoic siliciclastic and felsic volcanic rocks lie above Pb, amphibole and biotite Ar–Ar) and are exclusively re- the thrust in the Mountain Pass area (Evans, 1974). In ported in conference abstracts (DeWitt et al.,1987, 2000; the east, the 7 km wide structural block of Proterozoic Premo et al., 2013). Thus, these ages lack the information crystalline rocks is bound by the inferred normal-sense necessary to directly compare the dates, such as which Ivanpah fault (Olson et al., 1954). The structural block decay constants and monitor standards were used, and that exposes MPIS rocks is also cut by several trans- what sources of uncertainty are included in the quoted verse faults, none of which place the Proterozoic rocks data. Furthermore, DeWitt et al. (1987) noted U–Pb discord- against units of significantly different age or lithology ance and ‘anomalously young’ dates in some samples, (Olson et al., 1954; Fig. 1b). Despite local faulting, deci- suggesting that age interpretation may be complicated for meter- to meter-scale dike emplacement in the some rock types at Mountain Pass. Cenozoic, and minor, localized hydrothermal alteration Field relationships generally indicate that carbonatite and localized deposition of Th along faults (Shawe, is the youngest rock type in the MPIS in terms of cross- 1952), the Proterozoic rocks in the district are relatively cutting relationships, but at least one study noted that a well preserved and were less affected by Cenozoic ex- centimeter-scale vein of shonkinite appears to cut car- tension and deformation compared with neighboring bonatite in at least one location (Olson & Pray, 1954), in- areas in the Mojave Desert (Haxel, 2005). consistent with the geochronological data. Given that fractional crystallization and liquid immiscibility proc- esses are commonly thought to operate over time- GEOLOGICAL UNITS AT MOUNTAIN PASS scales 10 Myr (e.g. Williams et al., 1986; Schmitt et al., Paleoproterozoic host rocks 2010), the proposed 15–25 Myr age gap is irreconcilable The Proterozoic crystalline basement rocks that underlie with the hypothesis that the carbonatite and the most of the Mountain Pass area are part of the Mojave ora fPetrology of Journal 115° 114° Gold Butte (a) NW Explanation (b) N Las Vegas N Nevada 12 t s 35.495° u Alluvium r 1A,1B, BDAY, BDAY2 8 No. 57, Vol. 2016, , California h 26 T 36° e Birthday n

o t s y 15 e Sedimentary and K C A L I F O R N I A volcanic rocks N E V A D A Mountain Pass ( (1440 m) SQ1

115.545° ) Neoproterozoic & Phanerozoic

MESQUITE Sulphide Stocks Dikes I-15

LAKE A Queen N

Eldorado R

Carbonatite

KEANY E

I

Mountains

Z PASS V

A O MINERAL D N Syenite & alkali granite Searchlight Mexican HILL A A MOUNTAIN Well Mesoproterozoic PASS NEW TRAIL Newberry Shonkinite (shown in CANYON IVANPAH Mountains 2 Fig. 1b) Groaner Kingman Dead Davis 4 Hualapai 6 19 35.47 Mountains Dam 7, 21 Gneiss, schist, and BOBCAT Homer Mountains Corral 5° granite pegmatite HILLS Mountain 5A, 5B

525°C Paleoproterozoic 5. l 35° a 11 r 9B Tors k 2 M 10A, 10B, o Sample location u 10C, 10D, n 16 Needles t 17 a 23 in Wheaton F

a Intrusive or depositional

u 22A

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RATTLESNAKE Gradational igneous A CANYON C

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Mountains I Fault, dotted Mesoproterozoic Paleoproterozoic 25 km Mineral Mesoproterozoic where concealed ultrapotassic rock granitoids metamorphic rocks Hill 15 occurrence 35.455 Tors 5.505° 1 km 11 115.485° ° Name of stock

Fig. 1. (a) Map showing locations of Paleoproterozoic metamorphic rocks and Mesoproterozoic intrusive rocks in the southwestern USA. Modified from Castor (2008). (b) Generalized geological map of the Mountain Pass area. Alluvium contacts/exposures have been simplified for clarity. It should be noted that dike thickness is not to scale, and that north is not up.

Modified from Haxel (2005), after Olson & Pray (1954), Olson et al. (1954), Burchfiel & Davis (1971) and Evans (1971).

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Crustal Province (Wooden & Miller, 1990) and are pri- within a 7km 2 km zone in the host gneiss. Most marily Paleoproterozoic gneiss and schist. The dikes and stocks in the MPIS are oriented roughly paral- Paleoproterozoic rocks of the Mojave Crustal Province lel to the regional foliation of the basement rocks record a complex history of plutonism, metamorphism, (roughly north to NW), but some dikes also strike and sedimentation associated with multiple orogenic roughly NE. Dikes and stocks of both orientations cross- events. U–Pb zircon ages from the province reported in cut foliation on an outcrop scale and are undeformed; several studies have been interpreted to indicate sev- there does not appear to be an obvious structural rea- eral discrete episodes of widespread pluton emplace- son for the two different trends. In general, cross- ment between 179 and 164 Ga (Wooden & Miller, cutting relationships indicate that the sequence of intru- 1990; Barth et al., 2000, 2009; Strickland et al., 2013; sion in the MPIS was (1) shonkinite and melanosyenite, McKinney et al., 2015). Zircon and monazite U–Pb ages (2) syenite, (3) quartz syenite, (4) granite, (5) shonkinite indicate that metamorphism also took place in several dikes, and (6) carbonatite (e.g. this work, Fig. 2a–d; Downloaded from https://academic.oup.com/petrology/article-abstract/57/8/1555/2413438 by guest on 31 March 2020 discrete episodes, occurring locally between 179 and Olson et al., 1954; Haxel, 2005; Castor, 2008). However, 165 Ga (Wooden & Miller, 1990; Barth et al., 2000, 2009; as stated above, a centimeter-scale shonkinite vein ap- Strickland et al., 2013; McKinney et al., 2015). The pears to cut carbonatite in at least one location (Olson & Paleoproterozoic rocks of the Mojave Crustal Province Pray, 1954; Haxel, 2007). Magma mingling and mixing were widely intruded by 145–140 Ga granitoids textures are common in some areas (e.g. Tors stock, throughout the southwestern USA (e.g. Anderson & Figs 1b and 2e; Haxel, 2005), and some shonkinite dikes Bender, 1989; McKinney et al., 2015), including the within syenite are discontinuous, deformed, and back- Mountain Pass Intrusive Suite. The origin of the 145– intruded by fine-grained syenite (Fig. 2f), indicating that 14 Ga granitoids and the tectonic environment in which emplacement of some of the shonkinite and syenite they were emplaced remains debated, with some work- was synchronous (this work; Haxel, 2005). The Sulphide ers inferring an anorogenic setting (e.g. Anderson & Queen carbonatite stock, which is the principal ore Bender, 1989; Hoffman, 1989), whereas others have body at Mountain Pass (Fig. 1b), contains blocks of concluded that they were emplaced during an orogenic gneiss, shonkinite, syenite, and older carbonatite, the event at 149–135 Ga (Picuris orogeny of Daniel et al., last indicating that carbonatite was not emplaced dur- 2013; and see also Nyman et al., 1994; Daniel & Pyle, ing a single event (Olson et al., 1954). 2006; Whitmeyer & Karlstrom, 2007). The MPIS lies roughly at the northwesternmost end In the Mountain Pass area, the Paleoproterozoic of a zone containing Mesoproterozoic alkaline granit- rocks are quartzofeldspathic gneiss and schist, coarse- oid plutons that vary in areal exposure from 4to150 grained to pegmatitic alkali granite dikes, and minor km2, extending from southeastern California into mafic to ultramafic rocks (this work; Olson et al., 1954; southern Nevada and western Arizona (Haxel, 2007; Haxel, 2005; Castor, 2008; Fig. 1). K-feldspar augen Castor, 2008; Fig. 1a; also see above). Ultrapotassic gneiss and migmatitic K-feldspar þ plagioclase þ quartz rocks are known only from the western edge of this þ biotite 6 garnet 6 sillimanite gneiss are most com- zone and form an 130 km belt from Rattlesnake mon. Foliation in the gneiss and schist generally strikes Canyon in the south to Mesquite Lake in the north north to NW and dips 45–85 to the west (this work; (Castor, 2008; Fig. 1a), indicating that their occurrence Olson & Pray, 1954; Olson et al., 1954). Mineral ages at Mountain Pass is potentially not unique (e.g. from Paleoproterozoic rocks in the Mountain Pass area Gleason et al., 1994). However, the ultrapotassic rocks have not been reported, but are generally assumed to that compose the belt are not well studied outside the be similar to the rest of the Mojave Crustal Province Mountain Pass area, with the exception of the Barrel based on their similar field relationships, lithology and Spring Pluton in the Piute Mountains (Gleason et al., close proximity to well-studied occurrences (e.g. 1994; Fig. 1a). Generally, neither the relationships Ivanpah Mountains, Strickland et al., 2013; Fig. 1a). among ultrapotassic rock occurrences in the region nor the broader connections to Mesoproterozoic alka- line magmatism in the southwestern USA are well Mountain Pass Intrusive Suite studied (Haxel, 2007; Castor, 2008). Paleoproterozoic rocks in the Mountain Pass area are intruded by hundreds of sub-meter- to meter-scale dikes and eight map-scale stocks that compose the Shonkinite Mountain Pass Intrusive Suite (Fig. 1b). Six of the stocks The most abundant intrusive rock in the study area is consist of shonkinite and/or syenite, one contains sig- coarse-grained shonkinite—a distinctive, dark gray to nificant granite in addition to syenite (Mineral Hill), and greenish rock composed of microcline, clinopyroxene the Sulphide Queen stock is carbonatite (Fig. 1b). (diopside, augite, and/or aegirine–augite), and euhedral, Geochemical, mineralogical, and textural gradations Mg-rich biotite, with or without amphibole (riebeckite, between shonkinite–syenite and syenite–granite are arfvedsonite, and/or hornblende) and minor sodic plagio- common in single bodies. The stocks form ovoid to clase, perthite, and apatite (up to 3%). Accessory roughly tabular, sill-like bodies tens to hundreds of phases include quartz, zircon, titanite, magnetite, pyrite, meters wide and hundreds of meters long, and occur Fe-oxides, calcite, and fluorite. Some samples contain Journal of Petrology, 2016, Vol. 57, No. 8 1559 Downloaded from https://academic.oup.com/petrology/article-abstract/57/8/1555/2413438 by guest on 31 March 2020

Fig. 2. Examples of cross-cutting relationships within the MPIS. Contacts are indicated by dashed lines for clarity. (a) Granite xeno- liths in melanosyenite dike (MP-P0913-6) just west of Corral stock. (b) Porphyritic shonkinite enclave (MP-P0914-10A) in coarse sy- enite (MP-P0913-10B) in Tors stock. (c) Porphyritic shonkinite dike (e.g. MP-P0913-10C) in coarse syenite (MP-P0913-10B) in Tors stock. (d) Bastnasite–barite€ carbonatite dike (MP-L0613-BDAY2; Birthday Vein) that cuts coarse shonkinite (MP-L0613-BDAY, MP- P0913-1A) in Birthday stock. (e) Diffuse syenite inclusions in shonkinite at Tors stock. (f) Shonkinite dike that cuts coarse syenite and is back-intruded by fine-grained syenite (MP-P0714-17) in Tors stock (ex situ). c, coarse; f, fine. micrometer-scale veins composed primarily of barite, Syenite, quartz syenite, and granite quartz, and Fe-oxide with minor monazite. Based on min- Syenite is the second most common intrusive rock type eralogy, the rock is a melanosyenite; however, the name at Mountain Pass, occurring as a pinkish to purple to shonkinite is used here to maintain continuity with previ- brick-red rock that consists mostly of K-feldspar with ous work and to describe the most mafic rocks consisting variable amounts of biotite, sodic amphibole (riebeckite of K-feldspar þ biotite þ clinopyroxene 6 amphibole. and/or arfvedsonite) and quartz, with minor sodic plagio- Melanosyenite is used for rocks that are transitional from clase, clinopyroxene, and Fe-oxides. Accessory minerals shonkinite–syenite and contain 40–55% mafic minerals. include apatite, zircon, titanite, pyrite, fluorite, calcite, The shonkinite has MgO > 3wt %, K2O > 3wt %, and thorite, and galena. Rutile, epidote, monazite, and trace K2O/Na2O > 2, and is therefore considered ultrapotassic barite are present in some samples, but textural relation- based on the criteria of Foley et al. (1987). Some shonkin- ships suggest that these phases are secondary. Monazite ite is peralkaline [highest (K2O þ Na2O)/Al2O3 11], but and barite primarily occur in veins, as fine, disseminated most samples are metaluminous. Sample petrographic grains (this work; Stoeser, 2013), or as fine aggregates, descriptions and photographs of representative hand and are most abundant in samples with pervasive rust- samples are provided in Supplementary Data Electronic colored stains along grain boundaries and fractures (see Appendices 1 and 2, respectively (supplementary data Supplementary Data Electronic Appendix 1). Based on are available for downloading at http://www.petrology. these textures, we consider samples with abundant oxfordjournals.org). veins and/or fine, disseminated monazite and barite 1560 Journal of Petrology, 2016, Vol. 57, No. 8 grains to be altered (see also Stoeser, 2013). spatial association and similar geochemistry of the Leucosyenite, quartz syenite, and granite occur as dikes ultrapotassic rocks and carbonatites as evidence for a and pods in shonkinite or syenite, although the Mineral common parental magma. Hill stock (Fig. 1b) contains significant granite. The min- Available geochronological data do not support the eralogy of these rock types differs from that of syenite notion that the ultrapotassic rocks and carbonatites may only in the relative proportions of K-feldspar and quartz. have differentiated from a common parental magma. Some syenite and all leucosyenite at Mountain Pass is Early attempts to determine ages of the MPIS yielded potassic rather than ultrapotassic (see criteria above); data that were too imprecise to critically evaluate the ab- however, we use the term ultrapotassic here to refer to solute timing of the various intrusive phases (i.e. Jaffe, all K-rich igneous rocks for brevity. 1955; Lanphere, 1964). More recent studies by DeWitt et al. (1987, 2000) reported 40Ar/39Ar dates of 1400 6 8Ma and 1403 6 7 Ma from phlogopite in shonkinite and arf- Downloaded from https://academic.oup.com/petrology/article-abstract/57/8/1555/2413438 by guest on 31 March 2020 Carbonatite vedsonite in syenite, respectively; apatite from shonkin- Carbonatite at Mountain Pass varies in appearance, but ite yielded a U–Pb date of 1410 6 2 Ma; monazite from is generally porphyritic with centimeter-size barite carbonatite yielded a Th–Pb date of 1375 6 7 Ma. Zircons phenocrysts in a matrix of calcite, dolomite, quartz, Fe- from the syenite were reportedly highly discordant with oxide, and bastnasite€ ([LREE]CO F). Synchysite 3 respect to U–Pb. Bastnasite€ and parisite from carbonatite (Ca[LREE](CO ) F), parisite (Ca[LREE] (CO ) F ), anker- 3 2 2 3 3 2 contained abundant common Pb—both yielding dates of ite, siderite, monazite, thorite, zircon, strontianite, celes- 1330 Ma and younger, which DeWitt et al. considered tite, apatite, an unidentified Th-carbonate, and at least ‘anomalously young’ based on the monazite Th–Pb date two unidentified REE-bearing phases are common as of 1375 6 7 Ma for the same rock type. DeWitt et al. minor, accessory, or trace phases. The most abundant (1987, 2000) interpreted these dates as indicating em- types of carbonatite are bastnasite–barite€ beforsite placement of carbonatite 25 Myr after crystallization of (dolomite-dominated) and bastnasite–barite€ dolomitic the ultrapotassic intrusive rocks. so¨ vite (calcite-dominated with significant dolomite), but Premo et al. (2013) reported zircon U–Pb secondary so¨ vite (calcite-dominant) and silicio-carbonatite are also ionization mass spectrometry dates from shonkinite, sy- common (Castor, 2008). enite, and granite that overlap within uncertainty be- tween 1410 and 1422 Ma, supporting the interpretations of DeWitt et al. (1987, 2000), but did not specify whether SUMMARY OF PREVIOUS GEOCHEMICAL, there was a statistically distinguishable age difference GEOCHRONOLOGICAL, AND ISOTOPIC WORK between rock types. AT MOUNTAIN PASS DeWitt et al. (1987, 2000)andPremo et al. (2013) also Previous workers generally agreed that the MPIS car- reported Pb, Sr, and Nd isotopic data for the MPIS. bonatite and ultrapotassic rocks are genetically related Common Pb isotopic ratios from galena in carbonatite based on field relationships and similar geochemical and K-feldspar in the ultrapotassic rocks are as follows: characteristics (e.g. Olson et al., 1954; Woyski, 1980; 204Pb:206Pb ¼ 1:1608, 204Pb:207Pb ¼ 1:1532, 204Pb:208Pb ¼ Haxel, 2005, 2007; Castor, 2008). Haxel (2005, 2007) pro- 1:3561 (DeWitt et al., 1987). 87Sr/86Sr ratios vary from posed that a small degree (001–01%) of partial melt- 07046 to 07072 (DeWitt et al.,2000). Premo et al. (2013) ing of highly enriched lithospheric mantle source reported a slightly smaller range of whole-rock 87Sr/86Sr produced the shonkinite magma. In this model, shon- (07045–07068) and whole-rock eNdi values ranging kinite is the parental magma that evolved to the more from –15 to –6 for the ultrapotassic rocks and carbona- silicic rock types in the complex (syenite, quartz syenite, tite. DeWitt et al. (2000) interpreted their Pb, Sr, and Nd granite). Crow (1984) showed that the syenite could be isotope data to indicate derivation of the carbonatite produced by 20% crystal fractionation of the shonkin- magma from a hybrid source, produced by mixing of 14 ite. The relatively high abundance of compatible elem- Ga mantle and 175 Ga mantle or enriched lower crust. ents (e.g. Cr, Ni) but distinctively lower contents of Al, DeWitt et al. (2000) also noted that only the youngest Ca, and Na in the shonkinite in comparison with other shonkinite dikes at Mountain Pass have the same Nd iso- primitive mafic rocks was interpreted to indicate deriv- topic signature as the carbonatite. ation from harzburgitic peridotite that had previously Based on their U–Th/Pb data, DeWitt et al. (1987, been depleted in Al, Ca, and Na through extraction of 2000) and Premo et al. (2013) concluded that, although basaltic magma (Haxel, 2007). These geochemical char- the various intrusive phases in the MPIS appear to be acteristics coupled with extreme enrichment in incom- closely related based on petrological and geochemical patible elements (i.e. K, Ba, LREE) in the MPIS rocks evidence, the different rock types of the MPIS were not require that the depleted source was metasomatically contemporaneous. re-enriched (Haxel, 2007). The model of Haxel (2005) did not consider derivation of the carbonatite from a shon- kinite parental magma, partially owing to the implica- ANALYTICAL METHODS tions of the geochronological data reported by DeWitt Here, we report new U–Th/Pb, trace element, isotopic, et al. (1987, 2000). Conversely, Castor (2008) viewed the and/or textural data for a suite of U- and/or Th-bearing Journal of Petrology, 2016, Vol. 57, No. 8 1561 accessory minerals from 22 Mountain Pass shonkinite, to subhedral, dark brown to golden crystals up to syenite, and carbonatite samples and three host-rock 2mm(Fig. 5a and b). Interstitial grains are common in samples. Additionally, whole-rock geochemical data are coarse-grained samples, and euhedral matrix grains are reported for 14 ultrapotassic rock samples and one car- typical in fine-grained samples (Fig. 5a and b). Titanite bonatite sample. Three to four kilograms of each sam- is absent in most samples that contain significant ple was collected from suitable outcrops (e.g. Fig. 2), monazite and rutile (e.g. MP-P0913-2), with one excep- and representative samples were selected for mineral tion (MP-P0913-10B). Titanite grains in some samples analysis based on cross-cutting relationships, lithology, contain inclusions of clinopyroxene or apatite (Fig. 5a and spatial context (see Table 1 for summary of sam- and b) and small (2 mm) inclusions of monazite ples and sample locations). Zircon, titanite, monazite, (Fig. 5c); monazite inclusions are most common in sam- apatite, bastnasite€ and rutile were analyzed, but the ples that appear to have undergone alteration (see measured concentrations of U in the last three minerals above description of alteration textures in syenite). Downloaded from https://academic.oup.com/petrology/article-abstract/57/8/1555/2413438 by guest on 31 March 2020 were too low to provide reliable U–Pb dates; therefore Grains with small monazite inclusions were typically only zircon, titanite, and monazite data are utilized for avoided for analysis. Titanite in most samples appears geochronology in this study. Bastnasite€ and apatite are unzoned in BSE images, but some grains appear to be employed only for Nd isotopic data and trace element weakly zoned (Fig. 5d). Where identified, specific zones data, respectively. Accessory mineral U–Th/Pb isotopes were targeted for U–Pb, trace element and Nd isotopic and trace elements were measured by laser ablation analysis, but no consistent age, trace element, or iso- split-stream inductively coupled plasma mass spec- topic pattern with respect to zonation was observed. trometry (LASS-ICP-MS), accessory mineral Nd and Hf Similarly, core and rim regions of grains are indistin- isotopes were measured by laser ablation multi- guishable in terms of their age, trace element, and Nd collector (LA-MC)-ICP-MS, and whole-rock geochemis- isotopic composition. try was measured by X-ray fluorescence spectrometry Representative titanite geochronology data are pre- (XRF) and laser ablation quadrupole (LA-Q)-ICP-MS. sented in Table 3, and complete titanite LASS-ICP-MS Detailed analytical methods are presented in the data (including trace element data) are reported in Appendix, and representative SEM images of sample Supplementary Data Electronic Appendix 5. LASS-ICP- grains with labeled spot analyses are provided in MS U–Pb data from 10 titanite samples yielded single- Supplementary Data Electronic Appendix 3 (supplemen population [i.e. mean square weighted deviation tary data are available for downloading at http://www. (MSWD) 1] isochrons ranging from 1429 6 10 to petrology.oxfordjournals.org). 1385 6 18 Ma (Fig. 6). MP-P0714-17 has relatively little spread in U/Pb and 207Pb/206Pb ratios, leading to a rela- tively imprecise isochron age of 1358 6 71 Ma. Two RESULTS samples have MSWD ¼ 17, larger than expected for a Whole-rock geochemistry single age population, and yield apparent ‘errorchrons’ of 1410 6 17 and 1367 6 9 Ma (MP-P0714-16 and MP- Geochemical data for ultrapotassic rock samples are P0714-26). Subdividing the analyses into two single shown on variation diagrams in Fig. 3, and presented in populations based on their relative 207Pb-corrected full in Table 2.TiO,FeO ,MnO,MgO,CaO,andPO , 2 2 3 2 5 206Pb/238U date yields dates that broadly overlap within compatible trace elements (e.g. Cr, V), and incompatible uncertainty for MP-P0714-16, and yields dates of trace elements (e.g. Sr, Ba, LREE) decrease in abundance 1410 6 17 (n ¼ 8, MSWD ¼ 03) and 1363 6 7Ma(n ¼ 27, with increasing silica content; Al O and Na Ocontents 2 3 2 MSWD ¼ 10) for MP-P0714-26 (Fig. 6). In general, the increase with increasing silica content; K O, high-field- 2 position of each analysis in Tera–Wasserburg space strength elements (HFSE; e.g. Zr, Nb, and Ta), heavy rare correlates with U content (Fig. 6), suggesting that the earth elements (HREE), and actinides (Th, U) do not degree of discordance in single analyses is due to the show demonstrable trends. The ultrapotassic rocks are radiogenic Pb/common Pb ratio, as opposed to enriched in LREE by 500–1000 times chondritic values, inheritance. and in HREE by 10–50 times chondritic values (Fig. 4). Chondrite-normalized REE profiles for titanite are They display chondrite-normalized REE patterns with characterized by LREE enrichment, with a negative steep negative slopes (La/Yb ranging from 18 to 174), LREE to HREE slope and no notable Eu anomaly and without a significant Eu anomaly. Carbonatite from (Fig. 7a). Zr-in-titanite apparent temperatures form sin- the MPIS is highly enriched in LREE, containing 500 000 gle populations in only three samples (MP-P0913-10C chondritic values, with a La/Yb ratio of 15 000 (Fig. 4). weighted mean ¼ 859 6 9C; MP-P0714-17 weighted MPIS carbonatite whole-rock data are provided in mean ¼ 875 6 8C; MP-P0714-23 weighted mean ¼ Supplementary Data Electronic Appendix 4. 806 6 16C) (Ti and Si activities assumed to be 08 and unity, respectively; see the Appendix for analytical de- Titanite textures, U–Pb, trace elements, and Nd tails). Apparent temperatures in other samples range isotopes from 950–850 to 700–650C(Fig. 7b). There is no con- Titanite is present in most fresh ultrapotassic rock sam- sistent correlation observed between apparent Zr-in- ples collected at Mountain Pass as translucent, euhedral titanite temperature and either the trace element 1562

Table 1: Summary of sample lithology, location, geochronology data, Nd and Hf isotope data

Sample Rock type Location Coordinates Field context Titanite Zircon Monazite Accessory Zircon age (Ma) age (Ma) Th–Pb age mineral eHfi (Ma) eNdi (mean) (mean) MP-L0613-BDAY biotite Birthday mine 3548916 N, massive — 1425 6 5* — — 16 6 05 shonkinite 11553627 W body MP-L0613-BDAY2 bastnasite–€ Birthday mine 3548916 N, dike in — — 1396 6 16† — — barite 11553627 W shonkinite carbonatite MP-P0913-SQ1 bastnasite–€ Sulphide Queen 3546971 N, massive — — 1371 6 10 –29 6 013 — barite (parking lot near 11552961 W body (?) (monazite & carbonatite mine entrance) bastnasite)€ MP-P0913-1A biotite Birthday mine 3548911 N, massive body — — — — — shonkinite 11553628 W MP-P0913-1B bastnasite–€ Birthday mine 3548916 N, dike in — discordant — — — barite 11553627 W shonkinite carbonatite MP-P0913-2 amphibole Groaner stock 3546479 N, massive body — discordant 1376 6 36† — — syenite 11552261 W MP-P0913-4 garnet–biotite– 170 m SW of 3546244 N, host rock — 1800–1660 1693 6 8— — sillimanite Groaner stock 11552376 W (mean 207 Pb/ gneiss 206 Pb age) MP-P0913-5A leucosyenite Corral stock 3546213 N, massive body 1406 6 24 1416 6 9* — –96 6 1† 19 6 07 11552045 W (titanite) MP-P0913-5B granite Corral stock 3546213 N, m-scale host — 1820–1640 — — — 11552045 W rock xenolith MP-P0913-6 biotite dike 50 m W 3546218 N, dike in host 1429 6 10 1423 6 7‡ — –58 6 09† 2 6 05 melanosyenite of Corral stock 11552153 W rock (titanite) MP-P0913-7 syenite Mexican Well 3546608 N, massive body — discordant 1401 6 32† — — stock 11551612 W MP-P0913-9B leucosyenite dike at W edge 3545592 N, dike in host — discordant — — —

of Tors stock 11551462 W rock Petrology of Journal MP-P0913-10A biotite Tors stock 3545634 N, enclave in 1402 6 13 discordant — — — shonkinite 11551416 W syenite MP-P0913-10B biotite– Tors stock 3545634 N, massive body 1398 6 14 1411 6 8‡ — –47 6 0412 6 23† amphibole 11551416 W (titanite) syenite MP-P0913-10C biotite Tors stock 3545636 N, dike in syenite 1385 6 18 1458 6 7‡ — –4 6 0436 09 shonkinite 11551418 W (titanite)

MP-P0913-10D biotite– Tors stock 3545636 N, enclave in 1401 6 12 — — –41 6 05— 8 No. 57, Vol. 2016, , amphibole 11551418 W shonkinite dike (titanite) syenite

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Table 1: Continued Sample Rock type Location Coordinates Field context Titanite Zircon Monazite Accessory Zircon age (Ma) age (Ma) Th–Pb age mineral eHfi (Ma) eNdi (mean) (mean) MP-P0913-12 feldspar host rock 360 m 3549240 N, host rock — 1820–1765 — — — augen gneiss NNW of 11553518 W Birthday mine MP-P0714-15 quartz syenite Mineral Hill 3545159 N, massive body — discordant — — — stock 11548979 W MP-P0714-16 biotite Tors stock 3545551 N, massive body 1413 6 16 — — –73 6 13† — shonkinite 11551433 W (titanite) MP-P0714-17 leucosyenite Tors stock 3545664 N, dike intruding 1358 6 71 — — –55 6 06† — 11551430 W shonkinite dike (titanite) in syenite MP-P0714-19 biotite Corral stock 3546223 N, massive body 1414 6 19 discordant — –42 6 05† — shonkinite 11551896 W (titanite) MP-P0714-21 biotite– Mexican Well 3546631 N, enclave in — 1407 6 11* — — 08 6 09 amphibole stock 11551594 W syenite melanosyenite MP-P0714-22A biotite– Wheaton stock 3546063 N, massive body 1404 6 12 — — –46 6 07† — amphibole 11550265 W (titanite) shonkinite MP-P0714-23 leucosyenite Wheaton stock 3546153 N, massive body 1406 6 26 discordant — — — 11550457 W MP-P0714-26 biotite Birthday stock 3548844 N, massive body 1410 6 17 1434 6 10‡ — –56 6 17† 22 6 1 shonkinite 11553650 W (titanite)

*U–Pb concordia age. †MSWD 1. ‡Mean 207 Pb/206 Pb age. Stock names in the ‘location’ column are shown in Fig. 1b. Mean eNdi and eHfi values are weighted means of all analyses in a sample. Titanite ages are U–Pb isochron lower-intercept dates.

1563 Downloaded from https://academic.oup.com/petrology/article-abstract/57/8/1555/2413438 by guest on 31 March 2020 March 31 on guest by https://academic.oup.com/petrology/article-abstract/57/8/1555/2413438 from Downloaded 1564

16 (a) 9 (b) 9 (c) 2.5 (d) 14 8 8 12 7 7 2 10 6 6 5 5 1.5

8 (wt. %) 3 4 4 (wt. %) 2

6 O 1 3 3 2 MgO (wt. %)

4 TiO CaO (wt. %) 2 2 Fe 0.5 2 1 1 0 0 0 0 40 45 50 55 60 65 70 40 45 50 55 60 65 70 40 45 50 55 60 65 70 40 45 50 55 60 65 70 3 (e)18 (f) 5 (g) 14 (h) 16 12 14 4 10 2 12 10 3 8 (wt. %) 3 (wt. %) 8 5 6 O (wt. %) O 2 O (wt. %) 2 2 2 1 O 6 2

4 K Al Na P 4 1 2 2 0 0 0 0 40 45 50 55 60 65 70 40 45 50 55 60 65 70 40 45 50 55 60 65 70 40 45 50 55 60 65 70 1200 (i) 300 (j) 2000 (k) 9000 (l) 1000 250 1750 7500 1500 6000 800 200 1250 600 150 1000 4500

750 Sr (ppm) Cr (ppm) V (ppm) 400 100 3000 Ba (ppm) 500 1500 200 50 250 0 0 0 0 40 45 50 55 60 65 70 40 45 50 55 60 65 70 40 45 50 55 60 65 70 40 45 50 55 60 65 70 400 80 30 (m) (n) (o) (p) Petrology of Journal 350 70 4 25 300 60 250 50 20 3 200 40 15 2

150 Nd (ppm) 30 U (ppm)

Nb (ppm) 10 Yb (ppm) 100 20 1 50 10 5

0 0 0 0 8 No. 57, Vol. 2016, , 40 45 50 55 60 65 70 40 45 50 55 60 65 70 40 45 50 55 60 65 70 40 45 50 55 60 65 70 SiO (wt. %) SiO2 (wt. %) SiO2 (wt. %) 2 SiO2 (wt. %) Shonkinite Syenite Leucosyenite

Fig. 3. Whole-rock major and trace element variations versus silica for MPIS ultrapotassic rocks. (a–h) Major elements; (i–p) trace elements. Downloaded from https://academic.oup.com/petrology/article-abstract/57/8/1555/2413438 by guest on 31 March 2020 March 31 on guest by https://academic.oup.com/petrology/article-abstract/57/8/1555/2413438 from Downloaded Journal of Petrology, 2016, Vol. 57, No. 8 1565

Table 2: Whole-rock geochemical data

Sample: MP- MP- MP- MP- MP- MP- PO913-2 PO913-5A PO913-6 PO913-9B PO913-10A PO913-10B wt % SiO2 6001 6369 5999 6454 5187 5676 TiO2 057 054 083 041 118 081 Al2O3 1604 1450 1356 1681 1243 1310 Fe2O3 502 506 493 246 835 658 MnO 007 002 007 001 011 006 MgO 151 120 456 054 735 376 CaO 085 194 440 061 415 316 Na2O359 427 348 467 266 364

K2O1109 789 660 937 927 1015 Downloaded from https://academic.oup.com/petrology/article-abstract/57/8/1555/2413438 by guest on 31 March 2020 P2O5 032 022 087 005 136 080 Total 9907 9932 9929 9947 9873 9883 ppm Rb 444 347 228 446 658 568 Sc 3 4 12 0 22 13 V 111 95 107 27 199 178 Ni 10 4 72 0 106 64 Ga 17 22 17 19 17 17 Cu 53 38 64 38 75 100 Zn 119 78 111 45 183 106 Cr 352119 271047 3090534 Sr 697 381 645 687 1228 846 Y325677340259572568 Zr 508 1176 496 530 845 946 Nb 379709404293428655 Cs 3227302415870 Ba 3356 1874 3054 437 5622 5753 La 1485 2596 1604888 2246 1903 Ce 2894 4923 3075 1952 4815 4309 Pr 326568360247554524 Nd 1230 2094 1368946 2164 2136 Sm 212362229153390413 Eu 479 828 515 384 899 934 Gd 148268162115279296 Tb 159 311 164 131 282 307 Dy 730 1473 735 592 1252 1372 Ho 1146 2359 1194 0942 2021 2125 Er 282 565 282 238 459 495 Tm 0341 0707 0338 0301 0541 0589 Yb 212 427 210 192 337 365 Lu 0279 0569 0296 0226 0464 0514 Hf 112265128119198242 Ta 276223229113125161 Pb 1540608871361911570 Th 458 3019 1006428 1202 1843 U382431153184243

(continued) composition or the 207Pb-corrected 206Pb/238U date of within a single sample (2 to –11 in MP-P0714-26; single LASS-ICP-MS spots. However, the two youngest Fig. 8). For four of the seven samples, the majority of samples in terms of cross-cutting relationships and analyses (66 of 71 spots) fall between –35 and –7. The titanite U–Pb dates (MP-P0913-10C and MP-P0714-17) remaining three samples yield the most scattered, and have some of the highest (>800 C) and the most con- negative, eNdi values (as low as –13); one of these sam- sistent apparent Zr-in-titanite temperatures. ples (MP-P0913-5A) also contains abundant xenoliths of In situ Nd isotopic measurements on titanite from host granite and gneiss as well as abundant inherited three ultrapotassic samples from the Tors stock give (1700–1640 Ma) zircons. The other two samples single population weighted means of eNdi: MP-P0913- (MP-P0714-16 and MP-P0714-26) also do not yield 10B ¼ –47 6 04 (MSWD ¼ 13, n ¼ 20); MP-P0913- single-population U–Pb isochrons, indicating a possible 10C ¼ –4 6 04 (MSWD ¼ 05, n ¼ 20); MP-P0913-10D ¼ relationship between the disturbance of the U–Pb and –41 6 05 (MSWD ¼ 15, n ¼ 20), (Fig. 8). These three Nd isotopic systems. In general, more primitive and samples also yield the youngest U–Pb isochron dates in more consistent eNdi is associated with larger crystals; the MPIS, and contain some of the largest titanite crys- however, there is no apparent relationship between tals of the studied samples. In contrast, the other seven eNdi and position in crystals or location of samples in samples yield dispersed eNdi values from approxi- the field area. In the oldest (1430–1420 Ma) and young- mately –2 to –13, with a range of up to 9 epsilon units est (1380–1370 Ma) rocks, eNdi is typically lower with 1566 Journal of Petrology, 2016, Vol. 57, No. 8

Table 2: Continued

Sample: MP- MP- MP- MP- MP- MP- MP- MP- PO913-10C PO913-10D P0714-16 P0714-17 P0714-19 PO913-22A PO714-23 P0714-26 wt % SiO2 5144 5466 4990 6096 4856 4892 6747 4530 TiO2 113 085 137 016 167 181 056 234 Al2O3 1224 1350 1209 1556 1100 871 1416 987 Fe2O3 741 701 721 365 723 822 340 809 MnO 010 007 007 004 007 017 000 006 MgO 729 452 1047 117 967 1301 070 1381 CaO 577 369 438 165 795 620 045 721 Na2O280 303 210 323 196 251 429 214

K2O912 1039 943 1231 819 703 839 718 Downloaded from https://academic.oup.com/petrology/article-abstract/57/8/1555/2413438 by guest on 31 March 2020 P2O5 129 102 144 005 222 171 006 259 Total 9857 9874 9846 9878 9852 9829 9948 9860 ppm Rb 604 652 652 679 277 364 329 435 Sc 20 15 16 5 22 20 0 21 V 185 164 157 165 163 142 32 245 Ni 99 105 478 0 207 396 0 475 Ga 16 20 17 20 16 16 22 17 Cu 161 147 95 47 136 30 29 77 Zn 141 132 133 46 130 194 37 112 Cr 2993638 5064168 4554 644473 9987 Sr 1326 1079 1082 626 1322 1417 183 1783 Y509444507228477631391584 Zr 644 420 516 1235 420 423 1124 492 Nb 379308282173314397549284 Cs 1361071196076692678 Ba 7241 6402 7793 5243 7926 8214 943 4863 La 1912 2172 2485428 3548 6553 1971 3070 Ce 4074 4641 5555909 7224 10565 3560 6614 Pr 468544634119826 1000372745 Nd 1836 2151 2501489 3135 3395 1254 2912 Sm 327385426100479493194483 Eu 767 832 956 287 1143 1010 417 1089 Gd 23626230481311336134339 Tb 242 249 290 092 280 314 154 305 Dy 1112 1058 1181 436 1147 1351 748 1327 Ho 1754 1560 1727 0756 1720 2048 1296 1963 Er 414 340 385 196 375 495 356 438 Tm 0488 0384 0435 0322 0408 0600 0553 0529 Yb 308 222 255 229 238 377 355 296 Lu 0417 0310 0328 0384 0295 0521 0519 0380 Hf 156119134431100109262117 Ta 138133101149126145154113 Pb 2557834456 6870758234283455 Th 740643 1003 4076396813 1049588 U549576236305711461

Reported values for major element oxides and Rb to Zn were measured by XRF; reported values for Cr to U were measured by LA- Q-ICP-MS. increasing whole-rock silica content (Fig. 8). In general, rounded or anhedral to euhedral grains that range from samples with older dates have lower eNdi values 30 lm to 3 mm in size are also present (Fig. 9c), but are (Fig. 8). Representative examples of titanite Nd isotopic less common. Some show typical oscillatory or sector data are shown in Table 4, and the complete set of titan- zoning in CL (e.g. Fig. 9d), and others show irregular ite Nd isotopic data is presented in Supplementary Data zoning (e.g. Fig. 9e). Zoning patterns do not appear to Electronic Appendix 6. correlate with either whole-rock composition or age. Representative zircon geochronology data are pre- sented in Table 5, and the complete set of zircon LASS- Zircon textures, U–Pb, trace elements, and Hf ICP-MS data (including trace element data) is given in isotopes Supplementary Data Electronic Appendix 7. Zircon from Zircon in Mountain Pass rocks is diverse in morphology. 14 samples of Mountain Pass shonkinite and syenite Metamict, pseudo-octahedral, opaque grains are abun- are characterized by moderate to high U (300– dant in most samples, and do not luminesce in CL, but 10 000 ppm), radiogenic Pb loss, high common Pb con- some grains show irregular, blotchy CL-bright domains tent, high or variable LREE, highly variable Ti contents (Fig. 9a and b). Translucent, pink to reddish-brown, (10–450 ppm), and correspondingly variable apparent Journal of Petrology, 2016, Vol. 57, No. 8 1567

106 Chondrite-normalized Whole-rock REE SQ11 CarbonatiteC 105 26 19 4 22AA 10 SShonkinite 16 10CC 103 10AA 10DD 10BB 2 Syenite 10 6 2 Whole-rock/chondrite

10 17 Downloaded from https://academic.oup.com/petrology/article-abstract/57/8/1555/2413438 by guest on 31 March 2020 5A LeucosyeniteL 9B 1 23 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Element

Fig. 4. Chondrite-normalized REE patterns for all whole-rock samples analyzed in this study.

Fig. 5. Examples of typical titanite from MPIS ultrapotassic rocks. (a) Plane-polarized light (PPL) photomicrograph of euhedral titan- ite in shonkinite thin section (MP-P0714-19). (b) PPL photomicrograph of subhedral titanite in shonkinite thin section (MP-P0913- 10C). (c) Back-scattered electron (BSE) image of titanite grain in leucosyenite thin section with veins containing monazite (MP- P0913-9B). (d) BSE image of zoned titanite in grain mount (MP-P0714-19). Zones are outlined by dashed lines in the lower image. ap, apatite; bt, biotite; cal, calcite; cpx, clinopyroxene; kfs, K-feldspar; mnz, monazite; qtz, quartz; ttn, titanite. 1568

Table 3: Representative titanite LASS-ICP-MS geochronology data

Spot Pb Th U Th/U Measured isotopic ratios Apparent isotopic ages (Ma)

(ppm) (ppm) (ppm) 207 Pb/ 2r % 207 Pb/ 2r % 206 Pb/ 2r % Rho 207 Pb/ 2r abs 206 Pb/ 2r abs 207 Pb-corrected 2r abs 206 Pb 235 U 238 U 206 Pb age 238 U age 206 Pb/238 U age

MP-P0913-6 titanite (spot size ¼ 40 lm, shot frequency ¼ 4 Hz, shot count ¼ 100, laser energy ¼ 3 mJ at 100%) 1403 175 495355 05027 155 3199 266 04586 216 0812 4249 659 2435 526 1390 394 2453 247 592416 045142 152 2618 251 04207 200 0797 4090 620 2265 453 1417 347 3530 291 713408 044319 151 2539 253 0415 203 0803 4063 612 2240 455 1420 347 4683 419 992421 040812 151 2202 253 03912 203 0803 3939 593 2130 432 1429 332 5434 211 535393 04717 153 2908 257 04491 207 0804 4155 635 2393 496 1451 377 6 1103 1219 1770690 017531 151 6639 253 02748 202 0801 2609 395 1566 317 1428 283 7436 211 541391 047983 151 3003 254 0454 204 0804 4180 630 2415 493 1443 376 8859 949 1429666 015106 151 5609 255 02692 205 0805 2358 357 1538 316 1441 288 9839 947 1608590 015221 151 5458 255 02607 205 0804 2371 359 1495 306 1395 278 10 489 290 695420 041797 151 2298 256 03974 206 0807 3975 600 2159 445 1426 339 11 594 492 4421120 045881 151 2727 255 04312 205 0804 4114 623 2313 474 1431 361 12 693 365 892412 04522 157 2709 276 04353 227 0822 4092 643 2331 529 1462 393 13 941 1003 1477678 01904 161 7459 266 02837 213 0798 2746 441 1611 342 1445 302 14 875 989 1460677 0153 151 5624 253 02651 203 0802 2380 360 1517 308 1417 280 15 34394267353 06325 151 5699 259 06534 210 0811 4584 693 3244 680 1417 556 16 28473264277 05954 151 4912 257 05978 208 0810 4497 679 3023 630 1442 501 17 377 159 459347 04979 151 3239 257 04724 208 0809 4235 641 2496 519 1445 394 18 607 686 1171587 013786 152 4967 255 02614 205 0803 2201 335 1498 307 1423 283 19 637 721 1232588 013648 151 4927 256 02632 206 0806 2183 331 1507 311 1435 287 20 728 333 1497222 038206 151 1954 255 03718 205 0806 3840 579 2040 419 1423 323 21 661 395 722549 048209 151 2961 263 04443 215 0818 4187 634 2372 511 1406 380 22 316 123 367338 05182 151 3498 265 04901 218 0821 4294 650 2573 560 1433 418 23 416 188 534354 048186 151 2985 256 04509 206 0806 4187 634 2401 495 1427 376 24 466 293 969302 03327 154 1578 266 03464 217 0816 3630 558 1919 416 1438 327 Petrology of Journal 25 528 281 615458 047583 151 2916 256 04451 206 0808 4168 628 2375 491 1427 371 26 393 149 430347 05386 153 3761 261 05052 212 0810 4350 665 2638 558 1410 425 27 1031 1131 1749647 015656 157 5778 260 02693 207 0796 2419 380 1538 318 1432 289 28 863 922 994931 021349 152 8527 254 02897 203 0802 2932 445 1641 334 1432 288 29 529 296 720411 043585 151 2485 254 04128 204 0803 4038 611 2230 455 1432 348 30 526 257 677380 047042 151 2881 256 04427 207 0807 4151 627 2365 489 1435 370 31 838 605 790763 04611 156 2739 266 04317 216 0810 4121 644 2315 499 1426 375 06 o.5,N.8 No. 57, Vol. 2016, ,

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Table 3: Continued 8 No. 57, Vol. 2016, , Spot Pb Th U Th/U Measured isotopic ratios Apparent isotopic ages (Ma)

(ppm) (ppm) (ppm) 207 Pb/ 2r % 207 Pb/ 2r % 206 Pb/ 2r % Rho 207 Pb/ 2r abs 206 Pb/ 2r abs 207 Pb-corrected 2r abs 206 Pb 235 U 238 U 206 Pb age 238 U age 206 Pb/238 U age

MP-P0913-10C titanite (spot size ¼ 40 lm, shot frequency ¼ 4 Hz, shot count ¼ 100, laser energy ¼ 3mJat75%) 1301 166 619264 04358 152 2319 255 03824 204 0802 4037 614 2089 427 1383 339 2325 197 662292 03964 153 1981 256 03587 206 0804 3895 594 1978 408 1385 323 3237 104 390265 05671 151 3866 267 0493 220 0823 4426 670 2586 568 1380 455 4255 110 417261 05623 155 3906 276 04994 228 0827 4413 684 2613 596 1412 471 5281 153 551275 04463 152 2402 264 03876 216 0817 4073 619 2113 456 1376 355 6386 231 746307 04073 152 2074 261 03671 212 0812 3936 600 2017 427 1392 336 7357 178 653272 05012 152 3006 259 04329 210 0811 4245 643 2321 488 1389 388 8510 279 1021273 044484 151 2386 259 0387 210 0811 4068 616 2111 444 1378 348 9247 114 403283 05528 154 3621 261 04745 211 0807 4389 677 2505 528 1371 432 10 236 107 396268 05527 152 3681 266 04825 219 0821 4388 668 2540 556 1393 444 11 582 348 1828190 03231 151 1426 254 03191 205 0804 3585 542 1787 365 1378 296 12 412 161 570281 06325 152 5083 253 05815 203 0801 4584 695 2957 599 1391 538 13 253 123 475261 04999 155 2941 262 0428 211 0806 4241 656 2299 485 1377 386 14 262 134 436309 05112 153 3071 263 04353 214 0813 4274 653 2331 498 1370 395 15 257 110 424262 05547 152 3716 260 0485 212 0813 4394 667 2551 540 1394 440 16 470 269 1222223 03881 152 1907 249 03554 198 0792 3864 588 1962 388 1390 312 17 281 126 508251 05325 152 3394 262 04603 214 0815 4334 659 2443 522 1387 418 18 333 196 619321 041205 152 2128 267 03742 219 0823 3954 600 2051 450 1407 350 19 390 252 769331 03611 153 1729 257 0346 206 0803 3754 575 1917 396 1411 317 20 528 285 1077267 04378 153 2356 243 03882 189 0777 4044 618 2116 399 1398 328 21 621 312 1245256 04809 152 275241 04134 188 0778 4184 635 2232 419 1379 349 22 662 343 1299267 046914 151 2655 241 04083 187 0778 4147 627 2209 414 1391 343 23 377 189 663288 05079 153 3139 244 04455 191 0780 4264 652 2377 453 1410 382 24 274 134 430316 05392 152 3473 244 04665 190 0782 4352 661 2470 470 1387 404 25 411 176 1821098 035225 151 16345 242 03355 190 0783 3717 561 1866 354 1387 289 26 245 115 414280 05378 152 3488 247 04703 195 0788 4348 661 2487 484 1401 410 27 327 155 586266 05212 152 3269 244 04541 191 0783 4302 653 2415 461 1400 390 28 535 267 804336 054294 151 352246 04694 194 0789 4362 659 2483 482 1384 409 29 406 156 528298 06534 153 5524 247 0613 194 0787 4631 706 3084 600 1387 571

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Fig. 6. Titanite U–Pb concordia plots for MPIS ultrapotassic rocks. Analyses are colored by U content; warmer colors represent higher concentrations. It should be noted that there are two age populations shown for MP-P0714-26 (top right corner) and MP-P0714-16 (left of middle row); the younger age populations are shown by red isochrons; also that for MP- P0714-26 the U-content scale does not include one analysis that has much higher U content (450 ppm vs <130 ppm). BSE images of representative grains are shown in each plot; it

should be noted that scale is different for each plot. Downloaded from https://academic.oup.com/petrology/article-abstract/57/8/1555/2413438 by guest on 31 March 2020 March 31 on guest by https://academic.oup.com/petrology/article-abstract/57/8/1555/2413438 from Downloaded Journal of Petrology, 2016, Vol. 57, No. 8 1571

(a) (b) 60 105 Average Chondrite-normalized REE in Titanite 50 104 Relative Probability 40 103 30

102 10C 26 # Analyses 10D 16 20

Titanite/chondrite 10A 17 10 10B 19 10 6 22A Downloaded from https://academic.oup.com/petrology/article-abstract/57/8/1555/2413438 by guest on 31 March 2020 5A 23 1 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu 0 650 750 850 950 1050 Element Zr-in-ttn T (°C)

Fig. 7. Titanite trace element plots. (a) Average chondrite-normalized REE patterns for titanite from all analyzed samples. (b) Probability distribution plot of Zr-in-titanite temperatures for all titanite samples calculated using the calibration of Hayden et al. (2008) with adjustment of activities for Si and Ti according to the Appendix.

Decreasing Age 1425 Ma 1380 Ma

Increasing whole-rock SiO2 45% 64% 49% 57% 52% 61% MP-P0714-26 MP-P0714-19 MP-P0913-6 MP-P0913-5A MP-P0714-16 MP-P0714-22A MP-P0913-10D MP-P0913-10B MP-P0913-10C MP-P0714-17

-1 Mean monazite & bastnäsite -3 εNd i (carbonatite) -5 i -7 εNd

-9

-11

-13

Mean = Mean = Mean = Mean = Mean = Mean = Mean = Mean = Mean = Mean = -5.6 ± 1.7 -4.2 ± 0.5 -5.8 ± 0.9 -9.6 ± 1.0 -7.3 ± 1.3 -4.6 ± 0.7 -4.1 ± 0.5 -4.7 ± 0.4 -4 ± 0.4 -5.5 ± 0.6 MSWD = MSWD = MSWD = MSWD = MSWD = MSWD = MSWD = MSWD = MSWD = MSWD = 7.8 2.0 2.3 2.5 4.2 2.6 1.5 1.3 0.5 2.4 n = 14 n = 20 n = 9 n = 12 n = 10 n = 17 n = 20 n = 20 n = 20 n = 20

Fig. 8. Titanite Nd isotopic data for MPIS ultrapotassic rocks. Each set of analyses is from a single sample; weighted means are shown in boxes below each set of analyses. Samples are arranged by age, with age decreasing from left to right. Within each age group, samples increase in whole-rock silica content from left to right. Mean eNdi of monazite and bastnasite€ from MPIS carbonatite is shown by the gray bar.

Ti-in-zircon temperatures (800–1500C, Ti and Si activ- only discordant zircon U–Pb data (e.g. MP-P0913-10A, ities assumed to be 08 and unity, respectively; see the Fig. 10c and d). Appendix for analytical details). Most of the U–Pb data A minority of zircons (134 of 436 analyses) from are highly discordant on Tera–Wasserburg concordia seven out of 15 MPIS rocks yielded small populations of diagrams, defining broad arrays that appear to be the concordant or near-concordant U–Pb dates, primarily result of a mixture of radiogenic Pb loss and high or from low- to moderate-U grains (80–500 ppm). The variable common Pb content (Fig. 10a–f). In general, per dates primarily form two clusters; one at 1460– cent discordance correlates positively with U content, 1400 Ma, and another at 1800–1640 Ma (Fig. 10a and b), LREE content, and mass-204 signal intensity (a proxy although one sample additionally yielded sparse ana- for common Pb content) (e.g. Fig. 10c and d). Zircon is lyses from 1600 to 1430 Ma (Fig. 10b). Elongate or present in only one carbonatite sample (MP-P0913-1B), rounded, colorless to brown zircons from two samples and all crystals examined are metamict and do not yield that contain gneissic or granitic xenoliths (MP-P0913-5A meaningful U–Pb data. Of 15 samples, eight yielded and MP-P0913-6; e.g. Fig. 2a) yield dates >1460 Ma and 1572

Table 4: Representative LA-MC-ICP-MS Nd isotopic data

146 143 143 144 145 147 147 144 Spot Nd Total % Nd/ 2SE Nd/ Nd 2SE Nd/ 2SE Sm/ 2SE Sm/ Nd 2SE 2SE b (Sm) b (Nd) Age eNdi 2SE (V) Nd 144 Sm 144 Nd std corr’d 144 Nd 144 Nd std corr’d propagated (Ma)

MP-P0913-SQ1 monazite 11537 659 10129 0511105 37 0511168 37 0348420 20 0049067 76 0047754 76 2389 –1406 –1629 1380 –234 08 21533 657 10132 0511069 33 0511132 33 0348405 24 0049064 66 0047751 66 2388 –14–16267 1380 –305 08 31507 646 10042 0511036 43 0511099 43 0348385 26 0048656 65 0047354 65 2369 –1405 –16213 1380 –362 09 41518 651 10042 0511080 26 0511143 26 0348412 22 0048656 95 0047354 95 2370 –1406 –16256 1380 –276 07 51508 646 09964 0511080 36 0511143 36 0348404 21 0048255 63 0046964 63 2349 –1405 –16223 1380 –269 08 61531 656 10049 0511077 39 0511140 39 0348394 21 0048680 140 0047377 140 2373 –1407 –16223 1380 –283 09 71504 645 09962 0511057 33 0511120 33 0348414 19 0048250 140 0046959 140 2352 –1392 –16193 1380 –314 08 81532 657 10073 0511096 39 0511159 39 0348388 24 0048826 80 0047519 80 2377 –1411 –16188 1380 –248 09 91524 653 10016 0511060 32 0511123 32 0348414 22 0048531 90 0047232 90 2363 –1405 –16183 1380 –313 08 10 1511 647 09999 0511065 51 0511128 51 0348402 24 0048441 98 0047145 98 2359 –1405 –16192 1380 –302 11 11 1519 651 10189 0511080 38 0511143 38 0348403 24 0049362 95 0048041 95 2404 –1404 –1617 1380 –288 09 12 1506 646 10177 0511084 4 0511147 40 0348408 17 0049300 130 0047981 130 2403 –1401 –16166 1380 –2809 13 1521 652 10212 0511068 35 0511131 35 0348397 20 0049450 140 0048127 140 2410 –1383 –1615 1380 –313 08 14 1513 649 10086 0511081 29 0511144 29 0348409 22 0048878 98 0047570 98 2381 –1412 –16182 1380 –278 07 15 154 661 10247 0511092 44 0511155 44 0348423 28 0049666 74 0048337 74 2418 –1409 –16171 1380 –271 16 1527 654 09839 0511098 38 0511161 38 0348420 23 0047649 94 0046374 94 2321 –1404 –16262 1380 –224 09 17 1428 612 0976 0511036 34 0511099 34 0348418 25 0047320 120 0046054 120 2306 –1419 –16271 1380 –339 08 18 1497 641 09804 0511075 42 0511138 42 0348406 23 0047500 110 0046229 110 2314 –1419 –16264 1380 –266 09 19 1495 6409917 0511083 37 0511146 37 0348391 22 0048072 75 0046786 75 2340 –1417 –16257 1380 –2608 20 1499 642 09792 051106 33 0511123 33 0348398 18 0047434 62 0046165 62 2309 –1416 –16214 1380 –294 08 21 1456 624 09718 0511076 38 0511139 38 0348413 25 0047080 140 0045820 140 2295 –1397 –16217 1380 –257 08 22 1452 622 09718 0511048 38 0511111 38 0348411 26 0047080 130 0045820 130 2295 –1403 –16178 1380 –312 08 23 1454 623 09749 0511061 36 0511124 36 0348396 22 0047230 110 0045966 110 2301 –1408 –16169 1380 –289 08

24 1452 622 09759 0511070 41 0511133 41 0348393 21 0047257 99 0045992 99 2302 –1412 –16154 1380 –272 09 Petrology of Journal 25 1416 606 09632 0511045 37 0511108 37 0348409 21 0046700 120 0045450 120 2276 –143 –16136 1380 –311 08 26 1552 665 09778 0511050 37 0511113 37 0348404 25 0047386 70 0046118 70 2307 –1403 –16144 1380 –313 08 27 1575 675 09838 0511091 46 0511154 46 0348395 30 0047708 74 0046431 74 2323 –1431 –1616 1380 –238 1 28 1543 661 09874 0511063 38 0511126 38 0348405 18 0047850 150 0046570 150 2333 –1406 –16135 1380 –296 09 29 1568 672 09818 0511121 38 0511184 38 0348410 21 0047570 100 0046297 100 2317 –1406 –16169 1380 –177 08 30 1617 693 09843 0511089 35 0511152 35 0348402 18 0047696 71 0046420 71 2322 –1386 –16146 1380 –242 08 (continued)

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Table 4: Continued 8 No. 57, Vol. 2016, , 146 143 143 144 145 147 147 144 Spot Nd Total % Nd/ 2SE Nd/ Nd 2SE Nd/ 2SE Sm/ 2SE Sm/ Nd 2SE 2SE b (Sm) b (Nd) Age eNdi 2SE (V) Nd 144 Sm 144 Nd std corr’d 144 Nd 144 Nd std corr’d propagated (Ma)

MP-P0913-6 titanite 11047 462 2464 0511538 47 0511583 47 0348410 24 0119500 1500 0116473 1500 6014 –1492 –16725 1420 –6 14 2177 794 3327 0511842 39 0511887 39 0348401 23 0161300 1700 0157214 1700 8042 –14758 –16716 1420 –749 17 31616 722 3096 0511809 52 0511854 52 0348409 23 0150090 960 0146288 960 7377 –14865 –16711 1420 –614 17 41648 742 3534 0512005 39 0512050 39 0348415 22 0171200 1800 0166864 1800 8535 –14219 –16309 1420 –606 17 51572 705 3326 0512038 39 0512083 39 0348395 22 0161180 890 0157097 890 7905 –14239 –16243 1420 –363 16 61127 499 2643 0511612 44 0511657 44 0348418 23 0127900 1400 0124660 1400 6388 –1486 –16651 1420 –605 15 71189 526 2634 0511652 41 0511697 41 0348411 22 0127700 1300 0124465 1300 6358 –1454 –16497 1420 –523 14 81483 657 2722 0511691 26 0511736 26 0348404 18 0131940 930 0128598 930 6497 –148 –16668 1420 –522 13 92137 968 383 0512011 84 0512056 84 0348398 39 0185660 830 0180957 830 9086 –1455 –16428 1420 –852 23 MP-P0913-10C titanite 10428 1905 2892 0511893 59 0511938 59 0348429 38 0140110 540 0136561 540 6849 –1399 –1618 1380 –303 17 20438 1952 2923 0511815 79 0511860 79 0348375 38 0141600 1100 0138013 1100 6988 –1409 –16166 1380 –481 2 30617 2757 3126 0511919 51 0511964 51 0348428 31 0151470 620 0147633 620 7408 –1423 –16263 1380 –449 17 40576 2572 30983 0511967 47 0512012 47 034839 32 0150120 320 0146318 320 7323 –1408 –16237 1380 –331 16 50787 3529464 0511867 72 0511912 72 0348437 35 0142790 360 0139173 360 6968 –1425 –16246 1380 –4 19 6063 279 3025 0511913 44 0511958 44 0348415 33 0146740 730 0143023 730 7188 –1423 –1617 1380 –379 15 70383 1704 2875 0511852 67 0511897 67 0348415 33 0139400 100 0135869 100 6867 –1403 –16064 1380 –371 18 80567 2522 28853 0511859 51 0511904 51 0348391 24 0139820 160 0136279 160 6816 –1394 –16003 1380 –365 16 90555 2473 2947 0511858 50 0511903 50 0348411 33 0142800 1000 0139183 1000 7031 –1418 –16206 1380 –418 16 10 0527 2344 287 0511813 53 0511858 53 0348425 33 0139100 1200 0135577 1200 6884 –142 –1616 1380 –442 16 11 0377 1681 2933 0511816 77 0511861 77 0348381 35 0142200 1100 0138598 1100 7017 –1412 –1601 1380 –492 12 0418 1853 2751 0511795 56 0511840 56 0348418 43 0133350 170 0129972 170 6501 –1405 –15859 1380 –378 16 13 0398 177 2859 0511836 89 0511881 89 0348389 46 0138550 860 0135041 860 6807 –1386 –15959 1380 –388 21 14 0483 2151 2978 0511891 44 0511936 44 0348475 26 0144400 1000 0140742 1000 7108 –1418 –1591 1380 –381 15 15 0833 371 2941 0511839 44 0511884 44 0348418 25 0142500 1100 0138891 1100 7031 –1402 –16065 1380 –4515 16 0659 2948 3153 0511891 54 0511936 54 0348418 25 0152800 2400 0148930 2400 7824 –1406 –16136 1380 –526 17 17 0462 2056 28478 0511876 56 0511921 56 0348384 32 0137980 340 0134485 340 6733 –1393 –15868 1380 –299 16 18 0475 2116 2919 0511894 72 0511939 72 0348402 29 0141460 740 0137877 740 6933 –1388 –15926 1380 –324 19 19 0408 1821 2852 0511833 66 0511878 66 0348419 39 0138240 720 0134739 720 6775 –1392 –15869 1380 –388 18 20 0367 163 27958 0511813 76 0511858 76 0348382 44 0135430 150 0132000 150 6602 –1371 –15746 1380 –379 19

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Fig. 9. Examples of typical zircon from MPIS ultrapotassic rocks. (a) Pseudo-octahedral, metamict zircons separated from shonkin- ite (MP-P0714-22A). (b) Cathodoluminescence (CL) image of metamict zircon grain (MP-P0714-17). (c) Euhedral, pink to reddish- brown zircons separated from melanosyenite (MP-P0913-6). (d) CL image of zircon in grain mount showing oscillatory zoning (MP- P0714-26). (e) CL image of anhedral zircon with irregular zoning pattern in shonkinite thin section (MP-P0913-10A). have Th/U < 1; pink to reddish-brown, euhedral to anhe- Th/U is consistently < 1. Chondrite-normalized REE pro- dral zircon with low aspect ratios (2:1) yield files are highly variable, and Ti-in-zircon apparent tem- dates < 1460 Ma and chiefly have Th/U > 1. Mean peratures mostly range from 650 to 800C 207Pb/206Pb and concordia dates < 1460 Ma range from (Supplementary Data Electronic Appendix 8).

1458 6 7 to 1407 6 11 Ma (e.g. Fig. 10e and f; The eHfi values from Mesoproterozoic zircons typic- Supplementary Data Electronic Appendix 8). Only one ally range from 05 to 3. Five of seven samples ana- sample gave > 10 concordant, Mesoproterozoic dates lyzed yield single populations (MSWD 1; Fig. 12). Two out of 30–40 analyses (MP-L0613-BDAY), yielding a con- samples with fewer analyses have higher MSWD owing cordia age of 1425 6 5Ma(n ¼ 25; Supplementary Data to one or two outlying analyses (e.g. MP-P0913-10B, Fig. Electronic Appendix 8). Where titanite and zircon occur 12). Taken as a whole, data from all the samples give a in the same sample, concordant, Mesoproterozoic zir- weighted mean of 19 6 02 (MSWD ¼ 09; seven of 76 con dates overlap titanite dates within uncertainty in all analyses rejected; Fig. 12). eHfi values > 3 occur only in cases except one (MP-P0913-10C). Multiple dates within one sample—a shonkinite dike (MP-P0913-10C) that single crystals are consistently similar. cross-cuts syenite, which also yielded the oldest zircon Zircons that yield concordant and slightly discordant age (1458 6 7 Ma; mean 207Pb/206Pb age), the youngest data have similar REE concentrations to typical igneous titanite age (1385 6 18 Ma), and the most primitive eNdi zircon from a variety of igneous rocks (e.g. Belousova values (weighted mean ¼ –4 6 04). Despite the higher et al., 2002), and yield Ti-in-zircon apparent tempera- eHfi of this sample, excluding it does not affect the mean tures from 600 to 850 C(Fig. 11a and b). eHfi for all samples (weighted mean eHfi without MP- Zircons from three Mountain Pass host-rock samples P0913-10C ¼ 172 6 024). Representative examples of zir- are primarily colorless to brown and elongate or moder- con Hf isotopic data are shown in Table 6, and the com- ately rounded, and yielded zircon dates of 1820– plete set of zircon Hf isotopic data is provided in 1765 Ma (feldspar augen gneiss, sample MP-P0913-12), Supplementary Data Electronic Appendix 9. 1820–1640 Ma (coarse-grained alkali granite, sample MP-P0914-5B) and 1800–1660 Ma (garnet–biotite–silli- manite gneiss, sample MP-P0913-4) (Supplementary Monazite textures, U–Th/Pb, trace elements, and Data Electronic Appendix 8). U–Pb discordance is com- Nd isotopes mon in the host-rock zircons, and generally correlates Monazite in the ultrapotassic rocks primarily occurs in positively with U content and mass-204 signal intensity, veins and as inclusions in other phases (e.g. titanite, Fig. but 50–70% of the zircon analyses are concordant. 5c), and as fine, subhedral to anhedral grains in altered Table 5: Representative zircon LASS-ICP-MS geochronology data Petrology of Journal

Spot Used in % Pb Th U Th/U Measured isotopic ratios Apparent isotopic ages (Ma)

age interp.? discordance (ppm) (ppm) (ppm) 207 Pb/ 2r % 207 Pb/ 2r % 206 Pb/ 2r % Rho 207 Pb/ 2r abs 206 Pb/ 2r abs 206 Pb 235 U 238 U 206 Pb age 238 Uage

MP-L0613-BDAY zircon (spot size ¼ 20 lm, shot frequency ¼ 4 Hz, shot count ¼ 80, laser energy ¼ 4 mJ at 100%) 1 Yes 152 196 306 90 345 009 183 3045 250 02457 165 0683 1439 263 1417 234 2No 472 150 231 94 246 009248 179 3146 234 02463 145 0646 1491 267 1421 206 8 No. 57, Vol. 2016, , 3No 309 217 342 190 181 009065 163 3046 221 02439 143 0675 1453 237 1408 201 4No 035 127 180 146 125 009114 167 3191 222 02536 139 0659 1463 244 1458 203 5No 388 135 199 132 152 00926 221 3187 276 02492 160 0601 1493 330 1436 230 6 Yes 002 144 221 125 178 008937 182 307 236 02474 145 0639 1426 259 1426 206 7No 1646 223 307 171 181 01075 182 3873 227 02577 130 0601 1771 322 1479 192 8No 118 119 185 125 148 009058 166 3148 220 0249 138 0657 1452 241 1434 198 9 Yes 163 160 248 103 243 008965 175 3069 234 0244 149 0664 1432 251 1409 210 10 Yes 222 133 205 95 214 009041 167 3106 232 02454 155 0694 1448 242 1416 220 11 No 394 138 215 151 144 009142 176 3155 237 02445 152 0669 1469 258 1411 215 12 Yes 243 111 168 122 139 009094 174 3157 232 02469 148 0664 1459 254 1424 211 13 Yes 029 195 298 128 234 008943 163 3106 216 02468 136 0658 1427 233 1423 193 14 Yes 089 102 157 124 127 008957 167 3083 211 02457 121 0611 1430 239 1417 172 15 No 2625 141 188 87 216 01196 185 4203 240 02516 146 0636 1963 364 1448 212 16 No 1491 123 183 182 100 01015 185 3493 245 02456 155 0656 1665 308 1417 220 17 Yes 155 109 163 116 141 009039 184 3089 232 02472 135 0612 1448 266 1425 193 18 Yes 213 187 289 191 152 009027 164 3066 215 02451 133 0650 1445 236 1414 188 19 No 457 264 415 151 280 009265 166 3148 213 02474 127 0627 1494 248 1426 180 20 Yes 073 156 238 148 164 009017 166 3067 215 02486 129 0635 1443 239 1432 185 21 Yes 159 172 268 103 262 008995 173 3047 224 02453 136 0637 1438 248 1415 192 22 Yes 262 173 268 168 160 009052 165 3031 221 02447 139 0662 1450 240 1412 197 23 Yes 068 132 201 86 238 009021 177 3085 232 02489 144 0649 1444 255 1434 207 24 No 1626 553 738 311 238 01105 196 3982 312 02666 239 0778 1821 358 1525 364 25 Yes 001 94 141 119 119 00898 173 309 217 02491 123 0602 1435 249 1435 177 26 Yes 093 383 569 279 205 009041 158 313 225 0249 154 0710 1448 229 1434 220 27 No 665 100 148 100 148 009361 178 3198 249 02449 169 0700 1514 270 1413 239 28 No 2490 117 152 102 151 01199 190 4251 265 02574 180 0697 1968 374 1478 265 29 No 605 209 299 125 240 00955 196 3364 257 02535 162 0650 1552 304 1458 235 30 No 672 117 169 186 091 009414 163 3215 246 02466 178 0747 1525 249 1422 254 31 No 092 54 74 101 073 009095 176 3159 255 02512 179 0724 1459 257 1446 259 32 Yes 113 154 228 75 305 009067 179 3094 273 02495 202 0755 1453 261 1437 290 33 No 945 224 341 129 265 00973 194 3323 283 02494 202 0728 1587 308 1437 290 34 Yes 034 254 379 162 233 008998 161 3056 217 02489 138 0668 1439 232 1434 198 35 Yes 029 168 244 101 243 009011 174 3117 254 02512 179 0728 1442 250 1446 259 36 Yes 247 72 110 110 101 009061 174 3045 301 02455 241 0816 1452 253 1416 342 37 Yes 099 263 378 149 256 008969 170 3085 225 02514 140 0653 1433 244 1447 203 38 Yes 100 111 166 167 100 008994 165 3043 232 02469 158 0704 1438 237 1424 225 39 No 308 150 225 137 164 009105 172 3054 229 02455 145 0661 1461 252 1416 206 40 No 2633 165 146 286 052 01428 183 577 293 02965 225 0781 2274 416 1675 376 41 Yes 182 318 486 244 199 009023 159 3027 222 02458 148 0697 1444 230 1418 210 42 No 3930 176 183 114 162 01668 166 6118 233 02697 158 0702 2538 422 1541 243 43 Yes 041 98 152 82 185 00899 187 3033 301 02484 232 0784 1437 268 1431 331 44 Yes 064 262 408 147 279 008925 168 2999 280 02451 219 0799 1423 239 1414 310

45 Yes 009 142 221 132 168 00893 165 3014 233 02473 158 0703 1424 235 1426 225 1575

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Table 5: Continued Spot Used in % Pb Th U Th/U Measured isotopic ratios Apparent isotopic ages (Ma)

age interp.? discordance (ppm) (ppm) (ppm) 207 Pb/ 2r % 207 Pb/ 2r % 206 Pb/ 2r % Rho 207 Pb/ 2r abs 206 Pb/ 2r abs 206 Pb 235 U 238 U 206 Pb age 238 Uage

MP-P0714-21 zircon (spot size ¼ 25 lm, shot frequency ¼ 4 Hz, shot count ¼ 100, laser energy ¼ 5 mJ at 100%) 1No 7036 970 4677 710 658 010339 156 1169 252 00812 198 0785 1699 288 504 120 2No 3847 1001 2419 611 399 009951 180 23626 0168 599 0958 1628 335 1002 615 3No 1414 1215 2094 494 422 009527 163 3015 255 02286 197 0770 1547 307 1328 315 4No 1288 1188 2002 461 428 009505 166 3039 238 02316 170 0716 1543 313 1344 290 5No 8258 1434 9120 1121 814 013194 157 1086 265 00594 214 0806 2137 275 372 94 6No 1330 983 1578 328 478 009589 167 3097 274 02331 217 0793 1559 314 1352 344 7No 7521 1094 6160 804 769 010866 160 107 253 00712 196 0773 1790 293 444 105 8No 1221 1020 1648 341 482 009473 165 306 273 02325 218 0798 1536 310 1349 344 9No 3799 1094 2363 589 399 010075 160 237 607 0172 586 0965 1651 298 1024 615 10 No 1465 998 1623 392 411 009608 171 3024 269 02297 208 0772 1563 321 1334 329 11 No 1340 950 1502 387 384 009604 172 3095 250 02333 181 0725 1562 323 1353 304 12 No 6238 971 3923 699 556 010161 158 1432 240 01021 181 0752 1667 293 627 141 13 No 1228 853 1388 412 335 009473 162 3032 259 02323 202 0780 1536 305 1348 326 14 Yes 280 225 351 123 287 009044 158 3034 256 02439 201 0786 1449 302 1408 339 15 No 7736 811 4512 871 524 011252 162 1043 296 00672 248 0838 1854 293 420 118 16 Yes 267 146 227 101 223 009015 158 3013 247 02431 190 0769 1442 302 1404 326 17 No 1944 1340 2440 466 521 009645 162 2858 243 02166 180 0743 1570 305 1265 283 18 No 1072 1032 1649 359 460 009444 163 3039 258 02359 199 0773 1531 308 1366 327 19 Yes 258 110 176 79 226 008961 155 2959 238 02412 180 0757 1431 297 1394 312 20 No 3589 1076 2499 655 389 009719 169 2258 302 01705 250 0829 1584 317 1016 288 21 No 406 463 746 204 367 009109 156 3029 224 02429 160 0716 1462 297 1403 292 22 No 744 1129 1942 389 504 009234 156 3023 229 0238 167 0729 1488 297 1377 293

23 No 8108 972 4955 1045 478 014287 165 1343 305 0069 256 0841 2275 285 430 124 Petrology of Journal 24 No 4010 1060 3450 710 481 009458 154 198 675 0153 657 0974 1533 291 918 616 25 No 7921 955 6305 1008 639 010259 159 0788 320 00558 278 0868 1685 294 350 108 26 No 7020 1168 5561 799 711 011327 174 1397 326 009 276 0846 1866 314 556 170 27 No 1743 1437 2753 572 493 009643 156 2949 213 02225 144 0677 1570 294 1296 254 28 No 1300 1118 1830 466 401 009543 163 304 260 02325 202 0777 1550 308 1349 326 29 No 1587 827 1510 355 437 009549 167 298 469 02242 438 0934 1551 314 1305 598 30 No 858 602 956 254 384 009294 162 302 247 02369 187 0754 1500 308 1372 314 31 No 3038 933 2053 547 383 009512 158 2378 247 01813 190 0768 1544 298 1075 249

32 No 1180 1214 2045 491 431 009467 160 3055 239 02335 177 0743 1535 301 1354 300 8 No. 57, Vol. 2016, , Downloaded from https://academic.oup.com/petrology/article-abstract/57/8/1555/2413438 by guest on 31 March 2020 March 31 on guest by https://academic.oup.com/petrology/article-abstract/57/8/1555/2413438 from Downloaded Journal of Petrology, 2016, Vol. 57, No. 8 1577 Downloaded from https://academic.oup.com/petrology/article-abstract/57/8/1555/2413438 by guest on 31 March 2020

Fig. 10. Representative Tera–Wasserburg concordia plots for U–Pb analyses of zircon in MPIS ultrapotassic rocks. CL images of rep- resentative grains are shown in each plot except (d); it should be noted that scale is different for each plot. (a) Sample MP-P0913-6. (b) Sample MP-P0913-5A. (c) Sample with only discordant analyses, colored by U content (MP-P0913-10A). Greater discordance in high-U grains should be noted. (d) Sample with only discordant analyses, colored by Ce content (MP-P0913-10A). Greater discord- ance in high-Ce grains should be noted. (e) Sample MP-P0913-10C. (f) Sample MP-P0714-21. samples (Fig. 13a; see description of alteration textures in Supplementary Data Electronic Appendix 10. in syenite, above). Euhedral grains were found only in Monazite from the ultrapotassic rocks and carbonatites two samples that show abundant evidence for alteration typically contains 3–50 ppm U and has low (typic- (MP-P0913-2 and MP-P0913-7; Supplementary Data ally < 5 ppm) radiogenic 206Pb; thus, U–Pb analyses Electronic Appendices 1 and 3). In contrast, monazite is a have large uncertainties and do not yield meaningful common accessory mineral in carbonatite, occurring as dates. However, monazite contains significant Th (typic- euhedral to anhedral grains that vary from 25 to ally 2000–20 000 ppm) and has low common 208Pb (i.e. 200 mm in size in carbonatite sample MP-L0613-BDAY2 >85% is radiogenic); it is therefore possible to calculate (Fig. 13b), and as anhedral, porous, crumbly grains from 208Pb/232Th dates. Weighted mean, common-Pb cor- 50 mmto1 mm in carbonatite sample MP-P0913-SQ1 rected 208Pb/232Th dates for monazite in ultrapotassic (Supplementary Data Electronic Appendix 3). samples MP-P0913-2 and MP-P0913-7 are 1376 6 36 Ma Representative monazite geochronology data are (n ¼ 13, MSWD ¼ 45) and 1401 6 32 Ma (n ¼ 8, presented in Table 7 and the complete monazite LASS- MSWD ¼ 19), respectively (Fig. 13c). The weighted ICP-MS dataset (including trace element data) is given mean 208Pb/232Th date from carbonatite sample MP- 1578 Journal of Petrology, 2016, Vol. 57, No. 8

(a) (b) 4 10 Average Chondrite-normalized REE in Zircon 1000

103

800 102

5A 10 10B 600 6 Ti-in-zrn T (°C) T Ti-in-zrn 1 21 Zircon/chondrite 400 10C Downloaded from https://academic.oup.com/petrology/article-abstract/57/8/1555/2413438 by guest on 31 March 2020 0.1 26 BDAY 5A 10B 6 21 10C 26 BDAY -2 200 10 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Element

Fig. 11. Selected trace element plots for concordant zircon from MPIS ultrapotassic rocks. (a) Average chondrite-normalized REE patterns for all concordant zircons. (b) Ti-in-zircon apparent temperatures for all concordant zircons. High uncertainties are a prod- uct of poor precision as a result of low Ti content in zircon (<10 ppm).

Decreasing Age 1425 Ma 1380 Ma Increasing SiO2 content 45% 64% ~50% 57%

MP-P0714-26 MP-L0613-BDAY MP-P0913-6 MP-P0913-5A MP-P0714-21 MP-P0913-10B MP-P0913-10C

7

i 5 εHf

Mean εHf 3 i (all analyses) 1

-1 Mean = Mean = Mean = Mean = Mean = Mean = Mean = 2.2 ± 1 1.6 ± 0.5 2 ± 0.5 1.9 ± 0.7 0.8 ± 0.9 1.2 ± 2.3 3 ± 0.9 MSWD = MSWD = MSWD = MSWD = MSWD = MSWD = MSWD = 1.5 0.7 1.3 1.1 0.9 6.4 1.7 n = 10 n = 15 n = 20 n = 8 n = 5 n = 6 n = 12 Mean of all analyses = 1.9 ± 0.2, MSWD = 0.9, 7 of 76 rej.

Fig. 12. Weighted mean plot of eHfi for all measured MPIS zircons. Each set of analyses is from one sample. Analyses are only from zircon that yielded concordant U–Pb analyses. Samples are arranged by age, with age decreasing from left to right. Within each age group, samples increase in whole-rock silica content from left to right. Blue bars represent analyses that were rejected in the weighted mean calculation for all analyses. The mean of all analyses is shown by the gray bar.

L0613-BDAY2 is 1396 6 16 Ma (n ¼ 23, MSWD ¼ 42), and minor negative Eu anomalies (Fig. 13d). Monazite whereas carbonatite sample MP-P0913-SQ1 defines a from carbonatite sample MP-P0913-SQ1 shows greater more narrow range, giving a weighted mean of depletion in HREE. Nd isotopic analysis of monazite 1371 6 10 Ma (n ¼ 46, MSWD ¼ 16) (Fig. 13c). Multiple from carbonatite sample MP-P0913-SQ1 yielded con- analyses of single crystals indicate that some monazite sistent eNdi values, with a weighted mean of –28 6 02 grains are zoned with respect to Th, but Th content (MSWD ¼ 08; Fig. 13e). Monazite Nd isotopic data are does not correlate with the common-lead corrected reported in Table 4. 208Pb/232Th dates. U–Pb monazite dates from one host-rock sample, Chondrite-normalized REE patterns in monazite from a garnet–biotite–sillimanite gneiss (MP-P0913-4), yield a the ultrapotassic rocks and carbonatites show enrich- mean 207Pb/206Pb date of 1693 6 8Ma (MSWD¼ 22; ment in LREE, with steep negative LREE to HREE slopes, Supplementary Data Electronic Appendix 8). ora fPetrology of Journal 06 o.5,N.8 No. 57, Vol. 2016, , Table 6: Representative zircon LA-MC-ICP-MS Hf isotopic data

176 176 176 176 C Spot Hf beam bHf bYb Hf/ 62r Lu/ 62r Yb/ Analysis Hf/ eHfi 62r Propagated TDM TDM 177 177 177 177 (V) Hf Hf Hf age (Ma) Hfi 2r 2-stage MP-P0913-5A zircon 173–146 –198 0281927 0000049 0000562 0000007 0029 1420 0281912 101719 1835 2088 252–146 –196 0282021 0000067 0001744 0000072 0090 1420 0281974 332425 1751 1953 396–149 –199 0281958 0000059 0001699 0000086 0067 1420 0281912 112122 1834 2087 468–146 –198 0281939 0000043 0000631 0000012 0034 1420 0281922 141517 1821 2066 581–146 –201 0282020 0000100 0001380 0000120 0061 1420 0281983 363536 1739 1934 667–146 –199 0281956 0000055 0001173 0000021 0057 1420 0281924 152021 1818 2060 768–146 –198 0282003 0000045 0000872 0000023 0043 1420 0281980 351617 1743 1941 865–144 –194 0281928 0000063 0000572 0000028 0030 1420 0281913 112223 1834 2086 MP-P0714-21 zircon 161–145 –198 0281888 0000042 0000250 0000011 0012 1405 0281881 –041517 1876 2163 254–146 –195 0281928 0000064 0000477 0000004 0023 1405 0281915 082324 1830 2089 353–145 –198 0281973 0000055 0001211 0000024 0059 1405 0281941 172021 1796 2034 461–145 –202 0281951 0000052 0000375 0000005 0019 1405 0281941 171820 1795 2034 555–145 –198 0281933 0000050 0000749 0000027 0038 1405 0281913 071819 1833 2094 MP-L0613-BDAY zircon 147–145 –203 0281914 0000063 0000143 0000016 0007 1425 0281910 112223 1837 2088 259–145 –176 0281904 0000043 0000097 0000003 0005 1425 0281901 081517 1849 2107 361–146 –196 0281930 0000051 0000159 0000002 0008 1425 0281926 171819 1816 2055 472–144 –162 0281969 0000064 0000087 0000002 0004 1425 0281967 312324 1761 1966 572–145 –207 0281970 0000037 0000151 0000005 0008 1425 0281966 311315 1762 1967 661–145 –196 0281917 0000040 0000082 0000002 0004 1425 0281915 131416 1831 2078 760–145 –184 0281917 0000053 0000075 0000000 0004 1425 0281915 131920 1831 2078 862–145 –188 0281927 0000040 0000135 0000002 0007 1425 0281923 161416 1819 2060 957–144 –205 0281910 0000049 0000110 0000002 0005 1425 0281907 101719 1841 2095 10 57–145 –193 0281938 0000047 0000125 0000001 0006 1425 0281935 201718 1804 2035 11 68–144 –196 0281933 0000038 0000150 0000003 0008 1425 0281929 181315 1812 2048 12 67–145 –190 0281957 0000057 0000139 0000001 0007 1425 0281953 262021 1779 1995 13 71–145 –188 0281909 0000043 0000070 0000001 0003 1425 0281907 101517 1841 2095 14 55–145 –215 0281914 0000050 0000139 0000002 0007 1425 0281910 111819 1837 2088 15 71–145 –198 0281918 0000043 0000085 0000002 0004 1425 0281916 131517 1830 2076

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Fig. 13. Examples of monazite grains and monazite data from MPIS rocks. (a) BSE image of anhedral monazite in altered syenite (MP-P0913-7). (b) BSE image of euhedral monazite in carbonatite (MP-L0613-BDAY2). (c) Weighted mean plot of all common-Pb corrected Th–Pb dates from monazite in MPIS rocks. Blue bars represent analyses rejected from the weighted mean calculation. BSE images of representative grains are shown for each sample. It should be noted that scale is different for each sample. (d) Average chondrite-normalized REE patterns for monazite from MPIS ultrapotassic rocks and carbonatite. (e) Weighted mean plot of eNdi for monazite in carbonatite (MP-P0913-SQ1). dol, dolomite; Fe-ox, Fe-oxide; kfs, K-feldspar; mnz, monazite; qtz, quartz.

Chondrite-normalized REE profiles are flat among the samples. It occurs as tabular or elongate to acicular, LREE, have pronounced negative Eu anomalies, and subhedral to euhedral, honey-yellow to light brown show a range of HREE contents (10–100 times chondritic crystals up to 1 mm in sample MP-P0913-SQ1 values; Supplementary Data Electronic Appendix 8). (Fig. 14a), and as anhedral intergrowths with barite in samples MP-P0913-1B and MP-L0613-BDAY2 (Fig. 14b). It is occasionally intergrown with synchysite or parisite Bastnasite€ textures, U–Pb and Nd isotopes (Fig. 14c). The grains are compositionally heteroge- Bastnasite,€ the primary REE ore-bearing mineral at neous and contain abundant inclusions of calcite, dolo- Mountain Pass, was observed only in carbonatite mite, and barite. Table 7: Representative monazite LASS-ICP-MS geochronology data Petrology of Journal

Spot U Th Pb Th/U %208 Pb Measured isotopic ratios Apparent isotopic ages (Ma)

(ppm) (ppm) (ppm) common 208 Pb/ 2SE % 207 Pb/ 2SE % 206 Pb/ 2SE % Rho 208 Pb/ 2SE % 208 Pb/ 2SE 208 Pb*/ 2SE % 208 Pb*/ 2SE 204 Pb 206 Pb 238 U 232 Th 232 Th age abs. 232 Th 232 Th age abs.

MP-P0913-SQ1 monazite (spot size ¼ 9 lm, shot frequency ¼ 3 Hz, shot count ¼ 80, laser energy ¼ 4 mJ at 100%) 158 2660 2629 402 1247 1771 418 08188 192 054 172 1000 007895 310 1536 48 006910 326 1350 44

235 2962 2829 787 1059 195564 07726 195 033 239 1000 007628 311 1486 46 006820 327 1333 44 8 No. 57, Vol. 2016, , 341 2678 2644 599 1223 1873 368 08085 188 148571000 007882 311 1533 48 006918 327 1352 44 471 4920 460 654 251 2531150599 254 064 6100999 007458 313 1454 45 007271 328 1419 47 583 4287 4147 503 843 1998 455 0727 304 137690999 007722 322 1503 48 007071 338 1381 47 6111 5210 484 465 569 217461 06705 194 132 2350997 007408 313 1444 45 006987 328 1365 45 784 3820 3835 472 891 1918 396 07392 176 072 7221000 008007 315 1557 49 007294 331 1423 47 838 3462 3425 952 989 1985 413 07511 203 11 155 1000 007892 308 1535 47 007111 324 1389 45 974 2913 2808 442 746 22 545 06667 198 055 8181000 007684 313 1496 47 007111 328 1388 46 10 34 2395 2428 855 1016 1784 533 07772 197 016 475 1000 008083 310 1571 49 007262 325 1417 46 11 41 4624 4432 1087 846 2185 288 06812 173 217141000 007640 311 1488 46 006993 327 1366 45 12 35 3155 3099 909 1210 1786 353 08062 196 1 100 1000 007833 310 1524 47 006885 326 1346 44 13 64 2726 2544 446 970 265528 05913 229 296550999 007435 313 1450 45 006714 328 1314 43 14 7 2556 2672 417 1340 1958 403 07444 186 248331000 008324 307 1616 50 007209 323 1407 45 15 34 3468 329 1031 498 214607 06843 207 12 150 1000 007556 311 1472 46 007180 326 1401 46 16 36 4280 411 1370 667 1817 457 07697 185 –025 252 1000 007642 312 1488 46 007132 327 1392 46 17 47 3020 2997 641 927 20 420 0683 246 02 500 1000 007895 352 1536 54 007164 366 1398 51 18 23 2565 2558 1176 1132 168440 08047 185 –046 185 1000 007931 337 1543 52 007033 352 1374 48 19 75 2964 2893 410 982 1942 360 07396 192 065 8621000 007763 339 1511 51 007001 353 1368 48 20 16 2805 2854 182 944 2241 393 06615 200 0785 1260988 008096 343 1574 54 007332 357 1430 51 21 46 2378 2453 588 1291 1938 351 07612 190 05 460 1000 008204 354 1594 56 007144 368 1395 51 22 11 2789 2683 262 730 245531 06041 201 071 6481000 007648 355 1490 53 007090 368 1384 51 23 63 2761 2627 476 603 183710 0713 219 –006 1217 1000 007566 334 1474 49 007110 348 1388 48 24 89 3330 3185 431 814 22 545 0636 233 031 181 1000 007590 340 1479 50 006972 354 1362 48 25 98 2472 2339 269 716 208673 06725 196 061 9021000 007506 348 1463 51 006968 362 1362 49 26 87 2671 2559 324 1010 194423 07468 200 083 7471000 007598 347 1480 51 006831 361 1336 48 27 68 2908 2678 463 669 203591 0731 217 055 105 1000 007307 343 1425 49 006818 357 1333 48 28 104 2702 2612 270 790 1968 493 07128 204 071 5210999 007672 346 1494 52 007066 360 1380 50 29 103 3009 268 319 801 193570 07442 194 008 1188 1000 007066 333 1380 46 006500 348 1273 44 30 51 3530 3289 752 531 223628 0694 233 027 226 1000 007385 323 1440 47 006993 338 1366 46 31 156 3850 365 274 675 246447 062 224 097 2070994 007510 334 1464 49 007003 349 1368 48 32 88 3214 3019 397 678 199553 0716 219 019 321 1000 007439 349 1450 51 006935 363 1355 49 33 46 3523 3351 833 692 195564 07666 194 033 188 1000 007532 323 1468 47 007011 338 1370 46 34 54 3098 2942 625 1029 1884 425 077 212 06 183 1000 007524 348 1466 51 006750 362 1320 48 35 103 3803 354 394 791 213469 07104 183 113 3540999 007374 342 1438 49 006791 356 1328 47 36 31 3004 2881 1515 1051 1811 392 08117 185 023 430 1000 007598 339 1480 50 006800 354 1330 47 37 44 3560 3334 719 595 1973 497 0744 206 03 250 1000 007420 333 1447 48 006979 348 1364 47 38 68 3477 3277 568 963 1877 479 0782 202 112 7231000 007457 342 1454 50 006739 356 1318 47 39 255 5670 574 198 284 359836 0434 278 0423 1119 0970 007923 350 1541 54 007698 364 1499 55 40 81 3356 322 372 673 206485 0721 218 05 220 1000 007511 334 1464 49 007005 348 1369 48 41 3 3414 3393 1053 843 1953 384 07509 197 11 136 1000 007778 324 1514 49 007122 339 1391 47 42 27 4053 4025 1299 1010 1974 350 07739 179 225911000 007767 338 1512 51 006982 353 1364 48 43 3 3461 3569 1190 1093 1777 422 08131 185 –03 833 1000 008069 329 1568 52 007187 344 1403 48 44 83 2955 2704 312 906 231563 06728 188 13 100 1000 007158 351 1397 49 006510 365 1275 47 45 41 3334 3458 629 1041 2103 409 06866 195 — — — 008115 349 1577 55 007270 363 1419 51

46 97 3562 3463 337 634 254591 06383 208 042 110 1000 007605 339 1482 50 007123 354 1391 49 1581

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Table 7: Continued Spot U Th Pb Th/U %208 Pb Measured isotopic ratios Apparent isotopic ages (Ma)

(ppm) (ppm) (ppm) common 208 Pb/ 2SE % 207 Pb/ 2SE % 206 Pb/ 2SE % Rho 208 Pb/ 2SE % 208 Pb/ 2SE 208 Pb*/ 2SE % 208 Pb*/ 2SE 204 Pb 206 Pb 238 U 232 Th 232 Th age abs. 232 Th 232 Th age abs.

MP-L0613-BDAY2 monazite (spot size ¼ 9 lm, shot frequency ¼ 3 Hz, shot count ¼ 80, laser energy ¼ 4 mJ at 100%) 127 7570 758 3030 057 13 9230457 341 –014 143 1000 007129 211 1392 29 007089 234 1384 32 232 6790 690 2174 119 1131770586 254 014 7860999 007205 211 1406 30 007119 234 1390 32 3101 7730 803 781 113 18 6670604 249 008 238 1000 007352 213 1434 31 007269 235 1418 33 465 16290 1670 2364 054 18528603733 291 051 106 1000 007215 218 1408 31 007176 240 1401 34 577 17210 1717 2128 085 1331280533 255 037 4060998 007023 229 1372 31 006964 250 1361 34 6123 24100 2448 1880 051 27 37 0392 318 036 3890997 007154 223 1397 31 007118 244 1390 34 719 12940 1263 6536 031 1481820444 329 005 520 1000 007029 215 1373 30 007008 237 1369 32 875 15700 1516 2198 057 145897 0616 221 054 6110999 006994 222 1366 30 006954 244 1359 33 952 2161 227 420 270 14715064 240 035 8861000 007661 240 1492 36 007453 260 1453 38 10 49 3490 370 719 152 1631720598 251 014 393 1000 007749 247 1508 37 007631 266 1486 40 11 31 9310 939 3333 020 956530361 442 004 275 1000 007383 260 1440 38 007368 279 1437 40 12 57 19560 1830 3448 039 2243840467 354 024 313 1000 006844 389 1338 52 006817 401 1333 54 13 56 12840 1305 2252 045 1971520515 245 034 221 1000 007441 241 1451 35 007407 261 1444 38 14 43 11270 1136 2500 039 18817604625 258 –014 429 1000 007382 260 1440 37 007353 278 1434 40 15 262 8870 889 355 154 1195 569 0694 208 019 848 0971 007342 228 1432 33 007229 249 1411 35 16 8 8080 815 1087 102 1341870582 298 026 50 0998 007409 237 1445 34 007334 257 1431 37 17 236 10560 1012 448 105 1451590686 253 0203 1330982 007033 337 1374 46 006959 351 1360 48 18 372 16540 1538 441 069 1521120587 436 0135 1340951 006820 245 1333 33 006773 264 1325 35 19 10 20530 2006 1949 022 22 7270381 302 029 2970995 007176 255 1401 36 007160 274 1398 38 20 526 16050 1526 309 129 1322 499 0678 232 0158 829 0963 006978 236 1363 32 006888 256 1346 35 21 127 4590 466 373 132 236636 03244 206 0393 1380989 007455 241 1453 35 007357 261 1435 37 22 279 5720 5734 195 035 20 240 03513 243 01176 806 0958 007177 241 1401 34 007152 261 1396 36 23 105 3644 288 31 505 1704 393 05643 195 01233 529 0938 005588 311 1099 34 005306 327 1045 34 Petrology of Journal 24 234 2673 191111809 1643 365 0602 192 0089 1690994 005048 664 995 66 004640 672 917 62 25 215 4550 350 20 199 25114704443 258 00359 814 0953 005433 377 1069 40 005325 390 1049 41 26 11 3745 3989 327 115 32 37504651 247 057 5790999 007541 249 1470 37 007455 269 1453 39

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Fig. 14. Examples of bastnasite€ in MPIS carbonatite. (a) Plane-polarized light photomicrograph of bastnasite€ in a matrix of calcite and minor dolomite in carbonatite thin section (MP-P0913-SQ1). (b) BSE image of bastnasite–barite€ intergrowth in grain mount (carbonatite sample MP-P0913-1B). (c) BSE image of intergrowth between bastnasite€ and synchysite–parisite in carbonatite thin section. (d) Weighted mean plot of eNdi for bastnasite€ in carbonatite (MP-P0913-SQ1). brt, barite; bst, bastnasite;€ cal, calcite; dol, dolomite; syn, synchysite/parisite.

Bastnasite€ from two carbonatite samples was ana- attributed to differences in the modal abundance of lyzed for U–Pb and Nd isotopes. Both samples are very major and accessory phases. The trends observed be- low in U (<10 ppm), and have high and variable com- tween major elements and silica in MPIS rocks, such as mon Pb contents. The low U combined with the large increasing Al2O3 with increasing silica and lack of trend scatter in Pb and U/Pb isotopic ratios suggest that bast- between K2O and silica, are common in other ultrapo- nasite€ has behaved as an open system with respect to tassic suites (Foley et al., 1987; Castor, 2008). Pb; thus reliable dates could not be calculated and bast- Decreasing Mg and Ca contents with increasing silica nasite€ U–Pb data are not considered further. The open- content are controlled by the greater modal abundance system behavior described here for bastnasite€ was also of clinopyroxene (Mg, Ca) and apatite (Ca) in low-silica reported by DeWitt et al. (1987, 2000). In contrast, Nd samples. Decreasing Fe content with increasing silica is isotopic analysis of bastnasite€ from sample MP-P0913- probably a product of decreased abundance of clinopyr-

SQ1 yielded relatively consistent eNdi values with a oxene and biotite. Lower P2O5 is probably due to weighted mean eNdi of –32 6 03 (MSWD ¼ 18), con- decreased abundance of apatite in more felsic samples. sistent with eNdi from monazite in the same sample TiO2 also decreases with increasing silica, probably (Figs 13e and 14d). A date of 1380 Ma was used for bast- owing to the high abundance of Ti-bearing biotite in nasite€ eNdi age correction, based on the sample’s mean mafic samples, which has been reported in previous monazite Th–Pb date of 1371 6 10 Ma (see above). studies (Haxel, 2005; Castor, 2008). Higher Ti in mafic Bastnasite€ Nd isotopic data are reported in samples may also be due in part to the presence of Supplementary Data Electronic Appendix 6. clinopyroxene (Haxel, 2007). Increased levels of Al and Na in high-silica samples are probably due to greater abundances of K-feldspar and plagioclase, the latter of which is mostly absent in mafic samples. Potassium DISCUSSION does not show a marked trend, presumably because of Trace element sinks in an ultrapotassic silicate- the high abundance of biotite in mafic samples and K- carbonatite suite feldspar in felsic samples. Trends observed between whole-rock major element The higher abundance of compatible trace elements oxides in Mountain Pass shonkinite and syenite may be (i.e. Cr, V) in mafic samples is characteristic of most 1584 Journal of Petrology, 2016, Vol. 57, No. 8 igneous suites, and is probably controlled primarily by The HREE, Th, and U contents of the samples do not clinopyroxene and biotite. The high abundance of in- show a trend with major element composition. compatible trace elements in mafic MPIS samples (i.e. Metamict, high-Th, high-U, and HREE-rich zircon (e.g. Ba, Sr, and LREE) is atypical of most igneous suites, but Fig. 9a and b, Supplementary Data Electronic Appendix is common in ultrapotassic suites (Foley et al., 1987; 7) is ubiquitous in MPIS rocks of all compositions. Castor, 2008) and reflects high modal percentages of in- Zircon has been shown to be a significant host to HREE compatible element-rich phases in the mafic samples. in granitoids (Bea, 1996) and actinides in alkaline rocks Apatite analyzed in this study typically has Sr contents (e.g. Zacek et al., 2009), suggesting that zircon may be in the tens of thousands of ppm range (Supplementary an important host to HREE, Th, and U in the MPIS. Data Electronic Appendix 11), and is probably the pre- Concordant zircon grains utilized for geochronology dominant sink for Sr in the less evolved rocks, as it is data are less abundant and contain much lower Th, U, most abundant in these samples. High Ba content is and HREE than metamict grains (e.g. Table 5), and are Downloaded from https://academic.oup.com/petrology/article-abstract/57/8/1555/2413438 by guest on 31 March 2020 probably due to high-Ba biotite, which was noted by thus not considered. An approximate modal abundance Haxel (2005) and Castor (2008). The absence of plagio- of 0125–025% metamict zircon accounts for 30– clase in mafic MPIS rocks indicates that it is unlikely to 70% of whole-rock Yb content in felsic samples account for high Sr and Ba in mafic samples, and is (Supplementary Data Electronic Appendix 12). In mafic therefore not an important host to these elements. samples, however, zircon only contains 5–15% of the Chondrite-normalized REE patterns of the whole- whole-rock Yb. Apatite and titanite each account for rock samples analyzed in this study are typical of ultra- 8–20% of the whole-rock Yb in all samples, regardless potassic rocks, most notably the high LREE to HREE of composition. Sodic amphibole may account for the slope and lack of an Eu anomaly (e.g. Foley et al., 1987; bulk of the remaining HREE—it is common in mafic to Castor, 2008), the latter indicating that plagioclase frac- intermediate MPIS rocks, and is known to contain ap- tionation was not a significant factor in the magmatic preciable HREE (e.g. Bea, 1996). Approximately 5–30% history of the MPIS—consistent with the paucity of of whole-rock Th can be attributed to zircon, whereas plagioclase in these rocks. apatite accounts for 4–10% (but as much as 20%) and The higher abundance of LREE in low-silica samples titanite for <1–3% (Supplementary Data Electronic is most probably due to the abundance of apatite. Appendix 12). Thorite was also observed in some sam- Apatite has been noted as an important REE host in al- ples, and probably contains the bulk of whole-rock Th— kaline rocks (e.g. Zhou & Wang, 1989; Krenn et al., less than 002% modal thorite would be required to ac- 2012), and forms up to 3% of the mafic MPIS samples count for the whole-rock Th in most samples. For two in this study. Previous studies have reported up to 5% samples with the highest whole-rock Th content (MP- apatite in mafic MPIS samples (Woyski, 1980; Haxel, P0913-5A and MP-P0714-17), 003% and 005% modal 2005). For MPIS samples in which apatite was analyzed, thorite would be required, respectively. Where present, its modal abundance accounts for 30–40% and 40– monazite also accounts for some of the whole-rock Th, 70% of the whole-rock Ce content for felsic and mafic but occurs primarily in felsic samples and is not present samples, respectively (Supplementary Data Electronic in significant abundance. Most of the whole-rock U (65– Appendix 12). Titanite is also an important REE host—it 90% or more) can be attributed to zircon, whereas apa- is known to incorporate up to 4 wt % REE in alkaline tite and titanite each account for 1–15% igneous rocks (e.g. Vuorinen & Ha˚ lenius, 2005), and is (Supplementary Data Electronic Appendix 12). commonly considered an important REE sink in silicate Together, these data indicate that apatite is probably magmas (e.g. Gromet & Silver, 1983; Sawka et al., the most important host of LREE in these rocks, fol- 1984). Although the modal abundance of titanite is lowed by titanite. Apatite, titanite, and zircon are prob- poorly constrained in most samples, for samples in ably important HREE hosts, but amphibole may also which its approximate modal abundance is known, the host significant amounts of HREE in mafic samples. modal abundance of titanite accounts for 5–20% of Thorite and zircon are the most important hosts to Th whole-rock Ce (Supplementary Data Electronic and U, respectively. Monazite contains abundant REE Appendix 12). Titanite in apatite-poor samples contains and Th, but is not present in significant amounts in higher LREE—for example, titanite accounts for up to most samples and appears to occur only as a secondary 90% of whole-rock Ce in one apatite-poor leucosyenite phase. The presence of monazite in veins may indicate sample (MP-P0714-17). However, titanite modal abun- that monazite grew from a late-stage fluid, as suggested dance is similar in both mafic and felsic samples, and by Stoeser (2013)—in this case, monazite may be an therefore the observed trends in REE cannot be ex- additional primary REE host in some samples. plained by differences in its relative abundance be- Alternatively, the presence of monazite and rutile in tween rock types. Monazite is present in some felsic rocks with titanite that appears to be breaking down MPIS samples, and must account for some of the (Fig. 5c), and the lack of titanite in rocks that contain sig- whole-rock LREE in these samples. Sodic amphibole nificant monazite and rutile (e.g. MP-P0913-2), may indi- may also contain a significant amount of the whole-rock cate that the latter two phases grew at the expense of LREE (e.g. Bea, 1996); however, amphibole composition titanite, and thus host REE, Th, and HFSE that were for- was not determined. merly hosted by titanite. Journal of Petrology, 2016, Vol. 57, No. 8 1585

Based on these data, apatite, titanite, zircon, thorite, 1407 6 11 Ma, and, based on crystal morphology and and sodic amphibole are likely to have played the most oscillatory zonation, are regarded as reflecting igneous important roles in controlling the REE, HFSE, and actinide crystallization ages in five of these samples. budget of the ultrapotassic magmas as they crystallized. Concordant dates from 1635 to 1800 Ma are derived from multiple, discrete grains in MP-P0913-5A and MP- P0913-6; these are interpreted as xenocrystic zircon in- Geochronology of the MPIS herited from the host-rocks based on their similarity to Ultrapotassic silicate rocks zircon from the host-rock in terms of age, morphology, With the exception of two samples that have undergone and Th/U < 1 (see description of zircon above). MP- minor disturbance of the U–Pb system and therefore re- P0913-5A also yielded zircon dates from 1600 to cord two discrete age populations (samples MP-P0714-16 1430 Ma from three large, rounded grains that do not and MP-P0714-26), titanite in the ultrapotassic rocks of the overlap with either host-rock zircon dates, or zircon Downloaded from https://academic.oup.com/petrology/article-abstract/57/8/1555/2413438 by guest on 31 March 2020 MPIS consistently yields single-population U–Pb dates dates from other MPIS rocks (Fig. 10b, Supplementary that range from 1429 6 10 to 1385 6 18 Ma (Fig. 6). In con- Data Electronic Appendix 8). These are interpreted as trast, zircon yields complex and chiefly discordant U–Pb ‘mixed’ dates resulting from sampling of both data (Fig. 10), providing minimal age information for most Mesoproterozoic overgrowths and inherited, samples. Based on textural observations, monazite in the Paleoproterozoic cores of grains. Additionally, these zir- ultrapotassic rocks appears to be secondary, and yields con grains have Th/U ratios < 1(Supplementary Data scattered and imprecise Th–Pb and U–Pb dates (Fig. 13). Electronic Appendix 8), the same as for zircon older Several lines of evidence indicate that titanite is most than 1640 Ma in the sample and zircon from the host- probably a primary phase and that the titanite dates are rocks. In contrast, all zircon grains younger than best interpreted as the igneous crystallization ages of 1430 Ma in this sample (as well as most zircon grains the ultrapotassic rock samples: (1) detailed petrographic from all other MPIS samples except MP-P0913-10B) observation indicates that titanite is generally euhedral have Th/U > 1(Supplementary Data Electronic to subhedral and shows interlocking textures with pri- Appendix 8); therefore, zircon grains with Th/U < 1 are mary igneous mineral assemblages (Fig. 5a and b), sug- excluded from the age interpretation of MP-P0913-5A. gesting that it crystallized directly from a melt; (2) MP-P0913-10C is the only sample in which zircon where present, titanite alteration textures suggest that it yields only Mesoproterozoic dates, but titanite and zir- is primary and has partially reacted to form secondary con dates do not coincide (Figs 6 and 10e). This rock minerals (Fig. 5c), rather than grown in response to al- crosscuts another rock (MP-P0913-10B) that yields teration; (3) chondrite-normalized REE patterns in younger overlapping dates from titanite and oscillatory- Mountain Pass titanite are similar to chondrite- zoned zircon (e.g. Figs 2c and 6, Supplementary Data normalized REE patterns from Mountain Pass ultrapo- Electronic Appendix 8). We therefore infer that the U–Pb tassic rocks in terms of LREE enrichment and lack of an zircon age does not reflect the crystallization age of this Eu anomaly (Figs 4 and 7a), suggesting that both crys- rock; instead, we prefer the interpretation that these zir- tallized from the same melt; (4) Zr-in-titanite thermom- con grains are inherited, and a result of recycling of etry predominantly yields temperatures of 700C and older zircon within the MPIS. higher, consistent with an igneous origin (Fig. 7b); (5) dates obtained from oscillatory-zoned, concordant zir- Carbonatite con overlap the titanite dates within uncertainty in four The Th–Pb monazite crystallization ages of carbonatite of five samples. Taken together, and as stated above, samples MP-L0613-BDAY2 and MP-P0913-SQ1 are these data suggest that titanite is most probably a pri- inferred to be 1405–1395 Ma and 1380–1370 Ma, re- mary igneous phase in the Mountain Pass ultrapotassic spectively. We attribute the excess scatter (MSWD ¼ 42 rocks, and has recorded the same igneous crystalliza- and 16, respectively) observed in the samples to be a tion event that is sparsely recorded by zircon. We attri- result of variable but minor radiogenic Pb loss, which bute titanite U–Pb age sub-populations in MP-P0714-16 probably skews their mean dates toward younger ages. and MP-P0714-26 to subtle loss of radiogenic Pb, and Regardless, monazite Th–Pb dates from carbonatite the ages of the samples are interpreted to be repre- overlap titanite and zircon U–Pb dates < 1415 Ma from sented by their older age components (Fig. 6). The vari- the ultrapotassic rocks within uncertainty—indicating ability in apparent Zr-in-titanite temperatures in all but that carbonatite magmatism and the latter stages of the youngest samples (MP-P0913-10C and MP-P0714- ultrapotassic magmatism at Mountain Pass were 17) suggests that some titanite grains may have re- broadly coeval. corded lower apparent Zr-in-titanite temperatures in re- sponse to thermal disturbance during emplacement of later phases of magma, which may provide an explan- Timescales of ultrapotassic silicate–carbonatite ation for the subtle loss of radiogenic Pb in MP-P0714- magmatism at Mountain Pass 16 and MP-P0714-26. U–Pb zircon dates from ultrapotassic rocks in the Despite their scarcity, concordant zircon dates are Birthday and Corral stocks (Fig. 1b) constrain ultrapo- present in six samples, ranging from 1458 6 7to tassic magmatism in these locations to 1430–1420 Ma 1586 Journal of Petrology, 2016, Vol. 57, No. 8

(Fig. 15). Although less precise, titanite dates corrobor- these rocks that is now exposed. However, alkaline and ate these ages. Zircon and titanite dates from ultrapo- carbonatite magmatism in other locations around the tassic rocks in the Tors, Mexican Well, and Wheaton world is commonly long-lived within a restricted area stocks indicate a second stage of magmatism c. 1410– (Jones et al., 2013). For example, alkaline igneous rocks 1400 Ma (Figs 1b and 15). The mean Th–Pb date of in the Southern Granulite Terrain of India (Santosh 1396 6 16 Ma from monazite in carbonatite dike sample et al., 2014), the Parana–Etendeka large igneous prov- MP-L0613-BDAY2 suggests that this stage was accom- ince in Paraguay (Gibson et al., 2006), and the panied by emplacement of carbonatite dikes (Figs 2d Monteregian Hills of Quebec, Canada (Eby, 1984) were and 15). We interpret titanite dates of 1385 6 18 Ma and all shown to have been emplaced in two distinct epi- 1358 6 71 Ma from shonkinite and syenite dikes (MP- sodes 20 Myr apart. Additionally, alkaline igneous P0913-10C and MP-P0714-17), respectively, to record rocks in the Chilwa Alkaline Province of Malawi were the latest stage of ultrapotassic magmatism at c. 1380– emplaced in three phases 10 Myr apart (Eby et al., Downloaded from https://academic.oup.com/petrology/article-abstract/57/8/1555/2413438 by guest on 31 March 2020 1370 Ma, on the following basis: (1) the dikes cross-cut 1995). The 45 Myr age range of MPIS rocks is longer 1410–1400 Ma syenite; (2) the dikes are continuous and than these other occurrences, but regardless, the tem- do not appear to be deformed, suggesting that they poral range of ultrapotassic and carbonatitic magma- were not comagmatic with the rock they cross-cut (Fig. tism at Mountain Pass is consistent with temporal 2c); (3) previous work suggests that a shonkinite vein relationships observed in other alkaline rock occur- cross-cuts carbonatite (Olson & Pray, 1954), indicating rences, and indicates repeated partial melting of the relatively late emplacement of shonkinite dikes and source(s) of the MPIS during the Mesoproterozoic. veins. The mean monazite Th–Pb date of carbonatite sample MP-P0913-SQ1 indicates that this stage was accompanied by emplacement of the Sulphide Queen Petrogenetic processes recorded by mineral- stock (Figs 1b and 15). scale Nd and Hf isotopes

The geochronology results reveal that ultrapotassic Titanite eNdi values from the ultrapotassic rocks at silicate–carbonatite magmatism at Mountain Pass Mountain Pass predominantly range between –35 and occurred episodically over 45–50 Myr, a relatively –6, with the highest values in all but one sample over- long period of time considering the small volume of lapping the eNdi (25to–35) values obtained from

BDAY Birthday Stock U-Pb Titanite 26 U-Pb Zircon 6 Th-Pb Monazite Corral Stock 19

5A

Mexican Well Stock 21 23 Wheaton Stockk 22A

16

10A Tors Stock 10B

10D

Ultrapotassic 10C Dikes 17

Carbonatite Dike BDAY2 Sulphide Queen Stock SQ1 Carbonatite Ultrapotassic Silicate Rocks 1440 1420 1400 1380 1360 Age (Ma)

Fig. 15. Summary of titanite, zircon, and monazite dates from all samples analyzed in this study. Each date and associated uncer- tainty is from all analyses of a specific mineral interpreted as an age. It should be noted that the uncertainty of one sample extends to the right beyond the width of the plot (‘17’ ¼ MP-P0714-17, titanite date ¼ 1358 6 71 Ma). Locations of samples in the field area are indicated. Journal of Petrology, 2016, Vol. 57, No. 8 1587 monazite and bastnasite€ in MPIS carbonatite (Fig. 8). that this may be the case only for the earliest MPIS

The overlap of eNdi of the carbonatite and the most rocks (Fig. 8). primitive eNdi of the ultrapotassic rocks supports the The low eNdi in titanite from ultrapotassic rock sam- idea that they are related and probably derived from a ples may be explained by assimilation of crustal mater- common source. ial, either the host-rocks exposed at Mountain Pass or

In contrast to titanite Nd isotopic data, zircon eHfi unexposed basement rocks of the Mojave Crustal from Mesoproterozoic zircon in the same rocks does Province. This hypothesis is supported by the general not systematically differ within or between samples, decrease in eNdi with increasing SiO2 content in the old- with all data from all samples giving a weighted mean est (1430–1420 Ma) and youngest (1380–1370 Ma) of 19 6 02 (MSWD ¼ 09, Fig. 12). These data support MPIS samples (Fig. 8) — assimilation of felsic country the hypothesis that MPIS ultrapotassic rocks are all rock and accompanying fractional crystallization would Downloaded from https://academic.oup.com/petrology/article-abstract/57/8/1555/2413438 by guest on 31 March 2020 derived from the same source and, more regionally, co- have increased the SiO2 content of the magmas. A bin- incide with previously reported zircon eHfi for 14Ga ary mixing model demonstrates that the variability in granitoids throughout the Mojave Desert (Wooden Nd isotopic signatures of MPIS titanite can be explained et al., 2012, Fig. 1a). by assimilation (Fig. 16). In this model, mean titanite

Variable titanite eNdi and consistent zircon eHfi indi- eNdi values are used to approximate whole-rock eNdi cate that the Nd isotopic signature of some titanite has for each sample, parameters for the primary magma been influenced by a process that did not have a notice- are estimated based on the most primitive MPIS sam- able effect on zircon eHfi. Possible explanations include ples (45 wt % SiO2, 325 ppm Nd, eNd –3), and par- (1) magma mixing, (2) contamination of the magma by ameters for the contaminant are estimated using 70 wt wall-rock, and/or (3) post-crystallization disturbance of % SiO2 to approximate a granitic (sensu lato) compos- the Nd isotopic system in titanite. ition. Using these parameters, 70% contaminant and Pankhurst et al. (2011) attributed variable titanite 30% primary magma would be required to produce eNdi values in an A-type granite to mixing of parental the MPIS sample with the highest silica content (MP- magmas. DeWitt et al. (2000) interpreted Pb and Nd iso- P0913-5A, 65 wt % SiO2) through binary mixing. tope data to record mixing of melts derived from mantle Assuming that MP-P0913-5A is 70% contaminant, a and lower crustal sources of variable age to produce curve fit to the MPIS samples in eNd vs 1/Nd space de- the MPIS carbonatite magma. Thus, some variability in fines a contaminant end-member with eNd –15 and a titanite eNdi from MPIS ultrapotassic rocks may be at- Nd concentration of 150 ppm (Fig. 16). Nd abundance tributable to mixing between magmas from different and isotopic data have not been reported for Mountain source regions with different Nd isotopic signatures. Pass host-rocks, making the definition of this end- However, the same variability is not observed in car- member difficult. Additionally, a binary mixing model is bonatite or the youngest ultrapotassic rocks, suggesting perhaps an oversimplification, as fractional

1/whole-rock Nd (ppm) 0 0.005 0.010.015 0.02 0.025 0 Sample Location Carbonatite Model -2 0% contaminant 10C -4 17 Fractional Crystallization 10D Tors -6 10B

Assimi 16 i

-8 lation

εNd 19 6 Corral -10 5A

-12 26 Birthday

Typical εNdi uncertainty 22A Wheaton -14 99% contaminant SQ1 Sulphide Queen -16

Fig. 16. Nd isotope binary mixing model for MPIS rocks. Each data point is a mean titanite eNdi value from all analyses in one sam- ple, used here to approximate whole-rock eNdi. Typical uncertainty on the mean eNdi is shown by the vertical bar at the lower right (607 e-units). Each bar on the mixing model line represents 10% increments (i.e. addition of 10% contaminant to the original magma). 1588 Journal of Petrology, 2016, Vol. 57, No. 8 crystallization must have accompanied assimilation and provides a potential explanation for the subtle loss of would also have increased the SiO2 content of the radiogenic Pb observed in these two samples. We favor evolving magma. However, an assimilation–fractional this interpretation over disturbance to the U–Pb system crystallization model (DePaolo, 1981) with a ratio of as- in these samples by later intrusion of magma, because similation rate to fractional crystallization rate (r) ¼ 1 the latter scenario does not explain the variability in and relatively high bulk partition coefficient (D) ¼ 08– eNdi in these samples. 12 for Nd (estimated based on the high proportion of In summary, mineral-scale Nd and Hf isotopic data apatite and titanite in the most primitive samples) yields reported here are interpretedtoindicatederivationof a very similar result. In summary, a binary mixture of Mountain Pass ultrapotassic rocks and carbonatite 30% primary magma with 325 ppm Nd, eNd ¼ –3, and from the same source, potential mixing of magma 45 wt % SiO2 and 70% contaminant with 150 ppm from two or more sources to produce these magmas, Downloaded from https://academic.oup.com/petrology/article-abstract/57/8/1555/2413438 by guest on 31 March 2020 Nd, eNd ¼ –15, and 70% SiO2 would be required to and crustal contamination of some of the ultrapotassic produce MP-P0913-5A (mean eNdi –96 and 65 wt % magmas. Post-crystallization disturbance of the Nd SiO2, Figs 8 and 16). MP-P0913-5A contains abundant isotopic signature in titanite by a secondary fluid can inherited zircon grains (Fig. 10b) and xenoliths of host convincingly be demonstrated in only two of the ana- gneiss and granite, providing direct evidence of crustal lyzed samples. assimilation. Samples MP-P0913-6 and MP-P0714-17 fall off of the mixing curve and can be explained by a combination of fractional crystallization and assimila- Petrogenetic model for the Mountain Pass tion (40% assimilation; Fig. 16). Intrusive Suite The uniformity of the zircon eHfi in MPIS rocks indi- Despite the temporal overlap between ultrapotassic and cates that the Hf isotope compositions of the magmas carbonatite magmatism at Mountain Pass, available were unaffected by contamination. This could be ex- data suggest that the two are unlikely to have differenti- plained by a contaminant with a very low Hf concentra- ated from a common parental melt. Derivation of the tion relative to the primary magmas. Primitive MPIS carbonatite from a silicate melt by fractional crystalliza- samples contain 12 ppm Hf (Table 2), requiring the Hf tion implies either that the carbonatite is a cumulate content of the contaminant to be < 2 ppm based on that crystallized from a silicate melt (e.g. Gittins & the mixing parameters above. It is more likely that zir- Harmer, 2003; Downes et al., 2005) or that it is the most con was the dominant Hf reservoir in the contaminant evolved member of the suite and represents a late- and was not dissolved when incorporated into the ultra- stage, residual liquid. It seems unlikely that the car- potassic magma, resulting in contamination of the bonatite is a cumulate because it does not fall along an magma without a noticeable effect on the Hf isotopic evolutionary trend with the ultrapotassic rocks (Fig. 17a composition of the melt. This is feasible, given the high and b). Furthermore, a cumulate should be composed distribution coefficient (Kd) for Hf between zircon and primarily of crystals and is therefore unlikely to intrude common silicate melts (e.g. Mahood & Hildreth, 1983; subsequently as cross-cutting dikes or to differentiate Fujimaki, 1986), and because of the abundance of euhe- into different types of carbonatite (i.e. so¨ vite and befor- dral (apparently undissolved), inherited zircon in some site); additionally, it should contain trapped intercumu- MPIS samples. An alternative explanation is that the lus material that represents the silicate melt from which MPIS and its host-rocks have a similar Hf isotopic com- it crystallized, which is not observed. position. However, this would suggest that the host- Alternatively, if the carbonatite represents a late- rocks plot far above the eHfi–eNdi terrestrial array of stage, residual liquid, incompatible elements that are Vervoort et al. (1999); that is, they have eHfi between 2 abundant in the carbonatite (i.e. Ba, Sr, LREE) should be and 0, and eNdi –15 (from the model presented least abundant in primitive (i.e. low-silica) members of above); this is therefore an unlikely explanation. the MPIS ultrapotassic suite and most abundant in Post-crystallization disturbance of the Nd isotopic evolved (i.e. high-silica) members. However, the most system in titanite could also explain the source of Nd primitive members of the MPIS ultrapotassic suite have isotopic variation in at least two samples (MP-P0714-16 the highest Ba, Sr, and LREE, whereas the most evolved and MP-P0714-26). These samples yielded two sub- members have the lowest contents of these elements populations of U–Pb data (Fig. 6), and also have some (Fig. 3). Furthermore, the carbonatite and ultrapotassic of the lowest and most variable eNdi values (as low as – rocks do not occur in a concentrically zoned, plug-like 107; Fig. 8). Titanite from these samples may record intrusion with a relatively small core of carbonatite sur- the eNdi of an altering fluid that post-dated crystalliza- rounded by alkaline silicate rocks, as has been docu- tion (e.g. Shawe, 1952), in addition to potential mixing mented in other locations and cited as evidence for of magma from different sources and/or magma con- carbonatite representing a volumetrically small, late- tamination by wall-rock. The variable eNdi and relatively stage residual liquid (e.g. Juquia complex in Brazil, low silica content in these two samples alone suggests Beccaluva et al., 1992; Salmagorskii complex in Russia, that magma contamination may be a less likely scenario Korobeinikov et al., 1998). On the contrary, field evi- to account for their Nd isotopic composition. dence shows that the carbonatite and ultrapotassic Furthermore, post-crystallization disturbance of titanite magmas were emplaced separately (e.g. Fig. 2d), Journal of Petrology, 2016, Vol. 57, No. 8 1589

3e5 6e4 (a) (b) 5e4

5 2e carbonatite 4e4 carbonatite

3e4 Ba (ppm) Ce (ppm) 1e5 2e4 ultrapotassic rocks ultrapotassic rocks 1e4

0 0 0 10 20 30 40 50 60 70 0 10 20 30 40 50 60 70

SiO2 (wt. %) SiO (wt. %) 2 Downloaded from https://academic.oup.com/petrology/article-abstract/57/8/1555/2413438 by guest on 31 March 2020 ultrapotassic rocks bastnäsite-barite sövite (SQ1) Haxel (2005) bastnäsite-barite sövite Castor (2008) samples: bastnäsite-barite sövite (85-4) bastnäsite-barite sövite (HP20B) bastnäsite-barite beforsite (O-924)

Bastnäsite-barite Sövite/ultrapotassic Rock Partition Coefficients 103 (c)

102

10

1

10-2 Bastnäsite-barite Sövite/ultrapotassic Rock 10-3

Cs Rb Ba Sr P2O5 Th U Hf Zr TiO2 Ta Nb La Ce Pr Nd Sm Eu Gd Tb Yb Lu Element Martin et al. (2013) Experimental Melts: Mountain Pass Ultrapotassic Rocks:

H2O-bearing LM73 LM83 LM101 Shonkinite 10A 10C 19 22A Haxel (2005) Anhydrous LM108 Syenite 2 10B 10D Leucosyenite 9B 17 23

Fig. 17. Whole-rock variation diagrams and carbonatite/ultrapotassic rock trace element ratio plot for MPIS rocks. (a) Ba vs silica plot for MPIS carbonatite and ultrapotassic rocks. (b) Ce vs silica plot for MPIS carbonatite and ultrapotassic rocks. Sample SQ1 is not shown, but contains 138 000 ppm Ce and <1 wt % SiO2 and thus plots above the carbonatite samples shown. (c) Bastnasite–€ barite so¨ vite/ultrapotassic rock trace element patterns for MPIS rocks. Experimental data shown for comparison are from Martin et al. (2013). Bastnasite–barite€ so¨ vite data are from Haxel (2005); the plotted elements are all of those that are available from this dataset. Samples MP-P0913-5A, MP-P0913-6, MP-P0714-16, and MP-P0714-26 are interpreted as being contaminated based on Nd isotopes, and are not shown, but plot similarly to samples shown here. requiring that a parental magma would have had to bearing, potassic systems with MPIS data from Haxel undergo protracted fractional crystallization prior to (2005), Castor (2008), and this study to test the liquid im- mobilization and emplacement of its products. Taken miscibility hypothesis for MPIS ultrapotassic rocks and together, these data indicate that if the carbonatite and carbonatite. Although the experiments of Martin et al. ultrapotassic rocks were derived from a common paren- (2013) were performed at higher pressures (1–3 GPa) tal melt, liquid immiscibility and physical separation of than that of the MPIS emplacement (01 GPa), the ex- carbonatite and ultrapotassic melts would have to have periments of Veksler et al. (1998) were carried out at occurred at some point along its evolutionary path. lower pressures (<1 GPa) and yielded similar results for Partition coefficients between immiscible carbonatite melts of comparable composition. and silicate liquids have been determined for a number The ratios of trace elements between MPIS carbona- of elements by experimental studies (e.g. Veksler et al., tites [bastnasite–barite€ so¨ vite from Haxel (2005)] and 1998, 2012; Martin et al., 2012, 2013), providing a quan- ultrapotassic rocks (from this study) are significantly dif- titative method for evaluating whether or not carbona- ferent from the experimentally derived carbonatite– tite and alkaline silicate rocks at a given locality are silicate partition coefficients of Martin et al. (2013). The potentially related by liquid immiscibility. We compared MPIS carbonatite/ultrapotassic rock ratios are lower for the experimental data of Martin et al. (2013) from H2O- P2O5, TiO2, Cs, Rb, and HREE, and higher for Zr than the 1590 Journal of Petrology, 2016, Vol. 57, No. 8 experimental melts (Fig. 17). Additionally, ratios are Tectonic setting of the MPIS lower for Ta in all but one sample, and higher for LREE The spatial relationship and temporal overlap between in all but one sample. Carbonatite/ultrapotassic rock the MPIS and 14 Ga granitoids of similar age through- trace element ratios for bastnasite–barite€ so¨ vite ana- out the southwestern USA suggests that both were lyzed in this study show similar differences from the ex- emplaced in response to the same thermal event perimental data, and are also lower for K2O, Na2O, (Haxel, 2005) and in a similar tectonic regime (Gleason Al2O3, and Zr, and higher for Pb and LREE (see et al., 1994). Although some studies have postulated an Supplementary Data Electronic Appendix 13 for parti- anorogenic origin for the 14 Ga granitoids (e.g. tion coefficient plots). Carbonatite/ultrapotassic rock Anderson & Bender, 1989; Hoffman, 1989), more recent trace element ratios calculated with MPIS carbonatite studies have concluded that they were associated with samples from Castor (2008) (so¨ vite, dolomitic so¨ vite, a contractional orogenic event (e.g. Nyman et al., 1994; and beforsite) show similar discrepancies. Daniel & Pyle, 2006; Whitmeyer & Karlstrom, 2007; Downloaded from https://academic.oup.com/petrology/article-abstract/57/8/1555/2413438 by guest on 31 March 2020 A key difference between the MPIS carbonatite and Daniel et al., 2013). Alkaline and carbonatitic magma- experiments is that experimental immiscible carbonatite tism has been documented in contractional settings in liquids are invariably alkali-rich (e.g. Veksler et al.,1998, other locations (e.g. Tilton et al., 1998; Wang et al., 2012; Martin et al., 2012, 2013), whereas MPIS carbona- 2001; Ying et al., 2004), and commonly occurs peripher- tite is distinctly alkali-poor (Na2O þ K2O < 2). However, ally to orogens (e.g. Veizer et al., 1992; Jones et al., this may be due in part to late-stage alkali loss, which 2013) or larger subalkaline igneous provinces (e.g. some researchers have argued is common in carbona- Gibson et al., 2006; Ernst & Bell, 2010). The occurrence tites (Bell et al.,1999; Martin et al.,2013), and may also of a belt of ultrapotassic rocks (including the MPIS) have resulted in depletion of large ion lithophile elem- along the westernmost edge of a large, 14 Ga granit- ents (LILE) such as Cs and Rb. Regardless of alkali con- oid suite (Castor, 2008) that may have been emplaced in tent, carbonatite/ultrapotassic rock ratios for a multitude an orogenic environment is consistent with this scen- of elements differ from those of experimental immiscible ario. The ultrapotassic and carbonatite magmas and carbonatite–potassic silicate melts, and suggest that the granitoid magmas were probably derived from distinct MPIS rocks are unlikely to be related by liquid immiscibil- sources (Gleason et al., 1994; Haxel, 2005), which may ity. This suggests, then, that MPIS carbonatite was gener- have been juxtaposed to the east of the ultrapotassic ated as a primary carbonatitic melt. belt, and both partially melted during a widespread and A number of studies have contended that some, if not long-lived thermal event. most, carbonatites are primary, mantle-derived melts (e.g. Harmer & Gittins, 1998; Harmer, 1999; Gittins & Implications for carbonatite–alkaline magmatism Harmer, 2003; Ying et al.,2004). In cases where primary The MPIS does not have a direct analog because of its carbonatites are associated with alkaline silicate rocks, unique features (Haxel, 2005; Castor, 2008), including the two may still be closely related despite being gener- the wide range of silica contents in the ultrapotassic ated from separate melts. A number of studies have rocks, the association between ultrapotassic (rather shown that carbonatite melts may metasomatize mantle than potassic) rocks and significant amounts of car- rocks (e.g. Green & Wallace, 1988; Yaxley et al.,1991; bonatite, and the geochemistry of the shonkinite and Hauri et al.,1993; Rudnick et al.,1993), and Harmer carbonatite, preventing direct comparison with other (1999) presented a model in which these rocks may later carbonatite–alkaline rock occurrences worldwide. partially melt and produce alkaline silicate melts. In this However, the MPIS is most akin to the group of model, the alkaline silicate melts ascend with the assist- carbonatite–alkaline rock occurrences in which the two ance of volatiles, and subsequent carbonatite melt as- rock types are spatially and temporally associated but cends by exploiting the conduit formed by earlier silicate were generated as separate melts—for example, the melts and without significant interaction with country- Spitskof, Shawa, Dorowa, Kerimasi, and Napak com- rock. This model is consistent with the age and isotopic plexes in Africa (Harmer & Gittins, 1998; Harmer, 1999), relationships of the MPIS in that (1) the earliest and most and the Jacupiranga complex in southern Brazil (Huang voluminous stages of magmatism resulted in the em- et al., 1995). placement of alkaline silicate rocks, (2) alkaline silicate magmatism progressively decreased in volume and car- bonatite magmatism progressively increased in volume, and (3) the carbonatite and silicate rocks have similar iso- CONCLUSIONS topic characteristics owing to their derivation from the Whole-rock and mineral geochemistry data from the same source, although the silicate rocks have more vari- MPIS indicate that titanite, zircon, apatite, thorite, and able isotopic signatures as a result of crustal contamin- sodic amphibole are the most important hosts of REE ation. Although the composition of the MPIS and actinides in the ultrapotassic rocks. Titanite and zir- ultrapotassic rocks and carbonatite are somewhat differ- con from ultrapotassic rocks yield U–Pb ages of 1425– ent from those in the model of Harmer (1999),thismodel 1385 Ma and 1425–1407 Ma, respectively, and record provides an appropriate analog for the petrogenesis of three discrete phases of magmatism (1425, 1405, and the MPIS. 1380 Ma; Fig. 15). Trace element and textural data Journal of Petrology, 2016, Vol. 57, No. 8 1591 indicate that titanite grew as an igneous phase in a Nu Instruments HR Plasma high-resolution MC-ICP- Mountain Pass rocks. Titanite yields identical dates to MS system for collecting U–Th/Pb data and an Agilent demonstrably igneous zircon, and is therefore inter- 7700S quadrupole (Q)-ICP-MS system for collecting preted to accurately record the igneous crystallization major and trace element data. Methods used in this ages of the MPIS. Monazite Th–Pb ages from carbonatite study follow those outlined by Cottle et al. (2012, 2013) are 1400–1380 Ma (Fig. 15) and overlap titanite and zircon and Kylander-Clark et al. (2013), except that trace ele- dates from ultrapotassic rocks within uncertainty. Hf iso- ments were measured via Q-ICP-MS (method outlined topic data from zircon in the ultrapotassic rocks are uni- below). Different laser run conditions were used form, yielding a mean eHfi of 19 6 02 across all samples for each mineral depending on expected U, Th, and Pb (Fig. 12), indicating derivation from a common source. content, varying in the range of 9–40 mm spot size, 3–7 The most primitive Nd isotopic values measured in titan- mJ laser power at 75–100%, laser pulse frequency 3–4 Downloaded from https://academic.oup.com/petrology/article-abstract/57/8/1555/2413438 by guest on 31 March 2020 ite overlap eNdi from monazite and bastnasite€ in the car- Hz, and 80–120 shots per analysis (see Supplementary bonatite (Fig. 8), further supporting a genetic relationship Data Electronic Appendices 5, 7, 10 and 11). Bastnasite€ between the carbonatite and ultrapotassic rocks of the was ablated using line scans 25 mm wide and 100 mm MPIS. Whole-rock major and trace element data suggest long with a travel rate of 4 mms–1. Before each ablation that the carbonatite and ultrapotassic rocks are unlikely period, the laser was fired twice to remove contamina- to have differentiated from a common parental magma. tion from the surface of the analyte, followed by an 8 s Combined, data from this study effectively demonstrate delay to permit sample washout. Masses 204Pb þ Hg, and resolve—contrary to previous studies—that carbona- 206Pb, 207Pb, and 208Pb were measured on ion counters, tite and ultrapotassic rocks younger than 1410 Ma were and 232Th and 238U were measured on Faraday detec- contemporaneous, but were derived from separate melts tors. Typically, 20–40 analyses were performed per from the same source region. In this way, the MPIS is sample, according to the abundance of the mineral of most similar to carbonatite–alkaline rock suites in which interest per sample. Every set of eight sample analyses the two rock types were generated as separate melts. We was bracketed by analyses of a primary and a speculate that REE deposits of similar grade to the secondary natural reference material of the same Mountain Pass may be more likely to occur in primary mineral type as the sample. Information about the pri- carbonatites than in carbonatites that separated from mary and secondary reference materials used is parental silicate melts. provided in Supplementary Data Electronic Appendix 14. Bastnasite€ is the only exception, owing to the lack of a widely distributed and/or well-characterized APPENDIX: ANALYTICAL METHODS bastnasite€ reference material; therefore, zircon and Accessory minerals monazite were used as reference materials for Sample preparation bastnasite€ and all three minerals were ablated using line scans to avoid addition of systematic uncertainty Accessory minerals were analyzed either in epoxy grain owing to differences in downhole fractionation of U and mounts or in thin section. Sample thin sections were pre- Pb. Zircon data were acquired during seven separate pared by Spectrum Petrographics of Vancouver, analytical sessions, monazite data over four, rutile data Washington. To prepare mineral separates, 2–3 kg of over one, and titanite, bastnasite,€ and apatite over two each sample was crushed in a jaw crusher and disc mill, sessions each. sieved, separated by density using a gold pan or Raw U–Th–Pb and trace element data were reduced Rodgers Table and then in methylene iodide (q ¼ 332 g using Iolite v2.5 (Paton et al., 2011) to correct for instru- cm–3), and separated by magnetic susceptibility using a ment drift, laser-ablation-induced down-hole elemental Frantz isodynamic separator. Thirty to 50 grains of each fractionation, plasma-induced elemental fractionation, mineral were picked from samples by hand under a bin- and instrumental mass bias. Secondary reference mate- ocular microscope, mounted in epoxy pucks, and pol- rials were used to monitor accuracy and internal repro- ished to a 025 lm (titanite, monazite, apatite, bastnasite)€ ducibility from run to run, and between analytical or 1 lm (zircon, rutile) finish. Backscattered-electron sessions. External uncertainty was added to sample iso- (BSE) and cathodoluminescence (CL) images of the sam- topic ratios in quadrature, and was assigned based on ples were collected using scanning electron microscopy the additional uncertainty needed to produce an MSWD (SEM) to provide textural context for LASS-ICP-MS spot 1 in a population (n > 9) of secondary reference mate- analysis placement. Representative example SEM rial analyses from the same analytical session. images of sample grains with spot analyses shown are 206Pb/238U typically required the addition of 1–1 5% provided in Supplementary Data Electronic Appendix 3. external uncertainty. 207Pb/206Pb measurements usually required addition of <1% uncertainty. However, long- U–Th–Pb term 207Pb/206Pb reproducibility for this laboratory and Mineral analyses were carried out using LASS-ICP-MS setup is reported to be 15–2% (e.g. Kylander-Clark at the University of California, Santa Barbara (UCSB). et al., 2013; Spencer et al., 2013); thus, 15% external The LASS-ICP-MS system combines a Photon Machines uncertainty has been added to all 207Pb/206Pb ratios to 193 nm ArF Excimer laser and Hel-Ex ablation cell with account for long-term accuracy. U–Th/Pb data collected 1592 Journal of Petrology, 2016, Vol. 57, No. 8 from the secondary reference materials are shown in elements heavier than Mg. The Q-ICP-MS system was Supplementary Data Electronic Appendix 15. tuned such that ThOþ and CeOþ were less than 05%, Isoplot 4.1 (Ludwig, 2012) was used to plot U–Pb data and doubly charged ions less than 2%. Dwell time for on Tera–Wasserburg diagrams, to calculate lower- elements of stoichiometric abundance (e.g. Ca, Ti, and intercept and concordia dates, and to calculate weighted Si for titanite; Zr and Si for zircon) was 5 ms. A dwell means. Reported titanite ages are U–Pb isochron dates time of 10 ms was used for trace elements with derived from Tera–Wasserburg plots. Zircon ages are U– expected abundances of 10 000–1000 ppm (e.g. REE in Pb concordia dates derived from Tera–Wasserburg plots titanite; Hf in zircon). For trace elements with expected for samples that yield MSWD < 1 þ 2(2/f)1/2,wheref ¼ n – abundances of 1000 ppm or less, or those that were 2(Wendt & Carl, 1991). For samples that do not yield a used for geothermometry (e.g. Zr in titanite, Ti in zir- concordia age that meets this criterion, reported ages con), dwell time was set to 20 ms. Sweep time was are weighted means of 207Pb/206Pb dates calculated from 08 s. Analog detection mode was used for elements Downloaded from https://academic.oup.com/petrology/article-abstract/57/8/1555/2413438 by guest on 31 March 2020 measured 207Pb/206Pb ratios. Zircon dates were calcu- of stoichiometric abundance; pulse-counting mode was lated using data that are <3% discordant; per cent dis- used for most elements and allowed to switch to analog cordance was calculated as the per cent difference mode automatically. Masses monitored during analysis between the 207Pb/206Pb and 206Pb/238U dates from the are listed in Supplementary Data Electronic Appendix corresponding isotopic ratios measured in the sample. 16. All reported monazite ages are common-Pb corrected ‘GJ1’ zircon was used as the primary reference mate- 208Pb/232Th dates. Correction was applied according to rial for most trace elements in zircon samples [see Liu the following steps. (1) Raw 232Th, 208Pb, and 204Pb þ Hg et al. (2010) for trace element reference values]. ‘91500’ signals were corrected for instrumental drift and mass zircon was used as the primary reference material for bias in Iolite v2.5 (Paton et al.,2011). (2) Per cent com- Si, P, and Ta, because of the lack of ‘accepted’ values mon 208Pb in the 208Pb signal was calculated based on for these elements in ‘GJ1’ zircon. Trace element refer- the mass-204 signal and the 208Pb/204Pb at a reference ence values used for ‘91500’ are from the GeoReM data- age of 1400 Ma from the Stacey & Kramers (1975) model base, application version 18 (October 2009; http:// of Pb evolution. Changes in the reference age of 100 georem.mpch-mainz.gwdg.de/) (Jochum et al., 2005). Myr did not significantly change the final 208Pb/232Th ‘Stern’ monazite was used as the primary reference date (<1 Myr). The mass-204 signal was not corrected material for monazite trace elements (reference values for isobaric interference of 204Hg on 204Pb; however, the based on data compiled from UCSB LASS-ICP-MS and possible presence of Hg in the Ar gas supply should be electron probe micro-analyzer laboratories). NIST accounted for in the background mass-204 signal, and SRM610 reference glass (Jochum et al., 2011) was we therefore consider the effect of neglecting this correc- measured in conjunction with titanite, apatite, and tion insignificant. (3) Radiogenic 208Pb (208Pb*) was cal- metamict zircon and used as the primary trace element culated by subtracting the common 208Pb from the total reference material for these phases. Secondary trace measured 208Pb. (4) Corrected 208Pb*/232Th ratios were element reference materials used are the same as the used to calculate 208Pb/232Th dates. Uncertainties were primary U–Pb reference materials (Supplemetary Data calculated by the same method as was used to calculate Electronic Appendix 14). Trace element data were uncertainty for 206Pb/238Uand207Pb/206Pb ratios as reduced using the ‘trace elements’ data reduction described above, with an additional 1% added in quadra- scheme in Iolite v2.5 (Paton et al., 2011). Zircon and ture for uncertainty on the 208Pb/204Pb ratio from the monazite trace element data were reduced using the Stacey & Kramers (1975) model. Measured 208Pb/232Th ‘semi-quantitative’ standardization method of Iolite. Ca ratios of the reference materials typically required addi- was used as an internal standard for titanite and apatite, tion of 3% uncertainty. and Si was used as an internal standard for metamict zircon; Ca and Si values were assumed stoichiometric. These standardization schemes reproduced values in Trace elements the secondary reference materials to within 10% or less Trace elements of interest were measured in titanite, for most trace elements. REE plots are normalized to zircon, monazite, and apatite by LASS-ICP-MS simulta- the chondrite concentrations of McDonough & Sun neously with U–Th–Pb isotopes using an Agilent 7700 S (1995). Q-ICP-MS system; laser settings, ablation periods, and Zr-in-titanite apparent temperatures were calculated washout times used for trace element measurement using the calibration of Hayden et al. (2008). Pressure were therefore identical to those described above. was assumed to be 01 6 01 GPa based on the presence Zircon grains that appear to be metamict (see below) of K-feldspar pseudomorphs after primary leucite in were also measured using LA-Q-ICP-MS for U–Th–Pb some samples (Castor, 2008), which has been experi- isotopes and trace elements of interest; analytical meth- mentally shown to be unstable above 01 GPa (Barton ods were otherwise identical to those used for titanite, & Hamilton, 1978). Ti-in-zircon apparent temperatures zircon, monazite, and apatite. The Q-ICP-MS system is were calculated using the calibration of Ferry & Watson equipped with a second Edwards E2M 18 rotary inter- (2007). For both geothermometers, TiO2 activity was face pump, increasing sensitivity by about two-fold for assumed to be 08 6 02, based on the apparent lack of Journal of Petrology, 2016, Vol. 57, No. 8 1593

primary rutile, and SiO2 activity to be unity, based on (Wasserburg et al., 1981) was used to monitor the accu- the presence of minor quartz. For samples where quartz racy of the Nd mass bias correction. 147Sm/144Nd was 147 was not observed in thin section, SiO2 activity was calculated using the mass bias corrected Sm and the assumed to be 09 6 01. Uncertainty in elemental con- interference corrected 144Nd. Natural reference materi- 143 144 centration (2SE), TiO2 activity, and SiO2 activity are als ‘Trebilcock’ monazite ( Nd/ Nd ¼ 0512616 6 11; propagated into the reported temperature uncertainties. Fisher et al., 2011b) and ‘Hondo Canyon’ titanite Additionally, uncertainty in pressure is propagated in (143Nd/144Nd ¼ 0512211 6 9; Fisher et al., 2011b) were the reported Zr-in-titanite apparent temperatures. analyzed between every 10–20 unknowns and used to Temperatures quoted here are weighted means calcu- monitor the accuracy of the corrected 143Nd/144Nd and lated by Isoplot 4.1 (Ludwig, 2012), or are a range of val- 147Sm/144Nd ratios. The weighted mean corrected ues from non-single population groups of analyses. 143Nd/144Nd values (62SD) obtained for the reference materials are as follows: 0512612 6 18 (n ¼ 34) for Downloaded from https://academic.oup.com/petrology/article-abstract/57/8/1555/2413438 by guest on 31 March 2020 Trebilcock monazite; 0512218 6 59 (n ¼ 24) for Hondo Nd isotopes Canyon titanite.

Nd isotopes in titanite, monazite, and bastnasite€ were The ENdi values were calculated relative to the chon- measured in situ by LA-MC-ICP-MS at UCSB. The meth- dritic uniform reservoir (CHUR) parameters of Jacobsen & ods presented by Fisher et al. (2011b) were followed for Wasserburg (1980) (present-day 143Nd/144Nd ¼ 0512638 this study, and are summarized here. For titanite and and 147Sm/144Nd ¼ 01967) with 143Nd/144Nd re-normalized monazite, only grains that were analyzed for U–Pb geo- to 146Nd/144Nd ¼ 07219 (Hamilton et al., 1983; Bouvier chronology were analyzed for Nd isotopes, with the et al., 2008). Age corrections on the measured 143Nd/144Nd exception of one sample (MP-P0913-10B). All minerals were applied using the sample age interpreted from titan- were ablated with 5 mJ laser energy at 100% and pulse ite and/or zircon dates; changes of 20 Myr in the correc- rate of 6 Hz for each analysis. Laser spot sizes were tion age did not have a drastic effect on the epsilon values 65 lm for titanite, 20 mm for monazite, and 15 mm for (02 epsilon units). Uncertainties on the measured bastnasite.€ Reference glasses were ablated with a spot 143Nd/144Nd and 147Sm/144Nd ratios were propagated into size of 40 mm to obtain an optimal signal during mona- the eNdi values. zite and bastnasite€ analysis, and 65 mm during titanite analysis. Samples were ablated for 50 s, with a 30 s delay between analyses to allow washout. Masses 141– Hf isotopes 150 were measured on 10 Faraday cups with 1 a.m.u. Hf isotopes in zircon were measured in situ by LA-MC- spacing. Data were reduced using Iolite v2.5 (Paton ICP-MS at UCSB. Methods used in this study are similar et al., 2011). The data were carefully screened to detect to those used by Hagen-Peter et al. (2015),andaresum- the presence of inclusions; portions of analyses that marized here. Only zircon grains that were analyzed for appeared to record measurement of inclusions were U–Pb geochronology and yielded concordant dates rejected during data processing. younger than 1460 Ma were analyzed for Hf isotopes. During in situ measurement of Nd isotopes, the iso- Ablation spots were placed over the original U–Pb analy- baric interference on 144Nd by 144Sm has been noted to sis spots to obtain Hf compositions that correspond to the constitute between 15 and 46% of the mass-144 signal measured U–Pb date of the zircon. A laser spot size of in LREE-rich minerals (Fisher et al., 2011b); thus, cor- 50 lm, 4 mJ laser energy at 100%, and a pulse rate of 8 Hz recting this interference is vital to obtaining an accurate were used to ablate samples for 50 s, with a 30 s delay 143Nd/144Nd ratio. In addition, determination of between analyses to allow washout. Masses 171–180 147Sm/144Nd in the sample is necessary to accurately were measured on 10 Faraday cups with 1 a.m.u. spacing. calculate the initial 143Nd/144Nd of the sample. To make Data were reduced using Iolite v2.31 (Paton et al., these corrections, instrumental mass bias factors for 2011). Natural ratios of 176Yb/173Yb ¼ 0786847 (Thirlwall Sm and Nd must first be determined. ‘JNdi-1’ glass was & Anczkiewicz, 2004) and 176Lu/175Lu ¼ 002656 (Chu employed to evaluate Nd isotopic measurements with- et al., 2002) were used to subtract isobaric interferences out complication by isobaric interference of 176Yb and 176Lu on 176Hf. The Yb mass bias factor (143Nd/144Nd ¼ 0512115 6 7; Tanaka et al., 2000). ‘JNdi- was calculated using a natural 173Yb/171Yb ratio of 1’ doped with Ce, Pr, Sm, Eu, and Gd (‘LREE’ glass; 1123575 (Thirlwall & Anczkiewicz, 2004) and was used Fisher et al., 2011b) was employed to provide a homo- to correct for both Yb and Lu mass bias. A natural geneous 147Sm/144Nd reference material, and to evalu- 179Hf/177Hf ratio of 07325 (Patchett & Tatsumoto, 1980, ate Nd isotope measurements where Sm is included. 1981) was used to calculate the Hf mass bias factor. 147Sm/149Sm was used to calculate a Sm mass bias fac- To assess accuracy and precision, synthetic reference tor, using a natural 147Sm/149Sm ratio of 022332 (Isnard material zircons ‘MUNZirc1’ and ‘MUNZirc3’ et al., 2005). 144Sm interference was then calculated (176Hf/177Hf ¼ 0282135 6 7; Fisher et al., 2011a), as well using the measured 149Sm and a natural 144Sm/149Sm as natural reference material zircons ‘91500’ ratio of 108680 (Dubois et al., 1992). For Nd mass bias (176Hf/177Hf ¼ 0282308 6 6; Blichert-Toft, 2008), ‘GJ-1’ determination, a natural 146Nd/144Nd ratio of 07219 was (176Hf/177Hf ¼ 0282000 6 5; Morel et al., 2008), used. A canonical 145Nd/144Nd value of 0348415 ‘Plesovice’ (176Hf/177Hf ¼ 0282482 6 13; Slama et al., 1594 Journal of Petrology, 2016, Vol. 57, No. 8

2008), and ‘Mud Tank’ (176Hf/177Hf ¼ 0282507 6 6; dispersive system equipped with a standard set of crys- Woodhead & Hergt, 2005) were analyzed between tals, a duplex detector, and calibrations constructed every 5–10 sample analyses. The weighted mean cor- using 61 certified reference standards. Interference cor- rected 176Hf/177Hf values (62SD) obtained for the secon- rections are adapted from Johnson et al. (1999) and dary reference materials were 0282142 6 38 (n ¼ 8) for detection limits were described by Lackey et al. (2012). MUNZirc1 and MUNZirc3, 0282305 6 42 (n ¼ 5) for Whole-rock trace elements were measured by LA-Q- 91500, 0282000 6 56 (n ¼ 4) for GJ-1, 0282517 6 24 ICP-MS at UCSB. The analytical setup was the same as (n ¼ 5) for Plesovice, and 0282527 6 42 (n ¼ 5) for Mud that used for accessory phase trace elements, except Tank. that laser energy was set to 5 mJ and 30 s of washout

The eHfi values were calculated relative to CHUR was allowed between each analysis. Samples were (present-day 176Hf/177Hf ¼ 0282785; 176Lu/177Hf ¼ 00336; ablated using line scans 50 lm wide and 250 lm long at Bouvier et al.,2008) at the U–Pb age of each sample a travel rate of 2 lms–1 for 2 min of data each. Prior to Downloaded from https://academic.oup.com/petrology/article-abstract/57/8/1555/2413438 by guest on 31 March 2020 (concordia age if MSWD < 2 and mean 207Pb/206Pb age if each ablation period, surface contamination was MSWD > 2) using k 176Lu ¼ 1867 10–11 a–1 (Scherer removed from the sample surface by performing a line et al.,2001, 2003; So¨ derlund et al., 2004). Age corrections scan 50 lm wide by 250 lm long at a travel rate of on the measured 176Hf/177Hf were applied using the sam- 70 lms–1. Three line scans were performed on each ple age interpreted from titanite and/or zircon dates; sample. Whole-rock reference materials BCR-2, BHVO- changing the correction age by 20 Myr did not have a 2, AGV-2, Atho-G, ML3B-G, StHs6/80-G, GOR132-G, and drastic effect on the epsilon values (05 epsilon units). T1-G were measured at the beginning and end of the Uncertainties on the corrected 176Hf/177Hf ratios were analytical session, with Atho-G and AGV-2 measured propagated into the eHfi values; uncertainties on the between every nine unknowns. Reference values used 176Lu/177Hf have negligible effect. Depleted-mantle for these reference materials are from the GeoReM model ages (TDM) were calculated relative to a depleted- database, application version 18 (January 2015; http:// mantle model reservoir with present-day georem.mpch-mainz.gwdg.de/) (Jochum et al., 2005). 176Hf/177Hf ¼ 028325 (Nowell et al.,1998; Griffin et al., Atho-G was used as the primary reference material and 2000, 2002)and176Lu/177Hf ¼ 00384 (Griffin et al.,2000). AGV-2 as the secondary reference material to assess C Two-stage depleted-mantle model ages (TDM )werecal- accuracy of the measurements of each element individ- culated using the same depleted mantle parameters and ually. Three analyses were performed on each sample a crustal average 176Lu/177Hf of 0015 for the evolution of to assist in the recognition of outliers; the reported val- the crustal magma source after it separated from the ues are the arithmetic mean of the three values. LA-Q- depleted mantle (Griffin et al.,2002). ICP-MS data were reduced using the ‘Trace Elements IS’ data reduction scheme in Iolite v2.5 (Paton et al., 2011). Ca content was measured by XRF, and was used Whole-rock geochemistry as the internal elemental standard. Elements reported Samples representative of the range of rock types in the here were reproduced within less than 10% of the MPIS were selected for whole-rock geochemical analy- accepted values for AGV-2, except for Cs, Tb, Pb, and U sis, with emphasis on those that contained accessory (135%, 105%, 144%, and 109% difference from minerals that produced useful U–Pb age data. Between accepted values, respectively). 05 and 075 kg of the least weathered or altered por- tions of each sample was first gently hammered into 1– 2 cm fragments and powdered in a tungsten carbide ACKNOWLEDGEMENTS (WC) ring mill and thoroughly mixed. More material We thank Andrew Kylander-Clark for assistance with was powdered for coarse-grained samples to ensure LASS-ICP-MS data collection. Additionally, we thank accurate representation of sample mineralogical com- Graham Lederer for reconnaissance work on the proj- position. Sample powder (35 6 00001 g) was combined ect, for providing samples, and for assistance in Nd iso- with 7 6 00001 g of di-lithium tetraborate flux for a low- tope data collection. We are indebted to Callum dilution sample to flux ratio of 2:1, and thoroughly Hetherington and an anonymous reviewer, whose mixed by vortexer. The mixtures were then transferred insightful and constructive reviews led to substantial to graphite crucibles and heated at 1000C in a furnace improvements to the paper. for 30–40 min to ensure complete fusion. The resulting glass beads were then re-powdered in a WC ring mill to achieve optimal homogenization, and re-fused at FUNDING 1000C for an additional 30–40 min. The final glass bead This work was supported by funding from the was polished to an 15 lm finish, and 2–3 mm chips University of California, Santa Barbara. were then cut from the beads for trace element analysis. The chips were mounted in an epoxy puck and polished to a 1 mm finish. Whole-rock major elements were SUPPLEMENTARY DATA measured by XRF at Pomona College in Claremont, Supplementary data for this paper are available at California using a PanAlytical Axios wavelength- Journal of Petrology online. Journal of Petrology, 2016, Vol. 57, No. 8 1595

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