The Stillwater Complex: Integrating Zircon Geochronological and Geochemical Constraints on the Age, Emplacement History and Crystallization of a Large, Open-System Layered Intrusion

The Stillwater Complex: Integrating Zircon Geochronological and Geochemical Constraints on the... Abstract The Neoarchean Stillwater Complex, one of the world’s largest known layered intrusions and host to a rich platinum-group element deposit known as the J-M Reef, represents one of the cornerstones for the study of magmatic processes in the Earth’s crust. A complete framework for crystallization of the Stillwater Complex is presented based on the trace element geochemistry of zircon and comprehensive U–Pb zircon–baddeleyite–titanite–rutile geochronology of 22 samples through the magmatic stratigraphy. Trace element concentrations and ratios in zircon are highly variable and support crystallization of zircon from fractionated interstitial melt at near-solidus temperatures in the ultramafic and mafic cumulates (Ti-in-zircon thermometry = 980–720°C). U–Pb geochronological results indicate that the Stillwater Complex crystallized over a ∼3 million-year interval from 2712 Ma (Basal series) to 2709 Ma (Banded series); late-stage granophyres and at least one phase of post-emplacement mafic dikes also crystallized at 2709 Ma. The dates reveal that the intrusion was not constructed in a strictly sequential stratigraphic order from the base (oldest) to the top (youngest) such that the cumulate succession in the complex does not follow the stratigraphic law of superposition. Two distinct age groups are recognized in the Ultramafic series. The lowermost Peridotite zone, up to and including the G chromitite, crystallized at 2710 Ma from magmas emplaced below the overlying uppermost Peridotite and Bronzitite zones that crystallized earlier at 2711 Ma. Based on the age and locally discordant nature of the J-M Reef, the base of this sequence likely represents an intrusion-wide magmatic unconformity that formed during the onset of renewed and voluminous magmatism at 2709 Ma. The thick anorthosite units in the Middle Banded series are older (2710 Ma) than the rest of the Banded series, a feature consistent with a flotation cumulate or ‘rockberg’ model. The anorthosites are related to crystallization of mafic and ultramafic rocks now preserved in the Ultramafic series and in the lower part of the Lower Banded series below the J-M Reef. The Stillwater Complex was constructed by repeated injections of magma that crystallized to produce a stack of amalgamated sills, some out-of-sequence, consequently it does not constitute the crystallized products of a progressively filled and cooled magma chamber. This calls into question current concepts regarding the intrusive and crystallization histories of major open-system layered intrusions and challenges us to rethink our understanding of the timescales of magma processes and emplacement in these large and petrologically significant and remarkable complexes. INTRODUCTION Layered intrusions play a key role in illustrating compositional diversity in magmas in the Earth’s crust (Wager & Brown, 1967; Parsons, 1987; Cawthorn, 1996; Charlier et al., 2015). They represent a critical link in the differentiation pathway of basaltic magma from partial melting in the uppermost mantle, through storage in crustal reservoirs to eruption, in many cases as voluminous and extensive flood basalts (e.g. Duluth Complex, Paces & Miller, 1993; Stillwater Complex, Helz, 1995; Muskox intrusion, Mackie et al., 2009). These intrusions also host world-class ore bodies of chromium, platinum group elements (PGE), and vanadium (e.g. Jackson, 1961; Naldrett et al., 1987; Cawthorn et al., 2005). From a temporal perspective, layered intrusions constitute parts of Earth’s earliest greenstone belts (e.g. layered sills associated with the Ujaraaluk unit – O’Neil et al., 2007, 2012; Stella intrusion; Maier et al., 2003) and are present throughout geologic time to the Cenozoic intrusions associated with continental and oceanic flood basalts (e.g. Eocene Skaergaard intrusion, East Greenland, Wotzlaw et al., 2012; Oligocene Val intrusion, Kerguelen Archipelago, Scoates et al., 2007). Studies of the Neoarchean Stillwater Complex, a large mafic–ultramafic layered intrusion in the Beartooth Mountains of southern Montana (USA), have profoundly influenced our understanding of the origin and evolution of igneous processes and mineralization in crustal magma chambers (e.g. Howland et al., 1936; Peoples & Howland, 1940; Hess, 1960; Jackson, 1961, 1969; Conn, 1979; Page, 1979; McCallum et al., 1980; Todd et al., 1982; Campbell et al., 1983; Irvine et al., 1983; Raedeke & McCallum, 1984; Boudreau, 1988, 2016; Zientek & Ripley, 1990; McCallum, 1996; Meurer & Boudreau, 1996; Godel & Barnes, 2008; Selkin et al., 2008; Keays et al., 2012; Aird & Boudreau, 2013; Barnes et al., 2015). In contrast, a systematic geochronological framework from the base to the top of the intrusion for assessing the age and duration of magmatism of the Stillwater Complex has not been established, despite the intrusion being the focus of numerous geochronological investigations since the late 1960 s (e.g. Fenton & Faure, 1969; Nunes & Tilton, 1971; DePaolo & Wasserburg, 1979; Premo et al., 1990). Although historically considered as relatively poor candidates for dating studies, mafic and ultramafic rocks of layered intrusions may locally contain small pockets of late-crystallized interstitial minerals where U–Th–Pb-bearing phases (e.g. zircon, baddeleyite, apatite, rutile) occur with quartz, Na-rich plagioclase, K-feldspar, or biotite (e.g. Scoates & Chamberlain, 1995; Schwartz et al., 2005; Scoates & Friedman, 2008; Grimes et al., 2009; Morisset et al., 2009; Zeh et al., 2015; Scoates & Wall, 2015; Mungall et al., 2016). During the past decade, major advances in sample pretreatment, instrument sensitivities, and data reduction protocols for U–Th–Pb geochronology (e.g. chemical abrasion-ID-TIMS or CA-ID-TIMS, Mattinson, 2005; EARTHTIME tracers and synthetic standards, Condon, 2005; Condon et al., 2015; McLean et al., 2015) have led to significantly improved precision and accuracy of dates (Schmitz & Kuiper, 2013; Schoene, 2014). Combined with the ability to efficiently extract zircon from layered intrusions based on selecting appropriate samples (e.g. Scoates & Wall, 2015; Zeh et al., 2015; Mungall et al., 2016), these advances open up the possibility of determining ages of crystallization for multiple samples throughout a stratigraphic sequence of cumulates, specifically for those intrusions that show mineralogical and geochemical evidence for open-system behaviour and magma replenishment events like the Stillwater Complex (e.g. Jackson, 1961; McCallum et al., 1980; Irvine et al., 1983; Wooden et al., 1991; Lipin, 1993; McCallum, 1996). Together with its use as a mineral chronometer, the trace elements of zircon allow for evaluating the geochemical evolution of crystallizing melt as a function of temperature and time (e.g. Finch & Hanchar, 2003; Whitehouse & Platt, 2003; Ferry & Watson, 2007; Harley & Kelly, 2007; Schoene et al., 2012; Barboni & Schoene, 2014; DesOrmeau et al., 2015; Samperton et al., 2015; Deering et al., 2016). In this study, a high-precision geochronologic framework for the crystallization of the Stillwater Complex is provided based on U–Pb dating results for 22 samples, including mafic and ultramafic cumulates representing the entire ∼6000 m thick stratigraphic sequence, mineralized units, and cross-cutting mafic dikes and granophyres. Samples collected from many of the major petrologic units in the intrusion allow for the production of an unprecedented record of age variations for a major layered intrusion. The high-precision U–Pb dating includes ages from both of the enigmatic thick anorthosite horizons in the uppermost part of the exposed intrusion (e.g. Anorthosite-II or AN2; Wall et al., 2016) and from the PGE-rich J-M Reef (Wall & Scoates, 2016), which was sampled along strike to test for lateral variations in age. These dates, combined with trace element compositions of zircon, are used to determine the age and duration of magmatism in the Stillwater Complex and to assess the composition and evolution of late-stage interstitial melts. The results of this study contribute to understanding whether there is evidence for variations in the timescales of magma addition and magma flux (i.e. periods of enhanced or reduced magmatic activity) and whether the intrusion was constructed in a sequential stratigraphic order from oldest at the bottom to youngest at the top. GEOLOGIC SETTING AND STRATIGRAPHY OF THE STILLWATER COMPLEX The c.2·7 Ga Stillwater Complex is exposed within the Beartooth Mountains of southern Montana, one of the major exposed blocks of the Archean Wyoming Province. It crops out over an area of approximately 200 km2 and consists of a ∼45 km-long, 6·5 km-thick, relatively steeply dipping sheet of layered mafic–ultramafic rocks and an associated marginal suite of sills and dikes (Hess, 1960; Jackson, 1961; McCallum, 1996; Boudreau, 2016) (Fig. 1). Early regional gravity and magnetic surveys indicated that the Stillwater Complex extends at depth approximately 25 km to the northeast of the Beartooth Mountains front (Blakely & Simpson, 1984; Kleinkopf, 1985) and more recent analysis from a 3 D gravity model indicates that the buried extent of the Stillwater Complex extends 30 km to the north and 40 km to the east (Finn et al., 2013, 2016). Fig. 1. View largeDownload slide Geology of the Stillwater Complex. (a) Simplified stratigraphic section showing the major subdivisions of the layered mafic–ultramafic rocks and the position of the Ni–Cu sulphide deposits (Ni) in the Basal series, the chromitite seams (A–K) in the Peridotite zone, the PGE-rich J-M Reef near the top of Olivine-bearing zone I, and the PGE-rich Picket Pin mineralized zone (PPZ) at the top of Anorthosite zone II (stratigraphic section after McCallum, 1996). Abbreviations on the section: N1, Norite zone I; GN1, Gabbronorite zone I; OB1, Olivine-bearing zone I; N2, Norite zone II; GN2, Gabbronorite zone II; OB2, Olivine-bearing zone II; AN1, Anorthosite zone I; OB3, Olivine-bearing zone III; OB4, Olivine-bearing zone IV; AN2, Anorthosite zone II; OB5, Olivine-bearing zone V; GN3, Gabbronorite zone III. (b) Generalized geologic map of the Stillwater Complex showing the major series, the position of the J-M Reef (black solid line), and the sample locations of mafic–ultramafic rocks (yellow stars) and granophyres (red stars); map modified from Zientek et al. (2005). Also shown are the locations of the two PGE mines, Stillwater and East Boulder. Abbreviations on the map: PP, Picket Pin; IM, Iron Mountain; LM, Lost Mountain; CRM, Chrome Mountain; CTM, Contact Mountain; CP, Castle Point; WFC, West Fork Creek; BB, Benbow. Inset in the lower right shows the location of the Stillwater Complex in the southwestern part of the state of Montana (USA). Fig. 1. View largeDownload slide Geology of the Stillwater Complex. (a) Simplified stratigraphic section showing the major subdivisions of the layered mafic–ultramafic rocks and the position of the Ni–Cu sulphide deposits (Ni) in the Basal series, the chromitite seams (A–K) in the Peridotite zone, the PGE-rich J-M Reef near the top of Olivine-bearing zone I, and the PGE-rich Picket Pin mineralized zone (PPZ) at the top of Anorthosite zone II (stratigraphic section after McCallum, 1996). Abbreviations on the section: N1, Norite zone I; GN1, Gabbronorite zone I; OB1, Olivine-bearing zone I; N2, Norite zone II; GN2, Gabbronorite zone II; OB2, Olivine-bearing zone II; AN1, Anorthosite zone I; OB3, Olivine-bearing zone III; OB4, Olivine-bearing zone IV; AN2, Anorthosite zone II; OB5, Olivine-bearing zone V; GN3, Gabbronorite zone III. (b) Generalized geologic map of the Stillwater Complex showing the major series, the position of the J-M Reef (black solid line), and the sample locations of mafic–ultramafic rocks (yellow stars) and granophyres (red stars); map modified from Zientek et al. (2005). Also shown are the locations of the two PGE mines, Stillwater and East Boulder. Abbreviations on the map: PP, Picket Pin; IM, Iron Mountain; LM, Lost Mountain; CRM, Chrome Mountain; CTM, Contact Mountain; CP, Castle Point; WFC, West Fork Creek; BB, Benbow. Inset in the lower right shows the location of the Stillwater Complex in the southwestern part of the state of Montana (USA). The Stillwater Complex was emplaced at shallow depths in the crust (6–7 km), possibly as a sub-volcanic intrusion, into a c.3·3 Ga metasedimentary sequence consisting of pelitic schists, iron formations and quartzites (Page, 1977; Mogk & Mueller, 1990; Helz, 1995; Labotka & Kath, 2001). Contact metamorphism produced a large, high-temperature contact aureole along the base of the intrusion, including an inner 500–1000 m wide hypersthene hornfels zone at the contact (650–800°C), and an outer 500–1500 m wide cordierite + cummingtonite hornfels zone (Labotka & Kath, 2001; Thomson, 2008). The Mouat quartz monzonite, a coarse-grained quartz monzonite, occurs along the southeastern basal part of the Stillwater Complex and based on contact relations appears to be younger than the mafic–ultramafic rocks (Nunes & Tilton, 1971; Page & Nokleberg, 1972). The complex was intruded by many generations of mafic dikes and was subjected to regional metamorphism at c.1·7 Ga, an event that locally produced greenschist facies mineral assemblages (Nunes & Tilton, 1971; Page, 1977; McCallum et al., 1999). The Stillwater Complex was uplifted, tilted, and eroded prior to the Middle Cambrian. The uppermost part of the intrusion and its upper intrusive contact are missing and the complex is overlain by Paleozoic and Mesozoic sedimentary rocks. The Stillwater Complex has been divided into the Basal, Ultramafic, Lower Banded, Middle Banded, and Upper Banded series (McCallum et al., 1980; Raedeke, 1982; McCallum, 1996) (Fig. 1). The Basal series (60–400 m thick) is an irregular sheet-like body dominated by orthopyroxenite with subordinate norite and sulphide-bearing assemblages, and it contains xenoliths of cordierite–pyroxene hornfels (Page, 1979; McCallum, 1996). A sill/dike suite includes discontinuous sills and dikes of diabase, mafic norite, and massive sulphide that intrude metasedimentary rocks beneath the complex (Zientek, 1983); five chemical groups are distinguished in the sill/dike suite (Helz, 1985). The Ultramafic series (840–2000 m thick) is subdivided into the lower Peridotite zone and upper Bronzitite zone (Jackson, 1961; Raedeke & McCallum, 1984; McCallum, 1996) (Fig. 1). The Peridotite zone contains approximately 20 cyclic units, where a complete unit consists of peridotite–harzburgite–bronzitite (orthopyroxenite), with or without chromitite seams, that are attributed to multiple magma injections during open-system magmatism (Raedeke & McCallum, 1984; Cooper, 1997). The Banded series, ranging in thickness from 4468 m in the Contact Mountain area to 1935 m in the Picket Pin area, is dominated by plagioclase-rich rocks (norite, gabbronorite, troctolite, anorthosite). The 1800 m thick Middle Banded series contains two thick anorthosite zones, Anorthosite-I (AN1) and Anorthosite-II (AN2) (Fig. 1) (Haskin & Salpas, 1992; McCallum, 1996). The Stillwater Complex hosts a variety of mineral deposits, including Ni–Cu sulphide deposits associated with the Basal series (Howland et al., 1936; Page et al., 1985), chromitite seams (A–K) in the Peridotite zone (Jackson, 1961; Raedeke & McCallum, 1984; Lipin, 1993; Cooper, 1997; Horan et al., 2001; Lenaz et al., 2012), and the world-class platinum group element (PGE) deposit known as the J-M Reef in the Lower Banded series (Todd et al., 1982; Irvine et al., 1983; Barnes & Naldrett, 1985, 1986; Mann et al., 1985; Page et al., 1985; Lambert & Simmons, 1988; Boudreau, 1988; Godel & Barnes, 2008) (Fig. 1). Although volumetrically minor, small bodies of granophyre occur throughout the Banded series (Czamanske et al., 1991). They are present as white to pink veins, 1–12 cm thick and up to 100 m long, composed nearly exclusively of quartz and sodic plagioclase with some veins containing large splays (30 cm length) of tremolitic to actinolitic amphibole. A large ‘alaskite’ (coarse-grained alkali granite) body with quartz-rich segregations and veins occurs over an area of 130 x 210 m within AN1 of the Middle Banded series about 2 km south of Picket Pin Mountain (Fig. 1). The granophyres, typically oriented at high angles to layering, have sharp linear contacts and narrow tapering terminations indicating that they were emplaced after consolidation of the host mafic rocks. The granophyres have been interpreted as crystallization differentiates that evolved in equilibrium with a high-temperature aqueous chloride solution during the final stages of crystallization of the mafic cumulates of the Banded series (Czamanske et al. 1991). PREVIOUS GEOCHRONOLOGY OF THE STILLWATER COMPLEX The Stillwater Complex has been a focus of geochronology studies since the late 1960s (e.g. Fenton & Faure, 1969; Kistler et al., 1969; Nunes & Tilton, 1971). A variety of dating methods and isotopic systems (e.g. K–Ar, Rb–Sr, U–Pb, Sm–Nd, Pb–Pb, Re–Os, 40Ar/39Ar) have been applied and a wide range of ages have been reported, mostly between 2600 and 2750 Ma (Table 1). Nunes & Tilton (1971) separated zircon from a 100 pound (45 kilograms) sample of norite from the Basal series and published the first U–Pb zircon date from the Stillwater Complex based on the discordant U–Pb results from two multi-grain fractions that yielded 207Pb/206Pb dates of 2745 Ma and 2750 Ma (no uncertainty reported). Nunes (1981) subsequently re-analyzed three multi-grain zircon fractions (up to 1 mg) from the same sample and provided a revised age of 2713 ± 3 Ma, an upper intercept 207Pb/206Pb date from strongly discordant analyses. DePaolo & Wasserburg (1979) produced a relatively precise whole rock–mineral Sm–Nd isochron of 2701 ± 8 Ma for gabbronorite from the Gabbronorite zone II of the Lower Banded series that was generally accepted as the age of crystallization of the Stillwater Complex. Table 1: Summary of published geochronology dates for rocks related to the Stillwater Complex Unit  Isotope/Method  Sample Material  Age Method  Age (Ma, ±2σ)  References  Middle Banded series  K–Ar  Phlogopite, plagioclase  Plateau  2350 ± 350  Kistler et al., 1969  Middle Banded series  Rb–Sr  Whole rock  Isochron  2900 ± 300  Fenton & Faure, 1969  Metasedimentary rock  Rb–Sr  Whole rock  Isochron  2672 ± 150  Powell et al., 1969  Chill zone  U–Pb ID-TIMS  Zircon  Upper intercept*  2725 ± 25  Nunes & Tilton, 1971  Metasedimentary rock  Rb–Sr  Whole rock  Isochron  2692 ± 45  Mueller & Wooden, 1976  Lower Banded series  Sm–Nd  Whole rock, plagioclase, orthopyroxene, augite  Isochron  2701 ± 8  DePaolo & Wasserburg, 1979  Banded series  Sm–Nd  Whole rock  Isochron  2896 ± 34  Coffrant et al., 1980  Lower Banded series  Sm–Nd  Whole rock  Isochron  2742 ± 34  Coffrant et al., 1980  Middle Banded series  Sm–Nd  Whole rock  Isochron  2793 ± 21  Coffrant et al., 1980  Ultramafic series  Pb–Pb  Whole rock  Isochron  2662 ± 12  Manhes et al., 1980  Chill zone  U–Pb ID-TIMS  Zircon  Upper intercept*  2713 ± 3  Nunes, 1981  Lower Banded series  U–Pb ID-TIMS  Zircon  Upper intercept*  2683 ± 87  Lambert et al., 1985  Lower Banded series  U–Pb ID-TIMS  Zircon, baddeleyite  Upper intercept*  2705 ± 4  Premo et al., 1990  Group 1 dike  U–Pb ID-TIMS  Zircon  Upper intercept*  2711 ± 1  Premo et al., 1990  Group 2 dike  Pb–Pb  Whole rock, plagioclase, orthopyroxene, augite  Isochron  2704 ± 25  Premo et al., 1990  Group 2 dike  Sm–Nd  Whole rock, plagioclase, orthopyroxene, augite  Isochron  2731 ± 92  Premo et al., 1990  Group 3 dike  U–Pb ID-TIMS  Zircon  Upper intercept*  2703 ± 11  Premo et al., 1990  Group 4 dike  U–Pb ID-TIMS  Zircon  Upper intercept*  2712 ± 3  Premo et al., 1990  Group 6 dike  Sm–Nd  Whole rock, plagioclase, orthopyroxene, augite  Isochron  2706 ± 64  Premo et al., 1990  Peridotite zone  Re–Os  Chromitite  Isochron  2740 ± 80  Marcantonio et al., 1993  Middle Banded series  40Ar/39Ar  Amphibole  Plateau  2744 ± 11  Selkin et al., 2008  J-M Reef  U–Pb CA-TIMS  Zircon  Weighted Mean†  2709·11 ± 0·56  Wall & Scoates, 2016  J-M Reef  U–Pb CA-TIMS  Zircon  Weighted Mean†  2709·28 ± 0·32  Wall & Scoates, 2016  J-M Reef  U–Pb CA-TIMS  Zircon  Weighted Mean†  2709·00 ± 0·45  Wall & Scoates, 2016  J-M Reef  U–Pb ID-TIMS  Baddeleyite  Weighted Mean†  2708·85 ± 0·46  Wall & Scoates, 2016  AN2  U–Pb CA-TIMS  Zircon  Weighted Mean†  2710·44 ± 0·32  Wall et al., 2016  AN2  U–Pb ID-TIMS  Baddeleyite  Weighted Mean†  2709·73 ± 0·48  Wall et al., 2016  Unit  Isotope/Method  Sample Material  Age Method  Age (Ma, ±2σ)  References  Middle Banded series  K–Ar  Phlogopite, plagioclase  Plateau  2350 ± 350  Kistler et al., 1969  Middle Banded series  Rb–Sr  Whole rock  Isochron  2900 ± 300  Fenton & Faure, 1969  Metasedimentary rock  Rb–Sr  Whole rock  Isochron  2672 ± 150  Powell et al., 1969  Chill zone  U–Pb ID-TIMS  Zircon  Upper intercept*  2725 ± 25  Nunes & Tilton, 1971  Metasedimentary rock  Rb–Sr  Whole rock  Isochron  2692 ± 45  Mueller & Wooden, 1976  Lower Banded series  Sm–Nd  Whole rock, plagioclase, orthopyroxene, augite  Isochron  2701 ± 8  DePaolo & Wasserburg, 1979  Banded series  Sm–Nd  Whole rock  Isochron  2896 ± 34  Coffrant et al., 1980  Lower Banded series  Sm–Nd  Whole rock  Isochron  2742 ± 34  Coffrant et al., 1980  Middle Banded series  Sm–Nd  Whole rock  Isochron  2793 ± 21  Coffrant et al., 1980  Ultramafic series  Pb–Pb  Whole rock  Isochron  2662 ± 12  Manhes et al., 1980  Chill zone  U–Pb ID-TIMS  Zircon  Upper intercept*  2713 ± 3  Nunes, 1981  Lower Banded series  U–Pb ID-TIMS  Zircon  Upper intercept*  2683 ± 87  Lambert et al., 1985  Lower Banded series  U–Pb ID-TIMS  Zircon, baddeleyite  Upper intercept*  2705 ± 4  Premo et al., 1990  Group 1 dike  U–Pb ID-TIMS  Zircon  Upper intercept*  2711 ± 1  Premo et al., 1990  Group 2 dike  Pb–Pb  Whole rock, plagioclase, orthopyroxene, augite  Isochron  2704 ± 25  Premo et al., 1990  Group 2 dike  Sm–Nd  Whole rock, plagioclase, orthopyroxene, augite  Isochron  2731 ± 92  Premo et al., 1990  Group 3 dike  U–Pb ID-TIMS  Zircon  Upper intercept*  2703 ± 11  Premo et al., 1990  Group 4 dike  U–Pb ID-TIMS  Zircon  Upper intercept*  2712 ± 3  Premo et al., 1990  Group 6 dike  Sm–Nd  Whole rock, plagioclase, orthopyroxene, augite  Isochron  2706 ± 64  Premo et al., 1990  Peridotite zone  Re–Os  Chromitite  Isochron  2740 ± 80  Marcantonio et al., 1993  Middle Banded series  40Ar/39Ar  Amphibole  Plateau  2744 ± 11  Selkin et al., 2008  J-M Reef  U–Pb CA-TIMS  Zircon  Weighted Mean†  2709·11 ± 0·56  Wall & Scoates, 2016  J-M Reef  U–Pb CA-TIMS  Zircon  Weighted Mean†  2709·28 ± 0·32  Wall & Scoates, 2016  J-M Reef  U–Pb CA-TIMS  Zircon  Weighted Mean†  2709·00 ± 0·45  Wall & Scoates, 2016  J-M Reef  U–Pb ID-TIMS  Baddeleyite  Weighted Mean†  2708·85 ± 0·46  Wall & Scoates, 2016  AN2  U–Pb CA-TIMS  Zircon  Weighted Mean†  2710·44 ± 0·32  Wall et al., 2016  AN2  U–Pb ID-TIMS  Baddeleyite  Weighted Mean†  2709·73 ± 0·48  Wall et al., 2016  * upper intercept 207Pb/206Pb date. † weighted mean 207Pb/206Pb date. Table 1: Summary of published geochronology dates for rocks related to the Stillwater Complex Unit  Isotope/Method  Sample Material  Age Method  Age (Ma, ±2σ)  References  Middle Banded series  K–Ar  Phlogopite, plagioclase  Plateau  2350 ± 350  Kistler et al., 1969  Middle Banded series  Rb–Sr  Whole rock  Isochron  2900 ± 300  Fenton & Faure, 1969  Metasedimentary rock  Rb–Sr  Whole rock  Isochron  2672 ± 150  Powell et al., 1969  Chill zone  U–Pb ID-TIMS  Zircon  Upper intercept*  2725 ± 25&nbs