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Petersson, A. (2019) A new 3.59 Ga magmatic suite and a chondritic source to the east Pilbara Craton Chemical Geology, : 51-70 https://doi.org/10.1016/j.chemgeo.2019.01.021

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Permanent link to this version: http://urn.kb.se/resolve?urn=urn:nbn:se:nrm:diva-3263 Chemical Geology 511 (2019) 51–70

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Chemical Geology

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A new 3.59 Ga magmatic suite and a chondritic source to the east Pilbara T Craton ⁎ Andreas Peterssona,b, , Anthony I.S. Kempb, Arthur H. Hickmanc, Martin J. Whitehousea, Laure Martinb,d, Chris M. Grayb a Swedish Museum of Natural History, Box 50 007, SE-104 05 Stockholm, Sweden b School of Earth Sciences, The University of Western Australia, Perth, Australia c Geological Survey of Western Australia, 100 Plain Street, East Perth, WA 6004, Australia d Centre for Microscopy, Characterisation, and Analysis, ARC Centre of Excellence for Core to Crust Fluid Systems (CCFS), University of Western Australia, Australia

ARTICLE INFO ABSTRACT

Editor: Balz Kamber The Pilbara Craton, Western Australia hosts one of the best-preserved Paleoarchean granite-greenstone terrains Keywords: on Earth, and is inferred to have developed on an older (> 3.8 Ga), possibly Hadean, continental substrate. Such Pilbara Craton ancient crust has, however, never been identified in outcrop. Here, we show that metamorphosed gabbroic, Shaw Granitic Complex leucogabbroic and anorthositic rocks of the South Daltons area, in the western part of the Shaw Granitic Archean Complex, formed at 3.59–3.58 Ga and were intruded by granitic magma at 3.44 Ga. The 3.59–3.58 Ga gabbroic Zircon rocks, here named the Mount Webber Gabbro, represent the oldest, unambiguous igneous rock emplacement in Oxygen isotopes the Pilbara Craton and significantly predate the oldest volcanic activity of the 3.53–3.23 Ga Pilbara Supergroup Lu–Hf within the East Pilbara Terrane. We interpret the Mount Webber Gabbro samples to represent fragments of a Chondritic mantle dismembered layered mafic intrusion. Mantle-like zircon18 δ O and Hf isotope signatures indicate derivation from a chondritic to near chondritic mantle at ~3.59 Ga, and do not support the existence of a > 3.8 Ga basement to the East Pilbara Terrane. These results strengthen the notion of an approximately chondritic > 3.5 Ga mantle beneath the Pilbara Craton, and provide further evidence that recent estimates of Archean stabilised continental volumes, based on the assumption of crust extraction from a global, convecting depleted mantle reservoir, may be overestimated.

1. Introduction is based on several, largely indirect, lines of evidence. For example, in the Warrawagine Granitic Complex, tonalitic gneiss enclaves containing Granite-greenstone terranes are the hallmark of Archean cratons, 3.66–3.58 Ga zircon cores are found within younger ∼3.42–3.24 Ga but it is debated whether they formed entirely from juvenile mantle- monzogranite and granodiorite (see Kemp et al., 2015). Enclaves of ca. derived material (e.g. Hiess et al., 2009; Kemp et al., 2015; Fisher and 3.58 Ga gabbroic anorthosite in 3.43 Ga granitic rocks of the Shaw Vervoort, 2018) or developed through reworking of an older con- Granitic Complex have also been reported (McNaughton et al., 1988, in tinental substrate (e.g. Hickman, 1983; Collins et al., 1998; Kamber a conference abstract). Although the gabbroic enclaves are not ne- et al., 2005; Smithies et al., 2005; Van Kranendonk et al., 2007; cessarily of continental affinity, a magmatic event of this age iscorro- Gardiner et al., 2017). This debate has implications for the initial borated by trondhjemitic and granodioritic gneisses in the Muccan growth, volume and composition of the earliest continental nuclei on Granitic Complex, dated to 3.591 ± 0.036 Ga and 3.576 ± 0.022 Ga Earth. respectively (zircon U-Pb, Wiemer et al., 2018). A 3.724 Ga xenocrystic The East Pilbara Terrane of the Pilbara Craton, Western Australia, zircon in the Panorama Formation (Thorpe et al., 1992) and hosts one of the best preserved Paleoarchean granite-greenstone ter- 3.71–3.50 Ga detrital zircon grains in the Farrel Quartzite (Sheppard ranes on Earth (Fig. 1). The komatiitic to basaltic lavas, strati- et al., 2017) could suggest the existence of even older crust within the graphically part of the Pilbara Supergroup, are inferred to have ex- east Pilbara Craton. Sedimentary successions across the Pilbara Craton truded onto an older (> 3.53 Ga) substrate of continental crust (e.g. contain detrital zircon that predates the most ancient rocks in the area Van Kranendonk et al., 2007; Tessalina et al., 2010). This interpretation by up to 300 million years (Van Kranendonk et al., 2007; Hickman

⁎ Corresponding author at: Swedish Museum of Natural History, Box 50 007, SE-104 05 Stockholm, Sweden. E-mail address: [email protected] (A. Petersson). https://doi.org/10.1016/j.chemgeo.2019.01.021 Received 10 September 2018; Received in revised form 14 January 2019; Accepted 17 January 2019 Available online 24 February 2019 0009-2541/ © 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/). A. Petersson, et al. Chemical Geology 511 (2019) 51–70

Fig. 1. Simplified geological map of the Pilbara Craton in Western Australia (modified after Smithies et al., 2007 and Kemp et al., 2015). LWSC = Lalla Rookh–- Western Shaw structural corridor; MB = Mallina Basin; MCB = Mosquito Creek Basin; MSZ = Maitland Shear Zone; SSZ = Sholl Shear Zone. Key to lower right inset: PC = Pilbara Craton; GC = Gawler Craton; KC = Kimberley Craton; YC = Yilgarn Craton.

Table 1 Summary table.

Sample 12TKPB01 15TKPB23 15TKPB25 15TKPB26 15TKPB27 OGC/OG1

Easting (UTM, 50K) 731840 m E 731863 m E 731777 m E 731769 m E 731758 m E 797690 m E Northing (UTM, 50K) 7615411 m S 7615480 m S 7615452 m S 7615442 m S 7615408 m S 7643949 m S Age (Ga) 3.588 ± 0.005 3.438 ± 0.003 3.575 ± 0.007 3.580 ± 0.002 3.580 ± 0.001 3.4654 ± 0.006a b Weighted mean εHf(t) −0.3 ± 0.3 −0.4 ± 0.3 −0.4 ± 0.2 −0.2 ± 0.2 −0.4 ± 0.2 +0.6 ± 0.1/+0.6 ± 0.2 Mean δ18O +4.0 ± 0.1‰ +5.4 ± 0.4‰ +5.9 ± 0.2‰ +5.4 ± 0.1‰ +5.88 ± 0.08‰

All errors at 95% confidence level. a Stern et al. (2009). b Kemp et al. (2017). et al., 2010; Kemp et al., 2015). An ancient continental crust has also isotopes are also reported from granitic rocks of the Mount Edgar been inferred from trace element modelling (Green et al., 2000) and Batholith (Gardiner et al., 2017; see their Table 1). So far, the oldest from Nd isotope compositions of basaltic (Van Kranendonk et al., 2007; (> 3.6 Ga) Nd and Hf model ages have been reported from north- Tessalina et al., 2010; Gardiner et al., 2017) and granitic rocks (e.g. western Yule, northern Shaw, Mount Edgar, Muccan, and eastern Hamilton et al., 1981; Jahn et al., 1981; Gruau et al., 1987; Bickle et al., Warrawagine granitic complexes (see Van Kranendonk et al., 2007; 1989, 1993; Smithies et al., 2003; Smithies et al., 2007; Champion, Gardiner et al., 2017). Granitic rocks of > 3.22 Ga granitic supersuites 2013; Champion and Huston, 2016; Gardiner et al., 2017). Eoarchean in the East Pilbara Terrane (Callina, Tambina, Emu Pool and Cleland) (ca. 3.8–3.6 Ga) depleted mantle model ages based on zircon Hf yield Nd model ages of ~4.1–3.4 Ga (Smithies et al., 2003, 2007;

52 A. Petersson, et al. Chemical Geology 511 (2019) 51–70

Champion, 2013) and some of the more recent studies suggest that the thickness is unknown. However, no stratigraphic repetition has been cryptic ancient crust might even be Hadean (4.5–4.0 Ga; e.g. Tessalina demonstrated (Van Kranendonk et al., 2001, 2004, 2007; Hickman and et al., 2010). Van Kranendonk, 2004), despite numerous geochemical traverses It is notable, however, that outcrops of unambiguously Eoarchean to across the major greenstone belts (Glikson and Hickman, 1981a, 1981b; Hadean continental crust have never been identified in the Pilbara Glikson et al., 1986; Smithies et al., 2007) and geochronological map- Craton; the previous existence of such ancient crust is largely inferred ping of the Pilbara Supergroup (Van Kranendonk et al., 2002, 2006, from the older detrital zircon relics noted above (which are of uncertain Fig. 2). origin; Kemp et al., 2015), and model crust formation ages that hinge Multiple pulses of voluminous magmatic intrusions (3.49–3.46, on the interpretation of continent separation from a depleted (MORB- 3.45–3.42 Ga, 3.32–3.29 Ga, 3.25–3.22 Ga, 2.95–2.92 Ga, and source) mantle from 4.5 Ga to the present day (e.g. McCulloch and 2.85–2.83 Ga) over hundreds of millions of years produced the granitic Wasserburg, 1978; DePaolo, 1980). Such an assumption of continental complexes of the east Pilbara Craton. The oldest suites of voluminous formation from a > 3.5 Ga strongly depleted mantle, particularly for granitic magmatism (3.49–3.42 Ga) are mainly represented by tonali- Lu-Hf isotopes, is based on few robust observational data and might te–trondhjemite–granodiorite (TTG). They are especially well preserved lack justification (e.g., Kemp et al., 2015; Fisher and Vervoort, 2018). If within the Shaw Granitic Complex, but also form parts of the Carlindi, continental crust was extracted from a less depleted mantle source re- Yule, Mount Edgar, Corunna Downs, Warrawagine and Muccan Granitic servoir, such as convecting asthenosphere of chondritic composition, Complexes (Figs. 1 and 2; Buick et al., 1995; Nelson, 1999a; Bagas these model ages, and the inferences of ancient basement in the Pilbara et al., 2005; Van Kranendonk et al., 2007; Gardiner et al., 2017). These Craton, would be invalid. More widely, a number of studies (e.g., older Paleoarchean granitic rocks vary from being moderately de- Vervoort and Kemp, 2016) have noted that, for Hf isotopes, the as- formed (e.g. the northern parts of the Shaw Granitic Complex; Bickle sumption of a strongly depleted mantle source will produce a bias to- et al., 1993), to highly deformed migmatitic gneisses (e.g. Shaw and Mt. wards model crust formation ages that are too old, leading to erro- Edgar Granitic Complexes; Bickle et al., 1993; Collins, 1993). Such neously high estimates of ancient continental volumes. gneisses are found as numerous enclaves in younger granitic rocks. The Here, we report new zircon U–Pb-O-Hf isotope data from meta-ig- pre-3.42 Ga granitic rocks have been divided into the 3.49–3.46 Ga neous (mostly gabbroic) rocks within the South Daltons pluton in the Callina Supersuite and the 3.45–3.42 Ga Tambina Supersuite: the Cal- western part of the Shaw Granitic Complex, East Pilbara Terrane. With lina Supersuite is contemporaneous with the felsic extrusive units these results, we identify a new 3.59–3.58 Ga magmatic event in the within the Duffer Formation; the Tambina Supersuite and rhyolitic Shaw Granitic Complex, here named the Mount Webber event, which Panorama Formation also form a contemporaneous intrusive/extrusive predates the previously oldest known single component igneous rocks pair (Van Kranendonk et al., 2006). in the Pilbara Craton. The 3.59–3.58 and ca. 3.44 Ga intrusions have Younger Paleoarchean granitic intrusions of the East Pilbara zircon O and Hf isotope signatures that are consistent with generation Terrane became increasingly potassic, and are represented by grano- from a near-chondritic mantle source. These results add another sig- diorite and monzogranite of the 3.32–3.29 Ga Emu Pool Supersuite and nificant element to the magmatic history of the Pilbara Craton and by monzogranite and syenogranite of the 3.25–3.22 Ga Cleland provide no evidence for the existence of a > 3.8 Ga basement. Supersuite. Following rifting and breakup of the East Pilbara Terrane at ~3.20 Ga, margins of the separated terranes were locally intruded by 2. Regional geology tonalite and granodiorite of the Mount Billroth Supersuite (Hickman, 2012, 2016). 2.1. The Pilbara Craton Three younger terranes make up the West Pilbara Superterrane, the ∼3.28–3.25 Ga Karratha, the ∼3.20 Ga Regal, and the ∼3.13–3.11 Ga The Pilbara Craton consists of three main lithotectonic elements, the Sholl Terranes (Fig. 1). The Regal Terrane (basaltic crust formed in a East Pilbara Terrane (3.53–3.22 Ga), the West Pilbara Superterrane rift basin) was obducted onto the Karratha Terrane between 3.16 and (3.28–3.07 Ga), and the De Grey Superbasin (3.06–2.93 Ga; Figs. 1 and 3.07 Ga, and the Sholl and Karratha Terranes were tectonically juxta- 2). About 60,000 km2 of the craton is exposed in the northeast, but posed along the Sholl Shear Zone between 3.09 and 3.07 Ga. The West three-quarters of the craton is unconformably overlain to the south by Pilbara Superterrane then collided with the East Pilbara Terrane at volcanic and sedimentary Neoarchean–Paleoproterozoic (2.77–2.43 Ga) ~3.07 Ga (Van Kranendonk et al., 2010). rocks of the Mount Bruce Supergroup (Hickman, 2004). Two main extensional events have affected the Pilbara Craton. The The East Pilbara Terrane is composed of three volcano-sedimentary first was initiated at ~3.2 Ga, separating the Karratha Terrane fromthe groups (Warrawoona, Kelly, and Sulphur Springs) deposited between East Pilbara Terrane, and is associated with deposition of the ~3.19 Ga ~3.53 and 3.23 Ga, collectively known as the Pilbara Supergroup, that Soanesville Group clastic succession, which in turn is overlain by a flank and overlie large (35–120 km across), granitic complexes. Each substantial basaltic volcanic sequence. Following the amalgamation of individual domal structure of the East Pilbara Terrane comprises a the East Pilbara Terrane and West Pilbara Superterrane, a second ex- central granitic complex flanked by an adjacent greenstone belt (Van tensional event led to accumulation and deposition of volcanic and Kranendonk et al., 2002). Eleven such granite–greenstone domes make sedimentary rocks of the 3.06–2.93 Ga De Grey Supergroup (Fig. 1). up the East Pilbara Terrane, and are separated by boundary faults, most of which are located between the greenstone belts of adjacent domes 2.2. The Shaw Granitic Complex (Van Kranendonk and Collins, 1998). This crustal architecture forms the archetype of the dome-and-keel pattern of Archean granite-green- The Shaw Granitic Complex is a > 50 km diameter assemblage of stone terranes (Van Kranendonk et al., 2007). granitic intrusions with emplacement ages spanning ~650 Ma (Fig. 3A). The largely coherent eruptive units of the Pilbara Supergroup are Stratigraphic younging occurs within the greenstone sequences away dominated by tholeiitic and komatiitic basalts interlayered with thin from the granitic complex. At the present level of exposure, the complex sedimentary horizons and locally including thick felsic volcanic units; approximates to a cross-section in which the granitic margins grade komatiite units are less common, although locally up to 400 m thick downward into progressively more sheared variants forming a sheeted (e.g. Hickman et al., 1990; Smithies et al., 2007; Van Kranendonk et al., granitic complex, interleaved with older greenstones. 2007; Hickman, 2012). The 3.5–3.3 Ga calc-alkaline plutonic suite in the Shaw Granitic The lowermost contact of the supergroup is intruded out by younger Complex is predominantly highly sheared, folded and metamorphosed granitic bodies, and the uppermost parts of several of the eruptive units to the upper amphibolite facies (Bickle et al., 1980; Bickle et al., 1989; have been eroded, such that the maximum original depositional Pawley et al., 2004; Van Kranendonk et al., 2007). This has formed a

53 .Ptrsn tal. et Petersson, A. 54 Chemical Geology511(2019)51–70

Fig. 2. Space-time chart outlining the evolution of the exposed Pilbara Craton (modified after Hickman et al., 2010). The stratigraphic positions of samples analysed in this study are indicated by a red star symbol. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) A. Petersson, et al. Chemical Geology 511 (2019) 51–70

A B 1 km 119˚15’

South Dalton Plutons

15TKPB25/26 15TKPB23

15TKPB27 12TKPB01

21˚35’

1050 15 km Post-tectonic granites Eluvial and colluvial sediments Variably deformed intrusive granitic rocks Quartzofeldspathic sand Biotite granodiorite gneiss Soanesville Group Dalton Suite Banded hornblende gneiss Warrawoona Group Apex Basalt Migmatite Tambina Supersuite Undivided granites North Shaw Suite and gneisses Greenstone intercalations Sample locations

Fig. 3. A) Simplified geology of the Shaw Granitic Complex. Modified after Bickle et al. (1983). B) Simplified geology of the South Daltons area, western north Shaw Granitic Complex. Based on the interactive geological map of the Geological Survey of Western Australia (GeoVIEW.WA). greyish, largely migmatitic gneiss, which is intruded by several large 1992). plutons of variably deformed granitic rocks and post-tectonic granites Granitic rocks of the Tambina Supersuite constitute a major part of in the central part of the complex (Fig. 3). However, the northern part the Shaw Granitic Complex, and are interpreted to be derived through of the Shaw Granitic Complex includes relatively undeformed grano- melting of Callina Supersuite protoliths (Pawley et al., 2004). There is a diorite, with subordinate tonalite and monzogranite, usually with pre- close temporal relationship between the less deformed intrusive rocks served igneous textures (Bickle et al., 1983). These rocks also have of the North Shaw Suite and the grey gneisses within the complex sharp, discordant intrusive contacts with the surrounding greenstone (Williams et al., 1983; Pawley et al., 2004). Williams et al. (1983) dated formations. a zircon core from the grey gneiss to 3.485 ± 0.030 Ga (SIMS, zircon Bickle et al. (1983) determined the age of older, predominantly U-Pb). This suggests a common source to these rocks, and that the grey granodioritic rocks of the Shaw Granitic Complex to 3.499 ± 0.022 Ga gneisses are derived from the North Shaw Suite, a relationship other- using a Pb-Pb whole-rock isochron, and McNaughton et al. (1988) wise obscured by poor exposure. dated rocks from the same suite to 3.493 ± 0.004 Ga and Two regional scale metamorphic events have been identified in the 3.467 ± 0.006 Ga respectively using ion-microprobe U-Pb on zircon. A Shaw Granitic Complex (Bickle et al., 1985), a ~3.2 Ga high-P kyanite- granodiorite in the South Daltons area in the western part of the Shaw sillimanite event and a ~2.95–2.84 Ga lower-P andalusite-sillimanite Granitic Complex was dated by McNaughton et al. (1993) to event (Wijbrans and McDougall, 1987). 3.431 ± 0.004 Ga using ion microprobe on zircon. These igneous The gabbroic samples examined in the present study comprise a raft events are not restricted to the Shaw Granitic Complex, and the several hundred metres in length within the South Daltons pluton in the ~3.45 Ga event is coeval with formation of the Tambina Supersuite and western part of the Shaw Granitic Complex (Fig. 3). These rocks are eruption of the Panorama Formation. Furthermore, the North Pole described below. Monzogranite, which intrudes a greenstone belt 20 km north of the Shaw Granitic Complex, is virtually coeval with the older North Shaw Suite intrusions at 3.459 ± 0.018 Ga (U-Pb, zircon: Thorpe et al.,

55 A. Petersson, et al. Chemical Geology 511 (2019) 51–70

3. Analytical methods 3.3. Ion microprobe oxygen isotope analysis in zircon

3.1. Major and trace element analyses Oxygen isotope ratios (18O/16O) in zircon were determined using a Cameca IMS 1280 multi-collector ion microprobe hosted by the Centre All samples were analysed by Bureau Veritas Minerals Pty Ltd. in for Microscopy, Characterisation and Analysis (CMCA), University of Perth. The samples were cast using a 66:34 (Lithium Tetraborate 66%/ Western Australia (UWA). After U-Pb analyses, the gold coating was Lithium Metaborate 34%) flux with 4% Lithium nitrate added to forma removed by light polishing. The sample mounts were then carefully glass bead. Major elements were determined by X-ray Fluorescence. cleaned with detergent, distilled water and ethanol in an ultrasonic Trace elements were determined by Laser Ablation Inductively Coupled bath and re-coated with gold (30 nm in thickness) prior to SIMS O Plasma Mass Spectrometry on a fused bead. Loss on Ignition was de- isotope analyses. termined using a robotic TGA system with furnaces set to 110 °C and The sample surface was sputtered over a 10 × 10 μm area with a 1000 °C (LOI1000). A sub-sample was digested with sulphuric and hy- 10 kV, Gaussian Cs+ beam with an intensity of ~3 nA and total impact drofluoric acids and FeO determined via titration. Accuracy andpre- energy of 20 keV. An electron gun was used to ensure charge com- cision of the analyses were monitored using geochemical reference rock pensation during the analyses. Secondary ions were admitted into the powders (Kerba Monzogranite and Bunbury Basalt; Morris, 2007), in- double focusing mass spectrometer within a 110 μm entrance slit and serted as unknown samples. All major and trace element data, including focused in the centre of a 4000 μm field aperture (x 100 magnification). that obtained from the reference materials, are found in Supplementary Secondary ions were energy filtered using a 30 eV band pass with a5eV Table A. gap towards the high-energy side. 16O and 18O were collected si- multaneously in Faraday cup detectors fitted with 10 Ω (L′2) and 1011 Ω (H1) resistors, respectively, and operating at a mass resolution 3.2. Zircon U-Pb SIMS age determination of ~2430. The magnetic field was regulated using NMR control. Each analysis included pre-sputtering of a 15 × 15 μm area during Secondary ionisation mass spectrometry (SIMS) U–Th–Pb isotopic 30s, and automatic centring of the secondary ions in the field aperture, analyses were carried out using a large geometry Cameca IMS1280 contrast aperture and entrance slit. Each analysis then consisted of 20 instrument at the Swedish Museum of Natural History. The instrument four-second cycles, which gave an average internal precision of set up follows that described by Whitehouse et al. (1999), Whitehouse ~0.16‰ (2 SE). The analytical session was monitored in terms of drift – and Kamber (2005) and references therein. An O2 primary beam with using at least two bracketing standards every 5 to 6 sample analyses. 23 kV incident energy (−13 kV primary, +10 kV secondary) was used Instrumental mass fractionation (IMF) was corrected using zircon for sputtering. For this study, the primary beam was operated in Temora 2 (8.2‰; Black et al., 2004) following the procedure described aperture illumination (Köhler) mode yielding a ~15–20 μm spot. Pre- in Kita et al. (2009). The spot-to-spot reproducibility was 0.2–0.3‰ (2 sputtering with a 25 μm raster for 120 s, centring of the secondary ion SD) on Temora 2 during the analytical session. beam in the 4000 μm field aperture (FA), mass calibration optimisation, Uncertainty on each δ18O spot was calculated by propagating the and optimisation of the secondary beam energy distribution was per- errors in the instrumental mass fractionation determination, which in- formed automatically for each run, FA, and energy adjustment using the cludes the standard deviation of the average oxygen isotope ratio 90 16 + Zr2 O species at nominal mass 196. Mass calibration of all peaks in measured on the primary standard during the session, and internal the mono-collection sequence was performed at the start of each ses- uncertainty of each sample data point. Raw 18O/16O ratios and cor- sion; within run mass calibration optimisation was performed using the rected δ18O (quoted with respect to Vienna Standard Mean Ocean 90 16 + Zr2 O peak, applying a shift to all other species. A mass resolution Water, VSMOW) are presented in Supplementary Table C. (M/∆M) of ~5400 was used to ensure adequate separation of Pb isotope peaks from nearby HfSi+ species. Ion signals were detected using the 3.4. LA-MC-ICP-MS zircon Lu-Hf analyses axial ion-counting electron multiplier. All analyses were run in fully automated chain sequences. Data reduction assumes a power law re- Lu-Hf analyses were carried out during two sessions at the School of + + + + lationship between Pb /U and UO2 /U ratios with an empirically Earth Sciences, The University of Western Australia, using a 193 nm derived slope in order to calculate actual Pb/U ratios based on those in Cetac Analyte G2 excimer laser installed with a two-volume HelEx2 the 91500 reference zircon (206Pb/238U = 0.17917 ± 8; Wiedenbeck sample cell, and a Thermo-Scientific Neptune Plus Multicollector ICP- et al., 1995). U concentration and Th/U ratio are also referenced to the MS. Where possible, the Lu-Hf spot overlapped pits from both the U-Pb 91500 standard (81.2 ppm U, Th/U = 0.344; Wiedenbeck et al., 1995). and O isotope analysis. Circular spots with a diameter of 40 μm and A common Pb correction was applied if the 204Pb signal statistically 50 μm were used. Each analysis was initiated by a 30 s electronic on- exceeded average background and assumed a 207Pb/206Pb ratio of 0.83 peak baseline followed by an ablation period of 60 s involving 60 in- (equivalent to present day Stacey and Kramers, 1975, model terrestrial tegration cycles of one second each. A laser pulse repetition rate of 4 Hz Pb). Decay constants follow the recommendations of Jaffey et al. (1971) was used and the laser energy was held at ~5 J/cm2, which equates to and compiled by Steiger and Jäger (1977). All age calculations were an ablation rate of ~0.05 μm per pulse for zircon. Helium carrier gas done in Isoplot 4.15 (Ludwig, 2008) and quoted at 2σ uncertainty level. (1.0 l/min) was used to transport the ablated particles from the sample All internal errors and external uncertainty in the standard analyses are chamber. This was combined with argon gas (flow rate c. 0.6 l/min) and propagated into the final quoted uncertainties. Systematic uncertainties nitrogen (~0.012 l/min) further downstream before entering the argon highlighted by interlaboratory experiments are not propagated. Zircon plasma torch. Masses 171Yb, 173Yb, 175Lu, 176(Hf + Lu + Yb), 177Hf, OGC/OG1 from the Owens Gully Diorite (207Pb/206Pb 178Hf, 179Hf and 180(Hf + W + Ta) were measured simultaneously by age = 3465.4 ± 0.6 Ma; Stern et al., 2009) was also periodically ana- Faraday detectors. Isobaric interference of 176Yb and 176Lu on 176Hf was lysed to monitor data quality and accuracy. The Owens Gully Diorite, calculated using the measured intensities of 171Yb and 175Lu along with from the Mount Edgar Granitic Complex (Nelson, 2005), is one of the known isotopic ratios of 176Yb/171Yb = 0.897145 (Segal et al., 2003) oldest and best-studied intrusive rocks of the Pilbara Craton, and thus is and 176Lu/175Lu = 0.02655 (Vervoort et al., 2004). Mass bias correc- an appropriate comparator for the present study. Sixteen analyses from tions were calculated using the exponential law. For calculations of βHf, these zircons returned a 207Pb/206Pb age of 3465.0 ± 2.2 Ma (95% measured intensities of 179Hf and 177Hf and a canonical 179Hf/177Hf conf., MSWD = 1.5), identical to the values provided by Stern et al. ratio of 0.7325 were used (Patchett and Tatsumoto, 1980). βYb was (2009). All results are presented in Supplementary Table B. calculated using the measured intensities of 173Yb and 171Yb and a 173Yb/171Yb ratio of 1.130172 (Segal et al., 2003). Mass bias behaviour

56 A. Petersson, et al. Chemical Geology 511 (2019) 51–70 of Lu was assumed to be identical to Yb. Five zircon references were hornblende (see below). Variants range from coarser grained and rutile- used for quality control, FC-1, Mud Tank, Temora 2, 91500 and OGC. bearing to medium grained with spindly hornblende; some of these Analysed 176Hf/177Hf ratios of the sample zircon were normalised based show a diffuse layering and grade locally into anorthosite withde- on a comparison between the average of analysed 176Hf/177Hf of Mud creasing hornblende content (Fig. 5A). The tight packing of constituent Tank zircon measured in a given session and its reported 176Hf/177Hf of plagioclase prisms is visible on the pale brownish-orange weathered 0.282507 ± 0.000006 determined by solution analysis (Woodhead surfaces of these rocks. Coarser, pegmatitic domains (5–10 cm long) and Hergt, 2005), itself reported relative to JMC475 with hornblende prisms to several centimetres long occur in several 176Hf/177Hf = 0.282160. Session 1, yielded a Mud Tank zircon places in the leucogabbros, particularly immediately adjacent to the 176Hf/177Hf = 0.282491 ± 0.000011 (n = 18, 2SD) and session 2 ultramafic bodies, where the leucogabbros have a ‘hybrid’ appearance yielded a 176Hf/177Hf = 0.282491 ± 0.000011 (n = 26, 2 SD), both (Fig. 5B). Hornblende-rich metagabbro crops out as several prominent within the uncertainties of the recommended values of Woodhead and elongate lenses (up to 40 m long) in the central part of the outcrop tract Hergt (2005). All analyses were screened for within run variability to (Fig. 5C–D), whereas amphibolite defines small bodies (several m2). decipher any heterogeneities within the analysed zircon grains. Calcu- Boulders of the biotite granodiorite occur on the north and south sides lations of εHf were done using the interpreted pooled igneous crystal- of the ultramafic–leucogabbroic strip, although the nature of the con- lisation age of the rock (Whitehouse and Kemp, 2010), tact between these and the leucogabbros is not clear. On the south side λ176Lu = 1.867 × 10−11 a−1 (Scherer et al., 2001; Söderlund et al., of the ultramafic pods, dark, even-grained diorite to tonalite slabs crop 176 177 176 177 2004), ( Lu/ Hf)CHUR = 0.0336 and ( Hf/ Hf)CHUR out sporadically and contain thin felsic veins that are locally intensely = 0.282785 ± 11 (Bouvier et al., 2008). All zircon Lu-Hf data are folded (Fig. 5E). Dioritic-tonalitic rocks are interlayered with, and in- given in Supplementary Table D (unknowns) and Supplementary Table truded by, metre-scale leucogranitic sheets, and are covered by allu- E (zircon reference materials). vium further downslope (south) from the ultramafic pods. All gabbroic samples studied here are metamorphosed and contain deformation 4. Results fabrics, but the meta- prefix is omitted below for brevity. Photographs of representative hand-samples of each rock type analysed in this study 4.1. Field relations are in Supplementary Fig. A.

Five samples were collected from a small (200 m × 200 m) outcrop 4.2. Petrography area within the middle portion of the South Daltons pluton. Here, strongly foliated and complexly interspersed leucogabbroic, gabbroic 4.2.1. Leucogabbros (12TKPB01, 15TKPB26, 15TKPB27) and dioritic to tonalitic exposures flank thin strips of ultramafic rock, Sample 12TKPB01 is from the central part of the leucogabbroic strip which define a low, approximately E–W trending ridge that persists for and is representative of the main leucogabbroic rock type in the area. several hundred metres (Fig. 4). This lithological assemblage is crosscut This is a moderately coarse, even-grained rock dominated by altered, by sheets of leucogranite, and enclosed by a coarse-grained, foliated chalky to greenish plagioclase, with a strong foliation defined by prisms biotite granodiorite, in which pinkish K-feldspar phenocrysts impart a (to 6 mm) and clotty aggregates of hornblende (Fig. 6A). Sheaves and ‘speckled’ appearance. plates of coarse white mica line some foliation surfaces. In thin section, The leucogabbroic rocks appear to enclose the ultramafic pods and plagioclase is almost completely altered to sericite and includes gran- are restricted to within ~ 50 m either side of these (a sample from the ules of epidote and ragged plates of muscovite and chlorite (Fig. 6A). westernmost ultramafic pod is dominantly hornblende, with aligned Former blocky plagioclase laths show euhedral faces against horn- sheaves of clinochlore). Leucogabbros are lithologically heterogeneous blende masses (generally unaltered, pale yellow-green to blue-green), on outcrop scale, varying in grain size and in the proportions of mafic which comprise aggregates with polygonal internal grain boundaries. versus felsic minerals. The dominant variety is medium to coarse- Irregularly-shaped blebs of quartz and rutile are sparsely disseminated. grained and comprises ~60% altered plagioclase and 40% blocky Metamict allanite grains are enclosed within hornblende.

Fig. 4. Simplified sketch map of the sample area showing the general distribution of the various lithologies analysed inthisstudy.

57 A. Petersson, et al. Chemical Geology 511 (2019) 51–70

Fig. 5. Outcrop photos from the South Daltons area. All coordinates are UTM zone 50 K. a) Anorthositic leucogabbro. The diffuse layering is defined by spindly hornblende grains (073167E 7615403S). b) Heterogeneous leucogabbro outcrop, with radiating aggregates of coarse hornblende crystals (up to 3 cm long). This lithology is localised to the contact between leucogabbro and hornblende-rich gabbro or ultramafic pods (0731812E 7615461S). c) Low boulder of hornblende-rich gabbro (sampling site of 15TKPB23) that comprises a narrow lens along the northern margin of the leucogabbro body (0731845E 7615472S). d) Diffuse miner- alogical layering defined by variations in modal hornblende in a hornblende rich gabbro (sampling site of 15TKPB25) (0731777E 7615430S). e) Boulderoffoliated tonalite that outcrops sporadically south of the leucogabbro body. The clotty texture is defined by hornblende aggregates (0731751E 7615339S).

Zircon grains are subhedral to anhedral, equant to very elongate, orientation. Small pinkish granules (to 0.5 mm) and elongate ag- and vary in size from 50 × 100 μm to 150 × 400 μm. Many of the gregates of rutile are conspicuous. In thin section, these are seen to be grains are irregularly shaped without sharply defined crystal faces, al- associated with distorted, pale brown flakes of biotite, which are now though most appear to be fragments of larger grains. They vary in extensively replaced along cleavage traces to birefringent white mica colour from pale brown to dark brown and turbid, with fine growth and form clots with epidote. Rutile grains are interstitial to blocky zoning visible in the outer parts of the grain in transmitted light. In plagioclase grains, and metamict allanite is common (Fig. 6). Zircon cathodoluminescence (CL) images, these zircon grains have unzoned grains are subhedral to anhedral and vary in size from 70 × 100 μm to centres with faint oscillatory zonation preserved towards grain 150 × 350 μm. They show very similar characteristics to zircon grains boundaries. of 12TKPB01 (Fig. 7). Sample 15TKPB26 is lithologically similar to 12TKPB01 but is dis- Medium-grained leucogabbro 15TKPB27 differs from the other tinctly coarser grained, such that lenticular hornblende aggregates samples in containing a higher proportion of plagioclase (aligned pale reach 3 cm in length (Fig. 6A). In three dimensions, these aggregates are greenish polycrystalline masses up to 1 cm long) and in that hornblende discoid, and the constituent hornblende blades show no linear preferred occurs as small, strongly aligned spindly grains (rarely to 3 mm) or, less

58 A. Petersson, et al. Chemical Geology 511 (2019) 51–70

Fig. 6. Photomicrographs of representative thin sections of each rock-type analysed in this study (A: leucogabbros, B: gabbros). Images on the left are in plane polarised light, and those in the right column are under crossed polarised light. Chl: Chlorite, Ep: Epidote, Hbl: Hornblende, Ms.: Muscovite, Op: Opaque, Pl: Plagioclase, Ru: Rutile, Ser: Sericite, Ttn: Titanite, Qtz: Quartz. Mineral abbreviations according to Kretz (1983).

commonly, small clots of polygonal grains (Fig. 6A). Green-tinted mica flakes are sparse and oriented in the foliation – in thin section, theseare either chlorite or white mica strips replacing pale orange biotite (Fig. 6A). Elongate quartz grains, and to a lesser extent rutile, are common (Fig. 6A). The constituent zircon grains are euhedral to sub- hedral and commonly elongate, although equant grains occur. They vary in size from 50 × 130 μm to 200 × 450 μm and are light to dark brownish in colour. In CL, two main growth phases are evident, a darker primary phase (cores) and a brighter secondary phase seen as thick rims (e.g. #16, #19 and #25, Fig. 7), both unzoned.

4.2.2. Gabbro (15TKPB25, 15TKPB23) Gabbro 15TKPB23 comprises a discrete east–west oriented body ~40 m long and 5 m wide that lies along the interface between coarse leucogabbro and the biotite granodiorite. It is a dark grey to black, medium, even-grained rock dominated by hornblende, which defines a strong lineation, with subordinate plagioclase. In thin section, horn- blende is pale khaki to deep blue-green and forms blocky, irregular- shaped and unaltered grains (most ~1 mm) intergrown with heavily seritised plagioclase (Fig. 6B). Quartz is conspicuous as interstitial blebs and inclusions in hornblende. Irregularly shaped aggregates of titanite, commonly with small epidote granules and opaque blebs, are dis- seminated throughout (Fig. 6B). Zircon grains in 15TKPB23 are in general slightly rounded and subhedral, but euhedral grains with sharp terminations occur. Most grains are around 50 × 150 μm in size and colourless to slightly pale brown. In CL, the zircon grains are broadly zoned varying from con- centric (e.g. #04 and #05, Fig. 7) to irregular zoning (e.g. #09 and #11, Fig. 3). Thin (< 10 μm) CL bright rims are present in most grains and attributed to secondary growth. Gabbro 15TKPB25 occurs as a series of low (< 1 m) boulders that define a pod 10 m long and 6 m wide enclosed within the central partof the leucogabbro tract. It is a dark grey, medium-grained rock com- prising aligned hornblende prisms and plagioclase grains. A diffuse layering on the scale of ca. 5–6 cm is defined by variations in modal hornblende abundance (Figs. 5D and 6B). Rutile granules are con- spicuous and abundant in hand sample. In thin section, these form ir- regularly-shaped aggregates interstitial to both hornblende and plagi- oclase, that are intimately intergrown with an opaque mineral (Fig. 6B). Zircon grains are subhedral to anhedral with very few flat crystal planes. They vary in size from 70 × 100 μm to 200 × 450 μm and are light to dark brownish in colour. In CL images, a weakly luminescent broad zoning is preserved (e.g. #26, Fig. 7). Slightly more luminescent zircon occurs as thin rims (e.g. #15 and #26, Fig. 7) and patchy do- mains in most grains (e.g. #01 and #17, Fig. 3).

4.3. Geochemistry

A summary of sample locations and geochemical analyses is in Table 1. Major and trace element data are in Supplementary Table A, and zircon U-Pb, O and Lu-Hf data are in Supplementary Tables B–E.

4.3.1. Major and trace elements The South Daltons gabbroic samples have basaltic SiO2 contents, ranging from 48 to 51 wt%, low to moderate TiO2 of 0.3–1.3 wt% (except 15TKPB25, which has 4.1% TiO2), high Al2O3 of 14–24 wt%, MgO of 4.5–7.0 wt% and K2O ranging between 0.9 and 2.6 wt%. The MgO is slightly lower and K2O is markedly higher in the leucogabbros

59 A. Petersson, et al. Chemical Geology 511 (2019) 51–70

Fig. 7. Cathodoluminescence (CL) images of representative zircon grains. Small ellipses indicate spot locations for U-Pb (SIMS), small circles O isotopes and large circles indicate sites for Lu-Hf isotope analysis (LA-ICP-MS). Dashed line denotes second analyses (#b) in the same grain.

(12TKPB01, 15TKPB26 and 15TKBP27) compared to the hornblende- gabbro 15TKPB23) Sr (Fig. 9a). Rb varies over a considerable range rich gabbros 15TKPB23 and 15TKPB25. The bulk-rock Mg numbers (35–172 ppm), and Th is enriched (0.98–3.21 ppm), with all samples (Mg# = 100 × molar MgO/(MgO + FeOtot) are variable and range besides 15TKPB23 having Th concentrations > 2.5 ppm. All rocks are from 48 (15TKPB23) to 77 (15TKPB27), as are Cr (25–159 ppm) and Ni strongly enriched in Cs. Sample 15TKPB25 has a pronounced positive (52–98 ppm). The conspicuously low Mg# in hornblende-rich gabbro spike in Ti, and subtler negative Nb-Ta anomaly, in accordance with

15TKPB23 reflects greater Fe enrichment (12.1% FeOt) than in the conspicuous modal rutile. The enrichment in fluid-mobile elements is other samples, although this rock has the highest Cr content. Sr contents further highlighted on a MORB-normalised diagram using the ordering are variable, from 161 ppm in hornblende-rich gabbro 15TKPB23 to a scheme of Pearce (1983) (Fig. 9B), although here the depletion in Nb-Ta maximum of 535 ppm in coarse leucogabbro 15TKPB26. Based on their relative to adjacent elements is much less evident; the normalised low Zr/P2O5 all samples are tholeiitic in character (Winchester and abundance of these elements instead largely conforms to a sloping ne- Floyd, 1976). However, in an AFM plot all leucogabbros plot in the gative trend from Rb to Yb. calc-alkaline field, while 15TKPB23 has clear tholeiitic affinity and All leucogabbro samples, and gabbro 15TKPB25, show steeply 15TKPB25 falls on the dividing line between calc-alkaline and tholeiitic sloping chondrite normalised REE patterns with enrichment in the

(Irvine and Baragar, 1971). The FeO*/MgO versus SiO2 diagram of LREE and variable relative depletion in the HREE (Fig. 9); leucogabbros Miyashiro (1974) classifies all leucogabbros as calc-alkaline, while the 12TKPB1 and 15TKPB27 show the lowest middle to HREE abundances. hornblende-rich gabbros 15TKPB23 and 15TKPB25 plot as tholeiitic Gabbro 15TKPB23 is distinctly different in having a much flatter REE (Fig. 8A–B). pattern. 12TKPB01 and 15TKPB27 have minor positive Eu-anomalies Primitive mantle normalised trace element patterns are LILE en- with Eu/Eu* of 1.16 and 1.24, respectively, while remaining samples riched and have negative anomalies for Nb, Ta and P relative to have negligible Eu anomalies. neighbouring elements, and positive anomalies for K, Pb and (except

60 A. Petersson, et al. Chemical Geology 511 (2019) 51–70

Fig. 8. A: diagram of wt% FeO*/MgO vs. SiO2 (Miyashiro, 1974). B: AFM diagram (dividing line from Irvine and Baragar, 1971). Both A and B show a calc-alkaline compo- sition for the leucogabbros while the gabbros (15TKPB23 and

15TKPB25) yield tholeiitic signatures. C: εHf (t) (2σ error bars) vs. Nb/Th, which argues against formation of negative Nb anomalies by assimilation of substantially older con- tinental crust.

4.3.2. U-Pb, O and Lu-Hf isotopes suggests multiple Pb-loss events this date seems geologically 4.3.2.1. Leucogabbro 12TKPB01. Individual U-Pb analyses of twenty- insignificant. A regression of the 13 data points with the oldest five grains yield mainly discordant data (n = 21/25 at the 2σlevel). 207Pb/206Pb-dates defines an upper intercept of 3.582 ± 0.013 Ga One concordant analysis (#11) plots slightly below the main cluster of (MSWD = 4.9) and the five oldest analyses yield a 207Pb/206Pb- concordant analyses indicating slight Pb-loss. The five analyses with the weighted average of 3.580 ± 0.008 Ga (95% confidence, oldest 207Pb/206Pb dates, ranging between 3.593 and 3.583 Ga, MSWD = 12). including two < 2% discordant analyses (#05 and #12), yield a Among the concordant and nearly concordant data points, a group 207Pb/206Pb-weighted average of 3.588 ± 0.005 Ga (95% confidence, of three analyses (n5778-8, #18 and #26) form an overlapping cluster, MSWD = 1.4). This is interpreted as the igneous crystallisation age of which yields a 207Pb/206Pb-weighted average of 3.575 ± 0.007 Ga this sample (Fig. 10). (95% confidence, MSWD = 2.0). This age is within the uncertainty of Nine Lu-Hf isotope analyses, all in different zircon grains, yield the upper intercept age cited above and is interpreted to best represent 176Lu/177Hf < 0.003, 176Yb/177Hf < 0.07 and 176Hf/177Hf ranging the age of igneous crystallisation of this rock from the available data between 0.28050 and 0.28060. The εHf (3.588 Ga) values range between (Fig. 10). −0.7 and +0.2 with a weighted average of −0.3 ± 0.3 (95% con- Seven analyses on six different dated grains result inδ18O values fidence, MSWD = 0.4). between 4.5 and 5.8‰, with an arithmetic average of 5.3 ± 0.3‰ (2SD) and weighted average of 5.4 ± 0.4‰ (95% conf., MSWD = 7.1). 4.3.2.2. Gabbro 15TKPB23. Sixteen U-Pb analyses produced Twelve zircon Lu-Hf isotope analyses in the nine least discordant 176 177 176 177 exclusively concordant data-points. A 207Pb/206Pb-weighted average (0–17%) dated grains yield Lu/ Hf < 0.003, Yb/ Hf < 0.07 176 177 ε of all data points yields a 3.436 ± 0.004 Ga date (95% confidence, and Hf/ Hf ranging between 0.28050 and 0.28062. The Hf (3.575 MSWD = 1.8). However, the thirteen analyses with the oldest Ga) values range between −1.3 and +0.3 with a weighted average of 207Pb/206Pb-dates define a tight cluster while three analyses (n5782- −0.4 ± 0.2 (95% confidence, MSWD = 1.1). 2, #7 and #15) indicate slight Pb-loss as they plot below the main cluster. The thirteen coherent analyses result in a Concordia age of 4.3.2.4. Coarse-grained leucogabbro 15TKPB26. Twenty-seven U-Pb 207 206 3.437 ± 0.003 Ga (2σ, MSWD = 1.1) and a Pb/ Pb-weighted analyses in 23 different zircon grains yield 11 discordant and16 average of 3.438 ± 0.003 Ga (95% confidence, MSWD = 0.5), which concordant data points (at the 2σ level). The data loosely define a is interpreted as the igneous crystallisation age of this sample (Fig. 10). discordia indicating ancient Pb-loss and yield an upper intercept date of 18 Twenty analyses of 15 different dated grains yield δ O values be- 3.567 ± 0.012 Ga. Due to apparent multiple Pb-loss events and large tween 3.6 and 4.2‰, where all sampled domains produced concordant scatter in highly discordant analyses, the nominal lower intercept of U-Pb dates. All analyses have an arithmetic average of +3.9 ± 0.3‰ 0.897 ± 0.086 Ga probably lacks geological significance. The nine (2SD) and weighted average of +4.0 ± 0.1‰ (95% conf., analyses with the oldest 207Pb/206Pb-dates, all concordant and MSWD = 0.7). Fourteen zircon Lu-Hf isotope analyses from thirteen without obvious indication of Pb-loss, define a Concordia age of 176 177 176 177 grains yield Lu/ Hf < 0.002, Yb/ Hf < 0.05 and 3.580 ± 0.001 Ga (95% confidence, MSWD = 3.1) and a virtually 176 177 ε 207 206 Hf/ Hf ranging between 0.28061 and 0.28068. The Hf (3.436 Ga) identical Pb/ Pb-weighted average of 3.580 ± 0.002 Ga (95% values range between −1.7 and +0.4 with a weighted average of confidence, MSWD = 3.7). This is interpreted to best represent the −0.4 ± 0.3 (95% confidence, MSWD = 1.8). igneous crystallisation age of this rock from this dataset (Fig. 10). Eighteen zircon oxygen isotope analyses from seventeen different 4.3.2.3. Gabbro 15TKPB25. Thirty-four U-Pb analyses in 31 grains dated grains, all ≤5% discordant, yield δ18O values between 3.2 and yielded mostly discordant data (n = 31/34 at the 2σ level) with 6.2‰. Omitting one analysis (#21, discarded due to the ion beam 207Pb/206Pb-dates ranging from 3.587 to 2.141 Ga, indicating both sampling a crack), the δ18O range contracts to 5.4–6.2‰, with an recent and ancient Pb-loss. A regression of all 34 data points yields an average of 5.9 ± 0.4‰ (2SD) and a weighted average of 5.9 ± 0.2‰ upper intercept of 3.514 ± 0.058 Ga. However, as the data dispersion (95% conf., MSWD = 1.3).

61 A. Petersson, et al. Chemical Geology 511 (2019) 51–70

4.3.2.5. Leucogabbro 15TKPB27. Forty-four U-Pb analyses in 36 different grains yield ten concordant data points and 34 that are discordant at the 2σ level. The data indicate multiple Pb-loss events with concordant to almost concordant analyses with 207Pb/206Pb-dates ranging from 3.581 to 3.383 Ga. A regression using the entire data set results in an upper intercept date of 3.521 ± 0.028 Ga (MSWD = 141). This intercept is markedly younger than the concordant data points, so we interpret this date to be geologically meaningless. Given the scatter evident among the concordant analyses, presumably due to ancient Pb- loss, the five concordant analyses comprising the oldest and most coherent group are favoured and yield a Concordia age of 3.579 ± 0.001 Ga (95% confidence, MSWD = 7.6). This age is identical within uncertainty to the 207Pb/206Pb-weighted average of the same five spots at 3.580 ± 0.001 Ga (95% confidence, MSWD = 1.0), which is interpreted to best represent the age of igneous crystallisation of this rock (Fig. 10). No geological significance is attached to the lower intercept (0.768 ± 0.0039 Ga) of the scattered discordia array. Nineteen oxygen isotope analyses, all in different grains of which fourteen have been dated, yield δ18O values between 0.9 and 5.7‰. Sixteen of these analyses have coherent values between 5.3 and 5.7‰, and three analyses (15TKPB27-14, #25 and #30) deviate, with values of 0.9, 4.3 and 2.6‰ respectively. Analysis 15TKPB27-30 is considered unreliable as it overlapped a crack. It is also noteworthy that analysis 15TKPB27-14, yielding the anomalous δ18O value of 0.9‰ is from a 0.5% discordant grain that yields a 207Pb/206Pb-date that indicates slight Pb-loss. A weighted average of all remaining analyses yields 5.1 ± 0.5‰ (95% conf., MSWD = 38) while the coherent group, ex- cluding the two remaining statistical outliers, yields an average of 5.4 ± 0.3‰ and a weighted average of 5.4 ± 0.1‰ (95% conf., MSWD = 0.6). Fourteen zircon Lu-Hf isotope analyses in as many different dated grains yield 176Lu/177Hf ≤ 0.002, 176Yb/177Hf < 0.07 and 176Hf/177Hf

ranging between 0.28052 and 0.28060. The εHf (3.580 Ga) values range between −0.9 and +0.3 with a weighted average of −0.4 ± 0.2 95% confidence, MSWD = 1.0).

4.3.2.6. Owens Gully Diorite, OGC/OG1 Thirty-two O isotope analyses of the OGC zircon yield very coherent 18O/16O between 0.0020161 and 0.0020176 and a weighted average of 0.0020171 ± 1 and δ18O of 5.92 ± 0.07‰ (95% conf., MSWD = 1.2, Supplementary Table C). Although the oxygen isotope characteristics of zircon OGC have not previously been documented, these results can be compared with those generated during an earlier analytical session using a Cameca IMS1280 ion microprobe at the Swedish Museum of Natural History (instrument setup and analytical procedures similar to those of Whitehouse and Nemchin, 2009). Twenty-two analyses, of which one statistical outlier was discarded, provided very consistent 18O/16O between 0.0020159 and 0.0020177 and a weighted average of 0.0020169 ± 2 and δ18O of 5.83 ± 0.01‰ (95% conf., MSDW = 2.2). These values are similar to results obtained from the present study (Fig. 11, Supplementary Table C). Twenty-seven Lu-Hf isotope analyses of zircon OGC, obtained over two different analytical sessions, yield 176Lu/177Hf < 0.002, Fig. 9. Primitive mantle-normalised trace element patterns (upper panel), 176Yb/177Hf < 0.06 and 176Hf/177Hf ranging between 0.28060 and MORB normalised trace element patterns (middle panel) and chondrite-nor- 0.28070, with an average value of 0.280639 ± 0.000021. The age- 176 177 malised REE patterns (lower panel) for the South Daltons Suite samples. corrected Hf/ Hf(3.467 Ga) has an average of 0.280556 ± 0.000021 Chondrite and primitive mantle values from Sun and McDonough (1989), and weighted average = 0.280556 ± 0.000004, 95% confidence MORB values are from Pearce (1983). MSWD = 0.9, which is identical to the 0.280554 ± 0.000007 solution

value reported by Kemp et al. (2017). The εHf (3.467 Ga) values range Twelve zircon Lu-Hf isotope analyses yield 176Lu/177Hf ≤ 0.003, between +0.1 and +1.5 with a weighted average of +0.6 ± 0.1 (2σ, 176Yb/177Hf < 0.1 and 176Hf/177Hf ranging between 0.28052 and MSWD = 0.9). 0.28066. The εHf (3.580 Ga) values range between −0.9 and + 0.3 with a weighted average of −0.2 ± 0.2 (95% confidence, MSWD = 0.5).

62 A. Petersson, et al. Chemical Geology 511 (2019) 51–70

Fig. 10. Tera-Wasserburg diagrams showing SIMS (Secondary-Ion-Mass-Spectrometry) zircon spot data for all samples (2σ error ellipses). Red ellipses denote analyses not used in age calculations. Insets show weighted average plots of 207Pb/206Pb dates used in age calculations and magnification of Tera- Wasserburg diagrams. For clarity, the panel for sample 12TKPB01 does not show analyses with 207Pb/206Pb dates younger than 3.548 Ga. All data are in Supplementary Table B. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

5. Discussion constrained to be younger than 3.44 Ga. The 3.59–3.58 Ga South Daltons gabbroic rocks are coeval with the 5.1. 3.59–3.58 Ga magmatism in the Pilbara Craton 3.591 ± 0.036 Ga and 3.576 ± 0.022 Ga TTG gneisses reported from the Muccan Granitic Complex (zircon U–Pb; Wiemer et al., 2018). The 3.588 ± 0.005 Ga zircon U-Pb age obtained from leucogabbro Zircon grains of this age have also been previously recorded within two 12TKPB01 predates the oldest known (3.53 Ga) eruptive rocks in the gneiss samples separated by 15 km in the Warrawagine Granitic Com- East Pilbara Terrane by almost 70 Ma. Merging all data from the gab- plex (heterogeneously banded tonalitic gneiss GSWA 142870 and broic samples 12TKPB01, 15TKPB25, 15TKPB26 and 15TKPB27, and GSWA 180057, both forming enclaves in 3.42 Ga granodiorite of the calculating an age from the most coherent group of 207Pb/206Pb-dates Tambina Supersuite, Nelson, 1999b; Wingate et al., 2009; Kemp et al., (n = 25), yields a weighted average of 3.578 ± 0.001 Ga (95% con- 2015). As this group of igneous rocks comprises the oldest magmatic fidence, MSWD = 1.7). This is in line with the ages estimated fromthe episode recorded in the Pilbara Craton, we propose that the magmatic individual samples and, together with the similar major and trace ele- episode is referred to as the Mount Webber event, and that the South ment characteristics and Hf-O isotope signatures (see below) is con- Daltons gabbroic rocks are collectively named the Mount Webber sistent with a mutual igneous evolution to these rocks. The distinctly Gabbro (Fig. 2). Zircon crystals of this age are also commonly en- younger zircon age of 3.438 ± 0.003 Ga for 15TKPB23 is compatible countered in younger sedimentary successions across the Pilbara with the age range of the Tambina Supersuite, which is an extensive Craton. For example, the majority (n = 90/104) of the detrital and component of the Shaw Granitic Complex (Van Kranendonk et al., inherited pre-Pilbara Supergroup zircon grains analysed by Kemp et al. 2006). It also corroborates the 3.431 ± 0.004 Ga age (sample UWA (2015) yield 207Pb/206Pb-dates between 3.65 and 3.55 Ga. If these 98053) quoted by McNaughton et al. (1993) from the northern part of grains originate from within the Pilbara Craton, then they further the South Daltons area. The younger emplacement age, together with strengthen the notion of a widespread ~3.58 Ga magmatic event pre- the distinctly different REE pattern and zircon Hf-O systematics, pre- dating the oldest rocks in the East Pilbara Terrane. Such an event is in clude a simple petrogenetic link with the 3.59–3.58 Ga gabbros and line with a proto-continental basement for the Pilbara Craton, as pro- leucogabbros, despite the geographical proximity. Given that posed by Van Kranendonk et al. (2007). The limited available dataset 15TKPB23 also contains the pervasive, hornblende-defined foliation does not allow for areal delineation of such a putative basement except present in the older leucogabbros, the age of deformation and amphi- that it is represented in three of the East Pilbara Terrane granitic bolite facies metamorphism in this part of the Shaw Granitic Complex is complexes, at localities separated by 170 km.

63 A. Petersson, et al. Chemical Geology 511 (2019) 51–70

An alternative, that the gabbros and associated ultramafic rocks are cumulates from an intermediate magma derived by re-melting young, hydrated mafic crust, is not supported by the zircon O isotope data. The latter provide no evidence for such a hydrated (i.e., altered) protolith, and thus for the involvement of a hydrous fluid necessary to flux melting (Section 5.3). Although a basaltic progenitor magma is inferred, deducing the specific composition of this, and the tectonic setting in which itwas generated, is made ambiguous by the metamorphosed and altered nature of the gabbros, as presently preserved. It is also clear from the moderate MgO and compatible trace element contents, and cumulate features, that the gabbroic rocks do not represent primary mantle-de- rived melts. Strongly elevated LILE relative to the HFSE is the hallmark of subduction zone processes (e.g., McCulloch and Gamble, 1991), al- though in the case of the Mount Webber gabbro samples enrichment in the mobile elements Cs, Rb and K is probably at least partly due to alteration, evidenced by conversion of plagioclase to fine sericite and epidote, and secondary growth of white mica and chlorite. Rb and K abundances also closely correlate with LOI (Supplementary Table A). This LILE enrichment serves to accentuate the negative Nb-Ta anomaly, although it remains that the gabbroic samples do exhibit low Nb-Ta relative to the LREE and Th. If primary, the origin of this feature, common in Archean anorthositic rocks (e.g., Polat et al., 2018), does not have a clear-cut explanation in the present, somewhat limited da- Fig. 11. Comparison of the CMCA and Nordsim OGC zircon O-isotope data taset. Possibilities include (1) the involvement of a Nb-Ta sequestering expressed as δ18O. Red analyses denote CMCA and blue analyses denote phase at an earlier stage in the magmatic evolution, such as rutile or Nordsim. Black and grey horizontal lines mark the weighted average and the 2σ amphibole (Foley et al., 2002), (2) derivation from a mantle source error respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) overprinted by a LREE-rich component, perhaps derived from recycled oceanic crust (e.g. as argued for Archean anorthositic rocks by Polat et al., 2018), or (3) intra-crustal contamination, potentially accom- It is noteworthy that older (to 3.66 Ga) zircons in a gneiss enclave in panying fractional crystallisation of the parental magma to the gabbroic the Warrawagine Granitic Complex (Kemp et al., 2015), > 3.7 Ga xe- to anorthositic enclaves. Concerning the latter possibility, the lack of nocrystic zircon in the Panorama Formation and Farrel Quartzite correlation between Nb/Th and Hf isotope data precludes formation of (Thorpe et al., 1992; Sheppard et al., 2017) and ancient > 3.8–3.6 Ga the variable Nb-Ta anomalies solely by assimilation of significantly detrital zircon grains within sedimentary basins of the Pilbara Craton older continental crust (Fig. 8C). The zircon oxygen isotope data further (Van Kranendonk et al., 2007; Hickman et al., 2010; Kemp et al., 2015) argue that any contaminant was not of supra-crustal derivation. could imply the existence of even older crust within the Pilbara Craton. Other chemical features of the Mount Webber gabbros are easier to relate to those of the igneous protolith. The small positive Eu anomalies 5.2. Origin of the Mount Webber gabbroic rocks in leucogabbro 12TKPB1 and anorthositic sample 15TKPB27, in tandem with the Sr enrichment, are consistent with plagioclase accumulation The Mount Webber leucogabbros, gabbros and associated ultramafic and suggest that these samples do not represent melt compositions. This rocks are of limited outcrop extent and have been substantially mod- accords with the relict cumulate features in the leucogabbros noted ified by metamorphism, deformation and pervasive alteration. above, and could also explain the lower overall REE content of these Nonetheless, as described above, the 3.59–3.58 Ga samples locally re- two samples. Accumulation, perhaps magmatic, of a titaniferous phase tain relict igneous textural features and mineralogical variations re- is implicated by the prominent Ti anomaly and smaller negative Nb-Ta miniscent of cumulate igneous layering in both leucogabbro-anortho- anomaly in gabbro 15TKPB25. The high La/Yb and low Lu/Hf of all site and hornblende-rich gabbro, as well as an overall basaltic major samples (besides gabbro 15TKPB23) could reflect deep melting in the element composition, viz., 48–51 wt% SiO2 and 4.5–7.0 wt% MgO. garnet and/or hornblende stability field, or magmatic fractionation of Such lithological assemblages (i.e., leucogabbros, anorthositic gabbros these minerals, but geochemical trends from a larger compositional and ultramafic rocks) and bulk compositions are commonly en- range of samples are required to assess this further (e.g., Davidson et al., countered in layered mafic intrusions (e.g., Bushveld, Yuan et al., 2017; 2007). Sept Iles; Namur et al., 2010; Skaergaard, Irvine et al., 1998), where On a more definitive note, the markedly flatter REE pattern ofthe they are interpreted to form by differentiation of ponded basaltic par- younger (3.44 Ga) gabbro 15TKPB23, together with the lower Sr but ental magmas. This origin is favoured for similar rock types within large higher FeO and Cr, suggest that this younger intrusion was generated anorthosite complexes (e.g., Laramie; Frost et al., 2010) and Archean from a different mantle-source, or under different melting conditions. layered intrusions (e.g., Fiskenaesset, Polat et al., 2010; Manfred, This is supported by the Hf isotope data (see below). Myers, 1988, see also Ashwal and Bybee, 2017). The small, pegmatitic domains observed in the Mount Webber gabbro are also a feature of 5.3. A 3.59 Ga chondritic basement to the East Pilbara Craton layered anorthosite complexes, and attributed to trapping of evolved residual melts against less permeable layers in the cumulate pile (e.g., The 3.59–3.58 Ga age of the Mount Webber gabbroic samples de- Mitchell et al., 1996). fines emplacement of the oldest single component igneous rocks re- By analogy, we therefore consider it most plausible that the Mount cognised within the Pilbara Craton. As noted above, older zircon grains Webber gabbroic to anorthositic enclaves represent variably differ- (to 3.66 Ga) are recognised in a banded tonalitic gneiss enclave in the entiated fragments of a layered mafic intrusion (Mount Webber Gabbro) Warrawagine Granitic Complex. These zircons have fine oscillatory formed by crystallisation of mantle-derived basaltic parent magma. zoning, consistent with crystallisation from melt, but younger zircon This inference accords with the zircon O-Hf isotope data (Section 5.3). age populations are also present (3.61–3.57 Ga, 3.46–3.41 Ga), in part

64 A. Petersson, et al. Chemical Geology 511 (2019) 51–70 developed around the older cores (Kemp et al., 2015). This, together with the lithological heterogeneity, implies that the rock comprises different components that formed at different times; the actual empla- cement age of the igneous protolith to the gneiss is therefore uncertain (see Kemp et al., 2015). These 3.59–3.58 Ga gabbro and leucogabbro components of the Mount Webber Gabbro are significantly older than the surrounding South Daltons granodiorite. Hence, these gabbroic rocks provide the first direct evidence in the Shaw Granitic Complex for part ofthein- ferred basement on which the Pilbara Supergroup was deposited. Furthermore, in a progressively inflating granite dome where younger magmas exploit the same structural magma conduits, older components are displaced towards the batholith margins (Van Kranendonk et al., 2007); hence it is not surprising that the oldest constituent of the Shaw Granitic Complex is found along its outer margins, as is also true for the Mount Edgar and Muccan granitic complexes (Van Kranendonk et al., 2007; Wiemer et al., 2018). From an isotopic perspective, the 3.59–3.58 Ga Mount Webber Gabbro samples have zircon δ18O values consistent with crystallisation in equilibrium with mantle oxygen. These results corroborate pre- 18 viously presented zircon O isotope data from > 3.3 Ga Pilbara rocks Fig. 13. A plot of εHf (t) versus δ O for zircon of this study, showing primarily (Fig. 12, Kitajima et al., 2012; François et al., 2014; Van Kranendonk mantle-like O isotope and chondritic Hf isotope signatures, interpreted to re- et al., 2015). This argues against reworking of an 18O-enriched supra- flect a chondritic mantle composition at the time of zircon crystallisation. crustal source component by the 3.59–3.58 Ga magmas (i.e., materials Horizontal grey field denotes the 5.3 ± 0.6‰ mantle value of Valley et al. ε that have interacted with surface waters). Additionally, a weighted (1994, 2005). Vertical grey field denotes the Hf (t) = 0 ± 0.4 CHUR value of Bouvier et al. (2008). average of all zircon εHf (t) values from the Mount Webber gabbro samples yields −0.01 ± 0.13 (95% conf., MSWD = 2.4), within error of CHUR, as defined by Bouvier et al. (2008). There is no obvious derivation of the ~3.59–3.58 Ga Mount Webber gabbros from an ap- 18 correlation between zircon δ O and εHf (t) in any of the samples, but as proximately chondritic mantle, with minimal influence from older illustrated in Fig. 13, a majority of the analyses from the > 3.5 Ga source materials. In this interpretation, the emplacement of the samples yield isotopic signatures within the mantle range for both O ~3.59 Ga Mount Webber gabbroic rocks signifies an early crust gen- and Lu-Hf isotopes (Fig. 13). The zircon Hf isotope data from the eration episode within the nascent Pilbara Craton. younger gabbro 15TKPB23 also overlap CHUR, suggesting a single, homogeneous source to this rock. 5.4. Implications for cratonic evolution It could be argued that the zircon Hf isotope signatures of the Mount Webber gabbroic rocks reflect interaction between a depleted man- It is instructive to consider the new isotope data presented here in tle–like component (i.e., εHf > 0) and older crust (εHf < 0), where the the broader context of published radiogenic isotope data for igneous resulting mixture fortuitously straddles CHUR. This is, however, diffi- rocks from the Pilbara Craton. Whole rock Nd isotopes from granites, cult to reconcile with the consistent and tightly clustered zircon Hf granodiorites and tonalites from the North Shaw Suite (McCulloch, isotope signatures (Figs. 13–14), together with the mantle-like zircon 1987; Bickle et al., 1993) yield εNd (3.49–3.47 Ga) of +1.3 to −1.3 with an δ18O and the gabbroic bulk rock composition of these samples. We average of 0.4 ± 0.5 (2SD, n = 6). As with the zircon Hf isotope sig- instead interpret the geochemical and isotope data to indicate natures of the Mount Webber Gabbro, these values are in agreement with a chondritic source. Nebel et al. (2014) presented whole-rock Hf isotope data from ~3.5–2.9 Ga komatiites and basaltic komatiites from different stratigraphic units of the Pilbara Craton. These data havea

large spread of Hf isotope signatures ranging from εHf (3.46 Ga) = +8.2 to εHf (3.35 Ga) = −14.6 (Fig. 14A). Fisher and Vervoort (2018) argue that Hf isotope data for igneous rocks should be treated in similar fashion to U-Pb data and assigned a weighted average instead of being presented as a range. However, due to lack of coherence within the dataset of Nebel et al. (2014), no weighted average has been calculated for these data. Younger Paleoarchean granitic supersuites, e.g. Emu Pool (3.32–3.28 Ga) and Cleland (3.27–3.22 Ga), reported by Gardiner

et al. (2017), show scattered zircon Hf isotopic signatures, εHf (t) = +4.4 to −5.1, when recalculated using recent reference values for CHUR and 176Lu decay constant of Bouvier et al. (2008) and Söderlund et al. (2004) respectively. This has been interpreted to indicate sub- sequent reworking of older TTG crust, with trends towards slightly less radiogenic signatures with time (see Fig. 14A; Gardiner et al., 2017). It should, however, be noted that our recalculation of the Hf data in Gardiner et al. (2017) gives a weighted average of all 3.48–3.29 Ga zircon εHf (t) values of the Tambina and Emu supersuites of +0.17 ± 0.20 (n = 243, 95% conf., MSWD = 11), and 63% of in- dividual analyses (n = 152/243) yield values within error of CHUR. 18 Fig. 12. Pilbara zircon δ O versus age (Ga) showing homogeneous mantle-like Amelin et al. (2000) analysed zircon from different 3.45–3.32 Ga quartz values for > 3.4 Ga zircon grains. porphyries from the Marble Bar region that yield Hf isotope signatures

65 A. Petersson, et al. Chemical Geology 511 (2019) 51–70

Fig. 14. A) Zircon εHf versus crystallisation age (in Ga) showing the overall chondritic Hf isotope signatures of zircon grains from the Eo- and Paleoarchean Pilbara Craton. All data from the East Pilbara Terrane (solution data from Nebel et al., 2014). Note that data-points for 15TKPB27 (red circles) are obscured by the other South Daltons samples. B)

Average εHf ( ± 2 SD) versus crystallisation ages (in Ga), showing zircon data from East Pilbara Craton igneous rocks. Data from this study, Amelin et al. (2000), Kemp et al. (2015) and Gardiner et al. (2017). Dashed blue evolution array represents a 176Lu/177Hf of 0.008, calculated from an average value of the 3.59–3.58 Ga Mount Webber gabbro samples. The dashed line ‘MORB-depleted mantle’ represents an average Hf isotope composition of radiogenic modern MORB (e.g., Chauvel and Blichert-Toft, 2001) extrapolated

to εHf = 0 at 4.5 Ga, and is shown for comparative purposes only. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version ofthis article.)

indistinguishable from CHUR, with weighted average εHf (t)values of better knowledge of the isotope evolution of the Pilbara mantle over +1.0 ± 1.0 (Fig. 14A). Similarly, Guitreau et al. (2012) analysed this period. This can potentially be provided by the basaltic and ko- zircon from 3.48 to 3.30 Ga Pilbara TTGs yielding Hf isotope signatures matiitic rocks erupted in the East Pilbara Terrane from 3.53 to 3.3 Ga, with a εHf (t) weighted average of +0.4 ± 0.9. Kemp et al. (2017) since the Hf isotopic composition of these can constrain that of the characterised the 3.467 Ga zircon from Owens Gully Diorite of the mantle source. Mount Edgar Granitic Complex, obtaining an average εHf (t) of Van Kranendonk et al. (2015) argued for a ~35 km thick > 3.5 Ga +0.6 ± 0.7 (Fig. 14A, B). In fact, all available zircon Hf isotope data protocrust, a thickness approximately twice that of the entire Pilbara from Paleoarchean felsic igneous rocks of the East Pilbara (i.e., from 3.6 Supergroup (Hickman and Van Kranendonk, 2004; Hickman, 2012), as to 3.3 Ga) Terrane yield average values within error of CHUR. This a source to the Paleoarchean TTG of the Pilbara and Kaapvaal cratons. observation is further reinforced by the inherited and detrital zircon Similarly, Wiemer et al. (2018) inferred a ~43 km thick, ≥3.6 Ga ul- data presented by Kemp et al. (2015, Fig. 14B). The ca. 3.47–3.3 Ga tramafic/mafic protocrust as the source for Eo- to Paleoarchean TTG igneous rocks (including gabbro 15TKPB23 of the present study) cannot within the Pilbara Craton. Extraction of an equivalent of > 35 km of be derived solely from reworking of the 3.59–3.58 Ga Mount Webber crust prior to 3.6 Ga would lead to formation of an incompatible ele- gabbros and leucogabbros, which would be too unradiogenic at < 3.5 ment depleted mantle residue, which would evolve over time towards Ga (Fig. 14B). Instead, it appears that an overall chondritic Hf isotope superchondritic 176Hf/177Hf. Such a radiogenic signature is not evident signature prevailed in the eastern Pilbara Craton from 3.59 Ga to at within the Mount Webber gabbros (of inferred mantle derivation), or in least 3.3 Ga, suggesting that felsic magmatic episodes in this time span the Pilbara detrital and inherited zircon dataset (3.66–3.55 Ga) re- involved the addition of new crust. Quantifying the proportion of new ported by Kemp et al. (2015), or, more widely, in Hf isotope datasets versus reworked crust incorporated by the felsic igneous suites requires from the oldest, best preserved and best age-constrained igneous rocks

66 A. Petersson, et al. Chemical Geology 511 (2019) 51–70 from a number of Archean cratons, which cluster conspicuously around expected from post crystallisation alteration. CHUR (Kemp and Hawkesworth, 2013; Kemp et al., 2015; Hiess and Relatively constant magmatic zircon δ18O values from primitive Bennett, 2016; Fisher and Vervoort, 2018; Fig. 14A, B). Collectively, mantle melts though time compared to current mantle δ18O values led these sample sets (and see also Bolhar et al., 2017 for the Zimbabwe Valley et al. (2005) to propose a constant and homogenous δ18O of the Craton) therefore provide no evidence for a global depleted mantle mantle back to ~4.4 Ga. Major oxygen isotope fractionation is con- reservoir before 3.6 Ga. strained to near surface processes and so non-mantle-like values in ig- There are two different ways to reconcile this observation, de- neous rocks are commonly explained by reworking of crustal material pending on the prevailing mode of early Archean mantle dynamics. On in the magma source or interaction between mantle derived melts and a the one hand, the paucity of a super-chondritic signal in ancient felsic crustal component during magma ascent (Eiler, 2001). rocks could suggest that early continental crust was sourced from However, the lack of inherited zircon, combined with the tightly convecting mantle, where the complementary highly depleted residues clustered zircon Lu-Hf and O isotope data for sample 15TKPB23 suggest were physically isolated and not resampled by younger magmas. In this a homogeneous single source to this rock. Low-δ18O igneous zircons way, successive, large continental volumes could separate from a con- characteristically form in hydrothermal magmatic systems or at mid- tracting primitive mantle reservoir and not record the chemical/isotope ocean ridges, but rocks formed at such tectonic environments have low legacy of prior crust extraction. In such a model, the lack of super- preservation potential, and sub-mantle zircon δ18O values are very chondritic Hf isotope signatures in the Mt. Webber rocks would not uncommon in the Archean (Hammerli et al., 2018). As there is no ob- preclude the existence of local, highly depleted mantle domains, as vious evidence for post crystallisation zircon alteration in 15TKPB23, it could, for example, have been sequestered in a cratonic lithosphere can be inferred that the gabbroic magma assimilated low δ18O crust root. prior to zircon crystallisation. Byerly et al. (2017) identified 3.27 Ga

An opposing viewpoint, developed to account for positive εNd values komatiites, from the Weltevreden Formation in the Barberton green- in Archean rocks, is that Earth's stable continental crust has grown from stone belt, Kaapvaal Craton in Southern Africa, that yield sub-mantle a progressively depleting mantle reservoir through time (e.g., (~3‰) δ18O values from fresh olivine. These signatures are in line with McCulloch and Bennett, 1994; Bowring and Housh, 1995). Although sample 15TKPB23, ~2‰ below the 5.3 ± 0.6‰ mantle value of the veracity of some of these Nd data has been questioned (e.g. Valley et al. (1994, 2005) and suggest a low δ18O magma. This implies a Vervoort et al., 1996), it remains that the Hf isotope composition of heterogenous mantle derived source enriched by crustal material with juvenile felsic igneous rocks preserved on different cratons clearly di- sub-mantle δ18O-values. Oceanic crust affected by high-temperature verges from CHUR towards radiogenic values after ca. 3.6 Ga (Fisher hydrothermal alteration has most commonly been used as such com- and Vervoort, 2018). This indicates that the ultimate mantle source of ponent (Muehlenbachs, 1986). Assimilation of hydrothermally altered, at least some juvenile Archean continental crust (and Archean an- juvenile (i.e., chondritic for Hf) mafic crust could potentially lower the orthositic rocks; Ashwal and Bybee, 2017; Polat et al., 2018) recorded a δ18O of the source without markedly affecting the Hf isotopes, ex- long-term depletion in incompatible elements, as would be the case if plaining the combined zircon O-Hf isotope signatures of 15TKPB23. the residues of crust extraction were, at least in part, back-mixed into ambient source mantle; indeed, crust generation from such a global, 5.6. Oxygen isotope characterisation of the OGC/OG1 zircon convecting depleted reservoir has continued to the present day (e.g., Vervoort and Blichert-Toft, 1999). In this circumstance, the apparent The increased popularity and use of zircon micro-analyses, such as lack of positive εHf values before 3.6 Ga would be evidence that the U-Pb, O and Lu-Hf isotopes, increases the demand for high-quality and volume of crust extracted and stabilised above the Moho in the Hadean well characterised reference zircons as data quality monitors. The to early Archean was too small to discernably influence the isotope Paleoarchean Owens Gully Diorite (OGC/OG1) zircon has been char- evolution of a continuously tapped mantle source, which retained acterised for U-Pb isotope systematics by Stern et al. (2009), and Hf broadly chondritic 176Hf/177Hf (as reflected in the Mount Webber isotopes by Kemp et al. (2017). The oxygen isotope data presented Gabbro). For the Pilbara Craton, this scenario does not favour the herein show excellent agreement between two different laboratories models of Van Kranendonk et al. (2015) and Wiemer et al. (2018), both strengthening the validity of the results. Combining datasets from the of which argued for substantial crust extraction prior to the eruption of two different laboratories yields a weighted average δ18O of the Pilbara Supergroup. More broadly, applying a pre-3.5 Ga chondritic 5.88 ± 0.06‰ (n = 53, 95% conf., MSWD = 1.7, Fig. 11). This re- mantle source to recent continental growth curves based on compila- presents the first characterisation of the 18O/16O in the OGC/OG1 tions of depleted mantle Hf model ages from zircons (e.g. Belousova zircon. The homogeneity (at the sampling scale) and reproducibility of et al., 2010 and Dhuime et al., 2012), would markedly lower estimates the 18O/16O data (within the uncertainties of the techniques employed) of global crustal generation rates during the first billion years of Earth's suggests that the Owens Gully Diorite (OGC/OG1) zircon holds promise history. If such a chondritic mantle model for pre-3.5 Ga crust extrac- as an oxygen isotope quality monitor for ion microprobe (SIMS) micro- tion is correct, this would help sustain the view that the volumes of the analysis. earliest stabilised continents have been overestimated (Hiess and Bennett, 2016; Vervoort and Kemp, 2016; Fisher and Vervoort, 2018). 6. Conclusions

5.5. Interpreting the source of 3.44 Ga sub-mantle zircon δ18O Metamorphosed gabbroic, leucogabbroic and anorthositic rocks (Mount Webber Gabbro) of the South Daltons area in the western part Taken alone, the low zircon δ18O values of the 3.44 Ga gabbro of the Shaw Granitic Complex are here dated to 3.59, 3.58 and 3.44 Ga 15TKPB23 might suggest post crystallisation alteration involving high- by zircon U-Pb isotopes. The 3.59–3.58 Ga gabbros comprise the oldest grade metamorphism or hydrothermal alteration. All analysed grains single component igneous rocks yet identified in the Pilbara Craton, are, however, concordant at the 2σ level and the U-Pb systematics of significantly predating the eruption age of the oldest basaltic-komatiitic zircon from this rock indicate minimal disturbance. As shown by lavas. The recognition of these rocks thus adds another significant Petersson et al. (2015) and Rubatto and Angiboust (2015), meta- element to the magmatic history of the Pilbara Craton. Combined with morphic alteration of zircon δ18O can be traced using the zircon Th/U, existing U-Pb data from TTG gneisses of the Muccan Granitic Complex where a linear correlation between Th/U and δ18O indicate post crys- (Wiemer et al., 2018), detrital and inherited zircon (Kemp et al., 2015) tallisation disturbance. However, there is no correlation between Th/U and two gneissic tonalitic enclaves within the Tambina Superuite in the and δ18O within the 15TKPB23 zircons, and furthermore, the very co- Warrawagine Granitic Complex (Nelson, 1999b; Wingate et al., 2009; herent values with a spread of zircon δ18O less than one per mil are not Kemp et al., 2015), a widespread magmatic event across the Pilbara

67 A. Petersson, et al. Chemical Geology 511 (2019) 51–70

Craton at ~3.59 Ga is established. We collectively refer to these gab- Black, L.P., Kamo, S.L., Allen, C.M., Davis, D.W., Aleinikoff, J.N., Valley, J.W., Mundil, R., 206 238 broic rocks as the Mount Webber Gabbro, and interpret the combina- Campbell, I.H., Korsch, R.J., Williams, I.S., Foudoulis, C., 2004. Improved Pb/ U microprobe geochronology by the monitoring of a trace-element-related matrix ef- tion of field, petrographic, chemical and isotopic features to suggest fect; SHRIMP, ID–TIMS, ELA–ICP–MS and oxygen isotope documentation for a series that these represent variably differentiated fragments of a layered mafic of zircon standards. Chem. Geol. 205, 115–140. intrusion. Given that the parental magma of the Mount Webber Gabbro Bolhar, R., Hofmann, A., Kemp, A.I., Whitehouse, M.J., Wind, S., Kamber, B.S, 2017. 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