Formation of Early Archean Granite-Greenstone Terranes From

Chemical Geology 551 (2020) 119757

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Chemical Geology

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Formation of early Archean Granite-Greenstone Terranes from a globally chondritic mantle: Insights from igneous rocks of the Pilbara Craton, T Western Australia ⁎ Andreas Peterssona,b, , Anthony I.S. Kempa, Chris M. Graya, Martin J. Whitehouseb a School of Earth Sciences, The University of Western Australia, Crawley, Australia b Swedish Museum of Natural History, Box 50 007, SE-104 05 Stockholm, Sweden

ARTICLE INFO ABSTRACT

Editor: Balz Kamber The continental crust grows via juvenile additions from the mantle. However, the timing of initial continent fi Keywords: stabilisation and the rate of subsequent continental growth during the rst billion years of Earth history is widely Pilbara Craton debated, in part due to uncertainty over the composition of the mantle source of new crust. Well-preserved Chondritic mantle Archean granite-greenstone terranes, as present within the Pilbara Craton (Western Australia), provide insights Granite-greenstone belts into the sources of felsic magmas and the processes of continental growth and evolution in the distant geological Archean past at the regional scale. Here, we present zircon U-Pb, O and Hf isotope data from ancient gneissic and granitic Zircon rocks of the Pilbara Craton, to decipher magma sources and the timing and processes of craton growth. There is U-Pb, Lu-Hf, O-isotopes no evidence for depleted mantle compositions, and the simplest interpretation is that the crust of the Pilbara Crustal growth Craton was generated from mantle with a chondritic Hf isotope composition. Our results indicate crustal addition at ~3.59 Ga, represented by emplacement of gabbroic to anorthositic rocks. We suggest that the formation of these igneous rocks, and the foundering of the complementary residues, triggered extensive melting of hot, upwelling mantle, leading to the subsequent accumulation of the > 12 km thick greenstone belt eruptive sequences from 3.53 Ga, with emplacement of coeval felsic magmas at depth. This process shaped the initial crustal configuration of the proto-craton, which subsequently underwent gravitationally driven overturn and reworking to generate stable, cratonic continental crust with the distinctive dome and keel architecture. The zircon Hf and O isotope signatures of the Pilbara igneous rocks from ~3.59–3.4 Ga do not support remelting of an ancient (> 3.8 Ga) basement, and reinforce the overwhelmingly chondritic to near-chondritic zircon Hf isotope composition of Eoarchean meta-igneous rocks from a number of different Archean cratons. A corollary of this remarkable global consistency is that a significant volume of the mantle maintained a chondritic composition for the Lu-Hf system from the formation of the Earth into the Paleoarchean (up to 3.6–3.5 Ga), as would be the case if stabilised volumes of felsic continental crust prior to 3.5 Ga were relatively small. One implication is that the common assumption of a linear evolution of depleted mantle from 4.5 Ga to the present day is in- appropriate for determining the timing and volume of continental crust extraction in the Archean. The nearly identical early evolution of the Pilbara and Kaapvaal cratons suggests a common process to generate Archean granite-greenstone terranes that does not require extensive reworking of ancient crust, but rather involves juvenile crustal addition above persistent zones of upwelling, chondritic mantle.

1. Introduction Belousova et al., 2010; Dhuime et al., 2012). In calculating the growth rate of continental crust, such models typically adopt the conventional Due to the miniscule amount of preserved Hadean to Paleoarchean view (e.g., Hofmann, 1988) that the continental crust and incompatible rocks, models for the generation and evolution of continental crust element depleted mantle reservoirs are complementary, with the im- during the ﬁrst billion years of Earth history emphasise long-lived plicit assumption that the extent of mantle depletion, as registered by radiogenic isotope systems such as Sm-Nd (Bennett et al., 1993; Nd and Hf isotope compositions, can be used to constrain the mass of Bowring and Housh, 1995) and Lu-Hf (e.g. Vervoort et al., 1996; continental crust that was extracted and stabilised through time. Thus,

⁎ Corresponding author at: School of Earth Sciences, The University of Western Australia, Crawley, Australia. E-mail address: [email protected] (A. Petersson). https://doi.org/10.1016/j.chemgeo.2020.119757 Received 12 May 2020; Received in revised form 12 June 2020; Accepted 14 June 2020 Available online 22 June 2020 0009-2541/ © 2020 Elsevier B.V. All rights reserved. A. Petersson, et al. Chemical Geology 551 (2020) 119757 continental growth curves derived from large compilations of detrital been repeatedly proposed that the East Pilbara Terrane developed on an zircon U-Pb and Hf isotope data infer that 30–50% of the present even older, perhaps ≥3.8 Ga, continental substrate (Green et al., 2000; continental volume was stabilised prior to 3.5 Ga (Belousova et al., Van Kranendonk et al., 2007; Tessalina et al., 2010; Gardiner et al., 2010; Dhuime et al., 2012), an outcome that aligns with conclusions 2017; Wiemer et al., 2018). Critically, the evidence used to argue for based on other isotopic approaches (e.g. Armstrong, 1991; Bowring and this ancient basement is based on whole rock Sm-Nd model ages that Housh, 1995; Pujol et al., 2013; McCoy-West et al., 2019). assume crust separation from a strongly depleted mantle reservoir However, as pointed out by Kamber (2015), continental growth (Hamilton, 1981; Jahn et al., 1981; Gruau et al., 1987; Bickle et al., models based on detrital zircon U-Pb and Hf isotope data assume that 1989, 1993; Smithies et al., 2003; Smithies et al., 2007; Van the mass of the depleted mantle has remained constant and that the Hf Kranendonk et al., 2007; Tessalina et al., 2010), variations in zircon Lu- isotope evolution of the depleted mantle was linear, from chondritic at Hf isotope compositions (Gardiner et al., 2017), and modelling of trace ~4.5 Ga to strongly radiogenic at the present day, as represented by the element abundances in the oldest eruptive rocks (Green et al., 2000). upper mantle sampled by modern mid-ocean ridge basalts. If the mass Direct evidence for an Eoarchean substrate includes: (1) ~3.66–3.58 Ga of depleted mantle changed through time, then the radiogenic isotope gneiss enclaves within younger, ~3.42–3.24 Ga monzogranite and signatures of some Archean rocks, if primary, could indicate derivation granodiorite in the Warrawagine Granitic Complex (Kemp et al., 2015); from small, transient, highly depleted mantle domains (e.g., Bennett (2) pre-3.6 Ga zircon cores in ~3.46 Ga and younger felsic igneous et al., 1993), rather than reflect the existence of a global, convecting rocks (Thorpe et al., 1992; Kemp et al., 2015; Sheppard et al., 2017); mantle reservoir that was depleted by voluminous prior crust extraction and (3) detrital zircon grains found across the Pilbara Craton that (Bédard, 2018). Several studies highlight that a linear isotope evolution predate deposition of their respective sedimentary successions by up to of depleted mantle from 4.5 Ga to the present day is not supported by 300 million years (Van Kranendonk et al., 2007; Hickman et al., 2010; the growing Hf isotope datasets from well-preserved igneous rocks, and Kemp et al., 2015). These lines of evidence are consistent with that the depletion history of the early Archean mantle is not well Eoarchean crust predating the East Pilbara Terrane, however a base- constrained (Kemp et al., 2015; Vervoort and Kemp, 2016; Fisher and ment component that is older than 3.8 Ga has not been confirmed. More Vervoort, 2018). Small pockets of chondritic mantle may even have recently, based on Hf isotopic signatures of inherited zircon crystals in a persisted through to the present day (Woodhead et al., 2019). rhyolite, Petersson et al. (2019b) suggest a small, ~3.75 Ga felsic The above models for large volumes of ancient continental crust crustal nucleus to the Pilbara Craton, rather than a widespread felsic must also be reconciled with the paucity (< 1%) of > 3.5 Ga rocks ≥3.8 Ga ‘proto’-crust. Models for formation of the Pilbara Craton preserved on Earth today. It is important to recognise that the present therefore vary markedly regarding the timing of initial continental distribution of ancient crust reflects preservation biases and is not a crustal growth, ranging from Hadean (Tessalina et al., 2010) to ~3.6 Ga reliable proxy for the volumes of crust that were actually generated. (Kemp et al., 2015; Petersson et al., 2019a), for the most part based on However, if large continental volumes were indeed stabilised by 3.5 Ga, whether the upper mantle is assumed to have been depleted or chon- the question remains as to why early Archean continental materials, or dritic in composition. the geochemical signals of their former existence, do not feature more To address the controversy about the age of the putative continental prominently in the extant geological record. It has been suggested that, substrate for the Pilbara Craton, and the implications for global crust- globally, the low number of retrieved > 3.5 Ga detrital or inherited mantle evolution models, we report the results of a study designed to zircons is also inconsistent with numbers that might be expected if large identify and characterise the oldest igneous components of the Pilbara stabilised volumes of pre-3.5 Ga felsic continental crust were reworked Craton. These older, generally gneissic and multi-component rocks in and eroded through time (Stevenson and Patchett, 1990; Nutman, the granitic complexes have not received the same level of attention as 2001; Condie et al., 2011; Rollinson, 2017). We note that much smaller the older portions of the eruptive sequences, but contain important (< 10%) volume fractions of Eoarchean continental crust are inferred clues about the earliest history of the craton. We employ zircon U-Pb, O by other studies to satisfy constraints from Pb (e.g. Kramers and and Hf isotope data to test whether or not these rocks were derived by Tolstikhin, 1997; Kamber et al., 2005) and Nd isotopes (Nägler and remelting older crust, as would be suggested by the previous studies Kramers, 1998), and secular trace element variations (Collerson and noted above, or represent juvenile contributions of continental crust Kamber, 1999). linked to early craton growth. The resilient accessory phase zircon is A complementary approach to determining ancient crustal growth targeted for these ancient rocks because the isotope tracer data are histories, and thus the extent of early Earth crust-mantle differentiation, obtained from parts of zircon crystals that are directly dated and, based is to focus on the evolution of the best-preserved Archean granite- on U-Pb systematics and microstructures, are considered best retentive greenstone terranes, as in the Pilbara Craton (Western Australia) and of their primary chemical information. This circumvents uncertainties the Barberton Greenstone Belt of the Kaapvaal Craton (southern Africa). related to element mobility and isotope disturbance that plague whole These cratons provide, on a regional scale, a robust geological archive rock studies of Archean rocks (e.g. see Hammerli et al., 2019). to test the extent to which formation of Archean continental crust involves: (1) extensive reworking of ancient mafic to ultramafic ‘proto’- 2. The Pilbara Craton crust, a scenario that would be in line with substantial pre-existing volumes of stabilised Eo- to Paleoarchean crust; or (2) the progressive Recent descriptions of the Pilbara Craton interpret five main litho- emplacement and differentiation of juvenile mantle-derived additions. tectonic elements: pre-3.53 Ga crust of uncertain age, composition, and Studies of ancient, but well-preserved igneous rocks also offer insight original extent; the 3.53–3.22 Ga East Pilbara Terrane; 3.22–3.16 Ga into the composition of the early terrestrial mantle (e.g. Vervoort and Mesoarchean sedimentary basins; the 3.28–3.09 Ga West Pilbara Blichert-Toft, 1999), in particular to establish when depleted isotopic Superterrane; and the 3.05–2.93 Ga De Grey Superbasin (Figs. 1 and 2) signatures were developed that may signal the extraction and storage of (Hickman and Van Kranendonk, 2012; Hickman, 2016). Three large continental volumes. Chondritic isotope compositions of juvenile 3.53–3.23 Ga volcano-sedimentary groups, Warrawoona, Kelly and rocks, by contrast, could be taken to suggest that ancient crust sepa- Sulphur Springs, collectively forming the Pilbara Supergroup, and rated from a primitive mantle source. several (3.53–3.22 Ga) granitic complexes, make up the East Pilbara The granitic complexes in the Pilbara Craton (Fig. 1) record a 750 Terrane (Fig. 1). The structural configuration whereby domical granitic million-year magmatic evolution, from the emplacement of gabbros, complexes are flanked by low to medium grade, synclinal ‘greenstone’ anorthosites and TTG (tonalite-trondjhemite-granodiorite) suites at belts (volcanic and sedimentary formations) is the epitome of the dome- 3.59–3.58 Ga (Petersson et al., 2019a), to the intrusion of K-rich and-keel, Archean granite-greenstone terrane (Van Kranendonk et al., granites at 2.85–2.83 Ga (Smithies et al., 2009; Hickman, 2012). It has 2007).

2 A. Petersson, et al. Chemical Geology 551 (2020) 119757

Fig. 1. Simpliﬁed geological map of the Pilbara Craton in Western Australia (modiﬁed after Smithies et al., 2007; 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. Abbreviated versions of samples numbers are used for simplicity.

The Pilbara Supergroup is dominated by weakly metamorphosed Petersson et al., 2019a). Wiemer et al. (2018) suggested that similar- basalt and lesser komatiite, with interlayered horizons of felsic volcanic aged granodioritic to trondhjemitic rocks occur along the southwestern and sedimentary rocks (Fig. 2). The supracrustal succession includes no margin of the Muccan Granitic Complex, although this is not sub- recognised structural repetition of the stratigraphy and, at the present stantiated by the results of the present study (see below). A ~ 3.58 Ga exposure level, is delimited by younger felsic intrusions at the base and zircon population is also recognised in banded tonalitic gneiss enclaves an erosional surface at the stratigraphic top. The preserved strati- found in ~3.42–3.24 Ga monzogranite and granodiorite of the Warra- graphic thickness of the Pilbara Supergroup is 12–15 km, but the ori- wagine Granitic Complex (see Kemp et al., 2015), implying that the ginal maximum depositional thickness is unknown (Burke et al., 1976; igneous protolith to these enclaves may have been partly of Mount Van Kranendonk et al., 2002). Webber age. An extrusive equivalent of the 3.59 Ga igneous rocks has Granitic complexes of the eastern Pilbara Craton were assembled by yet to be recognised (Petersson et al., 2019a). coalescence of magmatic pulses emplaced over hundreds of millions of The 3.49–3.42 Ga granitic rocks are widespread within the Shaw years (3.49–3.42 Ga, 3.32–3.29 Ga, and ~3.25 to 3.22 Ga) (Fig. 3). Granitic Complex, but also occur in the Carlindi, Yule, Mount Edgar, Older (~3.59 Ga) meta-gabbroic to anorthositic rocks belonging to the Corunna Downs, Warrawagine and Muccan granitic complexes (Buick Mount Webber event (Petersson et al., 2019a), are identiﬁed in the et al., 1995; Nelson, 1999a; Bagas et al., 2005; Van Kranendonk et al., north-western-most part of the Shaw Granitic Complex (Fig. 3, 2007; Gardiner et al., 2017; Petersson et al., 2019a). These intrusions

3 A. Petersson, et al. Chemical Geology 551 (2020) 119757

Fig. 2. Time-space chart delineating the evolution of the exposed Pilbara Craton (modified after Hickman et al., 2010; Petersson et al., 2019a). The stratigraphic locations 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.) vary from slightly deformed tonalitic to granodioritic rocks (e.g. as in between 3.33 and 3.30 Ga (Emu Pool Supersuite) are the most wide- the northern parts of the Shaw Granitic Complex; Bickle et al., 1993)to spread at the present erosional level (Fig. 3). Between 3.25 and 3.24 Ga, strongly deformed migmatitic gneisses (e.g. Shaw and Mount Edgar a second, and the last, major pulse of monzogranite was emplaced Granitic Complex; Bickle et al., 1993; Collins, 1993). The 3.49–3.42 Ga (Williams, 2001). The younger magmas appear to have exploited pre- granites are subdivided into two supersuites, the 3.49–3.46 Ga Callina existing conduits upon ascent, as the older components are displaced Supersuite and the 3.45–3.42 Ga Tambina Supersuite (Fig. 3). The towards the margins of the complex, an arrangement that is also ob- Callina Supersuite is contemporaneous with the extrusive units within served for the Shaw, Mount Edgar and Muccan granitic complexes (Van the Duffer Formation, and the Tambina Supersuite and the Panorama Kranendonk et al., 2007; Gardiner et al., 2017; Wiemer et al., 2018; Formation form a contemporaneous intrusive/extrusive pair (Van Petersson et al., 2019a). Kranendonk et al., 2006). Younger igneous supersuites within the Pil- Enclaves of banded tonalite gneiss (GSWA 142870) within a foliated bara Craton include, with decreasing age, the Emu Pool, Cleland, Mount monzogranite yield some of the oldest zircon dates so far obtained from Billroth and Elizabeth Hill supersuites (Figs. 2–3). the Pilbara Craton at ~3.66–3.64 Ga and 3.60–3.58 Ga (Nelson, 1999a; Kemp et al., 2015). Based on the relations to other units within the 3. Early evolution of the east Pilbara granitic complexes region, Nelson (1999a) interpreted this gneiss to be the oldest phase of the Warrawagine Granitic Complex. Nelson (1999a) assigned the zircon Thus far, the oldest plutonic components in the Pilbara Craton have U-Pb analyses into five age groups based on their 207Pb/206Pb ratios: been located in the Warrawagine (Nelson, 1998, 1999a), Shaw 3.655 ± 0.006 Ga (n = 4), 3.637 ± 0.012 Ga (n = 3), (Petersson et al., 2019a), Muccan (Wiemer et al., 2018) and southern 3.595 ± 0.004 Ga (n = 2), 3.576 ± 0.006 Ga (n = 5) and Carlindi (Van Kranendonk, 2000) granitic complexes, raising the pos- 3.410 ± 0.007 Ga (n = 5). The 3.41 Ga date, obtained partly from sibility that even older rocks exist in these areas. We therefore targeted zircon rims, was interpreted to represent the time of deformation and these four granitic complexes for additional sampling in our study, and metamorphism that formed the gneissic fabric, while older dates were provide brief outlines of the geology of these in the sections below. interpreted to represent different magmatic components of this gneissic rock, consistent with the fine oscillatory zoning of these zircon domains 3.1. Warrawagine Granitic Complex (Kemp et al., 2015). This suggests early felsic plutonism and the generation of tonalite between 3.66 and 3.58 Ga, prior to eruption of the The earliest confirmed plutonism in the Warrawagine complex oc- oldest supracrustal rocks of the East Pilbara Terrane (the 3.52–3.50 Ga curred between 3.47 and 3.43 Ga, although granitic rocks emplaced Coonterunah Subgroup, Buick et al., 1995). The actual emplacement

4 A. Petersson, et al. Chemical Geology 551 (2020) 119757

Fig. 3. Simplified map of the exposed Pilbara Craton, demonstrating the distribution of granitic rocks of the 3.59–3.58 Ga Mount Webber event, the 3.53–3.49 Ga Mulgundoona Supersuite, the 3.48–3.46 Ga Callina Supersuite, the 3.45–3.42 Ga Tambina Supersuite, the 3.32–3.29 Ga Emu Pool Supersuite and the 3.27–2.92 Ga Cleland Supersuite. Sampling sites are shown as hexagons coloured to match their affiliated supersuite. Modified after Smithies et al. (2007) and Van Kranendonk et al. (2007). Abbreviated versions of samples numbers are used for simplicity. (For interpretation of the references to colour in this figure legend, the reader is referred to the online version of this chapter.) age of the igneous precursor to the tonalitic gneiss enclave remains (Nelson, 1998). uncertain, however (see Kemp et al., 2015) – existing data permit a 3.66 Ga magmatic protolith, with the younger zircon populations being due to U-Pb disturbance, or a 3.59–3.58 Ga protolith, with in- 3.3. Shaw Granitic Complex herited > 3.6 Ga zircon. These scenarios have different implications for the timing of initial crustal growth in the Pilbara Craton. The Shaw Granitic Complex is composed of granitic intrusions that young towards the centre of the complex. Most of the ~3.6–3.4 Ga 3.2. Muccan Granitic Complex plutonic rock suites are highly sheared, folded and locally metamorphosed to upper amphibolite facies. This generated a migmatitic gneiss The oldest rocks dated within the Muccan Granitic Complex are that was later intruded by several plutons of variably deformed post- ~3.59–3.46 Ga TTG, granite and diorite gneisses from the southwestern tectonic granite within the central parts of the complex (Bickle et al., margin of the granite complex (zircon U-Pb isotopes, LA-ICPMS; 1980; Bickle et al., 1989; Pawley et al., 2004; Van Kranendonk et al., Wiemer et al., 2018). Apart from their antiquity, the significance of 2007; Petersson et al., 2019a). these rocks is that they been used to constrain the timescales of maficto Metamorphosed gabbroic and leucogabbroic bodies in the north- ultramafic proto-crust formation, leading to density stratification and western part of the Shaw Granitic Complex, emplaced at 3.59–3.58 Ga subsequent overturn of gravitationally unstable crust on 100 Ma growth (the Mount Webber gabbros; Petersson et al., 2019a), are the oldest cycles (see Wiemer et al., 2018). It should, however, be noted that the rocks so far identified in the Pilbara Craton. The age of the younger, ages of the Muccan gneisses quoted by Wiemer et al. (2018) are from mainly granodioritic rocks in this area has been determined through a upper intercepts on U-Pb concordia diagrams, and that weighted Pb-Pb whole-rock isochron to 3.499 ± 0.022 Ga (Bickle et al., 1993), average 207Pb/206Pb dates of zircon grains in these rocks are younger. and to 3.493 ± 0.004 Ga and 3.467 ± 0.006 Ga, using ion-microp- This is elaborated in Section 6.1. robe U-Pb on zircon (McNaughton et al., 1988). Rocks of the Nelson (1998) obtained a zircon U-Pb date of 3.470 ± 0.004 Ga 3.45–3.42 Ga Tambina Supersuite make up extensive parts of the Shaw from a granodiorite gneiss towards the centre of the Muccan complex Granitic Complex. Employing ion microprobe zircon analysis, (GSWA 142828), interpreted to represent igneous crystallisation of the McNaughton et al. (1993) and Petersson et al. (2019a) dated a grano- gneiss precursor. However, five out of 26 U–Pb zircon analyses yield diorite and a gabbro in the Mount Webber area to 3.431 ± 0.004 Ga 207Pb/206Pb dates that are ≥3.5 Ga, indicating the presence of older and 3.438 ± 0.003 Ga, respectively. Granitic rocks of the Tambina components within the gneiss. Whole-rock Sm-Nd isotope data for the Supersuite have been interpreted to form by reworking rocks of the granodiorite gneiss yield a chondritic εNd(3.470 Ga) of 0.0 ± 0.4 Callina Supersuite (Pawley et al., 2004; Gardiner et al., 2017). (Gardiner et al., 2017), however, as noted above, zircon age data shows indications of mixed source components of unknown affiliation

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3.4. Carlindi Granitic Complex

The Carlindi Granitic Complex comprises ~3.49–2.86 Ga granitic (19/ rocks that intrude and locally metamorphose the lower stratigraphic ‰ units of the Warrawoona Group. The 3.49–3.46 Ga rocks of the Callina Supersuite are dominant, with younger intrusions, primarily belonging 15TKPB20 Shaw tonalite 3.469 ± 0.0021.3) (15/ 6.4 ± 0.1 to the 3.28–3.23 Ga Cleland Supersuite, present in the interior of the 1.2) granitic complex.

The oldest zircon populations from the Carlindi area are (17/

3.532 ± 0.009 Ga in a rhyolite (sample 70601) and ‰ 3.527 ± 0.006 Ga in a felsic volcaniclastic rock (sample 90649), both from the Coonterunah Formation adjacent to the southeastern part of 15TKPB19 Carlindi quartz dioritic gneiss 3.501 ± 0.0011.1) (15/ 5.9 ± 0.1 the granitic complex (Green, 2001). No ≥3.5 Ga granitic rock or pro- 0.4) tolith has been identiﬁed in the area, although it is notable that Nelson (1999a) reports the presence of gneissic rocks closer to the eastern margin of the granitic complex. (18/ Whole-rock Sm-Nd isotope data from the 3.48–2.85 Ga granitic ‰ rocks of the Carlindi Granitic Complex yield εNd(t) of +1.5 to −1.8 with a near chondritic mean value of +0.5 ± 0.5 (Smithies et al., 2007). 0.8 ± 0.2 (18/0.9) 0.2 ± 0.2 (13/0.6) 0.3 ± 0.2 (13/1.0) 18APPB05 Muccan (south) granodioritic gneiss 3.523 ± 0.0021.3) (15/ 5.0 ± 0.1 1.7)

4. Methods (18/

4.1. Fieldwork and sampling strategy ‰

Field study was undertaken in an attempt to locate the oldest 0.2 ± 0.3 (11/ − 0.5) 15TKPB12 Muccan (south) tonalitic gneiss 5.3 ± 0.2 3.502 ± 0.0032.6) (10/ granitic and/or gneissic rocks in the Warrawagine, Muccan, Shaw and 5.4) Carlindi granitic complexes. Emphasis was given to examining exposures close to the margins of the granitic complexes, given that the older igneous components tend to be localised along these margins (e.g. (23/ ‰ Van Kranendonk et al., 2002). Prospective outcrops were located by consideration of previous geological mapping and inspection of satellite and aerial photographic. For Warrawagine, this revealed large, water- 12TKPB17 Muccan (north) tonalitic gneiss 5.8 ± 0.1 1.9) 3.431 ± 0.0031.9) (23/ washed pavements and rock bars along the northern banks of the De Grey River, which were sampled. We also revisited the locality where the tonalitic gneiss enclave (GSWA142870) was collected, to determine (17/ if additional gneiss enclaves were available for sampling. Aerial pho- ‰ tography was used to identify large tracts of layered rocks in the

northern part of the Muccan Granitic Complex, and in the southern part 0.8 ± 0.6 (6/2.6) 0.2 ± 0.3 (20/0.7) 1.0 ± 0.5 (7/1.9) − 12TKPB16 Muccan (north) layered gneiss 2.9) 5.2 ± 0.3 3.312 ± 0.0048.1) (8/ of the complex, including the area previously studied in detail by 3.496 ± 0.0072.9) (9/ Wiemer et al. (2016). Prominent outcrops at the northern periphery of the Shaw Granitic Complex, and in the southeastern Carlindi Granitic (17/ Complex were also targeted based on satellite imagery. Sample site ‰ coordinates are presented in Table 1.

4.2. Sample preparation and analytical techniques 3.2 ± 0.4 (12/ − 3.2) 12TKPB12 Warrawagine amphibolitic gneiss 6.9 ± 0.1 1.1) 3.425 ± 0.0021.2) (13/

Details of sample preparation and the analytical methods are outlined in supplemental ﬁle S1. In brief, (U-Th)–Pb dating of zircon (10/ crystals was conducted using a Cameca IMS1280 ion microprobe at the ‰ NordSIMS facility in Stockholm, Sweden. Oxygen isotope ratios 18 16

( O/ O) in zircon were determined using Cameca IMS 1280 ion mi- 0.2 ± 0.5 (13/ − 5.0) 12TKPB06 Warrawagine banded gneiss 5.5 ± 0.1 1.3) croprobes hosted by the Centre for Microscopy, Characterisation and 3.580 ± 0.0033.2) (7/ Analysis (CMCA), University of Western Australia (UWA) and in the NordSIMS facility. Finally, Lu-Hf isotope analyses of zircon were carried (20/

out at the School of Earth Sciences at The University of Western ‰ Australia using a 193 nm Cetac Analyte G2 excimer laser and a Thermo- Scientiﬁc Neptune Plus multicollector ICP-MS. All analytical results ﬁ 0.8 ± 0.4 (4/0.4) 0.4 ± 0.7 (7/4.7) GSWA 142870 Warrawagine banded gneiss − 0.7 ± 0.4 (3/0.1) − 1.3) 3.577 ± 0.0052.2) (5/ 1.9) from sample and reference zircons are presented in supplementary les 0.1) (Tables S1–S5). O 6.1 ± 0.1 18 Hf(Cryst) Hf Hf ε ε δ 5. Results ε

Field photos and images of hand specimens of the nine rocks analysed in this study are found in supplementary File S2. The whole rock (Other 1) (Other 2) crystallisation age major and trace element geochemistry of each sample is reported in Sample Granitic complex rock-type Weighted mean Weighted mean UTM (m E)UTM (m S)Interpreted 51 K 246822 51 K 7696662 51 K 246893 51 K 7696759 51 51 K K 241280 7709439 50 50 K K 7743231 773747 50 K 7743280 50 K 773827 51 K 7688467 51 K 189087 50 K 7689312 50 K 784225 50 K 7673775 50 K 737532 50 K 7633254 50 K 755977 Other age 2Weighted mean 3.417 ± 0.003 (6/ Other age 1 3.654 ± 0.006 (3/ Weighted mean

Table S5, and provided for completeness. All weighted average zircon Table 1 Summary table with sample coordinates, ages, and zircon isotope results. Numbers in brackets denoteAll (number errors of at analyses/MSWD). 95%All conf. ages presented in Ga.

6 A. Petersson, et al. Chemical Geology 551 (2020) 119757

Fig. 4. Tera-Wasserburg diagrams showing SIMS (Secondary Ion Mass Spectrometry) zircon spot data (2σ error ellipses). Analyses shown in red ellipses were discarded and not used in any age calculations. (a) Warrawagine tonalitic gneiss sample GSWA142870. Solid ellipses denote U-Pb data from this study and dashed ellipses represent the data of Nelson (1999a). Blue ellipses are those analyses used in 3.654 Ga age calculation, purple ellipses denote analyses used in 3.577 Ga age calculation and green ellipses represent analyses used in 3.417 Ga age calculation. (b) Warrawagine tonalitic gneiss 12TKPB06; (c) Warrawagine amphibolitic gneiss 12TKPB12, (d) Muccan felsic gneiss 12TKPB17; note that the left panel shows results from analysis of cores, and the centre panel shows analyses of rims; (e) Muccan tonalitic gneiss 12TKPB16; (f) Muccan tonalitic gneiss 15TKPB12; (g) Muccan granodioritic gneiss 18TKPB05; (h) Carlindi quartz-dioritic gneiss 15TKPB19 (i) North Shaw tonalite 15TKPB20. The panels on the right show representative cathodoluminescence (CL) images of zircon grains. Small coloured ellipses indicate SIMS spot locations for U-Pb, small circles represent O isotope analytical sites and large circles indicate LA-ICP-MS spot locations for Lu-Hf. Spot colour corresponds to the colouring of data ellipses. ‘b’ corresponds to analyses labelled #b in supplementary tables. Dashed ellipses denote a second analysis of the zircon domain (#b in supplementary tables). (For interpretation of the references to colour in this ﬁgure legend, the reader is referred to the web version of this article.)

7 A. Petersson, et al. Chemical Geology 551 (2020) 119757

Fig. 4. (continued)

207 206 18 Pb/ Pb dates, δ O values and εHf(t) values are quoted at 95% zoning present in the central parts of most grains. Non-luminescent, confidence limits, unless otherwise stated. Concordance of U-Pb data is rounded cores are obvious in a few grains (Fig. 4a). assessed at the 2σ level of individual data ellipses. The zircon isotopic Forty zircon U-Pb analyses in twenty-nine zircon grains yield data are summarised on Figs. 4 (U-Pb), 5 (oxygen), and 6–8 (Hf). nineteen concordant data points. 207Pb/206Pb-dates range from 3.64–3.15 Ga and concordant analyses cluster into three tight groups at 5.1. Warrawagine Granitic Complex, banded tonalitic gneiss GSWA142870 ~3.64 Ga (n = 3), ~3.58 Ga (n = 5) and ~3.42 Ga (n = 6) (Fig. 4a). Four concordant analyses (−9, −21, −22b and −23) plot between 5.1.1. Field context and petrography these groups, suggesting partial ancient Pb loss or mixed sampling of This is the sample described and analysed by Nelson (1999a), which domains of different age, and one analysis (−29) has a large analytical comprises an isolated, metre-long enclave enclosed within a hetero- uncertainty. These five analyses are not considered further. The geneous assemblage of granitic and gneissic rocks. The contacts with youngest group gives a 207Pb/206Pb weighted average of the host granitic rocks are not exposed, but lithologically similar 3.417 ± 0.003 Ga (MSWD = 0.1). The ~3.58 Ga group yields a banded gneiss enclaves in adjacent outcrops are commonly sheathed 207Pb/206Pb weighted average of 3.577 ± 0.005 Ga (MSWD = 2.2), and crosscut by pegmatite, and show evidence for partial disaggrega- and the oldest group spread along concordia and return a poorly de- tion into elongate strips and diffuse schlieren in a granodioritic host. fined 207Pb/206Pb weighted average of 3.627 ± 0.027 Ga (MSWD = 16). When combining these data with those of Nelson 5.1.2. Zircon U-Pb, O and Lu-Hf (1999a), the oldest group of analyses defines a concordia age of We conducted further work on zircon crystals in the original epoxy 3.653 ± 0.005 Ga (MSWD = 3.7) and an identical 207Pb/206Pb grain mount analysed by Nelson (1999a) for U-Pb isotopes, and Kemp weighted average of 3.654 ± 0.006 Ga (MSWD = 1.9). These ages are et al. (2015) for Hf isotopes, the aim being to further unravel the pro- equivalent to the upper intercept age reported above. In summary, tracted thermal history of this rock, and to establish the magmatic three age groups have been identified from these complex zircon protolith age. Zircon grains are generally subhedral and slightly crystals, at 3.654 ± 0.006 Ga, 3.577 ± 0.005 Ga and rounded, and are microstructurally complex (see Kemp et al., 2015). 3.417 ± 0.003 Ga. They vary between 60 × 100 μm and 100 × 250 μm in size. CL-bright Twenty-nine O isotope analyses from 21 dated and three undated rims, between 10 and 80 μm wide, discordantly cut the oscillatory grains give δ18O values between 3.8 and 6.6‰ and a weighted average

8 A. Petersson, et al. Chemical Geology 551 (2020) 119757

Fig. 4. (continued)

9 A. Petersson, et al. Chemical Geology 551 (2020) 119757

Fig. 4. (continued)

of 6.09 ± 0.12‰ (2σ, MSWD = 0.5; Fig. 5). Discarding analyses of Analyses of rims give εHf(3.417 Ga) between −1.5 and +0.3 with a grains that are > 5% discordant for U-Pb eliminates discrepant δ18O weighted average of −0.4 ± 0.7 (MSWD = 4.7). values, resulting in a tightly clustered group with a spread between 5.7 and 6.6‰. This lowers the weighted average marginally to 5.2. Warrawagine Granitic Complex, banded tonalitic gneiss 12TKPB06 6.05 ± 0.08‰ (2σ, MSWD = 1.3). All age groups yield eﬀectively identical weighted averages: 6.14 ± 0.21‰, 6.09 ± 0.27‰ and 5.2.1. Field context and petrography 6.06 ± 0.21‰ for the 3.65, 3.58 and 3.42 Ga groups respectively. This sample was obtained from an elongate, partially disaggregated Fourteen new zircon Lu-Hf isotope analyses were obtained, three gneiss enclave hosted by pegmatite and granodiorite, which crops out from 3.65 Ga cores, four in 3.58 Ga cores and seven in 3.42 Ga rims ~120 m northeast of the GSWA142870 sampling site. The rock is (Figs. 6 and 7). The εHf(3.654 Ga) values cluster tightly between +0.6 and characterised by isoclinally folded, highly recrystallised felsic bands +0.8 (176Hf/177Hf = 0.28048–0.28050) with a weighted average of that are discontinuous and of irregular thickness, in a medium grey, 0.7 ± 0.4 (MSWD = 0.1). The εHf(3.577 Ga) values range from −0.5 to biotite-rich, tonalitic matrix. −1.0 with a weighted average of −0.8 ± 0.4 (n = 4; MSWD = 0.4). Plagioclase and quartz are the main minerals, with lesser (> 10%)

Fig. 4. (continued)

10 A. Petersson, et al. Chemical Geology 551 (2020) 119757

Fig. 4. (continued) amounts of biotite. In thin section, plagioclase grains are irregularly by the 207Pb/206Pb weighted average of 3.580 ± 0.003 Ga (95% shaped and vary in size from large > 5 mm grains down to much ﬁner conﬁdence, MSWD = 3.2) from the same analyses (Fig. 4b). One grains. Quartz has a more homogeneous size distribution around slightly reversely discordant analysis (12TKPB06–05, 1.2%) gives a 0.5 mm. The average plagioclase composition is estimated optically at 207Pb/206Pb date of 3.619 ± 0.001 Ga, older than the main concordant

An15 to An20. Biotite forms strongly oriented, splintery grains, with a cluster (Fig. 4b). lesser amount being partly or completely altered to chlorite, which Oxygen isotopic measurements are restricted to concordant and the deﬁne schlieren interwoven with conspicuous epidote. eight least discordant (< 10%) grains. Eighteen O isotope analyses Zircon forms whole crystals and fragments that are slightly elon- from thirteen grains yield δ18O values between +2.1 and +5.9‰. gate, subhedral to euhedral, and typically pale yellowish-brown to However, all analyses from discordant grains, which sample CL-bright brown. They vary between 60 × 150 μm and 250 × 500 μm in size. domains, returned exclusively sub-mantle δ18O, with a large spread; Most are fractured. The grains are homogeneously CL-dark, with CL- these are therefore discarded from further consideration. The remaining bright zircon superimposed as an irregular domainal texture (Fig. 4b). analyses of CL-dark concordant domains give δ18O values between +5.4 and +5.9‰ and a weighted average of +5.5 ± 0.1‰ (n = 10, MSWD = 1.3; Fig. 5). 5.2.2. Zircon U-Pb, O and Lu-Hf Fourteen Lu-Hf isotope analyses from eleven dated zircon grains, of Single U-Pb analyses on thirty-seven grains return mainly dis- which one was compromised by a large inclusion, yield 176Hf/177Hf σ cordant data (n = 29/37 at the 2 level). One concordant analysis plots ranging between 0.28053 and 0.28070 (Figs. 6 and 7). The εHf(3.58 Ga) slightly below the main cluster of concordant analyses, suggestive of values range between −1.1 and +2.1 with a weighted average of slight Pb loss (Fig. 4b). The remaining seven analyses generate a con- −0.2 ± 0.5 (MSWD = 5.0). cordia age of 3.580 ± 0.002 Ga (MSWD = 0.3), which is corroborated

Fig. 4. (continued)

11 A. Petersson, et al. Chemical Geology 551 (2020) 119757

Fig. 4. (continued)

5.3. Warrawagine Granitic Complex, amphibolitic gneiss 12TKPB12 grains (400 × 600 μm) are commonly darker while smaller grains (50 × 100 μm) tend to be more transparent. Sharp core-rim relations 5.3.1. Field context and petrography are visible in CL, with weak oscillatory zoning in the CL-dark core Amphibolitic gneiss 12TKPB12 was sampled from water-washed domains. This zonation is mostly visible in larger grains, while smaller rock pavements along the De Grey River about 15 km north–northwest grains have very weakly zoned to unzoned cores. Irregularly zoned CL- of GSWA142870. Here, the amphibolitic gneiss forms large (to 10 m bright to CL-dark rims discordantly cut the igneous zonation of the core across), lithologically-heterogeneous rafts that include decimetre-sized domains (Fig. 4c). Most of these rims are, however, too thin to be lozenges of ultramaﬁc rock and are interlayered with, and intruded by, successfully analysed. voluminous pegmatite. This assemblage is enclosed within a medium grained monzogranite, and cross-cut by dykes of similar-looking 5.3.2. Zircon U-Pb, O and Lu-Hf granitic material, which contain rotated angular fragments of the am- Thirty-four U-Pb analyses in thirty-one grains return data that phibolitic gneiss. mainly cluster around 3.42 Ga, but with some discordant analyses; one Sample 12TKPB12 is a dark grey, even medium grained, layered concordant grain (n5575–15) with a 207Pb/206Pb date of amphibolitic gneiss. The principal minerals are hornblende and plagi- 3.457 ± 0.003 Ga plots above the main cluster (Fig. 4c, Table S1). As oclase that show a distinct preferred orientation, with smaller quartz the majority of the analyses are slightly discordant, a concordia age grains and euhedral titanite also being common throughout the rock. cannot be calculated. Twenty-seven analyses return a weighted average The majority of the plagioclase is altered to sericite and epidote, but 207Pb/206Pb age of 3.421 ± 0.003 Ga (MSWD = 1.2) (Fig. 4c). fresh plagioclase is estimated optically to be about An30. However, some of these analyses, obtained from microstructural do- Zircon grains separated from this sample are commonly subhedral mains that are not visibly diﬀerent, suggest slight ancient Pb loss, most and transparent to brownish. They vary greatly in shape and size; larger clearly visualized by two concordant analyses that fall below the main

Fig. 5. Pilbara zircon δ18O versus age (Ga). Grey band denotes the 5.3 ± 0.6‰ (2 SD) mantle value of Valley et al. (1994, 2005).

12 A. Petersson, et al. Chemical Geology 551 (2020) 119757

range between 0.28048 and 0.28054, corresponding to εHf(3.425 Ga) values between −3.8 and −1.5, with a weighted average of −3.2 ± 0.4 (MSWD = 3.2; Figs. 6 and 7).

5.4. Muccan Granitic Complex (north) - layered gneiss 12TKPB17

5.4.1. Field context and petrography Prominent ridges and low pavements of layered quartzofeldspathic gneiss, crosscut by pegmatite dykes, crop out very close to the northwestern corner of the Muccan Granitic Complex. The gneiss consists of leucocratic and mesocratic components, deﬁned by variation in modal biotite and interlayered on the scale of several to tens of centimetres. It contains discrete, layer-parallel granitic leucosomes and wider pegmatite sheets that are commonly complexly folded and contain the biotite foliation of the host. The gneissic layering is further accentuated by dark, biotite-rich schlieren.

176 177 207 206 A sample of the pale variant of the gneiss (12TKPB17), selected to Fig. 6. Hf/ Hf(t) versus Pb/ Pb dates (Ga) of zircons from the ff GSWA142870 tonalitic gneiss, showing the resolvable difference between the be free of pegmatite and leucosome material, has a di use, centimetre- three different age groups (error bars are plotted at 2SE). Inset shows the scale layering and is dominated by plagioclase and quartz, with minor

GSWA142870 zircon Hf isotope data from this study as εHf versus Age (Ga). K-feldspar and biotite. The zircon grains in this rock are euhedral to Orange squares denote weighted averages of the analyses encapsulated by the subhedral and vary in size from 100 to 450 μm along their c-axis. Small dashed orange boxes (the 3.577 and 3.654 Ga age-groups) and horizontal grey grains are typically transparent while larger grains are darker and more solid lines show 2σ uncertainties. An evolution from the average 176Hf/177Hf of cracked. In CL, the zircon grains show oscillatory zoning with no signs 176 177 the 3.65 Ga group to the average Hf/ Hf of the 3.58 Ga group corresponds of overprinting microstructures or recrystallisation (Fig. 4d). to a 176Lu/177Hf of 0.008 (orange arrows), i.e. much higher than the value of zircon in the rock (~0.001) and so cannot be explained by Pb loss (black dashed lines). (For interpretation of the references to colour in this ﬁgure legend, the 5.4.2. Zircon U-Pb, O and Lu-Hf reader is referred to the online version of this chapter.) Twenty-one analyses were conducted on seventeen grains, of which ﬁve analyses are slightly discordant. All analyses yield a weighted 207 206 cluster. The thirteen analyses with the oldest Pb/ Pb dates (ex- average 207Pb/206Pb date of 3.431 ± 0.003 Ga (MSWD = 3.7, Fig. 4d). 207 206 cluding n5575–15) yield a weighted average Pb/ Pb age of Twenty-three O isotope analyses from twenty zircon grains return 3.425 ± 0.002 Ga (MSWD = 1.2). This result is indistinguishable from δ18O values between 5.3 and 6.2‰, with a weighted average of 207 206 that derived via the weighted average Pb/ Pb of all twenty-seven 5.8 ± 0.1‰ (MSWD = 1.9; Fig. 5). Lu-Hf isotope analysis of sixteen analyses in the main cluster, at 3.421 ± 0.003 Ga. 176 177 grains yields Hf/ Hf ratios from 0.28059 to 0.28064, and εHf(3.431 Seventeen O isotope analyses in thirteen dated grains have δ18O Ga) values from −0.1 to +0.9, with a weighted average of +0.2 ± 0.3 values between +6.7 and +7.0‰ and give a weighted average of (MSWD = 0.7; Figs. 6 and 7). +6.9 ± 0.1‰ (MSWD = 1.1; Fig. 5). The 176Hf/177Hf ratios (n = 12)

Fig. 7. Zircon εHf(t) versus crystallisation ages (in Ga) showing the overall chondritic Hf isotope signatures of Eo- and Paleoarchean meta-igneous rocks in the Pilbara Craton. Error bars represent the quoted 2SE uncertainties of individual analyses. The nominal depleted mantle curve (dashed line) was plotted using the parameters in Blichert-Toft and Puchtel (2010).

13 A. Petersson, et al. Chemical Geology 551 (2020) 119757

Fig. 8. Mean zircon εHf(t) versus crystallisation ages (in Ga), showing zircon data from East Pilbara Terrane igneous rocks, almost exclusively yielding values within errors of CHUR. Uncertainties are shown at the 2SD level.

5.5. Muccan Granitic Complex (north) – tonalitic gneiss 12TKPB16 cluster and generate a concordia age of 3.312 ± 0.003 Ga (95% confidence, MSWD = 1.7) and an identical weighted average 5.5.1. Field context and petrography 207Pb/206Pb age of 3.312 ± 0.004 Ga (95% confidence, MSWD = 2.9, Enclosed within the layered granitic gneiss exposures described Fig. 4e). One grain (n5784-8) returns a 207Pb/206Pb-date of 3.58 Ga and above are bands of a darker, blue-grey tonalitic gneiss. These bands are has a significantly higher Th/U (1.1) than that of the other concordant 1–2 m thick, contain a biotite-defined foliation parallel to that of the analyses (average Th/U = 0.45). host, and are persistent laterally for tens of metres aligned with the Fifteen different dated grains yielded δ18O values between 4.2 and layering of the surrounding granitic gneisses; contacts appear to be 5.9‰ (n = 19). Omitting analyses from two discordant grains results in sharp. The tonalitic bands also contain discrete, isoclinally folded leu- a weighted average δ18O of +5.2 ± 0.3‰ (n = 17, MSWD = 2.9; cosomes and transgressive felsic veins, but are less obviously layered Fig. 5). Thirteen Lu-Hf isotope analyses in as many dated zircon grains than the host gneiss. Sample 12TKPB16, obtained from one of these return 176Hf/177Hf ratios ranging between 0.28063 and 0.28074. Seven darker bands, is a lithologically uniform, even grained rock consisting analyses targeted > 3.4 Ga rounded cores and six analyses targeted of ~50% plagioclase, 40% quartz and 10% biotite, with sparse, small rims and zircon grains of the ~3.31 Ga population (Figs. 6 and 7). The blebs of microcline. Accessory minerals include titanite, muscovite εHf(3.496 Ga) values of the cores range between +0.5 and +1.9 with a (secondary), apatite and zircon. Weakly twinned plagioclase crystals weighted average of +1.0 ± 0.5 (MSWD = 1.9). Zircon rims yield εHf (0.5-1 mm) are mostly fresh, but show some alteration to sericite and (3.496 Ga) values ranging from −1.6 to +0.1, with a weighted average of epidote. Plagioclase and quartz are irregularly shaped and quartz has −0.8 ± 0.6 (MSWD = 2.6). strongly undulose extinction. Unaltered, well-shaped biotite defines the prominent planar fabric. 5.6. Muccan Granitic Complex (south) – tonalitic gneiss 15TKPB12 Zircon grains are subhedral and slightly rounded, varying significantly in size (between 20 × 50 μm to 400 × 600 μm) and shape. 5.6.1. Field context and petrography Smaller examples are commonly transparent while the larger are darker This sample was collected from low, slabby outcrops of diffusely and have more cracks. In CL images, the grains are seen to comprise banded tonalitic gneiss that contain isoclinally folded pegmatitic leu- broadly zoned to sector zoned cores that are rounded and outlined by a cosomes and thin biotite-rich schlieren. The rock consists of a mosaic of very thin zone of CL-bright zircon, mantled in turn by rims of variable plagioclase crystals ranging up to 3 mm in size, intergrown with thickness that truncate the zoning of the cores (Fig. 4e). polycrystalline quartz aggregates and some larger interstitial quartz masses. The plagioclase crystals have subhedral to weakly prismatic 5.5.2. Zircon U-Pb, O and Lu-Hf shapes and are altered slightly to sericite. Many of the quartz aggregates In contrast to the host gneiss (12TKPB17), this sample yields a more have elongate shapes with a subparallel orientation and together with complex spread of zircon isotope data. Thirty-six U-Pb analyses on the mafic minerals define a weak fabric in this rock. Small blebs of twenty-three grains return eleven data points that are discordant at the microcline occupy interstices between plagioclase and quartz. Biotite 2σ level. The concordant analyses define two main clusters, with five occurs together with blocky epidote, titanite and lesser amounts of analyses plotting between these. The eight analyses defining the older chlorite. Some epidote has relict cores of allanite. cluster are from large, rounded cores and have 207Pb/206Pb dates that Zircon crystals from this rock are euhedral and elongate with aspect range from 3.51 to 3.48 Ga, with a poorly defined weighted average of ratios of about 3:1 to 6:1. They vary from 150 and 300 μm in length and 3.496 ± 0.007 Ga (95% confidence, MSWD = 8.1). Nine analyses, are mostly a pale brownish colour. Broad oscillatory zonation, which is from both rims and individual small grains (Fig. 4e), define the younger commonly cut by thin CL-dark overgrowths, is visible in most grains

14 A. Petersson, et al. Chemical Geology 551 (2020) 119757

(Fig. 4f). crystals. The fragments have subhedral to euhedral features and are about 150 × 300 μm to 250 × 450 μm in size. Most grains are trans- 5.6.2. Zircon U-Pb, O and Lu-Hf parent with a reddish tinge ranging from very pale to dark. The zircon Sixteen U-Pb analyses, all in different grains, give six data points grains have clear oscillatory zonation in CL, with no signs of secondary that are discordant at the 2σ level. The remaining ten analyses generate alteration (Fig. 4h). a concordia age of 3.502 ± 0.002 (MSWD = 2.6) and an identical weighted average 207Pb/206Pb date of 3.502 ± 0.003 Ga 5.8.2. Zircon U-Pb, O and Lu-Hf (MSWD = 2.4; Fig. 4f). In accord with the microstructurally simple character, zircon grains Twenty-one O isotope analyses from sixteen dated grains yield δ18O of this sample have uncomplicated isotope systematics. U-Pb analyses values between 0.7 and 6.0‰. Discarding analyses that are > 5% dis- of fifteen grains yield fourteen concordant data points that return a cordant for U-Pb isotopes results in a restricted range of δ18O values weighted average 207Pb/206Pb date of 3.501 ± 0.001 Ga between +4.4 and +6.0‰, and a weighted average of +5.3 ± 0.2‰ (MSWD = 1.1) (Fig. 4h). Seventeen O isotope analyses from eleven (n = 18, MSWD = 5.4; Fig. 5). dated grains exhibit δ18O values between 5.7 and 6.1‰, with a Of the fourteen Lu-Hf isotope analyses, three were discarded due to weighted average of 5.9 ± 0.1‰ (n = 17, MSWD = 0.4; Fig. 5). sampling inclusions. The remaining eleven analyses range in Hafnium isotope analysis of eleven zircon grains returns 176Hf/177Hf 176 177 Hf/ Hf between 0.28053 and 0.28076. The εHf(3.502) Ga values from 0.28056 and 0.28069, and εHf(3.501 Ga) values from −0.5 to +0.6 range between −0.5 and +0.7 with a weighted average value of (weighted average = +0.2 ± 0.2; MSWD = 0.6; Figs. 6 and 7). −0.2 ± 0.3 (MSWD = 0.5; Figs. 6 and 7). 5.9. Shaw Granitic Complex, hornblende tonalite 15TKPB20 5.7. Muccan Granitic Complex (south) – granodioritic gneiss 18APPB05 5.9.1. Field context and petrography 5.7.1. Field context and petrography This sample was collected from an area of prominent, blocky This rock is a coarse-grained, bluish-grey granodioritic gneiss from boulders up to 1.5 m high in the northeastern part of the Shaw Granitic the same unit described by Wiemer et al. (2018) and collected within a Complex. Exposures are locally traversed by thin epidote veins, but few metres of the sampling site of those authors. Here, the granodioritic otherwise lithologically reasonably uniform and lack a strong foliation. gneiss forms an area of low, slabby boulders that host thin (1–8 cm) This rock is a spotty, light coloured, even- and medium-grained pegmatitic veins, but is otherwise reasonably homogeneous and un- tonalite with ~45% chalky plagioclase, 35% quartz, 10% hornblende, layered. The rock has a principal mineral assemblage of plagioclase, and minor amounts of chloritised biotite, titanite and an opaque phase. quartz, hornblende and titanite. Accessory minerals include zircon, Microcline forms rare, millimetre-sized interstitial masses. The plagio- epidote and opaque grains. Quartz is mainly recrystallised and inter- clase is lath-shaped and strongly altered to sericite and epidote, stitial. Minor amounts of chlorite are present. whereas quartz forms elongate interstitial grains. Blocky hornblende The zircon grains are transparent, 100 × 150 μm in size, and prisms commonly contain relict, twinned clinopyroxene. Hornblende commonly with pyramidal terminations. In CL, they show sharp oscil- and titanite are intergrown with biotite in small clots that show a slight latory zonation and some grains are sector zoned, all without obvious preferred orientation. signs of secondary alteration (Fig. 4g). Apatite inclusions are common. The zircon grains are light to dark brown, subhedral to euhedral and many crystals have sharp pyramidal terminations. They are relatively 5.7.2. Zircon U-Pb, O and Lu-Hf uniform in size, being between 50 × 100 μm to 100 × 200 μm. The Twenty U-Pb analyses of twenty grains yield nineteen concordant grains show faint and broad oscillatory zoning in cathodoluminescence data points. Fifteen analyses form a tight cluster, while one analysis images (Fig. 4i). (n6052–12) returns a slightly older, and four grains (n6052–06, −13, −16 and −17) clearly younger 207Pb/206Pb dates. The main cluster of 5.9.2. Zircon U-Pb, O and Lu-Hf fifteen analyses generates a concordia age of 3.523 ± 0.002 Ga (2σ, As with the Carlindi quartz diorite, zircon grains from this sample MSWD = 1.1) and an identical weighted average 207Pb/206Pb date of provide very simple data distributions. All fifteen U-Pb analyses are 3.523 ± 0.002 Ga (MSWD = 1.3) (Fig. 4g). concordant, and collectively yield a concordia age of 3.469 ± 0.002 Eighteen O isotope analyses from sixteen dated zircons yield δ18O (2σ, MSWD = 2.7) and an identical weighted average 207Pb/206Pb date values between +4.7 and +5.4‰. All analyses give a weighted of 3.469 ± 0.002 Ga (MSWD = 1.3) (Fig. 4i). Nineteen O isotope average of +5.0 ± 0.1‰ (n = 18, MSWD = 1.7; Fig. 5). The analyses from fifteen dated grains give δ18O values between 6.0 and 176Hf/177Hf of seventeen different dated grains ranges from 0.28057 6.7‰, with a weighted average of 6.4 ± 0.1‰ (n = 19, MSWD = 1.2; and 0.28062, corresponding to εHf(3.523 Ga) values between 0.0 and Fig. 5). Thirteen Lu-Hf isotope analyses from as many dated zircons +1.7 (weighted average = +0.8 ± 0.2, MSWD = 0.9; Figs. 6 and 7). range in 176Hf/177Hf between 0.28057 and 0.28060, corresponding to

εHf(3.469 Ga) values that cluster tightly between −0.1 and +0.9 5.8. Carlindi Granitic Complex – quartz dioritic gneiss 15TKPB19 (weighted average + 0.3 ± 0.2, MSWD = 1.0; Figs. 6 and 7).

5.8.1. Field context and petrography 6. Discussion of the formation of the Pilbara Craton A lithologically uniform quartz dioritic gneiss deﬁnes several prominent, but isolated, bouldery knolls in the southeastern part of the 6.1. Interpretation of geochronological data in gneissic rocks Carlindi Granitic Complex. Sample 15TKPB19, obtained from one of these, is a medium grained, equigranular rock with a homogeneous The U-Pb systematics of zircon crystals from ancient gneissic rocks texture. The principal minerals are plagioclase, khaki hornblende are typically highly complex. These complexities are commonly at- (partly altered to a pale, blue-green amphibole), interstitial quartz tributed to the presence of older (inherited) components and meta- masses, and distorted plates of orange-brown biotite. Plagioclase is the morphic growth domains, along with zircon crystallised from the ig- dominant mineral and is commonly strongly altered to sericite, but neous protolith to the gneiss, where all components may exhibit some fresh grains persist, euhedral against quartz. Using the Michel- evidence for one or more episodes of Pb loss (e.g. Corfu et al., 2003; Levy method, the average plagioclase composition is estimated at about Whitehouse et al., 1999; Nutman et al., 2001). In cases of complete 207 206 An40. Small veins of white mica and albite cut through the rock. ancient Pb loss, this leads to concordant analyses with Pb/ Pb The zircon in this rock was mainly retrieved as fragments of larger dates equivalent to the Pb loss event. If, however, the Pb loss was

15 A. Petersson, et al. Chemical Geology 551 (2020) 119757 ancient and partial, analyses can disperse along concordia and lack Webber gabbros and leucogabbros from the Shaw Granitic Complex specific geological significance. (Fig. 4a). This interpretation is based on two independent lines of evi- The interpretation of such datasets is commonly ambiguous, and so dence. Firstly, the ~3.58 Ga analyses are obtained from both individual it is necessary at the outset to establish some criteria on which inter- oscillatory zoned grains (e.g. #28 in Fig. 4a) and broad oscillatory pretations can be based. We consider that tight clusters defined by zoned rims that mantle the older cores and truncate the zoning in these multiple zircon analyses on a U-Pb concordia diagram are most likely to (e.g. #17 in Fig. 4a). This suggests that zircon of this age in the gneiss be geologically meaningful and to represent either igneous crystal- represents a discrete episode of igneous crystallisation, rather than re- lisation or a post-crystallisation Pb-loss event. Analyses that yield sulting from Pb loss from the ~3.65 Ga cores. We note that a ~ 3.58 Ga 207Pb/206Pb dates that are older than, and do not correspond to, any crystallisation age is also inferred for the lithologically similar tonalite cluster of data, and/or with Th/U ratios that are clearly higher or lower gneiss, 12TKPB06, sampled ~120 m to the northeast. The second line of than the main age populations, are interpreted as reflecting xenocrystic evidence concerns a re-evaluation of the Hf isotope systematics. Kemp zircon. Th/U ratios are, however, not used to define ‘metamorphic’ et al. (2015) suggested that the spread of zircon U-Pb data along con- versus igneous protolith zircon, as this discriminant cannot reliably be cordia between ~3.66–3.46 Ga in GSWA142870 represents the effects applied to ancient gneissic rocks (Whitehouse and Kamber, 2005). of variable ancient Pb loss from the ~3.66 Ga zircon component – an Zircon 207Pb/206Pb dates are also interpreted below in conjunction with interpretation that relied on the very similar 176Hf/177Hf ratios of these the physical characteristics of the analysed domains revealed by optical grains, which disperse along a Pb loss trend. However, the additional examination and cathodoluminescence imaging, following the ap- data reported by the present study allows a slight difference in the proach of Whitehouse et al. (1999). 176Hf/177Hf of the ~3.58 Ga and 3.65 Ga zircon age groups to be re- Based on this treatment, we can subdivide the results from the nine solved. The weighted average 176Hf/177Hf of the 3.58 Ga Pilbara samples into those that are relatively simple, with a single (0.280441 ± 5, n = 13, MSWD = 0.5) and the 3.65 Ga tightly clustered data population on a concordia diagram, and those (0.280426 ± 6, n = 12, MSWD = 0.8) groups fall outside of 95% that show variations suggesting a complex thermal history. These are confidence limits (Fig. 6). Evolution from the mean 176Hf/177Hf of the treated separately below, and summarised in Table 1. 3.65 Ga group to the mean 176Hf/177Hf of the 3.58 Ga group would correspond to an ‘upper crustal’ 176Lu/177Hf of 0.008 (orange arrow in 6.1.1. ‘Simple’ zircon U-Pb results Fig. 6), which is higher than the value of zircon in the rock (~0.001) Five of the nine analysed samples fall into this category, including and cannot be explained by Pb loss. the layered gneiss (12TKPB17) and tonalitic gneiss (15TKPB12) from On balance, we therefore favour igneous crystallisation of the to- the Muccan Granitic Complex, the quartz diorite gneiss from the nalitic precursor to GSWA142870 at ~3.58 Ga. The ~3.65 Ga rounded Carlindi Granitic Complex (15TKPB19), and the North Shaw tonalite cores are interpreted to reflect the presence of inherited components in (15TKPB20). We also include granodiorite gneiss 18APP05 from the the 3.58 Ga magma (Fig. 4a). Formation of the ~3.42 Ga age group is southern Muccan Granitic Complex, although the U-Pb data distribu- considered to define the age of high grade metamorphism and partial tion here is slightly more complex. In these five cases, the weighted melting of the tonalitic protolith, i.e., the time that the gneissic layering average 207Pb/206Pb date of the concordant data cluster is interpreted developed in the rock. The chondritic Hf isotope signatures of the to represent magmatic crystallisation of the igneous precursor to these 3.42 Ga zircon component preclude an origin by Pb loss from the older rocks. Three of these samples have igneous crystallisation ages that are zircons or in-situ melting and equilibration with the host rock (i.e. 3.5 Ga or older, making these amongst the oldest plutonic rocks re- closed system reworking), but requires an additional contribution from cognised in the Pilbara Craton. a relatively radiogenic, externally-derived melt phase, perhaps re- It is necessary to comment on the discrepancy between the zircon U- presented by thin granitic layers in the sample. Pb age of the granodiorite gneiss from the south Muccan Granitic The relatively simple zircon U-Pb distribution in the other Complex determined by the present study (sample 18APPB05: Warrawagine tonalitic gneiss sample (12TKPB06) suggests that the ig- 3.523 ± 0.002 Ga) and that inferred by Wiemer et al. (2018) neous protolith of this rock crystallised during the Mount Webber (3.576 ± 0.022 Ga). Both samples were collected within a few metres event, at 3.580 ± 0.003 Ga. Grain −05, with a 207Pb/206Pb date older of each other from boulders in a small outcrop area of reasonably than the main cluster at 3.619 ± 0.001 Ga, is interpreted to be xe- uniform granodioritic gneiss. The whole-rock geochemistry of the nocrystic (Fig. 4b, Supp. Table S1). This grain may have derived from samples is very similar (Table S5), and thus we regard our sample to be the same source as the > 3.6 Ga population recognised in equivalent to that of sample 309–2ofWiemer et al. (2018). The age GSWA142870, and provides further evidence for the existence of > quoted by Wiemer et al. (2018) was derived from an upper concordia 3.6 Ga crust within the Warrawagine Granitic Complex. intercept of largely discordant U-Pb data, but it is notable that the As discussed above (Section 5.3), zircon crystals within the Warra- weighted average 207Pb/206Pb date of the < 5% discordant analyses wagine amphibolitic gneiss 12TKPB12 show indications of ancient Pb from this dataset is 3.525 ± 0.020 Ga (n = 8; MSWD = 0.7). This loss, and therefore only the main cluster of the thirteen analyses with agrees with the result determined by the present study. It is also no- the oldest 207Pb/206Pb dates are used to define the 3.425 ± 0.002 Ga teworthy that the weighted average 207Pb/206Pb date of the seven least age of zircon crystallisation within the rock. The zircons from this discordant > 3.4 Ga zircon analyses from the trondhjemitic gneiss sample, having the lowest Th/U in the study with a mean of 0.29, and sample reported by Wiemer et al. (2018), from ~400 m south of the being hosted by banded amphibolitic gneiss, are considered to re- granodiorite gneiss outcrop, is 3.517 ± 0.019 Ga. We therefore con- present metamorphic growth. The analysed zircon core domains are CL- sider that the best estimate of the igneous protolith age of gneissic rocks dark and weakly zoned to unzoned, characteristics that are consistent from the southern Muccan Granitic Complex is ~3.52 Ga. with zircon formed by metamorphic processes (Corfu et al., 2003). Grain −15, with a 3.457 ± 0.002 Ga 207Pb/206Pb date, obtained from 6.1.2. ‘Complex’ zircon U-Pb results a microstructurally-similar CL-dark core, is clearly older than the main Samples in this category are the three rocks from the Warrawagine data cluster. This grain could be indicative of the crystallisation age of Granitic Complex, and the tonalitic gneiss 12TKPB16 from the northern the igneous precursor to the amphibolite (Fig. 4c, Supp. Table S1). The Muccan Granitic Complex. origin of this amphibolitic gneiss is further discussed below in Section The deconvolution of the zircon U-Pb data in tonalitic gneiss sample 6.4. GSWA142870 is not straightforward (see discussion in Kemp et al., The north Muccan tonalitic gneiss 12TKPB16 contains two main 2015). We here suggest that the igneous protolith to this gneiss crys- zircon age populations, at ~3.5 Ga and 3.3 Ga. As noted in Section tallised at 3.577 ± 0.002 Ga, contemporaneous with the Mount 5.4.2 the 3.3 Ga age group is defined by smaller individual zircon grains

16 A. Petersson, et al. Chemical Geology 551 (2020) 119757 and weakly zoned rims, while the older ~3.5 Ga population is recorded 3.503 ± 0.014 Ga;); and (3) the granodiorite gneiss 18APPB05, also in in larger, unzoned, rounded cores (Fig. 4e). Considering these micro- the southern Muccan Granitic Complex (3.523 ± 0.002 Ga). We note structural relationships, and given that the rock forms a distinct band in addition to this the occurrence of a ~ 3.5 Ga inherited zircon po- enclosed by 3.43 Ga layered gneiss (i.e., sample 12TKPB17) we cau- pulation in tonalitic gneiss 12TKPB16, occurring in the northernmost tiously attribute the ~3.5 Ga population to an inherited zircon com- parts of the Muccan Granitic Complex, as further evidence for a wide- ponent and consider that the younger 3.312 ± 0.004 Ga date re- spread magmatic event at this time. presents the igneous crystallisation age of the tonalitic protolith In view of this, we propose the name Mulgundoona Supersuite for (Fig. 4d, Supp. Table S1). The zircon crystal that returned a 207Pb/206Pb 3.53–3.49 Ga granitic rocks within the Pilbara Craton (Fig. 2), due to date of 3.583 ± 0.002 Ga (analysis n5784–8) has a Th/U ratio of 1.1, the proximity of tonalitic gneiss sample 15TKPB12 to Mulgundoona Hill higher than all other concordant analyses (Th/U:0–0.9) and is probably in the Muccan Granitic Complex. The identification of this episode fills xenocrystic. a gap in the plutonic history of the East Pilbara Terrane and indicates a more extensive intra-crustal differentiation and development of granitic 6.2. Updating the earliest evolution of the east Pilbara Craton crust at 3.5 Ga in the East Pilbara Terrane than is implied by the thin felsic eruptive units in the greenstone belts. The above results confirm that the areas considered on geological grounds as likely to contain the oldest rocks in the east Pilbara Craton 6.3. O-Hf isotope constraints on magma sources in the Pilbara Craton do so. The ages recovered overlap those of the basal Pilbara Supergroup and extend back in time to the realm of the Mount Webber gabbros, as Gardiner et al. (2017) presented zircon Lu-Hf data from four dif- summarised in Table 1. Several new occurrences of ≥3.5 Ga gneisses ferent ~3.45–2.83 Ga supersuites of the Mount Edgar Granitic Complex are identified, which provide evidence for formerly unknown igneous of the East Pilbara Terrane that showed a large spread in εHf(t) between episodes in the East Pilbara Terrane. This requires a revised chronology strongly positive and strongly negative values (Fig. 7). They interpreted for the earliest felsic magmatism within the craton. these data to represent both a MORB-like depleted mantle and older crustal components in the source to the Mount Edgar Granitic Complex. 6.2.1. Reappraising the Mount Webber magmatic event Such Hf isotope variation was not, however, noted in the solution and The ~3.58 Ga protolith age for the two tonalitic gneiss enclaves laser ablation zircon Hf isotope datasets reported by Kemp et al. (2017) (12TKPB06 and GSWA142870) confirms the existence of > 3.53 Ga from the slightly older (3.47 Ga, Stern et al., 2009) Owens Gully intrusive rocks in the Warrawagine Granitic Complex, as suggested by Diorite, which is part of the Callina Supersuite and occurs near the previous studies (Nelson, 1999a; Kemp et al., 2015). The inferred em- southwestern margin of the Mount Edgar Granitic Complex. A mean εHf placement age of the protoliths to these gneisses is coeval with the (3.467 Ga) value of +0.7 ± 0.6 (2 SD) was established for this rock. 3.59–3.58 Ga Mount Webber gabbros from the Shaw Granitic Complex A striking feature of the new Hf isotopic data of the present study is (Petersson et al., 2019a), hence broadening the geographical extent of its relative uniformity (Fig. 7). Besides the Warrawagine amphibolitic this magmatic episode across the eastern Pilbara Craton (Fig. 3). gneiss 12TKPB12, discussed further below, zircon grains of all samples The new data reported here, combined with a re-assessment of the yield consistently chondritic Hf isotope compositions from ~3.65 Ga to U-Pb data of Wiemer et al. (2018), do not confirm the occurrence of 3.3 Ga, and show limited to negligible internal variability (Fig. 7). Mount Webber aged igneous intrusions in the southern Muccan Granitic These data conform to the near-chondritic Hf isotope signatures re- Complex. However, the inherited concordant grain (207Pb/206Pb date of ported by Kemp et al. (2017), to the oldest dated component of the 3.58 Ga) in the tonalitic gneiss 12TKPB16 hints at a Mount Webber Mount Edgar and Shaw Granitic Complexes (Petersson et al., 2019a), as aged component in the northern part of the Muccan Granitic Complex well as to the > 3.5 Ga inherited and detrital zircon data of Kemp et al. (Fig. 3). The full areal extent of the Mount Webber aged igneous rocks (2015). Petersson et al. (2019a) noted that means of the more scattered remains to be established, but products of this magmatic episode clearly data of Gardiner et al. (2017) are also similar to CHUR, within un- extend beyond the Shaw Granitic Complex. certainty. Rocks with near-chondritic mean zircon Hf therefore extend over large parts of the East Pilbara Terrane, and span the first 350 6.2.2. Evidence for a 3.5 Ga granitic supersuite million years of the magmatic history of the craton (Fig. 8). As first noted by Williams and Collins (1990) (and see Van Considering the oxygen isotope data, the majority of zircon δ18O Kranendonk et al., 2006), a striking aspect of the East Pilbara Terrane is values, excluding a few altered grains with sub-mantle oxygen isotope that felsic volcanic units in greenstone belts have coeval plutonic signatures, plot within, or close to, the mantle field as defined by Valley counterparts in the adjacent granitic complexes, suggesting a shared et al. (1994, 2005) (Fig. 5). This is also the case for the Owens Gully magmatic history. One exception to this has been the 3.52–3.50 Ga Diorite of the Mount Edgar Granitic Complex, which has zircon δ18Oof Coucal Formation of the Coonterunah Subgroup, rhyolitic and volca- 5.9‰ (Petersson et al., 2019a). Slightly elevated zircon δ18O values are niclastic units of which occur in the East Strelley Greenstone Belt at the shown by the 3.65, 3.58 and 3.41 Ga zircon populations in the War- southern margin of the Carlindi Granitic Complex (Buick et al., 1995). rawagine tonalitic gneiss enclave (GSWA142870) and the North Shaw A contemporaneous intrusive suite has not yet been identified for this tonalite (15TKPB20), whereas zircon grains of the Warrawagine am- eruptive unit, even though the existence of this has been suggested by phibolitic gneiss show distinctly heavier oxygen (Fig. 5). abundant detrital and xenocrystic zircon grains of around 3.53–3.49 Ga Collectively, the zircon O-Hf isotope evidence suggests that the in several different greenstone belts and granitic complexes (e.g., parental magmas to the oldest rocks and zircon crystals in the Pilbara Nelson, 1999b; Green, 2001; Smithies et al., 2002; McNaughton et al., Craton were extracted from a source reservoir of chondritic (for Lu-Hf) 1988; Zegers et al., 2001; Nelson, 2001; Williams and Collins, 1990; composition, and were not significantly influenced by the reworking of Wingate et al., 2009). 18O-enriched supracrustal rocks. These data could be reconciled by The new data presented by this study enables the first definition of a crystallisation of melts from chondritic mantle, as would be appropriate time-equivalent plutonic episode to the Coucal Formation. This is re- for the Mount Webber gabbros and leucogabbros of the Shaw Granitic presented by: (1) the 3.501 ± 0.001 Ga quartz dioritic gneiss Complex (Petersson et al., 2019a; but see further discussion below), and 15TKPB19, which is the oldest igneous sample dated in the Carlindi perhaps the Carlindi quartz diorite (15TKPB19), or the remelting of Granitic Complex; (2) sample 15TKPB12 from the Muccan Granitic young, unweathered mafic crust, as might be appropriate for the to- Complex (3.502 ± 0.003 Ga), which is equivalent in age to a quartz nalitic to granodioritic rocks (e.g. Smithies et al., 2009; Wiemer et al., dioritic gneiss reported by Allen et al. (2016) and Wiemer et al. (2018) 2017). There is no requirement in the Hf isotope data presented here, or from the southwestern margin of the Muccan Granitic Complex (i.e., in the datasets of Amelin et al. (1999), Guitreau et al. (2012), Kemp

17 A. Petersson, et al. Chemical Geology 551 (2020) 119757 et al. (2017) or Petersson et al. (2019a, 2019b), for input from a reported here, as well as the data of Amelin et al. (1999), Guitreau et al. strongly depleted mantle source, or from substantially older crust, at (2012), Kemp et al. (2017) and Petersson et al. (2019a, 2019b), none of least before 3.3 Ga. The mantle-like zircon δ18O of some dioritic to which supports the remelting of ancient (> 3.6 Ga) felsic crustal pre- tonalitic rocks does not preclude a role for hydrous fluid in the gen- cursors. Rather, these data suggest juvenile growth of the Pilbara eration of the silicic melts, only that the 18O/16O ratio of this was not Craton from ~3.65 to 3.3 Ga, where crust was extracted episodically substantially different from that of the mantle, or that this component from chondritic mantle. was volumetrically insufficient to shift the δ18O of the bulk magma. The subtly elevated zircon δ18O of the North Shaw tonalite, and the War- 6.5. The ‘birth’ of an Archean granite-greenstone terrane rawagine tonalitic gneiss does require the participation of an 18O-enriched component, although the identity of this is not well constrained The formation of the distinctive granite-greenstone style of geology by the present data. The chondritic Hf of these slightly higher δ18O that characterises a number of Archean cratons has been long debated zircons suggests that the supracrustal component was not significantly (Macgregor, 1951; Anhaeusser et al., 1969; Kröner, 1977; Hickman, older than the time of magma generation. 1983, 1984; Kusky and Polat, 1999; Bédard et al., 2003; Cawood et al., 2006; Van Kranendonk, 2010; Hansen, 2015). Understanding the de- 6.4. Continental basement and its role in craton evolution velopment of this cratonic architecture may hold the key to unravelling the processes of ancient continental growth. As mentioned above, a number of studies have suggested that A number of studies employ geodynamic modelling to explore the the < 3.53 Ga volcano-sedimentary sequences of the Pilbara formation and evolution of Archean granite-greenstone terranes. Supergroup accumulated on an older continental substrate. Although Johnson et al. (2014) modelled a two-dimensional scenario where, at the ambiguities of model crust formation ages are well known (e.g. higher mantle potential temperatures (> 1500 °C), a thick primary Arndt and Goldstein, 1987; Vervoort and Kemp, 2016), the recognition MgO-rich crust would be gravitationally unstable, leading to dense of 3.75–3.55 Ga inherited and detrital zircons (Thorpe et al., 1992; crust ‘dripping’ down into the mantle, stimulating compensatory asth- Kemp et al., 2015; Petersson et al., 2019b), and 3.59 Ga igneous rocks enospheric mantle melting, and the generation of additional primary (this study, and Petersson et al., 2019a) do provide direct evidence for mafic to ultramafic crust. Silicic melts were subsequently generated by pre-Pilbara Supergroup crust within the East Pilbara Terrane. melting of the fractionated products of these primary magmas near the In this respect, it is instructive to return to the Warrawagine am- hot base of overthickened crust. Fischer and Gerya (2016) used high- phibolitic gneiss, 12TKPB12. The ~3.43 Ga zircon grains of this sample resolution 3D thermo-mechanical modelling under Archean (hotter) are distinctive in having clearly sub-chondritic Hf and a heavy O iso- mantle potential temperatures to explore crust-mantle evolution during tope composition. In igneous rocks, such zircon isotope signatures mantle plume-induced tectonic scenarios, with a range of different would be compatible with the reworking of supracrustal materials that crustal starting compositions and structures. Their results support a have exchanged oxygen at low temperature with the hydrosphere. An long and slow initial growth and crustal thickening phase, where a alternative, post-crystallisation alteration of the zircon O isotopic felsic nucleus is formed during the first few million years, and a second composition, is deemed improbable as there is no correlation between ‘catastrophic’ phase involving outpouring of mafic flood volcanism on Th/U and δ18O, or U-Pb discordance and δ18O, in any of the samples in top of the felsic nuclei, with emplacement of coeval plutonic rocks at this study (see Petersson et al., 2015; Rubatto and Angiboust, 2015). depth in the crust. This second phase is triggered by foundering of The amphibolite is, however, a highly metamorphosed layered gneiss, eclogitic roots, enhancing mantle convection, upwelling and melting, and the microstructures of the zircon grains are consistent with solid- and initiating voluminous eruption of basaltic and komatiitic lavas, state growth. Thus, we consider that the zircon grains formed during further discussed below. The resulting crustal stratification, in turn, metamorphic recrystallisation of an older, altered basaltic rock at encourages rapid gravitational overturn in response to Rayleigh–Taylor ~3.43 Ga, and inherited the isotopic characteristics of this supracrustal instabilities, a process considered capable of forming the typical dome- protolith. The actual age and affinity of the basaltic protolith is enig- and-basin structures seen today in both the East Pilbara Terrane and the matic, although we note that recalculating the Hf isotope composition Kaapvaal Craton (Fischer and Gerya, 2016). at 3.53 Ga, the age of the oldest supracrustal rocks in the East Pilbara Any model for the early formation of the Pilbara Craton must ac-

Terrane, yields an εHf value of −1.8 (based on the Lu/Hf of the whole- count for the following features: (1) the presence of a 3.75 Ga con- rock). This seems too unradiogenic for the Warrawoona Group basaltic tinental nucleus (Petersson et al., 2019b); (2) a widespread magmatic rocks (Nebel et al., 2014). The εHf value is, however, chondritic at event at 3.59 Ga that involved the emplacement of mantle-derived ~3.65 Ga, the age of the oldest zircon population within the Warra- gabbros and felsic igneous rocks (Petersson et al., 2019a, this study); (3) wagine tonalitic gneisses, which also have chondritic Hf isotope ratios a massive outpouring of basaltic and komatiitic lavas, with coeval in- (Kemp et al., 2015). The Warrawagine amphibolitic gneiss therefore trusion of felsic magmas in plutonic complexes, commencing at appears to represent a fragment of older crust entrained from depth by 3.53 Ga; and (4) a subsequent evolution that involved episodic and younger granitic magmas, forming another line of evidence for an an- protracted mantle-derived and felsic magmatism, and development of cient nucleus to the East Pilbara Terrane - although this crust was the dome and keel architecture. evidently not a significant melt source for the igneous precursors of the To accommodate these observations, we envisage a model of the gneisses analysed in this study, given the absence of sub-chondritic earliest crustal growth in the Pilbara Craton that incorporates elements zircon Hf. of the Johnson et al. (2014) and Fischer and Gerya (2016) simulations, Despite the accumulating evidence for Eoarchean crust beneath the and builds on the scenarios proposed by a number of authors for Ar- East Pilbara Terrane, the role of such a basement in the subsequent chean craton evolution (e.g. Collins et al., 1998; Bédard, 2006; evolution of the craton remains unclear. Gardiner et al. (2017) sug- Champion and Smithies, 2007; Smithies et al., 2009; Van Kranendonk gested that the > 3.53 Ga crust in the Pilbara Craton was reworked et al., 2015)(Fig. 9). At the outset, this model involves formation of a during younger magmatic episodes and provided a melt source to the thickened volcanic plateau, stabilised via an ancient continental nu- Paleoarchean granitic supersuites. Petersson et al. (2019b) suggested a cleus (Petersson et al., 2019b) above a zone of upwelling mantle more passive role for an older, but volumetrically limited, crustal nu- (Fig. 9a). Incipient rifting of the thickened protocrust at ~3.59 Ga and cleus in facilitating the survival of juvenile lithosphere formed above decompression melting of the underlying mantle generated the Mount zones of upwelling mantle, but not participating in younger magmatic Webber gabbros and leucogabbros in the northwestern Shaw Granitic events. The latter interpretation is most consistent with the new Hf Complex. We infer that melting of juvenile mafic crust at depth asso- isotope data for the oldest igneous components of the Pilbara Craton ciated with this mantle-derived plutonism also generated the first TTG

18 A. Petersson, et al. Chemical Geology 551 (2020) 119757

Fig. 9. A simplified cartoon summarising a model for the earliest crust formation in the Pilbara Craton. (A) incipient rifting of an oceanic plateau, including the ~3.75 Ga ancient nucleus inferred by inherited zircons from the North Pole Dome (Petersson et al., 2019b), caused by mantle upwelling at > 3.59 Ga; (B) De- compression melting of the mantle is initiated and generates the mantle derived rocks of the Mount Webber event; infracrustal melting produces TTGs, and dense eclogite residues start to drip; (C) The descending eclogite residues trigger more extensive melting in a zone of upwelling mantle (e.g., Bédard, 2018), initiating a ‘catastrophic’ phase of komatiitic and basaltic volcanism. The 3.53–3.50 Ga Coonterunah Subgroup is emplaced and infracrustal melting generates the 3.53–3.49 Ga Mulgundoona Supersuite (the intrusive pair to the extrusive felsic Coucal Formation of the Coonterunah Subgroup); (D) Gravitational overturn initiated by Ray- leigh–Taylor instabilities (e.g., Johnson et al., 2014) forms the typical dome-and-keel structures of the Pilbara Craton. The felsic intrusive Callina Supersuite and its extrusive pair, the Duffer Formation, is emplaced. magmas (e.g., the ~3.58 Ga tonalitic gneisses in the Warrawagine focussed, intense zone of mantle upwelling (see Davies, 2008), analo- Granitic Complex) and gave rise to a differentiated crustal structure. gous to the model forwarded by Bédard (2006), inducing a ‘cata- Retention of such melts at depth to form plutons (i.e., a low eruption strophic’ phase of mainly komatiitic and basaltic volcanism, manifest as efficiency) led to a steep geotherm, a warm and weak crust, and sub- the ~3.53–3.50 Ga Coonterunah Subgroup. Simultaneously, infra- sequently to detachment of the lower lithosphere (Rozel et al., 2017). crustal melting generated granitic plutons of the 3.53–3.49 Ga Mul- Hence, this ~3.59–3.58 Ga magmatic activity could potentially have gundoona Supersuite (Fig. 9c), leading to a differentiated and grav- left a complementary, residual eclogite root that started to founder into itationally-unstable crustal configuration. From ~3.50 Ga, intra-crustal the mantle at this time (Fig. 9b). Chemical and thermal heterogeneities overturn is initiated due to these density-driven instabilities (e.g. between the upper mantle and a delaminating eclogitic (garnetiferous) Johnson et al., 2014) and the archetypical ‘dome-and-keel’ structures root are potential fertile spots that eventually warm up past the melting start to take form, where the domical granitic complexes are flanked by point (Anderson, 2006). Delamination of the lower crust leads to in- sinking volcanic and sedimentary ‘greenstones’. This was magmatically creased mantle convection and, consequently, more subcrustal partial a very active period with the emplacement of the 3.49–3.46 Ga Callina melting (Johnson et al., 2014; Fischer and Gerya, 2016). An increase in Supersuite and eruption of the coeval Duffer Formation felsic volcanic convection accelerates foundering of eclogitic lower crust, and leads to sequences (Fig. 9d). In this model, the isotopic signature of the in- faster rise of the upward mantle return flow, an interplay that generates itial > 3.59 Ga basaltic protocrust is overwhelmed by the later juvenile buoyant mantle diapirs that undergo further adiabatic decompression additions, however the Warrawagine amphibolitic gneiss, with its un- and melting as they ascend (Fischer and Gerya, 2016). Decompression radiogenic zircon Hf and supracrustal δ18O, could represent a relict of upwelling mantle that is already near its melting point is a very ef- fragment of this ancient protocrust. fective way of producing large volumes of melt, as required to explain The scenario outlined above supports the timeline of magmatic LIP (Large Igneous Provinces) volcanism (Anderson, 2006). activity in both the Pilbara and Kaapvaal cratons, and explains the We therefore posit that coexisting mantle domains of contrasting apparent magmatic lull between ~3.59 and ~3.53 Ga in the east temperatures accentuated by the sinking eclogite roots triggered a Pilbara Craton.

19 A. Petersson, et al. Chemical Geology 551 (2020) 119757

7. Towards a working model for Eo- and Paleoarchean crust- As emphasised by Fisher and Vervoort (2018), Petersson et al. mantle evolution (2019a) and Kemp et al. (2019), these global datasets provide scant evidence for input from a strongly depleted mantle source with super- 7.1. Questions of scale and representativeness chondritic Lu/Hf into Archean continental crust. We now speculate about the significance of this observation, and build a working model Terrane scale studies provide insight into the geological processes for the isotopic evolution of the early Archaean mantle. Although the by which segments of ancient cratons were generated and evolved, but early evolution of Earth's mantle is enigmatic, evidence cited in support a key question concerns the extent to which such studies relate to, and for the existence of an incompatible element depleted reservoir comes can constrain, global-scale models for continental growth. Given the from 142Nd excesses in Eoarchean rocks, which nonetheless appear to fragmentary, and possibly biased, preservation of Archean crust, cau- be restricted to small areas in Greenland (e.g., Bennett et al., 2007) and tion is required in attempting to extrapolate findings from individual North China (Li et al., 2017), and, more controversially, super- terranes more broadly. Notwithstanding this, a critical first step in- chondritic 143Nd/144Nd and 176Hf/177Hf ratios in Archean mafic ig- volves the identification of the best geological archives from which neous rocks of mantle derivation (e.g., Puchtel et al., 2013; Nebel et al., robust inferences can be derived, and then pieced together at a larger 2014; Blichert-Toft et al., 2015). The size and origin of such a putative scale. For this reason, the exposed northern portion (60,000 km2) of the depleted reservoir, and its role in continent formation, are unresolved. Pilbara Craton has received considerable attention due to excellent It has to be borne in mind that during the Hadean to Paleoarchean state of preservation, minimal modification of cratonic architecture by the difference in Hf isotopic composition between CHUR and a depleted younger deformation, and the longevity of the thermal and magmatic mantle reservoir may be small and not easily resolved. The depleted history (750 Ma). Moreover, there is evidence that the Pilbara Craton mantle parameters of Blichert-Toft and Puchtel (2010), based on was originally part of a much larger cratonic entity, which included the modern MORB-source mantle and extrapolated to 4.56Ga, yield a dif-

Kaapvaal Craton (Cheney, 1996; Zegers et al., 1998; Byerly et al., 2002; ference between depleted mantle and CHUR of ~4.4 εHf units at 3.5 Ga, Smirnov et al., 2013). This correlation is reinforced by the results of the becoming smaller with increased age. This diﬀerence between chon- present study. Besides ~3.75 Ga inherited zircons (Petersson et al., dritic and depleted mantle evolution almost lies within the realm of 2019b), the oldest known components within the Pilbara Craton are analytical uncertainties for laser ablation techniques, which may 3.66 Ga zircon grains (Kemp et al., 2015; Sheppard et al., 2017; achieve reproducibility of ± 1.5 ε units (at 2SD) in REE-rich zircon Kiyokawa et al., 2019; this study). The next recorded episodes of plu- (Fisher and Vervoort, 2018). Nonetheless, if a depleted mantle com- tonic magmatism include rocks of the 3.59–3.58 Ga Mount Webber ponent was sampled, data points plotting 1.5 εHf units above a nominal event and the ~3.5 Ga Mulgundoona Supersuite. This temporal barcode depleted mantle curve in εHf versus time-space would be statistically compares well with the oldest igneous components in the Kaapvaal expected. Such resolvable super-chondritic data points are exceedingly Craton, which has zircon age populations at 3.64, 3.58 and 3.50 Ga rare in global datasets from > 3.5 Ga zircons. On Fig. 10, only 2.2% of

(Compston and Kröner, 1988). Notably, the oldest felsic volcanic rocks the data from > 3.5 Ga zircons (20 out of 894 points) yield εHf values from the Barberton Greenstone Belt yield mean εHf(3.53–3.46 Ga) values of above +1.8, that is, above the expected variation of a sample depicting −0.06 ± 0.15 (n = 84, MSWD = 3, Kröner et al., 2013), in line with a chondritic composition (0.3 units uncertainty of CHUR composition Hf isotope signatures from coeval rocks in the Pilbara Craton. The si- and 1.5 units of variability due to analytical reproducibility). Of course, milarities between these two large cratonic blocks provides some jus- this will be different if less extreme mantle depletion models are used, tification as to why the findings reported here for the Pilbara Craton such as that of Vervoort et al. (2015), where super-chondritic isotope may have wider importance for Archean crustal evolution, as discussed compositions diverging from CHUR are only developed after 3.8 Ga. below. In this context, it is pertinent to highlight the uncertainty in the isotope evolution of the early Archean mantle, and the complexities and 7.2. A globally chondritic (for Hf) mantle source for Eoarchean continental relevance of the datasets that have been used to constrain this. For crust example, Puchtel et al. (2013) establish mean εHf values of +1.9 for 3.48 Ga komatiites from the Barberton Greenstone Belt, which can A major finding of this study is that zircon crystals of well-con- reasonably be attributed to reflect the isotope composition of source strained age from the oldest igneous rocks of the Pilbara Craton have peridotite with a time-integrated history of depletion. On the other broadly chondritic Hf isotope compositions. This outcome is by no hand, Blichert-Toft et al. (2015) report a range of εHf values for ko- means endemic to the Pilbara-Kaapvaal cratons, but aligns well with matiites from the same terrane that reach +10; some of the variation zircon Lu-Hf data from Eoarchean meta-igneous rocks on a number of reflects isotope disturbance, but values of around +5 are obtained for different continents, which also yield dominantly chondritic to near samples preserving igneous mineralogy (Blichert-Toft et al., 2015). chondritic signatures (e.g. Kemp et al., 2015; Hiess and Bennett, 2016; Whole rock Hf isotope analysis of komatiites and basalts from the Pil-

Fisher and Vervoort, 2018, Fig. 10). Chondritic zircon initial bara Craton return strongly superchondritic εHf values of +6 at 3.52 Ga 176 177 Hf/ Hf ratios have been obtained from a variety of rock types, and +8 at 3.46 Ga (Nebel et al., 2014). Even higher εHf values ex- ranging from anorthosites and TTG gneisses to granites, from Archean ceeding +10 are reported for 3.72 Ga boninitic metabasalts from the terranes in India (Kaur et al., 2016; Pandey et al., 2019), Greenland Isua Greenstone Belt, Greenland (Hoffmann et al., 2010). Whether ko- (Kemp et al., 2009; Hiess et al., 2009; Hoffmann et al., 2011; Rizo et al., matiites provide meaningful information about the isotope composition 2011; Næraa et al., 2012; Hiess and Bennett, 2016; Fisher and Vervoort, and depletion history of the convecting mantle, as opposed to simply 2018; Kemp et al., 2019), Antarctica (Hiess and Bennett, 2016), Aus- reflecting cratonic peridotite mineralogy, has, however, been called tralia (Amelin et al., 2000; Guitreau et al., 2012; Kemp et al., 2015; into question (see discussion in Kamber and Tomlinson, 2019). The Hiess and Bennett, 2016; Petersson et al., 2019a, 2019b), the Superior relevance of the mantle source to komatiite magmas for continental Craton (Satkoski et al., 2013), Acasta Gneiss Complex of Canada growth is also unclear, particularly as the trace element systematics of (Amelin et al., 2000; Iizuka et al., 2009; Reimink et al., 2016; Bauer Archean felsic igneous rocks call for melting of a relatively enriched et al., 2017), the Ancient Gneiss Complex of South Africa (Zeh et al., basaltic precursor (Bédard, 2006, and see Smithies et al., 2009). These 2011) and the Tarim Craton of north-western China (Ge et al., 2018, observations underscore the complexities of determining what is a

Fig. 10). Running means of zircon εHf values of meta-igneous rocks from realistic model for the evolution of a mantle reservoir in the Eoarchean 3.9 to 3.0 Ga, computed at 100 Ma intervals, straddle CHUR (Fig. 11, that can meaningfully be compared with zircon Hf isotope datasets and see also Guitreau et al., 2012). We consider this striking isotopic from Archean felsic igneous rocks. consistency to be a robust signal of Archean continent formation. Given the foregoing, and the pronounced clustering of zircon εHf

20 A. Petersson, et al. Chemical Geology 551 (2020) 119757

Fig. 10. εHf versus crystallisation ages (in Ga), showing zircon data from Paleo–Eoarchean magmatic provinces from around the world. Note how the majority of data clusters around CHUR. Yellow ﬁeld denotes an assumed depleted mantle (from

Blichert-Toft and Puchtel, 2010) at ± 1.5 εHf-units.

Grey ﬁeld denotes the εHf(t) = 0 ± 0.4 CHUR (Chondritic Uniform Reservoir) value deﬁned by

Bouvier et al. (2008), plus 1.5 εHf-units analytical uncertainty. (For interpretation of the references to colour in this ﬁgure legend, the reader is referred to the web version of this article.)

values around zero, we support a simple hypothesis whereby juvenile mantle source for continental crust would not preclude the localised early Archean continental crust separated from a chondritic mantle development and survival of depleted mantle domains, only that these source (e.g., Guitreau et al., 2012) from 3.9 Ga and to at least 3.3 Ga in refractory materials were not a significant component in the mantle the Pilbara Craton. The most straightforward implication of this hy- source of new continental crust until some time after 3.5 Ga. The gra- pothesis is that the early Archean upper mantle was primitive and re- dual divergence to super-chondritic Hf isotope ratios in igneous rocks cords no isotopic evidence for prior melt extraction, as would be the from 3.6–3.5 Ga (e.g., Fig. 10 and see Fisher and Vervoort, 2018) may case for growth of limited volumes of Hadean to Eoarchaen continental signal the onset of progressive continental stabilisation, where the in- crust. A near chondritic Eoarchean mantle composition could, alter- compatible element depleted residues began to be entrained in zones of natively, have been maintained dynamically via extensive crustal re- hotter, upwelling mantle (e.g., Bédard, 2018), or because water-fluxed cycling and convective re-homogenisation of this crustal material and melting of refractory peridotite above foundering hydrous crustal slabs the complementary depleted residues, perhaps augmented by influx of and negative diapirs became viable. The picture emerging from zircon fertile lower mantle peridotite during episodic mantle overturns (e.g., Hf isotope studies of ancient igneous rocks is therefore one of incre- Bédard, 2018). The latter scenario would also imply that sequestration mental extraction and stabilisation of small volumes of continental of continental crust during this time period was insufficient to cause crust from the Eo- to Paleoarchean, similar to that proposed by Nägler long-term depletion in the upper mantle. Both chondritic mantle sce- and Kramers (1998). narios are compatible with the primitive mantle-like trace element The above interpretations of a chondritic mantle source for systematics of Archean basaltic rocks (Collerson and Kamber, 1999; Eoarchean continental crust readily explain the observed paucity of Bédard, 2018). As noted by Petersson et al. (2019a), a chondritic Hadean to early Archean rocks and mineral detritus. One caveat

Fig. 11. εHf(t) versus crystallisation ages (in Ga), showing 100 Ma running means of εHf(3.9–3.0 Ga). All means fall within error of CHUR. Data sources as for Fig. 10 (n = 1923).

21 A. Petersson, et al. Chemical Geology 551 (2020) 119757 concerns whether higher degrees of melting in a hotter Eoarchean upwelling zone, and to allow thickened mafic crust to form. The sub- mantle could have supressed fractionation of Lu/Hf ratios to the extent sequent rifting and collapse of this thickened protocrust attending that crust separation was not associated with formation of residual further mantle melting led to the emplacement of the 3.59–3.58 Ga peridotite with superchondritic Lu/Hf. In this case, the radiogenic igneous rocks of the Mt. Webber event, marking the first stabilisation of 176Hf/177Hf ratios as a legacy of crust extraction would not be devel- felsic crust associated with growth of the Pilbara Craton. Such rocks oped. Limited fractionation of Rb/Sr ratios between Archean mantle provided a substrate to facilitate the accumulation and stabilisation of and new continental crust was inferred by Dhuime et al. (2015). Trace voluminous eruptive products of the Warrawoona Group, commencing element modelling of suites of well-preserved Archean basalts, linked to with pronounced mantle upwelling from 3.53 Ga, which was also as- thermal models for upper mantle evolution, could be used to evaluate sociated with the emplacement of felsic magmas of the Mulgundoona this proposition. Mantle geodynamic modes must also be borne in mind, Supersuite into the deeper part of the crustal pile. Overturn of the re- as, for example the isotopic legacy of extraction of small volumes of sulting unstable configuration of dense basaltic crust overlying felsic continental crust would be more difficult to resolve in the case of whole middle crust, and accompanying infracrustal reworking of the foun- mantle convection as opposed to incremental crust removal from a dering basaltic piles, generated the felsic intrusive and extrusive rocks progressively depleting upper mantle reservoir (e.g., McCulloch and of the granitic complexes and flanking greenstone belts. This process Bennett, 1994). Irrespective of these uncertainties, a key finding was the critical step in stabilising the Pilbara Craton and led to the emerging from the present study is that the assumption of a linear distinctive dome and keel architecture. evolution of depleted mantle associated with production of Archaean In assessing the broader relevance of these results, we emphasise continental crust from 4.5 Ga to the present day is not required by the that the chondritic zircon Hf isotope compositions reported here for the existing data. This reinforces points made by several studies (e.g., Pilbara Craton are in line with zircon Hf isotope data from meta-ig- Kamber, 2015; Vervoort and Kemp, 2016; Bédard, 2018) that such a neous rocks of Eo- to Paleoarchean cratons on a number of other con- simple depleted mantle model cannot be used to reliably infer ancient tinents. The striking uniformity of these near chondritic Hf isotope periods of continental crust formation. signatures on a global scale suggest either that new continental crust Another clear consequence of the Hf isotope dataset reported here, separated from a primitive, undifferentiated mantle source from 3.9 to and reinforced by the results of Amelin et al. (1999), Guitreau et al. 3.3 Ga, leaving depleted mantle residues that were not re-sampled until (2012), Kemp et al. (2017) and Petersson et al. (2019a, 2019b), con- later in the Archean, or that the depleted signatures induced by limited cerns the lack of evidence for reworking of ancient crustal precursors by continent sequestration were largely erased by crustal recycling and igneous rocks in the Pilbara Craton, as would be evident in sub-chon- vigorous mantle convection, maintaining a near-chondritic mantle dritic 176Hf/177Hf. This finding does not, by itself, exclude the existence composition. Both scenarios are compatible with the prevalent primi- of stable continents in the Eoarchean, but suggests that the Pilbara tive mantle trace element composition of Archean basaltic rocks Craton developed remote from the isotopic influence of these, and/or (Bédard, 2018), although the latter situation would suggest that stabi- that any pre-existing continental material was small in volume. A si- lisation of continental crust prior to 3.5 Ga was not voluminous enough milar rationale could be extended to the other cratons noted above that to cause long-term depletion of the mantle source of new crust in in- have a prevalent chondritic Hf signature. compatible trace elements. If the mantle of the newly assembled Earth was chondritic for involatile elements, it is reasonable to propose that 8. Conclusions the history of the convecting chondritic mantle extends through the Hadean to the earliest stages of the Earth and is a fundamental aspect of Zircon U-Pb, oxygen, and hafnium isotope data reported here for the planet. The gradual slowing of convective activity into the Pa- nine gneissic samples of the Pilbara Craton provide new insights into leoarchean associated with mantle cooling may have contributed to the the earliest magmatic and growth history of this exceptionally well- initiation of a different tectonic regime that gave rise to the more vo- preserved section of Earth's Archean crust. Zircon age determinations of luminous continental crust preserved from that time. Newly identified a newly identified tonalite gneiss enclave (12TKPB06), and additional similarities between the initial growth phases of the Pilbara and zircon U-Pb data from the gneiss enclave GSWA142870, confirm the Kaapvaal cratons, arguably the best-preserved Archean terranes on existence of 3.58 Ga igneous protoliths within the Warrawagine Earth, suggest that granite-greenstone terranes may initially form via Granitic Complex. This is 160 km northeast of the first documented assembly and differentiation of juvenile crustal contributions sourced occurrence of rocks of this age in the Shaw Granitic Complex, and thus from chondritic mantle, and that reworking ancient ‘proto’-crust was markedly expands the geographical extent of the ~3.59–3.58 Ga Mount not an essential process in the growth of Archean cratons. Webber magmatic event across the Pilbara Craton. Igneous protolith Supplementary data to this article can be found online at https:// ages of 3.53–3.49 Ga have been established for gneisses of the Carlindi doi.org/10.1016/j.chemgeo.2020.119757. and Muccan Granitic Complexes. We propose the name Mulgundoona “ ” Supersuite for these rocks, which comprise the hitherto missing plu- Declaration of competing interest tonic complement to the felsic extrusive Coucal Formation of the Coonterunah Subgroup. Combined zircon U-Pb, O and Lu-Hf isotopes indicate that magmatic This manuscript contains original new data and these results are not events from ~3.58 Ga to 3.4 Ga in the Pilbara Craton were associated previously published or under consideration for publication elsewhere. with the emplacement of juvenile continental crust extracted from All authors who have contributed to the paper agree to its submis- chondritic mantle. The overwhelmingly chondritic zircon Hf isotope sion. signatures of meta-igneous rocks in the Pilbara Craton do not support the reworking of even older (> 3.8 Ga) crust within the region. Acknowledgements Generally, mantle-like zircon δ18O indicates a limited role for 18O-enriched sources in the production of these rocks, although slightly ele- Financial support by the Swedish Research Council (grant vated values (6–6.5‰) in the oldest zircons suggest minor incorpora- VR#2016-00261 to A. Petersson) is gratefully acknowledged. AK ac- tion of a juvenile supracrustal component. knowledges an Australian Research Council Future Fellowship Building on the recent findings of Petersson et al. (2019b),we (FT10010059) and field logistic support from the Geological Survey of propose a refined model for the earliest evolution of the Pilbara Craton. Western Australia. Hf isotope analysis at UWA was conducted with This involves the assembly of a basaltic plateau that was ‘seeded’ by a instrumentation funded by the Australian Research Council 3.76–3.65 Ga continental nucleus to localise melting above a mantle (LE100100203 and LE150100013). The authors acknowledge the

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