Archean Gravity-Driven Tectonics on Hot and Flooded Continents

Home , Eastern Pilbara Craton, Warrawoona Group

Precambrian Research 229 (2013) 93–104

Contents lists available at SciVerse ScienceDirect

Precambrian Research

journal homepage: www.elsevier.com/locate/precamres

Archean gravity-driven tectonics on hot and ﬂooded continents: Controls on long-lived mineralised hydrothermal systems away from continental margins

a, b N. Thébaud ∗, P.F. Rey a Centre for Exploration Targeting, University of Western Australia, School of Earth and Environment, M006, 35 Stirling Highway, WA 6009, Perth, Australia b Earthbyte Research Group, School of Geosciences, University of Sydney, NSW 2006, Sydney, Australia article info abstract

Article history: We present the results of two-dimensional numerical modelling experiments on the thermal evolution Received 1 February 2011 of Archean greenstones as they sink into a less dense, hot and weak felsic crust. We compare this thermal Received in revised form 17 February 2012 evolution to that obtained via the analysis of isotopic data and fluid inclusion microthermometry data Accepted 1 March 2012 obtained in the Paleoarchean to Mesoarchean Warrawoona Synform (Eastern Pilbara Craton, Western Available online 8 March 2012 Australia). Our numerical experiments reveal a two-stage evolution. In the first stage, cooling affects zones of downwelling as greenstone belts are advected downward, whereas adjacent domes become Keywords: warmer as deep and hot material is advected upward. We show that this is consistent with stable isotopes Sagduction Numerical simulation data from the Warrawoona Synform, which reveal an early episode of seafloor-like alteration (90–160 ◦C) Archean Tectonic strongly focused along steeply dipping shear zones. In a second long-lived stage, lateral heat exchanges Thermal evolution between domes and basins dominate the system as domes cool down while downwelling zones become Gold mineralisation increasingly warmer. In the Warrawoona greenstone belt, stable isotopes in gold-bearing quartz veins post-dating the sagduction-related vertical fabrics reveal that rock–fluid interaction occurred at much higher temperatures (234–372 ◦C) than seafloor-like alteration. We propose that emplacement of thick and dense continental flood basalts, on flooded hot and weak continental plates, led to conditions particularly favourable to hydrothermal processes and the formation of mineral deposits. We further argue that sagduction was able to drive crustal-scale deformation in the interior of continents, away from plate margins. On largely flooded continents, sagduction-related shear zones acted as fluid pathways promoting gold mineralisation far away from active plate boundaries, continental rift zones or collisional mountain belts. © 2012 Elsevier B.V. All rights reserved.

1. Introduction which, in combination to the thermal insulation effect of CFBs, provided the heat engine, in the form of a strong thermal gradient, Several features made the Paleo- to Mesoarchean landscape to power hydrothermal cells in the CFBs. Fourth, Paleoarchean (3.6–2.8 Gyr old) fundamentally different than it is today, and to Mesoarchean cratons throughout the world are characterised particularly favourable to the formation and preservation of gold by ovoid granitic domes, 40–100 km in diameter, encircled by deposits. First, most Archean cratons are buried under up to 15 km greenstone belts. Greenstone belts form strongly foliated ver- thick blanket of autochthonous Continental Flood Basalts (CFBs, tical sheets connected through vertical triple junctions where e.g. Maurice et al., 2009), which include high-magnesium basalts constrictional fabrics dominate (Bouhallier et al., 1995; Chardon and komatiites that have only few equivalents on modern Earth. et al., 1996; McGregor, 1951). This dome and basin pattern, which Second, despite their thicknesses most of these thick volcanic piles characterises many Archean cratons, is commonly interpreted in were emplaced below, and remained below, sea level for most terms of gravitational sinking (sagduction) of dense greenstone of their Archean history (Arndt, 1999; Flament et al., 2008, 2011; belts into hot, and therefore weak, felsic crust (Chardon et al., 1996, Kump and Barley, 2007). This characteristic is of great importance 1998; Collins et al., 1998; Dixon and Summers, 1983; Mareschal as it implies that an inﬁnite ﬂuid reservoir was available to feed and West, 1980; McGregor, 1951). This process is often wrongly hydrothermal circulations in the CBFs. Third, radiogenic heat pro- described as “vertical tectonics”. Indeed, during sagduction hor- duction in the continental crust was much higher than it is today izontal and vertical displacements are perfectly coupled. Domes can rise and greenstone keels can sink because horizontal displacements provide the necessary space for vertical mass transfer (e.g. Mareschal and West, 1980). This process is also often wrongly ∗ Corresponding author. Tel.: +61 86188 7139; fax: +61 86488 1178. E-mail address: [email protected] (N. Thébaud). opposed to plate boundary driven deformation. Indeed there is

0301-9268/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.precamres.2012.03.001 94 N. Thébaud, P.F. Rey / Precambrian Research 229 (2013) 93–104 no mechanical incompatibility between sagduction and plate (35–37 km) and the average erosion level (ca. 7 km) suggest that tectonics processes as both can coexist spatially and temporarily the greenstones accumulated on top of a 30–35 km thick conti- (e.g. Bloem et al., 1997; Rey et al., 2003). While Archean lode gold nental crust. Ubiquitous pillow-lava, hydrothermal cherts, VMS deposits are interpreted traditionally as hydrothermal systems mineralisation, epidote-chlorite-Ca–Na-plagioclase-Ca-amphibole developed along convergent margins (Groves et al., 1998), sagduc- secondary mineral assemblages and intense silicification through- tion provided a mechanism to deform the interior of continents, out the greenstone pile imply subaqueous emplacement (Barley, hence greatly enlarging the domain where mineral deposits could 1984; Barley and Pickard, 1999; Buick and Barnes, 1984; DiMarco form. Fifth, considerations on the rheology of Archean continental and Lowe, 1989; Van Kranendonk, 2006). lithospheres suggest that continents in the Archean were unable to The structural complexity of greenstone covers is a function of sustain elevated mountain belts and high orogenic plateaux (Rey their position with respect to the domes. On the NE flank of the and Houseman, 2006; Rey and Coltice, 2008). Elevations > 3000 m Mount Edgar dome, the Marble Bar greenstone belt lies directly would have been absent at the surface of the Earth, which would on top of the Mount Edgar granitic complex and is structurally have strongly limited erosion, providing an explanation for the part of the dome (Fig. 1b). In the Marble Bar greenstone belt, the remarkable preservation of Archean supracrustal environments. stratigraphy of the Warrawoona and Kelly Groups is relatively well We suggest that these Archean specificities provide some ratio- preserved and structurally simple with a monotonous dip of 20–60◦ nales to explain the exceptional endowment in mineral resources to the NE. In contrast, in the Warrawoona synform (white star in in general, and in gold in particular, of most Archean cratons when Fig. 1b1), greenstones are steeply dipping and strongly deformed compared to younger terranes (Goldfarb et al., 2001). against the near-vertical southwest margin of the Mount Edgar In this paper, we present results of two-dimensional coupled dome (Fig. 3). At this location, the greenstone cover belongs to thermo-mechanical numerical experiments on the sagduction of a basin pinched between the Mount Edgar dome to the north greenstones into a hot and weak felsic crust. We compare the mod- and the Corunna Down dome to the south. Compared to that of elled thermal evolution of greenstones to that obtained through the Marble Bar greenstone belt, the structure in the Warrawoona stable isotopes analyses and fluid inclusions microthermometry synform is more complex showing multiple phases of folding, a on shear zones from the Warrawoona Synform, in the East Pil- prominent horizontal to vertical stretching lineation, and numer- bara Granite-Greenstone Terrane (EPGGT, Fig. 1a). Results point to ous shear zones and quartz vein arrays (e.g. Thébaud et al., 2008). a protracted two-stage hydrothermal history involving: (1) low- Contrasting, yet predictable structural complexity, with complex temperature syn-deformation fluid–rock interactions during the structures above downwelling regions (e.g. Warrawoona syncline) early stage of sagduction, and (2) mid-temperature syn- to post- and simpler structures above rising domes (e.g. Marble Bar belt) is deformation fluid–rock interactions during thermal relaxation and a key attribute of sagduction settings (e.g. Bouhallier et al., 1995; warming of sagducted greenstone sheets. We argue that sagduc- Thébaud, 2006). tion of thick greenstone piles, with a large and near-permanent seawater reservoir above and a hot and weak basement below, led to efficient, crustal-scale and long-lived plumbing systems that 3. Numerical experiment setup favoured fluid–rock interactions and the genesis of gold deposits in the interior of Paleoarchean to Mesoarchean cratons. In order to interpret the thermal history derived from isotopic studies, and to understand fluid flow in a sagduction setting, we have performed a series of numerical experiments to document 2. The East Pilbara Craton: an example of the thermal and mechanical history of sagducted greenstones. sagduction-related “simple structural complexity” The process of sagduction has been tested through numerical and physical experiments (de Bremond d‘Ars et al., 1999; Dixon and The East Pilbara Granite Greenstone Terrane (Fig. 1a) is one of Summers, 1983; Mareschal and West, 1980; Robin and Bailey, the best documented examples of dome-and-basin pattern inter- 2009; West and Mareschal, 1979). This process is driven by the preted in terms of gravitational instability (Collins, 1989; Delor need for minimisation of internal gravitational potential energy et al., 1991; Hickman, 1983; Teyssier et al., 1990; Thébaud, 2006; of a system involving a density inversion (denser layer above a Van Kranendonk et al., 2004a), although alternative views exists layer of lower density). It is resisted by the viscosity of the sys- (Blewett, 2002; Kloppenburg et al., 2001). The East Pilbara Granite tem, in particular by that of the stronger layer involved in the Greenstone Terrane provides a Paleoarchean to Mesoarchean geo- sagduction. Hence, the timing of sagduction is inversely propor- logic backdrop to study tectono-thermal processes and fluid–rock tional to the density contrast and proportional to the viscosity of interactions in a sagduction setting. It preserves a geologic history the stronger layer. The emplacement of greenstones increases the often largely overprinted in many Neoarchean cratons. The bulk geothermal gradient (e.g. Rey et al., 2003; Sandiford et al., 2004; of the domes consist of 3324–3300 Ma old syn- to post-kinematic West and Mareschal, 1979), which in turn reduces the viscosity suites of high-K granitic suites that are derived from, and intru- and accelerates sagduction. To model this process, we use Ellip- sive in, older 3460–3430 Ma Tonalite–Trondjhemite–Granodiorite sis, a Lagrangian integration point finite element code capable of (TTG) gneisses and greenstones of the Warrawoona Group tracking time dependent variables in combination with an Eule- (Hickman, 1983; Hickman and Van Kranendonk, 2004; Smithies rian mesh. This coupled Lagrangian/Eulerian approach allows for et al., 2003; Van Kranendonk et al., 2002, 2007). The domes are the accurate tracking of density interfaces during large deformation themselves intruded by younger, mainly 3300–3240 Ma old, gran- (Moresi et al., 2001, 2002). We use viscoplastic rheologies mim- ites. The older TTG gneisses formed the basement of greenstones, icking standard rheological profiles for the continental lithosphere the emplacement of which is associated with two major volcanic (e.g. Brace and Kohlstedt, 1980). Our experiments include realistic cycles starting with the deposition of Warrawoona Group at ca. geotherms with self-radiogenic heating and partial melting with 3490 Ma and followed by the deposition of the Kelly Group at ca. feedback on viscosities and densities. 3335 Ma (Fig. 2)(Van Kranendonk et al., 2007). The thicknesses In our numerical experiments, the greenstone cover is 15 km of the greenstone covers show significant lateral variation from thick in average, consistent with the thickness of many Archean 8 to 12 km for the Warrawoona Group, and 4–9 km for the Kelly greenstone covers, and in particular the average cumulative thick- Group (e.g. Hickman, 1983; Van Kranendonk et al., 2007). Con- ness of the Warrawoona and Kelly Groups in the EPGG (e.g. Van siderations on both the present crustal thickness of the Pilbara Kranendonk et al., 2007) to which we will confront our results. N. Thébaud, P.F. Rey / Precambrian Research 229 (2013) 93–104 95

Fig. 1. (a) Simplified Geological Map of the North Pilbara Terrain modified after Van Kranendonk et al. (2002). EPGGT, East Pilbara Granite-Greenstone Terrane; CPTZ, Central Pilbara Tectonic Zone; WPGGT, West Pilbara Granite-Greenstone Terrane; MB, Marble bar greenstone belt. Legend: (1) Greenstones, (2) Granitoid complexes, (3) Sedimentary basins, (4) Hamersley basin, (5) Phanerozoic cover. (b1) Simplified structural sketch of Mount Edgar and Corunna Down granitic domes complexes and surrounding regions. (b2) The first derivative of the topography shows that domes (thick dashed lines) are characterised by gentle topography and include both granitic rocks (crossed region on the right panel) and greenstone cover (grey region). In contrast a rough topography characterises the basins surrounding the domes. The mining districts of Klondike in the Warrawoona Synform (white star), and the mining district of Bamboo Creek (white hexagon) are both located above downwelling regions.

Small thickness variations are introduced to allow for the initia- of a 30 km thick basement to which we assign a depth-independent 3 tion of sagduction. Gravity modelling in the East Pilbara Granite density of 2720 kg m− . In comparison to Robin and Bailey (2009), Greenstone Terrane (Blewett et al., 2004) constraints the bulk our experiments use a smaller density contrast between greenstone 3 3 3 density of our CFBs (2840 kg m− ). These CFBs are emplaced in and basement (120 kg m− vs 200 kg m− ), a thicker greenstone three successive 2.5 km thick layers at t0, t0 + 30 myr (lower War- cover (15 km vs 10 km), and the emplacement of the greenstone rawoona Group), t0 + 60 myr (upper Warrawoona Group), plus a cover follows a stepwise history over 140 myr rather than an instan- 7.5 km thick layer (the Kelly Group) emplaced at t0 + 140 myr. taneous emplacement. Assuming t0 = 3490 Ma, this emplacement history approximates Deformation in our models follows the mechanism that requires that of the Warrawoona and Kelly Groups (e.g. Bagas et al., 2002; the least differential stress amongst: frictional faulting (Coulomb Van Kranendonk et al., 2007). Our model CFBs are deposited on top criteria), plastic faulting and viscous creep. The frictional faulting is 96 N. Thébaud, P.F. Rey / Precambrian Research 229 (2013) 93–104

Fig. 2. Lithostratigraphy of the Pilbara Supergroup. Geochronology references: a (Buick et al., 1995), b (Thorpe et al., 1992), c (McNaughton et al., 1993) and d (Nelson, 1999, 2000, 2001).

Table 1 List of parameters used in the models.

Parameters Value(s) Unit

(a) Mechanical parameters 2 g: Acceleration of gravity ﬁeld 9.81 m s− 3 !atm: Air density 2.0 kg m− 3 !cfb: CFB density 2840 kg m− 3 !cc: Crustal density 2720 kg m− 3 !m: Mantle density 3310 kg m− Ccfb: CFB cohesion 10 MPa Ccc: Crustal cohesion 10 MPa Cm: Mantle cohesion 40 MPa εa: Strain weakening factor 0.2 ε0: Strain from which weakening is maximum 0.5 εn: Strain weakening sensitivity to accumulated strain 0.5 #cfb: CFBs maximum yield stress 100 MPa #cc: Crustal maximum yield stress 250 MPa #m: Mantle maximum yield stress 400 MPa $cfb: CFBs internal angle of friction 5 ◦ $cc: Crustal internal angle of friction 15 ◦ $m: Mantle internal angle of friction 25 ◦ Á : Air viscosity 5 1018 Pa s air × A : CFBs pre-exponential constant 5 10 5 MPa n s 1 cfb × − − − A : Crustal pre-exponential constant 5 10 6 MPa n s 1 cc × − − − A : Mantle pre-exponential constant 7 104 MPa n s 1 m × − − ncfb: CFBs stress exponent 3 ncc: Crustal stress exponent 3 nm: Mantle stress exponent 3 5 1 Qcfb: CFBs Activation enthalpy 1.9 10 J mol− × 5 1 Qcc: Crustal activation enthalpy 1.9 10 J mol− × 5 1 Qm: Mantle activation enthalpy 5.2 10 J mol− × 1 1 R: Gas constant 8.3145 J mol− K−

(b) Thermal parameters 5 1 ˛m: Mantle coefﬁcient of thermal expansion 2.8 10− K− × 6 2 1 Ä: Thermal diffusivity 0.9 10− m s− × 1 1 Cp: Heat capacity 1000 J kg− K− 7 3 Hcfb: CFBs heat production 1.335 10− Wm− × 6 3 Hcc: Crustal heat production 1.335 10− Wm− × 3 Hcc: Mantle heat production 0 W m− 2 qm: Basal mantle heat ﬂux 0.025 W m−

(c) Partial melting parameters Latent heat 250,000 J CFBs: Solidus (P) = 993–1.2 10 7 P + 1.2 10 16 P C × − · × − · ◦ Liquidus (P) = 1493–1.2 10 7 P + 1.2 10 16 aP × − · × − Basement: Solidus (P) = 983–9.37 10 8 P + 6.32 10 17 P C × − · × − · ◦ Liquidus (P) = 1393–9.37 10 8 P + 6.32 10 17 P × − · × − · a Non listed values for the CFB are taken equal to crustal values. N. Thébaud, P.F. Rey / Precambrian Research 229 (2013) 93–104 97

Fig. 3. Geological map of the Warrawoona Synform (after Collins et al., 1998; Kloppenburg et al., 2001), localisation on the bottom right panel in Fig. 1.

described by a yield stress: #yield =(C0 + tan (˚) #n) f(ε), in which C0 material fails and deformation is modelled by an effective viscos- is the cohesion at atmospheric pressure; ˚ is the angle of internal ity: nyield = 'yield/2E in which E is the second invariant of the strain friction, #n is the normal stress. Strain localisation is achieved via rate tensor. In our model, viscous rheologies are based on both strain weakening function f(ε), which reduces the yield stress as the dislocation creep and diffusion creep (Bürgman and Dresen, 2008). accumulated strain (ε) increases. The strain weakening function This choice seems justified given that (i) diffusion creep is favoured 1 (1 ε )(ε/ε )εn ,ε<ε by relatively small gravitational differential stress such as the one is given by: f (ε) − − a 0 0 where ε is the = ε ,ε ε initiating sagduction, and (ii) diffusion creep is an important flow a ≥ 0 accumulated strain,! taken as the second invariant of the deviatoric mechanism in feldspar-bearing assemblages prevalent in TTG. Therefore, viscous creep in our model crust is modelled using plastic strain tensor, and ε0 is the “saturation” strain from which the fastest of dislocation creep and diffusion creep mechanisms the yield stress is reduced by a proportion εa. εn modulates the 1 dependency between accumulated strain and strain weakening. (Turcotte and Schubert, 1982) following: Áeff = (1/Ádis + 1/Ádiff)− . Both creep regimes can be described by an Arrhenius formulation The yield stress has an upper limiting value #crit, which describes 1 n 1/n semi-brittle deformation independent of pressure (Ord and Hobbs, (Karato and Wu, 1993): Á =(N0Exp[Q/(RT)]E − ) with E the 1989). For differential stresses that attain the yield stress, the second invariant of the strain rate tensor, Q the activation energy 98 N. Thébaud, P.F. Rey / Precambrian Research 229 (2013) 93–104 of the creep mechanism, and n the power law stress exponent. 7.5 km thick Kelly group, partial melting affects a growing portion ndis For dislocation creep viscosity: N0dis = 1/(2 Adis), with Adis the of the lower crust facilitating the initiation of gravitational insta- pre-exponential factor in the power law (parameter values from bilities which strain the greenstone cover (Fig. 4b2). Convective Brace and Kohlstedt, 1980, Table 1). In the crust, diffusion creep motion affects the deep crust. From t0 + 150 myr to t0 + 158 myr Ma, becomes important under low differential stress, high tempera- a couple of gravitational instabilities develop, one at ca. 152 myr ture, and when partial melt is present (Mecklenburgh and Rutter, (Fig. 4b3), the other at ca. 158 myr (Fig. 4b6). Their length-scale (ca. 2003). For many rocks, parameters Q and A for diffusion creep 100 km) and time-scale (10 myr) are compatible with those of the are unknown. However, they must lead to faster strain rate at dome and basin pattern of the East Pilbara Granite Greenstone Ter- differential stress lower than a given transition differential stress rane (Collins et al., 1998), and consistent with the results of Robin (here #trs = 30 MPa) at which both mechanisms result in the same and Bailey (2009). In the upper crust, above downwelling regions, as strain rate. In our models, we assume that the activation energies much as 60% bulk horizontal shortening accommodates sagduction for dislocation creep and diffusion creep are the same and we drop downward flow. Upward motion in domes and downward motion the N0 factor for diffusion creep to a value for which the dislocation in basins are coupled through horizontal motions in the upper and and diffusion viscosities are equal at the transition differential lower part of the crust. Notably, enclaves of greenstones are caught stress. For diffusion creep, viscosity n = 1 and N0 is obtained by into the upward flow in domes and exhumed from the base of the matching both diffusive and dislocation strain rates at the transi- crust into the upper crust. tion differential stress (# ): N = N = 1/(2ndisA ) # 1 ndis. Thermal evolution: From the onset of sagduction at t + 148 myr, trs 0dif 0dis dis · trs − 0 We disregard diffusion creep in the mantle because the transition downward flow advects cool greenstone rocks in downwelling differential stress between dislocation creep and diffusion creep is regions where the temperature at 10 km depth decreases from ca. low (ca. 0.5 MPa, Turcotte and Schubert, 1982). In all experiments, 210 to ca. 90 ◦C(Fig. 5a). In contrast, in rising domes the temper- viscosities are clipped beyond a minimum of 5 1018 Pa s and a ature at 10 km depth increases from ca. 210 to ca. 660 C(Fig. 5a). × ◦ maximum of 5 1022 Pa s. All parameters are reported in Table 1. In about 12 myr, partial convective overturn leads to the build- × In most CFBs, magmatic mineral assemblages are deeply altered up of a long-wavelength (ca. 100 km) profound lateral thermal into micas and talc-bearing retrogressive assemblages. This alter- anomalies >500 ◦C through fast advective cooling and heating of ation reduces both the density and strength of mafic rocks (de downwelling and upwelling regions respectively (Fig. 5a). The max- Bremond d‘Ars et al., 1999). Hence, we assume an effective viscos- imum horizontal temperature gradient reaches 26 ◦C/km. From ity one tenth of that of the basement. This is achieved by imposing t0 + 158 myr onward, following the main sagduction stage, tem- that: N0cfb = 0.1N0Crust (i.e. Acfb = 10ACrust, Table 1). perature anomalies, around fully developed domes, decrease. This In our experiments, the radiogenic heat production in the crust second post-deformation stage, led to rapid conductive heating of is calculated backward in time to 3.5 Ga from the averaged crustal greenstone basins and conductive cooling of domes (Fig. 5b). The heat production of modern Archean crusts (Taylor and McLennan, temperature at 10 km depth in the basins increases from 90 to 1995). The total crustal radiogenic production is partitioned into 200 ◦C in ca. 8 myr, whereas the temperature at the same depth the pre-greenstone basement and we assume little heat produc- in the domes decreases from 660 to 310 ◦C(Fig. 5b). Our numeri- tion in the greenstone cover and none in the mantle (cf. Table 1). cal experiments also revealed that, because of the sheer size of the Assuming a basal heat flux of 25 10 3 Wm 2, the steady-state thermal anomalies, and in part because domes are diachronous, × − − Moho temperature is at 710 ◦C before the emplacement of the first deformation and thermal relaxation last over 150–200 myr after member of the CFB. A free-slip boundary condition is applied to the bulk of sagduction. This is compatible with the folding of the horizontal and vertical boundaries and no plate boundary force is Gorge Creek Group (<3240 Ma) as well as the hornblende Ar–Ar applied. Deformation is achieved through gravity-driven processes ages in the EPGGT that spread from 3350 to 3100 Ma (Davids et al., initiated by small variations in thickness of the greenstone cover 1997; Kloppenburg et al., 2001; Wijbrans and McDougall, 1987). (Fig. 4). Partial melting impacts significantly on the thermo-mechanical properties of the system (temperature, viscosities and densities). In 5. Comparison to temperature records derived from our numerical experiments, viscosities decrease over three orders petrological and isotopic data of magnitude as the melt fraction increasing from 0.2 to 0.3. In nature, the viscosity drop reaches many orders of magnitude To verify the 2-stage thermal history documented in our (Clemens and Petford, 1999), however only the first two or three numerical experiments we compare it to that derived from geo- orders are likely to have mechanical significance. As the temper- chemical and isotopic data from the Warrawoona Synform (Huston ature increases from the solidus to the liquidus, the density of et al., 2001; Thébaud et al., 2008). In the Warrawoona Synform, the partially melted region decreases by 13%, which increases its hydrothermal alteration has been described as strongly partitioned buoyancy. Solidus and liquidus parameters are reported in Table 1. into ductile shear zones that accommodated the sagduction of the greenstones cover (Fig. 3)(Thébaud et al., 2006). The Fielding’s Find Shear Zone, parallel to the northern border of the felsic vol- 4. Results canic Wyman Formation (Fig. 3), has been recently the focus of a detailed fluid–rock interaction study. A section across this shear Crustal flow: From t0 to t0 + 140 myr, the Moho temperature zone shows that the increase in strain correlates with enhanced increases from 704 ◦C to 772 ◦C leading to onset of partial melt- hydrothermal alteration, including strong silicification, coupled to ing, which is limited to only a few percent of melt at the very a pronounced zoning of major elements, trace elements and oxygen base of the crust (Fig. 4b1). This temperature increase is due to isotopes values (Thébaud et al., 2008). The bulk rock ı18O val- the thermal insulation of the radiogenic crust progressively buried ues increase towards the shear zone from 19 to 25‰, indicating a under the greenstone cover (Mareschal and West, 1980; Rey et al., fluid-buffered system (Fig. 6). This zonation is consistent with the 2003; Sandiford et al., 2004; West and Mareschal, 1979). There is shear zones having acted as fluids pathways. Similar extreme (up no mechanical disturbance of the greenstone-basement interface to 18‰) 18O-enrichments have been documented in metabasalts of (Fig. 4b1), which is consistent with the concordant contact between the Superior Province (Kerrich et al., 1981), and in association with the Kelly Group and the Warrawoona Group (Van Kranendonk Paleoarchean to Mesoarchean hydrothermal vents in Barberton (de et al., 2004b). At t0 + 150 myr, 10 myr after the deposition of the Wit et al., 1982; Knauth and Lowe, 2003; Lowe and Byerly, 1986). N. Thébaud, P.F. Rey / Precambrian Research 229 (2013) 93–104 99

Fig. 4. Initial settings (top box) and snapshots of the numerical experiment from t0 + 140 to t0 + 158 myr following the emplacement of the Kelly Group at t0 + 140 myr. The history from t0 is t0 + 140 myr is not shown. This period corresponds to the progressive emplacement of the greenstone cover, and to a warming up of the crust. During this time there is with little to no deformation. Thickness variations of the lower Warrawoona greenstone speedup initiation of sagduction. Two circular red markers record the horizontal versus vertical motions as well as the magnitude of shortening in the greenstone cover. From t0 + 140 to t0 + 158 myr, gravity-driven shortening above the downwelling region is >60%. Blue shading shows post yielding plastic strain. Arrows pointing at passive vertical markers in the basement document the deformation pattern. 100 N. Thébaud, P.F. Rey / Precambrian Research 229 (2013) 93–104

a) Temperature at 10 km depth, from t0+140 to t0+158 myr, building up of a thermal anomaly 800 Upwelling Upwelling Upwelling

Downwelling Downwelling

600 t0 + 158 myr 26 ºC/km T (ºC) T 400

200 t0 + 148 myr

t0 + 140 myr

0 0 100 200 300 Distance (km)

b) Temperature at 10 km depth, from t0+158 to t0+166 myr, relaxation of the thermal anomaly 800

26 ºC/km t + 162myr 600 0

t0 + 158myr

T (ºC) 400

200 t0 + 166 myr

0 0 100 200 300 Distance (km)

Fig. 5. Thermal evolution at 10 km depth. Upper graph shows the evolution from t0 + 140 to t0 + 158 myr following the emplacement of the Kelly Group. During this stage, the temperature in the basin decreases from 210 to 90 ◦C, whereas the temperature in the dome rises from 210 ◦C to 660 ◦C. This leads to a horizontal temperature difference of ca. 570 ◦C over ca. 65 km distance, with a horizontal gradients up to 26 ◦C per km. From t0 + 158 to t0 + 166 myr (lower graph), the temperature evolution changes with cooling of the granitic domes from 660 to 310 ◦C and heating in the greenstone basin from 90 to 200 ◦C.

18O-enrichment up to ca 14‰ have been also described in ophiolites fluid under near-isothermal conditions. These veins are there- and modern-day seafloor alteration zones and is interpreted as the fore in isotopic disequilibrium with their silicified host, which product of hydrothermal convection cells involving seawater at low implies an emplacement postdating the silicification (Thébaud temperature (e.g. Putlitz et al., 2001). In Archean sagduction sett- et al., 2008). Quartz-hosted CO2–NaCl–H2O–CH4 fluid inclusions ings, seafloor-like alteration would have been strongly enhanced by yielded homogenisation temperatures between 234 and 372 ◦C deformation and the development of faults and shear zones above (Huston et al., 2001; Thébaud et al., 2006). Using Zheng (1993) and within downwelling regions. In the Warrawoona Synform, syn- quartz/water fractionation equation, Thébaud et al. (2008) calcu- deformation alteration process was driven by interaction with large lated that the ı18O of the water from which these quartz veins volumes of low temperatures ( 90–160 C) seawater-derived flu- precipitated is in the range of +2.4 to +12.1‰. This oxygen iso- ∼ ◦ ids during the early stage of sagduction process (Thébaud et al., topic composition, together with the significant enrichment in 2008). This is consistent with our numerical modelling showing potassium and base metals revealed by Synchrotron radiation X- that downward advection of greenstones maintains a low temper- ray fluorescence fluid inclusion analyses, support a metamorphic ature environment (<100 ◦C) in the top 10 km of the downwelling and/or magmatic source (Thébaud et al., 2006). These hotter fluids greenstones. were mobilised at a later stage of the sagduction process, possibly In the Warrawoona Synform, ductile shear zones also host gold- during the devolatilisation of the greenstones keels and/or during bearing quartz calcite sulphide ankerite veins (Huston et al., the large production of potassic melt (Thébaud et al., 2008). ± ± ± 2001). These veins do not exist in the nearby less deformed Marble In summary, field observation, isotopic and fluid inclusions Bar Greenstone Belt, but they do in other downwelling regions such microthermometry data, and our numerical investigation pre- as the Bamboo Creek mining district on the NE margin of Mount sented here, document a long-lived two-stage hydrothermal Edgar (white hexagon in Fig. 1b1). These gold-bearing quartz veins history involving: (1) low-temperature (90–160 ◦C) syn- develop largely in association with shear zones in downwelling deformation seafloor-like fluid–rock interactions strongly focused regions and have a syn- to post-shearing origin (Thébaud et al., along steeply dipping ductile shear zones during the early stage of 2006). Across the Warrawoona Synform, quartz veins display a sagduction, and (2) moderate-temperature (234–372 ◦C) syn- to restricted range of ı18O values with a mean value of +13.2 2‰ post-deformation fluid–rock interactions, strongly focused along ± (Fig. 6). This suggests that quartz precipitated from a homogeneous shear zones, leading to the formation of gold-bearing quartz veins N. Thébaud, P.F. Rey / Precambrian Research 229 (2013) 93–104 101

18 18 Fig. 6. Whole rock ı O and wt% SiO2 and quartz vein ı O plotted against a synthetic lithological section of the Fielding’s Find shear zone from the Warrawoona Synform (modified after Thébaud et al., 2008). Whole Rock analyses across the Fielding’s Find Shear Zone reveal that he bulk rock ı18O values together with silica content, from both sides of the shear zone, increase progressively towards its centre. Such enrichment in heavy oxygen isotopes and silicification was proposed to be driven by interaction with large volumes of low temperature ( 90–160 C) hydrothermal fluids (sea-water) (Thébaud et al., 2008). The quartz veins are in isotopic disequilibrium with their altered ∼ ◦ host, which suggest that a fluid circulation event followed the silicification around the Fielding’s Find Shear Zone. Vein quartz display a relatively restricted range of ı18O values across the Fielding’s Find shear zone, with a mean value of +13.2 2‰. Together with fluid inclusion analyses the quartz veins are regarded as the signature of a ± composition involving magmatic and/or metamorphic fluid sources (Thébaud et al., 2006).

post-dating the vertical fabrics. The early alteration assemblages steeply dipping greenstone sheets, these hot crustal fluids would can be linked to the early stage of greenstone sagduction and the have been efficiently channelled toward the surface through the co-development of (i) crustal-scale thermal anomalies, during well-established network of shear zones formed at an early stage. which cooling affected the downwelling greenstone, and (ii) This is consistent with field observations showing that quartz vertical shear zones which acted as efficient fluid pathways for veins are restricted to downwelling regions. The slow conductive supracrustal porous flow (Fig. 7a). In the early stage of sagduc- relaxation of the thermal anomaly may have helped to maintain tion, large volumes of fluid, trapped in hydrated minerals during this plumbing system for several 10 s of myr. seafloor alteration, would have been transported deep into the crust. The second hydrothermal stage can be linked to the release 6. Discussion and implication for Paleoarchean to of metamorphic and/or magmatic fluid in association to the Mesoarchean gold mineralisation devolatisation of sagducted greenstone covers, and crystallisation of felsic magmas (Fig. 7b). These fluids would have been released To form a gold deposit, the average crustal gold content of at a time when sagduction-related lateral thermal anomalies 1.3 ppb must be concentrated by one thousand to one hundred entered the relaxation phase during which the temperature in thousand times (e.g. Walshe and Cleverley, 2009). Temperature the greenstone keels rapidly increased. In the strongly foliated gradients above shallow magmatic intrusions typically drive 102 N. Thébaud, P.F. Rey / Precambrian Research 229 (2013) 93–104

Fig. 7. Conceptual tectonograms illustrating Archean intracontinental mineral systems associated to gravity driven tectonics and fluid flow during sagduction of subaqueous greenstone covers (green envelops) into their hot and weak basement (red envelops). During the early stage of sagduction (upper panel), early deformation enhances low- temperature seafloor-like alteration along downwelling regions. Sagduction transfers cooler supracrustal rocks and large volumes of hydrated minerals deep into the crust. As shown in our numerical experiments, dome regions become a few hundred degrees hotter than the intervening downwelling regions. These lateral thermal gradients contribute to focus fluid circulations into downwelling regions. In a second stage (lower panel), the relaxation of this lateral temperature anomaly led to the rapid heating and devolatisation of hydrated steeply dipping greenstone sheets. Released metamorphic and magmatic fluids focus into pre-existing shear zones, which act as crustal-scale fluid pathways to the surface where mineral deposits form. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

efficient hydrothermal systems in which fluid–rock interaction silicification observed in greenstone belts. Numerical and phys- is the main concentration mechanism (Eldursi et al., 2009). It ical experiments have shown that density inversions related to is therefore not surprising that shallow magmatic activities in the emplacement of thick continental basalts onto less dense and back-arc settings, as well as sub-seafloor volcanism associated weak basement are large enough to drive the foundering of basaltic with continental rift systems, are the main settings of gold deposits piles into the continental crust (Chardon et al., 1998; de Bremond on the modern Earth (Goldfarb et al., 2001). In other words, miner- d‘Ars et al., 1999; Dixon and Summers, 1983; Robin and Bailey, alised hydrothermal systems are strongly partitioned onto ocean 2009; West and Mareschal, 1979; this study). Our numerical exper- floors and active continental margins. However, it is likely that iments show that sagduction is capable of promoting significant most Archean ocean floors and Archean continental plate margins horizontal temperature gradients up to 26 ◦C/km. These horizontal did not survive the past 3 Gyr. This suggests that the bulk of the gradients may have been large enough to force fluids, released dur- preserved Archean gold deposits may have formed in the interior ing devolatisation of greenstones and crystallisation of domes, back of continents, away from plate margins. On the modern Earth, into the cooler greenstone keels, where steeply dipping shear zones this setting is the least favourable for hydrothermal systems, but would have channelised these hotter fluids toward the surface. perhaps not so in the Archean. Importantly, sagduction is a mechanism that explains crustal-scale In the Archean, basaltic piles up to 15 km thick emplaced on deformation within the inner-part of continental plates, away from older felsic continental crusts were common (de Wit and Ashwal, their margins. Therefore in the Archean, the association of volcanic 1997), and most of them emplaced onto flooded continents (Arndt, rocks above a felsic crust but below sea level, in tectonically active 1999; Flament et al., 2008; Kump and Barley, 2007). Higher sea regions with high heat flow – critical to formation of gold deposits – level in the Archean would have maintained an infinite reservoir was not restricted to continental rifting or active continental mar- of water above the continental basalts, whereas higher surface gins. Sagduction on flooded continents would have promoted the heat flow, due to enhanced radiogenic heat production in the crust formation intracontinental hydrothermal systems expanding con- and due to CFBs thermal insulation, would have powered shallow siderably the prospectivity of Archean cratons, compared to that of hydrothermal cells at the interface between continent and sea- modern continents whose prospectivity is limited to active plate water, accounting for widespread low temperature alteration and margins and continental rifts. N. Thébaud, P.F. Rey / Precambrian Research 229 (2013) 93–104 103

7. Conclusions Blewett, R.S., Shevchenko, S., Bell, B., 2004. The North Pole Dome: a non-diapiric dome in the Archaean Pilbara Craton, Western Australia. Precambrian Research 5, 105–120. Using realistic rheologic and thermal parameters, our coupled Bloem, E.J.M., Dalstra, H.J., Ridley, J.R., Groves, D.I., 1997. Granitoid diapirism during thermo-mechanical numerical experiments confirm that, in the protracted tectonism in an Archaean granitoid-greenstone belt, Yilgarn Block, Archean, sagduction was a natural response to density inversions Western Australia. Precambrian Research 85, 147–171. Bouhallier, H., Chardon, D., Choukroune, P., 1995. Strain patterns in Archaean dome- and thermal weakening due to the emplacement of thick continen- and-basin patterns: the Dharwar craton (Karnataka, India). Earth and Planetary tal flood basalts onto radiogenic crusts. Our experiments show that, Science Letters 135, 57–75. during sagduction, the advection of cold rocks into downwelling Brace, W.F., Kohlstedt, D.L., 1980. Limits on lithospheric stress imposed by laboratory basins and advection of hot rocks into rising domes lead to crustal- experiments. Journal of Geophysical Research 85, 6248–6252. Buick, R., Barnes, K.R., 1984. Cherts in the Warrawoona Group: Early Archean Silici- scale, horizontal, thermal anomalies in excess to 500 ◦C over an fied Sediments Deposited in Shallow Water Environments, vol. 9. University of horizontal distance of ca. 30 km. Large temperature gradients up to Western Australia, Geology Department, University Extension Special Publica- 26 C/km and the availability of large volumes of fluids (i.e. (i) sea- tion, pp. 37–53. ◦ Buick, R., Thornett, J.R., McNaughton, N.J., Smith, J.B., Barley, M.E., Savage, M., 1995. water above flooded continents, (ii) metamorphic fluids released Record of emergent continental crust 3.5 billion years ago in the Pilbara Craton ∼ during dehydration of sagducting supracrustal greenstones, and of Australia. Nature 375, 574–577. (iii) magmatic fluids released during crystallisation of magma in Bürgman, R., Dresen, G., 2008. Rheology of the Lower Crust and Upper Mantle: evidence from rock mechanics, geodesy, and field observations. Annual Review of domes) would have powered crustal-scale hydrothermal systems. Earth and Planetary Sciences 36, 531–567. The formation around granitic domes of networks of sagduction- Chardon, D., Choukroune, P., Jayananda, M., 1996. Strain, patterns, decollement and related steeply dipping greenstone sheets and vertical shear zones incipient sagducted greenstone terrains in the Archaean Dharwar craton (south India). Journal of Structural Geology 18, 991–1004. connected through vertical triple junctions, would have facilitated Chardon, D., Choukroune, P., Jayananda, M., 1998. Sinking of the Dharwar Basin crustal-scale transfer of fluids, matter and heat. Fluid–rock inter- (South India): implications for Archaean tectonics. Precambrian Research 91, actions in these tectonically active plumbing systems would have 15–39. Clemens, J.D., Petford, N., 1999. Granitic melt viscosity and silicic magma dynamics in overprinted low-temperature fluid–rock interactions that occurred contrasting tectonic settings. Journal of the Geological Society 156, 1057–1060. during the emplacement of CFBs onto flooded continents and dur- Collins, W.J., 1989. Polydiapirism of the Archean Mount Edgar Batholith, Pilbara ing the early stage of sagduction. This is consistent with structural, Block, Western Australia. Precambrian Research 43, 41–62. petrological and isotopic data. Based on our numerical experi- Collins, W.J., Van Kranendonk, M.J., Teyssier, C., 1998. Partial convective overturn of Archaean crust in the east Pilbara Craton, Western Australia: driving ments, field observations and existing petrological and isotopic mechanisms and tectonic implications. Journal of Structural Geology 20, data, we propose that in the Archean, the sagduction of thick 1405–1424. 40 39 greenstone piles with a large and near-permanent seawater reser- Davids, C., Wijbrans, J.R., White, S.H., 1997. Ar/ Ar laser probe ages of metamorphic hornblendes from the Coongan Belt, Pilbara, Western Australia. voir above, and a hot and weak basement below, allowed for the Precambrian Research 83, 221–242. development of efficient and long-lived plumbing systems. This de Bremond d‘Ars, J., Lecuyer, C., Chardon, D., 1999. Hydrothermalism and diapirism plumbing systems favoured crustal-scale fluid–rock interactions in the Archean: gravitational instability constraints. Tectonophysics 304, 29–39. de Wit, M.J., Ashwal, M.D., 1997. Greenstone Belts (Oxford). and the formation of craton-wide gold deposits in the interior of de Wit, M.J., Hart, R., Martin, A., Abbot, P., 1982. Archaean abiogenetic and probable continents, far away from their margins, a context very different biogenetic structures associated with mineralized hydrothermal vent systems from the paleoproterozoic and Phanerozoic, where orogenic and and regional metassomatism, with implications for the greenstone belt studies. Economic Geology and the Bulletin of the Society of Economic Geologists 77, magmatic orebodies are closely associated with active margin sett- 1783–1802. ings. Delor, C.A., Burg, J.P., Clarke, G., 1991. Relations diapirisme-metamorphisme dans la Province du Pilbara (Australie Occidentale): implications pour les régimes ther- miques et tectoniques à l‘Archéen. Comptes Rendus de l‘Académie des Sciences Acknowledgements de Paris 312, 257–263. DiMarco, M.J., Lowe, D.R., 1989. Petrography and provenance of silicified early Archaean volcaniclastic sandstones, eastern Pilbara Block, Western Australia. This work was supported by the Australian Research Council’s Sedimentology 36, 821–836. Discovery funding scheme (ARC DP 0342933) and AUSCOPE- Dixon, J.M., Summers, J.M., 1983. Patterns of total and incremental strain in subsiding troughs: experimental centrifuged models of inter-diapir synclines. Canadian NCRIS and Computational Infrastructure for Geodynamics software Journal of Earth Sciences 20, 1843–1861. infrastructure. This paper has benefited from the reviews of Jean Eldursi, K., Branquet, Y., Guillou-Frottier, L., Marcoux, E., 2009. Numerical investiga- Bébard and Laurent Aillères, and from countless discussions with tion of transient hydrothermal processes around intrusions: heat-transfer and fluid-circulation controlled mineralization patterns. Earth and Planetary Science N. Flament, N. Coltice, P. Philippot, F. Wellmann, M. Fiorentini, K. Letters 288, 70–80. Gessner, M. Van Kranendonk and many other colleagues from the Flament, N., Coltice, N., Rey, P.F., 2008. A case for late-Archaean continental emer- University of Sydney, the University of Paris 6 and the Centre for gence from thermal evolution models and hypsometry. Earth and Planetary Exploration Targeting in Perth. Science Letters 275, 326–336. Flament, N., Rey, P.F.C.N., Dromart, G., Olivier, N., 2011. Lower crustal flow kept Archean continental flood basalts at sea level. Geology 39, 1159–1162. Goldfarb, R., Groves, D., Gardoll, S., 2001. Orogenic gold and geologic time: a global References synthesis. Ore Geology Reviews 18, 1–75. Groves, D.I., Goldfarb, R.J., Gebre-Mariam, M., Hagemann, S.G., Robert, F., 1998. Oro- Arndt, N., 1999. Why was flood volcanism on submerged continental platforms so genic gold deposits: a proposed classification in the context of their crustal common in the Precambrian? Precambrian Research 97, 155–164. distribution and relationship to other gold deposit types. Ore Geology Reviews Bagas, L., Smithies, R.H., Champion, D.C., 2002. Geochemistry of the Corunna Downs 13, 7–27. Granitoid Complex, East Pilbara Granite/Greenstone Terrane, Western Australia. Hickman, A.H., 1983. Geology of the Pilbara Block and its Environs. 1981/36 (Perth). Geological Survey of Western Australia - Annual Review Technical Papers Hickman, A.H., Van Kranendonk, M.J., 2004. Diapiric processes in the formation of 2001–02, 61–69. Archaean continental crust, East Pilbara Granite-Greenstone Terrane, Australia. Barley, M.E., 1984. Volcanism and hydrothermal alteration, Warrawoona Group, East In: The Precambrian Earth: Tempos and Events. Elsevier. Pilbara. In: Muhling, J.R., Groves, D.I., Blake, T.S. (Eds.), Archaean and Proterozoic Huston, D., Blewett, R., Mernaugh, T., Sun, S.-S., Kamprad, J., 2001. Gold Deposits of Basins of the Pilbara, Western Australia: Evolution and Mineralization Poten- the Pilbara Craton: Results of AGSO Research, 1998–2000. tial. University of Western Australia, Geology Department, University Extension Karato, S., Wu, P., 1993. Rheology of the upper mantle a synthesis. Science 260, Special Publication, pp. 23–36. 771–776. Barley, M.E., Pickard, A.L., 1999. An extensive, crustally-derived, 3325 to 3310 Ma Kerrich, R., Fryer, B., Milner, K.J., Peirce, M.G., 1981. The geochemistry of gold-bearing silicic volcanoplutonic suite in the eastern Pilbara Craton: evidence from the chemical sediments, Dickenson mine, Red Lake, Ontario: a reconnaissance study. Kelly Belt, McPhee Dome and Corunna Downs Batholith. Precambrian Research Canadian Journal of Earth Sciences 18, 624–637. 60, 185–241. Kloppenburg, A., White, S.H., Zegers, T.E., 2001. Structural evolution of the War- Blewett, R.S., 2002. Archaean tectonic processes: a case for horizontal shortening in rawoona Greenstone Belt and adjoining granitoid complexes, Pilbara Craton, the North Pilbara Granite-Greenstone Terrane, Western Australia. Precambrian Australia: implications for Archaean tectonic processes. Precambrian Research Research 113, 87–120. 112, 107–147. 104 N. Thébaud, P.F. Rey / Precambrian Research 229 (2013) 93–104

Knauth, L.P., Lowe, D.R., 2003. High Archaean climatic temperature inferred from Taylor, S., McLennan, S., 1995. The geochemical evolution of the continental crust. oxygen isotope geochemistry of cherts in the 3.5 Ga Swaziland Supergroup, Reviews of Geophysics 33, 241–265. South Africa. Geological Society of America Bulletin 115, 566–580. Teyssier, C., Collins, W.J., Van Kranendonk, M.J., 1990. Strain and kinematics dur- Kump, L.R., Barley, M.E., 2007. Increased subaerial volcanism and the rise of atmo- ing the emplacement of the Mount Edgar Batholith and Warrawoona Syncline, spheric oxygen 2.5 billion years ago. Nature 448, 1033–1036. Pilbara Block, Western Australia. In: Geoconferences (W.A.), Perth, Western Lowe, D.R., Byerly, G.R., 1986. Archaean flow-top alteration zones formed initially Australia, pp. 481–483. in a low-temperature sulphate-rich environment. Nature 324, 245–248. Thébaud, N., 2006. From Synchroton Characterisation of Single Fluid Inclusions to Mareschal, J.C., West, G.F., 1980. A model for Archean tectonism. Part 2. Numeri- Archaean Geodynamics: A Study on Fluid–Rock Interaction in a Triple Foliation cal models of vertical tectonism in greenstone belts. Canadian Journal of Earth Junction (The warrawoona Syncline, Western Australia). Paris VI University and Sciences 17, 60–71. The University of Sydney, 276 pp. Maurice, C., David, J., Bédard, J.H., Francis, D., 2009. Evidence for a widespread mafic Thébaud, N., Philippot, P., Rey, P., Brugger, J., Van Kranendonk, M.J., Grassineau, N., cover sequence and its implications for continental growth in the Northeastern 2008. Protracted fluid–rock interaction in the mid-Archaean and implication for Superior Province. Precambrian Research 169, 45–65. gold mineralization: example from the Warrawoona Syncline (WA). Earth and McGregor, A.M., 1951. Some milestones in the Precambrian of southern Africa. Planetary Science Letters 272, 639–655. Proceedings of the Geological Society of South Africa 54, 27–71. Thébaud, N., Philippot, P., Rey, P., Cauzid, J., 2006. Composition and origin of fluids McNaughton, N.J., Compston, W., Barley, M.E., 1993. Constraints on the age of associated with lode gold deposits in a mid-Archaean greenstone belt (Warra- the Warrawoona Group, eastern Pilbara Block, Western Australia. Precambrian woona Syncline, Pilbara, WA) using Synchrotron radiation X-Ray Fluorescence. Research 60, 69–98. Contributions to Mineralogy and Petrology 152, 485–503. Mecklenburgh, J., Rutter, E.H., 2003. On the rheology of partially molten synthetic Thorpe, R.I., Hickman, A.H., Davis, D.W., Mortensen, J.K., Trendall, A.F., 1992. U–Pb granite. Journal of Structural Geology 25, 1575–1585. zircon geochronology of Archaean felsic units in the Marble Bar region, Pilbara Moresi, L., Dufour, F., Muhlhaus, H.B., 2002. Mantle convection modeling with Craton, Western Australia. Precambrian Research 56, 169–189. viscoelastic/brittle lithosphere: numerical methodology and plate tectonic mod- Turcotte, D.L., Schubert, G., 1982. Geodynamics: Applications of Continuum Physics eling. Pure and Applied Geophysics, 2335–2356. to Geological Problems. Wiley, New York. Moresi, L., Muelhaus, H., Dufour, F., 2001. Particle in cell solution for creeping viscous Van Kranendonk, M.J., 2006. Volcanic degassing, hydrothermal circulation and the flows with internal interfaces. In: Muelhaus, H., Dyskin, A., Pasternak, E. (Eds.), flourishing of early life on Earth: a review of the evidence from c. 3490–3240 Ma Proceedings of the 5th International Workshop on Bifurcation and Localisation. rocks of the Pilbara Supergroup, Pilbara Craton, Western Australia. Earth-Science Balkema, Perth. Reviews 74 (February), 197–240. Nelson, D.R., 1999. Compilation of geochronology data, 1998, Geological Survey of Van Kranendonk, M.J., Collins, W., Hickman, A., Pawley, M.J., 2004a. Critical tests of Western Australia Record 1999/2, p. 222. vertical vs. horizontal tectonic models for the Archaean East Pilbara Granite- Nelson, D.R., 2000. Compilation of geochronology data, 2000, Geological Survey of Greenstone Terrane, Pilbara Craton, Western Australia. Precambrian Research Western Australia Record 2001/2, p. 205. 131, 173–211. Nelson, D.R., 2001. Compilation of geochronology data, 2000, Geological Survey of Van Kranendonk, M.J., Hickman, A.H., Smithies, R.H., Nelson, D.R., Pike, G., 2002. Western Australia Record 2002/2, p. 205. Geology and tectonic evolution of the Archaean North Pilbara Terrain, Pilbara Ord, A., Hobbs, B.E., 1989. The strength of the continental crust, detachment zones Craton, Western Australia. Economic Geology and the Bulletin of the Society of and the development of plastic instabilities. Tectonophysics 158, 269–289. Economic Geologists 97, 695–732. Putlitz, B., Katzir, Y., Matthews, A., Valley, J.W., 2001. Oceanic and orogenic fluid–rock Van Kranendonk, M.J., Smithies, R., Hickman, A., Champion, C., 2007. Review: secular interaction in 18O/16O-enriched metagabbros of an ophiolite (Tinos, Cyclades). tectonic evolution of Archean continental crust: interplay between horizontal Earth and Planetary Science Letters 193, 99–113. and vertical processes in the formation of the Pilbara Craton, Australia. Terra Rey, P., Houseman, G., 2006. Lithospheric scale gravitational flow: the impact of Nova 19, 1–38. body forces on orogenic processes from Archaean to Phanerozoic. In: Buiter, Van Kranendonk, M.J., Smithies, R.H., Hickman, A.H., Bagas, L., Williams, I.R., S.J.H., Schreurs, G. (Eds.), Analogue and Numerical Modelling of Crustal-Scale Farrell, T.R., 2004. Event stratigraphy applied to 700 million years of Processes. , pp. 153–167. Archaean crustal evolution, previous term Pilbara next term Craton, West- Rey, P., Philippot, P., Thébaud, N., 2003. Contribution of mantle plumes, crustal thick- ern Australia, Technical Papers Geological Survey of Western Australia, Perth, ening and greenstone blanketing to the 2.75–2.65 Ga global crisis. Precambrian pp. 49–61. Research 127, 43–60. Walshe, J.L., Cleverley, J.S., 2009. Gold deposits; where, when and why. Elements 5, Rey, P.F., Coltice, N., 2008. Neoarchean lithospheric strengthening and the coupling 288. of Earth’s geochemical reservoirs. Geology 36, 635–638. West, G.F., Mareschal, J.C., 1979. A model for Archean tectonism. Part 1. The thermal Robin, C.M.I., Bailey, R.C., 2009. Simultaneous generation of Archean crust and sub- conditions. Canadian Journal of Earth Sciences 16, 1942–1950. cratonic roots by vertical tectonics. Geology 37, 523–526. Wijbrans, J.R., McDougall, I., 1987. On the metamorphic history of an Sandiford, M., Kranendonk, M.V., Bodorkos, S., 2004. Conductive incubation and Archaean granite-greenstone terrain, East Pilbara, Western Australia using the origin of Granite-Greenstone dome-and-keel structure: the eastern Pilbara the 40Ar/39Ar spectrum technique. Earth and Planetary Science Letters 84, Craton, Australia. Tectonics 23. 226–242. Smithies, R.H., Champion, D.C., Cassidy, K.F., 2003. Formation of Earth’s early Zheng, Y.F., 1993. Calculation of oxygen isotope fractionation in anhydrous silicate Archaean continental crust. Precambrian Research 127, 89–101. minerals. Geochimica et Cosmochimica Acta 57, 1079–1091.