Gondwana Research 18 (2010) 213–221

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Stratigraphy-related, low-pressure metamorphism in the Hardey Syncline,Hamersley Province, Western Australia

Takazo Shibuya a,⁎, Kazumasa Aoki b, Tsuyoshi Komiya b, Shigenori Maruyama b

a Precambrian Ecosystem Laboratory, Japan Agency for Marine-Earth Science and Technology (JAMSTEC), 2-15 Natsushima-cho, Yokosuka 237-0061, Japanb Department of Earth and Planetary Sciences, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro, Tokyo 152-8551, Japan

⁎ Corresponding author. Tel.: +81 46 867 9647; fax:E-mail address: takazos@jamstec.go.jp (T. Shibuya).

1342-937X/$ – see front matter © 2010 International Adoi:10.1016/j.gr.2010.01.002

a b s t r a c t

a r t i c l e i n f o

Article history:Received 3 October 2009Received in revised form 18 December 2009Accepted 11 January 2010Available online 20 January 2010

Keywords:PaleoproterozoicLow-pressure metamorphismOphthalmian orogenyContinental riftingTectonicsHamersley Province

The late Archean Fortescue Group and Paleoproterozoic Hamersley Group are exposed in the HardeySyncline located at the southwestern edge of the Hamersley Basin, Pilbara Craton, Western Australia. Thesecondary mineral assemblages and compositions of the basaltic rocks of the Fortescue and HamersleyGroups reveal the metamorphic conditions of the study area. The estimated metamorphic grade ranges fromprehnite–actinolite facies (Hamersley Group), through greenschist facies (Fortescue Group), to a transitionbetween greenschist facies and actinolite–calcic plagioclase facies (Fortescue Group), indicating a low-pressure type metamorphic facies series. The metamorphic grade increases northward, which is opposite tothe general southward increase of the regional metamorphic grade. Furthermore, the change ofmetamorphic grade strongly correlates with stratigraphy, and the metamorphic temperature increaseswith stratigraphic depth. These observations suggest that the metamorphism of the study area was causedby a thermal event before the folding due to the Ophthalmian orogeny that affected most of the HamersleyProvince. Considering the presence of 2.2 Ga dolerite sills in the Hardey Syncline and the low-pressuremetamorphic condition, it is suggested that the metamorphism of the study area was caused by the 2.2 Gacontinental rifting, which is consistent with the reported metamorphic age.

© 2010 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

1. Introduction

Well-preserved Archean–Proterozoic cratons carry importantgeologic records for understanding the early history of the Earth(e.g., Condie, 1981; Van Kranendonk et al., 2007; Eriksson et al., 2009)because the present day Earth has lostmost of its old crust (e.g., Rino etal., 2008; Kawai et al., 2009). Metamorphic processes, including thoseongoing in active subduction zones (e.g. Omori et al., 2009), providecritical clues in evaluating the tectonic history. The Pilbara Craton ofnorthwestern Australia is known as one of the best preserved cratonswhich records accretion of oceanic crust, a large-scale continentalrifting, voluminous depositions of banded iron formation andcarbonate rock, and large igneous province (LIP) activity (Ohta et al.,1996; Barley et al., 1997; Müller et al., 2005; Van Kranendonk et al.,2007). Such processes have been intensely studied in various terranesto elucidate the early evolutions of solid earth, environment, and life(e.g., Ohta et al., 1996; Barley et al., 1997; Schopf, 2006; VanKranendonk, 2006; Shibuya et al., 2007a; Van Kranendonk et al.,2007; Komiya et al., 2008; Rino et al., 2008; Santosh and Omori, 2008).Subsequent metamorphic processes on a local and/or regional scaleharbor key clues for decoding the tectonic history of cratons.

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ssociation for Gondwana Research.

It was previously suggested that greenstones in the Pilbara Cratonregionally underwent post-depositional, low-grade burial metamor-phism and the metamorphic grade generally increases toward thesouth (Fig. 1) (Smith et al., 1982). However, geochronological studies(e.g., isochron age) indicate that the greenstones were isotopicallyreset after the eruption and register thermal events with various ages(Nelson et al., 1992; Alibert and McCulloch, 1993). In-situ dating ofmetamorphic phosphates in sedimentary rocks was conducted toconstrain the precise timing of the thermal events,which revealed thatPilbara Cratonwas affected by ca. 2.4 Ga and ca. 2.2 Ga thermotectonicevents (Rasmussen et al., 2001, 2005). Furthermore, the precise U–Pbages of the phosphates show a younging trend from south to north,leading to the model of a southward increasing metamorphic grade ofthe Pilbara Craton associatedwith themigration of metamorphic frontfrom the collisional zone in the southern margin of the Pilbara Craton(Fig. 1) (Rasmussen et al., 2005). However, Thorne and Tyler (1996)pointed out that themetamorphicmineral isograd pattern reported bySmith et al. (1982) appears to reflect the fold pattern, with lower graderocks in the synclines and higher grade rocks in anticlines. Thus, moredetailed studies on a local scale are essential to reveal the cause ofmetamorphic events. Here we present a stratigraphy-related investi-gation of the metamorphism in the Hardy syncline, HamersleyProvince, Pilbara Craton, the results from which indicate an oppositedirection of increasing grade as against the general southwardincreasing grade of the regional metamorphism.

Published by Elsevier B.V. All rights reserved.

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2. Geological setting

The Pilbara Craton is subdivided into two tectonic components: 3.5to 2.8 Ga granite–greenstone terrains and <2.8 Ga unconformablyoverlying volcano-sedimentary successions (Fig. 1). The latter is madeup of three stratigraphic components, the Fortescue Group, HamersleyGroup, and Turee Creek Group, in ascending order (Thorne, 1990;Thorne and Seymour, 1991). The Fortescue Group consists mainly ofmafic volcanics, volcaniclastic rocks, and sedimentary rocks. TheHamersley Group unconformably overlies the Fortescue Group, andconsists mainly of banded iron formation (BIF), shale, carbonate, andtuff, interlayered with mafic and felsic igneous rocks. After thedeposition, the southern Hamersley Province underwent folding anddeformation (mainly WNW–ESE trend) during the Ophthalmianorogeny associated with collision along the southern margin of thePilbara Craton (Fig. 1) (Blake and Barley, 1992). The metamorphic

Fig. 1. Simplified geological map of the Pilbara Craton (modified after Trendall, 1990). Broassemblages of greenstones in the Fortescue Group; ZI, prehnite–pumpellyite zone; ZII, preZIV, (prehnite)–epidote–actinolite zone (Smith et al., 1982). In-situ U–Pb ages of metamorp

grade of the Fortescue Group decreases northward on a regional scale(Smith et al., 1982).

The Hardey Syncline, southern Hamersley Basin is one of the mainfolds related to the Ophthalmian orogeny (Fig. 1). The study area islocated in the northern limb of the Hardey Syncline and exposes theFortescue and Hamersley Groups (Figs. 1 and 2). The lower part of thearea is composed mainly of pillow basalts, massive sheet flows,hyaloclastites, reworked hyaloclastites, basaltic komatiite, and minordikes, often intercalated with thin-bedded cherts and felsic tuffs,which correspond to the Fortescue Group (Figs. 2 and 3; BoongalFormation, Pyradie Formation, Bunjinah Formation, and JerinahFormation in ascending order). Sub-spherical to ellipsoidal pillowsare closed-packed and range from 50 to 200 cm across. They have adark-colored chilled margin without any trace of vesicles. The way-upstructure of the pillows points to the south (Komiya, 2004). Althoughthe greenstones of the Fortescue Groups are part of the continental

ken lines indicate metamorphic boundaries estimated mainly from secondary mineralhnite–pumpellyite–epidote zone; ZIII, prehnite–pumpellyite–epidote–actinolite zone;hic phosphate minerals in sedimentary rocks are also plotted (Rasmussen et al., 2005).

Fig. 2. Geological map of the study area (modified after Komiya, 2004). Secondary mineral assemblages of basaltic greenstones are also plotted. Grid number corresponds to thegeological map of Thorne et al. (1995).

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flood basalt, the greenstones in the study area erupted under the sea(e.g., Thorne and Trendall, 2001).

Although the contact between the Fortescue and HamersleyGroups is generally unconformable, the Hamersley Group in thestudy area conformably overlies the Fortescue Group (Thorne andTyler, 1996). The Hamersley Group in the study area consists ofmainly sedimentary rocks of BIF, chert, tuff, and felsic, basaltic, andkomatiitic volcanic rocks (Figs. 2 and 3). The BIF (Marra Mamba IronFormation, Mount McRae Shale, and Brockman Iron Formation) isintercalated with minor tuffaceous layers, shales, cherty conglomer-ate and a thin greenstone sheet (Komiya, 2004). Repeated basaltic andkomatiitic flows (Weeli Wolli Formation) conformably overlie the BIFand are intercalated with thin BIF layers, indicating the frequenthiatus and submarine eruption of basalt/basaltic komatiite volcanism.The overlying rhyolite (Woongarra Rhyolite) shows cooling joints,amygdules, flow banding, and breccias. The Weeli Wolli Formationand the Woongarra Ryholite are considered to have resulted from aLIP activity (Barley et al., 1997). The uppermost sedimentary rocks ofthe study area comprise alternating BIF and tuffaceous layers withmonolithic chert layers (Boolgeeda Iron Formation). In the study area,the strike of sedimentary bedding is NW–SE, while the dips aremostly50–60°S (Fig. 2). The depositional age of the Woongarra Rhyolite isconstrained by the U–Pb age of zircons, which is dated at 2449±3 Ma(Barley et al., 1997). The upper part of the Marra Mamba Formation(bottom of the Hamersley Group) and the Jeerinah Formation (top ofthe Fortescue) are dated at 2597±5 Ma (Trendall et al., 1998) and

2629±5 Ma (Nelson et al., 1999; Trendall et al., 2004), respectively.The depositional environment is interpreted as a sedimentary basin inan open rift valley (Thorne and Seymour, 1991).

3. Secondary mineral parageneses

The volcanic rocks in the study area were weakly metamorphosedto yield secondary minerals such as clay minerals, quartz (Qz), calcite(Cc), chlorite (Chl), K-feldspar, sericite, and biotite in felsic rock, andchlorite, epidote (Ep), actinolite (Act), prehnite (Prh), albite (Ab),oligoclase (Olg), biotite, quartz, sericite, and calcite in basaltic andkomatiitic rocks (Fig. 4A, B, and C). Under such low-grade metamor-phism, the original igneous textures are ubiquitously preserved.

Rhyolitehasquartz and feldsparphenocrysts, andaggregates of quartzand feldspar with felsitic texture in the groundmass (Fig. 4A). Quartzphenocrysts include melt inclusions that were replaced by chlorite/smectite and sericite. K-feldspar phenocrysts are replaced by sericite andsecondary K-feldspar, chlorite/smectite, and quartz (Fig. 4A). Plagioclasephenocrysts are replaced by albite, chlorite/smectite, and calcite. Minorbiotite phenocrysts are replaced by chlorite/smectite and/or secondarybiotite.

Among basaltic rocks, the fine-grained samples have intergranular,or intersertal textures, whereas coarse-grained samples display ophitictextures. The original igneous minerals in a groundmass of fine-grainedvolcanic rocks are partly or completely replaced by secondary mineralssuch as chlorite, prehnite, epidote, actinolite, quartz, sericite, biotite, and

Fig. 3. Simplified stratigraphic column section with metamorphic zones A to C of thestudy area. The lithologic patterns are same as in Fig. 2.

Fig. 4. Microscopic images of volcanic rocks in the study area, showing secondaryminerals. (A) Rhyolite in the upper part of the volcanic unit, (B) Basaltic rock in theHamersley Group, (C) Basaltic rock in the uppermost part of the Fortescue Group.

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calcite (Fig. 4B and C).Many igneous clinopyroxenes arewell-preservedin some coarse-grained samples. Olivine phenocrysts in basaltic rocksare completely replaced by aggregates of quartz, chlorite, and/orepidote. Igneous plagioclase is replaced by epidote, albite, prehnite,sericite, and calcite.

Basaltic komatiite has a spinifex texturewith clinopyroxene dendritesthat tends topreserve igneous clinopyroxene in the coreand is rimmedbychlorite and/or actinolite. Interstitial plagioclase is replaced by chlorite,albite, epidote, sericite, and minor K-feldspar and biotite.

We subdivided the study area into three zones from A to C on thebasis of the secondary mineral assemblages of basaltic greenstones(Figs. 2 and 4); note that the secondary minerals of rhyolite wereexcluded from the following description because of the differencein effective bulk composition for secondary minerals. Zone A ischaracterized by the assemblage Chl±Prh+Ep±Act+Qz+Ab±Cc(Fig. 5). The mineral assemblage of Zone B is Chl+Ep+Act+Qz+Ab±Cc and is characterized by the absence of prehnite compared toZone A (Fig. 5). Zone C is characterized by thefirst emergence of calcicplagioclase with oligoclase composition; the mineral assemblage isChl+Ep+Act+Ab+Olg+Qz±Cc (Fig. 5). The systematic changein the secondary assemblage from Zones A to C indicates that theassemblage changes stratigraphically downwards.

4. Mineral chemistry

The chemical compositions of secondary minerals in basalticgreenstones were analysed with an electron probe analyzer (JEOL-JXA-8800 M) at Tokyo Institute of Technology. All analyses wereperformed with an accelerating voltage of 15 kV, a 12 nA beamcurrent, and a counting time of 10–40 s.

Fig. 5. Secondary mineral parageneses of basaltic greenstones in the study area.

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4.1. Plagioclase

Secondary plagioclase is ubiquitous in all zones, whereas theigneous calcic plagioclases were completely replaced by secondarysodic plagioclase, sericite, calcite, and/or epidote. The anorthite (An)content of the plagioclases from Zones A to C is shown in Fig. 6.

In Zones A and B, all of the plagioclases are albite, and the Ancontent ranges from 0.4 to 6.5%; plagioclases in Zones A and B do notshow a clear compositional difference in An content. In Zone C, the Ancontent of plagioclase is clearly higher than those in Zones A and B,and albite and oligoclase coexist with a peristerite gap in some

Fig. 6. Frequency diagrams for anorthite (An) content of secondary plagioclase fromZonesA to C. A peristeritic gap between albite and oligoclase is also shown by broken lines.

samples; subhedral to irregular-shape oligoclase grain (<5 μm) issurrounded by host albite. For example, plagioclase in 95BR107 has abimodal composition of 2.2–8.3 and 15.2–23.8% of An content (Fig. 6).In summary, the plagioclases in Zones A and B are one phase withalbite composition, but two phases of plagioclase with a peristeritegap appears in Zone C (Fig. 6).

4.2. Prehnite

Prehnite occurs only in Zone A, replacing the interstitial glass and/orplagioclase. Assuming that the total iron is ferric, the variation of theXFe3+ (=Fe3+/(Fe3++Al)) is illustrated inFig. 7. TheXFe3+of prehnitetends to be lower than that of coexisting epidote.

4.3. Epidote

Epidote is one of the most common Ca–Al silicates in thegreenstones of the study area. It occurs as granular aggregates inthe groundmass with igneous textures and replaces plagioclasephenocrysts accompanied with chlorite in a few places. The variationof XFe3+ of epidote, assuming no ferrous iron and a formula of 12.5oxygens, is illustrated in Fig. 7.

Epidote in the study area shows a wide compositional variation interms of XFe3+ but has relatively homogeneous composition within asingle zone (Fig. 7). In Zone A, XFe3+ of epidote ranges from 0.27 to0.31, and epidote coexisting with prehnite shows higher XFe3+ than

Fig. 7. Frequency diagrams for XFe3+ in the prehnite and epidote from Zones A to C.

Fig. 9. Chemical composition of chlorite in the study area along with compositionalvariations of chlorites from the 3.2–3.0 Ga Cleaverville area, Pilbara Craton, WesternAustralia (Shibuya et al., 2007a).

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that of prehnite. All the epidotes in Zones B and C have lower XFe3+

(0.10–0.19) as compared to those in Zone A. The epidotes in theFortescue Group greenstones from the whole Pilbara Craton have asimilar XFe3+ variation (Smith et al., 1982).

4.4. Calcic amphibole

Colorless to pale-green Ca-amphiboles occur in Zones A, B, and C.The amphibole replaces the rim of relict clinopyroxene. Chemicalcompositions of Ca-amphiboles in Zones A to C are plotted on theamphibole classification diagram (Fig. 8A and B). The Fe3+ content ofamphibole was estimated from the stoichiometry based on a formulaof 23 oxygens (Terabayashi, 1993).

The Si content of amphibole in Zone A ranges from 7.71 to 7.95,and the XMg (=Mg/(Mg+Fe)) ranges from 0.54 to 0.62 (Fig. 8A). Theamphibole in Zone B has a Si content from 7.59 to 7.97 and an XMgfrom 0.51 to 0.77, while the composition of amphibole in Zone Cranges from 7.66 to 7.92 in Si content and from 0.50 to 0.64 in XMg.Based on the Ca-amphibole discrimination diagram of Leake et al.(1997), all amphiboles are classified as actinolite. Ca-amphiboleplotted on the Hallimond diagram (Hallimond, 1943) does not showsystematic change in the edenite, pargasite, and tschermakitecomponents from Zones A to C (Fig. 8B).

4.5. Chlorite

Chlorite replaces igneous olivine, clinopyroxene, plagioclase, andinterstitial glass. The chemical compositions of chlorites from Zones Ato C were plotted on Hey's diagram (Hey, 1954), assuming no ferriciron, on the basis of 28 oxygens in the formula (Fig. 9).

Fig. 8. Composition of Ca-amphiboles from Zones A to C, plotted on (A) Leake's diagram(Leake et al., 1997) and (B) Hallimond's diagram (Hallimond, 1943).

The chlorites have large compositional variations even within asingle zone, but are relatively homogeneous in terms of the XFe (=Fe/(Fe+Mg)) in each sample. Chlorite in Zone A has smaller composi-tional variation than that in Zones B and C. The XFe of chlorite in ZoneA ranges from 0.51 to 0.56, and the Si content from 5.38 to 5.99. TheXFe and Si content of chlorite in Zone B ranges from 0.39 to 0.62 and5.32 to 6.00, respectively. Chlorite in Zone C has XFe ranging from 0.39to 0.63 and Si content ranging from 5.47 to 5.92. Chlorite in the studyarea mostly overlaps the composition of chlorites from hydrother-mally metamorphosed greenstones in the 3.2–3.0 Ga Cleaverville area(Shibuya et al., 2007a).

5. Discussion

5.1. Low-pressure metamorphism

5.1.1. Mineral assemblage and compositionHere we discuss the compositional changes of secondary minerals

related to the metamorphic grade in the study area. In general, the Ancontent of plagioclase increases with metamorphic grade, and twoplagioclases have a peristerite gap at around 12% of An content underlow-pressure conditions (e.g., Maruyama et al., 1982). Similarcompositional changes of plagioclase were reported from theHorokanai ophiolite in Japan (Ishizuka, 1985) and Karmutsenmetabasites of the contact metamorphic aureole in Vancouver Island,Canada (Terabayashi, 1993), indicating that the increase in the Ancontent of plagioclase in the metabasite represents an increase inmetamorphic grade (Ishizuka, 1985; Terabayashi, 1993). In the studyarea, the plagioclases in Zones A and B are in one phase with albitecomposition, but two phases of plagioclase with a peristerite gapappear in Zone C (Fig. 6). Therefore, the increase in the An content ofplagioclase in the study area indicates that the metamorphic gradeincreases from Zone B to C under low-pressure conditions. Further-more, XFe3+ of epidote in Zone A is clearly higher than that in Zones Band C (Fig. 7). This indicates the lower metamorphic grade of Zone Acompared to that of Zones B and C because of the general decrease inXFe3+ of epidote with increasing metamorphic grade (e.g., Ishizuka,1985). In addition, the XFe of chlorite reflects not only themetamorphic grade but also the XFe of effective bulk compositionfor secondary minerals (e.g., Hayashi et al., 2000). Therefore, thevariation of the XFe within the single zone was probably generated bya difference in the effective bulk composition for secondary minerals.

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The transition from greenschist to amphibolite facies is subdividedinto a higher-pressure albite–epidote–amphibolite subfacies and alower-pressure actinolite–calcic plagioclase subfacies (e.g., Miyashiro,1973; Maruyama et al., 1983). Therefore, the appearance of calcicplagioclase precedes that of hornblende with increasing temperatureat low-pressure (<2 kbar; e.g., Maruyama et al., 1983). A calcicplagioclase-bearing assemblage without hornblende is also reportedfrom the prehnite–actinolite facies of low-pressure contact metamor-phism (Terabayashi, 1993) and on-land ophiolites (Liou and Ernst,1979; Ishizuka, 1985; Shibuya et al., 2007b), which also indicates low-pressure conditions. In the study area, amphibole is the only actinolite(Fig. 8a and b), but calcic plagioclase is present in Zone C (Fig. 6).Therefore, the assemblage of Olg+Act+Ep+Chl in Zone C indicateslow-pressure conditions (<2 kbar).

5.1.2. Metamorphic reactions and conditionsTwo metamorphic reactions that define the boundaries between

Zones A to C are investigated based on the calcite-free assemblages.The pressure–temperature (P–T) conditions of the following reactionswere calculated with THERMOCALC 3.25 (Holland and Powell, 1998and its update). A model CaO–MgO–Al2O3–SiO2–H2O system, in thepresence of excess quartz and H2O, is assumed.

The diagnostic mineral assemblage changes from Chl+Ep+Prh+Act+Ab+Qz in Zone A to Chl+Ep+Act+Ab+Qz in Zone B. Themetamorphic reaction between Zone A and B is defined by the disap-pearance of prehnite from Zone A to B. This can be represented throughthe following reaction:

5prehniteþ chlorite þ 2quartz ¼ 4clinozoisiteinepidote þ actinolite þ 6H2O

ð1ÞThis reaction has a steep Clapeyron slope on a P–T diagram

(Fig. 10), whichmeans that themetamorphic temperature of Zone A isless than approximately 300 °C. Furthermore, the prehnite-bearingassemblage in Zone A indicates prehnite–actinolite facies metamor-phism, which strongly suggests low-pressure conditions (<2–3 kbar;Liou et al., 1985; Beiersdorfer and Day, 1995).

Fig. 10. Schematic P–T diagram for metamorphism of basaltic rocks in the study area.Lines 1 and 2 indicate P–T conditions for the metamorphic reactions between Zones Aand B, and Zones B and C, respectively. The dark gray band indicates estimated P–Tconditions (A–C for Zones A to C). For comparison, a petrological grid is also shown (Liouet al., 1985; Beiersdorfer and Day, 1995). ZEO, zeolite facies; PA, pumpellyite–actinolitefacies; PP, prehnite–pumpellyite facies; PRA, prehnite–actinolite facies; BS, blueschistfacies; GS, greenschist facies; EA, epidote–amphibolite facies; AP, actinolite–calcicplagioclase facies; AM, amphibolite facies.

The reaction between Zones B and C, defined by the appearance ofoligoclase in the metabasite, can be attributed to the followingreaction:

chloriteþ 6clinozoisite þ 7quartz ¼ actinolite þ 10anorthite in plagioclaseþ 6H2O

ð2ÞThis reaction represents decomposition of epidote and formation of

anorthite in secondary plagioclase. It is therefore suggested that theassemblageChl+Ep+Act+Olg inZoneCwas formed through reaction(2) and at temperatures >330 °C (Fig. 10). However, this assemblageoccurs widely in Zone C, which is not consistent with the discontinuousreaction (2). This should be derived from the assumption of a FeO-freemodel system. The addition of FeO into the systemmakes reaction (2) acontinuous reaction, and results in awide temperature range dependingon the variation of the FeO/MgO ratio in the system. In addition, theassemblage of Chl+Ep+Act+Olg is generally formed under transitionfrom greenschist facies to actinolite–calcic plagioclase facies, whichindicates low-pressure conditions (<2 kbar; Maruyama et al., 1983;Liou et al., 1985).

5.2. Metamorphism of the study area

It iswell known that the Pilbara Cratonunderwentpost-depositionalmetamorphic and/or hydrothermal events inwhole or part (Smith et al.,1982; Macfarlane and Holland, 1991; Nelson et al., 1992; Alibert andMcCulloch, 1993; Erel et al., 1997; Rasmussen et al., 2001, 2005;Rasmussen, 2005). Based on the secondary mineral assemblages of theFortescue Group greenstones, Smith et al. (1982) suggested thatmetamorphic grade of the Pilbara Craton increases southward, whichwas considered to reflect an increasing depth of the burial metamor-phism (Fig. 1). However, the precise in-situ dating of metamorphicphosphates in sedimentary rocks revealed that the Pilbara Cratonrecorded two major thermal events at ca. 2.4 Ga and 2.2 Ga in regionalscale (Fig. 1) (Rasmussen et al., 2005). The cause of the former event hasbeen still uncertain but Rasmussen et al. (2005) suggested that the lattermetamorphic age corresponds to the Ophthalmian orogeny based onthe alignment of the dated phosphate fabrics. This eventwas consideredto be associated with the collision along the southern margin of thePilbara Craton (Blake and Barley, 1992). Thus, the general southwardincreasing metamorphic grade of the Pilbara Craton reported by Smithet al. (1982) was reinterpreted to be related to migration ofmetamorphic front from the collisional zone, spanning a 70 million-year period fromca. 2215 Manearest the collisionalmargin in the south,to ca. 2145 Ma toward the craton interior in the north (Fig. 1)(Rasmussen et al., 2005). However, this interpretation may be notnecessarily be consistent with the metamorphic zones reported by theSmith et al. (1982) because the metamorphic grade increases generallyincreases southward (fromZI to ZIII) but lowergrade zone (ZIII) appearsagain in the south of the highest zone (ZIV) (Fig. 1).

In the study area, the increase of themetamorphic grade from Zone Ato C indicate that the metamorphic grade increases northward, which isopposite direction to the general southward increasing grade of theregional metamorphism but probably corresponds to southward de-creasing metamorphic grade in the southernmost part of the PilbaraCraton (Fig. 1) (Smith et al., 1982). If the metamorphism had occurredduring the Ophthalmian orogeny due to the collision from the south,metamorphic grade would decrease toward the north. It is thereforesuggested that the metamorphism of the Hamersley basin does notcorrespond to a simple southward increasing grade. More importantly,the increasing metamorphic grade from Zone A to C also indicates anincrease in metamorphic temperature with stratigraphic depth. Thestratigraphy-related thermal structure could not be recorded if themetamorphism had occurred after folding due to the Ophthalmianorogeny. Therefore, the preservation of stratigraphy-related metamor-phismstrongly suggests that themetamorphismwas causedby a thermal

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event before the Ophthalmian folding. Previously, Thorne and Tyler(1996) presumed that the isograd pattern in the Hamersley Basinreported by Smith et al. (1982) appears to reflect the fold pattern, withlower grade rocks in the synclines and higher grade rocks in anticlines.This indicates that the stratigraphically lower units show highermetamorphic grades and that the metamorphic pattern was folded bythe Ophthalmian orogeny. Hence, the stratigraphy-related metamor-phism reported here supports the prediction by Thorne and Tyler (1996).

Although geochronological study on the metamorphic mineralsindicates that thermal event prior to the Ophthalmian orogeny wasalternatively the 2.4 Ga unknown thermal event (Rasmussen et al.,2005), there has been no geological evidence for the 2.4 Ga thermalevent in the Hardey Syncline. However, pre-Ophthalmian magmaticactivity was recorded in the Hardey syncline as dolerite sills thatintruded into the Turee Creek Group, which is an evidence for ca.2.2 Ga continental rifting before the beginning of the Ophthalmianorogeny (Müller et al., 2005). The dolerite sill has a baddeleyite age of2208±10 Ma (Müller et al., 2005), which is identical to the age of2216±13 Ma for metamorphic phosphate in sedimentary rocks nearthe study area (Fig. 1) (Rasmussen et al., 2005) within the errors.Furthermore, the mafic sill was folded by the Ophthalmian folding,indicating that the rifting event preceded the Ophthalmian folding(Müller et al., 2005). Therefore, the stratigraphy-related, low-pressuremetamorphism of the study area was likely caused by the 2.2 Gacontinental rifting. As pointed byMüller et al. (2005), the age of 2.2 Gais a global peak in LIP activity (French et al., 2004) as observed in ca.2210 Ma (2217.2±4 and 2209.6±3.5 Ma) Nippissing sills thatintruded the Huronian Supergroup of the Superior Province, Canada(Noble and Lightfoot, 1992) and in 2222±13 Ma volcanic rocks of theOngeluk and Hekpoort formations in the Transvaal Supergroup of theKaapvaal Craton, South Africa (Cornell et al., 1996). Such a vigorousmagmatic activity should have generated a high geothermal gradientin the continental crust, which could cause the stratigraphy-related,low-pressure metamorphism observed in the study area.

6. Conclusions

(1) The metamorphic grades of the Hamersley and the FortescueGroups in the Hardey Syncline are newly defined in this paper.The secondary mineral assemblages and compositions ofbasaltic rocks from the study area indicate that the metamor-phic grade ranges from prehnite–actinolite facies (HamersleyGroup), through greenschist facies (Fortescue Group), to atransition between greenschist facies and actinolite–calcicplagioclase facies (Fortescue Group).

(2) The metamorphic grade in the study area increases northward,namely stratigraphically downward, which is also not consistentwith the model that the regional metamorphism was caused bythe Ophthalmian orogeny (Rasmussen et al., 2005). However, thisfinding is consistent with a southward decreasing metamorphicgrade in the southernmost part of the Pilbara Craton (Smith et al.,1982). Furthermore, the low-pressure type metamorphic faciesseries and the correlation between the metamorphic grade andstratigraphy suggest that themetamorphismof the study areawascaused by a thermal event before the Ophthalmian folding.

(3) The mafic sills with an age of 2208±10 Ma in the HardeySyncline are considered tobe associatedwith a continental riftingevent immediately prior to the Ophthalmian orogeny (Mülleret al., 2005), which is interpreted as the most probable cause ofmetamorphism in the study area because the metamorphic agereported near the study area (2216±13Ma; Rasmussen et al.,2005) is identical to the age of intrusion. The low-pressuremetamorphic condition and the increasing temperature withstratigraphic depth also support this model.

Acknowledgements

We thank S. Johnson and an anonymous reviewer for improvingthe paper. This research was partially supported by the 21st CenturyCOE Program “How to build habitable planets,” Tokyo Institute ofTechnology, sponsored by the Ministry of Education, Culture, Sports,Technology and Science, Japan. T.S. is grateful for a Research Fellowshipfrom the Japan Society for the Promotion of Science for Young Scientists.

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