J. metamorphic Geol., 1999, 17, 145–156

Model for the development of kyanite during partial convectiveoverturn of Archean granite–greenstone terranes:the Pilbara Craton, AustraliaW. J. COLLINS AND M. J . VAN KRANENDONK*Department of Geology, University of Newcastle, Newcastle, NSW 2308, Australia

ABSTRACT Restricted occurrences of early, syn- and late-kinematic kyanite adjacent to large domal batholiths in theArchean granite–greenstone terrane of the east Pilbara craton, Australia, are considered to result frompartial convective overturn of the crust. The analogue models of Dixon & Summers (1983) and thermo-mechanical models of Mareschal & West (1980), involving gravitionational overturn of dense greenstonecrust that initially overlay sialic basement, successfully explain the geometry, dimension, kinematics andstrain patterns of the batholiths and greenstone rims. Application of these models suggests that andalusiteand sillimanite are the stable aluminosilicate polymorphs in domal crests and rims, where progradeclockwise P–T –t paths, with small pressure changes, should be recorded. Both aluminosilicates are pre-dicted to overprint kyanite, which is observed locally around the east Pilbara domes. Kyanite is thepredicted aluminosilicate polymorph in the deeper parts of domal rims and within sinking greenstonekeels, reflecting rapid, near-isothermal burial. The narrow zones of kyanite-bearing schists adjacent tosome batholiths in the Pilbara craton are metamorphosed, highly strained equivalents of altered felsicvolcanic rocks in the low-grade greenstone succession, dragged to mid-crustal depths (6 kbar) duringgreenstone sinking. The schists rebounded as an arcuate tectonic wedge along the southern Mount Edgarbatholith rim, during the later stages of doming, and were juxtaposed against regional, greenschist facies,low-strain greenstones. Thus, kyanite was preserved: if the walls had remained at depth, it would havebeen overprinted by the higher-temperature aluminosilicate polymorphs during thermal recovery.

Kyanite growth in the Pilbara craton is unlikely to have resulted from ballooning of plutons, mantledgneiss doming, metamorphic core complex formation, or early crustal overthickening. The typical sub-vertical foliations and lineations of the tectonic wedge suggest that subvertical fabrics extended tomid-crustal depths (c. 20 km) before rebound, providing a three-dimensional glimpse of Archean dome-and-keel structures. The general occurrence of large granitoid domes in Archean granite–greenstoneterranes, restriction of rare kyanite to the adjacent, high-strain batholith margins, and its absence fromthe batholiths, suggest that partial convective overturn of the crust may have been a common process atthis early stage of Earth history.

Key words: Archean; diapirs; granite–greenstone terranes; kyanite; P–T –t paths.

reported from Archean GGTs in Australia, Africa,INTRODUCTION

Canada, Greenland, India and Russia (see Percival,1979 for references), but the proposed tectonic settingsKyanite is one of the index minerals of modern

collisional orogens characterized by clockwise P–T –t for these terranes is conjectural, raising the need toestablish dynamic environments within which thispaths, where it forms part of an early assemblage in

response to crustal thickening (e.g. England & mineral will occur.In the Archean Pilbara craton of Australia, kyaniteThompson, 1984). A general absence of intermediate-

to high-pressure rocks, including those with kyanite- occurs in felsic schists at the margins of two large(>50 km diameter), multiphase granitoid batholiths,bearing assemblages, across large areas of generally

low-grade Archean granitoid–greenstone terranes the Mount Edgar (MEB) and Shaw batholiths (Fig. 1).These aluminous quartz–sericite±alumino-silicate(GGTs) has prompted suggestions that modern colli-

sonal orogeny did not affect these areas, or that schists are locally metasomatized and highly deformedequivalents of the felsic volcanic Duffer formation ofArchean geothermal gradients were too high to

preserve kyanite throughout the later stages of orogeny the Lower Warrawoona group (Hickman, 1983; Cullerset al., 1993). The kyanite schists are restricted to the(e.g. Choukroune et al., 1995). Rare kyanite has beensouthern and western sides of the MEB and Shawbatholith, respectively, and terminate abruptly at high-*Present address: Geological Survey of Western Australia, 100

Plain St., East Perth, WA, 6004, Australia. angle faults. Along strike, in the northern Shaw

145© Blackwell Science Inc., 0263-4929/99/$14.00Journal of Metamorphic Geology, Volume 17, Number 2, 1999

146 W. J . COLLINS & M. J . VAN KRANENDONK

Fig. 1. Regional geology of the eastern Pilbara craton showing restricted occurrence of kyanite near batholith rims in the inferredhigh-strain equivalents of the Duffer formation. Note facing of greenstones off the batholith domes, which contain granites andgneisses of the same age as the greenstone successions. Note the zone of Late Archean structural overprinting on western side ofShaw batholith. Flow lines in batholiths from Mackey & Richardson (1997). TB, Tambourah Dome.

batholith region and Marble Bar belt (Fig. 1), the crustal evolution, involving subhorizontal tectonics(e.g. Kusky, 1989; Percival & Williams, 1989; Kusky,precursor Duffer formation felsic volcanic rocks are

weakly deformed and metamorphosed. The general 1993; de Wit et al., 1992).Structural fabric elements in the kyanite schists ofabsence of kyanite and its restriction to highly strained

batholith margins places tight constraints on tectonic the southern MEB rim have been interpreted byCollins (1989) and Teyssier & Collins (1990) to resultmodels for the Pilbara craton.

Contrasting models of formation have been pro- from the diapiric rise of the batholiths, and concomitantsinking of the greenstones. Collins et al. (1998)moted for the two kyanite occurrences. Kyanite in an

area of tight, reclined, refolded folds on the western suggested that the former were produced by partialconvective overturn of the crust. Thus, a controversymargin of the Shaw batholith led Bickle et al. (1985)

to suggest that it developed in response to crustal exists about the tectonic significance of kyanite withinthe one craton.thickening during regional, Alpine-style tectonism, and

that the resultant Archean crust was of similar In this paper, we describe the occurrences of kyanitein the Archean Pilbara craton in an attempt to resolve(55–60 km) thickness to normal Phanerozoic orogenic

belts. The Alpine-style crustal thickening was viewed this dilemma. Results of thermo-mechanical modelsdeveloped by Mareschal & West (1980) have beenas the mechanism responsible for subsequent diapirism

of buoyant sialic crust that was buried under an applied to the MEB and Shaw batholith to determinewhether the distribution of aluminosilicates could haveanomalously thick pile of dense greenstones. This

hypothesis is similar to more recent models for Archean resulted from crustal overturn. Although rare, kyanite

KYANITE IN ARCHEAN GRANTITE–GREENSTONE TERRANES 147

occupies a similar structural position in other Archean tral strike–slip shear (Krapez & Barley, 1987; Krapez,1993; Van Kranendonk & Collins, 1998; Zegers et al.,GGTs, so the results of this work may have general

application. 1998). Isolated, syn-kinematic granitoid stocksaccompanied the shearing. Post-tectonic granites wereemplaced c. 2860–2830 Ma (Bickle et al., 1989; Collins

PILBARA GEOLOGY& Gray, 1990).

The greenstone sequence is relatively undeformedThe Archean Pilbara craton is a low-grade GGT witha characteristic dome-and-basin map pattern defined throughout the east Pilbara. Aside from the kyanite-

bearing, narrow high-strain zones around the batho-by multiphase granitoid domes 30–100 km in diameter,and intervening, arcuate, synformal-to-monoclinal gre- liths and the Late Archean strike–slip deformation

zone in the central Pilbara (Van Kranendonk &enstone belts comprising predominantly low-grade,weakly strained, ultramafic–mafic volcanic rocks, less Collins, 1998), the rocks typically lack tectonic fabrics.

Vesicles are preserved in undeformed pillow basalts,abundant felsic volcanic rocks and subvolcanic sills,and subordinate chemical and clastic sedimentary lenticles and shards are preserved in ignimbrites as

typical eutaxitic textures, and delicate growth struc-rocks (Hickman, 1983, 1984). Four unconformity-bound lithostratigraphic packages and three corre- tures are preserved in the North Pole stromatolites.

Greenschist facies metamorphism is widespread, exceptsponding periods of granitoid magmatism exist withinthe craton. near batholith margins where it reaches amphibolite

facies, and the sequences around the domes areThe lowermost stratigraphic package is theWarrawoona group, comprising mafic/ultramafic and invariably right-way-up. These features indicate that

much of the east Pilbara has not suffered the effects offelsic volcanic rocks ranging in age from 3515–3450 Ma, including the Duffer formation (e.g. tectonic reworking since dome formation.

The timing of dome-and-keel formation is variableMcNaughton et al., 1993; Buick et al., 1995). Awidespread suite of subvolcanic tonalite–trondhjemite– throughout the craton. Pre-, syn- and postkinematic

granitoid plutons in the MEB range from 3320–granodiorite (TTG) magmas was emplaced at thisstage, particularly in the Shaw batholith (Bickle et al., 3310 Myr old (Williams & Collins, 1990; Collins et al.,

1998), indicating that doming occurred at this stage.1993; Thorpe et al., 1992; McNaughton et al., 1993).The second package comprises a distinctive basal A similar timing of dome formation is evident in the

CDB (Collins et al., 1998), whereas most workerssuccession of metasediments and cherts of the Towersformation, which pass upwards into a thick section of consider that doming in the Shaw batholith occurred

at c. 3000–2950 Ma, based on 40Ar/39Ar age determi-interlayered mafic and komatiitic lavas of the Salgashsubgroup. The age of this subgroup is somewhat nations of hornblende from the western Shaw region

(Wijbrans & McDougall, 1987). In contrast, Zegerscontroversial. In one area, a felsic volcanic unitstratigraphically above the Duffer formation is et al. (1996) suggested that initial doming in the Shaw

batholith occurred at c. 3470 Ma, as part of an3454±1 Myr old and interpreted as part of the Salgashsubgroup (Thorpe et al., 1992), but the unit is not extensional core complex, although more recent

40Ar/39Ar age determinations of 3240–3170 Ma fromunderlain by the Towers formation and thus may bepart of the first package. In another area, several felsic the inferred major detachment zone (Davids et al.,

1997) suggest that doming occured closer to 3200 Ma.volcanic flows from above the Towers formation,interlayered with mafic volcanic rocks of the Salgash Thus, doming appears to have occurred diachronously

in the east Pilbara between 3400 and 2700 Ma, theSubgroup, are 3325±1 and 3324±3 Myr old(McNaughton et al., 1993), which we interpret as the MEB developing as one of the earliest domal structures

at 3320–3310 Ma.true age of this subgroup. This is the same age asconformably to unconformably overlying felsic volcanicrocks of the Wyman formation (Thorpe et al., 1992;

Kyanite adjacent to the Mount Edgar batholithMcNaughton et al., 1993). A widespread suite ofc. 3310 Myr old granitoid intrusions accompanied A narrow (<5 km) rim of high-grade supracrustal

rocks extends for over 50 km around the southernfelsic volcanism (Williams & Collins, 1990). Thegranitoids were voluminous in the MEB and probably margin of the MEB (Fig. 1). Nearest the batholith, the

rocks consist of amphibolites, quartzofeldspathicin the Corunna Downs batholith, but were much lesscommon in the Shaw batholith. schists and kyanite-bearing aluminous schists, rep-

resenting highly strained equivalents of the LowerThe third sequence comprises andesitic to felsicvolcanic rocks of the Strelley succession and con- Warrawoona group (Collins, 1989). Farther outward,

the rocks are dominantly cherts, metabasalts andformably overlying banded iron formation and clasticsedimentary rocks of the Gorge Creek group. These minor metapelites considered to be the Towers forma-

tion (Hickman, 1983). The outermost part of the high-were also involved in dome-and-keel formation. Thehighest stratigraphic sequence is the c. 2950 Myr old grade rim contains ultramafic schists of the Salgash

subgroup, which stratigraphically overlies the TowersDe Grey group, manifest primarily as the Lalla Rookhsandstone, which was deposited during regional sinis- formation. They are disrupted by the Klondyke fault,

148 W. J . COLLINS & M. J . VAN KRANENDONK

Fig. 2. Map showing wedge of high-grade, kyanite-bearing rimof supracrustal rocks, between low-grade greenstones and theMount Edgar batholith. The Salgash fault is the transferstructure between the bounding D4 shear zone and theKlondyke fault.

which separates the higher strained rocks from lower-grade, lower-strain greenstones of the subgroup in theWarrawoona syncline (Fig. 2). Greenstones in thelimbs of the syncline young inward and the synclinalaxis is occupied by the Wyman formation, whichstratigraphically overlies the Salgash subgroup.

Kyanite occurs in the high-grade rim in myloniticmuscovite–quartz-bearing schists and in late-tectonicpegmatitic to hydrothermal (quartz–kyanite–muscov-ite) veinlets that cut the mylonitic foliation. In themylonitic schists (Fig. 2), kyanite exists in threetextural modes:1 As distended, rotated porphyroblasts (Fig. 3a) withnumerous quartz inclusion trails;2 As highly aligned blades arranged with muscovitein an asymmetric mylonitic S1 foliation (Fig. 3a) thelong axis defining the steep down-dip (L1) lineationthat typifies the schists, and with an S–C asymmetry Fig. 3. (a) Early kinematic kyanite porphyroblast around whichthat indicates batholith-side up movement (Collins, a kyanite–muscovite–quartz mylonitic foliation is deflected.

Field of view is 5.5 mm. (b) Late-kinematic kyanite1989; Teyssier & Collins, 1990); andovergrowing kyanite–muscovite–quartz mylonitic foliation.3 As irregular grains cutting across the stronglyField of view is 1.2 mm. (c) Stellate clusters of kyanite–quartzfoliated kyanite blades (Fig. 3b), presumably reflectingin late-stage (post-D1, pre-D2) dykes. Retrogressive overprint

late kinematic growth. is sericite. Field of view is 5.5 mm.Late muscovite also overgrows the mylonitic foliation.These textures indicate P–T conditions within thestability field of kyanite throughout the recorded to tight F2 folds at this locality. Thus, the kyanite

veins are post-D1 and late-D2.deformation history of the rock. Kyanite also occursas clusters of stellate crystals in quartz–muscovite veins The high-grade rim exists as a tectonic wedge bound

by the Salgash fault to the west and the Klondyke(Fig. 3c), which cut S1 in the hinge of the major F2fold (centre of Fig. 2). However, the veins have a weak fault to the south (Fig. 2). The Salgash fault is a

steeply dipping, S- to SSW-striking structure, whichsubvertical foliation (S2) concordant with the F2 axialplane, and are openly folded, in contrast to the close bifurcates from the D4 rim shear zone of the MEB

KYANITE IN ARCHEAN GRANTITE–GREENSTONE TERRANES 149

and terminates at the axis of the Warrawoona syncline. the sinking greenstones and rising batholith, whichmigrates toward the keel as the dome-and-keel struc-The marked southward decrease of stratigraphic offset

and metamorphic grade change indicates that it is a ture evolves (Collins et al., 1998). As such, thedeformation mechanism is best described as convectivescissor fault, with upthrow on the eastern side.

The Klondyke fault is the southern bounding overturn of the crust, rather than diapirism, to highlightthe role of greenstones in the formation of the dome-structure of the kyanite-bearing tectonic wedge. S/C

fabrics in the fault indicate consistent NE block-up and-keel structure. However, the GGTs rarely (if ever)completely invert, principally because of the highsense of movement. The fault dips steeply NE and

separates amphibolite facies ultramafic/mafic and felsic viscosity of sialic crust which increases as the domesrise and cool. Rather, the process goes to partialL–S tectonites from greenschist facies metabasalt and

chert L–S tectonites to the south (Fig. 2), all of which completion; hence the term, partial convective overturn(Collins et al., 1998).are part of the Salgash subgroup. It also truncates

major sections of the north limb and part of the Collins et al. (1998), following Teyssier & Collins(1990), suggested that the structure, strain pattern andsynclinal axis of the Warrawoona syncline and can be

traced for more than 50 km eastward around the kinematics of the MEB and adjacent Warrawoonasyncline resulted from sinking of greenstones throughMEB, but terminates against the Salgash fault to the

west (Figs 1, 2). Thus, the rim of kyanite-bearing the sialic crust, similar to the analogue models forinterdiapir synclines generated by Dixon & Summersschists around the southern margin of the MEB is

separated from the regional greenschist-facies green- (1983). Cross-folding mechanisms were considered,including basement–cover disharmonic folding, butstone belts by the Klondyke and Salgash faults.rejected as possiblities for dome-and-keel formation(Collins et al., 1998), primarily because the domes lack

Aluminosilicates in the Shaw batholith domethe early fold generations necessary for fold inter-ference, lineations become progressively better devel-Kyanite occurs together with either chloritoid, stauro-

lite and chlorite (+quartz+muscovite) in lower-grade oped toward greenstone synclines (see below) and theirtrend lines do not pass through triple junctions.aluminous mylonitic schists, and with staurolite–

biotite±garnet (+quartz+muscovite) in higher-grade The strain pattern around the southern margin ofthe MEB and southward through the Warrawoonamylonitic schists around the margin of the Shaw

batholith (Bickle et al., 1985). Kyanite locally over- syncline is characterized by a progressive increase froma weakly inclined flattening (S-tectonite) to intensegrows a crenulated foliation and is also locally

pseudomorphed by andalusite (Morant, 1984). vertical constrictional strain (L-tectonite) as the syn-cline is approached (Fig. 4). The most lineated rocksTypically, the kyanite-bearing assemblages are syn- to

postkinematic with respect to a mylonitic schistosity, consistently lie in the axial trace of the syncline, andkinematic criteria from the S–L tectonites consistentlysimilar to that seen around the MEB. Closer to, and

within large supracrustal rafts in the batholith, eitherandalusite is replaced by sillimanite or sillimanite isthe only aluminosilicate phase present (Morant, 1984).The distribution of aluminosilicates indicates that thegeothermal gradients become steeper toward thebatholith.

MODELLED AND OBSERVED GRANITE–GREENSTONE TERRANES

The Dixon & Summers (1983) and Mareschal & West(1980) analogue and thermo-mechanical modelsdescribe convective overturn of the crust. They showthat deformation begins by subsidence of the densegreenstones into sialic basement, which proceedsrapidly, with domes rising more slowly and passively.Whereas the narrow greenstone synclines sink some

Fig. 4. Map distribution of tectonite types in the Warrawoona15 km below the 10 km deep basement–cover interface,syncline. Note progressive increase in constrictional strain

and show intense vertical constrictional strain and toward axis of syncline and convergence of lineation towardstrong horizontal shortening, the granitoid domes are central part of diagram. Using sense of shear criteria and

lineation orientation, the large arrows represent the particlemuch broader in diameter, rise only 3 km within thetrajectory relative to the Mount Edgar batholith. They outlinecrest, and have an accumulated subhorizontal flat-a zone of sinking within the synclinal axis, which closely

tening strain that is an order of magnitude lower than resembles a node of maximum vertical constrictional strainin the greenstones. The crustal flow pattern is a developed for analogue models of interdiapir synclines (Dixon

& Summers, 1983) (modified from Teyssier & Collins, 1990).convective cell centred approximately halfway between

150 W. J . COLLINS & M. J . VAN KRANENDONK

indicate a ‘MEB-up’ sense of shear (Teyssier & Collins,1990). Furthermore, the lineations converged towarda central point along the synclinal axis (Fig. 4), whichcorresponds to one of the two sites of preferentialsubsidence generated by sinking of a dense surfacelayer through a less dense substrate (Fig. 12 in Dixon& Summers, 1983). If the MEB is taken as a fixedreference point, the inferred particle trajectories(approximated by lineation attitude) converge into thezone of intense vertical constrictional strain. Giventhat the sense of shear in the Warrawoona syncline isconsistently batholith-up, greenstone-down, Teyssier &Collins (1990) described this area as a ‘zone of sinking’.

Although the analogue models for greenstone sinking(Dixon & Summers, 1983) closely match the strainpattern and kinematic history of the Warrawoonasyncline, they do not consider strain patterns withinthe domes; so it is difficult to establish the effect ofthis process on the batholiths. Nonetheless, incipientdevelopment of subsiding synclines appears to inducea ‘return flow’ in putative domes, resembling aconvective flow pattern (Dixon & Summers, 1983;Model A18). The strain patterns in domes were bettermodelled by Mareschal & West (1980), who showedthat gravitational subsidence of dense cover induceshigh-strain, rapidly developed greenstone keels, withlower-strain sialic crust rising relatively passively in itsplace. The strain pattern exhibited by the Mareschal& West models closely resembles those observed inthe Warrawoona syncline and the MEB.

The geometrical comparison between the GGTstructures modelled by Mareschal & West (1980) andthose observed in the Pilbara is also remarkable.Spacing between the central points of the major domesis 50–60 km in the Pilbara craton, similar to the 50 kmwavelength chosen for the modelling. The resultant12 km wide greenstone keel (at 10 km depth) in themodel is within the 10–15 km range of many interven-ing greenstone belts in the Pilbara. Furthermore,greenstone layers above the domes in the Pilbara

Fig. 5. (a) Cross-section of mature diapir adapted for theremain shallow- to moderately dipping (<45°) toPilbara GGT, showing instantaneous fields for aluminosilicatewithin approximately 5 km of the outer margin, wherepolymorphs, based on isotherms calculated by Mareschal &

the orientation rapidly changes to subvertical or West (1980; their Figure 5). Trajectories of rock particles areslightly overturned (e.g. the Marble Bar Belt and shown during convective overturn: (1) rising granitoid core; (2)

rising granitoid crest, (3) sinking synclinal keel. (b) InferredNorth Pole Dome: Hickman & Lipple, 1978), matchingP–T –t paths for particle trajectories in (a). Paths 2 and 3closely the modelled greenstone geometry (Fig. 5a).assume an initial greenstone thickness of 10 km. LimitingMareschal & West (1980) also suggested that thethermal conditions of the Archean crust during convective

greenstone keels should sink to a maximum of overturn are shaded. Initial isobaric heating is required toc. 25 km, an estimate that has been verified independ- trigger overturn. Light shading reflects P–T variation along

granite–greenstone contact.ently by Delor et al. (1991) for schistose rocks aroundthe margin of the MEB and CDB. At two separ-ate locations, they identified S1 assemblages of sillimanite overprints kyanite in metapelite and requires

a slightly higher temperature estimate of 575±50 °C.quartz+muscovite+kyanite±sillimanite in metapel-ites and quartz+plagioclase+hornblende±garnet in The P–T estimate for adjacent metabasites is

600±50 °C at 5.5±1 kbar. Taken together, the datametabasites. At locality 1, using the internally consist-ent dataset of Holland & Powell (1990), they calculated suggest that the domal walls equilibrated at 5.5–6 kbar

at 500–600 °C. As these rocks represent the domalP–T estimates of 5.5±1 kbar at 475±50 °C for themetapelites and 6.75±1 kbar at 520±50 °C for the wall, those in the synclinal axis should extend to

greater depth.metabasites. At locality 2, closer to the domal wall,

KYANITE IN ARCHEAN GRANTITE–GREENSTONE TERRANES 151

Another common feature of the modelled and In order to induce widespread anatexis at midcrustallevels, the stable Archean geotherm of c. <25 °C km−1observed domes is the short time span of formation.

The results of Mareschal & West (1980; their Fig. 4) must have been elevated to c. 40 °C km−1. West &Mareschal (1979) assumed it resulted from the blanket-indicate that once the viscosity of the granitic ‘middle

crust’ was reduced sufficiently to initiate deformation, ing effect of overlying greenstones and radioactivedecay in the underlying granitoids. We consider itit proceeded rapidly, within several million years.

Supporting this predicted result are U–Pb zircon more likely that heat was derived by conductivecooling of the mantle plume, which generated theSHRIMP analyses from the MEB, which indicate that

early, syn- and postkinematic granitoid intrusions are komatiite-bearing c. 3325 Myr old greenstone suc-cession of the Upper Warrawoona group (Collins et al.,all the same age, within experimental error (Williams

& Collins, 1990; Collins et al., 1998): The U–Pb zircon 1998). The ≤20 Myr time lag between greenstoneeruption and granite generation (and deformation) isage for a prekinematic granitoid is 3314±13 Ma,

whereas the age of a postkinematic pluton is the period required for conductive heating of the crustabove a plume head (Campbell & Hill, 1988).3324±7 Ma, which indicates that deformation associ-

ated with convective overturn was a very rapid process. The effects of a raised geotherm were calculatedusing the first set of Mareschal & West (1980) modelsand the aluminosilicate triple point of Bohlen et al.

PRESSURE–TEMPERATURE CONDITIONS(1991). The trajectories of rock particles from theDURING CONVECTIVE OVERTURNdomal core (1), domal crest (2) and synclinal axis (3)are shown in Fig. 5(a) and their predicted P–T –t pathsGravitational instabilities within the crust have been

suggested as the major cause of observed dome-and- in Fig. 5(b). The depth of the granite–greenstoneinterface is arbitarily taken at 10 km, but was probablykeel structures of Archean GGTs for almost half a

century (e.g. MacGregor, 1951; Anhaeusser et al., 1969; slightly greater in the Pilbara (12–15 km), as deducedfrom metamorphic studies (Morant, 1984; Delor et al.,Gorman et al., 1978; Ramberg, 1981). However, most

of these models rely on diapiric rise of the batholiths, 1991) and the average thickness of the greenstonesuccession (Hickman, 1983). The following featuresrather than the active role of sinking greenstones (with

more passive batholith rise) as the deformation mech- arise from the results.1 In the structurally deep core of the batholith, aanism. The strain pattern, kinematic evolution, geo-

metrical similarity and metamorphic comparison clockwise P–T –t path associated with near-isothermaldecompression is expected (path 1, Fig. 5b). Sillimanitebetween modelled GGTs and the east Pilbara GGT

confirm the suitability of the crustal overturn model is the inferred stable aluminosilicate and the maximumgeothermal gradient is c. 75 °C km−1. Partial meltingand suggest that the thermal modelling by Mareschal

& West (1980) is appropriate for understanding the of the deeper rocks within the granitic layer isinevitable (Weinberg, 1998), so late to postkinematicP–T –t evolution of the Archean GGTs.

Two factors are critical for initiation of convective granites should occur.2 In the crestal region of the granitoid dome andoverturn: (1) a negatively buoyant crustal configur-

ation, dense greenstones overlying less dense sialic directly above in the cover, isobaric heating followedby minor decompression should occur, producing acrust, and (2) a substrate of low viscosity, capable of

allowing the greenstones to sink through it. Both poorly developed clockwise P–T –t path (path 2,Fig. 5b; identical to that estimated by Morant, 1984).conditions are satisfied in the Pilbara:

1 A ≤18 km thickness of a dense mafic-ultramafic Andalusite is the inferred syn-kinematic mineral phase.However, the model does not account for decom-greenstone succession in the east Pilbara, representing

the lower and upper Warrawoona groups, overlies c. pression melting associated with convective overturn,which greatly facilitates the rise of granitoid magmas3450 Myr old, TTG-dominated sialic crust (Hickman,

1983). Up to 10 km of greenstone (Upper Warrawoona and increases heat input through advection (Weinberg,1997). This effect could stabilize late- to postkinematicand Gorge Creek groups) was deposited upon the

Pilbara crust 0–20 Myr before overturn (Collins et al., sillimanite, particularly adjacent to syn-kinematic plu-tons, generating a late isobaric-heating spike on path 2.1998); and

2 Collins et al. (1998) demonstrated from field evidence 3 Within the greenstone synclinorial keel, the rocksexperience a slight rise in temperature during conduc-that the old (c. 3450 Myr old) sialic crust was

undergoing anatexis during dome formation. tive heating prior to convective overturn, but thenundergo rapid near-isothermal loading during sinkingFurthermore, the voluminous early- to late-kinematic,

c. 3310 Myr old granites are recycled sialic crust to the base of the descending greenstone syncline (path3, Fig. 5b). Kyanite is the inferred stable aluminosilicate(Bickle et al., 1989, 1993; Collins, 1993), which confirms

that wholesale partial melting of the sialic crust throughout the burial history of the keel. Late-kinematic kyanite veins may occur in the deeper partsoccurred during crustal overturn. Indeed, Collins et al.

(1998) postulated that this widespread ‘thermal soften- of the sinking greenstone keel, if altered felsic metavol-canics (muscovite-bearing schists) are buried to depthsing’ of the crust was the necessary condition for

triggering convective overturn. >6–7 kbar. There, the wet granite solidus is exceeded

152 W. J . COLLINS & M. J . VAN KRANENDONK

and small peraluminous partial melts might develop. depth. If the batholiths are magmatically induced corecomplexes, as suggested by Zegers et al. (1996) for thePost-kinematic overprinting by sillimanite and/or

intrusion by granites is expected as the stable geother- Shaw batholith, kyanite is not expected within thesyndepositional volcanic cover overlying the domesmal gradient is re-established. Therefore, if the green-

stones remain at depth for an interval of time (cores) as most of the greenstones have not reachedmidcrustal depths, suggested by their regionally low(>10 Myr), thermal recovery is likely to generate

granites that will obliterate the greenstone keel. metamorphic grade and general lack of penetrativefoliations. Furthermore, following the Zegers et al.4 In the domal wall, a variety of P–T –t paths is

possible. In the upper domal rim, where marginal (1996) model, the cover rocks should not be subjectedto major deformation as they lie above the majorrocks might rise, a clockwise P–T –t path is expected,

similar to that in the crestal regions: andalusite is the detachment surface, but highly strained felsic schistsabove this decollement (Duffer formation equivalents)inferred stable aluminosilicate. In the lower domal rim,

the wall rocks might sink, so the rocks would follow a are those containing kyanite. Thus, this core complexmodel cannot explain the occurrence of kyanite aroundburial path similar to (3) and kyanite would be the

stable mineral. the MEB and the Shaw batholith in the east Pilbara.Collins et al. (1998) have also presented detailedstructural evidence why the batholithic domes cannot

DISCUSSIONbe metamorphic core complexes.

The presence of kyanite along the western marginArchean kyanite-bearing assemblages in wall rocksrimming granitoid domes have been reported from the of the Shaw batholith was used by Bickle et al. (1980,

1985) to develop an overthrust model for the PilbaraDharwar craton, India (Bouhallier et al., 1993), SlaveProvince, Canada (Percival, 1979), Zimbabwe craton craton, whereby inferred early crustal overthickening

led to heating and rise of buried, buoyant granitoid(Saggerston & Turner, 1976), and the Yilgarn craton,Australia (Binns et al., 1976). An exception is the material to produce batholithic gneiss domes. This is

analogous to early versions of gravitational collapsePontiac subprovince of Canada, where syn-kinematickyanite occurs in a linear zone that parallels the models for metamorphic core complex formation (e.g.

Coney & Harms, 1985). In theoretical models ofdominant (linear) structural grain of the SuperiorProvince. The kyanite zone occurs within a large-scale collapsed overthrust belts, kyanite ± sillimanite should

occur in the cores of structurally deep granitoid domesduplex structure that represents the deeper part of anoverthrust belt (Benn et al., 1994). This setting for (P>4 kbar), whereas andalusite might be expected in

the rims (cover). However, in the Shaw batholith,kyanite is entirely different from that of GGTscharacterized by dome-and-keel structures, and reflects kyanite occurs in the rim and sillimanite±andalusite

in the core. Nonetheless, these high-temperatureexhumation of midcrustal sections during lateralaccretion of distinct lithotectonic (microcontinental?) Al2SiO5 polymorphs could be due to a younger

magmatic event, during which earlier kyanite wasblocks (e.g. Sawyer & Benn, 1993).Few studies have attempted to incorporate kyanite obliterated. Only high-precision age determination of

the metamorphic assemblages will resolve thisinto structural models for dome-and-keel evolution ofArchean GGTs. Percival (1979) appealed to an initial possibility.

Kyanite should be restricted to an early kinematic‘thermal depression’ above a large granitoid core,which later rose as a dome, to account for its restricted phase in the overthrust model, but it postdates a

crenulation cleavage in the Shaw batholith.occurrence. Is such special pleading required?Batholithic domes of Archean GGTs have geometri- Nonetheless, whether or not this foliation represents

the early or late stage of thrusting is not clear, and socal similarities with ballooning plutons, metamorphiccore complexes and mantled gneiss domes, but the a thrust model remains a possibility. However, some

of the kyanite is late- or postkinematic around thethermal and structural response to these processes isdifferent. Ballooning plutons, if they exist, are likely to MEB (Figs 3b,c); so the overthrust model cannot

apply to this area.contain andalusite or sillimanite in their aureoles,rather than kyanite, as they must form at shallow The zone of inferred thrusting described by Bickle

et al. (1980, 1985) is restricted to a small, anomalouslevels in order to lift their roof rocks during expansion(cf. England, 1990). Also, strain should be focussed area along the western margin of the Shaw batholith,

and is not representative of the craton. Indeed, Vaninto the aureoles, where maximum fattening wouldoccur. However, the strain pattern around the sou- Kranendonk & Collins (1998) have demonstrated that

this zone is an area that has undergone complextherm MEB indicates that deformation is concentratedwithin the synclinal axis of the Warrawoona syncline, deformation involving crustal thickening during the

Late Archean, after (not before) doming (Fig. 1). Thenot within the aureole (Collins et al., 1998).The inferred isothermal decompression paths of complex deformation in this zone contrasts with the

simple dome-and-keel geometry throughout themetamorphic core complexes (e.g. Hill & Baldwin,1993) also indicate that kyanite might form in the wall remainder of the east Pilbara, where stratigraphic,

structural or metamorphic inversion in typically weaklyrocks of core complexes if exhumed from sufficient

KYANITE IN ARCHEAN GRANTITE–GREENSTONE TERRANES 153

deformed greenstones is not evident. Stratigraphy isinvariably right-way-up and greenstones dip and youngsystematically away from the domes. Unless the entiregreenstone belt was overthrust by other belts prior todoming (which have since been completely removedby erosion), crust loading models seem inappropriate.In addition, the earliest-recognized but non-penetrativefoliation developed in layer-parallel extensional faultsnot thrusts (Collins, 1989; Zegers et al., 1996).Therefore, the Pilbara is an area of general strati-graphic, structural and metamorphic simplicity.

The overprinting of, and spatial association between,the aluminosilicate polymorphs in the western Shawregion (Fig. 1), seem more compatible with crustaloverturn models. Andalusite overprinting kyanite inthe domal wall (Morant, 1984) is predicted by path 2(Fig. 5b) and for any wall rocks that do not exceed c.4 kbar during overturn. Also, sillimanite in the Shawbatholith and kyanite in the steeply dipping marginalgreenstones is the predicted arrangement in an overturnmodel (Fig. 5a). We consider that sillimanite pseudo-morphs after andalusite in both structural domainsreflects increasing advective heat from syn- and late-kinematic granitoid intrusions produced during over-turn (cf. Weinberg, 1997). Thus, the crustal overturnmodel provides a reason for ‘… the unexplained

Fig. 6. Model for convective overturn of the Pilbara crust,thermal anomaly associated with the Shaw batholith’followed by north-eastward tilting of the Mount Edgar dome

(Bickle et al., 1985). (from Collins et al., 1998). Note location of kyanite in domalOverprinting of kyanite by sillimanite in the margin wall and synclinal keel. Uplift and tilting of the dome to the

north-east, with major movement along the Klondyke fault, isof the MEB/CDB domes (Delor et al., 1991) is alsorequired to juxtapose kyanite schists against low-gradeconsistent with a crustal overturn model. Delor et al.greenstones. See text for discussion.

(1991) reported that sillimanite defined the myloniticS1 and rodded L1 fabric, indicating an increase intemperature before or during early deformation. This

Preservation of kyanite in greenstone beltsrelation is predicted by the model, as the crust mustfirst heat before convective overturn begins, as Application of the thermal modelling results of

Mareschal & West (1980) suggest that kyanite shouldshown in Fig. 5(b), which also shows that bothpolymorphs may coexist in the deeper parts of the not be readily preserved in Archean greenstone belts,

because convective overturn produces a perturbeddomal wall.Syn-kinematic kyanite in schistose rocks around the geothermal gradient, particularly within the keel of

sinking greenstones where a ‘cold finger’ exists. Thermalsouthern margin of the MEB further supports thecrustal overturn model. Based on the results presented recovery is likely to diffuse heat into these ‘cold finger’

greenstone keels, leading to kyanite breakdown. Thisabove, early, syn- and late-kinematic kyanite mayoccur in vertically lineated rocks adjacent to batholithic phenomenon is common along the western margin of

the Shaw batholith, where kyanite has been locallydomes (Fig. 5b, path 3), and is observed along thesouthern margin of the MEB (Fig. 3a–c). Pressure replaced by sillimanite or andalusite (Morant, 1984;

Bickle et al., 1985).estimates of 6 kbar for this margin (Delor et al., 1991)suggest that parts of the southern domal wall of the Greenstone keels commonly exist as partly disaggre-

gated and assimilated mafic–ultramafic lenses withinMEB descended to c. 20 km depth. Rare, post-D1,pre-D2 kyanite-bearing sweats and/or pegmatites in migmatitic gneisses or granitoids, described as amphi-

bolitic ‘xenolith trails’ by Glikson (1984). The xenoliththe hinge of F2 folds, part of the late-stage structuraldevelopment of the wall, are consistent with the 6 kbar trails are typically extensively migmatized, and rare

metasedimentary lenses within them usually containestimate. Therefore, structural and metamorphic stud-ies on the schistose rocks marginal to the MEB sillimanite. This is expected as partial melting generated

by either convective overturn (Weinberg, 1997) orsupport an origin for kyanite by convective overturn(Fig. 6). Indeed, restriction of kyanite to these rims of thermal recovery produces migmatites and granites in

the keels and walls of deep greenstone synclines. Adomal batholiths might be a diagnostic feature ofconvective overturn, because no other mechanism good example is the ‘xenolith trail’ along the western

margin of the Tambourah Dome (Fig. 1), composed ofappears capable of explaining this phenomenon.

154 W. J . COLLINS & M. J . VAN KRANENDONK

discontinuous amphibolite and agmatite lenses that fault to have acted as a transfer zone that linked theD4 shear zone with the Klondyke fault. Together,extend as a curvilinear belt for at least 30 km.

Therefore, greenstone keels are highly unlikely to these two faults facilitated uprise and juxtaposition ofthe deep-level marginal supracrustal rocks againstcontain kyanite.

Other factors that militate against kyanite preser- their lower-grade equivalents (Fig. 2) during the sec-ondary doming (D4) event.vation in Archean GGTs are the general lack of

peraluminous rocks and the typical higher crustal level Therefore, uplift of the kyanite-bearing schists isinterpreted to have occurred as a result of outwardof exposure (e.g. Powell et al., 1991). The very

mechanism of convective overturn is unlikely to migration of the main (D4) displacement zone in thewalls of the dome. In this model (Fig. 6), the supracrus-preserve mid-crustal rocks in greenstone belts, as

greenstones sink and granitoids rise to replace them, tal rocks were initially rapidly buried as the greenstonebelt sank into the underlying basement (Fig. 6a).providing a stable density configuration that is still

evident today in almost all Archean cratons (e.g. Subsequently, the MEB dome tilted about a fixedposition to the NE (Collins, 1989) and the SW marginKaapvaal: Anhaeusser et al., 1969; Abitibi: Gorman

et al., 1978; Pilbara: Hickman, 1984; Dharwar: was uplifted. Deformation associated with upliftinitially became localized within the MEB as the D4Bouhallier et al., 1993; Zimbabwe: Jelsma et al., 1993).

The deeper levels of sinking greenstone keels will be zone, which locally cut through the early formed dome(Fig. 1), but then the highly strained wall rocks wereexposed only if they are exhumed during a later event,

provided they were not already obliterated by partial uplifted as movement was transferred to the Klondykefault (Fig. 6b). The process is similar to the model formelting. However, along the southern margin of the

MEB, the kyanite-bearing keel is preserved. Why? high-P blueschists being buried in a subduction zoneand subsequently rebounded by plastering onto theCollins (1989) inferred two stages of uplift on the

southern margin of the MEB. The initial movement overriding plate during outboard migration of thesubduction front, e.g. the ‘eduction model’ of Dixon &produced the dome-and-keel pattern, including the

arcuate Marble Bar belt and Warrawoona syncline Farrar (1980); see also Wakabayashi (1992). In theMEB, the ‘burial fault’ of the rim supracrustal rocks(Fig. 1), and was the major fabric-forming event. The

syn- to late-D1, pre-D2 kyanite of the MEB southern is represented by the D1 shear zone along the southernmargin of the batholith, extending north-westwardrim formed at this stage, within the sinking wall of the

greenstone keel. The second event produced a ‘dome- beyond Marble Bar (Fig. 1) and outlined largely bythe Duffer formation, whereas the major D4 ‘upliftwithin-a-dome’ and was associated with ‘D4’ defor-

mation, which produced a concentric batholith-up fault’ is the Klondyke fault.deformation zone around the southern MEB margin(Fig. 1). This was the ‘polydiapirsim’ of Collins (1989),

CONCLUSIONSbut it is better described as the later stage of theconvective overturn process, when deformation became The significance of the 6 kbar, kyanite-bearing rim

around the MEB is that it allows the deeper parts oflocalized along the granite–greenstone (D4) shear zoneof Collins (1989). As the domal crest was hinged in Archean dome-and-keel structures to be recognized

and analysed, and allows P–T –t paths derived fromthe NE, the deeper level gneiss complex was upliftedin the SW quadrant during D4 (Collins, 1989). Our thermo-mechanical models to be compared. As such,

it has provided insights into the evolution of Archeanmore recent mapping shows that part of the greenstonewall, representing the kyanite-bearing schists of the GGTs. Models involving ballooning pluton develop-

ment, mantled gneiss doming, metamorphic coresouthern MEB rim, was also uplifted in this ‘D4’ event(see below). complex formation, or early crustal overthickening are

difficult to reconcile with kyanite rims around batho-Directly eastward from Marble Bar, the D4 shearzone separates high-level, non-foliated granites and liths. Convective crustal overturn models seem appli-

cable, but require that the kyanite rim be upliftedlow-grade greenstones from orthogneisses (Fig. 1).From this location, the Salgash Fault splays southward before thermal recovery. We infer that it occurred

during the later stages of dome-and-keel formationaway from the SE-trending D4 shear zone. This faultseparates the low-grade Marble Bar Belt from the around the MEB, when uplift (D4) faults that were

previously confined to the batholith margin migratedhigh-grade, high-strain schists of the Warrawoonasyncline, but it loses all expression (and displacement) outward into the keel. This process may have been

uncommon, rendering the preservation of kyanite inin the axis of the Warrawoona syncline (Fig. 2).Therefore, it is a major scissor fault. Within the greenstone synclines extremely unusual, evident by its

paucity in Archean GGTs.syncline, the Klondyke fault becomes the majordisplacement zone separating the high- and low-grade The kyanite-bearing rim also provides a three-

dimensional glimpse of deep-crustal levels withinrocks (Fig. 2) and extends eastward around thesouthern margin of the MEB, parallel to the D4 shear Archean GGTs. Most important is the concordance of

the higher-level, greenschist facies, subvertical sur-zone (Fig. 1), but cutting the Warrawoona synclinalaxis (Fig. 2). Thus, we interpret the ‘scissor’ Salgash rounding rim with the subvertical high-strain, deep-

KYANITE IN ARCHEAN GRANTITE–GREENSTONE TERRANES 155

continental crust ~3.5 billion years ago in the Pilbara Cratonlevel structures, which have been buried to c. 6 kbarof Australia. Nature, 375, 574–577.(Delor et al., 1991). It indicates that the entire

Campbell, I. H. & Hill, R. I., 1988. A two-stage model for thegreenstone keel had a similar subvertical orientation formation of the granite–greenstone terrains of the Kalgoorlie-to c. 20 km, suggesting that convective overturn in the Norseman area, Western Australia. Earth and Planetary

Science L etters, 90, 11–25.Pilbara was a crustal-scale process, similar in magni-Choukroune, P., Bouhallier, H. & Arndt, N. T., 1995. Softtude to that inferred for the Dharwar craton, where

lithosphere during periods of Archaean crustal growth orthe dome-and-keel structures can be observed through- crustal reworking. In: Early Precambrian Processes (edsout the crustal column, to depths consistent with Coward, M. P. & Ries, A. C.) Geological Society Special

Publication 95, 67–86.8–9 kbar pressures (Choukroune et al., 1995).Collins, W. J., 1989. Polydiapirism of the Archaean Mount

Edgar Batholith, Pilbara Block, Western Australia.Precambrian Research, 43, 41–62.ACKNOWLEDGEMENTS

Collins, W. J., 1993. Melting of sialic crust under high aH2OWe thank C. Teyssier for his inspiration in the early conditions: genesis of 3300 Ma old Na-rich granitoids in theMount Edgar Batholith, Pilbara Block, Western Australia.stages of this work. R. Offler critically read an earlyPrecambrian Research, 60, 151–174.version of the manuscript. This research was supported

Collins, W. J. & Gray, C. M., 1990. Rb–Sr isotopic systematicsby an ARC Postdoctoral Fellowship to M.V.K. and of an early Archaean granite–gneiss terrain: the Mount Edgarby ARC small grants. We thank M. Sandiford and Batholith, Pilbara Block, Western Australia. Australian Journal

of Earth Sciences, 37, 9–22.K. Hodges for helpful reviews, D. Kinder and S. DickCollins, W. J., Van Kranendonk, M. J. & Teyssier, C., 1998.for computer drafting, and R. H. Vernon for incisive

Partial convective overturn of Archaean crust in the easterneditorial comments (as usual ).part of the Pilbara Craton, Western Australia: 2. Drivingmechanisms and tectonic implications. Journal of StructuralGeology, 20, 1405–1424.REFERENCES

Coney, P. J. & Harms, T. A., 1985. Cordilleran metamorphiccore complexes: Cenozoic extensional relics of MesozoicAnhaeusser, C. R., Mason, R. & Viljoen, R., 1969. A reappraisalcompression. Geology, 12, 550–554.of some aspects of Precambrian shield geology. Bulletin of the

Cullers, R. L., DiMarco, M. J., Lowe, D. R. & Stone, J., 1993.Geological Society of America, 80, 2175–2200.Geochemistry of a silicified, felsic, volcaniclastic suite fromBenn, K., Miles, W., Ghassemi, M. R. & Gillett, J., 1994. Crustalthe early Panaroma Formation, Pilbara Block, Westernstructure and kinematic framework of the northwesternAustralia: an evaluation of depositional and post-depositionalPontiac Subprovince, Quebec: an integrated structural andprocesses with special emphasis on the rare-earth elements.geophysical study. Canadian Journal of Earth Sciences, 31,Precambrian Research, 60, 99–116.271–281.

Davids, C., Wijbrans, J. R. & White, S. H., 1997. 40Ar/39ArBickle, M. J., Bettenay, L. F., Boulter, C. A., Groves, D. I. &laserprobe ages of metamorphic hornblendes from theMorant, P., 1980. Horizontal tectonic interaction of anCoongan Belt, Pilbara, Western Australia. PrecambrianArchean gneiss belt and greenstones, Pilbara Block, WesternResearch, 83, 221–242.Australia. Geology, 8, 525–529.

Delor, C., Burg, J.-P. & Clarke, G. L., 1991. Relations diapirisme-Bickle, M. J., Bettenay, L. F., Chapman, H. J., Groves, D. I.,metamorphisme dans la province du Pilbara (AustralieMcNaughton, N. J., Campbell, I. H., & de Laeter, J. R., 1989.occidentale): implications pour les regimes thermiques etThe age and origin of younger granitic plutons of the Shawtectoniques a l’Archeen. Compte Rendu de l’Acadamie desBatholith in the Archean Pilbara Block, Western Australia.Sciences de Paris II, 312, 257–263.Contributions to Mineralogy and Petrology, 101, 361–376.

Dixon, J. M. & Farrar, E., 1980. Ridge subduction, eductionBickle, M. J., Bettenay, L. F., Chapman, H. J., Groves, D. I.,and the Neogene tectonics of southwestern North America.McNaughton, N. J., de Campbell, I. H. & de Laeter, J. R.,T ectonophysics, 67, 81–99.1993. Origin of the 3500–3300 Ma calc-alkaline rocks in the

Dixon, J. M. & Summers, J. M., 1983. Patterns of total andPilbara Archean: isotopic and chemical constraints from theincremental strain in subsiding troughs: experimental centri-Shaw Batholith. Precambrian Research, 60, 117–150.fuge models of inter-diapir synclines. Canadian Journal ofBickle, M. J., Morant, P., Bettenay, L., Boulter, C. A., Blake, T.Earth Sciences, 20, 1843–1861.S. & Groves, D. I., 1985. Archean tectonics of the Shaw

England, P. C. & Thompson, A. B., 1984. Pressure–tempera-Batholith, Pilbara Block, Western Australia: Structural andture–time paths of regional metamorphism I. Heat transfermetamorphic tests of the batholith concept. In: Evolution ofduring the evolution of regions of thickened continental crust.Archean Supracrustal Assemblages (eds Ayers, L. D. Thurston,Journal of Petrology, 25, 894–928.P. C. Card, K. D. & Weber, W.), Geological Association of

England, R. W., 1990. The identification of graniticCanada, Special Paper, 28, 325–341.diapirs. Journal of the Geological Society of L ondon, 147,Binns, R. A., Gunthrope, R. J. & Groves, R. I., 1976.931–933.Metamorphic patterns and development of greenstone belts

Glikson, A. Y., 1984. Significance of Early Archaean mafic–in the eastern Yilgarn block, Western Australia. In: T he Earlyultramafic xenolith patterns. In: Archaean Geochemistry (ed.History of the Earth (ed. Windley, B. F.), pp. 303–313. Wiley,Kroner, A.), pp. 262–281. Springer Verlag, Berlin.New York.

Gorman, B. E., Pearce, T. H. & Birkett, T. C., 1978. On theBohlen, S. J., Montana, A. & Kerrick, D. M., 1991. Precisestructure of Archean greenstone belts. Precambrian Research,determinations of the equilibria kyanite=sillimanite and6, 23–41.kyanite=andalusite and a revised triple point for the Al2SiO5 Hickman, A. H., 1983. Geology of the Pilbara Block and itspolymorphs. American Mineralogist, 76, 677–680.environs. Geological Survey of Western Austalia Bulletin,Bouhallier, H., Choukroune, P. & Ballevre, M., 1993. Diapirism,127, 268.bulk homogeneous shortening and transcurrent shearing in

Hickman, A. H., 1984. Archaean diapirism in the Pilbara Block,the Archean Dharwar craton: the Holenarsipur area, southernWestern Australia. In: Precambrian T ectonics Illustrated (edsIndia. Precambrian Research, 63, 43–58.Kroner, A. & Greiling, R.), pp. 113–127, E. Schweizerbart’sche-Buick, R., Thornett, J. R., McNaughton, N. J., Smith, J. B.,

Barley, M. E. & Savage, M., 1995. Record of emergent Verlagbuchhandlung, Stuttgart.

156 W. J . COLLINS & M. J . VAN KRANENDONK

Hickman, A. H. & Lipple, S. L., 1978. Explanatory notes on the Ramberg, H., 1981. Gravity, Deformation and the Earth’s Crust,2nd edn. Academic Press, London.Marble Bar Geological Sheet (SF50-8). Geological Survey of

Western Australia, p. 24. Saggerston, E. P. & Turner, L. K., 1976. A review of thedistribution of metamorphism in the ancient RhodesianHill, E. J. & Baldwin, S. L., 1993. Exhumation of high-pressure

metamorphic rocks during crustal extension in the Craton. Precambrian Research, 3, 1–53.Sawyer, E. W. & Benn, K., 1993. Structure of the high-gradeD’Entrecasteaux region, Papua, New Guinea. Journal of

Metamorphic Geology, 11, 261–277. Opatica Belt and adjacent low-grade Abitibi subprovince, anArchean mountain front. Journal of Structural Geology, 15,Holland, T. J. B. & Powell, R., 1990. An enlarged and updated

internally consistent thermodynamic dataset with uncertainties 1443–1458.Teyssier, C. & Collins, W. J., 1990. Strain and kinematics duringand correlations: the system K2O–Na2O–CaO–MgO–MnO–

FeO–Fe2O3–Al2O3–TiO2–C–H–O. Journal of Metamorphic the emplacement of the Mount Edgar Batholith andWarrawoona Syncline, Pilbara Block, Western Australia. In:Geology, 8, 89–124.

Jelsma, H. A., van der Beek, P. R. & Vinyu, M. L., 1993. T hird International Archaean Symposium, Extended Abstracts,pp. 481–483. Geoconferences (WA) Inc., Perth, WesternTectonic evolution of the Bindura-Shamva greenstone belt

(northern Zimbabwe): progressive deformation around diapiric Australia.Thorpe, R. A., Hickman, A. H., Davis, D. W., Mortensen, J. K.structures. Journal of Structural Geology, 15, 163–176.

Krapez, B., 1993. Archaean strike-slip faulting and related & Trendall, A. F., 1992. U–Pb zircon geochronology ofArchaean felsic units in the Marble Bar region, Pilbaraensialic basins: evidence from the Pilbara Block, Australia.

Precambrian Research, 60, 1–45. Craton, Western Australia. Precambrian Research, 56, 169–189.Van Kranendonk, M. J. & Collins, W. J., 1998. Timing andKrapez, B. & Barley, M. E., 1987. Archaean strike-slip faulting

and related ensialic basins: evidence from the Pilbara Block, tectonic significance of Late Archaean sinistral strike-slipdeformation in the central Pilbara structural corridor, PilbaraAustralia. Geological Magazine, 124, 555–567.

Kusky, T. M., 1989. Accretion of the Archean Slave province. craton, western Australia. Precambrian Research, 88, 207–232.Wakabayashi, J., 1992. Nappes, tectonics of oblique plateGeology, 17, 63–67.

Kusky, T. M., 1993. Collapse of Archean orogens and the convergence and metamorphic evolution related to 140 millionyears of subduction, Franciscan complex, California. Journalgeneration of late- to post-kinematic granitoids. Geology,

21, 925–928. of Geology, 100, 19–40.Weinberg, R. F., 1998. Diapirism-driven crustal flow: decom-MacGregor, A. M., 1951. Some milestones in the Precambrian

of southern Rhodesia. T ransactions of the Royal Society of pression melting, renewal of the magma source and the originof nested diapirs. T ectonophysics, 271, 217–229.South Africa, 54, 27–71.

Mackey, T. E. & Richardson, L. M., 1997. Airborne Gamma-Ray West, G. F. & Mareschal, J.-C., 1979. A model for Archeantectonism: the thermal conditions. Canadian Journal of EarthSpectrometry Colour Composite Pixel-Image Map of Marble

Bar 1:250 000, Western Australia. Australian Geological Survey Sciences, 16, 1942–1950.Wijbrans, J. R. & McDougall, I., 1987. On the metamorphicOrganisation.

McNaughton, N. J., Compston, W. & Barley, M. E., 1993. history of an Archaean granitoid greenstone terrane, EastPilbara, Western Australia, using the 40Ar/39Ar age spectrumConstraints on the age of the Warrawoona Group, eastern

Pilbara Block, Western Australia. Precambrian Research, technique. Earth and Planetary Science L etters, 84, 226–242.Williams, I. S. & Collins, W. J., 1990. Granite–greenstone60, 69–98.

Mareschal, J.-C. & West, G. F., 1980. A model for Archean terranes in the Pilbara Block, Australia, as coeval volcano-plutonic complexes; evidence from U–Pb zircon dating of thetectonism: numerical models of vertical tectonism in greenstone

belts. Canadian Journal of Earth Sciences, 17, 60–71. Mount Edgar Batholith. Earth and Planetary Science L etters,97, 41–53.Morant, P., 1984. Metamorphism of an Archaean granitoid–

greenstone terrain, Eastern Pilbara Block, the Western Shaw de Wit, M. J., Roering, C., Hart, R. J., Armstrong, R. A., deRonde, C. E. J., Green, R. W. E., Tredoux, M., Peberdy, E. &batholith and adjacent greenstones. PhD Thesis, University of

Western Australia. Hart, R. A., 1992. Formation of an Archaean continent.Nature, 357, 553–562.Percival, J. A., 1979. Kyanite-bearing rocks from the Hackett

River area, N.W.T. Implications for Archean geothermal Zegers, T. E., White, S. H., de Keijzer, M. & Dirks, P. G. H.M., 1996. Extensional structures during deposition of thegradients. Contributions to Mineralogy and Petrology, 69,

177–184. 3460 Ma Warrawoona Group in the eastern Pilbara craton,Western Australia. Precambrian Research, 80, 89–105.Percival, J. A. & Williams, H. R., 1989. The Late Archean

Quetico accretionary complex, Superior Province, Canada. Zegers, T. E., de Keijzer, M., Passchier, C. & White, S. H., 1998.The Mulgandinnah Shear Zone; an Archaean crustal scaleGeology, 17, 23–25.

Powell, R., Will, T. M. & Phillips, G. N., 1991. Metamorphism strike-slip zone, eastern Pilbara, Western Australia.Precambrian Research, 88, 233–247.in Archaean greenstone belts: calculated fluid compositions

and implications for gold mineralization. Journal ofMetamorphic Geology, 9, 141–150. Received 5 August 1997; revision accepted 22 June 1998.