Gneiss domes and extensional deformation in the highly mineralised Archaean Eastern Goldfields Province, Western Australia

Ore Geology Reviews, 8 (1993) 141-162

141

Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

Gneiss domes and extensional deformation in the highly mineralised Archaean Eastern Goldfields Province, Western Australia P.R. Williams and A.J. Whitaker Australian GeologicalSurvey Organisation, GPO Box 378, Canberra, ACT 2601, Australia (Received February 10, 1992; revised version accepted November 9, 1992 )

ABSTRACT Williams, P.R. and Whitaker, A.J., 1993. Gneiss domes and extensional deformation in the highly mineralised Archaean Eastern Goldfields Province, Western Australia. In: D.I. Groves and J.M. Bennett (Editors), Structural Setting and Controls on Mineral Deposits. Ore GeoL Rev., 8: 141-162. The recognition of large areas of gneiss within granitic complexes of the northeastern part of the Eastern Goldfields Province of Western Australia, based on analysis of regional aeromagnetic data, has resulted in a re-evaluation of the structural data for that region. Five magnetic domains have been established in the granitic complexes, which are marginal to the main greenstone belts. At least two of these domains have a distinctive domal form, and also have internal domeshaped regions. The Ballard and Laverton domains have extensive areas of layered gneissic and migmatitic rocks which flank the internal domes. Boundary relationships at the Laverton and Ballard domains show that the domes are concordant sheets, and the metamorphic conditions of the boundary rocks imply that the sheets were intruded at a middle crustal level. Strain conditions in the boundary zones indicate that emplacement of the gneiss domains to the crustal level of the surrounding lower-grade greenstones was initially along relatively gently-dipping shear zones which were active during uplift. Structural relationships around both the domal granitic complexes and the later granite batholiths, however, show that regional shortening deformation events were not caused directly by the emplacement of any of the granitic rocks. The tectonic hypothesis presented for the Eastern Goldfields is similar to that proposed for the formation of Cordilleran metamorphic core complexes. The uplift event responsible for the crustal extension required to allow the emplacement of the gneiss, may have been driven by the same thermal input responsible for the generation of the voluminous granites intrusive into both the gneiss complexes and the surrounding greenstone belts.

Introduction The Eastern Goldfields Province is one of the major tectonic subdivisions of the Archaean Yilgarn Block of Western Australia (Gee, 1979). It comprises extensive supracrustal "greenstone" rocks intruded by granite plutons, with marginal granitic complexes made up of a variety ofgranitoids and granitic gneiss. Correspondence to: P.R. Williams, Australian Geological Survey Organisation, GPO Box 378, Canberra, A.C.T. 2601, Australia.

The central geographical segment of the Province (between 28°S and 31°S), is characterised by extensive areas of granitic and gneissic rocks, which separate greenstone sequences of the northeastern sector (Hallberg, 1985 ) from the southern sector (Fig. 1 ). The structural history of this segment of the Eastern Goldfields Province appears to be straightforward (Table 1 ), and similar to the structural history documented for other parts of the province (Archibald et al., 1978). However, structural problems and granite-gneiss-greenstone relationships investigated during recent geological

142

P.R. WILLIAMS AND A.J. WHITAKER

Greenstone [ ~

Gneissandgrahite

Fig. 1. Division of the Eastern Goldfields Province into informal sectors showing the central east-west segment. Major tectonic subdivisions (Gee, 1979; Myers and Hocking, 1988 ) shown in inset. The major greenstone belt and enclosed granites constitute the Norseman-Wiluna Belt of Gee (1979).

mapping and regional geophysical interpretation suggest that the structural evolution of the central segment is more complex than is generally accepted. Several domal granitic bodies have been mapped in the extensive granitic complexes of the central segment. Metamorphic rocks adjacent to the granite domes are of high-pressure regional metamorphic origin and tectonically juxtaposed against low-grade sequences in the greenstones. Structural information from within and adjacent to the domes is not consistent with dome formation by diapiric ballooning of rising magma. The central segment is transected by broad, linear high-strain zones, first identified by Gower (1976) and Thom and Barnes (1977), and later described as tectonic zones by Hallberg (1985, 1986). The dominant feature of these zones is complex, multiphase deformation, which is not always apparent in the regions between the zones. Although transcurrent movement may have occurred on these zones late in the structural history, there is mounting evidence that the zones also represent reactivated early tectonic features (Barley et al., 1989; Swager et al., 1990). Hammond and Nisbet (1990) determined movement directions on a number of shear zones in the NE goldfields and concluded that the earliest set of ductile shears records an extensional event. We propose that several of these early shears in the central segment of the Province are indeed major extensional structures related to the evolution of regional gneiss and granite domes. Although the information derived from geological and aeromagnetic mapping does not provide sufficient constraints to forward a unique solution to the structural problems, we present a new hypothesis to explain the structural, metamorphic and granite emplacement data, based on the extensional tectonic models developed for the Basin and Range Province (Crittenden et al., 1980) and passive continental margins (Lister et al., 1989). We pres-

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GNEISSDOMESAND DEFORMATIONIN ARCHAEANEASTERNGOLDFIELDSPROVINCE,WESTERNAUSTRALIA TABLE 1 Sequence of deformation events and granite intrusions Event

Granite intrusion

Study Swager and Griffin (1990) (southern, central)

Williams et al. ( 1989 ) (local, Leonora)

D~

Dl

Thrust faults and sequence repeats

D2

ENE-WSW shortening; upright folds, NNW steep cleavage

D3

faults during regional shorten shortening, later faults

Sinistral wrench

E-W overturning and reclined folds, probable thrusts

ENE-WSW shortening; upright folds, regional cleavage

Hammond and Nisbet ( 1991 ) (northern)

This paper

Low-angle shears along granite/ greenstone margins

Shear zones on gneiss/granitegreenstone margins

NNW-directed thrusts,

Thrust faults and

sequence repeats ENE-WSW shortening; imbrication over a deep detachment

(central)

sequence repeats

ENE-WSW shortening; upright folds, cleavage, reactivation of earlier shears

NW-NNW sinistral faults

NS dextral faults and

NS dextral shears NNW

and shears; NE faults

shears, related to D2

sinistral reactivation

ent a structural model which explains the observed relationships between the major regional tectonic features and addresses the implications of the structural history for tectonic models of the Eastern Goldfields Province. Tectonic components of the Eastern Goldfields Province Gee (1979) identified the Norseman-Wiluna Belt as the major component of the Eastern Goldfields Province, located on the western edge of the Province. Recent mapping by the Geological Survey of Western Australia in the southern sector of the Eastern Goldfields

This paper

Mid-crustal granite sheets,equivalents

Linear zone of magnetic elongate plutons along De regional shear Large, generally ovoid granite plutons, Kfeldspar megacrysts common with deformed margins

Small fractionated plutons

province has led to the definition of five major terranes (Myers, 1990; Swager et al., 1990), defined as regions with different structural history or stratigraphy development bounded by major shear zones. Swager et al. (1990) subdivide the major terranes into subsidiary domains. Domains within the same terrane have a common structural history and similar stratigraphy, but are separated by shear zones across which major structures and stratigraphic units cannot be traced. The terrane bounding shears are commonly broad deformation zones, often with sharp boundary faults. In the northeast sector, the continuations of the terrane boundaries have variously been termed tectonic zones (Hall-

144

berg, 1985), or tectonic lineaments (Gower, 1976). Witt and Swager (1989) and Swager et al. (1990) argue that the terrane bounding shears (or tectonic zones) are likely to have been long active structures, possible controlling the original greenstone basin architecture. All of the tectonic components discussed above are based on the structure and stratigraphy of the greenstone sequences. In this paper we define five magnetic domains in the granitic complexes adjacent to the greenstone sequences.

Tectonic zones in the central segment The central segment of the Eastern Goldfields is transected by the Keith-Kilkenny, Celia and Laverton tectonic zones (Fig. 2), between which the rock sequences record only low strain. These zones extend beyond the central segment, with the Keith-Kilkenny zone recognised between Agnew in the north and the Kurnalpi area in the south. These zones have been variously interpreted as late strike-slip fault zones (Skwarnecki, 1987; Mueller et al., 1988), regional terrane boundaries (Myers, 1990; Swager et al., 1990) and (?reactivated) rift basin margins (Barley et al., 1989). Limited structural analysis has been attempted on these zones, but detailed mapping by the Geological Survey of Western Australia and BMR over the last two years has helped to clarify their structural history of some places (Passchier, 1990, 1993; Williams et al., 1989; Vanderhor, 1992 ), and Eisenlohr ( 1990 ) has provided a detailed study of the Keith-Kilkenny zone in the Agnew area. The zones record a complex deformation history. There are commonly early, variably oriented, isoclinal folds and in places a mylonitic bedding-parallel foliation. For example, east of Leonora intrafolial and sub-regional isoclinal folds are reclined on a gently north-dipping axial surface, whereas at Mount Margaret, in the Celia Tectonic zone, the isoclinal folds have axial surfaces parallel to steeply dip-

P.R. WILLIAMS AND A.J. WHITAKER

ping bedding. Individual shear zones associated with the isoclinal folds have been identified (Williams et al., 1989). The major folds of this event have a wavelength of at least 1.2 km (Williams and Currie, 1993 ). In the KeithKilkenny zone these early folds and fabrics are overprinted by the regional upright folding event; in places the early foliation was crenulated during the regional upright folding. Because the zones record intense D1 deformation, and where reactivated during D3,, they are not primarily late strike-slip faults. The observation that the early structures are tight to isoclinal, reclined to recumbent folds or bedding parallel shear zones indicates that the primary cause of these regional structural features was displacement between blocks adjacent to the tectonic zones on low-dip surfaces. The tectonic zones are also spatially associated with the major gneissic granite domes. The Keith-Kilkenny structure is a narrow zone of deformation in the Agnew area where it is close to the major outcrops of gneiss of the Ballard domain, and broadens out to a wide zone of deformation between Leonora and the boundary marked in Fig. 2. The Celia and Laverton tectonic zones follow the margins of the Laverton Dome, and both lineaments are difficult to trace to the south: they may in fact join. Late strike-slip movement is apparent on these lineaments, which obscures their early movement history.

Magnetic domains of the central segment Five major magnetic domains can be defined on the basis of the magnetic signature of the large areas of dominantly felsic igneous intrusives surrounding the southern and northeastern sectors of the Eastern Goldfields Province. Figure 3 shows a composite image of the magnetic intensity and gradient of the magnetic field over the central segment of the Eastern Goldfields Province. There are several distinctive features on the image which have not previously been identified as major tectonic

GNEISS DOMES AND DEFORMATION IN ARCHAEAN EASTERN GOLDFIELDS PROVINCE, WESTERN AUSTRALIA

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components of the region. Figure 4 is an interpretation of the image showing the major Archaean magnetic lineaments and five regional magnetic domains within the granitic complexes, and major ovoid granite plutons clearly intrusive into the greenstones.

Southern Cross magnetic domain The Southern Cross magnetic domain (Fig. 4), to the west of the Norseman-Wiluna Belt, is more highly magnetised than the other gneissic magnetic domains. It contains nebulous, low-contrast magnetic highs indicative of

P.R. WILLIAMSANDA.J.WHITAKER

146 120 °

123 ° 28 o

31 ° Fig. 3. East-west gradient-enhanced total magnetic intensity image map of part of the Eastern Goldfields Province, derived from 1.5 km line-spacing regional magnetic data from the BMR magnetic database. Grid-cell size is 500 m, grey shading from black to white corresponds with low to high magnetisation.

gneiss and migmatite. Migmatite and granite in the south of the magnetic domain is mapped eastward to the Ida Lineament and northward at least to Mount Ida. The magnetic domain encompasses several enclaves of greenstone belts which record lower to upper amphibolitegrade metamorphism. The major greenstone basin immediately to the east is metamorphosed to middle amphibolite facies adjacent to the magnetic domain. In common with the Laverton magnetic domain, regions of doming have not been identified within the Southern Cross magnetic domain.

Ballard magnetic domain The Ballard magnetic domain is a region characterised by elongate domal features surrounded by banded units which appear to be warped around the domes. The banded material has been identified as paragneiss (Rattenbury, 1990) with interleaved granitic (felsic volcanic?) and amphibolite layers. The domes are composite bodies of foliated porphyritic monzogranite, undeformed K-feldspar megacrystic granite and deformed gneissic granite. The common occurrence of gneiss in the belt indicates that a major part of the magnetic do-

GNEISS DOMES AND DEFORMATION IN ARCHAEAN EASTERN GOLDFIELDS PROVINCE, WESTERN AUSTRALIA

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148

main was at a high metamorphic grade (middle-upper amphibolite facies). Within the Ballard magnetic domain there are several poorly exposed domal bodies of varied size. These have varied magnetic signatures, from very fiat, low magnetism in the Raeside Gneiss domes to moderate magnetism with an uneven texture in the large unnamed dome in the south of the high-grade terrain (Fig. 4). Two large elongate domes on the southern margin of the Ballard magnetic domain, the Mount Pleasant Dome and the Scotia-Kanowna Dome (Witt and Swager, 1989 ) are continuous with the high-grade rocks and are likely to be related to them. Gneiss within the high-grade terrain is commonly strongly foliated, and is cut by undeformed pegmatites and porphyry dykes. The margins of the terrain are invariably strongly deformed, especially where the terrain is in direct contact with greenstone sequences. The deformed zones are characterised by noncoaxial deformation and mylonite formation.

Laverton magnetic domain Similar high-grade gneissic regions have been identified northeast of Laverton (Fig. 4), although there are no internal domal bodies currently identified in the Laverton magnetic domain. Rocks at the margin of the Laverton magnetic domain are strongly sheared, intercalated felsic porphyry and amphibolite, the margin forming a major regional mylonite zone which affects the surrounding greenstones. Internally, the magnetic domain comprises a composite body of gneissic granite, porphyritic granite and late, undeformed porphyry dykes and pegmatites (Fig. 5 ). Gower ( 1976 ) has indicated that the margins between the porphyritic granite and migmatitic gneiss within the Laverton magnetic domain are transitional, implying that the bulk of the granitic rocks in the dome are of high metamorphic grade.

P.R. WILLIAMS AND A.J. WHITAKER

Eastern magnetic domain To the east of the Norseman-Wiluna Belt (Fig. 4 ) is a magnetic domain of low to moderate magnetization and low anomaly contrast. Structurally complex, low-amplitude magnetic highs extend for tens of kilometers. These anomalies are characteristic of regions of gneiss and migmatite described in the other marginal magnetic domains. The region includes small greenstone enclaves which are evident in the imaged magnetic data as highly contrasting bands of low and high magnetic intensity. Greenstone terrane abutting to the west is metamorphosed to upper amphibolite facies (Binns et al., 1976). Except for the remnants ofgreenstone belts, the magnetic domain has a similar appearance to the Laverton magnetic domain in the magnetic image and a similar composition dominated by gneiss and granitic rocks of high metamorphic grade is inferred. Domes are also present in the Eastern magnetic domain, but the age of the domes relative to the migmatite and gneiss is not certain. Williams (1976) infers that the domes are later granite intrusions.

Boyce magnetic domain The Boyce magnetic domain is a linear belt dominated by granite intrusion. The magnetic domain is only 10-20 km in width, where it abuts the southeast margin of the Ballard magnetic domain• Further south the magnetic domain attains a width of 40 km, while its overall length is in excess of 250 km. Most of the component granites are moderately magnetised and a number are weakly zoned with more highlymagnetised rims. Individual plutons are generally ovoid with long dimensions oriented 165 °, parallel to the regional cleavage but weakly discordant to the 155 ° trend of the belt • Greenstone sequences incorporated in the magnetic domain or adjacent to the magnetic domain have been metamorphosed to lower

149

GNEISS DOMES AND DEFORMATION IN ARCHAEANEASTERNGOLDFIELDS PROVINCE, WESTERNAUSTRALIA

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amphibolite facies• The domain has a distinctive magnetic signature and unique geometry in this part of the Yilgam Block and possibly represents long-lived syn- and post-tectonic intrusion along a major deep-rooted structure.

Greenstone terranes

The major greenstone belts (Fig. 4) are poorly magnetised relative to the surrounding magnetic domains underlain largely by felsic intrusive rocks and the majority of component lithologies are similarly magnetised. These

150

characteristics, coupled with the poor resolution of the regional surveys do not lead to unequivocal subdivision of the basins, based on the magnetic data. Some indication of internal structure is, however, obtained from sparse, highly-magnetised units. In the northeast sector, magnetic units are dominantly thin, banded iron formation, commonly associated with ultramafic rocks, whereas south of the Ballard magnetic domain highly magnetic units are dominantly komatiite flows. The northeastern and southern sectors are separated by the BaUard magnetic domain and the adjacent Boyce magnetic domain. Barley et al. (1989) consider the northeastern and southern sectors to be distinct basins; the northeastern basin being characterised by calc-alkaline acid volcanism in a volcanic arc setting and the southern by komatiite volcanism in an extensional environment. While the southern sector is on average more highly magnetised than the northeastern sector, the difference in magnetization could equally be due to differences in either bulk composition or thickness of the greenstone sequences. Calc-alkaline volcanism is not restricted to the northeastern sector, nor komatiite flows to the southern sector. The occurrence of moderateto highly-magnetised units in both sectors is comparable and both sectors contain greater development of banded iron formation along their eastern margins, although the absolute abundance is greater in the northeastern sector.

Nature of magnetic domain boundaries Details of one of these contacts, the northern and eastern margin of the Raeside Gneiss (Fig. 2 ), have been presented by Williams and Currie (1993). A major characteristic of the boundary zones is the compressed metamorphic gradient associated with the transition from amphibolite facies mylonitic gneiss and amphibolite to greenstones of middle and lower greenschist facies in adjacent greenstone belts. Williams and Currie estimate that 3-5 kbars

P.R. WILLIAMS AND A.J. WHITAKER

equivalent of crustal section has been excised over a stratigraphic distance of only tens of metres. The Mount Pleasant and Scotia-Kanowna domes have similar high-grade assemblages and compressed metamorphic gradients on their margins. The Raeside Gneiss on the northeastern margin of the Ballard magnetic domain, was tectonically emplaced in the solid state. Feldspar and quartz deformation fabrics (Williams et al., 1989; Williams and Currie, 1993 ) suggest that emplacement took place under amphibolite facies temperatures, but that the deformation on the shear zone continued as the shear zone cooled. Amphibolite facies dynamic recrystallisation in the granite and the greenstone margins, together with.the presence of both deformed and undeforrned porphyry dyke swarms in the upper plate, which do not cut down into the underlying granite sheets, also shows that there was considerable translation on these sheared margins which continued from high to low metamorphic grades. The marginal mylonite has a strong pervasive mineral elongation and stretching lineation associated with a planar fabric parallel to the granite greenstone contact. The regional dip of the contact ranges from 20 ° on northwest-dipping margins to 70 ° on the east-dipping margin. The Laverton gneiss is bounded on its southern and southeastern margin by amphibolite intercalated with deformed quartz feldspar porphyry and is very similar in character to the observed margins of the Raeside gneiss. The pattern of mineral lineations and movement vectors on mylonite around the Laverton magnetic domain has not yet been established. In the amphibolite enclaves of the Laverton magnetic domain (Fig. 5 ), the strong amphibolite foliation is folded by the regional upright folding event. Stretching lineations in high-grade rocks around the Mount Pleasant Dome are generally shallowly plunging and north-directed (Witt, 1989), but movement indicators re-

GNEISS DOMES AND DEFORMATION IN ARCHAEAN EASTERN GOLDFIELDS PROVINCE, WESTERN AUSTRALIA

lated to dome emplacement may have been overprinted by later strike-slip faulting. Peak metamorphic conditions in the amphibolite adjacent to the gneiss domes at Leonora, and adjacent to the Laverton magnetic domain, were reached during mylonite formation. This is indicated by peak amphibole-plagioclase assemblages defining the penetrative fabric. However, later overgrowth of the foliation by amphibolite facies amphibole, andalusite and chloritoid porphyroblasts took place very late in the deformation episode (Williams et al., 1989; Williams and Currie, 1993 ), indicating that high-grade conditions were maintained for some time. In the lower-grade greenstone domains, peak metamorphism was reached during the later upright folding phase (Archibald et al., 1981 ), indicating that two temporally distinct metamorphic peaks are recorded in the greenstone sequences. A regional compilation of metamorphic zones by Binns et al. ( 1976 ) shows that the observations at the above-mentioned localities are representative of the regional metamorphic pattern around the gneissic terrains (Fig. 6 ). Regions of high metamorphic grade are located adjacent to the gneissic magnetic domains identified from regional magnetic data, except for the high-grade corridor in the Boyce magnetic domain. Figure 6 shows that there is a very poor correlation between the metamorphic zonation pattern and the position of the large late, to post-D2 granite plutons outside the granitic complexes. As these plutons are largely in F2 fold culminations, are broadly synchronous with D2 and have developed local staticthermal aureoles (Witt and Swager, 1989 ), the pattern of the high-grade metamorphic zonation cannot be related to D2 thermal or structural events. The foliation in the high-grade zones is overprinted by $2 (Williams et al., 1989 ); thus the high-grade metamorphism and emplacement of the gneissic domes must predate D2 and the associated regional lower-grade metamorphism.

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Fig. 6. Metamorphicisograds in greenstonesequences (from Binnset al., 1976) in relationto the positionof gneiss-dominatedmagneticdomains.

Granite emplacement and regional deformation One of the major late Archaean features of the Yilgarn Craton is the widespread occurrence of granitic complexes comprising up to 70% of the outcrop area of the Craton (Williams, 1976). Individual granite batholiths tend to be ovoid in shape, commonly with deformed margins or marginal phases, leading to the conclusion that the granite has intruded, initially as diapirs, and later probably by inflation (or ballooning) at their present crustal level (Gee, 1979). Witt and Swager (1989) have argued that

152

there are three phases of granite emplacement in the southern part of the Eastern Goldfields province, each phase with distinctive structural characteristics. Strongly deformed granites of the early emplacement phase pre-date the regional upright folding (D2, Table 1 ) in the Kalgoorlie area (Witt and Swager, 1989). In the northern sector the strongly deformed granites also pre-date the earliest deformation event (D~, Williams et al., 1989). The second phase of granite intrusion in the southern sector is considered to have taken place late, during the upright folding event (D2, Table 1 ). Petrographically similar granites in the Leonora area, may largely pre-date the D2 folding event. The third granite intrusion event postdates penetrative deformation events and produced small circular plutons, generally undeformed and strongly differentiated. The regional diapiric model for the emplacement of deformed granitic complexes appears attractive, partly because the assumed highdensity greenstone layer in the upper crust provides a strong inverse density gradient (Ramberg, 1970). There are remnants ofgreenstone sequences over a large part of the Yilgarn Craton (Myers and Hocking, 1988), but recent models suggested by Swager et al. (1990) and Barley et al. (1989) for localisation of greenstones in discrete depositional basins or terranes, and the significant age differences between greenstone belts (Pidgeon et al., 1988 ), indicate that there may not have been a continuous greenstone layer across the craton. There is a good correlation between the highest metamorphic grades recorded in greenstones and the margins of deformed granitic complexes (Binns et al., 1976 ). On a regional scale, the greenstones at these margins are intensely-deformed dynamic metamorphic domains, dissimilar to static high temperature-low pressure aureoles in high-level intrusions. The deformation pattern around the complexes is also not indicative of strong flattening due to ballooning of magmatic diapirs, but rather characterised by linear mylonitic

P.R. WILLIAMS AND A.J. WHITAKER

simple-shear fabrics. Thus, in the absence of conclusive evidence for regional diapirism, the nature of granite emplacement needs to be determined at a local scale because of the likely complexity and variability of the intrusion mechanisms (e.g., Witt and Swager, 1989).

Form of granite intrusions in the central segment of the Eastern Goldfields In the central segment of the Eastern Goldfields Province several ovoid granite bodies have been identified (Fig. 2; Kriewaldt, 1970; Gower, 1976; Williams, 1976; Thom and Barnes, 1977). In some places (e.g., Raeside Gneiss) there is a strong cleavage formed parallel to pluton margins; in others (e.g., Minerie Hill, Fig. 7) the stratigraphy and regional cleavage appear to be deflected around the plutons. The former bodies belong to the earlier phase of emplacement, the latter to the second major phase of granite intrusion identified by Witt and Swager (1989). The field evidence suggests that neither the "early" deformed granites nor the later granite batholiths cause the regional deformation, although the intrusions do cause local deformation in the surrounding greenstone sequences.

Laverton granitic complex Age relationships in the Laverton magnetic domain (Fig. 5 ) indicate that significant components of the dome were emplaced early in the structural history. Several sheets of gneissic granite are strongly foliated, in places with two distinct foliations developed. There is a strong foliation in the marie rocks adjacent to the dome, parallel to the strike of the contact, and that foliation is folded by the upright folding event which produced the regional Margaret Anticline (a D2 regional fold; Gower, 1976 ). Thus the regional S~ and $2 are present in both the amphibolite enclaves and the granite. At Mount Gooses the earlier foliation is oriented east-west and dips to the south, and porphyry dykes in the amphibolite structurally

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overlying the granitic complex are also deformed in a non-coaxial (shear) event parallel to the early shallowly dipping east-west foliation. Linear wedge-shaped apophyses and aligned enclaves of amphibolite extend into the Laverton domain in a northwesterly direction from its southern margin. A small body of domi-

153

nantly fine-grained strongly deformed porphyry forms an outlier of the main complex. The porphyry has a gently dipping concordant roof zone with a strong cleavage parallel to the margin. Greenstones adjacent to the porphyry are strongly lineated amphibolite intruded by abundant quartz- and quartz-feldspar porphyry dykes. The dykes are also strongly deformed, particularly where oriented parallel to the external foliation. Asymmetric pressure shadows developed around feldspar phenocrysts suggest that the movement direction was top-side-north. To the north of the porphyry body, the amphibolite shows excellent evidence of two phases of deformation. The strong foliation defined by aligned amphibole laths is folded into upright, shallowly plunging eastverging folds which are parasitic on the regional Mount Margaret Anticline. There is a persistent crenulation cleavage in the amphibolite belt which is oriented parallel to the axial surface of the regional D2 anticline. The northern apophysis of amphibolite, which hosts the Windarra nickel mine (Fig. 5 ), is structurally complex, but the southwestern margin of the apophysis is marked by a persistent unit of banded iron formation which dips 20 ° to 40 ° to the northeast, concordant with the margin of the granitic complex. The intensity of deformation in the banded iron formation increases towards the northwest. On the northeastern side of the apophysis, the amphibolite is also concordant with the foliated porphyry margin east of Windarra, where bedding and cleavage dip 40 ° to 50 ° to the southeast. Banded iron formation in this strongly deformed margin may correlate with the banded iron formation to the southwest. Thus, the Windarra apophysis is a synform which varies from tight in the northwest to relatively open in the southeast. The banded iron formation (and ultramafics) can be traced south from the Windarra apophysis to the South Windarra area and into the narrow apophysis to the south. The evidence suggests that the upper surface of the dome is a concordant sheet. There is no evi-

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P.R. WILLIAMS AND A.J. WHITAKER

Late Granitic lutrusions

Late (post tectonic) granite

Strongly foliated granitic rocks

K-feldspar phyric hornblende biotite monzogranite

Medium-coarse grained biotite ( +/- hornblende ) granite

Gneissic granite and migmafite

Amphibolite facies greenstones

Strongly deformed feldspar +/- quartx porphyry

Fig. 8. Schematic block diagram of southern part of the Laverton Dome showing the structural position of the greenstone apophyses and there interpretation as "mega-mullions" in the roof zone of the dome.

dence of stoping and no piercement structures as would be expected with diapiric emplacement. A model of the dome shape is shown in Fig. 8. The apophyses are concordant synclinal remnants of the overlying greenstone sequences. They are original features of the dome emplacement, not infolded during D2, because the synclinal axes of the amphibolite lying between the porphyry outlier and the main complex is at a high angle to and overprinted by the regional D2 anticline. The regional D2 overprint may, however, have steepened the dips on the earlier NW-trending structures, as these would have remained in the flattening field of D2 strain. The apophyses are interpreted as remnants of upper plate greenstones between lobes of the Laverton dome with slightly different uplift rate. The elongation direction of the apophyses may define the regional extension direction during dome emplacement.

Raeside Gneiss The pattern of deformation around the Raeside Gneiss at Leonora (Williams et al., 1989) is also not consistent with models which require the intrusion of the gneiss as a diapir. The gneiss-greenstone margin is characterised by a well-developedmylonitic foliation and a strong stretching lineation. Movement on the deformation zone took place at progressively decreasing metamorphic conditions ranging from amphibolite to lower greenschist facies. The deformation zone is also not everywhere restricted to the gneiss-greenstone margin. At Leonora, the deformation is concentrated in the overlying amphibolite, which suggests that the gneiss was emplaced with the surrounding amphibolite-grade rocks. Sweat zones containing partially melted amphibolite in the deformation zone is strong evidence that the gneiss was originally emplaced at a high pressure as

GNEISS DOMES AND DEFORMATION IN ARCHAEAN EASTERN GOLDFIELDS PROVINCE, WESTERN AUSTRALIA

well as high temperature (Winkler, 1979; Williams and Currie, 1993 ), and preservation of intrusive structures and associated aplite and porphyry dyke swarms indicate that the gneiss was initially intruded as a magma. Additional evidence provided by sense-of-shear indicators and analysis of deep diamond-drill holes (Williams and Currie, 1993 ) suggests that the granite domes are emplaced in an extensional environment, and Passchier (pers. commun., 1990) has identified evidence of overprinting of the earlier extensional structure by thrust faults.

Granitic rocks of the Eastern Gneiss domain The relationship between gneiss and migmatite of the Eastern Gneiss domain and the surrounding greenstones has been discussed by Williams (1976). The metamorphic gradient increases sharply as the boundary of the gneiss and migmatite terrane is approached. There is a marked increase in the number of porphyry and granite dykes and sills towards the gneiss domain, and the migmatite-greenstone boundary is regionally concordant. Lit-par-lit structure is replaced by banded migmatites with increasing metamorphic grade (Williams, 1977). These features are similar to those observed around the Raeside gneiss margins and the Laverton complex, except that the difference in metamorphic grade seems greater in the Eastern Domain. We infer that a similar emplacement history may be applicable.

15 5

alignment of quartz aggregates and a weak to poor alignment of feldspar megacrysts. This relationship suggests crystallisation of quartz in a stress field after partial solidification of the pluton margin. The equidimensionalshape and the fabric relationship indicates that the monzogranite was intruded late in the D2 deformation phase: there was insufficient flattening to re-align the K-feldspar megacrysts, but sufficient directed stress in the granite to control the preferred orientation of quartz grains. The foliation in the granite is parallel to the external foliation, which is deflected by the batholith. However, the external foliation can be traced across the whole of the Minerie 1:100 000 sheet area with a remarkably consistent orientation (165°); the granite body causes only local variations in the foliation pattern. Spectacular mylonite foliations in the aureole close to and parallel with the granite margin are not overprinted, but the granite foliation marked by quartz aggregates remains parallel to the external regional $2 foliation. The timing of the Minerie Hill Batholith in relation to the $2 foliation is ambiguous, but the internal fabric and equidimensionalshape indicate it is likely to have intruded late in the deformation event producing the regional foliation (D2). The batholith would have presented a regional strain inhomogeneity causing rotation of the local stress orientations during D2, and itself be deformed on the margins during D2. There is no evidence that the intrusion of the batholith caused the regional cleavage pattern.

Ovoid granite plutons The Minerie Hill Pluton is a typical example of an ovoid granite of the later intrusive event (Williams et al., 1992). It is a variably porphyritic biotite monzogranite typical of the 2.65 Ga intrusion event elsewhere in the Goldfields (R.I. Hill, pers. commun., 1992). The pattern of the regional $2 foliation around the granite shows some distortion adjacent to intrusion (Fig. 2 ), but is not particularly strongly developed adjacent to the granite. The western margin of the granite is deformed, with a good

Granite emplacement and deformation of the greenstones In some recent tectonic models, a significant amount of the deformation of the greenstone sequence, particularly the tight upright folding in some belts, has been related to granite intrusion, in some cases specifically to the ballooning phase of granite diapirs (Campbell and Hill, 1988; Skwarnecki, 1987). However, the inferred relationship between deformation and

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intrusion is not supported by the structural evidence. Granite diapirism is conceivably important at lower and middle crustal levels, because the granitic melt phase has a lower density than the surrounding rocks. However, Clemens and Mawer (1992) suggest that, even at deep levels in the crust, diapiric upwelling of large granite bodies is highly unlikely, and that magma sources are more likely tapped by fractures and magma transported in dykes. In the upper crust, brittle deformation associated with granite emplacement is more likely. Because of the physical properties of the crust away from the granite, there can be little regional ductile deformation caused by the intrusions. In examples of granite bodies where inflation of a stationary diapir has been implicated in causing local deformation (e.g., Ardara pluton; Meneilly, 1982 ), there is synchronous regional deformation in rocks encompassing the pluton. Indeed the Donegal examples of diapiric granite in Ireland, the Torries and Ardara plutons, are both anomalous in that local compression (transpression) in an otherwise tensional environment is the cause of the forceful emplacement of the bodies (Hutton, 1982 ). In the Criffel zoned pluton, where ballooning is shown to account for the internal deformation of the granite (Courrioux, 1987), the penetrative deformation in the country rock occurs only in the thermal aureole, and increases markedly in intensity towards the granite (Kafafy and Tarling, 1985 ). This situation is common in the contact aureoles of the pre- and syn-D2 plutons of the Eastern Goldfields Province (e.g., the Minerie Hill pluton ). Granite and gneiss intrusions in the central segment of the Eastern Goldfields conform to the pattern of deformation seen in these other examples. At the crustal level under consideration (equivalent to middle to lower greenschist), where the crust has significant strength, any buoyancy will be relieved by fracturing of the roof, stoping and lifting the roof, as this requires much less work than is done in penetra-

P.R. WILLIAMS AND A.J. WHITAKER

tively deforming a large body of rock by horizontal compression. This is expected because horizontal compression results in both ductile deformation of the whole rock body (folding), as well as lifting of the roof. Thus intrusion of the early granites as thick sheets at or near the base of the greenstones, and the later batholiths as roof lifting and stoping plutons causing intense local deformation in their thermal aureoles is consistent with the physical expectations and the available field evidence. There is no evidence that granite intrusion in the Eastern Goldfields Province caused the regional ductile deformation. Discussion

Extension related to basin formation The nature of the greenstone depositional basins remains conjectural (Barley et al., 1989), largely because of the absence of confirmed basement rocks and the deformed nature of nearly all granite-greenstone boundaries. There is circumstantial evidence that the mafic volcanic piles did not rise to any great height above sea level. Following the eruption of thick komatiites (Hill et al., 1989) evidence from pillow lavas and interflow chemical sedimentary rocks suggests that the greenstone basins remained below sea level during most of their history. Very few terrestrial sedimentary rocks have been recorded, and only minor terrigenous clastic detritus has been identified. In contrast, extensive subaqueous mafic volcanic rocks are present, and extensive thin chemical sedimentary horizons attest to quiet deposition over wide areas. In areas of more felsic volcanism (Kurnalpi Terrane; Swager et al., 1990), volcanic conglomerate horizons and cross-bedded volcaniclastic sandstone suggest that the felsic piles rose above sea level and was the source of significant detrital sedimentary rocks. Turbidites in some parts of the succession are generally quartz rich, and the late conglomerates are polymict, but largely composed

GNEISS DOMES AND DEFORMATION IN ARCHAEAN EASTERN GOLDFIELDS PROVINCE, WESTERN AUSTRALIA

of granitic and other quartz-rich detritus. Geochemical (Arndt and Jenner, 1986) and petrological (Compston et al., 1986) evidence suggests that the greenstone sequences of the Kalgoorlie Terrane are underlain by continental or continentally-derived crustal material. These factors suggest that the mafic sequences of the Kalgoorlie Terrane may have been deposited in extensional basins (Groves et al., 1978; Groves and Batt, 1984; Hallberg, 1986 ), and that post-rift extensional subsidence generally kept pace with deposition. However, no evidence of rift-phase sedimentation has been identified, possibly because the base of the greenstone sequences has never been found. The presence of abundant thick sequences of mafic dykes and sills within the basin also supports a continuing extensional environment for the depositional basin. The basins of the Kurnalpi Terrane appear to have a far more complex architecture. Greenstone basins west of the Eastern Goldfields Province probably record an earlier volcanic event, and may be explained by the diachronous extension of the pre-Late Archaean crust with localisation of magmatism in the area of "latest" rifting, or may represent a separate tectonic regime.

Emplacement of the granitic complexes It can be inferred from the structural and metamorphic evidence that several components of the regional gneiss domes had a significant residence time at a deeper crustal level than the bulk of the greenstone sequences in both the southern and northern sectors. Selvedges of high-grade, strongly-deformed amphibolite from a deeper crustal level, intercalated with porphyry, are juxtaposed against the greenschist-facies greenstone sequences (Williams and Currie, 1993 ). Emplacement of midcrustal rocks against higher-level sequences along mylonitic shear zones, and the preservation of steeply compressed metamorphic gradients at the level of the mylonite zones are features commonly used to infer a regional ex-

| 57

tensional tectonic environment (Crittenden et al., 1980). Binns et al. (1976) suggested that the thermal input from intruding granites and the synclinal nature of the greenstone basins may account for the gradients, but Archibald et al. (1981) discussed the difficulties of maintaining the required metamorphic gradients and establishing a focussed heat input to explain the metamorphic isograd patterns if the rocks where metamorphosed autochthons Local thermal metamorphic models do not explain the ubiquitous mylonitic, gently dipping margins, and also do not explain the difference in timing of peak metamorphism within the deformed gneiss dome selvedges and the lowergrade greenstone sequence rocks. P-T-t paths in the Eastern Goldfields are generally poorly constrained, but the available evidence suggests that peak metamorphic assemblages in areas away from the granitic complexes are synD2, whereas assemblages in the high-grade selvedges are syn- to late-De. A significant episode of regional extensional tectonics, which pre-dated the regional upright folding (D2) event would account both for differences in metamorphic grade and timing of peak metamorphism between the upper and lower plate. Rapid uplift of the hot lower-plate rocks from a higher-grade environment will result in syn-emplacement metamorphic fabrics, while at the same time providing a thermal input to the upper plate, resulting in peak metamorphism post-dating dome emplacement. All major domes are predicted to underlie the greenstones with a strongly sheared mylonitic margin in an extensional model, whereas in an intrusive model, high-grade and gently dipping mylonitic margins are unexplained. The structural features of the mylonitic boundaries and the universal dip of gneiss beneath greenstone are inconsistent with a diapiric origin for gneiss domes and granitic complexes. The extensional model proposed here also provides a predictive framework for further geological investigations. The main features of the extensional model are presented in Fig. 9. Crustal

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P.R. WILLIAMS AND A.J. WHITAKER

High-strain Zone

Thinned Greenstone Seauence

Attenuated High-strain Zon~

Early Extensional Geometry

Greenstone Ballard Domain I

avertenGneiss Domain Early GraniteSheets Eastern Gneiss Domain Lag Fault

Fig. 9. South-north cross-section through the Ballard and Laverton magnetic domains. The movement directions indicated are resolved in the plane of the section.

extension resulting in the separation of the northern greenstone sequence from the southern sequence across the Ballard magnetic domain is the major feature of the model. Uplift of the mid-crustal complexes now exposed as the Ballard, Laverton and Eastern magnetic domains is analogous with the formation of Cordilleran metamorphic core complexes. The lack of rotational block faulting in the upper plate in the Eastern Goldfields is a significant departure from the Cordilleran example. However, the scale of the extensional complex represented by the Ballard magnetic domain is almost identical to that of the Colorado River extension corridor (Howard and John, 1987 ), but differs in that the whole of the Ballard magnetic domain has been uplifted, rather than a number of smaller core complexes. These differences in geometry may relate to erosion level. Differential uplift of regions within the Ballard and Laverton extensional complexes is indicated by the presence of internal domes and composite gneissic and granitic material. The commonly transitional relationship of the granites with high-grade gneiss and migmatitic rocks in both the Ballard magnetic domain and the Laverton magnetic domain suggests that the "early" granites were intruded

into the middle crust and deepest parts of the greenstone sequences as thick sheets. The intrusion level is within the amphibolite facies, and presumably marks the shallowest level of diapiric granite activity. Myers and Watkins (1985) discussed evidence that the structural elements within the domes have been refolded, but the evidence in the Ballard and Laverton magnetic domains for a regional foldinterference pattern to explain the elongation of the domes is weak. Their basic observation that the domes predate the regional D: compressive deformation is however, correct. In the extensional model proposed here, the elongate domal nature of the granite is explained by the granites rising with the gneiss in a regional stress field with the maximum compression exceeding the vertical confining pressure and oriented perpendicular to the dome long axes and the extension direction (a~ > a2 > a3; in extension at a high crustal level, a ~ a 2 > a 3 ) . Several of the domes are aligned, and where evidence is available, the elongation direction is parallel to the stretching lineation. The alignment of the greenstone apophyses on the Laverton dome can also be related to dome emplacement in this regional stress field. Recent modelling by Yin (1991) suggests that aligned core complexes require a regional compressive stress component, and that core complexes are most likely to have been elongated during emplacement, rather than during later deformation. In the Eastern Goldfields, the flattening required to produce the dome shapes from initially sub-circular shapes (Fig. 3) is not recorded in the strain in the surrounding greenstones. The stretching lineation directions and dome elongation directions are parallel, and suggest a regional extension direction oriented north-south to north-northeast-south-southwest, with a maximum compression oriented east-west to east-southeast-west-northwest exceeding the vertical confining stress. The development of regional core complexes as the main response to crustal exten-

GNEISS DOMES AND DEFORMATION IN ARCHAEAN EASTERN GOLDFIELDS PROVINCE, WESTERN AUSTRALIA

159

sion is enhanced by the crustal conditions likely to have been present in the Archaean. The geophysical characteristics of crust in regions of core complex formation suggest that the lower crust was weak at the time of formation of the complexes. The lack of relief on the Moho beneath modern core complexes has been explained by rapid flow of the lower crust into the extending region (Block and Royden 1988; Buck, 1991). In the Eastern Goldfields, Campbell and Hill (1988) and Hill et al. (1989) suggest that the voluminous late granite is the result of lower crustal melting above a rising mantle plume. A lower-crustal heating event will provide ideal conditions for the formation of core complexes. The amount of extension will also be greater if the far-field extensional stress is enhanced by the effect of topographic uplift above a thermal plume. The presence of abundant undeformed, late granite bodies within the gneiss domes suggests that doming may have preceded intrusion, a relationship which would be consistent with a model incorporating a transition from core complex formation due to lower-crustal thermal weakening, proceeding to partial melting in the lower crust and granite intrusion over a short period of time (Campbell and Hill, 1988).

the southern extension of the Keith-Kilkenny zone, and consequently an abnormally high metamorphic grade with steep gradients to the surrounding greenstones in the zone south of the Ballard magnetic domain. Such a metamorphic pattern was mapped by Binns ( 1976; Fig. 6). Generation of granitic magma late in the extensional episode, due to a rapid increase in the thermal gradient in the crust, is predicted by the extensional model (Lister et al., 1986). Localisation of these intrusions in the Boyce magnetic domain is, therefore, structurally controlled: they are restricted to regions of high metamorphic grade (?close to their source) and localised by intrusion along active fractures (Clemens and Mawer, 1992) into the regional high-strain zone. The high magnetisation of the granites can be explained as a result of high-level melting in the detachment surface (e.g., Czamanske et al., 1981; Gastil et al., 1990). The extensional model suggests that the greenstone sequences in the Northeast Goldfields and those in the Kalgoorlie-Kambalda area were closer together prior to extension, and also that the abnormal width of the greenstone belt across the central segment of the goldfields may be due to a northeasterly crustal extension direction.

The Boyce magnetic domain

Mineralisation in the detachment zones

This distinctive belt of granitic intrusions (Fig. 4) is coincident with the western margin of the Keith-Kilkenny high-strain zone. In the Leonora area the Keith-Kilkenny zone represents a splay from the primary extensional fault related to the Raeside gneiss emplacement. Early movement on the low-angle faults is topside-north (Williams et al. 1989; Williams and Currie, 1993), whereas the later upright shears are dextral (Sckwarnecki, 1987; Passchier, 1991 ). Extension of the greenstone belt in a north to northeasterly direction to allow emplacement of the Ballard magnetic domain would require thinning of the greenstones along

Several significant ore bodies are located in the large structures associated with gneiss-amphibolite boundaries. These include the Sons of Gwalia, Tower Hill and Harbour Lights deposits near Leonora, several smaller deposits in the Sons of Gwalia shear, and several small prospects around the Laverton dome, and also the Lancefield mine adjacent to the Laverton dome. Williams et al. ( 1989 ) described the relationship between structures and shear-zone formation near Leonora, and concluded that the mineralisation at both Harbour Lights and Sons of Gwalia was synchronous with deformation in the shear zone. The steep plunge of

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the stretching lineation in both of these deposits indicates that the mineralisation did not take place in a predominantly strike-slip environment, but took place during development of the shear zones. Most mineralised veins are strongly deformed and, hence, where emplaced during movement on the shear (Finucane, 1965; Williams et al., 1989). However, some veins are also only weakly deformed or undeformed (particularly at Tower Hill). This relationship suggests that mineralisation in the Sons of Gwalia Shear was broadly synchronous with the emplacement of the gneiss domes and peak metamorphism in the shear zone. The later strike-slip shear zones in the Leonora area (such as the Keith-Kilkenny high-strain zone (Hallberg, 1985; Williams et al., 1989 ) are not mineralised. There are, however, numerous mines and prospects in the northeast sector of the Eastern Goldfields Province which do not appear to be related to extensional faulting and have formed later in the structural history (e.g., Mertondale, see Nisbet and Williams, 1990). The variable relationship between the timing of structures and gold mineralisation suggests that the higher-grade mineralisation may be earlier than the lower-grade mineralisation associated with the regional strike-slip events. Because these structural events may be temporally close (Campbell and Hill, 1988 ), there is a strong possibility that the mineralisation may have continued throughout the structural history, with rocks from deeper crustal levels recording earlier fluid ingress. The role of extensional faults in tapping deep fluid sources may have been significant.

P.R, WILLIAMS AND A.J. WHITAKER

major role for granites as the cause for upper crustal deformation are not supported by the structural (or tectonophysical) evidence. (3) Ballooning of late granite diapirs has resulted in local deformation, generally contained within the thermal aureoles. (4) Extensional tectonics was of major importance in construction of the Late Archaean crustal architecture of the Eastern Goldfields Province. ( 5 ) Major crustal extension took place after greenstone deposition, and followed the intrusion of "early" granite sheets at a middle crustal level. The extension resulted in emplacement of gneiss domes and granitic complexes as metamorphic core complexes. Conglomerate basins may have formed as a response to core complex uplift. (6) Cordilleran-type horizontal accretion models do not readily accommodate the sequence of early deformational events outlined here. ( 7 ) The dominance of batholithic late granite intrusions around the margins of the greenstones and to the west, the presence of compressed metamorphic gradients around gneissic or deformed granite cores, and the importance of reactivated early sub-horizontal structures are consistent with an asymmetrical extensional model (Lister et al., 1987) rather than a horizontal terrane-accretion model. The emplacement of gneiss domes in an extensional environment, followed by granite intrusion is consistent with the thermal evolution of the crust suggested by Campbell and Hill ( 1988 ).

Acknowledgements Conclusions (1) There is no structural evidence for regional diapirism at the crustal level now exposed in the NE goldfields. In contrast, there is strong evidence that the aerially extensive "early" granites were intruded as sub-horizontal sheets at a middle crustal level. (2) Vertical tectonic models which imply a

Thanks are due to M. Etheridge, L. Wyborn and B. Drummond for stimulating discussions over several years on the tectonics of ancient fold belts; to C. Swager and W. Witt for a free exchange of ideas on the geology of the Eastern Goldfields; to A.L. Jaques and M. Rattenbury for comments on the manuscript, and to C. Passchier and K. Currie for argument on the

GNEISSDOMESANDDEFORMATIONIN ARCHAEANEASTERNGOLDFIELDSPROVINCE,WESTERNAUSTRALIA

detailed structural and metamorphic history of the Leonora district. The manuscript was improved by careful reviews by R. Hammond, L. Wyborn and an anonymous reviewer. The National Geoscience Mapping Accord, Eastern Goldfields Project has provided the framework in which continued research has been possible. Western Mining Corporation provided access to the deep drill core from Leonora. Published with the permission of the Director, Australian Geological Survey Organisation, Canberra. References Archibald, N.J., Bettenay, L.F., Binns, R.A., Groves, D.I. and Gunthorpe, R.J., 1978. The evolution of Archaean greenstone terrains, Eastern Goldfields Province, Western Australia. Precambrian Res., 6: 103-131. Archibald, N.J., Bettenay, L.F., Bickle, M.J. and Groves, D.I., 1981. Evolution of Archaean crust in the Eastern Goldfields Province of the Yilgarn Block, Western Australia. In: J.E. Glover and D.I. Groves (Editors), Archaean Geology. Spec. Publ. Geol. Soc. Aust., 7: 491-504. Arndt, N.T. and Jenner, G.A., 1986. Crustally contaminated komatiites and basalts from Kambalda, Western Australia. Chem. Geol., 56: 229-255. Barley, M.E., Eisenlohr, B.N., Groves, D.I., Perring, C.S. and Vearncombe, J.R., 1989. Late Archaean convergent margin tectonics and gold mineralisation: A new look at the Norseman-Wiluna Belt, Western Australia. Geology, 17: 826-829. Barley, M.E. and Groves, D.I., 1990. Deciphering the tectonic evolution of Archaean greenstone belts: the importance of contrasting histories to the distribution of mineral deposits in the Yilgarn Block. Precambrian Res., 46: 3-20. Binns, R.A., Gunthorpe, R.J. and Groves, D.I., 1976. Metamorphic patterns and development ofgreenstone belts in the Eastern Yilgarn Block, Western Australia. In: B. Windley (Editor), Early History of the Earth. Wiley, London, pp. 303-313. Buck, W.R., 1991. Modes of continental lithospheric extension. J. Geophys. Res., 96: 20,161-20,178. Block, L. and Royden, L., 1990. Core complex geometries and regional scale flow in the lower crust. Tectonics, 9: 557-567. Campbell, I.H. and Hill, R.I., 1988. A two-stage model for the formation of the granite-greenstone terrains of the Kalgoorlie Norseman area, Western Australia. Earth Planet. Sci. Lett., 90:11-25. Clemens, J.D. and Mawer, C.K., 1992. Granitic magma

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