Geology and tectonic evolution of the Palaeoproterozoic Bryah, Padbury and Yerrida Basins (formerly Glengarry Basin), Western Australia: implications for the history of the south-central Capricorn Orogen

Premmbrinn Reseur(h ELSEVIER

Precambrian Research 90 (1998) 119 140

Geology and tectonic evolution of the Palaeoproterozoic Bryah, Padbury and Yerrida Basins (formerly Glengarry Basin), Western Australia: implications for the history of the south-central Capricorn Orogen F. Pirajno *, S.A. Occhipinti, C.P. Swager 1 Geological Survey o f Western Australia, 100 Plain Street, East Perth, WA 6004, Australia Received 17 June 1997; accepted 3 February 1998

Abstract

The Palaeoproterozoic Bryah, Padbury and Yerrida Basins are situated along the northwestern margin of the Archaean Yilgarn Craton, central Western Australia. These basins form part of the Capricorn Orogen, which developed between 2.0 and 1.8 Ga as a result of the collision between the Archaean Pilbara and Yilgarn cratons. The Bryah, Padbury and Yerrida Basins, which at the present day cover a total area of ca 20 000 km 2, were formerly considered as one geological entity, the Glengarry Basin. These three basins are characterized by distinct stratigraphy, igneous activity, structural and metamorphic history, and mineral deposit types. Igneous activity only affected the Bryah and Yerrida Basins, with voluminous eruptions of tholeiitic magma. In the Bryah Basin tholeiitic volcanic rocks are Mg-rich and have mixed MORB to oceanic island chemical signatures, but with a boninitic (subductionrelated) component. In the Yerrida Basin tholeiites are Fe-rich and have chemical signatures that suggest a mixed tectonic environment ranging from oceanic to continental. It is considered possible that this tholeiitic magmatism is related to a mantle plume. Two models for the tectonic evolution of the Bryah, Padbury and Yerrida Basins are proposed: (1) the Bryah and Yerrida Basins were formed in a back-arc setting, whilst the Padbury Basin developed as a retro-arc foreland basin over the Bryah Basin; and/or (2) strike-slip transtension, during and following the Pilbara Yilgarn collision, created the Bryah and Yerrida strike-slip pull-apart Basins. A change in regional stress regime resulted in the inversion of the basins and the development of a foreland basin in the northwest (Padbury Basin). © 1998 Elsevier Science B.V.

Keywords." Capricorn Orogen; Mafic volcanism; Mantle plume; Mineralization; Palaeoproterozoic basins; Tectonic evolution

* Corresponding author. Tel.: +61 9 2223155: Fax: +61 9 2223633: e-mail: [email protected] 1Present address: North Ltd. Exploration Division, P. O. Box 231, Cloverdale, WA 6105, Australia. 0301-9268/98/$19.00 (¢) 1998 ElsevierScience B.V. All rights reserved. Pll S0301-9268 (98) 00045-X

120

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1. Introduction

Volcano-sedilnentary basins of Proterozoic age are recognized as important geological entities because: (1) the study of their dcvelopment provides insights into Proterozoic gcodynamic, magmatic, metallogenic and sedimentological processes; and (2) they contain significant base and precious metal deposits. This paper outlines the results of recent field-based studies of Palaeoproterozoic basins in central Western Australia. The research on these basins is aimed at understanding their initial formation, their infilling with sedimentary and volcanic rocks, and their subsequent tectonic inversion and amalgamation into an orogenic zone. The Proterozoic basins studied are situated along the northern margin of the Archaean Yilgarn Craton. They form the central-southern domaiu of the Capricorn Orogen [Gee (1979); inset of Fig. 1], which also inchides the Earaheedy Basra. the Ashburton Basin, and the Gascoyne Complex. The Capricorn Orogen is a major zone of deformed, low-grade volcano-sedimentary belts. high-grade metamorphic belts and granitoid intrusions, t\~rmed during the continental collision between the Pilbara and Yilgarn Cratons. tit some time between 2000 1800 Ma (Tyler and Thornc, 1990: Myers, 1990, t993: Myers et al., 1996). The Capricorn Orogeny also afl'ected other tectonic units, such as the Archaean Narryer Terrane, the Marymia Inlicr, the Sylvania lnlier and parts of the Hamersley Basin (Tyler and Thorne. 1990: Myers eta[., 1996: Tyler et al.. in press). The Bryah, Padbury and Yerrida Basins were formerly described as a single geological entity, the Glengarry Basin (Gee and Grey, 1993), which formed the western part of the Nabberu Basin (Hall and (}oode, 1978: Bunting et al., 1977). The Glengarry Basin contained the Glengarry and Padbury Groups (Table 1 ). As a result of recent mapping, the volcano-sedimentary rocks of the

Glengarry Group

are now divided into the Bryah and Yerrida (h-oups (Tables 1 and 2 ), and some

formations prex,iously assigned to the Glengarry Group are re-assigned to tile Padbury (}roup. In

14(7

addition, several lines o1 evidence suggest that the (economically important) Peak Hill Metamorphic Suite, previously considered to be part of the Glengarry Group {Gee, 1987), constitutes a septirate unit, the Peak Hill Schist, derived from a protolith of probable Archaean age. Consequently the previous nomenclature (e.g. Glengarry Group and Basin) is no longer used (Tables I and 2). The Bryah and Padbury Groups (Fig. 1) make up the westcrn part of the former Glengarry Basin, and are n o w interpreted to have developed in rift and foreland basins, respectively (Pirajno et al., 1996; Martin, 1994). The Ycrrida Group (Fig. 1) makes up the eastern part of the former Glengarry Basin, and includes two subgroups, Windplain and Mooloogool (Fig. 1, Table 2), which developed in sag and rifl basins, respectively (Pirajno c t a l . , 1995, 1996). The Bryah, Padbury and Yerrida Groups tire characterized not only by different lithologies, but also by different regional structures, metamorphism, and mineral deposit types. The Bryah Group is in tectonic contact with the Yerrida Group along the northeast-trending Goodin Fault. which is interpreted as a high-angle reverse fault. Thus. based oil structural, stratigraphic and metamorphic criteria, the area occupied by the Bryah and Padbury Groups and the Peak Hill Schist, can be regarded as a single structural metamorphic domain; in this paper, where appropriate, this area is inl\)rmally referred to as the Bryah Padbury terrane ( Fig. 1 ). A companion paper in this vohime by Occhipinti el al. provides details on the structure and metamorphism of the Bryah and Padbury (h-oups (Occhipinti et al.. 1998, this volume). Interim accounts of the tectonic evolution, structural and stratigraphic relations in the former Glengarry Basin. have been reported by Pirajno (1996), Pirajno ctal. (I995, 1996)arid Occhipinti ct al. ( 1996. 1997a.b).

2. Stratigraphy and geochronology

The recognition of distinct lithostratigraphic domains resulted in the establishment of a new stratigraphy summarized in Table 2, where a comparison with previous works is also provided. A brief review of the new stratigraphy, geochronolog-

O30

F. Pira/no et al. Precamhrian Research 90 (1998) 119 140 I

121

I

118° 30'

30 km

BANGEMALL GROUP

+++I

Jr_ MLRYMIA

+ NARRYER GNEISS 4 TERRANE +

.....

INLIER

WESTERN AUSTRALIA

_{_

+

+

+

5 30'

Mt Padbl

-t-26 ° 00'

YILGARN CRATON :::::::::::::::::::::::::::::::::::::::::::::::::::::::: ::::::::::::::::::::::::::::::::::::::::::::::::::::::: "1:: ................................................... =====================================================

Earaheedy Group

-t-

o o o

o

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{

Horseshoe and " ~:ii Ravelstone Formations -INarracoota Formation: " sheeted dykes mafic-ultramafic schist/ metabasaltic hyaloclastite

i ::::::::::iiii -I-

+

+

Karalundi Formation

[Moo,oogoo, /Su0grou0 k[ ~ ~

~. (~ / Windplain [ ~ L Subgroup L

-I-

Doolgunna and Thaduna Formations with intercalated Killara Formation araloou Formation

Killara Formation uderina and Johnson

Cairn Formations

118o30' i SAO14B

F

==:===================================== ~o - o4~@~'~ oo • :: :::: o o ~ <:~-1-~4-====================== "

Padbury Group

m(.9

~

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~Peak -~

Hill Schist Archaean basement

- Fault - Geological boundary 119° 30' I

0705.98

Fig. 1. Simplified geological map of the Bryah, Padbury and Yerrida basins, central Western Australia: inset shows position of map area in relation to the Capricorn Orogen. The Earaheedy Group is unconformable on the Yerrida Group and is part of the PalaeoMesoproterozoic Earaheedy Basin in the east. To the north is the Bangemall Group (not shown in legend), which is part of the Mesoproterozoic Bangemall Basin.

122

[: Pirajm; el al. ' Precamhrian Re.search 90 (1998) 119 140

Table I Stratigraphy' of the former Glengarry' basin according to Gee (1987) and Gee and Grey {1993), compared to the revised stratigraphy of the Yerrida. Bryah and Padbury groups (this work) tectonic contact ]

'tectonic contact] Gee and Grey 0993)

Gee (1987) Glengarry Basin Glengarry Group

Padbury Group

Glengarry Group

This paper Yerrida Basin Yerrida Group

Bryah Basin Bryah Group

Labouchere Formation

, ?Marymia Inlier

Tectonic unit

Millidi¢ Creek Formation Robinson Range Formation Wilthorp¢ Formation (Heincs, and Beatty Park rncmbcrs) Labouchcre Formation

Millidie Creek Formation Robinson Range Formation Willhorp¢ Conglomerate unconformity ..............................................

Padbury Basin Padbury Group

,

Maraloou Formation unconformity

Horseshoe Formation Mooloogool Subgroup

Thaduna Greywack¢

Maraloou Formation

Horseshoe Formation Ravelstone Formation

succession)

Narracoota Volcanics Karalundi Formation ' Doolgunna Formation Johnson Cairn Shale Juderina Formation

Narracoota Volcanics Thaduna Grcywacke Doolgunna Formation Johnson Cairn Shale

Windplain Subgroup (sag basin)

Juderina Formation (Finlayson Sandstone Member)

I Crispin Conglomerate Mandoou Formation Finlayson Sandstone Peak Hill Mettamorphics

ical constraints and its significance in terms of basin settings are given below. Detailed descriptions of formations and their contact relationships are given ill Occhipinti et al. (1997a,b). 2.1.

Yerrida Grow)

The Yerrida Group is present in the east and southeast of the former Glengarry Basin (Gee and Grey, 1993). and includes siliciclastics, carbonate sedimentary rocks and mafic volcanic and hypabyssal rocks. The Yerrida G r o u p unconformably overlies, or is in tectonic contact with, Archaean granite greenstone rocks of the Yilgarn Craton and the Marymia and Goodin lnliers ( Fig. 1 ). Its northwestern boundary with the Bryah G r o u p is marked by the Goodin Fault. In the eastern part of the Yerrida Basin, rocks of the Yerrida Group are

Killara Formation (Bartle Member) Doolgunna Formation Thaduna Formation Johnson Cairn Formation

Narracoota Formation Karalundi Formation

Juderina Formation (Bubble Well, and Finlayson members)

Peak Hill Schist

unconformably overlain by the Proterozoic Earaheedy Group (Fig. 1 ). The age of the Yerrida G r o u p is less than 2.2 Ga, based on a Pb Pb isochron of 2173_+64Ma obtained from stromatolitic carbonate rocks of the Bubble Well Member (Windplain Subgroup) (Woodhead and Hergt, 1997). A 1785±11 Ma U Pb detoital zircon date from the overlying Earaheedy G r o u p (Nelson, 1997), gives a minimum age. The Yerrida G r o u p is divided into two subgroups: Windplain and Mooloogool: that are interpreted to have developed in different tectonic settings (Pirajno et al., 1995). The Windplain Subgroup contains the Juderina and Johnson Cairn Formations, the sedimentological characteristics of which suggest that they were deposited in mature fluvial and shallow marine environments

F Pirajno et aL /Precambrian Research 90 (1998) 119-140

123

Table 2 Stratigraphy of the Yerrida, Bryah and Padbury groups [after Pirajno et al. (1996), data for Padbury Group is from Martin (1994)] Basins/Groups

Subgroups

Formations

Rock types

Millidie Creek

Sericitic siltstone, chloritic siltstone, BIF, dolomitic arenite Ferruginous shale, BIF Quartz pebble conglomerate (mafic siltstone/wacke, and polymictic conglomerate, respectively) Turbidite sequence (quartz wacke, siltstone)

Padbury (retro-arc /oreland basin)

Padbury Group

Robinson Range Wilthorpe (Beatty Park and Heines members) Labouchere UnconJbrmable contact many places tectonized Bo,ah ( ri[~ basin)

&

Bryah Group

Horseshoe Ravelstone Narracoota

BIF, wacke, shale Quartz lithic wacke Mafic ultramafic volcanics and intercalated sedimentary rocks Conglomerate, quartz wacke

Karalundi Fault contact Yerrida (sag and r([t basin)

Yerrida Group

Mooloogool Subgroup Maraloou (rift succession) Killara Doolgunna Thaduna Windplain Subgroup Johnson Cairn (sag-basin succession) Juderina (Bubble Well and Finlayson members)

[sag basin or pre-rift depression; Pirajno et al. (1995, 1996)]. The Juderina Formation comprises siliciclastic, carbonate and evaporite rocks (significantly no basal conglomerates are present), and contains two members: Finlayson and Bubble Well. The former consists of a thin (< 100 m) and widespread basal quartz arenite unit, which commonly displays herring-bone, trough cross-bedding and multi-directional ripple marks. The Finlayson Member is overlain by and/or intercalated with chertified stromatolitic carbonate and evaporitic sedimentary units of the Bubble Well Member. The Juderina Formation is unconformable on, or faulted against the Archaean basement rocks of the Goodin and Marymia Inliers, and the Yilgarn Craton. This formation is conformably overlain by the Johnson Cairn Formation, which dominantly consists of argillaceous rocks. The Mooloogool Subgroup conformably over-

Black shale, siltstone, carbonate Mafic extrusives and intrusives Mixtite and clastic sedimentary rocks Lithic wacke, siltstone, shale, minor arkose Siltstone, shale, carbonate, minor lithic wacke Arenite, conglomerate, minor carbonate (silicified carbonate with evaporites, and arenite, respectively)

lies the Windplain Subgroup and its sedimentological characteristics suggest that it was deposited in a rift-basin setting (Pirajno et al., 1995, 1996). The Mooloogool Subgroup is divided into four formations: Thaduna; Doolgunna; Killara; and Maraloou. The sedimentary rocks of the Thaduna and Doolgunna Formations were deposited in a high-energy environment (conglomerates, turbidite facies rocks) and as such these rocks herald an abrupt change from the shallow and mature environment of the Windplain Subgroup. The distribution of these two formations is centred around the Goodin Inlier, forming a broad north-east-trending belt (Fig. 1). The Thaduna and Doolgunna Formations were largely sourced from the Goodin and Marymia Inliers and they interdigitate, testifying to a complex depositional environment. The Doolgunna Formation is represented by a succession of conglomerates, turbidite-facies rocks and

124

K Pirajm) el ell / Precambt'ian Research 90 (1998) 119 140

diamictite units, the latter being the result of masswasting sourced from rocks overlying and including the Goodin Inlier. These sediments accumulated in a northeast-trending graben-like structure the Doolgunna graben (Pirajno, 1996; Pirajno and Occhipinti, 1998), on the east side of the Goodin Fault (Fig. 1 ). The Thaduna Formation is typically a turbiditic succession dominated by coarse- to fine-grained wackes, containing lithic fragments of volcanic rocks, shale and siltstone. Some of these volcanic rocks were sourced from Archaean greenstones within the Marymia Inlier, others from the Killara and Narracoota Formations. The Doolgunna and Thaduna Formations also interdigitate with the tholeiitic volcanic and intrusive rocks of the Killara Formation, indicating that volcanism and clastic sedimentation were concurrent in rift-related depocentres [Mooloogool rift; Pirajno (1996)], with local 'cannibalization' and scouring of the volcano-sedimentary units. In the east and southeast, the mafic rocks of the Killara Formation (described in more detail later) overlie and intrude the Windplain Subgroup, and are overlain by the Maraloou Formation. The Maraloou Formation comprises carbonaceous argillite, marl, dolostone and minor chert, and represents a marked change in environmental conditions (deepening of the basin). In the west, peperite margins in the mafic lavas and sills of the Killara Formation indicate that the mafic melts were emplaced into unconsolidated wet sediments of the Maraloou Formation. The contact between the two formations is transitional over a stratigraphic thickness of ca 150 m with the volcanic component consistently decreasing with stratigraphic height. In the east, however, the Maraloou Formation is unconformable on the Killara Formation and conformable over the Thadunna and Doolgunna Formations. This may reflect onlap due to contemporaneous block faulting in the eastern areas.

the Bryah Group is poorly constrained between ca 2.0 and 1.8 Ga. Detrital zircons of uncertain provenance in the Ravelstone Formation (Upper Bryah Group) provide a maximum age of 2014_+22 Ma (Nelson, 1997). The Bryah Group must be older than the unconformably overlying Mount Leake Formation (outlier of the Earaheedy Group), which has a U Pb (detrital zircons) maximum age of 1785_+11 Ma (Nelson, 1997). Pb Pb isochron ages were obtained from the mesothermal Mikhaburra gold deposit [1.74 Ga: Pirajno and Occhipinti (1998)] and inferred syngenetic pyrite from the Narracoota Formation [1920_+35 Ma; Windh (1992)]. However, these Pb Pb isochron ages probably represent mineralizing events in the Bryah Basin, rather than the depositional age of the Bryah Group. The basal unit of the Bryah Group is the Karalundi Formation which is in faulted contact with the Doolgunna Formation (Yerrida Group) along the Goodin Fault. The Karalundi Formation consists of quartz conglomerate, quartz arenite, lithic wacke and shale. At one locality, on the southern margin of the Bryah Basin, the top of the Karalundi Formation is intercalated with volcaniclastic basal units of the Narracoota Formation. The Narracoota Formation [previously known as Narracoota Volcanics; Gee and Grey (1993)] is the dominant lithology in the Bryah Basin and consists of low-K tholeiitic volcanic and intrusive rocks with minor ultramafic units, intercalated with minor jasperoidal chert units, and clastic sedimentary rocks. The Narracoota Formation is disconformably overlain by, and locally interfingers with the Ravelstone Formation. The Ravelstone Formation comprises a succession of lithic and quartz wacke, shale and siltstone that was deposited by turbidity currents. This formation is in turn conformably overlain by the Horseshoe Formation, comprising quartz wacke, manganiferous shale and banded ironformation.

2.2. Bo~ah Group 2.3. Padburv Group The Bryah Group is divided into four formations comprising subalkaline mafic and ultramafic extrusive and intrusive rocks, and terrigenous and volcanogenic clastic rocks (Fig. 1, Table 1 ). The age of

The Padbury Group locally unconformably overlies the Horseshoe Formation of the Bryah Group, but in places is in fault contact with the

F. Pirajno et al. / Precambrian Research 90 (1998) 119 140

Bryah Group and the Archaean Narryer Terrane (Fig. 1 ). Considerable onlap of the Padbury succession onto the various formations of the Bryah Group can be inferred; these contacts were re-worked, possibly in several stages, during basin closure. The Padbury Group contains quartz wacke, siltstone, conglomerate, iron-formations, hematitic shale and minor mafic clastic rocks and dolomite (Martin, 1994; Occhipinti et al., 1997a,b). Martin (1994) interpreted the Padbury Group to have been deposited in a peripheral foreland basin, that was developed on top of the Bryah Group. The only precise age constraints on the Padbury Group are: ( 1 ) a maximum age of 1996_+ 35 Ma from U-Pb dating on detrital zircons (Nelson, 1997) in a sedimentary chert lens in the upper part of the Wilthorpe Formation; and (2) a minimum age of ca 1.8 Ga inferred from a cross-cutting leucogranite dyke (Windh, 1992). The basal Labouchere Formation consists of quartz arenite, quartz wacke, siltstone, minor conglomerate and banded iron-formation in an upward-coarsening succession. The Labouchere Formation locally appears to unconformably overlie the Horseshoe Formation (Bryah Group), although in most places this contact is tectonized (Martin, 1994, in pr; Occhipinti et al., 1996; Windh, 1992). The Labouchere Formation is conformably overlain by and grades into the Wilthorpe Formation (Martin, 1994), although this contact was previously described as an unconformity (Gee, 1979). The Wilthorpe Formation, including the Beatty Park and Heines members, comprises quartz and chert pebble conglomerate, quartz wacke, sericitic siltstone, chlorite-quartz shale, polymictic conglomerate, quartz-sericitehematite schist, dolomitic sandstone and finely laminated chert lenses. The Beatty Park Member contains clastic rocks that were partly sourced from the mafic volcanic rocks of the underlying Narracoota Formation (Bryah Group). The Heines Member consists of polymictic conglomerate, shale and wacke. The Wilthorpe Formation is conformably overlain by the Robinson Range Formation, which consists of granular and banded

125

iron-formations, and iron-rich shale. The Robinson Range Formation is in apparent conformable contact with the overlying Millidie Creek Formation, and is in faulted contact with the Bryah Group. 2.4. Peak Hill Schist

The Peak Hill Schist [formerly called Peak Hill Metamorphic Suite, Gee (1987)] is exposed in the 'Peak Hill Dome' or anticline, and constitutes a tectonic unit representing the southwestern tip of the Marymia Inlier [Fig. 1; Thornett (1995)]. Rocks of the Peak Hill Schist include phyllonite, quartz-muscovite schist, calc-silicate schist, sericite (quartz) schist and quartz-muscovitebiotite-(chlorite) schist locally with alkali feldspar porphyroblasts and minor metabasite. These units have been variously deformed, during the Capricorn Orogeny, to form a range of mylonitic textures. Some discrete mylonitic units form arcuate zones, interpreted as early, possibly thrust, fault zones. One of these units is the Peak Hill Mylonite, which is a refolded quartz-blastomylonite and quartz-mylonite lens within quartz-muscovite schist, associated with gold mineralization (see later). Other, less conspicuous quartz-mylonite lenses are common within the Peak Hill Schist; they were previously mapped as cherts or banded cherts [e.g. Windh ( 1992)]. The boundary between the Peak Hill Schist and the granitic rocks of the Marymia Inlier is a zone of intense deformation and metamorphism, characterized by tectonic interleaving and duplexing. Towards the northeast, the intensity of the Capricorn Orogeny deformation and metamorphism in the Marymia Inlier granites decreases to a point where they are undeformed.

3. Volcanism, volcanic geochemistry and eruptive palaeoenvironments 3.1. Yerrida Basin

Voluminous tholeiitic extrusive and intrusive rocks (Killara Formation) were emplaced during the Mooloogool rifting event, in the south, east

126

1~ Pirq/no et al. : Precambrian Research 90 (1998) 119 140

and southeast of the Yerrida Basin (Fig. 1). The thickness of the Killara Formation is uncertain, but is estimated to be in the order of 1000m (Pirajno et al., 1995). The Killara mafic rocks are unmetamorphosed, flat-lying or shallow dipping, have tholeiitic to calc-alkaline basaltic and basaltic andesite compositions, and were emplaced as subaerial and subaqueous lava flows, intrusive sheets, sills and dykes. Examination of drillcore in one area, revealed the presence of 15 individual lava flows in a 90 m-thick section, indicating a high rate of eruption. Volcaniclastic deposits are either absent or uncommon and no detectable volcanic centres have been observed. Pirajno et al. (1995) suggested that a number of subparallel, linear easterly trending aeromagnetic structures, recognized in the southern areas of the basin, can be correlated with microgabbroic dykes. These microgabbroic dykes may have fed the outpouring of the mafic volcanic rocks of the Killara Formation. The generally aphyric, variolitic and/or vesicular lavas, and associated intrusive rocks have remarkably uniform compositions; they contain augite and labradorite (An 59) as the main constituents. No olivine was observed in these rocks. In the southeast, the Killara Formation is overlain by laminated and cross-stratified volcaniclastic, chemical and evaporitic sedimentary rocks, which constitute the Bartle Member (Fig. 1 ). This unit developed during the closing phases of Killara volcanism, and was probably formed in an environment characterized by shallow salt lakes, with localized hot springs (Pirajno and Grey, 1997). Representative analyses of Killara Formation volcanic rocks are given in Table 3. The Killara mafic rocks are characterized by low rare earth element ( R E E ) abundances (10-80 times chondritic values) with a slight light REE ( L R E E ) enrichment and positive Eu anomalies [Fig. 2(a)]. The Killara REE patterns resemble those of the Pleistocene-Miocene basalts of Iceland (Schilling et al., 1982). Compared with the Narracoota Formation volcanic rocks (Bryah Group), the Killara volcanic rocks have higher TiO2 (average 0.86 wt%), lower Mg# (average 50; see Table 3 for definition), Ni (average 75 ppm) and Cr (average 112 ppm) abundances (see Table 4). Sun (1997), suggests that the light REE-enriched patterns and

the strong Nb depletion of the Killara volcanic rocks may relate to a back-arc basin environment. Also, the low Nb/La ratio and TiO2 values would suggest derivation from a mantle modified by subduction processes [e.g. Pearce and Parkinson (1993)]. However, the sedimentary rock associations (predominantly of a continental nature, such as siliciclastic and evaporite rocks of the Yerrida Group), petrological data and discriminant diagrams [Fig. 2(b)], indicate a mixed tectonic signature ranging from oceanic-island to continental settings for the Killara volcanic rocks. It is suggested that a probable setting for the Killara Formation is an intracontinental rifting environment (i.e. continental flood basalt), in which fissures were largely responsible for the eruption of lavas (Fig. 3 ). 3.2. Brvah Basin Mafic and ultramafic igneous rocks of the Narracoota Formation are the most voluminous component, and probably constitute the floor of the Bryah Basin (Fig. 1). This conclusion is supported by field mapping, exploration drilling, structural, aeromagnetic and gravity data (Pirajno and Occhipinti, 1998). The thickness of the Narracoota Formation is difficult to estimate owing to intense deformation, but preliminary modelling of gravity data indicates a thickness of up to 6.5km [Fig. 4: see also Fig. 6(A) in Occhipinti et al. (1998, this volume)], although this may include structural repetition. Hynes and Gee (1986) estimated a thickness of ca 4 km. The Narracoota Formation is metamorphosed to greenschist facies and consists of pillowed metabasalt, mafic and ultramafic schist, and metabasaltic hyaloclastite. Where preserved, the main primary igneous minerals of the Narracoota extrusive and intrusive rocks are augite, labradorite and, in places, olivine. Locally, olivine-cumulates are present. Metamorphic minerals include epidote, chlorite, pumpellyite, albite and calcite. Maficultramafic schist include actinolite-tremolite schist and chlorite schist, with the distinction between mafic and ultramafic largely based on geochemistry (e.g. MgO and SiO2 contents). The metabasaltic hyaloclastites are albite normative (13 23 wt%),

£ Pirajno et aL / Precambrian Research 90 (1998) 119-140

127

Table 3 R e p r e s e n t a t i v e m a j o r a n d t r a c e e l e m e n t analyses o f volcanic rocks f r o m the N a r r a c o o t a a n d K i l l a r a f o r m a t i o n s Sample Killara Formation

Narracoota Formation

127417 127423 127462 SiO2 TiO2 A1203 Fe203 FeO MnO MgO CaO Na20 KzO P205 Total Mg# Cr Ni Co Sc V Cu Pb Zn Cd w Mo As Sb Pd Ag Pt Au Rb Ba Sr Ga Ta Nb Hf Zr Y Th U La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

53.67 0.71 14.58 1.47 8.82 0.17 6.35 11.89 1.65 0.63 0.06 100 52.73 233 100 63 49 269 146 4 92 0.31 0.62 0.41 0.62 3.21 13.4 0.2 14.5 5.2 28 189 159

2.9

2.6

71 18 1.24 0.1 4.77 12.05 2.69 9.99 2.81 1.4 3.45 0.64 3.41 0.86 2.31 11.41 2.38 0.59

139 25 4.22 0.41 17.79 35.33 5.42 21.25 5.03 2.16 5.92 0.98 5.6 1.14 3.62 0.56 3.1 0.73

97 18 3.34 0.1 11.9 22.92 3.08 11.42 2.56 1.29 3.33 0.6 3.47 0.71 2.45 t).38 2.28 0.54

Hyaloclastites

127463 127480 132788 132789 132790 133033 133050

54.73 53.58 52.13 54.99 1.2 0.85 0.79 1 13.99 14.39 14.06 13.9 1.55 2.08 3.93 2.03 11.01 9.66 7.17 10.87 0.19 0.2 0.16 0.22 3.96 5.37 5.4 4.11 10.39 I 1.36 16.22 9.88 2.11 2.12 0.04 2.06 0.75 0.31 0.03 0.85 0.12 0.07 0.08 0.09 100 99.99 100.01 100 36.26 45.33 47.33 36.57 139 101 103 84 82 65 82 37 61 59 58 62 40 43 40 43 328 265 239 306 94 112 94 172 8 3 3 10 116 92 92 107 0.41 0.63 0.54 0.63 2.06 3.86 1.78 /).41 0.11 0.63 2.88 3.19 4.9 5.12 12.5 10.8 0.3 0.2 0.1 0.3 2.1 13.6 11.9 3.1 10.8 17 11 1 26 396 172 35 415 200 240 61 204

3.4

Mafic schist

2.5

54.24 0.28 14.22 2.48 5.98 0.17 9.29 11.06 2.18 0.07 0.03 100 66.87 509 203 55 50 196 168 65

1.65

1 55 64 l0

2.5

71 126 15 24 2.38 5.64 0.22 0.31 7.14 16.61 1 3 . 4 3 47.11 2.61 4.88 1 0 . 0 8 17.64 2.45 3.85 1.74 1.67 3.33 4.65 0.52 0.8 2.97 4.7 0.83 1.04 2.11 3 0.39 0.48 2.21 2.81 0.52 0.66

49.45 1.13 11.72 3.45 8.24 0.19 12.43 11.07 2.12 0.08 0.13 100.01 66.15 1201 548 86 41 277 124 2 94

1.57

140 149 16

52.77 46.3 0.31 0.19 14.66 9.48 1.84 6.61 7.05 7.41 0.18 0.25 10.36 15.17 9.62 14.51 2.74 0.03 0.46 0.01 0.02 0.03 100.01 99.99 67.95 66.92 686 2574 241 1274 58 223 51 31 202 138 63 327 68

0.52

6 79 64 10

45

225.51

27 54 8

6.3 13 13

0.89 2.18 0.37 1.66 0.8 0.42 1.66 0.4 2.68 0.71 2.22 0.39 2.29

68 18 1.04 9.36 22.64 2.9 11.93 3.28 1.3 4.09 0.69 4.08 0.87 2.31 0.33 1.96

49.117 0.26 7.94 6.35 1.01 1.98 7.78 22.62 0.09 2.84 0.07 100.01 67.36 72 91 51 10 78 20 7 50

Ultramafic schist

112643 116485 104256 132791 139138 139139 51.87 51.39 50.11 49.23 47.23 47.89 0.73 0.63 0.53 0.19 0.16 0.17 13.8 15.12 15.03 10.43 7.65 8.67 2.24 2.96 2.78 1.46 3.96 2.01 8.48 6.62 5.29 8.34 5.48 7.08 11.2 0.16 0.14 0.18 0.12 0.15 10.36 8.3 9.36 20.63 28.77 26.78 9.74 11.44 15.14 8.6 6.41 7.04 2.16 3.211 1.45 0.92 0.18 0.19 I).36 0.10 0.11 0.02 0.03 0.01 0.07 0.06 0.05 0.01 0.01 0.01 100.01 99.98 99.99 100.01 100 100 63.77 61.42 68.16 79.2 85 84.3 489 364 283 1836 2530 3146 258 143 164 866 1531 1230 99 113 111 40 34 38 250 249 217 144 115 143 108 101 39 55 9 11 85

73

55

62

130.32

85 2846 100 9

51

45

8.32 4.15

5.11

1

I

111.4 6 341 163 12

127 82 12

2 92 188 13

74 23 8

I 37 23 6

102 7 6

51 17

47 14

38 11

7 8

7 6

8 7

0,93 1.53 0.2 1.02 0~65 0.29 1.29 0.29 2.14 0.53 1.62 0.27 1.66

0.17 /).42 0.09 0.47 0.3 0.11 0.65 0.14 1.08 1/.27 0.83 0.14 0.86 0.13

0.56 0.82 0.19 0.79 0.34 0.11 0.71 0.16 1.16 0.3 0.91 0.15 0.9 0.14

5.2 15 12

0.43 1.29 0.2 1.08 0.65 0.32 1.41 0.31 2.33 0.57 1.73 0.27 1.82

17 8 0.64 0.54 0.84 0.2 1.02 0.56 0.28 0.85 0.18 1.2 0.28 0.94 0.16 1.15 0.19

50 16 7.17 1.3 18.19 44.45 4.18 15.11 3.06 2.23 3.05 0.4 2.02 0.37 1.15 0.17 1.25 0.2

3.6 9 1.2 5.7 1.5 0.9 2.3 0.4 2.7 1/.5 1.8 0.2 1.5 0.2

3.5 8.3 1.1 5.1 1.4 0.7 1.7 0.4 2.4 0.5 1.5 11.2 1.4 0.2

3.5 8.3 1.3 5.7 1.5 0.8 2 0.4 2.4 0.5 1.6 0.2 1.5 0.2

M g # is defined as 1 0 0 ( M g O / M g O + F e O t o t ) . Blanks, e l e m e n t not d e t e r m i n e d . A n a l y s e s were p e r f o r m e d at the C h e m i s t r y C e n t r e o f the D e p a r t m e n t o f M i n e r a l a n d E n e r g y , by X - r a y fluorescence for m a j o r elements, following i n c o r p o r a t i o n o f s a m p l e into a b o r a t e glass disk, or a pressed p o w d e r ; Co, Cr, Cu, N k V, Z n were d e t e r m i n e d by I C P - A E S following a m i x e d acid solution; all o t h e r analytic c o n c e n t r a t i o n s (e.g. R E E ) were quantified using I C P - M S a f t e r a m i x e d acid solution. M a j o r e l e m e n t s in weight percent, trace e l e m e n t s in p a r t p e r million ( p p m ) , A u , Pt a n d Pd in p a r t s per billion ( p p b ) .

128

K Pira/no el al. ,' Precamhrian Research 90 ( 19982 119 140

Table 4 Selected comparative data between Narracoota and Killara Formations

Olivine normative Diopside normative Hypersthene normative Quartz normative Metamorphic minerals Magnetic susceptibility (range) Plagioclase composition ( La/'Yb)N

Ti02 wt% (range): mean

Ni/Cr

Mg# mean

Mean A 1203/Ti02

Mg0 wt% (range): mean

Pyroclastics (explosive volcanism) present

Narracoota Formation

Killara Formation

Yes Yes Yes

No Yes Yes Yes

Yes, except where olivine normative Yes: albite, epidote, actinolite, pumpellyite. calcite, chlorite, titanite High (38 4 3 0 0 x l 0 ' SI) Difficult to determine due to metamorphism. but probably labradoritc Hyaloclastite: 1.44 Malic schist: 1.702 Ultramalic: 0.37 Hyaloclastite: (0.09 4.24); 0.86 Marie schist: (0.09 1.97); 0.68 Ultramafic:(0.11 0.33): 0.19 Hyaloclastite: 0.43 Marie schist: 0.39 Ultramafic: 0.42 Hyaloclastite: 75.45 Mafic schist: 60.0 Ultramalic: 78.04 Hyaloclastite: 16.2 Malic schist: 20.5 Uhramafic: 46.68 Hyaloclastite: ( 1.20 10.70): 7.31 Matic schist: (1.88 14.48): 7.80 Uhramafic: (12.61 28.90): 21.19 Yes

generally aphyric and composed of acicular crystals of actinolite, epidote, minor carbonate, prehnite, quartz, titanite in a fine-grained groundmass of albite microlites, chlorite and epidote. Volcaniclastic rocks, locally with well-preserved eutaxitic textures, are not uncommon. In one area felsic schist is present and is interpreted to be derived from acid volcanic precursor rocks (Pirajno et al., 1995; Occhipinti et al., 1996; Windh, 1992). This felsic schist is associated with the volcanogenic Horseshoe Lights C u ~ u deposit (see below). On the basis of the Jensen's cationic plot [Fig. 5(a)] the pre-metamorphic precursors of the Narracoota Formation may have been high-Mg basalt, basaltic andesite, basaltic komatiite and peridotitic komatiite. They were emplaced mainly as subaqueous lavas, and subvolcanic sills and dykes. In one area, the remnants of a sheeted dyke

No: minor chlorite, albite, epidote, ilmenite L o w ( H ) 7 8 x l 0 ~SI) An% 59 (labradorite) 2.92

(0.31 1.91):0.86

0.67

50.35

15.5

0.87 8.91 (6.08)

No

system were recognized (Fig. 1) and described by Pirajno and Occhipinti ( 1998 ). The Trillbar mafic ultramafic layered complex in the western portion of the Bryah-Padbury terrane is interpreted as part of the Narracoota Formation and may represent a remnant of layer 3 of oceanic crust. Ultramafic schist is characterized by low REE abundances ( 1 < 10 times chondrite values), with nearly flat patterns and slight positive Eu anomalies. The metabasaltic hyaloclastite is generally weakly LREE enriched, with distinct positive Eu anomalies [Fig. 5(b)]. The primitive nature of these rocks is shown by the REE abundances, which do not exceed 20 times the chondritic values. The REE patterns for the mafic schists show two distinct trends: one with LREE enrichment, the other with a LREE depletion in the eastern and western Bryah Basin, respectively [Fig. 5(c)]. These trends are remarkably similar to those of

F Pirqjno et al.

Precambrian Research 90 (1998) 119 140

fill° 1oo

I

[

I

I

I

I

L

I

I

I I

¢O t-

10 E ffl

La

b,

I

Ce

I

Pr

I

Nd

I

I

I

I

I

I

I

Eu Tb Ho Tm Lu Sm Gd Dy Er Yb

FeO*

MgO

A1203

FMP153

29.10.97

Fig. 2. ta) Chondrite-normalized REE diagram of mafic rocks from the Killara Formation (49 samples). Normalizing factors after Sun (1982). (b) Discriminant triangular plot, after Pearce et al. ( 1977 ), showing the tectonic environment(s) of the Killara Formation mafic volcanic rocks. Tectonic fields as follows (after Pearce et al., 1977): ( 1 ) Spreading centre island; (2) orogenic; (3) ocean ridge and floor; (4) ocean island; (5) continental.

129

seamounts and mid-ocean ridge (MOR) in the Lau spreading centres and Iceland (Pearce et al., 1995; O'Nions et al., 1976). The LREE depleted trend of the volcanic rocks in the western areas of the Bryah Basin, is indicative of very refractory material and may reflect a deeper, subvolcanic component of the mafic schist rocks, such as harzburgite or peridotite. Compared to the Killara Formation, the Narracoota Formation rocks have lower TiO2 abundances (average 0.61 wt%), with the exception of the metabasaitic hyaloclastite which have similar TiO2 values (0.86 wt%), higher Mg# (average 63), Ni (average 256ppm), and Cr (647ppm) contents. Representative analyses of Narracoota Formation lithologies are given in Table 3, whereas selected comparative data between the two formations are presented in Table 4. Hynes and Gee (1986) recognized the MORBlike signature of the mafic volcanic rocks in the Narracoota Formation. They also noted the high magnesium content of these rocks and suggested a possible boninitic, subduction zone-related, origin. However, on the basis of field observations, they concluded that a rift setting for the deposition of these rocks was more appropriate. Sun (1997), on the other hand, on the basis of REE patterns and trace element data, such as high A1203/TiO2 (>40), low Ti/V (7.1-8.6) and Ti/Y (100-160), obtained from the datasets of Hynes and Gee (1986) and present authors, suggests that the volcanic rocks of the Narracoota Formation show characteristics of boninites derived from a refractory mantle source, and modified by subduction processes. Sun (1997) also draws on spinel geochemistry, Cr number (89-96), low Ti ( ~ 200 ppm) (W.R. Taylor, personal communication, 1997), as supporting evidence for boninites derived from refractory mantle sources. Sun (1997) concluded that the geochemistry and petrological data from both suites of volcanic rocks (Narracoota Formation in the Bryah Basin and Killara Formation in the Yerrida Basin) are consistent with a subduction-related model. Pirajno et al. (1995) and Pirajno and Occhipinti (1998) propose that the Narracoota Formation has a MORB to oceanic island geochemical signature [Fig. 5(d)]. They suggest that the Narracoota

130

F. Pit'q/m) el al. ,'Precamhrian Researc# 90 (199b;) 119 140

GOODIN

S

N

MARYMIA

\ +

I

I I

I

WlNDPLAIN SAG-BASIN

MARALOOUKILLARA DOMAIN

SUCCESSION

l

*

Juderina Formation

1

Maraloou Formation

[-~

Killara Formation

DOOLGUNNATHADUNA DOMAIN ~

Doolgunna and Thaduna Formations [

Volcanic "feeders"

FMP64

Fig. 3. Tectonic setting for the Yerrida Basin and the emplacement of the Killara Formation. After Pirajno (1996).

o

g

Modelled gravity ++++ Observed gravity

- -

-25

r-s0 - -75 m

U)

.-0

25o0

E

¢.- 5000 ..[:

~

" 7500 0 I

FMPI61

20 I

40 I

60 i

Kilometres

80 I

1O0 I

,~.,, 9,

Fig. 4. Gravity model in BRYAH, obtained assuming a background density of 2.67 gem 3. Modelling was carried out by Mr S. Shevchenko(GSWA). The section shows the depth and extent of the Narracoota Formation rocks modelled in terms of the observed gravity data. Gravity data ['rom AGSO database. rocks were emplaced in a rift that evolved to form narrow strips of oceanic crust and associated volcanic islands. Pirajno and Occhipinti (1998) also suggested that the hyaloclastite rocks in the east

were basaltic lavas erupted in shallow waters, possibly near rift margins (hence their similar TiOz abundances with the Killara volcanic rocks), whereas basaltic pillow lavas, dykes and sills (now mafic schist) and ultramafic-mafic rocks (e.g. Trillbar Complex) to the west were emplaced at spreading centres. The inferred palaeoenvironmental setting for the emplacement of the Narracoota Formation in the Bryah Padbury terrane is shown in Fig. 6. In summary, it is envisaged that rifting and development of oceanic crust propagated from west to east, as suggested by the fact that more ultramafic rocks crop out in the western parts of the Bryah Basin (see Section 4). The precise relationship and/or correlation between volcanic rocks of the Narracoota and Killara Formations remains uncertain. They may represent different manifestations ofmagmatism related to melting of refractory mantle, perhaps with a subduction component, in a setting dominated by oceanic (Narracoota) and continental (Killara) rifling.

4. Deformation and metamorphism Polyphase regional deformation and prograde greenschist facies metamorphism affected the

F. Pirajno et al. / Precambrian Research 90 (1998) I 19 140

131

b.

a~

100

Feo* + Tio 2

[ 1 [ 1 1 1 1 [ 1 1 1 1 [ 1 1

-E "ID tO t-

e~

10

E

I

MgO

AI203

La

I

I

I

Pr Ce Nd

I

[

I

I

I

I

I

I

I

I

Eu Tb Ho Tm Lu Sm Gd Dy Er Yb

d.

C. 100

I

I

I

]

I

I

I

I

I

I

I

I

I

I

Feo*

I

1o

1 .6

La

Ce

Pr

Nd

Eu Tb Ho Tm Lu Sm Gd Dy Er Yb

MgO

A1203

Fig. 5. (a) Jensen (1976) cationic plot showing range of compositions of Narracoota Formation rocks, dominantly from high-Mg tholeiite ( H MT), through high-Fe tholeiite ( H FT), to basaltic komatiite ( BK ) and peridotitic komatiite ( PK ). Chondrite-normalized REE diagram of metabasaltic hyaloclastite (four samples) (b) and mafic schist of the Narracoota Formation (17 samples) (c); normalizing factors after Sun (1982). (d) Discriminant triangular plot, after Pearce et al. (1977), showing the tectonic environment(s) of the Narracoota Formation (see caption of Fig. 3 for explanation of tectonic fields).

Bryah P a d b u r y terrane a n d the s u r r o u n d i n g A r c h a e a n rocks ( M a r y m i a Inlier, Peak Hill Schist and N a r r y e r Gneiss Terrane) (Occhipinti et al., 1998, this volume). M e t a m o r p h i c grades o f u p p e r greenschist facies are recorded in n a r r o w zones

along the contacts with the r e w o r k e d A r c h a e a n rocks. Provisional g e o t h e r m o m e t r i c and g e o b a r o metric studies provide m a x i m u m P ~ T estimates (over-estimates?) o f 500-620°C a n d 6.5 6.7 k b a r for the core o f the Peak Hill Schist ( T h o r n e t t , 1995).

132

F. Piraflm et al.

l~

Precanthrian Re.searuh 90 f 1996) 119 140

Clasticsediments

~!~]

Pillowlava

Granite-greenstoneTerrane

O I

Gabbroicrocks Dykecomplex

Asthenosphericmantle

O ~--~ Volcanicedifice

=
~

Hyaloclastitelavas

Fig. 6. Tectonic setting for the origin and emplacement of the Narracoota Formation. Alter Pirajno [ 1996).

Intensity of deformation and metamorphism in the Yerrida Group decreases rapidly away from the contacts with the Bryah-Padbury terrane and the Marymia Inlier (Fig. 1). South and southeast of these contacts [Goodin and Jenkin faults; Bagas (in press)], tight to isoclinal folds give way to open folds and little-deformed, fiat-lying successions that were not affected by regional metamorphism. In the Bryah Padbury terrane four deformation events have been recognized. Occhipinti et al. (1998, this volume) suggest that the first three (DI D3) may have occurred during the same progressive compression event (Table 5 ). Although overprinting relations between different structural styles can be recognized in parts of the Bryah Padbury terrane, in many cases regional scale folds and fold orientations are limited to specific domains (Occhipinti et al., 1998). Faults and shear zones that developed early in the deformation history of the terrane, were probably

Table 5 Del\~rmation history of the Capricorn Orogeny in the Bryah and Padbury Basins [note that D2 and D3 are regarded as largely contemporaneous and restricted to certain domains. After Occhipinti et al. ( 1998, this volume)] D4 N N E S S W COmln'ussion Small-scale folds, subvertical foliation: shear zones, faults, with quartz blows; all trending 280 31(1 . D3 E IV compre.~sion N S trending folds; subvertical foliation; subvertical faults or shear zones (thrust faults) localized east of Narryer Terrane: eastwards: increasingly disharmonic NN E-trending ['olds D2 N S compru.~sion Upright tight-isoclinal E W folds and subvertical foliation: E W shear zones, south-verging thrust faults D I N S contprus,sion Subhorizontal mylonites, thrusts and folds; meso-scale recumbent folds: tight-isoclinal, rootless

F. Pim/no et aL / Precambrian Research 90 (1998) 119 140

re-activated with continued north south compression. Northwest-trending mesoscopic chevron folds, kinks, shear zones and faults (D4), locally accompanied by a foliation, have overprinted D1-D3 structures in the western part of the Bryah Padbury terrane. The history of D 1 - D 4 events is summarized in Table 5, and the relationships between metamorphic mineral growths and deformation in the area around Peak Hill (Fig. 1) are summarized in Table 6.

5. Mineralization

The Bryah Padbury terrane and Yerrida Basin differ not only in structure and stratigraphy, but also in style and type of mineralization. Mesothermal gold-only lode mineralization is present in the Bryah-Padbury terrane, and is associated with the deformation and metamorphism of the region. The Yerrida Group is characterized by the presence of epigenetic base metal concentrations, and absence of mesothermal gold mineralization. The mineral deposits in the Bryah Padbury terrane and Peak Hill Schist are structurally controlled, mesothermal gold-only lodes. A volcanogenic massive-sulphide-type Cu Au-Ag deposit (Horseshoe Lights) is present in the northernmost part. Other metal deposits in the Bryah-Padbury terrane are supergene Mn and iron ore (Pirajno and Preston, 1998), but these are not considered in this paper. The lode-Au deposits occur in greenschist-facies metamorphic rocks, and are hosted in ductile to brittle high strain zones, in metasedimentary and/or metavolcanic rocks. Zones of hydrothermal alteration, characterized by pyritization and alkali metasomatism (albite and biotite) are associated with these deposits. Other common alteration minerals are: titanite; chlorite: tourmaline: and sericite. The mesothermal gold mineralization in the Bryah Padbury terrane is generally syn- to post-peak metamorphism. In places, the gold mineralization was re-mobilized, and possibly concentrated, during late-stage deformation events [D2 D3; Occhipinti et al. (1998)]. The nature of the hydrothermal solutions responsi-

133

ble for the lode deposits is poorly known. Limited fluid inclusion studies in the Labouchere Fortnum group of deposits in the northwest of the Bryah Padbury terrane, suggest that the mineralization was caused by the mixing of two fluids of different density and salinity, that is, deeply sourced, hot, saline, CO2-bearing fluids mixed with cooler, less saline, near-surface aqueous fluids (Dyer, 1991 ). Kerrich and Cassidy (1994) and Groves et al. (in press) consider that the hydrothermal solutions responsible for the emplacement of mesothermal lodes are generated during thermal processes in accretionary and collisional orogens. The latter authors have proposed the term synorogenic, rather than mesothermal, for this type of mineralization. The Horseshoe Lights Cu-Au Ag deposit is a massive sulphide deposit in which the geometry of the ore zones (massive sulphides and stringer zone), alteration patterns (silicification, sericitic and chloritic), predominantly felsic composition of the host rocks, and the metal association suggest that Horseshoe Lights was originally a Kurokostyle, volcanogenic massive sulphide deposit (Pirajno and Preston, 1998). Epigenetic lead carbonate and oxide deposits (cerusite PbCO3, p[attnerite, PbO 2 and anglesite PbSO4) occur within the Yerrida Basin. The resources of one prospect are estimated at ca 210 x 10~' t of ore at 1.8% Pb (Pirajno and Preston, 1998). This mineralization is hosted by both silicifled sandstone and silicified stromatolitic carbonate rocks of the Juderina Formation and in an outlier of the unconformably overlying Yelma Formation of the Earaheedy Group. A Pb Pb model age of the carbonate ore gave a value of 1650 Ma (Le Blanc Smith et al., 1995). The origin of this mineralization is not known, but it may be speculated that low-temperature basinal fluids were driven south-eastward by compressive events in the west, along the contact with the Bryah Padbury terrane, and north, along the Marymia Inlier. The lack of sulphides and the unique presence of oxide minerals would suggest that the deposit is the result of palaeoweathering processes, under physico-chemical conditions which were conducive to the oxidation and subse-

134

F. Pim/no eta/.

Precambrian Research 90 (1998) 119 140

Table 6 Relationship between diagnostic metamorphic minerals of rocks of the Bryah Group and deformation Formation name

Rock type

Mineralogy

Peak Hill Schist

Pelite

Quartz Biotite M uscovite Chlorite Albitc Tourmaline Quartz Spessartine Magnetite Quartz Epidote Chlorite Actinolite Titanite Magnetite Quartz Muscovite Andesine Opaques Quartz Actinolite Epidote Chlorite Sericite Arvfedsonite Titanite Calcite Albite Pumpellyite Quartz M uscovite Tourmaline Garnet Feldspar Quartz Biotite Albite Sericite Tourmaline Quartz Biotite Grunerite Spessartine Chlorite Quartz Stilpnomalenc Biotite

Chemical sediment

Calc-silicate

Psammite

Narracoota

Metabasite

Volcanic breccia Ravelstone

Pelitic tourmalinite

Subarkosic wacke

Horseshoe

Banded iron-formation

Robinson Range

Banded iron-tbrmation

Pre$1 MI

S1

Post -S1

$2 $3 M2

Posttectonic

F Pirajno et al. / Precambrian Research 90 (1998) 119 140

quent mobilization of Pb (Pirajno and Preston, 1998 ). Other metal occurrences in the Yerrida Basin are present in carbonaceous argillite of the Maraloou Formation, within which a number of gossans contain anomalous abundances of Pd, Cu, Zn and Ba (Pirajno and Occhipinti, 1998), and the cherts of the Bartle Member (Killara Formation), which contain anomalous abundances of Au and Ba (Pirajno and Grey, 1997). A conceptual model that attempts to explain the origin of the epigenetic precious and base metal deposits in the Bryah Padbury terrane and Yerrida Basin is shown in Fig. 7. 5.1.

Tectonic evolution

Gee (1979), Hynes and Gee (1986), Windh (1992) and Gee and Grey (1993) interpreted the geodynamic evolution of the Glengarry Basin, as originally defined by them, in terms of an ensialic or intracontinental basin. Tyler and Thorne (1990), Myers (1993) and Martin (1994) proposed models in which the Glengarry Basin could have formed in a back-arc setting, during the convergence of the Pilbara and Yilgarn cratons. The development of the Bryah, Padbury and Yerrida Basins is linked with the geodynamic NW

SE

AreaofAu deposition

~-

-..{

YERRIDA BASIN

BRYAH-PADBURY TERRANE

/~~g'-. C

o

l

I-

"

~.::::'

"

*

Y*.*.+.*

-'~=

a~t~o,7ooN~

~ B r y a h and Padbury groups

r

"

°

'

BASEMENT

Bastn ~te e p fluids rletic

-I

~Mooloogool v.'.'.'.l

Subgroup

135

evolution of the Capricorn Orogen, which is interpreted to have resulted from the convergence of the Pilbara and Yilgarn cratons between 2000 and 1800 Ma (Myers, 1993). The closing of the ocean between the Pilbara and Yilgarn cratons was followed by the oblique collision between the rifted passive margin on the Pilbara side and an inferred active magmatic arc on the Yilgarn side. Myers (1993) speculated that a southward oceanic subduction system with a south-facing Andean-type magmatic arc, had developed off the northern passive margin of the Yilgarn Craton. Following collision, the southern side of the Pilbara was sliced up by major thrusts, whereas most of the tectonic transport of the inferred magmatic and oceanic crust rocks was towards the south (Myers, 1993). Remnants of arc magmatic suites have not yet been found, but may be buried under the Meso Proterozoic Bangemall Basin. In the light of the re-interpretation of the former Glengarry Basin, some modification of the above tectonic scheme is necessary. Lack of sufficient geochronological data poses the problem of the precise timing of events. This lack of information must be taken into account when modelling basin tectonics. Therefore, in formulating models of the geodynamic evolution of the basins studied, at least two scenarios must be considered: (1) the basins were formed during convergence and subsequent collision in a back-arc-foreland basin setting: they were opened and infilled during southward subduction of oceanic crust (extensional back arc) and subsequently overlain by sediments in a newly developed foreland basin (syn-collisional); or (2) the basins were formed at the time of the oblique collision between the Pilbara and Yilgarn cratons, as pull-apart structures in a strike-slip setting with transitions from extensional (transtension) to compressional (transpression) regimes.

Windplain

Subgroup

Direction of fluid movement

Fig. 7. Schematic illustrations showing a conceptual model for the origin of precious and base metal epigeneticdeposits in the Bryah, Padbury and Yerrida Basins. After Pirajno and Preston (1998).

5.2. B a c k arc basin-foreland basin m o d e l

Pirajno (1996) proposed that the development of the Yerrida and Bryah Basins may be related to the processes of back-arc opening during sub-

136

ki Pirafl*o el aL ' Precambrian Research 90 ~ 199~') 119 14(1

N

S

PILBARA CRATON

YILGARN CRATON

MICROCONTINENT AND VOLCANICARC

: -" 22 " -

-~:

BACKARC RIFTS

r

;-~ >

~

i

//

7 -- - Z.-...>, •

~

Continental crust

~]]

Oceanic crust

~

Magma

....

.,: ..

.

Ocean

Sediments and evaporites p ~,~ Volcanics Fig. 8 . S c h e m a t i c illustration s h o w i n g tectonics based on Tyler and T h o r n e (

a model of back-arc rifting to explain the origin of the Yerrida and Bryah basins. Regional 1990), Myers et al. (1996). After Pirajno (1996).

duction of oceanic crust beneath the northern margin of the Yilgarn Craton l¥om ca 2000 Ma onwards. This model is illustrated in Fig. 8. At ca 1850 Ma, closure of the intervening ocean and the Pilbara Yilgarn oblique collision resulted in reversion of the back-arc rifts. Rift propagation could have developed, progressively from west to easL through a sequence of crustal thinning, rupture and graben formation. Initial thinning of continental crust, caused by these first stages of back-arc rifting, resulted in a sag basin phase. The presence of a sag basin is inferred by the lack of evidence of detachment faults and of rifting in these early stages. Continued extension and rifting resulted in the development of horst and graben structures. The Archaean Goodin |niter, covered by shallow-water sediments, is interpreted as a horst structure, and therefore a topographic high, from which elastic sedimentation in surrounding areas was sourced. During this phase, continental tholeiitic lavas were erupted along east west trending fissures ( Killara Formation), and the sag basin, which had developed on continental crust, evolved into a rift basin,

into which the sediments of the Mooloogool Subgroup accumulated. Further west, rift lbrmafion became advanced enough to result in limited ocean floor spreading, with concomitant development of oceanic islands (Narracoota Formation, Bryah Group). The Padbury Basin is interpreted by Martin (1994) as et retro-arc foreland basin, which records the collision of the Yilgarn and Pilbara cratons. Following collision, back-arc volcanism ceased and et foreland basin developed on top of the back-arc succession (Bryah Group). This basin was filled by siliciclastic rocks only (Padbury Group). During continued regional compression, the Bryah Padbury terrane developed into a fold-andthrust belt, and was partly thrust over the Yerrida Basin along the Goodin Fault. The mesothermal Au-only lodes were probably formed at this time. 5.3.

Strike-slip Imll-aparr hash7 mode/

Pull-apart basins are associated with strike-slip displacements along overstepping en-echelon faults, or faults with 'releasing bends', leading to

F Pim/no et al. / Precarnbrian Research 90 (1998) 119 140

local extension (Rodgers, 1980; Allen and Allen, 1990). In a pull-apart model, two first-order basin structures, the Bryah and Yerrida Basins, would have formed at the same time during the progressive evolution of a pull-apart structure, resulting from an inferred sinistral strike-slip system (Gee and Grey, 1993). In the west, a proto-oceanic basin (Bryah Group) developed, whereas to the east thinning of continental crust occurred without break-up (Yerrida Group). The Goodin Inlier became a basement horst within the actively extending Yerrida Basin (Fig. 1 ), thus providing a source for clastic sedimentation in surrounding depocentres. A change in regional stress regime led to closure and inversion of the basins, and the development of a foreland basin (Padbury Group) in the northwest. Continuing compression (and accompanying dextral strike-slip), resulted in the deformation and metamorphism of the Bryah-Padbury terrane as it was thrust over the Yerrida Basin along the Goodin Fault.

6. Discussion and conclusions

The Bryah, Padbury and Yerrida Groups represent deposition (ca 2.0 1.8 Ga) into three distinct Palaeoproterozoic basins, defined on the basis of characteristic stratigraphic, structural, metamorphic and sedimentological features. These basins were developed along the northern margin of the Yilgarn Craton. Previous workers regarded the basin fill successions as two Groups, Glengarry and Padbury, deposited into a single basin, the Glengarry Basin (Gee, 1987; Gee and Grey, 1993). The Yerrida Basin contains voluminous tholeiitic lavas and sills characterized by lower Mg# and LREE-enriched patterns. These rocks are interpreted to be of continental flood basalt affinity, and are interleaved with turbiditic sedimentary rocks. This continental rift succession (Mooloogool Subgroup) overlies an epicontinental succession (Windplain Subgroup), which unconformably overlies the Archaean basement. By contrast the Bryah Basin is interpreted as a proto-oceanic basin, dominated by tholeiitic and ultramafic rocks characterized by high Mg#, low REE abundances with flat to LREE-depleted

137

chondrite-normalized patterns. The regional setting and the geochemical features suggest that rifting of continental crust led to limited ocean floor spreading. The Padbury Group is interpreted a foreland basin succession that developed over the Bryah Basin. This group contains mainly siliciclastic turbiditic sedimentary rocks. The geodynamic evolution of the Bryah, Padbury and Yerrida Basins is linked to the tectonic evolution of the Capricorn Orogen, which records the convergence and collision of the Archaean Pilbara and Yilgarn Cratons. The favoured model considers that the Bryah and Yerrida Basins were formed as back-arc rifts during convergence. The Padbury Basin formed in a retro-arc foreland basin during initial collision. Continued compression led to formation of the Bryah-Padbury fold-and-thrust belt, and transport of this belt over the Yerrida Basin, along the Goodin Fault. Alternatively the basins were formed as pull-apart structures in a transpressional regime during oblique collision between the Pilbara and Yilgarn cratons. Both tectonic models can explain the origin of mineral deposits in the Bryah, Padbury and Yerrida Basins. Mesothermal, syn-orogenic, Au-only lode deposits occur in the deformed Bryah-Padbury terrane, whereas the little deformed Yerrida Basin contains epigenetic base metal deposits, possibly linked to expulsion of low-temperature basinal fluids. In this work, in addition to field evidence, basalt geochemistry and discriminant diagrams were used in an attempt to constrain and understand the tectonic setting and evolution of the Bryah, Padbury and Yerrida Basins. It is recognized that the use of discriminant diagrams on a stand-alone basis is unsatisfactory [e.g. Duncan (1987); Wang and Glover (1992)] due to the complexities of tectonic processes and of source regions. The origin and setting of the Narracoota and Killara volcanic rocks remains somewhat uncertain, due to the ambiguity of the geochemical signals which indicate that they could be interpreted as one or more of the following: ( 1 ) boninite and subduction-related, (2) MOR:

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I~ Pira/no et al. /Precamhrian Research 90 (1998) 119 140

(3) oceanic island: and (4) continental (within plate). On the basis of field relations (e.g. sedimentary rock associations, such as undeformed and umetamorphosed evaporite and epicontinental facies rocks), we are confident that the Killara Formation tholeiites cannot be related to subduction processes, but were erupted in a mature intracontinental environment. In the case of the Narracoota Formation, the situation is more difficult to unravel, owing to the effects of deformation and metamorphism. Nevertheless, the following points must be borne in mind: (1) the metamorphism of the Narracoota Formation, strongly suggest interaction with sea water and is very similar to hydrothermal metamorphism typical of MOR settings (Pirajno and Occhipinti, 1998); (2) the presence of a remnant sheeted dyke complex (Fig. 1 ) and the layered mafic ultramafic Trillbar Complex: (3) high-Mg basaltic rocks: (4) common presence of volcaniclastics (Pirajno and Occhipinti, 1998): (5) absence of intermediate and felsic products such as andesites and dacites; (6) boninite geochemistry. With respect to the last point, the boninitic characteristics of the Narracoota Formation are comparable to the boninites of the Koh ophiolite in New Caledonia, studied by Mefre et al. (1996). These authors concluded that the Koh boninites were formed during rifting of oceanic crust in a back arc spreading centre. For the above reasons, we favour a spreadingcentre tectonic setting for the Narracoota volcanic rocks, created during advanced rifting of continental crust, perhaps in the same way as suggested by Breitkopf and Maiden (1987) for the Matchless Amphibolite Belt of the Damara Orogen (Namibia). However, the authors concur with Sun (1997) that the boninitic component in the chemistry of the Narracoota rocks needs explanation. An alternative, albeit speculative, possibility is that the Narracoota and Killara volcanism may be related to a mantle plume, contaminated by subduction material, that impinged onto or near the

northern margin of the Yilgarn Craton. The high MgO and low TiO 2 contents and depleted REE of the Narracoota and Killara Formations, notwithstanding the tectonic setting, support this view [e.g. Campbell and Griffiths ( 1992)]. Finally, the implications of the models presented in this paper, within the framework of the Capricorn Orogen, hinge on the question of whether the tectonic units of the Orogen were formed in oceanic-subduction settings or whether in an ensialic setting [see Tyler et al. (in press)]. Evidence in the Bryah Padbury and Yerrida Basins region suggests that along the northern margin of the Yilgarn Craton, the Proterozoic tectonic processes were largely ensialic (e.g. rifting of continental crust). In places, this rifting advanced enough to form narrow strips of oceanic crust (Bryah), whereas in other places rifting failed to produce oceanic crust and tholeiites were erupted on continental crust (Yerrida) (see Fig. 3 ). The authors' field work in this area of the Capricorn Orogen is continuing and as further data become available (e.g. geochronological and geochemical) they hope to gain further insights into the geological history of this fascinating region of Western Australia.

Acknowledgment The authors acknowledge the contribution of colleagues, who have participated in the mapping of the former Glengarry Basin: N.G. Adamides, L. Bagas, P. Dawes, D. Ferdinando and G. Le Blanc Smith. They thank John Myers, lan Tyler, David Nelson (GSWA) and Shen-su Sun (AGSO) for stimulating discussions. They are grateful to Peter Carroll and his staff for preparing the figures. The constructive criticism of Dr R. Shaw of AGSO and an anonymous reviewer, is gratefully acknowledged. This paper is published with the permission of the Director of the Geological Survey of Western Australia, Dr P. Guj.

References Allen. P.A., Allen. J.R., 1990. Basin Analysis Principlesand Applications. Oxtbrd ScientificPublications, Oxford.

1~ Pirajno et al. / Precambrian Research 90 (1998) 119-140

Bagas, L.B., in press. Geology of Marymia 1:100000 sheet, W.A. Western Australia Geological Survey, Explanatory Notes. Breitkopf, J.H., Maiden, K.J., 1987. Geochemical pattersn of metabasites in the southern part of the Damara Orogen, SWA/Namibia: applicability to the recognition of tectonic environment. Geo[gical Society Special Publication, No. 33, 33. Geolgical Society, pp. 355 361. Bunting, J.A., Commander, D.P., Gee R.D., 1977. Preliminary synthesis of Lower Proterozoic stratigraphy and structure adjacent to the northern margin of the Yilgarn Block. Western Australia Geological Survey Annual Report 1976. Western Australia Geological Survey, pp. 87 92. Campbell, 1.H., Griffiths, R.W., 1992. The changing nature of mantle hot spots through time: implications for the chemical evolution of the mantle. J. Geol. 92, 497 523. Duncan, A.R., 1987. The Karoo igneous province--a problem area for inferring tectonic setting from basalt geochemistry. J. Volcanol. Geotherm. Res. 32, 13 34. Dyer, F.L., 1991. The nature and origin of gold mineralisation at the Fortnum, Nathan's and Labouchere deposits, Glengarry Basin, Western Australia. Unpublished BSc Honours Thesis, University of Western Australia, Australia. Gee, R.D., 1979. The geology of the Peak Hill area. Western Australia Geological Survey Annual Report 1978. Western Australia Geological Survey, Australia, pp. 55--62. Gee, R.D., 1987. Peak Hill, W.A. Western Australia Geological Survey, 1:250 000 Geological Series Explanatory Notes, 2nd edn, Western Australia Geological Survey, Australia. Gee, R.D., Grey, K., 1993. Proterozoic rocks on the Glengarry 1:250 000 sheet: stratigraphy, structure, and stromatolite biostratigraphy. Western Australia Geological Survey Report No. 41. Groves, D.I., Goldfarb, R.J., Gebre-Mariam, M., Hagemann, S.G., Robert, F., in press, Orogenic gold deposits: a proposed classification in the context of their crustal distribution and relationship to other gold deposits. Ore Geol. Rev. Hall, W.D.M., Goode, A.D.T., 1978. The early Proterozoic Nabberu Basin and associated iron formations of Western Australia. Precambrian Res. 7, 129 184. Hynes, A., Gee, R.D., 1986. Geological setting and petrochemistry of the Narracoota Volcanics, Capricorn Orogen, Western Australia:. Precambrian Res. 31, 107 132. Jensen, L.S., 1976. A new cation plot for classifying subalkalic volcanic rocks. Ontario Division of Mines, MP 66. Ontario Division of Mines, Ontario, Canada. Kerrich, R., Cassidy, K.F., 1994. Temporal relationships of lode gold mineralization to accretion, magmatism, metamorphism and deformation Archaean to present: a review. Ore Geol. Rev. 9,263 310. Le Blanc Smith, G., Pirajno, F., Nelson, D., Grey, K. 1995. Base metal deposits in the Early Proterozoic Glengarry terrane, Western Australia. Western Australia Geological Survey, Annual Review 1993 94. Western Australia Geological Survey, Western Australia, pp. 59 62. Martin, D.M., 1994. Sedimentology, sequence stratigraphy, and tectonic setting of a Palaeoproterozoic turbidite complex,

139

Lower Padbury Group, Western Australia. Ph.D. thesis, University of Western Australia, Western Australia. Martin, D.M., in press. Lithostratigraphy and Structure of the Palaeoproterozoic Padbury Group, Milgun 1:100 000 sheet, Western Australia. Western Australia Geological Survey, Report. Western Australia Geological Survey, Western Australia. Mefre, S., Aitchison, J.C., Crawford, A.J., 1996. Geochemical evolution and tectonic significance of boninites and tholeiites from the Koh ophiolite, New Caledonia. Tectonics 15, 67-83. Myers, J.S., 1990. Capricorn Orogen. In: Geology and Mineral Resources of Western Australia. Western Australia Geological Survey Memoir No. 3. Western Australia Geological Survey, Western Australia, pp. 197-198. Myers, J.S., 1993. Precambrian history of the West Australian Craton and adjacent orogens. Annu. Rev. Earth Planet. Sci. 21,453-485. Myers, J.S., Shaw, R.D., Tyler, I.M., 1996. Tectonic evolution of Proterozoic Australia. Tectonics 15, 1431-1446. Nelson, D.R., 1997. Compilation of SHRIMP U-Pb zircon geochronology data, 1996. Western Australia Geological Survey Record 1997/2. Occhipinti, S.A., Swager, C.P., Pirajno, F., 1996. Structural and stratigraphic relationships of the Padbury Group, Western Australia--implications for tectonic history. Western Australia Geological Survey, Annual Review 1995-96. Western Australia Geological Survey, Western Australia, pp. 88 95. Occhipinti, S.A., Myers, J.S., Swager, C.P., 1997b. Padbury, W. A. Sheet 2546, Western Australia Geological Survey, 1:100 000 Geological Series. Western Australia Geological Survey, Western Australia. Occhipinti, S.A., Grey, K., Pirajno, F., Adamides, N.G., Bagas, L., Dawes, P., Le Blanc Smith, G., 1997b. Stratigraphic revision of Palaeoproterozoic rocks of the former Glengarry Basin. Western Australia Geolgical Survey Record 1997/3. Western Australia Geolgical Survey, Western Australia. Occhipinti, S.A., Swager, C.P., Pirajno, F., 1998. Structural metamorphic evolution of the Palaeoproterozoic Bryah and Padbury Groups during the Capricorn orogeny, Western Australia. Precambrian Res. 91, 141-158. O'Nions, R.K., Pankhurst, R.J., Gronvold, K., 1976. Nature and development of basalt magma sources beneath Iceland and the Reykjanes ridge. J. Petrol. 17, 315-338. Pearce, J.A., Parkinson, I.J., 1993. Trace element models for mantle melting: application to volcanic arc petrogenesis. In Prichard, H.M., Alabaster, T.H., Harris, N.B.W. (Eds.), Magmatic Processes and Plate Tectonics, Geological Society Special Publication No. 76. Geological Society, London, pp. 373 403. Pearce, J.A., Ernewein, M., Bloomer, S.H,, Parson, L.M., Murton, B.J., Johnson, L.E., 1995. Geochemistry of Lau Basin volcanic rocks: influence of ridge segmentation and arc proximity. Geological Society Special Publication, No. 81. Geological Society, pp. 53-75. Pearce, T.H., Gorman, B.E., Birkett, T.C., 1977. The relationship between major element chemistry and tectonic environ-

140

F Pirajno el al.

Precamhrian Research 90 (1998) 119 140

ment of basic and intermediate volcanic rocks. Earth Planet. Sci. Lett. 36, 121 132. Pirajno, F., 1996. Models for the geodynamic evolution of the Palaeoproterozoic Glengarry Basin, Western Australia. Western Australia Geological Survey, Annual Review 1995 96. Western Australia Geological Survey, Western Australia, pp. 96 103. Pirajno, E., Grey, K., 1997. A Palaeoproterozoic hot spring environment for the Bartle Member cherts of the Yerrida Basin, Western Australia. Western Australia Geological Survey, Annual Review 1996 97. Western Australia Geological Survey, Western Australia. Pirajno, E., Occhipinti, S.A., 1998. Geology of Bryah I : 100 0fl() sheet, W.A. Western Australia Geological Survey, Explanatory Notes. Western Australia Geological Survey, Western Australia. Pirajno, F., Preston, W.A.. 1998. Mineral deposits of the Padbury, Bryah and Yerrida basins. In: Geology ot" Australian and Papuan New Guinean Mineral Deposits. Australasian Institute of Mining and Metallurgy, Monograph 22. Australasian Institute of Mining and Metallurgy, Australia, pp. 63 69. Pirajno, F., Adamides, N., Occhipinti, S., Swager, C.P.. Bagas, L. 1995. Geology and tectonic evolution of the Early Proterozoic Glengarry Basin, Western Australia. Western Australia Geological Survey, Annual Review 1994 95. Western Australia Geological Survey, Western Australia, pp. 71 80. Pirajno, F., Bagas, L., Swager, C.P., Occhipinti, S.A.. Adamides, N.G., 1996. A reappraisal of the stratigraphy of the Glengarry Basin, Western Australia: Western Australia Geological Survey, Annual Review 1995 96. Western Australia Geological Survey, Western Australia, pp. 81 87.

Rodgers, D.A., 1980. Analysis of pull-apart basin development produced by en echelon strike-slip faults. International Association of Sedimentologists, Special Publication No. 4. International Association of Sedimentologists, pp. 27 41. Schilling, J-G., Meyer, P.S., Kingsley, R,H.. 1982. Evolution of the Iceland hotspot. Nature 296. 313-320. Sun, S.-S., 1982. Chemical composition and origin of the Earth's primitive mantle. Geochim. Cosmochim. Acta 46. 179 192. Sun, S-S., 1997. Chemical and isotopic features of Palaeoproterozoic marie igneous rocks of Australia: implications for tectonic processes. AGSO Record. 1997'4. pp. 119 122. Thornett, S.E., 1995. The nature, origin and timing of gold meneralisation in Proterozoic rocks of the Peak Hill District. M.Sc. thesis, W.A. University of Western Australia, Perth, Australia. Tyler, I.M., Thorne, A.M., I990. The northern margin of the Capricorn Orogen, Western Australia an example of an Early Proterozoic collision zone. J. Struct. Geol. 12, 685 701. Tyler, I.M.. Pirajno, F., Bagas, L., Myers, J.S., in press. Geology and mineral deposits of the Proterozoic in Western Australia. AGSO J. Geol. Geophys. 17. Wang, P., Glover, L., 1992. A tectonic test of the most commonly used geochemical discriminant diagrams and patterns. Earth Sci. Rev. 33, 111-131. Windh, J.. 1992. Tectonic evolution and metallogenesis of the Early Proterozoic Glengarry Basin, Western Australia. Ph.D. thesis, University of Western Australia, Western Australia. Woodhead, J.D.. Hergt, J.M., 1997. Application of the double spike technique to Pb-isotope geochronology. Chem. Geol. (IsotopeGeosci.) 138, 311 321.