Geological setting and petrochemistry of the Narracoota Volcanics, Capricorn Orogen, Western Australia

Precambrian Research, 31 (1986) 107- 132 107 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

GEOLOGICAL SETTING AND PETROCHEMISTRY OF THE NARRACOOTA VOLCANICS, CAPRICORN OROGEN, WESTERN AUSTRALIA

A. HYNES

Department of Geological Sciences, McGill University, Montreal H3A 2A 7 (Canada)

R.D. GEE

Geological Survey of Western Australia, Mineral Ho., 66 Adelaide Terrace, Perth, Western Australia 6000 (Australia)

(Received and accepted September 10, 1985)

ABSTRACT

Hynes, A. and Gee, R.D., 1986. Geological setting and petrochemistry of the Narracoota Volcanics, Capricorn Orogen, Western Australia. Precambrian Res., 31 : 107--132.

The Narracoota Volcanics, part of the early Proterozoic Glengarry Group on the northern margin of the Yilgarn Block, were deformed and metamorphosed between 1.7 and 1.8 Ga ago and then covered by sediments of the Padbury Group (1.6--1.7 Ga). Although most of the Glengarry Group is in-place on granitoid basement, pre-Padbury Group deformation produced large-scale south-vergent recumbent folds in parts of the region, so that much of the Glengarry Group is at least parautochthonous. It was probably deposited on substantially thinned continental crust.

The Narracoota Volcanics in the southern, autochthonous part of the region are dominated by thick (4 kin), chemically uniform successions of tholeiites, differing from MORB only in their slightly low Ti/Fe and Zr/Fe. Further north, autochthonous and parautochthonous Narracoota Volcanics are dominated by ultramafic schists of probably pyroclastic origin. Mafic volcanic rocks associated with these schists range up to 58% SiO2 and are very poor in Ti and Zr. They resemble some boninites of the western Pacific.

The southern volcanic rocks are interpreted to reflect open-system fractionation of mantle-derived magmas in large, subcrustal magma chambers, similar to those postulated at oceanic ridges. The high Si-values and low Ti- and Zr-values in more northerly mafic rocks may reflect hydrous melting and second-stage melting, respectively.

The geochemical similarities of some of the Narracoota Volcanics to boninites suggest an arc(subduction-related)-origin for these rocks, but there is little evidence from the regional geological setting to support this. Origin in a continental-rifting environment in which the sub-continental mantle was locally H20-rich is the favoured interpretation. The thick sequences of MORB-like volcanic rocks, their geochemical differences from continental tholeiites and the thin-skinned deformation suggest that continental rifting was of major dimensions in the region.

0301/9268/86/$03.50 © 1986 Elsevier Science Publishers B.V.

108

INTRODUCTION

Tectonic processes in the early Proterozoic remain controversial. Although there is mounting evidence that some foldbelts have characteristics compatible with plate-tectonics {e.g., Hoffman, 1980; Lewry et al., 1981; Hynes and Francis, 1982; Hoffman and Bowring, 1984), the geological evidence in many is less compelling, and it has been used, together with palaeomagnetic data in some cases, to suggest that the foldbelts were produced within cratons, rather than in association with the opening and closing of major ocean-basins (e.g., Glikson, 1980; Dimroth, 1981; KrSner, 1981).

The early Proterozoic Capricorn (or Ophthalmian) Orogen of Western Australia, separating the Archaean Yilgarn and Pilbara cratons (Gee, 1979a), has been interpreted as ensialic on the basis of a lack of evidence for subduction and the similarities of sedimentation history on its two sides (Gee, 1979a). This interpretation is consistent with the inferred contiguous relationship of the Yilgarn and Pilbara blocks in late Archaean time (Horwitz and Smith, 1978) and with available palaeomagnetic data (McElhinny and Embleton, 1976; Embleton, 1978; Dunlop, 1981) although the latter data admit of another interpretation (McWilliams, 1981). Even if the foldbelt is in the broadest sense 'ensialic' (reflecting thinning and only limited rifting of continental crust), and the data m u s t be considered equivocal at this stage, there is no evidence bearing on the degree of continental thinning or crustal separation that was experienced in the region.

One of the more profitable approaches to understanding tectonic evolution has been the study of volcanic successions. Although the characteristics of volcanic rocks cannot provide unique interpretations of tectonic environment, they may be used to constrain the physical conditions in the upper mantle and crust at the time of their formation. This, in turn, provides some constraints on tectonic environment. In this paper, the field and petrochemical aspects of the Narracoota Volcanics, on the southern edge of the Capricorn Orogen, are described, and used to infer some of the characteristics of the orogeny in which they were involved. The paper is based on the recent remapping of the Peak Hill Sheet (Gee, 1983), and some more detailed structural and petrochemical work by A. Hynes.

GEOLOGICAL SETTING AND AGE

The Narracoota Volcanics occur in the Glengarry Group, a thick sequence of clastic sediments and volcanic rocks in the Glengarry Sub-basin (Gee, 1979b). The Glengarry Sub-basin is at the western end of the Nabberu Basin, a complex of lower Proterozoic rocks flanking the northern margin of the Yilgarn Block. Stratigraphic relationships within the basin have been interpreted to reflect a facies change from 'shelf' conditions in the south to 'trough' conditions further north (Gee, 1979a,b).

The age of the Glengarry Group is bracketed by the 2.5 Ga age of the

109

Yilgarn Block, on which they rest unconformably, and the 1.7--1.8 Ga orogeny in the Gascoyne Province, which affected them (Gee, 1980). It is therefore lower Proterozoic. No more precise an age can be assigned at present. The Glengarry Group is overlain by rocks of the Padbury Group, probably 1.6--1.7 Ga old (Bunting et al., 1977), the contact being a regional unconformity (Gee, 1979b).

STRUCTURE AND REGIONAL GEOLOGY

Structures in the Glengarry Sub-basin are dominated by Archaean basement domes, between which the Padbury Group sediments are pinched into tight synclines (Gee, 1979b; Fig. 1). The underlying Glengarry Group has corresponding synformal structures, but in this paper large-scale over- turning of parts of the Glengarry Group, before deposition of the Padbury Group, is documented. This requires major horizontal transport of parts of the Glengarry Group and necessitates caution in interpretation of the original basin-geometry. In particular, it suggests that some rocks of the Glengarry Group are at least parautochthonous.

Rocks of the Glengarry Group are exposed over an area roughly 200 km X 90 km (Fig. 1). In the south, rocks of the Glengarry Group rest unconformably on Archaean granites. The stratigraphic sequence within the group here, in the neighbourhood of Mikhburra and Cashman, consists of a thin orthoquartzit ic sandstone (Juderina Sandstone) overlain by a thick, but highly variable, coarse-grained arkosic unit (Doolgunna Arkose), a mixed clastic--carbonate-chert unit (Karalundi Formation) and then by a thick mafic lava unit -- the Narracoota Volcanics. Units above the Narracoota Volcanics are buried under the extensive alluvial deposits of the Murchison River, although there are isolated exposures of greywacke. Further east along the southern boundary, both the Narracoota Volcanics and the Dool- gunna Arkose thin markedly, and are overlain by a thick greywacke unit (the Thaduna Greywacke, see Fig. 1). The basal sandstone may be traced almost continuously around a NNE trending synform onto the northern edge of the area, where Archaean basement is locally replaced by the Peak Hill Metamorphic Suite, of possible early Proterozoic age (Gee, 1979b). On this northern edge the arkosic unit is again moderately thick around the Marymia Dome and the Narracoota Volcanics are well developed, particularly in the vicinity of Quartz Bore.

The consistent stratigraphic relationships, and the spatial association of domes and thick arkosic units, suggest that the Glengarry Group is in-place on the northern and southern margins of the area, and that the domes were areas of positive relief even at the time of deposition of the group. Al- though it is conceivable, given the outcrop quality, that upper parts of the group, including the Narracoota Volcanics, are allochthonous, it is considered unlikely, especially since mafic sills occur in the lower parts of the succession in the east of the region. The Narracoota Volcanics of the southern

110

1 1 1

and northern boundaries are here referred to as the southern and northern ' au tochthonous ' successions, respectively.

The northern autochthonous succession is overlain by sediments of the Padbury Group, which are deformed into a tight, doubly plunging syn- clinorium, the Robinson Syncline (Fig. 1). These folds are evidenced further west in the Glengarry Group, off the western end of the Marymia Dome, by the outcrop pattern of the Thaduna/Narracoota contact.

Because of poor outcrop quality and cleavage intensity in the Thaduna Greywacke the most useful way in which to moni tor pre-Padbury Group deformation is with minor-fold facing directions (the 'facing' direction of a fold is used here in the restricted sense of Shackleton (1958) as the direction toward which beds become progressively younger along the hinge- surface). The axes of post-Padbury minor folds, measured in the Thaduna Greywacke at the western end of the Marymia Dome, occupy much of the axial surface, and face consistently to the NW, regardless of their plunge (Fig. 2a). Fold facing direction does not change as the fold plunge swings from NW to SE, i.e., the change in plunge is accompanied by a change from upward- to downward-facing folds. There are two possible simple explanations for this: (1) there was a pre-Padbury (F1) set of folds, producing some overturned strata in the region. Post-Padbury folding has then produced doubly-plunging folds. Fortuitously, all NW-plunging folds were measured in upward-facing limbs of F1 folds, and all SE-plunging folds were measured in overturned limbs. (2) There were no overturned strata in the region before folding of the Padbury Group. The variable fold-plunge was produced by

(a) N (b) N

Fig. 2. Fold axes (open circles, wi th ticks in the facing direct ion) and poles to axial surfaces (filled circles) in the Thaduna Greywacke. (a) At the western end of the Marymia Dome. (b) In the Narracoota Syncline.

Fig. 1. Geological map of the Peak Hill district; simplified after Gee (1983). Locat ions of major sampling traverses are shown as solid straight lines. The Rel ief line and Dimble Well are west o f the area shown, in the Robinson Range area (see Gee, 1979b).

112

rotat ion of the folds as they slid off the rising dome to the NW, producing major NW-vergent parasitic structures. Such structures would rotate NW- plunging, NW-facing minor folds through the vertical to produce SE-plunging, but still NW-facing, minor folds. The second explanation is favoured, because it is consistent with the domal emplacement mechanism for the gneisses and does not require the fortui tous collection relationships. However, since there is evidence for major pre-Padbury overturned structures further south (see below) the first explanation cannot be excluded.

To the south of the Robinson Syncline there is a broad, exposed area of Narracoota Volcanics and associated greywackes, deformed into a large, westward closing fold (Fig. 1). Structural evidence from this region is critical to the interpretation presented here. Minor folds with the main cleavage as their axial surface (Fig. 2b) again exhibit a broad range of plunges on the axial surface here, but, with one exception, the folds are consistent with the major westward closing fold being an east-plunging syncline (the 'Nar- racoota Syncline'). Support for this interpretation comes from approximate facing directions preserved in pillowed lavas in the nose of the fold, and in silicified pillow lavas on the two limbs. The northern limb of the syncline is then southward-facing, and is overlain by the northward-facing limb of the Robinson Syncline. Barring juxtaposi t ion of these two oppositely- facing successions due to major displacement on the fault that extends along the south-side of the Robinson Syncline, the Narracoota Volcanics here were upside down when the Padbury Group was deposited, and the Narracoota Syncline is pre-Padbury. Direct evidence for a pre-Padbury age of the Narracoota Syncline is the presence of two cleavages at several places in the Thaduna Greywacke, on its western edge. Here, a strong NW-striking cleavage, parallel to the axial surface of the Narracoota Syncline, is cut by a weaker WNW-striking cleavage, that is more compatible in att i tude with major folds in the Padbury Group.

Unfolding of the Robinson Syncline leaves the Narracoota Syncline a major recumbent fold-closure with a roughly southerly vergence. The succession of the Narracoota Syncline may, therefore, have been transported into the region from north of the Marymia Dome before the Padbury Group was deposited. Alternatively, it may be rooted south of the Marymia Dome, in which case, if it was deposited on sialic basement, development of the Narracoota Syncline involved decollement at or above the basement-cover boundary and substantial intracontinental shortening. Note that this applies regardless of the age of the Narracoota Syncline. There is no evidence that it was deposited on sialic basement, and if it was, then it has either been detached from its basement and transported into its current position or its basement has been removed by intracontinental thrusting.

The Narracoota Syncline is succeeded to the SW by an eastward closing fold. Exposure here is poor, but it is possible that this closure represents an anticlinal equivalent of the Narracoota Syncline. This interpretation is suggested in Fig. 1.

113

Further north and west exposure of the Glengarry Group is poor and its relationship to the surrounding basement blocks is uncertain. In this region one of the better exposed successions of the Narracoota Volcanics is in the Relief Bore-Dimble area, where the unit is sandwiched between Archaean gneisses to the north, and south-facing Padbury Group sediments to the south. Limited facing criteria in the Narracoota Volcanics support a southward facing direction for it too, but stratigraphic considerations suggest that its relationship to the gneisses to the north is tectonic. Since the rocks here are geochemically similar to those of the Narracoota Syncline they are treated with them, although there is no direct evidence that they are not autochthonous.

FIELD CHARACTERISTICS OF VOLCANICS

The Narracoota Volcanics are divided by Gee (1983) into a mafic and an ultramafic member. The mafic member consists almost exclusively of sheared mafic lava. Shearing is generally sufficiently intense that it imparts a spaced cleavage to the lavas, with spacings between cleavage surfaces of 1--4 cm. Between cleavage surfaces, however, the rocks exhibit little strain. Defor- mation has obliterated most primary flow features. Only in the Dimble area in the west, and in the core of the Narracoota Syncline, are there well- preserved pillowed and bulbous lava flows. However, there is a suggestion of pillowed forms elsewhere, and the uniformly fine-grained character of the lavas indicates that flows were, in general, thin. In the Dimble area, there are some flows up to 20 m thick, with clear internal differentiation and peridotitic cumulates. These flows, unlike any other rocks observed in the region, exhibit the irregular, polygonal jointing characteristic of high-Mg (komatiitic) flows (Arndt et al., 1979). In some places, particularly in the eastern part of the autochthonous succession but also on the eastern end of the northern limb of the Narracoota Syncline, there are sills up to 500 m thick. They exhibit minor internal differentiation in outcrop.

The ultramafic member consists predominantly of very fissile ultramafic schists. These schists have a clastic appearance, and probably originated as ultramafic pyroclastic rocks. They are generally poorly exposed and the land is low-lying. However, sporadically developed in the regions of ultramafic schist are linear ridges of highly silicified, pale-pink weathering, pillowed lava. In view of the degree of silicification the original character of these lavas -- whether mafic or ultramafic -- is uncertain. They provide the only means of monitoring structure within many of the ultramafic members. They are interpreted as a result of local outpourings of lava in a predominantly pyroclastic environment. At the western end of the autochthonous northern boundary, and near the core of the Narracoota Syncline, the ultramafic schists are interbedded with and laterally transitional into siliceous clastic rocks and cherts of the Karalundi Formation. An unusual feature of this member in the autochthonous succession, near Peak Hill, is the occurrence of some massive, dark unsilicified mafic igneous rocks.

114

PETROGRAPHY

Mineral assemblages in the region are in the greenschist facies of meta- morphism, with almost complete replacement of the primary mineralogy. Thus, mafic rocks typically carry the mineral assemblage albite--actinolite-- chlorite--clinozoisite (epidote)--quartz--leucoxene (sphene). In the present study, relict pyroxenes have been observed only in a few of the better preserved lavas from the Dimble area, in one medium-grained lava near Cashman, and in one of the dark lavas associated with ultramafic schists southwest of Peak Hill. Despite complete recrystallisation, primary igneous textures are clear in many samples. The rocks are commonly aphyric, but pseudomorphs of actinolite after probable pyroxene and more rarely of albite after plagioclase occur in small quantities in some rocks, indicating that pyroxene and plagioclase were probably liquidus phases. The groundmass of fine-grained metavolcanics consists of sheafs and plumose bundles of actinolite needles replacing former pyroxene, with random orientations, and rarer elongate albite prisms, in an equigranular matrix of albite, chlorite and epidote-family minerals. The textures are typical of quenched sub- marine basalts. Coarser-grained mafic rocks have sub-ophitic textures in some cases. The well-preserved pillow lavas and thicker flows with komatiitic characteristics near the southern end of the Dimble line carry abundant serpentine pseudomorphs after euhedral olivine grains, in a finer-grained groundmass including elongate, hopper-shaped pyroxenes.

Few of the ultramafic schists have been examined optically. They consist of a mixture of serpentine and chlorite, one of which has a pronounced preferred orientation. No primary textures are visible.

SAMPLING TRAVERSES

Several stratigraphic sections through the Narracoota Volcanics were sampled for geochemical study. Locations of these sections are given in Fig. 1. A brief description of the field characteristics of the sections is given below. 'Thicknesses' for these sections are estimates, because of the absence of primary features along most of them. The thicknesses have been adjusted for dip of sedimentary successions above and/or below them where possible. Otherwise they have been adjusted using the dip of the main cleavage {usual- ly about 80 ° ) as an indication of the dip of the succession.

Southern autochthonous border

(1) Mikhburra line The volcanic successions of the southern autochthon are the thickest

in the entire region. The Mikhbu~a line, oriented at 10 ° for m o s t of its length and passing through the Mikhburra pit, traverses a 5--6 km thickness of mafic igneous rocks before passing into a poor ly exposed zone of sedi,

115

ments and ultramafic schists just south of the Mikhburra pit itself. At the southern (lower) end of the line there is a 1.2 km unexposed zone between the lowermost volcanics and the uppermost (Karalundi) sediments, which may also be underlain by mafic igneous rocks. In view of the apparent structural simplicity of the southern border, these thicknesses are probably close to the true stratigraphic thicknesses.

The Narracoota Volcanics on this line are apparently devoid of sediments, although the lack of continuous exposure leaves open the possibility of some unexposed sedimentary horizons. No primary volcanic features were observed in the field. Two possible (100 m thick) sills were identified on the line, but the generally uniformly fine-grained character of the rocks suggests that most of the sequence is made up of thin lava flows.

(2) Cashman line The Cashman line, oriented at 0 ° (Fig. 1) begins in greywackes of the

Karalundi Formation. It is notewor thy for the intercalation of mafic pillow- lavas, 10--20 m thick flows and sills, and sandstones in the first 2000 m ('stratigraphic'). This mixed unit is overlain by a roughly 4 km stratigraphic thickness of mafic lavas, with almost complete exposure, exhibiting common pillow-form and 2--10 m thick flows. There are no sediments in this upper unit. Exposure dies at the northern end of the line, so that the location of the true top, and maximum thickness of the sequence, are unknown.

East of the Cashman line, exposure is poor. Traverses near Rubywell and Goodin Find revealed sporadic exposures of both siltstone and mafic volcanic rocks. It is probable that the major accumulation of mafic lavas, reflected in the Mikhburra and Cashman lines, thins going eastwards.

(3) John Bore Still further east, a line oriented at 315 °, beginning 2.5 km due east of

John Bore, transected about 2 km of poorly exposed lava, with sediments at the base and top. There is a thick (150 m) sill below the main volcanic unit, in the underlying sediments.

Northern autochthonous border

(4) Quartz Bore A line at 80 °, from a position on the track 3 km NNE of Quartz Bore

crossed 3.5 km (stratigraphic) of sheared mafic lavas between the Karalundi Formation and the Thaduna Greywackes. This line, because of its orientation, crossed back into the lavas further east, in an apparent lateral equivalent of the upper lava sequence, although they are chemically different (see below). A line due east out of Quartz Bore crossed rather poorly exposed mafic lavas and, near the top of the formation, a ridge of pink, silicified pillows. These pillows are in a region of poor exposure but, based on the general association of such pillows, they are probably surrounded by ultramafic schists.

116

Further west along this boundary, outcrop is poor. From the limited outcrop available south of St. Crispin Find it appears that lavas are not abundant in the section. Most of the float and small outcrops are of clastic sediments, although there are minor amounts of mafic lava.

(5) Peak Hill South and southwest of Peak Hill exposure is again poor. In places the

Narracoota Volcanics are less than 1 km thick. Along the telephone line running south out of Peak Hill townsite, however, the Narracoota Volcanics are exposed over a 2 kin-wide zone. They consist of ultramafic schists, and isolated ridges of pink, highly silicified lava. There are intercalated sediments in many places. As illustrated on Fig. 1, this band of largely ultramafic rocks opens into a broad zone to the west, probably as a result of post-Padbury folding. It is from within this refolded unit, about 11 km southwest of Peak Hill Townsite, that the dark lavas associated with the ultramafic schists were collected.

The Narracoota Volcanics on the northern boundary then, change, going westwards, from a thick mafic succession near Quartz Bore to a much thinner ultramafic succession, with abundant intercalated sediments, and associated, highly silicified lavas.

Narracoota Syncline

(6) Randell line A line 4.5 km due west of Randell Well tran~ected the northern limb

of the Narracoota Syncline. The lower 2800 m (assumed vertical dip) of the section consists of ultramafic schists, with local ridges of silicified pillow- lava. There are also rare mafic lavas. The upper 2 km consists predominantly of mafic lava, with small amounts of ultramafic schist. This section provides the best example of a true association of the ultramafic schists and mafic lava.

(7) Narracoota Nose Near the nose of the Narracoota Syncline the lower ultramafic unit is

similar to that on the Randell line, but contains iron-rich sandstones and siltstones as well as the silicified pillows. The mafic rocks, forming a prominent hill in the core of the fold, exhibit very clear pillowed morphology, and do not contain interbedded ultramafic schists.

Far west

Two short sections were run across the Narracoota Volcanics :in the west near Dimble.

(8) Relief line Three kilometres west of Relief Bore, running due south from the road

117

between Relief Bore and Kelly Well, highly sheared mafic lavas and ultramafic schists are exposed over about a 2 km thickness.

(9) Dimble Five kilometres NNE of Top Dimble Well, mafic lavas and ultramafic

schists are exposed over about 2.5 km N--S. A prominent hill near the southern end of this exposed zone consists of well-preserved pillowed lavas, and some flows, up to 20 m thick, with pronounced olivine cumulate zones and komatiitic jointing patterns.

GEOCHEMISTRY

The volcanic rocks from the southern autochthonous boundary - - o n the Mikhburra and Cashman lines and near John Bore -- are uniformly mafic. Those on the northern autochthonous boundary and in the Narracoota Syncline are both mafic and ultramafic. Since even the mafic rocks in the two regions are geochemically distinct, the two regions are treated separately.

Southern autochthonous volcanic rocks

The earliest volcanic rocks on the Mikhburra and Cashman lines are relatively rich in SiO: (52.23--56.65 wt.%, Table I). On the Cashman line these high-SiO2 rocks occur in the lower 500 m of the sequence, and are separated from the main lavas by 500 m of poorly exposed sediments and 750 m of sediments carrying sills. On the Mikhburra line they occur over a thickness of 400 m, separated from the main lavas by a 750 m unexposed zone which may also be underlain by sediments. Rocks from the main Mikhburra and Cashman sequences, each of which is about 4 km thick, and from the John Bore line (excluding the lower sill), have very uniform chemistry, with SiO2 ranging from 48.47 to 52.96, MgO from 7.25 to 8.74 and TiO2 from 0.77 to 1.03 wt.% (Table I). The early, high-SiO2, lavas differ from the 'main-sequence' lavas in having generally higher alkalis and lower CaO, and a broader range in Fe-content, but their compositions generally overlap the more uniform ones of the main sequences.

The limited chemical range of the main-sequence lavas is reflected in both major- (Fig. 3a,b,c) and trace-element levels (Fig. 4a,b). The rocks are strikingly similar in major-element chemistry to modern mid-ocean- ridge basalts (MORB), with Si, A1, Fe and Mg-values comparable with those of the most-evolved FAMOUS glasses (Fig. 3a,b). They differ from the FAMOUS rocks only in having lower Ti contents at a given Fe-level (Fig. 3c). Similarly low Ti contents are reported from some MORB such as those from DSDP Leg 37 (Blanchard et al., 1976) and the Somali Basin (Frey et al., 1980; Fig. 3c) although they do not appear to be common. Other incompatible elements also have relatively low concentrations in these rocks; Ti/Zr is essentially constant, and typical of chondritic values {Fig. 4a).

118

TABLE I

Chemical analyses of volcanic rocks from the southern authochthonous sequence

82160 82158 82155 82153 82150 82148 82146 82144 82224

SiO: 53.44 54.93 52.59 49.62 51.52 50.55 51.44 51.74 53.51 TiO 2 0.64 0.75 0.77 0.97 0.91 0.95 0.92 0.62 1.46 A120~ 14.80 14.51 14.58 15.15 14.58 14.80 14.53 14.10 15.23 MgO 6.81 7.45 7.84 8.15 7.10 6.71 6.89 9.98 5.95 FeO 10.54 9.51 10.96 12.16 11.33 11.17 10.78 9.97 12.75 MnO 0.18 0.18 0.20 0.22 0.20 0 .20 0.20 0.19 0.20 CaO 10.49 7.87 9.55 10.65 11.06 13.48 12.44 10.89 5.09 Na20 2.89 4.27 3.15 2.28 2.86 1.94 2.48 1.51 3.99 K~O 0.17 0.46 0.28 0.72 0.36 0.10 0.23 0.95 1.70 P205 0.05 0.07 0.07 0.09 0.08 0.09 0.09 0.06 0.13 Total 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00

Cr 157 289 281 338 214 289 265 619 181 Ni 72 79 72 86 65 64 79 174 68 Zr 26 40 44 55 51 52 53 31 88 Y 16 16 19 21 23 20 18 14 30 Sr 256 44 186 119 203 122 195 151 118 Rb 2 5 4 8 4 2 3 19 45

82225 82226 82228 82229 82231 82234 82236 82179 82180

SiO~ 53.99 56.09 52.23 56.65 48.74 48.47 51.35 51.36 52.96 TiO~ 0.83 0.74 1.11 0.90 0.81 0.87 0.87 1.03 0.91 Al~O 3 15.12 13.17 14.71 12.65 14.80 14.68 14.61 13.40 14.39 MgO 8.71 6.54 7.73 7.28 8.06 8.74 7.57 7.25 7.33 FeO 9.80 8.61 12.42 10.65 11.37 11.74 11,17 12.10 11.11 MnO 0.17 0.16 0.19 0.21 0.22 0.22 0.20 0.22 0.20 CaO 6.19 10.24 7.49 7.83 14.11 13.94 10.32 11.71 9.26 Na20 4.44 4.17 3.51 3.63 1.76 0.99 2.74 2.48 3.56 K20 0.68 0.21 0.52 0.11 0.05 0.27 1.09 0.34 0.19 P205 0.07 0.07 0.11 0.08 0.07 0.08 0.08 0.09 0.08 Total 100.00 100.00 100.00 100.00 100.00 100,00 100.00 100.00 100.00

Cr 484 257 276 240 231 332 228 Ni 155 80 68 68 58 83 70 Zr 48 41 62 51 45 48 49 55 49 Y 18 17 24 16 18 21 19 20 19 Sr 139 99 120 25 183 225 329 113 79 Rb 11 2 7 0 1 3 15 6 4

82160--82144: Mikhburra line; 82224--822236: Cashman line, in stratigraphic order base to top; 82179, 82180: John Bore line; 82155- -82146 and 82231- -82236 are main- sequence lavas; 82160, 82158 and 8 2 2 2 4 - 8 2 2 2 9 are early lavas. 82144 is at the top of the main sequence on the Mikhburra line, where u!tramafic schists appear. All analyses by XRF at McGill University. Major elements normalised to 100% volatile-free.

119

20-

AI

8 I i i I

40 $i

I I I I 6 o

Mg

~ b

i i i i I

o I Fe

C

/ -

i l ~ I I J IO

Fe

Fig. 3. Major-element characteristics of the Narracoota Volcanics of the southern autochthonous successions. Open circles: early lavas on the Mikhburra line; open squares : early lavas on the Cashman line; triangles: lavas from the John Bore line; half-filled circles and squares: main-sequence lavas from the Mikhburra and Cashman lines, respectively; filled circle: uppermost lava from the Mikhburra line. All diagrams are in cation per cent. Solid line encompasses most data from the MORB suites of the FAMOUS area (Bryan, 1979), DSDP Leg 37 (Blanchard et al., 1976; Byerly and Wright, 1978), 22--25 N on the Mid-Atlantic Ridge (Bryan et al., 1981) and the Somali Basin (Frey et al., 1980). Dashed line encompasses most data from the continental tholeiite suites of the Columbia River (Carlson et al., 1981), the northern Karroo (Cox, 1974) and the Deccan Traps (Krishnamurthy and Cox, 1977). In Fig. 3c the dotted line encompasses data from the early Proterozoic Labrador Trough (Dimroth et al., 1970) and the Cape Smith foldbelt (Francis et al., 1983; pyroxene- and plagioclase-phyric rocks only).

In view of the demonstrably 'ensialic' setting of these volcanic rocks it is instructive to compare their chemistry with that of flood basalts. Such basalts are in general similar in major-element chemistry to MORB, although they exhibit substantially higher Fe/Mg at high Mg-values (Francis, 1985) and range to much higher Fe and Si levels (Fig. 3a,b). At an Fe-level of 8 cation per cent, where there is substantial compositional overlap between basalts from the two settings (Fig. 3b}, flood basalts exhibit a broad range in Ti-content, from values comparable with MORB (in the Columbia River basalts), to much higher ones (Fig. 3c). They do not extend to the low Ti- values typical of the Narracoota suite. The unusual feature of the lavas is their low Ti/Fe. Similarly low Ti/Fe ratios (in comparison with most MORB) are observed in early Proterozoic volcanic rocks from the Cape

120

12000 t

8ooo

4000 ~

24--

y -

12-

0

0

0 O f /

/

/

0

/ /

/

I D

j . J

I t

/

/

/ O

1 /

D

b /

I I I I I I I I I '1 Z r 100

Fig. 4. Trace-element characteristics of the Narracoota Volcanics of the southern autochthonous successions. Symbols as for Fig. 3. Dashed lines are ratios for ordinary chondrites (Hickey and Frey, 1982 ; after Mason). All elements are measured in ppm.

Smith foldbelt and the Labrador Trough, in the circum-Ungava foldbelt of North America (Fig. 3c). They are also characteristic of many modern 'island-arc tholeiite' suites such as those of the Mariana island-arc {Dixon and Batiza, 1979).

The volcanic rocks that underlie the main sequence exhibit a broader ~.

chemmal range than the main sequence, both in major- and trace-elements. It is noteworthy that the most Si-rich rocks of this set have the lowest incompatible-element levels and the highest Mg/Fe, of any rocks in the suite, suggesting that the high Si-contents are not due simply to fractionation (see below).

The main volcanic sequence on the Mikhburra line is overlain by a poorly- exposed succession of ultramafic schists. This succession has not been extensively sampled. However, the uppermost mafic volcanic rocks of the Mikhburl~ main sequence, of which one analysis is plotted in Figs. 3 and 4, have high M ~ , high M~#(Mg/(Mg + Fe)) and low incompatibleelement contents. These are clmraeteristics of the mafic volcanic rocks associated with ultramafic schists in the Narracoota Syncline and on the northern autochthonous border. The transition observed on the Mikhburra line provides the only direct evidence of the temporal relationships of these two sequences, although regional mappiag (Gee, 1979) suggests the transition may characterise much of the. southern autochthonous succession.

121

Northern autochthonous volcanic rocks and Narracoota Syncline

Both the northern autochthonous volcanic rocks and the volcanic rocks of the Narracoota Syncline are characterised by thick units of mafic lava. On the Quartz Bore line of the northern autochthon there is an uninter- rupted 3.5 km-thick unit of such lavas, followed, after some sediments, by further lavas that may be lateral equivalents of the lower ones. These units are referred to as the 'lower' and 'upper' Quartz Bore. The sequence on this line is gradational westwards into one characterised mainly by ultramafic schists and associated silicified pillow lavas too altered for geochemical study. In the Narracoota Syncline a 1 km-thick sequence of pillowed mafic lavas overlies 1.5 km of ultramafic schists in the core of the fold. The mafic lavas from the Quartz Bore line and the upper sequence from the Narracoota Syncline are sufficiently well preserved to be amenable to chemical study. Lavas within the ultramafic succession are generally completely silicified, and have not been analysed. The only exception to this is some dark-coloured lavas sampled in a poorly exposed terrain on the autochthonous northern border. Several ultramafic schists were also analysed. It is clear that their chemistry cannot be treated rigorously, but the analyses (Table II) serve to illustrate the Mg-rich character of their protoliths.

Lavas from the lower Quartz Bore unit exhibit a broad range of Si contents, and low Fe/Mg compared with the main-sequence lavas of the southern autochthon. They are comparable geochemically with the earliest lavas of the southern autochthon, except that they have extremely low contents of Ti and Zr (Figs. 4 and 5). Lavas from the upper Quartz Bore unit do not share this low-incompatible-element feature. They have a very limited range in chemistry, comparable in many respects with that of the southern sequence, but with lower Fe contents (Fig. 5) and higher Ni and Cr (Tables I and II). Lavas from the upper part of the Narracoota Syncline are signifi- cantly lower in Fe than any other mafic lavas from the region, have a broad range in Si content , and again very low contents of Ti and Zr (Figs. 5 and 6).

The low Ti and Zr contents in the lower Quartz Bore unit and the Nar- racoota Syncline distinguish these rocks from those of the southern autochthon and indeed from most modern basaltic rock types. They are well below the levels typical of MORB, and even below those typical of most mafic island-arc rocks. The only modern mafic rocks that have comparably low incompatible-element levels are some boninites (e.g. Cameron et al., 1979; Hickey and Frey, 1982). These Narracoota Volcanics are similar to boninites in other respects too, such as in their relatively high Si-levels. On the other hand, there are MORB with similarly high Si (Frey et al., 1980) and, except for their low Ti and Zr, these rocks are still very similar to MORB or island- arc tholeiites. Unlike most boninites (Hickey and Frey, 1982) these rocks have chondritic Ti/Zr (Fig. 6a). It is noteworthy, however, that they,are not low in Y, so that they have very high Y/Zr and Y/Ti (Fig. 6b).

The dark lavas on the autochthonous northern border are similar in

122

<

e~

r ~

o

Z

" 0 e~

e~

0

0

s e~

o e~

o

R

0

d~do~o~od~

123

o

0 o

<

0.1

,,-4

c,.l

0 0

~ q ~ q q q q q

~ ~ ~ 0

N ~ ~ q q q q q 0 0 ~ 0 0 0 0 0 0 0

o-,.,"dOo ~ . ~ o o ° . o . c o

~ 0

~ 0 ~

o

..C

aO

0

o'

oO

¢q ~q oO

C~ oO

o

0 o

z

cx] cq

7~

124

20-

AI

J

8

~ ' ' • 0 ' Q , 0 ~

l l l l l l

Si

I I I l

6 0

I Mg !

0 "i 0

i i i ~ . . . . . . . . . i l

o I

Fe

T i

I

o_t o

i i i ] i p / 1 0

Fe

Fig. 5. Major-element characteristics of the mafic Narracoota Volcanics from the northern autochthon and the Narracoota Syncline. Open circles: lower Quartz Bore;filled circles: upper Quartz Bore; triangle: chilled margin of sill, John Bore line; open squares: Randell line; half-filled hexagons: western end of Narracoota Syncline; half-filled squares: dark lavas south of Peak Hill. Fields as for Fig. 3.

major-element chemistry to other mafic rocks of the region, but are dis- tinguished by very high levels of Ti, Zr and Y, and high Ti/Y and Zr/Y ratios.

Narracoota Volcanics in the far west

Few volcanic rocks from the far west have been analysed (Table III). Some of these rocks closely resemble the southern autochthonous main- sequence lavas, although there are lavas, both at Relief Bore and Dimble Well, that are highly depleted in Ti and Zr like those of the Narracoota Syncline. As was the case further east, there are high-Si, high-Mg lavas (82128) with very low Ti- and Zr-levels.

Of particular interest are the hish-Mg flows exposed near the southern end of th@ 'Dimble' line. Quenched, fine-~ained mat ins from these flows carry pseudomorphs after olivine, and have up to 20% M~O. These rocks are komatiitic basalts and are chemically similar to other Proterozoic and Archaean high-Mg basalts. Unlike them, however, they have Fe-values similar to the high-Mt~ rocks from MORB suites. Most Archaean and Protero- zoic komatiitic basalts have substantially higher Fe (Francis, 1985). In view of the chemical similarities of the associated mafic rocks to those

125

1 2 0 0 0 -

Ti -

8000

4 0 0 0

i 24

y --

1 2 -

0 /

/

/

,i

i ; 1 /

/

/

, P / b

0 / /

/

/

[3 0 ° 0

0 /

o /

I I 1 I

/ []

I I I I I I Z r 100

Fig . 6. T r a c e - e l e m e n t c h a r a c t e r i s t i c s o f t h e m a f i c N a r r a c o o t a V o l c a n i c s f r o m t h e n o r t h e r n a u t o c h t h o n and t h e N a r r a c o o t a S y n c l i n e . S y m b o l s as for Fig . 5. D a s h e d l ines as for Fig . 4.

further east, these komatiitic basalts are considered to be an integral part of the volcanic association of the Narracoota Volcanics, although their relative stratigraphic position is unknown.

P E T R O G E N E S I S

The main sequence of lava from the southern autochthon has a very limited range in bulk chemistry. Given the thickness of the sequence, this is most reasonably interpreted as a consequence of compositional buffering in one or several, large, periodically replenished magma chambers (cf. O'Hara and Mathews, 1981). Compositions cluster about the low-pressure olivine-- clinopyroxene--plagioclase eutectic (Fig. 7a,b), suggesting that such magma chambers were probably developed at shallow depth, perhaps within, or at the base of, the continental crust. The relatively constant Ti/Zr and the de- crease in Y/Zr with increasing Zr observed in the sequence (Fig. 4a,b) are consistent with fractionation controlled by olivine, clinopyroxene and plagioclase. The Ti/Zr value, which is close to chondritic, probably reflects the value in the source. Given the strong possibility of open-system fractionation, and the non-uniqueness of inversion methods for it, no attempt has been made to model fractionation within the sequence. The low values of Ni and Cr in the suite indicate, however, that none of these magmas is likely

26

o

o,I oo

o~ co

oo

co

oo

< 0 [--

o,I

C~ oo

co

O o~

,-4

:.~ oo

oo 0 o~

¢q .~ oo

o

oo o

o,I

o,I oo

o,I

i

c~o~oo o o o ~ o D O O O

Z N Z M ~ 4 Z ~ o

o o ° ° ° ° ° . ~qo

0 ~ C

0¢o

oo

o~

~.~ 0 J~

0

127

PI Di

\

s A

Fig. 7. Major-element analyses from the southern autochthonous succession, plotted on the isomolar pseudo-liquidus projections of Elthon (1983). Symbols as for Fig. 3. Primary- phase-field boundaries at 1 atm from Elthon on the olivine--diopside--silica diagram, and estimated from the analytical data of Walker et al. (1979) on the silica--plagioclase-- olivine diagram.

to have reached the surface without substantial fractionation, a conclusion that is also supported by their low Mg/Fe (cf. Hanson and Langmuir, 1978).

The broader chemical range of the precursors to the main-sequence lavas may reflect processes associated with the development of magma chambers, which allow greater opportunit ies for magma evolution (cf. Robson and Cann, 1982). Their generally higher Si-values could be at tr ibuted to crustal contamination, although the still-low incompatible-element-contents of these rocks make this unlikely. These low incompatible-element levels also preclude an explanation by greater degrees of crystal fractionation, especially since the rocks with the highest Si-levels have some of the lowest incompatible-element levels (Table I). It appears that some process at the source is responsible for the different chemistry of these rocks. High Si- values at a given degree of partial melting are favoured by the retention of greater proport ions of olivine in the source, i.e., by expansion of the olivine primary-phase-volume. Such an expansion is most simply achieved by high PH2 o (e.g., Green, 1976; Tatsumi, 1982), and indeed partial melting of hydrous peridotite is the most commonly invoked mechanism for producing primitive, high SiO2, magmas at the Earth's surface today. The relationship between the early lavas and the main-sequence lavas can only be a subject for speculation. It is possible that extraction of a partial melt from hydrous mantle led to dehydrat ion of mantle through a large region, thereby allowing more 'normal ' melts to develop the main sequence. Alternatively, melting of hydrous mantle may have been triggered by emplacement of a hot, dry mantle diapir, from which the main-sequence lavas were derived, in a manner similar to that invoked by Crawford et al. (1981).

128

The main-sequence lavas were followed by voluminous ultramafic, probably pyroclastic, rocks and associated, highly depleted tholeiites, and these rock-types dominate the northern autochthon and the Narracoota Syncline. As noted above, the tholeiites have some of the characteristics of boninites -- low Ti and Zr, and relatively high Si. Their origin is most simply attr ibuted to melting of hydrous peridotite. Their very low Ti and Zr, and low Fe may reflect a parent mantle that had already been melted, as is also assumed for many boninites {e.g., Cameron et al., 1979, 1983; Hickey and Frey, 1982). In view of their very low Ti/Y, and the chondritic Ti/Y in the main- sequence lavas, the main-sequence lavas cannot easily be considered as candidates for products of the earlier melting event, despite the field relationships. The only rocks with unusually high Ti/Y in the region are the dark-coloured lavas of the northern autochthon. They may be candidates for first-stage melts, although they do not appear to be widespread.

Extraction of a melt with low Ti/Y and Zr/Y could be achieved by leaving either garnet or amphibole in the source (cf. Pearce and Norry, 1979). Garnet is favoured, since hornblende should also fractionate Ti and Zr.

The generally high Cr and Ni values of mafic rocks on the northern autochthon and in the Narracoota Syncline indicate that these rocks did not undergo appreciable fractionation en r o u t e to the surface, in contrast to those of the southern autochthon.

Ultramafic schists associated with these tholeiites also have low Zr/Y and Ti/Y. It is possible, therefore, that they were derived from the same or similar parents. The apparently pyroclastic character of their volcanism could be viewed as evidence of the hydrous conditions under which they were generated, although it is equally possible that they were hydrated during passage through the upper crust. In either case it would appear that the ultramafic schists and associated tholeiites from the northern autochthon and the Narracoota Syncline reflect partial melting of a previous- ly depleted source-area under probably hydrous conditions, and that these conditions prevailed on the southern autochthon after eruption of the main- sequence lavas.

The uppermost lavas on the Quartz Bore line reflect a return to conditions similar to those of the main-sequence. They have similar Ti- and Zr-levels, and chondritic Ti/Y and Zr/Y. They differ from the main-sequence lava only in having higher Cr, Ni and lower Fe, which presumably reflect lower degrees of fractionation.

For the western region, it is no tewor thy that the low Ti, Zr lavas have very low Ti/Y and Zr/Y, whereas the rocks with more normal Ti and Zr have chondritic ratios, so that all the features observed in the stratigmphical- ly controlled situation further east are also observed here. The well-preserved komatiit ic basalts from the Dimble Well area fall into the low Ti, Zr group, with very low Zr/Y and Ti/Y. They therefore have close chemical affinities to the less magnesian of the ultramafic schists, and were presumably derived from an already strongly depleted mantle, which would explain their low Fe-contents.

129

The Narracoota Volcanics therefore appear to preserve evidence of two types of mantle source-region. One type, from which the early and main- sequence lavas of the southern autochthon were derived, had essentially chondritic ratios of Ti, Y, and Zr, but with levels perhaps marginally lower than those of the source-region for most modern MORB. The bulk of lavas derived from this source (the main sequence) have very limited chemical variations; their composit ions were probably buffered by open-system fractionation in large magma chambers at shallow depth, a plumbing system similar to that postulated at modern oceanic ridges. The earliest lavas from this source are more variable, probably reflecting immaturi ty of the plumbing system, and show tendencies towards basaltic andesites which may reflect partial melting under hydrous conditions.

The second type of source had suffered an important previous melting event, leaving it substantially depleted in Ti and Zr, but not in Y. Rare, dark-coloured lavas in the region may be products of this event. The second melting event gave rise to high-Mg melts, most of which were probably erupted explosively, and a suite of rocks in the basalt--basaltic andesite-- andesite range, but with relatively high Mg, Cr and Ni. The melting that gave rise to them was probably under hydrous conditions.

DISCUSSION AND CONCLUSIONS

It is clear from their geological setting that the Narracoota Volcanics were erupted on, and perhaps near, continental crust that was undergoing rapid subsidence. Their eruption was probably associated with a continental thinning/rifting event. The MORB-like characteristics of the main-sequence lavas are compatible with this setting, and similar to those of some other early Proterozoic continental-rifting environments (Dimroth et al., 1970; Hynes and Francis, 1982; Francis et al., 1983).

The basalt--basaltic andesite--andesite suite and associated ultramafics of the northern autochthon and the Narracoota Syncline are, however, dissimilar to the rocks typical of continental rifting environments, both in the Proterozoic and Phanerozoic. They have many more similarities to the boninite association of the fore-arc regions of the western Pacific than to any other modern associations. Considering that the boninites in these settings are commonly accompanied by voluminous low-K tholeiites (island- arc tholeiites), to which the main-sequence lavas bear some resemblance, one must consider the possibility that the NarracootaVolcanics were erupted during a rifting event in an island-arc (cf. Crawford et al., 1981). It is, however, probably overly simplistic to assume that particular magmatic associations are uniquely diagnostic of particular tectonic settings. Indeed, boninites have recently been reported in a demonstrably continental setting (Wood, 1980). There is no evidence in the pre-Narracoota geological history of this region to suggest the presence of an island-arc, but some rifting event associated with subduction cannot be precluded at this stage.

130

Rifting in this region involved large-scale melting of mantle material, some of which was similar to the source for MORB, and some of which was very refractory. Melting was probably hydrous in part. Where these conditions prevail in modern arc environments the refractory character of the mantle is at tr ibuted to a previous MORB- or island-arc-tholeiite-generating event and the hydrous character is explained by dehydrat ion of an underlying descending oceanic plate. It is possible, however, to envisage other mechanisms by which the mantle might become hydrated, especially given the now-widespread evidence for metasomatism in sub-continental mantle environments (e.g., Harte, 1983). Until more is known about the volcanism associated with catastrophic continental rifts, the possibility that these rocks represent a not-unusual feature of Atlantic-type margin development cannot be ruled out.

It is clear that the Narracoota Volcanics bear little geochemical resemblance to modern 'continental ' tholeiites. The geochemical characteristics of such continental tholeiites appear to have been constant at least back into late Proterozoic time (Green, 1976), and tholeiites with continental geochemical characteristics are preserved at an early stage of rifting in the early Protero- zoic Cape Smith foldbelt (Hynes and Francis, 1982; Francis et al.. 1983). Based on this contrast, it may be argued that rifting in the Capricorn Orogen was of major dimensions, leading to the development of truly oceanic crust, although there is no evidence bearing on the actual dimensions of an ocean-basin produced by it. This conclusion would clearly be strength- ened if the Narracoota Volcanics were, in fact, arc-related, although this is not the favoured interpretation. The Capricorn Orogen has, then, at least some of the hallmarks of a Phanerozoic orogen. A model in which it was produced by the formation and destruction of one or more major ocean- basins must still be considered a possibility.

ACKNOWLEDGEMENTS

Field and laboratory work by A. Hynes was supported by a grant from NSERC, Canada, on sabbatical leave from McGill University. We are grateful to both organisations, and to the Geological Survey of Western Australia, for their support.

REFERENCES

Arndt, N.T., Francis, D. and Hynes, A.J., 1979. The field characteristics and petrology of Archean and Proterozoic komatiites. Can. Mineral., 17:147--163.

Blanchard, P., Rhodes, J.M., Dungan, M.A., Rodgers, K.V., Donaldson, C.H., Brannon, J.C., Jacobs, J.W. and Gibson, E.K., 1978. The chemistry and petrology of basalts from Leg 37 of the Deep Sea Drilling Project. J. Geophys. Res., 81: 4231--4246.

Bryan, W.B., 1979. Regional variation and petrogenesis of basalt glasses from the FAMOUS area, mid-Atlantic Ridge. J, Petrol., 20: 293--325.

131

Bryan, W.B., Thompson, G. and Ludden, J.N., 1981. Compositional variation in normal MORB from 22--25 N: Mid-Atlantic Ridge and Kane Fracture Zone. J. Geophys. Res., 86: 11815--11836.

Bunting, J.A., Commander, D.P. and Gee, R.D., 1977. Preliminary synthesis of Lower Proterozoic stratigraphy and structure adjacent to the northern margin of the Yilgarn Block. Geol. Surv. W. Aust. Ann. Rep. for 1976, pp. 43--48.

Byerly, G.B. and Wright, T.L., 1978. Origin of major element chemical trends in DSDP Leg 37 basalts, Mid-Atlantic Ridge. J. Volcanol. Geotherm. Res., 3: 229--279.

Cameron, W.E., Nisbet, E.G. and Dietrich, V.J., 1979. Boninites, komatiites and ophiolitic basalts. Nature, 280: 550--553.

Cameron, W.E., McCuUock, M.T. and Walker, D.A., 1983. Boninite petrogenesis: chemical and Nd--Sr isotopic constraints. Earth Planet. Sci. Lett., 65: 75--89.

Carlson, R.W., Lugmair, G.W. and MacDougall, J.D., 1981. Columbia River volcanism: the question of mantle heterogeneity or crustal contamination. Geochim. Cosmochim. Acta., 45 : 2483--2499.

Crawford, A.J., Beccaluva, L. and Serri, G., 1981. Tectonomagmatic evolution of the West Philippine--Mariana region and the origin of boninites. Earth Planet. Sci. Lett., 54: 346--356.

Cox, K.G., 1974. The olivine-rich lavas of Nuanetsi: a study of polybaric magmatic evolution. J. Petrol., 15: 269--301.

Dimroth, E., 1981. Labrador Geosyncline: Type example of Early Proterozoic cratonic reactivation. In: A. Kr6ner (Editor), Precambrian Plate Tectonics. Elsevier, pp. 332-- 352.

Dimroth, E., Baragar, W.R.A., Bergeron, R. and Jackson, G.D., 1970. The filling of the Circum-Ungava geosyncline. In: A.E. Baer (Editor), Precambrian basins and geo- synclines of the Canadian Shield. Geol. Surv. Can. Pap. 70-40, pp. 45--142.

Dixon, T.H. and Batiza, R., 1979. Petrology and chemistry of recent lavas in the northern Marianas: implications for the origin of island-arc basalts. Contrib. Mineral. Petrol., 70: 167--191.

Dunlop, D.J., 1981. Palaeomagnetic evidence for Proterozoic continental development. Philos. Trans. R. Soc. London, Ser. A: 301 : 265--277.

Elthon, D., 1983. Isomolar and isostructural psuedo-liquidus diagrams for oceanic basalts. Am. Mineral., 68: 506--511.

Embleton, B.J.J., 1978. The palaeomagnetism of 2400 M.y. old rocks from the Australian Pilbara craton and its relation to Archaean--Proterozoic tectonics. Precambrian Res., 6: 275--291.

Francis, D.M., 1985. The nature of the Baffin Bay lavas and the value of picrites as analogues of primary magmas. Contrib. Mineral. Petrol., 89: 144--154.

Francis, D., Ludden, J. and Hynes, A., 1983. Magma evolution in a Proterozoic rifting environment. J. Petrol., 24: 556--582.

Frey, F.A., Dickey, J.S., Thompson, G., Bryan, W.B. and Davies, H.L., 1980. Evidence for heterogeneous primary MORB and mantle sources, NW Indian Ocean. Contrib. Mineral. Petrol., 74: 387--402.

Gee, R.D., 1979a. Structure and tectonic style of the Western Australian Shield. Tec- tonophysics, 58: 327--369.

Gee, R.D., 1979b. The geology of the Peak Hill Area. W. Aust. Geol. Surv. Ann. Rep. 1978, pp. 55--62.

Gee, R.D., 1980. Summary of the Precambrian stratigraphy of Western Australia. Geol. Surv. W. Aust., Ann. Rep. for 1979, pp. 85--90.

Gee, R.D., 1983. Peak Hill (Preliminary Printing). Geol. Surv. W. Aust. Sheet SG 50-8. Glikson, A.Y., 1980. Precambrian sial--sima relations: evidence for Earth expansion.

Tectonophysics, 63: 193--234. Green, D.M., 1976. Experimental testing of equilibrium partial melting of peridotite

under water-saturated, high pressure conditions. Can. Mineral., 14: 25--268.

132

Hanson, G.N. and Langmuir, C.H., 1978. Modelling of major elements in mantle-melt systems using trace element approaches. Geochim. Cosmochim. Acta, 42: 725--741.

Harte, B., 1983. Mantle peridotites and processes -- the kimberli te sample. In: C.J. Hawkesworth and M.J. Norry (Editors), Continental Basalts and Mantle Xenoliths, Shiva Publ., Nantwich, pp. 46--91.

Hickey, R.I~. and Frey, F.A., 1982. Geochemical characteristics of boninite series volcanics: implications for their source. Geoehim. Cosmochim. Acta, 46: 2099--2115.

Hoffman, P., 1980. Wopmay orogen: a Wilson cycle of early Proterozoic age in the northwest Canadian Shield. Geol. Assoc. Can. Spec. Pap., 20: 524--549.

Hoffman, P.F. and Bowring, S.A., 1984. Short-lived 1.9 Ga continental margin and its destruction, Wopmay orogen, northwest Canada. Geology, 12: 68--72.

Horwitz, R.C. and Smith, R.E., 1978. Bridging the Yilgarn and Pilbara blocks, Western Australia. Precambrian Res., 6 : 293--322.

Hynes, A. and Francis, D.M., 1982. A transect of the early Proterozoic Cape Smith foldbelt , New Quebec. Tectonophysics, 88: 23--59.

Krishnamurthy, P. and Cox, K.G., 1977. Picrite basalts and related lavas from the Deccan Traps of western India. Contrib. Mineral. Petrol., 62 : 53--75.

KrSner, A. (Editor), 1981. Precambrian plate tectonics. In: Precambrian Plate Tectonics. Elsevier, pp. 57--90.

Lewry, J.F., Stauffer, M.R. and Fumerton, S., 1981. A Cordilleran-type batholithic belt in the Churchill Province in northern Saskatchewan. Precambrian Res., 14: 277-- 313.

McElhinny, M.W. and Embleton, B.J.J., 1976. Precambrian and Early Paleozoic paleo- magnetism in Australia. Philos. Trans. R. Soc. London, Ser. A: 280: 417--431.

McWilliams, M.O., 1981. Palaeomagnetism and Precambrian tectonic evolution of Gond- wana. In: A. KrOner (Editor), Precambrian Plate Tectonics. Elsevier, pp. 649--687.

O'Hara, M.J. and Mathews, R.E., 1981. Geochemical evolution in an advancing, periodically replenished, periodically tapped, continuously fractionating magma chamber, Geol. Soc. London J., 138: 237--277.

Pearce, J.A. and Norry, M.J., 1979. Petrogenetic implications of Ti, Zr, Y and Nb variations in volcanic rocks. Contrib. Mineral. Petrol., 69: 33--47.

Robson, D. and Cann, J.R., 1982. A geochemical model of mid-ocean ridge magma chambers. Earth Planet. Sci. Lett. , 60: 93--104.

Shackleton, R.M., 1958. Downward-facing structures of the Highland border. Geol. Soc. London Q. J., 113: 361 -392 .

Tatsumi, Y., 1982. Origin of high-magnesian andesites in the Setouchi volcanic belt, southwest Japan, II. Melting relations at high pressures. Earth Planet. Sci. Lett. , 60: 305--317.

Walker, D., Shibata, T. and DeLong, S.E., 1979. Abyssal tholeiites from the Ocean- ographer Fracture Zone II. Phase equilibria and mixing. Contrib. Mineral. Petrol., 70: 111--125.

Wood, C.P., 1980. Boninite at a continental margin. Nature, 288: 692--694.

Documents

Geological setting and petrochemistry of the Narracoota Volcanics, Capricorn Orogen, Western Australia