The geochemistry of archaean clastic metasediments in relation to crustal evolution, Northeastern Yilgarn Block, Western Australia

Precambrian Research, 6 (1978) 157--175 157 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

T H E G E O C H E M I S T R Y O F A R C H A E A N C L A S T I C M E T A S E D I M E N T S IN R E L A T I O N TO C R U S T A L E V O L U T I O N , N O R T H E A S T E R N Y I L G A R N BLOCK, WESTERN A U S T R A L I A

R.J. MARSTON*

Department of Geology, University of Western Australia, Nedlands, W.A. 6009 (Australia) (Revision accepted January 14, 1977)

ABSTRACT

Marston, R.J., 1978. The geochemistry of Archaean clastic metasediments in relation to crustal evolution, northeastern Yilgarn Block, Western Australia. Precambrian Res., 6: 157--175. Clastic rudaceous metasedimentary rocks range from arkose to ultramafic para-schist in lithology and have high Na, Rb, Ni, Cr and V contents, except for pure arkose which has low Ni, Cr and V. The various arkoses are not comparable chemically with analyses of any Archaean or younger arkose or greywacke. The distinctive geochemistry and immature sedimentology of this clastic sequence (Jones Creek Conglomerate) results from: (a) derivation from sodic granitoid, low-K basaltic, peridotitic and gneissic source areas, (b) sedimentation in a high energy environment close to source areas, and (c) a lack of major post-depositional chemical alteration. Pebbles in the Conglomerate also attest to the local derivation of detritus from both sides of its very elongate outcrop. Following the emplacement (at 2689 +- 17 Ma) and unroofing of a sodic granitoid pluton, the Conglomerate was rapidly deposited in a graben-like basin. A n irregular unconformable contact between the Conglomerate and this pluton is preserved locally. Elsewhere contacts with granitoid or supracrustal rocks are tectonised, but the petrology of the Conglomerate indicates that these contacts were unconformities also. Contrary to previous suggestions, it is considered unlikely that the Conglomerate stratigraphically separates an older from a younger supracrustal sequence in this area. The Conglomerate probably represents the last depositional event before the onset of deformation and protracted regional metamorphism to the greenschist--amphibolite facies transition. Crustal

evolution from the emplacement of the sodic pluton to the cessation of metamorphism probably occupied some 100 Ma rather than 60 Ma as proposed elsewhere.

INTRODUCTION The n o r t h e a s t e r n Yilgarn B l o c k consists o f n a r r o w supracrustal ("greens t o n e " ) belts c o m p r i s i n g m e t a m o r p h o s e d volcanic and h y p a b y s s a l rocks, *Postal address: Geological Survey of Western Australia, 66 Adelaide Terrace, Perth, W.A. 6000, Australia.

158 felsic volcaniclastic sedimentary rocks and minor pelitic and iron-rich sedimentary rocks of volcanogenic origin. Polymictic clastic rocks are rare and appear to be better developed in the stratigraphically higher parts of more expansive supracrustal terrains such as in the Leonora-Laverton and Kalgoorlie areas (McCall et al., 1970; Glikson, 1971; Williams, 1974). Poorly known granitoid and gneissic terrain intervenes between the supracrustal belts which are conspicuously oriented north-northwest (Fig.l). Although most sedimentary rocks in Archaean regions are individually closely comparable compositionally and texturally with younger counterparts (e.g., Pettijohn, 1972), it is widely recognised that the relative proportions of the different sedimentary types making up the Archaean record may be characteristic. Orthoquartzites, K-rich pelites, limestones and microclinebearing arkoses are considered to be rare in the Archaean (Engel and Engel, 1964; Engel et al., 1974), whereas sodic greywackes, pelites and tuffaceous arenites are abundant. In this context the Na-rich clastic metasedimentary rocks at Jones Creek--Kathleen Valley in the Mt. Keith supracrustai belt (Fig.l) are unusual because the arkoses contain considerable microcline and amounts of K20 comparable with average arkose (Pettijohn, 1963, p.15), despite their sodic nature. The distinctive geochemistry of the Jones Creek-Kathleen Valley clastic rocks can be explored in relation to possible source rock geochemistry and conditions of deposition and diagenesis, before recrystallisation to low amphibolite facies mineral assemblages t o o k place. This is possible because regional metamorphism was largely isochemical as metamorphic differentiation and metasomatism were minor in extent. The purpose of this paper is (a) to present geochemical data on an unusual suite of Archaean clastic rocks, and (b) to add weight to geological evidence (Marston and Travis, 1976) that these rocks are unlikely to separate stratigraphically older and younger supracrustai sequences in this area, as has been proposed by Durney (1972), Glikson and Lambert (1973, 1976), Burt and Sheppy (1975) and Roddick et al. (1976). A re-interpretation of the Rb-Sr radiometric data of Roddick et al. (1976) is also offered, that proposes a more extended and geologically more feasible period of crustal evolution for the Mt. Keith supracrustal belt at this locality.

Fig. 1. A. Archaean blocks of Western Australia and location of Mt. Keith supracrustal belt. B. Mt. Keith supracrustal belt and known extent of the Jones Creek Conglomerate. C. Interpreted solid geology of the Jones Creek Conglomerate and adjacent rocks, Kathleen Valley, Jones Creek area. 1, Meta-arkose and granitoid clast arkosic matrix metaconglomerate; 2, granitoid clast amphibolite matrix metaconglomerate; 3, mafic to ultramafic clastic schists and amphibolites, minor arkose; 4, mafic to ultramafic igneous rocks within the Conglomerate; 5, adamellite and granodiorite, some porphyritic; 6, tholeiitic metabasalt, dolerite, gabbro and anorthositic gabbro; 7, mafic to ultramafic igneous rocks, felsic volcaniclastics, black slate; 8, unconformity, tectonically modified; 9, fault or flattening zone; 10, trend line.

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160

Fig.2. Na20 versus K20 versus Fe203 t + MgO plot for Jones Creek Conglomerate and associated (source) rocks, compared with clastic and igneous rocks from other Archaean and younger terrains. Jones Creek rocks (Tables I and II) indicated by dots (arkose), triangles (mafic arkose), crosses (para-amphibolite and ultramafic schist); full squares (adamellitegranodiorite); ThB = metabasalts; Gb = anorthositic metagabbro; U = metaperidotite. Other rocks are: 1, average Columbia River sand (Whetten et al., 1969); 2, average greywacke (Pettijohn, 1963); 3, average arkose (Pettijohn, 1963); 4, average lithic arenite (Pettijohn, 1963); 5, average Harz greywacke (Pettijohn, 1963); 6, average Kalgoorlie Archean metagreywacke (Glikson, 1971); 7, average Wyoming Archaean metagreywacke (Condie, 1967); 8, average Sheba Formation (Fig Tree Group) Archaean metagreywacke (Condie et al., 1970); 9, average of 9 Archaean Rhodesian greywackes (Phaup, 1973); 10, average Archaean Superior Province metagreywacke (Goodwin, 1972); 11, average of 6 Archaean Rhodesian meta-arkoses (Phaup, 1973); 12, average of 5 Proterozoic Rhodesian meta-arkoses (Phaup, 1973); 13, average granodiorite (Nockolds, 1954); 14, average of 6 leucotonalite gneisses, Ancient Gneiss Complex, South Africa (Hunter, 1975); 15, average of 4 leucotonalites, diapirs in Ancient Gneiss Complex (Hunter, 1975); 16 Mt. Keith Granodiorite, analysis 20, Table I; 17, Mt. Falconer Adamellite, analysis 21, Table I; 18, (only pfotted on Fig. 3), average orthoquartzite (Pettijohn, 1963). The subdivisions into fields of sodic and potassic sandstone and ferromagnesian potassic sandstone are after Blatt et al. (1972, p. 318). Gw = greywacke field.

GEOLOGICAL SETTING T h e J o n e s C r e e k - - K a t h l e e n Valley area is s i t u a t e d in a n a r r o w p o r t i o n o f t h e Mt. K e i t h s u p r a c r u s t a l b e l t ( F i g . l B ) , w h e r e felsic t o u l t r a m a f i c r u d a c e o u s m e t a s e d i m e n t a r y r o c k s ( J o n e s C r e e k C o n g l o m e r a t e o f D u r n e y , 1 9 7 2 ) unc o n f o r m a b l y overlie a sodic a d a m e l l i t e - - g r a n o d i o r i t e p l u t o n (Western adamellite) and n o r t h e a s t - s t r i k i n g m a f i c t o u l t r a m a f i c m e t a - i g n e o u s r o c k s (Western supracrustals) o f t h e w e s t e r n section o f t h e belt. A m o r e diverse a s s e m b l a g e of north- northwest-striking mafic to ultramafic rocks interspersed within felsic volcaniclastic m e t a s e d i m e n t a r y r o c k s ( E a s t e r n supracrustals) f o r m s the eastern s e c t i o n o f t h e belt. Hence, s o u t h o f J o n e s Creek, t h e C o n g l o m e r a t e o c c u p i e s t h e central z o n e o f t h e b e l t ( F i g . l C ) . Major strike faults and vertical f l a t t e n i n g z o n e s t y p i f y t h e e a s t e r n s e c t i o n b u t also o c c u r in t h e west, converging in a c o m p l e x a r e a (near t h e a b a n d o n e d K a t h l e e n Valley H o t e l ) referred t o as t h e " c e n t r a l m61ange" b y M a r s t o n and Travis (1976). Little def o r m e d intrusive u l t r a m a f i c r o c k s are p r e s e n t h e r e within t h e C o n g l o m e r a t e . H e t e r o g e n e o u s l y d e v e l o p e d p e n e t r a t i v e d e f o r m a t i o n a n d i n c o m p l e t e recrystallisation have a l l o w e d e x c e l l e n t p r e s e r v a t i o n o f p a r t s of t h e u n c o n f o r m i t y a n d p r i m a r y s e d i m e n t a r y and igneous t e x t u r e s in t h e s u p r a c r u s t a l r o c k s ( D u r n e y , 1 9 7 2 ; M a r s t o n a n d Travis, 1976). In c o n t r a s t t h e s t r a t i g r a p h i c rel a t i o n s h i p b e t w e e n t h e C o n g l o m e r a t e a n d t h e E a s t e r n s u p r a c r u s t a l s is obs c u r e d b y i n t e n s e d e f o r m a t i o n and the p r o b l e m is c o m p o u n d e d b y p o o r outc r o p in t h e c o n t a c t zone.

161

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PETROGRAPHY

OF THE CONGLOMERATE

Most of the clastic sequence is made up of metaconglomerate containing granitoid and mafic igneous clasts identical to rocks in the adjacent metaigneous terrain, enclosed by a poorly sorted, arkosic to amphibolitic matrix consisting of variable proportions of angular quartz, plagioclase and microcline, plus biotite, muscovite, hornblende, clinozoisite, diopside and carbonate. Arkose, para-amphibolite and magnesian amphibole--talc--chlorite schist with sparse clasts are less common. Mafic arkose completes the spectrum of

162

clastic compositions. In all rock types a range of recrystallisation textures from static (e.g., a rock which can be accurately termed a meta-arkose) to dynamic (e.g., siliceous blastomylonite) is observed. However, randomly oriented crystalloblasts of amphiboles and micas in particular, are commonly superimposed on both static and dynamic fabrics. Critical mineral assemblages indicate that greenschist-amphibolite transition facies regional metamorphic conditions prevailed (Marston and Travis, 1976). The Western adamellite (Mt. Keith Granodiorite) is also metamorphosed and its petrology is described in detail by Roddick et al. (1976). I

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Fig.3. A. Na20 versus Al203 plot for the Conglomerate and other rocks. Line B to C represents c o m m o n trend towards increasing maturity in clasticsedimentary environments from g~eywacke through lithicarenite to orthoquartzite. B. I ~ O versus A1203 plot for the Conglomerate and other rocks. Symbols and numbers on both plots are as for Fig.2.

163

ALTERATION FOLLOWING DEPOSITION OF THE CONGLOMERATE Before detailing and discussing the geochemistry of the Conglomerate in terms of likely source rocks it is necessary to demonstrate that the chemistry of the clastic rocks has remained essentially unchanged since deposition. The relevant evidence is of two kinds: chemical and mineralogical. Depletion of incompatible elements in rocks by metamorphic purging is a feature of highly metamorphosed terrains (e.g., Sheraton et al., 1973} but the plot of K versus Rb (Fig.4} indicates that the rocks in the study area have not been so affected. Consistency in geochemistry for rocks of similar mineralogy is to be expected if isochemical metamorphism was operative. In general this is so (see Table I), the exceptions being variable Sr (Fig.5B} and Ca, especially in the mafic clastic rocks, and more rarely variable alkalis. Reversed Na/K ratios coincident with high Y contents indicate that two arkoses (analyses 6 and 11 in Table I) may have been partly open systems geochemically. Arkose number 11 also has unusually high TiO2, FeO and MgO. Variable A1203/K20 ratios for the arkose in general suggests some selective loss of K (Fig.3B). Mineralogical evidence derives from the Conglomerate, the Western adamellite, and adjacent supracrustal rocks. The composition and structural state of feldspars from the basal arkose and the Western adamellite are indistinguishable. Roddick et al. (1976, p.70) note that the coincidence of plagioclase separates with whole rock Rb-Sr isochrons from the Western adamellite (Mt. Keith Granodiorite) indicates that this mineral has remained a closed system since magmatic crystallisation. Sodic igneous plagioclase and derived clastic plagioclase have evidently survived the low amphibolite facies regional metamorphism imposed on the granitoid and supracrustal rocks alike. Idiomorphic calcic plagioclase occurs in metabasalts and metagabbros of the Western supracrustals and brown coloured, elongate skeletal olivine is present in spinifex-textured metaperidotites of the Eastern supracrustals. These minerals, particularly the olivine, are interpreted as relict igneous phases by Binns et al. (1976) in a general study of Archaean metamorphism in the Yilgarn Block. This provides evidence, in addition to the feldspar in the arkose and granitoid, that certain metamorphic reactions were inhibited during the accumulation and subsequent regional metamorphism of the supracrustal rocks. It is concluded that the post-depositional chemical modification of the Conglomerate has been largely restricted to hydration, addition of small amounts of CO2 and limited mobility of Ca, Sr and more rarely alkalis. Open system behaviour seems more prevalent in the mafic clastic rocks perhaps because these rocks contain more metamorphically generated minerals (amphiboles, diopside, zoisite, calcite) than the arkose. Diagenetic reactions are common in some greywacke basins (e.g. Reimer, 1972} and likewise the reactive clastic mineral mixtures resulting in the mafic arkoses and para-amphibolites of Jones Creek would be expected to respond more readily to diagenetic alteration.

140

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Fig. 5. A. Ni versus Rb plot for the Conglomerate and associated rocks, with fields occupied by Archaean metagreywackes from the Sheba Formation, South Africa (Condie et al., 1970) and Wyoming (Condie, 1967) indicated by stippled areas. B. Rb/Sr versus Rb plot for the Conglomerate and associated rocks plus the greywacke fields as for A. The " c o m m o n igneous rock trend" encompasses the axial parts of the compositional fields of ultramafic rocks, submarine tholeiites, andesites and low-Ca granites (after Condie et al., 1970, p. 2768). Symbols on both plots are as for Fig.2 and stars are mafic igneous rocks (W. supracrustals).

8.21 335 215 400 13 120 50 209 14 67

0.24 0.97 141 1.43 7.3

Rb/Sr Sr/Ba K/Rb Na/K Al;O3/Na20

97.63

Total (volatile free)

Fe203t Ni Ba Cr Li V Rb Sr Y Zr

53.49 0.44 10.26 1.15 6.53 0.22 13.92 11.62 1.41 0.88 0.07

SiO: TiO 2 Al203 Fe203 FeO MnO MgO CaO Na;O K20 P2Os

0.75 0.30 173 0.24 30.7

9.82 215 385 305 33 270 88 117 17 96

97.33

53.27 0.92 15.77 1.38 7.85 0.14 6.26 11.68 0.51 1.90 0.31

0.42 0.93 150 0.40 18.1

8.69 520 185 265 20 150 73 172 15 60

95.86

51.78 0.53 10.94 1.58 6.81 0.24 13.32 12.74 0.61 1.37 0.08

9.38 32.5

0.12

10.36 1250 190 1165 4 ]40 1 23 19 20

92.25

51.60 0.38 7.40 1.51 8.35 0.18 24.16 5.68 0.23 0.02 0.00 93.70

54.02 0.19 5.27 1.00 7.81 0.15 23.72 7.69 0.11 0.00 0.02

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Ultramafic para-schist

Para-amphibolite

99.11

68.00 0.39 13.76 1.27 3.48 0.08 2.53 6.45 1.35 2.59 0.08

1.13 0.33 156 0.47 10.2

5.09 50 365 60 11 95 138 122 29 237

6

98.32

72.27 0.22 15.39 0.26 2.28 0.06 0.84 6.36 1.50 0.75 0.05

3.84 0.08 125 1.79 10.2

2.72 36 155 50 143 60 50 13 7 148

7

Mafic meta-arkose

99.31

74.03 0.17 14.98 0.99 0.94 0.04 0.70 2.04 4.25 1.80 0.05

0.10 0.69 256 2.11 3.5

2.01 5 845 20 22 25 58 581 6 110

8

Meta-arkose

98.87

71.40 0.22 15.86 0.64 1.02 0.03 0.95 3.12 4.39 2.30 0.08

318 1.71 3.6

0.06 0.52

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9

99.25

75.50 0.12 14.98 0.45 0.16 0.02 0.24 1.17 3.93 3.38 0.04

0.20 0.46 264 1.04 3.8

0.63 5 1155 15 76 35 105 529 5 81

10

1.64 0.18 204 0.10 37.5

3.04 5 475 15 24 35 138 84 40 319

98.63

74.29 0.57 13.70 0.65 2.19 0.03 1.42 3.21 0.37 3.43 0.15

11

0.08 1.21 255 3.06 3.5

1.70 22 420 35 18 30 43 508 3 85

99.09

72.07 0.15 15.99 0.05 1.51 0.05 0.68 3.50 4.60 1.34 0.05

12

0.14 0.49 302 1.32 3.1

1.53 18 1300 160 2O 2O 88 640 6 111

98.28

72.96 0.19 14.94 0.25 1.18 0.03 0.38 2.00 4.76 3.24 0.06

13

Major e l e m e n t (wt. %, recalculated volatile free) and trace e l e m e n t ( p p m ) analyses of rocks from the J o n e s Creek Conglomerate, and of rocks marginal to the C o n g l o m e r a t e

TABLE I

6.93 15.4

0.02 2.62 130 22 9.21

10.45 425 60 560 15 175 3 157 16 75

99.13

51.37 0.63 13.84 1.20 8.41 0.17 9.23 13.45 1.50 0.06 0.14

--

--

6.26 9.58

0.57

16.60 85 180 20 10 360 1 102 36 103

98.18

50.92 1.56 12.98 8.25 7.87 0.29 5.00 11.46 1.35 0.19 0.14

0.06 7.34 98 12.0 11.32

6.41 78 35 40 83 125 16 257 16 39

99.07

49.07 0.68 24.46 1.67 4.37 0.12 3.35 13.89 2.16 0.16 0.08

0.29 0.43 194 1.38 3.14

1.93 17 1030 30 46 25 129 448 13 153

98.08

72.38 0.28 14.83 0.67 1.14 0.04 0.60 2.16 4.73 3.07 0.09 97.32

74.76 0.15 14.59 0.37 0.79 0.03 0.37 1.32 3.60 3.96 0.06

0.18 0.56 297 0.81 4.06

1.24 13 1100 20 12 25 108 611 8 121

19

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.

1.30

1.17" 0.03 0.40 1.37 4.72 3.44 0.05

0.18 291 1.23 3.09

--

98 553

--

--

--

--

73.91 0.18 14.59

20B

237 1.35 3.30

0.22

145

105 485

33

1135

2.22

99.45

72.35 0.25 15.08 0.90 1.20 0.05 0.35 2.06 4.57 3.02 0.14

21

Fe203 t = total Fe expressed as Fe20 r Rocks from eastern contact of the Conglomerate are designated*. Analysis of SiO2, TiO 2, AI~O 3 total Fe, M n O , MgO, CaO, K ~ O and P20~ by X R F method of Norrish and Hutton (1969), F e O by titration, Na20, Ba, Ni, Cr, and V by AAS, Li by flame emission spectroscopy and Rb, St, Y and Zr by X R F using mass absorption coefficients of Reynolds (1963). Analyst: R.F. Lee. 14, spinifex textured meta-peridotite, Eastern supracrustals; 15 and 16, tholeiiticmetabasalts, Western supracrustals; 17, anorthositic metagabbro, Western supracrustals; 18, porphyritic Western adamellite; 19, aphyric Western adamellite; 20, average of 9 X R F analyses of" Mt. Keith Granodiorite" (Roddick et al., 1976; Table If); 2 0 A average of nos 4318, 831, 832, 834, 835 and 837; 20B, average of nos 827, 828, 840 and 842 (Roddick et al.,1976 Table II); 21, average of 2 analyses of "Mt. Falconer Adamellite" (Bunting and Williams, 1976). *Total iron as FeO, columns 2 9 A and 20B.

--

2.80

--

Rb/Sr Sr/Ba K/Rb Na/K Al2OJNa20

Ni Ba Cr Li V Rb Sr Y Zr

11.00 1660 15 1075 13 105 1 42 5 13

90.71

Total ( v o l a t i l e free)

F%O3 t

44.96 0.24 5.25 3.35 7.98 0.20 31.84 5.78 0.34 0.04 0.02

SiO7 TiO~ A1203 F%O~ FeO MnO MgO CaO Na20 K20 P=Os

17

18

16

14

15

A d a m e l l i t e -- granodiorite

Supracrustal rocks

168

GEOCHEMISTRY Geochemical data on the Jones Creek Conglomerate, the Western adamellite (Mt. Keith Granodiorite of Roddick et al., 1976; Mt. Falconer Adamellite of Bunting and Williams, 1976) and selected rocks from the Western and Eastern supracrustals are presented in Table I and Figs.2--5. Analyses have been recalculated volatile-free to allow more equable comparison. The range of clastic compositions from ultramafic to felsic and the dominant sodic nature of the Conglomerate are well summarised by Fig.2 on which the compositions of average sedimentary rock types (numbered 1--12) are added for comparison. Consistent A1203/Na20 ratios (Fig.3A) indicate that high Na is a primary feature of the Jones Creek arkose. The only rocks from Jones Creek comparable on Fig.2 with common Archaean and younger clastic rocks are some of the mafic arkoses. The Jones Creek arkoses are far more sodic than greywackes (Archaean and younger ones) and any available Precambrian arkose analyses, though data on the latter are meagre. The only igneous rocks plotted on Fig.2 of comparable composition to the various components of the Jones Creek Conglomerate are Archaean leucotonalites from South Africa (14, 15), the Western adamellite or its equivalents (full squares), and basalt or peridotite. High Rb content (>40 ppm) typifies all the Jones Creek clastics (Fig.4), except for the ultramafic para-schist. Low Sr contents in the para-amphibolite and mafic arkose are responsible for the high Rb/Sr ratios in these rocks: selective loss of Sr may be the ultimate cause (Fig.5B). High Rb/Sr ratios distinguish the para-amphibolite from local ortho-amphibolite but in this case the very low Rb content of the ortho-amphibolite is the important factor. Para-amphibolite is also characterised by high Ni ( > 2 0 0 ppm, Fig.5A), Cr and V contents, with the highest values for these elements being found in the ultramafic para-schist. Towards the eastern margin the Conglomerate is characterised by more mafic rocks. Samples 1 and 3 have Ni and MgO contents roughly double those of para-amphibolites further west (2, Table I). Mafic arkose contains amounts of Ni, Cr and V intermediate between para-amphibolite and arkose. Analyses numbered 14--21 (Table I) are of rocks marginal to the Conglomerate. The spinifex-textured metaperidotite (14) is a typical c o m p o n e n t of an ultramafic unit which occurs extensively along the eastern contact of the Conglomerate. Low-K tholeiitic metabasalt and metagabbro (including anorthositic varieties) make up the bulk of the Western supracrustals. Analysis 15 is probably most typical of the rocks in immediate contact to the west with the Conglomerate, the high content of incompatible elements in analysis 16 suggests it is an unusually fractionated tholeiite. The Western adamellite (18-21, Table I) can be divided into two chemical groups: (a) analyses 18, 20A and 21 and (b) analyses 19 and 20B, characterised by higher SiO2, K20 and K/Rb and lower Rb/Sr, TiO2, MgO and Fe203 t representing a more fractionated type than (a).

169

DISCUSSION

A range of clastic compositions from ultramafic para-schist to arkose is apparent. The "end members" of this range, ultramafic para-schist, para-amphibolite and arkose, reflect the composition of their source rocks (peridotite, basalt and adamellite respectively) in terms of most major and trace elements (Figs.2 and 5A, Table I). Intermediate compositions represented by granitoid clast-amphibolite matrix conglomerate and mafic arkose reflect a mixed provenance. This is comparable with many greywackes, as for example the Sheba greywackes of South Africa (Condie et al., 1970; Hunter, 1974, p.280). For the arkose, the geochemistry confirms the evidence from contained clasts and matching feldspars that the Western adamellite was a major source rock for it (and for the granitoid conglomerate). However, away from the contact with the Western adamellite foliated and gneissic clasts have been noted from the granitoid conglomerate. The nearest present-day source for such rocks would be a zone of deformed granitoid and gneiss immediately east of the Mt. Keith supracrustal belt. On the basis of AI203/Na20 ratio, which Pettijohn (1957, p.509) suggested as an index of sandstone maturity, arkose appears far more immature than average arkose but is comparable with the Rhodesian arkoses and the Kalgoorlie greywackes (Fig.3A). The high original Na content of the Jones Creek and other Archaean arenites is probably responsible for this contrast with average arkose which is largely based on Phanerozoic examples. Immaturity is confirmed by the poorly sorted, angular clastic texture and sedimentary structures of the arkose and associated conglomerate (Marston and Travis, 1976) and by the irregular nature of the unconformity (Fig.lC). All these factors point to the very local derivation of much of the arkose and granitoid conglomerate. Some arkoses bear the imprint of a small mafic clastic c o m p o n e n t in their high Ni and Cr contents (6, 7, 12 and 13, Table I). In addition to the presence of recognisable basalt, gabbro and rare ultramafic clasts, the high Ni, Cr and V contents of para-amphibolite and ultramafic para-schist indicate mafic to ultramafic igneous sources, supplemented by granitoid sources which accounts for a small admixture of high Rb, arkosic detritus as well as rare granitoid pebbles. Metaperidotite is rare in the Western supracrustals b u t is c o m m o n in the Eastern supracrustals, particularly close to the contact with the Conglomerate. Ultramafic para-schist is also most c o m m o n adjacent to this contact and there is some evidence suggesting that para-amphibolite here is richer in MgO and Ni than para-amphibolite from the western part of the Conglomerate. An important contribution of ultramafic detritus from sources in the Eastern supracrustals is implied by these observations. Granitoid source areas appear to have been much more important to the west, north of Jones Creek. A southward decrease in granitoid detritus is considered to result from this and the influence of mafic source areas on

170

both sides of the basin (Fig.lC). Deposition by alluvial fans in a graben-like terrestrial environment is envisaged by Marston and Travis {1976, p.153). In summary, the distinctive geochemistry of the Jones Creek Conglomerate is concluded to be dependent on the following: (1) Derivation from sodic adamellite-granodiorite, low-K basaltic, peridotitic, and granitoid gneiss source terrains. (2) Sedimentation in a high energy environment involving little transport and rapid deposition close to source areas, resulting in highly immature clastic rocks. (3) A lack of major post-depositional chemical alteration whether diagenetic or metamorphic. C R U S T A L E V O L U T I O N O F T H E MT. K E I T H B E L T

Besides providing factual data on an unusual suite of Archaean sedimentary rocks, the geochemistry gives further clues to their provenance and the nature of post-depositional processes affecting them and adjacent rocks. A review of the crustal evolution of the area is apposite, particularly for the following reasons. Firstly, Roddick et al. (1976) have recently published the results of Rb-Sr geochronological studies at Jones Creek, b u t their suggestion of a period of only 60 Ma, encompassing emplacement and unroofing of the sodic granitoid pluton, subsequent supracrustal rock accumulation, tectonism and regional metamorphism, is questionable. Secondly, the conclusion that the Conglomerate is older than the Eastern supracrustals and therefore stratigraphically separates two supracrustal sequences (Durney 1972; Glikson and Lambert, 1973, 1976; Butt and Sheppy, 1975; Roddick et al., 1976) is not consistent with the evidence presented here. Geological and geochemical evidence on crustal evolution are combined with the radiometric data of Roddick et al. (1976) in Table II. The Western adamellite (Mt. Keith Granodiorite) yielded a satisfactory isochron of 2689 + 17 Ma, with a low initial STSr/S6Sr ratio (0.70149 + 15), for the emplacement age of the pluton, despite variable metamorphic recrystallisation and deformation. Roddick et al. (1976) did not distinguish different phases within the pluton: these clearly exist as discussed above (Table I), but would appear to be essentially coeval. The geological and geochemical evidence for the provenance of the Jones Creek Conglomerate and the structures within the Conglomerate and Eastern supracrustals (Marston and Travis 1976, p.154), demonstrate that this age places an older limit on the Conglomerate but probably not on the Eastern supracrustals ("upper greenstone") as concluded by Roddick et al. (1976, p.72). A younger time limit for the Conglomerate is defined by the onset of prograde metamorphism.

*Radiometric data from Roddick et al. (1976)..

emplacement of lepidolite pegmatites, aplites and small granitoid intrusion

aplite, pegmatite, quartz veins

intrusion of ultramafics into Conglomerate?

minor mafic and felsic vulcanicity?

deposition of Jones Creek Conglomerate in graben-like basin

felsic volcaniclastic sedimentation ( E supracrustals)

emplacement of mafic to ultramafic volcanics and gabbroids (W and E supracrustals)

intrusion of granitoid pluton and cogenetic aplite and pegmatite veins (W adamellite)

Sedimentary events

Igneous events

uplift, cooling

tilting of unconformity, deformation of Conglomerate, other supracrustals and of pluton

periodic uplift of source areas

uplift and unroofing of pluton, uplift and tilting of supracrustals

Tectonic events

retrogressive metamorphism

prograde regional metamorphism, outlasts penetrative deformation

(absence of burial metamorphism)

Metamorphic events

Summary crustal evolution of the Mt. Keith supracrustal belt, Jones Creek--Kathleen Valley area

TABLE II

2540 ± 12 (aplite veins) 2535 ± 18 (pegrnatite veins)

2620 ± 20 (aplite cobbles) 2590 ± 18 (microcline ages?)

2689 -+ 1 7 (pluton)

Rb-Sr isochrons (Ma)*

b.a --3

172

Roddick et al. (1976, pp. 71--72) conclude that metamorphism had ceased by 2622 Ma on the basis of the ages of biotites from the pluton, and thus infer that the whole rock ages of 2620 Ma from aplite cobbles in the Conglomerate and 2540 Ma from aplite veins in the pluton are too low. However, in view of the fact that plagioclase separates from the pluton lie on the 2689 Ma isochron and that the K feldspar separates have younger ages (2590 Ma) than the biotites, the selection of the biotite ages as defining the close of metamorphism is open to question. Furthermore the pluton is only weakly deformed and recrystallised at distances greater than about 1 km from the Conglomerate contact. This suggests that metamorphic grade and/or deformation facilitating recrystallisation decreased rapidly westwards. The biotites used for age determinations by Roddick et al. (1976) were from samples 2.5--6 km west of the contact, therefore their ages may represent incomplete resetting by weak metamorphism and/or deformation. The biotite ages could represent in part the age of uplift--cooling following emplacement of the pluton. As K feldspar is c o m m o n l y more retentive than biotite, interpreting the mineral ages in terms of overprinting or uplift-cooling is problematical. An age of 2620 Ma is assigned by Roddick et al., (1976, p.71) to two aplite cobbles in the Conglomerate by assuming that their inital STSr/S6Sr is the same as the adamellite. This age could be lowered if the inital ratio were higher than that of the pluton due to resetting during metamorphism. The alternatives to this are to disregard the 2620 age on the basis of the biotite ages, as Roddick et al. (1976) do, or to assume that 2620 is an uplift ~cooling age. The latter interpretation requires very stable crustal conditions for some 70 Ma following the emplacement of the Western adamellite before deposition of the Conglomerate commenced. Geochronological and geological data from the rest of the eastern Yilgarn indicate that the period 2700 --2600 Ma was one of major crustal instability and thermal activity involving major batholithic emplacement, tectonism and regional metamorphism (Arriens, 1971; Archibald et al., 1978). The importance of major strike oriented tectonic dislocations in the development of the Mt. Keith belt in general (Marston and Travis, 1976) also militates against such a prolonged stable phase in crustal evolution. Therefore the aplite cobbles probably have a metamorphic age which is no greater than 2620 Ma. The superimposition of static recrystallisation fabrics over earlier dynamic fabrics characterises the Conglomerate and adjacent supracrustal rocks and is indicative of protracted regional metamorphism. Later retrogressive metamorphism is evidenced by the development of chlorite after biotite in the Conglomerate and the Western adamellite (Roddick et al., 1976, p.60) and by lizardite pseudomorphs after metamorphic or relict igneous olivine in metaperidotites (Binns et al., 1976). Regional metamorphism was probably accomplished in less than 50 Ma. Unrecrystallised lepidolite pegmatites place a younger limit of 2535 + 18 Ma on metamorphism. Although the geological

173

relationships of aplite veins dated by Roddick et al. (1976) are unknown, aplites and pegmatites cut the Conglomerate and a small adamellite intrusion is emplaced in the Conglomerate southeast of the Kathleen Valley Hotel (Marston and Travis, 1976 Fig.2). The 2540 Ma age for the aplite veins could be taken at face value, therefore isotopic readjustment of older rocks and minerals may have taken place at this time: it is interesting to note that the three K feldspars can be fitted to an isochron at 2540 Ma with an initial ratio of 0.703. That portion of crustal evolution between the emplacement of the Western adamellite and the cessation of regional metamorphism probably occupied some 100 Ma (Table II). This is comparable with the spread of radiometric ages from better known orogenic belts such as the British Caledonides where the emplacement of pre-metamorphic plutons, regional metamorphism and subsequent cooling occupied at least 110 Ma (Moorbath, 1975). The deposition of the Conglomerate probably took place in less than 40 Ma, allowing for the unroofing of the Western adamellite.

ACKNOWLEDGEMENTS

The analytical work performed by R.F. Lee is gratefully acknowledged. M.P. Gorton, D.I. Groves, R. Davy, R.D. Gee, L.S. Andersen and W. Libby are thanked for their comments and criticism of an early version of the paper.

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