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Chapter 4.2 The Oldest Well-Preserved Felsic Volcanic Rocks on Earth: Geochemical Clues to the Early Evolution of the Pilbara Supergroup and Implications for the Growth of a Paleoarchean Protocontinent

Chapter 4.2 The Oldest Well-Preserved Felsic Volcanic Rocks on Earth: Geochemical Clues to the Early Evolution of the Pilbara Supergroup and Implications for the Growth of a Paleoarchean Protocontinent

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340



Chapter 4.2: The Oldest Well-Preserved Felsic Volcanic Rocks on Earth



4.2-2. REGIONAL GEOLOGICAL SUMMARY

The Pilbara Craton is divided into the 3.53–3.17 Ga East Pilbara Terrane, the 3.27–

3.11 Ga West Pilbara Superterrane, and ∼3.2 Ga Kurrana Terrane, distinguished by unique

lithostratigraphy, structural map patterns, geochemistry and tectonic histories. The early

volcano-sedimentary history of greenstones within the East Pilbara Terrane is described by

the Pilbara Supergroup, which is composed of four demonstrably autochthonous groups,

of which the 3.53 to 3.426 Ga Warrawoona Group is the stratigraphically lowest group.

The felsic volcanic units described here are interlayered with a greater volume of typically

pillowed tholeiitic basalts at the preserved base of the Warrawoona Group (i.e., the Coucal

Formation of the Coonterunah Subgroup), or at a higher stratigraphic level (Duffer Formation) within the lower part of the Warrawoona Group. A detailed account of the regional

geology of these regions is provided elsewhere (Van Kranendonk et al., 2002, 2005, 2006a,

2007a, this volume).



4.2-3. ANALYTICAL METHODS

All rocks sampled have undergone at least lower greenschist-facies metamorphism and

samples in the lower part of the Coonterunah Subgroup have an amphibolite-facies thermal

overprint close to contacts with rocks of the Carlindi Granite Complex. Such processes

can result in mobility of some major elements (particularly Si, Na, K and Ca, but also Fe)

and trace elements (particularly the large ion lithophile elements (LILE)). The rare earth

elements (REE) and high field strength elements (HFSE), however, are generally immobile

under these conditions (e.g., Arndt et al., 2001) and so description of the trace elements

characteristics of the volcanic rocks presented here concentrates mainly on these trace

elements.

Major elements were determined by wavelength-dispersive XRF on fused disks using

methods similar to those of Norrish and Hutton (1969). Precision is better than ±1% of the

reported values. Loss on Ignition (LOI) was determined by gravimetry after combustion at

1100 ◦ C. FeO abundances were determined by digestion and electrochemical titration using

a modified methodology based on Shapiro and Brannock (1962). The trace elements Ba,

Cr, Cu, Ni, Sc, V, Zn and Zr were determined by wavelength-dispersive XRF on a pressed

pellet using methods similar to those of Norrish and Chappell (1977), while Cs, Ga, Nb,

Pb, Rb, Sr, Ta, Th, U, Y and the REE were analysed by ICP-MS (Perkin Elmer ELAN

6000) using methods similar to those of Eggins et al. (1997), but on solutions obtained by

dissolution of fused glass disks (Pyke, 2000).



4.2-4. COONTERUNAH SUBGROUP

Buick et al. (1995), Green et al. (2000), Van Kranendonk (2000) and Arndt et al. (2001)

have described the geology of the Coonterunah Subgroup of the Warrawoona Group from



4.2-4. Coonterunah Subgroup



341



Fig. 4.2-1. Schematic stratigraphic column of the lower part of the Coonterunah Subgroup (Coucal

Formation) in the Pilgangoora area.



along the southern margin of the Carlindi Granitic Complex, where it is best exposed

(Fig. 4.1-2). Although typically dominated by basaltic and gabbroic rocks, andesitic to

dacitic volcanic and volcaniclastic deposits are locally abundant and volumetrically dominate some sections (Fig. 4.2-1). Sampling of these felsic sequences was restricted to the

more massive, coherent, feldspar porphyritic parts of units and to thick units of feldsparphyric vitric tuff. Banded, sorted, graded, brecciated and agglomeratic portions of depositional units were avoided. Sampling traverses were across strike, broadly perpendicular to

bedding, which dips at a moderate to steep angle to the south and is right-way-up. There

was no indication from mapping, or from geochemical trends, of any large-scale structural

repetition of the sequence.



342



Chapter 4.2: The Oldest Well-Preserved Felsic Volcanic Rocks on Earth



The exposed base of the Coonterunah Subgroup includes a unit of interleaved vesicular tholeiitic basalts and thin (1–5 m scale) vesicular and spinifex-textured komatiite flow

units (Fig. 4.2-1). These units are overlain by a thick sequence of locally vesicular and pillowed tholeiitic flows (Fig. 4.2-2(a)) with rare interflow accumulations of carbonate-rich

sandstone and siltstones, chert and quartzite. This lower komatiite-basalt unit comprises

the Table Top Formation and is overlain by the Coucal Formation – a lithologically diverse

sequence of basalt and andesitic to dacitic rocks. The boundary between the Table Top and

Coucal Formations is locally marked by a transition zone in which there occurs geochemical and textural evidence for limited magma mingling and mixing between the tholeiites

and overlying magmas, producing hybrid compositions (Fig. 4.2-2(b)). Tholeiitic basalt

does not reoccur within the Coonterunah Subgroup above this transition zone.

The sequence overlying the transition zone is dominated by andesitic to dacitic volcanic

and volcaniclastic units. These include massive plagioclase porphyry, feldspar-phyric vitric

tuff (Fig. 4.2-2(c)) and, near the top of the unit, multiple ∼5 m thick bed sets of graded fineto coarse-grained volcaniclastic sandstones (Fig. 4.2-2(d)) each capped by a 0.5–1 m thick

layer of silicified ash (banded chert). Two geochemical series of felsic rocks are recognised;

Coonterunah F1 volcanics range from andesite to dacite and dominate the lower half of the

sequence. Coonterunah F2 volcanics range from basalt to andesite and dominate the upper

half.

4.2-4.1. Geochemistry of the Coonterunah Subgroup

4.2-4.1.1. Table Top Formation – basalts and komatiites

The presence of Mg-rich basaltic rocks (MgO up to ∼12 wt%) at the exposed base of the

Coonterunah Subgroup (Table Top Formation) was described by Green et al. (2000). Our

data include a basal unit of interleaved vesicular tholeiitic basalts and thin (1–5 m scale)

vesicular and spinifex-textured komatiite flow units. These are the oldest known komatiites

of the Pilbara Craton. They have SiO2 from 45.5 to 49.4 wt%, MgO from 22 to 30 wt% and

Mg# from 79 to 85 wt% (Table 4.2-1), and have flat normalised trace-element patterns with

values typically between 0.9 to 2.4 times primitive mantle values (∼3 to 6 × chondritic)

and [Ce/Yb]PM ∼0.9 (Fig. 4.2-3). With near-chondritic Gd/Yb and Al2 O3 /TiO2 ratios of

1.1–1.3 and 20–24, respectively, these rocks are Al-undepleted komatiites.

Basaltic rocks in the Table Top Formation are high-Ti tholeiites (TiO2 > 0.8 wt% –

Smithies et al., 2005b) (Table 4.2-1). They show very weakly fractionated mid-oceanic

ridge basalt (MORB)-like trace element patterns which typically vary from 4 to 10 times

primitive mantle values, with [La/Yb]PM from 0.8 to 2.0 (Fig. 4.2-3). Most incompatible trace-element ratios are correspondingly close to primitive mantle values, including

[La/Nb]PM values which range from 0.89 to 1.66. Mg# varies from 69 to values as low as

32 with very little change in SiO2 values, which lie around 50.5 wt%. The most primitive

rocks have Th/Nb and Nb/La ratios slightly lower than primitive mantle values, reflecting a slightly depleted source, but these ratios, and particularly Th/Nb, remain higher than

MORB values. Th/La ratios, which are reliable indicators of crustal input (e.g., Plank,

2005), are low (0.08 to 0.15) and close to primitive mantle values (∼0.12). A single Nd-



4.2-4. Coonterunah Subgroup



Fig. 4.2-2. Vesicular and pillowed tholeiitic flows (a); mafic globules within basalts at the ‘transition zone’ between the tholeiitic basalts and

the F1 series (b); plagioclase porphyry, feldspar-phyric vitric tuff (c); fine- to coarse-grained graded beds within the F2 series (d).

343



344



Chapter 4.2: The Oldest Well-Preserved Felsic Volcanic Rocks on Earth



Table 4.2-1. Representative analyses of komatiites and basalts from the Table Top Formation (Coonterunah Subgroup)

Series



Komatiites



Basalts



Hybrids



Sample No 179757 179756 179755 179763 179766 179769 179770 179811 179746 179776

SiO2

aSiO2

TiO2

Al2 O3

MnO

MgO

Fe2 O3 T

CaO

K2 O

Na2 O

P 2 O5

LOI

Total

Cr

Ni

Sc

V

Rb

Ba

Sr

Th

U

Nb

Hf

Zr

Y

La

Ce

Pr

Nd

Sm

Eu

Gd

Tb

Dy

Ho

Er

Yb

Lu



41.49 43.67 46.64 48.20 48.80 49.34 51.87 46.88 51.49 53.88

45.52 47.00 49.38 51.17 49.67 50.38 52.48 52.55 53.14 55.04

0.22

0.25

0.27

1.33

0.94

0.94

1.19

0.64

1.27

1.09

5.03

5.74

6.48 15.95 13.91 14.64 15.63 12.67 15.50 15.67

0.15

0.14

0.17

0.14

0.26

0.20

0.26

0.20

0.23

0.20

29.95 26.92 22.89

6.74

6.32

8.65

3.77

4.20

4.22

4.38

10.18 10.03

9.76 18.66 14.81 12.88 12.99 10.83 11.50 10.31

2.95

5.24

7.82

1.64 11.72

8.41 10.92

9.16

9.81

9.24

0.01

0.01

0.02

0.22

0.20

0.21

0.38

0.42

0.61

0.58

0.02

0.08

0.17

2.31

2.31

3.46

2.63

3.67

2.76

3.11

0.02

0.02

0.02

0.12

0.06

0.08

0.11

0.05

0.19

0.14

9.71

7.64

5.89

6.17

1.78

2.12

1.19 12.08

3.21

2.15

100.10 100.13 100.10 100.01 100.05 100.06 100.07 100.08 100.03 100.06

5034

4699

3636

309

188

1589

1286

1005

110

94

23

25

27

53

46

100

99

112

339

298

1.7

1.5

2.3

10.6

4.6

25

18

26

24

41

12.0

44.9

22.6

44.8

83.8

<0.1 <0.1 <0.1

0.5

0.3

<0.1 <0.1 <0.1

0.1 <0.1

0.4

0.6

0.7

4.3

2.2

0.3

0.4

0.5

2.6

1.5

10.6

13.8

15.4

86.6

43.7

3.7

4.8

5.6

14.8

17.0

0.12

0.22

0.60

3.58

2.64

1.51

2.20

2.44 11.56

7.62

0.26

0.34

0.40

1.82

1.19

1.27

1.71

2.06

8.92

6.26

0.33

0.63

0.65

2.52

1.89

0.10

0.14

0.28

0.55

0.82

0.55

0.70

0.87

2.75

2.65

0.48

0.10

0.13

0.15

0.53

0.67

0.84

0.98

3.31

2.98

0.16

0.20

0.24

0.75

0.68

0.46

0.61

0.65

2.02

1.91

0.48

0.61

0.71

2.17

1.87

0.11

0.34

0.30

0.08

0.10



424

128

314

156

65

93

44

42

37

298

323

294

5.0

4.4

12.6

50

93

105

88.7

66.8

84.0

0.3

0.9

2.7

<0.1

0.2

0.6

2.5

3.9

3.6

1.7

3.1

2.1

52.5

97.7

64.8

19.5

28.0

22.2

2.78

6.18

8.84

8.50 14.94 17.64

2.22

2.25

1.42

7.01 10.99

9.27

2.31

3.19

2.77

0.90

1.03

0.84

3.14

4.05

3.12

0.58

0.76

0.59

3.55

5.04

3.76

0.83

1.11

0.86

2.30

3.24

2.45

2.40

3.14

2.44

0.39

0.56

0.39



47

35

34

272

26.6

147

178.5

2.4

0.5

8.4

3.3

135.5

29.7

17.09

33.77

4.18

19.20

4.47

1.35

5.20

0.81

5.07

1.09

3.13

2.85

0.49



171

80

26

175

28.7

78

185.9

3.9

1.0

7.5

3.0

125.5

30.7

13.97

26.33

3.27

15.22

3.78

1.10

4.46

0.70

4.26

0.92

2.65

2.62

0.46



4.2-4. Coonterunah Subgroup



345



Table 4.2-1. (Continued)

Series



Komatiites



Basalts



Hybrids



Sample No 179757 179756 179755 179763 179766 179769 179770 179811 179746 179776

K2 O/Na2 O

Mg#

Sr/Y

La/Yb



0.21

85

3.24

0.25



0.12

84

9.35

0.36



0.09

82

4.04

0.85



0.10

42

3.03

1.65



0.08

46

4.93

1.41



0.06

57

4.55

1.16



0.14

37

2.39

1.97



0.11

43

3.78

3.62



0.22

42

6.01

6.00



0.19

46

6.06

5.33



Fig. 4.2-3. Primitive mantle normalised trace element diagram comparing Coonterunah tholeiitic

basalts, komatiites and basaltic members of the F2 series. N-MORB also shown. (N-MORB and

normalisation factors from Sun and McDonough (1989)).



isotopic determination gives an εNd of +1.39, is only slightly below the depleted mantle

value of +2.37 at 3.51 Ga, and is consistent with La/Yb and La/Nb values that bracket

primitive mantle values. These data indicate that the more primitive basalts in the lower

part of the tholeiitic pile underwent very little interaction with felsic crust.

4.2-4.1.2. Coucal Formation – basalts to rhyolites

The transition zone between the tholeiitic basalts and the overlying felsic-dominated sequence includes basalts with higher concentrations of the more highly incompatible trace

elements (Th, U, Nb, Zr and LREE) than the underlying tholeiites (Table 4.2-1). In terms of

all major and trace elements, and trace element ratios, these basalts invariably plot between

the compositional ranges for tholeiites and the interbedded and directly overlying Coon-



346



Chapter 4.2: The Oldest Well-Preserved Felsic Volcanic Rocks on Earth



Fig. 4.2-4. Major element variation diagrams for volcanic rocks of the Coonterunah Subgroup,

Duffer Formation and Panorama Formation. Panorama 1–4 refers to four geographically separated

volcanic centres of Panorama Formation rocks (see Smithies et al., 2007). Outlined field is for the hybrid rocks found near the transition between the Table Top and Coucal Formation of the Coonterunah

Subgroup. Note that SiO2 is calculated on an anhydrous basis (a SiO2 ).



4.2-4. Coonterunah Subgroup



347



Table 4.2-2. Representative analyses of volcanic rocks from the Coucal Formation (Coonterunah

Subgroup)

Series



Coonterunah F1



Coonterunah F2



Adakite



Sample No 179742 179787 179865 179740 179789 179792 179794 179791 179741

SiO2

aSiO2

TiO2

Al2 O3

MnO

MgO

Fe2 O3 T

CaO

K2 O

Na2 O

P2 O5

LOI

Total



51.26

56.18

0.88

15.34

0.14

2.61

9.04

7.19

0.48

4.00

0.17

9.59

100.07



58.28

60.44

0.84

15.70

0.12

2.49

8.17

6.59

0.70

3.69

0.27

3.71

100.06



59.47

60.08

1.24

15.26

0.13

2.93

10.28

5.46

1.37

3.01

0.24

1.03

100.05



67.28

70.74

0.85

15.87

0.01

0.82

0.80

2.32

1.83

4.88

0.18

5.15

100.08



46.47

47.49

2.00

14.97

0.24

7.22

15.35

9.24

0.91

2.30

0.14

2.20

100.01



53.44

56.02

1.43

15.24

0.19

4.27

10.74

5.38

0.54

4.23

0.44

4.82

100.06



54.79

55.55

1.46

15.80

0.24

4.02

11.64

5.79

0.63

4.46

0.56

1.40

100.06



56.60

57.53

1.34

15.95

0.19

3.41

10.37

5.60

0.53

4.67

0.36

1.64

100.04



66.06

69.02

0.48

15.04

0.05

1.14

3.58

3.28

0.90

5.06

0.12

4.48

100.05



Cr

Ni

Sc

V

Rb

Ba

Sr

Th

U

Nb

Hf

Zr

Y

La

Ce

Pr

Nd

Sm

Eu

Gd

Tb

Dy

Ho

Er

Yb

Lu



10

35

21

199

18.8

153

129.5

3.8

0.8

9.2

4.2

170.4

29.8

19.24

37.33

4.46

19.56

4.76

1.18

5.11

0.82

5.05

1.09

3.06

2.86

0.51



43

36

17

107

33.6

185

242.9

4.8

1.0

12.9

5.6

240.3

36.0

33.90

66.15

7.84

32.89

6.80

1.70

6.41

0.95

5.74

1.22

3.38

3.17

0.57



6

15

19

157

42.6

659

194.3

5.1

1.2

11.2

6.2

214.0

33.1

26.96

57.38

7.06

28.05

6.26

1.53

6.10

0.97

5.86

1.28

3.75

3.46

0.56



28

18

25

174

46.5

271

119.5

3.8

0.9

9.3

4.1

177.0

26.0

20.90

38.95

4.55

20.03

4.65

1.22

4.84

0.71

4.32

0.89

2.40

2.14

0.37



145

172

27

174

24.8

94

135.1

0.4

<0.1

8.6

3.2

108.7

27.4

7.62

19.91

3.29

16.45

4.69

1.71

4.98

0.87

5.09

1.04

2.67

2.40

0.40



80

57

20

114

13.1

94

138.3

2.7

0.6

20.9

7.7

308.5

47.7

31.46

71.09

9.59

40.09

9.02

2.49

8.68

1.48

8.48

1.84

5.11

5.04

0.84



61

40

19

102

13.9

118

176.7

3.9

0.8

21.2

8.0

309.7

48.9

51.29

109.50

14.51

57.07

10.99

2.73

9.57

1.55

8.51

1.90

5.17

4.98

0.82



38

45

19

113

11.2

102

156.3

3.1

0.7

18.0

7.3

290.7

43.6

29.87

63.51

8.48

35.26

7.61

2.31

7.84

1.30

7.71

1.65

4.60

4.37

0.71



12

18

7

42

17.3

334

174.3

5.7

1.3

7.9

4.1

190.0

11.5

26.22

44.45

4.78

19.16

3.28

0.90

2.68

0.35

1.89

0.36

0.98

0.98

0.16



348



Chapter 4.2: The Oldest Well-Preserved Felsic Volcanic Rocks on Earth



Table 4.2-2. (Continued)

Series



Coonterunah F1



Sample No



179742 179787 179865 179740 179789 179792 179794 179791 179741



K2 O/Na2 O

Mg#

Sr/Y

La/Yb



0.12

36

4.35

6.73



0.19

38

6.75

10.69



Coonterunah F2

0.45

36

5.87

7.79



0.38

67

4.60

9.77



0.39

48

4.93

3.18



0.13

44

2.90

6.24



Adakite

0.14

41

3.61

10.30



0.11

39

3.58

6.84



0.18

39

15.16

26.76



terunah F1 volcanic series (Fig. 4.2-4). They are almost certainly binary hybrid magmas.

The stratigraphic transition zone in which they occur provides common evidence favouring this interpretation, in the form of globules of basalt in andesite and of andesite in basalt

(e.g., Fig. 4.2-2(b)).

Rocks belonging to the two volcanic series in the upper sequences (Coonterunah F1

and Coonterunah F2 – Table 4.2-2) are not readily distinguishable from each other in the

field, but are compositionally distinct. The Coonterunah F1 rocks are dominantly andesites

with silica values typically between 55 and 65 wt% (one sample at 71 wt%), whereas

Coonterunah F2 rocks range from basalt to andesite, with a silica range of 48 to 58 wt%

(Fig. 4.2-4). The two groups overlap extensively in terms of Al2 O3 , MgO, K2 O and Na2 O.

The Coonterunah F1 rocks straddle the calc-alkaline-tholeiite transition, whereas the Coonterunah F2 rocks appear to be part of a tholeiitic series (Fig. 4.2-4). Both groups are lowto medium-K, with a K2 O/Na2 O range between 0.05 and 0.45, and with no evolutionary

trend to higher K2 O. The Coonterunah F1 rocks, however, differ from the Coonterunah F2

rocks in typically having lower concentrations of TiO2 , Fe (as Fe2 O3 ) and P2 O5 .

A single dacite (69 wt% SiO2 ), not belonging to either group, has compositions typical

of Archean TTG, including low Yb (0.98 ppm) and very high La/Yb (26.7) (Fig. 4.2-5),

reflecting a source with residual garnet.

Basaltic rocks within the Coonterunah F2 series show considerable overlap in major

element compositions with tholeiites in the lower part of the Coonterunah Subgroup, but

range to slightly more evolved members with MgO as low as 2.5 wt% (Mg# ∼ 24) and SiO2

as high as 58 wt%, and with significantly lower ranges in Cr and Ni. Concentration ranges

for Yb also overlap extensively, but the more incompatible trace elements are significantly

enriched in the Coonterunah F2 basalts (Fig. 4.2-3), with very little overlap in La and Th

concentrations. Values of La/Yb for the Coonterunah F2 basalts range from 3.2 to 6.5 (cf.

1.08 to 2.9 for the lower tholeiites), but La/Nb ratios are low (0.89 to 2.06) and similar to

those of the lower tholeiitic basalts (Fig. 4.2-6).



Fig. 4.2-5. Primitive mantle normalised trace element diagram comparing felsic volcanic rocks of

the Coucal, Duffer and Panorama Formations. The average composition of TTG (Martin, 1999) is

also shown for comparison (normalisation factors from Sun and McDonough, 1989).



4.2-4. Coonterunah Subgroup



349



350



Chapter 4.2: The Oldest Well-Preserved Felsic Volcanic Rocks on Earth



Fig. 4.2-6. Trace element variation diagrams for volcanic rocks of the Coonterunah Subgroup, Duffer

Formation and Panorama Formation. Symbols as for Fig. 4.2-4.



The Coonterunah F1 andesites and dacites show wide ranges in La/Nb (1.95–2.72)

and Th/Nb (0.34–0.76) ratios over a narrow range of Nb concentrations (8–13 ppm)

(Fig. 4.2-6). In contrast, Coonterunah F2 basalts and andesites have narrower ranges of

La/Nb (1.35–1.66) and Th/La (0.04–0.19, one value at 0.28), at significantly lower Nb values. They also have generally lower La/Yb ratios (3.2–6.8 cf. 6.6–10.7) but similar Gd/Yb

ratio (1.6–2.5) (Fig. 4.2-6).



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Chapter 4.2 The Oldest Well-Preserved Felsic Volcanic Rocks on Earth: Geochemical Clues to the Early Evolution of the Pilbara Supergroup and Implications for the Growth of a Paleoarchean Protocontinent

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