Geochemistry of Archaean metavolcanic rocks from the Holenarsipur and Shigegudda volcano—sedimentary belts of Karnataka, South India

Precambrian Research, 19 (1982) 119--139 Elsevier Scientific Publishing Company, Amsterdam -- Prmzed in The Netherlands

119

GEOCHEMISTRY OF ARCHAEAN METAVOLCANIC ROCKS FROM THE HOLENARSIPUR AND SHIGEGUDDA VOLCANO-SEDIMENTARY BELTS OF KARNATAKA, SOUTH INDIA

S.A. DRURY Dept. of Earth Sciences, The Open University, Milton Keynes (Gt. Britain) (Received September 22, 1981; revision accepted June 16, 1982)

ABSTRACT Drury, S.A., 1982. Geochemistry of Archaean metavolcanic rocks from the Holenarsipur and Shigegudda volcano--sedimentary belts of Karnataka, South India. Precambrian Res., 19: 119--139. Major and trace element data for ultramafic and mafic metavolcanic rocks from the volcano-sedimentary belts at Holenarsipur and Shigegudda are presented. Although the Holenarsipur belt has been regarded as representing two stratigraphic groups separated widely in time -- the Sargur (pre-3.4 Ga) and Dharwar (post-3.4 Ga) Groups -- and Shigegudda is clearly younger than the 3.4 Ga gneisses, representative samples from all three suites are part of the same geochemical population. This comprises komatiitic to tholeiitic lavas which are genetically related by progressive fractional crystallization of olivine + pyroxene -+ Cr-spinel. Some of the variability in the high-MgO rocks may reflect differential, partial melting of the mantle. There are two sub-populations, separable into light rare-earth-enriched and light rare~earth-depleted, which may reflect different depths of melting of compositionally homogeneous mantle. Many of the geochemical characteristics of the population bear a strong resemblance to modern basic volcanics formed in destructive plate margin environments.

INTRODUCTION T h e H o l e n a r s i p u r s u p r a c r u s t a l belt, l o c a t e d in t h e Hassan District o f K a r n a t a k a State, I n d i a (Fig. 1), has been t h e f o c u s o f c o n s i d e r a b l e geological a t t e n t i o n over a p e r i o d o f m o r e t h a n 80 years (see R a m a Rao, 1 9 6 2 , for details o f early w o r k ) . This interest reflects the relatively well-exposed n a t u r e o f t h e belt, t h e variability o f t h e s e d i m e n t a r y - - v o l c a n i c association w i t h i n it, a n d its c o m p l e x s t r u c t u r a l a n d m e t a m o r p h i c e v o l u t i o n . T h e s t r u c t u r e o f t h e belt has been described in r e c e n t years b y V i s w a n a t h a and R a m a k r i s h n a n ( 1 9 7 6 ) , Naqvi et al. ( 1 9 7 8 ) and C h a d w i c k et al. ( 1 9 7 9 ) . E a c h o f these a u t h o r s has expressed views o n t h e s t r a t i g r a p h y a n d s t r u c t u r e o f t h e belt, and Naqvi et al. ( 1 9 7 8 ) have discussed limited g e o c h e m i c a l data. T h e H o l e n a r s i p u r belt takes t h e f o r m o f a t r i d e n t - s h a p e d b o d y with several c u s p a t e a n d l o b a t e b o u n d a r i e s (Fig. 1), w h i c h is t y p i c a l o f m a n y Ar0301-9268/82/0000--0000/$02.75 © Elsevier Scientific Publishing Company

120 70

80

90

30 2 0 ~ Shlgegudda

oHassan

13°N -

Supracrustal rocks 3.1 Ga trondjhemites I

3.4 Ga gneiss complex *'~-*

Sampling localities 76°E

lOkm t

I

1

I

Fig. 1. Sketch map of Holenarsipur and Shigegudda belts showing sample locations (asterisks).

chaean supracrustal belts (Windley, 1977). The age of the belt is not known, but it is enveloped by tonalitic gneisses dated at 3.4 Ga (Beckinsale et al., 1980) and cut by deformed trondjhemite bodies dated at 3.1 Ga (unpublished Rb--Sr data of G.J. Reeves-Smith and R.D. Beckinsaie, 1981). Although an Archaean age is demonstrated, there are two main views of the age relationships between different components of the Holenarsipur belt. Viswanatha and Ramakrishnan (1976) suggested that there are two distinct stratigraphic units within the belt, separated by a basal conglomerate, which they assigned to a lower Sargur Group and an upper Dharwar group as defined by their view of regional stratigraphic relationships in the Archaean of western Karnataka. The details of this stratigraphic view are presently disputed (Naqvi et al., 1978). One of the main lines of evidence used to support this view is the supposed difference in metamorphic grade between those components designated Dharwar (garnet zone} and those of the Sargur (kyanite--staurolite zone) groups (Viswanatha and Ramakrishnan, 1976).

121 Field work and petrographic studies by the author and his colleagues do n o t support this dual stratigraphic hypothesis. Virtually every lithological comp o n e n t of one 'group' is to be ~ound in the other; both 'groups' have metamorphic assemblages containing garnet, staurolite and kyanite. No angular discordance is associated with the supposed basal conglomerate and the same sequence of structures is c o m m o n to both 'groups' (see also Chadwick et al., 1979). The difficulty in clarifying n o t only the stratigraphy within the belt but also its relationship to the enveloping gneisses lies in the high strains superimposed on the area by complex late-Archaean deformation. The Holenarsipur belt occupies the core of a westward-verging, partly overturned synform, b u t contains evidence of earlier major isoclinal folds which may be nappe-like. Similar rocks are present as narrow belts in gneisses to the east, where they t o o o c c u p y small synforms. These synforms occur within a major N--S zone of high late Archaean shear strain (Drury and Holt, 1980). Consequently, although primary features such as cross-bedded sediments, amygdular and pillowed lavas are found in the Holenarsipur belt, they have been highly deformed and new metamorphic fabrics have formed. Very similar lithologies can be found in the nearby Shigegudda belt (Fig. 1), b u t their strain is very low and the volcanic--sedimentary association lies on t o p of the 3.4 Ga gneisses with clear angular unconformity. My preliminary view is that many of the apparent differences d o c u m e n t e d between different supracrustal belts in Karnataka are reflections of late Archaean strain heterogeneity on a single post-gneiss volcanic--sedimentary association. There is also evidence that large variations in sedimentary and volcanic facies were present during the early evolution of the supracrustal sequences, further complicating stratigraphic analysis. The Holenarsipur metavolcanic samples described here were collected from the left and right bank canal excavations of the Hemavathi irrigation project and are therefore the freshest material available from the area. The Shigegudda samples were collected on a traverse across that belt and are from surface outcrops. At Holenarsipur metavolcanics from both the supposed Sargur and Dharwar groups are represented, so that, together with the Shigegudda data, they provide a limited test of the t w o stratigraphic views. However, that is a secondary objective of this paper. The samples represent a suite of Archaean volcanics ranging from ultramafic to mafic, which provide an o p p o r t u n i t y to examine evidence for the magmatic processes operative in the Archaean evolution of the Indian continental crust. Geochemical features of Archaean volcanic rocks have been compared with those of modern lavas to deduce possible ancient tectonic environments by several authors (e.g., Hart et al., 1970; White et al., 1971; Condie and Harrison, 1976). Much recent work has suggested that geochemical variations among modern lavas, with distinct relationships to plate boundaries, derive from source heterogeneities, variations in degree of melting and fractional crystallization and the influence of vapour phases during magmagenesis.

122

Their association with plate tectonic features is, therefore, not a direct reflection of tectonic processes. Variations in magmagenetic parameters in the past may have operated under quite different tectonic conditions to those of the present. For this reason, and because so little is known about the structure and evolution of the Indian Archaean, no speculation is made about tectonic setting. I shall emphasize the petrogenesis of different rock types, geochemical implications for the Archaean upper mantle and comparisons with other volcanic suites of similar age. ANALYSED SAMPLES All the volcanic rocks have been subject to metamorphism and no primary minerals were found in thin sections. Spinifex textures are reported to have been discovered as pseudomorphs in some of the ultramafic rocks of Holenarsipur (personal communication, S.M. Naqvi, 1979). Amygdular metabasalts have been avoided in sampling. The most common mineral assemblages are (a) in ultramafic rocks: tremolite--actinolite + chlorite + talc + antigorite + chromite + opaques; (b) in mafic rocks: hornblende + chlorite + epidote + plagioclase (An 20--35) + actinolite + garnet + opaques. These assemblages are stable in the lower amphibolite facies. The rocks are all thoroughly altered and, in view of the strong evidence for mobilization of elements such as K, Na, Rb, St, and Ba during alteration (e.g., Frey et al., 1974), these data are presented for completeness only. Some studies (Frey et al., 1974) suggest that rare-earth elements (REE), particularly Ce, can be affected by alteration too, but this generally disrupts chondrite-normalized patterns and regular patterns here are assumed to represent original igneous patterns (Sun and Nesbitt, 1978). Ti, Zr, Y, Sc, Ta, Th, Hf, Cr, Ni and Co are all elements with a high charge/ionic radius ratio and are unlikely to be transported by aqueous fluids unless they contain high activities of complexing agents such as F- (Pearce and Norry, 1979). They are assumed to have remained immobile in the rocks described. Among the major elements, SiO2 and CaO are commonly redistributed during low-grade metamorphism. Consequently only general significance is attached to data for these elements. ANALYTICALTECHNIQUES Except for REE, Sc, Hf, Ta and Th, which were determined by instrumental neutron activation analysis at the Open University (Paul et al., 1975), major and trace element abundances were estimated by X-ray fluorescence spectrometry at the Open University (Link Systems energy dispersive XRF on glass beads for major elements), Birmingham University (Phillips PW 1450) and Nottingham University (Phillips PW 1400). Details of accuracy, precision and limits of detection will be supplied by the author on request.

123 ANALYTICAL RESULTS, COMPARISONS AND DISCUSSIONS Major and trace element analyses for 37 rocks are given in Tables I and II, together with important inter-element ratios. Because of the high variability of SiO2 and CaO in altered rocks, norms have n o t beencalculated.

Major elements The overall affinities of the Holenarsipur and Shigegudda suites are clear on a triangular plot of (Na20 + K20) A: (total iron as FeO) F: (MgO) M (Fig. 2). They form a coherent trend similar to that of modern tholeiitic suites, with moderate iron enrichment, b u t no evidence of any calc-alkaline affinities. The t w o suites are indistinguishable on this basis from Archaean metavolcanics from southern Africa (Hawkesworth and O'Nions, 1977) and Finland (Jahn et al., 1980). Plots of A1203 and TiO2 against MgO (Fig. 3) reveal negative correlations although the scatter is high. These trends imply that the major controlling phases during magmagenesis were magnesian, and probably comprised olivine and orthopyroxene. Data for CaO are t o o scattered to give meaningful trends. The trends intercept the MgO axis at between 35--40% MgO, slightly less magnesian than for southern African Archaean metavolcanics, b u t F

(FeO)

A

(Na20

M

K20 )

(MgO)

Fig. 2. AFM diagram for metavolcanics from Holenarsipur (filled circles) and Shigegudda (open circles).

124 TABLE I Analyses o f w h o l e rocks: X R F data H -- H o l e n a r s i p u r ; S = Shigegudda; TS = talc schist; TAS = tremolite--actinolite schist; A = a m p h i b o l i t e ; n d = not d e t e c t e d ; - n o data. *Total F e expressed as Fe203. S a m p l e No. I12 Locality H R o c k t y p e TS

I269 H TAS

I270 H TAS

I275 H A

I278 H TAS

I505 H TS

I506 H TS

I507 H TS

I526 H A

SiO 2 A1203 Fe20* MnO MgO CaO Na20 K20 TiO2 P205 LOI

48.5 8.0 9.6 0.21 18.1 10.4 0.6 0.01 0.18 0.028 3.0

51.6 7.1 ]0.7 0.22 16.1 11.7 0.7 0.04 0.35 0.072 1.9

46.5 15.6 15.3 0.18 8.1 6.8 2.4 0.88 1.29 0.166 2.6

48.5 9.6 13.1 0.20 14.4 10.3 0.8 0.10 1.44 0.168 1.7

48.7 8.6 8.7 0.19 21.9 8.0 0.7 0.05 0.13 0.017 4.1

49.6 6.0 8.4 0.23 27.2 1.8 0.1 0.02 0.t0 0.008 6.4

51.8 5.4 7.5 0.08 27.6 0.1 0.1 0.01 0.05 0.020 6.8

48.2 15.7 12.6 0.19 6.9 12.2 2.1 0.13 0.71 0.108 2.1

Total

51.4 1.01 11.4 0.15 22.2 10.6 0.3 0.01 0.10 0.027 2.8 100.5

99.0

100.7

99.7

100.4

101.6

100.0

100.1

100.9

Trace e l e m e n t s in parts per million V Cr Co Ni Cu Zn Rb Sr Y Zr Nb Ba La Ce Th

88 2694 101 652 6 41 2 9 8 1] 1 nd 3 9 5

127 2141 81 528 4 105 4 49 11 24 3 nd nd 13 3

101 1950 92 587 ]l 86 3 36 10 42 6 nd nd 5 4

2.2 8

4.2 7

192 91 69 135 40 91 24 115 32 154 6 143 12 23 3

273 1932 85 498 10 100 5 19 22 111 3 19 3 22 5

132 2878 82 627 nd 59 2 4 9 14 2 nd 1 9 nd

64 4974 96 1237 4 90 nd 2 6 12 1 11 nd 36 nd

47 4813 85 1297 nd 64 nd 1 nd 12 nd 13 3 8 2

202 259 75 170 158 89 3 136 16 51 4 12 nd 7 2

Interelement ratios Zr/Y Zr/Nb

1.4 11

4.8 26

5.0 37

1.6 7

2.0 12

---

3.2 12.8

suggesting a source in the mantle. The simplest explanation is differential partial melting of a homogeneous source (Hawkesworth and O'Nions, 1977 ), but this cannot be distinguished from crystal fractionation effects by major elements alone. On a plot of TiO2 against A1203 {Fig. 3) Jahn et al. {1980, Fig. 4) were

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126 TABLE I (cont.) A n a l y s e s o f w h o l e r o c k s : X R F d a t a H = H o l e n a r s i p u r ; S = S h i g e g u d d a ; T S = talc s c h i s t ; T A S = t r e m o l i t e - - a c t i n o l i t e schist~ A = a m p h i b o l i t e ; n d = n o t d e t e c t e d ; - - n o d a t a . * T o t a l F e e x p r e s s e d as F % O 3.

Sample N o . I 5 4 6 Locality H Rock type

A

I563 H A

I564 H A

SiO: A1203 Fe203 MnO MgO CaO Na20 K20 TiO: P205 LOI

47.8 3.4 12.4 0.16 21.7 8.1 0.2 0.02 0.43 0.026 6.3

50.1 13.7 14.9 0.25 5.0 10.3 2.0 0.22 1.10 0.142 2.0

-----1.83 0.44 ----

I581 H A

I590 H A

I591 H A

$373 H TS

$378 H TAS

$424 H TS

51.7 13.3 12.9 0.20 7.6 10.8 2.4 0.26 0.98 0.086 1.7

51.4 14.7 12.5 0.17 7.6 10.1 2.3 0.18 0.86 0.084 0.99

49.9 14.2 12.7 0.18 6.3 10.6 2.2 0.21 1.07 0.119 2.4

51.5 3.8 13.4 0.25 25.7 3.7 0.07 0.02 0.24 0.010 2.0

49.3 7.2 9.3 0.16 21.4 8.9 0.33 0.17 0.23 0.010 3.2

54.5 1.7 15.9 0.34 24.9 0.7 nd 0.02 0.26 0.110 2.2

Total

101.0

99.7

--

101.9

100.8

99.9

100.7

100.1

100.6

V Cr Co Ni Cu Zn Rb Sr Y Zr Nb Ba La Ce Th

102 2521 94 1193 61 76 3 54 7 37 1 7 nd nd 1

263 122 67 116 135 135 5 296 33 140 6 32 16 30 8

242 779 71 217 34 112 16 137 23 84 7 106 9 16 7

321 158 94 133 99 83 9 176 29 76 5 50 6 9 1

232 248 79 143 195 89 6 139 21 66 4 39 4 14 6

232 194 66 132 265 94 5 252 28 85 5 21 1 21 4

-3353 -632 -89 nd 4 5 15 4 10 4 nd nd

-2208 -1102 -68 4 31 6 26 4 32 6 nd nd

-4006 -1088 -119 nd 5 nd 15 7 10 5 nd nd

Zr/Y Zr/Nb

5.3 37

3.0 3.8

4,3 6.5

-2.1

4.2 23

3.7 12

3.0 15.2

3.1 17

3.0 17

Western Australia (Naldrett and Turner, 1977). However, the MgO/FeO ratios cover the range o f both the komatiitic and tholeiitic types of Ja~hn et al. (1980). The trends among the major elements indicate that fractional crystallization may have played an important role in their magmatic evolution, but are unable to indicate whether this reflects one or several primary magmas.

127

$414 H A

$461 H TAS

I260 S A

I261 S TS

I261A S A

I264 S A

I265 S A

I266 S A

48.6 11.8 11.3 0.21 8.8 16.1 1.9 0.2 0.49 0.090 1.1

47.7 5.3 16.2 0.33 17.4 10.0 0.4 0.1 0.43 0.080 3.5

53.3 13.5 13.3 0.21 8.0 8.6 3.23 0.08 0.71

47.7 6.1 11.6 0.18 22.0 7.9 0.5 -0.55

49.6 14.7 13.6 0.20 10.2 7.4 2.6 0.13 1.17 0.087 1.1

52.2 13.9 14.4 0.16 6.4 9.9 2.2 0.15 1.19 0.097 0.6

50.6 11.9 10.9 0.18 10.5 11.0 3.2 0.61 0.90

--12.2 ----

-

-

-

-

0.7

4.0

99.7

101.3

101.8

101.2

335

1182

253

4921

180

374

168

1446

64 nd 96 14 39 6 28 3 8 nd

101 nd nd 13 25 9 12 nd nd nd

.

2.7 6.5

1.9 2.8

4.8 19

4 131 16 77 4 66 nd 17 2

.

. 5 8 11 51 4 40 8 24 nd

4.6 13

100.9

-

-

1.7

1.8 -

-

--

101.2

101.8

-

166

932

210

-

110

202

119

3 120 31 143 13 59 nd 21 3

27 205 17 83 6 138 9 14 nd

3 300 51 213 9 159 24 37 5

. 3 79 26 100 5 ---3

3.8 20

-

-

.

4.6 11

4.8 14

4.2 24

MgO, Cr, Co, Ni I n g e n e r a l , p l o t s o f C r , C o a n d N i a g a i n s t M g O ( F i g . 4) all s h o w d e c r e a s i n g abundances with decreasing MgO. In detail, the Cr data show a flat trend at h i g h M g O w i t h a m a r k e d b r e a k in s l o p e a t a b o u t 1 5 % M g O , N i c k e l d o e s n o t s h o w a clear s t e p p e d t r e n d d u e t o d a t a scatter, b u t a gradual increase in slope from high MgO to low may be inferred. Cobalt shows only a small decrease f r o m 9 0 t o 6 0 p p m w i t h d e c r e a s i n g M g O . T h e r e is a c l e a r c o m p o s i t i o n a l g a p

128 T A B L E II A n a l y s e s o f w h o l e rocks: I N A A data in parts per million Sample No.

I261

I264

I265

I270

I527

I546

I563

$414

$373

$46

La Ce Nd Sm Eu Gd Tb Tm Yb Lu

3.9 8.4 6.2 1.7 0.27 1.9 0.37 0.16 1.05 0.16

1.7 15.3 4.3 1.8 0.86 3.7 0.73 0.60 3.56 0.52

8.3 15.3 11.9 2.9 1.07 3.1 0.56 0.23 1.51 0.20

1.3 4.9 2.1 0.8 0.35 1.1 0.21 0.11 0.91 0.13

3.3 6.3 4.7 1.6 0.69 2.3 0.41 0.26 1.76 0.26

2.4 4.3 3.0 0.9 0.38 1.2 0.20 0.11 0.65 0.10

14.7 32.8 19.2 4.5 1.34 5.0 0.93 0.51 3.30 0.47

3.0 6.6 3.1 1.4 0.58 2.4 0.35 0.28 1.81 0.38

1.4 2.5 2.1 0.9 0.40 1.1 0.17 0.13 0.83 0.19

1.9 5.2 5.2 1.10 0.44 1.7 0.30 0.25 1.70 1.32

Th Ta Hf Sc

0.67 0.21 1.10 24

2.6 0.88 3.60 34

1.31 0.41 1.97 38

0.64 0.29 0.80 24

0.30 0.21 1.1 34

0.17 0.11 0.66 20

2.49 0.60 3.45 32

0.51 0.16 1.16 40

0.25 0.08 0.41 30.3

1.54 0.24 1.12 61

(La/Sm)N (Gd/Yb)N Sc/Hf

1.4 1.4 22

0,58 0.81 9.3

1.8 1.6 19

1.0 0.98 30

1.3 1.0 32

1.7 1.4 30

1.3 1.1 34

1.3 1.1 74

1.1 1.0 55

2.0 1.2 9

16.0

ee•

14.0 12.0 10.0 8.0

AI 0

%

6,0 4.0 2.0 0

1.6 o

1.4

1.4

1.2

1.2 o

1.0

1.0 ee~

TiO:, % 0.8

TiO

0.6

0.6

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%

0.8

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$

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6.0

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16.0

= 5.0

i 10.0

i 15.0

I

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r

20.0

25.0

30.0

0

MgO %

Fig. 3. Plots o f AI203 and TiO 2 versus MgO and T i O 2 versus AI=O3. S y m b o l s as in Fig. 2.

129

5000

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50

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5.0

10.0

15.0

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20.0

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25.0

30.0

MgO dry w e i g h t %

Fig. 4. Cr, Ni and Co versus MgO. Open symbols represent Holenarsipur rocks, closed symbols Shigegudda rock. No Co data for Shigegudda.

between 15 and 10% MgO. These trends are identical to those from Finland (Jahn et al., 1980), southern Africa (Hawkesworth and O'Nions, 1977) and Western Australia (Nesbitt and Sun, 1976). As discussed b y Nesbitt and Sun (1976), Hawkesworth and O'Nions (1977) and Jahn et al., (1980), accounting for trends such as those in Fig. 4 in a consistent manner is complicated by variations in mineral-melt partition coefficients (KD). The inflection in the Cr trend can be explained by increasing control of pyroxene or minor spinel over liquids with less than 15% MgO (Schreiber and Haskin, 1976). Nickel and Co have a KD of less than one in pyroxene and their trends reflect the control of olivine throughout the range. Variation in K~il~-liq from 1--10 in the temperature range 1600-1200°C (Leeman, 1974) can explain the curved trend for Ni by differential partial melting alone (Nesbitt and Sun, 1976), olivine dominating the residue for high degrees of melting and high MgO liquids. This, together with K~ 1-1iq of 0.6 (Akella et al., 1976), enables primary melts to have roughly mantle concentrations of Ni and enhanced Cr compared with the mantle when the residue is almost pure olivine. Failure to consume p y r o x e n e and/or

130

spinel with lower degrees of melting imparts a high bulk KCr to the residue and Cr in the melt falls sharply as the percentage of melting decreases. At the same time, decreasing temperature increases K ~ liq and Ni too falls more sharply in progessively less magnesian liquids. The less extreme fractionation displayed by Co is explicable by the lower r~ol--liq (Leeman, 1974). ~Co The same trends can be explained equally by olivine fractionation at high temperature, followed by olivine plus pyroxene as MgO in the residual liquid falls below 15% (Jahn et al., 1980). There is, therefore, no unambiguous means of isolating melting effects from crystallization using these elements. The trends permit derivation of the suite from a single primary, high-MgO liquid by progressive fractionation of olivine and then olivine + pyroxene -+ spinel, or differential partial melting of a source which is homogeneous with respect to Mg, Cr, Co and Ni. The compositional gap between 15 and 10% MgO coincides with the inflection in the Cr trend and could be used to argue for two parental magmas, one with high MgO from which only olivine crystallized, the other with 10--12% MgO which evolved by fractional crystallizal~on of olivine + pyroxene + spinel_ However, it could equally reflect incomplete sampling of these rather monotonous rocks. An interesting feature of high-MgO rocks is the very high scatter in Ni compared with regular variations in Cr and Co (Fig. 4). For MgO > 20% Ni varies from 600--1500 ppm, thereby combining normal values for komatiites with depleted values. One possibility is that Ni-depleted ultramafic lavas may reflect equilibration with nickeliferous sulphides {Duke and Naldrett, 1978), but their highly altered nature counsels caution. Ti, Zr, Nb and Y

The sensitivity of this group of incompatible elements to different petrogenetic processes, as reflected in modern basalts, has been demonstrated clearly by Pe~ce and Cann {1973) and Pearce and Norry (1979). The Holenarsipur and Shigegudda data are presented as log--tog plots of TiO2, Y and Nb against Zr (Fig. 5) and Zr/Y and Zr/Nb against Zr {Fig. 6), in which there are no distinctive differences between the two belts nor between the two supposedly contrasted components of the Holenarsipur belt. The high Y/Nb ratios and low TiO2 abundances of all the samples indicate a total lack of alkaline affinities (Pearce and Cann, 1973). The TiO2 and Zr values fall in the fields of komaliites (Nesbitt and Sun, 1976) and low potassium tholeiites (Pearce and Cann, 1973). These features appear to be common to all Indian Archaean metavolcanics for which there are suitable data (Ananta Iyer and Vasudev, 1979; Drury, 1981; Bhaskar Rao and Drury, 1982). There are strong positive correlations of TiO:, Y and Nb with Zr (Fig. 5), where all four elements show a roughly negative correlation with MgO concentration. The data for Nb are badly scattered, abundances being close to the detection limit. Like Archaean metabasalts from Western Australia

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crystals, which would cause depletion in Ti, Y and Nb as Zr increased (Pearce and Norry, 1979). Variation of the Zr/Y ratio with Zr {Fig. 6) displays a clear and interesting trend of increasing Zr/Y ratio with increasing Zr, from which only five samples deviate significantly. The field occupied by the Holenarsipur and Shige~dda samples is similar to that for many Archaean komatiitic to tholiitic metavolcanics from Australia (replotted data of Nesbitt and Sun, 1976) and West Greenland (replotted data of McGregor and Mason, 1979). All three suites bear some resemblance in this respect to modern island-arc and mid-ocean ridge basalts (Pearce and Norry, 1979), though no precise tectonic setting is presumed from this. Comparable Indian Archaean metavolcanics from the nearby Bababudan and Kudremukh belts have significantly higher Zr/Y ratios (Drury, 1981; Bhaskar Rao and Drury, 1982), implying relative Zr~enrichment and Y-depletion in their petrogenesis. Pearce and Norry (1979) have modelled carefully the theoretical variations of Zr/Y ratio relative to Zr abundance for various source compositions, degrees and residual mineralogy of source melting and fractional crystallization.The trend exhibited in Fig. 6 coincides very closely at low Zr with the Zr/Y ratio in C3 chondrites as quoted by Pearce and Norry (1979). Since the corresponding rocks have M g O concentrations between 25--30%, they reflect high degrees of mantle melting where ratios betweenincompatible elements are transfe~ed almost unchanged from source to melt. Thus the source

133

mantle for at least the ultrabasic c o m p o n e n t of the suite was chondritic with respect to Zr and Y. Both Zr/Ti and Zr/Nb show similar C3 chondritic values for these rocks, thereby helping confirm the very primitive nature of the source for the Holenarsipur suites. As with the other data discussed, t w o alternative explanations can account for the trend in Fig. 6. Assuming a C3 chondrite source for Zr and Y, the trend is encompassed by t w o genetic pathways for equilibrium partial melting of a source with minerals in the following proportions: olivine: orthopyroxene: clinopyroxene: plagioclase = 60: 20: 1 0 : 1 0 (curve 1) and olivine: orthopyroxene: clinopyroxene: garnet = 60: 2 0 : 1 0 : 10 (curve 2) (Pearce and Norry, 1979, Fig. 4a); that is, by varying degrees of melting and a homogeneous chondritic source at varying depths in the mantle. Part of the trend could be produced by open-system fractional crystallization (O'Hara, 1977) in which the magma chamber composition was periodically reset with new input of primary magma until steady state was achieved, involving separation of minerals in the proportion plagioclase: clinopyroxene: olivine = 50: 30: 20, with 80% of the melt remaining during each cycle, 22% of primary magma being added to this evolved fraction while 2% is erupted (Pearce and Norry, 1979, Fig. 4d). Similar trends but with more rapidly increasing Zr/Y could be developed with plagioclase-free cumulate fractions. Rare-earth elements, Sc and Hf Chondrite-normalized plots of REE (Fig. 7) reveal considerable diversity among the analysed samples. Two samples show aberrant Ce anomalies, suggesting some redistribution via hydrothermal solutions. However, t w o types of REE patterns seem to be present among b o t h ultramafic and basaltic populations. One is flat to depleted in light REE. The other, equally c o m m o n , indicates light REE enrichment. Unlike Archaean metavolcanics from the nearby Kudremukh and Bababudan belts (Drury, 1981; Bhaskar Rao and Drury, 1982), the light REE-enriched patterns do not exhibit flat heavy REE, instead the (La/Sm)N and (Gd/Yb)N ratios are approximately the same. Several samples show Eu anomalies but, in view of the evidence for Ce mobilization in the suite, this could indicate the disturbance of primary patterns by hydrothermal exchange (Hellman et al., 1979; Jahn et al., 1980) and no petrogenetic significance is attached to them. The division into a light REE-enriched group and a fiat to light REEdepleted group is sufficiently clear to suggest that the t w o groups reflect differences in primary magma composition within the Holenarsipur metavolcanics. In b o t h groups there is an increase in total REE as MgO, Cr and Ni decrease, partly confirming that t h e y evolved from primary ultrabasic magmas as a result of olivine + pyroxene + spinel fractional crystallization. On a plot of Ni versus total chondrite-normalized REE, the Holenarsipur samples plot, like the Finnish Archaean komatiites and tholeiites, much

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closer to trends modelled for fractional crystallization than to those for differential partial melting (Jahn et al., 1980). The two-fold division on the basis of light REE abundances may be explainecl in two ways. First, it may reflect two different mantle source regions, one enriched in light REE, the other with more primitive and possibly light REE-depleted REE abundances. Second, it may reflect differences in depth of melting and hence mineralogy of the source mantle. Light REEenrichment may reflect the presence o f garnet in the source (Sun and Nesbitt, 1978; Jahn et al., 1980), which has low ~ E and high K~iaR--~. In this regard it is possible to use the Sc data, since Sc is strongly partitioned into

135 garnet (Frey et al., 1974). Normalizing Sc relative to an element upon which the presence or absence of garnet has no effect, such as Hf, should distinguish magmas whose source was garnetiferous (low Sc/Hf ratio) from those originating in the shallower depths where garnet was not stable (higher Sc/Hf ratios). Inspection of Table II reveals that the light REE-enriched samples have lower Sc/Hf ratios than flat to light REE-depleted samples, although I264 and I546 do n o t obey this general rule. Like Jahn et al. (1980), rather than postulate coeval magmagenesis from two distinct mantle compositions, which is not borne out by other element patterns, m y tentative conclusion is that the REE bear witness to different depths of mantle melting. A garnetiferous mantle source for Archaean metavolcanics is evidenced also by data from the K u d r e m u k h and Bababudan belts (Drury, 1981 ; Bhaskar Rao and Drury, 1982) and, like Holenarsipur, the Archaean Kolar metavolcanics show a similar bimodal set of REE patterns (author's unpublished data on samples of K. Shiv Kumar).

Hf, Th and Ta On a triangular plot of Hf, Th and Ta (Fig. 8) most Holenarsipur metavolcanics, like others from India (Drury, 1981; Bhaskar Rao and Drury, 1982), show depletion in Ta and relative enrichment in Th. Consequently, when compared with modern basalts, they plot within the field of volcanic

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136

arc basalts (Wood et al., 1979). The usefulness of these elements is that Th and Hf are elements of low ionic potential whereas Ta has high ionic potential, and the pattern exhibited may be explained in modern arc and marginal basin basalts by metasomatic transfer of Th and Hf (together with light REE) from a subducted lithosphere to the overlying mantle wedge which forms their source. CONCLUSION The following conclusions are based on the assumption that abundances of the elements discussed have n o t been changed significantly during metamorphism. (1) All samples, the supposed Sargur and Dharwar metavolcanics of Holenarsipur and those of the post-gneiss Shigegudda belt, are a single geochemical population. They bear strong similarities to u n d o u b t e d post-gneiss metavolcanics from Kudremukh, Bababudan and Kolar. This tends to support the view of a single major volcanic association in the Karnataka craton, which varies only in its degree of deformation and metamorphism. (2) Major element data enable the metavolcanics to be classed as a komatiitic--tholeiitic suite, similar to those so c o m m o n in other Archaean cratons. (3) The high Cr content of the ultrabasic rocks (up to twice that in C3 chondrites) does n o t indicate a cumulate origin. The ultrabasics all contain more than chondritic abundances of Ti, REE, Zr, Y, Nb, Sc, Hf and Th, which cannot be envisaged in olivine--pyroxene--spinel cumulates with very low bulk K D for these elements. The high Cr reflects either a high-Cr mantle source or, more simply, a nearly pure olivine residue for which KCr m a y be around 0.6 and from which high degrees of melting may derive magmas with enhanced Cr. The presence of Ni-depleted high-MgO rocks may indicate separation of immiscible sulphide melts from ultrabasic magmas. (4) Data for Cr, Co and Ni permit the derivation of the compositional range either b y different degrees of mantle melting or fractional crystallization from primitive parents. In the latter case a compositional gap may indicate either t w o parent magmas, one peridotite, the other high-MgO basalt, or simply inadequate sampling. Total REE shows a marked increase with decreasing MgO, Cr and Ni, which favours fractional crystallization rather than differential partial melting as the main explanation for the observed compositional variations. However, plots of Zr/Y ratio versus Zr imply some variation in primary magma bulk composition through variable degrees of equilibrium partial melting followed by open-system fractional crystallization of olivine and pyroxene. (5) Ti, Zr, Y and Nb data all indicate that the source for the primary magma(s) was of C3 chondritic composition with respect to these elements.

137 However, there are t w o distinct populations on the basis of REE patterns in ultrabasic and basic samples, one m o d e r a t e l y enriched in light REE, the o t h e r with flat to light REE-depleted patterns. T he Sc and H f data, although s o m e w h at erratic, suggest t h a t rather than there having been t w o separate mantle sources, the light R E E - e n r i c h m e n t t r e n d derives f r o m deep melting of garnet-bearing mantle, whereas the o t h e r samples m ay have melted from a compositionally similar source but at shallower depths. (6) Both populations o f samples exhibit the same Th, Hf and Ta patterns and resemble m o d e r n basalts derived from destructive plate margins, being strongly depleted in Ta. The precise tectonic setting implied by this and o t h e r geochemical features must await a m or e comprehensive study of south Indian Archaean metavolcanics, taking into a c c o u n t details of stratigraphy and structural evolution of the volcanic--sedimentary associations in which t h e y are found. ACKNOWLEDGEMENTS The research was carried out u n d e r an NERC Research Grant ( G R 3 / 3 6 6 5 ) , field w or k being aided by a vehicle purchased by the Open University and in part by facilities provided by NGRI. Six of the rocks were kindly supplied b y Dr. S.M. Naqvi of NGRI. Thanks are due t o P.J. Potts, J. Watson, P.C. Webb, and O. Williams-Thorpe of the Open University, P.K. Harvey o f Nottingham University and B. Weaver of Birmingham University for their analyses of t he samples, to J. T a y l o r for help in drafting the figures and to D. Whyte for preparing t he t y p e d draft.

REFERENCES Akella, J., Williams, R.J. and MuUins, O., 1976. Solubility of Cr, Ti, and A1 in coexisting olivine, spinel and liquid at 1 atm. Proc. 7th Lunar Sci. Conf., 2: 1179--1194. Ananta Iyer, G.V. and Vasudev, V.N., 1979. Geochemistry of the Archaean metavolcanic rocks of Kolar and Hutti gold fields, Karnataka, India. J. Geol. Soc. India, 20: 419--432. Arndt, N.T., Naldrett, A.J. and Pyke, D.R., 1977. Komatiitic and iron-rich tholeiites of Munro Township, Northeast Ontario. J. Petrology, 18: 319--369. Beckinsale, R.D., Drury, S.A. and Holt, R.W., 1980. 3360 M yr old gneisses from the South Indian craton. Nature, 283: 409--470. Bhaskar Rao, Y.J. and Drury, S.A., 1982. Incompatible trace element geochemistry of Archaean metavolcanic rocks from the Bababudan volcanic--sedimentary belt, Karnataka. J. Geol. Soc. India, 23: 1--12. Chadwick, B., Ramakrishnan, M., Viswanatha, M.N. and Srinivasa Murthy, V., 1979. Structural studies in the Archaean Sargur and Dharwar supracrustal rocks of the Karnataka craton. J. Geol. Soc. India, 19: 531--549. Condie, K.C. and Harrison, N.M., 1976. Geochemistry of the Archaean Bulawayan Group, Midlands greenstone belt, Rhodesia. Precambrian Res., 3 : 253--271. Drury, S.A., 1981. Geochemistry of Archaean metavolcanic rocks from the Kudremukh area, Karnataka. J. Geol. Soc. India, 22: 405--416.

138 Drury, S.A. and Holt, R.W., 1980. The tectonic framework of the South Indian craton: a reconnaissance involving Landsat imagery. Tectonophysics, 65: T1--T15. Duke, J.M. and Naldrett, A.J., 1978. A numerical model of the fractionation of olivine and molten sulphide from komatiite magma. Earth Planet. Sci. Lett., 39 : 255--266. Frey, F.A., Bryan, W.B. and Thompson, G., 1974. Atlantic ocean floor: geochemistry and petrology of basalts from legs 2 and 3 of the deep-sea drilling project. J. Geophys. Res., 79: 5507--5527. Hart, S.R., Brooks, C., Krogh, T.E., Davis, G.L. and Nava, D.F., 1970. Ancient and modern volcanic rocks: a trace element model. Earth Planet. Sci. Lett., 10: 17--28. Hawkesworth, C.J. and O'Nions, R.K., 1977. The petrogenesis of some Archaean volcanic rocks from Southern Africa. J. Petrology, 18: 487--520. HeUman, P.L., Smith, R.E. and Henderson, P., 1979. The mobility of the rare~earth elements: evidence and implications from selected terrains affected by burial metamorphism. Contr. Mineral. Petrol., 71: 23--44. Jahn, B.M., Auvray, B., Blais, S., Capdevila, R., Cornichet, J., Vidal, F. and Hameurt, J., 1980. Trace element geochemistry and petrogenesis of Finnish greenstone belts. J. Petrology, 21: 201--244. Leeman, W.P., 1974. Experimental determination of partitioning of divalent cations between olivine and basaltic liquid. Ph. D. Thesis, Univ. Oregon, 231--303 (unpubl.). McGregor, V.R. and Mason, B., 1979. Petrogenesis and geochemistry of metabasaltic and metasedimentary enclaves in the Amitsoq gneisses, West Greenland. Am. Miner., 62: 887--904. Nakamura, N., 1974. Determination of REE, Ba, Fe, Mg, Na and K in carbonaceous and ordinary chondrites. Geochim. Cosmochim. Acta, 38: 757--775. Naldrett, A.J. and Turner, A.R., 1977. The geology and petrogenesis of a greenstone belt and related nickel sulphide mineralization at Yakabinde, Western Australia. Precambrian Res., 5 : 43--103. Naqvi, S.M., Viswanathan, S, and Viswanatha, M.N., 1978. Geology and geochemistry of the Holenarsipur schist belt and its place in the evolutionary history of the Indian Peninsula. In: B.F. Windley and S.M. Naqvi (Editors), Archaean Geochemistry. Elsevier, Amsterdam, pp. 109--126. Nesbitt, R.W. and Sun, S.S., 1976. Geochemistry of Archaean spinfex textured peridotites and magnesian and low magnesian tholeiites. Earth Planet. Sci. Lett., 31 : 433---453. O'Hara, M.J., 1977. Geochemical evolution during fractional crystallization of a periodically refilled magma chamber. Nature, 266: 503--507. Paul, D.K., Potts, P.J., Gibson, I.L. and Harris, P.G., 1975. Rate,earth abundances in Indian kimberlite. Earth Planet. Sci. Lett., 25: 151--158. Pearce, J.A. and Cann, J.R., 1973. Tectonic setting of basic volcanic rocks determined using trace element analyses. Earth Planet. Sci. Lett., 19: 290--300. 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. Rama Rao, B., 1962. A handbook of the geology of Mysore State, Southern India. Bangalore Printing and Publishing, Bangalore, 264 pp. Schreiber, H.D. and Haskin, L.A., 1976. Chromium in basalts: experimental determination of redox states and partitioning among synthetic silicate phases. Proc. 7th Lunar Sci. Conf., 2: 1221--1259. Sun, S.S. and Nesbitt, R.W., 1978. Petrogenesis of Archaean ultrabasic and basic volcanics from rare~earth elements. Contrib. Mineral. Petrol., 65: 301--325. Viljoen, M.J. and Viljoen, R.P., 1969. The geology and geochemistry of the lower ultramafic unit of the Onverwacht Group and a p r o p o s e d n e w class o f igneous rock. Spec. Publ. Geol. Soc. S. Afr.. 2 : 55--85.

139 Viswanatha, M.N. and Ramakrishnan, M., 1976. The pre-Dharwar supraerustal rocks of the Sargur schist complex in southern Karnataka and their tectono-metamorphic significance. Indian Mineral., 16: 48---65. White, A.J.T., Jakes, P. and Christie, D.M., 1971. Composition of greenstones and the hypothesis of sea-floor spreading in the Archaean. Spec. Publ. Geol. Soc. Aust., 3 : 47--56. Windley, B.F., 1977. The Evolving Continents. Wiley, London, 385 pp. Wood, D.A., Joron, J.-L. and Treuil, M., 1979. A re-appraisal and discriminate between magma series erupted in different tectonic settings. Earth Planet. Sci. Lett., 45: 326--336.