Petrochemistry of early Precambrian metasediments from the central part of the Chitaldrug schist belt, Mysore, India

Chemical Geology Elsevier Publishing Company, Amsterdam - Printed in The Netherlands

PETROCHEMISTRY OF EARLY PRECAMBRIAN METASEDIMENTS FROM THE CENTRAL PART OF THE CHITALDRUG SCHIST BELT, MYSORE, INDIA

S.M. NAQVI and S.M. HUSSAIN National Geophysical Research Institute, Hyderabad {India)~

(Received January 12, 1972)

ABSTRACT Naqvi, S.M. and Hussain, S.M., 1972. Petrochemistry of early Precambrian metasediments from the central part of the Chitaldrug schist belt, Mysore, India. Chem. Geol., 10: 109-135. Petrological and geochemical studies of the metasediments in the central part of the Chitaldrug schist belt, Mysore, India indicate that these metasediments, most probably were supplied from an area of predominantly basic composition. Their partial metasedimentary and plutonic provenance is also suggested by the pebble composition of the conglomerates and the modal and chemical compositions of their matrix and associated graywackes. The existence of metasedimentary rocks in the source area of Dharwar sediments suggests at least one more cycle of sedimentation, orogeny, metamorphism and granitization prior to the main Dharwar cycle (2,600 m.y.). The Dharwars are therefore, probably not the oldest sediments of the Indian Shield as was previously thought. Recent radiometric age data support this conclusion. The amphibolitic xenoliths, so frequently found in the peninsular gneisses, may be the relics of the basic component of the Dharwar basement. INTRQDUCTION The peninsular region o f the Indian subcontinent is one o f the major Precambrian shields of the world. Except for the areas occupied by the rocks of the Gondwana system, Deccan traps and the Mesozoic and Tertiary sediments along the coastal region, the rest of peninsular India consists o f various types o f gneisses, schists, granites and ancient unfossiliferous sediments. These rocks represent a great span of time from 3 , 0 0 0 - 6 0 0 m.y. (Aswathanarayana, 1968; Crawford, 1969; Venkatsubramaniam et al., 1971). The rocks of the Dharwar system o f the Indian Shield are believed to represent a major cycle o f sedimentation ( 2 , 3 0 0 - 3 , 0 0 0 m.y.) (Krishnan, 1967; Crawford, 1969). The Dharwars of Mysore state occupy an area o f nearly 8,000 km 2. They have been divided into five parts from east to west; the Chitaldrug schist belt forms the central part of the Dharwars. The insert in Fig. 1 shows the location of the central part o f the Chitaldrug schist belt (latitude: 14000 ' to 14°15'; longitude: 75015 ' to 75°30'). This area consists of gneisses, granites, various types of schists (metasediments) and a basic volcanic ~"N.G.R.I. Contribution No.258.

T6;F

d K.) MYSORE STATE

/~AP OF I N D I A ;HOWING LOCATION OF THE STUDIED A R E A ( ' )

400 KM i i

"~1 DHARWAR SCHIST BELT | STUDIED AREA

Score

0 I

MAP SHOWING THE STUDIED AREA IN RELATION "0 THE MYSORE DHARWAR SCHISTS

F FAULT

GRANODIORITE

GRANITE

DYKES

ALLUVIUM

AND SCHIST

LIMESTONE

AIMANGLA

CONGLOMERATE

KURMERDIKERE CONGLOMERATE CHITALDRUG GRAYWACKE

I

tiles O

SCALE

i

2

I

4 Miles

3EOLOGICAL MAP OF THE CENTRAL PART OF I-HE C H I T A L D R U G S C H I S T 3ELT- MYSORE

to •

SAMPLE NUMBERS

GRANITIC AND QUARTZITIC GNEISSES

I-~ SI

$43

ACTINOLITE-CHLORITE- QUARTZ SCHIST AND INTERBEDOEO QUARTZITE SERICITE- QUARTZ - FELDSPAR GNEISS 2-~

2~

3-~

05-'~ SERICITIC PHYLLITE AND MAGNETITE4 ~ I TALYA CONGLOMERATE QUARTZITE 4~'1 ~

6~

9~

,o~ I0~ CHLORITIC PHYLLITES

VOLCANIC AGGLOMERATE|CHITALDRUG ORTHOAMPHIBO',TE _]VOLCAN,CSU,TE

'14"~] SERICITIC AND FERRUGENous PHYLLITES WITH PYRITIFEROUS CHERT ,v I JOGIMARDI TRAP -1

F~

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MINOR FOLD AXIS DIP AND STRIKE SCHISTOSITY PLANE(S) DIP AND STRIKE OF BEDDING PLANES

f 4j

,..~f MAJOR FOLD AXIS(ANTICLINE)

/

Z

-r,

Z

~>

<_

Z >. 0

PRECAMBRIAN METASEDIMENTS FROM MYSORE

111

suite, which have been metamorphosed to the greenschist facies. The metasediments and metavolcanics rest on highly acidic peninsular gneisses and are believed to be the oldest formation of the Indian shield (Wadia, 1966; Krishnan, 1968; Pichamuthu, 1970). These gneisses and granites frequently exhibit xenoliths of amphibolites and schists (Rama Rao, 1940). The recent radiometric age dating has shown these gneisses to be younger than the Dharwar metasediments. However, the basement on which the Dharwars were laid down is not known or has not yet been identified (Pichamuthu, 1962, 1970; Radhakrishna, 1967a). The composition of the pre-Dharwar crust of this part of the peninsular shield may be revealed by the composition of these metasediments. Geochemical data, especially information about the trace-element composition of Precambrian formations of the Indian peninsular shield, is scanty. The geochemistry of granites, charnockites, and some basic intrusives and extrusives have been recently studied (Howie, 1955; Rao, 1968; Saha et al., 1968; Naqvi, 1969, 1970; Leelanandam, 1970; Hasnain and Qureshy, 1971 ). However, geochemical studies of the argillaceous metasediments, have not yet received attention. There are few data on the effect of metamorphic processes on the redistribution of elements. While metasomatism may involve the migration of elements over considerable distance, doubt genuinely exists on the amount and scale of chemical redistribution brought about by metamorphism when no introduction of material has taken place (Taylor, 1955). The transfer of material over a distance of a few centimeters is evidenced by the growth of garnet porphyroblasts. Evidence for the migration of elements on a large scale during metamorphism is however, not available (Taylor, 1965). It is believed that regional metamorphism, without notable introduction of chemical elements from external sources, causes little change in the bulk chemical composition of the original rock (Shaw, 1956; Turner and Verhoogen, 1960; Schwarcz, 1966). In such cases, by taking adequate precautions in sampling, the nature of the parent rock and the source from which it was derived can be deduced. Large samples from fresh outcrops were collected during geological mapping of the area carried out by Naqvi (1970). The locations of these samples are shown on the geologic map of the area (Fig.l). The present paper deals with the petrological and geochemical studies carried out on the metasediments of the central part of the Chitaldrug schist belt to deduce the composition of their source area, i.e., the pre-Dharwar crust. ANALYTICAL METHODS Forty-three representative samples of all metasediments exposed in the area (except limestone) were analysed for their major elements by rapid methods of silicate analysis (Shapiro and Brannock, 1962). To achieve reproducibility of the analyses, duplicate and triplicate runs were made on several different samples. In one case six parallel total analyses were made on a sample of graywacke. The probable error in the individual Fig.1. Geological map of the area showing surface exposures of the rock types, location of the samples, and the sequence of the formations.

112

S.M. NAQV1 AND S.M. HUSSAIN

d e t e r m i n a t i o n s a m o n g t h e six parallel r u n s was f o u n d t o be SiO2 = 0.1%, TiO2 = 0.02%, A I 2 0 3 = 0.1%, F e 2 0 3 = 0.08%, MgO = 0.05%, CaO = 0.12%, N a 2 0 = 0.05%, K 2 0 = 0.02%, P2Os = 0.05%, a n d M n O = 0.03%. A n analysis o f W.1 was carried o u t in four parallel runs to e s t a b l i s h c o m p a r a b i l i t y . TABLEI Details of analytical method Spectrograph: Arc source: Arc gap: Slit width: Electrodes: Shape of the sample electrode:

Wavelength: Exposure: Plates: Plate calibration: Standards: Internal standard: Sample: Processing: Photometry: Analytical lines:

Hilger and Watts Large Quartz Prism 6A 4 mm 10/a Johnson Matthey Specpure carbon rods depth of cavity: 3.397 mm inner diameter of cavity: 3.048 mm outer diameter of cavity: 3.406 mm 2718 ,~-4450/~ 40 sec Agfa-Gevaert "Scientia" No. 23056 7 Step Sector as suggested by Bastron et al. (1960) Pd (3421.24 ~ ) and In (3039.36 ,~) as suggested by Ahrens and Taylor (1961) 32 mg of 3/1 mixture of rock powder and powdered graphite 120 sec at 20 ° C in developer; 20 sec in water; 600 sec in fixer; washed and dried density measurements from Hilger and Watts Recording Microphotometer Co 3453.17 ,~ Ni 3414.76 Cr 4254.35 .~ V 3183.98.~ Cu 3273.96 ~ Y 3242.28 ,~ Ga 2943.64 ~ Zr 3391.98 .~ Ge 3039.06

TABLE II Precision and accuracy from replicate analysis of W-1 Element

Reported value in W-l, Ahrens and Taylor (1961)

Mean of 20 W-1 determinations

Standard deviation

Coefficient of variation

Co Cr Cu Ga Ge Ni V Y Zr

38 or 49 124 107 13.5 1.6 82 247 35 100

55.1 125.6 108.3 11.95 5 79.3 250.35 32.75 99.2

6.4 8.3 7.28 2.7 0.00 4.79 12.6 3.65 7.45

11.61 6.6 6.72 22.59 0.00 6.03 5.03 11.14 7.51

PRECAMBRIANMETASEDIMENTSFROM MYSORE

113

For quantitative trace-element analysis, the method suggested by Ahrens and Taylor (1961) was followed closely. The details of the analytical methods are given in Table I. To determine the accuracy and precision of the spectrochemical analyses, replicate analyses (20) of W-1 were carried out. The results of the replicate analyses are given in Table II. GENERALGEOLOGY The surface exposures in the area consist of gneisses, actinolite-chlorite-quartz schist, quartzites, Talya conglomerate, micaceous schist, sericitic phyllites, magnetite quartzite, limestone, Aimangala conglomerate, graywacke, Kurmerdikere conglomerate, chloritic schist, a basic volcanic suite, sericitic ferruginous phyllites, banded pyritiferous chert, granodiorite, granite, and various dykes (Fig.l). These metasediments and metavolcanics have been folded into isoclinal folds (Naqvi, 1970). It is believed that the Chitaldrug schist belt is a synclinal limb of a large anticlinorium which has been eroded away during the geologic past. The other limb of the anticlinorium is thought to be the Shimoga schist belt (Pichamuthu, 1951). The area was first studied by Sambasiva Iyer (1889). Subsequently it was mapped by Sampat Iyengar (1905), who classified the rocks into the Javanhalli, Chitaldrug and Guadda-Ranganahalli (G.R.) formations. He was of the opinion that the G.R. formation (sericitic ferruginous phyllites of the present paper) and the conglomerates were sedimentary. Smeeth (1916), however, did not agree with the field evidence offered by Sampat Iyengar, and concluded that the entire Dharwar group was of igneous origin. The igneous nature of the Dharwar rocks was generally thought to be true until Pichamuthu (1936), Rama Rao (1940) and Radhakrishna (1940) produced strong evidence for a sedimentary origin. Rama Rao (1940) classified the main rocks of the area into three divisions (modified by Pichamuthu, 1967). The three-fold classification of the Dharwar has been questioned by Radhakrishna (1967a). Narayanaswamy (1964) divided the Precambrian formations of the Indian Shield into different provinces. He has suggested that the Dharwar metasediments originated in an early Precambrian eugeosyncline. The eugeosynclinal sedimentation of the Dharwars is supported by the structural, petrological and geochemical studies carried out by Naqvi (1970). The stratigraphic sequence proposed by Naqvi (1970) is given in Fig.1. The petrology and geochemistry of the metasediments are discussed in their stratigraphical order, the oldest first. PETROLOGY The actinolite-chlorite-quartz schist appears to be the oldest schistose formation. Previous workers (Sampat Iyengar, 1905; Smeeth, 1916; Rama Rao, 1940) have described it as a hornblendic schist. It consists mainly of relatively large prismatic, tabular crystals of actinolite and fine-grained quartz. Chlorite is generally associated with the actinolite, and in some places fine-grained plagioclase and biotite was also noticed. The quartz grains show interlocking sutured boundaries. Two thin feebly cross-bedded bands of quartzite,

114

S.M. NAQVI AND S.M. HUSSAIN

some 20 ft. thick, are interbedded with the schist (Fig.l). These bands consist of coarsegrained interlocking quartz fragments having sutured boundaries. The Talya conglomerate consists mainly of well-polished pebbles of texturally mature to immature quartzites embedded in a micaceous matrix. A few pebbles of granite are also present. The rounded to sub-rounded pebbles are arranged, elongated, and flattened along the schistosity. Some current-bedded quartzite pebbles were also observed. A nearly contact framework is exhibited by the pebbles. This contact framework may be an original sedimentary structure, or may have been produced by intense deformation which brought the pebbles closer to each other. The modal analysis (Table III) of the matrix of the Talya conglomerate shows that its main constituents are quartz, micaceous minerals (sericite, muscovite, biotite), microcline, orthoclase and quartzite fragments with occasional plagioclase grains. The associated micaceous schist resembles the matrix of the Talya conglomerate in all its petrological aspects. Sericitic phyllites with interbedded magnetite quartzite overlie the micaceous schist. The sericitic phyllites consist of extremely fine-grained quartz, micaceous minerals (sericite), and clay minerals. The magnetite quartzite is made up of alternating layers rich in magnetite and fine-grained quartz. A small patch of limestone is exposed in the southwestern part of the area (Fig.l). It consists of alternating thin layers of limestone and argillaceous phyllites. The Aimangala conglomerate (Fig. 1), being a polymictic conglomerate, consists of rounded to sub-rounded pebbles of quartzite, granite, altered basalts, shales (schists), and angular to sub-angular pebbles of ferruginous chert and vein quartz. A large variation in the size of the pebbles is observable and a disrupted framework is present. The pebbles are randomly embedded in a highly unsorted matrix made up of angular grains of quartz, clouded orthoclase, plagioclase, ferruginous chert, chert, fragments of chloritic schists, quartzites, and chlorite (Table III). The schist exposed to the east of the Aimangala conglomerate (Fig.l) exhibits a typical graywacke (Pettijohn's definition, 1943, 1957) texture and composition. It is a thick, massive and widespread formation of the area, varying in colour from blackish green to greenish brown. At many places, alternating layers of graywackes and chloritic phyllites are found. The graywacke layers at certain places show graded bedding. Specks of pyrite and chalcopyrite have been noticed in them along the schistosity. Modal analysis of the graywacke (Table III) shows that angular fragments of quartz, plagioclase, and potash feldspar grains vary in shape from angular to sub-rounded and in size from 0.1 mm to 1 cm. Larger grains are much more angular than the smaller ones. Potash feldspars occur as clastic fragments in the form of microcline and orthoclase having sharp corners. Rock fragments were not considered separately because most of them are of phyllite, consisting of exceedingly fine-grained chlorite and quartz. All gradations between rock fragments and chloritic mineral content are present in the matrix. The matrix of the graywackes is made up mainly of a fine-grained mixture of chlorite (37-42%), quartz, and feldspars. Chlorite occurs as recrystallised flakes of variable size. An important feature of the Chitaldrug graywacke is the replacement of quartz by authigenic chlorite, which makes it difficult to

-

34.4

N.P. = Not present. - = Not determined. T.C. = Matrix of the Talya conglomerate.

29.8 6.0 3.0 8.0 12.01 30.5

41.5 9.5 N.P. N.P. -

Quartz Chert and quartzite Potash feldspars Plagioclase feldspars Rock fragments Micaceous minerals Rock fragments and micaceous minerals

A.C.

T.C.

Modes

-

34.5 14.5 N.P. 2.0 20.0 28.0

K.C.

39.6

39.5 4,9 3.5 10.5 . .

2

. . 37.3

42.0 5.8 3.3 11.6

3

. . 39.3

40.6 4.5 3.9 11.7

4

. . 39.4

39.5 5.0 5.4 12.0 . .

5

A.C. = Matrix of the Aimangala conglomerate. K.C. = Matrix of the Kurmerdikere conglomerate. 1 - 10 = Chitaldrug graywackes.

39.7

41.8 5.5 4.5 8.5 .

1

Modal analysis of matrix of the conglomerates and Chitaldrug graywackes

TABLE Ill

41.4

40.5 4.5 3.0 10.6 . .

6

38.7

50.3 4,7 4.5 11.8 . .

7

. . 39.8

39.0 5.6 4.4 11.2

8

. . 40.2

41.2 5.4 3.2 9.6

9

42.2

39.8 5.2 3.5 9.3

10

©

© ~r

Z

(/)

t-o

Z

¢3

116

S.M. NAQVI AND S.M. HUSSAIN

obtain sharp microphotographs. The fine and irregular flakes of chlorite pass out from the surrounding matrix and penetrate the boundaries of the quartz fragments. The original boundaries of the quartz appear to have partially disappeared. The Kurmerdikere conglomerate consists mainly of rounded to sub-rounded pebbles of quartzite, disc-shaped phyllites, schists, and angular to sub-angular chert. Pebbles of vein quartz and granites are seldom present. A large variation exists in the size of the pebbles. Graded bedding is commonly exhibited in good exposures. Repetition of the thin layers of conglomerate, graywackes and phyllites was observed at many places. The matrix is highly unsorted and immature. The modal analysis (Table III) shows that it is made up of angular grains of quartz, chert, quartzite, argillaceous rocks and a fine-grained mixture of quartz and chlorite. A minor amount of plagioclase was noticed, but potash feldspar could not be recognised. Chloritic schist and phyllite is the most widespread formation of the area. These are exceedingly fine-grained rocks, rendering mineral identification difficult. Samples from the coarser variety of the schists show quartz, chlorite, sericite, muscovite and altered plagioclase as their constituents. Sericitic ferruginous phyUites and pyritiferous bedded chert constitute the youngest formation of the area (the G.R. clays of Sampat Iyengar, 1905). This formation consists of grey, green, white, yellow, and brown coloured phyllites and interbedded pyritiferous chert. The phyllites consist mainly of extremely fine-grained quartz and sericite. The red varieties contain limonite and hematite together with clay minerals. The pyritiferous chert bands consist of chemically precipitated chert layers, which have been impregnated by hydrothermal pyrite and other sulfide minerals (Naqvi, 1967). It can be seen from the above petrological description of the rock types exposed in this area, that only minor amounts of sandstones (quartzite) are present in the area. GEOCHEMISTRY The major and trace-element composition of the Dharwar metasediments is given in Table IV and graphically represented in Fig.2 and 3. The individual rock formations show considerable variation in their composition. Composition o f Dharwar metasediments as compared with pefitic and argillaceous metasediments A comparison of the average composition of Dharwar metasediments with the average composition of low-grade pelitic rocks (Shaw, 1954, 1956) and shales (Turekian and Wedepohl, 1961) is given in Table V. This comparison shows that the most widespread metasedimentary formations (actinolite-chlorite-quartz schist, graywacked, and chloritic schist) of this part of the Indian shield are more basic (more MgO, Fe203, CaO, Co, Cr, Cu, and V etc.) than average pelitic rocks or shales. The matrix of the two conglomerates (Talya and Kurmerdikere), micaceous schist, and sericitic phyllite contain more K20 than

0.06

K20/Na20

(continued on page 118)

78 620 320 15 5 250 220 25 70

98.28

Total

Co (p.p.m.) Cr Cu Ga Ge Ni V Y Zr

51.64 1.03 12.31 10.27 12.04 8.60 1.40 0.08 0.69 0.22

2.69

94 750 210 15 5 370 130 20 75

98.84

53.69 1.95 13.53 10.64 8.98 4.43 1.28 3.45 0.70 0.19

0.20

80 750 600 12 5 160 260 32 75

98.84

52.47 2.04 12.19 10.87 8.61 8.74 2.40 0.49 0.73 0.30

0.60

55 285 170 11 5 145 170 24 125

97.03

55.38 1.76 12.14 10.64 9.10 2.43 2.92 1.76 0.80 0.10

(4)

0.38

65 305 205 11 5 210 145 30 85

98.27

57.83 0.80 14.38 9.56 5.17 7.98 1.30 0.50 0.43 0.32

(5)

0.67

74 542 301 13 5 227 185 26 86

98.21

54.20 1.51 12.91 10.39 8.78 6.43 1.86 1.25 0.66 0.22

Avg.

13.36 207.28 157.75 1.60 0.00 80.59 48.37 4.30 20.10

2.20 0.50 0.89 0.46 2.18 2.54 0.67 1.23 0.12 0.07

S.D.

1.71

10 28 15 12 '5 205 14 35 215

99.55

61.84 1.61 11.42 9.81 6.79 2.39 1.87 3.21 0.50 0.11

3.82

35 70 32 10 5 150 30 40 185

98.87

74.10 1.08 5.60 8.76 3.43 1.86 0.68 2.60 0.66 0.10

(7)

7.00

12 54 25 16 5 140 25 32 190

99.07

61.60 0.85 13.27 8.05 2.10 7.48 0.60 4.20 .0.80 0.12

(8)

3.17

19 50 24 12 5 165 23 35 196

99.13

65.84 1.18 10.09 8.87 4.10 3.91 1.05 3.33 0.65 0.11

Avg.

(6)

(3)

(1)

(2)

Matrix of the Talya conglomerate

Actinolite-chlorite-quartz schist

SiO2 TiO 2 A1203 *Fe203 MgO CaO Na20 K20 P2Os MnO

Wt. %

11.34 17.30 6.97 2.49 0.00 28.57 6.68 3.29 13.12

5.83 0.31 3.26 0.72 1.97 2.53 0.58 0.65 0.10 0.01

S.D.

Major, minor and trace-element composition of the Dharwar metasediments of the central part of the Chitaldrug schist belt, Mysore, India

TABLE IV

,< ©

©

Z

,..]

Z

('3 ;}

5.30

22.50

5.41

22 42 158 6 5 52 14 51 134

9.01 16.10 70.00 2.04 0.00 26.86 3.76 13.38 69.13

-

-

96.69

47.68 0.43 44.11 1.30 1.48 0.51 1.18

-

-

-

95.62

46.94 0.66 42.70 1.37 2.16 0.58 1.21 0.01 0.01

1.05 1.40 0.01 0.68

1.35

1.52

20 48 135 7 5 60 12 52 130

94.59

46.21 0.90 41.30 1.44 2.84 0.65 1.25

2.85

3.48

K20/Na20

35 58 190 5 5 87 15 60 125

98.52

3.09 0.39 1.23 1.68 0.49 0.74 0.33 1.64 0.10 0.03

85 62 105 5 5 115 32 34 138

25 47 250 10 5 52 20 65 172

99.96

71.63 0.69 11.53 5.07 2.89 2.18 0.61 3.30 0.54 0.08

40 22 135 5 5 75 28 42 165

10 15 60 5 5 12 10 30 210

Co (p.p.m.) Cr Cu Ga Ge Ni V Y Zr

97.64

72.60 0.75 13.27 2.25 2.10 3.48 0.20 4.50 0.69 0.12 95.47

97.70

66.40 1.29 12.14 6.72 3.06 1.78 0.89 4.72 0.59 0.05 96.14

98.93

Total

74.46 0.53 10.26 5.74 3.45 1.72 0.38 0.58 0.48 0.10

(16) 62.76 0.69 15.64 11.60 1.72 1.47 0.63 0.85 0.10 0.01

Avg. 71.89 0.27 13.52 1.17 1.95 2.62 1.20 3.42 0.05 0.05

73.08 0.21 10.48 5.58 2.95 1.75 0.98 3.42 0.43 0.05

(14) 0.73

S.D.

(15)

Avg.

S.D.

(t2)

(13)

(11)

(9)

(10)

Sericitic phyllites

Magnetite qua rt z i t e

Micaceous schist

(continued)

SiO 2 TiO2 A I2 03 *Fe203 MgO CaO Na20 K20 P205 MnO

Wt%

TABLE IV

2.34

62 42 120 5 5 95 30 38 151

95.77

67.32 0.48 14.58 6.38 1.83 2.04 0.91 2.13 0.07 0.03

Avg.

22.51 2 0 .0 0 15.00 0.00 0.00 20.00 2.00 4.00 13.49

4.57 0.20 1.06 5.21 0.11 0.57 0.28 1.28 0.01 0.01

S.D.

Z

O~

Z

_<

z

oo

~r

oo

0.24

K20/Na20

(continued on page 120)

35 160 450 9 5 60 50 25 50

97.47

Total

Co (p.p.m.) Cr Cu Ga Ge Ni V Y Zr

60.85 1.35 13.49 5.64 3.04 6.68 4.60 1.10 0.50 0.22

0.28

35 162 520 7 5 55 47 28 55

94.36

56.78 1.28 14.01 5.15 4.72 6.16 4.53 1.28 0.34 0.11

0.22

40 165 150 10 5 58 56 30 40

95.23

57.21 1.22 12.94 6.21 4.68 6.72 4.73 1.05 0.35 0.12

0.24

36 162 373 9 5 57 51 27 48

95.66

58.28 1.28 13.48 5.66 4.14 6.52 4.62 1.14 0.39 0.15

Avg.

S.D.

2.35 2.30 160.49 1.24 0.00 2.05 3.74 2.05 6.27

1.82 0.05 0.43 0.43 0.78 0.25 0.08 0.09 0.06 0.04

0.67

45 230 590 5 5 80 62 28 50

97.93

63.43 0.39 12.55 5.05 3.02 6.77 3.67 2.48 0.51 0.06

0.42

66 180 480 10 5 78 80 20 30

97.93

60.50 0.42 15.17 5.14 3.35 7.18 3.92 1.67 0.52 0.06

(21)

(20)

(19)

(17)

(18)

Graywackes

Matrix of the Aimangala conglomerate

SiO2 TiO2 AI203 *Fe203 MgO CaO Na20 K20 P205 MnO

Wt%

0.34

70 50 150 12 5 66 55 32 25

97.35

65.58 0.45 11.27 3.32 3.02 7.63 4.12 1.40 0.51 0.05

(22)

0.58

65 175 180 9 5 74 45 15 20

98.07

62.97 0.37 14.66 5.72 3.02 5.91 3.07 1.81 0.54 -

(23)

0.50

58 60 130 10 5 48 68 14 15

98.34

60.04 0.35 15.50 5.26 4.02 5.67 4.36 2.21 0.50 0.43

(24)

0.25

90 155 115 5 5 150 180 30 40

98.21

61.72 0.38 17.24 5.38 3.35 3.72 4.35 1.12 0.49 0.46

(25)

0.53

56 115 95 5 5 72 66 10 48

98.69

61.35 0.39 15.77 5.17 3.69 4.16 4.97 2.68 0.48 0.03

(26)

0.50

38 105 600 5 5 70 20 13 40

98.03

59.55 0.40 15.17 5.14 4.29 6.10 4.50 2.29 0.49 0.10

(27)

0.38

35 116 450 5 5 65 54 20 44

97.43

62.73 0.42 12.87 5.43 3.35 6.41 3.90 1.51 0.50 0.31

(28)

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Z

¢'3

38 228 350 5 5 75 62 15 30

0.44

KzO/Na20

97.74

Total

Co (p.p.m.) Cr Cu Ga Ge Ni V Y Zr

60.81 0.52 15.89 5.20 4.69 3.89 4.15 1.83 0.51 0.25

0.46

56 141 314 7 5 70 69 19 34

97.94

61.86 0.40 14.61 5.08 3.58 5.74 4.10 1.90 0.50 0.17

16.57 59.83 192.77 2.66 0.00 25.54 39.93 7.36 11.37

1.73 0.04 1.69 0.61 0.55 1.31 0.48 0.47 0.01 0.16

1.85

48 95 225 5 5 68 56 18 45

96.51

64.81 0.69 13.62 8.06 2.92 4.30 0.70 1.30 0.11

1.79

58 210 95 5 5 75 70 25 40

93.50

58.32 0.34 12.26 7.96 6.98 4.34 0.92 1.65 0.55 0.18

(31)

2.14

36 205 275 10 5 70 62 15 35

93.89

60.32 0.45 13.01 7.22 5.76 4.42 0.64 1.37 0.59 0.11

(32)

1.92

47 170 198 6 5 71 62 19 40

94.61

61.15 0.49 12.96 7.74 5.22 4.35 0.75 1.44 0.38 0.13

Avg.

8.99 53.07 75.86 2.35 0.00 2.94 5.73 4.18 4.08

2.71 0.14 0.54 0.3.7 1.70 0.04 0.12 0.15 0.26 0.03

S.D.

(30)

S.D.

(29)

Avg.

Matrix of the Kurmerdikere conglomerate

Graywackes

SiO2 TiO2 AI20 3 *Fe20 3 MgO CaO Na20 K20 P2Os MnO

Wt %

TABLE IV (continued)

0.50

52 105 800 10 5 96 90 20 18

96.93

55.58 0.51 15.01 6.54 4.36 7.54 4.25 2.15 0.52 0.47

(33)

0.83

52 120 270 13 5 70 78 25 12

96.90

54.65 0.33 15.27 7.69 2.68 10.20 2.72 2.28 0.52 0.56

(34)

0.18

72 195 185 12 5 76 95 23 16

95.86

55.42 0.40 14.04 7.05 7.04 6.72 3.48 0.64 0.51 0.56

(35)

Chloritic schist and phyllites

0.17

76 110 820 10 5 55 360 16 20

98.29

55.42 0.45 15.06 6.42 5.03 9.22 5.12 0.90 0.52 0.15

(36)

0.12

96 72 950 5 5 84 90 20 36

95.87

59.16 1.05 13.21 8.05 6.44 2.80 4.16 0.51 0.25 0.24

(37)

0.32

69 120 605 10 5 76 142 20 19

96.72

56.04 0.54 14.51 7.15 5.11 7.29 3.94 1.29 0.46 0.39

Avg.

16.51 40.62 139.85 2.75 0.00 13.71 108.84 3.05 8.23

1.59 0.25 0.77 0.63 1.54 2.55 0.80 0.76 0.10 0.16

S.D.

0.58 0.13 87.45

0.56 0.13 96.84

2.60

K20/Na20

3.02

45 92 720 6 5 75 35 20 105

99.71

1.27

86 65 600 8 5 250 235 15 42

97.76

64.26 0.69 15.64 12.55 1.51 1.34 0.73 0.93 0.10 0.01

2.73

160 120 180 6 5 210 180 21 50

95.92

73.64 0.62 7.32 8.55 1.51 2.36 0.48 1.31 0.10 0.03

2.47

85 75 410 6 5 142 122 20 83

97.41

70.85 0.59 12.71 6.66 1.80 1.74 0.84 2.08 0.09 0.05

45.96 35.01 251.33 1.08 0.00 89.75 87.69 4.26 38.59

3.82 0.17 3.35 4.30 0.31 0.86 0.26 0.97 0.02 0.03

*Fe2 03 = total iron oxide; Avg. = average; S.D. = standard deviation; - = not determined.

50 25 150 6 5 35 38 27 135

Co (p.p.m.) Cr Cu Ga Ge Ni V Y Zr

96.42

1.80 42.80 1.80 0.75

1.70 16.75 1.44 1.48

Total

39.59

72.54 0.31 15.44 4.47 2.25 0.53 0.98 2.96 0.12 0.11

S.D. 74.78

72.98 0.77 12.46 1.10 1.93 2.72 1.20 3.12 0.06 0.08

Avg.

(43)

(41)

(42)

(40)

(38)

(39)

Pyritiferous chert

Sericitic ferruginous phyUites

SiO2 TiO 2 A120 3 *Fe203 MgO CaO Na20 K20 P205 MnO

Wt%

92.13

0.57 0.13

1.75 29.77 1.62 1.11

57.18

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0.01 0.00

0.01 13.03 0.16 0.36

17.59

S.D.

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122

S.M. NAQVI AND S.M. HUSSAIN

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Na20, whereas the actinolite-chlorite-quartz schist, matrix of the Aimangala conglomerate, graywacke and the chloritic schist and phyllites contain higher amounts of Na20. The matrix of the Aimangata conglomerate, graywacke, and the chloritic schist are sodarich sediments (KzO/Na20 ratio less than one) compared to shales or pelitic rocks (ibid). The Fe2Oa, MgO and CaO content of the actinolite-chlorite-quartz schist is higher than the Sheba shales of South Africa (Condie et al., 1970), and the Canadian argillites of the

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54.11 1.00 17.54 8.88 6.97 0.22 2.14 2.72

11

* F e 2 0 3 = total iron oxide.

1 = W y o m i n g graywacke (Condie, 1967a); 2 = Precambrian graywacke (Pettijohn, 1963); 3 = Yellowknife graywacke (Boyle, 1961); 4 = Precambrian graywacke (Grout, 1922); 5 = Precambrian graywacke (Todd, 1928); 6 = Sheba graywacke (Condie et al., 1970); 7 = Belva Road graywacke (Condie et al., 1970); 8 = Gowganda argillite (Young, 1969); 9, 10, and 11 = Sheba shales (Condie et al., 1970);

64.43 0.62 15.48 6.65 3.12 2.22 3.74 2.44

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Average chemical composition o f Precambrian graywackes, shales and argillites

TABLE VI

t.~

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'X7

126

S.M. NAQVI AND S.M. HUSSAIN

same age (Macpherson, 1958). The composition of the Chitaldrug graywackes closely resemble the composition of other Precambrian graywackes (Table VI) reported by Todd (1928, quoted in Pettijohn, 1957, p. 306), Grout (1929, quoted in Pettijohn, 1957, p. 306), Pettijohn (1957, 1963), Boyle (1961), Condie (1967a) and Condie et al. (1970); CaO is, however, considerably higher. The chloritic schist contains more Na20 and CaO than the Sheba shales (ibid) and Fe203, MgO, and CaO are higher than the Canadian argillites (Macpherson, 1958; Young, 1969). When compared with Precambrian slates (Nanz, 1953), the chlorite schist contains relatively similar amounts of silica and iron and higher amounts of soda, magnesium and calcium; alumina and potash are lower. Provenance of the Dharwar metasediments

The composition of the Precambrian crust which supplied the debris for the early Precambrian sediments, or on which they were laid down, has fascinated many workers (Rubey, 1951 ; Pettijohn and Bastron, 1959; Condie, 1967b; Fahrig and Eade, 1968). Metasediments, specially the graywackes, being the most widespread and characteristic formations of the Precambrian era (Pettijohn, 1943) have been the subject of many studies. Early workers thought that the graywackes were derived from basalts (Kay, 1951). However, later investigations by Helmbold (1958), Mattiot (1960), Donaldson and Jackson (1965) and Condie (1967a) have shown that most of the Precambrian graywackes were derived from a source rock of interanediate composition. Certain compounds and elements such as FeO, MgO, CaO, TiO2, MnO, Co, Cr, Ni, and V are characteristically more highly concentrated in basic rocks than in acidic rocks, whereas Ba, Zr, Y, and other elements are more concentrated in acidic rocks. Low or high concentrations of these elements and compounds in sediments or metasediments have been considered as evidence for the acidic or basic nature of the source area (Macpherson, 1958; Weber and Middleton, 1961; Condie, 1967a; Condie et al., 1970; Young, 1969). The actinolite-chlorite-quartz schist deviates in its higher TiO2, Fe203, MgO, CaO, P2Os, MnO, Co, Cr, Cu, Ni, V and its lower SiO2, A12Oa, Na20, K20, and Zr content from the average composition of the crust (Table VII) which has been stated to be granodioritic (Taylor, 1967). Its overall composition resembles the average composition of basalts reported by Nockolds (1954), Turekian and Wedepohl (1961) and Taylor (1964). In the ternary compositional diagrams (Fig.4) it falls near the basaltic field. This suggests that the source area which provided this sediment probably h~t a basic composition. The matrix of the Talya conglomerate and the associated micaceous schist deviate from the average composition of the crust (Taylor, 1964) in their higher SiO2, TiO2, Fe203, MgO, K20 and Zr; and in their lower A1203, CaO, Na20, Co, Cr, Cu and V. They deviate in their composition from average granites (Taylor, 1964; Nockolds, 1954) in their lower A1203, K20 and Na20 contents. The sharp deviation, especially in the case of Na20, from a granitic composition appears to be the effect of provenance. The predominance of quartzite pebbles suggests a metasedimentary source area. It would appear that the original sediment, which after weathering and long transportation (suggested by rounded and well polished pebbles), that supplied the debris for the Talya conglomerate was most probably

PRECAMBRIAN METASEDIMENTSFROM MYSORE

127

TABLE VII Average composition of crust, basalt, and granite Wt% SiO2 TiO2 A1203 *Fe203 MgO CaO Na20 K20 P (p.p.m.) Mn Co Cr Cu Ga Ge Ni V Y Zr

1

2

3

4

5

6

60.25 0.95 15.55 8.05 3.86 5.81 3.18 2.52

69.13 0.38 14.55 3.86 0.27 2.21 3.73 4.02

67.21 0.57 15.50 4.23 1.56 3.54 3.83 3.04

74.27 0.20 13.61 2.03 0.27 0.71 3.48 5.06

51.37 1.50 16.56 12.24 7.46 9.40 2.62 1.00

49.23 2.30 14.74 12.37 7.63 10.63 2.43 1.00

1050 950 25 100 55 15 1.5 75 135 33 165

700 400 1 4 10 18 1.5 0.5 20 40 180

1400 1500 48 200 100 12 1.5 150 250 25 150

1100 1500 48 170 87 17 1.3 130 250 21 140

900 500 7 22 30 17 1.3 15 88 35 140

600 400 1 4.1 10 17 1.3 4.5 44 40 175

1 = crust (Taylor, 1964); 2 = granite (Taylor, 1964); 3 = granite-high Ca (Turekian, 1961); 4 = granite-low Ca (Turekian, 1961); 5 = basalt (Taylor, 1964); 6 = basalt (Turekian, 1961); *Fe20 3 = total iron oxide. a micaceous quartzite deficient in Na20. The high MgO and Fe203 contents suggest that a partial basic provenance should not be completely ruled out even in the case of the Talya conglomerates. In the ternary compositional diagrams (Fig.4) it falls well out of the region of all the igneous rocks. The sericitic phyllites are highly siliceous and contain more Co, Cr, and Ni, than normally found in granites. The highly siliceous character of the sericitic phyllites may be attributed to processes whereby silica was chemically precipitated along with fine-grained detrital quartz. The interbedded iron ore (magnetite quartzite) suggests precipitation of both iron and silica. Gross (1965) has discussed the concept of the origin of iron formations at length. Most of the existing theories agree that a source of iron and silica was either a deeply weathered landmass, or volcanic emanations, or reaction of sea water on hot extrusive rocks. The sericitic phyllites as shown in Fig.4 occupy a position outside the fields of the igneous rocks. The high content of Co, Cr, and Ni in the sericitic phyllite and its association with iron ores (magnetite quartzite) indicates a source area composed of quartzitic metasediments and basic igneous rocks for the phyllite. The composition of the matrix of the Aimangala conglomerate closely resembles the composition of the crust. However, its Na20 content is significantly higher than that of the crust and of granite (Table VII), and its K20 content is almost equal to that of basalt.

128

S.M. NAQVI AND S.M. HUSSAIN

Fe203(Total Iron) CaO /

~

~hitaldrug

Gronodior~t~ B . . , , ~ % . , * c - -

i

MgO

Na20

volcanic

suite

X

x

TiO 2

K20

• Average basalt (Nockolds). 1~ Average granodiorite (Nockolds). 3j~ Average granite (Nockolds).)~Wyoming graywacke(Condie). ~ P r e - e a m b r i a n graywacke (Pettijohn). + Yellowknife graywacke (Boyle). (~Graywacke (Grout). c] Graywacke(Todd).el Actinolite -Chlorite =Quartz s c h i s t . • 2 Matrix of Tafya c o n g l o m e r a t e . • 3 M i c a c e o u s s c h i s t . e 5 $ e r i c i t i c Phyllite. I) 6 Matrix of the A i m a n g l a c o n g l o m e r a t e . e B Matrix of the Kurmerdikere conglo m e r a t e . e 9 Chloritic s c h i s t and P h y l l i t e . e l O Sericitic Ferrugenous Phyllite.

Fig.4. Ternary compositional diagrams showing the position and fields of the Dharwar metasediments and other rock types. The Co, Cr, and Cu content of the matrix of this polymictic conglomerate is closer to that of basalts, whereas the Ni content is lower than the average in the crust. The average composition of the matrix occupies a place between the fields of basalt and granodiorite in the ternary compositional diagram (Fig.4). This, together with its petrological features, suggest a mixed source area composed of basalts, granites and metasediments (quartzites, schists, etc.) for the Aimangala conglomerate. There is not much difference in the composition of the matrix of the Aimangala conglomerate and the later graywackes. The Co, Cr, and Cu content of the graywackes, however, is higher than that found in the average continental crust. The ternary compositional diagram (Fig.4) and the mineralogy of the Chitaldrug graywackes rule out the possibility of basic volcanics being their only source rock, However, the low K20/Na20 ratio (Fig.5) and the trace-element contents of these graywackes indicate a predominance of basic volcanics in the source area. Considering all the data it would appear that the source area which supplied the detrital material for this graywacke probably consisted of basic volcanics, granite or granodiorite, quartzite, chert, and chloritic metasediments.

PRECAMBRIAN METASEDIMENTSFROM MYSORE

6

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129

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@ Graywackes - ~ M a t r i x of the K u r m e r d i k e r e C o n g l o m e r a t e •

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Fig.5. K20/Na2 O diagram showingthe predominance of basic volcanics in the source area of the Dharwar metasediments. This view is further strengthened by the composition of the matrix of the Kurmerdikere conglomerate, which deviates from the average crustal composition in its lower Na20, K20, A1203, Zr and higher MgO, Co, Cr and Cu content. In the ternary diagram it occupies a position outside that of the igneous rocks. This deviation in composition, together with the mineralogy of the matrix and the nature of the constituent pebbles, suggests a mixed source rock. The lower Na20 and K20 content suggests that granitic material was not predominant in the source area. The chloritic schists and phyllites deviate from the average crustal composition in their lower SiO2, TiO2, A12Oa, K20, Zr and Y, and their higher MgO, CaO, Na20, P2Os, MnO, Co, Cr, Cu and V contents. Their overall composition closely resembles that of the basalts (Table VII) except in Na20 and K20 which are higher. In the ternary compositional diagram (Fig.4) they occupy a place near the basalts. It would appear, therefore, that the source rock for this rock unit was mainly basic. However, the presence of detrital quartz grains suggests a partial acid plutonic provenance. The sericitic ferruginous phyllites appear to be chemical precipitates of amorphous silica. Later hydrochemical activity may also have affected these rocks. Their composition therefore does not appear to be useful in source-rock studies. DISCUSSION According to Vinogradov and Tugarinov (1968) a comparison of the compositions of

130

S.M. NAQVI AND S.M. HUSSAIN

sediments of different periods simultaneously developed on different continents clearly indicates a typical change in their composition, thus obeying a definite crustal evolution process. The composition of early Precambrian metasediments is considered to be due to the weathering and erosion of predominantly basic and ultrabasic effusives laid down on the early crust. During this era accumulation of basic extrusives, graywackes, and shales predominated (Vinogradov and Tugarinov, ! 968). Compositional similarities between the early Precambrian metasediments throughout the world have also been pointed out by Ronov (1964), Anhaeusser et al. (1969) and Glikson (1970). The results of the present study indicate that the early crust that supplied the sediments of the Dharwar eugeosyncline was composed of diverse types of igneous and metasedimentary rocks. Among these, the rocks of basaltic composition appear to have predominated. This is evidenced by the higher concentration of MgO, FeO, CaO, Co and Cr and occasionally by higher contents of Ni and V in the most common metasediments of the area. The Cu content of these rocks is also significantly high. The sulfide mineralization in the area, as described by Rama Rao (1936) and Radhakrishna (1967b), may explain the general increase of the Cu content in these rocks. Middleton (1960) has shown that eugeosynclinal graywackes differ from other sandstones especially in their low K20/Na20 ratios, a feature which can be correlated with the presence of basic volcanic detritus in the graywackes. The low K20/Na:O ratio (Fig.5) of the Dharwar metasediments may also be explained by the predominance of basaltic rocks in the source area. The high Na20 content in metasediments of the graywacke type from different shield areas has also been attributed to soda metasomatism or albitization (Pettijohn, 1957; Pettijohn and Bastron, 1959; Condie, 1967a; Young, 1969). The same phenomenon may also account for the high Na20 content in the Dharwar metasediments which had an original composition susceptible to such changes. Sediments which originally contain calcic plagioclase can undergo such a chemical change. Albitization of the graywackes, the matrix of the Aimangala conglomerate, and the chloritic schist took place because the original sediment contained calcic plagioclase. The matrix of the Talya conglomerate, the micaceous schist, and the sericitic phyllite could not be albitized because of the absence of calcic plagioclase in the original sediment. The high CaO content of the Chitaldrug graywackes and their associated rocks may be due to the high calcic plagioclase content of the original sediments, a feature that appears to be related to the predominance of basaltic rocks in the source area. The pebble composition of the conglomerates and the chemical and modal composition of their matrix suggests a partial acid plutonic and partial metasedimentary composition for the source area. Such source areas of mixed composition appear to have been relatively common during the early Precambrian. Studies of the Archaean sedimentary rocks in the North Sprit Lake area of Canada and of the Fig Tree group in South Africa have indicated that not only basic rocks, but also granitoid and older sedimentary rocks, were present in abundance in the source area of these sediments (Donaldson and Jackson, 1965; Condie et al., 1970). The presence of cross-bedded pebbles of quartzite in the Talya conglomerate, which is

PRECAMBRIAN METASEDIMENTSFROM MYSORE

131

considered to be the oldest conglomerate of the area, indicates probably another cycle of sedimentation prior to the main Dharwar cycle of 2,600 m.y. The well-polished and rounded shape of the pebbles of the texturally mature to immature quartzites suggest a long period of transportation. It also indicates that these pebbles do not belong to the quartzitic bands found interbedded with the actinolite-chlorite--quartz schist. The existence of a pre-Dharwar cycle of sedimentation is also supported by the presence of sedimentary rock fragments (chert, quartzites and chloritic schist) in the matrix of the Aimangala and Kurmerdikere conglomerates and as constituents of the grains in the graywacke. The composition and position of these conglomerates in the ternary diagrams (Fig.4) further strengthens this view. It is suggested, therefore, that the rocks belonging to the Dharwar system (2,600 m.y.) are probably not the oldest sediments of the Indian shield. Basic volcanics and metasediments older than the Dharwars in Mysore have not yet been recognised. However, Vinogradov and Tugarinov (1964, quoted by Pichamuthu, 1971, and Sarkar, 1968) have suggested that the charnockites are the oldest rocks in the Mysore state of India and that the Lower Dharwar sediments are older, having an age of 3,100 m.y. Hornblende schists from the Hutti gold mines and from Channagiri have given ages of 3295 ± 200 m.y. and 2631 m.y. respectively (Sarkar, 1968) and the Dharwar schist near Mysore has given an age of 3,000 m.y. (Crawford and Compston, 1967). According to Pichamuthu (1971) these data favour an older age for the Dharwar metasediments. Radhakrishna (1967a) has suggested that the islands or xenoliths of schists and amphibolites so frequently found in the peninsular gneisses are probably relics of an older series of basic igneous rocks and primitive sediments, which were caught up in the gneisses. Similarly, remnants of rock bodies found as enclaves in granites older than the Kalgoorlie system, have been recognised in Australia (Glikson, 1970). It is suggested that a systematic study of these xenoliths in the peninsular gneisses and granites may reveal the pre-Dharwar history of the Indian shield. The presence of granitic and quartzitic pebbles in the Dharwar conglomerates shows that all the acid plutonic material of the Indian peninsular shield is not post-Dharwarian. Sarkar (1968) has suggested three prominent orogenic-metamorphic cycles with granitic activities in the Dharwar region, one dated at 3,100 m.y., another at 2,600 m.y., and the last at 2,000 m.y. He believes that the pre-Dharwar basement complex is older than 3,100 m.y., and was affected by later metamorphic and granitization cycles. The granitic and quartzitic pebbles present in the conglomerates may possibly represent the 3,100 m.y. old cycle of orogeny, metamorphism and granitization. The evidence at hand suggests that the basement of the Dharwars consisted of basic volcanics, metasediments and some granites. The emplacement of granites and the formation of granitic gneisses during and after the Dharwar orogeny have probably obliterated the Dharwar basement and have made the composition of the crust more acidic. However, to prove this interpretation of the geochemical and petrological data, extensive radiometric data for the whole of the Indian Precambrian shield are required, and these are not available at present. Recently, Venkatsubramaniam et al. (1971) have found that the augen and banded gneisses and the associated amphibolitic xenoliths have an age of 2,950 ± 100 m.y.

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The age of the peninsular gneisses obtained by Venkatsubramaniam et al. (1971) is close to the ages obtained by Ramamurthy and Sadashiviah (1967, quoted in Venkatsubramaniam et al., 1971) and Crawford (1969). The evidence of a much older age for the amphibolite xenoliths or islands of amphibole are probably the relics of the basic component of the Dharwar basement or pre-Dharwar crust. CONCLUSIONS The interpretation of the petrological and geochemical data obtained from the early Precambrian Dharwar metasediments in the central part of the Chitaldrug schist belt of Mysore, India leads to the following conclusions: (1) Sandstones rich in quartz and feldspars constitute a minor amount of the metasediments. (2) The most widespread formations of the area are rich in Mg, Fe, Ca, Na, P, Co, Cr, Cu, and in places in Ni and V; they are poor in Zr and Y. This indicates that rocks of basic composition predominated in their source area. The composition of the conglomerates and their matrices suggests that in addition to predominant basic source rocks, the area was also composed of acid-plutonic (granite-granodiorite) and metasedimentary rocks (quartzite, chert and schists). (3) The existence of metasedimentary rocks (quartzite, chert and chloritic schist) in the source area of the Dharwar metasediments suggests at least one more cycle of sedimentation prior to the main Dharwarian cycle (2,600 m.y.), a feature which probably indicates that the Dharwars are not the oldest sediments of the Indian shield. Recent radioactive data support this view. (4) The amphibolitic xenoliths found in the gneisses may be the relics of a basic component of the Dharwar basement. ACKNOWLEDGEMENTS We are deeply indebted to Dr. M.N. Qureshy, Scientist, N.G.R.I., to Dr. B.P. Radhakrishna, Director, Department of Mines and Geology, Bangalore, to Prof. F. Abroad, Head of the Geology Department, Muslim University, Aligarh for their encouragement and guidance during this work; to Mr. R.D. Naidu, Scientist, D.M.R.L., and Mr. D. Gupta Sharma, Scientist, N.G.R.I. for their help during the spectrochemical investigation. We are grateful to Dr. R.W. Boyle, Canadian Geological Survey, Ottawa for the review and many valuable suggestions for the improvement of the manuscript. Thanks are also due to Dr. Z. Hasan, Mineralogisk-Geologisk Museum, University of Oslo for his guidance during the initial stages of this work, to Dr. D.N. Kanungo and Dr. K. Satyanarayana, Scientists, N.G.R.I. for many fruitful discussions during the preparation of this paper. We are thankful to Dr. Hari Narain, Director, N.G.R.I. for permitting the publication of this paper.

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