Geochemical Characteristics of Archean Clastic Metasediments of Gadag Gold Field, Southern India: Implications for Provenance and Tectonic Setting

Gondwana Research, c! 5, No. I, p p . 245-255. 02002 International Association for GOndWQnQResearch, Japan. ISSN: 1342-937X

Gondwana Research

Geochemical Characteristics of Archean Clastic Metasediments of Gadag Gold Field, Southern India: Implications for Provenance and Tectonic Setting A.G. Ugarkar and R.C. Nyamati Department of Geology, Karnatak University, Dharwad - 580 003, India (Manuscript received October 13,2000; accepted August 2,2001)

Abstract The Gadag Gold Field in the western Dharwar craton consists of a thick pile of sediments, apart from the volcanic rocks. These rocks have undergone metamorphism of greenschists to lower amphibolite facies. The clastic metasediments (metagreywacke, chlonte phyllite and quartz-sericite phyllite) constitute the prominent lithounits in the eastern half of the area. Metagreywackes consist of abundant lithic fragments and are designated as lithic greywackes. Their mineralogical maturity index is very low (0.4). Metagreywackes have higher SiO, and Ba contents, lower Al,O,, FeO', MgO and CaO contents than chlorite phyllites. Compared to chlorite phyllites, the quartz-sericite phyllites have higher SO,, YO, Ba, Zr and Sr contents, and lower FeO', MgO, CaO, Ni, Co, Cr and V contents. YO/Na,O ratios of metagreywackes and chlorite phyllites are typically less than 1, and are chemically immature. Further, these sediments are characterised by higher concentrations of SiO, and Zr, higher SiOJMgO and Zr/Y ratios, suggesting felsic component in the provenance. While higher abundance of Cr and Ni with K,O/MgO ratios less than 1and low &O/MgO ratios suggest a more basaltic nature of the source area. High L a m ratios (42), negative Eu anomaly and overall REE enrichment in metagreywackes indicate a component of granite-granodioritic rocks in the provenance. The polymictic metaconglomerate with pebbles of gneiss, granite, quartzite, andesite, rhyolite and the metagreywackes with lithic fragments like quartzites, chert, andesite and rhyolite suggest a sialic dominant, mixed felsic and mafic source in the provenance. The geological setting, associated rock types, their field relationships and primary sedimentary structures coupled with petrographic and geochemical characteristics plus their modeling through various tectonic discrimination diagrams indicate that metagreywackes have continental island arc + active continental margin, chlorite phyllites have continental island arc + oceanic island arc and quartz-sericite phyllites have active continental margin + passive margin tectonic settings. It is suggested that these clastic metasediments of Gadag Gold Field, with different tectonic settings, were probably brought together (juxtaposed ?) by accretion processes during convergence of plates. However, geochronolgic and rare earth element data on a large number of samples of volcano-sedimentary rocks of this area is required to test this accretionary model. Key words: Metasediments, geochemistry, provenance, Gadag belt, Karnataka.

Introduction The origin of Archaean greenstone belts is a topic of much debate, and it has been addressed by several geologists and geochemists from all over the world based on their field and laboratory data. Plate tectonic models like intra-continental rifts, mid oceanic ridge rifts, backarc basins, island arcs, ocean plateaus, plumes and subduction accretion complexes, or some combinations, can account for the variety of structural, lithological and geochemical characteristics of volcano-sedimentary sequences of Archaean greenstone belts (see Condie, 1981; Kroner, 1991; Windley, 1993; Abbott, 1996; Polat et al., 1998). The increased understanding of clastic Sedimentary rocks in recent years, has acquired a significant dimension

as they provide clues to trace back the Archaean plate tectonic setting, provenance and evolution of the continental crust. The method involved is nothing but the final step to reverse the process of sedimentation and infer ancient plate tectonic configurations and provenance from the types of sediments found. The overall geometry, stratigraphic sequence, lithology, petrography and geochemistry are the most diagnostic features. In many instances, the provenance regions get destroyed but their record still lies in the sediments derived from them. Similarly, clastic metasediments preserve some petrographic and geochemical signatures of their source rocks and possible plate tectonic setting of the continental crust. The petrographic and geochemical data has been successfullyutilized for discriminating tectonic setting and provenance for the Archaean clastic metasediments from

246

A.G. UGARKAR AND R.C. NYAMATI

Fig. 1. Geological maps of Gadag Gold Field showing the western (W), central (C) and eastern (E) Auriferous Zones (modified after Narayanaswami and Ahmed, 1963). Index: 1-Peninsular gneiss, 2-Metabasaltic andesite and metaandesite, 3-Chlorite schist, 4-Manganese ores, 5-Metabasalt, 6-Quartz porphyry, 7-Schistose amphibolite, &Metaconglomerate, 9-Argillite, 10-Chlorite phyllite, 11-Quartz-sericite phyllite, 12-Garnetiferous-quartz-mica schist, 13-Auriferous zones, 14-Carbonates, 15-Banded Iron Formation, 16-Metagreywacke, 17-Dykes.

Australia, Canada, North America, Greenland, South Africa and India (Bhatia, 1983; Dickinson et al., 1983; Taylor and McLennan, 1985; Bhatia and Crook, 1986; Naqvi et al., 1988; Feng and Kerrich, 1990; Fed0 et al., 1996; Manikyamba et al., 1997; Yibas, 1999). In Southern India, the Dharwar craton represents one of the most significant shields of the Archaeans and the most striking geological features are the greenstone belts of different ages comprising of both metavolcanic and metasedimentary sequences in different proportions. Gadag greenstone belt forms one of the important belts of the western block of the Dharwar craton by virtue of its diverse volcano-sedimentary lithological assemblage

and gold deposits (Gadag Gold Field, Fig. 1).In the present paper, petrographic and geochemical data on the clastic metasediments (metagreywacke, chlorite phyllite and quartz-sericite phyllite) of Gadag Gold Field has been presented in an attempt to understand the provenance and tectonic setting for their deposition. However, it is widely believed that caution must be taken while interpreting ancient metasediments for their provenance and tectonic influences. Ideally, the petrographic and geochemical data should be complemented by the data from associated rock types, field relations and structures so as to strengthen the modeling for tectonic setting and provenance, and accordingly we have made an attempt. Gondwana Research, V. 5,No. 1,2002

ARCHEAN CLASTIC METASEDIMENTS OF GADAG GOLD FIELD, INDIA

Geological Setting The late Archaean Gadag greenstone belt which hosts gold mineralisation, constitutes a part of the western Dharwar craton made up of older and younger greenstone belts, Tonalite-Trondhjemite Gneiss (TTG) and minor Kgranites. The younger greenstone belts were deposited over clearly recognisable sialic basement made up of older gneisses. These greenstone belts were formed in faultbounded ensialic intracratonic basins and contain metavolcanic suite at the base, succeeded by large thickness of metasediments (Radhakrishna and Naqvi, 1986). Gadag greenstone belt forms the northern continuation of the well-known Chitradurga greenstone belt of middle to late Archaean (2,600-2,400 Ma), and consists of metamorphosed volcano-sedimentary sequences belonging to the Chitradurga Group of the Dharwar craton (Swaminath and Ramakrishnan, 1981;. The eastern margin of the Gadag belt is a major thrustfault contact (Chitradurga thrust fault) marked by strong mylonite zone extending for about 400 km from Gadag in the north to Mysore in the south (Drury and Holt, 1980). The whole-rock Rb-Sr age of the metavolcanic rocks from Gadag belt is 2456+ 76 Ma, which indicates that this was formed during late Archaean (Bhaskar Rao et al., 1992). It is suggested that the metavolcanics were emplaced in an active continental margin or a continental island arc tectonic setting (Ugarkar et al., 2000). Gold mineralisation of vein type occurs in both metavolcanics and metasedimentary suites in three distinct shear zones trending NNW-SSE. Gold deposits of this belt as well as Archaean greenstone belts elsewhere in the world share many common geological characters. The regional strike of both schistosity and bedding veers between NW and NNW and dips are generally 45-6O0due NE-ENE, steepening to 75O at some places. The regional structure as described by Narayanaswami and Ahmed (1963) is an asymmetrical isoclinal syncline with axial plane dipping to the east. The Gadag Gold Field forms a part of the western limb of the regional symline where the belt is widened and the rocks are highly sheared due to refolding (cross-folds) along a northwest axis, plunging to the southeast. The bedding ascertained by colour and compositional banding or litho-contacts is NNW-SSE with 20-45O easterly dips. The schistosity and the regional tectonic stress associated with the deformation and tilting of the lithological sequence is developed both in metavolcanics and metasediments of the area. The schistosity generally trends parallel to sub-parallel to the bedding, but dips easterly at relatively steep angles. The metasediments at many places show primary sedimentary structures, like bedding stratification, cross bedding, graded bedding, ripple marks, convolute Gondwana Research, V. 5, No. 1,2002

247

laminations, load and flute structures (Chakrabarthi et al., 1993), while the metavolcanics exhibit pillow structures. These features indicate a normal order of superposition of various lithounits, i.e., younging in the dip direction from west to east.

Lithology The western half and parts of the northern and eastern areas of Gadag Gold Field are mainly occupied by the metavolcanics. The different lithounits of metavolcanics are metabasalt, metabasaltic andesite, metaandesite, felsic volcanics o r quartz porphyry (metadacite a n d metarhyolite), schistose amphibolite and chlorite schist. Metabasalt is interspaced with banded iron formations especially in the northern area. It exhibits well developed pillow structures. Within metabasalts lensoidal bodies of metabasaltic andesite and metaandesite occur. Metagabbros in the form of sills ( < 2 m thick) are seen interbedded within metabasalt at places. While sheared and rudely schistose felsic metavolcanics occur as bands within metabasalts along the western margin of the main central auriferous zone and along the contact of the western auriferous zone. North of Varvi village, a band of lensoidal shape of felsic metavolcanics is exposed overlying the mafic metavolcanics and underlying the metaconglomerate bed conformably Amphibolite schist is exposed in the eastern part. Elongated lensoidal bodies of chlorite schist are exposed within metabasalts and metaandesites. Metasediments are the next group of rocks and the sequence commences with polymictic metaconglomerate, which occurs as a prominent lithounit in southern part. It is followed by different sedimentary facies like metagreywacke-argillite suite, chlorite phyllite, quartzsericite phyllite, quartz-mica schist and garnetiferous quartz-micaschist and carbonates. Phyllites are commonly interbedded by banded iron formations. In the central auriferous zones, the contact of chlorite phyllite and argillite with metavolcanics is invariably marked by metagreywackes. Numerous interbedded sequences of metagreywacke-argillite occur within and around auriferous zones. Individual layers may consist of either thin bedded to laminated fine-grainedmetagreywacke and carbonaceous argillite or massive thick bedded medium to coarse-grained metagreywacke. Graded bedding is common in metagreywacke-argillite and is marked by gradation in grain size from coarse metagreywacke at the bottom to fine argillite/shale units at the top. Each complete graded bed unit ranges from 15 to 50 cm in thickness and represents normal graded beds. Chlorite phyllite is seen around the central and

A.G. UGARKAR AND R.C. NYAMATI

248

eastern auriferous zones. Quartz-sericite phyllite is seen resting on the chlorite phyllite. An arcuate shape band of gametiferous quartz-mica schist occurs in the amphibolite. At places, thin bands of carbonates occur within the fracture planes in highly sheared chlorite phyllite. Granites and granodiorites intrude the supracrustal units. The dykes have intruded all the rock types exposed in the study area. The rocks have undergone metamorphism of greenschist to lower amphibolite facies, as indicated by the mineral assemblages like chlorite, hornblende, actinolite, Na-plagioclase,biotite, muscovite, sericite, vein quartz, calcite, iron sulphides, etc. In general, the greenschist facies assemblages are predominantly restricted to the central parts of the area which gradually grade into lower amphibolite facies on either side near the contact of Peninsular gneiss. Medium grade metamorphic assemblage like garnetiferous quartz-mica schist is restricted to northeastern parts. The schistosity is more pronounced along with the secondary foliations in the metasedimentary lithounits.

Petrography of Clastic Metasediments (Metagreywackes and Phyllites) Metagreywackes are greenish grey, fine to coarse grained rocks. The clasts are angular, subangular to subrounded. These clasts are set in a matrix of chlorite, sericite and calcareous material. They have an average modal composition (Table 1) of 21% quartz, 29% lithic fragments (6% volcanic, 23% chert+quartzite), 11% feldspars (3% K-feldspar, 8%plagioclase) and 39% matrix (33% chlorite +sericite, 6% calcareous). Volcanic lithic fragments consist of andesite and rhyolite. Both mono and polycrystalline quartz grains have been noticed, and

are almost equal in abundance. Plagioclase is mostly andesine. The rock has undergone metamorphism that has induced schistosity, and the matrix minerals are oriented. The plots of these metagreywackes in QFL diagram (Fig. 2) of sandstone classification clearly indicate

A

Quartz wacke

I

L

F

Fig. 2. QFL diagram of sandstone classification for metagreywackes of Gadag.

that they are lithic greywackes. The plots are not scattered much, indicating almost uniform mineralogical composition of metagreywackes. Plagioclase is higher in abundance than K-feldspar and their ratio (plagioclase/ K-feldspar) is more than 1 (av. 2.2). Abundance of quartz and chert quartzite is almost equal (Fig. 3) and their ratio (quartz/chert+quartzite) is 0.9. Ratio of lithic fragments to feldspars is high (av. 2.7). There is a

+

Table 1. Modal composition of the metagreywackes. Minerals Quartz (Q) K-feldspar (K) Plagioclase (P) Lithic fragments (L) Volcanics+ glass (v) Chert (c)+quartzites(q) Matrix (M) Chlorite+sericite+biotite Carbonate+ opaque P/K Q/c+q) VK+ P c+q/v Mineralogical Maturity Index

1

2

3

4

5

6

7

8

9

Av.

Ch

14.50 2.40 8.60

21.85 4.55 6.75

24.00 3.65 7.75

22.52 3.10 8.30

21.30 3.30 7.50

16.75 2.30 5.90

16.95 2.25 8.80

18.75 2.35 7.95

20.90 6.25 5.60

21.33 3.35 7.46

37.70 3.90 8.00

7.75 17.40

3.00 21.10

4.30 26.45

4.35 25.40

5.95 31.45

8.70 15.55

9.40 17.75

6.80 28.10

6.50 26.60

5.77 23.31

13.40

44.85 3.85 3.60 0.80 2.30 2.30

32.20 9.35 1.50 1.00 2.70 7.00

27.75 5.25 2.10 0.90 2.70 6.20

28.68 6.70 2.70 0.90 2.60 5.80

25.60 4.05 2.30 0.70 3.40 5.30

38.85 10.95 2.60 1.10 2.90 1.80

40.95 4.15 3.90 09.0 2.50 1.90

31.30 3.85 3.40 0.70 3.40 4.10

28.95 4.30 0.80 2.80 4.10

33.21 5.83 2.20 0.90 2.70 4.20

31.30 4.50 2.00 2.80 1.10

0.20

0.40

0.50

0.40

0.50

0.30

0.30

0.40

0.40

0.40

1.50

0.90

-

-

Ch - Chitradurga metagreywackes (Naqvi et al., 1988).

Gondwana Research, V. 5, No.1,2002

ARCHEAN CLASTIC METASEDIMENTS OF GADAG GOLD FIELD, INDIA

Q

Fig. 3. Q-v-(c+q) diagram for metagreywackes of Gadag indicating dominance of sedimentary (c+q) rock fragments over volcanic (v) fragments.

dominance of sedimentary lithic fragments (chert -t quartzite) over volcanic lithic fragments (Fig. 3), and their ratio is high (av. 4.2). The mineralogical maturity index is very low (av. 0.4). Total quartz, namely quartz, chert and quartzite are around 44%, which shows reworking of the basin. The percentage of chert quartzite is around

+

249

23%, which further suggests that quartzite which was deposited within the basin has been reworked. Metagreywackes of Gadag are mineralogically distinct from the greywackes of Chitradurga which are quartzrich greywackes (Table 1). Gadag metagreywackes are relatively enriched in lithic fragments and matrix, and are depleted in quartz compared to Chitradurga. Gadag metagreywackes contain abundant volcanic lithic fragments whereas Chitradurga metagreywackes seldom contain volcanic lithic fragments. The Chitradurga sediments were probably derived from the Archaean upper crust and deposited on an active continental margin (Naqvi et al., 1988). Chlorite phyllites are well foliated and are composed of small, elongated grains of chlorite, quartz, biotite and plagioclase which have preferred alignment and are embedded in the clayee matrix of feldspars and chlorite. Quartz-sericite phyllites are greenish, medium grained and consist of quartz, sericite and chlorite with biotite and opaque iron oxides as accessories.

Geochemistry Representative samples of clastic metasediments (metagreywacke, chlorite phyllite and quartz-sericite phyllite) of Gadag Gold Field were analysed for major

Table 2. Chemical analyses of metaxrewackes. 2

3

4

5

6

7

8

9

10

Av.

Ch

62.18 0.71 14.13 5.82 2.38 4.11 3.18 3.01 0.16 0.49 2.97 99.14

59.86 0.72 14.65 8.91 2.60 3.96 3.00 2.96 0.16 0.18 2.44 99.08

63.48 0.69 13.99 6.23 2.18 3.94 3.08 2.97 0.12 0.31 2.84 99.83

66.82 0.56 12.42 9.12 2.40 3.04 2.34 2.10 0.16 1.12 100.26

61.86 1.50 17.92 7.03 2.48 1.78 1.72 2.98 0.12 0.34 3.20 99.93

61.76 0.58 14.81 8.11 1.59 3.24 2.94 2.52 0.15 0.17 3.04 98.91

71.36 0.46 12.28 7.01 1.78 2.21 2.48 1.26 0.08 0.07 1.22 100.21

66.94 0.52 13.83 8.46 1.88 2.65 2.58 1.87 0.11 0.08 1.26 100.18

67.04 0.48 13.79 8.64 1.96 2.84 2.83 1.61 0.09 0.11 0.84 100.23

63.14 0.66 16.00 6.98 2.31 1.43 2.03 2.20 0.15 0.08 4.05 98.95

64.44 0.69 14.38 7.63 2.16 2.82 2.62 2.35 0.12 0.20 2.30 99.67

64.52 0.42 12.68 7.72 2.82 2.21 2.86 1.50 0.16 0.11

70 34 148 57 456 112 139 14

65 30 161 61 502 108 143 12

75 29 147 67 478 119 138 16

80 43 168 170 313 88 68 21

78 29 175 188 426 113 298 8

63 41 264 191 480 146 196 23

62 32 196 200 278 100 96 29

75 46 170 180 294 108 60 24

71 39 168 180 300 98 58 25

102 52 191 208 43 1 112 334 8

74 38 179 150 396 125 153 18

101

21 244 113 275 132 125 18

26.13 4.44 1.26 0.95 2.1 1 8.00

23.02 4.88 1.14 0.99 2.48 9.00

29.12 4.54 1.36 0.96 1.96 7.44

27.84 5.31 0.88 0.90 2.10 4.19

24.94 10.42 1.20 1.73 2.24 14.13

38.84 5.04 1.58 0.86 4.19 6.35

40.09 4.95 0.71 0.51 3.16 3.45

35.61 5.36 0.99 0.72 2.27 4.50

34.20 4.87 0.82 0.57 2.37 3.92

27.33 7.88 0.95 1.08 1.87 14.00

29.83 5.49 1.09 0.90 2.42 6.94

22.88 4.43 0.53 0.52 2.30 7.90

1

SiO, TiO, AP, FeO(t) MgO CaO Na,O K,O

MnO P A LO1 Total Ni CO

Cr V

Ba Zr Sr

Y SiOJMgO AI,OJNa,O YO/MgO &O/Na,O Cr/Ni Zrff

0.10

Ch - Chitradurga metagreywacke (Naqvi et al., 1988).

Gondwana Research, V. 5, No.1,2002

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250

A.G. UGARKARAND R.C. NYAMATI

Table 3. Chemical analyses of chlorite phyllite.

SiO, TiO, A1203

FeO(t) MgO CaO Na,O

YO

MnO '2'5

LO1 Total

Ni co Cr V Ba Zr Sr Y SiOJMgO AI,OJNa,O K,O/MgO K,O/Na,O Cr/Ni ZrN

1

2

3

4

5

6

7

8

Av.

Ch

57.11 1.00 13.53 11.46 4.78 3.74 3.96 0.93 0.18 0.18 3.35 100.22

58.42 0.88 11.86 11.56 4.13 4.72 1.96 1.38 0.18 0.11 5.01 100.21

55.14 1.08 15.83 16.02 3.54 0.19 1.63 2.60 0.05 0.11 4.05 102.05

62.34 0.77 13.68 8.28 3.21 2.26 4.03 1.62 0.13 0.14 3.53 99.99

56.47 0.95 17.94 9.96 4.13 1.21 3.64 2.03 0.10 0.18 2.86 98.96

63.06 0.72 10.14 9.18 3.10 5.46 1.81 1.01 0.17 0.10 5.85 100.06

55.73 0.92 13.41 13.88 5.01 3.93 2.11 1.83 0.19 0.10 2.42 99.53

58.03 0.87 13.88 12.04 3.51 3.08 2.75 1.81 0.14 0.13 3.76 100.13

58.24 0.61 17.65 9.06 3.30 0.83 0.79 3.76 0.18 0.12 -

28 43 384 136 193 210 34 38

84 53 94 139 122 100 119 14

117 23 428 284 260 84 312 45

44 28 381 130 408 175 78 30

94 52 218 184 136 284 139 9

86 50 176 150 144 138 142 13

55.97 0.68 14.71 12.84 3.51 3.12 2.84 3.04 0.15 0.15 3.04 100.05 38 42 346 128 188 195 38 35

86 58 25 130 124 48 115 12

72 44 257 160 197 154 122 25

80 34 118 91 563 163 296 28

11.95 3.42 0.19 0.23 13.71 5.53

14.15 6.05 0.33 0.70 1.12 7.14

15.58 9.71 0.73 1.60 3.66 1.87

19.42 3.39 0.50 0.40 8.66 5.83

13.67 4.93 0.49 0.56 2.32 31.56

20.34 5.60 0.33 0.56 2.05 10.62

15.95 5.18 0.87 1.07 9.11 5.57

11.12 6.36 0.37 0.87 0.29 4.00

16.53 5.05 0.52 0.66 3.57 6.16

17.65 22.34 1.14 4.76 2.20 7.20

Ch - Chitradurga chlorite phyllite (Naqvi et al., 1988).

and selected minor elements by using Atomic Absorption Spectrometer (Varion, Australia). Standardisation was based on the USGS rock standards (Flanagan, 1973). To minimise the analytical errors, solutions of each sample were fed thrice and the average was taken. The analyses are presented in tables 2,3 and 4. As the Gadag belt forms the northern continuation of a well-known Chitradurga belt, we have compared the chemical data of Gadag with that of the published data on corresponding clastic metasediments of Chitradurga belt. However, chemical data on quartz-sericite phyllite of Chitradurga belt are not available. Metagreywackes have higher concentrations of SiO, and Ba and while lower, concentrations of Al,O,, FeO, MgO and CaO than the chlorite phyllites. As compared to chlorite phyllites, the quartz-sericite phyllites have higher SO,, K,O, Ba, Zr and Sr contents, and lower FeO', MgO, CaO, Ni, Co, Cr and V contents. Distinctions could be made in terms of average values of selected major and minor elements like SO,, FeO', MgO, Cr, Ba and Sr of these clastic metasediments (Tables 2, 3 and 4). Major and minor elements in clastic metasediments (metagreywackes and phyllites) are almost uniformly distributed. The chemical compositions of metagreywackes of Gadag and Chitradurga belts are comparable, excepting that Gadag metagreywackes have higher Y O contents and higher &O/Na,O, &O/MgO ratios, while chlorite phyllites

Table 4. Chemical analyses of quartz-sericite phyllite. ~~

SiO, TiO, FeO(t) MgO CaO Na,O

50

MnO p,o5 LO1 Total Ni co Cr V Ba Zr Sr

Y SiO,/MgO A1,03/Na20 K,O/MgO K,O/Na,O Cr/Ni Zr/Y

~

1

2

3

4

Av.

70.53 0.321 14.68 2.01 0.51 1.53 5.11 3.62 0.05 0.11 1.53 99.89

75.93 0.09 12.98 1.23 0.63 1.42 0.92 3.96 0.02 0.03 2.9 100.12

74.11 0.09 13.48 1.78 0.39 1.21 3.78 4.12 0.05 0.05 1.17 100.23

76.00 0.16 12.96 1.83 0.45 0.93 2.13 3.61 0.03 0.1 1.74 99.94

74.14 0.14 13.53 1.71 0.5 1.27 2.99 3.83 0.04 0.07 1.84 100.05

37 13 116 28 45 1 175 194 12

14 13 18 36 93 98 288 23

14 13 34 93 28 1 380 384 45

24 12 71 44 380 176 263 25

22 13 60 50 301 207 282 26

138.29 2.87 7.10 0.71 3.14 14.58

120.52 14.11 6.29 4.30 1.29 4.26

190.03 3.57 10.56 1.09 2.43 8.44

168.89 6.08 8.02 1.69 2.96 7.04

148.28 4.53 7.66 1.28 2.73 7.96

have higher contents of Al,O,, K20, Ba, Sr and higher Al,O,/Na,O, &O/MgO, &O/Na,O ratios than the phyllites Gondwana Research, V. 5, No. 1,2002

ARCHEAN CLASTIC METASEDIMENTS OF GADAG GOLD FIELD, INDIA

of Chitradurga. It is of interest to note that SiO, content of metagreywackes of Gadag and Chitradurga are equal. However, in Gadag greywackes the modal quartz is significantlyless and modal chert+quartzite is more when compared with metagreywackes of Chitradurga. Amount of total quartz (namely, quartz+chert+quartzite) is rather high (av. 44%) while SiO, content is moderate (av. 64.44%). Ratio of K-feldspar/plagioclase is very low (av. 0.38), while K,O/Na,O ratio is high (av. 0.9). It is believed that the ratio of &O/Na,O is controlled by relative proportion of K-feldspar to albitic plagioclase while the SiO, content is controlled by abundance of quartz. However, the role of matrix and its composition cannot be ruled out for understanding the bulk chemical composition of greywackes. Resistites (minerals which resist weathering) like quartz, chert and quartzites have retained their identity, whereas other rock fragments of less resistive composition (volcanic) have gone into the matrix. The abundance of matrix in Gadag samples is high (av. 39%) and it is mainly composed of chlorite, sericite, biotite along with minor amount of carbonates and opaque. Therefore, most of the FeO, MgO, K,O and CaO is contributed by the matrix. The K,O/Na,O ratios of metagreywackes and chlorite phyllites are typically less than 1, which is comparable to that of most of clastic metasediments formed prior to 2.5 Ga (Engel et al., 1974). Degree of weathering undergone by source rocks could be obtained by 'Chemical Index of Alteration' (CIA) proposed by Nesbitt and Young (1982). Accordingly, the plot of Al,O, against Al,O,+Na,O+CaO+K,O shows that the CIA values for Gadag metasediments are around 70 (Fig. 4), indicating relatively chemically unweathered source. 20.0

15.0

e 10.0 Q: 0

5.0

0.0

0

5

10

15

20

25

30

AI,O,+Na,O+CaO+K,O

Fig. 4. Plots of A1,0, vs. AI,O,+Na,O+CaO+K,O to estimate the Chemical Index of Alterations (CIA) affecting the clastic metasedimentary rocks of Gadag (figure after Nesbitt and Young, 1987).

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Except one, all samples of chlorite phyllites contain FeOt less than 13.88 wt% (av. 12.04) and quartz-sericite phyllites have FeO' less than 1.23 (av. 1.91), i.e., they have FeOt less than 15 wt %. This feature indicates that these phyllites cannot be designated as fenuginous shales (see Gross and McLead, 1980). The abundance of Cr and Ni, and Cr/Ni ratio in metagreywackes (Cr=179 ppm, Ni=74 ppm, Cr/Ni=2.4 ppm) and chlorite phyllites (Cr=257 ppm, Ni=72 ppm, Cr/Ni=3.6 ppm) are higher. Abundance of mafic group elements like Cr and Ni, which is characteristic feature of Gadag clastic metasediments, has also been recorded from other places (Feng and Kerrich, 1990; Manikyamba et al., 1997). These features also suggest for a more basaltic nature of the source area for these clastic metasediments (see Naqvi et al., 1988). These are characteristics of metasediments of Archaean supracrustal belts in general (Taylor and McLennan, 1985; Condie and Wronkiewicz, 1990). The Al,O,/Na,O ratios are less than 6, which according to Garrels and Mackenzie (1971) indicate chemical immaturity of the sediments. This is further supported by the lower ratios of K,O/Na,O of these metasedimens.

Provenance The clastic metasediments of Gadag Gold Field are characterized by higher concentrations of SiO, and Zr, higher SiO,/MgO and Zr/Y ratios, suggesting felsic component in the provenance. While higher abundance of Cr and Ni and low K,O/MgO ratios suggest a more basaltic nature of the source area for these rocks (see also Naqvi et al., 1988; Condie and Wronkiewicz, 1990). Naqvi et aI. (1988) have suggested that the metagreywackes of Gadag show REE patterns that resemble more the ones obtained for present day upper crust or the post-Archaean sediments. They are characterized by high L a m ratio (av. 42) and negative Eu anomaly and overall REE enrichment, suggesting for a component of granitegranodioritic rocks in the provenance which was a mixed mafic-felsic crust (see also Srinivasan and Naha, 1993). Further, the pebbles especially of gneiss, granite, quartzite and vein quartz in the metaconglomerate points to the sialic crustal component in the provenance. This is further substantiated by the associated metasedimentary litho units like quartz-sericite phyllite and garnetiferous quartzmica schist of the sequence. Metavolcanic sequence of Gadag, however, is older than the metasediments. The metaconglomerates of Chitradurga belt including Gadag belt are not indicative of unconformities, but denote a turbidite environment of sedimentation (see Naqvi et al., 1978). Volcanic fragments within metaconglomerate and metagreywacke clearly suggest that some contribution

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from the intrabasinal volcanic rocks for the clastic metasediments of Gadag cannot be ruled out. Thus these different parameters suggest a mixed felsic and mafic component in the provenance. It is worth to mention here that the provenance characters of the greenstone belts of the Dharwar craton has been a question of debate as to whether it was dominantly mafic or felsic. On the basis of higher Cr and Ni abundance in the clastic metasediments, Naqvi et al. (1988) have suggested for a mafic crust. While Chadwick et al. (1986) have argued for a sialic basement on the basis of fairly common detrital zircon in the clastic metasediments. The bulk of the sialic crust (i.e., Peninsular gneiss) in the western block of Dharwar craton evolved over a protracted period from -3.4 to 3.0 Ga through juvenile addition from mantle as well as through rapid differentiation of older crustal components (Bhaskar Rao et al., 1983; Radhakrishna and Naqvi, 1986; Rogers et al., 1986; Meen et al., 1992). This resulted in a highly evolved upper continental crust in the western block by -3.0 Ga. In fact, presence of K-rich granitoids -3.0 Ga ago has been identified by sedimentological criteria (Naqvi, 1983; Srinivasan and Ojakangas, 1986). Such an evolved crustal source provided debris for the late Archaean volcano-sedimentary basins of the western block of Dharwar craton. Based on the consistent negative Eu anomaly in the Archaean clastic sedimentary rocks, Srinivasan and Naha (1993) have suggested for a large component of granite-granodioritic rocks in the provenance, which was a mixed mafic-felsic crust.

Tectonic Setting The polymict metaconglomerate which conformably overlies the metavolcanics, contains pebbles of gneiss, granite, chert, andesite and rhyolite, and the immature metagreywackes with lithic fragments like quartzite, chert, andesite and rhyolite give credence to the sialic dominant provenance with an arc and active continental margin type setting (see Prothero and Schwab, 1996). Various sedimentary structures encountered in the area, indicate that these metasediments were deposited in an unstable environment like continental margin setting (geosyncline) (Chakrabarthi et al., 1993). While the pillow structures of metabasalts suggest a submarine volcanism. The characteristic bimodal volcanic association (maficintermediate-felsic) itself manifests an island arc and continental margin settings (see Boillot, 1981). Further, Ugarkar et al. (2000), based on the petrogenesis, geochemistry and lithological assemblages of metavolcanics of Gadag Gold Field, have suggested that these rocks were emplaced in an active continental margin or a continental island arc setting.

In QFL diagram of Dickinson et al. (1983) used for discriminating the tectonic provenance of sandstones, metagreywackes distribute within the dissected arc and transitional arc fields of magmatic arc (Fig. 5). Q

Fig. 5. QFL diagram (after Dickinson et al., 1983) for metagreywackes of Gadag indicating dissected and transitional arc tectonic provenance for them.

Bhatia (1983) has provided the discriminate functions based on the Fe,O,+MgO content along with &O/Na,O, Al,O,/SiO,, TiO, and Al,O,/CaO +Na,O ratios of sandstones of Phanerozoic and Recent ages to characterise various tectonic settings of sedimentation. But while 9-

6-

0

1

1

2

3

4

A D

5

Chlorite Phyllite Quartz-sericite phyllite

6

7

6

1

-..

9

MgO Fig. 6. K,O-MgO plots (after Naqvi et al., 1988) for clastic metasediments of Gadag. A-Granite, B-Gneiss, C-ACM, D-PM, E-CIA, F-OIA, G-Basalt.

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Table 5. Chemical characteristics of greywackes of various tectonic settings (after Bhatia, 1983) and clastic metasediments of Gadag.

TiO, Al,O$SiO, YO/Na,O Al,O$CaO+Na,O SiO,

Oceanic Island Arc

Continental Island Arc

Active Continental Marrrin

(OM) 1.06 0.29 0.39 1.72 58.33

(CIA) 0.64 0.20 0.61 2.42 70.69

(ACM) 0.46 0.18 0.99 2.56 73.86

applying these criteria it is necessary to remember that the Archaean clastic sediments have some special characteristics which are not normally seen in such clastic sediments of later ages (Srinivasan and Naha, 1993). Further, sedimentation in unstable basins of Archaean age often brings in considerable amount of iron from the underlying iron formations enhancing the Fe,O, content much higher than in sediments of later ages (see Naqvi et al., 1988). In view of this, Naqvi et al. (1988) have constructed a simple plot of K 2 0 vs. MgO and the fields of various tectonic settings from the data of Bhatia and Crook (1986) were marked (Fig. 6). In this diagram, metagreywackes exhibit Active Continental Margin (ACM) and Continental Island Arc (CIA) affinities, chlorite phyllites exhibit Oceanic Island Arc (OLA) affinity while quartz-sericite phyllites show Active Continental Margin (ACM) affinity. A comparison of geochemical parameters like TiO,, Al,O,/SiO,, K,O/Na,O and Al,O,/CaO +Na,O ratios with the sandstones of different tectonic settings (Bhatia, 1983) and Gadag clastic metasediments suggest that metagreywackes have ACM and CIA characteristics, chlorite phyllites have CIA and OIA while quartz-sericite phyllites have mostly Passive Margin (PM) characteristics (Table 5). Trace element parameters like Ce (81 ppm), La (50 ppm), La/” (1.79), Rb/Sr (0.9) and Ti/Zr (24) of Gadag metagreywackes (data from Naqvi et al., 1988) which are considered to be useful for tectonic setting discrimination are suggestive of tectonic settings like CIA and ACM (see Bhatia and Crook, 1986). In CaO-Na,O-K,O diagram (Fig. 7) of Bhatia (1983), the plots of greywackes distribute in CIA setting, chlorite phyllites distribute in CIA and OIA while quartz-sericitephyllites distribute mostly in ACM field. In SiO,/Al,O, K,0/Na20 diagram (Fig. 8) of Roser and Korsch (1986), the greywackes and chlorite phyllites mostly plot in ACM setting and quartz-sericite phyllites plot in ACM and PM settings. The clastic metasediments although occur together (juxtaposed ?) in Gadag Gold Field, they show different Gondwana Research, V. 5, No. 1,2002

Passive Margin (PM) Depleted 0.10 1.60 4.15 81.95 Enriched

Greywackes

Gadag chloritephyllite

Quartz-sencite phyllite

0.87 0.24 0.75 1.64 58.03

0.14 0.21 1.95 4.09 74.14

0.69 0.22 0.90 2.64 64.44

CaO

Na,O Fig. 7. CaO-Na,O-YO ternary compositional plot (after Bhatia, 1983) indicating tectonic setting fields for clastic metasedimentary rocks of Gadag. X-Metagreywackes,solid circle-chloritephyllite, open circle-quartz-sericite phyllite.

0.01

0.1

1

10

100

WNap

Fig. 8. Tectonic setting discrimination diagram using SiOJAl,O, and YO/Na,O relations (after Roser and Korsch, 1986) for Gadag clastic metasediments. PM-Passive Margin, ACM-Active Continental Margin, A 1 =Arc setting (basaltic and andesite detritus), A2=Evolved Arc setting (felsic-plutonic detritus).

tectonic settings. Metagreywackes have CIA+ACM, chlorite phyllites have CIA+ OIA, while quartz-sericite

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phyllites have ACM+PM tectonic settings. It is not possible to have all these tectonic settings in one subduction complex. These metasediments of Gadag Gold Field having different tectonic settings can be brought together by accretion processes during convergence of plates. Further greenstone belts are believed to be made up of different accretionary prisms brought into juxtaposition by converging processes (Lowe, 1994; Polat et al., 1998). Similarly the Archaean greenstone belts like Kolar, Ramagiri and Sandur belts of Dharwar craton are interpreted to be made up of accretionary prisms brought into juxtaposition by converging processes (see Krogstad et al., 1989; Zachariah et al., 1996; Manikyamba et al., 1997; Manikyamba et al., 2000).

Conclusions The clastic metasediments (metagreywacke, chlorite phyllite and quartz-sericite phyllite) of Gadag Gold Field had a sialic dominant, mixed felsic and mafic source in the provenance. Metagreywackes were deposited in the continental island arc + active continental margin, chlorite phyllites in the continental island arc + oceanic island arc and quartz-sericite phyllites in an active continental margin passive margin tectonic settings. It is suggested that these clastic metasediments which had different tectonic settings were probably brought together (juxtaposed ?) by accretion processes during convergence of plates. However, geochronogic and rare element data on a large number of samples of volcano-sedimentary rocks of this area is required to test this accretionary model.

+

Acknowledgments The authors are thankful to the Chairman, Department of Geology, Karnatak University, for providing facilities to work. This work is a part of CSIR-ResearchAssociateship of AGU, and he is thankful to the Chairman, Department of Geology, University of Poona, for providing facilities to analyse the samples on AAS. AGU is grateful to the CSIR, New Delhi for the support in the form of Research Associateship. We appreciate and thank both the referees of Gondwana Research for their critical and valuable comments and suggestions to improve our paper.

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