Geochemical characteristics of the Late Cretaceous

Geochemical characteristics of the Late Cretaceousradiolarian cherts from North Andaman Island,Bay of Bengal, India

S H JAFRI, D SRINIVASA SARMA*, TAVHEED KHAN and D K SINGH

CSIR-National Geophysical Research Institute, Uppal Road, Hyderabad 500 007, India.*Corresponding author. e-mail: [email protected]

MS received 4 July 2019; revised 2 January 2020; accepted 19 January 2020

The radiolarian cherts of Late Cretaceous age are reported in the Andaman-Nicobar ophiolites of Bay ofBengal, India. They are of chocolate-coloured (Type-I) and light pink-coloured (Type-II) varieties, juxta-posed and tectonically associated with phyllites, claystones and basalts and are exposed in eastern part ofNorth Andaman Island. These cherts are composed of radiolarian tests in a Bne-grained ferruginous matrix.Type-I radiolarian cherts have lowSiO2, and highAl2O3, Fe2O3

(T),MgO,Na2OandTiO2 and trace elements ascompared to Type-II cherts. Al–Fe–(

PREE–Ce) diagram of the studied cherts indicates a mixed terrigenous

and volcanogenic source. LaN/CeN ratios (0.76–0.89 for Type-I and 0.71–0.88 for Type-II) and Ce-anomalies(Ce/Ce* = 1.15–1.33 for Type-I and 1.07–1.38 for Type-II) and other elemental ratios in these cherts suggestthat they were deposited in continental margin environments. It has been suggested that the studied samplesof cherts were deposited at different places, were scrapped oA the subducting plate, became tectonicallyjuxtaposed. They were obducted onto the leading edge of the Eurasian continent during the Late Cretaceousprior to the currently activeAndaman–Java subduction, thatwas probably initiatedduring theLateMiocene.

Keywords. Radiolarian cherts; geochemistry; Andaman Island; Bay of Bengal; India.

1. Introduction

Radiolarian cherts in ophiolites are conventionallyconsidered to be pelagic deposits derived from abyssalradiolarian oozes (Kanmera 1976; Suzuki and Hada1979). However, the deposition of the radiolariancherts also taken place in varied oceanic environ-ments (Jenkyns and Winterer 1982). The trace, rareearth element (REE) abundances and their ratios arehelpful in understanding their depositional environ-ments (Sugisaki et al. 1982; Steinberg et al. 1983;Murray et al. 1991; Murray 1994). Radiolarian chertsinAndaman-Nicobar ophiolites are exposed in severalplaces. These cherts are of Late Cretaceous to EarlyEocene time (Roy et al. 1988) or Late Cretaceous to

Middle Eocene age (Ling et al. 1996) and are depositedin shallow marine environments (Jafri et al.1993). In the ophiolitic suite of rocks from NorthAndaman Island, the radiolarian cherts are exposed.There is no geochemical study carried out on theseradiolarian cherts. In this paper, geochemical studieshave been presented to better understand theirprovenance and depositional environments.

2. Regional geology

The Andaman-Nicobar Islands of Bay of Bengalconstitute a part of Sunda–Burmese double chainarc system, extending from the Indo-Burman

J. Earth Syst. Sci. (2020) 129:103 � Indian Academy of Scienceshttps://doi.org/10.1007/s12040-020-1368-2 (0123456789().,-volV)(0123456789().,-volV)

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ranges to the north with the Indonesian Islands inthe south (Hamilton 1979; Bgure 1). The part ofthe outer sedimentary arc is represented byAndaman-Nicobar Islands while the part of theinner volcanic arc is represented by the Barren andNarcondam Islands. In the outer sedimentary arc,the basement of the Tertiary sediments is charac-terised by the ophiolites and the overlying Tertiarysediments are classiBed into the Mithakhari,Andaman Flysch and Archipelago Groups (inascending order) and the ages of these groups are:(i) Cretaceous to Paleocene for Ophiolite suite, (ii)Lower to Middle Eocene for Mithakhari Group,(iii) Late Eocene to Oligocene for Andaman FlyschGroup, and (iv) Miocene–Pliocene for ArchipelagoGroup (Karunakaran et al. 1964; Ray et al. 1988;Pal et al. 2003; Bandopadhyay 2005).The ophiolites of the Andaman-Nicobar Islands

are well exposed in Andamans as compared to theNicobar group of Islands (Bgure 2a). These ophio-lites occur as dismembered units, and are consid-ered as accreted and uplifted oceanic basementrocks, and mainly consists of ultramaBcs(harzburgite, wherlite, pyroxenite and dunite),gabbro, East Coast Volcanics (basalts and

andesites), lower and upper pillow basalts, pla-giogranites, dacites, basic dykes and radiolariancherts (Vohra et al. 1989; Jafri et al. 2010; Jafri andSheikh 2013). In these ophiolites, the radiolariancherts are present as minor constituents, andbecause of their small exposures, it is difBcult tomap them separately (Karunakaran et al. 1964;Ray et al. 1988). Paleontological studies on thesecherts indicate their deposition to be of UpperCretaceous (Karunakaran et al. 1968; Vohra et al.1989), Upper Cretaceous–early Eocene (Roy et al.1988; Ling et al. 1996) or Late Cretaceous toMiddle Eocene age (Ling et al. 1996). Recent U–Pbdata for plagiogranites which intrude these ophio-lites (Jafri et al. 1995), show their magmatic crys-tallization age at 93.6± 1.3 Ma (Sarma et al. 2010),or at 95 ± 2 Ma (Pederson et al. 2010), suggestingthe age of these ophiolites to be of middle to upperCretaceous time.Ophiolites are also well exposed in North

Andaman Island (Bgure 2b), which mainly consistsof quartz–mica schists, ultramaBcs, gabbros,basalts, cherts, limestones and red clays (Pal et al.2003; Pal and Bhattacharya 2010). The studiedradiolarian cherts are exposed near Lamia Bay, in

Figure 1. Tectonic map of a part of the northeastern Indian ocean and location of the Andaman-Nicobar Islands. (Map afterCurray 2005).

103 Page 2 of 10 J. Earth Syst. Sci. (2020) 129:103

the eastern part of North Andaman Island. Thesecherts are massive and are associated with phyl-lites, claystones, and basalts. Based on colour andtexture; they can be grouped as chocolate and lightpink-coloured varieties, which are juxtaposed witheach other. The chocolate-coloured cherts areabout a meter thick (Bgure 3a), while the lightpink-coloured variety is about 3 m thick, are Bnelylaminated. Light pink-coloured radiolarian chertsare intercalated with phyllites and claystones andhave been aAected by shearing and are traversedby numerous silica veins (Bgure 3b).

3. Petrographic studies

The chocolate-coloured radiolarian cherts arecomposed of radiolarian tests in a Bne-grained darkcoloured ferruginous clayey matrix. In these cherts,along with the radiolarian tests, a few foraminiferatests are also noticed. The radiolarian tests areintact in nature (Bgure 4a) and the radiolariantests are mostly Blled by secondary silica, while

some of them are Blled with ferruginous material.In few sections, very thin silica veins are alsonoticed. While, the light pink-coloured radiolariancherts consist of radiolarian tests in a Bne-grainedlight-coloured ferruginous matrix, and the radio-larian tests are invariably ellipsoidal and are Blledwith ferruginous material and numerous silicaveins (Bgure 4b). They have also been intenselyaAected by shearing and faulting even on micro-scopic scale (Bgure 4c).

4. Methods

Major and trace element analyses of the represen-tative samples of radiolarian cherts were done byX-ray Fluorescence Spectrophotometer (XRF)(Philips MagiX PRO, model PW 2440) and Induc-tively Coupled Plasma mass Spectrometer (ICP-MS) (ELAN DRC II, Perkin Elmer, Inc., Waltham,MA, USA), at the CSIR-National GeophysicalResearch Institute, Hyderabad, India. The precisionand accuracy of data for major (Krishna et al. 2007)

Figure 2. (a) Generalized geological map of Andaman Islands showing the distribution of ophiolite and (b) geological map ofNorth Andaman (map after Pal et al. 2003). R denotes the location of radiolarian cherts.

J. Earth Syst. Sci. (2020) 129:103 Page 3 of 10 103

and trace elements (Balaram and Rao 2003) havebeen reported. Analytical data presented here havethe reproducibility of measurements better than±5% for all the trace and rare earth elements.

5. Discussion

Major and trace elements including REE data ofthe radiolarian cherts (both chocolate and light-pink coloured types) are presented in table 1. Thechocolate-coloured cherts have relatively low SiO2

(72.90–78.90 wt.%) and high Al2O3 (5.63–7.11wt.%), Fe2O3 (4.45–5.20 wt.%), MgO (2.26–5.14wt.%), Na2O (3.27–5.08 wt.%) and TiO2 (0.45–0.66wt.%) as compared to light pink-coloured cherts,which is higher in SiO2 (88.20–90.30 wt.%) and lowin Al2O3 (1.50–1.96 wt.%), Fe2O3 (1.27–1.73wt.%), MgO (1.12–1.84 wt.%), Na2O (0.27–0.92wt.%) and TiO2 (0.08–0.17 wt.%).

To understand the deposition of these cherts, thebehaviour of the major elements like Al2O3, TiO2,MnO, Fe2O3 and their ratios are most useful. Al2O3

Figure 3. (a) Field photo of radiolarian cherts showing bothdark-brown chocolate coloured (Type-I) and light-pinkcoloured (Type-II) varieties and (b) Type-II cherts aretraversed by numerous silica veins and have been aAected byshearing.

Figure 4. (a) Photomicrograph of Type-I cherts showingradiolarian tests in a dark, Bne-grained ferruginous matrix,in which the intact nature radiolarian tests are seen, (b) Pho-tomicrograph of Type-II radiolarian cherts showing radiolar-ian tests in a Bne-grained ferruginous matrix, in which all theradiolarian tests are ellipsoidal and have been traversed bynumerous silica veins, and (c) Photomicrograph of Type-IIradiolarian cherts showing the faulting of radiolarian tests.


Table

1.Major,Trace

andREEof

radiolariancherts

from

North

Anda

man

Island.

Major

oxides

inwt.%,tracean

dREEsin

ppm.Normalisingvalues

ofMcD

onou

ghan

dSun

(199

5).1–9=

Type-Iradiolariancherts,10

–16

=Type-II

radiolarian

cherts.

12

34

56

78

910

11

12

13

14

15

16

AN33/

10-1

AN33/

10-2

AN34/

10-1

AN34/

10-2

AN35/

10-1

AN35/

10-2

AN35/

10-3

AN50/

6-2

AN50/

6-3

AN36/

10-1

AN18/

7-1

AN18/

7-2

AN18/

7-3

AN20/

7-1

AN49/

6-1

AN49/

6-2

SiO

275.40

75.63

77.21

78.16

73.65

72.90

74.10

78.00

78.90

90.30

89.89

89.35

90.20

90.20

90.00

88.20

TiO

20.59

0.65

0.53

0.49

0.65

0.57

0.66

0.48

0.45

0.08

0.12

0.15

0.12

0.09

0.17

0.14

Al 2O

36.55

6.32

6.86

6.73

6.88

6.49

7.11

6.10

5.63

1.60

1.89

1.88

1.96

1.50

1.80

1.87

Fe 2O

34.50

4.90

4.60

4.90

5.10

5.20

5.10

4.86

4.45

1.27

1.62

1.55

1.73

1.31

1.40

1.30

MgO

3.14

3.27

2.60

2.26

5.14

4.78

4.37

2.59

2.50

1.77

1.12

1.55

1.41

1.48

1.78

1.84

CaO

0.38

0.58

0.38

0.26

0.50

0.56

0.46

0.27

0.41

0.28

0.31

0.49

0.44

1.65

0.48

1.13

Na2O

4.19

4.84

4.50

3.95

4.64

4.97

5.08

3.52

3.27

0.41

0.35

0.41

0.33

0.27

0.57

0.92

K2O

1.58

0.76

0.48

0.67

0.38

0.62

0.42

0.78

0.65

0.48

0.84

0.73

0.81

0.48

0.78

0.79

MnO

0.15

0.13

0.12

0.09

0.13

0.11

0.13

0.07

0.08

0.11

0.12

0.15

0.12

0.12

0.10

0.21

P2O

50.05

0.05

0.05

0.05

0.06

0.07

0.07

0.06

0.06

0.04

0.06

0.08

0.06

0.07

0.06

0.04

LOI

1.77

1.62

1.42

1.30

1.72

1.90

1.65

1.47

1.62

1.73

1.75

1.86

0.91

1.04

1.92

1.65

Total

98.30

98.75

98.75

98.86

98.85

98.17

99.15

98.20

98.02

98.23

98.07

98.20

98.09

98.24

99.06

98.89

Fe 2O

3/TiO

27.63

7.54

8.68

10.00

7.85

9.12

7.73

10.13

9.89

15.88

13.50

10.33

14.42

14.56

8.24

9.29

Fe 2O

3/Al 2O

30.69

0.78

0.67

0.73

0.74

0.80

0.72

0.80

0.79

0.79

0.86

0.82

0.88

0.87

0.78

0.70

MnO/TiO

20.25

0.20

0.23

0.18

0.20

0.19

0.20

0.15

0.18

1.38

1.00

1.00

1.00

1.33

0.59

1.50

Sc

12.58

12.52

9.06

9.16

10.38

11.32

11.33

9.23

8.49

3.09

3.73

4.47

2.99

3.24

8.87

7.48

V95.35

99.81

51.67

49.54

58.72

63.65

62.84

36.13

34.71

6.82

18.52

11.11

10.71

5.62

11.71

11.32

Cr

31.53

35.34

28.27

124.87

125.64

128.00

127.14

35.57

26.34

107.67

14.31

12.73

14.56

25.47

20.15

16.37

Co

17.33

20.64

15.12

12.91

11.99

12.27

12.21

9.91

9.69

4.92

9.29

5.80

5.72

4.88

9.15

9.14

Ni

47.77

57.34

45.75

39.88

36.83

38.11

41.06

58.66

49.83

10.62

15.12

48.92

16.68

29.53

98.22

30.15

Cu

34.92

31.42

33.17

28.23

33.22

28.80

29.76

51.39

45.75

17.12

17.59

29.92

25.28

28.91

37.84

37.38

Zn

38.39

34.55

30.46

27.43

36.52

31.76

49.58

35.29

26.03

17.98

17.61

44.90

31.19

12.99

48.70

82.21

Ga

9.33

13.31

8.79

7.46

9.14

9.51

9.49

5.68

5.64

1.74

2.76

2.21

2.21

2.04

3.02

3.13

Rb

18.38

43.20

19.41

14.98

7.59

6.95

7.02

13.80

13.59

8.16

18.15

12.50

12.94

10.98

13.82

14.03

Sr

60.38

55.62

49.86

46.15

43.49

49.70

51.10

36.65

37.29

10.63

12.87

10.88

10.05

13.16

14.70

14.23

Y16.86

19.27

15.55

14.01

16.61

18.94

18.56

15.26

14.92

5.02

6.39

5.02

4.98

13.34

10.47

9.92

Zr

195.83

244.19

246.77

195.86

237.92

243.34

263.91

149.64

143.52

60.78

120.08

56.44

57.41

53.41

88.88

79.66


Table

1.(C

ontinued.)

12

34

56

78

910

11

12

13

14

15

16

AN33/

10-1

AN33/

10-2

AN34/

10-1

AN34/

10-2

AN35/

10-1

AN35/

10-2

AN35/

10-3

AN50/

6-2

AN50/

6-3

AN36/

10-1

AN18/

7-1

AN18/

7-2

AN18/

7-3

AN20/

7-1

AN49/

6-1

AN49/

6-2

Nb

3.49

5.67

4.36

3.85

4.69

4.84

4.88

2.28

2.19

0.86

1.34

1.07

1.07

0.97

2.61

2.44

Cs

1.22

2.78

1.41

1.09

0.56

0.53

0.51

1.05

1.01

0.33

0.89

0.68

0.69

0.60

0.64

0.64

Ba

90.84

111.61

86.39

84.07

135.21

127.21

135.48

65.12

58.09

55.95

132.65

67.80

66.50

47.54

57.23

53.13

La

13.45

13.04

12.02

10.59

12.43

13.82

13.75

8.28

7.89

5.37

7.17

4.76

4.87

3.41

5.64

4.69

Ce

34.48

32.17

29.91

26.33

28.40

30.65

30.13

20.57

19.68

12.46

16.11

10.88

11.26

7.52

15.51

12.91

Pr

3.48

3.34

3.08

2.61

3.29

3.50

3.50

2.17

2.09

1.43

1.96

1.34

1.37

1.01

1.60

1.30

Nd

13.93

13.22

12.14

10.62

12.72

13.64

13.57

9.14

8.78

5.49

7.99

5.42

5.64

4.33

6.18

5.29

Sm

3.05

2.79

2.38

2.14

2.79

2.88

2.83

1.95

1.99

0.97

1.59

1.10

1.15

1.02

1.30

1.09

Eu

0.75

0.66

0.59

0.54

0.75

0.78

0.79

0.49

0.49

0.15

0.37

0.26

0.29

0.27

0.31

0.26

Gd

2.83

2.68

2.41

2.16

2.69

2.93

2.88

1.10

1.89

1.06

1.43

1.04

1.02

1.22

1.23

1.15

Tb

0.55

0.54

0.44

0.41

0.53

0.57

0.55

0.39

0.39

0.19

0.23

0.17

0.17

0.28

0.25

0.23

Dy

3.17

3.35

2.71

2.42

2.99

3.32

3.23

2.67

2.61

1.07

1.21

0.91

0.94

2.09

1.74

1.63

Ho

0.63

0.69

0.56

0.49

0.60

0.67

0.66

0.56

0.54

0.19

0.23

0.18

0.18

0.43

0.37

0.34

Er

1.83

2.05

1.64

1.43

1.78

1.94

1.89

1.57

1.48

0.51

0.69

0.54

0.54

1.14

1.01

0.92

Tm

0.32

0.37

0.29

0.25

0.30

0.34

0.33

0.34

0.26

0.07

0.12

0.09

0.09

0.21

0.17

0.16

Yb

1.89

2.28

1.68

1.51

1.84

2.11

2.10

1.58

1.55

0.46

0.77

0.65

0.62

1.15

1.08

0.97

Lu

0.31

0.40

0.28

0.26

0.32

0.37

0.35

0.25

0.25

0.07

0.14

0.12

0.12

0.18

0.16

0.15

Hf

6.97

8.75

8.84

6.44

8.11

8.27

8.63

4.34

4.13

1.99

3.64

0.45

1.42

1.50

2.35

2.35

Ta

0.27

0.47

0.39

0.25

0.31

0.41

0.33

0.16

0.16

0.07

0.09

0.07

0.06

0.06

0.17

0.17

Pb

8.83

8.32

8.89

8.02

10.77

10.98

11.41

6.75

5.72

5.73

6.41

5.32

4.03

4.04

6.39

7.23

Th

2.97

5.29

3.72

3.20

4.23

4.35

4.37

2.01

1.91

1.51

1.10

0.83

0.82

1.08

1.77

1.68

U0.78

1.20

0.79

0.72

0.72

0.79

0.81

0.44

0.42

0.21

0.32

0.47

0.46

0.52

0.44

0.45

PREE

80.67

77.58

70.13

61.76

71.43

77.52

76.56

51.06

49.89

29.49

40.01

27.46

28.26

24.26

36.55

31.09

Ce/Ce*

1.33

1.29

1.30

1.32

1.17

1.16

1.15

1.28

1.28

1.19

1.13

1.14

1.15

1.07

1.36

1.38

(La/Ce)

N0.76

0.79

0.78

0.78

0.85

0.88

0.89

0.78

0.78

0.84

0.87

0.85

0.84

0.88

0.71

0.71


and TiO2 contents are excellent indicators ofterrigenous input (Murray 1994), and are relativelyenriched in sediments deposited near continents orisland arcs (Sugisaki 1984; Murray 1994; Girtyet al. 1996). Elemental concentrations in thesecherts such as the higher abundance of MgO,Al2O3, Fe2O3, Na2O and TiO2 and the trace ele-ments may be due to higher abundance of clayminerals (Halamic et al. 1999). Further, it is sug-gested that Ni, Co, and Cu have precipitated fromsea water (Sugisaki 1984), while Al, Ti, Cr and Zrare used to show the proportion of the terrigenoussediments (Matsumoto and Ijima 1983; Sugisaki1984). The relatively higher abundance of Ni, Co,Cu associated with higher Al, Ti, Cr and Zr inthese cherts (table 1) suggest their derivation frommixed terrigenous and volcanogenic (basaltic)sources. The above observation is also substanti-ated on the Al–Fe–(

PREE–Ce) ternary diagram

(Bgure 5), as these cherts plot in the Beld repre-senting terrigenous and volcanogenic (basaltic)materials.To understand the depositional environment,

different major element ratio diagrams, viz.,1009Al2O3/SiO2 vs. 1009Fe2O3/SiO2 (Bgure 6a);Al2O3/100–SiO2 vs. Fe2O3/100–SiO2 (Bgure 6b);Al2O3/(Al2O3+Fe2O3) vs. Fe2O3/TiO2 (Bgure 6c);and LaN/CeN vs.Al2O3/(Al2O3+Fe2O3) (Bgure 6d);have been used for the radiolarian cherts whichindicates continental margin aDnity. Fe2O3/Al2O3

ratio in marine sediments increases with increasingdistance from the detrital sources (Bostr€om et al.1969; Baltuck 1982), as Al2O3 is aDliated with thealumino-silicate phases and is an excellent indicatorof terrigenous input while Fe2O3 is enriched in ridgeproximal sediments and can be used as an indicatorof the hydrothermal input in the oceanic spreadingcenter (Murray 1994). In the studied samples, theFe2O3 content is less than Al2O3 in Type-I chertsand are almost similar in Type-II cherts, suggestingthat they have not been aAected by hydrothermalprocesses (Halamic et al. 2001). It is also observedthat continental margin cherts have LaN/CeN*1,pelagic cherts about 2–3 and ridge cherts[3.5 andalso Ce/Ce* anomaly for continental margin*0.90to 1.3, ocean basin Coor *0.55 and spreading ridge*0.29 (Murray et al. 1991, 1993). Shale normalizedREE patterns of the Type-I and Type-II chertsshow an almost Cat patterns (Bgure 7) and have lowLaN/CeN ratios (Type-I = 0.76–0.89 and Type-II = 0.71–0.88) and weak positive Ce anomalies(Ce/Ce* = 1.15–1.33 for Type-I cherts and1.07–1.38 for Type-II cherts), substantiate theabove observation that these cherts were depositedat continental margin environment rather than byhydrothermal activity.As discussed above, the Type-I and Type-II

cherts were accumulated at the continental marginenvironment; their Fe2O3/Al2O3 ratios (avg. 0.75,avg. 0.81, respectively), suggest that Type-IIcherts were deposited away from Type-I cherts.Further, the Fe2O3/TiO2 and MnO/TiO2 ratiosare also useful indicators for the depositionalenvironment and it is suggested that the Fe2O3/TiO2 and MnO/TiO2 ratios are lower in nearshoresediments and higher in oAshore sediments(Aitchison and Flood 1990). The Fe2O3/TiO2

ratios for Type-I (avg. 8.73) and Type-II (avg.12.31) cherts, and MnO/TiO2 ratios for Type-I(avg. 0.20) and Type-II (avg. 1.11) cherts suggestthat Type-I cherts were deposited in nearshoreenvironment of continental margin as compared toType-II cherts.Total rare earth elements (

PREE) of the marine

sediments are inCuenced by continental contami-nation, exhibiting a decrease in

PREE due to an

increase in the continental input (Shimizu andMasuda 1977; Murray et al. 1991). Accordingly,the Type-II cherts, which are suggested to havedeposited away from the Type-I cherts at thecontinental margin environment, should have moreP

REE as compared to Type-I cherts. However,the

PREE in Type-II cherts (avg. 31 ppm) is lower

Figure 5. (a) Al–Fe–(P

REE–Ce) plot for radiolarian cherts.Fields correspond to (1) Pelagic red clays, (2) submarineweathering products of basalts, (3) hydrothermal deposits (4)and (5) represent terrigenous and volcanogenic materials, theformer prevailing in the Beld (4) Type-I and Type-II chertsplot in terrigenous and volcanogenic material Beld. The Beldsare after Steinberg et al. (1983).


Figure 6. (a) 1009Al2O3/SiO2 vs. 1009Fe2O3/SiO2 diagram, (b) Al2O3/100–SiO2 vs. Fe2O3/100–SiO2 diagram, (c) Al2O3/(Al2O3 + Fe2O3) vs. Fe2O3/TiO2 binary diagrams (d) Al2O3/(Al2O3+ Fe2O3) vs. LaN/CeN, Fields are after Murray (1994)

( =Type-I cherts, = Type-II cherts).

Figure 7. Shale-normalised REE patterns for (a) Type-I and (b) Type-II radiolarian cherts, showing positive Ce anomalies.


than Type-I cherts (avg. 68 ppm), which may beexplained as Type-II cherts were derived from amixed source relatively enriched in volcanogenic(basaltic) sediments. It is inferred that these chertswhich were deposited at different places, scrappedoA the subducting oceanic plate, became tectoni-cally juxtaposed, and were obducted onto theleading edge of the Eurasian continent during theLate Cretaceous prior to the currently activeAndaman–Java subduction, that was probablyinitiated during the Late Miocene (Acharyya et al.1990; Sengupta et al. 1990; Acharyya 2007).

6. Conclusion

1. Radiolarian cherts, a part of the Andaman-Nicobar ophiolite are preserved in the easternpart of the North Andaman Island.

2. Based on Beld, petrographic and geochemistrythese cherts can be divided into two categories:

• Chocolate-coloured radiolarian cherts.• Light pink-coloured radiolarian cherts.

3. Elemental abundances and their elementrelationship suggest that Type-I and Type-IIcherts were derived from the terrigenous andvolcanogenic (basaltic) source.

4. LaN/CeN ratios and Ce/Ce* anomaly of theseradiolarian cherts also suggest that they aredeposited in continental margin environment.

Acknowledgements

The authors are thankful to Director, CSIR-National Geophysical Research Institute, Hyder-abad for his permission to publish this work. Thiswork is supported from CSIR-NGRI GENIASproject funds. The authors are thankful to Dr AKeshav Krishna for XRF analysis, Dr M Satya-narayan for trace and REE analysis. We sincerelythank Prof. N V Chalapathi Rao for Editorialhandling and constructive comments by theJournal Reviewers.

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Corresponding editor: N V CHALAPATHI RAO


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Geochemical characteristics of the Late Cretaceous