Depositional environments of Cretaceous radiolarian cherts from Andaman-Nicobar Islands, northeastern Indian Ocean

Marine Geology, 112 (1993) 291-301 Elsevier Science Publishers B.V., Amsterdam

291

Depositional environments of Cretaceous radiolarian cherts from Andaman-Nicobar Islands, northeastern Indian Ocean S . H . Jafri, V. B a l a r a m a n d P . K . G o v i l National Geophysical Research Institute, Hyderabad-500 007, India (Received April 16, 1992; revision accepted December 16, 1992)

ABSTRACT Jafri, S.H., Balaram, V. and Govil, P.K., 1993. Depositional environments of Cretaceous radiolarian cherts from Andaman-Nicobar Islands, northeastern Indian Ocean. Mar. Geol., 112: 291-301. Radiolarian cherts of Cretaceous age are tectonically associated with pillow basalts, ultramafic rocks and turbidites in the outer sedimentary arc of the Andaman-Nicobar Islands. These radiolarian cherts are composed of radiolarian tests, quartz, albitic feldspar, basaltic rock fragments, montmorillonite and chlorite, and are classified into three different types: (I) massive tuffaceous radiolarian claystone, (II) bedded tuffaceous radiolarian claystone, and (III) bedded radiolarian argillaceous chert. Radiolarian cherts of Types I and II are similar in composition, characterised by the lower abundance of SiO2, total REE (~REE) and are rich in Fe203, MgO, TiO2 and A1203 and trace elements (e.g., Ni, Co, Cr, V, Rb, Sr, Cu and Zr) as compared to those of Type III radiolarian chert. Based on elemental abundance as well as petrological evidence, it is suggested that both Types I and II cherts have been derived from a mixed continental and basaltic source. In comparison, Type III chert seems to have been derived from a continental source. Low values of Fe203/A1203, Y-REE and weakly positive to negative Ce anomalies in Types I and II cherts further suggest that they accumulated close to continental margins as compared to Type III chert, which is suggested to have accumulated in a relatively distal oceanic (hemipelagic) environment. It is inferred that these radiolarian chert sequences, which were originally deposited in different oceanic environments, were scraped off the subducting Indian plate, became tectonically juxtaposed, and now constitute a part of the Andaman-Nicobar ophiolite complex.

Introduction Radiolarian cherts occurring in ophiolitic belts are conventionally considered to be pelagic deposits derived f r o m abyssal radiolarian oozes (AudleyCharles, 1965; K a n m e r a , 1976; Suzuki and H a d a , 1979). Recent studies, however, have shown that they m a y be deposited in varied oceanic environments a n d geochemical studies have helped in understanding their origin and depositional environrnents (Rangin et al., 1981; Baltuck, 1982; Barrett, 1982; Sugisaki et al., 1982; Steinberg et al., 1983; M u r r a y et al., 1990, 1991). The A n d a m a n - N i c o b a r islands, which constitute a part o f the outer sedimentary arc o f the S u n d a - B u r m e s e double chain arc system, have several exposures o f radiolarian cherts o f Cretaceous age (Jacob, 1 9 5 4 ; K a r u n a k a r a n e t a l . , 1964, 1968;Jafri, 1986). 0025-3227/93/$06.00

A l t h o u g h these chert sequences have been paleontologically studied, their petrology and geochemistry have n o t been investigated. In this paper, these latter aspects have been studied to understand their depositional environments before they were scraped off into the ophiolitic belt o f the A n d a m a n - N i c o b a r islands.

Geology The A n d a m a n - N i c o b a r Islands (6-12 ° N; 9 0 - 9 2 ° E ) in the Bay o f Bengal f o r m a part o f the S u n d a - B u r m e s e double chain arc system linking the I n d o - B u r m a ranges to the n o r t h with the I n d o n e s i a n islands in the south (Fig. 1). This arc system is c o m p o s e d o f an inner volcanic arc and an outer sedimentary arc, wherein the

© 1993 - - Elsevier Science Publishers B.V. All rights reserved.

292

S.H. JAFRI ET AL.

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Fig. I. Generalisedgeologicalmap of the southern part of South Andaman Island. I, 11, 111 represent Type I, Type II, and Type IIl radiolarian cherts (Map after Ray et al., 1988). Andaman-Nicobar Islands form an integral part of the outer sedimentary arc. These islands are thought to be accreted and uplifted during collisional events associated with subduction of the Indian plate under the Eurasian plate (Curray et al., 1978, 1982; Hamilton, 1979). The dominant rock types in these islands are (1) Cretaceous igneous rocks of the ophiolite suite, (2) Eocene-Oligocene turbidites comprising greywackes, siltstones and shales (also known as the Andaman Flysch), and (3) Neogene chalk and limestones of the Archipelago Group (Jacob, 1954; Karunakaran et al., 1964, 1968; Srinivasan, 1979; Ray et al., 1988)• In the southern part of the south Andaman Island (Fig. 1) tectonic slices of radiolarian cherts, igneous rocks, conglomerate and grit, and turbidites are exposed• The radiolarian cherts (chert) represent the lowermost member of the ophiolite suite in the area and occur near the eastern coast of the island (Karunakaran et al., 1968)• On these cherts, no structural or stratigraphic studies have been carried out so far, possibly because of their small exposures, which are difficult to map separ a t e l y ( K a r u n a k a r a n e t al., 1964; Ray et al., 1988). These cherts, which are lithologically different and located distinctly apart, a r e associated with

different rock types, probably belong to different stratigraphic horizons. They are of three different types (as per the classification of Jones and Murchey, 1986): (I)massive tuffaceous radiolarian claystone, (II) bedded tuffaceous radiolarian claystone, and (III) bedded radiolarian argillaceous chert• Type I chert is exposed about 11 km and 2 km south of Type II and Type III radiolarian cherts, respectively, and occurs west of Bedanabad, south of Port Blair• It is associated with pillow basalts and is very much assimilated with basic volcanic rocks; it is difficult to demarcate any diffinite boundary between them. Based on the assimilated nature, it is suggested that the volcanic activity started after the deposition of the cherts (Karunakaran et al., 1964). The bed is about 20 m in width and 200 m in length. The rock is dark brown in colour and at places is traversed by thin veins of quartz• Type II chert (Fig. 2a) is exposed in the intertidal zone, near Bambooflat, about 2 km north of Port Blair and is associated with conglomerate and grit. Here the bed is about 30 m long and 10 m wide, dark brown to light green in colour and has ribbon-like bands of claystone varying from 3 to 12 cm in thickness• Locally, the bands show lamin-

DEPOSITIONAL ENVIRONMENTS OF CRETACEOUS RADIOLARIAN CHERTS

293

Fig. 2. (a) Field photograph of bedded tuffaceous radiolarian claystone (Type II). (b) Field photograph of bedded radiolarian argillacous chert (Type III). ations of different colours. The bands of claystones are parallel and are rhythmically interbedded with shales which vary from 1 to 5 cm in thickness, Type III chert (Fig. 2b) is exposed near Bedanabad, south of Port Blair, and is associated with pillow basalts, conglomerate and grit. This

outcrop is about 300 m long and 15 m wide. The rock is reddish brown to pale green in colour, has well defined (3-7 cm thick) bands of argillaceous chert which are rhythmically interbedded with shales. Chert bands are uniform in thickness and are texturally fine-grained and homogeneous as

294

compared to those in Type II radiolarian chert, The thickness of the shale bands also varies from 1 to 5 cm. However, the individual bands of shale and chert show a constant thickness mostly of the order of 4 to 5 cm. The igneous rocks which structurally overlie these cherts, are mostly confined to the eastern coast of the island and include peridotite/serpentinite, pyroxenite, gabbro and anorthositic gabbro, in addition to the basaltic rock. The igneous sequence is overlain by conglomerate and grit of Late Cretaceous age (Karunakaran et al., 1964, 1968). The conglomerate and grit are overlain by a thick pile of turbidites of Eocene-Oligocene age. The turbidites cover most parts of the island and comprise alternate greywackes, siltstones and shales, which are intensely folded, faulted and sheared (Karunakaran et al., 1964, 1968).

Petrology Type I chert consists of radiolarian "ghosts" set in a fine-grained clay matrix, and is characterised by a hematite pigmentation which is uniformally distributed. A few samples contain lithic fragments of angular grains of quartz, albitic feldspars, ironoxide and basaltic rock. The mineralogy and rexture of these basaltic rock fragments is similar to the basaltic rock described above. Thin silica and hematite rich veins occur in these cherts, Type II chert is characterised by glass shards and distinct compositional laminations defined by quartz and albite-rich clay layers alternating with distinctly clay-rich layers. The skeletons as well as the broken tests of the radiolarians, which are confined mostly to the fine-grained clay-rich laminae, at places are observed to be completely replaced by silica. Sometimes angular quartz, albitic feldspar, glass shards and hematite grains are seen. These radiolarian cherts also show the occurrence of thin quartz and hematite-rich veins, Type III chert is texturally homogeneous and contains abundant radiolarian tests which are uniformally distributed in a fine-grained clay matrix, A few extremely small quartz and plagioclase crystals and specks of hematite are present. The abundant radiolarian tests are observed to be

S.rt. JAFRI ET AL.

invariably filled by secondary quartz, apart from thin quartz and hematite-rich veins. Whole-rock powder X-ray diffractometry indicates that the clays are dominantly montmorillonite and chlorite and the feldspars are albitic. Quartz and hematite are the other minerals present.

Geochemistry Thirteen rock samples representing these three types of cherts were analysed for major and minor (TiO2, MnO and P205) elements by using a Philips PW-1400 model microprocessor-controlled sequential X-ray fluorescence spectrophotometer (XRF). Precision and accuracy of X R F data have been reported in detail elsewhere (Govil, 1985). Reproducibility of data was documented to be within the range of + 1% relative for the major as well as the minor elements. All the trace and rare earth elements (REE) were determined using an ICP-MS. For trace and REE estimation, samples were dissolved using the following acid digestion procedure. 0.1-g sample splits in PTFE beakers were digested for about 12 hours using 7 ml of HF, 3 ml of HNO3 and 2 ml of HCIO4 after adding 1 ml of 1 ~tg/ml Indium to act as an internal standard. Then the contents were evaporated to near dryness on a hot plate at 180°C. The procedure was repeated with 3 ml of HF and 2 ml of HC104. Finally the residue was dissolved in 10 ml of 1 : 1 HNO3 and the volume was made with double distilled water to 100 ml giving a dilution of 1 : 1000. The Standard reference materials GSR-4 (sandstone) and GSR-5 (shale) from the Institute of Geophysical and Geochemical Prospecting, China and AWI-1 (shale) from the Universitk de Likge, Belgium were analysed along with the samples to check the analytical accuracy. A blank was run in parallel through the complete procedure. Blanks analysed by ICP-MS were below the detection limit of the instrument. The results of ICP-MS analyses are presented in Table 1. The precision and accuracy of the data have been reported in detail by Balaram et al. (1992). The analytical data presented here have the reproducibility of measurements better than ___7% RSD for all the trace and rare earth elements. Major, minor, trace, REE data and relevent

DEPOSITIONALENVIRONMENTSOF CRETACEOUSRADIOLARIANCHERTS

elemental ratios in the three different types of cherts are presented in Table 1. The major, minor and trace element composition of Type I and Type II cherts are similar. They are characterised by relatively low abundance of SiO 2 (67.50-72.30%) and high A120 3 (11.20-13.40%) compared to Type III chert, which is rich in SiO2 (84.90-86.71%) and poor in A1203 (3.60-4.15%). They are also characterised by the higher abundance of MgO, Fe203, TiO2 oxides and Ni, Co, Cr, V, Cu, Rb and Zr elements compared to Type III chert. Average total REE (EREE) in Type I and II cherts are 86 ppm and 82 ppm, respectively; much lower than Type III chert which is characterised by 142 ppm EREE. REE abundance in these cherts are normalised

295

to average shale values to minimize the effect of the odd-even variability in elemental abundances. Normalization values used here are of the North American Shale Composite (NASC; Gromet et al., 1984). We calculate the Ce anomaly, which is defined by the ratio Ce/Ce*=(Cesample/Ceshalc)/ Ce*, where Ce* is obtained by a linear interpolation between the shale-normalised La and Pr values (Murray et al., 1992). The Eu anomaly (Eu/Eu*) is calculated similarly, using values of the shalenormalised Sm and Gd. Also, the overall pattern of light REE (LREE)depletion or enrichment with respect to heavy REE (HREE) is calculated as La./Yb n = (Las.mplJLashal~)/(Yb~ample/Yb~hale). Shale-normalised REE patterns for these Andaman samples (Fig. 3) show that Types I and

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Ni (by ICP-MS) Co Cr Rb Sr V Cu Zr Y Ba

SiO2 (by XRF) TiO2 AI20 3 Fe20 3 MgO CaO Na20 K20 MnO P2Os LOI Total

Element

68.80 0.74 13.30 6.25 1.68 1.35 3.12 1.14 0.09 0.28 3.16 99.10

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67.50 0.78 13.40 5.83 2,16 1.56 3.00 0,54 0.13 0.02 3.34 98.26

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69.20 0.67 12.35 5.72 1.74 2.09 2.58 0.61 0.09 0.24 3.73 99.02

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68.90 0.73 12.60 5.64 2,14 2.30 2.60 1.04 0.09 0.19 3.74 99.97

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72.30 0.48 11.30 5.00 1.87 1.51 3.13 0.35 0.12 0.10 3.11 99.27

An40 71.50 0.45 11.20 5.20 2.87 1.85 2.98 0.21 0.14 0.13 3.68 100.21

An41

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Chemical composition of radiolarian cherts from south Andaman island

TABLE 1

9

50 22 31 41 36 47 68 62 20 85

70.45 0.61 11.90 5.70 2.51 2.40 2.02 0.56 0.10 0.24 3,32 99.81 35 13 28 35 33 47 58 56 19 79

70.30 0.58 12.15 5.85 2.74 2.36 2.14 0.88 0.16 0.24 2.98 100.38

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84.90 0.25 4.15 2.45 0.95 0.87 2.80 1.16 0.03 0.09 1.20 98.85

85.50 0.22 3.95 2.40 0.74 1.06 2.65 0.90 0.03 0.12 1.04 98.61

10 2 7 18 56 14 17 21 18 119

86.39 0.11 3.60 2.10 0.52 1.76 2.45 0.82 0.01 0.10 0.65 98.59

An10-86 A n l l - 8 6 An43

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8 1 8 17 54 22 18 37 14 112

86.71 0.22 4.00 2.36 0.73 1,52 2.10 0.63 0,02 0.18 0.84 99.31

An44

13

Average ___stdev. Type II

Average ___stdev. Type III

68.50+__0.69 71.14+__0.94 85.88+0.83 0.69+0.09 0.53+0.08 0.20+0.06 12.92+0.45 11.64+0.46 3.93+0.23 5.95___0.31 5.44+0.40 2.33__+0.16 1,99__+0.26 2.50+0.44 0.74+0.18 1.62+0.59 2.03___0.43 1.30+0,41 2.82__+0.24 2.57+0.57 2.50__+0.30 0.98+0.43 0.50+__0.29 0.88_+0.22 0.10+0.02 0.13+0.03 0,02__+0.01 0.18+0.10 0.18+0.07 0.12+0.04

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15.80 33.82 3.31 14.22 3.90 0.92 3.22 0.58 3.29 0.65 1.49 0.19 1.94 0.33

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17.31 36.32 4.07 14.93 3.57 0.83 2.95 0.54 3.46 0.68 1.73 0.24 2.13 0.31 0.63 0.44

67.44 0.62 0.46

0.61 0.48

96.80 79.46

8.86 13.19 14.06 2 4 . 6 2 40.31 30.48 2.80 4.56 3.30 1 2 . 4 4 18.13 14.22 3.70 4.36 3.91 0.80 0.79 0.85 3.52 3.53 2.91 0.64 0.67 0.55 4.08 4.47 3.66 0.85 0.90 0.68 2.12 2.32 1.73 0.27 0.29 0.30 2.32 2.77 2.51 0.42 0.51 0.30

*Values for Nd, Tb and Ho estimated from chondrite-normalised plots.

A1/(A1+ Fe) FezO3/AI203

REE

La Ce Pr Nd* Sm Eu Gd Tb* Dy Ho* Er Tm Yb Lu 0.61 0.48

85.56 0.56 0.59

147.07

1 6 . 4 3 33.74 3 3 . 6 7 44.36 3.63 7.08 1 4 . 9 3 29.86 3.63 8.16 0.88 1.74 3.46 7.04 0.60 1.16 3.53 6.59 0.69 1.I0 1.49 2.45 0.29 0.29 2.08 3.07 0.25 0.43 0.55 0.61

152.67

36.04 54.78 7.69 30.57 7.30 1.69 5.67 0.81 4.37 0.60 1.37 0.17 1.41 0.20 0.57 0.58

0.56 0.59

156.81 112.39 0.62__+0.01 0.46+0.02

0.62+0.01 0.47+0.02

37.71 29.64 14.35+4.29 13.14+3.16 58.81 42.44 31.46_+7.89 32.27_+6.54 9.28 5.92 3.51-1-0.57 3.57+0.74 29.15 19.98 15.43+0.95 14.93+2.38 6.53 3.80 4.71__+1.21 3.90+0.33 1.38 0.68 1.13+0.36 0.83+0.04 4.57 3.09 4.12+_1.34 3.36+0.30_ 0.70 0.49 0.73+0.23 0.62+__0.05 4.55 2.62 4.46+1.55 3.94+0.43 0.68 0.51 0.82+0.22 0.78+0.11 1.53 1.30 1.99+0.50 1.92+0.37 0.22 0.18 0.28_+0.09 0.29+0.01 1.54 1.58 2.65+0.73 2.42+0.29 0.16 0.16 0.40__+0.14 0.37___0.12 0.56-t-0.01 0.59+0.01

34.28+__3.50 50.10+7.95 7.49+ 1.40 27.39+4.97 6.45+__1.89 1.37___0.49 5.09+_1.67 0.79_+0.28 4.53-+1.62 0.72-+0.26 1.66+0.53 0.22-t-0.05 1.90-t-0.78 0.24+0.13

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II cherts exhibit a weakly positive to negative Ce anomaly as compared to type III chert, which is characterised by a moderate negative Ce anomaly, Type I chert has a weakly positive Eu anomaly as compared to Types II and III cherts which are characterised by both weakly positive to negative Eu anomalies. LREE/HREE fractionation in Types I and II cherts have near flat patterns with slight HREE enrichment as compared to Type III chert which shows a marked HREE depletion. Type I and Type II cherts are also distinguished by the average low values of FeEOa/A1203 ratios (Type I=0.46 and Type II=0.47) than those of Type chert, which has higher values of these ratios (0.59)(Fig. 4a). Bostr6m and Peterson (1969) have suggested that the A1/(AI+Fe+Mn) ratio in the oceanic sediments increases away from the spreading centres and their associated hydrothermal systems. Recent studies (Murray et al., 1992), have shown that Mn is fractionated during chert formation, resulting in MnO/Al20 a ratios that no longer record the depositional signal of the precursor sediment. Hence, in calculating the

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Discussion The major, minor, trace and REE abundance in Types I and II cherts, which are similar, are

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A1/(A1+ Fe + Mn) ratio of these rocks, Mn value is not used. The Andaman cherts are characterised by high A1/(Al+ Fe) ratios (Type I = 0.62, Type II = 0.62, Type III= 0.56). These ratios are higher than those found in hydrothermally influenced cherts (0.12) and porcellanite (0.32) depos*ted in the northern Pacific (Adachi et al., 1986), but are similar to the values reported for the continentally derived Triassic bedded cherts of Central Japan (0.60; Sugisaki et al., 1982), the Cretaceous DSDP cherts of Leg 62 (0.64; Hein et al., 1981), the average shales (0.62; Bostr6m, 1976; Baltuck, 1982)and the average deep-sea clay (0.54; Turekian and Wedepohl, 1961). Thus the higher values of A1/(Al+ Fe) in Andaman cherts indicate that they have not been influenced by hydrothermal activity (Fig. 4b). On the A1-Fe-(EREE-Ce) diagram (Steinberg et al., 1983), also Type I and Type II cherts fall in the field corresponding to terrigenous and volcanogenic materials while Type III chert falls in the field of pelagic red clays (Fig. 5).

a=

TYPE

HI

Fig. 4. (a) Fe203/AI203 and (b) A1/(AI+Fe) diagrams for A n d a m a n radiolarian cherts (filled symbols indicate average values for each Type).

AI

Fe

Fig. 5. Ternary plot of A1-Fe-(EREE-Ce) for A n d a m a n radiolarian cherts, fields corresponding to (1) pelagic red clays, (2) submarine weathering products of basalts, (3) hydrothermal deposits while fields, (4) and (5) represent terrigenous and volcanogenic materials, the former prevailing in field (4). The fields are after Steinberg et al. (1983).

299

DEPOSITIONAL ENVIRONMENTS OF CRETACEOUS RADIOLARIAN CHERTS

characterised by low abundance of SiO2, REE and high Fe203, MgO, TiO2 and A1203 as compared to Type III chert. These cherts are also characterised by a higher abundance of Ni, Co, Cr, V, Rb, Sr, Cu and Zr compared to the Type III chert. Ni, Co and Cu are believed to be precipitated from sea water (Sugisaki, 1984), while A1, Ti, Cr and Zr are used for estimating the terrigenous portion of sediments (Matsumoto and Iijima, 1983; Sugisaki, 1984). Though the trace element data on these radiolarian cherts are scanty, the observed average abundances of Ni, Co and Cu along with higher A1, Ti, Cr and Zr contents in Type I and II cherts suggest that Ni, Co and Cu content could have been derived from a mixed continental and basaltic source rather than from sea-water precipitation, This observation is also supported by the presence of lithic angular grains of quartz, albitic feldspar and basaltic rock fragments in these cherts, However, the Type III chert seems to have been derived from a continental source. The Eu anomaly present in radiolarian cherts is probably related to feldspars, the only minerals which can concentrate Eu during magmatic processes (Philpotts, 1970). This enrichment suggests the occurrence of detrital feldspars in radiolarian cherts (Steinberg et al., 1983). The Eu anomaly in Type I chert presents slightly higher values (Eu/Eu*= 1.13) than those of Type II and III cherts (Eu/Eu* = 1.02 and = 1.03), most likely reflecting relatively more incorporation of detrital feldspars in Type I chert, Types I and II cherts show LREE-depleted values relative to HREEs (Lan/Ybn=0.58 and = 0.53), while Type III chert shows LREE-enriched values relative to HREEs (LaJYbn = 1.93). The values of Lan/Yb~ in Types I and II cherts are depleted, while it is enriched in Type III chert as compared to the values proposed for a terrigenous input to oceans ( ~ 1.3, Sholkovitz, 1990; ~ 1 . 0 , Condie, 1991). The low La,/Yb~ ratio in Type I and Type II cherts is most likely due to a LREEdepleted component in the source of these rocks. The high LaJYb~ ratio in Type III chert suggests deposition in an environment dominated by a particulate terrigenous source, which may indicate the influence of an absorbed and preferentially

LREE-enriched component (e.g., Byrne and Kim, 1990). To understand the depositional setting of these Andaman cherts, the Fe2Oa/m1203 ratio has ~been used as this ratio in oceanic sediments increases with increasing distance from the detrital sources (Baltuck, 1982; Bostrrm and Peterson, 1969). In Type I chert, the average Fe2Oa/AI203 ratio is 0.46, identical to that of Type II (0.47). However, Type III preserves a distinctly higher Fe203/A1203 ratio (0.59), indicating accumulation farther from the continental source than Types I and II cherts. Rare earth element studies of the oceanic sediments have also shown that they are influenced by continental contamination, exhibiting a decrease in Ce anomaly and EREE due to an increase in the continental input (Murray et al., 1990, 1991; Shimizu and Masuda, 1977). Studies on the cherts and shales from the Mesozoic East Pacific Ocean show strong negative cerium anomaly (Ce/Ce*~0.29) suggesting hydrothermal influence (Murray et al., 1990). The Andaman cherts are characterised by a weakly positive to moderately negative cerium anomaly Ce/Ce* (Type I=0.91 to 1.01, Type 11=0.95 to 1.13, Type 11I=0.63 to 0.72; Fig. 3), suggesting that these cherts have not been influenced by hydrothermal activity. Further, Types I and II cherts show lower REE contents (86ppm and 82ppm) than Type III chert (142 ppm), suggesting their accumulation nearer to continental margins than Type III, which accumulated in a relatively distal oceanic (hemipelagic) environment. The above inferences are also substantiated by the fact that both Types I and II cherts are characterised by (1) the occurrence of glass shards and angular grains of quartz, albitic feldspar and basaltic rock fragments, and (2) their position in the field corresponding to terrigeneous and volcanogenic materials on the A1-Fe-(EREECe) diagram. In contrast, Type III chert, which is texturally homogeneous with a dominantly clay matrix, falls in the field of pelagic red clays (Fig. 5). Conclusions From the present study the following conclusions have been drawn: (1) The cherts from the Andaman-Nicobar Islands are classified into three different types:

300

S.H. JAFRI ET AL.

(I) massive tuffaceous radiolarian claystone, (II) bedded tuffaceous radiolarian claystone, and

Balaram, V., Manikyamba, C., Ramesh, S.L. and Anjiah, K.V., 1992. Rare earth and trace element determination in ironformation reference samples by ICP-MS. Atomic

(III) bedded radiolarian argillaceous chert. (2) High A1/(AI + Fe) ratios and weakly positive to moderately negative Ce anomalies suggest that these cherts have not been influenced by hydrother-

Spectroscopy, 13: 19-25. Baltuck,M., 1982. Provenance and distribution of Tethyan pelagic and hemipelagic siliceous sediments. Pindos Mountain,Greece. Sediment. Geol., 31: 63-88.

mal activity, (3) Types I and II cherts are similar in composition, characterised by a lower abundance of SiO2,

Barrett, T.J., 1982. Stratigraphy and sedimentology of Jurassic bedded chert overlying ophiolites in the North Apennines,

Italy. Sedimentology,29: 353-373.

and

Bostr6m, K., 1976. Particulate and dissolved matter as sources for pelagic sediments. Stockholm Contrib. Geol., 30: 16-79. Bostr6m, K. and Peterson, M.N.A., 1969. The origin of alumi-

trace elements than Type III chert. It is suggested that Types I and II cherts have been derived from

num-poorferromanganoan sediments in areas of high heat flow on the East Pacific Rise. Mar. Geol., 7: 427-447.

mixed continental and basaltic source, whereas

Byrne, R.H. and Kim, K.H., 1990. Rare earth element scavenging in seawater. Geochim. Cosmochim. Acta, 54: 2645-2656. Condie, K.C., 1991. Another look at rare earth elements in shales. Geochim. Cosmochim. Acta, 55: 2527-2531. Curray, J.R., Moore, D.G., Lawver, L.A., Emmel, F.J., Raitt,

and richer in

a

Fe203,

MgO, TiO2 and

A120 3

Type III chert seems to have been derived from a

continental

source, (4) L o w F e 2 0 3 / m l 2 0 3 ratios, E R E E and weakly

positive to negative Ce anomalies in Types I and II cherts further suggest that they accumulated closer to continental margins than Type III chert. (5) It is inferred that these radiolarian cherts, which were originally deposited in different oceanic environments, were scraped off the subducting Indian plate, became tectonically juxtaposed, and now constitute a part of the A n d a m a n - N i c o b a r ophiolite complex.

Acknowledgements The authors are thankful to the reviewers of this journal for their constructive comments and suggestions on this paper and to Dr. H.K. Gupta, Director, National Geophysical Research Institute, for his permission to publish this work. The authors are also thankful to Drs. R.N. Singh, S.M.

Naqvi, R. Srinivasan, S.N. Charan, S.M. Husain and O.P. Pandey for their valuable suggestions and to M r . Mir Raza Husain, Indian Institute of Chemical Technology, Hyderabad for X-ray diffraction analysis of the samples.

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