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Strontium-isotopic composition of Dhandhuka basalts, western India
Chemical Geology, 32 (1981) 129--138
129
Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
STRONTIUM-ISOTOPIC COMPOSITION OF DHANDHUKA BASALTS, WESTERN INDIA
P.O. ALEXANDER
Department of Applied Geology, University of Saugar, Sagar 470 003 (India) (Received November 15, 1979; revised and accepted January 23, 1981 )
ABSTRACT
Alexander, P.O., 1981. Strontium-isotopic composition of Dhandhuka basalts, western India. Chem. Geol., 32: 129--138. The initial 'VSr/'~Sr ratios of three representative, mildly alkaline basalts of the Dhandhuka borehole suite vary from 0.70620 to 0.70952, and are among the highest ratios reported from the Deccan b'asalts. St-isotopic ratios vary with the Sr contents rather than with the Rb/Sr ratios of each sample. The higher ratios for the Dhandhuka basalts correlate positively with the concentrations of incompatible elements and are considered to be significant. It is suggested that radiogenic Sr and other incompatible elements in the basalts of the Dhandhuka region have been introduced into the magma as a result of contamination due to wall-rock reaction.
INTRODUCTION
The isotopic composition of Sr, apart from being an important tool in geochronology, has also wider application to the problems of basalt petrogenesis. St-isotopic study of the voluminous Deccan basaltic rocks has been very limited so far. Faure and Hurley (1963) reported the first Sr-isotopic data for the basalts of Deccan region. More recently, Sr-isotopic data for the Girnar complex (Murali, 1974; Paul et al., 1977) and Sagar (Alexander and Paul, 1977) have been reported. The present work (Fig. 1) reports initial Sr-isotopic compositions for three rocks of the Dhandhuka sequence. This additional information on the previously much studied borehole suite (West, 1958; Krishnamurthy, 1974; Alexander and Gibson, 1977; Krishnamurthy and Cox, 1977) indicates that differentiation in this suite of basic rocks is due not only to fractional crystallization of the parent magma, but also to some degree of contamination.
0009-2541/81/0000--0000/$02.50 © 1981 Elsevier Scientific Publishing Company
130
• Delhi
~,,,~
r
L
26
F -22
.18 ------IDeccan Trap o
,.2?°
,
47<
Krn 74 \
78
Fig. 1. T h e D e c c a n t r a p o u t c r o p s h o w i n g t h e l o c a l i t i e s t h a t are r e f e r r e d t o in t h e t e x t : 1 = D h a n d h u k a ; 2 = G i r n a r ; 3 = S a g a r ; 4 = B o m b a y ; F - × -× - F = f a u l t s o f s u b - c r u s t a l o r d e r ( a f t e r R a j u et al., 1 9 7 1 ) .
STRONTIUM-ISOTOPIC
ANALYSIS
St-isotopic analyses were carried out on a modified AEI ® MS5 mass spectrometer with 30-cm radius, 90 ° sector analyser tube and Faraday collector. The rock samples were brought into solution in a mixture of HF--HC104, followed by separation of Sr by the ion-exchange technique. Further details of chemical purification, mass-spectrometric analysis and data processing were given by Burwell fi1975). The measured 87Srf16Sr ratio for the Eimer and Amend® standard SrCO3, was 0.7082 + 0.0002 (2a). All 87Sr/~6Sr ratios were normalised to a value of 8.375 for the 88Sr/S6Sr ratio and to a value of 0.7080 for the Eimer and Amend ® standard. Rb and Sr concentrations were determined by X-ray fluorescence spectrometry. Reproducibility for Sr and Rb was -+ 4 and 5% (20). Petrochemistry and Sr-isotopic data for the three flows under study are reported in Tables I and II.
131 TABLE I Petrochemistry of the three Dhandhuka basalts Flows rock type
OA/D-37 normal basalt
OA/D.32 picrite basalt
OA/D-3 porphyritic basalt
SiO 2 TiO 2 A1203 F%O3 FeO MnO MgO CaO Na20 K20 P2Os H~O*
47.10 2.56 16.08 1.62 9.20 0.15 5.44 10.48 2.77 1.07 0.32 1.97
47.75 1.85 11.21 1.54 8.74 0.14 10.19 11.43 1.54 0.65 0.27 4.07
45.70 2.62 14.26 1.65 9.33 0.21 7.93 9.86 2.12 1.80 0.43 4.31
Total
98.96
99.42
100.22
CIPW norms: Qz
Or Ab Ne An Di Hy Ol Mt I1 Ap
--
6.50 24.02 -29.34 18.35 3.94 9.54 2.42 5.01 0.78
--
-
4.02 13.62 -22.81 28.09 22.37 2.21 2.33 3.68 0.66
11.05 18.71 -25.11 18.74 1.35 16.31 2.49 5.19 1.04
Petrochemical data from Krishnamurthy (1974).
TABLE II Rb, Sr and aTSr/S6Sr ratios for the three Dhandhuka basalts, western India Sample
OA/D-37 OA/D-32 OA/D-3
Rock type
normal basalt picrite basalt porphyritic basalt
Rb (ppm)
Sr (ppm)
Rb/Sr
28 21 39
510 300 365
0.0549 0.070 0.1068
STSr/S~Sr
aTSr/"Sr
(measured)
(initial)
0.07634 0.70970 0.70925
0.70620 0.70952 0.70898
'~Sr/*'Sr have been calculated assuming an age of 63 Ma (K-At data of Alexander, 1977).
132 DISCUSSION The three representative rock types of the Dhandhuka basaltic suite, comprising picrite basalt (OA/D-32), porphyritic basalt (OA/D-3) and normal basalt {OA/D-37) are mildly alkaline. This makes them slightly different from the average Deccan basalt which is considered to be sub-alkaline. The average contents of Rb and Sr are 29 and 391 ppm, respectively, for the flows under study. Thus both Rb and Sr are within the normal range for basaltic rocks (Prinz, 1967) but somewhat higher than in basalts from elsewhere in the Deccan region (Table III). The initial 87Sr/S6Sr ratios of these rocks vary from 0.70620 to 0.70952 which appears to be higher than the range of initial Sr-isotopic ratios from other continental basaltic provinces (Table IV). Only Tasmanian dolerites (Heier et al., 1965) and Antarctic basalts (Faure et al., 1974) have higher ratios. The Sr-isotopic ratios of the former range from 0.708 to 0.718, with an average of 0.712, while the Kirkpatrick basalt, Antarctica also, has anomalously high Sr-isotopic ratios ranging from 0.7094 to 0.7133 and averaging 0.7112. However, both Tasmartian dolerites and Antarctic basalts have much lower Sr concentrations only ~ 130 ppm as against 390 ppm for Dhandhuka flows. There is more similarity between the Dhandhuka flows and the basalts of the Nuanetsi zone in the Karroo province (87Sr/86Sr ratio 0.706--0.710, Manton, 1968). Recently Cox and Krishnamurthy (1977) have also pointed out the similarity between the lava sequence of these two regions in their broadest compositional characters. Among the Deccan traps, the range of Sr-isotopic ratios of the Dhandhuka flows (0.70620--0.70952) is the highest among those reported. Because the number of samples is small, it is not justified to make generalisations between the petrochemistry--Sr-isotopic ratio relationship. Fig. 2 shows a plot of initial Sr-isotopic ratios vs. Rb/Sr ratios for the Dhandhuka flows. For comparison, the fields for oceanic and continental basaltic rocks are also shown together with fields for the Girnar and Sagar basalts of the Deccan region: (1) it is observed that while some of the Sagar and Girnar basalts lie within the oceanic field, the Dhandhuka flows are distinctly continental; and (2) in contrast to those of Sagar and Girnar, the basalts of Dhandhuka are more erratic, indicating the possibility of contamination by introduction of foreign radiogenic Sr. High Sr-isotopic values for basalts and gabbros of the Nuanetsi zone (0.706--0.710) are regarded as being due to isotopic exchange with the Archaean basement rocks. Manton (1968) has shown that the Sr-isotopic ratios for these basic rocks: "appear t~ vary with strontium content rather than with the Rb/Sr ratio of each sample, which suggests contamination". A similar relationship is observed for the Dhandhuka flows on the initial Srisotopic ratio vs. Sr plot (not shown). This can indicate contamination for the
0.7052--0.7067
S~Sr/S6Sr
0.7039--0.7084
2,406 183 6 208 155 12 1--3
*Mantle background (Harris, 1972).
2,181 292 6 264 --1.5
K Ba Rb Sr Zr Nb Th 0.7062--0.7095
10,293 534 31 411 197 39 5.44 0.7095
5,395 400 21 300 140 30 6.31
picrite basalt OA/D-32
400--500 5? ~1 10 30 1--5? 0.20
mantle background*
20 106 31 41 6 8 27
11 80 21 30 4 6 30
picrite basalt OA/D-32
average Dhandhuka basalt
average Dhandhuka basalt
Girnar
Sagar
E n r i c h m e n t factor c o m p a r e d to mantle b a c k g r o u n d
Values in p p m
Comparison o f incompatible elements and Sr-isotopic ratios in D h a n d h u k a , Girnar and Sagar basalts of the Deccan p r o v i n c e in relation to the mantle background
T A B L E III
C.o
134 TABLE IV Initial Sr-isotopic ratios of Dhandhuka flows in comparison to other Deccan areas and basaltic provinces Deccan basalts
Other continental basalts
Dhandhuka basalts, 0.7062--0.7095 (present work) Sagar basalts, 0.7039--0.7084 (Alexander and Paul, 1977) Girnar basalts, 0.7052--0.7067 (Murali, 1974) Bombay basalts, 0.7045--0.7057 (G. Faure, pets. commun., 1978 on the work of Faure and Hurley, 1963)
basalts, New South Wales, 0.7035--0.7042 (Heier et al., 1965) basalts, Oregon, Arizona, 0.7030--0.7040 (Faure and Hurley, 1963) dolerites, Karroo, S.A., 0.7065 (Compston et al., ] 968) basalts, Tasmania, 0.7027--0.7078 (Compston et al., 1968) dolerites, Tasmania, 0.711 (Heier et al., 1965) basalts, Nuanetsi zone, South Africa, 0.706--0.710 (Manton, 1968) Kirkpatrick basalt, Antarctica, 0.7094-0.7133 (Faure et al., 1974)
IDhk
-714
.709 CO
~ -704 CO
..
•6 9 9 0
_ o ",£..Ocn. g a s . Rks.
1 .1
! "2
'| "3
i "4
Rb/Sr Fig. 2. Plot of initial Sr-isotopic ratio vs. Rb/Sr ratio for Deccan trap basalts. Boundary curves for continental and oceanic basaltic rocks (after Faure and Powell, 1972) are also shown for comparison. Abbreviations used are as follows: D h k = Dhandhuka; G i r = Girnar; S g r = Sagar, C o n t . B a s . R k s . = continental basaltic rocks; and O c n . B a s . R k s . = oceanic basaltic rocks.
135
0"710
iDhk eGir -,i-Sgr
/
.+
0"708
4-
b.
¢ 4-
0.706 •.I-
+÷
.-... 8.5]
+e "f"
+-. 7.8|X10 -3 "F"* 15"4J
e'l"
+ 0-704
4- -i, 13.7 J
[ 1.0
I 2"0
I' 3"0
-1
1/Sr ppm
I 4 "0
I
5"0
61-0 X
10- 3
Fig. 3. A plot showing initial 87Sr/S~Sr ratio vs. 1/St for flows from Dhandhuka, Girnar and Sagar, indicating contamination for Dhandhuka basalts. The abbreviations used are the same as in Fig. 2.
two Dhandhuka samples with high Sr-isotopic ratios. A plot of STSr/S6Srratio vs. 1/Sr (Fig. 3) indicates contamination for the Dhandhuka flows more effectively, where correlation is observed between 878r/86Sr ratio and 1/Sr for the samples under study. There is no such correlation apparent for Girnar and Sagar flows. The Girnar samples show a nearly flat pattern, whereas the Sagar flows are more erratic. In the situation of a magma undergoing contamination with material with a higher 8~Sr/S6Sr ratio and incompatible elements, a lava flow with low Sr content should be affected the most. The fact that the sample with the highest 878r/86Sr ratio has the lowest Sr content from among the samples of the present study is consistent with the suggestion that they were contaminated. Not only has the picrite basalt (OA/D-32) the highest 8TSr/868r ratio of 0.70952 and the lowest Sr value (300 ppm) in contrast to the normal basalt (Sr = 510 ppm; 8VSr/8~Sr = 0.70620) and the porphyritic basalt (Sr = 365 ppm; 878r/8~Sr = 0.70898) but it is also the highest in silica content, giving further support to the idea of contamination. From his petrological, chemical and phase-equilibria studies of the Dhandhuka borehole suite, Krishnamurthy (1974) has demonstrated picrite rocks to be the most "primitive", and that other rock types can be derived from picrite basalt by the process of crystal fractionation. Rare-earth element data on t h e same suite of rocks (Alexande r and Gibson, 1977) also confirm this. Enrichment of incompatible elements in these primitive picrite basalts and the entire suite as whole, however, is remarkable and needs to be explained. Table III compares some incompatible elements (K, Ba, Rb, Sr, Zr,
136
Nb and Th) and the initial Sr-isotopic ratios of Dhandhuka basalts and the basalts of Girnar and Sagar. The data establish that the Dhandhuka suite as a whole is considerably richer in incompatible elements in comparison with basalts of the Girnar and Sagar regions. In comparison with the mantle background of these elements (Harris, 1972), the Dhandhuka suite shows remarkable enrichment (Table III). The enrichment factor ranges from 4 to 80 for primitive picrite basalt and 6 to 106 when the Dhandhuka suite is considered as a whole. The mean enrichment factor, on the other hand, for all the elements considered is 26 and 34 for primitive picrite basalt and average Dhandhuka basalt. It seems difficult to account for the enrichment of incompatible elements of this magnitude in the primitive picrite basalt by the degree of melting alone as has also been demonstrated by Krishnamurthy, 1974). Other processes by which incompatible trace elements could be enriched in the residual liquids, namely, high-pressure eclogite fractionation {O'Hara and Yoder, 1967) and subsequent olivine fractionation of the initial liquids en route (O'Hara, 1968) appear to have played an insignificant role in the present case (Krishnamurthy, 1974). However, there is some evidence for the operation of eclogite fractionation on the basis of K20/Na:O ratios of the borehole suite, but it will definitely not account for the observed manifold increase in the enrichment factors of incompatible elements in contrast to the mantle background. It is suggested therefore that incompatible elements such as K, Ba, Sr, Th, Pb, Rb and Zr and radiogenic Sr in the Dhandhuka borehole suite were enriched due to contamination. Unfortunately, there are no data for incompatible elements and Sr-isotopic ratios for the basement rocks underlying the Dhandhuka basalts. This makes it difficult to specify with certainty the mechanism and degree of contamination of basaltic magma. However, considering the overall petrochemistry of the Dhandhuka suite, bulk contamination is considered unlikely, since, unlike Antarctic Kirkpatrick basalts with anomalously high STSr/S6Sr and silica (Faure et al., 1974), the Dhandhuka flows are typically basaltic and do not seem to have resulted from the hybridisation of a normal basaltic magma with the Precambrian granitic rocks. If it were so, there should have been pronounced changes in the bulk petrochemistry of this suite. The mechanism of wall-rock reaction (Green and Ringwood, 1968) both at or near the source and also at shallower depths should have played a dominant role whereby radiogenic Sr and other incompatible elements were selectively introduced into the basaltic magma. The Dhandhuka borehole suite, falling within the tectonically active region of the Gulf of Cambay, on the western margin of the Indian shield, has been suggested as the focus of earliest Deccan trap eruptions (Auden, 1949). Preliminary K--Ar investigations (Alexander, 1977) also suggest that the flows comprising this borehole suite represent the earliest Deccan trap volcanism. Considering this, it seems reasonable to assume that the wall-rock reaction was the dominant process in modifying the incompatible element distribution. It seems more likely that combined Pb- and Sr-isotopic studies on a number of samples of this much studied borehole suite will lead to a better understanding of the problem discussed.
137 ACKNOWLEDGEMENTS
The author gratefully acknowledges the Commonwealth Academic Staff Scholarship award (1974--1976) during which time the present work was carried out at the Department of Earth Sciences, University of Leeds, England. For strontium-isotopic work he is grateful to E. Oegazi and to M.H. Dodson for general guidance. For the Dhandhuka borehole samples the author is thankful to P. Krishnamurthy and K.G. Cox. For comments on the manuscript he is thankful to D.K. Paul, P.E. Baker, W.D. West and S. Mallikarjunan. REFERENCES Alexander, P.O., 1977. Geochemistry and geochronology of Deccan trap lava flows around Sagar, M.P., India. Ph.D. Thesis, University of Saugar, Sagar (unpublished). Alexander, P.O. and Gibson, I.L., 1977. Rare-earth abundances in Deccan trap basalts. Lithos, 10: 143--147. Alexander, P.O. and Paul, D.K., 1977. Geochemistry and strontium isotopic composition of basalts from the eastern Deccan volcanic province. Ind. Mineral. Mag., 41: 165--172. Auden, J.B., 1949. Dykes in western India. Trans. Natl. Inst. Sci. Ind., 3: 123--157. Burwell, A.D.M., 1975. Rb--Sr isotope geochemistry of the lherzolites and their constituent minerals from Victoria, Australia. Earth Planet. Sci. Left., 29: 69--78. Compston, W., McDougall, I. and Heier, K.S., 1968. Geochemical composition of the Mesozoic basaltic rocks of Antarctica, South Africa, South America and Tasmania. Geochim. Cosmochim. Acta, 32: 1 2 9 - 1 4 9 . Faure, G. and Hurley, P.M., 1963. The isotopic composition of strontium in oceanic and continental basalts: application to the origin of igneous rocks. J. Petrol., 4: 31--50. Faure, G. and Powell, J.L., 1972. Strontium Isotope Geology. Springer, New York, N.Y., 188 pp. Faure, G., Bowman, J.R., Elliot~ D.H. and Jones, L.M., 1974. Strontium isotope composition and petrogenesis of the Kirkpatrick basalt, Queen Alexandra range, Antarctica. Contrib. Mineral. Petrol., 48: 153--169. Green, D.H. and Ringwood, A.E., 1967. The genesis of basaltic magma. Contrib. Mineral. Petrol., 15: 103--190. Harris, P.G., 1972. Geothermal environment and basalt magma type. J. Earth Sci,, 8(2): 275--285. Heier, K.S., Compston, W. and McDougall, I., 1965. Thorium and uranium concentrations and isotopic composition of strontium in different Tasmanian dolerites. Geochim. Cosmochim. Acta, 29: 643---659. Krishnamurthy, P., 1974. Petrological and chemical studies of Deccan trap lavas of western India. Ph.D. Thesis, University of Edinburgh, Edinburgh (unpublished). Krishnamurthy, P. and Cox, K.G., 1977. Picrite basalts and related lavas from the Deccan traps of western India. Contrib. Mineral. Petrol., 62: 53--75. Manton, W.L., 1968. The origin of associated basic and acid rocks in the L e b o m b o - Nuanetsi igneous province, South Africa as implied by strontium isotopes. J. Petrol., 19: 2 3 - - 3 9 . Murali, A.V., 1974. Some aspects of geochemistry of the Girnar igneous complex, western India. Ph:D. Thesis, University of Saugar, Sagar (unpublished). O'Hara, M.J., 1968. The bearing of phase equilibria and the origin and evolution of basic and ultrabasic rocks. Earth-Sci. Rev., 4: 69--133. O'Hara, M.J. and Yoder, Jr., H,S., 1967. F o r m a t i o n and fractionation of basaltic magma at high pressures. Scott. J. Geol., 3: 67--117.
138 Paul, D.K., Potts, P.J., Rex, D.C. and Beckinsale, R.D., 1977. Geochemical and petrogenetic study of the Girnar igneous complex, Deccan volcanic province, India. Geol. Soc. Am. Bull., 88: 2 2 7 - 2 3 4 . Prinz, M., 1967. Geochemistry of hasalts: trace elements. In: H.H. Hess and A. Poldervaart (Editors), Basalts, I. Interscience, New York, N.Y., pp. 271--323. Raju, A.T.R., Choube, A.N. and Choudhary, L.R., 1971. Deccan trap and the geologic framework of the Cambay basin. Bull. Volcanol., 35: 521--538. West, W.D., 1958. The petrography and petrogenesis of 48 flows of the Deccan traps penetrated by borings in western India. Trans. Natl. Inst. Sci. Ind., 4(1): 1--56.
129
Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
STRONTIUM-ISOTOPIC COMPOSITION OF DHANDHUKA BASALTS, WESTERN INDIA
P.O. ALEXANDER
Department of Applied Geology, University of Saugar, Sagar 470 003 (India) (Received November 15, 1979; revised and accepted January 23, 1981 )
ABSTRACT
Alexander, P.O., 1981. Strontium-isotopic composition of Dhandhuka basalts, western India. Chem. Geol., 32: 129--138. The initial 'VSr/'~Sr ratios of three representative, mildly alkaline basalts of the Dhandhuka borehole suite vary from 0.70620 to 0.70952, and are among the highest ratios reported from the Deccan b'asalts. St-isotopic ratios vary with the Sr contents rather than with the Rb/Sr ratios of each sample. The higher ratios for the Dhandhuka basalts correlate positively with the concentrations of incompatible elements and are considered to be significant. It is suggested that radiogenic Sr and other incompatible elements in the basalts of the Dhandhuka region have been introduced into the magma as a result of contamination due to wall-rock reaction.
INTRODUCTION
The isotopic composition of Sr, apart from being an important tool in geochronology, has also wider application to the problems of basalt petrogenesis. St-isotopic study of the voluminous Deccan basaltic rocks has been very limited so far. Faure and Hurley (1963) reported the first Sr-isotopic data for the basalts of Deccan region. More recently, Sr-isotopic data for the Girnar complex (Murali, 1974; Paul et al., 1977) and Sagar (Alexander and Paul, 1977) have been reported. The present work (Fig. 1) reports initial Sr-isotopic compositions for three rocks of the Dhandhuka sequence. This additional information on the previously much studied borehole suite (West, 1958; Krishnamurthy, 1974; Alexander and Gibson, 1977; Krishnamurthy and Cox, 1977) indicates that differentiation in this suite of basic rocks is due not only to fractional crystallization of the parent magma, but also to some degree of contamination.
0009-2541/81/0000--0000/$02.50 © 1981 Elsevier Scientific Publishing Company
130
• Delhi
~,,,~
r
L
26
F -22
.18 ------IDeccan Trap o
,.2?°
,
47<
Krn 74 \
78
Fig. 1. T h e D e c c a n t r a p o u t c r o p s h o w i n g t h e l o c a l i t i e s t h a t are r e f e r r e d t o in t h e t e x t : 1 = D h a n d h u k a ; 2 = G i r n a r ; 3 = S a g a r ; 4 = B o m b a y ; F - × -× - F = f a u l t s o f s u b - c r u s t a l o r d e r ( a f t e r R a j u et al., 1 9 7 1 ) .
STRONTIUM-ISOTOPIC
ANALYSIS
St-isotopic analyses were carried out on a modified AEI ® MS5 mass spectrometer with 30-cm radius, 90 ° sector analyser tube and Faraday collector. The rock samples were brought into solution in a mixture of HF--HC104, followed by separation of Sr by the ion-exchange technique. Further details of chemical purification, mass-spectrometric analysis and data processing were given by Burwell fi1975). The measured 87Srf16Sr ratio for the Eimer and Amend® standard SrCO3, was 0.7082 + 0.0002 (2a). All 87Sr/~6Sr ratios were normalised to a value of 8.375 for the 88Sr/S6Sr ratio and to a value of 0.7080 for the Eimer and Amend ® standard. Rb and Sr concentrations were determined by X-ray fluorescence spectrometry. Reproducibility for Sr and Rb was -+ 4 and 5% (20). Petrochemistry and Sr-isotopic data for the three flows under study are reported in Tables I and II.
131 TABLE I Petrochemistry of the three Dhandhuka basalts Flows rock type
OA/D-37 normal basalt
OA/D.32 picrite basalt
OA/D-3 porphyritic basalt
SiO 2 TiO 2 A1203 F%O3 FeO MnO MgO CaO Na20 K20 P2Os H~O*
47.10 2.56 16.08 1.62 9.20 0.15 5.44 10.48 2.77 1.07 0.32 1.97
47.75 1.85 11.21 1.54 8.74 0.14 10.19 11.43 1.54 0.65 0.27 4.07
45.70 2.62 14.26 1.65 9.33 0.21 7.93 9.86 2.12 1.80 0.43 4.31
Total
98.96
99.42
100.22
CIPW norms: Qz
Or Ab Ne An Di Hy Ol Mt I1 Ap
--
6.50 24.02 -29.34 18.35 3.94 9.54 2.42 5.01 0.78
--
-
4.02 13.62 -22.81 28.09 22.37 2.21 2.33 3.68 0.66
11.05 18.71 -25.11 18.74 1.35 16.31 2.49 5.19 1.04
Petrochemical data from Krishnamurthy (1974).
TABLE II Rb, Sr and aTSr/S6Sr ratios for the three Dhandhuka basalts, western India Sample
OA/D-37 OA/D-32 OA/D-3
Rock type
normal basalt picrite basalt porphyritic basalt
Rb (ppm)
Sr (ppm)
Rb/Sr
28 21 39
510 300 365
0.0549 0.070 0.1068
STSr/S~Sr
aTSr/"Sr
(measured)
(initial)
0.07634 0.70970 0.70925
0.70620 0.70952 0.70898
'~Sr/*'Sr have been calculated assuming an age of 63 Ma (K-At data of Alexander, 1977).
132 DISCUSSION The three representative rock types of the Dhandhuka basaltic suite, comprising picrite basalt (OA/D-32), porphyritic basalt (OA/D-3) and normal basalt {OA/D-37) are mildly alkaline. This makes them slightly different from the average Deccan basalt which is considered to be sub-alkaline. The average contents of Rb and Sr are 29 and 391 ppm, respectively, for the flows under study. Thus both Rb and Sr are within the normal range for basaltic rocks (Prinz, 1967) but somewhat higher than in basalts from elsewhere in the Deccan region (Table III). The initial 87Sr/S6Sr ratios of these rocks vary from 0.70620 to 0.70952 which appears to be higher than the range of initial Sr-isotopic ratios from other continental basaltic provinces (Table IV). Only Tasmanian dolerites (Heier et al., 1965) and Antarctic basalts (Faure et al., 1974) have higher ratios. The Sr-isotopic ratios of the former range from 0.708 to 0.718, with an average of 0.712, while the Kirkpatrick basalt, Antarctica also, has anomalously high Sr-isotopic ratios ranging from 0.7094 to 0.7133 and averaging 0.7112. However, both Tasmartian dolerites and Antarctic basalts have much lower Sr concentrations only ~ 130 ppm as against 390 ppm for Dhandhuka flows. There is more similarity between the Dhandhuka flows and the basalts of the Nuanetsi zone in the Karroo province (87Sr/86Sr ratio 0.706--0.710, Manton, 1968). Recently Cox and Krishnamurthy (1977) have also pointed out the similarity between the lava sequence of these two regions in their broadest compositional characters. Among the Deccan traps, the range of Sr-isotopic ratios of the Dhandhuka flows (0.70620--0.70952) is the highest among those reported. Because the number of samples is small, it is not justified to make generalisations between the petrochemistry--Sr-isotopic ratio relationship. Fig. 2 shows a plot of initial Sr-isotopic ratios vs. Rb/Sr ratios for the Dhandhuka flows. For comparison, the fields for oceanic and continental basaltic rocks are also shown together with fields for the Girnar and Sagar basalts of the Deccan region: (1) it is observed that while some of the Sagar and Girnar basalts lie within the oceanic field, the Dhandhuka flows are distinctly continental; and (2) in contrast to those of Sagar and Girnar, the basalts of Dhandhuka are more erratic, indicating the possibility of contamination by introduction of foreign radiogenic Sr. High Sr-isotopic values for basalts and gabbros of the Nuanetsi zone (0.706--0.710) are regarded as being due to isotopic exchange with the Archaean basement rocks. Manton (1968) has shown that the Sr-isotopic ratios for these basic rocks: "appear t~ vary with strontium content rather than with the Rb/Sr ratio of each sample, which suggests contamination". A similar relationship is observed for the Dhandhuka flows on the initial Srisotopic ratio vs. Sr plot (not shown). This can indicate contamination for the
0.7052--0.7067
S~Sr/S6Sr
0.7039--0.7084
2,406 183 6 208 155 12 1--3
*Mantle background (Harris, 1972).
2,181 292 6 264 --1.5
K Ba Rb Sr Zr Nb Th 0.7062--0.7095
10,293 534 31 411 197 39 5.44 0.7095
5,395 400 21 300 140 30 6.31
picrite basalt OA/D-32
400--500 5? ~1 10 30 1--5? 0.20
mantle background*
20 106 31 41 6 8 27
11 80 21 30 4 6 30
picrite basalt OA/D-32
average Dhandhuka basalt
average Dhandhuka basalt
Girnar
Sagar
E n r i c h m e n t factor c o m p a r e d to mantle b a c k g r o u n d
Values in p p m
Comparison o f incompatible elements and Sr-isotopic ratios in D h a n d h u k a , Girnar and Sagar basalts of the Deccan p r o v i n c e in relation to the mantle background
T A B L E III
C.o
134 TABLE IV Initial Sr-isotopic ratios of Dhandhuka flows in comparison to other Deccan areas and basaltic provinces Deccan basalts
Other continental basalts
Dhandhuka basalts, 0.7062--0.7095 (present work) Sagar basalts, 0.7039--0.7084 (Alexander and Paul, 1977) Girnar basalts, 0.7052--0.7067 (Murali, 1974) Bombay basalts, 0.7045--0.7057 (G. Faure, pets. commun., 1978 on the work of Faure and Hurley, 1963)
basalts, New South Wales, 0.7035--0.7042 (Heier et al., 1965) basalts, Oregon, Arizona, 0.7030--0.7040 (Faure and Hurley, 1963) dolerites, Karroo, S.A., 0.7065 (Compston et al., ] 968) basalts, Tasmania, 0.7027--0.7078 (Compston et al., 1968) dolerites, Tasmania, 0.711 (Heier et al., 1965) basalts, Nuanetsi zone, South Africa, 0.706--0.710 (Manton, 1968) Kirkpatrick basalt, Antarctica, 0.7094-0.7133 (Faure et al., 1974)
IDhk
-714
.709 CO
~ -704 CO
..
•6 9 9 0
_ o ",£..Ocn. g a s . Rks.
1 .1
! "2
'| "3
i "4
Rb/Sr Fig. 2. Plot of initial Sr-isotopic ratio vs. Rb/Sr ratio for Deccan trap basalts. Boundary curves for continental and oceanic basaltic rocks (after Faure and Powell, 1972) are also shown for comparison. Abbreviations used are as follows: D h k = Dhandhuka; G i r = Girnar; S g r = Sagar, C o n t . B a s . R k s . = continental basaltic rocks; and O c n . B a s . R k s . = oceanic basaltic rocks.
135
0"710
iDhk eGir -,i-Sgr
/
.+
0"708
4-
b.
¢ 4-
0.706 •.I-
+÷
.-... 8.5]
+e "f"
+-. 7.8|X10 -3 "F"* 15"4J
e'l"
+ 0-704
4- -i, 13.7 J
[ 1.0
I 2"0
I' 3"0
-1
1/Sr ppm
I 4 "0
I
5"0
61-0 X
10- 3
Fig. 3. A plot showing initial 87Sr/S~Sr ratio vs. 1/St for flows from Dhandhuka, Girnar and Sagar, indicating contamination for Dhandhuka basalts. The abbreviations used are the same as in Fig. 2.
two Dhandhuka samples with high Sr-isotopic ratios. A plot of STSr/S6Srratio vs. 1/Sr (Fig. 3) indicates contamination for the Dhandhuka flows more effectively, where correlation is observed between 878r/86Sr ratio and 1/Sr for the samples under study. There is no such correlation apparent for Girnar and Sagar flows. The Girnar samples show a nearly flat pattern, whereas the Sagar flows are more erratic. In the situation of a magma undergoing contamination with material with a higher 8~Sr/S6Sr ratio and incompatible elements, a lava flow with low Sr content should be affected the most. The fact that the sample with the highest 878r/86Sr ratio has the lowest Sr content from among the samples of the present study is consistent with the suggestion that they were contaminated. Not only has the picrite basalt (OA/D-32) the highest 8TSr/868r ratio of 0.70952 and the lowest Sr value (300 ppm) in contrast to the normal basalt (Sr = 510 ppm; 8VSr/8~Sr = 0.70620) and the porphyritic basalt (Sr = 365 ppm; 878r/8~Sr = 0.70898) but it is also the highest in silica content, giving further support to the idea of contamination. From his petrological, chemical and phase-equilibria studies of the Dhandhuka borehole suite, Krishnamurthy (1974) has demonstrated picrite rocks to be the most "primitive", and that other rock types can be derived from picrite basalt by the process of crystal fractionation. Rare-earth element data on t h e same suite of rocks (Alexande r and Gibson, 1977) also confirm this. Enrichment of incompatible elements in these primitive picrite basalts and the entire suite as whole, however, is remarkable and needs to be explained. Table III compares some incompatible elements (K, Ba, Rb, Sr, Zr,
136
Nb and Th) and the initial Sr-isotopic ratios of Dhandhuka basalts and the basalts of Girnar and Sagar. The data establish that the Dhandhuka suite as a whole is considerably richer in incompatible elements in comparison with basalts of the Girnar and Sagar regions. In comparison with the mantle background of these elements (Harris, 1972), the Dhandhuka suite shows remarkable enrichment (Table III). The enrichment factor ranges from 4 to 80 for primitive picrite basalt and 6 to 106 when the Dhandhuka suite is considered as a whole. The mean enrichment factor, on the other hand, for all the elements considered is 26 and 34 for primitive picrite basalt and average Dhandhuka basalt. It seems difficult to account for the enrichment of incompatible elements of this magnitude in the primitive picrite basalt by the degree of melting alone as has also been demonstrated by Krishnamurthy, 1974). Other processes by which incompatible trace elements could be enriched in the residual liquids, namely, high-pressure eclogite fractionation {O'Hara and Yoder, 1967) and subsequent olivine fractionation of the initial liquids en route (O'Hara, 1968) appear to have played an insignificant role in the present case (Krishnamurthy, 1974). However, there is some evidence for the operation of eclogite fractionation on the basis of K20/Na:O ratios of the borehole suite, but it will definitely not account for the observed manifold increase in the enrichment factors of incompatible elements in contrast to the mantle background. It is suggested therefore that incompatible elements such as K, Ba, Sr, Th, Pb, Rb and Zr and radiogenic Sr in the Dhandhuka borehole suite were enriched due to contamination. Unfortunately, there are no data for incompatible elements and Sr-isotopic ratios for the basement rocks underlying the Dhandhuka basalts. This makes it difficult to specify with certainty the mechanism and degree of contamination of basaltic magma. However, considering the overall petrochemistry of the Dhandhuka suite, bulk contamination is considered unlikely, since, unlike Antarctic Kirkpatrick basalts with anomalously high STSr/S6Sr and silica (Faure et al., 1974), the Dhandhuka flows are typically basaltic and do not seem to have resulted from the hybridisation of a normal basaltic magma with the Precambrian granitic rocks. If it were so, there should have been pronounced changes in the bulk petrochemistry of this suite. The mechanism of wall-rock reaction (Green and Ringwood, 1968) both at or near the source and also at shallower depths should have played a dominant role whereby radiogenic Sr and other incompatible elements were selectively introduced into the basaltic magma. The Dhandhuka borehole suite, falling within the tectonically active region of the Gulf of Cambay, on the western margin of the Indian shield, has been suggested as the focus of earliest Deccan trap eruptions (Auden, 1949). Preliminary K--Ar investigations (Alexander, 1977) also suggest that the flows comprising this borehole suite represent the earliest Deccan trap volcanism. Considering this, it seems reasonable to assume that the wall-rock reaction was the dominant process in modifying the incompatible element distribution. It seems more likely that combined Pb- and Sr-isotopic studies on a number of samples of this much studied borehole suite will lead to a better understanding of the problem discussed.
137 ACKNOWLEDGEMENTS
The author gratefully acknowledges the Commonwealth Academic Staff Scholarship award (1974--1976) during which time the present work was carried out at the Department of Earth Sciences, University of Leeds, England. For strontium-isotopic work he is grateful to E. Oegazi and to M.H. Dodson for general guidance. For the Dhandhuka borehole samples the author is thankful to P. Krishnamurthy and K.G. Cox. For comments on the manuscript he is thankful to D.K. Paul, P.E. Baker, W.D. West and S. Mallikarjunan. REFERENCES Alexander, P.O., 1977. Geochemistry and geochronology of Deccan trap lava flows around Sagar, M.P., India. Ph.D. Thesis, University of Saugar, Sagar (unpublished). Alexander, P.O. and Gibson, I.L., 1977. Rare-earth abundances in Deccan trap basalts. Lithos, 10: 143--147. Alexander, P.O. and Paul, D.K., 1977. Geochemistry and strontium isotopic composition of basalts from the eastern Deccan volcanic province. Ind. Mineral. Mag., 41: 165--172. Auden, J.B., 1949. Dykes in western India. Trans. Natl. Inst. Sci. Ind., 3: 123--157. Burwell, A.D.M., 1975. Rb--Sr isotope geochemistry of the lherzolites and their constituent minerals from Victoria, Australia. Earth Planet. Sci. Left., 29: 69--78. Compston, W., McDougall, I. and Heier, K.S., 1968. Geochemical composition of the Mesozoic basaltic rocks of Antarctica, South Africa, South America and Tasmania. Geochim. Cosmochim. Acta, 32: 1 2 9 - 1 4 9 . Faure, G. and Hurley, P.M., 1963. The isotopic composition of strontium in oceanic and continental basalts: application to the origin of igneous rocks. J. Petrol., 4: 31--50. Faure, G. and Powell, J.L., 1972. Strontium Isotope Geology. Springer, New York, N.Y., 188 pp. Faure, G., Bowman, J.R., Elliot~ D.H. and Jones, L.M., 1974. Strontium isotope composition and petrogenesis of the Kirkpatrick basalt, Queen Alexandra range, Antarctica. Contrib. Mineral. Petrol., 48: 153--169. Green, D.H. and Ringwood, A.E., 1967. The genesis of basaltic magma. Contrib. Mineral. Petrol., 15: 103--190. Harris, P.G., 1972. Geothermal environment and basalt magma type. J. Earth Sci,, 8(2): 275--285. Heier, K.S., Compston, W. and McDougall, I., 1965. Thorium and uranium concentrations and isotopic composition of strontium in different Tasmanian dolerites. Geochim. Cosmochim. Acta, 29: 643---659. Krishnamurthy, P., 1974. Petrological and chemical studies of Deccan trap lavas of western India. Ph.D. Thesis, University of Edinburgh, Edinburgh (unpublished). Krishnamurthy, P. and Cox, K.G., 1977. Picrite basalts and related lavas from the Deccan traps of western India. Contrib. Mineral. Petrol., 62: 53--75. Manton, W.L., 1968. The origin of associated basic and acid rocks in the L e b o m b o - Nuanetsi igneous province, South Africa as implied by strontium isotopes. J. Petrol., 19: 2 3 - - 3 9 . Murali, A.V., 1974. Some aspects of geochemistry of the Girnar igneous complex, western India. Ph:D. Thesis, University of Saugar, Sagar (unpublished). O'Hara, M.J., 1968. The bearing of phase equilibria and the origin and evolution of basic and ultrabasic rocks. Earth-Sci. Rev., 4: 69--133. O'Hara, M.J. and Yoder, Jr., H,S., 1967. F o r m a t i o n and fractionation of basaltic magma at high pressures. Scott. J. Geol., 3: 67--117.
138 Paul, D.K., Potts, P.J., Rex, D.C. and Beckinsale, R.D., 1977. Geochemical and petrogenetic study of the Girnar igneous complex, Deccan volcanic province, India. Geol. Soc. Am. Bull., 88: 2 2 7 - 2 3 4 . Prinz, M., 1967. Geochemistry of hasalts: trace elements. In: H.H. Hess and A. Poldervaart (Editors), Basalts, I. Interscience, New York, N.Y., pp. 271--323. Raju, A.T.R., Choube, A.N. and Choudhary, L.R., 1971. Deccan trap and the geologic framework of the Cambay basin. Bull. Volcanol., 35: 521--538. West, W.D., 1958. The petrography and petrogenesis of 48 flows of the Deccan traps penetrated by borings in western India. Trans. Natl. Inst. Sci. Ind., 4(1): 1--56.