K/Ar AGES OF SUCCESSIVE LAVA FLOWS FROM THE DECCAN ...

EARTH AND PLANETARYSCIENCELETTERS 18 (1973) 229-236. NORTH-HOLLAND PUBLISHING COMPANY

K/Ar AGES OF SUCCESSIVE LAVA FLOWS FROM THE DECCAN TRAPS, INDIA Ichiro KANEOKA Geophysical Institute, Faculty of Science University of Tokyo, Tokyo, Japan * and Hiroshi HARAMURA Geological Institute, Faculty of Science University of Tokyo, Japan

Received 14 September 1972 Revised version received 17 January 1973

K/Ar ages on rocks from the Deccan Traps include results on exposed lava flow sequences at Mahabaleshwar and Amboli. Several criteria were used to assess their reliability. The calculated ages range from 40 my to 66 my and are not concordant with the stratigraphy. This can be explained by the altered state of most samples, which is visible in hand specimen and in thin section. This conclusion is further supported by the measured H20 (+) contents, almost all of which exceed one percent. Consideration of these factors, in addition to the possible error magnification of a large air correction in some samples, leads to the conclusion that the age of these flows is at least 60-65 my.

1. Introduction The Deccan Traps, covering an area o f more than 500000 square kilometers in western and central India, are mainly composed of tholeiitic basalts with minor amount of alkaline rocks. They are estimated to be about 500000 to 1000000 km 3 [1] in volume. In spite o f their importance in understanding the mechanism of plateau formation, their ages are not yet settled definitely. Nonmarine sediments within the Deccan Traps are reported to contain fossil plants and fish Palaeocene age [2]. Field relations suggest that they were formed during the period late Cretaceous to lower Tertiary [ 2 - 4 ] . Published K / A r ages of basalts and alkaline rocks from the Deccan Traps range from 40 m y to 65 my [5, 6]. Although Rama's reported results [5] seem to indicate two main periods o f volcanic activity around * Present address: Laboratoire de G6ochimie, Institut de Physique du Globe, Universit6 de Paris VI, Paris V, France.

4 2 - 4 5 my and 6 0 - 6 5 my ago, Wellman and McElhinny rejected the idea on the ground that they could get several instances of ages concordant at 6 0 - 6 4 my after careful choice of samples [6]. However the number of dated rocks is limited and the criteria for sample choice are rather subjective. We felt therefore that the possible existence o f younger rocks in the Deccan Traps could not be excluded on the available data. During the winter o f 1 9 6 9 - 1 9 7 0 , two lava flow sequences and some alkaline rocks were collected from five localities in the Deccan Traps as part of the Indo-Japanese joint program of Deccan basalt studies. In this paper K / A r dating results are reported for these rocks, with particular emphasis on the flow sequences at Mahabaleshwar and Amboli. Since the field occurrence of these lava flows gives good stratigraphic control, they offer a good test for the occurrence of volcanic activity in the Deccan Traps later than about 60 my ago. A recent publication (7) suggests that H20(+)

230

1. Kaneoka, H. Haramura, K/Ar Ages of successive lava flo ws

content of a rock is a simple guide to the degree of weathering, and may therefore serve as another criterion for evaluation of K/Ar ages. This, the observed field relations, and other more conventional checks such as the degree of air contamination in the measured argon and the usual macro- and microscopic observations, are used in a critical evaluation of the new data. New measurements on alkaline rocks from other sites permit comparison with the published ages.

MA 1400 m '.'.'.'.' THICK .'.'.'.'. LATE RITE

.'.'.'.'2

Z12

1200

--~Z7

1000

~Z2 800

2. Samples and sampling localities

~W

Samples were collected from Mahabaleshwar (MA), Amboli (AB), Bombay (BO), Girnar Hill (GI) and Pavagarh Hill (PA) (fig. 1). Their general geological and petrographical descriptions will be reported elsewhere [8, 9]. At the Mahabaleshwar sections, 42 successive lava flows were distinguished in cliffs about 1200 m in height. These lava flows are composed of tholeiitic basalts. Eight were selected for K/Ar dating from the least weathered representatives of at least 4 - 5 suc-

----

-F 200 - -

-

-B

~001

~7/777~, BASEMENT

Fig. 2. Schematic diagram of the successive lava flows at Mahabaleshwar sections.

i

F

N 22 °

--L

400

0

I PAVAGARH HILL o

600

AB THICK LATERITE

.......

m

:.:.'.%.:':':':

100

04

-

02

-

20

BQMBAY

-- UNCONFORMITY ~

PRE CAMBRIAN AMPHIBOLITE

MoAHABALESHWAR

1B,

o

70

_

Fig. 3. Schematic diagram of the successive lava flows at Amboli sectio.n.

looKm

V

BOLl_

7~

74

Fig. 1. Sampling sites in the Deccan Traps.

76 ° E

cessive lava flows (fig. 2). Even so, some of them are still much altered. Furthermore, rocks containing large phenocrysts of plagioclase were discarded on the ground that they might include excess Ar. At the Amboli section, only five lava flows were observed resting directly upon the Precambrian base ment (fig. 3). Two of them were selected for K/Ar dating under the same criteria as those adopted for

L Kaneoka, H. Haramura, K/Ar Ages of successive lava flows

231

K-Ar AGE

AT

35 (m.y.)

30

(m.y.) MA Z12- 51

NA L-55

20

-t

t

z,5 W NA. - 51

MA

10

~AB 02-22

Z7-51 {

55

AB04-21

!

i

I

0.5

1.0

1.5

2.0

65

H20 (") (%) Fig. 4. K/Ar age and H20(+) content of successive lava flows from Mahabaleshwar and Amboli sections. Open circle: sample from Mahabaleshwar sections. Closed circle: sample from Amboli section. Errors in the age indicate one standard deviation and those in H20(+ ) content are assigned -+ 10% as the maximum. A T(apparent reduction in age) is calculated with the assumption that T (formation age) was 65 my ago for present rocks. Since the rocks from the upper layers should be younger than those from the lower layers, T was a little different among these rocks. AT should be therefore read as a rough indication. The indication of K/Ar age represents measured value. samples from the Mahabaleshwar sections. They also are altered to some degree. The amphibolite o f the underlying Precambrian basement complex was dated to establish the age difference between the volcanic activity o f the Deccan Traps and the metamorphism of the basement rocks. Alkaline rocks were collected from Bombay, Girnar Hill and Pavagarh Hill areas. Since the lack o f key beds made it difficult to determine their stratigraphic correlations, only one or two samples were dated from each site for comparison with the previously published age data [5, 6]. The same selection criteria were adopted as at the Mahabaleshwar and Amboli sites. Petrographic descriptions of samples are given in the appendix.

K-analyses were made with a single channel flame photometer with Na as buffer. Ar was analysed by the isotope dilution method with 38Ar as a tracer, using a 15 cm radius Reynolds type mass spectrometer. H20 contents were measured by the conventional ignition loss method. Experimental error in K-analysis is less than 2% and that in Ar-analysis is about 2%. Replicate analyses for U.S.G.S. muscovite standard sample P - 2 0 7 resulted in the value o f ( 1 . 2 6 3 + 0.026) X 10 - 9 moles/g for radiogenic 4°Ar. Experimental error in H20 measurement is less than 10%. More details about the experimental procedures have already been reported [10].

4. Results 3. Experimental

procedures

Samples were analysed as whole rocks and each fraction for K and Ar analysis was selected b y the quartile method. H20 was measured on separately crushed material.

4.1. Mahabaleshwar The K / A r data for rocks from Mahabaleshwar are given in table 1. They are arranged in flow sequence, with the upper layers at the top. The ages, ranging from about 40 my to 62 my, are not concordant with the

I. Kaneoka, H. Haramura, K/Ar Ages of successive lava flows

232

Table 1 K/At ages of successive lava flows from Mahabaleshwar, Deccan Traps, India. Sample No.

MAZ12-51 (basalt) MAZ7-51 (basalt) MAZ2-51 (basalt) MAW-51 (basalt) MAL-55 (basalt) MAF-61 (basalt) MAB-61 (basalt) MA001-51 (basalt)

K (%)

(4oAr) tad. (moles/g)

(40As) air (4°Ar)tot. (%)

Age+ (my)

H20(+)

0.216

24.8 83.8 30.6

40.6 -+0.8 42.0 +- 2.6 55.1 -+ 1.2

1.56

0.91

0.488

1.577 X 10-It 1.629 4.854

1.27

0.82

0.340

3.423

78.9

55.7 +- 2.7

-

-

0.249

2.170

32.3

48.4 +- 1.1

1.61

1.00

0.133

1.115

91.9

46.8 +-5.6

0.79

1.15

0.257

2.827

83.3

60.8 -+3.7

-

-

0.423

4.703

44.7

61.5 -+ 1.4

1.40

0.74

0.166

1.319

52.4

43.6 -+ 3.1

-

-

+he = 0.585 × 10-1° yr -1 , deviation.

k# = 4.72 × 10 -1° yr -1,

4°K/K = 1.19 × 10-4moles/mole.

sequence of lava flows. We attribute this to alteration by weathering, which can be observed both in hand specimen and in thin section. Such samples as M A 0 0 1 - 5 1 , M A L - 5 5 and MAW-51 are visibly more altered than the others and their apparent younger ages are therefore attributed to Ar loss. For some classes of volcanic rock, an anticorrelation has been observed between the apparent K/Ar age and H20(+) content of the rock [7]. The results for Mahabaleshwar samples are show in fig. 4.Unfortunately for the present study, different batches had to be used for K/Ar dating and H:O measurement because the initial preparation was insufficient. This may have affected the correlation to some degree. M A L - 5 5 is the only sample which shows H2 O(+) content less than one percent. This sample is exceptional and will be discussed separately. For the others, these is a good correlation between H20(+) and apparent reduction in age. This is strong evidence that these rocks have been hydrated [7]. The resulting clay minerals are derived mostly from olivine, interstitial glass and mesostasis. Most plagioclase remains intact and there is little K in olivine. Therefore error in Ar/K should result from alteration of the glass and mesostasis. It is worth noting that the young ages of M A Z 1 2 - 5 1 was not expected from the macro- and

H20(-)

(wt. %)

Error in the age : one standard

microscopic observations. Its high H20(+) seems therefore to be a better indication of argon loss. Sample M A L - 5 5 did seem altered on optical examination and showed low K/Ar age, but these results are not matched by a distinctly lower H20(+) which removes the point in fig. 4 out of the trend defined by the other samples. There are two possible explanations. On the one hand, this might be an extreme example of sampling error. The water content of the batch prepared for ignition may not have been the same as that prepared for K and Ar analysis. On the other hand, this sample had the highest air contamination in the analysed argon. The experimental error in the measured age is much larger for this sample than for the others. The significance of this age result must therefore remain ambiguous for the present.

4.2. A m b o l i The K/Ar data for the Amboli samples are displayed in table 2, again in stratigraphic order; the older layer ( A B 0 2 - 2 2 ) gives the younger age. Both samples have a high HiO(+) content, 1.16% and 1.53%. Both are therefore likely to have lost argon, even though 60 my for A B 0 4 - 2 1 is similar to the


233

Table 2 K/At ages of successive lava flows from Amboli, and some alkaline rocks Bombay, Girnar Hill and Pavagarh Hill, Deccan Traps, India. Sample No.

Amboli AB04-21 (basalt) AB02-22 (Basalt) AB58 (amphibolite) Bombay B0-07 (mugearite?) Girnar Hill GI-17 (diorite) Pavagarh Hill PA-05 (basalt) PA-15 (ankaramite)

K (%)

(4oAr) rad. (moles/gm)

(40Ar) air (4°At)tot. (%)

0.374

3.998 × 10 -11

70.2

0.257

2.341

88.8

0.174

1.123× 10 -9

4.0

1.20

8.136 × 10 -~a

47.2

2.56

3.023X 10 -1°

0.589 0.598

H 20 (+) (wt. %)

H 20 (-)

60.2 + 2.1

1.16

1.09

51.2 + 4.8

1.53

0.80

0.63

1.10

37.7 ± 0.9

1.93

0.48

16.7

65.3±

1.3

1.66

0.29

6.998 × 10 -1~

57.0

65.6 ± 1.7

2.26

0.63

7.161

31.0

66.2 +- 1.4

2.16

1.77

ages obtained for some of the Mahabaleshwar samples. Even this sample, however, clearly shows the presence of interstitial clay minerals under the microscope, indicating the alteration which is reflected by its H20(+) content. The basement amphibolite on the other hand is rather fresh in thin section and the H20(+) content for this rock is 0.63%. Hence the calculated age, about 2000 m y older than the Deccan Traps basalts, is expected to be more reliable than those for the younger rocks.

Age (my)

2030

±40

H20(+). Since we have no data about the correlation between H20(+) content and the reliability of K / A r age for such intrusive rocks, we cannot evaluate the value properly at present from H20(+) content only. Alkaline basalts P A - 0 5 and P A - 1 5 from Pavagarh Hill show similar ages of about 66 my. F r o m microscopic observations, they are not so altered as those from Bombay. However they contain comparatively high H20(+), more than 2%. Although the present H20 measurements were made on different crushings from those used for K / A r dating, we cannot exclude the possibility of hydration for these rocks.

4.3. Bombay, Girnar Hill and Pavagarh Hill The K/Ar ages for rocks from these sites are also included in table 2. Sample B O - 0 7 from Bombay appears to be very young, about 38 my. Under the microscope, clay minerals are observed to replace the interstitial glass. The high H20(+) content is definitely due to the presence of this clay mineral and we should consider the value as minimal. Sample G I - 1 7 is a diorite from Girnar Hill. F r o m microscopic observation, it is practically free from alteration products. The measured K / A r age is about 65 my. The sample contains, however, about 1.7% of

5. Discussion and conclusions In general, present samples contain less than ten percent glass, and no clear xenoliths or xenocrysts are observed. Hence the effect of excess Ar is likely to be negligible [11]. However the effect o f alteration on K / A r ages is more serious for all these rocks. All samples except AB58 and G I - 1 7 contain clay minerals. As a considerable part of the glass or mesostasis is altered to clay, the effect on original K and radiogenic Ar could have been serious. On the other hand, more than 40

234


whole rock analyses of tholeiitic basalts from Mahabaleshwar and Amboli sections indicate a very consistently low K content, making the whole tholeiite series one of the least K-enriched differentiation series known to date (our unpublished data). This may indicate the lack of any serious amount of K enrichment during alteration of the rocks to form clay minerals. It may be reasonable, then, that the only effect of alteration was to lower the Ar/K ratio and hence the apparent age. The result would be a minimum formation age. Among the successive lava flows from Mahabaleshwar and Amboli, the oldest indicated ages are about 60-62 my. These are similar to those obtained by Wellman and McElhinny [6] who accepted such concordant values as pointing to the age of the main volcanic activity. However in the present study, microscopic observation and a H20(+) content greater than one percent both show that even those rocks with older ages appear to have been somewhat altered. Hence we should consider even these values as minimum estimates of the ages of these lava flows. It is true that the younger ages obtained from these successive lava flows cover the age period which Rama reported for some volcanic rocks [5]. However these "younger" rocks are always more altered, and occur at no well-defined place in the sequence. Hence we agree with Wellman and McElhinny [6] that it is more probable that these young ages are caused by loss of radiogenic Ar through alteration of the samples. In the area around Bombay, although the present sample indicates very young age of about 38 my due to Ar loss, Rama dated an older age of trachyte (60 + 3 my) and a flow of basic rocks (42 + 6, 45 + 3 my [5] and Wellman and McElhinny reported the results of a basalt from near the base of the section (minimum age: 59 + 2 my) [6]. These scatterings in measured ages may be also explained in the different degree of alteration. The result for sample GI-17 from Girnar Hill (65 + 1 my) in the present study is almost the same as the value of 65 + 1 my obtained by Wellman and McElhinny [6]. Under microscope, this sample is rather fresh. Hence this value may be close to the formation age of the rock. Results for alkaline rocks from Pavagarh Hill (66 -+ 2 my and 66 + 1 my) agree with those of 65 +

5 my measured by Rama [5] and 62 + 2 my by Wellman and McElhinny [6]. However, since present rocks contain comparatively high H20(+) contents, it is more conservative to regard these values as minimal. Furthermore we must point out that the rock from Girnar Hill is intrusive rock, hence the surrounding volcanic rock area would be older than these rocks. As for the age range for volcanic activity, palaeomagnetic results for successive lava flows from Mahabaleshwar and Amboli sections show only two epochs [12, 13] and indicate a comparatively short duration of volcanic activity of less than 5 my [12]. Although the K/Ar ages for these rocks indicate only the minimum age for each lava flow, if we take K/Ar ages of samples with lower H20(+) content (excluding sample MAL-55), the ages range from 55 my to 62 my for successive lava flows from Mahabaleshwar sections. This may also suggest comparatively short volcanic activities for these lava flows. From these considerations, it is concluded that the most active period of volcanism in the Deccan Traps was 60 my to 65 my ago or possibly older, and the activity lasted for a relatively short period of time.

Acknowledgements We are very grateful to Dr. S. Aramaki who read the manuscript critically and gave many comments on the mineralogical properties of samples used in this study. He made the petrographical description of samples in the appendix. Dr. J. Richards also read the manuscript critically and corrected our English. Fig. 4 was modified into the present form according to his suggestion. We appreciate him very much for these works and advices. We are thankful to Drs. Y.S. Sahasrabudhe, S.S. Deshmukh, S.K. Roy, S. Aramaki, T. Konda, H. Kurasawa, H. Kinoshita, M. Kono, N. Shimizu and M. Yamakawa for their help in collecting samples. The expenses of the sampling trip for Japanese members were defrayed by the Ministry of Education.

L Kaneoka, H. Haramura, K/Ar Ages of successive lavaflows Appendix Description of samples (by S. Aramaki) P: phenocryst, G: groundmass, Minerals: O1: olivine, Cpx: clinopyroxene, PI: plagioclase, Opq: opaque oxides, GI: glass or mesostasis, Bi: biotite, Hb: hornblende, Alk. FI: alkali feldspar. Relative abundance: +++(abundant), ++ (medium), + (poor). Alteration: a(altered), p.a. (partly altered). Mahabaleshwar (All rocks are tholeiitic basalts) MAZ12-51 P: O1(+, a), P1(++). G: Cpx (+++), P1(++), Opq (+), G1(÷, p.a.). MAZ7-51 P: O1(+, a), P1(+). G: Cpx (+++), P1(+++), Opq (++), SiO2 (+), G1 (% p.a.). MAZ2-51 P: Cpx (+), P1(+, p.a,). G: Cpx (+++), P1(+++), Opq (++), SiO2 (+), G1(+, p.a.). MAW-51 P: O1 (+, a), Cpx (+), P1(+). G: Cpx (+++), P1(+++), Opq (++), SiO2 (+), G1 (+, a). MAL-55 P: O1(+, a), P1(++). G: Cpx (+++), P1(+++), Opq (+), SiOz (+), G1 (+, a). MAF-61 P: O1(+, a), Cpx (+), P1(+). G: Cpx (+++), P1(+++), Opq(+), GI(++, p.a.). MAB-61 P: O1(+, a), P1(+). G: O1 (+?, a), Cpx(+++), P1 (+++), Opq (+), SiO2(+?), G1(+, p.a.). MA001-51 P: O1(+, a), P1 (+). G: O1(+9., a), Cpx (+++), P1(+++), Opq(+), SiOz(+), G1(+, p.a.). Amboli AB04-21

AB02-22

tholeiitic basalt P: O1(+, a), Cpx (+), P1(+). G: Ol (+, a), Cpx (+++), P1(+++), Opq(+), GI(+, p.a.). tholeiitic basalt P: O1(+, a), Cpx (+), P1(+).

AB-58

Bombay BO-07

Girnar Hill GI-17

235

G: O1(+9., a), Cpx(+++), PI(+++), Opq (+), G1 (++, a). amphibolite Hb, P1, Opq, quartz (9.).

mugearite rock P: Cpx (+), P1(+). G: Cpx (++), P1(++), Opq (+), G1(++, a).

hornblende-biotite diorite Bi, Hb, augite, P1, Alk.FI., Opq, apatite, sphene, calcite.

Pavagarh Hill PA-05 picrite basalt P: O1(+, p.a.), Cpx (+), P1(+). G: Cpx (++), P1 (++), Opq(+), G1 (+, a). ankaramite PA-15 P: O1(+, p.a.), Cpx(+). G: Cpx (++), P1(++), Opq (+), G1(+, p.a.). References

[1] H. Kuno, Plateau basalts, Amer. Geophys. Union, Geophys. Monogr. 13 (1969) 495. [2] M.S. Krishnan,The geologyof India and Burma, Higginbothams,5th ed (Madras, 1968). [3] A.P. Subramanianand Y.S. Sahasrabudhe,Geologyof Greater Bombayand Aurangabad-EUora-Ajantaarea, Intern. Geol. Cong., 22nd, India, 1964, Guide to Exc. Nos. A-13 and C-10 (1964) 1. [4] E. Pascoe, A manual Of the geologyof India and Burma, Vol. III (Governmentof India Press, Calcutta, 1964) 1345. [5] S.N.I. Rama, 22nd Intern. Geol. Congr. 1964, New Delhi, Pt VII (1968) 139. [6] P. Wellmanand M.W.McElhinny,K-Arages of the Deccan Traps, India, Nature 227 (1970) 595. [7] I. Kaneoka,The effect of hydration on the K/Ar ages of volcanicrocks, Earth Planet. Sci. Letters 14 (1972 216. [8] Y.S. Sahasrabudhe,S.S. Deshmukh,S.K. Roy, S Aramaki, T. Konda, H. Kurasawa,H. Kinoshita, M. Kono, N. Shimizu,I. Kaneokaand M. Yamakawa, Deccan basalt from exposures along PoladpurMahabaleshwarand AmboliGhats sections in Western Ghats, India - a geologicalstudy (to be published).

236


[9] S.S. Deshmukh, S. Aramaki, H. Kurasawa, N. Shimizu and T. Konda, Petrography of the basalt flows exposed along Mahabaleshwar and Amboli sections in Western Ghats, India (to be published). [10] I. Kaneoka, The use of obsidian for K-Ar dating, Mass. Spectr. 17 (1969) 514. [ 11 ] M. Ozima, I. Kaneoka and S. Aramaki, Reply to the comments, Earth Planet. Sci. Letters 9 (1970) 311.

[12] M. Kono, H. Kinoshita and Y. Aoki, Paleomagnetism of Deccan Traps basalts, India, Geomag. Geoelectr. (in press). [13] H. Wensink and C.T. Klootwijk, Paleomagnetism of the Deccan Traps in the Western Ghats near Poona, India, Tectonophysics 11 (1971) 175.