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39Ar dating on volcanic rocks of the Deccan Traps, India
Earth and Planetary Science Letters, 46 (1980) 233-243 © Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
233
[1l
4°Ar/a9ArDATING
ON VOLCANIC ROCKS OF THE DECCAN TRAPS, INDIA ICHIRO KANEOKA
Geophysical hlstitute, Faculty of Science, University o f Tokyo, Bunkyo-ku, Tokyo 113 (Japan)
Received May 16, 1979 Revised version received September 11, 1979
4°Ar/a9Ar dating results on seven volcanic rocks from four areas of the Deccan Traps, India, suggest that volcanic activity more than 70 Ma ago might have occurred at least in limited areas. In the lgat Puri area, the uppermost flow shows an 4°Ar/39Ar age of 63 Ma, whereas a lower flow has an age of around 82-84 Ma. 4°Ar/39Ar ages of samples from the Bombay area also seem to favor the occurrence of volcanic activity more than 70 Ma ago. One rhyolite dyke from the Osam Hill in the Girnar Hill area shows a well-defined plateau age of 68 Ma, whereas two tholeiitic basalts from the Mahabalesliwar area indicate a total 40Ar/39Ar age of around 63-64 Ma, though they show the effect of secondary disturbance in the age spectra. The volcanic activity(ies) more than 70 Ma ago may correspond to precursory one(s) for the main volcanic activity around 65 Ma ago in the Deccan Traps.
1. Introduction The Deccan Traps, one of the largest lava plateaux in the world with a volume of about ( 0 . 5 - 1 ) X 106 km 3, are mainly composed o f tholeiitic basalts with minor amounts of alkaline rocks [ 1 ]. In order to understand the mechanism o f plateau formation, their eruption ages are crucial. Several investigators have reported K-Ar ages o f these rocks, which range from about 40 to 65 Ma [ 2 - 4 ] . Since some o f these rocks have lost Ar due to the alteration of samples, the younger ages have been regarded as unreliable. In effect, when little altered samples were selected, their K-Ar ages seem to concentrate around 6 0 - 6 6 Ma [3,4]. Furthermore, paleomagnetic results for some limited areas in the Deccan Traps also seem to support a relatively short period o f volcanic activity [5,6]. For these reasons, a period o f several million years has been assigned as the main volcanic activity in the Deccan Traps [ 3 - 6 ] . However, the areas investigated in these previous studies are limited. Furthermore, radiometric ages of these rocks were determined only by the K-Ar method. K-At ages are more affected by the condi-
tion of the rocks and/or minerals used for dating than some of the other dating methods. Deccan volcanic rocks are easily altered due to regional climatic conditions, which apparently results in lowering the K-Ar ages. Hence, the possibility that volcanic activity occurred in the Deccan Traps more than 70 Ma ago cannot be precluded. On the other hand, older ages could result if excess Ar occurs in a sample, which we cannot identify from a single K-Ar age unless other evidence exists. For this purpose, the 4°Ar/39Ar method with stepwise heating is suitable, since Ar loss and excess Ar can be identified sometimes by analysing the age spectrum. In the present study, the 4°Ar/39Ar method has been applied to samples from four localities in order to reexamine the ages o f volcanic activities in the Deccan Traps.
2. Samples and sampling localities Samples were collected during the winters o f 1 9 6 9 - 1 9 7 0 and 1 9 7 2 - 1 9 7 3 by the Indo-Japanese scientific team as a part of the Indo-Japanese joint program o f Deccan basalt studies. For 4°Ar/agAr
234 dating, seven samples from four localities were selected, which were collected from the Igat Puri (IG-02, IG-15), Bombay (BO-24, BO-30), Girnar Hill (GI 62-50) and Mahabaleshwar (MA A-51, MA Z1151 ) areas. At the Igat Puri area, fifteen successive flows of tholeiitic olivine-augite basalts with about 290 m thickness in total are exposed as a section located between Kasara (19°40'N, 73°29'E) and Igat Puri (19°42'N, 73°34'E) about 105 km north of Bombay. Samples were selected from the uppermost (IG-15) and from the second flow from the bottom (IG-02) to study the age difference between them. From the Bombay area, two lava flows were selected. Sample BO-24 is an olivine nephelinite, collected at Trombay, while BO-30 was a tholeiitic basalt, collected at a site northwest of Jogeshnari. However, their stratigraphical relationships are not clear. Sample GI 62-50 is a rhyolite dyke, collected at the eastern foot of the Osam Hill, about 23 km northeast of Junagadh, situated to the east of the Girnar Hill. This sample was selected to compare the result with the age of the Girnar Hill diorite determined by the conventional K-At method. Mallabaleshwar is located about 140 km southsoutheast of Bombay and 100 km south-southwest of Poona. At the Mahabaleshwar sections, 42 successive lava flows were identified in cliffs about 1200 m in height. From them, eight flows were previously dated by the K-Ar method [4]. For present purposes, two samples were selected from the uppermost (MA Z 1151) and the lowermost part (MA A-51), respectively. MA A-51 was taken from the fifth flow from the bottom and MA Z11-51 from the second flow from the top of these successive lava flows. They are tholeiitic basalts. Petrographic descriptions for these samples are given in the Appendix and exact sampling sites are described by latitude and longitude in Table 1 together with the results.
3. Experimental procedures Since most of the experimental procedures are similar to those reported before [7], only the characteristic points are described here.
Samples were crushed to - 3 5 , +60 mesh before irradiation and wrapped in Al-foil. They were stacked in air in quartz ampoules (10q~ × 70 mm) together with standard muscovites (Bern 4M) and irradiated with fast neutrons (about 2 X 1018 n cm -2) in the core of JMTR. From the irradiated samples, Ar was extracted and purified by conventional procedures. Each temperature step was taken for one hour. The extracted Ar was analysed on a 60 °, 15-cm-radius Reynolds-type mass spectrometer, which is separated from the extraction and purification systems. All necessary corrections were made including mass discrimination, memory effect, interference of Ar isotopes derived from neutron irradiated K and Ca as described before [7]. The amount of Ar was estimated by peak height comparison between the sample and the standard and it includes an ambiguity of about 20%. Uncertainties in the 4°Ar/39Arage are calculated by combining those from the scatter in the mass spectrometer runs and those in the J-value (refer to the legend in Table 1) including the uncertainty derived from the age of the standard. The uncertainties in the isochron age and the initial 4°Ar/a6Ar ration in the isochron diagram are calculated by the least squares fitting method developed by York [8].
4. Results The 4°Ar/39Arage results are given in Table 1 and Figs. 1-7. In each figure, both the age spectra and the 4°Ar/a6Ar-agAr/a6Ar isochron diagram are shown. As shown in these figures, only samples IG-15 and GI 62-50 show well-defined plateau ages, while IG-02 seems to show a plateau-like age at intermediate temperatures (800-900°C). On the other hand, several samples seem to fall roughly on isochrons with reasonable (4°Ar/a6Ar)i ratios. Furthermore, their total 4°Ar/39Ar ages range from 61 Ma (IG-15) to 88 Ma (BO-24), the latter of which is definitely older than the eruption ages estimated from the conventional K-At dating method. As shown in Fig. 1, the age spectrum of IG-02 seems to be a little disturbed, but a plateau-like age of 81.9 + 2.9 Ma for the intermediate temperature
235
TABLE 1 Ar data for neutron irradiated samples from the Deccan Traps Sample
IG-02, 1.293 g (19 ° 41'N, 73 Q29'E)
IG-15, 1.301 g ( 1 9 ° 4 2 ' N , 72 °3 I'E)
BO-24, 1.448 g (19°02'N, 7 2 ° 5 5 ' E )
BO-30,'1.268 g (19°02'N, 72°50'E)
GI 62-50, 1.288 g (21 °39'N, 7 0 ° 2 1 ' E )
4°Ar (× 10 - 8 cm 3 STP/g)
36 Ar/4 o Ar
37Ar/4OAr
39Ar/4OAr
4 ° A r / 3 9 A r age (Ma)
J = 0.01013 600 700 800 900 1000 1100 1300
61.8 164 109 115 73.4 77.6 86.0
0.003082 0.002656 0.001087 0.001201 0.001775 0.002512 0.003306
0.01881 0.00974 0.09376 0.3841 0.5363 0.2163 0.5540
0.01381 0.02738 0.1477 0.1411 0.1164 0.06886 0.00895
114.5 138.1 82.0 81.7 73.1 67.1 46.5
0.2345
0.07617
83.9
0.05234 0.1355 0.3277 0.6388 0.3735 0.7422
0.01.665 0.08595 0.1099 0.1694 0.2020 0.02386
0 73.0 63.1 62.5 63.7 63.4 45.1
T (°C)
Total
686.8
0.002173
J = 0.01013 600 700 800 900 1000 1100 1300
82.2 33.6 31.9 43.9 65.8 110 282
0.003402 0.003154 0.002362 0.002090 0.001347 0.000971 0.003182
~0
~0
Total
649.4
0.002534
0.4820
0.07427
60.9
J = 0.01013 600 700 800 900 1000 1100 1300
93.8 72.7 175 38.8 39.2 149 132
0.001438 0.002194 0.001207 0.001587 O.002558 0.002714 0.002269
0.07413 0.05326 0.1308 0.1288 0.07802 0.04491 0.08538
136.5 116.8 87.7 72.1 56.3 78.8 69.2
Total
0.08533
88.0
700.5
0.001959
0.03000 0.03883 0.02314 0.04184 0.04140 0.07964 1.108 0.2447
J = 0.01013 600 98.3 700 93.1 800 100 900 80.8 1000 134 1100 257 1300 407
0.003137 0.001529 0.001031 0.001383 0.001003 0.002899 0.003055
~0 0.04035 0.06467 0.1438 0.3591 0.04287 0.2563
0.00924 0.1133 0.1614 0.1674 0.1707 0.02623 0.02692
139.0 86.3 77.0 63.4 73.8 172.2 64.8
Total
1170.2
0.002383
0.1583
0.06980
82.2
J = 0.008703 600 262 700 96.5 800 294 900 98.7 1000 137 1100 42.4 1300 210
0.002195 0.001752 0.000666 0.001042 0.001191 0.003191 0.003221
0.03250 0.06433 0.03092 0.00205 0.00441 0.085 82 0.01277
0.09880 0.1259 0.1821 0.1545 0.1452 0.02990 0.01317
55.0 59.2 68.0 69.0 68.8 29.7 56.5
Total
0.001769
0.02712
0.1146
64.2
1140.6
± 4.6 -+ 7.0 ± 2.9 ± 5.1 _+ 2.8 ± 2.3 ± 1.3
_*8.4 ± 2.4 ± 2.8 ± 2.0 ± 2.2 _+ 2.0
± 5.9 _+ 3.6 ± 2.2 _+ 3.6 ± 3.0 ± 2.2 ± 3.0
± 6.1 ± 3.9 _+ 3.5 _+ 3.9 ± 2.9 ± 6.6 ± 1.9
± 1.8 ± 2.2 ± 1.6 ± 2.5 _+2.7 ± 2.3 ± 1.7
236 TABLE 1 (continued) 36Ar/4OAr
37Ar/40Ar
39Ar/40Ar
J = 0.008703 600 17.2 700 27.1 800 53.4 900 58.7 1000 40.2 1100 7.92 1300 8.40
0.003306 0.001103 0.000670 0.000655 0.000885 0.002675 0.003057
0.1092 0.2397 0.4574 0.8391 0.5024 3.380 5.896
0.04877 0.1448 0.1634 0.1903 0.1936 0.1100 0.1372
7.38 71.7 75.5 65.2 58.9 29.7 11.0
Total
T (°C)
Sample
MA A-51, 1.332 g ( l ? ° 5 6 r N , 73°32rE)
MA Z l l - 5 1 , 1.349 g (17 °56PN, 7 3 ° 3 8 ' E )
4°At (X 10 -8 cm 3 STP/g)
212.92
4 ° A r / a 9 A r age (Ma)
0.001143
0.8385
0.1618
63.1
J = 0.008703 600 24.8 700 26.2 800 34.5 900 64.8 1000 186 1100 16.9 1300 46.4
0.003095 0.002661 0.002401 0.002646 0.003125 0.002220 0.002899
0.1095 0.09087 0.4191 0.2321 0.1994 1.302 1.747
0.01883 0.07479 0.08168 0.02893 0.01857 0.1025 0.03719
69.8 44.3 55.0 114.7 63.7 51.9 59.6
Total
0.002888
0.4370
0.03509
64.4
399.6
_+0.45 _+ 3.0 _+4.0 -+ 3.0 _+ 3.8 -+ 1.5 _+ 0.8
-+ 5.0 _+ 2.0 _+ 2.2 _+4.7 _+ 1.6 _+ 3.1 _+ 3.3
All values were corrected for K-derived 4 ° A r and Ca-derived 36mr and 39Ar. 4 ° A r * is defined as ( 4 ° A r - 2 9 5 . 5 • 36Ar). Error figures: l a (standard deviation). The age (tin) of the standard sample Bern 4M (muscovite) was assumed to be (18.70 _+0.66) Ma. J ~ ~ [ e x p ( h t m ) - 1 ]/(4 OAr,/39Ar)m ) h e = 0.581 × 10 -10 yr -1 ; ht3 = 5.543 × 10 -10 yr -1 [10].
70o.
150
60¢ icl ,~,~too w 800" t.9 <
IG 02
9000
oo. /
I.-z LLI
, ~ "
/
84 Ma
D~ < 50 Q_
12. <
13o0"
I
0
o'.239Aro.4RELEASED°"6 0.e
i.o
oo
i
lo'o 39Ar/36A r
200
Fig. 1. Sample IG-02: 40Ar/39Ar age spectra as a function o f 39Ar released (left) and 4°Ar/a6Ar vs. 39Ar/36A~ isochron diagram (right). The band in the 4°Ar/39Ar spectra diagram indicates the -+l a (standard deviation) envelope about the calculated age o f each temperature fraction. All temperatures are given in degrees Celsius.
237 an anomalously old 4°Ar/a9Ar age in the 1100°C fraction, which may reflect the occurrence of excess 4°Ar in a special phase such as clinopyroxene or titanomagnetite. The disturbances in the age spectra for these samples may reflect at least partly the alteration state of mesostasis in the samples. Both BO-24 and BO-30 have relatively old total 4°Ar/39Ar ages of more than 80 Ma. To explain them, the occurrence of excess 4°Ar is one possibility. If it is the case, excess 4°At should have appeared mostly in the lower-temperature fractions as shown in the age spectra (Figs. 3 and 4). Non Ar-retentive sites such as imperfections and lattice defects in minerals and/or mesostasis are candidates to trap such excess 4°At. However, we have no evidence that these samples contain more such sites than the IG samples. Furthermore, it is not common that subaerial volcanic rocks trap large amount of excess 4°At, unless they contain large phenocrysts or abundant glassy parts. (If we assume that the true eruption ages for Bombay samples were around 6 5 - 7 0 Ma, the amount of excess 4°At in these samples should be about ( 6 - 8 ) X 10 -7 cm 3 STP/g. This amount is comparable to that observed in oceanic pillow basalts [9] and much larger than that observed in subaerial basaltic rocks [10] .) The Bombay samples show no sign of
fractions (800-900°C) contains more than 60% of the 39Ar released. The isochron age for the 8 0 0 1300°C fractions is 83.6 + 4.2 Ma and the total 4°Ar/39Ar age is about 84 Ma. These three kinds of 4°Ar/39Ar ages all agree with one another within their experimental uncertainties. As known in the Appendix, sample IG-02 has a coarse-grained subophitic texture with only a small amount of mesostasis. This may preclude a special property of almost glassy materials that zero age oceanic pillow basalts give apparent isochrons in the isochron diagram with atmospheric (4°Ar/a6Ar)i ratios [9]. Mesostasis is altered and olivine is partly altered. Such alteration might have caused the disturbance in the age spectrum due to Ar loss and/or K addition, both of which result in lowering the K-Ar age. Hence, it is less likely that this old age is due to excess 4°Ar in this sample. On the other hand, sample IG-15 shows a welldefined plateau age of 63.3 + 1.3 Ma for the 8 0 0 1100°C fractions, which represent about 85% of the 39Ar released (Fig. 2). This age agrees quite well with the isochron age of 63.8 + 1.1 Ma, which shows the atmospheric value for the initial 4°Ar/a6Ar ratio. As shown in Figs. 3 and 4, the samples from the Bombay area (BO-24, BO-30) show an inversed-staircase-type age spectrum. Furthermore, BO-30 shows
80
~
700*
6O
900o 1 0 0 0 "
1100°
i
°',ooo
v bJ
oo
I
~40' I-Z UJ rr
I0 ~
-j
S
f
IG 15 500
~ . 2C <
,oo~,B00,
64 Ma
5300"
600* o,2_^3~ArO.4RELEASEDO.6 0.8
to
°o
~oo 39ArI36A r 200
Fig. 2. Sample IG-15. The plateau age is 63.3 -+ 1.3 Ma for 800-1100°C fractions. The isochron suggests an 4°Ar/39Ar age of 63.8 -* 1.i Ma with the (4°Ar/36Ar)i ratio of 294.6 ± 2.7 for 600-1300°C fractions.
238 900 150
BO
L_
24
~
8o~*
7~
~oo"
90o"
~r loo
800"
uJ (.9 ~,,,z n.-
~ 'F~I~13oo" 9ooL~ I
~so o_
50o
lood
•':[
01
70o" / ~ ÷~3oo' ~/ '1000"
72 MO
300
012.~ na~Ar0,'4RELEASED0~6 0.8
1.0
1000
.~0 39Ar136Ar 1~)o
Fig. 3. Sample BO-24. The isochron indicates 72.0 -+ 6.9 Ma with the (4°Ar/36Ar)i ratio of 289.6 -+ 14.3 for 9 0 0 - 1 3 0 0 ° C fractions.
the old total 4°Ar/39Ar age is that secondary Ar distribution took place within these samples. In this case, the total 4°Ar/39Ar age would correspond to the approximate eruption age. The Columbia River Plateau basalts show such examples [11]. The age spectra for BO-24 and BO-30 are similar to those of
containing these phases. Hence, it is not likely that the old total 4°Ar/agAr ages for the Bombay samples are mainly caused by excess 4°At, except for the 1100°C fraction of the sample BOo30. If the 1100°C fraction is excluded, the total 4°Ar/39Ar age reduces to 73.9 Ma for B0-30. Another possibility to explain
'~oo" ~i_ 150
1
6oo"
BO 30
0 ~r
o~ "4"
1 O0
tO (,9
1
7O0" 00"
Z hl O-
n'SO
)0~
1000'
~30o"
7o0S .o
O.
500'
0
0.2 "~Ar~'0.4RELEASEDO.6 ~ 0.8
1.0
el
0
100 39Ar/36Ar 200
Fig. 4. Sample BO-30. The isochron of 74 Ma is drawn as a reference, which goes through the (4°Ar/36Ar)i ratio of 320.
239 80
500 I
800"
900" 1000°
60
,3o0"
o
O
:E
v
600"
LIJ
o <
ooot
900/
40
I-Z IJ.l
GI
62-50 5O0
Q.
1o/.o . ° 6°°.
69 Ma
0.6 o.~ L0 0 loo 2oo 39Ar/36Ar3OO 02 39AOrARELEASED Fig. 5. Sample GI 62-50. The 800-1000°C fractions indicate a plateau age of 68.3 -+ 1.2 Ma in the age spectra. An isochron of 68.8 -+ 1.3 Ma is defined with the (4°Ar/36Ar)i ratio of 292.8 -+ 3.6 for 800-1300°C fractions.
the Columbia River Plateau basalts. Hence, redistribution o f Ar seems more likely than excess At. In Fig. 5, sample GI 62-50 show a relatively welldefined plateau age o f 68.3 -+ 1.2 Ma for the 8 0 0 1000°C fractions, which cover about 68% of the agAr released. The plateau age agrees quite well with the
isochron age of 68.8 -+ 1.3 Ma as defined from the 8 0 0 - 1 3 0 0 ° C fractions. The intercept gives 292.8 -+ 3.6 for the (4°Ar/a6Ar)i ratio. These results suggest that the age o f 68 Ma probably indicates the time of eruption. Samples MA A-51 and MA Z11-51 also show dis-
80
10000 60 O :E v hi
(.9
40
I--" Z hi rr
<
MA A-51
,loo"
20
6OO'
13oo°
600*
o.~__ o~4
o76
0.'8
~UAr RELEASED
,.o
o
,oo
2oo3_#s_~oosA bar
Fig. 6. Sample MA A-51. No plateau nor isochron is observed for this sample. In the isochron diagram, a reference isochron of 60 Ma is drawn assuming the atmospheric ratio for the (4°Ar/36Ar)i ratio.
240
120 ~oc
9oo"
MA
Zll-51
~
100
--~
o
~
40C
,,,
o <80
/
.^^. ~'~3o
Z n< o_ 12-60
+900. yfoo. O"
P~O00"
1o0o* - - ~
3oo
40
200
012~P"J~Ar 0I~RELEASED 0.6 0.8
50 MQ
1.0
' 0
2'0
i
4'0 3__r/3--r609A bA
Fig. 7. Sample M A Z 11-51. In the isochron diagram, a reference isochron of 50 Ma is drawn with the (4°Ar/36Ar)i ratio of 300.
turbed age spectra. The former indicates an inversedstaircase type, whereas the latter may suggest excess 4°At in the 900°C fraction. Neither sample yields a good isochron, but in spite of such disturbed age spectra and scattered data points in the isochron diagram, both samples show similar total 4°Ar/39Ar ages of 63.1 and 64.4 Ma, respectively. These ages are similar to those determined for less altered samples from Mahabaleshwar and other areas by the K-At method [2-4]. Hence as discussed for samples from Bombay, a closed system redistribution of radiogenic 4°Ar in the sample is possible. From successive lava flows, sample MA Z 11-51 was collected from a higher part than sample MA A-51. An apparent older age of MA Z 11-51 may be due to the occurrence of excess 4°Ar in a special phase (probably plagioclase phenocrysts) as revealed in the 900°C fraction in the age spectrum. If we exclude this fraction for this sample, the total 4°Ar/39Ar age becomes 56.6 Ma.
5. Discussion
Results of 4°Ar/39Ar dating are summarized in Table 2, where the samples may be classified into two groups; one group has 4°Ar/a9Ar ages of around 6 0 70 Ma (IG-15, GI 62-50. MA A-51, MA Z11-51) and
the other shows the 4°Ar/39 Ar ages of more than 70 Ma (IG-02, BO-24, BO-30). The 4°Ar/39Ar ages of the younger group are similar to the conventional K-At ages reported for less altered samples from the Deccan Traps. For example, diorites from the Girnar Hill were reported to be 6 6 67 Ma old [3,4]. (All K-Ar ages in this paper are recalculated by using the decay constants recommended by the Subcommission on Geochronology [12] .) Although the rhyolite dyke GI 62-50 was collected at the Osam Hill, situated northeast of the Girnar Hill, it also shows an 4°Ar/agAr age of about 68 Ma. Since it has a total 4°Ar/39Ar age of 64 Ma, those rocks probably erupted during the same series of volcanic activity in this area. The total 4°Ar/a9Ar ages for Mahabaleshwar samples are also similar to the oldest K-Ar age obtained for a Mahabaleshwar sample (MA B-61,63.0 Ma) [4]. Sample MA B-61 was collected from just above a lava flow of MA A. Hence, the 4°Ar/39Ar dating results in this study also support a volcanic activity in the Mahabaleshwar area 6 0 - 6 5 Ma ago. In this respect, it is interesting to note that IG-15 also shows an 4°Ar/a9Ar age of about 63 Ma, while IG-02 shows a definitely older value of 8 2 - 8 4 Ma. Both samples were collected from the same succession of lava flows in the Igat Puri area. Apparent
241 TABLE 2 Summary of the 4°Ar/39Ar age data for samples from the Deccan Traps Sample
40 Ar/39 Ar age (Ma) total
plateau
IG-02
83.9
81.9 ± 2.9
IG-15
60.9
63.3 ± 1.3
BO-24 BO-30 GI 62-50
88.0 82.2 64.2
68.3 _+1.2
MA A-51 MA Zll-51
63.1 64.4
-
range (integrated 39 Ar)
4OAr/39Ar isochron age (Ma)
range
(40Ar/36Ar)i
800-900°C (0.617) 800-1100°C (0.849) 800-1000°C (0.678)
(83.6 ± 4.2)
800-1300°C
(258.3 ± 19.3)
63.8 ± 1.1
600-1300°C
294.6 ± 2.7
72.0 ± 6.9 (74.1 ± 3.3) 68.8 _* 1.3
900-1300°C 600-1300°C 800-1300°C
289.6 ± 14.3 (320.7 ± 28.5) 292.8 ± 3.6
(49.1 ± 3.0)
600-1300°C
(303.7 _+6.5)
Uncertainty in each value means lcr (standard deviation). Numerical figures in parentheses are less reliable than the other ones.
4°Ar/39Ar age relationships between both samples are concordant with the order o f the succession layers. These results suggest that there might possibly be a relatively large age gap between the lava flows for samples IG-02 and IG-15. Although several gaps are observed geologically among the intermediate lava flows in this section (K. Aoki, personal communication, 1978), we cannot identify which boundary corresponds to this presumed large age gap. Although due to the disturbed age spectra we cannot exclude the possibility o f excess 4°Ar completely, the results of IG-02 together with the Bombay samples give evidence favoring relatively old volcanic activity(ies) o f more than 70 Ma ago in the Deccan Traps. Although we do not know at present to what extent it ocurred, this fact may give us an important constraint on the clarification of the mechanism o f the Deccan Traps formation. It is reported by several investigators that paleomagnetic studies on volcanic rocks in the Deccan Traps have revealed only a few polarity epochs (e.g. [13]). Based on such paleomagnetic results together with the limited number o f K-Ar dating results, a relatively short period o f volcanic activity o f around 5 Ma has been suggested [ 3 - 6 , 1 4 ] . On the other hand, some investigators [15] propose a continuous igneous activity since the Rajmahal volcanic phase whose eruption ages were determined to be about
100 Ma [16]. Present results open a door for the latter possibility from the geochronological viewpoint, though it may not always mean a continuous igneous activity. Judging from the distribution of ages, however, we can still argue for the main volcanic activity around 6 0 - 7 0 Ma ago. Geomagnetic-reversal time scale based on the summary of marine-anomaly data [17] indicates that only a few polarity epochs are identified during the Upper Cretaceous time. 4°Ar/agAr age results for the present samples seem to suggest that older volcanic activity(ies) o f more than 70 Ma might have occurred during this Cretaceous magnetic quiet period. Hence, the limited number o f observed polarity epochs is not always incompatible with the occurrence o f the older volcanic activity(ies) in the Deccan Traps. Alkaline rocks from the Gimar Hill and the Pavagarh Hill seem to show almost similar or even slightly older K-At ages than tholeiitic rocks from the other localities [3,4]. If we follow a hypothesis that alkaline rocks extrude later than tholeiitic rocks during a series o f magmatic differentiation, the above mentioned alkaline rocks might have belonged to the series o f older tholeiitic volcanic activity o f more than about 65 Ma. Hence, the history o f the formation o f the Deccan Traps may not be so simple as has been suggested based on the limited number o f paleomagnetic and
APPENDIX
w i t h clay
w i t h clay
.
olivine nephelinite
+ (a.)
.
tholeiitic basalt
++ (a. to g r e e n c l a y )
9
+
++
++
* +++ = a b u n d a n t ; ++ = m e d i u m ; + = r a r e ; - = a b s e n t ; p.a. = p a r t l y a l t e r e d ; a. = a l t e r e d .
diktytaxitic cavities filled
olivine-augite basalt
++ (p.a.)
9
+
diktytaxitic cavities filled
+ + (a. t o clay)
brown hornblende mesostasis
++
+
+
-?
N.B.
+ +
titanomagnetite ilmenite
. ++
coarse-grained subophitic
-
+ subophitic
+ (micro phenocryst)
-
BO-30
-
+ (p.a.)
BO-24
++
olivine-augite basalt
++
nepheline plagioclase
-I-+
coarsegrained subophitic
++ -
+ (p.a.)
IG-15
Rock name
++
coarsegrained subophitic
++ -
+ (p.a.)
Groundmass olivine clinopyroxene
Texture
clinopyroxene plagioclase magnetite biotite
Phenocryst olivine
IG-02
P e t r o g r a p h i c d e s c r i p t i o n o f s a m p l e s * ( b y S. A r a m a k i )
.
quartz aggregates
secondary
rhyolite
+++
_
--
++ (alkalifeldspar?)
_
cryptocrystalline
+? ( a l t e r e d )
-
-
GI 62-50
tholeiitic basalt
+ (p.a. to c l a y )
+
+
++
++
intergranular
-
-
-
MA A-51
tholeiitic basalt
+ (p.a. to c l a y )
+
+
++
++
+? (a. to c l a y )
intergranular
++
+ (a. t o g r e e n clay)
MA Z 1 1 - 5 1
t,9
243 K-At age data. To clarify this point, additional 4°Ar/39AI dating is being tried for samples form the other areas in the Deccan Traps.
Acknowledgements The author appreciates Prof. S. Aramaki of the Earthquake Research Institute, University of Tokyo for microscopic observations of samples and comments on this study. He is also grateful to Prof. K. Aoki of Tohoku University and Dr. H. Kurasawa of the Geological Survey of Japan for their information on geological setting of sampling sites for IG and GI samples. Prof. M. Ozima of the University of Tokyo made suggestive comments on this study. The author thanks Prof. F. Begemann of the Max-Planck-Institut fi~r Chemie (Mainz) and two anonymous reviewers for their helpful comments to improve the manuscript. He also expresses his thanks to Prof. S. Yajima and Mr. M. Narui of Tohoku University for the neutron irradiation. Samples were collected by the Indo-Japanese team through the joint program on the Deccan basalt studies. The expenses for the sampling trip for Japanese members were defrayed by the Ministry of Education of Japan. This study was financially supported in part by the Ito Science Foundation.
References 1 H. Kuno, Plateau basalts, Am. Geophys. Union, Geophys. Monogr. 13 (1969) 495-501. 2 S.N.I. Rama, Potassium/argon dates of some samples from Deccan Traps, 22nd Int. Geol. Congr., New Delhi, 1964, Rep. VII (1964) 139-140.
3 P. WeUmanand M.W. McElhinny, K-Ar ages of the Deccan Traps, India, Nature 227 (1970) 595-596. 4 I. Kaneoka and H. Haramura, K]Ar ages of successive lava flows from the Deccan Traps, India, Earth Planet. Sci. Lett. 18 (1972) 229-236. 5 M. Kono, H. Konoshita and Y. Aoki, Paleomagnetism of Deccan Trap basalts, India, J. Geomag. Geoelectr. 24 (1972) 49-67. 6 M.W. McElhinny, Northward drift of India-Examination of recent paleomagnetic results, Nature 217 (1968) 342344. 7 I. Kaneoka and K. Aoki, 4°Ar/39Ar analyses of phlogopite nodules and phlogopite-bearing peridotites in South African kimberlites, Earth Planet. Sci. Lett. 40 (1978) 119-129. 8 D. York, Least squares fitting of a straight line with correlated errors, Earth Planet. Sci. Lett. 5 (1969) 320-324. 9 D. Seidemann, 4°Ar/39Ar studies of deep-sea igneous rocks, Geochim. Cosmochim. Acta 42 (1978) 17211734. 10 G.B. Dalrymple and M.A. Lanphere, Potassium-Argon Dating (W.H. Freeman and Co., San Francisco, Calif., 1969) 258 pp. 11 R.J. Bottomley and D. York, 4°Ar-a9Ar age determinations on the Owyhee basalt of the Columbia Plateau, Earth Planet. Sci. Lett. 31 (1975) 75-84. 12 R.H. Steiger and E. J~iger, Subeommission on geochronology: Convention on the use of decay constants in geoand cosmochronology, Earth Planet. Sci. Lett. 36 (1977) 359-362. 13 P.W. Sahasrabudhe, Paleomagnetism and the geology of the Deccan Traps, Semin. Geophys. Invest. Peninsular Shield, Osmania Univ., Hyderabad (1963) 226-243. 14 H. Wensink, Newer paleomagnetic results of the Deccan Traps, India, Tectonophysics 17 (1973)41-59. 15 R.N. Alhavale and R.K. Verma, Paleomagnetic results on Gondwana dykes from the Damodar valley coal-fields and their bearing on the sequence of Mesozoic igneous activity in India, Geophys. J. 20 (1970) 303-316. 16 I. McDougalland M.W. McElhinny, The Rajmahal Traps of India - K/Ar ages and Paleomagnetism, Earth Planet. Sci. Lett. 9 (1970) 371-378. 17 J.E. van Hinte, A Cretaceous time scale, Am. Assoc. Pet. Geol. Bull. 60 (1976)498-516.
233
[1l
4°Ar/a9ArDATING
ON VOLCANIC ROCKS OF THE DECCAN TRAPS, INDIA ICHIRO KANEOKA
Geophysical hlstitute, Faculty of Science, University o f Tokyo, Bunkyo-ku, Tokyo 113 (Japan)
Received May 16, 1979 Revised version received September 11, 1979
4°Ar/a9Ar dating results on seven volcanic rocks from four areas of the Deccan Traps, India, suggest that volcanic activity more than 70 Ma ago might have occurred at least in limited areas. In the lgat Puri area, the uppermost flow shows an 4°Ar/39Ar age of 63 Ma, whereas a lower flow has an age of around 82-84 Ma. 4°Ar/39Ar ages of samples from the Bombay area also seem to favor the occurrence of volcanic activity more than 70 Ma ago. One rhyolite dyke from the Osam Hill in the Girnar Hill area shows a well-defined plateau age of 68 Ma, whereas two tholeiitic basalts from the Mahabalesliwar area indicate a total 40Ar/39Ar age of around 63-64 Ma, though they show the effect of secondary disturbance in the age spectra. The volcanic activity(ies) more than 70 Ma ago may correspond to precursory one(s) for the main volcanic activity around 65 Ma ago in the Deccan Traps.
1. Introduction The Deccan Traps, one of the largest lava plateaux in the world with a volume of about ( 0 . 5 - 1 ) X 106 km 3, are mainly composed o f tholeiitic basalts with minor amounts of alkaline rocks [ 1 ]. In order to understand the mechanism o f plateau formation, their eruption ages are crucial. Several investigators have reported K-Ar ages o f these rocks, which range from about 40 to 65 Ma [ 2 - 4 ] . Since some o f these rocks have lost Ar due to the alteration of samples, the younger ages have been regarded as unreliable. In effect, when little altered samples were selected, their K-Ar ages seem to concentrate around 6 0 - 6 6 Ma [3,4]. Furthermore, paleomagnetic results for some limited areas in the Deccan Traps also seem to support a relatively short period o f volcanic activity [5,6]. For these reasons, a period o f several million years has been assigned as the main volcanic activity in the Deccan Traps [ 3 - 6 ] . However, the areas investigated in these previous studies are limited. Furthermore, radiometric ages of these rocks were determined only by the K-Ar method. K-At ages are more affected by the condi-
tion of the rocks and/or minerals used for dating than some of the other dating methods. Deccan volcanic rocks are easily altered due to regional climatic conditions, which apparently results in lowering the K-Ar ages. Hence, the possibility that volcanic activity occurred in the Deccan Traps more than 70 Ma ago cannot be precluded. On the other hand, older ages could result if excess Ar occurs in a sample, which we cannot identify from a single K-Ar age unless other evidence exists. For this purpose, the 4°Ar/39Ar method with stepwise heating is suitable, since Ar loss and excess Ar can be identified sometimes by analysing the age spectrum. In the present study, the 4°Ar/39Ar method has been applied to samples from four localities in order to reexamine the ages o f volcanic activities in the Deccan Traps.
2. Samples and sampling localities Samples were collected during the winters o f 1 9 6 9 - 1 9 7 0 and 1 9 7 2 - 1 9 7 3 by the Indo-Japanese scientific team as a part of the Indo-Japanese joint program o f Deccan basalt studies. For 4°Ar/agAr
234 dating, seven samples from four localities were selected, which were collected from the Igat Puri (IG-02, IG-15), Bombay (BO-24, BO-30), Girnar Hill (GI 62-50) and Mahabaleshwar (MA A-51, MA Z1151 ) areas. At the Igat Puri area, fifteen successive flows of tholeiitic olivine-augite basalts with about 290 m thickness in total are exposed as a section located between Kasara (19°40'N, 73°29'E) and Igat Puri (19°42'N, 73°34'E) about 105 km north of Bombay. Samples were selected from the uppermost (IG-15) and from the second flow from the bottom (IG-02) to study the age difference between them. From the Bombay area, two lava flows were selected. Sample BO-24 is an olivine nephelinite, collected at Trombay, while BO-30 was a tholeiitic basalt, collected at a site northwest of Jogeshnari. However, their stratigraphical relationships are not clear. Sample GI 62-50 is a rhyolite dyke, collected at the eastern foot of the Osam Hill, about 23 km northeast of Junagadh, situated to the east of the Girnar Hill. This sample was selected to compare the result with the age of the Girnar Hill diorite determined by the conventional K-At method. Mallabaleshwar is located about 140 km southsoutheast of Bombay and 100 km south-southwest of Poona. At the Mahabaleshwar sections, 42 successive lava flows were identified in cliffs about 1200 m in height. From them, eight flows were previously dated by the K-Ar method [4]. For present purposes, two samples were selected from the uppermost (MA Z 1151) and the lowermost part (MA A-51), respectively. MA A-51 was taken from the fifth flow from the bottom and MA Z11-51 from the second flow from the top of these successive lava flows. They are tholeiitic basalts. Petrographic descriptions for these samples are given in the Appendix and exact sampling sites are described by latitude and longitude in Table 1 together with the results.
3. Experimental procedures Since most of the experimental procedures are similar to those reported before [7], only the characteristic points are described here.
Samples were crushed to - 3 5 , +60 mesh before irradiation and wrapped in Al-foil. They were stacked in air in quartz ampoules (10q~ × 70 mm) together with standard muscovites (Bern 4M) and irradiated with fast neutrons (about 2 X 1018 n cm -2) in the core of JMTR. From the irradiated samples, Ar was extracted and purified by conventional procedures. Each temperature step was taken for one hour. The extracted Ar was analysed on a 60 °, 15-cm-radius Reynolds-type mass spectrometer, which is separated from the extraction and purification systems. All necessary corrections were made including mass discrimination, memory effect, interference of Ar isotopes derived from neutron irradiated K and Ca as described before [7]. The amount of Ar was estimated by peak height comparison between the sample and the standard and it includes an ambiguity of about 20%. Uncertainties in the 4°Ar/39Arage are calculated by combining those from the scatter in the mass spectrometer runs and those in the J-value (refer to the legend in Table 1) including the uncertainty derived from the age of the standard. The uncertainties in the isochron age and the initial 4°Ar/a6Ar ration in the isochron diagram are calculated by the least squares fitting method developed by York [8].
4. Results The 4°Ar/39Arage results are given in Table 1 and Figs. 1-7. In each figure, both the age spectra and the 4°Ar/a6Ar-agAr/a6Ar isochron diagram are shown. As shown in these figures, only samples IG-15 and GI 62-50 show well-defined plateau ages, while IG-02 seems to show a plateau-like age at intermediate temperatures (800-900°C). On the other hand, several samples seem to fall roughly on isochrons with reasonable (4°Ar/a6Ar)i ratios. Furthermore, their total 4°Ar/39Ar ages range from 61 Ma (IG-15) to 88 Ma (BO-24), the latter of which is definitely older than the eruption ages estimated from the conventional K-At dating method. As shown in Fig. 1, the age spectrum of IG-02 seems to be a little disturbed, but a plateau-like age of 81.9 + 2.9 Ma for the intermediate temperature
235
TABLE 1 Ar data for neutron irradiated samples from the Deccan Traps Sample
IG-02, 1.293 g (19 ° 41'N, 73 Q29'E)
IG-15, 1.301 g ( 1 9 ° 4 2 ' N , 72 °3 I'E)
BO-24, 1.448 g (19°02'N, 7 2 ° 5 5 ' E )
BO-30,'1.268 g (19°02'N, 72°50'E)
GI 62-50, 1.288 g (21 °39'N, 7 0 ° 2 1 ' E )
4°Ar (× 10 - 8 cm 3 STP/g)
36 Ar/4 o Ar
37Ar/4OAr
39Ar/4OAr
4 ° A r / 3 9 A r age (Ma)
J = 0.01013 600 700 800 900 1000 1100 1300
61.8 164 109 115 73.4 77.6 86.0
0.003082 0.002656 0.001087 0.001201 0.001775 0.002512 0.003306
0.01881 0.00974 0.09376 0.3841 0.5363 0.2163 0.5540
0.01381 0.02738 0.1477 0.1411 0.1164 0.06886 0.00895
114.5 138.1 82.0 81.7 73.1 67.1 46.5
0.2345
0.07617
83.9
0.05234 0.1355 0.3277 0.6388 0.3735 0.7422
0.01.665 0.08595 0.1099 0.1694 0.2020 0.02386
0 73.0 63.1 62.5 63.7 63.4 45.1
T (°C)
Total
686.8
0.002173
J = 0.01013 600 700 800 900 1000 1100 1300
82.2 33.6 31.9 43.9 65.8 110 282
0.003402 0.003154 0.002362 0.002090 0.001347 0.000971 0.003182
~0
~0
Total
649.4
0.002534
0.4820
0.07427
60.9
J = 0.01013 600 700 800 900 1000 1100 1300
93.8 72.7 175 38.8 39.2 149 132
0.001438 0.002194 0.001207 0.001587 O.002558 0.002714 0.002269
0.07413 0.05326 0.1308 0.1288 0.07802 0.04491 0.08538
136.5 116.8 87.7 72.1 56.3 78.8 69.2
Total
0.08533
88.0
700.5
0.001959
0.03000 0.03883 0.02314 0.04184 0.04140 0.07964 1.108 0.2447
J = 0.01013 600 98.3 700 93.1 800 100 900 80.8 1000 134 1100 257 1300 407
0.003137 0.001529 0.001031 0.001383 0.001003 0.002899 0.003055
~0 0.04035 0.06467 0.1438 0.3591 0.04287 0.2563
0.00924 0.1133 0.1614 0.1674 0.1707 0.02623 0.02692
139.0 86.3 77.0 63.4 73.8 172.2 64.8
Total
1170.2
0.002383
0.1583
0.06980
82.2
J = 0.008703 600 262 700 96.5 800 294 900 98.7 1000 137 1100 42.4 1300 210
0.002195 0.001752 0.000666 0.001042 0.001191 0.003191 0.003221
0.03250 0.06433 0.03092 0.00205 0.00441 0.085 82 0.01277
0.09880 0.1259 0.1821 0.1545 0.1452 0.02990 0.01317
55.0 59.2 68.0 69.0 68.8 29.7 56.5
Total
0.001769
0.02712
0.1146
64.2
1140.6
± 4.6 -+ 7.0 ± 2.9 ± 5.1 _+ 2.8 ± 2.3 ± 1.3
_*8.4 ± 2.4 ± 2.8 ± 2.0 ± 2.2 _+ 2.0
± 5.9 _+ 3.6 ± 2.2 _+ 3.6 ± 3.0 ± 2.2 ± 3.0
± 6.1 ± 3.9 _+ 3.5 _+ 3.9 ± 2.9 ± 6.6 ± 1.9
± 1.8 ± 2.2 ± 1.6 ± 2.5 _+2.7 ± 2.3 ± 1.7
236 TABLE 1 (continued) 36Ar/4OAr
37Ar/40Ar
39Ar/40Ar
J = 0.008703 600 17.2 700 27.1 800 53.4 900 58.7 1000 40.2 1100 7.92 1300 8.40
0.003306 0.001103 0.000670 0.000655 0.000885 0.002675 0.003057
0.1092 0.2397 0.4574 0.8391 0.5024 3.380 5.896
0.04877 0.1448 0.1634 0.1903 0.1936 0.1100 0.1372
7.38 71.7 75.5 65.2 58.9 29.7 11.0
Total
T (°C)
Sample
MA A-51, 1.332 g ( l ? ° 5 6 r N , 73°32rE)
MA Z l l - 5 1 , 1.349 g (17 °56PN, 7 3 ° 3 8 ' E )
4°At (X 10 -8 cm 3 STP/g)
212.92
4 ° A r / a 9 A r age (Ma)
0.001143
0.8385
0.1618
63.1
J = 0.008703 600 24.8 700 26.2 800 34.5 900 64.8 1000 186 1100 16.9 1300 46.4
0.003095 0.002661 0.002401 0.002646 0.003125 0.002220 0.002899
0.1095 0.09087 0.4191 0.2321 0.1994 1.302 1.747
0.01883 0.07479 0.08168 0.02893 0.01857 0.1025 0.03719
69.8 44.3 55.0 114.7 63.7 51.9 59.6
Total
0.002888
0.4370
0.03509
64.4
399.6
_+0.45 _+ 3.0 _+4.0 -+ 3.0 _+ 3.8 -+ 1.5 _+ 0.8
-+ 5.0 _+ 2.0 _+ 2.2 _+4.7 _+ 1.6 _+ 3.1 _+ 3.3
All values were corrected for K-derived 4 ° A r and Ca-derived 36mr and 39Ar. 4 ° A r * is defined as ( 4 ° A r - 2 9 5 . 5 • 36Ar). Error figures: l a (standard deviation). The age (tin) of the standard sample Bern 4M (muscovite) was assumed to be (18.70 _+0.66) Ma. J ~ ~ [ e x p ( h t m ) - 1 ]/(4 OAr,/39Ar)m ) h e = 0.581 × 10 -10 yr -1 ; ht3 = 5.543 × 10 -10 yr -1 [10].
70o.
150
60¢ icl ,~,~too w 800" t.9 <
IG 02
9000
oo. /
I.-z LLI
, ~ "
/
84 Ma
D~ < 50 Q_
12. <
13o0"
I
0
o'.239Aro.4RELEASED°"6 0.e
i.o
oo
i
lo'o 39Ar/36A r
200
Fig. 1. Sample IG-02: 40Ar/39Ar age spectra as a function o f 39Ar released (left) and 4°Ar/a6Ar vs. 39Ar/36A~ isochron diagram (right). The band in the 4°Ar/39Ar spectra diagram indicates the -+l a (standard deviation) envelope about the calculated age o f each temperature fraction. All temperatures are given in degrees Celsius.
237 an anomalously old 4°Ar/a9Ar age in the 1100°C fraction, which may reflect the occurrence of excess 4°Ar in a special phase such as clinopyroxene or titanomagnetite. The disturbances in the age spectra for these samples may reflect at least partly the alteration state of mesostasis in the samples. Both BO-24 and BO-30 have relatively old total 4°Ar/39Ar ages of more than 80 Ma. To explain them, the occurrence of excess 4°Ar is one possibility. If it is the case, excess 4°At should have appeared mostly in the lower-temperature fractions as shown in the age spectra (Figs. 3 and 4). Non Ar-retentive sites such as imperfections and lattice defects in minerals and/or mesostasis are candidates to trap such excess 4°At. However, we have no evidence that these samples contain more such sites than the IG samples. Furthermore, it is not common that subaerial volcanic rocks trap large amount of excess 4°At, unless they contain large phenocrysts or abundant glassy parts. (If we assume that the true eruption ages for Bombay samples were around 6 5 - 7 0 Ma, the amount of excess 4°At in these samples should be about ( 6 - 8 ) X 10 -7 cm 3 STP/g. This amount is comparable to that observed in oceanic pillow basalts [9] and much larger than that observed in subaerial basaltic rocks [10] .) The Bombay samples show no sign of
fractions (800-900°C) contains more than 60% of the 39Ar released. The isochron age for the 8 0 0 1300°C fractions is 83.6 + 4.2 Ma and the total 4°Ar/39Ar age is about 84 Ma. These three kinds of 4°Ar/39Ar ages all agree with one another within their experimental uncertainties. As known in the Appendix, sample IG-02 has a coarse-grained subophitic texture with only a small amount of mesostasis. This may preclude a special property of almost glassy materials that zero age oceanic pillow basalts give apparent isochrons in the isochron diagram with atmospheric (4°Ar/a6Ar)i ratios [9]. Mesostasis is altered and olivine is partly altered. Such alteration might have caused the disturbance in the age spectrum due to Ar loss and/or K addition, both of which result in lowering the K-Ar age. Hence, it is less likely that this old age is due to excess 4°Ar in this sample. On the other hand, sample IG-15 shows a welldefined plateau age of 63.3 + 1.3 Ma for the 8 0 0 1100°C fractions, which represent about 85% of the 39Ar released (Fig. 2). This age agrees quite well with the isochron age of 63.8 + 1.1 Ma, which shows the atmospheric value for the initial 4°Ar/a6Ar ratio. As shown in Figs. 3 and 4, the samples from the Bombay area (BO-24, BO-30) show an inversed-staircase-type age spectrum. Furthermore, BO-30 shows
80
~
700*
6O
900o 1 0 0 0 "
1100°
i
°',ooo
v bJ
oo
I
~40' I-Z UJ rr
I0 ~
-j
S
f
IG 15 500
~ . 2C <
,oo~,B00,
64 Ma
5300"
600* o,2_^3~ArO.4RELEASEDO.6 0.8
to
°o
~oo 39ArI36A r 200
Fig. 2. Sample IG-15. The plateau age is 63.3 -+ 1.3 Ma for 800-1100°C fractions. The isochron suggests an 4°Ar/39Ar age of 63.8 -* 1.i Ma with the (4°Ar/36Ar)i ratio of 294.6 ± 2.7 for 600-1300°C fractions.
238 900 150
BO
L_
24
~
8o~*
7~
~oo"
90o"
~r loo
800"
uJ (.9 ~,,,z n.-
~ 'F~I~13oo" 9ooL~ I
~so o_
50o
lood
•':[
01
70o" / ~ ÷~3oo' ~/ '1000"
72 MO
300
012.~ na~Ar0,'4RELEASED0~6 0.8
1.0
1000
.~0 39Ar136Ar 1~)o
Fig. 3. Sample BO-24. The isochron indicates 72.0 -+ 6.9 Ma with the (4°Ar/36Ar)i ratio of 289.6 -+ 14.3 for 9 0 0 - 1 3 0 0 ° C fractions.
the old total 4°Ar/39Ar age is that secondary Ar distribution took place within these samples. In this case, the total 4°Ar/39Ar age would correspond to the approximate eruption age. The Columbia River Plateau basalts show such examples [11]. The age spectra for BO-24 and BO-30 are similar to those of
containing these phases. Hence, it is not likely that the old total 4°Ar/agAr ages for the Bombay samples are mainly caused by excess 4°At, except for the 1100°C fraction of the sample BOo30. If the 1100°C fraction is excluded, the total 4°Ar/39Ar age reduces to 73.9 Ma for B0-30. Another possibility to explain
'~oo" ~i_ 150
1
6oo"
BO 30
0 ~r
o~ "4"
1 O0
tO (,9
1
7O0" 00"
Z hl O-
n'SO
)0~
1000'
~30o"
7o0S .o
O.
500'
0
0.2 "~Ar~'0.4RELEASEDO.6 ~ 0.8
1.0
el
0
100 39Ar/36Ar 200
Fig. 4. Sample BO-30. The isochron of 74 Ma is drawn as a reference, which goes through the (4°Ar/36Ar)i ratio of 320.
239 80
500 I
800"
900" 1000°
60
,3o0"
o
O
:E
v
600"
LIJ
o <
ooot
900/
40
I-Z IJ.l
GI
62-50 5O0
Q.
1o/.o . ° 6°°.
69 Ma
0.6 o.~ L0 0 loo 2oo 39Ar/36Ar3OO 02 39AOrARELEASED Fig. 5. Sample GI 62-50. The 800-1000°C fractions indicate a plateau age of 68.3 -+ 1.2 Ma in the age spectra. An isochron of 68.8 -+ 1.3 Ma is defined with the (4°Ar/36Ar)i ratio of 292.8 -+ 3.6 for 800-1300°C fractions.
the Columbia River Plateau basalts. Hence, redistribution o f Ar seems more likely than excess At. In Fig. 5, sample GI 62-50 show a relatively welldefined plateau age o f 68.3 -+ 1.2 Ma for the 8 0 0 1000°C fractions, which cover about 68% of the agAr released. The plateau age agrees quite well with the
isochron age of 68.8 -+ 1.3 Ma as defined from the 8 0 0 - 1 3 0 0 ° C fractions. The intercept gives 292.8 -+ 3.6 for the (4°Ar/a6Ar)i ratio. These results suggest that the age o f 68 Ma probably indicates the time of eruption. Samples MA A-51 and MA Z11-51 also show dis-
80
10000 60 O :E v hi
(.9
40
I--" Z hi rr
<
MA A-51
,loo"
20
6OO'
13oo°
600*
o.~__ o~4
o76
0.'8
~UAr RELEASED
,.o
o
,oo
2oo3_#s_~oosA bar
Fig. 6. Sample MA A-51. No plateau nor isochron is observed for this sample. In the isochron diagram, a reference isochron of 60 Ma is drawn assuming the atmospheric ratio for the (4°Ar/36Ar)i ratio.
240
120 ~oc
9oo"
MA
Zll-51
~
100
--~
o
~
40C
,,,
o <80
/
.^^. ~'~3o
Z n< o_ 12-60
+900. yfoo. O"
P~O00"
1o0o* - - ~
3oo
40
200
012~P"J~Ar 0I~RELEASED 0.6 0.8
50 MQ
1.0
' 0
2'0
i
4'0 3__r/3--r609A bA
Fig. 7. Sample M A Z 11-51. In the isochron diagram, a reference isochron of 50 Ma is drawn with the (4°Ar/36Ar)i ratio of 300.
turbed age spectra. The former indicates an inversedstaircase type, whereas the latter may suggest excess 4°At in the 900°C fraction. Neither sample yields a good isochron, but in spite of such disturbed age spectra and scattered data points in the isochron diagram, both samples show similar total 4°Ar/39Ar ages of 63.1 and 64.4 Ma, respectively. These ages are similar to those determined for less altered samples from Mahabaleshwar and other areas by the K-At method [2-4]. Hence as discussed for samples from Bombay, a closed system redistribution of radiogenic 4°Ar in the sample is possible. From successive lava flows, sample MA Z 11-51 was collected from a higher part than sample MA A-51. An apparent older age of MA Z 11-51 may be due to the occurrence of excess 4°Ar in a special phase (probably plagioclase phenocrysts) as revealed in the 900°C fraction in the age spectrum. If we exclude this fraction for this sample, the total 4°Ar/39Ar age becomes 56.6 Ma.
5. Discussion
Results of 4°Ar/39Ar dating are summarized in Table 2, where the samples may be classified into two groups; one group has 4°Ar/a9Ar ages of around 6 0 70 Ma (IG-15, GI 62-50. MA A-51, MA Z11-51) and
the other shows the 4°Ar/39 Ar ages of more than 70 Ma (IG-02, BO-24, BO-30). The 4°Ar/39Ar ages of the younger group are similar to the conventional K-At ages reported for less altered samples from the Deccan Traps. For example, diorites from the Girnar Hill were reported to be 6 6 67 Ma old [3,4]. (All K-Ar ages in this paper are recalculated by using the decay constants recommended by the Subcommission on Geochronology [12] .) Although the rhyolite dyke GI 62-50 was collected at the Osam Hill, situated northeast of the Girnar Hill, it also shows an 4°Ar/agAr age of about 68 Ma. Since it has a total 4°Ar/39Ar age of 64 Ma, those rocks probably erupted during the same series of volcanic activity in this area. The total 4°Ar/a9Ar ages for Mahabaleshwar samples are also similar to the oldest K-Ar age obtained for a Mahabaleshwar sample (MA B-61,63.0 Ma) [4]. Sample MA B-61 was collected from just above a lava flow of MA A. Hence, the 4°Ar/39Ar dating results in this study also support a volcanic activity in the Mahabaleshwar area 6 0 - 6 5 Ma ago. In this respect, it is interesting to note that IG-15 also shows an 4°Ar/a9Ar age of about 63 Ma, while IG-02 shows a definitely older value of 8 2 - 8 4 Ma. Both samples were collected from the same succession of lava flows in the Igat Puri area. Apparent
241 TABLE 2 Summary of the 4°Ar/39Ar age data for samples from the Deccan Traps Sample
40 Ar/39 Ar age (Ma) total
plateau
IG-02
83.9
81.9 ± 2.9
IG-15
60.9
63.3 ± 1.3
BO-24 BO-30 GI 62-50
88.0 82.2 64.2
68.3 _+1.2
MA A-51 MA Zll-51
63.1 64.4
-
range (integrated 39 Ar)
4OAr/39Ar isochron age (Ma)
range
(40Ar/36Ar)i
800-900°C (0.617) 800-1100°C (0.849) 800-1000°C (0.678)
(83.6 ± 4.2)
800-1300°C
(258.3 ± 19.3)
63.8 ± 1.1
600-1300°C
294.6 ± 2.7
72.0 ± 6.9 (74.1 ± 3.3) 68.8 _* 1.3
900-1300°C 600-1300°C 800-1300°C
289.6 ± 14.3 (320.7 ± 28.5) 292.8 ± 3.6
(49.1 ± 3.0)
600-1300°C
(303.7 _+6.5)
Uncertainty in each value means lcr (standard deviation). Numerical figures in parentheses are less reliable than the other ones.
4°Ar/39Ar age relationships between both samples are concordant with the order o f the succession layers. These results suggest that there might possibly be a relatively large age gap between the lava flows for samples IG-02 and IG-15. Although several gaps are observed geologically among the intermediate lava flows in this section (K. Aoki, personal communication, 1978), we cannot identify which boundary corresponds to this presumed large age gap. Although due to the disturbed age spectra we cannot exclude the possibility o f excess 4°Ar completely, the results of IG-02 together with the Bombay samples give evidence favoring relatively old volcanic activity(ies) o f more than 70 Ma ago in the Deccan Traps. Although we do not know at present to what extent it ocurred, this fact may give us an important constraint on the clarification of the mechanism o f the Deccan Traps formation. It is reported by several investigators that paleomagnetic studies on volcanic rocks in the Deccan Traps have revealed only a few polarity epochs (e.g. [13]). Based on such paleomagnetic results together with the limited number o f K-Ar dating results, a relatively short period o f volcanic activity o f around 5 Ma has been suggested [ 3 - 6 , 1 4 ] . On the other hand, some investigators [15] propose a continuous igneous activity since the Rajmahal volcanic phase whose eruption ages were determined to be about
100 Ma [16]. Present results open a door for the latter possibility from the geochronological viewpoint, though it may not always mean a continuous igneous activity. Judging from the distribution of ages, however, we can still argue for the main volcanic activity around 6 0 - 7 0 Ma ago. Geomagnetic-reversal time scale based on the summary of marine-anomaly data [17] indicates that only a few polarity epochs are identified during the Upper Cretaceous time. 4°Ar/agAr age results for the present samples seem to suggest that older volcanic activity(ies) o f more than 70 Ma might have occurred during this Cretaceous magnetic quiet period. Hence, the limited number o f observed polarity epochs is not always incompatible with the occurrence o f the older volcanic activity(ies) in the Deccan Traps. Alkaline rocks from the Gimar Hill and the Pavagarh Hill seem to show almost similar or even slightly older K-At ages than tholeiitic rocks from the other localities [3,4]. If we follow a hypothesis that alkaline rocks extrude later than tholeiitic rocks during a series o f magmatic differentiation, the above mentioned alkaline rocks might have belonged to the series o f older tholeiitic volcanic activity o f more than about 65 Ma. Hence, the history o f the formation o f the Deccan Traps may not be so simple as has been suggested based on the limited number o f paleomagnetic and
APPENDIX
w i t h clay
w i t h clay
.
olivine nephelinite
+ (a.)
.
tholeiitic basalt
++ (a. to g r e e n c l a y )
9
+
++
++
* +++ = a b u n d a n t ; ++ = m e d i u m ; + = r a r e ; - = a b s e n t ; p.a. = p a r t l y a l t e r e d ; a. = a l t e r e d .
diktytaxitic cavities filled
olivine-augite basalt
++ (p.a.)
9
+
diktytaxitic cavities filled
+ + (a. t o clay)
brown hornblende mesostasis
++
+
+
-?
N.B.
+ +
titanomagnetite ilmenite
. ++
coarse-grained subophitic
-
+ subophitic
+ (micro phenocryst)
-
BO-30
-
+ (p.a.)
BO-24
++
olivine-augite basalt
++
nepheline plagioclase
-I-+
coarsegrained subophitic
++ -
+ (p.a.)
IG-15
Rock name
++
coarsegrained subophitic
++ -
+ (p.a.)
Groundmass olivine clinopyroxene
Texture
clinopyroxene plagioclase magnetite biotite
Phenocryst olivine
IG-02
P e t r o g r a p h i c d e s c r i p t i o n o f s a m p l e s * ( b y S. A r a m a k i )
.
quartz aggregates
secondary
rhyolite
+++
_
--
++ (alkalifeldspar?)
_
cryptocrystalline
+? ( a l t e r e d )
-
-
GI 62-50
tholeiitic basalt
+ (p.a. to c l a y )
+
+
++
++
intergranular
-
-
-
MA A-51
tholeiitic basalt
+ (p.a. to c l a y )
+
+
++
++
+? (a. to c l a y )
intergranular
++
+ (a. t o g r e e n clay)
MA Z 1 1 - 5 1
t,9
243 K-At age data. To clarify this point, additional 4°Ar/39AI dating is being tried for samples form the other areas in the Deccan Traps.
Acknowledgements The author appreciates Prof. S. Aramaki of the Earthquake Research Institute, University of Tokyo for microscopic observations of samples and comments on this study. He is also grateful to Prof. K. Aoki of Tohoku University and Dr. H. Kurasawa of the Geological Survey of Japan for their information on geological setting of sampling sites for IG and GI samples. Prof. M. Ozima of the University of Tokyo made suggestive comments on this study. The author thanks Prof. F. Begemann of the Max-Planck-Institut fi~r Chemie (Mainz) and two anonymous reviewers for their helpful comments to improve the manuscript. He also expresses his thanks to Prof. S. Yajima and Mr. M. Narui of Tohoku University for the neutron irradiation. Samples were collected by the Indo-Japanese team through the joint program on the Deccan basalt studies. The expenses for the sampling trip for Japanese members were defrayed by the Ministry of Education of Japan. This study was financially supported in part by the Ito Science Foundation.
References 1 H. Kuno, Plateau basalts, Am. Geophys. Union, Geophys. Monogr. 13 (1969) 495-501. 2 S.N.I. Rama, Potassium/argon dates of some samples from Deccan Traps, 22nd Int. Geol. Congr., New Delhi, 1964, Rep. VII (1964) 139-140.
3 P. WeUmanand M.W. McElhinny, K-Ar ages of the Deccan Traps, India, Nature 227 (1970) 595-596. 4 I. Kaneoka and H. Haramura, K]Ar ages of successive lava flows from the Deccan Traps, India, Earth Planet. Sci. Lett. 18 (1972) 229-236. 5 M. Kono, H. Konoshita and Y. Aoki, Paleomagnetism of Deccan Trap basalts, India, J. Geomag. Geoelectr. 24 (1972) 49-67. 6 M.W. McElhinny, Northward drift of India-Examination of recent paleomagnetic results, Nature 217 (1968) 342344. 7 I. Kaneoka and K. Aoki, 4°Ar/39Ar analyses of phlogopite nodules and phlogopite-bearing peridotites in South African kimberlites, Earth Planet. Sci. Lett. 40 (1978) 119-129. 8 D. York, Least squares fitting of a straight line with correlated errors, Earth Planet. Sci. Lett. 5 (1969) 320-324. 9 D. Seidemann, 4°Ar/39Ar studies of deep-sea igneous rocks, Geochim. Cosmochim. Acta 42 (1978) 17211734. 10 G.B. Dalrymple and M.A. Lanphere, Potassium-Argon Dating (W.H. Freeman and Co., San Francisco, Calif., 1969) 258 pp. 11 R.J. Bottomley and D. York, 4°Ar-a9Ar age determinations on the Owyhee basalt of the Columbia Plateau, Earth Planet. Sci. Lett. 31 (1975) 75-84. 12 R.H. Steiger and E. J~iger, Subeommission on geochronology: Convention on the use of decay constants in geoand cosmochronology, Earth Planet. Sci. Lett. 36 (1977) 359-362. 13 P.W. Sahasrabudhe, Paleomagnetism and the geology of the Deccan Traps, Semin. Geophys. Invest. Peninsular Shield, Osmania Univ., Hyderabad (1963) 226-243. 14 H. Wensink, Newer paleomagnetic results of the Deccan Traps, India, Tectonophysics 17 (1973)41-59. 15 R.N. Alhavale and R.K. Verma, Paleomagnetic results on Gondwana dykes from the Damodar valley coal-fields and their bearing on the sequence of Mesozoic igneous activity in India, Geophys. J. 20 (1970) 303-316. 16 I. McDougalland M.W. McElhinny, The Rajmahal Traps of India - K/Ar ages and Paleomagnetism, Earth Planet. Sci. Lett. 9 (1970) 371-378. 17 J.E. van Hinte, A Cretaceous time scale, Am. Assoc. Pet. Geol. Bull. 60 (1976)498-516.