Geochronological studies on whole-rock basalts, Deccan Traps, India: evaluation of the timing of volcanism relative to the K-T boundary

EPSL ELSEVIER

Earth and Planetary Science Letters 121 (1994) 43-56

Geochronological studies on whole-rock basalts, Deccan Traps, India: evaluation of the timing of volcanism relative to the K - T boundary Ajoy K. Baksi Department of Geology and Geophysics, Louisiana State University, Baton Rouge, LA 70803-4101, USA

(Received June 4, 1993; revised and accepted October 25, 1993)

Abstract K - A r and 4°Ar/39Ar incremental heating studies have been carried out on whole-rock basalts spanning different sections of the Deccan Traps. K - A r dates typically fall in the range ~ 55-65 Ma and, for an individual section, are commonly out of stratigraphic order; indicating post-crystallization loss of 4°Ar* from some samples. Specimens from six lava flows spanning a composite Western Ghats section and another six from sections in the southern, western, and northern areas of the Deccan, were selected for 4°Ar/39Ar step-heating studies. Many of these specimens yield low temperature steps with apparent ages < 60 Ma, indicating post-crystallization loss of 4°Ar*. The following (low-intermediate temperature) steps commonly yield apparent ages ( > 70 Ma) older than crystallization values. Such spectra are characteristic of fine-grained basalts exhibiting 39Ar recoil loss and could involve: (1) 39Ar internal redistribution, out of K-rich sites into K-poor phases, a n d / o r (2) 39Ar recoil loss out of the sample. Comparison of total gas ages with K - A r dates permitted constraints to be placed on these two mechanisms. For some rocks, plateau ages were recovered from the intermediate-high temperature steps in the range 64-66 Ma, with l~y errors of 0.5-1.0 m.y. A dyke specimen from the western Deccan, with a K - A t date of ~ 97 Ma, shows a descending staircase type of age spectrum and its estimated crystallization age is ~ 65 Ma. Two lava flows from the central Deccan with low K - A r dates ( ~ 50-55 Ma) yield crystallization ages of ~ 65 Ma; K - A r dates deviating significantly from ~ 65 Ma, do not reflect crystallization ages. Integration of the age data with the magnetic polarity of the different sections of lavas suggests the main reversed polarity epoch trapped in the Deccan volcanic province is chron 29R, which includes the Cretaceous-Tertiary boundary (estimated age 64.5 Ma). The plateau ages presented herein tend to be slightly older ( ~ 1%) than the age of chron 29R; this probably results from minute amounts of 39Ar recoil loss out of the sites degassed in the intermediate-high temperature range. The Western Ghats section of the Deccan Traps, representing > 80% of the exposed material, was extruded in ~ 1 m.y.

I. Introduction

[CL]

T h e D e c c a n T r a p s cover ~ 500,000 km 2 of the I n d i a n s u b c o n t i n e n t , a n d their initial areal extent

0012-821X/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSD1 0012-821X(93)E0205-X

A.K. Baksi / Earth and Planetary Science Letters 121 (1994) 43-56

44

!

i.

I.

400

o

-

I

I

I

'

'

v

i

r~

~-.

v

v

v

v

vl

~,p

~ v

~

y v

v

~

v

v

BOMBA

v

v

6 l

v

v v

"5

v

,,

v

v v

v

,,

v

-

v

v

v

'-~

.5

v-'--~

MAHARASHTRA

,,rI ,,,/~,/

/'-'%

-7..x~,,2 / "2 v vr ---,-~ " ~ " ~ dv

v

'--7

"~ ,-"~-~

v'~

)~..I

~

"-,C..,-,

,,

-

,"

v

""

\LGUJARAT/

-

v

PRADESH

kv

v

I .

v

vMADHYA

r'

-~:-~^ ,, ~ / v

,e/d

v

vY>

2o,-

v vc,"3"

v

Iv °7 v r V v M ~.O~AP~R ~ v v v °v v EZ'~,HYDERABAD

~

~ _v

I~

~

~ o~.

L

~

9

~

v

D

v

v¢°_

E

,

C

C

,

A

N ,

TRAP ,

Fig. 1, Map of India showing surface exposures of Deccan Traps and sample location sites [modified from Lighffoot eta]., 3]. I = I g a t p u r i ; M = M a h a b a l e s h w a r - - t h e c o m p o s i t e W e s t e r n G h a t s section [1]; 1 = Panvel; 2 = Koyna; 3 = A m b a g h a t ; 4 = R a t n a g i r i ; 5 = D i v e g h a t ; 6 = Bhor G h a t ; 7 = S i n g h a g a r h ; 8 = Rajpipla.

may have exceeded 1.5 X 1 0 6 km 2. Over vast areas, the basalts are essentially horizontal, being thinnest in the east and thickening to the west. The thickest exposed sections are those at Igatpuri ( ~ 1.7 km) and at Mahabaleshwar ( ~ 1.2 kin) (Fig. 1). Detailed field work and petrochemistry have been used to work out the stratigraphy of these sections [1]. Summaries of the physical features, stratigraphy and geochemistry are available in the literature [1-3]. Paleontological evidence, primarily from intertrappean sediments around the periphery of the Traps, suggests eruption during the Late Cretaceous-Early Tertiary [4]. The disposition of the Traps in the western areas is consistent with the formation of the Deccan Traps as the Indian subcontinent drifted over the Reunion Hotspot [5-7]. The timing and duration of Deccan volcanism has been a subject of controversy [8-10] and it has been hypothesized that their rapid eruption led to the global faunal extinction at the Cretaceous-Tertiary ( K T) boundary [11].

Here, I present geochronological data for lavas from the western, central and southern areas, which represent the most voluminous sections of the Deccan Traps. A simplified stratigraphy of

SUBGROUP

Fo

r

m

a t

i0

n

Mahabaleshwar

20002

WAI

=:.

Poladpur

LONAVLA

[m~

Ambenali

Bushe

m

1000"

Bhimashankar

o m_ o

KALSUBAI

Thakurvadi Neral Igatpuri Jawahar

Fig. 2. S t r a t i g r a p h i c n o m e n c l a t u r e for D e c c a n Basalts, Western G h a t s [modified from B e a n e et al., 1]. All lava flows show r e v e r s e d m a g n e t i c polarity, except for a few at the top of the M a h a b a l e s h w a r F o r m a t i o n [12,13].

A.K. Baksi / Earth and Planetary Science Letters 121 (1994) 43-56

lavas for the Igatpuri-Mahabaleshwar area, the composite Western Ghats section (CWGS), based on the work of Beane et al. [1], is shown in Fig. 2. Paleomagnetic studies in these areas indicate a thick pile of lavas with reversed polarity, overlain by a few flows of normal polarity [12,13]. Sections of lavas in other areas of the Deccan reported on here can be stratigraphically correlated to the Subgroups seen in the CWGS. With one exception, all the samples dated here, belong to flows showing reversed magnetic polarity. Integration of geochronological and paleomagnetic results suggest the Deccan Traps were extruded within ~ 1 m.y. of the K - T boundary.

2. Review of previous geochronological work Numerous attempts have been made to date the Deccan Traps using the K - A r technique; these dates and preliminary 4°Ar/39Ar results have been critically evaluated elsewhere [8,14]. Over the last few years, further results have appeared in the literature, and these will be evaluated. Because problems exist regarding the ages of monitors used for 4°Ar/39Ar dating, all ages will be reported (recalculated where necessary) relative to 162.9 Ma for the US Geological Survey, Menlo Park standard SB-3 Biotite and 513.9 Ma for MMhb-1 [15]; although the latter is probably too inhomogeneous to serve as the primary interlaboratory standard [16]. Ages for the Deccan will be compared to that of the K - T boundary, for which I use an age of 64.5 _+ 0.3 Ma relative to 162.9 Ma for SB-3 Biotite. This estimate is based on the recent 4°Ar/39Ar dating results on K - T boundary bentonites, tektites and craters [ 17-20]. The studies of Duncan and Pyle [21,22] include a number of drawbacks. Among these are: (1) no account was taken of the fact that most of the age spectra showed features resulting from 39Ar recoil [23,24]; (2) plateau ages and errors were not always calculated correctly [23,24]; (3) numerous whole-rock plateau ages did not overlap at the 95% confidence interval (e.g., 67.5 _+ 0.6 Ma and 64.4 _+ 0.9 Ma for IGA-004 and CAT-034, respectively) and therefore an average plateau age

45

should not have been calculated for their suite of rocks. These authors quote a mean (isochron) age of 66.5 _+ 0.3 Ma for the Western Ghats section [22], considerably older than the best estimate of the K - T boundary (64.5 _+ 0.3 Ma). The whole-rock and plagioclase specimens of Courtillot et al. [25] do not appear to be affected by 39Ar recoil problems. The sections dated by these authors lie in the (north)eastern Deccan and are not volumetrically important. My concern focuses on attempts to tie in geochronological data with the paleomagnetism of the various sections of the Deccan [9,10]. 'Acceptable' plateau ages in the literature span a range > 5 m.y. [9], whereas paleomagnetic studies, indicating two reversals of the geomagnetic field, suggest all ages should span ~ 1 m.y. [23,24]. 4°Ar/3')Ar 'ages' have been presented for alkali basalts from Gujarat and lava flows near Nagpur [26,27]; for most specimens reliable (plateau) ages are not developed because: (1) the step ages utilized do not overlap at the 95% confidence level; or (2) < 50% of the total gas released lies on the plateau. In a recent review article, by V a n d a m m e et al. [10], all available 4°Ar/3')Ar ages are listed; a number of comments are pertinent. Their table 3 lists the monitor samples used by various laboratories but makes no attempt to reduce the data for reported interlaboratory differences in ages. Eliminating what were regarded as less reliable plateau ages, a mean value of 65.5 _+ 2.5 Ma was quoted for Deccan volcanism [10]. This wide range of ages is at variance with paleomagnetic results indicating that only two magnetic chrons are represented in the major areas of the Deccan Traps. In summary, it is evident that critical evaluation of data available in the literature shows that the timing and duration of Deccan volcanism have not unequivocally been determined, in particular with respect to the K - T boundary. The ages derived here, tied in to published paleomagnetic studies, permit closer constraints to be placed on Deccan volcanism.

3. Analytical techniques utilized Whole-rock basalt samples from the CWGS [1] were obtained in coarse-crushed form from Peter

46

A.K. Baksi /Earth and Planetary Science Letters 121 (1994) 43-56

H o o p e r of Washington State University. Specimens from flows in Western Maharashtra [28] were obtained as either hand specimens or coarse crushed material from Dalim Paul in India. Sample location sites are shown in Fig. 1. One of the specimens from the Western Ghats (IGA-009) contained giant plagioclase phenocrysts; these were removed by hand prior to dating. K - A r analyses were carried out on the 25-60 mesh size fraction of each rock. A total of 21 whole-rock basa|ts were analyzed; 12 of these specimens, 6 from the CWGS and 6 from Western Maharashtra, were selected for 4°Ar/39Ar dating. All (K-rich) specimens from the CWGS were dated because their relative stratigraphic positions were known accurately. For the remainder of the sections, specimens from Koyna, Panvel, Bhor Ghat, Diveghat and the Rajpipla areas (Fig. 1) were selected; in part, because K - A t dates for flows deviated significantly from the ~ 65-60 Ma range shown by most relatively unaltered whole-rock basalts from the Deccan [8]. Nine whole-rock specimens were loaded in a single capsule, along with splits of SB-3 Biotite as the monitor (age = 162.9 Ma) and analyzed at the US Geological Survey, Menlo Park (MP). Six specimens were analyzed at Queen's University (Q), utilizing Fish Canyon Tuff-3 Biotite as the monitor (age = 27.95 Ma with respect to 162.9 Ma for SB-3 Bio [16]). All fast neutron irradiations were carried out in position 5C of the McMaster University Reactor, and corrections for interfering reactions were applied following Onstott and Peacock [29]. At MP, following radio frequency induction heating and a two-stage clean up, the gas was analyzed on a multi-collector mass spectrometer [30]. At Q, step-heating was carried out with a Lindberg furnace and the gas analyzed on a modified MS10 mass spectrometer [31]. All heating steps were of 30 min duration.

4. K - A r results

The K - A r data, summarized in Table 1, are useful for a number of reasons. In sections where numerous flows were studied, dates out of stratigraphic order focus attention on critical samples.

Table 1 Summary of K - A r dates on whole-rock basalts, Deccan Traps, India. Sample No.

K (%)

Age _+lo" (Ma)

36At content a

Composite Western Ghats section (CWGS) MAP-052 MAP-037 BSH-008 KOP-021 JEB-013 JEB-339

0.512 0.164 0.524 0.910 1.475 0.838

57.4 _+0.5 59.8 + 0.8 58.8 _+0.8 63.6 _+0.8 58.1 _+1.0 61.8 _+0.5

I H I H I,H I

97.0 _+0.8 63.1 _+0.8

H I

61.0 ± 0.8 60.9 _+0.7 59.8 _+1.4 53.0 + 0.8 60.5 _+0.8

I L I H I

60.2 _+0.8 63.3 + 0.8

H I

60.8 + 0.7

I

56.2 _+0.8

I

Panvel (Kalsubai Subgroup) D921 D907

0.421 0.586

Koyna (Wai Subgroup) D963 D961 D957 D956 D958

0.259 0.341 0.121 0.341 0.199

Ambaghat (Wai Subgroup) D926 D930

0.323 0.141

Ratnagiri (Wai Subgroup) D941

0.257

Di~,eghat (Wai Subgroup) D970

0.265

Bhor Ghat (Lonac,la Subgroup) D949

0.491

48.1 + 0.6

H

0.394 0.224

60.9 _+0.8 58.7 + 1.0

H I

2.53

61.4+_0.7

H

Singhagarh (Wai Subgroup) D974 D972

Rajpipla RP81-19

Specimens from individual sections listed in stratigraphic order. a L = low, I = intermediate, H = high [8].

Since numerous authors have argued for protracted volcanism in the Deccan, some specimens with dates departing from the 60-65 Ma range were selected for 4°Ar/39Ar work. Further, the 36Ar contents of the rocks could be used to gauge the degree of alteration of the rock [8]. Finally, comparison of the 4°Ar/39Ar total gas age with the K - A r result would help pinpoint samples where a fraction of the 39Ar was lost from the specimen.

A.I~ Baksi / Earth and Planetary Science Letters 121 (1994) 43-56 Table 2 Summary of 4°Ar/39Ar step-heating runs on whole-rock basalts, Deccan Traps, India IGA-009Q--CWGS, Jawahar Formation, Kalsubai Subgroup 0.55 g, J = 0.004971, 9 steps, total gas age = 64.3 _+ 0.9 Ma Plateau age (steps 3-6, 670-850°C, 61% gas) = 65.6 _+ 0.6 Ma Isochron (steps 3-6): age = 66.0 _+ 1.0 Ma, (4°Ar/36Ar)i = 294 ± 4, MSWD = 2.8 JEB-339Q--CWGS, Neral Formation, Kalsubai Subgroup 0.64 g, J = 0.004993, 8 steps, total gas age = 65.9 _+ 0.8 Ma ( ~ 7% 39Ar lost) a Plateau age (steps 3-7, 670-1000°C, 70% gas) = 65.6 _+ 0.5 Ma Isochron (steps 3-7): age = 65.3 ± 0.5 Ma, (4°Ar/36Ar)i = 296.3 ± 1.6, MSWD = 0.97 JEB-339MP--CWGS, Neral Formation, Kalsubai Subgroup 0.52 g, J = 0.006520, 9 steps, total gas age = 66.7 _+ 0.4 Ma ( ~ 7% 39Ar lost) Marginal plateau (steps 4-6, 720-850°C, 52% gas) = 66.3 _+ 0.7 Ma Isochron (steps 4-6): age = 63.7 _+0.9 Ma, (4°Ar/36Ar)i = 487 _+ 70, MSWD = 0.59 JEB-013QT--CWGS, Thakurvadi Formation, Kalsubai Subgroup 0.38 g, J = 0.002391, 5 steps, total gas age = 66.1 _+ 0.4 Ma ( ~ 14% 39Ar lost) Plateau age (steps 3-5, 67% gas) = 65.9 ± 0.4 Ma Isoehron (steps 3-5): age = 66.5 _+0.7 Ma, (4°Ar/36Ar)i = 270 + 28, MSWD = 1.9 J E B - 0 1 3 Q - - C W G S , Thakurvadi Formation, Kalsubai Subgroup 1.02 g, J = 0.003449, seven steps, 'total gas' age = 68.5 +_ 0.2 Ma (degassed at ~ 480°C) No plateau; steps 5-7 (800-1170°C, 45% gas) ~ 65 Ma Isochron (steps 5-7): age = 67.5 +_ 0.5 Ma, (4°Ar/36Ar)i = 176 +_ 32, MSWD = 11 JEB-013MP--CWGS, Thakurvadi Formation, Kalsubai Subgroup 0.31 g, J = 0.006472, 9 steps, total gas age = 66.4 +_ 0.4 Ma ( ~ 14% 39Ar lost) Marginal plateau (steps 5-9, 800-995°C, 43% gas) = 65.1 _+ 0.6 Ma Isochron (steps 5-9): age = 67.3 +_ 1.4 Ma, (4°Ar/36Ar)i = 236 ± 42, MSWD = 2.2 KOP-021MP--CWGS, Khandala Formation, Lonavla Subgroup 0.33 g, J = 0.006400, 9 steps, total gas age = 66.2 ± 0.4 Ma ( ~ 4% 39Ar lost) No plateau; steps 5-7 (780-920°C, 46% gas), ~ 65 Ma Isochron (steps 5-7): age = 66.4 ± 0.5 Ma, (4°Ar/36Ar)i = 264 ± 11, MSWD = 2.0 BSH-008MP--CWGS, Poladpur Formation, Wai Subgroup 0.74 g, J = 0.006493, 9 steps, total gas age = 62.5 _+ 0.4 Ma ( ~ 6% 39Ar lost) No plateau; steps 5-8 (780-1000°C, 33% gas) ~ 64 Ma Isochron (steps 5-8): age = 66.0 ± 0.6 Ma, (4°Ar/36Ar)i = 245 _+ 13, MSWD = 0.71 MAP-052MP--CWGS, Mahabaleshwar Formation, Wai Subgroup 0.73 g, J = 0.006479, 9 steps, total gas age = 63.6 +_ 0.4 Ma ( ~ 11% 39At lost) Plateau age (steps 4-8, 720-1000°C, 57% gas) = 64.9 ± 0.5 Ma Isochron (steps 4-8): age = 66.9 ± 1.0 Ma, (4°Ar/36Ar)i = 271 _+ 12, MSWD = 0.91 D-907MP--Panvel, Kalsubai Subgroup 0.60 g, J = 0.006501, 9 steps, total gas age = 66.5 ± 0.4 Ma ( ~ 5% 39Ar lost) No plateau; steps 6-8 (850-1000°C, 27% gas) ~ 64 Ma Isochron (steps 6-8): age = 62.5 _+ 0.7 Ma, (4°Ar/36Ar)i = 306 _+ 7, MSWD = 0.68 D-921MP--Panvel, Kalsubai Subgroup 0.68 g, J = 0.006436, 10 steps, total gas age = 98.0 _+ 0.6 Ma ( < 1% 39Ar lost) No plateau; steps 7-9 (890-1040°C, 32% gas) ~ 70 Ma Isochron (steps 7-9): age = 65.1 _+ 3.1 Ma, (4°Ar/36Ar)i = 349 _+ 30, MSWD = 1.6 D-949MP--Bhor Ghat, Lonavla Subgroup 0.63 g, J = 0.006507, 10 steps, total gas age = 50.0 _+ 0.5 Ma ( < 4% 39Ar lost) Marginal plateau (steps 3-6, 600-790°C, 44% gas) = 64.5 +_ 0.5 Ma Isoehron (steps 3-6): age = 65.5 _+ 1.2 Ma, (4°Ar/36Ar)i = 278 ± 25, MSWD = 0.46

47

48

A.K. Baksi / Earth and Planetary Science Letters 121 (1994) 43-56

Table 2 (continued) D-961MP--Koyna, Wai Subgroup 0.006466, 9 steps, total gas age = 68.1 + 0.7 Ma (~ 11% 3 9 A r lost) No plateau; steps 4-6 (720-850°C, 31% gas) ~ 68 Ma lsochron (steps 4-6): age = 68.9 + 2.9 Ma, (4°Ar/3('Ar)i = 275 _+75, MSWD = 4.0 0.68 g, J

=

D-970Q--Diveghat, Wai Subgroup 0.50 g, J = 0.004512, 6 steps, total gas age = 57.6 _+ 1.7 Ma (< 2% 39At lost) Plateau age (steps 3-5,700-920°C, 52% gas) = 64.7 + 1.1 Ma lsochron (steps 3-5): age = 63.4 + 1.2 Ma, (4°Ar/36Ar)i = 325 + 38, MSWD = 0.90 RPSl-19Q--Rajpipla 0.21 g, J = 0.004731, 8 steps, total gas age - 61.0 + 0.5 Ma (< 1% 39Ar lost) Marginal plateau (steps 1-7, 500 935°C, 63% gas) = 60.8 + 0.6 Ma lsochron (steps 1-7): Age = 60.6 + 0.3 Ma, (4°Ar/36Ar)i = 297 + 2, MSWD - 1.9 A complete set of analytical data can be obtained on request from the author. Au errors quoted at the lcr level, internal precision. Calculated by comparing total gas age to K Ar data assuming no recoil loss of 41~Ar.

Six s a m p l e s t a k e n f r o m flows s p a n n i n g the C W G S (Fig. l ) w e r e d a t e d ; exact s a m p l e locations are available in B e a n e et al. [1]. T h e K - A r d a t e s fall in the r a n g e ~ 5 8 - 6 3 M a a n d are c o m m o n l y out of s t r a t i g r a p h i c o r d e r . T h e 3~Ar c o n t e n t s of t h e s e rocks suggests s o m e a l t e r a t i o n ; t h e s e d a t e s m a y be t a k e n as m i n i m u m values for t h e crystallization age a n d are in r e a s o n a b l e a g r e e m e n t with e a r l i e r results [12]. P a l e o m a g netic studies [12,13] i n d i c a t e that this section shows a few n o r m a l p o l a r i t y flows at t h e top, overlying a thick section of r e v e r s e d p o l a r i t y lavas. Flows from o t h e r a r e a s of the D e c c a n give a c o n s i d e r a b l y w i d e r r a n g e of K - A r dates. E a r l i e r field a n d g e o c h e m i c a l studies [1,3,32] h e l p to r e l a t e the s t r a t i g r a p h y o f t h e s e sections to t h a t in the C W G S . S a m p l e l o c a t i o n sites are shown in Fig. 1 a n d the g e o c h e m i s t r y a n d p e t r o g r a p h y o f t h e lavas is d e s c r i b e d e l s e w h e r e [28]. F o r sections n e a r the W e s t e r n G h a t s that display lavas with r e v e r s e d p o l a r i t y only [12,13], two s p e c i m e n s from Panvel (site 1, Fig. 1) yield d a t e s out of stratig r a p h i c o r d e r a n d a single lava from B h o r G h a t (site 6, Fig. 1) gives a d a t e of ~ 48 M a , c o n s i d e r ably y o u n g e r t h a n the d a t e s o f - 6 1 - 6 4 M a rep o r t e d e a r l i e r [33]. T o verify t h a t K - A r d a t e s d e p a r t i n g significantly from 65 M a d o not r e p r e sent crystallization ages all t h r e e of t h e s e specim e n s w e r e s e l e c t e d for 4 ~ J A r / 3 9 A r dating. In the s o u t h e r n D e c c a n , lava flows from K o y n a (site 2, Fig. 1) give d a t e s close to ~ 60 Ma, a n d a

single s p e c i m e n was s e l e c t e d for 4°Ar/39Ar d a t i n g b a s e d on its low 36Ar c o n t e n t (freshness). Lavas f r o m n e a r A m b a g h a t a n d R a t n a g i r i (sites 3 and 4, Fig. 1) give u n r e m a r k a b l e d a t e s of 6 0 - 6 3 Ma, in g e n e r a l a g r e e m e n t with p u b l i s h e d d a t e s for n e a r b y sections that d i s p l a y a few n o r m a l p o l a r i t y lavas overlying a thick r e v e r s e d section [12]. In t h e c e n t r a l D e c c a n , w h e r e a few n o r m a l p o l a r i t y lavas overlie thick r e v e r s e d sections, two lavas from S i n g h a g a r h (site 7, Fig. 1) give d a t e s of ~ 5 8 - 6 0 M a a n d a single s p e c i m e n f r o m Divegh a t (site 5, Fig. 1) gives a slightly y o u n g e r K - A r d a t e ( ~ 56 Ma). T h e l a t t e r was s e l e c t e d for 4 ° A r / 39Ar study since its relatively low 36Ar c o n t e n t (freshness) is at v a r i a n c e with its l o w e r e d K - A r result. Finally, a single alkali basalt f r o m n e a r R a j p i p l a (site 8, Fig. 1) in t h e n o r t h e r n D e c c a n , w h e r e i n t e r b e d d e d alkali basalts a n d t h o l e i i t e s occur [34], yields a K - A t d a t e o f ~ 61 Ma. A l t h o u g h t h e s t r a t i g r a p h i c e q u i v a l e n c e of this section to the C W G S is u n k n o w n , this s p e c i m e n , the only alkali b a s a l t available, was s e l e c t e d for 4 ° A r / 39Ar dating.

5. 4°Ar/39Ar dating results A s u m m a r y of the s t e p - h e a t i n g e x p e r i m e n t s c a r r i e d out on twelve d i f f e r e n t rocks is p r e s e n t e d in T a b l e 2. A p l a t e a u is d e f i n e d as r e p r e s e n t i n g t h r e e or m o r e c o n t i g u o u s steps o f an age spec-

A.K. Baksi /Earth and Planetary Science Letters 121 (1994) 43-56

trum, comprising > 50% of the total 39Ar released, whose ages agree within experimental errors at the 95% confidence level [cf. 35]. In addition, the term 'marginal plateau' is used to include sections of age spectra where: (1) only 40-50% of the total 39Ar is included in the steps under consideration; or (2) the step ages just fail to overlap at the 95% confidence level. Many of the age spectra show artifacts resulting from 39Ar recoil during or after fast neutron irradiation and yield low temperature step ages greater than crystallization ages [23,24,36]. In instances where the highest temperature step showed high ( > 20) C a / K ratios, the age was not considered for plateau sections. These low ages (Fig. 3), result from: (1) implantation of 39Ar recoiling out of the K-rich sites into refractory K-poor sites [cf. 36]; a n d / o r (2) possible recoil loss of 37Ar out of these Ca-rich sites. The latter would result in a measured value of 37Ar/39Ar that is too low, leading to an incorrect (low) correction for 36Arca and yield an incorrect, young, age [37]. All errors are quoted at the l~r level internal precision. The experimental data were also examined using isochron diagrams, and ages were calculated from straight lines fitted following York [38] through those points defining plateau sections on the age spectra. The isochron data were scanned for any three (or more) contiguous steps that defined 'good straight lines' (MSWD < 2.5) in an attempt to see if any further information regarding crystallization ages could be recovered from specimens containing excess 4°Ar or suffering post-crystallization loss of 4°Ar*. Straight line plots showing initial argon ratios significantly lower than the atmospheric argon value reflect samples suffering post-crystallization loss of 4°Ar*; in such instances, the resulting 'age' could serve as an upper estimate of its true age [cf. 35]. In general, the tholeiites from the CWGS contain quite high amounts of K (Table 1) and plateau steps exhibit C a / K ratios of ~ 4-6; in contrast, the quartz tholeiites from the central and southern Deccan [28] show higher ( ~ 8-16) C a / K values in the plateau steps. The gas appears to reside in the mesostasis of these relatively finegrained basalts.

49

5.1 Flows from the C W G S IGA-O09: This sample was taken from a flow in the Jawahar Formation near the bottom of the CWGS (Fig. 2). A nine-step run (Fig. 3) yielded a total gas age of 64 Ma. As there is no K - A r result, it is not possible to comment on 39Ar recoil loss from the specimen. Internal redistribution of 39At is indicated by the age of step 2 ( > crystallization value) and lowered step ages for steps 7-9. Steps 3-6 define a plateau age of 65.6 +_ 0.6 Ma; these steps define a straight line with essentially the same age (Table 2), although the MSWD value is marginally high. JEB-339: Splits of this rock taken from a flow in the Neral Formation near the bottom of the CWGS (Fig. 2) were analyzed at both MP and Q. They yield total gas ages of ~ 66 Ma, slightly older than the K - A r date of 62 Ma, indicating some loss of 39Ar out of the specimen by recoil. The two age spectra (Fig. 3) show similar features, with low temperature steps ages greater than crystallization values, followed by a section in the intermediate temperature region where steps approximate plateaux. The MP run yields a marginal plateau (step ages just fail to overlap at the 95% confidence level) of 66.3_+0.7 Ma, whereas the Q run gives a plateau of 65.6 + 0.5 Ma. (The latter specimen was reported earlier to have a plateau age of 64.6 + 0.6 Ma [24], due to the use an incorrect age for the monitor specimen FCT-3 B i o - - s e e above). Isochron plots give ages that are essentially in agreement (Table 2), although the run at MP yielded three gas steps lying very close together on the isochron plot. A crystallization age of ~ 65.5 Ma is inferred from these experiments. JEB-O13: Splits of this sample taken from a flow in the Thakurvadi Formation of the CWGS (Fig. 2), were analyzed once at MP and twice at Q. In the latter runs, one experiment (labelled QT) was carried out using a system where extraction temperatures are unknown; the other experiment was carried out with a Lindberg furnace with accurate temperature control and involved an initial degassing step at ~ 480°C. The Q T and MP experiments yielded similar results, with total

I

0

80

. ~

100

_J 20

40

20

I

i

60

SO

60

•

>

I

|

,

I

I

No plateau

80

i

[

100

100

65

70

75

KOP-021MP

i

8O

i

No plateau

60

41o ' 61o 20 Cumulative %39Ar

10

~ L

40

100

50

55

60

65

70

76

0

5O

i

20 40 Cumulative

I

~lNO

60

,

60 %39Ar

I

BSH-008MP

40

80

, ~ ,

plateau

80

100

50

55

60

65

70

7s

5O

0

500

55

60

65

o o

io

,

~

~

,

410

,

60 %39Ar

9+0.5

Ma

,

80

)

6 Ma

'

100

100

1 O0

)

plateau

80i

810

I

Plateau ago

610

'

Marginal

610

JEB-013MP

410

JEB-339MP

t

Marginal plateau 66.3±0.7 Ma >

20 40 Cumulative

210

I 20

L:~(

Fig. 3.4°Ar/39Ar age spectra for step-heating experiments on whole-rock basalts from the Deccan Traps. Q and MP refer to stepheating runs carried out at Queen's University and US Geological Survey, Menlo Park, respectively. All errors shown and listed at the l~r level, internal precision.

co el 5 5 ; Q. : so 0

®

,~, 75 =S 70

<

•.. C .o JEB-013Q

I

i

I

55

65

7O

75

1 0050

I

JEB-339Q i

i

70

75

55

I"

80

|

55

65

Plateau age 65.6±0.5 Ma

m ~

20

610

I

Plateau age

'

JEB-013QT

410

IGA-009Q

]

70

75

6o

'

i

Plateau age 65.6¢0.6 Ma

6O

65

70

75

<

200

i

60

Q

~;

A m

0

O. 4[ SO

6o

6s

4[

e

70

7S

Ao l :Z v

I tal

e~

m

A

6O

70

8o

70


m

4o


e

~

Q. ¢,1 < 4O

IQ

<

o

"~

IE

!

60

80

I

80

No plateau

60 %39Ar

D-961MP

40

D-907MP

~

20 40 Cumulative

20

'

No plateau

4O 100 0

5O

6O

7O

80

4O 100 0

50

6O

7O

80

o

•

.

.

.

,

,

80

60 %39Ar

8O

Plateau Age 64.7±1.1 Ma

|

100

100

I..~

-

No plateau

Fig. 3 (continued).

.

i

60

D-970Q

40

i

D-921MP

20 40 Cumulative

,

20

a

I

o

5o

54

58

62

66

70

0

SO i , ~

40

,

>

60

20 40 Cumulative

I

60 %39Ar

RP81-19Q

Marginal plateau 60.8±0.6 Ma

20

80

80

ginal plateau Margin~ 64.6+0.5 Ma

100

100

I

52

A.K. Baksi/Earth and Planetary ScienceLetters 121 (1994)43-56

gas ages ( ~ 66 Ma) substantially greater than the K - A r date (58 Ma), and ' p l a t e a u ' ages of 65-66 Ma with errors of ~ 0.5 m.y. Isochron plots are not entirely satisfactory; the MP experiment yields a slightly low initial argon ratio and an age that is marginally too old (Table 2); whereas the Q T run yields an isochron age (66.5_+ 0.7 Ma) that is acceptable, but with a marginally high M S W D value. The second run at Q furnished results that are quite different. First, the low t e m p e r a t u r e (480°C) degassing step led to a 'total gas' age of ~ 68.5 Ma, reflecting degassing of altered phases that show post-crystallization loss of 4°Ar*. The step ages were determined with high precision (Fig. 3), and show features typical of whole-rock basalts suffering 39Ar recoil loss. No plateau section was recovered, and only very poorly controlled isochrons could be plotted since all steps contained very low amounts of 'atmospheric' argon and plotted close to the 39Ar/4°Ar axis. KOP-021: This sample was taken from a flow in the Khandala Formation of the CWGS (Fig. 2). A split analyzed at MP yielded a total gas age (66 Ma), close to the K - A r value (64 Ma), suggesting little or no loss of 39Ar from the sample. The age spectrum is typical for a rock showing internal redistribution of 39Ar by recoil and yielded no plateau section. On the isochron diagram, steps 5 - 7 yield an 'age' of ~ 66.5 Ma, an upper bound for the crystallization age [cf. 35]. BSH-O08: This sample belongs to the Poladpur Formation of the CWGS (Fig. 2). The total gas age (62.5 Ma) is marginally higher than the K - A r value ( ~ 59 Ma) and step ages show lower precision than in the other experiments. Steps 5 - 8 give an age of ~ 64 Ma and the corresponding isochron diagram suggests ~ 66 Ma as an u p p e r limit for the crystallization age. MAP-052: This sample was taken from a lava flow in the Mahabaleshwar Formation in the uppermost part of the CWGS (Fig. 2). The stepheating experiment at MP indicates partial loss of 39Ar from the sample by recoil (total gas age > K - A r date). Steps 4 - 8 define a plateau with an age of 64.9 _+ 0.5 Ma in the intermediate-high t e m p e r a t u r e region. The corresponding isochron plot yields a slightly low initial argon ratio and hence an age that is somewhat too old (Table 2).

5.2 Flows from other sections of the Deccan Panvel: A relatively unaltered lava flow gives a total gas age (D-907MP) marginally greater than the K - A r date. The age spectrum (Fig. 3) shows a descending staircase pattern and three hightemperature steps (6-8) yield an isochron age of ~ 63 Ma, with an initial argon ratio of atmospheric composition and an acceptable MSWD value. A dyke specimen (D-921), containing excess argon ( K - A r date ~ 97 Ma), shows an age spectrum with very high step ages ( ~ 580 Ma) at the lowest temperatures which decreases monotonically with increasing temperature to values ~ 70-60 Ma at fusion (Fig. 3). Although no plateau was recovered, steps 7 - 9 show concordant ages of ~ 70 Ma and define an isochron with an age of ~ 65 Ma, with an initial argon ratio marginally higher than the atmospheric value (Table 2). These two age spectra suggest crystallization ages of ~ 63-65 Ma for this section belonging to the Kalsubai Subgroup (Fig. 2). Bhor Ghat: A single specimen (D-949MP) from a lava flow in the western Deccan that belongs to the Lonavla Subgroup (Fig. 2) gives a total gas age (50 Ma), in agreement with the K - A r value, indicating little or no recoil loss of 39Ar out of the sample. The age spectrum shows post-crystallization loss of 4°Ar * and yields a four-step 'marginal plateau' (carrying only 42% of the total 39Ar) of 64.5 _+ 0.5 Ma. The isochron plot gives an age of ~ 65 Ma, with initial argon of atmospheric composition and an acceptable M S W D value. This age is in agreement with ages reported herein for flows from the underlying Kalsubai Subgroup (IGA-009Q, JEB-339Q, JEB-013QT) and the overlying Wai Subgroup (MAP-052MP). Koyna: A single specimen (D-961) thought to be relatively fresh, based on its low 36Ar content, was analyzed at MP (Fig. 3). The total gas age (68 Ma) is older than the K - A r value (61 Ma) and demonstrates considerable 39Ar recoil loss out of the sample. Steps 4-6, carrying 31% of the total 39Ar show ages of ~ 68 Ma and yield an isochron age of 69 _+ 3 Ma, with a high MSWD value. A crystallization age of ~ 65-70 Ma is inferred for this section that belongs to the Wai Subgroup [321.

A.K. Baksi / Earth and Planetary Science Letters 121 (1994) 43-56

Diveghat: A single sample (D-970), taken from a lava belonging to the Wai Subgroup (Fig. 2), yields a total gas age in agreement with the K - A r result, suggesting little or no recoil loss of 39Ar out of the sample. The age spectrum (Fig. 3) is an inverted U shape, suggesting post-crystallization loss of 4°Ar*. Three intermediate temperature steps yield a plateau age of 64.5_+ 1.1 Ma, in agreement with the isochron value of ~ 63.5 Ma. This estimate of the crystallization age is in agreement with that obtained for a flow (MAP052) from the Wai Subgroup in the CWGS. In association with the results obtained for D-949 (Bhor Ghat), it is demonstrated that low K - A r dates for Deccan whole-rock basalts do not indicate prolonged volcanic activity, but rather reflect post-crystallization loss of 4°Ar * Rajpipla: This alkali basalt (RP81-19Q) from the northern end of the Deccan (Fig. 1) yields step ages with somewhat poor precision. Steps 1-7 define a marginal plateau of 60.8 _+ 0.6 Ma

53

(Fig. 3) and the corresponding isochron also suggests an age of ~ 60.5 Ma (Table 2); however, the highest t e m p e r a t u r e step (carrying ~ 38% of the total 39Ar) yields an age of ~ 62.5 Ma. These results suggest: (1) a crystallization age of ~ 60 Ma and a high t e m p e r a t u r e phase containing excess 4°Ar; or (2) a crystallization age of > 63 Ma, with the low t e m p e r a t u r e 'marginal plateau' reflecting a partial resetting event at ~ 60 Ma. The second alternative is preferred because this specimen was collected from a lava flow ~ 10 m above the contact with an ~ 8 m thick dyke [34, Fig. 2, J o h n Mahoney, pers. commun., 1993]; it is possible that D e c c a n volcanism in this area ext e n d e d to ~ 60 Ma. 6. Discussion Plateau ages for I G A - 0 0 9 Q , JEB-339Q, JEB013QT, M A P - 0 5 2 M P and D-970Q overlap at the 95% confidence level, and hence it is permissible

8O

Weighted m e a n ~ [ ~ P l a t e a u ['.~1J . _ _ ~_ _ _ _ .=. . .~. . .

7O

o

,¢

....~

E

i

iLL.__.

<( to O3

50 O.. Q.

40 Cumulative

60

80

0.002

0.001

0.000 u.0

40

20

1

Ma

Age (40Ar/36Ar}i

a g e = 6 5 . 5 , 0 . 5 Ma :---:---:.--:-- ~ _ _~_~ _ _ .

O~ 60

= 65.5.0.2

0.003

=

!595.5.1.2

i

0.1

0.2

39Ar/40Ar

%39Ar

80

Weighted m e a n Plateau age = 64.6~0.5

0.003

Ma

~ , ~

A g e = 64.2~0.8 Ma (40Ar/36Ar)i = 302~18

70 o~

O

,<

0.002

6O

E t~ O.. O.

<

50

to

0.001

, () o3

4o o

20

40 Cumulative 40

39

60 %39Ar

80

100

oooo u.0

i

0.1

0.2

39Ar/40Ar

Fig. 4. Integrated A r / - Ar age spectra and isochron plots for whole-rock specimens yielding plateau sections. (a) and (b) Results for rocks from the composite Western Ghats section (]GA-009Q, JEB-339Q, JEB-013QT and MAP-052MP). (c) and (d) Results for rocks from other areas of the Deccan Traps (D-949MP and D-970Q). All errors shown and listed at the lo- level, internal precision.

54

A.K. Baksi / E a r t h and Planetary Science Letters 121 (1994) 43-56

to use the weighted mean average age for Deccan volcanism at 65.5 + 0.5 Ma. (An earlier abstract [39] listed an average age of 64.9 + 0.6 Ma, due to the use of plateau ages obtained at Queen's University using an incorrect age for the monitor specimen FCT-3 Bio [16,40]). Addition of the marginal plateau ages (JEB-339MP, JEB-013MP and D-949MP), changes this average value to 65.2 _+ 0.6 Ma (six s a m p l e s - - o n e age per flow). Fig. 4 displays composite age spectra and isochron plots for specimens yielding plateau ages; the initial argon in each instance was of a composition indistinguishable from that of atmospheric argon. Therefore, I prefer to use the average of the plateau ages (65.5 _+ 0.5 Ma), which are in good agreement with high precision 4°Ar//39Ar dating results (65.2 _+ 0.5 Ma and 64.7 _+ 0.5 Ma) on plagioclase separates taken from lava flows in the Deccan [41], whose age spectra are unaffected by 39Ar recoil problems. All the lavas analyzed here were extruded either during the main Deccan reversed epoch [12] or within the ensuing normal epoch. Listed ages for the Deccan are plotted against a geomagnetic polarity time scale (GPTS) modified from Harland et al. [42] by placing the K - T boundary at 64.5 Ma (Fig. 5). The use of ages listed in earlier reports [10,22] does not permit unequivocal recognition of the reversed epoch trapped within thick sections of lavas of the CWGS. The mean age obtained here, suggests that the reversed epoch trapped within lavas in the lower ~ 2 km of the CWGS corresponds to chron 29R. Because most of the plateau ages obtained (at both Q and MP, using different monitors) correspond to chron 30N, evaluation of systematic errors is necessary. I suggest this results from small amounts of 39At recoil loss out of the relatively K-rich sites degassed in the plateau steps, producing slightly elevated 4°Ar*/39ArK ratios and ages. This hypothesis is supported by the observation that the two specimens yielding inverted-U age spectra (D-949MP and D-970Q) and suffering no overall loss of 39Ar (Table 2) yield plateau ages ~ 0.5-1.0 m.y. younger than rocks that show artifacts in the age spectra resulting from 39Ar recoil. It appears unlikely that serious errors exist in the GPTS in the time frame chron 30N-29N.

63

64

~65 V91 66

67

68

Fig. 5. 4°Ar/39Ar ages for Deccan Traps plotted against the geomagnetic polarity time scale (GPTS) at 68-63 Ma. Black = normal polarity; white = reversed polarity. GPTS modified from Harland et al. [41] using an age of 64.5 Ma for the K - T boundary (see text). DP88 = D u n c a n and Pyle [22], V91 = V a n d a m m e et al. [10], B93 = this work. A n age of 65.5_+ 0.5 Ma obtained here indicates that the thick section of reversed polarity lava flows in the lower sections of the Deccan were extruded during chron 29R (see text).

Finally, comments on the postulated link between flood basalt volcanism and global faunal extinctions are in order. It has been suggested that the eruption of the Siberian Traps, Russia, is genetically linked to the Permo-Triassic boundary extinction event [43,44]. 4°mr//39Ar plateau ages on whole-rock basalts and plagioclase separates from the Traps [40,45,46] yield ages ~ 2% younger than plateau ages on biotite and ( U - P b age) on zircon separated from ore bodies intrusive into the Traps [44,46]. This suggests partial loss of 4°Ar* from phases of low argon retentivity (mesostasis of whole-rock basalt and plagioclase), that is not unequivocally detected in step-heating experiments. A similar scenario cannot be ruled out for the Deccan Province; an undetected loss of ~ 3% of the 4°Ar* would shift whole-rock basalt and plagioclase crystallization ages to ~ 67.5 Ma and indicate the bulk of the Deccan Traps were extruded during chron 31R (Fig. 5).

A.K. Baksi / Earth and Planetary Science Letters 121 (1994) 43-56

(Eruption of the Deccan Traps in Chron 30R, spanning ~ 0.09 m.y., would give rise to rates of lava extrusion that appear to be too rapid ~ 107 kma/m.y.) Further, the existence of ~ 3 km of older flows below the exposed surface level [47], should be borne in mind whilst evaluating a genetic link between Deccan volcanism and the K - T boundary extinction event.

[4]

[5]

[6]

Acknowledgments

[7]

Peter Hooper generously supplied rocks from the Western Ghats section, made available chemical data on the rocks, and patiently answered numerous questions about the CWGS.D.K. Paul provided specimens from sections in the central and southern Deccan and an alkali basalt from the Rajpipla area was obtained from Doug Macdougall. Edward Farrar provided access to the argon dating laboratory at Queen's University, and Doug Archibald provided considerable help with the K - A r analyses. The staff at the McMaster University Reactor assisted with the fast neutron irradiations. I thank Brent Dalrymple for permitting use of the facilities at U.S. Geological Survey, Menlo Park, for most of the 4°Ar/39Ar dating; Malcolm Pringle and James Saburomaru extended considerable help in the laboratory. Marvin Lanphere, Edward Farrar and Robert Duncan offered a number of useful comments during formal review of this manuscript. This study was supported by grants from the American Chemical Society and the B. Bhattacharyya Foundation.

[8]

[9]

[10]

[11] [12]

[13]

[14]

[15]

[16]

References [1] J.E. Beane, C.A. Turner, P.R. Hooper, K.V. Subbarao and J.N. Walsh, Stratigraphy, composition and form of the Deccan Basalts, Western Ghats, India, Bull. Volcanol. 48, 61-83, 1986. [2] J.J. Mahoney, Deccan Traps, in: Continental Flood Basalts, J.D. Macdougall, ed., pp. 151-194, Kluwer, Dordrecht, 1988. [3] P.C. Lightfoot, C.J. Hawkesworth, C.W. Devey, N.W. Rogers and P.W.C. Van Calsteren, Source and differenti-

[17]

[18]

[19]

55

ation of Deccan Trap lavas: Implications of geochemical and mineral chemical variations, J. Petrol. 31, 1165-1200, 1990. E.H. Pascoe, A Manual of the Geology of India and Burma, 2130 pp., Government of India Press, Calcutta, 1950. W.J. Morgan, Hotspot tracks and the opening of the Atlantic and Indian Oceans, in: The Sea, Vol. 7, C. Emiliani, ed., pp. 443-487, Wiley, New York, 1981. K.G. Cox, Deccan Traps and the Karoo: stratigraphic implications of possible hotspot origins, IAVCEI Prog. XVIII IUGG General Assembly, Hamburg, p. 96, 1983. C.W. Devey and P.C. Lightfoot, Volcanological and tectonic control of stratigraphy and structure in the western Deccan traps, Bull. Volcanol. 48, 195-207, 1986. A.K. Baksi, Critical evaluation of the age of the Deccan Traps, India: Implications for flood-basalt volcanism and faunal extinctions, Geology 15, 147-150, 1987. J.J. Jaeger, V. Courtillot and P. Tapponier, Paleontological view of the ages of the Deccan Traps, the Cretaceous/Tertiary boundary and the India-Asia collision, Geology 17, 316-319, 1989. D. Vandamme, V. Courtillot, J. Besse and R. Montigny, Paleomagnetism and age determinations of the Deccan Traps (India): Results of a Nagpur-Bombay traverse and review of earlier work, Rev. Geophys. 29, 159-190, 1991. C.B. Officer and C.L. Drake, The Cretaceous-Tertiary transition, Science 219, 1383-1390, 1983. H. Wensink, N.A.I.M. Boelrijk, E.H. Hebeda, H.N.A. Priem, E.A.T. Verdurmen and R.H. Verschure, Paleomagnetism and radiometric age determinations of the Deccan Traps, India, in: IV Int. Gondwana Symp., Calcutta, B. Laskar and C.S. Raja Rao, eds., pp. 832-849, Hindustan, Delhi, 1977. S.F.R. Khadri, K.V. Subbarao and M.S. Bodas, Magnetic studies on a thick pile of Deccan Traps at Kalsubai, Geol. Soc. India Mem. 10, 163-179, 1988. A.K. Baksi, Critical evaluation of the age of the Deccan Traps, India: Implications for flood-basalt volcanism and faunal extinctions: Reply, Geology 16, 758-759, 1988. M.A. Lanphere, G.B. Dalrymple, R.J. Fleck and M.S. Pringle, Intercalibration of mineral standards for K-Ar and 4°Ar/39Ar age measurements, EOS Trans. Am. Geophys. Union 71, 1658, 1990. A.K. Baksi, E. Farrar and D.A. Archibald, Intercalibration of standards utilized for 4°Ar/39Ar dating, using an MS10 mass spectrometer, EOS Trans. Am. Geophys. Union 73, 328, 1992. G.A. Izett, G.B. Dalrymple and L.W. Snee, 4°Ar/39Ar age of the Cretaceous-Tertiary boundary tektites from Haiti, Science 252, 1539-1542, 1991. M. McWilliams, A.K. Baksi and H. Baadsgaard, New 4°mr/39Ar ages from K - T boundary bentonites in Montana and Saskatchewan, EOS Trans. Am. Geophys. Union 72, 301, 1991. C.M. Hall, D. York and H. Sigurdsson, Laser 4°Ar/39Ar step-heating ages from Cretaceous-Tertiary boundary

56

A.K. Baksi / Earth and Planetary Science Letters 121 (I 994) 43-56

glass spherules, EOS Trans. Am. Geophys. Union 72, 531, 1991. [20] C.C. Swisher et al., Coeval 4°Ar/39Ar ages of 65.0 million years ago from Chicxulub Crater melt rock and Cretaceous-Tertiary boundary tektites, Science 257, 954-958, 1992. [21] R.A. Duncan and D.G. Pyle, Rapid eruption of the Deccan flood basalts at the Cretaceous/Tertiary boundary, Nature 333, 841-843, 1988. [22] R.A. Duncan and D.G. Pyle, Rapid eruption of the Deccan flood basalts, western India, Geol. Soc. India Mere. 10, 1-9, 1988. [23] A.K. Baksi, Timing and duration of Mesozoic-Tertiary flood-basalt volcanism, LOS Trans. Am. Geophys. Union 71, 1835-1836, 1990. [24] A.K. Baksi and E. Farrar, 4°Ar/39Ar dating of the Siberian Traps, USSR: evaluation of the ages of two major extinction events relative to episodes of flood-basalt volcanism in the USSR and the Deccan Traps, India, Geology 19, 461-464, 1991. [25] V. Courtillot, G. Feraud, H. Maluski, D. Vandamme, M.G. Moreau and J. Besse, Deccan flood basalts and the Cretaceous/Tertiary boundary, Nature 333, 843 846, 1988. [26] K. Pande, T.R. Venkatesam K. Gopalan, P. Krishnamurthy and J.D. Macdougall, 4°Ar 39Ar ages of alkali basalts from Kutch, Deccan volcanic province, India, Geol. Soc. India Mem. 10, 145-150, 1988. [27] P.N. Shukla, T.R. Venkatesan and N. Bhandari, Chemistry and chronology of traps and intertrappeans from Takli, Nagpur, Geol. Soc. India Mem. 10, 213-223, 1988. [28] D.K. Paul, P. Kersten, T. Ray Barman, R.H. McNutt and A.O. Brunefelt, Geochemical and petrological relations in some Deccan basalts, western Maharashtra, India, J. Volcanol. Geotherm. Res. 21, 165 176, 1984. [29] T.C. Onstott and M.W. Peacock, Argon retentivity in hornblendes: a field experiment in a slowly cooled metamorphic terrane, Geochim. Cosmochim. Acta 51, 2891 2904, 1987. [30] J.S. Stacey, N.D. Sherill, G.B. Dalrymple, M.A. Lanphere and N.V. Carpenter, A computer controlled fivecollector mass spectrometer for precision measurement of argon isotopic ratios, US Geol. Surv. Open File Rep. 78-701,411-413, 1978. [31] A.K. Baksi, V. Hsu, M.O. McWilliams and E. Farrar, 4°Ar/3'Ar dating of the Brunhes/Matuyama geomagnetic field reversal, Science 256, 356 357, 1992. [32] C. Mitchell and K.G. Cox, A geological sketch map of the southern part of the Deccan Province, Geol. Soc. India Mere. 10, 27 33, 1988. [33] J.K. Agrawal and Rama, Chronology of Mesozoic volcanics of India, Proc. Ind. Acad. Sci. 84A, 157-179, 1976.

[34] J.J. Mahoney, J.D. Macdougall, G.W. Lugmair, K. Gopalan and P. Krishnamurthy, Origin of contemporaneous tholeiitic and K-rich alkalic lavas: a case study from the northern Deccan plateau, India, Earth Planet. Sci. Lett. 72, 39-53, 1985. [35] M.A. Lanphere and G.B. Dalrymple, The use of 4°Ar/ 39Ar data in evaluation of disturbed K - A r systems, US Geol. Surv. Open File Rep. 78-701,241-243, 1978. [36] G. Turner and P.H. Cadogan, Possible effects of 39At recoil in 4°Ar/39Ar dating, Geochim. Cosmochim. Acta Suppl. 5, 1601-1615, 1974. [37] A.K. Baksi, 4°Ar/39Ar incremental heating studies on terrestrial whole-rock basalts: an attempt to unravel the effects of recoil loss of 3'Ar and 37Ar, EOS Trans. Am. Geophys. Union 71,653-654, 1990. [38] D. York, Least squares fitting of a straight line with correlated errors, Earth Planet. Sci. Lett. 5, 320-324, 1969. [39] A.K. Baksi, Timing and duration of Deccan Trap volcanism: Results of 4°Ar/39Ar dating studies, EOS Trans. Am. Geophys. Union 70, 488, 1989. [40] A.K. Baksi and E. Farrar, 4°Ar/39Ar dating of whole-rock basalts (Siberian Traps) in the Tunguska and Noril'sk areas (USSR), EOS Trans. Am. Geophys. Union 72, 570, 1991. [41] R.A. Duncan and M.S. Pringle, K / T boundary events were synchronous with rapid eruption of the Deccan flood basalts, EOS Trans. Am. Geophys. Union 72, 301, 1991. [42] W.B. Harland, A.V. Cox, P.G. Llewellyn, C.A.G. Pickton, A.G. Smith and R. Walters, A Geologic Time Scale, 131 pp., Cambridge Univ. Press, New York, 1982. [43] W.J. Morgan, Flood basalts and mass extinctions, EOS Trans. Am. Geophys. Union 67, 371, 1986. [44] I.H. Campbell, G.K. Czamanske, V.A. Fedorenko, R.I. Hill and V. Stepanov, Synchronism of the Siberian Traps and the Permian Triassic boundary, Science 258, 17601763, 1992. [45] P.R. Renne and A.R. Basu, Rapid eruption of the Siberian Traps flood basalts at the Permo-Triassic boundary, Science 253, 176-179, 1991. [46] G.B. Dalrymple, G.K. Czamanske, M.A. Lanphere, V. Stepanov and V. Fedorenko, 4aAr/39Ar ages of samples from the Noril'sk-Talnakh ore-bearing intrusions and the Siberian flood basalts, Siberia, LOS Trans. Am. Geophys. Union 72, 570, 1991. [47] A.V. Murali and D.P. Blanchard, Deccan volcanism, India: A witness, but not the cause of the K-T biotic extinctions, EOS Trans. Am. Geophys. Union 71, 1713, 1990.