Origin of Decan Lava Nd and Sr Isotopic and Chemical Evidence

8/6/2019 Origin of Decan Lava Nd and Sr Isotopic and Chemical Evidence

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.Earth and Planetary Science Letters, 60 (1982) 41-60Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

[ 3 1

Origin of the Deccan Trap flows at Mahabaleshwar inferred fromNd and Sr isotopic and chemical evidenceJ. Mahoney I, J.D. Macdougall , G.W. Lugmair , A.V. Murali *,

M. Sankar Das * and IS. Gopalan 3 Scripps Institution of Oceanography, La Jolla, CA 92093 (U.S.A.)

Bhabha Atomic Research Center, Bombay (India) Physical Research Laboratory, Ahmedabad (Indra)

Received January 13, 1982Revised version received May 6, 1982

The Deccan flows at Mahabaleshwar are divisible into a lower and an upper group, based on Nd and Sr isotopicratios, which define two correlated trends. This distinction is supported by incompatible element ratios and bulkcompositions. The data reflect contamination in a dynamic system of magmas from an LIL-depleted, cJuv > + 8 mantleby two different negative flUV endmembers, one undoubtedly continental crust, the other either continental crust orenriched mantle. The depleted mantle source, anomalously high in (Sr/*%r), may have been in the subcontinentallithosphere or a region of rising Indian Ocean MORB mantle.

1. IntroductionThe late Cretaceous to early Eocene DeccanPlateau of central India is one of the most exten-sive continental flood basalt provinces of the world,

comprising more than 500,000 km of predomi-nantly tholeiitic basalt. This vast accumulation ofnearly flat-lying flows reaches its maximum thick-ness in western India, outcropping spectacularly inthe peaks and gorges of the Western Ghats. The1200 m sequence at Mahabaleshwar, which is dis-cussed in this paper, lies about 140 km southeastof Bombay (Fig. 1). With as many as 48 successiveexposed flows, it is one of the thickest of the Ghatsections (Fig. 2). The few available K-Ar and40Ar/39Ar dates place the entire sequence between61.5 and 64.4 m.y. in age; thus a tremendousoutpouring of tholeiitic lavas appears to have oc-curred in this region over a geologically very shortinterval [1,2]. Deshmukh et al. [24] and Najafi etal. [23] have given detailed petrographic summariesof these basalts.

A major problem in the study of continentaflood basalts is to identify the chemical characteof their mantle source regions. This is likely to bobscured by processes such as variable partimelting, fractional crystallization, and crustal cotamination. Clear evidence of fractionation is preent in many flood basalt sequences (e.g. [IS]while their eruption through varying thicknesses continental crust permits the possibility of assimlation and contamination. At present, the mopromising approach for disentangling characteristics of the mantle source from those produced petrogenesis involves collecting detailed chemicand isotopic data for a large number of relatelavas. In this paper we present such data for tMahabaleshwar basalts.

Recently it has been argued on the basis limited Nd isotopic evidence that the source rgions of flood basalts are chemically primitivunfractionated portions of the mantle (e.g. [loHowever, detailed investigations of Nd and isotopic and chemical compositions of the C

0012-821X/82/COOO-0000/%02.75 0 1982 Eisevier Scientific Publishing Company


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48

ARABSEA

---+ 7Z0 740

24

I8

16-

Fig. 1. Map showing Deccan Traps (stippled) and location of Mahabaleshwar.COLUMNAR SECTION OF MAHABALESHWAR GHAT

M53MBBI-12

600 mMB81-24

ME81-23M2MB8l-22M881-21MBBI- 20M881-I9

300m MBBI-18MB81-3 M881_I7

%~~-M881-I5M88l_2M97

M881_I M881-1446m MB8l-13Fig. 2. Simplified section of Mahabaleshwar. Elevations are onthe left side of the column and sample numbers and approxi-mate locations on the right. The five M samples were pro-vided by Dr. S.F. Sethna.

lumbia River basalts [5,6] and the North AtlantiTertiary Province (e.g. [7-91) suggest that insteathe major mantle components in these areas wedepleted. Further, the magmas appear to habeen variable modified by interaction with cotinental crust. The present work is in part attempt to compare similar data for the Deccawith the results from these two provinces. Tsection at Mahabaleshwar was chosen for initiastudy because it represents a classical sequence typical Deccan tholeiites whose great thicknesobviates the need for correlating flows at widelseparated exposures.

2. MethodsField sampling was done in the winter of 198

The main criteria for selection of samples wefreshness in hand specimen and the collection offairly representative sample suite. Additionally, fisamples were graciously provided by Dr. SF


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Sethna of St. Xaviers College in Bombay.Major element analyses were performed by

electron microprobe on fused powdered aliquantsof sample following Brown [12]. Trace elementswere determined by conventional X-ray fluores-cence techniques utilizing a W X-ray tube. Esti-mated precisions are given in Table 1. Cr and REEwere analyzed by instrumental neutron activation,the details of which will be described in a forth-coming paper.

Isotopic analyses were carried out on a subsetof samples chosen on the basis of their major andtrace element composition, and especially traceelement ratios. Additionally, an effort was made toobtain isotopic data on samples from throughoutthe section. Nd and Sr isotopic analyses, as well asNd, Sm, Rb, and Sr isotope dilution measure-ments, closely followed techniques described byLugmair et al. 1131 and Carlson [ 141. Isotopicfractionation corrections, estimated precisions, andstandard reference values are given in Table 1.

3. ResultsTable 1 shows the measured bulk and trace

element contents and Nd and Sr isotopic composi-tions of our samples. Nd and Sr isotopic values arepresented graphically in Fig. 3. For reference, theso-called mantle trend, defined mainly by mantle-derived oceanic rocks [ 15,161, and the mixing curvefitting the distribution of samples from the Co-lumbia River basalts [4] are also shown. There areseveral salient features of this diagram. First, avery wide range of isotopic values is observed,from ~,ov( T) = + 7.8, (Sr/ %r), = 0.7039, tocJvv(T) = - 16.2, (87Sr/86Sr), = 0.7179. Thehighest eJt,v(T) values approach those of oceanicisland arcs and certain oceanic island basalts[ 15,161, and are also similar to values for the leastcrustally-contaminated basalts of the Columbia .River Province [5,6]. However, the Mahabaleshwarrocks are somewhat higher in ( 87Sr/86Sr), thaneither oceanic island basalts or the Columbia Riverlavas, and lie outside the range of variation of themantle trend.

Secondly, the data appear to define two corre-lated trends, both diverging from the same high

* AVERAGE lND,ANOCEAN MORB

OOMAHABALESHWAR

-14-16. .

7020 7040 7060 7080 7100 7120 7140 7160 .7180 7200 7(*7Sr/B6Sr) I

Fig. 3. Nd vs. Sr isotopic composition of flows at Mahabaleshwar. Also shown are the trend defined by mantle-derivedrocks ([ 15,161, and others), the mixing curve of the ColumbiRiver basalts [5], average Indian Ocean MORB [14,38]. apostulated bulk earth values (BE). Solid circles (0) represengroup 1 samples, dotted circles (0) represent group 2 sampleThose with c,,,(T)? +6 where it is not possible to resolve two groups. are shown as belonging to group 1, although sommay well belong to group 2.

eJuv(T) region toward two different negative cJUregions. Clearly, both Mahabaleshwar trends adistinct from the Columbia River curve. Stratgraphically the trends correspond to two differengroups of flows. Samples belonging to the array othe right, henceforth called trend 1, are all fromthe lower levels of the section. Those falling on tleft-hand trend, trend 2, which closely parallels tmantle trend, are all from the upper portion of tsection. There appears to be a major divisionaround the flow represented by sample MB8 1-(Fig. 2). This boundary is only an upper limihowever, because resolution of the two trends not possible among the high cJuv(T) flows that beneath MB81-17, all of which we have somewhaarbitrarily placed in group 1. Within each groupadefined there is a general tendency toward increasing c,,,(T) with increasing elevation; in eacgroup the earliest flows appear to possess the monegative c&T), while later ones have the greatest e,uV( T) values.

The separation of flows into two groups is alapparent in Fig. 4, which shows K/Zr versuBa/Zr. As with the isotopic data, an extremely


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TABLE 1Chemical and isotopic data for basalt flows at MahabaleshwarSample No.: MBXI-1 M97 MBXl-2

SiO, (W)TiO,A,&FeO*MgOCaONa,O(P*O,)K (ppm)RbBaSrZrYNiCrVNdSme,,,(T) b

51.880.96

14.4510.38X.11

10.471.900.10

50.763.01

13.3514.605.119.032.690.32

4X.261.92

14.2213.817.21

10.851.970.15

MBXI-3 MBXl-4 MBXl-5 MBXl-6

4944 49.78 51.85 50.122.88 2.49 1.84 1.88

13.09 13.58 14.76 14.5715.32 14.19 10.81 12.965.87 5.87 6.38 5.889.32 9.77 10.79 11.022.29 2.28 2.16 2.340.29 0.27 0.21 0.21

466716.9 a

125176.1 9X29166

235271

12.553.03

639115.9 a

12019X.7 a253

5769456

26.777.18

18334.77 a

35204.8 a1253110093

37715.034.20

43331382

147216

497252

445

4167 2417 250015.4 a 10.8 a 5.71 a84 125 63

209.0 205.9 a 197.8 =196 135 11847 34 3180 117 7272 227 87

405 337 35724.11 17.84 14.72

6.47 4.6 1 4.14- 16.2kO.6

+3.3 +4.1 + 3.5-0.3 zo.4 kO.50.706 17 0.70567 0.70590-c2 22 *3

-3.5-to.6

+3.9-0.50.70581*2

0.71972~6

0.7083523

Rb and Sr measured by isotope dilution.- Sr measured by XRF generally agrees within 2%.- Rb is within 1.3 ppm on the average, when measured by both XRF+ I.D. (max. dev. 2.4 ppm).- Sm+Nd precision is estimated at


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MB81-7 MB81-8 MBBl-9 M94 MBSl-IO MB81-11 MB81-12 M53

49.16 49.16 49.64 49.11 49.51 49.65 49.22 49.912.13 2.43 2.84 2.56 3.59 3.19 2.31 2.6814.40 14.20 13.80 14.28 13.07 12.88 13.15 14.4812.62 13.83 14.5 1 14.00 14.38 14.81 14.04 13.056.56 6.14 5.95 6.10 5.19 5.67 6.32 5.94

11.53 11.10 10.27 10.48 10.11 10.17 10.84 10.512.34 2.19 2.39 2.58 2.53 2.30 2.24 2.610.23 0.24 0.26 0.31 0.36 0.38 0.24 0.31

1333 10001.59 0.61

40 37208.8 = 222.8 a132 14934 38

102 82127 66383 439

15.56 18.164.45 5.13

1333(238

218169418762

499

1.45 =38

214.9 a1744390

50221.85

6.35

21676.65 a

44220.4 =240

518492

57728.97

8.17

1750 loo04 3

40 34213 202193 14048 3888 8582 79

533 432

1333(1

4021517244

102492

+6.6 + 6.4 +7.8 + 7.00.2 to.5 i-o.5 20.40.70419 0.70433 0.70396 0.70391zr-4 24 *3 -t4 Samples with variable, but high amounts of sulfur, not quantitatively measured, but detected with EEDS X-ray attachment

microprobe. MB81-17 has sparse, tiny grains of probably secondary chalcopyrite.d Deviations for major elements are lo,,, values. For trace elements maximum deviations in ppm from duplicate XRF analyses

given. For Cr estimated precision is < 10%.

certain trace element concentrations and ratios for em Deccan basalts to MORB has also been susome of the Mahabaleshwar flows approach those gested by Chandrasekharam and Parthasarathyof abyssal tholeiites. The similarity of some west- [do]; moreover, Murali et al. [41] have discerned

Fig. 5. Correlation of (sSr/%r), with Ba/Zr.

lTl -4 -

- 1 6 -10 1 2 3 4 5

0 0 I YFig. 6. Correlation of eJUv( T) with Ba/Y.


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TABLE 1 (continued)Sample No.: MB81-13 MBSI-14 MB81-15 Ml00 MB81-16 MBSl-17 MB81-18

SiO, (%) 49.36 49.21 49.74 49.91 48.20 49.98 49.44TiO, 2.44 2.69 3.53 3.33 4.29 2.11 3.30A#, 13.72 13.43 12.63 13.10 12.51 14.25 12.82FeO* 13.80 14.33 15.42 15.10 16.34 12.44 15.38M@ 6.78 6.07 5.58 5.91 5.17 6.45 5.07CaO 10.45 10.61 9.36 9.48 9.43 10.78 9.64Na,O 2.13 2.23 2.3 1 2.66 2.38 2.25 2.25(P,O,) 0.23 0.27 0.34 0.38 0.46 0.24 0.38K (ppm) 2250 1750 3250 3486 3833 2917 4583Rb 5 7.97 B 9 6 8 10.6 11.9Ba 42 27 51 70 92 150 143Sr 194 201.1 = 209 225 253 292.4 a 242.6 aZr 143 165 226 227 254 170 236Y 36 38 52 53 59 33 53Ni 96 91 78 78 75 158 60Cr 118 89 78 26 241 47V 431 454 520 528 484 290 443

19.66 22.04 31.00Sm 5.63 5.15 7.75,lJ(T) b +6.2 - 1.5 +2.2

to.4 -0.3 -0.7( S7Sr/*6Sr), b 0.70423 0.70564 0.70490

*2 t2 *7

affinity of lavas from the extreme Northwest Dec-can with certain types of oceanic basalts. So far aswe are aware, this is true for rocks of only one

8-4-

o-%uv(T)

-4 -

-8 -

-12 -

-16 -

Sm/NdFig. 7. Correlation of c,,,(T) with Sm/Nd

other continental flood basalt province: the NortAtlantic Tertiary basalts from Baffin Bay [ 1Other continental flood basalts generally hamuch higher LIL element abundances (cf. [ 18,19

Unlike the chemically primitive Baffin Brocks, however, those from Mahabaleshwar aclearly quite fractionated. Major elements, showgraphically in Fig. 8, exhibit a fairly wide spreaof loosely grouped values, with high FeO* (10.3816.34%), relatively low MgO (8.1 l-4.94%), anvariable TiO, (0.96-4.29%), MgO/FeO*, a montor of olivine and/or clinopyroxene fractionation,ranges from 0.78 to 0.32. CaO/Al,O,, diagnosticof clinopyroxene fractionation, varies between 0.and 0.68. In fact, the groupings in Fig. 8 correspond rather closely to those of certain oceaniferrobasalts (cf. [20]), which are thought to resufrom as much as 75% fractional crystallization plagioclase, clinopyroxene, and olivine [20,21They are also quite consistent with the model Cox [22], who has recently proposed that man


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MB81-19 MB8 I-20 MB81-22 M2 MB81-23 MB81-24C Est.errors d

49.46 49.16 50.31 50.30 50.10 49.74 0.193.38 3.16 3.14 2.98 2.30 2.43 0.04

13.23 13.60 13.70 14.14 14.25 14.46 0.1114.85 14.05 13.21 12.57 12.28 12.61 0.204.94 5.02 5.26 5.60 6.04 5.58 0.099.87 9.79 10.32 10.27 10.95 10.96 0.072.26 2.57 2.65 2.85 2.23 2.44 0.120.37 0.41 0.36 0.33 0.23 0.27 0.03

3417 3249 2749 3237 2499 2666 0.0 123 II.38 12 11.5 a 4 7 1.389 121 90 83 68 99 5

234 234.8 a 226 224.3 a 214 226 6236 229 201 193 145 167 8

48 48 43 43 33 37 361 74 81 90 116 81 567 103 70 116 83

438 398 442 466 372 342 828.36 34.60

7.20 9.11+3.6 + 4.6kO.5 20.4

0.70499 0.70437*I -c4

- Isotopic fractionation corrections: s6Sr/s8Sr=0.1194, 48Nd0/44Nd0=0.242436.- Nd values given relative to 43Nd/ Nd=0.511859 for the La Jolla Nd Standard, such that Nd/Nd=0.512566 foresent-day Juvinas. This is defined as cJuv(0)=O.- Sr values given relative to *Sr/s6Sr=0.71025 for NBS 987 Sr.

ental flood lavas are the result of extensivefractionation from originally picritic parent

The two series of flows are distinguishable bycompositions as well (Fig. 8)

2 having generally lower FeO*, and higher, for a given MgO content than

1. Thus they are separable stratigraphically,the basis of major element composition, and

by trace element and isotopic characteristics.et al. 1231, relying primarily on trace ele-

cently argued for three groupsflows at Mahabaleshwar. Such a division is

dly practical for purposes of regionalcorrelation. However, taking into

the isotopic evidence presented here, pet-insight is improved by considering two

of flows, each with a broad range of chemi-and isotopic variability.

4. Discussion4.1. Two-contaminant model

The most straightforward interpretation of thisotopic and trace element data from Mahaba-leshwar is that an LIL-depleted, high r,,,(T)(a + 8), low ( 87Sr/ %r), (4 0.7039) mantle sourceregion generated magmas which became variablycontaminated at different times by two differenttypes of low zJUv material, probably continentalcrust. The material mixing in to form trend 2 mayhave been an ancient granulitic crust, long bereftof Rb and other mobile LILs, but with smalSm/Nd and therefore a very negative eJuv at thtime of contamination. The crustal endmember fotrend 1 must have been much less depleted iLILs, thereby possessing a much higher (*Sr/%r).The close juxtaposition of diverse Precambrian


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5 4

1 7

1 6

1 5

1 4

F 1 35

" 0 1 2G

1 1

1 0

9

8

7 7. 1

..

I 1 I I 13 4 5 6 7 8

MgO wt. % I4 5 6 7 6

MgO w t . %Fig. 8. Major element variation diagrams of Mahabaleshwar

terrains outcropping around the borders of theDeccan (e.g. (4]), and the vertically complex natureof the underlying Indian Shield [3] lend support tothe possibility of more than one crustal contami-nant. Unfortunately, no basement is exposed nearMahabaleshwar, so this hypothesis can be testedonly in a general way. In their study of trace

element variation Najafi et al. [23] suggestegranitic crustal contamination played an important role at Mahabaleshwar; this proposal wrecently supported in a more detailed investigationby Cox and Clifford [42]. The involvement ancient depleted granulitic crust in continentalflood basalt genesis has been demonstrated fcertain Scottish Tertiary basalts [7,8], and that more LIL-enriched crust for the Columbia Riv[5,6] and Scottish Provinces [7,8].

Although Wasserburg and DePaolo [lo] havrecently argued that a primordial undifferentiatedmantle source with zJuv = 0 is characteristic continental flood basalts, there is no evidence fit at Mahabaleshwar. Our results are thus in gooagreement with the findings of Carlson et al. [5,for the Columbia River basalts and those of Carteet al. [39] for the North Atlantic Tertiary ProvinceThe relatively well defined trends in Fig. 3 woulalso seem to rule out the involvement of multipleheterogeneous mantle source regions, although thparallelism of trend 2 with the mantle trend sugests the possibility of mixing between two different mantle sources; this will be considered belowAll of the trend 1 samples lie well outside the fieof measured values for mantle rocks.

If the observed trends indeed result from contamination, simple binary mixing models can constructed to help constrain the characteristics possible endmembers. We point out that the samples with the most contaminated-appearing istopic and trace element ratios, MBSI-1, 5, and 1are also among the feast fractionated in terms their Mg/Fe values, Ni, and Cr contents (Table Assuming that MB8 l- 1 results from a 20 : 80 mixture (e.g. [25]) of crustal material and a mantlmagma with the isotopic characteristics of M94, mixing trend can be generated which closely fithe data for trend 1 (Fig. 9). The mantle endmem-ber possesses Nd and SK concentrations similar MORB, while those of the crustal component awithin the range of continental crustal compositessuch as those from the Canadian Shield [26,27(Unfortunately, detailed data of a similar naturefor the Indian Shield are almost non-existant.) MORB-like values are assigned to other trace elments in the depleted mantle endmember, thmodel again requires concentrations for the crusta


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lN0lA.N OCEAN B A SA L T6ppmd, 20ppmr

b_l O-t B O O p p m S r5 4 0 p v m N d\ u '

- 3 o - \ \

- 2 o -

3 9SQQ t S C 39 QpmNd

-40 I !\ I I I I I I I I4M )QQmSr. Z?QQmNd7 0 3 0 7 1 1 0 7 1 9 0 7 2 7 0 7 3 5 0 7 4 3 0

P ' s r / * %r ) IFig. 9. (87Sr/86Sr), vs. cJuv(T) for flows at Mahabaleshwarshowing hypothetical endmembers and mixing curves. Nd andSr contents shown with depleted mantle endmember are initialvalues only. See text for explanation.

endmember similar to those of the crustal com-posites of Shaw et al. [26].

Other combinations are also possible, of course.Nevertheless, in order to have a contaminant withreasonable Nd content and isotopic value, thedepleted mantle magma is constrained to have alow Nd concentration, 5 8 ppm, since MBSl-1itself has only 12.6 ppm. Some of the other group1 samples must be fractionated up to a total of75% to produce their observed Nd contents by thismechanism. Alternatively, and perhaps more likely,the uncontaminated mantle magma may haveevolved continually, so long as the Sr/Nd ratio,and thus the curvature of the mixing trend, did notchange greatly. This could have taken place in agreat subcrustal magma chamber as Cox et al. [ 191have proposed. Another possibility is that the de-gree of partial melting of the mantle source mayhave decreased with time, gradually increasing themagmatic incompatible element contents but notthe Sr/Nd ratio as long as Dsr = D,, -=c1. It maynot be coincidence, for example, that the Nd con-centrations of the consecutive least-contaminatedflows MB81-7, 8, M94, and MB81-10 and 14,

show a general increase with time. In fact, thREE patterns of the consecutively less contaminated group 1 flows MB81-1, 5, 7, and 1(Fig. lo), are most easily explained by early majocontamination of a depleted magma by a LREE-enriched upper crustal component, followed bless contaminated but increasingly evolved magmas. Moreover, in any simple mixing scenario thpoints in Fig. 7 would form, two straight lines. Thscattered distributions actually observed, howevercan be explained if one or more endmembers werevolving compositionally through time and if significant post-mixing fractionation occurred.

To explain trend 2 by a similar mechanism,some additional assumptions must be made, because the most contaminated-appearing sample othis series, MB8 1- 17, is also somewhat fractionated.If the uncontaminated mantle magma had evolvedas discussed above until it contained 12 ppm Nand 250 ppm Sr, for example, then 11% or lescontamination with an ancient LIL-depletedcrustai endmember having 22 ppm Nd, eJUV =-40, and Sr = 400 ppm (87Sr/86Sr) = 0.7140

l- Lo Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb LuFig. 10. Chondrite-normalized REE patterns for several grou1 rocks from Mahabaleshwar.

I / I I I 1 I , I

l B81-10 MB81-50 MB81-7A MB81-IO


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56

would reproduce trend 2. Again, these postulatedvalues appear reasonable based on the compositesof Shaw et al. [26] and on analyses of Lewisiangranulitic gneisses [7,28] (mean Nd 18.5 ppm. meanSr 334-569 ppm). In this simple case, the mixturethat was to become MB81-17 would have had tofractionate subsequently an additional 20% or soto reach its present Nd value of 22 ppm.

An alternative possibility is that trend 2 re-sulted from contamination by an LIL-enriched,negative eJUv mantle endmember. As an example,a mix of 25% or less of enriched mantle havingNd =40 ppm, zJuv = - 10, and Sr = 800 ppm,(Sr/%r) = 0.7069, and a depleted mantle withNd = 12 ppm, Sr = 200 ppm would generate trend2. Here too Nd must increase somewhat aftercontamination to attain its level in MB81-17. Sucha hypothetical enriched mantle lies within the rangeof reported isotopic values of garnets and di-opsides from mantle xenoliths [29,30]. The traceelement contents of this contaminant would bebroadly similar to those of alkali basalts. It isinteresting to note that in the western Deccanvolumetrically minor alkalic volcanism occurredafter the main tholeiitic flood phase (e.g. [31]).Thus it is conceivable that the late flows compris-ing group 2 may have begun to sample an enrichedsource that would even later express itself in cer-tain alkalic suites.

None of these models is unique. However, if asimple closed system fractionation process oper-ated in the generation of these highly evolvedflows, Ni and Cr would quickly be drasticallydepleted. Yet MB81-17, for example, in mid-se-quence and the most contaminated sample of group2, is also very rich in Ni (158 ppm) and Cr (247ppm). The same is true for MB81-5, in the middleof group 1. A likely explanation is that the de-pleted magma evolved in an open system, perhapssomewhat similar to the recharged magma cham-ber envisioned by OHara [32].4.2. Combined assimilation and fractional crystalli-

zationSeveral workers have recently modelled in some

detail the isotopic and trace element effects ofsimultaneous fractional crystallization and assimi-

lation [6,33-351. In a general sense such a processeems eminently plausible in the genesis of tMahabaleshwar basalts. An important property these models is that they allow the generation more than one mixing array from a sing/e serendmembers. This effect can be achieved by varing either the ratio of crystallized mass to assimlated mass (C/A, after Carlson et al. [6]). or tvalues of bulk distribution coefficients of relevantrace elements (see references 34 and 6 for detaiand figures).

For the phases fractionating in the Mahabaleswhar magmas, olivine, clinopyroxene. anplagioclase, it may be assumed that D,, 2 D,,all cases, and that D,, was quite small (probablyG 0.1). Substantial plagioclase fractionation couincrease D,, to values greater than 1, but otherwisD,, K 1. An increase in C/A from group 2 group 1 would then produce two curves in Fig.qualitatively similar to those observed. In such case, Sr concentration would increase relativelymore slowly for trend 2 than for trend 1 (cf. fig. 71). The data in Fig. 11, however. show just treverse: for a given ( s7Sr/shSr), , trend 2 rockhave the higher Sr concentrations. Similarly, forgiven eJvv(T), lower Nd, Ba and Rb concentra-tions would be expected for group 2 basalts. yjust the opposite is observed. Therefore, this mechanism seems to be ruled out as a means of producing the two trends.

Another possibility is that D,, changed fro> 1 for group 1 to < 1 for group 2 (see DePaolo[34, fig. 61). This is equivalent to having relativelymore plagioclase fractionation for group 1. Wiconstant C/A, however, successively lower Nisotopic values would require successively largeamounts of fractionation in any coupled fractiona-tion-assimilation scheme, because greater total asimilation would be accompanied by greater totfractionation. Thus samples with the most cotaminated-appearing isotopic ratios should also the most fractionated. Such a situation appears obtain for lavas of the Columbia River Province[6], but at Mahabaleshwar just the opposite observed. MB8 1- 1, with the most extreme isotopivalues, is also clearly the least fractionated of osamples, with a very low Nd concentration. MB8 5 and 17 are also relatively unfractionated, wherea


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. 7 0 3 0 1 ' 1 ' 1 1 ' 1 ' ' ' 11 7 0 1 9 0 2 1 0 2 3 0 2 5 0 2 7 0 2 9 0Sr pm

Fig. 11. (sSr/*%r), vs. Sr for Mahabaleshwar basalts.

MB81-10, with e&T) = + 7.0, is quite evolved(Fig. 10). Furthermore, a change in D,, due toplagioclase fractionation should not greatly affectmost other incompatible element ratios. However,as Figs.4 through 7 illustrate, trend 2 has con-sistently different Sm/Nd, K/Zr, Y/Zr, andBa/Zr.

Variations in both C/A and D,, could producemore drastic changes in the mixing trajectories,but the above arguments would still be valid. Thusdifferences in conditions of coupled crystallizationand assimilation of a single enriched endmemberappear incapable of accounting for the evidence,although they may have played some role. Ourconclusion that contamination of the two groupswas affected by two fundamentally different nega-tive eJUv endmembers therefore appears valid.4.3. Mode of contamination

From Fig. 3 it seems that in each series ofbasalts there is a general tendency, going upward

in the section. to progress from most to leacontaminated flows with time. This could be thresult of decreasing assimilation of the wallrock two different conduit systems over time as successive eruptions gradually depleted available materiaand/or coated the wallrock over with chillebasaltic rinds. Alternatively, periodically rcharged magma chambers may have existed at thbase of or within the crust. The earliest magmawould probably be the most contaminated owinto disruption and erosion of wallrock during fomation of the chamber itself. As these were eruptedand the chamber flushed with fresh magmas, thdegree of contamination would tend to decreasewith time. It would even be possible to have single magma chamber which experienced two diferent periods of growth or disruption and thusampled more than one type of wallrock. Althoughat the limit of resolution of our measurements,there is some scatter in the eJLIV(T) and initia(sSr/ %lr) values of the least contaminated flowfrom Mahabaleshwar. One explanation for thcould be that their parental magmas experiencedsmall amounts of contamination from both eriched sources; thus MB81-10 and 14 would rcord the earliest assimilation of the group 2 eriched endmember which had become the domnant contaminant by the time of flow MB81-17.4.4. Mantle endmember

So far in our discussion little attention has beepaid to the depleted mantle source. From the M9data, it must have possessed eJuv 2 +8 an(Sr/ Sr) 4 0.7039. Although such values atypical of some island arcs, the tectonic conditionsin India at the time of Deccan activity were clearlof an extensional or rifting nature with no knowactive subduction zones nearby. Such a sourcthus seems out of the question. In fact, the Indianplate at that time was racing away from Africand Madagascar at as much as 18-35 cm/yr[36,37]. Some oceanic island basalts also have apropriate ~~~(2) values but none are so high ( 67Sr/86Sr), as the least contaminated samplefrom Mahabaleshwar. We may then postulate depleted source in the subcontinental lithospherewhich was enriched at some ancient date in R


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and other LILs, perhaps by an episode of deepmetasomatism. If so it must later have been de-pleted again, because the K and Rb contents ofthe least contaminated Mahabaleshwar lavas areexceedingly small, and their Ba/Zr and K/Rbratios very similar to those of MORB. In line withthis Jordan [ 1 l] has pointed out that for geophysi-cal reasons the subcontinental lithosphere must beeven more depleted than MORB-type mantle in itsmajor basaltic components and thus would beexpected to have high E,,, and low ( 87Sr/86Sr)ratios.

It is also conceivable that even M94 may besomewhat contaminated, with the ultimate mantlesource having a yet higher zJuv. In this context itis interesting to note that measured samples ofIndian Ocean floor basalts, even relatively unal-tered glasses, have anomalously high ( s7Sr/86Sr),values, ranging from 0.7028 to 0.7035 [ 14,381, Thishas been illustrated in Figs. 3 and 9. Norton andSclater [36] have argued that there was a largejump in the Indian Ocean spreading axis at thetime of major Deccan activity (- 65 m.y.) whichsundered the Seychelles Platform from the edge ofthe continent and formed the Chagos-LaccadiveTransform Fault. Therefore, based on the availa-ble tectonic, isotopic, and trace element evidence itseems possible that the source feeding the Deccanmagmas at Mahabaleshwar could have been anupwelling diapir of suboceanic mantle associatedwith the landward ridge jump. In their work onthe North Atlantic Tertiary Province, Carter et al.[39] arrived at a very similar conclusion. It is thenwithin the realm of possibility, at least for group 1rocks, that the subcontinental mantle may nothave been involved at all in their genesis.

Alltgre et al. [43] have recently argued, prim-arily on the basis of limited lead isotopic evidence,that a MORB-type mantle could not be responsi-ble for the Deccan lavas. However, they fail toconsider the complexities of contamination involv- .ing more than one type of old continental crust.Several of their samples appear to belong to ourgroup 2. For these at least, contamination of aMORB endmember with ancient Lewisiantypecrust can explain the observed Pb isotope values,which plot close to but slightly off the MORBarray.

5. Summary and conclusionsThe Nd and Sr isotopic ratios of the flows

Mahabaleshwar describe two correlated trendwhich correspond to lower and upper stratigraphicgroups. These groups are also separable on tbasis of trace element characteristics and majoelement composition. The isotopic and trace ement data are best explained as the result mixing of magma from a depleted mantle (e,v +S) with varying amounts of material from twdifferent enriched sources. One was probably Archean or early Proterozoic crustal rock wivery negative e,uv and moderate to hig( 87Sr/s6Sr). The second may have been either Precambrian granulitic rock with very negativzJuv and correspondingly low (*Sr/ 86Sr), or somtype of enriched mantle material with trace ement contents similar to alkali basalts. There no evidence at Mahabaleshwar for any primordialchondritic source. High degrees of fractionationoccurred for most lavas, but simultaneous frationation and assimilation of a single enrichedendmember could not have generated the two istopic trends, although combined fractionation anassimilation appear likely. The operation of a dnamic magma system appears necessary. Finallythe depleted mantle source region was anomalously high in (Sr/*%r). Either subcontinentalmantle with a complex history, or an Indian OceaMORB-type of mantle associated with a landwardridge jump around 65 m.y. ago are plausiblecandidates for this component.

AcknowledgementsWe are grateful to Dr. S.F. Sethna of St. Xavier

College in Bombay, who provided some of thsamples, and to James Holbrook, who assisted chemical separations for isotopic analysis. Peoplat B.A.R.C. and the Tata Institute in Bombay, anat P.R.L. in Ahmedabad helped in organizingfieldwork. We thank reviewers K.G. Cox and G.GGoles for their helpful comments. This researchwas supported by NSF grants INT-8105600 anEAR-8009231. S.I.O.-I.G.L. Contribution No. 4.


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Documents

Origin of Decan Lava Nd and Sr Isotopic and Chemical Evidence