NdSr isotopes, petrochemistry, and origin of the Siberian flood basalts, USSR

Gmchrmrca d Cosmochmica Acta Vol. 55, pp. 1183-l 192 Copyright 0 1991 Pergamon Press pk. Printed in U.S.A.

0016-7037/91/$3.00 + .I0

LETTER

Nd-Sr isotopes, petrochemistry, and origin of the Siberian flood basalts, USSR

MUKUL SHARMA,’ ASISH R. BASU,’ and G. V. NESTERENKO*

‘Department of Geological Sciences, University of Rochester, Rochester, NY 14627, USA *Vernadsky Institute of Geochemistry, Academy of Sciences, USSR, Kosygina 19, Moscow I 17075, USSR

(Received October 3 1, 1990; accepted in revisedform February 20, 199 1)

Abstract-The Siberian Flood Basalt Province (SFBP) of Permo-Triassic age is one of the largest flood basalt provinces with an estimated area of exposure of 337,000 km*, average thickness of 1 km, and a magma volume of 337,000 km3. Forty-seven basaltic rocks from two main subprovinces, Norilsk (5- 10% of area, thickness up to 3 km) and Putorana (90-95% of area, thickness of more than 2 km), were selected, on the basis of petrography and volcano-stratigraphic relation, for major-element analysis. Twenty- six of these basalts, twelve from Norilsk and fourteen from Putorana, were analyzed for Nd- and Sr- isotopic compositions. The Norilsk and Putorana basalts show some contrasting behavior in terms of the ratios of the highly incompatible elements of Ti, P, and K as a function of their Mg*. The Norilsk basalts are more variable, suggesting the role of fractional crystallization-assimilation in their evolution. In contrast, the Putorana basalts show remarkable uniformity in their bulk chemical compositions.

There is no overall apparent correlation between the Mg* and the initial tNd for the basalts. Most of the Putorana and some Norilsk lavas show a narrow range of initial tNd between 0 and $2.5 at Mg* = 0.53, suggesting that the bulk of the SFBP was derived from a mantle-source with near-chondritic Nd- isotopic signature. Our data and six previously published analyses define a hyperbolic array near the mantle correlation line in 87Sr/86Sr(t)-t,d(t) space. However, the majority (70%) of the volumetrically significant Putorana basalts fall in the ranges of 0 to $2.5 and 0.7046 to 0.7052 for the initial tNd and 87Sr/86Sr, respectively. In Nd- and Sr-isotopic space, most of the Siberian basalts also fall within the field defined by the ocean island basalts data, implying common mantle sources. It is concluded that the SFBP originated by hotspot volcanism due to the rise of a large and relatively primitive lower mantle-derived plume beneath the Siberian continent.

INTRODUCTION

CONTINENTAL FLOOD BASALT volcanism has received con-

siderable attention in recent years from geophysicists and geochemists (e.g., MORGAN, 198 1; WHITE and MCKENZIE, 1989; MACDOUGALL, 1988). It is generally believed that, at least for the Deccan of India, the Karoo of southern Africa,

and the Parana of South America, flood basalt effusions mark the initiation of subcontinental hotspots, followed by rifting and continental fragmentation. Flood basalt volcanism is

characterized by voluminous accumulations of relatively uniform basaltic magmas, which cover large areas (0.2 X 1 O6 to 2 X lo6 km2). For many of the flood basalt provinces, which are younger than 250 Ma, there is a large body of Nd, Sr, Pb, and to a lesser extent O-isotopic data. These include the Columbia River (NELSON, 1980; CARLSON et al., 198 1; CARLSON, 1984), the Deccan (MAHONEY et al., 1985; MA- HONEY, 1988), the Parana (HAWKESWORTH et al., 1986), and

the Karoo (HAWKESWORTH et al., 1984). The correlated Sr- Nd isotopic trends for both the Deccan and Columbia River basalts have been interpreted as resulting from variable amounts of assimilation of continental crust by heterogeneous mantle-derived magmas (e.g., MAHONEY, 1988; CARLSON et al., 1981). The Parana basalts of Brazil and the Karoo volcanic rocks from southern Africa have been suggested to be derived from heterogeneous lithospheric sources within the sub-con-

tinental mantle (HAWKESWORTH et al., 1984, 1986). DE-

PAOLO ( 1988), however, has maintained that a mantle source having primitive characteristics with respect to the Nd and

Sr isotopes was an important source in both Karoo and Co- lumbia River basalts, which subsequently mixed with other mantle and crustal reservoirs.

In this study, we report petrological and geochemical vari-

ations, the Nd- and Sr-isotopic compositions, and the Rb,

Sr concentrations through two major volcano-stratigraphic sections of the Siberian Flood Basalt Province (SFBP), USSR.

The purpose of this study is to document spatial and temporal geochemical variations in the SFBP and to address the important questions of petrogenesis, particularly with respect to the nature of mixing among the various mantle reservoirs and crustal components in response to a subcontinental hotspot activity.

GEOLOGIC AND VOLCANO-STRATIGRAPHIC RELATIONS OF NORILSK AND PUTORANA BASALTS

The diabase and basaltic traps of the Siberian platform in the USSR constitute one of the largest continental flood basalt provinces in the world. Originally, these volcanic rocks are believed to have covered an area of 1.5 X lo6 km* with an average thickness of 1 km. At present the extrusive basaltic rocks are estimated to occupy an area of 337,000 km2 with

1183

1184 M. Sharma. A. R. Basu, and G. V. Nesterenko

an average thickness of 1 km and hence a magma volume

of some 337,000 km3 (LURIE and MASAITIS, 1964). These

volcanic rocks occur principally in the great Tunguska basin east of the Enisei River (Fig. 1) where they erupted through the Archean-Proterozoic Siberian Platform. K-Ar age deter-

minations indicate that the volcanism began around 240 Ma and reached its peak around 235 Ma (ZOLOTUKHIN and

AL’MUKHAMEDOV, 1988); preliminary results of an “Ar-39Ar

age-spectra study (BAKSI, 1990) suggest that the Siberian Traps erupted during a relatively short period around 230- 235 my. The SFBP can be broadly divided into three regions

of fundamentally different lava sections. The largest of these

regions is the Putorana subprovince with two smaller sections,

the Meimecha-Kotui and the No&k-Kharelakh in the

northeast and northwest, respectively (Fig. 1). The Putorana

lavas constitute 90 to 95% of the bulk of the SFBP. The great majority of the Putorana rocks are relatively homogeneous,

aphyric and polyphyric tholeiitic basalts. The volcanic rocks from Meimecha-Kotui subprovince cover about 2% of the

total SFBP and are similar to Putorana basalts. The Norilsk basalts represent about 5 to 10% of the entire volume of the

SFBP; the volcanism in the Norilsk region is marked by widely varying rock types from picritic through tholeiitic to subalkaline basalts and basaltic andesites. Felsic rocks are con-

spicuously absent in the SFBP. For the purpose of the present study, we have concentrated our attention, principally, to the flood basalts of the Putorana and the Norilsk regions.

Areas A, B, C, D, and E in Fig. 1 represent the locations

of the samples selected for this study. The relative volcano- stratigraphic positions of these samples are given in Fig. 2,

which also demonstrates the stratigraphic correlation between the Norilsk and the Putorana regions (ZOLOTUKHIN et al.,

1986; SADOVNIKOV, 1981). Areas A, B, C, and D in Fig. 1

84’ 90’ 96O 102O 108 o 72 ’

FIG. 1. Schematic map of Permian-Triassic basaltic rocks from the Siberian Platform (modified from ZOLOTUKHINAND AL'MUKHAMEDOV, 1988): 1. basaltic trap: 2. intrusive trap and t&s; 3. platform sedimentary sequence. The dashed and solid lines refer to the boundaries between volcanic provinces. Samples analyzed are from areas A-E: A) 1-8; B) 9-20; C) 2 l-27; D. 28-3 1; E) 32-47.

are the locations of the Putorana-type basal& including sam-

ples from the stratigraphically measured sections on Ayan

Lake, Ayan River, Sobach’e Lake, and Kotui River. The Pu- torana samples include members of all three major suites and are believed to represent a maximum thickness of 1800

m of basalts. Area E in Fig. 1 is the location of the stratigraphic section at the Gluboke Lake for the Norilsk-type basalts. The

basalts from the Gluboke Lake stratigraphically underlie those

from the Putorana region. We also analyzed three samples

from the Morongovsky suite of the Norilsk subprovince, which are correlated with the Ayansky suite of the Putorana

region (Fig. 2). We have studied a total of 47 basaltic rocks, of which 3 1 are from Putorana and 16 from Norilsk (Table

1). All these rocks were remarkably fresh with only a few

samples showing minor amount, usually less than l%, of

chlorites.

ANALYTICAL METHODS

Major-element compositions of the 47 Siberian basalts were de- termined at the Vernadsky Institute of Geochemistry, Moscow, by X-ray fluorescence (XRF) spectrometry on fused glasses. Twenty-six of these basalts were analyzed for Sr- and Nd-isotopic compositions, and Rb, Sr concentrations by the isotope dilution method at the University of Rochester. For the isotopic analysis, about 500 g of each rock sample was crushed into gravel-size bits using a stainless steel hammer. A coarse fraction ofabout 40 g was obtained by coning and quartering. The splits were then washed in deionized water, dried and crushed to -400 mesh size in a Rock Labs Zirconia Ball Mill. Special precautions in cleaning the Ball Mill. including scrubbing with high purity silica sand followed by crushing a dummy sample. were taken to avoid cross-sample contamination. Approximately 100 mg of the crushed sample was dissolved for the separation of Sr and Nd for isotopic analyses. Another portion of the pulverized sample (20-30 mg) was mixed with a “Rb-%r tracer solution and dissolved for Rb and Sr concentration measurements. Details of clean-laboratory procedures followed at the University of Rochester Isotope Laboratory are given in BASU et al. (1990). The Rb-Sr and Nd-isotopic measurements were conducted with a VG Sector Mass Spectrometer equipped with five-collectors using procedures described by BASU et al. (1990). Procedural blanks for Rb, Sr. and Nd are 50, 200, and 150 pg, respectively.

PETROLOGY, MAJOR ELEMENTS, AND Sr-, Nd- ISOTOPIC GEOCHEMISTRY

The major-element compositions of the Siberian basalts

are presented in Table 1 along with a petrographic note de-

scribing each sample. Table 1 also shows a few selected petrogenetic parameters, including Mg*, K20/P205, and TiOz/ PzOs. The Rb and Sr concentrations and the Sr- and Nd-

isotopic ratios are given in Table 2. These data are plotted

in Figs. 3 through 6 to characterize the petrological and geochemical variations throughout the stratigraphic horizons of

the SFBP. The total alkali versus silica diagram (Fig. 3) demonstrates that some of the Norilsk group of rocks are more evolved than the Putorana basalts. The Norilsk rock types

range from picrobasalts through tholeiitic basalts to subalkaline basalts and basaltic andesite. All the Putorana samples, except one (# 19), plot in the basalt field. The major-element data presented here (Table 1) and by others (NESTERENKO,

1984; ZOLOTUKHIN and AL’MUKHAMEDOV, 1988; LIGHT-

FOOT et al., 1990; NESTERENKO et al., 1991) demonstrate that the bulk of the Siberian flood basalts fall on the classical Hawaiian tholeiite differentiation trend. To evaluate the pos- sible role of combined wall-rock assimilation and crystal

Petrogenesis of Siberian flood basalt 1185

NORILSK REGION PUTORANA REGION

cd O-200

t+i P2 sv O-240

1’ I/

1’ /’

--__ -_

\ \ \ \ ‘\ \ ‘\ ’ ‘\ \ ’ ‘\ \ \ \ \ \ \ \

1

3 a=

6

8.8s

23

ga

7

8

10

8.11.24

D

fa 26

m

14,z7,94

16

it7ff

18.f8.B.po

FIG. 2. Composite volcano-stratigraphic section of the Norilsk and Putorana regions showing stratigraphic positions of the analyzed samples in italics. Boundaries between the volcanic suites are tentative, based on field and textural criteria. Letters in the age column refer to P for Permian and Tl and T2 for Triassic.

fractionation (AFC) processes in the evolution of the Siberian

flood basal&, we have plotted in Fig. 4 the ratios of the highly incompatible elements K20/P205, TiOJP205, and tNd(t) against Mg*, where the last two parameters are as defined in

Tables 1 and 2. In general, most of the Siberian basalts (except #39 and #42 in Table 1) reflect some degree of fractional crystallization from “primary” melts, as indicated by their low Mg* (0.63-0.33; median = 0.5 l-0.53). An examination of Fig. 4a-c, however, indicates that the Putorana basalts are more uniform than the Norilsk lavas; the latter show wide variations in incompatible major-element ratios and ++,(t) values with respect to Mg*. We discuss below the petrogenetic implications for each of these figures separately.

Figure 4a shows that a majority of the Putorana basalts fall in a cluster with a K20/Pz05 ratio of about 2 and Mg* of around 0.53. Most mantle-derived magmas have K20/ P205 ratio of 2 or less (e.g., BASALTIC VOLCANISM STUDY

PROJECT, 198 1). Crustal assimilation and/or apatite fractionation can result in elevated K20/P205 ratios, as observed in the Columbia River Basalts of the northwestern United

States (CARLSON and HART, 1988). The low K20/P205 ratios combined with a high Mg* of the Putorana basalts suggest

that they did not evolve to a great extent by the AFC processes. In contrast, the Norilsk basalts show uniformly high KzO/ P205 ratios ( 15 out of 16 samples have K20/P205 ratios > 2) for a wide variation in Mg*, indicative of the AFC processes in causing the variation. These observations are also supported by Fig. 4c where a majority of the Putorana basalts show positive cNd values without any correlation with the Mg*; in contrast, most of the Norilsk lavas display negative tNd values and a crude positive correlation with the Mg*.

Ti02/P205 ratios of Ocean Island Basalts (OIBs) and Mid- Ocean Ridge Basalts (MORBs) are generally between 6 and 12 (BASALTIC VOLCANISM STUDY PROJECT, 1981), similar

1186 M. Sharma, A. R. Basu, and G. V. Nesterenko

Table 1. Major element analyses of basaltic rocks from regions of the Siberian Flood Basalt Province. Note refers to (1) aphyric tholeiite basalt, (2) PI-porphyric

tholeiite basalt, (3) PI-glomerophorphyric tholeiite basalt, (4) aphyric poikilophitic basalt, (5) PI-porphyric poikilophitic basalt, (6) PI-glomeroporphyritic poikilophitic

basalt, (7) polyphyric basalt, (8) picrite basalt (tholeiitic), (9) picrite basalt (subalkaline), and (10) subalkaline basalt. The Mg* refers to MgO/(MgO + FeO) in moles,

assuming Fe0 is 0.85 of FeO* in this table.

Sample Suite

NO.

SiO, TiO, A’203 FeO* MnO M@ cso Na20 K,O PZ% SUM Mg” KzO~2’35 Ti02/P20s Note

PUTORANA UASALTS: SECTION ON AYAN LAKE

1 Nr 48.53 1.32 15.11 12.48 0.20 7.35 11.50 1.86 0.12 0.11 98.58 0.55 1.09 12.00

2 Nr 47.96 I .28 15.42 11.81 0.20 7.74 10.63 1.73 0.66 0.09 97.52 0.54 7.33 14.22

3 NI 48.91 I .42 15.10 12.13 0.20 7.28 11.22 1.98 0.31 0.13 98.68 0.52 2.38 10.92

4 Nr 48.12 I .60 15.09 12.59 0.20 6.25 11.42 2.16 0.36 0.17 97.96 0.47 2.12 9.41

5 NT 48.12 1.30 15.25 11.72 0.20 7.60 10.59 2.02 0.44 0.13 97.37 0.54 3.38 10.00

6 Nr 50.21 1.34 15.23 12.23 0.20 7.72 11.04 2.04 0.38 0.11 100.5 0.53 3.45 12.18

I HI11 49.36 1.25 15.06 II.88 0.20 7.47 11.38 1.82 0.20 0.10 98.72 0.53 2.00 12.50

8 Hill 48.74 1.29 15.46 II.92 0.21 6.96 11.11 1.98 0.46 0.11 98.24 0.5 1 4.18 11.73

SECTION ON AYAN RNER

9 Ho1 48.72 1.30 15.32 11.79 0.19 7.44

10 Mm 48.67 1.20 15.02 11.75 0.20 7.08

II Hm 49.8 I 1.32 15.27 12.34 0.19 7.13

12 An so.49 1.07 15.56 I 1 .oo 0.18 7.48

13 An 48.04 I .39 14.70 12.84 0.20 7.04

14 An 48.72 I .22 14.97 II.65 0.20 7.09

15 An 49.55 1.08 15.44 11.38 0.18 7.33

16 All 49 00 1.14 15.17 11.60 0.18 7.66

17 An 50.20 1.06 15.28 I I .27 0.18 7.15

18 All 48.12 I .06 15.46 12.30 0.18 7.73

19 All 44.39 4.30 8.92 13.64 0.18 13.11 20 All 48.43 1.17 15.55 I I.15 0.18 7.08

10.98 1.90 0.25 0.11 98.00

11.68 1.94 0.20 0.11 97.85

10.95 2.01 0.35 0.11 99.48

12.02 I .72 0.20 0.09 99.81

I I .40 2.18 0.22 0.15 98.16

12.97 I .80 0.13 0.11 98.86

11.95 1.83 0.14 0.10 98.98

11.50 1.91 0.19 0.11 98.46

II.80 1.85 0.25 0.09 99.13

11.51 1.69 0.29 0.10 98.44

9.94 1.47 1.46 0.42 97.83

12.39 I .69 0.20 0.10 97.94

SECTION ON SOBACH’E LAKE

0.53 2.27 11.82

0.52 1.82 10.91

0.5 I 3.18 12.00

0.55 2.22 11.89

0.48 1.47 9.27

0.52 1.18 11.09

0.53 1.40 10.80

0.54 1.73 10.36

0.53 2.78 11.78

0.53 2.90 10.60

0.63 3.48 10.24

0.53 2.00 11.70

?I NI 48 58 I.52 I5.3R 12.45 0.20 5.x’)

22 Nr 49.10 1 so 15.10 12.78 0.20 6.70

23 Hm 48.19 1.37 15.54 12.43 0.19 7.06 24 Hill 48.68 1.30 15.24 12.26 0.20 6.73

25 An 46.92 1.15 15.07 11.14 0.20 7.35 26 An 48.26 1.08 15.66 11.32 0.23 7.42 27 An 48.46 1.13 15.00 12.26 0.19 6.96

i(l.92 2.12 0.35 0.19 97.60

10.70 2.08 0.54 0.19 98.89

10.92 I .97 0.32 0.15 98.14

11.07 2.24 0.38 0.15 98.25

11.47 1.94 0.22 0.15 95.61

11.20 1.76 0.38 0.15 97.46

11.50 1.89 0.24 0.12 97.75

SECTION ON KOTUI RNER

0.4b I .x4 8 .OO

0.48 2.84 7.89

0.50 2.13 9.13

0.49 2.53 8.67

0.54 1.47 7.67

0.54 2.53 7.20

0.50 2.00 9.42

28 Nr 49.15 I .32 15.52 11.60 0.18 7.57 10.68 1.86 0.44 0.14 98.46 0.54 3.14 9.43

29 Hm 48.41 1.37 15.52 12.01 0.19 7.49 10.84 2.11 0.42 0.14 98.50 0.53 3.00 9.79 30 An 49.33 1.12 15.25 11.26 0.18 7.87 11.75 1.69 0.20 0.13 98.78 0.55 1.54 8.62 31 Ao 50.35 0.98 14.88 10.20 0.16 7.46 11.78 1.70 0.55 0.12 98.18 0.57 4.58 8.17

NORILSK TYPE BASALT%

32 MO 47.68 1.19 15.35 12.09 0.20 7.22

33 MO 48.64 1.17 15.11 11.51 0.19 7.24

34 MO 49.87 I .34 13.89 12.02 0.23 6.62 35 Nd 48.69 1.19 14.65 11.44 0.20 7.14 36 Nd 51.30 1.18 14.68 11.94 0.18 5.88 37 Nd 50.05 1.33 14.23 12.01 0.21 6.35 38 Nd 53.66 1.10 15.06 10.97 0.17 6.00 39 TU 46.12 0.40 18.62 8.58 0.12 10.75 40 TU 47.66 0.95 14.69 10.98 0.17 9.01 41 TU 48.24 1.06 15.25 10.32 0.21 7.67 42 Gd 44.38 1.68 7.81 14.00 0.19 18.26 43 SV 51.80 1.73 15.13 9.84 0.15 6.21 44 sv 54.40 1.63 14.11 10.10 0.15 5.73 45 sv 55.72 1 .b6 16.33 8.78 0.12 4.31 46 Iv 5 I .95 2.46 14.5 I 12.40 0.19 3.44 47 Iv 47.33 2.39 15.90 13.55 0.19 5.30 8.83 2.94 1.13 0.37 97.93 041

SECTION ON GLUBOKE LAKE

11.14 1.97 0.42 0.14 97.40

11.17 1.86 0.37 0.15 97.41

11.07 2.10 0.72 0.16 98.02

11.00 2.03 0.69 0.14 97.17

9.99 2.36 1.03 0.16 98.70

11.07 2.10 0.78 0.17 98.30

9.90 2.34 1.40 0.12 100.7

10.84 1.40 0.23 0.06 97.12

11.54 1.56 0.22 0.09 96.87

12.05 1 .b6 0.25 0.10 96.81

7.40 0.89 0.12 0.13 94.86

6.37 2.53 3.74 0.24 97.74

6.31 2.90 2.62 0.24 98.19

8.75 2.47 0 93 0.23 99.30

7.15 3.04 2.40 0.80 98.34

0.52

0.53

0.50

0.53

0.47

0.49

0.49

0.69

0.59

0.57

0.70

0.53

0.50

0.47 0.33

3.00 8.50

2.47 7.80

4.50 8.38

4.93 8.50

6.44 7.38

4.59 7.82

11.67 9.17

3.83 6.67 2.44 10.56

2.50 10.60 0.92 12.92

15.58 7.21

10.92 6.79

4.04 7.22 3.00 3.08 3.05 6.46

5 4

2

3

1

1

1

3

4

1

3

I

1

1

5

1

1

4

9

4

s

3

4

3

4

1

1

4

1

2

1

1

4

2

1

1

2

7

8

4

4

8

1

10

1 10 10


Table 2. Rb, Sr concentrations and the isotopic compositions of Sr and Nd and the Mg* in 26 selected basalts of the SFBP.

Sample Mg* Rb Sr *‘Rb/=& (87~r/s6~r) a m (s’srWr) b (‘43Nd/‘44Nd) c m E,,(t) d No.

1 0.55 (PPm) 1.93

(ppm) 153.7 0.036 0.704877 * 28 0.704758 0.512737 * 12 +2.52

2 0.54 3 0.52 6 0.53 7 0.53 8 0.51 11 0.5 1

15 0.53

16 0.54 17 0.53

18 0.53

19 0.63 20 0.53

31 0.57

32 0.52 33 0.53

35 0.53

37 0.49

38 0.49 39 0.69 41 0.57 42 0.70 44 0.50

45 0.47

46 0.33 47 0.41

10.17 176.1 0.167 3.65 168.9 0.063 5.72 171.3 0.097 2.97 195.3 0.044 7.94 215.6 0.107

4.66 193.9 0.069

4.86 179.3 0.078

1.88 178.1 0.031 _- __

3.17 191.2 24.09 631.5

5.26 204.9

7.89 229.3

5.88 207.9

3.69 202.7

14.29 216.9 21.61 200.6 41.55 233.9

4.33 272.3 9.38 224.8 3.16 221.2

77.12 601.3 12.89 525.0 47.24 397.5 18.89 452.1

__

0.048 0.110 0.074

0.010 0.082 0.053

0.191 0.312 0.514 0.046 0.121 0.041 0.371 0.071 0.344 0.121

0.705509 z+ 28 0.704961 0.512727 f 12 +2.33

0.704831 f 37 0.704626 0.512714 * 12 +2.07 0.705411 zt 28 0.705095 0.512634 * 12 co.51

0.705232 + 38 0.705088 0.512677 + 12 +1.35

0.705568 f 28 0.705219 0.512658 I 29 +0.98 0.705338 *40 0.705111 0.512632+12 +0.47

0.705443 * 28 0.705186 0.512734 i 12 +2.46

0.705057 * 28 0.704967 0.512688 * 14 +1.57

0.706311 *28 0.706053# 0.512483 f 12 -2.43

0.705292 * 28 0.705135 0.512669 * 15 +1.19

0.704415 * 34 0.704053 0.512620 + 12 10.24

0.706028 * 28 0.705785 0.512509 f 12 -1.93

0.707953 * 33 0.707627 0.512252 * 30 -6.94

0.705397 * 28 0.705129 0.512690 i 12 +1.60

0.705249 * 37 0.705077 0.512691 + 12 11.62

0.707106 * 28 0.706482 0.512394 * 12 -4.17

0.706840 * 31 0.705819 0.512557 f 12 -0.99

0.710468 f 31 0.708786 0.512113 * 12 -9.65 0.705764 * 30 0.705613 0.512527 f 12 -1.58 0.705815 f 28 0.705420 0.512596 t 13 -0.23 0.703381 f 28 0.703246 0.512885 f 12 +5.41

0.706660 f 30 0.705445 0.512356 * 12 -4.91

0.705284 + 28 0.705051 0.512384 i 12 -4.37 0.707171 * 28 0.706045 0.512339 f 12 -5.24

0.705624 f 28 0.705228 0.512465 zt 12 -2.78

Reported errors are 20 of the mean. Uncertainties in Sr are less than 0.1%. and for Rb and *7Rb/*6Sr they are less

than 1%.

d measured ratio corrected for mass fractionation effects using s6sr/s4sr = 0.1194 NBS 987 Sr standard analyzed

dunng the course of this study gave 87SriB6Sr = 0.710235 ~28 (20).

b age corrected 87Sr/86Sr ratio assuming that the rocks crystallized at 235 Ma

c measured ‘43Nd/‘44Nd ratio corrected for mass fractionation using 146Nd/144Nd ratio of 0.7219. La Jolla Nd

Standard analyzed during this study yielded 143Nd/‘44Nd = 0.511854 ~12 (20).

d refers to 0.1 mil deviations from Chondritic Uniform Reservoir ( 143Nd/‘44Nd = 0.512638). age corrected using

E,,(t) = ~N~(0)-t.Q,+,,&fl~ Here, QNd = 25.10 Ga-‘; the enrichment factorfsfld is assumed to be -0.1 and

t = 0.235 Ga

# calculated by assuming 87Rb/86Sr ratio of 0.079 (average of other thirteen Putoraoa basalts).

SiO, ~1%

FIG. 3. Total alkali-silica diagram for the Siberian basalts. Rock fields are after LEBAS et al. (1986). Notice that most Putorana basalts plot in a tight cluster: the Nor&k-type basalts seem to be more dif- ferentiated.

to those of continental crustal rocks. It follows that crustal assimilation will cause a gradual decrease in Mg* accom- panied by a change in Ti02/P205 ratios, which may (i) increase following removal of apatite from the melt or (ii) de-

crease following fractionation of Fe-Ti oxides or (iii) decrease following fractional crystallization of olivine and pyroxenes because Ti is more compatible than P. As shown in Fig. 4b, the variation in Ti02/P205 in the Putorana basalts is essentially independent of Mg*, similar to that displayed by OIB and MORB. The Norilsk basalts, however, show a weakly positive correlation possibly reflecting AFC processes. TiOz,

P205, and Mg* are negatively correlated for the Norilsk lavas (Table I), implying that the weakly positive trend in Fig. 4b

may be due to fractional crystallization of olivine and py-

roxenes as in case (iii) above. Negative tNd values for the Norilsk basalts also require both assimilation and fractional crystallization.

We have plotted tprld(t) against Mg* in Fig. 4c for the 26 Siberian basalts analyzed (Table 2) in this study. We estimated tNd(t) by assuming an age of crystallization of 235 Ma and an enrichment factor, fSmlNd of -0.1 (Table 2), a number


16

14 (a)

n Putorana q Norilsk

with a majority of these rocks displaying negative t&t) values, indicating that the Norilsk lavas have evolved by incorpo- rating significant amounts of continental crust.

12

O" 10 CL"

0‘6

y" 6

14

12

0" 10 I?

2 6 I-

6

4

2

0

(b)

From the above considerations of the major-element and Nd-isotopic data, it is clear that, compared to the Norilsk lavas, crustal contamination accompanying crystal fractionation was minimal in the evolution of the Putorana basalts. It is important to note that the Putorana basalts erupted after the initial eruptions of the Norilsk-type lavas (Fig. 2) during the peak of the Siberian flood basalt volcanism (ZOLOTUKHIN et al., 1986). It is also interesting that the No&k and the Putorana subprovinces show strongly contrasting modes of eruption. The initial stages of volcanism in Norilsk are marked by central-type eruptions with short, irregular, cas- cade-like flows fed by fairly small magma chambers where AFC processes could have been operational. The Putorana basalts, in contrast, show evidence of fissure-type eruption with subaerial outpourings of large volumes of melts in reg- ular, subhorizontal flows; for example, one single Nadayanski flow of the Ayanski suite occupies an area of 270 X 250 km with a maximum thickness of more than 100 m. Thus, in the Putorana-type of eruptions, the magma volumes were relatively large, homogeneous, with quick subaerial eruptions, inhibiting AFC processes.

4

E 0

P w

-4

-0

-12

(4

0

We have observed that the volumetrically less significant Norilsk basalts may have evolved through AFC processes. We have also observed that the Putorana lavas comprising 90 to 95% ofthe bulk ofthe SFBP evolved mostly by fractional crystallization with insignificant amounts of crustal assimilation. To quantitatively evaluate these observations, we have plotted in Fig. 5 the (s7Sr/86Sr), ratios against Sr concentrations of the Siberian basalts. We have also used the AFC equations of DEPAOLO ( 198 1) to model the evolution of these basalts. From theoretical considerations, it has been shown that for given Sr isotopic compositions and Sr concentrations of initial magma and contaminant, different trajectories in Fig. 5 should reflect different bulk distribution coefficients (Dsr) of Sr, and different rate ratios of assimilation to fractionation, r (DEPAOLO, 198 1). With respect to the Siberian basalt data, the following observations can be made. A large number of samples, mostly from the Putorana region, cluster around (87Sr/86Sr)I values of 0.7045 and 0.7052 and Sr concentrations of 150 and 220 ppm. This clustering of Putorana basalts, even with the hint of a slight positive trend, is consistent with our conclusion that the Putorana lavas have evolved mainly by crystal fractionation. The Norilsk basalts and two Putorana basalts with CNd(t) < 0, on the other hand. define two arrays in Fig. 5, again conforming to our previous conclusion that the AFC processes were important in the evolution of these rocks.

0.20 0.30 0.40 0.60 0.60 0.70 0.60

w

FIG. 4. Plot showing variation of K20/P205, Ti02/P205 and C.&t) against the Mg* in Siberian basalts.

established from the six previously published analyses of the Siberian basalts (DEPAOLO and WASSERBURG, 1979a; SMITH and SCHMITT, 1981). A few unusual rock types, especially the picrobasalts (samples # 19 and #42), may have more frac- tionated REE patterns and an enrichment factor as low as -0.4. A lower enrichment factor of -0.4 could shift the CNd(t) values of these rocks by as much as +2.4 units. However, this shift would only strengthen our conclusions concerning the petrogensis of these two rocks. The data in Fig. 4c show that for Putorana lavas there is no apparent correlation between Mg* and eNd; also, only three of the sixteen Putorana samples show t&t) values less than zero. These observations indicate that the Putorana lavas have not evolved by assimilating a substantial amount of the Archean-Proterozoic crust of the Siberian Platform. In contrast, the tNd(t) values of the Norilsk basalts show a crude positive correlation with Mg*

For calculating the AFC curves in Fig. 5, we assumed (“Sr/ “Sr), = 0.7047 and [Sr] = 180 ppm for the basaltic precursor and (%r/?3r), = 0.7200 and [Sr] = 400 ppm for the Archean- Proterozoic Siberian continental crust as the contaminant. We have made no attempt in our AFC calculations to model the evolution of individual basalts; each basalt probably evolved along a unique AFC path. More importantly, the coherence of the basalt data and the calculated curves indicates that these individual paths were grossly similar (Fig. 5). The solid AFC curve was calculated for Dsr = 0.01 and r

Petrogenesis of Siberian flood basalt

0.708

1189

q #42

n #I9

0.702 /

0 100 200 300 400 600 600 700

Sr Concentration (ppm)

FIG. 5. (87Sr/86Sr)I plotted against Sr concentration, with model AFC curves calculated after DEPAOLO (198 1). The clustering of Putorana basalts in this plot implies their evolution without any significant assimilation of continental crust. The Norilsk basalts evolved by assimilating as much as 12% crustal Sr. The lower tick marks along the curves are values of F, the ratio of final magma mass to initial magma mass. The upper tick marks along the curves are values of Ma/Mm’, the ratio of mass assimilated to the initial magma mass. See text for discussion.

= 0.05. The No&k-type basalts that follow this curve probably evolved by crystallizing olivine and pyroxenes. Notice that a low r value suggests that these basaltic magmas en- countered cold country rocks. Calculations of Ma/Mm’, the ratio of mass assimilated to the initial mass of the magma (FARMER and DEPAOLO, 1983) suggest that the Norilsk basalts assimilated at the most 3.6% crustal Sr. Such low values of Ma/Mm’ would not generally be noticeable in the major element chemistry, but elements such as K appear to be sen- sitive indicators of crustal contamination, as seen in Fig. 4a. The dashed AFC curve in Fig. 5 is calculated for Dsr = 1 and r = 0.2. The basaltic magmas following this curve evolved dominantly by crystallizing plagioclase with subordinate amounts of olivine and pyroxenes. A higher r value for this curve implies that the magmas interacted with hot continental crust. Calculated Ma/Mm’ ratios fall between 0.02 and 0.12, suggesting that at the most 12% crustal Sr was assimilated by the most contaminated Norilsk magma.

Two samples of picrobasalts (# 19 and #42), shown in Fig. 5, do not follow the calculated AFC curves. Sample #42 has rNd(t) = +5.4 and (87Sr/86Sr)r = 0.7032, suggesting that it is not genetically related to other basalts. Moreover, the isotopic signatures imply that depleted mantle was tapped during Si- berian flood basalt eruption. The rare occurrence of this sample, however, suggests that the depleted mantle was not a significant source for the Siberian basalts. Sample #19 is an alkali-pi&e which has an +&t) = f0.24, (87Sr/86Sr)1 = 0.7041, and a high Sr concentration (631.5 ppm), causing it to deviate from the general field of Siberian basalts in Fig. 5. Petrographically, this sample shows a cumulate texture

with early accumulates of olivine, clinopyroxene (Ti-augite), and apatite enclosed in plagioclase. High concentration of Sr in this sample is clearly due to early crystallization of apatite, which is expected to have a high Ds,. In summarizing the observations of Fig. 5, it may be stated that the volumetrically insignificant Norilsk lavas of the SFBP show clear evidence of their magmatic precursors evolving along AFC paths. In contrast, the Putorana samples, which represent the bulk of the SFBP, do not show any significant evidence of continental crustal contamination. Thus, we conclude that the initial isotopic ratios of the majority of the Putorana basalts are di- agnostic of the mantle source from which the bulk of the SFBP was derived.

Figure 6 shows a plot of the (87Sr/86Sr)r and C&t) for the Siberian basalts. It also includes six previously published analyses of the Putorana basalts (DEPAOLO and WASSER- BURG, 1979a). In (87Sr/86Sr),-t,,(t) space, all the Siberian basalts define an array that lies near the mantle correlation line but is distinctly displaced to the right (Fig. 6). This deviation from the mantle array increases as the tNd(t) values become more negative. This relationship may be defined by a hyper- bola having three different components: depleted suboceanic mantle, undepleted primitive mantle, and old continental crust. However, the majority (70%) of the Putorana basalts in Fig. 6 show a clustering in the range of 0 to +2.5 and 0.7046 to 0.7052 for q&t) and (87Sr/86Sr),, respectively. The data presented here, therefore, warrant a less depleted mantle component, near-chondritic in Sm-Nd and Rb-Sr isotope characteristics, as the major source for the bulk of the Siberian flood basalts. It is also interesting to compare the ranges of


12

fi x South Siberia

n Putorana

II Norilsk Island

E

s 0

w

-4

-0

-12 1 ~~

0.702 0.703 0.704 0.705 0.706 0.707

initial *7Sr/*6Sr

FIG. 6. Plot of initial tNd and initial (“Sr/%r). Most of the Siberian basalts fall in the field of OIB. The majority of the Putorana basal& representing the bulk of the SFBP, lies in the ranges of 0 to +2.5 and 0.7046 to 0.7052 for the initial c~,, and *‘Sr/*‘?3r, respectively. Fields of Mid-Ocean Ridge Basalts (MORBs) and Ocean Island Basalts (OIBs) are from HART and STAUDIGEL (I 989). Nd-Sr mantle-correlation is shown by the solid line. Data from South Siberia are from DEPAOLO and WASSERBURG (1979a).

Nd-Sr isotopic compositions of present-day OIB with the Si- berian basalts. There is an excellent, overlapping correspon- dence in Fig. 6 between the Siberian continental flood basalts

and the basalts of the oceanic islands in terms of their Nd- and Sr-isotopic signatures. This feature was also noted by CARLSON et al. (1981) and by DEPAOLO (1988), particularly in the case of the Columbia River Basalts, and, in general, for all continental flood basalts by MACDOUGALL (1988). In

the following section, we discuss this resemblance in the con- text of the similarity in mantle sources for the SFBP and

the OIB.

ORIGIN OF THE SIBERIAN FLOOD BASALTS

A number of recent studies on various flood basalt provinces have suggested that the subcontinental lithosphere may be a viable source for the basalts because they display a wide range of Nd-, Sr-isotopic compositions (CARTER et al., 1979; ALLEGRO? et al., 1982; Carlson et al., 1981, 1983; CARLSON, 1984; LIGHTFOOT, 1985; HAWKESWORTH et al., 1986; HUGHES et al., 1986; CARLSON and HART, 1988; MAHONEY,

1988; FODOR et al., 1990). However, the available trace element and isotopic data also support the earlier suggestion of

DEPAOLO and WASSERBURG (1979b) that the continental flood basalt volcanism is the result of uprising mantle plumes from primitive mantle sources. The most important argument

in favor of DePaolo and Wasserburg’s model is the fact that volume-weighted, major portions of continental flood basalts, such as those of the Columbia River in the US, cluster around an t&t) = 0 and (87Sr/86Sr), value of0.705 (DEPAOLO, 1983, 1988). The major element and Sr-Nd isotopic data, as presented in this study, clearly demonstrate that the most voluminous portion of the SFBP in the Putorana region was derived from a mantle-source with a near-chondritic Sr-Nd isotopic signature.

It has been suggested (MORGAN, 1981) that the Siberian flood basalt volcanism was the first expression ofthe inception

of the present-day Jan Mayen hotspot. Following MORGAN (198 I), it is recognized that some continental margin flood

basalt provinces are related to the thermal anomaly created by a mantle plume or hotspot (e.g., WHITE and MCKENZIE,

1989; CAMPBELL and GRIFFITHS, 1990). Early geochemical studies (GAST et al., 1964; TATSUMOTO et al., 1965) dem- onstrated that the ocean island basalts, which are also man- ifestations of hotspot activity, were derived from different

mantle sources than those of MORB. This demonstration of

chemical heterogeneity in suboceanic mantle subsequently led to some new ideas on hotspots and mantle plumes, in-

cluding the characterization and recognition of geochemical signatures of the hotspots (e.g., SCHILLING, 1973; ZINDLER

and HART, 1986). In this perspective, a striking overlapping of the SFBP and OIB fields in Sr-Nd isotopic space (Fig. 6) and the evidence of the formation of the SFBP by the Jan

Mayen hotspot activity (MORGAN, 1981) lead us to support the mantle plume model for the origin of the Siberian flood basalts. This model is essentially similar to that proposed by DEPAOLO and WASSERBURG (1979b) where the flood basalts are derived from a lower mantle source having primitive Nd- and Sr-isotopic characteristics. We recognize, however, that

our data do not uniquely indicate the presence of a primitive lower mantle. This is due to the fact that in (87Sr/86Sr),-t,,(t) space, we cannot, a priori, estimate the degree of interaction

between the rising plume and the overlying subcontinental

mantle. We therefore prefer to call the lower mantle source component of the SFBP near-chondritic. The analytical data presented in this study thus support the generation of the Siberian flood basalts by the rise of a mantle plume of near- chondritic composition and its very limited interaction with the overlying subcontinental lithosphere.

A few important factors that determine the gross chemical characterization of the flood basalts are the size of the plume and the amount of heat it can potentially supply to its sur- roundings, and the thickness, composition, and age of the


overlying continental lithosphere. If the plume is large and can supply large amounts of heat and the subcontinental lithosphere is not very thick, the latter would stretch up dy- namically to compensate for the temperature increase and become thin (WHITE and MCKENZIE, 1989). The astheno- spheric plume material would then passively flow into the created gap and basalts would be generated by decompression melting (YODER, 1976; WHITE and MCKENZIE, 1989). This is probably what caused the Siberian flood basalt volcanism, where the formation of large amounts of primary melt from a near-chondritic mantle plume component is evident in the Putorana basalts.

CONCLUSION

We have reported here the geochemistry and petrology of 47 basal& representing two major stratigraphic sections of the Siberian Flood Basalt Province. The Norilsk region represents 5- 10% of the total volume of the SFBP, whereas the Putorana region represents the bulk, 90-95%, of the SFBP. The Norilsk basalts show considerable variation in major element and in Nd-, Sr-isotopic compositions. In contrast, the bulk of the volcano-stratigraphic volume of the Putorana region is remarkably uniform in major-element composition, and the initial Nd-, Sr-isotopic signatures are within the fields of ocean island basalts, but, more importantly, these signatures are near-chondritic in terms of Sm-Nd and Rb-Sr isotope systematics. A deep mantle-derived plume model, hotspot connection, and flood basalt volcanism can all be synthesized in the present study to support an origin for one of the largest flood basalt provinces of the world in Siberia by the rise and subsequent melting of a relatively large, near-chondritic mantle plume beneath the Siberian continent. The implications of the results of our study can be extended towards resolving the controversy over other continental basalt provinces, whether these basalts tapped a primitive, chondritic mantle or were derived from a depleted mantle that interacted with the continental crust. The arguments presented above, based on a fairly extensive set of data, support the proposal that the major continental flood basalt provinces of the world may have originated from either a chondritic or near-chondritic mantle reservoir which interacted to variable degrees with the overlying subcontinental mantle/lithosphere. Further implications of this conclusion can be extended to evaluate the nature of chemical heterogeneity, structure, and scale of convection in the mantle of the Earth, including a reappraisal of the two-layered mantle model in which a primitive lower mantle has remained virtually unfractionated with respect to the original abundances of the rare earth elements and Rb and Sr.

Acknowledgments-This research is partially supported by a grant from the National Science Foundation (EAR-880578 1) and by an instrumentation grant, also from NSF (EAR-8709503), which partially supported the acquisition of the VG Mass Spectrometer at the Uni- versity of Rochester. GVN is grateful to the Vernadsky Institute of Geochemistry for long-term support for his work on the Siberian flood basalts. We thank R. W. Carlson, W. P. Leeman, R. J. Stern, M. Tatsumoto, and an anonymous reviewer for thoughtful reviews of this manuscript.

Editorial handling: G. Faure

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NdSr isotopes, petrochemistry, and origin of the Siberian flood basalts, USSR

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