Enrichment of the continental lithosphere by OIB melts: Isotopic evidence from the volcanic province of northern Tanzania

EPSL ELSEVIER

Earth and Planetary Science Letters 130 (1995) 109-126

Enrichment of the continental lithosphere by OIB melts: Isotopic evidence from the volcanic province of northern Tanzania Cassi Paslick a, Alex Halliday a, Dodie James b, J. Barry Dawson b a Department of Geological Sciences, University of Michigan, Ann Arbor, MI 48109-1063, USA b Department of Geology and Geophysics, University of Edinburgh, Edinburgh, EH9 3Jcr! UK

Received 4 August 1994; accepted after revision 20 December 1994

Abstract

Alkali basalts and nephelinites from the southern end of the East African Rift (EAR) in northern Tanzania have incompatible trace element compositions that are similar to those of ocean island basalts (OIB). They define a considerable range of Sr, Nd and Pb isotopic compositions (87Sr/ %r = 0.7035-0.7058, eNd = -5 to +3, and 206Pb/ ‘04Pb = 17.5-21.3), each of which partially overlaps the range found in OIB. However, they occupy a unique position in combined Nd, Sr and Pb isotopic compositional space. Nearly all of the lavas have radiogenic Pb, similar 238U/ 204Pb coupled with unradiogenic Nd (+ 2 to -5) and radiogenic Sr to HIMU with high time-integrated ( > 0.7041, similar to EMI. This combination has not been observed in OIB and provides evidence that these magmas predominantly acquired their Sr, Nd and Pb in the subcontinental lithospheric mantle rather than in the convecting asthenosphere. These data contrast with compositions for lavas from farther north in the EAR. The Pb isotopic compositions of basalts along the EAR are increasingly radiogenic from north to south, indicating a fundamental change to sources with higher time-integrated U/Pb, closer to the older cratons in the south. An ancient underplated OIB melt component, isolated for about 2 Ga as enriched lithospheric mantle and then remelted, could generate both the trace element and isotopic data measured in the Tanzanian samples. Whereas the radiogenic Pb in Tanzanian lavas requires a source with high time-integrated U/Pb, most continental basalts that are thought to have interacted with the continental lithospheric mantle have unradiogenic Pb, requiring a source with a history of low U/Pb. Such low U/Pb is readily accomplished with the addition of subduction-derived components, since the lower average U/Pb of arc basalts (0.15) relative to OIB (0.36) probably reflects addition of Pb from subducted oceanic crust. If the subwntinental lithosphere is normally characterized by low time-integrated U/Pb it would appear that subduction magmatism is more important than OIB additions in supplying the Pb inventory of the lithospheric mantle. However, U/Pb ratios of xenoliths derived from the continental lithospheric mantle suggest that both processes may be important. This apparent discrepancy could be because xenoliths are not volumetrically representative of the subcontinental lithospheric mantle, or, more likely, that continental lithospheric mantle components in basalts are normally only identified as such when the isotopic ratios are dissimilar from MORB or OIB. Lithospheric enrichment from subaccreted OIB components appears to be more significant than generally recognized.

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110

C. Paslick et al. /Earth and Planetary Science Letters 130 (1995) 109-126

1. Introduction

mantle, lithospheric mantle and continental crust in the generation of continental basalts. Many continental basaltic volcanic provinces are associated with rifting [4,5]. In general, basaltic magmas erupted in areas of greatest extension within continental rift environments are similar in both incompatible trace element patterns and isotopic

The origin of basaltic magmas erupted in continental settings remains the focus of considerable debate despite three decades of research in mantle geochemistry [l-31. A critical issue is the relative importance of convecting asthenospheric

Modern Alluvium and/or Detritus (waraha

- - Fault

0

Volcanic Cone Road

-

l

Town or Village

Fig. 1. Map of the volcanic province of northern Tanzania (after [9]).

C. Paslick et al. /Earth and Planetary Science Letters 130 (1995) 109-126

compositions to ocean island basalt (OIB) [6,7]. This suggests that these continental basalts are derived from the convecting asthenosphere. However, in areas of less extension trace element patterns and isotopic signatures are more complex. This is believed to be the result of either passage through or generation within the continental lithospheric mantle [8]. The volcanic province of northern Tanzania (Fig. 1) is the youngest southern expression of the eastern branch of the East African Rift system [9]. It offers the possibility of testing various models for the generation of continental rift related volcanic rocks. We present the first comprehensive isotopic and chemical data for lavas from this region and show that although the trace element enrichment patterns are consistent with an asthenospheric (OIB) source, Sr, Nd and Pb isotopic data suggest the involvement of continental lithospheric mantle enriched at > 1 Ga. This can be explained if the ancient enrichment of the lithospheric mantle in this region was produced by underplating and metasomatism with OIB source magmas rather than components derived from a subduction zone.

2. Geologic background The volcanic province of northern Tanzania is the southern extension of the East African Rift and is superimposed on two major geologic provinces. The basement rock of western Tanzania is the Tanzanian Shield, an Archean craton [lo], while the Mozambique Belt, the basement rock of eastern Tanzania, is a late Proterozoic mobile belt that has been reworked during several erogenic events [ill. The present-day rift valley in northern Tanzania is a result of major faulting at ca. 1.2 Ma [9]. This modem rift valley was imposed on a pre-existing, broad tectonic depression that was the result of mid-Tertiary faulting and uplift of the Lake Victoria and Masai blocks. The earliest volcanic activity (beginning at about 8 Ma [12]) followed the mid-Tertiary faulting event and consisted mainly of large volumes of alkali basalt, trachyte and phonolite erupted from shield volcanoes. Several nephelin-

111

ite volcanoes were also active during this time (i.e., Essimingor, Mosonik and Sadiman [9]). Volcanic activity following the 1.2 Ma faulting consisted of nephelinite, phonolite and carbonatite magmas erupted explosively in small quantities from small centers [9]. The northern Tanzania volcanic province is the youngest area of volcanic activity in the Eastern Rift Valley, and there is no reason to suggest that activity will not continue in the future. Although Dawson [9] has highlighted the differences (petrographic types, volume of extruded material) between the pre-rift volcanics (OV) and the post-rift volcanics (WI, evolved nephelinites and carbonatites also form an early though minor part of the OV. The evolved nephelinites and carbonatites that dominate the YV may well represent the incipient melting of alkali and volatile rich mantle, and greater degrees of melting could give rise to more extensive alkali basalt magmatism similar to that dominating the OV. The volcanoes in this province are not well dated; we have listed the approximate age of each volcano (from [91> in Table 3. Although we see no relationship between isotopic composition and age of the volcano this may change with subsequent work. In contrast to the narrow graben in Kenya [13], the Rift Valley in northern Tanzania is a broad depression trending north-south [9]. It is bounded in the west by a major east-facing escarpment which runs from Lake Natron in the north to Lake Balangida in the south. This scarp cuts through the eastern flanks of Loolmalasin, Ngorongoro and Oldoinyo Sambu volcanoes. There is no eastern equivalent to this western escarpment; the eastern margin of the depression is formed by a series of small faults that are a southerly continuation of the Ngong-Turoka Fault in Kenya. Many minor faults occur in the floor of the depression. We have sampled twelve silicate volcanoes (Essimingor, Meru, Ketumbeine, Burko, Loolmalasin, Monduli, Mosonik, Hanang, Sadiman, Tarosero and Lemagrut) and one carbonatite volcano (Kerimasi). These range in age from 8 Ma to Recent. We also sampled three flows of uncertain origin and age exposed by the 1.2 Ma faulting event, and several small isolated cones.

112

C. Paslick et al. /Earth and Planetary Science Letters 130 (1995) 109-126

Table 1 Major elements (wt% oxide) for northern Tanzanian lavas Sample BU&O BD435 BD 4% BD 450

SiO2

A1203

Fe203

MgO

40.24 42.68 4531

11.46 14.51 17.16

13.45 12.03 7.19

6.42

45.75

11.88 11.29 11.23

cao

Ml0

Na20

K2O

::z

14.15 10.55 10.18

4.04 2.74 9.71

1.87 3.47 4.40

3.53 2.86 0.94

:-z

12.62 14.05 13.70

6.57 6.06 7.91

1037 15.31 12.70

4.34 5.29 3.91

2.48 233 2.03

3.29 4.87 4.14

0.18

7.94

14.32

13.11

14.47

1.63

1.59

11.64

15.66 13.99 12.49 12.20

8.17 10.59 6.78 6.53

9.54 8.55 8.50 8.21

3.03 2.80 4.97 4.85

0.91

sz ::g

14.37 13.82 12.91 14.78

5:96 10.07 9.52 8.99 10.34 10.65 7.74 6.34 8.43

10*06 10.25 8.37 8.55 9.83 12.33 14.01 12.89 12.47 12.40 10.55 9.45 12.69

t-z 5:88 5.75 5.07 2.52 1.72 2.47 237 2.41 3.80 4.10 233

1.15 1.40 0.83

0.78 5.16 0.67

53.96 50.20 53.83

TiQ

LQI

Total

Aft 0:67

1.87 4.49 249

9831 98.58 9931

::;

0.62 1.32 131

0.99 0.62 3.62

99.09 98.87 99.24

3.03

0.21

0.46

3.19

9638

3.79 2.70 2.67 2.65

0.18 0.16 0.19 0.19

0.54 0.53 0.57 0.55

-0.04 1.09 0.09 0.44

99.40 99.25 99.42 99.12

0:31

p205

EsshillgO~

BD 214

F% 37.42 3750 Kwaraha H93-3 39.62 Ketumbeiie BD 144 45.99 BD 333 :!-t: K93-Wl K93-W2 48:13 Oldez Extrusives RW93- 1 44.95 RW93-2 44.87 RW93-3 44.01 RW93-4 44.15 RW93-5 44.23 RW93-6 44.35 RW93-7 43.16 RW93-8 43.90 RW93-9 44.49 RW93-i0 44.76 OE93-3 46.20 oE93-4 46.87 OE93-7 45.21 Kdmase BD345 0.88 BD 892 0.58 BD 893 0.96 z?F

::-g 13:59 13.87 13.98 15.22 15.10 13.86 11.12 9.76 10.21 10.93 10.81 12.60 13.84 10.80

14.45 14.12 11.77 11.69 11.54 14.31

:E5 14:36

% 2122 1.74 1.78 3.74 3.94 2.45 0.97

3.35 3.31 3.08 3.08 2.71

0.61 0.62

% ;:g

E%! ;p;

!% oh 0.59 0.41

3:07 0.64 1.58 1.09 0.49 0.39

96:60 9933 98.54 98.94 99.47 99.86

2: Oh5

z% 98:69

:z 0.93 0.92 1.53 1.73 0.74

fzt 2:95 2.84 3.12

% 0:27 0.27 0.18 0.18 0.18 0.20 0.18 0.19 0.22 0.19 0.17

:g 0:20

:: 0:03

0.03 0.02 0.03

0.16 0.26 0.15

2.05 1.44 1.50

40.09 39.60 40.83

9933 98.88 99.00

:z 3:15

:z 0:77 0.50 0.45 0.39

57.10

17.85

7.79

1.52

3.72

6.05

3.06

1.41

0.18

0.44

0.49

99.11

!iiEM 45.94 Monduli

14.95

12.10

4.16

8.29

6.15

3.11

3.09

0.21

0.85

-0.16

98.69

46.08 44.47 47.88

15.05 9.59 10.90

11.02 12.38 13.68

3.78 15.47 10.72

8.29 10.04 10.49

5.72 2.73 2.29

2.85 1.27 0.85

3.50 2.41 2.33

0.22 0.18 0.22

:z 0:31

1.49 0.67 0.67

98.99 99.16 99.66

49.36

18.30

8.35

1.08

5.18

8.40

5.29

1.08

0.20

0.17

2.14

99.54

18.55 16.97

9.61 9.70

1.79 2.64

5.59 9.63

6.48 8.06

3.35 4.11

1.92 2.45

0.23 0.27

0.63 0.77

1.08 1.99

98.77 98.04

14.74 10.33 11.66

12.51

4.53 8.42 6.73

1:‘: 1157

4.41 2.77 2.31

2.04 1.26 1.61

2.95 3:;:

0.20 0:21 0.22

0.59 0.41 0.50

16.19 9.20 11.49 14.75

10.15 15.90 13.18 14.90

3.72 10.16 9.18 5.70

6.75 13.47 12.15 8.77

5.76 4.13 3.62 3.55

2.71 1.75 0.90 131

:z 3:93 3.63

0.18 0.25 0.20 0.18

0.63 0.94 0.86 0.59

0.97 0.87 236 -0.52

98.90 98.73 99.77 100.03

hD 621 h4D93-4 MD93-7 SaditIlsll S93-2 Talaren,

190 :;*z BD 515 Loolmalasin * BD 130 48.98

Isolated Cones BD 180 BD 652 c933 K93-1

49.50 3830 41.92 47.18

:::%

113

C. Paslick et al. /Earth and Planetary Science Letters 130 (1995) 109-126 Table 2 Traceelements(ppm) for northernTanzanianlavas Sample

Burke

Nb

198 BD 435 215 BD 436 300 BD 450 Esaimiqur 84 BD214 180 BD 216 161 BD 247

Kwsmba 98 H93-3 Ketumbeiw

BD 144 BD 333 K93-Wl K93-W2

Older Extrusives

Y

Sr

Rb

445 423 855

36 1701 44 39 2139 176 67 3650 88

309 344 413

29 %3 43 1107 43 1300

211

42 265 45 198 123 345 123 352

83 RW93-1 83 RW93-2 RW93-3 253 RW93-4 248 RW93-5 161 52 RW93-6 60 RW93-7 68 RW93-8 50 RW93-9 RW93-10 50 77 01393-3 87 OE93-4 OE93-7 86 Kerimaae 11 BD 345 48 BD 892 19 BD 893 Lemagrut

Zr

Pb

251

Cu

Ni

Cr

21 24 55

8 11 34

120 121

136

43 20 5

41 8 4

209

!z

Ce

Nd

La

V

Ba

121 107 310

152 317 4767 179 236 2703 85 1843 339

13 10 0

198 174 20 229 25 191

69 91 69

78 308 900 118 3% 633 98 443 871

20

81

39

47

276

910

32 24 19 16 12

47

121 146 122 270 162 267

19 1061

70

12

107

192 149 654

29 25 30 30

13 23 61 61

2

153 128 127 125

72 110 96 86

232 370 120 103

381 374 225 184

110 127 164

48 48 63 68

40 45 90 95

356 261 209 192

290 477 814 830

48 52 151 150 147 116 249 213 118 120 114 84 2tnJ

70 65 22 22 92 223 187 191 227 243 123 79 115

160 137 4 6 177 581 438 416 607 633 273 148 264

61 59 101 256 100 167 60 43 82 49 46 : 82. 40 84 114 zt 130 61 123 56

54 63 148 143 93 34 36 36 35 32 53 60 59

253 246 174 177 198 294 365 350 309 299 265 243 313

539 551 1576 1666 1130 322 512 448 317 311 675 692 708

281 282 531 530 325 166 1% 212 160 158 246 281 262

33 820 43 44 39 869 48 1694 106 47 1853 111 26 1882 43 25 23 846 29 24 831 25 687 21 800 z 22 789 29 726 : 29 848 43 27 836 14

121 127 115

46 4117 33 4127 33 3902

0 0 0

38 1045 74 115 446 L93-2 Mew 166 421 36 1289 82 BD 194 Mooduli 35 1606 132 189 532 BD 621 24 847 88 242 MD93-4 19 37 161 20 444 33 MD93-7 Sadimao 149 335 30 1374 158 s93-2 Tprrrscn, 50 1812 67 211 620 BD190 376 670 63 2131 % BD515 LMmalasin 81 304 33 815 BD 130 53 24 621 68 221 BD 252 62 226 28 795 BD 177 33 39 holated Cooes 31 1076 75 109 395 BD 180 138 368 35 1303 45 BD 652 143 360 c93-3 27 1176 67 63 227 25 933 29 K93-1 *Measurement madeby isotopedilution.

1: 15

7 (6.5)+ ; 7 9

96 16 21

SC

314 638

280

17 9 10 9 (8.8)* (6.9)+ 1016(13.0)*

626 651 973 944

!z

‘lh

12 9

10

7 3

9 79 12 9

: 7 6

141 138 139 137 101 103 108 119 104 104 108 121 112

0 0 0

2(1.3)* 2(22)* 2 (2.5)*

48 127 21

35 30 41

6 5 6

13 11 10

270 211 158

69 50 34

172 150 97

95 558 110 794 74 521

: 6

13

13

121

6

3

0

195

94

123

18 1222

3

16 10(9.9)+

119

50

14

14

252

94

132 239 1290

10

21

12

6 11

;

123 112 110

289 108 130 54 67 38

138 226 1276 68 236 677 25 278 316

2i 26

21

23

162

50

4

0

175

52

131

74 1948

1

26 42

14 9

148 165

21 25

:

0 8

325 330

115 127

171 190

14 1343 187 4256

0 -2

;

9

133 47 40 73 138 212 194 405 127 148 125 243

157 123 lm

71 50 50

70 45 44

267 692 413 499 382 588

8 31 2a

32 53 180 45 188 214 308 211 129 106 198 203 33 26 112 25

72

94 163 997 98 325 877 103 267 1024 43 318 477

7

11 10 25 23 16 8 10

7 (6.8)* 7 (6.2)* 0 (7.6)+ 5 (7.2)f 7 (6.2j* 4 (3.5)* . ‘5

4

3 (5.0):

16 12 14 8

11 11 4 5

117 137 101 116

46 117 7:; 9:: 61 202 562

z 57

:: 5 1: 24

U*

1.8 4.1 3.9

1.6 1.4 1.8 1.8 1.5 1.1

25 21 22

z 15

0.4 A::

3.5

1

114

C. Paslick et al. /Earth and Planetary Science Letters I30 (1995) 109-126

Table 3 Sr, Nd Pb isotopic compositions

CSd@jS~ BUlkO BD435

fran [ll] 1 Foidik

%?a3

0.704079 fll 0.704282 *21 0.705159 ill

0512706 k?8 0.512667 %I8 0.512630 US

19.891 19.744 m.234

15.684 15.682 15.793

39.801 40.084 40.257

0.512385 ti5

-4.94

0.512761 a9

+2.41

0.704263 215

0.512730 kll

+l.79

20.360 21.2m 21.233 21.158 21.134

15.841 15.813 15.823 15.868 15.848

40.349 40.730 40.798 40.603 40.553

0.703529 a7

0.512749 f10

+2.l7

m.937

15.755

40.509

cmimnuile cuborvtitc

0.703975 *13 0.703967 ill

0.512744 W 0.512730 ti

+2.07 +I.79

m.809

15.754

40.361

(hhutik

0.704oM f19

0.512706 %I9

+l.33

m.712 20.664 20.907

15.722 15.743 15.801

40.236 40.315 40.516

20.854

15.762

40.4m

Basmite FoidiIe

Bssimingor BD 214 BD 216

Basmlitc Foidite

0.705723 frl0 0.703566 ~32

BD 247

Foidite

BD893 Ketrunbdne BD144

4.0 - 8.0

1 Foidite 0.5

1.7 BUh

BD333 K93-Wl

Basalt Buanite

K93W2

Bumlile

Lcmsgn* L93-2

MondUli BD621 MD934 MD93-7

0.703852 *m 0.703941 +11 0.703743 L18 0.703649 90

0.512720 9 0.512617 %

+1.60 -0.41

18.461

15.506

38.525

18.335 19.214

15.500 us1

0.512623 212

-0.30

19.078

15.553

38595 39.210 39.127

0.705271 *m

0.512369 f23

-5.24

19.517

15.72rJ

39.055

0.703932 +I 1 0.704698 f14 0.703835 fl2

0.512599 %I7 0.512728 212 0.512698 axi

XI.76 tl.77 t1.17

18.812 m.366 m.196

15.554 15.768 15.808

40.928 40.390 40.588

0.703994 211

0.512640 an

18.305

15.439

38.492

0.7039% f10 0.703712 %!O 0.704101 *20

0.512611 %I7 0.512750 fl3

18.802 19.692 17.632

15.519 15.673

39.167 39.832

15.366

38.035

17.646

15.382

38.087

19.063

15.525

39.119

5 TIldlyte

LcalmslAsin BD 130 Tbytc ti.zn-bas& BD 252 Basalt BD 177 Mall BD194

& Nd

+1.33 to.57 4.16

BD 436 BD 450

KWdU H93-3 KedmuC BD 345 BD 892

143Nd1144Nd

*2a

0.09 - 1.5 B& 1.7 BualiIe Picn-baulr BaUlt

OlderBxtnuiva RW93-1 Blti

0.512607 a9

-0.53 a.26 -0.60

> 1.2 0.703884 U8 0.703969 +17 0.703933 f17

0.512733 U6

t1.85

0.512720 m 0.512718 %J7

t1.60

19.202

15.639

39.494

rn.028

15.665

39.954

0.512714 %I 0.512733 %I9 0.512709 %J5

+1.85 t1.38

19.980 19.807 20.083

0.512735 a5

+1.89

20.234

15.626 15.643 15.719 15.738

39.860 39.807 40.604

Picrvbult

0.703981 0.7elo56 0.704968 0.7Ml81

+1.56 +1.48

RW93-8

F’kro-bult

0.704673 f10

0.512729 m

+1.78

20.128

15.719

RW93-9 RW93-10

Picnbasah Picmhsalt

0.7Mo48 a3

0.512713 u5

+1.46

20.094

15.696

40.581 40.498

t152

20.378

15.787

40.468

T=+* Tnchyre

0.704967 a9 0.704313 in 0.704206 f15

0512716 %I8

OE93-3 OE93-4 OB93-7 S_ s93-2

Basalt

0.703787 ti8

0.512667 a08 0512692 f10

-1.05 tO.57 +1.05

19.547 19.085 19.104 19.754

15.621 15.547 15.565 15.612

39.867 40.524 40.562 39.620

0.705428 *35

0.512490 a1 0.512499 f10

-2.89 -2.7 1

20.079

15.759

39.998

0.7w108 210 0.704112 -8

0.512742 kX 0.512778 %8

t2.03 t2.73

m.080 20.155

15.717 15.723

40.M3

0.703781 %3

0.512717 ti

t1.54

0.704846 a83 0.704580 fll 0.703757 f10

0.512576 fll 0.512532 m 0.512734 w

-1.21 -2.07 +1.87

19.169 19.181 19.169 18.773

15.554 15.566 15.554 15.523

39.242 39.298 39.242 41.150

RW93-2 RW93-3

Bati Foidh

RW93-4 RW93-5 RW936

Foidiw Buanite Piuc-bnult

RW93-7

fl5 f16

0.512640 a08 0.512584 a7

40.335

4.5 sbidite

Twem

Phmcaile BD 190 BD515 Foiditc Youn~erculU c93-3 Plhnwlilc K93-1 BD 180 BD 652

fl 1 *30

Baa&It Ptnlndite Foidile

25


40.171

115

C. Paslick et al. /Earth and Planetary Science Letters 130 (1995) 109-126

stvlau

. Older Exmwves I

. Kwanna

o Tarasao

+ Essimingor

o Olda Exmrsivss II

. Sdiman

v Monduli

x Kelumbeine

q

. Burke

h Lnohlalnsin

Older ExtrusivesIII

l

3. Analytical methods

B Isolared Canes

Lemaguut

Major and trace elements were measured by XRF at the University of Edinburgh using the methods outlined by Fitton and Dunlop [14] (the major and trace element data are presented in Tables 1 and 2). The sample preparation and isotopic techniques have been previously described in Halliday et al. [15,16]. Isotopic compositions were measured on a VG Sector thermal ionization mass spectrometer equipped with six Faraday collectors. The Sr and Nd isotopic compositions were measured in multidynamic mode. The average 143Nd/ 144Nd ratio for the La Jolla standard was 0.511845 + 9 (n = 6). The average 87Sr/86Sr ratio for NIST standard SRM987 was 0.710247 + 15 (n = 101. The Sr and Nd measurements were normalized for mass fractionation to 86Sr/ **Sr = 0.1194 and 146Nd/ l”Nd = 0.7219 respectively. The Pb isotopic compositions were measured in static mode. Pb measurements were

a K&maw

12

9

6

&

Nd

3 0

-3

-6 0.704

0.703

0.702

0.707

0.706

0.705 86

87Sd

Sr

Fig. 2. cNd vs. 87Sr/ %r. Fields of data for Ascension, Azores, Gough, Tristan da Cunha, Cape Verdes and St. Helena from [62]; Mangaia and Tubuai from [64]; Hawaii from [65,66]; Kerguelen from [67,68]; Walvis Ridge from 1691; Samoa from [701; Madeira and Trinidade from [71].

12

“l’,“l’l”“,‘i”,I Madeira

‘7

9

6

&

Nd

16.0

3. Helena I 15.8 % w $

15.6

a

0.702 17

18

20

19 206

22

17

I8

19

20

206

204 Pbl

21

Pb

21

22

204 Pbl

Pb

Fig. 3. (a) M8Pb/ 204Pb vs. 206Pb/ 204Pb. (b) 207Pb/ 204Pb vs. 2”Pb/ ‘@‘Pb. (cl cNd vs. 206Pb/ 204Pb. (d) s7Sr/ 86Sr vs. *06Pb/ *“Pb. All symbols as in Fig. 2.

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corrected for fractionation and mass discrimination (O.l%/amu> based on replicate analyses of NIST standard SRM981. The Pb isotopic duplicate measurements on seven samples were within 0.08% for 2061”1204Pb, 0.1% for 207Pb/204Pb and 0.14% for Pb/ 2WPb.

9 6 &Nd

3 0 -3 -6

4. Sr-Nd-Pb isotopic variations In Table 3 and Figs. 2-4 the Sr, Nd and Pb isotope data are presented for whole-rock powders of basalts, nephelinites and trachytes. The ranges of Sr, Nd and Pb isotopic data for the Tanzanian volcanic rocks c8’Sr/ 86Sr = 0.70350.7058, eNd = -5 to +3 and 206Pb/ 2a4Pb = 17.5-21.3) overlap those of OIB. The Tanzanian data define a broad field in Nd-Sr space, similar to many OIB associated with the DUPAL anomaly [17] or the EM1 component [181 (Fig. 2). In contrast, on plots of 208Pb/204Pb and 207Pb/ 204Pb vs. “‘Pb/ 204Pb (Figs. 3a and b), the Tanzanian data follow the Northern Hemisphere Reference Line (NHRL) trending slightly above and towards more radiogenic values, similar to those found in HIMU basalts. Many Tanzanian lavas have HIMU Pb coupled with EM1 Nd and Sr and combined plots of 87Sr/86Sr and 143Nd/ 144Nd vs. “‘Pb/ 204Pb reveal that most of the Tanzanian data define a field distinct from those of any OIB (Figs. 3c and d). This combination of HIMU Pb with EM1 Nd and Sr has not yet been observed in OIB and provides evidence that the lavas did not acquire their Sr, Nd and Pb from an asthenospheric OIB source. Continental lithospheric mantle may have survived a complex history of enrichment events and is therefore a more convincing candidate for the source of Sr, Nd and Pb in the Tanzanian lavas. There is some overlap between the Tanzanian data and isotopic compositions of other African provinces, especially for Kenya and Uganda in the southern EAR (Fig. 4). The major distinction appears to be in the more radiogenic Pb compositions of the Tanzanian samples, similar to data for Napak Volcano in Uganda in the eastern branch of the EAR [19] (Fig. 4b). The Sr and Nd isotopic data for Tanzania, however, are distinct

16 .O

87Sri

86 Sr

19

20

% 15.8 2 a 15.6 a g

15.4 9 6

‘Nd

3 0 -3 -6_ 17

18

21

22

206 Pb/ 204 Pb Fig. 4. (a) E~VS. 87Sr/ %jr. (b) *“Pb/ zo4Pb vs. 2MPb/ ulzPb Pb/ 2MPb. All symbols as in Fig. 2. Fields of cc) EN,j vs. data for Red Sea, Ethiopia and eastern Afar from [39,41,42,72]; Uganda from [19,38,73]; Kenya from 174,751.

from the Sr and Nd isotopic data for Napak (Fig. 4a).

5. Discussion 5.1. Crustal contamination A major concern in dealing with volcanic rocks that have been extruded through the continental crust is the possibility of crustal contamination, which can give rise to enriched Sr and Nd isotopic compositions. All but three of the samples in this study are nepheline-normative, with SiO,

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and Planetary Science Letters 130 (1995) 109-126

117

fore modeled the change in Nd and Pb isotopic compositions as a result of mixing between lower crust [24] and an uncontaminated magma represented by our most radiogenic sample (BD216). The results are shown in Fig. 5, where r = (Nd/Pb),/(Nd/Pb), (m = uncontaminated magma and c = contaminant). The value of this ratio using the two end members specified above is 4.4, and much of the isotopic data cluster near this line, suggesting that some of the samples in this study have been contaminated by lower crust. This is particularly true for sample MD93-7 with a 206Pb/ 204Pb of 17.6. Three samples from Burko, BD435, BD436 and BD450, show a decreasing MgO with increasing SiO,. Of these three samples BD450, an evolved nephelinite with the lowest MgO and highest SiO,, also has the most radiogenic 87Sr/ ‘?Sr and the least radiogenic 143Nd/ 144Nd. This sample also has the most radiogenic Pb. However, there is no known crustal component in the area with such radiogenic Pb [22]. Sample BD214, from Essimingor, also shows elevated 87Sr/ “Sr (0.7057) coupled with low 143Nd/ ‘@Nd and very radiogenic Pb, but has 6.7 wt% MgO. The consistent radiogenic Pb composition despite

ranging from 37 to 57 wt%, and MgO ranging from 1 to 15 wt% (Table 1; rock names in Table 3 have been assigned using the total alkali vs. silica method outlined in [ZO]). The silica-undersaturated character of these rocks argues against significant amounts of assimilation of a high-SiO, component such as crustal material, although the MgO content suggests that most of the magmas experienced some olivine and/or pyroxene fractionation prior to eruption. The samples show no convincing correlations between 87Sr/ “?Sr and MgO or Sr concentration. We have modeled assimilation and fractional crystallization processes (AFC) with upper and lower crust compositions [21]. The isotopic composition of the upper crust in Tanzania is poorly known, but calculating forward in time (using a p of 8) using Pb isotopic compositions measured in 2.7 Ga ore deposits [22] results in Pb isotopic compositions very similar to the average values suggested by [23]. These values produce vectors with a higher slope than the Tanzanian Pb-Pb data. However, similar calculations using lower crustal Pb isotopic compositions [24] produces a vector that could explain the less radiogenic Pb isotopic compositions of several of the samples in this study. We have there4

2

0

& Nd

-2

-6

-8

206

Fig. 5. Calculated (Nd/Pb),/(Nd/Pb),

Pb/

204

Pb

mixing lines (m = primary magma, c = contaminant). Model parameters: 2”Pb/ u)4Pb and of 1, 4.4

lNd for primary magma = 21.23 and +2.41, for lower crust = 17.5 and - 10. Lines plotted for r = (Nd/Pb),/(Nd/Pb),

and 20.

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a range of very silica-undersaturated volcanic rocks suggests that the Pb is a feature of the mantle source and not of crustal contamination. Finally, trace element ratios such as Rb/Nb and Ba/Nb can be used to test for crustal contamination. However, several of the Tanzanian samples have Ba and Rb concentrations significantly higher than concentrations of these elements in the crust (Table 2). Trace element ratios in these samples have been buffered by high concentrations in the melt and any information about crustal contamination is obscured. The remaining samples have Rb/Nb and Ba/Nb ratios well within the range typical of OIB, and do not show evidence of crustal contamination. 5.2, Lithospheric source of the Tanzanian volcanic rocks

Two possible sources for the Tanzanian volcanic rocks are the convecting asthenosphere and the continental lithospheric mantle. Although in detail the source of OIB is still controversial [18,25,26] there is no doubt that the source is in the convecting asthenospheric mantle [17]. The Tanzanian data converge on what appears to be a common end-member composition with very radiogenic Pb but relatively unradiogenic Nd and radiogenic Sr compared with HIMU (Figs. 3c and 5). This appears to represent a lithospheric mantle component. The degree to which the samples with even less radiogenic Nd and Pb and more radiogenic Sr represent crustal contamination or involvement of other mantle reservoirs like EM1 cannot be uniquely determined from the data. Unlike HIMU sources, which require an enrichment in U and Th or depletion in Pb without a concurrent enrichment in Rb, the Tanzanian source must have been enriched in Rb relative to Sr, U relative to Pb, and in LREEs. This history agrees with the postulated history of continental lithospheric mantle in general; this area of the mantle is believed to have been depleted by melting and subsequently metasomatically enriched by small degree partial melts [2,27,281. However, U is commonly considered to be depleted relative to Pb in the continental lithospheric mantle 1291. There is evidence from mantle xenoliths for

both depleted and enriched mantle in Tanzania [24,30-321. Lherzolite, harzburgite and dunite xenoliths from Pello Hill (a small cratered tuff cone located about 10 km east of Oldoinyo Lengai) have been metasomatically enriched in K, Fe, Ti, OH and REEs, resulting in the formation of phlogopite and amphibole [30]. Isotopic data for this suite of xenoliths [30] are very similar to those of OIB. Lherzolite xenoliths from Lashaine (a small ankaramite cone near Monduli [33]), although depleted in both major and trace elements, have an unusually large spread in Sr, Nd and Pb isotopic compositions. One xenolith has very high 87Sr/ ‘%r (> 0.8) and an unradiogenic Pb isotopic composition similar to granulites. Cohen et al. [24] suggested an age of 2.0 Ga for the fractionation event that gave rise to these isotopic values in the mantle below Lashaine. Further evidence for a lithospheric origin comes from the thorogenic Pb data. On a plot of ‘08Pb/ 2”“Pb vs. 206Pb/ 204Pb three samples (OE93-4, BD130 and BD180) lie significantly above the NHRL. These samples have ~~~ values that range from 4.5 to 5.0. The ~~~ value of the mantle is believed to be about 3.9, [34] implying that some Tanzanian samples came from a source that experienced a preferential enrichment in Th over U some time in the past. Model calculations, using a Th/U ratio of 5, require a 232Th/ 2”Pb ratio of about 55 in order to generate the 208Pb/ 204Pb values measured in the above samples in 2 Ga. Similar time-integrated Th enrichment has been seen in mantle xenoliths from the northwestern Wyoming Craton 1351 and in lamproites from the Gausssberg Volcano in Antarctica [36]. Williams et al. [36] suggested that multistage metasomatism created a Th-enriched reservoir in the subcontinental lithosphere that complements the Th-depleted asthenosphere. 5.3. Comparison provinces

with other

east African

rift

The Pb isotopic data for the east branch of the rift (Kenya and eastern Uganda) and the west branch (western Uganda and Zaire) are similar (with the exception of the extremely radiogenic

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Pb values in nephelinites from Nyiragongo, Zaire, which are interpreted as indicating enrichment of the subcontinental lithosphere at 500 Ma [37]). The Tanzanian data overlap slightly with both (Fig. 4b), but are generally lower in 207Pb/ 204Pb for the same 206Pb/ 204Pb. There is considerably more overlap in Nd and Sr isotopic compositions between the fields of data for Kenya and Tanzania (Fig. 4a). Isotopic data for western rift volcanic rocks are limited, but there is some overlap with Tanzanian lavas. Samples from Bufumbira in western Uganda have Sr and Nd isotopic compositions that are very enriched. The Pb isotopic compositions, however, are significantly different from those for the Tanzanian samples. Specifically, the Tanzanian lavas are more radiogenic in 206Pb/ 204Pb while the Bufumbira lavas are more radiogenic in 207Pb/ 204Pb. The western rift volcanic rocks are similar to the Tanzanian samples in that both areas show trace element patterns similar to OIB [38]. Isotopic data for eastern Uganda volcanic rocks, however, are distinctly different from the Tanzanian data. The Pb isotopic compositions of Ethiopia and eastern Afar lavas are very unradiogenic, with some values lower than those for MORB. The Nd, Sr and Pb isotopic data for the Red Sea are very similar to MORB. There is a clear increase in 206Pb/ 204Pb along the EAR from north to south, while Sr and Nd isotopic compositions show little relationship to geography. There is no simple relationship between isotopic composition and extent of rifting in the EAR. The Red Sea province appears to be both geophysically and geochemically a new mid-ocean ridge [39,40]. Ethiopia and Afar, the next most northern and rifted areas, are not depleted, as would be expected if they were being produced by a combination of MORB and OIB mantle. In fact the isotopic data suggest that the continental lithospheric mantle played a major role in their genesis 141,421. It appears that away from the Red Sea spreading center the dominant source in the generation of basaltic magmas in the EAR is the continental lithospheric mantle. The fields of data for the EAR provinces are much broader than those for ocean islands. This is not surprising if the EAR samples have ob-

119

tained their isotopic signatures from the continental lithospheric mantle. If the continental lithosphere has indeed been isolated from the convecting asthenosphere for long periods ( > 500 Ma) one would expect it to have a very heterogeneous isotopic composition, dependent on its particular geochemical history. This history is likely to be similar in some aspects (i.e., Pan-African events affected large parts of East Africa), but there are bound to be differences in detail. The data presented here suggest that the continental lithospheric mantle beneath Tanzania experienced preferential enrichment in LREEs and U relative to Pb. The Tanzanian magmas may also contain a more substantial component of Nd, Sr and Pb from the lithospheric mantle, compared with the magmas from the northern end of the EAR. 5.4. CompanSon with the western United States There is a correlation between isotopic composition for basaltic rocks and lithosphere extension in the western United States [6,8]. These studies are thought to demonstrate the competing influences of continental lithospheric mantle and asthenospheric mantle in the genesis of intraplate continental basalts. In the Basin and Range province, where extension is greatest, isotopic compositions are very similar to OIB. In areas such as the Transition Zone [6] that have experienced less extension, however, the influence of the continental lithosphere is greater. The isotopic data for the Transition Zone define a field that is very similar to those for Ethiopia and eastern Afar (i.e., Sr and Nd isotopic compositions are enriched compared to fields for MORB and OIB, while Pb isotopic compositions are depleted or similar to MORB [S]). Trace element data for the western United States provide strong evidence that the source of volcanic rocks with enriched Sr and Nd isotopic compositions is continental lithospheric mantle. The western margin of the United States experienced subduction both in the Proterozoic and more recently from approximately 150 to 30 Ma. Parts of the overlying mantle appear to have been chemically modified during these episodes. The

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1”“““““” Rb Ba K Nb La Ce Sr Nd P Zr Ti Y

1

Fig. 6. Primitive mantle normalized trace element patterns (normalizing values from [76]). Shaded area represents the range of basaltic samples from this study. Average OIB (from [S]) plotted for comparison.

trace element signature of this subduction-modified mantle (i.e., Nb depletion) is apparent in recent extension-related basalts with enriched isotopic compositions [8,43,44]. A subductionmodified trace element signature is not seen in the Tanzanian samples (Fig. 6). Instead the Tanzanian trace element data require a source that has retained an OIB trace element chemistry. 5.5. The enrichment sphere

of the subcontinental

litho-

Both the mechanism of growth and the composition of the continental lithosphere have been intensively studied for decades [29,45-481. One set of models proposes that the continental lithosphere is both accreted and modified by subduction processes. Oxburgh and Parmentier [47] suggested that mantle depleted by production of the ocean crust becomes buoyant when heated by subduction and rises and accretes to the continental lithosphere. Menzies and Hawkesworth [48] suggested that subduction processes including slab-derived fluids are responsible in large part for enrichment of the continental lithosphere and the abundance of eclogitic xenoliths beneath the continents. The influence of subduction processes on the continental lithosphere is seen in trace elements for basalts and mantle xenoliths from the western United States 18,491.

Because of the distinctive trace element signature associated with subduction zone volcanism (i.e., Nb depletion), this type of modification is one of the easiest to identify in continental lavas. Trace element patterns that are similar to OIB are often attributed to the passive ascent of asthenospheric mantle in response to rifting [8]. The isotopic evidence in this study clearly illustrates that in the case of Tanzania this scenario is too simple. It is possible to generate the Pb isotopic compositions measured in the Tanzanian rocks from a source that is similar in trace element composition to OIB but which has been isolated from the convecting asthenosphere for approximately 2 Ga. Using isotopic data for modern OIB, isotopic ratios for a model OIB source have been calculated for 2 Ga with bulk earth parent-daughter ratios (CL= 238U/ 204Pb = 8). If such a source produced enriched components at that time (by melting) with higher parentdaughter ratios (,CL= 11-15, comparable to many OIB (Fig. 7)), the spectrum of modem compositions could completely encompass the values measured in the Tanzanian samples. Model calculations using Sr and Nd isotopic compositions are less well constrained than those for Pb. Using a 2 Ga OIB source, it is possible to generate the isotopic compositions measured today with slightly enriched Rb/Sr (0.03-0.05) and Sm/Nd

15.9

15.8

15.6

15.5

206

Pb/

204pb

Fig. 7. 2WPb/ 204Pb vs. 206Pb/204Pb. Symbols as in Fig. 2. Lines represent Pb isotopic compositions calculated for p = 11-15, for MORB [62] and OIB (represented by Azores [77]).

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and Planetary Science Letters 130 (1995) 109-126

(0.26-0.32). Trace element patterns in the Tanzanian samples require that these metasomatizing melts were generated in a source with OIB characteristics; partial melts from the MORB source would not have OIB-type trace element patterns

m. Because it is possible to generate values that fit the Tanzanian isotopic data using both ancient enriched OIB and MORB sources, the isotopic data alone cannot constrain the source evolution precisely. Trace element patterns of the Tanzanian samples, however, are very similar to those of OIB (Fig. 6). Partial melts of MORB-enriched lithosphere would be OIB-like but depleted in elements such as Ba unless subjected to submarine alteration and mixing with sediments [51]. The combination of isotopic and trace element data is therefore consistent with ancient OIB as the source of the Tanzanian volcanic rocks. Nevertheless, the possibility that the lithospheric enrichment is from plume material as opposed to ambient asthenospheric upper mantle is poorly constrained. An ancient mantle plume underneath East Africa could produce a melt that would percolate and freeze in the lithosphere [16]. This added plume material and its enriched melt would then become new lithospheric mantle, metasomatize existing lithosphere and become isolated from the convecting asthenosphere. After 2 Ga of isolation the isotopic compositions would evolve to values similar to those measured in northern Tanzania. The trace element patterns generated by remelting such a source would be very similar to but slightly more enriched than OIB. Trace element patterns in the Tanzanian samples range from OIB-like to slightly more enriched than OIB. One model for rifting in East Africa involves a mantle plume presently under the EAR [52-541. This relatively new heat source could be remelting the enriched lithosphere to generate the volcanic rocks we see today in northern Tanzania. The model proposed above for enrichment of the continental lithosphere as a result of underplating by plumes and metasomatism by asthenosphere-derived melts is similar to models previously suggested by several others. Ringwood [55] proposed that megaliths, his postulated source of

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OIB, may rise underneath continents and become part of the continental lithosphere. Menzies et al. [49] proposed that asthenospheric melts become trapped if they rise beneath the continents; these melts then interact with the continental lithosphere, producing hydrous veins like those seen in mantle xenoliths from Pello Hill in northern Tanzania [30]. Halliday et al. [16] suggested an underplated fossil plume to explain the Cameroon line Pb isotopic data. Supporting evidence for the importance of OIB components in the generation of continental lithosphere is found in the REE patterns [56] and Rb/Sr ratios [57] of the continental crust relative to those of OIB and arc basalts. In many continental provinces, unradiogenic Pb isotopic compositions are coupled with an enriched Sr and Nd signature as seen in Scotland [58], the western United States [6] and Ethiopia [41]. This combination requires that the source was enriched in Rb relative to Sr and Nd relative to Sm, but not in U relative to Pb, even though U is more incompatible than Pb in mantle melting [59]. The unradiogenic Pb found in these continental provinces is a good indication that the formation of the continental lithosphere in these areas was dominated by subduction processes. Uranium and Pb are fractionated during alteration of the ocean crust; Pb is leached from the crust and deposited in metalliferous sediments [601. Although only a small amount of sediment is subducted there is overwhelming evidence that Pb in the sediment and in fluids from the subducted crust dominates the Pb budget, resulting in low U/Pb and Ce/Pb ratios in arc basalts [51,61,62]. This implies that subduction-related additions to the continental lithosphere will also have a low U/Pb ratio. Sufficient time between underplating/metasomatism and the generation of basalts from this reservoir will result in unradiogenic Pb isotopic compositions. In contrast, isotopic data from Tanzanian volcanic rocks require long-term enrichment in Rb/Sr, U/Pb and Nd/Sm, as expected for enrichment by underplated OIB. Therefore, the continental lithospheric mantle in Tanzania has been modified by a process other than subduction. In Fig. 8 we have compiled U/Pb ratios for

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rdafic K-rich

basalts from island arcs, ocean islands, mafic potassic lavas from the western branch of the EAR and continental lithospheric xenoliths (see figure caption for references) and compared them to U/Pb ratios for Tanzanian lavas from this study (Table 1). Arc basalts consistently have a lower U/Pb ratio than OIB. The U/Pb ratios of the Tanzanian lavas are higher than those of arc lavas (average U/Pb for Tanzanian lavas is 0.25) and overlap with the lower end of the range for OIB. This further supports our suggestion that the modification of the mantle beneath Tanzania has been dominated by OIB melts rather than subduction processes. The range for the mafic potassic rocks is very similar to that for Tanzania. The U and Pb data presented for continental mantle xenoliths are restricted to those measured in clinopyroxenes, as the U/Pb ratio of this mineral is believed to be representative of the xenolith whole rock [63]. The range of U/Pb seen in clinopyroxene from mantle xenoliths may be explained by addition of both island arc and OIB components to the continental lithosphere. The spread in U/Pb in xenoliths suggests that the continental lithospheric mantle is enriched almost equally by metasomatism of both subduction-generated and plume-generated material. This conclusion must be tentative, however, because it is not known whether the xenoliths are volumetrically representative of the subcontinental lithospheric mantle. Sampling of Pb by basalts would suggest that subduction processes are more important in transferring U and Pb to the continental lithosphere. However, given that lithospheric mantle is only recognized as a contaminated source component of basalts when the Pb is unradiogenic, it is also likely that the xenolith U/Pb data are more meaningful in this respect than Pb isotopic data for continental basalts.

lavas,

western branch EAR

N

Ocean

IslandBasalls

6

N 4

Acknowledgements

0

0.2

0x4

a6

Qll

1

1.2

WPb

Fig. 8. Histogram plots of U/Pb ratios for island arc basalts [78,79], OIB [62,64,70,77,80,81], mafic potassic rocks from the western branch of the EAR 1731, and continental mantle xenoliths [24,63,82-841.

We would like to thank Andrew N. Nyblade and Aloyce Tesha for their help in planning and completing the fieldwork for this study. We would also like to thank Eric J. Essene, Rebecca A. Lange, Henry N. Pollack, Sally A. Gibson and an anonymous reviewer for helpful reviews of the manuscript, Der-Chuen Lee for allowing us to

C. Paslick et al. /Earth and Planetary Science Letters 130 (1995) 109-126

use unpublished U and Pb data for xenoliths from Cameroon, and Rick Keller for technical assistance. Some specimens (prefix BD) were collected by Dawson while he was a geologist with the Tanzanian Geological Survey. This work was supported in part by NSF grants EAR 91-04877 and EAR 92-05435 and a University of Michigan Rackham Thesis Grant. Dawson acknowledges travel support from the Carnegie Trust for the Universities of Scotland and the NERC. [CL]

References [l] D.H. Green, The origin of basaltic and nephelinitic magmas in the earth’s mantle, Tectonophysics 7, 409-422, 1969. [2] C.J. Hawkesworth and R. Vollmer, Crustal contamination versus enriched mantle: 143Nd/ ‘@Nd and “Sr/ ‘%r evidence from the Italian volcanics, Contrib. Mineral. Petrol. 69, 151-165, 1979. [3] C.J. Hawkesworth, J.S. Marsh, A.R. Duncan, A.J. Erlank and M.J. Norry, The role of continental lithosphere in the generation of the Karoo volcanic rocks: evidence from combined Nd and Sr isotopic studies, Geol. Sot. South Afr. Spec. Publ. 13, 341-354, 1984. [4] R. White and D. McKenzie, Magmatism at rift zones: the generation of volcanic continental margins and flood basalts, J. Geophys. Res. 94, 7685-7729, 1989. [5] F.V. Perry, W.S. Baldridge and D.J. DePaolo, Chemical and isotopic evidence for lithospheric thinning beneath the Rio Grande rift, Nature 332, 432-434, 1988. [6] P.D. Kempton, J.G. Fitton, C.J. Hawkesworth and D.S. Ormerod, Isotopic and trace element constraints on the composition and evolution of the lithosphere beneath the southwestern United States, J. Geophys. Res. 96, 1371313735, 1991. [7] F.V. Perry, W.S. Baldridge and D.J. DePaolo, Role of asthenosphere and lithosphere in the genesis of late Cenozoic basaltic rocks from the Rio Grande Rift and adjacent regions of the southwestern United States, J. Geophys. Res. 92, 9193-9213, 1987. [S] J.G. Fitton, D. James and W.P. Leeman, Basic magmatism associated with late Cenozoic extension in the western United States: compositional variations in space and time, J. Geophys. Res. 96, 13693-13711, 1991. [9] J.B. Dawson, Neogene tectonics and volcanicity in the North Tanzania sector of the Gregory Rift Valley: contrasts with the Kenya sector, Tectonophysics 204, 81-92, 1992. [lo] L. Cahen, J.J. Snelling, J. Delhal and J.R. Vail, The

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Geochronology and Evolution of Africa, Clarendon, Oxford, 1984. [ll] R.M. Key, T.J. Charsley, B.D. Hackman, A.F. Wilkinson and C.C. Rundle, Superimposed Upper Proterozoic collsion-controlled orogenies in the Mozambique Orogenic Belt of Kenya, Precambrian Res. 44, 197-225, 1989. [12] G.P. Bagdasatyan, V.I. Gerasimovski, A.I. Polyakov and R.K. Gukasyan, Age of volcanic rocks in the rift zones of East Africa, Geochem. Int. 10, 66-71, 1973. [13] B.H. Baker, R. Crossley and G.G. Goles, Tectonic and magmatic evolution of the southern part of the Kenya Rift Valley, in: Petrology and Geochemistry of Continental Rifts, E.R. Neumann and LB. Ramberg, eds., pp. 29-50, Reidel, Dordrecht, 1978. [14] J.G. Fitton and H.M. Dunlop, The Cameroon line, West Africa, and its bearing on the origin of oceanic and continental alkali basalt, Earth Planet. Sci. Lett. 72, 23-38, 1985. [15] A.N. Halliday, G.A. Mahood, P. Holden, J.M. Metz, T.J. Dempster and J.P. Davidson, Evidence for long residence times of rhyolitic magma in the Long Valley magmatic system: the isotopic record in precaldera lavas of Glass Mountain, Earth Planet. Sci. Lett. 94, 274-290, 1989. [16] A.N. Halliday, J.P. Davidson, P. Holden, C.P. DeWolf, D.-C. Lee and F.J.G., Trace-element fractionation in a plume and the origin of HIMU mantle beneath the Cameroon line, Nature 347, 523-528, 1990. [17] S.R. Hart, Heterogeneous mantle domains: signatures, genesis and mixing chronologies, Earth Planet. Sci. Lett. 90, 273-296, 1988. [18] S.R. Hart, DC. Gerlach and W.M. White, A possible new Sr-Nd-Pb mantle array and consequences for mantle mixing, Geochim. Cosmochim. Acta 50, 1551-1557, 1986. [19] A. Simonetti and K. Bell, Nd, Pb and Sr isotopic data from the Napak carbonatite-nephelinite centre, eastern Uganda: an example of open-system crystal fractionation, Contrib. Mineral. Petrol. 115, 356-366, 1994. [ZO] M.J. Le Bas, R.W. LeMaitre, A. Streckeisen and B. Zanettin, A chemical classification of volcanic rocks based on the total alkali-silica diagram, J. Petrol. 27, 745-750, 1986. 1211 D.J. DePaolo, Trace element and isotopic effects of combined wall-rock assimilation and fractional crystallization, Earth Planet. Sci. L&t. 53, 189-202, 1981. [22] P.G. Coomer and D.K. Robertson, Lead isotope study of Archean mineralized areas in Tanzania, J. Geol. Sot. London 130,449-460, 1974. [23] Y. Asmerom and S.B. Jacobsen, The Pb isotopic evolution of the Earth: inferences from river water suspended loads, Earth Planet. Sci. Lett. 115, 245-256, 1993. [24] R.S. Cohen, R.K. O’Nions and J.B. Dawson, Isotope geochemistry of xenoliths from East Africa: impIications for development of mantle reservoirs and their interaction, Earth Planet. Sci. Lett. 68, 209-220, 1984.

124

C. Paslick et al. /Earth and Planetary Science Letters 130 (1995) 109-126

1251A.W. Hofmann and W.M. White, Mantle plumes from ancient oceanic crust, Earth Planet. Sci. Lett. 57,421-436, 1982. 1261 D. McKenzie and R.K. G’Nions, Mantle reserviors and ocean island basalts, Nature 301, 229-231, 1983. [27] AJ. Erlank, H.L. Allsopp, C.J. Hawkesworth and MA. Menzies, Chemical and isotopic characterization of upper mantle metasomatism in peridotite nodules from the Bultfontein kimberlite, Terra Cognita 2, 261-263, 1982. [28] M.A. Menzies and V.R. Murthy, Nd and Sr isotope geochemistry of hydrous mantle nodules and their host alkali basalts: implications for local heterogeneities in metasomatically veined mantle, Earth Planet. Sci. Lett. 46, 323-334, 1980. [29] C.J. Hawkesworth, P.D. Kempton, N.W. Rogers, R.M. Ellam and P.W. van Calsteren, Continental mantle lithosphere and shallow level enrichment processes in the Earth’s mantle, Earth Planet. Sci. Lett. 96, 256-268, 1990. [30] J.B. Dawson and J.V. Smith, Metasomatised and veined upper mantle xenoliths from Pello Hill, Tanzania: evidence for anomalously light mantle beneath the Tanzanian sector of the East African Rift Valley, Contrib. Mineral. Petrol. 100, 510-527, 1988. [31] A.P. Jones, J.V. Smith and J.B. Dawson, Glasses in mantle xenoliths from Olmani, Tanzania, J. Geol. 91, 167-178, 1983. 1321 R.L. Rudnick, W.F. McDonough and B.W. Chappell, Carbonatite metasomatism in the northern Tanzanian mantle: petrographic and geochemical characteristics, Earth Planet. Sci. Lett. 114, 463-475, 1993. [33] J.B. Dawson, D.G. Powell and A.M. Reid, Ultrabasic xenoliths and lava from the Lashaine volcano, Northern Tanzania, J. Petrol. 11, 519-548, 1970. [34] S.J.G. Galer and R.K. G’Nions, Residence time of thorium, uranium and lead in the mantle with implications for mantle convection, Nature 316, 778-782, 1985. [35] R.W. Carlson and A.J. Irving, Depletion and enrichment history of subcontinental lithospheric mantle: An OS, Sr, Nd and Pb isotopic study of ultramafic xenoliths from the northwestern Wyoming Craton, Earth Planet. Sci. Lett. 126, 457-472, 1994. [36] R.W. Williams, K.D. Collerson, J.B. Gill and C. Deniel, High Th/U ratios in subcontinental lithospheric mantle: mass spectrometric measurement of Th isotopes in Gaussberg lamproites, Earth Planet. Sci. Lett. 111, 257268, 1992. [37] R. Vollmer and M.J. Norty, Unusual isotopic variations in Nyiragongo nephelinites, Nature 301, 141-143, 1983. [38] G.R. Davies and F.E. Lloyd, Pb-Sr-Nd isotope and trace element data bearing on the origin of the potassic subcontinental lithosphere beneath south-west Uganda, in: Proc. 4th Int. Kimberlite Conf., pp. 784-794, Perth, 1986. [39] R. Altherr, F. Henjes-Kunst and A. Baumann, Asthenosphere versus lithosphere as possible sources for basaltic

magmas erupted during formation of the Red Sea: constraints from Sr, Pb and Nd isotopes, Earth Planet. Sci. Lett. 96, 269-286, 1990. [40] F. Martinez and J.R. Cochran, Structure and tectonics of the northern Red Sea: catching a continental margin between rifting and drifting, Tectonophysics 150, l-32, 1988. [41] W.K. Hart, G. Wolde-Gabriel, R.C. Walter and S.A. Mertzman, Basaltic volcanism in Ethiopia: constraints on continental rifting and mantle interactions, J. Geophys. Res. 94, 7731-7748, 1989. [42] J.-A. Barrat, B.M. Jahn, J.-L. Joron, B. Auvray and H. Hamdi, Mantle heterogeneity in northeastern Africa: evidence from Nd isotopic compositions and hygromagmaphile element geochemistry of basaltic rocks from the Gulf of Tadjoura and southern Red Sea regions, Earth Planet. Sci. Lett. 101, 233-247, 1990. [43] D.S. Ormerod, C.J. Hawkesworth, N.W. Rogers, W.P. Leeman and M.A. Menzies, Tectonic and magmatic transitions in the Western Great Basin, USA, Nature 333, 349-351, 1988. [44] R.A. Lange, I.S.E. Carmichael and P.R. Renne, Potassic volcanism near Mono basin, California: evidence for high water and oxygen fugacities inherited from subduction, Geology 21, 949-952, 1993. [45] C. Brooks and S.R. Hart, Rb-Sr mantle isochrons and variations in the chemistry of Gondwanaland’s lithosphere, Nature 271, 220-223, 1978. [46] T.H. Jordan, Composition and development of the continental tectosphere, Nature 274, 544-548, 1978. [47] E.R. Gxburgh and E.M. Parmentier, Thermal processes in the formation of continental lithosphere, Philos. Trans. R. Sot. London 288,415-429, 1978. [48] M.A. Menzies and C.J. Hawkesworth, Upper mantle processes and composition, in: Mantle Xenoliths, P.H. Nixon, ed., pp. 726-738, Wiley, 1987. [49] M. Menzies, N. Rogers, A. Tindle and C. Hawkesworth, Metasomatic and enrichment processes in lithsopheric peridotites: an effect of asthenosphere-lithosphere interaction, in: Mantle Metasomatism, pp. 313-361, Academic Press, London, 1987. [50] D.J. DePaolo and G.J. Wasserburg, Nd isotopic variations and petrogenetic models, Geophys. Res. Lett. 3, 249-252, 1976. [51] B.L. Weaver, The origin of ocean island basalt end-member compositions: trace element and isotopic constraints, Earth Planet. Sci. Lett. 104, 381-397, 1991. [52] K. Burke and A.J. Whiteman, Uplift, rifting and the break-up of Africa, in: Implications of Continental Drift for the Earth Sciences, D.H. Tarling and S.K. Runcom, eds., pp. 735-755, Academic Press, London, 1973. [53] A.B. Kampunzu and P. Mohr, Magmatic evolution and petrogenesis in the East African Rift System, in: Magmatism in Extensional Structural Settings, A.B. Kampunzu and R.T. Lubata, eds., pp. 85-136, Springer, 1991.

C. Paslick et al. /Earth

and Planetary Science Letters 130 (1995) 109-126

[54] U. Achauer, A study of the Kenya rift using delay-time

tomography analysis and gravity modeling, Tectonophysics 209, 197-207, 1992. [55] A.E. Ringwood, Phase transformations and differentiation in subducted lithosphere: implications for mantle dynamics, basalt petrogenesis, and crustal evolution, J. Geol. 90, 611-643, 1982. [56] R.K. O’Nions and D.P. McKenzie, Melting and continent generation, Earth Planet. Sci. Lett. 90, 449-456, 1988. [57] R.M. Ellam and C.J. Hawkesworth, Is average continental crust generated at subduction zones?, Geology 16, 314-317, 1988. [58] M.A. Menzies, A.N. Halliday, Z. Palacz, R.H. Hunter, B.G.J. Upton, P. Aspen and C.J. Hawkesworth, Evidence from mantle xenoliths for an enriched lithospheric keel under the Outer Hebrides, Nature 325, 44-47, 1987. [59] H.E. Newsom, W.M. White, K.P. Jochum and A.W. Hofmann, Siderophile and chalcophile element abundances in oceanic basalts, Pb isotope evolution and growth of the earth’s core, Earth Planet. Sci. Lett. 80, 299-313, 1986. 1601 W.M. White, 238U/ z@‘Pb in MORB and open system evolution of the depleted mantle, Earth Planet. Sci. Lett. 115, 211-226, 1993. [61] D.M. Miller, S.L. Goldstein and C.H. Langmuir, Cerium/lead and lead isotope ratios in arc magmas and the enrichment of lead in the continents, Nature 368, 514-520, 1994. [62] S.-S. Sun, Lead isotopic study of young volcanic rocks from mid-ocean ridges, ocean islands and island arcs, Philos. Trans. R. Sot. London A297,409-445, 1980. 1631 S.J.G. Galer and R.K. O’Nions, Chemical and isotopic studies of ultramafic inclusions from the San Carlos Volcanic Field, Arizona: a bearing on their petrogenesis, J. Petrol. 30, 1033-1064, 1989. 1641 E.H. Hauri and S.R. Hart, Re-0s isotope systematics of HIMU and EM11 oceanic island basalts from the south Pacific Ocean, Earth Planet. Sci. Lett. 114, 353-371, 1993. [65] M.F. Roden, T. Trull, S.R. Hart and F.A. Frey, New He, Nd, Pb, and Sr isotopic constraints on the constitution of the Hawaiian plume: results from Koolau Volcano, Oahu, Hawaii, USA, Geochim. Cosmochim. Acta 58,1431-1440, 1994. [66] M.O. Garcia, B.A. Jorgenson, J.J. Mahoney, E. Ito and A.J. Irving, An evaluation of temporal geochemical evolution of Loihi Summit lavas: results from Alvin submersible dives, J. Geophys. Res. 98, 537-550, 1993. 1671 M. Storey, A.D. Saunders, J. Tarney, P. Lest, M.F. Thirlwall, R.N. Thompson, M.A. Menzies and G.F. Marriner, Geochemical evidence for plume-mantle interactions beneath Kerguelen and Heard Islands, Indian Ocean, Nature 336, 371-374, 1988. [68] J. Barling and S.L. Goldstein, Extreme isotopic variations in Heard Island lavas and the nature of mantle reservoirs, Nature 348, 59-62, 1990.

125

[69] S.H. Richardson, A.J. Erlank, A.R. Duncan and D.L. Reid, Correlated Nd, Sr and Pb isotope variation in Walvis Ridge basalts and implications for the evolution of their mantle source, Earth Planet. Sci. Lett. 59, 327342, 1982. [70] E. Wright and W.M. White, The origin of Samoa: new evidence from Sr, Nd, and Pb isotopes, Earth Planet. Sci. Lett. 81, 151-162, 1986/87. (711 A.N. Halliday, G.R. Davies, D.-C. Lee, S. Tommasini, C.R. Paslick, J.G. Fitton and D.E. James, Lead isotope evidence for young trace element enrichment in the oceanic upper mantle, Nature 359, 623-627, 1992. [72] P.J. Betton and L. Civetta, Strontium and neodymium isotopic evidence for the heterogeneous nature and development of the mantle beneath Afar (Ethiopia), Earth Planet. Sci. Lett. 71, 59-70, 1984. [73] R. Vollmer and M.J. Norry, Possible origin of K-rich rocks from Virunga, East Africa. by metasomatism of continental crustal material: Pb, Nd and Sr isotopic evidence, Earth Planet. Sci. Lett. 64, 374-386, 1983. [74] M.J. Norry, P.H. Truckle, S.J. Lippard, C.J. Hawkesworth, S.D. Weaver and G.F. Marriner, Isotopic and trace element evidence from lavas bearing on mantle heterogeneity beneath Kenya, Philos. Trans. R. Sot. London A297, 259-271, 1980. (751 G.R. Davies and R. MacDonald, Crustal influences in the petrogenesis of the Naivasha basalt-comendite complex: combined trace element and Sr-Nd-Pb isotope constraints, J. Petrol. 28, 1009-1031, 1987. (761 S.-S. Sun and W.F. McDonough, Chemical and isotopic systematics of oceanic basalts: implications for mantle composition and processes, in: Magmatism in the Ocean Basins, A.D. Saunders and M.J. Norty, eds., Geol. Sot. London Spec. Publ. 42, 313-345, 1989. [77] G.R. Davies, M.J. Norry, D.C. Gerlach and R.A. Cliff, A combined chemical and Pb-Sr-Nd isotope study of the Azores and Cape Verde hot-spots: the geodynamic implications, in: Magmatism in the Ocean Basins, A.D. Saunders and M.J. Norry, eds., Geol. Sot. London Spec. Publ. 42, 231-255, 1989. (781 M. Tatsumoto and R.J. Knight, Isotopic composition of lead in volcanic rocks from central Honshu with regard to basalt genesis, Geochem. J. 3, 53-86, 1969. [79] W.M. White and B. Dupre, Sediment subduction and magma genesis in the Lesser Antilles: isotopic and trace element constraints, J. Geophys. Res. 91. 5927-5941, 1986. [80] A.N. Halliday, D.-C. Lee, S. Tommasini, G.R. Davies, CR. Pashck, J.G. Fitton and D. James, Incompatible trace elements in OIB and MORB and melt enrichment in the sub-oceanic mantle, Earth Planet. Sci. Lett., in prep., 1994. [81] D. Weis, Y. Bassias, I. Gautier and J.-P. Mennessier, Dupal anomaly in existence 115 Ma ago: evidence from isotopic study of the Kerguelen Plateau (South Indian Ocean), Geochim. Cosmochim. Acta 53,2125-2131,1989.

126

C. Paslick et al. /Earth and Planetary Science Letters 130 (1995) 109-126

[821 J.D. Kramers, J.C.M. Roddick and J.B. Dawson, Trace element and isotope studies on veined, metasomatic and ‘MARID’ xenoliths from Bultfontein, South Africa, Earth Planet. Sci. Lett. 65, 90-106, 1983. [83] D.-C. Lee, A.N. Halliday, G.R. Davies, E.J. Essene, J.G. Fitton and R. Temdjim, Melt enrichment of shallow depleted mantle: a detailed petrological, trace element and isotopic study of mantle derived xenoliths and

megacrysts from the Cameroon line, J. Petrol., in prep., 1994. [84] A. Meijer, T.-T. Kwon and G.R. Tilton, U-Th-Pb partitioning behavior during partial melting in the upper mantle: implications for the origin of high mu components and the ‘Pb paradox’, J. Geophys. Res. 95433-448, 1990.