Geochemistry of mafic volcanic rocks from the Lake Kivu (Zaire and Rwanda) section of the western...

Journal of Volcanology and Geothermal Research, 39 (1989) 73-88 73 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

Geochemistry of mafic volcanic rocks from the Lake Kivu (Zaire and Rwanda) section of the western branch

of the African Rift

G. MARCELOT 1, C. DUPUY 1, J. DOSTAL 2, J.P. RAN~ON 3 and A. POUCLET 4

1Centre Gdologique et G~ophysique, Universit~ des Sciences et Techniques du Languedoc, Place Eugene Bataillon, 34060 MontpeUier Cddex, France

2Department of Geology, Saint Mary's University, Halifax, N.S. B3H 3C3, Canada 3Ddpartement de Gdologie, B.R.G.M., B.P. 6009, 45060 Orleans, France

4Ddpartement de G~ologie, Universitd d'Orldans, B.P. 6759, 45067 Orldans, France

(Received July 18, 1988; revised and accepted March 14, 1989 )

Abstract

Marcelot, G., Dupuy, C., Dostal, J., Ran~on, J.P. and Pouclet, A., 1989. Geochemistry of mafic volcanic rocks from the Lake Kivu (Zaire and Rwanda) section of the western branch of the African Rift. J. Volcanol. Geotherm. Res., 39: 73-88.

The volcanic rocks from the Lake Kivu volcanic province (southeastern Zaire and Rwanda) are part of the western branch of the east African Rift system and were emplaced from early Miocene to sub-Recent times. The lavas are mainly of basaltic composition and range from quartz-normative tholeiites through alkali basalts and basanites to melilite nephelinites. They were derived from a heterogeneous upper mantle source by variable degrees of melting. The inferred source compositions are highly variable in K, Rb, Ti, V, P, Th and Hf but relatively uniform in light REE, Sr and Zr. Compared with primordial mantle, the source beneath the western branch of the East African Rift is strongly enriched in highly incompatible elements. The enrichment and heterogeneity probably were caused by interaction of the mantle with a kimberlite melt.

Introduction

During the last decade, numerous studies have at tempted to decipher the chemical composition of upper mantle sources of various basaltic magmas. These studies focused mainly on oceanic areas, especially the source regions for mid-ocean ridge basalts (MORB) and oceanic island basalts (OIB). There is significantly more controversy about the origin of continental volcanics, particularly alkali basalts and related rocks, and the chemical nature of their sources. Numerous geochemical and petrological features of these rocks are frequently taken

as evidence of large-ion-lithophile element (LILE)-enriched mantle (e.g., Lloyd and Bai- ley, 1975; Frey et al., 1978) and raise questions about the nature of upper mantle sources and the relationship among the various magmatic associations within a single volcanic province. The western branch of the east African Rift, where several alkali mafic and mafic volcanic types of restricted age occur within a limited space, provides an opportunity to investigate these problems.

This study presents data on the major- and trace element compositions of the various mafic volcanic rocks occurring north and south of

0377-0273/89/$03.50 © 1989 Elsevier Science Publishers B.V.

74 G. MARCELOT ET AL.

Lake Kivu (eastern Zaire and Rwanda) in the Western valley of the East African Rift Zone in order to: (1) geochemically characterize the different volcanic rocks and their source regions; and (2) constrain the petrogenesis of these rocks.

G e n e r a l g e o l o g y

The Lake Kivu volcanic province is located in the Western Rift Valley of Central Africa (Fig. 1) and consists of two major volcanic

areas: Virunga and South Kivu Lake (Fig. 2). Virunga, a classic volcanic province (Holmes and Harwood, 1937) is dominated by eight large central volcanoes (Fig. 2) associated with transverse faults; two of them - Nyiragongo and Nyamuragira - are still active. The oldest volcanic rocks in the area which form the base- ment of the stratovolcanoes, range in age from 20 to 9 Ma and erupted contemporaneously with the early South Kivu volcanics. However, these lavas have basanitic compositions whereas the equivalent South Kivu volcanics are mainly

N

I 30°E

- - l O ° S

- ~ Z ~ l a k e ~ "T-: lake V ~ Victoria .G::

Kiv'u/ - z / -

lake o~ / / U J Tanganyika

\ x ~ % lake Rukwa

"~x.\ R

lake I 1 Malawi

300 km I I 4o!

I /I

GULF OF ADEN

,.~.~~.u.- lo" N _ !

o_

r

k

f '

Fig. 1. Generalized map showing distribution of volcanic rocks (dotted areas) of the east African rift. In the western branch, the volcanic fields are: T= Toko-ankole; V= Virunga; SK= South Kivu; R = Rungwe.

GEOCHEMISTRY OF MAFIC VOLCANIC ROCKS, LAKE KIVU VOLCANIC PROVINCE

~: 2

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R W A N D A

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BK 24 ( 7. OL

R W 81 6.

RW82 ~7.

BK20 (23.5}

BK 19 ( 9.6 )

-- RW86 (9.0) - - R W 8 7 (11.4) RW84 (14.1)

- - R W 8 8 (10.6)

- R W 8 3 ( 7 . 7 5 )

/ / \ 3 0 k m RW90 . ~ ' J RW 89 29"L- 3"5

- - { 8 . 1 ) ( 7 .7 ) I

Fig. 2. Generalized tectonic setting of the Lake Kivu zone showing the location and age (in Ma, in brackets) of the studied samples• The K-Ar determinations are from Bellon and Pouclet (1980), Rancon and Demange (1983) and H. Bellon (pers. commun., 1986 ). ( I ) volcanic zones; (2) stratovolcanoes; (3) major faults• N = Nyamuragira; N y = Nyiragongo; K = Karisimbi; V = Visoke; S = Sabinyio; G = Mgahinga; M = Muhauvura.

tholeiites. The overlying volcanic rocks of Virunga were emplaced at the end of the Pli- ocene (Guilbert, 1977). The volcanoes of Vi- runga exhibit a remarkable diversity of silica- undersaturated rocks, ranging from leucite- bearing to nepheline and/or melilite-bearing lavas.

In the South Kivu region, the volcanic se- quences contain basaltic flows with subordinate amounts of differentiated rocks. The vol-

canism was mainly fissure-fed with no central volcanoes and appeared as early as the Mio- cene. The South Kivu area (Bukavu) in Rwanda and Zaire is located in the main por- tion of the Western branch of the East African Rift (Figs. 1 and 2). The compositions of the lavas range from quartz-normative tholeiites to alkali basalts and basanites. Three volcanic cycles have been recognized in the South Kivu region (Bellon and Pouclet, 1980; Kampunzu


et al., 1983). The first volcanic cycle probably started in the early Miocene (about 25 Ma ago ) with the outpouring of tholeiitic basalts, post- dating the pre-rift stage, and lasted till about 9 Ma ago (Kampunzu et al., 1983). This earliest volcanic activity occurred mainly along the margins of the rift zone (Fig. 2 ). During the late Miocene, it was closely followed by alkaline volcanism of the second cycle. Volcanic activity reached its peak between 2 and 8 Ma ago with the eruption of basanite and alkali basalt lavas. After an apparent period of quiescence, a final volcanic cycle of Lower Pleistocene age occurred and is represented by alkali basaltic lavas emplaced in the axial part of the rift. The Lake Kivu volcanic rocks also show a distinct variation in chemical and mineralogical composition (Kampunzu et al., 1983; Auchapt et al., 1987) which appears to be typical of a continental rift stage (Kampunzu et al., 1983). In addition, the Lake Kivu volcanic suites include subordinate differentiated rocks which underwent extensive fractional crystallization.

Petrographic notes

On the basis of petrography and mineralogy, the analyzed mafic rocks can be divided into seven groups, each of which displays distinctive geochemical features. These are: tholeiites (quartz-normative); olivine tholeiites (olivine- normative); alkali basalts; nepheline-bearing basanites; leucite-bearing basanites (K-rich basanites); olivine nephelinites; and melilite nephelinites.

Quartz-normative tholeiites are usually composed ofplagioclase (An45_65), pigeonite, augitic clinopyroxene and subordinate Fe-Ti oxides and exhibit subophitic to ophitic textures. Oli- vine-normative tholeiites are subophitic or porphyritic and made up of phenocrysts of plagioclase (An60_6~), olivine (F050 85) and augitic clinopyroxene (Wo3a_50En40_46Fslo_17) set in a groundmass containing plagioclase, clinopyroxene and Fe-Ti oxides and glass. Alkali basalts have porphyritic textures with pheno-

crysts of olivine (Fo75_s6) clinopyroxene (Wo40_55En35 47Fs10_20) and minor plagioclase (An55_70) enclosed in a matrix containing olivine, clinopyroxene, plagioclase, Fe-Ti oxides and glass. Basanites are similar to alkali basalts but contain nepheline in the groundmasses. The K-rich basanites are porphyritic lavas with phenocrysts of leucite (up to 30 vol.% ), clinopyroxene (W046 51En~6_47Fs6_14), olivine (Foso_ 8~), plagioclase (An6~ 70) and phlogopite [Mg/ (Mg+Fe)=0.77-0.82] set in groundmasses composed of nepheline, clinopyroxene, Fe-Ti oxides, apatite and glass. The melilite nephelinites display porphyritic textures with phenocrysts of melilite, nepheline and minor clinopyroxene and Fe-Ti oxides and mesostases containing nepheline, clinopyroxene, Fe-Ti oxides and glass. The olivine nephelinites are porphyritic and contain phenocrysts of nepheline and subordinate leucite, clinopyroxene and Fe-Ti oxides, and groundmasses composed of nepheline, leucite, olivine, calcite, apatite and glass.

Sampling and analytical techniques

The locations of the analyzed samples are shown on Fig. 2, together with the K-Ar ages reported by Bellon and Pouclet (1980), Ran- con and Demange (1983) and H. Bellon (pers. commun., 1986). They have been selected from approximately one hundred large exposures cropping out in each volcanic region. The field relationships and petrographic descriptions of the volcanic rocks are given by Pouclet (1980), Rancon and Demange (1983) and Auchapt et al. (1987). Thirty-two selected samples were analyzed for major elements and Li by atomic absorption spectrometry. Rare-earth elements (REE), Hf, Th, Sc, and Co were determined by neutron activation analysis and the remaining trace elements were analyzed by X-ray fluores- cence spectrometry. The precision and accu- racy of the trace-element data have been reported by Dostal et al. (1986). Briefly, the precision is generally better than 10% of the

GEOCHEMISTRY OF MAFIC VOLCANIC ROCKS, LAKE KIVU VOLCANIC PROVINCE 77

amount present and results for s tandard rocks are in good agreement with recommended values.

G e o c h e m i s t r y

The various mafic rock types from the Lake Kivu area have distinct relationships in terms of the K 2 0 / N a 2 0 ratio and SiO2 (Fig. 3). The tholeiites, alkali basalts and basanites have K 2 0 / N a z O < 0 . 5 but differ in their SiO2 contents which decrease from the tholeiites to the basanites. On the other hand, the K-rich basanites and nephelinites have K20 /Na20 around 1. The K-rich basanites, which are the most abundant rock type of the Eastern Virunga area and occur at all stratovolcanoes, have higher SiO2 contents than melilite nephelinites which occur only in the Nyiragongo volcano.

The differences between the various mafic rocks are also shown clearly in their trace-element concentrations. Incompatible trace elements (Table 1 ) increase with the degree of silica-undersaturation from the quartz-normative tholeiites through the alkali basalts and basanites to the melilite nephelinites. The K-rich basanites have compositions similar to the basanites for light REE (LREE) , Th, Sr and Ba and to the nephelinites Ti, V and Rb. However, the K-rich basanites differ from the other alkali

1.5

0

O 4~

05 , "eel" ®

l o A A ° ° @

O O

I I I I

3 5 4 5 Si 02 % 55

Fig. 3. K20/Na20 vs. Si02 (as analyzed) in the mafic lavas of the Lake Kivu volcanic province. Empty circle = quartz- normative tholeiite; circle and dot=olivine-normative tholeiite; solid circle=alkali basalt and basanite; trian- gle=nepheline-bearing basanite; star=K-rich basanite; square = olivine nephelinite; asterisk = melilite nephelinite.

1000

100

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0

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®

x x

x \ x

\ x

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I I I [ I I I La Ce Nd Sm Eu Tb Yb Lu

,, ® "%

I I I I I I La Ce Nd Sm Eu Tb Yb Lu

Fig. 4. Rare earth elements normalized to chondrites in the mafic lavas of the Lake Kivu volcanic province. A. Tholei- ites (dashed-dotted lines), alkali basalts and basanites {solid lines) and nephelinites (dashed lines). B. K-rich basanites (solid lines). The heavy dashed line represents leucite-bearing lava RW 100 and the thin dashed lines de- lineate the range of K-rich basalts from California (Van Kooten, 1980 ).

mafic rocks by their lower P20~, AleQ, Na20 and heavy REE ( H R E E ) abundances. The chondrite-normalized REE patterns (Fig. 4 ) are enriched in L R E E with La /Yb increasing from the quartz-normative tholeiites to the nephelinites. The range of L R E E abundances is sig-


TABLE1A

Major- and trace-element compositions of representative basaltic lavas from Lake Kivu

QTH OTH NAB

RW86 RW87 TH KT1 TR N373 N378 LKA4

SiO~ 52.57 49.68 51.13 48.98 47.59 46.90 44.13 42.76

AI~O:~ 13.98 13.75 15.16 13.55 14.85 16.19 15.72 11.68

FezO.~ 10.86 13.26 11.60 10.57 12.86 11.62 11.08 10.47 MnO 0.14 0.30 0.17 0.16 0.20 0.21 0.22 0.17

MgO 6.90 7.00 6.41 10.68 8.10 5.05 6.52 12.11

CaO 8.23 8.86 9.48 10.10 8.98 8.81 10.92 12.90

Na20 2.64 2.61 2.79 2.56 2.90 3.66 3.80 3.64

K20 0.95 0.62 0.44 1.14 0.86 1.45 1.15 0.97

Ti02 1.57 1.59 1.65 1.17 1.72 1.66 1.53 1.77

P20~ 0.24 0.24 0.28 0.44 0.54 1.25 1.12 1.14 L.O.I. 1.14 1.54 - 0.31 2.30 2.84 2.00

99.22 99.45 99.11 99.66 98.60 99.10 99.03 99.61

[ Mg ] 0.586 0.540 0.695 0.489 0.563 0.720

Li (ppm) 6 6 5 6 6 9 8 9

Rb 27 16 12 26 22 40 49 32

Sr 254 247 279 495 499 1400 1890 1620

Ba 229 740 601 1482 1710 1311 Sc 21.8 24.4 25 216.1 25 13.7 16.0 20.8

V 182 198 206 168 218 192 204 168

Cr 237 248 235 413 284 29 168 388

Co 43 58 52 54 52 40 39 52

Ni 140 131 135 282 181 32 76 258

Cu 66 58 67 61 60 33 65 42 Zn 101 103 116 78 109 101 95 84

La 27.5 20.3 24.8 45.6 54.2 141 132 135

Ce 50.4 38.1 44.7 74.1 88.7 257 266 252 Nd 18.1 17.2 24.9 84.2 93.0 92.1

Sm 4.53 4.11 4.90 4.43 6.74 11.6 12.7 12.9

Eu 1.48 1.46 1.71 1.45 2.21 3.32 3.70 3.77

Tb 0.79 0.75 0.90 0.62 1.05 1.05 1.00 1.07

Yb 2.20 2.09 2.54 2.00 2.74 3.07 12.71 2.02

Lu 0.35 0.35 0.40 0.31 0.44 0.44 0.41 0.31 Hf 3.0 2.7 3.04 2.1 3.23 4.4 4.0 3.2

Th 6.0 3.0 3.2 9.8 7.5 22.1 17.3 16.8

Y 35 22 36 36 35 30

Zr 115 104 154 273 265 192 Nb 25 62 54 190 237 129

QTH = quartz-normative tholeiites; OTH = olivine-normative tholeiites; NAB = Na-rich basanites (nepheline-bearing); ON = olivine nephelinites; MN = melilite nephelinites.

TH and TR=averages of tholeiitic and transitional basalts, respectively, from Bukavu {south of Lake Kivu) given by Auchapt et al. (1987).

[Mg] = (Mg/Mg+Fe 2+ ) with Fe3+/Fe 2+ assumed to be 0.15.

South Kivu, phase 1" RW86=Bitare Bay, RW87=Ntendezi; phase 3: KTl=Cib inda . Virunga, phase 1: LKA4=Nord-Idjwi, N373 = Mushebele, N378 = Tongo; phase 2: RG 47, NY133, RW 73, NYB8, NY50, NY51A, NY45, NY46 = Nyiragongo, VII = Visoke. Fe203 = total Fe as Fe203. L.O.I. = loss-on ignition.


TABLE 1A (continued)

79

ON MN

RG47 NY133 VII RW73 NYB8 NY50 NY51A NY45 NY46

41,00 41.22 40.82 36.84 34.51 33.91 36.31 34.77 34.93 12.75 12.22 13.20 12.84 11.26 10.51 11.52 11.77 10.85

12.66 12.47 12.80 12.81 12.32 11.19 12.27 11.81 12.01

0.21 0.21 0,22 0.28 0,31 0.20 0.21 0.22 0.22

8.40 9.93 7.50 6.01 5.30 7.48 6.40 6.98 7.05

13.05 12.93 12.73 16.10 15.34 16.20 16.71 17.85 17.22

3.30 3.13 3.42 4.50 3.90 3.38 4.32 4.07 4.11

3.35 3.02 3.65 4.00 4.00 3,32 4.13 3.79 4.01 3.32 3.16 3.26 3.04 2,96 2.18 1.87 2.11 1.88

1.15 1.00 1,25 1.85 1.75 1.61 2.19 2.07 2.03

0.12 0.35 0.44 1.39 7.47 8.85 2.34 4.28 4.15

99.31 99.64 99.29 99.06 99.12 98.83 98.27 99.72 98.46

0.599 0.643 0.568 0.510 0.473 0.578 0.532 0.562 0.557

8 7 9 12 18 10 16 13 12

94 70 107 106 105 86 133 78 94 1420 1190 1515 2650 3115 2070 3370 3475 2670 1590 1680

20.4 26 16.8 11.1 10.5 16 9.5 8.6 9

345 342 415 403 278 326 389 280 284 412 222 13 18 105 22 20 31

51 54 48 42 41 42 42 42 44

142 116 33 35 42 41 25 33

86 101 156 135 113 151 139 112

101 111 93 112 326 110 113 110 118 107 128 241 258 170 286 232 211

235 209 251 418 485 322 526 435 400

96.7 84 102 156 171 123 190 160 149 14.4 13.5 14.97 25,9 24.5 18.3 27.5 23.0 21.6

4.08 3.60 4.16 6.55 6.76 4.80 7.60 6.20 5.80 1.22 1.02 1.35 2.05 1.98 1.20 1.80 1.50 1.41

2.40 2.20 2.53 3,79 4.25 2.59 4.23 3.50 3.06

0.33 0.33 0.37 0,56 0.61 0.40 0.62 0.50 0.45

6.8 6.1 6.5 5,8 6.5 5.4 6.1 5.0 4.6 14.3 12.7 15.4 22,2 36.1 19.5 39.0 27.0 25.0 31 35 44

353 368 415 166 190 319

nificantly greater than that of HREE, especially in the K-rich basanites where LaN (N = chondrite normalized) varies by a factor of 2.5 whereas YbN varies by a factor of 1.5.

Several inter-element correlations, particularly of compatible trace elements among sam-

ples of a single rock type, suggest that the mafic rocks underwent fractional crystallization. A distinct positive correlation between Ni and Cr usually accompanied by covariation of Mg/Fe ratios suggests fractionation of olivine and clinopyroxene, in accordance with the petro-


TABLE 1B

Major- and trace-element compositions of representative basaltic lavas from Lake Kivu

KB

RW3 RW5 RW29 RWl00 RW44 RW45 RW52 RW57

SiO2 46.28 46.12 44.40 39.86 43.81 45.91 45.44 44.38

Al2Oz 12.38 11.82 12.00 10.68 12.25 12.46 13.57 10.49

Fe203 12.14 11.24 13.47 14.30 13.80 12.48 12.60 12.34 MnO 0.18 0.17 0.19 0.22 0.20 0.18 0.19 0.18

MgO 9.23 10.41 8.92 6.37 8.43 8.24 6.70 11.60 CaO 11.00 9.90 11.52 12.62 11.55 10.07 9.00 13.15

Na20 2.43 2.46 2.21 3.67 2.14 2.68 3.18 1.98

K~O 1.60 2.89 2.71 5.25 2.82 3.27 4.17 1.55

TiO2 3.04 2.90 3.22 5.60 3.66 3.24 3.40 2.82

P205 0.56 0.62 0.54 0.88 0.56 0.66 0.80 0.46 L.O.I. 0.49 0.65 - 0.02 0.02 0.14

99.33 99.18 99.18 99.45 99.24 99.19 99.07 99.09

[ Mg ] 0.631 0.675 0.599 0.502 0.580 0.598 0.545 0.679

Li (ppm) 7 8 7 10 7 9 8 6 Rb 184 96 76 144 90 102 128 107

Sr 819 1026 855 1600 877 895 1193 1148

Ba 1070 1262 1142 1201 991 Sc 30.2 26.1 30.0 21.2 31.1 24.7 20.7 38.8

V 303 278 404 502 408 300 295 364

Cr 494 700 375 65 317 429 249 672

Co 51 50 60 50 59 51 50 62 Ni 48 156 124 55 99 138 84 108

Cu 27 23 66 132 58 64 64 29

Zn 92 86 95 111 96 93 96 81

La 66.7 95.2 71.1 150 75.1 87.6 102 56.9 Ce 134 184 141 285 148 172 192 122

Nd 55.2 69.3 57.6 107 59.6 62.2 68.7 50.5

Sm 9.87 11.21 10.2 15.4 10.6 10.1 10.2 8.52 Eu 2.74 2.85 2.67 4.03 2.79 2.71 2.82 2.45

Tb 1.02 1.01 0.97 1.10 0.97 0.96 0.95 0.88

Yb 2.21 2.06 1.96 2.16 1.98 1.93 1.94 1.77

Lu 0.36 0.35 0.33 0.31 0.32 0.33 0.31 0.29

Hf 5.8 6.6 5.8 8.7 6.1 6.1 6.3 4.8 Th 10.6 17.4 11.2 22.3 11.7 15.6 17.5 7.6

Y 31 27 26 27 23

Zr 251 308 258 266 216

Nb 80 118 93 95 72

KB = K-rich basanites, AB = alkali basalts and basanites. [Mg] = M g / ( M g + F e 2+ ) with Fe3+/Fe 2+ assumed to be 0.15.

South Kivu, phase 2:BK24 = Bukavu, RW88 = Bugarama graben, RW82, RW89 = Upper Ruzizi river, RW83 = Mushava; phase 3:

TB4 = Cibinda. Virunga, phase 2: RW3, RW5 = Muhavura, BW29, RWl00 = Visoke, RW44 = Sabyinio, RW45, RW52, RW57 = Karisimbi, RW64 = Mgahinga, N2 = Nyimuragira. Rungwe: T49 = Rungwe.

Fe203 = total Fe as Fe203. L.O.I. = loss-on-ignition.


TABLE 1B (continued)

81

KB AB

RW64 N2 TB4 T49 BK24 RW88 RW89 RW82 RW83

46.10 44.90 46.84 45.10 46.45

12.25 13.55 13.56 14.05 13.80 10.70 12.80 11.20 11.38 10.90

0.17 0.18 0.18 0.18 0.18

12.17 7.50 8.87 9.00 8.61

11.30 10.70 11.59 10.84 10.90

1.70 2.80 3.52 3.65 3.67

1.87 2.90 1.60 1.70 1.55 2.30 3.37 1.53 2.13 2.04

0.40 0.57 0.70 0.97 0.87

0.12 - 0.31 1.15

99.08 99.27 99.59 99.31 100.12

0.719 0.569 0.642 0.640 0.640

6 7 8 8 8

64 80 46 33 35 607 873 772 1125 1152

767 1079 1123 1261 1038

35.4 26.3 27,4 22.8 22.6

302 314 219 219 213 968 251 301 422 319

54 52 48 48 44

172 102 176 142 166

22 76 75 64 59 79 96 88 92 90

51.8 66.8 72.3 71.7 94.6

101 138 123 147 178

41.1 56.4 40.6 61.5 65.0

7.43 9.47 7.39 9.86 10.4

1.99 2.72 2.10 2.93 3.04

0.83 0.95 0.88 0.87 1.11

1.91 2.22 2.50 2.23 2.44

0.32 0.34 0.40 0.31 0.37 4.2 6.0 3.0 4.0 4.3

8.4 10.0 13.0 7.3 11.6 24 28 30 28 32

186 278 143 228 224

59 100 93 88 125

43.90 47.62 45.65 46.60 14.94 14.35 14.45 14.10

12.88 11.50 10.85 10.95

0.18 0.15 0.20 0.21

8.80 6.70 7.17 8.40

9.85 10.66 11.54 10.70

3.33 3.00 3.63 3.45 1.40 1.35 1.56 1.22

2.87 2.33 2.06 2.12

0.96 0.86 1.00 0.89

0.98 1.36 1.03 0.51

100.09 99.88 99.14 99.15

0.606 0.566 0.596 0,632

8 5 8 7

36 30 43 30

702 890 1107 1050 614 908 1340 996

23.3 26.6 25.3 23.4

234 262 252 220

213 272 138 296

48 44 40 48

147 129 80 171

53 42 61 63

110 96 89 89 45.4 79.2 111 81.2

94.3 152 197 151

46.8 60.9 67.2 59.1

10.2 10.3 11.5 9.64

3.27 3.04 3.31 2.89

1.35 1.20 1.21 1.05

2.85 2.81 2.78 2.40

0.40 0.40 0.44 0.38

5.8 4.9 4.9 4.2

4.7 10.6 14.2 9.6

36 34 37 31 282 235 243 210

67 137 170 109

graphic observations. The variations of incompatible elements within the individual rock types are minor compared with the variations between the various rock types. However, even for a single rock type, the trace-element varia-

tions cannot generally be explained by fractional crystallization alone. For example, the La/Yb ratio of the K-rich basanites varies between 24 and 69 and cannot be accounted for solely by clinopyroxene fractionation. Thus, the

82 O. MARCELOT ET AL.

variations of incompatible trace elements mainly reflect variable degrees of partial melting and a heterogeneous upper mantle source.

Rare earth elements

As shown in Figure 5A, several pairs of LREE (e.g., La vs. Ce, Ce vs. Nd, etc. ) display strong positive correlations among the various mafic

3000

1000

S r ( ppm)

A ~

100 300

"*" Q

I I

50O

Ce(ppm)

25

15

Sm ( p p m )

¢:." e~

I I

5O

. ®

I

150

N d ( p p m ]

500

300

100

C e ( p p m )

W~

® W~

~ La (ppm) l I I I I

50 150 250

Fig. 5. A. Ce vs. La; B. Sm vs. Nd; C. Sr vs. Ce for the mafic rocks from the Lake Kivu area. Symbols as for Fig. 3.

rocks, with the data plotting along lines passing through the origin, suggesting that LREE have identical bulk mineral/liquid partition coefficients (D) between the diverse lava types and their mantle residues, and that mantle sources are essentially homogeneous with respect to La and Ce concentrations. Thus the wide range of LREE abundances may be due to the different degrees of partial melting required to generate the various mafic lavas.

The middle and heavy REE (e.g, Nd vs. Sin, Sm vs. Eu, Yb vs. Lu) are also distinctly correlated (Fig. 5B ) but these linear trends do not extend through the origin, reflecting a slight difference in D values. On the other hand, the correlation is rather poor between LREE and HREE (e.g., La vs. Yb; Fig. 6 ). The abundances of HREE, particularly of Yb are relatively constant in the sequence from tholeiites to basa-

300 I l I I

E o. o.

210

120

®

© ©

0 i

15

i

#

Yb ppm I I I I

2.5 3.5 4.5

Fig. 6. La vs. Yb for the Lake Kivu area. Symbols as for Fig. 3.


nites but decrease in the K-rich basanites. This suggests Dyb values close to, or slightly higher than, 1. The REE distributions in the various lavas are consistent with an origin by variable degrees of partial melting from a common source leaving a residue with variable amounts of garnet (Kay and Gast, 1973). Regardless of mineralogy, the upper mantle source should have the following ratios: La/Ce = 0.52, Ce/Nd = 2.6, Nd /Sm=6.1 . These values are close to those for the sources of within-plate basalts (Wede- pohl, 1985 ) and oceanic-island basalts (Sun and Hanson, 1975; Clague and Frey, 1982) and, as demonstrated by Clague and Frey (1982), imply a LREE-enriched upper mantle source.

Alkali-earth elements

The strong correlation between Sr and Ce (Fig. 5C ) and the constancy of the Sr/Ce ratio (6.3) of the analyzed lavas indicate similar behaviour of these two elements during partial melting as already suggested by Sun and Han- son (1975) and Wood (1979). The value of the Sr/Ce ratio which is typical of continental alkali basalts (Wass and Rogers, 1980; Wede- pohl, 1985) is lower than that of oceanic island basalts (Clague and Frey, 1982; Liotard et al., 1986) and about half of the chondritic value. The Ba/La ratio varies between 10 and 14; a range characteristic of oceanic and continental alkali basalts and chondrites.

Alkali elements

Plots involving Rb vs. LREE show consid- erable scatter, although the K-rich basanites have the highest Rb /LREE ratios which sepa- rate them from the other mafic lavas (Fig. 7). A larger increase of Ce compared to Rb for all the undersaturated rocks {Fig. 7) suggests D~b > Dee and implies the presence of a K-rich phase in the residues. Assuming Dce -- 0.01 (ap- proximated from the partition coefficients of Irving and Price, 1981 ) and an upper mantle source with 9 times chondritic abundances, the

Rb(ppm) C

150 ~, / 1 .

,oo . f :

J .

50 87 ~ , ~..........~IA

•

2d o Ce ( ppm}

I I I

100 300 500

Fig. 7. Rb vs. Ce for the mafic rocks from the Lake Kivu area and calculation of the equilibrium partial melting model using the following parameters:

Line: A B C

Co ce (ppm) 8 8 8 C~ b (ppm) 3 5 5 Dce 0.01 0 . 0 1 0.01 DRb 0.05 0.04 0.02

The numbers along lines A, B, C refer to the degree of partial melting. DRb (bulk partition coefficient for Rb) is calculated taking KRb mica/liq. = 3.2 (Irving and Price, 1981 ) and assuming residual mica in the following proportion: 1.6%: 1.2%: 0.6%. C ce and C~ b were selected so that the model fit the data with a reasonable degree of partial melting. Symbols as on Fig. 3.

calculation of a simple batch partial melting process shows that the analytical data on Fig- ure 7 require both variable DRb and variable Rb content in the sources, suggesting the presence of variable amounts of a K-rich phase.

K and Rb are positively correlated and the K/Rb ratio varies from 200 to 450 with an average value of 300, typical of continental alkali basalts (Wedepohl, 1985). The Ba/Rb ratios of the K-rich basanites remain relatively constant (9-12 ) and similar to the primitive mantle value (Hofmann and White, 1983). On the other hand, this ratio is twice as high in the other undersaturated mafic rocks, corroborating the less


incompatible character of Rb relative to Ba in the most undersaturated rocks.

P, Ti, Th, Nb, Zr, Hf and Y

The incompatible elements - P, Ti, Th, Nb, Zr, Hf and Y - are correlated with different REE having similar D values {e.g., Th vs. La, Hfvs. Sm, Y vs. Tb, etc. ). Some of the corresponding ratios remain constant (within the limits of analytical uncertainty) while others vary. The Nb/La ratio increases from the tholeiites (Nb/ La--0.9) to the most undersaturated rocks ( N b / L a = l . 4 ) suggesting either the more incompatible character of Nb relative to La or a source heterogeneity. On the other hand, the Y/ Yb ratio shows little variation and the values are similar to those of chondrites. In agreement with Jochum et al. (1986), the data indicate that Y is intermediate between Tb and Yb in degree of incompatibility. The Hf /Sm ratio displays chondritic values in the tholeiites (Hf/ Sm=0.62) then decreases (Fig. 8A) in the K- rich basanites and basanites and attains its lowest values in the melilite nephelinites (Hf/ Sm=0.26) . It is noteworthy that the Hf /Sm ratio decreases with the increase of La and P (Fig. 8A and B). The decrease of Hf /Sm with the increase of La cannot be explained by variable degrees of partial melting from a common source. According to the partition coefficient values given by Irving and Frey (1984), the ratio should decrease with increasing degree of melting of a four-phase peridotitic source. The residual phase, which may produce a decrease of Hf /Sm during partial melting, is ilmenite which concentrates Hf more than Sm (McKay et al., 1986). Their amount in the residue should be highest in that of the nephelinites which have the lowest Hf /Sm ratios but this is rather un- likely since Ti and V display similar concentrations in the nephelinites and the K-rich basanites. Alternatively, the decrease of the Hf/ Sm ratio accompanied by an increase of P205 (Fig. 8A) suggests an influence of apatite in the upper mantle source. Using published partition

0.20

0.15

0.10

] ' h / L a

© ~

o ~

®

© * ~ ° o o A o • mmml A

I l I I I 50 150 250

©

La ( p p m )

0.6

0.4

0.2

0.6

0.4

0.2

H f / S m

© *

®

Oo m m O ° o o N

,I I I

50 150

H f / S m

",

I i I

250

®

La(ppm)

'$" .,~ • P205 (%) I I

2 I I

1

Fig. 8. A. H f / S m vs. P205; B. H f / S m vs. La; and C. T h / La vs. La for the mafic rocks from the Lake Kivu area. Symbols as on Fig. 3.

coefficients of Hf and Sm (Irving and Frey, 1984), a possible explanation for the trend shown in Figure 8 is an increase of apatite from the sources of tholeiites and K-rich basanites to the source of melilite nephelinites. The presence of apatite in these sources does not modify the value of ratios such as Sr/Ce since Ds, = Dee

in apatite (Irving and Frey, 1984 ). On the other


hand, the Zr /Hf ratio increases from the tholeiites which have chondritic values(Zr/ Hf=40) to the nephelinites with Z r /Hf> 50. The Zr /Hf ratio which is frequently assumed to be constant in most basaltic rocks (Jochum et al., 1986) increases simultaneously with the increase of La/Yb from the silica-saturated to the most undersaturated basalts (Clague and Frey, 1982; Wedepohl, 1985; Liotard et al., 1986). This suggests that Zr is more incompatible than Hf or heterogeneous sources. On average, the F h / La ratio (Fig. 8C) tends to decrease with the increase of La. Since Dwh < Dan in apatite (Ir- ving and Frey, 1984) the model postulated for the Hf /Sm ratio also applies to the T h / L a ratio. However, in a single basaltic rock-type, the T h / L a ratio increases with the increase of La, especially in the K-rich basanite and melilite nephelinite where the T h / L a ratio increases respectively from 0.13 to 0.18 and from 0.09 to 0.14 (Fig. 8C ). The difference between Dwh and Daa is tOO small to explain the variation of the T h / L a ratio by a variable degree of partial melting from a homogeneous source.

Titanium and vanadium

The abundances of Ti and V are highest in the K-rich basanites and nephelinites. How- ever, the lack of a distinct correlation of Ti and V with many incompatible elements, particularly HREE and middle REE, implies a deri- vation from a heterogeneous upper mantle source. The Ti /V ratio, which is about 50 in the tholeiites, increases up to 70 with the increase of REE in most other basaltic rock-types ex- cept the melilite nephelinites where Ti /V < 50. On the other hand, Ti and V are positively correlated with Rb suggesting that the partition- ing of these three elements is controlled by the same mineral phase. Such a phase cannot be amphibole which is relatively depleted in Rb and is always associated with a high Ti /V ratio (Ti / V> 70) in related basalts (Wass and Rogers, 1980). On the other hand, mica may account for the correlation of Rb with Ti and V and the

relatively low Ti /V ratio of the corresponding basalts.

Discussion

Mantle mineralogy and partial melting

In addition to phlogopite and apatite which might be present in variable amounts in the mantle source region of the mafic lavas studied, geochemical features point to the occurrence of residual garnet and clinopyroxene. The narrow range of HREE concentrations and fractionation in the different mafic rocks suggest the presence of residual garnet in the source. The occurrence of this phase is also consistent with the variation of A1203. The lower contents of HREE and A120~ in the K-rich basanites compared with the other mafic rocks suggest higher amounts of residual garnet and probably a higher garnet/clinopyroxene ratio in their sources.

The Sc/Yb ratio, which is not fractionated by garnet, puts some constraints on the role of clinopyroxene during partial melting. The presence of clinopyroxene in the residue should decrease the Sc/Yb ratio in the melt since Dsc 3 and Dvb 0.2. In this respect, the ratio in neephelinites ( Sc/Yb = 3), which is lower than in the K-rich basanites (Sc/Yb = 14), implies a higher proportion of residual clinopyroxene in the mantle source for the former rocks. This further supports the higher garnet/clinopyroxene ratio in the residual source of the K-rich basanites which were probably also derived from a deeper source (Kay and Gast, 1973). All these features, including variable proportions of garnet and clinopyroxene and variable contents of a fluid-bearing phase (e.g., apatite and/or phlogopite), point to significant mineralogical heterogeneity of the source region. The large range of LREE and other incompatible elements associated with a systematic variation of the La/ Yb and A12OffCaO ratios suggest that different mafic types were derived by variable degrees of partial melting. The degree of partial melting

8 6 G. MARCELOT ET AL.

may be evaluated by using Zr which is, among the incompatible elements, one of the least af- fected by the enrichment process in the upper mantle source. Thus, assuming a source rock with Zr abundances twice that of chondrites and Dzr = 0.05, the calculated degree of partial melting ranges from 9 to 11% for the tholeiites, to 4 to 6% for the K-rich basanites and ~ 2% for the nephelinites. These values are in agreement with those suggested for similar rocks from elsewhere (Van Kooten, 1980; Clague and Frey, 1982; Takahashi and Kushiro, 1983).

Several experimental studies (Brey and Green, 1977; Brey, 1978; LeBas, 1978) have shown that to generate strongly undersaturated magmas such as nephelinites, a vapour-rich source and high CO2/HeO ratios are required. The presence of fluid-bearing phases such as apatite and phlogopite are consistent with melting of a vapour-rich peridotite source.

Alternatively, the mafic lavas could have been derived from an alkali clinopyroxenite source as suggested by Lloyd et al. (1987). The observed variations of trace elements are consistent with such a source provided that it con- tained phlogopite and apatite. However, due to the limited amount of data on the partition coefficients of titanite, it is difficult to quanti- tatively evaluate the model. The frequent occurrence of clinopyroxenite nodules in this part of the African Rift (Dautria and Girod, 1987) supports the model of a clinopyroxenite source.

Composition of the upper mantle source

In the mafic lavas studied, the behaviour of LREE may suggest that their distribution results from variable degrees of partial melting from a common mantle source. The chondrite- normalized ratios [e.g., (La/Ce)N higher than 1 ] imply that the source was enriched in LREE. Some other incompatible elements (e.g., Sr, Zr) are closely related to different LREE as indi- cated by the constancy of ratios such as Sr/Ce or Zr/Sm. On the other hand, Ti, V, Rb, P, Hf and Th display a more complex distribution

which suggests, in addition to variable degrees of partial melting, the presence of residual phlogopite and apatite heterogeneously distrib- uted in the mantle source region. The deriva- tion from a heterogeneous (metasomatized) upper mantle source is favoured.

Such an interpretation is corroborated by isotopic data from the Nyiragongo nephelinites (Vollmer and Norry, 1983) and seems to have a more general implication for ultra-potassic magmas (Vollmer et al., 1984; Nelson et al., 1986). Some element ratios which are lower than the chondritic values, differ from those of oceanic basalts and seem to be characteristic of subcontinental mantle. This is the case for Sr/ Ce which averages between 5 and 8 in various intraplate continental basalts (Wass and Rog- ers, 1980; Wedepohl, 1985; Dautria et al., 1988). On the other hand, this ratio is generally higher than 9 in oceanic island basalts which have sub- chondritic values (Clague and Frey, 1982; Lio- tard et al., 1986). Similarly, K/Rb is lower in basalts generated in a continental environ- ment. Whereas the decrease of K/Rb may reflect the increasing influence of phlogopite in continental alkali basalt petrogenesis, the fractionation of Sr/Ce cannot have resulted from an upper mantle melting process since plagioclase is absent at the depth of generation of the lavas studied. Several other alternatives including incorporation of subducted continental crust and interaction of kimberlitic magma with the source can be considered. The addition of recycled continental crust to the mantle source cannot explain the decrease of this ratio since, on average, the crust has a chondritic Sr/Ce value (Taylor and McLennan, 1985). Alterna- tively, the addition of a carbonated kimberlite magma to the depleted refractory base of the lithosphere (Wyllie, 1987) is a possible process since kimberlites have low Sr/Ce associated with relatively high Nb/La.

The Pb, Sr and Nd isotope systematics of the nephelinites from Nyiragongo (Vollmer and Norry, 1983) indicate an enrichment event dated at 484 + 28 Ma which may reflect the em-


placement of kimberlite bodies at the base of the lithosphere. These kimberlites, which lo- cally crop out in the western branch of the Af- rican Rift (Demaiffe and Fieremans, 1981 ) may have given off fluids migrating upwards thus creating a concentration gradient along a ver- tical profile in the lithosphere (Menzies and Hawkesworth, 1987). Similar fluids are docu- mented from the micro-inclusion analyses in diamonds (Navon et al., 1988). They are enriched in K and LREE and may explain some of the geochemical features observed in the studied rocks.

Such a chemically and mineralogically zoned lithosphere would have been reactivated during the rift development and, according to the depth of the source, would have produced melts with the various compositions encountered, in this study. Tholeiites were probably generated at a higher level where the lithosphere was only slightly enriched, whereas nephelinites were produced at a deeper level where the enrichment was more extensive.

Conc lus ion

The trace-element characteristics of the Lake Kivu mafic rocks support the concept that the mantle region beneath the East African rift is extremely enriched in highly incompatible elements relative to the primordial mantle. This probable ancient enrichment may have resulted from the emplacement at the base of the lithosphere of a kimberlite magma which generated a highly heterogeneous column from which the mafic melts with variable compositions were extracted. The close isotopic simi- larity between nephelinite of Nyiragongo (Vollmer and Norry, 1983) and kimberlite from the same region supports such a hypothesis.

A c k n o w l e d g e m e n t s

The study was supported by the Centre Gdo- logique et Gdophysique, Montpellier (France) and the Natural Sciences and Engineering Re-

search Council of Canada (operating grant A 3782).

References

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8 8 G. MARCELOT ET AL.

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