Two volcanic zones (Bukavu and Kamituga) south of Lake Kivu (southeastern Zaire) are part of the western branch of the Eastern African rift. They were formed ...
Journal of Volcanology and Geothermal Research, 31 (1987) 33-46
33
Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
GEOCHEMISTRY AND PETROGENESIS OF RIFT-RELATED VOLCANIC ROCKS FROM SOUTH KIVU (ZAIRE) A. A U C H A P T 1, C. D U P U Y 1, J. DOSTAL 2 and M. K A N I K A 3 ~Centre Gdologique et Gdophysique, Universit~ des Sciences et Techniques du Languedoc, Pl. E. Bataillon, 34060Montpellier, France 2Department of Geology, Saint Mary's University, Halifax, N.S., B3H 3C3, Canada 3Department of Geology, University of Lubumbashi, Lubumbashi, Zaire (Received November 5, 1985; revised and accepted June 12, 1986)
Abstract Auchapt, A., Dupuy, C., Dostal, J. and Kanika, M., 1987. Geochemistry and petrogenesis of rift-related volcanic rocks from South Kivu (Zaire). J. Volcanol. Geotherm. Res., 31: 33-46. Two volcanic zones (Bukavu and Kamituga) south of Lake Kivu (southeastern Zaire) are part of the western branch of the Eastern African rift. They were formed during three volcanic cycles, one pre-rift ( 70-7 Ma old) and the other two syn-rift (7.8-1.9 Ma old and 14,000 y.-sub-Recent, respectively), and evolved from quartz tholeiites of the pre-rift period to alkali basalts of the rift stage. The basaltic rocks, which strongly predominate, are compositionally similar to other rift-related basalts and also to oceanic-island rocks. Most of the basalts have undergone only limited fractional crystallization ( ~ 5-10%) dominated by olivine and clinopyroxene. The distinct variations of incompatible elements even in rocks of very similar major-element composition imply that the basaltic rocks were derived from a heterogeneous source by variable degrees of melting. The inferred source composition closely resembles that of metasomatized peridotite xenoliths from alkali basalts.
Introduction In recent years, volcanic rocks of continental rifts have received relatively little attention from geochemists and petrologists. In fact, modern geochemical data are still lacking on rocks from a significant number of continental rift zones. In the East African rift zone, some parts are relatively well documented (Ethiopia - Treuil and Varet, 1973; Barberi et al., 1980; Piccirillo et al., 1979; Kenya - Baker et al., 1977; Brotzu et al., 1983) while others are poorly known. This study presents and discusses geochem-
0377-0273/87/$03.50
ical data on the different types of basalt in the western branch of the East African rift in the South Kivu ( southeastern Zaire ) (Fig. 1 ). The objective is to provide constraints on the source rock composition and to evaluate upper mantle heterogenetites.
Geological setting The South Kivu volcanic region belongs to the western branch of the East African rift zone. The geology and volcanology of the area have been described by K a m p u n z u et al. (1979, 1982, 1983), Guibert (1977) and Villeneuve (1978).
© 1987 Elsevier Science Publishers B.V.
34 ~ Edwar_~ake
]
E 1=
~
2"
UGANDA
~
R W A N DA
@il~;~Bukavu
3o
''~-
I/
I
p
|
~°
The region lies at the intersection of two tectonic systems and is composed of two volcanic zones ( Fig. 2 ) : (1) The Bukavu zone which spreads along the southwestern edge of Lake Kivu. (2) The Mwenga-Kamituga region 50 km farther southwest. In both regions, volcanic activity occurred in three eruptive cycles (Fig. 3). The first cycle corresponds to a pre-rift stage, whereas the second and third cycles represent the rift stage. The first stage started 70-60 Ma ago and ended around 7 Ma ago. It is related to the early development of the rift, when basaltic rocks poured out from fissures along the developing pairs of opposing continental margins. The basalts range from quartz tholeiites in the earliest stage to olivine tholeiites in the later stage. The second cycle (7.8-1.9 Ma old), also
i
-t i
Uvira . ~
/
Fig. 1. Simplifiedstructural map of Eastern Africa (after Thorpe and Smith, 1974). /=major faults; 2=volcanic zones; R=Rungwe zone; SK=South Kivu; V=Virunga; T=Toro Ankole.The location of the studied area is outlined by a rectangle.
:
w
~-~-
T.~. 28 °
i F
29"
,.k.
I 2 0 Km
~
!
I
30"
Fig. 2. M a p showing the distribution of volcanic rocks in the studied area (afterKampunzu et al.,1979). 1 = volcanic rocks of Bukavu and Mwenga-Kamituga; 2 = volcanic rocks of the Virunga belt.
involving fissure-type volcanic activities, produced alkali basalts accompanied by more differentiated products including trachytic and phonolitic domes. The volcanism shifted during this cycle toward the axis of the rift. Simultaneously, the composition of the lavas became gradually less undersaturated. A similar compositional evolution was observed in the rift zone of Kenya ( Lippard and Truckle, 1978) and is probably typical of continental rifts (Barberi et al., 1982). The third cycle, which started around 14,000 y. ago, includes mainly transitional and alkali hasalts emplaced in the axial part of the rift. The rocks are very similar to Plio-Quaternary basalts of the Ethiopian rift (Piccirillo et al., 1979). In addition to the evolution within each cycle, the progression from the tholeiitic basalts of the pre-rift period to alkali-basalt volcanism char-
35
ll~
lower cycle
1
upper
I-qF.I
~_31 . . . . . . . . . . .
I~1
cycle 2 , cycle 3
I
lower
Icyele
1 middle
I-
I........
upper
1
cycle 2 v
cycle 3
[--q
1
~H'~
2
I
3
Fig. 3. Relationbetweenmagmatictype and eruptive cyclein Bukavuand Kamitugazones. 1 = quartz-normativetholeiites; 2 = olivine-normativetholeiites (transitional basalts) ; 3 = alkali basalts and basanites. acteristic of the rift stage has been reported from the other parts of the African rifts (Piccirillo et al., 1979; Brotzu et al., 1981). A summary of the petrography and mineral chemistry of the volcanic rocks of the South Kivu is given in Table 1. Geochemistry
Analytical notes Ninety-four samples were analyzed for major elements and Li, Rb, Sr, V, Cr, Co, Ni, Cu and Zn by atomic absorption. Forty-eight samples were then selected for the determination of rareearth elements ( R E E ) , Sc, Hf and T h by instrumental neutron activation, and sixty-five samples for the determination of Y, Nb, Zr and Ba by X-ray fluorescence. The precision and accuracy of the trace element data were given by Dostal et al. (1983).
Major elements The analyzed rocks from all three cycles (Tables 2 and 3 ) have basaltic composition with
the differentiation index ranging from 17 to 35. As in the other parts of the African rift (e.g. Barberi et al., 1982), the rocks can be subdivided on the basis of the normative composition into: (1) Quartz-normative tholeiites; ( 2 ) Olivine-normative tholeiites; and (3) Alkali basalts and basanites. These magmatic types are also distinguished on the (Na20 + K20) vs SiO2 graph (Fig. 4), where olivine tholeiites plot in the transitional field of Piccirillo et al. (1979), more specifically in the part for transitional rocks with alkaline affinities. The three rock types also have distinct contents of K20 and P205 which increase from quartz tholeiites to basanites. According to TiO2, which increases with the degree of undersaturation, two populations can be recognized. The first group includes both types of tholeiites and has lower TiO2 content than the alkali basalts and basanites. The plot of TiO2 vs [Mg] ( = M g / ( M g + F e 2 + ) w i t h Fe3+/Fe2+=0.25) (Fig. 5) shows that each magmatic type has only a limited range of fractionation, and that the effect of mineral fractionation on Ti content was very small. Undersaturated rocks have concentrations of
14 to 17 P:
Hemo ~
8
Hem0_7
8 to 18
14 to 28
1 to 3
%
Ca43 51Mg40 a~Fe~2 18
Ca4o.,,Mg~ ~Fe~,, .,,
Fo~s 7o
Fo86 79
AnTo 4~"
AnT~ ~
p
Hem11
Usp6., 4o
M:
P:
M:
P:
M:
P:
II. Cycle
11 to 14
9 to 12
2 to 8
%
Ca4u-48Mg46 40Fe9 ~
C a ~ 46Mg4s 4~Fe9 14
Fo::/6s
Foss_s,)
An6.~4,~
An6s so
Hem6 1o
Usp6o ~s
M:
P:
M:
P:
M:
P:
p
Haute Ruzizi
III. Cycle
% - refers to phenocrysts; P = phenocrysts; M = mesostase. Textures: O-~ ophitic, SO = subophitic, P = porphyritic, A = aphanitic.
Glass
Usp6:~64
3 to
Ca36 45Mg48 38Fe~:~ 26
Ca:~74uMg46 4oFeH j~
UspTo 64
Ca:~ :lsMg44:~6Fe~ :~:~
Ca~62oMg59 41Fe2.~ 39
Fo62_:~6
M:
Ti-magnetite ilmenite
32 to 36
Ca7 l~Mg6:~ ~2Fe:,,:, ~7
Fo83 ~
An6~ :~6
An~4_6,
P:
M:
P:
M:
]
23 to 29
9 to 13
An6~ 46
6O to 64
Salite
Augite
Augite S. Ca.
Pigeonite
Olivine
Plagioclase
%
0
% SO
Upper sequence
Lower sequence
I. Cycle
Mineral composition of basaltic lavas from Bukavu
TABLE 1
to 70
60
(I
M:
F076 7~
Fo83 =7
An64 46
An67 ~4
A
Ca.,. ,~Mg4:~ e~Fe,,, ~
Ca= ~Mg6:~ ,,Fe~o 4~
P:
4
M:
P:
to 8
5
2 to
%
Idjwi
37 TABLE 2 Average major- and trace-element composition of basaltic rocks of Bukavu Transitional basalts
Tholeiites n: SIO2(%) A1203 Fe20~ MnO MgO CaO Na20 K20 TiO2 P~O5 Li (ppm) Rb Sr V Cr Co Ni Cu Zn
19
n: La Ce Sm Eu Tb Yb Lu Th Hf Sc
(0.9) (0.6) (0.6) (0.03) (0.7) (0.8) (0.13) (0.2) (0.2) (0.1) (1.3) (5.5) (40) (15) (24) (18) (38) (15) (26)
24.8 44.7 4.9 1.71 0.90 2.54 0.40 3.2 3.04 25
(1) (5) (150) (24) {125) (11) (97) (18) (12)
45.76 14.97 12.48 0.22 9.07 10.07 2.90 0.97 2.03 0.73
(7) (14) (1) (0.3) (0.15) (0.4) (0.05) (1.1) (0.6) (1)
54.2 88.7 6.74 2.21 1.05 2.74 0.44 7.5 3.23 25
(10) (21) (5) (63)
(0.4) (5) (92) (10) (67) (4) (38) (9) (6)
(21) (34) (1.4) (0.4) (0.15) (0.3) (0.04) (3.3) (0.5) (2)
58.1 103.5 7.27 2.26 1.06 2.71 0.42 8.3 4.11 28
7.1 34 838 238 273 51 192 70 108
34 202 81 719
(0.6)
(1) (0.04)
(1.5) (1.3) (0.5) (0.2)
(O.3) (O.3) (1) (7) (200)
(15) (84) (6) (53) (17) (22)
8 (13) (20) (0.7) (0.1) (0.05) (0.15) (0.03) (2.8) (0.4) (3)
86.1 154.1 10.35 3.15 1.32 2.98 0.45 11.4 4.94 26
3 (6) (30) (17) {196)
(1.3)
44.79 14.28 12.58 0.22 9.12 11.13 3.41 1.08 2.12 0.79
3
8 36 154 54 601
17 (1.3) (0.5) (0.5) (0.04) (1.6) (0.8) (0.2) (0.2) (0.2) (0.2)
6.8 28.2 649 249 285 50 189 74 90
5
15 35 115 25 229
(1) (1.4) (0.9) (0.04) (1.8) (0.8) (0.4) (0.2) (0.3) (0.1)
5.9 21.9 499 218 284 52 181 60 109
Basanites
5
47.59 14.85 12.86 0.20 8.10 8.98 2.90 0.86 1.72 0.54
7
n: Y Zr Nb Ba
12
51.13 15.16 11.6 0.17 6.41 9.48 2.79 0.44 1.65 0.28 5.3 12 279 206 235 52 135 67 116
Alkali basalts
(19) (33) (1.3) {0.4) (0.15) (0.5) (0.01) (3.7) (0.8) (3) 13
(1) (26) (21) (115)
38 231 88 871
(5) (47) (22) (190)
n = number of samples; values in brackets - 1 standard deviation.
TiO2 similar to those of equivalent rocks from Hawaii (Clague and Frey, 1982) whereas tholeiitic rocks ( including olivine-normative transitional basalts) plot into the field of mid-ocean ridge basalts (MORB) and/or continental tholeiites (CT). Each magmatic type from the South Kivu has contents of K20 lower than those of equivalent rocks from the Ethiopian Plateau (Piccirillo et
al., 1979) but comparable to those from Afar (Barberi et al., 1980). The ratios of A12OJTiO2 and CaO/TiO2 in the tholeiitic rocks range respectively from 12 to 8 and from 6.2 to 7.3, values typical of primitive MORB ( Sun et al., 1979). On the other hand, the undersaturated rocks have lower ratios (A12OJTiO2 ~ 7 and CaO/TiO2 ~ 5-4) due to their higher TiO2 contents.
38 TABLE 3 Average major and trace element composition of basaltic rocks of Kamituga
n: Si02%
AI203 Fe20~ MnO MgO CaO Na20 K20 TiO~ P20~ Li (ppm) Rb Sr V Cr Co Ni Cu Zn
3 51.00 15.06 10.50 0.17 7.02 9.84 2.65 0.33 1.39 0.32
n: La Ce Nd Sm Eu Tb Yb Lu Th Hf Sc
48.66 14.47 11.50 0.20 7.75 10.27 3.02 0.65 1.44 0.75 4.9 12.2 746 197 235 57 223 71 101
3
n: Y Zr Nb Ba
(0.6) (2.1) (10) (1) (36) (7) (44) (5) (5)
23.2 43.5 18.2 4.18 1.44 0.69 1.86 0.30 3.21 2.01 24
(0.5) (2) (1.4) (0.2) (0.1) (0.05) ( 0.02 ) (0.01) (0.2) (0.1) (1)
5 (1) (0.4) (0.8) (0.06) (0.5) (0.2) (0.2) (0.1) (0.2) {0.1)
(0.8) (2.1) (119) (14) (38) (13) (88) (32) (13)
89.6 150.9 51.1 8.37 2.50 1.07 2.49 0.39 13.7 3.03 24
46.19 13.47 12.51 0.21 8.35 11.02 2.93 0.75 2.04 0.97
(1) (0.5) (0.4) (0,01) (0.6) (0.4) (0.2) (0.1) (0.1) (0.1) (o.8) (4.4) (176) (2o) (51) (3) (16) (17) (5)
5.8 14.6 1055 215 211 48 155 83 103
43.38 13.29 13.41 0.21 9.99 11.46 2.98 0.99 2.68 0.90
105 175 62.6 10.16 2.98 1.27 2.82 0.45 14.4 3.38 25
33 146 69 850
(0.6) (0.5) (0.8) (0) (0.5) (0.2) (0.1) (0.1) (0.1) (0.1)
5.7 24.0 882 279 282 53 179 63 98
3 (11) (19) (7) (1) (0.3) (0.1) ( 0.3 ) (0.03) (2) (0.3) (1)
15
4
(0.5) (2.8) (34) (24) (88) (3) (22) (2) (4)
43.52 13.10 12.36 0.20 10.38 12.19 3.18 0.86 2.19 1.09 6.2 19.9 1174 240 258 53 214 78 93
(0.9) (0.6)
(0.4) (O.Ol) (1.2) (0.6) (0.3) (0.2) (0.1) (0.1) (0.8) (4) (96) (15) (74) (8) (45) (12) (5)
4
(21) (33) (6.6) (1.1) (0.3)
(o.1) (o.1) (o.o2) (3.7) (0.2) (1)
(5) (8)
65.2 129.3 58.4 10.94 3.3 1.33 2.82 0.45 7.4 5.17 29
(4.6)
(0.8) (0.2)
(o.1) (0.4) (0.04) (0.9) (o.3) (0.7)
124.5 216 73.5 12 3.48 1.34 2.77 0.43 17.5 3.80 26
(7) (lO) (7) (1) (0.1) (0.1) (O.2) (0.04) (1)
(o.1) (2
2
12 (2) (3) (1) (12)
Group II
Group I
7
3 26 80 21 234
Alkali basalts
14 (2) (0.7) (1.1) (0.01) (0.7) (0.2) (0.1) (0.04) (0.02) (0.02)
3.3 7.3 298 179 197 45 133 71 90
Basanites
Transitional basalts
Tholeiites
(3) (13) (9) (37)
38 178 95 1113
n = n u m b e r o f samples; v a l u e s i n b r a c k e t s - 1 s t a n d a r d d e v i a t i o n .
(15) (13) (38)
37 249 103 724
(5) (21)
(20) (45)
37 198 116 1261
t2 (16) (12)
(98)
39
Na20 + K20%
•
BUKAVU
•
•
/// /
• ~ • •
A
I I N"
•
.1¢II
i II f
/ I
~
l
+, +1:,,"'.'. i / / ,~*~.
/
•
ii
+1/+ + +1+ / i" t~A I I + / e II i + I I • i1++ / *11 & /+ • /eO • , ii
%~ KAMITUGA !
//
i II
Ti 0 2
f
BUKAVU
J
z~
,il;ll~
/
/
I
I
Na20 + K20 %
Si02 % KAMITUGA ~
TRANSITIONAL
ii
/
ii
// ALKALI •
II
ti•
•
t
A
A
ii 11
I1 / 1
/i///
~
ii
/ / //;"
• SiO2 %
I 50
~
/ THOLEIITIC ~o
i il/i
~
II
/ I 45
I~
ii
I 55
Fig. 4. (Na20 + K20 ) vs SiO2 plot for volcanic rocks from the Bukavu and Kamituga zones. The heavy line separates the alkali and tholeiitic domains (MacDonald and Katsura, 1964) the dashed lines delineate the field of transitional basalts (Piccirillo et al., 1979). Quartz-normative tholeiites (filled circles), transitional basalts (crosses), alkali basalts (empty triangles), basanites (solidtriangles).
T h e u n d e r s a t u r a t e d and s a t u r a t e d rocks also differ by t hei r c o n t e n t s of A1203, CaO and MgO. Tholeiitic rocks have higher A1203 but lower CaO and MgO contents. T h e analyzed rocks display a positive correlation of CaO-MgO and a negative one for CaO-AIeO3 and MgO-A1203. T h e s e correlations are stronger in t he rocks from the M w e n g a - K a m i t u g a area t h a n in those from th e B u k a v u zone.
~o
~o
[~1
Fig. 5. TiO2 vs [Mg] diagram for volcanic rocks from the Bukavu and Kamituga zones. Fields delineated by solid lines: 1 = basanites of Honolulu; 2 = alkali basalts of Honolulu (both after Clagueand Frey, 1982); 3 = tholeiites from Hawaii (Basaltic Volcanism Study Project, 1981). The discontinuous lines show the field of continental tholeiites of Nova Scotia and Morocco (Dupuy and Dostal, 1984). Solid symbols= Kamituga; empty symbols= Bukavu; circles=quartz-tholeiites; stars=transitional basalts; triangles=alkali basalts and basanites. [Mg] = Mg/( Mg+ Fe2+) assuming Fe'~+/Fe2+ =0.25.
Trace elements Transition elements. Several t r a n s i t i o n elem e n t s vary distinctly in the basalts. T h e contents of V and to a lesser ext ent Sc increase from quartz tholeiites to basanites but do no t show any obvious t r e n d with differentiation. T h e T i / V ratio ranges between 37 and 48 in tholeiites. T h i s ratio is higher in alkali basalts a n d basanites (51-64) because of higher T i abundances in the u n d e r s a t u r a t e d rocks. T h e ratios are higher t h a n those of M O R B (Shervais, 1982), but similar to those from oceanic island basalts (OIB) (Clague and Frey, 1982) and C T ( D u p u y and Dostal, 1984). Chromium, Ni and to certain degree Co
40
400
Sm ppm I 14
/
10 6
L
300 -
f /,,'.
... o ~ ° °
Ji
.oo///:
2
/ / / * ,/,,',
/
/
•
i
/
Zr V ~ ppm J
q *,,.#/
4
~ %%/ / /
,%
1 0%
T 10
Hf ppm
. . . . . . .
Lx I@ ['
6
/**'7-/ I / I
100
/! iI
1 I i
2
,~
200 I 10
16 MgO%
Fig. 6. Ni vs MgO diagram for volcanic rocks from South Kivu. Solid lines correspond to liquids produced by 5 and 25% partial melting of an upper mantle source (Hart and Davis, 1978); dashed lines represent variations of liquids produced by fractional crystallization of olivine (lines 1 and 3) and of an assemblage of olivine and clinopyroxene (in proportions of 1:4) from parents having Ni = 330 ppm and MGO=12.5% and Ni=270 ppm and MgO=8%, respectively. Other symbols as in Fig. 5.
decrease in the residual liquid with differentiation in all rock types. According to the relationship of MgO with Ni (Fig. 6), the rocks are separated into two groups. The first, with relatively low MgO, includes tholeiitic basalts whereas the second, with relatively high MgO, corresponds to the undersaturated basalts. In both groups, the fractionation trend may be accounted for by 5-10% fractionation of olivine _+clinopyroxene. The crystallization of clinopyroxene is further implied by a strong positive correlation between Cr and Ni. The fractionation of Fe-Ti oxide is probably negligible, as suggested by the lack of variation of Ti and V in a given magmatic type when plotted against an index of differentiation. Cu and Zn have scattered contents but seem on average to remain constant regardless of differentiation or magmatic type. Their abundances are similar to those of OIB (Clague and Frey, 1982), whereas compared to CT they are depleted in Cu.
Incompatible elements. Most of the incompati-
J
P 100 I
o~ o
a,o -J 50
L 100
Nb ppm
Fig. 7. Sm vs Hf and Zr vs Nb diagrams for the volcanic rocks from South Kivu. Symbols are the same as in Fig. 5.
ble trace elements display a large range of variation. In general, their abundances increase from quartz tholeiites through transitional basalts to alkali basalts and basanites. The degree of enrichment, however, increases with increasing incompatibility of trace elements. For example, compared to the quartz tholeiites, basanites have about 6 times more Ba, Nb and Th but less than 2 times more Y and Yb. The increase in the incompatible element content is gradual in the basalts of the Bukavu belt, while a gap appears between quartz and olivine tholeiites from the Kamituga area. The compositional variations with degree of silica-saturation are accompanied by inter-elem e n t correlations (e.g. Sm-Hf and Zr-Nb, Fig. 7) that are distinct for each region. In the same region, ratios of elements with the same degree of incompatibility (e.g. Ba/La, Th/La, and Zr/Eu) remain constant in rocks of the three magmatic types. Two exceptions are the K/Ba and P / C e ratios, which are lower in some of the undersaturated rocks. Such a relation, frequently encountered in OIB such as those from Hawaii, has been attributed to the presence of
41
400
400
KAMITUGA
300 200
200
~00
100
i.-
"~'
0.65, and the AI2OJTi02 and CaO/Ti02 ratios of the quartz and olivine tholeiites are in the range of primitive MORB (Sun et ak, 1979). Both features suggest that fractional crystallization was of limited extent (degree of crystallization ~ 5-10% ). Therefore, most basalts with [ Mg ] > 0.65 may be used to estimate the source composition of these rocks. Among several models of the genetic relations among the various rock types of continental rifts, the one most consistent with the observed trace-element distribution invokes a process of different degrees of partial melting at progressively shallower depths from either a homogeneous (Gass, 1972) or heterogeneous (Gast, 1968) upper mantle source. In agreement with experimental petrology, the increase of CaO and MgO accompanied by a decrease of A1203 in the sequence, quartz tholeiite-olivine tholeiite-alkali basalts and bas-
43
anite, corresponds to a decrease of the degree of partial melting. According to experimental data of Takahashi and Kushiro (1983), the quartz tholeiites and transitional basalts were probably generated at depths corresponding to 4-7 kbar and 7-9 kbar of pressure respectively, whereas the undersaturated basalts were formed at a pressure greater than 10 kbar. Likewise such ratios as La/Yb, La/Sm, Nb/Zr and Ba/Sr, involving incompatible elements where the numerator element has a larger degree of incompatibility, exhibit a regular increase from quartz tholeiite to basanite, in agreement with the decreasing degree of partial melting. In the Kamituga region, the ratios of trace elements of the same degree of incompatibility, such as Ba/La, Th/La, Nb/La and Nd/Sr, are almost constant for all basaltic rocks with the exception of basanites of group I which have higher Nb/La and lower T h / L a ratios. These basalts could have been derived from a single upper-mantle composition by different degrees of partial melting. This model has been tested using the inverse method (Allbgre et al., 1983a, b) applied to the equilibrium partial melting equation C1= Co/Dr(1-F) -{-Fwhere Dr is the bulk partition coefficient involving residual phases. In the first stage, the modelling has been limited to the following elements: Sr, Sm, Eu, Tb, Yb, Y, Zr, and Hf. The range of "a priori" Co ( concentrations in the source) and D values were taken from the data of Clague and Frey (1982) and Liotard et al. (1986). The relatively high value for Dyb (Table 4) implies, in agreement with the fractionated HREE pattern (Fig. 8), a proportion of 2-4% garnet in the residue. Clague and Frey (1982) have suggested that residual garnet played a similarly important role in the genesis of Hawaiian basalts. The "a priori" F (degree of partial melting) was taken as 15 __5% for quartz tholeiites and 5 + 5% for transitional and undersaturated basalts. The mineral proportions of the residue were assumed to be constant for quartz tholeiite-~basanite. The results (Table 4) show that as far as the content of this group of elements is concerned,
TABLE4 Calculation of source rock composition by total inverse method D*
Sr (ppm) Sm Eu Tb Yb Y Zr Hf
0.03 0.05 0.07 0.09 0.25 0.25 0.07 0.10
Quartz tholeiites Transitional basalts Basanites
C
(0.01) (0.02) (0.03) (0.03) (0.05) (0.05) (0.04) (0.03)
Co
1
2
48 0.85 0.32 0.19 0.65
60 0.71
34 0.77
0.60 6 20 0.50
F 16 % 6 2
(2) (2 ) (1)
52 0.84 0.30 0.16 0.67 7.6 19 0.48
(3) (0.06) (0.03) {0.02) (0.05) {0.7) (2) {0.05)
D* - bulk partition coefficients (data from Clague and Frey, 1982; Liotard et al., 1986). C - calculated source composition from Marquesas Archipelago (1 - Liotard et al,, 1986} and Honolulu (2 Clague and Frey, 1982) ; Coo - calculated source for volcanic rocks of Kamituga; F - calculated degree of partial melting; the values in brackets correspond to 'a priori' ( for D ) and 'a posteriori' (for Co and F) errors.
the three magmatic types may have been derived from the same source by variable degrees of partial melting (F=16+_2% for quartz tholeiites, 6+_2% for transitional basalts and 2 +_1% for basanites). The additional model calculations using the F value already fixed at 16 +_2% and 2 _ 1%, respectively, suggest that the quartz tholeiites and basanites (group II) could have been derived from sources with the same abundances of La, Ce, Ba and Th, assuming DLa=l~a=Wh~-O.O07 and Dce=0.01. However, basanites (group II) have lower K/Ba and P/Ce. This may suggest that for F~
