Geology Reference
In-Depth Information
0.5128
Nyiragongo
Eastern Birunga
Bulk Earth
0.5126
K-Basanite Series
Karisimbi
0.5124
0.5122
Sabinyo Latite Series
0.5120
0.704
0.706
0.708
0.710
/
86
87
Sr
S
r
Fig. 5.6 Conventional Sr
Nd isotope plot for the eastern Birunga lavas compared with the
eld
de
ned by K-basanites from Karisimbi and Nyiragongo in the western parts of the Birunga
province (Vollmer and Norry 1983a). All of the data plot in the enriched quadrant of the diagram,
implying source regions with higher Rb/Sr and lower Sm/Nd ratios than bulk Earth. The sabinyo
latites plot at more extreme values of the isotope ratios than the K-basantie series, in agreement
with two analyses of similar rocks from Vollmer and Norry (1983b) (after Rogers et al. 1985)
-
0.5124 to 0.51206. The Pb isotope ratios range in the following manner:
206
Pb/
204
Pb: 19.30
207
Pb/
204
Pb: 15.69
15.93 and
208
Pb/
204
Pb 40.28
19.51;
41.5).
-
-
-
The low-silica K
basanites re
ect interaction between a ma
cK
basanitic melt
-
-
(
143
Nd/
144
Nd
87
Sr/
26
Sr
0.51204,
0.707)
and
a
nephelinite
*
*
(
143
Nd/
144
Nd
0.51267 and
87
Sr/
86
Sr
0.7045). The isotope ratios of Sr and Nd
of selected eastern Birunga lavas are illustrated in a conventional isotope diagram
(Fig.
5.6
).
Rogers (1992) compared the mantle-normalized abundance of incompatible
elements of leucite-basanites from Karisambi (Birunga volcanic Province
(De Mulders et al. 1986) and Vulsini (Italy; Rogers et al. 1985). Despite similar
abundance, Vulsini leucite basanites are lower in Ta, and Nb and to a lesser extent
Zr and Nb contents than similar rocks from Karsimbi. Rogers (1992) thought that
K-rich basanites, representing average analyses of Karsimbi rocks is similar to
magmas from in-plate environment.
The Rb/Sr ratio in Karsimbi lavas is
*
*
cantly higher than non-
potassic basanites and basalts, which have this ratio close to 0.03. The Th /U ratio in
leucite basanites from Karsimbi is very high having an average of 5.5 compared
with the values of 4 or less in most crustal rocks and mantle-derived magmas.
The K-rich volcanic rocks from Toro-Ankole and Birunga have initial
87
Sr/
86
Sr
ratio varying between 0.7036 and 0.7076. The K
1 and signi
*
-
feldspar-bearing absarokite
Search WWH ::
Custom Search