Geology Reference
In-Depth Information
Gahinga and Muhavura are characterized by the presence of plagioclase, clinopy-
roxene, olivine,
ed as K-basanites and
K-hawaiites. More evolved samples from Muhavura and Gahinga include
K-benmorites. The lavas of Gahinga and Muhavura are shoshonitic.
The age of Birunga volcanism ranges from <100 ka to 10 Ma. The three ages
from the Sabinyo mineal separates have ages as follows: 64
leucite and Fe-oxide. They are classi
±
25, 113
±
35 and
176
30 ka. Mikeno lavas are more than 200 ka old.
All lavas are shoshonitic with 1 < K
2
O/Na
2
O < 2 and strongly enriched in
incompatible elements. The
87
Sr/
86
Sr ratios vary from 0.70586 in the K-basanites to
0.70990 in the latites. The
143
Nd/
144
Nd content ranges from 0.5124
±
0.51206, and
-
(
206
Pb/
204
Pb:
Pb isotopes
follow sub-vertical
trends on isotope diagrams
207
Pb/
204
Pb: 15.69
208
Pb/
204
Pb: 40.28
19.30
41.5). The
40
Ar/
39
Ar ages of leucite and phlogopite separates suggest that the latites have ages
between 100 and 200 ka and that of K-basanite <100 ka. The latites are hybrid
magmas produced by mixing between a K-basanite melt with a silicic melt from
the deep crust. The low-silica K-basanites re
19.51,
15.93 and
-
-
-
ect
interaction between a ma
c
K-basanitic melt (
143
Nd/
144
Nd ratio
0.5120 and
87
Sr/
86
Sr
*
*
0.707), and neph-
elinite (
143
Nd/
144
Nd
0.51267 and
87
Sr/
86
Sr ratio
0.7045). Both were derived
from the mantle lithosphere with source ages of 1 and 0.5 Ga, respectively, and the
youngest ages correspond to the deepest magma sources. The magma production
rate in the Birunga is low (
*
0.04 km
3
/year), and reflect prolonged (10
-
15 Ma)
heating of the lithosphere by the East African mantle plume.
The interesting feature of the Western Rift is the presence of Ruwenzori horst,
which is a nonvolcanic mountain in Africa (16,794 feet high). The massif narrows
down towards Lake Albert with fault scraps on either side. The volcanic products
are as follows. In the Western Rift a few thousand year old carbonatite lavas have
been recognized. An older carbonatite intrusion is seen in the northeast of Birunga,
north of Lake Kivu. The pyroclastic rocks and lavas at the Fort Portal region range
from carbonatites to K-rich silica-undersaturated lavas and lapillies containing
leucite, K-rich nepheline, kalsilite, pyroxene, olivine and melilite. Silicate bombs
and lapilies are found in the volcanic
*
*
ows are
observed only around isolated volcanoes of Katunga in the south and volcanoes of
Fort Portal in the north. Intrusive tongues and ejected blocks of carbonatites are
found in the Katwe area.
The lavas erupted from these volcanoes comprise mainly such feldspathoids as
leucite, kalsilite, nepheline and melilite. The lava
fields around Ruwenzori. Lava
flows in the volcanic
field south of
Lake Kivu are mainly constituted of olivine basalts, with a few
flows of trachytes,
rhyolites and pyroclastics.
Detailed petrological investigation of lavas from Mikeno, Magahinga and
Muhavura (Fig.
4.10
) and some smaller cones from the adjoining areas were made
by Ferguson and Cundari (1975). According to them the lavas range from olivine
basanite to leucite phonolite and slightly silica-saturated trachytes. The lavas
include xenoliths of mica pyroxenites, and peridotites with or without mica.
According to them, there are two distinct petrographic series originating from a
melabasanite. Earlier, melabasanite was termed as ugandite or olivine ugandite.
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