Download The Origin of Felsic Lavas in the East African Rift Gabriel Akec

The Origin of Felsic Lavas in the East African Rift
Gabriel Akec, Pennsylvania State University
Dr. Tanya Furman, Professor of Geosciences, Research Mentor
Abstract
East African Rift is a site of active continental rifting that will eventually lead to
the formation of new ocean. The origin of silica-poor rocks (basalts) in this rifting
environment has been studied fairly extensively but the origin of silica-rich phonolites
and evolved lavas has remained a mystery. There are two contrasting methods through
which evolved lavas may form, although a combination of two mechanisms are possible:
a) large volumes of basalt lavas may crystallize in near-surface magma chambers beneath
individual volcanoes. As minerals grow from the basalt lava, the remaining liquid
becomes more silica-rich, eventually leading to rhyolitic or phonolitic composition that
may get erupted. b) Alternatively, smaller volumes of basalt may melt the shallow crust
as the pass through on the way to eruption or below volcano storage. The crustal melts
will have silica-rich composition that can dominate the chemical signature of erupted
liquids. This research focuses on evolved lavas from Turkana (Northern Kenya) to
evaluate the competing roles of crustal melting and crystallization; regional comparison
is made to similar samples from Ethiopia (to the North of Turkana) and Tanzania (to the
South. We focus on the chemistry of these lavas as well as the minerals within them to
determine the way in which the lavas formed. The study will enable us to conclude
whether these lavas developed as result of fractional crystallization or from crustal
melting, and the degree to which these vary along the length of the East African Rift.
Keywords: East African Rift, Turkana, Silica-rich, Fractional crystallization, Crustal
Melting
Introduction
The East African Rift System is located in the eastern part of Africa (Fig. 1b) is a
site of active continental rifting that has been going on for millions of years. This process
will eventually lead to continental breakup and the formation of a new ocean or sea
between two continental masses and an Island. The focus of this paper is the Turkana Rift
which is located between the Ethiopian and East African (Fig. 1a) domes widely believed
to the surface manifestation of mantle hotspot (Karson & Curtis 1994). Turkana Rift
represents a relatively cool section of lithosphere along the Branch (Karson & Curtis
1994). The regions are important to the study of active rifting because of several wellestablished geophysical datasets available, and because the volcanic rocks are not so
voluminous that they obscure the tectonic structure of the rifts (Karson and Curtis 1989).
The Turkana area is markedly different from the rest of the African Rift. It has
low topography, in contrast to the Ethiopian dome to the north and the Kenya Dome to
the south. It is much wider than the rest of the Rift (~150 vs. ~50 km) and it has been
active volcanically for over 40 Million years . The results of these difference is that the
crust beneath Turkana volcanoes is widely believed to be thinner than beneath other
volcanic areas, providing greater opportunity to study thermal and chemical structures of
underlying mantle. We thus anticipate that Turkana felsic lavas have originated through a
different suite of processes than those from Rungwe and southern Ethiopia, where crust is
much thicker.
Geological Setting
The Turkana Rift (oldest manifestation of volcanic activity ~30-35 Ma) connects
the Gregory Rift of Kenya to the Gofa Basin and Range of Southern Ethiopian Dome
(Karson and Curtis 1994). It is approximately 200 km in length and up to ~150 km wide
and occupies a broad depression straddled between the Kenyan and Ethiopia Domes
(Karson & Curtis 1989). Localized alkalic volcanic activity in the late Oligocene (~40
Ma) was followed by widespread and voluminous basaltic flood volcanism in the
Miocene to Early Pliocene (<23 Ma) and continues through to presence (Karson & Curtis
1989). Turkana Rift seats between The Nubian Plate and Somalian Plate that currently
undergoing continental rifting. The eventual result of two plates pulling apart will be the
splitting of Africa and the formation of new ocean.
Regional geophysical and structural studies indicate that Turkana Region seats on
top of a series of elongated basins that decrease in age and depth from West to East
(Karson & Curtis). These basins appear to be structures that formed in the hanging wall
of major, east-dipping crustal detachments that flatten into the lower crust (Karson &
Curtis 1989). Under Lake Turkana lies the youngest of these basins. Rocks analyzed in
this study came primarily from the most recent volcanic episode as sampled on North,
South Islands, Central and the Barrier.
Sample Selection
Data used in this study were acquired from published and unpublished sources.
These studies were conducted on Turkana Islands (North, South, Barrier and Central
Island; Furman and Curtis unpublished data), the Ethiopian Main Rift (Ayalew, 2002)
and the Rungwe Province in southern Tanzania (Furman, unpublished data). These
samples were chosen for this study because of their well-documented geochemical
mineralogical data and can be compared easily across the region.
Methodology
This step involves data reorganization, manipulation and graphing. Normative
calculations of the thermodynamically most stable minerals expected to grow in each
sample. Samples were prepared for geochemical analysis, plotting of geochemical data,
and examining data to establish final analysis regarding relationship between samples’
location and chemical properties. As part of this research, there was extensive use of MS
excel worksheet to organize the volume of datasets that was available. Another key
method was the use of petrographic analysis to help in understanding chemical
characteristics of major elements such Si, Ti, Al, Mg, Ca, Na, K and Cr.
Sample Preparation
There are various steps that are necessary in order to prepare samples for analysis.
They involve selecting desired samples for cutting using brick saw. It is important to
keep samples labeled to avoid any contamination. A sample rock is cut at about 0.5-1.0
cm width for easy grinding at a later stage. It is then sanded to rid it of any saw blade
marks that could contaminate its chemical composition and it is then chipped using rock
chipper to approximately 1-4 mm fine fragments that are then picked for petrographical
analysis using light microscope. Prepared samples are then sent to Duke University
where they are heated using direct current (dc-plasma) for chemical analysis. Samples
emit light at certain frequencies which indicate the presence of elements and compound
in the sample. The chemical composition data of major and trace elements is then
collected and calculated using norm calculation program for corrected analysis and
normalized weight percentage data that is more representative and refined.
Results
The corrected analysis data from the Turkana samples yielded no normative
quartz except for the North Island samples which showed high quartz content. Ethiopia
samples on the other hand consistently exhibit 4.11-6.86 wt% quartz normative. This was,
however, not the case for Rungwe samples to the South of Turkana Rift which showed no
evidence of quartz. The graphical (fig. 1b) analysis of samples indicated a major break
indicative of the absence of intermediate rock or Daly Gap (Ayalew et al 2003). Major
element variations within the Rungwe and Ethiopia suites show an absence of
intermediate lava composition, known as a Daly Gap (Ayalew et al. 2003). In contrast,
the Turkana samples show a continuous distribution from silica-poor to silica-rich
composition (figure 1c).
Major and trace element variations (e.g. P2O5, Rb, Zr) plotted as a function of
MgO content show no analogous break. These variations show that chemical trends
among Turkana and Ethiopia samples are similar to one another, but different from
Rungwe lavas which show much greater enrichment in these lavas at low MgO contents
(figure 2a).
Discussion
From works done on Ethiopian Rift, the source of silicic melts, their relationship
with associated basalts and the nature of Daly Gap stands for several subalkaline domes
such as Iceland, Parana, and Deccan, Yemen etc. (Peccerillo, Barberio, Yirgu, Ayalew,
Barbieri and Wu, 2003). With the help of graph of major elements geochemical data from
the Turkana (N. Kenya), Ethiopia, and Tanzania, we accept the role of continuous
fractional crystallization starting from transitional basaltic parents, and (possibly with
role of crustal contamination) for lavas from Turkana and Ethiopia (Peccerillo et al.,
2003). This hypothesis is able to explain several geochemical and isotopic features of
mafic-silicic rocks. In contrast, we suggest that crustal melting plays a more important
role in the evolution of Rungwe silica-rich rocks. This hypothesis is able to explain
several geochemical features of both mafic and silicic rocks from these areas. The data
obtained in this study will be discussed with the aim of providing constraints on the
source of peralkaline rocks and to explore the implications for East African Rift
volcanism as well as overall constraints on the rifting process. The trends of SiO2 against
P2O5 indicated that Turkana samples showed (Fig. 4) increase in P2O5 content but
decrease >50 wt% which is indicative of the fractional crystallization of P2O5 as SiO2
approaches <55 wt.%.
Geochemical analysis and trends indicate that three regions with varying crustal
thickness: Ethiopia (~25 km), Turkana (~20 km), and Rungwe, Tanzania (~>35 km) have
varying geochemical signatures. Ethiopia and Turkana lavas have fairly the same
geochemical characteristics which is attributable to their nearly the same crustal thickness.
(Fig. 2) Shows that Turkana has undergone fractional crystallization because it exhibit a
linearly decrease in Alkalis content as SiO2 content increase at a checked phase.
Conclusion
The genesis of Turkana ultra-silicic rocks is through crystal fractionation which we were
able to ascertain in this study although a further study would be needed to firmly
establish this conclusion. Our findings were consistent with the initial hypothesis in that
fractional crystallization played a leading role in the evolvement of silica-rich lavas of
East African Rift System. Turkana and Ethiopia showed the geochemical characteristics
which means that they must be related by virtue of their crustal thickness.
References
Ayalew, D., Yirgu, G. & Pik, R. 1999. Geochemical and isotopic (Sr, Nd and Pb)
characteristics of volcanic rocks from southwestern Ethiopia. Journal of African
Earth Sciences, 29, 381-391.
Ayalew, D., Barbery, P., Marty, B., Reisberg, L., Yirgu, G. & Pik, R. Source, genesis and
timing of giant ignimbrite deposits associated with Ethiopian continental flood
basalts. Geochimica et Cosmochimica Acta, 66, 1429-1448, 2002.
Karson J. A. and Curtis C.P: “Quaternary volcanic centers of the Turkana Rift” Journal of
African Earth Sciences, vol. 18 No. 1, pp. 15-35, 1994
Peccerillo A., Barberio M. R., Yirgu G., Ayalew D., Barbier M., Wu T. W.,:
“Relationship between Magmatism in Continental Rift Settings: a Petrological,
Geochemical and Isotopic Study of the Gedemsa Volcano, Central Ethiopia Rift”:
Journal of Petrology Vol. 44, No. 11, pp. 2002-2032, 2003.
Ebinger C.J., Yemane T., Harding D.J., Tesfaye S., Kelley S., Rex D.C., : “Rift
deflection, migration, and propagation: Linkage of the Ethiopia and Eastern rifts,
Africa”, GSA Bulletin Vol.112; no. 2; pp. 163-167; February 2000
J.-P. Clement, M. Caroff, and J. Cotton, C. Hemond, J.-J. Tiercelin, and C. Bollinger;
“Pleistocene Magmatism in lithospheric transition area: petrogenesis of alkaline
and peralkaline lavas from Baringo-Bogoria Basin, central Kenyan Rift” Canadian
Journal of Earth Sciences, Vol. 40, pp. 1239-1257, 2003.
Fig. 1a Tectonic setting of East African Rift System. Lake Turkana lies in northern
Kenya within the half grabben depression between Ethiopian Dome and East African
Dome
Fig. 1b. Show is the location of Lake Turkana where most of our samples came from
Fig. 1c. Show the graph of [Alkalis] vs. SiO2 for Turkana Islands. Note that the trend is
disrupted by Daly Gap (Discussed above) is bridged by Turkana samples.
18
16
Rungwe
14
Ethiopia
Alkalis
12
Turkana
10
8
6
4
2
0
40
50
60
SiO2
70
80
Corrected Analysis data used
Fig. 2a. MgO vs. trace element Rb shows variations within Turkan and Ethiopia against
the Rungwe
300
Rb (ppm)
250
Rungwe
200
150
100
Ethiopia
50
0
0
Turkana
5
10
MgO
15
Fig. 3. Shows SiO2 vs. P2O5 which is indicates that as SiO2 content increase; the P2O5
decrease substantiates crystallization
2.5
P2O5
2.0
1.5
Fractionation
1.0
0.5
0.0
40
50
60
SiO2
70
80