The Origin of Felsic Lavas in the East African Rift

Transcription

1 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.

2 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.

3 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

4 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 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

5 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 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

6 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

7 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.

8 References Ayalew, D., Yirgu, G. & Pik, R Geochemical and isotopic (Sr, Nd and Pb) characteristics of volcanic rocks from southwestern Ethiopia. Journal of African Earth Sciences, 29, 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, , Karson J. A. and Curtis C.P: Quaternary volcanic centers of the Turkana Rift Journal of African Earth Sciences, vol. 18 No. 1, pp , 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 , 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 ; 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 , 2003.

9 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

10 Fig. 1b. Show is the location of Lake Turkana where most of our samples came from

11 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. Alkalis Rungwe Turkana Ethiopia SiO2 Corrected Analysis data used Fig. 2a. MgO vs. trace element Rb shows variations within Turkan and Ethiopia against the Rungwe 300 Rb (ppm) Rungwe Ethiopia Turkana MgO

12 Fig. 3. Shows SiO2 vs. P2O5 which is indicates that as SiO2 content increase; the P2O5 decrease substantiates crystallization P2O Fractionation SiO2