Eastern Kentucky University Encompass EKU Faculty and Staff choS larship 10-2006 Application of the QUIlF thermobarometer to the peralkaline trachytes and pantellerites of the Eburru volcanic complex, East African Rift, Kenya. MInghua Ren University of Texas at El Paso Peter Omenda Geothermal Development Corporation, Kenya Elizabeth Y. Anthony University of Texas at El Paso John C. White Eastern Kentucky University, [email protected] Ray Macdonald Lancaster University See next page for additional authors Follow this and additional works at: http://encompass.eku.edu/fs_research Part of the Geochemistry Commons, and the Geology Commons Recommended Citation Ren, M., Omenda, P.A., Anthony, E.Y., White, J.C., Macdonald, R., and Bailey, D.K., 2006, Application of the QUIlF thermobarometer to the peralkaline trachytes and pantellerites of the Eburru volcanic complex, East African Rift, Kenya. In: Peralkaline Rocks: A Special Issue Dedicated to Henning Sørensen, PERALK2005 Workshop (G. Markl, Ed.) Lithos, v. 91, p. 109-124. (doi: 10.1016/ j.lithos.2006.03.011) This Article is brought to you for free and open access by Encompass. It has been accepted for inclusion in EKU Faculty and Staff choS larship by an authorized administrator of Encompass. For more information, please contact [email protected]. Authors MInghua Ren, Peter Omenda, Elizabeth Y. Anthony, John C. White, Ray Macdonald, and D K. Bailey This article is available at Encompass: http://encompass.eku.edu/fs_research/199 Lithos 91 (2006) 109–124 www.elsevier.com/locate/lithos Application of the QUILF thermobarometer to the peralkaline trachytes and pantellerites of the Eburru volcanic complex, East African Rift, Kenya ⁎ Minghua Ren a, , Peter A. Omenda b, Elizabeth Y. Anthony a, John C. White c, Ray Macdonald d, D.K. Bailey e a Department of Geological Sciences, University of Texas at El Paso, TX 79968, USA b Olkaria Geothermal Project, P.O. Box 785, KenGen, Moi South Lake Road, Naivasha 20117, Kenya c Department of Earth Sciences, Eastern Kentucky University, Richmond, KY 40475, USA d Environment Centre, Lancaster University, Lancaster LA1 4YQ, UK e Department of Earth Sciences, University of Bristol, Bristol BS8 1RJ, UK Received 11 July 2005; accepted 13 March 2006 Available online 10 July 2006 Abstract The Quaternary Eburru volcanic complex in the south-central Kenya Rift consists of pantelleritic trachytes and pantellerites. The phenocryst assemblage in the trachytes is sanidine+fayalite+ferrohedenbergite+aenigmatite±quartz±ilmenite±magnetite ± pyrrhotite±pyrite. In the pantellerites, the assemblage is sanidine+quartz+ferrohedenbergite+fayalite+aenigmatite+ferrorichter- ite+pyrrhotite±apatite, although fayalite, ferrohedenbergite and ilmenite are absent from more evolved rocks (e.g. with SiO2 N71%). QUILF temperature calculations for the trachytes range from 709 to 793 °C and for the pantellerites 668–708 °C, the latter temperatures being among the lowest recorded for peralkaline silicic magmas. The QUILF thermobarometer demonstrates that the Eburru magmas crystallized at relatively low oxidation states (ΔFMQ +0.5 to −1.6) for both trachytes and pantellerites. The trachytes and pantellerites evolved along separate liquid lines of descent, the trachytes possibly deriving from a more mafic parent by fractional crystallization and the pantellerites from extreme fractionation of comenditic magmas. © 2006 Elsevier B.V. All rights reserved. Keywords: Kenya; Eburru volcanic complex; Trachyte; Pantellerite; Peralkaline; QUILF 1. Introduction salic magmas, they are enriched in FeO*, Na2O, HFSE, REE and halogens, and are relatively low in Al2O3,CaO, Peralkaline magmas form mainly in extensional P2O5,SrandBa(Noble, 1968; Macdonald, 1974). Trace environments and hot spots. Compared to metaluminous element and isotopic characteristics of peralkaline silicic rocks are most commonly interpreted to show that the ⁎ Corresponding author. Tel.: +1 915 747 5843; fax: +1 915 747 magmas are ultimately mantle-derived, either by fraction- 5073. ation of basaltic magmas (Barberietal.,1975;Baconetal., E-mail addresses: [email protected] (M. Ren), 1981; Harris, 1983; Novak and Mahood, 1986; Bloomer et [email protected] (P.A. Omenda), [email protected] (E.Y. Anthony), [email protected] (J.C. White), al., 1989; Caroff et al., 1993; Civetta et al., 1998; Kar et al., [email protected] (R. Macdonald), 1998) or by remelting of underplated mafic rocks (Bailey [email protected] (D.K. Bailey). and Schairer, 1966; Mahood et al., 1990; Lowenstern and 0024-4937/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.lithos.2006.03.011 110 M. Ren et al. / Lithos 91 (2006) 109–124 Mahood, 1991; Frost and Frost, 1997; Bohrson and Reid, rocks of the Proterozoic Gardar province (Markl et al., 1997, 1998; White et al., 2006-this volume). An exception 2001a,b; Marks and Markl, 2001; Marks et al., 2003). are the comendites of the Greater Olkaria Volcanic Complex (GOVC) in the south-central Kenya rift, where 2. Geological setting of the volcanoes of the Kenya Pb isotopes and U-series disequilibria and relatively high Dome LILE/HFSE ratios have been inferred to indicate an origin by crustal anatexis (Davies and Macdonald, 1987; The Kenya Rift is the segment of the East African Rift Macdonald et al., 1987; Black et al., 1997; Heumann System that extends from the Ethiopia–Kenya border into and Davies, 2002). However, the extreme depletion of Ba northern Tanzania. Geophysical studies have established and Sr in the Greater Olkaria Volcanic Complex (b5and that the depth to Moho varies from 20 km beneath b1 ppm, respectively) is not easily explained by anatexis Turkana in the north to 35 km beneath the Kenya Dome andseemstorequireanextendedperiodofcrystal (center) to 38 km in northern Tanzania (Keller et al., 1994; fractionation of the parental magmas, whatever their origin Mechie et al., 1997). The increased thickness of crust in (Davies and Macdonald, 1987; Heumann and Davies, the Kenya Dome reflects a 7 km thick lower crustal layer 2002). that has a P-wave velocity of 6.8 km/s. P-wave velocity in Petrogenetic information for individual systems can the uppermost mantle varies only slightly from 7.6 to be provided through the determination of magma 7.5 km/s in the Turkana to Kenya Dome segment. Seismic intensive parameters and mineral and whole-rock geo- velocity for the southernmost segment derives from an E– chemistry, which place constraints on parameters such W profile that passed through the Magadi area and thus is as pre-eruptive volatile concentrations of the magma less well constrained. A conservative estimate is 7.8 km/s ( f O2, f H2O) and magma temperature (Markl et al., (Birt et al., 1997). The existence of higher velocities south 2001a, 2003; Marks and Markl, 2001; Newman and of the Kenya Dome is consistent with gravity studies Lowenstern, 2002; White et al., 2005). Magma (Simiyu and Keller, 1997, 2001) that model a change in temperatures, for instance, provide a means to evaluate mantle densities from 3120 kg m−3 under the Kenya the competing hypotheses of fractional crystallization Dome to 3260 kg m− 3 in northern Tanzania. The from mantle-derived rocks vs. crustal anatexis. Since geophysical data thus imply that the Kenya Dome sits magmas as a rule do not achieve superheated conditions above a transition to thicker lithosphere as the Tanzanian (but rather generate more magma given more heat to the Craton is approached. East–west seismic profiles indicate system), temperatures substantially in excess of the a steep-sided velocity gradient between the low P-wave likely range of solidus temperatures for crustal rocks are velocities in the upper mantle of the axial region and more reasonably interpreted as resulting from crystalli- velocities of 8.1 km/s on the eastern flanks (Byrne et al., zation from mantle-derived magmas. On the other hand, 1997). Finally, the location of the Kenya Rift is temperatures at approximately the solidus for crustal structurally controlled; rift faults exploit weaknesses at compositions are permissive of crustal anatexis. Like- the contact between the Archean Tanzanian Craton to the wise, initial magma composition is often inherited or west and Proterozoic orogenic belts to the east (Smith and buffered by the oxidation state of the protolith, and thus Mosley, 1993; Smith, 1994; Stern, 1994). low oxidation states reflect direct derivation from A series of short wavelength gravity highs are mantle melting or anatexis of mantle-derived source superposed on the broad negative anomaly in the axial rocks (Frost and Frost, 1997; Anthony, 2005). region of the Kenya Rift. Swain (1992) modeled these Peralkaline rocks tend to lack mineral assemblages gravity highs as resulting from pervasive dike injection to a appropriate for determining intensive parameters; for depth of 22 km. The gravity interpretation of density bodies example, coexisting Fe–Ti oxides are rare in peralkaline is corroborated by seismic data from the KRISP experi- rhyolites. As a result, very few quantitative data for these ments (Simiyu and Keller, 1997, 2001). Simiyu and Keller parameters are available (Scaillet and Macdonald, 2001). modeled the high density bodies and their interpretation is However, the Eburru volcanic complex, which neigh- that dense mafic intrusions underlie the volcanic complexes bours the Greater Olkaria Volcanic Complex in the Kenya of Menengai, Eburru, Olkaria, and Suswa. The bodies rift, contains rocks with phenocryst assemblages contain- occur in the upper crust, perhaps as shallow as 7 to 12 km ing fayalite, clinopyroxene, ilmenite, and quartz, making and have densities of about 2900 kg/m3. This interpretation the use of the QUILF thermometer and oxygen barometer coincides with the interpretation by Swain (1992),who appropriate (Lindsley and Frost, 1992; Frost and Lindsley, envisaged each of the young (b2 Ma) complexes in the 1992). The application of QUILF is a similar approach to KenyaDomeasrepresentinganexus,wherearegionaldike that used recently in a series of papers on the peralkaline swarm has developed a shallow reservoir.