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 SiO 2 N 71%). 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 Peralkaline magmas form mainly in extensional environments and hot spots. Compared to metaluminous salic magmas, they are enriched in FeO*, Na 2 O, HFSE, REE and halogens, and are relatively low in Al 2 O 3 , CaO, P 2 O 5 , Sr and Ba (Noble, 1968; Macdonald, 1974). Trace element and isotopic characteristics of peralkaline silicic rocks are most commonly interpreted to show that the magmas are ultimately mantle-derived, either by fraction- ation of basaltic magmas (Barberi et al., 1975; Bacon et al., 1981; Harris, 1983; Novak and Mahood, 1986; Bloomer et al., 1989; Caroff et al., 1993; Civetta et al., 1998; Kar et al., 1998) or by remelting of underplated mafic rocks (Bailey and Schairer, 1966; Mahood et al., 1990; Lowenstern and Lithos 91 (2006) 109 – 124 www.elsevier.com/locate/lithos ⁎ Corresponding author. Tel.: +1 915 747 5843; fax: +1 915 747 5073. E-mail addresses: ren@geo.utep.edu (M. Ren), pomenda@kengen.co.ke (P.A. Omenda), eanthony@utep.edu (E.Y. Anthony), John.White@eku.edu (J.C. White), r.macdonald@lancaster.ac.uk (R. Macdonald), Ken.Bailey@bristol.ac.uk (D.K. Bailey). 0024-4937/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.lithos.2006.03.011