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Open System Evolution of Peralkaline Trachyte and Phonolite from the Suswa Volcano, Kenya Rift

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Open System Evolution of Peralkaline Trachyte and Phonolite from the Suswa Volcano, Kenya Rift Eastern Kentucky University Encompass EKU Faculty and Staff choS larship 11-2012 Open system evolution of peralkaline trachyte and phonolite from the Suswa volcano, Kenya Rift. John C. White Eastern Kentucky University, [email protected] Vanessa V. Espejel-Garcia Universidad Autonoma de Chihuahua Elizabeth Y. Anthony University of Texas at El Paso Peter Omenda Geothermal Research Corporation, Kenya Follow this and additional works at: http://encompass.eku.edu/fs_research Part of the Geology Commons, and the Volcanology Commons Recommended Citation White, J.C., Espejel-García, V.V., Anthony, E.Y., and Omenda, P., 2012, Open system evolution of peralkaline trachyte and phonolite from the Suswa volcano, Kenya Rift. nI : Peralkaline Rocks and Carbonatites with Special Reference to the East African Rift G( . Marks, M.A.W. Markl, and A. Zaitsev, Eds.) Lithos, v. 152, p. 84-104. (doi: 10.1016/j.lithos.2012.01.023) 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]. This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and education use, including for instruction at the authors institution and sharing with colleagues. Other uses, including reproduction and distribution, or selling or licensing copies, or posting to personal, institutional or third party websites are prohibited. In most cases authors are permitted to post their version of the article (e.g. in Word or Tex form) to their personal website or institutional repository. Authors requiring further information regarding Elsevier’s archiving and manuscript policies are encouraged to visit: http://www.elsevier.com/copyright Author's personal copy Lithos 152 (2012) 84–104 Contents lists available at SciVerse ScienceDirect Lithos journal homepage: www.elsevier.com/locate/lithos Open System evolution of peralkaline trachyte and phonolite from the Suswa volcano, Kenya rift John Charles White a,⁎, Vanessa V. Espejel-García b,1, Elizabeth Y. Anthony b, Peter Omenda c a Department of Geography & Geology, Eastern Kentucky University, 521 Lancaster Ave., Roark 103, Richmond, KY 40475, USA b Department of Geological Sciences, University of Texas at El Paso, 500 W. University Ave., El Paso, TX 79968, USA c Geothermal Development Company, Taj Tower, Upper Hill, P.O. Box 10076–00101, Nairobi, Kenya article info abstract Article history: Suswa is the southernmost volcanic center in the Central Kenya Peralkaline Province (CKPP) and represents Received 14 October 2011 the only salic center to have erupted significant volumes of peralkaline silica-undersaturated lavas and tuffs Accepted 25 January 2012 (trachyte, nepheline trachyte and phonolite). The eruptive products of Suswa can be clearly divided into two Available online 3 February 2012 series, which correspond closely to the volcano's eruptive history. The earlier series (C1) includes lavas and tuffs that built the initial shield volcano (pre-caldera, unit S1) and erupted during the first caldera collapse Keywords: (syn-caldera, units S2–S5); these rocks are dominated by peralkaline, silica-saturated to mildly under- Trachyte Phonolite saturated trachyte. The later series (C2) includes lavas and tuffs that erupted within the caldera structure Central Kenya Peralkaline Province following the initial collapse (post-caldera, units S6–S7) and during the creation of a second smaller, nested caldera and central “island block” (ring trench group, RTG, unit S8); these rocks are dominated by peralkaline phonolite. In this study, we combine mineralogical evidence with the results of major-element, trace- element, and thermodynamic modelling to propose a complex model for the origin of the Suswa volcano. From these results we conclude that C1 is the result of protracted fractional crystallization of a fairly “dry” alkali basalt (b1 wt.% H2O) under relatively high pressure (400 MPa) and low oxygen fugacity (FMQ to FMQ-1). Although C1 appears to be primarily the result of closed system processes, a variety of open system processes are responsible for C2. We propose that crystallization of C1 trachyte resulted in the formation of a syenitic residue, which was assimilated (Ma/Mc=0.1) during a later stage of recharge and differentiation of alkali basalt to produce post-caldera ne-trachyte. Post-caldera (S6-7) phonolites were in turn the result of fractional crystallization of this ne-trachyte. RTG phonolites, however, are the result of feldspar resorption prompted perhaps by magma recharge as evidenced by reverse zoning in alkali feldspar and linear compatible trace element patterns. © 2012 Elsevier B.V. All rights reserved. 1. Introduction (~1 km) amounts of uplift. The dome is apparently in isostatic equilib- rium, being supported by the loading of anomalous mantle within the Mount Suswa is a large (>700 km2) trachytic shield volcano with underlying lithosphere (Smith, 1994). Salic volcanism in the CKPP has two nested summit calderas that erupted peralkaline trachyte and pho- been focused on five centers. These volcanic centers include: (a) two nolite lavas and tuffs from approximately 240 to b100 ka (Baker et al., dome fields: the Eburru Volcanic Complex composed of trachyte and 1988; Johnson, 1969; Nash et al., 1969; Skilling, 1988, 1993). Suswa pantellerite (Omenda, 1997; Ren et al., 2006; Velador et al., 2003), is the southernmost volcano within the Central Kenya Peralkaline and the Greater Olkaria Volcanic Complex (GOVC), dominated by comen- Province (CKPP), an area within the Kenyan branch of the east African dites with minor outcrops of trachyte (Clarke et al., 1990; Macdonald et rift system (EARS) between approximately 0° and 1.5°S latitude al., 1987, 2008a; Marshall et al., 2009; Omenda, 1997); and (b) three (Fig. 1a; Macdonald and Baginski, 2009; Macdonald and Scaillet, trachytic caldera volcanoes: Menengai with a comenditic to pantelleritic 2006). The location of the CKPP coincides with the apical region of the trachyte composition (Leat et al., 1984; Macdonald et al., 2011a), Kenya dome, an area of crustal upwarping associated with minor Longonot composed of pantelleritic trachyte and mixed (trachyte/ hawaiite) lavas (Clarke et al., 1990; Heumann and Davies, 2002; Rogers et al., 2004; Scott and Bailey, 1984), and Suswa, with trachyte and phono- ⁎ Corresponding author. Tel.: +1 859 622 1276; fax: +1 859 622 3375. lite lavas and tuffs (Johnson, 1969; Nash et al., 1969; Scott and Skilling, E-mail addresses: [email protected] (J.C. White), [email protected] 1999; Skilling, 1988, 1993). The maficlavafields Elmenteita, Ndabibi, (V.V. Espejel-García), [email protected] (E.Y. Anthony), [email protected] and Tandamara, lie in the rift floor adjacent to Eburru, Olkaria and (P. Omenda). 1 Present address: Facultad de Ingeniería, Universidad Autónoma de Chihuahua, Suswa, respectively, and are composed of alkali and transitional basalt, Circuito No. 1, Campus Universitario 2, C.P. 31125, Chihuahua, Chih., México. basaltic trachyandesite and trachyandesite. 0024-4937/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.lithos.2012.01.023 Author's personal copy J.C. White et al. / Lithos 152 (2012) 84–104 85 Fig. 1. (a) Map of the Central Kenya Peralkaline Province (CKPP; Macdonald and Scaillet, 2006). The subject of this study, Suswa, is the southernmost volcano in the CKPP. (b) Ternary diagram indicating silica saturation for CKPP volcanic centers. GOVC, Greater Olkaria Volcanic Center; S1–S5 Suswa pre-and syn-caldera units (also referred to as C1 in this paper); S6-S8 Suswa C2 units. See Table 1 and text for further information. The Suswa volcanic system includes silica-saturated (hypersthene- the result of protracted fractional crystallization of Ndabibi-like alkali normative [hy-]trachyte) and silica-undersaturated (nepheline- basalt and the silica-undersaturated suite (C2) primarily the result of normative [ne-]trachyte and phonolite) lavas and tuffs, and represents coupled assimilation-fractional crystallization from a similar parent the only salic volcanic system in the CKPP not dominated by silica- (with the syenitic residue from C1 petrogenesis serving as the assimi- oversaturated materials (quartz-normative [q-]trachyte, comendite, lant) plus feldspar resoprtion triggered by magma recharge. This com- and pantellerite) (Fig. 1b). Coexisting oversaturated and undersaturat- plexity reflects the open-system behavior of EARS volcanoes, where ed alkaline igneous suites have also been described from several intra- mafic dikes periodically intrude compositionally evolved shallow plate magmatic provinces, including the Massif Central, France magma chambers. Examples of this phenomenon occurred in 2005 in (Wilson et al., 1995); the Cameroon line, west Africa (Fitton, 1987); the Afar region of Ethiopia, where an episode of recharge/diking opened the Marie Byrd Land province, west Antarctica rift system (LeMasurier a 60 km rupture in the crust (Sigmundsson, 2006; Wright et al., 2006; et al., 2003, 2011; Panter et al., 1997); the Canary Islands (Carracedo Yirgu et al., 2006), and in 2007, when a magma-driven earthquake et al., 2002); and the Trans-Pecos Magmatic Province, Texas USA swarm struck northern Tanzania for a period of two months and trig- (Potter, 1996; White and Urbanczyk, 2001). In each of these provinces, gered an eruption at Oldoinyo
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